Recoverable fluorescence chemosensors for Ni2+ ions based on hydrogen-bonded side-chain copolymers presenting pendent benzoic acid and pyridyl receptor units

Article abstract of DOI:10.1039/c2jm31367g

In this study we synthesized the novel fluorescent conjugated homopolymers P1 and P4 and the copolymers P2 and P3 containing proton-acceptor (pyridyl receptor) M1 monomer units and proton-donor (benzoic acid) M2 monomer units and investigated them as reco

Full text of DOI:10.1039/c2jm31367g

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PAPER
Recoverable fluorescence chemosensors for Ni2+ ions based on hydrogen-
bonded side-chain copolymers presenting pendent benzoic acid and pyridyl
receptor units†
*
Po-Jen Yang, Hsuan-Chih Chu, Te-Cheng Chen and Hong-Cheu Lin
Received 5th March 2012, Accepted 16th April 2012
DOI: 10.1039/c2jm31367g
In this study we synthesized the novel fluorescent conjugated homopolymers P1 and P4 and the
copolymers P2 and P3 containing proton-acceptor (pyridyl receptor) M1 monomer units and proton-
donor (benzoic acid) M2 monomer units and investigated them as recoverable chemosensors for Ni2+
ions (based on adjusting the fluorescence energy transfer between the M1 and M2 moieties through
varying the degree of hydrogen bonding). The levels of available (i.e., non-hydrogen-bonded) pyridyl
receptors in the copolymers P2 and P3 were modified using different molar ratios of the pyridyl
monomer M1 and the benzoic acid monomer M2. Significant chelation of the Ni2+ ions by the
fluorescent homopolymers P1 and P4 and the copolymers P2 and P3 was evident by their distinct
fluorescence quenching behaviors; the specific quenching constants (KSV) decreased in the order P1 >
P2 > P3 > P4, following the trend of the molar ratios of pyridyl receptors. When we added a third
component, cyclodextrin (CD), to cap the benzoic acid moieties and, thereby, disrupt the hydrogen
bonding between the M1 and M2 moieties in the copolymers P2 and P3, the levels of available pyridyl
receptors in the recoverable chemosensor copolymers increased, resulting in relatively higher quenching
constants for the P2 + CD, P3 + CD and blend (P1/P4) + CD systems. Fluorescence decay experiments
revealed that the addition of CD also increased the fluorescence quenching efficiencies of the
copolymers P2 and P3 towards Ni2+ ions. Such supramolecular (hydrogen-bonded) side-chain
fluorescent copolymers might have practical applicability as recoverable and/or adjustable
chemosensors.
Some polymeric fluorescence sensors were reported to explain their
Introduction
photo-physical phenomena, such as electron and/or energy transfer
processes.7 Furthermore, receptors featuring various functional
segments can be included in copolymeric sensors to extend their
applicability.8 The molecular design of copolymers offers the ability
not only to vary the fluorescence intensities and/or wavelengths of
the individual fluorophores to different degrees but also to add
additional logic signals (or functions) through the incorporation of
other responsive segments in the copolymer chains.9 Special pho-
tophysical properties can be produced by the twisted intra-
molecular charge transfer (TICT) phenomena of 4-(N,N-
dimethylamino)benzonitrile (DMABN)10 and its derivatives11 in
the hydrophobic cavities of cyclodextrin (CD). In particular, the
dependence of the TICT emissions on the environmental polarity,
viscosity and rotational mobility of the donor moieties makes these
molecules (e.g., DMABN) excellent candidates12 for examining
microscopic molecular environments. The excited state dynamics
of DMABN units positioned within the cavities of CDs13 vary with
respect to those in solution because of the low polarity of the cavity
and the geometric constraints. The reduced polarity and restricted
environment of the CD medium can be used to tune the position
and intensity of the distinct band of a fluorophore, due to the
Many chemo- and bio-sensor materials have been developed
recently to measure values of pH and detect metal ions and bio-
logical species based on changes in their fluorescence signals.1,2
Among them, chemosensors derived from conjugated polymers
have attracted considerable attention because their fluorescence
properties respond significantly to coupling between the polymer’s
receptors and the detected analytes. Polymeric chemosensors
typically exhibit an amplifying mechanism as a result of energy
transfer of excitons among the conjugated chains; as a result, most
conjugated polymeric sensors are more sensitive than their small-
molecule counterparts.3 Moreover, the responses of multi-receptor
polymeric systems featuring collective interactions are generally
stronger than those of mono-receptor small-molecule systems
featuring single interactions. Several fluorescent polymeric sensors
have been developed to interact selectively with particular ions.4–6
Department of Materials Science and Engineering, National Chiao Tung
University, Hsinchu, Taiwan (ROC). E-mail: linhc@cc.nctu.edu.tw;
Fax: +8863-5724727; Tel: +8863-5712121 ext. 55305
† Electronic supplementary information (ESI) available: Synthesis and
characterization of monomer M2. See DOI: 10.1039/c2jm31367g
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energy difference between the locally excited state and internal
charge transfer (ICT) state. For example, the photophysical
properties of p-dimethylaminobenzoic acid,14 p-aminobenzoic
acid,15 o-aminobenzoic acid16 and m-aminobenzoic acid17 have
been investigated in CD systems.
