J. Biomedical Science and Engineering, 2011, 4, 242-247 JBiSE
doi:10.4236/jbise.2011.44033 Published Online April 2011 (http://www.SciRP.org/journal/jbise/).
Published Online April 2011 in SciRes. http://www.scirp.org/journal/JBiSE
Immobilization of antibodies on the self-assembled monolayer
by antigen-binding site protection and
immobilization kinetic control
Myungok Yoon1, Hyun Jin Hwang2, Jeong Hee Kim3
1Department of Chemistry, Kyung Hee University, Seoul, Korea;
2R&D Center, Ahram Biosystems Inc., Seoul, Korea;
3Department of Oral Biochemistry, Kyung Hee University, Seoul, Korea.
Received 17 January 2011; revised 23 March 2011; accepted 28 March 2011.
ABSTRACT
The orientation of the biological molecule immobi-
lized on a solid surface has been critical in devel-
opment of various applications. In this study, ori-
entation of antibody was retained by protecting the
antigen-binding site of the antibody prior to immo-
bilization to -functionalized mixed self-assembled
monolayer (SAM) of 12-mercaptododecanoic acid
and 1-heptanethiol. More importantly, the number
of immobilization bonds formed between each an-
tigen-binding site protected antibody molecule and
the solid surface was controlled by optimizing the
mole fraction of the activated carboxyl group of the
linker molecules in the mixed SAM. The amount of
antibody used in this study was approximately
equivalent to the amount for one monolayer surface
coverage. The resulting activity of protected immo-
bilized antibody was about 10 fold higher than that
of random immobilized antibody.
Keywords: Antibody; Oriented Immobilization;
Antigen-Binding Site Protection;
Self-Assembled Monolayer; Kinetic Control
1. INTRODUCTION
Interests in the efficient immobilizations of biomolecules
such as enzymes, proteins, antibodies, and DNA [1-6] on
the solid surfaces for biomedical and biotechnological
applications have rapidly increased during the last two
decades. Immunoassays, affinity chromatography, and
DNA microarray [3,7,8] are all based on the immobiliza-
tion of biomolecules on the solid phases for the purpose
of clinical diagnostics, food industry and environmental
monitoring [9-11]. Widely used methods for the attach-
ment of antibodies to solid surfaces are physical adsorp-
tion, covalent coupling, cross-linking, or entrapment in a
gel network [4,12-14]. However, since these methods
may result in decreased binding activity and selectivity
of the antibodies after immobilization due to improper
orientations, or denaturing of the antibodies, much effort
have been recently put in the development of site-spe-
cific immobilization of antibodies [3,15-18]. Among these
are immobilizing protein A or G first to the solid surface
followed by immobilization of antibodies [16,19]. In an-
other method, azobenzene-containing polymers were used
to control antibody orientation [20]. It was reported that
the antigen-binding activities of immobilized Fab’ frag-
ments of rabbit anti-human IgG with proper orientation
were more than 2 fold increase than those with random
orientation [15].
In this study, in order to maximize the natural bio-
logical activity of an antibody after immobilization, the
antigen-binding sites of the antibody were protected by
incubating with its own antigen prior to immobilization.
More importantly, the number of chemical bond formed
between the antigen-binding site-protected antibody and
the solid surface was optimized by kinetic control of the
immobilization reaction (protected immobilization, PIM).
The resulting activity of the antigen-binding site pro-
tected immobilized antibody was significantly increased
compared to that of randomly immobilized antibody.
2. MATERIALS AND METHODS
2.1. Chemicals and Reagents
Chemicals for immobilization reaction are purchased
from the following sources; (D,L)-thioctic acid (Aldrich,
USA), 1-ethyl-3-[3-(dimethylamino)propyl]carbamide (EDC)
(Sigma, USA), and N-hydroxysulfosuccinimide (sulfo-
NHS) (Pierce, USA). Au-coated slides were purchased
from EMF, USA. Mouse anti-DNA monoclonal antibody
(IgM) recognizes both single- and double-stranded DNA

M. Yoon et al. / J. Biomedical Science and Engineering 4 (2011) 242-247 243
was obtained from Roche Diagnostics, Germany. Taq
polymerase, and deoxy nucleotide mixture (dNTP) were
obtained from Takara, Japan. [-35S] d-ATP (1250 Ci/
mmole) and scintillation cocktail solution were pur-
chased from NEN and ICN, USA, respectively. Univer-
sal and reverse primers were synthesized from Bioneer,
Korea. Other chemicals were purchased from Sigma,
USA, or from other common sources.
