Noble-metal nanoparticles labeled with fluorescent molecules are used in a variety of applications requiring the measurement of size and diffusion properties of single nanoprobes. We have successfully used intrinsic surface-plasmon-induced photoluminescence (SPPL) signatures of monodispersed bare gold and silver nanoparticles in water to detect and measure their precise diffusion coefficient, concentration and hydrodynamic radius by fluorescence correlation spectroscopy (FCS). Measurement of the effective hydrodynamic radius confirms particle size to be 80 ± 8 and 64 ± 14 nm for gold and silver, respectively, which is in excellent agreement with scanning electron microscopic measurements made on the same particles. Detection of bare gold and silver nanoparticles at the single-molecule level with moderately high value of “per particle brightness” (PPB) confirms those particles to be used as fluorescent probes in biological research and in different medical and biotechnology applications where fluorescence detection plays a vital role. Additionally, these results demonstrate an alternative method for measuring hydrodynamic properties, particularly the size-distribution of bare noble-metal nanoparticles in solution using data-fitting algorithm for FCS based on the maximum entropy method (MEMFCS).

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