Molecular Interactions Driving the Layer-by-Layer ... - ACS Publications

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Molecular Interactions Driving the Layer-by-Layer Assembly of Multilayers Joaõ Borges*,†,‡ and Joaõ F. Mano*,†,‡ †

3B’s Research GroupBiomaterials, Biodegradables and Biomimetics, University of Minho, Headquarters of the European Institute of Excellence on Tissue Engineering and Regenerative Medicine, AvePark, Zona Industrial da Gandra, S. Cláudio do Barco 4806-909 Caldas das Taipas, Guimarães, Portugal ‡ ICVS/3B’s − PT Government Associate Laboratory, Braga/Guimarães, Portugal 6.1. Effect of Hydrogen Bonding Donor and Acceptor Groups on the Amount of Adsorbed Layers and on the Responsive Properties of Assembled Systems 6.2. Halogen Bonding as an Alternative to Hydrogen Bonding for LbL Assembly 6.3. Computational Simulation Studies to Understand LbL Assembly Mechanism and Investigate the Structure and Morphology of LbL Multilayer Assemblies 7. LbL Assembly Driven by Charge-Transfer Interactions 7.1. Influence of Nonionic Molecules on the Growth and Properties of Multilayer Assemblies 8. LbL Assembly via Host−Guest Interactions 8.1. Influence of Several Host and Guest Molecules on the Assembly, Growth, and Responsive Properties of Multilayer Systems 8.2. Computational Simulation Approaches to Reveal LbL Assembly Mechanism and Investigate the Structure, Conformation, and Dynamics of LbL Multilayer Assemblies 9. LbL Assembly Driven by Biologically Specific Interactions 9.1. Avidin−Biotin Interactions to Build Up and Grow Multilayer Assemblies 9.2. Antibody−Antigen Interactions to Build Up and Grow Multilayer Assemblies 9.3. Lectin−Carbohydrate Interactions to Assemble and Grow Multilayer Assemblies 9.4. DNA Hybridization to Assemble and Grow Multilayer Assemblies 10. LbL Assembly through Coordination Chemistry Interactions 10.1. Solution-Based LbL Growth Methods 10.2. Vapor-Based LbL Growth Methods 10.2.1. ALD Growth Method 10.2.2. MLD Growth Method 10.2.3. Combination of ALD and MLD Growth Methods 11. LbL Assembly Based on Covalent Bonding 11.1. Fabrication of Highly Stable, Robust, and Functional Polymeric Multilayer Assemblies

CONTENTS 1. Introduction 2. Brief Overview of Bottom-Up Approaches 3. Comprehensive Overview of Layer-by-Layer Assembly Approach 4. LbL Assembly via Electrostatic Interactions 4.1. Understanding LbL Growth Mechanisms: Linear versus Exponential Growth 4.2. Switching Between LbL Growth Mechanisms: From Linear to Exponential Growth and Vice Versa 4.3. Parameters Controlling the LbL Growth, Structure, and Properties of Multilayer Assemblies 4.3.1. Effect of Solution pH 4.3.2. Effect of Temperature 4.3.3. Effect of Ionic Strength and Electrolyte Species 4.3.4. Effect of Solvent Quality and Adsorption Time 4.3.5. Effect of Polyelectrolyte Charge Density 4.3.6. Effect of Polyelectrolyte Molecular Weight 4.3.7. Effect of Polyelectrolyte Chain Architecture 4.4. Computational Simulation Strategies as Tools to Understand LbL Assembly Mechanism and Investigate the Structure, Stability, and Dynamics of LbL Multilayer Assemblies 5. LbL Assembly through Hydrophobic Interactions 5.1. Influence of Density of Hydrophobic Groups on Adsorbed Layers 5.2. Computational Simulation Approaches to Unravel LbL Assembly Mechanism and Investigate the Structure, Dynamics, and Morphology of LbL Multilayer Assemblies 6. LbL Assembly Based on Hydrogen Bonding

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Received: September 30, 2013 Published: August 20, 2014 © 2014 American Chemical Society

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Chemical Reviews 12. LbL Assembly via Stereocomplexation 12.1. Fabrication of LbL Poly(methacrylate) Stereocomplex Ultrathin Films and Hollow Capsules 12.2. Fabrication of LbL Poly(lactide) Stereocomplex Ultrathin Films and Hollow Capsules 13. LbL Assembly via Surface Sol−Gel Process 13.1. Preparation of LbL Metal Oxide Ultrathin Films and Metal Oxide-Based Nanocomposites 14. Conclusions and Future Perspectives Author Information Corresponding Authors Notes Biographies Acknowledgments List of Acronyms and Abbreviations References Note Added after ASAP Publication

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through the assembly of an unprecedented choice of materials. Moreover, its advantages, disadvantages, and capability to address several potential applications in the near future are also discussed (section 3). As the purpose of this Review, a systematic overview of the different types of intermolecular interactions driving the fabrication of multilayer assemblies using the LbL assembly technology is given in sections 4−13. Several examples are given of multilayer assemblies fabricated from electrostatic, hydrophobic, charge-transfer, host−guest, coordination chemistry, and biologically specific interactions, as well as from hydrogen bonding, covalent bonding, stereocomplexation, and surface sol−gel process. Moreover, the potential impact of the materials assembled by each type of interaction on the creation of advanced devices for promising practical applications is also discussed. In the last section, we provide a brief outcome of the current status, developments, and future challenges and perspectives of the prominent LbL assembly technology.

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2. BRIEF OVERVIEW OF BOTTOM-UP APPROACHES The design and fabrication of nanostructured functional materials through bottom-up surface modification methods has been a strategy widely used by the scientific and engineering communities to develop novel and promising two- (2D) and three-dimensional (3D) structures with desired properties (e.g., mechanical, thermal, electrical, optical, and catalytic) for a miscellaneous range of emerging applications from biology, medicine, and biotechnology to electronics, optics, catalysis, or energy. Bottom-up methods, which rely on the spontaneous selfassembly of small molecular components into more complex, larger, and functional 2D or 3D structures atom-by-atom or molecule-by-molecule have already been extensively explored by researchers from different research areas. Therefore, and because the purpose of this Review is not to describe the different bottom-up fabrication methods to achieve surface modification and functionalization, the reader is referred to some excellent reviews on this subject.4,5,11−17 Nevertheless, a brief description and comparison of the most common bottom-up surface engineering technologies is provided in this Review along with their main advantages and disadvantages. Bottom-up nanofabrication methods include Langmuir−Blodgett (LB)18−29 and self-assembled monolayers (SAMs).30−47 Both methods, which enable the fabrication of closely packed, well-ordered, and organized monolayers and allow for the immobilization of several functional molecules onto surfaces, present some drawbacks that limit their practical applications.14,48 In the case of the LB method, the expensive and specialized instrumentation, the long construction times, and the need for specific and limited molecules, namely, amphiphilic molecules, to prepare the films represent great shortcomings that limit its applicability. Moreover, although this method allows us to prepare multilayer films from oriented monolayers, the absence of a strong molecular interaction between the film and the solid support (no chemisorption is involved during the formation of the LB films) is a problem due to its mechanical instability, which limits the stability and robustness of the films under ambient and physiological conditions and, thus, makes difficult the transfer of the molecules of the film from the air−water interface to the solid support. This makes the process very slow and limits the incorporation of biological molecules into the films. On the other hand, the main disadvantages of SAMs are also the limited stability and robustness of the films under ambient and physiological conditions, the limited loading of biological

1. INTRODUCTION Over the last few decades there has been a huge interest in nanostructured functional materials and assembly techniques for preparing functional molecular assemblies with tunable compositions, structures, and enhanced properties. The welldefined control of the properties of the surface and directed assembled materials (e.g., biological macromolecules and inorganic materials) plays a key role in the design and creation of enhanced functional nanostructured and nanocomposite materials so commonly found in chemistry, physics, biology, medicine, engineering, or biotechnology. In this concern, the properties of the bulk materials, which necessarily differ from the physicochemical properties of nanostructured materials created by several surface modification techniques, and the ability to precisely tailor nanoscale surface properties such as polarity, energy, charge, and physical features such as morphology, topology, and roughness have been recognized as being of extreme importance, playing a key role in a wide range of research fields such as biomaterials, biosensing, drug/gene delivery, tissue engineering, implantable materials, diagnostics, detection, electronics, energy, and optics.1−3 Thus, significant efforts have been made over the years to develop bottom-up approaches that can be used to tailor in an effective way the surface properties by choosing the appropriate functionalities and to design functional molecular assemblies with precisely layered structures, compositions, and properties at the molecular level.4−14 In this Review, we focus our attention and systematically overview the different types of intermolecular interactions behind the fabrication of multilayer assemblies using the layerby-layer (LbL) assembly approach. Moreover, we also comment on the potential impact of each type of intermolecular interaction and materials assembled through them on the development of advanced functional systems or devices for several emerging applications. This Review begins with a brief overview of the most commonly used bottom-up methods to modify surfaces and fabricate functional multilayer thin films, with a special focus on their main advantages and disadvantages (section 2). Following this section, we provide a deep overview of LbL assembly approach and highlight its importance as a simple, versatile, flexible, and prominent bottom-up strategies to modify surfaces and fabricate novel and robust multifunctional systems 8884


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uniform LbL films due to the significant reduction of time and material needed for the deposition of each layer as well as to the large surface coatings being thus more suitable for industrial applications.91−96 Nevertheless, other promising deposition methods such as hydrodynamic-dip-coating,97,98 high-gravity field-99 and inkjet printing-assisted LbL,100−104 and LbL deposition on spherical particles105−109 have also been reported for the fabrication of multilayer assemblies. These deposition methods meaningfully improve the efficiency of the LbL assembly process and extend the fabrication of multilayer assemblies from simple 2D flat platforms to more convoluted 3D surfaces. LbL assembly can be performed in any type of substrate (e.g., planar, porous, colloidal particles, and cylindrical structures) of almost any surface chemistry (regardless of size and shape) and in confined environments, and it allows the incorporation of a large range of building blocks such as polymers, peptides, carbon nanotubes, clays, dyes, metal oxides,110−129 and other components including particles,130−138 nucleic acids,139−143 proteins,144−153 enzymes,154−156 and viruses157,158 into the multilayer films to add new functionalities and capabilities.159 Moreover, the assembly process can occur at room temperature, is relatively cheap (requiring only simple laboratory equipment), allows the integration of a large amount of biomolecules in the films, and can be performed under mild conditions with several inexpensive materials in entirely aqueous solutions requiring no exposure to organic and harmful solvents, high temperatures, or extreme pH values, being thus considered an environmentally friendly fabrication process. This is a very important feature especially when dealing with biomolecules, such as nucleic acids, polypeptides, polysaccharides, proteins, enzymes, viruses, and even cells, which have limited solubility in nonaqueous solutions and are susceptible to denaturation (i.e., loss of biological activity).160,161 The possibility of changing the nature of the building blocks that are deposited during each step also permits one to have a great control over the composition of the film along the vertical direction. Therefore, by comparing the LbL assembly approach with the above-mentioned bottom-up approaches, it seems clear that the LbL assembly technology is a much simpler, cost-effective, flexible, and versatile approach for modifying surfaces and fabricating robust multilayer assemblies, surpassing several limitations imposed by the former described techniques. However, as previously mentioned, LbL assembly technique also presents some constraints in terms of time and amount of materials needed for each deposition step, mainly when building up multilayer assemblies with a huge number of layers by dipassisted LbL assembly. Nonetheless, spin- and mainly sprayassisted LbL assembly approaches constitute valuable alternative deposition methods that tend to overcome such limitations. Moreover, the unprecedented choice of surfaces, the wide range of materials, the experimental assembly conditions, and the several different types of intermolecular interactions involved in the multilayer assembly process constitute key parameters that influence the LbL assembly process. Consequently, it is difficult to define and establish a priori the best assembly features for achieving a desired target, mainly for less used, tested, and explored materials and/or surfaces, thus deserving an in-depth preliminary screening process by combining several techniques, such as circular dichroism (CD), Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR), UV−visible (UV−vis) spectroscopy, X-ray diffraction (XRD), quartz crystal microbalance (QCM), surface plasmon resonance (SPR) spectroscopy, ellipsometry, X-ray reflectometry (XRR),

molecules in the films due to their monolayer nature, and the need for the presence of specific compounds, namely, thiols (in the case of noble metals surfaces), silane (in the case of titanium, silicon, and aluminum oxide surfaces), or organic acid (in the case of several metal or metal oxide surfaces) molecules, on the substrate in order to allow the formation of the monolayers. However, other surface engineering methods such as polymer grafting,49−53 thin polymer network films,54−59 pulsed laser ablation,60 plasma61,62 or chemical vapor deposition63−67 have also been employed to design functional thin films on solid substrates and to improve surface characteristics. Nevertheless, all these approaches have limitations, in terms of costs of the instrumentation and production procedures, number and variety of materials that can be assembled, and surfaces where the materials are deposited. Therefore, great efforts have been devoted in the last few decades to develop simple, inexpensive, and versatile surface modification tools that would enable the modification of surfaces as well as the fabrication of robust functional multilayer assemblies on virtually any kind of surface by engineering an unlimited number and variety of materials. In this regard, the layer-by-layer (LbL) assembly technology has emerged as a simple, cost-effective, flexible, and versatile bottomup strategy to modify surfaces and prepare functional multilayer assemblies with high capability to address several emerging applications, as described in the following section.

3. COMPREHENSIVE OVERVIEW OF LAYER-BY-LAYER ASSEMBLY APPROACH LbL assembly is an easy, efficient, reproducible, robust, flexible, and extremely versatile way of modifying surfaces and fabricating highly ordered nanostructured polymeric multilayer thin films and nanocomposites with tailored thicknesses, compositions, structures, properties, and functions over any type of substrate. Although this prominent method was first proposed by Iler in 1966,68 who reported for the first time the fabrication of multilayer films on solid surfaces through the sequential adsorption of positively and negatively charged colloidal particles, its importance was only recognized after the pioneering work of Decher and Hong on polyelectrolyte multilayers (PEMs) in the early 1990s.69 Since then, there has been a tremendous interest in this method from both the fundamental and practical points of view.48,70−77 LbL assembly consists of the sequential adsorption of complementary multivalent molecules on a surface, occurring either via electrostatic or nonelectrostatic interactions, as will be described later in this Review. To build an effective and reproducible film, washing steps are needed between the deposition of each molecule. This procedure avoids the contamination of the next adsorption solution by liquid adhering to the substrate from the previous adsorption step and stabilizes weakly adsorbed layers.78 Moreover, the deposition cycles are repeated in order to deposit the desired number of layers, yielding multilayer assemblies. The LbL buildup of multilayer assemblies generally occurs through a variety of deposition methods that have been already extensively reviewed, including dip-coating, spin-coating, spraying,79−88 and perfusion.89,90 Among these ways of preparing multilayers, the dip-coating is the most widely used to date. An advantage of this method is the possibility of coating substrates with more complex geometries. However, this method presents some constraints because it needs a relatively large amount of materials for each deposition step and is time-consuming. Thus, spin-coating and mainly spraying-assisted LbL methods appeared as good alternatives to generate low-cost, rapid, and 8885


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molecular imprinting, sensors, biosensors, nanoreactors, and nanocontainers. In the following sections of this Review, we focus our attention and systematically overview the different types of intermolecular interactions behind the fabrication of multilayer assemblies using the LbL assembly methodology and comment on the potential impact of the materials assembled by each type of interaction on the development of advanced functional systems or devices for several applications. Several examples are given of multilayer assemblies fabricated from electrostatic, hydrophobic, chargetransfer, host−guest, coordination chemistry, and biologically specific interactions, as well as from hydrogen bonding, covalent bonding, stereocomplexation, and surface sol−gel process. Such a multitude of intermolecular interactions, and combinations among them, may extend the number and variety of possible building blocks that can be processed into LbL assembled films and further stimulate the development of novel and highly promising nanostructured functional materials with enhanced properties, as well as complex and elaborated structures wellsuited to be applied in a wide range of applications, including in optics, electronics, energy, coatings and textiles, drug/gene delivery, diagnostics, separation, tissue engineering, and biomaterials and biosensors in biomedicine. One remarkable example is the incorporation of synthetic polymers in a multilayered fashion using the LbL assembly technology to develop in the near future smart nanostructured coatings for a variety of biomedical applications. There is no doubt that the existence of multiple intermolecular interactions will open new horizons for which the LbL assembly approach can be applicable, resulting in promising technological advances.

optical waveguide lightmode spectroscopy (OWLS), dual polarization interferometry, differential scanning calorimetry (DSC), X-ray photoelectron spectroscopy (XPS), confocal laser scanning microscopy (CLSM), atomic force microscopy (AFM), transmission electron microscopy (TEM), or scanning electron microscopy (SEM), as will be shown in the following sections. Nevertheless, it is worth mentioning that the selection of the appropriate characterization technique will depend on the film properties that researchers intend to investigate and on the substrate being coated. Hence, the LbL approach and its promises of tailoring the physicochemical properties and structure of several materials based on the number of deposited layers has opened new challenges and possibilities in a wide range of applications in the fields of sensing, biosensing, and bioelectronics,48,160,162−176 drug/gene/therapeutic delivery,48,75,160,166−173,175−212 food industry,160,213,214 biomineralization,215,216 protein adsorption, cell adhesion/differentiation/proliferation, cellular inflammation, tissue engineering and regenerative medicine,48,75,88,108,109,128,160,166,171,172,182,205,206,217−230 implantable materials, 4 8 , 7 5 , 1 6 0 , 1 6 6 , 1 7 1 , 1 7 2 adhesives, 2 3 1 , 2 3 2 catalysis,160,167,176,192,233−235 separation,236−239 energy storage and conversion,160,240−243 etc. The architecture and properties of the films deposited by this method (such as thickness, stiffness, chemical composition, structure, roughness, wettability, and swelling/shrinking behavior) can be well-controlled at the nanometer-scale level by varying the properties of the adsorbed species such as the charge density, composition, and structure; the physicochemical properties of the liquid medium such as the salt/buffer composition, solvent quality, ionic strength, and pH; and the external parameters such as temperature, exposure to light, mechanical stress, electrical field, adsorption time, and number of layers deposited during the assembly process.71,113,114,244−250 Generally, the film thickness can be well-controlled at the molecular level and increases with decreasing charge density and increasing ionic strength, temperature, and number of deposited layers. In addition, the chosen type of substrate and its morphology greatly influences the growth of the multilayer films. Conventionally, the main and most extensively studied driving force for the buildup of nanostructured multilayer thin films using the LbL assembly approach is the electrostatic interaction. However, besides electrostatics, nonelectrostatic interactions also play a very important role in the possible assembly of materials and further growth of nanostructured multilayer thin films due to the possibility of incorporating several charged or uncharged materials within the LbL assemblies, thus enlarging their potential applications.251,252 One may also combine different kinds of intermolecular interactions to produce multilayers under nonconventional routes.253 Those methods, which include electrostatic complex formation,253−260 hydrogen bonding complex,253,261 block copolymer micelles,253,262−267 inclusion complexes,268−270 coordination polyelectrolyte,271−277 and polymer-assisted complex,253,278−284 comprise two steps, i.e., a first supramolecular assembly of species in solution and further LbL deposition onto a surface. Nonconventional methods are used to fabricate multilayer assemblies that cannot be fabricated by well-established methods. For example, the conventional LbL assembly methods are not suitable for the fabrication of multilayer films based on single charged or waterinsoluble species. In addition, these methods endow the multilayer films with new structures and functionalities of surface

4. LBL ASSEMBLY VIA ELECTROSTATIC INTERACTIONS The electrostatic interaction is one of the most important, if not the chosen one, among the interactions between molecules and surfaces when both molecule and adsorbent surface are electrically charged.285 Moreover, this type of interaction was the first and is by far the most explored assembly mechanism within the LbL approach, as proved by the overwhelming majority of publications in the literature. LbL assembly based on electrostatic interactions allows us to obtain multilayer films with well-controlled composition, structure, and thickness by simply repeating the alternate immersion of a charged substrate into dilute solutions of oppositely charged molecules.73,114 This method, which occurs entirely in aqueous solutions, is well-suited for the fabrication of homogeneous multilayer ultrathin films from combined materials and for several applications, including biomedical applications. Figure 1 shows a schematic illustration of the LbL assembly process using polyelectrolytes. Since the pioneering work of Decher and co-workers,69−73 several publications incorporating polyelectrolytes and other building blocks have been reported.286−333 However, despite the increasing number of publications involving the LbL assembly process, the mechanisms behind recharging and the driving forces for the buildup of the multilayer films are still not fully understood, thus deserving further comprehensive information. In early publications, it was suggested that the electrostatic interaction between oppositely charged molecules was the main driving interaction for the buildup of the multilayer thin films. The finding that a minimum molecule charge density is needed for the successful layer growth further supported that theory.334 In this concern, the adsorption is likely to stop when the 8886


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linearly or exponentially or can experience a transition between both types of growth depending on the strength of the polyelectrolyte intermolecular interactions, i.e., on the combination of polyelectrolyte pairs and experimental deposition conditions. Although the linear regime, which generally leads to thinner films than the exponential growth does, has been extensively studied in the literature, the focus on the exponential growth regime is a more recent approach. Decher et al.70 reported the linear growth of both the mass and film thickness with the increase in the number of deposited layers. A linear growth is usually observed for films comprising strong polyelectrolyte pairs such as poly(styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH)162,334,380−382 and is due to the charge overcompensation that is required for the buildup of the multilayers. However, other systems based on polyelectrolyte pairs consisting of at least one polypeptide or polysaccharide, such as poly(L-lysine)/alginate (PLL/ALG), PLL/hyaluronan (PLL/HA), PLL/poly(L-glutamic acid) (PLL/ PGA), PLL/poly(acrylic acid) (PLL/PAA), PGA/PAH, chitosan/HA (CHT/HA), and poly(ethylenimine)/PAA (PEI/ PAA), have shown to grow exponentially.383−393 Exponentially growing systems comprise those films whose mass and thickness grow with the number of deposited layers faster than in the case of linear systems. Although the mechanism behind the exponential growth regime is not yet fully understood, the film roughness and the diffusion of the polycationic chains “in-andout” of the whole film during the buildup are pointed to as the main sources of this growth.384,394 Picart et al.394 studied the structure of PEM films composed of PLL/HA by several techniques such as CLSM, QCM, and OWLS and explained most of the exponential-like growth processes reported in the literature on a molecular basis. The authors reported that the film thickness increased exponentially with the number of deposited layers. Such an exponential growth regime was due to the fast diffusion of both polyelectrolyte types into PEM films.395 Therefore, one can reach micrometer thick films within a reasonable time, enabling the visualization of the polyelectrolyte diffusion by CLSM. Schaaf and co-workers396 proposed a model to explain the assembly mechanism behind the buildup of exponentially growing systems. The model is based on the previously mentioned “in-and-out” diffusion of at least one of the polyelectrolytes of the multilayer throughout the entire film as well as on the existence of an energetic barrier that prevents the complete diffusion of any polyelectrolyte outward from the film. When none of the polyelectrolytes that comprise the multilayer diffuse, the film grows linearly. Meanwhile, other researchers also reported simple models to elucidate the buildup of exponentially growing PEM systems. For instance, Salomäki and Kankare397 proposed an acoustic-based theoretical model which unveiled that the growth mode of the well-known CHT/HA and PLL/HA multilayers is mainly exponential. Moreover, Hoda and Larson392 proposed a simple model that accounts for the already mentioned diffusion of polycations “in-and-out” of the film, the existence of an energetic barrier at the film surface, and film dissolution. Currently, the fundamental understanding of the exponential growth mechanism provided new insights into the polyelectrolyte adsorption process and led to considerable technological developments. Podsiadlo et al.398 further extended the number of species capable of experiencing LbL exponential growth. They reported the exponential growth of LbL films incorporating inorganic sheets using a complex tricomponent film composed of PEI, PAA, and Na+-montmorillonite (MTM) (see Figure 2).

