Highly sensitive and selective fluorescent chemosensor for Ag+ based on a coumarin-Se2N chelating conjugate

Article abstract of DOI:10.1039/c0cc04589f

A highly sensitive and selective fluorescent chemosensor SC1 for Ag ⁺ based on a coumarin-Se₂N chelating conjugate has been synthesized and characterized. Due to inhibiting a photoinduced electron transfer (PET) quenching pathway, a fluorescent enhancement factor of 4-fold is observed under the binding of the Ag⁺ cation to the chemosensor SC1 with a detection limit down to the 10^-8 M range.

Full text of DOI:10.1039/c0cc04589f

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COMMUNICATION
Highly sensitive and selective fluorescent chemosensor for Ag+ based
on a coumarin–Se2N chelating conjugatew
Shanshan Huang,a Song He,a Yan Lu,*a Fangfang Wei,a Xianshun Zeng*a and
Liancheng Zhaoab
Received 25th October 2010, Accepted 23rd November 2010
DOI: 10.1039/c0cc04589f
A highly sensitive and selective fluorescent chemosensor SC1 for Ag+
based on a coumarin–Se2N chelating conjugate has been synthesized
and characterized. Due to inhibiting a photoinduced electron transfer
(PET) quenching pathway, a fluorescent enhancement factor of 4-fold
is observed under the binding of the Ag+ cation to the chemosensor
SC1 with a detection limit down to the 10À8 M range.
and selective optical sensors for Ag+ with simple instrumental
implementation and easy-operation have received much
attention.6–8 But, Ag+ belongs to the so-called ‘‘silent ions’’
since, unlike some biological metal ions (such as Cu2+ or Fe2+),
it does not have an intrinsic spectroscopic or magnetic signal
because of its d10 electronic configuration which is known as
fluorescence quencher, discrimination between Ag+ and
chemically close ions presents a challenge. The fluorometric
detection of such ions is based on the use of small-molecule
fluorescent chemosensors that consist of an ionophore–
chromophore system. These chemosensors may coordinate
selectively to the target ion and respond to changes in ion
levels with changes in their fluorescence profile that may be
translated to accurate measurements of the ion concentrations.
Most of the reported fluorescent chemosensors for Ag+ are
based on a fluorescence quenching mechanism.7 Only a few
fluorescence ‘‘turn on’’ chemosensors for Ag+ have been
described and developed by now.8
As heavy metal ions can cause severe risks for the environment
and human health, highly selective cation sensing is imperative
for many areas of technology, including environmental,
biological, clinical, and waste management applications.1
Silver ions are known to bind with various metabolites,
including amine, imidazole, and carboxyl groups, and
inactivate sulfhydryl enzymes.2 There are also many reports
on silver bioaccumulation and toxicity.3 On the other hand, the
widely used silver nanoparticles (AgNPs)4 are also known to
generate highly reactive oxygen species and may destroy the
environmentally benign bacteria by inhibiting their growth and
disturbing their reproductive ability.2 Thus, development of
sensitive and selective methods for the determination of trace
amounts of silver ions (Ag+) in various media is of considerable
importance for the environment and human health.
Traditional quantitative approaches,5 such as atomic
absorption spectroscopy and inductively coupled plasma-
mass spectroscopy, have been reported for the trace-quantity
determination of Ag+. Potentiometric methods for Ag+ assay
based on ion-selective electrodes (ISEs) are also reported with
a detection limit down to the ppb range. But, most of these
methods are expensive and time-consuming in practice.
Therefore, in addition to the sophisticated methods, sensitive
Herein, we report a new Ag+-ion-specific fluorescent
chemosensor Silver Coumarin
1
(SC1), which takes
advantage of both the high selenophilicity of Ag+ and high
photostability of coumarin fluorophore. The choice of the
Se2N ionophore was based on our previous reports using
similar pendant or crown ether systems for the coordination
of heavy metals.9 The chemosensor SC1 has excellent
selectivity for Ag+ ions over relevant competing metal ions
with a fluorescent enhancement factor of 4-fold by inhibiting a
photoinduced electron transfer (PET) quenching pathway,
thus resulting in the enhancement of fluorescence,1c and with
the detection limit reaching the 10À8 M range in ethanol/water.
