A highly selective colorimetric sensor to Fe3+ and Co 2+ in aqueous solutions

Article abstract of DOI:10.1007/s11164-012-0793-5

Five aromatic azo dyes with hydroxyl groups (1-5) were designed and synthesized by coupling reactions. The relationships between structures of the compounds and the spectroscopic properties were investigated. The absorption spectra of these compounds upon titration with K⁺, Ca²⁺, Al³⁺, Mg²⁺, Ni²⁺, Mn²⁺, Cd ²⁺, Cr³⁺, Fe³⁺, Cu²⁺, Zn ²⁺, Co²⁺, Hg²⁺, and Pb²⁺ ions in neutral aqueous solutions were reported. The results are coincident with the calculation results using the density functional theory method. The high selectivity, excellent water solubility and simple synthetic process make 1-[(2-Hydroxyl)phenylazo]-2-naphthol (5) a potential sensor for sensing Fe ³⁺ and Mn²⁺ with the naked eye. 1-[(2-hydroxyl)phenylazo]- 2-naphthol-6-sulfonic acid (3) shows high selectivity for the colorimetric detection of Fe³⁺ and Co²⁺ among the tested metal ions. The detection limitations of 3 for determining Co²⁺ and Fe ³⁺ were calculated to be 2.8 × 10^-7 and 5.6 × 10^-7 mol/L, respectively.

Full text of DOI:10.1007/s11164-012-0793-5

Res Chem Intermed (2013) 39:2723–2734
DOI 10.1007/s11164-012-0793-5
A highly selective colorimetric sensor to Fe3+ and Co2+
in aqueous solutions
•
Haiping Wang Juan Li Denghui Yao Qun Gao
•
•
•
•
Fengqi Guo Puhui Xie
Received: 26 July 2012 / Accepted: 27 August 2012 / Published online: 19 September 2012
Ó Springer Science+Business Media B.V. 2012
Abstract Five aromatic azo dyes with hydroxyl groups (1–5) were designed and
synthesized by coupling reactions. The relationships between structures of the com-
pounds and the spectroscopic properties were investigated. The absorption spectra of
these compounds upon titration with K?, Ca2?, Al3?, Mg2?, Ni2?, Mn2?, Cd2?
,
Cr3?, Fe3?, Cu2?, Zn2?, Co2?, Hg2?, and Pb2? ions in neutral aqueous solutions
were reported. The results are coincident with the calculation results using the density
functional theory method. The high selectivity, excellent water solubility and simple
synthetic process make 1-[(2-Hydroxyl)phenylazo]-2-naphthol (5) a potential sensor
for sensing Fe3? and Mn2? with the naked eye. 1-[(2-hydroxyl)phenylazo]-2-naph-
thol-6-sulfonic acid (3) shows high selectivity for the colorimetric detection of Fe3?
and Co2? among the tested metal ions. The detection limitations of 3 for determining
Co2? and Fe3? were calculated to be 2.8 9 10-7 and 5.6 9 10-7 mol/L, respectively.
Keywords Azo compounds Á Colorimetric sensing Á Metal ions Á Water-soluble Á
DFT calculations
Electronic supplementary material Theonlineversion of this article (doi:10.1007/s11164-012-0793-5)
contains supplementary material, which is available to authorized users.
H. Wang Á J. Li Á D. Yao Á Q. Gao Á F. Guo (&)
College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001,
Henan, People’s Republic of China
e-mail: fqguo@zzu.edu.cn
P. Xie (&)
College of Sciences, Henan Agricultural University, Zhengzhou 450002, Henan,
People’s Republic of China
e-mail: pxie2007@yahoo.com.cn
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H. Wang et al.
Introduction
Although some metal ions play important roles in various biological processes,
they can cause serious environmental pollution and healthy problems if high
concentrations are introduced into the soil, groundwater and the human body
[1–6]. Therefore, the design and synthesis of new sensors with high sensitivity
and selectivity for metal ions have attracted considerable attention [6–10].
