Optimized Model Surfaces for Advanced Atomic Force Microscopy

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Optimized Model Surfaces for Advanced Atomic Force Microscopy Studies of Surface Nanobubbles Bo Song,†,‡ Yi Zhou,†,‡ and Holger Schönherr*,† †

Physical Chemistry I, Department of Chemistry and Biology, and Research Center of Micro and Nanochemistry and Engineering (Cμ), University of Siegen, Adolf-Reichwein-Strasse 2, 57076 Siegen, Germany ‡ Suzhou Key Laboratory of Macromolecular Design and Precision Synthesis, Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China S Supporting Information *

ABSTRACT: The formation of self-assembled monolayers (SAMs) of binary mixtures of 16-mercaptohexadecanoic acid (MHDA) and 1octadecanethiol (ODT) on ultraflat template-stripped gold (TSG) surfaces was systematically investigated to clarify the assembly behavior, composition, and degree of possible phase segregation in light of atomic force microscopy (AFM) studies of surface nanobubbles on these substrates. The data for SAMs on TSG were compared to those obtained by adsorption on rough evaporated gold, as reported in a previous study. Quartz crystal microbalance and surface plasmon resonance data acquired in situ on TSG indicate that similar to SAM formation on conventional evaporated gold substrates ODT and MHDA form monolayers and bilayers, respectively. The second layer on MHDA, whose formation is attributed to hydrogen bonding, can be easily removed by adequate rinsing with water. The favorable agreement of the grazing incidence reflection Fourier transform infrared (GIR FTIR) spectroscopy and contact angle data analyzed with the Israelachvili−Gee model suggests that the binary SAMs do not segregate laterally. This conclusion is fully validated by high-resolution friction force AFM observations down to a length scale of 8−10 nm, which is much smaller than the typical observed surface nanobubble radii. Finally, correspondingly functionalized TSG substrates are shown to be valuable supports for studying surface nanobubbles by AFM in water and for addressing the relation between surface functionality and nanobubble formation and properties.

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INTRODUCTION

Surface nanobubbles have been initially observed at the solid−water interface,18,19 and only recently has evidence of their presence at the interface of solids with protic media, such as, formamide, ethylammonium nitrate, and propylammonium nitrate, been reported.20 Surface nanobubbles were also shown to possess an appearance in atomic force microscopy (AFM) height images similar to that of polysiloxane nanodroplets,21 motivating the quest for adequate alternative characterization techniques.22−25 In several AFM studies, we showed that AFM may under certain circumstances provide adequate height information that allows one to analyze, among others, the radii of curvature of surface nanobubbles.26−29 While the interaction forces and the deformation of the gas−water interface might influence the values obtained and the contact angle data derived from these, these errors were shown not to be the origin of the observed

Well-defined surfaces are a prerequisite for many fundamental studies at the solid−liquid interface. The recently controversially discussed cavities at the solid−liquid interface, the socalled “surface nanobubbles”, represent a relevant example.1−7 In addition to fundamental interest, surface nanobubbles are considered generally speaking to be relevant in several technological areas of (potential) application. Among these, froth flotation,8 manipulation of protein adsorption on surfaces,9,10 and surface cleaning11−13 have been discussed by various authors. This interest underlines the fact that an understanding of surface nanobubbles is essential for further developments. Among the many until recently unsolved problems for surface nanobubbles, in particular their unexpectedly long lifetimes and very high nanoscopic contact angles (measured through the condensed phase) stand out.14 These fundamental issues mandate a closer analysis of the phenomenon, from a theoretical as well as experimental point of view, also because of their relevance in the application areas of immersion lithography15,16 and hydrodynamic boundary slip.17 © 2016 American Chemical Society

Special Issue: Nanobubbles Received: May 9, 2016 Revised: June 13, 2016 Published: June 14, 2016 11179

