Microfabrication technology has been used to prepare a microchip sensor‐array with six sets of platinum electrodes. Chromium/platinum (10 nm/100 nm thick) were sputtered on a borosilicate wafer and patterned by wet etching method. The electrodes were designed with working electrode area of 700×400 μm in the middle and a 200 μm wide and 2600 μm long counter electrode surrounding it from three sides in a U‐shape. The connection pads (1000×1500 μm) were located at the edge of a sensor‐array chip. Silicon wafer was etched through to form holes with slanting side walls for immobilization cavities. The silicon and the borosilicate wafers were adhesion bonded with SU‐8 epoxy resin. The cyclic voltammetry and electrochemical impedance experiments were carried out in a three‐electrode electrochemical system to characterize the fabricated sensor‐array chip. The results show that the current density depends on the electrode potential sweep rate ν. Also, current density depends on the concentration of potassium hexacyanoferrate(III). At slow potential sweep rates (ν≤0.01 V s−1) the steady‐state signal is achieved and the electrodes behave as micro‐electrodes. Such an array is a promising candidate for fast and simple biochemical oxygen demand (BOD) measurements.

The microfabrication technique was used to prepare a six-channel sensor-array chip. The sensor-array has been fabricated using borosilicate wafer containing Pt electrodes and connection pads and the silicon wafer with through holes, which were bonded with SU-8 using adhesion bonding method. To characterize the prepared sensor-array chips the cyclic voltammetry and electrochemical impedance experiments were carried out in a three-electrode electrochemical system. The electrooxidation of ferrocyanide and electroreduction of oxygen were studied. 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. The pseudo steady-state conditions were achieved at slow potential sweep rates (>0.01 V s -1 ) and the electrodes behaved as the microelectrodes. The pseudo steady-state current density is proportional to the concentration of electroactive species. The recessed microdisc electrode model has been used to describe the system.

There is need to rapidly measure biochemical oxygen demand (BOD) to estimate organic pollution in wastewater. Biosensors are able to estimate BOD values within 5–30 minutes, but they have some limitations that can be overcome with biosensor-array. This work used sensor-array, which consists of 8 × 3 electrodes. The working electrode was inner Pt circle electrode, counter electrode was a Pt band electrode and the reference electrode was a silver wire. The potentiostat was used to record cyclic voltammetry and chronoamperometry. The pumping speed was set at 1.5 cm3 min−1 or higher, to avoid the interference. Next, sensor-array was tested to measure different oxygen amounts and calibrated accordingly. Lastly, Pseudomonas putida membranes were calibrated and used to estimate BOD value. The calibration gave linear range up to 85 mg L−1 of BOD and sensitivity from 0.0018 to 0.0068. Real industrial wastewater, from lignocellulosic bioethanol production, was used to test the biosensor-array. It underestimated BOD values from 8 to 37 %. This biosensor-array allows to measure BOD value in less than 15 minutes.

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).