Ultrathin Monomolecular Films and Robust Assemblies Based on

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Ultrathin Monomolecular Films and Robust Assemblies Based on Cyclic Catechols Markus M. Zieger, Ognen Pop-Georgievski, Andres de los Santos Pereira, Elisseos Verveniotis, Corinna M. Preuss, Matthias Zorn, Bernd Reck, Anja S. Goldmann, Cesar Rodriguez-Emmenegger, and Christopher Barner-Kowollik Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03419 • Publication Date (Web): 21 Dec 2016 Downloaded from http://pubs.acs.org on December 22, 2016

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Ultrathin Monomolecular Films and Robust Assemblies Based on Cyclic Catechols Markus M. Zieger,1 Ognen Pop-Georgievski,2 Andres de los Santos Pereira,2 Elisseos Verveniotis,3 Corinna M. Preuss,1† Matthias Zorn,4 Bernd Reck,4 Anja S. Goldmann,*1 Cesar Rodriguez-Emmenegger,*2,5 Christopher Barner-Kowollik*1,6

1

Preparative Macromolecular Chemistry, Institut für Technische Chemie und Polymerchemie,

Karlsruhe Institute of Technology (KIT), Engesserstr. 18, 76128 Karlsruhe, Germany and Institut für Biologische Grenzflächen, Karlsruhe Institute of Technology (KIT), Hermann-vonHelmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany 2

Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic v.v.i.,

Heyrovsky sq. 2, 162 06 Prague, Czech Republic 3

International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for

Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan 4

BASF SE, Registered Office, Ludwigshafen, Germany

5

DWI - Leibniz-Institute for Interactive Materials and Institute of Technical and Macromolecular

Chemistry, RWTH Aachen University, Forckenbeckstraße 50, 52074 Aachen, Germany 6

School of Chemistry, Physics and Mechanical Engineering, Queensland University of

Technology (QUT), 2 George Street, Brisbane, QLD 4000, Australia

KEYWORDS. Cyclic catechols; ultrathin films; macromolecular monolayers; functional coatings

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ABSTRACT. We introduce a newly designed catechol-based compound and its application for the preparation of homogeneous monomolecular layers as well as for robust assemblies on various substrates. The precisely defined cyclic catechol material (CyCat) was prepared from ortho-dimethoxybenzene in a phenolic resin-like synthesis and subsequent deprotection, featuring molecules with up to 32 catechol units. The CyCat’s chemical structure was carefully assessed via matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF), proton nuclear magnetic resonance (1H NMR), diffusion ordered spectroscopy (2D DOSY) and high resolution electrospray ionization mass spectrometry (ESI MS) experiments. The formation of colloidal aggregates of the CyCat material in alkaline solution was followed by dynamic light scattering (DLS) and further verified by dropcasting CyCat from solution on highly oriented pyrolytic graphite (HOPG), which was examined by Kelvin probe force microscopy (KPFM). The adsorption behavior of the CyCat to form monomolecular layers was investigated in real time by surface plasmon resonance (SPR). Formation of these thin CyCat layers (1.6–2.1 nm) on Au, SiO2 and TiO2 substrates was corroborated by spectroscopic ellipsometry (SE) and X-ray photon-electron spectroscopy (XPS). The prepared coating perfectly reflects the surface structure of the underlying substrate and does not exhibit CyCat colloidal aggregates as verified by atomic force microscopy (AFM). The functional nature of the prepared catechol monolayers was evidenced by reaction with 4-bromophenethylamine and bis(3aminopropyl)-terminated poly(ethylene oxide) (PEO). Multilayer assemblies were prepared by a simple procedure of iterative immersion in solutions of CyCat and a multifunctional amine on Au, SiO2 and TiO2 substrates forming thicker coatings (up to 12 nm). Post-modification with small organic molecules was performed to covalently attach trifluoroacetyl, tetrazole and 2bromo-2-methylpropanoyl bromide moieties to the amine groups of the multilayer assembly


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coating. Furthermore, the versatility of the novel multilayer coating was underpinned by ‘grafting-to’ of phenacyl sulfide–terminated PEO and ‘grafting-from’ of poly(methyl methacrylate) via surface-initiated atom transfer radical polymerization (ATRP).

