Microfluidic Surface Titrations of Electroactive Thin Films - Langmuir

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Microfluidic Surface Titrations of Electroactive Thin Films Morgan J. Anderson and Richard M. Crooks* Department of Chemistry, The University of Texas at Austin, 105 East 24th Street, Stop A5300, Austin, Texas 78712-1224, United States S Supporting Information *

ABSTRACT: We report the use of microfluidic surface titrations (MSTs) for studying electroactive self-assembled monolayers (eSAMs) and other thin films. The technique of MST utilizes a microfluidic generation-collection dual channel electrode (DCE) configuration to quantify the charge associated with electroactive thin films that might or might not be in direct contact with an electrode surface. This technique allows for quantitative measurement of surface coverages, Γ, as low as 30 pmol cm−2 for electrodeposited Cu thin films. Additionally, we show that it is possible to quantify Γ for ferrocene (Fc)-terminated alkylthiols in mixed-monolayer eSAMs. Interestingly, MSTs sometimes reveal a two-fold higher eSAM concentration compared to direct electrochemical measurements. This finding suggests that in these instances not all the constituent Fc-moieties of the eSAM are in sufficiently close proximity to the surface to be addressable via direct electrochemistry.

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INTRODUCTION Here, we report the use of microfluidic surface titrations (MSTs) for studying electroactive self-assembled monolayers (eSAMs) and other thin films. The technique of MST utilizes a microfluidic generation-collection dual channel electrode (DCE) configuration to quantify the charge (q) associated with electroactive thin films that might or might not be in direct contact with an electrode surface.1 This technique allows for quantitative measurement of surface coverages, Γ, as low as 30 pmol cm−2 for electrodeposited Cu on indium tin oxide (ITO) electrode surfaces. Additionally, we show that it is possible to quantify Γ for ferrocene (Fc)-terminated alkylthiols in mixedmonolayer eSAMs. Interestingly, MSTs sometimes reveal a two-fold higher eSAM concentration compared to direct electrochemical measurements. This finding suggests that, in these instances, not all constituent Fc-moieties of the eSAM are in sufficiently close proximity to the surface to be addressable via direct electrochemistry. DCE generation-collection systems consist of two coplanar electrodes placed on the floor of a rectangular channel.2 As illustrated in Scheme 1a, the upstream generator electrode (GE) drives an electrochemical reaction. The products of this reaction are carried downstream under laminar flow conditions where the reverse reaction takes place at the collector electrode (CE). The collection efficiency (N) of DCE systems is described by eq 1,. N=−

iLCE iLGE

Scheme 1

catalysis,3,4 homogeneous reaction kinetics,5−8 corrosion,9−14 the characteristics of flow profiles,15−17 and as a platform for selective detection of analytes.18,19 Most importantly, however, it has been shown that microfluidic DCE systems are highly flexible in that they can be easily tuned to access a broad range of mass transfer regimes ranging from thin-layer and pseudo-

× 100% (1)

Received: May 7, 2017 Revised: June 18, 2017

Here, iLGE and iLCE are the mass transfer-limited currents at the GE and CE, respectively. DCEs have been used to study © XXXX American Chemical Society

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DOI: 10.1021/acs.langmuir.7b01542 Langmuir XXXX, XXX, XXX−XXX

