Article pubs.acs.org/Langmuir
Effect of pH on Anodic Formation of Nanoporous Gold Films in Chloride Solutions: Optimization of Anodization for Ultrahigh Porous Structures Minju Kim and Jongwon Kim* Department of Chemistry, Chungbuk National University, Cheongju, Chungbuk 361-763, Korea S Supporting Information *
ABSTRACT: Nanoporous gold (NPG) structures have useful applications based on their unique physical and chemical properties; therefore, the development of NPG preparation methods is the subject of extensive research. Recently, the anodization of Au surfaces was suggested as an efficient method for preparing porous Au structures. In this work, the mechanistic aspects of the anodization of Au in Cl−-containing solutions for the preparation of NPG layers were investigated. The effects of the experimental parameters of the anodization reaction on the porosity of the NPG layers in terms of the roughness factor (Rf) were examined. The anodic formation of NPG was more effective in buffered solutions than in unbuffered electrolytes. The Rf of the NPG layer was sensitive to the electrolyte pH; this was ascribed to the efficient formation of protecting layers of gold oxide on the newly formed NPG structures. In buffer solutions at pH 8, ultrahigh porous NPG layers with Rf values of 1300 were obtained within 15 min. The ultrahigh porous NPG layers were used for the electrochemical detection of glucose; a high sensitivity of 135 μA mM−1 cm−2 was achieved in the presence of 0.1 M Cl−. This straightforward and time-saving preparation of NPG surfaces will provide new opportunities for applications of NPG structures. of the electrochemical detection of glucose.14 Although the anodization of Au in carboxylic acids is useful for fabricating NPG structures, this method requires a long reaction time, typically more than 1 h. Li and co-workers reported that NPG structures can be easily prepared by the anodization of Au in HCl solutions within 1 min.15 They also reported that NPG layers with greater surface areas were obtained by the anodization of Au in KCl solutions within 5 min, and these can be effectively used for the electrochemical detection of glucose.16 These investigations suggest that the anodization of Au in Cl−-containing solutions can be used as a time-saving method for preparing NPG structures. In the present work, we examined the mechanistic aspects of the formation of NPG structures during the anodization of Au in Cl−-containing media. Experimental parameters, such as applied potential and electrolyte pH, were systematically examined to elucidate the anodic formation of NPG structures. In particular, the pH of the electrolyte played a significant role during Au anodization, and pH control enabled the preparation of ultrahigh-porosity NPG layers. A detailed mechanism of the anodic formation of NPG was proposed on the basis of pH-dependent gold oxide formation, and the application of ultrahigh-porosity NPG to the electrochemical detection of glucose was demonstrated.
1. INTRODUCTION Nanoporous gold (NPG) structures have received much attention from various disciplines, from fundamental science to applied engineering. Ordered NPG structures with high surface areas can be used in various applications based on their distinctive physical and chemical properties.1−3 Specific applications of NPG include heterogeneous catalysts for gasphase reactions,4 surface-enhanced Raman substrates,5 and electrochemical sensors.6,7 In particular, NPG structures exhibit unique electrochemical properties, such as discriminative electrokinetics and nanoconfinement effects,8 which enables the application of NPG in electrocatalysis and electroanalysis.9−11 NPG structures have been fabricated using various methods, and selective dissolution of Ag from Ag−Au alloys in nitric acid has been extensively investigated.1,2 Although dealloying methods are useful for preparing NPG structures, this procedure is hazardous and time-consuming and the strongly corrosive nature of nitric acid may prohibit the use of this method in microfluidic applications. It was recently reported that anodization of Au offers an alternative route for fabricating NPG structures. Nishio and co-workers reported that NPG surfaces can be prepared by the anodization of Au in oxalic acid solutions.12 We have shown that these NPG electrodes can be effectively used for the electrochemical detection of glucose in the presence of Cl− ions.13 The expansion of NPG layers was achieved by the anodization of Au in citric acid solutions; the greater NPG surface area significantly enhanced the sensitivity © 2014 American Chemical Society
Received: February 24, 2014 Revised: April 3, 2014 Published: April 4, 2014 4844
