Enhanced peroxidase-like activity of porphyrin functionalized ZnFe2O4 hollow nanospheres for rapid detection of H2O2 and glucose

Article abstract of DOI:10.1039/c8nj00720a

Using a simple one-pot solvothermal method, binary metal oxide, magnetic and hollow ZnFe₂O₄ nanospheres functionalized with 5,10,15,20-tetrakis(4-carboxylphenyl)-porphyrin (Por-ZnFe₂O₄ HSs) were prepared and subsequently applied as a substitute for natural peroxidase. The Por-ZnFe₂O₄ HSs were characterized using powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). In comparison with bare ZnFe₂O₄ HSs, Por-ZnFe₂O₄ HSs exhibit enhanced intrinsic peroxidase-like activity. They demonstrate a fast colorimetric response to H₂O₂ with a detection limit as low as 0.86 μM. The catalytic mechanism of the system was proved to be a hydroxyl radical mechanism using electron spin resonance (ESR). By combining the Por-ZnFe₂O₄ HSs-catalyzed reaction and the enzymatic oxidation of glucose with glucose oxidase, a colorimetric platform for glucose detection was established. The Por-ZnFe₂O₄ HSs/glucose oxidase (GOx)/TMB system shows a good response toward glucose with a detection limit of 5.5 μM. Therefore, such nanocomposites have promising applications in biomimetic catalysis and environmental monitoring.

Full text of DOI:10.1039/c8nj00720a

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Enhanced peroxidase-like activity of porphyrin
functionalized ZnFe2O4 hollow nanospheres for
rapid detection of H2O2 and glucose†
Cite this: New J. Chem., 2018,
42, 18189
Bing Bian,ab Qingyun Liu *b and Shitao Yu*a
Using a simple one-pot solvothermal method, binary metal oxide, magnetic and hollow ZnFe2O4
nanospheres functionalized with 5,10,15,20-tetrakis(4-carboxylphenyl)-porphyrin (Por–ZnFe2O4 HSs)
were prepared and subsequently applied as a substitute for natural peroxidase. The Por–ZnFe2O4 HSs
were characterized using powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission
electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). In comparison with bare ZnFe2O4
HSs, Por–ZnFe2O4 HSs exhibit enhanced intrinsic peroxidase-like activity. They demonstrate a fast colori-
metric response to H2O2 with a detection limit as low as 0.86 mM. The catalytic mechanism of the system
was proved to be a hydroxyl radical mechanism using electron spin resonance (ESR). By combining the
Por–ZnFe2O4 HSs-catalyzed reaction and the enzymatic oxidation of glucose with glucose oxidase, a colori-
metric platform for glucose detection was established. The Por–ZnFe2O4 HSs/glucose oxidase (GOx)/TMB
system shows a good response toward glucose with a detection limit of 5.5 mM. Therefore, such nano-
composites have promising applications in biomimetic catalysis and environmental monitoring.
Received 12th February 2018,
Accepted 4th October 2018
DOI: 10.1039/c8nj00720a
rsc.li/njc
carbon-based nanosheets,23–25 core–shell nanoparticles,26–28
alloys29,30 and organic–inorganic nanocomposites31,32 have
1. Introduction
Due to their ease of preparation, tunable catalytic activity and been developed as peroxidase mimetics. Despite the great
high stability under harsh conditions, artificial enzymes, which progress, developing novel nanocomposites with improved
function as good substitutes for natural enzymes, have been peroxidase-like activity for biological applications is still
extensively applied in catalysis and bioassays in the past challenging.
decade.1 Nanomaterials have many unique properties such as
Among the various binary metal oxides, magnetic zinc ferrite
large surface-to-volume ratio, high catalytic efficiency and high magnetite (ZnFe2O4) has attracted enormous interest because
surface reaction activity. The pioneering work of Gao et al.,2 of its outstanding properties including high stability, good
who discovered the intrinsic peroxidase activity possessed by dispersibility, ease of separation, high catalytic efficiency, and
magnetic Fe3O4 nanoparticles, has paved a new way for exploiting low toxicity of zinc ions.33–35 Additionally, ZnFe2O4 nano-
novel enzyme mimetics. Since then, various nanoscale materials, particles have intrinsic peroxidase-like catalytic activity and
such as metals,3–6 metal oxides,7–9 metal sulfides,10–13 carbon have been successfully used for the colorimetric detection of
nanomaterials14–18 and organic compounds19–22 have been glucose in urine.36 Unfortunately, the poor separation effi-
reported to possess intrinsic enzymatic activity. Many single- ciency of electrons and holes in bare ZnFe2O4 nanoparticles
component catalysts usually demonstrate low catalytic effi- causes low catalytic activity. To solve this problem, one efficient
ciency, hampering their application in catalysis and biological solution is to further functionalize the ZnFe2O4 nanoparticles
fields. Subsequently, multi-component nanocomposites emerged and harness the synergistic effect between the different
with enhanced catalytic activity from the synergistic effect of their components to achieve high catalytic activity.
