A helical chiral polymer-based chromo-fluorescence and CD response sensor for selective detection of trivalent cations

Article abstract of DOI:10.1002/pola.26813

A novel chiral (S)-BINAM-based fluorescent polymer sensor was designed and synthesized by the polymerization of 4,4′-((2,5-dibutoxy-1,4-phenylene) bis(ethyne-2,1-diyl))-dibenzaldehyde (M-1) with (S)-2,2′-binaphthyldiamine (S-BINAM, M-2) via Schiff's base formation. The resulting helical chiral polymer sensor exhibited remarkable turn-on bright blue fluorescence color upon the addition of trivalent metal ions under a commercially available UV lamp; this change can be clearly observed by the naked eye for direct visual discrimination at low concentration. More importantly, the addition of trivalent metal cations can lead to a most pronounced change of CD spectra of the chiral polymer indicating this kind chiral sensor can also be used as a sole probe for selective recognition of trivalent metal cations based on CD spectra.

Full text of DOI:10.1002/pola.26813

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A Helical Chiral Polymer-Based Chromo-Fluorescence and CD Response
Sensor for Selective Detection of Trivalent Cations
Lu Wang, Fei Li, Xunhua Liu, Guo Wei, Yixiang Cheng, Chengjian Zhu
Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Chemistry department,
Nanjing University, Nanjing 210093, China
Correspondence to: Y. Cheng (E-mail: yxcheng@nju.edu.cn) or C. Zhu (E-mail: cjzhu@nju.edu.cn)
Received 7 March 2013; accepted 1 June 2013; published online
DOI: 10.1002/pola.26813
ABSTRACT: A novel chiral (S)-BINAM-based fluorescent polymer
sensor was designed and synthesized by the polymerization of
4,40-((2,5-dibutoxy-1,4-phenylene)bis(ethyne-2,1-diyl))-dibenzalde-
hyde (M-1) with (S)-2,20-binaphthyldiamine (S-BINAM, M-2) via
Schiff’s base formation. The resulting helical chiral polymer sen-
sor exhibited remarkable “turn-on” bright blue fluorescence
color upon the addition of trivalent metal ions under a commer-
cially available UV lamp; this change can be clearly observed by
the naked eye for direct visual discrimination at low
concentration. More importantly, the addition of trivalent metal
cations can lead to a most pronounced change of CD spectra of
the chiral polymer indicating this kind chiral sensor can also be
used as a sole probe for selective recognition of trivalent metal
C
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cations based on CD spectra.
2013 Wiley Periodicals, Inc.
J. Polym. Sci., Part A: Polym. Chem. 2013, 00, 000–000
KEYWORDS: chiral; CD spectra; fluorescence; helical polymer;
helical chiral polymer; sensor; trivalent metal cations
focused on small molecules,5 few studies were devoted to the
development of conjugated polymers as fluorescence chemo-
sensors. Conjugated polymers as fluorescence chemosensors
have several advantages over small molecule sensors, such as
enhancements associated with electronic interaction between
receptors and analytes within the conjugated p-electronic
polymer backbone, processability, and ease of structural
modification. Furthermore, another advantage of conjugated
polymer-based sensor over organic small molecules is that the
fluorescence responsive signal amplification can be arisen from
facile energy migration along the delocalizable p-electronic con-
jugated polymer backbone upon light excitation.6 More impor-
tantly, there has been no report on CD response behavior of
chiral fluorescence sensor as a sole probe for selective recogni-
tion of trivalent metal cations from a variety of mono- and
divalent metal cations up to now. As part of our research on
chiral polymers fluorescence chemosensors in metal cations
recognition7; herein, we firstly described a novel chiral (S)-
BINAM-based polymer with helical configuration in the main
chain backbone, which exhibited remarkable “turn-on” fluores-
cence enhancement response toward trivalent metal cations.
The chiral polymer sensor solution can appear bright blue fluo-
rescence color upon the addition of M31 under a commercially
available UV lamp at low concentration, the color change can
be clearly observed by the naked eye. Moreover, trivalent metal
cations can lead to obvious change of CD spectra of the chiral
INTRODUCTION Detection and quantification of metal cations
especially trivalent metal cations (M31) at low concentrations
is an active area of research due to their well-known impact
on the environment and human health.1 Traditional methods
like atomic emission spectrometry, atomic absorption spec-
trometry, and mass spectrometry (MS) depend on expensive
instruments and need rigorous testing requirements, mean-
while sometime they cannot distinguish metals with different
valence states, such as Fe31 and Fe21, these disadvantages
limit their extensive application. Recently, due to the simplicity,
high sensitivity and tenability, fluorescence chemosensors have
been widely used as tools in chemistry, biology, medicine, and
material science.2 Fluorescent probes for detection of biologi-
cally relevant metal cations not only in vitro detection but also
can in vivo with imaging applications,3 which could not be
realized by traditional methods. The most general strategy for
the fluorescent sensor design is to combine fluorescent dye
molecules with the arranged receptors for specific analytes,
the recognition event between receptors and analytes can lead
to a fluorescent property change of the dye moiety.4 So accord-
ing to different requirements, the researchers can design and
synthesize variety of fluorescent sensors with diversified struc-
ture and apply them to multifarious system.
