Near-Infrared Cell-Permeable Hg2+-Selective Ratiometric Fluorescent Chemodosimeters and Fast Indicator Paper for MeHg+ Based on Tricarbocyanines

Three tricarbocyanine dyes (IR-897, IR-877, and IR-925) with different thiourea substituents that function as dosimeter units through specific Hg2+-induced desulfurization have been demonstrated in a fast indicator paper for Hg2+ and MeHg+ ions. In comparison with available Hg2+-selective chemodosimeters, IR-897 and IR-877 show several advantages, such as convenient synthesis, very long wavelengths falling in the near-infrared (NIR) region (650-900 nm) with high molar extinction coefficients, a ratiometric response, and quite low disturbance with Ag+ and Cu2+ ions. They exhibit large redshifts, which result in a clear color change from deep blue to pea green that can be easily monitored by the naked eye for a convenient indicator paper. In emission spectra, they display a characteristic turn-off mode at 780 nm and turn-on mode at 830 nm with titration of Hg2+ ions. Remarkably, the signal/noise (S/N) ratio with other thiophilic metal ions (Ag+ and Cu2+) is greatly enhanced with ratiometric measurement of two channels: excitation spectra mode (I-810 nm/I-670 nm, monitored at 830 nm) and emission spectra mode (I-830 nm/I-780 nm, isosbestic absorption point at 730 nm as excitation). The distinct response is dependent upon the electron-donating effect of the thiourea substituents; that is, the stronger the electron-donating capability of the thiourea substituents, the faster the Hg2+-promoted cyclization. Additionally, experiments with living SW1116 cells show that these three tricarbocyanine dyes with low toxicity can exhibit special characteristics that are favorable for visualizing intracellular Hg2+ and MeHg+ ions in biological systems, including excellent membrane permeability, minimal interfering absorption and fluorescence from biological samples, low scattering, and deep penetration into tissues.