Photosensitive azopolymer brushes via atom transfer radical polymerization for protein sensing

Article abstract of DOI:10.1002/cjoc.201190044

Novel photosensitive azopolymer brushes were synthesized via surface initiated atom transfer radical polymerization using initiator self-assembled on Au surface. The chemical structures of azobenzene derivatives were confirmed by Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (NMR). The surface morphology of azopolymers via atom transfer radical polymerization (ATRP) for different time was investigated by atomic force microscopy (AFM). Additionally, the photoisomerization of azopolymer was measured by ultraviolet-visible spectroscopy (UV-Vis). The results indicate that such azopolymers can undergo trans-cis-trans photoisomerization efficiently by photo-irradiation with UV light. Furthermore, this photoisomerization property could also induce the reversible adsorption of bovine serum albumin (BSA) adsorption on azopolymer brush surfaces. This adsorption kinetics of the reversible process can be measured by surface plasmon resonance (SPR) spectroscopy in situ. It suggests that the protein biochips could be regenerated safely by UV irradiation rather than by being rinsed with chemical reagents. Novel photosensitive azopolymer brushes were synthesized via surface initiated atom transfer radical polymerization using initiator self-assembled on Au surface. Such azopolymers can undergo trans-cis-trans photoisomerization efficiently by photo-irradiation with UV light. It could lead to the reversible adsorption of bovine serum albumin (BSA) adsorption on azopolymer brush surfaces. It suggests that the protein biochips could be regenerated safely by UV irradiation rather than by being rinsed with chemical reagents. This photodeformable surface on the azopolymer warrants further investigation for protein chip application since the azopolymer can hold protein molecules without denaturation caused by one-step photoirradiation. Copyright

Full text of DOI:10.1002/cjoc.201190044

FULL PAPER
Photosensitive Azopolymer Brushes via Atom Transfer Radical
Polymerization for Protein Sensing
Zhang, Zhihong*,a(张治红)
Wang, Yaolia,b(王要丽)
Yan, Fufenga(闫福丰)
Peng, Donglaia(彭东来)
Ma, Zhi*,b(马志)
a Henan Provincial Key Laboratory of Surface & Interface Science, Zhengzhou University of Light Industry,
Zhengzhou, Henan 450002, China
b Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
Novel photosensitive azopolymer brushes were synthesized via surface initiated atom transfer radical polym-
erization using initiator self-assembled on Au surface. The chemical structures of azobenzene derivatives were con-
firmed by Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (NMR).
The surface morphology of azopolymers via atom transfer radical polymerization (ATRP) for different time was
investigated by atomic force microscopy (AFM). Additionally, the photoisomerization of azopolymer was measured
by ultraviolet-visible spectroscopy (UV-Vis). The results indicate that such azopolymers can undergo
trans-cis-trans photoisomerization efficiently by photo-irradiation with UV light. Furthermore, this photoisomeriza-
tion property could also induce the reversible adsorption of bovine serum albumin (BSA) adsorption on azopolymer
brush surfaces. This adsorption kinetics of the reversible process can be measured by surface plasmon resonance
(SPR) spectroscopy in situ. It suggests that the protein biochips could be regenerated safely by UV irradiation rather
than by being rinsed with chemical reagents.
Keywords azopolymer, photoisomerization, atom transfer radical polymerization (ATRP), adsorption, biosensor
Introduction
cently developed “living” or “controlled” radical po-
lymerization method.13,14 It is possible to prepare well
defined polymer brushes on various substrates via sur-
face-initiated ATRP.15,16 Consequently, the obvious ad-
vantage of surface-initiated ATRP over other techniques
is that the surface density of the polymer chains is so
high that the polymer chains are forced to stretch along
the direction normal to the graft surface.17,18
There is an important agreement that small chemical
or physical changes in the sensing environment trigger
macroscopically observable changes in material proper-
ties. Thereby the polymer brush containing responsive
azobenzene moieties on surface that undergo molecular
level changes, in turn, must translate into macroscopic
responses.19 Therefore, this signal amplification effect
of azobenzene derivate can be employed for bio-
molecules detection. In the present work, the photore-
sponsive azopolymer brush on Au surface was designed
via surface-initiated ATRP for the reversible bovine
serum albumin (BSA) adsorption. The potential of this
technique lies in its versatility and simplicity, with ex-
pected applications in the protein chips or biomolecules
immobilization and macromolecular-scale fabrications.
