Detection of glucose at 2 fM concentration

We report the amperometric detection of glucose at 2 fM concentration in a physiological buffer solution at 1 atm O2 pressure. The sensitive assay is based on the close to absolute electroreductive stripping of O2 from the solution near the glucose electrooxidizing anode. The glucose was detected by its electrooxidation on a stationary glassy carbon disk surrounded by an also stationary platinum ring. The disk was coated with a film of glucose oxidase (GOx), electrically "wired" with PVP-[Os(N,N'-dimethyl-2,2'-biimidazole)3]2+/3+ (polymer I), having a redox potential of -0.19 V versus Ag/AgCl. The ring was coated with bilirubin oxidase (BOD) "wired" with PAA-PVI-[Os(4,4'-dichloro-2,2'-bipyridine)2Cl]+/2+ (polymer II), having a redox potential of + 0.36 V versus Ag/AgCl. The ring-disk electrode was held facing up, and a 30-microL drop was placed on it for the assay, with the ring poised at -0.3 V/ AgAgCl and the disk poised at -0.1 V/ Ag/AgCl. Even though the atmosphere over the drop was O2 at 1 atm pressure, the wired BOD disk scavenged the O2 so effectively that the glucose-reduced FADH2 of GOx was not oxidized by O2, the natural cosubstrate of the enzyme.