Anodic stripping electrochemical analysis of metal nanoparticles

Anodic stripping electrochemical analysis of metal nanoparticles

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Anodic stripping electrochemical analysis of metal nanoparticles Dhruba K. Pattadar, Jay N. Sharma, Badri P. Mainali and Francis P. Zamborini Abstract

Traditional anodic stripping voltammetry (ASV) involves electrodeposition (reduction) of metal ions from solution over some time scale onto a working electrode followed by stripping (oxidation) of the deposited metal in a second step, where the stripping potential and quantity of charge passed provide information about the metal identity and solution concentration, respectively. ASV has recently been extended to the analysis of metal nanoparticles (NPs), which have grown popular because of their fascinating properties tunable by size, shape, and composition. There is a need for improved methods of NP analysis, and because metal NPs can be oxidized to metal ions, ASV is a logical choice. Early studies involved metal NPs as tags for the detection of biomolecules. More recently, anodic stripping has been used to directly analyze the physical, chemical, and structural properties of metal NPs. This review highlights the stripping analysis of NP assemblies on macroelectrodes, individual NPs in solution during collisions with a microelectrode, and a single NP attached to an electrode. A surprising amount of information can be learned from this very simple, low-cost technique. Addresses Department of Chemistry, University of Louisville, 2320 South Brook Street, Louisville, KY 40292, USA Corresponding author: Zamborini, Francis P ([email protected] edu)

Current Opinion in Electrochemistry 2019, 13:147–156 T h i s r evi ew c o m e s f r o m a t h e m e d i ss u e o n P hy s i c a l a n d Nanoelectrochemistry Edited by Michael V. Mirkin For a complete overview see the Issue and the Editorial Available online 26 December 2018 https://doi.org/10.1016/j.coelec.2018.12.006 2451-9103/© 2018 Elsevier B.V. All rights reserved.

Keywords Nanoparticles, Metal, Oxidation, Stripping, Electrochemistry, Impacts, Collisions, Analysis.

Introduction Anodic stripping voltammetry (ASV) is an electrochemical technique mainly used for quantitative and www.sciencedirect.com

qualitative detection of metal ions in solution present at trace levels [1e3]. The technique typically involves concentrating the metal onto an Hg working electrode by reduction of the metal ions over a determined time period followed by detection via oxidation (or stripping) of the deposited (reduced) metal. A currentepotential plot shows increased currents at specific potentials, where the potential indicates the identity of the metal ion, and the amount of current is proportional to the solution concentration, with ppb level detection limits. The technique has a long history and has been applied to measure environmentally important metals such as Pb [4], Hg [5], and As [6]. Metal nanoparticles (NPs) have grown popular over the past few decades because of their fascinating optical, electronic, and physical properties, which are tunable by size, shape, and metal composition. These tunable properties have led to important applications in catalysis [7,8], sensing [9e11], drug delivery [12,13], imaging [14e16], energy storage [17,18], photonics [19,20], and photothermal therapy [21]. Synthetic methods have been developed over the years, which provide excellent control over the composition, size, and shape of the metal NPs. Bimetallic and multimetallic NPs can also be synthesized with specific control over the atomic arrangement of the different metals (core/shell, segmented, and mixed). Analytical characterization is critical to determine their size, shape, and composition and the dispersity in these parameters within the sample population. This information can then be correlated to the properties and function of the metal NPs. Commonly available methods to characterize NPs include scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron spectroscopy, X-ray spectroscopy, and UVevis spectroscopy, which provide details about the size, shape, composition, oxidation state, and atomic arrangement in metal NPs. However, these methods are very expensive, time consuming, or require a synchrotron source, which greatly limits their availability and throughput of experiments. Electrochemistry is relatively simple, low cost, fast, and portable, making it attractive for metal NP analysis. Electrocatalysis measurements are most common on Current Opinion in Electrochemistry 2019, 13:147–156

