Block Copolymer Patterns as Templates for the Electrocatalyzed

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Block Copolymer Patterns as Templates for the Electrocatalyzed Deposition of Nanostructures on Electrodes and for the Generation of Surfaces of Controlled Wettability Chanchayya Gupta Chandaluri, Gilad Pelossof, Ran Tel-Vered, Roy Shenhar, and Itamar Willner ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10764 • Publication Date (Web): 23 Dec 2015 Downloaded from http://pubs.acs.org on December 30, 2015

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ACS Applied Materials & Interfaces

Block Copolymer Patterns as Templates for the Electrocatalyzed Deposition of Nanostructures on Electrodes and for the Generation of Surfaces of Controlled Wettability Chanchayya Gupta Chandaluri, Gilad Pelossof, Ran Tel-Vered, Roy Shenhar* and Itamar Willner* Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, 9190401, Israel E-mails: [email protected], [email protected] KEYWORDS: Electropolymerization, Prussian blue, Nanoparticles, Block copolymers, Contact angle

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ABSTRACT: ITO electrodes modified with a nano-patterned film of polystyrene-block-poly(2vinylpyridine), PS-b-P2VP, where the P2VP domains are quaternized with iodomethane, are used for selective deposition of redox-active materials. Electrochemical studies (cyclic voltammetry, Faradaic impedance measurements) indicate that the PS domains insulate the conductive surface towards redox labels in solution. In turn, the quaternized P2VP domains electrostatically attract negatively-charged redox labels solubilized in the electrolyte solution, resulting in an effective electron transfer between the electrode and the redox label. This phenomenon is implemented for the selective deposition of the electroactive Prussian blue on the nano-patterned surface and for the electrochemical deposition of Au nanoparticles, modified with a monolayer of p-aminothiophenol/2-mercaptoethane sulfonic acid, on the quaternized P2VP domains. The patterned Prussian blue-modified surface enables controlling the wettability properties by the content of the electrochemically-deposited Prussian blue. Controlled wettability is unattainable with the homopolymer-modified surface, attesting to the role of the nano-pattern.

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INTRODUCTION: Block copolymers (i.e., polymers consisting of sequences of chemically distinct comonomers) are promising materials for nanotechnology, as they give rise to periodic nanoscale structures with controlled morphologies due to microphase separation.1 Potential applications of block copolymers range from membranes for nano-filtration,2 through lithographic masks for storage media fabrication,3-4 to nano-photonics.5-7 Periodic arrays of block copolymers have been used as templates for organizing nanoparticles,8 either by co-assembly,7,

9-12

or by selective

deposition onto specific domains.13 Additionally, selective reactions of the patterns with environmental agents led to the selective electrical charging of domains, controlling their hydrophilic/hydrophobic properties.14 This allowed the selective deposition of surface-modified metallic nanoparticles on specific domains of the patterns.15-16 Previous studies have demonstrated that hydrophobic monolayers or thin films may impede electron transfer at electrodes,17-20 whereas charged monolayers or films may control the interfacial electron transfer of charged redox-active components in solution.21-23 This opens the opportunity to use the block copolymers as patterned masks for controlling electron transfer properties at the nanoscale. So far, however, site-selective chemistry using block copolymer nano-patterns was demonstrated only after removal of one domain of the copolymer by selective etching.24-25 Here we present a simple approach for performing site-selective electrochemistry on specific block copolymer domains that result in patterned surfaces. We further demonstrate that the patterned surfaces exhibit controlled wettability properties. We highlight the patterning of indium tin oxide (ITO) conductive glass electrode with polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) block copolymer, and the chemical quaternization of the pyridine groups via alkylation with iodomethane, leading to the formation

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of positively-charged P2VP domains. We further characterize the electron transfer features of ITO substrates coated with the homopolymers corresponding to the different blocks, and apply the different electron transfer features of the block copolymer domains to address the electropolymerization of chemical structures (Prussian blue26-28 or bis-aniline-bridged Au NPs29) on the polymer patterns. We further demonstrate that controlled deposition of Prussian blue on the patterned block copolymer surface enables the tuning of the wettability of the block copolymer-modified surface.

