Simultaneous optical and electrochemical recording ofsingle nanoparticle electrochemistry

Linlin Sun, Yimin Fang, Zhimin Li, Wei Wang (), and Hongyuan Chen ()

State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China

Received: 31 October 2016 Revised: 20 December 2016 Accepted: 26 December 2016 © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2017 KEYWORDS single nanoparticle collision, surface plasmon resonance microscopy, plasmonics-based electrochemical microscopy, Ag nanoparticles

ABSTRACT Single nanoparticle collisions have become popular for studying the electro-chemical activity of single nanoparticles by determining the transient current during stochastic collisions with the electrode surface. However, if only theelectrochemical current is measured, it remains challenging to identify andcharacterize the individual particle that is responsible for a specific current peak in a collision event; this hampers the understanding of the structure–activity relationship. Herein, we report simultaneous optical and electrochemical recordingof a single nanoparticle collision; the electrochemical signal corresponds withthe activity of a single nanoparticle, and the optical signal reveals the size andlocation of the same nanoparticle. Consequently, the structure (optical signal)–activity (electrochemical signal) relationship can be elucidated at the singlenanoparticle level; this has implications for various applications including batteries, electrocatalysts, and electrochemical sensors. In addition, our previous studieshave suggested an optical-to-electrochemical conversion model to independently calculate the electron transfer rate of single nanoparticles from the optical signal. The simultaneous optical and electrochemical recording achieved in the presentwork enables direct and quantitative validation of the optical-to-electrochemical conversion model.

1 Introduction

Recent advances in electronic, nano-fabrication, and electrochemical imaging techniques have led to significant progress in single nanoparticle electro-chemistry [1–4]; this enables the study of electrochemical reactions at the single nanoparticle level and elucidation of fundamental electron transfer at nano-scale interfaces.

More importantly, when combined with other in-situ characterization techniques, such as optical microscopy [5, 6], electron microscopy [7], and atomic force microscopy [8], the electrochemical current (activity of the nanoparticle) and morphology (structure of the nanoparticle) of the same individual nanoparticle can be measured independently; this provides the basis for a bottom-up strategy to effectively elucidate

Nano Research 2017, 10(5): 1740–1748 DOI 10.1007/s12274-017-1439-0

Address correspondence to Wei Wang, wei.wang@nju.edu.cn; Hongyuan Chen, hychen@nju.edu.cn

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the structure–activity relationship. In addition, single nanoparticle electrochemistry enables the identification of individual particles with outstanding performance [9], which would otherwise be difficult to isolate from the average signal associated with ensemble measurements.

There are two main approaches for studying single nanoparticle electrochemistry: electrical recording and optical recording. In electrical recording, the electro-chemical current is attributed to a single nanoparticle through spatial [8, 10, 11] or temporal [12, 13] separation so that only one nanoparticle is actively transferring electrons during the recording period. To spatially separate single nanoparticles, either an electro-active surface containing only one nanoparticle is fabricated [11] or the potential is locally applied using a glass pipette electrode with a tip as small as tens of nanometers [8, 10]. Bard’s group pioneered a single nanoparticle collision (SNC) approach that temporally separates individual nanoparticles by resolving their arrival times at the electrode [12, 13]. In a typical SNC experiment, a stable baseline of electrochemical current is continuously recorded by applying a constant potential to a blank electrode. In the presence of electro- active nanoparticles in the solution, the stochastic collision of a nanoparticle transiently forms an electro-active site and thereby increases the current signal. The subsequent departure or inactivation of the nanoparticle returns the current to baseline, resulting in a spike associated with the single nanoparticle. However, it is challenging to identify the location and morphology of the nanoparticle that is responsible for a spike in the current because the electrical recording lacks spatial resolution. Such correlation is critical to effectively elucidate the structure–activity relationship [14].

Optical recording has become an important alternative for studying single nanoparticle electrochemistry [15]. In this case, an optical microscopy that is capable of imaging single nanoparticles is used to monitor the optical intensity of each individual nanoparticle during electrochemical processes. To optically study single nanoparticle electrochemistry, the optical intensity of an individual nanoparticle must be quantitatively dependent on its oxidation state; this enables the generation of an optical-to-electrochemical conversion