2.2. Formation of the Mixed SAM on the Au
Surface
Au-coated glass slides (3 mm × 5 mm) were carefully
cleaned with Piranha solution (30% H2O2: Concentrated
H2SO4 = 1:3) for 15 - 30 sec and rinsed with d-H2O and
then ethanol. The cleaned bare Au surface was soaked in
10 mM thioctic acid in ethanol for overnight rinsed with
ethanol and dried. Mixed solution of 12-mercaptodode-
canoic acid [HS(CH2)11COOH] and 1-heptanethiol [HS-
(CH2)6CH3] was prepared in ethanol. Au-coated glass
slides were incubated with the mixed SAM solution for
1 hr at room temperature and then rinsed with ethanol.
The thiol groups were chemically adsorbed to the Au
surface, thereby creating a mixed monolayer of 12-mer-
captododecanoic acid and 1-heptanethiol. Then, Au-
coated glass slides were immersed in 5 mM sulfo-NHS
and 10 mM EDC in MES buffer (pH 6.0) for 1 hr to ac-
tivate the carboxyl groups on the surface and rinsed with
ethanol.
2.3. Radioactive Labeling of DNA by Polymerase
Chain Reaction (PCR)
Bacterial plasmid DNA, pBluescriptII KS(+) was used
as a template DNA. Concentration of DNA was meas-
ured by UV spectrophotometer (Pharmacia Biotech Ul-
traspec 2000, USA). A typical polymerase chain reaction
(PCR) mixture contained 200 ng of DNA, 0.4 M each
of universal and reverse primer, 50 M of dNTP, 2.5 U
of Taq polymerase and 0.1 vol. of 10x buffer in 100 l
final volume. For labeling purpose, 2 l of [-35S] d-
ATP was added to the reaction mixture. The reaction
mixture was heated to 94˚C for 5 min. The PCR profile
was 94˚C for 30 sec, 50˚C for 1 min, and 72˚C for 30 sec
for 30 cycles, followed by 72˚C for 10 min. We always
ran labeling reactions with the non-labeled standard con-
trol reaction side by side. After PCR, an aliquot of the
control reaction was analyzed on 1.2% agarose gel con-
taining 0.5 g/ml ethidium bromide to confirm the gen-
eration of PCR product.
2.4. Immobilization of Anti-DNA Antibody and
Immunoassay
In order to prepare protected antibody-DNA solution,
approximately 0.54 pmol of anti-DNA antibody was
incubated with approximately 1.07 pmol of labeled DNA
for 1 hr at 37˚C in 0.1 M phosphate buffer (pH 7.4).
Otherwise the concentration of antibody and DNA were
indicated in the text. The activated Au surface was incu-
bated with protected Antibody-DNA solution for 30 min
at 37˚C, rinsed with a buffer of 1.0 M potassium phos-
phate, pH 6.7. Randomly immobilized antibody was
prepared by the same procedure described above except
the incubation with labeled DNA. The antibody immobi-
lized Au-coated glass slides were incubated with ap-
proximately 2.0 pmol of labeled DNA for 2 hrs at RT
and then washed with TBST (20 mM Tris, pH 7.8, 150
mM NaCl, and 0.05% Tween-20) buffer three times. The
glass slides were dried and the -emission was measured
with a scintillation counter (Wallac, system 1400, EG&G
Co., Finland).
3. RESULTS AND DISCUSSION
3.1. Experimental Scheme
Immobilization scheme of anti-DNA antibody is shown
in Figure 1. In this experiment, Au surface was chosen
as a solid phase since it has an advantage over polysac-
Figure 1. Schematic representation of the steps of anti-DNA antibody immobilization to the mixed
SAM on Au surface. PIM; protected immobilization, RIM; random immobilization.

M. Yoon et al. / J. Biomedical Science and Engineering 4 (2011) 242-247 244
charides, polystyrene or silica which are most frequently
used solid phases for the immobilization of antibodies;
thiols form self-assembled monolayers (SAM) on Au
surface spontaneously due to the formation of strong
Au-S covalent bonds [15,21,22], which make follow-up
reactions for modification of the surface functional groups
easier. The mixed monolayer of 12-mercaptododecanoic
acid [HS(CH2)11COOH] and 1-heptanethiol [HS(CH2)6-
CH3] on Au surface was used in this work. Surface car-
boxyl groups on the SAM were activated using 1-ethyl
-3(3-dimethylaminopropyl)carbodiimide (EDC) and sulfo-
N-hydroxysuccinimide (sulfo-NHS) to form sulfo-NHS
esters. This coupling reaction was performed in 2-(N-
morpholino)ethane sulfonic acid (MES) buffer at pH 6.0
since it is reported that sulfo-NHS ester has longer life-
time at lower pH [23]. Then, anti-DNA antibodies were
reacted to be immobilized to the -functionalized SAM
through amide bonds.
In order to preserve the natural activity of the anti-
body after immobilization, the antigen-binding site of
antibody were protected before immobilization to the Au
surface by reactions with its antigen, DNA first to form
antigen-antibody complexes followed by the reactions
with sulfo-NHS esters (Protected immobilization, PIM).
