Toward an understanding of microalgae EPS-based selectivity and binding mechanisms for trace metal ions using Love wave acoustic and voltammetric sensors in seawater

This study investigates the interactions between microalgal extracellular polymeric substance (EPS) and six environmentally relevant trace metals (Cu²⁺, Pb²⁺, Hg²⁺, Cd²⁺, Co²⁺, and Ni²⁺) in natural sea water across a concentration range from 10^-14 to 10^-4 M. The focus is on evaluating the functionality of EPS as a bioinspired coating layer in the development of acoustic (Love wave type Surface Acoustic Wave, SAW) and electrochemical (Square Wave Voltammetry, SWV) sensors for <i>in situ</i> trace metal monitoring in the marine environment. Fourier-transform infrared (FTIR) spectroscopy and Scanning Electron Microscopy-Energy-Dispersive X-ray Spectroscopy Analysis (SEM-EDX) confirmed successful EPS immobilization on silicon dioxide and gold surfaces, with characteristic spectral shifts indicating coordination-based interactions with target metal ions. Acoustic measurements using variations of resonance frequency and amplitude as a function of metal ion concentration showed the highest sensitivity (amplitude attenuation of 6.29 and 5.69 dB per decade) with lead and mercury. Electrochemical characterization in seawater conducted with and without redox mediators revealed metal-specific differences in peak currents, redox potentials and overpotentials. Although the high ionic strength of seawater and metal-EPS reduced direct SWV responses in some cases, the use of a ferri/ferrocyanide mediator improved the sensitivity and selectivity of detection particularly for lead (12.76 µA per decade) and mercury (12.60 µA per decade). The results demonstrate that EPS-functionalized surfaces generate a distinct mechanical and electrochemical signature for each target metal, highlighting the potential of an EPS-based bioinspired coating for developing sustainable, selective, and environmentally relevant sensing platforms for monitoring and remediating marine trace metal pollution.