The thermoresponsive behaviour of a poly(N-isopropylacrylamide) hydrogel with a D-π-A type pyran-based fluorescent dye

Article abstract of DOI:10.1016/j.dyepig.2010.02.008

A thermoresponsive poly(NIPAM-co-fluorophore) hydrogel labeled with pyran-based fluorescent dye was prepared using conventional radical copolymerization. The low critical solution temperature behaviour was investigated using UV-vis spectroscopy which allowed the measurement of the phase transition over a wide range of temperature. The poly(NIPAM-co-fluorophore) in aqueous solution underwent a phase transition at 36-58?°C. The fluorescence intensities of the polymer in aqueous solution increased with increasing temperature across the low critical solution temperature transition range. The fluorophore displayed a blue shift in emission with increase in temperature due to destabilization of the excited state. Evidence from fluorescence and dynamic light scattering demonstrated that, >58?°C, the size of the polymer aggregates was 12,500?m. The morphology of the internal microstructure of the poly(NIPAM-co-fluorophore) hydrogel was observed using scanning electron microscopy.

Full text of DOI:10.1016/j.dyepig.2010.02.008

Dyes and Pigments 87 (2010) 84e88
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The thermoresponsive behaviour of a poly(N-isopropylacrylamide)
hydrogel with a D-p-A type pyran-based fluorescent dye
Sung-Hoon Kim a, , In-Jeong Hwang , Seon-Yeong Gwon , Young-A. Son
b
a
c
*
a
Department of Textile System Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea
Department of Advanced Organic Materials Science and Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea
Department of Advanced Organic Materials and Textile System Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 January 2010
Received in revised form
A thermoresponsive poly(NIPAM-co-fluorophore) hydrogel labeled with pyran-based fluorescent dye
was prepared using conventional radical copolymerization. The low critical solution temperature
behaviour was investigated using UVevis spectroscopy which allowed the measurement of the phase
transition over a wide range of temperature. The poly(NIPAM-co-fluorophore) in aqueous solution
underwent a phase transition at 36e58 C. The fluorescence intensities of the polymer in aqueous
solution increased with increasing temperature across the low critical solution temperature transition
24 February 2010
Accepted 26 February 2010
Available online 4 March 2010
ꢀ
range. The fluorophore displayed a blue shift in emission with increase in temperature due to destabi-
Keywords:
D-p-A dye
Pyran-based fluorescent dye
Poly(N-isopropylacrylamide)
Hydrogel
lization of the excited state. Evidence from fluorescence and dynamic light scattering demonstrated that,
ꢀ
>
58 C, the size of the polymer aggregates was 12,500 m. The morphology of the internal microstructure
of the poly(NIPAM-co-fluorophore) hydrogel was observed using scanning electron microscopy.
Ó 2010 Elsevier Ltd. All rights reserved.
Low critical solution temperature (LCST)
Hydrodynamic radius
1. Introduction
conjugate-electron acceptor (D-p-A) type fluorophores. D-p-A type
pyran-based fluorescent dyes such as 4-(dicyanomethylene)-2-
methyl-6-(p-(dimethylamino)styryl)-4H-pyran (DCM) have been
intensively developed for applications as photo- and electrolumi-
nescent materials in the fields of dye lasers [16], sensors [17] and dye-
sensitized solar cells [18]. Their fluorescence properties are highly
sensitive to changes in external environment such as polarity and pH
because of their intrinsic intramolecular charge transfer character
[19,20]. Previous work [21] by the present research group reported
the synthesis, photophysical properties and unusual aggregation-
induced fluorescence changes in some a novel pyran-based fluo-
rophore copolymer. This paper concerns fluorescence-based
investigations of the thermoresponsive behaviour of a poly(N-iso-
Temperature-responsive polymers and hydrogels have gained
scientific and technological importance and the characteristics of
these materials have been studied extensively for applications such
as drug delivery, fluorescent thermometer and separation systems
[1e4]. One of the most intensively studied polymers in this field is
poly(N-isopropylacrylamide) (PNIPAM) which exhibits a low critical
ꢀ
solution temperature (LCST) transition from 30 to 45 C [5,6]. The
polymer is hydrophilic and soluble in water < LCST but becomes
hydrophobic and forms a macroscopic coacervate phase above that
temperature, owing to the fluctuation of hydrophobic interactions
and hydrogen bonding [7]. This phase transition is accompanied by
a release of water bound to the polymer chain, which is an endo-
thermic process. In previous papers [8,9], we have reported bistable
photoswitching in poly(N-isopropylacrylamide) using a photo-
chromic spironaphthoxazine hydrogel for optical data storage. Some
copolymers with fluorophore have been reported [10e15], in which
the fluorescence intensity of the fluorophores was controlled by
temperature as external stimuli. To the best of our knowledge, there
propylacrylamide) hydrogel with D-p-A type pyran-based fluores-
cent dye.
