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Role of Solution and Surface Coverage on Voltage-Induced Response of Low-Density Self-Assembled Monolayers Mingxiang Luo, Aisley Amegashie, Alvin Chua, Gloria K. Olivier, and Joelle Frechette* Chemical and Biomolecular Engineering Department, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States S Supporting Information *

ABSTRACT: Low-density self-assembled monolayers (LD-SAMs) exhibit voltage-induced conformational changes leading to tunable wetting properties and ionic permeability. Here we investigate the role of solution conditions (pH, salt concentration, presence of acetonitrile) and surface coverage via nonfaradaic electrochemical impedance spectroscopy and contact angle measurements. We observe a large change in receding angle (32°) in an aqueous environment for an optimal combination of solution and film composition. This voltage-induced response in wetting properties is also correlated with a large change in dielectric properties as measured from electrochemical impedance spectroscopy. Our results demonstrate that the molecular nature of this responsive surface makes it particularly sensitive to solution conditions, a feature that could be exploited for sensing or detection.

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INTRODUCTION The submonolayer coverage and associated enhanced conformational freedom of low-density self-assembled monolayers (LD-SAMs) distinguish them from traditional SAMs, which tend to form a crystalline structure on a gold surface.1−3 This conformational freedom has recently been employed to create responsive films displaying switchable wetting,4,5 ion transport,6−8 and lubricating properties.9 LD-SAMs have also been employed as a platform to create films with mixed functionality10 and programmable adsorption.11,12 The applied potential is thought to control which functional groups are exposed at the SAM−solution interface, giving rise to the stimuli-dependent properties.13−16 For example, in the case of carboxyl-terminated alkylthiol SAMs, the applied potential between the gold substrate and the solution alters the electrostatic interactions within the film and leads to a conformational change (see Figure 1) whereby the film is more hydrophobic at positive potentials.4,8,11 As another example, bipyridinium-terminated SAMs also experience a

similar change in conformation but under opposite applied bias.16 While LD-SAMs have attracted much attention, the voltageinduced change in wetting properties in aqueous solutions has been modest (Δθ ∼ 9−20°) and limited to changes in the receding angle.4,8,16 For instance, Lahann et al. report a voltageinduced change in receding angle of 20° for LD-MHA films in pure d3-acetonitrile,4 which is not a very practical electrolyte. The work of Liu and co-workers,11 however, reports a change in static contact angle of more than 30° for large drops (100 μL) that is largely independent of surface coverage in PBS buffer solution. In addition to wetting properties, LD-SAMs have been found to exhibit a large voltage-induced change in ionic permeability consistent with the proposed conformational change.7,8,17 For instance, Lahann et al. report a voltageinduced change of 30% in imaginary impedance modulus and of 6° in phase angle at 1 Hz for a 0.4 V difference on LD-MHA films,7 and in our recent work we observed a change of 60% in impedance modulus and 10° in phase angle at 1 Hz.8 We also observed a small influence of the solution condition on the impedance response for LD-MHA with a larger surface coverage than the films produced by Lahann et al.8 Generally, the change in properties caused by the applied potential has only been characterized for a limited set of conditions, and the relationships between the surface coverage and the molecular reorganization has yet to be investigated. Finally, although the wetting properties and ionic permeability of LD-SAMs have

Figure 1. Illustration of the idealized conformational transition between a stretched (at negative potential) and a bent (at positive potential) state of LD-MHA monolayers. The schematic was adapted from Lahann et al.4 © 2012 American Chemical Society

