Quantifying the Selective Modification of Au(111) Facets via

Oct 23, 2017 - A related procedure was developed for the solution-based electroless removal of the SAM using NaBH4 achieving a similar selectivity at ...
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Quantifying the Selective Modification of Au(111) Facets via Electrochemical and Electroless Treatments for Manipulating Gold Nanorod Surface Composition Elizabeth A. Fisher,†,‡ Kaylyn K. Leung,†,‡ Jannu Casanova-Moreno,†,‡ Tamiko Masuda,§ Jeff Young,†,§ and Dan Bizzotto*,†,‡ †

AMPEL, ‡Department of Chemistry, and §Department of Physics and Astronomy, University of British Columbia, Vancouver, BC V6T 1Z4, Canada S Supporting Information *

ABSTRACT: Manipulating the composition of a mixed alkylthiol self-assembled monolayer (SAM) modified gold surface using both electrochemical and electroless methods is demonstrated. Through the use of fluorophore labeled thiolated DNA and in situ fluorescence microscopy with a gold single crystal bead electrode, a procedure was developed to study and quantify the selective desorption of an alkylthiolate SAM. This method enabled a self-consistent measurement of the removal of the SAM from the 111 surface compared to the 100 surface region at various potentials. A 20-fold increase in the electrochemical removal and replacement of the SAM from the 111 surface over the 100 surface was realized at −0.8 V/AgAgCl. A related procedure was developed for the solution-based electroless removal of the SAM using NaBH4 achieving a similar selectivity at the same potential. Unfortunately, in the electroless process fine control over the reducing potential was difficult to achieve. In addition, working in the presence of O2 complicates the solution potential measurement due to depolarization by the reduction of O2, resulting in a less clear relationship between selectivity and measured solution potential. Interestingly, the electrochemical method was not disturbed by the presence of O2. In preparation for work with Au nanorods, electrochemical measurements were performed in electrolyte that included 1 mM CTAB and was found to not interfere with this method. Preliminary results are promising for using this methodology for treatment of acid-terminated alkylthiol modified Au nanorods.

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INTRODUCTION

charge (pzc)) of the various metal surfaces in addition to the intermolecular interactions of the adsorbate molecules, the solubility of these molecules, and the competition of the produced thiolate with the electrolyte for the free metal surface.12 Electrochemical potential has also been used to remove the SAM from specific surface regions which were then backfilled with a different thiol. The resulting surface was analyzed using lateral force AFM which distinguished the shortand long-chained alkylthiolates.5,6 In addition to reductive desorption of alkanethiolate SAMs, oxidative desorption has been demonstrated, however, with less control over the

Control over the preparation of mixed component alkylthiolate self-assembled monolayers (SAMs) via electrochemical treatment has been demonstrated by manipulating the substrate potential during SAM deposition1−4 and via the reductive desorption of a portion of the monolayer.5−11 This is possible since the potential of SAM reductive desorption is sensitive to the substrate surface crystallography. Removal from Au(111) occurs at the least negative reduction potential while a more negative potential is required for more open surfaces (e.g., 210). This was demonstrated using single crystal electrodes12 as well as with a single crystal gold bead electrode using fluorophore labeled alkylthiolate SAMs and in situ fluorescence microscopy.13 This difference can be rationalized when considering the relative work functions (or potential of zero © XXXX American Chemical Society

Received: August 30, 2017 Revised: October 19, 2017 Published: October 23, 2017 A

DOI: 10.1021/acs.langmuir.7b03021 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. (a) Bright-field image of the single crystal bead electrode used in this work with the 111 and 100 regions outlined. (b) Schematic representation of the controlled removal of a MUA SAM and replacement with thiolated DNA bearing a fluorophore.

