Maleimido-terminated self-assembled monolayers

Article abstract of DOI:10.1002/chem.200400896

Four approaches have been explored for the preparation of maleimido-functionalized self-assembled monolayers (SAMs) on silicon. SAMs prepared by self-assembly of maleimido-functionalized alkyltrichlorosilanes (11-maleimido-undecyl-trichlorosilane) on oxide-covered silicon yield higher signals from maleimido functionalities in ATR-IR (attenuated total reflection IR) spectroscopy and XPS (X-ray photoelectron spectroscopy) than the other three methods. The surface composition of maleimido groups was tailored further by the formation of mixed monolayers with nonfunctionalized alkyltrichlorosilanes (decyltrichlorosilane). The order of the alkyl chains within the monolayers only slightly depends on the composition of the mixed monolayers. We utilized the maleimido-terminated SAMs to bind various nucleophilic compounds, alkylamines, alkylthiols, and thiol-tagged DNA oligonucleotides by means of conjugate addition.

Full text of DOI:10.1002/chem.200400896

Maleimido-Terminated Self-Assembled Monolayers
[
a]
[b]
[b]
[b]
Yayun Wang, Jun Cai, Hubert Rauscher, Rolf Jꢀrgen Behm, and
[
a, c, d, e]
Werner A. Goedel*
Abstract: Four approaches have been
explored for the preparation of male-
toelectron spectroscopy) than the other
three methods. The surface composi-
tion of maleimido groups was tailored
further by the formation of mixed
monolayers with nonfunctionalized al-
lane). The order of the alkyl chains
within the monolayers only slightly de-
pends on the composition of the mixed
monolayers. We utilized the maleimi-
do-terminated SAMs to bind various
nucleophilic compounds, alkylamines,
alkylthiols, and thiol-tagged DNA oli-
gonucleotides by means of conjugate
addition.
imido-functionalized
self-assembled
monolayers (SAMs) on silicon. SAMs
prepared by self-assembly of maleimi-
do-functionalized alkyltrichlorosilanes
kyltrichlorosilanes
(decyltrichlorosi-
(
11-maleimido-undecyl-trichlorosilane)
on oxide-covered silicon yield higher
signals from maleimido functionalities
in ATR-IR (attenuated total reflection
IR) spectroscopy and XPS (X-ray pho-
Keywords: conjugate addition
maleimide monolayers
self-assembly · silanes
·
·
·
Introduction
preparation of chemically and structurally well-defined or-
ganic monolayers. In these systems, organic molecules are
covalently bound to a surface through reactive groups. This
binding process is further enhanced by additional interac-
tions between comparatively long and densely packed alkyl
chains. Two types of self-assembled monolayers have been
extensively studied and have shown great promise as a
means of controlling surface properties: alkane thiolates on
Fundamental properties of solid materials, such as adhesion,
wettability, and chemical reactivity, are determined predom-
inantly by the first layer of atoms or molecules at the sur-
face. Surfaces can be modified at the molecular level by
Langmuir–Blodgett transfer, physical adsorption, or chemi-
[1]
sorption of monolayers to solid substrates. Self-assembled
monolayers (SAMs) offer one convenient route for the
[2]
gold and alkylsilanes on silica or glass. Advantages of
thiols on gold are their ease of preparation and the high
order of the obtained monolayers. On the other hand, alkyl-
silanes are advantageous because the monolayers are com-
paratively stable (chemically and thermally) and can be
formed on various inorganic oxides, including transparent
[
a] Y. Wang, Prof. Dr. W. A. Goedel
Organic and Macromolecular Chemistry, OCIII
University of Ulm, 89069 Ulm (Germany)
E-mail: werner.goedel@chemie.tu-chemnitz.de
[3]
dielectrics.
[
[
b] Dr. J. Cai, Dr. H. Rauscher, Prof. Dr. R. J. Behm
Surface Chemistry and Catalysis
Furthermore, desired functionalities can be introduced by
chemical modification of the terminal groups of the SAMs,
University of Ulm, 89069 Ulm (Germany)
[4]
for example by nucleophilic substitution,
hydrogena-
[
c] Prof. Dr. W. A. Goedel
[5,6]
[7,8]
tion,
and carbonyl reaction.
While a variety of useful
Material Science & Catalysis, ACII
University of Ulm (Germany)
[4,9,10]
[11–13]
functional groups, such as amine
and hydroxy,
have
d] Prof. Dr. W. A. Goedel
been well-studied previously, maleimido-functionalized self-
assembled monolayers have drawn increasing attention re-
cently. Maleimido groups are capable of reacting with thiol
or amino functionalities (Figure 1) under mild conditions
and thus have appealing properties, especially for the immo-
bilization of biomolecules, such as DNA-oligonucleotides or
proteins. A single-step preparation of maleimido-terminated
Polymer Physics, Polymer Research
BASF-Aktiengesellschaft (Germany)
[
e] Prof. Dr. W. A. Goedel
Physical Chemistry, Chemnitz University of Technology
Strasse der Nationen 62, 09111 Chemnitz (Germany)
Fax : (+49)371-531-1371
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
[14,15]
monolayers on gold surfaces was reported recently,
Fur-
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FULL PAPER
mented by hydrosilylation of a maleimido-functionalized
olefin (11-maleimido-undecene, MU) on a hydrogen-termi-
nated silicon surface. In method 4, maleimido-functionalized
trichlorosilane is reacted with oxide-covered silicon.
As shown in Scheme 2, the maleimido-terminated trichlo-
rosilane (w-maleimido-undecyltrichlorosilane, MUTS) nec-
essary for method 4 is synthesized from commercial starting
materials by a novel four-step route. Two compounds in the
route, PM and MU, are used in method 2 and 3, respective-
ly.
