Magnetic nanocrystals coated by molecularly imprinted polymers for the recognition of bisphenol A

Article abstract of DOI:10.1039/c1jm14139b

Molecularly imprinted polymers were coated on the surface of magnetic nanoparticles using an original and simple chemical strategy combining aryl diazonium salt chemistry and the iniferter method. This approach provides dispersed, highly soluble and BPA imprinted poly(methacrylic acid-co-ethylene glycoldimethacrylate) coated magnetic nanoparticles.

Full text of DOI:10.1039/c1jm14139b

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PAPER
Magnetic nanocrystals coated by molecularly imprinted polymers for the
recognition of bisphenol A†
a
b
a
a
a
a
N. Griffete, H. Li, A. Lamouri, C. Redeuilh, K. Chen, C.-Z. Dong, S. Nowak,
a
a
a
S. Ammar and C. Mangeney*
Received 24th August 2011, Accepted 3rd November 2011
DOI: 10.1039/c1jm14139b
Molecularly imprinted polymers were coated on the surface of magnetic nanoparticles using an original
and simple chemical strategy combining aryl diazonium salt chemistry and the iniferter method. This
approach provides dispersed, highly soluble and BPA imprinted poly(methacrylic acid-co-ethylene
glycoldimethacrylate) coated magnetic nanoparticles.
Introduction
We address this issue in the present paper by developing
a facile methodology to grow molecularly imprinted polymers
from individual metal oxide nanoparticles via an aryl diazonium
salt-derived initiator. Aryl diazonium salts have been shown to
be useful coupling agents for the grafting of polymer coatings on
Magnetic nanoparticles (NPs) have been the subject of extensive
research during the last decade because of their potential appli-
1
cations in many fields, such as magnetic ferrofluids, nucleic acid
2
separation, NPs/ligand targeting systems for drug delivery,
a,b
2c–g
7
8
carbon-based, metallic (gold, platinum, palladium, ruthenium
and titanium) planar or nanoparticle surfaces, affording strong
carbon or metal–carbon linkages. Several substrates could be
functionalized using this approach, by a range of grafting onto
3
removal of environmental pollutants. For many of these appli-
cations, surface modification of the NPs is a key challenge. One
promising approach consists of functionalizing the NPs by
biomimetic polymers such as molecularly imprinted polymers
9
and grafting from methods, which were reviewed recently. For
(
MIPs), in order to produce bifunctional materials with
example, surface-initiated photopolymerization was conducted
on gold surfaces modified by the electrochemical reduction of an
10
remarkable magnetic and recognition properties. Since the
4a
seminal work of Polyakov in the 1930s, using silica matrices,
aryl diazonium salt and gold-grafted ultrathin films of quer-
4b
the reports of Wulff and co-workers on covalent imprinting
cetin-imprinted polymers (Au-MIPs) were prepared by tandem
11
diazonium salt chemistry and surface-confined ATRP. As far as
protocols and those of Mosbach on the noncovalent imprinting
4
c
approach a decade ago, several examples have been reported in
oxide surfaces are concerned, their functionalisation using dia-
12
zonium salts has been reported for a long time but its extension
the literature on the preparation of magnetic molecularly
5
imprinted polymers (MMIPs). Usually, the MMIPs are
to the functionalization of iron oxide nanoparticles has only been
13
reported very recently, by taking advantage of the trans-
14,15
formation of diazonium species to diazoates in basic media.
composed of aggregates of superparamagnetic nanoparticles
embedded within a molecularly imprinted polymer matrix.
Although effective for extracting and sensing experiments, such
beads containing multiple magnetic nanoparticles remain rela-
tively large (with diameters larger than 60 nm), which severely
undermines their targeting specificity in biomedical applica-
These species are unstable and dediazonize along a homolytic
pathway to give aryl radicals which further react with the iron
oxide NPs during their formation and stop their growth. Using
this approach, our group could obtain monolayers of functional
6
tions. The use of individual ultrasmall magnetic nanoparticles
13
aryl groups grafted on the iron oxide NPs surface. However,
covered by a thin layer of molecularly imprinted polymer should
be a promising alternative offering several advantages over larger
conventional MMIPs: (i) control of the NPs size down to about
the possibility to grow molecularly imprinted polymers from the
aryl-modified NP surface has never been investigated so far. In
this paper, we fill this gap by exploring the capacity of aryl dia-
zonium coupling agents to initiate the growth of MIPs from
individual iron oxide NPs.
1
0–20 nm; (ii) higher surface-to-volume ratio; (iii) higher binding
capacities and faster binding kinetics.
