Surface-grafting of polyglutamate on Si wafer using micro contact printing

Article abstract of DOI:10.1080/15421400601031017

Surface-grafting of poly(-benzyl-, L-glutamate) (PBLG) on the 3-aminopropyl triethoxysilane (APS) pretreated silicone substrates was conducted by the monomer, -benzyl-L-glutamate N-carboxyanhydride (BLG-NCA) which was synthesized through the phosgenation procedure. PBLG, one of liquid crystalline polymers, micro-patterns were fabricated by micro-contact printing (CP) and surface-grafting method. Then, PBLG patterns were characterized with electron microscopy (SEM), atomic force microscopy (AFM), infrared spectroscopy (FT-IR), Raman spectroscopy and polarized optical microscopy. -helix conformation of PBLG was dominated on the patterned surface which was confirmed by FT-IR spectroscopy. From the polarizing microscope images, lyotropic liquid crystalline properties of the patterned surfaces were observed. The SEM and AFM images show that the height of PBLG patterns was 300nm and line width was 15m. The interval between lines was about 5m.

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Surface-Grafting of
Polyglutamate on Si Wafer
Using Micro Contact Printing
Jonghyub Kim a , Jaehyun Park a , Seongyeol Lee a
Daewon Sohn a
&
a Department of Chemistry , Hanyang University ,
Seongdong-gu, Seoul, Korea
Published online: 22 Sep 2010.
To cite this article: Jonghyub Kim , Jaehyun Park , Seongyeol Lee & Daewon
Sohn (2007) Surface-Grafting of Polyglutamate on Si Wafer Using Micro Contact
Printing, Molecular Crystals and Liquid Crystals, 464:1, 211/[793]-216/[798], DOI:
10.1080/15421400601031017
To link to this article: http://dx.doi.org/10.1080/15421400601031017
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Mol. Cryst. Liq. Cryst., Vol. 464, pp. 211=[793]–216=[798], 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400601031017
Surface-Grafting of Polyglutamate on Si Wafer using
Micro Contact Printing
Jonghyub Kim
Jaehyun Park
Seongyeol Lee
Daewon Sohn
Department of Chemistry, Hanyang University, Seongdong-gu,
Seoul, Korea
Surface-grafting of poly(c-benzyl-a, L-glutamate) (PBLG) on the 3-aminopropyl
triethoxysilane (APS) pretreated silicone substrates was conducted by the mono-
mer, c-benzyl-L-glutamate N-carboxyanhydride (BLG-NCA) which was synthe-
sized through the phosgenation procedure. PBLG, one of liquid crystalline
polymers, micro-patterns were fabricated by micro-contact printing (lCP) and sur-
face-grafting method. Then, PBLG patterns were characterized with electron
microscopy (SEM), atomic force microscopy (AFM), infrared spectroscopy
(FT-IR), Raman spectroscopy and polarized optical microscopy. a-helix confor-
mation of PBLG was dominated on the patterned surface which was confirmed
by FT-IR spectroscopy. From the polarizing microscope images, lyotropic liquid
crystalline properties of the patterned surfaces were observed. The SEM and
AFM images show that the height of PBLG patterns was 300 nm and line width
was 15 lm. The interval between lines was about 5 lm.
Keywords: lyotropic liquid crystalline property; micro-contact printing; PBLG; surface
grafting
1. INTRODUCTION
In recent years, ultra thin (from monolayer to 1 mm) organic films have
been considerably focused for their great potentials in electronic and
opto-electronical applications such as non-linear optics (NLO), liquid
crystal displays (LCD), solar cells and biosensors. There have been
This work was supported by the research fund from ABRL program of KRF. J. Kim
thanks the National R&D project for Nano Science & Technology in ITEP.
Address correspondence to Daewon Sohn, Department of Chemistry, Hanyang
University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea. E-mail: dsohn@
hanyang.ac.kr
211=[793]

