Salicylaldimine-functionalized poly(m-phenyleneethynylene) as turn-on chemosensor for ferric ion

Article abstract of DOI:10.1002/pola.28997

A new turn on fluorescent probe for ferric ion based on poly(m-phenyleneethynylene salicylaldimine) (PPE-IM) has been developed. The preparation of PPE-IM involves post-polymerization functionalization of the corresponding polymeric amine, PPE-AM, via the condensation with salicylaldehyde. The degree of polymerization of both PPE-IM and PPE-IM is 17 with polydispersity index of 1.5. In aqueous solution, the polymeric PPE-IM is highly stable unlike its small molecule analog which is gradually hydrolyzed. The weak fluorescence of initial PPE-IM (λ_em = 470) is greatly enhanced by 300 folds upon the addition of Fe³⁺. The ¹H NMR reveals that the fluorescence enhancement is caused by Fe³⁺-induced hydrolysis of the imine group. The sensing system shows a detection limit of 0.14 μM of Fe³⁺.

Full text of DOI:10.1002/pola.28997

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Salicylaldimine-Functionalized Poly(m-Phenyleneethynylene) as Turn-on
Chemosensor for Ferric Ion
Nopparat Thavornsin,1 Paitoon Rashatasakhon,2 Mongkol Sukwattanasinitt,2
2
Sumrit Wacharasindhu
1Program of Petrochemistry and Polymer Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
2Department of Chemistry, Faculty of Science, Nanotec-CU Center of Excellence on Food and Agriculture, Chulalongkorn
University, Bangkok 10330, Thailand
Correspondence to: S. Wacharasindhu (E-mail: sumrit.w@chula.ac.th)
Received 18 January 2018; accepted 27 February 2018; published online 24 March 2018
DOI: 10.1002/pola.28997
ABSTRACT: A new turn on fluorescent probe for ferric ion based
on poly(m-phenyleneethynylene salicylaldimine) (PPE-IM) has
been developed. The preparation of PPE-IM involves post-
polymerization functionalization of the corresponding poly-
meric amine, PPE-AM, via the condensation with salicylalde-
hyde. The degree of polymerization of both PPE-IM and PPE-
IM is 17 with polydispersity index of 1.5. In aqueous solution,
the polymeric PPE-IM is highly stable unlike its small molecule
analog which is gradually hydrolyzed. The weak fluorescence
of initial PPE-IM (kem 5 470) is greatly enhanced by 300 folds
upon the addition of Fe31. The 1H NMR reveals that the fluo-
rescence enhancement is caused by Fe31-induced hydrolysis of
the imine group. The sensing system shows a detection limit
of 0.14 lM of Fe31
.
2018 Wiley Periodicals, Inc. J. Polym.
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KEYWORDS: chemodosensor; ferric ion; hydrolysis; poly(m-phe-
nyleneethynylene salicylaldimine); Schiff base probe; turn on
fluorescence
turn on sensor from conjugated polymer is challenging since
a strong emissive background from initial highly conjugated
backbone of polymer must be suppressed. To date, a few
PPEs turn-on sensors were reported for metal sensing. These
are based on the inhibition of aggregation,23,24 photo-
induced electron transfer (PET),25,26 and intramolecular
charge transfer (ICT)27 mechanisms. It is still desirable to
enhance the specificity and sensitivity of this class of PPE-
based turn on sensor for important metal ion.
INTRODUCTION Application of conjugated polymer (CP)-
based optical sensory materials in environmental monitoring
and biomedical diagnostics has drawn great attention in
recent year.1–6 In comparison to small molecules, the CP-
based sensor exhibit greater absorption cross section, extinc-
tion coefficient and thermal stability.7–9 For fluorescent CP
such as poly(phenyleneethynylene) (PPE), the lateral attach-
ment of substituents on the aromatic ring with suitable
receptor unit provides efficient sensor-based PPE.10–14 The
p–p* conjugated electronic structure in PPE allows a fast
excitation energy transfer along the backbone to the energy/
electron acceptors which allows amplification of fluorescent
signal.15–18 With this property, numerous of highly sensitive
PPEs sensor was designed and reported as fluorescence
quenching (turn-off) mode via single quencher molecule
(analyte) causing an efficient quenching of the long polymer
segment.19–22 In principle, fluorescence “turn-on” sensor has
numerous advantages over “turn-off” sensors, for example:
(1) higher sensitivity as a result of contrasting fluorescence
signal with a dark background (2) higher selectivity as the
signal is generated by specific binding, whereas fluorescence
quenching can occur in multiple ways. Despite these advan-
tages, designing a highly selective and sensitive fluorescence
Iron is a crucial element in the biological systems. Elevated
level of Fe31 ion within the body has been associated with
increased incidence of cancer as well as dysfunction of critical
organs such as the heart, pancreas, and liver.28,29 Thus, moni-
toring and detection of Fe31 is important. Previously, most fluo-
rescent probes for Fe31 ion were reported as turn off mode
based on the paramagnetic nature of Fe31 30,31
However, in
.
