Nanophotonic Force Microscopy: Measuring Nanoparticle Interactions on the Thermal Energy Scale Using Near-field Optical Trapping and Light Scattering

Abstract

Nanoparticles are becoming ubiquitous in many applications including diagnostic assays, drug delivery and therapeutics, enhanced hydrocarbon recovery, and catalysis. However, there are challenges in the quality control of these products - it is necessary to ensure that nanoparticle suspensions contain particles of the appropriate size, within tolerable polydispersity, and that they maintain colloidal stability in face of potentially harsh environmental conditions. Through my doctoral research, I have developed Nanophotonic Force Microscopy, a technique for directly measuring nanoparticle interactions in the native suspension environment. This technique works by measuring the fluctuations in the intensity and position of scattered light as a nanoparticle of interest interacts with a nanophotonic optically trapping structure. In this dissertation, I demonstrate the use of Nanophotonic Force Microscopy to measure thermal energy scale interaction potentials and sub-pN scale interaction forces on dielectric and metallic nanoparticles with characteristic sizes of 50-800 nm. I then extend this technique to make simultaneous measurements of nanoparticle stability, diffusion coefficient, and sample polydispersity. This orthogonal measurement is accomplished by tracking the motion of nanoparticles in all three special dimensions as they interact with the evanescent field near an optical waveguide. This near-field interaction generates forces and results in motion in all three spatial dimensions. Along the propagation axis of the waveguide (x-direction) the nanoparticles are propelled by the optical forces which allow for a measurement of the sample polydispersity. Parallel to the plane of the waveguide and perpendicular to the optical propagation axis (y-direction) they experience an optical gradient force generated from the waveguide mode profile which confines them in a harmonic potential well, which can be used to provide a measurement of the diffusion coefficient. Normal to the surface of the waveguide (z-direction) they experience an exponential downward optical force balanced by the surface interactions that confines the particles in an asymmetric well, which is used to probe the suspension stability. The use of a waveguide integrated into a microfluidic channel allows for high throughput implementation of this technique, and the simultaneous measurement addresses several of the gaps left by current measurement technologies.