Morphology-and pH-dependent peroxidase mimetic activity of nanoceria

Article abstract of DOI:10.1039/c8ra00622a

The peroxidase mimetic properties of nanoceria have attracted extensive attention in recent years. In this work, the peroxidase mimetic properties of CeO₂ nanocrystals with different morphologies, namely, nanocubes and nanorods, were investigated. Two types of oxidative species, HO radicals and peroxide-like intermediates, were identified in the CeO₂/H₂O₂ systems. The formation of these oxidative species is strongly dependent on the pH value and the morphology of the CeO₂ nanocrystals. The origin of the peroxidase mimetic activity of nanoceria was mainly ascribed to the presence of HO under acidic conditions, whereas the peroxide-like species also played a major role under neutral and basic conditions. CeO₂ nanorods with excellent redox properties and higher concentration of Ce³⁺ and oxygen vacancies were more favorable for the generation of both HO and peroxide-like intermediates than that of CeO₂ nanocubes, exhibiting excellent peroxidase mimetic activity toward 2,2′-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), methylene blue (MB), and congo red (CR) in the presence of H₂O₂.

Full text of DOI:10.1039/c8ra00622a

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Morphology- and pH-dependent peroxidase
mimetic activity of nanoceria†
Cite this: RSC Adv., 2018, 8, 11764
Xiaoshu Wei, Xiaofeng Li, Yuqian Feng and Sen Yang
*
The peroxidase mimetic properties of nanoceria have attracted extensive attention in recent years. In this
work, the peroxidase mimetic properties of CeO nanocrystals with different morphologies, namely,
nanocubes and nanorods, were investigated. Two types of oxidative species, HOc radicals and peroxide-
like intermediates, were identified in the CeO /H systems. The formation of these oxidative species is
strongly dependent on the pH value and the morphology of the CeO nanocrystals. The origin of the
2
2
2 2
O
2
peroxidase mimetic activity of nanoceria was mainly ascribed to the presence of HOc under acidic
conditions, whereas the peroxide-like species also played a major role under neutral and basic
3+
conditions. CeO
2
nanorods with excellent redox properties and higher concentration of Ce
and
Received 21st January 2018
Accepted 21st March 2018
oxygen vacancies were more favorable for the generation of both HOc and peroxide-like intermediates
0
than that of CeO
2
nanocubes, exhibiting excellent peroxidase mimetic activity toward 2,2 -azino-bis(3-
DOI: 10.1039/c8ra00622a
ethylbenzothiazoline-6-sulfonic acid) (ABTS), methylene blue (MB), and congo red (CR) in the presence
of H
rsc.li/rsc-advances
2 2
O .
oxidation ability of nanoceria to some brown peroxide-like
1
Introduction
17–20
intermediates,
which oxidize organic substances through
17,21,22
Nanoceria has been extensively used in the eld of catalysis intermolecular rearrangement.
Although the exact struc-
ture of the active oxidative species responsible for the peroxi-
3
+
4+ 1–3
because of the facile redox cycles between Ce and Ce .
Among various catalytic applications of nanoceria, one inter- dase mimetic activity of nanoceria have not been fully
esting phenomenon is the peroxidase mimetic behavior, that is, elucidated, it is believed that the pH value of the reaction
some active oxidative species can be generated and some medium has a signicant inuence on the peroxidase mimetic
8
substrates can be oxidized concurrently during the catalytic properties of nanoceria. Most studies have shown that nano-
4
–6
decomposition of H
that CeO
nanoparticles exhibited excellent catalytic oxidation 2.0–6.0,
activity towards the classical peroxidase substrate 3,3,5,5-tetra- dase mimetic activity of nanoceria could be observed at even
2
O
2
.
For example, it has been reported ceria could be used as a peroxidase mimic in the pH range of
7,23
whereas a few studies have shown that the peroxi-
2
7
13,21
methylbiphenyl dihydrochloride in the presence of H O .
higher pH values.
Therefore, further study is still needed to
2
2
Because of their peroxidase mimetic properties, nanoceria and understand the relationship between the pH of the solution and
cerium-containing materials have been widely used in medical the nature of the oxidative species and their oxidation abilities.
diagnosis, biosensing, and Fenton-like processes for the
The peroxidase mimetic property of nanoceria originates
3
+
4+ 15,24
8
–11
degradation of organic pollutants.