different components. Recently, hybrid nanocomposites such as
Porphyrins, which have two-dimensional 18 p-electron aro-
matic macrocycle structures, can be synthetically modified by
introducing various functional groups or metal ions, leading
to unique optical and electronic properties.37–39 Therefore,
porphyrins can be adopted to functionalize nanostructures
applied in solar energy conversion and photocatalysis.40,41
Numerous studies involving porphyrin42 and porphyrin func-
a College of Chemical Engineering, Qingdao University of Science and Technology,
Qingdao 266042, China. E-mail: yushitaoqust@126.com; Fax: +86 532 84022719;
Tel: +86 532 84022719
b College of Chemical and Environmental Engineering, Shandong University of
Science and Technology, Qingdao 266510, China. E-mail: qyliu@sdust.edu.cn;
Fax: +86 0532 80681197; Tel: +86 0532 86057757
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nj00720a tionalized nanomaterials43 have been reported in the fields of
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biocatalysis and biosensors. Ju’s group have synthesized free- was carried out using a Rigaku D/Max2500PC powder X-ray
base-porphyrin-functionalized ZnO nanoparticles on indium diffractometer with Cu Ka radiation, a 2y range of 5–801 and a
tin oxide for the detection of cysteine.44 Our group previously scan rate of 51 minꢁ1. A JEM-2100 transmission electron
reported that porphyrin functionalized monometal oxides, NiO, microscope (JEOL, Japan) and a Nova Nano SEM450 scanning
Fe3O4 and CeO2,45 and metal sulfides ZnS and CdS46 demon- electron microscope (FEI, USA) were used to examine the
strated enhanced peroxidase-like activity to detect H2O2 and morphology of the Por–ZnFe2O4 HSs. UV-vis absorption spectra
glucose. Nevertheless, to the best of our knowledge, the use were recorded on a UV-3200PC spectrophotometer (Shanghai,
of porphyrin functionalized binary metal oxide, ZnFe2O4, as a China). Fourier transform infrared (FT-IR) spectra were
peroxidase mimic has not been reported yet.
recorded on a NICOLET 380 FT-IR spectrometer (Nicolet
In this work, 5,10,15,20-tetrakis(4-carboxylphenyl)-porphyrin Thermo, USA) in the range of 4000–500 cmꢁ1 using a KBr disk
functionalized hollow ZnFe2O4 nanospheres (Por–ZnFe2O4 HSs) method. X-ray photoelectron spectroscopy (XPS) analysis
were fabricated through a convenient one-pot hydrothermal was used to confirm the elemental composition using a PHI
process. The molecular structure of 5,10,15,20-tetrakis(4-carboxyl- Quantum 2000 Scanning ESCA Microprobe spectrometer with
phenyl)-porphyrin is shown in Fig. S1 (ESI†). The morphology and an Al Ka incident X-ray beam. The X-ray source was operated at
structure of the nanocomposites were systematically investigated. 35 W, and the spectra were recorded at 15 kV. The analysis
Owning to the synergistic effect of porphyrin and ZnFe2O4, the chamber pressure was 5 ꢂ 10ꢁ8 Pa. The electron spin resonance
Por–ZnFe2O4 HSs exhibited enhanced peroxidase-like activity (ESR) measurements were carried out with a Bruker ESP-300E
and rapidly catalyzed the colorless substrate 3,30,5,50-tetramethyl- spectrometer operating in the X-band at room temperature.
benzidine (TMB) into a blue product (oxTMB), observed by the
2.2. Preparation of magnetic Por–ZnFe2O4 HSs
naked eye in only 3 min (Scheme 1). The catalytic mechanism
of Por–ZnFe2O4 HSs was also investigated using electron spin
resonance. Meanwhile, a rapid, sensitive and selective colori-
metric method based on Por–ZnFe2O4 HSs for H2O2 and
glucose determination was established.
