Single Palladium Nanoparticle Collisions Detection through

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Single Palladium Nanoparticle Collisions detection through chronopotentiometric method: Introducing a New approach to improve the Analytical Signals Naser Daryanavard, and Hamid R. Zare Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b01362 • Publication Date (Web): 26 Jul 2017 Downloaded from http://pubs.acs.org on July 26, 2017

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Analytical Chemistry

Single Palladium Nanoparticle Collisions Detection through Chronopotentiometric Method: Introducing a New Approach to Improve the Analytical Signals

Naser Daryanavard, Hamid R. Zare*

Department of Chemistry, Faculty of Science, Yazd University, Yazd, 89195-741, Iran *

Corresponding author: Fax: +98 35 38210991 E-mail address: [email protected]

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Abstract

In the present research, the chronopotentiometric method and hydrazine, as a suitable probe, were used to detect single Pd nanoparticle (Pd-NP) collisions to the surface of a carbon fiber ultra-microelectrode (CFUME). The change in the potential, which is due to the electrocatalytic oxidation of hydrazine exactly at the time of Pd-NP collision to the CFUME surface, was used to detect each collide. It was shown that the amplitude and the frequency of the potential steps, produced through the nanoparticles collisions at the CFUME surface, are respectively proportional to their radius and concentration in an analytical solution. For the first time, a new approach is introduced for extraction of current-time plots (chronoamperograms) from experimental potential-time plots (chronopotentiograms). It is demonstrated that the signal-to-noise ratio (S/N) increases significantly based on the proposed method. Also, by using the chronoamperograms that resulted from the experimental chronopotentiograms, higher number of collisions are achievable and, thus, the collision frequency, f, increases and the limit of detection decreases. Interestingly, the collision frequency

resulted

from

the

chronoamperograms,

that

has

been

derived

from

chronopotentiograms, is closer to the collision frequency calculated by using the theoretical model.

Keywords:

Palladium

nanoparticles,

Single

collisions,

Distribution

Chronopotentiometry, Signal amplification, Carbon fiber ultra-microelectrode


histogram,

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Metal nanoparticles have a long history in terms of preparation, characterization, and applications. Characterization of metal nanoparticles (MNPs) has introduced new applications for them in various fields of practice such as catalysis,1,2 biology and medicine.3,4 Many properties of nanoparticles arise from their large surface area-to-volume ratio. Electrochemists immobilized MNPs on an inert electrode to make a new electrode and use it in electrocatalytic determination of organic3 and inorganic materials6,7 as well as fuel cells.8 There are a variety of techniques to determine the size of MNPs and their size distribution, such as electron microscopy, scanning probe microscopy, UV-visible spectroscopy, transmission electron microscopy (TEM), surface plasma resonance, mass spectrometry, dynamic light scattering, and X-ray absorption spectroscopy (XRD and EXAFS). Among these, transmission electron microscopy (TEM) is common and widely used to determine the size of MNPs with a diameter range of a few nanometers. Recently, electrochemical methods have been used to characterize metallic and nonmetallic nanoparticles and estimate their size distribution.9,10 These methods are based on detection of the collision of nanoparticles on the surface of electrodes. Experimental data obtained from nanoparticle collision reflects the complex interaction of various physical effects, which significantly include the mass transport of particle, its charge transfer specifications at the electrode, and the effect of the employed measurement equipment. Understanding the effect of these factors is important to describe the experimental data. In recent years a number of reviews has been published in the field of nano impact.11,12 Scientists have used several methods to observe the impact of nanoparticles on the electrode surface. These methods include electrocatalytic current amplification (ECA),13,14 potentiometric method,15 anodic particle coulometry (APC),16 cathodic particle coulometry (CPC),10 and blocking method.17 Beside determining the size distribution of nanoparticles, these methods are able to estimate their surface activity.18 ECA is the most frequently used method to study nanoparticle collisions.13,19 It was used to detect PtCl62- in a femtomolar concentration through nucleation of Pt ions and electrocatalytic reduction of H+ by the 3 ACS Paragon Plus Environment


