Local viscosity analysis of triblock copolymer micelle with cyanine dyes as a fluorescent probe

Article abstract of DOI:10.1021/la1028993

The local viscosity of Pluronic F127 triblock copolymer micelles in water was determined with cyanine dyes as fluorescent probes. These dyes show very weak fluorescence at a low temperature, but show enhanced fluorescence at a temperature higher than the

Full text of DOI:10.1021/la1028993

pubs.acs.org/Langmuir
© 2010 American Chemical Society
Local Viscosity Analysis of Triblock Copolymer Micelle with Cyanine Dyes
as a Fluorescent Probe
Yasuhiro Shiraishi,* Takuya Inoue, and Takayuki Hirai
Research Center for Solar Energy Chemistry and Division of Chemical Engineering, Graduate School of
Engineering Science, Osaka University, Toyonaka 560-8531, Japan
Received July 21, 2010. Revised Manuscript Received August 21, 2010
The local viscosity of Pluronic F127 triblock copolymer micelles in water was determined with cyanine dyes as
fluorescent probes. These dyes show very weak fluorescence at a low temperature, but show enhanced fluorescence at a
temperature higher than the critical micellization temperature (Tcm). This is because a viscous environment within the
micelle suppresses the formation of a nonradiative twisted intramolecular charge transfer (TICT) excited state of the
dyes. The good correlation between the fluorescence quantum yields of the dyes and the viscosity and the temperature of
the media allows a determination of local viscosity of micelle based on the fluorescence quantum yields. The local
viscosity of both core and corona regions of micelles increases at >Tcm and shows a maximum at a temperature 7-9 °C
higher than Tcm, and decreases at higher temperature due to the increased fluidity. The core viscosity is larger than that
of the corona, and the corona viscosity increases toward the micelle center. The polymer concentration has different
effects on the core and corona viscosity: the corona viscosity increases with a polymer concentration increase at the
entire temperature range, whereas the core viscosity increases only at a low temperature. The corona viscosity increase is
due to the condensation of a large number of polyethylene oxide (PEO) blocks. In contrast, the dehydration degree of
polypropylene oxide (PPO) blocks in the core scarcely changes, and the core has a similar composition regardless of
polymer concentration. The larger polymer concentration promotes a micelle formation at lower temperature where the
fluidity increase is very weak, resulting in larger core viscosity.
1. Introduction
The micellization of triblock copolymers in aqueous media is
driven by the heat-induced dehydration of PPO blocks.3a The
change in polymer structure is shown in Scheme 1. At a tempera-
ture below the critical micellization temperature (Tcm) of the
solution, both PPO and PEO blocks are hydrated and dissolved in
Pluronic polymers are an important class of triblock copoly-
mers consisting of two polyethylene oxide (PEO) blocks and one
polypropylene oxide (PPO) block with a general formula,
PEOx-PPOy-PEOx. These polymers have attracted much atten-
tion because of the phase transition behavior in aqueous media
such as the formation of micelles and gels,1 and broad industrial
applications as detergents, emulsifiers, and lubricants.2 In partic-
ular, micelle formation has attracted a great deal of attention.
Several kinds of micelle structures such as spheres, rods, and
lamellae are produced by a change in the composition and con-
centration of polymers and solution temperature.3 Due to the
variety of micelle structures, these copolymers have been employed
as a template for the synthesis of nanostructured materials.4 In
particular, spherical micelles are capable of encapsulating a variety
of chemicals within the interior; therefore, the application to drug
delivery materials has been studied extensively.5
solution;6 therefore, the polymer exists as an unimer. At >Tcm
,
the dehydration of PPO blocks leads to a solubility decrease and
promotes an aggregation of the blocks, while the PEO blocks
maintain their solubility.7 This results in the formation of micelles
consisting of a “core” dominated by the dehydrated PPO blocks
and a “corona” dominated by the hydrated PEO blocks.3 The
polymer concentration also affects the micelle properties; the
concentration increase leads to a decrease in Tcm and the forma-
tion of large micelles consisting of a larger number of polymers.8
The unimer-to-micelle phase transition and the micelle structure
have been studied by several analytical techniques such as neutron
scattering,3 differential scanning calorimetry,3c Fourier transform
infrared (FT-IR),6 NMR,7 dynamic light scattering,9 absorption
spectrometry,10 gel permeation chromatography,11 and electron
spin resonance (ESR).12
*To whom correspondence should be addressed. E-mail: shiraish@
cheng.es.osaka-u.ac.jp. Fax: þ81-6-6850-6273. Tel: þ81-6-6850-6271.
(1) (a) Chu, B.; Zhou, Z. Physical Chemistry of Polyoxyalkylene Block
Copolymer Surfactants. In Nonionic Surfactants; Nace, V. M., Ed.; Marcel Dekker:
New York, 1996; Vol. 60; pp 67. (b) Alexandridis, P.; Hatton, T. A. Colloids Surf. A
1995, 96, 1–46.
