B56 Journal of The Electrochemical Society, 166 (2) B56-B62 (2019)

0013-4651/2019/166(2)/B56/7/$37.00 © The Electrochemical Society

Nitrogen Doped Carbon Dots Derived from Natural Seeds and

Their Application for Electrochemical Sensing
Kunxia Li,1 Jinqiong Xu,1 Muhammad Arsalan,1 Ni Cheng,2 Qinglin Sheng, 1,2,3,z
Jianbin Zheng, 1 Wei Cao,2 and Tianli Yue2,z
1 College of Chemistry & Materials Science/Key Laboratory of Synthetic and Natural Functional Molecule Chemistry
of Ministry of Education/Shaanxi Provincial Key Laboratory of Electroanalytical Chemistry, Northwest University,
Xi’an, Shaanxi 710069, People’s Republic of China
2 College of Food Science and Technology, Northwest University, Xi’an, Shaanxi 710069, People’s Republic of China
3 State Key Laboratory of Analytical Chemistry for Life Science, Nanjing University, Nanjing, Jiangsu 210023, People’s
Republic of China

In this work, nitrogen doped carbon dots (N-CDs) derived from kiwi seeds, white sesame seeds, and black sesame seeds were
prepared by a simple, feasible and green route. Then a novel nitrite electrochemical sensor was successfully constructed. The
morphology and composition of N-CDs were characterized by Field emission transmission electron microscopy, Fourier transform
infrared spectra, Raman spectra and electrochemical methods. The particle size of the as-prepared N-CDs from the three kinds of
natural seeds were in the range of 1.4 ∼ 4.9 nm, 1.4 ∼ 4.6 nm, and 1.2 ∼ 4.7 nm, respectively. Moreover, these N-CDs nanomaterials
exhibited excellent electrocatalytic performances for nitrite sensing with a detection limit of 0.23 μM (S/N = 3) by electrochemical
methods. Additionally, the stability, anti-interference ability and real sample analysis of the sensors had been evaluated. Finally, the
electrochemical sensor was successfully applied for nitrite determination in real samples (ham sausages). Based on the present study,
more natural seeds could be expected as preferred candidates for N-CDs synthesis, and a general platform of novel electrochemical
sensors for nitrite detection is provided.
© 2019 The Electrochemical Society. [DOI: 10.1149/2.0501902jes]

Manuscript submitted November 15, 2018; revised manuscript received December 19, 2018. Published January 16, 2019.

Carbon nanomaterials, such as fullerence and carbon nanotubes, formation of nitrosamine which causes hypertension, potential cancer
have been widely used in the fields of energy, environment, medicine risk, and “blue baby” syndrome.21–24 Therefore, it is necessary to de-
delivery, and so on. However, the synthesis process of carbon nano- velop a reliable and sensitive method to monitor nitrite in food, drink-
materials is always a complex and time-consuming process, which ing water, and environmental samples for public health and safety. To
hinders their development in many terms of potential application. As a date, various analytical strategies have been used to determine nitrite,
new member of the carbon material family, carbon dots (CDs) have at- such as capillary electrophoretic methods,25,26 ion chromatography,27
tracted extensive attention because of their unique physical and chemi- ultra performance liquid chromatography – mass spectrometry,28 elec-
cal properties, and easier synthetic routes. The CDs, also called carbon trochemical method,29–31 etc. However, some methods still have lim-
quantum dots (C-QDs) or carbon nanoparticles (CNPs), were casually itations on sensitivity, selectivity, simplicity, and feasibility. In com-
discovered in 2004 while separating and purifying single-walled car- parison with other techniques, electrochemical method has aroused
bon nanotubes following the arc discharge method.1 The carbon nano- more interests due to its outstanding performances, including simple
materials exhibited quasi-spherical or spherical particles with sizes be- operation, rapid response, high accuracy, and low cost for the minia-
low 10 nm, which usually consist of sp2 conjugated core and contain turization and on-site detection.30–32 However, to some degree, the
generally oxygen, nitrogen or aldehyde based groups.2,3 Nowadays, surface of bare electrode is easily poisoned, which leads to a large
CDs have been synthesized by various methods, including chemical over-potential for the oxidation of nitrite, that restrict the sensing sen-
method,4 hydrothermal method,5 microwave-assisted method,6 elec- sitivity and accuracy.32 Thus, many researchers have been working
trochemical method,7 and calcination method.8,9 They have emerged hard to modify the surface of the electrode to obtain a better current
as promising nanomaterial due to the excellent optical properties, bio- response when nitrite is oxidized at a lower over-potential. Therefore,
compatibility, high electrochemical response, green synthetic routes, a variety of modified electrode layers have been reported, such as
smaller size, low cost, and abundant raw materials.3,4,10–12 Based on the metal organic framework (MOF),33,34 metal materials,13,35,36 etc), car-
unique advantages, CDs have emerged as attractive role for the devel- bon materials (porous carbons,19 carbon black,37 carbon nanotube,38
opment of bio-imaging, sensing, medical diagnosis and drug delivery, graphene,1,9 etc), and various combined composite materials,39,40 etc.
metal ion and small molecules detecting, environmental monitoring, Among them, carbon materials were widely used because of their low
food quality control, and other optoelectronic applications.3,4,12–15 In cost, good conductivity and stability.
most cases, CDs are formed from the precursors with fine carbon struc- In the present work, we demonstrated that different kinds of plant
tures like carbon nanotubes and graphite16 or conventional chemicals seeds can be used as the new carbon sources to obtain water soluble
such as hydroquinone,17 citric acid monohydrate (CA).18 Fortunately, CDs and no surface passive agent is required. The use of degrad-
the renewable resources or biomass material as carbon source has also able biomass materials to prepare CDs is beneficial to reducing food
achieved some success. For example, Gangaraju Gedda et al.,4 Madhu waste. We further demonstrated that N-CDs derived from plant seeds
et al.,19 have used prawn shells, banana-stem to form CDs and porous can serve as sensing materials by modifying it on electrode surface
carbon, respectively. and using the modified electrode for detecting nitrite selectively and
Nitrite is a typical inorganic pollutant from chemical fertilizers, sensitively. To evaluate the practical application in the sensing system,
food preservatives, corrosion inhibitors, and dyeing agents.20–22 It al- it is further applied to monitor nitrite in ham sausage. Here, we present
ways exists in human surroundings, including food, drinking water a facile preparation route to synthesize CDs and further applied for
and physiological systems. This pollutant has brought serious harms nitrite detection in real samples (Scheme 1).
to public safety of health and ecological environment especially. Ac-
cording to the report of World Health Organization, the lethal dose of
nitrite in the human body is between 8.7 μM and 28.3 μM.21,23 Long- Experimental
term cumulation and overdose of nitrite can result in irreversibly com- Regents and materials.—Sodium nitrite, dichloromethane, nafion
bination with hemoglobin in blood to produce methemoglobin and the dispersion (∼5% in a mixture of lower aliphatic alcohols and water)
were all purchased from Aladdin Industrial Corporation (Shanghai,
z
E-mail: qlsheng@nwu.edu.cn; yuetl@nwu.edu.cn China). Dialysis bag (MWCO: 500D); Different kinds of plant seeds:

