Generation and detection of single metal nanoparticles using scanning electrochemical microscopy techniques

Article abstract of DOI:10.1021/jp064434d

Different pathways towards the generation and detection of a single metal nanoparticle (MNP) on a conductive carbon support for testing as an electrocatalyst are described. Various approaches were investigated including interparticle distance enhancement, electrochemical and mechanical tip-substrate MNP transfer onto macroscopic surfaces, scanning electrochemical microscopy (SECM)-controlled electrodepositión, and the use of selective binding monolayers on carbon fiber electrodes (CFEs) for solution-phase-selective adsorption. A novel SECM technique for electrodepositing MNPs on CFE tips immersed 100-200 nm below the electrolyte level was developed and used to generate single Pt and Ni nanoparticles. Following their generation, we demonstrate electrocatalytic detection of Fe³⁺ on individual Pt particles with the CFE in a Fe³⁺/H₂SO₄ solution. We also describe an approach of attaching MNPs to CFEs by controlling the composition of monolayers bonded to the CFE. By employing a monolayer with a low ratio of binding (e.g., 4-aminopyridine) to nonbinding molecules (e.g., aniline) and controlling the position of the CFE in a colloidal Pt solution with a SECM, we attached a single 15 nm radius Pt nanoparticle to the CFE. Such chemisorbed Pt particles exhibited a stronger adhesion on surface-modified CFEs and better mechanical stability during proton reduction than MNPs electrodeposited directly on the CFE.

Full text of DOI:10.1021/jp064434d

J. Phys. Chem. B 2006, 110, 25279-25287
25279
Generation and Detection of Single Metal Nanoparticles Using Scanning Electrochemical
Microscopy Techniques†
Ran Tel-Vered and Allen J. Bard*
Department of Chemistry and Biochemistry and the Center for Nano and Molecular Science and Technology,
The UniVersity of Texas at Austin, Austin, Texas 78712
ReceiVed: July 13, 2006; In Final Form: September 15, 2006
Different pathways towards the generation and detection of a single metal nanoparticle (MNP) on a conductive
carbon support for testing as an electrocatalyst are described. Various approaches were investigated including
interparticle distance enhancement, electrochemical and mechanical tip-substrate MNP transfer onto
macroscopic surfaces, scanning electrochemical microscopy (SECM)-controlled electrodeposition, and the
use of selective binding monolayers on carbon fiber electrodes (CFEs) for solution-phase-selective adsorption.
A novel SECM technique for electrodepositing MNPs on CFE tips immersed 100-200 nm below the electrolyte
level was developed and used to generate single Pt and Ni nanoparticles. Following their generation, we
demonstrate electrocatalytic detection of Fe3+ on individual Pt particles with the CFE in a Fe3+/H2SO4 solution.
We also describe an approach of attaching MNPs to CFEs by controlling the composition of monolayers
bonded to the CFE. By employing a monolayer with a low ratio of binding (e.g., 4-aminopyridine) to nonbinding
molecules (e.g., aniline) and controlling the position of the CFE in a colloidal Pt solution with a SECM, we
attached a single 15 nm radius Pt nanoparticle to the CFE. Such chemisorbed Pt particles exhibited a stronger
adhesion on surface-modified CFEs and better mechanical stability during proton reduction than MNPs
electrodeposited directly on the CFE.
Introduction
Shifting from traditional multiple nanoparticle synthesis to
synthesis and study of single nanoparticles can minimize these
problems. While there have been extensive studies of single
molecules, including catalytic properties of single enzymes,
usually via fluorescence methods,62 the behavior of single MNPs
as catalysts remain relatively unexplored. In single molecule
studies, the high signal amplification required is provided by
the repeated excitation and emission of a suitable stable
fluorescent label or product.63 Amplification in electrocatalysis
studies on single MNPs occurs by the repeated electron-transfer
reactions that occur at the particle but require the detection of
currents in the picoampere or sub-picoampere region. Moreover,
a large variety of electrocatalytic reactions using solution phase
reactants can be studied at a single MNP.
This paper deals with electrochemical methods of preparing
and characterizing single MNPs. While single MNPs are more
difficult to prepare compared to multiparticle collections, they
provide several advantages, which are mainly related to reduced
structural complexity. Unlike multiparticle-covered surfaces,
single supported MNPs show no size or shape distributions and
allow complex variables such as interparticle distance or particle
surface density to be omitted from the analysis. From an
electrochemical standpoint, the diffusion associated with mul-
tiple nanoparticles can be spherical, planar, or a combination
of these, depending on the extent of overlap between the
diffusion fields, which is governed by the spatial distribution
of the particles on the support and is affected by several factors,
such as the duration of the experiment. The equation for a
diffusion-controlled reaction at a single nanoparticle is simpler
showing, to a good approximation, spherical (or hemispherical)
diffusion in accord with
In recent years, metal nanoparticles (MNPs), especially Ag
and Au, have been of interest because of their unique optical,1,2
electrical,3,4 magnetic,5,6 and chemical7-12 properties, often
remarkably different from their bulk counterparts.13-15 From a
commercial standpoint noble MNPs, typically supported on high
surface area carbons, are used in many heterogeneous catalytic
and electrocatalytic processes.16-24
Of special interest is the relationship between the particle
size and the structure with the chemical or electrochemical
reactivity.25-34 Several techniques have been used to investigate
MNPs and nanostructured surfaces, including electrochemical
methods,35-39 IR spectroscopy,40-42 X-ray absorption spectros-
copy,43,44 scanning tunneling (STM) and atomic force (AFM)
microscopies,45-47 differential electrochemical mass spectrom-
etry (DEMS),48 ultrahigh vacuum (UHV) techniques, and
others.49,50 Almost all studies of MNPs have involved large
arrays or collections of particles on surfaces or dispersed in
solution. These MNPs are typically prepared by bulk methods,
e.g., collection of metal colloid particles from suspension by
controlled adsorption51-53 or electrophoresis,54 vapor deposition
of metal at low coverage onto a surface in high vacuum,55 gas-
phase reduction of metal salt particles by H2,56 microwave
irradiation,57 or electroless or pulsed potentiostatic deposi-
tion.58,59 Significant problems that are typically encountered arise
from difficulties in preparing MNPs with a narrow size
distribution, surface agglomeration, occurrence of lattice defects,
and poisoning of the metal by impurities (e.g., sulfur) present
in the substrate support.60,61
† Part of the special issue “Arthur J. Nozik Festschrift”.
* Author to whom correspondence should be addressed. E-mail: ajbard@
mail.utexas.edu.