RESULTS AND DISCUSSION: The interfacial electron transfer properties of the homopolymers corresponding to the PS and P2VP blocks of the block copolymer were characterized by cyclic voltammetry and Faradaic impedance spectroscopy. PS (Mn 170 kDa, PDI=2.06) and P2VP (Mn 2.3 kDa, PDI=1.69) homopolymers were cast from chloroform solutions on ITO substrates, yielding films of ca. 28 nm thickness. Figure 1A shows the Faradaic impedance spectra (in the form of Nyquist plots) corresponding to the different homopolymers using Fe(CN)63-/4- as a redox probe. Faradaic impedance spectroscopy is a useful method for probing the interfacial electron transfer properties at chemically-modified electrodes.30-31 The semi-circle diameter of the Zim vs. Zre impedance plot corresponds to the interfacial electron transfer resistance, Ret, at the electrode. The results demonstrate that the PS homopolymer reveals the highest interfacial electron transfer resistance of ca. Ret=30 kΩ, whereas the P2VP homopolymer shows an electron transfer resistance of Ret=1.2 k. Alkylating the pyridine groups (forming poly(methyl-2-vinylpyridinium), denoted as P2VMeP), decreases the resistance for electron transfer to ca. Ret=0.3 kΩ, consistent with the electrostatic attraction of the negatively-charged redox label (Fe(CN)63-/4-) to the positively-

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charged polymer, which facilitates electron transfer at the interface. The cyclic voltammograms of the Fe(CN)63-/4- redox labels at the homopolymer-coated electrodes further support the Faradaic impedance measurements (Figure 1B). The PS coating insulates the electrode surface and blocks the electron transfer between the electrode and the redox label. In turn, the bare ITO surface shows a quasi-reversible redox wave with a peak-to-peak separation of 0.55 V, whereas the P2VMeP polymer shows a redox wave of an improved reversibility, peak-to-peak separation of 0.42 V, due to the electrostatic attraction of the redox label to the electrode through the polymer coating.

Figure 1. (A) Faradaic impedance spectra and (B) Cyclic voltammograms corresponding to: (a) PS on ITO, (b) P2VP on ITO, (c) a bare ITO electrode, and (d) P2VMeP on ITO. Inset in A shows expanded spectra. PS-b-P2VP patterned films were fabricated on ITO by spin coating of a chloroform solution that contained the block copolymer, followed by annealing the film under chloroform vapors. By treating the resulting films with iodomethane, quaternized PS-b-P2VP patterns were obtained (for a detailed description of preparation of the patterned block copolymer film, see

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experimental section). Figure 2A, B show the AFM images of the striped patterns of 28 nm-thick films of PS-b-P2VP (Mn 185 kDa, PDI=1.24, 67 wt% PS, 99 nm bulk period) and PS-b-P2VMeP on ITO substrates. The height difference between the PS and P2VP domains in the PS-b-P2VP film (Figure 2A) is 2.1±0.3 nm, based on the depth analysis of the AFM image (see Experimental Section for more details), and the separation between the domains is ca. 40 nm. The AFM image of the methylated copolymer film (Figure 2B) reveals a slightly increased height contrast (3.0±0.5 nm), which is attributed to the swelling of the charged P2VMeP domains by water, and, eventually, to repulsive inter-chain interactions of the positively-charged blocks that increase the volume of the domains. The patterned block copolymer-modified surfaces were further characterized by electrochemical means. Figure 2C presents the cyclic voltammograms of the Fe(CN)63-/4- redox label in the presence of the bare ITO, the block copolymer PS-b-P2VP and the PS-b-P2VMeP modified electrodes. Whereas the bare electrode shows a quasi-reversible wave of the Fe(CN)63-/4- label (peak-to-peak separation 0.55 V), the PS-b-P2VP copolymer-coated electrode is almost entirely insulated toward electron exchange with the redox label. However, alkylating the P2VP domains in the patterned electrode regenerates the electron transfer ability of the patterned coating, leading to a redox wave of improved reversibility (peak-to-peak separation 0.19 V). Faradaic impedance spectroscopy measurements (Figure 2D) further support the controlled interfacial electron transfer properties of the chemically-modified patterned block copolymer films. The interfacial electron transfer resistance of the patterned PS-b-P2VP film is Ret=8 kΩ, indicating a high barrier for electron transfer across the film. In turn, the positively-charged block copolymer-patterned surface shows a substantially lower interfacial electron transfer resistance of ca. Ret=0.55 kΩ, consistent with

