Nanogap Electrochemistry and Sensor Technology at the Molecular Limit

Sensors with electrochemical stimulus and read-out have found wide-spread use in gas, environmental, and medical trace analysis. Low cost and reliability as well as miniaturisation are major factors in commercialisation and mass production and in particular cheap screen printed sensors have dominated for example in the medical glucose sensing field. The low cost of these devices has been the secret to their commercial success and wide distribution. More powerful generator-collector electrode systems have been initially developed by Nekrasov and Frumkin and then further developed for rotating ring-disc systems, dual and interdigitated band electrode systems, in SECM and in STM, as well as in dual flow channel systems. Work with microelectrode arrays has been reported for electroanalysis and for patterning in DNA synthesisers. However, in these devices the inter-electrode gap has always been relatively large and the potential benefits of sub-micron gaps have not been exploited. In this project we propose (i) to develop extremely small gap systems to reach the limit of single molecule detection and (ii) to device novel bipotentiostat junction technology for powerful sensors for a wide range of applications.Our project hypothesis is that junction electrodes with 100 nm or less inter-electrode gap allow novel (electro-)chemical sensor processes to be exploited and investigated which have not been considered/realised in previous studies e.g. using conventional SECM and STM or in studies employing single-electrode electrochemical processes. The junction electrode formation is based on a robust (cheap, fast, & reproducible) electro-deposition approach which will need refinement and optimisation for particular applications. New experimental protocols will be developed to control surface roughness, improve growth & geometry, to introduce new junction materials, and to coat/fill sensor junctions. Diffusion within the junction will be investigated and short lived intermediates generated (for example HS., O2., O3., and other radical species) and modulator-sensor experiments conducted where pulses of reagents are generated at one (or more) modulator electrodes. Multi-dimensional pulse voltammetry with specifically optimised pulse sequences will provide higher sensitivity and higher selectivity and allow unusual detection modes (e.g. based on hydroxide pulses, see glucose detection in neutral solution aided by hydroxide pulses). The numerical simulation (based on new GPU methods ) of dual disc or band electrode systems and for junction electrodes is challenging and will provide important insight into physical phenomena, chemical mechanisms, and sensor optimisation.

Work described in this proposal is immediately relevant to the field of sensor development and in particular for environmental and for medical sensor technology. Over-the-counter sensors are now available for several areas of health monitoring and in particular for glucose sensing or for road-side testing. New technologies are constantly evolving and the market for these devices is massive (including the newly developing countries like China and India). Bipotentiostatic junction sensors could be cheap and robust and applicable in many fields. At this point in time junction processes are not well studied and the benefits from electrochemical processes in junctions have not been realised. With the fundamental work carried out in this project, fabrication tools will become accessible and new computer simulation methods will allow difficult sensor problems to be approached from a much more solid foundation. The impact of this project is likely to be high in both the academic exploitation and development of understanding of reactive intermediates (e.g. in biological contexts and in particular focusing on sulfur and oxygen radicals), and the commercial exploitation for sensors. A further impact of this study could be due to a step change in technology for example in biological research. Miniaturised sensing in very small volume (e.g. a cell) will be possible and improved selectivity achieved in 2D pulse methods could overcome traditional limits of electrochemical probes in this field. New instrumentation could be developed and a wider range of research areas could benefit (DNA analysis, biological, medical, environmental). In particular intriguing is the prospect of single molecule analysis will be revolutionary for the understanding of molecular properties and in many areas of sensor application.