Optical lithography and metal deposition (Ti 10 nm/Ni 70 nm/Au 200 nm) process was again undertaken to define a MicroSD pin-out, on-chip metal interconnection tracks, as well as counter and reference electrodes. Finally, a passivation Si3N4 layer was deposited on the chip by PECVD, with windows opened in this layer directly above the working, reference and counter electrodes and SD pinouts. The windows defined the length of the working electrode to be 45 µm. A PCB bearing a mounted microSD port connector structure was designed and fabricated to allow facile connection between the microSD primary contact pads with the potentiostat. In this manner, the microSD electrical pin-out enabled rapid and easy electrical connection to external electronics, enabling sensors to be used as a portable field sensor. Each silicon chip comprised six independent sensors containing two interdigitated electrode (IDE) structures, a platinum counter electrode, and a platinum pseudo-reference electrode. Gold contact pads and interconnection metallisation on two sides of the chip allowed electrical connection to both interdigitated structures. Figure shows an optical micrography of a fully fabricated chip (based on previous designs) the SEM image shows a highresolution image of a typical sensor device . IDEs were selected due to possibility of using localized pH control of the solutions for the ethidium bromide should this be required . Finally. a custom-made holder cell constructed from an aluminium base and a Teflon™ lid was fabricated to allow measurement in small electrolyte volumes (≈500 µL). The cell was included a Viton O-ring, chosen for their chemical resistance, embedded in the lid which formed a seal around the on-chip electrodes. The inner diameter of the O-ring was 7 mm with a cross section of 1.6 mm which was of sufficient size to expose all six sensors, counter, and reference electrodes on the device to the electrolyte , .