Revisiting Background Signals and the Electrochemical Windows of

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Revisiting Background Signals and the Electrochemical Windows of Au, Pt, and GC Electrodes in Biological Buffers Bo-Chang Lai, Jhih-Guang Wu, and Shyh-Chyang Luo ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b01249 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 14, 2019

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ACS Applied Energy Materials

Revisiting Background Signals and the Electrochemical Windows of Au, Pt, and GC Electrodes in Biological Buffers Bo-Chang Lai,†,‡ Jhih-Guang Wu,† and Shyh-Chyang Luo*,†,‡ †Department

of Materials Science and Engineering, National Taiwan University, No. 1, Sec.

4, Roosevelt Road, Taipei 10617, Taiwan ‡Advanced

Research Center for Green Materials Science and Technology, National Taiwan

University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan

KEYWORDS electrochemical window, biological buffer, background signal, cyclic voltammetry, electrode reaction, bioelectrode

ABSTRACT : For electrochemical experiments involving biological buffers, pH values, ions, and electrode materials play major roles in the electrochemical readout of the measurements. When conducting electrochemical experiments, background signals are sometimes mixed with true signals, easily leading to a wrong interpretation of the data. These background signals are easily induced by the reactions between buffers and electrode materials. However, these background signals have rarely been studied systematically. In response to rapid developments in the field and application of bioelectrodes, we conducted a much-needed systematic study of

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these background signals and the electrochemical windows in buffers—specifically, of the electrochemical windows gold, glassy carbon, and platinum in three most commonly used biological buffers, namely, Tris, HEPES, and phosphate. We examined the pH effect using HCl, H2SO4, and NaOH to modulate the pH values from 6.0 to 9.0 in the three buffers. Furthermore, through comparison of HCl and H2SO4, we were able to illustrate the reaction between Cl− ions and the metallic electrode. This reaction also led to clear redox peaks as background signals in cyclic voltammograms. When a high potential was applied, the formation of hydroxide was evident on the metallic electrode, which led to a clear reduction peak in cyclic voltammograms. In addition, we used an atomic force microscope to monitor the morphology of the electrode surface when a cyclic potential was applied. All tests were conducted in the presence of 100 mM LiClO4, which was used as the electrolytes. These characterization results yield critical insights into electrode surface reactions, insights which are crucial for precisely interpreting electrochemical results measured in biological buffers. This fundamental study provides comprehensive information, which is especially helpful for the development of bioelectrode materials and bioelectronics applications.

INTRODUCTION 　　 Electrochemical techniques have been under development since the 1800s.1-2 Because electrochemical setups are efficient and generally easy to apply, the electrochemical process has been extensively used in various applications such as electrochemical cells,3-5 bioelectrode,6-8 renewable energy resources,9-12 electrochemical machining,13-14 corrosion,15 and sensors.16-17 Recently, because of great advances in biomedical technologies,18 more attention has been paid to the use of electrochemical methods in bio-related applications.19-22 The emerging requirement for diagnostic devices for point-of-care testing (POCT)19 has recently stimulated interest in the development of portable electrochemical biosensors.23 For example, regular monitoring of their blood glucose concentration is critical to the health of


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patients with diabetes. Early detection is also very helpful for preventing fatal diseases. Thus, using portable biosensors for POCT can save time by making it unnecessary to frequently visit the hospital for regular checks. Similarly, using implantable bioelectronics24-25 for electrotherapy has demonstrated great success in curing many diseases. In these applications, the performance of these biosensors and bioelectronics primarily relies on the properties of their electrodes in physiological environments. 　　Studies have investigated the general electrochemical properties of the most commonly used electrodes in biological buffers, namely Au,26-29 Pt,30-31 and glassy carbon (GC)32 electrodes. Most of the early studies were not systematic. Moreover, because of the lack of modern analytical instruments, some electrode characteristics were proposed without direct measurement or observation. For example, the Cl− ions in buffers were determined to be able to cause erosion of electrodes when potentials are applied.33 Because the electrochemical properties of these different electrodes34 in biological buffers are crucial for their modern biological applications, we contend that the advances in analytical instruments enable these modern analytical tools to be usefully applied to confirm and clarify these characteristics. 　　In this study, we carefully examined the electrochemical properties of Au, Pt, and GC electrodes in three biological buffers, namely Tris, phosphate, and HEPES.35-36 This study specifically focused on how ions, pH values, and the redox phenomenon on electrode surfaces affect the electrochemical windows of the three electrodes. By mixing the solution with HCl, H2SO4, and NaOH solutions, we were able to adjust their pH values from 6.0 to 9.0, allowing us to illustrate the effects of pH levels and type of ions involved. Furthermore, we conducted electrochemical experiments with an atomic force microscope (AFM) to investigate the surface of these electrodes after a cyclic potential was applied. The ion and pH effects discussed not only elucidate the behavior of background signals and the electrochemical window of the three electrodes in biological buffers, they also provide useful information for the design of advanced


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electrode materials. This study is comprehensive, and thus aims to be an important reference for researchers who are interested in bioelectrodes and bioelectronics applications.

