Exploiting Spiropyran Solvatochromism for Heavy Metal Ion Detection in Aqueous Solutions.

Three spiropyran derivatives (-) were studied as colorimetric sensors for Cd, Pb, and Hg ions in nine solvents of varying polarity and hydrogen-bonding character based on the E(30) scale. We used solvatochromic analysis and UV-visible titrations to characterize each spiropyran's baseline absorption in each solvent and its changes upon metal interaction. Solvatochromic measurements enabled us to separate intrinsic solvent-induced color shifts from metal-specific responses. The results revealed a strong solvent dependence for sensing. and yielded distinct metal-induced colorimetric responses, with the former showing its strongest Hg response in tetrahydrofuran and acetone and the latter detecting Pb and Cd in methanol, whereas was comparatively less sensitive overall. Notably, Hg elicited the most selective and sensitive colorimetric response (inducing a purple to yellow transition), Cd presented clear blue shifts (with a color change from purple to dark pink and pink to colorless), and Pb caused only minor spectral changes in one system. These findings highlight that solvent polarity can be utilized to modulate spiropyran-merocyanine equilibria and interaction affinities. Selectivity tests in real contaminated water samples confirmed that the probe retains high specificity for Hg even in the presence of common interfering ions. Overall, this study highlights the potential of solvent-tuned spiropyran probes for rapid, selective detection of toxic heavy metals (particularly Hg) in environmental monitoring applications.