Malononitrile–derivative chromogenic devices for the detection of cyanide in water

Article abstract of DOI:10.1016/j.molliq.2016.08.093

Two compounds, 2–(4–hydroxybenzylidene)malononitrile (1a) and 2–[4–(dimethylamino)benzylidene]malononitrile (2), were synthesized with a view to their use as optical devices for anionic detection. Compound 1a was studied as a chromogenic chemosensor using an acid–base strategy. In purely aqueous medium, 1a was used in the detection of CN^? with high selectivity. This is possible because the colorless solution of the compound becomes yellow upon addition of the anion, which acts as a base and abstracts the hydrogen from the phenolic moiety of the molecule, generating the corresponding dye (1b). Optical detection and optical quantification limits were determined as 6.17?×?10^??7?mol?L^??1 and 2.06?×?10^??6?mol?L^??1, respectively. These values are much lower than the maximum level of the concentration of CN^? allowed by the World Health Organization in potable water (1.7?μmol?L^??1). Compound 2 was used in a chemodosimeter approach for the highly selective detection of CN^? in water with cetyltrimethylammonium bromide above its critical micellar concentration. Solutions of 2 are yellow but become colorless with the addition of the anion. This is because CN^? attacks the CH[dbnd]C double bond in the compound and disrupts the electronic conjugation between the electron donating and the electron accepting moieties of the dye. Thus, with the addition of CN^? to the medium compound 1a acts as an off–on optical device while dye 2 operates as an on–off chemodosimeter.

Full text of DOI:10.1016/j.molliq.2016.08.093

Malononitrile–Derivative Chromogenic Devices for the Detection of Cyanide
in Water
Adriana D.S. Schramm, Renata Menger, Vanderlei G. Machado
PII:
DOI:
Reference:
S0167-7322(16)31390-3
doi: 10.1016/j.molliq.2016.08.093
MOLLIQ 6254
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
31 May 2016
20 August 2016
24 August 2016
Please cite this article as: Adriana D.S. Schramm, Renata Menger, Vanderlei G.
Machado, Malononitrile–Derivative Chromogenic Devices for the Detection of Cyanide
in Water, Journal of Molecular Liquids (2016), doi: 10.1016/j.molliq.2016.08.093
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Malononitrile–Derivative Chromogenic Devices for the Detection of Cyanide in
Water
Adriana D. S. Schramm, Renata Menger and Vanderlei G. Machado
880 0−900 B z
*Corresponding author. Tel.: +55 48 3721 4542. Fax: +55 48 3721 6852.
E–mail: vanderlei.machado@ufsc.br

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Abstract
Two compounds, 2–(4–hydroxybenzylidene)malononitrile (1a) and 2–[4–
(dimethylamino)benzylidene]malononitrile (2), were synthesized with a view to their
use as optical devices for anionic detection. Compound 1a was studied as a
chromogenic chemosensor using an acid–base strategy. In purely aqueous medium, 1a
was used in the detection of CN– with high selectivity. This is possible because the
colorless solution of the compound becomes yellow upon addition of the anion, which
acts as a base and abstracts the hydrogen from the phenolic moiety of the molecule,
generating the corresponding dye (1b). Optical detection and optical quantification
limits were determined as 6.17×10–7 mol L–1 and 2.06×10–6 mol L–1, respectively. These
values are much lower than the maximum level of the concentration of CN– allowed by
the World Health Organization in potable water (1.7 mol L−1). Compound 2 was used
in a chemodosimeter approach for the highly selective detection of CN– in water with
cetyltrimethylammonium bromide above its critical micellar concentration. Solutions of
2 are yellow but become colorless with the addition of the anion. This is because CN–
attacks the CH=C double bond in the compound and disrupts the electronic conjugation
between the electron donating and the electron accepting moieties of the dye. Thus, with
the addition of CN– to the medium compound 1a acts as an off–on optical device while
dye 2 operates as an on–off chemodosimeter.
Keywords: chemosensors; chemodosimeters; anion sensing; naked–eye detection;
cyanide