specificity of the surface feature removed.14 Control over the surface coverage of thiolated DNA on gold was also demonstrated by reductive desorption in the presence of a short chain alkylthiol.7,9 The electrochemical reductive desorption approach for surface modification presented here could be adapted to achieve a similar controlled modification of alkylthiolateterminated gold nanoparticles (AuNP) or nanorods (AuNR), thereby addressing the challenge in specific modification of nanoparticles as outlined by Murphy.15 This may be possible through careful control of the reductive environment via interaction with immersed electrode surfaces.16,17 Precisely controlling the potential of nanostructures still presents a significant challenge since complete and reproducible modification of a solution of NPs is challenging when using electrodes.16 The development of a solution chemistry method for controlling NP potential can be accomplished through the use of a chemical reducing agent (so-called electroless approach). Under the right conditions, removal of the ligands from specific crystallographic regions can be achieved. Experimentally proving such changes have occurred is also challenging,15 so a well-defined set of control experiments are required which are realized in our work through the use of single crystal gold bead electrodes. The reductive desorption of alkylthiolate monolayers from gold substrates usually requires significantly negative (or reducing) potentials (∼−1 V/SCE).8,12,18,19 Sodium borohydride (NaBH4) has been reported to efficiently remove alkanethiolate SAMs from gold surfaces20 as well as from AuNPs21 where the authors suggest that the alkanethiolate is displaced by hydride adsorption, ignoring the possible BH4− oxidation driven reductive desorption of the SAM. BH4− oxidation is described by many reactions, and the mechanism is generally not well understood in aqueous solution.22,23 At basic pH (∼14), BH4− undergoes an eight-electron oxidation which is complicated by the pH-dependent hydrolysis of BH4−, generating H2 and BH3OH− which can also hydrolyze to form B(OH)4−. BH3OH− is also reported to be oxidized at a more negative potential than BH4− (more details provided in the Supporting Information).23 A metal electrode (or nanoparticle) in a solution containing a redox couple, at equilibrium, will adopt the potential of the solution (vs some reference electrode) as defined by the Nernst equation and the relative concentrations of the reduced and oxidized species.24,25 If more than one redox couple is present in solution, then a mixed potential will define the solution potential.26 The relative concentrations of BH4−, BH3OH−, and H2 in solution will dictate the potential of the solution as well as that of any immersed electrode or metal particle at electrostatic equilibrium. However, due to the unpredictable nature of the borohydride reactions, controlling the relative concentrations,

and by extension the solution potential, is challenging without direct measurement. This makes the electroless process envisioned more complex and the optimum conditions must be defined. In this contribution, the conditions for the controlled and specific removal of a MUA SAM from only the Au(111) facets on a single crystal gold bead surface are demonstrated using both electrochemical and electroless reduction conditions. The approach is shown schematically in Figure 1. The extent of desorption from the Au(111) facet is evaluated using capacitance measurements and by modifying the regions desorbed with a fluorescently labeled DNA alkylthiolate, enabling spectroelectrochemical study of the resulting mixed SAM surface. The specificity of the desorption from the 111 surface is determined by comparison with that from the 100 surface on the same electrode surface, ensuring an accurate characterization of the selectivity. In this way, the electrochemical and electroless methods are directly compared and the conditions which result in the same surface modification are determined. This electroless approach can be used to define the conditions under which Au NRs could be specifically modified so as to label only the Au(111) regions of the surface, which generally form the ends of the rods.15,27,28 Furthermore, the extent of modification can also be determined resulting in a high level of control over the local surface concentration. This method can also be used to evaluate the selectivity and degree of replacement in the complex solution compositions used with aqueous dispersed AuNR (e.g., the need for surfactants and low ionic strength buffers).

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MATERIALS AND METHODS

Gold Bead Electrode Preparation. Surface studies were performed on macroscopic monocrystalline gold beads hich were made in a manner similar to that described by Yu et al.13 (see Supporting Information). Immediately prior to each experiment, the Au beads cleaned with flame annealling for 30 s followed by rinsing with Milli-Q water (18.2 MΩ·cm, toc 20 μF cm−2), as expected.8 The capacitance of the modified electrode C

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gold electrode, indicating that the MUA has been entirely removed from the electrode surface. The partial reductive desorption of the MUA-modified gold bead electrodes was performed at a constant potential chosen from within the range of potentials where the SAM begins to become disrupted as indicated by the increase in capacitance. These electrochemical treatment potentials (Etreat) were each applied for 5 min, during which capacitance measurements were performed (Figure 3). As expected, the MUA SAM capacitance changes little for Etreat more positive than −0.700 V, indicating that the MUA SAM is not strongly perturbed at these potentials (Figure 3a) even after 5 min. For Etreat = −0.750 V (Figure 3b), a small but noticeable increase in capacitance is observed for the three electrodes studied, indicating that some MUA is removed from the electrode surface, likely from the 111 facets. A larger increase is measured for Etreat = −0.775 V (Figure 3c). Desorption of the MUA SAM at these potentials was not observed in the scanning potential measurements. The reductive desorption process is known to start at nucleation sites (e.g., defective regions in the SAM) which grow in size with time, explaining the sigmoidal changes in the capacitance.18 Since capacitance is an average measurement over the whole electrode surface, the changes observed cannot be conclusively assigned to the 111 facets since they may be due to desorption from other regions of the electrode surface as well. The sample-to-sample variation shown in Figure 3 is likely the result of small differences in the initial quality of the MUA SAMs. The rate of reductive desorption is dependent on the treatment potential (Figure 3d−f) where the time to reach a constant capacitance decreases with more negative Etreat. A comparison of the capacitance measured using the linear potential scan and the capacitance after these 5 min potential treatments is shown in Figure 2b. In addition, the capacitance from potential step scans where the potential is stepped negatively pausing at each step for a fixed time period is also shown in Figure 2b (the capacitance with the applied potential waveform is shown in Figure S1). The linear potential scan is comparable to the 15 s potential step scan while an increased

