8-Hydroxyquinoline-5-carbaldehyde Schiff-base as a highly selective and sensitive Al3+ sensor in weak acid aqueous medium

Article abstract of DOI:10.1016/j.inoche.2011.04.027

In the paper, a novel fluorescent sensor (1) based on 8-hydroxyquinoline carbaldehyde Schiff-base was synthesized and characterized. This fluorescent sensor exhibited high selectivity for Al³⁺ over other metal ions with the detection limit reaching below 10^{- 7} M under weak acid aqueous conditions. These suggested that 1 could be served as a highly selective and sensitive fluorescence sensor for aluminum ion in acid medium.

Full text of DOI:10.1016/j.inoche.2011.04.027

Inorganic Chemistry Communications 14 (2011) 1224–1227
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
8-Hydroxyquinoline-5-carbaldehyde Schiff-base as a highly selective and sensitive
Al3+ sensor in weak acid aqueous medium
b
a
a
Xin-hui Jiang a, Bao-dui Wang a, Zheng-yin Yang a, , Yong-chun Liu , Tian-rong Li , Zeng-chen Liu
⁎
a
College of Chemistry and Chemical Engineering, Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, Lanzhou University, Lanzhou 730000, PR China
College of Chemistry and Chemical Engineering, Key Laboratory of Longdong Biological Resources in Gansu Province, Longdong University, Qingyang 745000, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
In the paper, a novel fluorescent sensor (1) based on 8-hydroxyquinoline carbaldehyde Schiff-base was
synthesized and characterized. This fluorescent sensor exhibited high selectivity for Al3+ over other metal ions
with the detection limit reaching below 10−7 M under weak acid aqueous conditions. These suggested that 1
could be served as a highly selective and sensitive fluorescence sensor for aluminum ion in acid medium.
© 2011 Elsevier B.V. All rights reserved.
Received 16 March 2011
Accepted 29 April 2011
Available online 11 May 2011
Keywords:
Aluminum ion
Fluorescence sensor
Crystal structure
8-Hydroxyquinoline
Schiff-base
Aluminum is the third most prevalent element and the most
abundant metal in the earth's crust. But mounting evidences suggest
that aluminum has severe toxicity in the central nervous system, which
can cause idiopathic Parkinson's disease, impairment of memory and
Alzheimer's Disease [1–3]. In addition, the toxicity endangers freshwa-
ter fish species and influences agricultural production in acidic soils
(pH≤5.5) [4,5]. In acid condition, solubility of aluminum minerals raises
the amount of available Al3+, and it was found to be more toxic in weak
acid aqueous environment [6]. Toxicity of aluminum is often highly
concerned due to its wide distribution in acid environment. Conse-
quently, it is a challenging work to design and synthesize a highly
selective and sensitive chemosensor for Al3+ in acid aqueous medium.
Recently, fluorescent sensors for various metal ions have attracted
much interest because of the advantages such as high sensitivity,
selectivity, rapid response time and versatility [7]. Up to now, several
fluorescent sensors for Al3+ have been designed [8], but most of them
were operated in organic solvent, which limited their applications.
Furthermore, very few fluorescent chemosensors for Al3+ in acid
aqueous medium have been found in literature [8j]. 8-Hydroxyquinoline
(HQ) as a desirable fluorophore and binding moiety was employed
widely in designing chemsensors for detection transition metal ions [9].
But few Al3+ fluorescence sensors derived from HQ were synthesized up
to now [8c].
carbaldehyde (HQ5A) and 4-aminoantipryrine as shown in Scheme 1.
HQ5A was obtained conveniently from HQ by the Reimer–Tiemann
Reaction [10]. The single crystal of 1 was obtained by evaporated method
in ethanol (Fig. 1). The receptor could be dissolved in water when 10%
(v/v) of methanol was added. To obtain a selective Al3+ chemsensor
operated under acid aqueous solution, the value of pH was adjusted to
4.5 by acetate buffer and all the following studies were carried out in
aqueous solution containing 10% methanol (acetate buffer, pH 4.5) at
room temperature.
The binding properties of 1 with Al3+ were measured by UV–vis
titration study in the first place in the acid aqueous solution (Fig. 2).
Upon addition of Al3+, the absorbance bands at 251 nm, 283 nm and
362 nm enhanced and at 225 nm and 325 nm decreased in the UV–vis
spectra. These findings are characteristic of the HQ derivatives when a
complexation process is accompanied by the deprotonation of the
hydroxyl group [11]. Moreover, there are clear six isosbestic points at
242, 258, 271,298, 334 and 411 nm, which clearly indicated the presence
of a single complex in equilibrium with the free receptor [12]. Such
spectral changes of 1 in the presence of 0–2 equiv of Al3+ were ascribed
to a 1:2 complex formation.
The fluorescence intensity changes of 1 in the presence of Al3+ was
investigated in the acid aqueous medium. The lone pair electrons from
the Schiff-base and quinoline nitrogen atoms contributed to the
photoinduced electron-transfer (PET) phenomenon, which quenched
fluorescence emission of 1. Then the quenched fluorescence could
increase dramatically upon chelating metal ion by occurring highly
efficient chelation-enhanced fluorescence (CHEF) effect [13]. As
shown in Fig. 3, the free 1 (10 μM, λex =378 nm) emitted weak
fluorescence at 480 nm. Upon titration of 2 equiv of Al3+, as much as
30-fold fluorescence enhancement was detected. This demonstrated
Herein, a novel fluorescent Al3+ chemosensor, 4-(8′-hydroxyquino-
lin-5′-yl)methyleneimino-1-phenyl-2,3-dimethyl-5-pyzole (1), was syn-
thesized by Schiff-base condensation of 8-hydroxyquinoline-5-
⁎
Corresponding author.
E-mail address: yangzy@lzu.edu.cn (Z. Yang).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.04.027

