Glutamate sensing with enzyme-modified floating-gate field effect transistors

Neurotransmitter release is the key factor of chemical messaging in the brain. Fast, sensitive and in situ detection of single cell neurotransmitter release is essential for the investigation of synaptic transmission under physiological or pathophysiological conditions. Although various techniques have been developed for detecting neurotransmitter release both in vitro and in vivo, the sensing of such events still remains challenging. First of all, the amount of neurotransmitter released during synaptic transmission is unknown because of the limited number of molecules released and the fast diffusion and reuptake of these molecules after release. On the other hand, advances in microelectronic biosensor devices have made possible the fast detection of various analytes with high sensitivity and selectivity. Specifically, enzyme-modified field-effect (ENFET) devices are attractive for such applications due to their fast response, small dimensions and the possibility to integrate a large number of sensors on the same chip. In this paper, we present a floating-gate FET device coated with glutamate oxidase (GLOD) layer. The surface chemistry was optimized for maximal enzyme loading and long-term stability, and characterized by quartz crystal microbalance and colorimetric assays. Enzyme loading was largest on poly-L-lysin-based surfaces combined with glutaraldehyde. The surface chemistry showed excellent stability for at least one month in Tris buffers stored at 4 degrees C. A glutamate detection limit of 10(-7) M has been obtained with the GLOD-coated FET and our sensor proved to be selective to glutamate only. We show that this biosensor is a promising tool for the in vitro detection of glutamate and can be extended to other neurotransmitters.