A highly selective and sensitive fluorescent chemosensor for detection of Al3+ in totally aqueous media

Article abstract of DOI:10.1002/cjoc.201400121

A fluorescent chemosensor (1) based on 2-hydroxy-1-naphthaldehyde Schiff-base was developed for the detection of Al³⁺ in 100% aqueous solution. Upon addition of Al³⁺, a significant fluorescence enhancement was observed, which was not affected by other metal ions including Na⁺, K⁺, Ca²⁺, Mg²⁺, Mn ²⁺, Co²⁺, Ni²⁺, Cu²⁺, Cr ³⁺, Ag⁺, Pb²⁺, Zn²⁺, Cd ²⁺, Hg²⁺, Fe²⁺ and Fe³⁺ under weak acid conditions. Moreover, the specific response to Al³⁺ was visible under natural light. The binding mode between 1 and Al³⁺ was clarified by ESI-MS and ¹H NMR. The chemosensor exhibits a sensitive fluorescence response specific to Al³⁺ in water under weak acid conditions and the fluorescence response is visible under natural light. Copyright

Full text of DOI:10.1002/cjoc.201400121

FULL PAPER
DOI: 10.1002/cjoc.201400121
A Highly Selective and Sensitive Fluorescent Chemosensor for
＋
Detection of Al3 in Totally Aqueous Media
Xiaobo Li, Jiayi Chen, and Enju Wang*
School of Chemistry and Chemical Engineering, Hainan Normal University, Haikou, Hainan 571158, China
A fluorescent chemosensor (1) based on 2-hydroxy-1-naphthaldehyde Schiff-base was developed for the detec-
＋
tion of Al3 in 100% aqueous solution. Upon addition of Al3＋, a significant fluorescence enhancement was ob-
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served, which was not affected by other metal ions including Na＋, K＋, Ca2 , Mg2 , Mn2 , Co2 , Ni2 , Cu2 , Cr3
,
Ag＋, Pb2 , Zn2 , Cd2 , Hg2 , Fe2 and Fe3 under weak acid conditions. Moreover, the specific response to Al3
was visible under natural light. The binding mode between 1 and Al3 was clarified by ESI-MS and H NMR.
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1
Keywords fluorescence, sensor, aluminum ion, 2-hydroxynapthaldehyde, aqueous sensor
＋
Introduction
to Al3 in 100% aqueous solution were rarely re-
ported.[19-22] Herein, we report a Schiff-base based
chemosensor (1) derived from 2-hydroxy-1-napthalde-
Aluminum is the third most abundant element and
the most abundant metal in the earth’s crust. By the
weathering of aluminum-containing rocks, it is liable to
be released to aquatic environments. Aluminum con-
centrations in waters are increased further by industrial
discharges. In contrast with its ubiquity, it is not an es-
sential element for either plants or animals. Moreover,
aluminum is toxic to many aquatic species and exces-
sive aluminum intake can cause damage of the function
of human body, including muscle weakness, osteoporo-
sis, delayed growth in children, altered mental abilities,
etc.[1] The aluminum-containing food additives are the
major sources of aluminum in the diet. The aluminum-
containing medications such as antiperspirants, antiulcer
agents and antacids are more hazardous to human health
than the aluminum-containing food. Since aluminum is
closely associates with human health and our exposure
hyde. It exhibits a highly sensitive fluorescence turn-on
＋
response specific to Al3 in 100% aqueous solution. In
addition, it is a visible-excited fluorescent sensor and
the fluorescence response is visible under natural
light.[23] All these features make the chemosensor con-
venient and practical for use.
Experimental
Reagents and apparatus
All chemicals were purchased from commercial
suppliers and used without further purification. The
corresponding metal nitrates were employed as the
metal cation sources. HEPES buffer solution was used
1
as solvent in all spectral measurements. H NMR and
13C NMR spectra were recorded on a Bruker-400 spec-
trometer with TMS as the internal standard. ESI-MS
spectra were performed on a Bruker esquire HCT-
Agilent 1200 spectrometer. Fluorescence spectra were
taken on a Hitachi F-7000 fluorescence spectrometer.
