Probing protein surface interactions in nanoaperture optical tweezers

Nanoaperture optical tweezers (NOTs) can trap single proteins using local electromagnetic field enhancements at metal surfaces and therefore they are also subject to surface interactions. To probe these interactions, we consider the power dependence of the trapping stiffness at the limit of zero power, where an attractive or repulsive static force remains. We analyze various proteins with different charge and find that negatively charged proteins are attracted to the gold surface and positively charged proteins are repelled. We interpret this attraction as coming from local positive image charges on the gold adjacent to the negatively charged glass-water interface, as confirmed by finite-element Poisson equation simulations. This work shows a way to quantify the important impact of surface interactions in nanophotonic single molecule sensors of biomolecules and nanoparticles and to gauge their surface charge.