Abstract
Fluorescence correlation spectroscopy (FCS) is one of the most sensitive methods for enumerating low concentration nanoparticles in a suspension. However, biological nanoparticles such as viruses often exist at a concentration much lower than the FCS detection limit. While optically generated trapping potentials are shown to effectively enhance the concentration of nanoparticles, feasibility of FCS for enumerating field-enriched nanoparticles requires understanding of the nanoparticle behavior in the external field. This paper reports an experimental study that combines optical trapping and FCS to examine existing theoretical predictions of particle concentration. Colloidal suspensions of polystyrene (PS) nanospheres and HIV-1 virus-like particles are used as model systems. Optical trapping energies and statistical analysis are used to discuss the applicability of FCS for enumerating nanoparticles in a potential well produced by a force field.
Highlights
Sensitive detection of pathogenic nanoparticles, such as viruses, is key to prevention of infectious diseases
Optical trapping energies and statistical analysis are used to discuss the applicability of Fluorescence correlation spectroscopy (FCS) for enumerating nanoparticles in a potential well produced by a force field
Lacking an analytical expression for the FCS autocorrelation function, we need to examine experimentally the applicability of existing models for using FCS to measure the concentration of nanoparticles by enriching them in a potential well produced by a force field
Summary
Sensitive detection of pathogenic nanoparticles, such as viruses, is key to prevention of infectious diseases. Hosokawa et al [9] used optically biased diffusion to study the cluster formation of nanoparticles in a highly focused laser beam They demonstrated that it might be possible to quantify the relationships between the cluster size, trapping energy and the decay time of the FCS autocorrelation function (ACF). Ito et al [11] used a Brownian dynamics simulation to investigate the FCS ACF of colloidal nanoparticles in an optical gradient force field. They found the standard FCS ACF for free diffusing particles is applicable for particles in an optical trap with potential energies lower than 1 kBT. We further explain that the equality of the ACF amplitude to 1/ N under optical trapping requires that the particle number density fluctuation follows the Poisson statistics
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