Nonenzymatic Glucose Biosensor Based on Selective Reduction of Silver Ions

A nonenzymatic conductivity-based glucose biosensor, reported here, designed for potential noninvasive measurement in accessible biofluids such as saliva, sweat, and tears. The sensor exploits a classic Tollens' silvering process that deposits metallic silver on a sensitized surface via a quantitative, selective reduction of silver ions by glucose. Of the two surface sensitization methods used, the self-assembled bifunctional thiol/silane surfactant yields a more stable and sensitive sensor than tin-(II) chloride. The conductometric sensor consists of two spiral interdigitated silver electrodes fabricated by lithography and wet etching. The sensor impedance changes after selective metallic silver deposition between the electrodes. The sensor exhibits a sigmoidal response in a wide range of glucose concentrations from 200 mg/dL to less than 0.2 mg/dL and an ultralow limit of detection (ULOD) of 0.3 × 10^-5 mg/dL. The ULOD is attributed to the percolation network morphology of the deposited silver, connecting two electrodes (fractal dimension D = 1.5) that function like parallel nanoresistors. The log-log plot of conductivity vs glucose concentration yields a conductivity exponent that increases from 1 to 2, predicted for a 2D to 3D percolation transition in going from ultralow to high glucose concentration.