Received: March 3, 2012 Revised: June 1, 2012 Published: June 2, 2012 13964

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cations was monitored via FT-IR spectroscopy. The presence of LD-MHA monolayers was confirmed from contact angle measurements and from a shift of the symmetric and asymmetric CH2 peak to higher wavenumber. Contact Angle Measurements. Advancing and receding contact angle measurements under an applied potential were performed at room temperature using a FTA125 apparatus (First Ten Angstroms) by the captive drop technique.25 The LD-MHA/gold substrate acted as the working electrode, an Ag/AgCl wire as the reference electrode, and a platinum wire as the counter electrode. The potential of the gold substrate, relative to the Ag/AgCl wire, was controlled using a CHI 650B electrochemical workstation (CH Instruments). The potential of the Ag/AgCl wire relative to the standard calomel electrode (SCE) was measured, and all potentials are reported with respect to SCE. Expansion and contraction of the droplet at a rate of 6 μL/min was achieved using a syringe pump (World Precision Instruments) via a glass capillary inserted in the drop (the counter electrode was inserted in the glass capillary). Receding and advancing contact angles were denoted as the angles right before the three-phase contact line receded or advanced over the surface, respectively (see Figure S1). Samples were rinsed with ethanol, followed by a brief rinse with Milli-Q water, and dried with a stream of nitrogen gas prior to measurement. Results shown are averaged from at least nine locations on three samples; at least three measurements were made on each location. The reported values are the average ± standard deviation. Electrochemical Impedance Measurements. Electrochemical impedance measurements were conducted in the absence of a redox probe using 100 mM, 10 mM, 1 mM KCl, or 1 mM KCl with 25% acetonitrile (v/v) as the electrolyte solution. The ionic strength for 100, 10, and 1 mM KCl, pH 11 electrolyte solutions increased slightly to 101, 11, and 2 mM, respectively, in the process of adjusting the pH. The pH of the KCl electrolyte solution was adjusted using 5 M KOH or 5 M HCl standard solution. The pH of the KClO4 electrolyte solution was adjusted using 0.1 M KOH standard solution. Electrolyte solutions were prepared freshly prior to each measurement. All experiments were performed at room temperature after deaerating the electrolyte with watersaturated ultrapure nitrogen gas for 45 min. The electrolyte is covered by a constant blanket of ultrapure nitrogen during the entire measurement. Test samples were mounted on a flat-cell (Princeton Applied Research), with a fixed working electrode area of 1 cm2, Ag/AgCl/3 M KCl (aq.) reference electrode (CH Instruments), and platinum mesh counter electrode. A double junction reference electrode (Metrohm Inc.) was used when KClO4 was the electrolyte. Nonfaradic impedance measurements were performed at dc bias potentials, between +0.3 and −0.1 V with respect to the standard calomel electrode (SCE), using an ac amplitude of 5 mV on a CHI 650B electrochemical workstation (CH Instruments). After impedance measurement, the impedance spectra of the SAMs were fitted with a modified Randles circuit26 (see inset of Figure 2b) using the nonlinear least-squares fitting routine provided by the ZView software program. A constant phase element (CPE, with CPE = (Q(iω)n)−1, where Q and n are the magnitude and exponent parameter of CPE) was used to model the interfacial capacitance, to account for deviations from ideal capacitive behavior and to achieve a more accurate fit to the experimental data. Since both the magnitude and exponent of the CPE

been investigated separately, the relation between these two macroscopic properties has not been fully explored. We recently developed a method allowing us to systematically vary the surface coverage of a mercaptohexadecanoic acid (MHA) SAM on gold.18 We showed that the resulting LDSAMs displayed wetting and ionic permeability that depend on the applied potential.8 Here we show how combining nonfaradaic electrochemical impedance spectroscopy (EIS) with measurement of contact angle hysteresis allows us to investigate the role of solution conditions and surface coverage in the electrical response of LD-MHA films. Nonfaradaic EIS has been successfully employed to characterize conformational changes and defects within SAMs, allowing for the study of diffusion of ions and water molecules through the monolayers at different bias potentials over different time scales (frequencies).17,19−23 Measurement of contact angle hysteresis has the advantage of being highly sensitive to the chemical functionality present at the film−solution interface24 and therefore is well-suited to monitor a potential-induced conformational change for LD-MHA films. By combining EIS with measurements of contact angle hysteresis, we are able to capture a better picture of the electrical response in LD-SAMs. We show that both solution conditions and surface coverage influence the potential-induced conformational change in the films. More specifically, LD-MHA films show more pronounced switching at low surface coverage and low salt concentration. We also observe further enhancement in the voltage-induced change in wetting properties if acetonitrile is present in the solution. By optimizing the solution composition and surface coverage, we can achieve a larger (32°) change in receding angle with applied potentials. Finally, we observe a strong correlation between the values for the film capacitance and the measured receding angle of the films.