Figure 2. (a) Capacitance of a MUA-modified gold bead electrode (black) and a clean gold bead electrode (red) in 10 mM phosphate buffer (pH = 8) as the potential was scanned at a rate of 5 mV/s from 0 to −1.4 V vs Ag|AgCl. (b) Capacitances of the potential step scan for MUA-coated gold bead electrodes, a 5 mV/s linear scan of a MUAcoated gold bead electrode, and the capacitances measured after 5 min at various Etreat taken from Figure 3. The arrow indicates the scan direction.

increases sharply at −0.85 V, indicating the start of reductive desorption. It reaches a plateau, increasing again at −1.0 V where complete desorption begins. The gold bead electrode has 111 facets which occupy ∼20% of the surface. Since desorption from the 111 surface begins at the least negative potentials, this initial change is characteristic of removal from the 111 surfaces on the gold bead electrode. Near −1.2 V the capacitance of the MUA-modified electrode closely resembles that of the clean

Figure 3. Capacitance measured during the electrochemical potential treatment of MUA-coated gold bead electrodes. The capacitance-time results for various Etreat shown are (a) −0.725, −0.7, −0.65, −0.6, and −0.4 V, (b) −0.75 V, (c) −0.775 V, (d) −0.8 V, (e) −0.825 V, and (f) −0.85 V vs Ag| AgCl. Individual traces represent separate measurements performed on different bead electrodes. D

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Figure 4. 111/100 fluorescence intensity ratios of the mixed MUA/HSC6-DNA-AF488 SAMs. The y-error bars indicate the standard deviation at each Etreat, and the x-error bars show the highest and lowest OCP measured for the control experiments. The selected fluorescence images (1.56 mm × 1.56 mm) are of various bead electrodes following the application of Etreat (labeled a−d corresponding to the images) and HSC6-DNA-AF488 backfilling. Each image was taken in the spectroelectrochemical cell under an applied potential of −355 mV vs Ag|AgCl. The false color represents the fluorescence intensities shown on a log scale.

measurements at −0.355 V (−0.4 V/SCE). The fluorescence images of the beads are shown in Figure 4 for selected Etreat. Surface crystallography was indexed based on our previous work13 with the 111 and 100 surfaces specifically analyzed. The intensities of these regions were measured as detailed in the Methods section and in the Supporting Information. An intensity ratio for the two surfaces of interest was calculated and compared in Figure 4 as a function of the Etreat. Control measurements were also performed where the MUA-coated bead was immersed in the electrolyte used for Etreat but without applying a potential. The bead was also immersed in the DNA solution, and the resulting mixed SAM is shown in Figure 4d, revealing minimal thiol exchange. The 111 and 100 surfaces are both slightly modified, resulting in a ratio close to one for these control experiments. The same results were obtained for Etreat of −0.40, −0.60, −0.65, and −0.70 V, indicating that these potentials do not disrupt the MUA SAM. At more negative potentials, the 111 surface becomes increasingly covered with DNA, illustrating the selective removal of MUA with these potential treatments. The selective desorption of the 111 surface as compared to the 100 surface reaches a maximum at −0.8 V. This ratio decreases at more negative Etreat since the 100 surfaces are now becoming increasingly fluorescent while the 111 surface is at a maximum DNA coverage. Interestingly, the regions around the 111 facets show a slightly different behavior than the facet. In this region, the MUA seems to be desorbed at a slightly more positive

time at the applied potential results in an increased capacitance that eventually reaches a constant value. The capacitances for all Etreat positive of −0.775 V are independent of the time spent at the potential, indicating the MUA SAM stability. Capacitance represents an average measure of the entire electrode surface and is useful to indicate MUA desorption, but it is not capable of determining the regions (i.e., the surface crystallographies) from where the MUA was removed. In order to visualize the extent of removal of the MUA SAM, the beads were removed from the electrochemical cell after E treat application and immersed into a solution containing HSC6DNA-AF488. Adsorption of the fluorophore-labeled DNA is expected in regions on the bead surface that were desorbed or disturbed by the potential treatment since adsorption of the DNA is expected in these MUA free areas. Capacitance of the surface after Etreat and after modification with the DNA can still provide useful estimate of the extent of DNA SAM coverage after its formation measured at a stable potential (0 V, shown in Figure S2). The MUA/DNA SAMs have a capacitance that is very similar to the initial MUA SAM for Etreat positive of −0.8 V. For potentials more negative, the capacitance is larger than 2 μF/cm2 and indicates more defects in the MUA SAM, most likely due to the presence of DNA which results in a less efficient packing of the SAM and an increase of the adsorbed layer dielectric constant. A direct measure of the extent of DNA decoration on the treated gold bead electrodes is achieved with fluorescence E