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 Journal of The Electrochemical Society, 166 (2) B56-B62 (2019) B57

Scheme 1. Illustration of the preparation of N-CDs nanomaterials from natural seeds and their application in electrochemical sensing.

kiwi seeds, white sesame seeds, and black sesame seeds, were pur- water and washed with copious amounts of water by centrifugation,
chased from a store (Xi’an, China); Different ingredients of ham and the precipitates were collected. Next, pre-treated CDs were fully
sausage purchased from local supermarket (Xi’an, China). Nitrite so- purified by dialysis against doubly distilled water through a dialysis
lution is freshly prepared. Other reagents and chemicals were also bag (MWCO: 500 D) for 3 days, and the water was replaced every
all of analytical grade and used without further purification. Acetate four hours during dialysis. After the dialysis, the product was enriched
buffer solution (0.1 M) was used as the supporting electrolyte in elec- by centrifugation and then dried at room temperature naturally within
trochemical experiments. Doubly distilled water was used throughout 6 hours. Finally, nitrogen doped carbon dots were collected in powder
the experiments. All electrochemical experiments were conducted at form, and record it as N-CDs1, N-CDs2 and N-CDs3 for the further
room temperature (25 ± 2◦ C). applications.

Apparatus.—Field emission transmission electron microscopy

Electrode modification.—The glassy carbon electrode was pre-
(FE-TEM) images were obtained using JEM 2800 (JEOL, Japan).
pared by a sample casting method. Prior to modify, glassy carbon
UV-Vis spectra were obtained by T6 (Beijing Purkinje General Instru-
electrode surface was polished with 0.3 and 0.05 μm alumina powder
ment Co. Ltd, China). Fourier transform infrared spectra (FTIR) were
respectively to obtain a mirror-like surface. Next, the glassy carbon
recorded by VERTEX 70 (Bruker, German). Raman spectra analysis
electrode was rinsed with distilled water, and washed by ultrasoni-
were performed with a WITec Focus Innovations UHTS 300-RAMAN
cation in an ethanol solution and doubly distilled water successively.
spectrometer. X-ray photoelectron spectroscopy measurements were
Then, the GCE was allowed to dry in a stream of nitrogen. 1 mg the
carried on a PHI 5000 Versaprobe III (ULVAC-PHI, Japan). Zeta po-
as-prepared N-CDs were dispersed in doubly distilled water by ultra-
tential values were recorded by NanoPlus (Micromeritics Instrument
sonication for 30 min to form a homogeneous suspension. Finally, a
Ltd., Shanghai, China). Electrochemical measurements were carried
drop of N-CDs suspension (7 μL) and nafion disperision (0.5%) were
out using a conventional three-electrode electroanalysis system con-
spread onto the pre-polished GCE in turn, and then dried it at room
trolled by an IGS1130 electrochemical workstation (Guangzhou In-
temperature in air. The modified electrode was denoted as CDs/GCE.
sens Sensor Technology Co. Ltd., China). In the conventional three
electrodes system, the working electrode was a glassy carbon elec-
trode (GCE, 3 mm diameter), the auxiliary electrode and the reference
electrode were a platinum wire and a Ag/AgCl (3 M KCl, aq) elec- Results and Discussion
trode, respectively. Characterization techniques.—FE-TEM was used to characterize
the morphology and structure of the products as shown in Fig. 1. From
Synthesis of N-CDs.—To obtain the N-CDs derived from kiwi top to down, they are N-CDs1, N-CDs2, and N-CDs3 respectively;
seeds, white sesame seeds, and black sesame seeds, direct calcination and there are TEM images (left) and diameter distribution (right) of
method was used.9 All glassware, magneton, and porcelain crucible N-CDs. As shown in FE-TEM images (Fig. 1), the N-CDs have quasi-
were thoroughly cleaned in freshly prepared aqua regia (VHCl /VHNO3 spherical or spherical shape structure with uniform distribution and
= 3:1), rinsed in distilled water, and further oven-dried prior to use without apparent aggregation. Individual CD was marked by white
in this experiment. Exactly, 15 g seeds were placed in a porcelain dotted line, and from the HRTEM images (inset images of Figs. 1A–
crucible and individually marked. The porcelain crucible was allowed 1C), it can be seen that some CDs have obvious crystalline structure,
to naturally cool down to room temperature after heat treated using a which is justified the existence of D and G bands in Raman spec-
muffle furnace and last for 10 h after the temperature up to 350◦ C. And tra (Fig. 2C). HRTEM images showed the inter-planer spacing were
then, we can get black primary products. Next, these products were all around 0.22 nm, which is corresponded to the characteristics of
mechanically grinded to fine power and wash it in dichloromethane graphite.14 The results of diameter distribution revealed that diameter
to remove impurities of the CDs and then, dichloromethane need to of N-CDs from different carbon sources have slightly different. The
be evaporated under a stream of nitrogen. Interestingly, during the size distribution of N-CDs1, N-CDs2 and N-CDs3 spread in a narrow
solvent evaporating, the black solute will become yellowish brown range of 1.4 ∼ 4.6 nm, 1.4 ∼ 4.9 nm, and 1.2 ∼ 4.7 nm, and the aver-
and then disappeared quickly, and eventually a black residue was age sizes were found as 3.13 nm, 2.97 nm, and 2.68 nm, respectively,
obtained. Furthermore, the remaining products were re-dispersed in shown in Figs. 1D–1F.

