Orthogonal Nanoparticle Size, Polydispersity, and Stability Characterization with Near-Field Optical Trapping and Light Scattering

Here we present and demonstrate a new technique for simultaneously characterizing the size, polydispersity, and colloidal stability of nanoparticle suspensions. This method relies on tracking each nanoparticle's motion in three spatial dimensions as it interacts with the evanescent field of an optical waveguide. The Waveguide-driven nanoparticle transport Track 3D motion from scattered light motion along the optical propagation axis of the waveguide provides insight into the polydispersity of a nanoparticle suspension. Horizontal motion perpendicular to the propagation axis gives the diffusion coefficient and particle size. In the direction normal to the polydispersity size surface, statistical analysis of the scattered light intensity distribution gives a map of the interaction energy landscape and insight into the suspension stability. These three orthogonal measurements are made simultaneously on each partide, building up population level insights from a single-particle rather than ensemble-averaged basis. We experimentally demonstrate the technique using polystyrene spheres obtaining results consistent with the manufacturer's specifications for these suspensions. For NIST-traceable polystyrene size standard spheres, we measure a variability in the hydrodynamic radius of +/- 5 nm, compared with the manufacturer's certified measurement of 9 nm in the geometric diameter made using transmission electron microscopy.