Review pubs.acs.org/cm
Looking for Synergies in Molecular Plasmonics through Hybrid Thermoresponsive Nanostructures Mai Nguyen,†,‡ Nordin Felidj,*,† and Claire Mangeney*,† †
Laboratoire Interfaces, Traitements, Organisation et Dynamique des Systèmes, Université Paris Diderot, Sorbonne Paris Cité, CNRS UMR 7086, 15 Rue Jean-Antoine de Baïf, 75205 Paris Cedex 13, France ‡ USTH, University of Science and Technology of Hanoi, 18 Hoang Quoc Viet, 10000 Hanoi, Vietnam ABSTRACT: Gold nanoparticles (AuNPs) have been the subject of intensive work recently due to their outstanding optical, catalytic and electronic properties. Although the morphological characteristics (size and shape) of the AuNPs largely determine their properties, their functionalization or association to other materials play a primary role for optimizing their use in various applications such as drug delivery, theranostic, catalysis and (bio)sensing. Among the many possible options available by chemistry, the coupling of AuNPs with smart polymers is an exciting field offering the possibility to supply stimuli-responsive properties to the resulting nanocomposites. With regard to the photothermal properties of AuNPs, their combination with thermoresponsive polymers, such as poly(N-isopropylacrylamide) (PNIPAM) is particularly promising, likely to generate new synergies between the polymer component and the metallic nanoparticles. Despite such unique and intriguing advantages of AuNPs−PNIPAM nanocomposites, there is no exclusive review regarding this field at the interface between plasmonics and thermoresponsive polymers. To fill this gap, this review describes the general methods for preparing AuNPs−PNIPAM nanocomposites and their potential applications, highlighting the added-value properties emerging from the combination of AuNPs and PNIPAM in a single composite. New outlook for the near future to merge multiple functions at the nanometer scale and integrate building materials of various chemical nature (such as other stimulable polymers or the combination of AuNPs with other metals) into the same active plasmonic platform is described as well, emphasizing innovative approaches to improve their functionalities. AuNP size4 (see Figure 1) and shape, their chemical nature and the characteristics of their surrounding medium. This sensitivity of AuNPs to their close environment is greatly advantageous for chemical and biological sensing because the attachment of analysts to the particle surface can be detected through changes of the LSP wavelength.5 In addition, upon laser excitation in the LSP modes, an intense electromagnetic field is produced at the gold nanoparticle surface, allowing ultrasensitive detection of analytes by the socalled surface enhanced Raman spectroscopy (SERS) technique. AuNPs are also efficient photothermal convertors, i.e., upon exposure to a laser beam at the wavelength of their plasmon resonance, the absorbed photon energy is converted to heat, which represents a promising approach for biomedical hyperthermia and cancer treatment.6 In addition, AuNPs particles could act as catalysts in many reactions, such as oxidation of glucose, CO, or alcohol, opening up promising prospects in industrial applications.7 To obtain smart materials from AuNPs, recent works have focused on their combination with stimulable polymers, i.e., polymers which undergo large physical or chemical changes in response to small external
1. INTRODUCTION The recent years have witnessed a quantum jump in the number of publications pertaining to the design of hybrid nanostructures made of gold nanoparticles (AuNPs) and thermoresponsive polymers, such as poly(N-isopropylacrylamide) (PNIPAM). The association of these two materials generates advanced new functions for various applications from optics, biomedicine to catalysis. As many questions still remain to be addressed, this emerging field will continue to stimulate a wide range of interest in the near future. In this regard, this review aims at highlighting the outstanding properties of these hybrid nanocomposites made of AuNPs and PNIPAM and focuses on their chemistry, processing and applications. 1.1. State of the Art. Recently, gold nanoparticles have stimulated extensive research because of their remarkable properties and promising applications in biomedical materials, catalysis, electronics and sensors.1−3 Moreover, AuNPs exhibit high surface area and surface energy offering a suitable platform for a large variety of molecules and proteins. Their optical properties rely on the excitation, by electromagnetic waves in the UV to near-infrared range, of localized surface plasmon (LSP) due to collective oscillations of the conduction band electrons at the particle surface. This results in intense extinction bands (absorption and scattering) in the visible and near-infrared spectral region, which strongly depend on the © 2016 American Chemical Society
Received: January 18, 2016 Revised: May 10, 2016 Published: May 10, 2016 3564
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Chemistry of Materials
Figure 1. (a) Scanning electron microscopy image of a gold triangle array on indium−tin-oxide (ITO) substrate. The interparticle distance is D = 350 nm, the lateral particle size, a = 260 nm, and the height, h = 40 nm. (b) Experimental extinction spectra of gold submicrometer triangles. The spectra correspond to triangle arrays with five lateral sizes: (A) 50 nm, (B) 100 nm, (C) 150 nm, (D) 200 nm, and (E) 260 nm. Reproduced with permission from ref 4. Copyright 2008 American Institute of Physics.