In the following, we 1) present the details of the synthesis
of the individual compounds; 2) compare the properties of
maleimido-terminated surfaces obtained by the various
routes; 3) explore the preparation of mixed monolayers
comprising maleimido- and methyl-terminated trichlorosi-
lanes; 4) examine the reactions of maleimido-modified mono-
layers from method 4 with a variety of nucleophiles and
thiol-tagged DNA oligonucleotides.
Figure 1. General sketch of the use of maleimido-terminated monolayers
to bind thiols or amines to a surface by means of conjugate addition.
thermore, the monolayers were successfully utilized to im-
[14,16,17]
mobilize various peptides and carbohydrates,
If, on
the other hand, maleimido-terminated monolayers on glass
or oxide-covered surfaces are desired, the currently used
preparation involves a cascade of surface reactions, namely,
complex preparation of silane-based SAMs with nucleophil-
ic functionalities and a subsequent reaction with a hetero-
Synthesis of the necessary building blocks for the prepara-
tion of maleimido monolayers for methods 2, 3, and 4: The
classic method to produce N-alkylated maleimide deriva-
tives involves a condensation reaction between an appropri-
ately substituted amine and maleic anhydride followed by
acid-promoted dehydration and ring closure. However, the
ring closure is often inefficient and thus only poor yields of
the desired maleimide derivatives are obtained. A newly de-
[18–20]
crosslinker bearing the maleimido moiety.
The imple-
mentation of surface reactions has the disadvantage that
their completion is often hindered by interfacial effects,
[9]
such as steric hindrance. In addition, it is impossible to
purify unreacted groups or surface-bound side products
from the desired functionalities. Therefore, it is desirable to
decrease the number of surface reactions involved or, ideal-
ly, to prepare the monolayers in a single-step surface reac-
tion, as has already been demonstrated in the case of thiol
[21]
veloped approach
utilizes nucleophilic substitution of
alkyl bromide with a maleimide (PM) protected with furan
[14–17]
[22,23]
monolayers on gold.
by a Diels–Adler reaction
and subsequent deprotection
Herein, we develop three approaches for the preparation
of maleimido-functionalized SAMs (Scheme 1, methods 2–4)
and compare them with the conventional approach adopted
from reference [20] (Scheme 1, method 1). The properties of
such monolayers are tailored even further by deposition
from solutions of trichlorosilane mixtures. In addition, we
utilize the maleimido-functionalized surface to bind various
nucleophilic reagents and thiol-tagged DNA oligonucleo-
tides.
to afford a high yield of N-substituted maleimide. We used
w-bromoundecene and PM as starting materials (see
Scheme 2b), and obtained a high yield (80%) of 11-male-
imido-undecene (MU), which is comparable with the yields
reported. PM and MU are used for method 2 and method 3,
respectively.
We subjected the intermediate MU to hydrosilylation
with HSiCl and obtained the desired maleimdo-functional-
3
ized trichlorosilane (MUTS), which is then used to prepare
self-assembled monolayers in method 4.
Results and Discussion
Characterization of the self-assembled monolayers: The
maleimido-terminated monolayers on the surface of a silicon
crystal, prepared by the four methods illustrated in
Scheme 1, were characterized by attenuated total reflection
infrared (ATR-IR) spectroscopy, X-ray photoelectron spec-
troscopy (XPS), atomic force microscopy (AFM), and ellips-
ometry.
The established preparation of maleimido-functionalized
[20]
monolayers, as shown in Scheme 1 method 1, involves a
cascade of surface reactions: nucleophilic substitution of a
heterocrosslinker-bearing maleimido group (for example,
SSMCC sulfosuccinimidyl 4-(N-maleimidomethyl)cyclohex-
ane-1-carboxylate) by amino-terminated SAMs, which are
obtained by formation of bromo-terminated SAMs on
oxide-covered silicon, nucleophilic displacement of bromine
In the IR spectra, the cyclic imide carbonyl groups exhibit
À1
two bands in the region between 1700 and 1800 cm . One
À1
band is located between 1800 and 1740 cm (symmetric
[4]
by azide, and subsequent reduction. Three new routes, as
shown in Scheme 1 methods 2, 3, and 4 have been designed.
Method 2 utilizes the surface reaction between bromo-ter-
minated SAMs on silicon and a protected maleimide (3,6-
stretch) and a more intense band between 1740 and
À1
[24]
1700 cm (antisymmetric stretch). As Figure 2 shows, the
absorption band of the carbonyl antisymmetric stretch at
À1
about 1705 cm is present in all the maleimido-terminated
4
endoxo-D -tetrahydrophthalimide, PM) followed by depro-
tection. In method 3, the surface immobilization is imple-
monolayers obtained by the four methods, whereas the sym-
À1
metric stretching absorption band at 1770 cm shows up
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W. A. Goedel et al.
Scheme 1. The four methods used to synthesize the maleimido-terminated monolayers used here.
above the noise level only in the monolayers obtained by
method 4. The monolayers obtained from method 4 gives rise
to an absorbance at about 1706 cm , which is significantly
greater than that observed in the case of the other methods.
The elemental compositions measured by XPS of the sur-
faces obtained by using methods 1–4 are given in Table 1.
The starting Br-SAMs in the conventional method,
method 1, show a significant Br 3d signal, which dramatical-
ly decreased after the nucleophilic substitution by the azide.
