A novel fluorescent and colorimetric anion sensor based on thiourea derivative in competitive media

Article abstract of DOI:10.1016/j.saa.2007.10.047

A novel and simple fluorescent receptor bearing thiourea moiety as recognition site was described. The recognition behavior of the receptor toward different anions was investigated in DMSO/H₂O (95:5 v/v) and dry DMSO through two various channels: the colorless-yellow color change and a remarkable enhancement of the fluorescence. And the enhancement of the fluorescence was attributed to an anion-induced increase of the rigidity of the host molecule.

Full text of DOI:10.1016/j.saa.2007.10.047

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 70 (2008) 1217–1221
A novel fluorescent and colorimetric anion sensor based on
thiourea derivative in competitive media
Jie Shaoa, Ming Yua, Hai Linb, Huakuan Lina,∗
a Department of Chemistry, Nankai University, Tianjin 300071, China
b Key Laboratory of Functional Polymer Materials of Ministry of Education, Nankai University, Tianjin 300071, PR China
Received 21 September 2007; received in revised form 24 October 2007; accepted 31 October 2007
Abstract
A novel and simple fluorescent receptor bearing thiourea moiety as recognition site was described. The recognition behavior of the receptor
toward different anions was investigated in DMSO/H2O (95:5 v/v) and dry DMSO through two various channels: the colorless-yellow color change
and a remarkable enhancement of the fluorescence. And the enhancement of the fluorescence was attributed to an anion-induced increase of the
rigidity of the host molecule.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Colorimetric; Competitive media; Fluorescence; Enhancement
1. Introduction
for the selective recognition of anions accompanied with color
changes even in aqueous solutions [9].
The development of molecular sensors for anionic analyte,
especially for anions with biological interest, has always been
of particular importance and usually involves the design and
synthesis of molecules containing binding sites and a signal-
ing subunit able to display selective changes in color, UV–vis
absorption spectra and fluorescence emission intensity upon
guest binding [1–3]. Anions play a crucial role in biological
progresses, medicine, catalysis, and molecular assembly [4]. For
example, fluoride anion is associated with dental care and treat-
ment of osteoporoses [5]; the carboxylate anions exhibit specific
biochemical behaviors in the enzymes and antibodies and are
also critical components of numerous metabolic processes [6]
and iodide plays an important role in several biological activi-
ties such as neurological activity and thyroid function [7]. So, a
number of papers on the design and synthesis of fluorescent sen-
sors for the detection of anions, which offer distinct advantages
in the terms of sensitivity and response time, have been reported
in the past years [8]. However, most of these systems exhibit
fluorescence quenching but not an increase in the fluorescence
when bond to anions and meanwhile suffer from applicability
in the aqueous solutions [3]. Accordingly, progress in this area
would require novel strategies and simple molecular systems
The design of fluorescent chemosensors is mainly based on
photoinduced electron/energy transfer (PET) [10], intramolecu-
lar charge transfer (ICT) [11], excimer/exciplex formation [12],
fluorescence resonance energy transfer (FRET) [13], an increase
of the rigidity of the host molecules [14] and so on. In gen-
eral, the fluorescence-based sensors would induce a tremendous
enhancement of the fluorescence, making use of an increase of
the rigidity of the receptor as a signaling mechanism. Neverthe-
less, it should be noted that the fluorescent probes/chemosensors
with an increase of the rigidity of the receptor as a signaling
mechanism are quite limited, although it has been documented
previously [14]. With the objective of showing such a signal-
ing mechanism, we synthesized a novel neutral anion sensor
bearing thiourea [15,16] and –OH [17] functional groups for
colorimetric sensing of anions in competitive aqueous media.
The results from this investigation proved that there were sig-
nificant increases in the intensity of fluorescence when receptor
1 was bond to anions.