By this way the active sites are excluded from the sub-
sequent immobilization reaction, thus contribute for the
antibody to retain the proper orientation after immobili-
zation. For random immobilization (RIM), antibody was
immobilized as described above without protection of
the antigen-binding sites of the antibody.
3.2. Kinetic Control and Protection of
Antigen-Binding Site Increased the Activity
of Immobilized Antibody
We used the immobilization scheme presented in Figure
1 to immobilize antibodies on the SAM formed on Au
surface. Before immobilization, the amount of the anti-
body required to cover the Au surface to a monolayer
was calculated and approximately 0.54 pmol of anti-
DNA antibody was used. The number of surface car-
boxyl group involved in the cross-linking of the antibody
to the SMA was also considered. Due to the steric re-
quirement of large bio-molecules, high concentration of
surface carboxyl group was found to rather decrease the
activity of immobilized biomolecule [24]. Therefore,
considering the size of anti-DNA antibody (8.5 nm ×
14.5 nm) [2], mixed monolayer of 12-mercaptodode-
canoic acid and 1-heptanethiol was employed in our ex-
periment instead of using pure monolayer of 12-mer-
captododecanoic acid. Therefore, by controlling the mole
fraction of 12-mercaptododecanoic acid in the SAM, the
number of the carboxyl group involved in the immobili-
zation of antibodies can be controlled, subsequently the
number of antibody immobilized on the surface is con-
trolled.
In order to protect the antigen-binding site of the an-
tibody, a 65 bp double stranded DNA (ds-DNA) labeled
with 35S was prepared by polymerase chain reaction
(PCR) and about 1.07 pmole of the labeled ds-DNA was
used for protection of the two antigen-binding sites in
each antibody (Antibody:DNA ≈ 1:2). For RIM, prein-
cubation of the antibody with the labeled DNA step was
excluded. After immobilization, glass slides were incu-
bated with its labeled form of antigen, 35S-labeled ds-
DNA. The activities of the immobilized antibodies were
measured by counting -emission from antigen-antibody
complexes which were formed by incubating immobi-
lized antibodies with 35S-labeled DNA. Radioimmuno-
assay is very sensitive to a very small amount of 35S-
labeled DNA, thus enables us to measure a very small
amount of immobilized antibody on the surface.
The concentration of carboxyl group in the SAM was
varied from 0% to 100% and the activity of immobilized
antibody was measured as described above. The PIM
antibodies preserved their activity much better than the
RIM antibodies resulting -emission from these films
are larger throughout the carboxyl group ratio used in
this study (Figure 2(a)). It is very interesting to note that
the maximum activities of immobilized antibodies occur
at low surface carboxyl concentration of 5% (Figures 2
(a) and (b)). The activity of the immobilized antibodies
by PIM method at 5% of carboxyl group was approxi-
mately 10 times higher than the activity of RIM antibody
(Figure 2(b)).
In this experiment we labeled PIM antibodies with 35S
and then immobilization was performed. After these
treatments, part of antibodies would lose their activity
through kinds of modifications or other unknown me-
chanisms. If antibodies have been labeled by 35S, iso-
topes on inactivated antibodies would not be all released
from the Au surface, causing false conclusions. However,
it seems that these effects are negligible when we com-
pared the radioactivity acquired from pre-labeled PIM
antibodies and not pre-labeled RIM antibodies at higher
concentration of carboxyl group (25% - 100%) which
showed very low radioactivity in both PIM and RIM
antibodies (Figure 2(a)).
In the coupling reaction of antibody with sulfo-NHS
ester, either the amino group on the Fc region of the an-
tibody, or the one on the Fab’ fragment near the anti-
gen-binding site can react to form an amide bond; the
former will preserve the native structure of the antibody
and the latter may lose the native structure of the anti-
body. When the antibody was reacted randomly, it was
found that control over the orientation of the immobi-
lized antibody was difficult, and this random orientation

M. Yoon et al. / J. Biomedical Science and Engineering 4 (2011) 242-247 245
(a) (b)
Figure 2. The activity of immobilized anti-DNA antibody to the mixed SAM on Au surface. The activi-
ties of immobilized antibodies by PIM method (●) or RIM method (○) were obtained as a function of the
mole fraction of 12-mercaptododecanoic acid (a). The activity of the immobilized antibody either by PIM
or RIM method at 5% of 12-mercaptododecanoic acid was compared (b). A mixed SAM of 12-mercap-
tododecanoic acid and 1-hepthanethiol was used to introduce the carboxyl groups as the reactive group
for immobilization. The protection ratio of antibody to antigen was 1:2.
of the antibody results in the loss of the activity of the
immobilized antibody. However, when the antigen bind-
ing sites of the antibody were protected before immobi-
lization, the activity of the PIM antibody was signifi-
cantly increased compared to that of RIM antibody re-
sulting -emission from these films are much larger as
shown in Figure 2.
