NiCo-embedded in hierarchically structured N-doped carbon nanoplates for the efficient electrochemical determination of ascorbic acid, dopamine, and uric acid

Article abstract of DOI:10.1039/c5ra10937j

The development of a highly stable and efficient catalyst for sluggish electrooxidation in the electro-determination for ascorbic acid (AA), dopamine (DA) and uric acid (UA) is extremely important for the long-term operation and commercialization of a biosensor device, but it remains a challenge. Herein, we demonstrated an interesting structure of NiCo alloy nanocrystals embedded in hierarchically structured N-doped carbon nanoplates (NiCo-NPs-in-N/C), which is facilely synthesized via a one-step in situ reduction pyrolysis strategy. The two-dimensional N-doped porous carbon shells not only offered the effective confinement effect of NiCo nanocrystals avoiding detachment, dissolution, migration, and aggregation during catalysis process, but also allowed a fast transport pathway for the access of electrolyte to the NiCo surface. As a result, such an intriguing structure shows superior catalytic activity towards the electrooxidation of AA, DA, and UA. The well-separated voltammetric peaks between AA-DA, DA-UA, and AA-UA at the NiCo-NPs-in-N/C are up to 178, 122, and 300 mV, respectively, which is much better than graphene@N-doped carbon core@shell nanoplate (graphene@N/C) and NiCo alloy. Furthermore, the NiCo-NPs-in-N/C also exhibits good reproducibility and stability. The attractive features of NiCo-NPs-in-N/C make it a promising electrocatalyst for the simultaneous determination of AA, DA, and UA.

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ARTICLE
NiCo-Embedded in Hierarchically Structured N-Doped Carbon
Nanoplates for Efficiently Electrochemical Determination of
Ascorbic acid, Dopamine, and Uric Acid
Received 00th January 20xx,
Accepted 00th January 20xx
Xianglan Zhang,a Wenfu Yan,b Jianan Zhang,*,a,c Yuanyuan Li,a Wanyu Tanga, Qun Xu*,a
DOI: 10.1039/x0xx00000x
The development of highly stable and efficient catalyst for sluggish electrooxidation in electro-determination for ascorbic
acid (AA), dopamine (DA) and uric acid (UA) is extremely important for the long-term operation and the commercialization
of biosensor devise but still challenging. Herein, we demonstrated an interesting structure of NiCo alloy nanocrystals
embedded hierarchically structured N-doped carbon nanoplates (NiCo-NPs-in-N/C), which is facilely synthesized via a one
step in-situ reduction pyrolysis strategy. The two-dimensional N-doped porous carbon shells not only offer the effectively
confinement effect of NiCo nanocrystals avoiding from detachment, dissolution, migaration, and aggregation during
catalysis process, but also allowed the fast transport pathway for the access of electrolyte to the NiCo surface. As a result,
such intriguing structure shows superior catalytic activity towards the electrooxidation of AA, DA, and UA, the well-
separated voltammetric peaks between AA-DA, DA-UA, and AA-UA at the NiCo-NPs-in-N/C are up to 178, 122, and 300
mV, respectively, even to graphene@N-doped carbon core@shell nanoplate (graphene@N/C) and NiCo alloy.
Furthermore, the NiCo-NPs-in-N/C also exhibits good reproducibility and stability. The attractive features of NiCo-NPs-in-
N/C made it promising electrocatalyst for the simultaneous determination of AA, DA and UA.
www.rsc.org/
1. Introduction
graphene foams,18 Au@carbon dots-chitosan composite film,19
size-selected Pt@graphene/ nanocomposites.20 A challenging
issue for EC determination is that in the corrosive reaction
The precise dopamine (DA) determination is of great clinical
importance, because it is crucial catecholamine
conditions, metallic catalysts tend to show very limited
stability and activity owing to their aggregation and the
relatively weak interaction between the metallic NPs and the
supports. That means the simply combination of metallic NPs
and active substrates is still not satisfying to solve the
significant issues of EC determination.
a
neurotransmitter in the mammalian central nervous system
and low levels of it will cause heart disease, Parkinson's1 and a
variety of neurological diseases.2 Electrochemical (EC) method
was demonstrated to be a popular technology in DA
determination due to its fast response, simplicity, low cost,
and high sensitivity.3-5 However, a major problem facing the EC
detection for DA is the similar electrooxidation peak potentials
for ascorbic acid (AA), DA, and uric acid (UA) (coexist in body
fluids), resulting in poor selectivity and reproducibility.5
Therefore, the development of robust and durable catalysts
for the simultaneous separating their signal potentials with
improved activity and stability is highly desirable.6-17 Metallic
(such as Au, Pt, Ni, Co) nanoparticles (NPs) supported on active
substrates, representing an important class of catalysts for
different fields, especially for EC determination of AA, DA, and
UA, have emerged recently and have attracted intensive
investigation.2,6 Prominent examples include CuO@3D
Recent studies revealed that metallic-based NPs (such as Co,
Fe) confined inside the carbon architectures (such as carbon
nanotubes and so on) can exhibit enhanced catalytic activity
towards electrochemical reactions (such as Oxygen Reduction
Reaction, Hydrogen Evolution Reaction, and so on) due to their
high resistance against oxidation, improved electron and
charges transport, and excellent mechanical strength.21-23 In
this regard, the remarkable characteristics of metallic-
embedded carbon nanostructure motivate us whether we can
rationally design a new class of metal/C-based nanomaterials
for more robust and practical catalysts towards EC
determination of AA, DA, and UA.