Several oxidative species have been detected or proposed easier the redox cycles between Ce and Ce , the better the
from the redox cycles between Ce and Ce .
Generally, the
3
+
4+
5,12,13
12
during the catalytic decomposition of H
2
O
2
by nanoceria.
Liu et al. attributed the peroxidase mimetic activity of H
TCPP- strategies including doping ceria with heteroatom ions such as
CeO2 nanocomposites to the formation of HO$, which was Zr and decreasing the particle size of the ceria particles has
peroxidase mimetic activity of nanoceria. Up to now, several
8
2
4
+
14
conrmed with a uorescent probe technique. Lousada et al.
conrmed the formation of HO$ and studied the dynamics of ceria.
been used to enhance the peroxidase mimetic activity of
12,17,23
Zhou et al. reported that the redox performance of
formation of HO$. Besides HO$, the generation of other oxygen- nanoceria could also be optimized by tuning the crystal planes
ꢀ
25
derived free radicals such as HO
suggested.
2
$, O
2
$, and O
2
have also been exposed on the surfaces. Since then, a series of studies have
15,16
However, several researchers have attributed the been carried out to optimize the catalytic performance of
nanoceria for various reactions by tuning the morphology of
26–29
CeO
2
nanocrystals.
However, to the best of our knowledge,
Beijing Key Laboratory of Farmland Soil Pollution Prevention and Remediation,
College of Resources and Environmental Sciences, China Agricultural University, the inuence of the exposed crystal facet on the peroxidase
Beijing 100193, PR China. E-mail: syang@cau.edu.cn; Tel: +86 1062733470
mimetic activity of nanoceria has only been reported in a recent
paper, which found that the Fenton-like reactivity of CeO
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ra00622a
2
1
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30
nanocrystal was related to their exposed facets. Therefore, it is collected by a Raman spectrometer (Renishaw inVia, UK) with
worth applying nanoceria with different morphologies as a laser wavelength of 514.5 nm. Hydrogen temperature-
peroxidase mimetics to optimize the catalytic activity and programmed reduction (H -TPR) was conducted with a Chemi-
2
understand the underlying mechanisms of the nanoceria Sorb 2750 apparatus (Micromeritics, USA) equipped with a TCD
mimic enzyme.
detector. TPR was performed by heating the nanoceria samples
ꢁ
ꢀ1
ꢁ
In this work, CeO nanocubes and nanorods were prepared (50 mg) at 10 C min from 50–700 C in a 5 vol% H –N
by a hydrothermal method and their peroxidase mimetic mixture with a ow rate of 25 mL min
2
2
2
ꢀ1
.
properties were investigated under both acidic and basic
conditions. Our studies revealed
a strong morphology-
2.4 Electron paramagnetic resonance (EPR) measurements
dependent peroxidase mimetic activity of nanoceria. The
nature of the active oxidative species at different pH values and
their oxidation abilities were investigated to gain a deep
understanding of the mechanism of the peroxidase-mimetic
activity of nanoceria.
To monitor the potential radicals formed in the nanoceria/H O2
2
system, DMPO was used as the spin trap and EPR spectra were
recorded using a JES-FA200 EPR spectrometer (X-band) with
dual cavities (JEOL, Japan). Typical parameters were as follows:
modulation frequency, 100 kHz; microwave frequency, 9.05
GHz; sweep width, 50 G; EPR microwave power, 3 mW; modu-
lation amplitude, 1 G; time constant, 0.03 s; and sweep time,
2
Experiments
2
min.
2.1 Materials
Ce(NO ) $6H O, NaOH, HCl, H O (30%, w/w), n-butyl alcohol,
methylene blue (MB) and congo red (CR) were purchased from
3
3
2
2 2
2
.5 Peroxidase mimetics activity of nanoceria
ꢁ
ꢀ1
Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). The catalytic reaction was performed at 25 C using 1 g L CeO
2
ꢀ
1
ꢀ1
0
Catalase, 2,2 -azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) with 2 mM ABTS, 15 mg L MB or 70 mg L CR as substrate
ABTS) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) were ob- and 20 mM H , unless otherwise stated. Generally, catalyst
tained from Sigma-Aldrich Co. LLC. (Saint Louis, USA). All powder was added into a conical ask containing an aqueous
reagents were of analytical reagent grade and were used without solution of the substrate. Then, a H solution was added to
(
O
2 2
2 2
O
further purication.
