Hybrid Sol–Gel-Derived Films That Spontaneously Form Complex

Sep 8, 2016 - Film morphology, topography, surface charge, and chemical properties ... 1000 in intermittent contact mode with an Au-coated Si probe (k...
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Hybrid Sol−Gel-Derived Films That Spontaneously Form Complex Surface Topographies Joel F. Destino,† Zachary R. Jones,‡ Caitlyn M. Gatley,† Yi Zhang,† Andrew K. Craft,† Michael R. Detty,† and Frank V. Bright*,† †

Department of Chemistry, Natural Sciences Complex, SUNY-Buffalo, Buffalo, New York 14260-3000, United States Department of Chemistry, Ithaca College, Ithaca, New York 14850, United States

‡

S Supporting Information *

ABSTRACT: Surface patterns over multiple length scales are known to influence various biological processes. Here we report the synthesis and characterization of new, two-component xerogel thin films derived from carboxyethylsilanetriol (COE) and tetraethoxysilane (TEOS). Atomic force microscopy (AFM) reveals films surface with branched and hyper branched architectures that are ∼2 to 30 μm in diameter, that extend ∼3 to 1300 nm above the film base plane with surface densities that range from 2 to 77% surface area coverage. Colocalized AFM and Raman spectroscopy show that these branched structures are COE-rich domains, which are slightly stiffer (as shown from phase AFM imaging) and exhibit lower capacitive force in comparison with film base plane. Raman mapping reveals there are also discrete domains (≤300 nm in diameter) that are rich in COE dimers and densified TEOS, which do not appear to correspond with any surface structure seen by AFM.

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chemically tailored surface.14,19,20 However, to date, the range of topography control reported in the literature has been limited.21,22 Furthermore, it is well understood23−25 that different fouling organisms’ opposing settlement and release preferences have made it difficult to design a universal AF surface. Given this, we seek to diversify our toolbox and design hybrid xerogel-derived materials with self-forming micro/ nanotopograhy to create platforms that might simultaneously leverage all of the benefits of physical, chemical, and topographical approaches. We report the fabrication and characterization of hybrid ORMOSIL-based xerogel thin films prepared from carboxyethylsilanetriol (COE) and tetraethoxysilane (TEOS). COE was selected because it contains a −COOH residue to provide a variety of surface chemistries (e.g., free −COOH, −COO−, −COOH dimers, −COO− salts, −COOH hydrogen bonded to Si−OH residues). In addition to unique chemistries, the COE/ TEOS hybrids yield materials with topography over a range of length scales, a key AF characteristic. Film morphology, topography, surface charge, and chemical properties were assessed by using colocalized atomic force microscopy (AFM), scanning Kelvin probe microscopy (SKPM), and confocal Raman mapping.26 The relevant precursor chemical structures are presented in Scheme 1.

INTRODUCTION It is well known that micro- and nanotopography can significantly impact organism settlement1−6 as well as a variety of other biological processes including: cell adhesion, function and growth,7,8 and protein adsorption.9−11 Over the past decade, researchers have sought to design materials that provide a range of micro/nanotopographies, with strategies typically requiring engineered surface patterns.12 Although these methods allow a degree of control and precision superior to existing synthetic methods, they typically require the use of photolithography and clean room technology. For applications such as marine antifouling (AF) coatings, the practicality of engineered platforms on large aquatic structures (e.g., ships) is clearly limited. As a result, a coating that can spontaneously form micro/nanotopographic features without the need for any prepatterning is highly desirable. Research from our laboratories has focused on developing organically modified silane (ORMOSIL)-based, sol−gel derived hybrid xerogel coatings that exhibit AF, and fouling-release (FR) characteristics.13−15 ORMOSIL-based xerogel films are of particular interest because they are inexpensive, readily prepared by sol−gel processing, and are intrinsically crosslinked and the films can be covalently bonded to naturally stable metal oxide surfaces.16,17 In addition, xerogel chemistry and topography can be adjusted by controlling substitution at the −R′ group on the (RO)3Si−R′ precursor(s) and the precursor molar ratios,18 allowing one to synthesize materials that combine the benefits of a physically engineered and a © 2016 American Chemical Society

Received: July 18, 2016 Revised: September 7, 2016 Published: September 8, 2016 10113

