'Naked-eye' detection of fluoride and acetate anions by using simple and efficient urea and thiourea based colorimetric sensors

Article abstract of DOI:10.1016/j.molstruc.2013.04.077

Simple and efficient sensors 1 and 2 possessing azo and nitrophenyl as signaling units and urea and thiourea moieties as binding sites were designed and synthesized. These sensors were characterized by combination of ¹H, ¹³C, APT, CO

Full text of DOI:10.1016/j.molstruc.2013.04.077

Journal of Molecular Structure 1048 (2013) 392–398
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Journal of Molecular Structure
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‘Naked-eye’ detection of fluoride and acetate anions by using simple
and efficient urea and thiourea based colorimetric sensors
⇑
Ahmet Okudan, Serkan Erdemir , Ozcan Kocyigit
Selcuk University, Department of Chemistry, 42031 Konya, Turkey
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
Novel urea and thiourea-based on colorimetric sensors were synthesized.
These sensors were characterized with 1H, 13C, APT, COSY NMR, FTIR, elemental, UV–vis data.
It was found that the receptors are highly selective toward fluoride and acetate anions.
a r t i c l e i n f o
a b s t r a c t
Article history:
Simple and efficient sensors 1 and 2 possessing azo and nitrophenyl as signaling units and urea and
thiourea moieties as binding sites were designed and synthesized. These sensors were characterized
by combination of 1H, 13C, APT, COSY NMR, FTIR, elemental analysis, and UV–vis spectral data. The
interaction and colorimetric sensing properties of receptor 1 and 2 with different anions were investi-
gated by the naked eye, as well as UV–visible and 1H NMR experiments. It was found that the receptor
1 and 2 are highly selective toward fluoride and acetate anions in CHCl3.
Received 27 March 2013
Received in revised form 27 April 2013
Accepted 29 April 2013
Available online 23 May 2013
Keywords:
Colorimetric
Anion
Ó 2013 Elsevier B.V. All rights reserved.
Urea
Naked-eye
Detection
1. Introduction
of any spectroscopic instrumentation. Such receptors would be
more valuable if they could be obtained by a simple synthetic
The selective recognition and sensing of anions via artificial
organic chemosensor molecule/probe, containing a suitable recep-
tor site, have attracted considerable attention for chemists in past
decades [1]. Anions play significant roles in chemical, environmen-
tal and biochemical processes [2–8], hence their recognition is
important. The development of simple receptors capable of recog-
nizing biologically relevant anions such as fluoride, chloride, phos-
phate, and carboxylate has attracted considerable interest [9]. The
design of these receptors has focused on the ability to recognize
and sense selectively biologically important anions through naked
eye, electrochemical and optical responses [10]. The incorporation
of fluorescent chromophores into receptors has gained consider-
able attention owing to their high sensitivities and easy detection
[11]. The investigation of anion-selective receptors based on
colored chromophores is also studied [12]. In particular, the devel-
opment of colorimetric anion sensing is important and useful since
it allows so-called ‘naked-eye’ detection of anions without the use
method [13].
One successful approach for preparing chromogenic sensors
involves the formation of molecular architectures, which contain
one or more optical-signaling chromophoric groups that are cova-
lently or noncovalently linked to the receptor moiety, and thus
colorimetric sensing of anions with both temporal and spatial res-
olution would be achieved. Hydrogen-bonding sites used in chro-
mogenic or fluorogenic chemosensors are urea [14,15], thiourea
[16], amide [17], phenol [18], or pyrrole subunits [19]. The large
numbers of anion receptors containing these subunits have been
designed, synthesized and tested for anion recognition and sensing
during the past decades. For example, Liu and coworkers reported
synthesis of a simple and efficient chemosensor containing naph-
thalene signal moiety and thiourea recognition and that this sensor
has proven to be highly selective for fluoride and show
a
remarkable color change and fluorescence quenching [20]. A novel
colorimetric and fluorescent sensor possessing fluorenone and
naphthalene moieties as signaling groups for fluoride and pyro-
phosphate anions was prepared by Thangadurai and coworkers
[21].