recent years, fluorescent turn-on probes for Fe31 based on
small molecules were emerging as a new sensor design con-
cept. Generally, salicylaldimine Schiff base is incorporated as a
receptor into the chromophores.32–37 Owing to isomerization of
C@N and ESIPT from salicylaldimine group, those probes show
weak fluorescent signals in the initial state. The addition of
Additional Supporting Information may be found in the online version of this article.
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EXPERIMENTAL
Materials
All reactions were carried out under nitrogen atmosphere
using freshly anhydrous solvents. All reagents were pur-
R
chased from Sigma-Aldrich (USA), FlukaV (Switzerland), TCI
R
chemical (Japan), or MerckV (Germany) and used without
further purification.
FIGURE 1 Structure of PE-IM and PPE-IM. [Color figure can be
Measurements
viewed at wileyonlinelibrary.com]
Analytical thin-layer chromatography (TLC) was performed on
Kieselgel F-254 pre-coated plastic TLC plates from EM Science.
Visualization was performed with a 254 nm ultraviolet lamp.
Gel column chromatography was carried out with silica gel 60
(70–230 mesh) from Merck. Elemental (C, H, N) analysis was
performed on PE 2400 series II (Perkin-Elmer, USA). The 1H
and 13C NMR spectra were recorded on a Varian 400 or
Bruker 400 in CDCl3, THF-d8 or (CD3)2CO. Chemical shifts are
expressed in parts per million (d) using residual solvent pro-
tons as internal standards: CDCl3 (d 7.26 for 1H, d 77.00 for
13C), THF-d8 (d 1.72, 3.58 for 1H) and (CD3)2CO (d 2.05 for
1H, d 206.26 for 13C). Fourier transform infrared spectra were
acquired on Nicolet 6700 FTIR spectrometer equipped with a
mercury-cadmium telluride (MCT) detector (Nicolet, USA). All
polymer solutions were filtered through 0.45 lm syringe fil-
ters prior to use. Polymer molecular weights were determined
by Waters 600 controller chromatograph equipped with two
HR (waters), column (HR1 and HR4) at 35 8C and a reflective
index detector (waters 2414). Tetrahydrofuran (HPLC grade)
was used as the eluent with the flow rate of 1.0 mL/min
(4 mg/mL sample concentrations). Sample injection volume
was 50 mL. Polystyrenes (996–188,000 Da) were used as
standards for calibration. The UV–visible spectra were
Fe31 ion promotes the hydrolysis of salicylaldimine group and
return into the high emissive chromophores. This strategy cre-
ates the Fe31 chemodosensor as fluorescence turn-on probe in
which the fluorescence signal is not only irreversible but the
probes also act as scavenges of Fe31 in the physiological envi-
ronment. However, the only drawback of this strategy is that
the hydrolysis of salicylaldimine group may prematurely
undergo in aqueous media causing not only the instability of
probes itself but also false turn-on fluorescence signal.
In this study, we prepared a highly stable and selective fluo-
rescence turn on probe for Fe31 detection based on poly(-
phenyleneethynylene) containing salicylaldimine PPE-IM
peripheral group (Fig. 1). The salicylaldimine groups have
dual roles as a Fe31 receptor and fluorescence quencher. The
probe has a sufficiently dark fluorescence background which
is a prerequisite for turn-on sensing probe. The polymeric
structure facilitates both aggregation and self-coiling to pro-
tect the imine group from hydrolysis. This behavior results
in a highly stable probe in aqueous media. For comparison
purpose, we construct a small molecule analog, PE-IM, to be
studied alongside PPE-IM.
