Multifunctional photo- and thermo-responsive copolymer nanoparticles

Article abstract of DOI:10.1016/j.dyepig.2010.03.013

A novel amphiphilic photochromic copolymer P1-o consisting of a hydrophilic poly(N-isopropylacrylamide) backbone and thermally irreversible photochromic and hydrophobic 1,2-dithienylethene derivative pendants was designed, synthesized and used to fabricat

Full text of DOI:10.1016/j.dyepig.2010.03.013

Dyes and Pigments 89 (2011) 230e235
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Multifunctional photo- and thermo-responsive copolymer nanoparticles
*
Seon-Jeong Lim, Carl-Johan Carling, C. Chad Warford, Dennis Hsiao, Byron D. Gates, Neil R. Branda
4D LABS, Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia V5A 1S6, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel amphiphilic photochromic copolymer P1-o consisting of a hydrophilic poly(N-isopropylacrylamide)
backbone and thermally irreversible photochromic and hydrophobic 1,2-dithienylethene derivative pendants
was designed, synthesized and used to fabricate self-assembled copolymer nanoparticles in an aqueous
condition. The hydrodynamic diameters of the multifunctional copolymer P1-o nanoparticles reversibly
switched between two stable states with two stimuli: light and temperature in an aqueous suspension.
Graphical abstract:
Received 4 March 2010
Received in revised form
11 March 2010
Accepted 15 March 2010
Available online 21 March 2010
Keywords:
Amphiphiles
Copolymerization
Nanoparticles
Photochemistry
Self-assembly
Stimuli-sensitive polymers
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
stimulus due to the fact that its wavelength, intensity and focus can
be tuned to provide both spatial and temporal control.
Polymeric spherical nanostructures such as nanomicelles and
nanovesicles made from fully synthetic amphiphilic copolymers are
being regarded as one of the most attractive candidates for applica-
tions including encapsulation, drug delivery, nanoreactors and
templates for nanostructured materials [1e8]. Although copolymeric
nanostructures self-assembled in numerous aqueous conditions have
structures similar to natural phospholipid micelles and liposomes,
they can be optimized to possess enhanced stability and include
a more varied functionality than the phospholipid nanostructures
thanks to the structural and synthetic diversity available for organic
materials. When the amphiphilic copolymers or the soft surfaces
of the copolymer nanostructures are decorated with responsive
components, they offer an additional appealing feature: their prop-
erties can be precisely controlled by external stimuli such as light,
temperature, pH, and pressure [5e10]. Light is the most attractive
Photochromic molecules are among the most useful molecular
elements that respond to light by undergoing reversible photore-
actions generating two isomers having different molecular shapes,
and optoelectronic properties such as the absorption, emission and
refraction of light [11]. Among the various families of photochromic
compounds, 1,2-dithienylethene derivatives (DTEs) are the most
versatile because their reversible ring-closing reactions (Scheme 1)
tend not to be prone to photo-fatigue and do not proceed at ambient
temperatures in the dark [12]. Because DTEs not only have very
high thermal barriers to isomerization in the absence of light, but
also show predictable conformational shape and rigidity changes
between ring-open and ring-closed photoisomers, it is expected
that nanostructures made from them will also exhibit predictable
changes in conformation, shape, and rigidity as a consequence of
their photochromism [13e15]. Because the photoisomerization
tends not to be induced thermally, the good thermal stability of both
photoisomers of DTEs is a particularly attractive feature when using
these photoresponsive elements in multi-component, dual-mode
(more than one stimulus) systems in which light and temperature
* Corresponding author. Fax: þ1 778 782 3765.
E-mail address: nbranda@sfu.ca (N.R. Branda).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.03.013

S.-J. Lim et al. / Dyes and Pigments 89 (2011) 230e235
231
nanoparticles undergoing the photochromic reactions were taken on
a Sony Handycam (DCR-HC30) digital video camera.