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Langmuir thin layer behavior to the high mass transfer Levich regime.20 Indeed, DCEs can surpass the mass transfer coefficients and values of N accessible by the more common rotating ring-disk electrode (RRDE) configuration.1,4,8,20,21 Accordingly, DCE techniques are preferred in many cases due to their versatility, performance, and the low cost of instrumentation (compared to RRDEs). Previously, we showed that by decreasing the volumetric flow rate (VF) in a DCE system to a level where mass transfer is in the pseudo-thin layer regime, it is possible to obtain values of N higher than 97%.1 This is high enough to ensure that nearly all of the electrogenerated products at the GE react at the CE. Under these conditions it is possible to use an appropriate redox mediator to indirectly measure electrochemically active thin films located between the GE and CE, even in the absence of a direct electrical connection to the thin film under study. For example, we previously demonstrated the utility of this technique by measuring electrochemically generated Au2O3 on an unmodified Au substrate electrode (SE) interposed between the GE and CE using the Fe(CN)63−/4− redox couple as the mediator. These studies opened the door to examining other interesting systems and to determining the limit of detection (LOD) of the method. An electrochemical method known as surface interrogation scanning electrochemical microscopy (SI-SECM) is similar to MST.22−24 In SI-SECM, a SECM tip is brought into close proximity with the substrate surface to be analyzed. A redox reaction driven at the SECM tip results in products that diffuse to the substrate and either oxidize or reduce analyte species present thereon. This results in a detectable positive feedback loop, and a corresponding increase in current, that proceeds until all redox-active species confined to the substrate have been consumed. A key advantage of SI-SECM is that the tip and the substrate are in such close proximity that mass transfer is very fast. This makes it ideal for studies of short-lived intermediates confined to surfaces. This technique has recently been used to study adsorbed species on semiconductor photocatalysts and electrocatalysts,25−31 the properties of redox active polymers,32 and oxides on Au, Pt, and Ir.22,33,34 In the experiments reported here we focus on the interrogation of ferrocene-terminated alkylthiol eSAMs. Chidsey and co-workers reported on this class of eSAM in 1990,35,36 and they showed that it is possible to correlate the shift in the redox potential of the ferrocene moiety to the length of the alkyl chain. Subsequently, eSAMs have been extensively studied to better understand SAM structure and functionality,37−39 electrochemical desorption of alkylthiols,40−42 the formation and exchange of alkylthiol SAMs,36,43,44 and fundamental aspects of electron transfer.35,45−50 One conclusion that has emerged from these types of studies is that eSAMs containing exclusively ferrocene-terminated alkylthiols are disordered.49,50 Consequently, mixed monolayers, composed of ferrocene-terminated alkylthiols and non-electroactive, shorter chain alkylthiols, are typically employed to improve ordering within the monolayer.37,38 Additionally, the Schmittel and Plaxco groups have recently demonstrated that ferroceneterminated eSAMs can decompose over the span of just minutes due to the instability of the ferrocenium redox state.51−53 Despite this instability, eSAMs consisting of ferrocene-terminated alkylthiols are considered to be welldefined model systems. In the present article, we report the optimal geometry of the microelectrochemical flow system for MST and describe

guiding principles for lowering its (LOD) for electroactive thin films using electrodeposited Cu thin films on ITO as a model system. Additionally, we show that stable eSAMs can be formed in poly(dimethylsiloxane) (PDMS) microfluidic channels, and that their electrochemical properties can be evaluated by MST. The results indicate that sometimes Γ, as measured by MST, is twice as large as that determined by direct voltammetry. This suggests that not all of the ferrocene moieties within the monolayer are directly addressable by the underlying electrode in these instances.