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2. EXPERIMENTAL SECTION 2.1. Reagents and Instruments. All solutions were prepared using purified water (Milli-Q, 18.2 MΩ cm). KCl, glucose, ascorbic acid, and other reagents were obtained from Aldrich and used as received. The electrolytes for the anodization consisted of 1 M KCl in either purified water or 0.1 M phosphate buffer. Britton−Robinson buffers were used for the systematic investigation of pH dependence. Britton−Robinson buffers containing 0.04 M CH3COOH, 0.04 M H3PO4, and 0.04 M H3BO3 were prepared; the pH was adjusted with 0.2 M NaOH. Electrochemical measurements were performed using a CHI 660D (CH Instrument) potentiostat. Pt wire and Ag/AgCl electrodes (3 M NaCl) were used as counter and reference electrodes, respectively. All of the potentials are reported relative to the Ag/AgCl reference electrode. Scanning electron microscopy (SEM) characterization was performed using an ULTRA PLUS field emission microscope (Carl Zeiss). X-ray photoelectron spectroscopy (XPS) was performed using an Ulvac-PHI (PHI Quantera-II). The XPS excitation source was Al Kα, and the pass energy in the electron energy analyzer was 55 eV. 2.2. Preparation of NPG Electrodes by Anodization. NPG structures were prepared by the anodization of Au in solutions containing 1 M KCl, using a modified version of previously reported procedures.15,16 A commercially available Au rod electrode (CH Instruments, 2 mm in diameter) was mechanically polished with alumina powder from a lager particle size down to a smaller particle size (ca. 0.05 μm) on a microcloth pad (Buehler). Before the anodization reactions, the potential of the Au working electrode was swept at a rate of 5 mV s−1 from 0.7 V until a sudden drop in the anodic current occurred (passivation potential, Epass). The applied potential (Eappl) was set based on the potential gap (Egap), i.e., the difference between the passivation and anodizing potentials: Eappl = Epass − Egap. An Ag/AgCl reference electrode was inserted into the electrochemical cell via a double-junction reference electrode chamber to prevent contamination. The NPG electrode was electrochemically cleaned by cycling 5 times between −0.2 and 1.2 V in 0.1 M phosphate buffer (pH 7) solution before amperometric detection of glucose.
Figure 1. (A) Linear scan voltammogram of the Au electrode in 1 M KCl at a scan rate of 5 mV s−1. (B) Dependence of Rf of NPG prepared by anodizing in 1 M KCl for 300 s as a function of the potential gap.
upon the experimental conditions, such as electrolyte composition, and the initial state of the Au surface, the potential gap was used rather than applying a certain fixed potential. Figure 1B shows the potential gap dependence of the roughness factor (Rf, ESA divided by the geometric area of the Au substrate) of NPG prepared by anodizing in 1 M KCl. As indicated by the cyclic voltammograms obtained in 0.1 M H2SO4 (see Figure S1 of the Supporting Information), from which the ESAs of the anodized Au surfaces were obtained by integrating the charge consumed for the reduction of the surface oxide layer (400 μC cm−2),22 NPG formation was only possible with potential gaps between 0.025 and 0.045 V. The maximum NPG layer ESA (Rf = 560) was obtained using a potential gap of 0.03 V; a slight increase in the potential gap resulted in a significant decrease in Rf. In a previous report, NPG layers with Rf values of 220 were obtained in 1 M KCl in a reaction time of 300 s,16 which is similar to that obtained with a potential gap of 0.040 V in this work. Significantly greater Rf values were achieved by selecting an appropriate potential gap from the linear scan of the Au substrates; we therefore used this optimal potential gap in subsequent investigations. 3.2. Effect of the Electrolyte on NPG Formation. Figure 2 shows expansion of the NPG layers in terms of Rf as a function of time for the anodization reaction. In unbuffered aqueous 1 M KCl solution, the Rf of the NPG layers increased for reaction times up to 300 s, after which the Rf values were virtually constant. This behavior indicates that the formation of NPG layers does not occur after 300 s. A similar time dependence of NPG layer formation by the anodization of Au was previously observed, in which the finite expansion of NPG
3. RESULTS AND DISCUSSION 3.1. Optimization of Anodizing Potential for NPG Formation. Figure 1A shows a typical linear scan voltammogram obtained on Au surfaces in 1 M KCl; it can be divided into several regions.17,18 As the electrode potential moves to positive regions, surface Au atoms are electrochemically dissolved in either the +1 or +3 oxidation state, depending upon the potential and form complexes with Cl−.19,20 The anodic current reaches a plateau between 1.3 and 1.4 V, in which the reaction is controlled by the diffusion of Cl− to the Au surfaces.21 At the end of the plateau region, a sudden drop in anodic current occurs, indicating that the Au surfaces are passivated by the formation of oxide layers.17 After Au passivation, oxygen evolution or oxidation of Cl− occurs at the surfaces. The anodization of Au surfaces to form nanoporous structures is known to be possible when the electrode potential is maintained immediately before passivation of the Au surfaces.15,16 In this limited potential region, Au surfaces can be continuously electrochemically dissolved, followed by the redeposition of active Au atoms with thin protective layers of oxides (vide inf ra for a detailed discussion of mechanistic aspects). To examine the effect of applied potential on the formation of nanoporous structures, we monitored the changes in the electrochemical surface areas (ESAs) of the anodized NPG layers as a function of the potential gap. The potential gap is the difference between the passivation and anodizing potentials, as indicated in Figure 1A. Because the passivation potential of an Au surface in the linear scan differs depending 4845
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Figure 2. Dependence of Rf of NPG prepared in unbuffered (squares) and buffered (circles, 0.1 M phosphate buffer at pH 7) 1 M KCl solutions as a function of the reaction time. The potential gap was 0.030 V.
