A simple and effective fluorescent chemosensor for the cascade recognition of Zn2+ and H2PO4- ions in protic media

Article abstract of DOI:10.1016/j.tet.2013.10.099

The chiral Schiff base N-(2-hydroxy-1-naphthyl)methylidene-(S)-α- phenylethylamine (1) was designed and synthesized as a fluorescent chemosensor based on the photoinduced electron transfer (PET) and CN isomerization mechanisms. The receptor exhibited a highly selective switch-on fluorescence response toward Zn²⁺ and especially avoided the interference of Cd²⁺ when the detection was conducted in unbuffered methanol solution. Furthermore, the in situ generated 1·Zn^II-complex could be employed for the specific recognition of H₂PO ₄^- by fluorescence turn-off signaling through a ligand displacement process.

Full text of DOI:10.1016/j.tet.2013.10.099

Tetrahedron 70 (2014) 1011e1015
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
A simple and effective fluorescent chemosensor for the cascade
recognition of Zn2þ and H2POꢀ4 ions in protic media
*
Mengyu Zhang, Wei Lu, Jinting Zhou, Ganhong Du, Liming Jiang , Jun Ling,
Zhiquan Shen
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University,
310027 Hangzhou, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 July 2013
Received in revised form 12 October 2013
Accepted 28 October 2013
Available online 5 November 2013
The chiral Schiff base N-(2-hydroxy-1-naphthyl)methylidene-(S)-a-phenylethylamine (1) was designed
and synthesized as a fluorescent chemosensor based on the photoinduced electron transfer (PET) and
C]N isomerization mechanisms. The receptor exhibited a highly selective switch-on fluorescence re-
sponse toward Zn2þ and especially avoided the interference of Cd2þ when the detection was conducted
in unbuffered methanol solution. Furthermore, the in situ generated 1$ZnII-complex could be employed
for the specific recognition of H2POꢀ4 by fluorescence turn-off signaling through a ligand displacement
process.
Keywords:
Dihydrogen phosphate
Fluorescent chemosensor
Metal coordination
Schiff base
Ó 2013 Elsevier Ltd. All rights reserved.
Zinc ion
1. Introduction
recognition capability are cost effective and would be highly de-
sirable from the viewpoint of practical applications.
Ions are ubiquitous and play pivotal roles in biological and en-
vironmental processes.1 For example, zinc ions are involved in gene
transcription, regulation of metalloenzymes, neural signal trans-
mission, and apoptosis in living systems,2 while pollution from
inorganic phosphate anions and phosphorylated compounds are in
part responsible for the eutrophication of natural water sources.3
Accordingly, a considerable effort has been devoted to develop ef-
ficient and selective methods to detect Zn2þ or phosphate ions
using fluorescence probes.4,5 Although numerous chemosensors
for a single species have been developed, there are very few reports
on the sensing systems that allow the differential response to
multiple analytes.6 Recently, Yamato and co-workers described
a new type of fluorescent sensor based on a pyrene-linked triazole-
modified homooxacalix-[3]arene, which displayed ratiometric se-
lectivity for Zn2þ and H2PO4ꢀ ions in neutral solution.7 Bhalla et al.
synthesized a fluorescent probe utilizing terphenylphenanthroline
based macrocycle as a scaffold. The probe molecule showed high
selectivity for zinc ions in THF; the addition of H2POꢀ4 to the
probe solution containing Zn2þ produced a blue-shifted emission
band.8 Undoubtedly, such chemosensors with multiple analyte
Extensive research suggests that the rational use of nitrogen-
containing fragments as a receptor unit is an efficient strategy in
the design of fluorescent chemosensors for Zn2þ. Typical receptors
to construct Zn2þ chemosensors include di-2-picolylamine, quin-
oline, bipyridine, acyclic and cyclic polyamines, triazole, and so
on.4,7,8 The nitrogen of a Schiff base is known to have high affinity
for zinc. Therefore several fluorescent chemosensors with Schiff
base (imine) structures have been developed for detecting zinc ions
based on different signaling mechanisms, such as the photoinduced
electron transfer (PET), C]N bond isomerization, and excited-state
intramolecular proton transfer (ESIPT).9 On the other hand, zinc
also exhibit higher affinity toward phosphate functionality. Certain
zinc(II) complexes of cresolic, polyammonium macrocycle, or por-
phyrin derivatives have been successfully used to achieve selective
recognition of phosphate anions as the result of dative coordinative
interaction between the metal center and a negatively charged
oxygen atom present on the guest species.10 These facts motivated
us to explore the possibility of using a simple receptor molecule for
monitoring both zinc ion and phosphate anions of interest.
