A selective chromogenic reagent for cyanide determination

Article abstract of DOI:10.1039/b206500b

A chromogenic reagent for cyanide determination in water based on the reaction of this anion with a squaraine derivative functionalized with ether chains has been developed.

Full text of DOI:10.1039/b206500b

A selective chromogenic reagent for cyanide determination
Jose V. Ros-Lis, Ramón Martínez-Máñez* and Juan Soto
GDDS, Departamento de Química, Universidad Politécnica de Valencia, Camino de Vera s/n, 46071
Valencia, Spain. E-mail: rmaez@qim.upv.es; Fax: 963877349
Received (in Cambridge, UK) 4th July 2002, Accepted 28th August 2002
First published as an Advance Article on the web 11th September 2002
A chromogenic reagent for cyanide determination in water
based on the reaction of this anion with a squaraine
derivative functionalized with ether chains has been devel-
oped.
anion determination. One probable reason for this is because
one of the main drawbacks of working with squaraine dyes is
their very low solubility, both in organic solvents and in water.
For instance, a compound similar to L1 but synthesised from
N,N-dimethylaniline is just sparingly soluble in dichloro-
methane and completely insoluble in water. In order to avoid
this problem we have synthesised L1 which is a derivative
functionalised with ether chains to improve its solubility in
water. Ether chains have been extensively applied as polar
groups for instance in non-ionic surfactants and in crown
ethers.
Cyanide is probably one of the more toxic inorganic anions one
can find. Despite its acute toxicity, cyanide or hydrogen cyanide
are used in a large number of applications, for instance to make
certain synthetic fibres and resins, in herbicides, in gold mining,
etc. Due to its wide range of application and its serious toxicity
the development of new molecules for cyanide determination or
sensing may be of interest.1
L1 was synthesised (see Scheme 1) by a two step reaction. In
the first step, the mesylated derivative of 2-methoxyethanol
with N-phenyldiethanolamine gave N,N-di(ethyl-2-(2-meth-
oxyethoxy))aniline. Two equivalents of this compound react
with one equivalent of squaric acid with azeotropic removal of
water10 to yield L1 (18%). The absorption spectrum of L1 shows
a sharp and very intense band with a maximum at 641 nm in
acetonitrile (log e = 5.08).
There is increasing attention on the development of anion
chemosensors able to transform microscopic anion interactions
into a suitable macroscopic response.2 One of the most
interesting output signals is the visible absorption, because the
instrumentation is widely available, but also because it would be
possible to sense target species with the naked eye. Different
approaches have been envisaged for the design of chromogenic
probes for anion sensing. Most of them are based on the
coupling of dyes to anion binding sites3 or the use of
displacement reactions.4 More recently we have used anion-
induced cyclization processes for the colorimetric detection of
anions.5 However, many of these colorimetric chemosensors
only display colour changes in organic solution and can not be
used in aqueous environments. An alternative method is based
on the use of specific reactions (usually irreversible) produced
by target anions.6 This idea tries to take advantage of the
selective reactivity that certain anions display. Following this
approach we have selected a particular feature of the cyanide
anion: its nucleophilic character, and report here a selective
probe for cyanide determination in water using a colorimetric
method.
L1 has drastically changed its solubility properties when
compared with other reported squaraine derivatives. Thus, L1 is
soluble in a larger number of polar organic solvents; it is soluble
in water and in all the range of acetonitrile–water mixture
compositions. The ligand is stable for months as a solid or in
acetonitrile solution but it undergoes slow decomposition in
water-containing solutions. In a first step, the chromogenic
sensing ability of L1 was studied in acetonitrile in the presence
of the anions F2, Cl2, Br2, I2, NO32, H2PO42, HSO42, Ac2,
Bz2, CN2 and SCN2. Among these, only cyanide is able to
affect the band at 641 nm, as can be seen in Fig. 1. The
decolouration process is not instantaneous but it increases with
time and with the number of cyanide equivalents. 1H NMR data
for L1 in acetonitrile upon cyanide addition is consistent with
Among recent developments in the field of chromogenic
systems for anions two recent approaches to cyanide sensing
have been reported by Kim and Anzenbacher,7 however, these
only gave colour changes in dichloromethane and dichlor-
omethane+acetonitrile solutions and are not highly selective.
Our approach is based on the decolouration selectively
observed upon nucleophilic attack of cyanide to a functionalised
squaraine. We focused our attention on squaraine dyes because
of their peculiar chemical and physical properties. They are
known to show very intense (e up to 100 000) and sharp bands
near the IR which is a region of special interest for the
development of optical instrumentation and because the number
of potential coloured interferences is lower than when the
absorption is near the UV. The squaraine derivative we have
used (see Scheme 1) is built up by two N-functionalised
anilinium groups anchored to a central four-atom ring. The
colour in this dye is due to a charge transfer band between the
donor anilinium moieties and the central acceptor four-
membered ring.8 The atoms in that acceptor four-atom ring (see
Scheme 1) are electron-deficient and should be able to undergo
nucleophilic attack. As stated above cyanide is a well-known
nucleophile and we believed that a nucleophilic addition to the
ring could result in selective detection of this toxic anion.
Squaraine dyes have found several fields of application9 but
as far as we know, they have never been used as reagents for
Scheme 1 Synthesis of L1. i: CH3CN, reflux; ii: BuOH/tol.
2248
CHEM. COMMUN., 2002, 2248–2249
This journal is © The Royal Society of Chemistry 2002