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1. Introduction
The development of optical devices for the detection of anionic species [1–12]
represents an important challenge in the field recently named supramolecular analytical
chemistry [13]. Many strategies have been developed using organic compounds
specially designed to change their color in solution in the presence of a particular
analyte, according to a reading taken by a device of the parameters which are
characteristic of the anion, such as the geometry, acidity, and basicity. Of these
strategies, chromogenic chemosensors based on acid–base processes [9, 14–22] and
chemodosimeter approaches [19, 23–29] have been studied by many research groups in
order to provide access to systems for the detection of cyanide (CN–).
CN– is relevant in terms of detection due to its importance as a reactant in
industrial and chemical processes [30–33]. This species is released in the burning of
plastics generated from acrylonitrile [34–36] and in the hydrolysis of some nerve agents
[37, 38] and cyanoglycosides present in some seeds and roots [22, 39]. CN– is a very
toxic chemical agent, being lethal in minimal concentrations, due to its very efficient
action as a ligand at the iron sites of cytochrome c oxidase [40, 41]. This binding
prevents the electronic transport from the enzyme complex to the oxygen, leading to the
disruption of the electron transport chain.

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The fact that CN– is recognized as a strong base and a very efficient nucleophile
has led to the development of many optical devices for its detection based on acid–base
processes and strategies in which CN– acts as a nucleophilic species. Optical
chemosensors based on acid–base reactions make use of organic compounds that, by
means of interaction with a particular basic anion, generate a dye, indicating the
presence of the analyte. This strategy involves the use of a planned compound, having
groups such as NH (amides and amines), OH (phenols), or SH (thiols), associated
through a conjugated system to an electron–releasing group, and its interaction with the
anion can correspond to a partial or complete proton transfer [7, 9, 42, 43]. The
advantages of these chemosensors include low cost, real time analysis, and the reuse of
the system [44].
The chemodosimeter approach represents an interesting strategy for the design
of systems able to detect nucleophilic species [23, 24, 45]. In contrast to the acid–base
strategy, chemodosimeters are based on irreversible processes. They are comprised of
organic compounds in which a particular dye is masked with a covalently linked group.
When the latter reacts with a nucleophilic analyte the dye is released, and this can be
used for the detection of the analyte. Another related approach involves the use of a dye
with a conjugated bridge in its molecular structure. In this case, the nucleophilic action
of the anion disrupts the electronic conjugation of the system.

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Malononitrile is an organic reactant that is commonly used in organic synthesis for the
formation of C–C bonds [46–48]. This reactant can be used in Knoevenagel
condensation by reacting it with aldehydes in the presence of piperidine or hydroxide as
a base [49–52]. Recently, some derivatives from the reaction of malononitrile with
aldehydes were used to obtain chemodosimeters for the detection of CN– [53–61]. This
anion reacts with the C=C bond of the compound in solution, leading to the disruption
of the electronic conjugation of the molecular system. As a result, the previously
colored solution becomes colorless and the anion can thus be detected.
In
this
study,
we
synthesized
and
characterized
and
2–(4–
2–[4–
hydroxybenzylidene)malononitrile
(1a)
(dimethylamino)benzylidene]malononitrile (2) (Fig. 1) with a view to their use as
optical devices for anionic detection. Li et al. recently used compound 2 as a
chemodosimeter for the detection of CN– in a dimethyl sulfoxide–water mixture [62].
Herein, we show that compound 1a and also 2 can be employed in the detection of CN–
in a purely aqueous medium, using acid–base and chemodosimeter approaches. The
strategies demonstrated herein are very selective for CN– compared with other anions,
with very low detection and quantification limits.

