Mixed-Valence Ce-BPyDC Metal-Organic Framework with Dual Enzyme-like Activities for Colorimetric Biosensing

Article abstract of DOI:10.1021/acs.inorgchem.9b00661

Enzyme-like metal-organic frameworks (MOFs) are currently one type of starring material in the fields of artificial enzymes and analytical sensing. However, there has been little progress in making use of the MOF structures based on the catalytically acti

Full text of DOI:10.1021/acs.inorgchem.9b00661

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Mixed-Valence Ce-BPyDC Metal−Organic Framework with Dual
Enzyme-like Activities for Colorimetric Biosensing
Linpin Luo,† Lunjie Huang,† Xinnan Liu,† Wentao Zhang,† Xiaolin Yao,† Leina Dou,† Xu Zhang,†
Ying Nian,† Jing Sun,‡ and Jianlong Wang*,†
†College of Food Science and Engineering, Northwest A&F University, Yangling 712100, Shaanxi, China
‡Qinghai Provincial Key Laboratory of Qinghai-Tibet Plateau Biological Resources, Northwest Institute of Plateau Biology, Chinese
Academy of Sciences Qinghai, Xining 810008, China
S
* Supporting Information
ABSTRACT: Enzyme-like metal−organic frameworks (MOFs) are
currently one type of starring material in the fields of artificial
enzymes and analytical sensing. However, there has been little
progress in making use of the MOF structures based on the
catalytically active metal center with multiple valences. Herein, we
report a mixed-valence Ce-MOF (Ce-BPyDC) that can exhibit both
oxidase-like and peroxidase-like activities. Ce-BPyDC was synthe-
sized by a facile hydrothermal method, which preserves the rare
coexistence of Ce(III) and Ce(IV) in the MOF structure. The
enzymatic studies demonstrated the enzyme-like activities of Ce-
BPyDC follow the Michaelis−Menten kinetics and are strongly
dependent on temperature, pH, and reaction time. Ce-BPyDC was
also revealed to exert high catalytic activity that could transcend
horseradish peroxidase and other MOF nanozymes, due to the redox-active Ce(III)/Ce(IV) cycles inside. Furthermore, the
simple synthesis, high nanozyme activity, and great stability of Ce-BPyDC motivated us to establish a colorimetric biosensing
platform using 3,3′,5,5′-tetramethylbenzidine as a color reagent. Adopting this strategy, we established a visual, sensitive, and
selective colorimetric method for ascorbic acid (AA) detection, for which the linear interval and limit of detection were 1−20
and 0.28 μM, respectively. The successful AA detection in real juice samples implies the promising use of such mixed-valence
MOF nanozymes in food and biomedical samples.
nanomaterials with a high surface-to-volume ratio is the key
to optimizing the activities of nanozymes.
INTRODUCTION
■
Natural enzymes, such as the biomolecule catalysts, can
specifically catalyze chemical reactions under mild conditions.1
These biocatalysts, such as horseradish peroxidase (HRP),
glucose oxidase, acetylcholinesterase (AChE), and alkaline
phosphatase (ALP), were widely used in analytical applica-
tions, including in the enzyme-linked immunosorbent assay, as
colorimetric and electrochemical sensors, and in the lateral
flow immunoassay in previous studies. However, natural
enzymes have disadvantages, such as vulnerability to a harsh
environment, low operational stability, high costs, etc.2
Therefore, the artificial enzyme mimics, especially nanozymes,
which could make up for the shortcomings of natural enzymes,
have received widespread attention.2 Until now, a number of
nanomaterials, such as noble metal nanoparticles (NPs),3
metal oxides,4−6 oxy-hydroxide,7,8 bimetal oxides,9 carbon
materials,10 and transition metal disulfides,11 were found to
exhibit intrinsic enzyme-like activities. In particular, recent
works have proved that nanozymes with a high surface-to-
volume ratio always possess high catalytic activity, which is
attributed to the abundance of catalytic sites and affinity sites
toward colorimetric substrates.10−14 Hence, screening of
Metal−organic frameworks (MOFs) are a type of organic−
inorganic coordination compound formed by the assembly of
