Three bisphosphonate ligands improve the water solubility of quantum dots

Synthesised Quantum Dots (QDs) require surface modification in order to improve their aqueous dispersion and biocompatibility. Here, we suggest bisphosphonate molecules as agents to modify the surface of QDs for improved water solubility and biocompatibility. QDs_TOPO (CdSe/ZnS-trioctylphosphine oxide) were synthesised following modification of the method of Bawendi et al. (J. Phys. Chem. B, 1997, 101, 9463-9475). QDs surface modification is performed using a ligand exchange reaction with structurally different bisphosphonates (BIPs). The BIPs used were ethylene diphosphonate (EDP), methylenediphosphonate (MDP) and imidodiphosphonate (IDP). After ligand exchange, the QDs were extensively purified using centrifugation, PD-10 desalting columns and mini dialysis filters. Transmission electron microscopy (TEM) and fluorescent spectroscopy have been used to characterise the size and optical properties of the QDs. Cell toxicity was investigated using MTT (tetrazolium salt) and glutathione assays and intracellular uptake was imaged using confocal laser scanning microscopy and assessed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). QDs_TOPO and QDs-capped with BIPs (QDs_BIPs) were successfully synthesised. TEM showed the size and morphology of the QDs to be 5-7 nm with spherical shape. The stabilised QDs_BIPs showed significantly improved dispersion in aqueous solutions compared to QDs_TOPO. The cytotoxicity studies showed very rapid cell death for cells treated by QDs_TOPO and a minor effect on cell viability when QDs_BIPs were applied to the cells. Both EDP- and MDP-modified QDs did not significantly increase the intracellular levels of glutathione. In contrast, IDP-modified QDs substantially increased the intracellular glutathione levels, indicating potential cadmium leakage and inability of IDP to adequately cap and stabilise the QDs. EDP- and MDP-modified QDs were taken up by IGROV-1 (ovarian cancer) cells as shown by fluorescence microscopy, however, the IDP-modified QD signal was not clearly visible in the cells. Cellular uptake measured by intracellular cadmium levels using ICP-MS showed significant uptake of all three BIPs QDs. The structure of BIPs appears to play a significant role in the ability of these molecules to act as capping agents. Our findings demonstrate a novel approach to produce water-dispersible QDs through ligand exchange with certain types of BIPs molecules that can find application in bioimaging.