initiate the oxidation reaction. To avoid the inuence of ions on
the nanoceria activity, ultrapure water was used in the test, and
the initial pH of the reaction system was adjusted using 0.1 M
HCl and 0.1 M NaOH solution. The concentration of organics
2.2 Synthesis of CeO nanocubes and CeO nanorods
2 2
2
CeO nanocatalysts were synthesized through a hydrothermal
2 2
and H O in supernatant was analyzed by using a UV–vis
25,31
process.
3 3 2
Typically, Ce(NO ) $6H O (1.5 g) was dissolved in
spectrophotometer (Beijing Purkinje General, China). The
distilled water, and an appropriate amount of 10% NaOH
solution was rapidly added to give a nal NaOH concentration
of approximately 2.0 M. Aer stirring for approximately 15 min,
the slurry (approximately 40 mL) was placed into a 50 mL
Teon-sealed autoclave and heated. The morphology of the as-
prepared nanoceria depended on the hydrothermal tempera-
ture and reaction time, that is, nanocubes were formed by
+
maximum wavelengths (lmax) of H O , ABTSc , MB and CR are
2
2
240, 734, 664 and 497 nm, respectively.
3
Results and discussion
3
.1 Characterization of CeO nanocubes and nanorods
2
The morphologies of the as-prepared samples were examined by
ꢁ
heating at 180 C for 24 h and nanorods were formed by heating
transmission electron microscopy (TEM). The CeO nanocubes
2
ꢁ
at 120 C for 12 h. Aer cooling, the nal product was collected
had uniform cubic shapes with a size of 20–30 nm (Fig. 1a). The
HRTEM image in Fig. 1b shows an interplanar spacing of
by ltration, washed with deionized water, and then dried at
ꢁ
6
0 C.
0.27 nm, corresponding to the (200) lattice fringe of CeO
2
,
which indicates that the nanocubes were mainly enclosed by the
2.3 Catalyst characterization
31
2
(100) facets. As shown in Fig. 1c, CeO nanorods had diame-
The size and morphology of the nanoceria was characterized ters of approximately 15–20 nm and lengths of 100–200 nm. The
with JEM-2100F high-resolution transmission electron HRTEM analysis (Fig. 1d) revealed that the nanorods were
a
microscope (HRTEM, JEOL, Japan). The powder X-ray diffrac- single-crystalline and their growth direction was (110). This was
tion (XRD) patterns were obtained on an D8 Advance X-ray consistent with the previously reported results that nanorods
25
diffractometer (Bruker, Germany) with Ni-ltered Cu Ka radia- expose their (110) and (100) facets.
tion (l ¼ 0.1541 nm). The Brunauer–Emmett–Teller (BET) The XRD patterns of the CeO nanocubes and nanorods are
surface area was evaluated from nitrogen adsorption–desorp- shown in Fig. 2a. All the diffraction peaks of both samples can
tion data recorded at 77 K using a model NOVA 3200e auto- be indexed to the pure uorite structure of CeO (JCPDS 34-
2
2
28
mated gas sorption system (Quantachrome, USA). X-ray 0394). The sharper peaks of the nanocubes indicated their
photoelectron spectra (XPS) measurements were performed on higher crystallinity and bigger crystallite size than those of the
26
an ESCALAB 250Xi high-performance electron spectrometer nanorods, which was consistent with the TEM and HRTEM
Thermo Fisher, USA) using monochromatized Al Ka (hn ¼ analysis. The specic surface areas of the nanocerias were
486.6 eV) as the excitation source. Raman spectra were measured by a nitrogen gas adsorption–desorption isotherm
(
1
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2 2
Fig. 3 (a) Raman spectra and (b) H -TPR profiles of CeO nanocubes
and nanorods.
symmetry and a decrease in the full width at half maximum,
along with an evident shi towards higher wave numbers. This
was attributed to the higher degree of crystallinity and larger
3
5,36
crystallite size of the nanocubes.
The weak band at approx-
ꢀ
1
imately 590 cm was associated to the defect-induced mode
D). The degree of defect sites on CeO
35
(
2
was estimated using the
28,36
relative intensity of D/F2g
.