Jlim ) nFDC∞/a
(1)
10.1021/jp064434d CCC: $33.50 © 2006 American Chemical Society
Published on Web 10/28/2006

25280 J. Phys. Chem. B, Vol. 110, No. 50, 2006
Tel-Vered and Bard
where Jlim is the diffusion-limiting current density, n is the
number of electrons transferred, F is Faraday’s constant, D is
the mediator diffusion coefficient, C∞ is the mediator concentra-
tion, and a is the nanoparticle radius.
tip. Epoxy cement was injected into the top of the capillary to
attach the Ni-Cr wire to the glass tube and hold the CFE rigid.
The glass at the tip was then sealed around the CFE using a
cool low oxygen/natural gas flame by inserting the pipet tip
into the flame for approximately 0.5 s. The part of the CFE
that protruded from the glass pipet was then flame etched in an
oxygen-rich (bright blue) flame to a length of 50 µm or less
using periodic observation by optical microscopy to monitor
the progress of etching. During this step, it was very important
to etch the CFE alone, without melting the glass seal. This was
done by a very slow manual insertion of the CFE into the flame.
An orange glow from the CFE indicated that etching was
proceeding. Typically, three flame etches were performed, each
lasting approximately 1 s. The etched carbon fiber was insulated
with anodic electrophoretic paint. The etched CFE was im-
mersed into a 1:20 solution of the paint in water, and a Pt coil
surrounding the CFE was the reference/auxiliary electrode. A
direct current potential of 2.2 V was applied to the CFE for 40
s, after which time the oxidation current had decreased to a
small steady value of 0.2-1.0 nA. The electrode assembly was
removed and heated at 150 °C for 3 min. This process has been
shown to slightly shrink the deposited paint film, exposing the
tip. The entire deposition and heating process was repeated once
more to ensure good insulation of the sides of the fibers.
Instrumentation. A CH Instruments model CHI 900B (CH
Instruments, Austin, TX) was employed for electrochemical
measurements. Scanning electron microscopy (SEM) and energy-
dispersive spectrometry (EDS) analyses were carried out using
a LEO 1530 scanning electron microscope. Because the MNP
was frequently smaller than the tip apex and sometimes located
away from the apex, the CFEs were systematically SEM-imaged
in a horizontal position and then flipped over and probed from
the other side to cover the whole surface area.
The equation demonstrates that diffusion current densities
vary as a-1, and hence for smaller particles it is possible to
study fast interfacial reactions over a broad potential range, since
the limit of diffusion is not reached even at considerable over-
potentials. In synthesizing a nanoparticle for electrocatalytic
studies, one seeks a number of desirable characteristics: (i) well-
defined and known location of the nanoparticle; (ii) minimal
electrochemical interference from the substrate; (iii) good
adhesion to the substrate (high mechanical stability); (iv)
controlled particle size and shape; (v) good contact (efficient
bidirectional electron transfer) to the substrate; (vi) good
chemical stability.
Despite these significant advantages, only a few reports have
appeared on the electrochemistry of single MNPs.64-68 Stimming
and co-workers used an in situ method to prepare and measure
the reactivity of single Pd nanoparticles on a Au support,
reporting a strong size-dependent catalytic effect toward the
proton reduction reaction.64-66 In this study, particle preparation
involved a modification of a method originally developed by
Kolb et al. taking advantage of a fast response scanning
tunneling microscope to place a single nanoparticle on a
monocrystalline gold support.69 Here we seek alternative,
simpler techniques for both the generation and the electrochemi-
cal detection of single nanoparticles on electronically conductive
substrates.
Experimental Section
Materials and Syntheses. All chemicals used were reagent
grade and used as received. All aqueous solutions were prepared
with deionized water (Milli-Q, Millipore Corp.). Dichlo-
romethane (DCM, <0.001% H2O) was obtained from Aldrich;
dimethylformamide (DMF, 0.0037% H2O) was obtained from
Merck. Highly ordered pyrolytic graphite (HOPG, grade-1) was
obtained from SPI Supplies (West Chester, PA); Carbon yarn
(10 µm in diameter) was obtained from Strem Chemicals
(Newburyport, MA). Anodic electrophoretic paint (Glassophor
ZQ 84-43225) was from BASF (Mu¨nster, Germany) and diluted
1:20 with water prior to use. Silver epoxy was from Epoxy
Technology (Billerica, MA).
cis-PtPy2Cl2 was synthesized by the method of Kauffman.70
Au nanoparticle suspensions were prepared by a sodium citrate
reduction of HAuCl4 according to a method reported by Frens,71
and the Pt nanoparticle suspension was prepared by hydrogen
reduction of K2PtCl4 in the presence of the sodium salt of poly-
(acrylic acid), according to a method reported by Ahmadi et
al.72 The condensation reactions between oxidized CFEs and
functionalized molecules followed a procedure by Xiolin et al.,
who used a similar methodology to immobilize single-walled
carbon nanotubes on a gold substrate.73
CFE Tips. Glass pipets were prepared from borosilicate glass
tubes (o.d. 1.2 mm, i.d. 0.69 mm) using a laser pipet puller
(model P-2000, Sutter Instrument Co., Novato, CA), yielding
micropipet tips with inner diameters of approximately 20 µm
and tapers that were approximately 0.5 cm in length. Ap-
proximately 1 cm of the CFE that was 10 µm in diameter was
attached to one end of a piece of Ni-Cr (Nichrome) wire using
a small amount of silver epoxy and cured in an oven for 45
min at 100 °C. The wire/CFE assembly was then inserted into
the top of the pulled micropipet and pushed through so that
approximately 3 mm of the CFE protruded from the micropipet
Results and Discussion
Methods Involving Macroscopic Substrates. Bulk Deposi-
tion. MNPs can be prepared by direct synthesis on an inert
macroscopic substrate, which can then be probed directly as a
composite electrode, e.g., by scanning with a tip.74 Figure 1
illustrates several approaches for generating single nanoparticles
on larger (approximately millimeter) supports; this includes (A)
controlled electrodeposition of MNPs with a low surface density,
(B) on-contact mechanical transfer of a pre-deposited MNP on
a tip, and (C) MNP formation on substrate by electrochemical
oxidation at a tip.
The electrochemical generation of MNPs occurs through a
nucleation and growth mechanism.75 The process starts by
reduction and adsorption of solution phase cations at defect sites
of the substrate (e.g., edge planes on HOPG). In the next stage,
surface diffusion of the adatoms leads to competing processes
of aggregation and desorption, which primarily depend on the
adsorption energy and interactions with the substrate. During
aggregation, incorporating an increasing number of adatoms
forms stable nucleation centers, and three-dimensional particle
growth begins. On macroscopic substrates, a large number of
nucleation centers coexist due to many intrinsic surface defects.
Under such circumstances, generation of a single MNP is highly
improbable. However, when the surface density of the particles
is decreased, it is possible to separate the MNPs to a sufficient
distance that the spherical diffusion fields of individual particles
will not overlap (Figure 1A). At that stage single MNPs can be
examined by a scanning probe method. (For example, a carbon
nanofiber can be used to approach small entities by the scanning
electrochemical microscopy (SECM) collection/generation mode,
allowing their study at a short distance).