In the present work, we designed and synthesized a chiral Schiff
base N-(2-hydroxy-1-naphthyl)methylidene-(S)-a-phenyl-ethyl-
amine (1) (Scheme 1). The Schiff base is fluorescently silent in the
free-state owing to the PET from the imino nitrogen to the naph-
thalene fluorophore core and/or the C]N bond isomerization in
* Corresponding author. Tel.: þ86 571 87953727; e-mail address: cejlm@zju.edu.
cn (L. Jiang).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.10.099

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M. Zhang et al. / Tetrahedron 70 (2014) 1011e1015
the excited states.11 It can be expected that the formation of a metal
complex would suppress both the PET and the conformational
transition and thereby provoke fluorescence emission. In addition,
the chiral feature of the receptor 1 allows the exploration of the
possible hosteguest interactions through circular dichroism spec-
troscopy. The results of our investigation show that 1 is highly se-
lective to Zn2þ ion in switching-on-type fluorescence responses in
unbuffered methanol solution. Moreover, the emission of 1 induced
with Zn2þ ion could be selectively quenched by H2POꢀ4 . As a result,
this simple Schiff base 1 serves as an efficient fluorescence probe
for successively detecting Zn2þ and H2POꢀ4 ions. To the best of our
knowledge, similar examples are fairly limited in literature.6ꢀ8,12
Fig. 1. Fluorescence responses of 1 (50
m
M, lex¼400 nm) to various metal ions (25
mM)
in methanol. I451 is the fluorescence emission intensity at 451 nm for the free 1, and
I448 is the fluorescence intensity at 448 nm after adding metal ions. Inset: the corre-
sponding fluorescence spectra of 1 in the absence and presence of respective metal
ions.
Scheme 1. The synthetic procedure for receptor 1 and schematic representative of
fluorescent sensing toward Zn2þ and H2PO4ꢀ ions.
2. Results and discussion
2.1. The interaction of receptor 1 with various cations
The binding behavior of receptor 1 toward various metal ions
was examined by means of fluorescence in unbuffered methanol.
The free receptor 1 (50 mM) showed a weak band with emission
Fig. 2. Series (a) showing fluorescence response of 1 (50 mM) upon addition of various
maxima positioned around 451 nm on excitation at 400 nm (Fig. 1).
Upon addition of 0.5 equiv of Zn2þ, receptor 1 exhibited a remark-
able enhancement in emission accompanied by a slight blue shift
from 451 to 448 nm. The ratio of the emission intensity at 448 and
451 nm (I448/I451) reached 13.6. In contrast to what was seen with
Zn2þ, much smaller fluorescence increase was observed with other
metal ions. For example, the I448/I451 values were 3.1 and 2.0 for
Al3þ and Fe3þ, respectively. Little increase or a slight quenching in
the emission intensity was detected upon the addition of Agþ,
Mg2þ, Cd2þ, Ni2þ, Cu2þ, Mn2þ, and Ca2þ into the methanol solution
of 1.
metal ions (0.5 equiv) and series (b) showing competitive selectivity of 1 towards Zn2þ
in the presence of other metal ions (1 equiv) in methanol solution. Last four sets of data
are from the competition experiments conducted in CH3CN. lex¼400 nm.
detected by a threefold enhancement in the fluorescence intensity.
Such a low detection limit might fully meet the requirements in
biosensing. The fluorescence increase was saturated with 0.5 equiv
of Zn2þ
, indicating a 2:1 stoichiometry of the hosteguest
The expected preference for Zn2þ was observed, as inferred from
competitive experiments (Fig. 2). It is noteworthy that the receptor
1 displayed a considerable ability to distinguish Zn2þ from Cd2þ in
a common solution, which is usually difficult because both ions
have similar chemical properties.9j,13 The receptor binds Zn2þ more
effectively than Cd2þ when acetonitrile was used as the solvent.