F F
F F
F
F
F
F
UV
F
F
F
F
2.2. Materials
visible
R
R
All the chemicals were purchased from commercial suppliers
and used as received unless otherwise specified. Tetrahydrofuran
(THF) used as reaction solvent was dried over sodium metal.
N-isopropylacrylamide (NIPA, Aldrich Chemical Co., 97%) and
1,10-azobis(cyclohexane-1-carbonitrile) (VAZO 88, DuPont, >98%)
were recrystallized from ethyl acetate/hexanes (1/4) and ethanol
solutions three times, respectively. (4-Bromo-5-methylthiophen-2-yl)
trimethylsilane (2) and 2-methyl-3-(perfluorocyclopent-1-enyl)-5-
phenylthiophene (3) were synthesized according to the literature
procedures without modification [20,21].
S
S
S
S
R
R
ring-open
colourless
ring-closed
coloured
Scheme 1. Reversible photochemical reactions of dithienylethenes.
are used as independent triggers. Only with a thermally insensitive
photoactive component can temperature be used to induce
property changes independent of the function of the photoswitch.
An illustrative example is described in this communication using
a hybrid DTE-thermal polymer.
2.3. Synthesis of (4-(3,3,4,4,5,5-hexafluoro-2-(2-methyl-5-
phenylthiophen-3-yl)cyclopent-1-enyl)-5-methyl-
thiophen-2-yl)trimethylsilane (4)
One of the best behaved temperature-responsive polymers is
poly(N-isopropylacrylamide) (PNIPA) [16,17]. PNIPA self-assembles
in aqueous solutions at temperatures higher than the lower critical
solution temperature (LCST), which is typically around 33 ꢀC.
This characteristic behaviour of PNIPA can be harnessed as a ‘smart’
component in various thermal responsive nanostructures. Although
there exist some examples of PNIPA polymers incorporating
photochromic compounds such as azobenzenes and spirooxazines,
they suffer from some limitations. All were investigated for their
color changes by studying them in bulk hydrogels and not nano-
structures [18,19]. They also contain photochromic components
that spontaneously revert to their original photoisomer in the dark,
preventing the demonstration of specific bistable dual-mode (light
and temperature) response.
In this report, we introduce a novel amphiphilic photochromic
copolymer P1-o that is composed of a hydrophilic PNIPA backbone
decorated with thermally irreversible photochromic and hydro-
phobic DTE components. We demonstrate that in aqueous condi-
tions, these novel, multifunctional copolymers self-assemble into
nanoparticles capable of reversibly responding to two external
stimuli (light and temperature). To the best of our knowledge, these
copolymeric nanoparticles are the first example of multifunctional
photo- and thermo-responsive (dual-mode) structures of this type.
A solution of thiophene 2 (1.05 g, 4.21 mmol) in anhydrous Et2O
(17 mL) in a flame-dried round-bottom flask was treated with
n-BuLi (2.5 M in hexanes, 1.68 mL, 4.21 mmol) drop-wise over
10 min at ꢁ78 ꢀC under N2. The resulting solution was stirred for
additional 30 min, at which time it was treated with a solution of 3
(1.03 g, 2.81 mmol) in Et2O (3 mL) slowly. The resulting solution
was stirred for 1 h at ꢁ78 ꢀC, the cooling bath was removed and
the reaction stirred for additional 2 h at ambient temperature.
The reaction was quenched with NH4Cl, the layers were separated
and the aqueous layer was extracted with EtOAc (3 ꢂ 20 mL).
