Pyrylium-containing polymers as sensory materials for the colorimetric sensing of cyanide in water

Article abstract of DOI:10.1039/b502374b

A sensory polymeric material for the colorimetric sensing of cyanide in water has been developed based on the reactivity of this anion with the pyrylium cation. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b502374b

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Pyrylium-containing polymers as sensory materials for the colorimetric
sensing of cyanide in water
Fe´lix Garc´ıa,*a Jose´ Miguel Garc´ıa,a Beatriz Garc´ıa-Acosta,b Ramo´n Mart´ınez-Ma´n˜ez,*b Fe´lix Sanceno´nb
and Juan Sotob
Received (in Cambridge, UK) 16th February 2005, Accepted 8th April 2005
First published as an Advance Article on the web 21st April 2005
DOI: 10.1039/b502374b
have combined here the capabilities of polymers as materials and
suitable chromogenic organic groups in order to develop new
chromogenic devices with enhanced sensing properties.
A sensory polymeric material for the colorimetric sensing of
cyanide in water has been developed based on the reactivity of
this anion with the pyrylium cation.
The electrophilic character of the pyrylium ring has been
recently employed in the development of chromogenic probes for
charged and neutral molecules.10 With this in mind, we envisioned
that the reaction between cyanide and a pyrylium derivative to give
the corresponding cyano-enone11 could also be used as an output
signal in the development of novel sensory materials.
The development of new molecular chemosensors capable of
sensing target guests via changes in optical signals is a field of
interest. Despite the relatively large number of studies on cation
sensing, anions have only been targeted in the last decade mainly
due to new advances in receptor chemistry and the development of
new sensing principles.1 However, there are still particular
problems connected with the development of new selective
colorimetric probes for the chromogenic sensing of certain guests.
This is especially the case for sensitive reusable colorimetric sensory
materials capable of acting as probes in water for target anions.
Cyanide is a highly toxic anion that inhibits mitochondrial
cytochrome-oxidase and hence blocks electron transport, resulting
in decreased oxidative metabolism and oxygen utilization,2 and its
determination plays an important role in environmental control.
Several methods for cyanide determination in aqueous environ-
ments have been reported, but, among them, very few are based on
the use of chromofluorogenic chemosensors. Recent examples
involve the use of Ru(II)-containing pyrrolylquinoxalines,3 Zn(II)
complexes of porphyrin derivatives,4 boronic acid groups5 and
Re(I) polypyridyl complexes.6 However some of these systems
display colour changes in non-aqueous media or are not highly
selective. It has also recently been reported that the nucleophilic
attack of the cyanide anion on squaraine derivatives can be a
useful method for the colorimetric determination of this envir-
onmentally important anion.7 Using a somewhat similar design
principle we report here the use, for the first time, of pyrylium
derivatives as chromogenic reporters for the colorimetric detection
of CN2. As an additional effect to enhance its applicability, we
have covalently anchored the colorimetric probe into a polymer.
The connection between certain types of chromogenic receptors
for anions and polymers can be of interest. In fact, the
development of new, robust, sensitive sensory materials remains
as an emerging frontier and to date there are only few examples of
chromogenic and fluorogenic polymer-based anion chemosensors.8
Moreover, polymeric films can be easily prepared and are ideal
candidates for the development of sensory materials for in situ
sensing and rapid ‘‘naked eye’’ screening applications.9
Additionally, water insoluble sensing molecules anchored into
suitable polymers with good mechanical properties can be used as
sensory materials practically in any solvent or atmosphere. We
In order to incorporate the pyrylium chromophore into a
polymeric matrix, the monomer I was synthesized by a two step
reaction. In the first step, the reaction of methacryloyl chloride
with 4-hydroxybenzaldehyde gave 4-formylphenyl methacrylate.
The reaction of one equivalent of this compound with two
equivalents of acetophenone and two equivalents of boron
trifluoride diethyl etherate in 1,2-dichloroethane gave I as a yellow
powder that was crystallized from acetic acid to yield yellow
needles (see Scheme 1). Preliminary studies revealed that CN2
addition to acetonitrile solutions of I resulted in a remarkable
colour change from yellowish to red due to the formation of the
corresponding cyano-enone derivative II via nucleophilic attack of
the cyanide anion on the C1 carbon next to the oxygen atom in the
pyrylium ring (see Scheme 2). This reactivity was confirmed by
NMR studies. Thus, I shows characteristic pyrylium signals with
equivalent Ha and Hb protons at 8.6 ppm. Upon reaction with
tetrabutylammonium cyanide these two protons became non-
equivalent, with observed resonances at 6.9 and 7.6 ppm for Ha
Scheme 1 Synthesis of I. i: CH2Cl2, TEA,
0 uC; ii: BF3?OEt2,
*fegarcia@ubu.es (Fe´lix Garc´ıa)
rmaez@qim.upv.es (Ramo´n Mart´ınez-Ma´n˜ez)
CH2ClCH2Cl, rt.