Figure 1. Schematic representation of the buildup of multilayer thin films via electrostatic interactions: (A) immersion of the charged substrate in the correspondent polyionic solution, alternating with washing steps; (B) schematic representation of the construction of the polyionic architecture depicting film deposition starting, for example, with a positively charged substrate. The multiple immersions of the substrate in the corresponding polyion solution allow the buildup of a multilayer film at the molecular level (n layers).

molecule-coated substrate has a net neutral surface charge. Several experiments with weak polyelectrolytes supported this idea.78,335 However, this concept of a minimum value of charge density is oversimplified as some polymers with unexpectedly low charge density have been successfully grown in multilayer films by using the LbL assembly method.334,336,337 Then, it has been demonstrated that the buildup of the multilayer films is not restricted to materials that present charge, and thus, besides electrostatics, other driving interactions may play a key role in the fabrication and growth of multilayer thin films with well-defined properties, compositions, and structures for the integration of new functionalities.337 This observation is of critical importance as many materials are uncharged, and thus purely electrostatic interactions would not guide the formation of the films. Moreover, when using purely electrostatic interactions, changes in the applied external stimuli, i.e., variations in the surrounding environment such as pH,195,338−346 ionic strength,114,115,195,244,245,339,346−350 electrical field,195,346,351−360 light, 1 9 5 , 3 6 1 − 3 6 7 temperature, 7 1 , 1 9 5 , 2 4 5 , 2 4 6 mechanical stress,195,368−371 or the addition of specific biological moieties, namely, proteins,195,372−376 or ionic surfactants377 may alter the properties of the adsorbed layers or even induce their desorption, thus making other intermolecular interactions of great importance. 4.1. Understanding LbL Growth Mechanisms: Linear versus Exponential Growth

Over the last two decades, several researchers have focused their attention on the internal structure, dynamics, and growth mechanism of PEM assemblies.75,378,379 It is well-known that there is not a regular growth mechanism for all the polyelectrolyte systems reported to date. PEM films can grow 8887


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Those films could then be used for many applications, such as

4.2. Switching Between LbL Growth Mechanisms: From Linear to Exponential Growth and Vice Versa

In the past decade, several researchers have focused their attention on the transition from linear to exponential growth and vice versa during the buildup of PEM films. Although these transitions are still not yet fully understood, it is generally accepted that the switching from one regime to another depends on the strength of the polyelectrolyte intermolecular interactions, i.e., on the combinations of the polyelectrolyte pairs and on the parameters controlling the deposition process.391,399−409 For example, Ball and co-workers81 and Porcel et al.404 reported the fabrication of PAH/PGA and PLL/HA multilayers and observed a transition from exponential to linear growth after a given number of layers. They suggested that this type of transition was due to a film restructuring that progressively hindered the diffusion of one of the polyelectrolytes constituting the film into the restructured zone, which became inaccessible by diffusion. This “forbidden zone” then grew with the increasing number of deposition steps so that the outer zone of the film, which was still concerned with diffusion, kept a constant thickness and moved upward as the total film thickness increased. On the other hand, Ortega and co-workers405 built up PEMs comprising poly(diallyldimethylammonium chloride) (PDADMAC) and PSS and showed a transition from linear to exponential growth by increasing the salt concentration during the assembly process. The authors observed that, by reducing the salt concentration of the polyelectrolyte solution during the buildup, the films should become thinner (for a certain number of deposition cycles) and denser, thereby hindering polyelectrolyte diffusion into the film. Such a kind of transition requires an increase in the vertical mobility of the polyelectrolyte chains, which can be achieved by an increase in the flexibility and/or the formation of fewer complexes connection points. Thus, PEM films can growth linearly or exponentially or can switch from one regime to another depending on the combinations of polyelectrolyte pairs and on the applied external factors.

optical and sensing devices.

4.3. Parameters Controlling the LbL Growth, Structure, and Properties of Multilayer Assemblies

It has been clearly demonstrated that several physicochemical parameters such as pH, ionic strength and electrolyte species, temperature, solvent quality, adsorption time, and the intrinsic properties of polyelectrolytes such as charge density, concentration, architecture, and molecular weight strongly influence the stability, internal structure, dynamics, properties, and growth mode of PEM assemblies (refs 70, 113, 114, 244−247, 297, 334, 378, and 410−414). In this section, we discuss the effect of several factors and intrinsic properties of polyelectrolytes on the thickness, roughness, stability, wettability, permeability, swelling behavior, structure, and growth of multilayer systems. 4.3.1. Effect of Solution pH. The influence of the solution pH on the growth and stability of electrostatic LbL assemblies has been widely explored by several authors, as demonstrated by the vast number of reports in the literature. Bieker and Schönhoff406 and Shiratori and Rubner415,416 explored in detail the crucial role of solution pH in the LbL processing of multilayers comprising weak polyelectrolytes such as PAA and PAH. They showed that very small changes in the solution pH induced significant changes in the growth mechanism, thickness, level of interpenetration depths, and surface wettability of PEM films. By doing quite simple pH adjustments, it was possible to change the thickness of an adsorbed single polymer layer from very thin (ca. 0.4 nm) to very thick (ca. 8 nm), as well as to have

Figure 2. Compilation of thickness evolution of exponentially and linearly growing LbL films as a function of number of deposited layers and with different deposition intervals: (a) Comparison of linearly and exponentially growing LbL film growth with and without MTM grown on a silicon wafer with thickness measured using ellipsometry. The “exponential” upswing of the growth curve for (PEI/PAA/PEI/MTM)n can be clearly seen. The deposition interval for exponentially growing LbL films was 2 min, and for the (PEI/MTM)n it was 5 min. (b) Comparison of thicknesses from SEM for (PEI/PAA/PEI/MTM)n films with the specified deposition intervals prepared on microscope glass slides. The (PEI/MTM)n regression is based on the thickness of a 100- and a 200-bilayer film deposited on top of a glass slide with 5 min depositions. (c) Comparison of (PEI/PAA/PEI/MTM)n with and without MTM following 2 min depositions. Reprinted with permission from ref 398. Copyright 2008 American Chemical Society. 8888


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great control over the bulk and surface composition of the multilayer assemblies. For PAH/PAA assemblies, the film thickness was maximal when both polyelectrolytes were weakly charged. Moreover, it was also possible to control the number of nonionized carboxylic acid groups contained within the film and on its surface. The same research group also reported that multilayers assembled from PAH and PSS undergo, under specific pH conditions, abrupt pH-triggered discontinuous swelling/deswelling transitions417,418 similar to those exhibited by some polymer gels fabricated with weak polyelectrolytes.419 They highlighted the crucial role of the solution pH when it is close to or even higher than the pKa of PAH by revealing that multilayer films assembled at pH < 8.5 showed pH-independent swelling behavior over the pH range of 2.0−10.5, whereas remarkable discontinuous swelling behavior was observed when the assembly pH was greater than 8.5 (see Figure 3a). Moreover, in an attempt to inspect such behavior, they used FTIR

spectroscopy, which unequivocally demonstrated that the pHtriggered discontinuous swelling/deswelling transitions of PAH/ PSS multilayers assembled at pH > 8.5 were driven by changes in the degree of ionization of free amine groups of PAH (Figure 3b). Therefore, the assembly pH was found to play a vital role in determining the postassembly pH-dependent swelling behavior of multilayers containing these polyelectrolytes. Moreover, other research groups also revealed the influence of pH on the assembly of multilayer systems. For instance, Burke and Barrett420 described the formation of PEMs based on PLL and HA under different pH regimes. They examined by microelectrophoresis the acid−base equilibria of PLL/HA multilayer films as a function of the number of layers and the solution pH, and they observed significant shifts in the dissociation constants of PLL and HA upon incorporation into the multilayer film (from 1 to 3 pH units). The understanding of the acid−base dissociation behavior of polyelectrolytes in the films is essential to tailor the physicochemical properties of the multilayer films, such as film thickness, swelling behavior, surface wettability, and friction, which constitute valuable parameters for several applications involving biologically inspired materials. The morphology, stability, and pH-responsive behavior of other multilayer films and hollow microcapsules composed of weak polyelectrolytes, templated on several sacrificial cores by electrostatic LbL assembly, have also been thoroughly studied.338−346,421−423 These smart systems have important applications in several fields, including in the biomedical field, working as carrier vehicles of biomolecules for controlled drug delivery and tissue engineering. For instance, Déjugnat and Sukhorukov421 fabricated hollow microcapsules comprising PAH and PSS on several cores, namely, manganese (MnCO3) or calcium carbonate (CaCO3), or polystyrene (PS) latex particles, and they investigated the swelling/shrinkage behavior in response to changes in solution pH. They observed that the microcapsules experienced a dramatic swelling upon exposure to a basic solution (0.1 M NaOH), as confirmed by CLSM, which was due to electrostatic repulsions between highly negatively charged PSS layers. Moreover, such an increase in size remarkably increased upon increasing the time of exposition to NaOH solution. The capsules further experienced a shrinkage upon addition of an acidic solution (0.1 M HCl) to the NaOHswollen PS capsules (see Figure 4). These smart systems will be very promising for drug delivery applications. Sukhorukov and co-workers423 further investigated the morphology, stability, and pH-responsive behavior of hollow microcapsules incorporating other weak polyelectrolytes such as poly(methacrylic acid) (PMAA) and PAH in their shells. They found that the shells were stable over a large pH range but swelled and dissolved for pH values 11, respectively. Moreover, as one can easily tailor the ionization degree of PMAA as a function of the pH, the unbound carboxylic acid groups can be used as anchoring sites for the selective binding of specific ions such as calcium ions (Ca2+), further reducing the stability of the capsule shells at acidic pH and stimulating the shrinking in basic media. Therefore, these systems, which exhibit a pH-responsive behavior, may work as microreactors, being highly suitable for many practical applications. In addition, several other authors have studied the role of pH on the stability, growth, and internal properties of PEM assemblies. For example, Shen and co-workers424,425 reported a simple, yet straightforward pH-based method to enhance the exponential growth of LbL assemblies by sequential deposition

Figure 3. (a) Swelling level of PAH/PSS multilayer films during and after pH treatments. The circles, squares, and diamonds represent the films assembled at pH 9.3, 8.5, and 7.5, respectively. Each measurement in solution was carried out 3 min after immersion, and the immersion continued 5 min before drying. The pH of deionized water is ∼5.5. (b) Summary of the swelling level of PAH/PSS multilayer films assembled at various pH conditions in the range of pH 6.5−9.5. The squares, circles, and diamonds represent the swelling level during the first immersion in pH 2.0 water, the second immersion in pH 2.0 water, and immersion in pH 10.5 water, respectively (%). The triangles represent the degree of ionization of PAH in solution (%). All films started with a dry thickness of (50 ± 5) nm. Adapted with permission from ref 418. Copyright 2005 American Chemical Society. 8889


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unique chemical properties, namely, high charge density and multiple reactive sites of terminal groups, being considered extremely valuable components for LbL assemblies. Furthermore, other authors also attempted to scrutinize the effect of both solution pH and substrate on the buildup of multilayer assemblies. In this regard, Barrantes et al.427 unveiled the influence of both solution pH and two different substrates, silica and gold, on the buildup, internal structure, and viscoelastic properties of multilayers comprising PLL and heparin (HEP) by means of QCM and ellipsometry. The ellipsometric measurements indicated that the dry mass grew exponentially with the number of deposited layers, and it was larger as the pH values were raised. On the other hand, the wet mass, obtained by QCM, grew linearly or exponentially depending on the solution pH. Furthermore, the results also demonstrated that PEMs became more viscoelastic as the pH values increased for silica substrates, while for gold the highest viscoelastic behavior was obtained at neutral pH. 4.3.2. Effect of Temperature. Another widely studied factor that greatly affects the stability, internal structure, morphology, and growth of multilayer assemblies is the temperature. The effect of the temperature on the thickness of the PEMs was reported by several authors.71,245,246,399 For instance, Van Patten and co-workers246 studied the temperature dependence of PEM films comprising PDADMAC and PSS and unveiled that the assemblies fabricated at high temperatures were thicker than similar films built at room temperature because high temperatures induced the swelling of the films and allowed for increased interpenetration between the polyelectrolyte layers (see Figure 5).

Figure 4. Influence of pH on swelling/shrinking of PS (PAH/PSS)6 capsules: (a) in water, (b) 1 s after NaOH addition, (c) 1 min after NaOH addition, and (d) after 0.1 M HCl addition, as recorded by CLSM. Reprinted with permission from ref 421. Copyright 2004 American Chemical Society.

of PEI and PAA at high and low pH values, respectively. The alternate exposure of the PEI/PAA polyelectrolyte pair to a PEI solution at high pH and PAA solution at low pH switched the degree of ionization of the polyelectrolytes in the multilayer assemblies, thus enhancing their diffusion “in-and-out” of the films. Hence, the deposited mass of PEI and PAA per deposition cycle, and subsequently their concentration within the film, increased exponentially.425 This simple strategy supplies a new highly controllable way to easily fabricate multilayer assemblies containing simultaneously distinguishable hierarchical microand nanostructures in a small number of deposition cycles. Indeed, it was demonstrated that these structures could be further transformed into superhydrophobic surfaces that find widespread applications, including in microfluidics. A pHsensitive LbL growth of star-shaped polyelectrolytes, namely, poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) and PAA, exhibiting both linear and exponential growth was recently reported by Tsukruk and co-workers.426 They demonstrated the influence of the number of layers (up to 30 bilayers) and the assembly method (dip-coating vs spin-coating assisted LbL assembly) on the film growth and on its morphology. It was found that, upon increasing the number of bilayers, the spincoating method led to smoother and thinner films than the ones obtained by the dip-coating method. Although this behavior contradicts the results obtained for conventional strong polyelectrolytes, such as PAA and PSS components, such a difference was explained by the critical role of weaker interactions between PAA and PDMAEMA under certain pH conditions and the suppressed interdiffusion of both components during the fast removal of the solvent during spin-assisted LbL method. Accordingly, although the spin-assisted LbL assembly method always resulted in linear growth of LbL assemblies no matter the pH conditions under study, the dip-assisted LbL method led to exponential or linear growth depending on the pH conditions and the number of bilayers assembled. These materials present

Figure 5. Evolution of the thickness of hand-dipped (PDADMAC/ PSS)10 films deposited at different temperatures in the presence of 1 M NaCl, as measured by ellipsometry. Reprinted with permission from ref 246. Copyright 2003 American Chemical Society.

Moreover, Salomäki et al.399 also studied the effect of the temperature on the buildup of PEMs comprising PSS, PDADMAC, and PAH by means of QCM technique. They found that the temperature had a stronger effect on PDADMAC/PSS deposition than on the PAH/PSS deposition. In addition, the increase in the temperature of the deposition process was found to increase both the bilayer mass and the film thickness, as well as to extend the exponential buildup regime in all of the studied systems as compared with the linear one. They also proposed a model which predicted that every LbL buildup process is inherently exponential, turning linear whenever the 8890


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could be used for intracellular delivery of functional or active agents, thus constituting a promising system for biomedical applications.432 4.3.3. Effect of Ionic Strength and Electrolyte Species. Ionic strength and electrolyte species have also been widely explored parameters to study the stability, permeability, internal structure, function, and growth of multilayer systems assembled via electrostatic interactions.70,113,114,244,245,339,347−350 In this respect, Dubas and Schlenoff114,244 reported the growth of PEM films comprising PDADMAC and PAA or PSS and the dependence of the film thickness on the salt concentration. Film thickness increased until a salt concentration of 0.3 M and then quickly decreased, leading to the complete dissociation of the multilayer for a salt concentration higher than 0.6 M (see Figure 7). This apparent dissociation of the film is related to the competition for polyelectrolyte pairs by external salt ions.

diffusion rate is not fast enough to carry the polymer within the entire thickness of the layer. Recently, Helm and co-workers428,429 reported the effect of temperature on PEM films comprising PSS and PAH layers. It was found that, at low temperature, a top PSS layer was twice as thick as a top PAH layer (odd−even effect). When the temperature exceeded the crossover point, a top PAH layer was thicker than a top PSS layer (even−odd effect). Beyond a critical temperature, both the thickness and the surface roughness per deposited PSS/PAH bilayer increased. Moreover, the effect of the temperature on the stability, properties, and growth of multilayer assemblies has not been restricted to 2D assemblies. The fabrication and properties of 3D structures, such as microcapsules, have also been addressed. In this concern, Mano and co-workers430,431 reported the construction of polymeric multilayer microcapsules by sequential adsorption of oppositely charged layers of CHT and a temperature-responsive polypeptide, such as biomimetic elastin-like recombinamer (ELR), on spherical CaCO3 sacrificial microparticle templates preloaded with bovine serum albumin (BSA), and further studied the release profile of several biomolecules as a function of the temperature. It was found that the capsules were significantly smaller at 37 °C than at 25 °C (Figure 6a and b), probably due to rearrangements of the

Figure 7. Evolution of the thickness of hand-dipped (PDADMAC/ PAA)10 films, prepared with PAA with a molecular weight of either 84 500 (squares) or 5 200 (circles), as a function of NaCl concentration, as measured by ellipsometry. The deposition, soaking, and rising solutions were maintained at pH 11. Reprinted with permission from ref 244. Copyright 2001 American Chemical Society. Figure 6. CHT/ELR microcapsules exhibiting temperature-responsive properties. CLSM images of BSA-loaded (CHT/ELR)5 microcapsules at (a) 25 and (b) 37 °C. The final layer was tagged with rhodamine B. Cumulative release of BSA from (CHT/ELR)1, (CHT/ELR)3, and (CHT/ELR)5 microcapsules: ■□, ○●, and ▲Δ, respectively, in phosphate-buffered saline (PBS) (pH 7.4), at (c) 25 and (d) 37 °C. Adapted with permission from ref 430. Copyright 2013 Elsevier.

Moreover, in the last two decades other researchers also examined the influence of both salt type and concentration on the thickness, morphology, roughness, permeability, stability, and growth of multilayer assemblies. For instance, Fery et al.433 examined the influence of the salt concentration on the structure of PEMs comprising PAA/PAH systems. They demonstrated the formation of nanoporous PEMs through salt-induced structural changes, thus revealing that ionic strength can be used to control the permeability of the multilayer films. The authors also investigated the impact of salt on the morphology and mechanical properties of hollow capsules composed of PSS/ PAH layers and found that high salt concentrations (>3 M) induced a pronounced softening of the membrane of PSS/PAH capsules, thus leading to the capsules’ shrinkage.434 Similar studies, reporting the impact of several salt types and concentrations on the growth, structure, and stability of PSS/ PAH films and hollow capsules, were also performed by other researchers.435−437 As can be seen from Figure 8, the diameter of PSS/PAH capsules not only significantly changed by varying the

adsorbed layers as a result of the effect of different solvent temperatures on the temperature-sensitive ELR molecules, and that the multilayers demonstrated a more sustained drug release at physiological temperature (Figure 6c and d). Moreover, the cytotoxicity of those microcapsules was assessed to evaluate whether they could be used in biological studies. It was demonstrated that the microcapsules were noncytotoxic toward fibroblast-like cells, thus suggesting a broad range of applications in promising fields, including in biomedical and biotechnological fields, where great control in the release of drug/therapeutic molecules is required. Very recently, the same authors also demonstrated that an analogous multilayer CHT/ELR system 8891


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Figure 8. Influence of salt type and concentration on the diameter of PSS/PAH hollow capsules. (I) CLSM images of (PSS/PAH)4 capsules in water (a) and 10 min after addition to 1.7 M solutions of CsCl (b), Na2HPO4 (c) and NH4(HCO3) (d). The capsules are labeled by tetramethylrhodamine isothiocyanate (TRITC)-PAH in the fourth layer. (II) Dependence of capsule diameter on gradually increasing concentration of NaCl, NH4(HCO3), and NaHCO3. The time delay for each stepwise increase was 10 min. Adapted with permission from ref 435. Copyright 2005 Royal Society of Chemistry.

almost linear increase of the thickness with the number of deposited layers was observed by means of ellipsometric measurements. Moreover, it was found that the counterions in excess modified both the conformation and the amount of adsorbed polycation and strongly influenced the amount of the dye adsorbed on the polycationic layer. LbL films incorporating azo dyes constitute promising materials that will find promising applications in biomedical and biotechnological fields. Zan et al.443 also reported the influence of salt concentration on the addition of a supplementary PSS layer to existing PEMs consisting of PSS/PDADMAC swollen by NaCl. It was found that high salt concentration (salt concentration > 1 M for PEMs fabricated at a salt concentration of 0.5 M) induced a greater PSS mass uptake, full permeation, and faster diffusion of PSS across the PEM when compared to the analogous addition of PSS at low salt concentration (salt concentration < 1 M for PEMs fabricated at a salt concentration of 0.5 M). Kipper and co-workers444 reported the effect of ionic strength on PEMs built up from CHT and HEP. The authors showed that the film thickness increased by changing the pH at an optimal buffer concentration of 0.2 M. Higher and lower ionic strengths resulted in constricted ranges of accessible thickness. Moreover, several other researchers also investigated the effect of ionic strength on swelling/deswelling behavior of multilayer assemblies. For example, Boulmedais and co-workers445 studied the effect of ionic strength on the swelling process of exponentially growing PLL/HA films. It was shown that, over time, the increase of ionic strength led not only to the swelling and release of both polyelectrolytes constituting the film but also to the formation of spherical holes inside the film before its final dissolution. The formation of such holes was attributed to an increase of the osmotic pressure inside the film due to polyelectrolyte complex dissociation, enabling control of the permeability of the LbL films. The same research group extended this concept and reported that any increase or decrease of the

electrolyte species (see Figure 8I) but also decreased upon increasing the salt concentration (see Figure 8II). Goh and co-workers400 investigated the influence of salt concentration on the thickness, morphology, and roughness of PEMs formed from PDADMAC and PSS. Significant differences were observed between the films formed under low or high salt concentrations. Those differences can be accounted for by considering a conformational transition from extended rod (at low salt concentration) to globular coil (at high salt concentration) in the bulk polyelectrolyte solution at a salt concentration of 0.3 M. Furthermore, the thickness and roughness of PEMs increased with increasing salt concentration. Salomäki and co-workers438−440 explored the elastic properties of PEMs consisting of several layers of PDADMAC and PSS by QCM, and they revealed that, besides ionic strength, different counteranions greatly influence bilayer mass, stiffness, and swelling behavior of the PEM film. They reported a remarkable increase in both bilayer mass439 and stiffness438 of the PEM while changing the counteranion used in the deposition process. Boulmedais and co-workers441 investigated the influence of salt type on the buildup and structure of PSS/PAH films. They varied the nature of the anion by following the Hofmeister series from cosmotropic to chaotropic anions (F−, Cl−, NO3−, ClO4−) and found that for all the studied anions the film thickness increased with the number of deposited layers and followed the position of the counteranion in the Hofmeister series. Indeed, the interaction between ions and polyelectrolytes was larger for chaotropic ions, and these ions led to a larger increase of film thickness when compared with the cosmotropic ones. Dragan et al.442 reported the effect of counterion type (Cl−, Br−, SO42−) and its concentration on the growth of LbL assemblies comprising a strong polycation (containing 95 mol % of N,Ndimethyl-2-hydroxypropyleneammonium chloride units in the backbone (PCA5)) and azo dye Direct Red 80 (DR80). An 8892


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Figure 9. Influence of the ionic strength on the film structure. Vertical section images of (PLL/HA)50-fluorescein isothiocyanate (FITC)-labeled PLL films built up at 0.15 M NaCl and observed by CLSM. (a) Film in contact with the 0.15 M NaCl solution; (b) after 1 min in contact with a 0.04 M NaCl solution; (c) after 1 min in contact with a 0.01 M NaCl solution; and (d) after 16 h in contact with the 0.15 M NaCl solution again. The film was put step by step in contact with decreasingly concentrated NaCl solutions (1 min of contact at each step) before observation. The scale bars represent 10 μm. (e) Release of FITC-labeled PLL from a (PLL/HA)50-FITC labeled PLL film built at 0.15 M NaCl and brought into contact with a 0.01 M NaCl as a function of time. Reprinted with permission from ref 446. Copyright 2011 Royal Society of Chemistry.

