Three-Dimensional Super-resolution Imaging of Single Nanoparticles Delivered by Pipettes.

Controlled three-dimensional positioning of nanoparticles is achieved by delivering single fluorescent nanoparticles from a nanopipette and capturing them at well-defined regions of an electrified substrate. To control the position of single nanoparticles, the force of the pressure-driven flow from the pipette is balanced by the attractive electrostatic force at the substrate, providing a strategy by which nanoparticle trajectories can be manipulated in real time. To visualize nanoparticle motion, a resistive-pulse electrochemical setup is coupled with an optical microscope, and nanoparticle trajectories are tracked in three dimensions using super-resolution fluorescence imaging to obtain positional information with precision in the tens of nanometers. As the particles approach the substrate, the diffusion kinetics are analyzed and reveal either subdiffusive (hindered) or superdiffusive (directed) motion depending on the electric field at the substrate and the pressure-driven flow from the pipette. By balancing the effects of the forces exerted on the particle by the pressure and electric fields, controlled, real-time manipulation of single nanoparticle trajectories is achieved. The developed approach has implications for a variety of applications such as surface patterning and drug delivery using colloidal nanoparticles.