The D/F2g ratio of the CeO
2
2
Fig. 1 TEM images of (a) CeO nanocubes and (c) nanorods, and
HRTEM images of (b) CeO nanocubes and (d) nanorods.
2
nanorods was 0.056, which was higher than that of the nano-
cubes (0.023). This implied that CeO nanorods had more defect
2
sites such as oxygen vacancies, which was consistent with the
XPS results.
3
+
2
The presence of more Ce and defects in CeO nanorods
suggested that the nanorods could be more reducible than the
nanocubes. Subsequently, the redox properties of the nanoceria
samples were examined by H
2
-TPR. The TPR proles (Fig. 3b) of
both samples showed two reduction bands, one located below
ꢁ
ꢁ
5
80 C and one above 600 C. The low-temperature reduction
band is related to the surface oxygen reduction and is of
32
particular interest in catalytic reactions. The low-temperature
ꢁ
Fig. 2 XRD patterns (a) and Ce3d XPS spectra (b) of CeO
and nanorods.
2
nanocubes peak position of the CeO
nanorods was at 450 C, which was
2
ꢁ
much lower than that of the nanocubes (510 C), indicating that
the CeO
2
nanorods were much more reducible than the nano-
cubes. The facile redox performance of the CeO nanorods
nanorods should perform well as
28
2
and calculated by the BET method. The nanorods exhibited
2
ꢀ1
indicated that the CeO
peroxidase mimetics.
2
a larger specic surface area (89.9 m
nanocubes (39.7 m g ).
g ) than that of the
2
ꢀ1
The valence state of the Ce ions in the nanocerias was
determined by XPS and the results are shown in Fig. 2b. The
Ce3d XPS spectra revealed that a mixed valence state (Ce and
3
.2
species
The decomposition of H
was rst investigated from pH 3.0 to 9.0. This set of data is for
2 2
H O decomposition and probing the possible oxidative
3
+
4
+
4+
Ce ) was present and most of the Ce ions were Ce in both
2 2 2
O over CeO nanocubes and nanorods
32
3+
nanoceria samples. However, the peaks attributed to Ce in
the nanorods, were clearly stronger than those for the nano-
cubes, as marked in Fig. 2b, indicating that more Ce was on
the surface of the nanorods.
using PeakFit soware to calculate the Ce concentration on
ꢀ1
ꢁ
2
0 mM H
shows the H
The decomposition of H
rst-order kinetics as previous reported (Fig. S1†).
coefficients are given in Table S1.† It is clear that the effect of
the pH on the decomposition of H over nanoceria is not
obvious and only a slight increase in conversion can be
2
O
2
in the presence of 1 g L CeO
concentration changes as a function of time.
in the presence of CeO follows
The rate
2
at 25 C. Fig. 4a
3
+
2 2
O
33,34
The Ce3d spectra were tted
2
O
2
2
3
+
14,37

3
+
the surface of both samples. The relative concentration of Ce
on the surface of the nanorods was 34.7%, which was much
larger than that of the nanocubes (23.3%). The presence of more
2 2
O
3
+
Ce indicated that there were more oxygen vacancies in the
CeO nanorods, which was further veried by Raman analysis.
38
observed with increasing pH from 3.0 to 9.0. However, CeO
nanorods display higher activity over all reactivity towards
. When the amount of eliminated H was normalized to
2
2
Fig. 3a displays the Raman spectra of the samples; two bands
were observed for both samples. The presence of the prominent
H
2
O
2
2 2
O
the surface area of nanocubes and nanorods, CeO
still possessed higher level of activity (Fig. 4b).
It has been reported that nanoceria could facilitate the
decomposition of H to produce HOc through a series of
2
nanorods
ꢀ
1
F2g vibrational mode band at approximately 460 cm
was
ascribed to the symmetric mode of the oxygen atoms around the
28
cerium ions. Compared with that of CeO
2
nanorods, the
2 2
O
Raman band of the nanocubes displayed a clear increase in
15,39,40
reactions analogous to Fenton/Haber Weiss reactions:
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Fig. 5 DMPO spin trapping EPR spectra of CeO
Nanocubes/H at pH 3.0 (a) and 9.0 (b); nanorods/H
and 9.0 (d). The incubation time is 2 min, and [CeO
] ¼ 1 g L , [H
20 mM.