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metal NPs, but there have also been fundamental studies of the capacitance, electronic structure, and electron transport properties of metal NPs [22]. ASV analysis initially involved various Au, Ag, and Cu NPs as tags for biomolecule detection [23e26]. Direct anodic stripping analysis of the metal NPs themselves has been the focus of more recent studies. In this review, we focus on anodic stripping analysis of (1) multiple NPs assembled on a macroelectrode surface (Figure 1a), (2) individual NPs in solution diffusing to and colliding with a microelectrode (Figure 1b), and (3) a single individual NP electrode (Figure 1c). This review will show that anodic stripping analysis provides a surprising amount of information about the concentration, size, composition, atomic arrangement, aggregation state, size stability, reactivity, and surface area-to-volume ratio (SA/V) for a wide variety of single metal and multimetal NPs.

Anodic stripping analysis of multi-NP assemblies Figure 1a illustrates the analysis of multi-NP assemblies, which involves the attachment of many metal NPs to a macroelectrode surface followed by ASV analysis. Some common methods of attachment are chemical binding to functionalized electrodes, drop-cast deposition, and

electrophoretic deposition (EPD). With this setup, the data involve a peak in the current at a potential where the metal NPs become oxidized and the signal represents the ensemble average of millions of NPs. Size analysis

The peak oxidation potential (Ep) in ASV is the potential of maximum current during the oxidative stripping of a metal, which is related to the E0 of the M/Mnþ redox couple. It was predicted to be sensitive to the metal NP size by Plieth in 1982 because the standard redox potential of an M/Mnþ pair was calculated to shift negative as the metal NP diameter (for a sphere) decreases below about 40 nm on the basis of thermodynamics and the fact that the surface free energy is related to the surface stress and metal NP surface area [27]. This leads to a predictable difference between the peak oxidation potential of a metal NP (Ep,NP) and the bulk metal (Ep,bulk) in the ASV as follows [27]. Ep;NP ¼ Ep;bulk 2yV m =zFr Here, y is the surface stress, Vm is the molar volume of the metal, z is the number of electrons passed per oxidized atom, F is Faraday’s constant, and r is the NP radius.

Figure 1

Anodic stripping analysis of (a) assembly of multiple metal (M) nanoparticles (NPs) attached to an electrode surface, (b) individual metal NPs diffusing from solution to a microelectrode, and (c) an individual metal NP electrode structure. An illustration of what the data look like is included to the right of each situation.

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Anodic stripping analysis metal nanoparticles Pattadar et al.

Ward Jones et al. [28] published theoretical and experimental work on the effect of Ag NP size and coverage on the Ep in ASV. The simulations were based on diffusion effects only and did not consider the effect of the NP size on the thermodynamic value of E0. They found no size-dependent shift in Ep experimentally with similar coverage in the ASV of Ag NPs down to 25 nm in diameter, which was likely an error due to Ag NP aggregation as a result of drop-cast deposition of the NPs. Ivanova and Zamborini experimentally observed a sizedependent shift in Ep for the ASV of Ag NPs (40 nme 8 nm in diameter) [29] and Au NPs (250 nme4 nm in

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diameter) [30] attached chemically or by electrodeposition to glass/ITO electrodes. The Ep shifted negative by about 100 mV for Ag NPs oxidizing to Agþ in acid and 200 mV for Au oxidizing to AuBr-4 or AuBr-2 in KBr solution, and both were compared with Plieth theory. Later, different groups studied the size-dependent oxidation of Au [31e33], Ag [34], Bi [35], and Pd [36] NPs by ASV, showing general agreement with Plieth theory. Very recently, Pattadar and Zamborini [37] studied the ASV of 1.5e2.0 nm diameter tetrakis(hydroxymethyl)phosphonium chlorideestabilized Au NPs compared with citrate-stabilized 4-nm, 15-nm, and

