Rhodamine-based multianalyte colorimetric probe with potentialities as on-site assay kit and in biological systems

Herein, we report a new rhodamine-based multianalyte (Cu²⁺ and Hg²⁺) fluorescent sensor RH-FP. The synthesized probe acts as a dual chemosensor for recognition of Cu²⁺ and Hg²⁺ in acetonitrile-water (4:1 v/v, pH 7.2 with 10 mM, HEPES) in the presence of other miscellaneous metal cations i.e. Ag⁺, Al³⁺, Ba²⁺, Ca²⁺, Cd²⁺, Fe³⁺, Fe²⁺, K⁺, Li⁺, Mg²⁺, Mn²⁺, Cu⁺², Pb²⁺, Ni²⁺, Na⁺ and Zn²⁺. A highly selective and sensitive colorimetric (colorless to pink) response accompanied by a significant increase in absorption and emission maxima (250-folds) was observed by the interaction of RH-FP and Cu²⁺ or Hg²⁺. This makes RH-FP a suitable “bare-eye” detector. The Job's plot and ESI–MS confirm 1:1 binding stoichiometry between sensor RH-FP and metal cations. The calculated binding constant for the complexes (RH-FP-Cu²⁺ and RH-FP-Hg²⁺) are 3.37 × 10⁵ mol⁻¹ and 7.6 × 10⁵ mol⁻¹, with a detection limit of 3.9 nm and 2.36 nm respectively. The binding mechanism was confirmed by Fourier transform infra-red spectroscopy (FT-IR) ¹H, ¹³C NMR and density functional theory (DFT) calculations. Moreover, the fluorescent sensor RH-FP displayed potential in-field applications as detection kit and in fluorescent imaging of biological systems to detect Cu²⁺ or Hg²⁺.