layers was ascribed to limited mass transport through the inner nanopores after certain reaction times.15,16 In the present work, we noticed that the pH values of the electrolyte solutions increased during the anodization reactions. The pH change was assumed to be caused by reduction of water (2H2O + 2e− → H2 + 2OH−) at the counter electrode; this was verified by the color change of phenolphthalein and bubble formation (see Figure S2 of the Supporting Information). We speculated that the change in the electrolyte pH might affect the formation of NPG layers during anodization reactions (vide inf ra for a detailed discussion). To examine the effect of pH on the formation of NPG layers, we performed anodization reactions in 0.1 M phosphate buffers (pH 7) containing 1 M KCl. In contrast to anodization in unbuffered electrolytes, the Rf values of the NPG layers continuously increased after a reaction time of 300 s in buffered electrolytes, as shown in Figure 2. The Rf of the NPG layers obtained in a buffered electrolyte with a reaction time of 600 s was 950, which is significantly greater than that obtained in unbuffered electrolytes. We found that the pH of the unbuffered electrolyte increased to around 11 after the anodization reactions, whereas the pH of the buffered electrolyte remained unchanged (see Figure S3 of the Supporting Information). These phenomena suggest that pH control might play an important role in the continuation of NPG formation by the anodization of Au. Figure 3 shows the structures of NPG layers obtained by Au anodization. The NPG layers prepared in unbuffered 1 M KCl solution exhibit a typical three-dimensional nanoporous structure consisting of ligaments and pores (Figure 3A). The overall shapes and sizes of the NPG structures are similar to those previously reported.16 When buffered 1 M KCl solutions (phosphate buffer at pH 7) were used as electrolytes, the ligament/pore sizes of the NPG structures were smaller (Figure 3B). Cross-sectional SEM images (insets in Figure 3) show that the thickness of the NPG layers is ∼10 μm in both cases. The slightly greater Rf of NPG layers prepared in buffered KCl solutions may be attributed to the smaller nanopore dimensions. 3.3. pH Dependence of the Anodic Formation of NPG. Buffered electrolytes were more efficient for the anodic formation of NPG; therefore, we systematically investigated the effect of pH on Au anodization. Figure 4A shows the changes in Rf values of anodized NPG layers with electrolyte pH after anodization for 300 s. As the electrolyte pH increased, the NPG Rf gradually increased up to pH 8, indicating that the
Figure 3. SEM images of NPG prepared by anodization with a potential gap of 0.030 V for 300 s in (A) unbuffered and (B) 0.1 M phosphate buffer containing 1 M KCl. Insets show cross-sectional SEM images.
Figure 4. (A) Dependence of the roughness factor (Rf) of NPG prepared in buffered 1 M KCl solutions as a function of pH. The potential gap was 0.030 V (0.080 V at pH 10), and the reaction time was 300 s. (B) Linear scan voltammograms of Au electrodes in buffered 1 M KCl solutions at different pH values. Britton−Robinson buffers of pH 2 and 10 were used. Electrolyte solutions with pH 1 and 13 were prepared from H2SO4 and NaOH, respectively.