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EXPERIMENTAL METHODS Materials. 16-Mercaptohexadecanoic acid (MHA, 99.8%), carbon tetrachloride (99.99%), potassium chloride (99.0%), potassium perchlorate (+99.99%), acetonitrile (99%), and tetralkylammonium hydroxide salts of increasing chain lengths, [CH3(CH2)n]4N+OH− (TAAOH, received as 1 M aqueous solution, where the TAA+ cations are hereafter referred to as TEA+ for n = 1, TProA+ for n = 2, and TPeA+ for n = 4), were purchased from Sigma-Aldrich and used as received. Dimethyl sulfoxide, KOH pellets, HCl, H2SO4, and H2O2 were purchased from Fisher Scientific. Mica sheets were obtained from S&J Trading. Gold wire (99.99%) was purchased from Kurt J. Lesker. Chromium (99.99%) was purchased from Alfa Aesar. Ethanol (200 proof) was used as received from WarnerGraham Co. Purified water (18.3 MΩ-cm) was obtained from a Milli-Q Gradient system. Synthesis of Low-Density Monolayers. Low-density mercaptohexadecanoic acid (LD-MHA) films on gold-coated mica substrates were obtained by performing ion-exchange on monolayers made from TAA−MHA ion-pairs. In this work, 200 nm gold films were deposited onto freshly cleaved mica at the evaporation rate of 3 Å/s with 20 nm chromium as an adhesion layer. Cyclic voltammetry measurement indicates the roughness factor of our gold-coated mica substrate to be 1.1. We followed the synthesis and deposition of ion-pair monolayers as described in our previous work10,18 with the only difference being the use of a higher ratio of TPeAOH/MHA (6:1 vs 4:1) in solution to facilitate ion-pair formation. Removal of the TAA 13965

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both surface coverage and the electrolyte composition. The three LD-SAMs investigated here consist of the same MHA chain, but the spacing between the chains has been varied, giving rise to low-density MHA films (LD-MHA) with different surface coverage (see Table 1).10,18 Influence of Surface Coverage and Solution Conditions. The importance of the solution conditions and surface coverage are clearly evident in the Bode plots taken at two different bias potentials obtained for two LD-MHA films of different surface coverage and taken in different solution conditions (Figure 2, see Supporting Information Figure S2 for the same plots with the added data of a control MHA monolayer). When comparing the impedance spectra for the two films, we pay special attention to the phase angle obtained at low frequencies (0.1−10 Hz) because (1) it corresponds to the (RmC)−1 time constant for the films, where Rm and C are respectively the effective monolayer resistance and capacitance, and (2) Boubour and Lennox observed that changes in SAMs’ permeability are better captured by the phase angle than by the impedance modulus (|Z|).20,21 For these frequencies, we clearly see that the potential dependence of the impedance response varies drastically for the two films investigated (Figure 2). These LD-SAMs are essentially molecular switches, and it is therefore understandable that their responsive nature will be affected by chain−chain interactions (surface coverage) and chain−solution interactions (solution conditions). As such, we aim to understand the role played by the surface coverage and solution conditions on their potential-induced response and try to obtain the conditions that lead to a maximum observable change in properties upon a potential step. Throughout the remainder of this work, we compare the fitted values for the effective monolayer capacitance (Ceff), rather than the raw impedance data at a given frequency. The values for this effective circuit element is obtained from the equivalent circuit shown in the inset of Figure 2b. At low salt concentration (1 mM), the higher impedance modulus at moderate to high frequency (102∼105 Hz) and the bump in phase angle in the high-frequency region (104∼105 Hz) are due to the greater solution resistance. Effect of Surface Coverage and Solution pH−Film Permeability. The decrease in coverage between LD-MHA(TEA) and LD-MHA(TPeA) leads to important changes in the impedance response with applied potential. Shown in Figure 3 are the values of the effective capacitance obtained from the fits of the equivalent circuit for films with different surface coverage at pH 11 (see Supporting Information Figure S3 for the same figures taken at pH 3 and pH 8.5, as well as tabulated values in Table S1). Here we notice that the monolayer capacitance increases as the potential increases. The change in monolayer capacitance with applied potential also increases when the surface coverage decreases (Figure 3). This trend is consistent both with a reduction in the steric hindrance between the chains facilitating the conformational change with applied potential and with a decrease in the monolayer thickness (the capacitance is inversely proportional to the dielectric thickness in a parallel plate capacitor). We also compared the effective