The synthesis of SC1, Scheme 1, began with SeN2 receptor 1,
formed by the reaction of bis(2-chloroethyl)amine with the
sodium salts of phenylselenol, which was prepared in situ by
the reaction of 1,2-diphenyldiselenide with NaBH4 in the
a Key Laboratory of Display Materials & Photoelectric Devices,
Ministry of Education, School of Materials Science & Engineering,
Tianjin University of Technology, Tianjin 300384, China.
E-mail: luyan@tjut.edu.cn, xshzeng@tjut.edu.cn;
Fax: +86 22-60215226; Tel: +86 22-60216748
b School of Materials Science and Engineering, Institute of Information
Functional Materials & Devices, Harbin Institute of Technology,
Harbin 150001, China. E-mail: lczhao@hit.edu.cn;
Fax: +86 451-86418329; Tel: +86 451-86418329
w Electronic supplementary information (ESI) available: Experimental
details; synthesis of 1 and SC1; the emission spectra of SC1 with Ag+
salts with different counteranions (ClO4À, NO2À, PF6À, AcOÀ and
BF4À); Job’s plot; 1H and 13C NMR spectra of 1 and SC1 or other
electronic format. See DOI: 10.1039/c0cc04589f
Scheme 1 Synthesis of chemosensor SC1.
c
2408 Chem. Commun., 2011, 47, 2408–2410
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presence of NaOH. SC1 was prepared successfully via a
Mannich reaction by the condensation of 1 and Coumarin 4
with formaldehyde in THF/HOAc at 50 1C. All the reactions
were achieved in reasonable yields and characterized by MS,
1H- and 13C-NMR, and elemental analysis.
The spectroscopic properties of the chemosensor SC1 were
evaluated in a ethanol/water (1 : 1, v/v) solution. As shown in
Fig. 1, the chemosensor SC1 exhibits a maximal absorption at
320 nm (e = 1.035 Â 104 MÀ1 cmÀ1). Upon addition of silver
ions (0–3 equiv.), the absorbance at 320 nm decreases
gradually, and simultaneously, a significant absorption band
like a shoulder at 375 nm increases. The presence of three clear
isobestic points at 250, 271, 348 nm implies the conversion
of the free chemosensor SC1 to the only Ag+ complex.
Moreover, the absorbance at 375 nm remains constant in the
presence of more than 1 equiv. of silver ions, indicating the
formation of a 1 : 1 complex between the chemosensor SC1
and silver ion, which is in good agreement with 1 : 1
stoichiometry for the silver complex determined by Job’s plot
yielded from UV-vis absorption (Fig. S1, ESIw).
Fig. 2 Fluorescent titration spectra of SC1 (4 mM) in the presence
of different concentrations of Ag+ in ethanol/water (1 : 1, v/v),
[Ag+]/[SC1] = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.2,
1.7, 2.2. Inset: the fluorescence at 445 nm of SC1 (4 mM) as a function
of the Ag+ concentration. lex = 360 nm.
signifies the regeneration of the free SC1. The fluorescence can
be recovered by adding Ag+ again (Fig. S5, ESIw).
Subsequently, we evaluated the response of the chemosensor
SC1 to other metal ions. As shown in Fig. 3, the addition of 2
Fig. 2 shows the fluorescence spectrum of compound
SC1 and those in the presence of different amounts of Ag+
in H2O/ethanol (1 : 1, v/v). As displayed in Fig. 2, the titration
of Ag+ into SC1 gave a new emission band centered at 445 nm
which showed a linear enhancement with the increase of
[Ag+]total when the ratio of [Ag+]total/[SC1] is below or
equal to 1 : 1. When the ratio reached 1 : 1, however, higher
[Ag+]total did not lead to any further emission enhancement as
shown in the inset of Fig. 2. The enhancement of emission
intensity in Ag+-binding titrations saturated at the addition of
one equivalent of AgNO3 suggested that SC1 forms a 1 : 1
complex with Ag+ in H2O/ethanol (1 : 1, v/v), which is in
agreement with the UV/vis titration results. The association
constant is calculated as 1.61 Â 108 MÀ1(R = 0.999) by using
nonlinear least-square analysis (Fig. S2, ESIw). From the changes
in Ag+-dependent fluorescence intensity (Fig. S3, ESIw), the
detection limit was estimated to be 5.2 Â 10À8 M.10 Similar
fluorescence enhancement was observed for SC1 after
the addition of Ag+ salts with different counteranions
(ClO4À, NO2À, PF6À, AcOÀ and BF4À) (Fig. S4, ESIw).