Molecules with fluorophore and/or chromophore are the most frequently reported
sensors for metal ions based on the differences in their photophysical properties,
such as the absorption and fluorescence signals, before and after the interaction
with metal ions. Fluorescent sensors report the presence of metal ions via
fluorescence quenching, fluorescence enhancement or shift of the emission
maximum. But fluorescent sensors have some disadvantages such as the
interfering background fluorescence, the quenching from the competing metal
ions and even the incompatibility of the sensors with water [11–13]. In addition,
some fluorescent sensors need to be excited with ultraviolet light, which cannot
penetrate deeply into biological samples and can damage or trigger autofluores-
cence from such samples [14–16]. Colorimetric sensors infer the presence of
metal ions via change of absorbance at the absorption maxima. In theory, the
detection limitation of fluorescence sensors can be 106 times lower than that of
the colorimetric sensors because the fluorescence signal can be enhanced by
increasing the excitation energy. However, excellent colorimetric sensors can
recognize metal ions by using the cheap absorption spectrometers or even the
naked eye instead of expensive instruments. Work in this area is still of great
challenge and interest.
Aromatic azo dyes have been widely investigated as colorimetric sensors for
metal ions [17–21]. They are generally synthesized by a simple coupling reaction of
a diazonium salt and an aromatic compound. The absorption spectra of the aromatic
azo dyes are related to the two substituted aromatic rings conjugated with an azo
group. It is possible to design and synthesize colorimetric sensors with an absorption
maximum in the visible and near infrared regions, which is the requirement for
sensing in vivo. Most reported azo sensors use nitrogen- or sulfur-containing groups
as receptors of metal ions based on the interaction of metal ions with N or S atoms,
which possibly decrease the selectivity of the sensors due to the strong interaction
ability of N and S atoms with metal ions [22, 23]. In addition, the structurally
complex and hydrophobic properties of some aromatic azo sensors limit their
applications. It is still necessary to design and synthesize highly selective,
structurally simple and hydrophilic colorimetric sensors for metal ions. In
connection with our previous work on sensors for ions [15, 24, 25], five aromatic
azo dyes with hydroxyl groups (1–5, Fig. 1) were synthesized. The weak interaction
ability and hydrophilic property of the hydroxyl group make it possible to improve
the selectivity and solubility in aqueous solutions. And the colorimetric responses of
1–5 to K?, Ca2?, Al3?, Mg2?, Ni2?, Mn2?, Cd2?, Cr3?, Fe3?, Cu2?, Zn2?, Co2?
,
Hg2?, and Pb2?, which are frequently concerned with vital processes, in Tris–HCl
buffer solutions are investigated.
123

A highly selective colorimetric sensor to Fe3? and Co2? in aqueous solutions
2725
R3
N N
HO
OH
N N
HO
R2 = OH R3 = SO3H
R2 = H R3 = SO3H
R2 = OH R3 = H
R2
R1
1 R1 = OH
3
4
2 R1 = OCH3
5
Fig. 1 The structures of 1–5
Experimental
Materials
All chemicals are commercially available with AR grade and are used without
further purification. 6-Naphthenol-2-sulfonic acid was synthesized according to the
literature using naphthalene-2,6-disulfonic acid as a starting material [26, 27].
Syntheses
General procedures for preparation of 1–5 are as follows [28].
In a 100-mL beaker, 10.0 mmol compounds with an NH2 group were dissolved in
a mixed solvent of 10 mL deionized water and 4 mL concentrated hydrochloric
acid. Then, the solution was cooled 0 °C. A solution of 10.2 mmol sodium nitrite
dissolved in 2 mL deionized water was added dropwise into the above solution. The
temperature of the solution was controlled below 5 °C during the process.
The resulted solution was stirred for another 15 min after the addition was finished.
The excess sodium nitrite was decomposed by a proper amount of urea.
Next, 10.0 mmol of the coupling compound was dissolved in a mixed solvent of 10 mL
deionized water and 5 mL 10 % sodium hydroxide in a 200-mL beaker. The mixture was
cooled to 0 °C, and then the benzenediazonium solution of the above step was added
dropwise into the solution. The temperature of the system was controlled below 5 °C, and
the pH of the reaction solution was kept around 10 with 10 % sodium carbonate. After the
addition, the solution was stirred for another 1.5 h at room temperature. Then, the pH was
adjusted to 7 using 1 M hydrochloric acid and allowed to stand for 2 h. The red precipitate
was filtered out, washed with distilled water twice, and dried in vacuo. The crude product
was purified by column chromatography on silica gel, using methylene chloride as eluent.