Generation and Detection of Single MNPs
J. Phys. Chem. B, Vol. 110, No. 50, 2006 25281
Figure 1. Schematic presentation of different techniques employed to fabricate single or well-spaced individual nanoparticles on a macroscopic
area HOPG, with or without microtips: (A) decreasing nanoparticle surface density by controlled electrodeposition; (B) on-contact mechanical
particle transfer between tip/substrate; (C) electrochemical mass transfer between tip/substrate.
We have tested several approaches for MNP surface density
dilution, including: (i) minimizing surface defects on the
substrate (i.e., by using a freshly cleaved HOPG of the highest
available grade); (ii) decreasing the electrolyte concentration;
(iii) working in a diffusion-controlled overpotential region,
which increases the effect of the nucleation exclusion zones
around the growing nucleation centers.77 Our observations
indicate that a significant decrease in Pt nanoparticle surface
density was indeed evident using these guidelines, but in all
cases and even when shorter deposition pulses were applied,
the surface distribution of the generated particles still produced
a very short interparticle distance. The transition from spherical
to planar total diffusion happens within a very short time, given
by
td = R′2/2D
(2)
where td is the time for transition from spherical to planar
diffusion, R′ is the half distance between closest neighboring
nanoparticles, and D is the diffusion coefficient of the mediator
species.
Unfortunately, during the measurements we consistently
observed small interparticle distances. Figure 2A shows a SEM
image of Pt NPs on HOPG, electrodeposited from a 0.1 mM
H2PtCl4, 0.1 M H2SO4 solution with a 100 ms pulse to E )
-600 mV vs Hg/Hg2SO4. Although individual MNPs can be
discerned, R′ values on the order of a few tens of nanometers
were obtained. The deposition current, i(t), which accompanied
the measurement, fit a t-1/2 behavior in accord with planar
(Cottrell) diffusion. The transition from spherical to planar
diffusion occurred within a few milliseconds, too short an
experimental time to easily probe a single MNP with no
interference from neighboring particles. (Locating a probe tip
significantly closer to a single MNP than its neighbors is
technically challenging.76)
Figure 2. SEM images of HOPG following potentiostatic electrodepo-
sition for 100 ms at -600 mV vs Hg/Hg2SO4 from: (A) 0.1 mM H2-
PtCl6 in 0.1 M H2SO4; (B) 0.1 mM PtPy2Cl2 and 0.1 M THABF4 in
DCM.
tempted. Under similar conditions, reduction of PtPy2Cl2,
dissolved in 0.1 M tetrahexylammonium fluoroborate (THABF4)
in DCM exhibited an order of magnitude smaller deposition
currents compared to the aqueous solutions (Figure 3), appar-
ently because of lower electrolyte conductivity, a smaller Pt-
(II) diffusion coefficient, and different solvent/graphite/particle
In an effort to further increase the interparticle distance, Pt
electrodeposition on HOPG from organic solvents was at-

25282 J. Phys. Chem. B, Vol. 110, No. 50, 2006
Tel-Vered and Bard
Figure 3. Potentiostatic (E ) -600 mV vs Hg/Hg2SO4) electrodepo-
sition of Pt on HOPG from aqueous and organic Pt solutions. The
capacitance current was measured in 0.1 M THABF4/DCM. The
electrolyte was introduced and removed from the cell with the electrode
biased at E ) 0 mV vs Hg/Hg2SO4 before and after contact with
electrolyte to prevent spontaneous electroless Pt deposition. A large
surface Pt coil was the counter electrode mounted above the HOPG.
Figure 4. Electrochemical mass transfer from a Pd conical UME
(radius at apex 1 µm) to a HOPG substrate subject to a 60 s pulse. Etip
) 0.6 V; Esubstrate ) 0.1 V vs Ag/AgCl. Electrode gap, ∼200 µm.
Electrolyte, 4:16:80 HCl(conc)/CaCl2(sat)/H2O (v/v).
interactions that affect the surface tension and growth of the
particles. Despite the marginal increase in the estimated
interparticle distance to 100-200 nm, which is evident in Figure
2B, significant diffusion layer overlap still occurred for periods
longer than the millisecond range. We conclude that neither
aqueous nor organic phase electrodeposition on macroscopic
substrates are effective for the generation of separated MNPs
needed for a simple electrochemical ex situ detection.
Anodization of Tip Material. A different method involves
local deposition by anodization of an ultramicroelectrode (UME)
tip (Figure 1C) and deposition of a MNP on the substrate
(electrochemical patterning).78 In this method a carbon fiber
preloaded with a small amount of a metal or alternatively a
sharp conical tip of the desired metal is brought near a HOPG
substrate, e.g., using the feedback mode of the SECM with a
suitable redox mediator (e.g., ferrocene methanol (FcMeOH)).
A suitable etching solution of CaCl2(sat)/H2O/HCl(conc) (e.g., for
Pt 60:36:4 (w/w)) is then added to the cell, and both the substrate
and the tip are concurrently polarized. The tip is held at a
positive potential where it is anodically etched, and the metal
cations migrate to the surface of the substrate where the potential
is sufficiently negative for electrodeposition. Clearly, this
method is favorable for metals where electrochemical etching
is facile. Figure 4 shows chronoamperometric responses to a
60 s potential pulse simultaneously applied to a Pd metal conical
tip (0.6 V) and a HOPG substrate (0.1 V). The increase of both
the etching and the deposition currents with time results from
increasing exposure of the tip surface and the subsequent gradual
increase in the flux of cations migrating to the substrate. A
combined SEM/EDS analysis of the substrate following these
experiments confirmed the formation of Pd nanoparticles by
tip-substrate electrochemical mass transfer, densely distributed
around the tip position during pulsing. A possible solution for
this multi-MNP deposition may be to minimize the electrode
gap or to use smaller tips. This requires close control of the
tips over nanometer gaps. Although this is possible,76 it
frequently leads to tip-substrate crashes and significant damage.
Mechanical Transfer of the Nanoparticle from the Tip. Tip
and substrate crashes can be used for mechanical (rather than
electrochemical) particle transfer (Figure 1B). Here, a sharp
carbon microtip with an electrodeposited Pd NP was vertically
Figure 5. Approach curve to a point of contact between Pd-coated
conical carbon tip (radius at apex, ∼2 µm) and HOPG substrate in 1
mM FcMeOH/0.1 M KCl. Approach rate, 1 µm s-1
.
directed toward the surface of a HOPG substrate, while both
electrodes were immersed in FcMeOH electrolyte. An approach
curve, shown in Figure 5, was taken with the tip potential set
to oxidize the mediator and the substrate held at the potential
for FcMeOH+ reduction. When the tip reached the vicinity of
the HOPG, the positive feedback current became significant and
gradually increased. When the tip hit the surface of the graphite,
as indicated by a steep increase in the current, a small fraction
of the deposited Pd on the tip mechanically transferred to the
substrate. A fast retraction of the tip was immediately performed
upon detection of the contact current. In such experiments, we
consistently observed damage to both the tip and the substrate.
