A dual colorimetric and fluorescent sensor for lead ion based on naphthalene hydrazone derivative

Article abstract of DOI:10.1016/j.saa.2013.03.020

A new compound, 2-boronobenzaldehyde-(2′-hydroxyl-4′-sulfonic acid) naphthalene hydrazone (1), was synthesized and its structure was characterized by proton nuclear magnetic resonance, mass and element analyses. The presence of Pb²⁺ led 1 to undergo colorimetric and fluorescent changes, which were detectable with the naked eye. Thus, a dual spectral response for Pb²⁺ detection was introduced. In KH₂PO ₄-NaOH buffer aqueous solution (pH 6.0), 1 exhibited fluorescence enhancement at 568 nm and hyperchromicity at 595 nm upon the addition of Pb ²⁺. The fluorescent intensity change was proportionate to the concentration of Pb²⁺ with a dynamic working range of 5.0 × 10^-7 mol L^-1 to 1.0 × 10^-4 mol L ^-1 and a detection limit of 3.7 × 10^-8 mol L ^-1. The fluorometric method was successfully applied for the detection of Pb²⁺ water of Qianhu Lake and soil in Nanchang university campus. The recoveries were 111-116% for water and 97.6% for soil respectively, determined via the standard addition method.

Full text of DOI:10.1016/j.saa.2013.03.020

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 109 (2013) 221–225
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A dual colorimetric and fluorescent sensor for lead ion based on naphthalene
hydrazone derivative
⇑
Fang-Ying Wu , Hua Zhang, Ming Xiao, Bing-Xin Han
Department of Chemistry, Nanchang University, Nanchang 330031, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A 1:1 metal-ligand complex between lead ion and compound 1 was formed which resulted in color
change from red to blue and enhancement of fluorescence at 568 nm.
ꢀ
ꢀ
ꢀ
ꢀ
An easy-to-make colorimetric and
fluorometric sensor 1 for Pb2+ was
obtained.
The assay carried in aqueous solution
with low limit detection as
37 nM mol Lꢁ1
.
The high selectivity and sensitivity of
1 for Pb2+ was for 1’s distinctive
structure.
1ꢁPb2+ shows longer wavelength
absorption and emission and strong
fluorescence.
a r t i c l e i n f o
a b s t r a c t
A new compound, 2-boronobenzaldehyde-(20-hydroxyl-40-sulfonic acid) naphthalene hydrazone (1), was
synthesized and its structure was characterized by proton nuclear magnetic resonance, mass and element
analyses. The presence of Pb2+ led 1 to undergo colorimetric and fluorescent changes, which were detect-
able with the naked eye. Thus, a dual spectral response for Pb2+ detection was introduced. In KH2PO4–NaOH
buffer aqueous solution (pH 6.0), 1 exhibited fluorescence enhancement at 568 nm and hyperchromicity at
595 nm upon the addition of Pb2+. The fluorescent intensity change was proportionate to the concentration
of Pb2+ with a dynamic working range of 5.0 ꢂ 10ꢁ7 mol Lꢁ1 to 1.0 ꢂ 10ꢁ4 mol Lꢁ1 and a detection limit of
3.7 ꢂ 10ꢁ8 mol Lꢁ1. The fluorometric method was successfully applied for the detection of Pb2+ water of
Qianhu Lake and soil in Nanchang university campus. The recoveries were 111–116% for water and
97.6% for soil respectively, determined via the standard addition method.
Article history:
Received 18 December 2012
Received in revised form 27 February 2013
Accepted 3 March 2013
Available online 14 March 2013
Keywords:
2-Boronobenzaldehyde-(20-hydroxyl-40-
sulfonic acid) naphthalene hydrazone
Dual spectral response
Fluorimetry
Ó 2013 Elsevier B.V. All rights reserved.
Colorimetry
Lead ion
Aqueous solutions
Introduction
blood pressure; nerve disorders; and reproductive, cardiovascular,
muscular, and joint pains in adults and children [1–4]. Lead also
Among the metal ions, lead (Pb) is a highly toxic heavy metal
ion that causes various adverse health problems, such as high
causes environmental pollution. The main sources of lead pollution
are mining, metal smelting, coal combustion, and the use of Pb-
based paint, leaded gasoline, and Pb-containing pipes in water sup-
ply systems [5,6]. Thus, the concentration of Pb2+ in environmental
is restricted. The U.S. Environmental Protection Agency sets the
maximum contamination level for lead in drinking water at
⇑
Corresponding author. Tel.: +86 791 83969882 (O).
