CuFe2O4@PDA magnetic nanomaterials with a core-shell structure: Synthesis and catalytic application in the degradation of methylene blue in water

Article abstract of DOI:10.1039/c5ra09114d

In this paper, core-shell polydopamine (PDA)-encapsulated CuFe₂O₄ (CuFe₂O₄@PDA) magnetic nanoparticles (MNPs) were synthesized through in situ self-polymerization for the first time. The size of the core-shell product can be controlled by tuning the dopamine monomer concentration. The formation of a PDA layer effectively enhanced the catalytic performance and provided a large specific surface area which offered more active sites for the effective interaction. The as-synthesized CuFe₂O₄@PDA MNPs were characterized and their catalytic activity was evaluated using the degradation of methylene blue (MB) in the presence of H₂O₂ as a model reaction. The experimental results showed that MB could be degraded efficiently using CuFe₂O₄@PDA MNPs as a catalyst. Under the optimized conditions, the degradation efficiency of MB was above 97%. Furthermore, a possible reaction mechanism was discussed. Finally, the catalyst was used for effective degradation of MB in a Yellow River water sample, which indicates its potential for practical applications in water pollutant removal and environmental remediation.

Full text of DOI:10.1039/c5ra09114d

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Page 1 of 34
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CuFe2O4@PDA Magnetic Nanomaterials with Core-shell
Structure: Synthesis and Catalytic Application in Degradation of
Methylene Blue in Water Solution
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Suꢀdai Ma a,b, Jie Feng a,b, Wenꢀjie Qin a,b, Yuꢀyun Ju a,b, Xingꢀguo Chen a, b, c, ∗
a
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State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou, 730000,
China
b Department of Chemistry, Lanzhou University, Lanzhou, 730000, China
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c Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province,
Lanzhou University, Lanzhou, 730000, China
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∗ Corresponding author. Fax: +86ꢀ931ꢀ8912582 TEL: +86ꢀ931ꢀ8912763.
Eꢀmail address: chenxg@lzu.edu.cn.

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Abstract
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In this paper, coreꢀshell polydopamine (PDA)ꢀencapsulated CuFe2O4
(CuFe2O4@PDA) magnetic nanoparticles (MNPs) were synthesized through in situ
selfꢀpolymerization for the first time. The size of the coreꢀshell product can be
controlled by tuning the dopamine monomer concentration. The formation of PDA
layer effectively enhanced the catalytic performance and provided a large specific
surface area which offered more active sites for the effective interaction. The
asꢀsynthesized CuFe2O4@PDA MNPs were characterized and their catalytic activity
was evaluated using the degradation of methylene blue (MB) in the presence of H2O2
as a model reaction. The experimental results showed that MB could be degraded
efficiently using CuFe2O4@PDA MNPs as catalyst. Under the optimized conditions,
the degradation efficiency of MB was above 97%. Furthermore, a possible reaction
mechanism was discussed. Finally, the catalyst was used for effective degradation of
MB in Yellow River water sample, which indicated its potential for practical
applications in water pollutant removal and environmental remediation.
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Keywords: Coreꢀshell structure, CuFe2O4@PDA, Magnetic nanomaterial, Catalytic
degradation, Methylene blue

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1. Introduction
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Rapid industrialization has led to an increased amount of discharged wastewater
containing pollutant. Water pollution by organic dyes has become a serious
environmental issue and received remarkable attention. The undesirable and
immoderate release of wastewater containing organic dyes caused a pernicious impact,
including destroying the balance of ecological system, destruction of the food chain
existing in water ecosystem and endangering the animals and human beings,1ꢀ5 etc.
MB is an important member of the thiazine class of dyes. It is most widely used in
paper, textiles, plastics, food, leather and cosmetic to color products.6 However, acute
exposure to MB will cause increased heart rate, shock, vomiting, Heinz body
formation, jaundice, cyanosis, quadriplegia and tissue necrosis in humans.7ꢀ10 The
development of a process for the efficient treatment of MBꢀpolluted wastewater is
therefore of utmost importance. The various conventional treatment methods, such as
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physical adsorption,11,
chemical oxidation,13,
biological degradation,15
photocatalytic degradation,16, 17 and sonochemical technology18 have been developed
for the removal of such contaminants in wastewater. However, they suffer from some
disadvantages including incomplete removal, additional equipment and disposal
problem of sludge, etc. So it is still of great significance to discover a relatively low
cost, easy operation and higher activity materials for efficient removal of organic
dyes.
In recent years, the coreꢀshell nanomaterials were developed as novel