2. Experimental
2.1. Reagents and apparatus
In the titration experiments, all the anions were added in
the form of tetra-n-butylammonium (TBA) salts, which were
purchased from Sigma–Aldrich Chemical, stored in a vacuum
∗
Corresponding author. Tel.: +86 22 23502624; fax: +86 22 23502458.
E-mail address: hklin@nankai.edu.cn (H. Lin).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.10.047

1218
J. Shao et al. / Spectrochimica Acta Part A 70 (2008) 1217–1221
Scheme 1. The synthetic procedure for the anion receptor 1.
desiccator containing self-indicating silica and fully dried before
using. DMSO was dried with CaH2 and then distilled in reduced
pressure. Unless otherwise specified, all reagents for synthe-
sis were obtained commercially and were used without further
purification.
receptor 1 for anionic species were determined by non-linear
fitting analyses of the titration curves according to the equation
described by Connors [18].
Receptor 1 (1.0 × 10−2 M in DMSO-d6) was titrated against
AcO− (tetrabutylammonium salts) by incremental additions of
a concentrated solution of the anion in DMSO-d6 such that, at
the end of the titration, [1]:[AcO−] = 1:5.
1
The H NMR spectra were recorded on a Varian UNITY-
plus 400MHz spectrometer using tetramethylsilane (TMS) as an
internal standard. ESI-MS was performed with a Mariner appa-
ratus. C, H, N elemental analyses were made on an elementar
vario EL. UV–vis spectra and Fluorescent spectra were recorded
on a Shimadzu UV-2450 PC spectrophotometer and a Shimadzu
RF-5301PC Spectrophotometer, respectively, at 298.2 0.1 K.
3. Results and discussion
3.1. UV–vis spectral responses
The binding ability of the receptor 1 via hydrogen-bonding
interactions could be easily followed with the naked-eye of mon-
itoring the changes in the UV–vis absorption spectra upon the
addition of anions. Real-time color changes were observed upon
addition of F−, AcO−, or H2PO4− anions to DMSO/H2O (95:5
v/v) solutions of the host 1. On the contrary, addition of large
excesses of Cl−, Br−, or I− anions could not result in any
detectable color changes (see Fig. 1). These results observed
showed−that the receptor could strongly bind F−, AcO−, or
H2PO4 ions even in the protic solvent and had no binding
ability for Cl−, Br−, or I−. Next, the interaction of 1 with vari-
ous anions was evaluated through UV–vis titrations, by adding
a standard solution of the tetrabutylammonium salt of anions to
a dry DMSO solution of sensor at 298.2 0.1 K. Just as Fig. 2
shows, the sensor 1 was characterized by very strong absorption
band at 346 nm and a shoulder band at 360 nm. The peaks at
269 nm increased gradually while the peak at 346 nm decreased
gradually and a new broad band centered at 447 nm simultane-
ity appeared with increasing amounts of AcO− ions. This could
be explained based on the intramolecular electron-transfer inter-
actions [19] between the electron rich urea bond anion and the
electron deficient benzene ring. In addition, there were two well-
defined isosbestic points at approximately 320 nm and 360 nm,
which indicated that the stable complex having a certain stoi-
2.2. Synthesis
2.2.1. 2-Hydroxy-3-bromo-phenyl-thiosemicarbazone
A solution of 3-bromosalicylaldehyde (1 mmol, 0.201 g) in
methanol (20 ml) was added dropwise to a solution of thiosemi-
carbazide (1 mmol, 0.091 g) in methanol (60 ml) with stirring
at reflux (see Scheme 1). After being stirred for 12 h, the
solvent was removed by evaporation. The crude product was
recrystallized from DMF to give pale needle crystals. 1H NMR
(DMSO-d6) δH: 6.80(d, 1H), 7.31(d, 1H), 8.13(s, 2H), 8.20 (s,
1H), 8.35 (s, 1H), 10.20 (s, 1H), 11.39 (s, 1H), Anal. Calcd.
