A Ag+-selective "off-on" probe based on a naphthalimide derivative

Article abstract of DOI:10.1039/c2nj20974h

A new naphthalimide-based derivative L bearing an S-benzyldithiocarbazate moiety was synthesized as an "off-on" probe for Ag⁺. The new probe has high binding selectivity for Ag⁺ in water media with a detection limit of Ag⁺ down to 270 nM. Based on the Ag ⁺-induced inhibition of a PET process and restriction of CN rotation, the coordination mode of 1:1 between L and Ag⁺ was proposed. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012.

Full text of DOI:10.1039/c2nj20974h

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PAPER
A Ag+-selective ‘‘off–on’’ probe based on a naphthalimide derivativew
Jun Zhang,* Chunwei Yu, Gang Lu, Qiongyao Fu, Na Li and Yuxiang Ji
Received (in Victoria, Australia) 21st November 2011, Accepted 29th December 2011
DOI: 10.1039/c2nj20974h
A new naphthalimide-based derivative L bearing an S-benzyldithiocarbazate moiety was
synthesized as an ‘‘off–on’’ probe for Ag+. The new probe has high binding selectivity for
Ag+ in water media with a detection limit of Ag+ down to 270 nM. Based on the Ag+-induced
inhibition of a PET process and restriction of CQN rotation, the coordination mode of 1 : 1
between L and Ag+ was proposed.
Introduction
The silver ion (Ag+) plays a very important role in many
industrial processes and in biology. However, a major problem
is the subsequent pollution and harm to the environment and
humans because of its frequent use. Excessive Ag+ intake can
lead to the long-term accumulation of insoluble precipitates in
the skin and eyes.1 Accordingly, the recognition and sensing of
Ag+ has been an especially active research area.2
Scheme 1 Synthesis route of L.
The traditional analytical methods used for the detection of this
ion include various instrumental techniques such as atomic
Due to excellent photophysical properties, the naphthalimide
absorption, plasma emission spectroscopy and anodic stripping
derivatives have been extensively applied to the design of colori-
voltammetric as well as potentiometric methods based on ion
metric and fluorescent probes. The photophysical properties can
selective electrodes.3 However, the direct use of these techniques is
be easily fine tuned through judicious structural design either by
restricted owing to the interference caused by matrix elements and
the aromatic ‘‘naphthalene’’ moiety itself, or at the ‘‘N-imide
the lack of certified references for extreme trace levels of metals in
site’’, to make their absorption and fluorescence emission spectra
different materials. Therefore, reliable and efficient analytical
lie within the UV and visible regions.11 In general, compounds
methods are required for the detection of Ag+ at low levels in
containing acyclic CQN bonds show weak fluorescence, while
a wide variety of biological and environmental matrices.
compounds bearing cyclic CQN bonds through blocking the
Fluorescent probes are powerful tools for monitoring
CQN isomerization are significantly fluorescent. Therefore, it
biologically relevant species in vitro/in vivo, because of their
is reasonable to amplify fluorescence upon binding metal ions to
simplicity and high sensitivity,4 so the development of fluorescent
an elaborately designed molecular framework containing the
and colorimetric probes for the detection of Ag+ has attracted
acyclic CQN bond.6a,12,13 With this in mind, we can reasonably
significant attention. So far, probes reported for Ag+ have been
expect that the CQN isomerization may also be inhibited by
classified according to their structures as cyclic N, O, S-donor
Ag+ complexation to the recognition moiety linked to the
crown ethers based probes,5 acyclic N, O, S-donor crown ethers
fluorophore. Furthermore, an S-benzyldithiocarbazate motif
based probes,6 excimer based probes,7 Ag+–p interaction utilizing
with S and N donor atoms established high affinity for thio- or
probes,8 porphyrin or phthalocyanine based probes,9 reaction
aminophilic metal ions such as Ag+. Therefore, it is anticipated
based probes and probes based on polymers, quantum dots,
that a selective fluorescence ‘‘turn-on’’ probe for Ag+ can be
nanoparticles, DNAs or oligonucleotides.10 Although consider-
established with a naphthalimide derivative L. The synthesis
able efforts have been devoted to the development of fluorescent
route of probe L is shown in Scheme 1. The structures of L were
probes specific for Ag+, probes with a fluorescent enhancement
characterized by NMR and MS spectra (Fig. S1–S3, ESIw).
technique for Ag+ still remain rare.
Experimental
Hainan Medical University, Xueyuan Road 3,
Haikou city, Hainan Province, P.R. China.
Reagents and instruments
E-mail: jun_zh1979@163.com; Fax: +86 898 66989173;
Tel: +86 898 66973190
All reagents and solvents are of analytical grade and used
without further purification. The metal ions employed are
w Electronic supplementary information (ESI) available: Experimental
details and spectra of probe L. See DOI: 10.1039/c2nj20974h
c
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NaCl, AgNO3, MgCl2Á6H2O, PbCl2, CaCl2Á2H2O, FeCl3Á6H2O,
CrCl3Á6H2O, CdCl2, Zn(NO3)2Á6H2O, CoCl2Á6H2O, MnCl2Á4H2O,
CuCl2Á2H2O, HgCl2, NiCl2Á6H2O, respectively.