organic ligands and metal ions or clusters,15 which have shown
great potential in biosensing and imaging, adsorption and
separation, heterogeneous catalysis, and cancer therapy.16−19
Owing to their large surface area, abundant active sites, and
designable structures, MOF materials are well suited to being
candidates for nanozyme exploitation. In previous studies,
many kinds of MOFs have been explored for enzyme-like
catalysis,20 such as the peroxidase-like Fe-MOFs (MIL-53,
MIL-68, MIL-100, MIL-88A, MIL-88-NH2, and MIL-101),21
Cu-MOFs (Cu-BTC, Cu-hemin MOFs, HKUST-1, and
Hemin@HKUST-1),22 Zr-MOFs (UiO-66-NH2 and Pt NP@
UiO-66-NH2),23 Zn-MOFs (BHb@ZIF-8, GOx@mZIF-8, and
GOx/hemin@ZIF-8),24 Co-MOF (CoNPs/MC and Co/2Fe-
MOF),25 Ni-MOF nanosheets,26 Al-MOF [Hemin@MIL-
101(Al)NH2],27 and oxidase-like Co/2Fe-MOF.25 As such,
various high-performance sensing platforms have been
Received: March 6, 2019
© XXXX American Chemical Society
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Scheme 1. Illustration of Colorimetric Detection of AA by Using Ce-BPyDC as a Catalyst
established on such MOF nanozymes for the detection of ions,
small biomolecules, pesticides, DNA, and bacteria.28 To
summarize, these MOF nanozymes share the following
features. (1) The active catalytic centers are highly dependent
on the redox-active metal elements. (2) MOFs with more than
one metal element or a metal element of mixed valences tend
to process higher catalytic activities. (3) The chemical stability
of MOFs is a key parameter for reproducible catalytic
performance. However, the current MOF nanozymes process
either weak catalytic activities, limited dispersion stability, or
low environmental tolerance, which hinders their scalable uses
at present. In this regard, finding an excellent MOF enzyme
that mimics the positive features mentioned above would be
beneficial.
morphology and size of the as-prepared Ce-BPyDC. As
shown in Figure 1a and Figure S1, most of the obtained Ce-
Cerium, as an important rare earth element,29 has been
widely utilized in the field of catalysis. More and more cerium-
based nanomaterials have been developed as catalysts due to
their unique advantages.30 For instance, cerium dioxide
(CeO2) is one of the most popular industrial catalysts or
catalyst supports and plays a critically important role in the
development of three-way catalysts.31 Moreover, the superi-
ority of CeO2, which originated from a redox-active Ce
element, also strongly promoted the development of mimetic
enzymes.5,32 In contrast, the MOF structures with Ce centers
were rarely involved in nanozyme-related research projects,33,34
which could be a point of penetration for improving the
applicability of MOF nanozymes.
Figure 1. Structural characterizations of Ce-BPyDC: (a) SEM image,
(b) XRD pattern, (c) IR spectral analysis, and (d) XPS spectra for the
Ce 3d region of as-prepared Ce-BPyDC.
Herein, we synthesize a new type of MOF (Ce-BPyDC)
with a mixed-valence Ce element as the metal center that
possesses dual enzyme activities (peroxidase and oxidase). As
shown in Scheme. 1, Ce-BPyDC could catalyze the
chromogenic substrates, such as 3,3′,5,5′-tetramethylbenzidine
(TMB), o-phenylenediamine (OPD), and 2,2′-azinobis(3-
ethylbenzthiazoline-6-sulfonic acid) (ABTS), to produce
colorimetric oxidation products in the presence or absence of
H2O2. The catalysis of such a MOF nanozyme strongly
depends on the reaction temperature and solution pH and
follows Michaelis−Menten behavior. Moreover, the catalytic
mechanism studies demonstrated that the nanozyme proper-
ties of Ce-BPyDC were a consequence of the Ce(III)/Ce(IV)
redox cycles inside the Ce-BPyDC nanostructure. Finally, we
established a colorimetric method for highly sensitive detection
of ascorbic acid (AA) based on the oxidase activity of Ce-
BPyDC, which was further applied in the real fruit juice
samples.
BPyDC particles exhibit a cube-like structure with a size of
200−250 nm, while some other particles are smaller (∼100
nm). The crystalline structure of Ce-BPyDC was analyzed by
the powder X-ray diffraction (XRD) technique (Figure 1b).
