Polymeric temperature and pH fluorescent sensor synthesized by reversible addition-fragmentation chain transfer polymerization

Article abstract of DOI:10.1002/pola.25995

Simple-structured copolymer, poly(NIPMAM-co-CPMA), consisting of N-isopropylmethacrylamide (NIPMAM) and (Z)-4-(1-cyano-2-(4-(dimethylamino) phenyl)vinyl)phenylmethylacrylate (CPMA) units as thermo- and pH-responsive fluorescent signaling parts, respective

Full text of DOI:10.1002/pola.25995

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
Polymeric Temperature and pH Fluorescent Sensor Synthesized by
Reversible Addition–Fragmentation Chain Transfer Polymerization
Guofeng Liu, Wei Zhou, Jiaqi Zhang, Ping Zhao
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology,
Shanghai 200237, People’s Republic of China
Correspondence to: P. Zhao (E-mail: pzhao@ecust.edu.cn)
Received 13 September 2011; accepted 7 January 2012; published online 23 February 2012
DOI: 10.1002/pola.25995
ABSTRACT: Simple-structured copolymer, poly(NIPMAM-co-
CPMA), consisting of N-isopropylmethacrylamide (NIPMAM)
and (Z)-4-(1-cyano-2-(4-(dimethylamino) phenyl)vinyl)phenyl-
methylacrylate (CPMA) units as thermo- and pH-responsive
fluorescent signaling parts, respectively, has been synthe-
sized by reversible addition–fragmentation chain transfer po-
suppression of copolymer (PCN250) was observed with
high proton concentration. Moreover, the lower critical solu-
tion temperature of the copolymers, poly-(NIPMAM-co-
CPMA), with different component was also investigated.
C
V
2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym
Chem 50: 2219–2226, 2012
lymerization. The copolymer PCN250 (m/n
¼
250) shows
absorbance enhancement or decrease at different pH value.
However, the fluorescence intensity of this copolymer shows
KEYWORDS: living radical polymerization (LRP); N-isopropylme-
thacrylamide; pH; reversible addition–fragmentation chain
transfer polymerization; sensors; stimuli-sensitive polymers;
temperature
enhancement with
a rise in temperature regardless of pH
value in the range of pH ¼ 4–10. In addition, fluorescence
INTRODUCTION Among manifold stimuli-responsive poly-
meric materials, polymeric pH sensors have received increas-
ing attention in recent years.1–7 pH is an important factor
that is closely related to physiological activity, so quantita-
tively measuring pH is useful for cellular analysis or diagno-
sis. However, most proton sensors are limited to the narrow
pH response, ranging mainly 3–12.2,3,7 Sensors with high
proton concentrations are rarely studied,4 which restrict the
further application in some special fields, such as water pol-
lution evaluation, sewage treatment, and so on.
A current trend of optical sensor is the development of dual
sensors that respond simultaneously and independently to dif-
ferent stimuli, such as pressure and temperature,24 oxygen and
temperature,25,26 oxygen and carbon dioxide,27,28 oxygen and
pH value29,30 as well as temperature and pH value.13,31 We
aimed to prepare a dual fluorescent sensor that respond to
both pH and temperature, which would be beneficial, for exam-
ple to monitor chemical reactions and for biological diagnostics.
As the solubility transition of the polymer is dependent on
the corresponding molar mass distribution, a well-defined
polymer is required. While most of the previous polymer
sensors were synthesized by ordinary radical polymerization,
which was difficult to control the molecular weight, polydis-
persity, and molecular topology, and consequently negatively
affected the sensing capacity.32 Therefore, a controlled radi-
cal polymerization, reversible addition–fragmentation chain
transfer (RAFT) polymerization, was applied to prepare the
copolymer poly(NIPMAM-co-CPMA) (Scheme 1) to avoid or
reduce the aforementioned shortcomings. RAFT was one of
the most promising controlled/living radical polymerization
methods that can control the polymer molecular weight and
its distribution with the usage of chain transfer agent,33,34
such as esters disulfide compounds. This attractive method
for the synthesis of fluorescent polymers with desirable po-
lymerization degrees and low polydispersity was first used
Other stimuli-responsive polymeric materials, such as poly-
meric thermometers, also have attracted increasing research
interest.8–21 These fluorescent thermometers allow ‘‘remote’’
sensing of medium temperature, where no electric wires are
required to connect the thermometers. These thermometers
therefore have advantages over traditional thermometers in
applications where electromagnetic noise is strong, sparks
could be hazardous, and the environment is corrosive. Poly(N-
isopropylmethacrylamide) (PNIPMAM) materials have been
considered as a well-known member of molecular thermome-
ters in several years.14,22,23 Around a specified temperature,
commonly referred to as a lower critical solution temperature
(LCST), PNIPMAM in aqueous solutions undergo a coil-to-
globule phase transition due to the destruction of intermolec-
ular hydrogen bonding between polymer chains and water.
