Effect of the counterion on light emission: A displacement strategy to change the emission behaviour from aggregation-caused quenching to aggregation-induced emission and to construct sensitive fluorescent sensors for Hg2+ detection

Article abstract of DOI:10.1002/chem.201303251

In this paper, a simple strategy to change the emission behaviour of luminogenic materials was developed. Tetraphenylethene (TPE)-functionalised benzothiazolium salts with different counteranions (TPEBe-X; X=I^-, ClO₄^- and PF₆^-) were designed and synthesised. All the luminogens show weak red emission in the solution state that originates from intramolecular charge transfer from TPE to the benzothiazolium unit. Whereas aggregate formation enhances the light emission of TPEBe-ClO₄ and TPEBe-PF₆, that of TPEBe-I is quenched, thus demonstrating the phenomena of aggregation-induced emission and aggregation-caused quenching. TPEBe-I works as a light-up fluorescent sensor for Hg²⁺ in aqueous solution with high sensitivity and specificity owing to the elimination of the emission quenching effect of the iodide ion by the formation of HgI₂ as well as the induction in aggregate formation by the complexation of Hg²⁺ with the S atom of the benzothiazolium unit of TPEBe-I. A solid film of TPEBe-I was prepared that can monitor the level of Hg²⁺ in aqueous solution with a detection limit of 1 μM. From darkness to light! A series of tetraphenylethene-functionalised benzothiazolium salts with different counteranions (TPEBe-X) was developed. By varying the counteranion from I^- to ClO₄^- or PF ₆^-, TPEBe-X changes its emission behaviour from aggregation-caused quenching to aggregationinduced emission (see figure). TPEBE-I shows a light-up response to Hg²⁺. Copyright

Full text of DOI:10.1002/chem.201303251

DOI: 10.1002/chem.201303251
Full Paper
&
Sensors
Effect of the Counterion on Light Emission: A Displacement
Strategy to Change the Emission Behaviour from Aggregation-
Caused Quenching to Aggregation-Induced Emission and to
Construct Sensitive Fluorescent Sensors for Hg2+ Detection
Na Zhao,[a, b] Jacky W. Y. Lam,[a, b] Herman H. Y. Sung,[b] Hui Min Su,[c] Ian D. Williams,[b]
Kam Sing Wong,[c] and Ben Zhong Tang*[a, b, d]
Abstract: In this paper, a simple strategy to change the
emission behaviour of luminogenic materials was developed.
Tetraphenylethene (TPE)-functionalised benzothiazolium
nomena of aggregation-induced emission and aggregation-
caused quenching. TPEBeÀI works as a light-up fluorescent
sensor for Hg2+ in aqueous solution with high sensitivity
and specificity owing to the elimination of the emission
quenching effect of the iodide ion by the formation of HgI2
as well as the induction in aggregate formation by the com-
plexation of Hg2+ with the S atom of the benzothiazolium
unit of TPEBeÀI. A solid film of TPEBeÀI was prepared that
can monitor the level of Hg2+ in aqueous solution with a de-
tection limit of 1 mm.