In a typical process, hollow magnetic ZnFe2O4 nanospheres
functionalized with porphyrin were synthesized through a one-
step hydrothermal method. Briefly, ZnCl2 (0.34 g, 2.5 mmol)
and FeCl3ꢀ6H2O (1.35 g, 5 mmol) were dissolved in ethylene
glycol (40 mL) to form a clear solution, followed by the addition
of NaAc (3.6 g) and polyethylene glycol 20 000 (1.0 g). The color
of the solution turned from bright yellow to clear dark brown.
After addition of 5 mg 5,10,15,20-tetrakis(4-carboxylphenyl)-
porphyrin, the mixture was stirred vigorously for 30 min. The
mixture was then transferred into a 50 mL teflon-lined auto-
clave and incubated at 200 1C for 8 h. After cooling down, the
obtained black precipitate was separated from the solvent using
a magnet, washed several times with deionized water and
ethanol, and then dried at 60 1C in a vacuum drying oven
for 12 h.
2. Experimental
2.1. Reagents and apparatus
FeCl3ꢀ6H2O and ZnCl2 were purchased from Beichen Fangzheng
Reagent Co. (Tianjin, China). 3,30,5,50-Tetramethylbenzidine
dihydrochloride (TMBꢀ2HCl) was ordered from Solarbio Co.
(Beijing, China). Hydrogen peroxide H2O2 (30 wt%), sodium
acetate (NaAc), glucose, fructose, lactose, maltose and sucrose
were obtained from Guangcheng Reagent Co. (Tianjin, China).
Ethylene glycol and PEG 20 000 were obtained from BASF
Chemical Co., Ltd (Tianjin, China). 5,5-Dimethyl-1-pyrroline-N-
oxide (DMPO) was purchased from J&K Chemical Ltd (Shanghai,
China). Glucose oxidase (GOx, Z200 U mgꢁ1) was purchased from
Sigma-Aldrich and stored in a refrigerator at ꢁ18 1C. All reagents
Bare ZnFe2O4 hollow nanospheres were synthesized according
to the above route without adding porphyrin.
2.3. Peroxidase-like catalytic activity and kinetics analysis
study
were of analytical grade and deionized water was used in all Peroxidase-like catalytic activity and kinetics analysis were
experiments. 5,10,15,20-Tetrakis(4-carboxylphenyl)-porphyrin (Por) evaluated using the catalytic oxidation of 3,30,5,50-tetramethyl-
was synthesized according to a previously reported method.47
benzidine (TMB) in the presence of H2O2 at room temperature.
The samples were subjected to various physical charac- After adding 200 mL Por–ZnFe2O4 HSs stock solution, 200 mL
terization methods. Powder X-ray diffraction characterization 3 mM TMB and 200 mL 0.1 M H2O2 to 1400 mL 0.2 M acetate
Scheme 1 Schematic illustration of the colorimetric detection of glucose by combining GOx and Por–ZnFe2O4 nanospheres for catalyzed reactions.
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buffer solution at pH 4.0 to form a total 2 mL mixture, reactions
were monitored in time-scan mode at 652 nm. Kinetics mea-
surements were carried out in time course mode by monitoring
the absorbance change for 3 min on a UV-3200PC UV-vis
spectrophotometer. The Michaelis–Menten constant was calcu-
lated using Lineweaver–Burk plots: 1/n = Km/Vmax(1/[S] + 1/Km),
where n is the initial velocity, Vmax is the maximum reaction
velocity and [S] is the concentration of the substrate. Km is the
Michaelis–Menten constant, which is an indicator of enzyme
affinity for its substrate.
2.4. Electron spin resonance (ESR)
Fig. 1 XRD patterns of Por–ZnFe2O4 HSs (a) and bare ZnFe2O4 HSs (b).
The catalytic mechanism of the Por–ZnFe2O4 HSs was investi-
gated in the H2O2/UV/DMPO system in the presence and
absence of Por–ZnFe2O4 HSs. Samples containing 18 mM
DMPO, 25 mM H2O2, and different concentrations of the
Por–ZnFe2O4 HSs (0, 60, 100 mg mLꢁ1) were prepared in acetate
buffer (20 mM, pH 4.0). Each sample was UV irradiated at
365 nm for 10 min, transferred to a quartz capillary tube and
placed in the ESR cavity for spectrum recording.
(511) and (440) planes, respectively. Therefore, the charac-
terized material can be assigned as ZnFe2O4 (JCPDS 22-1012)
with a spinel structure. With the introduction of porphyrin
molecules, the XRD pattern shows negligible change, suggesting
that the formation of the nanocomposites does not influence the
crystal structure of raw ZnFe2O4.