Table 1. Topic, Scope and References of the Main Reviews Focusing on AuNPs, Responsive Polymers or Smart AuNPs− Polymer Composites topic gold nanoparticles
running title (date of publication)ref
scope of the review
Advances in localized surface plasmon resonance spectroscopy biosensing (2011).1 The golden age: gold nanoparticles for biomedicine (2012).2 Thermo-plasmonics: using metallic nanostructures as nanosources of heat (2013).6 Nanogold plasmonic photocatalysis for organic synthesis and clean energy conversion (2014).3 Self-assembled plasmonic nanostructures (2014).11
responsive polymers
Emerging applications of stimuli-responsive polymer materials (2010).8 Multiresponsive polymers: nanosized assemblies, stimuli-sensitive gels and smart surfaces (2011).9
AuNPs−polymer composites
Coating matters: the inf luence of coating materials on the optical properties of gold nanoparticles (2012).12 Tunable plasmonic nanostructures f rom noble metal nanoparticles and stimuli-responsive polymers (2012).13 Multifunctionality in metal@microgel colloidal nanocomposites (2013).14 “Multifaceted” polymer coated, gold nanoparticles (2013).15 Plasmonic nanocomposites: polymer-guided strategies for assembling metal nanoparticles (2013).16 Responsive hydrogels−structurally and dimensionally optimized smart f rameworks for applications in catalysis, micro-system technology and material science (2013).17 Nanocomposites of gold nanoparticles@molecularly imprinted polymers: chemistry, processing, and applications in sensors (2015).18
changes in their environment (such as variations of temperature, pH, ionic strength, etc.).8,9 In particular, thermoresponsive polymers such as poly(N-isopropylacrylamide) display a discontinuous coil-to-globule transition in aqueous solutions in response to a temperature stimulus. The transition temperature is called the lower critical solution temperature (LCST) and is about 32 °C for PNIPAM. Below this temperature, the polymer is hydrophilic, fully swollen in water and its chains are in the extended conformational state. When the temperature is raised above the LCST, the polymer undergoes a phase transition to a hydrophobic state and collapses, resulting in the shrinking of the hydrogel. Large
review on the recent advances in LSPR spectroscopy biosensing insights into the design, synthesis, functionalization, and applications of AuNPs in biomedicine overview of the recent progress in thermo-plasmonics survey on plasmon-based AuNPs photocatalysis using visible light and applications to organic reactions and clean energyconversion systems recent advances in solution-based self-assembly of plasmonic nanoparticles recent advances on stimuli-responsive polymeric materials, self-assembled from nanostructured building blocks emerging developments in polymer chemistry to design multiresponsive polymeric materials review on the effects of various types of coating materials on the optical properties of AuNPs progress in the field of tunable plasmonic nanostructured materials based on noble metal nanoparticles and stimuliresponsive polymers recent developments related to hybrid nanocomposites made of a metal core and a smart microgel shell state-of-the-art on AuNPs modified with a two (or multi-) component polymer brush coating overview of polymer-directed assembly of plasmonic nanoparticles and applications in enhanced spectroscopy and optical metamaterials review on responsive hydrogels and their applications review on nanocomposites composed of AuNPs and molecularly imprinted polymers
modifications of the polymer properties are generated by this phase transition, such as size or chain length, porosity, refractive index and colloidal stability. This characteristic swelling/ deswelling transition can be modified through copolymerization of NIPAM with organic comonomers, resulting in tailored shifts of the LCST. The association of temperature-responsive PNIPAM polymers with AuNPs opens up exciting perspectives to obtain smart multifunctional nanocomposites that ideally combine the responsiveness of the polymer with the optical, catalytic or photothermal properties of the metal nanoparticles. By this way, the properties of the AuNPs within the hybrids may be 3565
