Optically biased diffusion of single molecules studied by confocal fluorescence microscopy

We have used confocal fluorescence microscopy to study the diffusion of single molecules in solution, over a wide dynamic mass range. The results have been analyzed using both Poisson and autocorrelation methods and have been shown to be consistent with diffusion theory over extended time scales. We observe deviations from statistical behavior on short time scales due to a weak optically induced trapping effect. The power dependency of this effect has been measured and shown to be in good agreement with optical trapping theory. We find that for small molecules the optical biasing effect is only significant for molecules excitated at resonance.