Interferometric scattering for fluorescence-free electrokinetic trapping of single nanoparticles in free solution

Anti-Brownian traps enable the measurement of single particles in free solution for long times by actively applying feedback forces based on an observed particle position to counteract Brownian motion. However, current implementations of anti-Brownian traps generally rely on fluorescence emission to detect a particle's position. This reliance on fluorescence causes particles to be lost from the trap when they enter a fluorescence dark state by blinking or bleaching. Thus, there is a need for non-fluorescent methods of tracking for such traps. Scattered light provides a stable signal free of blinking and bleaching, but is very weak for small particles. However, interferometric scattering, a method of collecting the weak scattered field from a particle and interfering it with a strong reference field reflected from a nearby interface, allows particles to be tracked with sufficient speed and sensitivity. We combine interferometric scattering with our existing antiBrownian electrokinetic (ABEL) trap to create the interferometric scattering anti-Brownian electrokinetic (ISABEL) trap. This technique enables the trapping of single nanoparticles in free solution for extended durations regardless of fluorescence blinking or bleaching. We verify the scaling of the interferometric scattering signal with the diameter of the particle for gold nanoparticles as small as 20 nm. We also demonstrate the measurement of the fluorescence brightness signal of fluorescent beads as they photobleach, while continuing to trap them with the scattering signal. The ISABEL trap extends the ability of anti-Brownian traps to new samples and new measurements across multiple scientific communities.