1-[(2-Hydroxyl)phenylazo]-2,4-diphenol (1)
1
Yield: 85.4 %. H NMR (400 MHz, DMSO-d6, TMS): d = 12.65 (s, 1H), 11.39
(s, 1H), 10.46 (s, 1H), 7.73 (d, J = 8.0 Hz, 1H), 7.68 (d, J = 12.0, 1H), 7.30
123

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H. Wang et al.
(t, J = 8.0 Hz, 1H), 7.01 (d, J = 8.0 Hz, 1H), 6.96 (t, J = 8.0 Hz, 1H), 6.46
(d, J = 8.0 Hz, 1H), 6.36(d, J = 4.0 Hz, 1H) (Fig. S1, Supplementary material).
MS (EI): m/z = 229.0 (M-1) (100 %). IR (KBr, cm-1): 3,473, 1,629, 1,581, 1,425,
1,465 (N=N). Elem. Anal. C12H10N2O3: Calcd. C 62.60, H 4.38, N 12.17. Found: C
62.62, H 4.65, N 11.61.
1-[(2-Hydroxymethyl)phenylazo]-2,4-diphenol (2)
Yield: 80.5 %. 1H NMR (400 MHz, DMSO-d6, TMS): 14.10 (s, 1H), 10.63 (s, 1H),
7.72 (d, J = 8.0 Hz, 1H), 7.66 (d, J = 12.0 Hz, 1H), 7.45 (t, J = 8.0 Hz, 1H), 7.25
(d, J = 8.0 Hz, 1H), 7.06 (t, J = 8.0 Hz, 1H), 6.51 (d, J = 8.0 Hz, 1H), 6.29
(d, J = 2.0 Hz, 1H), 3.94 (s, 3H) (Fig. S2, Supplementary material). MS (EI):
m/z = 243.1 (M-1). IR (KBr, cm-1): 3,439, 1,627, 1,598, 1,502, 1,465. Elem.
Anal. C13H12N2O3: Calcd. C 63.93, H 4.95, N 11.47. Found: C 63.60, H 5.19, N
10.84.
1-[(2-Hydroxyl)phenylazo]-2-naphthol-6-sulfonic acid (3)
Yield: 65.5 %. 1H NMR (400 MHz, DMSO-d6, TMS): 16.44 (s, 1H), 10.75 (s, 1H),
8.52 (d, J = 12.0 Hz, 1H), 8.0–8.05 (m, 3H), 7.85 (d, J = 8.0 Hz, 1H), 7.24
(t, J = 8.0 Hz, 1H), 7.16 (m, 1H), 7.05 (t, J = 8.0 Hz, 1H), 6.90 (d, J = 12 Hz,
1H) (Fig. S3, Supplementary material). MS (EI) m/z = 343.5(M-1). IR
(KBr,cm-1): 3,416, 1,618, 1,546, 1,459, 1,466. Elem. Anal. C16H12N2O5S: Calcd.
C 55.81, H 3.51, N 8.14, S 9.31. Found: C 56.03, H 4.06, N 7.80, S 9.06.
1-Phenylazo-2-naphthol-6-sulfonic acid (4)
1
Yield: 85.7 %. H NMR (400 MHz, DMSO-d6, TMS): 15.76 (s, 1H), 8.50 (d, 1H,
J = 8.0 Hz), 8.03 (d, 2H, J = 8.0 Hz), 7.88 (d, J = 8.0 Hz, 2H), 7.81
(d, J = 8.0 Hz, 1H), 7.55 (d, J = 8.0 Hz, 2H), 7.39 (t, J = 8.0 Hz, 1H), 6.94
(d, J = 8.0 Hz, 1H) (Fig. S4, Supplementary material). MS (EI): m/z = 327
(M-1). IR (KBr, cm-1): 3,441, 1,620, 1,549, 1,499, 1,457. Elem. Anal.
C16H12N2O4S: Calcd. C 58.53, H 3.68, N 8.53, S 9.77. Found: C 57.99, H 4.01,
N 8.03, S 9.19.