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nanostructured catalyst to remove organic dyes. The coreꢀshell structural particles
showed some special properties such as large specific surface area, adjustable size and
devisable chemical composition. Moreover, the coreꢀshell structural particles which
may combine the advantages of “core” and “shell” showed enhanced physical and
chemical properties in optics, electronics, magnetics and catalysis.19ꢀ22 Thus, a lot of
researches have been reported on the functional shells, including polymers, ligands,
oxides or metals.23, 24 Dopamine (DA), one of the important natural chemical
neurotransmitters presented in various animals. It had the ability of adhering to almost
all material surfaces to form thin and surfaceꢀadherent polydopamine (PDA) coating
under mild condition.25ꢀ28 In addition, compared with other shell such as SiO2, Au,
PDA shell displays the following characteristics: (1) the process of preparing a PDA
coating seems to be simple, quick and ‘green’;29, 30 (2) a robust interfacial binding
force between the coating and the substrate through covalent bonds or other strong
intermolecular interactions is easily adapted for a variety of materials without surface
pretreatment; (3) PDA coating has plenty of functional groups such as amino and
hydroxyl groups as well as πꢀπ bonds, thus establishing further multiple
modification.31ꢀ34 (4) the good electrochemical behavior of a PDA coating with πꢀπ
stacking interaction can accelerate the electron transfer. On the basis of excellent
characteristics, PDA coating is an outstanding candidate to develop coreꢀshell
nanomaterials.
However, the application of some nanomaterials was restricted due to difficult
separation. This reason made it challenging to recycle and reuse the nanomaterials

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efficiently, further affecting their decontamination efficiencies in dye removal. It is
therefore crucial to look for stable, separated easily, and high catalytic activity
nanomaterials from the viewpoint of practical application. The integration of PDA
with superparamagnetic materials to form uniform coreꢀshell nanocomposite particles
has emerged as a viable solution. As an important kind of magnetic materials, copper
ferrites (CuFe2O4) with good superparamagnetic nature as a magnetic core can make
the catalysts efficiently separated from the reaction mixture with an external magnet,
which prevents the loss of catalyst and renders the catalyst cost effective. Furthermore,
CuFe2O4 nanoparticles possess excellent catalytic activity in various catalytic
applications.35ꢀ39
As far as we know, the research on nanocompounds combining the advantages of
the CuFe2O4 and PDA has not been reported. Hence, a facile and ecoꢀfriendly method
for the fabrication of magnetically recyclable CuFe2O4@PDA MNPs with coreꢀshell
nanostructures was reported (Fig. 1). The size of the coreꢀshell product was finely
controlled by tuning the DA monomer concentration to form varying shell thickness
of the PDA layer. Then, the CuFe2O4@PDA MNPs were employed to investigate its
catalytic potential in the degradation of MB. Our results demonstrated that
CuFe2O4@PDA MNPs possess excellent catalytic activity toward the degradation of
MB, with advantages such as wide working pH range, environmentally friendly
preparation, longꢀterm stability and easily removed from the reaction system. It is
expected to become a promising catalyst applied in water pollutant removal and
environmental remediation.

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2. Experimental
2.1. Materials
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FeCl3·6H2O was purchased from Tianjin Shuangchuan Chemical Reagent Factory
(Tianjin, China). Ethylene glycol and anhydrous ethanol were purchased from Tianjin
Rionlon Bohua Pharmaceutical Chemical Co., Ltd. (Tianjin, China). Polyethylene
glycol 20,000 was got from Xi'an Chemical Reagent Factory (Xi'an, China). Sodium
acetate (NaAc), hydrochloric acid (HCl) and sodium hydroxide (NaOH) were got
from Tianjin Guangfu Chemical Reagent Factory (Tianjin, China). 30% H2O2 (with
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extremely small amount of PO4 ) and MB were purchased from Tianjin Chemical
Reagent
Factory
(Tianjin,
China).
CuCl2·2H2O
and
tris(hydroxymethyl)aminomethane (Tris) were obtained from Sinopharm Chemica
Reagent Co. Ltd. Dopamine hydrochloride (DA) was obtained from Sigma Company.
All chemicals were of analytical grade and used without any further purification.
Ultrapure water was used throughout the whole experiment. The solution pH was
adjusted using diluted HCl and NaOH solutions.
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2.2. Preparation of CuFe2O4 magnetic nanoparticles
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CuFe2O4 magnetic nanoparticles (CuFe2O4 MNPs) were prepared according to the
literature procedures.37 Briefly, CuCl2·2H2O (2.5 mmol) and FeCl3·6H2O (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 mixture was

