Oxygen-sensing microdialysis probe for in vivo use

Electrochemical conditions were optimized to allow the metal tube used for the shaft of commercial microdialysis (MD) probes to be coated with gold. In in vitro tests with phosphate-buffered Ringer's solution using double differential pulse amperometry (DDPA), the gold-coated shafts were capable of specifically measuring the reduction of oxygen and the oxidation of ascorbic acid in the presence of high concentrations of potentially interfering endogenous substances. By using fixed-potential amperometry (FPA), the gold-plated shaft also measured oxygen with minimal interference from high concentrations of potentially interfering endogenous substances. Concentric design MD probes were constructed that used a metal shaft (O.D = 0.4 mm), fused silica inlet and outlet tubes, and a 1.5 mm dialyzing membrane (O.D = 0.2 mm). A 0.5-0.7 mm gold collar was electroplated onto the metal shaft approximately 0.5 mm above the dialyzing membrane. The nongold outer surface of the MD probe was coated with an insulating polymer. In vivo tests demonstrated that DDPA was not suitable for use with this gold microdialyzing electrode (GMDE). However, brain oxygen levels were satisfactorily measured using FPA. In urethane-anesthetized rats, the reduction current to oxygen in the striatum was increased by brief (1 min) inhalation of O2 or CO2 and decreased by inhalation of N2. Transient application of noxious stimuli (foot pinch) increased cerebral O2, whereas bilateral carotid artery occlusion and death decreased striatal O2. The responses of the GMDE were indistinguishable from the reduction current simultaneously measured from a conventional carbon fiber electrode implanted adjacent to the gold-plated area of the MD shaft. Basal levels of striatal O(2) were 20 +/- 5 microM (n = 4) for the GMDE and 30 +/- 11 microM (n = 3) for the carbon fiber. The GMDE was robust and could be used for at least three animals. This technique can be used to provide information about the oxygen status of the tissue adjacent to the dialyzing membrane without the need for implantation of an additional electrode.