Fluorescence emission spectra were obtained on a HORIBA
Fluoromax-4 spectrofluorometer. UV-vis spectra were
obtained on a Beckman DU-800 spectrophotometer (USA).
Nuclear magnetic resonance (NMR) spectra were measured
with a Bruker AVIII-500 spectrometer and chemical shifts
were given in ppm from tetramethylsilane (TMS). Mass (MS)
spectra were recorded on a Thermo TSQ Quantum Access
Agilent 1100.
General procedure for spectroscopic measurements
A stock solution of L (0.5 mM) was prepared in DMSO.
To 5 mL glass tubes, 0.10 mL L (0.5 mM) and a proper amount
of Ag+ stock solution (1.0 mM) were added subsequently and
then diluted with ethanol–water (8: 2, v/v, pH 6.5, 50 mM
HEPES). The resulting solution was mixed thoroughly. For all
measurements, excitation and emission slit widths were 5 nm,
excitation wavelength was 370 nm.
Fig. 1 Absorption spectrum of 10 mM of L at pH 6.5 in the
ethanol–water solution (8 : 2, v/v, 50 mM HEPES).
availability of the lone pairs of N of the –CQN group and O of
the OH group, which cause quenching.6a
Effects of pH on L and L with Ag+
Synthesis of probe L
The pH response of the new probe L to Ag+ was first
investigated as depicted in Fig. 2. The results revealed that
the fluorescence of the free L was insensitive to pH in the range
of 4.0–8.0. However, in the presence of the Ag+, there was an
obvious fluorescence off–on change of L in the pH range from
4.0 to 8.0. Probe L showed the highest fluorescence response
toward the Ag+ under pH 4.0–6.5, and pH 6.5 was chosen as
an optimum experimental condition in that L could work with
very low background fluorescence.
Compounds 1 and 3 were synthesized following the reported
method.13 Compound 2 was obtained according to our previous
work.14
Compound L: under N2 gas, compound 2 (582 mg,
1.5 mmol) and compound 3 (375 mg, 1.8 mmol) were dissolved
in ethanol (50 mL). The reaction solution was refluxed for 6 h
and stirred. The precipitate so obtained was filtrated and
washed with ethanol three times. The crude product was
purified by recrystallization from ethanol to give light yellow
L (626 mg, 75%). 1H NMR (d: ppm, DMSO-d6): 13.38
(s, 1H, OH), 10.63 (s, 1H, NH), 8.57 (s, 1H, ArH), 8.55–8.56
(d, 1H, ArH), 8.54 (s, 1H, ArH), 8.41–8.43 (d, 1H, ArH),
7.87–7.90 (t, 1H, ArH), 7.76–7.78 (d, 1H, ArH), 7.43 (d, 1H,
ArH), 7.41 (s, 1H, ArH), 7.31–7.34 (m, 2H, ArH), 7.25–7.28
(t, 1H, ArH), 7.20–7.22 (d, 1H, ArH), 6.75–6.77 (d, 1H, ArH),
6.71–6.72 (d, 1H, ArH), 4.49 (s, 2H, CH2), 4.02–4.05 (t, 3H,
CH2), 1.58–1.64 (m, 2H, CH2), 1.32–1.39 (m, 2H, CH2),
1.05–1.08 (t, 3H, CH3). 13C NMR (d: ppm, DMSO-d6):
195.91 (CQS), 163.83, 163.19 (CQO), 159.35, 158.63, 158.05
(ArC), 143.99 (CQN), 137.23, 132.94, 131.96, 129.70, 129.42,
129.34, 128.96, 128.53, 127.80, 127.70, 124.05, 122.72, 117.56,
117.02, 113.44, 111.87, 107.65 (ArC), 56.50, 30.13, 20.27,
19.02, 14.18. MS (ESI) 586.12 [M]À, 1136.45 [2M]À.
Fluorescent signaling of Ag+
The fluorescence spectra of L in the presence of different metal
ions in ethanol–water solution (8 : 2, v/v, 50 mM HEPES,
pH 6.5) were then recorded (Fig. 3a). The probe itself exhibits
a weak fluorescence emission band at lmax 523 nm as expected
upon excitation at lmax 370 nm. It is because of the efficient
photoinduced electron transfer (PET) quenching, which comes
from the availability of the lone pairs of N of the CQN group,
Results and discussion
Photophysical properties of L
L shows a broad absorption band (Fig. 1) at lmax 368 nm in the
absorption spectrum, which is due to an intraligand p–p*
transition. When excited at 370 nm (e = 25000 L cmÀ1 molÀ1),
it exhibits a weak fluorescence emission band at lmax 523 nm
(Fig. 3(a)). The band is typical of an excited state intramolecular
proton transfer (ESIPT) phenomenon. The band has low
intensity because of quenching due to a photoinduced electron
transfer (PET) mechanism, which comes into play due to the
Fig. 2 Influences of pH on the fluorescence spectra of L (10 mM) and
L (10 mM) plus Ag+ (50 mM) in the ethanol–water solution (8 : 2, v/v). The
pH was modulated by adding 1 M HCl or 1 M NaOH in HEPES buffers.
