Novel electrochemical sensors based on cuprous oxide-electrochemically reduced graphene oxide nanocomposites modified electrode toward sensitive detection of sunset yellow

Article abstract of DOI:10.3390/molecules23092130

Control and detection of sunset yellow is an utmost demanding issue, due to the presence of potential risks for human health if excessively consumed or added. Herein, cuprous oxide-electrochemically reduced graphene nanocomposite modified glassy carbon electrode (Cu₂O-ErGO/GCE) was developed for the determination of sunset yellow. The Cu₂O-ErGO/GCE was fabricated by drop-casting Cu₂O-GO dispersion on the GCE surface following a potentiostatic reduction of graphene oxide (GO). Scanning electron microscope and X-ray powder diffractometer was used to characterize the morphology and microstructure of the modification materials, such as Cu₂O nanoparticles and Cu₂O-ErGO nanocomposites. The electrochemical behavior of sunset yellow on the bare GCE, ErGO/GCE, and Cu₂O-ErGO/GCE were investigated by cyclic voltammetry and second-derivative linear sweep voltammetry, respectively. The analytical parameters (including pH value, sweep rate, and accumulation parameters) were explored systematically. The results show that the anodic peak currents of Cu₂O-ErGO /GCE are 25-fold higher than that of the bare GCE, due to the synergistic enhancement effect between Cu₂O nanoparticles and ErGO sheets. Under the optimum detection conditions, the anodic peak currents are well linear to the concentrations of sunset yellow, ranging from 2.0 × 10^?8 mol/L to 2.0 × 10^?5 mol/L and from 2.0 × 10^?5 mol/L to 1.0 × 10^?4 mol/L with a low limit of detection (S/N = 3, 6.0 × 10^?9 mol/L). Moreover, Cu₂O-ErGO/GCE was successfully used for the determination of sunset yellow in beverages and food with good recovery. This proposed Cu₂O-ErGO/GCE has an attractive prospect applications on the determination of sunset yellow in diverse real samples.

Full text of DOI:10.3390/molecules23092130

molecules
Article
Novel Electrochemical Sensors Based on Cuprous
Oxide-Electrochemically Reduced Graphene Oxide
Nanocomposites Modified Electrode toward Sensitive
Detection of Sunset Yellow
Quanguo He 1,2,†, Jun Liu 1,2,†, Xiaopeng Liu 2, Yonghui Xia 3, Guangli Li 1,2,*, Peihong Deng 4
and Dongchu Chen 1,
*
1
School of Materials Science and Energy Engineering, Foshan University, Foshan 528000, China;
hequanguo@126.com (Q.H.); liu.jun.1015@163.com (J.L.)
College of Life Sciences and Chemistry, Hunan University of Technology, Zhuzhou 412007, China;
amituo321@163.com
Zhuzhou Institute for Food and Drug Control, Zhuzhou 412000, China; sunnyxia0710@163.com
Department of Chemistry and Material Science, Hengyang Normal University, Hengyang 421008, China;
dph1975@163.com
2
3
4
*
Correspondence: guangli010@hut.edu.cn (G.L.); chendc@fosu.edu.cn (D.C.);
Tel./Fax: +86-731-22183382 (G.L. & D.C.)
†
These authors contributed equally to this work.
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 30 July 2018; Accepted: 21 August 2018; Published: 24 August 2018
Abstract: Control and detection of sunset yellow is an utmost demanding issue, due to the
presence of potential risks for human health if excessively consumed or added. Herein,
cuprous oxide-electrochemically reduced graphene nanocomposite modified glassy carbon electrode
(Cu2O-ErGO/GCE) was developed for the determination of sunset yellow. The Cu2O-ErGO/GCE
was fabricated by drop-casting Cu2O-GO dispersion on the GCE surface following a potentiostatic
reduction of graphene oxide (GO). Scanning electron microscope and X-ray powder diffractometer
was used to characterize the morphology and microstructure of the modification materials, such as
Cu2O nanoparticles and Cu2O-ErGO nanocomposites. The electrochemical behavior of sunset yellow
on the bare GCE, ErGO/GCE, and Cu2O-ErGO/GCE were investigated by cyclic voltammetry and
second-derivative linear sweep voltammetry, respectively. The analytical parameters (including
pH value, sweep rate, and accumulation parameters) were explored systematically. The results show
that the anodic peak currents of Cu2O-ErGO /GCE are 25-fold higher than that of the bare GCE, due to
the synergistic enhancement effect between Cu2O nanoparticles and ErGO sheets. Under the optimum
detection conditions, the anodic peak currents are well linear to the concentrations of sunset yellow,
ranging from 2.0
with a low limit of detection (S/N = 3, 6.0
successfully used for the determination of sunset yellow in beverages and food with good recovery.
This proposed Cu2O-ErGO/GCE has an attractive prospect applications on the determination of
sunset yellow in diverse real samples.
×
10−8 mol/L to 2.0
×
10−5 mol/L and from 2.0
×
10−5 mol/L to 1.0
×
10−4 mol/L
×
10−9 mol/L). Moreover, Cu2O-ErGO/GCE was
Keywords: cuprous oxide nanoparticles; reduced graphene oxide; modified electrode; sunset yellow;
second-derivative linear sweep voltammetry
1. Introduction
Sunset yellow, as a common azo colorant, has been widely added in several beverages (such as
carbonated beverage, orange juice, and Fanta drink) and food (i.e. candies, cakes, cheese) to improve
Molecules 2018, 23, 2130; doi:10.3390/molecules23092130
www.mdpi.com/journal/molecules