salts with different counteranions (TPEBeÀX; X=IÀ, ClO4
À
and PF6À) were designed and synthesised. All the lumino-
gens show weak red emission in the solution state that orig-
inates from intramolecular charge transfer from TPE to the
benzothiazolium unit. Whereas aggregate formation en-
hances the light emission of TPEBeÀClO4 and TPEBeÀPF6,
that of TPEBeÀI is quenched, thus demonstrating the phe-
Introduction
tion state, they become weakly emissive or even non-emissive
when aggregated in the condensed phase. This aggregation-
caused quenching (ACQ) effect has significantly limited the
high-tech application of conventional fluorophores.[2] Various
physical and chemical methods have been utilised to mitigate
aggregate formation but have met with little success as they
are working against the natural process.[3] In 2001, we ob-
served a phenomenon of aggregation-induced emission (AIE)
in some propeller-shaped molecules that is the exact opposite
of the ACQ effect.[4] These luminogens are non-emissive in the
solution state, but are induced to emit intensely upon aggre-
gate formation. Such a discovery has attracted much interest
and many research groups in the world are now conducting
AIE research.[5–7] Their enthusiastic research efforts have led to
the generation of new AIE fluorogens and exploration of new
technological applications. Incorporation of an AIE unit into
ACQ fluorophores has solved their ACQ problem and at the
same time imparted them with AIE characteristics without
causing any adverse effects, thus demonstrating a novel ap-
proach to widen the family of AIE luminogens with desirable
properties.[8,9]
The development of efficient luminescent materials in the ag-
gregated or solid state is a topic of current interest on account
of their potential applications in the fields of electronics, optics
and biological sciences.[1] In response to this demand, scientists
have prepared a large number and a wide variety of fluores-
cent materials. Whereas they are highly emissive in the solu-
[a] Dr. N. Zhao, Dr. J. W. Y. Lam, Prof. B. Z. Tang
HKUST-Shenzhen Research Institute, No. 9 Yuexing 1st RD
South Area, Hi-tech Park, Nanshan, Shenzhen, 518057 (P.R. China)
Fax: (+852)2358-1594
E-mail: tangbenz@ust.hk
[b] Dr. N. Zhao, Dr. J. W. Y. Lam, Dr. H. H. Y. Sung, Prof. I. D. Williams,
Prof. B. Z. Tang
Department of Chemistry, Division of Biomedical Engineering
Institute for Advanced Study and Institute of Molecular
Functional Materials, The Hong Kong University of
Science & Technology (HKUST), Clear Water Bay
Kowloon, Hong Kong (P.R. China)
[c] Dr. H. M. Su, Prof. K. S. Wong
Department of Physics, HKUST, Clear Water Bay
Kowloon, Hong Kong (P.R. China)
Sensitive and specific sensors for Hg2+ detection are in great
demand because of the serious deleterious effects of mercury
on the environment and human health.[10] To date, a variety of
fluorescent Hg2+ sensors have been developed from conven-
tional fluorophores based on the mechanism of fluorescence
resonance energy transfer,[11] photoinduced electron transfer[12]
and intramolecular charge transfer.[13] AIE luminogens have
also been utilised as sensory materials for Hg2+ detection.[14,15]
[d] Prof. B. Z. Tang
Guangdong Innovative Research Team
SCUT-HKUST Joint Research Laboratory
State Key Laboratory of Luminescent Materials and Devices
South China University of Technology
Guangzhou 510640 (P.R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201303251.
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Upon complexation with Hg2+, the intramolecular rotation of
the sensor is restricted, which blocks the non-radiative relaxa-
tion channel and thus converts the luminogens into strong
emitters. Since most AIE luminogens are hydrophobic in
nature and emit efficiently in the aggregated state, to maxi-
mise the sensitivity, the detection is normally carried out in
pure organic solvent that is miscible with the aqueous Hg2+
solution. Development of water-soluble AIE luminogens might
solve such problem, but it is not easy from the synthetic point
of view. On the other hand, for practical application, it is pref-
erable to perform the Hg2+ detection on a solid support be-
cause it requires no expensive spectrofluorometer and is thus
simple and convenient. Unfortunately, such sensing systems
are rare.
In this paper, we report a simple strategy to tune the emis-
sion behaviour of a tetraphenylethene (TPE)-functionalised
benzothiazolium salt (TPEBeÀX; Scheme 1). By varying the
À
counteranion from IÀ to ClO4 or PF6À, TPEBeÀX changes its
Scheme 1. Structure of TPEBeÀX and ORTEP drawing of TPEBeÀClO4. Anions
are omitted for clarity.
emission behaviour from ACQ to AIE. Addition of Hg2+ into
the aqueous solution of TPEBeÀI enhances the light emission
by 80-fold, which suggests that it is an excellent fluorescent
Hg2+ sensor with high sensitivity and selectivity. The sensing
process can also be performed in solid film with a detection
limit of 1 mm, thereby demonstrating the use of AIE com-
pounds for Hg2+ detection in real-world conditions.
Figure 1. Emission spectra of A) TPEBeÀI, C) TPEBeÀClO4 and E) TPEBeÀPF6 in
DMSO and DMSO/water mixtures with different water fractions (fw). Plots of
relative emission intensity (I/I0) versus the composition of the aqueous mix-
tures of B) TPEBeÀI, D) TPEBeÀClO4 and F) TPEBeÀPF6; I0 =emission intensity
in pure DMSO solution. Solution concentration: 20 mm; excitation wave-
length: 420 nm. Inset: Photographs of B) TPEBeÀI, D) TPEBeÀClO4 and
F) TPEBeÀPF6 in DMSO/water mixtures with fw values of 0 and 99 vol%
under 365 nm UV irradiation.