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produced Pt clusters at the surface of a 10-µm carbon fiber electrode.20 In the chronopotentiometric method, the OCP of the working electrode changes during nanoparticle collisions in the presence of an appropriate reagent. Chronopotentiometry is the most recent and sensitive method in comparison to other methods.21 In CPC and APC methods, current transient signals are produced by the direct electrochemical redox reaction of nanoparticles themselves. This technique has been used to detect silver nanoparticles.22 This kind of impact signal is known as a blocking signal. In this case, nanoparticles collision is detected by monitoring the blocking of the diffusion of a redox mediator to UME substrates.23 In recent years, detection of nanoparticles collision as well as production of current transient signals has been extended to many other applications. For example, the method is applied for ultrasensitive detection of single DNA molecules and antibodies,24 single enzyme molecules,25 detection of nanoparticles collision by electrogenerated chemiluminescence amplification26 and evaluation of kinetic parameters for single nanoparticles.27 Also, when the nanoparticles are injected into the solution, it is possible that aggregation or/and agglomeration occurs for the injected nanoparticle. The processes of aggregation and agglomeration are an irreversible and reversible processes, respectively. There are several techniques to investigate above processes such as dynamic light scattering (DLS), inductivity coupled plasma mass spectroscopy (ICP-MS), X-ray photoelectron spectroscopy and nanoparticle tracking analysis (NTA). In recent years, the impact processes has been used to study the aggregation and agglomeration of nanoparticles and the effect of solution ionic strength.28,29 In this study, the chronopotentiometric method was used to detect single palladium nanoparticles (Pd-NPs) using hydrazine as an indicator. The potential steps that are produced by nanoparticle collisions represent the size distribution of the nanoparticles, and the number of collisions is directly proportional to the Pd-NPs concentration. Also, for the first time, chronoamperometric data were extracted from experimental chronopotentiometric data. It 4 ACS Paragon Plus Environment

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was demonstrated that the analytical signals obtained from the chronoamperograms were significantly improved.

EXPERIMENTAL SECTION Instruments and Materials. A potentiostat Versa state 4 (from Princeton Applied Research) was used for all the electrochemical measurements All the electrochemical experiments were done with a conventional three-electrode system including a carbon fiber working ultramicroelectrode (UME), a platinum wire auxiliary electrode, and an Ag/AgCl/KCl (sat’d) reference electrode placed in a Faraday cage. The pH measurements were carried out with a Metrohm model 691 pH/mV meter. All the measurements were made at room temperature. All the potentials were reported with respect to the Ag/AgCl/KCl (sat’d) reference electrode. Pd(NO3)2 (40% Pd), sodium borohydride (NaBH4), trisodium citrate, and sulfuric acid (H2SO4) were purchased from Merck Company. TEM and SEM images were taken using Philips CM30 and Phenom ProX Netherland instruments, respectively. All the experimental solutions were prepared using double distilled water. Also, the applied eight µm carbon fibers were purchased from Goodfellow Company.

Pd Nanoparticles Synthesis. Palladium nanoparticles (Pd-NPs) were synthesized according to the literature.30 Briefly, Pd-NPs were synthesized by dissolving 0.009 g of Pd(NO3)2 in 50.0 ml of double distilled water. Then, 0.5 ml trisodium citrate 1% w/w in water and 0.2 ml NaBH4 1% w/w in water were added to the solution in a sequence under stirring for 30 min until Pd-NPs were formed. With the formation of Pd-NPs, the color of the solution was changed from yellow to black. The TEM analysis indicates the size of palladium nanoparticles (Pd-NPs) is in the range of 2 to 4 nm.



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CFUME Preparation and Electrodeposition of Pd-NPs on its Surface. To prepare a carbon fiber ultra-microelectrode (CFUME), 2 cm of an 8 µm carbon fiber was rinsed with ethanol and double distilled water and sealed in a glass tube using some epoxy adhesive. To make an electrical connection between the carbon fiber and a cooper wire, graphite powder was used. The prepared CFUME was polished with 0.3 and 0.05 µm alumina powder water suspension to obtain a mirror surface. The Pd-NPs were electrodeposited at the CFUME surface with the applied potential of -200.0 mV in a solution of 1.0 M H2SO4 containing 0.05 mM Pd(NO3)2 for 30 min.31 The SEM images of CFUME and Pd-NPs-CFUME are shown as Figure S1 in supporting information. A comparison of these Figures indicates the formation of Pd-NPs at the CFUME surface.

Recording the Collision of the Pd-NPs. The chronopotentiometry method was used to detect single Pd-NP collisions at the CFUME surface in the presence of hydrazine as a probe. The data acquisition rate and current step of the galvanostat were set at 5 ms and 300 pA. With applying this amount of the current step, there is no significant faradaic current related to hydrazine oxidation at the CFUME surface. Then, a few microliters of the freshly prepared Pd-NPs were injected into the analyte solution, and the solution was bubbled with N2 for 10 s. Nitrogen gas was used to distribute the nanoparticles homogenously in the solution.

