Metal-organic framework-derived copper nanoparticle@carbon nanocomposites as peroxidase mimics for colorimetric sensing of ascorbic acid

Article abstract of DOI:10.1002/chem.201404960

Metal-organic frameworks (MOFs) have emerged as very fascinating functional materials due to their diversity nature. A nanocomposite consisting of copper nanoparticles dispersed within a carbon matrix (CuNPs@C) is prepared through a one-pot thermolysis of

Full text of DOI:10.1002/chem.201404960

DOI: 10.1002/chem.201404960
Full Paper
&
Colorimetric Sensing
Metal–Organic Framework-Derived Copper Nanoparticle@Carbon
Nanocomposites as Peroxidase Mimics for Colorimetric Sensing of
Ascorbic Acid
Hongliang Tan,* Chanjiao Ma, Li Gao, Qian Li, Yonghai Song, Fugang Xu, Tao Wang, and
Li Wang*[a]
Abstract: Metal–organic frameworks (MOFs) have emerged
as very fascinating functional materials due to their diversity
nature. A nanocomposite consisting of copper nanoparticles
dispersed within a carbon matrix (CuNPs@C) is prepared
through a one-pot thermolysis of copper-based metal–or-
ganic framework precursors. CuNPs@C can catalyze the oxi-
dation of 3,3’,5,5’-tetramethylbenzidine (TMB) to form a col-
ored product in the presence of H2O2. As a peroxidase
mimic, CuNPs@C not only has the advantages of low cost,
high stability, and easy preparation, but also follows Michae-
lis–Menten behaviors and shows strong affinity to H2O2. As
the CuNPs’ surfaces are free from stabilizing agent,
CuNPs@C exhibited a higher affinity to H2O2 than horserad-
ish peroxidase. On the basis of the inhibitory effect of ascor-
bic acid (AA) on oxidation of TMB, this system serves as a col-
orimetric method for the detection of AA, suggesting that
the present work would expand the potential applications
of MOF-derived nanocomposites in biomedical fields.
stability and dispersion of nanomaterials.[8,10] Compared with
nanomaterials alone, such nanocomposites usually show im-
proved catalytic activity and/or some new properties owing to
enlarged effective surface area and the synergic effect of nano-
materials and the carbon matrix.[8] Nevertheless, the prepara-
tion of water-soluble carbon matrices and the loading steps of
nanomaterials are complicated and time-consuming. Therefore,
it is highly desirable to explore new nanocomposites with easy
preparation, excellent water-solubility, and high stability.
Introduction
In recent years, nanomaterial-based enzyme mimics have re-
ceived considerable attention due to their advantages over
natural enzymes, such as low cost, excellent stability, and tuna-
ble catalytic activity.[1] Various nanomaterials, including Fe3O4
nanoparticles (NPs),[2] Pt NPs,[3] Co3O4 NPs,[4] carbon dots,[5] CuO
NPs,[6] and single-walled carbon nanotubes,[7] have already
been evaluated as enzyme mimics. However, laborious proce-
dures are often required for the preparation of these nanoma-
terials. Moreover, most of them are unstable and prone to be
aggregated and settled in aqueous solutions, which might
affect their catalytic activity due to the decrease of specific sur-
face area.[8] To improve the stability and dispersion of nanoma-
terials, an often-used way is the introduction of surface ligands.