To the best of our knowledge, recently, most of the fluorescent
sensors specific for trivalent metal cations (M31) were mainly
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 Synthesis procedure of chiral polymer sensor.
ꢀ
polymer indicating this kind chiral polymer can be used as a
toluene. The solution was stirred at 80 C for 48 h. 20 mL of
methanol was added to precipitate the yellow solid polymer.
The resulting polymer was filtrated and washed with metha-
sole CD spectra sensor for selective recognition of M31
.
nol several times and dried in the yield of 85.0% (90.0 mg).
EXPERIMENTAL
25
GPC results: Mw 5 26,560; Mn 512,650; PDI 5 2.1 [a]D
5
1
Instrumentation and Materials
1299.0 (c 0.20, THF). H NMR (300 Hz, CDCl3) d 8.25 (s,
HC@N), 8.19–6.92 (m, ArH), 4.30–3.92 (t, CH2O), 1.92–
1.75(m, CH2), 1.67–1.50(m, CH2), 0.87–1.21 (m, CH3). FT-IR
(KBr, cm21): 2955, 2869, 1599, 1512, 1412, 1277, 813.
NMR spectra were obtained using a 300-Bruker spectrometer
1
300 MHz for H NMR and 75 MHz for 13C NMR and reported
as parts per million (ppm) from the internal standard TMS. FT-
IR spectra were taken on a Nexus 870 FT-IR spectrometer. Flu-
orescence spectra were obtained from a RF-5301PC spectrome-
ter. UV–vis spectra were obtained from a PerkineElmer Lambda
25 spectrometer. Specific rotation was determined with a Ruo-
lolph Research Analytical Autopol IV-T/V. Electrospray ioniza-
tion mass spectra were measured on a Thermo Finnigan LCQ
Fleet system and time-of-flight mass spectrometry (TOF-MS)
was determined on a Micromass GCT. C, H, N of elemental anal-
yses were performed on an Elemental Vario MICRO analyzer.
The gel permeation chromatography (GPC) analysis of the poly-
mers was conducted on a Shimadzu 10 Å with THF as the elu-
ent and polystyrene as the standard. The data were analyzed
by using the software package provided by Shimadzu Instru-
ments. Thermogravimetric analyses (TGA) were performed on
a Perkin-Elmer Pyris-1 instrument under N2 atmosphere. Circu-
lar dichroism (CD) spectra were carried out on a JACSCO J-810
CD Spectrophotometer. All solvents and reagents were commer-
cially available A.R. grade. THF and Et3N were purified by dis-
tillation from sodium in the presence of benzophenone.
RESULTS AND DISCUSSION
Synthesis and Structure Feature of Chiral Polymer
The synthetic routes of the polymer sensor are illustrated
in Scheme 1. The monomer M-1, 4,40-((2,5-dibutoxy-1,4-
phenylene)bis(ethyne-2,1-diyl))dibenzaldehyde was prepared
according to reported literatures,8 which reacted with the mono-
mer (S)-2,20-BINAM (M-2) afforded the corresponding polymer
as a yellow solid in 86.7% yield. Mw, Mn, and PDI of the chiral
polymer were determined by GPC using polystyrene standards
in THF, and the values of them are (26,560, 12,650, and 2.1),
respectively, indicating moderate molecular weight. The chiral
polymer is air stable solid and has good solubility in common
organic solvents, such as toluene, THF, CHCl3, and CH2Cl2, which
can be attributed to the nonplanarity of the twisted polymer
backbone and the flexible n-butoxy group substituents. In addi-
tion, the ethynyl linker can reduce steric hindrance between
phenyl groups and also have a beneficial influence on the stabil-
ity of the resulting chiral polymers. TGA result of the polymer
suggested that the chiral polymer has high thermal stability
The concentrations of the stock solution of metal salts
(nitrate) are 1 3 104 lM in H2O. Initially, each experiment
was started with a 3.0 mL polymer with a known concentra-
tion (10 lM). The adding amount of each experiment exam-
ple of metal salt is shown on the figure interpretation.
ꢀ
without loss weight before 440 C (Fig. 1). Therefore, the result-
ing chiral polymer can provide a desirable thermal property for
practical application as a stable fluorescence sensor material.