1-[(2-Hydroxyl)phenylazo]-2-naphthol (5)
Yield: 83.6 %. 1H NMR (400 MHz, DMSO-d6, TMS): 16.41 (s, 1H), 10.66 (s, 1H),
8.53 (d, J = 8.0 Hz, 1H), 8.01 (d, J = 8.0 Hz, 1H), 7.93 (d, J = 8.0 Hz, 1H), 7.77
(d, J = 8.0 Hz, 1H), 7.61 (t, J = 8.0 Hz, 1H), 7.46 (t, J = 8.0 Hz, 1H), 7.21
(t, J = 8.0 Hz, 1H), 7.06 (d, J = 8.0 Hz, 1H), 7.02 (d, J = 8.0 Hz, 1H), 6.86
(d, J = 8.0 Hz, 1H) (Fig. S5, Supplementary material). MS (EI): m/z = 263.2
(M-1). IR (KBr, cm-1): 3,441, 1,617, 1,594, 1,491, 1,446. Elem. Anal.
C16H12N2O2: Calcd. C 72.72, H 4.58, N 10.60. Found: C 72.69, H 4.64, N 10.53.
123

A highly selective colorimetric sensor to Fe3? and Co2? in aqueous solutions
2727
General methods
UV–Vis absorption spectra were recorded using a TU-1901 Ultraviolet–visible
1
double-beam spectrometer. H NMR spectra were measured on a Bruker advance
DPX-400 MHz resonance spectrometer. IR spectra were obtained using a NEXUS-
470 Fourier infrared spectrometer. Electrospray ionization mass spectra were
measured on a LC/MSD–Trap–XCT system. Elemental analyses were carried on a
Flash EA 1112 elemental analyzer.
Procedures of metal ion sensing
The nitrate salts of metal ions K?, Ca2?, Al3?, Mg2?, Ni2?, Mn2?, Cd2?, Cr3?
,
Fe3?, Cu2?, Zn2?, Co2?, Hg2?, and Pb2? were used to evaluate the sensing
properties of 1–5 to metal ions in Tris–HCl buffer (0.05 M, pH 7.1). Stock solutions
of the metal ions were prepared in deionized water with the concentration of
1 mmol/L. The solutions of 1–5 were prepared in Tris–HCl buffer with concen-
trations around 10-5 mol/L for spectroscopic measurements. In the titration
experiments, 3 mL solution of 1–5 was added into the quartz optical cell with an
optical path length of 1 cm, respectively. The stock solution of each metal ion was
added into the cell step by step via a pipette. The absorption spectra were collected
after stirring for 2 min of each addition.
Results and discussion
Absorption spectra studies
The absorption spectra of 1–5 in Tris–HCl buffer, as shown in Fig. 2, are dominated
by a band in the visible region (400–700 nm) due to the p–p* transitions of the
compounds along the conjugated long axes and a band in the ultraviolet region
(200–300 nm) due to the absorption of the phenyl or naphthyl group in these
compounds, respectively. The absorption maxima of 3–5 in the visible region are
red-shifted compared to those of 1 and 2 due to their larger p conjugated systems.
Fig. 2 Absorption spectra of
1–5 in Tris–HCl buffer (0.05 M,
pH 7.1) solutions
50000
1
2
3
4
5
40000
30000
20000
10000
0
200
300
400
500
600
700
800
Wavelength /nm
123

2728
H. Wang et al.
The absorption maximum in the visible region of 1 (462 nm) is 12 nm red-shifted
compared to that of 2 (450 nm) due to the less steric effect of 1 than 2. The
absorption maximum in the visible region of 3 (502 nm) is 18 nm red-shifted
compared to 4 (484 nm) due to the donating effect of the hydroxyl group on the
benzene ring of 3. The absorption maximum in the visible region of 3 is 40 nm blue-
shifted compared to that of 5 (542 nm) because of the strong electron-withdrawing
effect of the sulfonic acid group on naphthyl ring. These substituents effects of the
aromatic azo dyes on the absorption spectra could greatly help us to design sensors
to metal ions with proper absorption maxima.
Metal cations selectivity studies
The absorption spectra of 1 (15 lmol/L) in Tris–HCl (pH 7.1) buffer solution upon
titration of Fe3? and Pb2? are illustrated in Fig. 3a, b, respectively. Upon addition
of Fe3?, the absorbance at 462 nm decreased greatly (Fig. 3a). Additions of Ni2?
