A novel Schiff-base fluorescent probe based on 1,8-naphthyridine and naphthalimide for Al3+

Article abstract of DOI:10.1016/j.ica.2017.05.032

A novel Schiff-base, 7-acetamino-4-methyl-1,8-naphthyridine-2-carbaldehyde-(1′,8′-naphthalenedicarbonyl) hydrazone (HL) was designed, synthesized and evaluated as a fluorescent probe. The fluorescence properties of this probe towards various metal ions were investigated by UV–vis and fluorescence spectra in methanol. Test results indicated that the probe had high selectivity towards Al³⁺ over other commonly coexisting metal ions. Upon addition of Al³⁺, the fluorescence intensity at 414?nm increased significantly due to the inhibition of the PET process. The binding constant (Ka) of Al³⁺ binding to HL was calculated to be 5.64?×?10⁴?M^?1 from a Benesi-Hildebrand plot, and the detection limit (LOD) of HL for sensing Al³⁺ was calculated to be 0.13?μM. The binding stoichiometry between HL and Al³⁺ was determined as 1:1 by the Job's plot. Furthermore, the probe was chemically reversible for Al³⁺ in methanol by the addition of Na₂EDTA solution.

Full text of DOI:10.1016/j.ica.2017.05.032

Accepted Manuscript
Research paper
A novel Schiff-base fluorescent probe based on 1,8-naphthyridine and naph-
thalimide for Al3+
Xiao-li Yue, Chao-rui Li, Zheng-yin Yang
PII:
S0020-1693(17)30306-7
http://dx.doi.org/10.1016/j.ica.2017.05.032
ICA 17601
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Reference:
Inorganica Chimica Acta
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Please cite this article as: X-l. Yue, C-r. Li, Z-y. Yang, A novel Schiff-base fluorescent probe based on 1,8-
naphthyridine and naphthalimide for Al3+, Inorganica Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.
2017.05.032
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A novel Schiff-base fluorescent probe based on
1,8-naphthyridine and naphthalimide for Al3+
Xiao-li Yue, 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 novel Schiff-base, 7-acetamino-4-methyl-1,8-naphthyridine-2-carbaldehyde-(1’,8’-
naphthalenedicarbonyl) hydrazone (HL) was designed, synthesized and evaluated as a
fluorescent probe. The fluorescence properties of this probe towards various metal
ions were investigated by UV–vis and fluorescence spectra in methanol. Test results
indicated that the probe had high selectivity towards Al3+ over other commonly
coexisting metal ions. Upon addition of Al3+, the fluorescence intensity at 414nm
increased significantly due to the inhibition of the PET process. The binding constant
(Ka) of Al3+ binding to HL was calculated to be 5.64×104 M-1 from a Benesi-
Hildebrand plot, and the detection limit (LOD) of HL for sensing Al3+ was calculated
to be 0.13 μM. The binding stoichiometry between HL and Al3+ was determined as
1∶ 1 by the Job’s plot. Furthermore, the probe was chemically reversible for Al3+ in

methanol by the addition of Na2EDTA solution.
KEYWORDS: Fluorescent probe; 1,8-Naphthyridine; Naphthalimide; Aluminum ion;
Photo-induced electron transfer; Reversibility.
1. Introduction
Aluminum is the third most abundant metal element in the earth’s crust , whose
content is second only to oxygen and silicon [1-3]. Aluminum can be widely used in
aluminum alloy, aircraft, automobiles, trains, ships and other manufacturing industries
[4]. In addition, aluminum has a good ductility, so it is also extensively utilized in
food packaging [5-10]. However, improper use of aluminum has some side effects on
human health such as metabolic disturbance, cerebral injury, and even to the risk of
Alzheimer’s disease and Parkinson’s disease [11-13]. Hence, the World Health
Organization (WHO) stipulated that the daily human intake of aluminum should be
controlled within 10 mg and a weekly tolerable dietary intake should be controlled at
7 mg · kg−1 of the body weight [14]. The WHO also limited the concentration of
aluminum in drinking water to be 7.41 μM [15-17]. Additionally, high aluminum
contents in plants may affect the growth of root and seed [18, 19]. As a result, it’s
essential to develop some analytical methods for detecting and monitoring Al3+ to
maintain its concentration at a constant level.
In recent years, compared with the other traditional detection methods including
AMS, AAS, GFAAS, ICP-AAS, and IC-AES, the fluorescent chemosensors have
attracted significant interest due to their high sensitivity, good selectivity, short