Additional Supporting Information may be found in the online version of this article.
C
V
2012 Wiley Periodicals, Inc.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 2219–2226
2219

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
Fast-atom-bombardment mass spectrometer (FAB-MS) spectra
were obtained by a JEOL JMS 700 mass spectrometer. The pH
value was checked by pHS-25 with a range from pH 0 to 14.
The number-average molecular weights (Mn) and polydisper-
sity (Mw/Mn) of the copolymers were measured by gel permea-
tion chromatography (GPC) with RI (Infrared spectrometer)
detector at 30 ꢀC using THF as eluent and standard polysty-
rene as the reference (Waters 1515/Wyatt Technology Corp.).
Synthesis of CPA
A stirred mixture of 2-(4-hydroxyphenyl) acetonitrile (13.3 g, 0.1
mol) and acetic anhydride (30 mL) dissolved into pyridine (25
ꢀ
mL) was heated to 120 C for 4 h, after concentrated in vacuo,
cooled and poured into ice water (350 mL) slowly with stirring.
Sodium carbonate was added to neutralize the solution and it
was extracted twice with ethyl acetate (200 mL). After concen-
trated in vacuo, petroleum ether (150 mL) was added and a great
deal of white solid 4-(cyanomethyl)phenyl acetate (CPA) (17.2 g,
98.5%) was crystallized from the solution. m.p. 42–43 ꢀC.
1H NMR (400 MHz, CDCl3, d, ppm): d ¼ 7.35 (d, 2H, HAPh, J
¼ 8.5 Hz), 7.11 (m, 2H, HAPh, J ¼ 8.5 Hz), 3.72 (d, 2H,
CH2ACN, J ¼ 20.8 Hz), 2.26 (m, 3H, CH3ACO).
SCHEME 1 Synthesis of poly(NIPMAMm-co-CPMAn) (m/n
250/1 or m/n ¼ 51/1).
¼
Synthesis of DMPHA
in proton and fluoride sensing in our recent research.4,35,36
Up-to-date, only few studies that report on controlled
copolymers about pH and temperature sensing with chromo-
phores in the PNIPMAM exist.22
CPA (4.45 g, 25.4 mmol), 4-dimethylamino benzaldehyde (3.80
g, 25.4 mmol) and 1,8-diazabicyclo (5,4,0) undec-7-ene (DBU)
(3.87 g, 25.4 mmol) were dissolved in pyridine (30 mL) and
ꢀ
stirred for 18 h at 100 C under argon, then cooled, poured
into deionized water (180 mL). Here, the product had hydro-
lyzed to be a corresponding phenol. The solution was
extracted three times with dichloromethane (150 mL) and the
underlayer was washed three times with water (90 mL). After
drying with anhydrous MgSO4 and concentrated in vacuo, the
residue was applied to silica gel chromatography (petroleum
ether:ethyl acetate ¼ 5:1) to afford yellow compound (Z)-3-
(4-(dimethylamino)phenyl)-2-(4-hydroxyphenyl)acrylonitrile
(DMPHA) (2.69 g, 34.6%). m.p. 214–215 ꢀC.
Herein, smart copolymers of PCN250 and PCN51, prepared by
RAFT polymerization, have been designed for pH and temper-
ature sensing based on the hydrophobicity/hydrophilicity bal-
ance of copolymer chain. The molecular weights and the
monomer content copolymer were well controlled by the
RAFT method. The LCST and pH behavior of the resulted
copolymers with different pH have been investigated in detail.
EXPERIMENTAL
1H NMR(400 MHz, CDCl3, d, ppm): d ¼ 8.72 (d, 2H, HAPh, J
¼ 9.0 Hz), 7.51 (m, 2H, HAPh, J ¼ 8.5 Hz), 7.29 (s, 1H,
¼¼CHACN), 6.87 (m, 2H, HAPh, J ¼ 8.5 Hz), 6.72 (d, 2H,
HAPh, J ¼ 9.0 Hz), 3.05 (s, 6H, CH3AN). High resolution
mass spectrometer (HRMS) (m/z): calcd for C17H16N2O,
264.1263, found, 265.1335 [MþH]þ.
Materials
All solvents were of analytical grade and used without fur-
ther purification. NIPMAM was synthesized according to the
procedure described previously.22 NIPMAM monomer was
used after recrystallization from n-hexane. Water was puri-
fied by the MilliQ system. The synthesis route of the copoly-
mers is summarized in Scheme 1.
