Colorimetric naked-eye recognizable anion sensors synthesized via RAFT polymerization

Article abstract of DOI:10.1002/pola.23922

A polymeric sensor (PTH) containing naphthalimide signal moiety and thiourea recognition moiety for the detection of anions was synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization, which can guarantee controllable molecular weight, narrow molecular weight distribution, and precise polymer structure. Both PTH and its corresponding monomer (TH) showed naked-eye recognizable yellow-to-orange changes upon addition of fluoride (F^-, acetate (AcO^-, and dihydrogen phosphate (H₂PO₄^- of low concentration. However, only F^- can result in orange-to-purple change when the aforementioned anions were added at high concentrations, which was attributed to the deprotonation of the thiourea NAH groups and the mechanism was supported by the UV-Vis absorption spectra and ¹H NMR titration. The effect of these anions on thin PTH films was also investigated, and the addition of F ^- led to obvious spectra change. It was found that other halide anions (Cl^-, Br^-, and I^-) could hardly induce any variation of absorption spectra.

Full text of DOI:10.1002/pola.23922

Colorimetric Naked-Eye Recognizable Anion Sensors Synthesized via
RAFT Polymerization
JIANBING JIANG, XIAO XIAO, PING ZHAO, HE TIAN
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology,
Shanghai 200237, People’s Republic of China
Received 30 September 2009; accepted 4 January 2010
DOI: 10.1002/pola.23922
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: A polymeric sensor (PTH) containing naphthalimide
signal moiety and thiourea recognition moiety for the detection
of anions was synthesized by reversible addition-fragmentation
chain transfer (RAFT) polymerization, which can guarantee con-
trollable molecular weight, narrow molecular weight distribu-
tion, and precise polymer structure. Both PTH and its
corresponding monomer (TH) showed naked-eye recognizable
yellow-to-orange changes upon addition of fluoride (Fꢀ), acetate
(AcOꢀ), and dihydrogen phosphate (H2POꢀ4 ) of low concentra-
tion. However, only Fꢀ can result in orange-to-purple change
when the aforementioned anions were added at high concentra-
tions, which was attributed to the deprotonation of the thiourea
NAH groups and the mechanism was supported by the UV-Vis
absorption spectra and 1H NMR titration. The effect of these
anions on thin PTH films was also investigated, and the addition
of Fꢀ led to obvious spectra change. It was found that other ha-
lide anions (Clꢀ, Brꢀ, and Iꢀ) could hardly induce any variation
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of absorption spectra. 2010 Wiley Periodicals, Inc. J Polym Sci
Part A: Polym Chem 48: 1551–1556, 2010
KEYWORDS: anion; colorimetric; fluorescence; living radical
polymerization (LRP); RAFT polymerization; sensors
INTRODUCTION It is widely known that some biologically
important anions, such as fluoride (Fꢀ), acetate (AcOꢀ), and
dihydrogen phosphate (H2PO4ꢀ), have attracted great atten-
tion in the recent decades, because of their importance on
human health, environmental protection,1,2 and other
fields.3,4 Accordingly, quite a few chemosensors, monomeric
and polymeric, have been successfully created, developed,
and studied these years.5–7 Most of the interactions between
analytes and sensors are achieved by noncovalent binding.8
Thiourea moiety is widely used on chemosensors because
there are two NAH groups serving as recognition sites.9,10
However, most thiourea sensors are small molecules,11,12
polymer sensors with thiourea moiety are rarely reported.
What is more, since most of the polymers were synthesized
by traditional polymerization, the degree of polymerization
and polydispersity of previously reported anion polymer sen-
sors can hardly be controlled.13–15
also keep the polymer active, which can guarantee further
modification to improve polymer’s properties, such as solu-
bility, functionality, and so on.22–24 It was reported that for
some polymeric chemosensors, the degree of polymerization
of the polymer has effect of on the sensing ability. Therefore
we used RAFT method to control the degree of polymeriza-
tion herein. The RAFT method on fluorescent sensors synthe-
sis was reported in our recent research.25
During this research, different anions, such as Fꢀ, AcOꢀ, and
H2POꢀ4 , were used as analytes. A yellow-to-orange change hap-
pened upon the addition of these anions, which was due to
the intramolecular charge transfer (ICT) occurring on the NAH
site near the 4-amino-1,8-naphthalimide.26–28 Moreover, an or-
ange-to-purple change was observed at high Fꢀ concentrations,
which was due to the sufficient basicity of Fꢀ to deprotonate
the naphthalimide NAH. Similar deprotonation effect was also
reported by Gunnlaugsson and coworkers29,30 and Fabbrizzi
and coworkers.31,32 To confirm the above proposed mecha-
nism, UV-Vis absorption spectra and the 1H NMR titration
were conducted. In addition, the sensing properties of PTH
thin film on the aforementioned anions were also investigated.
