Surface functionalization strategies for cadmium selenide quantum dots using well-defined hydrophilic polymers

A general strategy is described that uses simple hydrophilic polymers to functionalize the surface of fluorescent CdSe quantum dots. Preliminary attempts of functionalization used highly defined copolymers that were developed using controlled polymerization methods. The relative content of hydrophilic and hydrophobic monomer units was varied to produce a range of polymers with various backbone compositions. The trends that were observed upon dispersion of the CdSe nanoparticles using these amphiphilic polymers were, however, not congruent with the interdigitation mechanism described in literature procedures. A thorough investigation of ligands attached to the surface of the CdSe nanoparticles was then conducted using 31P nuclear magnetic resonance (NMR) spectroscopy. The reagents, DMAP and propionate, were used to strip the native ligands from the surface and spectra of the supernatant showed sharp and distinct signals. The data were used to identify numerous surface-bound constituents including: n-octylphosphonic acid (OPA), TOPSe, TOPO. Additionally, pyrophosphonic acid (PPA) was identified as a new surface-bound ligand that has not been characterized in previous reports. The relative binding strength of the phosphorus ligands and their degree of susceptibility to carboxylate ligand exchange was also gauged using the resulting NMR data. The observed trend in binding strength was that phosphonic acids, namely OPA and PPA, are the strongest CdSe binding ligands. The most significant result of the NMR experiments is that a specific interaction with a carboxylate and the surface of the CdSe nanoparticle occurs. This interaction was utilized as the foundation for a new functionalization strategy which used poly(AA) to modify fluorescent CdSe, CdSe ZnS, and CdSe CdS, which were subsequently dispersed in water. The optical properties of the nanoparticles were analyzed, with specific attention to optimizing the quantum yield. Although the functionalization process resulted in decreases in the nanoparticle fluorescence, the addition of amines to the dispersions restored the quantum yield. Lastly, functional group tagging with FITC and electrophoresis was performed to assess the performance of the water-dispersed fluorescent particles. The functionalized material demonstrated the ability to host sites for covalent attachment, possess multiple functionalities, and have variable surface charge that can be manipulated by simple chemistry.