Fabrication and characterization of nanoporous gold on microelectrode

Fabrication and characterization of nanoporous gold on microelectrode

JEAC-02953; No of Pages 4 Journal of Electroanalytical Chemistry xxx (2016) xxx–xxx Contents lists available at ScienceDirect Journal of Electroanal...

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JEAC-02953; No of Pages 4 Journal of Electroanalytical Chemistry xxx (2016) xxx–xxx

Contents lists available at ScienceDirect

Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem

Fabrication and characterization of nanoporous gold on microelectrode Masiki Ikegami, Yu Hirano, Yasuhiro Mie, Yasuo Komatsu ⁎ Bioproduction Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 2-17-2-1 Tsukisamu-higashi, Toyohira, Sapporo 062-8517, Japan

a r t i c l e

i n f o

Article history: Received 21 July 2016 Received in revised form 21 October 2016 Accepted 11 November 2016 Available online xxxx Keywords: Nanoporous gold Nanostructure Microelectrode ORC treatment Anodization

a b s t r a c t We fabricated nanoporous gold (NPG) on a microelectrode by oxidization and reduction cycles (ORCs) in acidic media. The typical morphology of the nanostructures comprised tangled thickets of branched nanowires with diameters of 20–30 nm. Furthermore, the surface area was significantly increased compared with the bare electrode. The roughness factor (Rf) of NPG could be controlled by changing the number of ORCs. The NPGmodified microelectrode with a roughness factor of 49 could immobilize 11 times more thiol-modified oligodeoxynucleotides on its surface than a microelectrode without NPG (Rf = 1.8). The results suggest that NPG formation is an important method for enhancing the electrochemical sensitivity of microelectrodes. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Electrochemical systems with microelectrodes are of high interest owing to their potential application in electrochemical analysis, sensors, and microscopy [1–3]. Microelectrodes are becoming thinner in order to minimize the electrochemical systems and improve the response time and spatial resolution. Thus, the reduction in surface area creates problems, such as low sensitivity and low signal-to-noise ratio [2]. Introducing nanostructures into the electrode surface can increase the effective surface area and considerably enhance the catalytic activity. The methods for introducing nanostructures include the template technique [2,4,5], electrodeposition [6–8], nanoparticle modification [9–11], and alloying of multi-metal compounds followed by selective dealloying. The alloying–dealloying method provides nanoporous gold (NPG)modified electrode [12–15], which is biocompatible and easily functionalized, as well as colloidal gold nanoparticles [16,17]. Thus, it has been extensively applied to numerous biosensors and biofuel cells [18–20]. Recently, NPG was electrochemically fabricated via the one-step anodization of bare gold in acidic aqueous solutions instead of alloyingdealloying [21–23]. The advantage of this fabrication method is that it is relatively simple to prepare and the metallic material used in the fabrication is pure gold. We have also fabricated NPG-modified electrodes for the direct electrochemistry of cytochrome P450 [24]. On the other hand, the use of oxidation and reduction cycles (ORCs) in acidic electrolytes is a convenient method to fabricate nanostructures on a gold surface [10,25]. We applied the ORC treated electrodes to demonstrate

the direct electrochemistry of oxidoreductases, such as cytochrome P450 [26] and the flavin-containing monooxygenase [27]. Herein, we examined two fabrication methods of NPG microelectrodes: (1) anodization and (2) the ORC treatment. We concluded that the ORC treatment could successfully form NPG with high surface area on the microelectrode. The morphology of the fabricated microelectrode was observed by field emission scanning electron microscopy (FESEM) and the electrochemical behavior was assessed by voltammetry. We further confirmed the increase in the amount of oligodeoxynucleotides immobilized on the microelectrode. 2. Experimental 2.1. Reagents and chemicals Hexaammineruthenium(III) chloride ((Ru(NH3)6)Cl3, 98%) and potassium hexacyanoferrate(II) (K4[Fe(CN)6]·3H2O, 99.5%) were purchased from Sigma-Aldrich. Interstrand cross-linked oligodeoxynucleotides with a dithiol group (s-ODNs, Fig. S1) were prepared as previously reported [28,29]. All other chemicals were of analytical grade or of the highest purity available and were used without further purification. Double-distilled water was used throughout the experiments. Au wire was purchased from the Nilaco Corporation, Japan (99.95%), and borosilicate glass capillaries were purchased from (#1B150-6) from World Precision Instruments, USA. 2.2. Apparatus

⁎ Corresponding author. E-mail addresses: [email protected] (M. Ikegami), [email protected] (Y. Hirano), [email protected] (Y. Mie), [email protected] (Y. Komatsu).