The introduction of azide groups was indicated by the simul-
taneous appearance of N 1s signals. The reduction of the
azide groups to amino groups gives rise to a decrease of the
nitrogen signal. Maleimido groups, introduced by nucleo-
philic substitution of the hetero-crosslinker (SSMCC) by the
À1
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Self-Assembled Monolayers
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Scheme 2. Synthesis of the protected maleimide (PM) used in method 2, of 11-maleimido-1-undecene (MU) used in method 3, and of 11-maleimido-un-
decyltrichlorosilane (MUTS) used in method 4.
amino-terminated SAMs, give rise to an increase of the N 1s
signal. The two-step route of method 2, the surface nucleo-
philic substitution of Br-SAMs by the deprotonated protect-
ed maleimide (PM), also involves a reduction in the Br 3d
signal and the simultaneous appearance of N 1s signals. It is
worth nothing that in both methods, the Br 3d signal does
not vanish completely and thus indicates incomplete conver-
sion in the surface reactions.
High-resolution XPS of the monolayers obtained by using
methods 1–4 in the N 1s region are shown in Figure 3. The
N 1s peak of the monolayers obtained by using method 1 is
broader and has a higher intensity than those obtained from
Figure 2. ATR-IR spectra of maleimido-terminated monolayers prepared
by the four methods shown in Scheme 1.
Table 1. XPS elemental composition of SAMs prepared by the four
methods
[
a]
SAMs type
Elemental composition [atom%]
N 1s C 1s
Br 3d
[
b]
Br-SAMs
2.3
0.2
0.1
0.1
0.6
35.8
33.2
37.7
50.4
40.5
34.0
38.6
N
3
-SAMs
5.2
2.0
3.4
1.8
2.1
3.1
NH -SAMs
2
Ma-SAMs-Method1
Ma-SAMs-Method2
Ma-SAMs-Method3
Ma-SAMs-Method4
[
b]
[
b]
Figure 3. High-resolution XPS in the N 1s region of maleimido-terminat-
ed monolayers prepared by the four methods.
[
a] Ma refers to the maleimido-terminated SAMs. [b] No peak visible.
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W. A. Goedel et al.
the other methods. This is attributed to the fact that the
peak is composed of contributions assigned to the male-
imide, the amide formed in the last reaction step, and
amines that were not converted. A deconvolution of the
broad peak into the above-mentioned contributions is
shown by the two dotted lines.
In principle, the signal intensities in the ATR-IR spectra
and X-ray photoelectron spectra might be regarded as an in-
dicator of the amount of maleimido functionalities bound to
the surface. However, the signal intensities are influenced
by additional factors: in IR spectroscopy, the signal is influ-
enced by the relative orientation of the chromophores,
whereas in XPS the signal intensity is decreased if groups
are buried within the sample. Although the interpretations
of the results from both methods are not straightforward in
terms of surface concentrations, it is worth pointing out that
the intensities of the signals assigned to the maleimido
group increase from method 1 to method 4 both in the IR
spectra and the X-ray photoelectron spectra.
In this work, we tuned the properties of the monolayers
by mixing maleimido-functionalized silanes (MUTS) with
methyl-functionalized silanes (decyltrichlorosilane, DTS).
By varying the concentration ratio of MUTS and DTS (0:1,
1:2, 2:1, 1:0) in the original deposition solution, we influ-
enced the composition of the monolayers in a continuous
À1
manner: the absorption intensity at about 1706 cm , as-
signed to the carbonyl antisymmetric stretch vibration of the
maleimido carbonyl groups, increased with increasing molar
ratio of MUTS in the deposition solution (see Figure 4a and
Figure 5).
Ellipsometry indicates that for all four methods the ob-
tained thickness is largely in agreement with the formation
of monolayers. Methods 2–4 yield a thickness comparable to
that of the methyl-terminated monolayers prepared from
decyltrichlorosilane (DTS). Method 1 affords a significantly
greater thickness, which can be attributed to the binding of
the hetero-crosslinker (data given in the Supporting Infor-
mation).
The use of very carefully cleaned surfaces and precisely
controlled deposition conditions has shown that ultrasmooth
SAMs can be prepared with a root mean square (rms)
[25]
roughness of <1.0 ꢁ.
Less optimal conditions would
probably lead to imperfect monolayers with a higher rough-
ness. Therefore, it is of interest to use atomic force micro-
scopy to investigate the roughness of the surfaces involved
here. In our case, the two surfaces treated with trichlorosi-
lanes (the bromo-terminated SAMs after step 1 in method 1
and the maleimido-terminated SAMs in method 4) have a
rms roughness value of 2.8 and 2.0 ꢁ, respectively. These
values are similar to that of a plain silicon substrate (rms
roughness = 2.2 ꢁ). It appears that each step in method 1
introduces additional roughness: the root mean square
roughness increases from 2.8 to 5.0 ꢁ from Br-SAMs to Ma-
SAMs. Method 2 and method 3 produce an even rougher
surface than method 1 (rms roughness = 6.4 and 9.2 ꢁ, re-
spectively). The AFM images of the plain silicon substrates
and the silane-treated surfaces are given in the Supporting
Information.
Figure 4. ATR-IR absorbance spectra of mixed SAMs prepared from
deposition solutions with different molar ratio of MUTS and DTS, a) in
the carbonyl region, b) in the methylene region.
The frequencies of the symmetric and antisymmetric
methylene stretching vibrations are sensitive to the confor-
mational order of the alkyl chain in the self-assembled
monolayers. The value of the antisymmetric vibration at
À1
2918–2919 cm corresponds to all-trans alkyl chain confor-
À1
mations and a value of 2928 cm to liquid-like disordered
[27]
chains. It has been shown that the order of the SAMs de-
creases with decreasing chain length. Methyl-terminated
From an analysis of the above results, we conclude that
method 4 is more suitable for the generation of monolayers
bearing maleimido groups than the other three routes inves-
tigated here. Therefore, we utilized this route in further in-
vestigations.