Figure 2. Bode plots taken in different electrolyte solutions at −0.1 and +0.3 V (vs SCE) for LD-MHA(TEA) and LD-MHA(TPeA) SAMs. Sphere and square symbols are the measured values for LDMHA(TEA) and LD-MHA(TPeA). Lines are the result of the complex nonlinear least-squares fit to the data using the equivalent circuit model shown in the inset.

depend on the applied potential, we calculate the effective capacitance (C) using the following correction C=

n

Q (R s−1 + R m−1)1 − n

where Rs and Rm are the solution and effective monolayer resistance obtained from fitting the impedance spectra.27−29 More details of impedance measurement and electrical circuit fitting can be found in our previous report.8 The bump in the Bode phase plot observed in the high-frequency region (104− 105 Hz) is caused by solution resistance in 1 mM electrolyte solutions, and when present these frequencies were not used for the fitting. Reported values represent the mean of at least three independently prepared samples. Error bars indicate standard deviation from the mean, arising from sample-tosample variation.

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RESULTS AND DISCUSSION The results of this study show that the potential-induced conformational change of low-density SAMs is a function of

Table 1. Surface Coverage of the Films Investigated in This Work Obtained from Desorption10 surface coverage

MHA

LD-MHA(TEA)

LD-MHA(TProA)

LD-MHA(TPeA)

nm2/molecule % MHA

0.19 ± 0.01 100

0.32 ± 0.06 60 ± 11

0.45 ± 0.06 42 ± 6

0.57 ± 0.10 33 ± 6

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induced impedance response.8,30 Across the range of surface coverage for LD-MHA films investigated in this work, the same voltage-induced impedance response was again observed to occur under conditions where the MHA carboxylate group is uncharged (fully protonated) and under conditions where the terminal group is negatively charged (fully deprotonated). These findings reinforce the conclusion that the mechanism causing the conformational change within LD-MHA is not as simple as an electrostatic interaction (attraction/repulsion) between a negatively charged terminal group and a charged electrode to cause a change in the impedance response. We suspect, as discussed in our previous work, that polarization of the monolayer by the applied electric field is a sufficient driving force to cause a conformational change within the chains on the surface, as it has been proposed for other uncharged monolayers.8,19,31 Effect of Surface Coverage and Solution pHSurface Wettability. We opted to monitor the voltage dependence of the advancing and receding contact angles (especially the receding angle), instead of the static contact angle, as they better characterize the surface wettability, are less sensitive to drop history or volume, and can showcase the effect of chemical and physical heterogeneities.32,33 Consistently throughout our investigation, we observe that the effect of the applied potential is negligible on the advancing contact angle (unless mentioned otherwise) while large changes are observed for the receding contact angle,5 a feature that is consistent with another literature report for LD-SAMs4 and with the motion of the triple contact line on a heterogeneous surface.34,35 The receding angle measured on all the LD-MHA films increases with an increase in applied potential (Table 2). We also observe that decreasing the surface coverage leads to a larger change in the receding angle with applied potential (see Table 2), a trend similar to the observed increase in the monolayer capacitance at lower coverage and higher potential. This general trend was observed for the three solution pH values investigated. Also in agreement with the impedance data is that the solution pH does not play an important role in the change of receding angle with applied potential for a given LDMHA film. However, while the data at all three pH values is indistinguishable in the impedance measurements, we do observe a small effect of pH on the potential dependence of the receding angle. More specifically, we observe that increasing the pH from 3 to 11 leads to a larger change in the receding angle for the highest surface coverage investigated (LDMHA(TEA), see Table 2), but this effect disappears for the LD-MHA(TPeA) film, even when the salt concentration is lowered. Considering that the measured capacitance for LDMHA films is independent of solution pH (Table S1), these

Figure 3. Fitted results for the effective capacitance for SAMs of different surface coverage in 100 mM KCl, pH 11 electrolyte. Dashed lines are to guide the eye.

