Development of High-Sensitive Detection System for Redox Analytes Having a Standard Potential Higher Than O2 Evolution by Using Micropatterned Conductive Boron-Doped DLC Electrodes
Authors: Ohtomo Shinpei , Naragino Hiroshi , Okafuji Keigo , Kobayashi Ryutaro , Honda Kensuke 
Published: February 2020

1. Introduction

Recently, cobalt, manganese and cerium are widely used in mass-produced products such as a lithium ion battery. It has been reported that the excessive consumption of these metal ions causes various diseases like neurotoxicity. In industrial wastewater discharge standards, Co2+, Mn2+ and Ce3+contents of wastewater has been regulated below several micro-molar. These redox species are generally analyzed using photochemical method. Some time, however, is required for analysis using the method and initial measures cannot be taken promptly when problems such as the content of metal ions exceeding the discharge standard occur.
Therefore, electrochemical monitoring system which can shorten time of analysis should be developed.
The standard potential of Co2+, Mn2+ and Ce3+ are higher than that of O2evolution potential. Redox reactions of these analytes cannot be observed at conventional metal electrodes.
In this study, boron-doped diamond-like carbon (B-DLC) [1] with higher overpotential for O2evolution was selected as the electrode that redox reaction of the analytes can be observed.
The detection limit of Ce3+ (model analyte) at a flat-disk electrode of B-DLC was 39 mM.
Our research group tried to fabricate interdigitated microarray electrodes (IDA) because it shows one or two orders of magnitude higher sensitivity than that of the flat-disk electrode.
IDA are composed of two sets of microband array neighbored (called generator and collector). The potential of one microband pairs (collector) is set at reduction potential of redox analyte, and that of the other is swept toward oxidation potential (dual mode). Redox cycling occurs between two sets of the microband pairs. The generator exhibits higher amplification of oxidation current by redox cycling compared to the current without applying the potential on the collector (single mode).
We have reported that B-DLC-IDA with 2 mm/2 mm of bandwidth/gap exhibited ca.10 times amplification factor corresponding to the theoretical value [2]. The amplification factor of Ce3+, however, was half of the theoretical value. As a result, the detection limit was 8.8 mM.
The reduction of Ce4+ is not promoted at the collector because the sample solution is a reductant. Moreover, Ce3+generated at the collector is hardly supplied to generator by diffusion, which causes inefficient redox cycling.
When the collector is set at an oxidation potential, Ce3+ are constantly supplied to the collector by nonplanar diffusion. Ce4+generated at the collector is considered to be supplied to the generator at a higher diffusion rate.
In this report, amplification factor of Ce3+was tried to be improved by changing the potential set at the collector.

2. Experimental

B-DLC was synthesized on quartz substrates with RF plasma CVD method using a mixed solution of n-hexane and trimethyl borate as a source material. RF power was 175 W and the deposition time was 40 minutes.
The alminium pattern of IDA was fabricated by the lift-off technique using photoresist and alminium deposition. Alminium on B-DLC-IDA was removed in a sodium hydroxide solution after O2plasma etching of B-DLC.
The structure of B-DLC-IDA was designed to be 65 pairs of microbands with a length of 200 μm and bandwidth/gap of 2.0 mm.
Single-dual mode measurements were carried out using the B-DLC-IDA in 1 mM Ce(NO3)3 + 0.1 M HNO3solution.

3. Results and discussion

Figure1 shows the optical micrographs of the B-DLC-IDA. The bandwidths and gaps were observed to be 1.96 and 2.04 mm, respectively. The deviation between the actual gap and the designed value was less than 2%.
Raman spectra of B-DLC were identical before and after the processing of IDA. It was confirmed that amorphous structure and composition ratio of sp2/sp2+sp3 (= 75%) in B-DLC did not change during IDA processing.
The working potential range in 0.1 M sulfuric acid was constant after IDA processing. These results suggest that the redox reaction of Ce3+can be served at the B-DLC-IDA.
In the dual mode of Ce3+, the potential of the collector was set at 1.37 V, and the potential of generator was swept from 0.8 to 1.7 V. Amplification factor closed to the theoretical value could be obtained. The detection limit of Ce3+was decreased from 8.8 to 4.4 mM by changing the applied potential of the collector from 1.25 to 1.37 V.

4. Summary

Fabrication of the B-DLC-IDA and optimizing method of electrochemical measurement can establish highly sensitive electrochemical system that can analyze redox species with a standard potential higher than O2evolution.

5. References

[1] K. Honda et al., J. Electrochem. Soc, 161, pp207-215 (2014).

[2] S. Ohtomo et al., Proc. 96th CSJ Annual Meeting, 3B7-10 (2016).

Figure 1