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the electrostatic attraction of the negatively-charged redox label to the positively-charged P2VMeP domains.

Figure 2. AFM topography images of: (A) the PS-b-P2VP film, and (B) the PS-b-P2VMeP film. (C) Cyclic voltammograms and (D) Faradaic impedance spectra corresponding to a bare ITO electrode (a), and ITO electrodes modified with: (b) PS-b-P2VMeP, and (c) PS-b-P2VP. Inset in D shows expanded spectra.

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The control of the electron transfer at the different domains of the polymer enabled us to selectively electropolymerize functional nanostructures on the positively-charged P2VMeP domains of PS-b-P2VMeP. First, the electrochemically-driven generation of Prussian blue (Fe

Fe CN

) on the PS-b-P2VMeP was examined. Prussian blue is a redox-active

oligomer formed by the electrochemically-driven reaction between FeCl3 and K3[Fe(CN)6].32 The presence of Prussian blue on the surface after the electrochemical deposition was confirmed by XPS analysis (Figure S1). Furthermore, microscopic characterization (Figure 3A, B and C) shows an increase in the average height contrast between the domains to 7.3±0.8 nm (according to depth analysis; Figure 3D shows a representative cross-section). It should be noted that the observed non-homogeneous growth of the Prussian blue domains might be attributed to the fact that the deposition of the compound involves two consecutive steps, namely the reversible electrostatic association of the monomers on the domains and their irreversible electrochemical precipitation. That is, once a Prussian blue site is generated, its rapid growth will result in a nonhomogeneous aggregate. Indeed, a control experiment performed on a P2VMeP homopolymermodified surfaces show the non-homogeneous electrodeposition of Prussian blue (Figure S2). Figure 3E shows the absorption spectrum of the Prussian blue deposited on the patterned PS-bP2VMeP-coated electrode, which exhibiting the distinctive broad band at 725 nm.26 Cyclic voltammetry performed on this sample (Figure 3F) reveals two redox waves characteristic to Prussian blue,33 which represent the transitions from Prussian white (K Fe Fe CN Prussian blue and to Berlin green (Fe

Fe

CN

A

Fe

Fe CN

to

), according to equation 1 (where A is

either the chloride or the hydroxide counter ion): K Fe Fe CN

)

Fe

Fe

8


CN

A

(1)

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Figure 3. (A,B) SEM image of PS-b-P2VMeP-PB-coated electrode at different magnifications. (C) AFM topography image of PS-b-P2VMeP-PB-coated electrode. (D) Height profile recorded for: (a) the PS-b-P2VMeP-PB-coated electrode, along the white line in (C), and (b) the PS-bP2VMeP-coated electrode, along a similar distance line (taken from Figure 2B). (E) Absorption spectrum of the PS-b-P2VMeP-PB-coated electrode (against PS-b-P2VMeP coated ITO as a reference). (F) Cyclic voltammogram recorded in 0.1 M KNO3 solution at 100 mV s-1 corresponding to the PS-b-P2VMeP-PB-coated electrode.

