β-Cyclodextrin-modified hybrid magnetic nanoparticles for catalysis and adsorption

Article abstract of DOI:10.1039/c0jm03513k

β-Cyclodextrin-modified hybrid magnetic nanoparticles (Fe ₃O₄@SiO₂-PGMACD) were synthesized via the combination of atom transfer radical polymerization on the surfaces of silica coated iron oxide particles (Fe₃O₄@SiO₂) and ring-opening reaction of epoxy groups. The feasibility of using Fe ₃O₄@SiO₂-PGMACD as separable immobilized catalyst and adsorbent was demonstrated. It was found: (1) the prepared Fe ₃O₄@SiO₂-PGMACD could be used as catalyst in substrate-selective oxidation of alcohols system and the catalytic efficiency was close to pure β-Cyclodextrin of equal quantity; (2) the resulting particles appeared remarkably dominant adsorption capacity compared with poly(glycidyl methacrylate) grafted magnetic nanoparticles (Fe₃O ₄@SiO₂-PGMA) in the removal of bisphenol A from aqueous solutions. The results suggest that the novel fabricated nanoparticles could serve as bifunctional materials in catalysis or adsorption and subsequently become potential multifunctional materials. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c0jm03513k

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PAPER
b-Cyclodextrin-modified hybrid magnetic nanoparticles for catalysis
and adsorption
a
Yan Kang, Lilin Zhou, Xia Li and Jinying Yuan
a
b
a
*
Received 18th October 2010, Accepted 29th November 2010
DOI: 10.1039/c0jm03513k
b-Cyclodextrin-modified hybrid magnetic nanoparticles (Fe3O4@SiO2-PGMACD) were synthesized
via the combination of atom transfer radical polymerization on the surfaces of silica coated iron oxide
particles (Fe3O4@SiO2) and ring-opening reaction of epoxy groups. The feasibility of using
Fe3O4@SiO2-PGMACD as separable immobilized catalyst and adsorbent was demonstrated. It was
found: (1) the prepared Fe3O4@SiO2-PGMACD could be used as catalyst in substrate-selective
oxidation of alcohols system and the catalytic efficiency was close to pure b-Cyclodextrin of equal
quantity; (2) the resulting particles appeared remarkably dominant adsorption capacity compared with
poly(glycidyl methacrylate) grafted magnetic nanoparticles (Fe3O4@SiO2-PGMA) in the removal of
bisphenol A from aqueous solutions. The results suggest that the novel fabricated nanoparticles could
serve as bifunctional materials in catalysis or adsorption and subsequently become potential
multifunctional materials.
CDs-containing nanoparticles. In this work, we designed and
Introduction
fabricated the bifunctional b-Cyclodextrin (b-CD)-modified
hybrid iron oxide MNPs which could be used in catalysis and
adsorption efficiently and repeatedly.
Magnetic nanoparticles (MNPs) have the advantage of good
dispersibility in various solvents, high surface area, and strong
magnetic responsivity.1,2 MNPs have emerged as excellent
materials in many fields, such as immobilized catalysts, labeling
and sorting of biological species, targeted drug or gene delivery,
magnetic resonance imaging, and hyperthermia treatment.3–15
An important application among them is magnetic separation.
