Synthesis and characterization of colorimetric metal sensing properties based on azo chromophore moiety

Article abstract of DOI:10.1080/15421406.2011.564505

In this work, we have designed and synthesized a particular colorimetric dye sensor for the detection of heavy metal ions. Dye sensor, namely 4-hydroxy-3-(fluoro-phenyl-thiourea-N'-nitrilo-methylidynyl)-azobenzene was synthesized using 5-phenyldiazenyl sa

Full text of DOI:10.1080/15421406.2011.564505

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Synthesis and Characterization of
Colorimetric Metal Sensing Properties
Based on Azo Chromophore Moiety
Young-A Son a , Young-Sung Kim a , Sung-Hoon Kim b & Do-Hyun Lee c
a Department of Advanced Organic Materials and Textile System
Engineering , Chungnam National University , Daejeon, S. Korea
b Department of Textile System Engineering , Kyungpook National
University , Daegu, S. Korea
c Korea Dyeing Research & Technology Center , S. Korea
Published online: 16 May 2011.
To cite this article: Young-A Son , Young-Sung Kim , Sung-Hoon Kim & Do-Hyun Lee (2011) Synthesis
and Characterization of Colorimetric Metal Sensing Properties Based on Azo Chromophore Moiety,
Molecular Crystals and Liquid Crystals, 538:1, 310-319, DOI: 10.1080/15421406.2011.564505
To link to this article: http://dx.doi.org/10.1080/15421406.2011.564505
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Mol. Cryst. Liq. Cryst., Vol. 538: pp. 310–319, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2011.564505
Synthesis and Characterization of Colorimetric
Metal Sensing Properties Based on Azo
Chromophore Moiety
YOUNG-A SON,1 YOUNG-SUNG KIM,1
SUNG-HOON KIM,2 AND DO-HYUN LEE3
1Department of Advanced Organic Materials and Textile System
Engineering, Chungnam National University, Daejeon, S. Korea
2Department of Textile System Engineering, Kyungpook National
University, Daegu, S. Korea
3Korea Dyeing Research & Technology Center, S. Korea
In this work, we have designed and synthesized a particular colorimetric dye sensor
for the detection of heavy metal ions. Dye sensor, namely 4-hydroxy-3-(fluoro-
phenyl-thiourea-N’-nitrilo-methylidynyl)-azobenzene was synthesized using 5-
phenyldiazenyl salicylaldehyde and 4-phenylthiosemi carbazide. The dye sensor
has promising properties which are applicable to detect and recognize the presence
of heavy metal ions with colorimetric changing functions. Electron rich part in
dye sensor structure, which can be utilized as metal binding unit, such as NH, N,
S and OH (atom=group) provides the functions to coordinate metal ions. This metal
1
ion detection property was determined by using UV-Vis absorption, H-NMR peak
shift and computational calculation method analysis. Furthermore, we estimated
the electrochemical properties of dye sensor by using cyclicvoltammetry. In addition,
the formation type of metal binding complex was determined by Job’s plot
measurements.
Keywords Absorption; azo chromophore; colorimetric sensing; dye sensor;
heavy metal ion; HOMO=LUMO
1. Introduction
Recently, dye sensor researches have received much attention due to detection
properties of harmful materials such as heavy metal ions. Dye sensors for specific
chemicals are based on metal complex ligands such as crown ether, cryptand and
spherand in the structures. Many interesting works have been carried out for detec-
tion of heavy metal ions [1–4]. In the context of biological and environmental
aspects, heavy metal ions are considered to be highly dangerous in the human body
[5,6]. Thus, the design of effective dye sensors and their utilizations over heavy metal
Address correspondence to Young-A Son, Department of Advanced Organic Materials,
and Textile System Engineering, Chungnam National University, Daejeon, 305-764, S. Korea.
Tel.: þ82 42 821 6620; Fax: þ82 42 823 3736; E-mail: yason@cnu.ac.kr or Sung-Hoon Kim,
Department of Textile System Engineering, Kyungpook National University, Daegu, 702-701,
S. Korea. Tel.: þ82 53 950 5641; Fax: þ83 53 950 6617; E-mail: shokim@knu.ac.kr
310