E-mail address: fywu@ncu.edu.cn (F.-Y. Wu).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.03.020

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F.-Y. Wu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 109 (2013) 221–225
15
l
g Lꢁ1 (75 nM) [7]. The US Center for Disease Control set the
g Lꢁ1 (50–95 nM)
Synthesis of compound 1
standard contamination level for Pb at 10–19
l
in blood [8]. Many instrumentally intensive methods for the detec-
tion of lead have been developed to protect human health and the
environment, including inductively coupled plasma mass spec-
trometry [9,10], atomic absorption spectrometry [11,12], and ano-
dic stripping voltammetry [13,14]. However, these techniques
have some limitation, such as cumbersome sample preparation,
expensive instruments, and costly maintenance, which often limit
their use to the laboratory [15]. Therefore, the development of
alternative assays with easy operation and low cost has sparked
great interest. Fluorescent assays possess some advantages, such
as easy-to-operate, low cost, and sensitivity, which have been
widely used in the detection of metal ions analysis [16]. Some re-
search groups have exerted great effort in the design and synthesis
of many excellent fluorescent and colorimetric sensors for the
detection of Pb2+ [17,18]. However, only a few sensors in which
the binding of Pb2+ increases the fluorescence intensity have been
reported [19–22]. Colorimetric sensors are among the Pb2+-detect-
ing sensors that have received considerable attention [23–25].
However, major improvements can still be made in terms of devel-
oping suitable sensors that can function in aqueous systems.
With these considerations in mind, we extend our earlier inves-
tigations [26] and report a new successful development of a 2-bor-
2-Formylbenzeneboronic acid (0.15 g, 0.001 mol) was dissolved
with the appropriate amount of ethanol. 1-Amino-2-naphthol-4-
sufonic acid (0.24 g, 0.001 mol) was dissolved with NaOH solution
(0.1 mol Lꢁ1), and the pH was adjusted to 7.0 by adding acetic acid.
Above two kinds solution were mixed and refluxed for about 12 h
at 80 °C. The resulting mixture was cooled, filtered, and washed
with methanol and double-distilled water three times, and the
product was dried in a vacuum-drying oven to obtain 0.79 g in
20% yield, which was confirmed by the data. 1H NMR (D2O,
600 MHz) (ppm): 5.568 (s, 1H, ACH@NA), 7.068 (t, 3H, ArAH),
7.187–7.242 (m, 1H, ArAH), 7.355 (s, 1H, ArAH), 7.355 (s, 1H,
ArAH), 7.468 (t, 1H, ArAH), 7.561 (d, 1H, ArAH), 7.710 (d, 1H,
ArAH), 8.354 (d, 1H, ArAH);ESI mass: m/e calcd. for C17H13-
BNNaO6S [MAHꢁ] 392.05, found [MAHꢁ] 392.50; Anal.: calcd. for
C17H13BNNaO6S: C, 51.93; H, 3.33; N, 3.56; O, 24.42; S, 8.16, found
C, 51.63; H, 3.43; N, 3.76; O, 24.48; S, 8.06. (See Scheme 1)
Results and discussion
Fluorescent spectral responses of compound 1 to different metal ions
Compound 1 exhibits good solubility and unique fluorescent sta-
bility in water. Fig. 1 displayed the spectral change and intensity
change of 1 in the presence and absence of various metal ions. In
pH 6.0 buffer KH2PO4–NaOH aqueous solution, compound 1 emitted
weak fluorescence at 534 nm. Upon the addition of various metal
ions, such as Na+, K+, Mg+, Ca2+, Fe3+, Pb2+, Zn2+, Cu2+, Co3+, Hg2+, Ag+,
Mn2+, and Ni2+, only the presence of Pb2+ resulted in a remarkable
spectral change. The emission intensity at 568 nm was enhanced by
about eightfold upon the addition of Pb2+ (20 equiv.). However, the
other metal ions showed little fluorescence enhancement. Thus, com-
pound 1 could be used to detect Pb2+ with high selectivity.
onobenzaldehyde-(20-hydroxyl-40-sulfonic
acid)
naphthalene
hydrazone (1), which exhibits fluorescent and colorimetric re-
sponses to Pb2+ in aqueous solution. Compound 1 contains boric
acid and hydroxyl groups that are capable of chelating Pb2+ via
coordination. Meanwhile, 1 has good solubility in water due to
the sulfonic acid and boric acid groups. As described in our previ-
ous work, the quinoline group in compound 2 was replaced by the
naphthalene group in 1. 1ꢁPb2+ showed more superior character-
istics, such as longer wavelength of emission and absorption and
lower detection limit compared with 2ꢁPb2+
.