Figure 2

Anodic stripping voltammetry (ASV) of assemblies of nanoparticles showing (a) the effect of size on Ep from 2-nm up to 50-nm diameter Au NPs (reprinted with permission from Ref. [37], Copyright 2018 American Chemical Society), (b) the effect of pH-induced aggregation on the Ep (top) and absorbance spectrum (bottom) of 15-nm diameter citrate-stabilized Au NPs (reprinted with permission from Ref. [41], Copyright 2017 American Chemical Society, (c) the effect of atomic arrangement on the peak signature in the ASV for Cu/Au core/shell and CuAu mixed alloy NPs (submitted for publication), and (d) the size stability of 4-nm versus 2-nm diameter Au NPs during ozone exposure determined by monitoring the NP size change from the Ep (reprinted with permission from Ref. [37], Copyright 2018 American Chemical Society). NPs, nanoparticles.

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50-nm diameter Au NPs, where the Ep was about 450 mV negative of the 50-nm diameter Au NPs (See Figure 2a). It is well accepted that the Ep in the ASV of metal NPs shifts negative from the bulk metal for diameters of about 40 nm and below, and the shift is generally related to 1/radius, provided that the metal atom coverage is equivalent when making the comparison and there are no strong ligand stabilizers inhibiting the metal oxidation. Effect of substrate on Ep

The substrate material and surface charge can influence the Ep for metal NPs in ASV. Brainina et al. [38] considered both the change in Gibbs free energy of the NPs associated with decreasing size (Plieth theory), which leads to a negative shift in Ep, but also the Gibbs free energy associated with the metal NPesubstrate interaction, which leads to a positive shift in Ep if the work function of the substrate is more positive than the NP. Accordingly, the Ep of 20-nm diameter Au NPs was more positive when attached to a Pt electrode compared with a glassy carbon (GC) electrode. Similarly, Masitas et al. [39] found that the Ep values in the ASVs of 9-nm diameter Ag NPs increased in the order of Au z Pt > GC > glass/ITO, which was consistent with the order of the work function of the different electrodes. Aggregation analysis

NP aggregation may affect both current and Ep in ASV analysis. For example, Cloake et al. [40] observed that NP aggregation led to incomplete stripping because of an increasing number of cross-links between dopaminecapped Ag NPs as an increasing amount of melamine was added to them. This led to loss of contact between the Ag NPs and the electrode, which hindered oxidation. Allen et al. [41] showed a positive shift in the Ep of citrate-stabilized Au NPs following pH-induced aggregation, depending on the size. The Ep value was found to shift positive as a function of the extent of the aggregate size up to a maximum of 230 mV and 150 mV for 4-nm and 15-nm diameter NPs, respectively. Figure 2b (top) shows the ASVs of 15-nm diameter citrate-stabilized Au NPs attached to functionalized glass/ITO from their solutions at the pH values shown. As the pH decreased, aggregation occurred due to protonation/neutralization of the citrate stabilizer. Ep of up to 150 mV, which correlates well with the optical changes shown in the absorbance spectra (Figure 2b, bottom). Deposition studies

Kumar and Buttry [42] studied the effect of Pd NP size on Cu underpotential deposition (UPD) by depositing a Cu UPD layer on different size Pd NPs and monitoring the Ep for UPD Cu stripping in Clecontaining electrolyte. As the size of the Pd NPs decreased, the Ep for Current Opinion in Electrochemistry 2019, 13:147–156

stripping shifted negative, which they attributed to a decrease in the work function of the Pd NPs with decreasing size [42]. In a different type of study, Masitas et al. used ASV to study the size of citrate-coated Au NPs deposited onto glass/ITO electrodes by EPD in the presence of H2O2. At potentials near or positive of the oxidation potential for H2O2, the negatively charged Au NPs migrate to the working electrode where they catalyze the oxidation of H2O2 to form Hþ ions. The Hþ ions neutralize the carboxylate groups on the citrate stabilizer, leading to Au NP deposition onto the electrode [43]. ASV allowed detection of the amount and size of the deposited Au NPs by the stripping charge and Ep, respectively. The ASV showed that 4-nm diameter Au NPs deposited preferentially over larger Au NPs (15 nm and 50 nm) at lower EPD potentials because of the higher catalytic activity (lower overpotential) of the smaller Au NPs. This enabled size-selective EPD of Au NPs, which was easily determined by ASV from the Ep values. Surface area-to-volume ratio

