Ultrathin nickel-metal-organic framework nanobelt based electrochemical sensor for the determination of urea in human body fluids

Article abstract of DOI:10.1039/c9ra05716a

Ultrathin nickel-metal-organic framework (Ni-MOF) nanobelts, [Ni₂₀(C₅H₆O₄)₂₀(H₂O)₈]·40H₂O (Ni-MIL-77), have been exploited successfully for the fabrication of a non-enzymatic urea sensor. Ni-MOF ultrathin nanobelts in alkaline media can be used as an efficient catalyst for urea electrooxidation. As a non-enzymatic urea sensor, Ni-MOF ultrathin nanobelts exhibit a high sensitivity of 118.77 μA mM^-1 cm^-2, wide linear range of 0.01-7.0 mM, and low detection limit of 2.23 μM (S/N = 3). The selectivity, stability and reliability of ultrathin Ni-MOF nanobelts towards urea oxidation are also investigated. Moreover, Ni-MOF ultrathin nanobelts were further used to detect urea in human body fluids. All these findings confirm that the urea sensor based on Ni-MOF ultrathin nanobelts is successfully prepared and promising for applications in medical diagnostics and environmental monitoring.

Full text of DOI:10.1039/c9ra05716a

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Ultrathin nickel-metal–organic framework
nanobelt based electrochemical sensor for the
Cite this: RSC Adv., 2019, 9, 29474
determination of urea in human body fluids†
b
a
Cancan Bao,a Qiangqiang Niu,a Zi-Ang Chen,a Xiaowei Cao,
Hui Wang
a
*
and Wenbo Lu
Ultrathin nickel-metal–organic framework (Ni-MOF) nanobelts, [Ni20(C5H6O4)20(H2O)8]$40H2O (Ni-MIL-
77), have been exploited successfully for the fabrication of a non-enzymatic urea sensor. Ni-MOF
ultrathin nanobelts in alkaline media can be used as an efficient catalyst for urea electrooxidation. As
a non-enzymatic urea sensor, Ni-MOF ultrathin nanobelts exhibit a high sensitivity of 118.77 mA mMꢀ1
cmꢀ2, wide linear range of 0.01–7.0 mM, and low detection limit of 2.23 mM (S/N ¼ 3). The selectivity,
stability and reliability of ultrathin Ni-MOF nanobelts towards urea oxidation are also investigated.
Moreover, Ni-MOF ultrathin nanobelts were further used to detect urea in human body fluids. All these
findings confirm that the urea sensor based on Ni-MOF ultrathin nanobelts is successfully prepared and
promising for applications in medical diagnostics and environmental monitoring.
Received 24th July 2019
Accepted 4th September 2019
DOI: 10.1039/c9ra05716a
rsc.li/rsc-advances
and three-dimensional networks.13,14 Nickel-based metal–
organic frameworks (Ni-MOFs) have generated a large amount
Introduction
of interest because of their excellent structural and electro-
catalytic properties.13 In particular, Ni-MOF based catalysts have
exhibited excellent catalytic abilities for urea oxidation.15,16
However, few reports are available on the detection of urea by
Ni-MOFs. Recently, Yoon's group synthesized Ni-MOF micro-
particle and multiwalled carbon nanotube composites for urea
detection.17 Ni-MOF microparticles with a diameter of 1.2 mm
are too big to avoid many shortcomings.17 As a result, it is very
necessary to develop a new class of Ni-MOF nanomaterials for
the detection of urea. Ni-MOF nanobelts possess a rectangular
area on each ribbon strip, which exhibit high electrocatalytic
activity because of large surface-to-volume ratio and highly
active surface.17 As far as we know, Ni-MOF nanobelts for the
determination of urea have not yet been reported.
For the rst time, we describe our nding that Ni-MOF
ultrathin nanobelts, [Ni20(C5H6O4)20(H2O)8]$40H2O (Ni-MIL-77),
in alkaline media can be used as an efficient catalyst for urea
electrooxidation, as shown in this article. As a non-enzymatic
urea sensor, Ni-MOF ultrathin nanobelts exhibit superior urea
sensing performances, such as broad detection range, low
detection limit and high selectivity. Moreover, the detection of
urea in human body uids has been further explored for the
sensing performance of Ni-MOFs ultrathin nanobelts.