The Selective Fluorescence Recognition of the Chiral
Polymer Sensor on Metal Ions
The fluorescence spectra of the chiral polymer sensor were
carried out in a solution of THF. The concentration of the
Synthesis of the Chiral Polymer
A mixture of M-18 (100.0 mg, 0.19 mmol) and (S)-2,20-
BINAM (M-2, 21.0 mg, 0.19 mmol) was dissolved in 2 mL of
2
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the M31-containing polymer solution, respectively. This kind
chiral polymer in THF solution can exhibit bright blue “turn-
on” fluorescence change in the presence of Al31, Cr31, or
Fe31 under the excitation of 375 nm, and visual sensing
response could be clearly observed by the naked eye (Fig. 2,
inset). Based on the pronounced fluorescence difference, our
polymer can be used as an effective chromo-fluorescence
sensor for direct discrimination of trivalent metal cations
(M31, M 5 Al, Cr, Fe) over a variety of monovalent and diva-
lent metal cations at low concentration.
To more announcing the detail of the M31 on the fluorescent
behavior of our chiral polymer, Fe31 was chosen as the rep-
resentative and fluorescence responses of the polymer with
different concentration of Fe31 was showed in Figure 3.
Before the addition of Fe31, the color of the solution of the
free polymer itself emits weak blue. When Fe31 is added
into the solution, the color turns to light blue. The color
began to be obviously observed at [Fe31] of 20 lM and it
becomes brilliant blue when [Fe31] is higher than 30 lM.
FIGURE 1 TGA curves of the chiral polymer.
chiral polymer was 10 lM corresponding to 1,10-binaphthyl
moiety. The polymer sensor shows weak fluorescence situ-
ated at 457 nm with fluorescence quantum yield of 0.02.
The fluorescence response behaviors of the polymer sensor
on various metal ions have been investigated by fluorescence
spectra with the excitation at 375 nm. As evident from Fig-
ure 2, it was found that the fluorescence strength of the
polymer sensor has almost no change upon the addition of
CD and UV–vis Response Behaviors of the Chiral
Polymer toward Trivalent Metal Cations
The chirality of 2,20-binaphthyldiamine is derived from the
restricted rotation of the two naphthalene rings. The rigid
structure and C2 symmetry of the chiral binaphthyl mole-
cules can play an important role in inherently chiral induc-
tion.9 The dihedral angle between two naphthalene rings of
a binaphthyl molecule ranges from 60 to 120ꢀ, which leads
to the kinked or twisted polymer main chain backbone.10
Herein our synthesized (S)-BINAM-based chiral polymer can
adopt a stable helical conformation since the same configura-
tion of chiral (S)-binaphthyl unit is connected through rigid
group and a highly ordered structure of the main chain back-
bone are twisted to one-direction by the chiral binaphthyl
metal ions, such as Ag1, Ba21, Ca21, Cd21, Co21, Cu21, Fe21
,
Hg21, K1, Mg21, Mn21, Na1, Ni21, Pb21, Zn21, and the fluo-
rescent emission wavelengths of the metal-polymer com-
plexes do not show an obvious difference from the metal-
free polymer. Interestingly, upon the addition of trivalent cat-
ions, such as Al31, Cr31, and Fe31 (30 lM), the fluorescence
intensities of the mixtures show gradual enhancement as
high as 63-fold, 57-fold, and 70-fold, respectively (Fig. 2).
Quantum yields of 0.32, 0.24, and 0.57 were determined for
FIGURE 2 Fluorescence responses of the chiral polymer (10 lM in THF, kex 5 375 nm) to various metal ions (30 lM). [Inset: the
photograph of the fluorescence color of polymer sensor to various metal ions (30 lM)].
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FIGURE 3 Fluorescence spectra of the chiral polymer (10 lM in THF, kex 5 375 nm) with increasing amounts of Fe31 from 3 to 30
lM. [Inset: the photo of the fluorescence color of polymer sensor to different concentration of Fe31 (lM)].
linker.11 The helical conformation can be also confirmed by
the opposite specific rotations of (S)-BINAM (2157ꢀ) and
(S)-BINAM-based polymer (1299.0ꢀ).12 In our previous
work, we have reported this kind of chiral BINAM-based hel-
ical polymer can exhibit as an excellent fluorescence sensor
for enantioselective recognition of phenylalaninol enantio-
mers9(a) and N-Boc-protected alanine.13
changes as Fe31. On the contrary, CD spectrum of the chiral
polymer has almost no response for Fe21, as shown in Figure
4(b), Supporting Information Figure S5. Other monovalent or
divalent cations, such as Ag1, Ba21, Ca21, Cd21, Co21, Cu21
,
Hg21, K1, Mg21, Mn21, Na1, Ni21, Pb21, Zn21, showed similar
results as Fe21 [Fig. 4(b)]. The obvious highly selective CD
effect changes suggest that only trivalent metal cations M31
can produce an induced circular dichroism, which indicated
that the chiral BINAM-based polymer can be used as a sole
probe for selective recognition of trivalent metal cations
(Al31, Cr31, and Fe31) over monovalent and divalent metal
cations according to CD spectra.