2. Experimental
Melting points were determined using Electrothermal IA 900
apparatus and were uncorrected. Mass spectra were recorded on
are no reports of PNIPAM copolymers with electron donor-p-
a Shimadzu QP-1000 spectrometer using an electron energy of
1
7
0 eV and the direct probe EI method. H NMR spectra was recorded
in CDCl
3
using a Varian Inova 400 MHz FT NMR Spectrometer using
*
Corresponding author. Tel.: þ82 53 950 5641; fax: þ82 53 950 6617.
E-mail address: shokim@knu.ac.kr (S.-H. Kim).
TMS as internal standard. The weight-average molecular weight
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.02.008

S.-H. Kim et al. / Dyes and Pigments 87 (2010) 84e88
85
(
Mn) and polydispersity (Mw/Mn) of the polymer was measured on
2.2.2. Allyl-2-(2,6-bis((E)-4-(diphenylamino)styryl)-
4H-pyran-4-ylidene)-2-cyanoacetate 5
ꢀ
a PL-GPC model 210 chromatograph at 25 C using THF as the eluent
and standard polystyrene as the reference. Fluorescence spectra
were measured on a Shimadzu RF-5301PC Fluorescence spectro-
photometer. The UVevis spectra and transmittance were obtained
on an Agilent 8457 UVevis spectrophotometer. The LCST values
were also determined by spectrophotometric detection of the
A solution of allyl-2-cyano-2-(2,6-dimethyl-4H-pyran-4-yli-
dene)acetate 3 (1 g, 4.3 mmol), 4-(diphenylamino)benzaldehyde 4
(1.4 g, 5.2 mmol), and piperidine (0.3 ml) in 1-propane (50 ml) was
refluxed for 24 h using DeaneStark trap. The ensuing solution was
cooled to room temperature and the solid product was isolated and
washed well with 1-propanol and then dried. The reaction mixture
was evaporated to remove solvent and separated by column chro-
changes in transmittance (
l
max ¼ 300 nm) of aqueous polymer
ꢀ
ꢁ1
solutions heated at a constant rate (0.2 C min ) with a thermo-
statically controlled cuvette. Dynamic light scattering (DLS)
measurements was carried out on a Wyatt Technology Dynapro
Titan instrument at fixed with the 828.6 nm line of a laser as exci-
tation source.
ꢀ
matography using chloroform. Yield: 30%; mp 229e231 C; mass
þ
1
(m/z) 741 (M ). H NMR (400MHz, CDCl
3
):
d
4.69 (dt, J ¼ 1.48 Hz,
2H), 5.23 (dd, J ¼ 3.96 Hz, 1H), 5.37 (dd, J ¼ 4.64 Hz, 1H), 5.94 (m,
1H), 6.60 (d, J ¼ 6.44 Hz, 1H), 6.64 (d, J ¼ 6.36 Hz, 1H), 6.73 (s, 1H),
7
39 3 3
.02 (m, 30H), 8.04 (s, 1H). EA: Anal. calcd for C51H N O : C, 82.57;
H, 5.66; N, 5.30. Found: C, 81.84; H, 5.78; N, 5.44%.