EXPERIMENTAL SECTION　　Materials and Reagents. Trizma base crystalline, potassium phosphate dibasic powder, sodium hydroxide solution (NaOH, 1.0 N), and 4-(2-hydroxyethyl)-piperazine-1ethanesulfonic acid (HEPES) powder were purchased from Sigma-Aldrich Corp., America. Sulfuric acid (H2SO4, 97%) was purchased from SHOWA Corp., Japan. Hydrochloric acid (HCl, 37%) was purchased from Fisher Chemical, America. Lithium perchlorate (LiClO4, anhydrous) was purchased from Alfa Aesar, America. The deionic (DI) water used to prepare solutions was obtained using an Elga Micra Type II system. All chemical reagents used were of analytical grade and were used directly without further purification. 　Buffer Solution Preparation. All the pH values were measured using a pH meter (6100, EZDO, Taiwan) at 25°C, and 100 mM Trizma, HEPES, and potassium phosphate dibasic were first prepared individually in DI water. The pH value of the prepared 100 mM Trizma solution was initially 10.4. Both H2SO4 and HCl solutions were used to lower the pH value to 6.0, 7.0, 8.0, and 9.0. The pH value of 100 mM HEPES solution was 5.1, and 1.0 N NaOH solution was used to increase the pH value of HEPES solution to 6.0, 7.0, 8.0, and 9.0. The pH value of 100 mM potassium phosphate dibasic was 9.3. H2SO4 solution was used to lower the pH value of phosphate solution to 6.0, 7.0, 8.0, and 9.0. All solutions were freshly prepared before the measurements, and 100 mM LiClO4 was added as supporting electrolytes. 　Electrochemical Procedures. All electrochemical experiments were performed using an Autolab PGSTAT128N potentiostat (Metrohm, Netherlands). A three-electrode setup37-38 comprising a Pt wire as a counter electrode and a Ag/AgCl electrode saturated with NaCl as the reference electrode was used. The potentials were measured against the reference electrode


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and converted to the reversible hydrogen electrode (RHE). Au, GC, and Pt electrodes (dia. 3.0 mm) were used as working electrodes. All working electrodes were first polished using 5 × 10−2 µm of aluminum oxide powder and then ultrasonically polished in methanol for 30 s. Next, electrodes were rinsed with DI water and blown dry before used. Cyclic voltammograms (CVs) were obtained by applying a cyclic potential at a scan rate of 50 mV/s.39 All electrochemical experiments were conducted at 25 °C. XPS Spectra Analysis. The XPS signals were recorded on a PHI 5000 VersaProbe (ULVAC-PHI, Chigasaki, Japan) system, which use a microfocused Al X-ray beam (100 μm, 25 W) with a photoelectron take-off angle of 45 degree. A dual-beam charge neutralizer and ion gun (1 V electron beam and 7 V Ar+) are used to compensate the charge effect. The pressure of the chamber is lower than 10-7 Pa by evacuation using ion getter pumps. The pass energy is 117.4 eV. RESULT AND DISCUSSION 　 Tris Buffer (H2SO4 for pH Modulation). We prepared 100 mM Trizma base buffer, using 100 mM LiClO4 for supporting electrolytes. The initial pH value was 10.4. An aqueous solution containing 0.10 N H2SO4 was used to lower the pH values. Tris buffers with four pH values (6.0, 7.0, 8.0, and 9.0) were prepared. Figure 1 depicts the CV obtained using Au, GC, and Pt electrodes as the working electrodes. In these CV profiles, upward peaks indicate an oxidation process and downward peaks indicate a reduction process. For the Au electrode, we could clearly identify a minor redox behavior when the applied potential was between 0.40 and 1.10 V (vs RHE) at pH = 9.0. These redox peaks indicate a redox behavior of Au in the presence of H2O39-40, as shown in eq 1. Au(s) + n H2O ⇌ Au(OH)n + n H+ + n e-　(1)


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Figure 1. CV profiles of gold, GC, and platinum electrodes in Tris buffer solution. pH values modulated by H2SO4. 　According to this equation, the concentration of H+ (or OH-) ions affects the equilibrium potential. This is depicted in Figure 1. At pH = 6.0, a clear reduction peak was observed, although the oxidation peak was not readily apparent in the CV. This indicates that at a low pH value, Au can be oxidized to form Au(OH)n during the oxidation of water when a high oxidation potential is applied. Based on the Nernst equation, the reduction potential of Au(OH)n decreased as the pH values increased. We concluded that the potentials of these redox peaks— listed in Supporting Information Table S1—and the pH effect on this redox potential can be observed clearly and quantitatively. When the applied potential was higher than 1.10 V (vs RHE), a strong oxidation peak at approximately 1.5 to 1.6 V was observed. In the meantime, some bubbles appeared near the working electrodes, indicating the formation of O2 gas.41 Oxidation reactions can be represented in eqs 2 and 3. 4 OH- ⇌ O2(g) + 2 H2O + 4 e-　(2) 2 H2O ⇌ 4 H+ + O2(g) + 4 e-　(3)


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Figure 2. The CV profiles of gold electrode with upper potential at 1.0 V (orange), 1.2 V (olive), 1.4 V (magenta), 1.6 V (blue), 1.8 V (red), and 2.0 V (black) vs RHE in a buffer at pH = 8.0. 　For both reactions, the pH value also plays a key role. The pH value increased with an increase of OH− ion concentration. When the pH value was higher than 7.0, the oxidation of OH− ions was clearly revealed before the water was directly oxidized to form O2. Moreover, the increase of the pH value promoted both reactions in eqs 2 and 3. We also observed that the intensity of oxidation peaks increased with increasing pH values. This indicates a faster oxidation reaction. 　　The following is a key finding. As depicted in Figure 2, we obtained a series of CVs by continually changing the upper potential of the cyclic potential in a Tris buffer from 1.0 to 2.0 V (vs RHE) at pH = 8.0. We noticed that the intensity of reduction peaks increased with the increase of upper potentials. This was because, according to eq 1, the higher the upper potentials are, the more Au(OH)n forms on the electrode surface. This led to an increasing intensity of the reduction peak. We also used XPS to see the surface elemental composition after we applied linear sweep potential from -0.40 to 2.00 V (vs RHE) as shown in Figure S1.