model to calculate the electron transfer rates of single nanoparticles. So far, several optical imaging techniques, including dark-field [16, 17], fluorescence [18], Raman [19], holographic [6], and surface plasmon resonance microscopy (SPRM) [20, 21], have been adopted to resolve the electrochemistry of single nanoparticles. Among them, we are particularly interested in SPRM. First, SPRM is sensitive to the refractive index (RI) of single nanoparticles, which is an intrinsic property of all materials. Therefore, SPRM is suitable for imaging a broad variety of nanomaterials including metals [20–22], oxides [23], organic nanomaterials [23, 24], and even biological particles such as viruses [25], mitochondria [26], and bacteria [27]. Secondly, RI, or the dielectric constant, of nanomaterials is a function of its electronic structure, which is likely dependent on the oxidation state. Previous studies have shown that SPRM is capable of resolving the small difference in the RIs of Ru(NH3)62+ and its oxidized species Ru(NH3)63+ [28, 29]. For example, based on the quan-titative relationship between the volume (total number of atoms) of a single Ag nanoparticle (AgNP) and its SPRM intensity, we have shown that the electron transfer rate of a single AgNP can be determined optically with a detection limit as low as 100 fA [20]. However, only the optical signal was measured and used to calculate the electrochemical current in the previous study. This optical-to-electrochemical con-version model has yet to be experimentally validated.

Herein, we report the first quantitative validation of studying SNC using SPRM by comparing electro-chemical and optical signals that are simultaneously recorded for the same collision event. AgNPs stochastically strike the surface under a constant potential that is sufficient to induce oxidation of the AgNPs to soluble Ag+ ions. Consequently, the AgNPs shrink leading to a decreased SPRM signal from the same nanoparticle [20]. In the present work, the optical signal of a single AgNP was used to calculate its electron transfer rate, which was compared with the simultaneously recorded electrochemical current to determine the validity of the conversion. In addition, the optical signal revealed important information regarding the size and location of the collisions of each individual nanoparticle, which facilitates the investigation of the structure–activity relationship.

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2 Experimental

2.1 Materials

AgNPs were synthesized using a traditional chemical- reduction method. In the typical experiment, 50 mL of 1 mM aqueous silver nitrate was heated to boiling in a flask under vigorous stirring. After the addition of 1 mL of 1 wt.% trisodium citrate, the mixture was boiled with stirring for one hour. The flask was then removed from heat and cooled to room temperature with stirring. The average diameter of the prepared AgNPs was 61 ± 25 nm (Fig. S1(a) in the Electronic Supplementary Material (ESM)), as characterized by transmission electron microscopy (JEM-2100, JEOL). SiO2 nanoparticles were purchased from Janus New- Materials Co. as an aqueous solution. The average diameter of the SiO2 nanoparticles was 195 ± 30 nm (Fig. S1(b) in the ESM), and their zeta potential was −25 mV (Nano-ZS90, Malvern). 0.1 M KNO3 was used as the electrolyte solution throughout the work. All solutions were prepared using deionized water (DI H2O; 18.2 MΩ·cm) produced using a Smart2Pure 3 UF (Thermo Fisher).

2.2 Fabrication of Au microelectrodes

Transparent Au microelectrodes were fabricated using photolithography. A glass coverslip, which was used as the substrate of the Au microelectrode, was cleaned by DI H2O then dried under a stream of nitrogen. After pretreatment with hexamethyldisilazane for 10 min at 120 °C, the coverslip was coated with a 1.5-μm-thick positive photoresist (AZ5214) at 4,000 rpm for 30 s followed by soft baking for 90 s at 95 °C on a hot plate. The photoresist (MA-6, Karl Suss) was then aligned and exposed to UV light (365 nm) through a chrome mask for 6 s at 9.1 mW·cm−2. The chrome mask was divided into two sections for two lithography processes: One section comprised a 50 μm × 50 μm square connected to a 4 mm × 4 mm square via a 30-μm-wide band, and the other section comprised a 50-μm-wide band for SiO2 deposition. After exposure, the photoresist was baked at 110 °C for 120 s and developed for 45 s in a bath of developer (RZX-3038) to remove the photoresist of the unexposed areas (i.e., the opaque parts of the chrome mask). The glass coverslip was then coated with a 47 nm thick gold

film using high-power impulse magnetron sputtering (MSI50x6-L, GCEMarket) and stripped by sequential sonication for 30 min in acetone and isopropanol. Before proceeding to the second lithography process, a 300 nm SiO2 layer was coated onto the glass coverslip using plasma-enhanced chemical vapor deposition (PlasmaPro 100, Oxford Instruments). A negative photoresist (AZ5214) was used for the second lithography to remove the photoresist from the transparent parts. Finally, the glass coverslip was etched using reactive-ion etching (Tegal 903E) to remove SiO2 and then stripped. Thus, the 50 μm × 50 μm square (active area), 4 mm × 4 mm square (for connecting wires), and 30-μm-wide band were coated with a 47-nm-thick gold film, while only the band was coated with a 50-μm-wide SiO2 film (Fig. S2 in the ESM).