Results and Discussion
The TPE-functionalised benzothiazolium salts (TPEBeÀI, TPEBeÀ
ClO4 and TPEBeÀPF6) were prepared according to the synthetic
routes shown in Scheme S1 of the Supporting Information.
Whereas TPEBeÀI and TPEÀClO4 are new compounds, TPEBeÀ
PF6 was prepared in our previous work.[16] Briefly, the coupling
reaction of 3-ethyl-2-methyl-1,3-benzothiazol-3-ium iodide and
4-(1,2,2-triphenylvinyl)benzaldehyde in ethanol heated at reflux
generated TPEBeÀI in a high yield, which underwent counter-
anion exchange in the presence of sodium perchlorate and po-
tassium hexafluorophosphate, respectively, to give TPEBeÀClO4
tal data are summarised in Table S1 of the Supporting Informa-
tion.
As shown in Figure 1, photoexcitation of the dilute solutions
of all the luminogens in dimethylsulfoxide (DMSO) induces
a red emission peak at 682 nm that originated from the intra-
molecular charge transfer (ICT) from the electron-donating TPE
group to the electron-accepting benzothiazolium unit. The
fluorescence quantum yield (FF) using 4-(dicyanomethylene)-
2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (FF =82% in di-
methylformamide) as standard is all equal to approximately
2.00%, presumably due to the ICT effect as well as active intra-
molecular rotation, which consumes the energy of the excitons
through the non-radiative relaxation channel. The emission
was further weakened upon addition of water into the solution
as the ICT effect becomes intensified by increasing the polarity
of the solvent mixture. The higher the water content, the
1
and TPEBeÀPF6. All the final products were characterised by H
and 13C NMR and mass spectroscopies, from which satisfactory
results corresponding to their molecular structures were ob-
tained (see the Supporting Information for details). Single crys-
tals of TPEBeÀClO4 were grown by slow evaporation of its di-
chloromethane/ethanol mixture and analysed crystallograph-
ically. The ORTEP drawing is shown in Scheme 1, and the crys-
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lower the light emission (Figure 1B, D and F). Interestingly,
starting from a water fraction of 90 and 80%, respectively, the
TPEBeÀClO4 and TPEBeÀPF6 solutions become emissive again,
whereas that of TPEBeÀI remains weakly fluorescent. At 99%
aqueous solution, almost no light was emitted from the aque-
ous mixture of TPEBeÀI, and the appearance of the solution
changes from red in pure DMSO to dark in 99% aqueous mix-
ture under 365 nm UV illumination (inset in Figure 1B). In con-
trast, the aqueous mixtures of TPEBeÀClO4 and TPEBeÀPF6
show stronger emission peaks at 643 nm than their pure
DMSO solutions. Clearly, TPEBeÀI demonstrates an ACQ phe-
nomenon, whereas TPEBeÀClO4 and TPEBeÀPF6 are AIE-active.
The FF values of their powders determined by a calibrated in-
tegrating sphere are approximately 0.01, 8.20 and 9.48%, with
lifetimes of approximately 76 ps, 1.23 ns and 0.84 ns, respec-
tively, thus substantiating the conclusion stated above.
corresponding luminogens (TPEBeÀClO4 or TPEBeÀPF6) in the
aggregated state. Therefore this simple strategy provides an
opportunity to generate AIE luminogens with efficient solid-
state light emission.
Hg2+ is known to show a high affinity toward the iodide
ion.[20] With this in mind, it is envisioned that Hg2+ can displace
the cationic unit of TPEBeÀI, and the removal of the iodide ion
might affect the optical properties of the luminogen. Given
such a possibility, we thus tested the response of TPEBeÀI to
the presence of Hg2+ in aqueous buffer solution (pH 7.4,
20 mm 2-[4-(2-hydroxyethyl)piperazin-1-yl]ethanesulfonic acid
(HEPES) buffer with 1% DMSO) by both UV/Vis and fluores-
cence spectroscopies. Each sample was measured per minute.