View Article Online
probably due to the nucleophilic attack of the OH2 on the four-
atom ring of the squaraine dye.
We have also confirmed in preliminary studies that L1 can act
as a selective chromogenic reagent for the quantitative determi-
nation of cyanide in water samples. Thus in a typical experiment
a known amount of cyanide was added to water also containing
28.7 ppm of Na+, 23.4 ppm of K+, 44.3 ppm of Cl2, 12.3 ppm
Fig. 1 L1 in acetonitrile ([L1] = 3.0 3 1025 M) in the presence of 5
2
equivalents of anions, from left to right, F2, Cl2, Br2, I2, NO32, H2PO4
,
HSO42, Ac2, Bz2, SCN2, CN2 and no anion.
2
of NO3 and 19.3 ppm of SO422. This mixture containing a
concentration of cyanide of 2.2 ppm was mixed with acetonitrile
and following the method outlined above and using the
calibration curve in Fig. 3 a concentration of CN2 of 2.5 ppm
was determined. Further studies are being carried out in order to
determine the potential use of L1 as a chromogenic reagent for
easy-to-use colorimetric probes for cyanide determination in a
wide range of locations. Studies are also under way into the use
of other dyes also containing highly electrophilic acceptor units
as chromogenic reagents for cyanide.
In summary we have reported a new colorimetric method for
determination of cyanide in water samples based on the specific
reaction of this anion with a water soluble squaraine derivative.
The method is easy and allows detection by the naked eye of
low cyanide concentrations.
cyanide attack on a carbon of the four-atom squaraine ring next
to the phenyl group.11 This would produce both loss of the
acceptor character of the ring and rupture of the electronic
delocalisation with the consequent disappearance of the 641 nm
charge transfer band.
In order to examine the potential use of the molecular
chromogenic reagent L1 for the determination of cyanide in
aqueous solutions further experiments were carried out. The
studies were performed buffering to pH 9.5 (0.01 M of TRIS
[tris(hydroxymethyl)aminomethane]) aqueous cyanide solu-
tions containing different amounts of cyanide. Eight parts of
these aqueous solutions were mixed up with two portions of an
acetonitrile solution containing the L1 chromogenic reagent
(concentration of L1 in the water+acetonitrile 80+20 v/v final
mixture was 1025 M). Like in acetonitrile solutions it was
observed that the variation in the absorbance was proportional
to the cyanide concentration and that it changed with time. This
can be seen in Fig. 2 which shows the colour of L1 solutions in
the presence of different cyanide concentrations after 18
minutes once the mixture was set. Additionally Fig. 3 shows the
plot of the logarithm of the absorbance of the solutions in Fig.
2 versus the cyanide concentration. A linear response was
observed in the 1 3 1025–3 3 1024 M range. A detection limit
as low as 0.1 ppm was found for the detection of cyanide in
water.12
We should like to thank the DGICYT (PB98-1430-C02-02
and AMB99-0504-C02-01) for support. J.V.R.L. also thanks
the Ministerio de Educación, Cultura y Deporte for a Doctoral
Fellowship.
Notes and references
1 C. O. Ikediobi, L. Wen and L. M. Latinwo, Am. Environ. Lab., 1997, 9,
20.
2 F. P. Schmidtchen and M. Berger, Chem. Rev., 1997, 97, 1609; P. D.
Beer and P. A. Gale, Angew. Chem., Int. Ed., 2001, 40, 486; P. A. Gale,