Introduction Modern technology has benefited enormously from the extraordinary development of micro- and nanostructured materials and devices. A critical feature of these systems are solid/soft interfaces due to their decisive function for the device performance. In biomedical applications for instance, the successful operation of an implant relies not only on appropriate bulk material properties, yet also on an adequate physiological response at the implant site, which is modulated by the implant surface chemistry and morphology.1-3 In nanophotonic affinity sensing devices, the precise engineering of the transducer–matrix interface to promote analyte binding while rejecting non-specific adsorption of interfering substances is a prerequisite for effective detection in real settings.4-5 Thus, a major cornerstone of many applications is controlling the properties of solid surfaces and endowing them with desired functionalities, which can be generally achieved by employing functional coatings. These coating systems should be stable and facile to implement without interfering with the bulk properties of the underlying material. Moreover, substrate-independent approaches represent a key focus of research, maximizing the versatility of universally applicable coatings that do not require a specific chemical functionality of the substrate.6 Thus, the generation of functional or functionalizable coatings that can readily be applied on a large variety of substrates is essential to enable the engineering of their interface properties.


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In particular, bioinspired ad-layers based on catechol derivatives – particularly dopamine-based – have attracted significant attention.7-10 Nature has devised effective adhesion strategies even under very harsh conditions such as under marine environment, with mussels being a perfect example.11-12 The animals exploit proteins able to bind ions and form complexes with remarkable stability, which ultimately translate into the excellent adhesion these bivalves display.13 Various attempts aiming at mimicking mussel adhesion have been carried out with variable level of success. The most widely explored adhesion system is the use of poly(dopamine) ad-layers.7 Dopamine itself is able to complex various metal ions, which has been exploited to prepare physically crosslinked films. Analogously, dopamine − a small catechol-containing molecule − can bind to metal oxides surfaces, which is often used to link (macro)molecules to surfaces.14-16 The strength of the bond, however, is highly pH-dependent due to the variable speciation between the coordination number of the complex that dopamine forms with metal ions.17 The strongest bond is formed when three catechols bind a single metal. In turn, this only occurs in a limited pH window closer to alkaline conditions. At mildly alkaline pH, catechols exhibit irreversible oxidation to o-quinones which polymerizes to form melanin-like polymers.18 In ultrathin monomolecular films, the adhesion and stability is weakened by crosslinking. However, this almost ubiquitous reaction can be used to form rather stable polymeric films.7 Dopamine spontaneously polymerizes in mildly alkaline aqueous solution in the presence of oxygen forming oligomers and even nanoparticles which deposit on practically any solid surface, as the catechol moieties can bind to surfaces via multiple types of simultaneous covalent and noncovalent interactions. The formed poly(dopamine) ad-layers are highly adhesive and have been utilized in a plethora of applications including as anchoring point for antifouling polymer brushes,19 for water detoxification,20 surface immobilization of biomolecules,21-22 fabrication of functional,16,

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patterned,24 non-fouling19 surfaces and conducting polymers,25-26 antibacterial


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surfaces,27 drug delivery and photothermal therapy,28-29 dental applications,30 as well as functionalization of nanoparticles to name but a few.31-32 However, to date the mechanism of dopamine polymerization remains unclear and the contributions of the different processes participating in the formation of the poly(dopamine) adlayer have not been fully elucidated and are a subject of study.33-37 The formation of colloidal aggregates during dopamine polymerization results in coatings of variable thickness and roughness that fail to mimic the substrate topography and may compromise long-term stability.38 Interestingly, single catechol groups have been used as surface-binding groups in several methods that attempt to circumvent the disadvantages of the poorly controlled dopamine polymerization.39 To improve the stability of the binding, catechol-derived oligomers were favored to exploit the cooperative binding of the individual catechol moieties.15,

40-41

Several

strategies to precisely tune the chemical reactivity of the catechol groups were introduced, aiming to improve the adhesion and elucidate its mechanisms.42 By including electronwithdrawing groups in the catechol ring, the acidity of the catechol hydroxy groups can be increased and, thus, complexation with metal ions is improved.43 Hence, the formation of the reactive quinone intermediates is hindered, which reduces the self-polymerization of dopamine and promotes stronger surface adhesion.17 While bioinspired approaches based on dopamine have led to an impressive expansion of coating technologies, the need remains for simple methods that yield well-defined stable functional adlayers on multiple substrates while preserving the substrate topography. Furthermore, the capability to produce functional films with controlled thickness and defined chemical functionality is of high interest.44-45 In the present study, we tackle these challenges by introducing a novel family of catechol derivatives consisting of cyclic catechol oligomers (CyCat) with ring sizes up to 32 units and by investigating their ad-layer assembly process. We


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describe their synthesis and characterize their properties in solution of the obtained cyclic molecules. Their ability to form homogeneous monolayers was subsequently applied to several representative substrates (Au, SiO2, and TiO2) (Scheme 1). Aiming to shed light on the structure of these layers, they were thoroughly characterized via X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and surface plasmon resonance (SPR). Finally, the novel CyCat system was exploited for the introduction of a simple method to produce thicker covalent assemblies by alternating immersion in solutions of CyCat and a multifunctional amine [tris(2aminoethyl)amine, (TREN)]. These robust assemblies surface express amine groups, thus being amenable to further functionalization. The newly introduced cyclic catechol material, the associated monolayers and functional assemblies significantly expand the scope and versatility of catechol-based coating approaches.


