Graphitic carbon nitride (g-C3N4)/graphite nanocomposite as an extraordinarily sensitive sensor for sub-micromolar detection of oxalic acid in biological samples

Article abstract of DOI:10.1039/c9ra00982e

Nanosized graphitic carbon nitride (nano-g-C₃N₄) was synthesized using the thermal polymerization of melamine and utilized as a novel electrocatalyst for electrooxidation of oxalic acid (OA). The nano-g-C₃N₄ was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). The electrocatalytic performance of the g-C₃N₄-modified carbon paste electrode (g-C₃N₄/CPE) was investigated by cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The modified electrode showed excellent electrocatalytic activity towards the oxidation of OA. The effects of electrode composition, pH and scan rate on the electrooxidation response of OA were studied. Under optimized conditions, the differential pulse voltammetric response of the electrode was linearly related to OA concentrations between 1 and 1000 μM, with a limit of detection (LOD) of 7.5 × 10^-7 M. The electrode exhibited very high sensitivity of 1945 μA mM^-1 cm^-2 for OA assay. The developed method was successfully applied for the determination of OA in urine samples with satisfactory results.

Full text of DOI:10.1039/c9ra00982e

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Graphitic carbon nitride (g-C3N4)/graphite
nanocomposite as an extraordinarily sensitive
sensor for sub-micromolar detection of oxalic acid
in biological samples
Cite this: RSC Adv., 2019, 9, 13096
a
Sahar Nayerib and Negin Hamidia
*
Taher Alizadeh,
Nanosized graphitic carbon nitride (nano-g-C3N4) was synthesized using the thermal polymerization of
melamine and utilized as a novel electrocatalyst for electrooxidation of oxalic acid (OA). The nano-g-C3N4
was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and field
emission scanning electron microscopy (FE-SEM). The electrocatalytic performance of the g-C3N4-modified
carbon paste electrode (g-C3N4/CPE) was investigated by cyclic voltammetry and electrochemical
impedance spectroscopy (EIS). The modified electrode showed excellent electrocatalytic activity towards the
oxidation of OA. The effects of electrode composition, pH and scan rate on the electrooxidation response of
OA were studied. Under optimized conditions, the differential pulse voltammetric response of the electrode
was linearly related to OA concentrations between 1 and 1000 mM, with a limit of detection (LOD) of 7.5 ꢀ
10ꢁ7 M. The electrode exhibited very high sensitivity of 1945 mA mMꢁ1 cmꢁ2 for OA assay. The developed
method was successfully applied for the determination of OA in urine samples with satisfactory results.
Received 6th February 2019
Accepted 17th April 2019
DOI: 10.1039/c9ra00982e
rsc.li/rsc-advances
high thermal and chemical stability, non-toxicity and conve-
nient preparation as well as outstanding electronic proper-
ties.9–12 Generally, carbon nitride materials are fabricated by
directly condensation of nitrogen-containing organic precur-
sors for example urea,12 thiourea,13 melamine,10 dicyandia-
mide,14 cyanamide,15 guanidine hydrochloride.
Introduction
Oxalic acid is a toxic compound which exists extensively in
plants, animals and microbes. Since it can easily interact with
Ca2+ and Mg2+ to form less soluble oxalate salts, high levels of
OA in the digestive system cause kidney stones.1 Hence, the
determination of OA in food and urine has attracted much
interest in the management of food quality and arrangement of
a rational diet.2
So far, several techniques such as gas chromatography,3
liquid chromatography,4 spectroscopy,5 and enzymatic
methods6 have been developed for the determination of OA or
oxalate species. However, these instrumental methods have
suffered from some disadvantages such as high cost, low
sensitivity and insufficient selectivity compared to electro-
chemical techniques.7 Nevertheless, the oxidation of OA at
conventional electrodes usually requires a high overpotential.2,8
However, surface modication of the electrodes with various
compounds, reduces the oxidation potential for OA and
considerably increases the detection sensitivity.2,7
In spite of widely application of g-C3N4 as photocatalyst
material it has not been widely regarded as electrocatalyst
material. Literature survey indicated that a limited number of
works have been allocated to the study of electrocatalytic
application of this material.16–18 Some recent reviews have
summarized the various applications of g-C3N4.19 Therefore,
introducing g-C3N4 as the electrocatalyst material is interesting;
since, it can extend the application of this material in electro-
catalysis eld as a very important scientic eld.
