Diamond Based Ion-Sensitive Field Effect Transistors for Cellular Biosensing
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Diamond Based Ion-Sensitive Field Effect Transistors for Cellular Biosensing Andrew Bennett1, Olivier Gaudin2, Oliver A. Williams3, John S. Foord1, and Richard B. Jackman4 1 Department of Chemistry, University of Oxford, Mansfield Road, Oxford, OX1 3TA, United Kingdom 2 School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, KY16 9SS, United Kingdom 3 IMO, University of Hasselt, Wetenschapspark 1, Diepenbeek, B-3590, Belgium 4 London Centre for Nanotechnology, University College London, 17-19 Gordon Street, London, WC1H 0AH, United Kingdom
ABSTRACT The use of a chemically inert and essentially biocompatible material for cellular biosensing is an attractive idea. In this context we have studied the operation of diamond-based ion-sensitive field effect transistors (ISFETs) within solutions of varying pH, and alkali-halide concentrations. In particular, we report the use of an inexpensive diamond substrate material, often referred to as ‘black’ diamond. pH sensitivity was observed when devices based on hydrogen-terminated p-type surfaces where employed, provide some surface oxidation was performed prior to their use. Variation in the threshold voltage for ISFET operation of the order of 20mV/pH unit was determined. In terms of operation in potassium iodide solution, we have shown that the device is shows selective sensitivity to the iodide species, despite the equi-molar presence of both K+ and I- species. The origin of this selectivity is discussed. INTRODUCTION Cells represent the minimum functional and integrating communicable unit of living systems. Cultured cells both transduce and transmit a variety of chemical and physical signals, i.e., production of specific substances and proteins, throughout their life cycle within specific tissues and organs. Such cellular responses might be usefully employed as parameters to obtain chemical information for both pharmaceutical and chemical safety, and drug efficacy profiles invitro as a screening tool. However, such cellular signals are very weak and not easily detected with conventional analytical methods. An ISFET is an Ion Sensitive Field Effect Transistor used to measure ion concentrations in solution; when the ion concentration (or pH) changes, the current flowing through the transistor will change accordingly. Here, the solution is used as gate electrode. In the case of a conventional ISFET, the gate metal electrode of a silicon-based MOSFET is replaced by an electrolyte solution that is contacted by a reference electrode. The SiO2 gate oxide is then placed directly in an aqueous electrolyte solution [1]. The metal part of reference electrode can be considered as the gate of the MOSFET. When SiO2 is used as the insulator, the chemical nature of the interface oxide is reflected in the measured source-drain current. The surface of the gate oxide contains OH-functionalities, which are in electrochemical equilibrium with ions in the sample solutions (such as H+ and OH-). The hydroxyl groups at the gate oxide surface can be protona
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