As shown in Figure 2, upon gradual addition of Hg2+ into the
The luminogens must aggregate in aqueous mixture with
a large amount of water on account of their poor solubility in
water. However, all the solutions are homogeneous without
precipitates, which reveals that the formed particles are of
nanodimensions. This is indeed the case, as revealed by the ex-
istence of nanoparticles with average sizes of approximately
50–100 nm in 99% aqueous mixture by TEM analysis (Figure S1
in the Supporting Information). The emission spectra of thin
films of the luminogens are illustrated in Figure S2 of the Sup-
porting Information. The spectrum of TPEBeÀI is basically a flat
line parallel to the abscissa. However, intense emission signals
with peaks at 650 nm are recorded in TPEBeÀClO4 and TPEBeÀ
PF6. When observed under 365 nm UV irradiation, the thin film
of TPEBeÀI appears black, whereas those of TPEBeÀClO4 and
TPEBeÀPF6 show strong red emission. Such a distinctly differ-
ent appearance allows easy differentiation of TPEBeÀI among
the luminogens by the naked eye.
It is noteworthy that all the luminogens possess the same
cationic unit. Thus, the counteranion should play a key role in
their different emission behaviours in the condensed phase.
Iodine is a well-known effective fluorescent quencher due to
its heavy-atom effect, which helps to accelerate the rate of in-
tersystem crossing and hence promote non-radiative relaxation
by colliding with the luminogenic molecules.[17] In the solution
state, the TPEBeÀI molecules exist in the form of solvent-sepa-
rated ion pairs. Such a structure possesses a low rate of collid-
ing, and thus the quenching effect of IÀ is disabled.[18] In the
condensed phase, however, the drastic collision between the
iodide ion and the cationic unit in the state of closely contact-
ing ion pairs has annihilated the excitons, thus rendering
TPEBeÀI non-emissive in the aggregated or solid state. Chang-
Figure 2. A) UV/Vis and C) emission spectra of TPEBeÀI in aqueous buffer
solutions (pH 7.4, 20 mm HEPES buffer with 1% DMSO) with different con-
centrations of Hg2+ (0–2.0 mm). B) Plot of relative absorbance (A/A0) and
D) emission intensity (I/I0) versus the Hg2+ concentration. A0 =absorbance at
420 nm in the absence of Hg2+; I0 =the emission intensity at 640 nm in the
absence of Hg2+. Inset: Photographs of TPEBeÀI in aqueous buffer with
[Hg2+] of 0 and 2.0 mm taken under B) daylight and D) 365 nm UV illumina-
tion. Solution concentration: 20 mm; excitation wavelength: 480 nm.
aqueous solution of TPEBeÀI, the absorption peak at approxi-
mately 420 nm became weaker progressively and shifted
slightly to the redder region. Meanwhile, a level-off absorption
tail appears at the longer-wavelength region, thus revealing
the formation of aggregates.[5] The solution colour also
changes from yellow into colourless, thereby allowing rapid
detection of Hg2+ in aqueous solution by the naked eye. Cor-
respondingly, the emission of TPEBeÀI is turned on in the pres-
ence of Hg2+ (Figure 2C). At an Hg2+ concentration of 2.0 mm,
the emission intensity is 80-fold higher than that of aqueous
buffer without metal ions. When observed under 365 nm UV il-
lumination, the original non-emissive solution now shows in-
À
À
ing the counteranion to ClO4 or PF6 alters the emission be-
haviour in the solution state only a little, but recovers the in-
trinsic AIE characteristic of the TPE unit of TPEBeÀClO4 and
TPEBeÀPF6 in the condensed phase. Clearly, this displacement
strategy of counterion provides a simple and novel route for
changing the emission behaviour of a luminogen from ACQ
into AIE. According to previous reports[19] as well as crystal
structure of TPEBeÀClO4 and TPEBeÀPF6,[16] the size of the
À
À
ClO4 or PF6 and its interaction with the cationic unit seems
to exert a strong influence on the emission efficiency of the
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Figure 3. A,B) Bright-field and C,D) fluorescent images of TPEBeÀI (20 mm) in
aqueous buffer solutions with A,C) 0 and 2.0 mm B,D) of Hg2+. Scale bar:
100 mm.
tense red fluorescence, thereby enabling TPEBeÀI to serve as
a “light-up” fluorescent sensor for Hg2+ detection with a detec-
tion limit of 1 mm in aqueous solution.
Figure 4. Top: Photographs of TPEBeÀI in aqueous buffer solutions (20 mm)
with various metal ions (2.0 mm) taken under 365 nm UV illumination.