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stirred vigorously for 30 min at 60~70℃ and then sealed in a teflon lined
stainlesssteel autoclave (50 mL capacity). The autoclave was heated to and maintained
at 200 ℃ for 8 h and allowed to cool to room temperature. The generated black
CuFe2O4 MNPs were collected by magnetic separation, washed several times with
ethanol and ultrapure water. The resulting solids were dried at 60 ℃ for 3 h under
vacuum.
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2.3. Synthesis of CuFe2O4@PDA magnetic nanoparticles
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To prepare CuFe2O4@PDA MNPs, 0.4 g of the asꢀprepared CuFe2O4 MNPs was
dispersed in 200 mL of dopamineꢀTris solution (0.5 mg mLꢀ1, pH=8.5, 10 mM
TrisꢀHCl buffer), and allowed to proceed for 8 h under stirring at room temperature.
The resultant product was separated and collected with a magnet, and then washed
with ultrapure water three times and dried at 60 ℃for 3 h under vacuum.
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2.4. Apparatus and characterization
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A Tecnai G2F30 instrument was used to obtain the morphology and particle size of
the CuFe2O4 MNPs and CuFe2O4@PDA MNPs. A Nicolet Nexus 670 Fourier
transform infrared spectrometer (FTꢀIR) was used to demonstrate the chemical nature
of the products in KBr pellets. An Xꢀray diffraction (XRD) pattern of CuFe2O4 MNPs
were carried out with a X’pertpro Philips Xꢀray diffractometer with Cu Kα radiation
(λ=1.54056 Å) and scanning in the 2θ range of 10ꢀ80°. The magnetic property of the
nanoparticles was measured at room temperature with a vibration sample

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magnetometer (VSM, Lakeshore cryotronics 730, USA). A TUꢀ1901 double beam
UVꢀVis spectrophotometer (Beijing Purkine General Instrument Co. Ltd., China) was
used to implement scan spectrum and measure the absorbance for analysis.
Fluorescent measurements were carried out by a RFꢀ5301PC fluorescence (FL)
spectrophotometer with a 1ꢀcm quartz cell (Shimadzu, Kyoto, Japan). Mass spectra
were recorded on an Esquire 6000 Mass spectrometer (Bruker Daltonics, USA).
Inorganic ions in solution were analyzed using an DIONEX ICSꢀ1500 Ion
Chromatography (DIONEX, USA). Zeta potential measurements of CuFe2O4@PDA
MNPs were performed with a Malvern NanoꢀZS apparatus.
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2.5. Evaluation of catalytic performance
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The degradation of MB in the presence of H2O2 was chosen as a model reaction to
evaluate the catalytic properties of the asꢀprepared CuFe2O4@PDA MNPs. During the
degradation, the reactor of 50 mL capacity was immersed into a thermostated shaker.
For typical experimental runs, 4 mg freshly prepared CuFe2O4@PDA MNPs was
dispersed into 18 mL aqueous solution of MB (1.0×10ꢀ5 mol Lꢀ1). The degradation of
MB was initiated by rapid adding 2 mL H2O2 (5 mol Lꢀ1) to the reaction solution.
After the mixed solution was shaken for 30 min, a strong magnet was deposited at the
bottom of the conical flask, and the CuFe2O4@PDA MNPs were isolated from the
solution. The MB concentrations remained the aqueous solutions were measured at
λmax=656 nm. In addition, the adsorption experiment was carried out under the same
conditions just without H2O2. After oscillating 30 min, the change in absorbance of

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the mixture was measured again. The adsorption efficiency and the degradation
efficiency of MB in the CuFe2O4ꢀH2O2/CuFe2O4@PDA MNPs system were
calculated according to the equations:
η1 = [(A − A )/ A ]×100%
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η2 = [(A − A )/ A ]×100%
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Where η1 and η2 were the adsorption efficiency and the degradation efficiency of MB,
respectively; A0 was the absorbance value of original MB. A1 and A2 were the
maximum absorbance values of reaction solution at absence and present of H2O2,
respectively.
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For reusability, the catalysts were collected and washed with ultrapure water after
the catalytic reaction finished and the above catalytic process was repeated for several
times until obvious inactivation was acquired. The control experiments were carried
out under the same conditions without catalyst and/or H2O2.
Besides, a 3 mg portion of CuFe2O4@PDA MNPs were dispersed in 6 mL of
ultrapure water and stored for a month to test the stability of the PDA layer in aqueous
solution. A 3 mg portion of CuFe2O4@PDA MNPs were dispersed in 6 mL of
hydrochloric acid solution at pH 2 and pH 3 for 24 h to test the acid stability of the
sample.
The Yellow River water was chosen as a practical environmental water sample for
investigation. Before use, the sample was centrifuged (13,000 rpm) and filtered
through a 0.45ꢀꢁm membrane to remove any suspended particles. Stock solution was
prepared by dissolving MB in the water sample. The degradation experiment was

