Naphthalene-derived Al3+-selective fluorescent chemosensor based on PET and ESIPT in aqueous solution

Article abstract of DOI:10.1016/j.tetlet.2017.10.044

A simple fluorescent chemosensor HL based on naphthalene with high selectivity and sensitivity towards Al³⁺ over other commonly coexisting metal cations in fully aqueous solution to enhance the potential applications of the fluorescent chemosensor was developed. HL exhibited a significant fluorescence enhancement at 475 nm in the presence of Al³⁺ over other competitive metal ions with a low detection limit of 0.43 μM due to the inhibition of the photo induced electron transfer (PET) and the excited-state intramolecular proton transfer (ESIPT). The 1:1 binding stoichiometry between HL and Al³⁺ was corroborated by the Job plot and the ESI-MS spectrum. Importantly, the reversible recognition process of HL to Al³⁺ will make HL could be used circularly and repeatedly in practical applications by addition of Na₂EDTA. In addition, the binding behavior and sensing mechanism of HL to Al³⁺ were illustrated in detail by the ¹H NMR titration experiment.

Full text of DOI:10.1016/j.tetlet.2017.10.044

Accepted Manuscript
Naphthalene-derived Al3+-selective fluorescent chemosensor based on PET and
ESIPT in aqueous solution
Xiao-li Yue, Zhao-qing Wang, Chao-rui Li, Zheng-yin Yang
PII:
S0040-4039(17)31336-9
https://doi.org/10.1016/j.tetlet.2017.10.044
TETL 49406
DOI:
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
5 August 2017
15 October 2017
17 October 2017
Please cite this article as: Yue, X-l., Wang, Z-q., Li, C-r., Yang, Z-y., Naphthalene-derived Al3+-selective fluorescent
chemosensor based on PET and ESIPT in aqueous solution, Tetrahedron Letters (2017), doi: https://doi.org/10.1016/
j.tetlet.2017.10.044
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Naphthalene-derived Al3+-selective fluorescent chemosensor
based on PET and ESIPT in aqueous solution
Xiao-li Yue, Zhao-qing Wang, Chao-rui Li, Zheng-yin Yang*
College of Chemistry and Chemical Engineering, State Key Laboratory of Applied
Organic Chemistry, Lanzhou University, Lanzhou 730000, P.R. China
**Corresponding author. Tel: +86 931 8913515; Fax: +86 931 812582; e-mail:
yangzy@lzu.edu.cn (Z.Y. Yang)
ABSTRACT
A simple fluorescent chemosensor HL based on naphthalene with high selectivity and
sensitivity towards Al3+ over other commonly coexisting metal cations in fully
aqueous solution to enhance the potential applications of the fluorescent chemosensor
was developed. HL exhibited a significant fluorescence enhancement at 475 nm in the
presence of Al3+ over other competitive metal ions with a low detection limit of 0.43
ꢀM due to the inhibition of the photo induced electron transfer (PET) and the
excited-state intramolecular proton transfer (ESIPT). The 1∶1 binding stoichiometry
between HL and Al3+ was corroborated by the Job plot and the ESI-MS spectrum.
Importantly, the reversible recognition process of HL to Al3+ will make HL could be
used circularly and repeatedly in practical applications by addition of Na2EDTA. In

addition, the binding behavior and sensing mechanism of HL to Al3+ were illustrated
in detail by the 1H NMR titration experiment.
KEYWORDS: Fluorescent chemosensor; Naphthalene; Photo-induced electron
transfer; Excited-state intramolecular proton transfer.
1. Introduction
The design and synthesis of fluorescent chemosensors for Al3+ continues to be a
significant research subject. On the one hand, Al3+ has its impact on the environment
and human health. On the other hand, fluorescent techniques show simplicity, high
selectivity and sensitivity, on-site and real time monitoring, as well as low detection
limit [1-6] compared with traditional analytical methods for detecting Al3+ such as ion
selective membrane, atomic absorption spectrometry, voltammetry, inductively
coupled plasma mass spectrometry (ICP-MS), and liquid chromatography mass
spectrometry [7]. As the most prevalent metallic element (8.3% by weight) and the
third most abundant element (after oxygen and silicon) in the earth’s crust [8-10],
aluminum is widely used in modern society involving industrial fields and daily life.
In industrial fields, aluminum is diffusely used in water treatment, paper industry, dye
production, textile industry, cosmetic preparations, production of light alloy, as well
as manufacturing of cars and computers [11-16]. Additionally, in daily life, aluminum
is far and wide used in food additions, food packaging, aluminum-based
pharmaceuticals, storage/cooking utensils, and electrical equipment [17-24]. However,
aluminum in excessive amounts would lead to environmental contamination and be
toxic to human health [25,26]. Particularly, excess aluminum in the human body