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Table 1 Composition of the polymeric films
Film
I (x)a
III (y)a
IV (z)a
1
2
3
4
5
6
7
8
9
a
0.5
1.0
2.0
0.5
1.0
2.0
0.5
1.0
2.0
100
100
100
75
75
75
50
50
50
0
0
0
25
25
25
50
50
50
Molar composition ratio of the monomers used for the preparation
of the films.
Scheme 2 Reactivity of the monomer I with cyanide. A photograph
showing the colour of the polymer upon reaction with cyanide is also
included.
observed, probably due to an attack of the OH2 anion on the
pyrylium ring.16 To test the anion sensing ability of the conditioned
polymers, different anions were added to plain buffer solutions at
pH 11. The addition of cyanide into the cell containing the
pyrylium-doped films resulted in a change in the absorption
spectra with a gradual enhancement of the 537 nm band with the
cyanide concentration. In contrast, in the presence of the Cl2, Br2,
I2, NO32, H2PO42, SO422 and SCN2 anions, the films remained
silent.17 The response to cyanide can be seen in Fig. 1, which shows
the titration of the films with aqueous CN2. The inset shows the
corresponding binding isotherm. The best results in terms of
response time and colour variation were obtained with the most
hydrophilic film 9, which contains a larger percentage of IV and
also the higher content of the sensing probe I. Film 9 also showed
a suitable mechanical stability.
In preliminary studies a detection limit of 4.0 6 1023 mol dm23
has been determined with the polymeric material 9 at pH 11.0.
One particular feature of the cyanide reactivity with pyrylium
rings is that the process is reversible and it has been reported that
in the presence of acidic solutions the pyrylium derivative could be
retrieved. Therefore the attractive possibility of attaining reusable
colorimetric probes was tested via recuperation of the sensing
polymer by reaction of the cyano-enone derivative with acidic
solutions to obtain the former pyrylium cation. Red films saturated
with cyanide were soaked in 0.01M hydrochloric acid solutions
and Hb, respectively. Mass spectra studies also indicated the
formation of derivative II (base peak at 407 m/z). These data are
consistent with the formation of the cyano-enone.12
A series of methacrylic copolymer films containing the pyrylium
probe were prepared by radical copolymerization of the monomer
I, 2-ethoxyethyl methacrylate III, 2,3-dihydroxypropyl methacry-
late13 IV, small quantities of ethylene glycol dimethacrylate as
cross-linking agent and 2,29-azobis(isobutyronitrile) (AIBN) as
radical thermal initiator.14 Nine different polymer films were
prepared by varying two parameters: a) the concentration of I (see
Scheme 3 and Table 1) and b) the ratio of hydrophilic (IV) to
hydrophobic (III) monomers in order to optimize the character-
istics of the films. At this point, it must be taken into account that
more hydrophilic films (larger quantity of IV) would lead to a
higher diffusion rate15 of the water/cyanide into the membrane
where the sensing probe is homogeneously distributed. This would
result in faster response times, but also in a loss of mechanical
stability of the polymer film. Therefore, a set of films with different
hydrophilic/hydrophobic character, mechanical strength and con-
tent of the pyrylium probe were prepared in order to select that
showing a better performance.
The different polymers prepared containing the pyrylium
derivative are colorless or pale yellow (absorbance maxima
centered at 425 nm) and highly fluorescent (emission band
centered at 474 nm upon excitation at 394 nm). Prior to the
studies with cyanide, the films were soaked in plain buffer (NaOH
1023 mol dm23, pH 11) for 30 minutes to condition the polymer.
During this time a partial development of a band at ca. 530 nm is
Fig. 1 UV-Vis spectra of polymer 9 upon exposure to increasing
amounts of cyanide in water at pH 11. The inset shows the respective
binding isotherm by measurement of the 537 nm band.
Scheme 3 Structure of the sensing films and monomers III and IV used
in their synthesis.