In this work, the application of g-C3N4 nanomaterial,
prepared from melamine, was checked for electrocatalysis of OA
which exhibited very interesting results. The carbon paste
electrode, modied with g-C3N4 nanomaterial showed very
fascinating analytical characteristics such as high sensitivity,
low detection limit and also high selectivity, enabling use to
measure OA content of urine samples.
Graphitic carbon nitride (g-C3N4) which is a metal-free
semiconductor with a graphite-like layered structure has
drawn much attention as an effective photocatalyst, due to very
Experimental
Apparatus and chemicals
aDepartment of Analytical Chemistry, Faculty of Chemistry, University College of
Science, University of Tehran, P. O. Box 14155-6455, Tehran, Iran. E-mail:
talizadeh@ut.ac.ir
bDepartment of Applied Chemistry, Faculty of Science, University of Mohaghegh
The surface morphology of the prepared g-C3N4 was examined
by means of FE-SEM (TESCAN MIRA, Czech). The crystalline
Ardabili, Ardabil, Iran
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structure of the nanoparticles was studied by XRD (PHILIPS
PW1730, Netherlands) using Cu Ka radiation at 25 ꢂC. FT-IR
measurements were performed using a PerkinElmer spectro-
photometer RXI.
Results and discussion
Characterization of the synthesized g-C3N4
The morphology and structural properties of the synthesized g-
C3N4 were investigated using an FE-SEM. As shown in Fig. 1, the
g-C3N4 displays nano-sized particles-like structure.
Voltammetric measurements were carried out using
a potentiostat/galvanostat (Netherlands, Ivium VERTEX) and
controlled by Ivium So (Version 2.422). In all voltammetric
experiments, g-C3N4/CPE, saturated calomel electrode (SCE)
and a platinum wire were used as the working, reference and
auxiliary electrodes, respectively. All of the electrochemical
experiments were performed at room temperature.
The electrodes, explained previously, were also employed in
the EIS experiments. The EIS analysis were carried out in
a solution, containing 0.5 mmol Lꢁ1 of OA and 50.0 mmol Lꢁ1 of
ammonium acetate (pH ¼ 4.5), at frequency range of 1000 kHz
to 0.10 Hz, DEac ¼ 200 mV and dc potential of 1.2 V.
Stock solutions of OA were prepared by dissolving OA in
deionized water and diluting with ammonium acetate solution
(pH ¼ 4.5). Ammonium acetate solution was used as the sup-
porting electrolyte.
In order to study the crystalline property and phase purity of
the synthesized g-C3N4, powder XRD pattern of the material was
recorded in the 5–80ꢂ (2q) range which is displayed in Fig. 2(a).
As can be seen, g-C3N4 exhibits different peaks at 2q ¼ 13.1ꢂ and
27.3ꢂ. The weak peak (100) around 2q ¼ 13.1ꢂ is related to the in-
plane structural packing motif of tri-s-triazine repeating units
and the strong peak at 27.3ꢂ, corresponding to the (002)
reection, is a characteristic interlayer stacking reection of
conjugated aromatic systems, which can be well-indexed to the
interlayer diffraction of crystal planes of the hexagonal phase of
carbon nitride graphite-like structure. These data match well
with the values reported by the JCPDS (no. 87-1526).20–22 Ulti-
mately, it is apparent that no impurities peaks were observed in
the XRD patterns, which conrmed the high purity of the as-
prepared sample. It is generally accepted that g-C3N4 is based
on tri-s-triazine building blocks. Therefore, the graphitic
stacking structure is conrmed by XRD pattern.
All chemicals were of analytical grade and purchased from
Merck (Germany). These chemicals were used without further
purications.