,
Mn2?, Cd2?, and Hg2? to the solution of 1 decrease the absorbance at 462 nm in
different degrees without any red-shifting. Addition of Pb2? caused a simultaneous
decrease in the absorbance at 462 nm with red-shifting (Fig. 3b). Addition of Cu2?
,
Co2? and Zn2? to the solution of 1 resulted in the same absorption responses of 1 to
Pb2?, whereas additions of K?, Ca2?, Al3?, Mg2?, and Cr3? to the solution of 1
caused little change of the absorption spectrum of 1. The results show that 1 is not a
good sensor because the sensitivity was poor and no apparent color change occurred
upon addition of the tested metal ions.
The absorption properties of 3 upon titrations of the mentioned metal ions were
studied in order to determine the effect of the substitutes on the selectivity of
sensors. Figure 4a, b illustrates the effects upon addition of Fe3? and Co2? on the
absorption spectra of 3 (60 lmol/L). As shown in Fig. 4a, the absorption band at
502 nm of 3 was gradually decreased upon addition of Fe3? and finally disappeared.
At the same time, a new absorption band, accompanied by an isobestic absorption
point at 460 nm, at the blue side was gradually formed. As shown in Fig. 4b, the
0.6
0.4
0.2
0.0
0.6
0.5
0.4
0.3
0.2
0.1
0.0
a
b
200
300
400
500
600
700
800
200
300
400
500
600
700
800
Wavelength
Wavelength
Fig. 3 Absorption spectra of 1 (15 lmol/L) upon titration of Fe3? (0–30 lmol/L) (a) and Pb2? (b) in
Tris–HCl buffer solution
123

A highly selective colorimetric sensor to Fe3? and Co2? in aqueous solutions
2729
1.8
a
b
Co2+: 0 μmol/L
Co2+: 3.3 μmol/L
Co2+: 6.7 μmol/L
Co2+: 10 μmol/L
Co2+: 13 μmol/L
Co2+: 20 μmol/L
Co2+: 40 μmol/L
Co2+: 60 μmol/L
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Fe3+: 0 μmol/L
Fe3+: 4 μmol/L
Fe3+: 8 μmol/L
Fe3+: 12μmol/L
Fe3+: 16μmol/L
Fe3+: 20μmol/L
Fe3+: 24μmol/L
Fe3+: 32μmol/L
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
200
300
400
500
600
700
800
200
300
400
500
600
700
800
Wavelength
Wavelength
Fig. 4 Absorption spectra of 3 (60 lmol/L) upon titration of Fe3? (a) and Co2? (b) in Tris–HCl buffer
solution
absorption spectra of 3 upon titration of Co2? showed decreases at 502 nm and
270 nm; at the same time, the absorption band at 502 nm was split into two bands,
with the more intense transition shifting to longer wavelength. The red-shift
indicates that interaction of 3 and Co2? leads to a reduction of the HOMO–LUMO
gap [29]. The absorption isobestic points in the titration spectra indicate the
formation of the 1:1 complexes. The detection limitations of 3 for determining Co2?
and Fe3?, at which the value of A0/A equal to 1.1, were calculated to be 2.8 9 10-7
and 5.6 9 10-7 mol/L, respectively. The absorption spectra of 3 upon titration of
K?, Mg2?, Ca2?, Cr3?, Cd2?, Cu2?, Zn2?, Pb2?, and Hg2? changed little. The
results indicated that compound 3 shows high colorimetric selectivity to Fe3? and
Co2?, but almost no response to the other metal ions.
The absorption spectra of 2 and 4 did not change much upon the titration of even
7 equivalents of the tested metal ions, indicating weak or no interactions between 2
or 4 and the tested metal ions. The absorption spectra of 5 (30 lmol/L) in Tris–HCl
(pH 7.1) buffer solutions upon titration of metal ions afforded much different
behaviors of 1. The absorbance at 542 nm decreased and finally disappeared upon
addition of Fe3? to the solution of 5, while at the same time a new absorption peak
at the blue side was gradually formed (Fig. 5a) and four absorption isobestic points
were presented at 221, 253, 480, and 650 nm, respectively. The color of the solution
changed from orange to light yellow as shown in Fig. 6. The absorption spectra of 5
upon titration of Cu2? caused a blue-shift and an accompanying decrease of the
absorbance at 542 nm (Fig. 5b). At the same time, the color of the solution changed
from orange to purplish red. The blue-shift indicates that the interaction of 5 with
Cu2? increases the HOMO–LUMO gap. The absorption spectra of 5 upon titration
of Co2? showed a decrease in the absorbance of the visible band and an increase in
the absorbance around 270 nm, while at the same time the visible absorption band
was clearly split into two bands (Fig. 5c). The reason is probably that the interaction
between Co2? and the sensor weakens the conjugated properties of the molecule and
leads to a new blue-shift band. Similar phenomena have been reported [29, 30].