SAMs with a chain length between C10 and C11 have a
À1 [28,29]
CH -antisymmetric absorption at 2293–2294 cm ,
sug-
2
gesting a relatively disordered monolayer. In Figure 4b and
Figure 5, variation of the composition of the mixed mono-
layers (from maleimido-SAMs to methyl-terminated SAMs)
gave rise to a minor shift of the methylene antisymmetric
À1
Preparation of mixed monolayers: One great advantage of
preparing SAMs from trichlorosilane is that one can easily
tailor surface functionalization by mixing functionalized and
stretch absorption (from 2924.5 to 2923.0 cm ). We thus es-
timate that the degree of ordering of the alkyl chain in the
maleimido-terminated monolayers and mixed monolayers is
only slightly dependent on the composition.
[26]
nonfunctionalized trichlorosilanes.
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Self-Assembled Monolayers
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Figure 5. ATR-IR absorption intensity of the carbonyl peak and peak po-
sition of the CH
molar fraction of MUTS in the deposition solution ([MUTS]/([MUTS]+
DTS])).
2
-antisymmetric stretch vibration as a function of the
[
The mixed monolayers were further investigated by XPS.
The high-resolution C 1s spectrum of the maleimido SAMs
prepared by method 4 (Figure 6a) presents three compo-
nents at binding energies of 284.5 eV (67.8%), 285.5 eV
[30]
(
21.6%), and 288.4 eV (10.6%).
The dominant peak at
2
84.5 eV is assigned to the saturated hydrocarbon part of
[31,32]
the monolayers.
The other two components at higher
binding energies primarily originate from the carbon atoms
involved in various bonding arrangements with oxygen and
nitrogen (CÀN, NÀC=O). Additionally, the carbon atoms
Figure 6. a) Analysis of high-resolution C 1s spectra of maleimido-termi-
nated SAMs obtained with method 4 by fitting three Gauss–Lorentz
peaks. b) High-resolution XPS in the C 1s region of mixed monolayers
composed of maleimido and methyl end groups. c) High-resolution XPS
in the N 1s region of mixed monolayers composed of maleimido and
methyl end groups.
within the conjugated system of the maleimide ring are also
supposed to have a higher binding energy as a result of con-
jugation.
The high-resolution C 1s and N 1s spectra of the mixed
monolayers are shown in Figure 6b and c, respectively. The
corresponding integral intensities are presented in Table 2
and are plotted in Figure 7 versus the molar fraction of male-
imido-terminated trichlorosilane in the deposition solution.
The intensities of the peaks corresponding to the nitrogen
Table 2. XPS elemental composition of mixed monolayers composed of
maleimido and methyl end groups.
[
a]
Mixed
monolayers
Elemental
composition
C 1s
area
C 1s
area
C 1s
area
[b]
[b]
[b]
[
atom%]
C 1s
(
400.7 eV) and to the carbon atoms within or connected to
N 1s
at 284.5 eV at 285.5 eV at 288.4 eV
the maleimido group (285.5 eV and 288.4 eV) increase
roughly linearly with the composition of the deposition so-
lution. On the other hand, the signal corresponding to the
aliphatic carbons (284.5 eV) seems to be almost unaffected
by the mixing ratio.
Figure 8 shows a plot of the advancing and receding
contact angles of water on the mixed monolayers versus
the molar fraction of MUTS in the deposition solution
Ma:CH
Ma:CH
Ma:CH
Ma:CH
3
3
3
3
= 1:0 3.1
38.6
38.1
36.3
30.8
67.8
74.9
85.4
97.2
21.6
16.6
11.4
2.8
10.6
8.5
= 2:1 2.4
= 1:2 1.2
= 0:1 0.0
3.2
[c]
[d]
[
[
a] Ma refers to the maleimido-terminated SAMs. [b] Atom% of signal.
c] Peak at 286.5 eV. [d] No peak visible.
on the surface, which is in accordance with examples in the
literature.
[33]
(
[MUTS]/([MUTS]+[DTS])). The contact angles decrease
with increasing fraction of maleimido-functionalized alkyl-
trichlorosilanes in the deposition solution, which could be
explained by the increasing maleimido moieties deposited
on the surface, which have a higher hydrophilicity than
methyl groups. An increasing hysteresis was observed when
more maleimido moieties—polar groups—were deposited
All parameters measured above that, in principle, are sen-
sitive to the surface concentration of the maleimido groups
(intensity of the IR peak at 1706 cm , N 1s and C 1s XPS
signal, water contact angle) depend linearly on the molar
fraction of the maleimido functionalized silanes in the depo-
sition solution (Figure 5, Figure 7, Figure 8). Furthermore,
À1
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W. A. Goedel et al.
region of methylene stretching increases (Figure 9 and
Figure 10, a and b). The difference spectra (Figure 9 and
Figure 10, c) of maleimido-SAMs before and after reaction
with the alkylthiols show the binding of additional alkyl
chains to the underlying SAMs. In the case of decylthiol, the
difference spectrum shows a weak signal whereas that of oc-
tadecylthiol shows a strong peak. In the maleimide ring,
there is conjugation between the carbonyl groups, the
double bond, and the N atom. The addition of the double
bond might influence the carbonyl absorption position in
the IR spectrum. However, no systematic shift was observed
in our experiments. On the other hand, in previous research
carried out on similar reactions in homogeneous solutions,
[21,36]
no systematic shifts were reported either.
