Article pubs.acs.org/ac
Nanoscale Analysis of a Functionalized Polythiophene Surface by Adhesion Mapping Jae-Eun Lee,† Ju-Won Kwak,† Joon Won Park,*,† Shyh-Chyang Luo,‡,∥ Bo Zhu,‡ and Hsiao-hua Yu*,‡,§ †
Department of Chemistry, Pohang University of Science and Technology, San 31 Hyoja-dong, Pohang 790-784, Korea Responsive Organic Materials Laboratory, RIKEN 2-1 Hirosawa, Wako, Saitama 351-0198, Japan § Institute of Chemistry, Academia Sinica, 128 Academic Road Sec. 2, Nankang, Taipei 115, Taiwan ‡
S Supporting Information *
ABSTRACT: Functionalized ethylenedioxythiophene (EDOT) monomers, hydroxymethyl EDOT (EDOT-OH), and zwitterionic phosphorylcholine EDOT (EDOT-PC) were electropolymerized to prepare the homopolymers poly(EDOT-OH) and poly(EDOT-PC), and mixtures of these monomers were used to produce the copolymer poly(EDOTOH)-co-poly(EDOT-PC). Force−extension-curve-based atomic force microscopy (AFM) was utilized to analyze the surfaces of the films. The PEDOT-OH film yielded force− extension curves for short stretching, and the PEDOT-PC film yielded curves for long stretching. A dendron-modified AFM tip with anthracene groups tethered at the end resulted in adhesion maps with the highest contrast. The analytical data for the copolymer films correlated with the corresponding monomer composition, and the maps revealed that the average size for the copolymer nanodomains ranged from 10−14 nm. This approach can be applied to studies aimed at understanding the surface structure of other relevant polymers and copolymers at the nanoscale level.
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aqueous solutions. To satisfy the environmental requirements for creating an appropriate interface between the material and a biological system, functionalized PEDOT films are electropolymerized using an aqueous microemulsion to yield ultrasmooth films with tunable functionality, controlled nanomorphology, and outstanding biocompatibility.8−12 Through this process, the density of the side-chain functional groups can be controlled to modulate the chemical and physical properties of these polymers. However, only limited information is available regarding the nanoscale orientation and structure of these films. A full understanding of the nanodomain distribution throughout the electropolymerization process, which is governed by both thermodynamic and kinetic factors, remains a challenging, yet highly desirable, goal. In recent years, AFM has been used to study the stretching of a polymer chain and to distinguish and characterize polymeric surfaces. The desorption of polymer chains from solid substrates,13,14 the elasticity of a single chain under different solvent conditions,15 the impact of surface hydrophobicity on polymer adhesion,16 and the selection and stretching of a single polymer chain17 have been reported. A force map obtained via chemical force microscopy18 and the mapping of adhesion forces19 allowed for the observation of the lateral distribution of functional groups in a polymer film at various pH levels,20 the
he invention of atomic force microscopy (AFM) made possible the characterization of various surface properties of materials at the nanoscale level.1,2 In addition to monitoring the surface morphology of polymeric materials, it is now feasible to study the spatial arrangements of different domains on these materials when the domains present different physicomechanical properties. In particular, the functionalization of the tips used in AFM with certain molecules provides specific physicochemical interactions that turn these tips into adhesion probes, which allows for a greater contrast in the morphological and spatial characteristics of the surface. A new method for characterizing the surfaces of these materials is thus created.3 Among the polymeric materials, copolymers are of particular interest because they spontaneously assemble into nanoscale structures through a thermodynamic equilibrium. However, this characteristic may not be easily detected with conventional tools, including AFM. A close analysis of the surface with properly coated AFM probes is expected to reveal structures that have not been previously observed. In this paper, we report promising findings regarding the surface analysis of functionalized poly(3,4-ethylenedioxythiophene) (PEDOT) films via force−extension-curve-based AFM. The adhesion mapping results revealed the nanostructures of these polymers and copolymers. PEDOT films are one of the most promising conductive organic materials available and are currently used in various electronic applications.4−7 Recently, these applications have been extended to bioelectronics under ambient conditions in © 2014 American Chemical Society
Received: January 13, 2014 Accepted: June 17, 2014 Published: June 17, 2014 6865
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Scheme 1. Mapping of the Functionalized PEDOT Surface with an AFM Tip Functionalized with Anthracene Moietiesa
a The inset depicts the structures of the third-generation dendron and the second-generation dendron employed for the surface modification. The gray spheres on the chains represent the side-chain functional groups of the functionalized polymer.