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protocols, or template stripping. Gupta et al. showed that binary SAMs of dodecane-1-thiol and 11-mercaptoundecan-1-ol on ultraflat gold do not exhibit any measurable contact angle hysteresis, which suggests that the surface chemical heterogeneity does not possess a contribution to the hysteresis.58 A study by Brewer et al., who analyzed two-component SAMs by AFM pull-off force experiments, indicates that binary SAMs formed by the thiols mentioned above exhibit phase separation, but the phase domain is too small to be detected by friction force AFM.59 Their conclusion mirrors the STM results obtained by Strainick et al, who showed discrete phase domains only several nanometers in size.45,55 Compared to these length scales, the width of a typical surface nanobubble footprint is very large. The particular SAM system utilized in our previous study mentioned above comprises binary SAMs of 16-mercaptohexadecanoic acid (MHDA) and 1-octadecanethiol (ODT) obtained by solution codeposition on TSG.48,60 While the coadsorption of these thiols by rough gold was recently studied by us for the first time,48 the sample characteristics and in particular the potential phase separation into nanoscopic domains on TSG require attention as a basis for a companion study.39 For the formation, stability, and properties of surface nanobubbles on mixed SAMs, the composition, defect structures, and local length scale of phase segregation in the binary SAM are highly relevant. Hence, in the work presented here, we addressed experimentally the co-adsorption behavior of binary mixtures of MHDA and ODT from ethanol onto TSG, with a particular focus on the composition, the possible phase separation behavior, and the homogeneity of the resulting SAMs for systematic studies of surface nanobubbles at SAM−water interfaces.

discrepancies between macroscopic and microscopic contact angles. A recent explanation by Zhang and Lohse of the unusual long lifetimes and invariance of the nanoscopic contact angles observed by AFM is based on the assumption of pinned nanobubbles in gas-oversaturated media.30 Under these conditions, the outflow of gas due to the Laplace pressure is balanced by the gas oversaturation in the liquid, leading to a preferred bubble size and contact angle values that depend solely on the gas solubility. Pinning has indeed been observed experimentally by several groups.31−33 In addition, the dependence of the apparent nanoscopic contact angles on surface characteristics, such as exposed functional groups or materials leading to different macroscopic wetting, has been addressed.34−38 From these data acquired on samples with systematically varied wettability, no definite dependence of the nanoscopic contact angles on the macroscopic wettability could be concluded. To test the model mentioned above, we conducted detailed AFM investigations of surface nanobubbles on chemically different self-assembled monolayers (SAMs) deposited on template-stripped gold (TSG), in which the macroscopic contact angles were adjusted to the same value.39 The same macroscopic wettability with water was realized in binary SAMs, in which the underlying exposed functional groups were varied to include acidic and basic groups, hydrogen-bonding donors and acceptors of various quality, and polar and unpolar groups. These variations should influence, for instance, the adsorption of ions or the local water structure near the interface, as has been suggested recently, in very different ways. Because of the relevance of the surface properties of these model surfaces, the detailed analysis of the assembly process and the properties of binary SAMs on TSG are of central importance and are reported in this work, which forms the basis for the separate study addressing surface nanobubbles mentioned above.39 In general, SAMs of alkanethiols and disulfides40 have been extensively studied as model organic films with well-defined chemical and structural properties.41,42 Via the introduction of different terminal functional groups of the alkanethiols, the (macroscopic or microscopic) surface wettability, adhesion processes, and interfacial chemical properties can be modified.43−47 Importantly, the surface wettability can be tailored in a continuous manner by adjusting the surface composition of the mixed SAMs.40,41,48 In principle, an arbitrary composition of SAMs can be achieved by exposing the substrate in an adsorbate mixture solution with a defined concentration ratio. However, because of the kinetic control of the composition of the resulting binary SAM, which results in the preferential adsorption of one of the components,41,49−53 the surface composition often differs markedly from the solution composition and must be experimentally determined. In studies on rough gold, it has been established that binary SAMs may phase separate, depending on the differences in chain lengths and terminal functional groups.45,51,54,55 While systems with similar chains lengths did not phase separate on the experimental time scales chosen,56,57 Stranick et al. and others demonstrated that binary SAMs formed by two kinds of thiols with different ω-groups or with different alkyl chain lengths may very well phase separate.45,51,55 Very few studies have to date addressed the difference of mixed SAMs on rough (i.e., evaporated or sputtered) gold versus ultraflat gold obtained by using single crystals, annealing