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((Fig. 1 here))
2. Experimental
2.1. Materials
All chemicals used were high–purity commercial reagents. Acetonitrile (HPLC
grade, Sigma–Aldrich) was stored on molecular sieves (Aldrich). Tetra–n–
butylammonium hydroxide was purchased from Sigma–Aldrich. All tetra–n–
butylammonium salts were of purity greater than 95–99% and were purchased from
–
–
Fluka (Cl–, > 98%; NO3 , > 97%), Vetec (Br–, > 99%; I–, > 99%; HSO4 , > 99%), and
–
Sigma–Aldrich (H2PO4 , > 99%; CN–, > 95%; CH3COO–, > 97%; F–, > 98%). These
salts were dried over P4O10 under vacuum before use. Cetyltrimethylammonium
bromide (CTABr), Pb(NO3)2 (> 99%), Na2S (nonahydrate, > 98%), and NaN3 (> 99%)
were purchased from Sigma–Aldrich. The deionized water used in the measurements
was boiled and bubbled with nitrogen and then kept in a nitrogen atmosphere to avoid
the presence of carbon dioxide.
Compounds 1a and 2 were synthesized as described previously by Sheibani
and Saljoogi [63]. The detailed synthesis of the compounds and their corresponding
characterization data are shown in the Supporting Information.

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2.2. General methods
Melting points were obtained on a Kofler hot stage and were uncorrected. The
nuclear magnetic resonance (NMR) spectra were obtained using Varian NMR AS 200
MHz and NMR AS 400 MHz spectrometers. Chemical shifts were recorded in ppm
with the solvent resonance as the internal standard. Data are reported as follows:
chemical shift, integration, multiplicity (s = singlet and d = doublet), and coupling
constants (Hz). Infrared (IR) spectra were obtained on a Shimadzu spectrophotometer
(model Prestige–21), with KBr pellets. High–resolution mass spectra were obtained
with an electro–spray ionization (ESI) quadrupole time–of–flight (QTOF) mass
spectrometer. UV–vis experiments were carried out on an HP 8452A spectrophotometer
equipped with a thermostated bath and all measurements were performed at 25 °C,
employing a 1 cm quartz cuvette. The maximum wavelength (λmax) values of the UV–
vis spectra were calculated from the first derivative of the absorption spectrum. The pH
measurements were performed at 25.0 ± 0.1 °C, using a Beckman pHmeter (model ɸ
71).

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2.3. Naked–eye and UV–vis studies
Solutions of compounds 1a and 2 in acetonitrile were prepared in a
concentration of 1.0×10–5 mol L–1, and these were employed in the preparation of the
–
–
–
solutions of each anion (OH–, HSO4 , H2PO4 , NO3 , CN–, CH3COO–, F–, Cl–, Br–, and
I–) as tetra–n–butylammonium salts. The visual changes were recorded in digital
photographs and the UV–vis spectra at 25 oC were obtained.
The experiments in water involving compound 1a were performed using
2.0×10−5 mol L−1 of the compound, while the solutions of 2, in a concentration of
3.0×10−5 mol L−1 were prepared in the presence of CTABr (2.0×10−3 mol L−1). For
compound 1a the aqueous medium had its pH adjusted to 5.0 with the addition of a
small volume of HCl aqueous stock solution (1.0×10−5 mol L−1).

