Fluctuation correlation spectroscopy and photon histogram analysis of light scattered bygold nanospheres

Abstract

Fluorescence correlation spectroscopy (FCS) is a valuable tool in biological research. In recentyears there has been growing interest in using light scattered from metallic colloids in placeof organic fluorophores. Metallic colloids display optical cross sections for scattering thatare orders of magnitude brighter than fluorophores. We used the FCS method to study thescattering properties of varying sizes of gold colloids 38–100 nm in diameter. The opticalcross sections of the gold colloids increase rapidly with size, as can be seen by both theG(0) value of the autocorrelation function and the scattering intensity distributions. In mixturesof different size gold colloids the autocorrelation function is dominated by thelarger (brighter) colloids, even when present at a small fractional population.We show that it is possible to detect one 100 nm gold colloid in the presence of103–104 smaller 39 nm diameter colloids. Because the scattering cross sections of colloids willincrease with aggregation, we believe that FCS can be used to detect a small number ofassociated bio-labeled colloids in the presence of a much larger population ofnon-associated colloids.