RESULTS AND DISCUSSION

The open-circuit potential, OCP, of the working electrode demonstrated its potential with respect to the reference electrode where no external current existed in the electrochemical cell.15 Figure 1A shows the cyclic voltammogram of the prepared CFUME in a 10.0 mM phosphate buffer solution, PBS, (pH 7.0) containing 6.0 mM of K3Fe(CN)6. This Figure


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shows a steady state voltammogram. This voltammogram was used to estimate the performance of the prepared CFUME.32 Figure 1B shows the chronopotentiograms of the CFUME and the Pd-NPs modified CFUME (Pd-NPs-CFUME). Based on this Figure, at the surface of the Pd-NPs-CFUME, the oxidation of hydrazine starts at a more negative potential than that in the case of CFUME. A comparison of the oxidation potentials of hydrazine at the Pd-NPs-CFUME and CFUME surfaces indicates that the OCP difference of hydrazine redox reaction at the surfaces of these two electrodes is about 670 mV. Scheme 1A shows how a single nanoparticle collision at the CFUME surface is detectable based on the OCP changes of a suitable probe. When a Pd-NP collides with the CFUME surface, the anodic current increases due to the oxidation of hydrazine and the perturbation of dynamic charge equilibrium. It is noted that the oxidation overpotential of hydrazine in the presence of PdNPs is significantly decreased; therefore, hydrazine can be used as a suitable probe for detection of Pd-NP collisions. As Scheme 1A-I shows, if Pd-NP is far from the CFUME surface, the anodic and the cathodic currents are equal at the applied potential of E1. In other words, under these conditions, the net current is zero and OCP is E1. However, as indicated in Scheme 1A-II, when a single Pd-NP collides at the CFUME surface, under the potential of E1, the anodic current increases while the cathodic current decreases. Therefore, it is necessary for the potential of the working electrode to change to the E2 value to have a zero net current, as shown in Scheme 1A-III. In fact, E2 is the OCP value of the hydrazine redox reaction in the presence of Pd-NP at the surface of the CFUME. Scheme 1A-IV shows the OCP of the CFUME after Pd-NP, which is collided, away from the electrode surface. It has to be mentioned that Scheme 1A-IV is absolutely the same as Scheme 1A-I. Therefore, it is expected that a blip response is formed when a single Pd-NP collides with the CFUME surface and separates. Scheme 1B shows the OCP change which is due to the single Pd-NP collision with the CFUME surface as depicted in Scheme 1A. Figure 2A provides the chronopotentiogram of the CFUME in a 10.0 mM PBS (pH 7.0) containing 10.0 mM 7 ACS Paragon Plus Environment


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hydrazine in the absence of Pd-NPs. According to the inset of Figure 2A, the noise value is about 0.3 mV. For this reason, a potential step that is equal or more than 1.0 mV is considered as a signal produced through the collision of a single Pd-NP. Figures 2B-F shows the chronopotentiograms of the solutions containing different concentrations of Pd-NPs. These chronopotentiograms suggest that, with an increase in the Pd-NPs concentration, the number of collisions is increased too. Therefore, it is logical to conclude that the potential steps are due to the collision of single Pd-NPs with the CFUME surface. In addition, a comparison of voltammograms of Figure 2 indicates that the analytical signals size related to different concentrations of Pd-NPs is not similar. It is noted that during the different concentrations injection of Pd-NPs, the rate of injection and stirring rate of the solution are not quite identical. Therefore, the probability of nanoparticle aggregation in different injections related to different concentrations of nanoparticles is not exactly same. Consequently, the produced collision signals at different Pd-NPs concentrations have a difference. Figure 3A shows the variation in the number of Pd-NP collisions at the CFUME surface versus the Pd-NP concentration, which is taken from the data in Figures 2B-F. It is well understood from Figure 3A that, with an increase of the concentration of Pd-NPs, the number of collisions increases too. Figure 3B presents the distribution histogram of the potential changes which has been drawn using 304 of detected impacts. The Figure shows the highest frequency related to the potential changes of 2 mV to 4 mV. The number of impacts for the solution of 56.2 pM PdNPs during the chronopotentiometric experiment (1000 s) was 52. This concentration of palladium nanoparticles was used to calculate the collision frequency. Therefore, the collision frequency was found to be about 0.001 pM-1 s-1. Figure 4 shows the TEM image (Figure 4A) and the distribution histogram of the Pd-NPs size (Figure 4B) which is drawn based on the data of the TEM image. According to Figure 4B, the average size of the Pd-NPs was 2.98 ± 0.83nm. A comparison of Figures 3B and 4B indicates that the potential change of 2 mV to 4 mV is related to the collision of Pd-NPs with a diameter of 2-4 nm at the CFUME surface in 8 ACS Paragon Plus Environment

