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Electrochemical determination of selected
neurotransmitters at electrodes modified with
oppositely charged carbon nanoparticles†
Magdalena Kundys,
a
Katarzyna Szot,*
ab
Ewa Rozniecka,
a
Martin J¨
onsson-Niedzi´
ołka,
a
Ruth Lawrence,
c
Steven D. Bull,
c
Frank Marken
c
and Marcin Opallo
a
The electrocatalytic oxidation of neurotransmitters on the electrodes modified with oppositely charged
carbon nanoparticles has been investigated. These nanoparticles were deposited at the electrode from
the aqueous suspensions via a layer-by-layer method. The electrocatalytic response was evaluated by
cyclic voltammetry, differential pulse voltammetry, and chronoamperometry. The modified electrode
exhibited good electrocatalytic properties towards not only dopamine oxidation, but also for epinephrine
and serotonin oxidation. This allows us to separate their voltammetric signals from the signals of
interfering substances such as ascorbic acid or uric acid. The obtained calibration curves are in the range
0.4–350 mM, 1–49 mM and 0.8–100 mM with detection limits of 0.4 mM, 1.0 mM and 0.8 mM for
dopamine, epinephrine and serotonin, respectively. In addition these carbon nanoparticulate electrodes
showed excellent sample to sample reproducibility (the relative standard deviations for n¼7 equal 0.7%)
and, maintained 94% of electrochemical signal corresponding to dopamine oxidation after 18 month
storage.
1. Introduction
Neurotransmitters are chemical substances which are respon-
sible for communication between nerve cells. Dopamine (DA),
epinephrine (EP) and serotonin (5-HT) are among the most
abundant neurotransmitters. Although their concentration in
most of the body is very low (normal levels in serum are <8.9
10
10
mol dm
3
, 1.1–1.4 10
8
mol dm
3
, and 4.5–12 10
7
mol dm
3
for DA, EP and 5-HT, respectively
1
) they have a
signicant impact on human endocrine and immune systems.
The lack of balance between them may increase the risk of
developing diseases such as Parkinson's, Alzheimer's, Schizo-
phrenia, various neuroblastoma, adrenocortical carcinoma,
pituitary adenoma or depression.
2,3
In order to diagnose these
diseases an assay of neurotransmitters is highly desirable.
Currently, a wide range of techniques such as chromatographic
methods,
4
electrophoresis,
5
electrochemical methods,
6
uo-
rimetry,
7
and mass spectroscopy
8
are applied for detection of
DA, EP and 5-HT (or their metabolites
9
) in real samples (serum
or urine).
10–13
The electrochemical methods are simple, highly
selective and sensitive, cheap and the electrochemical devices
are easy to miniaturize. These advantages make them suitable
for clinical analysis. However, the main drawback of the elec-
trochemical methods is the overlap of the electrochemical
signal of neurotransmitters with the signal of some interfering
substances such as ascorbic acid (AA) or uric acid (UA) which are
present in high concentrations: AA (0.1–0.6 mM) in the extra-
cellular uid of the brain,
14
UA in the blood (0.15–0.45 mM) or
that excreted in urine (1.19–2.98 mmol per day).
15
This problem
can be solved by electrode modication with enzymes from the
oxidoreductase group e.g. laccase,
16
and/or nanomaterials
11,17–20
–which exhibit electrocatalytic activity towards oxidation of
phenolic compounds (DA, EP) or aromatic amines (5-HT). This
signicantly improves the selectivity of electrochemical sensors
for neurotransmitters. Although successful application of
unmodied edge plane pyrolytic graphite electrodes for
neurotransmitters' sensing
10
in the presence of interfering
compounds was reported, this material is expensive and not
suitable for thin lm preparation.
Among nanoparticulate materials successfully applied for DA,
EP and 5-HT electrochemical sensing one can nd mainly
metal,
17,21
metal oxide
18
and carbon-based nanostructures.
22–24
The
latter group has gained attention due to its remarkable electro-
chemical properties (electrocatalytic ability, superb electrical
conductivity and high surface area). Carbon-based materials like
carbon nanotubes
19,25–27
or graphene
20,28,29
have been widely applied
for selective electrochemical determination of DA, EP and 5-HT.
