Synthesis and characterization of photo- and pH-responsive nanoparticles containing amino-substituted azobenzene

Article abstract of DOI:10.1039/c0jm01235a

A novel dual responsive nanomaterial sensitive to both photo- and pH stimuli has been successfully developed. The polymer-based nanoparticles were fabricated from a specially designed photo- and pH-responsive amino-substituted azobenzene monomer, 4-amino-4′-methacrylatylazobenzene (AMAAB), via free radical polymerization with trimethylolpropane trimethacrylate (TRIM) cross-linkers. The trans-cis photoisomerization properties of AMAAB were retained after incorporation into the rigid three-dimensional cross-linked polymer matrix. The TRIM/AMAAB molar ratio significantly influenced the kinetics of the trans-cis photoisomerization. The nanoparticles also possessed good pH-responsive properties. The change in absorbance with media pH was monitored at 402 nm, and a "titration type" curve was obtained with a pK _a value of 0.61 ± 0.06. At this transition point, the color of the nanoparticle suspension changed colour from yellow to pink. Thermogravimetric analysis (TGA) indicated that the nanoparticles were thermally stable.

Full text of DOI:10.1039/c0jm01235a

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Synthesis and characterization of photo- and pH-responsive nanoparticles
containing amino-substituted azobenzene†
a
a
a
a
a
Qian Tang, Xianzhu Meng, Hongbo Jiang, Tianyou Zhou, Chengbin Gong, Xiangkai Fua
*
and Sanqiang Shib
Received 28th April 2010, Accepted 10th July 2010
DOI: 10.1039/c0jm01235a
A novel dual responsive nanomaterial sensitive to both photo- and pH stimuli has been successfully
developed. The polymer-based nanoparticles were fabricated from a specially designed photo- and pH-
responsive amino-substituted azobenzene monomer, 4-amino-40-methacrylatylazobenzene (AMAAB),
via free radical polymerization with trimethylolpropane trimethacrylate (TRIM) cross-linkers. The
trans-cis photoisomerization properties of AMAAB were retained after incorporation into the rigid
three-dimensional cross-linked polymer matrix. The TRIM/AMAAB molar ratio significantly
influenced the kinetics of the trans-cis photoisomerization. The nanoparticles also possessed good
pH-responsive properties. The change in absorbance with media pH was monitored at 402 nm, and
a ‘‘titration type’’ curve was obtained with a pKa value of 0.61 ꢀ 0.06. At this transition point, the color
of the nanoparticle suspension changed colour from yellow to pink. Thermogravimetric analysis (TGA)
indicated that the nanoparticles were thermally stable.
were much fewer than the other non-amino substituted azo-
benzene compounds.20–26
1. Introduction
Stimuli-responsive materials, also known as smart materials,
have aroused increasing interest in material science and tech-
nology for their good performance in energy transfer,1 drug
delivery,2 data storage3,4 and biotechnology.5 Azobenzene-con-
taining polymers are a very popular class of photo-responsive
materials because of their wide scope of applications in nonlinear
optics,6–9 optical storage,10,11 holography12 and liquid crystal
displays.13 Most of these applications utilized photo-responsive
materials that contained azobenzenes either as backbones or as
side-chains of linear polymers. On the other hand, reports on
materials with azobenzenes embedded into three-dimensional
network of polymers are scarce. Recently, multi-functional
materials14 have also come into being, supporting applications in
more extended domains. These multi-functional materials are
constructed from various smart elements that can respond to
different stimuli, such as light,1 temperature,15,16 pH17 and even
specific molecular species,18 in a co-operative manner.
In this paper, through a delicate molecular design, a tri-func-
4-amino-40-meth-
tional
amino-substituted
azobenzene,
acrylatylazobenzene (AMAAB), was successfully synthesized
and studied for its spectroscopic responses towards selected
external stimuli. It was observed that AMAAB underwent
reversible photoisomerization in the UV/vis spectral range which
has rarely been reported before. Besides its photo-response, the
amino functionality of AMAAB also brought about its pH-
responsive properties. Using AMAAB as the monomer, TRIM
as the cross-linker, and AIBN as the radical initiator, a novel
dual photo- and pH-responsive nanomaterial was fabricated.
