Robust Optical Detection of Ga by a Rhodamine- and Coumarin-Based Proficient Probe: Theoretical Investigations and Biological Applications.

The present investigation highlights a rhodamine-B- and coumarin-based efficient probe that selectively detects Ga over other metal ions. The active pocket of the ligand for trapping the metal ions and the binding stoichiometry of its Ga complex were discovered by single-crystal X-ray diffraction (SC-XRD) analysis. This binding stoichiometry was further confirmed in the solution state by mass spectrometry and Job's plot. The detection limit was found to be at the nanomolar level. Pyrophosphate being a well-known quencher could easily quench the fluorescence intensity of the RC in the presence of Ga and reversibly recognize Ga in the solution. The spiro ring opening of the ligand after Ga insertion is proposed to be the principal mechanism for the turn-on fluorescence response. This ring opening was confirmed by SC-XRD data and nuclear magnetic resonance (NMR) titration experiments. Both ground- and excited-state calculations of the ligand and complex have been carried out to obtain information about their energy levels and to obtain the theoretical electronic spectra. Furthermore, the live-cell imaging of the probe only and the probe after the addition of Ga have been carried out in HaCaT cells and satisfactory responses were observed. Interestingly, with the help of this probe, Ga can be tracked inside the intracellular organelle such as lysosomes along with other regions of the cell. The article highlights a rhodamine-coumarin-based probe for the detection of Ga over other metal ions with a nanomolar level detection limit. Structural characterization of the ligand and its Ga complex was investigated by SC-XRD. Density functional theory (DFT) and time-dependent DFT (TD-DFT) studies were carried out to explore the excited-state energies and electronic spectra. The application of the probe for the detection of Ga in live cells has been explored, and positive responses were observed.