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Accepted Manuscript
Title: Beneficial impact of oxygen on the electrochemical
performance of dopamine sensors based on N-doped reduced
graphene oxides
Authors: Piotr Wiench, Zoraida González, Rosa Menéndez,
Bartosz Grzyb, Grażyna Gryglewicz
SNB 23410
To appear in:
Sensors and Actuators B
Received date:
Revised date:
Accepted date:
Please cite this article as: Piotr Wiench, Zoraida González, Rosa Menéndez, Bartosz
Grzyb, Grażyna Gryglewicz, Beneficial impact of oxygen on the electrochemical
performance of dopamine sensors based on N-doped reduced graphene oxides, Sensors
and Actuators B: Chemical
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Beneficial impact of oxygen on the electrochemical performance of
dopamine sensors based on N-doped reduced graphene oxides
Piotr Wiencha, Zoraida Gonzálezb, Rosa Menéndezb, Bartosz Grzyba and Grażyna
Department of Polymer and Carbonaceous Materials, Faculty of Chemistry, Wrocław
University of Science and Technology, Gdańska 7/9, 50-344 Wrocław, Poland
Instituto Nacional del Carbón (INCAR-CSIC), Pintado Fe, 26, 33011 Oviedo, Spain
Keywords: dopamine, electrochemical sensor, nitrogen, reduced graphene oxide, selective
determination, sensitive detection
Corresponding author:
Graphical abstract
Nitrogen-doped reduced graphene oxides (N-rGOs) were synthesized hydrothermally.
N-rGOs as active electrode materials in electrochemical sensors of dopamine (DA).
Electrochemical sensor performance was influenced by oxygen content in N-rGOs.
Electronegative oxygen at the surface of N-rGOs attracts cationic DA (pH 7.4).
Oxygen influences the sensitivity, selectivity and limit of detection of DA sensors.
Graphene based electrochemical sensors are promising devices for direct and easy dopamine
determination. In this work, N-doped reduced graphene oxides (N-rGOs) were prepared via
hydrothermal treatment of graphene oxide (GO) and amitrole (3-amino-1,2,4-triazole) under
different experimental conditions (varying reaction temperature and time), which resulted in
each material having different oxygen content. Furthermore, the dopant nitrogen content was
comparable in the three N-rGOs and it was primarily pyridinic in nature. The three
synthesized N-rGOs were characterized by X-ray photoelectron spectroscopy, Fourier
transformed infrared spectroscopy, scanning electron microscopy, and nitrogen sorption at 77
K. Glassy carbon electrodes modified with N-rGOs were prepared and used as active
electrode materials in electrochemical sensors of dopamine (DA). The optimal operational pH
was determined to be 7.4, which is also the physiological value. During the electrochemical
detection of DA, it was found that there is a direct relationship between the oxygen content in
N-rGO and electrochemical performance of these sensors. Thus, the detection of DA in the
presence of ascorbic and uric acids was more sensitive and selective when the least reduced
N-rGO sample was used. The attraction between the electronegative oxygen in the graphene
structure and cationic DA facilitates the adsorption process which could explain these results.
However the limit of detection (LOD) was also higher in this case due to decreased
Graphene is a two-dimensional carbon material of single-atom thickness and has
attracted significant interest for applications in the fields of electrochemistry, electronics,
medicine, and optics [1, 2]. Due to its extraordinary properties such as high conductivity,
specific surface area, and simplicity of preparation, graphene has been applied in energy
storage [3], sensors and biosensors [4] and transistors [5]. Among the different methods of the
synthesis of graphene, the chemical oxidation of graphite by the modified Hummers method
[6, 7], with subsequent exfoliation and reduction is one of the most widely used procedures
that yields reduced graphene oxide (rGO). The oxygen functionalities present in the structure
of rGO facilitate the formation of its composites with metallic nanoparticles, leading to an
improved performance of the developed devices [8]. Furthermore, the properties of rGO can
be tailored by doping its structure with heteroatom dopants such as boron, phosphorus, sulfur
[9, 10], and nitrogen [11, 12]. In particular, nitrogen-doped reduced graphene oxides (NrGOs) have been used to replace platinum as catalysts in the oxygen reduction reaction (ORR)
taking place in fuel cells [13]. Additionally, N-rGOs have also been used as active electrode
materials in supercapacitors [14], lithium-ion batteries [15] and electrochemical sensors and
biosensors [16].