The properties of a local environment of triblock copolymer
micelles have been studied extensively. Several reports clarified
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2738–2744. (c) Su, Y.-l.; Wang, J.; Liu, H.-z. Langmuir 2002, 18, 5370–5374. (d) Guo,
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Langmuir 2010, 26(22), 17505–17512
Published on Web 10/13/2010
DOI: 10.1021/la1028993 17505

Article
Shiraishi et al.
Scheme 1. Structure Change from Unimer to Spherical Micelle for
Pluronic F127 Copolymer
conjugate15 and borondipyrrin (BODIPY) dyes16 have been
proposed, which are called molecular rotors. Upon photoexcita-
tion, these molecular rotors form a twisted intramolecular charge
transfer (TICT) excited state via a torsional motion of the
molecules. These rotors therefore exhibit two competing relaxa-
tion pathways: fluorescence emission and nonradiative deactiva-
tion from the TICT state.17 The formation of the TICT state is
suppressed with an increase in viscosity of the media due to a
suppression of the torsional motion,17 resulting in fluorescence
intensity enhancement. There are, however, no reports of molec-
ular rotors applied for determination of local viscosity of con-
densed matters.
Cyanine (CY) derivatives, such as hemicyanine and merocya-
nine, are one of the molecular rotors that form a TICT excited
state18 and show an increased fluorescence with a viscosity
increase.19 The fluorescence quantum yield (φF) of CY derivatives
depends on the viscosity and temperature of the media, and the
relationship is explained with the Debye-Stokes-Einstein
equation,19a as follows:
that the micelles have a less polar13 and viscous environment.14
Pandit et al.13a reported that the core polarity of a Pluronic F127
micelle is similar to that of ethanol based on a fluorescence
analysis with a pyrene probe. Grant et al.13b reported that the
corona is more polar than the core using coumarin probes.
Caragheorgheopol et al.12a and Ghosh et al.13c clarified that the
corona polarity becomes lower toward the micelle center based on
ESR and fluorescence analysis. Although the local polarity of
micelles has been revealed in detail, there are only a few reports
of local viscosity.14 Nivaggioli et al.14a reported that the core
viscosity is much larger than that of bulk solution by an excimer/
monomer emission intensity ratio of pyrene probes. Fluorescence
anisotropy analysis with rhodamine14b,c and coumarin probes14d
indicated that the corona viscosity is also larger than that of bulk
solution. Dutt et al.14e and Grant et al.14f reported that the core
viscosity is larger than the corona viscosity using dipyrrole and
coumarin probes. Although the local viscosity of micelles has
been outlined in these reports, detailed properties such as specific
viscosities for core and corona regions and their behaviors with
the change in temperature and polymer concentration remain to
be clarified. These data would be useful for better understanding
of local environment of micelles and may lead to a discovery of
new applications of Pluronic micelles.
ꢀ ꢁ
x
φF
η
¼ C ꢀ
ð1Þ
1- φF
T
where η is the solvent viscosity (cP), T is temperature (K), and x
and C are the constants. On the basis of this relationship, the local
viscosity of triblock copolymer micelles is determined using the
fluorescence quantum yields. In addition, the CY derivatives
exhibit a solvatochromic shift with a change in polarity of the
media.20 This therefore also allows a determination of local
polarity around the CY derivatives within the micelles by absorp-
tion analysis.
In the present work, the local viscosity of triblock copolymer
micelle was determined with CY derivatives. A popular Pluronic
F127 copolymer (PEO100-PPO65-PEO100) was employed be-
cause it undergoes a simple phase transition from a unimer to a
spherical micelle (Scheme 1), and other micelle structures such as
rod and lamellar do not form.3a,b We used four kinds of CY
derivatives containing an N,N-dimethylaniline moiety with dif-
ferent alkyl chain lengths (CY1, CY4, and CY8) and a phenol
moiety (CY-OH), as shown in Scheme 2. These derivatives have
different polarities and are located at different positions of the
micelle. This therefore allows the determination of the local
viscosity of different parts of the micelle solution by a simple
fluorescence analysis. We describe here the specific viscosity of the
respective parts and their properties as affected strongly by
temperature and polymer concentration.
One of the possible tools for determination of local viscosity of
micelles is fluorescent molecules showing a viscosity-sensitive
emission. Several molecules such as coumarin-julolidine
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17506 DOI: 10.1021/la1028993
Langmuir 2010, 26(22), 17505–17512

Shiraishi et al.
Scheme 2. Structure of Cyanine (CY) Dyes
Article
2. Experimental Section
2.1. Materials. Pluronic F127 triblock copolymer was sup-
plied from BASF, and other reagents were purchased from Wako,
Aldrich, and Tokyo Kasei. They were used without further
purification. Water was purified by a Milli-Q system. The CY1
dye was synthesized according to a procedure described previ-
ously.20a Other dyes (CY4, CY8, and CY-OH) were synthesized
as follows.
4-(4-Dimethylaminostyryl)-1-butylpyridinium Iodide (CY4).