UV-vis absorption spectra were measured on TU-1901
spectrophotometer. The pH measurement was carried
out on a PHS-3C digital pH meter.
to it is so common, the selective recognization and de-
＋
tection of Al3 are crucial for the environment, public
health and food safety.
＋
According to Pearson acid base concept, Al3 is one
of the hardest Lewis acids and H2O is a typical hard
＋
base. Consequently Al3 in water is apt to get involved
in hydration instead of coordinating with sensing recep-
＋
tors. The high binding affinity between Al3 and H2O is
＋
an insurmountable difficulty for sensing Al3 in 100%
Synthesis of propyl 4-(2,3-dihydroxypropoxy)benzo-
ate (3)
aqueous solution. In recent years, considerable efforts
have been made for fluorescent detection of metal ions
A mixture of 3-chloro-1,2-propanediol (3.4 g, 30.7
mmol), propyl 4-hydroxybenzoate (5 g, 27.7 mmol),
potassium carbonate (5.8 g, 42.0 mmol) and a particle of
potassium iodide in DMF (15 mL) was stirred at 90 ℃
under nitrogen for 30 h. After cooling, 80 mL water was
in biological systems[2-9] and environmental sys-
＋
tems[10-15] and a number of Al3 -selective fluorescent
chemosensors have been reported.[16-18] However, the
fluorescent sensors with high selectivity and sensitivity
*
E-mail: enjuwang@163.com
Received March 1, 2014; accepted May 4, 2014; published online May 22, 2014.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201400121 or from the author.
Chin. J. Chem. 2014, 32, 429—433
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Li, Chen & Wang
FULL PAPER
added. Then the result mixture was extracted with chlo-
roform and the organic extract was evaporated in vacuo
to afford 3 as a dark red viscous oil (6.7 g, 95%).
firmed the 1∶1 binding stoichiometry (Figure S5). The
association constant of 0.69×104 L•mol−1 was evalu-
ated from the Bensei-Hildebrand plot (Figure S6) and
the detection limit was calculated to be 6.57×10−8
Synthesis of 4-(2,3-dihydroxypropoxy)benzohydra-
zide (2)
mol/L according to the slope of calibration plot (Figure
＋
S7) between Al3 concentration and fluorescence in-
A solution of 3 (5.0 g, 19.7 mmol) and hydrazine
hydrate (2.1 g, 41.9 mmol) in ethanol (30 mL) was re-
fluxed for 50 h. The solvent was evaporated off in
vacuo and the residue was purified by silica gel column
chromatography (eluant: EtOAc∶MeOH, 4∶1, V∶V)
to give 2 as a white solid (2.4 g, 54%).
＋
tensity, which indicated a high sensitivity of 1 to Al3
in water. Figure S8 illustrated the change in the UV-Vis
＋
spectra of 1 with Al3 concentration. Upon the stepwise
＋
addition of Al3 , a dual peak gradually took shape and a
nearly linear dependence of the absorbance at 400 nm as
＋
a function of Al3 concentration (0－1.0 equiv.) was
observed.