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In the experiments of 1a using Pb2+, solutions of the compound were prepared
in water (2.0×10−5 mol L−1) containing each anion (6.0×10−4 mol L−1) and a volume of a
stock solution of Pb(NO3)2 (0.1 mol L–1) was added at a final concentration of 1.0×10−3
mol L−1 [64]. For compound 2, two studies were performed. In one study, aqueous
solutions of the compound (3.0×10−5 mol L−1) and CTABr (2.0×10−3 mol L−1)
containing each anion (6.0×10−4 mol L−1) were made and Pb(NO3)2 was added at a
concentration of 1.0×10−3 mol L−1. In the other study, firstly Pb(NO3)2 was added to a
final concentration of 1.0×10−3 mol L− 1 to the flask containing only the anionic species.
Subsequently, small volumes from stock solutions of 2 (1.0×10−2 mol L−1) and CTABr
(0.1 mol L−1) were added to reach final concentrations of 2 and CTABr of 3.0×10−5 mol
L−1 and 2.0×10−3 mol L−1, respectively.
Titration experiments were performed in the cases where the anions caused
changes in the color of the solutions of the compounds. The solutions of compounds 1a
and 2 were prepared as described in the previous paragraph. These solutions were
utilized in the preparation of 2 mL of the stock solution of the anions (0.1 mol L–1), in a
glass flask closed with a rubber stopper. An aliquot (1.2 mL) of the solution of the
compound was transferred to a quartz cuvette which was carefully closed with a rubber
stopper to avoid the entrance of water. The cuvette was placed in the cell holder of the
o
spectrophotometer and kept at 25 C using a thermostated bath. A small volume of the
anion stock solution was then added to the cuvette using a microsyringe. The mixture
was stirred, the UV–vis spectrum was obtained, and the absorbance value was collected
at the maximum wavelength (λmax) of the vis band. The anion was added until no
alterations in the absorbance were verified. The experimental data were transformed
into titration curves, in the form of plots of the absorbance values as a function of the
concentration of the anion added.

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The linear segment of the titration curve was used to estimate the optical
detection limit (ODL) and the optical quantification limit (OQL). Based on the linear fit
of the experimental data, the ODL and OQL were obtained from Eqs. (1) and (2)
according to a procedure reported in the literature [65], where Sb1 is the standard
deviation of the blank solution and S is the slope of the calibration curve. The B values
are 3 for ODL and 10 for OQL.
ODL = BSb1 / S
OQL = BSb1 / S
(1)
(2)

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2.4. Determination of pKa of compound 1a
A stock solution of compound 1a with a concentration of 1×10–2 mol L–1 was
prepared in dry acetone. Aliquots of this solution were transferred to glass flasks, the
solvent was left to evaporate and water at different pH values (adjusted with 0.1 mol L–1
aqueous NaOH or HCl solutions) was added to give solutions with concentrations of
o
2×10–5 mol L–1. The UV–vis spectra at 25 C were recorded for each solution and the
absorbance values for the λmax of the vis band were collected. A plot of the absorbance
as a function of the pH was constructed and the data were fitted to a sigmoid equation to
determine the pKa value of 1a.
2.5.1H NMR titrations
1
For the H NMR titrations 4.1 mg of 1a and 6.2 mg of 2 were placed in NMR
tubes and acetonitrile (0.6 mL) was added. The concentration of 1a was 3.9×10–2 mol L–
1
and of 2 was 5.1×10–2mol L–1. Aliquots of a 0.14 mol L–1 tetra–n–butylammonium
1
cyanide stock solution were added to the NMR tubes containing 1a and 2 and the H
NMR spectra were obtained at room temperature.

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2.6. Calculations
All constants were calculated through the fitting of least–squares regression
curves using the program ORIGIN® 6.1.
3. Results and Discussion
Compounds 1a and 2 were prepared according to Scheme 1, as described
previously by Sheibani and Saljoogi [63], through condensation of the corresponding
aldehyde with malononitrile in the presence of NaOH as a base, using water:ethanol
o
(4:1; v/v) as the medium and at 50 C. Products 1a and 2 were obtained with yields of
80 and 90 %, respectively, and the purity was sufficient for the use of the compounds in
the subsequent assays.
((Scheme 1 here))
An acid–base strategy using compound 1a
Fig. 2(A) shows the solutions of compound 1a in water in the presence and in the
absence of various anionic species. A solution of compound 1a is colorless but it
becomes yellow when HO– is added to the medium. This is due to the fact that
hydroxide abstracts the proton of the phenolic moiety of the compound, generating the
corresponding conjugate base 1b. Of the anions studied only CN– and sulfide caused the
deprotonation of 1a (Scheme 2).