In this study, we employed two conjugated monomeric fluo-
rophores—one featuring a pyridyl receptor and the other a benzoic
acid moiety—to prepare corresponding homopolymers and
copolymers as recoverable (and/or adjustable) chemosensors for
the detection of metal ions (see Fig. 1). Based on our previous
report,18 we prepared the proton-acceptor monomer M1 bearing
a conjugated pyridyl receptor for coordination with metal ions. As
illustrated in Scheme 1, we synthesized the proton-donor monomer
M2 bearing a conjugated benzoic acid unit for binding in the
cavities of b-CD moieties, and its detailed synthetic procedures and
characterization are demonstrated in the ESI†. Scheme 2 outlines
the preparation of the copolymers P2 and P3 with different molar
ratios of M1 and M2 units (x : y ¼ 1 : 0.1 and 1 : 0.8, respectively)
and the homopolymers P1 and P4 (from monomers M1 and M2,
respectively). The degrees of supramolecular (hydrogen-bonding)
interactions between the M1 and M2 units were tuned by varying
their molar ratios or by adding CDs. Because the benzoic acid
moieties in the M2 units of the copolymers P2 and P3 would bind
to the CDs, we could use the addition of CDs to control the degrees
of supramolecular interactions (between monomers M1 and M2) as
well as the metal ion coordination by the free (non-hydrogen-
bonded) pyridyl moieties of the M1 units in the copolymers P2 and
P3. Because the coordination of pyridyl receptors with different
metal ions affects their fluorescence properties, we could construct
a model to determine the sensing contributions of the pyridyl
receptors of the M1 units in the homopolymer P1 towards metal
ions and found the best sensing selectivity of Ni2+ ions in our
previous study.18 The intriguing photophysics of the polymeric
fluorophores upon complex formation in constrained CDs
provoked us to explore the photoluminescence (PL) emissions of
the benzoic acid moieties of the M2 units in the copolymers P2 and
P3 and the homopolymer P4. Based on characteristic changes in
PL emissions, we monitored the Ni2+ ion sensitivities of the
copolymers P2 and P3 in the presence and absence of CDs.
Moreover, as model chemosensors for specific metal Ni2+ ions or
biomolecules, we employed benzoic acid moieties in these
Scheme 1 Synthetic route towards monomer M2.
Scheme 2 Synthetic route towards polymers P1–P4.
copolymers to ensure the feasibility of energy transfer between
these proton donors and the pyridyl proton acceptors. In suitable
proton donor–acceptor combinations, we envisioned the tunable
sensing capabilities of chemosensors towards particular analytes
(e.g., metal ions and biomolecules). In addition, more-efficient
energy transfer, resulting in larger PL emission contrasts or broader
sensing concentration ranges, should ensue in the coordinated
(analyte-attached) states of polymeric chemosensors during the
sensing process.
Experimental
Fig. 1 Schematic representation of the release of pyridyl receptors, for
the sensing of metal ions in recoverable chemosensor copolymers bearing
pendent benzoic acid and pyridyl receptor units, upon adding CDs to cap
the benzoic acid moieties.
Materials
Chemicals and solvents of reagent grade were purchased from
Aldrich, ACROS, TCI, and Lancaster Chemical. Tetrahydrofuran
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(THF) and triethylamine (Et3N) were dried and distilled prior to
use. Azobisisobutyronitrile (AIBN) was recrystallized from MeOH
prior to use. The other chemicals were employed without further
purification. The monomer M1 was prepared according to the
literature.18
Metal ion-induced fluorescence quenching of polymers
The PL quenching behavior followed the Stern–Volmer (SV)
equation
I0/I ¼ 1 + KSV[Q]
where I0 and I are the emission intensities of the fluorescent
polymer in the absence and presence, respectively, of the
quencher Q (Ni2+ ions), KSV is the SV quenching constant, and
[Q] is the concentration of the quencher.
Measurements and characterization
1H NMR spectra were recorded using a Varian Unity 300 MHz
spectrometer with CDCl3 and DMSO-d6 as solvents. Elemental
analyses were performed using a HERAEUS CHN-OS RAPID
elemental analyzer. High-resolution electron impact mass spectra
were recorded using a Finnigan-MAT-95XL apparatus. Gel
permeation chromatography (GPC) was performed using
a Waters HPLC pump 510 connected to a Waters 410 differential
refractometer and three Ultrastyragel columns, with polystyrene
as a standard and THF as an eluent. UV-Vis absorption spectra
were recorded using an HP G1103A spectrophotometer; PL
spectra of dilute THF solutions (5 ꢀ 10ꢁ6 M) were recorded
using a Hitachi F-4500 spectrophotometer. Time-resolved pho-
toluminescence (TRPL) spectra were measured using a home-
built single photon counting system with excitation from
a 375 nm diode laser (Picoquant PDL-200, 50 ps fwhm, 2 MHz).
The signals collected at the excitonic emissions of polymer
solutions (l ¼ 480 nm) were connected to a time-correlated single
photon counting card (TCSPC, Picoquant Timeharp 200). The
emission decay data were analyzed with biexponential kinetics,
from which two decay components were derived; the lifetimes
(s1, s2) and pre-exponential factors (A1, A2) were determined.
Results and discussion
Synthesis and characterization of polymers
The proton-acceptor monomer M1 containing three conjugated
aromatic rings, including one pyridyl terminus for coordination
with metal ions, was prepared using Wittig and Pd-catalyzed
Heck couplings.18,19 The proton-donor monomer M2 bearing
three conjugated aromatic rings, including one terminal benzoic
acid moiety for interacting with CDs to tune the fluorescence,
was synthesized as indicated in Scheme 1. The homopolymers P1
and P4 and the copolymers P2 and P3 were prepared (Scheme 2)
through radical polymerizations of mixtures of the proton-
acceptor (pyridyl) monomer M1 and the proton-donor (benzoic
acid) monomer M2.20 Most importantly, the fluorescence of the
copolymers P2 and P3 could be recovered and/or adjusted
through coordination of the benzoic acid units of the M2
moieties with CDs, thereby allowing control over the chemo-
sensor’s sensitivities towards metal ions (through variations in
fluorescence energy transfer between the M1 and M2 moieties at
different degrees of hydrogen bonding). In a previous study, we
established the presence of hydrogen bonding between
carboxylic acid and pyridine units in polymers.21 In the present
study, we used the homopolymer P1 as a model compound
because we had demonstrated previously its effective fluores-
cence chemosensor capability, allowing it to distinguish the
presence of metal Ni2+ ions (with the maximum selectivity)
through interactions with its pendent pyridyl receptors.18 The
detailed synthesis of the proton-donor (benzoic acid) monomer
M2 is outlined in the ESI†. Compound 1 was prepared from
4-bromophenol and 3,4-dihydro-2H-pyran (DHP); subsequent
bromine–lithium exchange (using n-BuLi) and boronation (with
2-isopropoxy-4,4,5,5-teramethyl-1,3,2-dioxaborolane) gave 2.