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EXPERIMENTAL SECTION

Chemicals and Materials. All solutions were prepared using Milli-Q water (18.2 Ω·cm). K3IrCl6 (Sigma-Aldrich), 6-mercapto-1hexanol (MH, 97%, Sigma-Aldrich), 11-(ferrocenyl)undecanethiol (FcUDT, 95% Sigma-Aldrich), KNO3 (primary nitrogen standard, Fisher Scientific), and Cu(NO3)2·3H2O (p.a., Acros) were used asreceived. Glass slides coated with 60−100 nm of indium tin oxide (ITO) were purchased from Delta Technologies, Ltd. (Loveland, CO). Glass slides coated with a 100 nm-thick Au film (no adhesion layer) were purchased from Evaporated Metal Films (Ithaca, NY). AZP 4620 photoresist, AZP 1518 photoresist, AZ 421k developer, and AZ 400k developer diluted 1:4 were purchased from Integrated Micro Materials (Argyle, TX). SU-8 2025 photoresist was purchased from MicroChem (Westborough, MA). Sylgard 184 PDMS kits were purchased from Fisher Scientific. Instrumentation. Electrochemical measurements were obtained using a CH Instruments (Austin, TX) Model 700E bipotentiostat equipped with a Faraday cage. Experiments were performed in either the three-electrode or four-electrode bipotentiostat configuration using a Hg/Hg2SO4 (MSE) reference electrode and a Pt wire auxiliary electrode. All potentials are reported vs MSE. Au-coated glass substrates were evaluated using a Teflon electrochemical cell configured with a Viton O-ring to prevent leakage. Microelectrochemical measurements incorporated an electronically actuated switch interfaced with LabView to allow switching between an upstream flow measurement electrode (FE), and the GE and SE. All microelectrochemical measurements utilized the macro programming function of the potentiostat software interface to ensure reproducible timing of experiments. Glass/ITO and glass/Au electrodes were fabricated by standard photopatterning techniques (see the Supporting Information, SI).54 Devices were designed such that the separation between the downstream edge of the GE and the upstream edge of the CE was 800 μm. The SE was always centered between the downstream edge of the GE and upstream edge of the CE. This design was chosen to eliminate variations arising from spatial inconsistencies during device optimization. Additionally, an FE was patterned upstream of the other electrodes such that the distance from the downstream edge of the FE to the upstream edge of the CE was 2.50 mm. Flow Rate Measurements. VF was measured using a previously described electrochemical technique.1,15 Briefly, the upstream FE was pulsed from −0.20 to 0.60 V to oxidize IrCl63− to IrCl62−, while the CE was poised at −0.20 V. IrCl62− was carried to the CE by convection where it was reduced back to IrCl63−, thereby producing a detectable increase in the current at the collector electrode (iCE). The iCE continued to rise until it reached its mass-transport limited value. VF was then determined by measuring the time required for the iCE to reach 10% of the mass-transport limited value relative to the initial pulse at the GE. Microelectrochemical Measurements. The assembly of the microelectrochemical device is described in the SI. Briefly, it consisted of a PDMS monolith, having a 6 mm-long channel and 8.0 mmdiameter inlet and outlet reservoirs punched at each end. After mating the PDMS block to the glass/electrode substrate, the channel was filled with water until ready for use to maintain its hydrophilicity. For MSTs of eSAMs, 300 and 50 μL of the redox probe solution were B


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Langmuir injected into the inlet and outlet reservoirs, respectively, and an MSE reference electrode and Pt wire auxiliary electrode were inserted into the outlet reservoir. For device optimization, the value of VF is critical for obtaining reproducible MST measurements. Consequently, the inlet volume was adjusted by adding or removing solution until a VF of 330 ± 10 nL min−1 was achieved. SAM Preparation. Three types of Au substrates were used to prepare SAMs: macro disk electrodes, macro Au-coated glass slides, and on-chip Au microband electrodes. Macro disk electrodes were polished sequentially with 1.0 μm, 0.3 μm, and 0.05 μm alumina slurries. Mechanical polishing was followed by electrochemical cleaning scans, which consisted of cycling the electrode 10 times between −0.20 and 1.0 V in 0.10 M H2SO4. Au-coated glass slides and on-chip Au microband electrodes were not mechanically polished, but they were cleaned electrochemically as described above. SAMs on Au microband electrodes were formed by placing 300 and 50 μL of a 1.0 mM ethanolic thiol solution in the inlet and outlet reservoirs, respectively, and then covering the reservoirs with a glass slide to prevent evaporation. After SAM modification of the electrodes, the channel was rinsed with ethanol and subsequently filled with electrolyte solution. FcUDT/MH mixed SAMs were formed by first treating the Au substrate with 1.0 mM FcUDT, rinsing with ethanol, backfilling with 1.0 mM MH, and then rinsing again. SAM thicknesses were determined using a spectroscopic ellipsometer (J.A. Woollam M2000 model, Lincoln, NE) and Au-coated glass slides as proxies for the SE electrode in the microelectrochemical device. The electrochemically active surface area (AEC) of all Au substrates was determined by integrating the Au2O3 reduction peak of the final cleaning scans, and then dividing the resulting charge by the widely accepted conversion factor (390 μC cm−2) used for converting charge to AEC.55,56 Finite-Element Simulations. Modeling was performed using the COMSOL v4.3b commercial package and a Dell Precision T7500 workstation equipped with dual six core Intel Xeon Processors (2.40 GHz) and 24GB of RAM. A discussion of the simulation details is provided in our previous report.1 Briefly, simulations were performed in 2D, independently solving for fluid dynamics and diffusion of analyte. Fluid dynamics modeling assumed laminar flow and a steadystate (i.e., a fully developed) flow profile. For calculations of mass transfer, the Butler−Volmer kinetic model was used to describe the flux of molecules at the surface of the electrode.

