Colorimetric Calcium Probe with Comparison to an Ion-Selective Optode

Design strategies for small molecular probes lay the foundation of numerous synthetic chemosensors. A water-soluble colorimetric calcium molecular probe inspired by the ionophore-based ion-selective optode is presented here with a tunable detection range (around micromolar at pH 7). The binding of Ca²⁺ resulted in the deprotonation of the probe and thus a significant spectral change, mimicking the ion-exchange process in ion-selective optodes. The 1:1 exchange between Ca²⁺ and H⁺ was confirmed with Job’s plot. Computational studies revealed possible monomer and dimer forms of the probe–Ca²⁺ complexes.