The monolayers were also allowed to react with aliphatic
amines, isoindole, and thioacetamide. All of them were la-
beled with a cyano group as IR spectroscopic marker. As
Figure 11 shows, all spectra of the monolayers after expo-
Figure 7. Integral intensities of the N 1s signal (400.7 eV) and of the con-
tributions to the C 1s signals from carbon atoms within or connected to
the maleimido group (285.5 eV and 288.4 eV) and from aliphatic hydro-
carbons of the alkyl chain (284.5 eV) in XPS as a function of the molar
fraction of MUTS in the deposition solution ([MUTS]/([MUTS]+
sure to the nucleophiles show a peak in the region between
À1
2
300 and 2100 cm that can be assigned to the cyano group.
[
DTS])).
We thus conclude that these surface reactions also proceed
as shown in Scheme 3.
In further experiments, silicon surfaces that were covered
with maleimido-terminated monolayers were immersed in
aqueous buffer solutions of thiol-tagged DNA oligonucleo-
tides, thoroughly rinsed, and characterized with XPS
(
(
Figure 12). Before the exposure to DNA oligonucleotides
dashed line), the X-ray photoelectron spectra showed a
peak in the region of N 1s (at 400.7 eV), which is assigned
to the maleimide. However, the spectra showed no detect-
able signal in the P 2p region. After the exposure, the N 1s
peak increased in intensity (from 3.1 to 6.5) and a peak ap-
peared in the P 2p region at 133.9 eV. Because the backbone
of the DNA molecules comprises phosphate moieties, we
conclude that the immobilization of DNA oligonucleotides
occurred.
Figure 8. Water contact angles of mixed SAMs as a function of the molar
fraction of MUTS in the deposition solution ([MUTS]/([MUTS]+
Conclusion
[
DTS])).
Maleimido-functionalized alkyl trichlorosilanes can be pre-
pared with moderate synthetic effort. They can be used for
the generation of maleimido-terminated self-assembled
monolayers in a single step and give rise to higher surface
concentrations than those of monolayers prepared by a pre-
viously published multistep procedure. The properties of the
monolayers can be tuned by mixing with methyl-functional-
ized alkyltrichlorosilanes. The maleimido groups are effi-
cient in covalently binding nucleophilic heterocycles, al-
kylthiols, amines, and thiol-tagged DNA oligonucleotides.
IR spectroscopy of the methylene groups indicates only in-
significant changes in the packing order of the alkyl chains.
In combination, these observations can be interpreted as
nonpreferential uniform binding of both compounds from
solution.
Reactivity of the maleimido-terminated SAMs: To explore
the reactivity of the maleimido moieties on the surface, we
exposed the maleimido-terminated SAMs produced by
method 4 to various nucleophilic compounds. Based on simi-
[34,35]
lar reactions occurring in solution,
face reactions shown in Scheme 3.
we expected the sur-
Experimental Section
The conjugate addition of thiol was investigated by expo-
sure to decylthiol and octadecylthiol under mild conditions
Self-assembled monolayers were prepared on silicon surfaces. Si ATR
crystals (Korth GmbH, Berlin, parallelipeds with an angle of 458,
72 mmꢂ12 mmꢂ6 mm) were used for IR spectroscopy and contact-angle
(
558C in ethanol). In both cases, the absorption in the
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Self-Assembled Monolayers
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Scheme 3. Schematic representation of reactions between maleimido-terminated SAMs prepared by method 4 with various nucleophiles.
Figure 10. ATR-IR spectra of maleimido-SAMs prepared by method 4
before and after reaction with octadecylthiol: a) maleimido-SAMs,
Figure 9. ATR-IR spectra of maleimido-SAMs prepared by method 4
before and after reaction with decylthiol: a) maleimido-SAMs, b) male-
imido-SAMs + thiol, c) difference between spectra b and a.
b) maleimido-SAMs + thiol, c) difference between spectra b and a.
The reaction mixture was allowed to warm to room temperature, and
was then heated to 1008C for 2 h with stirring. After the mixture was al-
lowed to cool to room temperature, the supernatant liquid layer was de-
canted from the yellow precipitate. The precipitate was washed with a
small amount of toluene. The toluene used for washing was added to the
decanted liquid, and the toluene was removed by rotary evaporation
under reduced pressure. The oily residue was further dried at 0.13–
0.40 mbar, 258C. Afterwards, it was diluted with an equal volume of
ether. This mixture was washed (water, saturated sodium bicarbonate, sat-
urated sodium chloride), dried over sodium sulfate, and distilled. A yield
measurements. Si wafers (Crystec GmbH, Berlin, orientation 100, p type,
À1
>
1 ohmcm ) were used for XPS (size: 10 mmꢂ5 mm), AFM (size:
1
0 mmꢂ10 mm), and ellipsometry measurements (size: 10 mmꢂ10 mm).
[
37]
1
1-Bromo-1-undecene (BU): 10-Undecen-1-ol (85 g, 0.5 mol) was dis-
solved in a mixture of dry toluene (140 mL, dried through aluminum
oxide, neutral) and pyridine (27 mL). The solution was cooled to À108C,
and kept at À58C to À108C while a solution of phosphorus tribromide
(
54.2 g, 0.2 mol) in dry toluene (140 mL) was added over a period of 2 h.
Chem. Eur. J. 2005, 11, 3968 – 3978
www.chemeurj.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3975

W. A. Goedel et al.
1
H NMR (400 MHz, CDCl
.54 (s, 2H), 7.90 ppm (s, 1H); C NMR ([D
3
, 258C, TMS): d = 2.91 (s, 2H), 5.34 (s, 2H),
1
3
6
8
[
8
]THF): d = 176.64; 136.50;
+
0.97; 48.79 ppm; MS-CI (100 eV): m/z (%): 168 (1.6) [M+2] , 167 (1.4)
+
+
M+1] , 166 (14.3) [M] , 126 (22.7), 98 (100), 69 (41.67).