2.3 Apparatus

The SPRM setup was built on an inverted optical microscope (TIRFM, Nikon) equipped with a high numerical aperture oil immersion 60× objective (N.A. 1.49). A 680 nm super luminescent light-emitting diode light source (Q-photonics, operating power of 0.2 mW) was used as the light source. A polarizer was inserted in the optical path to generate p-polarized light to excite the surface plasmon wave at the Au film. The active area, i.e., 50 μm × 50 μm square of the Au microelectrode, was used as the working electrode and SPR sensor chip. Each gold microelectrode was rinsed with DI H2O and ethanol and dried under flowing nitrogen before use. The chip was further cleaned with a hydrogen flame to remove any remnant contaminants. A Ag/AgCl wire was used as the reference and counter electrodes in a two-electrode electrochemical system. Electrochemical experiments were performed with an Axon Multiclamp 700B amplifier and Axon Digidata 1550A digitizer (Axon Instruments). The low-pass filter bandwidth of the Axon amplifier is 20 Hz. The SPRM images were recorded using a charge-coupled device (CCD) camera (Pike F-032B, Allied Vision Technologies) with a frame rate of 100 frames per second (fps) at a pixel resolution of 320 × 240. A data acquisition card (USB-6251, National Instruments) was used to collect the camera transistor–transistor logic signal and electrical current simultaneously. The optical and electrochemical

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recordings were synchronized by extracting the current at each moment of CCD capture from data acquired with the digital acquisition card.

3 Results and discussion

As shown in the schematic in Fig. 1(a), the experimental setup consisted of an SPRM microscope for optical imaging, an electrochemical system to apply potential and record current, and a signal synchronization unit to synchronize the optical and electrical recordings. Details of the SPRM microscope have been previously published [21, 22, 30]. Briefly, a red beam (680 nm) is directed into an inverted microscope with a total internal reflection configuration. At a specific incident angle, parallel illumination towards the gold-coated coverslip results in a Krestchmann configuration, which excites the surface plasmon polaritons propagating in the gold-solution interface [30]. The reflected light is captured by a CCD camera to produce a background SPRM image. The presence of a nanoparticle on the gold film scatters the planar surface plasmon wave, leading to a characteristic parabolic pattern in the SPRM image, as shown in Fig. 1(a). The center point of the parabolic pattern (as indicated by the white arrow) represents the location of the nanoparticle. This pattern is considered to be the point spreading function of the SPRM microscope, which depends on the beam wavelength and dielectric constant of the gold film. Different nanomaterials exhibit the

same parabolic shape if they are smaller than the diffraction limit of the SPRM setup. However, the pattern intensity (image contrast) is a function of many useful factors, including the refractive index and volume of the nanoparticle. A nanoparticle with a higher RI and larger volume would exhibit higher SPRM intensity (i.e., greater contrast).

The electrochemical system includes a potentiostat and current amplifier to detect current as small as a few pA at a temporal resolution of 10 ms. The gold film serves not only as a substrate for SPRM imaging, but also as a working electrode for electrochemical recording. To minimize the charging current, a fabricated microelectrode with a size of 50 μm × 50 μm was used (Fig. S2 in the ESM). A two-electrode system was used, and the counter electrode (Ag/AgCl wire) was located close to the microelectrode in the solution. A constant potential (300 mV vs. Ag/AgCl) that is sufficient for triggering the electrochemical oxidation of AgNPs was applied to the gold film. In the absence of AgNPs, a polarization current was recorded as the baseline. After the addition of a small amount of AgNPs in the solution, individual AgNPs randomly strike and stick to the gold film due to van der Waals forces [20]. The random collisions were evidenced by the random distribution of collision locations on the gold film surface (Fig. S3 in the ESM), suggesting an unbiased sampling of SNC events. This type of collision followed by attachment has two consequences: From the electrochemical perspective,

Figure 1 (a) Schematic illustration of SPRM imaging of a single AgNP collision on a Au microelectrode. (b) Transient current spike associated with electro-oxidation of a single AgNP collision (black curve). (c) The collision initially results in increased SPRM intensity because SPR is a near-field phenomenon; this is followed by a rapid decrease in the optical signal because of oxidation-induced dissolution (blue curve).