Urea is one of the primary products of protein degradation and
metabolism of nitrogen containing compounds.1 The amount
of urea present in human body uids (such as blood and urine)
exceeds a certain level, which can cause damage to kidney or
liver of mankind.2 If the kidneys or livers are not working
properly, the concentration of urea will be abnormal in the body
uids.3 Abnormal levels of urea in human body uids cause
kidney or hepatic failure, nephritic syndrome and urinary tract
obstruction.4 Furthermore, urea in industrial waste water is
washed out in the environment, which can pollute the surface
and the ground water.5,6 Accordingly, it is essential to monitor
urea levels in the human body uids, environment, and
drinking water.
Metal–organic frameworks (MOFs) have some advantages,
such as high surface area and tuneable chemical structures.7
Nickel-based nanomaterials have been extensively applied for
the direct electrocatalytic oxidation of biochemical molecules.8,9
The electrocatalytic performance of the nickel-based catalysts
has been further improved by preparing various nano-
structures such as nanoparticles,10 nanorods,11 nanowires,12
aKey Laboratory of Magnetic Molecules and Magnetic Information Materials (Ministry
of Education), School of Chemistry and Material Science, Shanxi Normal University,
Linfen 041004, China. E-mail: luwb@sxnu.edu.cn; Fax: +86-357-2051192; Tel: +86-
357-2051192
Results and discussion
Physical characterization
bInstitute of Translational Medicine, Medical College, Yangzhou University, Yangzhou
225001, China
The morphology and structure of Ni-MOF ultrathin nanobelts
(the Ni-MOF nanobelts are synthesised by a hydrothermal
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra05716a
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method using nickel acetate and glutaric acid as main raw that Ni-MOFs have been successfully synthesized. A series of
materials, and the relevant experimental procedure is shown in visible peaks of Ni 2p region spectra can be viewed in Fig. 1F.
the ESI†) have been explored by scanning electron microscopy Two peaks at ca. 856.32 and 873.65 eV are assigned to Ni 2p1/2
(SEM), transmission electron microscopy (TEM) and other and Ni 2p3/2, respectively.19,20 Moreover, the broad peaks at
characterization methods. The phase purity and crystallo- 861.69 eV and 879.9 eV represent the satellite peaks of Ni 2p1/2
graphic structure of the experimental product was characterized and Ni 2p3/2, respectively.21 The results received from the Ni 2p
by XRD. As shown in Fig. S1† (before and aer electrochemical spectra testied the fact that nickel ions consisted in the
testing), the peak intensity is stronger before electrochemical prepared Ni-MOFs are present in the form of Ni(II). When
testing. However, the crystal plane of XRD did not change aer compared before and aer urea electrooxidation in alkaline
electrochemical testing. Therefore, we can draw the conclusion medium, it can be seen that the valence and peak position of Ni
that the two compounds before and aer testing are the same 2p regions on Ni-MOF nanobelts has not changed substantially
substance, and the structure of Ni-MOF nanobelts is stable.
aer a long-term detection. It again demonstrates that the
Fig. 1A shows the SEM images of Ni-MOF and reveals the structure of Ni-MOF nanobelts is stable. In conclusion, all of the
formation of ultrathin nanobelts. The nanobelt structure can be above characterizations conrm that the Ni-MOF nanobelts
further observed clearly in the inset of Fig. 1A. In addition, the have been prepared successfully.
SEM of Ni-MOF is also characterized aer a long-term detection
of 1 mM urea in 0.1 M KOH as displayed in Fig. S2.† It can be
seen that the morphology of Ni-MOF nanobelts has not changed
substantially aer long time detection, and its original nanobelt
morphology is basically maintained. It again conrms the
stability of Ni-MOF nanobelts. Fig. 1B exhibits the TEM images
of Ni-MOFs from which we can achieve a deeper understanding
of the structure of ultrathin nanobelts. The energy-dispersive X-
ray (EDX) spectrum exhibits the rational elemental presence of
Ni, C and O existing in the composites, which demonstrates that
no impurity substance exists in the Ni-MOF nanobelts (Fig. 1C).