The average crystallite size (D) of synthesized material is
calculated by the Debye–Scherrer equation:23,24
Synthesis of g-C3N4
kl
D ¼
(1)
b cos q
The bulk g-C3N4 was synthesized by thermal polymerization of
melamine. For this purpose, 5 g of melamine was placed into an
alumina crucible, covered with a lid, and heated in a furnace at
a heating rate of 3.3 ꢂC minꢁ1 up to 550 ꢂC and le for 2 h at this
temperature and then, slowly cooled down. Finally, a yellow
powder was obtained.
where l is the wavelength of the Cu-Ka radiation (light used for
the diffraction), D is the crystallite size, k is the Scherrer
constant and its value is taken as 0.9, q is the diffraction angle
and b is the full-width at half maximum (FWHM) of the sharp
peaks in radian.
The results revealed that the average crystallite size is
42.9 nm for synthesized material.
Preparation of g-C3N4-modied carbon paste electrode
For the construction of g-C3N4-modied carbon paste electrode
(g-C3N4/CPE), 65 mg of graphite powder was homogenized in
a mortar with 10 mg of g-C3N4 powder for 10 min. Subsequently,
ꢂ
n-eicosane (25 mg) was melted in a dish (at 50 C) and mixed
with the graphite/g-C3N4 blend by a stainless steel spatula. The
resulting paste was then packed into a hole (4.0 mm in diam-
eter), located at the end of an electrode. The electrical contact
was provided by a copper wire connected to the paste in the
inner hole of the tube. Then, the excess of solidied paste was
taken away by polishing of the electrode surface via a paper
sheet. Aer each determination stage, the modied electrode
could be reused, by smoothing of the electrode surface on
a paper sheet.
Preparation of real samples
Urine samples were taken from 2 healthy volunteers from Iran,
Tehran. For analysis of OA content of urine samples, 1 mL of the
described samples were diluted to 50 mL by adding 49 mL of
buffer solution (ammonium acetate buffer, pH ¼ 4.5).
Fig. 1 Scanning electron microscopy image of the synthesized g-
C3N4.
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Fig. 2 XRD pattern (a) and FT-IR spectrum of the synthesized g-C3N4
(b).
The presence of functional groups and molecular structure
information of the prepared g-C3N4 were further investigated
using FT-IR results. As shown in Fig. 2(b), the peaks at 806 cmꢁ1
is attributed to the breathing mode of the tri-s-triazine units
(the out-of-plane skeletal bending mode of triazine, s-triazine
ring modes).25–27 The prominent and broad set of peaks in the
region of 1200–1650 cmꢁ1 (1226, 1315, 1405, 1462, 1573,
1631 cmꢁ1) are characteristic peaks of the typical stretching
vibration modes of aromatic C–N heterocycles (the bands at
1226 and 1315 cmꢁ1 are attributed to the secondary (2C–N) and
tertiary (3C–N) amine fragments, respectively). The peak at
1631 cmꢁ1 and 1226 cmꢁ1 are attributed to C]N and C–N
stretching vibration modes, respectively.28 The prominent
bands in the region of 2900–3500 cmꢁ1 originate from the
stretching vibration modes for the –NH–, NH2– and hydroxyl of
the adsorbed H2O.29 The residual hydrogen atoms connected to
the edges of g-C3N4 sheets in the form of C–NH2 and 2C–NH
bonds.25 The results described above conrm the typical
chemical structure regarded usually for g-C3N4 material.
Fig. 3 Scanning electron microscopy image of g-C3N4/graphite/n-
eicosane nanocomposite, used as an electrode material for electro-
catalytic oxidation of OA (a); the magnified zone of the primary image
(b).
electrocatalytic oxidation of OA. As seen, the g-C3N4 nano-
materials are uniformly and tightly distributed on the graphite
sheets making the g-C3N4 covered graphite sheets which also
connected together, aiding binder material of n-eicosane.