The morphology and thermal properties, as well as the photo-
and pH-response of the nanoparticles were characterized via
atomic force microscopy (AFM), thermogravimetric analysis
(TGA) and UV/vis spectroscopy.
2. Experimental
At one stage, it was generally believed that amino-substituted
azobenzenes were not suitable for photo-responsive materials
because of the strong electron-donating effect of the amino group
that tended to induce the overlapping of the p–p* and n–p*
transitions of the chromophores. As a result, the weak n–p*
band could be masked by the p–p* absorption, resulting in the
irreversible photoisomerization of aminoazobenzenes.19 For this
reason, studies on the photoisomerization of aminoazobenzenes
2.1 Materials and apparatus
Methacryloyl chloride was prepared from methacrylic acid with
thionyl chloride. All the other reagents were commercially sup-
plied and purified by the usual methods. 1H NMR and 13CNMR
(300MHz) were performed on a Bruker AV-300 NMR instrument
at ambient temperature, with TMS as the internal standard.
Elemental analysis was performed by a Leco CHN-900 micro
carbon-hydrogen-nitrogen analyzer. Gas chromatography mass
spectrometry (GC-MS) chromatograms were obtained using an
Agilent 6890 GC system coupled to an Agilent 5973 quadrupole
mass selective detector. UV/cis spectra were recorded with an UV-
4802 spectrophotometer (UNICO(Shanghai)Instruments Co.,
Ltd.). A GY-13 50W high pressure Hg lamp (Tianjin Tuopu In-
strument Co., Ltd.) coupled with a DS-3 monochromator (Tianjin
Tuopu Instrument Co., Ltd.; slit: 4 mm) was used as the light
aCollege of Chemistry and Chemical Engineering, Southwest University,
Beibei, Chongqing, China 400715. E-mail: gongcbtq@swu.edu.cn; Fax:
+86-23-68254000; Tel: +86-23-68252360
bDepartment of Mechanical Engineering, The Hong Kong Polytechnic
University, Hong Kong, China
† Electronic supplementary information (ESI) available: NMR spectra,
UV/vis spectra and photograph of the AMAAB solution. See DOI:
10.1039/c0jm01235a
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2.4 Synthesis of 4-Amino-40-methacrylatylazobenzene
source in the photoisomerization studies. pH measurement was
performed with a PHS-3D pH meter (Shanghai Precision &
Scientific Instrument Co., Ltd.). The morphology of the nano-
particles was determined by a DI Multimode SPM (Veeco
Instruments Inc., USA) operated in tapping mode with a TESPT
cantilever. Spin-coating was carried out by spin coater SC-1B
(Beijing Chuangshiweina Instrument Co., Ltd.). The thermal
stability of the samples was determined by thermogravimetric
analysis (TGA) using a SDT Q600 thermal analyzer (USA) at
a heating rate of 10 ꢁC minꢂ1 up to 700 ꢁC under flowing N2
(100 mL minꢂ1). Melting points were measured on a X-6 melting
point apparatus (Beijing Fukai Instrument Co., Ltd.).
(AMAAB)
Acetone (60 mL), distilled water (15 mL), NMAAB (1.55 g,
0.005mol), zinc powder (2.6 g, 0.04 mol) and AlCl3ꢃ6H2O (0.4 g)
were placed in a 250 mL three-necked flask which was fitted with
a reflux condenser. The mixture was stirred at room temperature
for 1 h, then warmed to 40 ꢁC for 1 h, 50 ꢁC for 3 h, and finally
ꢁ
the mixture was refluxed at 60 C for 2 h. TLC was utilized to
monitor the reaction process. The resultant mixture was cooled
to room temperature and suction filtered to remove the excess
zinc powder, followed by evaporation under reduced pressure to
give the crude product in the form of a golden precipitate. The
product was obtained by filtration and recrystallization in
ethanol to produce 0.82 g of yellow powder. Yield: 58.5%. Mp:
178.5–180.1 ꢁC. 1HNMR(d, CDCl3): 2.08 (s, 3H), 4.06 (s, broad;
2H), 5.78 (s, 1H), 6.38 (s, 1H), 6.72 (d, 2H), 7.23 (d, 2H), 7.78 (m,
4H). 13CNMR(d, CDCl3):165.65, 152.02, 150.57, 149.62, 145.51,
135.82, 127.54, 125.16, 123.45, 122.10, 114.65, 18.42 GC-MS m/z:
281. Elemental analysis calculated (%) for C16H15N3O2: C 68.31,
H 5.37, N 14.94, found: C 68.19, H 5.339; N 14.99.