2
/H
2
O
2
system.
2
O
2
2 2
O at pH 3.0 (c)
ꢀ
1
2
2 2
O ]
¼
It should be noted that a rapid color change was observed
upon the addition of H into the nanoceria suspensions: the
2 2
O
nanocubes suspension became light yellow and the nanorods
suspension became deep yellow (Fig. 6). The color change was
attributed to the formation of another type of oxidative species
(
H
the peroxide-like intermediates) from the reaction between
12,19,38
2
O
2
and CeO
2
.
The small variations in the color change of
nanoceria from pH 3.0 to 9.0 were almost completely imper-
ceptible to the eye, indicating that the formation of peroxide-
like intermediates was independent of the pH value.
Fig. 4 Catalytic activity of CeO
a function of reaction time and (b) the eliminated H O
2 2
ized to CeO surface area (20 mM H O , 1 g L CeO , 25 C).
2 2 2 2
2
: (a) concentration changes of H
was normal- deeper color of the nanorods/H O system indicated more
2 2
O
2 2
as Compared with the suspension of the nanocubes/H O , the
2
2
ꢀ1
ꢁ
3
+
+
4+
Ce + H O + H / Ce + OHc + H O
(1)
(2)
(3)
2
2
2
OHc + H
2
O
2
/ HO + H
2ꢀ
2
O
4
+
ꢀ
3+
+
Ce + HO2 / O + Ce + H
2
To probe the possible generation of HOc during the catalytic
decomposition of H over nanoceria, EPR was performed
2 2
O
using the DMPO as a spin trap. As shown in Fig. 5, a 4-line EPR
spectrum characteristic of DMPO-OH adducts was detected for
a suspension of nanorods upon the addition of H
intensity of the DMPO-OH signal decreased from pH 3.0 to pH
.0. Similar results were obtained for a suspension of nano-
2 2
O , but the
9
cubes, except the intensity of the DMPO-OH peaks was signi-
cantly lower than those of the nanorods under the same
conditions. These results clearly show that HOc can be gener-
ated in the catalytic decomposition of H
is decreased with increasing pH from 3.0 to 9.0.
2 2
O and the yield of HOc
22,41
And CeO
2
nanorods with better redox properties were more favorable for
the generation of HOc than their nanocube counterparts. Then
the ability of the nanoceria to generate HOc strongly depends on
Fig. 6 Photographs of nanoceria suspensions after the addition of
H O at various pH (50 mM H O , 0.5 g L CeO ).
2 2 2 2 2
40
ꢀ1
its redox properties.
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peroxide-like species were generated owing to the presence of of the suspensions changes from dark green to lighter with pH
3
+
more Ce and oxygen vacancies, as proved by XPS and Raman value from 3.0 to 9.0, showing a strong pH-dependent peroxi-
3
+
4+
analysis. A generally accepted view is that the ease of Ce /Ce
dase mimetic activity of nanoceria. Nanoceria display higher
redox cycles plays a signicant role in the decomposition of peroxidase mimetic activity at acidic conditions and the time-
21
2 2
H O and the generation of peroxide-like intermediates. In dependent UV-vis spectra of the oxidized product of ABTS
3
+
+
this process, the rate of Ce oxidation is much faster than that (ABTSc ) at room temperature are shown in Fig. 8. To further
4
+
3+
of Ce reduction, and high concentration of Ce would understand the peroxidase mimetic behaviors of the ceria
enhance the reducibility of ceria.
32,38,42
Therefore, high peroxi- nanocubes and nanorods, decolorization of MB and CR were
dase mimetic activity of ceria nanorods can be attributed to the carried out.
3
+
high concentration of Ce and the better reducibility.
The above results show that there were two types of oxidative
species, namely HOc and peroxide-like intermediates, which
3
.4 Decolorization of MB and CR in nanoceria/H
2
O
2
system
First, decolorization of 15 mg L MB with 1.0 g L nanoceria
and 20 mM H at pH 3.0 was investigated. It turned out that
ꢀ
1
ꢀ1
can be generated in the nanoceria/H
of these oxidative species was strongly dependent on the pH
value and the morphology of the CeO nanocrystals, which
determined the peroxidase mimetic performance of nanoceria.