Synthesis of (Z)-4-(1-Cyano-2-(4-(dimethylamino)
phenyl)vinyl)phenylmethylacrylate
Methods
All of the spectroscopic measurements were carried out with a
1-cm path length quartz cell. Fluorescence spectra were meas-
ured on a HORIBA fluorescence spectrophotometer (kexc ¼ 380
nm, kem ¼ 480 nm) equipped with a temperature controller.
UV–vis spectra were obtained using a Varian Cary 500 spectro-
photometer (1 cm quartz cell). The measurements took place
in the temperature range from 15 to 65 ꢀC with a heating/cool-
ing rate of 1 ꢀC/min. The measurements were carried out after
the solution was left for 2 min at the designated temperature
with magnetic stirring. 1H NMR and 13C NMR in CDCl3 or
DMSO-d6 (dimethylsulfoxid) were measured on a Bruker AV-
400 spectrometer with tetramethylsilane as internal standard.
DMPHA (0.68 g, 2.59 mmol) was dissolved in dichlorome-
thane (10.0 mL) and triethylamine (0.4 mL, 2.59 mmol) and
stirred in ice bath under argon. Then methacryloyl chloride
0.25 mL (2.59 mmol), which was dissolved in dichlorome-
thane (10.0 mL), was dropped for 1 h. After stirring for 16 h
at room temperature, the solution was poured into deionized
water (100 mL) and extracted with dichloromethane (40.0
mL) four times. After drying with anhydrous MgSO4 and con-
centrated in vacuo, the crude product was recrystallized
two times by ethanol to afford brown compound (Z)-4-(1-
cyano-2-(4-(dimethylamino) phenyl)vinyl)phenylmethylacry-
late (CPMA; 0.75 g, 86.9%).
2220
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 2219–2226

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
TABLE 1 Molecular Weights and Polydispersity Indexes of the Copolymers Prepared by RAFT Polymerization
Polymer
[NIPMAM]/[CPMA]/[CDB]/[AIBN]
Mn(Theor.)
Mn(GPC)
PDI (Mw/Mn)
CPMA (%)
PCN51
300/6/2/1
13,000
13,900
11,500
11,900
1.29
1.24
1.9
0.4
PCN250
312.5/1.25/2/1
1H NMR (400 MHz, DMSO-d6, d, ppm): d ¼ 7.88 (d, 2H,
HAPh, J ¼ 9.0 Hz), 7.83 (s, 1H), 7.73 (m, 2H, HAPh), 7.29
(m, 2H, HAPh), 6.83 (d, 2H, H-Ph, J ¼ 9.0 Hz), 6.30 (s, 1H,
CH2¼¼, J ¼ 1.4 Hz), 5.93 (d, 1H, CH2¼¼, J ¼ 1.4 Hz), 3.03 (s,
6H, CH3AN), 2.02 (s, 3H, CH3AC). 13C NMR (100 MHz,
DMSO-d6, d, ppm): 165.2, 151.7, 150.2, 143.1, 135.2, 132.6,
131.1, 127.9, 126.1, 122.5, 120.8, 119.2, 111.6, 101.5, 39.9,
18.0. HRMS (m/z): calcd for C21H20N2O2, 333.1525, found,
333.1601 [MþH]þ.
pH sensing and molecular switching.37 Along with increasing
solvent polarity, the dipole–dipole forces in the system of
DMPHA derivatives become stronger, thus resulting in a
larger charge transfer from the donor moiety of an amino
group to the acceptor moiety of a DPAC ((Z)-2, 3-diphenyl-
acrylonitrile) group. Consequently, the fluorescence quantum
yield of CPMA is dramatically dependent upon solvent polar-
ity. In addition, PNIPMAM is a well-known member of
thermo-responsive polymer with a characteristic LCST. The
homopolymer of NIPMAM exhibits a coil-to-globule thermor-
eversible phase transition in dilute aqueous solution and has
a LCST at about 46 ꢀC.38,39 Taken into account in poly(NIP-
MAMm-co-CPMAn), the unit of NIPMAM is incorporated as a
thermo-responsive function, labeled with the moiety of
DMPHA as solvent polarity and pH sensing unit.
