Design, fabrication and characterisation of nanoelectrodes for electrochemical sensing
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In electrochemical sensing, the miniaturisation of electrodes leads to enhanced characteristics, including higher signal-to-noise ratio and lower detection limits and sensitivity to external convection due to more efficient mass transport. In recent years, this has generated considerable interest in both the manufacturing and characterisation of nanoelectrodes. However, the high-volume, commercial fabrication of integratable, low cost nanoelectrodes remains a challenge. This work presents a nanoelectrode architecture that can be manufactured using established and well-characterised microfabrication methods. Vertical ring electrodes are fabricated at hole edges using thin film deposition and microlithography techniques. A metal layer of nanometre thickness is sandwiched between two insulators on a substrate followed by the etching of micron scale holes through the stack of layers. This leads to the exposure of a metal nanoband around the hole perimeter and thus a nanoelectrode with the area defined by the hole perimeter and the deposited metal layer thickness. This work first reports a simulation study, which investigates the in uence of design parameters such as band and insulator thicknesses and hole size on the diffusive current. The results show a relative independence of the current to the band thickness and a linear dependence on the hole perimeter with a steady state current comparable to that of a microelectrode. For example, a nanoband electrode with a band thickness of 50nm produces up to approximately half of the limiting current measured on a planar microsquare electrode and a 25 nm electrode produces 88% of the current of a 50 nm electrode. This information contributed to the development of a process for the fabrication of arrays of platinum nanoband electrodes in microsquare holes on a silicon substrate with control over the critical geometric parameters. Electrodes with band thicknesses of 5 nm to 50 nm and a range of square side lengths have been fabricated for experimental validation. Their performance has been compared to microsquare electrode arrays, and was shown to give a similar response to established microdisc and square electrodes. An analysis procedure has been developed and inherent nanoelectrode behaviour and effects have been investigated. The relative importance of a range of nanoeffects on the electrodes has been assessed, indicating a contribution of migration to mass transfer. It has been demonstrated that these nanoband electrodes can be used to detect rapid processes such as the measurement of large electrochemical rate constants, unlike microsquare array electrodes.