Harnessing chromogenic response for photocurrent switching: A smartphone-aided, field-deployable optical platform for detection of heavy metal pollutants

A conjugated molecular sensing platform is presented that integrates selective Hg 2+ recognition with visible colorimetric response, charge-transfer photophysics, photocurrent modulation, and smartphone-assisted digital readout within a single system. The sensor is based on an oligo(p-phenylenevinylene) (OPV) backbone bearing terminal benzimidazole units, designed to couple intramolecular charge transfer (ICT) with coordination-driven supramolecular organization. Density functional theory (B3LYP/6-31G*) revealed a partially nonplanar geometry with asymmetric frontier orbital distribution. Hg 2+ binding in THF induced a pronounced bathochromic shift (∼25 nm) and a ratiometric fluorescence response with a detection limit of 5.6 ppb, while maintaining excellent selectivity over competing ions. A 1:1 binding stoichiometry was established with an association constant of K = 1.81 × 10 4 M −1 , and spectroscopic analyses confirmed metal-assisted aggregation and conformational rigidification as key amplification mechanisms. Notably, the probe also functioned as an active photodetector layer, exhibiting reproducible, wavelength-dependent photocurrent switching under blue (450 nm), green (555 nm), and red (650 nm) illumination at −1 V bias. The pristine device displayed maximum photosensitivity (∼180%) and highest responsivity under red excitation, while Hg 2+ coordination induced a ∼9.8-fold suppression in red-light photocurrent along with significant modulation of responsivity, detectivity (D*), and external quantum efficiency (EQE), consistent with ICT-state stabilization and trap-assisted recombination. Further, the sensing response was translated onto a paper substrate for reversible, naked-eye detection and quantitative smartphone-based RGB analysis (R 2 > 0.99). This unified optical–electronic–digital approach offers a robust, field-adaptable framework for monitoring toxic heavy metal pollutants.