Here, the CD spectra of the chiral polymer sensor in the
absence/ presence of metal ion have been measured in THF
[Fig. 4(a,b), Supporting Information Fig. S3–S5]. Before addi-
tion of metal ion, it can be found that the free polymer has
the band centered at 326 nm which is arisen from chiral
binaphthyl-imine base moiety, and another strong negative
Cotton effect at wavelengths of 375 nm, which can be
regarded as the extended conjugated structure in the repeat-
ing unit and a high rigidity of polymer backbone.14 In addi-
tion, it can also be clearly observed that the chiral polymer
shows a strongest positive Cotton effect at long wavelength of
425 nm, which is attributed to the significant chiral amplifica-
tion arisen from the helical conformation backbone.15 After
the addition of trivalent metal cations, such as Fe31, we found
that the negative Cotton effect of the chiral polymer sensor at
326 nm gradually turns the positive Cotton effect with 20 nm
blue shift; moreover, the negative Cotton effect at 375 nm and
the positive Cotton effects of long wavelength at 425 nm
show gradual reduction to complete disappearance as the
increase concentration of Fe31 from 5 to 35 lM, which can
be attributed to the break of the rigid chain backbone and
helical conformation of the chiral conjugated polymer struc-
ture. As shown in Supporting Information, other trivalent
metal cations, Al31 and Cr31 showed similar response
In this article, the UV–vis responses of the chiral polymer
sensor on metal cations were also carried out in THF solu-
tion. As a result, the intensity of UV–vis absorption maxima
of the chiral polymer shows the gradual reduction, and the
absorption wavelength arising from the conjugated structure
of the polymer main-chain backbone appears the gradual
blue shift from 399 to 390 nm upon the additions of
Fe31concentration from 10 to 40 lM [Fig. 4(c)]. In contrast,
UV–vis spectra of the polymer appear little change upon the
additions of various molar ratio of Fe21 and other metal cati-
ons [Fig. 4(d) and Supporting Information Fig. S9].
Then we turned our attention to why our chiral polymer with
binaphthyl-imine base moiety show selective recognition of
M31 rather than the metal cations like Zn21 and Cu21, which
have strong affinity to imine base moiety. As well known, the
trivalent metal cations M31 (Al31, Cr31, and Fe31) intensively
hydrolyzed in water and made aqueous solution acidic with
different extent. Thus the addition of M31 aqueous solution
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FIGURE 4 (a,b) CD spectra of the chiral polymer (10 lM in THF) with the increasing amounts of Fe31 or Fe21. (c,d) UV–vis spectra
of the chiral polymer (10 lM in THF) with the increasing amounts of Fe31 or Fe21
.
into our polymer THF solution is equivalent to the addition of
acid, and the H1 may be the key role leading to the enhance-
ment of the fluorescence and change of CD spectra. Mean-
while, the order of Ksp for Fe(OH)3, Al(OH)3, and Cr(OH)3 is
completely consistent with the order of I457 shown in Figure
2. So we further studied with the fluorescence response of
the polymer towards trifluoroacetic acid (CF3COOH). The
result shows that the fluorescence grows up as the addition
of CF3COOH, which coincide with our hypothesis (Fig. 5).
Tang and coworkers just reported a similar result using Py-
TPE as a fluorescent chemosensor for selective recognition of
trivalent metal cations based on the M31 hydrolyzed mecha-
nism.5(j) After the addition of CF3COOH, the CD spectra of the
chiral polymer show the similar tendency as M31 (Supporting
Information Fig. S6).
can be clearly observed by the naked eyes for direct visual
discrimination at low concentration. More importantly, the
addition of trivalent metal cations lead to the most pro-
nounced change of CD spectra indicating this kind chiral
polymer can be used as a sole probe for selective recognition
CONCLUSIONS
In summary, we have developed a novel BINAM-based helical
chiral polymer, which exhibited remarkable “turn-on” fluo-
rescence enhancement response toward trivalent metal cati-
ons (M31, M 5 Al, Cr, Fe) over monovalent and divalent
metal cations. The solution of the polymer turns on bright
blue fluorescence color upon the addition of trivalent metal
cations under a commercially available UV lamp, this change
FIGURE 5 Fluorescence spectra of the chiral polymer (10 lM in
THF, kex 5 375 nm) with increasing amounts of trifluoroacetic
acid from 5 to 30 lM.
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ACKNOWLEDGMENTS
The authors gratefully acknowledge the National Natural Sci-
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21172106) and National Basic Research Program of China
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