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Langmuir potential, resulting in a higher fluorescence intensity around the 111 facet. In addition, the MUA SAM is more easily exchanged with the HSC6-DNA-AF488 in this region as seen in the control measurement. SEM imaging of this region (shown in Figure S3) reveals large terraces with sizable steps which should behave similar to the 111 facet, but with many defects. These surrounding regions did not have an impact on the measured fluorescence intensity from the 111 region but may be an indication of the behavior of miscut 111 surfaces. Electroless Treatment Results. A similar methodology was used to determine the conditions for modifying the MUA SAM surface using a reducing agent in solution (e.g., NaBH4). This approach would be more appropriate for treating AuNRs; however, the choice of reducing agent is limited by many factors: need for a sufficiently negative potential on gold, products that are not redox active themselves and do not adsorb. In addition, the reducing agent must be easily quenched or decompose to products that do not influence the surface chemistry. These requirements make NaBH4 an ideal choice, but the inability to confidently predict the solution potential for a given concentration requires a direct measurement of the solution potential during the addition of NaBH4. The effect of an increasing amount of NaBH4 on the measured potential of an MUA SAM-modified Au bead in an O2-free electrolyte is shown in Figure 5a. The capacitance of the MUA SAM gold

bead was measured simultaneously (Figure S4) and is shown (Figure 5b) as a function of the measured potential and compared with the capacitance for linear and step potential electrochemical scans. The changes in the MUA SAM under these electroless conditions are very similar to what was measured using electrochemical treatments, suggesting that the manipulation of the solution potential with NaBH4 will result in a similar effect on the specificity of the alkylthiolate desorption. As seen in Figure 4, careful control over the potential experienced by the electrode is required to achieve the maximum selectivity, in addition to controlling the amount desorbed. This control is not easily achieved in the case of the NaBH4 treatment, as the potential can change quickly upon small additions of NaBH4 depending on the state of the surface (e.g., modified or bare gold). This is shown in Figure S4 where the potential continued to decrease even though no additional NaBH4 was added. In contrast to the electrochemical treatment, the potential experienced by the electrode will not be constant and more difficult to control by simply adding NaBH4. Therefore, for comparison, the most negative potential measured was designated as Etreat. Three examples of the change in capacitance and potential of MUA SAMs measured during the addition of NaBH4 to the electrolyte are shown in Figure 5c and compared to that from the many small additions of BH4− represented by the dashed line (the evolution of the potential and capacitance as a function of time for this data is shown in Figure S5a−c). After treatment via the use of NaBH4, the beads were immersed in the HSC6-DNA-AF488 solution and analyzed in the same manner as for the electrochemical treatment. The capacitance measured at 0.00 V after this procedure (shown in Figure S6) increased with more negative potentials similar to the electrochemical methods (Figure S2). The fluorescence images for MUA SAM beads that were exposed to NaBH4 (Etreat that ranged from −0.600 to −0.866 V) as well as a control sample (no added NaBH4) are shown in Figure 6. The control beads and beads that experienced potentials positive of −0.6 V reveal only thiol exchange activity and had little DNA on the surface (Figure 6c,d). The effect is more dramatic as the potentials approach −0.8 V: the 111 facet and the regions around the facet are heavily populated with the DNA SAM, revealing the large extent of MUA desorption (Figure 6c). More negative than −0.8 V reveals a heavily desorbed MUA SAM with the 100 and other regions modified with a large surface coverage of DNA (Figure 6d). The capacitance of these mixed layers measured at 0 V (Figure S6) is also larger than that measured for potentials positive of −0.8 V, another indication of the substantial disruption of the SAM via BH4− treatment. Interestingly, the most negative potential achieved (−1.1 V) results in a surface that is almost completely devoid of MUA, resulting in a large DNA SAM coverage that is highly heterogeneous (Figure S7). Analysis of the fluorescence images shows the influence of the NaBH4 and the resulting bead potential as being very selective for the removal of MUA from 111 as compared to 100 in a manner that matches the electrochemical treatment (Figure 6). This suggests that the mechanism for the reductive desorption reaction is equivalent in both cases and can be described similarly to an electrochemical reduction. The reducing agent effectively sets the potential of the gold surface by getting access through defects or through the SAM, being oxidized, transferring electrons to the gold which changes its Fermi level. The reductive desorption then proceeds once the

Figure 5. (a) Open circuit potential measured for a MUA SAM-coated gold bead electrode in 10 mM phosphate buffer, pH = 8, after addition of NaBH4 (+ and ∗ represent 1 μL additions of 30 mM or 0.3 M NaBH4, respectively), (b) a comparison of the capacitance measured during a negative potential scan (5 mV/s), during a 5 min potential step scan, and during the addition of NaBH4 shown in (a), and (c) comparison of the changes in potential and capacitance for three different MUA SAM-modified Au bead electrodes after additions of NaBH4. The electroless treatment during small frequent additions of NaBH4 is shown for comparison. More details are provided in the Supporting Information (Figure S5). F