Polymer brushes, defined as dense layers of
end-grafted polymer chains on a solid surface, provide
fascinating classes of flexible surfaces with various
functionalities. Therefore, it is employed in bio-related
areas, such as three-dimensional micro- and nanomem-
ory chips, DNA- and protein-based biochips and bio-
sensors.1-3 In addition, it is very important to control the
surface properties in the production of bio-related mate-
rials that can be used in biomedical and diagnostic ap-
plications. The introduction of light responsive proper-
ties of azobenzene into the surface-graft chains seems to
be of great interest in the fabrication of “smart” surfaces.
As known, azobenzene derivatives are well-known for
their reversible trans-cis isomerization upon light irra-
diation.4-6
On the basis of the photoisomerization, polymeric
thin films containing azobenzene chromophores exhibit
diverse photoresponsive properties, such as dipole mo-
ment, wetting, molecule size and surface energy.7,8
There are many methods to obtain the responsive
azobenzene layer, such as self-assembly,9,10 Langmuir-
Blodgett (LB),11 Layer-by-Layer (LBL),12 and so on.
Atom Transfer Radical Polymerization (ATRP) is a re-
*
E-mail: mainzhh@yahoo.com.cn; Tel.: 0086-371-63556510; Fax: 0086-371-63556510 (Zhang, Zhihong); mazhi728@mail.sioc.ac.cn (Ma, Zhi)
Received August 10, 2010; revised and accepted November 9, 2010.
Project supported by Program for New Century Excellent Talents in University of Henan Province (No. 2006HNC019) and International Coopera-
tion Project of Henan Province (No. 104300510070).
Chin. J. Chem. 2011, 29, 153— 158
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Zhang et al.
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Experimental
Scheme 2 Synthesis routes of the azopolymer
Regents and materials
HO
N N
COOH
Au substrates (2 nm chrome followed by 50 nm gold
on float glass) were purchased from the Institute of
Electronics, Chinese Academy of Sciences. Tetrahy-
drofuran (THF) was refluxed over sodium and distilled.
Dimethylformamide was purified by vacuum distillation
before used. Methacryloyl chloride, 6-chloro-1-hexanol,
2,2'-dipyridyl, phenol, and anisole were purchased from
Aladdin-reagent, Shanghai, China. Potassium hydroxide
(KOH), sodium nitrite (NaNO2), sodium hydroxide
(NaOH), anhydrous ethanol, cuprous chloride (CuCl),
chloroform (CHCl3), methol, and other chemicals were
used as received from Sinopharm Chemical Reagent Co.,
Ltd., China. The other chemicals were used without
further purification.
AZO1
Cl(CH2)6OH
acetone
KOH
KI
HO(CH2)6O
N N
AZO2
COOH
H3C C COCl
CH2
THF
O
O
O
H3C C CO(CH2)6O
CH2
N N
COC C CH3
CH2
AZO3
NaOH
THF
Synthesis of monomers and polymer
O
Schematic of experiment routes The initiator of
HS-(CH2)2OOCC(CH3)2Br was self-assembled onto Au
surfaces by immersing the slides into the initiator solu-
tion (20 mL ethanol of initiator, 2 mmol•L ) at room
temperature for 24 h. The modified substrates were
rinsed thoroughly with THF and ethanol, followed by
being dried in dry nitrogen stream and used for polym-
erization immediately. The whole routes were expressed
in Scheme 1. And the synthesis route was summarized
in Scheme 2.21-23
H3C C CO(CH2)6O
CH2
N N
COOH
AZO4
－
1
sure. Finally, the product was purified by column chro-
matography (Al2O3, hexane) and distilled to give the
product as colorless oil. H NMR (CDCl3, 400 MHz) δ:
1
1.5 (SH), 1.57—1.68 (m, 4H, CH2), 2.02 (s, 6H, CH3),
2.84 (2H, SCH2), 3.96, 4.14 (2H, CH2OCH2).