Synthesis of Poly(NIPMAMm-co-CPMAn)
(m/n 5 250/1, PCN250)
A Schlenk tube was filled with NIPMAM 316 mg (2.5 mmol),
CPMA 3.3 mg (0.0099 mmol), 2,2-azobisisobutyronitrile (AIBN)
1.3 mg (0.0080 mmol), cumyl dithiobenzoate (CDB) 4.3 mg
(0.016 mmol), and Nmethylpyrrolidone (NMP) 2.4 mL as sol-
vent. The Schlenk was sealed and the solution was degassed by
the freeze–pump–thaw method for three times to remove the
oxygen thoroughly. Then, the Schlenk tube was placed in oil
bath of 70 ꢀC to begin polymerization. After 36 h, the tube was
immerged in liquid nitrogen to quench the polymerization and
then opened. The copolymer was purified by precipitating twice
from tetrahydrofuran (THF) to diethyl ester, and once from
NMP to a mixture of diethyl ether/methanol (1:1, v/v). After
The copolymers were prepared by RAFT polymerization of
CPMA and NIPMAM at different molar ratio in the solution of
anhydrous N-methyl-2-pyrrolidone with CDB as the chain
transfer agent and AIBN as an initiator. Determined by GPC,
the number-average molecular weight for PCN250 is Mn
¼
11,900 g/mol (Mw/Mn ¼ 1.23), and for PCN51 is Mn ¼ 11,500
g/mol (Mw/Mn ¼ 1.29). Both the copolymers’ Mn determined
by GPC were slightly lower than the calculated values (Mn,thoer
¼ 13,900 for PCN250 and 13,000 g/mol for PCN51; See Table
1). The reason for this divergence was due to the calibration
of the GPC with respect to polystyrene standards. However,
the narrow molecular weight distribution for copolymers
PCN250 and PCN51 indicated the control of the polymeriza-
tion. CPMA was successfully incorporated into the copolymers
at approximately content of 0.4% for PCN 250 and 1.9% for
PCN51, respectively, as determined by UV/vis spectroscopy
(See Table 1 and Supporting Information Fig. S4). Apparently,
the molar ratio of CPMA units in the copolymer is much less
than that of NIPMAM units so as not to distinctly affect the
copolymeric thermosensitivity.
ꢀ
being filtrated and dried in a vacuum oven at 50 C overnight,
gray copolymer PCN250 was lastly obtained (223 mg, yield
68.8%), with the number-average molecular mass Mn of 11,900
g/mol and polydispersity Mw/Mn of 1.24.
Synthesis of Poly(NIPMAMm-co-CPMAn)
(m/n 5 51/1, PCN51)
The copolymer PCN51 was prepared using the same proce-
dure as that of PCN250. Determined by GPC, the number-av-
erage molecular weight of PCN51 is Mn ¼ 11,500 g/mol,
with the polydispersity of Mw/Mn ¼ 1.29.
RESULTS AND DISCUSSION
Design and Synthesis
Thermo-Responsive Behaviors
The fluorescent unit CPMA is depicted in Scheme 1. DMPHA
was functionalized with methacryloylchloride to gain the cor-
responding monomer CPMA. In 1H NMR of DMPHA and
CPMA, the characteristic chemical shift of alkene protons is
indicative of the predominant trans-isomer. Finally, the co-
polymer was prepared by RAFT copolymerization.
As poly(NIPMAM) contains both hydrophilic amide groups
and hydrophobic isopropyl groups on its side chains, most of
the polar water molecules can be repelled out of the poly-
mer when heating above the point of LCST,38–40 thus result-
ing in the characteristic thermosensitivity. The function of
PCN250 as a fluorescence thermometer with pH condition
was investigated in a mixture of methanol/water (1/5, v/v)
solution. The methanol/water mixture was chosen as solvent
system based on the copolymer solubility and fluorescence
response to temperature and pH.22
As
a derivative of DMPHA ((Z)-3-(4-(dimethyl-amino)-
phenyl)-2-(4-hydroxy-phenyl)acrylonitrile), the monomer of
CPMA is also a typical donor–p-acceptor (D–p-A) structure
with a broad absorption band resulting from an ultrafast
process of intramolecular charge transfer (ICT). Actually, the
ICT mechanism of DMPHA derivatives has been exploited for
As shown in Figure 1(a), the occurrence of a LCST fꢀor
PCN250 was distinctly observed during heating above 35 C
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 2219–2226
2221

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
nomenon at both acidic (pH ¼ 4) and alkali (pH ¼ 10) envi-
ronment, it presents less sensitivity of the copolymer to tem-
perature in the alkali environment [Fig. 1(b)]. Additionally,
the fluorescent emission of PCN250 shows a hypsochromic
shift by about 5 nm upon heating from 35 to 52 ꢀC (Sup-
porting Information Fig. S6) due to the decreased microen-
vironmental polarity near DMPHA unit.13,44 Also as a result
of the microenvironmental disturbance, the corresponding
fluorescence is enhanced to about 1.3-fold [Fig. 2(a)]. More
carefully, the fluorescence enhancement was observed in
acidic (pH ¼ 4) and alkaline conditions (pH ¼ 10) with an
ꢀ
increase of temperature from 35 to 52 C [Fig. 2(b)]. In addi-
tion, the emission intensity of monomer CPMA show obvious
temperature dependent change at different pH [Fig. 2(b)].
