Colorimetric detection of cyanide with phenyl thiourea derivatives

Article abstract of DOI:10.1016/j.saa.2011.04.087

Three structurally simple thiourea derivatives 1-3 were prepared, and their chromogenic behaviors toward various anions were investigated in aqueous solution. Among them 1 showed good sensitivity and selectivity for cyanide ion and also can distinguish it

Full text of DOI:10.1016/j.saa.2011.04.087

Spectrochimica Acta Part A 79 (2011) 1552–1558
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Colorimetric detection of cyanide with phenyl thiourea derivatives
Yi-Shan Lin, Jia-Xing Zheng, Yao-Kang Tsui, Yao-Pin Yen∗
Department of Applied Chemistry, Providence University, Taichung, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
Three structurally simple thiourea derivatives 1–3 were prepared, and their chromogenic behaviors
toward various anions were investigated in aqueous solution. Among them 1 showed good sensitiv-
ity and selectivity for cyanide ion and also can distinguish it from other anions by different color changes.
Besides that, the receptor 1 has a sensitive detection limit (1.27 ␮M) for cyanide ion accordingly it can
be used as a colorimetric sensor for the determination of cyanide ion. The use of the test strip of sensor
1 to detect cyanide ion was also reported.
Received 25 November 2010
Received in revised form 7 April 2011
Accepted 15 April 2011
Keywords:
Colorimetric sensor
Cyanide ion
© 2011 Elsevier B.V. All rights reserved.
Thiourea derivatives
1. Introduction
advantage of the particular feature of the cyanide ion: its good
nucleophilic character, and little competition from the aqueous
Anion recognition is an area of growing interest in supramolec-
ular chemistry due to its important role in a wide range of
environmental, chemical and biological applications. Recently, con-
siderable research attention has been focused on the design of host
molecules that can selectively recognize and sense anion species
through visible, electrochemical and optical response [1–3]. Among
the important anions, cyanide is of particular interest because it
is a detrimental anion causing poisoning in biology and environ-
ment [4–6]. Despite its toxicity, its application in various areas
as raw materials for synthetic fibers, resins, herbicides, and the
gold-extraction process is inevitable [7], which releases cyanide
into the environment as a toxic contaminant. Therefore, it is highly
desirable to develop selective, sensitive and convenient assays for
cyanide anion.
In recent years, many efforts have been devoted to design vari-
ous chemosensors specific for cyanide detection [8–26]. The most
attractive approach focuses on the research of novel colorimetric
cyanide anion sensors, which allow naked eyes detection of the
color change without resorting to the use of expensive instruments.
Moreover, simple and easily synthesized colorimetric sensors that
are cable of recognizing cyanide ion in aqueous environment may
still a great challenge.
media. Based on this idea, nucleophilic addition of cyanide to
oxazine [9,10], pyrylim [30], squarane [13], trifluoroacetophenone
[31,32], acyltrazene [33], acridinium [34], salicylaldehyde [35,36],
caboxamide [37,38], benzamide [39], fluorescein aldehyde [40],
and benzyl [41] have been reported in recent few years. However,
many receptors for cyanide reported so far have several limitations
such as poor selectivity over F− or AcO−, or utilization of expen-
sive instruments, complicated synthesis, and many of receptors are
reported to work only in organic media.
With these considerations in mind, we report herein three
simple yet effective colorimetric sensors 1–3. They utilize either
a nitronaphthyl or a nitrophenyl or a trifluoromethylphenyl
diazenylphenyl as a chromophore and a thiourea group as an elec-
trophile. It is well known that cyanide is a strong nucleophililic
reagent, and the nucleophilic attack to the carbon atom of an
electron-deficient thiocarbonyl (C S) group can achieve the so-
called ‘naked eye’ detection of cyanide anion in aqueous medium.
2. Experimental
2.1. Reagents
To date, many of the reported cyanide anion receptors have
relied on hydrogen-bonding motifs and, as a consequence, have dis-
played poor selectivities in aqueous media [27–29]. To overcome
this limitation, reaction-based receptors have been developed
recently [30–41]. This reaction-based recognition mode takes
Anions including F−, Cl−, Br−, I−, NO3−, CN−, SCN−, AcO−
and H2PO4 are in the form of tetrabutylammonium salts. These
−
anions were purchased from Sigma–Aldrich chemical company and
stored in desiccators under vacuum containing self-indicating sil-
ica. Solvents were purified prior to use and stored under nitrogen.
Dimethyl sulfoxide was dried with calcium hydride and distilled
in reduced pressure. Unless stated otherwise, commercial grade
chemicals were used without further purification.
∗
Corresponding author. Tel.: +886 4 2632 8001x15218; fax: +886 4 2632 7554.
E-mail address: ypyen@pu.edu.tw (Y.-P. Yen).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.04.087

