Two Colorimetric and Fluorescent Dual-Channel Chemosensors for the Selective Detection of pH in Aqueous Solutions

Fluorescent probe - which can detect metal ions, pH, and other analytical species through spectroscopic approaches - have recently attracted significant research attention in the fields of analytical and biological chemistry. For this study, two novel colorimetric and fluorescent probes, named RP1 and RP2, were designed and synthesized. Their structural confirmations were then achieved through elemental analysis, FT-IR, NMR (H-1 and C-13) spectroscopies, and tandem MS spectrometry. To gain full understanding of the photophysical properties and chemosensory performances of these two dyes, both the UV-Vis absorption and fluorescence titration experiments were carefully performed in aqueous media. Notably, upon treatment of RP1 and RP2 in solutions with each competitive metal ion, neither increases in absorption and fluorescence spectra above 400 nm nor any changes in color were observed. However, at lower pH (i. e.,<7.0), those two sensors exhibited a drastic change in color from colorless to pink, followed by broadened absorption bands. As expected, upon excitation at 510 nm, a strong enhancement of the fluorescence emissions was obtained. By conducting H-1 NMR and optical titrations, the spirolactam-ring opening reaction of these receptors, induced by H+ ions at lower pH, was approved. The results demonstrate that the fluorescent dyes RP1 and RP2 can be effectively used as reversible pH-responsive chromogenic and fluorogenic "Off-On" switches in real samples.