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anodic formation of NPG is more efficient at higher pH values. The NPG layer Rf decreased at pH 9 and dropped sharply at pH 10. In electrolytes of pH higher than 10, anodization could not be performed; therefore, no porous structures were obtained. It has been reported that a greater NPG Rf was obtained by anodization in neutral KCl solutions than in acidic HCl solutions;16 this is consistent with the present results. The effect of pH on the Rf of anodized NPG suggests that pH control plays a critical role in the formation of nanoporous structures. To elucidate the pH dependence of NPG formation by anodization, we first examined the effect of pH on the linear scans of Au surfaces in buffer solutions containing 1 M KCl. Figure 4B shows that the linear scan voltammograms of Au surfaces are not significantly affected by the electrolyte pH and the shapes of these voltammograms are identical to those obtained in unbuffered 1 M KCl solutions, as shown above (Figure 1A). At pH values higher than 10, the linear scans are different from those obtained at lower pH values, which indicates that the electrochemical dissolution of Au is not effective for the formation of NPG layers at these pH values. It is known that the anodic current plateaus followed by passivation of Au surfaces are controlled by the diffusion of Cl − to the Au surfaces. 21,23 We confirmed that the concentration of Cl− significantly affects the linear scan voltammograms of Au surfaces (see Figure S4 of the Supporting Information), whereas the pH does not have a significant effect. We therefore concluded that the electrochemical dissolution of Au during the anodic formation of NPG layers was not affected by the electrolyte pH; there are therefore other reasons for the pH-dependent variations in the NPG Rf. Although the detailed mechanism of NPG formation by the anodization of Au in Cl−-containing solutions has not been exactly established, Li and co-workers have suggested a plausible mechanism for the anodic formation of NPG in HCl media.15 In this mechanism, the Au surface undergoes electrochemical dissolution to form AuCl2− and AuCl4− and the surface becomes rough. The AuCl2− immediately takes part in disproportionation reactions to produce newly produced Au* atoms. 3AuCl 2− → AuCl4 − + 2Au* + 2Cl−
Figure 5. Cyclic voltammograms of NPG surfaces in 0.1 M H2SO4 at a scan rate of 50 mV s−1. NPG surfaces were prepared by anodization in 0.1 M phosphate buffer (pH 7) containing 1 M KCl for (A) 300 s and (B) 600 s.
complete removal of the gold oxides by the previous scan. When the anodic limit was expanded to 1.5 V (third scan), the formation and dissolution of gold oxides were observed. The amount of gold oxides present on the as-prepared NPG surface is ca. 4.6 times greater than that formed in the third cycle. The amount of gold oxides on NPG prepared for 600 s (Figure 5B) is 1.5 times greater than that on NPG prepared for 300 s (Figure 5A); this is consistent with the ratio of the ESAs of these two NPG layers. We also performed XPS measurements to examine the surface composition of the as-prepared NPG structures, as shown in Figure 6. Figure 6A shows XPS spectra of the Au 4f region; two doublets were observed for the as-prepared NPG surface (solid line). A doublet on the low-binding-energy side, 84.0 and 87.6 eV, corresponded to elemental Au,24 and another doublet appeared as shoulders on the high-energy side. Deconvolution peak fitting (see Figure S6 of the Supporting Information) revealed that the binding energies of these shoulders were 85.5 and 89.2, which is a characteristic of the Au3+ state in Au(OH)3 species formed by the electrochemical oxidation of Au.24−26 After potential cycling between −0.2 and 1.5 V, the doublet on the high-energy side disappeared, indicating oxide removal (dashed line in Figure 6A). XPS spectra of the O 1s region are shown in Figure 6B; a peak at 529.8 eV was observed for the as-prepared NPG surface (solid line). This binding energy can be attributed to the oxygen state in Au(OH)3 species.24,25 After potential cycling, the binding energy of O 1s shifted to 532.4 eV, originating from water adsorbed on the elemental Au surfaces (dashed line).25 The XPS and electrochemical results clearly indicate that the surfaces of the as-prepared NPG are covered with protective layers of gold oxides during Au anodization.