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EXPERIMENTAL DETAILS

Materials. 16-Mercaptohexadecanoic acid [MHDA, HS(CH2)15COOH, 90%], 11-mercaptoundecanoic acid [MUDA, HS(CH2)11COOH, 95%], and 1-octadecanethiol [ODT, HS(CH2)17CH3, 98%] were purchased from Aldrich and used as received. CZ-silicon wafers, type P/boron/(100) manufactured by OKMETIC (Vantaa, Finland) and gold grains (99.99%) bought from Allgemeine Gold- and Silberscheideanstalt AG (Pforzheim, Germany) were used for the gold surface preparation. The epoxy glue used in the substrate preparation was EPO-TEK 377 from Polytec PT GmbH Polymere Technologien (Waldbronn, Germany). This two-component glue contains no solvent and is resistant to many solvents, in particular alcohols. The glue was freshly mixed in a ratio of 1:1 (by weight) and left under vacuum for 1 h to remove possible air bubbles each time before use. Ethanol (97%, with 1% petrolether), used as a solvent for the thiols and for rinsing the substrates, and isopropanol for cleaning the prism in the surface plasmon resonance (SPR) experiment were purchased from J. T. Baker. Freshly prepared Milli-Q water from a Direct-Q 8 system (Millipore) with a resistivity higher than 18 MΩ cm and a pH of ∼5.5 was used in all experiments. Preparation of TSG Surfaces. The ultraflat gold surfaces were prepared according to the work of Stamou et al. (Scheme S-1).61 The silicon wafers were cut into 1 cm × 1 cm square pieces and pretreated with piranha solution [1:3 (v/v) mixture of 30% H2O2 and concentrated H2SO4 (CAUTION: Piranha solution should be handled with extreme caution. It has been reported to detonate unexpectedly)] until no gas bubbles evolved and then thoroughly washed with Milli-Q water and blown dry with nitrogen. Scheme S-1 shows the preparation procedure of TSG. Dried and dust-free wafers were directly placed into the vacuum chamber for evaporation of gold. At least 150 nm of gold was deposited in a thermal evaporator (MED 010/Balzers Union) operated at 10−6 Torr. The deposition speed was controlled to approximately 0.1 nm/s during the first 15 nm and then increased to 11180

