A Chromone-Derived Schiff-Base Ligand as Al3+ "turn on" Fluorescent Sensor: Synthesis and Spectroscopic Properties

Article abstract of DOI:10.1007/s10895-015-1720-0

In this study, a novel chromone-derived Schiff-base ligand called 6-Hydroxy-3-formylchromone (2′-furan formyl) hydrazone (HCFH) has been designed and synthesized as a "turn on" fluorescent sensor for Al³⁺. This sensor HCFH showed high selectivity and sensitivity towards Al³⁺ over other metal ions investigated, and most metal ions had nearly no influences on the fluorescence response of HCFH to Al³⁺. Additionally, the significant enhancement by about 171-fold in fluorescence emission intensity at 502 nm was observed in the presence of Al³⁺ in ethanol, and it was due to the chelation-enhanced fluorescence (CHEF) effect upon complexation of HCFH with Al³⁺ which inhibited the photoinduced electron transfer (PET) phenomenon from the Schiff-base nitrogen atom to chromone group. Moreover, this sensor formed a 1: 1 complex with Al³⁺ and the fluorescence response of HCFH to Al³⁺ was nearly completed within 1 min. Thus, this sensor HCFH could be used to detect and recognize Al³⁺ for real-time detection.

Full text of DOI:10.1007/s10895-015-1720-0

J Fluoresc (2016) 26:345–353
DOI 10.1007/s10895-015-1720-0
ORIGINAL ARTICLE
A Chromone-Derived Schiff-Base Ligand as Al3+ BTurn on^
Fluorescent Sensor: Synthesis and Spectroscopic Properties
Chao-rui Li1 & Jing-can Qin1 & Bao-dui Wang1 & Long Fan1 & Jun Yan1 & Zheng-yin Yang1
Received: 25 September 2015 /Accepted: 28 October 2015 /Published online: 6 November 2015
#
Springer Science+Business Media New York 2015
Abstract In this study, a novel chromone-derived Schiff-base
ligand called 6-Hydroxy-3-formylchromone (2′-furan formyl)
hydrazone (HCFH) has been designed and synthesized as a
Bturn on^ fluorescent sensor for Al3+. This sensor HCFH
showed high selectivity and sensitivity towards Al3+ over oth-
er metal ions investigated, and most metal ions had nearly no
influences on the fluorescence response of HCFH to Al3+.
Additionally, the significant enhancement by about 171-fold
in fluorescence emission intensity at 502 nm was observed in
the presence of Al3+ in ethanol, and it was due to the
chelation-enhanced fluorescence (CHEF) effect upon com-
plexation of HCFH with Al3+ which inhibited the photoin-
duced electron transfer (PET) phenomenon from the
Schiff-base nitrogen atom to chromone group. Moreover, this
sensor formed a 1 : 1 complex with Al3+ and the fluorescence
response of HCFH to Al3+ was nearly completed within
1 min. Thus, this sensor HCFH could be used to detect and
recognize Al3+ for real-time detection.
.
.
.
Keywords Chromone Fluorescent sensor Aluminum ion
.
.
Real-time detection PET Chef
Introduction
Fluorescent sensors for detecting and monitoring environmen-
tally and biologically important metal ions have found wide-
spread use in many fields, such as analytical chemistry, bio-
chemistry, medical science and environmental biology [1–5].
Among all the metals in the environment, aluminum is the
richest metal element and the third abundant element in the
earth’s crust [6, 7]. Although aluminum is not an essential
element for an organism’s survival and cell proliferation in
human body, it is generally used in our daily life like water
purification [8], food additives [9], clinical drugs [10] and
light alloys [11], which makes aluminum and its ionic state
widely distributed in the air, water and soil [12]. Thus,
Accumulation of Al3+ in human body can originate from food
chain [13]. However, when it is exposed to large amount of
Al3+, the neurogenic system and immune system can be dam-
aged [14, 15], leading to some neurodegenerative diseases,
such as Alzheimer’s disease [16], Parkinson’s disease [17]
and encephalopathy [18]. In addition, high concentration of
Al3+ in environment can also cause harm to plants and aquatic
fishes [19, 20]. Therefore, it is always a matter of great impor-
tance to establish efficient detecting and monitoring methods
with high sensitivity for the detection of Al3+ in environmental
and biological systems [21].