4
Protected maleimido-undecene (PMU): 11-(3,6-endoxo-D -tetrahydro-
[
21]
phthalimide)undecene: In an oven-dried (at 2008C for 12 h, closed by
a stopper and subsequently cooled to room temperature) round-bottom
flask, PM (0.131 g, 0.8 mmol) and BU (0.2 mL, 0.8 mmol) were dissolved
in anhydrous DMF (8 mL, dried over 4 ꢁ molecular sieve) along with
2 3 2
K CO (0.55 g, 5 equiv). The entire reaction mixture was stirred under N
at 558C for 2.5 h. The entire reaction mixture was diluted with ethyl ace-
tate (150 mL), washed (water, saturated sodium bicarbonate solution),
2 4
and dried over Na SO . The ethyl acetate was removed at reduced pres-
sure, and the residue was purified by column chromatography (CHCl
3
/
1
silica) to yield the pure product (0.2 mL; 80%). H NMR (400 MHz,
CDCl , 258C, TMS): d = 1.14–1.50 (m, 12H), 1.56 (m, 2H), 2.04 (m,
H), 2.84 (s, 2H), 3.50 (t, J = 7 Hz, 2H), 4.88–5.02 (m, 2H), 5.28 (s,
3
2
2
1
2
2
1
3
3
H), 5.80–5.92 (m, 1H), 6.52 ppm (s, 2H); C NMR (CDCl ): d =
76.27; 139.22; 136.54; 114.09; 80.90; 47.34; 39.03; 33.78; 29.37; 29.34;
9.07; 28.90; 27.58; 26.65 ppm; MS-CI (100 eV): m/z (%): 332 (1.0) [M] ,
Figure 11. ATR-IR spectra of maleimido-SAMs prepared by method 4
before and after reaction with nucleophiles bearing a nitrile group as
spectroscopic marker: a) maleimido-SAMs before reaction, b) male-
imido-SAMs + aminocapronitrile (ACN), c) maleimido-SAMs + 5-cya-
noindole (CI), d) maleimido-SAMs + 2-cyano-thioacetamide (CTA).
+
78 (18.0), 250 (100), 248 (19.3).
1
1-Maleimido-undecene (MU): 1-undecylenyl-1H-pyrrole-2,5-dione: A
mixture of PMU (0.2 mL, ꢀ0.5 mmol) and anisole (2 mL) was stirred
under reflux (1708C) for 2 h followed by removal of the anisole by Ku-
À2
gelrohr distillation (758C, 2.5ꢂ10 mbar) to afford the pure product.
Yield: 0.165 g (100%); H NMR (400 MHz, CDCl
1
3
, 258C, TMS): d =
1
.14–1.50 (m, 12H), 1.59 (m, 2H), 2.04 (m, 2H), 3.52 (t, J = 8 Hz, 2H),
1
3
4
.88–5.02 (m, 2H), 5.80–5.92 (m, 1H), 6.69 ppm (s, 2H); C NMR
): d = 170.87, 139.19, 134.02, 114.11, 55.12, 37.93, 33.77, 29.39,
9.34, 29.08, 28.89, 28.52, 26.72 ppm; MS-CI (100 eV): m/z (%): 251
(
CDCl
3
2
+
+
+
(
43.4) [M+2] , 250 (100) [M+1] , 249 (59.2) [M] , 248 (35.2), 194 (30.0),
80 (24.1), 166 (20.1).
Catalyst for hydrosilylation: dicyclopentadienylplatinum(ii) chloride
Cp PtCl ]: Method adopted from reference [38]: hydrated chloroplatinic
acid (0.69 g, 1.33 mmol) was dissolved in glacial acetic acid (1.5 mL) in a
5-mL flask. The solution was diluted with water (2.5 mL) and slowly
1
[
2
2
2
heated to 708C. Dicyclopentadiene (0.5 mL, 3.70 mmol) was then added
to the reaction vessel, and the mixture was stirred vigorously for 24 h at
this temperature. The crude product precipitated, was collected by filtra-
tion, then redissolved in THF, decolorized by charcoal, and finally recrys-
tallized from THF to yield the pure product (0.25 g; 59%; m.p. 2188C).
1
The structure was verified by H NMR (400 MHz, CDCl
3
, 258C, TMS): d
=
1.84 (m, 1H), 2.16 (m, 2H), 2.35 (m, 1H), 2.86 (m, 2H), 3.65 (m, 2H),
5
1
1
.53 (t, J = 36 Hz, 1H), 6.07 (t, J = 40 Hz, 1H), 6.47 (t, J = 32 Hz,
1
3
H), 6.85 ppm (t, J = 40 Hz, 1H); C NMR (CDCl
3
): d = 116.18,
05.31, 100.31, 97.42, 60.05, 56.14, 55.38, 43.67, 43.39, 33.69 ppm; MS-CI
+
+
(
(
100 eV): m/z (%): 399 (4.2) [M+1] , 398 (6.9) [M] , 365 (45.5), 364
41.7), 363 (89.6), 362 (92.1), 361 (100).
1
1-Bromo-undecyl-trichlorosilane (BUTS) and 11-maleimido-undecyltri-
chlorosilane (MUTS); hydrosilylation of BU and MU: BU (1 mL,
31 mmol), HSiCl (5 mL, 49.5 mmol), and the catalyst [Cp PtCl
10 mg, 0.018 mmol) were placed in a 25-mL three-neck-flask and re-
fluxed at 408C for 10–16 h under an Ar atmosphere. The disappearance
4
(
3
2
2
]
Figure 12. XPS spectra of maleimido-terminated SAMs (a) and DNA-
immobilized surface (c) in the region of a) N 1s, and b) P 2p.