Consequently, in the system of PCN250, the increase in the
microenvironmental hydrophobicity of N-alkylacrylamide
results in the high sensitivity toward the change in
FIGURE 1 (a) Single-run reversibility experiments of the UV/vis
absorption responses of PCN250 (1.0 mg/mL) in methanol/
water (1/5, v/v) solution at pH 7 to temperature variation at 380
nm. (b) Normalized ratio of absorbance of PCN250 (1.0 mg/mL)
at 380 nm in methanol/water (1/5, v/v) solution at various tem-
peratures at pH 4 and 10. HCl and NaOH methanol/water (1/5,
v/v) solution (1.0 mol/L) were used for the pH adjustments.
in a solution of methanol/water (1/5, v/v) mixture at pH 7,
ꢀ
which is decreased by 11 C than in aqueous solution due to
the addition of methanol.40,41 However, at pH 4 and 10, the
turbidity (A380 nm) curve of PCN250 is quite different from
that of at pH 7 (Fig. 1). It first increases gradually above 25
ꢀC and then decreases drastically above 30 ꢀC at pH 4 and
35 ꢀC at pH 10, respectively. The enhancement of the
absorption above 25 ꢀC can be attributed to the copolymer
phase transition from coil-to-globule (LCST). While at pH 4,
the precipitated polymer particles formed at the temperature
of 30 ꢀC, and therefore the turbidity (A380 nm) curve of
PCN250 decreases drastically above 30 ꢀC due to light scat-
tering by the particles [Fig. 1(b)].42,43 Also at pH 10, the pre-
cipitated polymer particles formed above 35 ꢀC and there-
fore the absorbance of PCN250 at 380 nm decrease above
FIGURE 2 (a) Single-run reversibility experiments of the fluo-
rescence responses of PCN250 (1.0 mg/mL) in methanol/water
(1/5, v/v) solution at pH 7 to temperature variation at 480 nm.
(b) Normalized ratio of fluorescence intensities of PCN250 (1.0
mg/mL) at 480 nm in methanol/water (1/5, v/v) solution at vari-
ous temperatures at pH 4, 7, and 10. HCl and NaOH methanol/
water (1/5, v/v) solution (1.0 mol/L) were used for the pH
adjustments.
ꢀ
35 C because of light scattering of thꢀe particles after a slight
enhancement upon heating above 25 C (LCST). Although the
thermo-responsive behaviors of PCN250 show similar phe-
2222
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 2219–2226

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
ascribed to the light scattering by the particles before it can
be absorbed by the dye [Fig. 3(a)].42,43 Therefore, at pres-
ent concentration of PCN51 solution system, there is no
LCST phenomenon observed because of the poor solubility
of the copolymer. Additionally, its fluorescent emission
shows enhancement upon heating from 27 to 42 ꢀC [Fig.
3(b)], and the corresponding fluorescence is enhanced to
about 2.5-fold [Fig. 3(b)]. While the corresponding fluores-
cence of PCN250 is enhanced only about 1.3-fold upon
ꢀ
heating from 35 to 52 C [Fig. 2(a)], just because of the dif-
ferent content of dye monomer (CPMA) incorporated into
the copolymer. Briefly, the different component of the
copolymers plays key role in the LCST for the optical and
fluorescent copolymeric thermometer.
pH Responsive Behaviors
The optical properties of PCN250 (1.0 mg/mL) are investi-
gated in methanol/water (1/5, v/v) solution. When com-
pared with the absorbance of the monomer CPMA and
FIGURE 3 (a) Single-run reversibility experiments of the UV/vis
absorption responses of PCN51 (1.0 mg/mL) in methanol/water
(1/5, v/v) solution at pH 7 to temperature variation at 380 nm.
(b) Single-run reversibility experiments of the fluorescence
responses of PCN51 (1.0 mg/mL) in methanol/water (1/5, v/v)
solution at pH 7 to temperature variation at 480 nm.
temperature at the pH rang of pH ¼ 4.0–10.0 because the
copolymer thermosensitivity is dominated by the hydropho-
bicity/hydrophilicity balance of N-alkylacrylamide units.
Briefly, the temperature change in the medium plays key
role in the ON/OFF switching for the optical and fluorescent
copolymeric thermometer at the neutral enviroment.