The SA/V of metal NPs is a very important parameter to measure because reactions involving NPs occur on the surface atoms, and it is, therefore, important to know the number of surface atoms relative to the interior volume atoms, which one would want to maximize for various applications. Sharma et al. [44] electrochemically measured the SA/V of spherical Au NPs by monitoring the surface Au oxide formation in acid solution to determine the SA followed by ASV of Au in KBr to determine V. This allowed the electrochemical analysis of Au NP size from 4-nm to 70-nm diameter because SA/ V is mathematically equal to 3/r for a sphere. The electrochemically measured radius matched the SEMmeasured radius well after cleaning the NPs by ozone treatment. This method can be applied to much larger NPs compared with using Ep to measure size because the Ep shift is insensitive to NPs >w40 nm in diameter. The method could be useful for measuring the SA/V of complex shapes, porous structures, and aggregates. Alloy analysis

Pattadar and Zamborini recently analyzed the composition of w5-nm diameter citrate-stabilized Cu/Au core/ shell alloy NPs attached to amine-functionalized glass/ ITO electrodes by ASV in acidic KCl electrolyte. The composition of Cu and Au could be measured accurately for Cu1/Aux core/shell NPs for x values of 0.1e1.0 by simply integrating the charge under the Cu and Au peaks separately [45]. They also observed that ASV is sensitive to different atomic arrangements in bimetallic CuAu NPs. Figure 2c shows a very different ASV signature in KCl electrolyte for a Cu1/Au2 core/shell structure (red) compared with a Cu1Au2 mixed alloy structure (blue) [45]. The peak for Cu at about 0.25 V is not very prominent in the core/shell structure because www.sciencedirect.com

Anodic stripping analysis metal nanoparticles Pattadar et al.

the Au shell blocks Cu from oxidation until the Au is oxidized at about 0.9 V. In contrast, both Cu and Au atoms are accessible on the mixed alloy NP surface, showing a prominent Cu peak at 0.25 V and Au peak at 0.9 V, whose integrations give the correct Cu:Au 1:2 ratio. Size stability analysis

Pattadar and Zamborini [37] recently used ASV to determine the size stability of very small 1.5- to 2.0-nm diameter Au NPs after electrochemical surface oxide formation, electrochemical Ostwald ripening, and ozone treatment. Interestingly, a single surface oxidation/ reduction cycle in acid, 1 min in ozone, and 5 min at 0.3 V in KBr solution leads to a significant increase in NP diameter to between 4 nm and 10 nm, as determined by a positive shift in the Ep in the ASV. This size change is undesirable, considering that 4-nm diameter NPs are not electrocatalytically active for CO2 reduction and the hydrogen evolution reaction but the 2-nm diameter tetrakis(hydroxymethyl)phosphonium chloride Au NPs are. Citrate-stabilized 4-nm diameter Au NPs were more stable under the same conditions, requiring much longer times for size change to take place. Figure 2d shows that the Ep in the ASV for 4-nm diameter Au NPs does not shift until about 90 min of ozone exposure (top), whereas the Ep for 2-nm diameter Au NPs shifted significantly positive because of an increased size after just 1 min in ozone (bottom). Observing these types of size changes directly on an electrode would be tedious and nearly impossible for very small size NPs using microscopic techniques. This is an important application for size analysis by ASV.

