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

Abstract

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.

Chemical Dissolution Pathways of MoS2 Nanosheets in Biological and Environmental Media

“…35 There are also chalcogen-based 0D nanoparticles such as CdSe quantum dots (QDs), and studies have shown quenching and chemical degradation during their use under physiologic conditions, which was ascribed to chemical oxidation of sulfur or selenium atoms on the QD surface. 39, 40 …”

Multiplexed In Vivo Imaging Using Size‐Controlled Quantum Dots in the Second Near‐Infrared Window

“…The BHMT may also offer additional spatial flexibility to the conjugation site because the amine sites should be further sticking away from the polymer backbones. 1080‐PQDs were conjugated to N ‐hydroxysuccinimide (NHS) ester‐biotin,19 and the conjugation was confirmed by streptavidin agarose bead (strep‐bead) assay (Figure S8, Supporting Information). The number of biotinylated 1080‐PQDs attached to the strep‐beads was much larger than those of the control experiment that used a mixture of unconjugated 1080‐PQDs and biotins.…”

“…The two FL spectra overlapped very little, which can be optimal for multiplexing with little interference between the two signals. PQD surfaces were modified by polymer encapsulation using crosslinked poly(ethylene glycol)‐conjugated poly(maleic anhydride‐alt‐1‐octadecene) (PMAO‐PEG) following a reported method with slight modifications . Figure d shows color CCD image of 1080‐PQD and 1280‐PQD phosphate buffered saline (PBS) buffer solution samples.…”

“…Compared to direct ligand‐exchange methods, encapsulation methods cause less perturbation to the QD surface state because they retain the as‐is surface ligands used for the synthesis . PbS/CdS QDs were encapsulated using amphiphilic polymers by using a reported procedure with modification ( Figure a) . Briefly, amphiphilic PMAO ( M n = 30 000–40 000) was reacted with methoxypolyethylene glycol amines (amine‐PEG, M n = 1000) to form amide bonds that yield PMAO‐PEG.…”

“…In the previous report by others,[12b] bioconjugations of PMAO‐PEG encapsulated QDs were demonstrated using the remaining carboxylic acids on QD surface. We intentionally used a significantly larger number of the BHMT crosslinkers per QD so that some of the BHMTs have remaining un‐crosslinked primary amine moieties on the PQD surface.…”