Surfactant-assisted chromogenic sensing of cyanide in water

Article abstract of DOI:10.1039/b909705h

Chromogenic cyanide recognition in water was achieved by the use of a hydrophobic dye in micellar containers.

Full text of DOI:10.1039/b909705h

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LETTER
www.rsc.org/njc | New Journal of Chemistry
Surfactant-assisted chromogenic sensing of cyanide in waterw
abc
abc
abc
Tatiana Abalos, Santiago Royo,abc Ramon Martınez-Manez,* Felix Sancenon,*
´
˜
´
´
´
´
Juan Soto,abc Ana M. Costero,ad Salvador Gilad and Margarita Parraad
Received (in Montpellier, France) 15th May 2009, Accepted 11th June 2009
First published as an Advance Article on the web 26th June 2009
DOI: 10.1039/b909705h
Chromogenic cyanide recognition in water was achieved by the
use of a hydrophobic dye in micellar containers.
leaves the zincon ligand free with a colour change.9 Some
other interesting approaches for the detection of cyanide in
water include the use of sensor materials. TiO2 supports
functionalised with hemoglobin protein10 resulted in a highly
selective and sensitive detection of CNꢀ by simply monitoring
the changes in the optical properties of the TiO2 films. The
same research group recently attached azophenyl thiourea
derivatives onto nanostructured Al2O3 films for the selective
chromogenic detection of cyanide in water.11 Organic
polymers bearing 2,4,6-triphenylpyrylium moieties have also
been used as chromogenic cyanide sensors at pH 11.0.12 Also
remarkable are the examples involving the use of QDs for the
detection of cyanide in aqueous environments.13 In addition
to these systems, other examples have been reported for colori-
metric signalling of cyanide in organic solvent—water mixtures,
usually containing a low percentage of water (frequently lower
than 20% by volume).14
Anion detection by chromogenic chemosensors and reagents is
an area of emerging interest in the field of supramolecular and
anion chemistry.1 However, despite remarkable advances,
there are a large number of chromogenic probes that are
reported to display sensing features only in organic solvents.
In fact, the preparation of probes for certain target anions in
pure water is still a challenge, since many receptor–anion
interactions suffer from strong solvation effects that impose
a highly effective energetic barrier that inhibits sensing
paradigms from occurring in aqueous solution.2,3 From the
different protocols that can be followed to achieve this goal,
we believe that the use of paradigms relying on a selective
anion reaction with a certain substrate in a chemodosimeter
fashion are especially appealing.4 Based on these concepts, and
also due to our interest in the development of chromogenic
anion probes,5 we report herein the use of a thiopyrylium
derivative for the chromogenic sensing of cyanide. Highly
selective optical detection of cyanide to ppm levels was
obtained in pure water containing a neutral surfactant.
Receptors 1 and 2 (see Scheme 1) were prepared by electro-
philic aromatic substitution between N,N-dimethylaniline and
N,N-dioctylaniline with 2,6-diphenylpyrylium perchlorate in
anhydrous DMF, followed by substitution of the oxygen atom
in the pyrylium ring by a sulfur atom in the presence of Na2S.
The 1H-NMR spectra of both receptors were characterized by
the presence of the aromatic resonances in the 6.80–8.60 range.
The most characteristic signals in this zone are a singlet at ca.
8.63 ppm, attributed to the equivalent protons of the thio-
pyrylium ring, and two doublets centred at 6.90 and 8.20 ppm
from the p-disubstituted aniline ring.
In spite of the fact that the cyanide anion is highly toxic to
living organisms, the use of cyanide salts in fields such as gold
mining, electroplating, resins, synthetic fibers, metallurgy, etc.
remains widespread.6 Cyanide can also be found in foods such
as bitter almonds, cassava roots, etc. Due to its high toxicity
and extensive use, the development of chromogenic probes for
cyanide detection is of interest for instance in undemanding
rapid screening applications. However, chromogenic systems
for cyanide in water or in water–organic solvent mixtures are
relatively scarce. For instance, cyanide probes have been
reported based on the nucleophilic attack of cyanide to blue
squaraine derivatives7 or by the selective reaction of cyanide
with subphthalocyanines in aqueous environments.8 A very
recent colorimetric chemosensor has been described involving
cyanide complexation to copper(II) from a CuII–zincon that
a
´
Instituto de Reconocimiento Molecular y Desarrollo Tecnologico,
Centro mixto Universidad Politecnica de Valencia-Universidad de
´
Valencia, Spain
b
´
´
Departamento de Quımica, Universidad Politecnica de Valencia,
Camino de Vera s/n, 46022 Valencia, Spain.