To investigate the electrocatalytic activity of the g-C3N4/CPE
towards OA oxidation, cyclic voltammetry experiments were
carried out at the bare CPE and g-C3N4/CPE in 0.05 M of
ammonium acetate, containing also 0.1 mM of OA, at scan rate
of 100 mV sꢁ1 (Fig. 4(a)). According to the CV results shown,
CPE exhibits a small peak around 1.4 V for OA. However, at the
Electrocatalytic oxidation of OA on the g-C3N4/CPE
Fig. 3 represents the SEM images of the g-C3N4/graphite/n-
eicosane nanocomposite, used as an electrode material for
same experimental conditions,
a
distinct and sharp
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Fig. 5 The effect of g-C3N4 (a) and binder (b) percent on the electrode
response; cyclic voltammetry conditions: [OA] ¼ 1 ꢀ 10ꢁ3 M in
50.0 mmol Lꢁ1 of ammonium acetate (pH ¼ 6), scan rate ¼ 100 mV sꢁ1
.
Fig. 4 Cyclic voltammetry responses of unmodified (solid line) and
modified (dashed line) carbon paste electrodes to 1 ꢀ 10ꢁ4 M of OA in
50.0 mmol Lꢁ1 of ammonium acetate (pH ¼ 6), obtained at potential
scan rate of 100 mV sꢁ1 (a); Nyquist plots of electrochemical imped-
ance spectroscopy experiment, obtained by unmodified and modified
carbon paste electrodes (b); EIS conditions: solution of 0.5 mmol Lꢁ1
of OA in 50.0 mmol Lꢁ1 of ammonium acetate (pH ¼ 6), frequency
range ¼ 1000 kHz to 0.10 Hz, DEac ¼ 200 mV, dc potential ¼ 1.22 V;
equivalent electrical circuit obtained for the electrodes based on the
impedance data (inset).
can be seen, the charge transfer resistance of OA at the electrode
surface is decreased about 3-fold in the presence of g-C3N4.
These results are in good accordance with the CV results shown
in Fig. 4(a) (described previously).
The effect of electrode composition
Since the composition of electrode signicantly affects the
electrode performance, the effect of electrode ingredients on its
performance was studied. For this purpose, several electrodes
containing different amounts of the modier and binder were
prepared individually and then their voltammetric signals for
OA were recorded. The results are represented in Fig. 5(a) and
(b). Either in the case of the modier agent or for binder
content, the related curves show optimal points. This means
that the controlled amount of these materials should be
included in the electrode composition to record appropriate
signal for OA. As can be seen, in the case of both tested ingre-
dients, the presence of the studied component higher than the
optimal value leads to a sharp decrease in the electrode signal.
This behaviour is related to the insulating characteristic of the
binder and high electrical resistance of g-C3N4, which at
elevated amounts results in unadoptable enhancement of the
composite electrode electrical resistance. However, based on
voltammetric signal at potential of about 1.2 V is created by the
g-C3N4/CPE. This means that in the presence of g-C3N4 not only
the peak current of OA is remarkably increased but also the
oxidation potential of OA is noticeably decreased, suggesting
that the electrooxidation of OA is facilitated when including g-
C3N4 in the electrode composition. Such effect can be attributed
to the electrocatalytic behaviour of g-C3N4 for electrooxidation
of OA at the modied electrode.
EIS experiments was also carried out to extract impedance
characteristics of the previously described paste electrodes,
utilized for the electrooxidation of OA. The obtained results are
depicted in Fig. 4(b) as the Nyquist plots. The diameters of the
depressed semicircles are assigned to the charge transfer
resistance magnitude of OA at the corresponding electrodes. As
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Fig. 6 Cyclic voltammograms of 5 ꢀ 10ꢁ5 M of OA in 50.0 mmol Lꢁ1
of ammonium acetate (pH ¼ 6) at g-C3N4/CPE recorded in various
scan rates (a); a graph of OA oxidation current vs. square root of scan
rate (b); a graph of normalized current vs. scan rate (c).
the results, depicted in Fig. 5, the modied electrode containing
10% of g-C3N4 and 25% of binder was adopted as the best
electrode in this research.