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Figure 1. Intermittent contact mode AFM height images (acquired in ambient air) and corresponding cross-sectional analysis of (a) evaporated gold and (b) template-stripped gold. approximately 2850 cm−1 (νs C−H stretching vibration of −CH2) and 2880 cm−1 (νs C−H stretching vibration of −CH3) were integrated. Quartz Crystal Microbalance (QCM). Labview was programmed to run a Stanford Research Systems (Sunnyvale, CA) QCM-100 5 MHz quartz crystal microbalance equipped with a liquid cell and to record the frequency and voltage changes versus time. The quartz crystal sensors (1 in. in diameter, optically polished, from Stanford Research Systems) were treated with piranha solution for 5 min and thoroughly rinsed with Milli-Q water each time before use. Gravitational flow afforded a flow rate of 0.2 mL/min. The QCM head was kept in a sealed water bath system comprising a Dewar vessel to maintain a stable temperature during the experiments. SPR. A Multiskop SPR system (Optrel GbR, Sinzing, Germany), with a laser wavelength of 633 nm, was utilized for recording SPR angular scans. The SPR sensors, with both rough gold and TSG surfaces, were prepared according to the procedure summarized above, but with chromium and gold thicknesses of ∼2 and ∼47 nm, respectively, and special glass slides (Schott D263 Borosilicate glass, refractive index of 1.518 at a 530 nm wavelength, purchased from SCHOTT AG, Grünenplan, Germany). The sensors were rinsed with ethanol after the silicon cover was peeled off and were used immediately. A triangular LFS 15 prism, with a refractive index of 1.699 at 587 nm, was employed in the measurement. The epoxy glue used to attach the gold to the glass has a refractive index of 1.519 at 589 nm. Reflectivity angular scans were recorded before and after the adsorption of the ODT/MHDA SAMs.62 AFM measurements were taken on MultiMode IIIa instrument (Bruker/Veeco/Digital Instruments, Santa Barbara, CA) and on an Asylum Research MFP-3D Bio instrument (Oxford Instruments/ Asylum Research, Santa Barbara, CA). The gold surface morphologies were scanned in tapping mode in air using Si probes (RTESP type, nominal spring constant of 40 N/m, and resonance frequency of 300 kHz, purchased from Veeco). Friction force AFM imaging was performed in contact mode using Si3N4 probes (MLCT type, also bought from Veeco), in which probes of type C with a nominal spring constant of 0.01 N/m and a resonance frequency of 7 kHz were chosen. The friction force measurements were taken at the same applied load. The surface nanobubbles were observed in tapping mode in Milli-Q water using Si3N4 probes, which were cleaned by oxygen plasma for 60 s (PlasmaPrep2, GaLa Instrumente, Bad Schwalbach, Germany). The surface nanobubbles were measured in conventional liquid cells operated without an O-ring. MLCT probes D (with a

0.4 nm/s. The substrates were allowed to cool for 15 min in the vacuum chamber and then quickly glued onto precleaned glass slides (Thermo Scientific, Gerhard Menzel GmbH), followed by curing at 150 °C for 2 h. The substrates were stored under argon immediately after preparation. Each time before use, the silicon wafer was carefully separated from the glass substrate using a scalpel knife. The fresh flat gold surface was immersed in an ethanolic solution of ODT and MHDA for 2 h to obtain overall covered SAMs on gold surfaces. The total concentration of the solutions was kept constant at 1 × 10−3 mol/L, and the ratio of ODT and MHDA was varied correspondingly. Finally, before the immediately subsequent analysis, a substrate was taken out, thoroughly rinsed with ethanol and water, and then blown dry with nitrogen. Stamp Fabrication and Microcontact Printing. Poly(dimethylsiloxane) (PDMS) stamps were fabricated using Sylgard 184 (Dow Corning) as directed by the manufacturer. First, 10 parts elastomer per 1 part curing reagent were well mixed and degassed under ∼15 mbar in a desiccator for ∼2 h. Next, the liquid PDMS mixture was cast on top of a patterned SU-8 photoresist master (fabricated in a home-built photolithography setup). After casting, PDMS stamps were cured for 1 h at 80 °C in an atmospheric oven for polymerization; the PDMS stamps were carefully peeled off from SU-8 photoresist master before being used. Next, the stamp was covered with an ink solution (ODT/MHDA in ethanol), kept this way for 1 min, and then blown dry with nitrogen. After this procedure had been repeated three times, the stamps were brought into contact with the surface of TSG for 10 min. After microcontact stamping, substrates were rinsed with ethanol and dried with nitrogen before analysis. Contact Angle (CA) Measurements. The contact angle measurements were taken on an OCA 15plus instrument (Data Physics Instruments GmbH). The static contact angle (θstatic) was analyzed by the sessile drop method; the dynamic contact angles (θadv and θrec) were calculated as reported previously.48 Grazing Incidence Reflection Fourier Transform Infrared Spectroscopy (GIR FTIR). FTIR spectra were recorded (1024 scans with a spectral resolution of 2 cm−1) in reflection mode on an IFS 66v spectrometer (Bruker, Ettlingen, Germany) equipped with a liquid nitrogen-cooled MCT detector and a VEEMAX-II grazing angle accessory (Pike Technologies, Fitchburg, WI). To deduce the surface composition of the binary SAMs, the peaks in the spectra were fitted with a Gaussian function and then the areas of the peaks at 11181

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Figure 2. (a) QCM frequency change upon adsorption of ODT (black) and MHDA (red) on rough gold. SPR angular shift upon adsorption on TSG of (b) ODT (0.36°) and (c) MHDA (0.61°).