On the other hand, many of the single-metal-catalysts holds
the drawbacks of unsatisfactory sensitivity, poor selectivity
and easy poisoning, which are critical issues for practical
applications in biosensors. Recently, Zhang et al. designed an
electrochemical biosensor for ascorbic acid based on carbon-
supported PdNi nanoparticles and good analytical results were
obtained.24 The bimetallic systems can bring interesting
physical and chemical properties into effect from the
a.College of Materials Science and Engineering, Zhengzhou University, Zhengzhou
450052 P. R. China.
b.State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of
Chemistry, Jilin University, Changchun 130012 P. R. China.
c.Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education),
Nankai University, Tianjin 300071, People’s Republic of China.
†Electronic Supplementary Information (ESI) available. See
DOI: 10.1039/x0xx00000x
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intermetallic combinations of different metals.25,26 Inspired by structure shows high electrocatalytic activity toward the
the above research, the bimetallic systematic catalysts are electrochemical oxidation of AA, DA, and UA, which benefits
expected to give a high performance for EC determination of from the synergistic effect of NiCo alloy NPs, N-doped
AA, DA, and UA. One of the most promising materials based on functionalities, and the hierarchical structures. As
a
bimetal is the NiCo alloy, which have been widely used for consequence, the well-separated voltammetric peaks between
catalysis,26 microwave absorption,27 etc.28 with enhanced AA-DA, DA-UA and AA-UA at the NiCo-NPs-in-N/C are up to
performance, due to their synergistic effect for catalytic 178, 122, 300 mV, respectively. For simultaneous sensing of
property between the two metals and active oxidation. three analytes, the linear response range for AA, DA and UA
However, the research concerning the NiCo alloy catalysts for are 50-1500, 0.5-900 and 10-500 μM, respectively, and the
EC determination of AA, DA, and UA has rarely reported.
detection limits (S/N=3) are 0.091, 0.080 and 0.014 μM,
Herein, we demonstrated one-step nanospace pyrolysis respectively. Furthermore, the NiCo-NPs-in-N/C also exhibits
strategy (Scheme 1) for the in situ synthesis of NiCo alloy NPs good reproducibility and stability.
embedded in hierarchical structured nitrogen-doped carbon
nanoplates (NiCo-NPs-in-N/C). With well-defined morphology, 2. Experimental
uniform NiCo alloy NPs with particle size of 15-20 nm were
homogenously distributed within the N-doped carbon 2.1. Materials
nanoplate. By changing the annealing temperature, the
formation process of NiCo alloy NPs in the N-doped carbon Dopamine Hydrochloride (DA, alladin), Uric acid (UA, sigma), L-
nanoplates was investigated. Such unique structure shows Ascorbic acid (AA, tianjin chemical reagent co., LTD ), N, N-
high electrocatalytic activity toward the electrochemical dimethylformamide (DMF, Guangdong guanghua science and
oxidation of AA, DA, and UA, which benefits from the technology co., LTD ), Nickel (
synergistic effect of NiCo alloy NPs, N-doped functionalities, 6H2O, Xilong chemical co., LTD), Cobalt(
and the hierarchical structures. As a consequence, the well- hexahydrate (Co(NO3)2 6H2O, Tianjin hebei sea crystal fine
Ⅱ) nitrate hexahydrate (Ni(NO3)2
•
Ⅱ
) nitrate
•
separated voltammetric peaks between AA-DA, DA-UA and chemical plant), Hexamethylenetetramine (HMTA, Afar Sally
AA-UA at the NiCo-NPs-in-N/C are up to 178, 122, 300 mV, (tianjin) chemical co., LTD), perchloric acid (HClO4, Tianjin
respectively. For simultaneous sensing of three analytes, the xinyuan chemical co., LTD), Aniline (ANI, Tianjin damao
linear response range for AA, DA and UA are 500-1500, 0.5-900 Chemical Reagent Factory), ammonium persulfate (APS, Sigma
and 10-300 μM, respectively, and the detection limits (S/N=3) Aldrich), and ethanol (CH3CH2OH, Its group chemical reagent
are 0.091, 0.080 and 0.014 μM, respectively. Furthermore, the co., LTD) are used as received without further purification.