Cyclic voltammetry at the tip before and after the collision with
the surface showed a significant increase of the FcMeOH steady-
state limiting oxidation current with occasional transitions from
an UME voltammogram to that typical of a macroscopic
electrode. Furthermore, the damage to the electrodes could be
observed optically or by SEM. Under such conditions detection
of the transferred single nanoparticle on the HOPG was
impossible to achieve. Efficient mechanical MNP transfer was
observed with fast response STM,64-66,69 considering also the
enhanced sensitivity of STM at nanometric tip/substrate gaps

Generation and Detection of Single MNPs
J. Phys. Chem. B, Vol. 110, No. 50, 2006 25283
compared to that of SECM. Nevertheless, STM is not easily
compatible with subsequent electrochemical measurements.
In our hands all of the methods discussed so far suffered from
various problems and hence seem less effective to the generation
of single MNPs as electrocatalysts. The use of a probing tip to
characterize the nanoparticle is challenging, and a reliable
analysis requires a bare, untreated tip. For this reason we
examined alternative techniques to generate and electrochemi-
cally detect a single MNP.
SECM Mediator Positioning Technique. As opposed to
macroscopic substrates, single nanoparticle formation on mi-
croscopic substrates such as the apex of a sharpened carbon
fiber offers several advantages. First, localization of the nano-
particle within a very limited area prevents the need to track it
down over a large substrate surface. Moreover, from an
electrochemical point of view, small UMEs support smaller
double-layer charging currents and shorter RC time constants
that significantly increase the sensitivity of detection.79
Carbon fiber electrodes provide several additional advan-
tages: (1) They can be easily flame etched to obtain a radius
of curvature of 50-250 nm; (2) after insulation, the CFE apex
shows a low (<0.5) aspect ratio that is useful in SECM
experiments since higher sensitivity to feedback current in-
creases with flat, disklike geometry;76 (3) carbon electrodes show
high intrinsic overpotentials for proton and oxygen reduction
so that CFEs can be used to electrodeposit MNPs from acidic
media and to study electrocatalytic effects of the nanoparticles
with respect to these reactions.
However, the biggest difficulty with a CFE tip is forming a
pinhole-free insulation layer with a well-defined exposed end.
Although we have occasionally observed tips with nearly perfect
insulation by coating with anodic electrophoretic paint (with
an electrochemically evaluated area approaching 160 nm2),
reproducing the same coating procedure of the CFE results in
the formation of a thin polymer-insulating layer with a random
distribution of tiny pinholes. The occurrence of such pinholes
is clearly evident in the SEM images of Figure 6 where
electrodeposition was performed on a fully immersed, electro-
phoretically insulated CFE in Pt and Pd solutions. In the absence
of electrodeposition control, MNPs grow at the pinholes (Figure
6A), and when the deposition charge is significantly increased,
surface aggregation leads to gradual coverage of the carbon by
metal (Figure 6B). These pinhole effects must be minimized
during the generation of the single nanoparticles, so a different
approach based on slow electrode displacement controlled by
SECM and sensitive detection of mediator redox currents at
the air/electrolyte with SECM was used.80,81 This method is
illustrated in Figure 7, with experimental results in Figure 8. A
CFE tip is held in air above a composite electrolyte containing
(1) cations of the metal (M) to be electrodeposited, (2) a redox
mediator, and (3) the supporting electrolyte. The tip is first held
at a potential where the mediator exhibits considerable redox
current at potentials where electrodeposition does not occur,
i.e., Etip (mediator redox) > EDEP (Mn+ + ne- f M).
Figure 6. Electrodeposition on CFE tips at full immersion. (A) Pt
from 1 mM K2PtCl6/0.1 M HCl for 10 s at -150 mV vs Ag/AgCl; (B)
Pd from 0.75 mM PdCl2/5 mM HClO4/50 mM NaClO4 by sweeping
the potential linearly five times from 0.6 to 0.0 V vs Ag/AgCl at 50
mV s-1
.
Figure 7. Schematic of the SECM mediator positioning technique.
tip is not deliberately halted at this stage, then an extended curve
in liquid is collected, which sometimes shows multiple current
spikes associated with pinholes in the CFE. At this stage, after
positioning via the mediator responses, a short (500 ms)
electrodeposition potentiostatic pulse is applied to the CFE. A
response to such a pulse (Figure 8, inset) shows a typical
increase in the deposition current, proportional to the square
root of the deposition time, indicating the growth of a single
nucleation center (discussed later). Unlike electrodeposition on
macroscopic HOPG, there is no overlap between the diffusion
fields of neighboring nucleation centers on the CFE, and the
overall diffusion is spherical.
With the right combination of metal cation and mediator the
tip contacts the solution at a potential where electrodeposition
does not occur.58 The tip is slowly directed toward the surface
of the electrolyte using piezoelectric control, where the tip in
air displays only the SECM potentiostat offset current (∼800
fA). Upon just touching the liquid surface, a current spike
combined from mediator redox Faradaic and double-layer
charging currents appears and quickly decays to a Faradaic
steady state. The tip is then stopped at 100-150 nm below the
solution level, exposing its small apex to the electrolyte. If the
Two difficulties in this electrochemical tip positioning method
arise from electrolyte evaporation and the formation of a

25284 J. Phys. Chem. B, Vol. 110, No. 50, 2006
Tel-Vered and Bard
sharp carbon edges are formed during the etching of the fiber
in the flame. These constrain the surface diffusion of the Pt
adatoms during nucleation and hence localize the nucleation
centers in their vicinity, leading to favored nanoparticle growth
at pinholes located along the carbon edges.
Electrochemical Detection of a Single Nanoparticle. Elec-
trochemical detection of single MNPs on conductive substrates
is challenging and requires several conditions to be met. One
must distinguish electrochemically the nanoparticle from its
surrounding support material from known electrochemical
behavior (Figure S1 of the Supporting Information) and be able
to detect the small redox currents associated with the tiny
particle. Moreover, we must ensure that the electrochemical
reaction and its products will not change the nanoparticle and
leave it intact. A benefit of Pt NPs is chemical stability and
electrocatalytic effects in several reactions. Electrochemical
detection of Pt can be carried out using selective mediators,
which are catalytically active on Pt and at the same time show
sluggish kinetics on carbon, e.g., H+ and O2 reduction reactions.
Figure S1 of the Supporting Information shows an overpotential
difference of more than 0.4 V between Pt and C toward proton
reduction reaction as well as irreversible and hindered kinetics
on the carbon. In addition, other distinctive features of Pt are
typically observed during cycling in strongly acidic solutions.