Synthesis of 1
2-Hydroxy-1-napthaldehyde (0.30 g, 1.7 mmol), 2
(0.58 g, 2.6 mmol) and 3 drops of glacial acetic acid
were dissolved in methanol (30 mL), and then refluxed
under nitrogen for 8 h. The reaction mixture was cooled
to room temperature to give a precipitate. The precipi-
tate was purified using silica gel column chromatogra-
phy (eluant: petroleum ether∶ethyl acetate, 1∶4, V∶V)
1
to afford 1 as a pale yellow solid (0.46 g, 70%). H
NMR (DMSO-d6, 400 MHz) δ: 12.88 (b, 1H), 12.12 (b,
1H), 9.49 (s, 1H), 8.21 (d, J＝8.4 Hz, 1H), 7.98 (d, J＝
8.0 Hz, 2H), 7.87－7.96 (m, 2H), 7.62 (t, J＝7.2 Hz,
1H), 7.42 (t, J＝7.2 Hz, 1H), 7.25 (d, J＝8.4 Hz, 1H),
7.13 (d, J＝8.0 Hz, 2H), 5.07 (d, J＝4.8 Hz, 1H), 4.77
(b, 1H), 4.13 (dd, J＝9.6, 3.6 Hz, 1H), 3.99 (dd, J＝9.6,
6.2 Hz, 1H), 3.81－3.89 (m, 1H); 13C NMR (DMSO-d6,
100 MHz) δ: 162.0, 161.9, 157.9, 146.2, 132.6, 131.6,
129.5, 129.0, 127.8, 127.7, 124.5, 123.5, 120.5, 118.9,
114.4, 108.6, 66.91, 69.87, 62.6; ESI-MS m/z: 381.5
[M＋H]＋, 403.4 [M＋Na]＋, 379.4 [M−H]−, 415.4 [M＋
H2O ＋ OH]−, 442.4 [M ＋ NO3]−. Anal. calcd for
C21H20N2O5: C 66.31, H 5.30, N 7.36; found C 66.18, H
5.23, N 7.32.
Figure 1 Fluorescence spectra of 1 (20 μmol/L) in the presence
＋
of different concentrations of Al3 (0－60 μmol/L) in HEPES
buffer solution (10 mmol/L, pH＝7.2), λex＝400 nm. Inset: the
＋
emission intensity as a function of Al3 concentration.
Results and Discussion
＋
Fluorescence response to Al3
Owing to a dihydroxypropoxy group in the sensor, 1
exhibits good solubility in water. All the optical proper-
ties of 1 were measured in HEPES buffered aqueous
Figure 2 The photos of fluorescence response of 1 to Al3＋, (a)
＋
in the absence (left) and presence (right) of Al3 excited at 395－
solution. The fluorescence responses of 1 to different
＋
concentrations of Al3 are shown in Figure 1. 1 in the
＋
400 nm, (b) in the absence (left) and presence (right) of Al3
＋
absence of Al3 hardly exhibits any fluorescence. The
under natural light.
＋
addition of Al3 leads to a remarkably enhanced fluo-
rescence emission with a dual-peak at 452 and 470 nm.
Meanwhile, bright blue luminescence is observed (Fig-
ure 2a). Significantly, 1 is a visible-excited fluorescent
Since the ionization of the phenolic hydroxyl group
＋
of 1 and the hydrolysis of Al3 are very sensitive to the
pH value, it is necessary to consider the pH effect on the
＋
fluorescence enhancement upon addition of Al3 . As
sensor with the excitation wavelength at 400 nm and the
＋
fluorescence response to Al3 can be observed under
shown in Figure 3, 1 exhibits good fluorescence sensing
＋
natural light (Figure 2b). During the titration with Al3 ,
＋
ability to Al3 over a wide range of pH values (4.5－
a rapid fluorescence enhancement (up to 109-fold) was
8.0). Nevertheless it shows no appreciable fluorescence
＋
observed when the concentrations of Al3 were below
＋
response to Al3 at a pH value above 10.0 or below 3.0,
＋
respectively due to the hydrolysis of Al3 and the fail-
20 μmol/L (1.0 equiv.) and soon afterwards the fluores-
cence saturation was observed, which suggested a
ure of the phenolic OH deprotonation. These results
＋
high-affinity binding between 1 and Al3 with 1∶1
＋
indicated that 1 is applicable to detect Al3 in weak
stoichiometry (Figure 1, inset). A Job’s plot further con-
acidic and neutral condition.