NiCo-NPs-in-N/C also exhibits good reproducibility and
stability.
2.2. The synthesis of the NiCo-NPs-in-N/C hybrid structure
NiCo2O4 sheets were prepared according our previous
method.29,30 NiCo2O4@PANI core@shell nanosheets were
prepared by one step polymerization of polyaniline (PANI) on
the surface of NiCo2O4 (Scheme 1). Typically, 3 mg of NiCo2O4
nanosheets was dispersed into 7 mL 1 M HClO4 aqueous
solution by ultrasonication. Then, 0.3 mL of ethanol and 9 μL
of 10 mM ANI were injected into the above mixture at 0 °C to
form a uniform solution. After 30 min, 3 mL of 1 M HClO4
solution containing APS (the molar ratio of ANI/APS was 1:1)
was dropwise added into the mixed solution, and further
reacted at 0 °C for 7 h. The obtained product was collected and
washed with deionized water and ethanol for several times,
followed by vacuum-drying at 60 °C. The NiCo-NPs-in-N/C
hybrid structure were obtained by carbonization of the as-
prepared NiCo2O4@PANI core@shell nanosheets at 750 °C for
2 h with a heating rate of 1 °C min-1 under N2 atmosphere. For
Scheme 1. Schematic illustration of the synthetic protocol of
NiCo-NPs-in-N/C.
comparison,
graphene@N-doped
carbon
core@shell
Herein, we demonstrated one step nanospace pyrolysis
strategy (Scheme 1) for the in-situ synthesis of NiCo NPs
emebed N-doped carbon nanoplate (NiCo-NPs-in-N/C). Well-
defined morphology, uniform NiCo alloy NPs with particle size
of 15-20 nm were homogenously distributed within the N-
doped carbon nanoplate. By changing the annealing
temperature, the formation process of NiCo alloy NPs in the N-
doped carbon nanoplates was investigated. Such unique
nanoplante (graphene@N/C) was prepared via the similar
synthetic protocol but using graphene instead of NiCo2O4
sheets and carbonized at 750 °C and NiCo alloy was also
prepared by annealing NiCo2O4 nanosheets at 750 °C under
reducing atmosphere..
2.3. Preparation of NiCo-NPs-in-N/C modified electrodes
2
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Glass carbon (GC) electrodes (diameter is 3 mm) were carefully sheet shows the average size of 2.5 μm in length and 1.5 μm in
polished to a mirror-like plane with 0.3 μm, 0.1 μm and 0.05 width with mesoporous structure formed by numerous
μm Al2O3 slurries, successively. Then the electrode were NiCo2O4 nanocrystals, which is in accordance with that
washed thoroughly with excess amounts of water and dried observed in the reported literature.29 All of the diffraction
under N2 gas. The active materials modified GC electrode was peaks of the NiCo2O4 sheets in the XRD pattern (Figure S1C)
prepared by casting 4 μL active materials suspension (1 mg mL- can be indexed to the cubic spinel NiCo2O4 phase (JCPDS card
1
NiCo-NPs-in-N/C in N, N-dimethylformamide) on the surface No. 20-0781). Then, NiCo2O4 sheets were employed as a planar
of GC electrode and dried at room temperature. For real support to grow PANI by one-pot in situ polymerization of ANI
sample analysis, no pretreatment process was performed. in the presence of APS as the oxidant (the product was noted
Human urine was diluted 100 times with 0.1 M PBS (PH=7.0) to as NiCo2O4@PANI core-shell nanoplates). As shown in Figure 1,
fit the calibration curve and reduce the matrix effect.
the PANI was uniformly coated on the whole NiCo2O4
nanosheets with the ordered PANI nanorods ensembles
assembled on the surfaces of the nanosheets, forming a 2D
sandwich structure of PANI- NiCo2O4-PANI.
2.4. Characterizations
The morphology and structure of the samples were studied by
a
field-emission scanning electron microscope FE-SEM
A)
B)
(JEORJSM-6700F) and a transmission electron microscope TEM
(FEI Tecnai G2 20) with an accelerating voltage of 200 kV. The
X-ray photoelectron spectroscopy (XPS) experiments were
performed with an ESCA LAB 250 spectrometer using a
focused monochromatic Al Kα (hν = 1486.6 eV) X-ray beam
with a diameter of 200 μm. The X-ray diffraction (XRD) data
were performed on a Y-2000 X-ray Diffractometer using
copper Kα radiation (λ =1.5406 Å) at 40 kV, 40 mA. All
electrochemical measurements were performed on a CHI660D
electrochemical workstation (Chenhua Instrument Company of
Shanghai, China). A conventional three-electrode system was
used, including a bare or modified GC electrode as working
electrode, a platinum wire as an auxiliary electrode, an
Ag/AgCl (saturated KCl) as the reference electrode.