Detailed discuss was processed in the following text.
To avoid or to reduce the aforementioned shortcomings,
monomer TH with thiourea moiety and the corresponding
polymer PTH (Scheme 1) were prepared. As an innovative
on colorimetric sensors, PTH was synthesized by reversible
addition-fragmentation chain transfer polymerization (RAFT),
which was one of the most promising controlled/living radi-
cal polymerization methods that can control the molecular
weight and distribution with the usage of chain transfer
agent, such as esters disulfide compounds.16–21 The use of
RAFT can not only control the degree of polymerization, but
EXPERIMENTAL
Characterization
1H NMR and 13C NMR measurements were conducted on a
Bruker AV-500 spectrometer with deuterated dimethyl
Additional Supporting Information may be found in the online version of this article. Correspondence to: P. Zhao (E-mail: pzhao@ecust.edu.cn)
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
7.64 Hz,1H), 8.06 (d, J ¼ 7.37 Hz, 2H), 8.27 (d, J ¼ 7.89 Hz,
1H), 8.49 (t, J ¼ 8.37 Hz 1H), 8.56 (dd, 2H), 11.96 (s, 1H),
12.95 (s, 1H). 13C NMR (125 MHz, DMSO-d6, ppm): d 17.9,
61.5, 120.4, 122.4, 125.1, 125.8, 127.0, 127.6, 128.2, 128.4,
128.8, 129.2, 130.7, 131.1, 132.0, 133.3, 135.7, 140.7, 163.0,
163.4, 166.5, 168.4, 181.4. MS (m/z): calcd for C26H21N3O5S,
487.1; found, 510.1 [MþNa]þ.
Polymerization of TH
TH (249.6 mg, 0.51 mmol), AIBN (0.3 mg, 0.0025 mmol),
CDB (1.36 mg, 0.005 mmol), and distilled NMP (3.0 mL)
were placed in a dry Schlenk tube. The tube was sealed and
immerged in liquid nitrogen for 5 min to make the solvents
frozen, evacuated to eliminate the small amount of residual
oxygen, and then recharged with argon. Repeat this proce-
dure three times to remove the oxygen thoroughly. Then, the
Schlenk tube was placed in a constant-temperature oil bath
of 70–75 ꢁC. The polymerization was terminated after 38 h
by immersion of the reactor in liquid nitrogen. The polymer
(PTH) was purified by precipitating twice from THF solution
to diethyl ester, and once from NMP solution to a mixture of
diethyl ether/methanol (1:1, v/v). Yellow polymer was
obtained after filtration and then dried at 50 ꢁC under vac-
uum overnight.
SCHEME 1 Synthetic scheme of TH and PTH.
sulfoxide (DMSO-d6) as solvent using tetramethylsilane
(TMS) as the internal standard. Mass spectra (MS) measure-
ments were carried out on ESI mass spectroscopy. The num-
ber-average molecular weight (Mn) and polydispersity indi-
ces (PDIs) of the polymers were determined with a Waters
1515 gel permeation chromatograph (GPC) equipped with a
refractive index detector and calibrated with polystyrene
(PSt) standard samples. Dimethylformamide (DMF) was used
ꢁ
as the eluent with a flow rate of 1.00 mL/min at 30 C. UV-
Vis absorption spectra were performed on a Varian Cray 500
spectrophotometer.
RESULTS AND DISCUSSION
RAFT Polymerization
The polymer PTH was prepared via RAFT polymerization
using CDB (Scheme 1) as chain transfer agent (CTA), AIBN
as initiator, and NMP as solvent at about 70 ꢁC. The main
advantage of RAFT polymerization was the controllability of
molecular weight and low polydispersity index.19,34–37 The
number average molecular weight of the resulted polymer is
49,700 g/mol, with narrow polydispersity of 1.17, deter-
mined by GPC. This GPC result reveals that the RAFT poly-
merization is successfully realized in this system.