E-mail: rmaez@qim.upv.es
c
´
CIBER de Bioingenierıa, Biomateriales y Nanomedicina (CIBER-BBN),
Spain
Departamento de Quımica Organica, Facultad de Ciencias Quımicas,
d
´
Universitat de Valencia, Valencia, 46100 Burjassot, Spain
´
´
Scheme 1 The structure of chromoreactands 1 and 2, and the synthetic
procedure for 2. (i) 1-Bromooctane–K2CO3, (ii) 2,6-diphenyl-
pyrylium perchlorate–DMF and (iii) Na2S–HClO4.
w Electronic supplementary information (ESI) available: NMR
spectra of receptors 1 and 2. See DOI: 10.1039/b909705h
ꢁc
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Fig.
1
Colour changes observed for acetonitrile solutions of
chromoreactand 1 (1.0 ꢂ 10ꢀ4 mol dmꢀ3) in the presence of 10 equiv.
of the correspondent anion (except for cyanide; 1 equiv.). From left to
right: no anion, Clꢀ, Brꢀ, Iꢀ, NO3ꢀ, AcOꢀ, H2PO4ꢀ, HSO4ꢀ, NCSꢀ
and CNꢀ.
Whereas 2,4,6-triphenylthiopyrylium is yellow, 2,4,6-tri-
arylthiopyrylium cations bearing an amine at the para position
of the 4-aryl group show a deep blue colouration. Thus,
acetonitrile solutions of chromoreactands 1 and 2 showed
intense absorption in the visible zone centred at ca. 575 nm.
This visible band most likely has a charge-transfer (CT)
character due to the presence of an electron donor aniline
group and an electron acceptor thiopyrylium moiety.
Fig. 2 Top: 1H-NMR spectrum of receptor 1 in CD3CN showing the
aromatic protons. Bottom: 1H-NMR spectrum of receptor 1 in
CD3CN after addition of 1 equiv. of cyanide anion showing the
aromatic protons.
In the first step, the colorimetric behaviour of acetonitrile
solutions (3.0 ꢂ 10ꢀ5 mol dmꢀ3) of 1 were tested in the
presence of equimolar quantities of certain anions. The addi-
tion of 10 equiv. of Clꢀ, Brꢀ, Iꢀ, NO3ꢀ, H2PO4ꢀ, HSO4
,
ꢀ
AcOꢀ and NCSꢀ to CH3CN solutions of 1 induced negligible
changes in the UV-visible spectra, whereas the addition
of 1 equiv. of CNꢀ anion resulted in complete bleaching
(see Fig. 1).
The disappearance of the visible band, instead of a shift, in
the presence of cyanide pointed to a chemical reaction between
this anion and 1 in a chemodosimeter fashion. Bearing in mind
the electrophilic character of the thiopyrylium moiety, we
assigned this reaction to an attack of the nucleophile cyanide
on the aromatic thiopyrylium ring, which would result in the
rupture of the electronic delocalization. The thiopyrylium ring
contains one positively charged sulfur atom in its structure,
which induces a certain electrophilic character on the C2 and
C4 carbons of the heterocycle. In fact, quantum chemical
calculations at the semiempirical level indicated that the
charge density on atoms C2, C3 and C4 is ꢀ0.06, ꢀ0.191
and 0.206, respectively.15 These calculations suggest that
the thiopyrylium ring is prone to suffer nucleophilic attack,
especially at C4 and to a lesser extent at C2. These calculated
charges, together with the fact that the C–S+ bond in the
thiopyrylium ring is somehow polarized by inductive effects,
should account for the reactivity observed.
Scheme 2 The proposed products obtained from the reaction of CNꢀ
with 1.