Bottom: Emission response of TPEBeÀI to various metal ions. Black bars rep-
resent the addition of different metal ions (2.0 mm) to the aqueous solution
of TPEBeÀI, whereas the red bars represent the subsequent addition of Hg2+
(2.0 mm) to the solution containing a specific metal ion. Excitation wave-
length: 480 nm; emission wavelength: 640 nm.
The Hg2+ sensing process was also examined under a fluo-
rescent microscope. As depicted in Figure 3A and B, nearly no
emissive particles are observed in the pure buffer solution of
TPEBeÀI. Addition of Hg2+ (2.0 mm) into the solution resulted
in the formation of more and larger particles with red emission
(Figure 3B and D). This result shows that Hg2+ induces the ag-
gregate formation of TPEBeÀI.
Excepting sensitivity, a fluorescent sensor should possess an
excellent selectivity for its targeting analyte. Thus, the fluores-
cence response of TPEBeÀI to various metal ions including
under acidic conditions. At pH 11, the non-fluorescent TPEBeÀI
solution alters little, even upon addition of Hg2+. The forma-
tion of Hg(OH)2 in strong basic buffer reduces the amount of
free Hg2+ in solution and thus led to no emission enhance-
ment. In addition, according to the previous report,[21] TPEBeÀI
undergoes hydrolysis in the presence of OHÀ ion, thus making
it incapable of sensing Hg2+. Through the above investigation,
it becomes clear that TPEBeÀI exhibits satisfactory sensing abil-
ity at neutral pH, thereby enabling it to work under a physio-
logical environment.
The Hg2+ sensing can also be performed in the solid state.
Thin solid films of TPEBeÀI were prepared by drop-casting its
solution in dichloromethane (2.0 mm, 10 mL) on cover glasses
followed by drying in air for 10 min. Aqueous solutions that
contained different Hg2+ concentrations (5.0 mL) were then
dropped onto the films. After being kept at room temperature
for 30 min, photographs of the tested films were taken under
Zn2+, Ni2+, Co2+, Cd2+, Cu2+, Mn2+, Hg2+, Ca2+, Fe3+, Mg2+
,
Ag+ and Pd2+ was carried out under the same experimental
conditions. The results show that none of these ions causes
a noticeable change in the emission of TPEBeÀI (Figure 4 and
Figure S3 in the Supporting Information), thus indicating that
TPEBeÀI possesses a higher selectivity to Hg2+ than other
metal ions. The interference of various metal ions on the sens-
ing process was also tested. Even when the buffer solutions
contained the same amount of other metal cations (2.0 mm),
a similar extent of emission enhancement was observed, thus
verifying the high anti-interference of the present Hg2+ sensor.
Relative to the previous sensor for Hg2+ based on the AIE
mechanism,[15] TPEBeÀI exhibits high sensitivity and water com-
patibility.
To check the effect of pH on the sensing property of TPEBeÀ 365 nm UV irradiation. As shown in Figure 5, the spots on the
I, we studied its emission in buffer solutions with different pH
in the absence and presence of Hg2+ (Figure S4 in the Sup-
porting Information). TPEBeÀI shows relatively stronger emis-
sion in acidic buffer solution (pH<7) without Hg2+, presum-
ably due to the removal of iodide ion by their combination
with the proton in the medium. At pH>7, the solution only
emits a faint light due to the quenching effect of the iodide
ion. Addition of Hg2+ into the basic aqueous solution en-
hanced the light emission but the extent was smaller than that
TPEBeÀI films that contained Hg2+ showed red light with vary-
ing degree, whereas the untreated parts remained dark. The
large contrast between the two portions enabled a detection
limit down to 1 mm (ꢀ200 ppm), which is 100-fold lower than
the value in aqueous solution. This provides an effective and
convenient tool for tracking the level of Hg2+ in aqueous solu-
tion for real-world applications. To the best of our knowledge,
this is the first example of utilising a solid film for detecting
Hg2+ in aqueous solution based on the AIE mechanism.
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the aromatic proton absorptions (Ha, Hb, Hc, Hd) upfield. The
higher the amount of Hg2+, the larger the peak shift. A similar
phenomenon was also observed in TPEBeÀClO4, thereby imply-
ing that the complex is really formed through the Hg2+ÀS in-
teraction, which diminished the shielding effect of the thiazole
ring and hence shifted the neighbouring aromatic proton reso-
nances to higher field.