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The successful electrodeposition of Prussian blue on the positively-charged domains of PS-b-P2VMeP suggested that other negatively-charged structures could be selectively electropolymerized on the surface as well. Previous studies have demonstrated that 2-4 nm diameter gold nanoparticles (Au NPs) functionalized with thioaniline and mercaptoethane sulfonate can be electropolymerized on electrodes, leading to the formation of bis-anilinecrosslinked Au NP matrixes.29 This suggested that electropolymerization of such NPs, which would be electrostatically attracted to the positively charged domains of the block copolymer film coating the ITO surface, would lead to the patterning of the domains with bis-aniline bridged Au NPs. Figure 4A shows an AFM image of the PS-b-P2VMeP template film after 30 min incubation in the Au NPs solution, upon the application of 40 cyclic voltage scans between -0.2 and 1.1 V vs. SCE at 100 mV s-1. Following the electrodeposition process, the presence of Au NPs on the surface was confirmed by XPS analysis (Figure S3). Additionally, cyclic voltammetry showed the characteristic quasi-reversible redox wave associated with the bisaniline dimers (Figure S4).34 The pattern of the block copolymer domains guides the Au NPs to selectively adsorb on the positively-charged P2VMeP regions, where they further undergo the electropolymerization process. The height of the positively-charged P2VMeP above the PS domains increased from 3.0±0.5 nm for the bare PS-b-P2VMeP to 8.0±0.4 nm after the deposition/electropolymerization process. Repeated experiments (4 measurements) revealed that the formation of the electrodeposited Au NPs pattern is fully reproducible. Upon analyzing different domains of the Au NPs patterns on a 1 cm2 scale, we estimate that ~80% of the electropolymerized Au NPs consisted of individual Au NPs, whereas the occurrence of two NPs and three Au NP aggregates amounts to ~18% and ~2%, respectively. It should be noted that in the experiments leading to the functionalized copolymer-patterned surfaces, we have

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implemented the covalent quaternization of the pyridine units via irreversible alkylation rather than the protonation of the pyridine sites under acidic conditions. This alkylation path is justified due to the following reasons: (i) the reversible protonation of the pyridine units in aqueous media is anticipated to dissociate the protonated sites under neutral conditions, thus perturbing the positively charged- induced functionalization of the positive domains. (ii) The reversible features of the protonated pattern suggest that the protons might interact with the electropolymerizable ligands, e.g. protonation of the p-aminothiophenol ligand associated with the Au NPs, thus eliminating the possibility to electrochemically deposit the desired structure.

Figure 4. (A) AFM topography image of PS-b-P2VMeP-Au NPs-coated electrode. (B) Height profile recorded for: (a) the PS-b-P2VMeP-Au NPs-coated electrode, along the white line in (A), and (b) the PS-b-P2VMeP-coated electrode, along a similar distance line. Besides the novel electrochemically-induced deposition of nanostructures on the microphase-separated patterns of the PS-b-P2VMeP block copolymer, we also aimed at introducing surface functionality through the electrochemically-deposited nanostructures. We find that the controlled electrochemical deposition of Prussian blue on the patterned PS-bP2VMeP enables one to control the wetting properties of the surface. The PS homopolymer 11



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exhibits a highly hydrophobic interface, whereas the methylated P2VMeP homopolymer provides a highly hydrophilic surface (water contact angles =97 and =8º, respectively). The block copolymer PS-b-P2VMeP-patterned surface reveals an intermediate wettability (contact angle =75º), consistent with the contribution of both polymer blocks to the surface properties. As the negatively charged Prussian blue is hydrophilic, it cannot be used to tune the wetting properties of the hydrophilic P2VMeP homopolymer surface (Figure S5). However, as the block copolymer surface is considerably less hydrophilic in nature, its wettability could be tuned by controlling the amount of the deposited Prussian blue through the number of electrodeposition cycles. Figure 5 shows that by increasing the amount of deposited Prussian blue through repetitive deposition cyclic voltammetry cycles, a gradual decrease in the contact angle of a water droplet placed on the electrode was evident, implying that the controlled deposition of Prussian blue on the block copolymer dictates the wettability of the resulting surface. The surface wettability properties of the coating should be preserved for a time-interval of at least two weeks upon storing the surfaces under an inert nitrogen atmosphere.

