Nanogap Electrochemistry and Sensor Technology at the Molecular Limit

Lead Research Organisation: University of Bath
Department Name: Chemistry

Abstract

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.

Planned Impact

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.

Publications

10 25 50
 
Description The nano-gap devices have been developed to allow sensitive detection of traces of analyte (e.g. nitrate, sulphide, thiols, chloride, etc.) and all of these were demonstrated. The new device concept provided further benefits of in situ removing oxygen during measurement and improving selectivity in the chemical sense. The ultimate aim of single molecule detection was not achieved due to fabrication challenges, but recently a group in the Netherlands also demonstrated this.

Overall, this research has allowed us to (i) develop a new class of "microtrench" sensors with interesting new functionality, (ii) explore new methods for the detection of really difficult analytes such as H2S, (iii) collaborate with other researchers and industry to explore better fabrication and application methods, and (iv) train electro-analytical chemists. The collaboration element in this research resulted in project which are still in progress. New theory for microtrench electrodes has recently been developed and published and collaboration with a group in Spain is continuing to give research output. A collaboration with the Department of electrical Engineering is also on-going and may lead to further sensor concepts.
Exploitation Route The concept of the nano-gap sensors has only just emerged and with better fabrication tools much better devices will be possible. Smaller gaps will also allow fundamentally different chemistry to be accessed and short-lived species could be employed in analytical detection. Finally, the concept of time-of-flight sensing in nano-gaps will develop into new hand-held low-cost devices that contain separation and detection in one system.
Sectors Environment,Healthcare

 
Description Our new sensor devices have been tested in a hospital in Exeter and shown to allow nitrate in blood serum detection (which is much faster than existing methods and therefore good for A&E patients with suspected bacterial infection). A patent has been obtained. Commercialisation efforts have been undertaken and a business plan has been designed.
First Year Of Impact 2012
Sector Healthcare
Impact Types Societal

 
Title Electrochemical sensor for sensing presence of specific material, has working electrodes that are provided with catalyst and spacer is dimensioned such that spacing of working electrodes is less than about specific value 
Description The electrochemical sensor has a first working electrode, a second working electrode and a spacer that is provided to hold the first and second working electrodes in a spaced relationship. The working electrodes are provided with a catalyst or having a layer of catalytic material. The spacer is dimensioned such that the spacing of the first and second working electrodes is less than about 4Opm. The spacing is smaller than the average cell diameter in a biological fluid sample being tested using the case. 
IP Reference WO2015082910 
Protection Patent granted
Year Protection Granted 2015
Licensed No
Impact (A) Serum testing at Exeter NHS hospital successful. (B) Business plan development for company "egapSense".