1-Butyl-4-methylpyridinium iodide (0.23 g, 1.02 mmol), 4-di-
methylaminobenzaldehyde (0.16 g, 1.07 mmol), and piperidine
(0.2 mL) were stirred in EtOH (20 mL) at 80 °C for 6 h. The
solution was cooled to 0 °C, and the precipitate formed was
washed with EtOH, affording a purple fluffy powder of CY4
1
(0.22 g, yield 53%): H NMR (270 MHz, deuterated dimethyl
sulfoxide (DMSO-d6), tetramethylsilane (TMS)): δ (ppm) = 0.93
(t, 3H, CH3), 1.30 (m, 2H, CH2), 1.87 (quintet, 2H, CH2), 3.02 (s,
6H, -N(CH3)2), 4.43 (t, 2H, NþCH2), 6.79 (d, 2H, ArH), 7.18 (d,
1H, -CHd), 7.60(d, 2H, ArH), 7.93(d, 1H, -CHd), 8.07(d, 2H,
ArH), 8.78 (d, 2H, ArH). 13C NMR (67.8 MHz, DMSO-d6,
TMS): δ (ppm) = 153.41, 151.60, 143.10, 141.86, 129.87,
122.25, 122.12, 116.84, 111.69, 58.73, 39.56, 32.29, 18.65, 13.21.
Fast atom bombardment mass spectrometry (FAB-MS): Calcd
for C19H25N2: 281.20. Found: m/z 281.22 (Mþ). 1H, 13C NMR,
and FAB-MS charts are shown in Figures S1-S3 (Supporting
Information).
Figure 1. Temperature-dependent change in (a) absorption and
(b) fluorescence spectra (λexc = 460 nm) of CY8 (30 μM) in water
containing 3 wt % F127.
2.2. Analysis. All spectroscopic measurements were carried
out with a 10 mm path length quarts cell. Fluorescence spectra
were measured on a JASCO FP-6500 fluorescence spectrophot-
ometer equipped with a temperature controller.21 Absorption
spectra were measured on an UV-visible photodiodearray spec-
trophotometer (Shimadzu, Multispec-1500) equipped with a
temperature controller (Shimadzu, S-1700).22 The spectra were
measured after the solution was left for 2 min at the designated
temperature. Fluorescence quantum yields (φF) were determined
with the following equation:20a
4-(4-Dimethylaminostyryl)-1-octylpyridinium Bromide (CY8).
1-Octyl-4-methylpyridinium bromide (0.31 g, 1.08 mmol), 4-di-
methylaminobenzaldehyde (0.16 g, 1.09 mmol), and piperidine
(0.2 mL) were stirred in EtOH (20 mL) at 80 °C for 6 h. The
solution was cooled to 0 °C, and the precipitate formed was
washed with MeCN, affording a purple crystal of CY8 (0.16 g,
yield 35%): 1H NMR (270 MHz, DMSO-d6, TMS): δ (ppm) =
0.86 (t, 3H, CH3), 1.26 (m, 10H, (CH2)5), 1.88 (quintet, 2H, CH2),
3.03 (s, 6H, N(CH3)2), 4.42 (t, 2H, NþCH2), 6.79 (d, 2H, ArH),
6.18 (d, 1H, -CHd), 7.60 (d, 2H, ArH), 7.94 (d, 1H, -CHd),
8.07 (d, 2H, ArH), 8.80 (d, 2H, ArH) 13C NMR (67.8 MHz,
DMSO-d6, TMS): δ (ppm) = 153.40, 151.59, 143.13, 141.85,
129.86, 122.21, 122.11, 116.85, 111.68, 58.94, 39.53, 30.97, 30.32,
28.29, 28.20, 25.30, 21.88, 13.79. FAB-MS: Calcd for C23H33N2:
ꢀ
ꢁꢀ
ꢁ
2
FAR
n
φF ¼ φFR
ð2Þ
FRA nR
where φFR is the fluorescence quantum yield of a reference
compound (anthracene, φFR = 0.297 in EtOH). F and FR are
the integrated values of fluorescence spectra for the samples and
reference, A and AR are the absorbances at the excitation wave-
length, and n and nR are the refractive indices of solvents. 1H and
13C NMR spectra were recorded on a JEOL JNM-GSX270
Excalibur with TMS as the standard. FAB-MS spectra were
obtained by a JEOL JMS 700 mass spectrometer. Viscosity of
the solution was measured with a TOKI SANGYO TVE-22 L
viscometer.
1
337.26. Found: m/z 337.45 (Mþ). H, 13C NMR, and FAB-MS
charts are shown in Figures S4-S6 (Supporting Information).
4-(4-Hydroxystyryl)-1-methylpyridinium Iodide (CY-OH).
4,4-Dimethylpyridinium iodide (0.13 g, 0.53 mmol), 4-hydroxy-
benzaldehyde (0.12 g, 0.98 mmol), and piperidine (0.2 mL) were
stirred in EtOH (20 mL) at 80 °C for 6 h. The resultant was cooled
to 0 °C, and the precipitate formed was washed with EtOH,
affording a purple fluffy powder of CY-OH (0.11 g, yield 60%):
1H NMR (270 MHz, DMSO-d6, TMS): δ (ppm) = 4.14 (s, 3H,
NþCH3), 6.67 (d, 2H, ArH), 7.05 (d, 1H, -CHd), 7.51 (d, 2H,
ArH), 7.86 (d, 1H, -CHd), 7.97 (d, 2H, ArH), 8.61 (d, 2H, ArH)
13C NMR (67.8 MHz, DMSO-d6, TMS): δ (ppm) = 165.93,
152.92, 143.69, 141.91, 130.60, 122.53, 121.41, 117.28, 115.93,
46.01. FAB-MS: Calcd for C14H14NO: 212.3. Found: m/z 212.1
(Mþ). 1H, 13C NMR, and FAB-MS charts are shown in Figures
S7-S9 (Supporting Information).