Figure 3. Static and dynamic water contact angles measured on binary SAMs on (a) TSG and (b) rough gold as well as (c) observed contact angle hysteresis. nominal spring constant of 0.03 N/m and a resonance frequency of 15 kHz) were used for these experiments. The free root-mean-square (rms) amplitude was set to 1.0 V; the images were collected by adjusting the set point amplitude as high as possible, so that the forces, which possibly cause deformation of the bubbles, were minimized. All the AFM images were analyzed by the software of the atomic force microscope (version 5.30 from Veeco, version AR 090909+1124 from Asylum), except for the distributions of the friction signals from the AFM images, which were analyzed using Gwyddion (developed by the Czech Metrology Institute, http://gwyddion.net).

ethanolic solution of the corresponding thiol. The frequency changes thus observed are summarized in Figure 2a. With the well-known Sauerbrey equation (eq 1), the mass increase on the quartz crystal due to layer formation can be deduced. Because a SAM can be considered nondissipative, as also shown by Basit et al. by QCM-D,63 the equation can be applied to estimate the mass change.

Δf = −Cf *Δm

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(1)

where Δf, Cf *, and Δm represent the frequency change, the sensitivity factor for the quartz crystal and detection system (which we calibrate via the well-established SAM formation of ODT according to Schlenoff as Cf * = 12.7 Hz μg−1 cm2), and the mass change of the quartz crystal, respectively.64 For ODT, the observed frequency change was 2.8 ± 1 Hz, which is by definition equal to a surface density of 0.76 ± 0.27 nmol cm−2.64 The adsorption rate extrapolated from the slope was 0.15 nmol cm−2 min−1; after 5 min, the mass equilibrium was established. For MHDA, a frequency decrease versus time curve with two distinct slopes was observed, with rates of 0.36 and 0.19 nmol cm−2 min−1. While the first regime takes only 3 min, the second regime is much longer. The total frequency change was 6.8 ± 1 Hz, corresponding to a surface density of 1.88 ± 0.27 nmol cm−2. This value, which is 1.5 times the value found for ODT, implies the formation of a (partial) double layer. The frequency changes of the first and second slopes are larger and smaller than that of ODT adsorption, respectively, which may indicate that initially MHDA is adsorbed as dimers to the gold surface. In the second phase, further adsorption and a reorganization of the layer might take place. The bilayer

RESULTS AND DISCUSSION The morphology and roughness of the freshly prepared TSG, as assessed by intermittent contact mode AFM in air, were compared with those of gold obtained by thermal evaporation onto glass under high vacuum. As shown in Figure 1a, the evaporated gold surface is composed of grains with apparent sizes of approximately 50−100 nm and corrugations on the order of 20 nm. Its rms roughness determined on a scan size of 5 μm × 5 μm was 4.4 ± 0.4 nm. The TSG surface was found to be substantially flatter with no discernible grains, corrugations of ≤1 nm, and an rms roughness of merely 0.22 ± 0.05 nm, which is close to the roughness of the silicon template [0.13 ± 0.05 nm (for an AFM image, see Figure S-1)]. Occasionally, depressions with a depth of approximately 1 nm were observed with a number density of ≈15 μm−2. The self-assembly kinetics and final coverages of ODT and MHDA on rough gold were first studied in situ by QCM to provide additional insight into the assembly process that will be important for interpreting the SPR data capture with rough gold and TSG (vide inf ra). After an initial equilibration period of the QCM setup, the ethanol was exchanged with an 11182