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The Au* atoms then aggregate and are deposited on the roughened Au surface, forming NPG structures. For the asformed NPG layers to survive from the electrochemical dissolution, the formation of protective layers is necessary, and the authors suggested that thin gold oxide layers serve as protective layers. If it is assumed that a similar mechanism operates in Cl−-containing buffer solutions in this work, different gold oxide formation behaviors, depending upon the electrolyte pH, could play a critical role in the evolution of NPG layers during the anodization reactions. Confirmation of the presence of gold oxide layers on the asprepared NPG structures is a prerequisite for verification of the mechanism of anodic formation of NPG described above. We first examined the presence of gold oxides on as-prepared NPG surfaces, and the results are shown in Figure 5. In the first cathodic scan, starting at 1.1 V, a large reduction peak corresponding to dissolution of gold oxide was observed. In control experiments, no oxide layers were observed on flat Au surfaces (see Figure S5 of the Supporting Information). In the next cathodic scan, no reduction peak was observed, indicating 4847
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At more positive potentials, adsorbed OH− is converted to Au(OH)3.24−26 AuOHad + 2H 2O → Au(OH)3 + 2H+ + 2e−
On application of more positive potentials, Au(OH)3 might be converted to AuOOH; however, oxygen evolution also occurs, as indicated by the sharp current increase seen in Figure 7. It was stated above that the anodization of Au surfaces to form nanoporous structures requires a narrow range of electrode potentials immediately before the passivation of the Au surfaces. The optimal potential for NPG formation decreased slightly from 1.39 to 1.36 V between pH 2 and 8 (see Figure S7 of the Supporting Information); this is marked by a gray box in Figure 7. The formation of NPG layers occurs continuously when the newly formed Au* deposit (eq 1) is effectively protected by Au(OH)3, as revealed by XPS measurements. In the case of electrolytes with low pH values, the applied potential at the Au surfaces corresponds to the initial stage of oxide formation; therefore, the formation of Au(OH)3 is not enough to protect the newly formed NPG layers effectively. As the electrolyte pH increases, the onset of oxide formation shifts to the negative potential regions, resulting in the formation of sufficient Au(OH)3 as protective layers. When the electrolyte pH is higher than 10, oxygen evolution occurs at the applied potentials, which hinders the anodic formation of NPG structures. We observed that the applied potential for anodization significantly decreased at pH 10 (see Figure S7 of the Supporting Information) and the NPG Rf at this pH was considerably smaller, as shown in Figure 4A. 3.4. Ultrahigh Porous NPG for Amperometric Detection of Glucose. The effect of pH on the Rf values of NPG layers, shown in Figure 4A, suggests that the optimal pH for the anodic formation of NPG is 8. At this pH, the applied electrode potential, immediately before passivation of the Au surface (ca. 1.37 V), effectively falls in the potential window in which the formation of Au(OH)3 protective layers is adequate but significant oxygen evolution does not occur. We attempted to expand the NPG layers using the optimized conditions for anodic formation of NPG by increasing the reaction time. Figure 8A shows that the NPG Rf increases with reaction time up to 1100 s during anodization in phosphate buffer (pH 8) containing 1 M KCl. The formation of NPG layers by anodization lasts significantly longer in buffered electrolytes than in unbuffered KCl solutions. However, the NPG layers were not further expanded after 1100 s, probably because of limited diffusion of electrolytes into the deeper NPG layers. Figure 8B shows a SEM image of NPG layers prepared by anodization for 1100 s. The structure of the NPG layer is similar to that shown in Figure 3B, obtained with a reaction time of 300 s. The thickness of the NPG layer is around 23 μm, indicating that the anodization reaction continued for a longer reaction time. The Rf value achieved in the present work (ca. 1300) is considerably greater than those obtained using previously reported methods, such as dealloying,11,27 anodization in oxalic acid,13 or hydrogen-templated electrodeposition.28 The formation of ultrahigh-porosity NPG layers with Rf values greater than 1000 has been reported for multicyclic electrochemical coalloying/dealloying of two sacrificial metals29 or anodization of Au in citric acid.14 However, these methods are complicated and require long reaction times or high-temperature conditions. In contrast, anodization in Cl−-containing buffer solution is a straightforward method and is complete in less than 15 min.
Figure 6. X-ray photoelectron spectra of NPG surfaces in (A) Au 4f and (B) O 1s regions. NPG surfaces were prepared by anodization in 0.1 M phosphate buffers (pH 7) containing 1.0 M KCl for 300 s.
