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Correlated Anodic–Cathodic Nanocollision Events Reveal Redox Behaviors of Single Silver Nanoparticles Mahmoud Elsayed Hafez, Hui Ma, Yue-Yi Peng, Wei Ma, and Yi-Tao Long J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b01369 • Publication Date (Web): 29 May 2019 Downloaded from http://pubs.acs.org on May 31, 2019

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The Journal of Physical Chemistry Letters

Correlated Anodic–Cathodic Nanocollision Events Reveal Redox Behaviors of Single Silver Nanoparticles Mahmoud Elsayed Hafez,†,‡ Hui Ma,† Yue-Yi Peng,† Wei Ma,*,† and Yi-Tao Long*,† †Key

Laboratory for Advanced Materials & Department of Chemistry, East China University of

Science and Technology, Shanghai 200237, P. R. China. ‡Department

of Chemistry, Faculty of Science, Beni-Suef University, Beni-Suef, 62511, Egypt.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

ACS Paragon Plus Environment

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ABSTRACT: We reported a novel method for the first time to real-time monitor the redox behaviors of single Ag nanoparticles (NPs) at a Au ultramicroelectrode between oxidizing and reducing pulse potentials using nanocollision electrochemical method. At fast pulse potentials, the instantaneous anodic–cathodic current transients of a single AgNP were observed for the electro-oxidation of AgNP followed by the electro-reduction of the newborn silver-oxide (AgO) NP in alkaline media via switching of redox potentials; however, only anodic oxidation signals of individual AgNPs were observed in neutral solution. Through this study, we have revealed the substantial different dynamic nanocollision electrochemical behaviors of single AgNPs on the electrode surface in various media. Our study offers a new view for clearly clarifying in situ tracking of the electron transfer process of single NPs by correlating electrochemical oxidation and reduction behaviors with the complementary information.

TOC GRAPHICS

KEYWORDS Single nanoparticle, Stochastic collision measurement, Pulse potential, Anodic– cathodic behavior, Reaction medium strategy


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Single entity collision electrochemical measurement provides new insight into the fundamental features and dynamic charge transfer properties of freely diffusing individual entities which is different from the traditional ensemble averaging studies.1 This stochastic collision electrochemistry has been successfully employed to investigate various of electrochemical processes of an individual, including NP,2–4 liposome,5 vesicle,6 emulsion,7 virus,8 and biological macromolecule,9 etc. The direct electrochemical detection of single-particle collisions is a rapidly developing field that strongly establishing itself as a powerful tool for the characterization of individual NPs.10 Among the most studied NPs, AgNPs have received the intensive attentions owing not only to their applications in many fields (electronics, sensors, catalysis, etc.),11–13 but also to their biological activity and environmental effect.14–16 Since the pioneer work of Compton’s group on single AgNPs electrochemistry,17 many related researches have been developed for the electrochemical size measurement of AgNPs via anodic particle coulometric analysis due to the destructive process of AgNPs dissolution.18–20 With the improvement of high-resolution electrochemical instruments using the optimal instrumental acquisition parameters to minimize the electronics effects on the measurement results, the dynamic electro-oxidation behaviors of individual AgNPs has been divulged out for a series of “discrete” sub-events, corresponding to the sequential partial stripping events during the random multi-collision processes of NP-ultramicroelectrode (UME).21–26 As demonstrated, by adjusting the dynamic movement of the particle adjacent to the electrode interface using a variable temperature nanocollision study, the partial versus complete oxidation processes of different sizes of AgNPs can be rationalized.27 Despite the significant progress in collision-based electrochemical measurements, the substantial studies for a clear understanding of the realistic dynamic behaviors and mechanisms of individual NPs at the electrode/solution interface have