The LCST values of the copolymers (poly-(NIPMAMm-co-
CPMAn)) with different component were also investigated.
For copolymer PCN51 (m/n ¼ 51), the occurrence of LCST
was not observed at the experiment conditions. On the con-
trary, there is significantꢀ UV–vis absorption decrease from
at temperature above 27 C in a solution of methanol/water
(1/5, v/v) mixture at pH 7, which is very different from
that of PCN250 (m/n ¼ 250) in methanol/water solution
(which shows obvious LCST above 35 ꢀC). For PCN51 solu-
tion of 1.0 mg/mL, there is precipitated polymer formed at
temperature 27 ꢀC, the significant UV/vis absorption
decrease of copolymer (PCN51) above 27 ꢀC can be
FIGURE 4 (a) UV–vis spectra of PCN250 (1.0 mg/mL) on addi-
tion of various concentrations of proton in methanol/water (1/
5, v/v) solution at 25 ꢀC. (b) Absorbance at 380 nm versus pH
for PCN250 (1.0 mg/mL) in methanol/water (1/5, v/v) solution at
25 ꢀC. HCl and NaOH methanol/water (1/5, v/v) solution were
used for the pH adjustments.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 2219–2226
2223

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
SCHEME 2 ICT (inter charge transfer) process of CPMA before
and after in acid.
tensity was decreased when changing pH from 2.8 to 0.6,
and the peak wavelength shifted little (Fig. 5).
The fluorescence quench observed in acidic environment can
be attributed to the ‘‘switching off’’ process of the ICT pro-
cess (Scheme 2) from the nitrogen atom (N1) of (dimethyla-
mino)phenyl donor to the DPAC fluorophore through the
phenyl ring in the system of PCN250 (Scheme 2). Accord-
ingly, upon decreasing pH, the fluorescence quench can be
achieved by the protonation of the nitrogen atom (N1) of
(dimethylamino) phenyl unit (Scheme 2).
The effect of proton on fluorescence in thin polymer film of
about 10 lm was also investigated. The polymer was
immersed in proton solution (pH ¼ 0.6), and after drying for
20 min the fluorescent spectra were monitored, the band at
480 nm dramatically decreased, as can be seen in Figure 6,
with quenching effect of 54.5%. The quenching effect could
also be observed in the absorbance spectra at 380 nm. 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 environmental researchers for detec-
tion of proton.
FIGURE 5 (a) Fluorescent spectra of PCN250 (1.0 mg/mL) on
addition of various concentrations of proton in methanol/water
(1/5, v/v) solution at 25 ꢀC. (b) Fluorescent intensity (excitation
wavelength: 380 nm; emission wavelength: 480 nm) versus pH
for PCN250 (1.0 mg/mL) in methanol/water (1/5, v/v) solution at
25 ꢀC. HCl and NaOH methanol/water (1/5, v/v) solution were
used for the pH adjustments.
CONCLUSIONS
Smart copolymers poly(NIPMAMm-co-CPMAn) (PCN250 and
PCN51), prepared by RAFT polymerization, have been
designed for fluorescent thermometer system based on the
hydrophobicity/hydrophilicity balance of copolymer chain
and pH sensing. The molecular weights and the monomer
copolymer PCN250 in methanol (Supporting Information Fig.
S4), they are superimposed to each other, indicating that the
characteristic absorption of copolymer poly(NIPMAMm-co-
CPMAn) is mainly resulted from the CPMA unit. Here, the
component of PNIPMAM has no absorbance or fluorescence.
Accordingly, the molar ratio (1/250) of monomer CPMA/NIP-
MAM in copolymer PCN250 was derived from standard
absorption working plot.
The PCN250 in methanol/water (1/5, v/v) solution was
used to study the pH effect on the absorption and fluores-
cence properties. When adjusting the methanol/water (1/5,
v/v) solution pH from 2.8 to 0.6, the corresponding absorp-
tion peak of CPMA unit was remarkable hypochromatic shift
with a clear isobestic point at 340 nm (Fig. 4). Notably, the
maximum absorbance peak at 380 nm was selected as exci-
tation wavelength. As expected, the fluorescence originating
from CPMA unit for PCN250 exhibited good sensitivity to pH
in methanol/water (1/5, v/v) solution. Its fluorescence in-
FIGURE 6 Fluorescent spectra of PCN250 thin film processed
on quartz.