The combined organic layers were dried (MgSO4), filtered and
concentrated under reduced pressure. The resulting dark orange oil
was purified by column chromatography (SiO2/hexanes) to yield
286 mg of a mixture of the title compound and starting material (3)
as a colorless waxy solid (8:2, 4:3) that becomes blue/purple when
exposed to UV light. This product mixture was used for subsequent
reactions without further purification (yield after correcting for
3 ¼ 286 mg; 246 mg of pure product 4,17%). Mp ¼ 39e42 ꢀC.1H NMR
(CD2Cl2, 600 MHz): d 7.58e7.57 (m, 0.5H, compound 3), 7.55e7.54 (m,
2H, compound 4), 7.41e7.37 (m, 3H, compound 4), 7.32e7.29 (m,
1.5H, compound 3), 7.26 (s, 1H, compound 4), 7.18 (s, 1H, compound
4), 7.09 (s, 0.25H, compound 3), 2.37 (s,1H, compound 3), 2.00 (s, 3H,
compound 4), 1.89 (s, 3H, compound 4), 0.30 (s, 9H, compound 4).
2. Experimental
2.1. Instrumentation
2.4. Synthesis of 5-bromo-3-(3,3,4,4,5,5-hexafluoro-2-(2-methyl-5-
phenylthiophen-3-yl)cyclopent-1-enyl)-2-methylthiophene (5)
1H NMR and 13C NMR spectra were recorded on a Bruker AMX 400
spectrometer and a Bruker Avance 600 spectrometer. Mass spectra
were measured on a Varian 4000 GC/MS by EI mode and an Agilent
6210 TOF LC/MS by ESI positive mode. Gel permeation chromatog-
raphy (GPC) analysis was carried out on a Waters 510 equipped with
A suspension of 4 (246 mg, 0.475 mmol) in MeCN (3.5 mL) in a vial
was treated with NBS (84 mg, 0.475 mmol) in one portion. The reaction
mixture was stirred at ambient temperature for 18 h, after which more
NBS (28 mg, 0.15 mmol) was added. After stirring for an additional 4 h,
the reaction mixture was concentrated onto silica and purified by
column chromatography (SiO2/hexanes) to yield 205 mg (82%) of 5 as
a colorless waxy solid that turns purple upon exposure to UV light.
m-Styragel columns by using THF and polystyrenes as the solvent and
the standard polymers, respectively. Differential scanning calorim-
etry (DSC) was performed on a PerkineElmer DSC 7 under nitrogen
atmosphere at a heating rate of 10 ꢀC/min and UVeVis absorption
spectra were acquired on a Varian Cary 300 Bio spectrophotometer.
The average hydrodynamic diameters and zeta potentials of P1-o
nanoparticles were measured on a Nano-ZS (Malvern Instruments,
Inc.) and transmission electron microscope (TEM) images were
acquired on an H-8000 (200 keV, Hitachi) TEM by dropping the
aqueous suspensions of P1-o nanoparticles on copper grids.
A 313 nm UV hand-held lamp (5.5 mW cmꢁ2) and visible light
(>500 nm) isolated from halogen white lamp (300 W) through
a color filter (Newport, FS-C) were used as the excitation light sources
for the photochromic ring-closing and the ring-opening reactions,
respectively. Photographs of an aqueous suspension of P1-o
Mp ¼ 81e84 ꢀC. 1H NMR (CD2Cl2, 600 MHz):
7.55 (td, J ¼ 8.4, 1.5 Hz,
d
2H), 7.39 (t, J ¼ 8.4 Hz, 2H), 7.32e7.29 (m, 1H), 7.27 (s, 1H), 7.09 (s, 1H),
1.97 (s, 3H), 1.90 (s, 3H). 13C NMR (CD2Cl2, 600 MHz):
d 144.36, 143.06,
142.23, 133.76, 129.91, 129.55, 128.55, 128.38, 126.10, 126.06, 125.95,
125.91, 122.78, 110.37, 14.91, 14.80.
2.5. Synthesis of 4-(4-(3,3,4,4,5,5-hexafluoro-2-
(2-methyl-5-phenylthiophen-3-yl)cyclopent-
1-enyl)-5-methylthiophen-2-yl)aniline (6)
A solution of bromide 5 (195 mg, 0.372 mmol) in toluene (5 mL)
was treated with an aqueous solution of Na2CO3 (2 M, 0.93 mL,

232
S.-J. Lim et al. / Dyes and Pigments 89 (2011) 230e235
1.86 mmol), EtOH (1 mL) and 4-(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolan-2-yl)aniline (98 mg, 0.446 mmol). The reaction mixture was
deoxygenated by bubbling dry nitrogen through it for 20 min.