Figure 10. Film thickness (a, b) and mass loading (c, d) as a function of the number of bilayers of branched PEI/MTM and branched PEI/LAP systems constructed through different deposition times. The data presented in (a, b) and (c, d) were obtained by ellipsometry and QCM, respectively. Adapted with permission from ref 448. Copyright 2010 American Chemical Society.

ionic strength of the solution in contact with the film induced a restructuring of the film with the formation of smaller and/or larger holes (Figure 9).446 While the direct increase or decrease of the ionic strength led to the formation of spherical holes, the increase of the ionic strength followed by a decrease to values lower than the initial assembly conditions led to the formation of nonspherical holes due to the release of ions and reinforcement of polyelectrolyte interactions within multilayer complexes. This

means that the formation and shape of the holes is strongly dependent on the strategy followed to apply ionic strength variations. Klitzing and co-workers447 addressed the influence of the concentration and type of ions (NaCl, NaF, and NaBr) on the structure and water content of PSS/PDADMAC multilayers by neutral reflectivity. It was shown that both thickness and water content of the multilayers increased with increasing salt 8893


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concentration and ion size. They also identified two types of water: “void water”, which filled the voids of the multilayers and contributed not to swelling but instead to a change in scattering length density, and “swelling water”, which directly contributed to swelling of the multilayers. The amount of void water decreased with increasing salt concentration and anion radius, while the amount of swelling water increased with the increase of the salt concentration and the anion radius. 4.3.4. Effect of Solvent Quality and Adsorption Time. Although pH and ionic strength have been called as the main parameters to tune the thickness, morphology, roughness, permeability, stability, structure, and porosity of PEMs, the solvent quality and adsorption time also play a key role in tailoring polyelectrolyte structure and properties and film growth.113 Caruso and co-workers247 studied the effect of the solvent quality on the growth and structure of PEMs comprising PSS and PAH by changing the amount of ethanol in the polyelectrolyte solutions. They showed that the increase in the amount of ethanol, i.e., decreasing the solvent quality, resulted in the increase of both film thickness and mass loading, as confirmed by SPR and QCM techniques. This increase in the multilayer thickness with increasing ethanol concentration is explained by the poor solvating effect of ethanol on the electrolyte ions when compared with water. Yang et al.448 investigated the influence of the adsorption time on the growth and structure of LbL films composed of MTM or laponite (LAP) clay paired and branched PEI. The authors varied the adsorption time of each solution from 5 to 300 s and studied its effect on the thickness, film structure, deposited mass, and oxygen barrier. Ellipsometry and QCM measurements demonstrated that a linear growth was observed for all the deposition times studied, and the branched PEI/LAP system showed to be more dip-time dependent and grew quicker than the branched PEI/MTM film (Figure 10). Moreover, as can be seen from Figure 10a and b, the thickness of the branched PEI/MTM system remained practically unchanged with increasing deposition time, whereas in the case of the branched PEI/LAP system it increased with longer deposition times. This behavior is explained by the much smaller diameter presented by the LAP clay platelet in comparison with the MTM platelet, which can be deposited in the defects of each PEI layer. In addition, microscopic images showed that the branched PEI/LAP system revealed a higher organizational level and uniformity for all the adsorption times in comparison with the branched PEI/MTM system (see Figure 11). Besides all the aforementioned parameters controlling LbL assemblies, the intrinsic properties of the polyelectrolytes such as charge density, concentration, molecular weight, and architecture (e.g., number of arms and arm length) also strongly influence the morphology, structure, properties, and growth of PEM films, as described in the following subsections. 4.3.5. Effect of Polyelectrolyte Charge Density. The influence of the polyelectrolyte charge density on the assembly of multilayer systems has also been studied and explored. Schoeler et al.334 and Klitzing and co-workers402,449 examined the influence of the polyelectrolyte charge density on the buildup of multilayer films composed of model random copolymers, such as positively charged monomer diallyldimethylammonium chloride (DADMAC) and nonionic monomer N-methyl-Nvinylacetamide (NMVA), and strong polyelectrolytes such as PSS. The authors selected strong polyelectrolytes in order to easily distinguish the change in the surface charge and the charge of the adsorbing polyelectrolyte chain. They also revealed that there was a critical minimum charge density (between 50% and

Figure 11. SEM images of (branched PEI/LAP)30 (a, b) and (branched PEI/MTM)30 (c, d) systems deposited on poly(ethylene terephthalate) substrate with 10 (a, c) and 300 s (b, d) deposition times for both constituents. Adapted with permission from ref 448. Copyright 2010 American Chemical Society.

75%) below which no multilayer growth was possible. Above the critical charge density limit, the rate of the film growth and the film morphology were strongly influenced by the polyelectrolyte’s solution structure. Thicker and rougher films were achieved by decreasing the polymer charge and increasing the salt concentration in the polyelectrolyte solutions. However, Caruso and co-workers337 reported the growth of similar multilayer assemblies at a polyelectrolyte charge density as low as 8%, thus suggesting the important contribution of other nonelectrostatic interactions on the LbL buildup. Moreover, several other researchers have been focusing on the study of the growth and internal structure of PEMs by varying the polymer charge density. In this regard, Glinel et al.450 also reported the effect of polyelectrolyte charge density on the growth of multilayer assemblies built up from strong polyanions, such as flexible PSS or rigid platelets of exfoliated synthetic hectorite (LAP), and cationic copolymers, such as DADMAC and N-methyl-Nvinylformamide (NMVF). The use of NMVF copolymer, which is less hydrophobic than NMVA copolymer, minimized additional hydrophobic interactions during polyelectrolyte deposition. They demonstrated that, in the case of the assembly of cationic copolymers with PSS, stable PEMs could be prepared from pure aqueous solutions above a critical linear charge density of ∼2.8 nm. However, and corroborating the findings of the previously mentioned work by Caruso and co-workers,337 the assembly of cationic copolymers with LAP could be performed over the entire range of DADMAC content, thus revealing the key contribution of other intermolecular interactions between the LAP platelets and the NMVF units on the film growth. Koetse et al.451 also studied the influence of the charge density of other strong polyanions, such as poly(vinyl sulfate) (PVS) or poly(vinylsulfonate) (PVSO), on the growth and internal structure of PEM assemblies, using a series of anionic cellulose derivatives. They showed that, as a general trend, the film thickness increased with decreasing charge density of the polyanion. Mermut and Barrett452 studied the impact of the polyelectrolyte charge density on the time-dependent assembly of PEMs. They reported the time-dependent thickness of weakly charged polymers to be sensitive to different variables such as pH, 8894


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much less understood. In this concern, Sui and co-workers456 reported the effect of polymer molecular weight on the assembly of PEMs in the presence of salt. It was observed that multilayer growth was inhibited by complete loss of low molecular weight polyanion on exposure to low molecular weight polycation. The authors also elucidated the key role of salt in moderating the strength of polymer−polymer interactions, as well as in the balance between multilayer growth and polyelectrolyte stripping. Kujawa et al.457 reported the influence of polysaccharide molecular weight on the thickness and morphology of multilayer assemblies based on CHT and HA. They reported that the film thickness increased twice when the molecular weight of the polysaccharides was higher, and this behavior was due to a difference in the film growth onset. Moreover, it was also found that such an increase of the film thickness followed an exponential trend. Lynn and co-workers458 further demonstrated the influence of polymer molecular weight on the assembly of LbL films composed of PAH and PAA. They observed important differences in the surface features and growth of PAH/PAA films fabricated by using well-defined low or higher molecular weight commercially available PAA. The use of low molecular weight PAA allowed its diffusion into those films over large distances, leading to thicker and exponentially growing films. Porcel et al.459 unveiled the effect of polyelectrolyte molecular weight on the linear growth of exponentially growing systems as well as on the diffusion of PLL in PLL/HA films. They found the increase in the film thickness to be independent of the polyelectrolyte molecular weights. In addition, low molecular weight PLL allowed the diffusion of these chains into the whole PLL/HA film (even for very thick films), while the PLL diffusion of high molecular weight chains was restricted to the upper part of the film. Shen et al.460 investigated the role of the pH-amplified method on the stability, growth, and internal properties of PEM films by alternate deposition of PLL and HA at high and low pH values, respectively. They focused their attention on the influence of HA molecular weight and on its concentration in solution and found that the film growth was faster for HA of high molecular weight. In addition, the more cohesive interactions in the case of PLLending films also led to more stable PLL-ending films when they were placed in PBS than their HA counterparts. Very recently, several authors have focused on the key role played by polymer molecular weight on the growth regime and internal structure of PEMs, as well as on the polymer chain mobility.461−464 They found that, as a general trend, the increase of polycation molecular weight induced an increase of the internal roughness of PEM films,461 and that the precise control of the polyanion molecular weight enabled the modulation of the swelling and disintegration behavior of LbL multilayer films in response to changes in pH.462 The work by Char and co-workers is rather relevant, showing that by tuning the polymer molecular weight one easily avoids the use of additional post-treatments, such as thermal or chemical cross-linking, or the incorporation of strongly binding molecules that could lead to the disruption between internal layers.462 The understanding of the disintegration mechanisms of LbL films as a function of polymer molecular weight is of great relevance to further design and develop advanced functional controlled release systems that could be used as coatings on implants. Moreover, other authors have also shown that, depending on the molecular weight of both the polyanion and polycation within PEM films, different growth regimes could be achieved. In this respect, Helm and coworkers463 have shown that there is a specific threshold value of polymer molecular weight, which is different for the polycation

concentration, charge fraction, and counterion species. Schoeler et al.453 reported the effect of polyelectrolyte charge density on the assembly and growth of multilayer films comprising a low but permanently charged copolymer composed of acrylamide and positively charged [3-(2-methylpropionamido)propyl]trimethylammonium chloride (AM-MAPTAC 10; 10 mol % of cationic monomers) and a weak polyacid (PAA). They changed the charge density of PAA, by tuning the pH of the adsorbing solutions, and observed an exponential growth behavior and the formation of thick and rougher films with increasing number of deposited layers at acidic pH (∼2−4). On the other hand, at basic pH (∼6−9), PAA is highly charged and, thus, the multilayer growth occurs through a series of adsorption−desorption steps due to the different conformation adopted by the polyelectrolytes. In this case, the adsorption of a highly charged PAA layer induced the partial desorption of the underlying partially charged AM-MAPTAC 10 layer. Choi and Rubner454 also studied the effect of the polyelectrolyte charge density on the growth of weak PEMs by assembling strong polyelectrolytes with either PAA or PAH and by using the pH of the adsorption solutions to tune the charge density. The degree of ionization of the weak polyelectrolytes in solution and in the multilayer system was modulated by adjusting the pH and quantified by FTIR spectroscopy measurements. The authors found that the degree of ionization of PAA or PAH in the multilayer film was strongly influenced by the chosen type of polycation or polyanion, respectively, and that, in all the studied multilayer systems, the film thickness increased upon decreasing the charge density of the weak polyelectrolyte. Kamburova et al.455 investigated the effect of pectin (PEC, a plant polysaccharide) charge density on the buildup of LbL films with CHT. It was found that after the deposition of the first few layers (∼3−4 layers) the film thickness increased linearly with the number of deposited layers; it was higher for the multilayer films built up with less charged PEC (see Figure 12) and ended with a CHT layer, as CHT might diffuse into the film interior. 4.3.6. Effect of Polyelectrolyte Molecular Weight. The effect of polymer molecular weight on the growth, internal structure, morphology, chain mobility, and properties of multilayer films has been the focus of few studies, being thus

Figure 12. Evolution of the hydrodynamic thickness of PEC (with different degree of ionization)/CHT multilayers as a function of the number of deposited layers (n). Multilayer films composed of PEC with a degree of esterification of 21% or 71% and CHT (open and filled squares, respectively) were studied. Reprinted with permission from ref 455. Copyright 2008 American Chemical Society. 8895


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HSPA-HCl could form stoichiometric polyelectrolyte complexes with the linear NaPA and had little influence on the complexation behavior. Very recently, Zhang and co-workers467−469 unveiled the influence of chain interpenetration, chain conformation, and electrostatic interactions at different salt concentrations on the growth of multilayers comprising different polyelectrolyte pairs. It was perceived, by QCM measurements, that the increase of salt concentration led to an increase of film thickness and chain interpenetration on both linear and exponential growth regimes when the multilayers comprised flexible or semiflexible polyelectrolyte chains. Moreover, it was found that at low salt concentration (CNaCl < 1.0 M) the multilayer growth is governed by the chain conformation, whereas at high salt concentration (CNaCl ≥ 1.0 M) it is dictated by polyelectrolyte interpenetration. The same authors also investigated the influence of the number of arms on chain interpenetration in the growth of PEMs based on star PDMAEMA/star PAA.470 The authors showed that the growth of the multilayers, in terms of chain interpenetration, proceeded via two different mechanisms, as confirmed by means of QCM technique. The arm chains of star PDMAEMA were introduced into a predeposited PAA layer to form a swollen multilayer, and the complex of star PAA with predeposited star PDMAEMA presented an “octopus-like” structure forming a dense multilayer. They also revealed that the increase of the number of arms of either PAA or PDMAEMA made the penetration of star PDMAEMA into the PAA layer more difficult. Under such conditions star PAA could more easily penetrate into the PDMAEMA layer.

and polyanion molecules due to their intrinsic differences in terms of linear charge density and persistence lengths, below or above which the thickness and growth regime change. They built multilayer films comprising PDADMAC and PSS and observed two opposing trends: (a) when the molecular weight of PDADMAC was decreased (below a threshold value of 80 kDa), while that of PSS remained large, then the thickness and number of layer pairs decreased; (b) when the molecular weight of PSS was reduced (below a threshold value of 25 kDa), while maintaining the large one for PDADMAC, the thickness and number of layer pairs increased. Moreover, they always observed, for all the deposited films, a well-defined transition from parabolic to linear growth after reaching a large number of deposited layer pairs (see Figure 13).

4.4. Computational Simulation Strategies as Tools to Understand LbL Assembly Mechanism and Investigate the Structure, Stability, and Dynamics of LbL Multilayer Assemblies

Molecular simulation studies have been reported in order to shed light on the role of the electrostatic interactions and better understand the stability, internal structure, dynamics, and mechanisms behind PEM growth.471 In this regard, Messina et al.472,473 investigated the adsorption of PEMs at charged planar surfaces and at charged spherical particles by means of a series of Monte Carlo (MC) simulations. The authors reported that, beyond electrostatics, specific short-range nonelectrostatic interactions were required to successfully initiate film growth. In addition, those interactions strongly influenced the stability of the PEMs. Nonetheless, as the MC simulations were limited to a few deposition steps, Dobrynin and co-workers performed the same experiments via molecular dynamics (MD) simulations.474−477 They confirmed that the buildup of the multilayers occurred through surface overcharging during each deposition step as well as that the film reached a steady-state regime after a few deposition cycles. The authors assessed the effects of the fraction of charged monomers, as well as the chain degree of polymerization, on the structure, stability, and mechanism behind multilayer formation by means of MD simulations. It was found that two deposition cycles of similarly charged polymers were effectively necessary to achieve the formation of one compact layer. In addition, the polyelectrolyte chains were not perfectly stratified within the multilayer structure, but instead, there was a substantial intermixing between polyelectrolyte chains adsorbed during different deposition cycles. Despite the high degree of chain intermixing, there was an almost perfect periodic oscillation of the density difference between positively and negatively charged chains after several deposition cycles. Overall, it was found that weakly charged

Figure 13. (a) From top to bottom: thickness of a PEM film (d), thickness of the top layer pair (Δd), and ratio f(N) = d/Δd vs number of deposited layer pairs (N) for PDADMAC and PSS with molecular weights of 159 and 16.8 kDa, respectively. The three different growth regimes (exponential, parabolic, and linear) can be distinguished by their slope (f ′). (b) Histogram of f ′, the slope of f(N) = d/Δd as determined from all PEMs investigated. Reprinted with permission from ref 463. Copyright 2013 American Chemical Society.

4.3.7. Effect of Polyelectrolyte Chain Architecture. The formation of the PEM films is also influenced by the polyelectrolyte chain architecture. Houska et al.465 investigated the role of the polyelectrolyte chain length on the buildup of protein/PEM assemblies. The authors studied systems comprising different globular proteins, such as BSA and human immunoglobulin G, and strong linear anionic polyelectrolytes, namely, PSS, dextran sulfate (DS), and HEP. It was found that the polyanion chain length significantly affected the LbL process and that, at lower molecular weights, the adsorbing protein molecule effectively competed with the adsorbed protein for the polyanion. A few years later, Zhou et al.466 reported the fabrication of polyelectrolyte complexes by hyperbranched poly(sulfone amine) hydrochlorate (HPSA-HCl) and sodium polyacrylate (NaPA). They found that the branched structure of 8896


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chains allocated significantly more polymer within the layers than the strongly charged ones. Those authors also reported MD simulations of LbL multilayer films assembled from oppositely charged polyelectrolytes, nanoparticles, and mixed nanoparticle/ polyelectrolyte layers.478−480 They concluded that the films assembled solely from nanoparticles showed better layer stratification within the multilayer film, with almost constant thickness of the layer composed of nanoparticles, and higher roughness than those assembled from polyelectrolytes. The film thickness and surface coverage increased almost linearly with the number of deposition cycles in all the studied systems. In addition, the same research group also reported MD simulations to study the effect of electrostatic and short-range interactions on the sequential assembly of oppositely charged polyelectrolytes at charged planar surfaces.481 Those simulations confirmed their previous studies showing that after each deposition step a charge reversal was crucial for stable film growth and that there was a linear increase in polymer surface coverage after the first few deposition steps. Furthermore, it was shown that the polymer surface coverage and the multilayer structure were strongly influenced by the strength of the electrostatic and short-range interactions. In addition, theoretical studies, performed to gather further insights into the internal structure and charge compensation of the PEM formed by both strongly and weakly dissociating polyelectrolytes, were in good qualitative agreement with most MD simulations.482 More recently, several researchers have performed MD simulations to study the influence of several parameters, such as the effect of the solvent quality for the polymer backbone, the strength of the electrostatic interactions, the chain degree of polymerization, the lubricating properties, and the brush grafting density, on the conformations of the polyelectrolyte brushes in salt or salt-free solutions.483−490 In summary, although still scarce, the information gathered from computational simulations and theoretical studies could then be used to corroborate the experimental data, thus enabling a better understanding of the parameters involved in the multilayer assembly process as well as the internal structure, dynamics, and growth mechanisms of multilayer films.471,482,491−497 Overall, LbL assembly via electrostatic interactions is a very simple, versatile, and reproducible method that enables the incorporation of a broad variety of charged materials into multilayer thin films to add new functionalities. This intermolecular interaction is by far the most extensive and powerful way of making LbL films, as demonstrated by the overwhelming majority of reports in the literature. Such electrostatic LbL assembled films can then be used for a wide range of applications, including drug/gene delivery, diagnostics, separation, biosensors, tissue engineering, coatings and textiles, catalysis, energy, optics, and electronics.

Figure 14. Fabrication process of ultrathin PVA films by the repetition of adsorption from aqueous solution and subsequent drying in air. Reprinted with permission from ref 501. Copyright 1999 American Chemical Society.

Kotov507 published a breakthrough paper that addressed the importance of the contributions of hydrophobic interactions between polyelectrolytes and charged surfaces in the formation of stable layers. Kotov reported the existence of several independent contributions to the Gibbs free energy of adsorption of a positively charged polyelectrolyte to a negatively charged polyelectrolyte surface. Those contributions included the removal of the ionic atmosphere around both positively and negatively charged polyelectrolytes, the reorientation of water molecules previously oriented by charged polyelectrolytes, the loss of mobility of the polyelectrolyte chains, and the partial removal of the hydration shell around both positively and negatively charged polyelectrolytes. All these contributions have a large component of entropic nature due to the release of water molecules when hydrophobic parts of polyelectrolyte chains establish a contact. Therefore, it has been demonstrated that, besides electrostatics, hydrophobic interactions must be necessarily taken into account when considering LbL multilayer formation, i.e., purely electrostatic interactions do not guarantee by themselves the formation of the multilayers. 5.1. Influence of Density of Hydrophobic Groups on Adsorbed Layers

Whitesides and co-workers508 reported the development of a model system comprising mixed SAMs of alkanethiols with a large number of hydrophobic groups for studying the interactions of proteins with well-defined hydrophobic surfaces that present organic groups of well-defined size and shape. The authors showed that the adsorbed amount of proteins could be controlled by the density of the hydrophobic groups predisposed at the surfaces. In addition, surfaces presenting high densities of hydrophobic groups promoted more conformational changes in the adsorbed proteins than analogous surfaces with low densities of hydrophobic groups. This issue can be explained by the higher propensity of proteins to spread and denature on very hydrophobic surfaces. Lojou and Bianco509 unveiled the interactions between proteins and polyelectrolytes on gold surfaces by QCM and electrochemical techniques and confirmed the assembly of such layers via hydrophobic interactions. Besides electrostatics, it was shown that hydrophobic effects play a vital role in the adsorption process. Guyomard et al.510 investigated the effect of the hydrophobicity of polysaccharide derivatives on the preparation of multilayer assemblies. They reported the assembly of amphiphilic carboxymethylpullulan (CMP), an anionic polysaccharide, with different polycations, such as PEI, PDADMAC, PLL, and CHT, and provided clear evidence that hydrophobic interactions strongly stabilize the adsorbed layers during multilayer buildup. In addition, they showed that the thickness of the adsorbed layers was determined by the

5. LBL ASSEMBLY THROUGH HYDROPHOBIC INTERACTIONS The hydrophobic interactions play a major role in the buildup of multilayer films mainly when the adsorbed molecules are uncharged.498−506 One such example is the fabrication of stable ultrathin films of poly(vinyl alcohol) (PVA) onto a gold substrate by repetitive physical adsorption from aqueous solution and subsequent drying processes, as demonstrated by Serizawa et al.501 The authors showed that surface reconstruction on drying facilitated the adsorption of PVA at the solid−liquid interface, as well as that the assembled amount of PVA increased with increasing its concentration. Figure 14 illustrates the fabrication of ultrathin PVA films via hydrophobic interactions. 8897


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Almost at the same time, Kaplan and co-workers511 reported for the first time the fabrication of biomaterial coatings by stepwise aqueous deposition of silk fibroin (SF) multilayers via mainly short-range hydrophobic interactions. They showed that those films were stable under physiological conditions and supported human bone marrow stem cell adhesion, proliferation, and differentiation, being thus suitable for bone tissue engineering (Figure 16). As can be seen from this figure, the hematoxylin

conformation and length of amphiphilic polyelectrolyte chains in aqueous solution because most intra- and/or intermolecular hydrophobic interactions between molecules were preserved during the adsorption process. As can be seen from Figure 15, the

Figure 16. Microscopy images (100×) of hematoxylin and eosin (H&E), alkaline phosphatase (ALP), and Alizarin Red-S (AR) staining of human bone marrow stem cell cultured for 1 day, 1 week, 2 weeks, and 3 weeks on 6-layered silk films on glass slides. The scale bars in each image are 200 μm. Reprinted with permission from ref 511. Copyright 2005 American Chemical Society.

and eosin (H&E, left column) and alkaline phosphatase (ALP, middle column) microscopic images show that the osteoblastlike cells increased with increasing culture time. Moreover, the red color of the Alizarin Red-S (AR, right column) stain further revealed the presence of calcium phosphate. In addition to the fabrication of LbL films, the construction of LbL microcapsules has been also reported. In this regard, Tsukruk and co-workers512 designed and engineered robust and highly permeable hollow microcapsules incorporating a single biocompatible component in their shells, namely, silk protein, making use of the LbL approach via hydrophobic interactions. The permeability of the silk microcapsules was evaluated by using fluorescein isothiocyanate (FITC)-labeled dextrans of different molecular weights as a fluorescent probe, as confirmed by CLSM (see Figure 17). As is clear from this figure, the multilayer shells remained open for the overwhelming majority of dextrans displaying different molecular weights, being closed only for the dextran of highest molecular weight. This behavior further confirmed the high permeability of such silk microcapsules, which make them highly suitable platforms for controlled drug/therapeutic delivery, biomedical, and biosensing applications. Very recently, Trojer et al.513 prepared well-defined charged polymeric microspheres consisting of hydrophobic polymers for controlled release of hydrophobic active dyes, such as 2-(4-(2-chloro-4-nitropheny-

Figure 15. (a) Thickness of LbL films assembled from PEI and CMPxCn derivatives (x ≈ 10%) versus number of dipping cycles: CMP (□), CMP-13C4(▲), CMP-8C6 (◊), CMP-8C8 (●), CMP-7C10 (▽), CMP9C12 (■). (b) Variation of the thickness of 10-bilayer [PEI/CMP-xCn] films versus the length of alkyl chains (n) of CMP-xCn derivatives (with x ≈ 10%). The lines are drawn as a guide for the eye. Reprinted with permission from ref 510. Copyright 2005 American Chemical Society.

increase of the length of the polyelectrolyte chains led to thicker films, thus meaning that a large amount of molecule is being adsorbed at each deposition step. This behavior is consistent with the reinforcement of the hydrophobic interactions between CMP-xCn chains themselves and between the CMP-xCn chains and the surface upon increasing the length of the hydrocarbon chains, and it clearly suggests that a better understanding of the structure of polyelectrolyte solutions is of relevance to understand the LbL assembly process. Hence, multilayer assemblies incorporating hydrophobic moieties constitute promising platforms for the immobilization of several small hydrophobic drugs and biomolecules, being thus of great interest for several research fields, including drug/therapeutic delivery and biosensing. 8898


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melanocyte stimulating hormone (α-MSH), onto the surface of the biodegradable polymer poly(lactic-co-glycolic acid) (PLGA) via hydrophobic interactions. The authors performed MD simulations to examine the structure and surface chemistry of α-MSH in solution and upon binding to PLGA with the main goal of determining the best conditions for LbL assembly. Furthermore, such a short peptide acted as a building block to assemble and stabilize multilayer systems comprising HA and CHT. The multilayer assembly was stable at physiological pH, and the optimal conditions would potentially allow the tunable release of α-MSH in order to modulate the inflammation triggered from biomaterials and tissue engineering implants. In summary, the hydrophobic interactions significantly extended the number of materials capable of experiencing LbL assembly and generated nanostructured functional materials that can be used in a wide range of applications such as drug/therapeutic delivery, biosensors, separation, protein and cell adhesion, or implantable coatings.