SCHEME 1 Synthesis routes of PE-IM and PPE-IM. [Color figure can be viewed at wileyonlinelibrary.com]
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1
powder (PE-IM) was obtained (yield: 118 mg, 93%) H NMR
(400 MHz, (CD3)2CO) d 13.51 (s, 1H), 9.12 (s, 2H), 7.83 (s,
2H), 7.68 (dd, J 5 8.0, 2.1 Hz, 10H), 7.62–7.58 (m, 4H), 7.50–
7.43 (m, 14H), 7.00 (dd, J 5 14.2, 7.6 Hz, 4H). 13C NMR (101
MHz, (CD3)2CO) d 165.10, 165.06, 162.56, 162.16, 149.83,
136.33, 134.68, 134.08, 133.45, 132.65, 132.42, 129.71,
129.65, 129.52, 129.44, 123.81, 123.77, 122.84, 120.78,
120.33, 120.10, 120.03, 119.27, 118.00, 117.90, 96.03, 91.45,
88.90, 86.67. FTIR (neat, cm21) 3055, 2359, 2201, 1606.
MALDI-TOF MS calcd. C29H19NO 397.1467, found 397.4780.
Synthesis of PPE-AM
In a typical experiment, a mixture of compound AM (100 mg,
0.29 mmol), 1,4-dibutoxy-2,5-diethynylbenzene39 (78.4 mg, 0.29
mmol), PdCl2(PPh3)2 (20.4 mg, 0.03 mmol), CuI (11.0 mg, 0.06
mmol), PPh3 (15.2 mg, 0.06 mmol) were stirred in the mixture
solvent of THF (2 mL) and DBU (1 mL). The reaction was car-
ried out under pressure of N2 filled in rubber balloons and
stirred at room temperature for 24 h. Then the solution was
added with dichloromethane (10 mL) to dissolve part of the
jelly polymer for 15 min. The solution was concentrated under
reduced pressure to a small volume (ꢀ1 mL) and the polymer
was precipitated by dropping the concentrated solution into
150 mL of cold methanol. The precipitate that formed was col-
lected by centrifuge, washed repeatedly with methanol (5 3
30 mL) and evaporated under vacuum to afford greenish yel-
low powder of PPE-AM (yield: 85 mg, 82%) Mw 5 6.3 3 103
FIGURE 2 FTIR spectra of PPE-AM and PPE-IM. [Color figure
can be viewed at wileyonlinelibrary.com]
obtained from Varian Cary 50 UV–Vis spectrophotometer (Var-
ian, USA) and the fluorescence emission spectra were
recorded on a Varian Cary Eclipse spectrofluorometer (Varian,
USA) using a quartz cuvette with 1 cm beam path length at
room temperature with excitation and emission slit widths of
10 nm and 10 nm, respectively.
Synthesis of 2,4-Diiodoaniline (AM)
This compound was prepared according to a previous litera-
ture report.20 1H NMR (400 MHz, (CDCl3) d 7.89 (s, 1H),
7.37 (d, J 5 8.4 Hz, 1H), 6.52 (d, J 5 8.4 Hz, 1H), 4.13 (s, 2H).
1
Da, DP 5 17, PDI 5 1.5, H NMR (400 MHz, CDCl3) d 7.86, 7.57,
Synthesis of (E)-2-((2,4-Diiodophenylimino)
Methyl)Phenol (IM)
7.29, 7.00, 6.71, 4.76, 4.05, 1.85, 1.58, 1.03. 13C NMR (101
MHz, CDCl3) d 155.08, 153.29, 148.29, 142.02, 134.52, 133.12,
132.65, 117.90, 116.94, 116.72, 116.41, 115.78, 115.16, 113.86,
107.93, 92.19, 84.92, 84.15, 82.84, 69.47, 68.92, 31.44, 19.22,
13.90. FTIR (neat, cm21) 3470, 3362, 2958, 2866, 2357, 2190,
1612, 1506, 1211.
A mixture of AM (100 mg, 0.29 mmol), salicylaldehyde
(33.87 lL, 0.32 mmol) in EtOH (3 mL) was stirred at 80 8C
for 3 h. The precipitated bright yellow solid was washed
with cold EtOH (3 3 25 mL) to obtain desired product
(yield: 118.2 mg, 91%) 1H NMR (400 MHz, (CD3)2CO) d
12.69 (s, 1H), 8.86 (s, 1H), 8.33 (s, 1H), 7.87 (d, J 5 7.4 Hz,
1H), 7.63 (m, 1H), 7.48 (m, 1H), 7.31 (d, J 5 7.0 Hz, 1H),
7.00 (d, J 5 7.7 Hz, 2H). 13C NMR (101 MHz, (CD3)2CO) d
166.08, 162.15, 147.57, 139.91, 134.99, 134.37, 121.65,
120.37, 120.17, 120.13, 118.05, 117.95, 98.40, 92.25. MALDI-
TOF MS calcd. C13H9I2NO 448.877, found 450.042.
