Article pubs.acs.org/JPCC
Nanoparticle Capping Agent Dynamics and Electron Transfer: Polymer-Gated Oxidation of Silver Nanoparticles Eden E. L. Tanner,† Kristina Tschulik,† Romilly Tahany,† Kerstin Jurkschat,‡ Christopher Batchelor-McAuley,*,† and Richard G. Compton*,† †
Department of Chemistry, Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, United Kingdom ‡ Department of Materials, Oxford University Begbroke Science Park, Sandy Lane, Yarnton, Oxford OX5 1PF, United Kingdom S Supporting Information *
ABSTRACT: Capping agent-controlled stability of nanoparticles tailors them for different applications, but the associated particle−solvent dynamics are poorly understood. Herein, previously unseen capping agent-gated nanoparticle redox activity is observed for poly(ethylene glycol)-coated silver nanoparticles. This is revealed by stochastic nanoparticle stripping, probing one individual nanoparticle at a time, from an ensemble of surface-immobilized nanoparticles. Thus, new and previously inaccessible understanding is gained on the crucial role of capping agent dynamics on nanoparticle reactivity.
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INTRODUCTION Silver nanoparticles (Ag NPs) are used in a wide variety of contexts to improve the functionality of a range of products, from sportswear and deodorant to medical implants.1 This is due primarily to their antimicrobial properties, which are arguably amplified when compared to those of bulk silver.2 Their increased toxicity over that of bulk silver means that relatively little expensive material is required to reap the benefits of its inclusion. In larger (≥1 μm) particles or bulk silver, silver undergoes a reaction with protons and oxygen to form Ag+ and water, progressing through a hydrogen peroxide intermediate. The increased toxicity seen in nanomaterials has been attributed3 to the reaction stopping at the hydrogen peroxide intermediate (eq 1) because of the increased surface area of the nanoparticle relative to that of bulk material, which results in increased mass transport of hydrogen peroxide away from the nanoparticle surface, and it is lost from the silver surface prior to its conversion to water. O2 + 2H+(aq) + 2Ag(s) ⇋ H 2O2 (aq) + 2Ag +(aq)
persistence and a greater opportunity for the uptake of nanosilver into biological systems. The stability of the NP is governed largely by the choice of the capping agent,12 which is added to prevent aggregation and agglomeration of the nanoparticles. One of the other advantages of this capping agent control is that it provides the opportunity to customize the NP for an application. Two of the most common capping agents are citrate and poly(ethylene glycol) (PEG) (Figure 1). Capping agents work to stabilize the
Figure 1. Capping agents examined in this study: citrate and poly(ethylene glycol) (PEG).
(1)
silver core in two primary ways, electrostatic and steric, with citrate capping agents being a typical example of electrostatic stabilization and PEG stabilizing the NP sterically. Citratecapped NPs are frequently used for a variety of applications, while PEG modification is particularly valued in contexts with high ionic strength. Specifically, PEG-capped NPs have been
With widespread use of Ag NPs also comes release into the environment, particularly through waterways, where their presence has a still yet unknown effect on biological systems.4−6 Typically, Ag NPs are expected to have a low environmental persistence, as dissolution or agglomeration/aggregation and subsequent sedimentation is assumed to occur on relatively short time scales.7 However, recent successful efforts to stabilize NPs in complex suspensions, especially those of high ionic strength, such as cell environments,8 seawater,9 blood,10 and stomach acid, 11 will result in increased environmental © XXXX American Chemical Society
Received: June 17, 2015 Revised: July 17, 2015