These fingerprints include characteristic hydrogen adsorption/
desorption waves, Pt oxide formation, and its corresponding
reduction peak. Assuming that the nanoparticle possesses a
hemispherical shape, eq 1 can be simplified and used to calculate
the expected diffusion-limited currents for reduction of 10 mM
H+ (5 mM H2SO4), according to
Figure 8. CFE approach from air to an electrolyte containing Ru-
(bpy)3Cl2 mediator and Pt(II) at Etip ) 1.0 V vs Ag/AgCl. The approach
was stopped at a depth of 150 nm, and a potentiostatic deposition pulse
was applied. The chronoamperometric response to a pulse at EDEP
-0.5 V vs Ag/AgCl is shown in the figure inset.
)
Figure 9. SEM images of two single Ni nanoparticles obtained from
1 mM NiCl2/0.85 mM FcMeOH/0.1 M KCl solution. For all cases,
τDEP ) 100 ms and EDEP ) -1.05 V vs Ag/AgCl. Figures 9A and 9B
are different magnifications of a single particle located ∼150 nm from
the CFE edge. Figure 9C presents a single particle deposited directly
at the CFE apex.
ilim ) 2πnFDC∞a
(3)
In accord with eq 3 and by disregarding possible size-dependent
catalytic effects for smaller-size particles, the expected diffusion-
limiting currents for 1-100 nm Pt nanoparticles in 10 mM H+
are on the order of 0.1-10 nA, which are readily measured.
The currents associated with the waves of hydrogen adsorption
and desorption on the Pt surface are much smaller. Considering
an adsorption of a full monolayer of H atoms on a a ) 70 nm
polycrystalline Pt nanoparticle taking QH ) 210 µC cm-2 for
cyclic voltammetry at 1 V s-1, the average current expected
for the hydrogen waves is on the order of sub-picoampere, which
makes detection by this route very difficult. The same problem
occurs with underpotential deposition (UPD) of metals such as
Cu, Pb, Sn, Ag, Tl, and Bi, which all support monolayer charge
densities below 400 µC cm-2 on Pt and hence do not
significantly increase the detection currents.
While performing cyclic voltammetry for CFEs deposited
with Pt single nanoparticles in 0.5 M H2SO4, we observed a
rapid detachment of the particle from the CFE surface at the
onset of proton reduction. This suggests that the individual
MNPs are held on carbon by weak physisorption58 not evident
in cases of larger metallic clusters such as those in Figure 6.
The use of O2 reduction for detection, which eliminates gas
evolution, although possible,82 is limited by the low oxygen
solubility (<2 mM) in water at 25 °C, which decreases the
detection current because of a smaller concentration term in eq
3.
meniscus between the tip and the liquid phase (schematically
presented in Figure S3 of the Supporting Information). The first
interference is overcome by fixing the approach rate above the
evaporation rate. This was measured by successive tip ap-
proaches at different rates, which detect the incremental change
in the level of solution with time, and was observed to be Vvap
≈ 22-28 nL s-1, equivalent in our 8 mm radius cell to 0.10-
0.14 µm s-1 z-movement of the tip, at T ) 20 °C and P ) 1
atm. Meniscus problems became less pronounced when the tip
was immersed less than 400 nm into the solution. SEM images
taken for tips electrodeposited at deeper immersion often
detected nanoparticles at heights exceeding 1 µm from the apex
of the CFE.
The mediator positioning technique was employed in the
preparation of single Ni and Pt nanoparticles (with either 1 mM
NiCl2/1 mM FcMeOH/0.1 M KCl or 1 mM K2PtCl4/0.25 mM
Ru(bpy)3Cl2/0.1 M K2SO4 solutions, respectively). Figure 9
shows two single Ni nanoparticles with radii of 60 ( 10 nm. A
focused EDS analysis was used to confirm the metallic
composition of these particles. While the particle in Figures 9A
and 9B is located ∼150 nm from the CFE edge, that in Figure
9C is at the CFE apex. Pt nanoparticles with similar, ∼60 nm,
radii were similarly obtained by applying 500 ms deposition
pulses at EDEP ) -0.50 V vs Ag/AgCl. Upon deposition at
deeper immersion (>400 nm) a significant meniscus was
formed, and the Pt nanoparticles showed a clear tendency to
align in straight lines along the CFE main axis (also shown in
Figure 6A), a phenomenon previously observed in the elec-
trodeposition of Pt NPs on HOPG,58 where the particles favor
growing on step edges in the graphite basal plane. Possibly, by
analogy to the existence of step edges on the surface of HOPG,
A good mediator, which supports preferential, enhanced
kinetics on Pt compared to carbon and no generation of a
disrupting gas is the Fe3+/Fe2+ couple.83,84 At pH 0, both species
of the iron are moderately soluble and do not cause any
mechanical or passivation hazards to the nanoparticle. At ∼5
mM in 1 M H2SO4, Fe3+ was first used to detect Pt NPs

Generation and Detection of Single MNPs
J. Phys. Chem. B, Vol. 110, No. 50, 2006 25285
Figure 10. Lower curve: Generation of Pt nanoparticles on a CFE
tip moving from air to a Pt(II) solution under active electrodeposition
tip potential. Upper curve: Detection of the Pt nanoparticles on the tip
by approach from air to a Fe3+/H2SO4 solution at a potential supporting
electrocatalytic Fe3+ reduction on Pt.
Figure 12. Schematic of the binding monolayer approach for adsorbing
Au and Pt MNPs from colloidal solutions on modified CFE tips and
controlling their surface concentration.
SEM analysis indicated a good geometrical agreement with
those observations.
Although detection of ensembles of NPs was demonstrated,
detection of a single Pt nanoparticle is more difficult. Figure
11 shows two air approach measurements in Fe3+/H2SO4
performed on the same CFE before and after the growth of a
single Pt nanoparticle using the mediator positioning technique
(under similar conditions to those presented in Figure 8). As
can be seen, in the presence of the single particle, a 20-fold
increase in the steady-state current was evident. To evaluate
the correlation between the detection current and the real
dimensions of the particle, we first estimated the radius
associated with the electrochemical generation. This can be done
by equating the hemispherical diffusion current in eq 3 to the
spatial growth rule provided by Faraday’s law
Figure 11. Approach from air curves for a CFE tip moving from air
to Fe3+/H2SO4 before and after the deposition of a single Pt nanoparticle
according to the mediator positioning technique (deposition from 1 mM
K2PtCl4/0.25 mM Ru(bpy)3Cl2/0.1 M K2SO4 solution for 0.5 s at EDEP
) -0.50 V vs Ag/AgCl).