The Fourier transform infrared (FT-IR) spectrum of the Ni-
MOFs is revealed in Fig. 1D. The intensive and broad peak
located at 3425 cmꢀ1 is attributable to the stretching mode of
the hydroxyl group (O–H) coupled to Ni(II), and the peaks
located at 1309 cmꢀ1, 1453 cmꢀ1 and 1413 cmꢀ1 represent the
C–H bond vibrations. Moreover, an intensive band lies at
1606 cmꢀ1 because of the representative pattern of alkene (C]
C). Above results are consistent with the literature.18 The
formation of Ni-MOFs is conrmed mediately indirectly by FT-
IR spectroscopy. We can identify the fundamental form and
composition of functional groups of Ni-MOFs through X-ray
photoelectron spectroscopy (XPS). As shown in Fig. 1E, the
survey of XPS shows distinct peaks located at 856.6 eV (Ni 2p3),
530.64 eV (O 1s) and 283.1 eV (C 1s), separately, demonstrating
Electrochemical behaviors towards urea
To estimate the electrocatalytic property of the Ni-MOF ultra-
thin nanobelts toward urea, cyclic voltammetric (CV) tests of
GCE, Naon/GCE and Ni-MOF/Naon/GCE are carried out with
and without 1.0 mM urea (scan speed: 50 mV sꢀ1). All electro-
oxidation behaviors of urea on the Ni-MOF/Naon/GCE elec-
trode in this experiment are studied in 0.1 M KOH electrolyte.
As displayed in Fig. 2, a couple of characteristic redox peaks
turned up immediately when the GCE surface was modied
with Ni-MOF nanobelts (curve a and b); in comparison, bare
GCE and Naon/GCE show no typical peak with 1.0 mM urea
(curve c and d). From the CV diagram, we can see that there is
no typical redox peak (curve d), indicating that bare GCE has no
catalytic performance towards urea. Aer modifying Naon to
the surface of GCE, there is almost no change in current,
manifesting that Naon acts only as a mediator for the trans-
port of electrons and immobilization.22,23 On the other hand, it
can be also discovered that there is a signicant oxidation peak
corresponding to the anode at about 0.60 V, which is attributed
Fig. 1 Low (A) and high (the inset) magnification SEM images of Ni- Fig. 2 Cyclic voltammograms of Ni-MOF/Nafion/GCE (curves a and
MOFs; (B) TEM image of Ni-MOFs; (C) EDX spectrum of Ni-MOFs; (D) b), Nafion/GCE (curve c) and GCE (curve d) electrodes with (curves a, c
FT-IR spectrum of Ni-MOFs; XPS patterns of survey (E) and Ni 2p and d) and without (curve b) 1.0 mM urea in 0.1 M KOH solution. Scan
regions before and after testing (F) on the Ni-MOFs.
speed: 50 mV sꢀ1
.
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to the reason that Ni(II) can be rapidly converted to Ni(III) in
presence of large number of OHꢀ anions. And the result is
accordant with the literature.17 At the cathode, however,
a visible reduction peak appears at about 0.30 V, which can be
considered as a process in which Ni(III) is reduced to Ni(II),
where the process occurs opposite to the anode. At the same
time, the peak current response heightens conspicuously in the
anode and weakens in the cathode when 1.0 mM urea was
injected (curve a). The above detecting results demonstrate that
Ni-MOF nanobelts possess an excellent electro-catalytic oxida-
tion property towards urea. The above electrochemical behav-
iours are in accordance with the characteristic catalytic
regeneration (EC' mechanism).24 Based on theory, urea is
chemically oxidized to corresponding substances accompanied
with the process of Ni(III) reduction to Ni(II), and the electro-
chemical regeneration of Ni(II) to Ni(III) occurs simultaneously.
In other words, it's the following redox transition process: Ni(II)
% Ni(III). The regeneration of Ni(III) catalytic agents leads to the
disappearance of invertibility and heightens the peak intensity
of Ni(III)/Ni(II) with the addition of urea.
Fig. 3 (A) CVs of the Ni-MOF/Nafion/GCE electrode with 1.0 mM urea
at distinct scan speeds: 20, 40, 60, 80, 100, 120, 140, 160 mV sꢀ1. (B)
The plot of variation of anode and cathode peak currents with scan-
ning rate. (C) DPV of the Ni-MOF/Nafion/GCE electrode at variant urea
concentrations: 0–0.2 mM. Test factors: amplitude 0.05 V, pulse cycle
time 0.2 s and impulse duration 0.05 s. (D) The correlation of associ-
ated response vs. the concentration.
