  Measuring the electrophoretic mobility of molecules is a powerful
experimental approach for investigating biomolecular processes. A frequent
challenge in the context of single-particle measurements is throughput,
limiting the obtainable statistics. Here, we present a molecular force sensor
and charge detector based on parallelised imaging and tracking of tethered
double-stranded DNA functionalised with charged nanoparticles interacting with
an externally applied electric field. Tracking the position of the tethered
particle with simultaneous nanometre precision and microsecond temporal
resolution allows us to detect and quantify electrophoretic forces down to the
sub-piconewton scale. Furthermore, we demonstrate that this approach is capable
of detecting changes to the particle charge state, as induced by the addition
of charged biomolecules or changes to pH. Our approach provides an alternative
route to studying structural and charge dynamics at the single-molecule level.