Fluorescent titration of 1 against Pb2+ was performed, and the
spectral changes are presented in Fig. 2a. The intensity change at
568 nm was proportionate to the Pb2+ concentration in the range
of 0.005 mol Lꢁ1 to 1.0 ꢂ 10ꢁ4 mol Lꢁ1. The linear regression equa-
Experimental
Materials and reagents
tion was C (
l
mol/L) = 0.5291
D
Fꢁ1.046 (n = 17, R = 0.9983). The
2-Formylbenzeneboronic acid, 1-amino-2-naphthol-4-sufonic
acid, and Pb(NO3)2 were purchased from Sigma–Aldrich (USA,
www.sigmaaldrich.com). Other metal salts were the products of
the Shanghai Qingxi Technology Co., Ltd. (Shanghai, China,
www.ce-r.cn/sites/qingxi/). All reagents and solvents were of ana-
lytical grade and used without further purification. The solution of
1 (5.0 ꢂ 10ꢁ4 mol Lꢁ1) was prepared with double-distilled water.
detection limit was calculated to be 3.7 ꢂ 10ꢁ8 mol Lꢁ1
. The
enhancement of fluorescent intensity was presumably due to
1ꢁPb2+ complex formation. Consequently, the binding ratio be-
tween 1 and Pb2+ was estimated as 1:1 via Job plot curve according
to the fluorescent intensity change (Fig. 2b).
Some similar methods for determination Pb2+ were listed in Ta-
ble 1. It is clear that the determination wavelength and linear
range of proposed method are better compared with the other
methods. Although the detection limit is not the best, it is fulfill
the requirements because the detecting limit is lower than the
maximum permitted amount of Pb2+ in drinking water defined
by the World Health Organization [7]. It is worth to point that
compound 1 as organic fluorescent probe is more easily obtained
than the others and possesses good water solubility.
The metal ion solutions of Na+, K+, Mg2+, Ca2+, Mn2+, Fe3+, Co2+
,
Ni2+, Cu2+, Zn2+, Cd2+, Cr3+, Hg2+, and Ag+ (0.01 mol Lꢁ1) were pre-
pared from their chlorides or nitrates. The buffer solution (pH 6.0)
was prepared using NaOH (0.1 mol Lꢁ1) and KH2PO4 (0.2 mol Lꢁ1).
Apparatus
Absorption spectra were collected on a Shimadzu-2550 ultravi-
olet–visible spectrophotometer (Japan,) using a 1.0 cm quartz cell.
All fluorescence measurements were performed with a scan speed
of 1200 nm minꢁ1 on a Hitachi F-4600 spectrofluorimeter (Japan)
equipped with a xenon lamp source and a 1.0 cm quartz cell. Pro-
ton nuclear magnetic resonance (1H NMR) spectra were obtained
with D2O on a Bruker Advance 600 MHz NMR spectrometer (Swit-
zerland) using tetramethylsilane as the internal standard. Electro-
spray ionization (ESI) mass spectrum was recorded using a Waters
ZQ4000/2695 LC–MS spectrometer (USA). The element analysis
data were obtained on Perkin-Elmer-240(II) (USA). All measure-
ments were operated at room temperature (about 298 K).
3.2 Absorption spectral change for 1 upon the addition Pb2+
Compound 1 exhibits a red color in aqueous solution. As shown
in Fig. 3, a solution of 5.0 ꢂ 10ꢁ4 mol Lꢁ1 1 in aqueous solution at
pH 6.0 KH2PO4–NaOH buffer solution exhibited an absorption band
centered at 528 nm (
e
= 2.03 ꢂ 103 molꢁ1 L cmꢁ1). With increasing
Pb2+concentration, the absorbance of the maximum wavelength at
528 nm decreased, and a new absorption band centered at 603 nm
emerged. Three isobestic points appeared at 419, 450, and 578 nm,
which also suggested a stoichiometric complex formation between
1 and Pb2+. Simultaneously, the change in intensity ratio (A603nm
/

F.-Y. Wu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 109 (2013) 221–225
223
Scheme 1. Syntheses of compound 1 and control compound 2.
Fig. 1. (a) The fluorescence spectra of 1 (5.0 ꢂ 10ꢁ6 mol Lꢁ1) and upon addition of
various metal ions (1.0 ꢂ 10ꢁ4 mol Lꢁ1) with an excitation at 450 nm. (b) Fluores-
cence density at 568 nm of 1 upon the addition of 1 equive. of different metal ions.