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A Highly Selective and Sensitive Fluorescent Chemosensor for Detection of Al3
＋
selectively recognize Al3 from the mixture of Al3
＋
and each competing metal ion. As shown in Figure 5, at
＋
pH 6.0 the Al3 -induced fluorescence enhancement was
little affected by other coexisting metal ions excluding
＋
Cu2 which lead to a 30% quenching of fluorescence.
＋
But fortunately, the depressing effect of Cu2 can be
entirely eliminated via adding 3 mol equiv. of
L-cysteine. When the pH value rises above 7.0, the
＋
Al3 -induced fluorescence was still not affected by al-
kali and alkaline earth metal ions, but most of the transi-
tion metal ions caused fluorescence quenching to dif-
ferent levels (Figure S10). The negative effect of alka-
＋
line medium on the Al3 -selectivity is because HO− is a
harder base than H2O, and consequently, exhibits a
Figure 3 pH-dependent emission intensities of 1 in the absence
＋
higher affinity to the hard acid ion Al3 than to transi-
＋
(black) and presence (red) of Al3 in aqueous solution.
tion metal ions.
＋
Selectivity to Al3
＋
For investigating the selectivity of 1 for Al3 , the
fluorescence spectra in the presence of different mental
ions were performed under same experimental condi-
tions. As shown in Figure 4, no significant fluorescence
responses were observed towards Na＋, K＋, Ca2 , Mg2 ,
＋
＋
＋
＋
＋
＋
＋
＋
＋
＋
Mn2 , Co2 , Ni2 , Cu2 , Cr3 , Ag＋, Pb2 , Zn2 , Cd2 ,
＋
＋
＋
＋
Hg2 , Fe2 and Fe3 except a Fe3 -induced faint en-
＋
hancement effect. In contrast, the addition of Al3 gives
rise to a remarkable fluorescence enhancement. How-
ever, UV-Vis spectral changes were observed when
＋
＋
added some metal ions (such as Zn2 , Ni2 , Ag＋ and
＋
＋
Pb2 ), indicating that besides Al3 some other metal
ions could also coordinate with 1 (Figure S9). The
＋
Al3 -specific fluorescence response of 1 might be at-
＋
tributed to the strong Lewis acidic character of Al3 ,
which leads to a more ionic Al—O bond and thereby
reinforces the propensity towards a quinoid-type reso-
nance structure that emits strong fluorescence. For fur-
＋
Figure 5 The selective responses of 1 to Al3 (2.0 equiv.) in
the presence of other different cations (2.0 equiv.) at pH 6.0.
ther evaluating the effects of complex backgrounds on
＋
the Al3 -selectivity, competing experiments were car-
Binding mode and signaling mechanism
＋
ried out by measuring the fluorescence response to Al3
Toward understanding the binding mode between 1
＋
and Al3 , ESI-MS spectra of 1 in the presence of Al3
＋
in the presence of other metal ions. It was found that pH
＋
value was crucial for the Al3 -selectivity against other
were performed (Figure 6). The spectrum in positive ion
mode exhibits the base peak at m/z 468.3 which was
assigned to [Al(1)−H＋NO3]＋ (calcd 468.11). That
confirmed the 1∶1 binding stoichiometry and indicated
coexisting metal ions. When the pH is below 6.0, 1 can
the deprotonation of the phenolic hydroxyl group upon
＋
coordination with Al3 . In the negative ion mass spec-
trum, the base peak was observed at m/z 529.2 corre-
sponding to [Al(1)−2H＋2NO3]− (calcd 529.09), which
implied the deprotonation of the amido NH besides the
phenolic OH.
＋
To further illustrate the binding mode of 1 with Al3 ,
1H NMR spectra of 1 in the absence and presence of
＋
Al3 (1.0 equiv.) were carried out in CD3OD/D2O (9∶
1). It is pity that the signals of phenolic hydroxy proton
(Hj) and amido proton (Hk) do not appear as a result of
the fast proton-exchange between 1 and solvent. How-
ever, the spectral changes in the aromatic region upon
Figure 4 Fluorescence spectra of 1 in the presence of various
cations (2.0 equiv.) in HEPES buffer solution (10 mmol/L, pH＝
7.2).
