660d potentiostat

Acetonitrile was purchased from Sigma-Aldrich and purified on an MBraun solvent purification system prior to use. Tetrabutylammonium tetrafluoroborate was purchased from Alfa Aesar, recrystallized from boiling 50% water/ethanol, and dried under high vacuum prior to use. UV-visible spectra were recorded using ~30 μM Acetonitrile solutions on a Varian Cary 5000 spectrophotometer at a resolution of 1 nm. Cyclic voltammograms were recorded using a CH Instruments 660D potentiostat at room temperature using a standard three-electrode configuration (working electrode: 2 mm diameter Pt disc; reference electrode: RE-5B Ag/AgCl electrode in saturated aqueous potassium chloride (BASi Inc.), referenced externally to ferrocene/ferrocenium (0.630 V vs NHE);^{75 (link)} counter electrode: Pt wire) in 0.1 M tetrabutylammonium tetrafluoroborate Acetonitrile solutions.

The electrochemical measurement was performed using a CHInstruments model 660D potentiostat (Austin, TX, USA) with the three-electrode cell system placed in a Faraday cage. A Pt wire and a Ag/AgCl (1 M KCl) electrode were used as the counter and the reference electrode, respectively. All potentials are reported vs. Ag/AgCl. TEM images were obtained from the National Center for Inter-University Facilities (NCIRF, Seoul, Korea) using JEM-F200 (JEOL, Tokyo, Japan). The solution of the electrochemical cell contained 50 mM of a phosphate buffer (PB, pH 6.8) with 15 mM of hydrazine. The 50 mM of electrolyte concentration was optimized by many experiments in the single-NP collision experiment (Figure S1, see the Supporting Information, SI). When the concentration of electrolyte is too low, the solution resistance increases and the buffer capacity decreases, thus the observation of the current signal is difficult. If the concentration of electrolyte is high, the aggregation of NPs is acerated.