Figure 7 shows the changes in the electrochemical oxidation formation and dissolution behaviors of Au surfaces with
Figure 7. Cyclic voltammograms of Au surfaces in supporting electrolytes with different pH values at a scan rate of 50 mV s−1. Britton−Robinson buffers of pH 2 and 10 were used. Electrolyte solutions of pH 1 and 13 were prepared from H2SO4 and NaOH, respectively.
electrolyte pH. In a strongly acidic electrolyte (pH 1), the formation of gold oxides starts at ∼1.1 V. The onset potential of oxide formation shifts to negative potential regions as the electrolyte pH increases. In the initial stage of oxide formation, the water adlayer is first oxidized to form an adsorbed layer of hydroxyl groups at the Au surface.24 Au·H 2Oad → AuOHad + H+ + e−
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Figure 9. (A) Amperometric responses of NPG surfaces (Rf = 1300) to added glucose (3, 6, 9, 12, and 15 mM) at 0.2 V in 0.1 M phosphate buffers (pH 7) in the presence of 0.1 M Cl−. (B) Calibration plot obtained from amperometric responses at NPG surfaces.
Figure 8. (A) Dependence upon the reaction time of Rf of NPG prepared by anodization in 0.1 M phosphate buffers (pH 8) containing 1 M KCl. (B) SEM image of NPG layers prepared by anodization for 1100 s. The inset shows a cross-sectional SEM image.
glucose in the presence of 0.1 M Cl− linearly increased with increasing NPG Rf (see Figure S9 of the Supporting Information), demonstrating the efficacy of ultrahigh-porosity NPG layers. After three consecutive amperometric measurements, the decrease in the sensitivity of the calibration plots was less than 7% (see Figure S10 of the Supporting Information), which confirms the stability of the NPG layers during the amperometric detection of glucose.
As a representative application of ultrahigh-porosity NPG structures, we performed amperometric detection of glucose using NPG surfaces in the presence of Cl−. It is well-known that the electrooxidation of glucose at Au surfaces is severely inhibited by the presence of Cl−.30,31 We previously showed that increasing the Rf of the NPG layer significantly enhanced the sensitivity of the electrochemical detection of glucose in the presence of high concentrations of Cl−, but a larger Rf does not necessarily result in higher electrode efficiency in the absence of Cl−.13,14 Figure 9A shows the amperometric response of a NPG surface to glucose in the presence of 0.1 M Cl−; the amperometric current at the NPG electrode increases linearly with the glucose concentration. The addition of ascorbic acid (AA), one of the most significant interfering species in the amperometric detection of glucose, did not produce a significant change in the amperometric response, considering the normal physiological level of glucose and AA, because of the well-known porous nature of NPG structures.11,32 From a calibration plot obtained from amperometric responses (Figure 9B), the sensitivity was measured to be 135 μA mM−1 cm−2, which is the highest value previously reported for nanoporous structures.16 The sensitivity of amperometric response in the absence of 0.1 M Cl− was 395 μA mM−1 cm−2 (see Figure S8 of the Supporting Information), indicating that the sensitivity decreased to 34% by the presence of Cl−. The degree of decrease in sensitivity at NPG layers anodized in Cl−containing media is smaller than that observed at NPG layers anodized in oxalic and citric acids.13,14 This behavior implies that NPG formed in Cl− media possesses adsorbed Cl− on the surface of NPG layers, which may block active sites on Au surfaces for glucose oxidation even in the absence of Cl−. We observed that the sensitivity of the amperometric detection of
4. CONCLUSION We investigated the mechanistic aspects of the anodization of Au in Cl−-containing solutions for the preparation of NPG layers with ultrahigh porosity. Among the various experimental parameters, the applied potential and electrolyte pH significantly affected the anodic formation of NPG layers. The NPG layers formed by anodization were expanded by extending the reaction time, when a buffered electrolyte was used rather than an unbuffered electrolyte. In particular, the porosity of the NPG layer, in terms of Rf, was sensitively dependent upon the electrolyte pH, and pH control enabled the formation of ultrahigh-porosity NPG layers. The electrochemical dissolution of Au during the anodic formation of NPG layers was not affected by the electrolyte pH. In contrast, the formation of protective gold oxide layers on the newly formed NPG structures was pH-dependent; this is responsible for the dependence of the Rf of the NPG layer on the electrolyte pH. Under optimized anodizing conditions, ultrahigh-porosity NPG layers were prepared, and these were used for the sensitive electrochemical detection of glucose in the presence of 0.1 M Cl−. This straightforward and time-saving method for NPG layer preparation will provide new opportunities for future applications based on nanoporous structures. 4849