2πnFDC∞a ) (nFdPt/MPt)(dVPt/dt) )
nFdPt2πa2(da/dt)/MPt (4)
electrodeposited along an insulated CFE (Figure 10). In the
beginning of this experiment the CFE was held at the deposition
potential of Pt, and an air approach measurement was performed,
allowing the tip to travel more than 18 µm below the solution
level. The current spikes observed in the figure along with the
tip displacement correspond to the generation of Pt deposition
centers through occasional pinholes. The current decay that
follows the spikes suggests an overlapping of close particle
diffusion fields. At the end of the generation stage, the tip was
backed out, the solution was replaced by Fe2(SO4)3 in 1 M H2-
SO4, and a second approach from air was performed, with the
tip potential at Etip ) 0.64 V vs the standard hydrogen electrode
(SHE). As can be seen in the upper portion of Figure S1 in the
Supporting Information, this potential allows one to distinguish
between the Pt nanoparticle and the carbon substrate, since it
solely supports the electrocatalytic reduction of Fe3+ to Fe2+
on Pt. Figure 10 demonstrates a good correlation between the
generation and the detection peaks along the CFE from which
it is clear that the Fe3+ mediator provides good selectivity in
tracking the deposited Pt particles on the composite substrate.
where VPt is the volume of the Pt nanoparticle, dPt is the density
of Pt, and MPt is the molar mass of Pt.
Hence, under potential supporting diffusion-limited current,
the time-dependent deposition current conditions and nanopar-
ticle radius are, respectively
i(t) ) nπF((2DC∞)3MPtt/dPt)1/2
r(t) ) (2DC∞MPtt/dPt)1/2
(5)
(6)
An electrodeposition pulse width of t ) 0.5 s yields a radius
of 70 nm, which is in good agreement with SEM observations.
According to eq 3 with DFe(III) ) 1.2 × 10-6 cm2 s-1 85
,
a
≈
detection current corresponding to this radius would be iss
25 pA.
Binding Monolayers and Particle-Adsorption Methods.
The interaction between the electrodeposited NPs and the carbon
support can be strengthened by the use of bifunctional molecular

25286 J. Phys. Chem. B, Vol. 110, No. 50, 2006
Tel-Vered and Bard
binding layers. As shown in Figure 12, a CFE can chemically
react and subsequently attach to a layer of binder (e.g.,
4-aminothiophenol) with a high affinity toward MNPs (e.g., Au).
The process of attaching the binder to the CFE requires it to
have surface carboxylic groups, which are formed during the
oxidation of the carbon fiber in the oxygen-enriched flame86-88
or by electrochemical oxidation for 180 s at E ) 1.7 V vs SHE
in 1 M H2SO4. The oxidized CFEs were then heated for >12 h
at 50 °C in DMF solutions containing 1 mM of 4-aminothiophe-
nol (for binding Au nanoparticles) or 4-aminopyridine (for
binding Pt nanoparticles). The organic amine molecules are
relatively short and contain a conjugated aromatic ring to provide
facile electron transfer between the MNP and the CFE. The
condensation reaction is carried out in the presence of 1,3-
dicyclohexylcarbodiimide (DCC), which facilitates dehydration
and binding via amide bonds. The modified CFE tips were then
immersed into a solution of colloidal Au or Pt nanoparticles,
where strong adsorption forces attach them to the S (Au) or to
the pyridine nitrogen (Pt) termini. Shown in Figure S2 of the
Supporting Information CFE tips in the absence (A) or in the
presence (C) of 4-aminopyridine surface modification following
immersion for 15 min in a unstirred Pt nanoparticle (∼30 nm
diameter) suspension and rinsing in water. As demonstrated,
the presence of a molecular binding layer is essential for the
linkage of the Pt nanoparticles to the CFE, and in such cases a
high surface density with an estimated nanoparticle radius of a
≈ 15 nm is evident.
Figure 13. SEM image of a single Pt nanoparticle, chemisorbed on
the surface of a CFE modified by 1 mM 4-aminopyridine and 200 mM
aniline. The tip was immersed at 350 nm below the surface level of a
Pt colloidal suspension using SECM air approach while monitoring
the capacitance current at Etip ) 0.02 V vs Pt reference/counter
electrode.
cohesion forces are evident by chemisorption of the particle on
the monolayer. This interaction between the pyridine nitrogen
and the Pt metal89 is significantly stronger than the net
physisorption associated with a Pt particle generated by elec-
trodeposition on carbon.
Conclusions
However, to reduce the surface density of the NPs, desirably
to the single MNP level, one must dilute the binder by similar
molecules that do not bind to the metal, i.e., ”blocking”
molecules such as aniline (Figure S3 of the Supporting
Information). The dilution effect is evident in Figure S2B of
the Supporting Information for a CFE incubated for 16 h in the
presence of 1:200 4-aminopyridine/aniline solution in DMF,
showing fewer Pt nanoparticles on the surface compared to part
C. A further decrease in the surface density can be accomplished
by decreasing the binder/blocker molar ratio, decreasing the
concentration of DCC, and decreasing the temperature or
shortening the condensation period. This can also be ac-
complished by careful control of the SECM tip positioning at
the air/liquid (NP solution) interface. Here, the chemically
modified apex of the CFE tip is immersed only a few hundred
nanometers below the metal NP solution level (Figure S3 of
the Supporting Information). The tip is held at this position for
a short time (the time is delayed until electrolyte evaporation
forces the apex outside the liquid, gradually leaving more contact
time for the apex edge) in which the randomly moving MNPs
can attach. Care must be taken not to oxidize the immobilized
molecules, and hence the approach scan is made with the
modified tip only slightly polarized with respect to a combined
counter/reference electrode (0.02 V). Under such conditions the
modified CFE approaches a mediator-free colloidal solution
without generating Faradaic current but only a double-layer
charging current that indicates the point of penetration. Upon a
successful adsorption shown in Figure 13, we were able to obtain
a single Pt nanoparticle bound to a pyridine on the CFE. As
opposed to the case of the electrodeposited Pt single nanopar-
ticle, repeated cyclic voltammetry performed in sulfuric acid
showed a proton reduction wave at the onset of E ) 0.0 V vs
SHE, with no loss of the Pt NP (Figure S4 of the Supporting
Information). Thus efficient electron transfer supporting minimal
potential losses occurs between the particle and the CFE through
the monolayer, probably because of the relatively short and
conjugated structure of the 4-aminopyridine molecule. Enhanced
While methodologies such as particle separation by pulsed
electrodeposition, electrochemical, and mechanical (SECM-
assisted) material transfer from tip to a macroscopic carbon
substrate show several problems that hinder the generation of
remote or single nanoparticles, a combination of a carbon
nanofiber tip and SECM piezoelectric-controlled positioning
demonstrates a fast and novel solution. Generation of isolated
a ) 60 ( 10 nm Pt and Ni particles is facile by short pulse
electrodeposition on CFE tips, which are immersed, using
controlled electrochemical positioning, about 100-200 nm
below the surface level of a combined electroplating and
mediator solution. An alternative method is to attach a binding
monolayer to the CFEs via condensation of amine-terminated
molecules and surface carboxylic acid groups. Strict control over
the ratio of binding to blocking functionalities in the monolayer
directly affects the surface density of Au and Pt nanoparticles
adsorbed on the surface upon immersion in metal colloidal
solutions. Combining the SECM-assisted positioning technique
with this method allows the chemisorption of a single Pt
nanoparticle onto the pyridine-terminated molecules, which
demonstrates favored mechanical stability toward hydrogen
evolution compared to that of the physisorbed electrodeposited
particle. A further advantage of this method is that the particle
size here is determined by the synthetic procedure of the
suspension (that typically involves electroless reduction of the
metal salt to a finely monodispersed colloid), which is signifi-
cantly easier to control compared to both electrochemical
generation and mechanical particle transfer.