Metal Sensing Azo Chromophore
311
ions are of great importance for chemical, biological, and environmental research
areas [7]. In this study, we have designed and prepared of new dye sensor and their
heavy metal ions detection properties were examined and determined. Structure of
1
the synthesized dye sensor was identified by H-NMR, Mass and EA (elemental
analysis). The metal ion sensing property of the dye sensor was determined by
UV-Vis absorption behaviors and metal binding position was also investigated by
computational calculation method, 1H-NMR peak shift [8–13]. Electron density dis-
tribution and 1H-NMR could provide the estimation of complex characteristic func-
tions between electron rich interacting positions and electrophile metal ions.
Furthermore, we also measured cyclicvoltammetry (CV) experiments to estimate
the energy potentials, namely HOMO and LUMO. We have used the oxidation
onset potential for calculation of HOMO energy levels and the UV absorption fit
curve calculation of band gap energy for determined the corresponding LUMO
energy levels. Finally, the complex formation types of metal detection were examined
by Job’s plot measurements [14–17].
2. Experimental
2.1. Synthesis
All the reagents and solvents, used for synthesis of azo chromophore dye sensor,
were purchased from Aldrich and used without further purification. As presented
in Scheme 1, precursor (a) synthesized by diazotization using NaNO2 (60 mmol,
4.2 g) in the presence of HCl (54 ml) and 4-fluoroaniline (60 mmol, 6.7 g), the sol-
ution heat to 70ꢀC. The solution was cooled to 0ꢁ5ꢀC and stirred for 30 min. Second
precursor (b) synthesized by salicylaldehyde (60 mmol, 6.4 ml) in 110 ml water con-
taining 2.4 g NaOH and Na2CO3 22.2 g the 5-Phenyldiazenyl salicylaldehyde dye
(b) in good yields. 5-phenyldiazenyl salicylaldehyde (2 mmol, 0.488 g) and 4-phe-
nylthiosemi carbazide (2 mmol, 0.334 g) were dissolved in 40 ml of ethanol. 3 drops
Scheme 1. Synthetic route for dye sensor.

312
Y.-A. Son et al.
of H2SO4 was added during the reaction. The reaction was continued for 7 h at room
temperature. The reaction product was filtered and dried in the vacuum.
1H-NMR (400MHZ, DMSO-d6) d 11.85 (s, 1H), 10.93 (s, 1H), 10.22 (s, 1H), 8.72
(s, 1H), 8.54 (s, 1H), 7.88 (m, 2H), 7.78 (m, 1H), 7.52 (m, 2H), 7.38 (m, 4H), 7.18 (m,
1H), 7.04 (m, 1H). 13C-NMR (DMSO-d6) 176.0, 164.5, 162.1, 159.6, 148.8, 145.3,
139.2, 138.9, 128.0, 126.2, 125.4, 125.2, 124.5, 124.4, 122.5, 116.9, 116.4, 116.2,
40.1, 39.9, 39.7, 39.5, 39.2, 39.0, 38.8. MS: 393 (Mþ). Anal. Calculated for
C20H16FN5OS: C, 61.06; H, 4.10; N, 17.80; S, 8.15: found; C, 63.28; H, 4.27; N,
18.04; S, 8.49.
2.2. Measurements
The spectroscopic characteristics of the prepared dye sensor were examined using
Agilent 8453 UV-Vis spectrophotometer respectively. 1H NMR spectra and elemen-
tal analyses were recorded with JNM-AL400 spectrometer operated at 400 MHz
NMR and a Carlo Elba Model 1106 analyzer, respectively. Mass spectra were
recorded on a Shimadzu QP-1000. HOMO=LUMO and electron distributions were
calculated with Material Studio 4.3 [12,13]. Electrochemical properties were exam-
ined with Versa STAT3 using cyclicvoltammetry method. Cyclicvoltammetry
measurement carried out in an acetonitrile solution containing tetrabutlyammonium
hexafluorophosphate electrolyte. The reference electrode, Ag=Agþ was directly
immersed in the reaction cell. The working electrode was a glassy carbon. The coun-
ter electrode was a platinum wire. The scan rate used 100 mV=s.
3. Results and Discussion
In this work, the dye sensor, 4-hydroxy-3-(fluoro-phenyl-thiourea-N’-nitrilo-
methylidynyl)-azobenzene was prepared. UV-Vis absorption spectra were checked
to determine the sensing functions of dye sensor toward various metal ions (Cd2þ
,
Hg2þ, Ni2þ, Zn2þ, Cu2þ, Fe3þ and Al3þ). The sensing investigations were conducted
to Cd2þ, Hg2þ, Ni2þ, Zn2þ, Cu2þ, Fe3þ and Al3þ for dye sensor and the correspond-
ing results were summarized in Figure 1. As shown in Figure 1, upon the addition of
divalent metal ions to the solution of the prepared dye sensor, the absorption band at
331 nm progressively decreased and a new peak at 390 nm appeared. The appearance
of this isosbestic point suggests that at least one stable dye sensor-divalent metal ion
moiety is present. However, divalent metal ions showed the different isosbestic point
at 357 nm and 361 nm. Namely, Cd2þ and Zn2þ were observed at 361 nm and Hg2þ
,
Ni2þ and Cu2þ indicate at 357 nm. We assumed that the absorption spectral changes
are belonging to the different metal binding positions in dye sensor structure. This
behavior causes significant increase in the charge density on the NH or S with asso-
ciated enhancement in the push-pull effect of the mixture of intermolecular charge
transfer (ICT).
The detection properties of dye sensor showed selectivity for metal ions
especially divalent metal ions (Cd2þ, Hg2þ, Ni2þ, Zn2þ and Cu2þ). Other metal ions
such as Fe3þ, Al3þ did not indicate the optical change. Dye sensor showed higher
selective functions to Cu2þ ion, which resulted in great absorption enhancement at
390 nm. The detection properties for all applied metal ions were summarized in
Figure 2.