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carried out under the same conditions and same operation.
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3. Results and Discussion
3.1. Characterization of CuFe2O4 and CuFe2O4@PDA MNPs
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XRD analysis was used to identify the crystal structure of the CuFe2O4 MNPs. As
shown in Fig. S1, except some Cu impurity peaks, all peaks were indexed to be
CuFe2O4 (JCPDS 77ꢀ0010). There were five obvious diffraction peak at 2θ values of
29.98°, 35.22°, 57.10°, 62.28° and 74.06°, corresponding to the (220), (311),
(422), (511) and (440) crystal plane of the spinel structure with CuFe2O4 MNPs. The
reason of existence of metallic copper was that the strong reducing capability of
ethylene glycol used as solvent in the preparation of the CuFe2O4 MNPs.40
The morphology and size of the asꢀprepared CuFe2O4 MNPs and CuFe2O4@PDA
MNPs were determined by TEM. Fig. 2a clearly displayed that the CuFe2O4 MNPs
were spherical and dispersive with an average size of about 150±20 nm. As shown in
Fig. 2b, a continuous layer, which exhibited a fine increment in brightness in
comparison to the dark inner core, was clearly observed on the outer shell of the
CuFe2O4 core. From Fig. 2a and 2b, it was very clear that the CuFe2O4@PDA MNPs
were encapsulated with a typical coreꢀshell structure. There was a clear interface
between the PDA shell and the CuFe2O4 core, indicating a tight encapsulation. In this
process, the size of the CuFe2O4@PDA particle can be easily controlled by tuning the
DA monomer concentration. As shown in Fig. 3, when the size of the CuFe2O4
microspheres was fixed and the DA monomer concentration was changed from 0.5

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mg mLꢀ1 to 4 mg mLꢀ1, the average thickness of the PDA shell was 15 nm, 24 nm, 32
nm, 55 nm and 95 nm, respectively. The typical EDX pattern of CuFe2O4@PDA
MNPs in Fig. 3f illustrated the fractions of all the elements in the MNPs. The
appearance of N element indicated the successful modification of PDA on CuFe2O4
MNPs.
FTꢀIR was employed to examine the surface composition of the synthesized the
CuFe2O4 and CuFe2O4@PDA MNPs. As shown in Fig. 4a, the absorption band at 596
cmꢀ1 and 417 cmꢀ1 were ascribed to stretching vibration of tetrahedral complexes and
octahedral complexes, respectively. The metal ions in ferrite were situated in two
different sublattices owing to the geometrical configuration of the oxygen nearest
neighbors.41 The adsorption peak at 3433 cmꢀ1 represented the stretching mode of
H2O molecules and OH groups. The absorption peak at 1626 cmꢀ1 on spectrum
referred to the vibration of remainder H2O in the sample.37 In Fig. 4b there were
several new peaks compared with CuFe2O4 MNPs. The new peak at 1487 cmꢀ1
belonged to the C=C stretching vibrations of aromatic ring in the PDA polymer. The
absorption peak at 1576 cmꢀ1 was attributed to N–H stretching and 1298 cmꢀ1 was
assigned to the C–O stretching of phenolic hydroxyl group.28, 42 The peak at 3433 cmꢀ1
of CuFe2O4@PDA MNPs was broader than that of CuFe2O4 MNPs, resulting from the
overlapping of hydroxyls, water adsorbed in PDA polymer and amines of PDA. The
FTꢀIR spectrum indicated that the PDA was indeed coated on the surface of the
CuFe2O4 MNPs and the main structure of CuFe2O4 was not changed by the
modification.

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Moreover, VSM measurement was employed to investigate the magnetic properties
of the nanostructure. As shown in Figure 5, a hysteresis loop of typical CuFe2O4 and
CuFe2O4@PDA MNPs measured by sweeping the external field between 1.5 and 1.0
T at room temperature. In the VSM magnetization curves of the CuFe2O4 and
CuFe2O4@PDA MNPs, there were no hysteresis, and the remanence and coercivity
were negligible, indicating the superparamagnetism of these nanomaterials. Although
the saturation magnetization value of CuFe2O4@PDA MNPs (39.15 emu/g) was lower
than CuFe2O4 MNPs (44.80 emu/g) due to the existence of nonꢀmagnetic PDA
coating, the CuFe2O4@PDA MNPs were readily separated from solution with a
magnet due to their superparamagnetism and large saturation magnetization. All the
above results revealed the successful synthesis of the CuFe2O4@PDA MNPs.
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In order to investigate the catalytic activity of the CuFe2O4@PDA MNPs, MB was
used as a typical dye pollutant in the removal of organic pollutants for wastewater
treatments. The degradation curve of the MB was shown in Fig. 6 by measuring the
change of the absorbance at 665 nm. It showed that after the addition of the
CuFe2O4@PDA MNPs, the characteristic absorbance of MB at 665 nm nearly
disappeared, and the solution became colorless. It indicated that the CuFe2O4@PDA
MNPs exhibited excellent catalytic activity in the degradation of MB.
Moreover, the degradation efficiency of the CuFe2O4@PDA MNPsꢀH2O2 was 99%
after 30 min (Fig. 7a), which was more effective than the CuFe2O4 MNPsꢀH2O2. The