But, complicated surface modification chemistry is required
and the presence of surface ligands may suppress the catalytic
activity of nanomaterials.[9] Very recently, the emergence of
nanocomposites constructed by loading nanomaterials onto
carbon matrices with large surface area, such as graphene and
carbon nanotube, provides an alternative way to enhance the
Metal–organic frameworks (MOFs) have recently emerged as
fascinating materials due to their tunable nature. The practical-
ly limitless choice of metals and organic bridging ligands af-
fords an essentially infinite number of possible combinations
for diverse applications.[11] To date, MOFs have been explored
extensively in the applications of gas storage, heterogeneous
catalysis, drug delivery, and sensing.[12] Besides, it has also been
proven that MOFs are ideal templates for the construction of
carbons, metal nanoparticles, or their composites under ther-
molysis conditions.[13] Due to the confinement effect of MOFs,
the regular arranged metal nodes can be transformed in situ
into metal nanoparticles during the process of thermolysis-in-
duced carbonization, while the abundant organic bridging li-
gands tend to form highly porous carbon materials. Therefore,
such an in situ approach leads to high dispersion of the metal
nanoparticles throughout carbon matrix with less aggregation.
Since the resultant metal nanoparticles were only stabilized by
the carbon matrix and without the involvement of extra stabi-
lizing agent, the shortcoming of surface oxidation of metal
nanoparticles caused by stabilizing ligands would be over-
come. Therefore, in light of the superiority of MOFs as precur-
sors, many nanocomposites have already been synthesized
[a] Dr. H. Tan, C. Ma, L. Gao, Q. Li, Prof. Y. Song, Dr. F. Xu, Prof. T. Wang,
Prof. L. Wang
Key Laboratory of Functional Small Organic Molecules
Ministry of Education, Key Laboratory of Chemical Biology of Jiangxi Prov-
ince College of Chemistry and Chemical Engineering
Jiangxi Normal University, Nanchang, 330022 (P.R. China)
Fax: (+86)791-88120861
E-mail: hltan@jxnu.edu.cn
lwanggroup@aliyun.com
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404960.
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and shown promising applications in energy and environmen-
tal fields.[14] Nevertheless, rational design and fabrication of
nanocomposites based on MOF precursors are still in an imma-
ture primary stage. Currently, most of the studies on MOF-de-
rived nanocomposites focus mainly on their controllable prep-
aration and exceptional porosity properties, whereas few stud-
ies have centered on their applications as catalysts, especially
in enzyme mimicking processes.[15] To our knowledge, there
has been no report to date of the investigation of MOF-derived
nanocomposites as enzyme mimics.
Scheme 1. Illustration of CuNPs@C catalyzed oxidation of TMB in the pres-
ence of H2O2 and the inhibition effect of AA, as demonstrated by the color
change.
Ascorbic acid (AA) is vitamin C naturally occurring in fruits
and vegetables. As a biological cofactor, it is necessary for
human health and plays an important role in many biochemi-
cal processes. The deficiency or maladjustment of the level of
ascorbic acid is linked to the symptoms of many diseases, such
as cardiovascular disease or cancer.[16] To date, various methods
have been developed for the detection of ascorbic acid, includ-
ing electrochemical,[17] chromatographic,[18] fluorometric,[19] and
chemiluminescence methods.[20] These methods are sensitive
and highly specific, but the requirement of expensive and so-
phisticated instruments limits their further applications. Colori-
metric detection would be a more desirable method due to its
low cost, simplicity, and practicality. In particularly, enzyme-like
nanomaterials-based sensors have already emerged as an im-
portant colorimetric tool owing to their advantages of excel-
lent stability and lower cost over natural enzymes.