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Figure 6. Comparison of the fluorescence intensity ratio from the 111 and 100 regions of many bead electrode surfaces following time spent at a treatment potential, realized either through the electrochemical application, or through the use of a reducing agent NaBH4. The fluorescence images (1.56 mm × 1.56 mm) are selected from electrodes after the electroless potential treatment for potentials indicated: (a) −0.866 V, (b) −0.742 V, (c) −0.619 V, and (d) −0.051 V/AgAgCl. The false color represents the fluorescence intensities shown on a log scale to increase the contrast.

of dissolved O2, but the treatment of AuNPs may be more easily performed in its presence. Therefore, as a prelude to these AuNP electroless treatments, the same NaBH4 measurements were performed on the gold bead electrodes in the presence of O2. Results are shown in Figure 7 and compared to the electrochemical results in the absence of O2. Selective removal from the 111 surface was observed, but at a measured potential more positive than without O2. This mixed potential is dependent on the [O2] in solution, and a decrease in [O2] results in a more negative OCP potential (Figure S8). Importantly, this mixed potential was reversibly responsive to the [O2] showing that O2 was not reacting with BH4− in the electrolyte (Figure S8). The OCP should not be influenced by the phosphate species in the electrolyte as it adsorbs to gold but only at potentials more positive than −0.2 V.35,36 The potential therefore depends on the [O2] which acts as a depolarizer significantly influencing the measured OCP. Interestingly, the electrochemical treatment resulted in the same modification independent of the presence of O2 (Figure S9). These observations reveal that the mixed potential measured on the Au electrode reflects the balance between the BH4− oxidation and the two reductions taking place on the gold electrode: the reduction of O2 (ORR) and reductive SAM

potential (or Fermi level) reaches the appropriate value. This suggests that the BH4− does not chemically reduce the Au− thiol bond, but rather the reduction is mediated through the gold. This is further supported by experiments that hold the potential of the bead at −0.35 V/AgAgCl before and during the addition of NaBH4 into the solution. These surfaces show some small amount of DNA exchange similar to what is observed in the absence of NaBH4 treatment. The reactivity of the gold− sulfur bond in SAMs on surfaces is complex,34 and more study is needed to address this issue. Unfortunately, it is not easy to achieve the conditions to realize a potential of −0.8 V/SCE via NaBH4 treatment. It seems that as soon as the potential is close to −0.8 V, defects are created which enable more reductant to get access to the surface which thereby quickly changes the potential to be more negative, accelerating the reductive desorption. This suggests that the desorption starts from regions on the surface, creating defects around which the thiol is easily removed, and the process cascades toward a completely MUA free surface, in keeping with the mechanism of reductive desorption (an example is shown in Figure S4). Influence of Dissolved O2. The electrochemical and electroless treatments shown were all performed in the absence G

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Figure 7. Influence of dissolved O2 on the fluorescence intensity ratio from the 111 and 100 regions of many bead electrode surfaces following the electroless treatment using NaBH4 of MUA SAM gold beads without the removal of dissolved O2 (▲) compared to results without O2 (■). The fluorescence images (1.56 mm × 1.56 mm) are selected from potentials indicated: (a) −0.694 V, (b) −0.609 V, and (c) −0.468 V/AgAgCl. The false color represents the fluorescence intensities shown on a log scale.

alkylthiol-capped AuNRs, the electrochemical treatment (−0.80 V) of two MUA-coated gold beads in the presence of CTAB (1 mM, ≥99.0%, Sigma-Aldrich) was performed (after removal of dissolved O2). The modified beads were removed from the electrochemical cell and rinsed with Milli-Q water. One bead was placed into a solution of 1 μM HSC6-DNA-AF488 in IB as previously done, and the other bead was immersed into a similar DNA solution which additionally contained 1 mM CTAB. The beads were incubated in these solutions for 20 h, removed, and rinsed in electrolyte before fluorescence imaging. As before, the fluorescence images (after removal of a significant amount of nonspecifically adsorbed DNA via potential step scans following our previous work30) are shown in Figure 8. The 111 facets on both beads are decorated with DNA at approximately the same surface coverage. Significantly, no other regions on the bead surface were modified with DNA, indicating that the application of −0.80 V only desorbed the MUA from the 111 surface even in the presence of CTAB. We also found that the presence of CTAB in the DNA deposition solution resulted in a significant amount of nonspecific DNA adsorption on regions other than the 111 facets. This was not observed for beads in the CTAB free electrolyte. It is possible that the formation of a complex between DNA and CTAB38 enhances the interaction with MUA-covered gold surface via electrostatics, and the removal of MUA from the 111 facet resulted in less of this nonspecific

