A Study of Fluorescent Nanoparticles: Quantum Dots and Silica Dots
Optical techniques have here been employed for the study of two types of nanoscopic fluorescent probes, quantum dots and silica dots. Basic characterization of these fluorescent nanoparticles? photophysics and their capabilities have been explored utilizing the following techniques: fluorescence correlation spectroscopy, FCS, time resolved single photon counting lifetime decay measurements, and steady state absorbance, emission, and excitation measurements. The unique properties of QDs, unmatched by any available organic fluorophore, have the potential to allow the development of FRET-based nanoscale biosensors. Unambiguous assignment of a quenching mechanism between partners as FRET may only occur through the measurement of the rate of transfer from the donor to the acceptor, this rate manifests itself as a negative amplitude component in the overall lifetime decay of the accepting partner. Two experimental regimes were executed in an attempt to identify the presence of FRET between fluorescent donor semi-conductor quantum dots surrounded by streptavidin with peak emission at 525nm (525-CdSe-ZnS QD-streptavidin) and biocytin dye acceptors chosen to have varying degrees of absorption overlap with the 525-CdSe-ZnS QD-streptavidin emission. Significant quenching of the donor, along with the acceptor, was seen when the donor was bound to the acceptor, signifying some form of energy transfer. The degree of donor quenching, illustrated by the decrease in average lifetime, corresponded with the loading ratio of the acceptor?the more acceptors present, the larger the quenching. An unambiguous assignment of the rate of transfer of excited state energy from the donor to the acceptor, signified by a negative amplitude component in the lifetime decay of the acceptor, was not accomplished. Organic fluorescent dyes encapsulated in silica to create nanoscopic core-shell particles for use in biological applications have presented an interesting alternative to semiconductor fluorescent probes. In this experiment, an expanded core-shell morphology was surrounded by increasingly thicker layers of the siliceous shell to determine if this could control the photophysical properties of the nanoparticles. Sixteen nanoparticles were synthesized, all of distinct sizes and their photophysical properties investigated. No correlation between the thickness of the silica shell and the brightness of the particle was found. However, as the size of the particle increased the siliceous shell acted as a protective coating from potential quenchers for the TRITC dye in the center allowing molecules to stay in the triplet state for longer periods of time.
quantum dots; silica nanoparticles; FCS; FRET
dissertation or thesis