We herein report the optical and cytotoxicity properties of highly luminescent water soluble mercaptopropanoic acid (MPA) capped CdTe/CdSe core shell nanoparticles (NPs). The synthesis of the CdTe/CdSe NPs was carried out via a simple, one pot and economical route, involving the use of greener materials under ambient environment in the absence of an inert atmosphere. The temporal evolution of the size and optical properties of the nanomaterials was investigated by varying the reaction time and stability of the as-synthesised material at pH 12. The as-synthesised nanomaterials were characterised using UV-vis absorption and photoluminescence (PL) spectroscopy. The nanoparticles obtained were of high quality with high absorption and emission features. Addition of Se precursor to produce CdSe layer on the CdTe NPs core surface resulted in significant red shirt of both the absorption and emission maxima. The stability study showed that the emission maximum peak positions and FWHM remain the same with increase in emission intensity for all the NPs during the aging period. The cytotoxicity assay showed very high cell viability for the CdTe/CdSe NPs produced at 7 h compared with those produced at 30 mins as the concentration increased from 0.1 to 60 ug/ml. The lower cytotoxicity at the higher reaction time was attributed to the higher stability of the material and hence lower release of Cd2+.