It has been reported that a cost-effective nickel-based catalyst
can convert urea into non-poisonous and harmless nitrogen gas
products under alkaline conditions by electro-oxidation,25 and
the electrocatalytic process of this material towards urea may
occur as follows:8,17,20,26
continually raised scanning speed. In addition, the redox peak
current signals are linearly dependent on the the square root of
the scan rates, as displayed in Fig. 3B. There is not only an
excellent mutual dependence between the peak current of anodic
oxidation and the radical sweep velocity, for which R2 is 0.968,
but also an analogous relevance for the peak response to cathodic
reduction, for which R2 ¼ 0.995. And the proportional relations
existing between peak current and sweep velocity demonstrates
that the redox reaction of urea on the Ni-MOF modied electrode
is a representative diffusion – controlled process.17 In addition,
the oxidation peak moves towards more positive direction cor-
responding to a series of increases in the scanning speed in the
range of 20 to 160 mV sꢀ1, while the reduction peak is completely
opposite.
Moreover, the distance (DEp) between the peak potential of
anode (Epa) and cathode (Epc) moves further away from each
other ranging from about 248 mV to 449 mV. The reason why
the oxidation potential moves in the positive direction may be
due to the extraordinary sorption to urea at the position of
Ni(III).27 Meanwhile, we can also observe that the peak signal
response ratios (Ipc/Ipa) between the cathode and the anode are
explicitly departed from 1, and the peak potential discrepancy
(DEp) is larger than 0.059/n, manifesting the quasi-reversibility
of the redox reaction.
Ni(II)-MOF / Ni(III)-MOF + eꢀ
(1)
Ni(III)-MOF + CO(NH2)2 + 6OHꢀ/
Ni(II)-MOF + N2 + 5H2O + CO2 + 6eꢀ
(2)
The above equations manifest that Ni(II) could be rapidly
oxidized into active Ni(III) in an alkaline environment, which
promotes the electrocatalytic oxidation procedure of urea.21
The structure of Ni-MOF is [Ni20(C5H6O4)20(H2O)8]$40H2O. It
can be seen that the ligand C5H6O4 is connected to Ni succes-
sively, and nally forms a series of nanobelt tunnels. When urea
is added, it is absorbed on the Ni-MOF nanobelts. Under
a certain voltage, urea undergoes an electrooxidation process in
the Ni-MOF nanobelt tunnel. In the tunnel, Ni(II) could be
rapidly oxidized into active Ni(III) under alkaline conditions,
and the urea was oxidized into N2, CO2 and H2O.
Furthermore, we studied the kinetic parameters of the
process through a series of electroanalytical measurement
techniques, including cyclic voltammetry (CV), linear sweep
voltammetry (LSV), differential pulse voltammetry (DPV) and
amperometric response curve (I–T).
On the other hand, there is a logarithmic relationship
between the oxidation peak potential (Epa) and sweep rate. The
linear relationship (Fig. S3†) between Epa and log v can be
demonstrated as follows:
Evaluation of electrochemical performances
Effect of scanning speed. For an electrocatalytic reaction, it is
exceedingly practicable to realize how the electrons are trans-
ferred in the course of the redox reaction, which helps us to have
a deeper comprehension of the reaction process. Therefore, we
investigated the inuence of scanning speed on the peak current
response of Ni(II)/Ni(III) redox couples by cyclic voltammetry (CV)
and the result is shown in Fig. 3A. The response currents to the
anode and cathode strengthened regularly in the wake of the
Epa (V) ¼ 0.794 + 0.136 log v (V sꢀ1), (R2 ¼ 0.983)
The electron transfer coefficient (a) can be obtained from the
inferior formula between Epa and log v:17
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0:03
an
Epa ¼ constant þ
log v
According to the slope of Fig. S3,† a can be gured out as
0.45. The transferred electrons (n) are calculated to be close to 6.
This indicates that 6 electrons are transferred on the anode
during the electrochemical oxidation of urea. The total reaction
is as follows:
CO(NH2)2 + 6OHꢀ / N2 + 5H2O + CO2 + 6eꢀ
(3)
which is consistent with the mechanism of reactions described
in the eqn (1) and (2).