Because of strong magnetism, MNPs can be the separable
supports for catalyst and adsorbent, which make profound
contributions to green chemistry.16–19 The surfaces of these
particles are often modified by capping agents such as polymers,
inorganic metals or oxides, and surfactants to make them stable,
biocompatible, and suitable for further functionalizations and
applications.20–26
The route for synthesizing the b-CD-modified MNPs is shown
in Scheme 1. The iron oxide MNPs (Fe3O4) were prepared
by thermal decomposition methods capping with a layer of
oleylamine.36 Sol–gel process was adopted for coating iron oxides
with silica to form core-shell structures (Fe3O4@SiO2).37 Silanes
were employed to provide amino functional end groups
(Fe3O4@SiO2-NH2) and then 2-bromopropionyl bromide was
linked to the surfaces of MNPs to form atom transfer radical
polymerization (ATRP) initiators (Fe3O4@SiO2-Br).38 Glycidyl
methacrylate (GMA) containing an epoxy group was used as
a monomer and the poly(glycidyl methacrylate) (PGMA) was
synthesized by ATRP on the surfaces of MNPs (Fe3O4@SiO2-
PGMA). The ethylenediamine modified CDs were appended to
MNPs by ring-opening reaction of epoxy groups with amino
groups. The resulting nanoparticles, Fe3O4@SiO2-PGMACD,
were obtained eventually. Further studies demonstrated the
feasibility of using Fe3O4@SiO2-PGMACD as catalyst and
adsorbent. The substrate-selective oxidation of alcohols system
was investigated utilizing the resulting nanoparticles as the
catalyst. Benzyl alcohol and 1-octanol were used as the substrate
molecules, which have different behaviors under mild condition
using b-cyclodextrins (b-CDs) as catalyst.39 In addition, the
adsorption capacity of Fe3O4@SiO2-PGMACD in the removal
of bisphenol A was evaluated. The newly fabricated nano-
particles could serve as multifunctional materials, including the
promising applications in catalysis and environmental pollution
cleanup.
Cyclodextrins (CDs) are natural products resulting from the
action of the enzyme cyclodextrinase on starch.27 CDs, known as
their interior cavities structure composing of cyclic oligosac-
charides, are promising for applications in drug carrier systems,
nanoreactors, bioactive supramolecular assemblies, molecular
recognition, and catalysis.28–33 The combination of CDs and
inorganic nanoparticles has especially attracted increasing
attention. These particles can be utilized for sensing, chirose-
lective analysis, controlled hydrophobic drug delivery and
so on.34,35 However, there are few reports on multifunctional
aKey Lab of Organic Optoelectronic & Molecular Engineering, Department
of Chemistry, Tsinghua University, Beijing, 100084, China. E-mail:
yuanjy@mail.tsinghua.edu.cn
bCollege of Polymer Science and Engineering, Sichuan University,
Chengdu, 610065, China
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Scheme 1 Synthetic route for b-CD-modified MNPs.
3 times and collected by external magnetic field. The product was
dried under vacuum overnight.
Experimental
Materials
b-CD was procured from Kermel (China) and recrystallized
from water. Tosyl chloride (TsCl), iron(III) acetylacetonate
(Fe(acac)3), 1-octadecene, oleylamine, ethyl 2-bromobutyrate
(EBB), and N,N,N0,N0,N00-pentamethyldiethylenetriamine
(PMDETA) were purchased from Acros (USA) and used as
received. 3-Aminopropyl-trimethoxysilane (APTS) was obtained
from Fluka (Germany). 2-Bromopropionyl bromide was got
from Xinxiang Hongchen Technology Co. Ltd (China). GMA
was obtained from Tokyo Chemical Industry Co. Ltd (Japan)
and distilled in vacuum before use. Methanol, N,N-dime-
thylformamide (DMF), and toluene were received from Beijing
Chemical Reagent Co. Ltd (China) and purified before use. All
other reagents of analytical grade were commercially available
and used without further purification. Distilled water was
utilized throughout the studies.
Preparation of Fe3O4@SiO2-NH2 nanoparticles
0.5 g of Fe3O4@SiO2 nanoparticles, 2.5 mL of APTS, and 5 mL
of toluene were placed in a dried 25 mL flask. After evacuating
and filling with Ar for 3 times, the flask was immersed into an oil
ꢀ
bath at 40 C under Ar atmosphere with stirring for 24 h. The
solution was cooled down to room temperature, and then
exposed to air. The precipitation was washed with ethanol
3 times and collected using magnet. The product was dried under
vacuum overnight.
Preparation of Fe3O4@SiO2-Br nanoparticles
0.4 g of Fe3O4@SiO2-NH2, 20 mL of toluene, and 8 mL of
triethyl amine (TEA) were placed in a dried 100 mL flask. The
mixture of 8 mL of toluene and 4 mL of 2-bromopropionyl
bromide was dripped slowly into the solution in an ice-water
bath with stirring. After further stirring for 4 h, the precipitation
was washed by toluene, water and ethanol and collected using
a magnet. The product was dried under vacuum overnight.