Acknowledgment. This work was supported by a grant from
the National Science Foundation (Grant No. CHE 0451494).
R.T. thanks the Israel Science Foundation and the Technions
Israel Institute of Technology for a postdoctoral fellowship.
Supporting Information Available: Additional cyclic vol-
tammetry and SEM figures. This material is available free of
charge via the Internet at http://pubs.acs.org.

Generation and Detection of Single MNPs
J. Phys. Chem. B, Vol. 110, No. 50, 2006 25287
(44) Zhang, J.; Lima, F. H. B.; Shao, M. H.; Sasaki, K.; Wang, J. X.;
Hanson, J.; Adzic, R. R. J. Phys. Chem. B 2005, 109, 22701.
(45) Henry, C. R. Prog. Surf. Sci. 2005, 80, 92.
References and Notes
(1) Haes, A. J.; Van Duyne, R. P. J. Am. Chem. Soc. 2002, 124, 10596.
(2) Pivin, J. C.; Garcia, M. A.; Hofmeister, H.; Martucci, A.; Vassileva,
M. S.; Nikolaeva, A.; Kaitasov, O.; Llopis, J. Eur. Phys. J. 2002, 20, 251.
(3) Graf, H.; Vancea, J.; Hoffmann, H. Appl. Phys. Lett. 2002, 80, 1264.
(4) Braun, M.; Link, S.; Burda, C.; El-Sayed, M. Chem. Phys. Lett.
2002, 361, 446.
(5) Shemer, G.; Markovich, G. J. Phys. Chem. B 2002, 106, 9195.
(6) Chen, D. H.; Hsieh, C. H. J. Chem. Mater. 2002, 12, 2412.
(7) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729.
(8) Liu, J.; Ong, W.; Kaifer, A. E.; Peinador, C. Langmuir 2002, 18,
5981.
(9) Rodriguez, J. A.; Liu, G.; Jirsak, T.; Hrbek, J.; Chang, Z. P.; Dvorak,
J.; Maiti, A. J. Am. Chem. Soc. 2002, 124, 5242.
(10) Lou, Y. B.; Maye, M. M.; Han, L.; Luo, J.; Zhong, C. J. Chem.
Commun. 2001, 473.
(11) Yonezawa, T.; Imamura, K.: Kimizuka; N. Langmuir 2001, 17,
4701.
(12) Templeton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J.
Am. Chem. Soc. 1998, 120, 1906.
(13) Feldheim, D. L.; Foss, C. A., Jr. Metal Nanoparticles: Synthesis,
Characterization, and Applications; Marcel Dekker: New York, 2002.
(14) Trindade, T.; O’Brien, P.; Pickett, N. L. Chem. Mater. 2001, 13,
3843.
(15) Schmid, G. Chem. ReV. 1992, 92, 1709.
(16) Wang, J. X.; Markovic, N. M.; Adzic, R. R. J. Phys. Chem. B 2004,
108, 4127.
(46) Bowker, M.; Stone, P.; Morrall, P.; Smith, Ru.; Bennett, R.; Perkins,
N.; Kvon, R.; Pang, C.; Fourre, E.; Hall, M. J. Catal. 2005, 234, 172.
(47) Gaynutdinov, R.; Vainshtein, D. I.; Hak, S. J.; Tolstikhina, A.; Den
Hartog, H. W. Radiat. Eff. Defects Solids 2003, 158, 77.
(48) Forti, J. C.; Manzo-Robledo, A.; Kokoh, K. B.; De Andrade, A.
R.; Alonso-Vante, N. Electrochim. Acta 2006, 51, 2800.
(49) Rupprechter, G. Annu. Rep. Prog. Chem., Sect. C: Phys. Chem.
2004, 100, 237.
(50) Lewera, A.; Zhou, W. P.; Vericat, C.; Chung, J. H.; Haasch, R.;
Wieckowski, A.; Bagus, P. S. Electrochim. Acta 2006, 51, 3950.
(51) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem.
Soc. 1992, 114, 5221.
(52) Doron, A.; Katz, E.; Willner, I. Langmuir 1995, 11, 1313.
(53) Chumanov, G.; Sokolov, K.; Gregory, B. W.; Cotton, T. M. J. Phys.
Chem. 1995, 99, 9466.
(54) Teranishi, T.; Hosoe, M.; Miyake, M. AdV. Mater. 1997, 9, 65.
(55) Roeder, H.; Hahn, E.; Brune, H.; Bucher, J. P.; Kern, K. Nature
1993, 366, 141.
(56) Bartholomew, C. H.; Boudart, M. J. Catal. 1972, 25, 173.
(57) Wei, X.; Chen, J.; Yang, L.; Zhaolin, L. Chem. Commun. 2002,
2588.
(58) Zoval, J. V.; Lee, J.; Gorer, S.; Penner, R. M. J. Phys. Chem. B
1998, 102, 1166.
(59) Liu, H.; Favier, F.; Ng, K.; Zach, M. P.; Penner, R. M. Electrochim.
Acta 2001, 47, 671.
(17) Kabbabi, A.; Gloaguen, F.; Andolfato, F.; Durand, R. J. Electroanal.
Chem. 1994, 373, 251.
(18) Freelink, T.; Visscher, W.; Van Veen, J. A. J. Electroanal. Chem.
1995, 382, 65.
(19) Yahikozawa, K.; Fujii, Y.; Matsuda, Y.; Nishimura, K.; Takasu,
Y. Electrochim. Acta 1991, 36, 973.
(20) Watanabe, M.; Sei, H.; Stonehart, P. J. Electroanal. Chem. 1989,
261, 375.
(60) Cortie, M. B.; Van der Lingen, E. Mater. Forum 2002, 26, 1.
(61) Kuhn, H.; Koch, J.; Hergenroeder, R.; Niemax, K.; Kalberer, M.;
Guenther, D. J. Anal. At. Spectrom. 2005, 20, 894.
(62) Barbara, P. F. Acc. Chem. Res. 2005, 38, 503 and references therein.
(63) Engelkamp, H.; Hatzakis, N. S.; Hofkens, J.; De Schryver, F. C.;
Nolte, R. J. M.; Rowan, A. E. Chem. Commun. 2006, 935.