Differential pulse voltammetry response of urea. The tech-
nique of differential pulse voltammetry (DPV) not only has
higher sensitivity and resolution, but also has lower detection
limit on account of the depressed background current, which
can be employed in the situation where the concentration is as
low as about 1.0 mg Lꢀ1. Fig. 3C reveals DPV response curves of
the Ni-MOF decorated electrode in 0.1 M basic electrolyte
involving different urea concentrations: 0–2.0 mM. Urea
concentration was rst increased stepwise by 0.1 mM until it
reached 0.8 mM, followed by 0.2 mM in each step until it nally
reached 2.0 mM. As presented in Fig. S4,† a clearer DPV curve
can be examined when urea is not present in the potential range
of 0.55–0.85 V vs. Ag/AgCl. Fig. 3D exhibits a linear response of
oxidation peak in the entire urea concentration range of 0–
2.0 mM, and its liner relevancy can be stated as:
Fig. 4 LSV curves of the Ni-MOF/Nafion/GCE electrode at varying
urea concentrations rang of 0–8 mM in the potential of 0.2–0.8 V.
Inset: Mutual coherence of anode peak current responses vs. the
variable concentrations.
Fig. S5A.† Fig. S5B† indicates the linear relationship between
the peak current and the corresponding concentration at
distinct potentials. We can conclude that the potential of
+0.60 V was chosen as the most favorable potential because of
the highest response.
The performance of the Ni-MOF/Naon/GCE electrode as
a urea sensor is evaluated by I–T measurements at an oxidation
peak potential of +0.6 V vs. Ag/AgCl. As shown in Fig. 5A, the
oxidation peak current heightens instantly and arrives at
a plateau in about 10 seconds aer the addition of urea solution
each time. A calibration curve plot of the oxidation peak current
response to the urea concentration is presented in the Fig. 5B.
Dependency of the oxidation peak current on urea concentration
(0.01–1.0 mM) is linear with a regression equation of Ipa (mA) ¼
1.08 + 3.74C (mM), and the R2 is 0.992. When the urea concen-
tration is more than 1.0 mM, the linear analysis chart is pre-
sented in Fig. S6.† Dependency of the oxidation peak current on
urea concentration (1.0–7.0 mM) is linear with a regression
equation of Ipa (mA) ¼ 4.11 + 0.893C (mM), and the R2 is 0.956.
The Ni-MOF/Naon/GCE electrode exhibits a sensitivity of 118.77
mA mMꢀ1 cmꢀ2. And the lowest detection limit obtained is 2.23
mM with S/N ¼ 3, as calculated from the slope of the above linear
calibration. Compared to earlier urea sensors including NiO and
other electrodes (Table 1), this Ni-MOF nonenzymatic sensor
exhibited a series of great performance, such as super sensitivity,
wider linear range, and even lower detection limit.
I (mA) ¼ 16.44 + 68.71C (mmol Lꢀ1), (R2 ¼ 0.998)
The DPV measurements manifest that the electrooxidation
course keeping accordance with the switch from Ni(II) to Ni(III) is
affected by the increasing concentration of urea and it is
simultaneously electro-oxidized to nitrogen gas in this
process.25
Linear sweep voltammetry response of urea. Linear sweep
voltammetry (LSV), similar to cyclic voltammetry (CV), is also
a kind of common electrochemical test means, which is oen
used for quantitative determination of adsorbent substances. For
instance, the LSV curves received on varying the concentrations
of urea are shown in Fig. 4, and the red arrow head points out the
orientation in which urea concentration increases constantly.
The LSV map indicates that the current response strengthened
progressively when various urea concentrations from 0 to 8 mM
are added. The linearly correlation can be gained from the
illustration in Fig. 4, and the R2 is as high as 0.997, proving that
the Ni-MOF self-made electrode is practicable and impactful for
the electrooxidation of urea.