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the presence of hydrazine. In other words, the experimental results reported in Figure 3 are confirmed by the TEM data given in Figure 4. Figure 5 shows the chronoamperogram of the solution used for recording the chronopotentiogram in Figure 2F. A comparison of Figures 2F and 5 indicates that the signal-to-noise ratio (S/N) resulting from the Pd-NP collisions at the CFUME surface in the chronopotentiometric method is higher than that in the chronoamperometry. In addition, as described below, the number of signals related to nanoparticles collision that distinguishable from the extracted chronoamperograms are more than those obtained from the chronopotentiograms. Sensitivity is defined as the slope of variations the number of collision against Pd-NPs concentration at a specified interval time. Consequently, if the current-time data (i.e. chronoamperometric data) are extracted from the experimental potential-time data (i.e. chronopotentiometric data), the sensitivity of the chronoamperograms is expected to be significantly improved. The following discussion clarifies how it is possible to extract a current-time plot (chronoamperogram) from a potential-time plot (chronopotentiogram). Figure 6A shows the linear sweep voltammogram of the CFUME in a PBS 0.01 M (pH 7.0) containing 10.0 mM hydrazine. As it can be seen, the faradaic current related to hydrazine oxidation at the CFUME surface is negligible if the applied potential is less positive than 0.3 V. Also, as suggested by Figure 6B, for a small potential range, which equal or even more than the observed potential steps of Figure 2, the variation of the current against the potential is linear. Considering Figure 1 and the above discussions, it is logical to conclude that the typical plots of the current versus the potential at the CFUME surface and a single Pd-NP attached to the CFUME in the presence of hydrazine are like voltammograms (a) and (b or c) in Figure 6C respectively. Voltammogram (c) is corresponded to a palladium nanoparticle with the larger size in comparison with voltammogram (b). A comparison of the three voltammograms indicates that, at a single Pd-NP attached to the CFUME surface (b and c), the current is increased, which is due to hydrazine oxidation at the surface of that Pd-NP. In 9 ACS Paragon Plus Environment


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addition, the current is larger at the case of (c) as compared with (b), because the attached palladium nanoparticle is larger, therefore, the electrocatalytic current related to hydrazine oxidation is more.32 In the other words, larger nanoparticles produce more impact current. Figure 6D shows that, in the chronopotentiometric method with an applied current of i1 at the surface of the CFUME, the potential of E1 is needed. In chronopotentiometric method under constant current of i1 when two single Pd-NPs with different sizes collides at the surface of the CFUME (case b and c), the potential of the electrode are changed to E2 or E3 which is dependent on the Pd-NP size. Accordingly, larger nanoparticles produce more changing in potential. In other words, the potential change from E1 to E3 is related to the collision of a larger Pd-NP in comparison with the potential change from E1 to E2. Based on above discussion regarding Figure 6, the potential changes from E1 to E2 or E1 to E3 are corresponding with the current changes from i1 to i2 or i1 to i3. Therefore, for chronoamperometry method with applied potential E1 to the CFUME surface, it is expected during collision single Pd-NP with two different sizes, the current of i1 changes to i2 or i3. It is noted that the value of i2 or i3 are dependent to the single Pd-NP size. Also, as shown in Figure 6D, i2 or i3 are the currents corresponding with the potentials of E2 or E3. These potentials (E2 or E3), in the chronopotentiometric method with applied current i1, are obtained when the two single Pd-NPs with different sizes collide to the CFUME surface. Consequently, the current i2 or i3 can obtain from the corresponding potentials (E2 or E3) based on the following equation.

