Chemically-Resolved Transient Collision Events of Single Electrocatalytic Nanoparticles. Zhihui Guo, Stephen J. Percival, and Bo Zhang*

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1 Supporting Information for Chemically-Resolved Transient Collision Events of Single Electrocatalytic Nanoparticles Zhihui Guo, Stephen J. Percival, and Bo Zhang* Department of Chemistry, University of Washington, Seattle, Washington United States Present Addresses School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi an , PR China S1

2 Chemicals and Materials: All chemicals and materials were used as received from the manufacturers. Deionized water (>18 MΩ cm, Barnstead Nanopure Systems), methanol (Sigma Aldrich), potassium chloride (KCl, Fisher Scientific), Ferrocene methanol (FcMeOH, Aldrich 97%), ferricyanide (K 3 Fe(CN) 6, Sigma-Aldrich 99%), ferrocyanide (K 4 Fe(CN) 6, Fluka 99.5%), hydrazine (anhydrous 98%, Sigma Aldrich), monobasic potassium phosphate (KH 2 PO 4, J. T. Baker), dibasic potassium phosphate (K 2 HPO 4, J. T. Baker), Gold Chloride trihydrate (HAuCl 4 3H 2 O, Sigma Aldrich), Chloroplatinic acid hexahydrate (H 2 PtCl 6 6H 2 O, Sigma Aldrich), sodium citrate dihydrate (J. T. Baker), citric acid (Fisher Scientific), Sodium Borohydride (Sigma Aldrich), 2-allylphenol (Sigma Aldrich), ammonium hydroxide (Fisher Scientific), 30 nm Pt nanoparticles (nanocomposix, Inc. San Diego, CA), were all used as received from the manufacturer. Synthesis of 12 nm Au Nanoparticles: Gold nanoparticles were synthesized by the reduction of HAuCl 4 with sodium citrate. 1 All glassware was thoroughly cleaned with fresh piranha solution prior to use and rinsed thoroughly before use. 50 ml of 0.01% m/v of HAuCl 4 was refluxed while stirring and 1.8 ml of sodium citrate solution (1% m/v) was then rapidly injected into the flask. The solution was allowed to reflux for 15 minutes and then allowed to cool to room temperature. Synthesis of 4 nm Pt Nanoparticles: Platinum nanoparticles were synthesized according a previous report. 2 Using thoroughly cleaned glassware (as described previously), 36 ml of a 0.2% chloroplatinic acid hexahydrate was added to 464 ml of DI water. The solution was brought to a boil and 11ml of 1% sodium citrate and 0.05% citric acid was added. After a half of a minute, 5.5 ml of freshly prepared 0.08% sodium borohydride with 1% sodium citrate and 0.05% citric acid was very quickly injected into the boiling solution and allowed to boil while stirring under reflux. After 10 min the product was cooled to room temperature. Preparation of Carbon Fiber Electrodes: The preparation of the carbon fiber electrodes was described in detail elsewhere 3 but briefly, 5 um carbon fibers were sealed in borosilicate capillaries (Outer diameter: 1.2 mm, inner diameter 0.69 mm) by thermal pulling on a micropipette puller (Sutter instruments P-97). Electrical contact was then made using Silver conductive paste (Dupont) and tungsten wires. The carbon fibers lengths would extend out of the borosilicate capillary after the pulling procedure and would need to be insulated. To insulate the carbon fibers the electrodeposition and cross linking of 2-allylphenol copolymer was used. Electrodeposition of the 2-allylphenol was done using a 0.90 mm 2-allylphenol in a 1:1 H 2 O:Methanol solution where the ph was adjusted to using ammonium hydroxide. The S2

3 electrodes were submerged in the 2-allylphenol solution and the copolymer was deposited by electroxidation where positive 4 volts was applied to the carbon fiber vs. a platinum wire counter electrode for ~10 minutes. The electrodes were then heated to 150 o C for 30 minutes in an oven to cure the copolymer coating. Electrochemical Measurements: Steady-state voltammograms were recorded using a computercontrolled Dagan Chem-Clamp voltammeter/amperometer and data was recorded using an in-house virtual instrumentation program written in LabView (National instruments) on a desktop PC equipped with a PCI-6251(National Instruments) data acquisition card. A commercially available Ag/AgCl reference electrode (Bioanalytical Sciences, Inc.) was used as reference electrode for all electrochemical data. Fast-Scan Cyclic Voltammetry (FSCV) data was obtained using a home-built breakout box (UNC Chemistry Electronics Design Facility) equipped with a low noise current amplifier (Electro Optical Components, Inc. model: LCA-10K-500M) running the FSCV software developed by the Wightman s group at the University of North Carolina. 4 Scanning Electron Microscopy (SEM): SEM images were obtained using a field-emission SEM (FEI Sirion). Samples were sputter coated with a 2-3nm thick layer of Au/Pd prior to SEM imaging. Transmission Electron Microscopy (TEM): TEM imaging was performed on a FEI Technai G2 F20 S- Twin operating at 200kV using a single tilt sample holder and carbon coated Formvar copper TEM grids (Ted Pella). S3