Mn2? induced a great decrease of the absorption band of 5 (Fig. 5d) with obvious
color change. However, the addition of K?, Mg2?, Ca2?, Ni2?, Al3?, Zn2?, Pb2?
,
Cd2?, Cr3?, and Hg2? induced only a little change of the absorption spectrum of 5
123

2730
H. Wang et al.
a
b
1.6
1.2
0.8
0.4
0.0
2.0
1.6
1.2
0.8
0.4
0.0
Cu2+: 0 μmol/L
Cu2+: 10 μmol/L
Cu2+: 20 μmol/L
Cu2+: 30 μmol/L
Cu2+: 60 μmol/L
Fe3+: 0 μmol/L
Fe3+: 10μmol/L
Fe3+: 20μmol/L
Fe3+: 30μmol/L
Fe3+: 60μmol/L
200
300
400
500
600
700
800
200
300
400
500
600
700
800
Wavelength /nm
Wavelength /nm
1.5
c
d
1.6
1.2
0.8
0.4
0.0
Mn:0 μmol/L
Mn:5.0 μmol/L
Mn:10 μmol/L
Mn:15 μmol/L
Mn:20 μmol/L
Mn:25 μmol/L
Mn:30 μmol/L
Mn:40 μmol/L
Mn:50 μmol/L
Co2+: 0 μmol/L
Co2+: 10μmol/L
Co2+: 20μmol/L
Co2+: 30μmol/L
Co2+: 60μmol/L
1.0
0.5
0.0
200
300
400
500
600
700
800
200
300
400
500
600
700
800
Wavelength /nm
Wavelength
Fig. 5 Absorption spectra of of 5 (30 lmol/L) upon titration of Fe3? (0–60) lmol/L (a), Cu2? (b), Co2?
(c), and Mn2? (d) in Tris–HCl buffer solutions
Fig. 6 Colors of the solutions of 5 when 1 equiv. metal ions were added
without obvious apparent color change. The colors of the solutions of 5 are shown in
Fig. 6 when 1 equiv. metal ions were added. The color of 5 changed to yellow upon
addition of Fe3? and light brown upon addition of Mn2?. We can see that compound
5 is suitable for detection of Fe3? and Mn2? in aqueous solutions by the naked eye.
DFT calculations
In general, the binding of metal ions with the conjugate molecules will enlarge the
conjugation length of the systems, which will induce a red-shift of the absorption
123

A highly selective colorimetric sensor to Fe3? and Co2? in aqueous solutions
2731
spectra. The absorption spectra of 1 upon titration of Co2?, Cu2?, Zn2?, and Pb2?
generated large red-shifts, whereas no shift were observed upon titration of Fe3?
and Hg2?. However, the absorption spectra of 5 upon titration of Co2?, Cu2? and
Fe3? afforded blue-shifts, and a similar phenomenon was observed in the absorption
spectra of 3 upon titration of Fe3?. In addition, the absorption band at 542 nm of 5
upon titration of Co2? was split into two bands. According to these facts, we
presume that the binding of the metal ions to the sensors has two effects: (1)
enlarging the conjugation length of the systems, and (2) weakening the conjugated
properties of the molecule. The absorption spectra afford red-shift if the former
effect dominates the overall effects, or the absorption spectra afford blue-shift if the
latter effect dominates the overall effects. To verify this presumption, the molecular
structures and the electronic density distributions of the HOMO orbitals of 1, 3 and
5 before and after the binding of Cu2? were calculated using the DFT method at
B3LYP level. The basis set used for C, H, N and O atoms was 6–31 g, and Lanl2DZ
Fig. 7 Electronic density distributions of the HOMO orbitals for 1 (a, b), 3 (c, d) and 5 (e, f) before and
after binding to Cu2?