2.2 Synthesis of 4-Nitro-40-hydroxylazobenzene (NHOAB)
4-nitroaniline (2.76 g, 0.020 mol) was dissolved in a solution of
concentrated sulfuric acid (4.4 mL) and distilled water (10 mL)
by gently heating and stirring. Then the solution was cooled with
ice water and the temperature was kept below 1 ꢁC. A cooled
solution (12 mL) of NaNO2 (1.52 g, 0.022 mol) was then added
dropwise. The resultant diazonium salt solution was stirred for
a further 15 min at 0–1 ꢁC. An ice cold mixture of phenol (1.88 g,
0.020 mol) and distilled water (15 mL) was then poured into the
diazonium salt solution, followed by neutralization (pH ¼ 8–10)
with cooled NaOH solution (10%). The mixture was stirred for
2.5 General preparation of nanoparticles
The nanoparticles were fabricated via free radical polymerization
of AMAAB and TRIM in the presence of AIBN as a radical
initiator. A series of nanoparticles with TRIM/AMAAB molar
ratios of 50 : 1, 20 : 1, 10 : 1, 5 : 1, 3 : 1, and 1 : 1 were prepared.
A typical procedure for the fabrication of NpT5A1 (a nano-
particle with a TRIM:AMAAB ¼ 5 : 1) was as follows: a solution
containing AMAAB (0.14 g, 0.0005 mol), TRIM (0.84 g, 0.0025
mol), AIBN (0.08 g), DMF (1.0 mL) and acetonitrile (50 mL)
were deoxygenated with nitrogen for ꢁ30 min, the resulting solu-
tion was placed in an oil bath at 70 C for 12 h in the dark to
minimize isomerization of the azobenzene residues. The precipi-
tate was obtained by centrifugation and washed several times
with hot methanol. Finally, the product was dried under vacuum
to constant weight and afforded a yellow powder.
ꢁ
about 2 h at 0–3 C and then was acidified with a few drops of
concentrated HCl. The crude product was obtained by filtration
and purified by recrystallization in ethanol–H2O (v/v ¼ 5 : 1) to
produce 4.16 g of red crystal. Yield: 85.6%, 1H NMR (d, CDCl3):
5.42 (s 1H), 7.10 (m, 2H), 8.06 (d, 4H), 8.50(d, 2H), 13CNMR(d,
CDCl3):159.49, 156.13, 147.23, 125.98, 125.52, 124.81, 123.17,
116.22, GC-MS m/z: 243.1. Elemental analysis calculated (%) for
C12H9N3O3: C 59.26, H 3.73, N 17.28, found: C 58.976, H 3.722;
N 17.665.
2.3 Synthesis of 4-Nitro-40-methacrylatylazobenzene
(NMAAB)
2.6 Spectroscopic characterization and photoisomerization
studies
A mixture of NHOAB (1.22 g, 0.005 mol), dichloromethane (30
mL) and triethylamine (acid scavenger, 1.05 mL, 0.0075 mol) was
purged with nitrogen for 30 min and was then maintained in an
ice bath. A solution of methacryloyl chloride (0.53 mL, 0.0055
mol) and dichloromethane (10 mL) was then added dropwise
from a dropping funnel and triethylammonium salt in the form
of smog wasꢁobserved immediately. The resultant mixture was
heated to 40 C (reflux) for 4 h and the reaction was monitored
by thin layer chromatography (TLC). Afterward, the mixture
was washed thoroughly with distilled water to remove the trie-
thylammonium salt. The organic layer was dried by anhydrous
Na2SO4, and evaporated by rotary evaporation. The crude
product was purified by recrystallization in ethyl acetate to
produce 1.40 g of an orange powder. Yield: 89.7%. Mp: 126.7–
128.0 ꢁC. 1H NMR (d, DMSO-d6): 2.09 (s, 3H), 5.92 (s, 1H), 6.38
(s, 1H), 7.46 (m, 2H), 8.16 (d, 4H), 8.44(d, 2H). 13CNMR(d,
DMSO-d6):165.43, 155.44, 154.21, 150.13, 149.11, 135.64,
128.92, 125.86 125.02, 124.43, 123.61, 18.56 GC-MS m/z: 311.
