Following fabrication, each chip was inspected using optical microscopy to identify any visible defects or faults and any defective chips discarded. Prior to electrochemical characterisation, chips were first cleaned by rinsing with acetone followed by de-ionized water. The chips were subsequently dried using nitrogen and inserted into a microfluidic holder. Electrochemical analysis was performed using an Autolab Bipotentiostat (MAC80150 with BA Module, Metrohm), within a Faraday cage. Cyclic voltammograms (CV) were performed in the voltage range of -0.15 V to 0.45 V at 50 mV/s at the first interdigitated comb (WE1) in 1 mM/L ferrocene carboxylic acid (FCA, Sigma Aldrich, 97%) while the second interdigitated comb of electrodes, (WE2) was biased at -0.15 V; for the duration of the scans.