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Chemistry of Materials tuned by temperature changes as an external stimulus, while the stimulus can be sensed through the detection of the modification of their properties.10 The hybrid AuNPs− PNIPAM nanomaterials have therefore promising applications in various fields such as nanosensors, smart nanoreactors for catalysis, targeted drug delivery and controlled release, biomedical diagnosis, therapeutics, etc.8,9 Depending on the morphological structure of the thermoresponsive composite (nano/microgels, core−shell particles, bulk material, 2D or 3D arrays, etc.) and on the targeted applications, the combined properties of AuNPs and PNIPAM can result either in the simple juxtaposition of both properties or may lead to new synergetic properties, each component being modified by the presence of the other one. This review particularly addresses these possible synergies between AuNPs and PNIPAM in AuNPs−PNIPAM nanocomposites. 1.2. Scope of the Review. Many comprehensive surveys of scientific papers on the topic of gold nanoparticles,1−3,6,11 smart polymers8,9 or the combination of both,12−17 have already been reported in the literature (see review articles with their corresponding scope in Table 1). Nevertheless, although a large amount of works has focused on the use of PNIPAM as a stimulable polymer to produce advanced plasmonic thermoresponsive AuNPs−PNIPAM composites, the advanced properties of these new hybrids and the possible synergies emerging from the combination of both materials (i.e. AuNPs and PNIPAM) has never been the main focus of a review so far. In the present review, we consider the combination of PNIPAM and AuNPs from the point of view of materials science in general and new advanced functionalities in particular, focusing on the synergetic properties emerging from these hybrid nanocomposites and examining the most recent developments in the area. The first part of this review will describe the general methods for the preparation of AuNPs−PNIPAM nanocomposites, distinguishing colloidal hybrid particles, 2D nanostructures or 3D materials. The added-value properties obtained from the combination of AuNPs and PNIPAM in the nanocomposites are then covered in separate sections, depending on the type of interaction between the properties of each component. The first section concerns the AuNPs−PNIPAM materials providing a simple juxtaposition of each component properties. The following sections deal with the positive synergies arising from the combination of both materials, each component exercising an effect on the other one (see illustration in Figure 2). An example of new advanced nanostructures supplying reciprocal synergetic properties between AuNPs and PNIPAM is then described separately. The last section emphasizes innovative approaches to improve the functionalities of AuNPs−PNIPAM materials by merging multiple functions or integrating various building materials into the same active plasmonic platform.
Figure 2. Illustration of the synergy between AuNPs and PNIPAM in AuNPs−PNIPAM hybrid structures, their properties and applications.
superlattices. Examples of these various nanostructures are illustrated in Table 2. Table 2. Description of the Different Types of AuNPs− PNIPAM Nanocompositesa
2. GENERAL METHODS FOR THE PREPARATION OF HYBRID AUNPS−PNIPAM STRUCTURES The combination of AuNPs with PNIPAM has been performed following various approaches, which can be classified according to the final format of the hybrid nanostructures: (i) colloidal materials based on micro/nanogels or core−shell particles; (ii) 2D materials, obtained by either bottom-up methods (such as the self-assembly of colloidal particles on a surface) or by topdown approaches (such as lithography); and (iii) 3D materials in the form of bulk amorphous hydrogels or ordered
a
Figures adapted with permission from refs 19−26. Copyright 2015 American Chemical Society, 2009 Wiley-VCH Verlag GmbH & Co., 2010 American Chemical Society, 2014 Elsevier, 2015 American Chemical Society, 2015 American Chemical Society, 2007 American Chemical Society, 2011 Wiley-VCH Verlag GmbH & Co., respectively.
