A highly sensitive and selective schiff base fluorescent chemodosimeter for aluminum(III)

Article abstract of DOI:10.1080/00958972.2013.792333

A fluorescent probe for Al³⁺, 4-(1'-phenyl-3'-methyl-5'- hydroxypyrazole)-1-acetone-(2'-hydroxybenzoyl) hydrazone (L), was prepared through a simple synthetic route. The fluorescent probe can detect Al ³⁺ ions sensitively, in ethanol

Full text of DOI:10.1080/00958972.2013.792333

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A highly sensitive and selective Schiff
base fluorescent chemodosimeter for
aluminum(III)
Xiao-Ying Cheng a , Ming-Fang Wang a , Zheng-Yin Yang a , Yong Li a
, Tian-Rong Li a , Chun-Jiao Liu a & Qiao-Xia Zhou a
a State Key Laboratory of Applied Organic Chemistry , College
of Chemistry and Chemical Engineering, Lanzhou University ,
Lanzhou , P.R. China
Accepted author version posted online: 04 Apr 2013.Published
online: 30 Apr 2013.
To cite this article: Xiao-Ying Cheng , Ming-Fang Wang , Zheng-Yin Yang , Yong Li , Tian-Rong
Li , Chun-Jiao Liu & Qiao-Xia Zhou (2013) A highly sensitive and selective Schiff base fluorescent
chemodosimeter for aluminum(III), Journal of Coordination Chemistry, 66:11, 1847-1853, DOI:
10.1080/00958972.2013.792333
To link to this article: http://dx.doi.org/10.1080/00958972.2013.792333
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Journal of Coordination Chemistry, 2013
Vol. 66, No. 11, 1847–1853, http://dx.doi.org/10.1080/00958972.2013.792333
A highly sensitive and selective Schiff base fluorescent
chemodosimeter for aluminum(III)
XIAO-YING CHENG, MING-FANG WANG, ZHENG-YIN YANG*, YONG LI,
TIAN-RONG LI, CHUN-JIAO LIU and QIAO-XIA ZHOU
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical
Engineering, Lanzhou University, Lanzhou, P.R. China
(Received 26 August 2012; in final form 4 February 2013)
A
fluorescent probe for Al3+
, 4-(1′-phenyl-3′-methyl-5′-hydroxypyrazole)-1-acetone-(2′-hydrox-
ybenzoyl) hydrazone (L), was prepared through a simple synthetic route. The fluorescent probe
can detect Al3+ ions sensitively, in ethanol solution with the detection limit of 2.5 Â 10À8 M via
photoinduced electron transfer progress. The presence of interfering metal ions and chelators had
little influence on the selectivity of Al3+. The probe could serve as an excellent chemodosimeter
for Al3+ in ethanol.
Keywords: Schiff base; Chemodosimeter; Al3+ Ions; PET Progress
1. Introduction
Concern for human health with metal cations has stimulated active research on chemodosi-
meters for toxic ions. Among cations, investigations on designing probes for aluminum
cations (Al3+) are attracting attention due to its toxicity and its effect on human health [1].
Neurotoxicity of Al3+ can damage the human nervous system and lead to Alzheimer’s
disease [2]. The accumulating evidence suggests that Al3+ can cause damage to certain
human tissues and cells, because Al3+ plays an important role in potentiating oxidative and
ammatory events, resulting in diseases such as dialysis dementia, osteomalacia, microcytic
anemia [3], lung cancer, bladder cancers, etc. [4, 5]. In the ecosystem, toxicity of Al3+ is a
grave concern. Almost 40% of the world’s acidic soils are caused by phytotoxic Al3+
[6, 7]. Al3+ ions become more soluble in acidic media, and hence, Al3+ ions are potentially
more toxic to aquatic biota [8].
The adverse health effects caused by Al3+ have stirred considerable attention on
detection of Al3+. Among all the Al3+ ion detection techniques, graphite furnace atomic
absorption spectrometry [9] and inductively coupled plasma-atomic emission spectrometry
[10] are developed, but expensive, complicated, and time consuming [11]. A number of
experiments indicate that fluorescent probes for Al3+ can solve such problems. Thus the
design and preparation of fluorescent probes with high selectivity and sensitivity are
*Corresponding author. Email: yangzy@lzu.edu.cn
Ó 2013 Taylor & Francis