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Selective determination of CN– in mixture of the latter in the presence of sulfides
can be in principle performed with the removal of sulfides from the medium by
precipitation with the use of Pb2+ [64, 66]. Fig. 2(B) shows the influence of the addition
of Pb(NO3)2 on the solution of 1a with the anions used in the study. Only the solution
containing sulfide changed its visual aspect, with the appearance of a brown precipitate
of PbS. The content of that flask was centrifuged, being verified that the resulting
solution is colorless (Fig. 2(C)). If 1a is placed in contact with a solution of CN– and
sulfide, and Pb2+ is added to the mixture, being after centrifuged, the resulting solution
is yellow (Figs. S21 and S22), suggesting that this strategy can be used for the naked–
eye discrimination of CN– over sulfide in a mixture containing these species.
Buffered aqueous solutions proved to be a not adequate medium to be utilized in
this strategy. If the buffer is used to keep the pH above the pKa of the chemosensor, the
compound is in its deprotonated form and it is unable for anion sensing. On the other
hand, if the buffer is used in pH values below the pKa of 1a, the anion is found in its
protonated form, being unable to act as base. In buffered media, increasing amounts of
the anion are required to be utilized in order to break the buffer and to have the anionic
species in the medium to abstract the proton of 1a (Figs. S16–S18).

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((Fig. 2 here))
((Scheme 2 here))
The UV–vis spectra in water (Fig. 3(A)) show that the band of 1a at max = 350
nm is replaced by other band with a maximum at 416 nm in the presence of CN– and
sulfide. This spectrum is the same as that obtained in the presence of HO– and typical of
the formation of 1b. Data also show a small influence of azide and acetate ions on the
spectrum of 1a. The pKa value of 1a was estimated as 7.08±0.02 (Fig. S15).The pKa of
o
HCN at 25 C is 9.21 [67], while the pKa values of H2S and HS– are 7.24 and 14.92
[68], respectively. Their conjugated bases are sufficiently strong to abstract the proton
o
of 1a, in contrast to the other anions (pKa values at 25 C of H3PO4, CH3COOH, HN3,
and HF are 2.12, 4.75, 4.73, and 3.14, respectively) [67, 69, 70].
Fig. 3(B) shows the relative absorbance for 1a in water with various anions in the
absence and in the presence of CN–. The data show that in water 1a behaves as an
optical device which is, in general, highly selective for the detection of CN–. The use of
Pb(NO3)2 in the samples containing a mixture of CN– and sulfide revealed to be a very
interesting strategy to make the system able to detect only CN–.
((Fig. 3 here))
Experiments were also made for the detection of CN– in aqueous medium using
three different cations (K+, Na+, and tetra–n–butylammonium) and no influence of these
species was verified on the performance of the chemosensor (Fig. S19).

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Fig. 4(A) shows the influence of the addition of increasing amounts of CN– on
the UV–vis spectra for 1a in water. With the addition of the anion, the absorbance of the
band corresponding to 1a, with a maximum at 350 nm, decreases with the simultaneous
appearance of other band related to 1b, with a maximum at 416 nm. An isosbestic point
was verified at 374 nm, which suggests that 1a and 1b are in equilibrium in solution.
The corresponding titration curve is shown in Fig. 4(B). A linear increase in the
absorbance at 416 nm occurs with the addition of CN– up to a 1:2 1a:CN– stoichiometry
and after that the absorbance remains constant with an increase in the anion
concentration. A Job’s plot was made (see insert in Fig. 4(B)) and the result obtained
also suggested a 1:2 1a:CN– stoichiometry. These results for a chemosensor:anion ratio
of 1:2 suggests that Eq. (3) could be used to fit the experimental data.