= where >

(1)

During the collision of Pd-NPs at the CFUME surface, Eq. (1) can be used to obtain the expected current ( ), from the chronopotentiograms of Figure 2, when potential is applied. In other word, based on Eq. 1 and the chronopotentiograms of Figure 2, the corresponding chronoamperograms can be drawn. Figure 7 shows the chronoamperograms derived from the 10 ACS Paragon Plus Environment

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chronopotentiograms of Figure 2. As it can be seen, the S/N ratio grows significantly, and a horizontal baseline is obtained. Based on Eq. (2), the nanoparticles diameter and nanoparticles size distribution can be determined using the extracted transient currents.33 = 4ln 2

(2)

In the above equation, i is the transient current corresponding to the nanoparticle impact, D and C are respectively the diffusion coefficient and the concentration of hydrazine that serves as a probe, r is the radius of a single Pd-NP, n is the total number of electrons transferred in the oxidation/reduction reaction of the probe, and F is the Faraday constant. As previously explained, in the chronopotentiometric method, when the single Pd-NP impacts at the CFUME surface, it is essential the potential decrease to value of E2 (less positive) until the applied current is constant. This reduction in potential in chronopotentiometric method has being used as the analytical signal. In the following, when the nanoparticle moves away from the electrode surface, the electrode potential changes from E2 to E1 again. Later potential changing (potential changing from E2 to E1) is due to the nanoparticle getting away from the electrode surface. In the proposed model, only the currents corresponding with the potential changes at the moment of nanoparticle collision are considered as the analytical signal and their corresponding currents are calculated as shown in chronoamperograms of Figure 7. In other words, for drawing the chronoamperograms, the current changings related to the potential changings which are due to the nanoparticles getting away from the electrode surface are not considered. In addition, in the proposed model, by multiplying the ratio of

potential changing during nanoparticle collision in the applied constant current (i1), the

resulted analytical signals in chronoamperograms are amplified. This amplifying in analytical signals increases the assay sensitivity. Moreover, amplifying of the analytical signals causes the tiny potential changing due to collision of very small nanoparticles, which are not detectable in chronopotentiograms, are appeared in the chronoamperograms. Therefore, it is 11 ACS Paragon Plus Environment


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expected that the numbers of detectable collisions in the extracted chronoamperograms are more than those observable in the experimental chronopotentiograms. Hence, it is logical that the collision frequency and sensitivity obtained from the chronoamperograms are improved. Also, in the proposed model, if in some collisions, the nanoparticle before returning to the bulk solution re-enter to electron transport layer (tunneling layer) or move on different routes in this layer and create small signals, they are also reinforced.34 It is interesting to note that comparing data of chronoamperograms of Figure 5 and Figure 7-f shows that the magnitude of observed impact currents from experimental chronoamperograms are in good agreement with the value of impact currents resulted from extracted chronoamperogram. By using the chronoamperograms that resulted from the experimental chronopotentiograms, higher number of collisions are achievable and, thus, the collision frequency, f, increases and the limit of detection decreases. It should be noted that the collision frequency, f, resulting from the chronopotentiograms is significantly less than the value derived from Eq. (3):35 = 4

(3)

In Eq. (3), f is the collision frequency, D and C are respectively the diffusion coefficient and the concentration of nanoparticles, and r is the radius of the electrode. Interestingly, the collision frequency resulted from the extracted chronoamperograms, that has been derived from chronopotentiograms, is closer to the collision frequency calculated by using Eq. (3). The lower value of the collision frequency resulting from the experimental data (i.e. chronoamperograms and chronopotentiograms), in comparison to the value calculated from Eq. (3), can be caused by the following factors: a) Aggregation of small nanoparticles and formation of large nanoparticles causes a decrease in the nanoparticles concentration and collision frequency. b) Limitation in bandwidth of amplifier.36 c) Some nanoparticles might adhere to the cell wall or the precipitate. 12 ACS Paragon Plus Environment

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d) Due to the small size of nanoparticles or their deficient collision, very small transient currents are undetectable because of instrumental limitation. This limitation can be rectified using the i-t data extracted from the E-t plots as described above.

CONCLUSIONS

In this study, it was shown that the oxidation overpotential of hydrazine in the presence of PdNPs is significantly decreased, and, therefore, hydrazine can be used as a suitable probe for detection of single Pd-NP collisions. The detection is made once a blip response is observed through the collision and separation of a single Pd-NP at the surface of a CFUME. The potential steps produced by nanoparticle collisions represent the size distribution of nanoparticles, and the number of collisions is directly proportional to the Pd-NPs concentration. In addition, the distribution histogram of the Pd-NPs size taken from chronopotentiograms is confirmed by the TEM data. Also, for the first time, chronoamperometric data were extracted from experimental chronopotentiometric data. It was shown, when such a data extraction occurs, the analytical signals obtained from chronoamperograms are significantly improved. In addition, using the chronoamperograms that resulted from the experimental chronopotentiograms, a higher number of collisions are detectable; consequently, the collision frequency, f, increases while the limit of detection decreases.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:



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Scanning electron microscopy (SEM) image of carbon fiber ultramicroelectrode (CFUME) and carbon fiber ultramicroelectrode modified with palladium nanoparticles (Pd-NPsCFUME) are provided as noted in the text.


