1,3,5-Triphenylbenzene fluorophore as a selective Cu2+ sensor in aqueous media

Article abstract of DOI:10.1039/c1cc16148b

Three water-soluble fluorophores are synthesized from 1,3,5-tris-(4′- iodophenyl)benzene. The compound with salicylate groups exhibits a selective fluorescent quenching by Cu²⁺via a static quenching mechanism. The addition of Triton X-100 impro

Full text of DOI:10.1039/c1cc16148b

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1,3,5-Triphenylbenzene fluorophore as a selective Cu2+ sensor in
aqueous mediaw
Sakan Sirilaksanapong,a Mongkol Sukwattanasinittb and Paitoon Rashatasakhon*b
Received 3rd October 2011, Accepted 9th November 2011
DOI: 10.1039/c1cc16148b
Three water-soluble fluorophores are synthesized from 1,3,5-
tris-(40-iodophenyl)benzene. The compound with salicylate
groups exhibits a selective fluorescent quenching by Cu2+ via
a static quenching mechanism. The addition of Triton X-100
improves the quantum efficiency of the fluorophore as well as the
detection limit for Cu2+ from 6.49 to 0.19 ppb.
With the environmental concern on water pollution as well as
the fact that most of the physiological species exist in aqueous
media, water-soluble fluorophores are desirable for sensor
development.1 Copper (Cu2+) is one of the most essential
trace transition metals that is important for life but could be
toxic to organisms such as algae, fungi, bacteria, or viruses.2 In
Fig. 1 Structure of fluorophores 1–3.
the human body, copper which is distributed in liver, muscle,
and bone can facilitate the process of iron uptake. Therefore,
sensing application. During our continuing research program
copper deficiency often results in anemia-like symptoms as
on the design and synthesis of water-soluble dendritic conjugated
well as hypopigmentation, bone abnormalities, impaired
polyelectrolytes as fluorescent signal transducers, we have
growth, and irregularity in glucose and cholesterol meta-
bolisms.3 In contrast, excessive accumulation of copper in
demonstrated the synthesis of triphenylamine-cored fluorophores
as a selective Hg2+ probe in aqueous media14 and elements in a
tissues can cause Wilson’s disease—an autosomal recessive
protein array sensor.15 We herein report our synthesis of 1,3,5-
triphenylbenzene derivative 1–3 (Fig. 1), and the application of 3
as a selective Cu2+ sensor.
genetic disorder.4 Several techniques such as atomic absorption
spectroscopy,5 inductively coupled plasma mass spectrometry,6
absorption spectroscopy,7 and voltammetry8 have been applied
for trace analysis of copper ions. However, the high cost of
instruments, complicated set-up of equipment, or tedious sample
preparation procedures have limited the use of these techniques
for real-time and in vivo analysis of copper. For the fluorescence
spectroscopy which is a non-destructive and highly sensitive
technique, a relatively more economical instrument is required.
A number of fluorescent materials based on calcein,9
rhodamine,10 coumarin,11 calix[4]arene,12 and polyfluorene
ethynylene derivatives13 with high sensitivity and specificity
toward Cu2+ have been developed. Nevertheless, most of
these materials are not well soluble in water and organic
solvents are required to achieve a suitable concentration for
1,3,5-Triphenylbenzene was selected as the core unit due to
its highly fluorescent nature as well as good thermal and
photochemical stabilities.16 Its derivatives have recently been
studied as linkers for microporous coordination polymers,17
multi-branched flavone-based chromophores,18 cores of triple-
stranded polymeric ladderphanes,19 and ligands in metal–
organic framework materials.20 In our work, 1,3,5-tris-
(40-iodophenyl)-benzene (4) was first synthesized by a modified
cyclotrimerization procedure (Scheme 1). It was found that the
use of most catalysts21 reported for this transformation failed
to give an appreciable yield of 4, while the reaction of
4-iodoacetophenone with trifluoromethanesulfonic acid in
refluxing toluene afforded 4 in 70% yield within 3 hours.
The Sonogashira couplings between 4 and the peripheral
units—ethyl 4-ethynylbenzoate,22 and diethyl 5-ethynyliso-
phthalate23—successfully produced tri-ester 6 and hexa-ester 724
in 78 and 87% yield, respectively. Unfortunately, attempt to
couple ethyl 5-ethynylsalicylate25 with 4 gave rise to a complex
mixture with a little formation of 8. To circumvent this problem, 4
was coupled with trimethylsilylacetylene followed by a base-
a Program of Petrochemical and Polymer Science, Faculty of Science,
Chulalongkorn University, Pathumwan, Bangkok 10330, Thailand
b Organic Synthesis Research Unit, Department of Chemistry,
Faculty of Science, Chulalongkorn University, Pathumwan,
Bangkok 10330, Thailand. E-mail: paitoon.r@chula.ac.th;
Fax: +66 2 218 7598; Tel: +66 2 218 7620
w Electronic supplementary information (ESI) available: Full experi-
mental procedure, photophysical data, and spectroscopic data. See
DOI: 10.1039/c1cc16148b
catalyzed desilylation to cleanly afford
5 in 85% yield.
c
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Chem. Commun., 2012, 48, 293–295 293

