Anti-Brownian traps enable the measurement of single particles in free solution for long times by actively applyingfeedback forces based on an observed particle position to counteract Brownian motion. However, current implementationsof anti-Brownian traps generally rely on fluorescence emission to detect a particle’s position. This reliance on fluorescencecauses particles to be lost from the trap when they enter a fluorescence dark state by blinking or bleaching. Thus, there isa need for non-fluorescent methods of tracking for such traps. Scattered light provides a stable signal free of blinking andbleaching, but is very weak for small particles. However, interferometric scattering, a method of collecting the weakscattered field from a particle and interfering it with a strong reference field reflected from a nearby interface, allowsparticles to be tracked with sufficient speed and sensitivity. We combine interferometric scattering with our existing anti-Brownian 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 offluorescence blinking or bleaching. We verify the scaling of the interferometric scattering signal with the diameter of theparticle for gold nanoparticles as small as 20 nm. We also demonstrate the measurement of the fluorescence brightnesssignal of fluorescent beads as they photobleach, while continuing to trap them with the scattering signal. The ISABEL trapextends the ability of anti-Brownian traps to new samples and new measurements across multiple scientific communities.
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