Molecules 2018, 23, 2130
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the appearance and appetite [
1
,2]. Sunset yellow contains an azo group (-N=N-) and aromatic ring
structure, which may cause mutagenic and carcinogenic risk for human [
many serious health problems, such as hepatocellular damage, kidney failures, headache, cancers,
and attention deficit hyperactivity disorder (ADHD), when it is excessively consumed [ ]. Hence,
3,4]. As a result, it can bring
5
the additive amount of sunset yellow in beverages and food demands strict control and regulation.
European Food Safety Authority (EFSA) recommends an acceptable daily intake (ADI) for sunset
yellow of 1.0 mg kg−1 body weight [
6
]. In some countries, it was explicitly stated that the permitted
maximum content of sunset yellow in nonalcoholic beverages is 100 ]. Moreover, Finland
g mL−1
and Norway even has already banned the use of sunset yellow in foods [ ]. While considering the
µ
[7
6
food quality and safety, it is urgent to develop reliable analytical techniques for the quick detection of
this food dye.
At present, various techniques have been developed for sunset yellow, including high performance
liquid chromatography (HPLC) [8], thin layer chromatography [9], spectrophotometry [10],
fluorometry [11], and capillary electrophoresis [12]. Although these methods have been proved
to be reliable, they still have some disadvantages, such as time-consuming, complicated preprocessing,
and expensive equipment. Recently, electrochemical analysis has been widely used for the detection
of bioactive molecules, nutrients, food additives, as well as contaminants, due to its considerable
merits such as low cost, simple operation, rapid response, high sensitivity and sensitivity. As we all
know, the key issue for electrochemical detection toward sunset yellow is to develop ultrasensitive
modified electrodes.
Nanostructure precious metals or alloys have become the preferred electrode modification
materials for the sensitive detection of sunset yellow, due to their superior electrocatalytic
activity [7,13–17]. For example, Wang and coworkers developed a promising electrochemical sensor
based on Au-Pd and reduced graphene oxide nanocomposite decorated electrode (Au-Pd-RGO/
GCE) [17]. The Au-Pd-RGO/GCE exhibited good stability, superior electrocatalytic performance,
low detection limit (1.5 nmol/L), and wide response range (0.686–331.686
µmol/L). Pd-Ru
nanoparticles incorporated carbon aerogel nanocomposites (Pd-Ru/CA) have been successfully
used for electrochemical detection and catalytic degradation of sunset yellow [16]. The Pd-Ru/CA
nanocomposite decorated screen printed carbon electrode (Pd-Ru/CA/SPCE) showed a low detection
limit (7.1 nmol/L) and high sensitivity (3.571 µA/(µ
mol/L cm2). Electrochemical sensing platform
based on Au NPs and reduced graphene modified GCE (Au NPs/RGO/GCE) was constructed for
the quantitative analysis of sunset yellow, and the Au NPs/RGO/GCE had excellent catalytic activity
toward the oxidation of sunset yellow [15]. The proposed electrode showed wide linear response
range of 0.002–109.14
carbon-paste electrode (Au NPs/CPE) was also fabricated for the simultaneous detection sunset yellow
and Tartrazine [ ]. The Au NPs/CPE displayed low detection limits of 3.0
10−8 and 2.0 10−9 mol/L
µmol/L and low detection limit of 2 nmo/L (S/N = 3). Au NPs decorated
7
×
×
for sunset yellow and Tartrazine, respectively. These precious metals-based modified electrodes can
detect sunset yellow at the nanomole level; however, the scarcity and high cost of precious metals or
alloys have seriously hindered the broad practical applications.
When compared to precious metals or alloys, transition metals and metal oxides have outstanding
advantages in terms of abundance and cost. What is more, transition metals and metal oxides also
have excellent catalytic activity. In our previous work, Cu2O-RGO nanocomposite [18], NH2-Fe3O4-
RGO nanocomposite [19], MnO2-RGO nanocomposite [20], TiO2-RGO nanocomposite [21], and
α
-MnO2/N-doped ketjenblack carbon composite [22,23] demonstrated excellent electrochemical
sensing performance toward the detection of dopamine and Tartrazine and superior electrocatalytic
activity in oxygen reduction reaction (ORR) & oxygen evolution reaction (OER). Recently, few
researchers have devoted to developing transition metal oxide modified electrodes for the
determination of sunset yellow. For example, Dorraji et al. [24] developed ZnO/cysteic acid
nanocomposite modified electrode (ZnO/CA/GCE), and successfully used for simultaneous
determination of sunset yellow and Tartrazine. The ZnO/CA/GCE exhibited two linear response