Metal Sensing Azo Chromophore
313
Figure 1. UV-Vis absorption spectra in the presence of 1 ꢂ 10ꢃ5 M of metal ions (Cd2þ, Hg2þ
,
Ni2þ, Zn2þ, Cu2þ, Fe3þ and Al3þ).

314
Y.-A. Son et al.
Figure 2. Comparison of absorption ratio for various metal ions (A and A0 are the absorbance
in the presence and the absence of metal ions, respectively at 390 nm).
Further to experimental determination of metal binding position, we estimated
1
this binding property using the computational calculation and H-NMR peak shift.
Through computational calculation method, we calculated and determined the opti-
mum molecular geometrical structure. In the Figure 3 showed that the optimization
of molecular geometries structure and molecular orbital of dye sensor were
Figure 3. Geometry optimization: optimum structure and electron density distribution.

Metal Sensing Azo Chromophore
315
computationally optimized with various possible structures. Compared with
HOMO=LUMO energy levels and energy gap, we selected the three structures. These
three structures showed the different geometrical conformation. The resulting com-
putational calculation indicated that the energy levels were HOMO=LUMO (ꢃ
5.110=ꢃ3.427 eV), DE (1.683 eV) of structure (I), HOMO=LUMO (ꢃ5.140=ꢃ
3.115 eV), DE (2.025 eV) of (II) and HOMO=LUMO (ꢃ5.362=ꢃ3.365 eV), DE
(1.997 eV) of (III), respectively. The structure (I) indicates the lowest HOMO energy
value and energy gap. In this reason, we proposed that the structure (I) is the most
stabilized structure. In addition metal binding reactions could be made in electron
rich HOMO state of dye sensor. Figure 3 clearly showed electron density of dye sen-
sor that the electron density in HOMO was mainly localized in sulfur and -NH,
where the possibility of metal binding reactions cab be expected.
To determine metal binding site using metal-ligand effects, we estimated the
1
interaction of ligand with divalent metal ions (Zn2þ and Hg2þ) by H-NMR shift
behavior. Metal binding effects were observed in NMR peak shift of dye sensor in
the presence of divalent metal ions. When the metal binding reaction occurred, these
corresponding metal ions give an influence to the electron density distribution of
1
dye sensor. Consequently, H-NMR peaks were shifted due to different electron
distribution.
It was found (Fig. 4) that the NH and OH proton signal on dye sensor
(11.86 ppm, 10.22 ppm) disappeared with increasing the concentration of Hg2þ
ion. And the upfield shift of the proton a in dye sensor was observed, which can
be attributed to the increasing electron density. This behavior showed that Hg2þ
ion could make efficient deprotonation of –NH and –OH of dye sensor. In contrast,
Figure 4. 1H NMR changes of dye sensor in DMSO-d6 (1) dye sensor, (2) and (3) in the pres-
ence of (2) Zn2þ and (3) Hg2þ
.

