EPD-iSCAT: Electrophoretic Mass Photometry

18 December 2023, Version 2
This content is a preprint and has not undergone peer review at the time of posting.


Interferometric scattering microscopy (iSCAT) has rapidly developed as a quantitative tool for the label-free detection of single macromolecules and nanoparticles. In practice, this measurement records the interferometric scattering signal of individual nanoparticles in solution as they land and stick on a coverslip, exhibiting an intensity that varies linearly with particle volume, and an adsorption rate that reflects the solution-phase transport kinetics of the system. Together, such measurements provide a multidimensional gauge of particle size and concentration in solution over time. However, the landing kinetics of particles in solution also manifest a measurement frequency limitation imposed by the slow long-range mobility of particle diffusion to the measurement interface. Here we introduce a new technique that offers a novel means to overcome the inherent diffusion-controlled sampling limitation of spontaneous mass photometry. We term this methodology, electrophoretic deposition interferometric scattering microscopy (EPD-iSCAT). This approach uses a coverslip supporting a conductive thin film of indium tin oxide (ITO). Charging this ITO film to a potential of around 1 V electrophoretically draws charged nanoparticles from solution and binds them in the focal plane of the microscope. Regulating this potential offers a direct means to control particle deposition. Thus, we find for a 0.1 nM solution of 50 nm polystyrene nanoparticles that the application of +1 V to an EPD-iSCAT coverslip assembly drives a electrophoetic deposition rate constant of 1.7 s−1 μm−2 nM−1. Removal of the potential causes deposition to cease. This user control of EPD-iSCAT affords a means to apply single-molecule mass photometery to monitor long-term changes in solution owing to slow kinetic processes. In contrast with conventional coverslips chemically derivatized with charged thin films, EPD-iSCAT maintains a deposition rate that varies linearly with bulk concentration.


Mass Photometry
Indium Tin Oxide

Supplementary materials

Supporting Information
Additional details, specifications and discussion regarding device construction, image classification, metrics and limitations.

Supplementary weblinks


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