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DOI: 10.1021/acs.jpcc.5b05789 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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sample of the respective additive. The experimental samples were prepared by, in the case of the citrate NPs, mixing a 0.5 mL aliquot of commercial NPs with a 20 mM solution up to a 3 mL volume and, in the case of the PEG NPs, diluting 1 mL of the NPs to a 2 mL total volume. In all cases, the absorption was recorded from 800 to 250 nm. UV−vis spectrophotometry in RTIL was carried out on a Thermo Scientific Evolution 60 using 50 μL quartz cells. [Bmim][BF4] was used as a reference in all cases, with the experimental cell containing a solution of either citrate- or PEG-capped NPs in RTIL. These solutions were prepared by combining 32 μL of NPs solvated in water with 80 μL of [Bmim][BF4] and placing under vacuum overnight before being pipetted into the cell for analysis. The absorption was recorded from 250 to 800 nm. DLS. Dynamic light scattering (DLS) was used to measure the hydrodynamic diameter of nanoparticles capped in both PEG and citrate. DLS measurements were performed using a Malvern Zetasizer Nano ZS instrument, with disposable solvent-resistant microcuvettes. The sample was filtered before being placed in a cuvette, using a sterile Whatman 0.2 μm filter to remove any dust particles or other large contaminants. Before any measurements were taken, the sample was equilibrated at 25 °C inside the DLS machine for 2 min, and then test measurements of the attenuation of light through the sample were taken in order to ensure that the sample was of an appropriate concentration for accurate sizing measurements from light scattering to be obtained; dilution was not necessary. Three sets of 12 light scattering measurements were taken using 570 nm light and averaged to give three size distributions for the nanoparticles; the data was output by Malvern Zetasizer software. All three distributions were found to be in reasonable agreement, and an average size distribution was generated from the results. TEM. To confirm the silver particle sizes, experiments were carried out using a JEOL 2010 analytical transmission electron microscope (TEM; Herts, U.K.), which comprises a LaB6 electron gun and can be operated at accelerating voltages of between 80 and 200 kV. The TEM samples were prepared by depositing a drop of the silver nanoparticle suspension (as received from the supplier) on a carbon-coated copper TEM grid and left to dry at room temperature for several hours before examination in the TEM. Figure 3 shows the TEM images of the spherical silver nanoparticles as an inset. ImageJ software developed at the National Institutes of Health, USA
used in medical applications, which has been proposed to allow for the targeted delivery of drugs.13 Although it is known that the alteration of the capping agent can dramatically affect the nanoparticlesize, shape, and interaction with the solvent effects of the capping agent on microscopic solution dynamics are still rarely understood. Electrochemical analysis has been shown to provide insight into the properties of individual nanoparticles, allowing the sizing of individual particles,14 the estimation of aggregation15 and electrostatic effects,16 and the measurement of concentrations of NPs in various complex media.9 A recent publication by Toh et al.17 observed reactivity differences of NPs in water, but no new insight into the dynamics of the different capping agents in that aqueous medium was gained. To examine the effect of the capping agent on the particle−solution dynamics, extending existing approaches to examine NPs in an entirely different, high-ionic strength solvent such as room-temperature ionic liquids (RTILs) is highly advantageous. RTILs are made up of a bulky, asymmetric cation and an inorganic anion18 and are molten at room temperature.19 They are excellent solvents for examining electrochemical reactions due to their wide electrochemical windows20 and ability to be tailored to this purpose by altering the ionic components.21 The RTIL used in this study is 1-butyl-3-methylimizolium tetrafluoroborate ([Bmim][BF4]) (Figure 2), chosen for its ease of synthesis and hydrophilicity.
Figure 2. RTIL used in this study: 1-butyl-3-methylimizolium tetrafluoroborate ([Bmim][BF4]).
In this article, we first analyze nanoparticles with two different capping agents, citrate and PEG, of the same nominal core size and establish reactivity differences of each with acid using UV−vis spectrophotometry. The NPs are then electrochemically assessed, and an ionic liquid is used to explore ensembles of NPs using both dropcast and in situ modification methods. It is found that PEG-coated NPs show never-before seen stochastic stripping of individual NPs even from a surfaceimmobilized NP ensemble. A polymer-gated mechanism, unobserved in the citrate-capped case, is proposed to explain the oxidation of the silver PEG nanoparticles.
