A new ratiometric Ag+ fluorescent sensor based on aggregation-induced emission

Article abstract of DOI:10.1016/j.tetlet.2011.11.107

The novel tetraphenylethylene(TPE)-based sensor 1 bearing bis(2-pyridin-2-ylmethyl)amine (BPA) units linked with triazole moieties could be obtained by click reaction efficiently. The results show that 1 can demonstrate a Ag⁺-specific emission

Full text of DOI:10.1016/j.tetlet.2011.11.107

Tetrahedron Letters 53 (2012) 593–596
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A new ratiometric Ag+ fluorescent sensor based on
aggregation-induced emission
Jia-Hai Ye a, , Lingjun Duan a, Congcong Yan b, Whenchao Zhang a, Weijiang He b,
⇑
⇑
a School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
b State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The novel tetraphenylethylene(TPE)-based sensor 1 bearing bis(2-pyridin-2-ylmethyl)amine (BPA) units
linked with triazole moieties could be obtained by click reaction efficiently. The results show that 1 can
demonstrate a Ag+-specific emission shift and highly sensitive fluorescent enhancement with a 1:2 bind-
ing ratio based on the aggregation-induced emission mechanism. Compound 1 is shown to behave as a
ratiometric sensor.
Received 16 August 2011
Revised 16 November 2011
Accepted 22 November 2011
Available online 29 November 2011
Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.
Keywords:
Ratiometric
Fluorescent sensor
Aggregation
Silver detection
Tetraphenylethylene
Introduction
detection of Ag+.3–5 Nevertheless, most of the reported fluorescent
chemosensor for Ag+ are based on quenching mechanism.4
The design and synthesis of highly sensitive and selective fluo-
rescent chemosensors used for detection in the presence of heavy
and transition metal (HTM) ions continues to grow at an unabated
pace due to their important functions or produce toxic effects in
environmental chemistry and biology.1 Among the important pre-
cious metal ions, silver(I) (Ag+) has received considerable attention
due to its toxic effects and bioaccumulation. For instance, silver
ions inactivate sulfhydryl enzymes and combine with amine, imid-
azole, and carboxyl groups of various metabolites. Thus, the devel-
opment of sensitive methods for the determination of trace
amounts of silver ion in various media is of considerable impor-
tance for environmental protection and human health.2
The traditional analytical methods, such as atomic absorption
spectroscopy, inductively coupled plasma mass spectroscopy and
electrochemical analysis, have been reported for the trace-quantity
determination of Ag+. But, most of those methods are expensive and
time-consuming in practice. In comparison, fluorescence spectros-
copy is widely used because of its high sensitivity and facile opera-
tion. More importantly, most fluorescent sensors are ready for
in vivo and in vitro cellular imaging to make the fluorescence ap-
proach superior to other analytical methods. Therefore, much effort
has been devoted to design various fluorescent sensors for the
Recently, the fluorescence ‘turn on’ chemosensors for Ag+, based
on photoinduced electron transfer (PET) mechanism, internal
charge transfer (ICT) mechanism, metal chelation enhancement
and excimer/exciplex, are attracting increasing attention.2g,4–7 In
addition, various strategies have been adopted for the design of
ratiometric fluorescent chemosensors because of such a ratiometric
fluorescent sensor would offer an advantage over the intensity-
based probes such as less sensitivity to the errors associated with
the sensor concentration, photobleaching, instrument’s sensitivity,
and environmental effects. However, ratiometric fluorescent
chemosensors for the detection of Ag+ have remained rare up to
now, due to the scarcity of suitable fluorophore prototypes display-
4,6–11
ing silver chelation-induced emission/excitation.
Thus, it is
highly desirable to propose a novel sensor with ratiometric module
for silver ions. To our best knowledge, ratiometric fluorescent sen-
sor for Ag+, especially with fluorescent enhancement technique
based on aggregation-induced emission (AIE) mechanism still re-
mains rare.4a,12–14 It is well known that the fluorogen in tetraphen-
ylethylene (TPE) derivatives is nonemissive when dissolved but
becomes highly emissive when aggregated due to its intramolecu-
lar rotations by the aggregate formation, thereby showing a novel
effect of AIE.15 Herein, inspired by the ‘abnormal’ AIE behavior of
TPEs, we reported the synthesis via click reaction and the
fluorescent properties of a new ratiometric fluorescence ‘turn on’
chemosensor 1 for Ag+ by making use of AIE feature of tetraphenyl-
ethylene motif with bis(2-pyridin-2-ylmethyl)amine (BPA).
⇑
Corresponding authors. Tel.: +86 25 8430 3116; fax: +86 25 8431 5857 (J.-H.Y).
E-mail address: yejiahai@mail.njust.edu.cn (J.-H. Ye).
0040-4039/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.11.107