1848
X.-Y. Cheng et al.
needed. However, many probes still have disadvantages, such as tedious synthetic routes
[12], poor coordination ability with Al3+ [13], high detection limit [14, 15], etc. Thus, we
pursue new Al3+ probes with easier synthetic procedure, higher stability, and lower
detection limits.
Previous reports indicate that structures of most fluorescent probes for Al3+ contain
nitrogen and oxygen-rich coordination environments providing a hard base environment
for the hard acid Al3+ [16]. Previously, our group developed a series of acylhydrazone
compounds as fluorescent sensors. Chunjiao Liu et al. have done experiments with
7-methoxychromone-3-carbaldehyde-(3′,4′-dimethyl) pyrrole hydrazone (MCPH) as fluo-
rescent sensor for Al3+ in acetonitrile, and the detection limit of MCPH for Al3+ was
2.5 Â 10À7 M [17]. Xiulong Jin et al. synthesized 8-hydroxyquinoline-5-carbaldehyde-
(benzotriazol-1′-acetyl) hydrazone as Mg2+ sensor with the detection limit of 9 Â 10À8
M
in acetonitrile [18]. In order to improve on these systems, we have synthesized a Schiff
base ligand, 4-(1′-phenyl-3′-methyl-5′-hydroxypyrazole)-1-acetone-(2′-hydroxybenzoyl)
hydrazone (L) to detect Al3+ through fluorescence spectra; ions such as Mn2+, Fe3+, Co2+,
Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, Ba2+, Mg2+, Pb2+, Cr3+, and Ca2+ have little influence on
the detection of Al3+. Moreover, the probe can detect Al3+ in ethanol without sodium tar-
trate dibasic dehydrate (STDD), sodium citrate (SC), or ethylenediamine tetraacetate
(EDTA) altering the fluorescence intensity. The detection limit of the probe was considered
to be 2.5 Â 10À8 M, indicating that the Schiff base ligand is a promising sensitive
chemodosimeter.
2. Experimental
2.1. Materials and instrumentation
1
All chemicals were obtained from commercial suppliers and used without purification. H-
NMR spectra were measured on Bruker 400 MHz instruments using TMS as an internal
standard. ESI-MS were determined on a Bruker esquire 6000 spectrometer. UV–vis
absorption spectra were determined on a Shimadzu UV-240 spectrophotometer. Fluores-
cence spectra were recorded on a Hitachi RF-4500 spectrophotometer equipped with quartz
cuvettes of 1 cm path length.
2.2. Synthesis of the probe
The synthetic route of L is shown in figure 1. 1-Phenyl-3-methyl-4-acetyl-pyrazolone-5
(PMAP) was prepared according to the literature [19]. The Schiff base was synthesized by
simple Schiff base condensation of PMAP and 2-hydroxybenzene-1-carbohydrazide. The
mixed solution of PMAP and 2-hydroxybenzohydrazide was refluxed for eight hours at
85 °C, giving bright yellow powder, m.p. 226–227 °C. The Schiff base was confirmed by
1H NMR spectra (figure S1) and IR spectra (figure S2). 1H NMR of L (400 MHz in
DMSO-d6): δ: 12.543 (s, 1H, H15), 11.193 (s, 1H, H9), 7.996 (d, J = 8.0 Hz, 2H, H1,5),
7.822 (d, J = 9.2 Hz, 1H, H10), 7.455 (t, J = 7.6, 4.8 Hz, H13), 7.399 (m, 2H, H11,12), 7.135
(t, J = 6.8 Hz, 1H, H3), 6.968 (dd, J = 8.0, 6.8 Hz, 2H, H3,4), 2.446 (s, 3H, CH3, H8), and
2.372 (s, 3H, CH3, H6).