Molecules 2018, 23, 2130
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ranges in the concentration ranges of 0.1–3.0
µmol/L, and 0.07–1.86
µmol/L, and detection limits of
0.03 µmol/L and 0.01 µmol/L for sunset yellow and Tartrazine, respectively. However, the linear
dynamic response range is limited for trace detection of sunset yellow. The detection capacity
of sunset yellow has been improved with graphene and mesoporous TiO2 composite [25] and
ZnO/RGO/ZnO@Zn [26] modified electrodes, and they showed superior sensing performance (i.e.,
linear ranges, detection of limit) that was comparable to precious metal modified electrode. Although
some progress has been made, there are only few related reports concerning the transition metal oxide
modified electrodes. Therefore, it is still worthwhile to develop novel transition metal oxide modified
electrodes for sensitive detection of sunset yellow.
Among transition oxides, cuprous oxide (Cu2O) is an environmentally friendly p-type
semiconductor material, which has been widely used in solar cells and photo catalysis [27,28], due to
its unique electronic structure and excellent catalytic performances. However, its electrical conductivity
is poor due to the nature of semiconductor. To resolve this problem, Cu2O nanoparticles are often
composited or hybrided with conductive materials [29–33], to decrease the charge transfer resistance
and eventually enhance the electrochemical performance. Graphene, as an emerging two-dimensional
(2D) carbon material, has been usually used as conductive materials in modified electrodes, owing to its
high specific surface area, excellent electrical conductivity, superior electrochemical performance, and
fast heterogeneous electron transfer rate. It has been reported that graphene-based modified electrodes
have been widely employed for the determination of azo dyes, such as sunset yellow, Tartrazine, and
Amaranth [34–36]. However, to our best knowledge, Cu2O/reduced graphene oxide nanocomposite
modified electrode toward sensitive detection of sunset yellow has not been reported.
Graphene usually prepared from graphene oxide (GO) by electrochemical reduction method in
the field of electrochemical analysis. The chemically reduced graphene oxide is hydrophobic, due
to the removal of most oxygen-containing functional groups. As a result, the chemically graphene
oxide tends to agglomerate resulting in a degradation on sensing performance. The agglomeration
issue can be overcomed by introducing the surfactants [37], which can effectively improve the
dispensability. However, the electrical conductivity also declined due to the use of surfactants.
Electrochemically reduction method is a green and efficient method to obtain reduced graphene
oxide that not require any reductants. Moreover, the residual oxygen-containing functional groups
can be tuned by facial adjusting the electrochemical parameter, such as reduction potential, reduction
time, and scanning cycles [18,19,21]. In other words, the property of reduced graphene oxide can be
tailored by electrochemical parameters. For these reasons, the electrochemically reduced graphene
oxide (ErGO) have been widely used for constructing diverse sensors.
Inspired by the foregoing reports, herein ErGO was composited with low cost and excellent
electrocatalytic activity Cu2O nanoparticles, aiming to develop a cost-effective, high sensitive, and
good selective modification materials to substitute the precious metal-based materials. Meanwhile,
the Cu2O-ErGO nanocomposites are expected to exert their synergistic sensitizing effects to improve
the sensing performance. Then, Cu2O-ErGO nanocomposites was modified on the surface of the
glassy carbon electrode (GCE) to construct a novel sensor toward sunset yellow. The Cu2O-ErGO
modified glassy carbon electrode (Cu2O-ErGO/GCE) was prepared while using a facile drop-casting
technique in combination with electrochemical reduction. The electrochemical behavior of sunset
yellow on the Cu2O-ErGO/GCE were investigated by cyclic voltammetry and second-derivative
linear sweep voltammetry. The effect of detection conditions (such as pH value, sweep rate, and
accumulation parameters) on the electrochemical response were also explored. Finally, the proposed
Cu2O-ErGO/GCE was used to detect the content of sunset yellow in soda drinks, orange juice, and
candies samples while using second-derivative linear sweep voltammetry.

Molecules 2018, 23, 2130
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2. Results and Discussion
2.1. Morphology and Microstructural Characterization
The surface morphologies of Cu2O nanoparticles (Cu2O NPs) and Cu2O-ErGO nanocomposites
were characterized by scanning electron microscope (SEM, Hitachi S-3000N, Tokyo, Japan). The SEM
images are shown in Figure 1A,B, respectively. The Cu2O NPs exhibit cubic-like structure with uniform
size, and the particle size is estimated to about 150 nm. Obviously, the thin layer ErGO sheets were
successfully coated on the surface of Cu2O nanoparticles. Moreover, the particle size of Cu2O-ErGO
nanocomposite increases slightly, which facilitates the adsorption of sunset yellow. The Cu2O NPs
were further characterized by X-ray diffraction (XRD, JEOL JEM-2010 (HT, Tokyo, Japan) and the XRD
pattern of Cu2O NPs is plotted in Figure 1C. The diffraction peaks of Cu2O nanoparticles are clearly
indexed into the pure cubic phase of Cu2O (JSPDS78-2076), suggesting that the cubic phase of Cu2O
nanoparticles was prepared.
Figure 1. The scanning electron microscope (SEM) photos of cuprous oxide (Cu2O) nanoparticles (A)
and cuprous oxide-electrochemically reduced graphene oxide (Cu2O-ErGO) nanocomposites (B); and,
(C) The X-ray diffraction (XRD) pattern of Cu2O nanoparticles.
2.2. Electrochemical Behavior of Sunset Yellow on Modified Electrodes
Second-derivative linear sweep voltammetric responses of 1.0
different electrodes are presented in Figure 2. On the bare GCE, a weak anodic peak of sunset yellow
appears at 798 mV with the anodic peak current (ipa) of 0.725 A. On the GO/GCE, the oxidation
peak current of sunset yellow decreases to 0.497 A, which is mainly due to the present of the poor
electrical conductivity of GO. On the Cu2O-GO/GCE, an apparent anodic peak of occurred at 770 mV,
and the ipa increases to 0.925 A, probably owing to the electrocatalytic activity of Cu2O nanoparticles.
When the GO was electrochemically reduced to ErGO, the anodic peak appears at 792 mV and the
pa increases to 16.93 A. This phenomenon may be related to the high electrical conductivity, large
specific surface area, and rapid heterogeneous electron transfer rate of ErGO. Moreover, the adsorption
capacity of sunset yellow on the electrode surface is improved greatly by the interaction, because
the conductive carbon-conjugated networks are restored after the reduction process. When the GCE
was modified with Cu2O-ErGO nanocomposites, the ipa is the largest (18.08 A), which is about 25 fold
×
10−5 mol/L sunset yellow on
µ
µ
µ
i
µ
π-π
µ
greater than that of bare GCE. It mainly due to the synergistic enhancement effect between Cu2O
nanoparticles and ErGO sheets, which significantly improves the sensitivity of sunset yellow detection.