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J.-H. Ye et al. / Tetrahedron Letters 53 (2012) 593–596
Results and discussion
The synthesis route of 1 is shown on Scheme 1. Initially, tetra-
phenylethylene derivate 2 was chosen as the starting material
which was converted to dibromo 3 by reaction with NBS in the
presence of catalytic amount of benzoyl peroxide (BPO) under re-
flux in CCl4. The target compound 1 was obtained from the further
reaction of dibromo 3 with NaN3 followed by the click reaction
with BPA derivate 4. Similarly, another tetraphenylethylene deri-
vate 5 with BPA unit was also prepared similarly (Scheme 1). The
chemical structures of these new compounds were established
by spectroscopic and elemental analysis data.
First, we attempted to determine the selective fluoroionophoric
properties of 1 toward representative alkali (Li+, Na+, K+), alkaline
earth (Mg2+, Ca2+), and transition-metal ions (Ni2+, Cu2+, Zn2+
,
Hg2+, Pb2+, Ag+, Cd2+). After systematically looking for selective sig-
naling toward a specific target metal ion, we found that aqueous
THF solutions are relatively well optimized sensing media. There-
fore, all the fluorescence measurements were carried out in
THF:H2O (1:2,v/v) solvent ([1] = 1.0 Â 10À5 mol/L, [M] = 4.0 Â 10
À5 mol/L), where the most pronounced selectivity toward Ag+ ion
was realized (Fig. 1). Figure 2 shows the fluorescence spectrum
of compound 1 and those in the presence of different amounts of
Ag+ in THF/H2O. Free compound 1 shows rather weak emission
at 435 nm. However, after addition of Ag+, the emission band at
485 nm emerged and its intensity increased gradually when the ra-
tio of [Ag+]total/[1] is below or equal to 2:1. Moreover, the fluores-
Figure 1. Variation of the fluorescence intensity at 485 nm (kex = 320 nm) of
compound 1 (1.0 Â 10À5 M) in THF/H2O (1:2, v/v) in the presence of 4.0 equiv of the
respective metal ions.
cence intensity of
1 at 485 nm increases linearly with the
concentration of Ag+ as shown in the inset of Figure 2, where a
nearly linear plot of I485nm versus the concentration of Ag+ in the
range of 0.2–2.0
lM was displayed (with an enhancement factor
Br
N3
Figure 2. Fluorescence titration spectra of compound
1
(1.0 Â 10À5 M) in the
Br
presence of different concentrations of Ag+ in THF/H2O (1:2, v/v), [Ag+]/[1] = 0, 0.2,
0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.4, 2.5. Inset the plot of the fluorescence
intensity at 485 nm (kex = 320 nm) vs the concentration of Ag+.
N3
2
3
N
of about fourfold, linear dependency coefficient
c = 0.997). The
Click reaction
N
N
detection limit of this Ag+ assay can reach 0.2
lM. When the ratio
N
HN
N
reached 2:1, higher [Ag+]total did not lead to any further emission
enhancement. The enhancement of emission intensity in Ag+-bind-
ing titration saturated at the addition of two equivalent of Ag+ sug-
gested that 1 forms a 1:2 complex with Ag+ in THF:H2O (1:2, v/v).
Furthermore, the Job plot measurement was carried out by varying
the concentration of 1 and the Ag+ ion (see Supplementary data
Fig. S1). The maximum point appears at the mole fraction of
0.67, close to the typical ligand mole fraction (0.66), indicated
the 2:1 stoichiometry between Ag+ and 1. The remarkable batho-
chromic shift made compound 1 a potential ratiometric sensor
for Ag+. Similar fluorescence enhancement was observed fÀor 1 after
the addition of Ag+ salts with different counteranions (NO2 , FÀ, BFÀ4
and OAcÀ) (Supplementary data Fig. S2). In contrast, the titration
result indicates that compound 5 reveals no fluorescent detective
behavior toward Ag+ and other metal ions (see Supplementary data
Fig. S3).
4
BPA
N
N
N
N
N
N
N
N
N
5
1
N
N
N
N
N
N
N
Such fluorescence enhancement observed for 1 in the presence
of Ag+ is attributed to the coordination of BPA moieties of 1 with
Ag+ ions leading to the formation of coordination complexes which
may further aggregate due to the low solubility; as a result, the
N
N
fluorescence due to the tetraphenylethylene unit of
1 in-
Scheme 1. Synthesis of 1.

