Trapping a blinking nanoparticle: decoupling detection and position estimation with the ISABEL trap

Abstract

Precision spectroscopic measurements of particles in free solution are greatly aided by use of an Anti-Brownian ELectrokinetic (ABEL) trap, which counteracts the effects of diffusion by means of closed-loop feedback to hold a single particle in a diffraction-limited spot for extended-time measurements. Generally, fluorescent emission from the trapped object is used to produce position estimates for the feedback circuit. However, many objects of interest may fluoresce only dimly (such as native fluorescence from a pigment-protein complex) or intermittently (such as a quantum dot). Here we report the development and demonstration of a new trapping modality that incorporates interferometric scattering to produce particle position estimates, called the Interferometric Scattering Anti-Brownian ELectrokinetic (ISABEL) trap. Using the ISABEL trap, we are able to completely decouple trapping from fluorescence detection, permitting trapping of dim and completely dark nanoparticles.