100 nm
500 nm
Ni
C)
D)
Co
(111)
d=0.204 nm
100 nm
5 nm
5 nm
Co 2p
Ni 2p
2p3/2
sat.
2p3/2
2p1/2
E)
2p1/2
sat.
sat.
sat.
770
780
790
800
810
850
860
870 880 890
Binding energy/eV
Binding energy/eV
C 1s
O 1s
Co 2p
Ni 2p
N 1s
0
200
400
600
800
1000
Binding energy/eV
Figure 2. (A) SEM, (B, C) TEM and (D) HRTEM images of NiCo-
NPs-in-N/C. (the inset of D shows the corresponding FFT image
of NiCo crystal with scale bar being 5 nm-1). (E) XPS spectra of
the hybrid structure. Insets of E are the XPS spectra of Co2p
and Ni 2p regions of NiCo-NPs-in-N/C, respectively.
Figure 1. SEM images of NiCo2O4@PANI core@shell
nanoplates.
3. Results and discussion
The as-prepared NiCo2O4@PANI core-shell nanoplates
underwent the carbonization at high temperature of 750 °C in
N2 atmosphere for 2 h at a heating rate of 1 °C min-1, allowing
the partly “escape” of the core from the carbon shell, resulting
the well-distributed NiCo alloy NPs embedded in N-doped
carbon nanoplates. Figure 2&S2 shows the SEM and TEM
images of NiCo-NPs-in-N/C nanostructures. The SEM image
(Figure 2A&S2A) shows that the hybrid structures maintain 2D
planar morphology with N-doped carbon nanorods ensembles
3.1. Characterization of NiCo-NPs-in-N/C
Two dimensional (2D) NiCo-NPs-in-N/C hybrid nanoplates
were synthesized through a one step in situ nanospace
confined pyrolysis of as-prepared NiCo2O4@PANI core@shell
nanosheets (see the details in experimental section). The
whole experimental flowchart is schematically illustrated in
Scheme 1. Firstly, NiCo2O4 sheets were prepared by a
hydrothermal method.29 As seen in Figure S1A&B, NiCo2O4
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on their surface. It is interestingly observed from TEM images completely reduced to NiCo alloy by carbon at 550 °C and the
(Figure 2B, C, and S2B) that numerous NPs with a size of 15-20 nanoscaled metallic NiCo fused and flow out from the porous
nm are confined in the hierarchical carbon planar structures. carbon shell.37 As seen from the XRD, the samples completely
As a further confirmation, the large magnified TEM images in transformed to NiCo alloy and the peak intensity becomes
Figure S3, reveal the light carbon sheath with numerous stronger. With the increase of annealing temperature to 750
micropores and a thickness of 3-5 nm around the dark central °C (Figure S4C), uniform the NiCo alloy NPs generally became
nanocrystal, indicating the NiCo NPs embedded in the carbon homogenous and smaller with the size of 15-20 nm. The
layer. Figure 2D shows the well-identified inter planar spacing corresponding XRD result of NiCo-NPs-in-N/C at 750 oC
of 0.204 and 0.203 nm for the distance of the (111) and (111) confirms that NiCo alloy NPs are reduced by carbon with
plane of face centered cubic (fcc) NiCo crystals, respectively.31- increases to the carbonization temperature. While the NPs
33
The corresponding fast Fourier transformation (FFT) image agglomerated into large bulks at 950 °C (Figure S4D).
(inset of Figure 2D) further confirms the fcc structure of NiCo
crystal.
To reveal the effect of nitrogen functionalities on capacitive
performance, the types of N group introduced to NiCo-NPs-in-
N/C were further studied by XPS (Figure S5). The
deconvoluting N1s region spectrum could provide three-type
peaks correlated to different electronic states of nitrogen
functional groups: pyridinic (N-6, 398.6 eV), pyrrolic (N-5,
400.0 eV), and quaternary nitrogen (N-Q, 401.1 eV).38 As
shown in Figure S5, the NiCo-NPs-in-N/C nanostructure from
different temperature exists different electronic states of
nitrogen functional groups, which indicates the influence of
temperature on the component of materials. There only N-Q
and N-6 are dominated in the structure of NiCo-NPs-in-N/C
from 750 °C, while no N-5 is observed. This could be due to the
higher stability of N-Q and N-6 than N-5. These accessible
species of N-6 and N-Q may play the important role to improve
the electrocatalytic activity of the NiCo-NPs-in-N/C
nanostructure.5,39
Co
Ni
JCPDS No. 15ꢀ0806
JCPDS No. 04ꢀ0850
350
550
750
950
NiCo2O4
JCPDS No. 20ꢀ0781
20
30
40
50
60
70
80
2 Theta/degree
Figure 3. XRD patterns of NiCo2O4@PANI core-shell nanoplates
carbonization at different temperature: 350, 550, 750, and 950
oC; and the standard XRD patterns of NiCo2O4, Ni, and Co.