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catalytic performance of CuFe2O4@PDA MNPs was about 668% and 434% higher
than that of CuFe2O4 MNPs at 2 min and 5 min, respectively (Fig. S2). However, no
obvious decolorization was observed under the conditions of H2O2 only. The above
results indicated that the core of the CuFe2O4 MNPs itself owned catalytic property.
But the PDA layer can dramatically enhance the catalytic performance in MB
degradation. The enhanced catalytic activity of CuFe2O4@PDA MNPs was probably
caused by the synergistic effect between CuFe2O4 MNPs and PDA layer. In this
process, MB was more easily absorbed on the surface of the CuFe2O4@PDA MNPs
than CuFe2O4 MNPs.
In order to further verify the adsorption effect of PDA layer, the adsorptions
experiment of CuFe2O4@PDA MNPs and CuFe2O4 MNPs were also studied (Fig. 7b).
There were nearly no decolorization in the presence of CuFe2O4 MNPs after 30 min,
which demonstrated the CuFe2O4 MNPs could not adsorb MB. However, in the
CuFe2O4@PDAꢀMB system, MB could be removed about 40% in the same time.
Consequently, in these two parallel tests, the significant discrepancy was mainly
caused by adsorption capacity of PDA layer. It is noted that there were three aspects
may contribute to the adsorption of PDA layers and MB: (a) because of the large
amount of functional groups (amino and catechol groups) on the surface of PDA layer,
πꢀπ interaction and hydrogen bonding induced the adsorption of MB toward the PDA
surface.43ꢀ45 (b) the PDA layer also provided a large specific surface area and active
sites for this process. (c) the electrostatic interactions favoured the efficient
enrichment of the MB molecules on the PDA surfaces. To demonstrate this effect, the

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zeta potential of CuFe2O4@PDA MNPs was determined to be negatively charged in a
wide pH range (Fig. S3), suggesting strong electrostatic interactions with positively
charged MB (Fig. S4). To further confirm the presence of electrostatic interactions,
the influence of pH on the adsorption performance of CuFe2O4@PDA MNPs was
investigated (Fig. S5). It was evident that MB was absorbed by the CuFe2O4@PDA
MNPs faster at higher pH, which was most likely attributed to the stronger
electrostatic interactions under higher pH. As confirmed by zeta potential
measurement of CuFe2O4@PDA MNPs, the MNPs showed more negatively charged
at higher pH. All of the evidences suggested that the PDA layer showed efficient
adsorption of MB in addition to the high catalytic activity. As a result, the local
concentration of MB at PDA surfaces was much higher than that in bulk solution.
Therefore, the rate of the catalytic reaction and the performance of the catalysts were
significantly improved.
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3.3. Effect of operating parameters on MB degradation.
3.3.1. Effect of dopamine monomer concentration
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To confirm the difference in catalytic performance of catalysts with different shell
thickness, we made a comparison of MB degradation efficiency. As illustrated in Fig.
8a, the CuFe2O4@PDA MNPs showed a distinct increase in MB degradation
efficiency compared to CuFe2O4 MNPs of which degradation efficiency is only
69.18%, clearly demonstrating that the CuFe2O4@PDA MNPs showed much higher
catalytic activity. However, the degradation efficiency almost reached to the plateau