In addition to the protection of antigen-binding sites,
the increased activity at lower carboxyl group concentra-
tion can be explained in terms of number of bonds
formed between antibody and the supporting surface.
Since there are multiple reaction groups exist on the
surface of the antibody as well as the supporting surface,
multiple immobilization bonds can be formed between
the antibody and the supporting surface. Such non-spe-
cific formation of multiple bonds in various region of the
antibody can induce structural change and destruction of
the biologically active molecule upon immobilization,
thereby causing substantial reduction of the antigen-
binding activity of the antibody. It seems that it is critical
to minimize the number of bonds formed between the
antibody and the surface. Our results revealed that about
5% of 12-mercaptododecnoic acid is appropriate to form
a minimal number of covalent bond between the anti-
body and the surface (Figure 2(a)). At higher concentra-
tion of carboxyl groups, where multiple immobilization
bonds were expected to formed, the activity of PIM an-
tibody was dramatically reduced and it was similar to
that of RIM antibody. This observation supports that the
multiple bond formation cause the destruction of anti-
body’s natural structure.
Considering the maximum density of thiol groups on
Au is about 0.5 nm [21,25], there will be about 100 - 200
thiol molecules under the antibody to be immobilized.
Since our results showed the maximum activity of the
immobilized antibody was acquired with about 5% of
reactive group in the SAM on the Au surface. Theoreti-
cally, 5% of the reactive group can produce at maximum
of 5 immobilization bonds. Considering that in many
available reaction conditions, especially in aqueous solu-
tion, the reaction probability of the reaction group is
substantially lower than 100%. Therefore, our results
suggest that it is likely that only 1 immobilization bond
or at most a few bonds are formed between the antibody
and the SAM.
3.3. Activity of I mmobilized Antibody as a
Function of the Concentration of the
Antibody Used and the Protection Ratio
We prepared different amount of antibodies ranged from
0.07 pmol to 0.68 pmol which is sub- to near-monolayer
concentration of antibody. Prepared antibodies were an-
tigen-binding site protected and immobilized on Au sur-
face as described above. The mole fraction of the 12-
mercaptododecanoic acid used to introduce carboxyl
reaction group on the Au surface with respect to the total
moles of the thiol molecules was 5%. The activity of the
immobilized antibody was compared and plotted. As
shown in Figure 3, the activity of immobilized antibody
was linearly proportional to the concentration of anti-
body used in this experiment.
The activity of the immobilized anti-DNA antibody
was measured at different protection ratio from 1:
0.0625 to 1:4 (Figure 4). The mole fraction of the

M. Yoon et al. / J. Biomedical Science and Engineering 4 (2011) 242-247 246
Figure 3. Antibody concentration dependence of the ac-
tivity of antigen-binding site protected antibody after
immobilization to Au-coated slide glass. The concentra-
tion of antibody used was from sub- to near-monolayer
to cover the immobilization surface. PIM method was
used at 5% of 12-mercaptododecanoic acid. The ratio of
anti-DNA antibody to DNA was 1:2. Radioactivity of
PIM antibody on Au surface (3 × 5 mm2) was measured
and plotted.
Figure 4. The activity of antibody immobilized to the
SAM on Au surface as a function of antigen-binding site
protection ratio. The antibody concentration was 0.54
pmol and the protection ratio of anti-DNA antibody to
DNA was ranged from 1:0.0625 to 1:4. The activity of
immobilized antibodies by PIM method (●) or RIM
method (○) were depicted.
12-mercaptododecanoic acid in the SAM was 5%. The
activity of the immobilized antibody was increased as
the protection ratio increased. PIM antibodies revealed
much higher binding activity compared to RIM antibod-
ies throughout the protection ratio used in this experi-
ment. The saturation phenomenon was observed in the
PIM case when the molar ratio of the anti-DNA antibody
to the ds-DNA used for protection was in the range of
1:1 ~ 1:2. Since there are two antigen-binding sites for
each antibody, it is very reasonable that the binding ac-
tivity of immobilized antibody reaches maximum when
the protection ratio increased from 1 to 2. This data
support the previous results described above that the
antigen-binding sites were protected by formation of the
antigen- antibody complex.
In conclusion, we report on the development of a
novel method to immobilize an antibody on the solid
surface by using a PIM method and kinetically control-
ling the number of chemical bond formed between the
protected antibody and the solid surface. The resulting
antigen-binding activity of protected immobilized anti-
body was about 10 fold higher than that of random im-
mobilized antibody. This method could have wide ap-
plication in production of various bio-chips.
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