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Scheme 1. Overview and schematic representation of the investigation of the cyclic catechol (CyCat) in the present study: (A) Monomolecular layer formation on various substrates (Au, SiO2, TiO2), chemical structure of CyCat (x = 1–30) (B) Formation of functional multilayer CyCat assemblies on various substrates (Au, SiO2, TiO2) after 6 cycles showing the proposed chemical structure of the assemblies.

Materials and Methods All materials, chemicals and reagents were used as received unless stated otherwise. The detailed synthetic procedures, characterization methods and further supporting analysis are provided in the Supporting Information (SI). In the following, a short summary of the sample preparation and the characterization methods is presented below.

Preparation of the cyclic catechols and their in-depth characterization. The oligomeric cyclic catechols (CyCat) were prepared by reacting ortho-dimethoxybenzene with formaldehyde under acidic conditions, followed by deprotection of the oligo(ortho-1,2-dimethoxy-4methylbenzene) using boron tribromide. The chemical structure of CyCat (protected and deprotected) was assessed by 1H NMR, 2D DOSY, ESI MS and MALDI-TOF analysis. Determination of aggregates in CyCat solutions. A solution of 1 mg·mL-1 of CyCat in aqueous 0.1 M NaHCO3 was prepared.46 Dynamic light scattering (DLS) measurements were performed using a Nicomp 380 DLS spectrometer from Particle Sizing Systems, Santa Barbara, USA, laser diode: 90 mW, 639 nm. The samples were loaded into 10 mm glass cells and the diffusion coefficient was determined at 25 °C. The hydrodynamic radii (RH) of the present species were determined from the calculated diffusion coefficient using the Stokes-Einstein


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equation. In addition, the formed aggregates were studied using KPFM. A drop of the same solution utilized for the DLS measurement was deposited on top of ultra-flat highly oriented pyrolytic graphite (HOPG) and the solvent was allowed to evaporate. The remaining aggregates on the surfaces in the dry state were studied using KPFM (in air) and XPS. Preparation and study of monomolecular films of CyCat. A solution of CyCat (1.0 mg·mL–1 in 0.1 M NaHCO3) was prepared and filtered after 15 min. The activated substrates, gold coated silicon wafers (Au), SiO2/silicon wafers and TiO2 wafers, were immersed in the CyCat solution at 25 ºC for 45 min. The thickness of the films was determined by ellipsometry, while the chemical composition was accessed by XPS. AFM was used to study the morphology of the surfaces. Surface plasmon resonance (SPR) was carried out by injecting the same solution described above in a custom made SPR (refer to SI) at 25 µL·min-1 while the resonant wavelength of the plasmon was constantly monitored. This allowed for the direct observation of the adsorption kinetics of the CyCat on Au. Study of the formation of functional CyCat assemblies. CyCat multilayers were formed by a simple iterative immersion approach. The substrates (Au, SiO2, TiO2) were first coated with a monomolecular layer as described above. Additional layers were built up by alternatingly immersing the substrates in an aqueous 5 mM solution of tris(2-aminoethyl)amine (TREN) and a CyCat (1.0 mg·mL–1 in 0.1 M NaHCO3) solution. The procedure was repeated 6 times. The formation of the crosslinked films was confirmed using XPS and the thickness was obtained by spectroscopic ellipsometry.


