Electrophoretic Deposition Interferometric Scattering Mass Photometry

• Published in: ACS nano Vol. 18; no. 15; pp. 10388 - 10396
• Main Authors: Kowal, Matthew D., Seifried, Teresa M., Brouwer, Carraugh C., Tavakolizadeh, Hooman, Olsén, Erik, Grant, Edward
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 16.04.2024
• Subjects: quantitative imaging; electrophoretic deposition; single-nanoparticle detection; machine learning; interferometric scattering microscopy; mass photometry
• ISSN: 1936-0851; 1936-086X
• DOI: 10.1021/acsnano.3c09221

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 the 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 an effective 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 of controlling 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 an electrophoretic 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 photometry 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 the bulk concentration.