18. Diamond Microelectrodes
H. Olivia, B. V. Sarada, T. N. Rao and A. Fujishima
18.1. Introduction
Microelectrodes have attracted much attention recently in
electrochemistry due to their superior properties, which enable
them to outperform conventional macroelectrodes and extend the
experimental range to several new fields, such as fast-scan
measurements and analysis in poorly conducting media [1-4]. The
history of microelectrodes actually started more than 60 years ago,
when 1942, Davies and Brink [5] reported the use of platinum
microdisk electrodes for the measurement of oxygen in muscle
tissues. In their work, microelectrodes were used to minimize the
damage to the muscle, and to limit the current flowing through the
electrode. Since then, several reviews [6-8] and books [9,10] about
microelectrodes have been published.
Denuault,
in
his
review
[8],
defined
the
term ”microelectrode” as an electrode with at least one dimension
in the range of 0.1 to 50 m. The small size of microelectrodes
makes them possible to be used for in vivo detection, which is
usually performed with very small volumes of samples, such as
those for neurotransmitter monitoring in the brain. Moreover, due
to its small size, at relatively long experimental timescales, the
thickness of the diffusion layer is comparable to the dimensions of
the microelectrode, and a spherical (or hemi-spherical) diffusion
field controls the fast mass transport of reactants and products to
and from the electrode surface. Accordingly, a steady-state
response (or pseudo-steady state response) can be observed with
cyclic voltammetry at low sweep rates.
Another interesting feature of microelectrodes is their small
interfacial capacitance. Capacitance decreases with electrode area,
and therefore, due to its small area, microelectrodes have a
reduced capacitance and hence small charging current, allowing
fast and sensitive response. Furthermore, voltammetry using
microelectrodes often completely eliminates iR drop, which
enhances the use of media such as organic solvents [11], nonelectrolyte solutions [12-14], and even gases and solids [15,16],
which are generally excluded from any measurements using
macroelectrodes.
The various geometries of microelectrodes include microdisks,
microfibers, microarrays, microbands, and microrings. Among
these, the microdisk is the most popular geometry, because of its
simple fabrication and the possibility of treatment by polishing.
However, the current response at microdisk electrodes is often
small enough that it limits the range of measurements,
augmenting the need for techniques to fabricate microfiber,
microband and microarray electrodes, which provide larger signals.
The
most
commonly
used
electrode
materials
for
microelectrodes include platinum, gold and carbon. Carbon fiber
microelectrodes are widely used for electroanalysis in aqueous
media, as they exhibit a relatively wide potential window. However,
similar to metal electrodes, carbon has several serious limitations,
including high background current and deactivation via fouling,
especially during the detection of compounds in complex biological
fluids, as reported by Baur et al.[17]. It is an inherent property of
carbon to undergo deactivation upon exposure to the laboratory
environment or working solution, which is due to factors such as
surface oxidation and adsorption of contaminants and reaction
products.
Diamond is one of the more recent of the carbon allotropes
that has been examined as an electrode material. It exhibits
several superior properties, including low background current,
wide potential window, long-term stability, relative insensitivity
towards the presence of dissolved oxygen in the solution, and
biocompatibility [18-20]. Thus, diamond is becoming an interesting
material to consider for electroanalysis.
Cooper et al. (1998) reported for the first time the fabrication
and the use of boron-doped diamond (BDD) microelectrodes in nonaqueous electrolytes [21]. Considering the advantages of BDD
mentioned above, the Fujishima group undertook the application
of BDD microelectrodes, especially BDD microdisk [22], microfiber
[23] and microdisk array [24] electrodes in aqueous solutions.
18.2. Preparation of Diamond Microelectrodes
18.2.1. Fabrication of diamond microdisk and
microfiber electrodes
Diamond microfibers were prepared by depositing boron-doped
diamond on electrochemically polished tungsten fibers. Diamond
deposition was carried out using a microwave plasma chemical
vapor deposition (MPCVD) system at a hydrogen pressure of 50-80
Torr and microwave power of 1500-3000 W for 3-8 h on tungsten
fibers. Different powers and deposition times resulted in the
variation of the crystal size and the film thickness, respectively.
The crystal size varies from 5 to 40 m, while the film thickness
varies from 5 to 20 m. Prior to deposition, the tips of the tungsten
wires (= 30 m) were etched in 2 M NaOH at 3 V for 45 s in order
to reduce the diameter of the fiber to ~10 m, and these tips were
nucleated by ultrasonicating in solution containing a suspension of
100-nm diamond particles for 60 min.