In order to determine the composition of the nanocomposites,
XPS was used to further explore the chemical composition and
surface electronic state of the Por–ZnFe2O4 HSs. The narrow-
scanned XPS spectrum indicates the coexistence of Zn, Fe, O, C
and N elements in the nanocomposites. The peak at 1021.2 eV
was ascribed to Zn 2p3/2 (Fig. 2a). Fe 2p signals located at
711.1 eV and 725.2 eV should be attributed to the characteristic
features of Fe 2p3/2 and Fe 2p1/2, respectively (Fig. 2b). Furthermore,
the O 1s spectrum displayed in Fig. 2c shows a peak at 530.0 eV
that could be assigned to typical surface lattice oxygen. The C 1s
(Fig. 2d) spectrum of Por–ZnFe2O4 HSs could be divided into three
peaks, the main peak at 284.80 eV was due to adventitious carbon,
which was used to calibrate binding energies, together with carbon
from the porphyrin molecules. The peaks at 285.8 eV and 288.2 eV
were attributed to the CQN and O–CQO bonds of the porphyrin
molecules, respectively. Fig. 2e shows the spectrum of the N 1s
regions with a peak at a binding energy of 399.7 eV. From the XPS
spectra of C and N from the porphyrin molecules, it can be seen
that porphyrin molecules exist in the Por–ZnFe2O4 HSs.
2.5. H2O2 and glucose detection using Por–ZnFe2O4 HSs
The colorimetric detection of H2O2 was performed as follows:
200 mL TMB (3 mM), 200 mL Por–ZnFe2O4 HSs stock solution
(0.4 mg mLꢁ1) and 200 mL H2O2 with different concentrations
were added into 1400 mL acetate buffer (pH 4.0) and incubated
for 3 min at room temperature. The Por–ZnFe2O4 HSs were
removed from the reaction mixture using an external magnetic
field. UV-vis spectra were recorded by wavelength scanning.
To examine the influences of pH and experimental temperature
on the catalytic activity of Por–ZnFe2O4 HSs, buffer solutions
from pH 2.0 to 9.0 and temperatures from 20 to 60 1C were
investigated.
Glucose detection was carried out in the following steps:
(1) 100 mL of 0.5 mg mLꢁ1 GOx and 500 mL of glucose with
different concentrations in 0.01 M phosphate buffered saline
(PBS, pH 7.0) were incubated at 37 1C for 30 min; (2) 200 mL of
3 mM TMB, 200 mL of the Por–ZnFe2O4 HSs (40 mg mLꢁ1) stock
solution, and 1 mL of acetate buffer (pH 4.0) were added into
the above incubated reaction; (3) the mixture was incubated in
a 37 1C water bath for 10 min, and kept in an ice-water bath for
another 10 min to terminate the reaction; (4) the Por–ZnFe2O4
HSs were then removed from the reaction solution using an
external magnetic field and the final reaction solution was used
to perform the absorption spectroscopy measurements at
652 nm. In control experiments, 5 mM maltose, 5 mM lactose,
5 mM sucrose and 5 mM fructose were used instead of glucose.
The functionalization of the ZnFe2O4 HSs by porphyrin was
verified using the FT-IR spectra, shown in Fig. S2 (ESI†). The
FT-IR of porphyrin (curve a) showed a signal at 1682 cmꢁ1
,
corresponding to the vibration of the carboxylic groups, and at
1423 cmꢁ1, corresponding to the vibration peak of an aromatic acid.
Notably, from curve b, it can be found that the vibration of the
carboxylic groups from the porphyrin molecule shifted to 1590 cmꢁ1
with a broad band, indicating that a coordination interaction
occurred between the carbonyl groups of porphyrin and ZnFe2O4.
The morphology of the samples was analyzed using SEM and
TEM. Fig. 3 shows typical SEM and TEM images of bare ZnFe2O4
HSs and Por–ZnFe2O4 HSs. Clearly, the obtained samples are both
3. Results and discussion
3.1. Characterization of the Por–ZnFe2O4 HSs
XRD was employed to analyze the crystallographic nature of composed of hollow nanospheres with uniform shape and
bare ZnFe2O4 HSs and Por–ZnFe2O4 HSs. Fig. 1 shows the diameters from ca. 100 to 200 nm. Furthermore, it can be seen
characteristic diffraction peaks at 29.91, 35.21, 42.81, 53.11, that the hollow nanospheres were composed of nanoparticles,
56.61 and 62.21, corresponding to the (111), (220), (400), (422), as shown in Fig. 3(a and c). The hollow structure can be clearly
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Fig. 2 XPS survey spectra of the Por–ZnFe2O4 HSs nanocomposites.