Evaluation of the effect of scan rate
The effect of the scan rate on the voltammograms prole was
evaluated. Cyclic voltammograms of 50 mM of OA at the g-C3N4/
CPE at different scan rates are presented in Fig. 6(a). It can be seen
that as the potential scan rate is increased, the oxidation peak
currents increase gradually and at the same time the oxidation
peak shi towards positive potentials. As shown in Fig. 6(b), there
is a linear relationship between the oxidation peak current and the
square root of scan rate in the range of 10–150 mV sꢁ1 (correlation
coefficient of 0.9973). These results suggest a mass transfer
(diffusion) controlled process rather than surface controlled. Also,
it can be concluded that the electrochemical reaction studied, is
a totally irreversible electrochemical reaction.
Fig. 7 The effect of pH on the cyclic voltammetry responses of g-
C3N4/CPE to OA (1 ꢀ 10ꢁ3 M) at various pH values, recorded at
potential scan rate of 100 mV sꢁ1 (a); variation of oxidative current as
a function of pH value of the electroanalysis media (b); variation of
peak potential versus the pH condition of electroanalysis (c).
The dependence of the oxidation peak potential with pH of
the supporting electrolyte is presented in (Fig. 7(c)). As can be
seen, the peak potentials shi to less positive direction with
increasing of the solution pH at the range of 2–4.5. However, the
peak potentials shi to inverse direction (more positive poten-
tial direction) when increasing pH value from 4.5 to higher
values (from pH ¼ 4.5 to pH ¼ 7). In the pH range of 2–4.5
a linear relationship exists between the oxidation peak potential
and pH value, exhibiting linear regression equation of Ep (V) ¼
1.31–0.035pH (R2 ¼ 0.9936), suggesting a curve slope of 35 mV
per pH. Furthermore, between pH values of 4.5–7, the E/pH
curve is linear and shows curve slope of about 28.5 mV per pH.
The inection point of the Ep versus pH in Fig. 7(c) appears at
pH 4.5, which is meaningfully coincident with the pKa2 value of
OA (4.2). The slope values of 35 and 28.5 mV per pH for the E/pH
curve are close to the half of the theoretical value of ꢁ59/2 mV
per pHꢁ1, which suggests that the proton numbers is half the
electron number, involved in the OA electrooxidation reaction.
Moreover, the presence of two distinct linear curves in the E/pH
curve suggests two different routes for the electrooxidation of
The electrochemical reaction of OA at g-C3N4/CPE was further
studied by plotting of the scan rate normalized current (I/n1/2
)
against the scan rate. This was made from the scan rate of 10 to
150 mV sꢁ1. The obtained results are depicted in Fig. 6(c). As can
be seen, the normalized current decreases as the scan rate
increases up to about 100 mV sꢁ1 and then it keeps relatively
constant. This is the typical shape of an electrocatalytic
mechanism.30
Evaluation of the effect of pH
The effect of pH value on the electrooxidation of OA on the
surface of g-C3N4/CPE was investigated between pH 2 to 7 in
ammonium acetate buffer solution (0.05 M) (Fig. 7(a)). The
current signal of OA at the g-C3N4/CPE increases, when
elevating pH value from 2 to 4.5 and then it decreases at further
pH values (higher than 4.5) (Fig. 7(b)). Thus, pH ¼ 4.5 was
chosen as optimum pH value for further studies.
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Fig. 9 Square wave voltammetric responses of the g-C3N4/CPE to
different concentrations of OA, recorded in the optimized conditions:
E-step ¼ 0.01 V, E-pulse ¼ 90 mV, frequency ¼ 25 Hz, pH ¼ 4.5 (a); the
linear range of the calibration curve (b).
Fig. 8 Cycle voltammograms recorded using the modified electrode
in a solution containing 10 mmol Lꢁ1 of K3Fe(CN)6 and 1.0 mol Lꢁ1 of
KCl at different scan rates including 10, 25, 50, 75 and 100 mV sꢁ1; plots
of anodic peak (Ip,a) and cathodic peak (Ip,c) currents versus the square
Table 1 Interference levels for some tested molecules in the deter-
mination of OA by the sensor
Interference
level
Interference
level
root of scan rate (mV s )
ꢁ1 1/2 (inset).
Species
Species
Glucose
Fructose
Urea
Lactic acid
Maleic acid
<1000
<1000
<1000
<500
Dopamine
Tartaric acid
Ascorbic acid
Citric acid
Uric acid
<100
<100
<30
<5
OA at the electrode surface depending on the pH conditions of
the solution.
