Mapping nanoscale forces and potentials in live cells with microsecond 3D single-particle tracking

3D single-particle tracking has the potential to resolve the molecular level forces which dictate particle motion in biological systems. However, the information gleaned from 3D single-particle tracking often cannot resolve underlying nanoscale potentials due to limited spatiotemporal resolution. To this end, we introduce an active-feedback 3D tracking microscope that utilizes silver nanoparticles (AgNPs) as probes to study intricate biophysical events in live cells at the nanometer and microsecond scales. Due to this extremely high and durable scattering photon flux of the plasmonic particles, 1 MHz sampling frequency at nanometer precision in all three dimensions can be achieved over an unlimited observation times. In this work, we applied microsecond-sampling, active-feedback 3D single-particle tracking to investigate the interaction between AgNPs and nanoscale filopodium on the live-cell surface. The nanometer precision and microsecond sampling revealed that TAT peptide modified particles visit and dwell at local "hot spots" on the filopodium surface. The high sampling rate further enabled the calculation of the local forces and potentials within these nanoscale hotspots on the cylindrical surface of live cell filopodia. This study presents a promising tool to investigate intracellular biophysical events with unprecedented spatiotemporal resolution and a pipeline to study nanoscale potentials on three-dimensional cellular structures.