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Raman Spectroscopy and Raman Mapping. Single-point Raman spectra and Raman map data were recorded by using a Horiba JY LabRam HR confocal Raman microscope interfaced to the AIST-NT SmartSPM 1000. A 532.06 nm laser (Laser Quantum) was used as an excitation source with a 100× LWD objective (NA = 0.70) (Mitutoyo), 500 μm diameter confocal hole, 300 grooves/mm grating, and a one electrode thermo-electrically cooled CCD detector (Andor Technology). The voxel depth for these experiments is ∼600 nm.28 Raman maps were acquired by using 3.5 mW of laser power with a 60 s integration time at each pixel and 250 nm × 250 nm pixel spacing. Repetitive AFM, SKPM, and Raman mapping experiments were reproducible (RSD < 2%), and there was no evidence of any hysteresis during multiple scans over the same areas. Data Analysis and Statistics. Multiple data sets were acquired for each sample, and typical results are presented. Statistical significance was assessed by using one way ANOVA at the 95% confidence level with pairwise comparison (Holm−Sidak test) (p < 0.05 being significant). Classical least-squares (CLS)29 (LabSpec 6.1, JY Horiba) was used to assess the position-dependent Raman spectra and create species maps. Toward this end, all position-dependent Raman spectra were first baseline-corrected and treated to suppress cosmic events by nearest neighbor averaging. The pure component Raman spectra were then used as the primary CLS inputs. Raman K-matrix coefficient maps were subsequently prepared from the resulting position-dependent normalized coefficient “K-matrix” data.

Scheme 1. ORMOSIL Precursor Chemical Structures

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EXPERIMENTAL SECTION

Chemical, Reagents, and Materials. The following were used: carboxyethylsilanetriol disodium salt (25% in water) (COE) and tetraethoxysilane (TEOS) (>98%) (Gelest); isopropylalcohol (IPA) (Sigma); HCl (ACS grade) (J.T. Baker); EtOH (200 proof) (Decon Laboratories); and Al-coated (50 nm) glass slides (EMF Corp.). Deionized water (DI H2O) (>18 MΩ cm) was prepared by using an Ameri-Water purification system (Metro Group). Sample Preparation. Individual sols of each precursor were prepared at room temperature. A TEOS sol was prepared by adding EtOH (15.9 mL, 14 equiv to TEOS) and TEOS (4.35 mL, 19.5 mmol) into a vial while stirring. Slowly a solution of DI H2O (3.69 mL, 10.5 equiv to TEOS) and HCl (12.0 M, 82.0 μL, 0.05 equiv to TEOS) was added to the vial that was then capped and stirred for 1 h. A COE sol was prepared by weighing COE (0.39 g, 0.50 mmol) in a vial, followed by the addition of EtOH (1.22 mL, 42 equiv to COE). While stirring, a solution of DI H2O (45.0 μL, 5 equiv to COE) and HCl (12.0 M, 8.50 μL, 2.05 equiv to COE) was added to the vial dropwise and stirred for 1 h. Upon completion, NaCl crystals that had precipitated out of solution were removed by filtration and the COE sol was mixed in appropriate mole ratios with the TEOS sol, sealed, and left to stir overnight under ambient conditions before coating. The as-received, Al-coated slides were initially oxidized in DI H2O for 24 h prior to xerogel film deposition on the freshly formed Al2O3 surface. Xerogel films were formed by spin coating (Specialty Coating Systems, model P6700) 130 μL of the two-component sol onto a 25 mm × 25 mm slide. Film deposition was carried out at 100 rpm for 10 s for delivery and at 3000 rpm for 30 s to coat. Instrumentation and Characterization Protocols. Scanning Electron Microscopy. Scanning electron micrographs (SEMs) were recorded by using a Hitachi model SU-70 field emission-SEM with a zirconium oxide/tungsten Schottky electron emission source, three state electromagnetic lens system, octapole electromagnetic type stigmator coil, 2-stage electromagnetic deflection type scanning coil, Everhart Thornly secondary electron detectors, and SEM Data Manager software 1.0. AFM and SKPM. AFM (height and phase imaging) and SKPM (contact potential difference (CPD) and capacitive force (CF)) measurements were recorded simultaneously by using an AIST-NT model SmartSPM 1000 in intermittent contact mode with an Aucoated Si probe (k = 5.3 N/m, 130 μm length) (fpN11Au, AIST-NT) operating at a 154 kHz resonant frequency, with a free amplitude of ∼24 nm. Images were recorded by using a 50 nm × 50 nm pixel spacing at 0.5 Hz. Frequency-modulated (FM) SKPM was performed with a 3.0 V applied bias and a 10 nm lift providing CPD and capacitive force CF signals.27