A condenser was fitted to the flask and the system was purged. Solid
Pd(PPh3)4 (22 mg, 0.019 mmol) was added in one portion and the
reaction was heated at reflux for 21 h. The reaction mixture was then
diluted with water and EtOAc, the layers were separated and the
aqueous layer was extracted with EtOAc (3 ꢂ 20 mL). The combined
organic layers were dried (MgSO4), filtered and concentrated under
reduced pressure to a dark brown oil. Purification by column chro-
matography (silica gel, 1:1 hexanes:CH2Cl2, 1% Et3N) yielded 168 mg
(84%) of 6 as a light yellow oil that becomes blue after exposure to
41.49, 35.76, 29.25, 27.08, 22.80,14.74,14.68,11.61. Mn ¼ 5097 g molꢁ1
,
Mw ¼ 5957 g molꢁ1 (PDI ¼ 1.17).
2.8. Synthesis of P1-o nanoparticles
A solution of copolymer P1-o (0.5 mg) in THF (1 mL) was placed
in a 50 mL round-bottom flask and irradiated with 313 nm light until
the photostationary state (PSS) was reached. The dark blue solution
was quickly treated with distilled water (25 mL) while the flask was
continuously irradiated with 313 nm light. A portion of the mixed
solvent (5e10 mL) was removed using a rotary evaporator at 50 ꢀC in
the dark. The mixture was then diluted with distilled water until
a final volume of 25 mL was reached. This step was also carried out
while irradiating the mixture with 313 nm light. The final aqueous
suspension was stirred vigorously for 5 min with a magnetic stir bar
in the dark. 1H NMR spectroscopy (acetone-d6, 400 MHz, 25 ꢀC) of
the final P1-o nanoparticles suspension confirmed that no residual
THF remained in the aqueous suspension as no peaks corresponding
to the protons of the THF solvent were observed in the spectrum.
The ring-open form of the P1-o nanoparticles were fabricated using
the same methods described above except that the PSS was not
generated and all synthetic procedures were performed under
visible light (wavelengths greater than 500 nm).
UV light. 1H NMR (CD2Cl2, 600 MHz):
d 7.60e7.58 (m, 2H), 7.40 (t,
J ¼ 7.8 Hz, 2H), 7.38e7.36 (m, 3H), 7.32 (t, J ¼ 7.8 Hz, 1H), 7.17 (s, 1H),
6.70e6.78 (m, 2H), 3.86 (s, 2H), 2.01 (s, 3H), 1.97 (s, 3H). 13C NMR
(CD2Cl2, 600 MHz):
d 171.23, 147.29, 143.45, 142.58, 142.03, 140.10,
133.73, 129.36, 128.26, 127.29, 127.10, 126.22, 125.90, 125.85, 123.88,
122.85, 120.51, 116.74, 115.03, 113.41, 111,61, 109.82, 60.63, 21.14,
14.72. HRMS (ESIþ) Calculated for C27H20NF6S2 [M þ Hþ]: 536.0941.
Found: 536.0944.
2.6. Synthesis of N-(4-(4-(3,3,4,4,5,5-hexafluoro-2-
(2-methyl-5-phenylthiophen-3-yl)cyclopent-1-enyl)-
5-methylthiophen-2-yl)phenyl)acrylamide (1-o)
2.9. TEM sample preparations of P1-o nanoparticles
A solution of amine 6 (160 mg, 0.299 mmol) in THF (2 mL)
and Et3N (46 ml, 0.329 mmol) in a vial was treated drop-wise over
10 min with a solution of acryloyl chloride (27 ml, 0.329 mmol) in
THF (0.75 mL) at 0 ꢀC. After the addition was complete, the ice bath
was removed and the reaction mixture was allowed to stir at
ambient temperature for 22 h. The suspension was diluted with THF
and vacuum filtered, the supernatant was concentrated and purified
several times by column chromatography (SiO2/1:1 hexanes:CH2Cl2)
to yield 70 mg (39%) of monomer 1-o as a white solid that becomes
blue upon exposure to UV light. Mp: 208e210 ꢀC. 1H NMR (CD2Cl2,
TEM samples of P1-o nanoparticles were prepared by dropping
their aqueous suspensions on copper grids, followed by drying
them overnight at room temperature.