⇑
Corresponding author. Tel.: +90 3322233858; fax: +90 3322232499.
E-mail address: serdemir82@gmail.com (S. Erdemir).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.04.077

A. Okudan et al. / Journal of Molecular Structure 1048 (2013) 392–398
393
Herein, we have designed a new class of urea and thiourea-
based receptors 1 and 2 containing azophenol–imine platform
and investigated their anion sensing properties towards FÀ, BrÀ,
ClÀ, IÀ, AcOÀ, ClO4À, NO3À, HSO4À, and H2POÀ4 anions. The spectro-
scopic data showed that receptor 1 and 2 are selective colorimetric
sensor for fluoride and acetate anions in CHCl3.
Receptor 1; Orange solid, Yield: 85%; Mp = 252-254 °C; 1H NMR
(400 MHz, d6-DMSO): d 7.00 (t, 1H, J = 7.4 Hz, ArH), 7.08 (d, 1H,
J = 8.8 Hz, ArH), 7.28 (t, 2H, J = 7.4 Hz, ArH), 7.62 (d, 2H,
J = 7.6 Hz, ArH), 7.83 and 7.85 (dd, 1H, J = 2.5 Hz, J = 8.8 Hz, ArH),
8.02 (d, 2H, J = 8.9 Hz, ArH), 8.32 (s, 1H, ArH), 8.40 (d, 2H,
J = 8.9 Hz, ArH), 8.66 (d, 1H, J = 2.5 Hz, CHN), 9.01 (s, 1H, NH),
10.74 (s, 1H, NH), 11.25 (br s, 1H, OH). 13C NMR (100 MHz, d6-
DMSO): d 160.68, 155.85, 153.42, 148.34, 145.90, 139.52, 137.36,
128.87, 125.65, 125.55, 123.71, 123.56, 122.97, 121.90, 120.55,
117.46; Anal. Calcd. for C20H16N6O4 (404.38): C, 59.40; H, 3.99;
N, 20.78. Found: C, 59.67; H, 4.05; N, 20.82.
2. Experimental
2.1. General
Receptor 2; Orange solid, Yield: 86%; Mp = 235–237 °C; 1H NMR
(400 MHz, d6-DMSO): d 7.07 (d, 1H, J = 8.8 Hz, ArH), 7.19 (t, 1H,
J = 7.4 Hz, ArH), 7.36 (t, 2H, J = 7.4 Hz, ArH), 7.52 (d, 2H,
J = 7.6 Hz, ArH), 7.82 and 7.84 (dd, 1H, J = 2.5 Hz, J = 8.8 Hz, ArH),
7.98 (d, 2H, J = 9.1 Hz, ArH), 8.37 (d, 2H, J = 9.1 Hz, ArH), 8.54 (s,
1H, ArH), 8.81 (d, 1H, J = 2.2 Hz, CHN), 10.22 (s, 1H, NH), 11.19
(br s, 1H, OH), 11.86 (s, 1H, NH). 13C NMR (100 MHz, d6-DMSO):
d 176.53, 161.17, 155.82, 148.29, 145.96, 139.60, 138.90, 128.49,
126.81, 126.77, 125.88, 125.49, 123.52, 123.33, 121.71, 117.52;
Anal. Calcd. for C20H16N6O3S (420.44): C, 57.13; H, 3.84; N, 19.99.
Found: C, 57.25; H, 3.91; N, 20.02.
