Nano-Optical Tweezing of Single Proteins in Plasmonic Nanopores

Abstract

Single-molecule sensing technologies aim to detect and characterize single biomolecules, but generally need labeling while the measurement times and throughput are severely restricted by a lack of positional control over the molecule. Here, a plasmonic nanopore biosensor is reported where single molecules can be electrophoretically delivered into a nanopore sensor with a plasmonic nanoantenna that is used to optically trap single molecules for extended measurement times. Using the light transmission through the antenna as read-out, optical trapping of 20 nm diameter polystyrene nanoparticles and individual beta-amylase proteins, a 200 kDa enzyme, in the plasmonic nanoantenna are demonstrated. Application of an electrical bias voltage allows the increase of the event rate over an order of magnitude as well as shorten the residence time of the proteins in the plasmonic nanopore as they can controllably be drawn out of the trap by electrical forces. Trapping is found to be assisted by protein–surface interactions and trapped proteins can denature on the nanopore surface. The integration of two single-molecule sensors, a plasmonic nanoantenna and solid-state nanopore, creates independent control handles at the single-molecule level—the optical trapping force and electrophoretic force—which provides augmented control over single molecules.