Rhodamine-Based Dual Chemosensor for Al3+ and Zn2+ Ions with Distinctly Separated Excitation and Emission Wavelengths

A rhodamine-based compound, 2-(2-((3-(tert-butyl)-2-hydroxybenzylidene)amino)ethyl)3'-6'-bis(ethylamino)-2',7'-dimethylspiro[indoline-1,9'-xanthen]-3-one, (HL-t-Bu), is reported here as a dual chemosensor for Zn2+ and Al3+ ions. This compound has been synthesized under mild conditions with high yield and characterized by elemental analysis and different standard spectroscopic methods. Its structure has been confirmed by single-crystal X-ray diffraction analysis. It acts as a fluorescent dual sensor for Zn2+ and Al3+ in 10 mM HEPES buffer in the methanol/water mixture (9:1, pH = 7.4) with well-separated excitation and emission wavelengths. The fluorescence intensity at 457 nm of HL-t-Bu increases by similar to 7 folds in the presence of 1 equiv of Zn2+ when it is excited at 370 nm. With the excitation at 500 nm, the emission intensity of the probe at 550 nm increases massively (similar to 650 times) in the presence of 1 equiv of Al3+. No other metal has any significant effect on the enhancement of the emission intensity of the probe in the detection process for either of the metal ions. The quantum yield of HL-t-Bu enhances significantly in the presence of Zn2+ and Al3+. Rhodamine derivatives are generally colorless and nonfluorescent when the spirolactam ring is closed, and they are pink in color and highly fluorescent when it exists in the ring-open form. In the presence of Al3+, the probe is pink and highly fluorescent, indicating that this metal ion is able to the open spirolactam ring of the probe. However, Zn2+ is not able to open the spirolactam ring, but it is coordinated through phenolic oxygen and imine nitrogen (salicylaldehyde unit) of the Schiff base restricting PET (photoinduced electron transfer) and imposing CHEF (chelation-enhanced fluorescence) to enhance the emission intensity of the probe. H-1 NMR, FT-IR, elemental analysis, and pH-dependent studies support these mechanisms. Limit of detection values are in the nanomolar range for both the metal ions, confirming very high sensitivity of the probe. The probe has been used in cell imaging studies for both of the metal ions.