Aqueous ZnS quantum dots (QDs) were synthesised via a facile, cost-effective one-pot synthesis method in the ambient environment. To achieve most favourable ratios of thioglycolic acid (TGA):zinc (Zn):sulphide (S), the concentration optimisation by optical spectra (COOS) method was adopted. The x-ray diffraction (XRD) spectra confirmed the zinc-blende (cubic) phase of ZnS quantum dots (QDs), and Scherrer's method determined that on average, the QDs exhibit a size of 2 nm. The Fourier-transform infrared spectroscopy (FTIR) study shows that characteristic peaks are consistent with the vibrational modes of the TGA ligands, confirming the capping. The quantum dots (QDs) synthesised displayed a significant effect of quantum confinement with blue shift in absorption peaks, compared with bulk ZnS absorption peak. Interestingly, on increasing the concentration of TGA, it resulted in red shift at the absorption edge, shifting from 300 nm to 314 nm. On the other hand, increasing the concentration of cation Zn and anion S ratio led to a blue shift, ranging from 315 to 295 nm, and a red shift, ranging from 280–320 nm, at absorption edges respectively. A prominent emission peak appeared at 428 nm. The application of Brus equation allowed estimating the size of the quantum dots (QDs) to be within the range of 2 to 2.7 nm, which correlated well with the Bohr's radius of ZnS. This favourable property proposes the use of QDs in various optoelectronic devices.