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Scheme 1. The synthetic procedure for the receptors 1–3.
2.2. General methods
7.48 (d, 2H, J = 8.0 Hz), 7.82 (d, 2H, J = 8.8 Hz), 8.20 (d, 2H, J = 8.8 Hz),
10.28 (s, 1H), 10.38 (s, 1H). 13C NMR (DMSO-d6):122.0, 124.2, 124.9,
125.5, 129.1, 139.5, 142.7, 146.7, 179.8. FAB MS m/z = 274.0648
[M+H]+, calc. for C13H12O2N3S = 274.0650.
Unless otherwise specified, all of the UV–Vis titration experi-
ments were carried out at 298 K. The 1H NMR spectra were recorded
on a Bruker 400 MHz and VARIAN INOVA 600 MHz spectrometer.
UV–Vis spectra were performed on a Cary 300 spectrophotometer.
2.5.3. Synthesis of N-(4-(4-trifluoromethyl)phenyldiazenyl
phenyl)-Nꢀ-phenyl-thiourea (3)
2.3. UV–Vis titration studies
The preparation of 3 followed the above-mentioned procedure
using aniline and 4-trifluoromethylphenyldiazenylphenyl isoth-
iocyanate in the same molar ratios; yield: 0.17 g (24%); Mp:
184.5-185.2◦ C. 1H NMR (400 MHz,DMSO-d6): ı 7.14 (dd, 1H, J = 7.6,
7.2 Hz), 7.35 (dd, 2H, J = 7.6, 8.0 Hz), 7.50 (d, 2H, J = 8.0 Hz), 7.81 (d,
2H, J = 8.8 Hz), 7.93 (d, 2H, J = 8.8 Hz), 7.94 (d, 2H, J = 8.4 Hz), 8.02
(d, 2H, J = 8.4 Hz), 10.13 (s, 1H), 10.24 (s, 1H). 13C NMR (DMSO-d6):
123.8, 124.2, 124.9, 126.0, 126.6, 127.9, 129.8, 131.5, 131.8, 140.4,
145.0, 149.0, 155.5, 180.5. FAB MS m/z = 401.1056 [M+H]+, calc. for
The binding ability of the receptor 1 for anions (as tetrabuty-
lammonium salts) was investigated by UV–Vis spectroscopy in
DMSO/H2O (6/4, v/v) solution using a constant host concentration
(1.0 × 10−4 M) and increasing concentrations of anions. The change
in absorbance at 506 nm for receptor was plotted against anion
concentration and fitted by the equation as described by Connors
[42].
C20H16N4F3S = 401.1045.
2.4. 1H NMR titrations
3. Results and discussion
1H NMR titration experiments were carried out in the DMSO-
d6 or DMSO-d6/D2O solution. A 10 mM solution of the receptor 1
in DMSO-d6 or DMSO-d6/D2O was prepared. Then, the increased
amount of cyanide anion was added to the above-mentioned solu-
tion and 1H NMR of the host–guest system was tested.
Preparation of receptors 1–3 is depicted in Scheme 1. Reaction
of aniline with 4-nitronaphthyl isothiocyanate or 4-nitrophenyl
isothiocyanate or 4-trifluoromethylphenyldiazenylphenyl isothio-
cyanate afforded the corresponding receptors 1–3, respectively. All
of these compounds were characterized by 1H NMR, 13C NMR and
HRMS.
2.5. Synthesis
2.5.1. Synthesis of N-(4-nitronaphthyl)-Nꢀ-phenylthiourea (1)
To a stirred solution of aniline (0.17 g, 1.77 mmol) in CH2Cl2
(50 mL), 4-nitronaphthyl isothiocyanate (0.41 g, 1.77 mmol) in
CH2Cl2 (30 mL) was added at room temperature. The resulting mix-
ture was stirred at room temperature for 18 h. The solution was
then concentrated in vacuum. The solid product was recrystal-
lized from ethanol to give 1 (0.45 g, 78%) as a yellow solid. Mp:
185–186 ◦C. ꢀmax/nm (ε/M−1 cm−1) 372 (5000). 1H NMR (600 MHz,
DMSO-d6): ı 7.16 (t, 1H, J = 7.8 Hz), 7.53 (t, 2H, J = 7.2 Hz), 7.54
(d, 2H, J = 7.8 Hz), 7.75-7.85 (m, 3H), 8.22 (d, 1H, J = 8.4 Hz), 8.35
(d, 1H, J = 8.4 Hz), 8.44 (d, 1H, J = 9.0 Hz), 10.11 (s, 1H), 10.20 (s,
1H). 13C NMR (DMSO-d6): 122.6, 122.8, 124.0, 124.1, 124.4, 125.9,
125.3, 127.4, 128.5, 129.6, 139.2, 141.5, 143.5, 181.0. FAB MS
m/z = 323.0796 [M]+, calc. for C17H13O2N3S = 323.0808.
3.1. Anion binding studies
In order to gain systematic knowledge on the effect of water
on the binding affinity and selectivity for cyanide, we chose
DMSO/H2O mixtures as solvents (9/1, 7/3, and 6/4 of DMSO/H2O
(v/v), respectively). The anions of interest, namely, F−, Cl−, Br−, I−,
NO3−, SCN−, AcO−, H2PO4− and CN− as their tetrabutylammonium
salts were added to the solution of 1, respectively. The correspond-
ing UV–Vis absorption spectra upon addition of different anions in
DMSO/H2O mixtures are shown in Fig. 1. In 9/1 DMSO/H2O mix-
ture, receptor 1 displayed an absorption band at 521 nm, and CN−,
F− and AcO− showed significant effect to the band, whereas less
basic anions such as Cl−, Br−, I−, NO3−, SCN−, and H2PO4 showed
−
less or no effect to the band (Fig. 1a). In 7/3 DMSO/H2O mixture,
though only CN−, F− and AcO− showed obvious interaction with the
receptor 1 (Fig. 1b), but the selectivity for CN− was still affected by
the interference of F− and AcO−. While in 6/4 DMSO/H2O mixture,
a remarkably higher selectivity for CN− over the other anions were
obtained, and other anions did not cause any significant changes
in the absorption intensity (Fig. 1c). These results indicate that
2.5.2. Synthesis of N-(4-nitrophthyl)-Nꢀ-phenylthiourea (2)
The preparation of 2 followed the above-mentioned pro-
cedure using aniline and 4-nitrophenyl isothiocyanate in the
same molar ratios; yield: 0.39 g (80%); Mp: 145–146 ◦C. 1H NMR
(400 MHz,DMSO-d6): ı 7.16 (t, 1H, J = 7.2 Hz), 7.36 (t, 2H, J = 7.6 Hz),

