Electrophilicity modulated targeted luminescence of MOF-coated cotton composite for dual analyte detection in aqueous medium

Abstract

Sulfur is a soft Lewis base and thiocarbonyl has moderate electrophilicity. Our probe's properties are adjusted in such a way that it could simultaneously detect a Lewis acid (Hg²⁺) and a strong nucleophile (NH₂–NH₂). Considering the above fact, a thioureido-functionalized robust MOF material was prepared, which was utilized for the selective fluorometric detection of environmentally significant toxic pollutants (Hg²⁺ and NH₂–NH₂) in an aqueous medium. The probe detected mercury by quenching the fluorescence emission intensity in a static pathway by soft-soft interaction with the S-atom of the probe. In contrast, the detection of hydrazine was furnished by a reaction-based pathway by the attack of hydrazine to the moderate electrophilic (thiocarbonyl) part of the probe, which resulted in an enhancement in the fluorescence emission intensity of the probe. A MOF-coated cotton composite was developed for the naked-eye detection of Hg²⁺ and NH₂–NH₂ for real-life applicability. The MOF was highly sensitive towards detecting Hg²⁺ and NH₂–NH₂ with very low detection times, i.e., 10 s and 50 s, respectively. The probe's sensitivity also remained unaltered under a significantly lower concentration of the targeted analytes, i.e., the detection limits for NH₂–NH₂ and Hg²⁺ were 1.9 nM and 4.0 nM, respectively. Our probe's response time and LOD are much lower than the other previously reported probes for Hg²⁺ and NH₂–NH₂ to date. A 92% fluorescence quenching and 28-fold fluorescence enhancement of the MOF were observed after the interaction of the probe with Hg²⁺ and NH₂–NH₂, respectively. The probe has excellent selectivity over the competitive analytes of Hg²⁺ and NH₂–NH₂. The MOF could also sense hydrazine in various environmental water specimens. Systematic mechanistic investigations were conducted to know the phenomena behind the fluorescence quenching and enhancement processes.