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EXPERIMENTAL SECTION Chemical Reagents. Sodium nitrate (>99.5%) and concentrated nitric acid (>70%) were provided by Fischer Scientific, Loughborough, U.K. and used as received. Silver nanoparticles, capped with PEG and citrate (nominally 30 nm diameter, NanoComposix, San Diego) were used as received. Ultrapure water from Millipore with a resistivity of not less than 18.2 MΩ cm at 25 °C was used to prepare all aqueous solutions. [Bmim][BF4] was prepared according to standard literature methods.21 UV−Vis. UV−vis spectrophotometry in water was carried out on a Hitachi U-2001 spectrophotometer with quartz cells. A reference contained either water or, in the case of samples containing 20 mM sodium nitrate or nitric acid, a 20 mM
Figure 3. Dynamic light scattering analysis of citrate-capped silver nanoparticles (red) with PEG-capped silver nanoparticles (black) used in this study. Insets: TEM images of citrate- and PEG-capped NPs. B
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nanoparticles using voltammetry and chronoamperometry was performed. Sizing. Transmission electron microscopy (TEM) was first used to size the silver core of the nanoparticles and returned values of 29.2 ± 4.2 nm for the citrate-capped NPs (109 particles analyzed) and 19.2 ± 4.0 nm for the PEG-capped NPs (43 particles analyzed). (Histograms can be found in Figure S1 in the Supporting Information.) The TEM sizing thus confirms that the silver cores of the NPs are of similar sizes. Due to the low atomic number and hence the low TEM contrast of the capping agent, the size of the capping agent cannot be properly resolved by TEM. Thus, to establish the extent of the capping agent layer surrounding the core, DLS was undertaken to measure the hydrodynamic diameter of the particles (their overall size). This analysis revealed that the hydrodynamic diameter of the NPs differed significantly, with the size of the citrate-capped nanoparticles calculated as ca. 35 (+40, −20) nm, and the PEG-capped nanoparticles as ca. 70 (+80, −33) nm. Figure 3 visually displays a 2-fold-larger hydrodynamic diameter of the PEG-capped nanoparticles (red) compared to that of the citrate-capped NPs (black), revealing the much increased thickness of the PEG over that of the citrate capping layer. The TEM images (inset in Figure 3) corroborate this, with the distance between the nanoparticles being twice as great in the case of the PEG-capped NPs. Reactivity toward Oxidative Dissolution. To establish the relative NP reactivities for the different capping agents, an acid reactivity test, whereby the NPs react with protons in the presence of oxygen to produce hydrogen peroxide as elucidated in the Introduction, was undertaken and monitored by UV−vis spectrophotometry. UV−vis absorption spectrophotometry was chosen as the preferred method for this time-dependent analysis due to the presence of a surface plasmon resonance absorption peak that appears at ca. 400 nm for the silver NP size range under study. Changes in the size of this peak enable the monitoring of changes in the population of nanoparticles relative to the control sample in the absence of acid. Nitric acid, to a 20 mM concentration, was added to two samples of citrateand PEG-capped Ag NPs. The maximum absorption over time was then monitored, and the normalized and baseline-corrected maximum absorbance over time is shown in Figure 4, alongside examples of the plasmon peak in the absence of acid for both the citrate- and PEG-capped NPs, which was found to be unaltered over the experimental time scales used. (The normalized maximum absorbance over time was calculated by dividing the experimental absorption by the maximum absorption in the absence of acid.) In the 20 mM nitric acid
was used to size the Ag citrate- and Ag PEG-capped nanoparticles recorded on the TEM images. The size distribution for the citrate-capped NPs is found in the SI in Figure S1, while the size distribution for the PEG-capped NPs is found in Figure 9 in the text. Electrochemical Apparatus and Sample Preparation. Electrochemical experiments (cyclic voltammetry and chronoamperometry) were conducted using a low-noise potentiostat, which was built in-house. This potentiostat comprises three main sections; the first is an analog-to-digital converter within the computer interface, the second is a current amplifier circuit, and the third is a stabilized potentiostat. The computer interface utilized a Labjack U6 (Labjack Corporation, Lakewood, CO, USA) with a Labjack tickDAC. A standard USB was used to connect to the Labjack but with the ground isolated from that of the PC (USB-ISO OLIMEX, Farnell, Leeds, U.K.). A script was written in Python 2.7 and run through the IDE