4-[(4'-Hydroxyphenyl)diazenyl] benzoic acid
(AZO1) 4-Aminobenzoic acid (2.74 g, 0.02 mol) was
dissolved in 10 mL of aqueous HCl and kept in the ice
bath at ice/salt. NaNO2 (1.5 g, 0.22 mol) in water (5 mL)
was added dropwise and stirred for 30 min. NaOH (1.2
g, 0.03mol) and phenol (1.88 g, 0.02 mol) were dis-
solved in water (40 mL) at 0 ℃. The former solution
was added and stirred for 2 h. The crude product was
filtered and recrystallized twice by ethanol.
Scheme 1 Schematic of the whole experiment routes
4-((4-((6-Hydroxyhexyl)oxy)phenyl)diazenyl)benz-
oic acid (AZO2) AZO1 (2.42 g, 0.01 mol) and KOH
(1.2 g, 0.22 mol) were dissolved in ethanol (100 mL),
0.02 mol 6-chloro-1-hexanol and a trace of KI were then
added, and the solution was heated at reflux for 24 h.
The mixture was poured into water and extracted with
CHCl3. The crude product was filtered and recrystal-
lized twice by ethanol.
4-((4-((6-Hydroxyhexyl)oxy)phenyl)diazenyl)benz-
oate (AZO3) AZO2 (0.342 g, 0.01 mol), TEA (0.03
mol), were dissolved in dry THF (20 mL) at 0 ℃ and
then methacryloyl chloride (0.02 mol) dissolved in THF
(5 mL) was added dropwise under vigorous stirring. The
mixture was stirred for 14 h at room temperature. The
crude product was washed several time with water and
recrystallized twice from CHCl3.
Synthesis of the initiator, mercaptomethyl
2-bromo-2-methylpropanoate20 BSA was purchased
from Shanghai Biolife Science & Technology Co., Ltd.,
with a molecular weight of 67000. Bromoisobutyryl
bromide (3.34 mmol, 0.41 mL) diluted by THF (5 mL)
was added dropwise to a stirred solution of 2-mercapto-
ethanol (3.67 mmol, 0.48 mL) and pyridine (3.34 mmol,
0.27 mL) in dry dichloromethane (30 mL) at 0 ℃. The
reaction was stirred at 0 ℃ for 1 h and then at room
temperature for 16 h. Then the mixed solutions were
poured into 100 mL of water. And the product was ex-
tracted with toluene, which was removed under reduced
pressure. The extract was dissolved again in ether,
washed with saturated sodium chloride and dried over
Na2SO4. Volatiles were evaporated under reduced pres-
4-((4-((6-(Methacryloyloxy)hexyl)oxy)phenyl)di-
azenyl)benzoic acid (AZO4) The compound AZO4
was obtained by the hydrolysis of AZO3 in an aqueous
solution catalyzed with sodium hydroxide and a small
amount of ethanol. The yield was 92%, melting point
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Photosensitive Azopolymer Brushes for Protein Sensing
1
was 125 ℃. H NMR (CDCl3) δ: 11.0 (s, 1H, OH),
prism onto the gold film in the Kretshmann configura-
tion. The intensity of the reflected light was measured as
a function of the angle of incidence, θ, using a photodi-
ode with a chopper/lock-in amplifier technique. The
baseline of the SPR binding curve was obtained after
8.12 (d, J＝8.9 Hz, 2H), 7.93 (d, J＝7.5 Hz, 2H), 7.85
(d, J＝9.6 Hz, 2H), 6.93 (d, J＝7.5 Hz, 2H), 6.32 (s, 1H,
HC＝C), 5.73 (s, 1H, HC＝C), 4.36 (t, 2H, CH2OCO),
4.04 (t, 2H, CH2O), 2.01 (s, 3H, CH3), 1.33—1.38 (m,
4H); 13C NMR (d6-DMSO) δ: 167.5 (C＝O), 131.6
(C＝C), 126.3, 124.9, 123, 117 (benzene ring), 40.5
(O＝CCH2), 18.5 (CH2).