response time, good reproducibility, operational simplicity and real-time monitoring
[20-23]. However, the relatively poor coordination ability blocks the development of
Al3+ chemosensors. As a hard acid, aluminum ion prefers to coordinate with hard
bases such as N and O atoms [24-26]. Owing to the intriguing structures and bonding
properties, 1,8-naphthyridine (napy) and its derivatives are widely used in the co-
ordination chemistry, pharmaceutical and molecular recognition fields [27-29].
Therefore, 1,8-naphthyridine group could be used as a binding unit for developing a
Al3+ fluorescent probe [30]. 1,8-Naphthalimide (Naph) and its derivatives have high
fluorescence quantum yield, moderate fluorescence emission, large Stokes shift,
perfect photostability and feasible structural modifications, so they can be widely util-
ized as fluorophores in the field of fluorescence sensing [31]. Thus, 1,8-Naphthalimi-
de group could be also used as the fluorophore of a fluorescent probe [32].
Taking these factors into account, we designed and synthesized a simple fluo-
rescent probe based on 1,8-naphthyridine and Naphthalimide, which was highly selec-
tive and responsive to Al3+. The fluorescence intensity of HL exhibits significant
enhancement upon the interaction of HL with Al3+ in methanol due to the inhibition of
the PET process. The binding stoichiometry between HL and Al3+ (1∶ 1) was studied
by the Job’s plot titration curve. The binding constant was calculated to be 5.64×104
M-1 by the Benesi-Hildebrand plot. Additionally, the fluorescence probe showed a
quite low detection limit (0.13 μM) of a micromolar concentration level. Significantly,
the probe was chemically reversible in the presence of Na2EDTA, which offered a
potentiality for detecting Al3+ in environmental and biological system circularly and

reversibly.
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 spectra 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 spectra were measured with a Shimadzu UV-240 spectrophotometer.
Fluorescence spectra 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
2.2.1. Synthesis of 7-amino-2,4-dimethyl-1,8-naphthyridine
7-Amino-2,4-dimethyl-1,8-naphthyridine was synthesized and purified from 2,
6-diaminopyridine according to a modified version of the literature procedure [33a,b].
2.2.2. Synthesis of 7-acetamino-2,4-dimethyl-1,8-naphthyridine
7-Acetamino-2,4-dimethyl-1,8-naphthyridine was synthesized from 7-amino-2,4-
dimethyl-1,8-naphthyridine according to a modified version of the literature method
[33c,d].
2.2.3. Synthesis of 7-acetamino-4-methyl-1,8-naphthyridine-2-carbaldehyde [34]
7-Acetamino-4-methyl-1,8-naphthyridine-2-carbaldehyde (4g, 0.0186mo) and
selenium dioxide (3.2g, 0.0288mol) were added to 1,4-dioxane (200 ml), and the

reaction mixture was refluxed for 6h with stirring . The mixture was filtered while hot,
and the crude product was obtained by concentration in vacuum. The final product
was purified by column chromatography using 50∶ 1 dichloromethane / ethanol as the
1
eluent (0.58g ). Yield: 29%; mp: 224-226 ℃. H NMR (400 MHz, CDCl3, TMS)
(Figure S1): δH 10.20 (m, 1H), 8.67 (d, 1H, J=8.8Hz), 8.46 (d, 1H, J=9.2Hz), 7.87(s,
1H), 2.80 (s, 3H), 2.32 (s, 3H).
2.2.4. Synthesis of N-aminonaphthalimide
Synthesis of N-aminonaphthalimide was synthesized from 1,8-naphthalicanhydr-
ide according to a modified version of the literature method [35]. Yield: 57%; mp:
262-264 ℃. 1H NMR (400 MHz, CDCl3, TMS) (Figure S2): δH 8.63 (d, 2H, J=7.2Hz),
8.23 (d, 2H, J=8.4Hz), 7.77 (t, 2H, J=7.2Hz, J=8.4Hz), 5.54 (s, 2H, -NH2).
2.2.5. Synthesis of 7-acetamino-4-methyl-1,8-naphthyridine-2-carbaldehy-
de-(1’,8’-naphthalenedicarbonyl) hydrazone (HL)
The synthetic route of the probe (HL) was shown in Scheme 1. The solution of
7-acetamino-4-methyl-1,8-naphthyridine-2-carbaldehyde (0.2292 g, 1.0 mmol) in
ethanol (20 ml) was added to another solution containing N-aminonaphthalimide
(0.2546 g, 1.2 mmol) in ethanol (20 ml). The mixture was stirred and refluxed for 24
h. After the reaction was completed, an excess amount of solvent was removed under
reduced pressure. Then the yellow solid was precipitated after cooling to room
temperature and the solid was filtered, washing 5 times with ethanol. Then obtained
product was then dried in vacuum. Yield: 54 %; mp: 259-260 ℃. 1H NMR (400 MHz,
DMSO-d6, TMS) (Figure S3): δH 11.17 (s, 1H), 8.90 (s, 1H), 8.65 (d, J=9.2Hz, 1H),