Based on the TEM, XRD, and XPS data of the NiCo-NPs-in-
N/C at different annealing temperature, the significant
o
optimization of the nanostructure is obtained at 750 C. The
NiCo NPs with the uniform size are completely reduced by the
carbon and confined in the hierarchical carbon shells. Such
interesting nanostructure is expected to show highest
electrocatalytic activity than other samples.
The elemental composition and chemical state of NiCo-NPs-
in-N/C are characterized by X-ray photoelectron spectroscopy
spectrum (XPS) (Figure 2E). By using a Gaussian fitting method,
the Co 2p spectrum (Fig. 2b) can be well fitted with two spin-
orbit doublets, characteristic of Co2+ and Co3+, and two shake-
up satellites (identified as “Sat.”).22, 23 Similar case happens on
the Ni 2p spectrum fitting into two spin-orbit doublets,
characteristic of Ni2+ and Ni3+, and two shake-up satellites. 34-36
The Ni/Co molar ratio obtained from the spectrum was found
to be close to the initial 1:2 Ni:Co molar ratio. About 2.23%
nitrogen is introduced into the NiCo-NPs-in-N/C nanostructure.
In order to verify the effectiveness of NiCo NPs as a new
catalysis toward EC determination of AA, DA, and UA, we also
prepared a 2D N-doped carbon coated graphene oxide
(graphene@N/C) nanoplates through the similar synthetic
protocol to NiCo-NPs-in-N/C by using graphene oxide instead
of NiCo2O4 nanosheets and carbonized at 750 °C. NiCo alloy
was also prepared based on the previous literature.40 The TEM
and SEM images of graphene@N/C (Figure S6) display that a
great number of small humps are uniformly coated on the
surface of graphene, indicating that the similar nanostructure
of graphene@N/C to NiCo-NPs-in-N/C.
The temperature-dependent structure change of NiCo-NPs-
in-N/C nanostructures under high temperature annealing
condition in N2 was used to study the formation mechanism of
NiCo-NPs-in-N/C (Figure S4). When the NiCo2O4@PANI was
carbonized at 350 °C, the NiCo2O4 sheets broke into smaller
NPs and were simultaneously reduced by carbon precursor
and partially converted to metallic NiCo phase (Figure S4A). It
is noteworthy that the cubic spinel NiCo2O4 phase in the
parent nanosheets partially converted into face centered cubic
NiCo alloy (JCPDS cards No. 15-0806 for Co and No. 04-0850
for Ni) at 350 °C (Figure 3). When the annealing temperature
went up to 550 °C, NiCo-NPs-in-N/C nanostructure were
obtained (Figure S4B). This is because that the NiCo2O4 were
3.2. Electrochemical properties
3-/4-
We used Fe(CN)6
as the electrochemical probe to evaluate
the electrochemical properties of NiCo-NPs-in-N/C,
graphene@N/C and GC electrodes (Figure 4). As we all know,
the peak potential separation (∆Ep) is the function of the
electron transfer rate and the lower ∆Ep, the higher electron
transfer rate. From the cyclic voltammetry (CV) curves of the
3-/4-
samples collected in 5 mM Fe(CN)6
(1:1) + 0.1 M KCl
solution (Figure 4A), the ∆Ep at the NiCo-NPs-in-N/C electrode
4
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is 86.2 mV, which is the lowest ∆Ep. Furthermore, the redox samples. No peak is observed at the electrodes, which show
peak current at the NiCo-NPs-in-N/C electrode is higher than irreversible electrode processes for AA electro-oxidation.44 For
the value at graphene@N/C and GC electrodes due to the DA (Figure 5B), it can be found a couple of redox peaks at
good electronic conductivity. The smaller value of ∆Ep and the every electrode, which corresponds to two-electron oxidation
higher redox peak currents indicate that the NiCo-NPs-in-N/C of DA to dopamine quinone and the subsequent reduction of
electrode exhibit the better electrochemical properties than dopamine quinone to DA. (Scheme 2.) However, a couple of
the graphene@N/C and GC electrodes.
the reversible and well-defined redox peaks with a lower ∆Ep
of 55 mV can be observed at the NiCo-NPs-in-N/C electrode,
indicating better reversibility for the redox process, while the
NiCo-NPs-in-N/C electrode has a much stronger response to
the oxidation of DA compared to graphene@N/C, NiCo alloy
and bare GC electrodes. Figure 5C shows the oxidation
behavior of UA, the CV curves reveal that UA is first oxidized to
quinonoid, and then undergoes a rapid chemical reaction,
which is considered as an EC mechanism (Scheme 2).45 NiCo-
NPs-in-N/C electrode exhibit most negative oxidation peak
potential and a significant enhancement in the oxidation
current when compared to that of graphene@N/C, NiCo alloy
and bare GC electrodes. These results reveal a fact that the
NiCo-NPs-in-N/C possesses the highest activity to
graphene@N/C, NiCo alloy and bare GC.
