Chromo-fluorogenic probes and sensing materials based on asymmetrically substituted dipyrromethenes for detection of Zn2+ ions

A simple chemosensors of the “integrated” type based on the asymmetrically substituted dipyrromethene ligands were proposed for selective detection of biologically significant Zn²⁺ ions in DMF and water. The dipyrromethenes act as a absorbance-ratiometric sensors due to the responses simultaneously on two absorption bands: ligand and the resulting complex. The probes exhibit a very weak fluorescence emission due to intramolecular migration (transfer) of a proton between heteroatoms and conformational mobility, that is, the ease of rotation of the pyrrole nuclei relative to the carbon atom of the methine spacer under the light influence. Intriguingly, upon the addition of Zn²⁺ ions to the ligands solutions, significant fluorescence enhancement response (FEF = 65–100) in solution was observed. Instant сolor responses were observed with the “naked eye” under visible light and UV irradiation. The chromophoric and fluorescent detection limits of Zn²⁺ ions by the asymmetrically methylsubstituted dipyrromethene ligands are 1.7 × 10⁻⁸–5.0 × 10⁻⁸ and 3.2 × 10⁻⁹–7.4 × 10⁻⁹ mol/L, respectively. It has been shown that ligands selectively detect Zn²⁺ ions in the presence of background and competing metal ions (Na⁺, Ca²⁺, Mg²⁺, Al³⁺, Cr³⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Cd²⁺, Hg²⁺, and Pb²⁺). Reversibility of spectral responses due to regeneration of the chemosensors in presence of Na₂EDTA was studied. The possibility of “naked-eye” rapid detection of Zn²⁺ ions in situ using solid state sensing materials based on test strips, cotton pads, cotton swabs and SiO₂ powder doped with dipyrromethene dye was demonstrated.