Based on the above mentioned data as well as considering
the chemical structure and characteristics of g-C3N4, the
following equations (eqn (2)–(5)) are proposed as the electro-
oxidation mechanism of OA at the g-C3N4/CPE.
<300
<5
According to the mechanism proposed, g-C3N4 is oxidized to
produce a radical cation compound (eqn (1)) which then
Table 2 Determination of OA in two different urine samples by the
modified electrode (n ¼ 3)
ꢁ
interact with HC2O4 (as the dominant species of OA in pH
Samples
[OA] by HPLC
[OA] by sensor
RSD%
Recovery%
range of 2–4.5) to give a radical species of HCO2ꢃ , carbon
1.58 ꢀ 10ꢁ4
2.88 ꢀ 10ꢁ4
1.51 ꢀ 10ꢁ4
2.79 ꢀ 10ꢁ4
3.5
4.1
95.57
96.87
monoxide and an electron (eqn (2)). In this process g-C3N4 is
also regenerated. The radical agent generated previously is Urine. 2
highly unstable and decompose to give second CO2, proton and
Urine. 1
another electron. We think that the described mechanism is
dominant at pH range of 2–4.5. However, at elevated pH (above
4.5) the dominant species of OA is oxalate and thus we propose
eqn (4) as the electrooxidation mechanism of OA at the elec-
trode. Herein, the radical cation species g-C3N4 c, created via
applied potential, attacks oxalate species, in the presence of
water as coregent, to oxidize electrocatalytically OA.
Electroactive surface area of the modied electrode
In order to study the active surface area of the g-C3N4/CPE, cyclic
voltammetry experiment was executed using this electrode in
the presence of Fe(CN)63ꢁ/Fe(CN)64ꢁ, as a well-known reversible
redox couple. The relationship between redox peak currents and
square root of scan rates (Randles–Sevcik equation) was applied
to calculate the electrochemical surface areas of the
electrode.31–33
+
g-C3N4 / g-C3N4+c + eꢁ
(2)
(3)
ꢃ
ꢃ
g-C3N4 þ HC2O4ꢁ/g-C3N4 þ CO2 þ HCO
HCO2ꢃ /CO2 þ Hþ þ eꢁ
þ
Ip ¼ 2.69 ꢀ 105n3/2AD1/2Cn1/2
(6)
2
(4)
where Ip is the current peak; n the number of electrons trans-
ferred in the redox evꢁe3nt; A the electrode area (cm2); C the
2ꢁ
concentration (mol cm ); D the diffusion coefficient (cm2 sꢁ1
)
g-C3N4+c + C2O4 + H2O / g-C3N4 + 2CO2
and n the scan rate (V sꢁ1). Fig. 8 shows cyclic voltammograms
obtained using the modied electrode. The plots, representing
+ 12O2 + 2H+ + 4eꢁ
(5)
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Table 3 Comparison of the g-C3N4-CPE with some previously reported g-C3N4-modified electrode
LOD (mol Lꢁ1
)
LR (mol Lꢁ1
)
Sensitivity (mA mMꢁ1 cmꢁ2
)
Ref.
2
DPV
2 ꢀ 10ꢁ4
(2–130) ꢀ 10ꢁ4
(13–45) ꢀ 10ꢁ3
(0.1–50) ꢀ 10ꢁ3
(0.03–5) ꢀ 10ꢁ3
Up to 0.3 ꢀ 10ꢁ3
N.R
105.45
80.02
295.82
295.82
32.59
N.R
CV
CV
0.1 ꢀ 10ꢁ3
0.2 ꢀ 10ꢁ4
1 ꢀ 10ꢁ6
34
35
36
37
Amperometry
CV
SWV
SWV
Down to 2 ꢀ 10ꢁ5
2.38 ꢀ 10ꢁ6
(0.05–3.4) ꢀ 10ꢁ4
N.R
1945
38
0.75 ꢀ 10ꢁ6
(1–1000) ꢀ 10ꢁ6
This work
the anodic peak (Ip,a) and cathodic peak (Ip,c) currents of the
cyclic voltammetry versus the square root of the scan rates, are
also shown in inset of Fig. 8. From the linear dependence of
peak current on square root of the scan rate, it can be found that
the currents obtained at modied electrode are diffusion
controlled. Considering n ¼ 1 and D ¼ 7.18 ꢀ 10ꢁ6 cm2 sꢁ1 and
utilizing the curves described, the surface area of the modied
electrode was found to be 0.072 cm2.
