Up to now, a variety of Zn2þ-selective fluorescent
sensors have been reported. Most of them are based on
photoinduced electron transfer (PET),8 internal charge
transfer (ICT),9 and fluorescence resonance energy trans-
fer (FRET),10 respectively. Nevertheless, most of these
chemosensors for Zn2þ have some limitations. For in-
stance, some exhibit low water-solubility, and thus, organic
solvents are needed for Zn2þ detection;11 interferences
Scheme 1. Chemical Structure of 1 and the Synthetic Approach
from other metal ions still exist, especially from Cd2þ 8d,12
.
Herein we report a new fluorescence turn-on sensor with
compound 1 for Zn2þ by taking advantage of the aggrega-
tion-induced emission (AIE) feature of tetraphenylethy-
lene compounds (Scheme 1).13,14 As will be discussed
below, the sensing mechanism of 1 toward Zn2þ is attrib-
uted to two facts: (1) the coordination between Zn2þ and
ꢀN(CH2COOꢀ)2 groups in 1 should inhibit the intra-
molecular PET; (2) the intermolecular coordination of 1
with Zn2þ will lead to aggregation and fluorescence en-
hancement. Additionally, compound 2, the ester precursor
of 1, is potentially applicable for intracellular Zn2þ imaging.
The syntheis of 1 started from tetraphenylethylene 3 as
shown in Scheme 1. In the presence of HNO3 and HOAc, 3
was nitrated into tetra-p-nitrophenylethylene 4, which was
reduced to tetra-p-aminophenylethylene 5 with Raney Ni
90% yield. The synthetic details and characterization data
are provided in the Supporting Information.
and N2H4 H2O.15 Compound 5 was allowed to react with
3
excess ethyl bromoacetate in the presence of NaH, leading
to 2 in 38% yield. Hydrolysis of 2 with KOH afforded 1 in
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2003, 103, 3899. (b) Komatsu, K.; Urano, Y.; Kojima, H.; Nagano, T.
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He, W.; Gao, X.; Hu, P.; Guo, Z. J. Am. Chem. Soc. 2009, 131, 1460.
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Figure 1. Fluorescence spectra (λex = 340 nm) of 1 (10 μM) upon
addition of 0ꢀ6.0 equiv of Zn2þ [ZnSO4] in HEPES buffer (50
mM, pH = 7.4, 0.1 M NaCl); inset shows the linear increase of
the fluorescence intensity at 485 nm for the ensemble of 1 and 1.0
equiv of Zn2þ upon further addition of Zn2þ
.
Compound 1 can be well dissolved in aqueous solution,
and the HEPES buffer solution (pH = 7.4, HEPES = N-
2-hydroxyethylpiperazine-N0-2-ethanesulfonic acid) of 1
was prepared for the absoprtion and fluorescence spectral
studies. To maintain the ionic strength, NaCl (0.1 M) was
added to each solution. As depicted in Figure 1, the
solution of 1 was almost nonemissive in the absence of
Zn2þ [ZnSO4] as expected.13,14 However, the fluorescence
started to increase gradually after addition of Zn2þ. Figure
S1a (Supporting Information) shows the variation of the
fluorescence intensity at 485 nm of 1 vs the amount of Zn2þ
added to the solution. Clearly, the fluorescence enhance-
ment was rather minor when less than 1.0 equiv of Zn2þ
was present, but it became much more obvious when more
than 1.0 equiv of Zn2þ was added. Compound 1 can be
regarded as a new off-on fluorescent sensor for Zn2þ. In
fact, the fluorescence intensity at 485 nm for the ensemble
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