monolayer resistance Rm at different surface coverage and solution pH (see Figure S3 in the Supporting Information). The interpretation of Rm is not simple as it can have multiple physical origins such as diffusion of ions through the film or pseudo-charge transfer caused by ion adsorption on the gold surface. Moreover, despite the fact that the electrolyte solution has been thoroughly deaerated prior to and during the impedance measurement, we cannot exclude the presence of a small amount of faradaic reaction from oxygen, especially at low frequency. Interestingly, all the LD-MHA films investigated display a reversible order-of-magnitude change in Rm with applied potential (Figure S3 in the Supporting Information), and this change in resistance is the same for the three LD-MHA films investigated (within experimental errors). For comparison, monolayer capacitance and resistance for fully packed MHA monolayers do not vary for the potential window investigated here (Figure 3). We also observe that the pH of the solution does not play a role in the impedance response. For the three LD-SAMs investigated here, the monolayer capacitance and resistance are largely independent of the solution pH (between pH 3 and pH 11) in 100 mM KCl electrolyte (see Table S1 for the values of the monolayer capacitance and resistance at pH 3, 8.5, and 11). These observations are consistent with our initial work with LD-MHA(TEA) monolayers for which we noted that 100 mM KCl electrolyte at pH 8.4 (well above the pKa of the carboxylic acid group) and at pH 3.3 (below the pKa of the acid group) caused the monolayer to exhibit the same reversible, voltage-

Table 2. Role of Solution pH on the Receding Angle Taken at Two Applied Potentialsa LD-MHA (TEA) θrec (deg) 10 mM

100 mM

pH 3 pH 8.5 pH 11 pH 3 pH 8.5 pH 11

−0.1 V 25 26 25 − 21 −

±2 ±1 ±2 ±2

LD-MHA (TProA)

+0.3 V 30 36 37 − 31 −

±0 ±1 ±1 ±2

5 10 12 − 10 −

LD-MHA (TPeA)

Δ

−0.1 V

+0.3 V

Δ

±1 ±1 ±2

35 ± 1 31 ± 1 31 ± 1 − − −

40 ± 1 40 ± 0 42 ± 1 − − −

5±1 9±1 11 ± 1 − − −

±2

−0.1 V 28 33 29 32 31 29

± ± ± ± ± ±

1 0 1 1 2 2

Δ

+0.3 V 41 46 45 39 39 38

± ± ± ± ± ±

1 0 3 2 2 2

12 12 15 8 9 10

± ± ± ± ± ±

2 1 1 1 2 2

The three LD-MHA films investigated have different surface coverage. The difference between the values measured at +0.3 and −0.1 V is denoted as Δ. a

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chains with the applied potential.39 The absence of a monolayer resistance in KClO4 while it is definitely present in KCl (see Supporting Information Figure S3) implies that specific adsorption of Cl− ion contributes significantly to the monolayer resistance. We cannot, however, exclude the possibility that the adsorption of Cl− is also influenced by the conformational change within the LD-MHA films and thus affects the potential dependence of the monolayer resistance. A decrease in the ionic strength (I) increases the Debye screening length κ−1 = 0.304/√I, where κ−1 is the Debye length in nm and I is the ionic strength in mol/L.40 A longer Debye length means that the electric field is felt farther away from the surface, and as such we would expect that decreasing the salt concentration would favor a voltage-induced conformational change. In our previous work, however, we found that a decrease in the bulk salt concentration (from 1 to 0.01 M KCl) did not lead to significant change in the impedance response of the LD-MHA(TEA) monolayers.8 Here we investigate how lower salt concentration (from 100 to 1 mM KCl) influences the impedance response of the LD-MHA(TPeA) film. Shown in Figure 4a are the Bode plots at two applied potentials taken at two salt concentrations (pH 11). We see from the Bode plot that decreasing the solution concentration leads to significant difference in the impedance spectra (Figure 4a). Interpretation of the data, however, is challenging because the low concentration also causes an increase in the capacitance of the electrical double layer of the solution, which becomes comparable to the capacitance of the monolayer. The increase in the double layer capacitance, combined with the high capacitance of the LD-MHA films investigated, prevents us from determining if the change in the impedance spectra is caused by a potential-induced change within the SAMs or due to the electrical double layer of the solution. When looking at the change in effective capacitance with potential (Figure 5), we observe that reducing the salt concentration did not lead to large changes in the monolayer capacitance. We also investigated how the addition of acetonitrile to the solution would influence the impedance response of the LDMHA(TPeA) monolayer. We observed that the addition of acetonitrile led to significant changes in the impedance spectra

