1185
Microelectrodes Modified with [MIII(bpy)3](ClO4)3
(M ¼ Co and Fe) as Analytical Sensors for Fullerenes in
Flow Injection Analysis
Marta E. Plonska, Piotr Diakowski, Tadeusz Krogulec, and Krzysztof Winkler*+
Institute of Chemistry, University of Bialystok, Pilsudskiego 11=4, 15443 Bialystok, Poland; e-mail: [email protected]
Present address: University of California, Davis, CA, USA
+
Received: September 13, 2000
Final version: January 8, 2001
Abstract
Catalytic electroreduction of fullerene (C60) at Au or Pt microelectrodes covered with [MIII(bpy)3](ClO4)3 (M ¼ Co and Fe, and bpy ¼ 2,20 bipyridine) was investigated. Under constant potential oxidation in acetonitrile=toluene (1:4, v=v) solution containing [MII(bpy)3]2þ
complexes and (n-Bu)4NClO4 as a supporting electrolyte, the microelectrodes were modified with electrochemically inactive
[MIII(bpy)3](ClO4)3 films. The chemical reduction of electrochemically formed films by Cÿ
60 results in enhancement of fullerene reduction
current enabling thus for sensing of C60. Analytical performance of electrodes modified with [CoIII(bpy)3](ClO4)3 was better than those
III
III
covered with [Fe (bpy)3](ClO4)3. Microelectrodes modified with [Co (bpy)3](ClO4)3 were used to quantify the fullerene concentration in
flow injection analysis. In the proposed analytical procedure the solution containing both the [CoII(bpy)3]2þ complex and the C60 analyte
was injected to the flowing stream of an acetonitrile=toluene (1:4, v=v) mixture containing (n-Bu)4NClO4. The triangular potential
waveform was applied to the gold microelectrode. During the positively going potential scan the electrode was covered by an electrochemically inactive [CoIII(bpy)3](ClO4)3 layer. In the cathodic scan, the catalytic current of C60 reduction was recorded. A dependence of a
current signal of catalytic C60 reduction on fullerene concentration was linear in a concentration range 5610ÿ7 to 2610ÿ5 mol dmÿ3.
Reproducibility of the electrode response under FIA conditions was good.
Keywords: Fullerene electrochemistry, Chemically modified microelectrodes, Flow injection analysis, Electrocatalysis
1. Introduction
Physical and chemical properties of fullerenes have been
extensively studied since they had become available in large
quantities. A large number of reactions has been developed for
functionalization of fullerenes [1, 2]. Significant progress has
been made in production of higher fullerenes [3] and in synthesis
of endohedral fullerenes [4, 5]. Separation and isolation of the
fullerene products is very often a limiting factor in fullerene
derivative synthesis. High performance liquid chromatography
(HPLC) has proven to be a versatile and effective method for the
separation and purification of functionalized fullerenes. UVvisible spectroscopy is by far the most widely used detection
technique employed in the HPLC separation of fullerenes [6–9].
This technique has a detection limit of a few mmol dmÿ3 [10].
Fullerenes and their derivatives show very rich electrochemistry [11–15]. For C60 and C70 up to six one-electron
reversible reduction steps have been reported [11, 12]. This welldefined electrochemistry could be coupled with highly sensitive
electrochemical techniques to produce a very powerful detector
for a variety of fullerene separations. To date however, there are
only two accounts on the use of electrochemical detection for
analytical fullerene determination. That is, fast scan-rate cyclic
voltammetry at a microelectrode was used as a detection technique in the HPLC of fullerenes [10]. Also, electrodes modified
with electrochemically inactive [MIII(bpy)3](ClO4)3 (M ¼ Co and
Fe) films were used for quantification of fullerenes [16]. Electrodes modified with this solid phase catalyze reduction of fullerenes according to the mechanism shown in Scheme 1.
Chemical reaction between the solid film and fullerene anions
results in the increase of reactant concentration at the electrode
surface and, hence, significant increase of the fullerene reduction
current. The lower detection limit for C60 was of the order of
0.1 mmol dmÿ3 [16].