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oxidation of the Ag nanoparticle leads to electron transfer to the gold film and an oxidative current (Fig. 1(b)). The electrochemical current rapidly returns to the baseline because of depletion of Ag atoms. From the SPRM perspective, the attachment of nanoparticles scatters the surface plasmon wave and generates a characteristic pattern in the SPRM images (Fig. 1(c), t0 → t1). Subsequent oxidation of AgNP shrinks the nanoparticle, leading to a decrease (t2) and eventual disappearance (t3) of the SPRM pattern. As a result, a peak appears in the optical intensity curve (Fig. 1(c)). Our previous results have shown that the optical intensity is proportional to the volume of AgNP, i.e., the total number of Ag atoms. Accordingly, its first-order derivative imparts the electron transfer rate, i.e., oxidative current, of a single AgNP [20]. By recording the optical and electrochemical signals simultaneously, the present work enables direct validation of the optical-to-electrochemical conversion model that was previously proposed.

The upper left image in Fig. 2(a) shows the SPRM image of the micro-electrode (50 μm × 50 μm). The light intensity on the gold film was lower because

of the reduced reflectivity at the SPR angle. The surrounding glass was much more reflective because of the total internal reflection at the glass–solution interface. The Au microelectrode was connected to the potentiostat through a gold band, which was covered with a thin layer of SiO2 to ensure that all the electron transfer occurred within the view window. The remainder of the images in Fig. 2(a) show time- lapsed SPRM snapshots during the collision and dissolution of three sequential AgNPs. The complete video is provided in the ESM (Movie S1). Note that the background SPRM image has been subtracted to emphasize the presence of a single nanoparticle. The SPRM was recorded at a rate of 100 fps. At 0.29 s, the first nanoparticle (NP1) collided at the upper right corner of the micro-electrode and dissolved in 60 ms, leading to a peak in the local SPRM intensity curve that was evident upon selecting a region of interest (ROI) within the parabolic tail (blue curve). The collisions of the other two nanoparticles occurred at 2.81 s (NP2) and 22.65 s (NP3). We also recorded the average electrochemical current over the entire micro- electrode during the same period, as shown in Fig. 2(b)

Figure 2 (a) Time-lapsed SPRM snapshots of three sequential AgNPs during collision and dissolution on a Au microelectrode. (b) Electrochemical current (top panel) and transient plasmonic image intensity curve (bottom panel) of the same three nanoparticles shown in (a). (c) Comparison between the optical current and electrochemical current of each nanoparticle.

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(top panel). The temporal resolution of the electro-chemical recording was the same as that of the optical recording (10 ms) for convenience of synchronization. Three peaks were observed at the three corresponding moments, which clearly demonstrates that electron transfer occurred during the collision. Spatial resolution of SPRM allowed us to choose three ROIs at the locations of the three collisions, leading to an SPR intensity curve for each collision, as shown in Fig. 2(b) (bottom panel). These curves reveal the quantitative information regarding the volume during the dynamic collision and dissolution process for each nanoparticle. Note that the SPR intensity curves of NP1 and NP2 were shifted up by 40 and 20 intensity units, respectively, to resolve the three curves. It was found that the optical recording exhibited a much better signal-to-noise ratio than the electrochemical recording for the same collision event, underscoring the value of studying single nanoparticle electrochemistry optically.

We further examined the optical-to-electrochemical conversion model. The theoretical current was con-sidered to be the first-order derivative of the SPR intensity, as the SPR intensity is proportional to the number of silver atoms and the current is the rate of silver atom consumption. A calibration curve was generated in our previous study to calculate the sizes of spherical AgNPs from their SPRM intensity [20]. Spherical AgNPs were chosen (λmax = 413 nm, Fig. S1(c) in the ESM) to avoid plasmonic coupling with the surface plasmon polaritons (λ = 680 nm), which com-plicates the correlation between size and SPRM intensity. Subsequently, the theoretical current (referred to as the optical current herein) of each single nano-particle was calculated using the conversion model (see S5 in the ESM for details): The results for each nanoparticle are shown in Fig. 2(c). Overlapping the theoretical current with the experimental current confirms the validity of the optical-to-electrochemical conversion. The timing and shapes of the spikes in the optical current are consistent with those in the electrochemical current. For instance, a plateau appeared at 22.75 s in the electrochemical current curve of NP3 (black arrow), and the same feature was observed in the optical current curve (red arrow).