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　　 Compared with the Au electrode, the redox behavior of the GC electrode was rather simple. No redox peak was evident when the applied potential was between 0.40 and 1.10 V (vs RHE), thus indicating inert behavior from the GC electrode within this range. Only when the applied potential was higher than 1.20 V (vs RHE), an oxidation peak at approximately 1.50 to 1.60 V was clearly observed. Similar to the Au electrode, this peak corresponds to the oxidation of hydroxide ions represented in eq 2. When the pH value increased, the concentration of OH− increased, and the peak area was also larger. 　　For the redox behavior of the Pt electrode, we also clearly observed reduction behavior when the applied potential was between −0.20 and 0.50 V (vs RHE) and when the potential was swept from 2.0 to −0.4 V (vs RHE). This reduction behavior can be described by the reaction represented in eq 4.42 Pt(s) + H2O ⇌ Pt-OH + H+ + e-　(4) 　Compared with the Au electrode, the oxidation of Pt at pH = 9.0 was not clearly observed, thus indicating more inert behavior from Pt. However, Pt could still be oxidized to form Pt-OH on the Pt electrode when a high oxidation potential was applied. According to the Nernst equation, the concentration of OH− or H+ ions affects the reduction potential. Similar to the Au electrode, the reduction potential of Pt-OH decreased as the pH values increased. We also used XPS to monitor the surface composition after a linear sweep potential from -0.40 to 2.00 V (vs RHE) was applied as shown in Figure S2. When the applied potential was higher than 1.00 V (vs RHE), a strong oxidation peak occurred, thus indicating the formation of O2 gas. Concurrently, some bubbles appeared near the Pt electrode surface. The oxidation reactions are represented in eqs 2 and 3. 　In summary, for the electrochemical properties of the three electrodes in Tris buffers, we determined that the increase of pH values promoted water oxidation and O2 gas formation. Conversely, Au and Pt could react with H2O to form hydroxide. Oxidation potential decreased


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as pH values increased, in accordance with the Nernst equation. Compared with the Au and Pt electrodes, the GC electrode was rather inert and had high overpotential to the oxidation of water when the pH value was lower than approximately 7.0. All redox potentials mentioned previously are summarized in Supplementary Information Table S1.

　Tris Buffer (HCl for pH Modulation). In addition to H2SO4, HCl is another common acid used to modify the pH value of buffers. In this section, we discuss the change in redox behavior of the three electrodes in Tris buffer when HCl rather than H2SO4 is used to modify the pH value. Similar to our previous experiment using H2SO4, we prepared 100 mM Trizma base solution with 100 mM LiClO4 used as the supporting electrolytes. A 37% HCl solution was used to modify the pH values and Tris buffers. Four pH values (6.0, 7.0, 8.0, 9.0) were prepared. The CV plots obtained using Au, GC, and Pt electrodes as the working electrode are depicted in Figure 3. Similar to the redox behavior in Tris buffer containing SO42− anions, we also observed minor redox behavior in the Au electrode, represented in eq 1, between 0.40 and 0.90 V. The concentration of OH− ions affected the redox potential, and the potential decreased as the pH values increased. When the applied potential was higher than 1.50 V (vs RHE), a strong oxidation peak was observed, indicating the formation of O2 gas through the oxidation of OH− ions and water. 　As depicted in the CV (Figure 1), the main difference between this experiment and the last is the strong oxidation peak between 1.0 and 1.5 V at pH values of 6.0 and 7.0. This observation was primarily because of the reaction represented in eq 5. Specifically, a Au electrode reacts with Cl− ions to form AuCl2−.43 Au(s) + 2 Cl- ⇌ AuCl2- + e-　(5)


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Figure 3. CV profiles of gold, GC, and platinum electrodes in Tris buffer solution. pH values modulated by HCl.

Figure 4. The optical images, AFM images and cross-section of (a) pristine Au surface, (b) Au surface after 3 cycles of cyclic potential applied, and (c) Au surface after 5 cycles of cyclic potential applied.