3. Results and Discussion
3.1. Fluorescence Behavior of the Probes. Figure 1 shows
the temperature-dependent change in absorption and fluores-
cence spectra of CY8 (30 μM) measured in water containing 3 wt %
F127. Distinctive absorption and fluorescence spectra, assigned
to CY dyes, appear at ca. 340-580 nm and 520-680 nm,
respectively.20a As shown in Figure 1b, the fluorescence intensity
is very weak at low temperature, but a rise in temperature leads to
an intensity increase. Figure 2a shows the change in fluorescence
quantum yield (φF) of CY8 with temperature. The φF value is very
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Langmuir 2010, 26(22), 17505–17512
DOI: 10.1021/la1028993 17507

Article
Shiraishi et al.
small at 10-22 °C (ca. 0.001), but increases at >22 °C. The
maximum φF (ca. 0.008) is attained at 29 °C, and the intensity
decreases at higher temperature. As reported,10 the Tcm of a
3 wt % F127 solution is ca. 22 °C, indicating that the φF increase
of CY8 at >22 °C is associated with the micellization of F127
polymer.
fluorescence spectra with a decrease in polarity of the media;20
therefore, the Stokes shift (cm-1) represents the local polarity
around the CY dyes, where more polar media show a larger
Stokes shift. The Stokes shift is defined as follows:23
!
1
1
Stokes shift ðcm- 1Þ ¼ 107
-
ð3Þ
3.2. Polarity Change of Micelle. As reported,13,14 the tri-
block copolymer micelle has a less polar and viscous environment
than the bulk water. It is well-known that the fluorescence of CY
dyes is enhanced with an increase in viscosity19 and a decrease in
polarity of the media.20a,b Both the polarity decrease and viscosity
increase upon micellization are therefore the possible factors
for fluorescence enhancement of CY8 at >Tcm (Figure 2a). The
polarity decrease, however, does not contribute to the fluores-
cence enhancement. This is supported by the polarity of polymer
solution determined by a Stokes shift measurement. The CY dyes
show a red shift of absorption spectra and a blue shift of
λabs
λem
,max
,
max
where λabs,max and λem,max are the maximum wavelengths (nm) of
absorption and fluorescence spectra of CY dyes. Figure 3a shows
the Stokes shift of CY8 measured in a 3 wt % F127 solution. At
<22 °C, the Stokes shift is almost constant (ca. 4600 cm-1);
however, the shift decreases at >22 °C and becomes constant (ca.
3500 cm-1) at >26 °C. The decrease in Stokes shift at >22 °C is
consistent with the Tcm of a 3 wt % F127 solution,10 indicating
that the local polarity around CY8 indeed decreases upon
micellization. It must be noted that the micelle concentration in
a 3 wt % F127 solution is ca. 140 μM,8 which is much larger than
the CY8 concentration (30 μM) in the solution. In addition, a
distinctive absorption band, assigned to an H-aggregate of CY
dyes, does not appear at 350-400 nm (Figure 1a).24 These suggest
that all of the CY8 molecules are highly dispersed and contained
in the micelles, and the Stokes shift values critically represent the
local polarity around the CY8 molecule.
To determine the absolute polarity around CY8 within the
micelle, absorption and fluorescence spectra of CY8 were mea-
sured in various solvents with different polarity. Table 1 shows the
λmax data for the spectra. Figure 4 (open circle) shows the
relationship between the Stokes shift of CY8 and the Reichardt’s
solvent polarity parameter (ETN).25 A linear relationship clearly
indicates that the Stokes shift of CY8 strongly depends on the
polarity of the media. As shown in Figure 3a, the Stokes shift of
Figure 2. Temperature-dependent change in φF of (a) CY8,
(b) CY4, (c) CY1, and (d) CY-OH in 3 wt % F127 solution. The
fluorescence spectra for samples b-d are shown in Figures S10
(Supporting Information).
CY8 measured in polymer solution at <22 °C is ca. 4600 cm-1
,
;
which is similar to that obtained in pure water (5097 cm-1
Table 1). This indicates that, at <22 °C, the CY8 molecule exists
in bulk water. In contrast, the Stokes shift decreases at >22 °C
and is saturated to ca. 3500 cm-1, which is similar to the value
obtained in ethanol (3394 cm-1; ETN = 0.65; Table 1). Figure 5
(open circle) shows the relationship between ETN and φF for CY8
measured in various solvents. The φF values of CY8 in solvents
N
with ET > 0.65 are very small (<0.001), but are larger in
solvents with ETN < 0.60. This indicates that φF increases with a
decrease in polarity of the media.20a,b However, the Stokes shift of
CY8 in the polymer solution (Figure 3a) is similar to that
N
measured in ethanol (ET = 0.65). As shown in Figure 5, the
φF of CY8 in ethanol is only 0.001, whereas the φF of CY8 in the
polymer solution is much larger (0.008; Figure 2a). The result
clearly indicates that the decreased polarity within the micelle is
not the major factor for fluorescence enhancement, leaving the
increased viscosity within the micelle as the major factor.
