Calix[4]arene-modified silica nanoparticles for the potentiometric detection of iron (III) in aqueous solution

Article abstract of DOI:10.1016/j.crci.2011.12.008

This article aims to present the synthesis and the characterization of a novel hybrid material, based on silica nanoparticles, and which can be used as a sensitive element for the electrochemical detection of iron (III). This material was obtained by graf

Full text of DOI:10.1016/j.crci.2011.12.008

C. R. Chimie 15 (2012) 290–297
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´
Full paper/Memoire
Calix[4]arene-modified silica nanoparticles for the potentiometric
detection of iron (III) in aqueous solution
M. Becuwe a,e, P. Rouge b,e, C. Gervais c, D. Cailleu d,e, M. Courty a,e, A. Dassonville-Klimpt b,e
,
P. Sonnet b,e, E. Baudrin a,b,e,
*
a UMR CNRS 6007, laboratoire de re´activite´ et chimie des solides, universite´ de Picardie Jules-Verne, 33, rue Saint-Leu, 80039 Amiens, France
b UMR CNRS 6219, laboratoire des glucides, faculte´ de pharmacie, universite´ de Picardie Jules-Verne, 1, rue des Louvels, 80037 Amiens, France
c
`
´
`
´
UMR CNRS 7574, laboratoire de chimie de la matiere condensee de Paris, college de France, UPMC universite Paris 06, 11, place Marcelin-Berthelot,
75231 Paris cedex 05, France
d Plateforme analytique, universite´ de Picardie Jules-Verne, 80039 Amiens, France
e FR3085 CNRS, universite´ de Picardie Jules-Verne, institut de chimie de Picardie, 33, rue Saint-Leu, 80039 Amiens cedex, France
A R T I C L E I N F O
A B S T R A C T
Article history:
This article aims to present the synthesis and the characterization of a novel hybrid
material, based on silica nanoparticles, and which can be used as a sensitive element for
the electrochemical detection of iron (III). This material was obtained by grafting an iron
ligand-based calix[4]arene structure onto nanosized silica particles. The covalent
anchoring of the chelator was obtained by nucleophilic coupling reaction between the
modified calix[4]arene and the chloropropyl functionalized silica. The grafting reaction
was confirmed by FTIR (DRIFT), solid-state 13C Cross-Polarization MAS (CPMAS) NMR and
thermal analysis. The interaction between the material and the dissolved iron (III) was
demonstrated though potentiometric measurements. A linear evolution of the open circuit
potential, which can be used analytically for iron (III) detection, has been obtained.
Received 14 October 2011
Accepted after revision 22 December 2011
Available online 7 February 2012
Keywords:
ICL670
Iron sensor
Potentiometry
Calix[4]arene
´
ß 2012 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
´
´
R E S U M E
Mots cle´s :
`
´
`
´
´
Cet article vise a presenter la synthese et la caracterisation d’un nouveau materiau hybride
ICL670
a` base de nanoparticules de silice et qui peut eˆtre utilise´ comme e´le´ment sensible pour la
Capteur de fer
Potentiome´trie
Calix[4]arene
´
´
´
´ ´
detection electrochimique du fer (III). Ce materiau a ete obtenu par greffage d’un
`
complexant organique du fer a base de calix[4]arene sur des particules de silice
nanome´triques. L’ancrage covalent du complexant e´te´ re´alise´ par substitution
a
´
´
´ ´ `
nucleophile entre le complexant et un fragment reactif insere a la surface des
´
´ ´
´
nanoparticules. La reaction de greffage a ete confirmee par IRTF (DRIFT), RMN solide
13C Cross-Polarization MAS (CPMAS) et par analyses thermiques. L’interaction entre le
mate´riau et du fer (III) dissous a e´te´ de´montre´e par mesures potentiome´triques. Une
´
´
´ ´
a ete obtenue, permettant ainsi
evolution lineaire du potentiel en circuit ouvert
´
d’envisager une detection analytique du fer (III) en solution.
ß 2012 Acade´mie des sciences. Publie´ par Elsevier Masson SAS. Tous droits re´serve´s.
1. Introduction
Due to its major role in various biological processes,
iron is one of the most vital elements [1]. To quote a few,
iron complexes are involved as oxygen carrier within the
body (haemoglobin, myoglobin) and are the reactive
* Corresponding author.
E-mail address: emmanuel.baudrin@u-picardie.fr (E. Baudrin).
1631-0748/$ – see front matter ß 2012 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.crci.2011.12.008