Coord. Chem. Rev., 2000, 199, 181; P. A. Gale, Coord. Chem. Rev.,
2001, 213, 79.
3 D. Jimenez, R. Martinez-Mañez, F. Sancenon and J. Soto, Tetrahedron
Lett., 2002, 43, 2823; H. Miyaji, W. Sato and J. L. Sessler, Angew.
Chem., Int. Ed., 2000, 39, 1777; D. H. Lee, H. Y. Lee, K. H. Lee and J.
I. Hong, Chem. Commun., 2001, 1188; F. Sancenón, R. Martínez-Máñez
and J. Soto, Angew. Chem., Int. Ed., 2002, 41, 1416; H. Miyaji and J. L.
Sessler, Angew. Chem., Int. Ed., 2001, 40, 154.
4 S. L. Wiskur, H. Ait-Haddou, J. J. Lavigne and E. V. Anslyn, Acc.
Chem. Res., 2001, 34, 963; H. Ait-Haddou, S. L. Wiskur, V. M. Lynch
and E. V. Anslyn, J. Am. Chem. Soc., 2001, 123, 11296.
5 F. Sancenón, A. B. Descalzo, R. Martínez-Máñez, M. A. Miranda and J.
Soto, Angew. Chem., Int. Ed., 2001, 40, 2640.
6 A. B. Descalzo, D. Jiménez, J. El Haskouri, D. Beltrán, P. Amorós, M.
D. Marcos, R. Martinez-Mañez and J. Soto, Chem. Commun., 2002,
562.
7 Y. H. Kim and J. I. Hong, Chem. Commun., 2002, 512; P. Anzenbacher,
Jr., D. S. Tyson, K. Jursíková and F. N. Castellano, J. Am. Chem. Soc.,
2002, 124, 6232.
8 R. W. Bigelow and H. J. Freund, J. Chem. Phys., 1986, 107, 159.
9 See for example: A. C. Tam and R. D. Balanson, IBM J. Res. Dev., 1982,
26, 186; A. P. Piechowski, G. R. Bird, D. L. Morel and E. L. Stogryn,
J. Phys. Chem., 1984, 88, 934; V. P. Jipson and C. R. Jones, J. Vac. Sci.
Technol., 1981, 18, 105; G. J. Ashwell, G. Jefferies, D. G. Hamilton, D.
E. Lynch, M. P. S. Roberts, G. S. Bahra and C. R. Brown, Nature, 1995,
375, 385.
Fig. 2 H2O+ACN 80+20, from left to right [CN2] = 0, 1, 3, 5, 8, 11, 14, 18,
22, 26, 30 and 40 3 1025 M, in presence of TRIS 0.01 M after 18 minutes
of reaction, [L] = 1.00 3 1025 M.
The fact that the reaction between L1 and cyanide is not
instantaneous but a function of the time and the linear response
found between the logarithm of the absorbance and the
concentration of cyanide (see Fig. 3) indicates a second-order
process related to the reaction between L1 and cyanide anion.
The reaction has a rate constant in water+acetonitrile 80+20 v/v
mixtures of 2.73 M21 s21 at 20 °C. The reaction can not be
carried out when the pH is more basic because then there is
rapid decolouration of L1 even in the absence of cyanide
10 H. E. Sprenger and W. Ziegenbein, Angew. Chem., Int. Ed. Engl., 1968,
7, 530.
11 1H NMR of L1 in acetonitrile gave in the aromatic region two doublets
(at 8.25 and 7.60 ppm) whereas upon cyanide addition four doublets are
observed (centred at 7.60, 7.24, 6.75 and 6.72 ppm). The 13C NMR of
L1 in the aromatic region gave as expected 6 signals at 188.5, 183.1,
153.7, 133.1, 119.9 and 112.5. Upon cyanide addition there is a loss of
symmetry. The most characteristic feature upon cyanide addition is that
the signals of the central four menber ring at 188.55 and 183.13 ppm
disappear and appear a total of 4 signals at 176.3, 131.5, 111.9, 52.1 and
a signal at 126.3 attributable to the carbon of the CN group.
12 The US Environmental Protection Agency (EPA) has set the so called
Maximum Contaminan Level Goals for cyanide in drinking water to 0.2
ppm.
Fig. 3 Log (Abs) versus the concentration of cyanide in water+acetonitrile
80+20 v/v, at pH 9.5 (TRIS 0.01 M) after 18 minutes of reaction, [L1] =
1.00 3 1025 M (20 °C).
CHEM. COMMUN., 2002, 2248–2249
2249