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Results and Discussions The preparation of thin films able to bind to various substrates is usually dominated by poly(dopamine) (PDA) films. While these films have demonstrated to be very versatile, their chemically ill-defined nature and the poor control over their morphology and functionality expose the need for improved coating strategies able to overcome these obstacles. In the current work we introduce a novel cyclic molecule (CyCat) that contains up to 32 catechol units. CyCat in 0.1 M NaHCO3 solution can readily be adsorbed via its catechol units and can be assembled on various substrates. Our strategy resulted in monomolecular films which perfectly copy the topography of the underlying substrate. Using a simple iterative alternating immersion technique, it was possible to prepare clear functional coatings of 11–12 nm thickness. Synthesis and characterization of CyCat. Protected CyCat was synthesized by the reaction of ortho-dimethoxybenzene, aqueous formaldehyde solution and sulfuric acid in acetic acid (Figure 1). At a reaction temperature of 90 °C, high molecular masses of close to 5000 g·mol-1 were determined. From the MALDI-TOF experiments, the highest molecular weight species with a repeating unit (m/z 150.07 Th) of x = 30 were clearly detected (Figure S3, detailed peak assignments in Table S2 and Table S3) with an average mass of m/z 4828.65 Th. 1H NMR analysis (Figure S1) shows an upfield shift of the aromatic protons suggesting the coexistence of CyCat with variable ring sizes. To further investigate the reaction product and understand the molecular structure, selected resonances of the 1H NMR were investigated with respect to their diffusion coefficients. In the DOSY measurements we focused on singlets and doublets arising from aromatic protons and non-aromatic protons and their upfield shifted multiplets (refer to the primary DOSY data in the SI). For both investigated ppm ranges a clear trend was observed: The singlet at 6.8 ppm, the doublet at 4.8 ppm and the singlet at 3.8 ppm possess similar diffusion


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coefficients and can be attributed to the same species. The analysis of the upfield shifted protons exhibits a decreasing diffusion coefficient, which can be explained by the presence of larger ring sizes. The actual number of repeating units of the smaller ring species cannot be determined via DOSY. Furthermore, high resolution Orbitrap ESI MS analysis was carried out giving a distribution in the mass range from m/z 375–1000 Th and m/z 1000–2200 Th with a repeating unit of m/z 150.0681 Th (Figure S2, detailed peak assignments in Table S1). ESI MS analysis clearly confirms the results of the MALDI-TOF measurements, giving a molar mass distribution with repeating units of m/z 150.07 Th (Figure S3).


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Figure 1. Synthesis protocol for the preparation of CyCat from ortho-dimethoxybenzene and subsequent deprotection of the methoxy groups to obtain CyCat with variable ring sizes as well as the corresponding high resolution Orbitrap ESI MS spectrum identifying cyclic () and linear () oligomers (refer to the supporting information Figure S5 for the complete ESI MS spectrum).

Subsequently, the catechol protecting methoxy groups were cleaved employing boron tribromide under reflux in dichloromethane overnight (Figure 1). Recovery of the cyclic product was carried out by simple extraction with ethyl acetate. 1H NMR analysis (Figure S4) shows the successful deprotection of the methoxy groups by the appearance of a broad peak at 8.5 ppm that can be assigned to the emerging catechol units. Only very small amounts of methoxy groups remain in the product which can be identified by the resonance at 4.5 ppm. Aromatic protons and


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those of the methylene unit connecting two CyCat molecules are slightly shifted upfield. Moreover, the mass spectrometric analysis (ESI MS) additionally supports the almost complete deprotection of the cyclic molecules (Figure S5, detailed peak assignments Table S4). The Orbitrap ESI MS of the deprotected CyCat was carried out in negative ion mode. The ESI MS profile measured in the mass range between m/z 300 Th to m/z 960 Th (Figure 1) indicates a single charged distribution with a repeating unit of m/z 122.0368 Th and the most abundant species can be clearly assigned to unprotected CyCat. Less abundant species can be assigned to linear polymers. Study of CyCat aggregates in solution. The oxidation of catechol groups to o-quinones is a well-known step in the polymerization of dopamine leading to the formation of PDA layers. The presence of these groups is a critical factor in the covalent bonding and aggregation behavior of catechol-based systems. As aggregate particles have an impact on the ability of the system to generate stable homogenous coatings by reducing the availability of dissolved CyCat molecules and obstructing the substrate surface, we first investigated the formation of CyCat aggregates in aqueous alkaline solution. A solution of CyCat (1.0 mg·mL–1 in 0.1 M NaHCO3) was prepared and the particle size distribution was examined by means of dynamic light scattering (DLS, Figure S6). Within the time frame of 30 to 60 min at ambient temperature, the presence of aggregates with a hydrodynamic diameter of Dh = 150–300 nm is readily apparent in the weight intensity plots. In contrast, this observation cannot be confirmed by the number weight intensity plots due to the yet low number of aggregates in the first hour. These aggregates are of much larger size than the molecularly dissolved CyCat molecules. Molecular characterization of the drop-casted material was performed via XPS. The same CyCat solution described above (a drop of 4 µL) was deposited on ultra-flat HOPG, allowing the solvent to evaporate. The chemical