Highlights • A novel chromone-derived Schiff-base ligand HCFH was
designed and synthesized.
• High selectivity and sensitivity of HCFH towards Al3+ was studied and
explained.
• BTurn on^ fluorescence response of HCFH to Al3+ for real-time detec-
tion was observed.
• Binding stoichiometry between HCFH and Al3+ was determined
systematically.
Electronic supplementary material The online version of this article
(doi:10.1007/s10895-015-1720-0) contains supplementary material,
which is available to authorized users.
* Zheng-yin Yang
yangzy@lzu.edu.cn
Owing to its relevant advantages like good selectivity, high
sensitivity, real-time detection and non-invasive properties,
fluorescence detection has attracted a great deal of attention
1
College of Chemistry and Chemical Engineering, State Key
Laboratory of Applied Organic Chemistry, Lanzhou University,
Lanzhou 730000, People’s Republic of China

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J Fluoresc (2016) 26:345–353
among scientists [22–24]. Some fluorescent sensors for de-
tecting Al3+ with high selectivity over other heavy and
transition metal ions (HTM) have been reported in re-
cent years [25–27]. Nevertheless, the poorer coordina-
tion ability and stronger hydration ability of Al3+ com-
pared to transition metal ions like Zn2+, Cu2+, Fe3+ and
Hg2+ make it difficult to design and develop Al3+ sen-
sors [28]. Furthermore, some fluorescent sensors for
Al3+ suffer from disadvantages like laborious organic
synthesis, complicated structure and low sensitivity, which
restrict their applications in environmental monitoring and
biological assays [29, 30]. Thus, it is a great challenge to
design and synthesis Al3+ selective and sensitive fluorescent
sensors with simple structures [31, 32].
data were obtained in ethanol from a Bruke Esquire 6000
spectrometer. UV–vis absorption spectra were recorded
on a Shimadzu UV-240 spectrophotometer. Fluorescence
emission spectra were generated on a Hitachi RF-5301
fluorimeter equipped with quartz cuvettes of 1 cm path
length at 298 K. Melting points were determined on a
Beijing XT4-100× microscopic melting point apparatus
without correction.
Synthesis
Hydroquinone diacetate (1) and acetylquinol (2) were synthe-
sized according to the method reported [35]. The synthetic
route of the compound HCFH is shown in Scheme 1.
Chromone derivatives have been extensively applied in
antitumoral agents, cardio cerebrovascular drugs and
fluorophores due to their excellent pharmacological and spec-
troscopic properties [33]. However, the reports about fluores-
cent sensors based on chromone to detect and sense metal ions
are relatively few [34]. Bearing these in mind, we have de-
signed and synthesized a novel chromone-derived Schiff-base
ligand called 6-Hydroxy-3-formylchromone (2′-furan formyl)
hydrazone (HCFH) (Scheme 1), which exhibited high selec-
tivity and sensitivity with Bturn on^ fluorescence response at
502 nm in the presence of Al3+. Hence, this compound HCFH
could serve as a fluorescent chemosensor to detect and recog-
nize Al3+ in ethanol.
Synthesis of Compound 3 (6-Hydroxy-3-Formylchromone)
Phosphorus oxychloride (10 mL, 16.45 g, 107 mmol) was
added dropwise to a solution of acetylquinol (1.52 g, 10 mmol)
in N,N-dimethyl formamide (DMF) (20 mL) under stiring in
an ice bath, and then placed overnight. The brown solution
was stirred and heated at 40 °C for 1 h, and was then cooled to
room temperature, subsequently pouring into ice slurry and
the reddish brown solid was precipitated from the solution.