J.-H. Ye et al. / Tetrahedron Letters 53 (2012) 593–596
595
creases.14,15 The UV–vis spectrum of the titration of Ag+ into com-
N
N
N
Hd
N
pound 1 supports this assumption. Compound 1 exhibits the vari-
ation of the absorption spectrum of 1 after the addition of Ag+ (see
Supplementary data Fig. S4). The absorption around 320 nm de-
creased gradually in the presence of Ag+, leading to the isobestic
point at 300 nm which implies the aggregation of the complex be-
tween 1 and Ag+. Additionally, TEM analysis with the solution of 1
containing 1.0 equiv of Ag+ indicates the formation of aggregates
with fiber-like structure (Supplementary data Fig. S5). Moreover,
the variation of 1H NMR spectra of 1 in the presence of Ag+ also
indicates the aggregation of 1. 1H NMR titration experiments were
carried out in THF-d8/D2O (1:2, v/v). The partial spectral changes
are shown in Figure 3. Upon addition of Ag+ ion (from 0.4 to
2.0 equiv) to the solution of compound 1, as expected, the chemical
shifts of pyridyl proton Ha and proton Hb on the triazole ring and
the acyclic protons Hc show downshift. In contrast to these, the
acyclic proton Hd undergoes significant upshift. Furthermore, the
characterization by 1H NMR spectroscopy indicated the desired
coordination of 1 with Ag+ ion by broadened signals of Ha, Hb, Hc,
and Hd protons. Very notably, when the Ag+ ion concentration
was not more than 1 equiv (e.g., from 0 to 0.8 equiv) the chemical
shifts of Hb and Hd exhibit upshift from 7.90 to 7.95 ppm and from
7.75 to 7.84 ppm, respectively. But when the ion concentration is
increasing from 0.8 to 2.0 equiv, the chemical shift of Hb undergoes
downshift process. Meanwhile the singlet peak of Hd was split to
doublet peaks. This result implies that the intramolecular coordi-
nation types between 1 and Ag+ at different ion concentrations.
When the Ag+ ion was of lower concentration (61 equiv of 1),
two molecules of 1 coordinate with one Ag+ ion at head to tail type.
As the Ag+ ion concentration increases, one molecule 1 coordinate
with two Ag+ ions with its two chelating BPA groups (Scheme
2).5b,16 These two different binding modes can explain the change
of the wavelength of maximum emission. As shown in Figure 2,
N
Hb
N
Ag
N
N
N
Ha
N
N
N
N
1:1Ag+
(a)
N
N
N
Hd
N
Ag
N
N
N
Ag
Hb
N
N
N
N
1:2Ag+
Ha
(b)
Scheme 2. Proposed binding modes of 1 with Ag+.
upon addition of Ag+ from 0.2 to 0.8 equiv, the wavelength of max-
imum emission changed from 435 nm to 485 nm, while upon addi-
tion of Ag+ from 1.0 to 2.5 equiv, a little bathochromic shift was
observed.
An important feature of many prospective metal ion sensors is
its ability to function in competition with other relevant metal
ions. The competition experiments between Ag+ and the selected
metal ions were shown in Figure S6 (Supplementary data). When
4.0 equiv of Ag+ was added into the solution of 1 (10
lM) in the
presence of 4.0 equiv of other metal ions, the emission spectra dis-
played a pattern similar to that with Ag+ only with most of the me-
tal ions studied (kem = 485 nm, kex = 320 nm) except Hg+/Zn2+
which also gave the clear fluorescence enhancement (threefold/
onefold, respectively) and Cu2+ which quenched the fluorescence
due to the metal-to-ligand charge transfer upon excitation or the
high affinity for the competition metal ion with BPA and triazole
unit.16c,17 Thus sensor 1 can be used as a selective fluorescent sen-
sor for Ag+ in the presence of most competing metal ions. In addi-
tion, we evaluated the effect of pH change on the fluorescence
intensity of 1 in the absence and presence of Ag+ ion. The fluores-
cence intensity measurements were made in the absence and pres-
ence of a 4.0 equiv of Ag+ in 1.0 Â 10À5 M solution of 1 at different
pH values. The pH of solution was adjusted by either HClO4 or
NaOH. As shown in Figure S7 (Supplementary data), sensor 1 dis-
played good fluorescence sensing ability to Ag+ and in the absence
of Ag+ over a wide range of pH, and the intensity was almost stable
around pH = 4.0–12.0, which makes it suitable for application in
physiological conditions. These results indicate that 1 can be em-
ployed as a selective fluorescent probe to recognize and distinguish
Ag+ in the presence of various interfering and biologically relevant
metal ions.
Conclusion
In conclusion, a novel tetraphenylethylene (TPE)-based sensor 1
incorporating bis(2-picoly)amine (BPA) motifs and triazole units
was prepared via click reaction successfully. Compound 1 demon-
strates a Ag+-specific ratiometric and highly sensitive turn-on
sensing behavior based on the aggregation mechanism. The spec-
troscopic properties of compound 1 indicate that it is a potential
powerful sensor which is likely to image free silver ions in extre-
mely low range in biological system.
Figure 3. Partial 1H NMR spectra (500 MHz) of compound 1 (1 Â 10À3 M) in THF-
d8/D2O (1:2, v/v) in the presence of increasing amounts of Ag+: 0; 0.4; 0.8; 1.6;
2.0 equiv [from (1) to (5)].

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Supplementary data
Supplementary data associated with this article can be found, in
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