Article pubs.acs.org/accounts
Electrochemistry at One Nanoparticle Published as part of the Accounts of Chemical Research special issue “Nanoelectrochemistry”. Michael V. Mirkin,*,†,‡ Tong Sun,†,‡ Yun Yu,†,‡ and Min Zhou† †
Department of Chemistry and Biochemistry, Queens College, City University of New York, Flushing, New York 11367, United States ‡ The Ph.D. Program in Chemistry, The Graduate Center of City University of New York, New York, New York 10016, United States CONSPECTUS: Electrochemistry at metal nanoparticles (NPs) is of significant current interest because of its applications in catalysis, energy conversion and storage, and sensors. The electrocatalytic activity of NPs depends strongly on their size, shape, and surface attachment. The use of a large number of particles in most reported kinetic experiments obscured the effects of these factors because of polydispersity and different NP orientations. Recent efforts to probe electrochemistry at single NPs included recording of the catalytically amplified current produced by random collisions of particles with the electrode surface, immobilizing an NP on the surface of a small electrode, and delivering individual NPs to electrode surfaces. Although the signals recorded in such experiments were produced by single NPs, the characterization issues and problems with separating an individual particle from other NPs present in the system made it difficult to obtain spatially and/or temporally resolved information about heterogeneous processes occurring at a specific NP. To carry out electrochemical experiments involving only one NP and characterize such an NP in situ, one needs nanoelectrochemical tools with the characteristic dimension smaller than or comparable to those of the particle of interest. This Account presents fundamentals of two complementary approaches to studying NP electrochemistry, i.e., probing single immobilized NPs with the tip of a scanning electrochemical microscope (SECM) and monitoring the collisions between one catalytic NP and a carbon nanopipette. The former technique can provide spatially resolved information about NP geometry and measure its electron transfer properties and catalytic activity under steady-state conditions. The emphasis here is on the extraction of quantitative physicochemical information from nanoelectrochemical data. By employing a polished disk-type nanoelectrode as an SECM tip, one can characterize a specific nanoparticle in situ and then use the same NP for kinetic experiments. A new mode of SECM operation based on tunneling between the tip and nanoparticle can be used to image the NP topography with a lateral resolution of ∼1 nm. An alternative approach employs carbon nanoprobes produced by chemical vapor deposition of carbon into quartz nanopipettes. One metal NP is captured inside the carbon nanocavity to probe the dynamics of its interactions with the electrode surface on the microsecond time scale. The use of high-resolution transmission electron microscopy is essential for interpreting the results of single-NP collision experiments. A brief discussion of the nanoelectrochemical methodology, recent advances, and future directions is included.
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INTRODUCTION
Recent progress in nanoelectrochemistry produced nanometer-sized probes suitable for electrochemical experiments at single NPs.3,8,9 Three different experimental approaches are illustrated in Figure 1. A single NP can be immobilized on a nanometer-sized electrode (Figure 1A) to investigate electron transfer (ET) and electrocatalytic reactions at its surface by voltammetry. In this way, single Au10,11 or Pt12 NPs were attached to Pt,10 carbon,11 or passivated Pt/TiO2 nanoelectrodes, and their activities toward the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) were investigated. These experiments were plagued by several
Nanoparticles (NPs) find a wide range of applications in electrocatalysis and sensing1−5 because of their large surface-tovolume ratio, high density of active sites, and remarkable physicochemical properties. The activity of NPs depends strongly on their size, shape, and surface morphology. Most published electrochemical studies involved a large number of NPs and particle ensembles. The data obtained in such experiments is averaged over a large population of particles; it may be affected by polydispersity, different NP orientations, and other phenomena that make it difficult to interpret. Electrochemical experiments at the level of a single NP can help clarify the structure−activity relationships.6,7 © 2016 American Chemical Society
Received: June 14, 2016 Published: September 14, 2016 2328
DOI: 10.1021/acs.accounts.6b00294 Acc. Chem. Res. 2016, 49, 2328−2335
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Figure 1. Electrochemical experiments with single NPs. (A) TEM image of an ∼15 nm AuNP electrostatically attached to the surface of a Pt nanoelectrode. Reproduced from ref 10. Copyright 2010 American Chemical Society. (B) Schematic representation of conductive AFM imaging of immobilized Fc-PEGylated nanoparticles and tip-current-based image of two NPs. Adapted from ref 16. Copyright 2013 American Chemical Society. (C) Current transient showing ∼60 pA steps produced by collisions of ∼3.6 nm Pt particles with a 10 μm Au disk electrode in solution containing ∼36 pM NPs and 15 mM hydrazine. Adapted from ref 22. Copyright 2008 American Chemical Society.