phase-separated polymer blend surface,21 the microdomains of a block copolymer film,22 and the aging effect and spatial distribution of additives on the polymer surface.23,24 Recently, the height image and the adhesion map were combined to characterize a conductive polymer surface and to observe changes after the surface was electrochemically charged.25 This approach would prove more useful if it could provide information that is inaccessible with other analytical tools, such as nanodomain formation. Moreover, the contrast in the maps can be enhanced if appropriate functional groups are introduced onto the probe tips, utilizing stronger, specific characteristic interactions between the tip and the polymer chain rather than relying on the hydrophobicity/hydrophilicity of the tips.
modification steps or used without further treatment for the PEDOT homopolymer mapping. For silylation, the probes were placed in 20 mL of anhydrous toluene containing 0.20 mL TPU under a nitrogen atmosphere for 4 h. Subsequently, the probes were briefly washed with toluene, baked at 110 °C for 30 min, and thoroughly rinsed sequentially with toluene and methanol. Finally, the probes were washed with methanol and dried under vacuum. To introduce the anthracene moiety, the probes prepared as above were placed in a methylene chloride solution with a small amount of dimethylformamide (DMF) containing 1.0 mM of the third-generation dendron (i.e., 27acid) or the second-generation dendron (i.e., 9-acid) with a 9anthrylmethoxycarbonyl group at its apex and the coupling agent, 1,3-dicyclohexylcarbodiimide (DCC) (27 mM for the 27-acid or 9 mM for the 9-acid), in the presence of 4dimethylaminopyridine (DMAP) (0.90 mM) for 12 h. After the ester-forming reaction, the probes were rinsed thoroughly with methylene chloride, methanol, and deionized water in a sequential manner and finally washed with methanol and dried under vacuum. Through the esterification, the dendron with multiple carboxylic groups at its periphery was covalently linked to the silylated surface. Therefore, the anthracene moiety was placed at the apex of the self-assembled dendron (Scheme S-1d,f, Supporting Information). For generating amine groups at the apex of the dendron, the dendron-modified probes were immersed into a methylene chloride solution dissolving trifluoroacetic acid (TFA) (1.0 M), and the solution was stirred for 1 h at room temperature. After the reaction, they were soaked in a methylene chloride solution dissolving diisopropylethylamine (DIPEA) (20% (v/v)) for 10 min. The probes were rinsed thoroughly with methylene chloride and methanol in a sequential manner and dried under vacuum. After removing the protecting group, the amine moiety was placed at the apex of the dendron (Scheme S-1e, Supporting Information). For the other silylations, the probes were reacted with a 0.10% (v/v) APDES solution or a 0.10% (v/v) n-BTMS solution in toluene for 4 h. These probes were then treated as
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MATERIALS AND METHODS Materials. The silane coupling agents, N-(3-(triethoxysilyl)propyl)-O-poly(ethylene oxide) urethane (TPU), (3aminopropyl)diethoxymethylsilane (APDES), and n-butyltrimethoxysilane (n-BTMS), were purchased from Gelest, Inc. and were stored under nitrogen. All the other chemicals and solvents were reagent grade and were purchased from SigmaAldrich unless otherwise noted. All the solvents used to wash the probes were HPLC grade and were obtained from Mallinckrodt Baker. Ultrapure deionized (DI) water (18.2 MΩ·cm) was obtained using a Milli-Q purification system (Millipore). The silicon nitride probes (PEN-0012-03; k = ∼16 pN/nm) were purchased from NanoInk, Inc. AFM Probe Modification. The dendron-modified tip was prepared as previously reported26 and used without the deprotection step to maintain the anthracene moiety at the top of the tip (Scheme S-1a, Supporting Information). The silicon nitride probes were first oxidized by dipping the probes in an 80% nitric acid solution and heating at 80 °C for 20 min. The probes were then washed and thoroughly rinsed with deionized water. The clean probes were dried in a vacuum chamber for 20 min and used immediately for the next 6866
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Figure 1. Representative force−extension curves categorized into five types.