2.1. Materials
2.3. Synthesis of poly(NIPAM-co-fluorophore)
2
,6-Dimethylpyran-4-pyrone, allyl cyanoacetate and 4-(diphe-
N-Isopropylacrylamide 6 (2.26 g, 20 mmol), allyl-2-(2,6-bis((E)-
nylamino)benzaldehyde were purchased from Aldrich. N-Iso-
propylacrylamide (Aldrich) was purified by recrystallization from
a mixture of toluene/hexane (1/4) and dried in vacuum. 2,2 -Azo-
bis-(isobutyronitrile) (AIBN) was recrystallized from methanol. The
other chemicals were of the highest grade available and were used
without further purification. All employed solvents were analyti-
cally pure and were employed without any further drying or
purification.
4
(
0
-(diphenylamino)styryl)-4H-pyran-4-ylidene)-2-cyanoacetate
0.15 g, 0.2 mmol), and 2,2 -azobis-(isobutyronitrile) (0.032 g,
5
0
0
.2 mmol) were dissolved in anhydrous THF (20 ml) under dry N
2
.
ꢀ
After being heated for 3 days at 65 C, the resultant mixture was
precipitated from ether. The ensuing polymer was dried in vacuum
to give satisfactory yield as orange-colored powder. Yield: 85%; Gel
permeation chromatography (GPC): Mn: 21,733, Mw: 25,342, MP:
2
8,225, Mw/Mn: 1.16.
3. Results and discussion
2.2. Synthesis of monomer 5
The organic functional dyes with D-
p
-A charge transfer are the
2.2.1. Allyl-cyano-2-(2,6-dimethyl-4H-pyran-4-ylidene)acetate 3
A mixture of 2,6-dimethylpyran-4-pyrone (3 g, 24 mmol) 1, allyl
most important class of compounds for elucidating the relation-
ships between push-pull chromophores and their optical proper-
cyanoacetate 2, (3 g, 24 mmol), and acetic acid (12.5 ml) was
refluxed for 24 h. The crude solid obtained was filtered, washed
with methanol, and recrystallized from methanol several times.
ties [22]. Despite the broad application of these and other D-p-A
chromophores [23,24] for the elucidation of reaction rates, chem-
ical equilibria, nonlinear optical properties, and so on, accurately
explaining the relationship between substituted groups and optical
properties remains the subject of active debate [25]. The synthetic
route of an monomer 5 and poly(NIPAM-co-fluorophore) is depic-
þ
1
Yield: 32%; mass (m/z) 231 (M ). H NMR (400MHz, CDCl
3
):
d 2.27
(
(
s, 6H), 4.65 (d, 2H), 5.20(d, 1H), 5.36 (d, 1H), 5.94 (m, 1H), 6.59
s, 1H), 7.87 (s, 1H). EA: Anal. calcd for C13
3
H13NO : C, 67.52; H, 5.67;
N, 6.06. Found: C, 67.17; H, 5.72; N, 6.03%.
ted in Scheme 1. We designed a new D-p-A charge transfer dye 5,
O
N
O
CN
O
CN
O
O
CHO
2
O
1
4
O
3
O
H
2
H
2
O
CN
C
C
O
m
n
CN
O
HN
O
HN
O
O
6
O
N
N
N
N
5
Poly(NIPAM-co-fluorophore)
Scheme 1. Synthesis of a D-p-A charge transfer dye, 5, and poly(NIPAM-co-fluorophore).

86
S.-H. Kim et al. / Dyes and Pigments 87 (2010) 84e88
500
400
300
200
100
0
o
20 C
Energy:
o
0 C
V
1
V
2
5
V
2
V
1
500
550
600
650
700
750
Increasing temperature
Wavelength(nm)
Increasing hydrophobicity
Decreasing matrix polarity
Fig. 1. Fluorescence spectra of monomer
5
at CHCl
3
5
ꢂ
10ꢁ7M at various
temperatures.