The result showed that the as-obtained Ce-BPyDC exhibited
the same crystalline structure that can be found in the
literature.35 The infrared (IR) spectrum of as-prepared Ce-
BPyDC contains high-intensity peaks at around 1701, 1587,
1536, and 1391 cm−1 (Figure 1c), which are associated with
asymmetric stretching vibrations for the carbonyl group (C
O), asymmetric stretching vibrations for the carbonyl group
(COO−) of the linker H2BPyDC, aromatic rings (CC), and
stretching vibrations for the carbonyl group (COO−) of the
linker, respectively.35 Finally, the high-resolution X-ray photo-
electron spectroscopy (XPS) spectra of Ce 3d, C 1s, and O 1s
were obtained. As shown in Figure S2, the C 1s peaks centered
at 284.7 and 288.6 eV are ascribed to C−C and CO,
respectively, while the O 1s peaks centered at 529.6, 531.4, and
533.0 eV are attributed to lattice oxygen (Ce−O), CO/OH,
and C−O, respectively.36 More importantly, as shown in
Figure 1d, the peaks at 917.4, 905.5, 901.6, 889.3, and 883.1
eV are related to the Ce(IV) state while the peaks at 904.7,
899.0, and 885.8 eV are related to the Ce(III) state. In
RESULTS AND DISCUSSION
■
Characterization of Ce-BPyDC. Scanning electron
microscopy (SEM) and transmission electron microscopy
(TEM) images were collected to investigate the surface
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addition, the ratio of peak areas of Ce(III) and Ce(IV) states
obtained from the Ce 3d spectrum is ∼1:1.2. The
thermogravimetric curves were obtained to evaluate the
thermal stability of Ce-BPyDC. As shown in Figure S3, Ce-
BPyDC kept losing H2O molecules when the heating
temperature was below 320 °C, and the linker molecules of
Ce-BPyDC started to degrade over 320 °C, which demon-
strated that Ce-BPyDC exhibits excellent thermal stability
below 320 °C. The specific BET surface area and pore size
calculated from the N2 adsorption−desorption isotherm
(Figure S4) are 1950 m2 g−1 and 0.82 cm3 g−1, respectively,
in good agreement with the parameters in ref 35. This result
illustrates that the Ce-BPyDC particle has a large surface area
and a high porosity. These results proved the successful
preparation of Ce-BPyDC with Ce(III)/Ce(IV) mixed
valences.
idase-like activity of this material. Figure S5a shows the colors
of the Ce-BPyDC/TMB/H2O2, Ce-BPyDC/ABTS/H2O2, and
Ce-BPyDC/OPD/H2O2 solutions are blue, green, and yellow,
respectively, with the corresponding absorption peaks located
at 652, 415, and 447 nm, respectively, while the rest of the
systems did not show color or absorbance changes in
comparison with the experimental group. Therefore, Ce-
BPyDCs can be developed as oxidase-like and peroxidase-like
mimics.
To further optimize the enzyme activities of Ce-BPyDC, we
explored the effects of reaction time, catalyst concentration,
pH, and temperature on the enzymatic activity of Ce-BPyDC.
Figure 2b and Figure S5b show the absorbance was time-
dependent and changed with the concentration of Ce-BPyDC
at 652 nm. The increasing concentration of Ce-BPyDC could
enhance the reaction rates. Meanwhile, the absorbance
increased with time and then remained constant after 10
min. Figures S6 and S7 show the relationship between
activities and the concentrations of Ce-BPyDC, indicating
that the enzymatic activity could be dramatically improved
with an increase in the concentration of Ce-BPyDC. Similar to
other nanomaterial-based enzyme mimics, the enzymatic
activity of Ce-BPyDC relied greatly on pH and temperature.
As shown in Figure 2c and Figure S5c, the high activities of
Ce-BPyDC could be maintained over a pH range of 3.0−6.0
(oxidase-like) or 3.0−5.0 (peroxidase-like), in which a pH
value of 4 was chosen for the catalytic reactions. The oxidase-
like activity showed a slow increase as the reaction temperature
increased, and the activity showed a downward trend when the
temperature was above 40 °C (Figure 2d). The peroxidase-like
reaction system (Figure S 5d) showed an activity−temperature
relationship similar to thta of the oxidase reaction system. As
such, the following studies were performed at 25 °C. The
optimal reaction time was 10 min according to the time-
dependent absorbance curve. In conclusion, the dual activities
of Ce-BPyDC strongly depend on the catalyst concentration,
pH, and solution temperature.
