Selective and sensitive chromofluorogenic detection of the sulfite anion in water using hydrophobic hybrid organic-inorganic silica nanoparticles

Article abstract of DOI:10.1002/anie.201306688

In water and wine: Chromofluorogenic detection of the sulfite anion in pure water was accomplished by using a new hybrid organic-inorganic material that contained a probe entrapped in hydrophobic biomimetic cavities. This material was used for the detecti

Full text of DOI:10.1002/anie.201306688

Angewandte
Chemie
DOI: 10.1002/anie.201306688
Sensors
Selective and Sensitive Chromofluorogenic Detection of the Sulfite
Anion in Water Using Hydrophobic Hybrid Organic–Inorganic Silica
Nanoparticles**
Luis Enrique Santos-Figueroa, Cristina Gimꢀnez, Alessandro Agostini, Elena Aznar,
Marꢁa D. Marcos, Fꢀlix Sancenꢂn, Ramꢂn Martꢁnez-MꢃÇez,* and Pedro Amorꢂs
Nowadays, sulfites or sulfiting agents such as sodium, calcium,
and potassium sulfite (SO32À); metabisulfite (S2O52À); and
bisulfites (HSO3 ) as well as sulfur dioxide (SO2) are
especially in fresh products as salads, fruit, mincemeat, or
sausages, is prohibited.[1,7]
À
Sulfur dioxide is an important and very common air
pollutant. When SO2 is dissolved in aqueous media a pH-
dependent equilibrium occurs and it favors the formation of
sulfite and bisulfite at neutral pH value.[8] Many studies
suggested that extended exposition to SO2 and/or its deriv-
atives could produce different toxicological effects such as
cancer, cardiovascular diseases, neurological disorders, and
the change of the characteristics of voltage-gated sodium and
potassium channels.[9]
compounds widely used as preservative and antimicrobial
agents to prevent browning of foods and beverages (E220–228
additives).[1] However, several studies had associated topical,
oral, or parenteral exposure to high doses of sulfite with
adverse reactions as dermatitis, urticaria, flushing, hypoten-
sion, abdominal pain, and diarrhoea.[2] In fact, several reports
confirmed that some people can be extremely sensitive even
to very low sulfite levels[3] and that bronchoconstriction can
occur in many asthmatic patients[4] or in people exposed to
high doses.[5] Exposure to high doses of sulfite can occur for
consumption of food and drinks that contain this additive (as
fruits, vegetables, salads, meat, gelatine, juices, vinegar, soft
drinks, beer, wine, and others), through the use of several
drugs (adrenaline, phenylephrine, corticosteroids, and local
anaesthetics), some cosmetics (hair colors and bleaches,
creams, and perfumes) or in some occupational settings
(leather, textile, mineral, pulp, rubber, agriculture, and
chemical industries).[2a] The addition of low levels of sulfite
(as low as 0.7 mgkgÀ1 of body weight dictated by FAO/
WHO)[6] are permitted in beer, wine, and some food under
rigorous control, but in many countries their addition,
Taking into account the above-mentioned facts, the
interest in the development of fast and efficient methods for
sulfite detection has increased in the last years. In particular,
methods based on electrochemistry,[10] spectrophotometry,[11]
chromatography,[12] capillary electrophoresis,[13] and titra-
tion[8b,14] have been extensively used for the detection and
quantification of sulfite. Recently, the development of
chromofluorogenic sensors for anion detection has become
a field of interest, since they usually offer several advantages
in terms of sensitivity, selectivity, and simplicity of operation
over classic, nonportable, and expensive instrumental analy-
sis.[15] In spite of these advantages, few chemosensors for the
chromofluorogenic detection of the sulfite anion have been
described. In this field, specific reactions of sulfite with
aldehydes,[16] levulinate esters,[17] Michael-type additions,[18]
and coordinative interactions[19] have been recently used.
However, some of those reported probes show certain
drawbacks such as low sensitivities and selectivities, poor
performane in pure water, the need for using acidic environ-
ment (pH < 5.5), or large response times. Moreover, very
recently some chemosensors for sulfite detection with good
stability based on the use of carbon quantum dots,[20] gold
nanoparticles,[21] and polymers[22] have been reported. In
addition, some sulfite biosensors have been described based
on the aerobic oxidation of sulfite by immobilized sulfite
oxidase and its electrochemical breakdown under high
voltage.[23] These biosensors are sensitive even in pure
water, however, have generally low stability, significant
metabolite interference, short life, and high cost.