2 2
O systems. The formation
2 2
O
the decolorization of MB over nanorods was 11.6% aer 30 min,
while only 4.8% was achieved over nanocubes. To investigate
the role of HO$ and peroxide-like intermediates in the decol-
orization process, we performed this experiment by adding
2
3.3 Peroxidase mimetic activity of CeO
2
nanocubes and
200 mM n-butanol as a radical scavenger before the reaction was
nanorods
initiated. Excess n-butanol in solution can scavenge all the HO$
The peroxidase mimetic activity of the CeO
2
samples was eval- produced in the H
uated semiquantitatively by the oxidation of ABTS in the pres- degradation of MB. As expected, the decolorization of MB was
2
4
O
2
decomposition process and inhibit the
0
ence of H O2 from pH 3.0 to 9.0. ABTS is typically used as completely inhibited in the presence of n-butanol. This result
2
a horseradish peroxidase substrate, which develops a green veried that the peroxidase mimetic activity of nanoceria was
+
color (ABTSc ) upon oxidation in aqueous solution. Control mainly owing to the formation of HO$ under acidic condi-
13,14
experiments showed that the color change of the systems was tions.
negligible without the presence of nanoceria or H
(Fig. S2†). can non-selectively oxidize most organic compounds at near
As shown in Fig. 7, a distinct and rapid change of the nanoceria/ diffusion-controlled rates under acidic condition. Although
It is well known that HOc exhibits high reactivity and
2 2
O
43
H O
2 2
suspension to a green color was observed upon the the peroxide-like intermediates were also present, it seemed
44 12
addition of ABTS, indicating that nanoceria did exhibit high they did not work. In fact, Chen et al. and Hamoud et al.
7,10
peroxidase-mimetic activity, as reported previously.
reported that the peroxide-like intermediates were prone to
Compared with the nanocubes, the nanorods exhibited a higher decompose in to HOc under acidic conditions. This can be used
activity under the current conditions, which could be explained to explain the complete inhibition decolorization of MB in the
by the generation of more HO$ and peroxide-like intermediates presence of n-butanol.
owing to their better redox property and high concentration of
Subsequently, the decolorization of MB at pH 9.0 was carried
Ce and oxygen vacancies. Moreover, it is evident that the color out. Because of the strong adsorption on ceria arising under
these conditions (pH 9.0), both adsorption and oxidative
3
+
degradation contributed to the total decolorization. Aer
Fig. 7 The pH effect on the production of colored product owing to
ꢀ
1
ABTS oxidation in the nanoceria/H
, 5 mM ABTS, t: 2 min).
2 2 2
O system (0.2 g L CeO , 10 mM Fig. 8 Time-dependent absorbance changes at 734 nm of ABTS in
ꢀ1
H
2
O
2
2 2 2
different systems (0.1 g L CeO , 5 mM H O and 2 mM ABTS).
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eliminating the interference arising from adsorption, the
degradation of MB was obtained (more information is provided
in the ESI†). Aer 0.5 h, the degradation of MB in the presence
of CeO2 nanorods and nanocubes was only 5.3% and 3.4%,
2 T. Montini, M. Melchionna, M. Monai and P. Fornasiero,
Chem. Rev., 2016, 116, 5987.
3 A. Trovarelli, Catal. Rev.: Sci. Eng., 1996, 38, 439.
4 M. Das, S. Patil, N. Bhargava, J. F. Kang, L. M. Riedel, S. Seal
and J. J. Hickman, Biomaterials, 2007, 28, 1918.
5 W. Lin, Y.-w. Huang, X.-D. Zhou and Y. Ma, Int. J. Toxicol.,
2006, 25, 451.
6 C. Xu and X. G. Qu, NPG Asia Mater., 2014, 6, e90.
7 X. Jiao, H. J. Song, H. H. Zhao, W. Bai, L. C. Zhang and Y. Lv,
Anal. Methods, 2012, 4, 3261.