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composition of the film was analyzed using XPS (Figure 2A). The signals in the high resolution C 1s XPS spectrum at 284.2 and 285.7 eV can be assigned to the C=C aromatic ring moieties and C−O catechol groups, respectively. The minor signal assigned to the π→π* shake up structure can be detected at 291.0 eV (Figure 2A). In order to directly observe the size and morphology of the aggregates, we additionally utilized scanning probe microscopy, i.e. KPFM, of the material deposited by drop casting (aggregates and CyCat molecules) on molecularly flat HOPG sheets. Notably, KPFM allows analyzing the aggregates in air after drying on the surface, whereas DLS is used to assess the hydrodynamic radius distribution in solution. The height image reveals that − besides molecularly adsorbed CyCat (discussed in the following section) – the same aggregates, which were observed by DLS, are present (Figure 2B). These aggregates were distributed across the surface, while simultaneously a continuous film of CyCat was detected. The mean grain size parameters of the aggregates could be determined from analysis of the height image. The mean height of the colloidal aggregates is hgrain = 68.7±1.5 nm and the radius is Req = 75.5±5.6 nm. The presence of these surface-adherent colloidal structures leads to a root mean square roughness (RRMS) of 11.4±1.1 nm. Colloidal particles and aggregates of similar size and morphology have been observed in natural eumelanins of Sepia officinalis and in the polymerization solutions and surface-adsorbed layers of synthetic melanins based on dopamine.8, 47 In the case of PDA, these structures have been proposed as a potential source of instability of the films.38 The potential maps also clearly proved the coexistence of adsorbed CyCat molecules and their aggregates on the HOPG (refer to the Supporting information Figure S7). The maximum surface potential of the aggregate particles is 100–120 mV higher than the HOPG substrate (work function is 100–120 meV lower). On the other hand, the inter-particle region shows the presence


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of a homogenous layer with only 10–15 mV higher surface potential than the HOPG carrier (10– 15 meV lower work function).

Figure 2. (A) High resolution C 1s XPS spectrum of CyCat adsorbed on highly oriented pyrolytic graphite (HOPG). (B) Surface height topography from Kelvin Probe Force Microscope (KPFM) measurements of CyCat deposits on atomically flat HOPG when drop-casted from a 0.1 M NaHCO3 solution and subsequent evaporation of the solvent.

Generally, the presence of aggregates on a surface weakens the films as a result of the instability of the anchoring layer and ad-layers. However, they cannot be easily avoided in the solution. The suitability of the material to generate a homogenous coating will rely on its preference to diffuse and rapidly adsorb on the surface.


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Ultrathin film formation of CyCat on various substrates. For the preparation of ultrathin films, CyCat (1 mg·mL–1) was dissolved in an aqueous 0.1 M NaHCO3 solution. For all substrates (Au, SiO2, TiO2), the surfaces were immersed into the catechol solution for 45 min and rinsed afterwards with 0.1 M aqueous NaHCO3 solution and deionized water to obtain colorless monomolecular films. We studied the formation of these films in real time via SPR on Au substrates. SPR could only be carried out on gold and not on oxidic surfaces such as TiO2 or SiO2 due to the need of a metal interface for the generation of plasmons. Figure 3 shows the adsorption kinetics monitored by SPR. After a stable baseline was achieved by flowing 0.1 M NaHCO3 in water, a solution of CyCat (1 mg·mL–1 in aqueous 0.1 M NaHCO3) was injected for 45 min. The sensogram exhibits the rapid adsorption of the CyCat on the Au surface, reaching a plateau in about 15 min. The slight decrease in the resonant wavelength observed after replacing the CyCat solution with a 0.1 M NaHCO3 solution is caused by the difference between the refractive indices of the solutions. The net resonant wavelength shift observed between the newly obtained and the original baselines indicates that CyCat forms a monomolecular layer on the surface.


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Figure 3. SPR experiment depicting the formation of catechol monomolecular layers on a Au surface. After stabilization of the baseline in aqueous 0.1 M NaHCO3, a 1 mg·mL–1 CyCat solution in 0.1 M NaHCO3 was injected for 45 min.

