Electrically controlled nanoparticle synthesis inside nanopores

Abstract

From their realization just over a decade ago, nanopores in silicon nitride membranes have allowed numerous transport-based single-molecule measurements. Here we report the use of these nanopores as subzeptoliter mixing volumes for the controlled synthesis of metal nanoparticles. Particle synthesis is controlled and monitored through an electric field applied across the nanopore membrane, which is positioned so as to separate electrolyte solutions of a metal precursor and a reducing agent. When the electric field drives reactive ions to the nanopore, a characteristic drop in the ion current is observed, indicating the formation of a nanoparticle inside the nanopore. While traditional chemical synthesis relies on temperature and timing to monitor particle growth, here we observe it in real time by monitoring electrical current. We describe the dynamics of gold particle formation in sub-10 nm diameter silicon nitride pores and the effects of salt concentration and additives on the particle's shape and size. The current versus time signal during particle formation in the nanopore is in excellent agreement with the Richards growth curve, indicating an access-limited growth mechanism.

Figure 1

Electric-field-driven nanoparticle synthesis in a nanopore. (a) TEM image of a 6.5 nm diameter nanopore in a SiN_x membrane. The scale bar is 5 nm. (b) Zoomed out TEM image of a SiN_x membrane. Membrane is prepatterned and thinned to form 200 nm × 200 nm square regions (light gray) used as markers. The scale bar is 100 nm. (c) Schematic of the membrane and support structure (not to scale). (d–f) Schematics of the particle growth process: (d) a SiN_x membrane with a single nanometer-size pore separates two chambers, A and B, of electrolyte. For Au synthesis, negatively charged gold(III) chloride is injected in chamber B, and positively charged hydrazine is injected in chamber A. At first, a potential applied across the chambers prevents the solutions from reacting. (e) The reaction is triggered by reversing the sign of the voltage difference, which drives the reagents into the pore where they react. (f) As the reagents react, a gold nanoparticle forms in the pore and in the process stops further reaction by preventing the reagents from mixing.

Figure 2

Current–voltage traces and current vs time traces during Au nanoparticle formation. (a) Current–voltage (I–V) traces for a nanopore before (red) and after (black) particle formation. For the empty pore trace, the ion current was measured in a solution of 5 mM KCl, without any hydrazine, gold chloride, or α-lipoic acid. (b) Current vs time trace and corresponding voltage vs time trace for a nanoparticle formation experiment on a 4.2 nm diameter pore in 5 mM KCl solution. (1) Hydrazine is injected to chamber A. The ion current shifts due to the chemical gradient that has formed. (2) Gold chloride is added to the chamber B, and the current again shifts. (3) Voltage polarity is reversed to electrically drive the reagents into the pore. The vertical dashed lines after (3) represent the time delay, t_d, before particle formation. The current then drops to zero when the particle forms. (c) Zoom-in of trace from voltage change to particle formation. The current spike is due to a capacitive response in our system. (d) Zoom-in from the dashed square in (c) highlighting the particle formation event. In over 80 experiments, these events display this characteristic sigmoid shape.

Figure 3

Richard’s model fit of gold nanoparticle formation event inside a nanopore. (a) Fit to eq 3 of a particle formation event. The experiment was performed on a 4.2 nm diameter pore in 5 mM KCl. Voltage applied during formation was 300 mV. (b) Derivative of the fit, dI/dt, is shown in panel a. The full width at half max gives a quantitative measure of the duration of particle growth.

Figure 4

Histogram of the measured time delays, t_d; t_d is the time elapsed between the time when voltage polarity is reversed and the time when ion current goes to zero and the particle fills the pore completely. Data are shown for three different salt (KCl) concentrations: 5 mM (gray, top), 50 mM (blue, middle), and 1 M (red, bottom). The concentrations of reactants were held constant. From Poisson fits: for 1 M, t_d = 15 ± 1 s; for 50 mM t_d = 2.3 ± 0.5 s; and for 5 mM t_d = 0.8 ± 0.2 s.

Figure 5

(a) Current vs time trace and corresponding voltage vs time trace for a nanoparticle formation experiment in the presence of α-lipoic acid. This experiment was performed on a 4.2 nm diameter pore in 50 mM KCl solution. (1), (2), and (3) represent the same experimental steps as in Figure 2b. (b) Zoom-in of (3) demonstrating that particle formation occurs faster than the limits of detection.

Figure 6

Transmission electron micrographs of particles synthesized in nanopores. Particles were formed using reagents in (a) 1 M KCl, (b) 50 mM KCl, and (c) 50 mM KCl with α-lipoic acid. Insets are zoomed in images of the particles. The scale bar is 10 nm in panel a, 20 nm in panels b and c, and 5 nm in the insets. For low salt concentrations (≤50 mM), the chemical reaction is tightly confined to the nanopore, and gold is observed inside the pore only with no additional Au present in the vicinity, as seen in the larger views of the particle and surrounding SiN_x surface of panels b and c. The white dashed lines mark the boundary of the etched regions in SiN_x.

Figure 7

Determination of lattice spacing. (a) TEM image of a gold nanoparticle. The area inside the white box is used to create the (b) profile. The particle is crystalline and shows lattice planes within the crystal, whereas the SiN_x membrane surrounding the particle does not show lattice planes. The intensity profile data averaged over the depth of the white box in (c) is Fourier transformed to create (d). The peak value in panel d is at 3.95 nm⁻¹. The scale bar is 5 nm in panels a and c.