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We expected changes in the surface morphology of Au after this reaction. We thus used an AFM and XPS to monitor the Au surface after a cyclic potential from −0.4 to 2.0 V (vs RHE) was applied on it. A silicon wafer coated with a thin 200-nm thick Au layer was prepared and used. Figure 4 displays the optical images, AFM images, and cross-sections of the three Au-coated substrates; one substrate was a pristine Au surface, another had three cycles of cyclic potential applied to it, and the third had five cycles of cyclic potential applied to it. The arithmetic average roughness Ra of the three samples was 2.8 nm, 19.1 nm, and 8.4 nm, respectively. According to the cross-sections of the three samples, the initial Au surface was clearly smooth and homogeneous. After three cycles of cyclic potential were applied, some Au was dissolved into the buffer and the Ra increased. After five cycles of cyclic potential were applied, most of the Au dissolved into the buffer, and only Au nanodots remained on the substrates. The XPS data of pristine Au surface and Au surface after 5 cycles of cyclic potential applied are shown in Figure S3. By contrast, compared with the Au electrode, the CV profiles of the GC electrode were rather simple. No redox behavior of GC itself was observed when the applied potential was between 0.40 and 0.90 V (vs RHE), indicating inert behavior from GC. A characteristic peak was observed in the oxidation scans when the applied potential was higher than 1.40 V (vs RHE). This oxidation peak corresponded to the oxidization of OH− and H2O. The reaction can be represented by eqs 2 and 3. We also observed that the intensity of oxidation peaks increased with the increase of pH values. For the Pt electrode, we also clearly observed oxidation of OH− and H2O when the applied potential increased. When the pH value was higher than 7, the contribution of this oxidation peak was dominant in the oxidation of OH−, as represented in eq 2. The peak intensity increased with increasing pH values. A reduction peak at approximately −0.10 and 0.40 V (vs RHE) was also observed for the Pt electrode, indicating a similar reduction behavior of Pt. The intensity of the reduction peak was higher when the pH


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Figure 5. CV profiles of gold, GC, and platinum electrodes in HEPES buffer solution. pH values modulated by H2SO4. value was below 7.0. We suspect that the reduction of PtCl42- , contributed to this reduction behavior. In the presence of highly concentrated Cl− ions in the buffer, Pt can be oxidized to form PtCl42-,44 as represented in eq 6. A clear oxidation peak at approximately 1.6 V (vs RHE) was also evident when pH = 6.0, corresponding to the oxidation of Pt to form PtCl42-. In Supporting Information Table S2, we present the precise potentials. Pt(s) + 4 Cl- ⇌ PtCl42- + 2 e-　(6) 　In summary, most CVs of Au, GC, and Pt electrodes displayed similar behavior when we tuned the pH values of the buffers by replacing H2SO4 with HCl. The primary difference between them is the complex ions formation. It is caused by the complexation reaction of metals with Cl− ions. In the presence of Cl− ions, the Au and Pt electrodes tended to be oxidized into AuCl4− and PtCl6−, respectively, when a high potential was applied. We observed this redox behavior more clearly when we added HCl, which lowered the pH values of the buffers. Specifically, the lower pH values led to an increasing concentration of Cl− ions in the buffers. Compared with Pt, the Au working electrode had a more intense oxidation peak, thus indicating


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that AuCl2− formed much faster than PtCl42-. Compared with Au, the Pt electrode was relatively stable in the presence of Cl− ions.

　HEPES Buffer (NaOH for pH Modulation). We prepared 100 mM HEPES solution with 100 mM LiClO4 as supporting electrolytes. The initial pH value was 5.1, and a 0.10 N NaOH solution was used to modulate the pH values. We prepared four HEPES buffers of pH values 6.0, 7.0, 8.0, 9.0. Figure 5 depicts the CV profiles obtained using Au, GC, and Pt electrodes as working electrodes. For the Au electrode, we could clearly observe minor redox behavior when the applied potential was between 0.50 and 1.20 V. The redox peaks indicate the redox behavior of Au, which can be represented by eq 1. At high potential, the oxidation peak was caused by the oxidation of OH− ions and H2O, which can be represented by eqs 2 and 3. Compared with the Au electrode, the redox behavior of the GC electrode was simple. The GC electrode exhibited no redox behavior when the applied potential was between −0.40 and 1.00 V (vs RHE), indicating inert behavior from GC when in HEPES buffers. When the applied potential was higher than 1.00 V (vs RHE), oxidation of OH− and H2O was observed. We also observed bubbles being formed near the GC electrode. Finally, regarding the redox behavior of the Pt electrode, we also observed small redox peaks when the applied potential was between −0.20 and 0.50 V (vs RHE). This redox behavior can be described by eq 4. When the applied potential was higher than 0.90 V (vs RHE), we also observed the oxidation of OH− and H2O. This oxidation can be represented by eqs 2 and 3. We also observed bubbles formed on the Pt electrode. 　 In summary, the characteristics of CV measured in the HEPES buffer were generally similar to those in the Tris buffer. However, the redox potentials were slightly shifted, mainly because of different ions in the buffers. All potentials are presented in Supporting Information Table S3.


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Figure 6. CV profiles of gold, GC, and platinum electrodes in phosphate buffer solution. pH values modulated by H2SO4. Phosphate Buffer (H2SO4 for pH Modulation). We prepared 100 mM potassium phosphate dibasic (K2HPO4) solution with 100 mM LiClO4 as supporting electrolytes. The initial pH value was 9.3, and a 0.10 N H2SO4 solution was used to modulate the pH values of the phosphate buffers. Four buffers of pH values 6.0, 7.0, 8.0, and 9.0 were prepared. Figure 6 depicts the CV profiles obtained using the Au, GC, and Pt electrodes as the working electrodes. For the Au electrode, we clearly observed a small reductive peak when the applied potential was lower than 0.80 V (vs RHE). We also observed a mild oxidation process when the applied potential was between 1.0 to 1.5 V (vs RHE). These redox peaks indicate the redox behavior of Au in the presence of OH−. This redox behavior is represented by eq 1. Moreover, as depicted in Figure 7, the intensity of the reduction peak increased when the upper potential of the CV increased. We applied a series of CVs by changing the upper potential from 1.0 to 2.0 (vs RHE). Different from the CV in Tris and HEPES buffers, the reduction of H2O was clearly observed in a phosphate buffer when the applied potential was lower than 0.2 V (vs RHE). Interestingly, when the upper potential of CV increased, the reduction of water was enhanced, which might