Preparation of Fe3O4 nanoparticles
0.71 g (2 mmol) of Fe(acac)3 was dissolved in a mixture of 10 mL
of 1-octadecene and 10 mL of oleylamine. The solution was
dehydrated at 110 ꢀC for 1 h. Then the solution was quickly
heated to 300 ꢀC and kept at this temperature for 1 h under argon
atmosphere. 50 mL of ethanol was poured into the solution after
it was cooled down to room temperature. The precipitation was
collected by external magnetic field and washed with ethanol
3 times. The product was dried under vacuum overnight.
Preparation of Fe3O4@SiO2-PGMA nanoparticles
0.3 g of Fe3O4@SiO2-Br as initiator, 30 mg (0.15 mmol) of
EBB as co-initiator to estimate the molecular weight and
polydispersity index (PDI), 24 mL of methanol, 24 mL (0.18 mol)
of GMA, 240 mg of CuBr, and 348 mL of PMDETA were placed
in a dried flask. The flask was immersed into liquid-N2 bath,
followed by evacuating and filling with Ar. The freezing-evacu-
ating-filling process was repeated 3 times. The flask was
immersed into an oil bath at 50 ꢀC with stirring for 4 h. Then the
solution was cooled and exposed to air. The particles were
washed with tetrahydrofuran (THF) and ethanol in turn,
collected using a magnet, and dried under vacuum overnight.
The THF solution was passed through a neutral alumina column
Preparation of Fe3O4@SiO2 nanoparticles
200 mg of Fe3O4 was added to a mixture of 80 mL of cyclo-
hexane, 16 mL of 1-hexanol, 20 mL of triton X-100, and 3.4 mL
of water. 2 mL of tetraethyl orthosilicate (TEOS) was added and
then the solution was stirred for 6 h at room temperature. After 6
h, 2 mL of ammonia was added and the solution was stirred
continuously for 24 h. The precipitation was washed with ethanol
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to eliminate copper salt and then precipitated in the mixture of
methanol and water. The polymer was dried at 50 ꢀC in a vacuum
oven. ResultingdataofPGMA:Mn,gpc ¼ 13180, Mw/Mn ¼ 1.11. 1H
NMR(CDCl3, d, ppm):4.29(1H, COOCH2),3.80(1H, COOCH2),
3.21 (1H, CH2CHOCH2), 2.82 (1H, CH2CHOCH2), 2.62
(1H, CH2CHOCH2), 1.88 (2H, CH2CCH3), 1.22 (3H, CH2CCH3,
isotactic), 1.07 (3H, CH2CCH3, heterotactic), 0.92 (3H, CH2-
CCH3, syndiotactic).
10 mL of ethyl acetate 3 times and ethanol in turn. The nano-
particles were dried under vacuum overnight. The ethyl acetate
solution was concentrated and the crude product was analyzed
by GC-MS with ethyl acetate as the solvent.
Adsorption of bisphenol A
The solution of bisphenol A (0.2 mM) was prepared by
dissolving the drug in the mixture of methanol and water
(methanol : water, v/v
¼
1 : 19). Determined amounts of
Preparation of mono-6-deoxy-6-(p-tolysulfonyl)-b-CD
Fe3O4@SiO2-PGMA or Fe3O4@SiO2-PGMACD were added
into 5 mL of bisphenol A solution respectively. The solutions
were stirred for 20 h and then the MNPs were removed by
magnet. The residual aqueous solutions were characterized by
UV-vis spectra.