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Figure 5. (A) Photographs of water droplets deposited on PS-b-P2VMeP-PB-coated ITO substrate containing increasing amounts of electrodeposited Prussian blue (0-5 electrodeposition cycles, consisting of 2 s incubation at -0.2 V vs. SCE followed by 50 ms at +1.3 V vs. SCE). (B) Dependence of the contact angles of the water droplets in (A) on the number of the electrodeposition cycles of Prussian blue. Error bars correspond to a set of 3 measurements. (C) Cyclic voltammogram recorded in 0.1 M KNO3 solution, at 100 mV s-1, corresponding to the PS-b-P2VMeP and PS-b-P2VMeP-PB-coated films with increasing amounts of Prussian blue deposited by repetitive cycles.

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CONCLUSIONS: We have demonstrated that the patterning of conductive ITO glasses with PS-b-P2VP followed by the alkylation of the pyridine groups yields positively-charged polymer domains that control the interfacial electron transfer properties at the electrode surface. The electrostatic attraction of negatively-charged electropolymerizable redox active units to the charged domains allows the domain-selective electron transfer between the associated redox active units and the electrode, thus enabling the electrochemical deposition of nanostructures on the domains. This is exemplified by the deposition of Prussian blue and negatively-charged Au NPs on the polymer patterns. Lastly, tuning the amounts of electrochemically-deposited Prussian blue on the block copolymer films yields surfaces with tunable wettability. Taking advantage of the nanoscale pattern of the electrode coating, surfaces of controlled wettability and surfaces of gradient wettability may be envisaged. Furthermore, our study paves the way for a further development of the functionalized patterned copolymer surfaces: the application of other copolymer compositions and, specifically, the use of hydrophobic/negatively charged copolymers for the electrodeposition of positively charged species could lead to surface coatings exhibiting new functionalities. Furthermore, by shortening the block copolymer chains, the width of the resulting electrodeposited patterns could be shortened as well. The effects arising from the resulting modified patterns, and especially the surface properties of the new coatings, would be an interesting path to follow.

EXPERIMENTAL SECTION: Materials: P2VP (Mn 2.3 kDa, PDI=1.69) and PS-b-P2VP (Mn 185 kDa, PDI=1.24, 67 wt% PS, 99 nm bulk period), were synthesized using standard anionic polymerization. PS (Mn