Quite recently hydrophilic carbon nanoparticles (CNPs) with
a
Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52,
01-224 Warszawa, Poland. E-mail: kszot@ichf.edu.pl; Fax: +48 22 343 3333
b
Department of Molecular Biology, University of Gdansk, ul. Wita Stwosza 59, 80-308,
Gdansk, Poland
c
Department of Chemistry, University of Bath, Bath, BA2 7AY, UK
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ay01344a
Cite this: Anal. Methods,2014,6,7532
Received 5th June 2014
Accepted 8th July 2014
DOI: 10.1039/c4ay01344a
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phenyl sulfonate functionalities as one of the nanoparticulate
forms of carbon were introduced for electrode modication.
30–33
This material is commercially available from Cabot Corporation
(Emperor 2000) and its production is based on diazonium
34
chemistry or controlled vapour-phase pyrolysis of hydrocarbons.
35
Unlike carbon nanotubes or graphene, it has been known for many
years and it is widely used in industry, for example as a ller or a
pigment. It offers most of the advantages of nanocarbons like
extremely high surface area, high level of interfacial edge sites,
reactive surface sites and good electrical conductivity.
In contrast to other nanocarbon materials CNPs form a
stable suspension in water, because of their hydrophilic func-
tionalities. The negative charge of phenylsulphonate functional
groups makes them suitable for electrode surface modication
with a layer-by-layer procedure.
33
Their deposition at the electrode surface has been achieved
for example by encapsulation in the polymer,
36
using electro-
static interactions with polyelectrolytes
30,37,38
or with positively
charged objects (gold nanoparticles
39
and functionalized sili-
cate submicrometer particles
40,41
), sol–gel processed silicate
lms,
31
or just by drop-coating the CNP suspension on the
electrode surface.
42,43
Commercially available CNPs can be
further functionalized to replace the sulfonate functionalities
with positively charged ammonium groups. This modication
allows the production of three-dimensional nanoparticulate
lm electrodes,
44
built entirely from carbon.
The CNP-based electrodes
45
were already applied for elec-
trochemical sensing of biologically important substances:
acetaminophen and tramadol simultaneously,
42
naltrexone,
43
azathioprine,
46
piroxicam,
47
dopamine in the presence of
ascorbate,
30,41
and benzophenone or triclosan.
48
These elec-
trodes also provide favorable conditions for efficient electron
exchange between the electrode substrate and a wide range of
redox enzymes.
32,33,49,50
Recently one of these electrodes has
been successfully applied as an anode in a self-powered sensor
for ascorbic acid detection.
51
Although lms consisting of CNPs linked with ionomers
30
or
functionalized silicate submicroparticles
41
were earlier employed
for DA detection, here we propose to apply a lm electrode
entirely composed of oppositely charged carbon nanoparticles
for electrochemical determination of selected neurotransmitters:
DA, EP and 5-HT in the presence of interfering substances.
Although the detection limit of the obtained electrodes is too
high for analysis of real human (serum) samples these electrodes
can still, thanks to the wide analytical window, be applied for
monitoring of neurotransmitter release from cells in the near
eld.
52,53
In fact there are no reported instances of DA detection
using electrochemical methods that can measure DA level found
in serum, with the exception of the ow injection analysis. But
this is a method that is not discriminating and requires prior
separation of analytes using e.g. HPLC.
2. Experimental section
2.1. Chemicals and materials
DA, EP, UA, and 5-HT were purchased from Sigma–Aldrich, and
AA was from Riedel-de Ha¨
en. H
3
PO
4
and NaOH were purchased
from Chempur. Negatively charged CNPs (ca. 7.8 nm mean
diameter, with a typical bulk density of 320 g dm
3
, Emperor
2000) were supplied by Cabot Corporation (Dukineld, United
Kingdom). These nanoparticles were used for preparation of
positively charged CNPs following a procedure described
earlier.
44
Indium tin dioxide (ITO) coated glass plates (resis-
tivity: 8–12 Ucm) were obtained from Delta Technologies Ltd.,
USA. Water was ltered and demineralized with an ELIX system
(Millipore). All reagents were used as received.
2.2. Electrode modication
The carbon nanoparticles were immobilized onto indium tin
oxide (ITO) covered glass sheets via layer-by-layer assembly.
33
Before the preparation the substrate was cleaned with ethanol,
then with deionized water and nally heated for 30 minutes in a
tube furnace (Barnstead International) at 500 C in air to
remove organic impurities. Suspensions of both types of parti-
cles were obtained by mixing 3 mg of particles with 1 ml of
deionized water followed by sonication of the mixture for
1 hour. ITO slides were immersed alternately into the positively
and negatively charged CNP suspension for 1 minute. Every
such step was followed by drying and immersion in pure water
for 2 s to remove weakly bonded particles. The above procedure
will be called one immersion and withdrawal step in this paper.
