Click functionalized poly(p-phenylene ethynylene)s as highly selective and sensitive fluorescence turn-on chemosensors for Zn2+ and Cd 2+ ions

Article abstract of DOI:10.1039/c1cc14113a

Side-chain functionalized poly(p-phenylene ethynylene)s (PPEs) carrying triazole linkers, amino donors/receptors, and solubilizing groups have been found to yield remarkably high efficiency of fluorescence turn-on sensing for Zn²⁺ and Cd²⁺

Full text of DOI:10.1039/c1cc14113a

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Click functionalized poly(p-phenylene ethynylene)s as highly selective and
sensitive fluorescence turn-on chemosensors for Zn2+ and Cd2+ ionsw
Yousef Pourghaz, Prateek Dongare, David W. Thompson and Yuming Zhao*
Received 8th July 2011, Accepted 25th August 2011
DOI: 10.1039/c1cc14113a
Side-chain functionalized poly(p-phenylene ethynylene)s (PPEs)
carrying triazole linkers, amino donors/receptors, and solubiliz-
ing groups have been found to yield remarkably high efficiency of
fluorescence turn-on sensing for Zn2+ and Cd2+ ions in THF,
and for H+ and Cd2+ ions in water.
displacement with Fe2+, this inorganic/organic hybrid system
gave fluorescence enhancement as great as 150-fold.
To our knowledge, solely CP-based chemosensors that
exhibit sizable fluorescence turn-on responses to metal ions
have not been reported. To address this challenge, we inves-
tigated the approach of incorporating amino groups as metal
ion receptors into the side chains of poly(phenylene ethyny-
lene)s (PPEs). In our study, the highly emissive nature of PPEs
imposed a major obstacle,7 given that a sufficiently ‘‘dark’’
fluorescence background is a key prerequisite to efficient turn-
on sensing. However, a fortuitous discovery we made in an
effort of using facile ‘‘click’’ chemistry8 to generate some
functionalized PPEs (e.g. PPE-1 in Fig. 1) offers a satisfactory
solution to this problem. In PPE-1 the 1,2,3-triazole linker
resulting from the Cu-catalyzed alkyne azide cycloaddition
(click reaction) not only acts as a structural element (linkage)
but plays a unique functional role in quenching the fluores-
cence of PPEs. The effect of triazole on fluorescence
Conjugated polymers (CPs) are chromophores exhibiting
significant advantages over other small-molecule based chemo-
sensors in the field of chemical sensory devices.1 In the current
literature, most of the CP fluorescence sensors are designed to
use fluorescence quenching (i.e. turn-off) as the signal readout
mechanism, whereas CP sensors showing fluorescence
enhancement (turn-on) sensing behavior are relatively few.2
Nevertheless, turn-on sensors do have some advantages not so
easily attainable by turn-off sensors, including the ease of
detecting low concentration contrast relative to a dark back-
ground, reduced false-positive signals, and enhanced sensitivity.
In this respect, the pursuit of new CP based fluorescence turn-
on sensors still remains a topic of considerable interest.
A common design motif for fluorescence turn-on sensors is
based on fluorophore-donor ensembles, in which the donor
group usually acts as the receptor and binds with analytes to
result in fluorescence turn-on based on photoinduced electron
transfer (PET) or internal charge-transfer (ICT) mechanisms.3
This strategy has been successfully applied to the construction
of numerous small-molecule sensors.4 However, it is not trivial
for CP fluorophores to achieve good fluorescence turn-on
performance due to the high background emission of most
CPs as well as the inherent fluorescence quenching effect of
some metal ions.5 Indeed, CP-based fluorescence turn-on
sensors for metal ions reported in the literature have only
attained weak to moderate fluorescence enhancement (ca. 1 to
3-fold).2c–f To make further improvement requires new design
concepts. For instance, Jones Jr. and co-workers6 in 2006
devised a tmeda-PPETE sensor preloaded with Cu2+ to gain a
substantially quenched fluorescence background. Upon
Fig. 1 (A) Molecular structures, and (B) fluorescence spectra of
PPE-1, PPE-2, and PPE-3. Spectra were measured in deoxygenated
THF at room temperature. Polymer samples were prepared at a
concentration of 100 mg/mL. Inset: photographic images of THF
solutions of PPE-1 (left), PPE-2 (middle), and PPE-3 (right) under
the irradiation of a UV lamp. (C) Proposed fluorescence quenching
mechanism for PPE-1.
Department of Chemistry, Memorial University of Newfoundland,
St. John’s, NL, Canada A1B 3X7. E-mail: yuming@mun.ca;
Fax: 1 709 864 3702; Tel: 1 709 864 8747
w Electronic supplementary information (ESI) available: Detailed
synthetic procedures of functionalized PPEs and related precursors,
as well as spectroscopic characterization data. See DOI: 10.1039/
c1cc14113a
c
11014 Chem. Commun., 2011, 47, 11014–11016
This journal is The Royal Society of Chemistry 2011