4 Figure S1 shows TEM images of (A) 4 nm platinum and (B) 12 nm gold nanoparticles we synthesized using the procedures described in the experimental section. (C) and (D) are TEM analysis of the nanoparticles showing that the average size of the platinum nanoparticles is 4.37 ± 1.07 nm with 173 counted and the Au NPs average size was ± 1.61 nm with 201 counted. Figure S2 is a typical current-time trace from a 5 µm carbon UME with 0.1 nm 4 nm Pt nanoparticles in 15 mm hydrazine, 5 mm phosphate at ph 7.4 with the potential fixed at 0.1 V vs Ag/AgCl. Figure S3 shows a CV from a single 12 nm gold nanoparticle taken from the FSCV plot shown in Figure 3 of the main text. The CV is taken from the event at 13 s and shows a peak current of approximately 80 pa with a half-wave potential, E 1/2, ~0.2 V vs Ag/AgCl. Figure S4a shows the current-time trace taken from the FSCV color plot at a potential of 0.2 V and is similar in appearance to the fixed potential (0.2 V vs Ag/AgCl)) amperometric current-time trace shown in Figure S4b. Figure S5a is a histogram showing how the scan rate does not alter the nanoparticle collision current step. The data was obtained using a freshly exposed 5 µm carbon fiber electrodes in 15 mm hydrazine with 5 mm phosphate buffer at ph 7.4. The event current steps were measured at 0.1 V vs Ag/AgCl. Figures S6a and S6b show a freshly exposed 5 µm carbon fiber electrode where the surface is relatively clean and void of particles. Figures S6c and S6d show a 5 µm carbon fiber that has 12 nm Au nanoparticles that can be seen on the surface of the electrode. Estimate of the diffusion layer thickness using the Einstein equation d = (2Dt) 1/2 = ( cm/s s) 1/2 = cm, or 422 nm Here, D = cm/s, t = RT/Fν = /( ) = s S4

5 Estimate of the collision frequency from diffusive flux - For the 4-nm Pt nanoparticle, collision frequency on the 5-µm carbon electrode can be estimated by calculating the diffusion flux: 5 f = 4DCr = 4 ( cm = 331particles / s 2 / s) [( mol / cm 3 ) ( / mol)] Here, f (particles/s) is the estimated frequency of particles reaching the electrode surface, D (cm 2 /s) is the diffusion coefficient of the nanoparticles, C (particles/cm 3 ) is the bulk concentration of particles, r (cm) is the radius of the electrode. The diffusion coefficient is calculated from Stokes-Einstein equation, kt D= 6πηr This is much greater than the measured frequency of nanoparticle collision indicating that only a small fraction of nanoparticles colliding on the electrode are measured. 4 cm S5

6 Figure S1: TEM images showing (A) 4 nm platinum nanoparticles and (B) 12 nm gold nanoparticles. (C) and (D) are histograms showing the nanoparticle size distributions taken from TEM images for both the 4 nm Pt NPs and the 12 nm Au NPs. The Pt NPs average size was 4.37 ± 1.07 nm with 173 particles counted, the Au NPs average size was ± 1.61 nm with 201 particles counted. (C) (D) S6

7 Figure S2: A typical current-time trace from a 5 µm carbon fiber with 0.1 nm 4 nm Pt nanoparticles in 15 mm hydrazine, 5 mm phosphate at ph 7.4 with the potential fixed at 0.1 V vs Ag/AgCl. S7

8 Figure S3: CV from a 12nm Au nanoparticle taken from the FSCV trace shown in Figure 3 of the main text at 13 s in 15 mm hydrazine in 5 mm phosphate at ph 7.4. S8

9 Figure S4: (a) Current-time trace taken from the FSCV color plot seen in Figure 3 of the main text at 0.2 V vs Ag/AgCl. (b) a typical current-time trace from a 5 µm carbon fiber with 0.1 nm 12 nm gold nanoparticles in 15 mm hydrazine in 5 mm phosphate at ph 7.4 with the potential fixed at 0.2 V vs Ag/AgCl. S9

10 Figure S5: Histogram showing the current step distributions for 4 nm Pt nanoparticles using FSCV at two different scan rates using a 5 µm carbon fiber UME in 15 mm hydrazine with 5 mm phosphate at ph 7.4. S10

11 Figure S6. SEM images of carbon fiber electrodes without nanoparticles present on the surface (A) and (B) and with 12 nm Au nanoparticles present on the surface (C) and (D). (B) and (D) are the magnified portions outlined by the red squares in (A) and (C) respectively. References: (1) Frens, G. Nature 1973, 241, (2) Bigall, N. C.; Härtling, T.; Klose, M.; Simon, P.; Eng, L. M.; Eychmüller, A. Nano Lett. 2008, 8, (3) Strein, T. G.; Ewing, A. G. Anal. Chem. 1992, 64, (4) Bucher, E. S.; Brooks, K.; Verber, M.; Keithley, R. B.; Owesson-White, C.; Carroll, S.; Takmakov, P.; McKinney, C. J.; Wightman, R. M. Anal. Chem. 2013, 85, (5) Boika, A.; Thorgaard, S. N.; and Bard, A. J. J. Phys. Chem. B 2013, 117, S11