The mono-6-deoxy-6-(p-tolysulfonyl)-b-CD (mono-6-OTs-b-
CD) was prepared according to reference 40. 20 g of b-CD (17.6
mmol) was suspended in 167 mL of water, and 2.19 g of NaOH
(54.7 mmol) in 7 mL of water was added dropwise. The
suspension turned to be homogeneous. The solution was
immersed into an ice-water bath, and then 5.04 g of TsCl (26.4
mmol) in 10 mL of acetonitrile was dripped slowly, causing the
formation of white precipitates. After further stirring for 2 h at
25 ꢀC, the suspension was refrigerated overnight at 4 ꢀC. The
resulting precipitate was recovered by suction filtration and
recrystallized from hot water 3 times. The product was dried
Characterization
Fourier transform infrared (FT-IR) spectra were recorded on an
AVATAR 360 ESP FT-IR spectrometer. The molecular weight
and molecular weight distribution of polymer were measured on
a gel permeation chromatograph-Wyatt DAWN HELEOS II
18-angle light scattering detector equipped with two columns
ꢀ
under vacuum at 50 C. Resulting data of mono-6-OTs-b-CD:
ꢀ
ꢀ
1H NMR (DMSO, d, ppm): 7.70 (2H, aromatic protons), 7.39
(2H, aromatic protons), 5.47–5.90 (14H, OH-2,3), 4.80
(4H, H-1), 4.71 (3H, H-1), 4.17–4.49 (6H, OH-6), 3.48–3.74
(28H, H-3, 5, 6), 3.18–3.43 (14H, H-2,4, overlaps with HOD),
2.35 (3H, –CH3).
(MZ-Gel SDplus 10E4 A, MZ-Gel SDplus 500 A). THF was
1
utilized as eluent at a flow rate of 1.0 mL minꢁ1 at 35 ꢀC. H
NMR spectra were obtained from a JEOL JNM-ECA300 NMR
spectrometer with DMSO-d6, D2O, and CDCl3 as the solvent.
The chemical shifts of protons were relative to tetramethylsilane
at d ¼ 0 ppm. Thermogravimetric analysis (TGA) was carried
out on a TGA 2050 thermogravimetric analyzer with a heating
rate of 10 ꢀC minꢁ1 from 30 ꢀC to 900 ꢀC under a nitrogen
atmosphere. Transmission electron microscope (TEM) observa-
tions were performed on a JEOL JEM-2010 electron microscopy
operated at an acceleration voltage of 120 kV. Vibrating sample
magnetometry (VSM) was implemented on a LakeShore 7307
vibrating sample magnetometer. The BET surface area of
Fe3O4@SiO2 was measured by nitrogen adsorption on a Micro-
meritics ASAP 2010C instrument. XPS spectra were measured
with a PHI-5300 ESCA X-ray photoelectron spectroscopy (XPS)
with a monochromatic Al X-ray source, the power is 250 W.
Matrix-assisted laser desorption ionization Time-of-flight
(MALDI-TOF) mass spectrum was recorded on Bruker Autoflex
Preparation of mono-6-deoxy-6-ethylenediamine-b-CDs
The mono-6-deoxy-6-ethylenediamine -b-CD (EDA-b-CD) was
synthesized according to the previous report.41 5.0 g of mono-6-
OTs-b-CD was reacted with an excess amount of ethylenedi-
amine (EDA) at 80 ꢀC for 4 h. After the reaction was completed,
the solution was cooled down and most of the unreacted EDA
was removed by rotary evaporation. EDA-b-CD was dissolved
in water–methanol mixture and precipitated in acetone 3 times.
The product was dried at 50 ꢀC for 24 h in a vacuum oven.
1
Resulting data of EDA-b-CD: H NMR (D2O, d, ppm), 4.92
(7H, H-1), 3.67–3.83(28H, H-3,5,6), 3.31–3.51 (14H, H-2,4), 2.89
(2H, –CH2NH-b-CD), 2.70 (2H, NH2CH2–CH2). MALDI-TOF
MS m/z: 1199.27 [M + Na+].
MALDI-TOF mass spectrometry, using
a nitrogen laser
(337 nm) and a-cyano-4-hydroxycinnamic acid (CCA) as matrix.
Gas chromatography-mass spectrometry (GC-MS) measure-
ments were carried out on a Shimadzu GCMS-QP2010. UV-vis
spectra were recorded by using a Hitachi U-3010 spectropho-
tometer.