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tional encoded DNA−Au bio bar codes. Anal. Chem. 2008, 80, 9124− 9130. (8) Park, S.; Kim, H. C.; Chung, T. D. Electrochemical analysis based on nanoporous structures. Analyst 2012, 137, 3891−3903. (9) Yu, C. F.; Jia, F. L.; Ai, Z. H.; Zhang, L. Z. Direct oxidation of methanol on self-supported nanoporous gold film electrodes with high catalytic activity and stability. Chem. Mater. 2007, 19, 6065−6067. (10) Zeis, R.; Lei, T.; Sieradzki, K.; Snyder, J.; Erlebacher, J. Catalytic reduction of oxygen and hydrogen peroxide by nanoporous gold. J. Catal. 2008, 253, 132−138. (11) Seo, B.; Kim, J. Electrooxidation of glucose at nanoporous gold surfaces: Structure dependent electrocatalysis and its application to amperometric detection. Electroanalysis 2010, 22, 939−945. (12) Nishio, K.; Masuda, H. Anodization of gold in oxalate solution to form a nanoporous black film. Angew. Chem., Int. Ed. 2011, 50, 1603−1607. (13) Jeong, H.; Kim, J. Electrochemical oxidation of glucose at nanoporous black gold surfaces in the presence of high concentration of chloride ions and application to amperometric detection. Electrochim. Acta 2012, 80, 383−389. (14) Jeong, H.; Kim, J. Fabrication of nanoporous au films with ultrahigh surface area for sensitive electrochemical detection of glucose in the presence of Cl−. Appl. Surf. Sci. 2014, 297, 84−88. (15) Deng, Y. P.; Huang, W.; Chen, X.; Li, Z. L. Facile fabrication of nanoporous gold film electrodes. Electrochem. Commun. 2008, 10, 810−813. (16) Xia, Y.; Huang, W.; Zheng, J. F.; Niu, Z. J.; Li, Z. L. Nonenzymatic amperometric response of glucose on a nanoporous gold film electrode fabricated by a rapid and simple electrochemical method. Biosens. Bioelectron. 2011, 26, 3555−3561. (17) Frankenthal, R. P.; Siconolfi, D. J. The anodic corrosion of gold in concentrated chloride solutions. J. Electrochem. Soc. 1982, 129, 1192−1196. (18) Wang, J.; Shao, Y.; Jin, Y.; Wang, F.; Dong, S. Electrochemical thinning of thicker gold film with qualified thickness for surface plasmon resonance sensing. Anal. Chem. 2005, 77, 5760−5765. (19) Loo, B. H. In situ identification of halide complexes on gold electrode by surface-enhanced Raman spectroscopy. J. Phys. Chem. 1982, 86, 433−437. (20) Diaz, M. A.; Kelsall, G. H.; Welham, N. J. Electrowinning coupled to gold leaching by electrogenerated chlorine: I. Au(III)− Au(I)/Au kinetics in aqueous Cl2/Cl− electrolytes. J. Electroanal. Chem. 1993, 361, 25−38. (21) Heumann, T.; Panesar, H. S. Dissolution mechanism of gold with formation of chlorine complexes and its passivation. Z. Phys. Chem. 1965, 229, 84−97. (22) Trasatti, S.; Petrii, O. A. Real surface-area measurements in electrochemistry. Pure Appl. Chem. 1991, 63, 711−734. (23) Nicol, M. The anodic behaviour of gold. Part IOxidation in acidic solutions. Gold Bull. 1980, 13, 46−55. (24) Peuckert, M.; Coenen, F. P.; Bonzel, H. P. On the surface oxidation of a gold electrode in 1 N H2SO4 electrolyte. Surf. Sci. 1984, 141, 515−532. (25) Juodkazis, K.; Juodkazyte, J.; Jasulaitiene, V.; Lukinskas, A.; Sebeka, B. XPS studies on the gold oxide surface layer formation. Electrochem. Commun. 2000, 2, 503−507. (26) Xiang, C.; Guell, A. G.; Brown, M. A.; Kim, J. Y.; Hemminger, J. C.; Penner, R. M. Coupled electrooxidation and electrical conduction in a single gold nanowire. Nano Lett. 2008, 8, 3017−3022. (27) Jia, F. L.; Yu, C. F.; Ai, Z. H.; Zhang, L. Z. Fabrication of nanoporous gold film electrodes with ultrahigh surface area and electrochemical activity. Chem. Mater. 2007, 19, 3648−3653. (28) Cherevko, S.; Chung, C.-H. Direct electrodeposition of nanoporous gold with controlled multimodal pore size distribution. Electrochem. Commun. 2011, 13, 16−19. (29) Jia, F.; Yu, C.; Zhang, L. Hierarchical nanoporous gold film electrode with extra high surface area and electrochemical activity. Electrochem. Commun. 2009, 11, 1944−1946.