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Figure 4. (a) Surface xODT values of binary SAMs adsorbed on TSG analyzed from FTIR spectra (squares) and calculated from the Cassie model (filled triangles) and the Israelachvili−Gee model (empty triangles) plotted vs solution xODT. (b) Two examples of FTIR spectra and the peaks of interest for integration.

formation of α,ω-mercaptoalkanoic acids is well-documented in the literature65,66 and was attributed to hydrogen bonding interaction between the carboxylic acid groups. The double-layer adsorption of MHDA is also evident in the SPR angular shifts recorded before and after adsorption of the thiols. As indicated in panels b and c of Figure 2, the angular shift upon MHDA adsorption on TSG is 0.61°, 1.7 times that of ODT (0.36°). If the refractive indices of ODT (1.465) and MHDA (1.540) are taken into account,67,68 these data indicate that the thickness of MHDA is approximately 1.5 time that of ODT. The SPR results on rough gold (Figure S-2) showed the same tendency. The angular shift of MHDA was 0.58° versus a shift of 0.29° for ODT. Because the molecules comprising the second layer of MHDA are attached by hydrogen bonding interactions to the molecules of the first layer, they are easily washed off with an intensive water rinse. The formation of double layers is further confirmed by the water CA values measured prior to and after the water rinse: the double-layered structure showed a relatively high CA of 60 ± 3°, while the −COOH-exposed surface showed a very low CA, i.e., ∼13 ± 3°, in agreement with a SAM exposing −COOH groups.69,70 Thus far, no significant differences in SAM formation on rough gold versus TSG were observed. In the following, the surface wettability data of binary SAMs on the different gold surfaces are discussed. The molar fraction of ODT in the mixed ODT and MHDA ethanol solutions, defined as xODT, was varied (0, 0.05, 0.10, 0.20, 0.30, 0.50, 0.75, and 1.00). The results are shown in Figure 3. It is clear that θstatic increases with increasing xODT and reaches 107 ± 1° for pure ODT at an xODT of 1.00. MHDA SAMs showed a θstatic of 13 ± 3°. As seen for the data on rough gold (see also ref 48), ODT was found to adsorb preferentially when ethanol is used as the solvent. The dynamic contact angles showed the same tendency as θstatic when xODT was varied. The contact angle hysteresis (difference between advancing and receding θ) was found to decrease for an increasing fraction of ODT in the binary SAMs for both types of substrates. Interestingly, on TSG, the hysteresis decreased to very low values, when the MHDA fraction in the binary SAM increased. However, even when considering the error bars, a marked hysteresis remains on TSG. This result does not agree with the data reported by Ulman and coworkers.58 These authors used dodecane-1-thiol (DoDT) and 11-mercaptoundecan-1-ol (MUO) instead of MHDA and ODT, respectively, and employed a different way to measure the contact angle. However, even when using the tilted stage method or the same molecules and the tilted stage method, we always observed a non-zero contact angle hysteresis for SAMs formed on TSG in our experiments (see Table S-1).

The preferential adsorption of ODT observed on TSG was quantitatively estimated by analyzing the GIR FTIR spectra of the two-component SAMs. Here, the peaks at approximately 2850 cm−1 (νs C−H, CH2, denoted as A) and approximately 2875 cm−1 (νs C−H, CH3, denoted as B) were chosen to calculate the surface composition, as indicated in Figure 4b. The calculation was based on the following procedure. (i) The peak areas of the background-corrected spectra were obtained by integration. (ii) The B1/A1 ratio in spectra of pure ODT was used as reference value k to determine the −CH2 portion contributed by ODT to A in spectra of the other (binary) SAM films, and then the rest of the −CH2 portion of A was attributed to MHDA. (iii) Because ODT and MHDA possess different numbers of −CH2− groups, the two portion areas were divided by 17 and 15 separately, and finally, the surface xODT of the specific binary SAM was calculated using eq 2.