After placing the reaction mixture into refrigerator for 5 h,
the reddish brown solid was filtered, washing five times with
distilled water (10 mL). The obtained crude product was re-
crystallized from DMF (25 mL) to give 3 as a gray solid
1
(Scheme 1). Yield: 1.33 g (70.00 %). m.p. 243–244 °C, H
Experimental
Materials
NMR (400 MHz, DMSO-d6) (Fig. S1): 10.24 (s, 1 H, H4),
10.07 (s, 1 H, −CHO), 8.82 (s, 1 H, −OH), 7.58 (d, 1 H,
J = 8.8 Hz, H3), 7.35 (d, 1 H, J = 2.8 Hz, H1), 7.24 (dd, 1 H,
J = 8.8 Hz, J = 2.8 Hz, H2).
Hydroquinone, acetic anhydride, aluminum chloride, phos-
phorus oxychloride, 2-furan formic acid, hydrogen peroxide,
concentrated sulfuric acid, hydrazine hydrate, absolute etha-
nol, N,N-dimethyl formamide (DMF), dimethyl sulfoxide
(DMSO) and salts of Al3+, Ba2+, Ca2+, Cd2+, Co2+, Cr3+,
Cu2+, Fe2+, Fe3+, K+, Mg2+, Mn2+, Na+, Ni2+, Pb2+ and Zn2+
were obtained from commercial suppliers, and used as
received without further purification. Stock solution of
compound HCFH (10 mM) was prepared in DMSO.
Stock solutions (10 mM) of the salts of Al3+, Ba2+,
Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Fe2+, Fe3+, K+, Mg2+,
Mn2+, Na+, Ni2+, Pb2+ and Zn2+ in absolute ethanol
were also prepared. Distilled water was used throughout
all experiments.
Synthesis of HCFH (6-Hydroxy-3-Formylchromone
(2′-Furan Formyl) Hydrazone)
Ethyl 2-furan formate (4) and 2-furan formylhydrazine (5)
were prepared by the reported method [36]. A solution of
2-furan formylhydrazine (0.630 g, 5 mmol) in absolute etha-
nol (10 mL) was added to a solution containing 6-Hydroxy-
3-formylchromone (0.950 g, 5 mmol) in absolute ethanol
(50 mL). The reaction mixture was stirred at room temperature
for 5 h, and yellow solid was precipitated from the solution.
Then the solid was filtered, washing five times with absolute
ethanol (10 mL). The obtained crude product was recrystal-
lized from absolute ethanol (30 mL) to afford the desired
product HCFH as an earth yellow powder (Scheme 1).
Yield: 0.28 g (18.79 %). m.p. 195–197 °C, 1H NMR
(400 MHz, DMSO-d6) (Fig. S2): 11.96 (s, 1 H, −NH−),
10.18 (s, 1 H, H4), 8.78 (s, 1 H, H7), 8.61 (s, 1 H, H5), 7.96
(s, 1 H, H6), 7.61 (d, 1 H, J = 8.8 Hz, H3), 7.39 (d, 1 H,
J = 2.8 Hz, H1), 7.33 (s, 1 H, −OH), 7.28 (dd, 1 H,
Apparatus
1H NMR spectra were measured on the JNM-ECS 400 MHz
instruments spectrometers using TMS (tetramethylsilane) as
an internal standard and DMSO-d6 as a solvent. The ESI-MS

J Fluoresc (2016) 26:345–353
347
Scheme 1 The synthetic route of
compound HCFH
J = 8.8 Hz, J = 2.8 Hz, H2), 6,71 (dd, 1 H, J = 3.2 Hz,
J = 1.6 Hz, −CH = N−). MS (ESI) (Fig. S3): m/z [M + H+]+
calcd 299.2447, found 299.1161; [M + Na+]+ calcd
321.2266, found 321.1001; [2 M + Na+]+ calcd 619.4634,
found 619.1924.