Molecules 2018, 23, 2130
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−5
10 mol/L sunset yellow on the
Figure 2. Second-derivative linear sweep voltammograms of 1.0
different electrodes.
×
The electrochemical behavior of 1.0
×
10−5 mol/L sunset yellow on the GCE (a), GO/GCE (b),
Cu2O-GO/GCE (c), ErGO/GCE (d), and Cu2O-ErGO/GCE (e) were further investigated by cyclic
voltammetry (Figure 3). All of the electrodes appear a pair of redox peaks, meaning that sunset
yellow undergoes a quasi-reversible process. Obviously, a pair of sharp redox peaks occurs on the
ErGO/GCE and Cu2O-ErGO/GCE. Furthermore, the order of anodic peak currents obtained from
cyclic voltammograms is consistent with the second-derivative linear sweep voltammograms, which
further confirms that Cu2O-ErGO nanocomposites can significantly enhance the electrochemical
response toward sunset yellow.
Figure 3. Cyclic voltammograms of 1.0
×
10−5 mol/L sunset yellow recorded on the glassy carbon
electrode (GCE) (a), graphene oxide/glassy carbon electrode (GO/GCE) (b), Cu2O-graphene oxide
nanocomposite modified glass carbon electrode (Cu2O-GO/GCE) (c), electrochemically reduced
graphene oxide modified glass carbon electrode (ErGO/GCE) (d) and Cu2O-ErGO modified glassy
carbon electrode (Cu2O-ErGO/GCE) (e). The inset is the magnification of the cyclic voltammograms
recorded on the GCE (a), GO/GCE (b) and Cu2O-GO/GCE (c).

Molecules 2018, 23, 2130
6 of 15
2.3. Effect of pH Value
Since proton (H+) plays an important role on the redox of sunset yellow, so it is worthwhile
investigating the influence of pH value on the response peak current of sunset yellow. The ipa of
sunset yellow recorded in various pH PBS solution are depicted in Figure 4A. The ipa of sunset yellow
increases gradually as the pH value increases. When the pH value increases to 3.8, the largest ipa is
obtained. Afterwards the ipa decreases slowly with the pH value further increasing. Hence, the pH 3.8
PBS solution was employed as supporting electrolytes on the subsequent experiments. Furthermore,
the anodic peak potential (Epa) of sunset yellow is negatively shifted with the increase of pH value.
As plotted in Figure 4B, there is a good linear relationship between Epa and pH value, confirming that
protons are involved in the oxidation of sunset yellow. The linear regression equation can expressed as
E
pa (V) =
approaches to the theoretical value (
electron involve in the electrochemical oxidation of sunset yellow [38].
−
0.0570 pH + 1.0167 (R2 = 0.998). According to Nernst equation, its slope (
0.0590 V/pH), suggesting that the equal amounts of proton and
−
0.0570 V/pH)
−
Figure 4.
Cu2O-ErGO/GCE; and, (
against pH value.
(
A
) Effects of pH value on the anodic peak currents of 1.0
×
10−5 mol/L sunset yellow on the
−5
B
) The plot of the anodic peak potential of 1.0
×
10 mol/L sunset yellow
2.4. Effect of Sweep Rates
Sweep rate is a crucial parameter that directly affects the electrochemical response of analysts
on the modified electrodes. Moreover, it is a powerful tool to reveal the electrochemical reaction
mechanism. Cyclic voltammograms at various sweep rates were recorded at 0.1 mol/L PBS solution
−5
containing 1
×
10 mol/L sunset yellow while using the Cu2O-ErGO/GCE, and their corresponding
cyclic voltammograms are shown in the Figure 5A. As expected, with sweep rates increasing, the anodic
peaks shift toward positive direction, while the cathodic peaks shift negatively, indicating that the
oxidation of sunset yellow on the Cu2O-ErGO/GCE is quasi-reversible. Both the anodic peak currents
(ipa) and cathodic peak currents (ipc) increase with the sweep speeding up, however, the background
currents also increase. To purist high signal-to-noise (S/N), a suitable sweep rate is recommended as
100 mV/s. It can be clearly seen from Figure 5B that both the anodic peak currents (ipa) and cathodic
peak currents (ipc) of sunset yellow is nearly linear with the sweep rates (v). Their linear equations are
expressed as: ipa
(
µ
A) = 0.2968 v (mV/s)
−
4.791 (R2 = 0.998) and ipc
(
µ
A) =
−
0.1471 v (mV/s) + 1.659
(R2 = 0.997), suggesting that the oxidation of sunset yellow on the Cu2O-ErGO/GCE is controlled by
the adsorption process [39].