(Figure S1). Flow was initiated by loading a differential height of solution into the reservoirs at either end of the microchannel. Specifically, a solution containing 0.20 mM Cu(NO3)2, 0.20 mM K3IrCl6, and 0.10 M KNO3 was injected into the inlet and outlet reservoirs, respectively, and then slight adjustments to these volumes were made until a VF of 330 ± 10 nL min−1 was achieved. The procedure for measuring VF is discussed in the Experimental Section. The first step for the MST of Cu is to reduce Cu2+ to zerovalent Cu metal on the SE by stepping its potential from 0.40 to −0.60 V. Next IrCl63− is oxidized to IrCl62− (species R and O, respectively, in Scheme 1b) by stepping the GE from −0.20 to 0.50 V. The MST can also be carried out using a potential sweep, rather than a potential step, to oxidize IrCl63−, and this alternative approach is discussed in the SI (Figure S2). Regardless of the method used to generate IrCl62−, it is carried downstream by PDF until it encounters the electrodeposited Cu metal (A in Scheme 1b) on the SE (the reaction scheme is shown in Figure S3a). At this point, the reaction shown in eq 2 occurs spontaneously due to the potential difference between the two relevant half reactions: ΔE° = 0.525 V.57 2IrCl 6 2 − + Cu → 2IrCl 6 3 − + Cu 2 +

(2)

2−

The IrCl6 that does not undergo the reaction in eq 2 is reduced to IrCl63− when it reaches the CE, which is held at a constant potential of −0.20 V for the duration of the experiment. The criteria required for an appropriate redox probe (e.g., IrCl63−) are important and are discussed in the SI (Figure S4). Figure 1a shows plots of iCE as a function of time in the presence (red trace) and absence (black trace) of a Cu thin film on the SE. Consider the black trace first. At t = 0, which corresponds to the time at which the GE is pulsed from −0.20 to 0.50 V to initiate oxidation of IrCl63−, iCE = ∼0 nA. As the measurement proceeds, however, iCE rises as IrCl62− begins to arrive at the CE. The initial rise in iCE is observed at 1.4 s. After 3.7 s, iCE achieves a steady-state, mass transfer-limited value that is determined by N and the GE current. The red trace is a plot of iCE vs t when Cu is present on the SE. In this case, the initial rise in iCE is observed at 2.9 s and steady state is not achieved until 6.6 s. This offset in t is due to oxidation of the intervening Cu thin film by IrCl62−. Note that IrCl63− is electroinactive at the potential set at the CE, and therefore it does not contribute to iCE. The difference between the iCE transients in Figure 1a is presented in Figure 1b as a plot of net current vs time. Here, a well-defined peak is observed, and the area beneath it corresponds directly to the q associated with the amount of Cu originally deposited onto the SE. The value of q measured by MST (119 ± 2 nC) is in good agreement with the value measured by cyclic voltammetry (118 ± 15 nC) for several nominally identical films. To begin the process of optimizing the geometry of the device to minimize the LOD of the method, we first examined the effect of varying the length of the GE using fixed lengths of the SE and CE (250 and 500 μm, respectively). To carry out these experiments, 1.3 ± 0.2 pmol of Cu was electrodeposited onto the SE by stepping the potential from 0.40 to −0.60 V for 5.0 s. Next, the Cu titration measurement was performed, and then the background scan was obtained after all Cu was removed from the SE. The resulting background-subtracted iCE transients are shown in Figure 1c. The titration curve obtained using a device having a 50 μm GE is broad, exhibits a flat response for ∼5 s in the middle of

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RESULTS AND DISCUSSION Optimization of MST by Titration of a Cu Thin Film. The ultimate goal of this project is to detect the smallest possible number of electroactive species present on a surface. Moreover, the surface may be an insulator or a conductor, and if the latter the electroactive species may or may not be within electron-transfer distance of the underlying electrode. Accordingly, our first step was to optimize the microelectrochemical system for maximum sensitivity. For example, a key parameter for obtaining quantitative MSTs is a high value of N.1 We previously showed that N values >97% can be achieved by increasing the length of the CE and lowering VF,1 but other key parameters have not thus far been optimized. Consequently, in our previous report the lowest value of Γ that could be measured was 1.2 nmol cm−2.1 The present device configuration is shown in Scheme 1b. It consists of a PDMS channel having a width of 500 μm and a height of 17 μm. All electrodes are fabricated from ITO. The lengths of the GE and SE range from 50 to 500 μm and the length of the CE is 500 μm. Previously, we used a mechanical syringe pump to induce flow of electrolyte solution through the channel. However, that resulted in a high level of noise, and therefore gravity-induced, pressure driven flow (PDF) was used in the present study. The relative noise levels associated with pump- and gravity-induced PDF are discussed further in the SI C