－
1
injecting buffer, i.e., 0.01 mol•L PBS (pH＝7.4) into
the cuvette. Afterwards, the protein was injected into the
flow cell and time resolved kinetic measurements of
protein adsorption were performed. Once the optical
properties of the interface were stable (i.e., no more
protein was adsorbed), the cell was rinsed with large
amounts of PBS buffer to remove any loosely adhered
protein. The last reflectivity scan was recorded. As for
the experiment of the effect of the azopolymer photoi-
somerization on protein adsorption, each protein ad-
sorption kinetic scan was conducted immediately after
the azopolymer was irradiated by UV light in the black
box of SPR equipment. All experiments were carried
out at room temperature.
ATRP of the AZO4 In a typical surface initiated
ATRP reaction, a solution of AZO4 and SAMs Au sub-
strate in anisole was degassed for 30 min to reduce the
amount of O2 present in the reaction system, followed
by another 10 min degassing upon the addition of the
catalyst mixture of CuCl, CuCl2, and 2,2'-bipyridyl at
1∶0.3∶2.9 in weight ratio.
Preparation of PBS solutions and BSA solution
Solution A: 9.465 g of Na2HPO4•12H2O dissolved in
1000 mL of DI water; solution B: 9.07 g of KH2PO4
dissolved in 1000 mL of DI water. 2.0 mL of solutions
A and 8.0 mL of B were mixed to produce PBS with pH
7.4. The protein solutions were prepared immediately
before used by dispersing commercially available BSA
in PBS at a concentration of 1 wt%.
Results and discussion
FTIR spectra of azobenzene derivatives
FTIR spectra of AZO1, AZO2, AZO3 and AZO4
were summerized in Figure 1. Complete conversion of
all reaction had been detected by IR-spectrum. As an
example, the IR spectra of AZO1 polymer and AZO2
show that full conversion of this reaction could be ob-
served due to the appearance of the methylene (-CH2-)
Property characterization of azobenzene monomers
and polymer
Fourier transform infrared spectroscopy (FTIR)
spectra of pure PPy films were recorded with a Nicolet
5700 FTIR spectrometer (Thermmo Electron Corpora-
tion) using the KBr pellet method. The spectra were
－
1
band at 2926 cm . The profile of AZO3 shows the
complete vanishing of the activated —COOH bands at
－
－
recorded from 4000 to 400 cm , with a 4 cm 1 resolu-
tion for 32 scans. Ultraviolet-visible absorption spectra
(UV-Vis) were taken with a UV-1800 spectrophotome-
ter. The photoisomerization of azopolymer film is tested
by UV absorption spectra. A series of UV absorption
spectra is obtain via irradiation with 365 nm UV light
for 5, 10, 15 , 20 and 30 s. Similarly, the different irra-
diation time of visible light is 10, 30, 60, 120 and 180
min. The wavelength domain in this UV measurement is
200—600 nm. Changes in the azopolymer surface
morphology were probed by atomic force microscopy
(AFM). The surface was observed by contact-mode
AFM (Digital Instruments Nanoscope E) and a silicon
nitrite cantilever (the typical tip radius of curvature is 15
nm, Nanoworld). In this experiment the experimental
error in the contact angle (CA) measurements is ±2°.
The deionized (DI) water was used as the test liquid for
all measurements.
1
－
1
2664 and 2540 cm , which means that the —COOH of
AZO2 has changed into ester completely. While the
－
1
profile of AZO4 shows an intense peak at 3400 cm
compared with the curve of AZO3, which tells us that
the ester of AZO3 is in a hydrolysis progress. All of
above discussed in the FTIR spectra could confirm the
structure of azobenzene derivatives basically.
BSA adsorption onto azopolymer surfaces measured
by surface plasmon resonance spectroscopy (SPR) in
situ
Figure 1 FT-IR spectra of AZO1, AZO2, AZO3 and AZO4.
A glass slide covered with gold film suitable for the
SPR apparatus (SPR 2005, Electronic Institute of the
Chinese Academy of Sciences) was pressed onto the
base of a half-cylindrical lens (n＝1.61) using an in-
dex-matching oil. Linearly p-polarized light of wave-
length 670 nm from a diode laser was directed through a
UV-Vis absorption spectra of AZO4 (in THF solu-
tion)
The light-induced isomerization of azobenzene
－
group of the AZO4 was investigated in 10 5 mol/L THF
Chin. J. Chem. 2011, 29, 153— 158
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Zhang et al.