We also noticed that the optical current spikes were consistently narrower than the electrochemical current spikes because of the broadening induced by the electronic filter (20 Hz) in the current amplifier, which highlights the importance of optical recording (see S4 in the ESM for details). The differences in the currents were primarily attributed to inaccurate calibration of the conversion from the optical intensity to the volume of the nanoparticle, particularly for anisotropic nanoparticles, due to plasmonic coupling. The influence of plasmonic coupling could be further minimized by adopting incident light with a longer wavelength in the near-infrared region (e.g., 840 nm). For dielectric nanomaterials, the SPRM intensity is determined by their size and is less sensitive to the geometry.

To examine the independence of the optical and electrochemical recordings, two control experiments were performed. The first experiment was performed in 0.1 M KNO3 at a potential of 300 mV and involved observing the collisions of electrochemically inactive SiO2 nanoparticles. Figure 3(a) shows several snapshots of the collision of three individual SiO2 nanoparticles onto the microelectrode. Figure 3(b) shows a step in the SPRM intensity curve with no peaks present in the electrical recording. In the second experiment, 1 mM Fe(CN)63− solution was injected into 0.1 M KNO3 at a potential of 300 mV. Figure 3(c) shows that the oxidative current increased immediately (top panel), but no distinct change was observed in the SPRM intensity curve (bottom panel). Note that the tiny increase in the RI of the bulk solution was too small to be detected with the present SPRM setup. The fluctuation of the curves in Fig. 3(c) resulted from the disturbance during liquid injection.

Thirty AgNP collision events were observed and analyzed to demonstrate the general applicability of this method. We examined the statistical correlation between the electrochemical and optical current at a single nanoparticle level, as shown in Fig. 4(a). The maximum values of the electrochemical and optical currents for the same single nanoparticle were found to correlate. Similarly, a positive correlation was observed between the electric quantity, i.e., integration of the current spike, and maximum SPR intensity

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(Fig. 4(b)). The former reflects the total number of Ag atoms in the AgNP, and the latter is proportional to the nanoparticle volume. The observed consistency further supports the validity of the optical-to- electrochemical conversion model.

4 Conclusions

We proposed a simultaneous optical and electro-chemical recording approach to study the electro-chemical activity of single nanoparticles. Optical

Figure 3 (a) Time-lapsed SPRM snapshots of three sequential SiO2 nanoparticles during the collision processes on a Au microelectrode at a potential of 300 mV in 0.1 M KNO3. (b) Collision of inactive SiO2 nanoparticles increases the local optical signal (bottom panel), but does not affect the electrochemical current (top panel). (c) The injection of 1 mM Fe(CN)6

3− solution into 0.1 M KNO3 significantly increases the electrode current under a potential of 300 mV, while no obvious increase was observed in the optical signal.

Figure 4 (a) Correlation between the electrochemical and optical currents of 30 individual AgNPs. (b) Relationship between the electric quantity (i.e., integration of the current spike) and the maximal SPRM intensity of the same 30 nanoparticles.

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recording provides sufficient information to quan-titatively resolve the electron transfer rate associated with a single nanoparticle using an optical-to- electrochemical conversion model, which was built on the sensitive dependence of single nanoparticle optical intensity on its volume or RI. For the first time, this model was directly and quantitatively validated by simultaneously recording the electrochemical current; this not only strengthens the theoretical basis of the SPRM-based optical-to-electrochemical conversion, but also encourages the further adoption of other optical imaging techniques to optically study nano-electrochemistry. Optical recording of single nanoparticle electrochemistry provides important information that is complementary to that obtained using traditional electrochemical recording. Spatial resolution of optical microscopy enables resolution of the contributions from each nanoparticle when two or more nanoparticles collide on the surface at the same time. In addition, the optical images reveal the size and location of each single nanoparticle collision event, allowing for combination with other in situ characterization techniques such as scanning electron microscopy. These advantages could facilitate elucidation of the structure–activity relationship of electro-active nanomaterials.

Acknowledgements

We thank financial support from the National Natural Science Foundation of China (Nos. 21327902, 21527807, 21522503, and 21327008), and the Natural Science Foundation of Jiangsu Province (Nos. BK20150013, BK20140592, and BK20150570).

Electronic Supplementary Material: Supplementary material (TEM images, further details of the conversion model and the movie of the oxidation of AgNPs) is available in the online version of this article at http:// dx.doi.org/10.1007/s12274-017-1439-0.

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