Enzyme-like Activities of Ce-BPyDC. The oxidase-like
activity of Ce-BPyDC was tested by measuring the catalytic
reaction of the TMB substrate without the participation of
H2O2. As shown in Figure 2a, the Ce-BPyDC/TMB system
Stability and Reusability of Ce-BPyDC. The stability of
Ce-BPyDC was evaluated in our research. To investigate the
water dispersion of Ce-BPyDC, a certain amount of material
was dispersed in water and stored for 7 days at room
temperature. As shown in Figure S8, Ce-BPyDC exhibited no
obvious aggregation in solution, proving its excellent aqueous
dispersity. The catalytic stability of Ce-BPyDC was examined
by measuring the enzymatic activity changes over time. As
shown, the Ce-BPyDC could remain over 90% of the oxidase-
like activity (Figure S9a) and peroxidase-like activity (Figure
S9b) after being stored for 20 days. In addition, the
physicochemical stability was also examined by TEM analysis,
XRD patterns, and Fourier transform infrared (FT-IR)
measurements before and after the catalytic reaction. The
results (Figure S10) show that there are no significant changes
in TEM images, XRD patterns, or FT-IR spectra of Ce-BPyDC
before and after catalysis, which indicated that Ce-BPyDC can
remain stable during the catalytic reaction.
We also evaluated the enzyme activities of five different
batches of Ce-BPyDC (Figure S11). As shown, Ce-BPyDCs
exhibited small differences (<10%) in their enzyme activities
between batches; thus, we can conclude that Ce-BPyDC
possesses excellent batch-to-batch reproducibility. After that, to
evaluate the cycle performance of Ce-BPyDC as a catalyst, the
enzymatic activities were examined during repetitive cycles
(Figure S12). The results show that >85% of dual nanozyme
Figure 2. Oxidase-like catalytic properties of Ce-BPyDC. (a)
Ultraviolet−visible absorption spectra from recording the oxidation
of 1 mM ABTS (green line), TMB (blue line), and OPD (yellow line)
catalyzed by Ce-BPyDC (25 μg mL−1). The corresponding dashed
line indicates the incubation of Ce-BPyDC with ABTS, TMB, or OPD
only. Reaction time t = 10 min. (b) Time- and catalyst concentration-
dependent absorbance at 652 nm measured from the reaction
solutions containing 1 mM TMB and Ce-BPyDC at different
concentrations in 0.1 M acetate buffer (pH 4.0) at room temperature.
The oxidase-like catalytic activity of Ce-BPyDC vs (c) pH and (d)
temperature. The maximum activity in each graph was set as 100%.
changed from colorless to blue and generated a high
absorbance at 652 nm. In contrast, TMB only did not show
a distinct difference in the color change or the absorption peak
at 652 nm. These results indicated that Ce-BPyDC could
catalyze the TMB. Meanwhile, several different chromogenic
substrates like 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic
acid) (ABTS) and o-phenylenediamine (OPD) were also
tested. The results depicted in Figure 2a show that Ce-BPyDC
also could catalyze the oxidation reaction of ABTS or OPD to
give a green or yellow color and exhibit the corresponding
absorption peaks at 415 and 447 nm, respectively.37 This
evidence proved that Ce-BPyDC exhibited intrinsic oxidase-
mimicking activity. Furthermore, we investigated the perox-
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activities were maintained even after eight repetitive cycles,
indicating the good reusability of Ce-BPyDC and its detection
platform. The excellent stability and reusability of Ce-BPyDC
are greatly significant for the establishment of a colorimetric
probe.
Catalytic Mechanism of Ce-BPyDC. The possible
catalytic mechanisms of Ce-BPyDC were also investigated.