Fig. 2. (a) Fluorescence emission spectra of 1 (5.0 ꢂ 10ꢁ6 mol Lꢁ1) in aqueous
solution upon addition of Pb2+ in concentrations of 0, 2.0, 5.0, 10.0, 15.0, 20.0, 25.0,
30.0, 40.0, 45.0, 50.0, 55.0, 60.0, 70.0, 80.0, 90.0, 100.0 ꢂ 10ꢁ6 mol Lꢁ1. (b) The Job-
plot for the complex of 1 and Pb2+, the total concentration of 1 and Pb2+ was
5.0 ꢂ 10ꢁ5 mol Lꢁ1
.
A528nm) was proportionate to the Pb2+ concentration in the range of
0.05 mol Lꢁ1 to 8.0 ꢂ 10ꢁ6 mol Lꢁ1. The linear regression equation
was C (
l
mol Lꢁ1) = 7.87A603nm/A528nm + 13.2, R = ꢁ0.9859, n = 11.
ln½ðA0 ꢁ AeÞ=ðAe ꢁ A Þꢃ ¼ nlnC0 þ lnKs
1
The linear dynamic range of absorption spectrometry was nar-
rower than that of fluorimetry.
where A0 is the absorbance intensity of 1, Ae is the absorbance
intensity of 1ꢁPb2+, and C0 is the initial molar concentration of Pb2+
.
Determining the binding constant of 1ꢁPb2+
KS and n can be derived from the intercept and slope obtained
through the plot of In[(A0ꢁAe)/(AeꢁA )] against lnC0. The fitted
1
The binding constant (Ks) and the binding site numbers (n) can
be calculated using the Benesi–Hildebrand equation [27] according
to the absorption spectral change, as follows:
curve is shown in Fig. 4. The binding constant and the number of
binding sites were 43028.16 and 1, respectively. The value of n
was approximately equal to 1, indicating the existence of one

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F.-Y. Wu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 109 (2013) 221–225
Table 1
Comparison of fluorescence enhancement sensor for Pb2+
.
Material
Monitor wavelengths (nm)
Conditions
Linear range
0.02–10
0–6
0–0.96
0–4 g/L
0–8.8
0.5–100
Detection limit (nm)
Reference
DNA device originates
1,8-Naphthalimide
DNA-based biosensor
Peptide Tryptophan-Dansyl
Bis(2-pyridylmethyl)amine
Naphthalene hydrazone derivative
594
485
587
495
560
568
pH 6.5, MOPS-NH2OH buffer
CH3CN–H2O
Aqueous solution
Aqueous solution
Aqueous solution
Aqueous solution
l
M
20
50
6
50
19
38
[21]
[29]
[30]
[31]
[32]
This method
l
M
l
M
l
l
M
l
M
Conclusions were drawn based on the structures of compounds
1 and 2. First, the N atom in the quinoline group did not participate
in the coordination with Pb2+ because the binding constant of
1ꢁPb2+ was much larger than that of 2ꢁPb2+. Second, the position
of the sulfonic acid groups affects the wavelength change of the Pb
complex. The sulfonic acid group in compound 1 containing naph-
thalene is positioned at the para-position of oxime, which is bene-
ficial for charge transfer, resulting in longer wavelength absorption
and emission. Thus, according to the obtained results, chemosen-
sor 1 is the most likely to chelate with Pb2+ via its O and N atoms
and the AOH of the boric acid group (Scheme 2). The electron den-
sity of 1 was changed due to chelation with Pb2+, which resulted in
a remarkable color change from red to blue. The combination of
atoms and space position maybe lead to the high selectively inte-
gration with Pb2+ over the other heavy metal ions like Hg2+, Cd2+
Cu2+, and Ag+.
,
Fig. 3. UV–vis spectra of 1 (5.0 ꢂ 10ꢁ5 mol Lꢁ1) in different concentrations of Pb2+
(a) 0, (b) 0.05, (c) 0.10, (d) 0.15, (e) 0.25, (f) 0.30, (g) 0.40, (h) 0.50, (i) 0.60, (j) 0.70,
Analytical application
(k) 0.80 ꢂ 10ꢁ5 mol Lꢁ1
.