ure 1b). By careful observation, it can be seen that these nano-
particles are highly dispersed. Nevertheless, no changes in the
morphology of the octahedron were detected after thermoly-
sis, implying that MOFs exhibit high stability as supports for
the synthesis of metal nanostructures.[13b] To discern the nature
of the nanoparticles, the calcined octahedrons were character-
ized by powder X-ray diffraction (PXRD), energy dispersive X-
ray spectroscopy (EDS), and Fourier-transform infrared (FTIR)
spectroscopy. The calcined octahedron gave rise to three sharp
diffraction peaks at 2q=44, 51, and 748 and an obscure broad
peak at 2q=208 (Figure 1c). As compared with Cu-MOF pre-
cursors, the three sharp peaks are new, and can be assigned to
the (111), (200), and (220) crystal planes of solid copper phase
(PDF card no. 04-0836). The peak at 2q=208 corresponds to
carbon diffraction.[23] The detection of both the Cu phase and
carbon diffraction peaks suggests that the thermolysis prod-
ucts of Cu-MOF precursors may be a hybrid of CuNPs and
carbon matrix. The obvious decrease in the content of organic
ingredients of the hybrid (CuNPs@C) confirms that the over-
whelming majority of Cu-MOF precursors have decomposed
into CuNPs and carbon matrix (see the Supporting Informa-
tion, Figure S1).[14a] Additionally, no peaks of impurity were
found in the PXRD pattern of the CuNPs@C, reflecting that the
as-prepared CuNPs are comprised of a purely Cu phase with-
out the presence of any trace of CuO, which is further support-
ed by the analytical result of FTIR spectrum of CuNPs@C (Fig-
ure 1d).[6] The transmission electron microscopy (TEM) image
of calcined Cu-MOF showed that the pure CuNPs are highly
dispersed in carbon matrix (see the Supporting Information,
Figure S2). The average size of CuNPs is 20.58 nm, which is in
agreement with the result estimated from the diffraction peak
(111) by using the Scherer formula (about 21.14 nm).
Herein, we report the fabrication of a nanocomposite
(CuNPs@C) by employing [Cu3(BTC)2] (BTC=1,3,5-benzene tri-
carboxylate) as a precursor, denoted as Cu-MOF, through
a one-pot thermolysis method, and its application as a perox-
idase mimic for colorimetric detection of AA. Compared with
conventional methods for nanocomposite construction, the
thermolysis method using MOFs as precursors has the advan-
tages of easy preparation and low cost. Meanwhile, the con-
finement effect of MOFs precursors enable the CuNPs to be
highly dispersed in carbon matrix.[21] CuNPs@C is found to ex-
hibit excellent intrinsic peroxidase-like activity and can catalyze
the oxidation of peroxidase substrate 3,3’,5,5’-tetramethylben-
zidine (TMB) to form a blue-colored product in the presence of
H2O2. The kinetic behavior and catalytic mechanism of
CuNPs@C as a peroxidase mimic were further investigated.
Furthermore, it is known to us that AA has some reducibility
due to the existence of phenol hydroxyl group. Thus, the pres-
ence of AA could consume H2O2, and thus suppress the
CuNPs@C-catalyzed oxidation of TMB by H2O2. On the basis of
these findings, a simple colorimetric method was developed
for quantitative analysis of AA (Scheme 1).
The specific surface area and the porosity of CuNPs@C were
then evaluated by employing Brunauer-Emmett-Teller (BET)
and Barrett-Joyner-Halenda (BJH) methods, respectively. From
Figure 2a, the BET surface area of Cu-MOF and CuNPs@C can
be calculated to be 574.85 and 285.03 m2gꢀ1, respectively. The
results indicate that CuNPs@C still remain a porous structure
as well as Cu-MOF precursor, despite that its BET surface area
is smaller than that of Cu-MOF precursor. By contrast, the pore
size of CuNPs@C showed a relatively wider distribution than
that of Cu-MOF precursor (Figure 2b). The average pore size
centers of CuNPs@C and Cu-MOF were calculated to be 6.81
and 2.80 nm, respectively. The changes of CuNPs@C in the BET
surface area and pore size are consistent with previous reports
of using MOFs as precursors to synthesize metal oxide nano-
particles and porous carbon, and might bring about the signifi-
Results and Discussion
The Cu-MOF precursors were prepared by a solvothermal reac-
tion.[22] The morphology of Cu-MOF was tested by scanning
electron microscopy (SEM). From Figure 1a, we can see that
the as-prepared Cu-MOF precursors are typical octahedral
structures with a smooth surface. After treating at 4008C for
2 h under N2 atmosphere, however, the surfaces of the octahe-
dron were covered with lots of nanoscaled particles (Fig-
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Figure 1. SEM images (a and b), PXRD patterns (c) and FTIR spectra (d) of Cu-MOF and CuNPs@C.