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RESULTS AND DISCUSSION Film Surface Morphology. Figure 1 presents typical, composition-dependent AFM images (50 × 50 μm2 area) for COE/TEOS xerogel films formed on Al2O3-coated glass. The COE mole fraction (2.5 to 20 mol percent) is shown in the blue boxes in the upper left corner of each panel. The calculated root-mean-square (RMS) roughness values are provided in the green boxes in the upper right corner of each panel and the relative surface densities, in the form of percent area, are given in the lower right corner of each panel. (For reference, AFM images for pure TEOS (100 mol %) and pure COE (100 mol %) containing xerogel thin films are provided in the Supporting Information, Figure S1.) For the 2.5:97.5 COE/ TEOS film, the surface morphology is similar to pure TEOS (results not shown) with no obvious topographical features, and no significant surface density is detectable at this length scale. For the 5:95 COE/TEOS film, the film surface exhibits dendritically shaped features with heights (h) of >1 μm at the distal ends of the branches and diameters (d) of ∼5 μm. These features are also sparse and only represent 2% of the film surface area, resulting in a 79 nm RMS. In the 7.5:92.5 COE/ TEOS film, the AFM images show features that exhibit lower heights (h < 100 nm) in comparison with the 5:95 COE/TEOS film (p < 0.001) with d of ∼3−7 μm. These features are more dense in comparison with the 5:95 COE/TEOS film (p
20 μm) (p < 0.001), and Z (h ≈ 130 nm), and these structures are very dense (77% film surface coverage) (p < 0.001). The dendritically shaped features are now hyperbranched, clearly exhibiting three generations of branching, with a 16 nm RMS. As a series, the evolution of asymmetric, branched, dendritic, or fractal structures is observed for films composed of ≥5 mol % COE. Asymmetric fractal and dendritic growth patterns are commonly observed in nonequilibrium, diffusion-limited crystal growth30−33 and in nanoparticle aggregation/self-assembly.34,35 Xerogels are materials that form from colloidal particle aggregations with fractal dimensions.36−38 Tian et al. used a surfactant-stabilized colloidal method with 1,2-bis(triethoxysilyl)ethane as a silane precursor and cetyltrimethylammonium chloride (CTAC)/cetyltrimethylammonium bromide (CTAB) surfactants to produce silicate-based materials that spontaneously self-assemble to form dendritically shaped features on a micrometer scale.39 These authors noted several key points that provide valuable insight into the morphological evolution seen in Figure 1. First, they observed distinct “dendritically” shaped structures that exhibited a highly ordered 4-fold symmetry with d of ∼10−80 μm, increasing in size with percent CTAC. This observation closely parallels the general trend shown for COE/TEOS in Figure 1, with few micrometer features seen in the 5:95 COE/TEOS films evolving to much larger (d > 20 μm) features in the 20:80 COE/TEOS film. The major difference between our materials is that our features appear to form a less ordered morphology by comparison. Second, Tian et al. found that the largest dendritically shaped features exhibited the most complex fractal dimensions, and intermediate CTAB levels lead to reduced complexity to the point where simple “crosses” were formed at the lowest CTAB levels. This behavior closely parallels the observation for COE/ TEOS, which shows simple three and four-armed features in 5:95 COE/TEOS film and more complex branching in the 20:80 COE/TEOS film. Finally, Tian et al. concluded that chloride ion played a crucial role in dendritic assembly. In our system the observed feature formation is likely influenced by residual NaCl within the sol that is derived from the COE disodium salt precursor acidification. Colocalized COE/TEOS Xerogel Film Analysis. Colocalized AFM/Raman characterization experiments focused on a 7.5:92.5 COE/TEOS xerogel film. This particular formulation was selected because the dendritically shaped feature dimensions were amenable to AFM/Raman. Figure 2 presents a typical cross-sectional SEM image for a 7.5:92.5 COE/TEOS xerogel film formed on Al2O3/Al-coated glass. The film exhibits a 600−700 nm thick base that is consistent with other xerogel thin films we have produced by spin coating19,21,40 with micron-sized dendritically shaped features and other features in the 100 nm range. The dendritically shaped feature is ∼100 nm above the film base. Figure 3 presents typical scanning probe height, phase, CPD, and CF images for an individual dendritically shaped feature recorded from a 7.5:92.5 COE/TEOS xerogel film formed on

Figure 2. Typical SEM cross-sectional image of a 7.5:92.5 COE/ TEOS xerogel film formed on Al2O3/Al-coated glass.