The electrodes prepared by 1, 3, 5 and 10 alternative immersion
and withdrawal steps will be marked CNP (+/), CNP (3+/),
CNP (5+/) and CNP (10+/). The electrode surface was dened
by masking the electrode with scotch tape so as to expose a
circular area of 0.2 cm
2
. Electric contact was assured by a piece
of copper tape between a crocodile clip and the conducting side
of the ITO glass.
2.3. Instrumentation and cell
Electrochemical experiments were performed with an Autolab
PGSTAT 30 (Metrohm Autolab) electrochemical system with GPES
soware in a conventional three electrode cell. Modied ITO,
platinum wire (d¼0.5 mm) and Ag|AgCl|KCl
sat.
were used as the
working, counter and reference electrodes, respectively. All
experiments were carried out at ambient temperature (22 2C).
3. Results and discussion
3.1. Electrochemical behaviour of dopamine at the electrode
modied with carbon nanoparticles
Fig. 1 shows cyclic voltammograms of dopamine at the elec-
trodes modied with different numbers of layers of carbon
nanoparticles (curve (b–e)) and at a bare ITO electrode (curve
(a)) in a 0.1 M phosphate buffer solution (pH 5.0) in the pres-
ence of interfering substances (AA and UA). The concentration
of dopamine in this case is very high; therefore these experi-
ments were performed at pH 5.0 in order to avoid effects of DA
polymerization. At the bare ITO electrode, only two poorly
dened anodic peaks are visible indicating that the signals of
DA and AA overlap. In contrast, three well dened oxidation
peaks at about 0.2 V, 0.48 V and 0.58 V –corresponding to the
oxidation of AA, DA, and UA, respectively –are seen on
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voltammograms recorded at the carbon nanoparticulate elec-
trodes. Obviously this material catalyzes the oxidation of AA and
DA as can be judged by the negative shiof their peak poten-
tials. This leads to better separation of their electrochemical
signals. Reversible voltammetry is recorded for DA (see ESI
Fig. S1 and S2(A)†). Similar results were observed earlier on
other CNP modied electrodes where the protonated form of
DA probably interacted via electrostatic interactions with
negatively charged CNP, promoting accumulation of dopa-
mine.
14,41
Moreover, the value of the current density, at the CNP
modied electrodes, is signicantly higher than that on a bare
ITO electrode. This result together with voltammetric signal
separation indicates that CNPs create a well developed electrode
surface suitable for (bio)sensing.
33,49
For the number of deposition steps greater than three the
magnitude of the anodic peak current density increases only
slightly and the peak positions stay basically the same (Fig. 1).
This conrms that already the CNP (3+/) electrode exhibits
a strong electrocatalytic effect offering high current density and
signal separation and therefore it was selected for further
experiments. From SEM images it was noted that this was the
smallest number of steps that covers the whole electrode
surface without leaving bare ITO
33
(Fig. 2).
Additionally, voltammetric experiments which were per-
formed in the presence of a simple redox probe such as
Fe(CN)
63
does not indicate any accumulation effect (see ESI
Fig. S3†). This indicates strong electrostatic interactions
between both components of the lm contributing to its
stability. Also, as can be observed from the cyclic voltammo-
grams obtained for the electrode modied by CNP (3+/) and
bare ITO electrode the presence of a carbon nanoparticulate
material increases the capacitive current demonstrating a well
developed electro-active surface area (see ESI Fig. S3†). However
the increase of the faradaic current is not observed because the
electrochemical reaction occurs only at the outer layer of carbon
nanoparticles.
The electrochemical oxidation of dopamine and the studied
interfering compounds is two-electron coupled with two-proton
reactions.
10,30,54
Therefore, the behaviour of the CNP (3+/)
electrode was additionally studied in a wider pH range (Fig. 3).
Indeed, at pH 5.0 the peak potentials are shied towards more
positive values as compared with pH 8.0, showing that protons
participate in the electrochemical reaction. As can be seen in
the inset of Fig. 3 the peak potential for DA (E
p
¼0.082 pH +
0.84, R
2
¼0.91) oxidation varies linearly with pH with a slope
value diverging from the theoretical value of 0.059 V per pH
unit. This is probably due to adsorption of polymerised dopa-
mine formed during the electrooxidation reaction at the pH
higher than 5 that blocks the electrode surface.