General Information
Test solutions were prepared by placing 10 μL of the probe
stock solution into cuvettes, adding an appropriate aliquot of
each metal ion stock, and diluting the solution to 2 mL with

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J Fluoresc (2016) 26:345–353
ethanol. For all fluorescence measurements of compound
HCFH, the fluorescence intensity of the test solution was
recorded at 502 nm, and the excitation wavelength was set at
428 nm. The excitation and emission slit widths were both
3 nm in fluorescence emission spectra of HCFH.
NMR and mass spectrometry. The details of the characteriza-
tion data of intermediate product and compound HCFH were
presented in the Supporting Information (Fig. S1-S3).
UV-vis Titration of Compound HCFH with Increasing
Concentrations of Al3+
The binding constant value for complex HCFH-Al3+ was
determined on the basis of the nonlinear filtting of the fluores-
cence titration curve assuming a 1 : 1 stoichiometry by the
Benesi–Hildebrand method (1) [37, 38]:
Firstly, we investigated the UV-vis titration spectra of com-
pound HCFH with increasing concentrations of Al3+ in etha-
nol. As shown in Fig. 1, compound HCFH in the absence of
any metal ion displayed relatively weak band centered at
208 nm, which was attributed to the absorption band of
chromone. However, the enhancement in this absorbance with
a slight red-shift to 215 nm was observed upon addition of 1.0
equiv. of Al3+. Furthermore, the absorbance at 215 nm reached
plateau in the presence of 1.0 equiv. of Al3+ and had almost no
change with further addition of Al3+ (Fig. 1), which indicated
that HCFH could bind Al3+ in a 1 : 1 ratio. From the results
above, it was concluded that the chromone unit in compound
HCFH participated in the coordination with Al3+.
1
1
1
Â
Ã
¼
þ
ð1Þ
KðFmax− FminÞ Al3þ
F−Fmin
Fmax−Fmin
Where Fmin, F, and Fmax are the emission intensities of the
organic moiety considered in the absence of aluminum ion, at
an intermediate aluminum concentration, and at a concentra-
tion of complete interaction, respectively, and where K is the
binding constant concentration.
The detection limit was estimated from the fluorescence
titration. The emission intensity of the complex HCFH-Al3+
without any anion was measured to determine the S/N ratios
[39, 40], and the standard deviation of blank measurements
was calculated. The detection limit was calculated based on
3 × σblank/k, where σblank is the standard deviation of the blank
solution and k is the slope of the calibration plot.
Time Responses of Compound HCFH to Al3+
Then the effect of reaction time on the fluorescence responses
of compound HCFH to Al3+ was studied by adding 1.0 equiv.
of Al3+ into the ethanol solution of HCFH at different time.
As can be seen from Fig. S4, it was shown that the fluores-
cence emission intensity at 502 nm enhanced remarkably in
1 min and increased less with more reaction time. These re-
sults suggested that this compound HCFH could be utilized as
a Al3+ fluorescent sensor for real-time detection.