Al2O3/Al-coated glass. Inspection of these results shows several interesting points. In the center of each image is a typical dendritically shaped structure. In this case the feature has six (6) ∼1 to 2 μm long “main” branches extending from the center with additional rounded branches extending from the mains. The AFM height image shows that maximum feature height is 98 nm above the film base. In addition to the central structure, various smaller features (h ≈ 2−10 nm, d ≈ 100−200 nm) can be seen atop the xerogel film base. The phase image provides insight into viscoelasticity properties across the film surface.41,42 In this image, areas with large phase shifts represent regions that are qualitatively less rigid in comparison with those of lower phase shifts. The phase image shows that the main dendritically shaped feature is measurably more rigid (φ = 115°) in comparison with the xerogel base layer surrounding the feature (φ = 110°) (p < 0.001). On the contrary, the smaller features provide conflicting insight with some more rigid and others less rigid in comparison with the xerogel base layer. This ambiguity may be the result of a technical aberration or under-sampling.43,44 In the CPD image, surface potential is assessed.27,27,45 Fluctuations in CPD can be related to surfacecharge heterogeneity.46 The dendritically shaped feature perimeter and the smaller features exhibit ∼200−300 mV higher CPD (p < 0.001) in comparison with the film base. Other researchers have shown that high CPD indicates the presence of charged surface groups.47 Last, the CF image provides insight into relative differences in capacitance across a sample.48 In strong contrast, it is shown that the dendritically shaped feature and smaller features exhibit a 4-fold reduction in CF (p < 0.001) in comparison with the film base. This suggests the dendritically shaped feature is a different material possessing different capacitive properties in comparison with the film base. To investigate these dendritically shaped feature chemistries in more detail, we performed colocalized Raman experiments. Toward this end, we recorded individual Raman spectra in a pixel-by-pixel manner at 250 nm increments across the exact same area assessed by AFM (Figure 3). We then used the pure component Raman spectra (Figure 4) in concert with a CLS framework to create K-matrix coefficient maps for COE, TEOS, and a residual component. (A complete list of Raman band assignments is provided in the Supporting Information in Table S1.) CLS multivariate analysis with Raman mapping has been 10115

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Figure 3. Typical AFM and SKPM images for an individual dendritically shaped surface feature from a 7.5:92.5 COE/TEOS film formed on Al2O3coated glass slides.

COE map shows that COE is largely localized within the dendritically shaped feature, providing up to a 67% spectral contribution. There are, however, substantially smaller areas, within the film base, away from the dendritically shaped feature, that show COE contributions, too. This is consistent with small COE-derived features dispersed throughout the film. These features may be subsurface because the Raman voxel probes below the film (see Experimental Section and a previous discussion26,28) as these regions do not appear to correspond universally to surface structures detected by our scanning probe experiments (cf., Figure 3). The residual map shows that CLS using only the pure COE and TEOS spectra can describe this film to within, on average, 1.5%. However, outside the dendritically shaped feature area, where the fit using only COE and TEOS is poorer, the residuals are statistically greater in comparison with the noise (p < 0.001). Furthermore, there are ≤300 nm features outside the dendritically shaped feature with especially high residual values dispersed across the film base. The areas associated with higher residuals were explored in more detail. To establish a chemical identity for these high residual features, we explored the Raman difference spectrum. Here the Raman difference spectrum (SDiff) is given by

Figure 4. Typical pure component Raman spectra for xerogel films derived from TEOS and COE formed on Al2O3-coated glass slides.

reported previously to generate component maps in a range of hybrid matrices49−51 including hybrid xerogel thin films.26 Figure 5 presents typical Raman K-matrix coefficient maps for TEOS, COE, and a residual component. The TEOS map shows that it is in the greatest abundance in the film base region in the area surrounding the dendritically shaped feature, ranging from 80 to 98%. TEOS also contributes within the dendritically shaped feature but to a lesser extent in comparison with the levels seen in the base region (p < 0.001); in some cases the contribution is only 1/3 the level seen in the base region. The

SDiff = SRaw − SCLS

(1)

where SRaw is the raw Raman spectrum and SCLS is the Raman spectrum produced by using only COE and TEOS pure components. Figure 6 presents the average Raman difference