8.57 (d, J=7.2Hz, 2H), 8.51-8.54 (m, 1H), 8.47 (d, J=7.2Hz, 2H), 8.20 (s, 1H), 7.93 (t,
J=7.6Hz, 2H), 2.81 (s, 3H), 2.20 (s,3H). FT-IR (KBr, cm-1) (Figure S4): 1692 (C=N).
ESI-MS (Figure S8) calculated for [M＋H]+ 424.4382, found 424.1904.
2.3. UV-vis and fluorescence spectra measurements
Stock solutions of 1×10-3 M various metal ions and HL were prepared in ethanol
and DMF, respectively. Additionally, the stock solution of Na2EDTA was also prepar-
ed in ethanol. All absorption and emission spectra 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 360 nm, and the emission was recorded from 370
to 600 nm. Both excitation and emission slit widths were 5 nm.
3. Results and discussion
The experimental emission spectra of HL after the addition of Al3+ in different
solvents are shown in (Figure S5). It can be seen in Figure S5 that the fluorescence
intensity of HL after the addition of Al3+ is the highest in methanol compared with
that in other solvents. Thus, we choose methanol as the buffer solution to perform
experiments.
3.1．UV-vis studies of HL towards Al3+
The UV-vis absorption spectra of HL towards Al3+ were shown in Figure 1a. The
absorbances at 318 nm, 333 nm, and 356 nm were gradually increased with the addit-
ion of Al3+ ion, while the absorbance at 385 nm was gradually decreased. Meanwhile,
an isobestic point was observed at 380 nm. These changes which were compared to

the original absorbance demonstrated the formation of a stable complex between HL
and Al3+ ion.
In addition, the UV-vis absorption spectra of HL towards various metal ions
(Ag+, Ca2+, Co2+, Cu2+, Fe3+, In3+, Mg2+, Na+, Pb2+, Ba2+, Cd2+, Cr3+, Fe2+, Ga3+, K+,
Mn2+, Ni2+, Zn2+ and Al3+) was also investigated as shown in Figure 1b. Upon the ad-
dition of Al3+ to the HL solution, the absorbances at 318 nm, 333 nm and 356 nm
were increased, while the other cations did not give rise to the enhancement of these
absorbances. These results suggest that HL as a fluorescence probe has high selectiv-
ity towards Al3+ over other commonly coexistent metal ions by UV-vis method.
3.2. Fluorescence studies of HL towards Al3+
HL alone exhibited weak fluorescence emission with excited at 360 nm in meth-
anol (Figure 2a). However, upon addition of Al3+, the fluorescence intensity at 414 nm
was enhanced remarkably. Under the identical conditions as used for Al3+, no signific-
ant change of the fluorescence spectrum was observed in the presence of other cations
except for Ni2+, Fe3+ and Cd2+, which resulted in small changes in fluorescence intens-
ity compared with Al3+. These results indicated that HL showed an efficient Al3+
selective OFF-ON fluorescent behavior.
To obtain more insight into the fluorescent properties of HL as a chemosensor
for Al3+, fluorescence titration experiments of HL towards increasing amounts of Al3+
were carried out (Figure 2b). The fluorescence intensity at 414 nm was gradually enh-
anced with the progressive addition of Al3+. Furthermore, this probe showed a nice
linear relationship between the fluorescence intensity (F/Fmin) and the concentration of

