Novel fluorescent sensor for detection of Cu(II) in aqueous solution

Article abstract of DOI:10.1016/j.saa.2006.01.009

A novel fluorescent chemosensor based on aminonaphthol, which can selectively recognize copper(II) over other metal ions in aqueous solution within a broad pH span, was synthesized.

Full text of DOI:10.1016/j.saa.2006.01.009

Spectrochimica Acta Part A 65 (2006) 749–752
Novel fluorescent sensor for detection of Cu(II) in aqueous solution
a,b
a,∗
a,∗
a
Jia-Sheng Wu , Peng-Fei Wang , Xiao-Hong Zhang , Shi-Kang Wu
a
Nano-Organic Photoelectronic Laboratory, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100101, PR China
b
Graduate School of Chinese Academy of Sciences, Beijing, PR China
Received 2 November 2005; received in revised form 20 December 2005; accepted 6 January 2006
Abstract
A novel fluorescent chemosensor based on aminonaphthol, which can selectively recognize copper(II) over other metal ions in aqueous solution
within a broad pH span, was synthesized.
2006 Elsevier B.V. All rights reserved.
©
Keywords: Molecular recognition; Aminonaphthol; Chemosensor; Copper(II); Aqueous solution; Fluorescence quenching
1
. Introduction
for development of fluorescent sensors for detection of diva-
lent copper with improved selectivity and sensitivity in aqueous
solution by using new fluorophores. As part of our efforts to
develop fluorescent chemosensors by using new fluorophores
Detection of metal ions by fluorescent chemosensor in aque-
ous solution has been receiving much attention in recent years.
This is mainly due to the potential application in clinical bio-
chemistry and environment. There are already many fluorescent
chemosensors developed so far [1–4], and many of them are
commercially available [5,6]. However, fluorescent chemosen-
sors for selective detection of transition metal ions, especially
2
+
[16–20], herein, we now wish to report a new Cu -sensitive
fluorescent sensor (aminonaphthol derivative), in which the flu-
orescent moiety itself acts as a ligand for metal cations. In this
intrinsic fluorescent ligand, metal cation binding may affect the
fluorescent properties dramatically. This fluorescent chemosen-
sor also shows a good selectivity and sensitivity for divalent
2
+
Cu , in aqueous solution are rather rare [7–9] although cop-
per is third in abundance (next for iron and zinc) among the
essential heavy metals in the human body and plays an impor-
tant role in various physiological processes. On the other hand,
it was also known that copper ion, despite being an essen-
tial element in biological systems, has a toxic impact on the
microorganisms at even submicromolar concentrations. Thus,
it is highly desirable to design and synthesize novel sensors
for copper ions with high selectivity and sensitivity in aqueous
solution. Indeed, some sensors for copper ions have been devel-
oped recently [10,11]. They are mainly based on anthracenyl
and dansyl moieties, most notably anthracenylated peptide and
dansylated peptide [12–14]. In these cases, the fluorescent moi-
ety is covalently linked to the recognition moiety via a spacer
to form PET system, and the change of fluorescent intensity can
be observed through PET process between an acceptor (ligand)
and a fluorophore generally [15]. Therefore, there is still a need
2
+
2+
2+
3+
copper cation over other cations, such as Zn , Co , Ni , Fe ,
2+
3+
2+
2+
Fe , Cr , Cd , Pb and alkali and alkaline earth cations as
well in aqueous solution within a broad pH range.
2. Experimental
2.1. Apparatus
1H NMR spectra were recorded on a Varian GEMANA-
00 spectrometer. Fluorescence spectra were recorded with a
3
Hitachi F4500 spectrophotometer. Mass spectra were recorded
on a Finnigan 4021C MS-spectrometer.
2.2. Reagents
2-Hydroxy-l-naphthaldehyde, 4-aminoantipyrine and Na-
BH4 were purchased from Aldrich and used as received. All
cationic chlorides used were analytical grade and were from
Beijing Chemical Works.
∗
Corresponding author. Tel.: +86 10 64888169; fax: +86 10 64879375.
E-mail addresses: wangpf@mail.ipc.ac.cn (P.-F. Wang),
xhzhang@mail.ipc.ac.cn (X.-H. Zhang).
1
386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.01.009