Molecules 2018, 23, 2130
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Figure 5.
recorded at various sweep rates (30 mV/s–300 mV/s); (
currents (ipa and ipc) of sunset yellow against sweep rates (v); and (
(
A
) Cyclic voltammograms of 1.0
×
10−5 mol/L sunset yellow on the Cu2O-ErGO/GCE
) Linear plots of anodic and cathodic peak
) Linear plots of andic and
B
C
cathodic peak potential (Epa and Epc) of sunset yellow against Napierian Logarithm of sweep rates
(lnv). Supporting electrolytes: 0.1 mol/L PBS.
It is observed that both the anodic peak potential (Epa) and the cathodic peak potential (Epc) is
well linear to the Napierian Logarithm of sweep rates (lnv). Their corresponding linear equations are
E
pa (V) = 0.0289lnv (V/s) + 0.8286 (R2 = 0.990) and Epc (V) =
−
0.0412lnv (V/s) + 0.7135 (R2 = 0.990).
As for an adsorption-controlled and quasi-reversible process, according to the Lavrion equation [40],
the peak potential and the sweep rate follows the following relationship:
0
RT
(1 − α)nF
D1/2
Ks
1
) − ln
2
RT
(1 − α)nF
RT
2(1 − α)nF
Epa = E0 +
[0.78 + ln(
] +
ln v
(1)
0
RT
αnF
D1/2
Ks
1
2
RT
αnF
RT
Epc = E0 +
[0.78 + ln(
) − ln
] −
2αnF
ln v
(2)
where Epa (V) and Epc (V) represents the anodic peak potential and the cathodic peak potential,
respectively; v (V/s) denotes the sweep rate; is the charge transfer coefficient; ks is the heterogeneous
electron transfer rate; n is the electron transferred number; T is Kelvin temperature; F is Faraday
constant (96,480 C/mol); and, R is molar gas constant (8.314 J/(mol K)). Combining the slopes of
α
·
Equations (1) and (2) with the Epa/Epc vs. lnv equations, the charge transfer coefficient α is estimated
to be 0.45 and the electron transferred number n is around 1. Since the equal amount of proton
and electron participates in the oxidation process, the electrochemical oxidation of sunset yellow is
1 electron and 1 proton process, which is in accordance with the previous studies [39,41]. Hence,
the electrochemical oxidation mechanism of sunset yellow on the Cu2O-ErGO/GCE can be inferred in
Figure 6.
Figure 6. The mechanism for the electrochemical processes of sunset yellow on the Cu2O-ErGO/GCE.
2.5. Effect of Acumualtion Parameters
Accumulation is a simple and effective technique to improve the electrochemical response.
Since the electrochemical oxidation of sunset yellow is an adsorption-controlled process,
so accumulation were performed before second-derivative linear sweep voltammetry. As we all know,

Molecules 2018, 23, 2130
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accumulation potential as well as time are two important parameters that affect the response peak
current greatly, so it is a worthwhile optimization. The Cu2O-ErGO/GCE was accumulated at different
accumulation potential for 240 s firstly. Then, their anodic peak currents (ipa) of sunset yellow were
recorded in 0.1 mol/L PBS solution (pH 3.8) while using second-derivative linear sweep voltammetry.
The effect of the accumulation potential on the ipa of sunset yellow is presented in Figure 7A. The ipa
increases gradually with the rising of accumulation potential. When the accumulation potential reaches
0.4 V, the strongest ipa is obtained. Afterwards, the ipa decreases with the accumulation potential further
increasing. Therefore, 0.4 V was selected in the subsequent experiments. Furthermore, the influence
of accumulation time was also explored. Similarly, the Cu2O-ErGO/GCE was accumulated at an
optimized accumulation potential for various time. Then, their ipa of sunset yellow were recorded and
compared. As shown in Figure 7B, the ipa increases with the prolong of the accumulation during the
first 180 s; then ipa keep stable with the accumulation time further prolonging, demonstrating that
the adsorption of sunset yellow achieved saturated. Hence, 180 s is recommended as the optimum
accumulation time.
Figure 7. Influence−s5of accumulation potential (
A), and accumulation time (B) on the anodic peak
currents of 1.0 × 10 mol/L sunset yellow on the Cu2O-ErGO/GCE.
2.6. Standard Curves, Linear Range and Limit of Detection
With the optimal analytical parameters, the ipa of different concentrations of sunset yellow
standard solution was determined by second-derivative linear sweep voltammetry. Figure 8A
shows the second-derivative linear sweep voltammograms of various concentrations of sunset
yellow. There are two linear response ranges for the detection of sunset yellow, namely
2.0
Their corresponding linear equations are ipa
A) = 0.0775c (
estimated to be 6.0
×
10−8 ~2.0
×
10−5 mol/L (Figure 8B) and 2.0
×
10−5 mol/L ~1.0
×
10−4 mol/L (Figure 8C).
(
µA) = 1.597c (µ
mol/L) + 2.628 (R2 = 0.973) and ipa
(µ
µ
mol/L) + 30.36 (R2 = 0.992), respectively. The limit of detection (LOD, S/N = 3) is
×
10−9 mol/L. The linear response range is lower than the permitted maximum
−4
content of sunset yellow (2.2
×
10 mol/L), so that the Cu2O-ErGO/GCE can be applied to detection
sunset yellow by a direct or the dilution method. A comparison on sensing performances toward
sunset yellow between the existing modified electrodes and Cu2O-ErGO/GCE is summarized on
Table 1.