Preparation of Fe3O4@SiO2-PGMACD nanoparticles
0.4 g of Fe3O4@SiO2-PGMA and 2.5 g of EDA-b-CD were
added to 15 mL of DMF. The flask was immersed into an oil
bath at 60 ꢀC with stirring for 24 h. The mixture was cooled
down, washed in ultrasonic with DMF, water–methanol, and
ethanol in turn, and collected using a magnet. The product was
dried under vacuum overnight.
Results and discussion
Characterization of MNPs
The monodisperse MNPs Fe3O4 were synthesized by thermal
decomposition methods with a lay of oleylamine as both
reducing agent and stabilizer.42 TEM image of Fe3O4 is shown in
Fig. 1a. The diameter of Fe3O4 is about 9 nm and the particles are
soluble in hexane and other nonpolar organic solvents. Fig. 1b
shows the Fe3O4@SiO2 nanoparticles with a diameter of 60 nm,
which were prepared by sol–gel process according to the
Oxidation of benzyl alcohol and 1-octanol
0.3 g of Fe3O4@SiO2-PGMACD (or 0.3 g of Fe3O4@SiO2-
PGMA, or 32 mg of b-CD) was suspended in 5 mL of water at
50 ꢀC. Then 0.2 mmol of substrate (20 mL of benzyl alcohol or 17
mL of 1-octanol) and 1 mL of 10% NaOCl were added dropwise.
After stirring for 24 h, the mixture was washed in ultrasonic with
3706 | J. Mater. Chem., 2011, 21, 3704–3710
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Fig. 1 TEM images of (a) Fe3O4, (b) Fe3O4@SiO2, (c) and (d)
Fe3O4@SiO2-PGMACD.
published procedure.37 The process relied on the well-known
Fig. 2 FT-IR spectra of (a) Fe3O4@SiO2, (b) Fe3O4@SiO2-NH2, (c)
43
Stober method, in which silica was formed though the hydro-
€
Fe3O4@SiO2-Br, (d) Fe3O4@SiO2-PGMA, (e) Fe3O4@SiO2-PGMA-CD.
lysis and subsequent condensation of TEOS in basic alcohol/
water mixture. The dark point is Fe3O4 and the light shell is SiO2.
Silica can prevent core from unfavorable interactions and
aggregation, lead to simple surface functionalization, and make
the MNPs more biocompatible and hydrophilic compared to
Fe3O4 capping with oleylamine. Silanes were anchored to the
surfaces of SiO2, which could provide functional end group for
further modification. ATRP macroinitiator was synthesized by
the reaction of Fe3O4@SiO2-NH2 with 2-bromopropionyl
bromide.38 ATRP, a kind of controlled/‘‘living’’ polymerization,
is often used to prepare well-defined polymers. Hybrid nano-
particles Fe3O4@SiO2-PGMA were prepared by surface-initiated
ATRP of monomer GMA. The co-initiator of EBB was utilized
to synthesize the PGMA homopolymers to estimate the molec-
ular weight and PDI of the polymers on the surface of the MNPs,
which were characterized by GPC and NMR. The molecular
weight was 13180, which indicated the degree of polymerization
was 93. The PDI was 1.11 and the narrow distribution was the
character of controlled polymerization. EDA-b-CD was appen-
ded to GMA by ring-opening reaction of epoxy group. Fig. 1c
shows the Fe3O4@SiO2-PGMACD nanoparticles, while Fig. 1d
is the high resolution TEM image of Fe3O4@SiO2-PGMACD.
The undertone outer shell is the PGMACD part with a thickness
of 10 nm, which hints of successful modification by organics.
FT-IR could confirm the synthesis of modified MNPs. From
Fig. 2a, for Fe3O4@SiO2, the peak at 3416 cmꢁ1 is due to the
O–H stretching vibrations of hydroxyl group on the surface of
silica; the band at 1092 cmꢁ1 is related to Si–O–Si stretching
vibration; peak at 956 cmꢁ1 is associated to Si–OH vibration.