Dopamine (DA) is an important neurotransmitter that plays a crucial role in maintaining
hormonal balance and the central nervous system [17]. Schizophrenia and Parkinson’s disease
are known to be related to abnormal levels of DA. For these important reasons, there is
significant interest in the development of detection methods for DA. While conventional
techniques of DA determination such as spectrophotometry, electrophoresis, and highperformance liquid chromatography require sophisticated equipment [18−20], electrochemical
methods in contrast are simpler, more effective and enable a more rapid determination [21].
However, the coexistence of ascorbic acid (AA) and uric acid (UA) in the human body
together with DA affects its electrochemical detection because of the similar oxidation
potentials of these compounds [22, 23]. Sensors based on N-rGOs deal with this problem by
providing a reliable peak separation, which allows selective determination of DA, AA, and
UA [24].
Electrochemical detection of DA using N-rGOs as active electrode materials has been
thoroughly investigated [25]. Li et al. [26] have obtained N-rGOs by thermal annealing of a
mixture of graphene oxide (GO) and melamine and developed a DA sensor with a linear
working range of 0.12−0.22 mM in the presence of AA and UA. Sheng et al. [27] applied a
similar synthesis method of N-rGOs to the development of a sensor that was able to detect
DA in the range of 0.5−170 µM with a limit of detection (LOD) of 0.25 µM. Feng and Zhang
[28] prepared N-rGOs by chemical vapor deposition and used ethylenediamine as the carbon
precursor and nitrogen dopant. The detection of DA was linear in the range of 3−10 µM with
a LOD of 1 nM. A composite of N-rGO with manganese monoxide has been developed by
Chen et al. [29]. The N-rGO/MnO-based sensor exhibited a linear operational range of
10−180 µM with a LOD of 3 µM in the presence of AA and UA. Tadayon et al. [30]
synthesized a composite of N-rGO with the spinel CuCo2O4 by a solvothermal method and
using ammonia as the dopant. Detection of DA in the presence of melatonin and tryptophan
was linear in the range of 0.01−3 µM with a LOD of 3.3 nM.
The adsorption of cationic DA molecules on the surface of carbon materials is
facilitated by the presence of electronegative oxygen functionalities owing to the electrostatic
attraction between these moieties. The extent of this attraction phenomenon depends directly
on the amount of the electronegative oxygen groups, which in turn determines the electrical
conductivity and influences the electrochemical performance of these materials toward DA
detection [31, 32]. This phenomenon has been confirmed for oxidized carbon nanotubes
(CNTs) by Jacobs et al. [33]. The CNTs were functionalized with carboxylic, amide, and
octadecylamine groups and tested for DA sensing. It was found that carboxylic- and amidefunctionalized CNTs displayed an increased sensitivity and a shorter response time. Roberts et
al. [34] have demonstrated that hydroxyl groups present on the surface of carbon fiber
microelectrodes are responsible for even a 6-fold increase in the sensitivity of these materials
towards DA. On the other hand, functionalization of carbon fibers with carbonyl groups did
not result in a similar enhancement, although their sensitivity had improved in comparison
with the non-oxidized electrodes.
Recently, a tremendous number of papers concerning DA detection by means of NrGOs based sensors have emerged. It has been demonstrated a remarkable improvement in the
sensitivity and response time of DA sensors when oxygen-enriched electrode material is used.
However, to the best of our knowledge, the influence of such oxygen content (in terms of its
amount and functionalities) on other important parameters related to DA detection using NrGOs, such as LOD and linear range, has not been deeply examined to date. The novelty of
this work lies in the proper identification of the oxygen effect on the main parameters of the
N-rGO-based DA sensors, in particular, on their LOD (with values in the nanomolar scale),
sensitivity, selectivity and operational linear range. Moreover, it has been found that although
the enhancement of sensitivity and selectivity of the developed DA sensors is oxygendependent, it is not directly related to the improvement operational linear ranges and LOD
Furthermore, the N-rGO-based sensors have been synthesized by means of an easy and
environmentally friendly procedure involving hydrothermal reduction of GO and amitrole (3amino-1,2,4-triazole) as the nitrogen dopant [35]. Under specific reduction parameters, a
series of N-rGOs containing markedly different oxygen contents (11.9, 9.4, and 2.3 at.%)
have been obtained and tested towards DA detection, either with or without AA and UA.
A coal-tar pitch-based graphite was synthesized in INCAR-CSIC (Oviedo, Spain)
[36]. All chemicals used in the synthesis of N-rGOs and electrochemical sensing experiments
were of analytical grade (Sigma Aldrich) and used without further purification. Ultrapure
Milli-Q water was used in order to wash all N-rGOs after the synthesis and prepare all
analytical solutions. The supporting electrolyte of electrochemical measurements was
prepared from dipotassium hydrogen phosphate and potassium dihydrogen phosphate.