Suzuki coupling between 2 and 1,4-dibromo-2,5-dimethoxy-
benzene yielded 3. Compound 4 was synthesized from 3 through
Sonogashira coupling with 4-ethynylbenzoic acid methyl ester;
deprotection of the THPO protecting group (with p-TsOH)
provided 5, which we reacted with 10-bromodecanol to give 6;
hydrolysis of the ester yielded the carboxylic acid 7. Finally, M2
was obtained through the acylation of 7 with acryloyl chloride.
The polymerization of M1 and M2 was performed through
conventional free radical polymerization in THF at 60 ꢂC, using
AIBN as the initiator. The product molar ratios of the monomer
units M1 to M2 in P1–P4 were verified, using NMR spectros-
copy, to be 1 : 0, 1 : 0.1, 1 : 0.8, and 0 : 1, respectively. In the 1H
NMR spectra of the polymers, the disappearance of the signals
for the vinyl (i.e., acrylate) protons at 5.5–6.0 ppm indicated the
complete consumption of the monomers. We estimated the
General procedure for the synthesis of the copolymers P2 and P3
and the homopolymer P4
A mixture (1.5 g) of the monomers M1 and M2 was dissolved in
dry THF (7.5 mL) in a Schlenk tube, with a monomer concen-
tration of 20 wt%, and then AIBN (2 mol% of total monomer
concentration) was added as an initiator. The solution was
degassed through three freeze/pump/thaw cycles and then the
tube was sealed off. The mixture was stirred at 60 ꢂC for 24 h and
then precipitated into Et2O. The precipitated polymer was
collected, washed with Et2O and dried under high vacuum.
P2 (M1–M20.8). 1H NMR (300 MHz, DMSO-d6): d (ppm) 8.47
(br, 2H), 7.96–6.76 (br, 16H), 3.88 (br, 18H), 1.78–1.51 (br, 32H).
P3 (M1–M20.1). 1H NMR (300 MHz, DMSO-d6): d (ppm) 8.46
(br, 2H), 7.93–6.75 (br, 11H), 4.33–3.83 (br, 11H), 1.73–1.52 (br,
23H).
1
P4 (M2). H NMR (300 MHz, DMSO-d6): d (ppm) 8.08 (br,
2H), 7.67–6.87 (br, 8H), 4.15–3.74 (br, 10H), 1.80–1.54 (br, 14H).
Metal Ni2+ ion and CD titrations
The metal Ni2+ ion and CD titrations were performed using
polymer solutions (5.0 ꢀ 10ꢁ6 M) in THF (150 mL) and solutions
of Ni2+ ions (1 ꢀ 10ꢁ3 M of NiCl2 in THF containing 3 vol%
H2O) and b-CD (3 ꢀ 10ꢁ3 M in THF containing 25 vol% EtOH
and 25 vol% H2O). The fluorescence titration spectra were
recorded after titration for 7 min to allow complete formation of
complexes between the Ni2+ ions and the polymers.
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Table 2 Absorption and PL emission spectral data of polymers P1–P4
in THF solutions
copolymer compositions of P1–P4 by comparing the relative
integration of the signals at 8.4 ppm (corresponding to two
protons of the pyridyl groups of the M1 units) and 6.8–7.9 ppm
(corresponding to the other overlapped aromatic protons of the
M1 and M2 units). The product compositions of copolymers P2
and P3 were very close to the feeding ratios of the monomers. All
of the polymers were soluble in common organic solvents,
including THF, DMSO, and DMF. The weight-average molec-
ular weights (Mw) and polydispersity indexes (PDIs) of the
polymers P1–P4, determined through GPC, were in the ranges
9.7–37.4 kg molꢁ1 and 1.18–2.11, respectively. Table 1 summa-
rizes the compositions (input and output molar ratios), molec-
ular weights (Mw), PDIs, and yields of P1–P4.
a
labs,sol (nm)
ab
lPL,sol (nm)
c
Polymer
FPL
P1
P2
P3
P4
332, 400
333, 399
327, 402
320
467
0.41
0.43
0.49
0.57
401, 463
404, 462
403
a
b
Measured in dilute THF solutions. Excited at the maximum
c
absorption wavelength of each polymer. Solution PL quantum yields
(FPL) were measured in THF, relative to 9,10-diphenylanthracene
(FPL ¼ 0.90).
Photophysics of polymers P1–P4
As reported previously, the fluorescent conjugated homopolymer
P1 containing the proton-acceptor (pyridyl) monomer M1 is an
ideal reversible chemosensory material.18 In this study, we prepared
the homopolymer P4 and the copolymers P2 and P3 to investigate
their abilities to function as recoverable chemosensors for metal
ions (through tunable fluorescence energy transfer between the M1
and M2 moieties upon varying the degrees of hydrogen bonding);
Table 2 summarizes the photophysical properties of these poly-
mers. The PL quantum yields (FPL) of the polymers P1–P4 in
dilute THF solutions were in the range 0.41–0.57, depending on the
content of M2 moieties. The fluorescence of the copolymers P2 and
P3 could be tuned by disrupting the hydrogen bonding between the
M1 and M2 moieties through the stronger interactions of the M2
(benzoic acid) moieties with CDs, thereby controlling the sensitiv-
ities of these chemosensors towards metal ions; therefore, we
investigated the adjustable fluorescence energy transfer between the
M1 and M2 moieties at various degrees of hydrogen bonding in the
copolymers P2 and P3.
