Pico-electrochemistry in humidity-equilibrated electrolyte films on nano-cotton: Three- and four-point probe voltammetry and impedance

Cotton-extracted cellulose nanocrystals are spin-coated from aqueous suspension (0.6 wt%) onto glass slides to give ca. 40 nm thick films. Impregnation with LiCl and redox active Fe(CN)₆ ^3-/4- into this film gives extremely thin redox active layers (typically 170 nm at 60% relative humidity), which were investigated with a 4-point or 3-point probe electrochemical system based on 250 μm diameter platinum wire probes. Both voltammetry and impedance measurements were performed and effects from humidity, concentrations, and time domain on measurements are reported. Only a pico-litre volume under the working electrode was "active" to give a novel electroanalytical "spot test".