1
of the olefinic protons was monitored with H NMR spectroscopy. HSiCl
was distilled off, and the BUTS product was isolated by Kugelrohr distil-
lation (1708C–1808C, 3ꢂ10 mbar). H NMR (400 MHz, CDCl
3
of 83.77 g (71.9%) of colorless 11-bromo-1-undecene was obtained.
À1
1
3
, 258C,
1
H NMR (400 MHz, CDCl
3
, 258C, TMS): d = 1.14–1.50 (m, 12H), 1.86
TMS): d = 1.14–1.50 (m, 16H), 1.59 (m, 2H), 1.87 (m, 2H), 3.41 ppm (t,
(
m, 2H), 2.06 (m, 2H), 3.41 (t, J = 7 Hz, 2H), 4.88–5.02 (m, 2H), 5.80–
13
J = 7 Hz, 2H); C NMR (CDCl
3
): d = 33.93, 32.84, 31.78, 29.44, 29.37,
1
3
5
3
2
.92 ppm (m, 1H); C NMR ([D
8
]THF): d = 139.14, 114.14, 33.93, 33.78,
2
3
9.29, 28.98, 28.75, 28.17, 24.33, 22.25 ppm; MS-CI (100 eV): m/z (%):
69 (6.7) [M+2] , 367 (11.9) [M] , 365 (9.4), 291 (34.1), 289 (100), 287
2.84, 29.37, 29.07, 28.91, 28.75, 28.17 ppm; MS-CI (100 eV): m/z (%):
+
+
+
+
35 (29.6) [M+2] , 233 (28.5) [M] , 111 (38.4), 97 (100), 85 (32.3), 83
(
7
90.2), 235 (23.8), 231 (24.4), 219 (21.4), 113 (25.5), 99 (31.7), 85 (61.0),
1 (67.7).
(
82.8), 71 (39.9), 69 (31.4).
4
Protected maleimide (PM): exo isomers of 3,6-endoxo-D -tetrahydro-
phthalimide: Maleimide (0.71 g, 7.31 mmol) was dissolved in fifty times
its weight of diethyl ether (50 mL) and 50% excess of furan (0.8 mL,
1
9
The hydrosilylation conditions for MU were similar to those of BU: MU
[
22]
(0.2 mL, 0.6 mmol) was allowed to react with HSiCl (0.3 mL, 2.7 mmol)
3
À4
and [Cp PtCl ] (2ꢂ10 mg). Samples were extracted at regular intervals,
and the reaction was monitored by H NMR spectroscopy. HSiCl was re-
moved by distillation. H NMR (400 MHz, CDCl , 258C, TMS): d =
2
2
1
0.9 mmol). The reaction solution was heated in an autoclave for 10 h at
08C. The white crystalline product that separated after cooling to 48C
3
1
3
was recrystallized with ethyl acetate to yield the product (0.25 g; 20.7%).
1.28–1.53 (m, 16H), 1.66 (m, 4H), 3.52 (t, J = 6.5 Hz, 2H), 6.69 ppm (s,
3976
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2005, 11, 3968 – 3978

Self-Assembled Monolayers
FULL PAPER
1
3
2
2
3
2
H); C NMR (CDCl
3
): d = 170.89; 134.02; 37.95; 31.79; 29.10, 29.08,
2:1, 1:2, and 0:1, while the total molar amount of MUTS and DTS were
1 mm.
+
6.73; 22.25; 21.45 ppm; MS-CI (100 eV): m/z (%): 386 (86.4) [M+2] ,
84 (89.6) [M] , 348 (60.0), 278 (75.5), 252 (65.5), 251 (62.8), 250 (100),
49 (77.7), 248 (59.3), 208 (32.7), 194 (53.4), 180 (41.4), 166 (33.8).
+
Reaction of maleimido-terminated SAMs with decylthiol and octadec-
ylthiol: Maleimido-terminated SAMs obtained with method 4 were im-
mersed in a solution of alkylthiol (3 mL, 100 mm decylthiol and octade-
cylthiol) in EtOH at 508C overnight. The sample was then washed with
EtOH and sonicated with acetone, THF.
Cleaning process of Si ATR (attenuated total reflection) crystal for mono-
layer coating: The silicon ATR crystal obtained from Korth GmbH,
Berlin (paralleliped with an angle of 458, 72 mmꢂ12 mmꢂ6 mm, 12 re-
flections) was consecutively sonicated in acetone, deionized water, and
isopropanol. It was then blow-dried with N
plasma in a plasma cleaner (Tepla AG, Germany, 100-E) at 100 W,
.5 mbar, 60 min on each side. The crystal was then immersed in deion-
ized water for 0.5 h and blow-dried with N
Reaction of maleimido-terminated SAMs with 5-cyanoindole (CI): Male-
imido-terminated SAMs obtained with method 4 were immersed in a so-
lution of 5-cyanoindole (3 mL, 100 mm) in AcOH and refluxed under an
N atmosphere for 96 h. The sample was then washed with AcOH and
2
sonicated with acetone, THF.
2
and exposed to an oxygen
0
2
.
Preparation of bromo-terminated SAMs (method 1): A freshly cleaned
silicon ATR crystal was immersed in a solution of BUTS (3 mL, 1 mm) in
Reaction of maleimido-terminated SAMs with 2-cyano-thioacetamide
(
CTA): Maleimido-terminated SAMs obtained with method 4 were im-
toluene (dried with Al
2 3
O , neutral) for 16 h at 258C. The crystal was then
mersed in a solution of 2-cyano-thioacetamide (3 mL, 100 mm) in EtOH
under refluxing conditions in the presence of a catalytic amount of tri-
ethylamine (0.05 mL) for 2 h. The sample was then washed with EtOH
and sonicated with acetone and THF.
washed with dry toluene and sonicated with acetone and THF.