Fig. 2 Absorption spectra of the polymers P1–P4 (5 ꢀ 10ꢁ6 M in THF).
In Fig. 2, we assigned the absorption bands of the polymer P1
near 332 and 400 nm to the M1 (pyridyl receptor) units. In addi-
tion, the absorption intensities of the polymers P2–P4 (in THF)
near 320 nm (attributed to M2) increased gradually relative to
those near 400 nm (attributed to M1), proportionally to the content
of M2 (benzoic acid) units. Fig. 3 presents the PL emission spectra
of the polymers P1–P4 in THF; Table 2 lists the pertinent data. The
homopolymers P1 and P4 (containing M1 and M2 moieties,
respectively) exhibited their maximum PL emissions near 467 and
401 nm, respectively. The PL emission intensities of the copolymers
P2 and P3 (in THF) near 463 and 403 nm are related to their M1
and M2 contents, respectively. The PL emission peaks near 403 nm
Fig. 3 Normalized PL spectra of the polymers P1–P4 (5 ꢀ 10ꢁ6 M in
THF).
(from M2 moieties) were weaker than those near 463 nm (from M1
moieties) for the copolymers P2 and P3, which implied efficient
energy transfer occurred from the M2 moieties to the M1 moieties.
In order to probe this energy transfer effect, the photoluminescent
Table 1 Compositions, yields, molecular weights, and degradation temperatures of polymers P1–P4
Molar ratio
b
Polymer
Feeding (M1/M2)
Product (x/y)a
Yield (%)
Mw (g molꢁ1
)
PDI (Mw/Mn)b
P1
P2
P3
P4
1/0
1/0
65
64
69
40
11 200
9707
11 950
37 441
1.70
2.11
1.42
1.18
1/0.1
1/1
0/1
1/0.1
1/0.8
0/1
a
b
Determined from H NMR spectra. Weight-average molecular weight (Mw) and polydispersity index (PDI) determined by GPC in THF using
1
polystyrene standards.
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excitation (PLE) and absorption spectra of the copolymer P3 in
THF solutions were recorded in Fig. S1 (ESI†). In contrast to P3,
the absorption and PL emission spectra of homopolymer P4 in
THF solutions are illustrated as well. The absorption spectrum of
copolymer P3 overlapped significantly with the PL emission spec-
trum of P4, suggesting that energy transfer from the M2 moieties to
the M1 moieties should be expected. The feature in the PLE
spectrum of P3 appears to match that in the corresponding
absorption spectra of P3, where the PLE spectrum was monitored
at the corresponding maximum PL emission. These measurements
conclusively revealed the energy transfer from the benzoic acid
(M2) moieties to the pyridyl receptors (M1) in the copolymer P3.
Furthermore, we might expect supramolecular interactions
between the M1 and M2 moieties (via hydrogen bonds) to further
facilitate energy transfer in the copolymers P2 and P3 (in THF),
providing a window to study the recoverable and/or adjustable
sensitivities of the chemosensors and photoinduced energy transfer
between proton donor and acceptor moieties.
(i.e., monomer M2 units), the quenching effect of P2 was greater
than that of P3 when titrated with Ni2+ ions. We prepared
a polymer blend (P1/P4) from the homopolymers P1 and P4 such
that it featured a molar ratio of its monomer unit similar to that
of the copolymer P2 (x : y ¼ 1 : 0.1). Fig. 4(c) reveals that the
quenching effect of P2 was greater than that of the blend (P1/P4)
when titrated with Ni2+ ions, indicating that the hydrogen
bonding interactions among the homopolymers P1 and P4 of the
blend (P1/P4) were stronger than those in the copolymer P2. In
contrast to the situation for the homopolymer P1, no complex-
ation occurred between P4 and Ni2+ ions [Fig. 4(d)], due to the
lack of pyridyl receptors in the homopolymer P4 (featuring only
benzoic acid moieties); therefore, no obvious fluorescence
quenching occurred upon the addition of Ni2+ ions. Hence,
according to the fluorescence quenching results, the excitons of
the homopolymer P1 were more readily trapped through charge-
transfer to the Ni2+ ions than were those of the copolymers P2
and P3. Fig. 5(a)–(d) present Stern–Volmer (SV) plots—replot-
ted from Fig. 4(a)–(d), respectively—of the copolymers P2 and
P3, the blend (P1/P4), and the homopolymers P4 titrated with
the same concentration (up to 1.1 ꢀ 10ꢁ6 M in THF) of Ni2+ ions;
Fluorescence quenching of polymers upon titration with Ni2+ ions
As in our previous report,18 among the variety of tested metal
ions (including Cu2+, Fe3+, Fe2+, Ni2+, Au3+, Mn2+, Mg2+, Cr2+,
and Hg2+), the homopolymer P1 (containing only pyridyl
receptors for metal ions) exhibited its best chemosensing capa-
bility towards Ni2+ ions, as measured through fluorescence
quenching titration experiments; the fluorescence intensity of P1
decreased dramatically upon the addition of Ni2+ metal ions as
a result of complexation of the fluorescent pyridyl units with the
Ni2+ ions. Therefore, in this study we chose18 to investigate the