Materials
All anions, in the form of tetrabutylammonium salts, were
purchased from J&K Chemical Company, stored in a desicca-
tor under vacuum containing self-indicating silica (desic-
cant), and used without any further purification. 1-Methyl-2-
pyrrolidinone (NMP) was used immediately after it was dis-
tilled under vacuum. Dimethyl sulfoxide (DMSO) was dis-
tilled over calcium hydride. Azobisisobutyronitrile (AIBN,
97%) was recrystallized from ethanol twice, dried at room
temperature under vacuum, and stored at ꢀ10 ꢁC in the
fridge. Compound 1 (Scheme 1) was obtained according to
our previous research.25 Cumyl dithiobenzoate (CDB) was
synthesized according to the method described previously,33
with purity above 99%. All other reagents were of analytical
purity and used without further treatment.
Colorimetric Properties
The interactions of TꢀH and PTH with anions (Fꢀ, AcOꢀ,
H2PO4ꢀ, Clꢀ, Brꢀ and I ) were investigated in DMSO at 25 C.
ꢁ
The concentration of TH was 10ꢀ5 mol/L and the naphthali-
mide segment of PTH was equivalent to 10ꢀ5 mol/L, too.
Standard solutions of tetrabutylammonium anions (5 ꢂ 10ꢀ3
and 0.1 mol/L) were added stepꢀwise to these solutions. On
addition of Fꢀ, AcOꢀ, and H2PO4 at the concentration of 2
mmol/L (modulated by the standard anion solutions), the light
yellow solutions of TH and PTH turned to orange, as shown
in the graphic abstract and Figure S2, which were very
obvious and naked-eye recognizable. While continuing modu-
lating the solution to higher concentrations of 6 mmol/L, onlꢀy
Fꢀ can lead to the orange-to-purple change, AcOꢀ and H2PO4
can hardly result in any further color change. This can be
explained by the deprotonation of the thiourea NAH by the
Fꢀ with sufficient basicity and can be supported by the UV-Vis
Preparation of TH
Benzoyl isothiocyanate (2.77 g, 17.00 mmol) was added to
the acetone solution (160 mL) of compound 1 (1.74 g, 5.37
mmol). After stirring for 4 h at 50 ꢁC, some yellow suspen-
sion appeared. Remove about half of the acetone under vac-
uum, add ethanol (80 mL) to the yellow residue, and then
filter to get the rude yellow solid product. Monomer TH was
obtained as a red brownish product after purification by col-
umn chromatography on silica gel with dichloromethane-
ethyl acetate (9:1, v/v) as the eluent.
1
absorption spectra and H NMR titration.
Yield: 39.40% (1.03 g). 1H NMR (500 MHz, DMSO-d6, ppm):
d 1.78 (s, 3H), 4.42 (dd, 4H), 5.60 (s, 1H), 5.94 (s, 1H), 7.59
(t, J ¼ 7.81 Hz, 2H), 7.70 (t, J ¼ 7.41 Hz, 1H), 7.96 (t, J ¼
The effect of the anions on the UV-Vis absorption spectra
was examined by adding a few lmol of the anions stock to a
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FIGURE 1 Absorption spectra of PTH in DMSO on addition of
AcOꢀ of various concentrations (0–8.0 mmol/L).
FIGURE 3 Absorption spectra of TH in DMSO on addition of Fꢀ
of various concentrations (0–8.0 mmol/L).