Fig. 3. A proposed transition energy diagram of poly(NIPAM-co-fluorophore) along
with the aggregation.
containing a polymerizable functional allyl group. As shown in
1
Scheme 1, the synthetic method was very simple. The H spectrum
found at the higher temperature). Poly(NIPAM-co-fluorophore) is
hydrophilic at low temperature, but a rise in temperature leads to
a formation of hydrophobic domain inside the polymer, along with
an increase in the size of polymer particles [26,27]. As proposed by
the transition energy diagram shown in Fig. 3, the aggregation of
polymer reduces the polarity of environment surrounding fluo-
rophore, and thus, the energy of the CT excited state will increase,
leading to a shorter wavelength light emission.
UVevis spectrometry is an easy and widely used method for the
determination of the LCST. In Fig. 4, we show the absorption spectra
of aqueous poly(NIPAM-co-fluorophore) solution at different
temperature. As solution is heated, it becomes turbid; these
temperature-responsive changes are entirely reversible. This kind
of gelation behaviour induced by temperature is well-known as an
LCST, resulting from a balance between hydrogen-bond formation
with water and intramolecular hydrophobic forces [28].
of 5 verified its well-defined chemical structure. Characteristic
NMR peaks due to the allyl group appeared at 5.23(dd), 5.37(dd)
and 5.94(m). Reaction of a monomer 5 and N-isopropylacrylamide
6
in THF affords a orange-colored powder of poly(NIPAM-co-
fluorophore).
Fig. 1 shows the fluorescence spectra of the monomer 5 in CHCl
3
ꢀ
at different temperature from 20 to 50 C. It can be seen that the
fluorescent intensities of this compound decreased with increasing
temperature. It can be assigned to increase in non-radiative decay
process with increase of temperature.
The temperature-dependent emission spectra of the poly
ꢀ
(
NIPAM-co-fluorophore) in temperature range from 20 to 65 C is
shown in Fig. 2.
The emission intensity of the poly(NIPAM-co-fluorophore)
increased with rising temperature when excitation wavelength was
4
45 nm. The emission enhancement at higher temperature is due
The values of transmittance (A300) of aqueous solution con-
taining poly(NIPAM-co-fluorophore) are plotted against the
temperature of the solution in Fig. 5; an obvious transmittance
to the formation of hydrophobic domain near the iso-
propylacrylamide units. As shown in Fig. 2, the emission spectra of
the fluorophore unit linked to NIPAM hydrogel in water show blue
shift with the increase in temperature (the maximum emission
wavelength is 599 nm at the lower temperature, while 569 nm is
ꢀ
ꢀ
decrease at >36 C:LCST ¼ 36e58 C. The results of thermo-
response of poly(NIPAM-co-fluorophore) solution characterized by
6
5
4
3
2
1
00
4
3
3
.0
.5
.0
00
00 65°C
6
5°C
6
0ºC
2.5
6
0ºC
00
00
Temperature(°C)
2
.0
.5
1
20°C
2
0°C
1.0
.5
0.0
00
0
0
20ºC
800
20ºC
5
00
550
600
650
700
750
300
400
500
600
700
800
Wavelength(nm)
Wavelennth(nm)
Fig. 2. Fluorescence spectra and images of poly(NIPAM-co-fluorophore) in aqueous
Fig. 4. Absorption spectra of poly(NIPAM-co-fluorophore) in aqueous solution
solution (0.025 g/ml) at various temperatures.
(0.025 g/ml) at various temperatures.

S.-H. Kim et al. / Dyes and Pigments 87 (2010) 84e88
87
UV-vis spectrometry are comparable with those from fluorescence
measurement.
100
Fig. 6 shows the temperature dependence of a hydrodynamic
radius of the polymer particle. It is clear that the R
h
increased with
80
60
40
20
0
ꢀ
increasing temperature. At 58 C, the size of the polymer hydrogels
is estimated to be 12,500 nm, due to a formation of the hydrophobic
polymer aggregates near LCST. This is the reason why the fluores-
cent intensities were increased at temperatures over the LCST.
Fig. 7 shows a few representative SEM image of the internal
matrix structure of poly(NIPAM-co-fluorophore) hydrogel which
has irregular fibrous aggregation and homogeneous coral-like
microstructure with a three dimensional network structure. This
morphology indicated that poly(NIPAM-co-fluorophore) hydrogel
could form stable hydrogel when the temperature was above the
LCST of poly(NIPAM-co-fluorophore).