interferes with the calcium metabolism, causing Osteomalacia, influences the
ingestion of iron in blood, causing microcytic hypochromic anemia, and also
decreases liver and kidney function. Moreover, excess accumulation of Al3+ leads to
malfunction of the central nervous system, which cause human illnesses such as
encephalopathy, myopathy, dementia, Guamanian amyotrophic lateral sclerosis,
Parkinson’s disease, and Alzheimer’s disease [27-33]. Therefore, it’s essential and
crucial to design and synthesize some fluorescent chemosensors for detecting Al3+ in
the environmental and biological systems.
The development of Al3+ fluorescent probe is rather slow compared with other
common transition metal ions due to the lack of spectroscopic characteristics and poor
coordination ability [34]. Recently, many various fluorescent chemosensors for
detecting Al3+ have been reported. However, many of them have suffered from many
drawbacks such as synthesis based on single mechanism causing poor selectivity and
sensitivity [35], tedious synthetic route [36], poor water solubility [36], and
fluorogenic response in organic solvents making them difficult for practical
applications [17]. Thus, it’s highly desirable to develop some fluorescent
chemosensors based on multi-mechanism for the detection of Al3+ in aqueous solution.
Owing to the short fluorescence life time [34], the low fluorescence quantum yield
[34], and the ability to act both as a donor as well as an acceptor [37], Naphthalene
and its derivatives have been chosen as ideal components of some fluorescent
chemosensors.
Hence, taking these factors into account, we have designed and synthesized a

simple fluorescent chemosensor (HL) based on PET (photo induced electron transfer)
[34] and ESIPT (excited-state intramolecular proton transfer) [38] for the detection of
Al3+ in aqueous media, which enhanced the practical application of the chemosensor.
The structure of HL was validated by 1H NMR spectrum and the ESI mass spectrum.
The 1∶1 binding stoichiometry between HL and Al3+ was confirmed by the Job’s
plot titration curve and the mass spectrum data. In addition, the recognition process of
HL towards Al3+ was chemically reversible in the presence of Na2EDTA with a quite
low detection limit (0.43 ꢀM) of a micromolar concentration level.
2. Experimental section
2.1. Materials and instruments
The materials used for this study were obtained from commercial suppliers and
used without further purification. 1H NMR spectrum were measured on the JNM-ECS
400 MHz spectrometer. Chemical shifts are reported in ppm using TMS as an internal
standard. ESI-MS were determined on a Bruker esquire 6000 spectrometer. UV–vis
absorption spectrum were measured with a Shimadzu UV-240 spectrophotometer.
Fluorescence spectrum were determined on a Hitachi RF-4500 spectrophotometer
equipped with quartz cuvettes of 1 cm path length. The melting point was determined
on a Beijing XT4-100x microscopic melting point apparatus.
2.2. Synthesis
The synthetic route of HL (2-hydroxy-1-naphthaldehyde-(4-pyridinecarboxylic)-
hydrazone) was shown in Scheme 1.