Canopy (Enthought, Austin, TX, USA) to control the Labjack. The current at the working electrode (running to ground) was measured with an LCA-4K-1G low-current amplifier (FEMTO, Messtechnik GmbH, Germany), and the bandwidth of the output of the current amplifier was limited using a 100 Hz twopole passive RC filter, Linear Technology DC338A-B (Farnell, Leeds, U.K.). The Labjack digitized, at a stream rate of 4 kHz, the resulting analog signal, which was oversampled. A highly stabilized (1 kHz bandwidth) classic adder potentiostat was used to provide potentiostatic control. To maintain control of the reference electrode, a high-quality LMC6001 operational amplifier (Farnell, Leeds, U.K.) with ultra-low-input bias (25fA) was used. Control of the potential at the counter electrode was maintained by a high-quality, low-noise AD797 operational amplifier (Farnell, Leeds, U.K.). All experiments were conducted inside a temperature-controlled Faraday cage.22 For the dropcasting experiments, 2 μL of commercial NP solution was pipetted onto a glassy carbon macroelectrode, which was polished prior to use with white lapping pads with diamond spray (3.0, 1.0, and 0.1 μm, 5 min on each size from Kemet, Kent, U.K.). A plastic pipet tip was cut to serve as a reservoir, to which 50 μL of RTIL was added. A silver wire was used as a reference electrode, and a platinum wire was used as a counter electrode. The working carbon microdisc electrode (IJ Cambria Scientific Ltd., U.K.), 10 μm nominal diameter, was polished prior to use using a water−alumina slurry (1, 0.3, and 0.05 μm, 5 min on each grade) on soft lapping pads (Buehler, IL, USA).23 A 0.5 mm silver wire was used both as a counter and as a quasi-reference electrode. Solutions were prepared by mixing 8 μL of silver nanoparticle solution as received with 20 μL of RTIL and placed under a 0.2 mbar vacuum overnight to remove residual water. A solution containing 10 μL of the aforementioned stock was placed in a plastic collar fixed on top of the working electrode, and a T-cell was used for structural stability, as described previously.24
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RESULTS AND DISCUSSION Initially, physical characterization of the commercially sourced nominally 30-nm-diameter nanoparticles (NPs), in the form of TEM and DLS analysis and stability experiments using UV−vis spectrophotometry, was performed on silver nanoparticles capped with both citrate and PEG in an aqueous solution. The transfer of both citrate- and PEG-capped NPs into a RTIL from an aqueous stock was attempted; subsequently, electrochemical analysis of the successfully transferred PEG-capped silver
Figure 4. (a) Normalized maximum peak absorbance with time for citrate-capped silver nanoparticles with 20 mM nitric acid (red circles) and PEG-capped silver nanoparticles with 20 mM nitric acid (black squares). (b) Plasmon peaks of citrate (red)- and PEG (black)-capped NPs in the absence of acid. C
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The Journal of Physical Chemistry C solution, the citrate-capped nanoparticles degrade very quickly, with a two-thirds loss of absorption signal after 5 min and nearly complete loss after 60 min, indicating the NP dissolution. In contrast, the signal corresponding to the PEGcapped NPs showed very slow degradation, decaying an equivalent amount only after 24 h. Complete disappearance of the absorption peak at 400 nm for the PEG-capped NPs occurred after 5 days, representing a dissolution rate ca. 100 times slower than that seen in the citrate-capped NPs. To ensure the degradation observed with the citrate-capped NPs was not an ionic-strength-induced agglomeration or aggregation, an equivalent timed experiment was performed with 20 mM sodium nitrate (Figure S2 in the Supporting Information), where the peak observed at 400 nm remained present after 60 min, demonstrating that the degradation of the signal is a result of higher reactivity, that is, NP dissolution, and is unrelated to the stability of the NP. Note that in this context “stability” means that the NPs are resistant to aggregation and agglomeration, while “reactivity” relates to their proton- and oxygen-driven chemical dissolution to hydrogen peroxide (eq 1). Further evidence for the decrease in the absorbance being due to the dissolution of the Ag NPs instead of their association is that the baseline of the scan did not increase over time (as would be observed for large NP aggregates due to light scattering). This experiment thus suggests that in 20 mM nitric acid, the citrate-capped NPs are approximately 2 orders of magnitude more reactive than the PEG-capped NPs. As outlined in the Introduction, this lack of reactivity in the case of the PEG-capped NPs might have