6. LBL ASSEMBLY BASED ON HYDROGEN BONDING Besides electrostatics, hydrogen bonding is one of the most studied driving forces to date and allows the incorporation of many uncharged materials into multilayer films provided that they present moieties that can act as hydrogen bonding donors and hydrogen bonding acceptors.

Figure 17. CLSM images of (silk)5 microcapsules exposed to FITClabeled dextran solutions with varied molecular weights. Reprinted with permission from ref 512. Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

lazo)-N-ethylphenylamino)ethanol (Disperse Red 13). Such systems would find promising applications as controlled-release devices in pharmaceutical, biomedical, biosensing, and biotechnological applications. Hence, despite the fact that hydrophobic forces can be posted as very important driving forces to guide the adsorption of molecules via the LbL approach, the role of these interactions on the stability, permeability, structure, and growth of multilayers is not fully understood, thus deserving further comprehensive attention.

6.1. Effect of Hydrogen Bonding Donor and Acceptor Groups on the Amount of Adsorbed Layers and on the Responsive Properties of Assembled Systems

The first studies employing LbL assembly based on hydrogen bonding were reported independently by Stockton and Rubner518 and Zhang and co-workers.519−524 Stockton and Rubner fabricated hydrogen bonded LbL assembled films by simple alternate deposition of polyaniline (PANI) and a variety of nonionic water-soluble polymers such as poly(vinylpyrrolidone) (PVPON), PVA, poly(acrylamide) (PAAm), and poly(ethylene oxide) (PEO).518 The assembly of the layers was confirmed through FTIR spectroscopy. Moreover, the effects of both solution pH and polymer molecular weight on the deposition process were examined using ellipsometry and UV−vis spectrophotometry. It was found that both parameters strongly influenced the bilayer thickness and electrical conductivity of the multilayer films. With the exception of PEO, lowering the pH resulted in an increase in the bilayer thickness. In addition, the large density of loops and tails of high molecular weight polymer chains, such as PVPON chains in a PANI/PVPON bilayer system, produced thicker layers than those of lower molecular weights, namely, PSS chains in a PANI/ PSS bilayer system. The bilayer thickness of a PANI/PVPON bilayer system increased with increasing the molecular weight of PVPON, reaching a plateau at a molecular weight of ∼350 000. In the case of PSS, the bilayer thickness of the PANI/PSS system remained practically constant even though the molecular weight of PSS had been changed from 5 000 to 990 000. Moreover, the authors also compared hydrogen bonded PANI films with analogous films assembled via electrostatic interactions. They showed that PANI films assembled via hydrogen bonding resulted in higher amounts of PANI adsorbed per deposition cycle and exhibited higher conductivities than those obtained through electrostatic interactions. Zhang and co-workers519−524 reported that hydrogen bonding LbL assembly guided the alternate deposition of a variety of polymers such as PAA (hydrogen bonding donor), p-(hexafluoro-2-hydroxylisoprop-

5.2. Computational Simulation Approaches to Unravel LbL Assembly Mechanism and Investigate the Structure, Dynamics, and Morphology of LbL Multilayer Assemblies

The deposition processes of such LbL assemblies were also theoretically simulated. MD simulations have been carried out to provide new insights on the stability and structural properties of multilayer films, as well as to better understand the LbL assemblies. For example, Haynie and co-workers514 have reported MD simulations to study the interactions between two polypeptides, PLL and PGA, in physical conditions that resemble those found in PEMs systems. The results obtained suggest that, besides electrostatics, hydrophobic interactions play a significant role in the structure and stability of PLL/PGA multilayer assemblies. Furthermore, it was also shown that the preferred orientation of peptides in the β-sheet structures is antiparallel within sheets and parallel between sheets. Those polypeptides could be used in a wide range of applications such as drug delivery systems, cell/tissue scaffolds, artificial cells/viruses, and implantable device coatings. Horinek et al.515 performed allatomistic MD simulations to determine the force necessary to remove a hydrophobic spider-silk peptide molecule from a flat hydrophobic surface in the presence of water. They compared desorption force values obtained theoretically and experimentally and found that those values were very similar. This examination clearly indicates that the MD simulations are suitable to study the hydrophobic attraction of peptides to surfaces. Furthermore, Go and co-workers516,517 reported the adsorption of a tridecapeptide anti-inflammatory hormone, α8899


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Figure 18. LbL assembly of PVP and DEN-COOH on a quartz substrate based on hydrogen bonding: (I) adsorption of PVP and (II) adsorption of DEN-COOH. Reprinted with permission from ref 523. Copyright 2003 American Chemical Society.

yl)-α-methylstyrene and styrene (PSOH, hydrogen bonding donor), carboxyl-terminated polyether dendrimer (DENCOOH, hydrogen bonding donor), and poly(4-vinylphenol) (PVPh, hydrogen bonding donor) with poly(4-vinylpyridine) (PVP, hydrogen bonding acceptor). The multilayer formation, structure, and morphology and the interaction between the two polymers were characterized by UV−vis and FTIR spectroscopy, XRD, and AFM measurements. Figure 18 shows a schematic representation of the LbL assembly process based on hydrogen bonding interactions between PVP and DEN-COOH. In the study concerning the LbL assembly of PVP/PVPh multilayer films, the absorption band that appeared at 256 nm was assigned to the π−π* transition of the pyridine ring of PVP, whereas the absorption bands at 230 and 278 nm were attributed to the π−π* transition of the benzene ring of PVPh, thus confirming the presence of both components in the multilayers.524 Moreover, the linear increase of the absorbance, at the three different wavelengths, with the number of deposited layers further indicated that the amounts of PVP and PVPh adsorbed at each deposition cycle were almost the same, thus confirming a uniform assembly process (Figure 19a). FTIR spectroscopy further confirmed the LbL buildup of the (PVP/PVPh)n multilayer films (Figure 19b), as confirmed by the PVP peaks appearing at 1598, 1558, and 1418 cm−1, which were assigned to the ring vibration of pyridine, and by the PVPh bands at 1512 and 1448 cm−1, which were attributed to the vibration of the benzene ring. XRD and AFM measurements further suggested the formation of a smooth film with constant thickness and homogeneous coverage, with low surface roughness (∼1.9 nm), and without regular electron density modulation across the film. Sukhishvili and Granick525,526 also constructed polymeric multilayer films via hydrogen bonded LbL assembly. They demonstrated that hydrogen bonded LbL films were very sensitive to their environment and could be destroyed by applying external stresses, such as pH, ionic strength, temperature, or an electrical field. Specifically, the authors assembled multilayer films comprising polyacids such as PAA and PMAA and polybases, namely, PVPON and PEO, and showed that multilayers based on PMAA/PVPON and PMAA/PEO could be stable up to pH 6.9 and 4.6, respectively. Over those pH values

Figure 19. (a) UV−vis spectra of (PVP/PVPh)8 multilayer film, measured after each deposition cycle. The solid spectra were recorded after PVPh adsorption, and the dotted spectra, after PVP deposition. The inset figure shows the absorbance at 230, 256, and 278 nm as a function of the number of deposition cycles. (b) FTIR spectra of (PVP/ PVPh)n multilayer film with n = 0, 2, 4, 6, and 8 (from bottom to top) assembled on a PEI-modified CaF2 plate. The inset figure shows the absorbance at 1598, 1558, 1512, 1448, and 1418 cm−1 as a function of the number of deposition cycles. Reprinted with permission from ref 524. Copyright 2004 American Chemical Society.

the hydrogen bonded multilayers were destroyed as a result of the increase in the ionization degree of the polyacid components. So, hydrogen bonded LbL assembly could be used to fabricate 8900


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Figure 20. (I) CLSM images of cross-linked (PGAAlk+DOX)6 capsules obtained from 3 μm diameter silica particles. (a, b) CLSM fluorescence and brightfield images. (c, d) Magnified CLSM image of two capsules and their corresponding fluorescence profile. (II) Enzymatic degradation of (PGAAlk+DOX)6 capsules obtained from 3 μm diameter silica particles, as monitored by flow cytometry. Degradation with 0.1 mg/mL protease was carried out in PBS at 37 °C. A control sample of capsules was incubated in the absence of protease. Adapted with permission from ref 537. Copyright 2010 American Chemical Society.

biomedical applications, including drug/therapeutic delivery and bioreactors. Huo et al.539 reported hydrogen bonding LbL assembly by deposition of multilayers of the same building block, namely, dendrimers, presenting carboxyl groups on its periphery, thus acting as hydrogen bonding donor as well as hydrogen bonding acceptor. Cecchet et al.540 described the linking of a benzylic amide macrocycle and a related benzylic amide macrocycle-based molecular shuttle with a SAM of 11-mercaptoundecanoic acid (MUA) via hydrogen bonding, as confirmed by XPS. Quinn and Caruso118,541 investigated hydrogen bonding based multilayer assemblies by alternate deposition of poly(N-isopropylacrylamide) (PNIPAAm) and PAA,118 and hydrophilic−hydrophobic copolymers such as poly(styrene-alt-maleic acid) (PSMA) and PEO at different salt concentrations, respectively.541 The use of hydrophilic−hydrophobic copolymers proved to greatly influence the value of these films as thermoresponsive materials. The multilayer growth was followed by QCM, and the surface morphology was examined by AFM. It was found that, for the same number of bilayers, films assembled at higher ionic strengths (0.2 M NaCl) exhibited higher thicknesses than the ones fabricated at lower ionic strengths (0.02 M NaCl). Furthermore, the multilayer films fabricated at higher ionic strength and ending with a PSMA layer showed higher surface roughness than the films assembled without salt and with a terminal PEO layer. The same authors also reported the alternate deposition of poly[(styrene sulfonic acid)-co-(maleic acid)] (PSSMA) and PNIPAAm at pH 2.5 via hydrogen bonding based LbL assembly.542 The authors also incorporated and assessed the influence of multivalent ions (e.g., Ce4+ or Fe3+) on the stabilization of hydrogen bonded LbL assemblies at high pH (7.1), and they found that the film loss was negligible. Yang et al.543 reported the fabrication of nanocomposite LbL films by hydrogen bonding of a spherical polymer brush and PVPON, as confirmed by FTIR spectroscopy. They also unveiled the influence of pH on the hydrogen bonding LbL assembly of

layered and erasable polymeric multilayers. Therefore, to stabilize hydrogen bonding based multilayer assemblies at high pH, Yang and Rubner527 developed thermal and photochemical cross-linking approaches using PAAm and PAA as building blocks. To avoid the dissolution of the film, the multilayers were deposited by the alternate dipping of a PAH-coated substrate into dilute aqueous solutions of PAA and PAAm at pH 3.0. Those approaches could then be used to create water-based micropatterned multilayer films using both photolithography and inkjet printing techniques. Kozlovskaya et al.528 demonstrated the fabrication of pH-sensitive polymeric multilayer capsules via hydrogen bonding LbL assembly. Furthermore, several authors have also reported the fabrication of multilayer capsules comprising different polymeric systems and responding to different stimuli through hydrogen bonding.529−538 For instance, Caruso and co-workers537 constructed biodegradable multilayer capsules encapsulating the well-known anticancer therapeutic doxorubicin hydrochloride (DOX) through hydrogen bonding interactions between alkyne-functionalized PGA (PGAAlk) and/ or DOX-conjugated PGAAlk (PGAAlk+DOX) with PVPON on planar and colloidal sacrificial silica templates, respectively. Such capsules were further covalently stabilized by using a diazide cross-linker, and PVPON was released from the multilayer assemblies by changing the solution pH to yield singlecomponent PGAAlk capsules that exhibit reversible swelling/ shrinking behavior. Further incorporation of PGAAlk+DOX in the multilayer shell enabled the fabrication of drug-loaded polymer capsules with precise control over drug dose and location in the multilayer assembly (see Figure 20I). Moreover, it was shown that cross-linked drug-conjugated polymer capsules (PGAAlk+DOX) could be degraded enzymatically, thus leading to the controlled release of the active DOX after ∼2 h (see Figure 20II). Further incubation of the capsules with cells showed a decrease in cell viability. These polymer−drug conjugates constitute very promising candidates as carrier systems for 8901


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Figure 21. (I) Optical fluorescence images of nontreated (a), PEI(TA/PVPON)3-coated cells (b), and (PAH/PSS)3-coated cells (c) after expression of yEGFP was induced. The inset figure shows a transmittance optical image of the same area for (c). (II) Growth of PEI(TA/PVPON)0,(3),(4)-coated YPH501 yeast cells after yEGFP expression was induced (a). CLSM images of PEI(TA/PVPON)3-coated cells after 10 h (b) and 46 h (c) of yEGFP expression. Adapted with permission from ref 555. Copyright 2011 Royal Society of Chemistry.

multilayer films composed of PVPON and PAA.544 The authors demonstrated that the solution pH greatly affected the thickness and morphology of the hydrogen bonded LbL PVPON/PAA film as a consequence of the ionization degree and conformation change of PAA. It was found that those films could be easily prepared at pH < 4.0 (critical pH value for film buildup) and disintegrated at pH 5.5. Hao and Lian545,546 and Jiang et al.547 fabricated hydrogen bonded LbL films based on polymer/ nanoparticle systems. Hao and Lian545 prepared hydrogen bonded LbL films via two different routes: by sequential adsorption of PVP and gold nanoparticles (AuNPs) stabilized with 4-mercaptobenzoic acid (MBA), and by using PAA and PVP-stabilized AuNPs. They also reported the fabrication of hybrid multilayer thin films comprising PVP and CdSe nanoparticles through hydrogen bonding between the pyridine group of PVP and the carboxylic acid group on CdSe, as confirmed by infrared spectroscopy.546 Jiang et al.547 used poly(3-thiophene acetic acid) (PTAA) and PAA to form hydrogen bonds with PVP-stabilized AuNPs. They investigated the release behavior of gold-containing multilayers at different pH values and found that the polymers were difficult to be removed from the substrate when compared to their gold-free counterparts. Moreover, it was found that, beyond the interaction between the S atom of PTAA and the PVP-stabilized

AuNPs, the interaction between neighboring bilayers was possibly induced by the AuNPs acting as physical cross-linking points.547 Hammond and co-workers548 fabricated hydrogen bonded LbL assemblies comprising PAA and biodegradable poly(ethylene oxide)-block-poly(ϵ-caprolactone) (PEO-b-PCL) micelles as drug delivery vehicles for hydrophobic drugs. The authors demonstrated the possibility of generating flexible freestanding films by using the weak interactions of the hydrogen bonded film on hydrophobic surfaces. Erel-Unal and Sukhishvili549 described the fabrication of purely hydrogen bonded hybrid multilayers using polymer pairs that decomposed at acidic (poly(N-vinylcaprolactam)−poly(L-aspartic acid), PVCL− PLAA) and basic (poly(N-vinylcaprolactam)−tannic acid, PVCL−TA) pH values. Thus, the dissolution and release of the film components could be effectively controlled by using purely hydrogen bonded multilayer films, as confirmed by FTIR spectroscopy and ellipsometry. The possibility of tuning the disassembly properties through film architecture, and the biocompatibility of the film components, made these films highly promising candidates for applications in controlled release. Almost at the same time, Zhuk et al.550 also fabricated LbL films of different neutral temperature-responsive polymers such as PVCL, PNIPAAm, or PAAm with a poly(carboxylic acid) such as PAA or PMAA via hydrogen bonding. They found that 8902


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the temperature had a very pronounced effect on the film stability while changing the solution pH and showed that the use of a deposition or postassembly temperature close to the lower critical solution temperature (LCST) resulted in enhanced film stability on the pH scale. Moreover, the authors also explored the interplay between the temperature and the pH response of such LbL assemblies as a crucial factor contributing to their temperature-responsive function. It was found that hydrogen bonded release films could be used in solutions at neutral pH and moderate temperatures. It was also shown that the working pH value for such temperature-triggered release films could be controlled and adjusted by simply choosing different pairs of hydrogen-bonding polymers, from a mixture of neutral polymers and poly(carboxylic acids). The authors reported that LbL assemblies of polymers with LCST properties showed an increasing critical pH of film disintegration in the temperature range from 10 to 37 °C, whereas polymers with an upper critical solution temperature (UCST) behaved the opposite way. This characteristic made this approach highly promising for drug delivery, biomedical, and tissue engineering applications. The same research group also published a very good review about the fabrication of polymeric LbL films via hydrogen bonding interactions, their properties, and further applications.551 The authors highlighted the potential of hydrogen bonding LbL assembly for surface modification and functionalization, the construction of responsive functional containers and membranes, the production of highly conductive solid-state matrices, and the applications in biotechnology and biomedicine. Manna et al.552 reported the fabrication of highly biocompatible and biodegradable LbL films comprising CHT/HA modified with DNA pairs via hydrogen bonding. Such LbL assemblies hold high potential for controlled drug delivery, electronics, optics, biosensing, and tissue engineering applications. Almost at the same time, Lin et al.553 prepared hydrogen bonded LbL assemblies based on PVPON and PAA and investigated the effect of salt concentration on the stability and structural properties of such films. They found that salt solutions of low concentration induced microphase separation in the films, which was explained by the enhanced dissociation of PAA leading to the partial disruption of hydrogen bonds in the films. Moreover, microphase separation in the films increased by increasing the solution pH and the salt concentration. Besides the fabrication of 2D multilayer films, the fabrication of 3D systems though hydrogen bonding has been also studied and explored. In this regard, Kozlovskaya et al.554 studied the responsive properties of hollow multilayer shells of TA assembled with several biocompatible and nontoxic neutral polymers, such as PVPON, PNIPAAm, and PVCL, through hydrogen bonding. The authors found that the properties of the multilayer shells could be tuned by changing the ionic strength of a neutral polymer with TA, as well as by a change in the counterpart hydrophobicity. They also demonstrated that the permeability of the multilayer films could be controlled by changing the pH, which provided an opportunity for loading and release of a functional cargo under mild conditions. The same research group also took advantage of highly permeable hydrogen bonded LbL assemblies incorporating PVPON and TA for encapsulation of yeast cells with preserved integrity and functions.555 They showed that hydrogen bonded multilayer shells comprising biocompatible and nontoxic TA and PVPON polymers and PEI monolayer as precursor did not interfere with the expression of the green fluorescent protein reporter (yEGFP) produced by the cells, as shown by the fluorescence emission from the yeast cells

(Figure 21I). The same behavior was not observed for multilayer shells incorporating PAH/PSS coatings. Moreover, such TA/ PVPON polymeric shells not only enabled the exponential growth of the multilayer shell-coated cells after several hours of the yEGFP expression but also allowed cells to break the polymeric shell, thus suggesting that the coating delays but does not suppress cell division (Figure 21II). Such hydrogen bonded LbL assemblies hold great potential for living cell surface engineering for biomedical and biosensing applications.251 Rubner and co-workers556 reported the fabrication of anisotropic CHT/SF LbL films with highly oriented fibers via hydrogen bonding. They suggested the potential of such biocompatible biopolymer-based assemblies for surface functionalization in a wide range of biomaterial applications. Moreover, the excellent mechanical properties assured by SF together with the biocompatibility of the multilayer system make these naturalbased highly oriented systems desirable to improve surface biocompatibility and for guiding cell adhesion, growth, and spreading. Such et al.557 published an excellent review in which they focused on the recent developments in the LbL assembly and engineering of hydrogen bonded films/capsules with enhanced stimuli-responsive features for biomedical applications.558 Recently, Kharlampieva and co-workers 559 also fabricated hydrogen bonded SF multilayer films with synthetic macromolecules, such as PMAA or PVCL, and a natural polyphenol, such as TA. They found the multilayer growth and stability to be strongly dependent on the solution pH. Although SF assembled with PMAA and TA at pH 3.5, it disintegrated at pH 5. On the other hand, SF/PVCL systems were stable at low and high pH values but resulted in thinner films at high pH. Those LbL assemblies could then be used as biomimetic materials with tunable properties. Irmukhametova et al.560 fabricated hydrogen bonding LbL assemblies of PEGylated organosilica nanoparticles and PAA under acidic conditions (pH < 3.0) for biomedical and pharmaceutical applications. They revealed that, when doing the assembly process in solution, larger nanostructures were formed. However, when doing it in a silicon wafer surface, through the LbL assembly process, multilayered coatings were achieved. These multilayer assemblies hold great promise as drug delivery systems for several biomedical and biotechnological applications. Sung et al.561 constructed PEO/ PAA and PEO/PMAA hydrogen bonded LbL assemblies and investigated the influence of the film thickness on the thermal properties of such LbL assemblies, using modulated DSC and temperature-controlled ellipsometry. In the PEO/PAA LbL films, the glass transition temperature (Tg) increased 9 °C in comparison with the bulk Tg as the film thickness decreased to 30 nm. On the other hand, the PEO/PMAA LbL films showed only a single Tg, which was unaffected by any changes in the film thickness. At the same time, Lee and co-workers562 fabricated hydrogen bonded LbL films incorporating PAA and PAAm and studied the effect of thermal treatment on cross-linking, structure, and swelling behavior of such films. The authors found that the well-defined control of the temperature allowed tailoring the properties, structure, and functionality of such LbL assemblies. A complete cross-linking of those films was only achieved at temperatures higher than 150 °C. Furthermore, they tested the adhesion of human mesenchymal stem cells (hMSCs) on such films pretreated at two different temperatures (90 and 180 °C). It was found that at 90 °C the films showed higher swelling behavior and, thus, great cell adhesion resistance, whereas at 180 °C they revealed lower swelling behavior and high cell adhesion properties (see Figure 22). Therefore, one can 8903


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temperature, in the range of 20−40 °C, on the surface topography and film thickness, as revealed by AFM and ellipsometry, respectively (Figure 23). As can be seen from this figure, these films exhibited reversible temperature-triggered variation owing to the contracted and expanded PNIPAAm configuration induced by temperature. Moreover, the influence of the temperature on the electrochemical response of such films and on the switchable electrocatalytic performance of the films toward the oxidation of glucose was also studied (Figure 24). It was found that, with

Figure 22. Fluorescence images of human mesenchymal stem cells (hMSCs) (5 days after seeding, 5000 cells/cm2) on hydrogen bonded PAA/PAAm films thermally treated at 90 °C for 8 h (a) and at 180 °C for 8 h (b). hMSCs were stained with rhodamine phalloidin for actin and DAPI for nuclei. Adapted with permission from ref 562. Copyright 2012 American Chemical Society.

easily conclude that the cross-linking temperature plays a crucial role in the cell adhesion properties. Hence, partially cross-linking LbL films, below their complete cross-linking temperature, could be useful for biomedical and energy storage and conversion applications. Rubner and co-workers563 prepared multilayer assemblies encompassing PVA and weak polyacids such as PAA and PMAA via hydrogen bonding based LbL assembly. The authors unveiled the influence of both degree of hydrolysis and molecular weight of PVA on the film growth behavior and pH stability. It was found that the degree of hydrolysis of PVA became significant only when using weak hydrogen bonding pairs such as PVA and PAA, i.e., LbL films incorporating PVA and PAA could be effectively assembled only by using a high molecular weight sample of partially hydrolyzed PVA and low solution pH. In addition, the degree of hydrolysis of PVA and the presence of free hydroxyl (−OH) and carboxylic acid (−COOH) groups on the film, via chemical and thermal methods, could be used to modulate the pH stability of the PVA/PAA films. The potential of such LbL assemblies is foreseen mainly for biological and optical applications. Dou et al.564 fabricated thermal-responsive layered double hydroxide (LDH) nanoparticle/PNIPAAm ultrathin films on an indium tin oxide (ITO) electrode via hydrogen bonding LbL assembly. The authors assessed the effect of the