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Figure 7. The CV profiles of gold electrode with upper potential at 1.0 V (orange), 1.2 V (olive), 1.4 V (magenta), 1.6 V (blue), 1.8 V (red), and 2.0 V (black), in a phosphate buffer at pH = 8.0. have been caused by the refresh of the Au surface when applied potential increased. We also used AFM to check if the surface morphology would be changed or not. The difference between Au films before and after a cyclic potential applied was not so obvious as shown in Supporting Information Figure S4. When the applied potential was higher than 1.50 V, the oxidation of OH− and H2O was observed and is described by eqs 2 and 3. Compared with the Au electrode, the redox behavior of the GC electrode was not clearly observed in the phosphate buffer when the applied potential was between 0.0 V and 1.5 V (vs RHE). Even when the applied potential reached 2.00 V (vs RHE), no oxygen bubbles near the working electrode were observed. This might have been attributable to the high overpotential of the GC electrode in phosphate buffer. Using the Pt electrode as the working electrode, we could see a reductive peak when the applied potential was lower than 0.40 V (vs RHE). However, the oxidation of Pt was a slower process, and sharp peaks did not readily appear. Finally, the oxidation of OH− and H2O could be observed when the potential was higher than 1.30 V (vs RHE). The oxidation is represented by eqs 2 and 3. All potentials mentioned are listed in Table S3.


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Table 1. CV data of some particular potential mentioned in this article. (Note. Ox: oxidation; Red reduction.) pH

6.0

Buffer

Tris

Acid/base Surface redox for control pH (V vs RHE) H2SO4

OH- oxidation

Other reaction

(V vs RHE)

(V vs RHE)

Ox : Not obvious Not obvious Red : 0.88

7.0

Tris

H2SO4

Ox : 0.91

1.56

Red : 0.68 8.0

Tris

H2SO4

Ox : 0.89

1.74

Red : 0.58 9.0

Tris

H2SO4

Ox : 0.72

2.10

Red : 0.49 6.0

7.0

6.0

Tris

Tris

Tris

HCl

HCl

HCl

Ox : Not obvious Not obvious

Ox : 1.54

Red : 0.70

Au(s) + 4 Cl- ⇌ AuCl4- + 3 e-

Ox : 1.08

Not obvious

Ox : 1.39

Red : 0.70

Au(s) + 4 Cl- ⇌ AuCl4- + 3 e-

Ox : Not obvious

Ox : 1.54

Red : 0.28

Pt(s) + 6 Cl- ⇌ PtCl62- + 4 e-

7.0

HEPES

NaOH

N/A

1.72

7.0

Phosphate

H2SO4

Ox : Not obvious 1.82 Red :0.13/ 0.64

7.0

HEPES

NaOH

Ox : 0.56

0.99

Red : 0.52


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CONCLUSIONS 　We have illustrated that the background signals and electrochemical window were different under various experimental conditions. Conditions such as buffer types, pH values, working electrodes, and the use of an acid or base to modulate the pH values were examined. To emphasize the main difference, the CV data mentioned in this article are compiled in Table 1. Our findings are four-fold. First, the oxidation of Au to form Au(OH)n is pH dependent and governed by the Nernst equation; as the pH values increase, the OH− ions promote the oxidation of Au and lower oxidation potential. Second, the Cl− ions can react with and dissolve Au and Pt to form AuCl2− and PtCl42- and dissolve. This underscores that the acid used to modulate the pH values is very crucial. When the electrochemical experiments are conducted in buffer solutions at low pH value, HCl should not be used for modulating the pH value. Third, the different electrode materials exhibited different chemical activity in the same buffer solutions. Compared with the Au and Pt electrodes, the GC electrode had a relatively high overpotential for the oxidation of OH−. Fourth, the same electrode material exhibited different electrochemical windows in different buffers. For the GC electrode, the overpotential of oxidation of OH− and H2O in the phosphate buffer solution was higher than in the Tris and HEPES buffer solutions. We also summarize the electrochemical windows of three electrodes in three buffers listed in Table S4. We expect this fundamental study to be an important reference for researchers who are interested in the development of bioelectrode materials and bioelectronics applications.

ASSOCIATED CONTENT Supporting Information


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The Supporting Information is available free of charge on the ACS Publications website at DOI: Detailed electrochemical data of Au, GC, and Pt electrodes in Tris, HEPES, and phosphate buffers. The AFM images of Au electrodes. The XPS data of Au and Pt electrodes. AUTHOR INFORMATION Corresponding Author *TEL: +886-2-3366-1875. E-mail: [email protected] ORCID Shyh-Chyang Luo: 0000-0003-3972-1086 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Ministry of Education and The Ministry of Education and the Ministry of Science and Technology of Taiwan. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge the financial support provided by the “Advanced Research Center for Green Materials Science and Technology” from The Featured Area Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (108L9006) and Ministry of Science and Technology of Taiwan under grant MOST 106-2113-M-002-017-MY2 and 108-3017-F-002-002. We also gratefully acknowledge the assistance provided by Dr. Cheng-Hung Hou and Dr. Jing-Jong Shyue from Research Center for Applied Science, Academia Sinica for XPS analysis.