Results and Discussion
Synthesis and Characterization of Compound HCFH
Compound HCFH was synthesized according to the synthetic
route outlined in Scheme 1. Hydroquinone diacetate (1),
acetylquinol (2), ethyl 2-furan formate (4) and 2-furan
formylhydrazine (5) were synthesized according to the report-
ed methods [35, 36]. Firstly, hydroquinone was reacted with
acetic anhydride under stiring at room temperature using con-
centrated sulfuric acid as a catalyst, and the obtained com-
pound called hydroquinone diacetate (1) was then changed
into acetylquinol (2) by a solid-phase reaction with anhydrous
aluminum chloride. Subsequently, the reaction between
acetylquinol (2) and phosphorus oxychloride in N,N-dimethyl
formamide (DMF) solution gave compound 3 (6-hydroxy-3-
formylchromone) as a gray solid. In another synthetic route,
ethyl 2-furan formate (4) was prepared by reacting 2-furan
formic acid with ethanol using concentrated sulfuric acid
and hydrogen peroxide as catalysts, then ethyl 2-furan formate
(4) was further reacted with hydrazine hydrate under refluxing
in absolute ethanol, and 2-furan formylhydrazine (5) was ob-
tained. Finally, the reaction between 6-hydroxy-3-
formylchromone (3) and 2-furan formylhydrazine (5) in abso-
lute ethanol afforded compound HCFH as an earth yellow
Selectivity of Compound HCFH for Al3+ over Other Metal
Ions
The change in fluorescence spectra of compound HCFH in
ethanol in the presence of various metal ions (Ba2+, Ca2+,
Cd2+, Co2+, Cr3+, Cu2+, Fe2+, Fe3+, K+, Mg2+, Mn2+, Na+,
Ni2+, Pb2+, Zn2+) was examined and the results were shown
in Fig. 2. With excited at 428 nm, the free HCFH exhibited
almost no fluorescence emission in the range of 440–690 nm.
Nevertheless, the addition of Al3+ and Mg2+ to the solution of
HCFH caused fluorescence enhancement in a certain degree,
but the fluorescence emission intensity at 502 nm of
HCFH-Al3+ was far higher than that of HCFH-Mg2+, this
was because of the similar ionic radius of Mg2+ with Al3+,
which induced the coordination environment of HCFH-Mg2+
similar with that of HCFH-Al3+. However, the addition of
Zn2+ caused little fluorescence enhancement and other metal
ions investigated induced nearly no change in the fluorescence
spectra of compound HCFH (Fig. 2). It was probably attrib-
uted to the Hard and Soft Acid and Base (HSAB) theory. As a
hard acid, Al3+ tends to bind with hard bases such as
1
powder. The structure of HCFH was characterized by H

J Fluoresc (2016) 26:345–353
349
compounds containing nitrogen or oxygen atom [41, 42]. This
compound HCFH was a Schiff-base ligand which provided
nitrogen-oxygen-rich coordination environments. Thus, this
compound HCFH could selectively bind with Al3+ and give
specific fluorescence response in the presence of Al3+.
In order to validate the practical application of compound
HCFH as a Al3+ selective fluorescent sensor, competition
experiments were conducted by adding Al3+ into the ethanol
solution of HCFH mixing with other coexisting metal ions
(Ba2+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Fe2+, Fe3+, K+, Mg2+,
Mn2+, Na+, Ni2+, Pb2+, Zn2+). As depicted in Fig. 3, the fluo-
rescence emission intensity at 502 nm decreased in some de-
gree in the presence of Cu2+, Fe2+ and Fe3+ but it was rela-
tively detectable, which was due to the magnetic property of
these three metal ions. However, upon addition of other metal
ions tested to the mixture of HCFH and Al3+ under identical
conditions, the fluorescence emission intensity at 502 nm did
not change (Fig. 3). Therefore, most metal ions had nearly no
influence on the fluorescence response of compound HCFH
to Al3+, and the selectivity was high for HCFH towards Al3+
in the presence of most of other metal ions.
Fig. 2 Fluorescence emission spectra of HCFH (50 μM) upon addition
of Al3+ (1 equiv.) and other metal ions (5 equiv.) in ethanol with an
excitation at 428 nm
nitrogen atom to chromone group which quenched the emis-
sion completely [43, 44]. However, the gradual enhancement in
fluorescence with an emission maximum at 502 nm was ob-
served with increasing concentrations of Al3+, and it was by
about 171-fold enhancement in the presence of 1.0 equiv. of
Al3+. This was because of the chelation-enhanced fluorescence
(CHEF) effect that was caused by the formation of a rigid
system upon complexation of HCFH with Al3+, which
inhibited the PET phenomenon and resulted in significant fluo-
rescence enhancement [45, 46] (Scheme 2). Interestingly, upon
addition of further Al3+ to the solution above, the fluorescence
intensity at 502 nm did not change further (Fig. 4), which
proved the 1 : 1 stoichiometry between HCFH and Al3+.