Figure 24. Current−time curves measured at 0.5 V for the (LDH/ PNIPAAm)10 films deposited on ITO electrode with successive addition of glucose in 0.1 M NaOH at 20 and 40 °C. The schematic illustrations show the expanded (“off”) and contracted (“on”) PNIPAAm configurations at 20 and 40 °C, respectively. Reprinted with permission from ref 564. Copyright 2012 American Chemical Society.

the successive additions of glucose, a gradual increase in the amperometric signals upon increasing the temperature from 20 to 40 °C was noticed, as well as an enhancement of response sensitivity. In addition, it was also demonstrated that the films exhibited a reversible temperature-dependent electrochemical on−off behavior, respectively, at 40 and 20 °C, which was assigned to the fast−slow interfacial charge transfer rate induced by the contraction−expansion transition of the PNIPAAm with low−high impedance in the film. These ultrathin films, displaying

Figure 23. (I) Surface topography change of (LDH/PNIPAAm)10 ultrathin film recorded by AFM images at various temperatures: from (a) 20 °C to (b) 40 °C and (c) back to 20 °C. Before AFM measurement, the film was immersed in water for 1 h. The scanned area was 5 μm × 5 μm for all images. (d) Reversible variation of root-mean-square (rms) roughness for the (LDH/PNIPAAm)10 ultrathin film at 20 and 40 °C for 5 heating−cooling cycles. (II) Reversible variation of the film thickness up to 10 cycles based on ellipsometry measurements for the (LDH/PNIPAAm)10 ultrathin films at 20 and 40 °C. Adapted with permission from ref 564. Copyright 2012 American Chemical Society. 8904


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Figure 25. LbL assembly of PIPBA and PVP via halogen bonding. (a) UV−vis absorption spectra of PIPBA/PVP multilayer films assembled on a quartz slide. The inset figure shows the growth of absorbance at 256 nm as a function of the number of bilayers of PIPBA/PVP films. (b) QCM frequency shift as a function of the number of bilayers of PIPBA/PVP multilayer film. Adapted with permission from ref 583. Copyright 2007 American Chemical Society.

well-suited to be applied in a broad range of research fields, including materials science, crystal engineering, liquid crystals, optics, photonics, synthetic chemistry, medicinal chemistry, supramolecular chemistry, and molecular recognition.

reversible switching behavior, would find promising applications in electroanalysis, switchable sensors, and information storage devices. 6.2. Halogen Bonding as an Alternative to Hydrogen Bonding for LbL Assembly

6.3. Computational Simulation Studies to Understand LbL Assembly Mechanism and Investigate the Structure and Morphology of LbL Multilayer Assemblies

In the past decade a few researchers have been focusing on an emerging intermolecular interaction driving the LbL assembly, which is based on halogen bonding. This intermolecular interaction, which presents similarities to the more familiar and aforementioned hydrogen bonding, involves halogen atoms such as bromine, iodine, or chlorine (halogen-bond donors, Lewis acids) and neutral or anionic Lewis bases, namely, amine or pyridine derivatives (halogen-bond acceptors). Furthermore, it has widespread applications in the fields of crystal engineering, separation, materials science, polymer sciences, liquid crystals, catalysis, medicinal chemistry, molecular recognition, supramolecular chemistry, and conductive materials.565−581 However, despite the similar features presented by both intermolecular interactions, the tunable interaction strength, hydrophobicity, larger atom size, and high directionality presented by halogen bonding interaction opened new and remarkable possibilities in the design and creation of advanced nanostructured functional materials with great potential for numerous applications.582, In this sense, Wang et al.583 described for the first time the fabrication of LbL assemblies comprising water-insoluble poly(4(4-iodo-2,3,5,6-tetrafluorophenoxy)-butyl acrylate) (PIPBA) and PVP via halogen bonding, as confirmed by UV−vis spectroscopy and QCM measurements (see Figure 25). As is clear from the visualization of Figure 25a, there are two main adsorption bands in the UV−vis absorption spectra. The absorption band at ∼233 nm was assigned to PIPBA, whereas the absorption band at ∼256 nm was attributed to the π−π* transition of the pyridine ring in PVP. Moreover, the inset figure clearly shows that the absorbance at 256 nm increased linearly with increasing the number of bilayers of PIPBA/PVP multilayer films, thus revealing that the amounts of both polymers adsorbed at each deposition cycle were the same. In addition, the frequency shift increased while increasing the number of bilayers (Figure 25b), thus further confirming the LbL assembly of both polymers. Very recently, Priimagi et al.584 also explored the key features, namely, the high directionality, presented by halogen bonding interaction in order to assemble with high efficiency and reliability stimuli-responsive supramolecular polymers, namely, azobenzene−polymer systems, for light-induced surface patterning. Therefore, although halogen bonding is still in its early stages, there is absolutely no doubt that it has great potential for designing advanced functional systems with unique properties

Hydrogen bonded LbL assemblies have been also theoretically simulated. For example, Yu et al.585 carried out MD simulations to investigate the morphology and structure of multilayers of the chiral molecule N-stearoy-L-glutamic acid (C18-L-Glu) assembled on a mica surface. They analyzed the changes in the energy during the MD simulation run and found that hydrogen bonding effects were the major driving forces behind the formation and self-organization of C18-L-Glu layered structures. On the basis of the results gathered from the MD simulation, they also proposed an atomic model for the assembling of C18-L-Glu molecules. Within every layer, C18-L-Glu molecules could assemble close to each other due to the formation of a lateral hydrogen bonding network between molecules, which might resemble the β-sheet structure of proteins in some extent. Overall, the LbL assemblies based on hydrogen bonding provide us with countless advantages regarding the preparation of stimuli-responsive materials and stabilization of the films, and they have shown to decisively contribute to enlarge the number and variety of materials that can be processed in the multilayer films, opening new challenges and opportunities for which LbL assemblies can be applied.

7. LBL ASSEMBLY DRIVEN BY CHARGE-TRANSFER INTERACTIONS The growth of multilayer films can also be achieved by the alternate adsorption of two types of nonionic molecules, which present electron-accepting and electron-donating groups, respectively, in the side chains. 7.1. Influence of Nonionic Molecules on the Growth and Properties of Multilayer Assemblies

Shimazaki and co-workers586−590 were the pioneers of this new concept for building up multilayer films. For example, multilayer films were constructed onto a gold surface based on chargetransfer (CT) interaction between poly[2-(9-carbazolyl)ethyl methacrylate] (PCzEMA, electron-donor polymer) and poly[2[(3,5-dinitrobenzoyl)oxy]ethyl methacrylate] (PDNBMA, electron-acceptor polymer).586−589 Thus, the obtained films have periodic layers of CT complexes, as shown in Figure 26, and such periodicity throughout the films may provide unique physical 8905


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In addition, the advantage of using CT interactions is that the films can be prepared in organic solvents, which considerably increases the possibility of incorporating hydrophobic functional groups in the films and the potential for making nanostructured films based on organic materials. Such films can be used to develop interesting materials for applications in electronics, photonics, and optics devices. Zhang and Cao591 reported the successful preparation of LbL films based on CT interaction between diazoresin (DAR) and poly(maleic anhydride-costyrene) (PMS). It was found that the multilayer thin films were very sensitive to light, and their stability toward polar solvents increased as a result of UV irradiation. Later, Wang et al.592,593 constructed anisotropically conductive LbL films composed of poly(dithiafulvene) (PDF, electron-donor polymer) and poly(hexanyl viologen) (6-VP, electron-acceptor polymer) through CT interactions between the polymer backbones. The authors investigated the incorporation of AuNPs into those LbL films either by reducing the gold ions with the previous deposited PDF layers or by directly depositing PDF-stabilized AuNPs as the electron-donor layers.593 Both inclusion methods have ensured good compatibility of the AuNPs with the LbL films and are applicable to other metal or semiconducting nanoparticles. Recently, Zhang and co-workers594 reported the generation of CT LbL films assembled from a two-component supramolecular amphiphile (8-hydroxypyrene1,3,6-trisulfonic acid trisodium salt−1′,1″-(butane-1,4-diyl)-bis(1-methyl-4,4′-bipyridine-1,1′-diium)dibromide diiodide (HPTS−diMV)) and diazoresins (photosensitive agent, DAR) followed by stabilization of their structure by photochemical

Figure 26. Side-view illustration of the LbL deposited film via chargetransfer interaction. For simplicity the carbazolyl groups and 3,5dinitrobenzoyl groups are represented as D and A, respectively. Pairs of D and A forming charge-transfer complexes are circled. Reprinted with permission from ref 586. Copyright 1997 American Chemical Society.

properties that are characteristic of the polymers used and are not expected for the films containing homogeneously dispersed CT complexes.

Figure 27. Schematic representation of the LbL assembly process comprising DAR and HPTS−diMV. (b) UV−vis spectra monitoring the LbL assembly of a (DAR/HPTS−diMV)5DAR multilayer film (i.e., five bilayers of DAR and HPTS−diMV plus an extra DAR layer). The inset figure shows the growth of absorbance at 380 nm as a function of the number of deposited layers. (c) UV−vis spectra of a (DAR/HPTS−diMV)5DAR multilayer film before and after UV irradiation. (d) AFM image of a (DAR/HPTS−diMV)5DAR multilayer film after UV irradiation (3 × 3 μm2). Adapted with permission from ref 594. Copyright 2011 American Chemical Society. 8906


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polymer films via strong host−guest interactions between a neutral β-cyclodextrin (β-CD) dimer and positively charged ferrocene-appended poly(allylamine hydrochloride) (Fc-PAH). Crespo-Biel et al.599 further demonstrated the possibility of building up inorganic/organic multilayer films with controlled thickness based on selective host−guest interactions between host CD-modified AuNPs and guest adamantyl-functionalized dendrimers, as shown in Figure 28.

cross-linking (see Figure 27). The stepwise assembly process, which is schematically depicted in Figure 27a, was monitored by UV−vis spectroscopy (Figure 27b and c). As is clear from the visualization of Figure 27b, a well-defined absorption band appears at ∼380 nm. Such an absorption band is characteristic of the π−π* transition of the diazonium groups of DAR, thus indicating that DAR was deposited on the multilayer film. In addition, the absorbance at 380 nm increased linearly with the number of deposited layers, thus indicating a regular and uniform deposition process. After the deposition process, the multilayer films were stabilized by UV irradiation-induced crosslinking (Figure 27c). As shown in this figure, the absorption band that appeared at 380 nm disappeared after UV irradiation, and a new band emerged at ∼290 nm, which was due to the ester linkage. This behavior indicated that the diazonium groups were decomposed and that the photoreaction between the diazonium and the sulfonate or hydroxyl groups occurred. Moreover, the structural characteristics of such assemblies were studied by AFM (Figure 27d). A roughness of ∼10 nm was obtained for the multilayer films, thus revealing that the surface of the assembled films is rather smooth. Very recently, the same research group also fabricated highly stable LbL assembled films composed of an azulene-based supramolecular amphiphile (PAL-Py) and a photoreactive polyanion (PAAN3) via CT interactions.595 They reported, for the first time, the successful fabrication of functional surface-imprinted multilayer films with special binding to polycyclic aromatic hydrocarbons (PAHs) upon removal of one hydrophobic component of the supramolecular amphiphiles, pyrene, from the multilayer films. The removal of pyrene was assured by washing with organic solvent after cross-linking by UV irradiation. It was envisaged that those LbL films could be used for the separation of PAHs and for water purification. Surface molecular imprinted LbL films comprising DAR and PAA− porphyrin complex have also been successfully fabricated on porous membranes made of poly(ether sulfone) (PES) for selective filtration of ions in solution.596 The combination of these or other LbL assemblies containing imprinting materials and porous membranes result in composite membranes that hold great promise for the development of advanced functional selective filtration systems for the separation of several compounds. In summary, CT interactions between nonionic electron-donor and electron-acceptor molecules allow the generation of supramolecular LbL assemblies that can be used to develop smart functional materials for applications in electronic, photonic, optical, and sensing devices.

Figure 28. LbL assembly scheme for the alternating adsorption of adamantyl-terminated dendrimer and CD-AuNPs onto CD SAMs via host−guest interactions. Reprinted with permission from ref 599. Copyright 2005 American Chemical Society.

The growth of multilayer films with the number of layer pairs was monitored by several characterization methods, such as SPR and UV−vis spectroscopy and ellipsometry. It was shown that the CD-adamantyl host−guest interactions were essential for the successful and well-defined multilayer buildup and for the accurate control of the thickness of the adsorbed layers at the nanometer scale level, measured using ellipsometry and AFM. These techniques showed that the film thickness grew linearly with the number of deposited layers and that such an increase was ∼2 nm per deposited bilayer (Figure 29I). A similar behavior was also found through UV−vis spectroscopy measurements (Figure 29II). The absorption band that appeared at 525 nm, which was assigned to the adsorption of CD-AuNPs onto the film, increased linearly with the number of deposited bilayers, thus validating the ellipsometry data. Van der Heyden et al.600 constructed multilayer films based on host−guest interactions between two derivatized chitosan biopolymers, one with β-CD cavities and the other with adamantyl moieties. They showed that the assembly stability was conferred by multivalent complexation at each step and that the assembly growth was mainly governed by the availability of the complexation sites supplied by each layer. Because the fabrication process is stimuli-responsive, promising devices can be fabricated through host−guest interactions between biopolymers, which would find promising applications in several research fields, including biomedical and biotechnological fields. Dubacheva et al.601 revealed the possibility of unlimited growth of host−guest multilayer films comprising the neutral poly(Nhydroxypropylmethacrylamide) (PHPMAAm) bearing Fc or βCD groups on SAM-β-CD modified Au surfaces. In addition to the fabrication of 2D films, the construction of 3D multilayer assemblies has been the focus of a few researchers. In this sense,

8. LBL ASSEMBLY VIA HOST−GUEST INTERACTIONS Multilayer assemblies can also be fabricated using the LbL assembly approach based on highly selective and specific host− guest interactions, by exploring the strong interactions between host (e.g., cyclodextrins, cucurbiturils, calixarenes, pillararenes, crown ethers, porphyrins) and guest (e.g., ferrocene, adamantane, azobenzene) molecules. 8.1. Influence of Several Host and Guest Molecules on the Assembly, Growth, and Responsive Properties of Multilayer Systems

Zhang and Cao597 revealed that the synergistic effect of LbL assembly and host−guest interactions could be used to build up multilayer films based on water-soluble calixarenes (cavityshaped cyclic molecules made up of benzene units) and lipophilic dyes. Almost at the same time, Anzai and co-workers598 unveiled the possibility of fabricating sensing devices by forming LbL 8907


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Figure 29. (I) Evolution of the ellipsometric thickness as a function of the number of bilayers of adamantyl-terminated dendrimer/CD-AuNPs deposited onto CD SAM modified silicon oxide. (II) UV−vis absorption spectra (a) and absorbance at 525 nm (b) as a function of the number of bilayers of adamantyl-terminated dendrimer/CD-AuNPs deposited onto CD SAM modified glass substrate. Adapted with permission from ref 599. Copyright 2005 American Chemical Society.

Figure 30. Schematic illustration of the LbL fabrication process via host−guest interactions and structural characterization of resulting microcapsules. (a) Host−guest driven LbL assembly of the same polyelectrolyte on calcium carbonate (CaCO3) particles followed by core dissolution with ethylenediaminetetraacetic acid (EDTA) to obtain hollow microcapsules. The chemical structures of PAH-g-β-CD, PAH-g-ferrocene, and β-CD/ ferrocene inclusive are shown in the second row. (b) The thickness of the PAH-g-β-CD/PAH-g-ferrocene multilayers assembled on silicone wafer as a function of layer number. (c) SEM, (d) AFM, and (e) TEM images of the prepared (PAH-g-β-CD/PAH-g-ferrocene)3 microcapsules; the scale bar is 5 μm. Inset in (c) is a higher magnification image of one capsule; the scale bar is 2 μm. Reprinted with permission from ref 602. Copyright 2008 American Chemical Society.

Gao and co-workers602 fabricated multiresponsive hollow microcapsules delimited by multilayer shells of the same polyelectrolyte, PAH, functionalized with β-CD or Fc via

host−guest interactions (Figure 30a). To verify the multilayer assembly of such grafted polyelectrolytes on sacrificial particles via host−guest interactions, the fabrication process was also 8908


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Figure 31. Swelling and shrinking behavior of the host−guest microcapsules in response to pH and ionic strength triggers. Evolution of the size of the microcapsules as a function of the (a) pH value and (b) salt concentration. CaCO3 sacrificial particles with a size of 4 μm were used. The inset figures in (a) are CLSM images of the host−guest microcapsules at pH 3, 6, and 11 as indicated by the arrows. (c) Reversible shrinking and swelling behavior of the microcapsules for three cycles of treatments in water and salt solution at pH 3. CLSM images of the microcapsules after being treated for three cycles in water (upper) and in 1 M salt solution (lower). The scale bar is 10 μm. Adapted with permission from ref 602. Copyright 2008 American Chemical Society.

Figure 32. CLSM images of (A, B) (PAA-C12-Azo)5/(CMD-g-α-CD and α-CD-RhB)5 and (C, D) (PAA-C12-Azo)5/(CMD-g-α-CD and RhB)5 microcapsules incubated in 0.4 M EDTA solution at pH 7.4 for 24 h in the dark. The scale bar is 10 μm. The curves in (E) and (F) correspond to the fluorescent intensities of the dotted lines (a, b) and (c, d), respectively. Reprinted with permission from ref 604. Copyright 2011 American Chemical Society.

accomplished on a 2D planar silicon wafer. As is clear from Figure 30b, the thickness of the multilayer film increased in a stepwise fashion while increasing the number of deposited layers, thus confirming the LbL growth driven by host−guest interactions. Therefore, the same LbL mechanism was assigned for the successful fabrication of the hollow microcapsules.

Further characterization of the structure and morphology of such microcapsules was performed by SEM (Figure 30c), AFM (Figure 30d), and TEM (Figure 30e) measurements. These microscopic images reveal the hollow nature of such microcapsules. 8909


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Figure 33. (I) Snapshots of the photodissociation of (PAA-C12-Azo)5/(CMD-g-α-CD and α-CD-RhB)5 microcapsules. The time interval between the snapshots was 20 min. The scale bar is 15 μm. (II) Drug release profiles of (PAA-C12-Azo)5/(CMD-g-α-CD and α-CD-RhB)5 microcapsules in the dark (A) and under 365 nm UV light irradiation (B). Adapted with permission from ref 604. Copyright 2011 American Chemical Society.

Furthermore, the responsive properties of such microcapsules when submitted to several stimuli, namely, pH and ionic strength, were evaluated. The capsules showed reversible shrinking and swelling behaviors for at least three deposition cycles at high and low pH and ionic strength, respectively (Figure 31). Consequently, such microcapsules showed high potential for the controlled loading and release of desired biomolecules. Some years later, Li et al.603 took advantage of the host−guest interactions between β-CD and adamantane (AD) to fabricate pH-responsive degradable microcapsules comprising two neutral biocompatible polymers such as dextran-graf t-β-CD (Dex-g-βCD) and poly(aspartic-graf t-adamantane) (PASP-g-AD). These pH-sensitive microcapsules exhibited controlled drug release behavior and, thus, could be used for applications in medicine and drug delivery. Almost at the same time, Zhang and coworkers604 fabricated photosensitive hollow microcapsules by assembling host carboxymethyl dextran-graf t-α-CD containing α-CD-rhodamine B or free rhodamine B (CMD-g-α-CD and αCD-RhB or CMD-g-α-CD and free RhB, respectively) and guest poly(acrylic acid) N-aminododecane p-azobenzeneaminosuccinic acid (PAA-C12-Azo) molecules on CaCO3 core particles via host−guest interactions, followed by dissolution of the CaCO3 sacrificial core template with EDTA. They demonstrated that both types of microcapsules, incorporating α-CD-RhB or free RhB, were very similar and could successfully load the model drug in the layers as proved by the regular, spherical, and red fluorescence, as shown by CLSM images (see Figure 32A and C). However, remarkable differences were observed after 24 h of dark incubation of both types of microcapsules in EDTA. While the background of the microcapsules fabricated by host−guest drug loading strategy (capsules containing α-CD-RhB) was still black (Figure 32B) and the fluorescent intensity was practically unchanged (Figure 32E), the red fluorescence could be clearly noticed in the background of the microcapsules constructed through physical adsorbing drug loading approach (capsules containing free RhB) (Figure 32D). In addition, the fluorescent intensity increased greatly (Figure 32F), and thus, a large amount of drug was released. Such a difference in the microcapsules’ behavior was assigned to the weak interaction established between free RhB and multilayers, which promoted the larger and uncontrollable release of the model drug RhB from the microcapsules.

The photodissociation behavior of hollow microcapsules containing α-CD-RhB and dipped into EDTA solution was further evaluated through CLSM by illuminating them with UV light. As shown in Figure 33I, the increase in the UV light irradiation time led to the decrease of the red fluorescence of regular shells until disappearance, as well as to an increase of the red fluorescence in the background. After 80 min of UV light irradiation, the core−shell structure of the microcapsules was destroyed and the α-CD-RhB loaded on the shells of the capsules got spread, thus suggesting that UV light irradiation promoted the dissociation of the microcapsules with the release of the model drug α-CD-RhB. Figure 33II further confirmed that the drug release dramatically increased with increasing UV irradiation time. 8.2. Computational Simulation Approaches to Reveal LbL Assembly Mechanism and Investigate the Structure, Conformation, and Dynamics of LbL Multilayer Assemblies

The LbL assemblies constructed via host−guest interactions have been also theoretically studied. Teobaldi and Zerbetto605 performed MD simulations to elucidate the conformation, structure, and dynamic properties of a fourth-generation poly(propylene amine) dendrimer dissolved in chloroform, as well as of the same dendrimer (host molecules) with subsequent encapsulation of several eosin Y dyes (guest molecules). It was found that the guest molecules compacted and made the dendrimer structure more spherical. Moreover, the simulations also showed that there were not static cavities in the dendrimer with eosin molecules inside. Chang and co-workers606 also investigated the structural and dynamical properties of host AD− urea poly(propyleneimine) dendrimers with carboxylic acid− urea guests using MD simulations and X-ray crystallography. Both methods suggested that guest molecules could bind to the dendrimer in several different ways. Nevertheless, most of those ways involved hydrogen bonding interactions between the host dendrimer urea groups with urea and/or carboxylic acid groups presented by the guest. Moreover, it was also found that acid− base interactions between the carboxylic acid groups of the guest and the tertiary amine in the interior of the dendrimer were also presented. Raffaini and Ganazzoli607 used MD simulations to obtain new insights about the conformation and dynamics of the host β-CD, as well as the stability and conformational properties of their inclusion compounds formed by β-CD and a glycoconjugate ((−)-menthol-β-D-glucoside, guest molecule). 8910


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Figure 34. Oversimplified schematics of a film deposition process based on specific biotin−streptavidin interactions. (1) Adsorption of biotinylated PLL on a precursor multilayer composed of a few layers of PSS and PAH. (2) Adsorption of streptavidin to biotin groups exposed to the solution interface of the film. Multilayer films are grown by cyclic repetition of steps (1) and (2). Reprinted with permission from ref 614. Copyright 1998 Elsevier.

Figure 35. (a) UV−vis absorption spectra of (avidin/ib-PEI)1−10 multilayer films deposited from 10 mM borate buffer solution at pH 12 on a dichlorodimethylsilane-modified quartz surface. The inset figure shows the absorbance of the films at 280 nm as a function of the number of deposited layers (a1) and the data obtained for the films prepared by using an avidin solution containing 10 mM biotin (a2). (b) Effect of the solution pH on the evolution of the absorbance at 280 nm of the films as a function of the number of depositions. A higher loading of avidin was found at pH 12, as well as an exponential growth of the film as a result of the diffusion of the constituents during the deposition. (c) Stability of (avidin/ib-PEI)10 multilayer films in different buffered solutions as a function of the immersion time. The avidin percent retained in the film was maximized at pH ≥ 8, and thus no deterioration was observed for up to 60 min. In opposition, at pH 5 and 6 the film was completely decomposed within a few minutes due to the reduced binding constant between ib and avidin. The fraction retained was calculated from the decrease of UV−vis absorption of the film at 280 nm. A 10 mM phosphate buffer was used for the media of pH 6, 7, and 8, whereas pH 5 buffer was prepared using 10 mM acetate. Adapted with permission from ref 624. Copyright 2005 American Chemical Society.