desorption. The relative rates of these two processes are of a very different magnitude since the [O2] is much greater than the limited amount of the surface bound SAM. The measured potential is a weighted average,26 essentially controlled by the [O2]. This coupled redox system is very similar to a corrosion process but mediated by the gold surface where BH4− oxidation drives both ORR and thiol SAM reduction. The potential of the gold electrode is dominated by the ORR and does not reliably reveal the thiol reduction process, in contrast to what was measured in the absence of O2. Therefore, removing the dissolved O2 from the solution containing AuNPs is important so as to realize a process that can be monitored and possibly controlled via an accurate reductive solution potential. Toward AuNR Modification: The Influence of CTAB. The synthesis of AuNRs relies on the use of cetyltrimethylammonium bromide (CTAB).15 Its presence is required in many cases so that the AuNRs remain dispersed in the aqueous electrolyte. The surface activity of CTAB has been linked to difficulties in the surface modification of AuNRs, even though modification of AuNRs can occur in its presence.15 Electrochemical studies have also shown that CTAB can adsorb onto the gold surface, specifically studied on Au(111) and Au(100) by Burgess.37 They showed that at −0.8 V/SCE the CTA+ and Br− were both desorbed from the Au(111) surface, in addition to being removed from the 100 at slightly more negative potentials. In preparation for work with CTAB stabilized H

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and further studies are planned that will allow for a measurement of the extent and location of the DNA modification.

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CONCLUSION Through the use of fluorescence microscopy and electrochemical potential treatments on gold single crystal bead electrodes, a method was developed that quantifies the selective desorption of an alkylthiolate SAM from the 111 surface as compared to other surface crystallographies. The specificity of SAM removal from the 111 surface and replacement with another thiolate can be controlled using appropriate electrochemical potential treatments. A 20-fold difference in the selective removal of the SAM from the 111 surface as compared to the 100 surface was realized at −0.8 V/AgAgCl. This selectivity was sharply peaked at this particular potential in agreement with other studies showing that the 111 surface is the first to be reductively removed. Using the same approach, a method was developed for realizing the same selectivity but through the use of solution-based redox speciesan electroless process with the use of NaBH4. The NaBH4 controlled the solution potential which was adopted by the gold surface at electrostatic equilibrium, enabling the reductive desorption of the SAM from the gold surface. A similar potential-dependent selectivity for the 111 surface was demonstrated, however, with less control over the solution potential and therefore the extent of removal. Nevertheless, the use of a reducing agent rather than an electrochemical reducing potential was shown to result in the same surface modification, strongly suggesting the same mechanism: reduction occurring electrochemically rather than as a chemical reduction of the SAM by the NaBH4. Also shown is that performing the same electroless measurements in the presence of O2 complicates the solution potential measurement due to depolarization by oxygen reduction, resulting in an uncertain correlation of selectivity and potential measured on the electrode surface because of an uncontrolled amount of dissolved O2. No such difference was observed for the electrochemical treatment in the presence or absence of O2. The electrochemical treatment in the presence of 1 mM CTAB resulted in a similarly modified electrode surface as when performed without CTAB. This electroless method was applied to CTAB-stabilized mercaptohexanoic acid-capped AuNRs. The treated AuNRs were not stable in solution when DNA was present in the buffer, in contrast to those in a solution without DNA, suggesting some modification of the AuNRs due to the electroless treatment. This work shows that control over the potential of the substrate has a strong influence over the modification of specific regions of the gold surface. The same surface modification is realized whether the potential is applied via a potentiostat or controlled via a reducing agent. Preliminary results using this control parameter to specifically modify AuNRs suggest a possible methodology for manipulating AuNR surface composition.

Figure 8. Fluorescence images of the electrochemically treated MUAcoated gold bead in a 1 mM CTAB and 10 mM phosphate buffer, pH = 8 electrolyte. (a) Bead immersed in DNA IB without CTAB and (b) bead immersed in DNA IB with 1 mM CTAB. The false color represents the fluorescence intensities shown on a log scale. These images were taken after potential step scan treatments to remove nonspecifically adsorbed DNA and were not background corrected.