55,56
As a result,
the studied electrode reaction at pH above 5 might be rather
quasi- than reversible.
The stability of the voltammetric response of the CNP (3+/)
electrodes was also evaluated (Fig. 4). This experiment was
carried out at pH 7.0 in order to simulate physiological condi-
tions. The magnitude of the peak current density corresponding
to DA (and also UA) oxidation decreased only by ca. 6% aer
18 months on shelf in air (778.89 mAcm
2
29.14 mAcm
2
for
n¼6). The signal corresponding to AA oxidation is more
affected by storage, but this is less important if DA is an analyte.
3.2. DA, EP and 5-HT sensing
The DPV method offers improved sensitivity in both the elec-
trochemical signal and the detection limit as compared to cyclic
voltammetry;
57
therefore the response of the DA in a phosphate
Fig. 1 Cyclic voltammograms obtained with (a) bare ITO and (b) CNP
(+/), (c) CNP (3+/), (d) CNP (5+/) and (e) CNP (10+/) electrodes
in 2 mM AA, 2 mM DA, and 1 mM UA in 0.1 M phosphate buffer at pH
5.0. Scan rate: 20 mV s
1
.
Fig. 2 SEM image of the ITO electrode coated by three immersion and
withdrawal steps to positively and negatively charged carbon nano-
particles aqueous suspensions alternately.
Fig. 3 Cyclic voltammograms obtained with a CNP (3+/) electrode
in 2 mM AA, 2 mM DA, and 1 mM UA in a 0.1 M phosphate buffer at pH
(a) 5.0, (b) 6.0, (c) 7.0, and (d) 8.0. Scan rate: 20 mV s
1
.
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buffer solution with pH 5.0 and 7.0 in the presence of the
interfering substances was investigated via application of that
technique (Fig. 5). Additionally, a CNP (3+/) electrode was
utilized for detecting other neurotransmitters such as EP and
5-HT in the presence of UA and AA (Fig. 6). DA, EP and 5-HT are
implicated in several neurological diseases, and at the same
time they coexist in biological systems inuencing each other.
Therefore, in terms of better diagnostics and studies under
chromaffin cells, it is useful to detect them separately in their
mixture.
10,12,13
Unfortunately, the simultaneous determination
of DA, EP and 5-HT with the CNP (3+/) electrode was not
possible (see ESI Fig. S4†) because of the similar oxidation
potentials of DA and EP (see ESI Fig. S1 and S2†) leading to
overlap of those peaks (see ESI Fig. S4 and S5†). Therefore they
have been studied separately.
As is seen in Fig. 5, the peak current corresponding to elec-
trooxidation of DA increases with the increase of the target
biomolecule concentration. However the peak current vs. DA
concentration (c
DA
) dependence is clearly not linear. This is
most likely due to adsorption of polymerised dopamine formed
during the electrooxidation reaction that blocks the electrode
surface
55,56
suggesting that maintaining pH at 5 is not enough to
prevent the polymerization of DA. By modeling the adsorption
using a Hill isotherm
14,58–60
the peak current vs. c
DA
dependence
can be tted to the function:
IDA ¼ScDA1
cDA
m
KAþcDAmþB;(1)
where Bis an offset, Sis the sensitivity, K
A
is related to the
adsorption strength and mis the Hill cooperativity coefficient.
In this case m¼0.75; a value less than unity, which means that
the adsorption is negatively cooperative. This model was
recently used to model the response curve of dopamine deter-
mination in a microuidic system. At low concentrations the
adsorption has a minor effect, and the response is simply given
as I
DA
¼Sc
DA
+B. For pH 5.0 and 7.0 respectively, the
detection limits are estimated to be 0.4 mM and 0.3 mMatS/N¼
3, and the relative standard deviations (RSD%) for n¼7 equal
0.7% and 1.1%. The effect of DA polymerization is more
pronounced at higher pH, which is clearly seen by the blocking
effect on the peak currents of UA and AA as in Fig. 5B.
The detection limit of the CNP (3+/) electrode is slightly
higher than that reported earlier for other CNP lms obtained
by the layer-by-layer method.
30,41
However, the proposed carbon
nanoparticulate electrode showed signicant advantage over
above-mentioned electrodes
30,41
and other nanocarbon based
electrodes in terms of reproducibility and stability (Table 1).