major implications in its persistence in various liquid media, and it is hence illuminating to study the PEG-capped nanoparticles in a different medium to better understand the role of this capping agent in terms of solvent−particle interaction. RTILs are ideal solvents for examining the effects of the capping agent on the microscale and nanoscale, as they allow new insights to be gained into how a solvent with charged components interacts with the capping agent material and allow verification that NPs can be studied electrochemically in an ionic solvent. Reactivity Differences in Ionic Liquids. Electrochemical NP studies performed previously in water provided limited insight into the effect of the PEG coating on the reactivity of Ag NPs.17 Therefore, electrochemical analysis was undertaken in [Bmim][BF4] in order to explore whether PEG-related reactivity differences can be studied in a suitable alternative solvent. Initially, PEG-coated Ag NPs were dropcast on a glassy carbon electrode and submerged in [Bmim][BF4] and a cyclic voltammogram (CV) was measured from 0 to 1.5 V vs a silver wire (blue line, Figure 5). The CV shows two features: the first, at ca. 0.1 V, is most likely trace chloride in the RTIL reacting with Ag to form AgCl,25 and the second feature at 0.65 V is attributed to the oxidation of Ag to Ag+(aq). A subsequent second scan did not show a stripping peak (Figure S3, SI). Dropcasting the capping agent material alone gave voltammetry without any stripping features, indicating the electrochemical inertness of PEG under the experimental conditions. This evidences the first successful electrochemical stripping of dropcast Ag NPs in RTIL (Figure 5, shown in blue). However, as this ex situ NP immobilization technique is known to cause significant alteration of NPs upon evaporation of the solvent26,27 and the effect of this drying on the capping agent is unknown, an in situ surface modification of NPs on the electrode is preferred. The successful use of this has been
Figure 5. Cyclic voltammetry: dropcast NPs on glassy carbon (blue) and in situ modification (20 min immersion time) both in the absence of NPs (black) and with PEG-capped silver NPs in [Bmim][BF4] (red) on a carbon fiber microelectrode.
reported in aqueous media28,29 and is herein adapted for RTILs. Transfer into Ionic Liquid. In order to undertake reactivity studies of NPs in RTILs, transfer of the NPs into the new solvent needs to be achieved, a protocol hitherto unreported in the literature. To this end, the transfer of NPs capped in both citrate and PEG into an RTIL was attempted. The method chosen for NP transfer was adapted from a commonly used method30 for transferring materials into RTILs, whereby the material of interest, dissolved in a volatile solvent, is mixed thoroughly with an RTIL that is highly miscible with that solvent and then exposed to vacuum overnight to remove the volatile solvent. Both samples of Ag NPs were originally solvated in water, hence the ionic liquid chosen was 1-butyl-3methylimidazolium tetrafluoroborate ([Bmim][BF4]), selected for its ease of synthesis and high water miscibility. UV−vis analysis was performed on samples of both citrate- and PEGcapped silver nanoparticles solvated in [Bmim][BF4] to verify the successful transfer. The absence of a clear plasmon peak associated with the Ag nanoparticles in Figure 6 and the overall increase in the background, suggesting that agglomeration has occurred, demonstrate that this transfer method was unsuccessful in the case of the citrate-capped NPs. This is consistent with the expected screening of the ligand charge in high-ionic-strength solvents, which according to the DVLO theory destabilizes the NPs, causing their aggregation. In contrast, the expected Ag plasmon peak at ca. 400 nm, as shown in Figure 6, is seen in the case of the PEG-capped NPs, illustrating that the vacuum-transfer method was successful and possibly emphasizing steric over electrostatic stabilization in a high-ionic-strength environment. On the basis of this, in situ modification studies with PEGcoated Ag nanoparticles were undertaken for the first time in RTILs. In Situ Modification. Initially, to confirm that any electrochemical observations are due to the presence of NPs, an electrode was submerged in a blank aliquot of [Bmim][BF4] for 10 min and a CV was recorded, sweeping the potential from 0 to 1.5 V (vs a silver wire). As the voltammetry of Figure 5 shows in black, in the blank RTIL, no cyclic voltammetric features were observed. In contrast, after 10 min of immersion D
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Figure 6. UV−vis spectrum of citrate-capped silver nanoparticles (red) and PEG-capped silver nanoparticles (black) solvated in [Bmim][BF4].