20
30
40
50
C)
60
Temperature(
°
4. Conclusion
Fig. 5. Transmittance of a solution of poly(NIPAM-co-fluorophore) in aqueous solution
0.025 g/ml). C, heating; B, cooling.
In conclusion, we synthesized a thermoresponsive poly(NIPAM-
(
co-fluorophore)
hydrogel
composed
of
poly(N-iso-
propylacrylamide) partly modified with pyran-based fluorescent
dye 5 and observed that this polymer hydrogel exhibited a low
ꢀ
critical solution temperature (LCST) transition from 36 to 58 C. The
phase transition temperature for poly(NIPAM-co-fluorophore) was
determined by UVevis spectroscopic method. The fluorescence
intensity increased and the fluorescence maximum emission
wavelength was blue-shifted with increasing the temperature due
to increasing hydrophobicity around the fluorophore. The aggre-
gation behaviour of the polymer particle was studied by dynamic
light scattering. The particle size of the aggregates was estimated to
1
1
1
1
6000
4000
2000
0000
ꢀ
8
6
4
2
000
000
000
000
0
be 12500 nm at 58 C.
Acknowledgements
This work was supported by Basic Science Research Program
through the National Research Foundation (NRF) grant funded by
the Korea government (MEST) (No. 2009-0063408).
2
0
30
40
50
60
70
o
References
Temperature( C)
[
[
[
[
1] Umeno D, Kawasaki M, Maeda M. Water-soluble conjugate of double-stranded
DNA and poly(N-isopropylacrylamide) for one-pot affinity precipitation
separation of DNA-binding proteins. Bioconjugate Chemistry 1998;9:719e24.
2] Hayashi H, Kono K, Takagishi T. Temperature sensitization of liposomes
using copolymers of N-isopropylacrylamide. Bioconjugate Chemistry 1999;10:
Fig. 6. Hydrodynamic radius(R
h
) of poly(NIPAM-co-fluorophore) in aqueous solution
(
0.025 g/ml) at various temperatures.
412e8.
3] Uchiyama S, Matsumura Y, de Silva AP, Iwai K. Modulation of the sensitive
temperature range of fluorescent molecular thermometers based on ther-
moresponsive polymers. Analytical Chemistry 2004;76:1793e8.
4] Iwai K, Matsumura Y, Uchiyama S, de Silva AP. Development of fluorescent
microgel thermometers based on thermoresponsive polymers and their
modulation of sensitivity range. Journal of Materials Chemistry 2005;15.
2896e2800.
[
5] Wu C, Zhou S. Thermodynamically stable globule state of a single poly(N-
isopropylacrylamide) chain in water. Macromolecules 1995;28:5388e90.
6] Akiyoshi K, Kang EC, Kurumada S, Sunamoto J, Principi T, Winnik FM.
Controlled association of amphiphilic polymers in water: thermosensitive
nanoparticles formed by self-assembly of hydrophobically modified pullulans
and poly(N-isopropylacrylamides). Macromolecules 2000;33:3244e9.
[
[
7] Otake K, Inomata H, Konno M, Saito S. Thermal analysis of the volume phase
transition with N-isopropylacrylamide gels. Macromolecules 1990;23:283e9.
8] Wang S, Choi MS, Kim SH. Bistable photoswitching in poly(N-iso-
propylacrylamide) with spironaphthoxazine hydrogel for optical data storage.
Journal of Photochemistry and Photobiology A: Chemistry 2008;198:150e5.
9] Wang S, Choi MS, Kim SH. Multiple switching photochromic poly(N-iso-
propylacrylamide) with spironaphthoxazine hydrogel. Dyes and Pigments
[
[
2008;78:8e14.
[
[
10] Xiao X, Fu YQ, Zhou JJ, Bo ZS, Li L, Chan CM. Reversible thermally responsive
and luminescent coil-rod-coil triblock copolymers. Macromolecuar Rapid
Communications 2007;28:1003e9.
