Long-Term Colloidal Stability and Photoluminescence Retention of Lead-Based Quantum Dots in Saline Buffers and Biological Media through Surface Modification

Lead-based quantum dots (QDs) can be tuned to emit in the transparent region of the biological tissue (700 to 1100 nm) which make them a potential candidate for optical bioimaging. However, to employ these QDs as biolabels they have to retain their luminescence and maintain their colloidal stability in water, physiological saline buffers, different pH values, and biological media. To achieve this, four different surface modification strategies were tried: (1) silica coating; (2) ligand exchange with polyvinylpyrrolidone; (3) polyethyleneglycol-oleate (PEG-oleate) intercalation into the oleate ligands on the surface of the QDs; and (4) intercalation of poly(maleicanhydride-alt-1-octadecene) (PMAO) into the oleate ligands on the surface of the QDs and further cross-linking of the PMAO. The first two methods exhibited excellent dispersion stability in water, but did not retain their photoluminescence. On the other hand, the intercalation strategy with PEG-oleate helped the QDs retain their luminescence but with poor colloidal stability in water. The fourth and final strategy involving intercalation and cross-linking of the amphiphilic polymer PMAO provided the QDs with colloidal stability in water but also resulted in the QDs retaining their luminescence as well. This process involved two steps; (1) the intercalation between octadecene chains of PMAO with the oleates on the surface of the QDs with some of the anhydride rings opened with PEG-amine; (2) the anhydride rings were cross-linked with bis(hexamethylene)triamine (BHMT) to avoid detachment of the polymer from the surface of QDs because of the polymer's dynamic nature in solvents. The presence of PEG molecules potentially improves the biocompatibility of the QDs and the presence of carboxylic acids after reaction with BHMT makes them suitable for further surface functionalization with antibodies, proteins, and so forth. The surface-modified QDs have excellent dispersibility in water, saline buffers, and in various pH conditions for more than 7 months and more than 20 days in serum-supplemented growth media. In addition to the colloidal stability, the QDs retained their photoluminescence even after 7 months in the aforementioned aqueous media. The intercalation and cross-linking process have also made the QDs resistant to oxidation when exposed to ambient atmosphere and aqueous media.