Selectivity, long-term stability and repeatability assay
To assess the selectivity of the self-assembly sensor, we tested
the current response of Ni-MOF/Naon/GCE electrode for
Amperometric response of urea
0.05 mM urea and various interfering species, such as creati-
2ꢀ
The electrocatalytic capacity of urea is discriminatory with nine, oxalic acid, uric acid (UA), cytosine, thymine, Clꢀ, SO4
different amperometric response to 0.2 mM urea being injected and adenine that coexist in human urine throughout during
into 0.1 M KOH consecutively, under the distinct potentials urea estimation. The results for the mentioned study are dis-
(0.50, 0.55, 060 and 0.65 V, respectively), as displayed in played at an oxidation peak potential of +0.6 V in Fig. 6. With
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Fig. 6 The current response of the Ni-MOF/Nafion/GCE electrode to
the addition of urea and different interfering species, 0.1 M KOH
solution as supporting electrolyte.
Another remarkable feature of the electrodes in practical
application is its prolonged stability. As displayed in Fig. S8,† the
sensor response to urea was measured for one month. We keep
the self-prepared urea sensor in the air and perform CV test to
record the change in current value. Aer keeping it in the air for
one week, the cathodic peak current attenuation does not reach
more than 5% of the original signal. And the current response
maintains 92.5% of its original current signal even aer one
month of air storage (I/I0 ¼ 92.5%). These results demonstrate
that the urea sensor based on Ni-MOF nanobelts has superior
stability for sensing applications. The RSD is 2.41%.
Fig. 5 (A) Amperometric i–t curve for unceasing additions of urea into
the electrolyte solution at oxidation peak potential of +0.6 V. (B) The
calibration curve is shown about Ipa (mA) and C (mM).
In addition, the reproducibility of the sensor is also evaluated
in our study. We investigate the CV responses of ve electrodes
prepared with the same method in 0.1 M electrolyte containing
1.0 mM urea, as displayed in Fig. S9.† The ve electrodes achieve
illustrious consistency, where the RSD of their current response
is 2.74%. The excellent stability and repeatability of the Ni-MOF
nanobelt electrode make them a promising candidate as a prac-
tical non-enzymatic urea sensor.
respective stepwise addition of creatinine, oxalic acid, UA,
2ꢀ
cytosine, thymine, Clꢀ, SO4
and adenine in 0.1 M KOH
solution every 60 seconds, it is clear that the current barely
changed even though the interfering species were present in 2-
fold or 0.2-fold concentration levels in comparison to urea
(creatinine: 10 mM; oxalic acid: 10 mM; UA: 0.1 mM; cytosine:
0.1 mM; thymine: 0.1 mM; Clꢀ: 0.1 mM; SO42ꢀ: 0.1 mM;
adenine: 0.1 mM). From Fig. S7,† we can evaluate that the
current responses of all the interfering species are less than
12.5% of the urea signal. The above results are enough to prove
the high selectivity of this self-assembly sensor towards urea
detection.
Determination of urea in human body uids
Real urine samples were analysed by linear sweep voltammetry
(LSV) to examine the practical feasibility of the Ni-MOF/Naon/
Table 1 Comparison of analytical performances of other reported urea sensor
Electrode material
Linear range (mM)
LOD (mM)
Sensitivity mA mMꢀ1 cmꢀ2
Ref.
Rh/urease
0.1–1.75
50
5
6
1.85
166
685.1
371
226.9
—
21.3
—
28
17
8
27
29
30
1
31
3D-graphene/NiCo2O4
Ni-MOF/MWCNT
NiO/cellulose/CNT
Graphene-PANi/GCE
Vitamin C based NiO
NiO-NPs/urease
0.06–0.30
0.01 ꢁ 10ꢀ3 to 1.12 ꢁ 10ꢀ3
0.01–1.4
0.01–0.2
0.1–1.1
0.83–16.65
0.1–1.2
0.01–7
7
5.88
10
830
8
NiO/CTAB/GO/GCE
Ni-MOF/Naon/GCE
2.719
118.77
This work
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Therefore, the conclusions drawn from the Ni-MOF sensor
have a certain degree of condence. The Ni-MOF sensor shows
excellent electrochemical performance towards urea. Recoveries
of 92.0–115.0% verify the suitability of the developed enzyme-
free urea sensor in real urine samples, where the presence of
various impurities has negligible effect on the authenticity of
the method. In view of this, the foregoing sensor can be
extended to urea determination in biological samples such as
milk and serum.