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Y.-S. Lin et al. / Spectrochimica Acta Part A 79 (2011) 1552–1558
Fig. 1. UV–Vis spectra of 1 (0.1 mM) on addition of various anions in different DMSO/H2O (v/v). (a) 9:1 DMSO/H2O, 1.0 equiv of anions; (b) 7:3 DMSO/H2O, 1.0 equiv of
anions; (c) 6:4 DMSO/H2O, 1.0 equiv of anions.
the selectivity for CN− is greatly influenced by water. Anions such
as F− and AcO− could interact with the water through Hydrogen-
bonding, and this leads to a large decrease in their nucleophilicity.
In contrast, cyanide has much weaker Hydrogen-bonding ability
in comparison with F− and AcO− and has stronger thiocarbonyl
carbon affinity, which results in the addition reaction of CN− to
thiocarbonyl carbon. Subsequent proton transfer of thiourea hydro-
gen to the developing alksulfide anion of 1 produces an active state
(its anionic state) of 1, which enhanced the charge-transfer inter-
actions between the electron-rich and electron-deficient moieties,
resulting in a visible color change (Scheme 2).
In order to investigate the interaction mechanism between the
receptor 1 and cyanide, 1H NMR titration experiments were carried
out by addition of cyanide to either the deuterated DMSO solution
or the mixture of deuterated DMSO/D2O (6/4, v/v). Upon addition
of 0.2 equiv of the cyanide, there was significant line broadening of
both the NH peaks of the thiourea. This indicates interaction of the
cyanide with both the NH protons (N–Ha and N–Hb). When more
of the CN− anion was added, all the N–H peaks disappeared com-
pletely (Fig. SI-1). This evidence supports that the cyanide anion
participates in the deprotonation of the NH in the deuterated DMSO
system. However, with the increasing addition of cyanide in the
Scheme 2. The proposed sensing mechanism of 1 for cyanide.

