Selective Glutamate Biosensor Arrays Integrated on Silicon Microprobes for In vivo Brain Recordings

Real-time monitoring of glutamate, being the primary excitatory neurotransmitter, is of utmost importance for understanding the functioning of the central nervous system. Since expected concentrations and its variation in the brain are in the range of 0.5 to 5 μM and interfering substances are present, constraints on the sensitivity and selectivity of the dedicated biosensors are challenging. Moreover, their size determines the spatial resolution of the measurement and tissue damage during insertion and removal. We realized microprobe arrays (2x4 needles with lengths up to 8 mm and a cross-section of 40x100 μm) with several integrated Pt microelectrodes (50x150 μm) using standard silicon microtechnology. This is the basis for simultaneous detection of the neurotransmitter at different locations in the brain and the interpretation of local circuitry. The functionalization of the electrodes towards selective detection of glutamate is done by the immobilization of the enzyme glutamate oxidase. Peroxide, one of the products involved in the enzyme reaction, is detected amperometrically. An array-compatible method for spatially controlled and parallelized membrane deposition is therefore required and found in electrochemically aided adsorption using also bovine serum albumin (BSA) and glutaraldehyd. This is followed by electro-polymerisation of m-phenylenediamine which generates a semi permeable membrane rejecting electroactive interferents as ascorbic acid endogenously present in the extracellular fluid at relatively high concentrations. The in vitro and in vivo calibration curves of the biosensor arrays show satisfying characteristics in means of reproducible sensitivity, selectivity and limit of detection and allow the investigation of more complex mechanisms in brain tissue.