Microfluidic flow control for inductive detection of nanoparticles

In recent years, micro- and nanofluidic channels for single nanoparticle analysis have attracted much attention. However, it is difficult to control the transport velocity of target particles introduced into the sensing part because of thermal fluctuations in liquids. In this study, we have developed a novel technique for controlling microfluidic flows to precisely induct single nanoparticles into micro- and nanofluidic channels. Herein, we fabricate a nanofluidic channel with a width of about 500 nm on a quartz-glass surface that crosses a pair of parallel microfluidic channels printed on a PDMS surface. Nanoparticles dispersed in an electrolyte solution on one side of the microchannel are inducted along the microchannel and into the nanochannel by flow control of the other microchannel. The nanoparticles gradually migrate toward a nanochannel opening, driven by the drag force and electrostatic force and experiencing thermal fluctuations. Transport of these nanoparticles is recorded using a high-speed CMOS camera, and the trajectories are analyzed by sing particle tracking technique. As a result, the target nanoparticles are effectively attracted to the nanochannel opening with appropriate transport velocity overcoming thermal fluctuations. Furthermore, the nanoparticle detection frequency is improved by the voltage and pressure differences between the microchannels. The present method is expected to contribute to the optical manipulation technology of single nanoparticles in liquids in micro- and nanofluidic channels.