212=[794]
J. Kim et al.
many efforts to bottom up control the thickness and the properties of
these thin films, which are the key issues for their applications. The
molecular orientation in thin films with respect to the substrate is
very important for several applications. Since the oriented organic
layers can exhibit optoelectronic and piezoelectric properties, they
can be applied to electronic and opto-electronic devices.
Polyglutamate is a well known thermotropic and lyotropic liquid crys-
talline polymer which has a-helix conformation. In our experiments,
poly(c-benzyl, L-glutamate) was synthesized and grafted on the silicon
wafer to generate the polymer brushes having rodlike characteristics.
Polyglutamate can easily be synthesized by the ring-opening polymeriza-
tion of N-carboxyanhydrides (NCAs) [1]. N-carboxyanhydrides were pre-
pared by phosgenation of the corresponding L-glutamic acid esters [2–7].
The polymerization process was initiated with primary amines through
the ‘‘amine mechanism’’ [8,9]. Most polymerizations are performed in
polar solvents such as acetonitril, dioxane, and N,N0-dimethylformamide
(DMF) in order to keep the formed polypeptide material in solution for a
long time. In particular, at temperatures between 40 ꢀ 60ꢁC, the DMF
molecules prevent the aggregation of the a-helices structure. At the high
concentration of the polypeptide, lyotropic liquid crystalline behaviors
could be observed [10]. Aims of these works are making the patterned
rodlike brushes and investigate the liquid crystalline behaviors on the
silicon wafer surface, which could be used as opto-eletronical devices.
2. EXPERIMENTS
2.1. Preparation of Polyglutamate Thin Film on Si Wafer
Prepolymer, BLG-NCA, was synthesized with L-glutamate, c-benzyl
ester (BLG) and triphosgene. 2 g of BLG (0.08 mol) was added to
15 ml of THF under N2 atmosphere, and then it was set on the reflux
process. 1.5 g of triphosgene (0.05 mol) was added to 15 ml of THF. And
it was moved to BLG=THF mixture by cannulation under N2 atmos-
phere. Reaction was performed for 5 hrs and product was obtained
by recrystallization method using n-hexane. Obtained product was
performed rephosgenation for 5 hrs one more time.
A silicon surface was covered with patterned 3-aminopropyl-
triethoxysilane (APS) or 4-aminobutyltriethoxysilane (ABS) layer
using soft lithography method. For the soft lithography method, pat-
terned PDMS stamp was drenched with 0.5 mol initiator solution.
Then, pretreated Si wafer (hydroxylated Si wafer) was covered with
PDMS stamp for 2 hrs. The PDMS stamp was peel off, and initiated
Si wafer was rinsed with toluene, acetone, and absolute ethanol
to remove untethered initiator unit. At the beginning, 0.2 M of

Surface-Grafting of Polyglutamate using Micro Contact Printing
213=[795]
BLG-NCA solution was prepared in DMF solution. A silicon wafer
which covered with patterned APS or ABS layer was immersed into
BLG-NCA solution for 5 hrs at 60ꢁC. APS or ABS was used as an
initiator for the polymerization processes. Any amine terminal
organo-silane could be used. Temperature must be under 60ꢁC.
Decomposition of polymer started at over 60ꢁC. At the end, a silicon
wafer which covered with polyglutamates was rinsed by DMF and
finally ethyl alcohol. BLG-NCA was characterized by FT-IR, in which
a-helix structure of PBLG on Si was observed from no absorption
peaks of b sheet conformation. Figure 1 presents the scheme for the
surface grafting processes of polyglutamate on Si wafer.
FIGURE 1 Scheme of surface grafting of polyglutamate on Si wafer.

