br Conflict of interest br Acknowledgements The authors

Conflict of interest
Acknowledgements The authors would like to thank Sherri Friend for her valuable assistance with microscopy. This work was supported by the National Institute for Occupational Safety and Health, Nanotechnology Research Center Fund and by a grant from the National Institutes of Health (R01-ES022968).
Introduction Generally, quantum dots (QDs) are referred to as the zero-dimensional colloidal crystals that possesses strong size dependence and multi-colored luminescence properties [1,2] along with its intrinsic features, such as the sharp and symmetric emission, photostability and high quantum yields. Owing to these inherent inimitable properties, QDs play a vital role in various avenues namely the identification of the chemical moieties [3], clinical diagnostics [4–9], optoelectronics [10,11], bio-imaging and bio-sensing [12–19]. QDs are chemically unstable and have poor solubility in water and due to these deficits it was not preferred for biomedical applications. But it associates the promising features needed for the biomedical studies and hence, a number of attempts were put forth, to make it water-soluble. Primary works relied on the usage of different thiol stabilizers to promote the water solubility and chemical stability in the water phase. The thiol linkage serves as the protection layer to the core and thus prevents it from surface oxidation. Thus, the solubility of the QDs in water can be enhanced. Thiols are often sorted as the ideal choice for the modification of QDs due to its facile functionality [20]. A wide variety of quantum dots are available namely cadmium selenide (CdSe), cadmium sulphide (CdS), zinc sulfide (ZnS), etc., [21,22]. The syntheses of these materials require high temperature, non-aqueous medium and possess low efficiency. These discrepancies can be rectified using cadmium telluride (CdTe) QDs as they are expected to show a high fluorescence efficiency and good stability [2]. In order to obtain an optimum quantum efficiency the following stabilizers are used namely mercaptoacetic hippo signaling pathway (MAA), glutathione (GSH) and thioglycerol (TGA) [23,24]. Few of the works reported the toxicity of CdTe QDs in relation to the metal ions and stabilizers used for its synthesis [25–31]. Hence the toxicity level of the stabilizers has to be considered as a very important factor for biomedical application, where the best choice is confined to the low toxicity. Recent studies relied on the preparation of CdTe using cysteine as a surfactant [32,33]. l-Cysteine (Cys) is a simple amino acid that possesses the thiol(-SH) group and two other active functionalities (−NH2, −COOH) which is considered to be the essential component of human metabolism and enzyme reactions. Specifically, the presence of thiol group functions as a nucleophile and thus makes the complex structure of proteins and is subjected to oxidation during the alteration of the disulfide bond [34]. Yang et al. attempted the direct labeling of Escherichia coli (E. coli) using CdTe QDs by the following treatments namely chloroform–SDS, lysozyme–EDTA and osmotic shock and, the results obtained showed an enhancement in the sensitivity [35]. Our present work is based on the synthesis of Cys capped CdTe QDs in an aqueous medium for labeling the cell and the reactivation of FL intensity using additional Cys. Li et al. also reported the recovery of FL intensity of CdTe to its initial level by the addition of Cys [36]. But our research differs from the other reported works in many aspects. Herein, we carried out a systematic investigation on the influence of the additional Cys to CdTe@Cys in the facet of biomolecular protection. The results obtained proved the optical superiority of the adopted technique. Addition of Cys to higher concentration significantly improved both the stability of bacterial communities and FL intensity of CdTe@Cys QDs. Furthermore, the reactivation of the CdTe@Cys QDs resulted in a mitigated activity of CdTe in the antibacterial test. Along with the lower cytotoxicity, reactivated CdTe@Cys QDs labeled E. coli were found to be more adequate. MTT assay was carried out for the determination of the cytotoxicity of reactivated CdTe@Cys to HeLa cells. The results derived out of the study proved the complementary evidences for its strong implication as a novel labeling method. Thus it will have a promising future and give an unprecedented insight by the way of an efficient labeling of cell using QDs.