Figure 7. Frequency of impacts vs time at open circuit in between scans (black squares) and predicted cumulative number of impacts according to a steady-state approximation (red). The inset shows a representative chronoamperogram, both with no nanoparticles present and with nanoparticles present after a 10 min wait, offset by 0.1 nA.
of the electrode in an RTIL at open circuit potential with silver nanoparticles suspended in the RTIL (red), sharp spikes in current were observed once a potential of 1 V was applied. These features were detected only at potentials more positive than 0.6 V on both the forward and reverse scans. These observations allow us to surmise, first, that the features observed are the result of the presence of nanoparticles in the solution. Second, the switching on and off of the features in a potential-dependent way evidences that the “jumps” in current are related to a Faradaic reaction due to the oxidation of the Ag NPs. Third, the noncontinuous spikelike current features, herein referred to as stochastic stripping, with durations on the order of 10 ms, are distinct from the continuous, smooth, and singular stripping peak observed in aqueous solvents31 or from dropcast PEG Ag NPs in RTIL, suggesting a more complex stripping mechanism of PEGcapped NPs in RTILs. There are at least two possible options for the origin of this stochastic stripping. The first is a “nanoimpact”32,33 process whereby nanoparticles diffuse to the surface of the electrode and then immediately undergo a redox reaction when they arrive at the electrode. The second is a stochastic stripping whereby NPs diffuse to and adsorb at the electrode to form an ensemble, with each individual nanoparticle undergoing a redox reaction once a sufficiently high potential is applied. To distinguish between the two, the time between successive oxidative scans can be altered. In the former case, there is no trend expected, whereas in the latter case, an increased number of oxidative features with an increased waiting time between scans is expected. To explore which mechanism may have led to the stochastic stripping features seen in Figure 5, time-dependent chronoamperometry was performed by holding the potential at 1 V (vs a silver wire) for 50 s. Following this, the length of in situ modification was systematically varied before applying a potential of 1 V. Herein the modification time denotes the duration the electrode was immersed in the NP suspension under open circuit conditions before stripping. Time Dependency. Figure 7 shows the number of individual spikes in current observed experimentally with different in situ modification times. It shows that with increased time between scans, the number of oxidative events also increased, demonstrating that the process is time-dependent, indicating
that the first mechanism proposed above, so-called nanoimpacts, is unlikely to be responsible for the results shown, suggesting that there is an assembly of NPs arranged at the electrode surface that are stripped stochastically upon applying a sufficiently oxidative potential. If each spike in current is assumed to be the result of one complete NP undergoing oxidation (vide infra), then the frequency of spikes with varied in situ modification time establishes if the NPs are adsorbing. Taking a given size for the radius of the nanoparticles from DLS sizing above (36 nm), the diffusion coefficient can be estimated from the Stokes−Einstein equation,34 giving a value of 6.6 × 10−14 m2 s−1, which then allows the estimation of the theoretical cumulative number of impacts per minute (red squares, Figure 7) using a steady-state approximation, which is an approximation of flux to a disc surface,35 shown in eq 2, where N is the number of impacts, NA is the Avogadro constant, D is the diffusion coefficient as calculated above, C is the NP bulk concentration (ca. 80 pM), r0 is the radius of the electrode, and t is the time elapsed.