However, in both cases this assay was subject to a severe in-
terference from Cu2þ and Fe3þ ions. In general, Cu2þ can be con-
sidered as a fluorescence quencher, like other heavy metals with
partially filled d-shells.4d,f;14 As for the interference of Fe3þ, it may
be attributed the possibility that the ligand was preferentially li-
gated by ferric ion due to its stronger complexing ability compared
with Zn2þ
Fluorescence titration spectra of 1 with Zn2þ are shown in Fig. 3.
In the titration experiments employing 1 of 50 M, the blue
emission intensity around 448 nm increased with the ion concen-
tration. Minimum 2.5
M concentration of Zn2þ can be easily
.
m
Fig. 3. Fluorescence titration profiles of 1 (50
methanol (lex¼400 nm). Inset shows plots of the relative emission intensity (I448/I451
as a function of Zn2þ concentration (0e50
M).
mM) with Zn(NO3)2 (0e25 mM) in
)
m
m

M. Zhang et al. / Tetrahedron 70 (2014) 1011e1015
1013
interactions. This binding stoichiometry in solution was further
confirmed by the Job’s plot of 1 with Zn2þ (Fig. S1). From the
spectral titration data, the association constant (log Kass) was de-
termined to be 3.58.
complexes.16 The computed results show that there is a small
conformational difference for the ligand before and after co-
ordinating to Zn2þ ion. For the bound receptor, the lone pair of the
imine nitrogen is engaged, electron transfer to the excited naph-
thalene fluorophore, i.e., the PET effect, does not take place. This
may be responsible for the observed enhancement on lumines-
cence response for receptor 1 on binding to Zn2þ. In addition, the
formation of 1$ZnII-complex could inhibit the C]N isomerization,
which is also expected to contribute to the observed switched-on
emission. That is, the emission enhancement in the 1:2 Zn2þ$1
complex is probably due to a combination of the retardation of PET
and imine isomerization by chelation of both imine nitrogen atom
and phenolic hydroxyl to metal ions.
Fig. 4 displays the UVevis titration spectra of 1 with Zn2þ in
methanol. The free ligand in solution exhibits characteristic ab-
sorption bands due to the p/p* transitions of the intramolecularly
hydrogen-bonded naphthaldimine chromophore at about 310 and
238 nm as well as two peaks around 400 and 418 nm, which resulted
from the C]N isomerization.9i Upon addition of Zn2þ the two long
wavelength peaks gradually merged to one, which indicates the
imine cisetrans isomerization was interrupted. Although similar
changes in absorption also happened with Cu2þ or Fe3þ (Fig. S2), only
Zn2þ can lead to a large fluorescence enhancement (see: Fig. 1). In-
terestingly, the addition of Zn2þ ions did not seem to produce
a substantial change in the CD spectra of 1 (the inset of Fig. 4).15
To gain a better understanding of the binding mode of Zn2þ to 1,
we also performed 1H NMR titration (Fig. S3). The results showed
that the proton signals of phenolic hydroxyl and imino groups were
observable throughout the titration. This means that the complex-
ation process of 1 with Zn2þ does not involve the deprotonation.
2.2. The sensing behavior of 1$ZnII-complex toward anions
It is conceivable that the Zn2þ-containing complex of 1 is labile
and thus an anion binding to the metal center would displace the
neutral ligand and thereby change or recover its spectroscopic
behavior. This coordination complex-based displacement approach
has been used for detection of certain anionic species including
phosphate anions.7,8,10 In accord with the expectation, a dramatic
decrease at the 448 nm emission was observed upon the addition of
0.15 equiv of NaH2PO4 into the solution of 1$ZnII (Fig. 6a). Both
acetate and chloride induced a small increase for the fluorescence,
whereas other anions (Fꢀ, Brꢀ, Iꢀ, NOꢀ2 , SO42ꢀ, HPO42ꢀ, PO34ꢀ, or
P2O47ꢀ) caused a slight fluorescence reduction. Compared with the
sensing systems making using of hydrogen-bonding inter-
action,5d,f,h the in situ generated sensor appears to be attractive
owing to the simplicity and particularly providing an effective
discrimination of H2POꢀ4 from AcOꢀ and Fꢀ anions in protic media.