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50
J.-S. Wu et al. / Spectrochimica Acta Part A 65 (2006) 749–752
Fig. 1. Synthesis route of compound 1.
2
.3. Preparation of compound 1
Compound 1, aminonaphthol derivative, was synthesized
according to the route [21] shown in Fig. 1 and characterized
1
1
by H NMR, MS and elemental analysis. H NMR (300 MHz,
DMSO) δ 10.86 (s, 1H, ArOH), 7.92 (d, 2H, ArH), 7.78 (d, 2H,
ArH), 7.64 (d, 2H, ArH), 7.49 (m, 2H, ArH), 7.33 (m, 2H, ArH),
7
.10 (m, 1H, ArH), 5.03 (s, 1H, NH), 3.52 (s, 2H, ArCH2), 2.54
(
m, 3H, C CCH3), 2.13 (s, 3H, NCH3); MS (m/e): 359 (358,
356, 265, 236, 188, 121, 57); anal. calcd. for C22H21N3O2: C
73.52%, H 5.89%, N 11.69%, O 8.90%. Found: C 73.79%, H
5.38%, N 11.66%, O 9.17%.
Fig. 2. Variation of fluorescence spectra of compound 1 (10 ␮M) in different
pH conditions in aqueous solution at 25 C.
2
.4. Sample preparation
◦
The fluorescence spectra of compound 1 were measured in
3
66 nm decreases with increasing pH value, and disappears ulti-
aqueous solution. The concentration of solution of compound
−5
−1
mately at pH 13 due to the deprotonation of naphthol (pKa ∼ 10).
The fluorescence of compound 1 in aqueous solution was
quenched gradually upon addition of CuCl2. A titration experi-
ment was performed to determine the binding ratio and constant
between compound 1 and Cu2 (Fig. 3). The reason of flu-
orescence quenching of compound 1 is probably due to the
occurrence of either an electron transfer or an electronic energy
transfer involving the transition metal and the excited fluo-
1
was 10 mol L , and the concentration of stock solution
of different metal salts used for the fluorescence titration is
− −1
3
1
0
mol L . Two milliliter solution of compound 1 was filled
in a quartz cell of 1 cm optical path length. Different stock solu-
tions of metal salts were added into the quartz cell gradually by
using a micro-syringe. The volume of metal salts stock solution
added was less than 100 ␮L with the purpose of keeping the total
volume of testing solution without obvious change. Excitation
wavelength was 320 nm and the slit widths were 5 and 2.5 nm,
respectively.
+
2+
rophore as observed in other Cu -recognition sensors [22,23].
Job’s plot shows a complexation at a molar ratio of 1:1 occurred
between compound 1 and Cu (inset of Fig. 3). According to
2+
3
. Results and discussion
The spectral and photophysical properties of compound 1
in organic solvents have ever been investigated in our group
21]. In this article, we will discuss its property in aqueous
[
solution. From fluorescent spectra of compound 1 in aque-
ous solution (Fig. 2), two emission bands centered at 366 and
4
36 nm were observed, which could be assigned to the emission
of the naphthol moiety and its deprotoned form, respectively.
The assignment could be confirmed by its pH-sensitive fluores-
cence spectra. As shown in Fig. 2, the fluorescence spectrum
of compound 1 shows no obvious change between pH 4 and
1
0. However, when the pH value of the solution is less than 4,
the peak at 366 nm increases and that at 436 nm decreases con-
comitantly and vanishes completely at pH less than 0.9. This
quenching process under strong acidic condition is related to the
protonation of secondary amine between naphthol (pKa ∼ 4). On
the other hand, when pH is more than 10, the emission peaked at
Fig. 3. Variation of fluorescence spectra of compound 1 (10 ␮M) upon addi-
tion of CuCl2 (from 5 to 200 ␮M) in aqueous solution (pH 6.8, 25 C) with
an excitation of 320 nm. Inset: Job’s plot for the binding of compound 1 with
CuCl2.
◦