The spectroscopic ellipsometry (SE) characterization of the catechol cycle adsorption on Au, SiO2 and TiO2 surfaces reveals the formation of 1.6±0.5 nm, 1.8±0.4 nm and 2.1±0.4 nm thick monolayers, respectively. The assembly of CyCat from an aqueous NaHCO3 solution on various substrates is accompanied by distinct changes of the chemical structure of the CyCat as evidenced by XPS (Figure 4). In addition to C=C and C−O contributions, the CyCat surface is characterized by a signal at 287.8 eV originating from the C=O quinone functionality that can be attributed to an oxidation step that occurs through the partial autoxidation of catechol units under basic conditions during the layer formation. Notably, the high resolution XPS C 1s spectra of the


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monomolecular layers formed from the self-assembly of CyCat from an aqueous NaHCO3 solution on different surfaces lack of the presence of O−C(=O)−O carbonate contribution at 289.3 eV, thus verifying that the bicarbonate ions are not incorporated within the structure of the CyCat layers.

Figure 4. Left: High resolution C 1s XPS spectra of the monomolecular layer formation of CyCat on Au (A), SiO2 (B) and TiO2 (C) surfaces. Right: Representative AFM surface height topographic map (500 × 500 nm2) of CyCat-coated Au (A), SiO2 (B) and TiO2 (C) surfaces, perfectly replicating the underlying morphology of the individual substrates.


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The morphology of the layers resulting from the CyCat self-assembly process was further examined by means of AFM (Figure 4 (right), Supporting information Figure S8, S10 and S12). Analysis of the AFM surface topography shows for all substrates (Au, SiO2 and TiO2) the formation of confluent defect-free layers that perfectly replicate the topography of the underlying substrate. The complete absence of large particles and aggregates on the surfaces and the only marginal changes in particle surface density, hgrain, Req and RRMS (Table 1, Figure S9, S11 and S13) corroborates the formation of surface confluent layers. The monomolecular layer surface confluency and absence of defects revealing bare substrate regions is further supported by the small phase shifts observed in the AFM phase data (Figure S8, S10 and S12). The formation of monomolecular CyCat layers represents an innovative and simple approach to modify inert metal and oxidic surfaces, while circumventing colloidal aggregates that are usually associated with impaired stability of the anchoring films. The rapid adsorption of the CyCat molecules on the surface occurs preferentially compared to the deposition of the larger aggregates, probably due to enhanced diffusion as consequence of their much smaller size (refer to the Stokes-Einstein equation). This observation is of major importance as the monomolecular layer can be generated from highly diluted solutions and form priming coatings on various materials that differ significantly from known melanin-based anchor layers. In addition to the surface homogeneity and preservation of substrate topography, the versatility of a particular anchor layer depends on the presence of reactive groups on the surface for subsequent reactions. The availability and accessibility of remaining functional groups on the ultrathin CyCat monomolecular layer was demonstrated by exploiting them in post-modification reactions with 4-bromophenethylamine and NH2-PEO-NH2. Analysis by SE showed thickness increments of 0.6±0.1 nm and 1.5±0.2 nm, after reaction with the small organic molecule or the NH2-PEO-NH2 polymer of 1500 Da, respectively. XPS measurements further confirm the success of the reactions (refer to


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the Supporting Information Figures S15 and S16). The C 1s and N 1s spectra indicate the formation of Michael addition and Schiff base products between the primary amine groups of the modifiers and the catechol and quinone groups in the film. Notably, the high resolution Br 3d spectrum of CyCat monolayers functionalized with 4-bromophenethylamine clearly shows the presence of the surface immobilized bromine.

Table 1. Surface characteristics of monomolecular layer formation of CyCat adsorbed on Au, SiO2 and TiO2 surfaces, respectively. Grain density

hgrain

Req

RRMS

[µm-2]

[nm]

[nm]

[nm]

Pure Au

1192

4.6 ± 0.1

6.2 ± 0.5

1.4 ± 0.3

CyCat on Au

1256

3.7 ± 0.1

4.7 ± 0.3

1.1 ± 0.1

Pure SiO2

952

0.9 ± 0.1

5.7 ± 0.9

0.2 ± 0.2

CyCat on SiO2

1060

0.9 ± 0.1

6.2 ± 2.1

0.3 ± 0.1

Pure TiO2

1604

2.0 ± 0.1

6.3 ± 1.1

0.6 ± 0.1

CyCat on TiO2

1812

2.0 ± 0.2

8.6 ± 0.4

0.6 ± 0.2

Functional Multilayer Assembly Formation. As a further improvement of the versatility of the approach, we demonstrate the preparation of highly functional multilayer films of the CyCat by reaction with a multifunctional amine (tris(2-aminoethyl)amine, (TREN)) with the aim of increasing the surface density of functional groups. The amine effectively acts as a covalent crosslinker by forming Michael addition products and Schiff bases with the accessible catechols and quinones, as well as providing amine groups to the assembled layer, making it more