Figure 5. Typical COE, TEOS, and residual component K-matrix coefficient maps for a dendritically shaped feature within a 7.5:92.5 COE/TEOS xerogel film formed on Al2O3-coated glass. Images are scaled from 0 (purple) to the maximum value (red). 10116

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are 3−7 μm in diameter and ≤100 nm high. These features are stiff and COE-rich in comparison with the film base areas. These COE-rich features exhibit a lower CF in comparison with the average film base. Smaller, diffuse features (h ≤ 10 nm) appear within the base in strong contrast in CF and CPD signals but were unidentifiable by Raman due to their small size (d ≈ 100−200 nm). Additionally, Raman mapping and CLS show there are microenvironments throughout the film surface that correspond to fully cross-linked TEOS in the presence of COE in the carboxylic acid dimer form. We were unable to correlate the physical location of these species with topographical features. Additional analysis studying the formation of the interactions of COE/TEOS and the chemistry of features which are smaller than diffraction-limited spatial resolution are currently underway.

Figure 6. Average Raman difference spectrum 7.5:92.5 COE/TEOS films formed on Al2O3-coated glass.

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CONCLUSIONS COE/TEOS materials spontaneously form thin films that exhibit compositionally dependent dendritically shaped features. Colocalized Raman, AFM, and SKPM analysis of a 7.5:92.5 COE/TEOS films shows that COE/TEOS materials are topographically and chemically complex with heterogeneity varying on nanometer and micrometer length scales in 2D and 3D. Additional research to tune the surface structures (RMS and feature density, in particular) and incorporate different surface chemistries is underway. Further analysis on shorter length scales is also being explored to better understand the relationship between surface chemistry, which drives the formation of these 3D dendritic structures.

spectrum for those film areas with high residuals (top 2.5% to ensure they are well above the noise at >99% confidence). For ease of viewing, the 99.7% confidence level is highlighted by the shaded blue box, and a zero point baseline is drawn in as a dashed line. Raman bands that are positive going, extending above the range of uncertainty, reflect species that are enriched. In contrast, Raman bands that are negative going, extending below the range of uncertainty, reflect species that are depleted. Inspection of Figure 6 with the band assignments from Table S1 (Supporting Information) is very revealing. To simplify this discussion those Raman bands labeled with a red asterisk are derived uniquely from some form of TEOS species. In contrast, those Raman bands labeled with a green asterisk are derived uniquely from some form of COE species. The Raman bands labeled “Ambiguous” cannot be assigned uniquely to TEOS or COE due to spectral overlap. The first revelation is that the high residual areas within the Raman map are, in comparison with the TEOS control, depleted in alkoxide (Si-OEt) and silanol (SiOH), but enriched in fully cross-linked siloxane (SiO4). The second revelation is that the high residual areas within the Raman map are also, in comparison with the COE control, enriched in carboxylic acid dimer ((COOH)2). Figure 7 presents a model that summarizes the 7.5:92.5 COE/TEOS xerogel film AFM, SKPM, and Raman results. In brief, the film is characterized by a 600−700 nm thick base. The film surface is decorated with dendritically shaped features that

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b02664. AFM height images for pure COE and pure COE films formed on Al2O3-coated glass slides and a complete listing of all Raman spectroscopic band assignments. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: 716-645-4180. E-mail: chefvb@buffalo.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is based in part on work supported by the National Science Foundation under grant numbers CHE0848171, CHE-1126301, and CHE-1411435, the Office of Naval Research through awards N0014-09-1-0217 and N001413-1-0430, and the New York State Pollution Prevention Institute. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of these funding agencies.

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ABBREVIATIONS AF, antifouling; AFM, atomic force microscopy; CCD, chargecoupled device; CF, capacitive force; CLS, classical leastsquares; COE, carboxyethylsilanetriol; CPD, contact potential

Figure 7. Model illustrating the distribution of COE and TEOS species within hybrid xerogel films formed on Al2O3-coated glass. 10117

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difference; CTAB, cetyltrimethylammonium bromide; CTAC, cetyltrimethylammonium chloride; deionized water, DI H2O; EtOH, ethanol; IPA, isopropylalcohol; FM, frequency modulated; FR, fouling release; LWD, long working distance; NA, numerical aperture; ORMOSIL, organically modified silane; RMS, root-mean-square; RSD, relative standard deviation; SE, standard error; SEM, scanning electron microscopy; SKPM, scanning Kelvin probe microscopy; TEOS, tetraethoxysilane

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DOI: 10.1021/acs.langmuir.6b02664 Langmuir 2016, 32, 10113−10119