Additionally, fluorescence titration results indicate that sub-
millimolar concentrations of dihydrogen phosphate could be sen-
sitively detected, and the relative emission intensity exhibited
a good linear response to the change of H2POꢀ4 concentrations in
Fig. 4. UVevis spectral changes of 1 (100
m
M) upon successive addition of Zn2þ in
M. Inset: the corresponding CD change
methanol solution, [Zn2þ]¼0, 10, 20, 30, 40, 50
m
upon addition of 0.5 equiv of Zn2þ
.
On the basis of above observations, the selective recognition of
receptor 1 for Zn2þ over a variety of other cations is probably at-
tributed to the fact that the chiral Schiff base as a bidentate chelate
better satisfy the geometrical requirements of tetrahedral or
pseudo-tetrahedral zinc(II) complexes.16a As depicted in the ge-
ometry optimized structure according to the density functional
theory using B3LYP/6-31G(d) (Fig. 5), the in situ generated 1,ZnII-
complex with two N,O-donor ligands coordinating to Zn2þ center is
very similar to the closely related bis(salicylideneiminato)zinc(II)
the 0.5e4.5 mM range (Fig. 6b).
Competition experiments were conducted for examining the
selectivity of 1$ZnII-complex toward the detection of H2POꢀ4 . As
shown in Fig. 7, little interference was observed in the presence of the
other tested anions, evenwhen certain anionic species were added at
a threefold excess relative to H2POꢀ4 . Interestingly, comparison of
spectral responses of 1,ZnII-complex and of its pre-synthesized an-
alog bis{1-[1-phenylethyliminomethyl]-2-naphtholato-N,O}zinc(II)
(2)17 toward H2POꢀ4 revealed that the former is a more sensitive
sensor for the target anion under identical conditions (Figs. S4, S5).
Both zinc(II)-complexes have almost the same fluorescence proper-
ties. Upon addition of 0.15 equiv of H2POꢀ4 the emission of 2 was
approximately quenched by 30%, whereas for the in situ formed
1,ZnII-complex its fluorescence was almost completely quenched. A
set of comparable experiments without Zn2þ ions showed that al-
most no distinct spectral variations could be observed with the free
ligand 1 (Fig. S6) upon addition of anions, suggesting that the for-
mation of zinc(II) complex is indispensable for the anion detection.
Nevertheless, the anion recognition effect of the metal complex
decreases strongly in aqueous media (Fig. S7), which is related to its
lower association constant (log Kass¼3.58).
3. Conclusions
Fig. 5. Calculated energy-minimized structure for the in situ formed 1$ZnII-complex
In the present studies, we demonstrated that the chiral Schiff
(left) and receptor 1 (right).
base 1 displays a highly selective fluorescence enhancement

1014
M. Zhang et al. / Tetrahedron 70 (2014) 1011e1015
achieved by using the simple Schiff base derivative. This research
suggests that the rational use of dynamical metal coordinative in-
teraction may be a practicable strategy in the design of sensing
systems for multiple analytes.
4. Experimental
4.1. Reagents, instruments, and methods
2-Hydroxyl-1-naphthaldehyde, (S)-a-phenylethylamine, and all
of the nitrate and sodium salts as analytes were purchased from
Aladdin Shanghai Reagent Company. Methanol were strictly
dehydrated and distilled before use. Other reagents were all used
without any further purification.
1H NMR and 13C NMR spectra were recorded on a BruAvance
AMX-400 or 500 NMR instrument. High-resolution mass spectrum
(HRMS) was measured on a TripleTOF 4600 system with a Duo-
Spray ion source. Steady-state fluorescence spectra were recorded
on a PerkineElmer LS-55 fluorescence spectrometer in the right-
angle geometry (90ꢁ collecting optics, lex¼400 nm). UVevis spec-
tra were obtained in methanol at 20 ꢁC using a quartz cell of 1 cm
on MOS-450 (Biologic Company, France).
