Quantification of high-efficiency trapping of nanoparticles in a double nanohole optical tweezer.

We measure the dynamics of 20 nm polystyrene particles in a double nanohole trap to determine the trap stiffness for various laser powers. Both the autocorrelation analysis of Brownian fluctuations and the trapping transient analysis provide a consistent value of ∼ 0.2 fN/nm stiffness for 2 mW of laser power, which is similar to theoretical calculations for aperture trapping. As expected, the stiffness increases linearly with laser power. This is comparable to the stiffness obtained for conventional optical traps for trapping, but for ten times smaller dielectric particles and less power. This approach will allow us to quantitatively evaluate future aperture-based optical traps, with the goal of studying the folding dynamics of smaller proteins (∼ 10 kDa) and small-molecule interactions.