Al3+ (Figure 2b, inset). Thus this fluorescent probe HL could detect Al3+ quantitative-
ly.
3.3. Interference studies from other metal ions.
As we all know, high selectivity of a chemosensor for sensing the analytes in the
presence of other species is one of the most important characterizations. To further
confirm the selectivity of HL towards Al3+, the fluorescence spectrum of HL towards
Al3+ in the presence of other metal ions (1 equiv.) was also measured. It could be
noted from Figure 3 that all the coexistent metal ions did not influence the emission
intensity of HL-Al3+ obviously. These results demonstrate that HL displayed a high
selectivity for Al3+ even in the presence of other metal ions.
3.4. Reversible test of HL towards Al3+ by Na2EDTA
It’s significant for a fluorescence probe to detect the analytes reversibly. To conf-
irm the reversibility of HL towards Al3+, we added 1 equiv of Na2EDTA to the solut-
ion of HL and Al3+. It can be seen from Figure 4 that the fluorescence intensity at 414
nm decreased upon addition Na2EDTA to the mixture, while the fluorescence of the
system was recovered with addition of Al3+ again. These results illustrated that HL
was a reversible fluorescent probe for detecting Al3+ in methanol.
3.5. Binding constant, stoichiometry and detection limit
The association constant of HL with Al3+ and the detection limit (LOD) of HL
for sensing Al3+ were obtained from the fluorescence titration experiments. The assoc-
iation constant (Ka) of HL with Al3+ was determined by the Benesi-Hildebrand Eq. (1)
[36]:
＝
＋
(1) where F is the fluorescence intensity at
－
－

414 nm at any given Al3+ concentration, Fmin is the fluorescence intensity at 414 nm in
the absence of Al3+, and Fmax is the maximum fluorescence intensity at 414 nm in the
presence of Al3+. The binding constant was calculated by plotting 1/ (F-Fmin) against
1/ [Al3+] (Figure S6). The data was linearly fitted well according to Eq. (1) and the as-
sociation constant (Ka) value was 5.64×104 M-1. The detection limit for sensing Al3+
was calculated to be 0.13 μM by the Stern-Volmer plot [37] (Figure S7): DL＝3σ/K
where σ is the standard deviation of the blank solution and K is the slope of the calib-
ration curve. Additionally, the binding stoichiometry was determined by the Job’s plot
(Figure 5). As shown in Figure 5, the fluorescence emission intensity at 414 nm reac-
hed maximum when the molar fraction of HL in HL- Al3+ was 0.5, which gave a solid
evidence for the formation of a 1∶ 1 complex between HL and Al3+. To better
understand the binding mode of HL towards Al3+, the ESI mass spectra of HL in the
presence of Al3+ was carried out (Figure S8). The m/z peak for HL in the presence of
- +
Al3+ at 573.2957 corresponding to [HL＋Al3+＋2NO3 ] . The result further confirm
the 1∶1 binding stoichiometry between HL and Al3+.
3.6. Proposed sensing mechanism of HL towards Al3+
The proposed reaction mechanism of HL with Al3+ was illustrated in Scheme 2
1
based on the optical properties, Job’s plot and the H NMR titration experiments of
HL towards Al3+. The nitrogen atom (1,8-naphthyridine ring) could transfer an electr-
on to the excited fluorophore (naphthalimide ring) to quench its fluorescence emission.
Upon the addition of Al3+ to the HL solution, this nitrogen atom participates in the
coordination process between HL and Al3+. Thus the PET process was restrained and