Chemodosimeter for aluminum(III)
1849
Figure 1. Synthetic route of L.
3. Results and discussion
The binding of L and Al3+ was investigated by UV–vis titration in ethanol at room
temperature (figure S3). Upon addition of Al3+ (0–2 equiv.) the absorbance at 365 nm for
L decreased gradually, while the absorbance at 381 nm increased with an isosbestic point
at 373 nm, indicating a balance between L and Al3+–L. Therefore, we conclude that
complexation of L and Al3+ occurred [20]. IR spectra (figure S4) also proved formation of
a complex between L and Al3+. The ν(O–H) for L is at 3261 cmÀ1 and shifts to
3309 cmÀ1 in the complex. The ν(C=O) of L is at 1627 cmÀ1 and the complex is shifted
to 1597 cmÀ1; Δν (L-complex) is 30 cmÀ1. In the complex, the band at 542 cmÀ1 was
assigned to ν(M–O). These demonstrate that oxygen of carbonyl forms a bond with alumi-
num ions. A weak-band at 413 cmÀ1 was assigned to ν(M–N) in the complex, confirming
that nitrogen of the imino-group bonds to aluminum. The бNNH of the free L (1496 cmÀ1
)
and that of the complex (1498 cmÀ1) shows that the active hydrogen of the –NHN– still
exists in the complex.
Fluorescence titration experiments were also performed in ethanol solution (figure 2).
The fluorescence spectra of L have a weak peak at 452 nm, but with addition of increasing
concentration of Al3+, a significant increase in fluorescence intensity at about 452 nm
occurs. Upon addition of Al3+, there was a dramatic fluorescence enhancement at 396 nm
and 419 nm, due to promotion of an emissive excimer channel formation, and the
fluorescence enhancement factors at 396 and 419 nm were determined as 53- and 56-fold,
respectively. Excitation spectra of the Al3+-bound probe (figure S5) suggest that the three
emission bands at 396, 419, and 452 nm originate from the same ground state [21]. The
fluorometric titration reaction curve showed two different peaks close to each other,
attributed to the –NH–N= structure. We also found that water can quench one peak and
make it become more intense due to coordination. When the amount of Al3+ reached
two equiv., the fluorescence intensity remains constant, indicating that the probe and
Al3+ are coordinated with 1:2 stoichiometry. This is also in agreement with Job’s plot
(figure S6). Job’s plot for the emission changes was applied by keeping the total
concentration of L and Al3+ at 10 μM, varying the molar fraction of Al3+ ions. From Job’s