cant growth of the CuNPs crystal size through solid-state diffu-
sion and coalescence at high calcination temperature.[24] Con-
sequently, the CuNPs derived from Cu-MOF precursors are em-
bedded into porous carbon matrix during the process of ther-
molysis.
fluorescence can be observed with increased concentrations of
CuNPs@C (see the Supporting Information, Figure S3). The
result confirms the production of ·OH by the system of H2O2
and CuNPs@C. Therefore, the nature of peroxidase-like activity
of CuNPs@C for the oxidation of TMB in the presence of H2O2
can be attributed to the generation of ·OH from the decompo-
sition of H2O2.
The peroxidase-like activity of CuNPs@C was tested for the
oxidation of TMB in the presence of H2O2. TMB itself is color-
less and displays no absorbance, even in the presence of H2O2
(Figure 3). However, upon the addition of CuNPs@C, the TMB
solution exhibited a deep blue color and two maximum ab-
sorption peaks at 370 and 652 nm. These results reveal that
CuNPs@C exhibits peroxidase-like catalytic ability and can con-
vert TMB to a colored product in the presence of H2O2 (see the
Supporting Information, Scheme S1). To demonstrate the cata-
lytic mechanism of CuNPs@C, we then studied the fluores-
cence changes of terephthalic acid (TA) in the presence of
H2O2 and CuNPs@C. TA is a highly sensitive and selective fluo-
rescent probe for hydroxyl radicals (·OH). TA itself is nonfluor-
escent, but it can easily react with ·OH and convert to highly
fluorescent 2-hydroxyterephthalic acid (see the Supporting In-
formation, Scheme S2).[25] The fluorescence of TA was switched
on upon the introduction of H2O2 and CuNPs@C and enhanced
The absorbance change of oxidized TMB at 652 nm against
the concentrations of CuNPs@C is time-dependent (see the
Supporting Information, Figure S4). The absorbance enhanced
with the increase of CuNPs@C concentrations, which further
confirms the catalytic property of CuNPs@C as a peroxidase
mimic for the oxidation of TMB. The absorbance changes of
oxidized TMB were then measured at different pH (from pH 2
to pH 12), reaction temperature (from 258C to 608C), and
CuNPs@C amounts. The maximum absorbance of oxidized
TMB was found at pH 4.0, 458C, and 50 mg of CuNPs@C, re-
spectively (see the Supporting Information, Figure S5). Under
the optimal conditions, the absorbance of oxidized TMB en-
hanced with increasing concentration of H2O2 up to 8 mm, and
then remained almost constant beyond 8 mm of H2O2
(Figure 4). This implies that excess H2O2 had no influence on
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Figure 4. Dependence of the absorbance of oxidized TMB at 652 nm on the
concentration of H2O2 at 0–100 mm concentration. Inset: The corresponding
linear calibration plot.
limit is lower than that of other peroxidase mimics based on
CuS NPs (0.12 mm),[25] CoFe layered double hydroxide nano-
plates (0.4 mm),[26b] Fe3O4 NPs (3 mm),[27] and gold NPs
(0.5 mm).[28] The lower detection limit may ascribe to the fact
that the CuNPs surfaces were free from stabilizing/conjugation
agents.
