Passivation of Quantum Dots and Nanoparticles with Short Lipoic Acid Derivatives to Afford Biocompatibility

Abstract

The use of quantum dots (QDs) and medium-to-large nanoparticles (NPs) (20-100 nm diameter) in biosensing applications is hindered by issues concerning stability and biocompatibility. In this thesis I address this concern by developing a new procedure with which to assess how coating conditions for these particles are able to confer colloidal stability in various pH ranges and different NaCl concentrations, and resistance to nonspecific protein adsorption. The particles to be tested include CdSe QDs, Ag NPs (20, 40, and 60 nm in diameter), and Au NPs (20, 40, 60, 80, and 100 nm in diameter). Coating conditions consist of combinations of three lipoic acid derivatives, one of which contains a polyethylene glycol end group and the other two containing complementary zwitterionic groups. I found that the ability of the coating layer to confer the desired properties is strongly dependent not only on the composition of the particle, but also on its size. Aggregation was observed in larger NPs and was more prone to occur in the case of Au NPs. I also discovered that, for the ligands tested in this thesis, coatings consisting of a mixture of ligands is required for biocompatibility optimization, as the use of just one ligand for any case cannot bestow both colloidal stability and resistance to protein adsorption. The colloidal stability of the NPs is further convoluted by dependencies upon the type of salt utilized, the concentration of the salt, and the time allowed for aggregation. Thus, this thesis provides the procedural foundation for future tests to optimize these variables for QDs/NPs.