The commonly required properties of diffusive electron mediators for point-of-care testing are rapid dissolubility, high stability, and moderate formal potential in aqueous solutions. Inspired by nature, various quinone-containing electron mediators have been developed; however, satisfying all these requirements remains a challenge. Herein, a strategic design toward quinones incorporating sulfonated thioether and nitrogen-containing heteroarene moieties as solubilizing, stabilizing, and formal potential-modulating groups is reported. A systematic investigation reveals that di(thioether sulfonate)-substituted quinoline-1,4-dione (QLS) and quinoxaline-1,4-dione (QXS) display water solubilities of ≈1 m and are rapidly dissoluble. By finely balancing the electron-donating effect of the thioethers and the electron-withdrawing effect of the nitrogen atom, formal potentials suitable for electrochemical biosensors are achieved with QLS and QXS (−0.15 and −0.09 V vs Ag/AgCl, respectively, at pH 7.4). QLS is stable for >1 d in PBS (pH 7.4) and for 1 h in tris buffer (pH 9.0), which is sufficient for point-of-care testing. Furthermore, QLS, with its high electron mediation ability, is successfully used in biosensors for sensitive detection of glucose and parathyroid hormone, demonstrating detection limits of ≈0.3 × 10 −3 m and ≈2 pg mL −1 , respectively. This strategy produces organic electron mediators exhibiting rapid dissolution and high stability, and will find broad application beyond quinone-based biosensors.
It is still challenging to detect glucose selectively in serum and blood, and quickly using nicotinamide adenine dinucleotide (NAD-GDH). In this paper, we report a selective and rapid glucose sensor, based on electrochemical-enzymatic-enzymatic (ENN) redox cycling involving bis(2,2-bipyridyl)dichloroosmium(II)[Os(bpy)2Cl2], diaphorase (DI), NAD+, NAD-GDH, and glucose. DI and Os(bpy)2Cl2 are used to obtain fast mediated oxidation of NADH that is generated as a result of glucose oxidation by NAD-GDH. DI and NAD-GDH are co-immobilized via affinity binding on an avidin-modified indium tin oxide electrode to obtain fast and stable ENN redox cycling. Two enzymes (DI and NAD-GDH) and two electron mediators [Os(bpy)2Cl2 and NAD+] are efficient. The reason is that they are insensitive to oxygen. The applied potential (0.0 V vs Ag/AgCl) is low enough to minimize interfering electrochemical reactions, and the redox reactions of Os(bpy)2Cl2 with interfering species are slow. NAD-GDH is much less reactive to problematic monosaccharides such as xylose, fructose, galactose, and mannose than glucose. Artificial serum containing 5% (w/v) human serum albumin shows a similar electrochemical back-ground level in serum. All results enable us to obtain selective and reproducible glucose detection. The fast ENN redox cycling allows sensitive glucose detection with a wide range of concentrations in artificial serum with a short measuring time (5 s) without an incubation period. Keywords: Glucose sensor, Glucose dehydrogenase, Diaphorase, Osmium complex
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