Determination of OA in urine samples
In order to evaluate applicability of the modied electrode, it
was applied for the determination of OA in urine samples.
The validity of the method was also assessed by comparing
the determination results of the developed method with
those obtained by HPLC as reference method. The results are
listed in Table 2. Based on the results depicted the method
shows good RSD and satisfactory recoveries for OA deter-
mination in urine samples. Moreover, there was no signi-
cant difference between the determination results of the
method and those obtained by HPLC method (condence
level of 95%).
Determination of OA by SWV on the g-C3N4/CPE
Aer evaluation of the electrochemical behaviour of OA at the
electrode surface, square wave voltammetry technique was
utilized to determine OA. As shown in Fig. 9(a), the intensities of
square wave voltammograms increase linearly with increasing
of OA concentration over the concentration range of 1–1000 mM
(calibration Fig. 9(b)). The detection limit of the method was
calculated to be 0.75 mM (based on the 3Sb/m formula, at which
Sb and m stand for standard deviation of the blank and slope of
the calibration curve, respectively). The relative standard devi-
ation (RSD) of three replicated determinations of OA with
a fabricated electrode was calculated to be 3.8% (n ¼ 3), indi-
cating a good repeatability for the sensor. Moreover, the
reproducibility of four electrodes independently made, showed
a RSD of 4.1% for the determination of 500 mM of OA. Mean-
while, it was found that the signal of g-C3N4/CPE was stable even
aer 12 months. This is interesting result and can be assigned
to the stability of g-C3N4.
Comparison of the sensor with some previously reported
sensors
The analytical characteristics of the proposed electrode were
compared to those of some electrochemical methods, reported
previously for the determination of OA. The results are
summarized in Table 3. This comparison shows acceptable
results for the proposed electrode. More interestingly,
compared to other methods represented, this sensor exhibits
the highest sensitivity for OA determination.
Conclusion
In this work, a nano-sized g-C3N4-modied carbon paste
electrode was synthesized and utilized for the determination
of OA by square wave voltammetry. The electrode showed
excellent electrocatalytic characteristics for OA electro-
oxidation. It was shown that the new introduced OA sensing
platform is the most sensitive sensing method, compared to
the previous OA electrochemical sensors. Moreover, the
method exhibited very low detection limit as well as high
selectivity, enabling us to use the sensor for OA determination
in urine samples. Furthermore, the modied electrode can
also be applied for determination of OA in urine samples with
excellent sensitivity.
Interference study
The interference effect of several compounds, that may be
present in biological matrix were checked and studied under
optimum conditions on the response of the prepared electrode
to OA. The tolerance limit was dened as the maximum
concentration of the interfering species that caused an error of
ꢄ5% in the determination of OA. It was found that 1000-fold
molar excess of glucose, fructose and urea, 500-fold molar
excess of lactic acid, 300-fold molar excess of maleic acid, 100-
fold molar excess of dopamine and tartaric acid, 30-fold molar
excess of ascorbic acid, 5-fold molar excess of citric acid and
uric acid had no inuences on the electrooxidation response of
100 mM of OA. The obtained results, presented in Table 1,
indicate that the proposed electrode has acceptable selectivity
and resistance against interfering agents.
Conflicts of interest
There are no conicts to declare.
13102 | RSC Adv., 2019, 9, 13096–13103
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RSC Advances
19 A. Wang, C. Wang, L. Fu, W. Wong-Ng and Y. Lan, Nano-
Micro Lett., 2017, 9, 47–53.
Acknowledgements
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The authors thank the University of Tehran and University of
Mohaghegh Ardabili, for all supports.
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