NMR spectra were recorded at room temperature on a Varian
400 MHz spectrometer in d6-DMSO and CDCl3. FT-IR spectra were
obtained on a Perkin Elmer Spectrum 100 FTIR spectrometer. UV/
Vis spectra were measured with a Perkin Elmer Lambda 25
spectrometer. Elemental analyses were performed using a Leco
CHNS-932 analyzer. Melting points were determined on an Elec-
trothermal 9100 apparatus in a sealed capillary and are uncor-
rected. Analytical TLC was performed using Merck prepared
plates (silica gel 60 F254 on aluminum). Flash chromatography
separations were performed on a Merck Silica Gel 60 (230–400
Mesh). All reactions, unless otherwise noted, were conducted un-
der nitrogen atmosphere. All starting materials and reagents were
of standard analytical grade from Fluka, Merck, and Aldrich and
used without further purification.
2.3. UV–vis experiments
The solutions of the receptor 1 and 2 (4.0 Â 10À5 M) and the
guest anions (2.0 Â 10À3 M) were prepared in CHCl3. The volume
of the receptor 1 and 2 solutions used in the UV–vis measurements
was 3 mL. Absorption spectra were recorded by adding different
amounts of anion solution to the receptor 1 and 2 solutions. The col-
orimetric studies of 1 and 2 towards various anions can be easily ob-
served by the naked eye in CHCl3 at concentration of 4.0 Â 10À5 M.
2.2. Synthesis
2.2.1. Synthesis of 5-(4-nitro-phenylazo)-salicylaldehyde (S1)
Synthesis of S1 was carried out according to known procedure
[22] For this, to a solution of 4-nitroaniline (3.5 g, 0.025 mol) in
water (2 mL) was slowly added 3 ml of 37% aq HCl solution at 0–
5 °C. 10 ml of 20% aq NaNO2 solution was added to this mixture
and the resulting solution was stirred for 1 h, affording a yellow
solution. Salicylaldehyde (2.5 ml, 0.025 mmol) was dissolved in a
solution comprising 9 g Na2CO3 and 75 ml H2O and the resulting
solution of salicylaldehyde was added dropwise to the bright yel-
low colored solution over 1 h. After stirring for 4 h, the reaction
mixture was neutralized with HCl, the brown crude solid was
filtered and recrystallized from ethanol to afford a pure yellow
product. Yield: 90%. 1H NMR (400 MHz, CDCl3): d 7.16 (d, 1H,
J = 8.8 Hz, ArH), 8.02 (d, 2H, J = 8.9 Hz, ArH), 8.11 and 8.14
(dd, 1H, J = 2.3 Hz, J = 8.8 Hz, ArH), 8.34 (d, 1H, J = 2.3 Hz, ArH),
8.37 (d, 2H, J = 8.9 Hz, ArH), 10.30 (s, 1H, CHO).
2.4. 1H NMR experiments
1H NMR titrations were performed on a Varian 400 MHz spec-
trometer at 298 K. The solution of the receptors
1 and 2
(0.0255 M in d6-DMSO) was titrated by adding known quantities
of concentrated solution of tetrabutylammonium fluoride and ace-
tate (0.02 M). The chemical shift changes of the receptors 1 and 2
were monitored. All titrations were repeated at least twice to get
the consistent values.
3. Result and discussion
2.2.2. Synthesis of the receptors 1 and 2
3.1. Synthesis of novel receptors
To a solution of S1 (0.2 g, 0.74 mmol) in EtOH (20 mL) was
added a solution of 4-phenylsemicarbazide or 4-phenylthiosemic-
arbazide (0.74 mmol) and a catalytic amount of p-toluenesul-
phonic acid in dry EtOH (10 mL). The mixture was refluxed for
24 h under nitrogen. The product, which was precipitated during
stirring, was filtered off, washed with ethanol and dried in vacuo.
As can be seen in Scheme 1, the receptors 1 and 2 were obtained
in 85% and 86% yields, respectively by reacting 5-(4-nitro-pheny-
lazo)-salicylaldehyde S1 with 4-phenylsemicarbazide or 4-phenyl-
thiosemicarbazide in dry EtOH. Their molecular structures and
purities were established from spectroscopic studies including 1H
Scheme 1. Synthesis of the receptors 1 and 2; Reagents and conditions: (i) Na2CO3, NaNO2/HCl, H2O, 0–5 °C; 90%; (ii) 4-phenylsemicarbazide or 4-phenylthiosemicarbazide,
EtOH.

