Sizing Individual Au Nanoparticles in Solution with Sub-Nanometer Resolution

Abstract

Resistive-pulse sensing has generated considerable interest as a technique for characterizing nanoparticle suspensions. The size, charge, and shape of individual particles can be estimated from features of the resistive pulse, but the technique suffers from an inherent variability due to the stochastic nature of particles translocating through a small orifice or channel. Here, we report a method, and associated automated instrumentation, that allows repeated pressure-driven translocation of individual particles back and forth across the orifice of a conical nanopore, greatly reducing uncertainty in particle size that results from streamline path distributions, particle diffusion, particle asphericity, and electronic noise. We demonstrate ∼0.3 nm resolution in measuring the size of nominally 30 and 60 nm radius Au nanoparticles of spherical geometry; Au nanoparticles in solution that differ by ∼1 nm in radius are readily distinguished. The repetitive translocation method also allows differentiating particles based on surface charge density, and provides insights into factors that determine the distribution of measured particle sizes.

Scanning Electrochemical Microscopy of Single Spherical Nanoparticles: Theory and Particle Size Evaluation

“…Thus, several methodologies were developed for electrochemical experiments at single NPs, including optical techniques, such as surface plasmon resonance imaging 8 and single molecule fluorescence imaging, 9 and electrochemical measurements at a metal NP either landing at or attached to a small electrode. 5a,10− 16 We showed recently 17 that electrochemical activity of single gold nanoparticles (AuNPs) attached to the catalytically inert carbon surface can be mapped by using small (≥3 nm radius) polished nanoelectrodes as tips in the scanning electrochemical microscope (SECM). Unlike the techniques based on immobilization of single NPs on nanoelectrodes, this approach is potentially useful for studying the effects of nanoparticle size, geometry, and surface attachment in a real-world application environment.…”

“…26 In our recent experiments, the apparent size of 10−20 nm Au NPs was much closer to their nominal diameter in SECM images than in AFM images. 17 However, an SECM image of an NP is relatively hard to obtain and analyze. A more straightforward and accurate approach developed in this Article is based on the fitting of an experimental current vs distance curve to the theory, using the NP radius (r p ) as an adjustable parameter.…”

“…This approach can be useful for investigating intermediates and mechanisms of electrocatalytic reactions occurring at the NP surface. 17 The steady-state diffusion problem for the SG/TC mode experiment with a finite-size spherically shaped conductive substrate is formulated in the Supporting Information. If the tip process is diffusion controlled and the species produced at the substrate is stable on the experimental time scale, the collection efficiency (i.e., i T /i S ) is determined by two dimensionless parameters: the normalized separation distance (d/r p ) and the ratio of the particle and tip radii (RP).…”

“…SECM experiments were carried out using a home-built previously described instrument. 17 The nanoelectrode used as an SECM tip was positioned a few tens of micrometers above the substrate surface. A long-distance video microscope was used to monitor the initial approach of the SECM tip to the substrate.…”

Nanoparticle displacement assay with electrochemical nanopore-based sensors

“…While the size resolution of nanopore-based resistive pulse sensors can reach subnanometer levels [13], their analytical use, i.e. selective detection of target species in complex samples, still requires the use of selective receptors, either added to the sample solution [10,14] or confined to the nanopore interior [15,16].…”