Figure 3. Temperature-dependent change in Stokes shift of (a)
CY8, (b) CY4, (c) CY1, and (d)CY-OHina 3wt% F127solution.
The absorption and fluorescence spectra for the samples b-d are
shown in Figure S10 (Supporting Information).
Table 1. Solvent Parameters and Spectral Data for CY8 in Various Solvents at 25 °Ca
Nc
entry
solvent
water
methanol
ethanol
1-propanol
1-butanol
1-hexanol
2-propanol
1-octanol
nb
ET
λabs,maxd/nm
λem,maxe/nm
Stokes shift/cm-1
φF
a
b
c
d
e
f
g
h
1.333
1.328
1.361
1.386
1.399
1.416
1.377
1.429
1.000
0.762
0.654
0.617
0.586
0.559
0.546
0.537
453
481
486
489
490
489
486
480
589
588
582
579
578
576
577
568
5097
3783
3394
3179
3107
3089
3245
3228
0.0002
0.0004
0.0011
0.0024
0.0039
0.0086
0.0024
0.0146
a The absorption and fluorescence spectra for CY8 measured in respective solvents are summarized in Figure S11 (Supporting Information). b Solvent
refractive index from refs 20h and 20i. c The Reichardt’s solvent polarity parameter from ref 25. d Maximum absorption wavelength. e Maximum
fluorescence wavelength.
17508 DOI: 10.1021/la1028993
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Scheme 3. Possible Locations of CY Probes around the F127
Polymer Micelle
Figure 4. The relationship between the Reichardt’s solvent polar-
ity parameter (ET )
N 25 and the Stokes shift for CY8, CY4, CY1, and
CY-OH measured in various solvents (a-h) at 25 °C. The
solvents, a-h, correspond to those in Table 1. The absorption
and fluorescence spectra of the probes are summarized in Figure
S11 (Supporting Information). The detailed Stokes shift of CY4,
CY1, and CY-OH are summarized in Table S1 (Supporting
Information).
smaller. As shown in Figure 4, the Stokes shift of CY1 and CY4
measured in various solvents shows an ETN-dependence as does
CY8. The results indicate that these probes are located at a more
polar environment than CY8. The corona of micelle has a higher
polarity than the core because the PEO blocks are hydrated even
at >Tcm.
7a,13b This suggests that CY1 and CY4 are located at the
corona. It is noted that the Stokes shift of CY4 is lower than that
of CY1 (Figure 3). As reported,12a,13c the corona polarity
decreases toward the micelle center. This suggests that the less
polar CY4 containing a longer alkyl chain is located at the inner
corona, while the more polar CY1 is at the peripheral (Scheme 3).
In contrast, as shown in Figure 3d, the Stokes shift of CY-OH is
almost constant (ca. 7100 cm-1) within the entire temperature
range. As shown in Figure 4, the Stokes shift of CY-OH measured
in various solvents also shows an ETN-dependence. The Stokes
shift of CY-OH in pure water is 7200 cm-1, which is similar to
the value in the polymer solution. This indicates that CY-OH
with a polar -OH group is located in bulk water (Scheme 3).
3.4. Calibration of Local Viscosity. The above results
indicate that the CY probes are located at the different positions
of polymer solution. As shown in Figure 5, the CY1, CY4, and
CY8 probes do not show fluorescence enhancement in solvents
N
Figure 5. The relationship between ET and φF of CY8, CY4,
CY1, and CY-OH measured in various solvents (a-h) at 25 °C.
The solvents, a-h, correspond to those in Table 1. The absorption
and fluorescence spectra are shown in Figure S11 (Supporting
Information). The detailed φF for CY4, CY1, and CY-OH are
summarized in Table S1 (Supporting Information).
3.3. Location of the Probes in a Micelle. As shown in
Scheme 1, the F127 micelle consists of a core dominated by
dehydrated PPO blocks and a corona dominated by hydrated
PEO blocks.3b,c As reported,13a the core polarity of an F127
micelle is similar to that of ethanol. As shown in Figure 3a, the
Stokes shift of CY8 in the polymer solution (ca. 3500 cm-1) is
similar to that obtained in ethanol (3394 cm-1). This indicates
that, as shown in Scheme 3, the CY8 probe is located at the core of
micelle. The fluorescence behaviors of other CY probes (CY4,
CY1, and CY-OH) in a 3 wt % F127 solution were studied. As
shown in Figure 2, these probes show fluorescence behaviors
different from that of CY8. CY4 and CY1 (Figure 2b,c) show
fluorescence enhancement similar to CY8 at >Tcm; however, the
increase is much smaller. In the case of CY-OH (Figure 2d),
fluorescence enhancement is not observed at the entire tempera-
ture range.