387 and 493 nm. As for PTH solution, the spectra were
somewhat different after the addition of equal quantities of
Fꢀ, mainly from 4.0–8.0 mmol/L. The appearance of band at
513 nm was not as obvious as that of the TH solution, which
was probably due to the mutual influence of the sensing
moiety on the polymer bone. These results were in agree-
ment with the naked-eye recognizable phenomena, with the
assumption of deprotonation of NAH and the formation of
HF2ꢀ. To confirm this assumption, 1H NMR titrations of TH
known volume of the sensor solution (3 mL). As can be seen
from the spectra, upon the addition of AcOꢀ (Fig. 1) and
H2PO4 (Fig. S3) to the PTH solution, both from 0–8.0
ꢀ
mmol/L, there was slight change centered at 450 nm, this
was due to the ICT mechanism upon recognition at the re-
ceptor site. Similar behavior was aꢀlso observed after the
addition of AcOꢀ (Fig. 2) and H2PO4 (Fig. S4) into the TH
solution. In the case of Fꢀ for both TH and PTH, as shown in
Figures 3 and 4 at lower concentrations, mainly from 0–4.0
mmol/L, the absorption spectra were as similar as those
under the addition of AcOꢀ and H2PO4ꢀ. With respect to Clꢀ,
Brꢀ, and Iꢀ, there was hardly any change as displayed in Fig-
ures S5–S10. However, different behavior happened after the
addition of Fꢀ at higher concentrations to the TH solution,
mainly from 4.0–8.0 mmol/L, the band at 450 nm decreases
progressively, while new bands with two peaks at 342 nm
and 560 nm form and develop, with two isosbestic points at
were carried out upon addition of Fꢀ, AcOꢀ, and H2PO4
ꢀ
(Figs. 5–7).
The binding of TH to Fꢀ, AcOꢀ, and H2PO4 was investigated
by monitoring the changes in the 1H NMR spectra with
DMSO-d6 as solvent. Upon addition of different concentra-
tions of Fꢀ, the two thiourea NAH signals became obviously
broadened after the addition of only small quantities of 0.05
eq. (Fig. 5) and nearly disappeared after the addition of 1.0
ꢀ
FIGURE 2 Absorption spectra of TH in DMSO on addition of
AcOꢀ of various concentrations (0–8.0 mmol/L).
FIGURE 4 Absorption spectra of PTH in DMSO on addition of
Fꢀ of various concentrations (0–8.0 mmol/L).
COLORIMETRIC NAKED-EYE RECOGNIZABLE ANION SENSORS, JIANG ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 7 Plot of 1H NMR spectra in DMSO-d6 after the addi-
ꢀ
tion of various quantities of H2PO4 (0–3 eq.).
changes in the chemical shifts of thiourea NAH, but no
bifluofide peak wasꢀobserved. This indicated that the basicity
of AcOꢀ and H2PO4 were not strong enough to deprotonate
the thiourea NAH, which was consistent with the color
change and the UV-Vis absorption spectra.
FIGURE 5 Plot of 1H NMR spectra in DMSO-d6 after the addi-
tion of various quantities of Fꢀ (0–5 eq.). ~ indicates the
The effect of Fꢀ, AcOꢀ, and H2PO4 on absorption spectra of
ꢀ
ꢀ
appearance of HF2
.
thin polymer films was also investigated. The polymer films
were immersed in anion solution of 5 ꢂ 10ꢀ3 mol/L, and af-
ter drying for 20 min the absorption spectra were moni-
tored. The band at 408 nm decreased dramatically for Fꢀ,
while slightly for AcOꢀ and H2PO4ꢀ, as can be seen in Fig-
ures 8, 9, and S11. On the basis of the present investigation,
it can be assumed that the polymer sensors can be made
and applied as convenient detecting apparatus for environ-
mental researchers for selective detection of biologically im-
portant anions, such as Fꢀ.
eq. Furthermore, after the addition of 4.0–5.0 eq. of Fꢀ, a
new triplet appeared at about 16.1 ppm, which was assigned
to the formation of bifluoride [FHF]ꢀ anion.24 Both the strik-
ing color change and the detection of bifluoride can well
support the deprotonation of thiourea NAH.
On the other hand, successive addition of AcOꢀ (0–5 eq.; Fig.
ꢀ
6) and H2PO4 (0–3 eq.; Fig. 7) resulted in significant
FIGURE 6 Plot of 1H NMR spectra in DMSO-d6 after the addi-
tion of various quantities of AcOꢀ (0–5 eq.).
FIGURE 8 Absorbance spectra of PTH processed on polymer
films after the addition of Fꢀ.
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A new polymeric colorimetric sensor (PTH) for biologically
important anions (Fꢀ, AcOꢀ, and H2PO4ꢀ) was synthesized
by RAFT polymerization. The polymer and the corresponding
monomer showed yellow-to-orange change at low anion con-
centrations and only Fꢀ resulted in further orange-to-purple
change at higher anion concentrations, which was attributed
to the protonation of the thiourea NAH. UV-Vis spectra and
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mechanism. Thin PTH films were also capable of sensing Fꢀ,
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