Dopamine hydrochloride was used to prepare analytical solutions. Prior to each experiment,
DA mixtures were protected from air and light exposure and examined immediately after
Hydrothermal preparation of the N-rGOs
For the preparation of N-rGOs, graphite was first oxidized following the modified
Hummers method [7]. In this procedure, 2 g of graphite was mixed with 2 g of NaNO3 and 96
mL of H2SO4. Next, 12 g of KMnO4 was added to this mixture with continuous stirring in an
ice-cooled bath. Subsequently, the mixture was heated to 35 °C and kept at this temperature
for 3 h with continuous stirring. After the reaction was complete, 400 mL of H2O2 was added
to the flask and the mixture was decanted and washed with Milli-Q water until a neutral pH
was reached. The resultant graphite oxide (GrO) was sonicated using an ultrasound bath to
obtain a water suspension of GO.
The nitrogen-doping of GO and its simultaneous deoxygenation was performed in an
autoclave under hydrothermal conditions at different temperatures and for different reaction
times with continuous stirring (500 rpm). Amitrole was used as the nitrogen dopant according
to the method previously described in detail in [35]. Briefly, 200 mL of GO (1 mg mL-1) was
mixed with 2 g of amitrole and reduced at 150 °C for 2 h and at 180 °C for 8 h to obtain the
N-doped reduced graphene oxides N-rGO-150-2 and N-rGO-180-8, respectively. After the
reaction was complete, the products were cooled, washed five times with Milli-Q water and
isopropanol, and dried overnight at 60 °C. The resultant samples have been labeled as N-rGOT-t, where T is the temperature (°C) of hydrothermal reduction and t is the time (h).
Additionally, the latter sample was annealed in ammonia at 700 °C for 30 min (N-rGO-1808/NH3).
Characterization of the N-rGOs
The elemental surface composition and functional groups distribution of the N-rGOs
were investigated by X-ray photoelectron spectroscopy (XPS) using a PHI 5000 VersaProbe
spectrophotometer. The deconvolutions of C1s core-level spectra were performed into four
individual peaks with a Gaussian-Lorentzian (70/30) peak shape using CasaXPS software.
The components of the C1s signal were attributed to the sp2 hybridized carbon (284.5 eV),
hydroxyl, epoxy groups, and C−N bonds (286.5 eV), carbonyl bonds and quinones (287.6
eV), and carboxyl groups (288.9 eV). N1s spectra were deconvoluted into pyridinic (N6,
398.7 eV), pyrrolic (N5, 400.3 eV), and quaternary nitrogen (NQ, 401.4 eV), amines and
amides (NC, 399.7 eV) and pyridine-N-oxide (NX, 402−405 eV). The morphologies of NrGOs were characterized by scanning electron microscopy (SEM, FEI model Quanta FEG 650
operating at 25 kV). Fourier transform infrared spectroscopic analysis (FTIR) was performed
with a Nicolet 8700 spectrometer in attenuated total reflectance mode (ATR-FTIR) using the
Pike MIRacle accessory equipped with a Ge crystal. The textural parameters of N-rGOs were
determined from the N2 adsorption/desorption isotherms at 77 K using an Autosorb iQ gas
sorption analyzer (Quantachrome). Prior to these measurements, the samples were outgassed
at 120 °C for 11 h. The specific surface areas (SBET) were calculated using the Brunauer-
Emmett-Teller equation and the amount of nitrogen adsorbed at the relative pressure of pp0-1
= 0.96 was used to determine the total pore volume (VT). The Dubinin-Radushkevich equation
was applied to estimate the micropore volume (Vmic). The mesopore volume (Vmeso) was
calculated as a difference between VT and Vmic. The pore size distribution was determined
from the isotherm using the quenched-solid density functional theory method (QSDFT).
Preparation of the working electrodes and electrochemical detection of DA
A glassy carbon electrode (GCE, diameter 10 mm, ALS Co., Japan) was polished with
0.3 µm aluminum oxide slurry and washed with Milli-Q water. The suspensions of N-rGO150-2, N-rGO-180-8, and N-rGO-180-8/NH3 were prepared by adding 12 mg of the
corresponding graphene material to 3 mL of a 1:1 mixture of dimethylformamide and Milli-Q
water followed by ultrasonication for 3 h. Subsequently, 2.5 µL of each of the prepared NrGO suspensions were drop-cast onto the GCE surface using a Hamilton microsyringe.
Finally, the three modified electrodes were dried in an oven at 60 °C.