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 B58 Journal of The Electrochemical Society, 166 (2) B56-B62 (2019)

sities of functional groups in as prepared CDs are slightly different.

The strong and broad peaks in the region of 3300∼3500 cm−1 ap-
peared due to the stretching vibrations of O-H & N-H.5 And there
are C-H absorption bands around 2800∼3000 cm−1 , which could be
due to C–H stretching vibrations of –CH2 , –CH3 .11 In addition, the
peaks at 1621 cm−1 , 1646 cm−1 , and 1617 cm−1 are attributed to the
C=O stretching vibration from N-CDs1, N-CDs2, and N-CDs3.9,42
The peaks at 1558 cm−1 , 1542 cm−1 , and 1563 cm−1 are assigned
to –COO– & C=C stretching of N-CDs1, N-CDs2, and N-CDs3
respectively.12,18 The characteristic peaks at 1087 cm−1 , 1096 cm−1 ,
and 1087 cm−1 can be assigned to the asymmetric stretching vibra-
tion of C–O–C43 of N-CDs1, N-CDs2, and N-CDs3, respectively.
Whereas, compared with N-CDs2 and N-CDs3, N-CDs1 has a strong
absorption peak at 728 cm−1 , which is related to bending vibration of
=C–H of aromatic rings, and the weak absorption peak at 1249 cm−1
indicates the presence of C-N stretching of aromatic amine.11 In ad-
dition, the prepared CDs nanomaterials showed gradually increased
solubility in both polar and non-polar solvents because they have both
hydrophobic (C=C) group and hydrophilic (–OH, C–O–C, –COO–)
groups in their surface, and absorption peaks of hydrophobic group
attenuated, as evident in the FTIR spectra (Fig. 2B). FTIR spectra
of N-CDs show absorption bands or peaks at different wavenumbers
which not only indicates that a great number of functional groups
exist in the surface of the N-CDs, such as hydroxyl, carboxyl and
ether groups, but are also consistent with the results of UV-Vis
(Fig. 2A) analysis described above. Moreover, the FTIR spectra of
Figure 1. Left: TEM images of (A) N-CDs1, (B) N-CDs2, (C) N-CDs3, Inset:
HRTEM images; Right: Particles size distribution of (D) N-CDs1, (E) N-CDs2, N-CDs from different plant seeds show that their chemical compo-
(F) N-CDs3. sitions are similar and independent of the carbon source, indicating
that N-CDs can be obtained from different plant seed precursors by a
simple and conventional method and classification is not required.
The electronic transition of the synthesized N-CDs was studied Raman spectroscopy is widely recognized as an important tool for
by UV-Vis absorption spectrum. Fig. 2A represents the UV spectra characterizing carbon-based nanomaterials because it can provide in-
of N-CDs dispersed in water. An obvious broad absorption band at formation on the degree of defects, structural order and disorder, and
275 nm was observed, which can be ascribed to the π-π∗ transition of degree of graphitization in carbon-based materials.44 Raman spec-
C=C bond.41 The weak absorption band around 325 nm is relevant to trometer was used to characterize structural information of N-CDs1,
n-π∗ transition of C=O.2 Moreover, the absorption bands of N-CDs1 N-CDs2, and N-CDs3. As it was shown in Fig. 2C, all N-CDs show
to N-CDs3 at 275 nm and 325 nm decreased gradually, which may be two main peaks in the Raman spectra: an apparent G band and a weak
ascribed to the differences in carbon sources. D band, all G bands were located around 1575 cm−1 and all D bands
FTIR were used to characterize the presence of functional groups were located near 1347 cm−1 . The G bands of N-CDs1 and N-CDs2
in the prepared CDs of N-CDs1, N-CDs2, and N-CDs3. As revealed appeared at 1583 cm−1 . For N-CDs3, the G band was at 1572 cm−1 .
in Fig. 2B, there are various functional groups on the surface of the In addition, the D bands of N-CDs1 and N-CDs2 were located at
N-CDs. According to the FTIR spectrum, the peak positions and inten- 1345 cm−1 , and N-CDs3 at 1353 cm−1 respectively. The D band was
caused by the vibration of the sp3 hybridized carbon atoms of the dis-
ordered graphite carbon, indicating the degree of defects and disorder
of the carbon material, whereas the G band was due to the vibration
of the sp2 hybridized carbon atoms of graphite, which was indicative
of the presence of crystalline graphite carbon.11,39 The intensity ratio
of the D and G bands (ID /IG ) can be used to estimate the degree of
disorder and the defects in these N-CDs. The intensity ratio of ID /IG
for N-CDs1, N-CDs2, and N-CDs3 was calculated to be 0.61, 0.80,
and 0.74, respectively. Besides, the degree of surface defects could
be improved by the doping of nitrogen atoms. A larger ID /IG value
exhibited a higher degree of surface defects of the N-CDs. In addition,
N-CDs1, N-CDs2 and N-CDs3 also had bands at 1425 cm−1 and 1487
cm−1 which were assigned to symmetric and anti-symmetric Cα =Cβ
stretching.18
The chemical compositions of the resultant N-CDs were also in-
vestigated by means of X-ray photoelectron spectroscopy (XPS). As
shown in Fig. 2D, the full scan XPS spectra of the CDs display three
peaks at 285.1 eV, 397.8 eV and 531.1 eV, which are ascribed to
the presence of carbon (C 1s), nitrogen (N 1s) and oxygen (O 1s)
respectively.8,18 The percentages of the elements corresponding to C,
N and O peaks are 78.74%, 3.12% and 18.14% of N-CDs1. Simi-
larly, the elements contents of C, N and O in N-CDs2 and N-CDs3 is
77.75%, 3.61% and 18.64; 81.94%, 3.16% and 14.9% in turn. Also,
Figure 2. (A) UV-Vis spectra of the three types of N-CDs; (B) FT-IR spectra zeta potential values of CDs were measured as ∼ −45.4 mV. The nega-
of the three types of N-CDs; (C) Raman spectra of the three types of CDs. tive zeta potential of CDs indicated the presence of hydroxyl, carboxyl
(D) XPS spectra of the three types of N-CDs. Line of a, b, and c represents and carbonyl on CDs surface. This result was consistent with the above
N-CDs1, N-CDs2, and N-CDs3 respectively. spectrum results. In addition, the carboxylate anion causes a strong