214=[796]
J. Kim et al.
2.2. FE-SEM Measurement
Samples were observed with FE-SEM (JEOL Ltd., JSM-6300). At
an emission current 12 ꢂ 10ꢃ6 A and a probe current of 1 ꢂ 10ꢃ11 A.
Samples were observed with non-conductive over-coating. Clear FE-
SEM contrasts for the micropatterned APS and PBLG samples were
obtained at 0.5 kV, Vacc, where the contrast did not depend on Vacc
.
3. RESULTS AND DISCUSSION
3.1. c-benzyl, L-glutamate N-carboxyanhydride (BLG-NCA)
The BLG-NCA was synthesized by phosgenation of c-benzyl L-gluta-
mate (20 g, 0.084 mol) using triphosgene (10 g, 0.034 mol) in anhydrous
THF. The crude product was purified by rephosgenation in the ratio of
crude BLG-NCA=triphosgene (8=1; w=w). The BLG-NCA was recrys-
tallized from a mixture of THF=n-hexane (1=5; v=v). The recrystallized
product was filtrated and dried completely in vacuum. The weight of
product was 19 g, 88% yield.
The stretching mode of carbonyl group always has two bands at
1830 ꢀ 1800 cmꢃ1. Conjugation moves the absorption to a lower fre-
quency. On the other hands, the ring strain of cyclic anhydride moves
the absorptions to a higher frequency. From the two absorptions at
1782 cmꢃ1 (anhydride band 2) and 1878 cmꢃ1 (anhydride band 1), it
can be concluded that cyclic anhydride, NCA ring was formed. There
are no absorptions at 1725 ꢀ 1700 cmꢃ1 which means no carboxylic
acid residue exits. And no absorption of hydroxyl group was shown
FIGURE 2 The FT-IR spectra. (a) BLG-NCA: two band of anhydride carbonyl
group were shown at 1878 cmꢃ1 (band 1) and 1782 cmꢃ1 (band 2). (b) PBLG on
Si wafer: there is no trace of b-sheet, amide A at 1630 cmꢃ1 and amide II at
1530 cmꢃ1. a-helix is the most stable conformation for PBLG molecule.

Surface-Grafting of Polyglutamate using Micro Contact Printing
215=[797]
FIGURE 3 (a) FE-SEM image of PBLG pattern. The line width of PBLG
patterns was 10 mm. The distance between the pattern lines was about 5 mm.
(b) Polarizing Optical Microscope image of PBLG pattern on APS initiated
Si wafer. Images were 500 magnifications.
in Figure 2(a). In Figure 2(b), we could verify the a-helix conformation
of grafted PBLG. [11] amide A band and amide II band should be
observed at 1630 cmꢃ1 and 1530 cmꢃ1 for the b-sheet conformation,
but there is no these peaks.
Figure 3(a) shows FE-SEM image of patterned PBLG on Si
wafer surface. After polymerization PBLG line width became 10 mm.
Diffusion of APS and orientation of PBLG increase its width more
than APS monolayer pattern. There are several small spots on PBLG
pattern due to the APS residues.
The lyotropic liquid crystalline property of the PBLG pattern on
the silicon wafers was investigated in DMF solvent system. A drop
of DMF was added to the PBLG patterned surface and the solvent
was evaporated for 30 min. at 50ꢁC. It was observed by polarizing
microscope and shown in Figure 3(b). When we rotate the angle of
the polarizer, we could observe anisotropic image on the PBLG pat-
terned surface.
PBLG pattern was also observed by AFM. Figure 4 shows the edge
region is higher than the middle region. The edge region on the pattern
is 500 nm tall and the middle region is 250 nm height. The edge effect
covers around 3 mm width on both ends. We could obtain relatively uni-
form pattern with less than 5 mm width which has no edge effect.
4. CONCLUSION
We report the preparation and the characterization of PBLG pattern
on Si wafer by surface-initiated grafting of polyglutamate, which could

216=[798]
J. Kim et al.
FIGURE 4 AFM images of PBLG pattern. The edge region on pattern is
500 nm tall and the center region is 250 nm.
be used as an optoelectronic device. The first step was to prepare pri-
mary amine functionalized Si surface with 3-aminopropyltriethoxysi-
lane (APS) and to fabricate initiator patterns on the solid substrates
by the micro-contact printing (mCP) method. The primary amine
groups initiate the ring opening polymerization to produce patterned
polyglutamate thin films on the Si surface.
The SEM and AFM images showed that the PBLG pattern on the Si
wafer were prepared from 1 ꢀ 20 mm width successfully. In cases, no
other conformation than a-helix was detected by FT-IR spectroscopy.
The lyotropic liquid crystalline behaviors were shown in polarizing
microscope images due to the molecular orientation of PBLG.
PBLG has been known as liquid-crystalline materials, and soft litho-
graphy method has been used as the tool in designing of electronic
circuit and optoeletronical devices.
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