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when the concentration of DA monomer was 0.5 mg mLꢀ1 to 2 mg mLꢀ1. As the DA
monomer concentration was more than 2 mg mLꢀ1, only slight decrease of degradation
efficiency may be caused by the increase of PDA shell thickness. The PDA shell
thickness was too thick, which would limit the catalytic activity of inner core
CuFe2O4. Therefore, 0.5 mg mLꢀ1 of DA monomer concentration (i.e., the
CuFe2O4@PDA MNPs with 15 nm thickness of the PDA shell) was selected for
further experiments.
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3.3.2. Effect of the CuFe2O4@PDA MNPs amount
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Fig. 8b illustrated the MB degradation in the CuFe2O4@PDAꢀH2O2 system with
various loads of CuFe2O4@PDA MNPs. Compared with the H2O2 system (i.e., the
zero load of CuFe2O4@PDA MNPs), the introduction of CuFe2O4@PDA MNPs
significantly enhanced the degradation efficiency of MB. Because the CuFe2O4@PDA
MNPs acted as catalyst to accelerate the decomposition of H2O2, the consequence of
strong oxidizing radical species were easier generated. When the CuFe2O4@PDA
MNPs amount range from 0 to 4 mg, the degradation efficiency of MB was greatly
increased. While the amount of the CuFe2O4@PDA MNPs exceeded 4 mg, the
degradation efficiency essentially achieved constant value. Hence, 4 mg was chose as
optimal CuFe2O4@PDA MNPs amount.
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3.3.3. Effect of H2O2 concentration
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with different concentrations of H2O2. As the H2O2 concentration increased to 0.5 mol
Lꢀ1, the degradation efficiency of MB increased rapidly. Nonetheless when the H2O2
concentration enhanced from 0.5 to 1.0 mol Lꢀ1, the degradation efficiency reduced
gradually. The degradation efficiency of MB reached a maximum when the H2O2
concentration was 0.5 mol Lꢀ1. To our knowledge, H2O2 was not stable under high
concentration. Considering the above factors, 0.5 mol Lꢀ1 of H2O2 was selected in
subsequent experiments.
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The solution pH was a very important operation parameter in catalytic reaction.
The effect of pH on the degradation efficiency of MB was researched in the range of
2ꢀ10. As can be seen from Fig. 8d, the MB degradation worked very effectively over a
wide pH range from 4.0 to 10.0. It was very favorable to the practical treatment of
wastewater because there was no need to adjust the pH of the solution. The
degradation of MB would be very favorable to occur at pH 4.0 to pH 6.0; however, it
was unlike the other typical reaction for MB degradation, which was usually
performed at approximately pH 4.0.46 This signified that the robust PDA layer could
effectively protect the CuFe2O4 cores and the CuFe2O4@PDA MNPs could improve
the pH tolerance. This work chose 6.0 as optimal pH value, because it was the most
close to the pure water pH.
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Temperature played an important role in most chemical reactions. The influence of
temperature on the degradation of MB was investigated between 298 K and 313 K.
The extent of the degradation reaction of MB by time at different temperatures was
plotted. Fig. S6 demonstrated that the degradation efficiency of MB was higher at
higher temperatures within the tested temperature range. Furthermore, the reaction
time needed for complete removal of MB dropped from 30 min to 15 min along with
the temperature increasing from 298 K to 313 K. The effective interaction of reactant
could be accelerated at high temperature, resulting in enhancement of reaction rate.
Also, the catalytic activity of CuFe2O4@PDA MNPs kept unchanged even at a
highꢀtemperature (313 K). The results demonstrated the highꢀtemperature tolerance of
the CuFe2O4@PDA MNPs, which was impossible for some catalyst due to
denaturation at higher temperatures. So in the treatment of wastewater, high
temperature could be taken to accelerate the removal of pollutant. For operational
convenience, a temperature of 303 K (room temperature) was selected for all the
experiment. In addition, the complete degradation of MB took place in 30 min and the
color of the reaction mixture solution changed from blue to colorless.
In summary, the optimized conditions for MB degradation using the
CuFe2O4@PDAꢀH2O2 system were as follows: 4 mg CuFe2O4@PDA MNPs, 0.5
mol/L H2O2, pH 6.0, temperature 30 ℃, oscillation time 30 min. Under optimal
conditions, the MB could be removed completely, and the degradation efficiency was
above 97%.
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The reusability and stability of catalyst are crucial properties to evaluate the
catalytic performance. Therefore, the reusability of the asꢀprepared CuFe2O4@PDA
MNPs was explored by checking the cycle number dependence of MB degradation
loading the same CuFe2O4@PDA MNPs. Then the CuFe2O4@PDA MNPs were
carefully collected after each cycle and reversibly reused in the identical catalytic
system. As shown in Fig. S7, the catalyst could be successfully recycled and reused
for five successive cycles with a degradation efficiency of >85%.
Furthermore, as shown in Fig. S8a, after a month of soaking in aqueous solution,
the PDA layers were still intact coated on the CuFe2O4 cores and the morphology
remained unchanged. Even dispersed under a strong acid environment (pH 2 and pH 3)
for over 24 h, the TEM images (Fig. S8b, c) showed that the CuFe2O4@PDA MNPs
still maintained the distinct coreꢀshell structure. It is indicated that the robust PDA
layer could effectively protect the CuFe2O4 core under strong acid conditions and
show good stability in aqueous environment over a reasonably long period of time.
Therefore, the magnetically separable CuFe2O4@PDA MNPs have outstanding
recyclable and stable performance. It is expected to be used for a promising efficient
catalyst in wastewater treatment.
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3.5. Kinetics of the degradation of MB
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Fig. 9 showed the timeꢀdependent UVꢀVis spectra change of MB catalyzed with
CuFe2O4@PDA MNPs. The absorbance peak at 665 nm gradually attenuated during
the degradation process and finally disappeared. Furthermore, the degradation rate