N
with ET > 0.65, which is similar to the polarity of polymer
solution. In addition, the φF of CY-OH is independent of solvent
polarity. These suggest that the φF of these CY probes in the
polymer solution depends solely on the viscosity of the environ-
ments; in other words, φF represents the local viscosity of different
positions of polymer solution. As reported,19a the φF of CY dyes is
correlated with viscosity and temperature of the media based on
the Debye-Stokes-Einstein equation (eq 1). The local viscosity
of micelle can therefore be determined with the φF of the re-
spective CY probes in polymer solution (Figure 2). To clarify the
relationship between φF, viscosity, and temperature, absorption
and fluorescence spectra of the probes were measured in glycerol/
MeOH mixtures with different compositions (10/90, 40/60, 60/40,
70/30, 95/5 wt/wt) at different temperatures (5, 15, 25, 35, and
45 °C) as the standard solutions. It is noted that, as shown in
Table S2 (Supporting Information), the Stokes shift of the probes
The different fluorescence behavior is because these probes are
located at different positions of the solution (Scheme 3). As shown
in Figure 3, the Stokes shift of CY1 and CY4 in the polymer
solution decreases at >Tcm as does CY8, but the decrease is much
(23) Bagchi, B.; Oxtoby, D. W.; Fleming, G. R. Chem. Phys. 1984, 86, 257–267.
(24) Huang, Y.; Cheng, T.; Li, F.; Luo, C.; Huang, C.-H.; Cai, Z.; Zeng, X.;
Zhou, J. J. Phys. Chem. B 2002, 106, 10031–10040.
(25) Reichardt, C. Chem. Rev. 1994, 94, 2319–2358.
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Shiraishi et al.
Figure 6. Plots of ln[φF/(1 - φF)] against ln(η/T) for (a) CY8, (b) CY4, (c) CY1, and (d) CY-OH in glycerol/MeOH mixtures with different
compositions (10/90, 40/60, 60/40, 70/30, and 95/5 wt/wt) measured at different temperatures (5, 15, 25, 35, and 45 °C). The detailed data are
summarized in Table S2 (Supporting Information).
Scheme 4. Change in Structure and Properties of an F127 Micelle
with Polymer Concentrationa
Figure 7. Temperature-dependent change in the local viscosity of a
micelle determined with (a) CY8,(b) CY4,(c) CY1,and(d) CY-OH
in a 3 wt % F127 solution. The data (e) denote the macroscopic
viscosity of polymer solution measured with a viscometer.
N
in these mixtures is similar to that obtained in ethanol (ET
>
0.65), suggesting that the polarity effect on φF is negligible. Figure 6
shows the plots of ln(η/T) with ln[φF/(1 - φF)] for the respective
probes in the mixtures. A linear relationship for all probes
indicates that the viscosity around the respective probes is clearly
explained with φF and temperature. The constants C and x for
eq 1 are therefore determined, as shown in the inset.
3.5. Determination of Local Viscosity of a Micelle. On the
basis of the calibration data (Figure 6), the temperature-φF
profiles for the respective probes obtained in a 3 wt % F127
solution (Figure 2) can be converted to a temperature-viscosity
profile (Figure 7a-d). The local viscosities determined with all
probes at <Tcm are less than 2 cP, which is similar to the macro-
scopic viscosity of polymer solution determined with a viscometer
(Figure 7e). This suggests that the unimer solution does not
contain a viscous environment.14e,f The local viscosity determined
a The values of Nagg and core size are from ref 8.
analysis to be 90 cP at 20 °C, 60 cP at 30 °C, and 30 cP at 45 °C,
respectively. These viscosities are roughly similar to that of an F127
micelle core (Figure 7a). Grant et al.14f specified the core viscosity of
a Pluronic F88 micelle (PEO109-PPO41-PEO109), which contains a
number of PPO blocks smaller than F127. The core viscosity of
5 wt % F88 solution (Tcm = 34 °C) is determined by a fluorescence
anisotropy analysis to be 20 cP at 38 °C, 13 cP at 50 °C, and 8 cP at
60 °C, respectively; these values are smaller than that of the F127
micelle core, as expected. These viscosity data for related systems
support the accuracy of local viscosity determined by the present
system. It is noted that, as shown in Figure 7a, the core viscosity
determined with CY8 decreases at >30 °C. This is because a rise in
temperature leads to an increase in fluidity of the core due to the
heat-induced mobility enhancement of the polymer chains and water
molecules. Similar core viscosity decrease at higher temperature is
observed in the above P123 and F88 systems.14e,f These indicate that
the core viscosity increases at >Tcm and attains a maximum at a
temperature 7-9 °C higher than Tcm, and then decreases at a higher
temperature due to the heat-induced fluidity increase.
with the CY8, CY4, and CY1 probes, however, increases at >Tcm
,
indicating that the micelle viscosity is indeed larger than the bulk
solution. The viscosity attains a maximum at around 29 °C and
decreases at higher temperature, resulting in an off-on-off
profile against the temperature window. The core viscosity of
the micelle determined with CY8 shows a highest value; the
viscosity at 29 °C is determined to be 62 ( 7 cP.
