Angular Trapping of Spherical Janus Particles

Xiaoqing Gao Yali Wang Xuehao He Mengjun Xu Jintao Zhu Xiaodong Hu Xiaotang Hu Hongbin Li Chunguang Hu

Developing angular trapping methods, which enable optical tweezers to rotate a micronsized bead, is of great importance for studies of biomacromolecules in a wide range of torque‐generation processes. Here a novel controlled angular trapping method based on model composite Janus particles is reported, which consist of two hemispheres made of polystyrene and poly(methyl methacrylate). Through computational and experimental studies, the feasibility to control the rotation of a Janus particle in a linearly polarized laser trap is demonstrated. The results show that the Janus particle aligned its two hemispheres interface parallel to the laser propagation direction and polarization direction. The rotational state of the particle can be directly visualized by using a camera. The rotation of the Janus particle in the laser trap can be fully controlled in real time by controlling the laser polarization direction. The newly developed angular trapping technique has the great advantage of easy implementation and real‐time controllability. Considering the easy chemical preparation of Janus particles and implementation of the angular trapping, this novel method has the potential of becoming a general angular trapping method. It is anticipated that this new method will significantly broaden the availability of angular trapping in the biophysics community.

DOI

Nano‐Optical Tweezing of Single Proteins in Plasmonic Nanopores

Daniel Verschueren, Xin Shi, Cees Dekker

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.

DOI