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.