results indicate that for our experimental system contact angle measurements can pick up more subtle changes in surface properties than impedance spectroscopy. Finally, we see that the response of the LD-MHA(TPeA) film is more robust: less affected by the pH and tending to display larger changes in the receding angle and capacitance with potential. As such, we selected LD-MHA(TPeA) at pH 11 in our investigation on the role of salt concentration and addition of acetonitrile. Effect of Electrolyte Composition and ConcentrationFilm Permeability. We investigated the change in impedance response of LD-MHA films in different electrolytes. Chlorine ions coming from the KCl solutions used throughout these experiments are known to specifically adsorb to the gold surface,36,37 while perchlorate salts are considered to not adsorb specifically to the gold surface. Therefore, we investigated the possible role played by specific adsorption of Cl− ion by performing impedance measurements in 0.1 M pH 8.5 KClO4 solution. The impedance spectra in KClO4 were best fitted with an R-CPE circuit (instead of the Randles circuit illustrated in the inset of Figure 2b).38 Our results show a similar effective capacitance in KClO4 as the one obtained in KCl (see Figure 4) with a change in effective capacitance from 5.6 ± 0.1 μF/cm2 at −0.1 V to 8.9 ± 0.1 μF/cm2 at +0.3 V. The change in capacitance in the absence of specific adsorption suggests its physical origin is likely a change in conformation of the MHA

Figure 4. Bode plots for LD-MHA(TPeA) films at different potentials: (a) effect of salt concentration and (b) effect of the addition of acetonitrile along with the circuit used for fitting the data (inset). Filled and open symbols correspond to the phase angle measured at −0.1 and +0.3 V, respectively. Lines are the result of the complex nonlinear least-squares fit to the data using the equivalent circuit model.

Figure 5. Effective capacitance for LD-MHA(TPeA) films at different potentials. The impedance measurements are taken in 100 mM pH 8.5 KClO4 (spheres), 100 mM pH 11 KCl (squares), 1 mM pH 11 KCl (diamonds), and 1 mM KCl with 25% acetonitrile pH 11 (triangles) electrolyte solutions. Dashed lines are to guide the eye. 13968

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(Figure 4b). Notably, an R-CPE had to be employed when working with an acetonitrile solution, likely due to the large capacitance of the circuit. The presence of acetonitrile (25% by volume) leads to an increase both of the baseline value and of the overall change in capacitance with potential (Figure 5). Similar to N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), acetonitrile is also expected to adsorb at the SAM surface as well as at gold defect sites by intercalating within the monolayer and replacing water molecules.41,42 Acetonitrile molecules adsorbed at the gold−electrolyte solution interface are known to display potential-induced orientation with the −CN group pointing toward substrate at positive potential and toward solution at negative potential.43,44 For acetonitrile present at the SAM−electrolyte solution interface, acetonitrile is expected to break the strong hydrogen bond between water and carboxylate groups and therefore lower the energy penalty required to move the carboxylate groups from SAM−electrolyte solution interface with higher dielectric constant to the inner space of hydrocarbon groups of the SAMs with lower dielectric constant.45 The switching process of MHA molecules from a stretched to a bent state is an energy consuming (uphill) process; as such the addition of acetonitrile is expected to facilitate potential-induced conformational change. Acetonitrile, however, also influences the electrical properties of the solution (resistance and capacitance) and is also known to display potential-dependent orientation on gold substrates at the potentials investigated here.43,44 It is therefore challenging to decouple the potential dependence of the solution from that of the films based on impedance measurements alone. Effect of Electrolyte Composition and ConcentrationSurface Wettability. Contact angle measurements, on the other hand, clearly show that decreasing the salt concentration and adding acetonitrile lead to a significant enhancement of the potential-dependent wetting properties of the films. As shown in Table 3, the change in the receding angle when the potential is switched between −0.1 and +0.3 V increases from a change of 10 ± 2° to a change of 26 ± 1°. The

advancing angle is generally insensitive to the solution condition (Table 3), consistent with drop motion on a chemically heterogeneous surface.33−35 As a result, we have a system that displays significant change in contact angle hysteresis (CAH) with applied potential, a feature that can be employed to control the motion of droplets on surfaces.5 In addition, the results show a similar change in the receding angle in KClO4 as the ones obtained in KCl, which supports that the change in capacitance obtained from impedance measurements is due to a conformational change of LD-MHA films. Correlation between Wettability and Impedance Properties. As shown in the previous sections, the changes both in impedance and in receding angles are capable of capturing the potential-induced conformational changes within the LD-MHA SAMs. The strong correlation between the dielectric and wetting properties of the films at positive potential (+0.3 V) is shown in Figure 6a. At positive potential we clearly observe that an increase in contact angle leads to monotonous increase in the capacitance. This relationship is consistent with the schematic of Figure 1 where, for example,