ASSOCIATED CONTENT
S Supporting Information *
Cyclic voltammograms of NPG electrodes in 0.1 M H2SO4 as a function of potential gaps (Figure S1), changes in color of electrolytes containing phenolphthalein during the anodization of the Au surface in 1 M KCl solution with reaction times (Figure S2), pH of unbuffered (squares) and buffered (circles, 0.1 M phosphate at pH 7) electrolytes containing 1 M KCl after anodization reactions as a function of reaction times (Figure S3), linear scan voltammogram of the Au electrode as a function of concentration Cl− (Figure S4), cyclic voltammograms of bare Au surfaces in 0.1 M H2SO4 at a scan rate of 50 mV s−1 (Figure S5), deconvolution peak fitting of XPS spectra of as-prepared NPG surfaces in the Au 4f region shown in the solid line of Figure 6A (Figure S6), optimized potentials for NPG formation depending upon pH values of electrolytes (Figure S7), amperometric responses of NPG surfaces (Rf = 1300) to added glucose (3, 6, 9, 12, and 15 mM) at 0.2 V in 0.1 M phosphate buffers (pH 7) in the absence of Cl− (Figure S8), sensitivity of amperometric detection of glucose in the presence of 0.1 M Cl− at NPG surfaces with different Rf (Figure S9), and (A) Three consecutive amperometric detections of glucose in the presence of 0.1 M Cl− at the same NPG surfaces (Rf = 1300) and (B) calibration plots obtained from amperometric responses at NPG surfaces (Figure S10). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +82-43-261-2284. Fax: +82-43-267-2279. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (2012R1A1A2041671). This research was financially supported by the Ministry of Education (MOE) and NRF through the Human Resource Training Project for Regional Innovation (2012H1B8A2026112).
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REFERENCES
(1) Ding, Y.; Kim, Y. J.; Erlebacher, J. Nanoporous gold leaf: “Ancient technology”/advanced material. Adv. Mater. 2004, 16, 1897− 1900. (2) Ding, Y.; Erlebacher, J. Nanoporous metals with controlled multimodal pore size distribution. J. Am. Chem. Soc. 2003, 125, 7772− 7773. (3) Fujita, T.; Guan, P.; McKenna, K.; Lang, X.; Hirata, A.; Zhang, L.; Tokunaga, T.; Arai, S.; Yamamoto, Y.; Tanaka, N.; Ishikawa, Y.; Asao, N.; Yamamoto, Y.; Erlebacher, J.; Chen, M. Atomic origins of the high catalytic activity of nanoporous gold. Nat. Mater. 2012, 11, 775−780. (4) Wittstock, A.; Zielasek, V.; Biener, J.; Friend, C. M.; Baumer, M. Nanoporous gold catalysts for selective gas-phase oxidative coupling of methanol at low temperature. Science 2010, 327, 319−322. (5) Zhang, L.; Lang, X.; Hirata, A.; Chen, M. Wrinkled nanoporous gold films with ultrahigh surface-enhanced raman scattering enhancement. ACS Nano 2011, 5, 4407−4413. (6) Liu, Z.; Du, J.; Qiu, C.; Huang, L.; Ma, H.; Shen, D.; Ding, Y. Electrochemical sensor for detection of p-nitrophenol based on nanoporous gold. Electrochem. Commun. 2009, 11, 1365−1368. (7) Hu, K. C.; Lan, D. X.; Li, X. M.; Zhang, S. S. Electrochemical DNA biosensor based on nanoporous gold electrode and multifunc4850
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(30) Hsiao, M. W.; Adzic, R. R.; Yeager, E. B. Electrochemical oxidation of glucose on single crystal and polycrystalline gold surfaces in phosphate buffer. J. Electrochem. Soc. 1996, 143, 759−767. (31) Cho, S.; Shin, H.; Kang, C. Catalytic glucose oxidation on a polycrystalline gold electrode with an amalgamation treatment (TM 05092). Electrochim. Acta 2006, 51, 3781−3786. (32) Park, S.; Chung, T. D.; Kim, H. C. Nonenzymatic glucose detection using mesoporous platinum. Anal. Chem. 2003, 75, 3046− 3049.
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