Scheme 1. The synthetic route of 2-hydroxy-1-naphthaldehyde-(4-pyridinecarboxylic
) hydrazone (HL). Reagents and conditions: (a) HMTA, glacial acetic acid, reflux, 12
h; (b) EtOH, reflux, 10 h.
2.2.1. Synthesis of compound 1 (2-hydroxy-1-naphthaldehyde) [39]
A 10.5 g of 2-naphthol was added to a solution of a 12 g of
Hexamethylenetetramine (HMTA) in 20 ml of acetic acid and then the reaction
solution was stirred and heated for 1 h at 50-60 °C. After rising the temperature to
90 °C, H2SO4 (98%, 10 ml) was added dropwise to the above solution within 40 min.
Upon adding completely, the temperature of the system was raised to 96 °C rapidly
and then the reaction solution was refluxed for 10 h with stirring at 96 °C. After
cooling to room temperature, 100 ml of ice-water mixture was added to precipitate the
crude product. The crude product was collected and washed with cold water until the
pH of the filtrate was neutral. The final product was recrystallized from ethanol and
dried in vacuum at low temperature to obtain a yellow solid. Yield: 76%; mp:
1
79-80 °C. H NMR (400 MHz, CDCl3, TMS) (Figure S1): ꢁH ppm 13.16 (s, 1H),
10.81 (s, 1H), 8.34 (d, 1H, J=8.8Hz), 7.98 (d, 1H, J=9.2Hz), 7.80 (d, 1H, J=8Hz), 7.62

(t, 1H, J=8Hz), 7.44 (t, 1H, J=8Hz), 7.14 (d, 1H, J=9.2Hz).
2.2.2. Synthesis of 2-hydroxy-1-naphthaldehyde-(4-pyridinecarboxylic)hydrazon-
e (HL)
2-hydroxy-1-naphthaldehyde (0.69 g) and 4-pyridinecarboxylic hydrazide (0.55
g) were mixed in 20 ml ethanol and refluxed for 10 h under N2. After cooling to room
temperature, the precipitate was collected and washed 3 times with ethanol. Then the
final product was recrystallized from ethanol and dried in vacuum to give a yellow
solid. Yield: 79%; mp: 257 °C. 1H NMR (400 MHz, DMSO-d6, TMS) (Figure S2): ꢁH
ppm 12.49 (s, 1H), 12.37 (s, 1H), 9.44 (s, 1H), 8.80 (d, 1H, J=2.0Hz), 8.78 (d, 1H,
J=2.0Hz), 8.28 (d, 1H, J=8.0Hz), 7.92 (d, 1H, J=8.0Hz), 7.88 (s, 1H), 7.86 (d, 1H,
J=2.0Hz), 7.84 (d, 1H, J=2.0Hz), 7.58 (m, 1H), 7.38 (t, 1H, J=3.6Hz), 7.21 (d, 1H,
J=8.0Hz). 13C NMR (400 MHz, DMSO-d6, TMS) (Figure S3): ꢁC ppm 109.05, 119.36,
121.48, 121.97, 124.15, 128.39, 128.44, 129.53, 132.13, 133.66, 140.32, 148.51,
151.02, 158.72, 161.55 (C=O). ESI-MS (Figure S4) calculated for [M＋H]+ 292.3166,
found 292.1458.
2.3. UV-vis and fluorescence spectrum measurements
Stock solutions of 5×10-3 M various metal ions and HL were prepared in ethanol.
Additionally, the stock solution of 5×10-3 M Na2EDTA was prepared in distilled water.
All absorption and emission spectrum were performed in a quartz optical cell of 1 cm
optical path length at room temperature. All fluorescence measurements were carried
out upon excitation at 415 nm. Both excitation and emission slit widths were 3 nm.
3. Results and discussion