chemosensor capabilities of the homopolymers P1 and P4 and
the copolymers P2 and P3. In the fluorescence quenching
experiments in Fig. 4, the PL spectra of the homopolymers P1
and P4 and the copolymers P2 and P3 (each 5 ꢀ 10ꢁ6 M in THF)
were monitored in the presence of Ni2+ ions with the same
concentration (1.1 ꢀ 10ꢁ6 M). The fluorescence spectra of the
homopolymers P1 and P4 exhibited maximum PL emissions at
464 and 403 nm, respectively. In Fig. 4(a) and (b), the copolymers
P2 and P3 with benzoic acid moieties titrated with Ni2+ ions
exhibited PL quenching trends similar to those of the homo-
polymer P1 suggesting that the complexation of the pyridyl units
in the copolymers P2 and P3 with the Ni2+ ions was similar to the
interactions of P1. Because of hydrogen bonding of the proton-
accepting pyridyl receptors of the M1 units with the proton-
donating benzoic acid moieties of the M2 units in the copolymers
P2 and P3, the content of available pyridyl receptors (i.e., not
hydrogen bonded to benzoic acid moieties and, therefore, free to
complex with Ni2+ ions) in P2 and P3 would be much lower than
that in the homopolymer P1. Consequently, the PL quenching
effect of the Ni2+ ions on the copolymers P2 and P3 was much
weaker than that on the homopolymer P1. In a previous study,
we reported a similar PL quenching phenomenon for gold
nanocomposites interacting with fluorescent homopolymers and
copolymers; in that case, hydrogen bonding of the monomer
moieties in the copolymer resulted in a lower quenching effi-
ciency of the gold nanoparticles towards the copolymer.21
Table 3 lists the corresponding quenching constants (KSV)
acquired from these SV plots. From the SV plots, the detection
limits of polymers without CDs for Ni2+ ions were in the range of
0.11–0.16 ꢀ 10ꢁ6 M. According to Fig. 5(a) and (b), the
quenching constants for the copolymers P2 and P3 (1.67 ꢀ 106
and 4.91 ꢀ 105 Mꢁ1, respectively) titrated with Ni2+ ions were
lower than that of the homopolymer P1 (5.65 ꢀ 106 Mꢁ1), due to
the lower availability of metal Ni2+ ion receptors (i.e., non-
hydrogen-bonded pyridine units) in the copolymers P2 and P3
for complexation with Ni2+ ions. Based on Fig. 5(c), the
quenching constant (3.72 ꢀ 105 Mꢁ1) of the blend (P1/P4) was
lower than that (1.67 ꢀ 106 Mꢁ1) of the copolymer P2 [at
a similar molar ratio of monomer units (x : y ¼ 1 : 0.1)] when
titrated with Ni2+ ions, confirming that the lower availability of
the free pyridyl receptors occurred in the blend (P1/P4) because
of its stronger hydrogen bonding interactions. Moreover, the
lack of pyridyl receptors in the homopolymer P4, featuring only
proton-donating benzoic acid units, provided a quenching
constant of 3.62 ꢀ 104 Mꢁ1, calculated from the SV plot in
Fig. 5(d), that was much lower than that of the homopolymer P1.
Hence, both (i) complexation of the pyridyl receptors with the
metal ions and (ii) hydrogen bonding between the proton-
acceptor pyridyl receptors (of monomer M1 units) and proton-
donor benzoic acid moieties (of monomer M2 units) played
important roles in the fluorescence quenching of the copolymers
P2 and P3. In general, we could distinguish the significant
chelation between the metal ions and the fluorescent
homopolymers P1 and P4 as well as the copolymers P2 and P3 in
terms of distinct fluorescence quenching behaviors with specific
quenching constants (KSV).
Fluorescence quenching of polymers in the presence of CDs upon
titration with Ni2+ ions
In contrast to the behavior of the fluorescent homopolymer P1,
which does not undergo self-complexation through hydrogen
bonding, the introduction of proton-donor benzoic acid moieties
(of monomer M2 units) to the copolymers P2 and P3 resulted in
Because of the greater availability of the pyridyl receptors (i.e.,
non-hydrogen-bonded and, therefore, free to complex with metal
ions) in P2, which featured a lower ratio of benzoic acid moieties
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Fig. 4 Fluorescence quenching spectra of polymers P1–P4 titrated with the same concentration (up to 1.1 ꢀ 10ꢁ6 M in THF) of Ni2+ ions: (a) P2, (b) P3,
(c) blend (P1/P4), and (d) P4 in THF solutions (5 ꢀ 10ꢁ6 M).
hydrogen-bonded interactions between the proton-accepting pyr-
idyl receptors (of monomer M1 units) and proton-donating ben-
zoic acid moieties (of monomer M2 units). As a result, the content
of available (i.e., non-hydrogen-bonded) pyridyl receptors could be
adjusted by varying the copolymer design to feature different molar
ratios of the proton-acceptor and proton-donor units, as we have
Fig. 5 SV plots of polymers, in THF solutions (5 ꢀ 10ꢁ6 M), titrated with the same concentration (up to 1.1 ꢀ 10ꢁ6 M in THF) of Ni2+ ions: (a) P2, (b)
P3, (c) blend (P1/P4), and (d) P4.