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Fig. 2 Fluorescence spectra of 3 (1 mM) in the presence of various
metal ions (10 mM).
this may result from the higher ICT between carboxylate
groups at the meta position and hydroxyl groups at the para
position.
To study the sensing properties of fluorophores 1–3 toward
metal ions, we observed changes in their fluorescent signals
upon mixing their solutions in phosphate buffer pH 8.0 with
each of the 19 metal ion solutions including Li+, Na+, K+,
Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+
,
Cu2+, Zn2+, Zr2+, Ag+, Cd2+, Pb2+, and Hg2+. It was
found that the fluorescent signal of 3 was selectively quenched
by the Cu2+ ion (Fig. 2) whereas no significant change was
observed for 1 and 2 (Fig. S2, ESIw). The selectivity of the
fluorescence quenching of 3 is probably due to the highly
Scheme 1 Reagents and conditions: (i) TfOH, PhMe, reflux, 70%; (ii)
TMSCCH, PdCl2(PPh3)2, CuI, NEt3, THF, then K2CO3, THF,
MeOH, 85%; (iii) ethyl 4-ethynylbenzoate or diethyl 5-ethynyl-
isophthalate, PdCl2(PPh3)2, CuI, NEt3, THF, 6: R1, R3 = H, R2
=
COOEt, 78%, 7: R1, R3 = COOEt, R2 = H, 87%; (iv) ethyl
5-iodosalicylate, PdCl2(PPh3)2, CuI, NHi-Pr2, THF, 8: R1 = COOEt,
R2 = OH, R3 = H, 73%; (v) KOH, THF, H2O, EtOH, then HCl, 1:
R1, R3 = H, R2 = COOH, 90%, 2: R1, R3 = COOH, R2 = H, 96%,
3: R1 = COOH, R2 = OH, R3 = H, 98%.
selective chelation of salicylate groups with Cu2+ 26
.
The selectivity and sensitivity of fluorophore 3 toward Cu2+
were examined by interfering experiments using mixed solutions
of 3 (1 mM), Cu2+ (1 mM) and each of the other metal ions
(10 mM) (Fig. S3a, ESIw). It was found that the fluorescent
intensities in the presence of interfering metal ions were not
significantly different from the control experiment, even in the
presence of biologically relevant ion at 1 mM (Fig. S3b, ESIw). The
The Sonogashira coupling between 5 and ethyl 5-iodo-salicylate25
yielded 8 in 73%. Finally, hydrolysis of 6–8 resulted in fluoro-
phores 1–3 in 90, 96, and 98% yield, respectively.
The absorption and fluorescence properties of 1–3 are
presented in Table 1 and Fig. S1 (ESIw). The three compounds
show maximum absorption wavelength (lmax) around 310–318 nm,
while the maximum emission wavelengths of 1–3 were 426,
365, and 465 nm, respectively. The effect of substitution
patterns at the peripheries on the photophysical properties
could be observed. In comparison with fluorophore 2, the
emission band of 1 and 3 appeared at a slightly longer
wavelength due to the para substituents extending the
p-conjugation. The Stokes shifts of 1 and 3 were also much
larger indicating the greater difference between the most stable
geometries of the ground and excited states. In the case of 3,
results indicated that 3 has an excellent selectivity toward Cu2+
.
In order to determine the sensitivity of fluorophore 3 toward
Cu2+, a Stern–Volmer plot for the fluorescence quenching at room
temperature (30 1C) was constructed (Fig. S4a and S5, ESIw). With
a KSV of 1.62 ꢀ 106 Mꢁ1, a detection limit at three-time noise of