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Figure 1. Experimental and simulated MST data obtained using Cu thin films. (a) iCE transients measured with a device having ITO microband electrodes. The transients were obtained by holding the CE at −0.20 V while the GE was stepped from −0.20 to 0.50 V in a solution containing 0.20 mM K3IrCl6, 0.20 mM Cu(NO3)2, and 0.10 M KNO3. The black trace is a background transient obtained with no Cu present on the SE. The red trace shows the response when Cu was previously deposited on the SE by stepping its potential from 0.40 to −0.60 V for 10.0 s. (b) The difference transient obtained by subtracting the red trace from the black trace in (a). (c) Background-subtracted iCE transients obtained using microelectrochemical devices having GE lengths varying from 50 to 500 μm and fixed SE and CE lengths of 250 and 500 μm, respectively. (d) Background-subtracted iCE transients obtained using microelectrochemical devices having SE lengths varying from 50 to 500 μm, and fixed GE and CE lengths of 250 and 500 μm, respectively. (e) Finite-element simulations analogous to the experimental results in (c). The SE was assumed to have a surface concentration of Cu equivalent to q = 257 nC. (f) Finite-element simulations analogous to the experimental results in (d). In accordance with experimental results, the SE is assumed to have values of q for Cu ranging from 92 nC for a 50 μm SE to 284 nC for a 500 μm SE.

and width of the peak increase due to the increased amount of exposed Cu. In other words, more IrCl62− is required to titrate the larger surface area substrate, and this in turn means more time is required to complete the titration. This longer time is manifested as a broader peak. The total amount of Cu titrated varies from 0.48 ± 0.03 pmol for a 50 μm-long SE to 1.5 ± 0.1 pmol for a 500 μm-long SE. On the basis of these findings we conclude that when the SE is lengthened, a higher, narrower peak results. As mentioned earlier, this shape is easier to integrate, and therefore should lead to a lower LOD than a shorter, broader titration curve. An SE length of 250 μm yields the highest peak height and consequently this is the optimal length for MST. Simulations were performed to confirm the experimental findings. The simulations in Figure 1e,f correspond to the experimental results in Figure 1c,d, respectively. Specifically, the GE length was varied in the simulations shown in Figure 1e, and the results are in near-quantitative agreement with the experiments (Figure 1c) with respect to the characteristics of the peaks: as the GE length increases, the peak width decreases and the peak height increases. The simulations in Figure 1f were obtained as a function of the length of the SE. These results relate to the net current transients shown in Figure 1d, and although the correspondence is not quite as good as that found for the GE length, there is still good qualitative agreement with the experiments. The important conclusion is that the close correlation of the experimental and simulated results indicates that the flow and electrochemical characteristics of the microfluidic system are well-defined. For surface titrations, the dynamic range over which Γ can be determined is an important parameter. Accordingly, we

the experiment, and is characterized by relatively low current. As the GE length is increased to 100 and 250 μm, the height of the peak increases in magnitude and the width decreases. This change in peak shape occurs because a larger fraction of the total inflowing IrCl63− is oxidized as the GE is lengthened. This results in an increase in the GE current and a corresponding increase in the flux of IrCl62− to the SE. As the GE is further lengthened to 500 μm, there is only a 14% increase in peak height compared to the trace obtained with a 250 μm GE. From these experiments, we conclude that increasing the GE length to the point where it operates in a thin-layer, mass transfer regime, where essentially all inflowing IrCl63− is oxidized, leads to a higher, narrower current peak. Peaks with this type of high aspect ratio are easier to integrate than shallower peaks, and therefore this shape is desirable and leads to lower LODs. The geometry of the device was further optimized by varying the SE length from 50 to 500 μm while maintaining fixed GE and CE lengths of 250 and 500 μm, respectively. The Cu thin film was electrodeposited onto the SE as described previously, but because the deposition time was fixed at 5 s the amount of Cu deposited scaled with the length of the SE. After Cu deposition, the titration experiment was performed, followed immediately by a background scan. The resulting titration curves are shown in Figure 1d. The titration for a device having a 50 μm-long SE exhibits a narrow, shallow peak. As the SE length is increased to 100 μm the peak height doubles but the width stays approximately the same because the amount of Cu deposited varies with SE length. That is, the total q under the transient increases due to the additional Cu. As the SE length is further increased to 250 and 500 μm, the height D