A)
B)400
bare GCE
NiCoꢀNPsꢀinꢀN/C
graphene@N/C
bare GCE
NiCoꢀNPsꢀinꢀN/C
graphene@N/C
80
40
300
200
100
0
0
C
ꢀ40
ꢀ80
Rs
Rct
Zw
ꢀ0.2 0.0 0.2 0.4 0.6
E/V (vs. Ag/AgCl)
0
200
400
Z'/ohm
Figure 4. A) CVs and B) EIS curves obtained for 5 mM Fe(CN)6
600
800
3-
/4- (1:1) + 0.1 M KCl at bare GC electrode, NiCo-NPs-in-N/C and
graphene@N/C electrodes. Scan rate for CVs: 50 mV s-1.
The electron transfer kinetics at different electrodes is also
investigated and the corresponding results are shown in Figure
4B. The distinct semicircular curve at the higher frequency
region and linear portion in the low frequency region
correspond to the electron transfer (Rct) and diffusion limited
process of the electrochemical reaction, respectively.41 Rs is
the electrolyte resistance between the working and reference
electrodes, while C is a constant phase element and Zw is
Warburgh impedance.42,43 The impedance diagram was fitted
based on the electrical equivalent circulation the inset of
Figure 4B. The Rct value at NiCo-NPs-in-N/C electrode is 49
ohm, which is smaller than graphene@N/C electrode (65 ohm)
and GC electrode (345 ohm). The result is in accordance with
that observed in Figure 4A and reconfirms that NiCo-NPs-in-
N/C possesses good conductivity and electrochemical
properties. All these results imply that the NiCo-NPs-in-N/C is
ꢀ2eꢀ ꢀH+
AA
ꢀ
ꢀ2eꢀ ꢀ2H+
DA
ꢀ2eꢀ ꢀ2H+
UA
O
N
C
H
expected to be
a
promising material to construct
Scheme 2. Electrochemical oxidation of AA, DA and UA.
electrochemical biosensors.
3.3. Electrocatalytic oxidation of AA, DA and UA
In order to study the electrocatalytic properties and
selectivity of the NiCo-NPs-in-N/C for the simultaneous
determination of UA, AA and DA, the DPV response of NiCo-
NPs-in-N/C, graphene@N/C, NiCo alloy and bare GC electrodes
in the co-existence system of AA, DA and UA is also collected
(Figure 5D). NiCo-NPs-in-N/C electrode exhibit the highest
activity, even than graphene@N/C and NiCo alloy. Typically,
the separated voltammetric peaks between AA-DA, DA-UA,
and AA-UA at the NiCo-NPs-in-N/C are up to 178, 122, and 300
mV, respectively. On contrary, the oxidation peaks of AA and
DA overlap with each other at the graphene@N/C electrode
and NiCo alloy, and the DPV obtained at bare GC electrode
exhibits inconspicuous peaks. Therefore, NiCo-NPs-in-N/C
nanostructure hold superior electrocatalytic activity and
selectivity to graphene@N/C,NiCo alloy and bare GC
electrodes.
In order to reveal the electrochemical performance of NiCo-
NPs-in-N/C nanostructure towards the oxidation of AA, DA and
UA, CV and differential pulse voltammetry (DPV)
measurements based on NiCo-NPs-in-N/C, graphene@N/C,
NiCo alloy and bare GC electrodes were carried out (Figure 5).
In the case of AA (Figure 5A), the oxidation peak corresponds
to the two electrons and one proton oxidation of hydroxy
groups to carbonyl groups in furan ring of AA (Scheme 2). In
detail, the oxidation potential at the NiCo-NPs-in-N/C
electrode is negative than that at the other electrodes, and the
corresponding peak current of the NiCo-NPs-in-N/C electrode
is higher than that of the graphene@N/C, NiCo alloy and bare
GC electrodes, indicating that the electron transfer kinetics of
AA oxidation was quicker at NiCo-NPs-in-N/C than other
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DOI: 10.1039/C5RA10937J
ARTICLE
Based on CVs and DPV data, the excellent electrocatalytic
RSC Advances
of DA concentration from 0.5 to 900 μM. The linear regression
equation is calibrated as IDA (μA) = 0.1313 + 0.0212 CDA (μM)
with the correlation coefficient of R = 0.9945. Similarly, the
peak current of UA increases linearly with the increase of the
UA concentration in the range of 10-500 μM with the linear
function IUA (μA) = 0.13281 + 0.01399 CUA (μM) with the
correlation coefficient of R = 0.9983. The detection limits for
AA, DA and UA are 0.091 μM, 0.080 μM and 0.014 μM,
respectively at S/N = 3. We further compared the catalytic
performance of our NiCo-NPs-in-N/C with various reported
metal-carbon-based catalyst (Table S1). Obviously, NiCo-NPs-
in-N/C exhibit excellent catalytic performance with lowest
detection limit and widest linear range.
properties and perfect selectivity of the NiCo-NPs-in-N/C
electrode is probably attributed to the synergistic effect of
NiCo alloys, nitrogen groups and the hierarchical structures.