The catalytic activities of Ce-BPyDC after centrifugation were
measured, and the obtained values were compared with those
of the original solutions. The results show no obvious
difference in catalytic activities (Figure S13), which manifested
that the dissolved ions in solution made no contribution to the
catalytic reaction of Ce-BPyDC. As previously reported, the
possible mechanism of oxidase-like activity for Ce-based
materials might originate from the Ce(III)/Ce(IV) system
that the Ce-based materials possess.33,36 In our work, as shown
in Figure S14, the possible mechanism behind the oxidase-like
activity of Ce-BPyDC might originate from the Ce(III)/
Ce(IV) couple that the cerium materials retain, in which these
species can balance between the two Ce(III)/Ce(IV) valences
in redox-active reactions. The Ce(IV) ions were reduced to
Ce(III) ions during the TMB oxidation reaction. Then, the
generated Ce(III) ions “spontaneously” recycle back to the
Ce(IV) ions. The possible mechanisms of peroxidase-like
activity were also explored. The hydroxyl radical (•OH), a
common intermediate during the peroxidase-like catalytic
reaction, is usually deemed to be the key to peroxidase-like
catalytic activity. Terephthalic acid (TA) could combine with
•OH and form a fluorescent product called 2-hydroxytereph-
thalic acid (TAOH).38 Therefore, the •OH possibly generated
during the peroxidase-like reaction was monitored by
fluorescence experiments. Figure S15 presents the fluorescence
image of the mixed solution of Ce-BPyDC, TA, and H2O2 after
incubation for 12 h at 37 °C. The Ce-BPyDC, TA, and H2O2
mixed solution exhibited a high fluorescence intensity at 425
nm compared with those of control experiments. tert-Butyl
alcohol can capture •OH radicals and scavenge them.
Therefore, colorimetric tests were also carried out to further
Figure 3. Steady-state kinetic analysis of Ce-BPyDC as an oxidase
mimetic. (a) Oxidase-like activity system with different concentrations
of H2O2. (b) Double-reciprocal plots of 25 μg mL−1 Ce-BPyDC in
acetate buffer (pH 4.0) at room temperature.
affinity of Ce-BPyDC for a catalytic substrate. In addition, Ce-
BPyDC possessed a Vmax value equal to or even higher than
those of other reported oxidase mimics. To gain more
information about the Ce-BPyDC peroxidase-like property,
the steady-state kinetic parameters for Ce-BPyDC as a
peroxidase mimetic were also determined (Figure S17). The
values of Km and Vmax for Ce-BPyDC, HRP, and other
peroxidase mimics are listed in Table S2. Ce-BPyDC exhibited
Km values lower than that of HRP and other peroxidase
mimics, suggesting that Ce-BPyDC has a higher affinity for
both TMB and H2O2. All of our results prove that Ce-BPyDC
was an excellent artificial enzyme with dual enzyme-like
activities.
Colorimetric Detection for Ascorbic Acid. Ascorbic acid
(AA) is usually added to food sources as an artificial
antioxidant in the food industry. AA is an indispensable type
of vitamin for the good health of humans.40 For instance, large
doses of AA function as a pro-oxidant anticancer agent and can
kill colorectal cancer cells selectively. Thus, precise detection of
AA is of great significance in the fields of human health, food
safety, and medical diagnosis.41−43 The blue oxTMB could be
reduced to the colorless TMB with the participation of AA.
Therefore, a colorimetric method for detecting AA was
established with the advantages of simple operation and
rapid, sensitive, and accurate detection. Under the optimized
experimental conditions, a range of concentrations of the AA
solution were added to the Ce-BPyDC/TMB system and the
mixture was incubated for 10 min at room temperature. As one
can see in Figure 4a, the more ascorbic acid is added (0−30
39
•
prove the generation of OH during the reaction. As shown
in Figure S16, tert-butyl alcohol could attenuate the absorption
intensity at 652 nm within a concentration range of 0−400 mg
•
mL−1. These results indicated the generation of OH radicals
in the peroxidase system.
The steady-state kinetic testing was performed on the Ce-
BPyDC/TMB system to gain more details about the oxidase
catalytic properties. A typical Michaelis−Menten curve was
obtained by plotting the corresponding initial reaction
velocities versus the substrate concentration (Figure 3a). By
taking the double reciprocal of the data in Figure 3a, we
obtained the Lineweaver−Burk plot (Figure 3b), in which the
slope and intercept of the curve represent Michaelis−Menten
constants (Km) and maximum initial reaction rates (Vmax),
respectively. Km was considered a parameter for evaluating the
affinity between enzymes and substrates. The enzymes with a
strong affinity for a substrate always have a low Km value. As
shown in Table S1, the Km and Vmax of Ce-BPyDC with TMB
as a substrate were 0.12 mM and 20.6 × 10−8 M/s,
respectively. Ce-BPyDC exhibits a Km value that is lower
than that of other oxidase-like nanozymes, indicating that the
smaller amount of TMB was needed to achieve maximum
catalytic activity for Ce-BPyDC. The low Km value, the
Ce(III)/Ce(IV) balance, accompanied by the adsorption
nature of MOF structure, may contribute to a high binding
Figure 4. AA detection assay using Ce-BPyDC as an oxidase mimetic.