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been quite limited27 because of the complex electrochemical reaction of NPs in various media environment.26,28 For instance, the dissolution mechanism of AgNPs is driven not only by their oxidation but also by the chemical environment owing to high affinity of Ag+ ions with different anions.29,30 Thus, exploring myriads of single NPs collision dynamics at an UME surface, including reaction kinetics and chemical or phase transformation, need more valuable electron transfer information at the same time by in situ electrochemical measurements. To resolve these situations in a practical system, complementary techniques have been combined with the stochastic collision electrochemical measurement to generate a full picture of single NP electrochemical behaviors with sufficient information.3,24,31 Recently, the simultaneous electrochemical measurement and optical visualization have emerged to assist the identification of the electrochemical collision behaviors of individual NPs on an UME. However, the fast diffusional motion of NPs and the nanoscale interfacial area at the electrode/solution interface make it very challenging if not impossible for optical-based methods with sufficient spatiotemporal resolutions. Identifying these limitations, correlating redox reaction of single NPs is a good strategy to clearly clarify the electron transfer processes using nanocollision electrochemical measurements coupling with their advantages of high temporal resolution and sensitivity.32 Herein, we introduced a novel method for the first time to investigate the electrochemical oxidation and reduction responses of a single AgNP in alkaline media using anodic–cathodic nanocollision events (Figure 1). By combining the potential control method and the reaction environment strategy, our results demonstrated that fast pulsing experiment allowed the detection of the electro-oxidation of a single AgNP followed by an instantaneous electroreduction of the newborn AgO NP in alkaline media. It was found that only anodic current


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transient events were observed at both oxidizing and reducing potentials in neutral solution, suggesting that Ag+ ions cannot be collected by electrochemical reduction.

Figure 1. Schematic illustration of anodic–cathodic electrochemical redox behaviors of a single AgNP at 12.5 µm Au UME in alkaline media by fast switching pulse potential between oxidizing (+600 mV vs. Ag/AgCl) and reducing (-200 mV vs. Ag/AgCl) potentials, respectively. AgNP is oxidized to Ag+ ions and reacted with OH- ions, shown as green pentagonal, to produce the newborn AgO NPs. The electrochemical behaviors of AgNPs were firstly investigated by cyclic voltammograms using a drop casted 40 nm AgNPs modified gold electrode (diameter of 2 mm) over a range of +0.7 V to -0.7 V vs. Ag/AgCl wire at a scan rate of 50 mV s-1 in both neutral solution and alkaline media. As expected, a pair of redox wave was observed in 20 mM phosphate buffer (PB) of pH 7.4, suggesting the formation of Ag+ ions and its reverse reaction (black color, Figure 2). In a pH 11.4 of alkaline media containing 10 mM PB and 10 mM NaOH, three anodic peaks (A1, A2 and A3) and two cathodic peaks (C1 and C2) were apparent (red color, Figure 2), indicating different electrochemical redox behaviors compared with the result in a neutral system. According to the previous work,29,33,34 the anodic peak A1 attributed to the electro-


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dissolution of Ag to [Ag(OH)2]- and the anodic peak A2 ascribed to the nucleation and crystal growth of Ag2O as shown in Equation 1 and 2: Ag + 2OH ― ↔[Ag(OH)2] ― + e ― 2Ag + 2OH ― ↔Ag2O + H2O + 2e ―

(1) (2)

Moreover, the anodic peak A3 attributed to further electrochemical oxidation of AgNPs and formation of AgO NPs by Equation 3: Ag + 2OH ― ↔AgO + H2O + 2e ―

(3)

Figure 2. cyclic voltammograms of 40 nm AgNPs modified Au electrode (diameter 2 mm) in neutral solution (20 mM PB, pH 7.4, black) and in alkaline media (10 mM PB and 10 mM NaOH, pH 11.4, red) over a range of +0.7 V to -0.7 V vs. Ag/AgCl wire. The scan rate is 50 mV s-1. While the cathodic peaks C1 and C2 represented the reverse reactions in Equation 2 and 3, indicating electrochemical reduction of Ag2O and AgO, respectively. The cyclic voltammetric results demonstrated that the presence of NaOH preceded a reaction of Ag+ ions, resulting from the oxidation of AgNPs, with OH- ions to allow the formation of [Ag(OH)2]-. Furthermore, [Ag(OH)2]- tended for undergoing further oxidation to produce Ag2O and AgO.34,35