Anodic stripping analysis of individual NPs in solution There has been a recent surge in single NP electrochemical analysis methods that involve NPs in solution catalyzing or undergoing electron transfer reactions as they collide with a working microelectrode (Figure 1b) [46]. These studies typically involve measuring the current as a function of time at a constant potential, although Guo et al. [47] recently used fast scan cyclic voltammetry to measure a full voltammogram of individual NPs as they collide with a microelectrode. This technique was originally demonstrated for NPs that catalyze a redox reaction [48], but later, researchers studied the NPs themselves being oxidized (stripping) [49]. In this section, we will review recent literature on the anodic stripping of individual NPs as they collide with a microelectrode, which has been used for the analysis of Ag [49], Au [50], Cu [51], and Ni [52] NPs as well as alloys [53] and metal oxide NPs, such as LiCoO2 [54]. The method provides information about the NP concentration, size, aggregation state, composition, and dynamics of the NPeelectrode interactions.

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Concentration analysis

Early metal NP analyses by oxidation during electrode collisions measured pM concentration levels of NPs, where each collision results in a current spike in a currentetime plot, and the specific concentration is proportional to the collision frequency (the number of spikes per unit time) [49]. More recent studies reported pM to zM concentrations by modifying the electrode materials and methods [55,56]. For example, Ellison et al. [57] used a cylindrical microwire electrode to measure Ag NPs at a concentration of 0.1 pM accurately. Yoo et al. [58] detected Ag NPs linked to conductive magnetic microbeads via DNA hybridization by individual NP collision events. The magnetic nature of the beads increased the flux, which helped them improve the detection limit of the NPs to aM concentration. Later, Batchelor-McAuley et al. [59] analyzed zM concentrations with very small 5.7  2.7 nm diameter Ag NPs at a carbon fiber microelectrode. Bonezzi et al. [60] increased the collision frequency by dielectrophoresis and electrothermal fluid flow using high-frequency AC current instead of DC as shown with the increased number of spikes in Figure 3a. They observed a significant increase in the collision frequency from 0.0002 s1 to 0.99 s1 of single Ag NPs and could detect fM concentrations. Size analysis

In 2011, Zhou et al. [49] first reported the oxidation of individual spherical Ag NPs ranging from 20 to 50 nm and 80 to 120 nm during collisions with a 11-mm radius GC electrode in citrate buffer electrolyte solution. The collisions resulted in single current spikes in currente time plots, where the radius of each colliding NP is r ¼ ð3MQ=4pFrÞ1=3 where M is the atomic mass of the metal, Q is the charge measured for the oxidation event, F is Faraday’s constant, and r is the density of the metal. The electrochemically determined sizes very closely matched with the SEMdetermined sizes. The frequency of oxidation collisions varied with electrode potential, and the onset potential was consistent with the oxidation potential of Ag NPs as shown in the plot of collision frequency vs. potential in comparison to the ASV in Figure 3b. In follow-up work, they determined the large diameter limit of 100  8 nm for citrate-coated Ag NPs [61]. More recently, Pumera et al. analyzed single Ag NPs [62] of 10e107 nm in diameter and Mo NPs [63] in the 10e100 nm size range using a screen printed electrode, which offers advantages such as portability, cost-effectiveness, and convenience. They found the size of the spherical Mo NPs to be higher than 100 nm. The study of non-noble metal NPs with applications as a catalyst for fuel cells was an important advancement in the field.