for C8H8N3O1S: C 35.05%, H 2.94%, N 15.33%, Found C
35.00%, H 3.00%, N 15.00%, MS–EI m/z 273.03 [(M + H)−,
Calcd. 272.96]
2.3. General methods
The binding ability of receptor 1 for anions was investi-
gated by UV–vis titrations and fluorescence titrations by adding
a standard solution of the tetrabutylammonium salt of anions
to DMSO:H2O (v/v 95:5) or dry DMSO solution of sensor
(5.0 × 10−5 M) at 298.2 0.1 K. Then affinity constants of
Fig. 1. Color changes observed for the receptor 1 in DMSO:H2O (95:5 v/v) upon the addition of anions as TBA salts at room temperature, [1] = 5.0 × 10−5 M,
[anion] = 5.0 × 10−5 M: (a) 1 only, (b) F−, (c) AcO−, (d) H2PO4−, (e) Cl−, (f) Br− and (g) I−.

J. Shao et al. / Spectrochimica Acta Part A 70 (2008) 1217–1221
1219
Table 1
Binding constants (Kass, M−1)a for receptor 1 with anionsb at 298.2 0.1 K.
100% DMSO
Absorption
DMSO:H2O (95:5)
Absorption
Emission
F−
5000
6667
952
ND
ND
ND
4900
6660
900
ND
ND
ND
NDc
2273
ND
ND
ND
AcO−
H2PO4
Cl−
−
Br−
I−
ND
a
All errors are 10%.
The anions were added as their tetrabutylammonium salts.
The binding constant cannot be determined by the spectra.
b
c
the receptor for AcO− is improved significantly in protic solvent
compared with aprotic solvent, which can be seen from Table 1.
3.2. Fluorescence titrations
Fig. 2. UV–vis spectra of 1 (5.0 × 10−5 M) in presence of different concen-
trations of AcO− in DMSO at 298.2 0.1 K. Inset: Job’s plot for 1–AcO−
interactions.
To corroborate well with those obtained during the UV–vis
titrations described above, the fluorescence titration experiments
was carried out. The fluorescent spectral properties of the host
1 (c = 5 × 10−5 M) were determined in DMSO showing no fluo-
chiometric ratio between the receptor 1 and AcO− ion formed.
The job plot of 1 with AcO−, which was shown in the inset of
Fig. 2, also indicated that the 1:1 stoichiometry host–guest com-
plex with an association constant Kass = 6667 M−1 is formed.
To examine the possibility of application of the sensor 1
to analytical chemistry, the interaction of 1 with anions tested
was investigated by spectrophotometric titrations in aqueous
solutions (DMSO:H2O = 95:5 v/v). Obviously shown in Fig. 3,
addition of AcO− resulted in similar changes in the UV–vis
spectrum with those changes shown in Fig. 2. And the bind-
ing constant (Kass) was calculated to be 2273 M−1, which was
shown in Table 1, based on the UV–vis titration data. However,
the addition of the other anions merely induced negligible spec-
tral changes during the titrations and the credible association
constants could not be obtained. Interestingly, the selectivity of
−
rescence. However, upon the addition of F−, AcO− and H2PO4
to DMSO solutions of 1, the fluorescence emission intensity
of the sensor enhanced (fluorescence-turn-on) drastically at
about 500 nm (λexc = 392 nm). This observation was illustrated
in Fig. 4, which showed the fluorescence spectra of the recep-
tor 1 in the absence and presence of various concentrations of
AcO−. Fluorescence spectral profiles and the intensities of 1
were found to be unsusceptible to the addition of other anions
such as Cl−, Br−, or I− ions (see Fig. 5). As expected, the analyt-
ical results of the fluorescence titration were very consistent with
those of the UV–vis titrations, which can be seen from Table 1.