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170 kDa, PDI=2.06, 95%), iodomethane (99%), K4[Fe(CN)]6 (99%), K3[Fe(CN)]6 (99%), HEPES buffer (99.5%, 0.1M, pH=7), FeCl3 (97%), HAuCl4 (99.9%), p-aminothiophenol (97%) and 2-mercaptoethanesulfonic acid (98%) were purchased from Sigma Aldrich. Au NPs capped by 2-mercaptoethane sulfonic acid and p-aminothiophenol ligands were synthesized as previously reported.29 Sample preparation: Polymer stock solutions (0.4 wt%) were prepared in chloroform and stirred for 3 h. The solution was filtered twice through a 220 nm PTFE filter. Films were cast by spin coating at 3000 rpm for 40 s. The concentration of the polymer solution was adjusted to obtain 28 nm-thick films on a silicon wafer (pre-cleaned in a sulfuric acid-NoChromixTM bath and extensively rinsed with deionized water). The same solution was, then, used to coat the ITO substrates (cleaned in ethanol at 75ºC for 15 min). Block copolymer films were annealed in a closed petri dish under saturated chloroform vapor for 30 min at ambient temperature to obtain a striped pattern on the ITO substrate. The P2VP domains were quaternized in the presence of methyl iodide vapor for 3 h under vacuum at 75ºC in a glass tube oven (Büchi GKR-50). Films were stored in a desiccator and used within two days after the methylation treatment. Characterization of the polymer-coated electrodes: Polymer films were dipped in HEPES buffer solution containing K3[Fe(CN)6] and K4[Fe(CN)6] (each at 2 mM concentration) for 10 min prior to measurement. Cyclic voltammetry was performed at a scan rate of 100 mV s-1. Impedance measurements were taken at +0.17 V vs. KCl-saturated calomel electrode (SCE). Deposition of Prussian blue: Polymer-coated ITO electrodes were dipped in a solution containing 25 mM FeCl3 and 25 mM K3[Fe(CN)6]. Prussian blue was deposited at -0.2 V vs. SCE for 2 s followed by the application of a chronoamperometric pulse at +1.3 V vs. SCE for 50 ms. To determine the effect of Prussian blue deposition on the wetting properties of the block

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copolymer coated surface, contact angle measurements were performed following the application of variable number of potential pulses corresponds to -0.2 V for 2 s and +1.3 V for at least 50 ms. Deposition of Au NPs: The polymer films were immersed in HEPES solution (0.1 M, pH 7.2) contains 2 mg ml-1 Au NPs for 30 min. Electropolymerization of the Au NPs was then carried out by the application of 40 potential scans between −0.2 and +1.1 V vs. SCE, at a scan rate of 100 mV s-1. Instrumentation: All electrochemical experiments were carried out using an Autolab electrochemical system (ECO Chemie, The Netherlands) driven by GPES software. A KClsaturated calomel electrode (SCE) and a carbon electrode were used as the reference and counter electrodes, respectively. The morphology of the polymer films was examined using HR-SEM Sirion (FEI company) and MagellanT XHR SEM, microscope as well as a Dimension 3100 scanning probe microscope with Nanoscope V controller (Veeco, Santa-Barbara, USA). The average height between the copolymer domains was determined using the depth analysis implemented in NanoScope Analysis, version 1.40, as the difference between the locations of the two Gaussians fitted to the height histogram. X-ray photoelectron spectroscopy data were acquired using an Axis Ultra X-ray Photoelectron Spectrometer (XPS/ESCA; Kratos Analytical). Contact angle measurements were performed using a CAM 200 optical angle analyzer (KSV instruments). A 10 µL droplet of milli-Q water was placed on the surface using a syringe. Contact angle was determined by applying the Young-Laplace analysis to the droplet photograph. Absorption spectra were recorded on a Shimadzu UV-2401PC spectrophotometer.

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ASSOCIATED CONTENT Supporting Information. Additional figures showing XPS of the PS-b-P2VMeP samples with Prussian blue, Au NPs; SEM images of homopolymer P2VMeP-PB deposited films; cyclic voltammogram of Au NPs during the electropolymerization on PS-b-P2VMeP-coated ITO electrode; contact angle photographs of P2VMeP homopolymer film before and after deposition of Prussian blue. The Supporting Information is available free of charge on the ACS Publications website AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], Fax: +972-2-6585 345, Tel: +972-2-6586311 *E-mail: [email protected], Fax: +972-2-6527715, Tel: +972-2-6585272 Funding Sources This project is supported by the Israel National Nanotechnology Initiative (INNI), Focal Technology Areas Program. ACKNOWLEDGMENT CGC thanks the Israel Planning and Budgeting Committee for a postdoctoral fellowship.

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Table of Contents Graphic Nano-patterned block copolymer-modified conducting surfaces enable the guided electrodeposition of redox-active materials on the patterned domains leading to surfaces with tunable wettability

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Block Copolymer Patterns as Templates for the Electrocatalyzed

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