Mapping Hindered Mass Transport through Electrogenerated Chitosan Films Using Electrochemiluminescence Microscopy

Electrogenerated chitosan films, commonly described as electrodeposited, underpin a broad class of electrochemical biosensors, but they suffer from irreproducibility and film-to-film heterogeneity. Film deposition necessitates an electrogenerated alkaline environment near the electrode surface, and the alkalinity can be obtained through either water electrolysis or the reduction of benzoquinone at milder potentials. Classically, films are characterized by electrochemical measurements, which lack spatial resolution, as well as by optical, force, and electron micrographs, which do not detail the chemical microenvironment within the films with appropriate spatiotemporal resolution. Here, we introduce electrochemiluminescence (ECL) microscopy via luminol/H2O2 to probe film homogeneity, and mass transport within the film, owing to the diffusive nature of molecules involved in the ECL mechanism. Because ECL emission requires luminol to reach the electrode, the film locally suppresses the emission, generating negative contrast that reports local differences in electrode access across the entire electrode surface. ECL microscopy resolves the comparatively uniform response of films obtained via benzoquinone-mediated processes and the pronounced heterogeneity of one obtained via the hydrogen-evolution pathway. Because benzoquinone retained in the chitosan film quenches the ECL signal, we demonstrate that a reductive potential step enhances the ECL signal by 13.2 ± 2.6-fold. This potential step allowed us to differentiate the quenching of the ECL signal resulting from hindered mass transport to that caused by chemical quenchers retained in the film, offering previously unreported mass transport insights. Our work extends ECL imaging to the characterization of continuous, hydrated polymer coatings, allowing one to carefully characterize and control the quality of chitosan-modified electrodes in real time.