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RESULTS AND DISCUSSION To analyze the surfaces of PEDOT homopolymer and copolymer films, force−extension curves were recorded for a 60 nm × 60 nm area at a resolution of 3.0 nm with 10 curves obtained at each pixel (4000 measurements per area). Functionalized ethylenedioxythiophene (EDOT) monomers, hydroxymethyl EDOT (EDOT-OH), and zwitterionic phosphorylcholine EDOT (EDOT-PC) were electropolymerized to prepare the homopolymers, poly(EDOT-OH) and poly(EDOT-PC), and the copolymer, poly(EDOT-OH)-co-poly(EDOT-PC) (Scheme 1). To understand the adhesion behavior between the tip and the polymeric films, the following six tips were prepared for the mapping study (Scheme S-1, Supporting Information): (a) bare tip (static water contact angle = 5−7°); (b) (3-aminopropyl)diethoxymethylsilane (APDES)-modified tip (60−62°); (c) n-butyltrimethoxysilane (n-BTMS)-modified tip (65−70°); and (d−f) dendronmodified tips (27-acid anthracene tip (53−56°), 27-acid amine tip (51−53°), and 9-acid anthracene tip, respectively). To measure the contact angles, silicon wafers were modified in the same way as the AFM tips: (1) hydroxyl groups should be abundant on the clean oxidized bare tip; (2) primary amine groups were attached to the end of the APDES-modified tip; (3) short alkyl groups were present on the surface of the nBTMS-modified tip; (4) anthracene groups were tethered at the top of the 27-acid anthracene tip (low density); (5) amine groups were tethered at the top of the 27-amine tip (low density); and (6) anthracene groups were tethered at the top of the 9-acid anthracene tip (at a higher density than that of the 27-acid anthracene tip). For the purpose of observing the
described above, without the dendron immobilization step. The use of APDES or n-BTMS generated a molecular layer with a terminal amine or alkyl group on the surface (Scheme S-1b,c, Supporting Information). Electropolymerization of the Functionalized PEDOT Films. A previously reported method for the electropolymerization of a PEDOT homopolymer and copolymer was used.8 Briefly, the functionalized PEDOT films were prepared via potentiostatic methods on ITO electrodes by electropolymerization from aqueous solutions of 0.01 M functionalized EDOT monomer and 0.1 M LiClO4 dissolved in the presence of 0.05 M SDS (1.2 V vs Ag/AgCl for 5 s). AFM Mapping Experiment. The force−extension curves were recorded with a ForceRobot 300 automated force spectroscope (JPK Instruments AG, Germany) and a NanoWizard I AFM (JPK Instruments AG, Germany). The spring constant of each AFM probe was calibrated in solution using the thermal fluctuation method, which yielded values ranging from 15−25 pN/nm. The images were obtained by recording force−extension curves during the raster scanning of 60 nm × 60 nm areas, and the scan area was divided into 20 × 20 (400) pixels. To determine the statistics and stochastic behavior, 10 curves were recorded at each pixel with a retraction velocity of 540 nm/s. The AFM tip was programmed to contact the surface with a force of 350 pN and move a vertical distance of 450 nm. The measurements were performed in a PBS buffer solution (10 mM phosphate buffer, 2.7 mM KCl, 137 mM NaCl, pH 7.4) at room temperature. The collected force− extension curves were analyzed using the JPK data processing software. 6867
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Figure 2. (a) and (c) Schematic drawings of the short and the long stretching upon retraction of the anthracene tip. The pink and green dots represent hydroxyl groups and PC groups, respectively. (b) Ratios of pink pixels and green pixels from the three sets of five maps including the images below. Each set of maps is obtained with the same anthracene tip. The value on the x-axis is the percentage of the feed composition of the EDOT-PC monomer. Representative adhesion maps of (d) PEDOT-OH, (e−g) poly(EDOT-OH)-co-poly(EDOT-PC) with 25%, 50%, and 75% EDOT-PC, and (h) PEDOT-PC.