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amenable to further functionalization. The multilayer formation procedure is simple, as the substrates (primed with a CyCat monomolecular layer) only need to be immersed alternatingly in solutions of TREN and CyCat. After 6 of these immersion cycles SE measurements revealed the formation of 10.9±1.0 nm, 12.9±0.6 nm and 12.1±0.3 nm thick layers on Au, SiO2 and TiO2 surfaces, respectively. XPS measurements were carried out for each sample of the different substrates. An increase of the signal at 286 eV can be attributed to the presence of C−N bonds originating from the Schiff base, formed by the reaction of the catechol-derived quinones and TREN (Figure 5). In contrast, for the monomolecular layers only signals assigned to the C−O of the catechols could be observed (Figure 4). In addition, the high resolution N 1s spectra show the presence of N−C and N+ species at 399.0 and 401.0 eV, respectively (Figure S14 in the Supporting Information). Only a minor decrease in thickness (8 %) was measured after incubation of CyCat–TREN multilayer films prepared on Si/SiO2 in water for 24 h, confirming the stability of the attachment (Table S5 in the Supporting Information). The preparation of the functional assemblies by reaction of the CyCat and TREN further demonstrates the simplicity and versatility of the approach, highlighting the potential of the coating for applications to substrate-independent functionalization. Moreover, it provides amine groups on the surface, which enhance the possibilities of functionalization, as well as allowing to achieve thicker films. In contrast to PDA ad-layers, which are prepared in a one-pot approach by the selfpolymerization of dopamine, the formation of CyCat multilayer assemblies requires more work as the substrates need to be dipped alternatingly in two solutions for five minutes per layer.


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Figure 5. High resolution C 1s XPS spectra of a multilayer assembly coating of CyCat with tris(2-aminoethy)amine (TREN) performed on Au (A), SiO2 (B) and TiO2 (C) surfaces.


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Postmodification of multilayer assemblies. In order to highlight the potential of the CyCat−TREN multilayer assemblies for different applications, we carried out their functionalization by reacting the available amines and catechol groups present in the film with various functional moieties. The performed post-modifications resulted in covalent binding of trifluoroacetyl, tetrazole and 2-bromo-2-methylpropanoyl groups to the multilayer coatings. The SE analysis of the functionalized films showed an increase of 0.9±0.2 nm, 1.1±0.1 nm and 0.9±0.3 nm for the trifluoroacetyl-, tetrazole- and 2-bromo-2-methylpropanoyl-bearing CyCat−TREN assemblies, respectively. The high-resolution XPS measurements further verified the presence of new ester and amide contributions as well as signals characteristic for the newly incorporated groups in the respective high resolution F 1s, N 1s and Br 3d spectra (refer to Supporting information Figures S17-S19). Thus, through the successful post-modification we proved the robustness of the multilayer assemblies and their suitability for imparting functionality to an interface. Notably, tetrazole groups have been employed for photo-patterning through nitrile-imine mediated tetrazole-ene cycloaddition (NITEC) reactions, while 2-bromo-2propionyl groups are commonly employed as initiators for the growth of polymers via SurfaceInitiated Atom Transfer Radical Polymerization (SI-ATRP). The surface grafting of polymer chains is a valuable tool for the engineering of interface properties. For this purpose, we demonstrate the immobilization of polymer chains from the CyCat–TREN multilayer films via both ‘grafting-to’ and ‘grafting-from’ approaches. The photoinduced grafting of phenacyl sulfide-terminated PEO to the available amines of the anchor layer resulted in 1.3±0.4-nm-thick polymer brush films. Furthermore, SI-ATRP of methyl methacrylate was performed from the initiating 2-bromo-2-methylpropanoyl groups immobilized on the CyCat−TREN multilayer coating to achieve 11.5±0.5-nm-thick poly(methyl methacrylate)


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brushes. The success of both polymer grafting processes was further verified by the pronounced changes in the high resolution XPS spectra (Figure 6). The immobilization of phenacyl sulfidePEO induced an increase in the C–O contributions at close to 286.0 eV and the appearance of the thioamide C(=S)–NH contribution at 288.1 eV. The covalent grafting of the PEO chains was further confirmed by the appearance of signals in the high resolution S 2p spectrum (Figure S20 in the Supporting Information). The presence of the poly(methyl methacrylate) brushes grown by SI-ATRP strongly dampened the signals originally observed for the anchoring in the C 1s region, while giving rise to contributions of aliphatic C–C and C–H, the carbon atoms of the C̲–O– (C=O) and ester C(=O)–O at 285.0, 286.5 and 288.9 eV. The preparation of the multilayer assemblies by reaction of the CyCat and TREN and their subsequent facile functionalization demonstrates the simplicity and versatility of our approach, highlighting the potential of the coating for applications to substrate-independent functionalization.