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Legends of Schemes and Figures:

Scheme 1. (A) Schematic representation of the chronopotentiometric response monitoring during the collision of single Pd-NPs and getting away from the CFUME surface. (B) Polarization plots at the CFUME surface and at the time of single Pd-NP collisions to the CFUME surface in the presence of a suitable probe. Figure 1. (A) Cyclic voltammogram of the fabricated CFUME with a diameter of 8 µm in a 10.0 mM PBS (pH 7.0) containing 6.0 mM K3Fe(CN)6. The potential scan rate was 20 mV s1

. (B) Chronopotentiogram of CFUME and Pd-NP-modified CFUME in a 10.0 mM PBS (pH

7.0) containing 10.0 mM hydrazine. The applied current was 300 pA. Figure 2. Chronopotentiogram of CFUME in a 10.0 mM PBS (pH 7.0) containing 10.0 mM hydrazine in the presence of (A-F) 0.0, 37.50, 42.50, 46.90, 51.90 and 56.20 pM Pd-NPs respectively. The applied current was 300 pA. Figure 3. (A) Plot of the collision counts vs. the concentration of Pd-NPs in the solution. (B) Statistical distribution of the collision counts vs. the potential steps which is drawn using 304 detected impacts. Figure 4. (A) TEM image and (B) distribution histogram of Pd-NPs size drawn based on the data of the TEM image. Figure 5. Chronoamperogram recorded at the CFUME surface in a 10.0 mM PBS (pH 7.0) containing 10.0 mM hydrazine and 56.2 pM Pd-NPs. Figure 6. (A) Linear sweep voltammogram of the CFUME in a 0.01 M PBS (pH 7.0) containing 10.0 mM hydrazine, (B) magnification of the same plot for a small potential range, (C) typical plots of current vs. potential at the surface of (a) a CFUME and (b,c) a single PdNP with two different sizes attached to the CFUME surface in the presence of hydrazine, and (D) magnification of the same plot for a small potential range.


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Figure 7. Chronoamperograms derived from the chronopotentiograms of Figure 2. The current data were calculated based on Eq. (1). Labels of A, B, C, D, E and F are related to 0.0, 37.50, 42.50, 46.90, 51.90 and 56.20 pM Pd-NPs, respectively.



Scheme 1

(A) Pd-NPs solution

Pd-NPs solution Pd-NP

Pd-NP

icathodic

icathodic

ianodic

inet = 0

inet = 0 E1

CFUME surface

a

c

ianodic CFUME surface

OCP of CFUME after

before Pd-NP collision

Pd-NP collision

E (+)

OCP of CFUME (I) Pd-NPs solution

( IV ) Pd-NPs solution

E2

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Pd-NP

icathodic

b

icathodic

Pd-NP

ianodic

inet = 0

inet ≠ 0 Time (s)

CFUME surface

ianodic

CFUME surface

icathodic and ianodic of CFUME

OCP of CFUME at the time

at the time of Pd- NP collision at E1 potential

of Pd-NP collision

( II )

(III)

(B)

Cathodic branch

Anodic branch

(icathodic)

(ianodic)

inet │i│

Polarization plots at the time of single Pd-NP collisions to the CFUME surface in the presence of a probe

Polarization plots at the CFUME surface in the presence of a probe

∆(OCP)

E2

E1


E (+)

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Figure 1



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Figure 2


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Figure 3



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Figure 4


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Figure 5

1,250

Current (pA)

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750

250 0

500

1000

Elapsed time (s)



45

Current (nA)

Figure 6

(A)

20

0

0.6

0.8

(B)

R² = 0.9946

0.5

0.2 0.21

-5 1.2

0.235

0.26

Potential (V)

i1 i2 i3 Current (nA)

Potential (V)

Current (nA)

Current (nA)

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(C) cb a

(D) c b a

E3

Potential (V)


E2

E1

Potential (V)

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Figure 7



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Single Palladium Nanoparticle Collisions Detection through

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