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Collisions of Ir Oxide Nanoparticles with Carbon Nanopipettes: Experiments with One Nanoparticle Min Zhou, Yun Yu, Keke Hu, Huolin L. Xin, and Michael V. Mirkin Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b04140 • Publication Date (Web): 03 Feb 2017 Downloaded from http://pubs.acs.org on February 8, 2017
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Collisions of Ir Oxide Nanoparticles with Carbon Nanopipettes: Experiments with One Nanoparticle
Min Zhou,† Yun Yu,† Keke Hu,† Huolin L. Xin,*,§ and Michael V. Mirkin*,† †
Department of Chemistry and Biochemistry, Queens College - CUNY, Flushing, New York 11367 The Graduate Center, CUNY, New York, NY 10016
§
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973
*Corresponding Author E-mail:
[email protected] FAX: 718-997-5531
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Abstract Investigating the collisions of individual metal nanoparticles (NPs) with electrodes can provide new insights into their electrocatalytic behavior, mass transport and interactions with surfaces. Here we report a new experimental setup for studying NP collisions based on the use of carbon nanopipettes to enable monitoring multiple collision events involving the same NP captured inside the pipette cavity. A patch clamp amplifier capable of measuring pA-range currents on the microsecond time scale with a very low noise and stable background was used to record the collision transients. The analysis of current transients produced by oxidation of hydrogen peroxide at one IrOx NP provided information about the origins of deactivation of catalytic NPs and the effects of various experimental conditions on the collision dynamics. High-resolution TEM of carbon pipettes was used to attain better understanding of the NP capture and collisions.
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INTRODUCTION Electrochemical experiments at single metal nanoparticles (NPs) can help clarify the structureactivity relationships essential for NP applications in sensing, electrocatalysis, and energy storage.1-7 One way of probing electrochemical processes at single NPs is by monitoring their collisions with the electrode surface.8-10 A catalytic NP colliding with the catalytically inert collector surface can act as an active nanoelectrode that switches on an electrochemical reaction during this transient event.11-13 The current is produced by the diffusion of dissolved electroactive species to the NP and electrocatalytic reaction at its surface (e.g., hydrogen evolution11), and the resulting catalytic amplification makes the collision event detectable. A number of recent studies of NP collisions have focused on the evaluation of NP size and geometry12,14 and catalytic activity,15-18 measuring ultralow concentrations,19,20 biosensor design,21-23 particle transport,24-26 and tunneling issues.27-31 Nevertheless, the understanding of some fundamental aspects of NP collisions such as the shape of a single impact transient and its relationship with the NP catalytic activity is incomplete.27,28,32 Although the signal recorded in reported collision experiments was produced by single NPs, the large number of particles simultaneously present in the system complicated the analysis of processes occurring at a specific NP and its interactions with the electrode surface. The very recent work by Oja et al.33 presents the first experiment in which several successive collisions between the same NP and the electrode surface were recorded on the millisecond time scale. In most published studies, after colliding with the electrode surface, a NP became attached to it (i.e., the collisions were not elastic but sticking), producing a step in the current transient. Separating the contributions of numerous particles to the measured current is challenging especially because the current at each individual NP typically decreases with time (deactivation
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effect). In some experiments this "staircase" response was avoided, e.g., by using a Hg/Pt electrode to poison the Pt NP, thus quickly deactivating the catalytic reaction.15 In a few studies, the NP collisions produced current spikes rather than steps.34,35 Nevertheless, quantitative analysis of transients produced by a number of polydisperse NPs was not straightforward. Here, we employ carbon nanoprobes36,37 produced by chemical vapor deposition of carbon into quartz nanopipettes38,39 to capture a single NP, thus, physically separating it from other particles in the system. After capturing a NP inside the conductive nanocavity, the nanopipette is transferred to the solution containing no NPs (Figure 1A).
2e-
+ 2H+
Figure 1. Experimental setup for monitoring collisions of a NP with a carbon nanopipette. (A) After capturing a NP inside the nanocavity, the pipette is transferred to the solution containing no NPs. (B) Electrocatalytic oxidation of H2O2 occurs at an IrOx NP during its collision with the wall of the carbon pipette. The carbon surface is catalytically inert. The electrocatalytic oxidation of hydrogen peroxide produced amplification of the collision events. The potential applied to the carbon pipette was such that this reaction occurred at the iridium oxide (IrOx) NPs,16 but not at the catalytically inert carbon surface (Figure 1B). The high catalytic activity of iridium oxide NPs toward H2O2 oxidation and their resistance to activity loss have been reported previously.40,41
EXPERIMENTAL SECTION Chemicals and materials. K2IrCl6 (99.98 %), H2O2 (30 wt %) solution, and sodium
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citrate were purchased from Sigma-Aldrich (St. Louis, MO) and NaOH, NaH2PO4·xH2O, and Na2HPO4 were supplied by Fisher Scientific. 0.1 M PBS solution was prepared using deionized water from the Milli-Q Advantage A10 system equipped with Q-Gard T2 Pak, a Quantum TEX cartridge and a VOC pak. The total organic carbon (TOC) was