group III (type d), and group IV (type e). Group II consists of curves featuring the characteristics of both groups I and III. The characteristic shapes of the force−extension curves were analyzed to obtain information regarding the interchain and chain-to-substrate interactions.28−31 Because most curves with a large linear peak were measured in air for both polymers (Figure S-1, Supporting Information), the buffer solution certainly plays a role in the aforementioned phenomena. With the dendron-modified (or anthracene) tip, the force−extension curves for short stretching (group I) were observed mainly for PEDOT-OH, while the curves for long stretching (group III) were largely recorded for PEDOT-PC. The PEDOT-OH resists the long stretching whereas the polymer chain certainly binds the anthracene on the AFM tips (Figure 2a). Frequent occurrence of group I supports the binding between them. In contrast, the weaker interchain adhesion of the PEDOT-PC is believed to allow the extended stretching and/or the water and ions stabilize the stretched state of the polymer chain that contains zwitterions tethered to the side-chain moieties (Figure 2c). In addition to the interaction between the single anthracene moiety and the single polymer chain, multiple interactions are expected to occur within this measurement, and the interaction mode can vary during measurements at the same pixel. Therefore, in some cases, curves of different groups coexist within a pixel, with one group typically prevailing. In particular, when examining various positions of the copolymer, an AFM tip could be positioned at a boundary area so that the anthracene groups simultaneously contacted the PEDOT-OH and PEDOT-PC (vide infra). The color of the prevailing group was used for the maps. For example, pink was used for the pixel when six curves belonged to group I (type a or b), one was group II, and three were group IV. In this way, each pixel was colored according to the prevailing grouppink for group I, gray for group II, green for group III, and black for group IV. An examination using other tips provided similar maps, but the contrast decreased. As indicated in Table 1, the ratios between the pink pixels and the green pixels for PEDOT-OH and
characteristic interaction between the tip and the conjugated backbone of PEDOT, we employed the commercially available dendron molecules because anthracene, a common polycyclic aromatic hydrocarbon, presents at the apex and their selfassembly on the surface is well-characterized. Facile chemical modification of the surface-bound dendrons is advantageous. For example, the deprotection step can provide the primary amine group, and other modifications are also possible. As reported previously, surface modification with the dendrons generated an absorption peak at 257 nm, which indicated the presence of the anthracene groups on the surface. Ellipsometry showed an increase in the thickness of the molecular layer of 1.1 nm upon the self-assembly of the dendron on the surface. In addition, a gold nanoparticle was conjugated at each apex of the immobilized dendron following deprotection, and high resolution scanning electron microscopy (SEM) showed the expected lateral spacing between the particles.27 To prepare the dendron-modified tips, multiple carboxylic groups at the dendron periphery were covalently conjugated on top of the silane layer with a terminal hydroxyl group through esterification. Although the backbone of the dendron comprises an ethereal linkage and an amide linkage, an anthracene group is present at each apex of the dendron (Scheme 1). A deprotection step generated a primary amine group at the apex of the dendron for comparison study. Stretching Behavior of PEDOT Homopolymer Films. An examination revealed force−extension curves of various shapes that could be categorized into five types: (a) curves with a linear profile(s) (more than one peak), (b) curves displaying a nonlinear and short stretching peak (only one peak; distance from the contact point to the last peak position
