The new role of dipyrromethene chemosensor for absorbance-ratiometic and fluorescence "turn-on" sensing Zn2+ ions in water-organic solutions and real water samples

Abstract

This study presents a dipyrromethene-based sensitive and selective probe for Zn²⁺ ions detection in aqueous and water-organic media. The probe demonstrates absorbance-ratiometric and "off-on" fluorescent sensing for Zn²⁺ in a DMSO/H₂O (9:1, v/v) mixture. The 3,3',4,4',5,5'-hexamethyl-2,2'-dipyrromethene (HL), similar to its analogs, exhibits weak fluorescence (with a quantum yield of less than 0.001). However, upon the presence of Zn²⁺ ions in the sensor HL solution, there is a remarkable increase (up to 200-fold) in fluorescence intensity due to the formation of a stable intramolecular chelate complex [ZnL₂]. This complex formation induces a significant hyperchromic effect and a red shift (57 nm) in the characteristic absorption bands. The sensing mechanism of the probe towards Zn²⁺ ions was thoroughly investigated through absorbance and fluorescent titrations, molar ratio plots, ¹H NMR, and DFT/TDDFT studies. The fluorescence response exhibited a strong linear relationship with Zn²⁺ concentration within the range of 0 to 5.7 × 10^-6 mol/L. The detection limit (LOD) and limit of quantitation (LoQ) for Zn²⁺ were determined as 2 × 10^-8 mol/L and 6.6 × 10^-8 mol/L, respectively. Moreover, the probe demonstrated high selectivity for Zn²⁺ ions over other metal ions (Na⁺, Mg²⁺, Al³⁺, Cr³⁺, Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Pd²⁺, Cd²⁺, Hg²⁺, Pb²⁺). Test systems in the form of test-strips and cotton-pads were developed based on the dipyrromethene sensor for rapid "naked-eye" detection of zinc ions in water. The sensor was successfully applied for detecting Zn²⁺ ions in real water samples.

Keywords: Detection limit; Dipyrromethene; Practical application; Ratiometric and fluorescent response; Selectivity; Zinc.