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 Journal of The Electrochemical Society, 166 (2) B56-B62 (2019) B59

It is worth nothing that the oxidation current gradually increased with

increment of nitrite concentration. These results indicated that CDs
are the owner of good catalytic performance toward nitrite.

Optimization of reaction conditions.—The effects of buffer

solution.—The influences of different kinds of buffer solutions to-
ward nitrite sensing were investigated. In this work, different types of
0.1 M buffer systems such as phosphate buffer system (PBS), acetate
buffer system (HAc-NaAc), and citrate buffer system (HOAc-NaOAc)
are prepared for this sensing system. Then, CVs under different buffer
systems were compared under the same conditions. The results of the
obtained CVs show that acetate buffer system has the best peak effect.
The peak shape is obvious and the peak current is moderate, and the
background current is small. Therefore, all the analytical studies are
performed with an acetate buffer system.

The effects of pH.—The effects of buffer pH on the electrochem-

ical response of nitrite were investigated to obtain the suitable pH
value. We selected some pH values as research objects and they are
3.0, 3.4, 3.8, 4.2, 4.6, 5.0, 5.4, and 5.8, respectively. Fig. 5 showed
Figure 3. CV curves of electrode modified by (A) N-CDs1, (B) N-CDs2, (C) the effect of pH values on the oxidation peak current of 2 mM nitrite.
N-CDs3, and (D) nafion comparing with that of bare GCE in the absence and For N-CDs1 and N-CDs3, the peak current increases gradually with
presence of 2 mM nitrite in 0.1 M acetate buffer (pH 5.0) at a scan rate of 20 the increased in the pH range from 3.0 to 4.2 and then peak current
mV · s−1 .
decreased in the pH range from 4.2 to 5.8. That’s the peak current
reached maximum at pH 4.2. Thus, the pH 4.2 is optimum for N-
electrostatic repulsion between the CDs.42 Therefore, the CDs were CDs1 and N-CDs3 in the next studies. While for N-CDs2, the peak
homogenously dispersed and without apparent aggregation, which is current reached maximum at pH 3.8, but its potential is larger in this
also confirmed by TEM images. pH value. So, pH 3.4 was selected as the appropriate pH value. At this
pH, the CDs as an electron donor supplies electrons to the nitrite ions
Cyclic voltammetric (CV) analysis of nitrite on CDs/GCE.—To and oxidize them. Moreover, nitrite is relatively stable under weakly
testify the sensing application of CDs, a nitrite sensor based on car- acidic conditions.40 Therefore, different suitable pH values of acetate
bon dots was fabricated. As shown in Fig. 3, it can be obviously seen buffer solution were selected for the determination of nitrite in the fol-
that in 0.1 M acetate buffer solution (pH 5.0), the bare GCE (curve lowing experiments. In addition, the experimental results showed that
a) and CDs/GCE (curve b) exhibit almost no eletrochemical response the solution pH has no effect on the peak potential of nitrite oxidation.
in the absence of nitrite at a scan rate of 20 mV · s−1 . However, after This feature has been also verified by other authors.29
adding 2 mM nitrite, the current responses are dramatically increased
at various modified electrodes and bare GCE. All the peaks of catalytic The effects of scan rate.—In order to investigate the electrochem-
current on CDs/GCE were enhanced and the over-potential for nitrite ical behaviors of CDs modified electrodes toward the nitrite oxidation
oxidation was decreased. Compared with that on bare GCE (curve c), at different scan rates, CVs of CDs/GCE are conducted and the results
the response current and potential were all improved, indicating that are shown in Fig. 6. It is observed that the oxidation peak current of
the presence of N-CDs exhibited excellent catalytic performance for nitrite increases with increasing the scan rate from 10 to 200 mV · s−1 .
nitrite oxidation. In addition, the response of the nafion-GCE (curve The oxidation peak current of nitrite was plotted against the square
a and b) and GCE (curve c and d) in the absence (curve a and c) root of the scan rate, and the peak current increased linearly in the
and presence (curve b and d) of 2 mM nitrite under the same con- scan rate range of 10 ∼ 200 mV · s−1 with a correlation coefficient
ditions were also recorded. According to the results of Fig. 3D, it of 0.9927, 0.9978, and 0.9988 of N-CDs1, N-CDs2 and N-CDs3, as
is obvious that the electrocatalytic effect of nafion film on nitrite is shown in the inset of Fig. 6, indicating this process was controlled by
extremely weak, suggesting that nafion is an ideal film for electrode diffusion.
modification.
Catalytic activity of N-CDs was evaluated via changing the con- Differential pulse voltammetric (DPV) response of nitrite on
centration of nitrite, as shown in CVs of Fig. 4. In the pattern, no CDs/GCE.—The DPV current responses of nitrite on the CDs/GCE
characteristic peaks were observed when nitrite was not introduced were examined by adding the nitrite into 0.1 M acetate buffer solution.
into this system. When nitrite was injected into the 0.1 M acetate All of the anodic peak potentials of nitrite at CDs/GCEs were about
buffer solution (pH 5.0), a well-defined oxidation peak was observed. +0.79V vs. Ag/AgCl. After certain amount of nitrite was added, the