(64) Meier, J.; Friedrich, K. A.; Stimming, U. Faraday Discuss. 2002,
121, 365-372.
(21) Kinoshita, K. J. Electrochem. Soc. 1990, 137, 845.
(22) Hirai, H.; Wakabayashi, H.; Komiyama, M. Chem. Lett. 1983, 1983,
1047.
(65) Meier, J.; Schiot, J.; Liu, P.; Norskov, J. K.; Stimming, U. Chem.
Phys. Lett. 2004, 390, 440.
(23) Brugger, P. A.; Cuendet, P.; Gratzel, M. J. Am. Chem. Soc. 1981,
103, 2923.
(66) Eikerling, M.; Meier, J.; Stimming, U. Z. Phys. Chem. 2003, 217,
395.
(24) Reetz, M. T.; Quaiser, S. A.; Breinbauer, R.; Tesche, B. Angew.
Chem., Int. Ed. Engl. 1995, 34, 2728.
(25) Friedrich, K. A.; Henglein, F.; Stimming, U.; Unkauf, W. Elec-
trochim. Acta 2001 47 689.
(26) Park, S.; Wasileski, S. A.; Weaver, M. J. J. Phys. Chem. B 2001,
105, 9719.
(27) Park, S.; Tong, Y. Y.; Wieckowski, A.; Weaver, M. J. Electrochem.
Commun. 2001, 3, 509.
(28) Schmidt, T. J.; Gasteiger, H. A.; Staeb, G. D.; Urban, P. M.; Kolb,
D. M.; Behm, R. J. J. Electrochem. Soc. 1998, 145, 2354.
(29) Takasu, Y.; Ohashi, N.; Zhang, X. G.; Murakami, Y.; Minigawa,
H.; Sato, S.; Yahikozawa, K. Electrochim. Acta 1996, 41, 2595.
(30) Yahikozawa, K., Fijii, Y., Matsuda, Y., Nishimura, K., Takasu, Y.
Electrochim. Acta 1991, 36, 973.
(31) Friedrich, K. A.; Henglein, F.; Stimming, U.; Unkauf, W. Elec-
trochim. Acta 2000, 45, 3283.
(32) Cherstiouk, O. V.; Simonov, P. A.; Savinova, E. R. Electrochim.
Acta 2003, 48, 3851.
(33) Schiffrin, D. J.; Kiely, C. J. Electroanal. Chem. 1996, 409, 137.
(34) Schiffrin, D. J. Platinum Met. ReV. 1999, 43, 2.
(35) Paulus, U. A.; Veziridis, Z.; Schnyder, B.; Kuhnke, M.; Scherer,
G. G. J. Electroanal. Chem. 2003, 77, 541.
(36) Kongkanand, A.; Kuwabata, S. J. Phys. Chem. B 2005, 109, 23190.
(37) Kucernak, A. R.; Chowdhury, P. B.; Wilde, C. P.; Kelsall, G. H.;
Zhu, Y. Y.; Williams, D. E. Electrochim. Acta 2000, 45, 4483.
(38) Antoine, O.; Bultel, Y.; Durand, R. J. Electroanal. Chem. 2001,
499, 85.
(39) Cherstiouk, O. V.; Simonov, P. A.; Savinova, E. R. Electrochim.
Acta 2003, 48, 3851.
(40) Calandra, P.; Giordano, C.; Longo, A.; Turco Liveri, V. Mater.
Chem. Phys. 2006, 98, 494.
(67) Chen, S.; Kucernak, A. J. Phys. Chem. B 2004, 108, 13984.
(68) Chen, S.; Kucernak, A. J. Phys. Chem. B 2003, 107, 8392.
(69) Kolb, D. M.; Ullmann, R.; Will, T. Science 1997, 275, 1097.
(70) Kauffman, G. B. Inorg. Synth. 1963, 7, 249.
(71) Frens, G. Nature 1973, 241, 20.
(72) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed,
M. A. Science 1996, 272, 1924.
(73) Xiaolin, N.; Zhennan, G.; Zhongfan, L. J. Colloid. Interface Sci.
2002, 245, 311.
(74) Penner, R. M. J. Phys. Chem. B 2001, 105, 8672.
(75) Scharifker, B. R.; Mostany, J. In Interfacial Kinetics and Mass
Transport; Bard, A. J., Stratmann, M., Calvo, E. J., Eds.; Encyclopedia of
Electrochemistry 2; Wiley-VCH: Weinheim, Germany, 2002; p 512.
(76) Tel-Vered, R.; Walsh, D. A.; Mehrgardi, M. A.; Bard, A. J. Anal.
Chem. 2006, 78, 6959.
(77) Serruya, A.; Mostany, J.; Scharifker, B. R. Faraday. Trans. 1993,
89, 255.
(78) Mandler, D. In Scanning Electrochemical Microscopy; Bard, A.
J.; Mirkin, M. V., Eds. Marcel Dekker: New York, 2001; p 593.
(79) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Funda-
mentals and Applications, 2nd ed.; Wiley: New York, 2001; p 216.
(80) Walsh, D. A.; Fernandez, J. L.; Mauzeroll, J.; Bard, A. J. Anal.
Chem. 2005, 77, 5182.
(81) Lee, A.; Bard, A. J. Anal. Chem. 2002, 74, 3626.
(82) Chen, S.; Kucernak, A. J. Phys. Chem. B 2004, 108, 3262.
(83) Wipf, D. O.; Bard, A. J. J. Electrochem. Soc. 1991, 138, 469.
(84) Wipf, D. O.; Bard, A. J. J. Electrochem. Soc. 1991, 138, L4-L6.
(85) Belqat, B.; Laghzizil, A.; Elkacimi, K.; Bouhaouss, A.; Belcadi,
S. J. Fluorine Chem. 2000, 105, 1.
(86) Kozlowski, C.; Sherwood, P. M. A. Carbon 1987, 25, 751.
(87) Fitzer, E.; Geigl, K. H.; Huttner, W.; Weiss, R. Carbon 1980, 18,
265.
(88) Boehm, H.-P.; Diehl, E.; Heck, W.; Sappok, R. Angew. Chem.,
Int. Ed. Engl. 1964, 3, 669.
(41) Chen, M.; Cai, Y.; Yan, Z.; Goodman, D. W. J. Am. Chem. Soc.
2006, 128, 6341.
(42) Stamenkovic, V.; Arenz, M.; Ross, P. N.; Markovic, N. M. J. Phys.
Chem. B 2004, 108, 17915.
(43) Vad, T.; Haubold, H. G.; Waldoefner, N.; Boennemann, H. J. App.
Crystallogr. 2002, 35, 459.
(89) De-Yin, W.; Bin, R.; Yu-Xiong, J.; Xin, X.; Zhong-Qun, T. J. Phys.
Chem. A 2002, 106, 9042.