2224
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 2219–2226

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
6 Uchiyama, S.; Makino, Y. Chem. Commun. 2009, 19
2646–2648.
content copolymer were well controlled by the RAFT
method. PCN250 displays specific UV–vis signals when
changing the temperature and pH of the copolymer solution
in methanol/water (1/5, v/v). When pH is 4 or 10, the ab-
sorbance of the copolymer at 380 nm shows selective
decrease at a specific temperature range because of the co-
polymer solubility at these pH values, which cause the pre-
cipitation of the polymer and thus results in the light scatter-
ing. While at pH ¼ 7, the absorbance of the copolymer at
380 nm presents enhancement when the temperature rises,
which indicates the LCST value of the copolymer (LCST 35
ꢀC). However, the fluorescence intensity of this copolymer
shows enhancement with a rise in temperature regardless of
pH value. This copolymer dissolved in methanol/water (1/5,
v/v) solution shows weak fluorescence below 35 ꢀC, while
showing fluorescence enhancement above 35 ꢀC and satura-
tion at 52 ꢀC. The fluorescence intensity measured at 52 ꢀC
7 Shen, L. J.; Lu, X. Y.; Tian, H.; Zhu, W. H. Macromolecules
2011, 44, 5612–5618.
8 Wang, Y.; Li, X.; Hong, C. Y.; Pan, C. Y. J. Polym. Sci. Part A:
Polym. Chem. 2011, 49, 3280–3290.
9 Shiraishi, Y.; Miyarnoto, R.; Hirai, T. Langmuir 2008, 24,
4273–4279.
10 Liras, M.; Paris, R.; Quijada-Garrido, I.; Garcia, O. Macromo-
lecules 2011, 44, 80–86.
11 Chen, C. T.; Chen, C. Y. Chem. Commun. 2011, 47,
994–996.
12 Uchiyama, S.; Gota, C.; Okabe, K.; Funatsu, T.; Harada, Y. J.
Am. Chem. Soc. 2009, 131, 2766–2767.
13 Uchiyama, S.; Kawai, N.; de Silva, A. P.; Iwai, K. J. Am.
Chem. Soc. 2004, 126, 3032–3033.
14 Uchiyama, S.; Matsumura, Y.; de Silva, A. P.; Iwai, K. Anal.
Chem. 2004, 76, 1793–1798.
ꢀ
is 1.3-fold higher than that at 35 C.
15 Aoshima, S.; Sugihara, S. J. Polym. Sci. Part A: Polym.
Chem. 2000, 38, 3962–3965.
Moreover, for copolymers with comparable molecular
weights, the thermo-responsive properties of poly(NIP-
MAMm-co-CPMAn) changed with various monomer ratios
incorporated in copolymers, that is, with different m/n.
When m/n ¼ 51 (for PCN51), there is no LCST was observed
at the applied solution system. On the contrary, the absorb-
ance of the copolymer PCN51 at 380 nm decreases when the
temperature rises from 27 to 42 ꢀC, however, which shows
enhancement for PCN250 (m/n ¼ 250). However, the copoly-
mer shows fluorescence enhancement with a rise in temper-
ature regardless of PCN51 and PCN250 in different tempera-
ture range. In one word, the different component
incorporated in the copolymers also plays key role in the
LCST for copolymeric thermometers.
16 Pietsch, C.; Schubert, U. S.; Hoogenboom, R. Chem. Com-
mun. 2011, 47, 8750–8765.
17 Can, A.; Hoeppener, S.; Guillet, P.; Gohy, J. F.; Hoogen-
boom, R.; Schubert, U. S. J. Polym. Sci. Part A: Polym. Chem.
2011, 49, 3681–3687.
18 Lee, R. S.; Wu, K. P. J. Polym. Sci. Part A: Polym. Chem.
2011, 49, 3163–3173.
19 Pan, T. T.; He, W. D.; Li, L. Y.; Jiang, W. X.; He, C.; Tao, J. J.
Polym. Sci. Part A: Polym. Chem. 2011, 49, 2155–2164.
20 Medel, S.; Garcia, J. M.; Garrido, L.; Quijada-Garrido, I.; Paris,
R. J. Polym. Sci. Part A: Polym. Chem. 2011, 49, 690–700.
21 Tu, Y. L.; Wang, C. C.; Chen, C. Y. J. Polym. Sci. Part A:
Polym. Chem. 2011, 49, 2866–2877.
22 Zhu, W. H.; Guo, Z. Q.; Xiong, Y. Y.; Tian, H. Macromole-
cules 2009, 42, 1448–1453.
What’s more, PCN250 (m/n ¼ 250) shows fluorescence sup-
pression in the solution with high proton concentration. The
pH change in the medium plays a key role in the ON/OFF
switching for the fluorescent copolymeric thermometer. Thin
PCN250 film also showed fluorescence changes upon addi-
tion of proton, which indicates that this copolymer can be
made and applied as convenient detecting apparatus for
environmental detection of proton.
