Physical Society Colloquium

Interview for Faculty Position

Laser Tweezers in Nanowire Manipulation and Nanopore Based Spectroscopy

Aleksandra Radenovic

University of California, Berkeley

In my talk I will discuss the use of optical tweezers in nanowire manipulation and as a part of novel nanopore based force spectroscopy. The use of nanowires in scientific, biomedical, and microelectronic applications is greatly restricted due to a lack of methods to assemble nanowires into complex heterostructures with high spatial and angular precision. Here we show that an infrared single-beam optical trap can be used to individually trap, transfer, and assemble high-aspect-ratio semiconductor nanowires into arbitrary structures in a fluid environment. Nanowires with diameters as small as 20 nm and aspect ratios of above 100 can be trapped and transported in three dimensions, enabling the construction of nanowire architectures which may function as active photonic devices. Moreover, nanowire structures can now be assembled in physiological environments, offering novel forms of chemical, mechanical, and optical stimulation of living cells.

The ability to control the capture rate and translocation behavior of biopolymers is an important prerequisite for numerous nanopore experiments, including studies of RNA structure, RNA folding kinetics, protein-DNA interactions, and DNA sequencing. We study the electrophoretic threading of DNA through synthetic nanometer-diameter pores. We use double stranded lambda DNA attached at one end to an optically trapped microsphere. Once the DNA has successfully threaded the pore, it acts as a mechanical transducer, exerting a force on the optically trapped bead. For very high applied voltages, the bead is pulled out of the optical trap as the DNA translocates the nanopore. In the regime of high electric force and low optical force, the bead acts as a large frictional volume to slow the progression of the DNA polymer.

In the opposite limit, the bead is held in the optical trap and the electric field exerts a small tension along the length of the DNA polymer. Varying the applied potential (both magnitude and sign) as well as the position of the optical trap controls electrical and mechanical forces exerted on the threaded polymer. These control parameters dictate the position of the DNA molecule with respect to the pore as well as translocation and retraction rates of DNA through the pore.