So far, only two reports have specified the local viscosity of the
micelle core for Pluronic polymers.14e,f As reported,14a the core
viscosity depends on the number of PPO blocks, while that of
PEO blocks scarcely affects the core viscosity. Dutt et al.14e
specified the core viscosity of a P123 micelle (PEO20-PPO70-
PEO20), which contains a number of PPO blocks similar to F127
(PEO100-PPO65-PEO100). The core viscosity of a 1 wt % P123
solution (Tcm = 16 °C) is determined by fluorescence anisotropy
17510 DOI: 10.1021/la1028993
Langmuir 2010, 26(22), 17505–17512

Shiraishi et al.
Article
Figure 8. Temperature-dependent changein (top) Stokes shift and (bottom) local viscosity determined with (a) CY8, (b) CY4, and (c) CY1 in
water with different F127 concentrations (1, 3, 5, and 10 wt %). The absorption and fluorescence spectra for probes are shown in Figures
S12-14 (Supporting Information).
The corona viscosity has been studied by only a few re-
ports.13c,14b-14f They report that the corona viscosity is lower
than that of the core; however, the specific viscosity is not
determined. Figure 7b,c shows the temperature-viscosity profiles
of the corona determined by the CY4 and CY1 probes. The
profiles are similar to that of the core (Figure 7a); the viscosity
increases at >Tcm and attains a maximum at a temperature
7-9 °C higher than Tcm, and then decreases at higher temperature.
The viscosity values are, however, much smaller than the core; the
viscosity at 30 °C is 15 cP (CY4) and 10 cP (CY1), respectively.
The result is consistent with previous reports,13c,14e,14f indicating
that the corona is indeed less viscous than the core. Note that the
viscosity of the inner corona determined with CY4 is larger than
that of the periphery determined with CY1. The PEO chains in the
corona become denser toward the micelle center.12a The rota-
tional motion of CY4 located at the inner corona is suppressed
more significantly due to the condensed PEO chains, thus show-
ing higher viscosity. As shown in Figure 7d, the viscosity of bulk
solution determined with CY-OH scarcely increases at the entire
temperature range, suggesting that the micelle formation does not
affect the viscosity of bulk solution.14f
3.6. Effect of Polymer Concentration on Core Viscosity.
The polymer concentration in solution strongly affects the prop-
erties of micelle. The increase in polymer concentration leads to a
decrease in Tcm and a formation of large micelles consisting of
a larger number of polymers (aggregation number: Nagg).26
Ruthstein et al.8 reported that, as shown in Scheme 4, a 3 wt %
F127 solution produces a micelle of Nagg = 13-17 with a 5 nm core
diameter, while a >7.5 wt % F127 solution produces a micelle of
Nagg >59 with a >9 nm core diameter. Some reports have described
the effects of polymer concentration on the local polarity of
micelles;13b,d however, the effects on local viscosity remain unclear.
The effect of polymer concentration on the local environment
of micelle core was studied. Figure 8a (top) shows the change in
Stokes shift of CY8 in water with different F127 concentrations
(1, 3, 5, and 10 wt %). At low temperature where the polymer
exists as a unimer, the Stokes shift decreases with an increase in
polymer concentration, because the solution becomes less polar
even without the micelle formation.7a A rise in temperature leads
to a decrease in the shift for all solutions, where the decrease starts
at lower temperature with a polymer concentration increase: 18
°C (10 wt %), 20 °C (5 wt %), 22 °C (3 wt %), and 24 °C (1 wt %),
respectively. These temperatures are consistent with the Tcm of
F127 solution.10,13e However, at higher temperature (>32 °C),
the Stokes shift for all solutions becomes similar (3500 cm-1).
Nivaggioli et al.13d reported that the polarity of a micelle core is
not affected by the polymer concentration. The similar Stokes
shift indicates that the core polarity is indeed independent of
polymer concentration (Scheme 4). Figure 8a (bottom) shows the
core viscosity determined with CY8. The viscosity for all solutions
increases at >Tcm and attains a maximum at a temperature 7-9
°C higher than Tcm. The maximum viscosity increases with an
increase in polymer concentration, indicating that the core of the
micelle consisting of larger aggregation number is more viscous.
The core viscosity for all solutions, however, decreases at higher
temperature and becomes similar at >32 °C.
The viscosity of a micelle core depends on the dehydration
degree of PPO blocks;14a therefore, the ratio of PPO units and
water molecules is the crucial factor for viscosity. To clarify the
effect of polymer concentration on the PPO/water ratio within the
core, the viscosity of a mixture of PPO (average molecular weight,
400) and water was measured with the CY8 probe. Figure 9 (lines)
shows the viscosity of the mixtures with different compositions. It
is noted that the Stokes shift of CY8 in the mixtures is almost
constant (ca. 3500 cm-1) within the entire temperature range,
which is similar to that of the F127 micelle core (Figure 8a (top)),
indicating that the polarity effect on the viscosity is negligible. The
mixtures with larger PPO amount show higher viscosity, and all
solutions undergo a viscosity decrease with a rise in temperature
due to the fluidity increase.27 Figure 9 (symbols) shows the core
viscosity of F127 solutions with different concentrations at a
(26) Lam, Y.-M.; Grigorieff, N.; Goldbeck-Wood, G. Phys. Chem. Chem. Phys.