xODT =

kA n 17 kA n B − kA + n 15 n 17

(2)

where An and Bn are the integrated areas at ∼2875 and ∼2850 cm−1 for the specific binary SAMs, respectively. Consistent with the contact angle data shown above, the surface xODT is always higher that the solution xODT in the binary SAMs (Figure 4a). In addition, the surface composition of the SAMs was analyzed according to the Cassie and Israelachvili−Gee equations (eqs 3 and 4, respectively).71,72 These models have been frequently used to describe the relation between the measured contact angle and the fractional surface coverage for mixed SAMs.73,74 While the Cassie model assumes a patchy surface (in terms of this study a phaseseparated SAM structure), the Israelachvili−Gee model describes a molecularly mixed scenario. cos θ = f1 cos θ1 + f2 cos θ2

(3)

(1 + cos θ )2 = f1 (1 + cos θ1)2 + f2 (1 + cos θ2)2

(4)

where f1 and f 2 are the fractional surface coverages, θ1 and θ2 are the contact angles of the individual components, and θ is the measured static contact angle of the binary SAM. These data are also included in Figure 4a. It is clearly seen from the plot in Figure 4a that the surface composition of the binary SAM on TSG as determined by FTIR spectroscopy is better described by the contact angle data analyzed with the Israelachvili−Gee model. A similar trend was observed previously on rough gold.48 Hence, on the basis of these data, we can conclude that a phase-separated patchy SAM structure is unlikely for both types of gold substrates. 11183

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retrace images, which are comparable in size to smaller nanobubbles. We can conclude at this point that TSG modified with binary SAMs of ODT and MHDA represents an ideal candidate for surface nanobubble studies.60 (1) The optimized surfaces are practically as smooth as silicon wafers. (2) The surface composition and hence the microscopic contact angle can be systematically varied from the very hydrophilic to the very hydrophobic range. (3) Because of the absence of lateral phase separation, the surfaces are indeed homogeneous on the typical length scales observed for surface nanobubbles. To demonstrate the feasibility of this application, surface nanobubbles on ODT-modified TSG, which has a typical macroscopic water CA of 107°, were investigated by intermittent contact mode AFM. The nanobubbles were nucleated by air entrainment during the setup of the AFM liquid cell.29 Because the experiment was also not conducted with a sealing elastomer O-ring, potential contamination originating from ethanol (used in the typically applied ethanol−water exchange procedure) or the elastomer (in case of silicone-based O-rings) was circumvented. During the scanning, the amplitude set point value was set as high as possible to minimize the deformation of bubbles.26−28 As shown in Figure 6a, the surface nanobubbles exhibited apparent

To confirm this indirect proof of the absence of phase separation and to establish an upper bound for the length scale for patches of one thiol in the binary SAMs, friction force AFM was employed. Microcontact printed patterns of ODT and MHDA as well as phase-separated binary SAM of ODT and mercaptoundecanoic acid (MUDA) with a pronounced chain length difference were used as a reference, as shown in Figure 5

Figure 5. Friction force AFM images of ODT/MHDA on TSG. Micropatterns with (a) MHDA in the circular areas, (b) ODT SAM, (c) binary SAM with an xODT of 0.5, (d) binary SAM with an xODT of 0.2, (e) MHDA SAM, and (f) binary ODT/MUDA SAM (1:1 molar ratio) on TSG. The phase patches have apparent sizes of 10−30 nm.

