Turning metals cations into catalytic nanoparticles on modified electrode: a general strategy for their electrochemical quantification

• Cu and Fe pre-accumulated on modified graphite electrodes form a nanoelectrode array by electrochemical reduction. • Cu and Fe nanoelectrode array dimensions tune the catalytic HER peak potential in cyclic voltammetry. • Peak shifts in electrocatalytic HER voltammetric curves enable accurate nanomolar Cu and Fe quantification with higher sensitivity than pulsed voltammetry. • Conceptual advance from conventional current-based to potential-based sensing, enabled by nanometric-scale diffusion regime shifts in cyclic voltammetry. • New detection concept easily added in the workflow of electrochemical metal sensor development. An innovative electrochemical sensing method has been presented in the present study. Instead of designing a specific high-affinity or highly selective surface, a generalizable approach is proposed, that complements conventional electrochemical stripping methods and can be readily integrated into existing sensor development workflows without additional cost or complex fabrication steps. Diazonium-modified graphite electrode were used as a robust platform to demonstrate that sparsely pre-accumulated copper and iron cations can be reduced into nanodeposits, which behave as a nanoelectrode array. For the first time, it is shown that the macroscopic electrochemical response of this array towards the hydrogen evolution reaction depends on the size of the nanoelectrodes: higher metal concentrations in the incubation solution lead to more extensive metal deposition and larger nanostructures, which shift the hydrogen evolution reaction catalytic peak potential positively in cyclic voltammetry. This shift continues until the metal becomes detectable by standard pulsed analytical methods. Importantly, the proposed approach achieves enhanced sensitivity up to ten times greater compared to standard pulsed reference methods. Moreover, this gain in sensitivity does not rely on a specific surface chemistry. Therefore, the more affine a given accumulation layer is for its target metal, the lower the detection limits that can be achieved with this method. This study points towards a promising route for next-generation electrochemical sensors for trace metal detection, offering sensitivity beyond that of traditional faradaic detection schemes while remaining simple, scalable, and broadly adaptable.