Molecules 2018, 23, 2130
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Table 1. Comparison the sensing performances toward the detection of sunset yellow between the
existing modified electrodes and the proposed Cu2O-ErGO/GCE.
Linear Range
(µmol/L)
LOD
(µmol/L)
Modified Electrodes
Method
Metrological Parameters
Reference
0.1 M phosphate-citrate buffer solution (pH 7.0);
accumulation for 60 s
PLPA/GCE
DPV
0.04–14
0.040
[42]
0.1 M CBS solution (pH 5.0); accumulation for 30 min;
scanned at 10 mV/s
MIP/f-MWCNTs/GCE
Au-Pd-RGO/GCE
CTAB-Gr-Pt/GCE
DPV
DPV
DPV
0.05–100
0.005
0.0015
0.0042
[43]
[17]
[44]
0.1 M PBS (pH 4.0); scanned at 50 mV/s
0.69–332
0.0085–1.0;
1.0–30
0.1 M PBS (pH 3.0); accumulation for 3 min
0.1 M PBS (pH 5.5); accumulation for 7 min; scanned at
50 mV/s
0.1–0.6;
0.6–12
0.004–1.0
GO/AgNPs-MIPs/GCE
ILRGO-Au/GCE
Au NPs/CPE
LSV
SWV
DPV
0.02
0.00052
0.03
[45]
[13]
[7]
0.1 M BR buffer solution (pH 7.0); accumulation for 300 s
0.1 M PBS (pH 4.0); accumulation for 1 min; modulation
amplitude = 60 mV and scan rate = 60 mV/s
0.1 M PBS (pH 7.0); accumulation at open circuit
potential for 2 min; potential increment of 0.004 V, pulse
amplitude of 0.05 V, and pulse period of 0.2 s
0.1 M PBS (pH 3.0); accumulation for 5 min
0.1 M PBS (pH 8.0); pulse amplitude = 0.05 V; pulse
interval time = 0.05 s, and scan rate = 0.02 V/s for
differential pulse voltammetry
0.1–2.0
MWCNT/GCE
DPV
DPV
DPV
0.55–7.0
0.01–10
0.02–20
0.12
0.002
0.0055
[46]
[47]
[48]
PDDA-Gr-Pd/GCE
Fe3O4@SiO2-
NPs@MIP/Gr/GCE
0.1 M PBS (pH 3.8); accumulation at 0.4 V for 180 s;
scanned at 100 mV/s
0.02–20;
20–100
Cu2O-ErGO/GCE
SDLSV
0.006
This work
The linear ranges and LOD of the proposed Cu2O-ErGO/GCE are at least comparable to and
even better than most of the previous reports. Moreover, Cu2O-ErGO/GCE have outstanding
advantages over noble metal modified electrodes (such as Au NPs/CPE [7], Au-Pd-RGO/GCE [17],
CTAB-Gr-Pt/GCE [44], GO/AgNPs-MIPs/GCE [45] and PDDA-Gr-Pd/GCE [47] in terms of the cost
and electrode fabrication.
Figure 8.
(A) Second-derivative linear sweep voltammograms of sunset yellow with various
concentrations on the Cu2O-ErGO/GCE; Calibration curves between the anodic peak current and the
−8
−5
−5
10
concentrations of sunset yellow ranging from 2.0
to 1.0 × 10 mol/L(C).
×
10 to 2.0
×
10 mol/L (
B) and from 2.0
×
−4
2.7. Interference and Reproducibility Investigation
Prior to the detection of real samples, the anti-interference and reproducibility was also
investigated to validate the practicability of the proposed Cu2O-ErGO/GCE. The response peak
current of pure sunset yellow solution and potential interfering compounds mixture solution were
recorded and compared. It is observed that the change of ipa of 10 µmol/L sunset yellow is less
than 5% in the presence of a 100-fold concentration of glucose, benzoic acid, citric acid, Na+, K+,
Fe3+, and 10-fold concentration of Tartrazine, quinoline yellow (Figure 9). It is demonstrating that
our proposed Cu2O-ErGO/GCE exhibits good selectivity toward sunset yellow. The reproducibility
of Cu2O-ErGO/GCEs were examined by continuous measurement the response peak currents of
10
µmol/L sunset yellow seven times. The result shows that the ipa remain stable with relative standard
deviation (RSD) of 2.78% (Table 2), indicating that the Cu2O-ErGO/GCE has good reproducibility.