The voltammetry experiments were performed on an HZ-5000 electrochemical analyzer (Hokuto Denko Co. Ltd., Japan). A piece of Pt wire and an Ag/AgCl electrode with saturated KCl were used as the counter

http://dx.doi.org/10.1016/j.jelechem.2016.11.023 1572-6657/© 2016 Elsevier B.V. All rights reserved.

Please cite this article as: M. Ikegami, et al., Fabrication and characterization of nanoporous gold on microelectrode, Journal of Electroanalytical Chemistry (2016), http://dx.doi.org/10.1016/j.jelechem.2016.11.023

2

M. Ikegami et al. / Journal of Electroanalytical Chemistry xxx (2016) xxx–xxx

and reference electrodes, respectively. All potentials were reported relative to the Ag/AgCl reference electrode. All experiments were carried out at room temperature. FESEM images were acquired with a Hitachi S-4300. Energy-dispersive X-ray spectroscopy (EDX) was performed by using an EDAX Genesis system with FESEM capabilities. 2.3. Fabrication of the nanostructures We first fabricated the bare Au microelectrode by heat-sealing a piece of Au wire (50 μm in diameter) in a borosilicate glass capillary. The diameter of the electrode including the glass sheath was ca. 80 μm. The tip of the Au wire encased in the capillary was polished with #4000 and #15,000 polishing films, and cleaned electrochemically in 0.5 M H2SO4 solution by scanning from 0 V to 1.6 V at 100 mV/s for 15 cycles. Then, the NPG was fabricated on the electrode surface by the ORC treatment. Potentials of 1.22 V and 0.28 V were applied for 30 s in 25 mM HCl aqueous solution, and that was repeated between 1 and 14 times. Constant-voltage anodization (CVA) was carried out at 1.22 V for 2 min in 25 mM HCl aqueous solution. 2.4. Measurement procedures The real surface area of the microelectrodes was electrochemically measured from the amount of charge consumed during the reduction of the gold oxide monolayer on the surface. Voltammetry was carried out in 0.5 M H2SO4 solution by scanning from 0 V to 1.6 V at 100 mV/s and the reported value of 386 μC/cm2 was used in the calculations [30]. The characterization of the microelectrode was performed in 4 mM K4[Fe(CN)6] in 0.1 M KCl with a scan rate of 50 mV/s. The sODNs modification was performed by dipping the microelectrode in 0.2 μM s-ODNs solution (3 μL, 0.5 M phosphate buffer, pH 7) overnight in a humidity chamber at room temperature, and then washing in stirred distilled water (150 rpm) for 10 min at 30 °C. After the modification, the electrode was masked with 1 mM 6-mercapto-1-hexanol reagent for 1 h and washed in stirred phosphate buffer (150 rpm) for 1 min. The immobilized s-ODNs were quantified with redox-active hexaammineruthenium(III) chloride [31]. 3. Results and discussion 3.1. Morphology of nanostructures We previously fabricated nanoporous gold (NPG) on a macro electrode by CVA [24]. CVA was applied for 2 min in 25 mM aqueous hydrochloric acid (HCl) to form NPG on the microelectrode surface. However,

Fig. 2. FESEM images of nanostructures obtained after 10 ORCs in (a) 15 mM and (b) 55 mM HCl.