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solution. The trans-cis photoisomerization was achieved
by irradiation with UV light of 365 nm and the back-
ward isomerization of cis-from to trans-form was
achieved by visible light of 450 nm. Figure 2a shows
the UV absorbance changes of the AZO4 irradiated by
UV at different time, 0 s, 15 s, 30 s, 1 min, 2 min, 5 min,
and 10 min. It is clear that an intense absorption peak
locates at 355 nm and a weak peak at about 445 nm. As
one knows well, the peak at 355 nm corresponds to the
π-π* electronic transition of the trans azobenzene side
chain and the peak at about 445 nm originates from the
weak n-π* electronic transition of the cis isomers. The
absorption at 250 nm is due to the π-π* and the n-π*
transition of the phenylbenzoate fragments and the aro-
matic Φ-Φ* transition of azobenzenes.24 As the UV ir-
radiation time going on, the intense absorption of π-π*
electronic transition of the trans azobenzene side chain
at 355 nm decreases. As while the peak of the weak
n-π* electronic transition of the cis isomers at 445 nm
becomes more pronounced, indicating that azobenzene
chromophores undergo a trans-cis photoisomerization
process in the AZO4. Figure 2b shows the changes in
the absorption spectra of AZO4 upon the visible light
irradiation for 0, 1, 2, 6, 15, 40, and 220 min at 450 nm.
The subsequent visible light irradiation leads to recover
of the shape of the absorbance spectra with the visible
light irradiation time. The isomerization of cis-form
achieved to trans-form gradually, and even recovered to
the state before UV irradiation.
The kinetic constants of the photoisomerization of
AZO4 were calculated with the intense of UV absorp-
tion at different irradiated time by formula (1).
A∞－A
t
ln
＝－Kt
(1)
A∞－A0
The slope of the curve formed by ln[(A －A )/(A －
∞
∞
t
A0)] to time was denoted as the kinetic constants of
AZO4 photoisomerization. The results of the calculation
shows that the constants of trans-cis photoisomerization
－
Kt-c was 4.00 min 1, and the constants of cis-trans
－
1
photoisomerization Kc-t was 0.02 min . This means the
AZO4 also has the distinctive properties that was re-
sponded quickly and recovered slowly as previously
reported.
Surface wettability of azopolymer thin film
In this work, the photoresponses of CA of water on
the azopolymer were studied under UV-Vis irradiation.
CA changes at various stages of irradiation are given in
Figure 3 by the sessile drop technique. Before UV light
irradiation, the average CA of azopolymer thin films is
approximately 77°. After irradiation by 365 nm UV
light, the average CA increased to 83°. Under the visible
light irradiation again, the average CA recovered to 77°.
The difference of the two CAs is up to 7°.24,41 It is re-
lated to the existence of carboxyl group on surface and
the photoisomerization of azo chromophores. The two
factors would cause the change from 0 (trans) to 3 D
(cis) of dipole moment of azopolymer and surface en-
ergy during the process of photoisomerization from
trans to cis.45 This contact angle change is almost com-
pletely reversible after visible light irradiation. On the
other hand, the results of CA determination indicated
the change of surface free energy of spin-coating films.
It is obvious that the azopolymer film has higher surface
free energy before UV irradiation, and then its surface
energy decreased after UV irradiation. Moreover, this
contact angle change is almost completely reversible
Figure 2 Changes in the absorption spectra of polymer in THF
upon irradiation by UV at (a) 365 nm and (b) visible light above
450 nm.
Figure 3 The contact angle of azopolymer film under UV and
visible light irradiation.
156
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Chin. J. Chem. 2011, 29, 153— 158

Photosensitive Azopolymer Brushes for Protein Sensing
after several cycles. This implies photoisomerization of
this unit in surface of azopolymer film under UV-Vis
light irradiation.
Surface morphology of azopolymers via ATRP
The morphologies of the AZO4 polymer brush sur-
faces on silicon substrates were examined under AFM
by means of a tapping mode. Figure 4 (b, c, d) shows
AFM images of AZO4 polymers with the polymeriza-
tion time of 16, 24 and 24 h. Before the preparation of
azopolymer by ATRP, the surface of silicon wafers
were extremely smoothed with the average roughness
(Ra) of 0.9 nm. However, it became rougher during the
formation of azopolymer. When the polymerization de-
gree reached to a certain constant, the polymer brushes
could turn to balls, forming AZO4 polymer nanoparti-
cles. It demonstrates that the surface become rougher
and rougher. When polymerization time was 24 h, the
vertical height, i.e., the thickness, approximately is 30
nm.