Elemental analysis calculated (%) for C16H13N3O4: C 61.73, H
4.21, N 13.50, found: C 61.835, H 4.441; N 13.615.
Spectroscopic characterization of the functionalized azobenzene
monomer and the subsequent nanoparticles were performed in
DMF. Air-tight screw-capped quartz cells of 1.0 cm optical path
length were used in all experiments. The suspension of nano-
particle material in the solvent media was maintained with the
help of a magnetic stirrer. For the trans to cis photoisomerization
studies, solutions or suspensions were stirred and irradiated at
405 nm (monochromator slit width: 4 mm), UV/vis spectra of
the suspension were measured at regular time intervals. Stirring
and irradiation was suspended during spectroscopic measure-
ment. For the cis to trans photoisomerization studies, irradiation
at 456 nm was adopted.
2.7 Atomic force microscopy
Thin films of nanoparticles on mica for AFM characterization
were prepared by spin-coating (at 4000 rpm) of nanoparticle
suspensions at a loading of 2.0 mg in 5 mL of ethanol. The
suspensions were sonicated for 20 min before spin-coating.
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2.8 Studies on pH responses
0.0042 g of NpT5A1 was suspended in 6.0 mL ethanol, followed
by ultrasonic stirring for 20 min. Then, 500mL of the suspension
was diluted into 5.0 mL with ethanol and distilled water (v/v ¼
1 : 1). pH of the media was adjusted by the addition of HCl.
Accurate pH of the media was measured by pH meter. UV/vis
spectra were obtained 5 min after each pH adjustment.
3. Results and discussion
3.1 Preparation of monomer and nanoparticle
The synthesis of AMAAB was tedious due to the fact that it
contains three different functional moieties in its molecular
structure. Using p-nitroaniline as the starting material, AMAAB
was synthesized via diazotization coupling, esterification and
nitro reduction reactions. The yield of nitro reduction reaction
was 58.5%, which was much higher than previously reported26
(only 15% with the reduction reaction). The synthesis route is
summarized in Scheme 1.
Scheme 2 The preparation of nanoparticles with different TRIM/
AMAAB molar ratios.
AMAAB is not easily soluble in acetonitrile, chloroform,
methanol or THF, but soluble in DMF and DMSO. Thus,
a mixture of acetonitrile and DMF was employed in the subse-
quent polymerization process. Without acetonitrile, the polymer
particles obtained were too big to suspend in solvents. They had
to be crushed, milled and wet sieved before UV/vis character-
ization, and the resultant particles had an irregular morphology.
In the presence of acetonitrile as the porogen, regular AMAAB-
based nanoparticles were formed. TRIM and other crosslinkers
have been attempted. Nevertheless, nanoparticle materials
fabricated from TRIM were found to show the best properties.
In addition, different monomer : cross-linker mole ratios have
been explored. The preparation process of the nanoparticles is
shown in Scheme 2.
3.2 Photoresponsive properties
AMAAB was dissolved in DMF and the nanoparticles were
suspended in DMF for the UV/vis measurements. Before the
measurement, the sample solutions were kept in the dark at room
temperature overnight to ensure that all of the azobenzene units
were in the trans configuration.
Fig. 1a shows the UV/vis spectra and spectral changes of
the AMAAB monomer in DMF solution (1.0 ꢄ 10ꢂ5 mol Lꢂ1
)
upon irradiation at 405 nm. Before irradiation, the maximum
Fig. 1 UV/Vis spectra and spectral changes of AMAAB solution (1.0 ꢄ
10ꢂ5 mol Lꢂ1, DMF) upon irradiation with 405 nm light for 0.5, 1, 2, 3, 5,
7, 9, 12, 15, 18, 22, 26, 30 min (a) and then upon irradiation with 456 nm
light for 1, 2, 4, 8, 13, 18, 25, 32, 39, 49, 59, 69, 79, 94, 109, 124, 144, 164,
184, 204, 224 min (b). Insets: the kinetics of the photoisomerization of
AMAAB.