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Notes and references
1 R. Mart´ınez-Ma´n˜ez and F. Sanceno´n, Chem. Rev., 2003, 103, 4419;
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8 H. A. Ho, M. Boissinot, M. G. Bergeron, G. Corbeil, K. Dor´e,
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Fig. 2 Absorbance of the 537 nm band upon exposure to cyanide at pH
11 and to acidic HCl aqueous solutions (0.01 M). A photograph showing
the colour of the polymer after each process is also included.
and recovery of the original colour of the pyrylium-containing
films was observed (see Fig. 2). This acid treatment also reverted
the ring opening caused by the plain buffer conditioning. This
cyclic process was repeated at least 10 times without significant
degradation of the sensing ability of the sensory polymer.
In summary we have shown here a new colorimetric probe for
cyanide sensing in water based on the reactivity of the cyanide
anion with pyrylium cation anchored into a polymer. The reaction
used is reversible and the sensory polymer can be easily recovered
by washing the film with aqueous acidic solutions. The combina-
tion of suitable chromogenic systems and polymeric materials
could lead to new sensory systems for the development of reusable
colorimetric easy-to-use in situ naked-eye chemosensors for target
species of interest in water.
9 G. J. Mohr, Chem. Eur. J., 2004, 10, 1082; G. J. Mohr, Chem.
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2262; F. Sanceno´n, R. Mart´ınez-Ma´n˜ez, M. A. Miranda, M.-J. Segu´ı
and J. Soto, Angew. Chem. Int. Ed., 2003, 42, 647; M. Comes,
M. D. Marcos, R. Mart´ınez-Ma´n˜ez, F. Sanceno´n, J. Soto,
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J. Guzma´n, Macromolecules, 2004, 37, 4156.
14 The films were prepared by radical polymerization of a mixture of I, III
and IV using 1,2-ethanedioldimethacrylate and AIBN (2.0 wt%) as
initiator. The nominal cross-linking ratio, i.e., the ratio of the number of
moles of monomers to the cross-linking agent, was y100:1, and the film
thickness was y110 mm. The addition of a cross-linking agent is
necessary in order to obtain membranes with adequate mechanical
properties. The reaction was carried out in a silanized glass mold of
100 mm thickness in an oxygen-free atmosphere at 65 uC for 5 h.
15 For a study of the diffusion process in similar membranes, see:
V. Compan˜, P. Tiemblo, F. Garc´ıa, J. M. Garc´ıa, J. Guzman and
E. Riande, Biomaterials, 2005, 26, 3783.
We thank the Ministerio de Ciencia y Tecnolog´ıa (projects
REN2002-04237-C02-01,
MAT2003-08568-C03-02
and
MAT2002-02502) and Junta de Castilla y Leo´n (project BU16/
02) for support. B. G.-A. thanks the Generalitat Valenciana for a
Doctoral Fellowship and F. S. also thanks the Ministerio de
Educacio´n y Ciencia for a Ramo´n y Cajal contract.
Fe´lix Garc´ıa,*a Jose´ Miguel Garc´ıa,a Beatriz Garc´ıa-Acosta,b
Ramo´n Mart´ınez-Ma´n˜ez,*b Fe´lix Sanceno´nb and Juan Sotob
aDepartamento de Qu´ımica, Universidad de Burgos, Plaza de Misael
Ban˜uelos s/n 09001, Burgos, Spain. E-mail: fegarcia@ubu.es;
Fax: +34 947 258831; Tel: +34 947 258800
16 Layers stored in buffer sodium hydroxide (1023 mol dm23) for 24 hours
showed a similar response upon cyanide addition to those only soaked
for 30 minutes.
17 Under the experimental conditions employed in the cyanide detection,
sulfide anion also gives a response, most likely due to a nucleophilic
attack on the pyrylium ring. See: D. Jime´nez, R. Mart´ınez-Ma´n˜ez,
F. Sanceno´n, J. V. Ros-Lis, A. Benito and J. Soto, J. Am. Chem. Soc.,
2003, 125, 9000.
bCentro de Investigacio´n en Qu´ımica Molecular Aplicada, Departamento
de Qu´ımica, Universidad Polite´cnica de Valencia, Camino de Vera s/n,
46071-Valencia, Spain. E-mail:rmaez@qim.upv.es;Fax:+34 963879349;
Tel: +34 96 3877343
2792 | Chem. Commun., 2005, 2790–2792
This journal is ß The Royal Society of Chemistry 2005