The diamond-deposited tungsten wire was then inserted into
a pre-pulled glass capillary (= 50-100 m) and was sealed using
epoxy. The ohmic contact to the diamond fiber was made using a
copper wire with either mercury or silver paste. In the case of the
microdisk electrode [22], the diamond fiber was preliminarily fully
sealed by the use of epoxy, and the tip was then polished until the
diamond was just exposed, while for the microfiber electrode [23],
a ~300-m length of fiber was left exposed.
18.2.2. Characterization
Successfully fabricated diamond fibers were characterized by use
of scanning electron microscopy (SEM) and Raman spectroscopy,
while the roughness factor of the diamond fiber was calculated
based on double-layer capacitance measurements. SEM images of
diamond fibers are shown in Fig 18.1. Figure 18.1(a) shows a
suitable diamond fiber for microdisk electrode fabrication, while
for
the
microfiber
electrode,
full
coverage
of
diamond
polycrystallites on the tungsten fiber was necessary (Fig 18.1(b)).
Raman spectra (not shown) indicated the high quality and purity
of the diamond.
Fig 18.1. Suitable diamond for (a) microdisc electrode an d (b)
microfiber electrode
The double layer capacitance of a diamond microelectrode is
calculated based on the equation
Ic =  C d
where Ic is the charging current,  is the potential sweep rate, and
Cd is the double layer capacitance. By plotting Ic as a function of ,
the double-layer capacitance Cd can be obtained from the slope.
The Cd value obtained for a diamond microfiber electrode was 8 nF,
and the capacitance density was calculated to be 7.02 F cm-2.
Considering
the
capacitance
density
of
a
smooth
(100)
homoepitaxial diamond electrode (ca. 3 F cm-2), the roughness
factor of the diamond fiber was estimated to be 2.34.
18.3. Electrochemical Behavior
18.3.1. Electrochemical behavior of diamond
microdisk electrodes
The simplest way to investigate the electrochemical behavior of an
electrode is by studying its cyclic voltammetric curves. Figure 18.2
shows cyclic voltammograms for the oxidation of ferrocyanide at
BDD microdisk electrodes with two different radii in aqueous
electrolyte.
Fig 18.2. Cyclic voltammograms at diamond micro-electrodes for the
oxidation of 1 mM K4Fe(CN)6 in 0.1 M KCl (potential sweep rate, 10
mV s-1); electrode radii: (a) 20 and (b) 6 μm.
The sigmoidal shapes of the curves and lack of hysteresis, i.e.,
steady state-type behavior, is characteristic of voltammetry at low
potential sweep rates for microelectrodes [1,3]. The half-wave
potential was +0.210 vs. SCE. This value agrees well with that
reported at conventional macro-type diamond electrodes by Jolley
et al. (+0.230 V) [25]. The radius of each microelectrode was
calculated from the equation
ilim = 4nFDCr
18.1
where ilim is the limiting current, C is the concentration, D is the
diffusion coefficient, r is the radius of the electrode, F is the
Faraday constant, and n is the number of electrons, in this case,
one. The radii of the microelectrodes were calculated to be 20 and 6
m using a value of 6.5  10-6 cm2s-1 for the diffusion coefficient for
ferrocyanide [26]. Similar steady-state type voltammograms were
also obtained for the oxidation of Ru(NH3)63+, for which a diffusion
coefficient of 6.0  10-6 cm2s-1 was used [27].
Owing to the steady-state nature of the spherical diffusion at
the microelectrode, the limiting current should be independent of
potential sweep rates at lower sweep rates. As the sweep rate
increases, the contribution of planar diffusion increases. The value
of sweep rate at which planar diffusion begins to significantly
interfere depends on the size of microelectrode.
One of the most promising features expected for BDD
microelectrodes is very low background current, due to a
combination of the effect of the microelectrode size [1] plus the
intrinsic properties of diamond [28]. One way this effect can be
tested is by examining the detection limit for a relatively simple
redox couple at slow sweep rates.
Figure
18.3(a)
shows
a
voltammogram
for
a
BDD
microelectrode (r=20 m) in a 200 nM ferrocyanide (0.1 M KCl)
solution,
compared
with
the
background
current.
The
voltammogram is very well defined, even at this low concentration,
indicating its potential use for electrochemical sensor applications.
Limiting currents increased linearly with increasing ferrocyanide
concentration up to 1.2 M (Fig 18.3(b)).