M. Becuwe et al. / C. R. Chimie 15 (2012) 290–297
291
catalytic centres in several enzymes. Thus, deregulation of
its metabolism can have major impacts. Abnormal iron
storage, observed in genetic hemochromatosis disease, can
cause liver and kidney damage [2]. Similarly, there seems to
be a correlation between the increased amount of iron in the
brain and cases of Huntington’s, Parkinson’s and Alzhei-
mer’s diseases [3]. On the other hand, deficiencies or a
problem to use iron leads to anaemia [4]. Consequently, the
conception ofnew analyticaltoolstoaccuratelymeasurethe
iron concentration, in a selective and rapid manner, would
be very helpful both as a diagnostic tool and as a research
tool to probe iron metabolism for a better understanding.
For such applications, the design of functionalized micro-
electrodes is appealing as it would allow following intra-
and extra-cellular ion concentrations [5]. Recently, different
studies aiming at developing potentiometric iron sensors
have been reported. These are mostly PVC-based mem-
branes containing specific ionophores such as crown ether
[6], EDTA derivatives [7], benzylthiocarbohydrazide [8],
imidazolidine [9] or cyclam [10].
Formation of silica-based hybrid materials (either class
I or class II) was largely reported, during the last 10 years,
as a promising route towards new sensitive elements for
the development of efficient ion sensing tools [11–14]. The
main advantage of this kind of material is the possibility of
truly tailoring the inorganic matrix formed using the very
versatile sol-gel route [15–17]. Such chemistry allows the
chemical modification of silica by either a co-condensation
or a post-grafting process with organosilane reactive
groups, which permit the anchoring of ionophores or
molecule-selective receptors. Consequently, the domain
has become very prolific and many hybrid materials have
been created and designed for the electrochemical or
optical detection of various species such as, for instance,
DNA [18], phenol [19], trinitrotoluene [20], pesticides [21]
or ionic species [22–24].
To the best of our knowledge, while the detection of
cations, including some transition metal ions, was reported
(for instance, Hg2+ [25] Cu2+ [26] and Cr3+ [27]), no hybrid
material was reported for iron sensing. First, we studied
the potentiometric response of cyclam-modified silica
[28]. This study demonstrated that an optimized iono-
phore-modified silica matrix can allow the formation of
potentiometric ion sensors (similar materials were previ-
ously reported for sensing but in an amperometric mode
[29]). We report here the use of specifically designed
ligands for the detection of iron (III).
The idea was to first design a supramolecular receptor
incorporating a known specific tridentate iron chelator: 4-
[3,5-bis(2-hydroxyphenyl)-1,2,4-triazol-1-yl]-benzoic acid
called ICL670. This was recently developed as a therapeutic
agent (Deferasirox, commercial designation) for the treat-
ment of iron overload and approved in 2005 by the US
Federal and Drug Administration (FDA). The macromolecu-
lar receptor was thus prepared and consists of a calix
[4]arene platform onto which the chelator was grafted
[30]. Association of two ICL670 chelators within the same
molecule should support the octahedral complexation
from the two tridentate ligands (calix-bisICL) [31].
Due to the specific chemistry of calixarenes, we were
able to introduce an amino group fragment onto the
calix[4] arene, in partial cone conformation, to enable the
grafting process onto an organo-modified silica material
(Fig. 1).
Fig. 1. Representation of the organo-modified silica particles prepared for iron (III) sensing.