11] Uchiyama S, Matsumura Y, de Silva AP, Iwai K. Fluorescent molecular ther-
mometers based on polymers showing temperature-induced phase transition
Fig. 7. SEM micrographs of poly(NIPAM-co-fluorophore).

88
S.-H. Kim et al. / Dyes and Pigments 87 (2010) 84e88
and labeled with polarity-responsive benzofurazans. Analytical Chemistry
003;75:5926e35.
12] Uchiyama S, Kawai N, de Silva AP, Iwai K. Fluorescent polymeric AND logic
gate with temperature and pH as inputs. Journal of the American Chemical
Society 2004;126:3032e3.
[20] Zucchero AJ, Wilson NJ, Bunz UHF. Cruciforms as functional fluorophores:
2
response to protons and selected metal ions. Journal of the American Chem-
ical Society 2006;128:11872e81.
[
[
[
[
[21] Kwak G, Kim H, Kang IK, Kim SH. Charge transfer dye in various polymers with
different polarity: synthesis, photophysical properties, and unusual aggrega-
tion-induced fluorescence changes. Macromolecules 2009;42:1733e8.
[22] Reichardt C. Solvents and solvent effects in organic chemistry. 3rd ed. Wein-
heim: Wiley-VCH; 2003.
13] Guo Z, Zhu W, Tian H. Hydrophilic copolymer bearing dicyanomethylene-4 H-
2þ
pyran moiety as fluorescent film sensor for Cu and pyrophosphate anion.
Macromolecules 2010;43:739e44.
14] Guo Z, Zhu W, Xiong Y, Tian H. Multiple logic fluorescent thermometer system
based on N-isopropylmethacrylamide copolymer bearing dicyanomethylene-
[23] Reichardt C. Solvatochromic dyes as solvent polarity indicators. Chemical
Reviews 1994;94:2319e58.
4H-pyran moiety. Macromolecules 2009;42:1448e53.
[24] Hartman H, Eckert K, Schroder A. New solvatochromic dyes of the 5-dime-
0
0
15] Zhu LL, Li X, Ji FY, Ma X, Wang QC, Tian H. Photolockable ratiometric viscosity
sensitivity of cyclodextrin polypseudorotaxane with light-active rotor graft.
Langmuir 2009;25:3482e6.
thylamino-5 -nitro-2,2 -bithiophene type. Angewandte Chemie International
Edition 2000;39:556e8.
[25] Ishchenko AA, Kulinich AV, Bondarev SL, Knyukshto VN. Phodynamics of
polyene-polymethine transformations and spectral fluorescent properties of
merocyanine dyes. Journal of Physical Chemistry A 2007;111:13629e37.
[26] Koizumi H, Kimata Y, Shiraishi Y, Hirai T. Temperature-controlled changeable
oxygenation selectivity by singlet oxygen with a polymeric photosensitizer.
Chemical Communications; 2007:1846e8.
[16] Antonov VS, Hohla KL. Dye stability under excimer-laser pumping. Applied
Physics B 1983;B32:9e14.
[
17] de Silva AP, Gunaratne HQN, Gunnlaugsson T, Huxley AJM, McCoy CP,
Rademacher JT, et al. Signaling recognition events with fluorescent sensors
and switches. Chemical Reviews 1997;97:1515e66.
[
18] Sansregret J, Drake JM, Thomas WRL, Lesiecki ML. Light transport in planar
luminescent solar concentrators: the role of DCM self-absorption. Applied
Optics 1983;22:573e7.
[27] Koizumi H, Shiraishi Y, Tojo S, Fujitsuka M, Majima T, Hirai T. Temperature-
driven oxygenation rate control by polymeric photosensitizer. Journal of the
American Chemical Society 2006;128:8752e3.
[19] Burland DM, Miller RD, Walsh CA. Second-order nonlinearity in poled-poly-
mer system. Chemical Reviews 1994;94:31e75.
[28] Garcia A, Marquez M, Cai T, Rosario R, Hu Zhibing, Gust D, et al. Photo-,
thermally, and pH-responsive microgels. Langmuir 2007;23:224e9.