Electroanalysis 2001, 13, No. 14
The introduction of microelectrodes has revolutionized electrochemical measurements during the last few decades [17]. They
are widely used in electrochemical detectors employing a variety
of different flow techniques, such as flow injection analysis [18,
19], supercritical fluid chromatography [20, 21] or capillary
electrophoresis [22, 23]. Significant elimination of the ohmic drop
is one of the main advantage of microelectrodes over conventional-size electrodes. Therefore electrochemical measurements in
high resistance media, such as solutions with little or no
supporting electrolyte content [24, 25], in systems under critical
conditions [26, 27] or solutions of very low dielectric constant
solvents [28, 29] are possible. Chemical modification of microelectrodes provides a very powerful tool for electroanalysis [30].
Taking into account the advantages of microelectrodes, we
realized that obvious extension of the work published previously
[16] is to use these electrodes in quantitative determination of
fullerenes at the [MIII(bpy)3](ClO4)3 (M ¼ Fe and Co) modified
electrodes. It can be expected that the radial nature of the
diffusion profile at the microelectrode may influence the process
of solid phase formation on the electrode surface and catalytic
current of fullerene reduction. Our studies are focussed on both
of these aspects. We have also compared the analytical ability of
Scheme 1.
# WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2001
1040-0397/01/1410–1185 $17.50þ.50=0
1186
[FeIII(bpy)3](ClO4)3 and [CoIII(bpy)3](ClO4)3 in fullerene detection. In addition, we have examined the possibility of using the
[MIII(bpy)3](ClO4)3 modified microelectrodes for fullerene
determination in flow systems.
M. E. Plonska et al.
were controlled by personal computer equipped with a PCL-818
(B&C Microsystems Inc., Sunnyvale, CA) interface card for data
acquisition. Solutions used in FIA were deaerated with argon
purge before running the experiment. The flow-through cell was
also placed in Faraday cage.
2. Experimental
2.1. Materials
The C60 was purchased from MER Corp. (Tuscon, AZ) and
used without additional purification. Tetrabutylammonium
perchlorate, (n-Bu)4NClO4, (Sigma Chemical Co., Germany)
was dried under reduced pressure at 70 C for 24 h. Perchlorate
salts of the 2,20 -bipyridine (bpy) metal complexes were prepared
according to procedures describes in literature [31]. Acetonitrile
(99.8 %) and toluene (both Aldrich Chemical Co., Germany)
were used as received.
2.2. Apparatus
Cyclic voltammetry and square-wave voltammetry were
performed by using an EG&G=PAR 283 potentiostat (EG&G
Instruments Co., Princeton, NJ). All electrochemical measurements were carried out by using a three electrode system. A silver
wire immersed in 0.01 mol dmÿ3 AgClO4 and 0.09 mol dmÿ3 (nBu)4NClO4 in acetonitrile and separated from the analyzed
solution by a ceramic frit (Bioanalytical System Inc, West
Lafayette, IN) served as the reference electrode. The AgClO4
solution was replaced daily because of the instability of Agþ to
photoreduction. The stability of the reference electrode was
examined by measuring the ferrocene redox potential under
cyclic voltammetry conditions in the studied solution as a function of time. The formal potential of the ferrocene=ferrocenium
system was stable for about 12 h. The counter electrode was
made from a platinum wire. The 12.5 mm radius platinum or 5
and 25 mm radius gold microelectrodes, which served as working
electrodes, were manufactured by sealing the Pt or Au wire
(Goodfellow Metals Ltd., UK) into a soft glass capillary by using
a Bunsen burner flame. The capillary was then cut perpendicularly to its longitudinal axis and the surface was polished by
using an extra fine carborundum paper followed by a 0.3 mm
alumina slurry. Electrical contacts were made using silver epoxy
(Johnson Mattey Ltd., UK). The electrochemical cell was placed
in Faraday cage. The solution was deaerated with argon for
20 min prior to the electrochemical measurements.
A flow injection analysis (FIA) system used for the fullerene
determination consists of a 3-way valve (Cole Parmer Instrument
Co., Vernon Hills, IL), peristatic pump, Model 77120-70
(Barnant Co., Barrington, IL), and a laboratory-made flowthrough cell. Teflon tubing of 0.5 mm internal diameter was used
for all manifold lines. The cell construction was described in
detail elsewhere [32]. A two-electrode configuration cell was
used for detection because of simplicity of the flow-through cell
structure and ohmic drop elimination. Gold microelectrodes of 5
and 25 mm diameter served as working electrodes and a stainlesssteel tubing was used as counter electrode. Potential of the
working electrode was controlled against the counter electrode.