Figure 4. CV curves of CDs/GCE using (A) N-CDs1, (B) N-CDs2, and (C) N-CDs3 modified in 0.1 M acetate buffer (pH 5.0) at different concentrations of nitrite
(a-g: 0, 0.5, 1, 1.5, 2, 2.5, 3 mM).

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 B60 Journal of The Electrochemical Society, 166 (2) B56-B62 (2019)

Figure 5. Current response curves of CDs/GCE with (A) N-CDs1, (B) N-CDs2 and (C) N-CDs3 in 0.1 M acetate buffer toward 2.0 mM nitrite at different pH
(3.0, 3.4, 3.8, 4.2, 4.6, 5.0, 5.4, and 5.8).

Figure 6. CV curves of CDs/GCE using (A) N-CDs1, (B) N-CDs2 and (C) N-CDs3 modified in 0.1 M acetate buffer containing 2.0 mM nitrite at different scan
rates (a-k: 10, 20, 40, 60, 80, 100, 120, 140, 160, 180, and 200 mV · s−1 ). Inset: the plots of anodic peak current versus square root of the scan rates.

peak current of differential pulse voltammetry increased as the nitrite lent catalytic activity of N-CDs to nitrite. The nitrite sensor based on
concentration increased obviously in Fig. 7 and the inset shows the CDs displayed excellent performances than that of similar sensors re-
sensitive current response toward lower nitrite concentrations. Fig. 7D ported previously. Additionally, the analytical performances of sundry
presented the corresponding calibration curve between anodic current the earlier reported nitrite sensors were also shown in Table I.
and nitrite concentration. From the calibration plots, the N-CDs1 and
N-CDs2 modified electrodes exhibited a wide linear range toward Repeatability and stability study.—The repeatability and stabil-
nitrite detection of 0.7 μM ∼ 6.0 mM and 0.7 μM ∼ 8.0 mM, respec- ity of the CDs/GCE were evaluated. Six parallel electrodes with the
tively. While, for the N-CDs3 modified electrode, its linear range is CDs modification were chosen for the repeatability experiment in
relatively narrow from 0.7 μM to 2.0 mM. The correlation coefficients buffer system with 2 mM nitrite solution under optimized conditions.
of N-CDs1, N-CDs2, and N-CDs3 are 0.9994, 0.9980, and 0.9975 re- A relative standard deviation (RSD) was calculated, and the parallel
spectively. The limit of detection (LOD) of the nitrite determination test of the three modified electrodes were lower than 4%, indicating
was calculated all to be 0.23 μM (S/N = 3), indicating the excel- that CDs/GCE displayed good repeatability. And the stability of the
modified GCE was also estimated each three days, and three types
CDs/GCE still maintained more than 92% of its original current re-
sponse to nitrite after three weeks. Thus, the experimental results
showed that the modified electrode possessed good repeatability and
stability.