Table 3. Role of Electrolyte Composition and Concentration on the Advancing and Receding Anglesa solutions 100 mM KClO4

100 mM KCl

10 mM KCl

1 mM KCl

1 mM KCl+25% ACN

−0.1 V +0.3 V Δ −0.1 V +0.3 V Δ −0.1 V +0.3 V Δ −0.1 V +0.3 V Δ −0.1 V +0.3 V Δ

θadv (deg)

θrec (deg)

CAH (deg)

70 ± 2 77 ± 1 7±2 67 ± 2 72 ± 2 5±2 64 ± 4 68 ± 3 4±2 65 ± 4 73 ± 2 8±3 64 ± 1 66 ± 3 3±1

31 ± 1 39 ± 1 8±1 29 ± 2 38 ± 2 10 ± 2 29 ± 1 45 ± 3 15 ± 1 29 ± 0 49 ± 1 20 ± 1 30 ± 2 56 ± 2 26 ± 1

38 ± 2 39 ± 3 1±3 38 ± 2 34 ± 3 4±2 35 ± 3 23 ± 3 12 ± 3 36 ± 1 24 ± 1 12 ± 2 34 ± 1 10 ± 2 24 ± 2

Figure 6. Correlation between the capacitance and the receding angle for the values measured at (a) +0.3 V and (b) −0.1 V. The solution concentration for the capacitance obtained at different pH values (circles, squares, and triangles) is 100 mM, while the corresponding concentration for the receding angle measurements is 10 mM. The surface coverage is the same for the impedance and receding angle measurements when investigating the role of solution condition (diamonds). Impedance and receding angle measurements are also performed in 0.1 M pH 8.5 KClO4 solutions (blank squares).

a

The KClO4 solution is at pH 8.5, and the KCl solution is at pH 11. The contact angle hysteresis is obtained from CAH = θadv − θrec. The difference between the θadv, θrec, and the CAH values measured at +0.3 and −0.1 V is denoted as Δ. 13969

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more “bent” chains lead to thinner films (higher capacitance) and more hydrophobic groups exposed (larger contact angle). Interestingly, we see from Figure 6a that the data points coming from films of different coverage (and pH) as well as from LDMHA(TPeA) in different solution conditions roughly follow the same Ceff vs θrec relationship. This suggests that the increase in capacitance obtained at different salt concentration and in the presence of acetonitrile is mainly caused by an enhanced change in the conformation of the chains (and not mainly due to change in the double layer properties or solvent adsorption). In addition, we clearly see from Figure 6a that the KClO4 data fall on the same line as the data obtained with KCl at different pH values or solution conditions. This agreement supports that the Ceff measured at +0.3 V is very sensitive to the chain conformation. On the other hand, a similar correlation between the capacitance and the receding angles is not observed at negative potential (see Figure 6b). At negative potential the smaller increase in receding angles when the surface coverage is decreased is not accompanied by a similar increase in capacitance for the LD-MHA films investigated here. The fact that the capacitance does not increase suggests that the different films keep a relatively constant εr/d ratio at negative potential (when the chains are more “up”) or that at negative potential the electric field has a similar effect for the three surface coverages investigated here. Also at negative potential we see that the change in solution conditions (green diamonds) does not cause any change in the receding angles. This could mean that, at negative potential, the change in capacitance mostly comes from changes in the solution properties such as the double layer thickness or solvent adsorption and that these changes are not affected by the increase in potential (Figure 6a, b). Therefore, by comparing impedance and contact angle measurements, it might be possible to decouple the contributions of the solution properties from the one coming from a conformational change of the films. We also note that the three films all have a higher capacitance and receding angle than a MHA monolayer, in agreement with their low surface coverage. Finally, our results also show that contact angle measurements appear to be more sensitive than the impedance measurements to the potential-induced change in surface chemical functionality brought about by a change in pH. Optimization. By controlling the surface coverage and solution conditions, the potential-induced change in the receding angle can be increased by a factor of 6 (from 5° to 32°, as shown in Figure 7). Indeed the smallest measured change in the receding angle is Δθrec = 5 ± 1° for LDMHA(TEA) observed at the highest surface coverage at pH 3, while we observe Δθrec = 26 ± 1° for the lowest coverage, LDMHA(TPeA) at lowest salt concentration (1 mM, pH 11) and with acetonitrile present (25% v/v). However, the presence of more acetonitrile, for example 50%, does not improve further the change in receding angle as it significantly lowers the surface tension and reduces both advancing/receding and static contact angles. Interestingly, a small increase in positive bias from +0.3 to +0.4 V (vs SCE) seems to facilitate switching and leads to a further increase in Δθrec to 32 ± 1°. A further increase in negative bias, however, does not lead to more change in the receding angle. Though not explored here, a larger change in receding angle might be achievable on modified rough substrates (see, for example, prior work with mixed SAMs46 or polymers47−49). In this context, the measured change in receding angle is fairly large, especially for aqueous solutions.

