ZnAl₂O₄ Nanomaterial as a Naked-Eye Arsenate Sensor: A Combined Experimental and Computational Mechanistic Approach

Raising public awareness over the emerging health risk due to intake of arsenic-contaminated potable water is a matter of great concern. Exploration of cost-effective, self-testing kits is a substantial way to reach out to the masses and detect the presence of arsenate in water. With this agenda, a photoluminescent Mannich base Zn(II) complex (ZnMC = [Zn₂(ML)₂]·(ClO₄)₂·(H₂O); HML = Mannich base ligand) has been synthesized, and its dinuclearity was verified with single-crystal X-ray diffraction structural analysis. Among a range of anions, ZnMC was found to detect arsenate selectively by showing a turn-off emission with a color change from bright green to dark under UV light. The real-life applicability of the ZnMC probe is somewhat restricted to only sensing of arsenate, but not its removal owing to the fact of its homogeneity. Considering the efficacy of ZnMC as well as a need for its easy removal from water, slight modification has been done with chloride ions in the form of ZnMC″ (=[Zn₂(ML)₂(Cl)₂]), and finally, an interface between homogeneous and heterogeneous solid support has been explored with a strategic fabrication of ZnMC″ grafted ZnAl₂O₄, named as ZAZ nanomaterial. This not only imparts successful segregation of arsenate from drinking water but also provides naked-eye detection under ambient light as well as UV light. Thermodynamic parameters associated with the binding of arsenate to ZnMC and ZAZ have been evaluated through isothermal calorimetric (ITC) measurements. Steady-state and time-resolved fluorescence titration study, absorption titration study, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and computational calculations have been performed to get deep insights into the sensing properties. Proper justification of the sensing mechanism is the highlight of this work. ZAZ nanomaterial has been exploited to produce a self-test paper kit for arsenate detection with a limit of 9.86 ppb, which potentially enables applications in environmental monitoring.