N = 4NADCr0t
(2)
An uncertainty was estimated in the theoretical value from the calculation of the number of impacts expected within the mean ±1σ standard deviation of the DLS sizing, with 19 nm representing the lower boundary (resulting in a diffusion coefficient of 1.25 × 10−13 m2 s−1 and ca. 7 impacts/min) and 75 nm representing the upper boundary (giving a diffusion coefficient of 3.2 × 10−14 m2 s−1 and ca. 4 impacts/min predicted). The experimental error is given by the standard deviation of the mean, calculated for both the 10 and 1 min time points, which was extrapolated as a percentage across the other time points. The close agreement of the experimental number of oxidative features per scan with the predicted cumulative number indicates that diffusion-limited NP adsorption occurs throughout the waiting time with a sticking coefficient of ca. 1, causing an in situ modification of the electrode with NPs at open circuit potential. (The sticking coefficient is herein defined as the proportion of particles that are immobilized at the surface with respect to the number that arrive at the electrode surface) A typical adsorption-stripping E
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The Journal of Physical Chemistry C mechanism, with a preaccumulation with waiting time and a simultaneous reaction once a potential is applied, is inconsistent with the appearance of spikes in current in the chronoamperogram (as shown in the inset of Figure 7 at 10 min at open circuit) as we would expect the oxidation of silver to occur rapidly after the application of a sufficiently positive potential. However, this observation can be understood in light of capping-agent-gated NP stripping, evidencing the crucial effect of the PEG ligand to NP stripping as will be described below. To explore any possible thermodynamic effect of the capping agent on the oxidation and to further confirm that the features are due to the oxidation of silver, a study considering the relationship between the charge (obtained by integrating each current feature over time) with the potential was undertaken. Potential Dependency. Chronoamperograms at 12 potentials within the range of 0 to 1.1 V were measured, and a time of 10 min was fixed as the in situ modification time prior to each scan. The same characteristic spike features as before were observed at all potentials greater than 0.6 V. Figure 8 displays
Figure 9. Sizing histogram of the electrochemical sizing method (red) compared to TEM sizing (black); the inset is an image from TEM sizing.
TEM, in black. The histograms are in excellent agreement with one another. The average radius of the impacts is calculated from impacts as 22.0 ± 9.2 nm, which is the same within uncertainty with the average radius from TEM sizing of 19.2 ± 4.0 nm. This close agreement strongly suggests that each feature represents a complete oxidation of a single silver NP. Note that this is the first experiment to demonstrate the ability to accurately size nanoparticles in an ionic liquid using an electrochemical sizing method. Proposed Mechanism for Stochastic Stripping. In previous experiments conducted in aqueous media, stochastic features that allowed the sizing of individual nanoparticles were observed only during individual diffusional nanoparticle impacts at microelectrodes. During nanoimpact experiments in aqueous media, individual nanoparticles travel via Brownian motion, and as they make contact with the electrode surface, they react almost instantaneously, undergoing electron transfer. In contrast, the stochastic features observed in both the chronoamperogram and the voltammogram in RTIL do not require that individual NPs arrive at the electrode at any given time, but out of an ensemble of NPs immobilized on the electrode surface, individual NPs can become electroactive and, following this, undergo a redox reaction. The following mechanism is suggested to describe this observation.
Figure 8. Average charge of individual oxidative current features at different potentials (blue) compared to a CV of Ag PEG NPs dropcast on a glassy carbon electrode (black) and blank RTIL with a bare GC (red).
the average charge of every current feature at varying potentials from 0 to 1.1 V. The charge per feature is negligible at potentials close to 0 V, increasing to a maximum at potentials greater than or equal to 0.6 V. These observations are consistent with a dynamic process involving a partial unwrapping and wrapping of the polymer coating, prior to the oxidation of the silver core as elucidated below. However, first, to further confirm that it is the complete oxidation of individual silver NPs that is being seen in the chronoamperometry, the charge measured per feature was converted into a radius. Sizing of Nanoparticles. Each spike was charge integrated, and we applied Faraday’s law to provide the number of atoms, assuming the exchange of one electron per silver atom, knowing the density of silver, and finally assuming that the particles are spherical (an assumption that holds on the basis of the TEM images), converting the charge to a radius.17 All features from 0.8 to 1.1 V were included in the electrochemical sizing method due to their equivalence in average charge, as illustrated in Figure 8. Figure 9 shows a sizing histogram of the electrochemical sizing, in red, compared to the sizing from
ka
kox
NPdeactive ⇄ NPactive → NPox kd
(3)
The above polymer-gated mechanism may involve a reversible partial unwrapping process to activate the nanoparticle, followed by an irreversible oxidation of the activated nanoparticle. This relationship supports the notion that the polymer capping agent gates the redox reaction occurring at the nanoparticle. If the capping agent were involved in a change in the thermodynamics of the process, then it would be expected that this switching-on potential would be altered relative to the dropcast CV seen in Figure 8, shifting it closer to zero whereby the process is made thermodynamically more favorable than silver alone or shifting it to a higher potential whereby this process is made more burdensome due to the increased overpotential as the energy barrier to undergo the oxidation becomes higher. However, what is seen is that the average F
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strated to allow for up until now inaccessible insights into the effect of nanoparticle capping agents on electron-transfer dynamics.