Electrochemical measurements were performed in a three-electrode cell with modified
GCE, Ag/AgCl/3.5 M KCl, and graphite rod as the working, reference, and counter
electrodes, respectively. Cyclic voltammetry (CV) tests were performed in a potential range
between -0.8 V and 1.0 V at a scan rate (vscan) of 100 mVs-1. Differential pulse voltammetry
(DPV) measurements were performed with pulse amplitude, width, period and increment of
0.10 V, 0.02 s, 0.10 s and 0.02 V, respectively. Electrochemical impedance spectroscopy
(EIS) experiments were performed from 0.1 to 105 kHz at the formal potential of DA
oxidation. A solution of 0.1 M phosphate buffer solution PBS (pH 7.4) containing 100.0 µM
DA was used as electrolyte. All experiments were carried out at room temperature by using a
VMP3 potentiostat-galvanostat (Bio-Logic, France).
Results and discussion
Characterization of the N-rGOs
The morphology of the different N-rGOs was investigated by using SEM. N-rGO-150-2
and N-rGO-180-8 exhibit similar structures consisting of large, sharp-edged agglomerated
sheets (Fig. 1). Under higher magnifications, crumpled graphene nanosheets could be
observed in both samples. However, the surface of N-rGO-180-8/NH3 was found to consist of
small irregular granules that had formed on etching of the material by ammonia. At the high
annealing temperature of 700 °C, NH3 penetrates into the N-rGO and causes fragmentation of
the nanosheets into granules. As a result, aggregated regions of diminished graphene flakes
are formed. At higher magnification, the presence of randomly distributed graphene
nanosheets is also revealed.
The surface elemental compositions and oxygen/nitrogen functional group distributions
of the N-rGOs were determined by XPS. It was found that the carbon content of the prepared
materials increased with an increase in the temperature of the hydrothermal treatment (Table
1). At the reaction temperature of 150 °C (N-rGO-150-2), the hydrothermal reduction with
amitrole results in 11.9 at.% O, 11.2 at.% N, and the lowest C/O atomic ratio of 6.5 among the
studied N-rGOs. When the temperature was raised to 180 °C, the oxygen content reduced to
9.4 at.% and slightly more nitrogen incorporated into the structure of N-rGO-180-8 (13.4
at.%). However, the sample annealed in ammonia at 700 °C (N-rGO-180-8/NH3) underwent
significant reduction, as evident from its higher C/O atomic ratio (36.1), and also had the
highest N content (14.6 at.%). These results can be attributed to the simultaneous
decomposition of thermally unstable oxygen groups such as carboxylic [37] and the reaction
of the oxygen functionalities with ammonia in this sample [38, 39].
Deconvolutions of C1s and N1s core-level XPS spectra are shown in Fig. 2 and the
distributions of the oxygen- and nitrogen-containing functional groups are summarized in
Table 2. It is evident that the sp2 hybridized carbon was predominant in all the N-rGOs
samples. The content of hydroxyl groups and C−N bonds in the structure of the more reduced
N-rGOs changed from 16.2 to 17.2 at.%, implying that a simultaneous removal of oxygen and
incorporation of nitrogen atoms takes place. Furthermore, carbonyl groups were present in
these samples to a lesser extent while the carboxyl functionalities could be considered as
residual groups in N-rGOs. Deconvolution of the N1s spectra revealed that the incorporated
nitrogen atoms were mainly in a pyridinic form and constitute 47.6–64.8% of the total
nitrogen content. At higher temperatures of hydrothermal treatment, the pyridinic nitrogen
content of the samples increases due to the ongoing restoration of the aromatic domains. The
amine nitrogen content in N-rGO-180-8 was lower than that of N-rGO-150-2 as a result of the
higher temperature treatment, which suggests its thermal decomposition. The pyrrolic
nitrogen content was comparable in all the studied N-rGOs (2.0−2.3 at.%). An increase in the
reaction temperature from 150 to 180 °C promotes the formation of quaternary nitrogen (1.9
vs. 0.2 at.%). It is interesting that annealing in ammonia at 700 °C led to a further decrease in
the content of carbonyl, carboxyl, and amide groups, a result that is consistent with the
rationale that thermal decomposition of these functionalities takes place (Table 2). Therefore,
as a result of the hydrothermal reduction of GO with amitrole and further annealing in
ammonia, a series of N-doped materials with comparable amount of nitrogen (11.2−14.6
at.%) but markedly different oxygen content (11.9-2.3 at.%) were obtained. The composition
of the produced N-rGOs is crucial for evaluating the influence of the oxygen content on their
performance as DA sensors.