Fig. 2b shows the FI-IR spectrum of Fe3O4@SiO2-NH2. The
peak at 956 cmꢁ1 becomes weak which shows most of the
hydroxyl groups on the surface of silica were substituted; C–H
stretching vibrations of alkyl at 2942 cmꢁ1 occur; the bands at
1632 cmꢁ1 and 1550 cmꢁ1 are the bendings of N–H, characteristic
of the presence of NH2 group. From Fig. 2c, for the Fe3O4@
SiO2-Br, the peak at 1650 cmꢁ1 is attributable to C]O stretching
vibrations of acrylamide. The XPS results of Fe3O4@SiO2-NH2
and Fe3O4@SiO2-Br are listed in Table 1. The presence of N and
Br can clarify the successful synthesis of Fe3O4@SiO2-NH2 and
Fe3O4@SiO2-Br. Fig. 2d is the spectrum of Fe3O4@SiO2-
PGMA, in which the prominent peak at 1724 cmꢁ1 is related to
C]O stretching vibration of ester. The results show PGMA is
grafted. The FT-IR spectrum of Fe3O4@SiO2-PGMACD shows
in Fig. 2e. The characteristic peaks of CDs at 945 cmꢁ1 and 1157
cmꢁ1 are visible, but the one at 1028 cmꢁ1 is overlapped by the
Si–O–Si stretching vibrations. The peak at 945 cmꢁ1 is due to the
R-1,4- bond skeleton vibration of CDs, while the peaks at 1028
cmꢁ1 and 1157 cmꢁ1 are attributable to the antisymmetric
glycosidic C–O–C vibration and the coupled C–C/C–O stretch-
ing vibration.35
The synthesis of MNPs is also substantiated by TGA. The
results of TGA are shown in Fig. 3, in which the weight loss is
due to the decomposition of organics. The grafting percentage,
grafting ratio and grafting density could be calculated by the
margin of weight loss percentage. The grafting percentage is
defined as the weight percentage of newly grafted component.
The grafting density of component a (ra, chains/nm2) is
calculated according to eqn (1):
ꢀ
NA
ma
ra ¼
ꢂ
(1)
Ma
So
where ma is the mass of component a for 1 g of inorganic
component which is calculated by TGA data; Ma is the molar
mass of a; So is the specific surface area of magnetite nano-
particles, for Fe3O4@SiO2, So is 45.39 m2 gꢁ1 measured by BET.
The grafting ratio is defined as rproduct/rreaction, where the rproduct
Table 1 XPS results of Fe3O4@SiO2-NH2 and Fe3O4@SiO2-Br
Element
Molar ratioa
Molar ratiob
Si
N
Br
2.4
1.0
0
14
4.2
1.0
a
b
The element molar ratio of Fe3O4@SiO2-NH2. The element molar
ratio of Fe3O4@SiO2-Br.
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Fig. 3 TGA curves of (a) Fe3O4@SiO2, (b) Fe3O4@SiO2-NH2,(c)
Fe3O4@SiO2-Br, (d) Fe3O4@SiO2-PGMA, (e) Fe3O4@SiO2-PGMACD.
Fig. 4 Magnetic curves of (a) Fe3O4@SiO2, (b) Fe3O4@SiO2-NH2, (c)
Fe3O4@SiO2-Br, (d) Fe3O4@SiO2-PGMACD. Inset: the separation and
redispersion of Fe3O4@SiO2-PGMACD in distilled water in the absence
(left) and presence (right) of an external magnetic field.
is the grafting density of product and rreactant is the grafting
density of reactant. The results are listed in Table 2. From the
TGA results, the amount of CD grafted on the MNPs could be
calculated as 108.2 mg gꢁ1. Based on these data from FT-IR, and
TGA, it can be concluded that the resulting product Fe3O4@
SiO2-PGMA-CD was prepared affirmatively.