single NPs immobilized on the substrate surface. The lateral resolution (ca. 100 nm) in SICM and SECCM experiments reported to date was not sufficiently high to image individual small NPs. Figure 1C illustrates another way to probe electrochemical processes at single NPs: monitoring their collisions with the electrode surface.22 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.23,24 The measured current is produced by the diffusion of dissolved electroactive species to the NP and electrocatalytic reaction at its surface (e.g., hydrazine oxidation in Figure 1C). The resulting catalytic amplification makes the collision event detectable. A number of recent studies of NP collisions have focused on the evaluation of the amplification factor,25 NP size and geometry22,26 and catalytic activity,27,28 measuring ultralow concentrations,29,30 and particle transport and tunneling issues.31−33 Nevertheless, the understanding of some fundamental aspects of NP collisions such as the shape of a single impact transient32−34 and its relationship to the NP catalytic activity is lacking. In most published studies, after colliding with the electrode surface, an NP became attached to it (i.e., the collisions were not elastic, but sticking), producing a step in the current transient (Figure 1C). Separating the contributions of numerous particles to the measured current is challenging especially because the current at each individual NP decreases with time (deactivation effect). In some experiments this “staircase” response was avoided (e.g., by using a Hg/Pt electrode to poison the PtNP, thus quickly deactivating the catalytic reaction27) to obtain current spikes instead of steps.34,35 Despite these advances, the quantitative analysis of transients produced by a number of polydisperse NPs is not straightforward. In this Account, we discuss electrochemical experiments involving only one NP. Nanometer-sized electrochemical
technical issues, including difficulties in characterizing the geometry of the nanoelectrode/NP system,11 modeling charge propagation across the passivating film,12,13 and separating the NP response from the background current produced by the underlying electrode surface.10 Moreover, the NP can be quickly deactivated by the intermediates/products of the electrocatalytic process, adsorption of impurities, or electroetching,14 thus diminishing the payoff from the laborious preparation of the nanoelectrode/NP arrangement. Another approach to single-NP electrochemistry makes use of different scanning probe microscopes (SPMs) equipped with electrically conductive tips or nanopipettes. In the earliest of such studies, a scanning tunneling microscope (STM) tip was used to form a PdNP by electrodeposition, deposit it onto an Au substrate, and then detect H2 produced by the HER at its surface.15 In another electrochemical SPM experiment, a conductive atomic force microscope (AFM) tip was used to measure the size and statistical distribution in grafting density of poly(ethylene glycol) (PEG) on NPs modified with a redoxlabeled ferrocene/PEG (Fc-PEG) capping agent (Figure 1B).16 The obtained sigmoidal tip voltammograms suggested that the tip selectively detected the Fc-PEG chains immobilized onto individual nanoparticles. One of the difficulties in these experiments is that a conical STM or AFM tip is not an ideal probe for quantitative electrochemical measurements because its geometry is typically imperfect and hard to characterize. By contrast, a polished, flat nanoelectrode suitable for quantitative studies of heterogeneous processes at single NPs can be used as a tip in the scanning electrochemical microscope (SECM),17 as discussed below. Using a nanopipette as a scanning probe, one can deliver single NPs18 and monitor their landing on the substrate surface.19 Two SPM techniques employing nanopipette tips scanning ion conductance microscopy (SICM)20 and scanning electrochemical cell microscopy (SECCM)21can address 2329
DOI: 10.1021/acs.accounts.6b00294 Acc. Chem. Res. 2016, 49, 2328−2335