292
M. Becuwe et al. / C. R. Chimie 15 (2012) 290–297
The formation of hybrid materials containing calixarene
which can be quantified by elemental analysis. Such a
method allows the quantification even in the presence of
residual ethoxy groups coming from either the native Si-
Nps or from the incomplete hydrolysis of the organo-
silane. Typically, we performed the reaction of the CP-Si-
Nps with a large excess of sodium azide in the presence
of a quaternary salt (TetraButylAmmoniumBromide) to
entirely transform the chloromethylene entities into
azidomethylene. After this step and after verifying the
total absence of residual chlorine by EDX, the nitrogen
content of the azidopropyl fragment (visible by FTIR at
2100 cm–1), was determined using elemental analysis.
This way we obtained 0.45% of nitrogen by gram of
material indicating 0.11 mmol.g–1 of azide and
by deduction 0.11 mmol of chlorine per gram of
CP-Si-Nps.
entities has been previously reported, for either the
preparation of stationary phases for chromatographic
applications [32,33] or for the use of the calixarene
derivative as support to conceive selective caesium
recovering material [34,35].
Here, we present the detailed study of the synthesis and
the characterization of such type of new hybrid material
specifically designed for the potentiometric detection of
iron (III) in aqueous media. This new material was obtained
by grafting the modified calix[4]arene (calix-bisICL) on
nanosized spherical silica particles prepared using the
Sto¨ber’s process (Fig. 1). We finally report on the
potentiometric properties of this new sensitive material
towards dissolved iron (III).
2. Experimental
2.2.4. Synthesis of the calixarene-modified silica
nanoparticles (calix-Si-Nps)
2.1. Chemicals
The immobilization of the calixarene platform (calix-
bisICL) on the chloropropyl-silica nanoparticles (calix-Si-
Nps) was carried out in dry acetonitrile in the presence of
TriEthanolAmine (TEA). Typically, one equivalent of
functionalized calix[4]arene (per chloropropyl linker)
and TEA were added to a suspension of CP-Si-Nps in
20 mL of acetonitrile. The mixture was heated under reflux
and magnetically stirred for 16 h; the sample was
recovered by centrifugation at 12,000 rpm. Finally, after
three washing cycles with dichloromethane, ethanol and
diethylether, the calix-Si-Nps were dried under vacuum at
100 8C for 5 h.
TetraEthOxySilane (TEOS, 98%), ammonia (28%) and
absolute ethanol were purchased from Acros Chemicals and
used as such. Chloropropylethoxysilane, acetonitrile,
TriEthanolAmine (TEA) and all the washing solvents were
obtained from Aldrich and used as received. The calix[4]-
arene-modified with the ICL670 iron (III) chelator was
synthesized following the recently reported procedure [30].
2.2. Synthesis
2.2.1. Synthesis of the silica nanoparticles (Si-Nps)
Silica nanoparticles (Si-Nps) were obtained by using the
Sto¨ber’s process [36]. The synthesis consists of a tempera-
ture-controlled hydrolysis-condensation of TEOS in alco-
holic media. Briefly, to a mixture of 100 mL of absolute
ethanol and 7.5 mL of ammonia (28%) stirred at 400 rpm,
we added 3 mL of TEOS at 70 8C. After 15 min stirring, a
translucent colloidal suspension is formed and maintained
under stirring for a further 12 h to complete the siliceous
precursor hydrolysis; the nanoparticles were recovered by
centrifugation (12,000 rpm) and abundantly washed with
absolute ethanol and distilled water.
2.3. Characterization
The particle morphologies were investigated using an
Environmental Scanning Electron Microscope (ESEM) FEI
Quanta 200 FEG equipped with an EDS analysis system
(Oxford Link Isis). FTIR spectra, in a diffuse reflectance
mode (DRIFT), were recorded at room temperature with a
Nicolet AVATAR 370 DTGS spectrometer provided by
Thermo Electron Corporation. The spectra were acquired at
room temperature from 400 to 4000 cm–1 with a resolu-
tion of 2 cm–1
Thermo-Gravimetric Analyses (TGA) were performed
Netzsch thermal analyzer STA 449 C Jupiter
equipped with Differential analysis microbalance.
.
2.2.2. Synthesis of the chloropropyl-silica nanoparticles (CP-
Si-Nps)
on a
a
Typically, 1 mL of chloropropyltriethoxysilane was
added to a suspension of 1 g of silica in dry toluene
(30 mL). The resulting mixture was heated at 90 8C for
16 hours under magnetic stirring. Then, the reactive
nanoparticles were recovered by centrifugation
(12,000 rpm) and washed successively with dry toluene
and ethanol. The as-synthesized material was finally dried
under vacuum at 90 8C for 5 h.
The samples (10 to 15 mg) were heated, in an alumina
crucible, under air from RT to 1000 8C with a heating rate
of 5 8C/minute. Elemental analyses (C, H and N) were
recorded on a Thermo Finnigan EA 1112 with a Sartorius
MC balance with a precision of Æ0.2% at the Spectropole
(Marseille, France). All the analyses were reproduced three
times to confirm the organic content of the materials.
Solid-state 13C MAS experiments were performed at
7.05 T on a Bruker AVANCE 300 spectrometer using a 4 mm
MAS Bruker probe. The Cross-Polarization MAS (CPMAS)
pulse sequence was used with a contact time of 1 ms,
spinal-64 1H decoupling and a spinning speed of 14 kHz.
13C chemical shifts were referenced to external adaman-
tine (used as a secondary reference), the high frequency
peak being set to 38.5 ppm.
2.2.3. Determination of the chloropropyl content in
chloropropyl-silica nanoparticles (CP-Si-Nps)
To determine the amount of chloropropyl linkers
grafted onto the silica surface, we first carried out a
quantitative chemical transformation of this function
(checked by EDX) into a nitrogen-containing linker,