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Chemistry of Materials 2.1. Colloidal AuNPs-PNIPAM Based on Micro/Nanogels or Core−Shell Particles. The colloidal form based on either micro/nanogels loaded with AuNPs,19,27−33 or core− shell particles12,14,20,34−42 in solution offers several advantages over 2D or 3D materials, including their capacity to diffuse in a liquid medium, their high specific surface area, the particle being in full contact with the solution in which it is dispersed and their possible use for in situ environmental analysis or in vivo biomedical applications. Nevertheless, controlling the synthesis process in order to obtain nonaggregated colloidal AuNPs−PNIPAM hybrid system is a critical stage, the particles being strongly prone to aggregation. Slight variations of the physicochemical parameters (pH, ionic force, temperature) of their surrounding medium can disturb their colloidal stability and therefore limit their reusability or provoke adverse toxicity effects in biomedical applications. To overcome these limitations, the hybrids may be confined on surfaces. 2.2. 2D Hybrid Structures of AuNPs−PNIPAM. By confining the hybrid nanostructures on a surface, one could avoid the problems of instability due to colloidal aggregation. Depending on the approach, the nanoparticles can be regularly organized in two-dimensions, in the form of a regular array or randomly deposited on the surface of polymer brushes.22 Obtaining organized structures requires either top-down approaches, such as electron-beam and soft lithography or particles self-assembly, giving rise to regularly spaced gold nanoparticles on the surface.43−45 The main advantage of these approaches relies on the perfect control over the structural parameters of the particles (size, shape, spacing between particles) favoring systematic and in-depth studies of their optical properties and thermoresponsive behavior.46 Compared to disorganized systems, the low amount of defects and heterogeneities in such samples is expected to minimize sample-to-sample variability of optical response and to improve the reproducibility of SERS response, one critical issue for sensing applications using AuNPs. In contrast, disorganized 2D hybrid AuNPs−PNIPAM materials provide a lower control of structural parameters but such systems generate randomly distributed hot-spots that offer a strong amplification of the SERS signals enhancing the sensitivity at the expense of the reproducibility.21 2.3. 3D Hybrid Structures of AuNPs−PNIPAM. A similar distinction between organized and randomly distributed AuNPs can be made for three-dimensional hybrid systems. Disorganized hybrid systems are simple to prepare via the polymerization of a mixture of the hydrogel precursor and AuNPs, leading to nanocomposites.47 The main difficulty for obtaining AuNPs dispersed within the hydrogel matrix lies in their poor colloidal stability inducing nanoparticle aggregation. To ensure a high colloidal stability and to avoid the formation of aggregates in the polymer matrix, strategies based on the grafting of hydrophilic polymer chains on the AuNPs surface has been proposed. For example, gold nanorods (AuNRs) coated by poly(ethylene glycol) were used for the preparation of AuNPs−PNIPAM composites, showing marked dispersion stability in the gel.25 Although more difficult to prepare, largescale AuNPs−PNIPAM 3D structures with high crystal ordering open up the possibility to couple the optical response of nanocrystals to the optical modes of the superlattice, i.e., the surface plasmon polariton response of small metal nanocrystals to the collective photonic modes of the macroscopic crystal.26 Indeed, for large spacing between AuNPs (ranging from 50 to 500 nm), the ordered plasmonic structure displays a
pronounced diffraction peak in the visible, in addition to the LSP band. Moreover, these hybrid plasmonic−photonic crystals exhibit a thermoresponsive behavior due to the PNIPAM shell, causing a fast melting or recrystallization response (
