Exploiting Spiropyran Solvatochromism for Heavy Metal Ion Detection in Aqueous Solutions

Three spiropyran derivatives (SP1–SP3) were studied as colorimetric sensors for Cd²⁺, Pb²⁺, and Hg²⁺ions in nine solvents of varying polarity and hydrogen-bonding character based on the E_T(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. SP1 and SP2 yielded distinct metal-induced colorimetric responses, with the former showing its strongest Hg²⁺response in tetrahydrofuran and acetone and the latter detecting Pb²⁺and Cd²⁺in methanol, whereas SP3 was comparatively less sensitive overall. Notably, Hg²⁺elicited the most selective and sensitive colorimetric response (inducing a purple to yellow transition), Cd²⁺presented clear blue shifts (with a color change from purple to dark pink and pink to colorless), and Pb²⁺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 SP1 probe retains high specificity for Hg²⁺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 Hg²⁺) in environmental monitoring applications.