Dielectrophoretic nanotweezers for single-particle force spectroscopy

Studying the structural and functional properties of biological molecules using single-molecule techniques has been fundamental in establishing a comprehensive understanding of the mechanisms that govern complex living systems. The aim of this thesis was to develop a dielectrophoretic nanotweezers setup to provide a new approach for applying and measuring dielectrophoretic forces on individual particles in aqueous solution. To achieve this goal, dual-barrel quartz nanopipettes were filled with pyrolytic carbon, forming a pair of nanoelectrodes at their tip. Their size and shape were characterised both through SEM imaging and electrochemical cyclic voltammetry. By applying low AC voltages (< 1 V) between these nanoelectrodes, very strong and highly inhomogeneous electric fields were generated at the nanopipette’s tip to form dielectrophoretic nanotweezers. The frequency of the electric field was tuned so that individual entities were either attracted or repelled depending on their dielectric properties. In this work, experimental measurements of the dielectrophoretic force acting on single polystyrene beads (2 μm) were acquired by single particle tracking on an inverted fluorescence microscope. The spatial coordinates of individual beads were extracted from their trajectories as a function of time under trapping conditions. The force magnitude, measured from their velocity over distance from the nanotweezers tip, was found in the femtonewtons range for a set of applied voltages and frequencies. In addition, the electric field distribution was simulated close to the nanotweezers tip by a finite element model developed for this system. Estimations of the dielectrophoretic force magnitude for different nanotweezers geometries were also performed. Overall, the simple operational mechanism and design of these dielectrophoretic nanotweezers combined with their ability to be controlled in three-dimensions, make them a versatile and promising platform for single-particle manipulation and force probing.