X. Jiang et al. / Inorganic Chemistry Communications 14 (2011) 1224–1227
1225
Scheme 1. Synthesis of fluorescence sensor 1.
Fig. 2. UV–vis spectra of 1 (10 μM) with addition of increasing amount of Al3+ (0, 2.5, 5,
7.5, 10, 12.5, 15, 17.5 and 20 μM) in 10% (v/v) methanol aqueous medium (acetate
buffer, pH 4.5). Inset: the absorption of 1 at 361 nm as a function of Al3+ concentration
in 10% (v/v) methanol aqueous medium (acetate buffer, pH 4.5).
that the 8-hydroxyquinoline-5-carbaldehyde Schiff-base, which could
be used as off–on type chemsensor, had potential application in
sensing Al3+. And the stoichiometry of 1-Al3+ complex was
determined by changes in the fluorogenic response in the presence
of varying Al3+ concentrations, and the results indicated the
formation of a 1:2 complex (Fig. 3(b)).
fluorescence intensity of 1 only slightly changes in the presence of 2
equiv other metal ions, respectively. Then 2 equiv Al3+ was added to
the above solutions, the fluorescence intensities were instantly and
remarkably enhanced. Despite Fe3+ and Cu2+ could quench fluores-
cence of the ligand via energy or electron transfer [14], the quenched
fluorescence emissions could be enhanced 8.6-fold and 11.5-fold
upon addition 2 equiv of Al3+, respectively. Therefore, 1 showed a
high selective chemosensor for Al3+ in the presence of these
coexistent metal ions in acid aqueous medium.
The selectivities of 1 to various metal ions were examined in acid
aqueous. As shown in Fig. 4, upon addition of 2 equiv Al3+, Ag+, Ca2+
,
,
Cd2+, Co2+, Cr3+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Mg2+, Mn2+, Na+, Ni2+
Pb2+ and Zn2+, sensor 1 (10 μM, λex =378 nm) showed large
fluorescence enhancement with Al3+. And the other metal cations
could not enhance the fluorescence intensity efficiently. The large
difference of the fluorescence spectra of 1 in the presence of different
metal ions might be explained as follows. On the one hand, transition
metal ions (except Zn2+ and Cd2+) often quenched fluorescence via
energy or electron transfer [14] or the heavy atom effect [15]. On the
other hand, Zn2+ and Cd2+ could increase fluorescence intensity of
some 8-hydroxylquinoline derivatives in organic or weak base aqueous
medium have been reported in many paper, but the enhanced
fluorescence was quenched significantly in acid aqueous [16]. In this
paper, weak fluorescence enhancements of the sensor induced by Zn2+
and Cd2+ were also observed in acid aqueous environment. The HQ
derivative chelating aluminum ions with high affinity displayed strong
CHEF effect. Therefore, large fluorescence enhancement was observed.
To validate the high selectivity of 1 in practice, the systems of other
metal ions and Al3+ coexisted were examined in 10% (v/v) methanol
aqueous medium (acetate buffer, pH 4.5). As shown in Fig. 5, the
The detection limit of 1 in recognizing Al3+ was also evaluated
using fluorescence spectra (Fig. S1). Upon addition of 0.5 μM Al3+ to
the sensor 1 (1 μM, with an excitation of 378 nm), 3.2-fold emission
enhancement demonstrated that the chemsensor could detect Al3+ at
10−7 M level. So 1 illustrated highly sensitive for Al3+ under the weak
acid aqueous medium.
Fig. 3. Fluorescence spectra of 1 (10 μM) with addition of increasing amount of Al3+ (0,
2, 4, 6, 8, 10, 12, 14, 16, 18, 20, and 22 μM) in 10% (v/v) methanol aqueous medium
(acetate buffer, pH 4.5), (λex =378 nm,
λem =470 nm, slit: 3 nm/3 nm). Inset:
(a), Fluorescence emission intensity of 1 at 488 nm as a function of Al3+ concentration.
(b), Fluorescence changes of 1 (10 μM) upon addition of Al3+ (20 μM) in acid aqueous
solution containing 10% methanol (acetate buffer, pH 4.5).
Fig. 1. X-ray single crystal structure of sensor 1.