Fig 18.3(a) Cyclic voltammogram for a diamond microelectrode of
radius 20μm for the oxidation of 200 nM K4Fe(CN )6 in 0.1 M KCl
(sweep rate, 2 mVs-1). (b) Calibration curve for K4Fe(CN)6 oxidation in
0. 1M KCl
In
contrast,
the
high
background
at
glassy
carbon
microelectrodes did not allow well-defined voltammograms to be
observed at low analyte concentrations. For example, for a
ferrocyanide concentration of 200 nM, the increment in the current
due to the analyte was only ~25% of the background current,
whereas for the BDD microelectrode of similar radius, the
corresponding value was ~200%.
18.3.2. Electrochemical
microfiber electrodes
behavior of diamond
A BDD microfiber (BDDMF) electrode was characterized by
performing voltammetric experiments using an outer-sphere redox
couple. Figure 18.4 shows the cyclic voltammogram for 1 mM
ruthenium hexaamine trichloride at a BDDMF electrode in 0.1 M
phosphate buffer (pH 7.1) at a sweep rate of 10 mV s-1. The
voltammogram shows the pseudo-steady state response, a
characteristic of microfiber electrodes. For a sweep rate of 100 mV
s-1, a peak-shaped voltammogram was observed, indicating that
planar diffusion is dominating the mass transport in the vicinity of
the electrode at relatively high scan rates.
Potential (vs SCE) /
-400 -300 -200 mV
-100
0
100
200
300
400
Current /nA
(b)
90 nA
(a)
Fig 18. 4. Cyclic voltammogr am for (A) 1 mM ruthenium hexaamine
trichloride in 0.1 M phosphate buffer and (B) 0.1 M phosphate buffer
at a diamond microfiber elec trodes. Sweep rate 10 mV s-1.
The current density of the fiber electrode was estimated from
the following equation, given for linear sweep voltammetry at
cylindrical microelectrodes[29,30]:
I = (n2F2Ca/RT)(0.446p-1 + 0.335p-1.85)
18.2
where I is the diffusion current density, a is the microelectrode
radius,  is the potential sweep rate, and p=(nFa2/RTD)1/2 is a
dimensionless parameter that characterizes the type of diffusion.
In the theoretical calculation, the value of 6.0 × 10-6 cm2 s-1 was
used [27] for the diffusion coefficient of ruthenium hexaamine
trichloride and 25 μm for the fiber radius, giving a current density
of 1380 nA mm-2. The experimental current density, calculated by
considering the fiber length of 0.8 mm, is 3916.34 nA mm-2. The
difference between measured and calculated current density (ca.
2.8) can be mainly attributed to the roughness factor. The
roughness factor of the diamond fiber calculated from the doublelayer capacitance measurement was 2.34. The other possibility is
that the rough surface of the electrode does not conform to the
microfiber model, and therefore, the formula above is not strictly
valid for diamond microfiber electrodes.
18.4. Electroanalytical Applications of Diamond
Microelectrodes
18.4.1. Detection of H2O2 at metal-modified
diamond microelectrodes
Despite its several superior properties, as mentioned above,
diamond has several limitations compared to metal electrodes,
such as slow kinetics for reactions involving adsorption and multielectron transfer processes, including hydrogen and oxygen
evolution reactions. However, since the low rates of the hydrogen
and oxygen evolution reactions result in the wide potential window
[31,32], this can be considered to be an advantage of using
diamond, especially in aqueous media. Another important multielectron transfer reaction is the oxidation and reduction reaction of
H2O2, which is generally enzymatically generated from the
oxidation reactions of biological materials, such as glucose, lactate,
pyruvate, and cholesterol. Therefore, the detection of H2O2 is
important for a wide range of applications in the electroanalytical
field.
Since diamond is inactive for the oxidation and reduction
reactions of H2O2, modification of the electrode is required to make
diamond suitable for the
enzyme-based biosensor application.
Tatsuma, et al. [33] reported the use of heme peptide and
horseradish peroxidase, types of redox enzymes, based on the
direct electron transfer between the diamond electrode and the
redox enzyme. Another promising approach is the deposition of
metal nanoparticles that have catalytic activity for the H2O2
oxidation- reduction reaction.
The modification of a BDDMF electrode with platinum
nanoparticles and its use for H2O2 detection are discussed in the
present chapter, based on the following reactions:
H2O2
H2O2+2H++2ePlatinum
deposition
Pt
Pt
on
O2+2H++2e2H2O
diamond
microelectrodes
was
performed electrochemically in 0.1 M H2SO4 containing 100 µM
K2PtCl6 by cycling between the potentials of –0.2 V and 1.2 V at 50
mVs-1. The electrode was then dipped into 0.1 M H2SO4, and the
same cycling potentials were applied until a stable cyclic
voltammogram was achieved, indicating the complete cleaning of
the Pt active area. The Pt active area was calculated from the
charge density for the hydrogen desorption reaction [34] between 0
and –0.2 V, using a standard value of 210 C cm-2 for
polycrystalline Pt [35]. We found that the Pt active area increased
linearly with deposition time (Fig 18.5), and the signal-tobackground ratio (s/b) for 1 mM H2O2 achieved its maximum value
for a 20-min Pt deposition (Fig. 18.6).