Fluorescence Titration of Compound HCFH
with Increasing Concentrations of Al3+
To understand the spectroscopic properties of compound
HCFH towards Al3+ further, we explored the fluorescence ti-
tration spectra of HCFH in the presence of increasing concen-
trations of Al3+ in ethanol, and the results were depicted in
Fig. 4. Upon excitation at 428 nm, compound HCFH in the
absence of any metal ion displayed almost no emission in the
range of 440–690 nm. It was probably due to the photoinduced
electron-transfer (PET) phenomenon from the Schiff-base
Fig. 3 The fluorescence intensity at 502 nm of HCFH (50 μM) with
Al3+ (1 equiv.) in the presence of other metal ions (5 equiv.) under the
identical conditions in ethanol. ((1) Al3+; (2) Al3+ + Ba2+; (3)
Al3+ + Ca2+; (4) Al3+ + Cd2+; (5) Al3+ + Co2+; (6) Al3+ + Cr3+; (7)
Al3+ +Cu2+; (8) Al3+ + Fe2+; (9) Al3+ + Fe3+; (10) Al3+ + K+;
(11) Al3+ + Mg2+; (12) Al3+ + Mn2+; (13) Al3+ + Na+; (14) Al3+ + Ni2+;
(15) Al3+ + Pb2+; (16) Al3+ + Zn2+) (λex = 428 nm)
Fig. 1 Change in UV-vis spectra of HCFH (100 μM) measured in
ethanol in the presence of various amounts of Al3+ (0.1, 0.2, 0.3, 0.4,
0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0 equiv., respectively).
Insert: the titration curve based on the absorbance at 215 nm

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J Fluoresc (2016) 26:345–353
Based on the fluorescence titration measurement, the de-
tection limit of compound HCFH for recognizing Al3+ was
evaluated. As illustrated in Fig. 5, the fluorescence response
could be easily observed in the Al3+ concentration range of
0.1–0.5 μM, which indicated that HCFH could respond Al3+
in low concentration. Additionally, a linear relationship be-
tween the fluorescence intensity at 502 nm and Al3+ concen-
tration was obtained in the range of 0.1–0.4 μM (Fig. S5). The
detection limit of compound HCFH for Al3+ was calculated
based on 3 × σblank/k, and it was found to be 3.87 × 10−8 M,
which was lower than US EPA defined tolerable levels of Al3+
in drinking water [47].
Binding Stoichiometry Between Compound HCFH
and Al3+
Fig. 4 Change in fluorescence emission spectra of HCFH (50 μM) upon
addition of increasing Al3+ (0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0,
1.2, 1.4, 1.6, 1.8, 2.0 equiv., respectively) in ethanol with an excitation at
428 nm. Insert: the titration curve based on the fluorescence emission
intensity at 502 nm
A Job’s plot was carried out to determine the binding stoichi-
ometry between compound HCFH and Al3+. For this pur-
pose, the total concentration of HCFH and Al3+ was kept
constant at 100 μM with the molar ratio of Al3+ varied from
0.1 to 0.9, and the fluorescence emission intensity at 502 nm
Scheme 2 The proposed binding
mechanism of HCFH with Al3+

J Fluoresc (2016) 26:345–353
351
was recorded. As can be seen from Fig. 6, the fluorescence
intensity at 502 nm reached maximum at the molar ratio of
0.5, indicating a 1 : 1 stoichiometry between compound
HCFH and Al3+ in ethanol, and it was consistent well with
the UV-vis and fluorescence titration spectra.