Our strategy relies on a bifunctional initiator, 4-(((diethyl-
a
carbamothioyl)thio)methyl)
ꢀ
benzenediazonium
tetra-
Univ Paris Diderot, Sorbonne Paris Cit eꢀ , ITODYS, UMR 7086 CNRS,
1
5 rue J-A de Ba €ı f, 75205 Paris Cedex 13, France. E-mail: mangeney@
univ-paris-diderot.fr; Fax: +33 157277263; Tel: +33 157276878
+
fluoroborate (BF
4
,
N
2
–C
6
H
4
–CH –DEDTC) containing (i)
2
a diazonium end group for surface anchoring and (ii) a N,N-
16
diethyldithiocarbamate (DEDTC) function able to activate
b
Faculty of chemical engineering and light Industry, Guangdong University
of Technology, Guangzhou, 510090, China
surface-initiated photoiniferter-mediated polymerization (SI-
PIMP), which is a versatile and robust method widely used in
†
1
Electronic supplementary information (ESI) available. See DOI:
0.1039/c1jm14139b
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J. Mater. Chem., 2012, 22, 1807–1811 | 1807

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1
7
20
spectroscopy evidenced a maghemite Fe O phase. As previ-
‘
‘graft-from’’ strategies. We demonstrate this approach, sche-
2
3
matized in Fig. 1, by grafting cross-linked molecularly imprinted
co-polymers of methacrylic acid and ethylene glyco-
ldimethacrylate on magnetic iron oxide nanoparticles, in the
presence of bisphenol A (BPA) as the template molecule.
BPA is a xenoestrogen that can disrupt endocrine function and
adversely affect the reproductive systems of wildlife and
ously observed for the in situ functionalization of iron oxide NPs
21
by diazonium salts, the crystal size appears to be dependent on
the presence of the diazonium salt, with lower diameters
measured after functionalization (ꢁ30% diameter decrease),
compared to bare NPs. This result indicates that the coating
limits the crystalline growth, in agreement with previously
18
22
humans. The MIP-coated magnetic nanoparticles developed in
this work should therefore open up new opportunities for rapid
extraction of trace amounts of BPA.
reported works.
As shown in Fig. 2, transmission electron microscopy (TEM)
revealed the production of quite isotropic particles with sizes
around 8 and 16 nm for NP-DEDTC and NP@MIP, respec-
tively. Although the particles seem to be aggregated after
DEDTC grafting, they appear well separated after polymeriza-
tion, suggesting a steric stabilization provided by the polymer
overlayer. HRTEM images of NP@MIP nanoparticles (dis-
played in the ESI†) show a crystalline core corresponding to the
iron oxide, covered by an amorphous layer corresponding to the
polymer coating. Note that in the case of NP-DEDTC, the
proximity between the crystal size inferred from XRD and
the inorganic core diameter deduced from TEM (see Table 1)
suggests that the produced particles are single crystals and that
their coating is very thin, close to a single molecular shell.
Results and discussion
Iron oxide cores functionalized with benzyl DEDTC groups on
the surface (NP-DEDTC) were synthesized by one-step copre-
2+
3+
cipitation of Fe /Fe under alkaline conditions, in the presence
ꢀ
+
of the diazonium salt BF
4
,
2 6 4 2
N –C H –CH –DEDTC (see
details of the procedure in the ESI†). The synthesis was per-
formed within a few minutes at room temperature in slightly
basic aqueous medium (pH 9). Then, the polymerisation could
proceed, mixing the NP-DEDTC particles with methacrylic acid
(
(
MAA) as the monomer, ethylene glycoldimethacrylate
EGDMA) as the crosslinking agent and bisphenol A (BPA) as
Fig. 3 shows the IR spectra of the Fe
O NPs recorded after
3
2
the template molecule. The deoxygenated mixture was irradiated
under UV light for 4 h. The final product consisting of nano-
particles coated by a molecularly imprinted polymer (NP@MIP)
was washed several times and separated by magnetic separation.