Figure 5 (a) The adsorption kinetic caves and (b) BSA adsorp-
tion amount on AZO4 polymer films.
amount decreased to 40 RU after being irradiated by
UV light for 1 h. Additionally, the adsorption amount
recovered to 130 RU again, when irradiating by Vis
light for 24 h on the polymer film. It demonstrates that
the trans-cis isomerization exchange induced by UV
irradiation led to the remarkable decrease of BSA.
Actually, with the photoisomerization of azobenzene
moieties from trans to cis state, the carboxyl groups on
the film surface would transfer into the inner of film, so
that the density of carboxyl groups on the surface of
azopolymer thin films will be changed correspondingly.
Before UV irradiation, azobenzene moieties appear in
the planar trans state and stand vertically onto the film
before UV light irradiation. The density of carboxyl
groups (—COOH) on the surface of azopolymer thin
films will be relatively large. Therefore, the surface ten-
sion will be relatively large. By absorbing UV light, the
planar azobenzene moieties switch into cis state and
assemble parallel to the film. Hence the density of car-
boxyl groups decreases on the surface, which means
that surface tension binding sites for BSA adsorption
will decrease. BSA is a protein with high surface energy.
When the azobenzene moieties present trans state be-
fore UV irradiation, the carboxyl groups with high sur-
face tension should have stronger interaction between
carboxyl groups and amino in BSA molecules. In con-
trary, when the azobenzene moieties present cis state
after UV irradiation, low surface tension of the film
Figure 4 The 3 D AFM images of (a) bare silicon wafer and
AZO4 brush films at the polymerization time of (b) 16, (c) 24 and
(d) 48 h. The surface Ra is 0.9, 7, 13 and 23 nm, respectively.
BSA adsorption before and after UV irradiation
Once the reflectivity units of SPR kinetics behavior
of AZO4 films in PBS reaches a stable state, BSA solu-
tion was introduced into the flow cell. The SPR kinetics
of BSA adsorption on AZO4 polymer films was shown
in Figure 5 (a). The systems were allowed to stabilize
approximately over a period of 15 min but showed no
further significant changes. After this stabilization pe-
riod, the samples were rinsed with PBS which, in all
cases, always caused a loss of some unbound protein
from the surface. Figure 5 (b) shows the adsorbed
amounts of BSA on polymer surfaces before and after
being rinsed by PBS. Before UV irradiation on polymer
surface, the amount of BSA adsorbed on the AZO4
polymer films was 130 RU. However, the adsorption
Chin. J. Chem. 2011, 29, 153— 158
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Zhang et al.
FULL PAPER
surface will reduce the interaction between BSA and
azopolymer films surface, which leads to less BSA ad-
sorption on the film. The film above was irradiated
again with visible light, azobenzene molecules were
isomerized from cis to trans state. The surface energy of
azopolymer film increases correspondingly.11 After UV
irradiation, the above films are exposed with visible
light. It is obviously the binding curve of BSA almost
recovered to the initial curve.
6
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Conclusions
The polymeric brushes containing azobenzene moie-
ties were prepared via ATRP as the adhesion layers for
the reversible adsorption of BSA. With the polymeriza-
tion going on, the surface roughness of azopolymer
brushes increased due to the formation of the polymer
particles. The surface wettability of azopolymer was
photo responsive since there was a substantial cycle of
CAs of the azobenzene polymer under UV-Vis irradia-
tion. Furthermore, the reversible photoisomerization of
azobenzene with a first-order isomerization rate was
detected. Since the —COOH groups in AZO4 polymer
chains point in or out from the films irradiated by UV
and visible light, the trans-cis-trans photoisomerization
properties can affect the BSA adsorption. The photode-
formable surface on the azopolymer warrants further
investigation for protein chip application since the
azopolymer can hold protein molecules without denatu-
ration caused by one-step photoirradiation.
19 Mart, R. J.; Osborne, R. D.; Stevens, M. M.; Ulijn, R. V.
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