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Inspired by the cyclic voltammetry results, we performed a stochastic collision electrochemical experiment to investigate the redox process of single AgNPs with the diameter of 40 nm in alkaline media at both oxidizing and reducing potentials. Significant anodic collision events of individual AgNPs were observed in a 4 s pulsing amperometric current-time curve at an oxidizing potential of +600 mV vs. Ag/AgCl; however, a smooth amperometric curve was observed when the applied potential was switched to the reducing potential of -200 mV vs. Ag/AgCl (Figure S1). We proposed that the AgO NPs as the product resulting from the electrooxidation of AgNPs in alkaline media may diffuse away from the surface of the Au UME to the bulk solution, decreasing their possibility to diffuse back to the electrode surface during the long timescale of 4 s. Therefore, fast pulse potential experiments were designed to record in situ redox events through rapidly switching the oxidizing and reducing potentials while measuring the current. Figure 3b(i) showed a 0.6 s section of current-time curve of individual 40 nm AgNPs at a pulsing frequency 10 Hz in alkaline media. Anodic current transients were still observed when Au UME was at an oxidizing potential of +600 mV vs. Ag/AgCl. Interestingly, switching the holding potential at Au UME electrode to a reducing potential of -200 mV vs. Ag/AgCl allowed tracking the cathodic collision behavior of the produced AgO NP. These obtained anodic– cathodic current transients were attributed to the fact of the formation of newborn AgO NPs during single AgNPs collision at the Au UME held at +600 mV vs. Ag/AgCl in alkaline system rich of hydroxide ions. Moreover, fast switch pulse potentials at ultrashort timescale of 0.1 s guaranteed the rapid capturing of the newborn AgO NPs due to the slower diffusion rate of particles than ions, resulting in high probability to be present in the tunneling region of the electrode. Typically, a spike with an undulating terrain was observed for the electro-oxidation of single AgNPs in alkaline solution, yielding a current height of 272.9 ± 4.8 pA and a duration


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Figure 3. Anodic–cathodic nanocollision redox behaviors of individual AgNPs with different sizes of a diameter 20 nm (a), 40 nm (b), and 60 nm (c) in alkaline media. (i) Current–time curves represent the electrochemical redox behaviors of Ag/AgO NPs at a Au UME at fast pulse frequency of 10 Hz by switching the potential between oxidizing potential (+600 mV vs. Ag/AgCl) and reducing potential (-200 mV vs. Ag/AgCl). The rectangles show close-ups of the representative time-resolved current traces corresponding to the oxidative and reductive spikes marked with the same legend in (i). (ii) Histograms show the distributions of the peak current height, the duration time, and the integrated charge for anodic (grey) and cathodic (red) collision events of individual AgNPs. Black and red curves show Gaussian statistics fits. Data were recorded in alkaline media (10 mM PB and 10 mM NaOH, pH 11.4) containing 30 pM AgNPs at a Au UME.


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time of 18.4 ± 0.5 ms (grey, Figure 3b(ii)). Compared with the electrochemical results in neutral solution, the dwell time was almost increased to tens of times in alkaline media (Figure S2). This is because that the adsorptive interactions between the AgO NP and the Au UME significantly enhanced, thus resulting in localizing AgO NPs near the surface of the electrode.36 Additionally, the integrated charge was found to be 0.34 ± 0.03 pC (grey, Figure 3b(ii)) that was almost 2 times from the results in neutral solution (Figure S3b). This is consistent with our hypotheses of a 2e- oxidation occurred in the process of Ag oxidation, corresponding to the AgO formation at +600 mV vs. Ag/AgCl in alkaline media. However, the cathodic peaks of AgO NPs showed a current height of 144.5 ± 3.4 pA and a much shorter duration time of 5.1 ± 0.2 ms (red, Figure 3b(ii)). The substantial difference in duration time of individual anodic–cathodic collision events resulted from different motion behaviors of NPs because of different interaction of Ag/AgO NPs with the Au UME surface at switching redox potentials.24 This observation indicated the newborn particle receded from the electrode surface to the bulk solution during the electrochemical reduction process of AgO NPs, resulting in incomplete reduction of AgO NPs. Indeed, it is worthy of note that the cathodic integrated charge was 0.11 ± 0.01 pC, which was one-third of that compared with the anodic measurement results (Figure 3b(ii)). To further elucidate individual AgNPs collision behaviors at a Au UME, we investigated the anodic–cathodic events of different sized AgNPs. As expected, we successfully obtained the anodic and the cathodic current transients for both 20 nm and 60 nm diameter of AgNPs at fast pulse potentials of 10 Hz frequency (Figure 3a(i) and 3c(i)). For smaller particles, 20 nm AgNPs showed a short tailed anodic trace with a current height of 112.6 ± 3.5 pA and a duration time of 0.9 ± 0.1 ms (grey, Figure 3a(ii)), while the cathodic single peak events yielded a current height of 65.9 ± 2.6 pA and a duration time of 0.7 ± 0.1 ms (red, Figure 3a(ii)). Interestingly, this