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quenching is manifested by comparison with two other model
PPEs (PPE-2 and PPE-3, Fig. 1).
Of the three polymers, only the fluorescence of PPE-1 is
substantially quenched as clearly evidenced by both visual
check (inset of Fig. 1B) and emission quantum yield measure-
ments (F = 0.038 for PPE-1, F = 0.74 for PPE-2, and F =
0.29 for PPE-3 in THF). It is also noted that the emission
spectral profile of PPE-1 is markedly different from those of
the other two polymers (Fig. 1B). The broad and structureless
fluorescence band of PPE-1 is characteristic of an emissive
charge-transfer (CT) excited state.9 Based on such spectral
features, a quenching mechanism is proposed as depicted in
Fig. 1C. Upon irradiation, the PPE unit first captures a
photon to reach the first excited state (S1). Rapid energy
transfer (ENT) sensitizes the donor (amino) group, triggering
an ICT process from the donor (amino) to the acceptor
(triazole) group. As such, the fluorescence is quenched by a
relay of energy transfer, sensitization, and charge transfer. It
should be noted that the combination of aminophenyl-triazole
is of indispensible necessity to fluorescence quenching, since
high emissivity is retained for the model polymers where only
triazole (PPE-2) or amino (PPE-3) groups are present on the
side chains.
Fig. 2 (A) Fluorescence spectral changes of PPE-1 (100 mg mLÀ1
,
l
ex = 380 nm) upon titration of Zn(OTf)2 in THF at concentrations of
0.0, 0.0033, 0.010, 0.030, 0.17, 0.83, 1.10, 1.50, 3.45, and 6.20 mM. (B)
Trend of fluorescence enhancement (Fs/Fo) for different cations at
maximum emission wavelength (Fs: fluorescence intensity at the
saturation point of titration; Fo: initial fluorescence intensity). Inset:
photographic images of THF solutions of PPE-1 without and with
various cations under the irradiation a UV lamp.
With the nearly non-emissive PPE-1 in hand, its turn-on
sensing function towards various metal ions was then tested.
THF solutions of PPE-1 were titrated with various metal ions,
ranging from Zn2+, Cu2+, Cd2+ to Ba2+, Na+, and Li+. The
spectral responses of PPE-1 were monitored by both UV-Vis
absorption and fluorescence spectroscopy (see ESI). In
addition to metal ions, PPE-1 was also subjected to the
titration of a strong Brønsted acid, trifluoroacetic acid
(TFA). The titration results indicate that PPE-1 shows
prominent fluorescence turn-on responses to certain metal
ions in the concentration range of 10À6 to 10À3 M. Fig. 2A
depicts the steady fluorescence enhancement of PPE-1 upon
addition of Zn2+ ion. It is worth noting that at the saturation
point of titration, the fluorescence of PPE-1 gains a 51-fold
enhancement (at lem = 462 nm), which is the greatest among
other tested ion species. Besides Zn2+, PPE-1 also affords a
very high sensitivity towards Cd2+ ion (ca. 34-fold enhancement).
Fig. 2B outlines the decreasing trend of sensitivity of PPE-1
for various cations. Clearly PPE-1 acts as a highly selective
and sensitive chemosensor for Zn2+ and Cd2+ ions (detection
limit: 1 Â 10À6 M) in THF.
Apart from THF, sensing properties of click-functionalized
PPEs in aqueous media were also explored. It was found that
PPE-1 can be partially solubilized in water/DMSO (1 : 1, v/v)
with the aid of surfactants such as sodium dodecylsulfonate
(SDS), and the resulting aqueous solution of PPE-1/SDS
showed selective fluorescence turn-on sensing behavior for
Cd2+ ion. The sensitivity however was relatively low (o 3-fold
enhancement, see Fig. S-8 in ESI), presumably due to the poor
solubility and significant aggregation of PPE-1 in water. To
circumvent this problem, a water-soluble polymer analogue,
PPE-4, was subsequently designed and tested (Fig. 3).
Fluorescence spectra of PPE-4 as a function of [Cd2+] in water
are shown in Fig. 3A. Analysis of these emission spectral data
reveals two distinct steps. At [Cd2+] B 2.0 to 3.2 mM an emission
band at 620 nm grows in, while at [Cd2+] B 2.1 to 12 mM
Fig. 3 (A) Fluorescence spectral changes of PPE-4 (100 mg mLÀ1
ex = 400 nm) upon titration of Cd(ClO4)2 in water at concentrations
,
l
of 0.0, 2.0, 2.4, 2.8, 3.2, 3.6, 4.0, 4.8, 6.0, 8.0, 12.0, 20.0, and 28.0 mM.
(B) Trend of fluorescence enhancement (Fs/Fo) for different cations at