this yielded thin and heterogeneous NPG on the microelectrode surface (Fig. S2). We have also previously shown that nanoparticles (20–30 nm in diameter) were formed on the macro electrode by the ORC treatment in 100 mM HCl [10]. Next, a microelectrode was subjected to the ORC treatment in 25 mM HCl. Interestingly, NPG actually different from that observed in the CVA treatment was homogeneously formed over an entire area of the microelectrode surface (Fig. 1). The nanostructures were tangled thickets with diameters in the range of 20–30 nm which can be categorized into mesoporous materials. They were completely different from the nanoparticles fabricated by using the ORC treatment for the macro electrodes [10]. Although the NPG could be observed on the microelectrodes from the ORC treatment in 20–50 mM HCl, the polished surface partially remained in 15 mM HCl (Fig. 2a) and the nanoparticles that had been seen in the previous report [10] were observed in 55 mM HCl (Fig. 2b). Deng et al. explained the formation of the NPG in aqueous HCl [21]. During oxidation, the Au substrate electrochemically dissolves to Cl− form [Au Cl− 2 ] near the electrode surface depending on the concentration of Cl− (Eq. (1)). −

Au þ 2Cl →½Au Cl2 −  þ e−

ð1Þ

− [Au Cl− 2 ] immediately transforms to Au* atoms and [Au Cl4 ] at the growth site of the nanostructures (Eq. (2)).



3½Au Cl2 − →½Au Cl4 −  þ 2Au þ 2Cl

ð2Þ

According to these equations, the dissolution of the Au substrate proceeds slowly in low concentrations of Cl− (Eq. (1)). On the other hand, high concentrations of Cl− disturb the growth of the Au nanostructures (Eq. (2)). These reactions could explain the formation of the Au structures in different Cl− concentrations. In our results, the ORC treatment could fabricate thicker branched nanowires of NPG than CVA. In the case of ORC treatment, [Au Cl− 4 ] is reduced to Au* atoms at a low potential (0.28 V) (Eq. (3)). This reductive deposition of Au on the nanowires might be the reason behind the thicker nanowires than those fabricated by CVA. ½Au Cl4 −  þ 3e− →Au þ 4Cl



ð3Þ

3.2. Characterization of NPG nanostructures

Fig. 1. (a) FESEM images of the tip of microelectrode with 50 μm diameter surrounding glass shield obtained by 10 oxidization and reduction cycles (ORCs) in 25 mM HCl. NPG surface was depressed 3–5 μm from glass shield. (b, c) High-magnification images of NPG.

EDX was used to characterize the chemical composition of the NPG that was fabricated on the microelectrode by using 10 ORCs (Fig. 3). The results suggest mass percentages of 91.6% Au, 6.8% C, and 1.6% Al. The carbon was a surface contaminant from the atmosphere and the aluminum was attributed to stray radiation from the specimen holder. Hence, the nanostructures were 99.95% Au. The NPG on the microelectrode was also electrochemically characterized in aqueous sulfuric acid (H2SO4) (Fig. 4a). Because the voltammogram of the NPG provided a nearly identical profile to that of the bare microelectrode, the NPG comprised polycrystalline gold, like the

Please cite this article as: M. Ikegami, et al., Fabrication and characterization of nanoporous gold on microelectrode, Journal of Electroanalytical Chemistry (2016), http://dx.doi.org/10.1016/j.jelechem.2016.11.023

M. Ikegami et al. / Journal of Electroanalytical Chemistry xxx (2016) xxx–xxx

NPG microelectrode Bare microelectrode

Au

Current (nA)

Counts ( x 103)

4 3

2 1

3

Al C

30

10

Au

0 0

5 10 Energy (ke V)

15

-10 -0.1

Fig. 3. Energy dispersive X-ray spectroscopy spectrum of 3D nanostructures on microelectrode obtained after 10 ORCs in 25 mM HCl.

NPG microelectrode Bare microelectrode

0

-250

Current (nA)

Current (nA)

a)

10 0 -10 -20 0.0 0.5 1.0 1.5 Potential (V)

-500

Fig. 5. Cyclic voltammograms of bare microelectrode (dashed line) and NPG microelectrode after 10 ORCs (solid line). Both were performed in 4 mM K4[Fe(CN)6] in 0.1 M KCl solution at 50 mV/s scan rate.

area but on the geometric surface area [2]. The NPG microelectrode showed a larger double-layer capacitance, which is proportional to the real surface area of the electrode, under these conditions [2,34]. These results suggest that the NPG microelectrode has the characteristics of a microelectrode with high surface area. It is noted that the fabricated NPG was sufficiently stable in these electrochemical analyses including cyclic voltammogram in 0.5 M H2SO4, indicating applicability to the biomolecules modification.