In order to confirm the functional groups distribution of oxygen and nitrogen in the NrGOs, their ATR-FTIR spectra were collected (Fig. 3). The intensity of the broad band at
3600−4000 cm-1 that corresponds to hydroxyl groups was observed to decrease for samples
synthesized at higher temperature, which implied that the -OH groups were removed on
increasing reaction temperature. The peak at 1560 cm-1 corresponding to the stretching
vibrations of C=C and/or C=O bonds and N-H bending vibration [40] had the lowest intensity
for the N-rGO-180-8/NH3 sample. This result can be attributed to a larger degree of
elimination of C=O groups during the decomposition of the organic matrix of N-rGO-180-8
as the reaction temperature increases from 180 to 700 °C and leads to the formation of more
aromatic carbon domains. The intensity of the band at 1710 cm-1 decreases for N-rGO-180-8
and completely disappears in the spectra of N-rGO-180-8/NH3, which indicates the removal
of carboxyl groups. The broad band that has a maximum at 1110 cm-1 is attributed to the
overlapped response of hydroxyl and epoxy functionalities and is lowest in intensity for the
most reduced sample [41].
The textural properties of the N-rGOs were investigated by nitrogen sorption at 77 K.
Fig. 4 shows the adsorption-desorption N2 isotherms. According to the IUPAC classification,
isotherms of the type IV were obtained for all the three samples, indicating a micromesoporous structure [42]. Among the studied N-rGOs, N-rGO-150-2 exhibited the largest
hysteresis loop, indicating the highest contribution from mesopores (Table 3). Furthermore,
the BET surface area and a total pore volume of N-rGO-150-2 were found to be 390 m2g-1 and
0.329 cm3g-1, respectively, which are higher than those of N-rGO-180-8 (354 m2g-1 and 0.212
cm3g-1, respectively). N-rGO-180-8/NH3 exhibited an increased porosity development due to
etching with ammonia, resulting in the highest SBET and VT values (584 m2g-1 and 0.351 cm3g1
, respectively).
Electrochemical performance of the N-rGOs towards DA detection
3.2.1 Characterization of electrodes
The electrochemical response of DA on the different N-rGOs electrodes was
investigated by means of CV experiments. Fig. 5a shows the CVs obtained on the bare GCE
and GCE modified with rGO-180-8, N-rGO-150-2, N-rGO-180-8, and N-rGO-180-8/NH3 in
0.1 M PBS (pH 7.4) containing 100.0 µM of DA. The bare GCE showed a negligible DA
oxidation signal (260 mV, 9.5 µA). After the modification of GCE with rGO-180-8, a minor
improvement in DA oxidation signal was revealed (150 mV, 42.6 µA). However, a significant
enhancement in the electrochemical performance can be observed after its modification with
N-rGO-150-2. A reversible redox process was developed with an anodic peak at 210 mV
(66.5 µA) corresponding to reversible oxidation of DA to o-dopaminoquinone. Moreover, an
additional faradaic process could also be observed at more negative potentials related to the
leucodopaminochrome/dopaminochrome redox pair [43, 44]. Modification of GCE with NrGO-180-8 resulted in a slightly lowered DA oxidation overpotential and an enhanced anodic
current (80.7 µA). A markedly enhanced electrochemical performance was achieved by
modifying GCE with N-rGO-180-8/NH3. In this case, DA was oxidized at 200 mV (145.6 µA)
with a significant capacitive current contribution. This last result could be explained by the
higher BET surface area developed during ammonia etching at high temperatures [45]. In
addition, Fig. 5b shows the Nyquist plots recorded on the bare GCE and after its modification
with the different N-rGOs. According to previous studies, the first point of impedance spectra
might be related to the material resistivity. An expanded view of the high frequency region
indicates that the modification with N-rGOs significantly decreased this value. The bare GCE
electrode exhibits a high resistivity (142.4 Ω). After its modification with rGO-180-8, the
resistivity was slightly decreased to 111.9 Ω and it was higher than that of N-rGO electrodes.
For the latter, the resistivity was found to follow the order consistent with that of decreasing
oxygen content as follows: N-rGO-180-8/NH3 (49.9 Ω) < N-rGO-180-8 (53.6 Ω) < N-rGO150-2 (57.1 Ω). The results of the CV and EIS measurements suggest that N-rGOs can be
proper active materials towards DA detection due to the enhancement in the measured current
densities and the reduced overpotential of the DA oxidation.