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2
Abs= [Abso + Abs12K12CA–2 ] / [1 + K12CA–
]
(3)
In this equation, Abs is the absorbance after each addition of the anion, Abs0 is the initial
absorbance without the anion added, Abs12 is the maximum absorbance obtained after
addition of the anion considering a 1:2 1a:anion stoichiometry, CA– is the anion
concentration for each addition, and K12 is the equilibrium constant. The fitting of the
data using Eq. (1) provided a K12 value of (3.08±0.1)×108 L2 mol–2 (S.D. = 1.05×10–2).
The ODL and OQL values were determined as 6.17×10–7 mol L–1 and 2.06×10–6 mol L–1,
respectively. The titration of 1a with CN– was also carried out in acetonitrile (Fig. S13),
which also provided a 1:2 1a:anion stoichiometry and the K12 value was (1.13±0.2)×1011
L2 mol–2 (S.D. = 7.8×10–2).
((Fig. 4 here))
1
Fig. 5 shows H NMR spectra for 1a in CD3CN in the absence and presence of
different equivalents of CN–. With the addition of one equivalent of the anion, the
singlet at δ 7.91 ppm of the CH double bond was shifted to δ 7.58 ppm and the doublets
centered at δ 7.87 and 6.99 ppm (related to aromatic hydrogens) were shifted to δ 7.14
and 6.66 ppm. In addition, the solution of the NMR tube was changed from colorless to
yellow (Fig. S24). These data are consistent with the previous results which indicated
that the anion deprotonates 1a generating 1b. It was also observed that the solution
becomes colorless again when two equivalents of CN– are mixed with the compound in

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the NMR tube. The analysis of the final NMR spectrum shows that the singlet at δ 7.58
ppm disappeared, being shifted upfield to δ 4.25 ppm. In addition, the doublets at δ 7.14
and 6.66 ppm were replaced by two other doublets at δ 7.19 and 6.78 ppm. These data
suggest that after it has acted in the formation of 1b the anion present in excess acts as a
nucleophile. CN– attacks the CH=C double bond and disrupts the electronic conjugation
between the electron donating and the electron accepting moieties of 1b, which
determines the color of the solution.
((Fig. 5 here))
Compound 2 as a chemodosimeter for the detection of cyanide
Compound 2 is not soluble in water but we verified that the dye can be solubilized
when CTABr is present in the medium in a concentration above its critical micellar
concentration. Fig. 6(A) shows that, of the anions tested, the yellow solutions of 2 are
changed to colorless only with the addition of CN– and sulfide. The addition of
Pb(NO3)2 in each flask revealed that only the solution containing sulfide are changed to
brown (Fig. 6(B)) and, differently from the assays using 1a, the PbS formed could not
be removed by centrifugation. Thus, a study was performed by adding Pb(NO3)2 on an
aqueous solution containing CN– and sulfide, but without 2 and CTABr. In this
situation, a reaction occurred with formation of PbS as precipitate. After the
centrifugation of the PbS formed, 2 and CTABr were added in the flask, being verified
that CN– could make colorless the solution (Fig. 6(C)). Thus, the results showed that
CN– could be detected in a highly selective fashion.

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((Fig. 6 here))
The UV–vis spectra for 2 in an aqueous CTABr medium show a band with max
=
438 nm, which disappeared only in the presence of CN– and sulfide (Fig. 7(A)). Fig.
7(B) shows UV–vis spectra for the solutions of Fig. 6(C), which reinforces the fact that
the use of the strategy using Pb(NO3)2 makes compound 2 highly selective for the
detection of CN–. The relative absorbances of dye 2 in water with various anions in the
absence and in the presence of CN– is shown in Fig. 7(C), which demonstrate the highly
selective detection of CN– in these experimental conditions.
((Fig. 7 here))
Spectrophotometric titrations of compound 2 with CN− were performed in
acetonitrile and in water/CTABr. The titration in acetonitrile shows that the band with a
maximum at 430 nm, corresponding to 2, decreases with the addition of the anion and a
complete disappearance of the band was verified for a 1:1 anion:2 stoichiometry (Fig.
S14). Fig. 8(A) shows a set of UV–vis spectra for the titration of 2 in water/CTABr
with CN–. The absorbance of the band in the vis region decreased with the addition of
the anion, until the complete disappearance of the band. Fig. 8(B) shows the
corresponding titration curve, which was constructed from the absorbance values at 438
nm as a function of c (CN–). The curve has an inflection at ca. 5.7×10–5 mol L–1 of the
anion and after this point a nearly linear decrease occurs up to c (CN–) = 2.5×10–5 mol
L–1. The inflection suggests that some event occurs simultaneously with the reaction of