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A. Okudan et al. / Journal of Molecular Structure 1048 (2013) 392–398
NMR, 13C NMR, APT, COSY NMR, elemental analysis, and FT–IR
analysis (Supplementary data). The formation of receptors 1 and
2 was confirmed by the disappearance of aldehyde protons
(CHO) belong to S1 at d 10.30, and the appearance of Schiff base
(CH@N), urea and thiourea protons (NH) at d 8.66, d 9.01 and d
10.74 ppm for 1 and d 8.81, d 10.22 and d 11.86 ppm for 2 in 1H
NMR spectra, respectively. The synthesis of receptors 1 and 2
was also confirmed by the disappearance of the characteristic alde-
hyde carbonyl band belong to S1 at about 1656 cmÀ1 and by the
appearance of the urea carbonyl bands at about 1690 cmÀ1 for 1
and 1631 cmÀ1 for 2 (Supplementary data).
color remained unchanged after the addition of BrÀ, ClÀ, IÀ, ClO4À,
NOÀ, HSOÀ, and H2POÀ. This is probably due to high negative
3
4
4
charge density on FÀ and AcOÀ which bring about the strong
hydrogen bonding with NH and OH in 1 and 2. The formation of
these hydrogen bonds affects the electronic properties of the chro-
mophore, concluding a color change with a subsequent new intral-
igand or internal charge transfer (ICT) band including between the
FÀ or AcOÀ -bound hydroxyl group and electron deficient azo moi-
ety [22].
3.3. UV studies
3.2. Naked eye detection
The anion binding affinities of receptors 1 and 2 were investi-
gated using UV–vis spectroscopy in CHCl3 solution at 4.0 Â 10À5
concentration. The receptors 1 and 2 were titrated by successive
increment of number of equivalents for acetate and fluoride ion
separately and monitored by UV–vis absorption spectra. The titra-
tions were carried out with all anion and UV–vis spectral changes
are depicted in Figs. 3 and 4. The UV–vis absorption spectra of the
receptors 1 and 2 in CHCl3 are dominated by strong absorption
bands at 325 and 390 nm for 1 and 348 nm for 2. Upon the addition
of FÀ to 1 and 2, prominent changes was observed in UV–vis
absorption spectra due to complexation between host–guest
The colorimetric sensitivity of 1 and 2 (4.0 Â 10À5 M)Àtowards
various anions such as FÀ, BrÀ, ClÀ, IÀ, AcOÀ, ClOÀ4 , NO3 , HSOÀ4 ,
and H2POÀ in their tetrabutylammonium form, were monitored
4
visually concomitantly. As depicted in Figs. 1 and 2, the color of
the receptor 1 solution changed from yellow to blue and pale pur-
ple upon addition of FÀ and AcOÀ anions (5 equiv.), respectively,
which could be easily observed by the naked eye. Under similar
conditions, the color of the receptor 2 changed from yellow to pur-
ple upon addition of FÀ and AcOÀ anions. At the same time, their
Fig. 1. Colorimetric response upon addition of 5.0 equiv. of TBA salt of anions into CHCl3 solutions of 1.
Fig. 2. Colorimetric response upon addition of 5.0 equiv. of TBA salt of anions into CHCl3 solutions of 2.
Fig. 3. UV–vis titration spectra for receptors 1 (a) and 2 (b) (4.0 Â 10À5 M) at room temperature on increasing the concentration of tetrabutylammonium fluoride in CHCl3.