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Table 3 Values of KSV for polymers titrated with Ni2+ ions
Because the concentration of available (i.e., non-hydrogen-
bonded) pyridyl receptors for the sensing of metal ions could be
adjusted by adding CDs, we investigated the fluorescence
quenching effects of the polymers (with different molar ratios of
M1 and M2 units) when titrated with Ni2+ ions in the presence of
CDs. From the SV plots, the detection limits of polymers with
CDs for Ni2+ ions were calculated to be in the range of 0.06–0.2
ꢀ 10ꢁ6 M. Fig. 7(a)–(d) display the fluorescence quenching
spectra of the P1 + CD, P2 + CD, P3 + CD, and blend (P1/P4) +
CD systems, in THF, when titrated with the same concentration
(up to 1.1 ꢀ 10ꢁ6 M) of Ni2+ ions. Compared with the fluores-
cence quenching spectra of the copolymers P2 and P3 in Fig. 4(a)
and (b), the corresponding copolymer + CD systems (i.e., P2 +
CD and P3 + CD) were quenched more effectively upon titration
with Ni2+ ions, because of the larger concentrations of available
(i.e., non-hydrogen-bonded) pyridyl receptors to sense the metal
ions when CDs were present. Fig. 8(a)–(d) present SV plots—
replotted from Fig. 7(a)–(d), respectively—of the P1 + CD, P2 +
CD, P3 + CD, and blend (P1/P4) + CD systems (in THF)
titrated with the same concentration (up to 1.1 ꢀ 10ꢁ6 M) of Ni2+
ions; Table 3 lists their quenching constants (KSV) extracted from
the SV plots. After the addition of appropriate amounts of CDs
to capture the benzoic acid moieties, the sensitivities of the
fluorescent copolymers P2 and P3 towards Ni2+ ions improved,
due to the increased concentration of non-hydrogen-bonded
pyridyl receptors for metal ion sensing. Apart from the P1 + CD
system, which exhibited a decreased value of KSV of 5.11 ꢀ 106
(because it possessed no benzoic acid moieties to interact with
CDs), the addition of CDs caused the quenching constants of the
P2 + CD, P3 + CD, and blend (P1/P4) + CD systems to increase
to 4.76 ꢀ 106, 2.81 ꢀ 106, and 5.02 ꢀ 105 Mꢁ1, respectively, due
to the release of hydrogen-bonded pyridyl receptors. Compared
with the blend (P1/P4) + CD system (i.e., featuring a mixture of
the homopolymers P1 and P4), however, the copolymer P2 + CD
system [featuring a similar molar ratio of monomer units (x : y ¼
1 : 0.1)] had a larger Ni2+ ion sensitivity (KSV ¼ 4.76 ꢀ 106),
suggesting a greater concentration of available (i.e., non-
hydrogen-bonded) pyridyl receptors in the copolymer P2 as
a result of its weaker hydrogen bonding relative to that in the
blend (P1/P4) + CD.
a
Polymer
Components (x : y)
KSV (Mꢁ1
)
P1b
P2
P3
P4
1 : 0
5.65 ꢀ 106
1.67 ꢀ 106
4.91 ꢀ 105
3.62 ꢀ 104
3.72 ꢀ 105
5.11 ꢀ 106
4.76 ꢀ 106
2.81 ꢀ 106
5.02 ꢀ 105
1 : 0.1
1 : 0.8
0 : 1
Blend (P1/P4)
P1 + CD
P2 + CD
P3 + CD
Blend (P1/P4) + CD
1 : 0.1
1 : 0
1 : 0.1
1 : 0.8
1 : 0.1
a
The PL quenching efficiency can be evaluated through the static SV
quenching constant (KSV), monitoring the corresponding decreases in
PL intensity upon increasing the metal ion concentration in the linear
region, according to the well-known static SV equation: I0/I ¼ 1 +
KSV[Q], where I0 is the PL intensity in the absence of the quencher, I is
the PL intensity in the presence of the quencher, and [Q] is the
b
quencher concentration. The value of KSV for P1 was obtained from
our previous report.18
demonstrated herein with the copolymers P2 and P3. An alterna-
tive approach is to add a third component, namely a CD, to cap the
benzoic acid moieties and, thereby, weaken the hydrogen bonding
between the M1 and M2 moieties to different extents in the
copolymers P2 and P3. Accordingly, the levels of free pyridyl
receptors could be increased through capture of the benzoic acid
moieties (of monomer M2 units) by the CDs, allowing the devel-
opment of the first recoverable chemosensor copolymers P2 and
P3. To prove the capabilities of the recoverable chemosensors, we
studied the fluorescence quenching properties of the copolymers P2
and P3 and the blend (P1/P4) titrated with Ni2+ ions in the presence
of CDs. Fig. 1 displays the mechanism behind the release of the
pyridyl receptors in the hydrogen-bonded copolymers P2 and P3
after the addition of CDs.
Upon the addition of CDs at certain concentrations, the
benzoic acid moieties (of monomer M2 units) were captured by
the CDs, such that some of the quenched hydrogen-bonded
pyridyl receptors were released; thus, both (i) the non-hydrogen-
bonded pyridyl receptors and (ii) the benzoic acid units capped
by CDs exhibited enhanced fluorescence to different degrees. As
illustrated in Fig. 6(a), the fluorescence intensity of the polymer
P1 (lacking benzoic acid units) was not modified upon titration
of CDs (up to 0.12 mM), which did not interact with the pyridyl
receptors. Fig. 6(b)–(e) reveal that addition of CDs increased
slightly the fluorescence intensities of the copolymers P2 and P3,
the blend (P1/P4) and the polymer P4 in THF solutions. The
optimal concentrations of CDs to capture the maximum number
of hydrogen-bonded benzoic acid moieties in the polymer solu-
tions—and, thereby, provide the highest PL intensities—were
0.02, 0.12, 0.05, and 0.12 mM for the copolymers P2 and P3, the
blend (P1/P4) and the homopolymer P4, respectively (PL emis-
sion enhancements: 1.09, 1.12, 1.08, and 1.16, respectively).
Hence, the PL emission enhancements (1.16- and 1.12-fold) for
the homopolymer P4 and the copolymer P3 were larger than
those for the copolymer P2 and the blend (P1/P4), due to the
greater release of benzoic acid moieties after the addition of CDs
to P4 and P3, because the hydrogen-bonded benzoic acid
moieties (bound as dimeric acids in the homopolymer P4 or to
pyridyl moieties in the copolymer P3) were capped with CDs to
the largest extents.