To find out the effect of solvent in the fluorogenic response of HL towards Al3+, we
recorded the emission changes in different solvents (Figure S5). The fluorescence
intensity of HL enhanced about 500-fold accompanied with the split of the
fluorescence emission spectrum upon addition of Al3+ in methanol. The fluorescence
intensity of HL strengthened approximately 400-fold and the phenomenon of the split
still existed after the addition of Al3+ in ethanol. Furthermore, the fluorescence
intensity of HL had different extent of enhancement and the phenomenon of the split
still existed upon addition of Al3+ in other solvents. It was notable that the
fluorescence intensity of HL enhanced about 550-fold and those bands just merge
together due to increased inhomogeneity after the addition of Al3+ in water. Thus,
further UV/vis and fluorescent studies were carried out in aqueous solution.
3.1．UV-vis studies of HL towards Al3+
The UV/vis spectrum of HL towards various metal ions (Na+, K+, Ag+, Ca2+,
Mg2+, Ba2+, Zn2+, Cu2+, Ni2+, Cd2+, Co2+, Pb2+, Mn2+, Hg2+, Cr3+, Fe3+, and Al3+) was
illustrated in Figure 1a. In the absence of Al3+, HL exhibited characteristic absorption
bands at 325 nm and 366 nm which should be assigned to ꢂꢃꢂ* transitions of
naphthalene ring [40-42]. Upon addition of Al3+ to HL solution, a new absorption
peak at 432 nm appeared, while other metal ions showed no absorption peak at 432
nm under the identical conditions. Thus, HL could serve as a highly selective
fluorescent probe for Al3+.
The absorption titrations of HL towards Al3+ were carried out in aqueous
solution (Figure 1b). Upon gradual addition of Al3+ to HL solution, the absorption

bands at 325 nm and 366 nm gradually decreased while the new absorption band at
432 nm appeared with increasing intensity. Meanwhile, an isobestic point at 400 nm
was observed. These results indicated that the formation of a new stable complex
between HL and Al3+. Moreover, the absorbance spectrum of HL exhibited no
remarkable changes and the intensity of absorption remained constant more than 1
equiv. of Al3+, demonstrating the 1 1 binding stoichiometry between HL and Al3+.
Figure 1. (a) UV-vis spectrum of HL before and after of 1 equiv. of various metal
cations in aqueous solution. (b) UV-vis absorption titration spectrum of HL (50 ꢀM)
with Al3+ (0-1.5 equiv.) in aqueous solution. Inset: Changes of absorbance intensity at
432 nm.
3.2. Fluorescence studies of HL towards Al3+
As we all know, an ability of achieving highly selective fluorogenic response to
the target analyte over other competitive species is an important characterization for a
chemosensor. The optical properties of HL towards various metal ions were
investigated by fluorescence emission spectrum in aqueous solution (Figure 2a). HL
exhibited no fluorescence emission at 475 nm with a low fluorescence quantum yield

(ꢄ=0.5%) upon excitation at 415 nm. Upon addition of 1 equiv. of different metal ions
(Na+, K+, Ag+, Ca2+, Mg2+, Ba2+, Zn2+, Cu2+,Ni2+, Cd2+, Co2+, Pb2+, Mn2+, Hg2+, Cr3+,
Fe3+, and Al3+) to the system of HL, only Al3+ could cause a significant fluorescence
enhancement at 475 nm with a high fluorescence quantum yield (ꢄ=26.4%),
indicating that HL was a highly selective fluorescent probe for Al3+.
Figure 2. (a) The fluorescence emission spectrum of HL (50 ꢀM) upon addition of 1
equiv. of various metal ions (Na+, K+, Ag+, Ca2+, Mg2+, Ba2+, Zn2+, Cu2+,Ni2+, Cd2+,
Co2+, Pb2+, Mn2+, Hg2+, Cr3+, Fe3+, and Al3+) in aqueous solution (ꢅex=415 nm;
ꢅem=475 nm). (b) The fluorescence emission spectrum of HL (50 ꢀM) upon addition
of Al3+ (0-1.5 equiv.) in aqueous solution (ꢅex=415 nm). Inset: Changes of
fluorescence emission intensity at 475 nm.
The optical properties of HL towards Al3+ were also studied by fluorescence
emission spectrum in aqueous solution (Figure 2b). The fluorescence intensity of HL
gradually increased with the progressive addition of Al3+. Furthermore, further
increasing concentrations of Al3+ more than 1 equiv, the fluorescence intensity of HL
remained steady, which also suggested that 1 1 binding model between HL and

