Dual-Mode Colorimetric and Fluorescent Detection of Cu2+ Based on a Two-Component Sensing Platform Integrating an Imine Derivative and Rhodamine B

Abstract

Copper is an essential trace element involved in key biological and environmental processes; however, its imbalance can lead to severe health and ecological consequences, making its accurate quantification crucial. In this work, a two-component colorimetric and fluorescent sensing platform based on the imine derivative 2-(((6-bromopyridin-2-yl)methylene)amino)-4-(tert-butyl)phenol (BPTP) and rhodamine B (RhB) was developed for the selective detection of Cu2+ in aqueous media. The BPTP ligand, synthesized via a Schiff base condensation reaction and fully characterized by NMR spectroscopy, enables rapid and highly selective colorimetric recognition of Cu2+ at pH ≥ 7.0, producing a distinct visible transition from yellow to red associated with the formation of a 2:1 BPTP–Cu2+ complex. This binding mode was confirmed by UV–vis titration, ESI-MS analysis, and density functional theory (DFT) calculations, which also elucidate the structural and electronic changes responsible for the observed bathochromic shift. To enhance analytical performance, RhB was incorporated as an independent fluorescent reporter, generating a complementary optical channel governed by an inner filter effect (IFE) induced by the absorption band of the BPTP–Cu2+ complex. This dual-mode strategy significantly improves both sensitivity and selectivity in the presence of competing metal ions, achieving detection limits of 2.1 μM (colorimetric, BPTP), and 1.9 μM (fluorescence, RhB + BPTP). The platform was successfully applied to Cu2+ quantification in tap water, yielding concentrations within World Health Organization (WHO) limits. Overall, the RhB–BPTP platform represents a robust, sensitive, and cost-effective strategy for real-time copper monitoring in environmental samples.