This study demonstrates a new electrochemical method for in vivo measurements of ascorbic acid (AA) in rat brain with multiwalled carbon nanotube (MWNT)-modified carbon fiber microelectrodes (CFMEs) based on the electrochemical property of MWNTs for facilitating AA oxidation. Cyclic voltammetry results indicate that the prepared MWNT-modified CFMEs possess a marked electrocatalytic activity toward AA oxidation and can be used for its selective measurement in the presence of other kinds of electroactive species coexisting in rat brain, such as 3,4-dihydroxyphenylacetic acid, uric acid, and 5-hydroxytryptamine. The selectivity of the MWNT-modified CFMEs toward AA measurement is further studied in vivo by exogenously infusing ascorbate oxidase into the brain, and the results confirm that the prepared electrodes are selective and can thus be used for reliable in vivo measurements of AA in rat brain, combined with their good stability during in vivo measurements. The basal level of striatum AA is determined to be 0.20 +/- 0.05 mM (n = 3). The application of the voltammetric method with the MWNT-modified CFMEs is preliminarily demonstrated for in vivo observation of homeostatic regulation of striatum AA with exogenous infusion of AA into the brain.

Using as-synthesized vertically aligned carbon nanotube-sheathed carbon fibers (VACNT-CFs) as microelectrodes without any postsynthesis functionalization, we have developed in this study a new method for in vivo monitoring of ascorbate with high selectivity and reproducibility. The VACNT-CFs are formed via pyrolysis of iron phthalocyanine (FePc) on the carbon fiber support. After electrochemical pretreatment in 1.0 M NaOH solution, the pristine VACNT-CF microelectrodes exhibit typical microelectrode behavior with fast electron transfer kinetics for electrochemical oxidation of ascorbate and are useful for selective ascorbate monitoring even with other electroactive species (e.g., dopamine, uric acid, and 5-hydroxytryptamine) coexisting in rat brain. Pristine VACNT-CFs are further demonstrated to be a reliable and stable microelectrode for in vivo recording of the dynamic increase of ascorbate evoked by intracerebral infusion of glutamate. Use of a pristine VACNT-CF microelectrode can effectively avoid any manual electrode modification and is free from person-to-person and/or electrode-to-electrode deviations intrinsically associated with conventional CF electrode fabrication, which often involves electrode surface modification with randomly distributed CNTs or other pretreatments, and hence allows easy fabrication of highly selective, reproducible, and stable microelectrodes even by nonelectrochemists. Thus, this study offers a new and reliable platform for in vivo monitoring of neurochemicals (e.g., ascorbate) to largely facilitate future studies on the neurochemical processes involved in various physiological events.

This study demonstrates a facile electrochemical method for simultaneous and selective on-line measurements of glucose and lactate in the brain of freely moving rats through integration of selective electrochemical detection with in vivo microdialysis. The selective electrochemical detection is accomplished by using oxidases as the specific sensing unit and prussian blue (PB) as the electrocatalyst for the reduction and thus for the determination of H(2)O(2) generated from the oxidase-catalyzed reactions in terms of its excellent electrocatalytic activity toward H(2)O(2) reduction. The uses of "artificial peroxidase" (i.e., PB) in this work to replace "natural peroxidase" (i.e., horseradish peroxidase) used in the previous on-line electrochemical methods essentially enables the method developed here to be facile but selective for the simultaneous and on-line measurements of glucose and lactate virtually interference-free from ascorbic acid and other electroactive species coexisting in the brain. Moreover, the dual oxidase/PB-based biosensors suffer from little cross-talk and exhibit a good stability and reproducibility. The basal levels of glucose and lactate in the microdialysate from the striatum of the freely moving rats are determined to be 200 +/- 30 and 400 +/- 50 microM (n = 3), respectively. The method demonstrated here is facile but reliable and durable and may find some interesting physiological and pathological applications.