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HOPG across this film occurred, as discussed previously.37,38 The NPs (i.e., citrate-capped AuNPs or CTAB-stabilized Pd nanocubes) were either electrostatically attached to negatively charged polyphenylene films17,39 or anchored by thiol groups.40 Two modes of the SECM operation were used in experiments with NPs (Figure 2). In a feedback-mode experiment (Figure 2A), a nanometer-sized SECM probe approached a metal NP whose radius (rp) was either larger than or comparable to that of the tip (a). The electrolyte contained an electroactive mediator (e.g., ferrocenemethanol; Fc), and the tip potential (ET) was such that oxidation of the mediator occurred at a rate governed by diffusion. If the separation distance (d) is comparable to a, the oxidized form of the mediator (Fc+) produced at the tip surface gets reduced at the substrate, and the tip current increases with decreasing d (positive feedback; the tip current near the surface is higher than its value in the bulk solution, iT > iT,∞). The tip current was recorded as a function of d (current−distance curve) or tip x−y position (imaging). If no mediator regeneration occurred at the sample or the regeneration rate was low, iT decreased with decreasing d because of the hindered diffusion of redox species (negative feedback; iT < iT,∞). In substrate generation/tip collection (SG/TC) mode (Figure 2B), d is too long for efficient SECM feedback, and a relatively large tip (a > rp) collects redox species generated at the NP surface (e.g., H2 in Figure 2B). While the feedback mode provides higher spatial resolution and faster mass transfer, SG/TC mode is more useful for probing weak signal sources (e.g., very small NPs) and investigating mechanisms of electrocatalytic reactions. A low density of AuNPs attached to the polyphenylene film (Figure 3A) is essential for addressing individual immobilized NPs with an SECM tip. The 20 nm AuNPs appear to be about
probes have been employed in these experiments to attain sufficiently high spatial resolution in topographic and reactivity SECM images of an NP or to monitor multiple collisions involving the same NP by physically separating it from other nanoparticles in the system.
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SECM OF SINGLE NANOPARTICLES To investigate NPs by SECM, they have to be attached to a flat, uniform, and electrochemically inert substrate that would provide an electrical connection to the particle. In the SECM experiments discussed here, the substrate was highly ordered pyrolytic graphite (HOPG) whose surface was passivated by a polyphenylene multilayer produced by electrochemical reduction of an aryl diazonium salt36 (Figure 2). The passivating film
Figure 2. Schematic representation of (A) feedback-mode and (B) SG/TC SECM experiments at a single NP.
was a few nanometers thick and sufficiently compact to block ET between the HOPG and dissolved electroactive molecules.17 At the same time, efficient ET between the NPs and
Figure 3. Probing NPs electrostatically attached to an HOPG/polyphenylene film substrate by AFM and SECM. (A) Noncontact-mode AFM image of 20 nm AuNPs and (B, C) feedback-mode SECM images of (B) 10 and (C) 20 nm AuNPs. (D, E) Experimental (symbols) current−distance curves obtained with same tip as in (C) approaching either the same NP (D) or a passivated portion of the substrate (E) and fitted to the theory (solid lines). The solution contained 1 mM Fc and 0.1 M KCl. iT in (D) and (E) is normalized by iT,∞ = 4.5 pA. ET = 400 mV vs Ag/AgCl; the substrate was unbiased. a = 3 nm in (B) and 14 nm in (C−E). Adapted with permission from ref 17. Copyright 2014 Wiley-VCH. 2330
DOI: 10.1021/acs.accounts.6b00294 Acc. Chem. Res. 2016, 49, 2328−2335
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Accounts of Chemical Research 10−15 nm high and ∼50 nm in diameter in the AFM image (Figure 3A). The significantly larger lateral NP size in the image is an artifact caused by the tip convolution effect.41 The NP size in topographic SECM images is closer to the nominal values and those obtained by transmission electron microscopy (TEM). For instance, the apparent NP diameter at the base (12−14 nm) in the feedback-mode image (Figure 3B) obtained with a very small SECM tip (a ≈ 3 nm) is reasonably close to the nominal value (10 nm). Besides imaging a single NP, one can probe the ET reaction occurring at its surface. The constant-height SECM image in Figure 3C shows significant positive feedback over the NP surface and negative feedback over the polyphenylene film. Accordingly, the current−distance curve obtained with the same Pt tip approaching the same 20 nm AuNP (symbols in Figure 3D) fits the theory for pure positive feedback (solid line in Figure 3D). The tip radius value obtained from the fit (13.6 nm) is in excellent agreement with a = 14 nm calculated from the diffusion-limiting current of Fc (iT,∞) as well as a = 14.4 nm obtained by fitting the experimental iT versus d curve (symbols in Figure 3E) to the theory for pure negative feedback (solid line). This indicates that ET at a single NP can be quantitatively probed by SECM. Although the apparent diameters of AuNPs in SECM images (e.g., Figure 3B,C) are reasonably accurate, a more straightforward and reliable approach for size evaluation is to fit an experimental current−distance curve to the theory using the NP radius (rp) as an adjustable parameter.39 In Figure 4, the
