Axial electrokinetic trapping of label-free nanoparticles using evanescent field scattering

Anti-Brownian electrokinetic trapping enables the confinement of individual nanoparticles in liquids by applying electric fields. This technique facilitates the long-term observation of nanoscopic objects, allowing for detailed studies of their physical, chemical, and biomolecular properties. However, this method has been largely restricted to nanoparticles that can be visualized by photoluminescence. While some techniques avoid fluorescent labeling by using dark-field or interferometric scattering microscopy, they are limited to two-dimensional particle trapping and lack control over the axial direction. Here, we demonstrate the axial electrokinetic trapping of fluorescence-free nanoparticles that scatter the evanescent field induced by total internal reflection. The distance between the particle and the glass surface is directly related to the intensity of scattered light, and controlled by an applied electric field. Consequently, nanoparticles can be trapped and monitored in response to applied voltages at kilohertz rates without the need for fluorescent labeling. In addition, we utilize this approach to investigate how surface proximity impacts the diffusion and mobility of the trapped nanoparticles. Our method paves a new way to study a broad range of nano-objects that can be trapped at the single-particle level, relying solely on their light-scattering properties, which offers significant potential for advancing research in surface chemistry, single-molecule biophysics, and cell membrane biology.