adsorption. This test shows that the electrochemical approach described here can be successful even in the presence of a large amount of CTAB, simply by controlling the potential. Given the demonstrated similarity between the electrochemical and electroless approaches, a similar outcome is anticipated for AuNRs even in the presence of CTAB though caution is warranted given the large extent of nonspecifically adsorbed DNA onto the MUA-modified gold surface. Preliminary NaBH4 Electroless Treatment of AuNRs. Based on the above results, the electroless treatment protocol developed for the gold beads was performed on a solution of AuNRs which were capped with 6-mercaptohexanoic acid (40 × 112 nm, Nanopartz). In this preliminary set of results, the AuNRs were modified with a shorter thiol to ensure their solubility in aqueous solutions (unfortunately, the MUA-coated AuNRs could not be used as they were not soluble in this solution even with the CTAB). The AuNRs were diluted into a pH 8 phosphate buffer (10% PBS) which included 1 mM CTAB (the minimum concentration needed to keep the AuNRs in solution). Nevertheless, to ensure consistency with the previous work, the solution potential was monitored using a MUA-coated Au bead electrode as described in the electroless treatment protocol. After the removal of O2, NaBH4 was added, and the OCP decreased to a value of −0.90 V/Ag|AgCl (Figure S10) before being quenched by the addition of HNO3, resulting in an increase in potential. The solution containing the AuNRs was separated into two aliquots. HSC6-DNA-AF488 (1 mM) was added to only one of the aliquots. Both samples were stored in the dark for 48 h. The sample without added DNA remained soluble. The AuNR aliquot with added DNA became insoluble in the solution and could not be resuspended. In a separate measurement, a control sample of AuNRs (in the same buffer with CTAB) that was not treated with NaBH4 but had DNA added to solution remained soluble. These preliminary results suggest that the AuNRs were modified via the NaBH4 treatment, and the DNA SAM was most likely formed. Unfortunately, the DNA-modified AuNRs were no longer stable in the solution, prohibiting further analysis. Moreover, given the fact that a significant amount of nonspecific adsorption was observed on the bead electrode measurements for the DNA/CTAB containing solutions, the presence of nonspecifically adsorbed DNA/CTAB may be also present on the acid-terminated AuNR surface. This further complicates any analysis as a simple rinsing will not guarantee removal of the nonspecifically adsorbed DNA. These preliminary results show that the electroless approach can be used to modify AuNRs,

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b03021. An outline of NaBH4 electrochemistry and detailed methods; potential step scan waveform used to measure capacitance; capacitance measured at 0 V after potential I

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(13) Yu, Z. L.; Casanova-Moreno, J.; Guryanov, I.; Maran, F.; Bizzotto, D. J. Am. Chem. Soc. 2015, 137, 276−288. (14) Musgrove, A.; Kell, A.; Bizzotto, D. Langmuir 2008, 24, 7881− 7888. (15) Burrows, N. D.; Lin, W.; Hinman, J. G.; Dennison, J. M.; Vartanian, A. M.; Abadeer, N. S.; Grzincic, E. M.; Jacob, L. M.; Li, J.; Murphy, C. J. Langmuir 2016, 32, 9905−9921. (16) Ung, T.; Giersig, M.; Dunstan, D.; Mulvaney, P. Langmuir 1997, 13, 1773−1782. (17) Novo, C.; Funston, A. M.; Gooding, A. K.; Mulvaney, P. J. Am. Chem. Soc. 2009, 131, 14664−14666. (18) Doneux, T.; Nichols, R. J.; Buess-Herman, C. J. Electroanal. Chem. 2008, 621, 267−276. (19) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687−2693. (20) Yuan, M.; Zhan, S.; Zhou, X.; Liu, Y.; Feng, L.; Lin, Y.; Zhang, Z.; Hu, J. Langmuir 2008, 24, 8707−8710. (21) Ansar, S. M.; Ameer, F. S.; Hu, W.; Zou, S.; Pittman, C. U.; Zhang, D. Nano Lett. 2013, 13, 1226−1229. (22) Morris, J. H.; Gysling, H. J.; Reed, D. Chem. Rev. 1985, 85, 51− 76. (23) Santos, D.; Sequeira, C. Electrochim. Acta 2010, 55, 6775−6781. (24) Fisher, A. C. Electrode Dynamics (Oxford Chemistry Primers); Oxford University Press: 1996. (25) Scanlon, M. D.; Peljo, P.; Méndez, M. A.; Smirnov, E.; Girault, H. H. Chemical Science 2015, 6, 2705−2720. (26) Park, J. H.; Zhou, H.; Percival, S. J.; Zhang, B.; Fan, F.-R. F.; Bard, A. J. Anal. Chem. 2013, 85, 964−970. (27) Indrasekara, A. S. D. S.; Wadams, R. C.; Fabris, L. Particle & Particle Systems Characterization 2014, 31, 819−838. (28) Zhang, Q.; Han, L.; Jing, H.; Blom, D. A.; Lin, Y.; Xin, H. L.; Wang, H. ACS Nano 2016, 10, 2960−2974. (29) Casanova-Moreno, J. R.; Bizzotto, D. Electrochim. Acta 2015, 162, 62−71. (30) Yu, Z. L.; Yang, C. W. T.; Triffaux, E.; Doneux, T.; Turner, R. F. B.; Bizzotto, D. Anal. Chem. 2017, 89, 886−894. (31) Chance, R. R.; Miller, A. H.; Prock, A.; Silbey, R. J. Chem. Phys. 1975, 63, 1589−1595. (32) Bizzotto, D.; Yang, Y.; Shepherd, J. L.; Stoodley, R.; Agak, J. O.; Stauffer, V.; Lathuilliere, M.; Akhtar, A. S.; Chung, E. J. Electroanal. Chem. 2004, 574, 167−184. (33) Casanova-Moreno, J.; Yu, Z. L.; Massey-Allard, J.; Ditchburn, B.; Young, J. F.; Bizzotto, D. Luminescence in Electrochemistry; Springer International Publishing: Cham, 2017; pp 21−77. (34) Reimers, J. R.; Ford, M. J.; Halder, A.; Ulstrup, J.; Hush, N. S. Proc. Natl. Acad. Sci. U. S. A. 2016, 113, E1424−E1433. (35) Yaguchi, M.; Uchida, T.; Motobayashi, K.; Osawa, M. J. Phys. Chem. Lett. 2016, 7, 3097−3102. (36) Niaura, G.; Gaigalas, A. K.; Vilker, V. L. J. Phys. Chem. B 1997, 101, 9250−9262. (37) Vivek, J. P.; Burgess, I. J. Langmuir 2012, 28, 5040−5047. (38) Trewavas, A. Anal. Biochem. 1967, 21, 324−329.