It was also found that the addition of small quantities of
water (less than 5%) to acetonitrile solutions of receptor 1
inhibited bleaching in the presence of cyanide. However, we
considered that an attractive possibility for overcoming the
severe limitation represented by the absence of response of
chromoreactand 1 in water could be the use of micelles as
nanocontainers, in which 1 and cyanide could find a suitable
hydrophobic reaction environment.
The use of functionalized chemosensors embedded in
micellar systems for the chromo- and fluorogenic sensing of
chemical species in water is a well stabilised field. Very
recently, several authors showed that certain binding sites
for metal cations and certain fluorophores can be arranged
in micelles of surfactants in water, allowing the presence of
certain metal cations to be detected by changes in fluores-
cence.16 However, to our knowledge, the use of micellar
systems for enhanced sensing of anions has only been reported
by Anslyn et al. for the chromo-fluorogenic recognition of
inositol triphosphate in aqueous environments.17
In order to confirm this possible explanation for the
observed bleaching, 1H-NMR studies were carried out on
mixtures of probe 1 and cyanide in acetonitrile-d3. The addi-
tion of 1 equiv. of CNꢀ to 1, results in the formation of a
mixture of two different compounds. This can be observed
through monitoring the singlet at 8.63 ppm for compound 1,
which turned into three new singlets at 5.96, 6.17 and 7.26 ppm
(see Fig. 2). The singlet centred at 6.17 ppm was assigned to
the product obtained from nucleophilic attack of the cyanide
on C4 of the thiopyrylium ring (structure I in Scheme 2),
whereas the other two signals were attributed to the product
formed by cyanide addition at C2 (structure II in Scheme 2).
From the areas of the three singlets, a ratio of 75 : 25 for
structures I : II was determined. These results are in agreement
with the quantum calculations (vide ante).
With this aim, we prepared receptor 2, which is similar to 1,
but additionally has its aniline group functionalized with
highly hydrophobic n-octyl chains in order to facilitate
inclusion of the probe into micellar superstructures. As a
surfactant, we opted for Triton X-100, which is neutral
(average molecular weight of 647 g molꢀ1), in order to prevent
any electrostatic interaction of the surfactant with the charged
2 and cyanide species. In a typical experiment, water solutions
were used with a Triton X-100 concentration of 6.47 g Lꢀ1
(0.01 mol dmꢀ3), which corresponds to an average micelle
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Fig. 3 Changes of the visible band (575 nm) of 2 (1.0 ꢂ 10ꢀ4 mol dmꢀ3
in water at pH 9.5 with 0.01 mol dmꢀ3 Triton X-100) in the presence of
10 equiv. of CNꢀ vs. time.
Fig. 4 Changes of the visible band of chromoreactand 2 (1.0 ꢂ
10ꢀ4 mol dmꢀ3 in water at pH 9.5 with 0.01 mol dmꢀ3 Triton
X-100) in the presence of 10 equiv. of anions Fꢀ, Clꢀ, Brꢀ, Iꢀ, AcOꢀ,
NO3ꢀ, CNꢀ, NCSꢀ, H2PO4ꢀ, HSO4 and CNꢀ.
ꢀ
concentration of ca. 1.0 ꢂ 10ꢀ4 mol dmꢀ3 (for Triton X-100
other aꢀnions, includinꢀg Fꢀ, Clꢀ, Brꢀ, Iꢀ, AcOꢀ, NO3ꢀ, NCSꢀ,
H2PO4 and HSO4 in aqueous Triton X-100 micellar
solutions at pH 9.5. In all cases, the presence of these anions
resulted in no variation of the band at 575 nm, indicating
that the reaction with cyanide is highly selective in water
(see Fig. 4). From plots of absorbance at 575 nm versus
increasing quantities of cyanide added to aqueous micellar
solutions of chromoreactand 2 at pH 9.5, a detection limit of
ca. 1 ppm of cyanide was determined (see Fig. 5).
critical micellar concentration is ca. 2.0 ꢂ 10ꢀ4 mol dmꢀ3
,
aggregation number 111) and adjusted to pH 9.5. Probe 2 is
not soluble in water, but readily dissolves in aqueous solutions
containing the surfactant, suggesting the rapid and effective
inclusion of 2 into the micelles.