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the SAMs on Au macro disk electrodes was evaluated. These experiments were carried out by immersing the SAM-modified electrodes in a 0.10 M HClO4 solution and cycling them three times from −0.40 to 0.35 V at a scan rate (ν) of 50 mV s−1. The cycling was repeated at 4 min intervals over the course of 3 h. Figure 3a shows the first and last cyclic voltammograms (CVs) for a FcUDT/MH eSAM. The initial scan (t = 0) reveals Gaussian-shaped oxidation and reduction waves having current peaks at −0.060 and −0.078 V, respectively (ΔEP = 18 mV). After scanning for 3 h, only slight differences are observed in the shape of the curves, the q associated with both the anodic and cathodic waves is unchanged (1.4 μC), and the capacitance is also the same. On the basis of these observations we conclude that mixed FcUDT/MH eSAMs are more stable and better behaved as compared to recent reports of singlecomponent Fc-terminated eSAMs.51,52 The FcUDT/MH-modified electrodes were further characterized by cycling from −0.40 to 0.35 V over the range of ν = Figure 2. (a) Background subtracted iCE transients obtained in a channel having 250 μm GE and SE, and a 500 μm CE. Transients were obtained by depositing Cu on the SE by stepping the potential from 0.40 to −0.60 V for the indicated deposition times in a solution containing 0.20 mM K3IrCl6, 0.20 mM Cu(NO3)2, and 0.10 M KNO3 and then stepping the GE to from −0.20 to 0.50 V while the CE is held at −0.20 V. (b) Charge (q) and surface coverage (Γ) as a function of time corresponding to the transients in (a). The red line is the best linear fit (R2 = 0.99) for t > 2.5 s data points. The error bars represent triplicate measurements obtained using the same microelectrochemical device.

examined Γ as a function of Cu deposition time. Figure 2a shows a series of representative titration curves obtained using a device having GE and SE lengths of 250 μm and a CE length of 500 μm. As the Cu deposition time increases from 0 to 10 s, the peak height increases due to the higher coverage of Cu on the SE. As the deposition time is further increased to 15 s, however, the peak height decreases slightly and a tailing shoulder is observed. These shape differences may be related to different morphologies of the electrodeposited Cu.58 Figure 2b is a plot of q, measured under titration curves like those shown in Figure 2a, as a function of the Cu electrodeposition time. This plot is linear for deposition times >2.5 s, which is expected for constant potential electrodeposition under mass transfer-limited flow conditions.59 For deposition time ≤2.5 s the amount of Cu deposited onto the electrode surface is roughly constant. This observation may suggest that Cu deposition under these conditions proceeds by a nucleation-dominated growth mechanism.58,60,61 The LOD for the Cu MST can be estimated from the data in Figure 2b and eq 3. Taking the background q at t = 0 s to be q = 0 ± 1.4 nC, the LOD is 4.3 nC (Γ = 17.9 pmol cm−2). The lowest q actually measured in this study was 7.3 ± 2.1 nC (Γ = 30.3 ± 9.0 pmol cm−2). These values of Γ are 2 orders of magnitude lower than in our previous report (1.2 nmol cm−2),1 suggesting that the optimization procedure was effective. LOD = q ̅ blank + 3σqblank

Figure 3. (a) Initial (t = 0) and final (t = 3 h) CVs for FcUDT/MH eSAMs obtained using a macro Au disk electrode. CVs were obtained periodically during the 3 h duration of the experiment with each successive CV starting 4 min after the conclusion of the previous scan. The scan rate (ν) was 50 mV s−1. (b) CVs of a FcUDT/MH eSAM on a macro Au disk electrode obtained at the indicated values of ν. (c) Plot of anodic and cathodic peak currents for the data shown in (b). The solid lines represent the best linear fits of the data points (R2 = 0.99). In all cases the electrolyte contained 0.10 M HClO4.