3.2.2 Optimization of the working pH value
The effect of the pH of the supporting electrolyte on the electrochemical response of the
different electrodes towards DA was examined by CV (Fig. 6a−c). Fig. 6d−f shows the
influence of pH (5.8−8.0) on the DA anodic peak potential and current intensities on the three
N-rGOs in 0.1 M PBS containing 100.0 µM of DA. In terms of the anodic peak currents of
66.8, 80.7, and 145.5 µA for N-rGO-150-2, N-rGO-180-8, and N-rGO-180-8/NH3,
respectively, the optimal pH value for DA oxidation on the tested electrodes was determined
to be 7.4, which is also the physiological value [46]. Moreover, the anodic peak potential
values were also observed to shift negatively with an increase in the pH of the supporting
electrolyte. This observation could be explained due to an improvement in the reversibility of
the investigated faradaic process that involves the deprotonation of DA during the oxidation
[47], followed by the protonation of amine group in DA to form a cation [48]. The ionized
form of DA is attracted to the electronegative oxygen functionalities present on the surface of
N-rGOs, resulting in an enhanced adsorption and improved electrochemical response in terms
of selectivity and sensitivity.
3.2.3 Influence of the scan rate
The influence of the scan rate (scan) on the electrochemical performance of the different
N-rGOs towards DA detection was also studied. Fig. 7a−c shows the CVs recorded at
increasing scan rate (from 2 to 250 mVs-1) in 0.1 M PBS containing 100.0 µM of DA. The
relationship between the measured anodic currents and the square root of the scan rates is
illustrated in Fig. 7d−f. All N-rGOs electrodes exhibit a linear relationship between these
electrochemically active surface area (ECSA) for these electrodes was estimated by means of
the slope of the regression line using the Randles-Sevcik equation [50]. With increase in the
temperature of the hydrothermal treatment, the ECSA increases from 8.9 m2g-1 for the NrGO-150-2 sample to 16.8 m2 g-1 for N-rGO-180-8. However, the best results were obtained
for the N-rGO-180-8/NH3 electrode that has an ECSA of 30.4 m2g-1, which is related to its
highest BET surface area, and the largest micropore volume. The high number of
electroactive sites, good electrical conductivity, and diffusion based mechanism of DA
oxidation indicate that N-rGOs are very promising materials for obtaining active electrodes to
detect DA.
3.2.4 Calibration curves, LODs, and sensitivities of the N-rGOs
DPV was employed for the detection of DA as this technique is highly sensitive and
responsive towards faradaic processes [51]. Fig. 8a–c shows the baseline corrected DPVs of
N-rGOs. As expected, the anodic peak currents increase linearly with an increase in the
concentration of DA. Fig. 8d–f shows the calibration curves with corresponding equations
and R2 values. The LOD for the three N-rGO electrodes (signal-to-noise ratio, S/N = 3) was
calculated according to equation:

where S is the standard deviation of the blank sample and b is the slope of the calibration
curve. Table 4 shows LOD values, linear ranges and sensitivities determined for N-rGO-1502, N-rGO-180-8, N-rGO-180-8/NH3 and other comparable electrode materials. It can be
remarked that the hydrothermally produced N-rGOs exhibit enhanced sensitivity and operate
in a wider linear range, maintaining a very low LOD when compared with other DA sensors
presented in literature. These good results represent a significant improvement, as highly
sensitive sensors work usually in a very limited linear range (Table 4) [24, 26, 27, 28, 30, 52].
In addition, the developed sensors operate in physiological pH, are mono-component and the
synthesis of active material is simple and scalable. The LODs of 1500.0, 630.0, and 410.0 nM
were calculated for N-rGO-150-2, N-rGO-180-8 and N-rGO-180-8/NH3, respectively, which
are comparable to previously reported values [23, 25, 52]. The lower LOD of N-rGO-1808/NH3 can be attributed to its improved electrical conductivity as a consequence of a higher
reduction degree. However, the N-rGO-150-2 electrode exhibits the highest LOD as it
possesses the highest amount of oxygen (11.9 at.%). Thus it may be concluded that the
removal of oxygen at elevated temperatures of hydrothermal treatment result in a lower LOD.
The linear ranges determined for the detection of DA were 3.0−70.0, 2.5−100.0, and
0.5−150.0 µM using electrodes N-rGO-150-2, N-rGO-180-8, and N-rGO-180-8/NH3,
respectively. The markedly wide linear range for N-rGO-180-8/NH3 can be explained
attending to its large SBET, ECSA, and micropore volume (0.213 cm3g-1), thus providing a
large number of electroactive sites which can adsorb more DA molecules.