alcohol which had proper size, shape, and hydrophobicity, such
as benzylic primary alcohols and pyridinemethanol, would be
oxidized in the presence of b-CD and no product was formed
without the addition of b-CD. Meanwhile, some alcohols,
including benzylic secondary alcohols, 2-adamantanol, and
1-octanol, would not be conversed to aldehyde even if b-CDs
were added and the time was enough. The reason was that
2-adamantanol and b-CD could form inclusion complex with
a high complex constant which led to a more rigid structure and
made the hydroxyl group of 2-adamantanol lack activity, while
the long chain aliphatic alcohol and b-CD had very weak inter-
actions and hardly formed an inclusion complex. In addition, the
existence of hydrogen bonds between CDs’ secondary hydroxyl
group and substrates’ hydroxyl group impacted the oxidation
dramatically. In our work, benzyl alcohol and 1-octanol were
considered and the results list in Table 3. Oxidation of benzyl
alcohol was compared under different conditions. Only a few
benzaldehyde were obtained in the absence of b-CD. The
conversion of oxidation using Fe3O4@SiO2-PGMACD
(containing 32 mg b-CD) as the catalyst is close to the one using
pure b-CD (32 mg). Interestingly, using Fe3O4@SiO2-PGMA as
the catalyst, low reactivity towards oxidation was found, which
was due to the load of benzyl alcohol by polymer chains and the
hydrogen bond between polymer and substrate’s hydroxyl
group. As we all know, the catalyst immobilizing on MNPs
The magnetic property of Fe3O4@SiO2-PGMACD was
proved in Fig. 4. The MNPs can be separated quickly and facilely
under a magnetic field in 2 min, and redisperse immediately once
the outer magnetic field disappeared. VSM curves in Fig. 4 reveal
that the samples have low coercivity and no obvious hysteresis,
which indicate the MNPs have superparamagnetism. Super-
paramagnetism means that when the outer magnetic field with-
drew, there is no residual magnetism for nanoparticles. If the
nanoparticles have residual magnetism, it is very possible for
these nanoparticles to aggregate irreversibly. The saturation
magnetization of Fe3O4@SiO2-PGMACD is 5.50 emu/g.
Catalysis in substrate-selective oxidation of alcohols
The CDs could be utilized as the catalyst in the substrate-selec-
tive oxidation of alcohols system, in which NaOCl was the
oxidant and water was the only solvent.39 It was found that the
Table 2 The grafting results calculated by TGA
Grafting Grafting
TGA/ percentage/ density/
Grafting
ratio/%d
a
b
c
Samples
%
%
chain$nmꢁ2
Fe3O4@SiO2
82.16
—
—
—
—
53.57
4.51
3.35e
Table 3 Oxidation of alcohols under different conditions
Fe3O4@SiO2-NH2
Fe3O4@SiO2-Br
Fe3O4@SiO2-PGMA
76.10 7.38
69.23 9.03
48.06 30.58
18.21
9.75
0.44
1.37
Conv.
(%)
Entry Condition
Substrate
Product
Fe3O4@SiO2-PGMACD 42.86 10.82
a
b
The remaining weight percent after TGA. The grafting percentage is
defined as the weight percentage of newly grafted component.
1
2
3
4
5
No CD
CD
Fe3O4@SiO2-PGMA
Fe3O4@SiO2-PGMACD Benzyl alcohol Benzaldehyde 84
Fe3O4@SiO2-PGMACD 1-Octanol 1-Octanal Trace
Benzyl alcohol Benzaldehyde
Benzyl alcohol Benzaldehyde 89
Benzyl alcohol Benzaldehyde 34
5
c
d
Calculated by eqn (1). The grafting ratio is defined as rproduct
/
The grafting ratio of CD is defined as rCD/rGMA and rGMA
e
rreaction.
represents the grafting density of GMA units.
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Scheme 3 The schematic representation of the selective adsorption and
separation of bisphenol A by Fe3O4@SiO2-PGMACD.