A. Okudan et al. / Journal of Molecular Structure 1048 (2013) 392–398
395
Fig. 4. UV–vis titration spectra for receptors 1 (a) and 2 (b) (4.0 Â 10À5 M) at room temperature on increasing the concentration of tetrabutylammonium acetate in CHCl3.
Fig. 5. UV–vis absorption spectra of 1 (4.0 Â 10À5 M) in CHCl3 after the addition of 5 equiv of each of the different guest anions.
molecules. The ICT bands at 325 nm for 1 and 348 nm for 2 as
shown in Fig. 3, disappear gradually with the formation of new
peaks centered at 577 and 555 nm (bathochromic shift). A clear
isosbestic points were observed at 295 and 410 for 1 and 440 nm
for 2. On the other hand, it can be seen in Fig. 4 that as the number
of equivalents of AcOÀ increases, the absorption bands, appeared at
325 and 348 nm wavelengths gradually reduced and new bands
appeared at 555 and 551 nm. On addition of other anionic species
as their tetrabutylammonium salts no significant modulation in
the UV–vis absorption spectra was observed thus, showing the
specificity of the chemosensor for selective binding interaction
with AcOÀ and FÀ anions (Fig. 5). The data showed that receptors
Fig. 6. The Job plots of receptors 1 (a) and 2 (b) with tetrabutylammonium fluoride using UV–vis.

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A. Okudan et al. / Journal of Molecular Structure 1048 (2013) 392–398
AcOÀ anions were calculated in CHCl3 based on the UV–vis titra-
tion experiments by Benesi–Hildebrand plots [23] and the results
were presented in Table 1. Benesi–Hildebrand plots of 1 and 2
for FÀwere depicted in Fig. 7 (see also supplementary data). Also,
the detection limits of receptors 1 and 2 towards FÀ and AcOÀ
anions were calculated by using UV titration data and found to
Table 1
Association constants (Kass, MÀ1) of 1 and 2 with FÀ and AcOÀ anions in CHCl3.
Receptor
1
Anion
Kass (MÀ1
)
FÀ
1.27 ( 0.20) Â 104
AcOÀ
7.04 ( 0.35) Â 104
2
FÀ
1.08 ( 0.12) Â 104
6.13 ( 0.25) Â 104
be 1.05 Â 10À6
M
and 9.43 Â 10À6
M
for receptor
1
and
AcOÀ
1.37 Â 10À5 M and 1.35 Â 10À6 M for receptor 2, respectively.
3.4. 1H NMR titration
1 and 2 have higher selectivity for fluoride and acetate than other
anions. In addition, the fluorescence behavior of the receptors to
anions was examined, but any fluorometric change could not be
observed.
On the basis of spectrophotometric experiments it is difficult to
predict whether the noticeable changes include formation of
complex between host and guest through hydrogen bonding or
deprotonation of host by sufficient basic strength of guest
molecules. To affirm the real phenomenon we performed 1H
NMR titration experiments with receptors 1 and 2 in DMSO-d6
(c = 2.55 Â 10À2 M) by stepwise addition of equivalents of FÀ and
AcOÀ as their tetrabutylmmonium salts and displayed in Figs. 8
and 9. Before the addition of FÀ and AcOÀ, the 1H NMR chemical
shifts of the OH protons of receptors 1 and 2 were d 11.25 and d
11.19 ppm, respectively. The phenolic OH protons in 1 and 2 disap-
peared after the addition of 0.5 equiv of tetrabutylammonium fluo-
ride and acetate to the receptor solutions. On the other hand,
In order to determine the stoichiometric ratio between recep-
tors and anionic guests, the method of continuous variation (Job’s
plot) was used. The total concentration of 1 or 2 and anionic guest
was constant (1.0 Â 10À4 M) with continuous variation of mole
fraction of 1 and 2 ([host]/[host] + [guest]). Fig. 6 shows the Job’s
plots of 1 and 2 with FÀ. The receptors 1 and 2–anions complex
concentration approaches a maximum when the mole fraction of
1 and 2 is 0.5, which means 1 or 2 and anions form 1:1 complexes.