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observation of the integrated charge showed a complete anodic oxidation (0.04 ± 0.01 pC) for 2etransfer while the cathodic events (0.03 ± 0.01 pC) were three-fourth of that for the anodic events, revealing particles with the small size of 20 nm permitted a larger probability for the total electrochemical reduction of AgO NPs in the timescale of a single encounter. For larger sized AgNPs, typically 60 nm, a spike with an undulating terrain was still observed for anodic events with a current amplitude of 1163.9 ± 145.7 pA and a duration of 35.0 ± 4.7 ms (grey, Figure 3c(ii)). However, the significantly different current traces appeared for the cathodic events in this case, in which the amplitude was decreased (219.5 ± 6.5 pA) and the duration was shortened (6.1 ± 0.2 ms) (red, Figure 3c(ii)). Correspondingly, there was a very low reduction ratio compared with the anodic oxidation charge, approximately one-fifth as can be seen in Figure 3c(ii). This observation could be attributed to different motion trajectories of Ag/AgO NP at the Au UME surface during the anodic–cathodic collision processes, resulting in the distinct redox behaviors. Moreover, an electrostatic interaction between the charged electrode surface and the charged AgNPs may play a role in the particle collision behaviours. At the initial potential of +600 mV vs. Ag/AgCl, there is the electrostatic effects between the negatively charged AgNPs and the positive probe electrode, thus enhancing the dwell time of NPs and the observed collision frequency at the Au UME. However, the electrostatic interaction between the AgNP and the electrode will quickly be changed from strong attraction to strong repulsion as the electrostatic potential on the probe electrode changes from positive to negative.26,37 This repulsive force drove the NP away from the electrode surface leading to a quick decrease in duration time and current magnitude of the of the cathodic events of single AgO NP.38 Indeed, it is worth noting that the extremely low detection frequency was observed at the negative potential of -200 mV vs.


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Ag/AgCl in all applied models. We did find about a 3 times or greater decrease in collision frequency when switching to the negative potential. We also investigated the anodic–cathodic collision behaviors of individual 40 nm AgNPs at both oxidizing and reducing potentials in neutral solution. Obviously, the cathodic signal transients almost disappeared by switching the holding potential to -200 mV vs. Ag/AgCl in neutral solution (Figure S4), suggesting the formed Ag+ ions during the oxidation of AgNPs in neutral solution cannot be collected by nanocollision electrochemical reduction. We proposed that the Ag+ ions in neutral solution diffused away from the electrode surface even in an ultrashort time interval, which is consistent with the highly rapid diffusion of Ag+ ions to the bulk solution.26 Moreover, as the pulsing frequency decreased to 2 Hz, the repetitive redox behaviors were still obtained for further demonstrating the instantaneous anodic–cathodic current responses from the collision events of AgNPs and AgO NPs in alkaline media, respectively (Figure S5). All results demonstrated that the detection of these cathodic transients was due to the high probability of newborn AgO NPs to be near the surface of the electrode within several hundred millisecond timescale based on the enhanced interactions and the relatively high chemical stability of AgO NPs in alkaline media.39 To better understand the correlated anodic-cathodic nanocollison behaviors, we investigated a control experiment aiming to the instant detection of single newborn AgO NP upon formation through holding the Au UME at switching potential under pulsing frequency 10 Hz and concentration down to 2 pM AgNPs in alkaline media. We found that the transient signals were almost deficient in the recorded amperometric current-time curves either in electro-oxidation or in electro-reduction sides that referred to lack of collision events. This was because that the low concentration of AgNPs caused decreasing in the experimental detection frequency, which is