Since the current is very low at microelectrodes, polarization of
the counter electrode can be neglected. The working electrode
potential was controlled with a laboratory-made potentiostat
described elsewhere [33]. Both injection system and potentiostat
Electroanalysis 2001, 13, No. 14
3. Results and Discussion
In a previous article [16], we described both a procedure for
preparation of electrodes covered with [MIII(bpy)3](ClO4)3
(M ¼ Ru, Fe and Co) and preliminary results relating to the use
of these electrodes for determination of fullerenes. In the course
of the present work, similar studies were carried out by using
microelectrodes. Microelectrodes modified with [CoIII(bpy)3]
(ClO4)3 were used for fullerene quantification in FIA.
3.1. Microelectrodes Modified with [MIII(bpy)3](ClO4)3
(M¼Co and Fe) Films for Determination of C60
Since a [CoIII(bpy)3](ClO4)3 film deposited on the electrode
surface dissolves during chemical reduction by fullerene anions
present in solution, two possible cases can be considered [16]:
i) if the amount of the deposited [CoIII(bpy)3](ClO4)3 salt is
large and=or the concentration of the fullerene anion is relatively low, the catalytic current of the C60 reduction is controlled by the concentration of the fullerene in solution;
ii) if the amount of the deposited [CoIII(bpy)3](ClO4)3 salt is
relatively low and=or the C60 concentration is high, the catalytic current depends on the amount of the solid phase
covering the electrode surface.
Figure 1 shows voltammograms obtained at microelectrodes
covered with the electrochemically inactive [CoIII(bpy)3](ClO4)3
film at a low potential scan rate. The electrode surface was
modified by potentiostatic (0.2 V vs. Agþ=Ag) oxidation of
[CoII(bpy)3]2þ in acetonitrile=toluene (1:4, v=v) mixture
containing (n-Bu)4NClO4 as a supporting electrolyte. Qualitatively, the results are similar to those reported previously for
conventional-size electrodes [16]. However, the potential
separation between peaks of catalytic and diffusion controlled
processes of C60 reduction is larger for microelectrodes than for
standard-size electrode. At microelectrodes, this larger potential
difference allows better discrimination of the background current.
For high concentration of fullerene in solution (ii), the R2
catalytic peak of C60 reduction grows with the increase of the
amount of material deposited on the electrode surface. The peak
width at its half height is independent of the time of electrode
modification and equal to 104 mV. Additionally, the modification
time for fullerenes determination at microelectrodes can be
significantly reduced, which is particularly important for studies
in flow systems.
For a relatively low C60 concentration (i), the current signal
linearly depends on the amount of fullerene in solution for the
concentration range 5610ÿ7 to 2610ÿ5 mol dmÿ3. The lower
detection limit was 2610ÿ7 mol dmÿ3 for the signal to noise
ratio equal to 3. Therefore, microelectrodes show comparable
analytical performance to standard-size electrodes [16].
One of the advantage of microelectrodes is the possibility of
using high scan rates even for voltammetric experiments carried
out in solutions with relatively high resistance. This is particu-
Fullerenes in FIA
1187
Fig. 1. Cyclic voltammograms recorded in acetonitrile=toluene (1:4)
containing 5610ÿ4 mol dmÿ3 [CoII(bpy)3]2þ (a), 5610ÿ4 mol dmÿ3
[CoII(bpy)3]2þ and 2610ÿ5 mol dmÿ3 C60 (b), and 2610ÿ5 mol dmÿ3
C60 (c) at Au(25 mm) electrode. 0.1 mol dmÿ3 (n-Bu)4NClO4 was used as
supporting electrolyte. Prior to CV measurements, the microelectrode
was modified with [CoIII(bpy)3](ClO4)3 at a constant potential of
þ 200 mV for 10 s. Sweep rate was 200 mV sÿ1.
larly important from the point of view of further analytical
application of the modified microelectrodes in FIA measurements. Usually, this technique requires the use a relatively narrow
time window for electrochemical detection. However, the
amplification of the catalytic C60 reduction current significantly
depends on the scan rate. At high scan rates, the height of the
catalytic R2 reduction peak becomes smaller and the width of the
peak is larger presumably due to the kinetic limitation of the rate
III
of chemical reaction between Cÿ
60 and [Co (bpy)3](ClO4)3. Our
results indicate that the sweep rate of voltammetric detection
should not exceed about 1 V sÿ1 in order to obtain high sensitivity of fullerene detection.