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was evaluated. For the MB degradation reaction, the ratio of the concentration ct of
MB at time t to its initial value c0 at t= 0 were directly given by the ratio of the
respective absorbance At/A0 (A represents the absorbance at 665 nm). Because H2O2
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was in excess (
) and its concentration was considered as a
cMB cH O = 9 500000
2
2
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constant during the reaction process, the reaction kinetics could be treated as
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pseudoꢀfirstꢀorder:
dct dt = −kappct
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or ln c c = ln A A = − kappt
(
)
(
)
t
0
t
0
where the apparent rate constant kapp could be obtained from the plot of ln(At/A0)
versus the reaction time. Linear relationships between ln(At/A0) and reaction time
were obtained using CuFe2O4@PDA MNPs as catalyst, and the rate constant, kapp was
calculated as 9.89×10ꢀ2 minꢀ1 for the reaction.
Although the previous reported catalysts loading of gold or silver nanoparticles
could also exhibit great catalytic activity for the reduction of MB,43, 47 using
expensive and scarce noble metals limited the widespread application of these
catalysts.48 By contrast, the CuFe2O4@PDA MNPs were lowꢀcost and affordable.
Furthermore, the CuFe2O4@PDA MNPs exhibited other advantages, such as the wider
working pH range, longꢀterm stability, operational stability and higher catalytic
efficiency compared with the analogous catalysts.18, 49, 50
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3.6. Possible reaction mechanism
According to the above discussions, the degradation mechanism of MB was

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investigated and proposed. Obvious decolorization of MB was observed under the
conditions of CuFe2O4@PDA MNPs and H2O2. In order to validate the reason of
decolorization, the UVꢀvis spectra were first inspected. Fig. 10 showed that after the
addition of the CuFe2O4@PDA MNPs, the adsorption peak of 665 nm and 291 nm,
which respectively represented the characteristic adsorption of conjugated structure
and substituent benzene of MB, both vanished; a new strong adsorption peak (λ= 251
nm), which represented the characteristic adsorption of aromatic structure of catalytic
degradation product, appeared.43, 44 It indicated that the destruction of the whole MB
molecular or the chromophore of MB. In order to further determine intermediates and
final products, solutions of the MB and the MB was catalyzed by CuFe2O4@PDA
MNPs were analyzed using ESIꢀMS (Fig. S9 and S10). It is noteworthy that new
fragments of m/z 228, 191, 131 were detected, demonstrating 3,
7ꢀdiaminoꢀphenothiazineꢀ5ꢀium, 2, 5ꢀdiaminobenzenesulfonic acid and dlꢀnorleucine
were formed in degradation process (Table S1). The generation pathways of these
fresh intermediates were speculated,51ꢀ53 as shown in Fig. S11. Most Cl− may be
ionized during the dissolution of MB and existed in the detached state. N−CH3 with
the lowest bond energy was first broken. Then C−S and C−N were broken, C−S
transformed into C−SO3H. C−NH2 bond in the remaining structure was broken and
C−SO3H transformed into C−OH in the following degradation. The DLꢀNorleucine
generated as the ring in 1ꢀaminoꢀ3,4ꢀdihydroxybenzene opened. Mass spectra data
provided overwhelming evidence of the MB degradation, which was in accordance
with the results of UVꢀVis spectra. Additionally, the ion chromatography was used to

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further determine ions in resultant degradation solution. As shown in Fig. S12, Clꢀ,
ꢀ
ꢀ
ꢀ
ꢀ
2ꢀ
NO2 , NO3 , SO42ꢀ were detected. The generation of NO2 , NO3 and SO4 further
manifested CꢀS and CꢀN bond gradually cleavage in the degradation process.52, 53
These overall results confirmed that decolorization of MB was due to the MB was
degraded by CuFe2O4@PDA MNPs as a catalyst in the presence of H2O2.
Based on the intermediate and final products detected, the degradation mechanism
for MB is analyzed and described, as shown in Fig. S13. The MB degradation in
CuFe2O4@PDAꢀH2O2 system was mainly due to the synergistic effect of the CuFe2O4
core catalysis and PDA shell adsorption capacity. When the CuFe2O4@PDA MNPs
were added and used for catalysis, H2O2 molecules and MB were adsorbed on the
surface of CuFe2O4@PDA MNPs. The surfaceꢀadsorbed H2O2 molecules were
activated to generate the ·OH, which further react with adsorbed MB to initiate the
degradation and/or diffuse into the solution to attack MB molecules near the
CuFe2O4@PDA/solution interface. Furthermore, the PDA layer with πꢀπ stacking
interaction and the CuFe2O4 core with ‘dn’ (n= 5ꢀ9) electronic configuration could
accelerate the electron transfer.37, 54 The faster electron transferred on catalyst surface,
the faster the reaction processed. To evidence this assumption, a fluorescence
technique was used to detect of the produced ·OH radicals by adding the fluorescent
probe terephthalic acid into the CuFe2O4@PDAꢀH2O2 system, where terephthalic acid
could easily react with ·OH to yield a strongly fluorescent product 2ꢀhydroxy
terephthalic acid.55, 56 It was clearly shown in Fig. S14 that fluorescence intensity
enhanced gradually with the increasing concentration of the CuFe2O4@PDA MNPs,