We found similar stoichiometric ratio also in case of 1 and 2 with
AcOÀ (Supplementary data). On the basis of 1:1 stoichiometry, the
corresponding binding constants (Kass) of 1 and 2 for FÀ and
Fig. 7. Benesi–Hildebrand plots from UV–vis titration data of receptor 1 (a) and 2 (b) (4.0 Â 10À5 M) with FÀ.
Fig. 8. Changes in 1H NMR titration spectra for receptor 1 (2.55 Â 10À2 M) in DMSO-d6 solution with gradual addition of equiv. of tetrabutylammonium fluoride and acetate.

A. Okudan et al. / Journal of Molecular Structure 1048 (2013) 392–398
397
Fig. 9. Changes in 1H NMR titration spectra for receptor 2 (2.55 Â 10À2 M) in DMSO-d6 solution with gradual addition of equiv. of tetrabutylammonium fluoride and acetate.
additions of FÀ and AcOÀ (2.0 equiv.) upon the receptor 1 and 2 in-
duce shifts upfield of the Schiff base protons from 8.66 and
8.81 ppm to 8.34 and 8.43 ppm for FÀ and to 8.32 and 8.38 ppm
for AcOÀ, respectively. At the same time, while the some signals
of the aromatic protons in receptors 1 and 2 after the addition of
tetrabutylammonium fluoride and acetate shifted upfield, the
other signals shifted downfield. The deprotonation of the phenolic
OH subunits in receptors 1 and 2 can induce two distinct effects on
the aromatic substituents: (i) it increases the electron density on
the phenyl rings with through bond propagation which generates
a shielding effect, and should produce an upfield shift of the CAH
protons; (ii) it induces polarization of the CAH bonds via a
through-space effect, where the partial positive charge causes a
deshielding effect and produces a downfield shift. On the other
hands, the characteristic urea (NH) proton signals in receptors 1
appeared at d 9.01 and d 10.74 (Fig. 8) and the thiourea (NH) sig-
nals in receptor 2 appeared at d 10.22 and d 11.86 (Fig. 9). On addi-
tion of 0.5–2.0 equiv. of FÀ and AcOÀ to a solution of 1, the signal of
urea NH at d 10.74 shifted towards upfield with broadening in sig-
nal (Fig. 8). On addition of 0.5–2.0 equiv. of FÀ and AcOÀ to a solu-
tion of 2, both singlet signals for NH protons shifted towards
upfield with broadening and weakening in signals and also ob-
served new signals at ꢁ9.8 and ꢁ8.9 ppm (Fig. 9). We think that
the new signals at ꢁ9.8 and ꢁ8.9 ppm belong to four identifiable
NH resonances, which integrate for two protons. These resonances
were clearly indicative of the formation of new species, which was
structurally different from that seen for receptor 2 and the forma-
tion of a host–guest hydrogen-bonding complex and an overall
change of the electron distribution [24,25].
The possible complexes with the anion are depicted in Fig. 10.
The binding between host and guest may be attributed to possible
double hydrogen bonding of receptors with anions causing strong
binding due to high basicity of FÀ and AcOÀ.
4. Conclusion
In summary, we have synthesized two novel colorimetric anion
sensors which selectively recoÀgnize fluoride and acetate over other
anions (ClÀ, BrÀ, IÀ, NO3À, ClO4 , HSO4À, and H2PO4À) in chloroform.
More importantly, these sensors show naked-eye detection for
fluoride and acetate anions at room temperature. The nature of
the anion receptor interaction has been defined by UV–vis and
NMR spectroscopy, and we concluded that in the case of 1 and 2,
hydrogen bonding interactions are most likely involved in the rec-
ognition process.
Acknowledgements
We thank the Research Foundation of Selcuk University (BAP)
for financial support of this work.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molstruc.2013.04.
077.
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