Additionally its preparation is fast and straightforward.
Further we explored the possibility of determination of the
neurotransmitters EP and 5-HT in the presence of UA and AA
separately with the CNP (3+/) electrode (Fig. 6). Fig. 6A and B
demonstrate results of DPV measurements in micromolar
solutions of epinephrine or serotonin in the presence of 2 mM
ascorbic acid and 1 mM uric acid. Under these conditions the
EP oxidation peak is seen at ca. 0.40 V whereas the signal of
5-HT is split into two oxidation peaks at ca. 0.05 V and 0.25 V.
Such results were earlier reported by Yao et al.
64
and explained
by the formation of electro-active intermediate products of 5-HT
oxidation.
64
From differential pulse voltammetry experiments (Fig. 6A) it
is clearly visible that the EP peak current is proportional to the
epinephrine concentration, and the CNP (3+/) electrode shows
a linear range from 1 to 49 mM with the detection limit (S/N¼3)
equal to 1 mM (Table 2). In the case of serotonin a similar effect
as that for DA is observed where the dependence of peak current
of DA on concentration is not linear (Fig. 6B). The obtained
Fig. 4 Cyclic voltammograms obtained with (a) fresh and (b) 18 month
stored CNP (3+/) electrodes in 2 mM AA, 2 mM DA, and 1 mM UA in a
0.1 M phosphate buffer at pH 7.0. Scan rate: 20 mV s
1
.
Fig. 5 DPV voltammograms obtained with a CNP (3+/) electrode immersed in (A) 2 mM AA, 0.4–350 mM DA and 1 mM UA in a 0.1 M phosphate
buffer at pH 5.0; (B) 2 mM AA, 0.3–160 mM DA and 1 mM UA in a 0.1 M phosphate buffer at pH 7.0. DPV parameters –scan rate: 20 mV s
1
, pulse
interval: 100 ms, pulse amplitude: 50 mV, and pulse width: 50 ms.
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calibration curve for serotonin is in the range 0.8–100 mM with
the detection limit (S/N¼3) equal to 0.8 mM (Table 2). Also
similar to the case of DA the intensities of the signals related to
AA and UA oxidation evolve with the increase of EP and 5-HT
concentration (Fig. 6A and B). This might result from adsorp-
tion of EP and 5-HT during their oxidation.
The proposed electrochemical sensor exhibits a higher
detection limit of epinephrine in comparison with the other
CNT
25,27,65,66
and graphene
28
based sensors, but a wider linear
dynamic range than that of CNT modied basal plane pyrolytic
graphite.
25
In the case of serotonin, the detection limit of the
CNP-based sensor is higher than that at a carbon ionic liquid
electrode modied with Co(OH)
2
nanoparticles and multi-wal-
led carbon nanotubes
26
and a graphene modied glassy carbon
electrode
67
but lower than that at multimembrane carbon ber
microelectrodes.
24
In order to evaluate response time and stability of the
obtained CNP-based sensor chronoamperometry was per-
formed. These experiments were carried out with stirring at
0.45 V, 0.40 V and 0.25 V in dopamine, epinephrine and
serotonin solutions, respectively. In order to avoid the impact
of the signals from interfering substances on the detection of
5-HT the 0.25 V potential was chosen instead of 0.05 V. Aer
successive addition of 182 ml of 1 mM neurotransmitter
solutions to 6.5 ml of phosphate buffer solution, a stepwise
growth of the oxidation current is observed (Fig. 7). The
current stabilizes aer 55 s, 55 s, and 16 s for DA, EP and
5-HT,respectively.InthecaseofDAandEPthesignalisquite
stable for 40 minutes; however for 5-HT it disappears aer 7
minutes. This may be due to the fact that serotonin reaches
saturation earlier than DA and EP, but this can also be
attributed to the fact it breaks down and therefore the
Fig. 6 DPV voltammograms obtained with a CNP (3+/) electrode immersed in (A) 2 mM AA, 1–49 mM EP and 1 mM UA in a 0.1 M phosphate
buffer, pH 5.0 and (B) 2 mM AA, 0.8–100 mM 5-HT and 1 mM UA in a 0.1 M phosphate buffer, pH 5.0. DPV parameters –scan rate: 20 mV s
1
, pulse
interval: 100 ms, pulse amplitude: 50 mV, pulse width: 50 ms.