treatment at Etreat and backfilling with HSC6-DNAAF488; SEM of Au bead electrode around the 111 facet; OCP and capacitance measured for a MUA SAM gold bead electrode during the sequential additions of NaBH4; three examples of addition of NaBH4 and the change in potential and capacitance compared to continuous addition of NaBH4; capacitance at 0 V after NaBH4 treatment and backfilling with DNA; example of fluorescence image after treatment at −1.1 V resulting from NaBH4 addition; changes in OCP with addition of NaBH4 with and without O2; comparison of electrochemical treatment at the same Etreat in the absence or presence of O2; the changes in OCP of a MUA SAM gold bead electrode in a solution of AuNRs during the addition of NaBH4 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.B.). ORCID

Dan Bizzotto: 0000-0002-2176-6799 Present Addresses

́ J.C.-M.: Laboratorio Nacional de Micro y Nanofluidica, Centro ́ de Investigación y Desarrollo Tecnológico en Electroquimica, Parque Tecnológico Querétaro s/n, Pedro Escobedo, México. T.M.: Department of Physics, University of Calgary. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Thanks are extended to Isaac Martens for SEM imaging of the bead electrodes, Jonathan Massey-Allard for initiating the AuNR work, Prof. Ian Burgess (Univ. of Saskatchewan), and Prof. David Harrington (Univ. of Victoria) for helpful discussions, funding provided by NSERC (Canada) through Discovery and equipment grants.

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REFERENCES

(1) Haag, A.-L.; Toader, V.; Lennox, R. B.; Grutter, P. Nanotechnology 2016, 27, 455501. (2) Jambrec, D.; Gȩbala, M.; La Mantia, F. M. F.; Schuhmann, W. Angew. Chem., Int. Ed. 2015, 54, 15064−15068. (3) Meunier-Prest, R.; Legay, G.; Raveau, S.; Chiffot, N.; Finot, E. Electrochim. Acta 2010, 55, 2712−2720. (4) Arakawa, T.; Hobara, D.; Yamamoto, M.; Kakiuchi, T. Electrochem. Commun. 2005, 7, 848−852. (5) Stolar, R. B.; Guerra, E.; Shepherd, J. L. Langmuir 2015, 31, 2157−2166. (6) Smith, S. R.; Han, S.; McDonald, A.; Zhe, W.; Shepherd, J. L. J. Electroanal. Chem. 2012, 666, 76−84. (7) Arinaga, K.; Rant, U.; Knezevic, J.; Pringsheim, E.; Tornow, M.; Fujita, S.; Abstreiter, G.; Yokoyama, N. Biosens. Bioelectron. 2007, 23, 326−331. (8) Shepherd, J. L.; Kell, A.; Chung, E.; Sinclar, C. W.; Workentin, M. S.; Bizzotto, D. J. Am. Chem. Soc. 2004, 126, 8329−8335. (9) Takeishi, S.; Rant, U.; Fujiwara, T.; Buchholz, K.; Usuki, T.; Arinaga, K.; Takemoto, K.; Yamaguchi, Y.; Tornow, M.; Fujita, S.; Abstreiter, G.; Yokoyama, N. J. Chem. Phys. 2004, 120, 5501−5504. (10) Satjapipat, M.; Sanedrin, R.; Zhou, F. Langmuir 2001, 17, 7637− 7644. (11) Imabayashi, S.-i.; Hobara, D.; Kakiuchi, T.; Knoll, W. Langmuir 1997, 13, 4502−4504. (12) Doneux, T.; Steichen, M.; De Rache, A.; Buess-Herman, C. J. Electroanal. Chem. 2010, 649, 164−170. J

DOI: 10.1021/acs.langmuir.7b03021 Langmuir XXXX, XXX, XXX−XXX