9. LBL ASSEMBLY DRIVEN BY BIOLOGICALLY SPECIFIC INTERACTIONS Biologically specific interactions, which are the interactions that have a high steric demand, are composed of many different molecular interactions, such as electrostatic and hydrophobic interactions, and hydrogen bonding, and they ensure high specificity and functionality to the target molecules. Some strategies based on this kind of intermolecular interaction have appeared over the years, such as avidin−biotin, antibody− antigen, and lectin−carbohydrate interactions, as well as DNA hybridization.

In addition, MD runs were also useful to establish the geometry of the host−guest compound in solution. The methodology involved the optimization of many trial adducts with the guest molecules placed outside the β-CD cavity. In conclusion, the combination of LbL assembly and high binding affinity, specificity, selectivity, and reversible nature of host−guest interactions allows the creation of oriented 2D and 3D supramolecular multilayer assemblies and stimuli-responsive multifunctional biointerfaces,608 which can offer benefits over, for example, the traditional electrostatic LbL assembly in terms of control over the growth of multilayers and layer thickness at the nanometer scale level, and the incorporation of neutral (bio)molecules into the multilayer assemblies. These assemblies can potentially be used for a wide range of applications, such as in information storage, drug delivery, and bionanotechnology and on optics, electronics, biomedical, and sensor devices.

9.1. Avidin−Biotin Interactions to Build Up and Grow Multilayer Assemblies

The first example of LbL assembly driven by biologically specific interactions employed the well-known avidin−biotin pair interaction.609 Müller et al.609 reported the assembly of protein multilayers induced by specific molecular recognition. The 8911


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Figure 36. Fabrication of mucin/WGA multilayer films. (a) Evolution of the hydrated film thickness as a function of the number of deposited layers for (mucin/WGA) films prepared in buffer containing 20 mM Hepes (pH 7.4) and 0, 200, or 400 mM NaCl. (b) Evolution of the hydrated and dry thicknesses and of the corresponding hydration for (mucin/WGA)12 films prepared in buffer containing different NaCl concentrations. (c) Multilayer film resistance to degradation. (Mucin-Alexa488/WGA)12 and (Mucin/WGA-Alexa488)12 films were tested for their resistance to a large range of pH values and ionic strengths. The reported values are the percentage of remaining mucin and WGA inside the films after the indicated treatments. The films remained largely intact under the tested conditions, except at pH 12, which promoted the destruction of the film structure. (d) (Mucin/WGA)12 multilayer films observed in the hydrated state by AFM. Larger aggregates with ∼1 μm could be observed after the deposition of the layers. The image size is 10 μm × 10 μm. Adapted with permission from ref 643. Copyright 2012 American Chemical Society.

They reported the fabrication of stable multilayer assemblies in the pH range 8−12 (Figure 35b) and the spontaneous decomposition of such assemblies at pH 5−6 (Figure 35c) due to the low affinity of the protonated iminobiotin residue in ib-PEI to avidin. Furthermore, the addition of biotin or analogues in the solution without changing the pH also led to the disruption of such assemblies due to the preferential binding of biotin or analogues to the avidin binding site. These stimuli-responsive films would find promising applications in biosensing and as drug delivery systems in order to control the release of desired biomolecules. The same research group also reported the disintegration of avidin/ib-PEI LbL films assembled on platinum (Pt)-coated quartz resonator by applying an electric potential to the Pt layer, without the need for chemical reagents.625 Very recently, Lehnert et al.626 fabricated LbL films incorporating streptavidin and biotinylated fibronectin (bFn) in order to build up well-defined nanoscale protein architectures. They showed that the degree of labeling of bFn influenced the assembly and stability of the protein multilayers.

authors mimicked the assembly of proteins by using streptavidin as a docking matrix. Almost at the same time, Decher and coworkers610,611 described the immobilization of multilayers of biological molecules (such as polyelectrolytes, proteins, or DNA) in their native state via biologically specific recognition. They also used the well-established biotin−streptavidin system to construct such multilayers. A few years later, Anzai and Nishimura612 revealed the formation of multilayer films based on the alternate deposition of avidin and biotin-labeled polymers and demonstrated that the multilayer structure of such films depends on the molecular geometry of the polymers. Almost at the same time, Spaeth and co-workers613 prepared protein multilayer systems by alternate deposition of a biotin−protein conjugate and polymerized streptavidin on hydrophobic silica surfaces, as confirmed by ellipsometry measurements. Following the same line of research, Cassier et al.614 and Anzai et al.615 produced hybrid multilayer films composed of streptavidin and biotinlabeled poly(amide)s via biologically specific recognition, as shown in Figure 34. It was found that the strong and highly specific affinity of streptavidin and biotin was responsible for the multilayer assembly. More recently, avidin and biotin have also been conjugated to enzymes, such as glucose oxidase (GOx) or horseradish peroxidase (HPR), in order to fabricate multilayer films using their strong affinity interaction.616−620 Indeed, immunosensing using multilayer films constructed from avidin and biotin-labeled antibody has also been reported.621,622 Inoue et al.623,624 constructed LbL films comprising avidin and 2-iminobiotinlabeled poly(ethylenimine) (ib-PEI) on a dichlorodimethylsilane-modified quartz surface (Figure 35a) and demonstrated the linear growth of the multilayer films after four deposition cycles. Moreover the sensitivity of such multilayer films toward the solution pH and the addition of biotin was also investigated.

9.2. Antibody−Antigen Interactions to Build Up and Grow Multilayer Assemblies

The fabrication of multilayer assemblies through antibody− antigen specific interactions has been the focus of a few studies. In this regard, Bourdillon et al.627,628 reported antibody−antigen specific interactions to successfully build up multilayers of GOx onto a glassy carbon electrode, as well as to fully preserve the catalytic activity of the enzymes and improve their stability against denaturation. In addition, Zhou and co-workers629 reported that the antigen binding affinity on antibodyimmobilized CHT/ALG multilayer film can be tailored by the assembly pH. They found that the increase of the assembly pH of alginate led to a decrease of the antigen binding affinity. These 8912


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Figure 37. LbL buildup of DNA multilayer films on planar surfaces via DNA hybridization. (a) Schematic illustration of LbL assembly of a polyAG and polyTC DNA film with PEI as the first adsorbed layer and polyT as the electrostatically adsorbed DNA layer. (b1) Evolution of the QCM frequency and mass change as a function of the number of deposited layers. (b2) Absorbance at 260 nm as a function of the number of layers. For all the samples, PEI was the first layer. For polyAG/polyTC multilayer assemblies, layer 2 was polyT; layers 4, 6, and 8 were polyTC; and layers 3, 5, and 7 were polyAG. For the polyA/polyT multilayer system, even layers were polyT and odd layers were polyA. For the PSS/PAH film, even layers were PSS and odd layers were PAH. (c) AFM images of the polyA20G20/polyT20C20 film before urea (left image, smooth surface) and after 6 M urea (right image, rough surface) treatments. The image size is 500 nm × 500 nm. Adapted with permission from ref 647. Copyright 2005 American Chemical Society.

One year later, Mahon et al.644 fabricated Con A-AuglycoNPs multilayers through specific lectin−carbohydrate interactions. The authors reported that such multivalent intermolecular interactions could be used to fabricate multilayer composite architectures and to enhance the detection and quantification of lectin−carbohydrate interactions. Very recently, Takahashi, Sato, and Anzai645 reviewed in detail the construction and properties of LbL protein architectures for biosensor applications making use of avidin−biotin and lectin−carbohydrate interactions.

highly specific interactions are particularly important for the design of biosensors, immunosensors, and immunoassays with enhanced detection capabilities.153,629,630 9.3. Lectin−Carbohydrate Interactions to Assemble and Grow Multilayer Assemblies

LbL assemblies constructed through lectin−carbohydrate interactions have been receiving increasing attention in the past decade, being more extensively reported. For instance, Lvov et al.,631 Anzai and co-workers,632−636 Yao and Hu,637−639 Zhu et al.,640 Gou et al.,641 and Hu et al.642 assembled 2D and 3D LbL architectures of Concanavalin A (Con A) or Con A/peanut agglutinin (PNA) via highly specific lectin−carbohydrate interactions. In those works, Con A and PNA molecules, i.e., plant lectins, were assembled as biospecific receptors in conjunction with well-defined synthetic glycopolymers, glycoenzymes such as GOx, HPR, and mannose-labeled lactate oxidase, or polysaccharides such as glycogen (a multibranched storage glucose-based polysaccharide) or dextran (a branched glucose-based neutral polysaccharide). More recently, Crouzier et al.643 and Mahon et al.644 also generated multilayer films through specific lectin−carbohydrate interactions. Specifically, Crouzier et al.643 showed that lectin wheat germ agglutinin (WGA) could be used to cross-link mucin-bound sugar residues and that, after the deposition of the first two layers, these films grew almost linearly in a highly hydrated state (Figure 36a). In addition, the hydrated thickness showed to be independent of the ionic strength used during the fabrication process (Figure 36b), and the films exhibited great resistance to extreme salt conditions and a wide range of pH (Figure 36c). Further information on the morphology of the mucin/WGA multilayer films deposited on gold-coated QCM crystals was obtained by AFM and revealed the presence of large aggregates (Figure 36d).

9.4. DNA Hybridization to Assemble and Grow Multilayer Assemblies

In the past few decades, the highly biocompatible and biodegradable DNA has been widely used as an attractive building block for the generation of smart responsive materials for biomedical, therapeutics, diagnostic, and biosensing applications. This biopolymer has been predominantly incorporated into LbL films via electrostatic interactions, being used as an anionic polyelectrolyte assembled with polycations. However, the incorporation of DNA multilayers into LbL assembled films can also be achieved via biologically specific interactions, namely, by DNA hybridization, thus allowing the assembly of species displaying the same charge. This specific interaction, which is based on hydrogen bonding, exploits the high specific interaction between complementary DNA base pairs, thus allowing us to finely design and engineer the composition and structure of DNA-based multilayer films. In this regard, Hou et al.646 reported the preparation of LbL DNA nanotubes using a template synthesis-based method. The DNA nanotubes have been assembled by specific hybridization of complementary DNA strands. However, to assemble those nanotubes, an outer layer of α,ω-diorganophosphate Zr(IV) was required to provide structural integrity. Furthermore, Caruso and co-workers647−649 fabricated DNA multilayer films on both planar and colloidal 8913


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supports and further hollow DNA capsules based solely on the composition and sequences of the DNA nucleotides by hybridization of complementary DNA strands. They reported the assembly of DNA films using DNA oligomers composed of building blocks of repeating homopolymeric units of nucleotides such as polyadenosine (polyAn) and polythymidine (polyTn) and two-block homopolymeric nucleotides such as polyadenosineblock-guanosine (polyAnGn) and polythymidine-block-cytidine (polyTnCn) (Figure 37a). The buildup of the multilayer films, which was followed by QCM and UV−vis spectroscopy by following the absorbance of DNA at 260 nm, increased linearly with the number of deposited layers, thus confirming the fabrication of DNA multilayer films (Figure 37b). The mass and frequency obtained for the polyAG/polyTC multilayer system were very similar to the ones obtained for a PSS/PAH film adsorbed under the same conditions with the same number of layers, thus revealing that DNA hybridization can be used to assemble films containing molecules of the same charge. In addition, those homopolymeric blocks allowed a great control over the morphology of the LbL films, which was significantly changed by a urea treatment (Figure 37c). As can be seen from this figure, before urea treatment the film surface was relatively smooth, whereas after urea treatment the surface of the film was rougher with morphology resembling the one obtained for multilayer assemblies containing DNA alternately deposited with a positively charged polyelectrolyte. Moreover, the exposure of the DNA films to low ionic strength solutions led to film disintegration, thus revealing that such films represent promising platforms for controlled delivery and sensing applications. Besides the fabrication of DNA multilayer films, the construction of hollow DNA capsules was also investigated by assembling 8 layers of (polyA20G20/polyT20C20) film on silica core particles coated with an initial PEI layer, followed by dissolution of the template core with hydrofluoric acid (HF)/ammonium fluoride (NH4F) buffer (pH 5) (Figure 38). As can be seen from Figure 38a and b, the core−shell particles displayed a highly fluorescent ring on the surface of the particles, which confirms the hybridization of the DNA strands and, thus, the formation of a double-stranded DNA (dsDNA) film onto the surface of the particles. Furthermore, after the dissolution of the silica template, core hollow capsules were obtained. Although the capsules were not visible by using bright-field illumination (Figure 38c), their treatment with PicoGreen, which is a fluorescent dye specific for dsDNA, enabled one to clearly see the shell of the capsules (Figure 38d), thus confirming the formation of DNA hollow capsules. These DNA films and capsules, built up from polymers displaying the same charge, show great potential for drug/gene delivery, biosensing, diagnostics, and separation. The modification and functionalization of PEI-coated PS electrospun fibers with DNA oligonucleotides, namely, polythymidine, and further formation of DNA fibers via LbL assembly of two block oligonucleotides, such as polyA15G15 and polyT15C15, on the modified PS fiber surface was also reported.650 The buildup of the DNA multilayers on the PS fiber surface was followed by CLSM, showing a linear increase in fluorescence intensity with increasing DNA bilayer number (Figure 39a). Therefore, this result demonstrated that there was a linear DNA multilayer growth on the modified PS fibers. In addition, it was found by SEM measurements that the small pores that appeared on the uncoated PS fibers (Figure 39b), as the result of the electrospinning process, were mostly covered after coating the fibers with DNA layers (Figure 39c). Furthermore,

Figure 38. Optical microscopy images of hollow DNA capsules constructed on colloidal supports via DNA hybridization. Bright-field (a, c) and fluorescence (b, d) images obtained for PEI-modified silica core particles coated with 8 layers of polyA20G20/polyT20C20 (a, b) and corresponding polyA20G20/polyT20C20 hollow capsules (c, d) obtained after silica core dissolution. The scale bar is the same for all the images shown. Reprinted with permission from ref 647. Copyright 2005 American Chemical Society.

the comparison between the uncoated and DNA-coated PS fibers revealed a smooth DNA surface coating. Caruso’s research group also studied the incorporation of DNA into polymer capsules,651 as well as the formation of nanostructured DNA capsules with well-controlled shrinkage properties652 to explore their permeability through DNA sensing. Some years later, Cavalieri et al.653 reported the synthesis and further LbL assembly of DNA−polymer conjugates into stable and functional multilayer films and microcapsules through DNA hybridization. They showed that DNA-grafted PNIPAAm microcapsules were less permeable than single-component DNA microcapsules, obtained by the LbL assembly of diblock oligonucleotides. It was also shown that those microcapsules could be functionalized with poly(ethylene glycol) (PEG), improving their stability, biocompatibility, and permeability. The authors also envisaged the potential applications of those capsules for the controlled delivery of therapeutics. Noh et al.654 fabricated highly ordered 2D patterned DNA nanoarrays by using uniform oligonucleotide films and lithographic processes. Caruso and co-workers655,656 also investigated the effect of the oligonucleotide length on the assembly of LbL DNA systems. It was found that short (30 bases) did not show an optimal film growth. They suggested that this behavior could be associated with the poor accessibility of the bases on the surface owing to the formation of self-protected interactions that prevent an efficient hybridization. They also performed MD simulations in conjunction with experimental data in an attempt to unveil the molecular interactions behind the efficient hybridization of DNA films. It was found that the degree of those interactions was dependent on the length and sequence of the oligonucleotides, corroborating the experimental findings. The same group also 8914


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Figure 39. (a) Evolution of the fluorescence intensity as a function of the number of (polyA15G15/polyT15C15) bilayers deposited on electrospun PS fibers. PolyT15C15 was labeled with the fluorescent dye teramethylrhodamine. The fluorescence intensity was measured after each deposition of polyT15C15 by CLSM. (b) SEM image of PS electrospun fibers before coating with polyA15G15 and polyT15C15. Small pores are evident in the fiber structure originating from the electrospinning process. (c) SEM image of PS fibers coated with five bilayers of (polyA15G15/polyT15C15). Adapted with permission from ref 650. Copyright 2006 American Chemical Society.

creation of SAMs incorporating OH-, COOH-, or N-functional end-groups. Then the surfaces functionalized with these organic linkers, which present several accessible functionalities and longrange 2D order, direct the orientation, nucleation, structure, stability, and quality of further deposited thin films. Mallouk and co-workers672,673 were the first to report the LbL assembly of ordered metal−organic multilayer thin films, namely, zirconium 1,10-decanebisphosphonate (Zr(O3PC10H20PO3)) multilayers, by sequential deposition of aqueous solutions of ZrOCl2 and 1,10-decanebisphosphonic acid on silicon and gold surfaces. This insoluble salt was chosen as the building block because it spontaneously crystallizes when soluble metal and phosphonic acid components are mixed, thus presenting a high thermodynamic stability. Since then, similar systems674,675 and more complex LbL functional molecular architectures also have been explored by several researchers. For example, Xiong et al.676 assembled multilayer films comprising transition metal neutralized polyelectrolytes, namely, poly(copper styrene 4-sulfonate) (PSS(Cu)1/2), and organic ligands, such as PVP, by exploring the coordination chemistry interactions between the metal ion Cu2+ and the reactive organic ligand pyridine, as confirmed by FTIR and UV−vis spectroscopy. Furthermore, such an LbL PSS(Cu)1/2/PVP multilayer film was incubated in H2S gas, by an in situ chemical reaction, to build up PSS-Cu2S nanoparticle/PVP hybrid composite systems. Analogous LbL systems have also been explored by other research groups.677,678 Rubinstein and co-workers679,680 constructed branched coordination multilayer films onto a Au surface functionalized with a SAM of bishydroxamate disulfide molecules by alternate deposition of an organic ligand possessing three bis-hydroxamate arms and a metal ion such as Zn4+, Ce4+, and Ti4+. Between the deposition of each layer, washing steps were performed to remove unbounded or physisorbed molecules, to stabilize weakly adsorbed materials, and to avoid the contamination of the next dipping solution by liquid adhering to the substrate. Wöll and co-workers681 also reported the LbL growth of highly oriented metal−organic polymeric films on a functionalized organic surface on the basis of coordination chemistry interactions. Multilayer films were successfully assembled by stepwise deposition of alternating layers of 1,3,5-benzenetricarboxylic acid ligand (C6H3(COOH)3, BTC) and zinc(II) acetate (Zn(CH3CO2)2, (Zn(OAc)2)) onto a carboxylic acid-terminated 16-mercaptohexadecanoic acid (MHDA)-SAM surface, as confirmed by infrared reflection absorption spectroscopy (IRRAS) (see Figure 40) and XPS.

investigated the role of salt concentration on the LbL assembly and morphology of DNA multilayer films built up from oligonucleotides composed of two homopolymeric diblocks (polyAnGn and polyTnCn).657 It was found that DNA films assembled at high salt concentrations (2 M) were denser than those assembled at low salt concentrations (1 M). In addition, high salt concentrations tuned the viscoelastic properties of DNA and triggered the formation of highly ordered DNA structures in solution, such as triplexes. Thus, biologically specific interactions further enlarge the materials that can be assembled into functional LbL films and mediate their appropriate immobilization, with improved stability and orientation, for the fabrication of sensing, biosensing, and immunosensing devices with high sensitivity and specificity. Furthermore, the materials assembled by these intermolecular interactions can be used for several other applications, including diagnostics, drug/gene delivery, and tissue engineering applications.

10. LBL ASSEMBLY THROUGH COORDINATION CHEMISTRY INTERACTIONS Coordination chemistry interactions are strong molecular interactions established between a wide variety of metal ions and organic ligands that enable the design and preparation of novel, well-ordered, highly oriented, versatile, and robust 2D functional multilayer thin films and even 3D nanoarchitectures comprising several materials, namely, organic polymers, activated carbon, metal oxides, metal nitrides, zeolites, and advanced inorganic−organic hybrid polymeric films, such as metal− organic frameworks (MOFs). For example, organic−inorganic hybrid microcapsules for efficient enzyme immobilization have been prepared through the metal−organic coordination based LbL assembly.658 Such materials, which have widespread applications in sensing, separation, porous smart membranes, catalysis, drug/gene delivery, optoelectronics, luminescence, energy, and gas adsorption and storage,659−671 can be grown by two main strategies: the solution-based LbL growth methods and the vapor-based LbL growth methods. 10.1. Solution-Based LbL Growth Methods

Solution-based LbL methods allow us to achieve precisely tailored surfaces with a variety of functionalities by prefunctionalization of different substrates (e.g., Au, Si, SiO2, glass), for instance, with thiol or organosilane molecules, to enable the 8915


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Figure 41. Schematic illustration of the LbL growth of the metal− organic polymer on the MHDA SAM-modified Au substrate obtained by repeated immersion of the substrate in solutions of the metal precursor (Zn(CH3CO2)2 = (Zn(OAc)2)) and the organic ligand (C6H3(COOH)3 = BTC). Reprinted with permission from ref 681. Copyright 2007 American Chemical Society.

Figure 40. (Plot a) (a) IRRAS of an MHDA-SAM and (b) an MHDASAM after immersion in a BTC solution for 24 h, (c) after immersion in a Zn(OAc)2 solution for 30 min, (d) after immersion in a Zn(OAc)2 solution for 30 min and in a BTC solution for 60 min (one cycle), and (e, l) after two to eight cycles of immersion in Zn(OAc)2 and BTC solutions, respectively. (Plot b) Correlation between the intensity of the COO band at 1654 cm−1 and the intensity of the CH2 band at 2923 cm−1 with the increase in the number of cycles. (Plot c) AFM image of a SAM laterally patterned by microcontact printing (μCP) consisting of (MHDA-SAM) COOH-terminated squares and (HDT-SAM) CH3terminated stripes after one cycle of immersion in Zn(OAc)2 and BTC solutions. (d) Height profile along the white line depicted in the AFM image. Adapted with permission from ref 681. Copyright 2007 American Chemical Society.

well-defined control of the orientation of MOFs thin films were published in the past few years.696−699 Although the sequential deposition of solutions of metal ions and organic functional ligands on organic surfaces is a wellestablished method, the multilayer assembly is commonly a very slow process, which, depending on the type of ligand, metal ions, and solvent used, can take much time to be completed. Therefore, such a method would not be suitable for practical industrial applications. In this concern, some strategies were attempted in order to accelerate the adsorption process. Rubinstein and co-workers700 described and compared a new and faster method for assembling metal−organic coordinationbased multilayers on organic functionalized surfaces, termed accelerated self-assembly procedure (ASAP), with standard procedures (typical overnight adsorption). In this novel method, which reports the rapid binding of organic ligand layers within a few minutes, a small volume of the organic ligand solution is spread on the surface modified with the metal-ion layer and evaporated under natural convection conditions, leaving the surface covered with an excess of organic ligand. Further extensive rinsing in pure solvents leads to the removal of weakly adsorbed molecules from the surface, leaving only the new coordination layer adsorbed (Figure 42). The authors performed and compared the results obtained by ellipsometry and water contact angle measurements for (Zn/ tetrahydroxamate)5 and (Zn/hexahydroxamate)5 coordination multilayer systems fabricated on bishydroxamate-modified Au surface through standard and ASAP approaches (Figure 43). Although both methods yielded similar results for the (Zn/ tetrahydroxamate)5 multilayer system (Figure 43a, c), i.e., the thickness increased regularly per added ligand layer and the oscillatory behavior of the water contact angles were similar in both methodologies, the fact that the ASAP method provides an effective and much faster ligand binding and multilayer fabrication than the standard method makes it highly promising for industrial purposes. However, it was postulated that the lower ellipsometric thickness measured for the (Zn/hexahydroxamate)5 multilayer system prepared through ASAP methodology was assigned to the more flexible and lower organization of such multilayers prepared in such a rapid way (Figure 43b). Hence, in such cases highly organized multilayers probably would be achieved if one assembles the organic ligand in a slower

However, it is clear from Figure 40a that there was no adsorption of BTC on carboxylic acid-terminated SAM surface when the SAM was only immersed in a BTC solution without the presence of Zn(II) ions. In addition, the thickness per cycle was practically the same up to a thickness of eight layers, as shown in Figure 40b. Furthermore, organic molecules were laterally patterned on the substrate surface, prior to the immersion in the Zn(OAc)2 and BTC solutions, by microcontact printing (μCP) to yield a surface consisting of COOH-terminated squares (made from MHDA) and CH3-terminated stripes (made from hexadecanethiol (HDT) (Figure 40c and d). Then, a highly ordered, oriented, and selective growth of metal−organic polymer structures on the patterned substrate can be achieved by repeating the adsorption steps in a stepwise fashion. Figure 41 shows a schematic illustration of the LbL growth of such metal− organic films onto MHDA-SAM modified surface. Moreover, beyond the growth of LbL metal−organic films on carboxylic acid-terminated SAM surfaces, the highly oriented growth of LbL multilayer films comprising MOFs by alternatively immersing a pyridyl-, trifluoromethyl-, hydroxyl-, or aminoterminated SAM surface in solutions of the metal ion and the organic ligand has also been reported.682−695 As aforementioned, rinsing steps were also needed between each immersion cycle to remove unreacted molecules, stabilize weakly adsorbed layers, and avoid the contamination of the next dipping solution by liquid adhering to the substrate. It was found that precise control over the orientation of such layers was dependent on the chemical composition of the underlying organic template surface. In this respect, very good reviews focusing on the influence of surface chemistry on the selective anchoring and 8916


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Figure 42. Schematic presentation illustrating the various stages of multilayer construction on gold using the ASAP method. The anchor layer is not shown. Reprinted with permission from ref 700. Copyright 2010 American Chemical Society.