Scheme
1
The synthetic route for trans-4-amino-40-meth-
acrylatylazobenzene (AMAAB).
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absorption was observed at 402 nm, which was attributed to the
p–p* transition of the azobenzene chromophore in its trans
configuration. UV/Vis irradiation at 405 nm induced trans/cis
photoisomerization, resulting in a decrease in the absorbance at
402 nm, with twin isosbestic points at 360 and 482 nm.
After irradiation at 405 nm for 30 min, spectral changes of the
AMAAB solution levelled off, indicating that the system reached
a photostationary state. The original absorption peak at 402 nm
disappeared and a new medium-strength peak was observed at
360 nm, which was attributed to the p–p* transition of the
azobenzene chromophore in its cis configuration. Subsequent
irradiation of the solution at 456 nm reversed the spectral change
of the solution back to its original state within 224 min (Fig. 1b).
This indicates that the photoinduced isomerization processes are
fully reversible. Furthermore, it is worth mentioning that the
absorption peak of the n–p* transition of the azobenzene chro-
mophore of AMAAB was not observed because it was very weak
and was masked by the strong and broad p–p* transition.
Kinetics of the photoisomerization processes were analyzed by
the first-order kinetics expression in eqn (1):
ðA0 ꢂ ANÞ
Ln
¼ kt
(1)
ðAt ꢂ ANÞ
where A0, At and AN are the absorbances at 402 nm of the
azobenzene chromophore at time 0, t and the photostationary
state, respectively.
Rate constants for the trans/cis and cis/trans isomerization
of AMAAB were found to be 23.3 ꢄ 10ꢂ4 and 2.6 ꢄ 10ꢂ4 sꢂ1
,
respectively. It is interesting to notice that the trans/cis rate was
almost 10 times faster than the cis/trans rate. This result might
be also attributed to the superimposition of the p–p* transition
on the n–p* transition. Irradiation at 456 nm excites both the
n–p* and, to a lesser extent, p–p* transitions of the azobenzene
chromophore simultaneously. This slows down the overall cis/
trans conversion.
Fig. 2 UV/Vis spectra and spectral changes of NpT5A1 suspension (1.2
ꢄ 10ꢂ4 g mLꢂ1, r.t., in DMF) upon irradiation with 405 nm light for 1, 4,
7, 12, 18, 26, 36, 46, 56, 76 min (a) and then upon irradiation with 456 nm
light for 2, 4, 7, 12, 17, 22, 29, 39, 49, 62, 81, 96 min (b). Insets: first order
plots for photoisomerization of the azobenzene chromophores in
NpT5A1.
Fig. 2 shows the spectroscopic responses of the AMAAB-
based nanoparticles NpT5A1, toward irradiation at 405 nm and
then 456 nm. Responses of the nanomaterial to photo-excitation
were quite similar to that of the AMAAB monomer. This indi-
cates that the azobenzene chromophore of the monomer has
retained their photo-switching properties after being incorpo-
rated into the rigid three dimensional cross-linked polymer
matrix. The extent of the spectral change of NpT5A1 was smaller
than that of AMAAB, probably due to steric hindrance within
the nanomaterial. Rate constants of the trans/cis and cis/
trans isomerization of NpT5A1 were found to be (14.4 ꢀ 0.5) ꢄ
10ꢂ4 and (7.4 ꢀ 0.3) ꢄ 10ꢂ4 sꢂ1, respectively. Other nanoparticles,
NpT50A1, NpT20A1, NpT10A1, NpT3A1, and NpT1A1
exhibited similar photo-responsive behavior (see ESI†).
latter was probably caused by the inconspicuous p–p* transition
due to the low proportion of AMAAB. Secondly, the rate of
trans/cis photoisomerization increased with increasing molar
ratio of TRIM to AMAAB and eventually approached the value
of the free AMAAB monomer (Table 1). A reasonable explanation
was as follows:27 a high content of TRIM in the polymer improved
the porosity of the nanoparticles, which was beneficial to the
sterically demanding conformational switching of the azobenzene
chromophores. When the cavities within the nanomaterial were
large enough, the azobenzene chromophores were able to undergo
photoisomerization as if they were freely dissolved in solution. To
summarize, NpT5A1 was selected for further evaluation on the
basis of its overall satisfactory photo-responsive properties.