The steady-state kinetic experiments were performed to fur-
ther investigate the catalytic property of CuNPs@C as a perox-
idase mimic. In a certain range of substrate concentration, the
plots of initial rate versus TMB and H2O2 concentrations both
follow typical Michaelis–Menten behaviors (Figure 5). The maxi-
mum initial velocity (Vm) and Michaelis–Menten constant (Km)
were calculated from Lineweaver–Burk plots (see the Support-
ing Information, Table S1). It is known that Km is an indicator of
enzyme affinity to substrates and a smaller Km value reflects
a stronger affinity of enzyme for substrate and vice versa.[10a] In
this case, the Km value for CuNPs@C with TMB as substrate
was 1.65 mm, which is higher than that of 0.434 mm for horse-
radish peroxidase (HRP),[2] suggesting that CuNPs@C had lower
affinity for TMB than HRP. In contrast, the Km value of
CuNPs@C with H2O2 (1.89 mm) as substrate is lower than that
for HRP (3.7 mm),[2] indicating that CuNPs@C have higher affini-
ty for H2O2 than HRP and that the requirement of CuNPs@C
for H2O2 concentration is lower than HRP when the maximum
activity was obtained.[10a] Moreover, the decreased Km value
also implies that CuNPs@C exhibits an improved catalytic ac-
tivity toward H2O2 as compared to HRP.[6]
Figure 2. N2 adsorption–desorption isotherms (a) and pore size distribution
(b) of Cu-MOF and CuNPs@C.
Figure 3. Absorption spectra of TMB (a), TMB in the presence of H2O2 (b),
and the mixture of H2O2 and CuNPs@C (c). Inset: The corresponding photo-
graphs of the three samples.
The inhibitory effect of AA on the CuNPs@C-catalyzed oxida-
tion of TMB is shown in Figure S6 (see the Supporting Informa-
tion). In the presence of H2O2 alone, typical absorption peaks
and a deep blue color of oxidized TMB can be observed for
the solution containing CuNPs@C and TMB. Upon the addition
of H2O2 and AA, however, the absorbance of the solution de-
creased drastically and its corresponding solution color
changed from deep blue to very light blue. Moreover, no ab-
sorbance could be observed from the solution contained AA
alone, indicating that AA is not capable of oxidizing TMB.
Therefore, the AA-induced changes in the absorbance and so-
lution color of oxidized TMB can be attributed to the reaction
the peroxidase-like activity of CuNPs@C, in contrast to natural
HRP, which was usually inhibited by H2O2 at high concentra-
tion.[26] Since the absorbance of oxidized TMB is highly depen-
dent on H2O2 concentration, the method could be used to
quantitatively determine H2O2 concentration. Based on the per-
oxidase-like activity of CuNPs@C, a linear relationship was ob-
tained between the absorbance of oxidized TMB and the H2O2
concentration ranging from 1 mm to 1 mm (Figure 4, inset). The
detection limit was calculated to be 64.12 nm. This detection
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Figure 6. a) Effects of AA on the absorption spectra of the TMB–H2O2–
CuNPs@C system; b) linear calibration plots of A0Aꢀ1 against AA concentra-
tions (b). Inset: Color changes of TMB–H2O2–CuNPs@C system in the pres-
ence of AA with different concentrations. A0 and A represent the absorbance
of oxidized TMB in the absence and presence of AA, respectively.
Figure 5. Steady-state kinetic assays of the CuNPs@C. a) The concentration
of TMB was 840 mm and H2O2 concentration was varied; b) The concentra-
tion of H2O2 was 8 mm and the TMB concentration was varied. Insets: Line-
weaver–Burk plots of the double reciprocal of the Michaelis–Menten equa-
tion.
ance responses of the TMB–H2O2–CuNPs@C system to amino
acids, carbohydrates, and the substances that may be used as
excipients to accelerate drug absorption. Only the addition of
AA caused the decrease in absorbance and no remarkable ab-
sorbance changes were observed upon the addition of inter-
ferential substances (Figure 7). To further assess the potential
effects of the interferential substances on the detection of AA,
the absorbances of the TMB–H2O2–CuNPs@C system alongside
AA and different interferential substances were measured, re-
spectively. The results (gray bars in Figure 7) indicated that the
presence of these interferential substances has virtually no in-
fluence on the detection of AA and further demonstrated the
high specificity of the TMB–H2O2–CuNPs@C system for AA de-
tection. Therefore, the TMB–H2O2–CuNPs@C system can be
used to detect AA in tablets.