As demonstrated in Scheme 2, the conformational restriction
of the sensor 1 was induced upon interacting with acetate ion
through hydrogen-bonding. Therefore, the astonishingly strong
enhancement of the fluorescence intensities observed could be
Fig. 3. UV–vis spectra of 1 (5.0 × 10−5 M) in presence of different concentra-
Fig. 4. Fluorescence spectra changes of 1 (5.0 × 10−5 M) upon addition of
AcO− in DMSO (λexc = 392 nm) at 298.2 0.1 K.
tions of AcO− in DMSO:H2O (95:5 v/v) solutions at 298.2 0.1 K.

1220
J. Shao et al. / Spectrochimica Acta Part A 70 (2008) 1217–1221
Fig. 5. Fluorescence spectra changes of 1 (5.0 × 10−5 M) upon addition of
10 equiv. different anions. (λexc = 392 nm) at 298.2 0.1 K.
rationalized on the result of an increase of the rigidity of the
receptor molecules upon anion complexation [14].
The trends of binding ability of the sensor 1 with anions tested
were AcO− > F− > H2PO4− > Cl− ∼ Br− ∼ I−, which could be
obviously observed from Table 1. The selectivity for special ana-
lyte of the host molecule could be rationalized on the basis of not
only the guest basicity but also shape complementarity between
the host and the anionic guests [20]. Consequently, the acetate
anion with an O–C–O angle of ca. 120◦ was a plane and triangu-
lar and might be the fittest for the two hydrogen atoms on binding
sites of the receptor among the anions tested and form mul-
titopic hydrogen-bonding interactions with 1 (see Scheme 2),
whichwouldbefurtherverifiedfromthe1HNMRtitrations. And
therefore acetate ion could be selectively differentiated from the
anions studied.
Fig. 6. Plot of 1H NMR spectra of the receptor 1 (1.0 × 10−2 M) on addition of
tetrabutylammonium acetate in DMSO-d6 at 298.2 0.1 K.
upon hydrogen-bond formation, would be considered [21]: (1)
through-bond effects, which increase the electron density of the
phenyl ring and promote an upfield shifts, and (2) through-space
effects, which polarize C–H bond in proximity to hydrogen
bond, create the partial positive charge on the proton and cause
a downfield shifts. Upon addition of 0.5 equiv. of acetate ions,
the peaks at 10.201 and 11.390 ppm, which were assigned to
–NH␤ and–OH␣, respectively, shifteddownfield, broadenedand
finally the signal for –OH disappeared, which might be ascribed
to the occurrence of deprotonation of –OH. This result indi-
cated that there was decrease in the electron density on –NH␤
and –OH␣ protons causing a deshielding effect, which were
involved in hydrogen-bonding interactions with anions. In addi-
3.3. 1H NMR titrations
To further shed light on the nature of the interactions between
1 and the anions, 1H NMR titrations were carried out in DMSO-
d6 (see Fig. 6). Before discussing the spectroscopic features in
detail, two effects, which were responsible for 1H NMR changes
Scheme 2. The proposed host–guest binding mode in solution.

J. Shao et al. / Spectrochimica Acta Part A 70 (2008) 1217–1221
1221
tion, owing to the through-bond effects, the signal of –NH2 (Hγ:
[2] V. Thiagarajan, P. Ramamurthy, Spectrochim. Acta
772–777.
[3] J. Shao, H. Lin, H.K. Lin, Spectrochim. Acta A 70 (2008) 682–685.
[4] J.-L. Fillaut, J. Andriès, J. Perruchon, J.-P. Desvergne, L. Toupet, L.
Fadel, B. Zouchoune, J.-Y. Saillard, Inorg. Chem. 46 (2007) 5922–
5932.
[5] K.L. Kirk, Biochemistry of the Halogens and Inorganic Halides, Plenum
Press, New York, 1991, p. 58.
[6] T. Gunnlaugsson, A.P. Davis, J.E. O’Brien, M. Glynn, Org. Lett. 4 (2002)
2449–2452.