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2 with CN–, which could be interaction of the anion with the hydrogen of the CH=C
double bond of the dye before the attack on the double bond.
((Fig. 8 here))
1
Fig. 9 shows the influence of increasing amounts of CN– on the H NMR
spectrum for dye 2 in CD3CN. With the addition of one equivalent of the anion the
singlet at δ 7.68 ppm of the CH double bond was shifted upfield to δ 4.25 ppm and the
doublets centered at δ 7.82 and 6.79 ppm (related to aromatic hydrogens) were shifted
upfield to δ 7.23 and 6.74 ppm. The solution of the NMR tube was changed from
yellow to colorless (Fig. S25). Evidence of the interaction of the anion with the
compound prior to the nucleophilic attack could not be obtained, in contrast with the
observations in the UV−vis titrations. The data demonstrate that the anion acts in a
fashion similar to that observed for 1b in the presence of an excess of CN– using the
NMR technique. It is important to note that a comparison of the final spectra for the two
processes (Figs. 5(C) and 9 (C)) shows that the singlet corresponding to the CH in the
products, originating from the double bond, appears in exactly at the same position (δ
4.25 ppm). Thus, the decolorization of the solution in the presence of the anion is a
result of the nucleophilic attack of CN– on the CH=C double bond (Scheme 3).

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((Fig. 9 here))
((Scheme 3 here))
4. Conclusions
Compounds 1a and 2 were synthesized and applied in two different strategies for
the detection, with high selectivity, of CN– in pure water. Compound 1a was employed
in an off–on acid–base strategy, in which the phenol is colorless in solution but turns
yellow due to the deprotonation of the compound, generating its corresponding
phenolate (1b) as a dye. The high selectivity of the system was demonstrated and
explained considering the basicity of the anionic species in aqueous medium. In
addition, compound 1a showed detection and quantification limits of 6.17×10–7 mol L–1
and 2.06×10–6 mol L–1, respectively, which are much lower than the maximum CN–
concentration in potable water allowed by the World Health Organization (1.7 mol
L−1) [71].

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A chemodosimeter approach could be applied for compound 2 in the highly
selective detection of CN– in water, with the use of CTABr above its critical micellar
concentration as an agent to solubilize the dye. Solutions of 2 are yellow but were
changed to colorless with the addition of the anion. This can be explained by CN– acting
as a nucleophile, attacking the CH=C double bond in the compound and disrupting the
electronic conjugation between the electron donating and the electron accepting
moieties of the dye. This system is also highly selective for the detection of small
amounts of CN– in water and the explanation is similar to that proposed for the action of
1a, i.e., it could be related to the differences in the hydration energies of the anions
studied. Thus, with the addition of CN– to the medium, compound 2 operates as an on–
off chemodosimeter.
Acknowledgements
The authors are grateful for the financial support provided by the Brazilian
governmental agencies Conselho Nacional de Desenvolvimento Científico e
Tecnológico (CNPq), Fundação de Amparo à Pesquisa e Inovação do Estado de Santa
Catarina (FAPESC), and Coordenação de Aperfeiçoamento de Pessoal de Nível
Superior (CAPES), as well as the Laboratório Central de Biologia Molecular
(CEBIME/UFSC) and UFSC.

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Supplementary data
Supplementary data (methodology for the synthesis of the compounds,
characterization data, spectrophotometric data, data for the determination of the pKa of
1a, spectrophotometric titrations, and solutions of 1a and 2 in CD3CN in the absence
and in the presence of CN−) associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/

ACCEPTED MANUSCRIPT
23
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