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Voltammetric Evaluation of FcUDT/MH eSAMs. We used FcUDT/MH SAMs as a model system to judge the effectiveness of MST for titrating organic thin films. Before commencing with MST measurements, however, the stability of E


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Langmuir 25 to 500 mV s−1 (Figure 3b). These CVs also exhibit Gaussian-shaped oxidation and reduction waves. Figure 3c reveals that there is a linear relationship between peak height (iP) and ν (R2 ≈ 1.00), which is characteristic of surfaceconfined electroactive thin films.59 In addition to studying the properties of FcUDT/MH SAMs on Au macro disk electrodes, we also examined the same SAMs on macroscopic Au-coated glass slides. This was done for two reasons. First, this type of electrode is a closer analog of the evaporated Au microband electrodes used in the microelectrochemical MST devices. Second, due to the larger size of these substrates, the SAMs can be more thoroughly characterized than on the smaller electrodes. For example, in addition to electrochemical measurements, the thicknesses of the eSAMs can be determined by ellipsometry. The results of these studies are discussed in detail in the SI. The important point is that the results are consistent with previous literature results.62−64 MST of FcUDT/MH eSAMs. To evaluate the utility of MST for quantification of Γ for eSAMs, FcUDT/MH monolayers were assembled onto the SE microbands in the microelectrochemical cell as described in the Experimental Section. Note, however, that during this process the eSAMs also adsorb to the other electrodes in the cell, but those monolayers are removed electrochemically by cycling the electrode potential 10 times between −0.20 to 1.00 V in 0.10 M H2SO4.42,65,66 Removal of the eSAMs from these other electrodes permits unhindered electrochemical reactions to occur thereon. H2SO4 was then removed from the cell and replaced with a solution containing 0.20 mM K3IrCl6 and 0.10 M HClO4. Once in place, the eSAM remaining on the SE was characterized by CV. The SE was cycled from −0.35 to 0.15 V to measure Γ and ensure that all FcUDT was in the reduced form. The CV for a FcUDT/MH eSAM prepared in this way (Figure 4a) is very similar to the one shown in Figure 3a for a macroscopic Au electrode: the shape is roughly Gaussian and ΔEP = 14 mV. The value of q obtained after baseline subtraction and integration of the reduction wave is 54.0 nC, which corresponds to Γ = 289 pmol cm−2. After determining Γ by the well-known cyclic voltammetric method, we measured exactly the same eSAM by MST. "(Scheme 1b, where A is the Fc-terminated eSAM, also see Figure S3b.) This was done by stepping the GE from −0.20 to 0.50 V while holding the CE at −0.20 V. The resulting iCE transient, obtained after appropriate background subtraction, is shown in Figure 4b. The value of q was obtained by integrating this peak and found to be 56.2 nC, which corresponds to Γ = 301 pmol cm−2. This value can be compared to that obtained by integrating the CV in Figure 4a: 289 pmol cm−2. Figure 4c compares Γ measured by MST and CV for three independently prepared devices and eSAMs. The maximum deviation between pairs of independent measurements is ±7.5%, and the average Γ values measured by MST and CV are 320 ± 38 pmol cm−2 and 327 ± 25 pmol cm−2, respectively. From these data, we conclude that the two methods are in agreement at the 95% confidence interval based on the paired ttest. Several control experiments were performed to ensure that no artifacts were present in the MST measurements. These experiments included MSTs of naked SE microbands and SE microbands modified with MH only. Additionally, we prepared a microelectrochemical device that had no Au SE, but then went through the procedure for forming the mixed eSAM to

Figure 4. CV and MST data for FcUDT/MH-modified Au microband electrodes. (a) Representative voltammogram (ν = 60 mV s−1) and (b) titration curve for FcUDT/MH eSAMs. (c) Histograms showing the relationship between Γ for three independent MST and CV measurements obtained using three different devices. All measurements were obtained using a solution containing 0.20 mM K3IrCl6 in 0.10 M HClO4.

