RUTHENIUM CATALYST FOR AMPEROMETRIC DETERMINATION OF INSULIN AT PHYSIOLOGICAL PH

A ruthenium-oxide-type catalytic film (RuOx) was produced on carbon fi ber microelectrodes by cycling the electrode potential between 0.65 an d -0.85 V vs. SSCE at 100 V s(-1) in an air-equilibrated acidic soluti on of RuCl3. The film catalyzes oxidation of insulin in a saline buffe r at pH 7.4. The minimum number of electrons transferred during the in sulin oxidation at 0.65 V is 6.7. The analytical performance of the mo dified electrode as an amperometric detector for insulin was character ized using flow injection analysis. Linear least squares calibration c urves over the range 0.10 to 1.0 mu M (five points) had slopes of 72+/ -2 pA mu M-1 and correlation coefficients of 0.999 or greater. The det ection limit, calculated as the concentration that would yield a signa l equal to three times the root mean square noise, was 23nM and respon se time (t(90%)) was 40ms or less. The electrode response to 0.2 mu M insulin was stable for 3 days, The modified electrode was used for amp erometric detection of exocytosis from individual pancreatic beta-cell s.