A
67 (2007)
8.13 ppm) shifted upfield and stopped after addition of 1 equiv.
of acetate ion, indicating that –NH2 did not act as a binding site.
1
So, the results of H NMR titration also further corroborated
the above supposition of the interactions between the host and
acetate ion. Moreover, the strong H-bonds interactions between
the receptor 1 and AcO− might enhance ␲ delocalization, which
was expected to reduce the energy of the ␲–␲* transition and
therefore account for the appearance of the new absorbance band
at 447 nm and the generation of a yellow color. The proposed
binding mode in solution was demonstrated in Scheme 2.
[7] N. Singh, D.O. Jang, Org. Lett. 9 (2007) 1991–1994.
[8] R. Martinez-Manez, F. Sancenon, Chem. Rev. 103 (2003) 4419–4476 (and
references therein).
[9] T. Gunnlaugsson, P.E. Kruger, P. Jensen, J. Tierney, H.D.P. Ali, G.M.
Hussey, J Org. Chem. 70 (2005) 10875–10878.
[10] T. Gunnlaugsson, A.P. Davis, J.E. O’Brien, M. Glynn, Org. Lett. 4 (2002)
2452–2499.
[11] Zh.Ch. Xu, Y. Xiao, X.H. Qian, J.N. Cui, D.W. Cui, Org. Lett. 7 (2005)
889–892.
[12] J.S. Wu, J.H. Zhou, P.F. Wang, X.H. Zhang, S.K. Wu, Org. Lett. 7 (2005)
2133– 2136.
[13] M.H. Lee, D.T. Quang, H.S. Jung, J. Yoon, C.-H. Lee, J.S. Kim, J. Org.
Chem. 72 (2007) 4242–4245.
[14] D.H. Lee, J.H. Im, J.-H. Lee, J.I. Honga, Tetrahedron Lett. 43 (2002)
9637–9640.
[15] D.A. Jose, D.K. Kumar, B. Ganguly, B.A. Das, Org. Lett. 6 (2004)
3445–3448.
[16] D.A. Jose, D.K. Kumar, B. Ganguly, A. Das, Tetrahedron Lett. 46 (2005)
5343–5346.
4. Conclusion
In conclusion, a novel fluorescent and colorimetric anion
sensor, which allows so-called “naked-eye” detection in a
straightforwardandinexpensivemanner, offeringqualitativeand
quantitative information without using expensive equipment,
hasbeendeveloped. Andwhatismoreattractiveisthatthesensor
is based on anion-induced enhancement of intrinsic fluorescence
(turn-on), which offers the potential for high sensitivity, in aque-
ous solution. In addition, the sensor is expected to have many
opportunities in detection of acetate ion in real life owing to the
simplicity and sensitivity of the analysis.
[17] D.A. Jose, P. Kar, D. Koley, B. Ganguly, W. Thiel, H.N. Ghosh, A. Das,
Inorg. Chem. 46 (2007) 5576–5584.
Acknowledgment
[18] K.A. Connors, Binding Constants, 1st ed., John Wiley & Sons, New York,
1987.
[19] J. Shao, H. Lin, X.F. Shang, H.M. Chen, H.K. Lin, J. Incl. Phenom. Macro-
cycl. Chem. 59 (2007) 371–375.
This project was supported by the National Natural Science
Foundation of China (20371028, 20671052).
[20] I.V. Korendovych, M. Cho, P.L. Butler, R.J. Staples, E.V. Rybak-Akimova,
Org. Lett. 8 (2006) 3171–3174.
References
[21] M. Bonizzoni, L. Fabbrizzi, A. Taglietti, F. Tiengo, Eur. J. Org. Chem.
(2006) 3567–3574.
[1] V. Amendola, D. Esteban-Gomez, L. Fabbrizzi, M. Licchelli, Acc. Chem.
Res. 39 (2006) 343–353.