[*] L. E. Santos-Figueroa, C. Gimꢀnez, Dr. A. Agostini, Dr. E. Aznar,
Dr. M. D. Marcos, Dr. F. Sancenꢁn, Prof. R. Martꢂnez-MꢃÇez
Instituto de Reconocimiento Molecular y Tecnolꢁgico
Centro Mixto Universidad Politꢀcnica de Valencia-Universidad de
Valencia (Spain)
and
Departamento de Quꢂmica, Universidad Politꢀcnica de Valencia
Camino de Vera s/n, 46022 Valencia (Spain)
and
CIBER de Bioingenierꢂa, Biomateriales y Nanomedicina (CIBER-
BBN)
E-mail: rmaez@qim.upv.es
Homepage: http://idm.webs.upv.es/
Prof. P. Amorꢁs
Instituto de Ciencia de los Materiales (ICMUV)
Universidad de Valencia (Spain)
[**] Financial support from the Spanish Government (project MAT2012-
38429-C04) and the Generalitat Valenciana (project PROMETEO/
2009/016) is gratefully acknowledged. We also thank the Fundaciꢁn
Carolina and UPNFM-Honduras for a doctoral grant to L.E. Santos-
Figueroa and to the Spanish Ministerio de Ciencia e Innovaciꢁn for
a doctoral grant to C. Gimꢀnez.
We herein report the development of a simple material for
the selective and sensitive chromofluorogenic recognition of
sulfite in aqueous solution. We have used functionalized
MCM-41 nanoparticles containing a suitable sulfite probe
within highly hydrophobic mesopores. The structure of the
used organic probe 2 and the synthetic procedure for the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201306688.
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The final hybrid nanoparticles N1 were prepared by
suspending N0 in an acetone solution of 1 (red) and allowing
its diffusion into the hydrophobic cavities for 24 h. During the
preparation of N1 the color of the solid changes from white to
blue. This is a consequence of the inclusion of probe 1 into the
hydrophobic cavities, through a simple adsorption process,
which transformed the red pyrylium stilbene 1 into the blue
quinone 2 (formed by the spontaneous deprotonation of
probe 1). By thermogravimetric and elemental analysis,
a content of 0.038 mmol of 2 per g SiO2 in the final N1
sensing material was determined.
To characterize this red-to-blue color change, a solution of
probe 1 in acetonitrile was reacted with 1,8-diazabicyclo-
[5.4.0]undec-7-ene (DBU, a non-nucleophilic base) to give an
immediate color modulation from red to blue (attributed to 2,
see the Supporting Information). This final blue color was the
same as that observed in the sensing N1 nanoparticles.
Moreover, this color transformation went along with changes
1
in the H NMR spectrum: the signals of the double bond
protons (Ha and Hb) of 1 showed significant upfield shifts with
a reduction of the coupling constant (from 16 to 12 Hz) upon
addition of DBU to give 2. Moreover, the hydroxy proton
signal of 1 centered at 10.8 ppm disappeared.
The nanoparticles N0 and N1 were characterized by
standard procedures. Powder X-ray diffraction (PXRD) of as-
synthesized siliceous MCM-41 (see the Supporting Informa-
tion) shows four low-angle reflections typical of a hexagonal
array that can be indexed as (100), (110), (200), and (210)
Bragg peaks. The PXRD patterns of N0 and N1 (see also the
Supporting Information) clearly preserve the reflections
(100), (110), and (200), thereby evidencing that the surface
functionalization and the further loading process with 1 did
not damage the mesoporous scaffold. The presence of the
mesoporous structure in the starting MCM-41 samples,
hydrophobic nanoparticles N0, and final sensory material
N1 was also observed by using TEM analysis (see the
Supporting Information).
To further explore different formats and to potentially
enhance applicability of N1, the nanoparticles were included
(as a coating) in a rigid monolith with the aim to design ready-
and simple-to-use dipsticks for the rapid “in situ” chromo-
fluorimetric screening of sulfite. In particular, a ceramic foam
monolith was selected because of its trimodal hierarchical
pore system with a high external surface, good mechanical
properties, and the possibility to achieve a high degree of
coverage with silica nanoparticles.[26]
The ceramic foam monolith was prepared through the
replication of commercially available and inexpensive poly-
urethane foam with a ceramic slip. Then, the surface area of
the ceramic foam was activated by using an alkaline-hydro-
thermal treatment. After the activation process, the coating of
the monolith with MCM-41 was carried out by succesive
impregnation cycles (4 times) in water suspensions of the
nanoparticles followed by a soft thermal treatment. Then, the
MCM-41-coated ceramic foam was hydrophobized, thereby
yielding the monolith M0. Finally, their pores were loaded
with derivative 1 resulting in the final blue ceramic foam
sensing monolith M1 (see Figure 1).