Fig. 5 Recycling and reuse of the nanocatalyst Fe3O4@SiO2-PGMACD
for oxidation.
hydrophobic parts. The molecules with proper size, shape, and
polarity could fit the cavity to form inclusion complex through
host–guest interactions, which contribute to the application of
CDs in environmental pollution cleanup (see Scheme 3). CDs can
remove typical organic pollutants, such as bisphenol A and
aromatic molecules, from large volumes of water.44,45 In our
work, bisphenol A, a key monomer in production of epoxy resins
and in the most common production process of polycarbonate
plastic, was selected as the research model. Bisphenol A was
adsorbed by different amounts of MNPs. Fig. 6 is the UV-vis
spectra of bisphenol A in the existence and presence of MNPs. It
was found that Fe3O4@SiO2-PGMACD had higher adsorption
capability than Fe3O4@SiO2-PGMA. Taking 5 mg Fe3O4@
SiO2-PGMACD as consideration, it contained 4.6 ꢂ 10ꢁ4 mmol
of b-CD. The absorbance of bisphenol A (5 mL, 10 ꢂ 10ꢁ4 mmol)
decreased apparently and near 67% of bisphenol A was adsor-
bed. Compared with Fe3O4@SiO2-PGMACD, only 24% was
adsorbed by 5 mg of Fe3O4@SiO2-PGMA. In addition, there is
a linear relation between the amount of MNPs and the decrease
always have less activity than the free catalyst, but Fe3O4@SiO2-
PGMACD is similar with unmodified CD, which is related to the
cooperation of both CDs and polymers. In the meantime,
1-octanol could not be conversed under the condition of
Fe3O4@SiO2-PGMACD. The results proved that the resulting
nanoparticles could be utilized in the substrate-selective oxida-
tion of alcohols system with obvious catalytic capability.
Recycling of the MNPs was conducted for oxidation of benzyl
alcohol. After each cycle, Fe3O4@SiO2-PGMACD was washed
in ultrasonic with ethyl acetate and ethanol in turn, magnetically
separated, dried under vacuum, and reused in the system of
oxidation of benzyl alcohol. The separation was easy and high
yielding. Fig. 5 illustrates the conversion from benzyl alcohol to
benzaldehyde in each cycle. After 5 cycles, the catalyst is still
active and efficient. The schematic representation was shown in
Scheme 2.
Adsorption of bisphenol A
CDs have hydrophilic outer surface and hydrophobic interior
cavity, where poorly water-soluble molecules can shelter their
Fig. 6 UV-vis spectra of bisphenol A (2 ꢂ 10ꢁ4 M) upon addition of
(a) 0 mg MNPs; (b) 2 mg Fe3O4@SiO2-PGMA; (c) 5 mg Fe3O4@SiO2-
PGMA; (d) 2 mg Fe3O4@SiO2-PGMACD; (e) 5 mg Fe3O4@SiO2-
PGMACD.
Scheme
2 The schematic representation of the substrate-selective
catalysis and recycling of the immobilized catalyst Fe3O4@SiO2-
PGMACD.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 3704–3710 | 3709

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15 I. Willner, Z. Cheglakov, Y. Weizmann and E. Sharon, Analyst, 2008,
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16 S. Shylesh, V. Schunemann and W. R. Thiel, Angew. Chem., Int. Ed.,
2010, 49, 3428–3459.
of absorbance. The results indicate Fe3O4@SiO2-PGMACD is
a potential material in the enrichment and removal of bisphenol
A from the aqueous solution.
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Conclusion
Fe3O4@SiO2-PGMACD was successfully fabricated by
a sequence of methods including thermal decomposition, sol–gel
process, ATRP and ring-opening reaction of epoxy group. The
resulting nanoparticles inherit the catalysis and inclusion prop-
erties of CD, the reactivity and assistance properties of PGMA,
and the magnetic separation property of MNPs. Fe3O4@SiO2-
PGMACD can be used in catalysis and adsorption with high
efficiency. The average conversion of oxidation of benzyl alcohol
is up to 84%, close to free CD, while the adsorption capability
has a linear relation with the quantity of MNPs. Furthermore,
due to the high activity of CDs in the resulting product, they have
promising applications in much more extensive fields.
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Acknowledgements
The authors gratefully acknowledge the financial support of the
National Natural Science Foundation of China (Nos. 20836004,
20974058 and 51073090), and the National Basic Research
Program of China (2009CB930602). We thank Mr. Nathan
Glover (Arizona State University) for help with our English
writing skills.
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