maximum emission wavelength. Inset: photographic images of
aqueous solutions of PPE-4 without and with Cd2+ and H+ ions.
an intense emission centered at 492 nm grows in. Saturation
behavior is observed at [Cd2+] 4 12 mM. Over the course of
titration a 14-fold enhancement of emission intensity is observed
(detection limit: 3 mM). The two different stages of spectral
changes suggest multiple binding modes are at work in the
c
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To determine the stoichiometry of metal ion binding, monomer
1 (Fig. 4) was prepared as a model compound and its
coordination mode with Cd2+ ion was examined by
1H NMR and UV-Vis spectroscopy. From Fig. 4A, it can be
seen that the NMR signals of the triazolyl and aminophenyl
protons (Ha, Hb, Hc, and CH3) of 1 are significantly shifted to
the down-field direction upon titration with Cd2+ ion. The
protons on the central phenyl ring (Hd) however show only a
slight up-field shift. The results indicate that the amino and
triazole groups are both metal ion receptors to effectively
interact with Cd2+. This binding mode concurs with the 1 : 4
binding stoichiometry (1ꢀ[Cd2+]4) manifested by the Job plot
analysis shown in Fig. 4B. Furthermore, the UV-Vis titration
data of 1 with Cd2+ (Fig. 4C) was subjected to SPECFIT
analysis10 (Fig. S-17, ESIw), the result of which validates the
1 : 4 binding ratio and gives a binding constant of logb = 1.10 Æ
0.06 MÀ4
.
In conclusion, we have found that incorporation of the
‘‘click’’-generated triazole linker into the side chain of PPEs
enables excellent fluorescence turn-on sensing function for
Zn2+ and Cd2+ ions in THF, H+ and Cd2+ ions in water.
The remarkable sensitivity and selectivity displayed by sensors
PPE-1 and PPE-4 bode well for a wide range of applicability;
in particular, the water-soluble PPE-4 is expected to be a
useful sensor for Cd2+ ion detection in various applications.
This work was supported by NSERC, CFI, IRIF, and
Memorial University of Newfoundland.
Fig. 4 (A) 1H NMR titration of compound 1 (6.8 mM) with
Cd(ClO4)2 in DMSO-d6. (B) Job plot of compound 1 in DMSO-d6
(Ds: shift of the CH3 signal; w: molar fraction). (C) UV-Vis spectro-
scopic titration of compound 1 (15 mM) in DMSO with Cd2+ at
concentrations of 0.0, 5.0, 9.9, 15, 20, 24, 29, 34, 38, 43, 48, 52, 57, 65,
74, 83, 91, 110, 130, 170, 200, 260, and 330 mM.
interaction of Cd2+ ion with PPE-4. Upon titration of PPE-4
with TFA in water, much stronger fluorescence enhancement
(29-fold) and higher sensitivity (detection limit: 0.3 mM) in
comparison to Cd2+ sensing were observed. The prominent
spectral responses to TFA in water indicate that PPE-4 is more
prone to protic acids than metal ions under aqueous conditions.
Unlike the two-stage scenario in Cd2+ titration, the titration of
PPE-4 with TFA showed only a monotonic emission increase at
492 nm (Fig. S-9, ESIw). The different titration curves suggest that
Cd2+ might be preferentially bound to the triazole ligand over the
amino group at the early stage of titration. In a sharp contrast to
PPE-1, the sensitivity of PPE-4 for Zn2+ in water was rather
diminutive. The statistical graph in Fig. 3B clearly reveals the
ability of PPE-4 to differentiate Cd2+ and Zn2+ ions in water.
On the side chains of the two PPE sensors, N-containing
amino and triazole groups were both expected to act as
receptors (ligands) for metal ions. In theory the binding of
metal ions to the amino groups should reduce their electron-
donating ability. The interaction of Cd2+ with either the
amino or the triazolyl groups would raise the energy of the
ICT state such that excited-state quenching of the PPE back-
bone becomes endergonic. The proposed mechanism differs
from the mechanistic arguments made by Jones Jr. and
co-workers.2c In the meantime, the binding of triazole with
metal ions can also alter non-radiative deactivation steps in the
mechanism. In this respect, characterization of the polymer-
metal ion interactions is of great value to further unraveling
the detailed photophysical mechanisms of the click-functionalized
PPE fluorescence turn-on sensors.
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c
11016 Chem. Commun., 2011, 47, 11014–11016
This journal is The Royal Society of Chemistry 2011