(Figure 5A), the tip current is due to oxidation of hydrogen generated at the AuNP. Unlike topographic images (Figure 3B,C), Figure 5A shows a map of the hydrogen flux that looks much larger than the 20 nm NP diameter because of the diffusion broadening. Quantitative information about HER kinetics can be obtained from plots of the tip current versus the substrate potential (iT vs ES) recorded at constant ET values (Figure 5B). The hydrogen oxidation current in Figure 5B was observed at the tip at ES ≤ −500 mV vs Ag/AgCl when the HER occurred at the AuNP. It was small at ET = 100 mV (curve 3), higher at ET = 400 mV (curve 2), and much higher at 500 mV (curve 1). The collection efficiency in SG/TC mode (i.e., iT/iNP) is determined by the geometry of the tip/substrate system and is independent of the substrate potential. Thus, iT is directly proportional to iNP, and the iT versus ES curves in Figure 5B represent the potential dependence of the proton reduction rate at a single AuNP. The linear portion of the Tafel plot for this process (dashed line in the inset) obtained from curve 1 in Figure 5B exhibits a slope of 116 mV/decade, consistent with literature data for the HER at polycrystalline gold. The theoretical treatment of SG/TC data at spherical NPs39 suggests that the collection efficiency in Figure 5B was ∼75%. SECM can also be used to characterize nonspherical NPs. A 25 nm × 25 nm image obtained with a 10 nm Pt tip in a 1 mM Fc solution (Figure 6A) shows a Pd nanocube anchored to the HOPG/polyphenylene surface by thiol groups. Similar to the images of AuNPs, the tip current increased over the Pd nanocube and decreased over the polyphenylene film (iT,∞ = 3.9 pA). The image of a parallelepipedal NP with an ∼13 nm × 13 nm base is in very good agreement with the TEM images of similar nanocubes (edge length of 14 ± 2 nm; Figure 6B). The lateral resolution in Figure 6A (∼1 nm) is extremely high, but a more detailed analysis revealed that the image of the cube is much sharper than can be expected according to the existing SECM theory. Finite-element simulations (Figure 6C) showed that no sharp edges or corners should be visible in the image of such an NP obtained with a 10 nm tip because of the diffusion broadening effect, and the top surface of the cube should not look flat. This discrepancy suggests that the tip current in Figure 6A is not due to the conventional diffusion-based SECM feedback. The electron tunneling between the tip and the nanocube (Figure 7A) is the likely source of the signal. Unlike a feedbackmode SECM experiment (Figure 1A), no feedback process (i.e., oxidation of Fc at the tip and its regeneration at the NP surface) occurred in the gap between the tip and the nanocube. While in a feedback experiment the potential of the NP is determined by the underlying conducting substrate, in tunneling mode the tip comes sufficiently close (d ≈ 1−2 nm) to the particle surface to control its potential. At very short separation distances, the tunneling resistance becomes small in comparison with the electron-transfer resistance at the NP− solution interface, thus diminishing the potential drop across the tip−nanocube gap. When this happens, the NP’s potential becomes similar to that of the tip, but the tunneling current still can flow because of the very small tunneling resistance. The nanocube in this case acts as a part of the tip electrode, and oxidation of Fc occurs at its surface. The tip current is determined by the diffusion flux of Fc to the nanocube. Most likely, electron tunneling occurs between the NP and a very small, sharp feature on the tip surface occurs (callout in Figure 7A). When this feature is moved away from the cube surface,
Figure 4. Evaluating rp from SECM approach curves. The current− distance curve (symbols) was obtained with an 11 nm radius tip approaching a 10 nm radius AuNP and fitted to the theory (blue curve). Other theoretical curves (solid lines) bracket the experimental data. The solution contained 1 mM Fc and 0.2 M KCl. iT,∞ = 3.4 pA. Adapted from ref 39. Copyright 2015 American Chemical Society.