Al3+.
3.3. Interference studies from other metal ions.
To evaluate the ability of resisting interference from other metal ions, we
recorded the fluorescence emission changes of HL to Al3+ in the presence of other
coexistent metal ions (Na+, K+, Ag+, Ca2+, Mg2+, Ba2+, Zn2+, Cu2+,Ni2+, Cd2+, Co2+,
Pb2+, Mn2+, Hg2+, Cr3+, Fe3+, and Al3+) (Figure 3). It could be seen from Figure 3 that
the competitive metal ions did not interfere with the detection of HL to Al3+ at the
same conditions, which may due to the formation of a stable complex of HL with Al3+
compared with other competitive metal ions. The experimental results illustrated that
HL still exhibited high selectivity for Al3+ over other competitive metal ions.
Figure 3. The fluorescence intensity at 475 nm of HL (50 ꢀM) upon addition of
various metal ions (black bars: HL with other metal ions; red bars: HL with other
metal ions and Al3+) in aqueous solution (ꢅex=415 nm).
3.4. Reversible test of HL towards Al3+ by Na2EDTA
To estimate the practical applicability of HL as a selective fluorescent probe for
Al3+, the reversible experiment was carried out in aqueous solution (Figure 4). It was
notable that HL showed significant fluorescence enhancement at 475 nm in the

presence of 1 equiv. of Al3+, while the fluorescence intensity at 475 nm decreased
remarkably upon addition of 1 equiv. of Na2EDTA due to the weak coordination
ability of HL to Al3+ compared with that of Na2EDTA to Al3+. What’s more, HL still
exhibited a good fluorescence response towards Al3+ at least 5 cycles with small
changes in fluorescence intensity at 475 nm when we added alternately Al3+ ion
solution (5 × 10-3 M) and aqueous Na2EDTA solution (5 × 10-3 M). The reversible test
results demonstrated that the recognition process of HL to Al3+ was chemically
reversible. Thus, HL could serve as a highly selective reversible fluorescent probe for
detection and recognition of Al3+ in both biological assays and the environmental
systems.
Figure 4. (a) The reversible experiment of HL towards Al3+ by adding Na2EDTA in
aqueous solution (ꢅex=415 nm). (b) The fluorogenic response of HL to Al3+ in the 5 ×
10-3 M of Na2EDTA over five complex/stripping cycles (ꢅex=415 nm).
3.5. Binding constant, stoichiometry and detection limit
The association constant (Ka) of HL to Al3+ was determined by the Benesi-Hild-
ͥ
ͥ
ͥ
ebrand equation:
＝
＋
where F is the fluorescence
ʞ
3+ʟ
ͿƳͺmax－ͺminƷ ͵Κ
ͺmaxͯͺmin
ͺ－ͺmin

intensity at 475 nm at any given Al3+ concentration, Fmin is the fluorescence intensity
at 475 nm in the absence of Al3+, and Fmax is the maximal fluorescence intensity at
475 nm in the presence of Al3+ [43]. The Ka value was calculated to be 5.17 × 104
M-1 according to the plotting 1/ (F-Fmin) against 1/ [Al3+] (Figure S6). The detection
limit (LOD) of HL to Al3+ was calculated to be 0.43 ꢀM according to the equation:
DL＝3ꢆ/K where ꢆ is the standard deviation of the blank solution and K is the slope
of the calibration curve (Figure S8) from the fluorescence titration experiments
(Figure S7) [43]. Additionally, the binding stoichiometry of HL to Al3+ was
determined by the Job’s method on the basis of fluorescence emission spectrum. It
could be seen from Figure 5 that the fluorescence intensity at 475 nm exhibited a
maximum when the molar fraction of HL was 0.5 demonstrating a possible 1
1
binding stoichiometry between HL and Al3+. To better understand the coordination
mode of HL to Al3+, the ESI mass spectrum of HL in the presence of Al3+ was
carried out (Figure S9). The m/z peak for HL in the presence of Al3+ at 380.1554
appeared, which was assigned to [HL＋Al3+＋H2O＋CH3CH2OH－2H+]+. The mass
spectrum analysis gave another solid evidence for the 1 1 binding model between
HL and Al3+.