23 Uchiyama, S.; Matsumura, Y.; de Silva, A. P.; Iwai, K. Anal.
Chem. 2003, 75, 5926–5935.
24 Stich, M. I. J.; Nagl, S.; Wolfbeis, O. S.; Henne, U.; Schae-
ferling, M. Adv. Funct. Mater. 2008, 18, 1399–1406.
25 Wolfbeis, O. S.; Borisov, S. M. Anal. Chem. 2006, 78,
5094–5101.
26 Jorge, P. A. S.; Maule, C.; Silva, A. J.; Benrashid, R.; Santos,
J. L.; Farahi, F. Anal. Chim. Acta 2008, 606, 223–229.
27 Wolfbeis, O. S.; Borisov, S. M.; Krause, C.; Arain, S. Adv.
Mater. 2006, 18, 1511–1516.
This work was financially supported by NSFC/China
(50673025), National Basic Research 973 Program
(2006CB806200), Scientific Committee of Shanghai, and ECUST
Funds for Excellent Youth Faculties (YJ0157116).
28 Klimant, I.; Schroeder, C. R.; Neurauter, G. Microchim. Acta
2007, 158, 205–218.
29 Klimant, I.; Schroder, C. R.; Polerecky, L. Anal. Chem. 2007,
79, 60–70.
30 Wolfbeis, O. S.; Kocincova, A. S.; Nagl, S.; Arain, S.; Krause,
C.; Borisov, S. M.; Arnold, M. Biotechnol. Bioeng. 2008, 100,
430–438.
REFERENCES AND NOTES
1 Andresen, T. L.; Sun, H. H.; Almdal, K. Chem. Commun.
2011, 47, 5268–5270.
31 Hoogenboom, R.; Pietsch, C.; Schubert, U. S. Angew.
Chem.-Int. Ed. 2009, 48, 5653–5656.
2 Allard, E.; Larpent, C. J. Polym. Sci. Part A: Polym. Chem.
2008, 46, 6206–6213.
32 Hua, J. L.; Qu, Y.; Jiang, Y. H.; Tian, H. J. Polym. Sci. Part A:
Polym. Chem. 2009, 47, 1544–1552.
3 Zhu, W. H.; Shen, L. J.; Meng, X. L.; Guo, Z. Q.; Tian, H. Sci.
China Ser. B-Chem. 2009, 52, 821–826.
33 Aamer, K. A.; Tew, G. N. J. Polym. Sci. Part A: Polym.
Chem. 2007, 45, 5618–5625.
4 Tian, H.; Jiang, J. B.; Leng, B.; Xiao, X.; Zhao, P. Polymer
2009, 50, 5681–5684.
34 Gujraty, K. V.; Yanjarappa, M. J.; Saraph, A.; Joshi, A.; Mog-
ridge, J.; Kane, R. S. J. Polym. Sci. Part A: Polym. Chem. 2008,
46, 7249–7257.
5 Chiu, D. T.; Chan, Y. H.; Wu, C. F.; Ye, F. M.; Jin, Y. H.; Smith,
P. B. Anal. Chem. 2011, 83, 1448–1455.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 2219–2226
2225

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
35 Tian, H.; Zhao, P.; Jiang, J. B.; Leng, B. Macromol. Rapid
Commun. 2009, 30, 1715–1718.
40 Winnik, F. M.; Ringsdorf, H.; Venzmer, J. Macromolecules
1990, 23, 2415–2416.
36 Zhao, P.; Jiang, J. B.; Xiao, X.; Tian, H. J. Polym. Sci. Part
A: Polym. Chem. 2010, 48, 1551–1556.
41 Schild, H. G.; Muthukumar, M.; Tirrell, D. A. Macromole-
cules 1991, 24, 948–952.
37 Zhu, L.; Ji, F.; Wang, Q.; Ma, X.; Chen, Z.; Tian, H. Front.
Chem. China 2009, 4, 278–291.
42 Shiraishi, Y.; Miyamoto, R.; Zhang, X.; Hirai, T. Org. Lett.
2007, 9, 3921–3924.
38 Iwai, K.; Matsumoto, N.; Niki, M.; Yamamoto, M. Mol. Cryst.
Liquid Cryst. Sci. Technol. Sect. A 1998, 315, 53–58.
43 Hoogenboom, R.; Pietsch, C.; Schubert, U. S. Polym. Chem.
2010, 1, 1005–1008.
39 Fujishige, S.; Kubota, K.; Ando, I. J. Phys. Chem. 1989, 93,
44 Ritter, H.; Koopmans, C. J. Am. Chem. Soc. 2007, 129,
3311–3313.
3502–3503.
2226
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 2219–2226