between H2O2 and AA, which leads to the suppression of the
TMB oxidation process catalyzed by CuNPs@C. On the basis of
these observations, a colorimetric method for the detection of
AA could be readily developed. The absorbance of oxidized
TMB decreased with the increase of the concentration of AA
(Figure 6a). There is a good linear relationship between the ab-
sorbance of oxidized TMB and AA concentrations in range of
10 mm to 1 mm (Figure 6b). The detection limit is estimated to
be 1.41 mm. Compared with other nanostructure-based detec-
tion methods for AA, the proposed colorimetric method not
only showed a very comparable detection limit (see the Sup-
porting Information, Table S2), but also possessed the advan-
tages of simple and low-cost detection procedures. Additional-
ly, AA-induced absorbance changes can be observed by naked
eyes (Figure 6a, inset). Therefore, the TMB–H2O2–CuNPs@C
system appears to be a useful colorimetric method for AA de-
tection.
The practicability of the presented method was examined
by determining the concentration of vitamin C in commercial
tablets. The standard addition method was used to detect the
level of AA in commercial tablets (see the Supporting Informa-
tion, Table S3). The recoveries for AA measurement at three
spiked samples ranged from 96.0 to 117.33%. The relative stan-
dard deviation (RSD, n=3) values at each concentration level
were less than 5.11%. The results indicate that the presented
TMB–H2O2–CuNPs@C system can be used to detect vitamin C
As a powerful water-soluble antioxidant, AA has been used
for the prevention and treatment of the common cold, cardio-
vascular disease, and cancer.[29] Therefore, the analysis of AA
content in tablets is a very important task. Accordingly, to eval-
uate its selectivity for AA detection, we investigated absorb-
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Peroxidase-like activity of CuNPs@C
The peroxidase-like activity of CuNPs@C was examined by using
CuNPs@C (50 mg) in NaOAc buffer (10 mm, pH 4.0) in the presence
of TMB (840 mm) as substrate, or H2O2 (8 mm) and a total reaction
volume of 500 mL. The catalytic reaction of CuNPs@C was carried
out for 20 min at 458C and monitored by a spectrophotometer.
The concentrations of oxidized TMB were quantified by UV/Vis ab-
sorption spectroscopy at 652 nm by using a molar extinction coef-
ficient of e=3.9ꢁ104 mꢀ1 cmꢀ1 [30]
The kinetic analysis of perox-
.
idase-like activity of CuNPs@C was studied by monitoring the ab-
sorbance with a 3 min interval after changing the concentration of
TMB and H2O2. The Michaelis–Menten constant was calculated by
using Lineweaver–Burk plots of the double reciprocal of the Mi-
1
n
Km 1
Vm ½Sꢁ Km
chaelis–Menten equation,
= ( +
1 ), in which n is the initial ve-
locity, Vm represents the maximal reaction velocity, [S] corresponds
to the concentration of substrate and Km is the Michaelis constant.
Figure 7. Absorbance response of the TMB–H2O2–CuNPs@C system to AA
(2 mm) and interferential substances (2 mm). Black and gray bars represent
the addition of interferential substance and AA together, respectively.
Detection of AA
The detection of AA was performed according to the following
steps: Firstly, H2O2 (4 mL, 1m) and different amounts of AA with
final concentrations from 0 to 2 mm were mixed in NaOAc buffer
solution (10 mm, pH 4.0). Then, CuNPs@C suspension (50 mL,
1 mgmLꢀ1) and TMB (10 mL, 42 mm) dissolved in absolute alcohol
were added to above reaction mixtures. The absorption spectra of
the reaction solutions were measured after incubating for 20 min
at 458C.
in tablets and this method has no obvious system error of de-
tection.
