Fluorescence sensing and intracellular imaging of Pd2+ ions by a novel coumarinyl-rhodamine Schiff base

Abstract

Coumarinyl-rhodamine, HCR, served as an extremely selective sensor for Pd²⁺ ions in ethanol/H₂O (8 : 2, v/v, HEPES buffer, pH 7.2) solution and the limit of detection (LOD) was 18.8 nM (3σ method). The free sensor, HCR, was weakly emissive and in the presence of Pd²⁺, the colour changed from straw to pink with very strong emission at 598 nm in the presence of eighteen other cations. A plausible mechanism involved opening of the spirolactam ring of rhodamine on interaction with Pd²⁺, which was justified by structure optimization and transition energy calculations using the DFT technique. HCR underwent 1 : 1 complexation with Pd²⁺, which was confirmed via the Job's plot, mass spectra and Benesi–Hildebrand plot (association constant K_a, 9.1 × 10⁴ M⁻¹). A separate in vitro experiment showed that HCR could specifically sense Pd²⁺ in MCF7 (human breast adenocarcinoma) cell lines.