seen from a broken sphere, indicated by the arrow in Fig. 3c. From significant blue color. To investigate the peroxidase-like cata-
the SEM and TEM images of both bare ZnFe2O4 HSs and lytic activity of the Por–ZnFe2O4 HSs, catalytic oxidation of
Por–ZnFe2O4 HSs, it can be suggested that the morphology of the chromogenic substrate TMB with Por–ZnFe2O4 HS nano-
the ZnFe2O4 HSs was not affected by the 5,10,15,20-tetrakis(4- composites in the presence or absence of H2O2 were investigated.
carboxylphenyl)-porphyrin, owing to a small quantity of porphyrin With the addition of Por–ZnFe2O4 HSs into the TMB–H2O2
molecules attached on the surface of the hollow nanoparticles.
solution, an obvious change to a blue color occurs (Fig. 4e),
suggesting the formation of Ox TMB catalyzed by Por–ZnFe2O4
HSs. In contrast, in the absence of H2O2, no obvious color
change was observed in the TMB/Por–ZnFe2O4 HSs solution.
3.2. The peroxidase-like catalytic activity of magnetic
Por–ZnFe2O4 HSs
In the presence of H2O2, natural peroxidases can catalyze the All the results suggest that neither H2O2 nor the Por–ZnFe2O4
oxidation of 3,30,5,50-tetramethylbenzidine (TMB) to produce a HSs alone could effectively oxidize TMB. It is the obtained
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Table 1 Time comparison of the color response time of substrates
observed by the naked eye when adding Por–ZnFe2O4 HSs and other
peroxidase mimetics
Catalyst
Reaction time (min)
Ref.
Fe3O4 MNPs
10
5
10
15
15
10
3
8
36
48
49
45
46
ZnFe2O4 MNPs
C-dots/NiAl-LDHs
GO–Fe3O4
H2TCPP–CeO2
H2TCPP–ZnS
Por–ZnFe2O4 HSs
This work
MNPs, ZnFe2O4 MNPs, C-dots/NiAl-LDHs, and GO–Fe3O4,
Por–ZnFe2O4 HSs exhibit the highest catalytic activity.
3.3. Mechanism of the peroxidase-like activity of magnetic
Por–ZnFe2O4 HSs
Generally, the catalytic mechanisms of peroxidase mimics
could be classified into two types. One is from electron transfer,
as seen in Ir and Co3O4.5,7 The other is from hydroxyl radicals,
which is seen in catalysts including Fe3O4, CoFe2O4 and
Fig. 3 (a) SEM and (b) TEM images of ZnFe2O4 HSs; (c) SEM (the arrow
marks a broken hollow sphere) and (d) TEM images of Por–ZnFe2O4 HSs.
Por–ZnFe2O4 nanocomposites that have excellent peroxidase-
like activity.
To compare the catalytic performance with that of bare
ZnFe2O4 HSs, control experiments were performed. As shown
in Fig. 4e, the solution in the presence of Por–ZnFe2O4 HSs,
TMB and H2O2 shows an intense absorption at 652 nm and a
deep color change. In contrast, under identical experimental
conditions, the TMB + H2O2 + ZnFe2O4 HSs system shows slight
color variation (Fig. 4d). The catalytic activity of Por–ZnFe2O4
HSs was about four times higher than that of bare ZnFe2O4 HSs.
The time taken for different colorimetric reactions catalyzed
by nanomaterials has been summarized and listed in Table 1.
Clearly, compared with other nanomaterials such as Fe3O4
Fig. 5 Spin-trapping ESR spectra of ꢃOH radicals in the systems of (A)
H2O2–DMPO, (B) 60 mg mLꢁ1 Por–ZnFe2O4 HSs–H2O2–DMPO and
(C) 100 mg mLꢁ1 Por–ZnFe2O4 HSs–H2O2–DMPO. Reaction conditions:
18 mM DMPO, 25 mM H2O2.
Fig. 4 Time-dependent absorbance changes at 652 nm of TMB in different
reaction systems: (a) TMB + ZnFe2O4 HSs, (b) TMB + Por–ZnFe2O4 HSs,
(c) TMB + H2O2, (d) TMB + ZnFe2O4 HSs + H2O2, and (e) TMB + Por–ZnFe2O4
HSs + H2O2 in a pH 4.0 acetate buffer at 35 1C. Inset are photographs of the
color changes of the different samples.