1850
X.-Y. Cheng et al.
Figure 2. Changes in fluorescence emission spectra of L (10 μM) upon titration by Al(NO3)3 in ethanol. Insert:
fluorescence intensity as a function of Al(NO3)3 concentration.
plot, when the molar fraction reached 0.67, the fluorescence intensity was maximum. From
the plot of the fluorescence intensity changes at 452 nm, the association constant was
determined from the intercept log K, which was 6.39, and the slope was n = 2.0783, indi-
cating that L and Al3+ coordinated with the stoichiometry of 1 : 2 [22]. Literature reported
on coordination of Al3+ in similar complexes helped us to conclude the present Al–L
complex [23]. The ESI-mass (figure S7) peaks at 453 and 466 were also proof of the 1 : 2
stoichiometry.
The detection limit of the fluorescent probe was determined from fluorescence titration
data at 452 nm with lower concentration (figure S8). When 0.5 μL (1 Â10À4 M) Al3+ ions
were added to solution of 10 μM L, the fluorescence intensity increased 2.35-fold, showing
that the probe can detect Al3+ below 2.5 Â 10À8 M. The detection limit was considered to
be 2.5 Â 10À8 M, indicating that the Schiff base is a promising sensitive chemodosimeter.
Figure S9 shows that in ethanol or water–ethanol, the ligand can detect Al3+ ions with
high sensitivity. In other organic solvents or mixed solvents, the Schiff base cannot detect
Al3+ ions efficiently. STDD, sodium citrate SC, and EDTA are good chelators. However,
when STDD, SC, and EDTA (10 equiv.) were added into solution of Al3+–L (figure 3), its
fluorescence intensity remains almost constant, indicating that the complex Al3+–L was
very stable and difficult to be damaged by other chelators.
To investigate the selectivity of the Schiff base, fluorescence titration experiments were
performed (figure 4a). In the experiment, two equiv. of nitrate salts of Hg2+, Na+, K+, Li+,
Ca2+, Cd2+, Co2+, Ni2+, Fe3+, Mn2+, Mg2+, Pb2+, Cu2+, Cr3+, Ba2+, and Zn2+ ions were
added to L in ethanol solution, and we found that the fluorescence intensities change sub-
tly, showing that the ligand exhibited excellent fluorescence selectivity towards Al3+ over
other alkali and alkaline earth metal ions, transition, and heavy metal ions. Competition
experiments of Al3+ with other metal ions were also performed. As shown in figure 4b,

Chemodosimeter for aluminum(III)
1851
Figure 3. Fluorescence responses of L (10 μM) upon addition of 20 equiv (200 μM) of Al(NO3)3 in ethanol.
Chelators (STDD, SC, and tetrasodium EDTA) were added to the Al3+–L mixture to investigate reversible
binding of Al3+ with L.
Figure 4. (a) Fluorescence spectra of L (10 μM) with addition of interfering metal ions (20 μM) in ethanol;
(b) Fluorescence responses of L (10 μM) to various metal ions (20 μM) in ethanol. Black bars represent the
addition of metal ions written below the bars to a 10 μM solution of L. Red bars represent emission intensity of a
mixture of L (10 μM) with the metal ions written below the bars (20 μM) following addition of 20 μM Al3+ to the
solutions.
when Al3+ ions were added into solution of the interfering ion (Hg2+, Na+, K+, Li+, Ca2+,
Cd2+, Co2+, Ni2+, Fe3+, Mn2+, Mg2+, Pb2+, Cu2+, Cr3+, Ba2+, or Zn2+) and L, the fluores-
cence intensity increased greatly, indicating that Al3+ can bind to L strongly and the Schiff
base ligand can detect Al3+ ions in the presence of other interfering ions.
According to the above discussion, L possess efficient probe properties due to the
emission of the exciplex formed between excited acetophenone and lone pair electrons on

1852
X.-Y. Cheng et al.
Figure 5. Possible coordination of Al3+–L and the mechanism of fluorescence enhancement for L chelating with
Al3+
.
N from –C=N group [24]. The fluorescence band at 452 nm with low intensity is due to
quenching by the –OH receptor through a photoinduced electron transfer (PET), induced
by a lone pair electrons from the Schiff base and quinoline nitrogens [25]. After addition
of Al3+, the fluorescence intensity increased dramatically, attributed to decrease of the
PET effect and the highly efficient chelation-enhanced fluorescence effect [26, 27].
The conceptual architecture and mechanism for fluorescence enhancement of L upon the
addition of Al3+ are shown in figure 5.
4. Conclusion
We have synthesized and characterized a Schiff base ligand. The Schiff base showed high
selectivity for Al3+ over other metal ions with 60-, 53-, and 56-fold enhancement at 452,
396, and 419 nm in ethanol. The presence of interfering metal ions had little influence on
the binding of Al3+. These properties suggest that L could be an excellent chemodosimeter
for Al3+ ions in ethanol solution.
Supplementary data
Supplementary material (some supporting figures) associated with this article can be found
in the online version.
Acknowledgment
This work is supported by the National Natural Science Foundation of China (20975046,
81171337 and J1030416).
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