(a) AA concentration-dependent ultraviolet−visible (UV−vis)
absorption change. The inset photograph shows the visually
recognizable color changes of the reaction system. (b) AA assay
curve obtained from UV−vis spectral measurements. The inset shows
the calibration curve. Conditions: 25 μg mL−1 Ce-BPyDC and 1 mM
TMB, incubation in acetate buffer (pH 4.0) at room temperature for
10 min.
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μM), the lower the absorption peak at 652 nm. The inset in
Figure 4a shows the AA-dependent color changes of the test
solutions, which could be discerned directly by the naked eye.
The absorbance change at 652 nm as a function of the
concentrations of AA changes is displayed in Figure 4b, and
the inset shows a good linear correlation (R2 = 0.9976) from 1
to 20 μM. On the basis of the 3S/N formula, the detection
limit of this colorimetric method for AA detection could reach
0.28 μM. Compared with other colorimetric methods for AA
determination (Table S3), our work has a lower detection
limit, which indicated that this detection system was highly
sensitive.
TMB oxidation reaction. It can be a readily available substitute
for HRP and many similar nanozymes owing to its superior
catalytic performance. The dual activities originated from the
Ce(III)/Ce(IV) redox cycle system inside. On the basis of the
oxidase-like catalytic activity, a colorimetric method for
detecting AA was established with the advantages of simple
operation and rapid, sensitive, and accurate detection, which
could reach a linear range from 1 to 20 μM with a LOD of 0.28
μM. The detection of AA can be easily achieved by visible
color fading without any instrumentation. Our work revealed
the enzyme-like Ce-MOFs exhibited potential in food analysis
and biosensing.
To explore the selectivity of our colorimetric probe for AA
detection, some potential interfering substances, including K+,
Na+, Ni2+, Mn2+, Cu2+, Mg2+, Ca2+, Fe2+, citric acid (CA),
starch, fructose, sucrose, tartaric acid (TA), alanine (Ala),
glycine (Gly), glutamate (Glu), and aspartic acid (Asp), were
tested. The results (Figure 5) show that the interfering
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00661.
■
S
Materials and methods, characterization (TEM, XPS,
thermogravimetric analysis, and N2 sorption isotherm),
absorbance curves, optimization of conditions, optical
photograph, stability tests, cycle experiments, scheme of
the reaction mechanism, PL spectra, enzymatic kinetics,
comparison of enzymatic activities, and comparison of
methods (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: wanglong79@nwsuaf.edu.cn. Fax: +86 29-8709-2275.
Telephone: +86 29-8709-2275.
■
Figure 5. Selectivity test of AA over other coexisting substances in
fruit juice. Condition: 20 μM AA, 2 mM other interfering substances.
substances have no apparent influence on absorbance even if
the concentration was 100 times that of AA, indicating that the
method exhibited excellent selectivity for AA. To further
illustrate the real application of this method, this assay was
used to analyze real samples (orange juice and grapefruit
juice). As shown in Table 1, the detection results of the Ce-
ORCID
Jianlong Wang: 0000-0002-2879-9489
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors appreciate the kind support from the National
Natural Science Foundation of China (21675127), the Shaanxi
Provincial Science Fund for Distinguished Young Scholars
(2018JC-011), the Development Project of Qinghai Provincial
Key Laboratory (2017-ZJ-Y10), and the Capacity Building
Project of Engineering Research Center of Qinghai Province
(2017-GX-G03).
Table 1. Detection of AA Contents in Fruit Juice Samples
added
(μM)
measured
(μM)
AA test kit recovery
RSD (%)
(n = 3)
sample
(μM)
(%)
orange
juice
10
9.81
9.84
98.1
1.264
20
10
20.33
10.14
19.68
10.26
101.7
101.4
2.236
0.925
grape
juice
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CONCLUSION
■
In summary, we successfully synthesized a Ce-BPyDC by a
facile method and found this material has dual enzyme
activities (oxidase-like and peroxidase-like activities) in the
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