Similar studies using microelectrodes were carried out in
solution containing a [FeII(bpy)3]2þ complex. Similarly to the
behavior observed for a [CoIII(bpy)3](ClO4)3 modified electrode,
the fullerene catalytic reduction current is significantly affected
by the sweep rate. The comparison of the catalytic current of the
C60 reduction at the [CoIII(bpy)3](ClO4)3 and [FeIII(bpy)3](ClO4)3
modified microelectrodes is presented in Figure 2. Figure 2a
shows the voltammetric behavior for a solution containing fullerene in the high concentration. Both bipyridine complexes films
were grown under similar conditions. The modification time and
the concentration of bipyridine complexes of Fe(II) and Co(II)
were the same. The modification potential was about 100 mV
more positive than the peak potential of [MeII(bpy)3]2þ (M¼Fe
or Co) oxidation. The integration of the surface under the peak of
the catalytic current in both cases gives approximately the same
charge density equal to 750 mC cmÿ2 indicating that the yield of
solid layer deposition on the electrode surface is similar for both
bipyridine complexes. However, the peak current of the catalytic
C60 reduction process, recorded at the electrode modified with
[CoIII(bpy)3](ClO4)3, is much higher and its width is lower.
Figure 2b shows square-wave voltammetric peaks of the catalytic
process recorded for solution of low C60 concentration. For the
same fullerene concentration, the signal obtained with the
[CoIII(bpy)3](ClO4)3 modified electrode is about twice as high as
the peak recorded at the electrode covered with [FeIII(bpy)3]
(ClO4)3. Both experimental results presented in Figure 2 indicate
that the cobalt bipyridine complex based layers exhibit much
Fig. 2. a) Cyclic voltammograms for 0.1 mol dmÿ3 (n-Bu)4NClO4 in
acetonitrile=toluene (1:4) solution of 1610ÿ3 mol dmÿ3 [CoII(bpy)3]2þ
and 3610ÿ4 mol dmÿ3 C60 (1), and 1610ÿ3 mol dmÿ3 [FeII(bpy)3]2þ
and 3610ÿ4 mol dmÿ3 C60 (2) at the 25 mm Au microelectrode. Prior to
CV measurements, the microelectrode was modified with [CoIII
(bpy)3](ClO4)3 (1) and [FeIII(bpy)3](ClO4)3 (2) for 15 s at a constant
potential of þ200 and þ900 mV, respectively. Sweep rate was
200 mV sÿ1. b) Square-wave voltammetric curves for 0.1 mol dmÿ3
(n-Bu)4NClO4 in acetonitrile=toluene (1:4) solution of 56
10ÿ6 mol dmÿ3 C60 at the 25 mm Au microelectrode modified with
[CoIII(bpy)3](ClO4)3 (1) and [FeIII(bpy)3](ClO4)3 (2). Background
current for curve (1) was obtained at bare 25 mm Au microelectrode.
The microelectrode was modified with [CoIII(bpy)3](ClO4)3 and
[FeIII(bpy)3](ClO4)3 for 15 s at a constant potential of þ 200 and
þ 900 mV, respectively. The potential step was 8 mV, the square-wave
amplitude 25 mV, and the square-wave frequency 15 Hz.
better analytical performance. Importantly, long-time usage of the
microelectrode results, however, in a decrease of the efficiency of
solid layer catalysis. This effect may be related to the contamination of the small microelectrode surface and inhibition of the
electrode processes.
3.2. Effect of the Electrode size on the Catalytic
Efficiency of the C60 Electroreduction at
[CoIII(bpy)3](ClO4)3 Modified Microelectrodes
The voltammetry and chronoamperometry at microelectrodes
differs from similar processes at conventional-size electrodes in
Electroanalysis 2001, 13, No. 14
1188
that the mass transport at microelectrodes is dominated by radial
diffusion [17]. This diffusion leads to rapid transport of the
electrode reaction products away from the electrode surface.
