Expanding the optical trapping range of gold nanoparticles.

We demonstrate stable three-dimensional (3D) single-beam optical trapping of gold nanoparticles with diameters between 18 and 254 nm. Three-dimensional power spectral analysis reveals that, for nanoparticles with diameters less than 100 nm, the trap stiffness is proportional to the volume of the particle. For larger particles, the trap stiffness still increases with size, however, less steeply.

Novel optical and statistical methods reveal colloid-wall interactions inconsistent with DLVO and Lifshitz theories.

We present an experimental method based on video microscopy to perform nanometer scale position detection of a micrometer bead in the direction along the propagation of the detection light. Using the same bead for calibration and detection significantly improves the in depth resolution in comparison to video microscopy methods from literature. This method is used together with an optical trap to measure interaction potentials between a glass surface and colloids made of polystyrene or silica at different electrolyte concentrations.