Selectivity and anti-interference study.—To further investigate

the feasibility of nitrite detection on three types CDs/GCE, the se-
lectivity and anti-interference ability experiments were carried out.
Interfering substances of various organic and inorganic matters, such
as lactose, ascorbic acid (AA), uric acid (UA), glucose, potassium
chloride (KCl), and disodium hydrogen phosphate (Na2 HPO4 ) were
successively added and detected by testing the DPV responses on
CDs/GCE in 0.1M acetate buffer containing 0.2 mM nitrite. After
the interfering substance equivalent to five times the concentration
of nitrite were successively added into the acetate buffer solution,
the responses of modified electrodes to nitrite were not changed sig-
nificantly. Therefore, it demonstrated that those possibly coexisted
chemicals had no influence on determination of nitrite when used as
an electrode modification material.
Figure 7. DPV curves of CDs/GCE using (A) N-CDs1, (B) N-CDs2, and (C)
N-CDs3 in 0.1 M acetate buffer at different concentrations of nitrite. (A) a∼l:
0.7, 3, 7, 20, 50, 80, 200, 500, 800, 2000, 4000, 6000 μM, Inset: 0.7∼200 Practical applications in ham sausages samples.—To study the
μM. (B) a∼m: 0.7, 3, 7, 20, 50, 80, 200, 500, 800, 2000, 4000, 6000, 8000 practicality of the fabricated sensor, the concentrations of nitrite
μM, Inset: 0.7∼80 μM. (C) a∼j: 0.7, 3, 7, 20, 50, 80, 200, 500, 800, 2000 in real samples had also been investigated. Before experiment, the
μM, Inset: 0.7∼80 μM. (D) The plots of anodic peak current versus nitrite extraction of nitrite from the ham sausages samples was accom-
concentrations. plished according to the Chinese National Food Safety Standard

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 Journal of The Electrochemical Society, 166 (2) B56-B62 (2019) B61

Table I. Comparison of several other matrix for nitrite sensors with N-CDs.

Electrode Materials Method(s) Linear Range (μM) Detection Limit (μM) References
HACa DPV/Amperometry 1.0∼127 0.07 19
TOABb /ZnPP c -C60 Amperometry 2.0∼164 1.44 30
CRd -GO Amperometry 8.9∼167 1.0 31
ERGOe /AuNPs DPV 1.0∼6000 0.13 32
α-Fe2 O3 NAsf /CFg Amperometry 0.5 ∼1000 0.12 36
ZrO2 /MWCNTs/Au Amperometry 0.5∼1115.5 0.3 38
N-CDs1 DPV 0.7∼6000 0.23 This work
N-CDs2 DPV 0.7∼8000 0.23 This work
N-CDs3 DPV 0.7∼2000 0.23 This work

a HAC: highly porous and heteroatom-enriched activated carbon.

b TOAB: Tetraoctylammonium bromide.
c Zn-PP: para-Zn porphyrin.
d CR-GO: chemically reduced graphene oxide.
e ERGO: electrochemically reduced graphene oxide.
f NAs: nanorod arrays.
g CF: carbon foam.

Table II. Determination of nitrite in ham sausage of different ingredients samples* .

Electrode Materials Sample No. Measured (mg/Kg) Added (mg/Kg) Found (mg/Kg) Recovery (%)
1 13.29 10.00 22.87 95.8
N-CDs1 2 12.02 50.00 64.12 104.2
3 14.39 100.00 116.49 102.1
1 13.58 10.00 23.31 97.3
N-CDs2 2 12.43 50.00 64.03 103.2
3 14.68 100.00 113.48 98.8
1 13.47 10.00 23.12 96.5
N-CDs3 2 12.30 50.00 65.15 105.7
3 14.51 100.00 116.81 102.3

* Values include mean values of three determinations.

(GB 5009.33-2016). The concentration of nitrite in different ingre- (SKLACLS1811), and the Natural Science Fund of Shaanxi Province
dient ham sausages were obtained and listed in Table II. It can be seen in China (2017JM2036).
that the recovery values of the samples using N-CDs1, N-CDs2, and
N-CDs3 were 95.8%∼104.0%, 97.3%∼103.2%, 96.4%∼105.0%, re-
spectively, showing that the fabricated sensors were suitable for the ORCID
determination of nitrite in real applications. Moreover, the content of
nitrite in different ingredients of ham sausage is lower than that pre- Qinglin Sheng https://orcid.org/0000-0002-2580-7922
scribed hygienic standards for food additives according to National Jianbin Zheng https://orcid.org/0000-0002-2114-6434
Food Safety Standards – Food Additive Use Standard (GB 2760-
2007).
References
Conclusions 1. X. Xu, R. Ray, Y. Gu, H. J. Ploehn, L. Gearheart, K. Raker, and W. A. Scrivens,
Electrophoretic analysis and purification of fluorescent single-walled carbon nan-
In summary, the CDs were successfully prepared via a simple one- otube fragments, J. Am. Chem. Soc, 126, 12736 (2004).
step method and a novel nitrite sensor was further fabricated. The 2. J. Zhang and S. H. Yu, Carbon dots: large-scale synthesis, sensing and bioimaging,
experiment results showed that CDs had a fairly narrow nanoparticle Mater. Today, 19, 382 (2016).
size distribution and the sensor exhibited an excellent comprehen- 3. X. Sun and Y. Lei, Fluorescent carbon dots and their sensing applications, Trac Trends
Anal. Chem, 89, 163 (2017).
sive performance. While for nitrite detection, the as-prepared CDs 4. G. Gedda, C. Y. Lee, Y. C. Lin, and H. F. Wu, Green synthesis of carbon dots from
showed enhanced electrocatalytic activity, which can be ascribed to prawn shells for highly selective and sensitive detection of copper ions, Sens. Actua-
the improved electric conductivity by multiple functional groups on tors B, 224, 396 (2016).
the surface of CDs. In addition, the nitrite sensor based CDs exhibited 5. W. Liu, H. Diao, H. Chang, H. Wang, T. Li, and W. Wei, Green synthesis of carbon
dots from rose-heart radish and application for Fe3+ detection and cell imaging, Sens.
a low detection limit, good stability as well as rapid detection of nitrite Actuators B, 241, 190 (2017).
in real samples. Therefore, it is expected that more biomass materials 6. X. Qin, W. Lu, A. M. Asiri, A. O. Al-Youbi, and X. Sun, Microwave-assisted rapid
could be used as preferred candidates for CDs synthesis, on which a green synthesis of photoluminescent carbon nanodots from flour and their applications
general platform for novel electrochemical sensors can be provided for sensitive and selective detection of mercury (II) ions, Sens. Actuators B, 184, 156
(2013).
for food safety evaluation. 7. X. Zhang, H. Ming, R. Liu, X. Han, Z. Kang, Y. Liu, and Y. Zhang, Highly sensitive
humidity sensing properties of carbon quantum dots films, Mater. Res. Bull, 48, 790
(2013).
Acknowledgments 8. L. Zhu, Y. Yin, C. F. Wang, and S. Chen, Plant leaf-derived fluorescent carbon dots
for sensing, patterning and coding, J. Mater. Chem. C, 1, 4925 (2013).
The authors gratefully acknowledge the financial support of this 9. S. S. Jones, P. Sahatiya, and S. Badhulika, One step, high yield synthesis of am-
project by the National Science Fund of China (21575113), the phiphilic carbon quantum dots derived from chia seeds: a solvatochromic study, New
State Key Laboratory of Analytical Chemistry for Life Science J. Chem, 41, 13130 (2017).