120
100
s/b
80
60
40
20
0
10
20
30
40
deposition time/min
50
60
Fig 18. 5. Plots of s/b value at 0.6 V as a function of Pt deposition
tim e, calculated from the cyclic voltammogram for 1 mM H 2O2 at
Pt-BDDMF electrode.
1.2
y = 0.0236x - 0.3995
2
R = 0.9946
Pt active area /cm 2
1
0.8
0.6
0.4
0.2
0
-0.2
10
20
30
40
50
60
deposition tim e /m in
Fig 18.6. Plots of Pt active area as a function of Pt deposition time.
Pt active area was calculated from the charge density of hydrogen
desorption reaction in 0.1 M H 2SO4.
This can be explained from the SEM images below. Figures
18.7(a) and (b) show SEM images of Pt-modified diamond
microfiber electrodes after 1 h and 20 min deposition, respectively.
It can be seen from both images that Pt nanoparticles were
distributed uniformly on the diamond surface. From Fig. 18.7(a),
after a 1-h deposition time, the amount of Pt loading was large,
with an average diameter (D1h) of 500 nm, and the number of Pt
particles deposited per unit real area (N1h) of ca. 8.0 107 particles
cm-2 [taking into account the roughness factor of the diamond
microfiber electrode [23] (ca. 2.34)]. The number of exposed surface
Pt atoms was estimated to be 5.82 × 1014 Pt atoms cm-2 from the
background CV. As the average distance between each particle
(estimated to be 1.12 m from N1h) is becoming small, the diffusion
layers of reactant were assumed to overlap each other [Fig 18.8(a)],
resulting in decreased electrochemical activity.
a
b
Fig 18.7. SEM images of Pt-BDDMF electrodes for (a) 1 hour and (b)
20 min Pt deposition time.
(a)
(b)
Fig 18.8. (a) Overlapping diffusion layers and (b) Ideal spherical
diffusion layers
In contrast, after a 20-min deposition [Fig. 18.7(b)], the
amount of Pt loaded on BDDMF was less (3.22 × 1013 Pt atoms
cm-2) with D20min = 400 nm and  = 6.0  106 particles cm-2. The
average distance between each particle for this electrode (4.09 m
from ) is 3.7 times greater than that for a 1-h deposition, and
this distance is considered to be close to the optimum value [36],
allowing an ideal spherical diffusion field to occur in the vicinity of
each particle. In this case, the electrode can be considered as a
micro-array of platinum particles that are uniformly distributed on
the diamond microfiber electrode (Fig. 18.8(b)).
Comparison experiments were performed using a bare
platinum microelectrode, a Pt-deposited diamond macroelectrode,
and a bare Pt macroelectrode. The s/b values for each electrode
taken for 1 mM H2O2 in 0.1 M PBS are summarized in Fig 18.9.
Fig 18.9. Comparison of S/B values at 0.6V calculated from cyclic
voltammogram for 1 mM H 2 O2 at Pt-diamond microfiber, Pt microfiber
electrodes, Pt-diam ond macroelectrodes, and Pt macroelectrodes.
The Pt-modified and bare Pt microelectrodes show higher s/b
values than the Pt-modified and bare Pt macroelectrodes,
respectively. This can be attributed to
the properties of
microelectrodes: non-planar diffusion profile, low background
current, and high sensitivity. Depositing the proper amount of
metal catalyst on the microelectrode further effectively enhances
the sensitivity of the microelectrode. Moreover, with the use of
diamond as a stable supporting material for metal deposition, the
resulting
microelectrode
overwhelmingly
outperforms
the
platinum microelectrodes.
18.5. Summary
Microelectrodes, due to their unique properties, such as small size,
non-planar diffusion, small capacitance, and small current, have
provided a major breakthrough in electrochemistry. Taking
advantage of the superior properties of diamond, the diamond
microelectrode is a promising new device for several applications
in electrochemistry. Furthermore, modification of the diamond
microelectrode with appropriate metal nanoparticles increases the
quality of the measurements, in the terms of sensitivity, stability,
and selectivity.
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