3. Results and discussion
The synthetic pathway to acrylamide-functionalized photo-
chromic monomer 1-o and its multifunctional copolymer P1-o is
illustrated in Scheme 2. The key intermediate is the non-symmetric
dithienylethene 6, which possesses an appropriate amine to convert
it to the required acrylamide group. Compound 6 was prepared
from thiophene 2 in 3 steps as shown in the scheme. The resulting
monomer 1-o was copolymerized with N-isopropylacrylamide (NIPA)
using free radical methods and VAZO 88 (1,10-azobis(cyclohexane-1-
carbonitrile)) as the initiator. In the present case, the ratio of mono-
mers was chosen to produce a random copolymer with substantially
less of the photochromic component (1-o) than NIPA (‘m’/‘n’ ¼ 12) in
order to ensure there is enough microscopic flexibility to allow the
photo-induced cyclization of the DTE components [22]. As illustrated
by the structure shown in Scheme 2, P1-o possesses an amphiphilic
structure, with a hydrophilic PNIPA backbone and hydrophobic DTE
pendant groups [23]. The 1H NMR spectrum of an acetone-d6 solution
of the P1-o demonstrates that approximately 8 mol-% of the polymer
is the photochromic component (1-o) corresponding almost exactly
to the co-monomer feed ratio. The polymer’s molecular weight
(Mw) and glass transition temperature (Tg) were measured to be
5957 g molꢁ1 (PDI ¼ 1.17) and 128 ꢀC, respectively.
600 MHz):
d
7.64 (d, J ¼ 7.8 Hz, 2H), 7.60 (s, 1H), 7.56 (d, J ¼ 7.9 Hz,
2H), 7.53 (d, J ¼ 7.8 Hz, 2H), 7.34 (t, J ¼ 7.8 Hz, 2H), 7.34e7.25 (m, 3H),
6.41 (d, J ¼ 16.8 Hz, 1H), 6.28 (dd, J ¼ 16.8, 10.2 Hz, 1H), 5.77 (d,
J ¼ 10.2 Hz,1H),1.98 (s, 3H),1.97 (s, 3H). 13C NMR (CD2Cl2, 600 MHz):
d
163.65,142.59,142.09,141.98,141.68,138.08,136.55,136.39,133.58,
131.35, 129.69, 129.30, 128.23, 128.09, 126.42, 126.03, 125.83, 125.61,
122.68, 122.25, 120.45, 118.28, 116.74, 116.58, 114.88, 111.64, 111.47,
111.30, 14.72, 14.69. HRMS (ESIþ) Calculated for C30H21F6NOS2
[M þ Hþ]: 590.0970. Found: 590.1037.