In order to verify this observation, FTIR and PL measure-
ments of the copolymer P2 and blend (P1/P4) are compared as
shown in Fig. S2 and S3 of the ESI†. In contrast to the O–H
bands of the pure donor homopolymer P4 (H-bonded dimer) at
2660 and 2580 cmꢁ1 in Fig. S2†, two new broad O–H bands
centered at 2465 and 1920 cmꢁ1 were observed in the H-bonded
copolymer P2 and blend (P1/P4), which revealed the existence of
hydrogen bonding between the pyridyl and the carboxylic acid
groups in the H-bonded networks of copolymer P2 and blend
(P1/P4). On the other hand, a stretching vibration of C]O at
1686 cmꢁ1 in the pure homopolymer P4 was shifted towards
higher wavenumber and overlapped with the band of the ester
carbonyl groups at 1725 cmꢁ1 in the copolymer P2 and blend (P1/
P4), which showed that the carbonyl group was in a less asso-
ciated state than that in the pure H-bonded dimer state of
proton-donor P4. These FTIR results suggested that hydrogen-
bonds were formed between M1 and M2 units in both the
H-bonded copolymer P2 and blend (P1/P4). Moreover, as
illustrated in Fig. S3†, the stronger H-bonded effect of the blend
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Fig. 6 Fluorescence quenching spectra of polymers in THF solutions (5 ꢀ 10ꢁ6 M), titrated with CDs (up to appropriate concentrations in paren-
theses): (a) P1 (0.12 mM), (b) P2 (0.02 mM), (c) P3 (0.12 mM), (d) blend (P1/P4) (0.05 mM), and (e) P4 (0.12 mM).
(P1/P4) could induce a more red-shifted PL emission in blend
(P1/P4) than P2. Overall, the copolymer P2 + CD system
exhibited the largest Ni2+ ion sensitivity (KSV ¼ 4.76 ꢀ 106)
among all of our studied polymer + CD systems; it recovered
almost all of its hydrogen-bonded pyridyl receptors after adding
the CDs, reaching a Ni2+ ion sensitivity (KSV ¼ 5.65 ꢀ 106) close
to that of the homopolymer P1. Therefore, the sensitivity of Ni2+
ion detection becomes recoverable and/or adjustable through: (i)
copolymer design, varying the molar ratio of hydrogen-bonding
pyridyl receptors (proton-acceptor M1 units) and benzoic acid
moieties (proton-donor M2 units) and (ii) the addition of CDs.
we observed that the ligation of the receptors in the polymers P2–
P4 and the P2 + CD and P3 + CD blends with metal ions (2.5 ꢀ
10ꢁ6 M) resulted in obvious quenching. Thus, we probed the
time-resolved fluorescence (TRF) signals of the polymers P2–P4
and the P2 + CD and P3 + CD systems at 480 nm upon exci-
tation at 375 nm. Table 4 summarizes the fluorescence lifetimes
obtained after deconvolution of the instrumental response
function25 and exponential fittings. The TRF traces of the
polymers P2–P4 in the absence/presence of metal ions, plotted in
Fig. S4(a)–(c) of the ESI†, reveal that Ni2+ ions affected the
fluorescence lifetimes of the copolymers P2 and P3 as a result of
coordination with the pyridyl receptor moieties. In the absence of
Ni2+ ions, a single exponential fitting provided fluorescence life-
times (s2) for P2–P4 of 1.296, 1.250, and 1.234 ns, respectively,
corresponding to the lifetimes of their S1 states. Adding metal
ions to the solutions caused another ultrafast decay time constant
(s1) to appear in the results of bi-exponential decay fittings. The
fluorescence lifetimes (s1) of P2–P4 were 0.131, 0.173, and
Time-resolved photoluminescence (TRPL) spectra
Heavy metal ions tend to quench the luminescence of conjugated
receptors, including polymers and monomers, through electron-
transfer and energy-transfer processes.21–24 Comparing the fluo-
rescence quenching spectra titrated with Ni2+ ions in Fig. 4 and 6,
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Fig. 7 Fluorescence quenching spectra of: (a) P1 + CD, (b) P2 + CD, (c) P3 + CD, and (d) blend (P1/P4) + CD systems (in THF) titrated with the same
concentration (up to 1.1 ꢀ 10ꢁ6 M) of Ni2+ ions.
0.208 ns, respectively. In the complex P1 + Ni2+, the ratio of the
faster decay component increased up to 78.8% in the presence of
Ni2+ ions,18 reflecting the different interactions between the
pyridyl receptor moieties of P1 and Ni2+ ions. More importantly,
the longer decay time constant (s2) of P1 remained (27.8% for
Ni2+ ions) even if Ni2+ ions were added to the solutions. The
Fig. 8 SV plots of: (a) P1 + CD, (b) P2 + CD, (c) P3 + CD, and (d) blend (P1/P4) + CD systems (in THF) titrated with the same concentration (up to
1.1 ꢀ 10ꢁ6 M) of Ni2+ ions.
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fluorescence quenching efficiencies of the Ni2+ ions in the fluo-
rescence decay experiments, thereby facilitating the use of
supramolecular side-chain copolymers as recoverable and/or
adjustable chemosensors.