Y.-S. Lin et al. / Spectrochimica Acta Part A 79 (2011) 1552–1558
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Fig. 2. 1H NMR spectra of 1 (1 × 10−2 M in DMSO-d6/D2O = 6/4) upon successive addition of cyanide.
Fig. 3. Family of spectra taken in the course of the titration of 1 (1.0 × 10−4 M) in aqueous solution (DMSO/H2O = 6/4, v/v) with cyanide anion at 25◦ C. Titration profiles
(insert) indicate the formation of a 1:1 stoichiometry.
mixture of deuterated DMSO/D2O (6/4, v/v), not only the disap-
pearance of N–H peaks but also the existence of upfield shift of the
aromatic protons (H2–H4, H7, and H8) to different extent (Fig. 2).
These upfield shifts may due to the fact that after the addition reac-
tion of CN− to thiocarbonyl carbon and subsequent proton transfer
of thiourea hydrogen to the developing alksulfide anion of 1. Thus,
the electron density in the aromatic ring increases, which results
in an upfield shift of the protons. While the phenyl proton H1 and
the naphthyl protons (H6 and H9) exhibited downfield shifts to
different extent. These downfield shifts may caused by the elec-
Fig. 4. The corresponding color changes when receptor 1 (0.5 mM) was treated with various anions (1.0 eqiv.) in DMSO/H2O (6/4, v/v).

