Micro-to-Nano Bimodal Single-Particle Sensing Using Optical Tweezers

Recently, electrical sensing techniques for single objects, such as nanoparticles, biomolecules, and viruses, have attracted a great deal of attention. To achieve both high throughput and high measurement accuracy, target objects need to be quickly transported to a small sensing section embedded in a fluidic channel. In the present study, we propose a novel method to improve the signal-to-noise (S/N) ratio of electrical signals of single particles, using optical tweezers and a microchannel. Optically trapping a 2 μm microparticle in a micro-orifice that has a comparable dimension of 3.0 μm (W), 2.5 μm (H), and 3.0 μm (L), the electrical signal from a small target particle that passes by the microparticle is sharpened and separated from the background noise. By irradiation with near-infrared light, the micro-orifice can be switched between opening and closing by optical tweezers, which works effectively to bring target particles to the sensing section using liquid flows and electrophoretic transport. As a result, the S/N ratio of electrical sensing of the smaller particle is improved by a factor of 5. The present microfluidic chip enables us to electrically detect particles of several hundreds of nanometers. Based on the present method, identification of single nanoparticles will also be feasible by using machine learning in the near future.