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 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 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 particle,
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.