316
Y.-A. Son et al.
Figure 5. Cyclicvoltammogram of dye sensor in acetonitrile.
in the case of Zn2þ, the metal complexed dye sensor did not indicate the NMR peak
shift. This behavior showed that Zn2þ metal biding position was not related to the
deprotonation of –NH and –OH. If Zn2þ metal biding position was –NH or
–OH, and then 1H-NMR must show the peak shift. However, Zn2þ did not indicate
any peak shift. Thus, we proposed that the metal binding positions of dye sensor
were S and –NH. This behavior was well agreement with our expectation. Formally,
we described the optimum structure of dye sensor with computational calculation.
The optimum structure (I) showed that the sulfur atom formation was not influenced
by other hydrogen. If optimum structure was (II) or (III), the peak shift can be mon-
itored. In this reason, it can be proposed that this dye sensor had two metal binding
positions as S and –NH.
Cyclicvoltammetric measurements of dye sensor were carried out in a conven-
tional three-electrode system. The oxidation and reduction potential values were also
used to estimate the HOMO=LUMO energy levels. Figure 5 showed the oxidation
potential peak of dye sensor.
From the oxidation potential and UV fit curve, HOMO and LUMO energy
levels of dye sensors were determined. HOMO and LUMO energy levels were calcu-
lated using the formula [18],
HOMOðorLUMOÞðeVÞ ¼ ꢃ4:8 ꢃ ðEpeak=onsetpotential ꢃ E1=2ðFerroceneÞÞ
Table 1. HOMO=LUMO values and energy gap of dye sensor
Optimization structure
Cyclicvoltammetry
I
II
III
and UV fit curve
HOMO
LUMO
DE
ꢃ5.110
ꢃ3.427
1.683
ꢃ5.140
ꢃ3.115
2.025
ꢃ5.362
ꢃ3.365
1.997
ꢃ5.114
ꢃ2.189
2.955

Metal Sensing Azo Chromophore
317
Figure 6. Plot of Job’s method for dye sensor.
Where, Epeak=onset potential are the maximum and minimum peak potential values and
E1=2(Ferrocene) is half-wave potential of Ferrocene (0.42 V).
The comparison of computational calculation and experimental determination
showed the similar values of HOMO=LUMO and DE (Table 1). Thus, the computa-
tional calculation results can be used as good supporting for the determination of
metal binding reaction and position.
Finally, we determined the composition of the metal binding reaction using the
Job’s method [19]. For Job’s plot measurements, 1 ꢂ 10ꢃ5 M solutions of dye sensor
and different mole concentrations of metal ions were prepared in various molar
Figure 7. Stoichiometric binding images between dye sensor and divalent metal ion.

318
Y.-A. Son et al.
ratios of dye and metal ions (1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, 9:1, 10:0). The
relationship for maximum absorption peak versus mole fraction of the metal ions
is summarized in Figure 6. The finding exhibited that the molecular fraction was
close to 0.6, which indicated 1:2 complex composition between metal ion and dye
1
sensor. Electron density distribution, HOMO=LUMO energy level and H-MNR
result were considered to estimate the metal binding positions in dye sensor molecule
(Fig. 7). In Figure 7, we propose stoichiometric binding images with two metal bind-
ing positions between dye sensor and divalent metal ions.
4. Conclusions
In this study, we investigated the effects of dye molecular chemosensing behaviors
and metal binding position. It showed the metal detections properties of divalent
metal ions. Especially, the dye sensor showed the divalent metal ion detection
(Cd2þ, Hg2þ, Ni2þ, Zn2þ and Cu2þ). Other metal ions such as Fe3þ and Al3þ did
not indicate the optical change. Especially, dye sensor showed higher selective func-
tions to Cu2þ ion, which resulted in great absorption increase at 390 nm. The struc-
ture (I) showed the lowest HOMO energy value and energy gap. It is proposed that
the structure (I) is the most stabilized structure where dye sensor-metal ion bindings
are expected. The comparison of computational calculation and experimental deter-
mination showed the similar values of HOMO=LUMO and DE (Table 1). Further-
more, dye sensor-metal ion bindings were formed by M:L (1:2) stoichiometry
complex type.
Acknowledgment
This research was supported by a grant from the Fundamental R&D Program for
Core Technology of Materials funded by the Ministry of Knowledge Economy,
Republic of Korea. This work was supported by the Basic Science Research Program
through the National Research Foundation (NRF) grant funded by the Korea
government (MEST) (No. 2010–0001885).
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