Table 4 Fluorescence decay time constants for the polymers P1–P4 and
the P2 + CD and P3 + CD systems in the absence and presence of Ni2+
ions
Polymer
A1
s1 (ns)
A2
s2 (ns)
P1a
100%
1.375
1.578
1.296
1.631
1.250
1.542
1.234
1.234
1.516
1.622
1.325
1.576
a
P1 + Ni2+
P2
78.8%
64.2%
40.1%
0.131
0.173
0.208
21.2%
100%
35.8%
100%
59.9%
100%
100%
100%
Conclusions
P2 + Ni2+
P3
We have synthesized a series of fluorescent conjugated homo-
polymers (P1 and P4) and copolymers (P2 and P3) containing
proton-acceptor (pyridyl) M1 monomer units and proton-donor
(benzoic acid) M2 monomer units and studied their recoverable
chemosensor sensitivities for metal ions (via adjustable fluores-
cence energy transfer between the M1 and M2 moieties with
various degrees of hydrogen bonding). Among the tested metal
ions, the homopolymer P1 (containing only pyridyl receptors)
exhibited its best chemosensing capability towards Ni2+ ions in
fluorescence quenching experiments. Hence, we modified the
concentrations of available (i.e., non-hydrogen-bonded) pyridyl
receptors through the design of copolymers P2 and P3 with
different molar ratios of pyridyl M1 and benzoic acid M2
monomer units. We could distinguish the significant chelation
interactions between the Ni2+ ions and the fluorescent homo-
polymers P1 and P4 and the copolymers P2 and P3 through their
distinct fluorescence quenching behaviors; the specific quenching
constants (KSV) decreased in the order P1 > P2 > P3 > P4,
consistent with the trend of the decreasing molar ratios of their
pyridyl receptors. Moreover, we introduced a third component,
CDs, to cap the benzoic acid moieties and, thereby, disrupt the
hydrogen bonding between the M1 and M2 moieties to different
extents in the copolymers P2 and P3. Accordingly, the concen-
trations of available pyridyl receptors in the recoverable che-
mosensor copolymers P2 and P3 were regained after capture of
the benzoic acid moieties (of monomer M2 units) by the CDs; as
a result, the KSV values of the P2 + CD, P3 + CD and blend
(P1/P4) + CD systems all increased from 1.67 ꢀ 106, 4.91 ꢀ 105,
and 3.72 ꢀ 105 Mꢁ1, respectively, in the absence of CDs to 4.76 ꢀ
106, 2.81 ꢀ 106, and 5.02 ꢀ 105 Mꢁ1, respectively, in the presence
of CDs, due to the release of hydrogen-bonded pyridyl receptors.
With the addition of CDs, the disrupted hydrogen bonding
between the pyridyl receptor (M1) and benzoic acid (M2)
moieties in the copolymers also enhanced the fluorescence
quenching efficiencies of the Ni2+ ions in fluorescence decay
experiments. Therefore, the developments of supramolecular
(hydrogen-bonded) side-chain fluorescent copolymers (contain-
ing H-bonded moieties of M1 and M2) might facilitate the future
applications of recoverable (and/or adjustable) chemosensors by
adding the third component of CDs.
P3 + Ni2+
P4
P4 + Ni2+
P2 + CD
P2 + CD + Ni2+
P3 + CD
70.2%
55.2%
0.231
0.221
29.8%
100%
44.8%
P3 + CD + Ni2+
a
Lifetimes of P1 and P1 + Ni2+ were obtained from our previous
report.18
emergence of the s1 decay component in the P1 + Ni2+ system
clearly indicates that its TRF traces featured two contributing
factors: one (s2) from free P1 and the other (s1) from its complex.
The quenching of the TRF signals upon addition of Ni2+ implied
that the metal ions formed a charge-transfer emitting state with
P1. For the copolymers P2 and P3 in the presence of Ni2+ ions,
we estimated their fluorescence lifetimes from bi-exponential
fittings. Relative to that for the homopolymer P1, Table 4 reveals
that the Ni2+ ions did not significantly affect the fluorescence
decay times of the polymers P2–P4. We suggest that hydrogen
bonding between the proton-acceptor pyridyl moieties (of
monomer M1 units) and proton-donor benzoic acid moieties (of
monomer M2 units) explains the similar fluorescence quenching
effects on both the copolymers P2 and P3. The decay time of the
original decay component for the homopolymer P4 could be
approximately 100% of the longer decay time constant (s2) upon
adding metal ions.26 Moreover, as revealed in Fig. S4(d) and (e)
of the ESI†, after adding appropriate amounts of CDs to the
solutions of P2 and P3, we used single-exponential fitting to
estimate the fluorescence lifetimes of the P1 + CD and P2 + CD
systems to be 1.516 and 1.325 ns, respectively; these values
correspond to the lifetimes of the polymers. Relative to the
copolymer P2 + Ni2+ and P3 + Ni2+ systems (without CDs), the
faster components of the P2 + CD + Ni2+ and P3 + CD + Ni2+
systems (with CDs) increased from 64.2 and 40.1%, respectively,
to 70.2 and 55.2%, respectively. The phenomena also proved that
the quenching efficiencies of the copolymers P2 and P3 titrated
with Ni2+ ions could be improved by adding CDs, and that these
systems could be used as recoverable and/or adjustable chemo-
sensors. Apparently, the interactions of the pyridyl receptors of
the polymers with the Ni2+ ions created another fluorescence
quenching pathway through which the photoexcitation energy of
the pyridyl receptors was effectively transferred from the
conjugated side chains to the Ni2+ ions. The observed fluores-
cence quenching phenomena of the ligands presumably reflect
excitation energy transfer from the ligands to the metal ion’s
d orbital.23,24 Therefore, with the addition of CDs, the disrupted
hydrogen bonding between the pyridyl receptor moieties (of
the proton-acceptor M1 units) and the benzoic acid moieties (of
the proton-donor M2 units) in the copolymers enhanced the
Acknowledgements
We thank the National Science Council of Taiwan (NSC
99-2113-M-009-006-MY2) and National Chiao Tung University
(97W807) for supporting this study financially.
Notes and references
ꢀ
ꢀ~
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