Synthesis of PPE-IM
Polymer PPE-AM (67 mg, 0.18 mmol based on repeating
unit) was dissolved in 2 mL of EtOH at 70 8C for 15 min.
Then salicylaldehyde (540 mg, 0.47 mL) was added and the
reaction was stirred for 24 h. The reaction mixture was
cooled to room temperature and concentrated to small vol-
ume by rotary evaporator before precipitation in cold
Synthesis of 2,4-Bis(Phenylethynyl)Aniline (PE-AM)
A typical procedure was followed according to the litera-
ture.38 A yellow powder of PE-AM was obtained. 1H NMR
(400 MHz, (CD3)2CO) d 7.62–7.57 (m, 2H), 7.49 (d, J 5 8.2
Hz, 3H), 7.43–7.33 (m, 6H), 7.28 (d, J 5 8.5 Hz, 1H), 6.83 (d,
J 5 8.5 Hz, 1H), 5.57 (s, 2H). FTIR (neat, cm21) 3465, 3373,
3029, 2362, 2202, 1612, 1500. MALDI-TOF MS calcd.
C22H15N 293.1204, found 292.2350.
Synthesis of (E)-2-((2,4-Bis(Phenylethynyl)
Phenylimino)Methyl)Phenol (PE-IM)
Compound PE-AM (94 mg, 0.32 mmol) was dissolved in
2.5 mL of EtOH and the solution was stirred at 50 8C for 15
min after the addition of salicylaldehyde (40.8 lL, 0.38
mmol). The reaction mixture was stirred at reflux for 2 h.
After filtration, the filter was collected and washed with cold
EtOH (5 3 10 mL). After drying under vacuum, an orange
FIGURE 3 Normalized spectra of absorption and emission of
PE-AM, PE-IM, PPE-AM, and PPE-IM. [Color figure can be
viewed at wileyonlinelibrary.com]
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polymeric material with only low degree of polymerization.
This is probably due to strong chelation of salicylaldimine to
Pd species.41,42 Fortunately, PPE-IM could be obtained via
post-polymerization functionalization (PPF) of PPE-AM via
the condensation with salicylaldehyde. Conversion of PPE-
AM into PPE-IM is clearly observed by FTIR (Fig. 2). The
NAH stretching of amine group in PPE-AM at 3470 and
3365 cm21 disappeared. Further confirmation was provided
by 1H NMR spectra which showed complete disappearance
of the amine protons (ANH2), at 4.76 ppm, and the aromatic
proton vicinal to the amine, at 6.98 ppm, that indicated an
efficient PPF [Supporting Information Fig. S1(A)]. Moreover,
the aromatic proton from both salicylaldehyde (d 5 8.12–
8.57 ppm) and aniline (d 5 8.79 ppm) was also observed.
Interestingly, two high chemical shifts at 12.83 and 13.44
ppm corresponding to the intramolecular hydrogen bonded
phenolic protons of PPE-IM in the possible head–head and
head–tail orientation [Supporting Information Fig. S1(B)].
The disappearance of both peaks upon addition of CD3OD
confirms that both peaks are exchangeable protons.43 We
also would like to note that unlike IM or PE-IM that gradu-
ally undergoes hydrolysis upon storage in CDCl3, PPE-IM is
stable for more than a week without detectable hydrolysis
products. From GPC measurement, the molecular weight
(Mw) of PPE-IM was 8455 Da corresponding to ꢁ100% con-
densation of salicylaldehyde on PPE-AM (Mw 5 6253 Da)
(Supporting Information Figs. S2 and S3). To the best of our
knowledge, this is the first report on post-polymerization
functionalization of PPE via imination that the methodology
is very effective and convenient.
