Sensitive single particle method for characterizing rapid rotational and translational diffusion and aspect ratio of anisotropic nanoparticles and its application in immunoassays.

In this article, we reported a new and sensitive method for characterizing rapid rotational and translational diffusion of gold nanoparticles (GNPs) and gold nanorods (GNRs) by resonance light scattering correlation spectroscopy (RLSCS). The RLSCS is a new single nanoparticle method, and its principle is based on measuring the resonance light scattering fluctuations in a highly focused volume due to Brownian motion of single particles, which resembles fluorescence correlation spectroscopy (FCS). On the basis of the theory of FCS, we first developed a model for rotational and translational diffusion and aspect ratio of nanoparticles in the RLSCS system. Then, we investigated the effects of certain factors such as the wavelength of illumination light and viscosity of solution using GNPs and GNRs as model samples and discovered that the polarization anisotropy and the scattering light intensity of GNPs and GNRs were significantly dependent on the wavelengths of illumination light. Using the 632.8 nm He-Ne laser as a light source, which was close to the resonance scattering band, we successfully obtained the translational and rotational diffusion coefficients and aspect ratios of anisotropic nanoparticles by the RLSCS method. The results obtained by this new method were in good agreement with transmission electron microscopy and theoretical calculation. Furthermore, the homogeneous sandwich immunoreaction was investigated using the antibody-modified GNPs as the probes. The changes in translational diffusion behaviors and aspect ratios of GNPs in immunoreaction were observed by the RLSCS method. By these changes, we can develop a new homogeneous immunoassay. Our preliminary results illustrated that the RLSCS method was a powerful tool for characterizing rapid rotational and translational diffusion behaviors of anisotropic nanoparticles in solution. We believe that the RLSCS method exhibits the wide applications in biological science especially in vivo study on the interaction of nanoparticles and biomolecules.