And the interactions between NiCo-NPs-in-N/C layers and
these biomolecules can promote the charge transfer of three
molecules. More significantly, the uniform NiCo alloy NPs play
the critical role on the high catalytic activity toward to the
oxidation of AA, DA and UA.
bare GCE
bare GCE
AA
DA
A)
B)60
NiCoꢀNPsꢀinꢀN/C
graphene@N/C
NiCo
NiCoꢀNPsꢀinꢀN/C
graphene@N/C
NiCo
20
10
0
30
0
4
10
A)
AA
3
ꢀ30
ꢀ0.2 0.0
0.2
0.4
0.6
ꢀ0.2 0.0
0.2
0.4 0.6
2
1
0
12
E/V (vs. Ag/AgCl)
E/V (vs. Ag/AgCl)
R=0.9952
600 1200
bare GCE
NiCoꢀNPsꢀinꢀN/C
graphene@N/C
NiCo
bare GCE
NiCoꢀNPsꢀinꢀN/C
graphene@N/C
NiCo
UA
C)30
15
D)
12
0
a
n @
a
@
9
6
CAA/µM
8
4
0
0
ꢀ115
ꢀ0.
2
0
.
0
0.2
0
4
0.
6
ꢀ0
2
0.
0
0
.
2
00.4
ꢀ0.2 0.0 0.2 0.4 0.6 0.8
E/V (vs. Ag/AgCl)
E/V
(vs.
A
/A
g
C
l
)
E/V (vs. Ag//AgCl)
Figure 5. CVs of bare GC electrode, NiCo-NPs-in-N/C, NiCo
alloy, and graphene@N/C electrodes in 0.1 M PBS (PH 7.0)
containing (A) 1.0 mM AA, (B) 1.0 mM DA, (C) 1.0 mM UA, and
(D) 1.0 mM AA + 0.05 mM DA + 0.1 mM UA.
B)
18
DA
12
20
15
10
5
6
R=0.9945
0
0
400 800
CDA
/µM
3.4. Effects of scan rate
To investigate the reaction kinetics, the influence of scan rate
on the CV responses of AA, DA and UA at the NiCo-NPs-in-N/C
electrode was investigated. As shown in Figure S7, the
oxidation peak currents of AA, DA and UA as a function of the
square root of scan rate in the range of 5-500 mV/s, which
indicates the electrochemical oxidation are diffusion-
controlled processes.
ꢀ0.2 0.0 0.2 0.4 0.6 0.8
E/V (vs. Ag/AgCl)
C)18
UA
6
15
3
R=0.9983
0
12
9
0
200 400
CUA
/µM
3.5. Simultaneous determination of AA, DA and UA
6
3
Since DPV technique has much higher current sensitivity and
better resolution compared to CV technique, simultaneous
detection of UA, AA and DA was carried out using DPV method.
As shown in Figure 6, series of DPV curves are obtained by
changing the concentration of one biomolecule while keeping
the other two biomolecules at a constant value. Three well-
defined oxidation peaks were observed and their currents
increased while the oxidation potential remained steady. The
electrochemical response of AA increases linearly with the
increase of its concentration in the range of 50-1500 μM, and
the corresponding linear function is IAA (μA) = 0.13075 +
0.00247 CAA (μM) with the correlation coefficient of R = 0.9952.
The oxidation peak current of DA increases with the increase
ꢀ0.2 0.0 0.2 0.4 0.6 0.8
E/V (vs. Ag/AgCl)
Figure 6. DPVs at the NiCo-NPs-in-N/C electrode in 0.1 M PBS
(PH 7.0) (A) containing 80 μM DA, 160 μM UA and different
concentrations of AA from 0 to 1500 μM; (B) containing 1000
μM AA, 160 μM UA and different concentrations of DA from 0
to 900 μM; (C) containing 1000 μM AA, 80 μM DA and
different concentrations of UA from 0 to 500 μM. Insets: plots
of I (peak) vs. concentration for AA, DA and UA, respectively.
