Deep UV photopatterning of biphenyl thiol self-assembled monolayer and its physical properties

Article abstract of DOI:10.1080/15421400601027981

Aromatic thiol such as a 4'-mercapto-biphenyl-4-carboxylic acid was synthesized, and then the self-assembled monolayer (SAM) was fabricated on a gold substrate. The pattern formation of biphenyl thiol and alkanethiol SAMs was obtained by oxidation reaction due to the irradiation of deep UV light. The resulting positive pattern image and current-voltage (I-V) characteristics of biphenyl thiol and alkanethiols were investigated through the measurements of Atomic Force Microscope (AFM) and Scanning Tunneling Microscope (STM). In addition, cytochrome c protein was immobilized onto the patterned gold substrate by self-assembly technique and electrochemical activity was studied with a cyclic voltammetry.

Full text of DOI:10.1080/15421400601027981

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Deep UV Photopatterning of
Biphenyl Thiol Self-Assembled
Monolayer and its Physical
Properties
Se Young Oh a b , Hee Jeong Kim a & Hyung Seok Choi
a
a Department of Chemical and Biomolecular
Engineering , Sogang University , Seoul, Korea
b Interdisciplinary Program of Integrated
Biotechnology, Sogang University , Seoul, Korea
Published online: 12 Mar 2007.
To cite this article: Se Young Oh , Hee Jeong Kim & Hyung Seok Choi (2007) Deep
UV Photopatterning of Biphenyl Thiol Self-Assembled Monolayer and its Physical
Properties, Molecular Crystals and Liquid Crystals, 463:1, 229/[511]-236/[518], DOI:
10.1080/15421400601027981
To link to this article: http://dx.doi.org/10.1080/15421400601027981
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Mol. Cryst. Liq. Cryst., Vol. 463, pp. 229=[511]–236=[518], 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400601027981
Deep UV Photopatterning of Biphenyl Thiol
Self-Assembled Monolayer and its Physical Properties
Se Young Oh
Department of Chemical and Biomolecular Engineering,
Sogang University, and Interdisciplinary Program of Integrated
Biotechnology, Sogang University, Seoul, Korea
Hee Jeong Kim
Hyung Seok Choi
Department of Chemical and Biomolecular Engineering,
Sogang University, Seoul, Korea
Aromatic thiol such as a 40-mercapto-biphenyl-4-carboxylic acid was synthesized,
and then the self-assembled monolayer (SAM) was fabricated on a gold substrate.
The pattern formation of biphenyl thiol and alkanethiol SAMs was obtained by
oxidation reaction due to the irradiation of deep UV light. The resulting positive
pattern image and current-voltage (I-V) characteristics of biphenyl thiol and alka-
nethiols were investigated through the measurements of Atomic Force Microscope
(AFM) and Scanning Tunneling Microscope (STM). In addition, cytochrome c pro-
tein was immobilized onto the patterned gold substrate by self-assembly technique
and electrochemical activity was studied with a cyclic voltammetry.
Keywords: biphenyl thiol; current-voltage characteristic; cytochrome c; electrochemi-
cal activity; self-assembled monolayer; UV photopatterning
INTRODUCTION
Over the past several years, self-assembled monolayers (SAMs) have
attracted much attention because of their scientific interest in two-
dimensional molecular assemblies and their potential applications
in molecular device, sensor and surface engineering etc. [1–4]. The
This work was supported by the Korean Research Foundation Grant (KRF-2004-005-
D00003).
Address correspondence to Se Young Oh, Department of Chemical and Biomolecular
Engineering, Sogang University, Shinsoo-dong 1, Mapo-gu, Seoul 121-742, Korea.
E-mail: syoh@sogang.ac.kr
229=[511]