2 2
respectively. These results suggested that nanoceria/H O
system could degrade MB at pH 9.0 but the peroxidase mimetic
reactivity of nanoceria was much lower than that at pH 3.0. To
further understand the peroxidase-mimetic activity of nano-
ceria under basic conditions, the degradation of CR, a typical
ꢀ
1
anionic azodye, was investigated at pH 9.0. The CR (70 mg L
)
could be completely degraded aer 1 h in the presence of
nanorods, whereas the degradation was only 8.2% in the pres-
ence of nanocubes. Interestingly, this time, the addition of n-
butanol could not completely inhibit the degradation of both
8 Q. Y. Liu, Y. Y. Ding, Y. T. Yang, L. Y. Zhang, L. F. Sun,
P. P. Chen and C. Gao, Mater. Sci. Eng., C, 2016, 59, 445–453.
9 X. M. Li, L. Sun, A. Q. Ge and Y. S. Guo, Chem. Commun.,
2011, 47, 947.
MB and CR under basic conditions, although the degradation 10 L. F. Sun, Y. Y. Ding, Y. L. Jiang and Q. Y. Liu, Sens. Actuators,
rates of dye were decreased with a certain extent. For example, B, 2017, 239, 848.
the complete degradation time of CR in nanorods/H O system 11 H. Issa Hamoud, G. Finqueneisel and B. Azambre, J. Environ.
was delayed from 1 h to 2.5 h upon addition of n-butanol. These Manage., 2017, 195, 195.
results suggested that both HO$ and the peroxide-like inter- 12 H. I. Hamoud, B. Azambre and G. Finqueneisel, J. Chem.
mediates were acting as the oxidative species in the nanoceria/ Technol. Biotechnol., 2016, 91, 2462.
system under basic conditions. According to previous 13 E. Aneggi, V. Cabbai, A. Trovarelli and D. Goi, Int. J.
reports, under neutral and basic conditions, the peroxide-like Photoenergy, 2012, 2012, 694721.
intermediates can directly function as peroxidase-like active 14 C. M. Lousada, M. Yang, K. Nilsson and M. Jonsson, J. Mol.
2
29
2 2
H O
2
3,45
sites
or induce an intermolecular rearrangement to achieve
Catal. A: Chem., 2013, 379, 178.
21,22
the oxidation of organics.
Because oxidation ability of HOc 15 T. S. Wu, Y. Y. Zhou, R. F. Sabirianov, W. N. Mei, Y. L. Soo
was much weaker under basic conditions than acidic condi-
and C. L. Cheung, Chem. Commun., 2016, 52, 5003.
the importance of the peroxide-like intermediates was 16 S. M. Hirst, A. S. Karakoti, R. D. Tyler, N. Sriranganathan,
S. Seal and C. M. Reilly, Small, 2009, 5, 2848.
12,46
tions,
raised.
17 W. D. Cai, F. Chen, X. X. Shen, L. J. Chen and J. L. Zhang,
Appl. Catal., B, 2010, 101, 160.
4
Conclusions
18 S. Y. Hao, J. Hou, P. Aprea and F. Pepe, Appl. Catal., B, 2014,
160, 566.
Nanoceria possesses strong morphology- and pH-dependent
peroxidase-like activity for the degradation of ABTS, MB and 19 F. H. Scholes, A. E. Hughes, S. G. Hardin, P. Lynch and
CR. Two types of oxidative species were generated in the
P. R. Miller, Chem. Mater., 2007, 19, 2321.
nanoceria/H O systems, namely, HOc and peroxide-like inter- 20 E. Grulke, K. Reed, M. Beck, X. Huang, A. Cormack and
2
2
mediates. Under acidic conditions, HOc are primarily respon-
sible for the peroxidase mimetic activity of nanoceria; under 21 P. F. Ji, L. Z. Wang, F. Chen and J. L. Zhang, ChemCatChem,
neutral and basic conditions, HOc and peroxide-like interme- 2010, 2, 1552.
diates are responsible for activity. Compared with CeO nano- 22 F. Chen, X. X. Shen, Y. C. Wang and J. L. Zhang, Appl. Catal.,
cubes, ceria nanorods exhibited a higher peroxidase activity B, 2012, 121, 223.
owing to their excellent redox property and more Ce and 23 Z. M. Tian, J. Li, Z. Y. Zhang, W. Gao, X. M. Zhou and
S. Seal, Environ. Sci.: Nano, 2014, 1, 429.
2
3
+
oxygen vacancies.