Figure 43. Change of the ellipsometric parameter Δ (a, b) and water contact angles (c, d) during the construction of six-layer multilayers comprising bishydroxamate/(Zr4+/tetrahydroxamate)5 (a, c) and bishydroxamate/(Zr4+/hexahydroxamate)5 (b, d) on gold. The first point in (a, b) corresponds to bare Au and in (c, d) to Au/bishydroxamate. Results obtained using the standard procedure (□) and ASAP (●, red) are shown. Reprinted with permission from ref 700. Copyright 2010 American Chemical Society.

atomically flat all over the entire surface. In this concern, other strategies focusing on vapor-based LbL growth methods, such as atomic layer deposition (ALD) and molecular layer deposition (MLD), have emerged that attempted to solve some of these problems, as described in the following section.

evaporating solvent, thus providing enough time for achieving high molecular organization. Overall, it seems clear that multilayer assemblies comprising hexahydroxamate are significantly different, in terms of rigidity and structural organization, from the ones incorporating tetrahydroxamate, thus suggesting that the applicability of the ASAP approach to LbL systems depends on the specific features of the system being studied as well as on the desired molecular organization. As mentioned above, the preparation and growth of LbL coordination multilayers through solution-based methods requires the prefunctionalization of the substrate surface with organic linkers, such as SAMs. However, the functionalization of the substrate surface with organic SAMs requires a well-defined control over the surface properties and limits the whole assembly process to suitable substrates for deposition of densely packed, well-ordered, oriented, high-quality SAMs. Moreover, even when using appropriate substrates for accurate deposition of SAMs, we cannot ensure the formation of pinhole-free films, i.e., films without defects, because in many cases the substrate is not

10.2. Vapor-Based LbL Growth Methods

Over the last two decades, several approaches focusing on vaporbased LbL growth methods, such as ALD and MLD, have appeared as very promising ways to chemically deposit multilayer thin films in a LbL fashion for a variety of applications such as low leakage dielectric films, transparent conductive coatings, and diffusion barrier coatings. Both emerging dry methods are based on sequential self-limiting surface reactions between different precursor molecules in order to grow pinhole-free, high-quality, uniform, and conformal LbL thin films with precise layer control on high-aspect-ratio structures and porous materials.701−709 These strategies provide an unprecedented level of control over the chemical composition, structure, conformation, and film 8917


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platinum and different gases were also performed by other researchers.732,733 For example, Detavernier and co-workers733 reported ALD of Pt thin films by using trimethyl(methylcyclopentadienyl)platinum (CH3C5H4Pt(CH3)3) and ozone as precursor molecules. By using ozone as reactant gas instead of oxygen, it is possible to obtain Pt films at considerably lower deposition temperatures (∼100 °C), thus making the deposition process highly suitable for Pt deposition on thermally fragile substrates. Bent’s research group also compared the areaselective ALD processes for HfO2 and Pt thin films deposited on different substrates modified with SAMs-based patterned resists.730,734,735 In such cases, SAMs were used to direct ALD growth and to unveil the effect of chain length, tailgroup structure, and functional end-group, as well as the role of the SAM adsorption time on its ability to block the ALD process. The linear dependence of Pt film thickness, measured by ellipsometry and XRR, on the number of ALD cycles is shown in Figure 45a, thus supporting the self-limiting growth mechanism

thickness at the atomic level, as well as good capability for upscaling. 10.2.1. ALD Growth Method. ALD growth has been extensively demonstrated for a wide variety of inorganic materials such as metal oxides (e.g., Al2O3, HfO2, TiO2, ZrO2, ZnO, Ta2O5, and SiO2), noble metals and their oxides (e.g., Ru, RuO2, Os, OsO2, Rh, RhO2, Ir, IrO2, Pd, PdO, Pt, PtO, Ag, AgO, Au, and Au2O3), and metal nitrides (e.g., TiN, Ta3N5, and Si3N4) by using two different precursors.706,708,710−724 Furthermore, the design and fabrication of nanolaminate composite structures, through ALD of multiple layers of two or more metal oxide materials (e.g., Al2O3−ZnO, Al2O3−HfO2, Al2O3−ZrO2, Ta2O5−Al2O3, TiO2− Al2O3, and Al2O3−TiO2−Al2O3), and their applications have been more recently investigated.725−729 Nanolaminates are composite films that have attracted intense interest within the scientific and engineering communities due to their unique properties such as high dielectric constants and great mechanical, optical, and electrical properties. In a typical ALD process, the precursor A is introduced on the substrate and reacts until reaching saturation. Then, any unreacted precursor and volatile byproducts are removed by the introduction of an inert gas. Furthermore, the precursor B is introduced on the surface modified with precursor A and also reacts until saturation. Subsequently, unreacted precursors and gaseous byproducts are removed by purge with an inert gas. The ALD cycle, which typically involves four steps, is repeated until the desired number of layers or film thickness is achieved. A schematic representation of the ALD growth method is shown in Figure 44.730

Figure 45. (a) Evolution of Pt film thickness as a function of the number of ALD cycles. (b) Film density of the as-deposited Pt film as a function of the film thickness. ALD conditions: substrate temperature, 285 °C; precursor temperature, 50 °C; dosing time, 2 s; purge time, 12 s. Reprinted with permission from ref 735. Copyright 2008 American Chemical Society.

of the ALD Pt fabrication process. The evolution of the asdeposited Pt film density as a function of the film thickness is shown in Figure 45b, revealing that the film density increased until a film thickness of ∼18 nm was achieved. For films thicker than 18 nm, the film density reached a plateau value at ∼21.4 g/ cm3, thus revealing that films thinner than 18 nm were porous and discontinuous. This behavior suggested an island growth mechanism during the early stages of ALD growth,714,735,736 i.e., the new material layers were preferentially deposited on the ALD-grown material, which was recently perceived for several O 2-based Pt ALD processes on a variety of inorganic surfaces.733,735,737,738 To get further insights on the microstructure of the films, the morphology of ALD Pt films deposited on patterned Si surfaces functionalized with octadecyltrichlorosilane (ODTS) SAM was evaluated by AFM and SEM measurements (Figure 46). It was shown that the grain size increased while increasing the film thickness (Figure 46a−c), thus revealing that the surface roughness increased with the increase of the film thickness. These results demonstrated the dependence of the Pt microstructure on the film thickness. SEM images (Figure 46d−h) further corroborated the change in surface morphology observed by AFM and confirmed the presence of island-like Pt structures on the surface. 10.2.2. MLD Growth Method. The MLD approach is closely related to ALD and further extends the ALD growth strategy to include organic and hybrid organic−inorganic

Figure 44. Schematic illustration of the atomic layer deposition (ALD) process. One ALD cycle consists of a sequential pulse of precursor A, purge by an inert gas, pulse of precursor B, and purge by an inert gas again. The ALD cycle will be repeated a number of times until the desired film thickness is achieved. Reprinted with permission from ref 730. Copyright 2009 American Chemical Society.

For instance, Chen and Bent731 reported high-resolution areaselective ALD of Pt thin films on patterned SiO2/Si surfaces functionalized with 1-octadecene SAMs. The selective adsorption behavior of the SAM was used to attach a monolayer resist to the hydride-terminated silicon surfaces of an oxide-patterned silicon substrate. Afterward, ALD of a Pt thin film was selectively deposited only onto the nondeactivated oxide regions by using a reactor containing high-purity trimethyl(methylcyclopentadienyl)platinum (CH3C5H4Pt(CH3)3) and dry air as precursor molecules. Similar studies reporting ALD of Pt thin films by using trimethyl(methylcyclopentadienyl) 8918


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Figure 47. Schematic of the molecular layer deposition (MLD) method based on sequential, self-limiting surface reactions: (a) reaction of A− L−A bifunctional monomer with surface terminated with −B species (squares); (b) reaction of B−M−B bifunctional monomer with surface terminated with −A species (triangles). “A” and “B” are chemical functional groups, and “L” and “M” are organic fragments. Reprinted with permission from ref 707. Copyright 2007 American Chemical Society.

48a). However, in the steady-state regime, the TMA exposure led to a large mass gain, whereas the EG exposure produced a much smaller mass change. The smaller mass change for the EG reaction may result from an EG molecule reacting with two −AlCH3 species both on the alucone polymer surface and on TMA molecules in the alucone polymer film, as it has been postulated that “double reactions” are predicted to display a mass loss. Moreover, it is clearly seen that the TMA mass change increased quickly during TMA exposure and then decreased slowly (Figure 48b). This behavior has been assigned to unreacted TMA slowly desorbing from the polymer film after the TMA exposure. The alucone MLD film thickness showed a linear dependence on the number of reaction cycles and decreased while increasing the temperature (Figure 48c). The deposition of conformal and uniform alucone MLD films on Al2O3 ALD-coated BaTiO3 particles was further confirmed by TEM (Figure 48d). 10.2.3. Combination of ALD and MLD Growth Methods. ALD and MLD growth methods can be also combined to extend the number of possible sequential selflimiting surface reactions; expand the number and variety of materials and hybrid organic−inorganic polymeric thin films or nanocomposites with adjustable composition that can be fabricated in a LbL approach; enable an accurate control of film thickness, conformality, and uniformity; and introduce new functionalities into the LbL thin films.764−769 Such methods not only constitute highly valuable tools for coating and multilayer device fabrication but also open new avenues in a wide range of fields, from sensing, separation, catalysis, and drug delivery to optoelectronics, microelectronics, electrochemistry, and energy, being highly suitable for industrial applications. Therefore, coordination chemistry provides highly stable and strong interactions between metal ions and organic ligands, being of special interest for assembling novel, robust, well-ordered, highquality, uniform, conformal, and functional organic, inorganic, and hybrid organic−inorganic multilayer thin films for a wide range of applications.

Figure 46. AFM images of (a) 5, (b) 10, and (c) 30 nm thick asdeposited ALD Pt films. SEM images of (d) plane view and (e) 45° cross-sectional view of a 10 nm as-deposited ALD Pt film. SEM image of (f) 30 nm thick as-deposited ALD film. SEM images of (g) plane view and (h) cross-sectional view of an 80 nm dc-sputtered Pt electrode showing the nanoporosity of the films. The vertical ranges of AFM images a, b, and c are 20, 8, and 18.7 nm, respectively. Reprinted with permission from ref 735. Copyright 2008 American Chemical Society.

polymeric thin films, such as MOFs.739−742 However, in the MLD approach, a “molecular” linker, which is an organic material and can contain inorganic elements, is deposited onto the surface during the surface reactions between bifunctional and/or multifunctional precursors, as shown in Figure 47.707 The advantage of using multifunctional precursors is that such materials can introduce cross-linking and binding sites into the organic thin films, allowing for better thermal stability and higher film density. MLD growth with bifunctional and/or multifunctional precursor molecules has been demonstrated for several organic polymers such as polyimide,701,743−745 polyamide,707,746,747 polyimide−polyamide,748 polyurea,749−753 polythiourea,754 and polyurethane,755 as well as hybrid organic−inorganic polymeric thin films.709,756−763 For instance, hybrid organic−inorganic metal alkoxide MLD systems known as “metalcones”, such as “alucone” MLD films grown using trimethylaluminum (TMA) and various organic alcohols such as ethylene glycol (EG) as precursors, have been demonstrated (Figure 48).756 As can be seen from this figure, large mass gains were recorded for TMA and EG exposure during the first reaction cycle (Figure 8919


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Figure 48. MLD of hybrid organic−inorganic “alucone” polymer films. QCM mass gain and reactor pressure as a function of time during sequential TMA and EG exposures at 135 °C: (a) the initial TMA and EG reaction cycle and (b) three sequential TMA and EG reaction cycles in the steady-state regime showing the reproducibility of the mass gain for sequential TMA and EG exposures. (c) XRR measurements of film thickness as a function of the number of reaction cycles for alucone growth on Al2O3 ALD layers on Si wafers using TMA and EG at a variety of deposition temperatures. Before the deposition of the alucone MLD films, at least 40 cycles of Al2O3 ALD were deposited on the crystal to ensure a reproducible starting surface. (d) TEM image of alucone MLD film deposited on a BaTiO3 particle after 40 cycles of Al2O3 ALD and 50 cycles of alucone MLD at 135 °C. The Al2O3 ALD and alucone MLD resulted in an ∼13 nm bilayer film thickness. Adapted with permission from ref 756. Copyright 2008 American Chemical Society.

11. LBL ASSEMBLY BASED ON COVALENT BONDING Covalent bonding is another driving force to assemble LbL films that significantly increase the stability and strength of the stratified multilayer structure. In addition, this intermolecular interaction enables the incorporation of several other functional groups in the films by reacting through excess reactive groups within the multilayer structure, thus enabling the design and fabrication of tailored multifunctional assemblies. 11.1. Fabrication of Highly Stable, Robust, and Functional Polymeric Multilayer Assemblies

The first example of covalent LbL assembly of polymers was performed by Crooks and co-workers,770 who reported the LbL assembly of multilayer thin films based on functional dendrimers and a reactive copolymer of maleic anhydride that could covalently attach to a surface. A few years later, Kohli and Blanchard771 reported a different approach for the covalent LbL growth of multilayer films using diphenylmethane derivatives where the interlayer bonding was assured through a urea moiety. They suggested two different ways to achieve polyurea multilayer growth: (a) using alternating copolymerization chemistry where isocyanate-containing monomers react with amine-containing monomers, and (b) using isocyanate-functionalized monomers and further hydrolysis. Although both methods yielded films with identical chemical structures, the first one led to better control over layer growth. The linear and nonlinear growth of the films assembled by the first and second approaches, respectively, was confirmed by ellipsometry (Figure 49), showing that, for the same number of bilayer cycles, the films assembled by the second approach were considerably thicker. The nonlinear growth observed for the films assembled by the second approach was expected due to the conditions employed in this reaction. In spite

Figure 49. (a) Ellipsometric thickness of bilayer assemblies as a function of the number of growth cycles using the alternating copolymerization of diisocyanates and diamines. (b) Ellipsometric thickness of multilayer assemblies as a function of the number of reaction cycles using isocyanate/hydrolysis polymerization chemistry. Reprinted with permission from ref 771. Copyright 2000 American Chemical Society.

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of the differences in film thickness observed for the two growth approaches, the same urea interlayer linking chemistry led to the formation of the multilayers. This work was rather different from most of the works performed on the basis of covalent bonds because in this case the polymer was grown by step-by-step alternating copolymerization chemistry and terminated by adding a single monomer unit per cycle. In addition, the fabrication of bilayer films, consisting of PEI and poly(ethylene-co-maleic acid) (PMAE) and thermoresponsive ultrathin hydrogels using covalent bonds based LbL assembly was also described.772,773 Those ultrathin hydrogels were prepared by using poly(vinylamine-co-N-vinylisobutyramide) [poly(VAm-co-NVIBA)] and PAA. The carboxyl group of PAA was activated by 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide hydrochloride (EDC) for the reaction with the amine groups of poly(VAm-co-NVIBA) to yield the amide linkage that stabilized the film. Moreover, combinations of similar films were also tested and developed under a wide range of conditions (e.g., effect of EDC amounts, pH, ionic strength, etc.) to prepare ultrathin films with different structural properties.774 Furthermore, Zhang et al.775 fabricated 3D systems, such as highly stable N-methyl-2-nitrodiphenylamine4-diazoresin/m-methylphenol formaldehyde resin (NDR/MPR) core−shell particles and hollow capsules, by taking advantage of covalent LbL assembly in organic solutions, as confirmed by TEM measurements (see Figure 50). As shown in this figure, no rupture or change in the capsule morphology was observed, thus confirming the formation of robust and highly stable capsules.

Figure 51. Fabrication of PEI/PTCDA nanotubes via covalent LbL assembly approach. (a) Infrared spectra of PEI, PTCDA, and (PEI/ PTCDA)10 nanotubes, respectively. The peaks at 1635.5 and 1566.1 cm−1 represent the newly formed covalent amide bond, R-CO-NH-R′, from (PEI/PTCDA)10. (b) SEM image of (PEI/PTCDA)6 nanotubes with a diameter ≈ 330 nm, corresponding to the diameter of the template pores. The wall thickness is (100 ± 10) nm. (c, d) CLSM images of (PEI/PTCDA)6 fluorescent nanotubes wrapped along different directions, showing that the nanotubes have a very high flexibility and stability. Adapted with permission from ref 778. Copyright 2006 American Chemical Society.

d) measurements. These nanotubes may be used for applications in catalysis or drug delivery, as well as for the design of sensing platforms. Liang and co-workers779,780 fabricated highly stable, robust, and functional thin films comprising polymers and polymer− nanoparticle conjugates, such as poly(p-phenylenevinylene) (PPV) polymers and CdSe nanoparticles, through covalentbased LbL assembly. These conjugates hold great promise for the fabrication of organic devices. More recently, Duan et al.781 reported the fabrication of hemoglobin protein hollow microcapsules by covalent LbL assembly. Those microcapsules, which presented good electroactivity, catalytic properties, and improved permeability comparing to the conventional polyelectrolyte microcapsules, were held together by the cross-linker glutaraldehyde (GA). These microcapsules have great potential for being used in biomedical and biotechnological fields. Such and co-workers782,783 introduced a new concept to achieve covalent reactions with high yields and under extremely mild conditions, based on click chemistry (Figure 52a). This method, which generally involves the copper(I)-catalyzed click reaction of azide (Az) and alkyne (Alk) functionality to form an extremely stable 1,2,3-triazole linkage, enables one to build up multilayer assemblies based on the sequential deposition of multilayers of a single polymer, which cannot be accomplished by traditional LbL assembly methods. In their first work, the authors reported the linear growth of sequentially assembled PAA multilayers, namely, (PAA-Az/PAA-Alk) multilayer films with a 14% component of Az and a 16% component of Alk functionality, by monitoring the absorption peak at 240 nm (Figure 52b). Such a peak corresponds to the complex formation between copper and PAA. In addition, covalent bonding LbL

Figure 50. TEM micrographs of (NDR/MPR)5NDR hollow capsules, fabricated by covalent LbL assembly approach, before (a) and after (b) being exposed to dimethylformamide (DMF) for 40 h. Reprinted with permission from ref 775. Copyright 2003 American Chemical Society.

A few years later, Puniredd and Srinivasan776,777 reported the fabrication of robust ultrathin films of oligoimide through the covalent LbL assembly of pyromellitic dianhydride (PMDA) and diaminodiphenyl ether (DDE) on a supercritical medium. The use of an environmentally friendly medium facilitated a solventfree environment and avoided problems related to the use of organic solvents, thus improving the film growth and its quality. Tian et al.778 employed a pressure-filter-template approach to successfully fabricate fluorescent nanotubes of PEI and 3,4,9,10perylenetetracarboxylicdianhydride (PTCDA) via covalent bonding based LbL assembly. The formation of covalent amide bonds between PEI and PTCDA was observed by FTIR spectroscopy (Figure 51a), and the preparation of the nanotubes was confirmed by SEM (Figure 51b) and CLSM (Figure 51c and 8921


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Figure 52. (a) Schematic illustration of LbL assembly of PAA-Az/PAA-Alk multilayer films using click chemistry. (b) UV−vis absorption spectra for (PAA-Az/PAA-Alk) multilayer films assembled on quartz with increasing bilayer number. The arrow indicates increasing bilayer number. The inset figure shows the evolution of absorbance as a function of the number of bilayers monitored at 240 nm. Adapted with permission from ref 782. Copyright 2006 American Chemical Society.

stepwise deposition of CHT or adenine-modified CHT (ACHT) on flat surfaces via LbL covalent bonding using GA as the crosslinking agent. It was found that the additional aromatic primary amine presented by adenine induced a faster reaction with GA than that of the protonated primary amine of CHT. Therefore, ACHT could be regarded as a better biopolymer for the fabrication of multilayer thin films based on single polymers. Furthermore, Gill et al.793 fabricated LbL films comprising homobifunctional poly(dimethylsiloxane) (PDMS) and poly(ethylene-alt-maleic anhydride) (PEMA) via covalent bonding. Although at that time there have been several reports on the assembly of multilayers on PDMS platforms, this was one of the first reports on the fabrication of multilayer thin films containing PDMS as a constituent of the films. Hu and Ji794 proposed a simple method to functionalize surfaces based on the alternate deposition of p-nitrophenyloxycarbonyl group-terminated hyperbranched polyether (HBPO-NO2) and PEI via covalent bonding LbL assembly. They suggested the potential application of the HBPO-NO2/PEI multilayer films in local drug delivery of multiple therapeutic agents, as well as in the control of the cell function. Furthermore, Allen et al.795 constructed smart temperature-responsive grafts of PNIPAAm/silica nanoparticles on both flat and topologically complex glass surfaces via covalent bonding based LbL assembly. Bechler and Lynn796 and Feng et al.797 used covalent LbL assembly of reactive polymer multilayers to create robust, tailorable functional biointerfaces, and Li et al.798 used click chemistry based LbL assembly to fabricate uniform hybrid nanocomposite multilayer films based on AuNPs and polymers with well-controlled thickness. Those assemblies could be used as promising materials for applications in biosensing, drug delivery, and biofuel cells. Covalent bonding is an alternative and very effective way to fabricate highly stable, robust, and functional ultrathin polymeric LbL films and nanocomposites capable of resisting a wide range of harsh conditions. The materials assembled via strong covalentbased LbL assembly could be used for applications in emerging areas such as biomedicine, controlled drug release, biosensing, catalysis, and nanobiotechnology.

assembly via click chemistry further extended the number and diversity of materials that can be incorporated in LbL films (such as noncharged polymers) and was found to be insensitive to the incorporation of other functional groups and suitable to study biologically based systems. In addition, Buck and co-workers784−787 fabricated reactive covalently cross-linked ultrathin films of poly(2-alkenyl azlactone)s, such as poly(2-vinyl-4,4-dimethylazlactone) (PVDMA), and appropriately functionalized polyamines, namely, PEI, for applications in sensing, therapeutics, and separation. Kinnane et al.788 also prepared multilayer films comprising other polymers such as poly(ethylene glycol)acrylate (PEGA) multilayers via click chemistry. These films were further functionalized with the adhesion-promoting peptide arginineglycine-aspartate (RGD), thus rendering cell-responsive films. Therefore, these films are very appealing to promote the adhesion and growth of cells onto surfaces. At the same time, Bergbreiter and Liao789 presented a highlighted paper in which they focused on the different types of chemical reactions involved in covalent bonding based LbL assembly and on its great potential to build up functional and more robust multilayer ultrathin films and nanocomposites. Since then, there has been a growing interest by the scientific community in covalent LbL assembly and in its potential toward the design and fabrication of advanced organic−inorganic hybrid materials exhibiting multiple functionalities. For instance, Amigoni et al.790 constructed superhydrophobic organic−inorganic hybrid films by sequential deposition of different layers of amino-functionalized silica nanoparticles and epoxy-functionalized smaller silica nanoparticles via covalent bonding based LbL assembly. The use of such assemblies was suggested for the fabrication of highly active materials for sensing and for catalytic applications. Zhang et al.791 functionalized multiwalled carbon nanotubes (MWNTs) with clickable polymers poly(2-azidoethyl methacrylate) and poly(propargyl methacrylate) and further modified such functionalized MWNTs with fluorescent dye and PS by click chemistry based LbL assembly. These materials are very promising for the immobilization of various functional molecules. Manna et al.792 reported a simple approach for the fabrication of singlecomponent multilayer thin films on surfaces via GA-mediated covalent bonding. They fabricated multilayer thin films by 8922


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12. LBL ASSEMBLY VIA STEREOCOMPLEXATION The stepwise stereocomplex assembly enables the preparation of stable, highly ordered molecularly regulated polymeric structures with great potential for applications in polymer surface chemistry and biomedicine. These stereocomplexes are the result of combinations of structurally well-defined synthetic polymers in certain solvents and are fabricated through weak van der Waals interactions between the polymers. This emerging driving force includes not only specific interactions between polymers but also their structural rearrangements at the film surface. Although the association of structurally regulated synthetic polymers into stereocomplexes has been extensively reported in the literature,799−802 only a few researchers have been focusing on the stepwise stereocomplex assembly of such structurally welldefined synthetic polymers on a substrate. In this regard, the stereocomplex assembly of polymer complexes, such as poly(methacrylates) and enantiometric poly(lactides), onto a substrate has been receiving increasing attention.803−815

efficiently detecting antibody−antigen interactions and for enzyme immobilization supports.816,817 The successful fabrication of well-defined 3D systems, namely, hollow capsules, composed of nonionic multilayers of it-PMMA/ st-PMMA stereocomplex films or it-PMMA/st-poly(methacrylic acid) (st-PMAA) stereocomplex films by combination of LbL assembly and silica template method has been recently pointed out (Figure 55).818,819 For instance, silica nanoparticles were coated with it-PMMA/st-PMAA stereocomplex films by alternate immersion in solutions of it-PMMA and st-PMAA. Furthermore, the dissolution of the silica sacrificial core with aqueous HF led to the formation of stereocomplex hollow capsules. The preparation of such hollow capsules was carried out according to the process schematically shown in Figure 55a. The construction of it-PMMA/st-PMAA stereocomplex films on the surface of the silica nanoparticles and the successful fabrication of it-PMMA/stPMAA stereocomplex hollow capsules were confirmed by TEM measurements (Figure 55b−d). These images provide clear evidence that the spherical shape of the particles was maintained after silica core dissolution and that the resulting hollow capsules were fabricated without destroying the stereocomplex films. Such stereocomplex hollow capsules further enabled the controlled release of encapsulated dyes, namely, rhodamine 6G, by combining the selective extraction and incorporation of stPMAA from and into the it-PMMA/st-PMAA stereocomplex hollow capsule shells, respectively (Figure 55f). The selective extraction of st-PMAA from the capsule shells was achieved by immersion in an alkaline aqueous solution (pH 6−9). A great potential is envisioned for these stereocomplex hollow capsules to act as drug carriers and nanocontainers.

