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 0(2) pressure. The sensitive assay is based on the close to absolute electroreductive stripping Of O-2 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)(2)Cl](+/2+) (polymer II), having a redox potential of +0.36 V versus Ag/AgCI. The ring-disk electrode was held facing up, and a 30-muL drop was placed on it for the assay, with the ring poised at -0.3V/AgAgCl and the disk poised at - 0.1 V/Ag/AgCl. Even though the atmosphere over the drop was O-2 at 1 atm pressure, the wired BOD disk scavenged the O-2 so effectively that the glucose-reduced FADH(2) of GOx was not oxidized by O-2, the natural cosubstrate of the enzyme.