2.7. Synthesis of photochromic copolymer P1-o
A
solution of monomer 1-o (65 mg, 0.11 mmol) and
N-isopropylacrylamide (150 mg, 1.33 mmol) in anhydrous N,N-dime-
thylformamide (5 mL) in a sealable glass tube was treated with
1,10-azobis(cyclohexane-1-carbonitrile) (VAZO 88) (17 mg, 0.07 mmol)
in one portion. The resulting solution was degassed by freeze-pump-
thawing and then heated to 100 ꢀC and stirred there for 48 h. After
cooling, the solvent was evaporated and the residue was dissolved
in acetone. This solution was treated with distilled water at room
temperature to precipitate the product (three times). The final residue
was collected by filtration and re-dissolved in EtOAc. This solution was
treated with excess cold hexanes to precipitate high molecular weight
copolymer product (three times) until no oligomers were observed on
TLC plate. P1-o was obtained as a white amorphous powder (190 mg,
Through the selective evaporation of THF solvent from a THF/
water solution of photochromic, polymeric P1-o, quite uniform-
sized (150 ꢃ 50 nm, 2 ꢂ 10ꢁ3 wt-% in water) and highly stable
(over several weeks) copolymeric nanoparticles were successfully
fabricated. Fig. 1(a, b, d and e) show the transmission electron
microscope (TEM) images of these P1-o nanoparticles. As shown in
these figures, every polymeric nanoparticle has a hole in its soft
skin. We suggest that the defect-like holes in the skins of the
nanoparticles are formed as a consequence of rapid evaporation of
the water captured within the nanoparticles during the high
vacuum sample processing and/or imaging using TEM [24].
82%). 1H NMR (acetone-d6, 400 MHz):
d 7.76e7.07 (br, 53H), 3.99
(br, 29H), 2.91e2.88 (br,16H), 2.22 (br, 32H),1.66 (br, 64H),1.42 (s, 7H),
1.29 (s, 6H), 1.13 (br, 177H), 0.88e0.83 (m, 9H). 13C NMR (CDCl3,
600 MHz):
d 174.30, 171.98, 142.41, 142.18, 141.45, 140.90, 133.48,
129.18, 125.77, 122.54, 121.86, 120.35, 118.03, 116.35, 114.66, 42.66,

S.-J. Lim et al. / Dyes and Pigments 89 (2011) 230e235
233
O
B
O
F
F
F
F
F F
1) n-BuLi
F
F
F
F
F
F
Br
CH3
NH2
Et2O, –78°C
NBS
F
F
F
F
F
F
TMS
2)
F
F
CH3CN
Pd(PPh3)4
Na2CO3
toluene/EtOH
S
F
F
Ph
TMS
Ph
Br
Ph
S
S
S
S
S
S
F
F
2
NH2
6
4
5
F
(84%)
(17%)
(82%)
Ph
CH3
S
3
m
O
n
O
m
O
n
O
O
HN
HN
HN
HN
F
F
O
N
H
F
F
Cl
F
F
313 nm
Et3N
THF
0°C – rt
VAZO 88
O
DMF, 100°C
> 500 nm
Ph
S
S
S
S
F
N
H
F
F
F
F
F
F
S
1-o
(39%)
S
F
F
F
F
F
P1-o
(82%)
P1-c
Scheme 2. Synthesis and photochromic reactions of copolymers P1-o. The mixture of P1-o and P1-c produced with UV light is referred to as P1-pss in this manuscript.
Fig. 2(a) shows the UVeVis absorption spectra of an aqueous
suspension containing the P1-o nanoparticles, in which light scat-
tering features are clearly observed as a result of nanoparticles
formation. When the suspension of the nanoparticles in their ring-
open form (P1-o) was irradiated with UV light (313 nm,
5.5 mW cmꢁ2), the suspension turned blue and new absorbances in
the visible region (lmax ¼ 590 nm) appeared in its UVeVis absorption
spectrum. This notable coloration clearly suggests that the dithie-
nylethene components in the nanoparticles are undergoing photo-
isomerization from their ring-open form to their ring-closed form
when irradiated with 313 nm light (P1-o / P1-c). The 313 nm
photostationary state (P1-pss) was attained in 2 min, and there
were no further spectral changes in the UVeVis absorption spectrum.
