Fig. 2: Detection of H₂O₂ using confocal fluorescence microscopy.

Representative fluorescent depth profile images of the viologen-modified films (top panel) at open circuit potential (OCP) and of H₂O₂-sensitive indicators (lower panels: 0–15 min) at a constant potential of −0.1 V vs SHE a in the absence and b in the presence of KI (0.1 M). c O₂-reduction current in absence and presence of KI (0.1 M) at constant potential of −0.1 V vs SHE. d The relative intensity of fluorescent H₂O₂ reporter, derived from a and b, normalized to the maximum value. An overlay plot of the depth profile of the fluorescence of the viologen and of the fluorescence of the H₂O₂-sensitive indicators is given in Supplementary Fig. 5 for better visualization of their respective positions. For all measurements, the glassy carbon electrodes (GCE, 3 mm in diameter) were coated with viologen-modified polymer with a surface coverage of 2.2 mg cm⁻². Electrolyte: 2 ml PB (0.1 M, pH 7) with 10 µL stock solution of Amplite™ Fluorimetric Hydrogen Peroxide Assay Kit. The fluorescence of the oxidized viologen-modified films was collected around its maximum emission of 590 nm upon excitation at 488 nm. The emission of the H₂O₂ fluorescent probe was determined around its maximum emission of 660 nm upon excitation at 633 nm. The scale is the same for all images. All measurements were performed under ambient air and at 300 K.