and discussed below, respectively. The height images (not shown here) indicate a flat, featureless morphology for all samples because of the similarity of the molecular length between ODT and MHDA. To ensure constant experimental conditions, the same tip was used for scanning the binary SAMs. Figure 5a shows AFM data acquired on the micropatterned SAM, while panels b−e of Figure 5 are the friction AFM images of an ODT SAM, binary SAMs with xODT values of 0.5 and 0.2, and finally a MHDA SAM, respectively. The MHDA areas are characterized by a high friction force, in agreement with the literature.75,76 The images acquired on binary SAMs do not show obvious phase separation in the form of patches on the length scale imaged (pixel width of ≈2 nm). The slight contrast variations observed in Figure 5c are very minor and correspond to those detected on pure MHDA (panel e). Hence, these AFM data confirm that the binary SAMs of ODT/MHDA possess down to length scales of 8−10 nm no phase separation on the flat gold surface. This conclusion is also evident from the distributions of the friction signal values of the corresponding images. As shown in Figure S-3, unlike the micropatterned surface that displays a distinct bimodal distribution, the friction force distributions for the neat SAMs and binary SAMs are clearly monomodal. This result fully supports the conclusion of the contact angle measurements. Although the two thiols possess different terminal groups, their alkyl chains length are very similar and very long. Thus, the van der Waals interactions between the alkyl chains become dominating and lead to the formation of homogeneous, practically molecularly mixed monolayers. Finally, binary SAMs of ODT with the much shorter acid-terminated adsorbate MUDA, prepared under identical conditions, showed clear evidence of phase separation (Figure 5f). The patches have an apparent size in range of 10−30 nm in both trace and

Figure 6. (a) AFM image of surface nanobubbles imaged in intermittent contact mode in water. (b) AFM image of the same region in panel a scanned in contact mode in water (the white circles mark the positions of the nanobubbles in panel a).

radii of 100−220 nm and heights of 7−10 nm. Without tip radius calibration, the nanoscopic contact angles were 174 ± 2.5°. Subsequently, the imaging mode was switched to contact mode without changing the scanning position. In agreement with reports by Holmberg et al.,77 the nanobubbles were no longer detectable (Figure 6b). However, the topographic defects in the TSG mentioned above were clearly resolved. A colocalization analysis showed 18 nanobubbles of 65 (in Figure 6a) reside at or very near these apparent pinholes in the substrate, while the remaining bubbles are dispersed randomly. In addition, the majority of pinholes are free of nanobubbles. A comparison of this experiment on TSG and on rough gold (Figure S-4) underlines the necessity to use TSG for AFM experiments on surface nanobubbles on alkanethiol SAMs on gold. On the rough substrate, it is practically impossible to identify surface nanobubbles in topographic AFM scans.

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CONCLUSION Binary SAMs formed by co-adsorption of 16-mercaptohexadecanoic acid and 1-octadecanethiol on TSG surfaces were characterized in detail to determine the wettability as a function of the molar fraction of the thiols in the assembly solution. No significant differences between rough gold and TSG were observed. As shown by GIR FTIR spectroscopy and contact angle data analyzed with the Israelachvili−Gee model and in 11184

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particular friction force AFM, the binary SAMS did not segregate laterally down to a length scale of 8−10 nm, which is much smaller than the typical observed surface nanobubble radii. Correspondingly functionalized TSG substrates were thus shown to be valuable model surfaces for AFM studies of surface nanobubbles to be able to address the relation between surface functionality and nanobubble properties.

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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.6b01776. Scheme of the TSG preparation, AFM data of substrates used, SPR angular scans acquired on evaporated gold, histograms of friction force AFM data, AFM data on surface nanobubbles on TSG and rough gold, and additional contact angle data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +49 271 740 2806. Fax: +49 271 740 2805. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Davide Tranchida for help with the FTIR data analysis and Dipl.-Ing Gregor Schulte and Brigitte Niesenhaus for excellent technical assistance. The authors gratefully acknowledge financial support from the Alexander von Humboldt Foundation (BS), a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, the European Union (FP7 Project BacterioSafe, Grant 245500), the European Research Council (ERC Project ASMIDIAS, Grant 279202), the Deutsche Forschungsgemeinsschaft (DFG, Grant INST 221/87-1 FUGG), and the University of Siegen.

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REFERENCES

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DOI: 10.1021/acs.langmuir.6b01776 Langmuir 2016, 32, 11179−11187