A pH of 9.5 was selected in order to overcome the com-
petition of the OHꢀ anion in the bleaching mechanism, but at
the same time remaining at a pH where the nucleophilic
species CNꢀ can still occur (lower pHs result in the formation
of the protonated HCN derivative). In fact, 2 can not be used
at very high pH values (higher than 11) because of the rapid
and effective nucleophilic attack of the OHꢀ anion on the
thiopyrylium ring. This reaction with OHꢀ, however, is not
effective in aqueous Triton X-100 solutions at pH 9.5, in which
2 remains blue for hours without signs of decomposition.
In the first step we decided to study the kinetic behaviour of
chromoreactand 2 (1.0 ꢂ 10ꢀ4 mol dmꢀ3) adjusted to pH 9.5
in aqueous Triton X-100, in the presence of 10 equiv. of
cyanide anion. As can be seen in Fig. 3 only partial bleaching
of the solution was observed. However, a remarkable 60%
reduction of the intensity of the 575 nm band at 18 min could
be observed. This fact is clearly in contrast with the complete
bleaching observed in acetonitrile solution and could be
ascribed to the energetic barrier imposed by the solvation–
desolvation process suffered by cyanide anions in aqueous
environments. This solvation process reduces the nucleo-
philicity of the cyanide anion and, as a consequence, the rate
of the reaction with thiopyrylium ring. Fig. 4 shows the visible
spectra of chromoreactand 2 (3 mL of micellar aqueous
solution 1.0 ꢂ 10ꢀ4 mol dmꢀ3) before and after addition of
10 equiv. of cyanide anion (visible spectra were recorded after
18 min of cyanide addition). The observed bleaching is
consistent with inclusion of the cyanide anion into the micelles
and cyanide attack on the thiopyrylium ring. The inclusion of
cyanide anions into the micelles excluded the water molecules
from the microenvironment of the micellar surface, enabling
the chromogenic reaction.
In conclusion, we have reported the use of anilinium–
thiopyrylium scaffolds as probes for the colorimetric detection
of cyanide, based on the nucleophilic addition of CNꢀ to the
electron deficient aromatic thiopyrylium ring. Sensitive and
selective sensing of cyanide has been obtained by using an
undemanding surfactant-assisted protocol. This is one of the
very few colorimetric probes described for cyanide detection in
pure water. From an alterative viewpoint, the results suggest that
surfactants could be a suitable general method of transferring
anion-sensing paradigms from the chemistry of organic solvents
to the world of colorimetric signalling in aqueous environments.
Fig. 5 Changes in the absorbance at 575 nm in micellar solutions of 2
(1.5 ꢂ 10ꢀ4 mol dmꢀ3 in water at pH 9.5 with 0.01 mol dmꢀ3 Triton
X-100) vs. increasing quantities of cyanide anion.
Bearing in mind these favourable characteristics, the reac-
tivity of chromoreactand 2 was tested towards a number of
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down to room temperature, and then diethyl ether (30 mL)
was added and the crude mixture stirred for another 16 h. The
final product was isolated by filtration as a dark magenta solid
(0.42 g, 0.77 mmol, 62.3% yield). 1H NMR (300 M Hz,
Experimental
Chemicals
Triton X-100, aniline, N,N-dimethylaniline, 1-bromooctane,
triethylorthoformate, acetophenone, sodium sulfide, perchloric
acid, potassium carbonate, diethyl ether, acetone, hexane,
anhydrous dimethylformamide (DMF) and anhydrous aceto-
nitrile (ACN) were purchased from Aldrich and were used as
received. Tetrabutylammonium salts of fluoride, chloride,
bromide, iodide, nitrate, dihydrogen phosphate, hydrogen
sulfate, acetate, cyanide and isothiocyanate were also
purchased from Aldrich and used as received.
DMSO): d = 0.82 (6H, t, -(CH2)5-CH3), 1.19–1.59
(24, N-CH2-(CH2)5-CH3) 3.33 (4H, t, N-CH2), 6.88 (2H, d,
ꢀ
ꢀ
ꢀ
C6H4), 7.72 (6H, m, C6H5 C6H4), 8.26 (2H, t, C6H5), 8.41
(4H, m, C6H5), 8.59 (2H, s, C5H2O). 13C NMR {1H} (300 M Hz,
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
DMSO): d = 13.9, 22.1, 26.4, 28.4, 28.7, 29.0, 31.3, 42.4,
109.1, 118.4, 127.5, 129.5, 129.8, 133.4, 135.4, 143.9, 146.9,
156.8, 158.6, 164.7.