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Figure 9. The response peak current of Cu2O-ErGO/GCE for 1
the presence of interfering substances (n = 3). The concentration of glucose, benzoic acid (BA), citric
×
10−5 mol/L sunset yellow (SY) in
acid (CA), Na+, K+, Fe3+, were 1
(QY) were 1 × 10 mol/L.
×
10−3 mol/L. The concentrations of Tartrazine, quinoline yellow
−4
Table 2. The reproducibility of Cu2O-ErGO/GCE for detection of sunset yellow.
No.
1
2
3
4
5
6
7
ipa (µA)
Average value (µA)
RSD (%)
21.07
22.34
21.36
22.04
22.02
2.78
22.29
22.85
22.24
2.8. Detection Sunset Yellow in Real Samples
Finally, Cu2O-ErGO/GCE was applied to determinate sunset yellow in real samples, including
carbonated drinks, orange juice, and candy samples. The detect results are listed in Table 3. No anodic
peak current is presented in the carbonated beverage sample near 798 mV, indicating that the
concentration of sunset yellow is low than the limit of detection. The concentrations of sunset yellow
in orange juice and candy samples are detected by 0.085
The content of sunset yellow in these samples is lower than the permitted maximum content that is
recommended by national standard (100 g/mL, namely 2.2
10−4 mol/L). Then, an appropriate
µmol/L and 0.162 µmol/L, respectively.
µ
×
amount of sunset yellow standard solution was added to the above three samples. Standard addition
test results suggests that the recoveries of sunset yellow were 98.75~102.0%, with the relative standard
deviation being less than 2.85%, indicating that satisfactory results were obtained using the proposed
Cu2O-ErGO/GCE. Together with cost-effective, quick response, high sensitive and good selectivity,
the Cu2O-ErGO/GCE exhibits great prospects on the determination of sunset yellow in different real
samples, including but not limiting to beverages, food, and nutrients.
Table 3. The results of determination of sunset yellow in soft drink and candies (n = 3).
Samples
Original (µmol/L)
Added (µmol/L)
RSD (%)
Found (µmol/L)
Recovery (%)
RSD (%)
ND 1
0.085
0.162
Sodas
Orange juice
Candies
4
1.28
2.46
3.25
4.06
0.082
0.158
102.0
102.5
98.75
1.50
2.31
2.85
0.080
0.160
1
ND: Not detected.
3. Materials and Methods
3.1. Chemical and Solution
Graphite powder, NaNO3, concentrated H2SO4, NaOH, KMnO4, H2O2, CuSO4
NaH2PO4, hydrochloric acid (HCl), polyvinylpyrrolidone (PVP), hydrate hydrazine (N2H4 H2O), and
·
5H2O, Na2HPO4,
·

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ethyl alcohol were analytic grade and supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai,
China). Sunset yellow was purchased from Aladdin (http://www.aladdin-e.com). All of these
chemicals were directly used without further purification. A series of sunset yellow standard solutions
were prepared by diluting appropriately the stock sunset yellow (1
solutions were kept in 4 C refrigerator when not in use. Deionized water (18.2 M
throughout the experiments.
×
10−3 mol/L). These standard
◦
Ω) was used
3.2. Synthesis of Cu2O Nanoparticles
Cu2O nanoparticles were synthesized by the hydrothermal method referred to our previous
work [18]. Specifically, 50 mg of CuSO4 5H2O and 24 mg of PVP were added into 10 mL deionized
·
water and then stirred with ultrasonication for 30 min. Afterwards, 2 mL of 0.2 mol/L NaOH
solution was added and stirred for 30 min at room temperature to obtain blue Cu(OH)2 precipitates.
Subsequently, 6
µ
L of N2H4 H2O was added as reductant and then stirred for 20 min at room
·
temperature to form a brick red suspension. The precipitate was separated by centrifugation at
5000 rpm, and washed repeatedly with deionized water and ethanol for three times, and vacuum-dried
at 60 ◦C to obtain Cu2O nanoparticles.
3.3. Preparation Cu2O-GO Nanocomposite Dispersion
Graphene oxide (GO) is prepared from cheap graphite powder by a modified Hummers method
according to our previous reports [18,19,21]. The as-prepared GO was dispersed in 100 mL of deionized
water under ultrasonication for 2 h, and then centrifuged twice to obtain a golden yellow GO solution
(1 mg/mL). 2 mg of Cu2O nanoparticles were added to 5 mL of the above GO solution, and then
ultrasonically dispersed for 2 h to obtain a Cu2O-GO nanocomposite dispersion.
3.4. Preparation of Cu2O-ErGO/GCE
The glassy carbon electrode (GCE,
m alumina slurry. Then, the electrode was rinsed by deionized water and ethanol alternately
(each for 1 min), and then dried by ultrapure N2 gas. Firstly, 5 L of Cu2O-GO dispersion were
ϕ = 3 mm) was polished to a form mirror-like surface with
0.05
µ
µ
transferred and coated on the surface of the GCE, and then dried under an infrared lamp to obtain
Cu2O-graphene oxide nanocomposite modified glass carbon electrode (Cu2O-GO/GCE). Then, the GO
component was electrochemically reduced by potentiostatic method. Specially, the Cu2O-GO/GCE
was immersed into 0.1 mol/L PBS solution, and electrochemically reduced at
−
1.2 V for 120 s.
For comparison, GO modified glass carbon electrode (GO/GCE), electrochemically reduced graphene
oxide modified glass carbon electrode (ErGO/GCE) and Cu2O-GO nanocomposite modified glass
carbon electrode (Cu2O-GO/GCE) were also fabricated by a similar method.
3.5. Electrochemical Measurements
All of the electrochemical measurements were carried out using a standard three-electrode
assemble, comprising of bare or modified electrodes as working electrode, saturated calomel electrode
(SCE) as reference electrode, and platinum wire electrode as counter electrode. 0.1 M PBS solution was
used as supporting electrolytes in all electrochemical experiments. The electrochemical performances of
1
×
10−5 mol/L sunset yellow on various modified electrodes were investigated by cyclic voltammetry
(CV) and second-derivative linear sweep voltammetry (SDLSV), with the potential scanning range of
0.4 ~1.2 V. Prior to all electrochemical measurements, an accumulation was performed under stirring
at 500 rpm to improve the sensitivity. After 5 s rest, the CV or SDLSV were recorded at a scan rate
of 100 mV/s, except where stated otherwise. All of the electrochemical tests were carried on the
electrochemical workstation (CHI 760E, Shanghai Chenhua Inc., Shanghai, China).