demonstrate that this process does not interfere with the other electrodes. These experiments are described in the SI (Figure S5). The interesting result is that MSTs carried out on naked Au electrodes exhibit a background signal on the order of 22% of a typical eSAM experiment of the type represented by Figure 4b. This is likely due to partial oxidation of the Au surface by IrCl62− electrogenerated at the GE. The formation of an oxygenated Au surface has been reported to occur at potentials as low as 0 V,22 which is lower than the EO of IrCl62− reduction (0.23 V). The measured level of background charge corresponds to about 0.02 monolayers of Au2O3. When the SE is coated with a monolayer of electrochemically inactive MH, a smaller background persists: ∼52% of the background signal of the naked Au SE. We ascribe this lower background signal to pinholes in the MH monolayer, which is short (∼1.0 nm)62 and known to be defective.67 Because of the excellent agreement between the CV and MST measurements of the eSAMs, we do not think the effects of these control experiments are important for longer SAMs. The results discussed thus far are typical of our findings and in good agreement with prior literature.37,68 However, we did observe some anomalies. For example, ∼20% of the nine experiments we carried out revealed significantly higher values of Γ for FcUDT/MH eSAMs than were obtained by CV. One such case is described in detail in the context of Figure S6. F


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Langmuir Here, an eSAM was prepared on the SE and evaluated by CV (Γ = 633 pmol cm−2) and MST (Γ = 1143 pmol cm−2). Note that the maximum reported monolayer coverage of a FcUDTonly eSAM is 460 pmol cm−2.43 The large discrepancy between these three coverages suggests that in some cases multilayers of the eSAM formed on the Au surface.69,70 We hypothesize that in these instances not all Fc-functionalities are accessible to the SE surface, and consequently they cannot be detected by direct electrochemistry (i.e., CV). This important result highlights the fact that MST is capable of electrochemically addressing molecules that are not in direct contact with an electrode surface, which of course is where its value lies.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (R.M.C.). ORCID

Richard M. Crooks: 0000-0001-5186-4878 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy (Contract: DE-FG02-13ER16428). We thank the Robert A. Welch Foundation (Grant F-0032) for sustained support of our research.

CONCLUSIONS Here, we have shown that MST can be used to detect electroactive species at surface concentrations as low as 30 ± 9 pmol cm−2. Moreover, the experimental results are in good agreement with expected trends based on finite-element simulations. Interestingly, we found that in some cases more than a single monolayer of the thiolated ferrocene derivative forms on the electrode surface even after extensive rinsing. In these cases normal electrochemical methods, such as CV, do not accurately report the total coverage. This is presumably because some of the electroactive groups are not within tunneling distance of the electrode. This latter requirement is relaxed in the MST method. The value of MST is two-fold. First, any electroactive surface layer that can be prepared on a glass or quartz substrate can be evaluated by MST. This includes films such as those made by the Langmuir−Blodgett and layer-by-layer methods.71−78 Second, thin films on macroscopic, planar substrates are easily characterized using analysis tools like electron microscopy and X-ray photoelectron spectroscopy before or after MST.79,80 Therefore, it is possible to obtain a complete chemical and structural picture of a film that can be combined with electrochemical MST measurements. Other methods used for electrochemical surface titrations, such as SI-SECM, are limited in terms of the methods used for synthesizing the film and the extent to which films can be characterized by nonelectrochemical methods. This is a consequence of the complex geometry of the electrode surface.24 The results described here lay the groundwork for forthcoming studies of low Γ surface adsorbed species, such as films containing electroactive catalysts and large biological redox molecules like proteins. In these cases, Γ typically ranges from 1 to 10 pmol cm−2. Moreover, the electroactive groups in such films are often too far from the electrode surface for methods, like CV, that require direct electron transfer.81−83

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eSAMs; and an example of an anomalously high q arising from an FcUDT/MH eSAM (PDF)

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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.7b01542. Details of the device fabrication; MST results obtained using pump- and gravity-driven flow; comparison of MST results obtained for Cu thin films using CV and chronoamperometry; discussion of potential redox probe titrants; illustration showing the specific MST reactions used in this work; CVs of K3IrCl6, Cu(NO3)2, and FcUDT/MH eSAMs; ellipsometry of Au-coated glass substrates; control experiments for MST of FcUDT/ G


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Microfluidic Surface Titrations of Electroactive Thin Films - Langmuir

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