Downloaded on 2019-01-21 to IP 130.70.8.131 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).
 B62 Journal of The Electrochemical Society, 166 (2) B56-B62 (2019)

10. Y. Wang, Y. Zhu, S. Yu, and C. Jiang, Fluorescent carbon dots: rational synthesis, 28. M. R. Khan, S. M. Wabaidur, Z. A. Alothman, R. Busquets, and M. Naushad, Method
tunable optical properties and analytical applications, RSC Adv, 7, 40973 (2017). for the fast determination of bromate, nitrate and nitrite by ultra performance liquid
11. M. Algarra, A. González-Calabuig, K. Radotić, D. Mutavdzic, C. O. Ania, chromatography–mass spectrometry and their monitoring in Saudi Arabian drinking
J. M. Lázaro-Martı́nez, J. Jiménez-Jiménez, E. Rodrı́guez-Castellón, and water with chemometric data treatment, Talanta, 152, 513 (2016).
M. del Valle, Enhanced electrochemical response of carbon quantum dot modified 29. A. Rahim, L. S. Santos, S. B. Barros, L. T. Kubota, R. Landers, and Y. Gushikem,
electrodes, Talanta, 178, 679 (2018). Electrochemical detection of nitrite in meat and water samples using a mesoporous
12. A. Kumar, A. R. Chowdhuri, D. Laha, T. K. Mahto, P. Karmakar, and S. K. Sahu, carbon ceramic SiO2 /C electrode modified with in situ generated manganese (II)
Green synthesis of carbon dots from ocimum sanctum for effective fluorescent sensing phthalocyanine, Electroanalysis, 26, 541 (2014).
of pb2+ ions and live cell imaging, Sens. Actuators B, 242, 679 (2017). 30. H. Wu, S. Fan, X. Jin, H. Zhang, H. Chen, Z. Dai, and X. Zou, Construction
13. P. Balasubramanian, R. Settu, S. M. Chen, T. W. Chen, and G. Sharmila, A new of a zinc porphyrin–fullerene-derivative based nonenzymatic electrochemical sen-
electrochemical sensor for highly sensitive and selective detection of nitrite in food sor for sensitive sensing of hydrogen peroxide and nitrite, Anal. Chem, 86, 6285
samples based on sonochemical synthesized calcium ferrite (CaFe2 O4 ) clusters mod- (2014).
ified screen printed carbon electrode, J. Colloid Interface Sci, 524, 417 (2018). 31. V. Mani, A. P. Periasamy, and S. M. Chen, Highly selective amperometric nitrite
14. Y. Y. Yao, G. Gedda, W. M. Girma, C. L. Yen, Y. C. Ling, and J. Y. Chang, Mag- sensor based on chemically reduced graphene oxide modified electrode, Electrochem.
netofluorescent carbon dots derived from crab shell for targeted dual-modality Commun, 17, 75 (2012).
bioimaging and drug delivery, ACS Appl. Mater. Interfaces, 9, 13887 (2017). 32. J. M. Jian, L. Fu, J. Ji, L. Lin, X. Guo, and T. L. Ren, Electrochemically reduced
15. F. Yuan, S. Li, Z. Fan, X. Meng, L. Fan, and S. Yang, Shining carbon dots: synthesis graphene oxide/gold nanoparticles composite modified screen-printed carbon elec-
and biomedical and optoelectronic applications, Nano Today, 11, 565 (2016). trode for effective electrocatalytic analysis of nitrite in foods, Sens. Actuators B, 262,
16. H. Tao, K. Yang, Z. Ma, J. Wan, Y. Zhang, Z. Kang, and Z. Liu, In vivo NIR 125 (2018).
fluorescence imaging, biodistribution, and toxicology of photoluminescent carbon 33. C. W. Kung, Y. S. Li, M. H. Lee, S. Y. Wang, W. H. Chiang, and K. C. Ho, In-situ
dots produced from carbon nanotubes and graphite, Small, 8, 281 (2012). growth of porphyrinic metal-organic framework nanocrystals on graphene nanorib-
17. L. Han, S. Li, J. Dong, J. Liang, L. Li, N. Li, and H. Luo, Facile synthesis of bons for electrocatalytic oxidation of nitrite, J. Mater. Chem. A, 4, 10673 (2016).
multicolor photoluminescent polymer carbon dots with surface-state energy gap- 34. B. Yuan, J. Zhang, R. Zhang, H. Shi, N. Wang, J. Li, F. Ma, and D. Zhang, Cu-based
controlled emission, J. Mater. Chem. C, 5, 10785 (2017). metal–organic framework as a novel sensing platform for the enhanced electro-
18. L. Sun, S. Li, W. Ding, Y. Yao, X. Yang, and C. Yao, Fluorescence detection of choles- oxidation of nitrite, Sens. Actuators B, 222, 632 (2016).
terol using a nitrogen-doped graphene quantum dot/chromium picolinate complex- 35. Y. Seo, S. Manivannan, I. Kang, S. W. Lee, and K. Kim, Gold dendrites co-deposited