FIGURE 4 (A) Time dependence of fluorescence intensity, at
kem, of PE-IM (25 lM) and PPE-IM (25 lM) in 10% MilliQ water/
THF (kex 5 340 nm). Photographic images under black-light of
(B) PE-IM and (C) PPE-IM at 0 and 120 min, respectively. [Color
figure can be viewed at wileyonlinelibrary.com]
methanol. After precipitated polymer was formed, there was
collected by centrifuge and washed repeatedly with cold
methanol (5 3 30 mL) to afford orange powder of PPE-IM
(yield: 75 mg, 85%). 1H NMR (400 MHz, (CDCl3) d 13.44,
12.83, 8.79, 8.57, 8.12, 7.77, 7.54, 7.43, 7.29, 6.98, 4.05, 1.83,
1.55, 1.02. 13C NMR (101 MHz, CDCl3) d 162.63, 161.49,
153.67, 153.32, 149.13, 148.59, 142.04, 136.79, 135.55, 133.41,
132.58, 132.43, 132.33, 119.15, 118.94, 117.38, 117.14, 96.05,
92.04, 91.22, 69.35, 68.88, 31.15, 19.11, 13.75, FTIR (neat,
cm21) 2955, 2929, 2866, 2193, 1609, 1497, 1205.
UV–Vis and Fluorescence Analysis
The stock solutions of all polymers (PPE-AM, PPE-IM) in
THF and small molecules (PE-AM, PE-IM) in acetone were
prepared at a concentration of 1.0 3 1023 M and diluted
with a THF-MilliQ mixture (9/1, v/v) as required before use.
The absorption spectra of all compounds were determined
from 250 nm to 600 nm. The emission spectra were
recorded from 350 nm to 700 nm using an excitation wave-
length at 300 to 410 nm at ambient temperature. The metal
ion stock solutions were prepared by dissolving the metal
salts in deionized water at a concentration of 10.0 3 1023
M and were diluted as required before use. A solution of the
test sample (1.0 mL, 10 lM for fluorescence and UV–vis)
was placed in a quartz cell (10.0 mm width), and the UV–vis
and fluorescence spectra were recorded respectively by mea-
suring the changes of intensity after mixing for 60 min.
The UV–vis absorption and fluorescence emission spectra of
fluorophore PE-AM, PE-IM, PPE-AM, and PPE-IM are shown
in Figure 3 and the photophysical properties are summarized
in Table S1. In comparison with the small molecule model
(PE-AM), the polymeric PPE-AM exhibited a significantly
longer absorption and emission wavelength that can be
RESULTS AND DISCUSSION
Synthesis of Imine Derivatives (PE-IM and PPE-IM)
The synthetic routes of PE-IM and PPE-IM are depicted in
Scheme 1. The model compound PE-IM was prepared in
high yield from diiodoaniline (AM) via two straightforward
steps. The Sonogashira coupling reaction between AM with
phenylacetylene gave PE-AM followed by the condensation
with salicylaldehyde to yield the desired PE-IM. For the syn-
thesis of PPE-IM, our initial attempt to condense AM with
salicylaldehyde led to a hydrolytically unstable imine, IM.40
Moreover, an attempt to polymerize IM into PPE-IM gave
FIGURE 5 (A) Fluorescent spectra (kex 5 410 nm) of PPE-IM (25
lM) upon the addition of various metal ions (5 equiv.) in 10%
MilliQ water/THF, (B) corresponding fluorescence intensity ratio
(I/I0) at kem 5 476 nm and (C) photographic images of corre-
sponding solution of PPE-IM in the presence of different cati-
ons. All data were recorded after 30 min of mixing. [Color
figure can be viewed at wileyonlinelibrary.com]
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attributed to the longer p conjugation system in our polymer
backbone. The imination of PE-AM to PE-IM and PPE-AM to
PPE-IM resulted in two emission bands (469 and 545). The
first band at 469 nm in PE-IM and PPE-IM corresponds to
the relaxation of the normal phenol tautomer while the sec-
ond band at 545 nm corresponds to the relaxation of the
keto tautomer formed by the excited induced proton transfer
process (ESIPT). The ESIPT is usually accompanied by a
large stroke shift values as large as 140–180 nm.44 The for-
mation of salicylaldimine Schiff base also significantly sup-
presses fluorescent quantum yields of PE-IM and PPE-IM
(Uf ꢂ 0.01) in comparison with the corresponding amines
(Uf > 0.20). The low fluorescent quantum yields are attrib-
uted to the combination of ESIPT and C@N isomerization
processes. Importantly, these results confirm the success of
our structural design in using salicylaldimine to suppress the
initial fluorescence signal of the turn-on fluorescent sensor.