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Fig. 3 (a) Fluorescence spectra of 10 mM of L in the presence of
50 mM of various metal ions at pH 6.5 in the ethanol–water solution
(8 : 2, v/v, 50 mM HEPES). (b) Fluorescence response of 10 mM of L to
10 mM of Ag+ or to 50 mM of other metal ions (the black bar portions)
and to the mixture of 50 mM of other individual metal ions with 10 mM
of Ag+ (the gray bar portions).
Fig. 4 (a) Fluorescence response of 10 mM of L with various
concentrations of Ag+. (b) Linear fluorescence intensity (F/F0) of
L (10 mM) upon addition of Ag+ (0.5–9 mM).
experimental results, it is suggested that L was a good Ag+-
specific probe under aqueous conditions.
S of the CQS group and O of the OH group,15 and rotation of
the acyclic CQN bond.12 However, the fluorescence spectrum
of the probe L shows enhancement in the intensity of the
signal at 523 nm upon binding with Ag+, the increase in the
fluorescence intensity can be attributed to the PET process
being inhibited upon complexation of the Ag+, N of the CQN
group, S of the CQS group and O of the OH group, and
fluorescence is restored from the off to on signal (about
10 fold), and meanwhile this significantly enhanced fluores-
cence is somewhat due to the formation of a complex L–Ag+
in which the rotation of the acyclic CQN bond is frozen.
Among other tested cations, under the identical conditions,
only Hg2+ caused the ignored fluorescence change. The low
background fluorescence of L and the large enhancement in the
fluorescence of the Ag+-bound form of this probe would make
it more advantageous than the reported on–off type fluorescent
silver probes in terms of sensitivity and selectivity concerns.
We then examined the fluorescence responses of 10 mM of
L to 1 equiv. of Ag+ in the presence of 5 equiv. of other
metal ions, respectively. No significant variation in fluores-
cence intensity is found by comparison with that without
other metal ions besides Ag+ (Fig. 3b). From the above
To learn more about the electronic properties of L as a
probe for Ag+, fluorescence titration was carried out. Fig. 4
illustrates the emission response of probe L with increasing
Ag+ concentration. The fluorescence intensity of a 10 mM
solution of L at 523 nm enhanced with a slight ‘‘red shift’’
upon the increasing addition of Ag+, while the shape of the
emission band is not changed. Furthermore, the F/F0 was well
proportional to the amount of Ag+ (5.0 Â 10À7–9.0 Â 10À6 M)
with a good linear correlation (R = 0.999). The detection limit
was 270 nM (based on S/N = 3). The result showed that the
probe L was capable of detecting Ag+ both qualitatively and
quantitatively.
Possible sensing mechanism
The method of continuous variation (Job’s method) was used
(Fig. 5) to determine the stoichiometry of the L–Ag+ complex.
As expected, the result indicated a 1 : 1 stoichiometry of Ag+
to L in the complex, which was also supported by the
Benesi–Hildebrand method (Fig. S4, ESIw).16 The association
constant K was determined from the slope to be 5.4 Â 105 MÀ1
,
by plotting the fluorescence intensity 1/(F À F0) against 1/[Ag+].
This K value is comparable to the affinities of the reported
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and found to be stable in both alkaline and acidic solutions. The
design strategy and remarkable photophysical properties would
help to extend the development of the fluorescent probe for
metal ions. Further studies including the design of new analogues
of L with good solubility in water which enable the practical
application of these types of Ag+ probes will be implemented.
Acknowledgements
This work was financially supported by the Research and
Training Foundation of Hainan Medical University
(No. HY2010-004) and the National Natural Science Foundation
of China (No. 21007087).
Notes and references
Fig. 5 The Job’s plot indicating the 1 : 1 stoichiometry for the L–Ag+
complex, the total concentration of L and Ag+ was kept fixed at 20 mM.
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1
nature of L–Ag+ interactions. The comparison of H NMR
spectra of L and L mixed with 1.0 equiv. of Ag+ is shown
in Fig. 6. Apparently, the addition of Ag+ into the solution of
L led to an apparent downfield shift of the signals of –OH and
–NH to certain degrees.
On the basis of the above results, the plausible binding
mechanism of L in the present system is schematically depicted
in Scheme 2. N of the CQN group, S of the CQS group and
O of the OH group are engaged in complexation.
Conclusions
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´ ´
16 M. I. Rodrıguez-Caceres, R. A. Agbaria and I. M. Warner,
In conclusion, we have developed a new fluorescent probe for
Ag+ to induce enhanced fluorescence change. The fluorogenic
probe was selective and sensitive for Ag+, capable of detecting
the metal ion down to 270 nM. The probe was easily prepared
J. Fluoresc., 2005, 15, 185.
c
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This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012