Dense Carbon Coated Electrode for Biosensing Application Made By Platingin Molten Salt Bath
Authors: Warigaya Ryo , Okubo Toshikazu , Sasa Tatsuro , Tsujimura Hiroyuki , Nishikiori Tokujiro , Ito Yasuhiko 
Published: February 2020

1. Introduction
Carbon electrodes are widely used for various electrochemical bio-sensing applications. This is due to the electrochemical stabilities and biological adaptabilities. Furthermore, from the viewpoints of business, it is considered that carbon is reasonable in price compared with precious metals such as platinum or gold and supposed stabilities of material supply. There are several kinds of materials and methods for carbon electrode fabrication. Glassy carbon and boron doped diamond are relatively difficult to be fabricated in desired stereo shapes. Thin carbon films made by sputtering or CVD (usually less than 1μm) are difficult to cover the substrate surface thoroughly and not so conductive when being deposited on insulating substrate. Carbon paste coating can make relatively thick carbon layer, but it is not dense enough to prevent from holes, pores and voids, which disturb precise measurements. Therefore, it is practically impossible to make electrodes with thick and dense carbon layer in complexed shape using the conventional methods.
We found that the carbon electroplating method developed by I’MSEP 1) can solve the problems to form the dense and thick (>1μm) carbon on metal substrates. Carbon films are electrochemically deposited on metal substrates in molten salt bath. The deposited carbon films are sufficiently dense, thick and conductive to be used for the electrodes for bio-sensing applications. We describe on the characters and electrochemical performances of the dense carbon coated electrode plated in the molten salt bath.
2. Formation of dense carbon coated electrode by plating
The reaction of the carbon plating in the molten salt bath is the electrochemical oxidation of carbide ions dissolved in bath on anode.
C2
2-　→　2C　＋　2e-

Various types of metal substrate can be used. We used SUS304 as the substrate because it is able to be precisely fabricated in many ways including wet etching combined with photo-resist. Carbon layer can be made on the SUS surface uniformly even on the back side, edge and recesses.
The electrode used in this experiment was plated under the following conditions:
The molten salt bath LiCl-KCl mixed, Temperature : 500°C　Cathode : Carbon plate,
Current density : 3.3 mA/cm2

3. Electrochemical measurements using the dense carbon coated electrodes
Figure 1 shows the cyclic voltammetry curves for the electrodes; dense carbon coated by plating (C plating, 2μm) , SUS304 as the substrate (SUS base) and carbon paste coated on the substrate (C paste, 2μm). The curves shown were recorded after ten times sweeps from －0.9 to +1.4V vs.Ag/AgCl in 0.1M HCl at 25°C . This indicates that C plating electrode has very wide potential window, where the base current of C plating electrode is almost zero, in comparison with C paste electrode. The characters of SUS was hidden due to the perfect coverage. The anodic current of C paste electrode may be caused by the oxidation of organic components or contaminants of the paste.
Furthermore, we investigated the possibility that C plating electrode can be used for bio-sensing application. Figure 2 shows the results of glucose concentration measurements. In this case, with the existence of oxygen (continuously bubbled) as the mediator, 100mg/L of glucose oxidase (GOX) dissolved in the solution (phosphate buffer pH 5.8) generated H2O2 when glucose is replenished. Any modification of C plating electrode was not made to show the original properties. The steady current at +0.9 V vs.Ag/AgCl, where H2O2 is detected, was measured depending on the glucose concentration in the solution. The linear relationship between glucose concentration and current values were obtained. It is supposed that dense carbon coated electrode made by plating is capable to apply to various electrochemical measurements of biomarkers because it has very wide potential window. Additionally, some modification or arrangement of measuring methods should enhance the selectivity and sensitivity.
Reference
1) Yasuhiko Ito ; ECS Transactions, 41 (46) 43-55 (2012)

Figure 1