M. Becuwe et al. / C. R. Chimie 15 (2012) 290–297
293
Fig. 2. Synthesis route of the modified ICL670-based calix[4]arene designed for iron (III) complexation; i: bromide, Cs2CO3, DMF; ii: aqueous hydrazine 35%,
EtOH; iii: ICL670, HOBt, EDCI, CH2Cl2; iv: TFA, CH2Cl2.
For electrochemical characterization, electrodes were
prepared using the plastic technology initially developed
by Bellcore for Li battery electrode formation and
implemented for sensor developments [37]. Typically,
the sensitive material is manually ground with 17 wt.% of
SP type carbon black (conducting additive), 25 wt.% of
PVDF-HFP copolymer and 48 wt.% of DiButylPhtalate (DBP)
plasticizer. Acetone is then added to the mixture so as to
obtain a paste that can be cast over a glass plate. After
drying, a plastic composite film is obtained, and can be cut
to the required size according to the measurement to be
performed. The film is then laminated on a platinum grid
under a 20 psi pressure at 135 8C. Prior to being used, the
DBP plasticizer is extracted using diethyl ether (two
washings, each of 15 min). Open circuit voltage and
potentiostatic measurements were carried out using a
three-electrode cell configuration with a saturated calomel
electrode (SCE) and Pt wires as reference and counter
electrodes, respectively. For the potentiometric measure-
ments, the three electrodes were immersed in aqueous
FeCl3 solutions with concentrations ranging from 10–1
3. Results and discussion
As described above, the first step was to obtain the
ionophore designed to interact with dissolved iron (III). For
that, we used a bisICL670 substituted calix[4]arene in
partial cone conformation to chelate the iron (III). The
synthesis process was extensively described in a previous
paper [30], and we will here only summarize the procedure
(Fig. 2). The calix[4]arene was used as a platform with the
incorporation of, firstly, two ICL670 molecules on the same
side (for the formation of an hexadentate complex) and,
secondly, an alkylamine function on the other side for the
grafting on the silica particles. Starting with a calix[4]arene
syn-1,3-diether protected with phtalimides, we intro-
duced a single boc-protected alkylamine with a 30% yield
leading to a 1,3-alternate conformation. Note that this
conformation is ‘‘ideal’’ keeping in mind the grafting of the
ionophore. After removing the phtalimides groups, pep-
tidic couplings with ICL670 were realized and followed by
the Boc group cleavage. The formed supramolecular
system enables the complexation of iron (III) using the
to 10–6 M after a 2-hour ageing period in a 10–2
solution.
M
two tridentate ICL670 in
(L. Dupont, Private Communication) (Fig. 3).
a preformed conformation
Fig. 3. Representation of iron (III) coordination with the bisICL modified calix[4]arene.