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consistent with the theoretical collision events frequency, f, based on AgNPs diffusion coefficient, size, and concentration, and the radius of the UME by Equation 4:40 𝑓 = 4𝐷𝑁𝑃𝐶𝑁𝑃𝑟𝑒𝑙𝑒𝑁𝐴

(4)

where CNP is the concentration of AgNPs (2 pM), rele is the radius of Au UME (6.25 µm), and NA is the Avogadro’s number constant (6.02 × 1023 mol-1). DNP is the diffusion coefficient of AgNPs, which can be determined from the Stokes-Einstein equation by Equation 5: 𝐷𝑁𝑃 =

𝑘𝐵𝑇 6𝜋𝜂𝑟𝑁𝑃

(5)

where kB is the Boltzmann constant (1.381 × 10-23 J·K-1), T is the temperature (298 K), η is the dynamic viscosity of water (8.94 × 10−4 Pa·s at 298 K), and rNP is the radius of AgNPs (20 nm). As a result, the calculated frequency was dropped to 0.3 particle/s for 2 pM AgNPs, suggesting it was very difficult to track a single NP within the time interval of 0.1 s.41 Because of ignoring the role of electrostatic effects –repulsive or attractive– on the collision frequency of the particle, the experimentally observed frequencies could slightly deviate from the theoretically calculated ones by Equation 4 within a typical variation. However, occasionally in long recorded section of a current-time curve, we can observe a cathodic transient only after an anodic oxidation collision event (Figure 4). This observation suggested that the instant cathodic collision event did correspond to the electrochemical reduction of the oxidation product of a single AgNP in alkaline media. Additionally, we further investigated the nanocollision behaviors of the switch phenomena by designing experiments in which we fixed the oxidizing potential at +600 mV vs. Ag/AgCl and the pulsing frequency at 10 Hz while the more positive reducing potential was applied at -100 mV, 0 mV and +200 mV vs. Ag/AgCl. As can be seen in Figure S6, normal anodic current transients were recorded for the electro-oxidation of individual AgNPs in our experiments while


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there was no observation of any cathodic collision signal in each case. We ascribed that shifting the applied reducing potential toward more positive led to decreasing the reducing power and therefore absence of the electrochemical reductive spikes under these potentials. As expected, switch electrochemical redox phenomena occurred only within reducing potential at more negative of -200 mV vs. Ag/AgCl, which was consistent with our cyclic voltammetric measurement results.

Figure 4. Current–time curves represent the anodic–cathodic nanocollision behaviors of individual 40 nm AgNPs in alkaline media at fast frequency of 10 Hz (left side). Right side shows close-ups of the representative time-resolved current traces corresponding to the oxidative and reductive spikes marked with same legend in the left side. Data were acquired in alkaline media with 2 pM AgNPs at pulse potential between oxidizing (+600 mV vs. Ag/AgCl) and reducing (-200 mV vs. Ag/AgCl) potentials. In summary, we demonstrated here real-time and in situ monitoring anodic-cathodic collision events of a single AgNP at a Au UME between oxidizing and reducing pulse potentials using the stochastic nanocollision electrochemical method. At fast pulse potentials, the electro-oxidation of single AgNPs followed by the electro-reduction transients because the formation of newborn AgO NPs near the Au UME surface in alkaline media led to detecting the cathodic spikes by


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switching to the reduction potential. In neutral system, we confirmed the absence of such cathodic reduction behaviors. Substantially different collision electrochemistry behaviors of single AgNP in both neutral and alkaline media ascribed to different diffusion coefficients between and Ag+ ions and AgO NP products. In this respect, our method can be extended to in situ monitoring of the chemical or phase transformation processes of individual NPs, such as electrocrystallization.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx/acs.jpclett.xxx. Experimental section and additional figures showing statistical analyses and comparisons of the parameters. AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT


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This research was supported by the National Natural Science Foundation of China (21775043, 21421004, and 21327807), the Program of Introducing Talents of Discipline to Universities (B16017), Innovation Program of Shanghai Municipal Education Commission (2017-01-07-0002-E00023), and the Fundamental Research Funds for the Central Universities (222201718001, 222201717003). REFERENCES (1)

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