Observing single nanoparticle events at the orifice of a nanopipet† †Electronic supplementary information (ESI) available: Experimental details, zeta potential distribution, finite-element simulations, CLSM, and equation derivation. See DOI: 10.1039/c6sc02241c Click here for additional data file.

Single nanoparticle (NP) events are successfully observed at the orifice of a nanopipet by blocking the ionic current with a single NP.


Chemicals and Materials
Potassium Chloride (KCl, ≥ 99.5%) was bought from Sinopharm Chemical Reagent Beijing Co., Ltd.) Different radius polystyrene (PS) particles with surface carboxylic acid (-COOH) functional group were purchased from Alfa Aesar. All the chemicals were used as received without further purification. All aqueous solutions were prepared using Milli-Q water (>18 MΩ·cm) from a Milli-pore purification system.
The nanopipettes prepared by type A were used as pulled, with average radius 69 nm.

Nanoparticle Characterization
ζ-potential and size measurements were performed using a Malvern Zetasizer Software v7.02 (Malvern Instruments Ltd.).

Cell Configuration and Data Acquisition
Current-time responses were obtained with an electrochemical workstation (CHI-660e,

S4 Finite-Element Simulations
The finite-element simulations were carried out with COMSOL Multiphysics 4.2a (Comsol, Inc.) using a high-performance desktop computer. Herein, the finite-element method was used to figure out the Poisson-Nernst-Planck (PNP) partial differential equations and compute the electric field distribution and current transients of particle events.
The Nernst-Planck equation, equation S1, describes the fluxes of the ionic species, where, J i , D i , c i and z i are, respectively, the flux, diffusion coefficient, concentration and charge of species i. Φ and u are the local electric potential and fluid velocity, and F, R and T are the Faraday constant, the gas constant and the absolute temperature, respectively.
The relationship between the electric potential and ion concentration is described by the Poisson's equation, equation S2, Here, ε is the dielectric constant of the medium.
Considering the symmetry along the centerline Z direction, half of the cross-section was used in the simulation. The number of degree of freedom was 2,521,497 for a simulation of the Poisson and Nernst-Planck without electroneutrality equations. The computation domain was assumed as potassium chloride solutions with different concentrations. The following parameters were used: T = 298 K, D(K + ) = 1.957 × 10 -9 m 2 /s, D(Cl -) = 2.032 × 10 -9 m 2 /s, relative dielectric constant ε = 78, η = 1 × 10 -3 Pa•s, and ρ = 1000 Kg/m 3 . The surface charge density was defined at -1 mC/m 2 . C 0 = 0.1 M. Rigorous mesh-refinement tests were performed to make sure that the solutions obtained during simulation were convergent and grid independent.

S5 Synchronous Electrochemical and Optical Experiments
To further confirm the correlation between two current transients (Scheme 1B and C) and the positions of particles, confocal laser scanning microscope was conducted simultaneously when the bias potential was applied across the nanopipet. Confocal laser scanning microscopy (CLSM) images were performed on an Olympus FV1000-IX81 CLSM and a Leica TCS SP confocal system (Leica, Germany). The i-t traces were recorded by electrochemical workstation (CHI 660e). Fluorescent dye encapsulated polystyrene NPs purchased from Alfa Aesar was used as the model.  The polystyrene NPs concentration was 3.37 fM. The applied potential was 0.5 V. The data acquisition time was 10 ms.

S6 Derivation of Equation 1
Equation 1 in the main text was derived as follows.
The transference number of a NP (t ps ) represents the relative flux of charged NPs in the electrolyte solution.
The sum runs over all charged species in solution. In the present condition, this sum is completely dominated by the salt ions K + and Clsince the NPs concentration is fM, which allows to simplify equation S1 to obtain equation S4.