experimental current−distance curve obtained with an 11 nm radius tip approaching a 10 nm radius AuNP (NP diameter of 19.8 ± 1 nm from TEM images) is fitted to the theory. From the best fit, obtained with RP = 0.9 (blue curve; RP = rp/a), and the analysis of the current−distance curves bracketing the experimental data (yellow and red curves), the value rp = 10 ± 1 nm was found, in excellent agreement with the TEM results. After characterizing the shape and size of a specific NP in situ, the same SECM tip can be used to probe electrocatalytic processes occurring at its surface. The SG/TC mode of SECM operation (Figure 2B) was used to investigate the HER kinetics at single AuNPs.17 With the separation distance sufficiently long to essentially eliminate the SECM feedback, the tip acted as a passive probe interrogating the catalytic process without affecting its rate. In the SG/TC-mode image obtained by scanning the Pt tip in the xy plane above a 20 nm AuNP 2331
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Figure 5. (A) SG/TC map of the HER at a 20 nm AuNP and (B) tip/substrate voltammograms obtained in solution containing 10 mM HClO4 and 0.1 M NaClO4. In (A), ET = 500 mV, ES = −750 mV, and a = 15 nm. In (B), ET = 500 (1), 400 (2), and 100 (3) mV; the potential sweep rate was 100 mV/s; a = 60 nm; d ≈ 80 nm. The inset shows the Tafel plot obtained from curve 1. Adapted with permission from ref 17. Copyright 2014 Wiley-VCH.
Figure 7. (A) Schematic representation of tunneling between the SECM tip and nanocube in a solution containing a redox mediator (Fc) and (B) simulated tunneling-mode SECM image of a 14 nm conductive cube with a 10 nm tip. Figure 6. (A) SECM image of a Pd nanocube obtained with a 10 nm radius Pt tip, (B) TEM image of similar nanocubes, and (C) simulated feedback-mode SECM image of a 14 nm conductive cube. a = 10 nm. Adapted from ref 40. Copyright 2016 American Chemical Society.
it. This methodology can enable mechanistic studies of processes at individual NPs constituting real-world macroscopic catalysts.
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SINGLE NP COLLISIONS Monitoring of NP collisions can provide temporally resolved information about transient interactions of particles with the electrode surface. To extract this information, one has to deconvolute the time-dependent contributions from multiple NP collisions that complicate the analysis of experimental current transients (Figure 1C). It was shown recently42 that collision experiments at the level of a single catalytic NP can be done using carbon nanopipettes prepared by chemical vapor deposition of carbon into a prepulled quartz pipette.43,44 In such an experiment, the nanopipette is used to capture an NP in its cavity and then transferred to the solution containing no NPs (Figure 8A). The electrocatalytic oxidation of hydrogen
the tunneling current decreases sharply. The simulated image of the nanocube based on the tunneling model (Figure 7B) agrees reasonably well with that in Figure 6A, thus explaining the abnormally high lateral resolution and sharp rectangular shape of the cube in the experimental image. Apparently, if the NP connection to the substrate is weak, it produces no SECM feedback, and only the tunneling response can be observed when the tip comes sufficiently close to the NP. Although our understanding of switching from the feedback response to the tunneling mechanism is incomplete, the tunneling approach has the potential for high-resolution electrochemical imaging and offers the possibility to measure the current flowing at a single NP without physically touching 2332
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Figure 8. Experimental setup for monitoring collisions of one NP with a carbon nanopipette. (A) After capturing an 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. (C) TEM image of the carbon nanosampler.
Figure 9. Current transients produced by collisions of one IrOx NP with the carbon nanosampler wall. (A) Capture of an NP inside the nanosampler cavity (Figure 8C) in solution containing 1 pM IrOx NPs. (B) Current recording obtained after the transfer of the nanosampler with one NP to the NP-free solution. (C) Zoomed view of a single current transient from (B). The inset shows a 25 ms long portion of the recording obtained with multiple IrOx NPs inside a 250 nm open carbon pipet. (D) Scatter plot of the peak current vs halfwidth for current traces in (B). Solutions contained 1 mM H2O2 and 0.1 M PBS (pH 7.4). The carbon electrode potential was 0.5 V vs Ag/ AgCl. The sampling and low-pass filter frequencies were 100 and 10 kHz, respectively.