230=[512]
S. Y. Oh et al.
structure of SAMs can be finely controlled and modified by attaching a
wide range of molecules that provide different functions to the SAMs.
SAMs allow the formation of a variety of surfaces with specific inter-
actions and fine structural control. Especially, modified SAMs have
been used in studies of molecular recognition [5], biomolecular recep-
tor [6], cell growth [7], crystallization [8], and many other systems [9].
In general, alkanethiols having carboxylic acid in the tail group
have been used in the fabrication of self-assembled biomolecule mono-
layers [10–12]. However, the electroactivity and sensitivity of biosen-
sor or bioelectronic device using the alkanethiol as biomolecular
receptor were very low due to the flexible nature of carbon chains
and poor electrical conductivity. The electrical conductivity of aro-
matic thiol is high due to the conjugated backbone system, and also
aromatic thiol SAMs show good packing structure because of inter-
chain interaction between benzene groups. In the present work, we
have synthesized 40-mercapto-biphenyl-4-carboxylic acid as biomolecu-
lar receptor, and then fabricated pattern of biphenyl thiol SAM by
photochemical conversion of thiol to sulfonate with a deep UV light.
In addition, cytochrome c protein was immobilized onto the patterned
gold substrate. Also, we report the pattern image and the electro-
chemical activity of biphenyl thiol with a high electrical conductivity
and conventional alkanethiols.
EXPERIMENTAL
4-carboxyphenylboronic acid, 4-bromothioanisole, tetrakis(triphenyl-
phosphine)palladium(0) (Pd(PPh3)4), sodiumthiomethoxide, 3-mercap-
topropanoic acid (3-MPA), 11-mercaptoundecanoic acid (11-MUDA)
and cytochrome c (extracted from horse heart muscle) were purchased
from Aldrich Chemical Co., (USA). Au (111) substrate was purchased
form INOSTEK Inc. (Korea). N,N-dimethylformamide (DMF), ethyle-
neglycoldimethylether (DME), N-methyl-2-pyrrolidinone (NMP),
methanol and ethanol were distilled from first grade solvents pur-
chased on the market.
Synthesis of 40-mercapto-biphenyl-4-carboxylic acid (MBCA) was
carried out according to the synthetic routes as shown in Scheme 1.
40-methylsulfanyl-biphenyl-4-carboxylic acid (1) was synthesized by
Suzuki coupling using approximately 6.12 g (0.03 mol) of 4-
bromothioanisole and 5.0 g (0.03 mol) of 4-carboxyphenylboronic acid
in the presence of 5 mol% of Pd(PPh3)4, 2 M K2CO3 aqueous solution
and 100 mL DME solvent. The reaction was carried out under nitrogen
atmosphere at 80ꢀC for 18 hrs. The reaction mixture was filtered
through a glass filter and washed with distilled water. The crude

Photopatterning of Biphenyl SAM and its Physical Properties
231=[513]
SCHEME 1 Synthetic routes of MBCA.
product was recrystallized from methanol (yield: 94%). 40-sodiummer-
capto-biphenyl-4-carboxylic acid (2) was synthesized by reaction of 1 and
sodiumthiomethoxide (5 equiv.) in NMP solvent under nitrogen. The
reaction ran at 100ꢀC for 48 hrs and the NMP solvent was vacuum dis-
tilled. The final product, MBCA, was prepared by the reaction of 2 and
hydrochloric acid in aqueous solution for 1 hr. The reaction mixture
was filtered through a glass filter and washed with distilled water. The
residual MBCA powder was dried under vacuum at 40ꢀC for 24 hrs (yield:
25%). Elemental analysis; Calculated C: 67.80%; H: 4.38%; S:13.92%;
Found C: 67.51%; H: 4.50%; S: 14.13%.
Gold substrate was immersed into 5 mM MBCA DMF solution for
48 hrs. The substrate was washed with DMF and ethanol, and
then dried in nitrogen stream. Successful formation of MBCA SAM
was identified by FT-IR analysis (Infiniti gold FT-IR 60AR, Thermo-
mattson, USA).
Photopatterning of MBCA SAM was carried out with an exposure
system of Spectral Energy Co., equipped with a 500 W high-pressure
mercury lamp in conjunction with a narrow band pass filter for
254 nm. The self-assembled MBCA monolayer on a gold substrate
was oxidized by deep UV light irradiation, and then developed with
deionized water. The pattern image was investigated using an AFM
(AutoProbe CP, Park Scientific Instruments, USA). Cytochrome c