Edge diffusion, which dominates for microelectrodes, may affect
both efficiency of the [CoIII(bpy)3](ClO4)3 layer formation and
the studied catalytic currents. Possibly, the higher transport rate,
expected for very small electrodes, may result in depositing the
electrochemically inactive salt away from the electrode surface,
changing thus the properties of this solid phase. Such a behavior
was reported for modification of microelectrodes with electroactive polymers [34].
In order to examine the influence of the electrode size on the
formation of the [CoIII(bpy)3](ClO4)3 phase, the square-wave
voltammetry of catalytic C60 reduction in solutions of low
concentrations of fullerene were recorded at gold disk microelectrodes of different sizes. Figure 3 presents the dependence of
the catalytic current (iR2) on the time of the electrode surface
modification (tm) under potentiostatic conditions at 200 mV for
different size disk microelectrodes. Plots are similar for all
electrodes. For a modification time larger than about 10 s, the
catalytic current is practically independent of the time of solid
phase formation. For the 12.5 mm Pt electrode, the experiment
was also repeated in solution of twice as high fullerene
concentration. In this case, the catalytic current was constant for a
modification time close to 15 s. This limiting current was twice as
large indicating that it is limited by the fullerene concentration in
solution. Moreover, for a larger modification time (tm > 30 s), the
experiment can be repeated several times for the same modified
electrode in a solution containing C60 of the concentration of the
order of 10ÿ5 mol dmÿ3 with almost no changes of the catalytic
current. Hence, the same modified electrode can be repeatedly
used for the determination of fullerenes.
Fig. 3. Dependence of the square-wave voltammetric peak current of
catalytic C60 reduction on time of the microelectrode modification with
[CoIII(bpy)3](ClO4)3 at the 25 mm Au (u), 12.5 mm Pt (s and d) and 5 mm
Au (e) electrode. 2610ÿ5 (empty marks) and 4610ÿ5 (filed marks)
mol dmÿ3 C60, 0.1 mol dmÿ3 (n-Bu)4NClO4 in acetonitrile=toluene (1:4).
The potential step was 4 mV, the square wave amplitude 25 mV, and the
square wave frequency 30 Hz.
Electroanalysis 2001, 13, No. 14
M. E. Plonska et al.
Currents of the diffusion control process of the C60 reduction
recorded at the bare microelectrode and the catalytic reduction
process obtained at the same size modified electrode were
compared. For a concentration range 2610ÿ6 to
1610ÿ4 mol dmÿ3, linear relations were observed for both
processes. The ratio of the slopes of the ip–c relations obtained
for modified and bare electrodes determines the enhancement
effect of the catalytic process. This value was 5.1, 7.1, and 6.3 for
the 5 mm Au, 12.5 mm Pt, and 25 mm Au microelectrodes,
respectively. These results indicate that the catalytic current is not
significantly influenced by the size of the microelectrode.
3.3. Microelectrodes Modified with [CoIII(bpy)3](ClO4)3
as Sensors for Fullerene Determination in Flow
Injection Analysis
Two procedures were developed for the FIA measurements.
i) The microelectrode was modified, separately, with the
[CoIII(bpy)3](ClO4)3 film before injecting the solution containing only C60 and a supporting electrolyte.
ii) Solution containing both C60 and [CoII(bpy)3]2þ was injected
to the flowing stream of 0.1 mol dmÿ3 (n-Bu)4NClO4, in
acetonitrile=toluene and the electrode surface was modified
with a [CoIII(bpy)3](ClO4)3 film in every positively-going
potential scan as long as [CoII(bpy)3]2þ and C60 were present
in the vicinity of the electrode surface.
In both cases, the linear potential waveform was applied to
the electrode in the flow-through cell. In order to increase the
sensitivity of fullerene quantification, the cathodic part of
the valtommograms was integrated in the potential range corresponding to the first reduction step in the range ÿ200 to
ÿ500 mV yielding a curve of charge as a function of time. Since
the magnitude of the analytical signal of the fullerene catalytic
electroreduction and a peak shape depend on the sweep rate, the
sweep rate affects quantitative determination of the fullerene.