The morphology, adsorption, and recognition properties of these
magnetic molecularly imprinted nanomaterials were investigated
by IR, DRX, TGA and TEM. A reference non-imprinted poly-
mer sample (NP@NIP) was prepared using the same procedure,
but without addition of the template.
each treatment step. Bare NPs display one intense band at
ꢀ
1
ꢁ590 cm , characteristic of the Fe–O stretching vibration. The
IR spectrum of NP-DEDTC shows the appearance of new bands
ꢀ1
at ꢁ1630 (dC]C), 1450 (nC–N), 950 (nC–S) and 890 (nSCS) cm due
23
to the phenyl and thiocarbamate group vibrations. It is note-
ꢀ1
worthy that the N^N stretching mode near 2280 cm appearing
in the IR spectrum of the free diazonium salt (see Fig. S3†) is not
visible in the spectra of the functionalized NPs, indicating the
2
release of the N group as a consequence of the cleavage of the
The X-ray diffraction (XRD) patterns of the powders are
characteristic of the spinel phase with broadened peaks (see
Fig. S2†). The cell parameter and the size of coherent diffraction
diazonium moieties during the grafting process. Strong modifi-
cations appear in the spectrum of NP@MIP: a very intense band
ꢀ
1
is seen at ca. 1730 cm due to the C]O stretching mode together
with two medium bands at ꢁ3400 cmꢀ1 (nO–H) and 2950 cmꢀ1
19
domain (hL
i) were determined with the MAUD software
XRD
which is based on the Rietveld method combined with Fourier
analysis. The Rietveld refinement analysis agrees well with the
formation of nanocrystalline spinel iron oxide while Raman
Fig. 2 Transmission electron micrographs and histograms of the
particle size distributions for (a) NP-DEDTC and (b) NP@MIP after 4 h
polymerization.
Fig. 1 Scheme depicting the synthesis of iron oxide nanoparticles
functionalized by bisphenol A-imprinted polymer films.
1
808 | J. Mater. Chem., 2012, 22, 1807–1811
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Table 1 Characteristics of the NPs
amount of BPA molecules bound to the NP@MIP which is
around ꢁ13%.
a
Org. (wt%)
Sample
hLXRDi/nm
hDTEMi/nm
The produced NP@MIPs were dispersed in water at different
pH values (Fig. 4). After sonication, the suspension in basic
medium (pH 8) appears more stable than that in deionized water
Bare NPs
11
8
11 ꢄ 1
8 ꢄ 1
—
4
18
43
60
47
6 4 2
NP–C H –CH –DEDTC
NP@NIP
NP@MIPwith BPA
NP@MIP without BPA
—
—
—
(
pH 5.5). This result indicates a self-aggregation process caused
16 ꢄ 1
b
by the pH-induced charge property change of the particle
surface. This self-assembled behavior can be readily reversed by
adjusting the pH. Furthermore, the NP-MIP particles are
magnetically responsive in both tested media. They can be
attracted under magnetic field under more or less aggregated
state, Fig. 4c.
—
a
Organic weight percents determined from TGA.
The binding properties of NP@MIP nanoparticles towards
bisphenol A were determined by measuring the uptake of
bisphenol A in ethanol over a range of concentrations from 50 to
ꢀ1
4
00 mmol L (see Fig. 5). The results show that the NP@MIP
nanoparticles have much higher binding capacity than the
control NP@NIP, suggesting that the molecular recognition sites
were generated on the surface of the MIP particles by the BPA
template involved in the polymerization process.
To gain further insight into the phenomenon of bisphenol A
binding by the NP@MIP nanoparticles, the binding data were
fitted into a Langmuir isotherm model. In the Langmuir model,
the equilibrium dissociation constant (K ) and saturation
d
capacity (Bmax ¼ B + Bunbound) can be estimated graphically from
a linearized version of the model by plotting the adsorption
26
isotherm in a Scatchard format:
Fig. 3 FT-IR spectra of (a) bare iron oxide NPs, (b) NP-DEDTC and (c)
NP@MIP.
B/[BPA] ¼ (Bmax ꢀ B)/Kd
The Scatchard plot (inset of Fig. 5) was not linear indicating
that the binding sites in NP@MIP are heterogeneous with respect
to the affinity for BPA. Because there are two distinct sections
within the plot which can be regarded as straight lines, it reveals
that two classes of binding sites were produced in NP@MIP. In
contrast, the plot was almost linear for NP@NIP, in agreement
with the presence of nonspecific binding sites only. From the
slopes and intercepts of the straight area in the range of 0–
(nC–H). These features, characteristic of poly(methacrylic acid),
indicate that the polymerization step has been effective.