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M. Becuwe et al. / C. R. Chimie 15 (2012) 290–297
Fig. 4. Scanning Electron Microscope (SEM) pictures of native silica particles (a), chloropropyl-modified silica particles (b), bisICL670 calix[4]arene-
modified silica particles (c).
The second step for the building of hybrid materials
sensitive to iron (III), consisted in the preparation of the
inorganic matrix. For that, we have chosen to prepare the
silica nanoparticles using the well-known Sto¨ber’s process
[32]. Using a 70 8C temperature for the inorganic polyme-
rization process, we obtained well-defined monodispersed
silica nanoparticles (Fig. 4a). The average diameter,
determined from image analysis, is 50 nm and consistent
with previous studies of hydrolysis-condensation process-
es performed at 70 8C [38]. The following step was to link/
couple the modified calix[4]arene with these nano-objects.
The particle was organically modified with chloropropyl
linkers (Fig. 5), by a reaction of a silylated precursor with
the surface hydroxyl groups of silica. As expected, no
modification of the particle size or morphology was
observed (Fig. 4b). The final step consisted in a SN coupling
Fig. 5. Synthesis scheme of the formation of the calix[4]arene-modified silica particles.

M. Becuwe et al. / C. R. Chimie 15 (2012) 290–297
295
2
1.8
1.6
1.4
1.2
1
1.9
1.8
(c)
1.7
1.6
(b)
(a)
(b)
(a)
1.5
1.4
1.3
1.2
Free
OH
2000
1800
1600
1400
1200
1000
4000 3800 3600 3400 3200 3000 2800
Wavenumber (cm-1)
Wavenumber (cm-1)
Fig. 7. Comparison of the DRIFT spectra for bisICL670 calix[4]arene (a),
bisICL670 calix[4]arene-modified silica particles (b), native silica
particles (c).
Fig. 6. DRIFT spectra of native silica particles (a), bisICL670 calix[4]arene-
modified silica particles (b).
of the propylamino modified calix[4]arene, with the
chloropropyl-modified silica. The functionalization of the
silica particles was first examined by FTIR analyses in
DRIFT mode which gives enhanced chemical information
from the surface as compared to classical transmission
FTIR. As generally observed for a grafting process on
preformed silica particles, we noticed the complete
disappearance of the free silanol vibration (Fig. 6b) initially
located at 3750 cm–1 (Fig. 6a). This indicates that these free
silanols have totally reacted with the organosilane to form
siloxane bridges. Then the calixarene derivatives were
coupled with this surface organosilane, inducing quite
small changes in the FTIR spectra (Fig. 7b). We similarly
measured the spectrum of the native silica and the
modified-calixarene prior to the grafting. From these
spectra, we have a first clue of the grafting of the
calixarene onto the silica spheres as underlined by the
vibration bands between 1650 cm–1 and 1400 cm–1
corresponding to aromatic vibration bands of the calix[4]-
arene platform and ICL670 fragment.
Fig. 8. Solid-state 13C Cross-Polarization MAS (CPMAS) NMR spectra of the chloropropyl-modified silica (a), bisICL670 calix[4]arene (b), the new hybrid
material bisICL670 calix[4]arene-modified silica (c); *: residual ethoxy fragment.