6
| RSC Adv., 2012, 00, 1-3
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3.6. Stability and reproducibility
In conclusion, an intriguing composite of sandwich-structured
NiCo nanoparticles-N-doped carbon nanoplate was facilely
Meanwhile, the stability and reproducibility of NiCo-NPs-in- synthesized a one step in situ nanospace confined pyrolysis
N/C nanosheets electrode was also investigated in the mixture strategy, and exhibits great potential for the simultaneous
solution containing 1.0 mM AA, 0.05 mM DA and 0.1 mM UA. determination of AA, DA and UA. In such a structure, the
Five electrodes modified by NiCo-NPs-in-N/C were used to homogeneous NiCo NPs (15-20 nm) are embedded in the
measure the three target molecules with DPV method, the compartment of carbon nanoplates. Initial study on the
current response remained almost constant and the relative formation mechanism of NiCo-NPs-in-N/C was carried out
standard deviations (R. S. D.) were 3.5%, 2.9%, 3.7% for AA, DA through changing the annealing temperature. The control
and UA, respectively. The current signals of the electrode also experiments reveal that the proper annealing temperature is
showed no obvious decreases (4.6%, 3.9%, and 4.5% for 1.0 the key to the in situ formation of well-defined NiCo-NPs-in-
mM AA, 0.05 mM DA and 0.1 mM UA, respectively) relative to N/C nanoplates. Significantly, the electrochemical features
the initial response after storage in air at room temperature could be efficiently regulated by the confinement effect of
for a month. It can be speculated that the carbon shell around NiCo NPs and the hierarchical structure of N-doping carbon for
the NiCo alloy NPs in NiCo-NPs-in-N/C acts as an effective NiCo-NPs-in-N/C. As a consequence, the advanced overall
protection for the NiCo NPs from dissolution in the corrosive performance has been optimized for NiCo-NPs-in-N/C, which
conditions, and thus leading to the excellent stability and exhibits
excellent
electrooxidation of AA, DA, and UA. As a consequence, the
NiCo-NPs-in-N/C nanoplates shows well-separated
catalytic
activity
towards
the
reproducibility.
3.7. Analysis of real samples
voltammetric peaks between AA and DA, and DA and UA at the
NiCo-NPs-in-N/C are up to 178 and 122 mV, respectively. For
To illustrate the feasibility of NiCo-NPs-in-N/C electrode for simultaneous sensing of three analytes, the NiCo-NPs-in-N/C
simultaneous determination of AA, DA and UA in biological nanoplates exhibit widest linear response range for AA, DA and
fluids, the electrode was applied to select the three molecules UA are 500-1500, 0.5-900 and 10-500 μM with lowest
in the human urine. The human urine sample was diluted 100 detection limits of 0.091, 0.080, and 0.014 μM, respectively,
times with 0.1 M PBS before the measurement and certain which is even better than ever reported relevant catalysts.
amounts of AA, DA and UA were added into the diluted sample. Additionally, the NiCo-NPs-in-N/C also exhibits good
The spike recoveries are 98.7%, 102.3%, 101.6% for 1.0 mM AA, reproducibility and stability. Therefore, we expect that the
0.05 mM DA and 0.1 mM UA (Table S2), respectively, implying new conception of confining metal NPs in the carbon
the NiCo-NPs-in-N/C has great potential for simultaneous architecture can be extended to produce various inorganic
determination of AA, DA and UA in real samples.
NPs–carbon based materials for catalytic and other
The results from the electrocatalytic studies toward applications.
simultaneous determination of AA, DA and UA indicate that
the confinement effect of NiCo NPs and the hierarchical
structure of N-doping carbon have the important roles in
Acknowledgements
optimizing the electrocatalytic properties of NiCo-NPs-in-N/C.
We found that such particular NiCo-NPs-in-N/C plate
architectures can well address several important challenging
issues related to electrocatalyst: (a) The synergistic effect of
bimetallic system encourages the electrocatalytic property of
the NiCo NPs in NiCo-NPs-in-N/C. (b) N-doped carbon shell can
act as an armour/or buffer for protecting the NiCo NPs from
detachment, dissolution, migration, and aggregation after
long-term use. (c) The 2D hierarchical structure coupled with
N-doped functionalities with high electron conductivity
provides perfect pathways for electrons and mass transport,
thus accelerating the charge transform kinetics of the target
molecules at the electrode.46,47 Accordingly, it is shown that
the catalytic performance of our NiCo-NPs-in-N/C is superior to
most reported metal-carbon-based catalysts (Table S1). Taking
their robust electrocatalytic performance into account we
conclude that rationally design of the architectures of the
metal-carbon-based catalytic can significantly realize the
optimization of the electrocatalytic performance for biosensor.
This work was financially supported by the National Natural
Science Foundation of China (Nos. 21101141 and 51173170),
Program for New Century Excellent Talents in Universities
(NCET), and the Open Project Foundation of Key Laboratory of
Advanced Energy Materials Chemistry of Nankai University
(2015-32).
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