123

2732
H. Wang et al.
Fig. 8 Electronic density distributions of the HOMO orbitals for 1 (left) and 3 (right)
was used for the Cu atom. All the calculations were carried out using the Gaussian
03 program [31].
The calculation results showed that the molecules with trans-configurations of
compounds 1, 3 and 5 have lower energy than their cis-configurations. Therefore,
most of the molecules take trans-configurations at normal conditions. The electronic
density of the HOMO orbital is localized around the –N=N– group before binding to
Cu2? (Fig. 7a), and delocalized around the molecule after binding to Cu2? (Fig. 7b)
for 1. Cu2? greatly involves the conjugation system, which enlarges the conjugation
length of 1. This result is in agreement with the absorption spectra of 1 exhibiting a
red-shift upon addition of Cu2?. The electronic density of the HOMO orbital of 3 is
delocalized around the molecule with a little more around the –N=N– group than the
other position (Fig. 7c). The electronic density of the HOMO orbital of 3 after
binding to Cu2? is localized on the sulfonic acid group (Fig. 7d), and Cu2? is not
involved in the conjugation of the system. The electronic density of the HOMO
orbital of 5 is delocalized around the molecule with a little more around the –N=N–
group than the other position (Fig. 7e), which indicates a great conjugation bridge
for the –N=N– group, whereas the electronic density of the HOMO orbital of 5 after
binding to Cu2? was clearly transferred to the phenyl and naphthanyl groups from
the –N=N– group (Fig. 7f). Also, Cu2? is slightly involved in the conjugation
system, indicating the weakening of the conjugation length. This is the reason that
the absorption spectra of 5 afforded a blue-shift upon addition of Cu2?
.
One of the N atoms in the –N=N– groups of 1–5 can be involved in coordinating
with metal ions together with the hydroxyl groups on the aromatic rings. Two
hydroxyl groups of 1, 3 and 5 can be involved in coordinating with the equivalent
metal cation. Only one hydroxyl group of 2 and 4 can be involved in coordinating
with one equivalent of the metal cation. Therefore, 1, 3 and 5 have better sensitivity
than 2 and 4. The sulfonic acid group of 3 reduces the electronic density of the
hydroxyl group on the naphthalene ring, thus reducing its metal cation coordinating
ability compared to that of 5. Thus, 3 has better metal ions selectivity than 5.
NMR analysis
In general, the coupling constants for aromatic signals are typically 0–1 Hz for para
coupling, 1–3 Hz for meta coupling, and 7–9 Hz for ortho coupling, respectively.
123

A highly selective colorimetric sensor to Fe3? and Co2? in aqueous solutions
2733
However, the hydrogen atoms on the benzene or naphthyl ring, which have been
circled in Fig. 8, show a J value of 12 Hz for coupling constant. The electronic
density on these carbons by DFT calculations is found to be very high, which makes
1
the connected hydrogen show a large coupling constant in their H NMR spectra.
The other hydrogen atoms show normal coupling constants of 8 Hz for ortho
coupling and 2 Hz for meta coupling.
Conclusions
Five aromatic azo dyes with hydroxyl groups were designed and synthesized by
coupling reactions. The absorption spectra of the sensors upon titration with K?,
Ca2?, Al3?, Mg2?, Ni2?, Mn2?, Cd2?, Cr3?, Fe3?, Cu2?, Zn2?, Co2?, Hg2?, and
Pb2? ions, which are frequently concerned with vital processes, in neutral aqueous
solutions were reported. The relationship between their structures and spectroscopic
properties were investigated. Compounds 1 and 5 could afford proper responses
upon titration of different metal ions, but the selectivity of them was not
outstanding. Compound 3 shows high selectivity for the detection of Fe3? and Co2?
among the tested metal ions. The results are also supported by the calculation results
using the DFT method. The high selectivity, excellent water solubility and simple
synthetic process make 3 applicable in vivo for sensing Fe3? and Co2?
.
Acknowledgments The authors are grateful for the financial support from National Natural Science
Foundation of China (No. 21102037) and from the Scientific Research Foundation for the Returned
Overseas Chinese Scholars, State Education Ministry of People’s Republic of China [No. 2008(890)].