Scheme 1. Representation of the preparation of N0 nanoparticles, the
final sensing material N1 (with probe 2 located inside the hydrophobic
cavities), and proposed chromofluorogenic reaction with sulfite anion.
preparation of the hybrid sensing nanoparticles (N1) are
shown in Scheme 1.
The design of the probe involves the preparation of
a sensing material that should ideally be hydrophobic enough
to maintain the probe in the nanopores but not too hydro-
phobic in order to obtain stable suspensions of the nano-
particles (note that highly hydrophobic nanoparticles tend to
float in water, which inhibits fast reaction with the analyte).
After several attempts (see the Supporting Information for
a detailed discussion) the following procedure was selected
for the preparation of N1. MCM-41 mesoporous nanoparti-
cles (diameter of ca. 100 nm) were selected as inorganic
scaffolds.[24,25] Before the extraction of the cetyltrimethylam-
monium bromide (CTAB; used as structure-directing agent),
the external surface of the silica nanoparticles was function-
alized with propyltrimethoxysilane. Then the CTAB located
inside the pores was extracted with hydrochloric acid. Finally,
the inner sides of the pore walls were hydrophobized with
hexamethyldisilazane (see the Supporting Information for
details). These experimental procedures yield inorganic
nanoparticles (N0) containing hydrophobic pockets. The
organic content in N0 was determined by thermogravimetric
and elemental analysis and amounts to 0.22 mg of organic
matter per mg SiO2.
2
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enhancement of the emission (Figure 2) with a concomitant
visible color change of the solid from blue to pale-yellow (see
the Supporting Information). The HSÀ anion also induced the
appearance of the fluorescence band at 460 nm, but of less
intensity. Remarkably, the addition of the other selected
analytes induced negligible changes in the emission of N1
nanoparticles. The same selective chromofluorogenic
response was observed when using the blue ceramic foam
monolith M1 (data not shown).
The change in color and in emission of 2 upon addition of
sulfite anions is ascribed to a 1,6-conjugated addition
reaction, which yielded the phenol 3 (Scheme 1). This
addition was confirmed by NMR experiments (Figure 3)
Figure 1. SEM images of A) the ceramic foam before coating and
B) after four impregnation cycles with MCM-41 nanoparticles.
C) Monolith M1 and D) SEM image of M1 macropores.
A preliminary study demonstrated that aqueous solutions
of sulfite in the presence of N1 or M1 were able to change the
color of the solids from blue to pale yellow and at the same
time the solids became fluorescent. The pH dependence of
color stability of N1 was also evaluated. N1 nanoparticles
were suspended in water at different pH values and the blue
color remained in the 6–9 pH range, with an optimum blue
color at pH 7.5 (see the Supporting Information for details).
Figure 2 shows the emission behavior of aqueous buffered
(HEPES 30 mm, pH 7.5) suspensions of N1 nanoparticles in
Figure 3. 1H NMR spectra of 2 (A) and 3 (B, obtained upon addition
of an excess of sulfite to probe 2) in [D6]DMSO.
carried out with quinone 2 (obtained upon deprotonation of
1 with DBU). As shown in Figure 3, the most remarkable
signals of quinone 2 are two doublets centered at 8.1 (proton
Hb in Scheme 1) and at 6.7 ppm (proton Ha in Scheme 1).
Addition of sulfite to 2 induced a marked upfield shift of both
Ha and Hb protons to 5.45 (Ha) and 4.65 ppm (Hb) attributed
to a loss of aromatic character upon reaction with sulfite
anion. HSQC studies (see the Supporting Information)
indicated the existence of a clear correlation between
proton Hb and a benzylic carbon (at 63.4 ppm) and also
between proton Ha and an olefinic carbon (at 116.8 ppm).
The selective response toward sulfite, shown by 2 when
incorporated in N1 nanoparticles, is ascribed to the prefer-
ential inclusion of sulfite into the highly hydrophobic pockets
in N1, which favors the 1,6-conjugated addition reaction. The
more hydrophilic HSÀ anion was partially included in the
hydrophobic cavities, whereas nucleophilic biological thiols
(such as Cys, Hcy, and GSH) are too polar and too large to be
internalized in the porous network of N1 nanoparticles. In
fact parallel assays carried out using DMSO solutions of 2
demonstrated that 2 is poorly selective and reacted similarly
with different nucleophiles such as sulfite, sulfide, GSH, Cys,
and Hcy.