Optimal experimental conditions
The effects of pH, reaction time, reaction temperature, and vol-
ume of ligand on the detection for Pb2+ were investigated. Optimal
experimental conditions were obtained under room temperature
in pH 6.0 KH2PO4–NaOH buffer solution and maintained for 5 min
reaction time. The concentrations of compound 1 were 5.0 ꢂ 10ꢁ5
and 5.0 ꢂ 10ꢁ6 mol Lꢁ1 for absorption and emission, respectively.
The fluorescent intensity and absorbance of 1ꢁPb2+ in testing
condition kept constant for at least 8 h.
Effect of foreign substances
Under optimal experimental conditions, the presence of the fol-
lowing amounts of foreign substances in the mixture solution of
5.0 ꢂ 10ꢁ6 mol Lꢁ1 Pb2+ resulted in less than 5% error compared
with the Pb2+concentration: 1000-fold Na+, K+, Clꢁ, NO3ꢁ, SO24ꢁ
,
and CO23ꢁ, 100-fold Ca2+, Mg2+, Co2+, Ni2+, and CH3COꢁ, 50-fold
2
Hg2+, Mn2+ and Zn2+, 10-fold Cu2+ and Fe3+, 5-fold Cd2+ and Ag+.
Fig. 4. Plot of fluorescence intensity change against the concentration of Pb2+
.
Analysis of real samples
The preliminary analytical application of this method was eval-
uated by determining Pb2+ in Qian Lake water and soil samples in
Nanchang university campus. The results obtained are given in
Table 3. The lake water samples were used without any treatment.
The soil was treated as reference.[28] The recovery range was be-
tween 97.6% and 116%, which was in good agreement with Pb2+
obtained between spike and measured Pb2+. The concentration of
Pb2+ in soil obtained by our method is accordant with the determi-
nation value from atom absorption spectrum.
binding site for 1, which is in agreement with the results of the Job
plot. The obtained binding constant further suggested a strong
binding force existing between 1 and Pb2+, possibly from the strong
force of the boric acid group. The binding parameters of 1ꢁPb2+
and control complex of 2ꢁPb2+ [26] are listed in Table 2 to explain
the binding model. Compound 1 exhibits a highly sensitive spectral
response to Pb2+ compared with control compound 2.
Table 2
Parameters of complexes of 1 and 2 with Pb2+
.
Complex
kax (nm)
kex/kem (nm/nm)
Ka (Mꢁ1
)
Detection limit (M)
Linear range (M)
1ꢁPb
2ꢁPb
595
590
450/568
360/477
43028
1089
9.7 ꢂ 10ꢁ8
0.50–100 ꢂ 10ꢁ6
2.3 ꢂ 10ꢁ7
0.36–10 ꢂ 10ꢁ6

F.-Y. Wu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 109 (2013) 221–225
225
Scheme 2. Proposed mechanism for the fluorescence changes of 1 upon addition of Pb2+
.
Table 3
Results for determination of Pb2+ in lake water and soil samplesa.
Sample
Pb2+ found (10ꢁ6 mol Lꢁ1
)
Pb2+ added (10ꢁ6 mol Lꢁ1
)
Total Pb2+ (10ꢁ6 mol Lꢁ1
)
Average (10ꢁ6 mol Lꢁ1
)
RSD (%)
Recovery (%)
2.409
2.409
2.409
6.620b
1.500
2.500
5.000
5.000
4.478
6.186
8.187
10.82
4.049
5.832
8.160
11.05
4.615
4.381
5.718
8.231
11.34
0.295
0.535
0.101
0.72
112
116
111
97.6
Lake water
Soil
5.134
8.346
12.16
a
The determination results of Pb2+ were obtained by fluorometric method.
b
The concentration of Pb2+ in soil was 6.793 ꢂ 10ꢁ6 mol Lꢁ1 obtained via atom absorption spectrum.
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4. Conclusions
A simple-structured and fluorescence-activating sensor 1,2-
boronobenzaldehyde-(20-hydroxyl-40-sulfonic acid) naphthalene
hydrazone, was easily synthesized through a one-step reaction.
However, the structure was distinctive. The boronic acid group guar-
anteed good water solubility and a bindingsite in thecoordination of
1ꢁPb2+. The sulfonic acid group and its position play key functions.
The sulfonic acid group in compound 1 was placed at the para-posi-
tion of oxime, which enhanced charge transfer, resulting in longer
wavelength absorption and emission. Meanwhile, the sulfonic acid
group is also hydrophilic. Therefore, a novel Pb2+ sensor based on
dual spectral response was introduced, and emission and color
changes were easily detected with the naked eye in aqueous
solutions.
Acknowledgment
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