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which suggested that the amount of generated ·OH increased as CuFe2O4@PDA
MNPs increased. However, no fluorescence intensity was observed in the absence of
H2O2. All of the above results indicated that H2O2 molecules were adsorbed on the
surface of the CuFe2O4@PDA MNPs and then activated by the CuFe2O4@PDA
MNPs to generate reactive oxygen species. All the evidences suggested that the
synergistic mechanism was feasible.
438
3.7. The application
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Finally, in order to verify whether the nanocomposite could be applied to
environmental water, the Yellow River water in Lanzhou section was collected and
used as a practical sample. The Yellow River water formed a blue solution after being
spiked with MB as a pollutant. Then 4 mg of the CuFe2O4@PDA MNPs was added to
this water in the presence of H2O2. Satisfactory results were obtained that the solution
became colorless, and the degradation efficiency was still above 97%. The
CuFe2O4@PDA MNPs were then easily separated by a magnet and could be reused
for further reactions. Therefore, the CuFe2O4@PDA MNPs were successfully used as
a catalyst for the degradation of MB for complex environmental water samples and
the proposed method was reliable.
449
4. Conclusions
450
451
In summary, a simple method to prepare polydopamine modified CuFe2O4 MNPs
was proposed. The PDA was directly grafted onto the CuFe2O4 using one step

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selfꢀpolymerization reaction to form wellꢀdefined coreꢀshell nanostructures. The
CuFe2O4@PDA MNPs exhibited excellent catalytic activity for the degradation of
MB in the presence of H2O2. The CuFe2O4@PDA MNPs could be easily separated by
a magnet after the catalytic reaction. In addition, the significant synergistic effect of
the CuFe2O4 core catalysis and PDA shell adsorption capacity was observed. MB and
H2O2 could be absorbed on the surface of the CuFe2O4@PDA MNPs. The
surfaceꢀadsorbed H2O2 molecules were activated to generate the ·OH, then it further
reacted with adsorbed MB to initiate the degradation. The formation of PDA layer
effectively enhanced the catalytic performance and protected the CuFe2O4 core to
improve the stability. The versatile PDA polymer coating on the CuFe2O4 core also
allowed the further surface functionalization for the development of multifunctional
nanomaterials. Because of its merits such as wide working pH range, simple
preparation, longꢀterm stability, good recyclability and high catalytic performance, the
CuFe2O4@PDA MNPs as potential catalysts will be facilitated to apply in various
fields such as environmental protection, bioseparator, biosensor, and so on.
467
Acknowledgements
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The authors are grateful for financial support from the National Natural Science
Foundation of China (No. 21375053) and Special Doctorial Program Fund from the
Ministry of Education of China (No. 20130211110039).

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Figure captions:
Fig. 1. Schematic diagram of the synthetic strategy and application of CuFe2O4@PDA
magnetic nanoparticles.
Fig. 2. TEM images of CuFe2O4 MNPs (a) and CuFe2O4@PDA MNPs (b).
Fig. 3. TEM images of the CuFe2O4@PDA MNPs based on CuFe2O4
subꢀmicrospheres with different shell thicknesses: 15 nm (a), 24 nm (b), 32 nm (c), 55
nm (d) and 95 nm (e), corresponding to 0.5, 1, 2, 3, 4 mg mLꢀ1 of DA monomer
concentration, respectively. The EDX pattern of the CuFe2O4@PDA MNPs (f).
Fig. 4. FTꢀIR spectra of the CuFe2O4 MNPs (a) and CuFe2O4@PDA coreꢀshell MNPs
(b), respectively.
Fig. 5. Roomꢀtemperature magnetization hysteresis loops of the CuFe2O4 MNPs (a)
and CuFe2O4@PDA MNPs (b).
Fig. 6. (a) UVꢀVis absorption spectrum of MB degradation before and after addition
of H2O2 and CuFe2O4@PDA MNPs. (b) Photograph of the reduction of MB by H2O2
in the presence of CuFe2O4@PDA MNPs.
Fig. 7. (a) Timeꢀdependent degradation efficiency of MB in systems of MB + H2O2,
CuFe2O4@PDA + MB + H2O2 and CuFe2O4 + MB+ H2O2. (b) Timeꢀdependent
adsorption efficiency of MB in systems of CuFe2O4 + MB and CuFe2O4@PDA + MB.
Fig. 8. Effects of reaction conditions on MB degradation efficiency. (a) dopamine
monomer concentration. (b) the CuFe2O4@PDA MNPs amount. (c) H2O2
concentration. (d) pH.
Fig. 9. The timeꢀdependent UVꢀVis absorption spectra change for the reduction

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process of MB. The illustration shows the relationship between ln(At/A0) and reaction
time (t) for the MB reduction.
Fig. 10. Successive UVꢀvis spectra of MB solution (black curve) and resultant
solution after degraded by CuFe2O4@PDA MNPs (red curve).