The slope of the calibration curve is related to the sensitivity of the sensor [53]. The
sensitivities of N-rGO-150-2, N-rGO-180-8, and N-rGO-180-8/NH3 were determined to be
3.14, 2.22, and 1.82 µAµM-1, respectively. N-rGO-150-2 displays the highest sensitivity
because the DA molecules are attracted to the surface of this electrode owing to the large
number of electronegative oxygen result in the most intense anodic peaks related to DA
oxidation. The attraction is weaker in the case of the most reduced sample, which results in a
lower sensitivity. Therefore, DPV results suggest that the selection of active electrode
material of DA sensors needs to be optimized in terms of their oxygen content in order to
achieve not only a good LOD but also a desirable sensitivity.
3.2.5. Selectivity of DA detection
In order to evaluate the selectivity of the different N-rGOs towards DA detection, DPV
measurements were also performed in presence of AA and UA [54]. The baseline corrected
DPVs of N-rGOs in 0.1 M PBS with different concentrations of DA and 300.0 µM of AA and
UA and illustrated in Fig. 9. The sample N-rGO-150-2 (Fig. 9a) showed a clear response
signal towards DA (280 mV) in the concentration range of 3.0−70.0 µM, corroborating the
results obtained without interferences. Furthermore, no peaks related to redox processes of
AA were observed although a signal of UA oxidation can be observed at 410 mV. The NrGO-180-8 showed an enhanced response towards DA (300 mV) and UA (475 mV) and a
weak signal of AA at 110 mV could also be noticed. The lowest concentration of DA that can
be detected was 5.0 µM, which is higher than that detected in the absence of interferences (2.5
µM). The presence of AA and UA significantly influence the detection of DA when N-rGO180-8/NH3 (275 mV) is used as the sensor. Distinct responses towards AA and UA are
developed at 5 mV and 440 mV, respectively. Furthermore, it was possible to clearly
distinguish between DA and UA only at a concentration of 30.0 µM. The highest selectivity
of the least reduced sample towards the detection of DA can be explained by the enhanced
attraction of the cationic DA to its surface due to the presence of a high amount of
electronegative oxygen and the repulsion of anionic AA and UA [55, 56]. These results
indicate that oxygen functionalities considerably affect the selectivity of the DA sensor. Fig. 9
d-f shows the calibration curves of N-rGOs towards DA detection determined in the presence
of AA and UA (300.0 µM). The sensitivity of N-rGO-150-2 was reduced to ~77% of the
initial value (2.4 vs. 3.1 µAµM-1). N-rGO-180-8 exhibited a decrease to ~91% (2.0 vs. 2.2
µAµM-1). At this point it is important to remark that the lowest drop in sensitivity was
obtained for N-rGO-180-8/NH3, i.e. ~94% of the initial value (1.7 vs. 1.8 µAµM-1). To sum
up, all developed N-rGO-based sensors are able to detect DA at very low concentrations in
the presence of AA and UA. N-rGO-150-2 exhibits relatively high LOD and sensitivity with a
linear range similar to that of N-rGO-180-8 which has over twice lower LOD value (Table 4).
However, from the analytical point of view, N-rGO-180-8/NH3 is the most suitable active
electrode materials for DA sensing due to a combination of low LOD, a wide linear range and
reasonably high sensitivity. In addition, N-rGO-180-8/NH3 exhibits the lowest drop in
sensitivity in the presence of AA and UA. The high capacity current is a consequence of a
highly developed surface area of N-rGO-180-8/NH3, resulting in extension of the linear range
by providing more active sites for DA adsorption.
The above findings prove the influence of oxygen in N-rGOs on the electrochemical
performance of DA sensors. A highly electronegative oxygen content enhanced sensitivity
and selectivity of N-rGOs towards DA sensing, as cationic DA is electrostatically attracted to
the electrode material, while anionic AA and UA are repelled. However, better LOD values
and linear ranges were achieved for a more reduced sample due to higher electrical
conductivity and increased ECSA. The electronegativity of N-rGO is crucial in tailoring DA
sensor, being necessary to reach a compromise between the specific parameters of the sensor.
3.2.6 Reproducibility, stability and repeatability
The reproducibility of the GCE/N-rGOs was determined by preparing five different
modified electrodes. The relative standard deviation (RSD) of the DA oxidation signal was
3.13, 5.72 and 6.22% for N-rGO-150-2, N-rGO-180-8 and N-rGO-180-8/NH3, respectively.