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 but not at the catalytically inert carbon surface34,45 (Figure 8B). Two types of pipettes were used in these experiments: an open carbon pipette with a thin carbon layer coated on the inner quartz wall43 and a “nanosampler”a quartz pipette almost completely filled with carbon having a nanometer-sized cavity at the tip44 (Figure 8C). In the experiment represented in Figure 9, the nanosampler was initially filled with 1 mM H2O2 solution in 0.1 M phosphate-buffered saline (PBS). The current measured immediately after its insertion into a solution of the same composition containing 1 pM IrOx NPs was stable and very low (Figure 9A); no detectible NP collisions occurred during the first 52 s. After the first current blip recorded at ∼52 s, subsequent spikes occurred at a much higher frequency (i.e., every 3−6 s), suggesting that an NP entered the nanosampler cavity. The current spikes continued to appear at about the same frequency after the pipette was transferred to the solution containing no NPs (Figure 9B) until the NP left the cavity. The first arrival time of 52 s is in line with the estimate based on the theory developed in ref 30. The very low background current in Figure 9A,B is due to the small surface area of carbon inside the nanocavity (Figure 8C) and the use of a patch-clamp amplifier (Multiclamp 700B, Molecular Devices) for current recording. The high signal-tonoise ratio facilitated quantitative analysis of the current transients. All of the collisions observed with 20−40 nm diameter citrate-capped IrOx NPs appeared to be elastic, with the catalytic current dropping to zero within ≤1 ms (Figure 9C).42 This time is incomparably shorter than the time scale of NP deactivation (seconds). The shape of transients obtained with the same NP (as opposed to transients produced by different NPs34,42) is highly reproducible. The scatter plot in Figure 9D shows that the variation in the peak half-width for all of the spikes in Figure 9A,B is less than 10%, which is in sharp contrast to the variation in the peak current by a factor of ∼3. Unlike the recordings obtained with a number of NPs colliding with the electrode surface (Figure 9C inset), no multiple closely spaced peaks were observed with a
single NP captured in the nanocavity. Closely spaced peaks occurring on the time scale of a few milliseconds were previously interpreted as multiple impact events and attributed to the hydrodynamic trapping of an NP near the electrode surface.34 The shape of the current spikes (Figure 9C) is asymmetric, with the time of the decay significantly longer than the rise time. It was suggested previously that the apparent shape of the collision spike may reflect instrumental limitations and that the true shape should be rectangular because the current switches between zero and the limiting value when the NP touches the surface.33 There is, however, no reason to think that the spikes in Figure 9 are affected by slow instrumentation. The Multiclamp 700B amplifier can resolve current pulses shorter than 0.1 ms. The signal-to-noise ratio was increased by using a Bessel filter that was recently shown to conserve the spike shape and the measured charge.46 The current spikes produced by NPs of different size had similar shapes, while their halfwidths varied from ∼0.1 to >1 ms.42 Clearly, the shape of longer transients could not be significantly affected by instrumental limitations. Moreover, no significant change in the spike shape was found when the low-pass filter frequency was varied between 5 and 30 kHz and the acquisition rate was increased from 100 to 500 kHz. The ∼30 nm NP diameter is comparable to the nanocavity dimensions in Figure 8C. The complicated nature of the interactions of the NP with the carbon surface manifests itself in the profound effect of nanoconfinement on the NP dynamics. One NP contained in an ∼10 −18 L cavity corresponds to a particle concentration of ∼1.7 μM, i.e., 6 orders of magnitude higher than the NP concentration in the bulk solution (1 pM). Thus, the probability of finding an NP 2333
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nanoelectrochemical probes employed in such experiments include stable background current and high signal-to-noise ratio that diminish the need for filtering and improve the overall data quality. By localizing an NP inside the nanocavity, one can attain better control over the collision experiment, eliminate problems caused by NP polydispersity and aggregation, and investigate the evolution of the NP response in a series of collisions. Additionally, this setup enables the investigation of intriguing effects of nanoconfinement on the NP dynamics. The combination of nanoelectrochemical tools with high-resolution TEM and computer simulations is useful for quantitative analysis of the experimental data.
within the nanosampler cavity is negligibly small, and after entering the cavity the NP is expected to immediately leave it. The long residence time of the NP inside the cavity (e.g., ∼100 s in Figure 9A,B) suggests that the NP motion in it is extremely slow. This assumption is also consistent with a low collision frequency (