296
M. Becuwe et al. / C. R. Chimie 15 (2012) 290–297
Table 1
A further proof of the successful functionalization
Elemental analysis obtained for the chloropropyl and calixarene-
modified silica particles and subsequent estimation of the calixarene
content.
process of the silica nanoparticles is brought by 13C solid-
state NMR experiments. Fig. 8 gathers the spectra of the
chloropropyl modified silica (a), the calix-functionalized
particle (c) and for comparison the spectrum of the pure
organochelator (b) (that is the bisICL670 propylamine
calix[4]arene, represented on Fig. 3). These spectra were
recorded at 14 kHz using a CPMAS sequence. Signals
related to the chloropropyl fragment of the chloropropyl
functionalized silica (Fig. 8a) are clearly observed at
11 ppm (CH2-Si), 28 ppm (central CH2) and 44 ppm
(CH2-Cl) respectively, in agreement with previous works
[39]. Additional peaks at 17 and 60 ppm arise from residual
ethoxy groups resulting from incomplete hydrolysis of the
sol-gel precursors (either TEOS, or the chloropropyl silane).
After the coupling reaction, the obtained calix-Si-Nps
material spectrum (Fig. 8c) shows the main signals of pure
calixarene bisICL (Fig. 8b). At high chemical shift values,
three characteristic broad peaks clearly observed at 170 to
152 ppm, 146 to 138 ppm and 136 to 113 ppm can be
assigned to the carbonyl/phenolic quaternary carbon,
triazole fragment resonance and the different ICL670
aromatic carbons, respectively. Furthermore, we can also
distinguish a broad peak in the 65 to 82 ppm range
corresponding to the CH2-O carbon of the alkyl oxide
chains linked to the calixarene unit [30]. Besides, within
the alkyl carbon region, we can distinguish a new peak at
35 ppm attributed to the secondary amine resulting from
the anchoring of the calixarene species on the chloropropyl
group of the organosilane. The covalent anchoring was also
corroborated with the decrease of the Cl-CH2 peak initially
observed at 44 ppm. Thus these spectra clearly demon-
strate the successful binding of the modified calix[4]arene
on the silica matrix.
%C
4.84
%H
1.23
%N
–
CP-Si-Nps
Calix-Si-Nps
Difference
10.32
5.48
55
1.71
0.48
0.74
0.74
58
Calixarene content (
m
mol/g)
64
(C, H, N) from which we also estimated the calixarene
content using either C, H or N composition. Using the
nitrogen content, which is not initially present in the
chloropropyl modified silica (Table 1), we can thus
unambiguously conclude that we have grafted about
58
mmol of calixarene per gram of material, that is to
say a grafting yield of 52% on the chloropropyl liner (about
half of the present chloropropyl).
Finally, we determined the ability of this original hybrid
material to interact and detect iron (III) in solution. This
was realized using the plastic Bellcore technology for the
formation of the electrodes [40,41]. First, the electrodes
were conditioned in a 10–2 mol.L–1 solution for 2 h prior to
the measurements. Such a conditioning step is common for
ion sensing and leads to
a stable and reproducible
electrode state. The open circuit potentials were then
measured in solution with concentrations ranging from
10–2 to 10–6 mol.L–1. As seen on Fig. 10, the potential
stabilized with a response time function of the concentra-
tion, namely the lower the concentration, the higher the
response time. The reversibility of the equilibrium taking
place during the measurement is good as the measure of
the potential in a 10–2 mol.L–1 Fe (III) solution after the test
gives back the pristine value. The open circuit voltage
evolves linearly along the all tested concentration range
showing that the prepared material can be used for
quantitative determination of dissolved iron (III) with a
sensitivity of 53 mV/decade of concentration (Fig. 11). The
next step will be to evaluate the properties of this system
in more complex media to evaluate, for instance, the
We estimated the amount of calix[4]arene grafted on
the silica nanoparticles by thermo-gravimetric analysis
(Fig. 9). For the calculation, we subtracted the weight loss
observed for the chloropropyl-silica from that of the final
material. The 7.74% extra-loss corresponds to the replace-
ment of the chlorine atom by the bisICL amine-modified
calixarene and leads to 56 mmol of calixarene per gram of
silica. This is confirmed by the elemental analysis results
0.65
10-1
M
10-1
M
10-2
M
10-3
M
0.55
0.45
0.35
10-4
M
10-5
M
10-6
M
0
2000
4000
6000
8000
Time (s)
Fig. 9. Comparison of the thermo-gravimetric curves obtained for a
heating rate of 10 8C/min for the chloropropyl-modified silica (a),
bisICL670 calix[4]arene-modified silica (b).
Fig. 10. Response at 30 8C of bisICL670 calix[4]arene-modified silica-
based plastic composite electrode as a function of iron (III) concentration.

M. Becuwe et al. / C. R. Chimie 15 (2012) 290–297
297
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We presented, in this work, the preparation and
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of forming new analytical tools for the potentiometric
detection of iron (III) in solution. This material is made of
silica nanoparticles obtained by the Sto¨ber’s process onto
which an original iron’s chelator was grafted. The chelator
consists in a calixarene unit in a partial cone conformation
modified with two iron (III) ligands (ICL670, Deferasirox).
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in aqueous solution. We observed a good linear potentio-
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the iron (III) complexation equilibrium taking place at the
electrode surface. These promising results pave the way to
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