The 1 : 1 stoichiometry between compound HCFH and
Al3+ was further confirmed by the electrospray ionization
mass spectra (ESI-MS), and the results were illustrated in
Fig. S6. Two peaks in the ESI-MS spectra of complex
HCFH-Al3+ at m/z 387.3153 and m/z 415.0082 which were
assignable to [HCFH + Al3+ + C2H5OH + H2O - 2 H+]+
(calcd m/z 387.2860) and [HCFH + Al3+ + 2C2H5OH -
2 H+]+ (calcd m/z 415.3377) respectively gave another reliable
proof that the 1 : 1 complex was formed between HCFH and
Al3+. On the basis of 1 : 1 stoichiometry determined by UV-vis
titration, fluorescence titration, Job’s plot and ESI-MS spectra,
the binding constant (K) of compound HCFH with Al3+ was
calculated from fluorescence titration experiment by the
Benesi–Hildebrand equation (1) and found as 1.74 × 107
M−1 (Fig. S7), which was within the range 103–109 M−1 of
those reported Al3+ fluorescent sensors [48–51].
Fig. 6 Job’s plot for determining the stoichiometry between HCFH and
Al3+ in ethanol (XAl = [Al3+]/([Al3+] + [HCFH]), the total concentration
of HCFH and Al3+ was 100 μM)
chromone group (at δ 10.177 ppm) was also shifted downfield
to δ 10.202 ppm. Nevertheless, almost no spectral changes
were observed in other proton signals with addition of Al3+
(Fig. S8). From the results above, we could conclude that
the oxygen atom of the amido-carbonyl group, the ni-
trogen atom of the Schiff-base and the oxygen atom of
the keto-carbonyl group in chromone participated in the
coordination of compound HCFH with Al3+ (Scheme 2), and
it agreed well with the conclusion obtained from UV-vis titra-
tion spectra.
1H NMR Experiments
To have more insight into the cation binding properties of
compound HCFH, 1H NMR experiments were conducted in
the presence of Al3+ in DMSO-d6, and the spectral changes
upon addition of 1.0 equiv. of Al3+ were shown in Fig. S8. The
addition of 1.0 equiv. of Al3+ to the solution of compound
HCFH caused downfield shift in the proton signal of the
imino group from δ 11.959 ppm to δ 11.971 ppm.
Simultaneously, the signal of the fourth proton H4 of the
Conclusion
In conclusion, a novel chromone-derived Schiff-base ligand
HCFH was successfully synthesized and characterized, and
the spectroscopic properties of HCFH towards metal ions
were studied by using UV-vis and fluorescence spectra. The
compound HCFH could selectively bind with Al3+ and give
specific fluorescence enhancement by about 171-fold at
502 nm, which was due to the chelation-enhanced fluores-
cence (CHEF) effect that inhibited the photoinduced
electron-transfer (PET) phenomenon from Schiff-base nitro-
gen atom to chromone group upon complexation of HCFH
with Al3+. Furthermore, most of other metal ions had nearly
no influence on the fluorescence response of compound
HCFH to Al3+. In addition, HCFH had high selectivity and
sensitivity towards Al3+ for real-time detection, and the 1 : 1
stoichiometry between HCFH and Al3+ was determined
by UV-vis titration, fluorescence titration, Job’s plot and
ESI-MS spectra. Therefore, this compound HCFH can be
utilized as a fluorescent sensor for detecting and recognizing
Al3+ and might accelerate the development of other
chromone-derived sensors.
Fig. 5 Change in fluorescence emission spectra of HCFH (50 μM)
measured in ethanol upon addition of various concentration of Al3+
(0.1, 0.2, 0.3, 0.4, 0.5 μM, respectively) with an excitation at 428 nm

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J Fluoresc (2016) 26:345–353
Acknowledgments This work is supported by the National Natural
Science Foundation of China (81171337). Gansu NSF (1308RJZA115).
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