Stability Test
For the stability test, fluorescence of PPE-IM and PE-IM were
monitored in 10% MilliQ water/THF for 2 h [Fig. 4(A)]. The
fluorescence intensity at 415 nm of PE-IM increased along with
bright blue appearance [Fig. 4(B)]. This result suggested that
the imine was hydrolyzed back to PE-AM confirmed by match-
ing of the emission spectrum with the original PE-AM [Support-
ing Information Fig. S4(A)]. However, no fluorescence change of
PPE-IM under the same condition and its appearance remained
dark under black-light [Fig. 4(C)]. Moreover, the fluorescence of
PPE-IM is stable under broad pH range of 4–8 (Supporting
Information Fig. S5). This result supports our hypothesis that
salicylaldimine Schiff base could be stabilized by incorporating
into the polymer chain.
Metal Cation Detection
The sensing ability of PPE-IM was screened with various metal
ions investigated in 10% MilliQ water/THF. The absorption
spectra of PPE-IM changed only slightly with the addition of
each metal ion (Supporting Information Fig. S6). In fluorescence
measurement, however, the addition of Fe31 (125 lM) caused
FIGURE 6 (A) and (B) Fluorescence titration (kex 5 410 nm) of
PPE-IM (25 lM) with Fe31 in 10% MilliQ water/THF. The inset
shows the calibration curves of ratio of PPE-IM to Fe31 concen-
tration. Each spectrum was obtained after 60 min of mixing.
(C) Time dependence of fluorescence intensity of PPE-IM (25
lM) in the presence and absence of Fe31 (5 equiv.) at pH 3.5.
(D) FL intensity of PPE-IM (25 lM) before (blue dots) and after
(red dots) addition of Fe(NO3)3 (125 lM) after 30 min of mixing
at various pH. [Color figure can be viewed at wileyonline-
library.com]
FIGURE 7 Relative fluorescence of PPE-IM (25 lM) in 10%
MilliQ water/THF in the presence of Fe31 (125 lM) plus another
metal ion (625 lM) tested for interference. Each spectrum was
obtained after 60 min of mixing. [Color figure can be viewed at
wileyonlinelibrary.com]
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was insignificantly changed. This result strongly supports
that the fluorescence enhancement of PPE-IM is caused by
Fe31 [Fig. 6(C)] induced hydrolysis. We also examined pH-
tolerance limit in PPE-IM sensing ability in the pH range
between 4 and 9. The fluorescent enhancement remained
the same as shown in Figure 6(D).
To evaluate the interference from other metal ions, competi-
tion experiments were carried out by addition of Fe31 and
another metal cation (5 equiv. of Fe31) tested for interfer-
ence to the PPE-IM solution (Fig. 7). In the presence of other
competing metal cations including Li1, Na1, K1, Ag1, Mg21
Ca21, Co21, Ni21, Cu21, Zn21, Cd21, Ba21, Pb21, Hg21, Fe21
,
,
Al31, and Cr31, no significant interference was observed. The
results indicate that PPE-IM is very selective for Fe31 detec-
tion and quantification.
FIGURE
8
1H NMR of PPE-IM in THF-d8 after addition of
Fe(NO3)3 (10 equiv.) at 30 min. [Color figure can be viewed at
wileyonlinelibrary.com]
To gain more mechanistic information of this turn-on phe-
nomenon, the 1H NMR were used to monitor the titration
between Fe(NO3)3 with PPE-IM in THF-d8 (Fig. 8). Upon
the addition of 10 equiv. Fe(NO3)3, imine proton signal (j)
at 9.01 ppm disappeared along with the increase in peak
at 10.02 ppm (j0) corresponding to the proton of aldehyde
group of salicylaldehyde. Furthermore, the proton signal
of aromatic region at 6.71 ppm (i) emerged, which is
match with otho-position of aniline proton in original
PPE-AM. In its absorption spectra, the new broaden band
at 380 nm is increased which can be assigned to the sali-
cylaldehyde residue (Supporting Information Fig. S10).
