In this paper, we have synthesized surface active ionic liquid (SAIL)-capped zinc sulfide (ZnS) quantum dots (QDs) with an average size of approximately 3.03 nm using a systematic and facile microwave-irradiation method in aqueous solution. The imidazolium-based SAIL, 1-dodecyl-3-ethylimidazolium bromide was acted as a capping agent and was found to modulate the morphology and properties of ZnS QDs. The synthesized ZnS QDs were examined via different techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDAX), transmission electron microscopy (TEM), Fourier transform-infrared spectroscopy (FT-IR), thermal gravimetric analysis (TGA), diffuse reflectance spectroscopy (DRS), photoluminescence spectroscopy (PL) and cyclic voltammetry (CV). TGA confirmed the thermal stability of capped ZnS QDs. To investigate the fluorometric application, the synthesized ZnS quantum dots were tested for the sensing of ascorbic acid (AA). The fluorescence intensity was found to be directly proportional to the concentration of AA in the range of 50–476 µM and the limit of detection was 63.11 µM. It has been observed that the synthesized capped ZnS QDs selectively sense AA and calculated limit of detection (LOD) of AA was found to be lower than that of other reported systems using the fluorescence technique. To understand the role of capping agent in the synthesis of ZnS QDs, density functional theory (DFT) using B3LYP (Becke’s three-parameter exchange functional with Lee–Yang–Parr correlation energy) and 6-311 + G (d, p) basis set was employed. The DFT results showed that the SAIL played a vital role in the size-controlled synthesis of ZnS QDs.