Moreover, the UVeVis absorption spectrum in the 313 nm PSS is
almost the same as that for the suspension of P1-pss nanoparticles
containing over 98% of ring-closed DTEs (this nanoparticle suspen-
sion was prepared from a THF solution of P1-pss already in its 313 nm
photostationary state, in which the photochromic conversion from
ring-open to ring-closed form (P1-o / P1-c) was greater than 98% as
determined by 1H NMR spectroscopy). Accordingly, it was estimated
that the photochromic ring-closing conversion in the P1-o nano-
particles was also over 98%. This highly efficient photoisomerization
of the photochromic molecules in the P1-o nanoparticles suggests
that they have enough free volume and conformational flexibility to
accommodate the structural changes that accompany the photo-
isomerization. This consideration was also supported by the fact
that the molar concentration of the hydrophobic DTE component
(co-monomer 1-o) was much lower than that of the hydrophilic
acrylamide backbone components in P1-o (w8 mol-%). More inter-
estingly, the light scattering by the ring-closed nanoparticles in the
313 nm P1-pss was higher than that observed for the ring-open form
P1-o nanoparticles in near-infrared (NIR) region (750e850 nm) as
shown in the Fig. 2(a). On the basis of the fact that both ring-open
and ring-closed forms of the photochromic component have no
absorbances in the NIR (Fig. 2(b)), these characteristic light scattering
features indicated that the volumes occupied by photochromic
nanoparticles are larger in the 313 nm photostationary state [25,26].
The blue-coloured 313 nm PSS suspension of the nanoparticles could
Fig. 1. Transmission electron microscope (TEM) images of the copolymeric nanoparticles P1-o with the DTE in its ring-open form. Frame (c) shows a representative particle-size-
distribution spectrum of open-form P1-o nanoparticles in an aqueous suspension at room temperature.

234
S.-J. Lim et al. / Dyes and Pigments 89 (2011) 230e235
Fig. 2. (a) UVeVis absorption spectra of a THF solution of P1-o (2.27 ꢂ 10ꢁ3 wt-%) and an aqueous suspension of P1-o nanoparticles (2 ꢂ 10ꢁ3 wt-%) at room temperature before and
after irradiation with UV light. (b) UVeVis absorption spectra of THF solutions of 1-o (2 ꢂ 10ꢁ5 M) and P1-o (2.27 ꢂ 10ꢁ3 wt-%) before and after irradiation with UV light. (c) Average
hydrodynamic diameter changes of P1-o and P1-pss nanoparticles at various temperatures. (d) Reversible switching of average hydrodynamic diameters of P1-o nanoparticles with
alternate exposure to UV light (313 nm, 5.5 mW cmꢁ2, 2 min; dashed line) and visible light (>500 nm, 300 W, 5 min; solid line) at 20 ꢀC and with alternating high (60 ꢀC; solid line)
and low (20 ꢀC; dashed line) temperatures at the 313 nm photostationary state.
be bleached by irradiation with visible light (>500 nm, 300 W) to
fully recover the original UVeVis absorption profile of the suspension
of ring-open P1-o nanoparticles.
nanoparticles at all investigated temperatures (ꢁ44 mV at 20 ꢀC,
ꢁ39 mV at 40 ꢀC, and ꢁ30 mV at 60 ꢀC at the 313 nm PSS). Moreover,
the average hydrodynamic diameters of P1-pss nanoparticles in the
313 nm photostationary state recovered to those for the ring-open
P1-o nanoparticles at each temperature, within experimental error,
Fig. 2(c) shows the characteristic average hydrodynamic diame-
ters of P1-o nanoparticles in an aqueous suspension as determined
by particle-size analysis (PSA) at various temperatures [27]. The
average hydrodynamic diameters shown in Fig. 2(c) were measured
over 30 times at each data point to reduce the experimental
error and a representative particle-size-distribution spectrum of an
aqueous suspension containing ring-open P1-o nanoparticles at
room temperature is illustrated in the inset in Fig. 1(c). As presented
in Fig. 2(c), the average hydrodynamic diameters of ring-open P1-o
nanoparticles decreased continuously with increasing temperatures,
with a change in the temperature dependence near the lower critical
solution temperature (LCST) of PNIPA (w33 ꢀC), and the diameters
were highly reproducible when the temperature was varied either