Table 1 Comparisons of analytical parameters of different nanocarbon-based electrodes applied for determination of dopamine
a
Electrode material Interferences
Calibration
range (mM)
Detection
limit (mM)
RSD
(%) Stability Method Ref.
ITO/functionalized silicate particles/CNP AA, UA, AC, citric acid,
NADH, tryptophan
0.3–18 0.1 1.64 50% aer
10 days
DPV 41
ITO/CNP/PDDA AA 0.1–10 0.05 —— DPV 30
ITO/polyaminoamine-MWCNT/Ni
tetrasulfonated phthalocyanines
AA 2.5–240 0.54 —— CV 61
GC/SWCNT/cetylpyridinum bromide AA, UA, citric acid, glucose,
cysteine, hippuric acid
4–120 0.6 —— DPV 19
GC/OMC/Naon AA, UA 1–90 0.5 —— DPV 23
CFE/GEF AA, UA 1.36–125.69 1.36 1.8 96.3% aer
20 days
DPV 20
GC/CNO/PDDA AA, UA 50–4000 10 1.5 —DPV 22
GC/b-cyclodextrin-MWCNT/chitosan —0.1–25 0.06 4.6 85% aer
30 days
DPV 62
GC/functionalized-OMC/IL AA 0.1–500 0.0041
b
6.4 97.1% aer
1 week
DPV 54
Graphite/PDDA/MWCNT-polystyrene
sulfonate
AA, UA 50–350 0.15 2.5 90% aer
3 weeks
AC 63
ITO/CNPs AA, UA 0.4–350 0.4 0.7 94% aer
18 months
DPV This
work
a
OMC, ordered mesoporous carbon; GC, glassy carbon; MWNT, multi-walled carbon nanotube; SWNT, single-walled carbon nanotube; PDDA,
poly(diallyldimethylammonium chloride); IL, ionic liquid; CFE, carbon bre electrode; GEF, graphene owers; CNO, carbon nanoanion.
b
The
lowest concentration actually measured is 0.1 mM.
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concentration does not increase between measurements.
The serotonin calibration curve is also markedly non-linear,
and can be tted with the same model as used above
for DPVs.
4. Conclusions
In the current study lm electrodes composed of oppositely
charged carbon nanoparticles were utilized as electro-
chemical CNP-based sensors for detecting selected neuro-
transmitters. These electrodes exhibit electrocatalytic
oxidation of DA, EP and 5-HT. The coexisting AA and UA had
not interfered in detection of the above mentioned analytes,
due to an electrocatalytic effect. The constructed CNP-based
sensor exhibits a wide calibration range with good low
detection limit and stability. Even though the simultaneous
determination of each neurotransmitter from its mixture was
not possible with the CNP (3+/) electrode, this electrode can
be successfully applied for distinction of the two of them.
Also it is worth noting that the obtained CNP-based sensor is
reproducible from sample to sample and stable over 18
months' storage. Additionally, its preparation is fast,
straightforward, and precludes the usage of volatile organic
solvents. It uses very cheap, commercially available
substrates as compared to pyrolytic graphite, carbon nano-
tubes or graphene. Therefore, it seems to be a promising
candidate for sensing neurotransmitters and other biologi-
cally important molecules difficult to oxidize at standard
electrodes.
Acknowledgements
This project was funded by the European Union within Euro-
pean Regional Development Fund, through grant Innovative
Economy (POIG.01.01.02-00-008/08).
Table 2 Calibration curve parameters for the determination of DA, EP and 5-HT at a CNP (3+/) modified ITO electrode in 2 mM AA and 1 mM
UA in a 0.1 M phosphate buffer at pH 5.0
Biomolecule
Oxidation potential
(V vs. Ag/AgCl)
Calibration
range (mM)
Detection
limit (mM)
Sensitivity
(mAmM
1
)r
RSD (%)
for n¼5
DA 0.40 0.4–350 0.4 0.038 0.974 0.7
EP 0.40 1–49 1.0 0.063 0.930 1.8
5-HT 0.25 0.8–100 0.8 0.055 0.985 2.1
Fig. 7 Amperometric response of the CNP (3+/) electrode in a 0.1 M phosphate buffer (pH 5.0), after subsequent addition of (A) dopamine, (B)
epinephrine and (C) serotonin samples to a stirred solution. Every addition step corresponds to increase of DA, EP, and 5-HT concentration by 28
mM. The potentials were kept at (A) 0.45, (B) 0.40 and (C) 0.25 V.
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