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Figure 3

Single metal NP collision stripping analyses showing (a) an increased Ag NP collision frequency by applying an AC electric field (Reprinted with permission from Ref. [60], Copyright 2017 American Chemical Society), (b) a plot of spike frequency versus potential showing that the onset of the spikes matches the potential for Ag oxidation in the ASV (Reprinted with permission from Ref. [49], Copyright 2011 Willey-VCH), (c) the mechanism of multiple oxidation peaks for a single Ag NP interacting with a microelectrode (Reprinted with permission from Ref. [64], Copyright 2017 American Chemical Society), (d) a histogram of the number of atoms in the measured spikes correlated to the size of aggregated Ag NPs (Reprinted with permission from Ref. [71], Copyright 2013 Willey-VCH), (e) a TEM image of an Au/Ag core/shell NP (1), the oxidation spikes at 0.9 V for Ag oxidation (2), the oxidation spikes at 1.3 V for Ag + Au oxidation (3), and the measured charge of the spikes as a function of potential (4) (Reprinted with permission from Ref. [73], Copyright 2011 Willey-VCH), and (f) ASV plots for individual Ag nanoelectrodes of different size (Reprinted with permission from Ref. [75], Copyright 2018 American Chemical Society). NP, nanoparticle; ASV, anodic stripping voltammetry; TEM, transmission electron microscopy.

NP–electrode dynamics and kinetics

Oja et al. [64] also studied the oxidation of Ag NPs at the single NP level, where in contrast to the work of Compton, they observed multiple peaks and incomplete oxidation on the collision of single Ag NPs. The individual Ag NP collision event consisted of a series of 1e10 discrete subevents over a 20-ms time interval. The first peak always had the largest current signal due Current Opinion in Electrochemistry 2019, 13:147–156

to a strong electrostatic attraction between the positively charged electrode and negatively charged Ag NPs. After the first impact, the Ag NP leaves the electrode with the same potential as the electrode. The NP undergoes partial oxidation to Agþ and can return to the electrode and go through the process multiple times, leading to multiple current spikes over a short time span. The NP may eventually be fully www.sciencedirect.com

Anodic stripping analysis metal nanoparticles Pattadar et al.

oxidized, but in most cases, it diffuses away before full oxidation occurs, which leads to an inaccurate calculation of the NP size from the measured charge. Figure 3c illustrates the mechanism for multipeak oxidations. Random walk simulations of the NP diffusion behavior reproduce the data well, indicating that the movement of the NPs is largely driven by Brownian motion. Replacing the microelectrode with an Au nanoband working electrode ranging from 60 to 180 nm in width led to fewer collisions, more single-peak collisions, and fewer subpeaks in multipeak collisions, as would be expected for a smaller electrode [65]. The charge measured for a single-peak event was 50% less than the first peak of a multipeak event, indicating very different NPeelectrode interactions with electrodes having a size similar to the NP size. Near the same time, Ma et al. [66] and Ustarroz et al. [67] studied the oxidation of Ag NPs of various sizes, also observing the multipeak behavior observed and simulated by Robinson et al. [68]. Zhou et al. [69] studied electron transfer kinetics for Ag and Ni NPs by oxidation impact electrochemistry, showing fast, reversible 1 electron transfer for Ag and slow, irreversible 2 electron transfer for Ni. More recently, Saw et al. [70]studied the oxidation of single 29-nm diameter Ag NPs by applying a staircase potential step experiment using a Pt microdisk electrode in various electrolyte solutions. They stepped the potential in increments starting before the oxidation potential to after the oxidation potential, allowing a measure of the oxidation kinetics. The appearance of oxidation collision peaks matched the known potential for oxidation, and the kinetics were reflected in the width and height of the peaks. At low overpotentials, the NPs fully dissolved but showed small peak height and large width because of the slower oxidation kinetics during the collision. This indicated that the electron transfer kinetics controlled the rate of oxidation. At high overpotentials, the peaks were larger and narrower, indicative of fast electron transfer kinetics and a mass transferelimited reaction. NP aggregation

Compton et al. studied the stripping of isolated and aggregated Ag NPs at the single particle/aggregate level in seawater media with a carbon microelectrode in a potassium chloride/citrate solution [71]. The presence of chloride in seawater led to aggregation of Ag NPs added to it. A larger current and charge measured during an event correlated with the degree of aggregation and showed excellent agreement with NP tracking for size and aggregation analysis. Figure 3d shows the calculated distribution of monomers and well-defined aggregates (dimer, trimer, and so on) determined by the different impact oxidation spikes and the measured stripping charge under these spikes [71,72]. www.sciencedirect.com