Figure 2. Fluorescence intensity at 460 nm (lex =355 nm) of N1 nano-
particles in HEPES buffer (30 mm at pH 7.5) in the presence of
10 equivalents of selected anions, biological thiols, and oxidants.
Hcy=homocysteine, GSH=glutathione, Cys=cysteine.
Having assessed the selective response of N1 nanoparticle
suspensions to sulfite anions, the sensitivity of the probe was
studied by monitoring the emission changes of aqueous
buffered (HEPES 30 mm, pH 7.5) suspensions of N1 nano-
particles upon addition of increasing quantities of sulfite.
Increasing the concentration resulted in a progressive and
immediate enhancement of fluorescence intensity at 460 nm
(Figure 4). From these studies, a remarkable limit of detection
(LOD) of 8 mm (0.32 ppm) was calculated. Besides, N1
the absence and in the presence of selected anions (HSÀ, ClÀ,
BrÀ, IÀ, FÀ, AcOÀ, N3À, NO3À, HCO3À, SO42À, SO32À, S2O3
,
2À
HPO42À, and citrate), biological thiols (Cys, Hcy, and GSH)
and oxidants (H2O2). In a typical experiment N1 (5 mg) was
suspended in buffered water (200 mL) containing the corre-
sponding analyte. Then the solid was isolated by centrifuga-
tion and the fluorescence at 460 nm (lex = 355 nm) was
measured. The addition of sulfite anions induced a selective
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In summary, we have reported here a sensing hybrid
material (N1) for the simple chromofluorogenic detection of
sulfite anions in pure water. The hybrid material was based in
MCM-41 mesoporous nanoparticles with hydrophobic cav-
ities able to encompass the chromofluorogenic probe 2. Of all
the anions tested, only sulfite was able to induce a remarkable
color change (from blue to pale yellow) with a high emission
enhancement. The sulfite detection was selective and sensi-
tive with a limit of detection of 0.32 ppm. In addition, N1
nanoparticles and M1 monolith were used for sulfite detec-
tion in a complex matrix such as wine. Especially the use of
systems such as M1 opens the possibility of sensing sulfite
with the naked eye by using a simple dipstick assay, which
may find applications in food and environmental analysis.
Moreover the study also demonstrated that the inclusion of
chromofluorogenic probes into hydrophobic biomimetic
cavities of mesoporous supports is a very simple and promis-
ing approach to design sensing materials for anions able to
display sensing features in pure water.
Figure 4. Emission intensity at 460 nm (lex =355 nm) of buffered
(HEPES 30 mm, pH 7.5) suspensions of N1 nanoparticles upon
addition of increasing quantities of sulfite.
presents an accurate sensitivity for its potential application in
food and in environmental analysis, based on both U.S.A.[6–7]
and E.U.[1] standards.
Received: July 31, 2013
Revised: September 26, 2013
Published online: && &&, &&&&
Based on this promising proof-of-principle and taking into
account the favorable spectroscopic response of N1 and M1,
we explored the possibility of using these materials for the
detection of sulfite in complex real samples. In particular we
selected sulfite-free red wine that was bleached by simple
addition of active carbon and spiked with 20 ppm of sulfite.
Then, M1 monolith was dipped in the doped wine samples
and a remarkable and immediate color change from blue to
pale yellow was observed (Figure 5A), whereas when the
Keywords: food analysis · mesoporous nanoparticles · organic–
.
inorganic hybrid materials · sensors · sulfite
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Figure 5. A) Photograph showing the M0 and M1 monoliths before
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(355 nm).
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4
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[27] Concentrations of less that 10 ppm were selected because this is
the maximum concentration limit allowed for red wine that is
[19] a) J. Xu, K. Liu, D. Di, S. Shao, Y. Guo, Inorg. Chem. Commun.
2007, 10, 681 – 684; b) R. C. Rodrꢁguez-Dꢁaz, M. P. Aguilar-
2À
sold as SO3 free wine according to E.U. regulations.
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Sensors
L. E. Santos-Figueroa, C. Gimꢁnez,
A. Agostini, E. Aznar, M. D. Marcos,
F. Sancenꢂn, R. Martꢃnez-MꢄÇez,*
P. Amorꢂs
&&&& — &&&&
Selective and Sensitive
Chromofluorogenic Detection of the
Sulfite Anion in Water Using
Hydrophobic Hybrid Organic–Inorganic
Silica Nanoparticles
In water and wine: Chromofluorogenic
detection of the sulfite anion in pure
water was accomplished by using a new
hybrid organic–inorganic material that
contained a probe entrapped in hydro-
phobic biomimetic cavities. This material
was used for the detection of sulfite in red
wine.
6
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!