Optical Trapping Effect and Its Calibration Method in Resonance Light Scattering Correlation Spectroscopy of Gold Nanoparticles in Solution

Bocheng Zhang, Tao Lan, Xiangyi Huang, Chaoqing Dong, and Jicun Ren

In this work, we reported an efficient method for eliminating the optical trapping effect on characterization of nanoparticle diffusion parameters by resonance light scattering correlation spectroscopy (RLSCS). The RLSCS represents a new single nanoparticle method and its principle was based on measuring the resonance light scattering fluctuations in a highly focused laser beam due to the Brownian motion of single nanoparticles such as gold nanoparticles (GNPs), which resembled fluorescence correlation spectroscopy (FCS). In RLSCS analysis, the polarizability of nanoparticles are much higher than fluorescent molecules in FCS, and the sizes of them are larger, therefore, the optical trapping force significantly affects the diffusion behaviors of nanoparticles under a highly focused laser beam. In this study, we used the 632.8 nm He—Ne laser as the light source, which was close to the resonance scattering band of GNPs, and chose GNPs (from 20 to 100 nm) as model samples. We theoretically and experimentally investigated the optical trapping effect of GNPs in RLSCS, and observed a good linear relation between the characteristic diffusion times of GNPs and laser intensity in the certain condition (below 100 μW). This result was in line with the theoretical deduction. By the extrapolation strategy, we effectively eliminated the optical trapping effect and accurately obtained the diameter of GNPs, which was in good agreement with that obtained by transmission electron microscopy. The method described here can extend to FCS analysis of fluorescent nanoparticles as well.

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