The stability of the sensors was evaluated after storage of modified electrodes immersed in
PBS (pH 7.4) in 25°C for 14 days. The GCE/N-rGOs retained 85.7, 84.2 and 74.4% of the
initial signal for N-rGO-150-2, N-rGO-180-8 and N-rGO-180-8/NH3, respectively. The
repeatability was resolved by measuring the 100.0 µM DA oxidation signal of five
independent solutions. The RSD was 2.73, 3.44 and 3.79% for N-rGO-150-2, N-rGO-180-8
and N-rGO-180-8/NH3, respectively, demonstrating respectable sensor performances.
In this work, three samples of N-rGOs were obtained by the hydrothermal reduction of
GO using amitrole as the nitrogen dopant. The reaction was performed at different
temperatures for a varying duration in order to obtain N-rGOs with desirable oxygen content
but similar amounts of nitrogen. The highest degree of GO reduction was obtained by thermal
annealing of N-rGO in ammonia. The N-rGOs prepared in this manner were used as active
electrode materials in DA sensors. After the modification of GCE with N-rGOs, a significant
enhancement of its electrochemical performance was observed. All sensors worked at
physiological pH, their LOD values, linear ranges, sensitivity and selectivity were found to be
dependent on the oxygen content of the active material. Due to its cationic form at
physiological pH, the adsorption of DA at the surface of the less reduced N-rGO is facilitated
by the higher amount of electronegative oxygen, ultimately resulting in a high sensitivity of
N-rGO-150-2 towards DA detection. Moreover, the selectivity towards DA determination in
the presence of AA and UA is also affected by the electronegative oxygen functionalities that
repel the anionic AA and UA. However, the LOD and linear range is improved by the
removal of oxygen in N-rGOs with the highly reduced N-rGO-180-8/NH3 material,
performing the best in this regard and detecting DA at nanomolar levels. These results suggest
that, in order to obtain a suitable electrode material based on N-rGOs for DA detection, an
optimization of the oxygen content is essential to reach a compromise among LOD,
sensitivity, and selectivity.
This project was supported by the Wrocław Centre of Biotechnology, a program at The
Leading National Research Centre (KNOW) for the years 2014−2018, and financed by a
statutory activity subsidy from the Polish Ministry of Science and Higher Education for the
Faculty of Chemistry of Wrocław University of Science and Technology.
Conflict of interest
Authors declare no conflict of interest
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Author Biographies
Piotr Wiench is a PhD candidate in the Faculty of Chemistry at Wrocław University of Science and
Technology. He received a MSc degree from the same university in 2015. His current research activity
is focused on the synthesis of graphene materials and their application as electrochemical sensors.
Zoraida González received her PhD in Electrochemistry from University of Oviedo (Asturias, Spain) in
2006. Since 2010 she has worked at the Instituto Nacional del Carbón (INCAR-CSIC, Oviedo). Her
current research interests include carbon materials used in electrochemical energy storage devices
and desalination systems, and graphene-based (bio)electrochemical sensors.
Rosa Menéndez is a research professor at INCAR-CSIC in Oviedo. She has supervised 20 PhD theses
and 22 MSc in the fields of materials, chemistry and energy. She has published more than 200 papers
in recognized journals and 9 patents. She has been director of INCAR and Vicepresident of CSIC.
Bartosz Grzyb received his PhD in chemical technology from the Institute of Chemistry and
Technology of Petroleum and Coal of Wrocław University of Technology (Poland) and Laboratoire de
Chimie et Applications at University of Metz (France). His recent research is focused on developing
nitrogen-doped graphene materials for supercapacitors and sensors.
Grażyna Gryglewicz is a professor of the Faculty of Chemistry at Wrocław University of Science and
Technology (Poland). Her current research is focused on the synthesis of activated carbons, carbon
nanofibers, carbon nanotubes and graphene materials and their applications in water treatment,
hydrogen and methane storage, supercapacitors and sensors.
Fig 1
Fig 2
Fig 3
Fig 4
Fig 5
Fig 6
Fig 7
Fig 8
Fig 9
Table 1. Surface composition of N-rGO-150-2, N-rGO-180-8, and N-rGO-180-8/NH3 determined by
XPS (at. %)
Table 2. Results of C1s and N1s spectra deconvolution (at. %)
C1s peak deconvolution
N1s peak deconvolution
Table 3. Textural properties of N-rGOs
SBET (m2 g-1)
VT (cm3g-1)
Vmic (cm3g-1)
Vmeso (cm3g-1)
Table 4. Comparison of DA electrochemical sensors based on different active electrode
Electrode material
GCE/N-graphene (CVD)
Linear range
0.1 - 2.0
0.05 - 1.0
0.1 – 30
0.01 – 3
0.5 – 340
1 – 130
This work
graphene nanobelts
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