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X. Jiang et al. / Inorganic Chemistry Communications 14 (2011) 1224–1227
of Fig. S2, the pH dependence of fluorescence intensities has been
analyzed, and it was plotted according to the following equations:
F max−F
pKa = pH– log
F−F min
where Fmax, Fmin and F are the maximum, the minimum and the
measured fluorescence intensities at 370 nm, respectively. The
average pKa value calculated of 1 is 6.8.
At last, determination of binding constant and stoichiometry was
determined from the fluorescence titration spectra of 1 with Al3+ in
the acid solution. As shown in Fig. S3, the total binding constant B (log
β) is deduced to be 8.012 and the fluorescence response fits to a Hill
coefficient of 2 (2.0171). The formation of a 1:2 stoichiometry
complex is illustrated [17]. Then the stoichiometry of the complex
was also determined by the Job's method from the fluorescence
intensity (Fig. S4), indicating a 1:2 stoichiometry complex that was
formed in the presence of 2 equiv Al3+. The result was consistent with
data from UV–vis and fluorescence spectra titration experiments for
1-Al3+ complex in solutions. The calculation formula of total binding
constant and Hill coefficient was given as follows,
Fig. 4. Fluorescence spectra of 1 (10 μM, λex =378 nm) with addition of various general
metal ions (2 equiv, respectively) in 10% (v/v) methanol aqueous medium (acetate
buffer, pH 4.5).
F−F min
F max−F
log
= n log½Mꢀ + B
where Fmax, Fmin and F are the same as that in above equation,
respectively.
According to the above discussion, the coordination formation of
the complex 1-Al3+ and the mechanism of electron or energy transfer
for the sensor 1 in sensing Al3+ were put forward in this paper, as
shown in Scheme 2. Upon binding with Al3+, the PET phenomenon of
Schiff-base and quinoline nitrogen atoms was inhibited efficiently, so
the fluorescence intensity was increased dramatically.
In conclusion, a highly selective and sensitive fluorescence sensor for
Al3+ was synthesized. And the Schiff-base receptor exhibited high
selectivity for Al3+ over other metal ions with 30-fold fluorescence
enhancement and high sensitivity with the detection limit reaching at
10−7 M level under weak acid aqueous conditions (acetate buffer, pH
4.5). These suggested that 1 could be served as an excellent chemsensor
for highly toxical aluminum ion in weak acid aqueous medium.
Fig. 5. Fluorescence responses of 1 (10 μM, λex =378 nm) to various metal ions (20 μM)
in methanol/water (1:9, v/v, acetate buffer, pH 4.5). Light gray bars represent the
addition of metal ions to a 10 μM solution of 1, respectively. Black bars represent
emission intensity of a mixture of 1 (10 μM) with other metal ions (20 μM) followed by
addition of 20 μM Al3+ to the solutions, respectively.
Acknowledgment
This work is supported by the National Natural Science Foundation
of China (20975046) and the Specialized Research Fund for the
Doctoral Program of Higher Education (20100211120010).
To check the pH effecting on the fluoresence response, the
fluorescence spectra of 1 with Al3+ under different pH conditions were
examined. It was found that the fluorescence intensity of 1-Al3+ could
almost remain constant under weak acid system (Fig. S2). From the data
Appendix A. Supplementary data
Synthetic procedure, spectroscopic data and figures (SEI3);
crystallographic data of 1 (SEI2) and Check CIF data report (SEI1).
Scheme 2. The possible coordination formation of the complex 1-Al3+ and the mechanism of significant fluorescence enhancement for the sensor 1 chelating with Al3+
.

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1227
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Supplementary materials related to this article can be found online at
doi:10.1016/j.inoche.2011.04.027.
Supplementary data to this article can be found online at doi:10.1016/j.
inoche.2011.04.027.
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