2: Pyrylium derivative 2b (0.42 g, 0.77 mmol) was dissolved
in acetone (50 mL) and then Na2S (0.37 g, 1.5 mmol) in 25 mL
of water solution was added, and the crude reaction was
allowed to react at room temperature for 20 min. Finally,
perchloric acid (1 mL, 20% water solution) was added and the
crude reaction mixture stirred for another 40 min at the same
temperature. The final product was isolated by vacuum filtra-
tion, and successive washings with water and diethyl ether, as
dark blue solid (0.23 g, 0.4 mmol, 52% yield). 1H NMR
Synthesis
All the reactions were carried out with glass material cleaned
with nitric acid (in order to avoid contamination from organic
matter) and in an inert atmosphere (Ar).
1a: N,N0-dimethylaniline (0.5 mL, 3.95 mmol) was dissolved
in dry DMF (10 mL) and then 2,6-diphenylpyrylium perchlorate
(2.63 g, 7.9 mmol) was added. The mixture was heated at
150 1C for 3 h. After this, the mixture was allowed to cool
down to room temperature, and then diethyl ether (30 mL)
was added and the crude mixture stirred for another 16 h. The
final product was isolated by filtration as a dark magenta solid
(1.09 g, 2.4 mmol, 61% yield). 1H- and 13C-NMR data,
and mass spectra are consistent with those reported in the
literature.
(300 M Hz, CD3CN): d = 0.92 (6H, t, -(CH2)5-CH3),
1.32–1.39 (24H, m, N-CH2-(CH2)5-CH3) 3.55 (4H, t,
ꢀ
ꢀ
N-CH2), 6.97 (2H, d, C6H4), 7.68–7.76 (6H, m, C6H5),
ꢀ
ꢀ
ꢀ
ꢀ
8.00 (4H, d, C6H5), 8.29 (2H, d, C6H4), 8.70 (2H, s, C5H2S).
ꢀ
ꢀ
13C NMR {1H} (300 M Hz, DMSO): d = 13.5, 22.4, 26.5,
27.3, 28.9, 29.0, 31.5, 51.1, 113.5, 121.7, 125.7, 128.0, 129.9,
132.6, 133.5, 135.2, 154.4, 156.9, 161.0. HRMS calc. for
C39H50NS, 564.3663, found 564.3660.
1: Pyrylium derivative 1a (0.36 g, 0.8 mmol) was dissolved in
acetone (50 mL) and then Na2S (2 mL, 10% water solution)
was added, and the crude reaction mixture was allowed to
react at room temperature for 20 min. Finally perchloric acid
(2 mL, 20% water solution) was added and the crude reaction
mixture stirred for another 40 min at the same temperature.
The final product was isolated by vacuum filtration, and
successive washings with water and diethyl ether, as dark blue
solid (0.19 g, 0.4 mmol, 50% yield). 1H NMR (300 M Hz,
CDCl3): d = 3.16 (6H, s, N-(CH3)2), 6.89 (2H, d, C6H4), 7.60
Acknowledgements
The authors wish to express their gratitude to the Spanish
Government for financial support (projects CTQ2006-15456-
C04-01, CTQ2006-15456-C04-02,). T.A. is grateful to the
Spanish Government for the fellowship awarded. S.R. would
like to thank Generalitat Valenciana for his fellowship.
References
ꢀ
ꢀ
(6H, m, C6H5), 7.88 (4H, m, C6H5), 8.19 (2H, d, C6H4), 8.49
(2H, s, C5H2S). 13C NMR {1H} (75 M Hz, CDCl3): d = 40.5,
ꢀ
ꢀ
ꢀ
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´ ´
´
´
´
´
´
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ꢀ
´
´
´
CH3) 3.07 (4H, t, N-CH2), 6.75 (3H, m, C6H5), 7.06 (2H, t,
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ꢀ
ꢀ
´
ꢀ
´
´
´
27.8, 30.0, 30.1, 30.3, 32.5, 44.1, 112.9, 116.65, 129.6, 150.0.
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´
´
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This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009 New J. Chem., 2009, 33, 1641–1645 | 1645