Conclusion
In summary, CuNPs@C has been successfully prepared by
using Cu-MOF as precursor under thermolysis conditions. We
have for the first time demonstrated that CuNPs@C exhibits
peroxidase-like activity and can catalyze the oxidization of TMB
by H2O2 with high efficiency. As a peroxidase mimic, CuNPs@C
not only has the advantageous features of low cost, high sta-
bility, and easy preparation, but also follows Michaelis–Menten
behavior and shows strong affinity to H2O2. Because the
CuNPs surfaces were free from stabilizing/conjugation agents,
CuNPs@C exhibited a higher catalytic activity to H2O2 than
HRP. On the basis of the inhibitory effect of AA on the perox-
idase-like activity of CuNPs@C, a simple colorimetric method
for AA detection was developed. The proposed AA detection
method exhibited high sensitivity and was successfully applied
to detect the level of AA in tablets. We believe that the pres-
ent work could improve the understanding of catalytic behav-
iors of other MOF-derived nanocomposites as enzyme mimics
and find wider application in bioanalysis.
For the detection of AA in tablets, a standard addition method was
used and the tablet sample was prepared as follows: Five tablets
of commercial vitamin C labeled with 100 mg per tablet were
finely powdered and dissolved in 0.01m HCl (100 mL) and then fil-
tered through an ordinary filter paper and washed with ultrapure
water. The filtrate and washings were collected in a beaker and di-
luted to 500 mL with water. An aliquot of vitamin C solution was
diluted for analysis to a final concentration of vitamin C 250 mm. Fi-
nally, different amounts of thiamine standard solutions were added
to the as-prepared vitamin C solution to obtain spiked tablet sam-
ples. The vitamin C concentrations in the tablet samples were de-
termined by using same procedures mentioned above.
Acknowledgements
This work was supported by the Natural Science Foundation of
China (No. 21305054), start-up funding of Jiangxi Normal Uni-
versity, Specialized Research Fund for the Doctoral Program of
Higher Education of China (No. 20133604120002), and the Sci-
entific Research Foundation of Education Commission of Jiang-
xi Province (No. GJJ14258).
Experimental Section
Preparation of CuNPs@C nanocomposite
Keywords: ascorbic acid
·
biosensors
·
metal–organic
frameworks · nanocomposites · peroxidase mimics
To prepare the CuNPs@C nanocomposite, Cu-MOF precursor
[Cu3(BTC)2] was first synthesized according to a previously reported
method.[22] Then, the as-prepared Cu-MOF precursors were heated
from room temperature to 4008C with a heating rate of 58C minꢀ1
under N2 atmosphere. The materials were maintained at a target
temperature of 4008C for 2 h. After cooling down to room temper-
ature (258C, as below), the final product was obtained as a black-
colored powder. For further details of the chemicals and instru-
ments used for the preparation and characterization of CuNPs@C
nanocomposites, see the Supporting Information.
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Received: August 22, 2014
Published online on && &&, 0000
Chem. Eur. J. 2014, 20, 1 – 8
www.chemeurj.org
7
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
FULL PAPER
Sensors and sensitivity: A nanocompo-
site consisting of copper nanoparticles
dispersed within a carbon matrix
(CuNPs@C) is prepared through a one-
pot thermolysis of copper-based metal–
organic framework precursors.
&
Colorimetric Sensing
H. Tan,* C. Ma, L. Gao, Q. Li, Y. Song,
F. Xu, T. Wang, L. Wang*
&& – &&
Metal–Organic Framework-Derived
Copper Nanoparticle@Carbon
Nanocomposites as Peroxidase Mimics
for Colorimetric Sensing of Ascorbic
Acid
CuNPs@C catalyzes the oxidation of
3,3’,5,5’-tetramethylbenzidine to form
a colored product in the presence of
H2O2. The inhibitory effect of ascorbic
acid on the reaction enables the use of
CuNPs@C as a colorimetric sensor (see
figure).
&
&
Chem. Eur. J. 2014, 20, 1 – 8
www.chemeurj.org
8
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!