by heating or cooling. These characteristic changes of the hydrody-
namic diameter of the ring-open P1-o nanoparticles depending on
the temperature likely results from the reversible degree of swelling
of water molecules into the P1-o nanoparticles [27]. Intramolecular
hydrogen bonding within each PNIPA chain is more favorable than
the intermolecular interaction between the PNIPA chains and
the water molecules at higher temperatures [16,17,28,29], thus the
number of water molecules swelled in the P1-o nanoparticles
decreases with increasing temperatures, which results in the smaller
average hydrodynamic diameters of the P1-o nanoparticles at higher
after irradiation with visible light (>500 nm, 300 W), and the
z
values as well as the light scattering features at the NIR regions
precisely corresponded to these reversible changes in hydrodynamic
diameters. Taken as a whole, these results suggest that the change in
the shape and rigidity of the DTE molecules in P1-o nanoparticles
followed by photochemical ring-closure should expand the diame-
ters of the P1-o nanoparticles mechanically or that the ring-closed
form of the DTE components in the 313 nm photostationary state
could enhance the degree of swelling of water molecules into the
amphiphilic nanoparticles [12e15]. On the basis of these results,
the average hydrodynamic diameters of P1-o nanoparticles could
be switched reversibly between two bistable states not only with
alternate UV and visible light irradiation at 20 ꢀC but also with
alternate high (60 ꢀC) and low (20 ꢀC) temperatures in the 313 nm
photostationary state as shown in Fig. 2(d).
4. Conclusion
A novel kind of amphiphilic photochromic copolymer P1-o
consisting of a hydrophilic PNIPA backbone and hydrophobic DTE
pendants was designed, synthesized and used to fabricate self-
assembled copolymer nanoparticles in an aqueous condition.
The hydrodynamic diameters (or volumes) of these multifunctional
P1-o nanoparticles reversibly switched between two bistable states
with the dual stimuli of light and temperature in an aqueous
suspension.
temperatures. In addition, the zeta potential (
open P1-o nanoparticles followed these characteristic changes
precisely. The values of the ring-open P1-o nanoparticles were
z) values of the ring-
z
ꢁ37 mV at 20 ꢀC, ꢁ35 mV at 40 ꢀC, and ꢁ27 mV at 60 ꢀC, and these
values were also temperature-dependent and highly reproducible
in the same way as the average hydrodynamic diameters of the ring-
open P1-o nanoparticles. The higher degree of swelling of water
molecules into the ring-open P1-o nanoparticles at lower temper-
Acknowledgements
atures is the likely cause of the higher negative
z values (or the
This research was supported by the Natural Sciences and Engi-
neering Research Council (NSERC) of Canada, the Canada Research
Chairs Program, and Simon Fraser University (SFU) through the
Community Trust Endowment Fund (CTEF). This work made use of
4D LABS shared facilities supported by the Canada Foundation for
Innovation (CFI), British Columbia Knowledge Development Fund
(BCKDF), and SFU. The authors specially thank Dr. Bronwyn Gillon
and Usama Al-Atar (SFU, Chemistry Department) for the helpful
discussions about copolymerizations and PSA measurements.
higher stabilities) of the P1-o nanoparticles. On the other hand,
the average hydrodynamic diameters of the P1-o nanoparticles
increased by several nanometers upon switching to the 313 nm PSS
as shown in Fig. 2(c). Given that the photochromic conversion from
ring-open form to the ring-closed form of the DTE photoisomers in
the nanoparticles was over 98% at the 313 nm PSS, and that the ring-
closed photoisomers of DTE derivatives generally have more rigid
structures [12e15], the larger diameters of the P1-pss nanoparticles
in the 313 nm PSS can be attributed to the change in the shape and
rigidity of the pendant DTE groups within the P1-o nanoparticles.
This conversion into the larger diameters was also consistent with
the higher light scattering features at the NIR regions in their UVeVis
absorption spectra at the 313 nm photostationary state (Fig. 2(a)). In
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