These evidences indicated that irreversible binding of
Fe31 ion with PPE-IM induced the reformation of PPE-
AM. This is caused by the cleavage of imine by Fe31-cata-
lyzed hydrolysis process.
a marked increase in fluorescence intensity at 476 nm by over
300-fold after 30 min (Fig. 5). The increase in water contents
in the system resulted in lower fluorescent sensing ability (Sup-
porting Information Fig. S7). This may be caused by the water
induced aggregation-caused quenching (ACQ)45,46 of both PPE-
AM and PPE-IM. Notably, the addition of Fe31 to PPE-AM gave
insignificant fluorescent change under the same condition
(Supporting Information Fig. S8). This result suggested the
hydrolysis of imine moiety of PPE-IM responds for increase in
fluorescent signal upon the addition of Fe31. Also, the PPE-
IM/Fe31 mixed solution showed bright blue luminescence
which remained unchanged for several hours [Fig. 5(C)]. For
other metal ions such as Li1, Na1, K1, Ag1, Mg21, Ca21
Co21, Ni21, Cu21, Zn21, Cd21, Ba21, Hg21, Pb21, Fe21, Al31
,
,
and Cr31, the fluorescence intensity of PPE-IM remained rela-
tively unchanged except slight increase with Al31. Therefore,
PPE-IM could serve as a stable and selective turn-on fluores-
cent probe for Fe31 sensing.
To investigate the effect of intramolecular hydrogen bonding
interaction on conformation change of our polymer, we fur-
ther measured the particle size of PPE-IM before and after
treatment with Fe(NO3)3 solution. The increase in particle
size was observed upon addition of Fe31 to the polymer
solution (Supporting Information Fig. S11). The result sug-
gested that the addition of Fe31 not only resulted in the
hydrolysis of PPE-IM to PPE-AM but also caused some
aggregation48 of the resulting PPE-AM (Fig. 9).
The time-dependence investigation showed that the fluores-
cence signal increased and became saturated after 60 min
after the addition of 250 lM Fe31 (Supporting Information
Fig. S9). The measurements of fluorescence intensity of all
subsequence experiments were thus performed after 60 min
of mixing to ensure the completion of the reaction and signal
stability. The fluorescence quantum efficiency (Uf) was
increased from <0.01 to 0.38. For quantitative analysis, the
titration of Fe31 at variable concentration was performed
and the fluorescence emission was measured at 60 min to
ensure the signal saturation [Fig. 6(A)]. A good linear fluo-
rescence response was obtained in the Fe31 concentration
range of 2–12 lM [Fig. 6(B)]. The detection limit at threefold
standard deviation (3r/K whereas r 5 standard deviation of
the fluorescence intensity of the PPE-IM solution in the
absence of Fe31; and K 5 slope of the calibration line)47 was
0.14 lM [Fig. 6(B inset)]. We would like to emphasize that
the acidity of Fe(NO)3 in aqueous solution is not responsible
for the hydrolysis of imine which is a probable cause of the
fluorescence enhancement. In control experiment at the
same pH of 3.5 but without Fe31, the fluorescence intensity
FIGURE 9 Schematic representation of the plausible behavior
of the PPE-IM before and after treatment with Fe31. [Color fig-
ure can be viewed at wileyonlinelibrary.com]
1160
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16 L.-J. Fan, Y. Zhang, C. B. Murphy, S. E. Angell, M. F. L. Parker,
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CONCLUSIONS
In conclusion, a new conjugated polymeric sensor based on
salicylaldimine-functionalized poly(m-phenyleneethynylene)s
was successfully synthesized in excellent yield via Sonoga-
shira coupling reaction and post-polymerization functionali-
zation. A well define Schiff base polymer provides high
hydrolytic stability in a wide pH range. This is caused by
strong intramolecular hydrogen bonding and hydrophobic
interaction among the side chains. The probe displays
remarkably sensitive and selective fluorescence turn-on
towards Fe31, through metal-promoted hydrolysis process,
with low detection limit of 0.14 lM and without significant
interference from other metal ions. Thus, PPE-IM developed
herein could be a promising material for Fe31 selective
detection. It can be useful in several biological and environ-
mental analysis as well as early rust detection.
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This study is financially supported by the Thailand Research
Fund (TRF-RSA6080018) and Nanotechnology Center (NANO-
TEC), NSTDA, Ministry of Science and Technology, Thailand,
through its International Research Integration: Chula Research
Scholar, program of Center of Excellence Network, Ratchadapi-
seksomphot Endowment Fund of Chulalongkorn University,
Center of Excellence on Petrochemicals and Materials Technol-
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