the fluorescence emission intensity at 414 nm enhanced [38-42]. This sensing
1
mechanism was further confirmed by the H NMR titration experiments of HL
towards Al3+ (Figure S9). Upon addition of Al3+, the proton signals of Ha and Hb were
shifted downfield by 0.032 ppm and 0.051 ppm respectively. Similarly, the proton
signal of methyl hydrogen of the acetamide group was shifted downfield by 0.053
ppm. Additionally, the signals of protons of the aromatic rings were also shifted
downfield. These changes demonstrated that the nitrogen atom of the 1,8-naphthy-
ridine ring and the CN group as well as the oxygen atom of the carbonyl group might
participate in the coordination of HL with Al3+.
4. Conclusion
In conclusion, we have designed and synthesized a simple fluorescence probe
based on 1,8-naphthyridine moiety and naphthalimide unit, which showed an high
selectivity for Al3+ over other metal ions through the inhibition of the photo-induced
electron transfer (PET) process. The binding ratio of HL-Al3+ complex was determin-
1
ed to be 1∶ 1 according to the Job’s plot and H NMR titration experiments. In addi-
tion, this probe may have potential applications to detect micromolar concentrations
of Al3+ in both biological assays and the environmental systems reversibly due to its
low detection limit.
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 1a. The UV-vis absorption titration spectra of HL (10μM) with Al3+ (0-1
equiv.) in methanol.
Figure 1b. Absorption spectra of HL before and after addition of 1 equiv. of various
metal ions in methanol.
Figure 2a. Fluorescence spectra of HL (10μM) before and after addition of 1 equiv.
of various metal ions of Ag+, Ca2+, Co2+, Cu2+, Fe3+, In3+, Mg2+, Na+, Pb2+, Ba2+, Cd2+,
Cr3+, Fe2+, Ga3+, K+, Mn2+, Ni2+, Zn2+ and Al3+ in methanol.

Figure 2b. Fluorescence emission titration spectra of HL (10μM) upon addition of
Al3+(0-1 equiv.) in methanol. Inset: Linear fluorescence intensity (F/Fmin) of HL upon
addition of Al3+.
Figure 3. Fluorescence response of HL and HL+Al3+ upon the addition of various
metal ions in methanol. The black bars represent the fluorescence intensities of HL in
the presence of various metal ions. The red bars represent the changes in the
fluorescence intensities that occur upon the subsequent addition of 1 equiv. of Al3+ to
the HL-metal cation solutions.
Figure 4. The reversibility of HL towards Al3+ in methanol.
Figure 5. Job plot of HL with Al3+ at 414 nm in methanol. The total concentration of
HL and Al3+ is 1.0×10-5 mol/L.
Scheme 1. The synthetic route of 7-acetamino-4-methyl-1,8-naphthyridine -2-carbal-
dehyde-(1’,8’-naphthalenedicarbonyl) hydrazone (HL).
Scheme 2. The proposed sensing mechanism of HL towards Al3+.

Figure 1a. The UV-vis absorption titration spectra of HL (10μM) with Al3+ (0-1
equiv.) in methanol. Figure 1b. Absorption spectra of HL before and after addition of
1 equiv. of various metal ions in methanol.
Figure 2a. Fluorescence spectra of HL (10μM) before and after addition of 1 equiv.
of various metal ions of Ag+, Ca2+, Co2+, Cu2+, Fe3+, In3+, Mg2+, Na+, Pb2+, Ba2+, Cd2+,
Cr3+, Fe2+, Ga3+, K+, Mn2+, Ni2+, Zn2+ and Al3+ in methanol. Figure 2b. Fluorescence
emission titration spectra of HL (10μM) upon addition of Al3+(0-1 equiv.) in methanol.
Inset: Linear fluorescence intensity (F/Fmin) of HL upon addition of Al3+.

Figure 3. Fluorescence response of HL and HL+Al3+ upon the addition of various
metal ions in methanol. The black bars represent the fluorescence intensities of HL in
the presence of various metal ions. The red bars represent the changes in the
fluorescence intensities that occur upon the subsequent addition of 1 equiv. of Al3+ to
the HL-metal cation solutions.
Figure 4. The reversibility of HL towards Al3+ in methanol.

Figure 5. Job plot of HL with Al3+ at 414 nm in methanol. The total concentration of
HL and Al3+ is 1.0×10-5 mol/L.
Scheme 1. The synthetic route of 7-acetamino-4-methyl-1,8-naphthyridine -2-carbal-
dehyde-(1’,8’-naphthalenedicarbonyl) hydrazone (HL).

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

Highlights
 A novel fluorescent probe based on 1,8-naphthyridine and naphthalimide was
designed and synthesized.
 The probe exhibited high selectivity towards Al3+ over commonly coexistent
metal ions.
 The probe was chemically reversible in the presence of Na2EDTA.
 The proposed binding mechanism between HL and Al3+ was investigated in
detail.