Dipyrromethene-based colorimetric and fluorescent sensors for the monitoring Zn2+ ions in aqueous media and food additives

Using 3,3′,5,5′-tetramethyl-2,2′-dipyrromethene (1), its meso-phenyl derivative (2) and 3,3′,5,5′-tetraphenyl-2,2′-dipyrromethene (3), the influence of the arylation effects of the meso-spacer and pyrrole cores of dipyrromethene bidentate chelate ligands (HL) on their colorimetric and fluorescent sensory characteristics towards Zn²⁺ ions in solutions was studied. The action of the sensors was based on the coordination reaction of dipyrromethene ligands by Zn²⁺ ions with the formation of stable (lgK^o = 5.2–8.9) intramolecular chelate complexes [ZnL₂]. It was found that the spectral responses of the reactions of HL with Zn(AcO)₂ in DMF/H₂O (9:1, v/v) differ greatly depending on the peripheral substitution nature of the dipyrromethene core. The analytical reaction of 3,3′,5,5′-tetramethyl-2,2′-dipyrromethene 1 with Zn(AcO)₂ is accompanied by bright color responses and high sensitivity of chromophore (LOD_abs = 1.7 × 10⁻⁸ M) and fluorescent (LOD_fl = 4 × 10⁻⁹ M) detection of the analyte. Arylation of the meso-spacer of 3,3′,5,5′-tetramethyl-2,2′-dipyrromethene leads to almost complete suppression of the fluorescence response (fluorescence enhancement factor ∼ 4) of reaction of 2 with the analyte. Sensitivity of chromophore-ratiometric detection of Zn²⁺ ions: LOD_abs is 1 × 10⁻⁸ M. Arylation of the pyrrole rings of ligand 3 leads to a significant decrease in the sensitivity of both spectrophotometric and fluorescent detection of the analyte. For the first time, encouraging results of practical application of dipyrromethene sensor 1 in the analysis of Zn²⁺ ions in samples of distilled and atmospheric water from melted snow, as well as extracts from food additives, were demonstrated.