Photophysical insights into acridine orange–polyglutamic acid interaction and its application in ratiometric turn-on heavy metal ion recognition

Abstract

Polyglutamic acid (PGA), a biodegradable and multifunctional biopolymer rich in carboxylate groups, holds strong promise for designing supramolecular sensing systems to address the urgent problem of heavy metal contamination. This study presents a systematic investigation of the detailed photophysical behaviour of acridine orange (AO) upon interaction with PGA, combined with the application of an indicator displacement assay (IDA) strategy using a PGA-based complex for heavy metal ion recognition. Spectroscopic investigations revealed distinct spectroscopic alterations, including a ∼ 34 nm blue shift in absorption, pronounced fluorescence quenching, a bisignate CD signal, and longer fluorescence lifetime, indicative of AO aggregate formation on the PGA scaffold. Harnessing these photophysical changes, the aggregated AO–PGA complex was applied for ratiometric turn-on sensing of Cd²⁺, Pb²⁺, and Cu²⁺ via indicator displacement mechanism, wherein metal ion binding triggers the dissociation of AO aggregates and restores the monomeric AO spectral features. The system exhibited detection limits of 0.19 μM for Cd²⁺, 0.29 μM for Pb²⁺, and 0.47 μM for Cu²⁺ in nanopure water, with the enhanced sensitivity towards Cd²⁺. Its practical applicability was further validated in tap water and 2% seawater, demonstrating the potential of this complex for environmental monitoring of heavy metal ion contamination.