3.3. Immobilization of oligodeoxynucleotides (s-ODNs) on NPG microelectrode Gold is an excellent substrate for self-assembled monolayers (SAMs) of thiol-modified molecules. The SAMs can be used as intermediate layers for the coupling of a wide range of molecules, such as antigens and enzymes [35,36]. Thiol-modified oligodeoxyribonucleotides (sODNs) can be scaffolds for enzymes [28,29,37]. To examine whether an NPG microelectrode would be compatible with s-ODNs modification, we immobilized s-ODNs on an NPG microelectrode that had 27 times higher real surface area than the bare microelectrode. Fig. 6 showed that the NPG microelectrode attached 11.4 times more s-ODNs than the bare one. The increase in s-ODNs did not reach the increased rate of the surface area (27 times). The branched nanowires have diameters of 20–30 nm and the size of the s-ODNs of 46 nucleobases is estimated at 15 nm. Thus, it might be difficult for s-ODNs to access the inside of the NPG layer owing to the tangled nanowires. However, this result confirms that the NPG microelectrode with extended surface area has the potential to immobilize significant amounts of s-ODNs.

Roughness factor

untreated bare electrode [3,32]. The cathodic peak of the bare electrode at 0.95 V slightly shifted to a negative potential at 0.93 V for the NPGmodified electrode, as observed in the previous report using nanostructures [33]. The real surface areas of the electrodes (averaged over three electrodes each) were calculated from the charge consumption of the reductive single peak. The NPG on the microelectrode fabricated with 10 ORCs yielded an Rf value of 49, which was almost 27 times higher than that of the bare electrode (Rf = 1.8). We prepared several microelectrodes by changing the ORCs and determined their Rf values. As shown in Fig. 4b, the Rf values increased as the numbers of ORCs increased. From the FESEM analysis, NPG was found to be formed on a part of the electrode surface for one or two ORCs; the NPG-formed area increased as the number of ORCs increased. The morphology of the branched nanowires did not significantly change through these ORC treatments (data not shown). More than 15 ORCs led to the microelectrodes breaking because of the erosion between the gold wire and glass capillary. We also evaluated the NPG microelectrode by cyclic voltammetry in the presence of 4 mM ferricyanide ions. Under this condition, the microelectrodes produced a sigmoidal cyclic voltammogram because the mass transport was dominated by radial diffusion, whereas the macro electrodes showed voltammograms with two current peaks because of linear diffusion. In Fig. 5, both NPG and the bare microelectrodes have typical sigmoidal shapes and the characteristics of the microelectrode are clearly seen. The current values in the plateau region were the same for NPG as for the bare microelectrodes; nevertheless, the cathodic peak current of the NPG microelectrode in H2SO4 solution was 24 times higher than that of the bare electrode (Fig. 4a). A possible explanation is that most ferrocyanide ions from the bulk solution reacted at the NPG outer layer before penetrating deeply into the nanostructure. In other words, the quantity of ferricyanide ions that diffused from the bulk solution to the electrode surface did not depend on the real surface

0.1 0.3 0.5 Potential (V) vs. Ag/AgCl

b)

60

40 20 0

0.0

0.5 1.0 1.5 Potential (V) vs. Ag/AgCl

0

5

10

15

ORCs

Fig. 4. (a) Cyclic voltammograms of bare microelectrode (solid line) and NPG microelectrode after 10 ORCs (dashed line). Both were performed in 0.5 M H2SO4 at 100 mV/s. The inset is a scaled version showing details of solid line. (b) Relation between Rf and number of ORCs in 25 mM HCl.