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Y.-S. Lin et al. / Spectrochimica Acta Part A 79 (2011) 1552–1558
reaches to a maximum when the molar fraction of ([1]/[CN−] + [1])
is 0.5, confirming the 1:1 stoichiometry (Fig. 3c) [43].
The selectivity of the system for cyanide was then examined by
competition experiments. Fig. 5 shows UV–Vis absorption inten-
sity changes upon addition of cyani−de when 2 equiv. of F−, Cl−, Br−,
I−, NO3−, AcO−, SCN−, and H2PO4 are also present. It is notice-
able that the absorption intensity was almost identical to that
obtained in the absence of anions. The miscellaneous competitive
anions did not lead to any significant spectral and color changes.
The colorimetric detection limit of 1 for CN− ion was also tested.
A plot of (A − A0) versus the cyanide concentration in DMSO/H2O
(6/4, v/v) solution gave a linear relationship (Fig. 6). The detection
limit was calculated on the basis of 3 ꢂ/K [44] is 1.27 ␮M, which
makes receptor 1 as a powerful tool for the detection of cyanide
ion. (The WHO suggested that the maximum allowed cyanide
concentration in drinking water is 1.90 ␮M). In order to investi-
gate how the different substituent group on the thiourea moiety
can influence the binding affinity and selectivity for cyanide, the
4-nitronaphthyl group was replaced by 4-nitrophenyl group or
4-trifluoromethyl-phenyldiazenylphenyl group; receptors 2 and 3
were examined. We repeated these measurements using receptors
2 and 3 in 6/4 DMSO/H2O and 7/3 DMSO/H2O (due to its solubility
problem), respectively. In fact, 2 shows different titration profiles
to 1 upon CN− ion introduction. The intensity of the absorption
peak at 350 nm was gradually decreased, while the intensity of the
shoulder peak at 425 nm evolved (Fig. 7a). The Job’s plot analysis
indicates 1:1 stoichiometrybetween 2 and CN− ions (Fig. SI-4b). The
color of the solution changes from colorless to gold color in the pres-
ence of CN− ions (Fig. 7b). The high selectivity for cyanide was also
observed for receptor 2 (Fig. 7a). However, the detection limit of 2
toward cyanide was determined to be 6.84 ␮M (Fig. SI-5). It seems
that the nitrophenyl group has less effect on the charge–transfer
interaction compare with the nitronaphthyl group in receptor 1.
From 1H NMR titration experiment, it was found that except H3
all the other aromatic protons underwent upfield shifts after the
increasing concentration of CN− anion was added (Fig. SI-6). These
results are compatible with the above proposed mechanism. The
Fig. 5. UV–Vis absorption responses of
1
(1.0 × 10−4 M) to various anions
(2.0 × 10−4 M) in aqueous solution (DMSO/H2O = 6/4, v/v). Bars represent the inten-
sity ratios of absorption at 506 nm. A0 represents the responses of 1 + CN−, A
represents either the responses of 1 + X− or the responses of 1 + (CN− + X−).
trostatic through-space effect [42]. These results are compatible
with the proposed mechanism shown in Scheme 2. The formation
of cyanide adduct was further confirmed by mass spectroscopy.
The electrospray ionization mass spectrum of the cyanide adduct
showed a molecular mass of 350.8, which corresponds to the for-
mula of [1 + CN + H] (calculated m/z 350.1, see Fig. SI-2). The above
results clearly demonstrated that a highly selective sensing sys-
tem for CN− can be developed on the basis of the specific chemical
reaction.
To quantitatively study the cyanide-sensing ability of 1 in 6/4
DMSO/H2O mixture, UV–Vis monitoring was performed by using
0.1 mM solution of 1. Upon addition of cyanide anions, the intensity
of the absorption peak at 372 nm was gradually decreased, while
the intensity of the peak at 506 nm evolved and after the addition of
2.0 equiv. of CN− anion, reached to their limiting values (Fig. 3a). The
isosbestic point at 395 nm indicated a clean conversion throughout
the titration process. Interestingly, the color of the solution changed
from its initial orange color to red color (Fig. 4). The signaling was
completed within 2 min (Fig. SI-3). By plotting the changes of 1 in
the absorbance at 506 nm as a function of CN− anion concentration,
a sigmoidal curve was obtained and is shown in the inset of Fig. 3b.
To corroborate 1:1 ratio between 1 and CN−, Job’s plot analysis was
also executed. The measured absorbance variation (ꢁA = Aabs − Ai)
Fig. 6. (a) Photograph of 1 in aqueous solution (DMSO/H2O = 6/4, v/v) in the presence of the CN− ion. (b) Absorption intensity of 1 versus CN− concentrations. [1] = 0.1 mM.

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Fig. 7. UV–Vis spectra (a) and color changes (b) of 2 (1.0 × 10−4 M) in the presence of various anions (1.0 equiv) in DMSO/H2O (6/4, v/v).
formation of cyanide adduct was also confirmed by mass spec-
troscopy. The electrospray ionization mass spectrum showed a
molecular mass of 299.3, which corresponds to the formula [2 + CN]
(calculated m/z 299.1, see Fig. SI-7).
Subsequently, the titration experiments were repeated for
receptor 3 in the 7/3 DMSO/H2O system. Receptor 3 shows
similar titration profiles to 2 upon CN− ion introduction. An
intense band for the ˘ − ˘* transition of the 4-trifluoromethyl-
−
Fig. 8. (a) UV–Vis spectra of 3 (5.0 × 10−5 M) in the presence of the CN− (2.5 × 10−4 M) and miscellaneous anions (X) including F−, Cl−, Br−, I−, SCN−, AcO−, NO3− and H2PO4
respectively. (b) Color changes of 3 (5.0 × 10−4 M) upon addition of tetrabutylammonium salt of X in aquesous solution (DMSO/H2O = 7/3, v/v).
,

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CN−. Therefore, the colorimetric sensor 1 can be used practically to
determine CN− ion.
Acknowledgements
We thank the National Science Council of Republic of China for
the financial support (NSC 98-2119-M-126-001-MY2).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2011.04.087.
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4. Conclusion
In conclusion, three colorimetric anion receptors 1–3 were syn-
thesized. Among them, receptor 1 shows high selectivity toward
CN− ion over miscellaneous competitive anions and also distin-
guishes CN− from other anions by color changes. In DMSO/H2O
(6/4, v/v) solution, the receptor 1 can selectively detect cyanide ion
with a detection limit of 1.27 ␮M. While the test strip containing 1
also exhibits a good selectivity and sensitivity (ca. 1.0 × 10−4 M) to