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Fluorescence Correlation Spectroscopy of Nanoparticles
The possibility of using nanoparticles as superior labels and sensors that do not photobleach in environmental
and biological studies has sparked wide-spread interest. In order to apply nanoparticles in these areas, it is
necessary to understand the diffusion of nanoparticles in a liquid environment. We are applying and developing
fluorescence correlation spectroscopy (FCS) methods to extract the diffusion constants and size distributions of
nanoparticles in solution. FCS measures spontaneous intensity fluctuations caused by small deviations from
equilibrium when molecules enter and leave a detection area. The largest fluctuations are observed when only a
few molecules are present in a small detection volume with the ultimate limit being a single molecule at a given
time. Sufficient signal to noise for single molecule FCS can be achieved through minimizing the detection volume
by focusing a laser beam to a diffraction limited spot combined with high quantum yield photodetectors.
The figure above shows autocorrelation curves for 40 (red) and 100 nm (blue) dye beads in water (left graph). Fits (lines) to the data (symbols) give the time it takes the beads to diffuse through the confocal excitation volume. With the known dimensions of the excitation volume the diffusion constant is calculated, which is directly related to the size of the beads by the Stokes – Einstein equation. A fluorescence transient of 100 nm beads (inset, left graph) shows individual fluorescence bursts, which confirm that less than one molecule is present in the excitation volume. Additional information can be obtained by calculating an intensity histogram of single bursts from the fluorescent transients (right graph). We are applying FCS to measure the diffusion of magnetic nanoparticles in an applied magnetic field. This work is carried out in collaboration with the Colvin lab, which found that magnetite nanoparticles can selectively adsorb arsenic from waste water. The nanoparticles together with the contaminants are then separated with a magnetic field in a magnetic separator. Our goal is to determine the mechanism by which the nanoparticles move in a low magnetic field gradient using FCS. We are also interested in developing FCS methods that allow us to study heterogeneous systems, which contain mixtures of different nanoparticles with varying sizes. This part of our FCS studies is done in close interaction with the Landes lab at the University of Houston. Group members involved: Publications:
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