Figure 5. Job’s plot for determining the binding stoichiometry of HL and Al3+ in
aqueous solution. The total concentration of HL and Al3+ was kept 100 ꢀM (ꢅex=415
nm).
3.6. Proposed sensing mechanism of HL towards Al3+
It is well known that the proton chemical shift of coordination atom of ligands will
change when metal ion coordinates with ligands. To better understand the binding
1
behavior and sensing mechanism of HL to Al3+, the H NMR titration experiment of
HL was performed in the presence of 1 equiv. of Al3+ (Figure 6). It was prominent
that the proton signals of Ha (-OH) and Hb (-NH) disappeared upon addition of Al3+.
Moreover, the proton signal of Hc (-CH=N) shifted downfield from ꢁ 9.44 ppm to ꢁ

1
Figure 6. The H NMR spectrum of HL with and without Al3+ in DMSO-d6 (400
MHz): HL (2); HL with 1 equiv of Al3+ (1). Inset: The image of the structure of HL.
9.51 ppm in the presence of Al3+. The above findings demonstrated that the probable
binding mode (Scheme 2) was that the oxygen atoms of the hydroxyl group and the
carbonyl group as well as the nitrogen atom of the imine group coordinated with Al3+.
Furthermore, the proposed sensing mechanism of HL to Al3+ was illustrated in
Scheme 2 according to the above results. In the absence of Al3+, the nitrogen atom of
imine could transfer an electron to the naphthalene ring (PET ON) and HL could
transfer a hydroxyl proton to a neighboring imine nitrogen along with the formation
of intramolecular hydrogen bond (OH ┈ N) (ESIPT), which resulted in no
fluorescence emission at 475 nm [44]. Upon addition of Al3+ to the system of HL, the
oxygen atoms of hydroxyl group and the carbonyl group as well as the nitrogen atom
of the imine group participated in the coordination of HL with Al3+, which inhibited

the process of PET and ESIPT accompanied with remarkable fluorescence
enhancement at 475 nm.
Scheme 2. The proposed sensing mechanism of HL with Al3+.
4. Conclusion
In summary, we have designed and synthesized a simple naphthalene-based
fluorescent probe HL which exhibited high selectivity for Al3+ in the aqueous solution.
Compared with other fluorescent probes for Al3+ previously reported, HL showed a
specific fluorogenic response towards Al3+ based on multi-mechanism (PET and
ESIPT) in the fully aqueous solution, which enhanced potential practical application
of HL for the detection of Al3+ in both biological assays and the environmental
systems. What’s more exciting, the sensing process of HL to Al3+ was chemically
reversible by adding Na2EDTA. The reversible phenomenon indicated that HL could
be used circularly and repeatedly for the detection of Al3+.

Acknowledgments
This work is supported by the National Natural Science Foundation of China
(81171337). Gansu NSF (1308RJZA115).
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List of Figure captions
Figure 1. (a) UV-vis spectrum of HL before and after of 1 equiv. of various metal
cations in aqueous solution. (b) UV-vis absorption titration spectrum of HL (50 ꢀM)
with Al3+ (0-1.5 equiv.) in aqueous solution. Inset: Changes of absorbance intensity at
432 nm.
Figure 2. (a) The fluorescence emission spectrum of HL (50 ꢀM) upon addition of 1
equiv. of various metal ions (Na+, K+, Ag+, Ca2+, Mg2+, Ba2+, Zn2+, Cu2+,Ni2+, Cd2+,
Co2+, Pb2+, Mn2+, Hg2+, Cr3+, Fe3+, and Al3+) in aqueous solution (ꢅex=415 nm;
ꢅem=475 nm). (b) The fluorescence emission spectrum of HL (50 ꢀM) upon addition
of Al3+ (0-1 equiv.) in aqueous solution (ꢅex=415 nm) Inset: Changes of fluorescence
emission intensity at 475 nm.
Figure 3. The fluorescence intensity at 475 nm of HL (50 ꢀM) upon addition of
various metal ions (black bars: HL with other metal ions; red bars: HL with other
metal ions and Al3+) in aqueous solution (ꢅex=415 nm).
Figure 4. (a) The reversible experiment of HL towards Al3+ by adding Na2EDTA in
aqueous solution (ꢅex=415 nm). (b) The fluorogenic response of HL to Al3+ in the 5 ×
10-3 M of Na2EDTA over five complex/stripping cycles (ꢅex=415 nm).
Figure 5. Job’s plot for determining the binding stoichiometry of HL and Al3+ in
aqueous solution. The total concentration of HL and Al3+ was kept 100 ꢀM (ꢅex=415
nm).
1
Figure 6. The H NMR spectrum of HL with and without Al3+ in DMSO-d6 (400