Please cite this article as: M. Ikegami, et al., Fabrication and characterization of nanoporous gold on microelectrode, Journal of Electroanalytical Chemistry (2016), http://dx.doi.org/10.1016/j.jelechem.2016.11.023

ODN sODN/NPG

Microelectrode

NPG

(x1013 molecule/cm2)

M. Ikegami et al. / Journal of Electroanalytical Chemistry xxx (2016) xxx–xxx

Immobilized ODNs

4

6 magnification:

4

11.4

2 0 Bare

NPG

Fig. 6. Schematic drawing of the s-ODNs modified NPG microelectrode and amount of sODNs immobilized on bare and NPG microelectrodes. Error bar were calculated from three electrodes.

4. Conclusions We developed a novel fabrication method of NPG on the tip of microelectrodes by ORCs of bulk gold electrode in acidic aqueous solution. The ORC treatment yielded 27 times greater real surface area and the Rf of the electrode surface could be easily controlled by changing the number of ORCs. The microelectrode showed sigmoidal voltammograms in ferricyanide ions, which are characteristic of microelectrodes with high surface area. In the functionalization of the electrode, the nanostructured microelectrode was able to immobilize 11 times more thiol-modified ODNs on its surface, compared with the bare microelectrode. This suggests that the NPG microelectrode with high surface area can find use in biomolecule modification. The facile fabrication method of the NPG microelectrode with high surface area further enhances the application potential of microelectrodes in the field of sensors, biosensors, and microscopy. Supplementary data to this article can be found online at doi:10. 1016/j.jelechem.2016.11.023. Conflict of interest The authors declare no conflict of interest to this study. Acknowledgements We thank N. Kibe of AIST for performing the SEM experiments, and Dr. Akiyoshi Nakamura and Mr. Honma of AIST for helpful discussions. Part of this study was conducted at the Nano-Processing Facility, AIST, supported by the IBEC Innovation Platform. Financial support was received, in part, from the Ministry of Education, Culture, Sports, Science and Technology, Japan (Grant-in-Aid for Scientific Research (No. 25620121)) and from Akiyama Life Science Foundation. References [1] M. Ciobanu, D.E. Taylor, J.P. Wilburn, D.E. Cliffel, Glucose and lactate biosensors for scanning electrochemical microscopy imaging of single live cells, Anal. Chem. 80 (2008) 2717–2727. [2] R. Szamocki, A. Velichko, C. Holzapfel, F. Mucklich, S. Ravaine, P. Garrigue, N. Sojic, R. Hempelmann, A. Kuhn, Macroporous ultramicroelectrodes for improved electroanalytical measurements, Anal. Chem. 79 (2007) 533–539. [3] J. Jiang, X. Wang, L. Zhang, Nanoporous gold microelectrode prepared from potential modulated electrochemical alloying-dealloying in ionic liquid, Electrochim. Acta 111 (2013) 114–119. [4] S. Cherevko, C.H. Chung, Gold nanowire array electrode for non-enzymatic voltammetric and amperometric glucose detection, Sensors Actuators B Chem. 142 (2009) 216–223. [5] M. Ikegami, Y. Mie, Y. Hirano, M. Suzuki, Y. Komatsu, Size-controlled fabrication of gold nanodome arrays and its application to enzyme electrodes, Colloids Surf. A Physicochem. Eng. Asp. 384 (2011) 388–392. [6] X.D. Zeng, X.F. Li, L. Xing, X.Y. Liu, S.L. Luo, W.Z. Wei, B. Kong, Y.H. Li, Electrodeposition of chitosan-ionic liquid-glucose oxidase biocomposite onto nano-gold electrode for amperometric glucose sensing, Biosens. Bioelectron. 24 (2009) 2898–2903. [7] L.P. Xu, Y.S. Ding, C.H. Chen, L.L. Zhao, C. Rimkus, R. Joesten, S.L. Suib, 3D flowerlike alpha-nickel hydroxide with enhanced electrochemical activity synthesized by microwave-assisted hydrothermal method, Chem. Mater. 20 (2008) 308–316.

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Please cite this article as: M. Ikegami, et al., Fabrication and characterization of nanoporous gold on microelectrode, Journal of Electroanalytical Chemistry (2016), http://dx.doi.org/10.1016/j.jelechem.2016.11.023