12.1. Fabrication of LbL Poly(methacrylate) Stereocomplex Ultrathin Films and Hollow Capsules

Akashi and co-workers were the first to make use of the potential of weak van der Waals interactions for the fabrication of highly regular polymeric LbL ultrathin films on a substrate. In their first studies, they reported the stepwise stereocomplex assembly of stereoregular isotactic (it-) and syndiotactic (st-) poly(methyl methacrylate) (PMMA) multilayers onto a gold substrate from different organic solvents through alternate repetition of physical adsorption and stereocomplexation, as shown in Figure 53.803

12.2. Fabrication of LbL Poly(lactide) Stereocomplex Ultrathin Films and Hollow Capsules

Besides the fabrication of poly(methacrylate) stereocomplex films and hollow capsules, the construction of poly(lactide) stereocomplex films and hollow capsules has also been reported. In this respect, Akashi and co-workers demonstrated the stepwise stereocomplex assembly of enantiometric poly(lactide) (i.e., poly(lactic acid), PLA) films, which exhibited a high degree of crystallinity, by stereocomplex formation,806,807,812 and further extended this method to prepare artificial enzymes.809 More recently, an interesting approach for the preparation of PLA hollow nanotubes through the one-dimensional fusion of LbL hollow nanocapsules composed of poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) stereocomplex films was proposed (Figure 56a).820 The PLA stereocomplex hollow nanocapsules were prepared in an analogous way to the aforementioned poly(methacrylate) hollow capsules, i.e., by combining the LbL assembly technology and silica template method. Although the formation of those nanotubes was found to be greatly affected by the molecular weights of the PLAs comprising the hollow capsules (Figure 56b and c), the mechanism behind it still remains unknown, thus deserving further comprehensive attention. As is clear from the visualization of Figure 56b and c, PLA hollow tubes were only fabricated after evaporating water from a water dispersion of hollow capsules composed of PLAs with lower molecular weights. Nowadays, the development of easier and faster approaches to fabricate stereocomplex films has been the focus of some research groups. The rapid fabrication of enantiometric PLA stereocomplex films on a substrate by combining the LbL deposition of polymers and inkjet printing technology has been recently

Figure 53. Schematic representation of the PMMA stereocomplex assembly on surfaces. Reprinted with permission from ref 803. Copyright 2000 American Chemical Society.

The LbL buildup of the synthetic polymers from various organic solvents was followed by QCM technique, which revealed that the assembly process followed a linear growth, mainly after the first two steps in which there was a direct influence of the gold substrate. Therefore, precise control over the amount of PMMA adsorbed at each assembly step was accomplished. Moreover, the assembly process was very sensitive to the selected solvent species, with acetonitrile being the best organic solvent for the assembly process (see Figure 54A). The stepwise stereocomplex assembly process occurred by starting with the physical adsorption of it-PMMA and then of st-PMMA, and it included a molecular rearrangement that started with the incorporation of st-PMMA into the it-PMMA layer. Moreover, the amount of stereocomplex assembled was significantly affected by the PMMA concentration (Figure 54B) and molecular weight (Figure 54C), and by the addition of water to the organic solutions (Figure 54D).803,804,808,810,811,815 Such stereocomplex PMMA LbL films can be used, for example, for 8923


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Figure 54. Frequency shift of QCM by stepwise assembly from (A) various organic solutions of it- and st-PMMA at the concentration of 1.7 mg mL−1 from (a) acetonitrile, (b) acetone, and (c) N,N-dimethylformamide solutions; (B) acetonitrile solutions of it- and st-PMMA at various concentrations: (a) 2.6, (b) 1.7, (c) 0.85, (d) 0.34, and (e) 0.17 mg mL−1; (C) acetonitrile solutions of it- and st-PMMA with different molecular weights: (a) 72 430, (b) 34 100, (c) 22 700, (d) 10 600, (e) 7 230, and (f) 5 540; (D) water mixed-organic solutions of it- and st-PMMA at the concentration of 1.7 mg mL−1: (a) acetonitrile/water, 10/1.5 (v/v), and (b) acetonitrile. The open and closed circles illustrate it-PMMA and st-PMMA adsorption steps, respectively. Adapted with permission from ref 803. Copyright 2000 American Chemical Society.

Figure 55. (a) Schematic illustration of the fabrication process of it-PMMA/st-PMAA stereocomplex hollow capsules and it-PMMA hollow capsules. TEM images of silica nanoparticles (b), silica nanoparticles coated with it-PMMA/st-PMAA stereocomplex films (c), it-PMMA/st-PMAA stereocomplex hollow capsules (d), and it-PMMA hollow capsules (e) obtained after the extraction of st-PMAA from it-PMMA/st-PMAA stereocomplex hollow capsules. (f) Controlled release of encapsulated rhodamine 6G from it-PMMA/st-PMAA hollow capsules by using the selective extraction and incorporation of st-PMAA. Adapted with permission from ref 819. Copyright 2012 American Chemical Society.

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Figure 56. (a) Schematic illustration of the formation of PLA hollow nanocapsules and nanotubes. (b, c) SEM images of nanostructures obtained after evaporation of water from the water dispersion of PLA hollow nanocapsules on a polyethylene terephthalate substrate: (b) Mw PLLA = 5 500, Mw PDLA = 5 800; (c) Mw PLLA = 30 000, Mw PDLA = 26 000. Adapted with permission from ref 820. Copyright 2010 American Chemical Society.

demonstrated.821 By using the fast and low-cost inkjet printing technology, one can easily obtain well-defined control over the amount, thickness, and structure of printed polymers, as well as avoid intermediate rinsing steps, making it thus highly suitable for practical industrial applications. It is envisaged that the fabrication of LbL stereocomplex films using inkjet printing technology would lead to the creation of advanced functional systems with higher performance and detection capabilities for practical applications. Furthermore, the stereocomplex films of the biocompatible, biodegradable, and nontoxic PLAs display desired features and properties, namely, enhanced thermal resistance and mechanical properties, being thus widely used for several biomedical applications, such as tissue engineering and drug delivery systems, as suggested in a number of reports.160,822−826 For further information on the fabrication of LbL stereocomplex assemblies and their biomedical applications, the reader is referred to a very interesting review by Akashi and co-workers.826 In summary, the stepwise stereocomplex assembly of structurally regular polymers allows great control over the surface properties and enables the fabrication of novel, stable, highly ordered molecularly regulated LbL assemblies for applications in the biomedical field, namely, in tissue engineering and controlled drug delivery systems.

Figure 57. Schematic representation of the LbL assembly process based on the adsorption of metal oxides via surface sol−gel process. Reprinted with permission from ref 829. Copyright 1997 American Chemical Society.

after hydrolysis (Figure 58a). The fabrication of metal oxide films, namely, titanium dioxide (TiO2)-based films, onto goldQCM surfaces was confirmed by SEM measurements, as shown in Figure 58b. Moreover, the authors also addressed for the first time the application of the surface sol−gel process for molecular imprinting in LbL films.830 Kunitake and co-workers831 further extended the stepwise surface sol−gel process to consider the alternate adsorption of molecular layers of metal oxides and polymers. It was found that polymers and similar molecules strongly adsorbed onto metal oxide layers prepared by the stepwise surface sol−gel process. In addition, the polymer layer was active enough for the subsequent chemisorption of metal alkoxides. A few years later, Acharya and Kunitake832 proposed a simple and efficient method for the fabrication of biocompatible surfaces from inert surfaces on the basis of surface sol−gel synthesis of a biocompatible titania layer. The titania layer could then be used as an active surface for the adsorption of bioactive molecules, such as collagen, insulin, fibrinogen, and heparin, as demonstrated by the QCM technique, being extremely useful for the fabrication of biocompatible surfaces on implantable devices. Yan and coworkers833,834 developed hydrolytic and nonhydrolytic surface sol−gel processes for the functionalization of mesoporous silica

13. LBL ASSEMBLY VIA SURFACE SOL−GEL PROCESS 13.1. Preparation of LbL Metal Oxide Ultrathin Films and Metal Oxide-Based Nanocomposites

The stepwise surface sol−gel process is a very simple, reproducible, and unique method to fabricate metal oxide ultrathin films and metal oxide-based nanocomposites with accurate control over film thickness at the nanometer-scale level. The stepwise surface sol−gel process, which is applicable to several metal alkoxides, was first introduced independently by Kleinfeld and Ferguson827 and by Kunitake and co-workers.828,829 This process is composed of four steps: chemisorption of the alkoxide, rinsing, hydrolysis of the chemisorbed alkoxide, and drying. Kunitake and co-workers828,829 reported the preparation of ultrathin films of metal oxide layers with molecular precision by means of stepwise adsorption of a variety of alkoxides of titanium, aluminum, zirconium, niobium, silicon, and their combinations, as schematically shown in Figure 57. They demonstrated the regular film growth at nanometer scale by repetition of chemisorption of alkoxides on uniform surface hydroxyl groups and subsequent regeneration of hydroxyl groups 8925


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Figure 58. (a) QCM frequency shifts (−ΔF) as a function of the number of deposition cycles for the adsorption of Zr(OnPr)4 (100 mM in toluene/ propanol = 1/1, 10 °C), and TiO(acac)2 (20 mM in methanol containing 300 mM of acetic acid, 60 °C). (b) Cross-sectional SEM image of TiO2-based film deposited on Au-coated quartz crystal resonator. The measured film thickness was (900 ± 100) Å, and the total frequency shift resulting from the deposition of the film was 5221 Hz. Adapted with permission from ref 829. Copyright 1997 American Chemical Society.

Figure 59. (a) UV−vis absorption spectra of titanium phosphate films with different layers. The number of layers is 1−9 from the bottom to the top. The inset figure shows the absorbance at 232 nm as a function of the number of titanium phosphate layers. (b) QCM frequency decrease (−ΔF) of successive growth of titanium phosphate ultrathin films. (c, d) SEM images of plane view (c) and cross-sectional view (d) of the as-prepared titanium phosphate films deposited on Ag-coated quartz crystal resonator. Adapted with permission from ref 837. Copyright 2005 American Chemical Society.

spectroscopy and QCM measurements (Figure 59a and b).837−839 In this regard, UV−vis spectroscopy measurements revealed a linear increase of the absorbance at 232 nm, which is characteristic of titanium phosphate films, with the increase of the number of deposition cycles, thus indicating a regular deposition of titanium phosphate in each layer (Figure 59a). In addition, QCM measurements also confirmed the regular deposition of the titanium phosphate layers while increasing the number of deposition cycles (Figure 59b). The surface morphology of these films was investigated by SEM, which revealed the deposition of uniform titanium phosphate films (Figure 59c and d). Moreover, silver ions can be incorporated into these ultrathin titanium phosphate films by a very simple ion-exchange process.840 These materials were demonstrated to be effective in inhibiting the growth of bacterias, such as Escherichia coli, and thus can be used as promising antibacterial coatings. More recently, Li and co-workers841 reported the fabrication of TiO2/PAA nanocomposite nanotubes via the sol−

materials with TiO2. This LbL approach allowed tuning mesoporous diameters with monolayer precision and further enabled the assembly of ultrasmall AuNPs.833,835 Generally, the design, functionalization, and further incorporation of ultrasmall AuNPs within LbL assemblies represents a great promise toward the development of novel nanomaterials that can deeply penetrate, for example, into tumors, thus showing a great potential as anticancer vehicles that can carry specific therapeutic agents more efficiently into tumors. A similar LbL surface sol−gel approach was employed to prepare thin layers of TiO2 over a silver island film as a unique surface-enhanced Raman spectroscopy (SERS) substrate to monitor the adsorption of several molecules on dielectric titania surfaces. Such a TiO2 layer has the capability to protect silver islands from oxidation and aggregation, thus improving the stability of SERS substrates.836 Titanium and zirconium phosphate ultrathin films were also prepared by repetitive adsorption of titanium or zirconium alkoxides and phosphate groups, as monitored by UV−vis 8926


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Table 1. Molecular Interactions Driving the LbL Assembly of Multilayers and Their Corresponding Advantages and Disadvantages

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Table 1. continued

phate/Prussian blue films, have also been fabricated to address several applications.842,843 Thus, the simplicity of the surface sol−gel process constitutes a great advantage in the production of well-defined ultrathin metal oxide films and nanocomposites and can be used for the fabrication of bioactive and biocompatible surfaces, antibacterial coatings, photovoltaic

gel process of titanium alkoxide in the inner pores of an alumina template followed by LbL assembly with PAA. The PAA component then could be completely removed by calcination and etching of the alumina template with a concentrated aqueous solution of sodium hydroxide. Such nanotubes could be used as carriers for biomolecules and catalysis. Meanwhile, other composite films, such as TiO2/protein and titanium phos8928


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detection capabilities for addressing several practical applications. We hope that this comprehensive review triggers increasing interest of a broad audience in surface and interface science and further stimulates scientists to keep on working and exploring the potentialities of the prominent LbL assembly technology in several fields.

cells, optical, and electronic and biosensing devices, as well as in photocatalysis.

14. CONCLUSIONS AND FUTURE PERSPECTIVES LbL assembly is a simple, inexpensive, efficient, robust, reproducible, and highly versatile method to modify surfaces and fabricate nanostructured polymeric thin films or nanocomposites with tailored compositions, structures, properties, and functions. It allows the incorporation of a large set of materials (e.g., polymers, proteins, nucleic acids, polysaccharides, enzymes, viruses, colloids, nanoparticles, clays, dyes, metal oxides, etc.) into multilayer films with promising applications in emerging areas such as medicine, sensing, biosensing, bioelectronics, separation, tissue engineering, drug delivery, catalysis, energy storage and conversion, electronics, and optics. In this Review we have systematically summarized the different intermolecular interactions associated with the assembly of the layers such as electrostatic, hydrophobic, charge-transfer, host− guest, coordination chemistry, and biologically specific interactions, hydrogen bonding, covalent bonding, stereocomplexation, and surface sol−gel process. Although the overwhelming majority of studies reported in the literature focused on the fabrication of LbL multilayer assemblies via electrostatic interactions, we have shown in this Review that there has been increasing attention and significant development on the LbL assembly of multilayer films through several other intermolecular interactions. It is envisaged that, despite the fact that each intermolecular interaction has its own advantages and disadvantages (Table 1), the concept of LbL assembly based on multiple intermolecular interactions may provide new insights for the assembly of a great variety of building blocks and fabrication of tailored multifunctional nanostructured multilayer assemblies of uncharged and uniformly charged materials with well-defined properties and architectures (namely, thickness, composition, structure, wettability, roughness, etc.) for a wide range of interesting applications. For example, such LbL assemblies could be used to fabricate hierarchical 2D and 3D architectures for applications in tissue engineering and regenerative medicine, biosensors, bioelectronics, drug delivery, catalysis, energy, and information storage. Such a variety of driving forces may also provide a way to precisely control the growth of multilayer films. In particular, one can increase the stability and quality of generated films, as well as add new functionalities. Moreover, the integration of the highly versatile and prominent bottom-up LbL assembly methodology with other well-developed approaches, for instance, currently existing bottom-up and top-down micro- and nanofabrication methods such as photolithography and printing techniques, might lead to the creation of functional, hierarchical, and well-defined 2D and 3D architectures with high mechanical stability and, thus, high performance at a relatively low cost. Therefore, the combination of top-down and bottom-up approaches is of great importance for advancing the current existing knowledge in nanoscience and nanotechnology and to accommodate the need for device applications. Hence, the coupling of bottom-up LbL assembly technology with other bottom-up and top-down strategies will help researchers to extend the LbL assembled systems from the research laboratories to practical applications. In summary, while remarkable progress and development has been achieved to date in the area of LbL assembly of multilayer systems, stimulating challenges and opportunities remain still open for the development, in the near future, of next-generation smart multifunctional devices with superior performance and

AUTHOR INFORMATION Corresponding Authors

*E-mail (J.B.): [email protected]. Phone: +351 253 510912. Fax: +351 253 510909. *E-mail (J.F.M.): [email protected]. Phone: +351 253 510904. Fax: +351 253 510909. Notes

The authors declare no competing financial interest. Biographies

João Borges studied Chemistry at Faculty of Sciences of the University of Porto, Portugal, where he received his first and Ph.D. degrees in Chemistry in 2008 and 2013, respectively. His doctoral research, under the supervision of Prof. António Fernando Silva, focused on studying the adsorption of proteins on gold surfaces modified with self-assembled monolayers (SAMs) of alkanethiols and biopolymeric materials. Currently, he is a postdoctoral researcher at 3B’s Research Group Biomaterials, Biodegradables and Biomimetics of the University of Minho, Portugal, working with Prof. João F. Mano. His current research interest focuses on the design and development of advanced biomimetic nanostructured coatings based on the layer-by-layer (LbL) assembly technology, combining different biologically inspired materials, for biomedical applications, especially aimed at being used in nanomedicine, bone tissue engineering, and in the control of the behavior of bioactive molecules and cells.

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MD MHDA MLD MOFs MPA α-MSH MTM MUA NaPA NMVA NMVF OWLS

molecular dynamics 16-mercaptohexadecanoic acid molecular layer deposition metal−organic framework(s) 3-mercaptopropionic acid α-melanocyte stimulating hormone montmorillonite 11-mercaptoundecanoic acid sodium polyacrylate N-methyl-N-vinylacetamide N-methyl-N-vinylformamide optical waveguide lightmode spectroscopy PAA poly(acrylic acid) PAAm poly(acrylamide) PAH poly(allylamine hydrochloride) PAHs polycyclic aromatic hydrocarbons PANI polyaniline PDADMAC poly(diallyldimethylammonium chloride) PDF poly(dithiafulvene) PDLA poly(D-lactic acid) PDMAEMA poly[2-(dimethylamino)ethyl methacrylate] PDMS poly(dimethylsiloxane) PEG poly(ethylene glycol) PEI poly(ethylenimine) PEM(s) polyelectrolyte multilayer(s) PEMA poly(ethylene-alt-maleic anhydride) PEO poly(ethylene oxide) PEO-b-PCL poly(ethylene oxide)-block-poly(ϵ-caprolactone) PGA poly(L-glutamic acid) PHPMAAm poly(N-hydroxypropylmethacrylamide) PLA poly(lactic acid) PLGA poly(lactic-co-glycolic acid) PLL poly(L-lysine) PLLA poly(L-lactic acid) PMAA poly(methacrylic acid) PMAE poly(ethylene-co-maleic acid) PMDA pyromellitic dianhydride PMMA poly(methyl methacrylate) PMS poly(maleic anhydride-co-styrene) PNIPAAm poly(N-isopropylacrylamide) poly(VAm-co-NVIBA) poly(vinylamine-co-N-vinylisobutyramide) PPV poly(p-phenylenevinylene) PSMA poly(styrene-alt-maleic acid) PSOH p-(hexafluoro-2-hydroxylisopropyl)-αmethylstyrene and styrene PSS poly(styrenesulfonate) PSS(Cu)1/2 poly(copper styrene 4-sulfonate) PSSMA poly[(styrene sulfonic acid)-co-(maleic acid)] PTAA poly(3-thiophene acetic acid) PTCDA 3,4,9,10-perylenetetracarboxylicdianhydride PVA poly(vinyl alcohol) PVAm poly(vinylamine) PVCL poly(N-vinylcaprolactam) PVP poly(4-vinylpyridine) PVPh poly(4-vinylphenol)

João F. Mano is an Associate Professor with Habilitation at the University of Minho, Portugal, and is a vice-director of the 3B’s Research GroupBiomaterials, Biodegradables and Biomimetics, from the same university. He received his Ph.D. in Chemistry from the Technical University of Lisbon. His current research interests include the development of new materials and multidisciplinary concepts for tissue engineering and regenerative medicine, where he has been developing bioinstructive and biomimetic biomaterials and surfaces. João F. Mano has coauthored about 400 papers in international journals (+8000 citations, h-index of 43) and published three books.

ACKNOWLEDGMENTS This work received funding from the European Union’s Seventh Framework Programme (FP7/2007-2013) under grant agreement no. REGPOT-CT2012-316331-POLARIS. The work was also funded by FEDER through the Competitive Factors Operational Program (COMPETE) and by National funds through the Portuguese Foundation for Science and Technology (FCT) in the scope of the projects PTDC/FIS/115048/2009 and PTDC/CTM-BIO/1814/2012. The authors gratefully acknowledge Dr. Luca Gasperini (3B’s Research Group, University of Minho, Portugal) for his help with the figures. LIST OF ACRONYMS AND ABBREVIATIONS AD adamantane AFM atomic force microscopy ALD atomic layer deposition ALG alginate AuNPs gold nanoparticles bFn biotinylated fibronectin BTC 1,3,5-benzenetricarboxylic acid C18-L-Gu N-stearoy-L-glutamic acid β-CD β-cyclodextrin CHT chitosan CLSM confocal laser scanning microscopy Con A concanavalin A CT charge-transfer DADMAC diallyldimethylammonium chloride DAR diazoresin DEN-COOH carboxyl-terminated polyether dendrimer DSC differential scanning calorimetry Fc ferrocene Fc-PAH ferrocene-appended poly(allylamine hydrochloride) FTIR Fourier transform infrared spectroscopy GA glutaraldehyde GOx glucose oxidase HA hyaluronan HBPO-NO2 p-nitrophenyloxycarbonyl group-terminated hyperbranched polyether HEP heparin HPR horseradish peroxidase ib-PEI 2-iminobiotin-labeled poly(ethylenimine) it-PMMA isotactic-poly(methyl methacrylate) LAP laponite LB Langmuir−Blodgett LbL layer-by-layer LCST lower critical solution temperature MBA 4-mercaptobenzoic acid MC Monte Carlo 8930


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NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on August 20, 2014 with an incomplete Table 1 due to production error. The corrected version was reposted on August 26, 2014.

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