A rate of chemical reduction of [CoIII(bpy)3](ClO4)3 by Cÿ
60
limits the upper level of the sweep rate to be used. In order to find
optimum conditions for C60 quantification, the dependences of
the electrode response on sweep rate, flow rate, and volume of the
injected sample were examined under the FIA conditions by
using the procedure (ii). The signal to noise ratio (S=N) was a
maximum for the flow rate of about 1 mL minÿ1 and a sweep rate
close to 1 V sÿ1.
For the procedure (i), the microelectrode was modified either
ex situ or in situ. The ex situ modified microelectrode was
inserted into the flow-through cell and used as a sensor for the
fullerene determination. However, in this case the [CoIII
(bpy)3](ClO4)3 film was often damaged during transfer and
manipulation with the microelectrode in the cell. Since the film is
dissolved in the course of its reduction by Cÿ
60, this procedure
requires frequent disconnecting of the cell and external modification of the electrode. In a much more convenient in situ
approach, the microelectrode was modified in a flow-through
cell. For that purpose, acetonitrile=toluene (1:4) containing
[CoII(bpy)3]2þ and (n-Bu)4NClO4 was pumped through the cell
while a constant potential equal to 200 mV was applied to the
microelectrode. In order to increase the efficiency of the solid
phase deposition on the electrode surface, a very low flow rate
(0.25 mL minÿ1) was applied. Typically, the modification time
was equal to 20 s. However, the [CoIII(bpy)3](ClO4)3 film is not
stable under FIA conditions. The peak height decreases for
Fullerenes in FIA
1189
Fig. 4. Voltammetric response of the 12.5 mm Au microelectrode under
FIA conditions. Carrier solution was 0.1 mol dmÿ3 (n-Bu)4NClO4 in
acetonitrile=toluene (1:4). The flow rate was 0.8 mL minÿ1. Three samples of 27 mL of 2610ÿ5 mol dmÿ3 C60, 2610ÿ4 mol dmÿ3
[CoII(bpy)3]2þ and 0.1 mol dmÿ3 (n-Bu)4NClO4 in acetonitrile=toluene
(1:4) were injected. Sweep rate was 0.8 V sÿ1.
consecutive injections. Even for low concentrations of C60 and
thick [CoIII(bpy)3](ClO4)3 films, the charge of the catalytic
electroreduction of fullerene decreased significantly starting from
the third injection. This behavior is attributed to the dissolution of
the [CoIII(bpy)3](ClO4)3 film during reduction of the fulleride.
For the time window of the FIA peak, the potential is periodically
sweeped about 25 times over the potential range of the C60
reduction. Under these conditions, even a relatively thick film is
removed from the electrode surface.
Much more promising results were obtained when a solution
containing both C60 and [CoII(bpy)3]2þ was injected into the
flowing stream of an acetointrile=toluene mixture, according to
procedure (ii). Figure 4 shows changes of current as a function of
potential and time recorded under FIA conditions. At the
beginning of the experiment, a mixture of acetonitrile=toluene
containing only the supporting electrolyte was flowing through
the cell. Next a 27 mL sample of 2610ÿ5 mol dmÿ3 C60 and
2610ÿ4 mol dmÿ3 [CoII(bpy)3]2þ solution was injected. Injection causes changes of the voltammetric response. That is, in the
positive potential range, oxidation of the cobalt complex results
in formation of the electrochemically inactive film on the electrode surface (peak O1). At negative potentials, the current peak
(R2), related to the catalytic reduction of C60, is observed. The
current quickly returns to its original value once the injected
solution leaves the cell.
