(157e) Implantable, Νear-Real-Time Microelectrode Array Biosensor for Simultaneous Monitoring of Glutamate and Dopamine

A Nafion and over-oxidized polypyrrole (OPPy)-modified, silicon-based platinum (Pt) microelectrode array biosensor was developed with optimal spatial and temporal resolution for simultaneous monitoring of glutamate (Glut) and dopamine (DA) by constant potential amperometry. A 2×2 microelectrode array located at the tip of a silicon-based microprobe with 100 μm and 40 μm separation between the microelectrodes on the same column (top and bottom sites) and the microelectrodes on the same row (neighboring sites), respectively, was patterned for maximum spatial resolution and minimum diffusional interaction of analytes. For each microelectrode, the average area was ~5000 μm². The microprobe shaft, measuring 120 μm in width and 150 μm in thickness, was designed to inflict minimum tissue damage during implantation while still providing sufficient mechanical stability. Microelectrodes modified with polymeric films provide a high degree of selectivity in the presence of electroactive interferents. Thin films of OPPy have been shown widely to have permselectivity against anions and excellent selectivity towards cations. Based on our previously reported results, however, a thick film of OPPy rejects both ascorbic acid (AA) and positively charged DA effectively. The total charge was controlled during the electrodeposition of polypyrrle (PPy) for an optimum film thickness. Selective deposition of thin PPy films at the top sites and thick PPy films at the bottom sites were followed by PPy over-oxidation. Nafion was dip-coated on all OPPy-modified microelectrodes to enhance the discrimination against negatively charged species, such as AA and uric acid (UA). A glutaraldehyde cross-linked glutamate oxidase (GlutOx) and bovine serum albumin (BSA) matrix was immobilized manually on the bottom two sites (with thick OPPy films). GlutOx turns over the non-electroactive analyte, Glut, in the presence of oxygen to give the electrooxidizable product, hydrogen peroxide, which undergoes a two-electron transfer reaction at the Pt surface at sufficiently oxidizing potential. The final configuration of the dual Glut/DA sensor was Pt/thick OPPy/Nafion/GlutOx for the Glut microsensor sites at the bottom and Pt/thin OPPy/Nafion for the DA microsensor sites at the top. Good selectivity, detection limit, sensitivity, and temporal resolution of the dual Glut/DA sensor were achieved in vitro. A constant oxidation potential was maintained at 0.7 V vs. Ag/AgCl. There was no detectable response from AA over the physiological concentration range. In the presence of both Glut and DA, the Glut microsensor sites responded to Glut with a detection limit of 5 μM and a 1 second response time with no DA response; whereas the DA microsensors detected DA with a detection limit of 100 nM and a 1 second response time without interference from Glut. The sensitivity of the dual sensor was ~50 nA?μM^-1?cm^-2 and ~1280 nA?μM^-1?cm^-2 toward Glu and DA, respectively. Studies of Glut and DA changes in the brains of live rodents using this microelectrode array biosensor currently are underway.