The field of application of micro-electrodes is very wide: from electroanalytical applications (detection of ultra-low quantities of analyte, biosensors, etc.), kinetic studies of very fast and complex reactions, to measurements in solutions of very low conductivity, etc.^1-3 Often several micro-electrodes are combined into sensor-arrays for simultaneous analysis of various parameters.

In this work the microfabrication technique was used to prepare a six-channel sensor-array chip.⁴ The sensor-array has been fabricated of borosilicate wafer containing Pt electrodes and connection pads and the silicon wafer with through holes, which were bonded using adhesion bonding method with SU-8. Although the fabrication process of two wafers is simple, the extensively perforated silicon wafer is brittle and can break down during bonding step. However, with the customized bonding process, a good bonding was achieved and all the chips were applicable after dicing.

The cyclic voltammetry and electrochemical impedance experiments were carried out in a three-electrode electrochemical system to characterize the prepared sensor-array chips. Sputtered platinum was used for micro-electrodes, and Ag/AgCl/sat. KCl as a reference electrode. The high surface area Pt counter electrode has been used for studies.

It was shown that the measured current density depends on the electrode potential sweep rate, however both linear and radial diffusion mass transfer components are important at high electrode potential scanning rate. When using slow potential sweep rates (v = 0.005 V s^‒1) the pseudo steady-state has been achieved and the electrodes behaved as the micro-electrodes. It was shown that the pseudo steady-state current density depends linearly on the concentration of potassium hexacyanoferrate(II) in the 1.0 M KCl aqueous solution.

Based on the electrochemical characterization data it can be concluded that the produced sensor array is suitable for future research including BOD biosensor-array construction. The sensor-array developed can be modified with microorganisms by immobilizing the bacteria into the cavities fabricated onto/into the chip.

Acknowledgements

This research was supported by the EU through the European Regional Development Fund (Centre of Excellence, 2014-2020.4.01.15-0011), Institutional Research Grant IUT20-13, and Estonian Science Foundation (Grant ETF 9136).

References

1. J. Heinze, Angew. Chem. Int. Ed. Engl., 32, 1268 (1993).

2. A.M. Bond, Analyst, 119, 1R (1994).

3. A. J. Bard and L. R. Faulkner, Electrochemical methods: fundamentals and applications, John Wiley & Sons Inc., New York (1980).

4. K. Pitman, M. Raud, G. Scotti, V.P. Jokinen, S. Franssila, J. Nerut, E. Lust, and T. Kikas, Electroanalysis (2016). Published online (DOI: 10.1002/elan.201600559).