Abstract
We describe the application of three-dimensional (3D) scattering interferometric (iSCAT) imaging to the measurement of spatial interaction potentials for nano-objects in solution. We study electrostatically trapped gold particles in a nanofluidic device and present details on axial particle localization in the presence of a strongly reflecting interface. Our results demonstrate high-speed (~kHz) particle tracking with subnanometer localization precision in the axial and average 2.5 nm in the lateral dimension. A comparison of the measured levitation heights of trapped particles with the calculated values for traps of various geometries reveals good agreement. Our work demonstrates that iSCAT imaging delivers label-free, high-speed and accurate 3D tracking of nano-objects conducive to probing weak and long-range interaction potentials in solution.
Highlights
Techniques to measure forces between discrete entities in fluids, ambient air, or vacuum have been vital in advancing our understanding of condensed matter
We describe the application of three-dimensional (3D) scattering interferometric imaging to the measurement of spatial interaction potentials for nano-objects in solution
Our work demonstrates that iSCAT imaging delivers label-free, high-speed and accurate 3D tracking of nano-objects conducive to probing weak and long-range interaction potentials in solution
Summary
Techniques to measure forces between discrete entities in fluids, ambient air, or vacuum have been vital in advancing our understanding of condensed matter. While the methods listed above rely on the mechanical motion (e.g. bending or displacement) of a sensing element in response to the action of an intermolecular force, optical microscopy has emerged as a sensitive, non-invasive, and often calibration-free tool, that offers a thermodynamic method to directly measure interaction energies [4]. The technique has been successfully used to study a variety of interactions in soft matter: electrostatic interactions of charged colloids, polymers and lipid membranes [5,6,7] as well as van der Waals [8], gravitational [9], depletion [10] and critical-Casimir [11] forces to name a few These measurements are founded in the Boltzmann principle for a system in thermodynamic equilibrium, where the potential ascribed to a state depends exclusively on the probability of occupancy of that state.
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