To quantify the organic coating of the produced hybrids,
thermal gravimetric analyses were performed, in the temperature
ꢂ
ꢂ
ꢀ1
range of 20–800 C with a heating rate of 10 C min under
ꢀ
1
a flow of air at 80 mL min . The observed total weight losses
reported in Table 1) were assumed to be due to desorption and/
(
or decomposition of the organic coating shell leading to a direct
measurement of the organic weight in the studied hybrids. A
ꢀ1
1
2 mmol g of binding amounts (corresponding to the higher
binding amount region for the NP@MIP), the values of affinity
for the NP@MIP were
total weight loss of 18% was observed on heating NP-DEDTC to
ꢂ
constants KA1 (KA ¼ 1/K ) and B
8
00 C while the uncoated NPs showed a weight loss of only 4%
D
max
found to be ꢁ3 times higher than the values calculated for the
NP@NIP (see Table 2). From results shown in Table 2, it is clear
that although the chemical compositions of both NP@MIP and
NP@NIP are similar, their spatial structures are different with
the presence of specific rebinding sites for BPA in the polymer
(
probably due to some contamination species). This result
suggests a high grafting density of initiators on the nanoparticle
surface. Taking into account the average inorganic core diameter
D
TEM z 8 nm of single functionalized particles obtained from
ꢀ3
TEM, and the density of ferric oxide of ꢁ4.9 g cm , a specific
24
2
surface area of 153 m g is obtained which yields a surface
ꢀ1
18
ꢀ2
coverage G of ꢁ3.6 ꢃ 10 aryl molecules m . It is noteworthy
that this value is in the same order of magnitude as the surface
concentration of a close-packed monolayer GCPML of phenyl (or
25
4
-substituted phenyl) groups estimated from molecular models,
ꢀ2
18
G
CPML ¼ 8.1 ꢃ 10 aryl molecules m . In comparison, the
weight losses measured for NP@NIP and NP@MIP are sharply
higher, reaching 43 and 60%, respectively (see the TGA profiles
in Fig. S4†). This strong increase in organic weight percent after
polymerization indicates a high polymer surface coverage. It is
worth noting that the 60% organic weight loss measured on
NP@MIP falls to 47% after BPA extraction. From the difference
between these two values, one could deduce approximately the
Fig. 4 Numerical photographs of aqueous dispersions of NP@MIP
particles (a) in deionized water, (b) in basic medium (pH ¼ 8), and (c) in
the presence of a magnet.
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Fig. 6 Curve of adsorption kinetics for BPA to NP@MIP (CBPA
ꢀ
¼
1
ꢂ
0
.1 mmol L , mMIP ¼ 3 mg, T ¼ 20 C).
Fig. 5 Binding isotherms of NP@MIP (squares) and NP@NIP (circles)
for BPA. The inset shows the Scatchard plot for the binding of BPA.
good mass transport and thus overcoming some drawbacks of
traditional packing imprinted materials. Such fast kinetics is
consistent with the thin shell imprinted layers generated over the
iron oxide nanocores.
Table 2 Affinity constant (K
(
a
) and maximum number of binding sites
max) for NP@MIP and NP@NIP
B
4
/10 L mol
ꢀ1
ꢀ1
Sample
K
a
Bmax/mmol g
a
Conclusion
NP@MIP
1.5 ꢄ 0.2
17.3
26.0
5.3
b
0
.47 ꢄ 0.01
In summary, we have successfully synthesized ultra-small
dispersed magnetic nanocrystals coated by a thin layer of
a molecularly imprinted polymer, using a combination of aryl
diazonium salt chemistry and the iniferter method. Our approach
offers several advantages over conventional methods: (i) ease and
rapidity of NPs surface functionalization using diazonium salts;
NP@NIP
0.43 ꢄ 0.06
a
b
Value of KA1 corresponding to the high affinity binding sites. Value of
K
A2 corresponding to the low affinity binding sites.
overlayer of the MIP particles. These specific rebinding sites offer
a steric and an electronic microenvironment complementary to
that of the compounds of interest to be bound.
(
ii) stability of the covalent link between the inorganic core and
the organic coating; (iii) formation of small and dispersed
NP@MIP. We do believe this synthetic approach will not only
pave an additional way for the preparation of BPA imprinted
nanoparticles but also provide a new general nanomaterial
strategy design for the detection of toxic molecules.
To determine the binding selectivity of NP@MIP for BPA,
BPF was selected as an interfering molecule since its molecular
structure is quite similar to BPA. The results, displayed in the
ESI (Fig. S6†), show that NP@MIP exhibit good adsorption
selectivity for the template BPA with a higher binding capa-
ꢀ1
city for BPA (Bmax ¼ 17.3 mmol g ) than for BPF (Bmax
.5 mmol g ).
Binding experiments from variable-temperature runs were
used to evaluate thermodynamic properties according to the
¼
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ꢀ1
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This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 1807–1811 | 1811