charge increased to a maximum at 0.6 V, the same maximum observed in the dropcasting CV, suggesting that the capping agent effect is not thermodynamic but is consistent with the polymer-gated mechanism proposed above. To assist with further validation of this mechanism, a count of features was performed with an equal number of NPs on the electrode at various potentials to determine whether there is a relationship between the presence of spikes in the current and the stage (beginning, middle, or end) of the 50 s chronoamperogram. (This was attempted through using equal immersion times but cannot be proven numerically due to capacitative charging at short times.) The recorded current− time responses are shown in Figure 10. The number of features
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ASSOCIATED CONTENT
S Supporting Information *
Histogram from TEM sizing of citrate-capped silver nanoparticles and PEG-capped silver nanoparticles. UV−vis spectra of citrate-capped Ag NPs in 20 mM nitric acid, PEG-capped Ag NPs in 20 mM nitric acid, and citrate-capped Ag NPs in 20 mM sodium nitrate. CV of Ag PEG NPs dropcast on a glassy carbon electrode in RTIL. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpcc.5b05789.
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AUTHOR INFORMATION
Corresponding Authors
*Phone: +44(0) 1865 275957. Fax: +44 (0) 1865 275410. Email:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest. E.E.L.T. and K.T. are joint first authors. Figure 10. Chronoamperograms recorded after 10 min of in situ modification time: 0 V (black), 0.2 V (blue), 0.7 V (red), and 1.1 V (green). The inset shows the number of features with the time of the experiments undertaken at increasing potentials: 0.2 V (blue), 0.7 V (red), and 1.1 V (green).
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ACKNOWLEDGMENTS
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REFERENCES
The research leading to these results has received partial funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/20072013)/ERC Grant Agreement no. 320403. K.T. was supported by a Marie Curie Intra European Fellowship within the Seventh European Community Framework Programme (grant no. 327706). E.E.L.T. thanks the Clarendon Fund and St. John’s College for funding.
according to the potential were then analyzed, with lower (blue), moderate (red), and higher (green) potentials chosen (inset of Figure 10). With increasing potential, the number of features decrease and tend toward occurring at shorter times after applying a potential in the chronoamperogram. The representative chronoamperograms shown in Figure 10 at each potential reveal that the high count in the lower potentials is due to smaller features, corresponding to incomplete oxidation (in agreement with the charge per spike as shown in Figure 8). These results tentatively support the hypothesis of a polymergated mechanism, involving an unwrapping activation of the nanoparticle being in competition with the electron-transfer process, as elucidated in eq 3. The presence of features less than 0.025 pC in size at potentials lower than 0.6 V suggests that there is insufficient overpotential to ensure complete oxidation within the lifetime of the activated (unwrapped) species. At potentials higher than 0.6 V, a constant stripping charge is observed, in agreement with the full oxidation of particles once they are unwrapped. This process is therefore limited by the activation of the particle, as evidenced in the oxidation occurring within shorter times after the potential step.
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CONCLUSIONS The oxidation of only one individual silver nanoparticle at a time was detected via sharp individual current spikes and explained by a polymer-gated redox mechanism. This mechanism involves the dynamic equilibrium between a wrapped and unwrapped nanoparticle, allowing charge transfer to the nanoparticle to occur only in the latter, activated state. This first observation of the stochastic stripping of individual nanoparticles from a surface-immobilized ensemble is demonG
DOI: 10.1021/acs.jpcc.5b05789 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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