Y. Q. Qu, Biomaterials, 2015, 59, 116.
2
4 L. Artiglia, S. Agnoli, M. C. Paganini, M. Cattelan and
G. Granozzi, ACS Appl. Mater. Interfaces, 2014, 6, 20130.
5 K. B. Zhou, X. Wang, X. M. Sun, Q. Peng and Y. D. Li, J. Catal.,
2005, 229, 206.
Conflicts of interest
2
There are no conicts to declare.
26 T. Naganuma, Nano Res., 2017, 10, 199.
27 L. Wang, G. Lu, D. Yang, J. Wang, Z. B. Zhu, Z. X. Wang and
K. B. Zhou, ChemCatChem, 2013, 5, 1308.
Acknowledgements
The authors are grateful for the nancial support provided by 28 H. Y. Tan, J. Wang, S. Z. Yu and K. B. Zhou, Environ. Sci.
the National Natural Science Foundation of China (Grant No.
1476251).
Technol., 2015, 49, 8675.
29 T. S. Sreeremya, A. Krishnan, K. C. Remani, K. R. Patil,
D. F. Brougham and S. Ghosh, ACS Appl. Mater. Interfaces,
2
2
015, 7, 8545.
References
3
0 C. Zang, X. Zhang, S. Hu and F. Chen, Appl. Catal., B, 2017,
1
M. Tamura and K. Tomishige, Angew. Chem., Int. Ed., 2015,
4, 864.
216, 106.
5
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 11764–11770 | 11769

View Article Online
RSC Advances
Paper
3
3
3
3
3
1 Z. Yang, K. Zhou, X. Liu, Q. Tian, D. Lu and S. Yang, 39 E. G. Heckert, S. Seal and W. T. Self, Environ. Sci. Technol.,
Nanotechnology, 2007, 18, 185606. 2008, 42, 5014.
2 X. W. Liu, K. B. Zhou, L. Wang, B. Y. Wang and Y. D. Li, J. Am. 40 L. J. Xu and J. L. Wang, Environ. Sci. Technol., 2012, 46,
Chem. Soc., 2009, 131, 3140. 10145.
3 D. Yang, L. Wang, Y. Z. Sun and K. B. Zhou, J. Phys. Chem. C, 41 P. Zhang, D. Sun, M. Wen, J. Yang, K. Zhou and Z. Wang, Adv.
010, 114, 8926. Synth. Catal., 2012, 354, 720.
4 Y. Xue, Q. F. Luan, D. Yang, X. Yao and K. B. Zhou, J. Phys. 42 R. N. McCormack, P. Mendez, S. Barkam, C. J. Neal, S. Das
Chem. C, 2011, 115, 4433. and S. Seal, J. Phys. Chem. C, 2014, 118, 18992.
2
5 J. E. Spanier, R. D. Robinson, F. Zheng, S. W. Chan and 43 A. D. Bokare and W. Choi, J. Hazard. Mater., 2014, 275, 121.
I. P. Herman, Phys. Rev. B: Condens. Matter Mater. Phys., 44 Y. C. Wang, X. X. Shen and F. Chen, J. Mol. Catal. A: Chem.,
2
001, 64, 245407.
6 W. H. Weber, K. C. Hass and J. R. McBride, Phys. Rev. B: 45 H. Zhao, Y. M. Dong, P. P. Jiang, G. L. Wang and J. J. Zhang,
Condens. Matter Mater. Phys., 1993, 48, 178. ACS Appl. Mater. Interfaces, 2015, 7, 6451.
7 A. Hiroki and J. A. LaVerne, J. Phys. Chem. B, 2005, 109, 3364. 46 S. H. Bossmann, E. Oliveros, S. Gob, S. Siegwart,
2014, 381, 38.
3
3
3
8 Y. J. Wang, H. Dong, G. M. Lyu, H. Y. Zhang, J. Ke, L. Q. Kang,
J. L. Teng, L. D. Sun, R. Si, J. Zhang, Y. J. Liu, Y. W. Zhang,
Y. H. Huang and C. H. Yan, Nanoscale, 2015, 7, 13981.
E. P. Dahlen, L. Payawan, M. Straub, M. Worner and
A. M. Braun, J. Phys. Chem. A, 1998, 102, 5542.
11770 | RSC Adv., 2018, 8, 11764–11770
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