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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, [email protected];
Hongyuan Chen, [email protected]
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Research
1741 Nano Res. 2017, 10(5): 1740–1748
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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1742 Nano Res. 2017, 10(5): 1740–1748
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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1743 Nano Res. 2017, 10(5): 1740–1748
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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1744 Nano Res. 2017, 10(5): 1740–1748
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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1745 Nano Res. 2017, 10(5): 1740–1748
(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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1746 Nano Res. 2017, 10(5): 1740–1748
(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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1747 Nano Res. 2017, 10(5): 1740–1748
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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