Figure 5. Photographs of thin films of TPEBeÀI containing different concen-
trations of Hg2+ (5.0 mL) in aqueous solution taken under 365 nm UV illumi-
nation.
Through the above experimental results, the mechanism for
the Hg2+ sensing is now clear and is illustrated in Figure 7.
For the purpose of mechanistic understanding, the emission
response of TPEBeÀClO4 to Hg2+ was also investigated. Addi-
tion of Hg2+ into the aqueous buffer solution of TPEBeÀClO4
gave a similar change in the UV/Vis spectrum to that in
TPEBeÀI. Simultaneously, the emission of TPEBeÀClO4 is en-
hanced progressively as the amount of Hg2+ increases. It is
noteworthy that at the same Hg2+ concentration, the magni-
tude of emission enhancement in TPEBeÀClO4 is lower than
TPEBeÀI owing to the stronger fluorescence background of the
former dye molecule. This result implies that the stronger
emission response or higher sensitivity of TPEBeÀI to Hg2+
stems from the presence of iodide ion. It also suggests that
the cationic unit of the luminogens plays a role in the Hg2+-in-
duced emission enhancement. Except for its high affinity to
the iodide ion, Hg2+ is a well-known thiophilic metal ion and
can interact preferentially with a soft base such as S atom on
the basis of Pearson’s hard/soft acid/base theory.[22] The coordi-
nation of Hg2+ with the S atom of the benzothiazolium unit of
TPEBeÀI and TPEBeÀClO4 generates complexes with even
worse solubility in aqueous buffer. This induces aggregate for-
mation and hence enhances the light of the luminogens.
Figure 7. Schematic illustration of the mechanism for detecting Hg2+
.
TPEBeÀI shows weak fluorescence in solution due to the ICT
effect as well as intramolecular rotation of the TPE unit. Due to
its hydrophobic nature, the TPEBeÀI molecules aggregate in
the presence of a large amount of water. The formed aggre-
gates are non-emissive because of the formation of close-con-
tacting ion pairs and the heavy-atom effect contributed from
the iodide ion. Addition of Hg2+ into the aqueous solution
forms HgI2 and thus eliminates the quenching effect and indu-
ces more aggregate formation by means of its complexation
with the cationic unit. These recover the light emission of the
luminogen, thereby enabling it to function as an excellent
To confirm the above speculation, the coordination process
1
was analysed by means of H NMR spectroscopy. As shown in
Figure 6, addition of Hg2+ into the DMSO/H2O mixture shifted
“light-up” fluorescent sensor for Hg2+
.
Conclusion
In summary, TPE-functionalised benzothiazolium salts with dif-
À
À
ferent counteranions (TPEBeÀX; X=IÀ, ClO4 and PF6 ) were
designed and synthesised. All the luminogens show faint red
emission in solution. Whereas aggregate formation enhanced
the light emission of TPEBeÀClO4 and TPEBeÀPF6, that of
TPEBeÀI is quenched owing to the heavy-atom effect of the
iodide ion. TPEBeÀI functions as a light-up fluorescent sensor
for Hg2+ detection in aqueous solution with high sensitivity
and specificity, which is attributed to the strong affinity of
Hg2+ toward IÀ and S atoms. A handy solid tool was prepared
that can monitor the level of Hg2+ in aqueous solution with
a detection limit of 1 mm. This work provides a novel and
simple strategy to change the emission behaviour of lumino-
genic materials and at the same time opens a new route for
detecting Hg2+ in aqueous solution based on the AIE mecha-
nism.
Figure 6. 1H NMR spectra of TPEBeÀI in D2O/DMSO mixtures with A) 0, B) 2,
C) 5 and D) HgCl2 (10 equiv).
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The work reported in this paper was partially supported by the
National Basic Research Program of China (973 Program,
2013CB834701), the Research Grants Council of Hong Kong
(604711, 602212, HKUST2/CRF/10 and N_HKUST620/11) and
the University Grants Committee of Hong Kong (AoE/P-03/08).
B.Z.T. thanks the support of the Guangdong Innovative Re-
search Team Program of China (201101C0105067115).
Keywords: aggregation · fluorescence · iodide · mercury ·
sensors
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Received: August 17, 2013
Published online on December 4, 2013
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