1.03 ꢀ 10ꢁ7 M (6.49 ppb) could be estimated. Interestingly, the
K
SV decreased to 0.68 ꢀ 106 Mꢁ1 when the experiment was carried
out at 50 1C (Fig. S4b and S5, ESIw). The decrease suggests that the
mechanism for quenching is a static mode, presumably via a
complexation between Cu2+ and 3, and therefore, the temperature
escalation causes dissociation of such a complex. This complexation
is reversible as proven by the addition of EDTA to the mixture of 3
and Cu2+ that caused a complete fluorescent signal restoration
(Fig. S8, ESIw).
Table 1 Photophysical properties of 1–3 in 10 mM phosphate buffer
(pH 8.0) without and with Triton X-100 (30 mM)
Several examples in the literature have demonstrated
fluorescence enhancements of conjugated polyelectrolytes by
addition of surfactants.13,27 In our study, it was found that the
addition of Triton X-100 could improve the quantum efficiency
of 3 (Fig. S6, ESIw), but the use of Triton X-100 at 30 mM could
provide the best sensitivity for Cu2+ sensing (Fig. S7, ESIw). This
improvement is depicted in Table 1, Fig. 3, and Fig. S4c (ESIw),
where the quantum yield of 3 is 13.1% and the KSV is 1.50 ꢀ
107 Mꢁ1. Since the concentration of Triton X-100 used was much
lower than the Critical Micelle Concentration (CMC) of the
surfactant (0.2 mM), we believed that the fluorescent
Absorption
abs/nm
Without Triton X-100
Emission
ems/nm
l
e/Mꢁ1 cmꢁ1
l
F
Compound
1
2
3
318
310
316
47 000
19 600
19 000
426
365
465
0.214a
0.427a
0.007b
With Triton X-100
3
316
71 200
465
0.131b
a
b
2-Aminopyridine in 0.1 M H2SO4 (F = 0.6) and Quinine sulfate in
0.1 M H2SO4 (F = 0.54) were used as the standard.
294 Chem. Commun., 2012, 48, 293–295
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Fig. 3 Stern–Volmer plots for the fluorescence quenching of 3 by
Cu2+ with and without Triton X-100.
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In conclusion, we have demonstrated the synthesis of three
water-soluble fluorophores containing a 1,3,5-triphenylbenzene
moiety and one of them showed a selective fluorescence quenching
toward Cu2+. A comparison of the KSV at different temperatures
indicated a static mode of quenching and the detection limit could
be improved from 6.49 ppb to 0.19 ppb by the addition of Triton
X-100. To the best of our knowledge, this sensing system
represents one of the most sensitive fluorescent sensors for
Cu2+ in aqueous media. This sensitivity is comparable to the
electrochemical analysis of Cu2+ 8
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The authors gratefully thank financial supports from the
National Nanotechnology Centre (P-11-00-00370 and NN-
B-22-FN9-10-52-06) and the Centre for Petroleum, Petrochemicals
and Advanced Materials, Chulalongkorn University. The research
instruments were partially supported by the Thai Government
Stimulus Package 2 (TKK2555, SP2) and the National Research
University Project (AM 1006A). SS thanks the 90th Anniversary of
Chulalongkorn University Fund for his scholarship.
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Chem. Commun., 2012, 48, 293–295 295