Surfactant-Free Green Synthesis of ZnS QDs with Active Surface Defects for Selective Nanomolar Oxalic Acid Colorimetric Sensors at Room Temperature

Quantum dots (QDs) are synthesized using surfactants to enable their viable applications with minimum surface defects. The environmentally benign method without a surfactant is a way forward for sustainable applications. Sunlight-mediated synthesis of uniform ZnS QDs without a surfactant is achieved with pristine surface defects that facilitate visible light absorption. The cubic phase of QDs and their size are revealed using transmission electron microscopy and X-ray diffraction. The unique absorption property is utilized to demonstrate a cost-effective colorimetric sensor for biological toxic substances such as oxalic acid using absorption spectroscopy. The unparalleled performance among the enzymatic or nonenzymatic processes for oxalic acid detection is realized, however, without any noble metal, for a wide range of 1 nM to 0.5 mM concentrations with a limit of detection of 0.2 nM. Typical inferences from organic acids, water-soluble salts, and metal ions are also investigated. Further, the role of surface defects in QDs is unfurled with photoluminescence, infrared, and Raman spectroscopic measurements, and the underlying mechanism of OA detection is elaborated. Thus, green synthesis and unmasking the influence of surface defects in ZnS QDs as demonstrated for selective detection open up for various other applications such as solar cells and catalysts.