Photoisomerization of the azobenzene chromophores within
the nanoparticle matrix was reversible. Fig. 3 illustrates the
modulation of the absorbance of a NpT5A1 suspension upon
Nonetheless, significant differences in the photoisomerization
properties of the AMAAB-based nanoparticles were observed.
Firstly, the extent of spectral change upon irradiation was
affected by the content of AMAAB in the nanoparticles. Both
high proportion (as in NpT1A1 and NpT3A1) and low proportion
(as in NpT50A1 and NpT20A1) of AMAAB diminished the
extent of spectral change upon irradiation. The former was prob-
ably caused by the steric hindrance between adjacent azobenzene
chromophores, resulting in incomplete photoisomerization. The
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Table 1 The effect of different cross-linker: monomer mole ratios on the
photoisomerization kinetics of the resultant nanoparticles
Molar ratio
(TRIM:AMAAB)
Isomerization rate of
trans/cis (ꢄ 10ꢂ4 sꢂ1
)
NpT1A1
NpT3A1
NpT5A1
NpT10A1
NpT20A1
NpT50A1
AMAAB
1 : 1
3 : 1
5 : 1
10 : 1
20 : 1
50 : 1
8.40 ꢀ 1.1
10.8 ꢀ 0.5
14.4 ꢀ 0.5
15.0 ꢀ 1.2
20.3 ꢀ 0.7
22.6 ꢀ 1.6
23.3 ꢀ 0.0
10mmol Lꢂ1 DMF solution
alternate irradiation at 405 nm and 456 nm. The extent of re-
versible isomerization did not significantly decay after 11 cycles,
indicating that the photo-responsive properties of NpT5A1 were
stable and repeatable.
3.3 pH-responsive properties
The pKa of AMAAB was measured to be 2.15 ꢀ 0.06 (see ESI†),
which was consistent with the literature value.26 Fig. 4a shows the
UV/vis spectra of a NpT5A1 suspension at different pH. At pH ¼
6.86, the absorption peak at 402 nm was predominant. As the
pH decreased, the 402 nm peak diminished and two new peaks at
328 and 506 nm grew dramatically. The color of the suspension
changed from yellow to pink. Two isosbestic points at 360 and
475 nm were observed. Fig. 4b shows a plot of absorbance at
402 nm versus pH. Beyond pH 3, there was no significant change
in the UV/vis spectra. However, large changes in the absorbance
at 402 nm were observed when pH was in the ranged of 0 to 2.
This is attributed to the protonation of the amino group in AMAAB
into an ammonium cation. From its spectroscopic responses,
the pKa of NpT5A1 was estimated to be 0.61 ꢀ 0.06, which was
smaller than that of AMAAB. This lowering of pKa may be due to
the fact that the amino azobenzenes were embedded in a rigid
environment of the polymer matrix in the nanoparticles, which
prevented protons from diffusing into the polymer to reach the
AMAAB, or the interior of the polymer nanoparticles is too
hydrophobic for protons to diffuse freely. By switching the media
Fig. 4 (a) UV/Vis spectra of the NpT5A1 suspension (0.1 mg mLꢂ1, in
50% v/v ethanol- dilute HCl) at pH 6.86, 3.13, 2.06, 1.23, 0.86, 0.64, 0.41,
0.21, 0.00, ꢂ0.50. The arrow indicates the direction of decreasing pH.
Inset: the color of the suspension at different pH. (b) Change in absor-
bance at 402 nm of NpT5A1 suspension. Inset: the plot of absorbance at
402 nm of NpT5A1 as a function of pH.
pH between 0.7 and 1.6 with 4.0 mol Lꢂ1 HCl and KOH solutions,
the reversibility of the spectroscopic responses was demonstrated
(inset of Fig. 4(b)).