References
1. W. Liu, L. Xu, R. Sheng, P. Wang, H. Li, S. Wu, Org. Lett. 9, 3829–3832 (2007)
2. C.A. Bell, S.V. Smith, M.R. Whittaker, A.K. Whittaker, L.R. Gahan, M.J. Monteiro, Adv. Mater. 18,
582–586 (2006)
3. C.R. Goldsmith, S.J. Lippard, Inorg. Chem. 45, 555–561 (2006)
4. X. Peng, Y. Wang, X. Tang, W. Liu, Dyes Pigments 91, 26–32 (2011)
5. F. Zhang, Y. Zhou, J. Yoon, Y. Kim, S. Kim, J. Kim, Org. Lett. 12, 3852–3855 (2010)
6. X. Zhang, Y. Xiao, X. Qian, Angew. Chem. Int. Edit. 47, 8025–8029 (2008)
7. B.S. Gaylord, A.J. Heeger, G.C. Bazan, Proc. Natl. Acad. Sci. 99, 10954–10957 (2002)
8. M.L. Ho, F.M. Hwang, P.N. Chen, Y.H. Hu, Y.M. Cheng, K.S. Chen et al., Org. Biomol. Chem. 4,
98–103 (2006)
9. S.J. Kim, E.T. Kool, J. Am. Chem. Soc. 128, 6164–6171 (2006)
10. J. Tian, H. Li, Y. Luo, L. Wang, Y. Zhang, X. Sun, Langmuir 27, 874–877 (2011)
11. H.J. Kim, J.E. Park, M.G. Cho, S. Ahn, S.K. Chang, Dyes Pigments. 84, 54–58 (2009)
12. I. Miletto, A. Gilardino, P. Zamburlin, S. Dalmazzo, D. Lovisolo, G. Caputo et al., Dyes Pigments.
84, 121–127 (2009)
13. M. Yu, Z. Li, L. Wei, D. Wei, M. Tang, Org. Lett. 10, 5115–5118 (2008)
14. P. Jiang, Z. Guo, Coordin. Chem. Rev. 248, 205–229 (2004)
15. P. Xie, F. Guo, D. Li, X. Liu, L. Liu, J. Lumin. 131, 104–108 (2011)
16. J.F. Callan, A.P. Silva, D.C. Magri, Tetrahedron 61, 8551–8588 (2005)
17. D.H. Lee, K. Hee, J. Hong, Org. Lett. 3, 5–8 (2001)
18. D.H. Lee, J.H. Im, S.U. Son, J. Am. Chem. Soc. 25, 7752–7753 (2003)
123

2734
H. Wang et al.
19. F. Wu, X. Tan, Y. Wu, Y. Zhao, Spectrochim. Acta 65, 925–929 (2006)
20. T.S. Lee, C. Yang, Synth. Met. 117, 135–136 (2001)
21. H. Dincalp, F. Toker, I. Durucasu, N. Avcıbasi, S. Icli, Dyes Pigments. 75, 11–24 (2007)
22. E.E. Nesterov, J. Skoch, B.T. Hyman, W.E. Klunk, B.J. Bacskai, T.M. Swager, Angew. Chem. Int.
Ed. 44, 5452–5456 (2005)
23. A. Caballero, A. Tarraga, M.D. Velasco, P. Molina, Dalton Trans. 11, 1390–1398 (2006)
24. P. Xie, F. Guo, W. Wang, X. Liu, Chem. Pap. 64, 723–728 (2010)
25. P. Xie, F. Guo, C. Li, Y. Xiao, Can. J. Chem. 89, 1364–1369 (2011)
26. J.M. Griffing, R.C. Elderfield, J. Org. Chem. 11, 123–135 (1946)
27. S.N. Chakravarti, V. Pasupati, J. Chem. Soc. 1859–1862 (1937). doi:10.1039/JR9370001859
28. G. Hallas, A.D. Towns, Dyes Pigm. 35, 219–237 (1997)
29. K. Ogawa, F. Guo, K.S. Schanze, J. Photochem. Photobiol. A. 207, 79–85 (2009)
30. F.S. Manas, L.X. Chen, Chem. Phys. Lett. 331, 299–307 (2000)
31. M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, et al. (eds.),
Gaussian 03 revision B05 (Gaussian Inc, Pittsburgh, 2003)
123