Figure 7. Change in receding angle taken at different surface coverage and solution conditions. Blue, red, and green columns correspond to LD-MHA monolayers made from TEA, TProA, and TPeA cations, respectively. Columns 1−9 are measurements in 0.1 M KCl at different pH values. Columns 10−12 are measurements in pH 11 KCl solution at different salt concentrations on LD-MHA(TPeA). The last two columns are measurements in 1 mM KCl with 25% acetonitrile, pH 11 solution. The receding angles were measured at two applied potentials −0.1 and +0.3 V, except for the one in the last column at −0.1 and +0.4 V.

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CONCLUSIONS In summary, we combined nonfaradaic EIS with contact angle measurements to investigate the role of solution conditions (pH, composition, salt concentration, and addition of acetonitrile) and surface coverage on the potential-induced conformational change of LD-MHA monolayers. Our experiments demonstrate that large change in the receding angle with applied potential can be achieved in an aqueous environment. We also observe that the maximum change in wetting and ionic permeability occurred for the lowest surface coverage investigated at low salt concentration (1 mM) and in the presence of 25% v/v acetonitrile. In these conditions, a large change in receding angle with applied potential is observed (Δθrec = 32° upon a change in potential of 0.5 V, and Δθrec = 26° upon a change in potential of 0.4 V). We also find that while the addition of acetonitrile enhances the response to the external stimulus, a significant change in receding angles (Δθrec = 20°) is still observed in purely aqueous solution (no acetonitrile). We also observe that the solution pH has no effect on the impedance response (salt concentration of 100 mM) and only has a small effect on wetting properties (salt concentration 10 mM). Finally, we find that the stimuliresponsive wetting properties and capacitance are strongly correlated at positive potential but this correlation disappears at negative potential. Particularly, the largest change in receding angle was achieved for the same film and solution conditions as for the largest change in capacitance. These results show that both contact angle measurement and impedance spectroscopy measurement are sensitive techniques to probe the electrical response of the macroscopic properties of LD-SAMs. However, contact angle measurement appears to be slightly more sensitive and is more direct as it does not involve the interpretation of fitted parameters that can also depend on the solution’s conditions. The large change in receding contact angle, along with the associated large change in CAH, could lead to effective control of surface properties in applications such as controlled adsorption and control of drop motion. 13970

dx.doi.org/10.1021/jp3020996 | J. Phys. Chem. C 2012, 116, 13964−13971

The Journal of Physical Chemistry C

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ASSOCIATED CONTENT

S Supporting Information *

(1) Advancing/receding contact angle measurement setup, (2) Bode plots taken in different solutions for different SAMs, (3) monolayer capacitance and resistance for LD-SAMs with different packing densities in 0.1 M KCl electrolyte solutions at pH 3 and pH 8.5, and (4) impedance measurement of fully packed MHA monolayer. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], phone (410) 516-0113. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is supported by the National Science Foundation under Grant No. CMMI-0748094. We also thank Christian Pick for generating the schematic illustration in Figure 1.

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dx.doi.org/10.1021/jp3020996 | J. Phys. Chem. C 2012, 116, 13964−13971