232=[514]
S. Y. Oh et al.
was adsorbed on the MBCA SAM by immersing the patterned gold
substrate in a 30 mmol phosphate buffer solution (pH 7.2) containing
50 mmol cytochrome c for 12 hours, followed by thorough rinsing with
the buffer solution and deionized water. Cyclic voltammetry measure-
ment was carried out with a potentiostat measurement analyzer (IM6
system, Zahner Elektrik Co., Germany). The Ag=AgCl (3.0 M KCl) for
reference electrode, a gold working electrode and a 1-cm2 Pt-gauze
counter electrode were used for the cyclic voltammetry measurements.
The I-V characteristics of biphenyl thiol and alkanethiol SAMs were
studied by a STM (BT00642, Nano Surf AG, Switzerland) technique.
The set point for Au tip approaching was 0.5 nA and the scan range
for conductivity measurement was ꢁ1ꢂ1 V. Cyclic voltammetry was
also carried out for measuring electrochemical activities of the fabri-
cated SAMs in potassium ferricyanide solution (0.01 M Phosphate
buffered saline, 0.138 M NaCl, 0.0027 M KCl and 1 mM K3Fe(CN)6 in
deionized water).
RESULTS AND DISCUSSION
The synthesized MBCA compound was identified through the mea-
surements of elemental analysis and FT-IR. The FT-IR spectrum of
MBCA was shown in Figure 1a. The spectrum of MBCA showed the
absorption peaks corresponding to thiol and carboxylic groups, e.g.,
OꢁH stretching at 3000 cmꢁ1, SꢁH stretching at 2550 cmꢁ1 and C¼O
stretching at 1700 cmꢁ1. The bands near 1650 cmꢁ1 and 1450 cmꢁ1 cor-
responded to C¼C stretching of a benzene ring. Figure 1b showed the
FT-IR spectrum of MBCA SAM fabricated on a gold substrate. The FT-
IR measurement was carried out by attenuated total reflectance
method. The spectrum of MBCA SAM was similar to the result of
MBCA compound. It can be concluded that the successful preparation
of MBCA SAM was confirmed through FT-IR analysis.
Photopatterning of MBCA SAM onto the gold substrate was carried
out with a 7 mm positive mask by the irradiation of deep UV light. Sul-
fonates oxidized in the exposed regions were developed thoroughly
with deionized water. The UV irradiation time for patterning of alka-
nethiol and biphenyl thiol SAMs was 2 hrs and 5 hrs, respectively.
This result was obtained from the measurements of contact angle after
UV irradiation. In order to explore the patterned image, the gold sub-
strate was etched in an aqueous solution of KCN 0.1 M and KOH 1 M.
Figure 2 showed the AFM image. The patterned MBCA SAM surface
regions exhibited bright contrast while exposed regions exhibited dark
contrast because of the difference in percolation rate of etching
solution. It can be found that positive pattern of MBCA monolayer

Photopatterning of Biphenyl SAM and its Physical Properties
233=[515]
FIGURE 1 FT-IR spectra of MBCA and MBCA SAM. a) MBCA, b) MBCA
SAM.
FIGURE 2 AFM image of photopatterned MBCA SAM.

234=[516]
S. Y. Oh et al.
with a 7 mm line was obtained by the photochemical conversion of
thiols to sulfonates.
Figure 3 showed the I-V characteristics for an Au probe in contact
with biphenyl thiol and alkanthiol SAMs. The electron transfer
between the SAMs surface and STM probe has occurred, when a volt-
age was biased in the range ꢁ1 ꢂ 1 V. The current of biphenyl thiol
was higher than that of alkanethiol because of low gap resistance
due to the conjugated backbone structure. Figure 4 showed the cyclic
voltammograms of cytochrome c protein immobilized onto the each
SAMs. The electrochemical activity of cytochrome c immobilized onto
the biphenyl thiol SAM was higher than that of alkanethiol. In order
to explore the effect of biphenyl thiol biomolecular receptor having
high electrical conductivity on the performance of electoactivity of
cytochrome c, cyclic voltammetry measurements of each SAMs and
bare gold substrate in potassium ferricyanide solution was carried
out. The biphenyl thiol SAM in potassium ferricyanide solution
allowed high electrochemical activity compared to the alkanethiol
SAMs as shown in Figure 5. Both results derived from cyclic voltam-
metry measurements indicated that high electrochemical activity of
cytochrome c immobilized onto the biphenyl thiol SAM is mainly due
to the difference in charge transport between cytochrome c and self-
assembled molecular wire.
FIGURE 3 STM I-V characteristics of MPA, MUDA and MBCA monolayer
junctions.

Photopatterning of Biphenyl SAM and its Physical Properties
235=[517]
FIGURE 4 Cyclic voltammograms of cytochrome c immobilized onto the
biphenyl thio and alkanethiol SAMs.
FIGURE 5 Cyclic voltammogram of Bare gold, MBCA SAM and MUDA SAM
in potassium ferricyanide solution.

236=[518]
S. Y. Oh et al.
CONCLUSIONS
Micropattern of self-assembled biphenyl thiol monolayer on a gold sub-
strate was obtained by photochemical oxidation using a 7 mm positive
mask. Cytochrome c molecules were well self-assembled onto the
positive patterned gold substrate. It can be found that cytochrome c
protein immobilized onto the biphenyl thiol SAM was very effective
in a view of electrochemical activity. Thus, it should be noted that aro-
matic thiol as a conducting bioreceptor exhibited the feasibility of
application in biosensor or bioelectronic device using electrochemical
activity.
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