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REFERENCES (1) Devilliers, D.; Mahe, E. Electrochemical Cells: Thermodynamics and Kinetics of Electrochemical Reactions. Actual. Chim. 2003, (1), 31-40. (2) Chen, G. H. Electrochemical Technologies in Wastewater Treatment. Sep. Purif. Technol. 2004, 38 (1), 11-41, DOI: 10.1016/j.seppur.2003.10.006. (3) Rosa, T. M.; Roveda, A. C.; Godinho, W. P. D.; Martins, C. A.; Oliveira, P. R.; Trindade, M. A. G. Electrochemical Cell Designed for in situ Integrate Microextraction and Electroanalysis: Trace-level Determination Of Norfloxacin in Aqueous Samples. Talanta 2019, 196, 39-46, DOI: 10.1016/j.talanta.2018.12.028. (4) Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J. M.; Van Schalkwijk, W. Nanostructured Materials for Advanced Energy Conversion and Storage Devices. Nat. Mater. 2005, 4 (5), 366-377, DOI: 10.1038/nmat1368. (5) Sun, D.; Shen, Y.; Zhang, W.; Yu, L.; Yi, Z. Q.; Yin, W.; Wang, D.; Huang, Y. H.; Wang, J.; Wang, D. L.; Goodenough, J. B. A Solution-Phase Bifunctional Catalyst for LithiumOxygen Batteries. J. Am. Chem. Soc. 2014, 136 (25), 8941-8946, DOI: 10.1021/ja501877e. (6) Holmberg, S.; Rodriguez-Delgado, M.; Milton, R. D.; Ornelas-Soto, N.; Minteer, S. D.; Parra, R.; Madou, M. J. Bioelectrochemical Study of Thermostable Pycnoporus sanguineus CS43 Laccase Bioelectrodes Based on Pyrolytic Carbon Nanofibers for Bioelectrocatalytic O-2 Reduction. ACS Catal. 2015, 5 (12), 7507-7518, DOI: 10.1021/acscatal.5b01600. (7) Hsiao, Y. S.; Liao, Y. H.; Chen, H. L.; Chen, P. L.; Chen, F. C. Organic Photovoltaics and Bioelectrodes Providing Electrical Stimulation for PC12 Cell Differentiation and Neurite Outgrowth. ACS Appl. Mater. Interfaces 2016, 8 (14), 9275-9284, DOI: 10.1021/acsami.6b00916. (8) Ahreu, C.; Nedellec, Y.; Gross, A. J.; Ondel, O.; Buret, F.; Le Goff, A.; Holzinger, M.; Cosnier, S. Assembly and Stacking of Flow-through Enzymatic Bioelectrodes for High


20

Page 21 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Power Glucose Fuel Cells. ACS Appl. Mater. Interfaces 2017, 9 (28), 23836-23842, DOI: 10.1021/acsami.7b06717. (9) Carrasco, J. M.; Franquelo, L. G.; Bialasiewicz, J. T.; Galvan, E.; Portillo, R.; Prats, M. M.; Leon, J. I.; Moreno-Alfonso, N. Power-electronic systems for the grid integration of renewable energy sources: A survey. IEEE Trans. Ind. Electron. Control Instrum. 2006, 53 (4), 1002-1016, DOI: 10.1109/tie.2006.878356. (10) Kim, J.; Chae, S.; Yi, A.; Hong, S.; Kim, H. J.; Suh, H. Syntheses and Optical, Electrochemical, and Photovoltaic Properties of Polymers with 6-(2-Thienyl)-4h-Thieno 2,3B Indole with a Variety of Electron-Deficient Units. J. Appl. Polym. Sci. 2019, 136 (23), DOI: 10.1002/app.47624. (11) Liang, Y. Y.; Wang, H. L.; Diao, P.; Chang, W.; Hong, G. S.; Li, Y. G.; Gong, M.; Xie, L. M.; Zhou, J. G.; Wang, J.; Regier, T. Z.; Wei, F.; Dai, H. J. Oxygen Reduction Electrocatalyst Based on Strongly Coupled Cobalt Oxide Nanocrystals and Carbon Nanotubes. J. Am. Chem. Soc. 2012, 134 (38), 15849-15857, DOI: 10.1021/ja305623m. (12) Pankratova, G.; Pankratov, D.; Di Bari, C.; Goni-Urtiaga, A.; Toscano, M. D.; Chi, Q. J.; Pita, M.; Gorton, L.; De Lacey, A. L. Three-Dimensional Graphene Matrix-Supported and Thylakoid Membrane-Based High-Performance Bioelectrochemical Solar Cell. ACS Appl. Energy Mater. 2018, 1 (2), 319-+, DOI: 10.1021/acsaem.7b00249. (13) Schuster, R.; Kirchner, V.; Allongue, P.; Ertl, G. Electrochemical Micromachining. Science 2000, 289 (5476), 98-101, DOI: 10.1126/science.289.5476.98. (14) Kirchner, V.; Cagnon, L.; Schuster, R.; Ertl, G. Electrochemical Machining of Stainless Steel Microelements with Ultrashort Voltage Pulses. Appl. Phys. Lett. 2001, 79 (11), 17211723, DOI: 10.1063/1.1401783.