The spectral analyses were accomplished in unbuffered meth-
anol solution at room temperature. The concentration of 1 for
fluorescence measurement was 50
mM and for UVevis measure-
ment was 100 M. All the nitrate and sodium salts were prepared in
m
aqueous solutions. UVevis and fluorescence spectrophotometric
titration were conducted directly in 2 mL cuvette by successive
addition of corresponding chemical reagent using a microliter sy-
ringe. Upon addition of every aliquot, the solution was well mixed
then the spectrum was measured.
Fig. 6. (a) Fluorescence spectra of 1 (50 m
M) in the presence of Zn2þ (0.5 equiv) in
methanol upon addition of 0.15 equiv various anions (Fꢀ, Clꢀ, Brꢀ, Iꢀ, NO2ꢀ, AcOꢀ, SO24ꢀ
,
4.2. Synthesis and characterization of receptor 1
H2PO4ꢀ, HPO24ꢀ, PO34ꢀ, or P2O47ꢀ). lex¼400 nm. (b) Fluorescence titration spectra of 1
(50.0 m
M) in the presence of Zn2þ (0.5 equiv) upon successive addition of H2POꢀ4 in
To
a solution of 2-hydroxyl-1-naphthaldehyde (1.72 g,
methanol solution (lex¼400 nm). Inset: relative emission intensity (I2/I1) versus H2PO4ꢀ
10.0 mmol) in ethanol (50 mL) was added (S)-
a
-phenylethylamine
concentration (0e7.5 mM). I1 and I2 represent the emission intensity of the solution of
(1.21 g, 10.0 mmol). After refluxing at 80 ꢁC for 12 h, the solvent was
removed under reduced pressure to give the crude product. The
product was recrystallized from n-hexane to give a yellow solid in
1,ZnII before and after adding NaH2PO4, respectively.
toward Zn2þ over other metal ions of interest including Cd2þ in
unbuffered methanol. Moreover, the in situ generated ZnII-complex
can be used as a probe for specifically detecting H2POꢀ4 by fluo-
rescence ‘turn-off’ signaling based on a ligand displacement
mechanism. Thus, the cascade recognition of both ions was
80% yield (2.45 g). 1H NMR (400 MHz, DMSO-d6)
d
14.96 (1H, m,
C10H6OH), 9.33 (1H, d, J 8.2 Hz, HC]N), 8.14 (1H, d, J 8.4 Hz,
C
C
10HaH5), 7.77 (1H, d, J 9.2 Hz, C10HfH5), 7.67(1H, d, J 8.0 Hz,
10HdH5), 7.38e7.48 (m, 5H, C6H5), 7.31 (m, 1H, C10HbH5), 7.23 (m,
1H, C10HcH5), 6.80 (1H, d, J 9.2 Hz, C10HjH5), 5.01 (m,1H, CHMe),1.67
(3H, d, J 6.8 Hz, Me), see: Fig. S8; 13C NMR (125 MHz, DMSO-d6)
d
175.4, 158.2, 143.3, 137.3, 134.4, 129.4, 129.3, 128.4, 128.1, 126.7,
126.1, 124.9, 123.0, 119.4, 106.7, 61.4, 23.7; HRMS (TOF): MHþ, found
20
276.1371. C19H18NO requires 276.1383. [
MeOH).
a]
þ202.7 (c 0.5 g/dL,
D
4.3. Computations
Geometry optimizations and zero-point energies of the com-
pounds were performed in gas phase by the density functional
theory (DFT) calculations at the B3LYP level18 with a 6-31G(d)
basis set using ‘Gaussian program’.19 Vibration frequencies
were used to confirm intermediates (number of imaginary
frequencies¼0).
Acknowledgements
Fig. 7. Series (a) showing fluorescence response of 1$ZnII-complex (50
mM) toward
We gratefully acknowledge the support from the Natural Sci-
ence Foundation of China (Grant No. 21074107).
various anions (0.15 equiv) and series (b) showing the competitive selectivity toward
H2POꢀ4 in the presence of different anions in methanol. lex¼400 nm.

M. Zhang et al. / Tetrahedron 70 (2014) 1011e1015
1015
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Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2013.10.099.
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