Molecules 2018, 23, 2130
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3.6. Analysis of Real Samples
Carbonated drinks, orange juice, and candy samples were purchased from a local supermarket.
The same amount of candies was taken out from five packages and carefully grounded into fine
powder. Then, the candies powder (about 1.0 g) was accurately weighed and dispersed in 10 mL
deionized water under sonication for 1 h. Subsequently, the mixture was centrifuged at 4000 rpm for
10 min to remove insoluble substances. The 0.5 mL as-obtained supernatant was diluted to 10 mL with
0.1 mol/L PBS solution. The liquid samples (carbonated drinks and orange juice) were added into
beaker and ultrasonicated for degasification. Then, 1.0 mL of the liquid samples was diluted to 10 mL
with 0.1 mol/L PBS solution. Prior to electrochemical detection, accumulation step was performed
in the sample solution to enhance the electrochemical response. Then, these sample solutions were
detected by second-derivative linear sweep voltammetry while using our proposed Cu2O-ErGO/GCE.
After each test, Cu2O-ErGO/GCE was scanned by cyclic voltammetry a 0.1 mol/L blank PBS solution
for serval cycles to remove any adsorbents. The modified electrode can reused only when the response
peak disappeared in the blank PBS solution.
4. Conclusions
In this study, a promising electrochemical sensor based on the Cu2O-ErGO/GCE was developed
toward sensing sunset yellow. The Cu2O-ErGO nanocomposites not only possess the advantages from
individual component materials, but also exhibit obvious synergistic enhancement effects toward
sunset yellow. The anodic peak current of sunset yellow on the Cu2O-ErGO/GCE increases by about
25 times as compared to that on the bare GCE. The proposed Cu2O-ErGO/GCE exhibits two linear
regions, namely 2.0
limit of detection is 6.0
×
10−8 mol/L–2.0
×
10−5 mol/L and 2.0
×
10−5 mol/L–1.0
×
10−4 mol/L, and the
−9
×
10 mol/L (S/N = 3). The sensing performances in terms of linear response
ranges and detection limit are comparable to, and even exceed the most reported modified electrodes,
such as precious metal-based modified electrodes. Obviously, the Cu2O-ErGO/GCE have outstanding
advantages over precious metal-based modified electrodes in term of the cost. Moreover, the response
current is basically not affected by potential interfering compounds, suggesting the Cu2O-ErGO shows
good selectivity. Besides, the good reproducibility was also obtained on the Cu2O-ErGO/GCE. Finally,
Cu2O-ErGO/GCE have been successfully used for the quantitative detection of sunset yellow in real
samples (i.e., carbonated beverage, orange juice, and candies) while using second-derivative linear
sweep voltammetry. The satisfactory results are obtained with recovery rate is 98.75 ~102.5% and RSD
is less than 2.85%. When compared with conventional analytical techniques (Table 4), the proposed
method does not require expensive equipment, and time-consuming and complicated pretreatment
procedures. Considering the considerable merits including low cost, rapid response, high sensitivity,
as well as good selectivity and good reproducibility, the Cu2O-ErGO/GCE will have broad application
prospects in the detection of sunset yellow in diverse beverages, foods, and nutrients.
Table 4. Advantages and drawbacks of previous reported protocols and our proposed protocol.
Protocols
Advantages
Drawbacks
Reliable; good repeatability;
high sensitivity; low LOD
Limited separation ability; Time-consuming;
expensive equipment
Liquid chromatography
Organic solvents are often used; they have
intensive disagreeable smell and
cancerogenic activity
Thin layer chromatography
Spectrophotometry
Low cost apparatus
Low sensitivity; extraction separation is
needed for detection of dyes in complex
product composition
Simultaneous identification and
quantification; simple technique
High column efficiency, short analysis time Limited sensitivity & selectivity; severe
Capillary electrophoresis
and minimal amounts of samples
matrix interferences
Low cost, rapid response, facile operate,
high sensitivity and good selectivity
Portability needs to be improved;
not disposable
Electrochemical analysis (This work)

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Author Contributions: Q.H., G.L. and D.C. conceived and designed the experiments; J.L., X.L., and Y.X. performed
the experiments; G.L., P.D., and X.L. analyzed the data; Q.H., J.L. and G.L. wrote the manuscript; Q.H. and D.C.
contributed reagents and materials; All authors read and approved the final manuscript.
Funding: This research was funded by the NSFC (61703152), Hunan Provincial Natural Science Foundation
(2016JJ4010, 2018JJ3134), Project of Science and Technology Department of Hunan Province (GD16K02), Project of
Science and Technology Plan in Zhuzhou (201706-201806), Opening Project of Key Discipline of Materials Science
in Guangdong (CNXY2017001, CNXY2017002 and CNXY2017003) and the key Project of Department of Education
of Guangdong Province (2016GCZX008).
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are not available from the authors.
©
2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).