21


Page 22 of 34

(15) Cho, S.; Kim, S.; Jo, Y.; Kim, S. Electrical, Optical and Electrochemical Corrosion Resistance of AZO, GZO, AGZO Films Depending on the Hydrogen Content. J. Nanosci. Nanotechnol. 2019, 19 (7), 3854-3858, DOI: 10.1166/jnn.2019.16268. (16) Chen, X.; Cheng, X. Y.; Gooding, J. J. Detection of Trace Nitroaromatic Isomers Using Indium Tin Oxide Electrodes Modified Using beta-Cyclodextrin and Silver Nanoparticles. Anal. Chem. 2012, 84 (20), 8557-8563, DOI: 10.1021/ac3014675. (17) Huang, P. C. S., M. Y.; Yu, H. h.; Wei, S. C.; Luo, S. C. Surface Engineering of Phenylboronic Acid-Functionalized Poly(3,4-ethylenedioxythiophene) for Fast Responsive and Sensitive Glucose Monitoring. ACS Appl. Bio Mater. 2018, 160-167, DOI: 10.1021/acsabm.8b00060. (18) Chen, C. H.; Luo, S. C. Tuning Surface Charge and Morphology for the Efficient Detection of Dopamine under the Interferences of Uric Acid, Ascorbic Acid, and Protein Adsorption. ACS Appl. Mater. Interfaces 2015, 7 (39), 21931-21938, DOI: 10.1021/acsami.5b06526. (19) Lee, H.; Choi, T. K.; Lee, Y. B.; Cho, H. R.; Ghaffari, R.; Wang, L.; Choi, H. J.; Chung, T. D.; Lu, N. S.; Hyeon, T.; Choi, S. H.; Kim, D. H. A Graphene-based Electrochemical Device with Thermoresponsive Microneedles for Diabetes Monitoring and Therapy. Nat. Nanotechnol. 2016, 11 (6), 566-+, DOI: 10.1038/nnano.2016.38. (20) Sun, S. C.; Dou, H. Y.; Chuang, M. C.; Kolpashchikov, D. M. Multi-labeled Electrochemical Sensor for Cost-efficient Detection of Single Nucleotide Substitutions in Folded Nucleic Acids. Sens. Actuators, B 2019, 287, 569-575, DOI: 10.1016/j.snb.2019.02.073. (21) Jiang, C.; Alam, M. T.; Silva, S. M.; Taufik, S.; Fan, S. J.; Gooding, J. J. Unique Sensing Interface That Allows the Development of an Electrochemical Immunosensor for the


22

Page 23 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Detection of Tumor Necrosis Factor alpha in Whole Blood. ACS Sens. 2016, 1 (12), 14321438, DOI: 10.1021/acssensors.6b00532. (22) Wu, J.-G.; Chen, J.-H.; Liu, K.-T.; Luo, S.-C. Engineering Antifouling Conducting Polymers for Modern Biomedical Applications. ACS Appl. Mater. Interfaces 2019, DOI: 10.1021/acsami.9b04924. (23) Tang, W. Z.; Yang, J. X.; Wang, F.; Wang, J. L.; Li, Z. H. Thiocholine-triggered Reaction in Personal Glucose Meters for Portable Quantitative Detection of Organophosphorus Pesticide. Anal. Chim. Acta 2019, 1060, 97-102, DOI: 10.1016/j.aca.2019.01.051. (24) Choi, C.; Lee, Y.; Cho, K. W.; Koo, J. H.; Kim, D. H. Wearable and Implantable Soft Bioelectronics Using Two-Dimensional Materials. Acc. Chem. Res. 2019, 52 (1), 73-81, DOI: 10.1021/acs.accounts.8b00491. (25) Leung, D. P.; McCormick, D. J.; Malpas, S. C.; Budgett, D. M. Reducing Drift in Implantable Pressure Sensors. IEEE Sens. J. 2019, 19 (7), 2458-2465, DOI: 10.1109/jsen.2018.2889097. (26) Erusalimchik, I. G. Electrochemical and Corrosion Behavior of Gold and its Alloys .1. Electrochemical Behavior of Gold in Hydrochloric Acid Solutions. Prot. Met 1995, 31 (6), 520-524. (27) Liu, G. Z.; Luais, E.; Gooding, J. J. The Fabrication of Stable Gold NanoparticleModified Interfaces for Electrochemistry. Langmuir 2011, 27 (7), 4176-4183, DOI: 10.1021/la104373v. (28) Tavallaie, R.; McCarroll, J.; Le Grand, M.; Ariotti, N.; Schuhmann, W.; Bakker, E.; Tilley, R. D.; Hibbert, D. B.; Kavallaris, M.; Gooding, J. J. Nucleic Acid Hybridization on an Electrically Reconfigurable Network of Gold-coated Magnetic Nanoparticles Enables