Scheme 2 Schematic energy level diagram and electron-transfer path
from the porphyrin to ZnFe2O4.
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Fe3(PO4)2ꢀ8H2O.50 Based on previous reports, it is speculated spin trap. Fig. 5A shows four weak characteristic peaks of the
that the excellent intrinsic peroxidase-like activity of Por– typical DMPO–ꢃOH adduct with an intensity ratio of 1 : 2 : 2 : 1.
ZnFe2O4 HSs could be ascribed to the generation of hydroxyl In contrast, strong ESR signals of the spin adduct are observed
radicals (ꢃOH) from H2O2 in the process of the rapid oxidation in Fig. 5B and C. It can be seen that the ESR signal of the spin
of TMB. As a powerful method to characterize the hydroxyl adduct in the presence of 100 mg mLꢁ1 Por–ZnFe2O4 HSs was
radicals, the ESR technique was adopted to verify the genera- stronger than that of 60 mg mLꢁ1 Por–ZnFe2O4 HSs, suggesting
ꢃ
tion of OH with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a that the spin adduct ESR intensity was dependent on the
Fig. 6 The dependence of the peroxidase-like activity of the Por–ZnFe2O4 HSs on (a) pH and (b) temperature in the presence of TMB and H2O2.
The maximum point in each curve is set as 100%.
Fig. 7 Steady-state kinetics assay of Por–ZnFe2O4 HSs. (a) The concentration of TMB was 0.3 mM while the H2O2 was varied; (c) the concentration of
H2O2 was 0.1 M while the TMB was varied; (b) and (d) double reciprocal plots of the activity of Por–ZnFe2O4 HSs at a fixed concentration of one substrate
versus varying concentration of the second substrate for H2O2 and TMB, respectively.
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concentration of Por–ZnFe2O4 HSs. Following the adsorption of and temperature on the catalytic activity of Por–ZnFe2O4 HSs
H2O2 onto the surface of Por–ZnFe2O4 HSs in the catalytic was investigated. By varying the pH from 2.0 to 9.0 and
system, the O–O bond was broken into highly reactive ꢃOH, the temperature from 20 1C to 60 1C, the experiments were
catalyzed by the nanomaterials. The generated ꢃOH oxidized carried out by incubating a solution containing 40 mg mLꢁ1
TMB into the typical blue color. As a result, it was verified that Por–ZnFe2O4 HSs, 300 mM TMB and 0.01 M H2O2 for 3 min.
the peroxidase-like activity of Por–ZnFe2O4 HSs actually originates Por–ZnFe2O4 HSs show pH and temperature-dependent catalytic
from ꢃOH radical generation, as shown in Scheme 2.
activity (Fig. 6), and the optimal pH and temperature for
Our results indicate that the peroxidase-like activity of the peroxidase mimetics were 4.0 and 35 1C, respectively. From
Por–ZnFe2O4 nanocomposites is higher than that of ZnFe2O4 Fig. S3 (ESI†), it was found that the absorption intensity increased
HSs. The reason is attributed to the synergistic effect of with increasing TMB concentration, but no obvious increase was
porphyrin molecules and the ZnFe2O4 HSs, similar to our previous observed when the concentration was higher than 0.3 mM. In the
study on porphyrin functionalized Fe3O4.45b Scheme 2 shows the subsequent experiments, 0.3 mM was suggested as an optimized
synergistic effect in the enhanced peroxidase-like activity of the TMB concentration.
Por–ZnFe2O4 HSs. Firstly, the photo-induced electrons can
be easily transferred from the LUMO of the porphyrin to the
conduction band (CB) of ZnFe2O4 owing to the lower CB level of
3.5. Kinetics analysis
ZnFe2O4. Since the high conductivity of ZnFe2O4 suppresses Under the optimal conditions, the kinetics mechanism of
the direct recombination of photo-induced electron–hole pairs Por–ZnFe2O4 HSs was studied by changing the concentrations
in Por–ZnFe2O4 HSs, the concentration of the active electrons of TMB and H2O2 respectively. The apparent steady-state
in the CB of ZnFe2O4 increased continuously. Therefore, the kinetics data for this catalytic reaction were collected by varying
active electrons could efficiently reduce the H2O2 adsorbed on the concentration of one substrate while keeping the
Por–ZnFe2O4 HSs into hydroxyl radicals to achieve enhanced other constant. Fig. 7 shows typical Michaelis–Menten curves
peroxidase-like activity.
recorded by varying the concentration of TMB or H2O2. It was
observed that the oxidation reaction catalyzed by the Por–
ZnFe2O4 HSs follows typical Michaelis–Menten behavior
towards both TMB and H2O2 (Fig. 7a and c). The Lineweaver–
Burk double reciprocal plots (Fig. 7b and d) show a good linear
relationship between 1/v and 1/[S].