Reversible binding of SC1 with Ag+ was also carried out.
Addition of Na2S to a mixture of SC1ÁAg+ results in
diminishing of the fluorescence intensity at 445 nm, which
equiv. of Na+, K+, NH4+, Mg2+, Ca2+, Pb2+, Co2+, Cd2+
,
Zn2+, Ni2+, Cr3+, Fe3+, and Al3+ has no obvious effect on
the fluorescence emission, whereas Hg2+, Cu2+ responded
with a significant decrease in the fluorescent intensity. In
contrast, the addition of Ag+ resulted in an enhancement of
the emission intensity (with an enhancement factor of 4-fold)
positioned around 445 nm as shown in Fig. 3. On the other
+
hand, upon the addition of 50 equiv. of Na+, K+, NH4
,
Mg2+, Ca2+, Pb2+, Co2+, Cd2+, Zn2+, Ni2+, Cr3+, Fe3+ to
the chemosensor SC1 solution, only Na+, NH4+, Mg2+ and
Ca2+ produced a much weaker fluorescence enhancement
compared to Ag+. Pb2+ and most of transition metal ions
responded with a significant decrease in the fluorescent
intensity (Fig. S6, ESIw); thus, SC1 can function as an highly
selective fluorescence chemosensor for the Ag+ cation.
To explore further the utility of SC1 as an ion-selective
fluorescent chemosensor for Ag+, the competition experiments
are conducted in which SC1 (5.0 mM) is first mixed with five
Fig. 3 Fluorescence spectra of SC1 (5.0 mM) upon the addition of the
nitrate salts (2.0 equiv.) of Na+, K+, NH4+, Ag+, Mg2+, Ca2+
,
Pb2+, Hg2+, Co2+, Cd2+, Zn2+, Ni2+, Cu2+, Cr3+, Fe3+, and Al3+
in ethanol/H2O (1 : 1, v/v). Inset: histogram representing the
fluorescence enhancement and quenching of SC1 in the presence of
metal ions. For the entire test, excitation and emission were performed
at 360 and 445 nm.
Fig. 1 UV-vis spectra of chemosensor SC1 (10.6 mM) upon titration
of Ag+ (0–3 equiv.) in ethanol/water (1 : 1, v/v), [Ag+]/[SC1] = 0,
0.08, 0.16, 0.25, 0.32, 0.42, 0.5, 0.58, 0.66, 0.75, 0.83, 0.92, 1, 1.5, 2,
2.5, 3. Inset: absorbance of SC1 at 375 nm as a function of CAg+/[CSC1].
c
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Fig. 4 Change ratio ((Fi À F0)/(FAg À F0)) of fluorescence intensity
of SC1 upon the addition of 1 equiv. Ag+ in the presence of 5 equiv.
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chemosensor SC1 is able to discriminate between Ag+ and
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common interfering anions in many cases7f–i,8a–c are eliminated.
In summary, we have prepared a fluorescence chemosensor
SC1 for silver ions in ethanol/water, which demonstrates a unique
ability to discriminate between Ag+ and chemically close ions
and with a detection limit down to the 10À8 M range. The highly
selective and sensitive Ag+-chemosensor SC1 may have wide
applications for quantitative measurement of silver ions.
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This work was sponsored by the NCET-09-0894, NNSFC
(20972111, 21074093, 21004044), the NSFT (08JCYBJC-26700),
SRF for ROCS, SEM and the State Key Lab. Elemental-
Organic Chemistry at Nankai University (NO. 0802, 0908).
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