＋
the addition of Al3 clearly demonstrated the binding
＋
mode between 1 and Al3 (Figure 7). The imine peak
Chin. J. Chem. 2014, 32, 429—433
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431

Li, Chen & Wang
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Figure 7 1H NMR spectra of 1 in the absence (a) and presence
＋
(b) of Al3 (1.0 equiv.) in CD3OD∶D2O (9∶1).
O
OH
Al3+
O
N
N
+ Al3+, - H+
NH
NH
O
Figure 6 ESI-MS spectra of 1 in the presence of Al3＋: (a) in
positive ion mode, [Al(1)−H＋NO3]＋ m/z＝468.3. Inset: ob-
served (left) and calculated (right) isotopic patterns for the
[Al(1)−H＋NO3]＋ ; (b) in negative ion mode, [Al(1)−2H＋
2NO3]− m/z＝529.2. Inset: observed (left) and calculated (right)
isotopic patterns for the [Al(1)−2H＋2NO3]−.
HO
O
HO
HO
O
HO
O
Al3+
N
N
- H+
O
(Hg) is markedly shifted downfield region supporting
＋
the coordination of the imine nitrogen to Al3 . In addi-
HO
O
tion, the inductive effect of coordinative imine group
causes a downshift of the Hb and Hf peaks. The peak for
ortho hydrogen (Ha) of hydroxyl group moves towards
the upfield region which confirmed the deprotonation of
the phenolic hydroxyl group. It is worth noting that the
Hh and Hi peaks are shifted downfield. This observation
HO
＋
Figure 8 The proposed binding mode between 1 and Al3
.
＋
It is generally known that Al3 shows very high
binding affinity to O-donor atom.[24] Consequently, the
sensor 1 containing O, N, O donor atoms displays the
indicated that the carbonyl group strongly coordinates to
＋
higher affinity to Al3 than to the other test metal ions.
＋
Al3 and the deprotonation process of the amido NH
Thanks to the much stronger Lewis acidic character of
does not occur. However, the deprotonation of the
amido NH was suggested in the negative ion mass spec-
trum. It may be attributed to only trace amount of
NH-deprotonated complex that is detectable for ESI-MS
＋
Al3 , the Al－O bond takes on a more ionic character
and thereby the sensor 1 is likely to take on a quinoid-
type resonance structure. The quinoid-type structure
probably is the reason why the complex between 1 and
1
＋
Al3 shows high absorption and emission ability.
but undetectable for H NMR. In order to directly see
the changes of the signals of Hj and Hk, DMSO-d6 was
1
used as solvent for H NMR experiments. As shown in
Conclusions
Figure S11, the OH (Hj) and NH (Hk) signals in free 1
appear at δ 12.8 and 12.1, respectively. The presence of
In summary, we have developed a fluorescent
chemosensor based on 2-hydroxy-1-naphthaldehyde
＋
Al3 results in a rapid disappearance of the Hj signal
and a gradual decrease of the Hk signal. The observa-
Schiff-base, which shows a highly sensitive response to
＋
Al3 in water over a wide range of pH values (4.5－
tions affirmed the possibility of NH deprotonation upon
＋
coordination with Al3 . The solvent may be a critical
8.0). Other coexisting metal ions have little or no effect
＋
on its specific response to Al3 under weak acid condi-
influence factor on the NH deprotonation process. In
＋
tions. Moreover, the specific response to Al3 is visible
view of the above, we would propose the binding mode
＋
between 1 and Al3 in water as shown in Figure 8.
1
under natural light. Based on ESI-MS and H NMR,
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© 2014 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2014, 32, 429—433

＋
A Highly Selective and Sensitive Fluorescent Chemosensor for Detection of Al3
＋
The binding mode between 1 and Al3 was proposed.
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