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C 1s

O HN

C=C

Br

C-O

N

N

C=O

N

C=S-NH

HO

N

HO

Normalized intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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N

O O

SI-ATRP

π

π*

A) C-C C-O

C(=O)-O

O HN

Br n

N

N HO HO

CO2Me

N

B) N

294 292 290 288 286 284 282 N

Binding energy (eV)

Figure 6. Left: Schematic representation of the SI-ATRP of methylmethacrylate. Right: High resolution C 1s XPS spectra of the photoinduced ‘grafting-to’ of phenacyl sulfide-terminated PEO to the CyCat–TREN coatings (A) and poly(methyl methacrylate) brushes synthesized by SIATRP (B).


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Conclusion In summary, we introduce a coating system from a novel cyclic catechol material (CyCat) containing up to 32 catechol units able to adhere to various surfaces. The chemical structure of the CyCat molecules was examined in detail utilizing MALDI-TOF, 1H NMR, 2D DOSY and ESI MS analysis. The aggregation behavior of the CyCat in alkaline solutions was assessed using DLS. Complementary KPFM analysis was performed to determine the surface morphology and size of the CyCat aggregates that form in solution. The molecular assembly of the CyCat was monitored in real time by SPR. The assembly process on different surfaces resulted in the formation of thin homogeneous monomolecular layers with a defined covalent structure as determined by SE and XPS. A detailed AFM analysis showed that the CyCat monolayers perfectly replicated the topography of the underlying substrates and were free of any surface adherent colloidal aggregates. The reactivity of the catechol monolayers was evidenced utilizing amine compounds. Moreover, a simple procedure consisting only of immersing the substrates alternatingly in CyCat and multifunctional amine solutions yielded thicker covalently assembled functional multilayers. Furthermore, the accessible functional groups were used for the creation of fluorinated surfaces, reactive surfaces suitable for photo-patterning reactions, SI-ATRP polymerizations, and the photoinduced grafting of PEO. The herein presented well-defined coatings based on cyclic catechol oligomers introduce a new simple avenue to the fabrication of bioinspired substrate-independent functional coatings, with potential applications in a wide range of fields. Ultimately, the precise characterization of the structural features of the newly introduced layers makes it possible to advance in the design of coating systems with improved performance.


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AUTHOR INFORMATION Corresponding Authors [email protected] or [email protected]; [email protected]; [email protected]; [email protected] Present Addresses †

Department of Chemistry, University of Warwick, Coventry, UK

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXX.

ACKNOWLEDGMENT C.B.-K. acknowledges continued funding from the Karlsruhe Institute of Technology (KIT), the Helmholtz Association via the BioInterfaces in Technology and Medicine (BIFTM) and the Science and Technology of Nanosystems (STN) programs. The work was additionally supported by the Program of Project Based Personal Exchange ASCR-DAAD (no. 14/08). C.B.-K., A.G. and M.Z. are grateful to the BASF SE (Ludwigshafen, Germany) for co-funding the project. This work was partially performed at the Center for Chemical Polymer Technology (CPT) under the support of the EU and the federal state of North Rhine-Westphalia (Grant EFRE 30 00 883 02).


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C.R.E., O.P.G and A.d.l.S.P. acknowledge funding from the Grant Agency of the Czech Republic (15 - 09368Y) and the OPPK3 (CZ.2.16/3.1.00/21545). E. V. is grateful for support from the World Premier International Research Center Initiative (WPI) and the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). M.Z. is grateful for a PhD scholarship from the Evangelisches Studienwerk Villigst. We are grateful to Dr. Steffen Weidner (Bundesanstalt für Materialforschung und -prüfung Berlin, BAM) for providing MALDI-TOF measurements. Jan Steinkönig (KIT) is thanked for supporting Orbitrap ESI MS and 2D DOSY NMR measurements, while Kilian Wust (KIT) is thanked for the provision of phenacylsulfide-PEG and tetrazole acid chloride. The authors further thank Dr. T. Nakayama and Dr. Y. Okawa (MANA, NIMS) for access to the SPM equipment.

For Table of Contents Only


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Ultrathin Monomolecular Films and Robust Assemblies Based on

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