Figure 5a shows a typical dependence of the charge peaks
corresponding to the catalytic C60 reduction on the fullerene
concentration in injected samples. The calibration plot obtained
on the basis of these peaks is linear in the fullerene concentration
range 5610ÿ7 to 2610ÿ5 mol dmÿ3 (Fig. 5b). The correlation
coefficient was 0.9968 and the sensitivity determined from the
slope of the calibration plot was 2610ÿ5 mC dm3 molÿ1. The
positive intercept of the calibration plot is attributed to changes of
capacity currents due to modification of the electrode surface
with a film of low conductivity. The behavior of the electrode is
also reproducible. Insert plot in Figure 5a shows analytical
signals, corresponding to ten consecutive injections of the solution containing C60. Well-defined peaks are observed over a
Fig. 5. a) Dependence of the charge signal of catalytic C60 reduction at
the 12.5 mm Au microelectrode modified with a [CoIII(bpy)3](ClO4)3 film
under FIA conditions on C60 concentration. Carrier electrolyte was
0.1 mol dmÿ3 (n-Bu)4NClO4 in acetonitrile=toluene (1:4). The flow rate
was 0.8 mL minÿ1. Samples of 27 mL of acetonitrile=toluene (1:4) of
different concentrations of C60, 2610ÿ4 mol dmÿ3 [CoII(bpy)3]2þ, and
0.1 mol dmÿ3 (n-Bu)4NClO4 were injected. Voltammograms were integrated in the potential range ÿ200 to ÿ500 mV. Sweep rate was
0.8 V sÿ1. The insert plot shows FIA assay for ten consecutive injections
of 2610ÿ5 mol dmÿ3 C60, 2610ÿ4 mol dmÿ3 [CoII(bpy)3]2þ in acetonitrile=toluene (1:4) at Au (12.5 mm) electrode. Carrier solution was
0.1 mol dmÿ3 (n-Bu)4NClO4 in acetonitrile=toluene (1:4). The flow rate
was 0.8 mL minÿ1, sample volume 27 mL. Voltammograms were integrated in the potential range ÿ200 to ÿ500 mV. Sweep rate was
0.8 V sÿ1. b) Calibration plot of analytical signals shown in Figure 5a.
relatively flat baseline. The relative standard deviation for ten
consecutive injections was 4.17 %. The charge obtained by
integration of voltammetric curves is by about six times larger at
the modified electrode than at the bare microelectrode.
4. Conclusions
The [MIII(bpy)3](ClO4)3 (M¼Co or Fe) modified electrodes
exhibit a marked electrocatalytic effect with respect to the
reduction of C60. This effect is related to the chemical reaction
between a solid electrochemically inactive phase of bipyridine
complex of MIII and electrochemically formed Cÿ
60 . Regardless of
a high rate of mass transport at the microelectrode, which may
Electroanalysis 2001, 13, No. 14
1190
hinder formation of the solid phase on the electrode surface, an
electrochemically inactive [MIII(bpy)3](ClO4)3 film can be
deposited successfully on the electrode. The efficiency and rate
of the microelectrode modification is relatively high. Since stirring of the solution is not necessary during the solid phase
deposition, microelectrodes also provide more reproducible
conditions for modification than standard-size electrodes. The
size of microelectrode does not influence significantly the current
of catalytic fullerene reduction. In stationary solutions, the
linear relation between catalytic current and fullerene concentration is observed in the concentration range 1610ÿ6 to
2610ÿ5 mol dmÿ3. Comparison of the electrochemical behavior
of fullerenes at cobalt and iron bipyridine complex based films
shows that microelectrodes modified with [CoIII(bpy)3](ClO4)3
exhibit superior analytical performance.
Although modified microelectrodes show comparable analytical performance and similar behavior in terms of catalytic
activity toward electroreduction of fullerenes in stationary solutions to the standard-size electrodes [16], they offer advantages of
miniaturization of the flow-through cell and detection under short
time window conditions. Both these aspects are particularly
important from the point of view of the use of the FIA technique.
Under FIA conditions, the current response of the microelectrode
modified with the [CoIII(bpy)3](ClO4)3 film due to the catalytic
C60 reduction linearly depends on the fullerene concentration in
solution. The results show that these modified microelectrodes
can be applied to other flow solution techniques (e.g., HPLC)
frequently used for the fullerene separation and purification.
Although the method presented here is addressed to quantify
fullerenes, it could be easily well-adopted to other compounds
with formal potential more negative than that of the potential of
[CoII(bpy)3]2þ oxidation.
5. Acknowledgements
Authors express their gratitude to Professor A.S. Baranski for
providing the electronic system and the computer program used
in this work, and to Doctor D.A. Costa for helpful discussion.
This work was supported by the State Committee for Scientific
Research (KBN), Poland, project no. 3 T09 A 182 19.
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