Conclusions
An ultrathin Ni-MOF nanobelt decorated electrode was evalu-
ated as a novel non-enzymatic urea sensor. The urea sensor
based on Ni-MOF ultrathin nanobelts exhibits superior sensing
performances such as broad detection range, low detection
limit and high selectivity. Furthermore, our prepared urea
sensor also shows acceptable anti-interference ability, stability
and reproducibility. Overall, the urea sensor based on ultrathin
Ni-MOF nanobelts is successfully fabricated and found to be
promising for applications in medical diagnostics and envi-
ronmental monitoring.
Fig. 7 LSV responses of Ni-MOF nanobelts modified GCE in diluted
human urine sample containing different urea concentrations: 0, 1, 2,
3, 4, 5, 6, 7, and 8 mM. Inset: Corresponding calibration plot of urea
concentration vs. current response.
GCE electrode, as shown in Fig. 7. For large urea concentration
ranging from 0 to 8 mM, the oxidation peak currents are
strengthened gradually, with a correlation coefficient of the
Ethical statement
corresponding linear equation being 0.987. In the meantime, The human urine samples were kindly provided by graduate
the peak potential shis towards the positive direction with students of our group in Shanxi Normal University (Linfen,
increasing urea concentration, which indicates that other China). In addition, the main purpose and application of this
molecules in the urine have little effect on the detection of urea
for this sensor. The results are satisfactory for the detection of
urea in routine samples.
research have been informed, and the urine samples used in
this experiment was obtained with the consent of the graduate
students (Qiangqiang Niu and Zi-Ang Chen, respectively).
To further estimate the functional characteristics of Ni-MOF/
Naon/GCE electrode in real urine samples, this developed
sensor was applied to three real urine samples using the
recovery test. The actual sample was diluted 100 times in 0.1 M
KOH solution. Then, a series of known concentration of urea
solution were added to the diluted urine sample and colori-
metric methods were used for a comparative study with the
proposed sensor. The results are presented in Table 2. The t-test
results show that there is no signicant difference between the
two methods.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 21705103), the Applied Basic
Research Project of Shanxi Province (No. 201801D221392), the
Table 2 Determination and recovery results of urea in urine samples by the developed sensor
Determined by
sensor (mM)
Urea spiked
sample (mM)
Sample no.
Sample-1
Determined by standard methoda (mM)
3.19a
Founded by sensor (mM)
Recovery (%)
2.98
3.20
2.71
1.00
3.00
4.00
1.00
3.00
4.00
1.00
3.00
4.00
3.97
6.15
7.34
4.12
6.21
7.66
3.86
5.94
7.00
99.5
105.7
109.0
92.0
100.3
111.5
115.0
107.7
107.25
Sample-2
Sample-3
3.75a
2.43a
a
Determined by colorimetric method.32
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Graduate Education Innovation Project of Shanxi Province 14 H. Liu, W. Ding, S. Lei, X. Tian and F. Zhou, Selective
(2018SY057), Collaborative Innovation Center for Shanxi
Advanced Permanent Materials (2019-05) and Technology and
adsorption of CH4/N2 on Ni-based MOF/SBA-15 composite
materials, Nanomaterials, 2019, 9, 149.
the 1331 Engineering of Shanxi Province. We appreciate Prof. 15 D. Zhu, C. Guo, J. Liu, L. Wang, Y. Duand and S.-Z. Qiao,
Dr Xiufang Qin (Shanxi Normal University) for her help on
TEM and XRD characterizations. At the same time, I sincerely
deliver my thanks to graduate students of our group in Shanxi
Two-dimensional metal–organic frameworks with high
oxidation states for efficient electrocatalytic urea oxidation,
Chem. Commun., 2017, 53, 10906–10909.
Normal University (Linfen, China) for providing human urine 16 J.-Y. Zhang, T. He, M. Wang, R. Qi, Y. Yan, Z. Dong, H. Liu,
samples.
H. Wang and B. Y. Xia, Energy-saving hydrogen production
coupling urea oxidation over bifunctional nickel-
a
molybdenum nanotube array, Nano Energy, 2019, 60, 894–
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17 T. Q. N. Tran, G. Das and H. H. Yoon, Nickel-metal organic
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83.
18 X. Xiao, S. Zheng, X. Li, G. Zhang, X. Guo, H. Xue and
H. Pang, Facile synthesis of ultrathin Ni-MOF nanobelts
for high-efficiency determination of glucose in human
serum, J. Mater. Chem. B, 2017, 5, 5234–5239.
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