3.4 Morphology studies
The morphology of the AMAAB-based nanoparticles was visu-
alized by AFM. Fig. 5a and 5b show the 2D and 3D AFM images
of NpT5A1, respectively. The two images show that NpT5A1
has regular spherical morphology and in the nanometre size
range. As seen in Fig. 5c, the height of nanoparticles varies
between 4.3 and 12.5 nm, with an average height of 7.2 nm.
Fig. 5d shows the size (in diameter) distribution of the nano-
particles, 60% of the particles were of 120–280 nm, and > 85% of
the particles were in the size range of 80 to 360 nm. The averaged
size (in diameter) of the particles was 187 nm.
Fig. 3 Reversibility of the photoisomerization processes of the azo-
benzene chromophores in the nanoparticles upon alternate irradiation at
405 nm and 456 nm respectively.
In addition, the suspension of NpT5A1 showed the Tyndall
effect (Fig. 5e), which confirmed the nano-size of the particles.
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Fig. 5 (a) A 2D-AFM image (3 ꢄ 3 mm2) of NpT5A1 spin-coated on mica. (b) A 3D-AFM image (3 ꢄ 3 mm2) of NpT5A1 spin-coated on mica. (c) A
typical height profile of the cross section. (d) Size (diameter) distribution of NpT5A1 taken from the area of 5 ꢄ 5 mm2. Inset: the AFM image (5 ꢄ 5 mm2)
of NpT5A1. The number of nanoparticles was 73. (e) A photograph of the NpT5A1 suspension (0.2 mg mLꢂ1, in 50% v/v ethanol-water) when a light
beam was irradiated from the side.
Neither the solvent (DMF) nor the AMMAB solution showed
the Tyndall effect (see ESI†).
around 308 ꢁC and 373 ꢁC, respectively, which indicated that
NpT5A1 possessed good thermal stability. The nanoparticle
exhibited a two-step weight loss process, which was within the
3.5 Thermal stability
ꢁ
ꢁ
temperature range of 280–398 C and 398–495 C, respectively.
The derivative TG curves in Fig. 6 exhibited maximum weight
The TG-DTG curves of NpT5A1 are shown in Fig. 6. From the
TGA curve, the 5% and 50% weight loss temperatures were
ꢁ
loss rate at 320 ꢁC for the first loss range, and at 417 C for the
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(20090182120010), Southwest University Doctoral Fund
(SWUB2008075), Postdoctoral Fund of Southwest University
and the national university student innovation test plan of
Southwest University (081063505).
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Fig. 6 TGA thermogram (curve a) and derivative of thermograms
(DTG, curve b) of NpT5A1.
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second, which suggested that the nanoparticles were made up of
two different compositions. Near 100% weight loss occurred
ꢁ
when temperature was above 650 C.
4. Conclusion
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2009, 50, 4512–4519.
An amino-substituted azobenzene monomer, 4-amino-40-meth-
acrylatylazobenzene (AMAAB) and a series of its cross-linked
polymer-based nanoparticles have been successfully synthesized
and characterized. The reversible and repeatable photo-
isomerization of AMAAB and the AMAAB-based nanoparticles
were monitored by UV/vis spectroscopy. The TRIM : AMAAB
molar ratio of the nanoparticles was found to govern the kinetics
of the trans/cis and cis/trans isomerization of the embedded
azobenzene chromophores. NpT5A1 showed high pH sensitivity
between media pH of 0 and 2. The spectroscopic transition point
was observed at pH 0.61 ꢀ 0.06. The particle size distribution
of NpT5A1 was within the range of 80–360 nm with a regular
spherical morphology. Thermogravimetric analysis indicated
that NpT5A1 possesseꢁd good thermal stability with no obvious
weight loss below 300 C in nitrogen atmosphere. The potential
applications of this kind of dual responsive nanomaterial in
biotechnology, chemical sensing, pH indication and wavelength-
specific photo-responding are under investigation in our labo-
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The work described in this paper was supported by the National
Natural Science Foundation of China (20872121), Natural
Science Foundation Project of CQ CSTC (CSTC, 2009BB4001),
the Fundamental Research Funds for the Central Universities
(XDJK2009C011, XDJK2009C178), Research Funds for the
Doctoral Program of Higher Education of China
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 9133–9139 | 9139