Ruthenium purple-containing zeolite modified electrodes and their application for the detection of glucose

Zeolite-entrapped ruthenium purple particles (denoted NaY \ RP) were prepared by exchanging Fe3+ into zeolite Y (NaY) and reacting further with ruthenium hexacyanide (Ru(CN)(6)(4-)). Although XRD and diffuse-reflectance UV-vis absorption analyses suggested that the RP(NaY) particles were basically amorphous, the resulting electrode prepared from these particles and glucose oxidase (GOx) displayed a remarkable sensitivity to glucose. The limiting current showed a linear relationship with the bulk activity of glucose up to 4 mM (pH 5.1, phosphate), covering a range from 10(-6) to 10(-3) M. Flow-injection analysis, in addition, showed the detection limit reaching a level as low as 0.1 mu M. Data simulation showed that the electrode sensitivity followed an electrocatalytic (EC') mechanism based on the reduction of H2O2 by the reduced ruthenium purple. Accordingly, the reaction rate constant (pseudo-first-order) and the effective activity of RP were determined to be 110 M-1 s(-1) (pH 5) and 1.5 x 10(-9) mol cm(-2), respectively. Electrochemical impedance spectroscopic (EIS) analysis showed that the charge-transfer resistance of the cm zeolite-electrode decreased systematically with the stepwise addition of glucose into the system. The exchange rate constant (k(o)) and the diffusion coefficient of electrons (D-e) in the zeolite film were estimated to be 5 x 10(-6) cm s(-1) and 6 x 10(-10) cm(2) s(-1), respectively. These experimental results suggested that the RP sites in the NaY \ RP particles were separated widely with the average distance between the adjacent RP sites estimated to be about 1 x 10(-4) cm or equivalent to 500 supercages, which agreed well with the results obtained from data simulation. (C) 1999 Elsevier Science S.A. All rights reserved.