MHz): HL (2); HL with 1 equiv of Al3+ (1). Inset: The image of the structure of HL.
Scheme 1. The synthetic route of 2-hydroxy-1-naphthaldehyde-(4-pyridinecarboxylic
) hydrazone (HL). Reagents and conditions: (a) HMTA, glacial acetic acid, reflux, 12
h; (b) EtOH, reflux, 10 h.
Scheme 2. The proposed sensing mechanism of HL with Al3+.
Figure 1. (a) UV-vis spectrum of HL before and after of 1 equiv. of various metal
cations in aqueous solution. (b) UV-vis absorption titration spectrum of HL (50 ꢀM)
with Al3+ (0-1.5 equiv.) in aqueous solution. Inset: Changes of absorbance intensity at
432 nm.

Figure 2. (a) The fluorescence emission spectrum of HL (50 ꢀM) upon addition of 1
equiv. of various metal ions (Na+, K+, Ag+, Ca2+, Mg2+, Ba2+, Zn2+, Cu2+,Ni2+, Cd2+,
Co2+, Pb2+, Mn2+, Hg2+, Cr3+, Fe3+, and Al3+) in aqueous solution (ꢅex=415 nm;
ꢅem=475 nm). (b) The fluorescence emission spectrum of HL (50 ꢀM) upon addition
of Al3+ (0-1 equiv.) in aqueous solution (ꢅex=415 nm) Inset: Changes of fluorescence
emission intensity at 475 nm.
Figure 3. The fluorescence intensity at 475 nm of HL (50 ꢀM) upon addition of
various metal ions (black bars: HL with other metal ions; red bars: HL with other
metal ions and Al3+) in aqueous solution (ꢅex=415 nm).

Figure 4. (a) The reversible experiment of HL towards Al3+ by adding Na2EDTA in
aqueous solution (ꢅex=415 nm). (b) The fluorogenic response of HL to Al3+ in the 5 ×
10-3 M of Na2EDTA over five complex/stripping cycles (ꢅex=415 nm).
Figure 5. Job’s plot for determining the binding stoichiometry of HL and Al3+ in
aqueous solution. The total concentration of HL and Al3+ was kept 100 ꢀM (ꢅex=415
nm).

1
Figure 6. The H NMR spectrum of HL with and without Al3+ in DMSO-d6 (400
MHz): HL (2); HL with 1 equiv of Al3+ (1). Inset: The image of the structure of HL.
Scheme 1. The synthetic route of 2-hydroxy-1-naphthaldehyde-(4-pyridinecarboxylic
) hydrazone (HL). Reagents and conditions: (a) HMTA, glacial acetic acid, reflux, 12
h; (b) EtOH, reflux, 10 h.

Scheme 2. The proposed sensing mechanism of HL with Al3+.

Graphical abstract

Highlights
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ꢀ
ꢀ
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A simple naphthalene-derived Al3+-selective fluorescent chemosensor was
designed and synthesized.
The binding stoichiometry between HL and Al3+ was determined to be 1 :1 by the
Job’s plot and ESI-MS spectrum data.
The recognition process of HL towards Al3+ was chemically reversible by adding
Na2EDTA.
HL exhibited a specific fluorogenic response towards Al3+ based on
multi-mechanism (PET and ESIPT) in fully aqueous solution.
1
The proposed sensing mechanism was investigated by the H NMR titration
experiment in detail.