1999, 1, 3331–3334.
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Shiraishi et al.
higher temperature. Within the entire temperature range, the
larger polymer concentration solution shows larger corona vis-
cosity. This is because the PEO blocks become condensed with a
polymer concentration increase (Scheme 4).7a,12b,12c As shown in
Figure 8b,c (bottom), the coronaviscosity determined with CY4 is
higher than that with CY1 within the entire polymer concentra-
tion range, suggesting that the inner corona of the micelle is more
viscous than the periphery due to the condensed PEO chains.
4. Conclusion
The CY dyes were used as fluorescent probes for determination
of local viscosity of Pluronic F127 micelles. These probes show
very weak fluorescence at low temperature, but show an enhanced
fluorescence at >Tcm due to the increased viscosity within the
micelle. The respective CY probes are located at the different
positions of micelle. This thus allows the determination of local
viscosity of different positions of micelle with the fluorescence
quantum yields of the probes. The local viscosity of both the core
and corona regions of the micelle shows off-on-off profiles
against the temperature window: the viscosity increases at >Tcm
Figure 9. Temperature-dependent change in viscosity of (lines)
the mixtures of PPO (average molecular weight, 400) and water
with different compositions (60/40, 65/35, 70/30, and 80/20 wt/wt)
and (symbols) F127 solutions with different concentrations, deter-
mined with CY8. The absorption and fluorescence spectra of CY8
in the mixtures are shown in Figure S15 (Supporting Information).
temperature above the maximum viscosity (Figure 8a (bottom)).
The core viscosities of F127 solutions are sandwiched in between
the viscosity profiles for the 65/35 and 70/30 PPO/water mixtures.
This suggests that the PPO/water ratios within the core of F127
solutions are almostconstant regardless of polymer concentration
(Scheme 4). The similar decrease in core viscosity for F127
solutions at >32 °C (Figure 8a (bottom)) is because the core com-
positions are similar and the viscosity depends just on the fluidity
induced by temperature. In contrast, the larger core viscosity for
F127 solutions with larger concentration (Figure 8a (bottom)) is
because the micelle is produced at lower temperature where the
fluidity effect is weaker.
3.7. Effect of Polymer Concentration on Corona Viscos-
ity. Change in polymer concentration also affects the properties
of corona (Scheme 4): the corona formed at larger polymer
concentrations contain concentrated PEO chains.7a,12b,12c The
ratio of PEO units to water molecules therefore increases and
leads to a polarity decrease.7a,13b Figure 8b,c (top) shows the
Stokes shift of CY4 and CY1 in water containing different F127
and attains a maximum at a temperature 7-9 °C higher than Tcm
,
and then decreases at a higher temperature due to the heat-
induced fluidity increase. The core viscosity is larger than that of
the corona, where the corona viscosity increases toward the
micelle center.
The polymer concentration in solution has different effects on
the core and corona viscosity. The concentration increase leads to
a condensation of PEO blocksinthe coronaregion, resultingin an
increase in corona viscosity at the entire temperature range. In
contrast, the polymer concentration does not affect the dehydra-
tion degree of the PPO blocks in the core region, and, hence, the
core viscosity depends on the fluidity induced by temperature. At
larger polymer concentration, the micelle core forms at a lower
temperature where the fluidity effect is weaker. This results in
larger core viscosity for larger polymer concentration solution at
low temperature. A rise in temperature leads to an increase in
the fluidity of the core, thus resulting in similar core viscosity
regardless of the polymer concentration.
concentrations. The Stokes shift for all solutions decreases at >Tcm
,
but the decreases at higher temperature (>32 °C) are different
from each other. The larger polymer concentration solution
shows a lower Stokes shift, suggesting that the corona polarity
indeed decreases with a polymer concentration increase.7a,13b
Figure 8b,c (bottom) shows the corona viscosity determined with
CY4 and CY1. They show a similar off-on-off viscosity profile:
the viscosity increases at >Tcm and attains a maximum at a
temperature 7-9 °C higher than Tcm, and then decreases at a
Acknowledgment. This work was supported by a Grant-in-
Aid for Scientific Research (No. 21760619) from the Ministry of
Education, Culture, Sports, Science and Technology, Japan
(MEXT). We thank the Division of Chemical Engineering for
the Lend-Lease Laboratory System.
Supporting Information Available: Supplementary data
(Figures S1-S15 and Tables S1 and S2). This material is
available free of charge via the Internet at http://pubs.acs.org.
(27) Williams, M. L.; Landel, R. F.; Ferry, J. D. J. Am. Chem. Soc. 1955, 77,
3701–3707.
17512 DOI: 10.1021/la1028993
Langmuir 2010, 26(22), 17505–17512