A typical micrograph of one such chip is shown in Figure , with the sensor array shown in Figure (b). A custom-made cell holder was fabricated to allow measurement with small electrolyte volumes (~50 µL to 5 mL.). The cell was constructed from an aluminium base and a Teflon lid. Spring loaded probes (Coda Systems Ltd. PM4J Plain Radius Microprobes) were inserted into the lid in position above the peripheral contact pads, to permit electrical connection to external potentiostats. The cell was assembled with a Viton O-ring embedded in the lid to form a seal around the on-chip electrodes. Viton O-rings were chosen for their chemical resistance. The inner diameter of the O-ring was 7 mm with a cross section of 1.6 mm to allow an opening large enough to expose all six sensors, counter, and reference electrodes on the device to the electrolyte. Typical volumes measured were 5 mL. Each chip was inspected using optical microscopy to identify any obvious defects or faults. Prior to any electrochemical characterisation chips were cleaned by rinsing for 20 seconds with acetone, iso-propyl alcohol and finally de-ionized water. The chips were dried in a flow of nitrogen and placed in the chip holder. Electrochemical analysis was performed using an Autolab Bipotentiostat (MAC80150 with BA Module, Metrohm). Cyclic voltammograms (CV) were performed from 0 V to 0.6 V at 50 mV/s in 1 mM ferrocene carboxylic acid (FCA, Sigma Aldrich, 97%) dissolved in 10 mM phosphate buffered saline (PBS, Sigma Aldrich). During these scans, the second interdigitated comb of electrodes was held at 0 V. All electrochemical measurements were recorded versus a commercial Ag/AgCl (CH Instruments). Optical micrographs were acquired using a calibrated microscope (Axioskop II, Carl Zeiss Ltd.) with a charge-coupled detector camera (CCD; DEI-750, Optronics). SEM images were obtained using a calibrated field emission FEI Quanta FEG 650 at a beam voltage of 10 kV.