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Alloy studies Holt et al. [73] characterized core/shell Au/Ag NPs on the basis of the difference in the oxidation potential of Au and Ag. They observed spikes due to oxidation of the Ag shell of individual core/shell NPs when they applied a potential below 0.9 V. The 0.3 pC of charge corresponded to a 6.1-nm Ag shell thickness. At 1.3 V, oxidation spikes correlating to both Ag and Au showed a charge value of 0.7 pC, which corresponded to a 35.1-nm diameter Au core when subtracting the charge for the Ag and considering Au as 1.9 electrons/atom. These collision-determined values matched well with the transmission electron microscopy images as shown in Figure 3e. Saw et al. [74] studied the composition and size of bimetallic Au-Ag NPs in 10-mM HCl electrolyte solution by the nanoimpact technique. To record the current transients, they set the potential at 0.6 V vs Ag/ AgCl in KCl solution, where only Ag should oxidize. The total charge expected for 14 nm Au0.27-Ag0.73 bimetallic NPs was 1.67  1014 C (Au þ Ag), and they measured 0.98  1014 C for the Ag only, which is in good agreement with the amount of Ag in the NP.

ASV analysis of individual NPs on an electrode Recently, Hua et al. [75] performed ASV of individual Ag nanoelectrodes (Figure 1c) of varied size fabricated by an improved laser-assisted pulling method with radii down to 10 nm. They observed the thermodynamics and kinetics of electro-oxidation of the Ag nanostructures successfully at the single NP/nanoelectrode level. As shown in Figure 3f, with decreasing size, there is a negative shift in the oxidation potential, as was observed with ensemble measurements, and faster electron transfer kinetics.

Conclusions and future outlook This review shows that stripping analysis provides a great deal of information about metal NPs, including their size, aggregation state, SA/V, surface coverage, concentration, and both composition and atomic arrangement for alloy NPs. In addition, these studies reveal fascinating fundamental information about the reactivity of metal NPs and the factors affecting their anodic stripping, such as the size-dependent thermodynamics of oxidation, size-dependent UPD chemistry, effect of substrate, electron transfer kinetics, and dynamic interactions between metal NPs and the electrode surface during collisions. Finally, anodic stripping can be used to study metal NP chemistry in terms of size, stability, and size-selective EDP processes. One major benefit of ASV for size analysis of metal NP assemblies is that it is simple, fast, cost-effective, and has high throughput compared with electron microscopy. It is an excellent screening method for high

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throughput characterization of metal NPs synthesized by different procedures. It has a great future for monitoring changes in the size, composition, and atomic arrangement of metal NPs attached to electrode surfaces after various electrochemical, chemical, and thermal treatments or after their use as a catalyst. The drawbacks are that it is not useful for metal NPs containing strong ligand stabilizers because they will mask the size and composition information. A major benefit of the NP collision experiments is that one can gain direct analysis of individual NPs. This allows one to measure the unique behavior of certain NPs that would be missed in ensemble measurements. It also provides direct analysis of the NPs in their native electrochemical environment. One drawback is that only electrochemical data are available, and there are uncertainties on what is actually colliding with the electrode to give the signal and what the nature of the NPe electrode interaction is. Combined optical and electrochemical measurements are beginning to appear at the single NP level that will allow correlations between specific NP structural features and the electrochemical response [76]. It is likely that these types of experiments will increase in the future. Improved time response in the future will also aid in a better understanding of NPeelectrode interactions. Improvements in instrumentation will be needed to detect the stripping of individual sube5-nm diameter NPs with sufficient signal-to-noise levels.

Conflict of interest statement Nothing declared.

Acknowledgements The authors gratefully acknowledge financial support from the National Science Foundation through research grant CHE-1611170.

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