J.-S. Wu et al. / Spectrochimica Acta Part A 65 (2006) 749–752
751
Scheme 1. Possible interaction mode between compound 1 and Cu2+
.
Fig. 4. Fluorescence spectra of compound 1 (10 ␮M) in aqueous solution (pH
◦
6
.8, 25 C) upon addition of different metal chlorides (200 ␮M) with an excita-
tion of 320 nm.
1
:1 stoichiometery, we further determined the stability constant
2+
−1
between compound 1 and Cu to be 4205 M in aqueous solu-
tion by using a binding model reported in the literature [24].
2+
Compound 1 exhibits an excellent selectivity for Cu over
2
+
2+
2+
3+
2+
3+
2+
other cations, such as Zn , Co , Ni , Fe , Fe , Cr , Cd ,
2
+
Pb , and alkali and alkaline earth cations. (No obvious change
of fluorescence spectra of compound 1 upon addition of alkali
and alkaline earth cations was observed, and these cations were
not listed in Fig. 4 for clarity.) From Fig. 4, when 20 equivalents
of Cu2 were added to an aqueous solution of compound 1
Fig. 5. Fluorescence intensity of compound 1 recorded at 366 nm at different
◦
pH conditions at 25 C (a) 10 ␮M free 1, (b) 10 ␮M 1 and 160 ␮M CuCl2, (c)
10 ␮M 1 and miscellaneous metal ions including ZnCl2, CoCl2, NiCl2, FeCl3,
FeCl2, CrCl3, CdCl2 and PbCl2, and each concentration is 20 ␮M, respectively.
+
(
10 ␮M), the fluorescence intensity was substantially quenched.
However, when several competitive transition metal ions includ-
secondary amino and oxygen atom of carbonyl group. Moreover,
the counter anion Cl may also participate in the coordination as
2+
2+
2+
3+
2+
3+
2+
2+
−
ing Zn , Co , Ni , Fe , Fe , Cr , Cd , Pb were added,
respectively, no obvious changes were observed. Relative fluo-
rescence intensities of compound 1 binding these metal ions at
the fourth chelate group. The possible interaction mode between
compound 1 and Cu2 was shown in Scheme 1.
+
3
60 and 436 nm are shown in Table 1. The stability constants
It is important to investigate the influence of the fluorescence
spectra of compound 1 at different pH range in order to find a
suitable pH span in which compound 1 can selectively detect
−
1
between 1 with these metal ions are all less than 300 M . The
selectivity for Cu2 might be interpreted to the geometrical dif-
+
2+
2+
ference between Cu -complex and other metal complexes as
observed in other metal–ligand complexes [25]. In aqueous solu-
tions, these ions generally form octahedral complexes. Due to
the Jahn–Teller effect, coordinating interaction of the four equa-
torial sites is stronger than that of the remaining axial positions
Cu efficiently. The fluorescent titration curve of compound 1
was measured with the change of pH. The fluorescence spectra
of free 1 (Fig. 5, Curve A) exhibits a good plateau between pH
4 and 10, in which compound 1 is not affected by pH variation.
The effect of pH on the fluorescence of 1–Cu2 solution (Curve
B) exhibits a quite different feature from that of free 1. When the
pH is less than 4, the fluorescence seems to be the same as that
+
[
26]. Consequently, four-coordinated Cu2+ complexes are gen-
erally characterized by a square planar geometry if there is no
special geometrical constraints in ligands, just like in the present
ligand—compound 1. In contrast, there is usually no intrinsic
preference for axial over equatorial coordination in other metal
ions in aqueous solution. Therefore, compound 1 can be supe-
2
+
of curve A, which illustrates that the dissociation of 1–Cu is
superior to complexation in strong acidic conditions. When pH
is larger than 4, the fluorescence is deeply quenched, which is
different from that of free 1. Curve C shows the influence of sev-
eral miscellaneous transition and heavy metal cations including
rior to bind Cu2 over other metal ions. In this case, Cu cation
may locate at the center of the pseudocavity of compound 1 and
coordinates with oxygen atom of naphthol, nitrogen atom of a
+
2+
2+
2+
2+
2+
2+
3+
2+
2+
Zn , Co , Ni , Fe , Fe , Cr , Cd and Pb . Only small
variation is observed comparing with Curve A between pH 4 and
Table 1
Relative fluorescence intensity of compound 1 upon addition of different metal ions
None
Fe2+
Co2+
Ni2+
Zn2+
Pb2+
Cd2+
Fe3+
Cr3+
Cu2+
RIf (360 nm)
RIf (436 nm)
1
1
0.87
1.07
1.13
0.89
1.06
0.85
0.71
0.86
0.89
0.97
0.84
1.05
0.84
0.89
0.85
1.04
0.27
0.089
RIf denote relative fluorescence intensity.

7
52
J.-S. Wu et al. / Spectrochimica Acta Part A 65 (2006) 749–752
2
+
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A Cu -sensitive fluorescent sensor based on a novel fluo-
rophore (ligand), aminonaphthol, was synthesized. It shows a
good selectivity for divalent copper over other metal cations in
aqueous solution within a wide pH range. Development of other
novel fluorescent chemosensors based on similar ligands is in
progress.
[
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