3.4. Optimization of experimental conditions
Like other nanomaterial-based peroxidase mimetics and HRP,
the catalytic activity of Por–ZnFe2O4 HSs was also dependent on
the pH and temperature. Consequently, the effect of pH value
Km is commonly identified as an indicator of enzyme affinity
for substrates and a smaller value of Km means a stronger
affinity between the enzyme and the substrate and vice versa.
The Km values of Por–ZnFe2O4 HSs with H2O2 and TMB as
substrates were calculated to be 0.045 mM and 0.026 mM,
respectively. For comparison, Table 2 lists the Km and Vmax
values of Por–ZnFe2O4 HSs, HRP and other nanomaterials.
Clearly, it can be seen that the Km values of Por–ZnFe2O4 HSs
with both H2O2 and TMB as substrates were much smaller than
those for reported HRP,2 Fe3O4 MNPs,2 ZnFe2O4 MNPs36 and
MoS2–Pt74Ag26,51 suggesting that Por–ZnFe2O4 HSs exhibited
Table 2 Comparison of the apparent Michaelis constant (Km) and max-
imum reaction rate (Vmax
)
Km [mM]
Vmax [10ꢁ8 msꢁ1
]
H2O2
TMB
H2O2
TMB
Catalyst
Ref.
HRP
3.7
154
1.66
0.386
0.045
0.434
0.098
0.85
25.71
0.026
8.71
9.78
7.74
3.22
1.4
10.00
3.44
13.31
7.29
2
2
36
51
Fe3O4 MNPs
ZnFe2O4 MNPs
MoS2–Pt74Ag26
Por–ZnFe2O4 HSs
2.88
This work
Fig. 8 (a) A dose–response curve for H2O2 detection using Por–ZnFe2O4 HSs as catalysts. (b) A linear calibration plot for H2O2 detection. Error bars
represent the standard deviation based on three repeated measurements.
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Table 3 Comparison of the linear range and LOD of this work with other materials as peroxidase mimics for the colorimetric detection of H2O2 and
glucose
Linear range (mM) for
LOD (mM) for
H2O2 detection
Glucose detection
H2O2 detection
Glucose detection
Peroxidase mimics
Ref.
Fe3O4 MNPs
5–100
20–50
18–1800
10–1000
5–600
50–1000
100–2000
0.16–1600
100–2000
80–1200
6–90
3
30
50
8
100
30
5.5
8
52
53
4
Cu2(OH)3Cl–CeO2
MnSe-g-C3N4 nanosheets
Cu NCs
Dichlorofluorescein
Por–ZnFe2O4 HSs
10
1.8
10
2
21
1–10
0.86
This work
Fig. 9 (a) A dose–response curve for glucose detection using Por–ZnFe2O4 HSs as peroxidase mimics and (b) the linear calibration plot for glucose. The
error bars represent the standard deviation of three measurements.
higher affinity toward both substrates than natural peroxidase HRP 6 to 90 mM (R2 = 0.998) with a detection limit as low as 5.5 mM. The
and the nanomaterials as peroxidase mimics listed in Table 2.
comparative results indicate great potential for the rapid and
sensitive analysis of glucose. In addition, the small error bars of
the data show the high working reliability and good repeatability
of the colorimetric method.
To further test the specificity of the proposed colorimetric
method for the detection of glucose, other saccharides, such as
maltose, lactose, sucrose and fructose, were used to replace glucose.
Fig. 10 shows that the absorbance at 652 nm of oxidized TMB with
glucose is obviously higher than that of other glucose analogs,
3.6. Detection of H2O2 and glucose
Based on the intrinsic peroxidase-like activity of Por–ZnFe2O4
HSs, we designed a simple colorimetric method for the deter-
mination of H2O2 and glucose. Since the absorbance at 652 nm
of oxidized TMB is H2O2 concentration-dependent, it is
straightforward to detect H2O2 using a UV-vis spectrophot-
ometer or only the naked eye. Fig. 8a shows a H2O2 concen-
tration–response curve in the range from 1 to 300 mM. A linear
relationship between the absorbance and H2O2 concentration
from 1 to 10 mM (R2 = 0.996) was obtained with a detection limit
of 0.86 mM (Fig. 8b). Furthermore, compared to other nano-
materials with peroxidase-like activity shown in Table 3, this
method exhibits a reasonable linear range and a lower detec-
tion limit for H2O2.