based sensor, J. Mater. Chem. B, 5, 9006 (2017). with m13 virus as a biosensor platform for nitrite ions, Biosens. Bioelectron, 94, 87
19. R. Madhu, V. Veeramani, and S. Chen, Heteroatom-enriched and renewable banana- (2017).
stem-derived porous carbon for the electrochemical determination of nitrite in various 36. Y. Ma, X. Song, X. Ge, H. Zhang, G. Wang, Y. Zhang, and H. Zhao, In situ growth
water samples, Sci. Rep, 4, 4679 (2014). of α-Fe2 O3 nanorod arrays on 3D carbon foam as an efficient binder-free electrode
20. M. Parsaei, Z. Asadi, and S. Khodadoust, A sensitive electrochemical sensor for for highly sensitive and specific determination of nitrite, J. Mater. Chem. A, 5, 4726
rapid and selective determination of nitrite ion in water samples using modified (2017).
carbon paste electrode with a newly synthesized cobalt(II)-Schiff base complex and 37. S. I. R. Malha, J. Mandli, A. Ourari, and A. Amine, Carbon black-modified electrodes
magnetite nanospheres, Sens. Actuators B, 220, 1131 (2015). as sensitive tools for the electrochemical detection of nitrite and nitrate, Electroanal-
21. A. Abo-Hamad, A. H. Alsaadi, and M. A. Hashim, Eutectic mixture-functionalized ysis, 25, 2289 (2013).
carbon nanomaterials for selective amperometric detection of nitrite using modified 38. Z. Meng, J. Zheng, and Q. Li, A nitrite electrochemical sensor based on electrodepo-
glassy carbon electrode, J. Electroanal. Chem, 812, 107 (2018). sition of zirconium dioxide nanoparticles on carbon nanotubes modified electrode, J.
22. L. Cui, J. Zhu, X. Meng, H. Yin, X. Pan, and S. Ai, Controlled chitosan coated. Iran. Chem. Soc, 12, 1053 (2015).
Prussian blue nanoparticles with the mixture of graphene nanosheets and carbon 39. C. Rajkumar, B. Thirumalraj, S. M. Chen, and S. Palanisamy, Novel electrochemical
nanoshperes as a redox mediator for the electrochemical oxidation of nitrite, Sens. preparation of gold nanoparticles decorated on a reduced graphene oxide–fullerene
Actuators B, 161, 641 (2016). composite for the highly sensitive electrochemical detection of nitrite, RSC Adv, 6,
23. Y. Zhang, Y. Zhao, S. Yuan, H. Wang, and C. He, Electrocatalysis and detection of 68798 (2016).
nitrite on a reduced graphene/Pd nanocomposite modified glassy carbon electrode, 40. G. Ma, M. Yang, F. Xu, and L. Wang, Three dimensional, macroporous hybrid of
Sens. Actuators B, 185, 602 (2013). polyoxometalate and reduced graphene oxide with enhanced catalysis for stable and
24. A. Shaikh, S. Parida, and S. Böhm, One step eco-friendly synthesis of Ag–reduced sensitive nonenzymatic detection of nitrite, Anal. Methods, 9, 5140 (2017).
graphene oxide nanocomposite by phytoreduction for sensitive nitrite determination, 41. A. A. Ensafi, S. H. Sefat, N. Kazemifard, B. Rezaei, and F. Moradi, A novel one-step
RSC Adv, 6, 100383 (2016). and green synthesis of highly fluorescent carbon dots from saffron for cell imaging
25. Ö Ö Erol, B. Y. Erdoğan, and A. N. Onar, Nitrate and nitrite determination in gunshot and sensing of prilocaine, Sens. Actuators B, 253, 451 (2017).
residue . samples by capillary electrophoresis in acidic run buffer, J. Forensic Sci, 62, 42. R. Ramar and I. Malaichamy, Highly selective and sensitive biosensing of dopamine
423 (2017). based on glutathione coated silver nanoclusters enhanced fluorescence, New J.
26. M. Greguš, F. Foret, and P. Kubáň, Portable capillary electrophoresis instrument with Chem,41, 15244 (2017).
contactless conductivity detection for on-site analysis of small volumes of biological 43. Y. Fu, Q. Sheng, and J. Zheng, The novel sulfonated polyaniline-decorated carbon
fluids, J. Chromatogr. A, 1427, 177 (2016). nanosphere nanocomposites for electrochemical sensing of dopamine, New J. Chem,
27. C. Lopez-Moreno, I. V. Perez, and A. M. Urbano, Development and validation of an 41, 15439 (2017).
ionic chromatography method for the determination of nitrate, nitrite and chloride in 44. S. Gao, H. Fan, and S. Zhang, Nitrogen-enriched carbon from bamboo fungus with
meat, Food Chem, 194, 687 (2016). superior oxygen reduction reaction activity, J. Mater. Chem. A, 2, 18263 (2014).

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