One-Shot SERRS Detection of Iron and Acidity in Aqueous Systems

Optical strategies for ion sensing hold promise for portable and in situ analysis, yet their reliability is frequently limited by pH-dependent interferences that alter metal-ligand interactions. Herein, we present a multiprobe surface-enhanced resonance Raman scattering (SERRS) platform that enables simultaneous quantification of Fe(II) and direct in situ pH readout in aqueous media. The sensor is constructed from polystyrene beads (PS) densely coated with silver nanoparticles and functionalized with two probes: phenanthroline (Phen), a metal-selective dye, hydrophobically entrapped within a cetrimonium bromide (CTAB) bilayer to retain its Fe(II) binding activity, and 4-mercaptobenzoic acid (MBA), covalently anchored to the silver surface to provide a reliable pH response. Resonant excitation at 532 nm maximizes SERRS sensitivity, yielding Fe(II) detection down to 30 ppb with strong selectivity against competing metal ions. Crucially, the MBA readout decouples pH effects from Fe(II) quantification, avoiding false results at alkaline pH where iron hydroxide precipitates dominate. This dual-sensing strategy provides a robust concept to overcome pH interference in optical ion sensing and paves the way for portable, time-resolved monitoring of metal ions in biological and environmental systems.