Since H2O2 is the main product of the GOx-catalyzed reaction
of glucose oxidation, the colorimetric detection of glucose can
be realized when replacing HRP with the Por–ZnFe2O4 HSs.
Considering the instability of GOx in pH 4.0 buffer solution,
glucose detection can be performed as follows: (1) glucose is
catalyzed by GOx to produce H2O2 and glucose acid in a pH 7.0
buffer solution; (2) Por–ZnFe2O4 HSs catalyze the oxidation of
TMB in the presence of H2O2 at pH 4.0 to produce a blue colored
mixture. Fig. 9 shows the standard response curve and the
corresponding calibration plot for glucose. Compared with other
materials shown in Table 3, this assay shows a linear range from
Fig. 10 Selectivity analysis for glucose detection by monitoring the
absorbance at 652 nm with the colorimetric method using GOx and the
as-prepared the Por–ZnFe2O4 HSs (from left to right: 1 mM glucose, 5 mM
maltose, 5 mM lactose, 5 mM sucrose, and 5 mM fructose). Inset: The color
change for the different samples.
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Fig. 11 Relative catalytic activity of the Por–ZnFe2O4 HSs after incubation at a range of temperatures (10–90 1C) (a) and a range of pH values (2.0–9.0)
(b) for 2 h. The maximum point in each curve is set as 100%.
HSs were collected using a magnet and after being washed with
deionized water, the Por–ZnFe2O4 HSs samples were reused in the
next cycle. From the data shown in Fig. 12, it can be concluded that
only a slight decline in relative activity was observed after 5 cycles.
In conclusion, the good stability and reusability of the Por–ZnFe2O4
HSs is suitable for wide use in the biochemistry and environmental
protection fields.
4. Conclusion
In summary, we have reported the preparation of porphyrin
functionalized magnetic ZnFe2O4 HSs through a facile one-pot
hydrothermal process. They could rapidly catalyze the oxida-
tion of TMB by H2O2 to produce a typical blue colored solution
and the catalytic activity was strongly dependent on pH and
temperature. As a peroxidase mimic, the nanocomposites show
several advantages over natural enzymes and other nanozymes.
Furthermore, owing to the synergistic effect of porphyrin
Fig. 12 Relative catalytic activity of the Por–ZnFe2O4 HSs in 5 successive
catalysis cycles.
although the concentration of these analogs is about 5 times higher
than that of glucose. This indicates that the proposed method is
extremely selective for glucose. The detection of glucose can be
realized conveniently through the color change that is observable
by the naked eye, and the good selectivity of Por–ZnFe2O4 HSs
towards glucose is confirmed.
ꢃ
molecules and ZnFe2O4 HSs, the OH radicals played a major
role in the peroxidase mimetic catalytic reaction, as indicated
by the ESR results. The synthesized organic–inorganic nano-
composite based glucose colorimetric method is simple, cheap,
convenient, highly selective, sensitive, and easy to handle.
Based on our investigations, it is expected that great potential
applications of the nanocomposites may be found in bio-
technology, environmental chemistry and other related fields.
3.7. Robustness of the peroxidase-like activity of Por–ZnFe2O4 HSs
For practical applications, the stability of peroxidase mimetics
over broad pH and temperature ranges is of great importance.
Therefore, we herein investigated the robustness of Por–
ZnFe2O4 HS nanocomposites. Por–ZnFe2O4 HSs were incubated
at different temperatures (10–90 1C) and pH values (2.0–9.0) for
2 h and their activities were then measured. Fig. 11a shows that
the activity of Por–ZnFe2O4 HSs remains stable over a wide
temperature range from 10 1C to 90 1C. When incubated in
solutions with different pH values, the maximum activity of
Por–ZnFe2O4 HSs exhibits no significant change (Fig. 11b). This
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
is in contrast to HRP where, after being incubated at a pH lower Financial support provided for this research by the National
than 5.0 or a temperature higher than 40 1C for 2 h, the catalytic Natural Science Foundation of China (Grant No. 21271119), the
activity decreased sharply.
Natural Science Foundation of Shandong Province (Grant
To investigate the recyclability of Por–ZnFe2O4 HSs, experiments No. ZR2018MB002 and ZR2018MEE003) and the Taishan
were performed by conducting the peroxidase mimetic experi- Scholars Projects of Shandong (No. ts201511033) is gratefully
ments successively for 5 cycles. After each cycle, the Por–ZnFe2O4 acknowledged.
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