23


Page 24 of 34

MicroRNA Detection in Blood. Nat. Nanotechnol. 2018, 13 (11), 1066-+, DOI: 10.1038/s41565-018-0232-x. (29) Le Saux, G.; Ciampi, S.; Gaus, K.; Gooding, J. J. Electrochemical Behavior of Gold Colloidal Alkyl Modified Silicon Surfaces. ACS Appl. Mater. Interfaces 2009, 1 (11), 24772483, DOI: 10.1021/am900427w. (30) Gao, W.; Sattayasamitsathit, S.; Orozco, J.; Wang, J. Highly Efficient Catalytic Microengines: Template Electrosynthesis of Polyaniline/Platinum Microtubes. J. Am. Chem. Soc. 2011, 133 (31), 11862-11864, DOI: 10.1021/ja203773g. (31) Clavilier, J.; Armand, D.; Durand, R. Electrochemical Study of the Adsorption of Hydrogen on Platinum(111), Platinum(100), Platinum(110), Changes of Induced Superficial Structure, Polyoriented Surfaces, and Local Characterization by Electrochemical Microsound. Vide: Sci., Tech. Appl. 1983, 38 (216), 145-146. (32) Nam, E.; Lee, C. Y.; Jun, M. B. G.; Min, B. K. Ductile Mode Electrochemical Oxidation Assisted Micromachining for Glassy Carbon. J. Micromech. Microeng. 2015, 25 (4), DOI: 10.1088/0960-1317/25/4/045021. (33) Alorro, R. D.; Hiroyoshi, N.; Kijitani, H.; Ito, M.; Tsunekawa, M. Electrochemical Investigation of Gold Uptake From Chloride Solution by Magnetite. Miner. Process. Extr. Metall. Rev. 2015, 36 (5), 332-339, DOI: 10.1080/08827508.2015.1004406. (34) Berggren, M.; Malliaras, G. G. How Conducting Polymer Electrodes Operate. Science 2019, 364 (6437), 233-234, DOI: 10.1126/science.aaw9295. (35) Engelbrekt, C.; Sorensen, K. H.; Zhang, J. D.; Welinder, A. C.; Jensen, P. S.; Ulstrup, J. Green Synthesis of Gold Nanoparticles with Starch-glucose and Application in Bioelectrochemistry. J. Mater. Chem. 2009, 19 (42), 7839-7847, DOI: 10.1039/b911111e.


24

Page 25 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(36) Cuculic, V.; Mlakar, M.; Branica, M. Influence of the HEPES Buffer on Electrochemical Reaction of the Copper(II)-salicylaldoxime Complex. Electroanalysis 1998, 10 (12), 852856, DOI: 10.1002/(sici)1521-4109(199809)10:123.0.co;2-o. (37) Zhang, M. Q.; Sun, P.; Chen, Y.; Li, F.; Gao, Z.; Shao, Y. H. Studies of Effect of Phase Volume Ratio on Transfer of Ionizable Species Across the Water/1,2-Dichloroethane Interface by a Three-electrode Setup. Anal. Chem. 2003, 75 (16), 4341-4345, DOI: 10.1021/ac0263824. (38) Ender, M.; Weber, A.; Ivers-Tiffee, E. Analysis of Three-Electrode Setups for ACImpedance Measurements on Lithium-Ion Cells by FEM simulations. J. Electrochem. Soc. 2012, 159 (2), A128-A136, DOI: 10.1149/2.100202jes. (39) Hamelin, A. Cyclic Voltammetry at Gold Single-crystal surfaces .1. Behaviour at Lowindex Faces. J. Electroanal. Chem. 1996, 407 (1-2), 1-11, DOI: 10.1016/00220728(95)04499-x. (40) Hamelin, A.; Martins, A. M. Cyclic Voltammetry at Gold Single-crystal Surfaces .2. Behaviour of High-index Faces. J. Electroanal. Chem. 1996, 407 (1-2), 13-21, DOI: 10.1016/0022-0728(95)04500-7. (41) Wang, J.; Li, K.; Zhong, H. X.; Xu, D.; Wang, Z. L.; Jiang, Z.; Wu, Z. J.; Zhang, X. B. Synergistic Effect between Metal-Nitrogen-Carbon Sheets and NiO Nanoparticles for Enhanced Electrochemical Water-Oxidation Performance. Angew. Chem., Int. Ed. 2015, 54 (36), 10530-10534, DOI: 10.1002/anie.201504358. (42) Jerkiewicz, G.; Vatankhah, G.; Lessard, J.; Soriaga, M. P.; Park, Y. S. Surface-oxide Growth at Platinum Electrodes in Aqueous H2SO4 Reexamination of Its Mechanism Through Combined Cyclic-voltammetry, Electrochemical Quartz-crystal Nanobalance, and Auger Electron Spectroscopy Measurements. Electrochim. Acta 2004, 49 (9-10), 1451-1459, DOI: 10.1016/j.electacta.2003.11.008.


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Page 26 of 34

(43) Horikoshi, T.; Yoshimura, S.; Kubota, N.; Sato, E. The Anodic-dissolution Behavior of Gold in Sulfuric-acid-solutions Containing Chloride-ions. Nippon Kagaku Kaishi 1983,

(8),

1118-1123, DOI: 10.1246/nikkashi.1983.1118. (44) Hudak, E. M.; Mortimer, J. T.; Martin, H. B. Platinum for Neural Stimulation: Voltammetry Considerations. J. Neural Eng. 2010, 7 (2), DOI: 10.1088/17412560/7/2/026005.


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Revisiting Background Signals and the Electrochemical Windows of

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