[
4]
Preparation of amino-terminated SAMs: The amino-terminated SAMs
were prepared from bromo-terminated SAMs by adopting the procedure
described in reference [4]. Briefly, the bromo-terminated substrate was
immersed in a solution of sodium azide (3 mL, 10 mm) in DMF at room
temperature for 24 h or 72 h. The substrate was then washed with DMF
Reaction of maleimido-terminated SAMs with aminocapronitrile (ACN):
Maleimido-terminated SAMs obtained with method 4 were immersed in
a solution of aminocapronitrile (3 mL, 100 mm) in EtOH at 508C over-
night. The sample was then washed with EtOH and sonicated with ace-
tone, THF.
and sonicated (acetone, Millipore H
SAMs were further reduced with a saturated solution of LiAlH
for 1 h. The substrate was copiously rinsed with THF, Millipore H
0% HCl, and Millipore H O, and then immersed in a 5 mm NaOH so-
lution for 1 min and rinsed with Millipore H O. The surface transforma-
2
O, and THF). The azide-terminated
in ether
O,
4
2
Immobilization of DNA oligonucleotides on maleimidio-terminated
SAMs: Oligonucleotides with a thiol-modification at the 3’-end were ob-
tained from MWG (Germany). The oligonucleotides are 31 nt in length
with a 15 mer dT spacer and a specific 16 mer sequence, that is, 3’-HSC6-
T15-AA-CGA-TCG-AGC-TGC-AA-5’. The oligonucleotides were used
immediately after delivery by the company. The silicon wafer (5 mmꢂ
1
2
2
tions were characterized and controlled with IR spectroscopy and contact
angle measurements. The nucleophilic displacement of bromo groups on
À1
surface by the azide groups gave to a strong IR absorption at 2098 cm
.
This peak disappeared when the azide groups were reduced to amino
groups by LiAlH . The conversions were also confirmed by the contact
5
mm), modified with maleimido-terminated SAMs, was placed face-up
polished side) in a chamber saturated with water vapor. A DNA so-
lution (ꢀ100 mL of 5 mm) in an aqueous buffer (NaH PO 10 mm, NaCl
m, pH 7) was added onto the surface. After 16 h, the wafer was rinsed
with a copious amount of the buffer solution, immersed in distilled water
for half an hour, and blow-dried with N
4
(
angle measurement. The advancing contact angles alter from 908 (bromo-
terminated surface) to 858 (azide-terminated) and finally to 648 (amino-
terminated one), in accordance with reference [4].
2
4
1
Reaction of amino-terminated SAMs with hetero-crosslinker sulfosuc-
2
.
[
20]
cinimidyl-4-(N-maleimidomethyl)cyclohexane-1-carboxylate (SSMCC):
The obtained amino-terminated SAMs were reacted with SSMCC
100 mL, 1.5 mm) in triethanolamine buffer solution (addition of NaOH/
IR spectroscopy of the monolayers: FT-IR measurements of the mono-
layers were recorded with a Bruker IFS 66 V spectrophotometer with a
DTGS detector. All measurements were made until the sample chamber
(
HCl to adjust to pH 7) at 258C for 16 h, and then washed with H
2
O, and
À4
À1
was evacuated to 4ꢂ10 mbar. Spectra were recorded with 4 cm reso-
lution, 1000–3000 scans, and application of a Blackman–Harris 3-term
apodization function in the Fourier transformation. Reference spectra
were recorded with freshly cleaned ATR spectra. The absorption spectra
of the corresponding functionalized SAMs in the process of formation,
mixed monolayers formation, and reactivity verification were obtained
by subtracting the reference spectrum from the absorbance spectrum of
the coated substrate.
sonicated with acetone, THF.
Preparation of maleimido monolayers from bromo-SAMs (method 2):
CO (3 g) and DMF (10 mL) were added to a flask and vigorously stir-
red for 16 h. The undissolved excess K CO was filtered off. PM (10 mm)
was dissolved in saturated K CO /DMF solution (3 mL), and allowed to
K
2
3
2
3
2
3
react with the obtained bromo-SAMs at 708C for 16 h. The ATR crystal
was then intensively washed with DMF, sonicated with acetone and THF,
and blow-dried with N
2
. The ATR crystal was heated in an oven at
X-ray photoelectron spectroscopy: X-ray photoelectron spectra were re-
corded with a PHI 5800 ESCA System (Physical Electronics) and mono-
chromatized AlKa radiation (1486.6 eV) as the excitation source and a
hemispherical analyzer with 150 mm radius. The takeoff angle was set to
1
708C for 2 h to transform the protected maleimido-terminated SAMs
into the maleimido-terminated analogue.
Preparation of maleimido SAMs from H-terminated silicon (method 3):
The silicon ATR crystal was sonicated with acetone, deionized water, and
isopropanol, respectively, and then blow-dried with N . In a plasma clean-
2
er, a hydrogen plasma (Tepla AG, Germany, 100-E) at 100 W, 0.5 mbar
was applied to each side of the ATR crystal for 60 min. The H-terminat-
ed silicon ATR after the measurement of IR background was immediate-
ly immersed in a solution of MU (3 mL, 100 mm) in mesitylene at 1758C
for 2 h under an argon atmosphere. ꢃThe ATR was then washed with me-
sitylene and sonicated with acetone, THF.
4
58. The silicon wafer samples with a size of 1 cmꢂ0.5 cm were mounted
on sample stubs with conductive carbon tape. Spectra were recorded with
a pass energy of 187.85 eV (survey scans) or 29.35 eV (high-resolution
scans). Atomic concentrations of elements within the electron escape
depth were determined by evaluating the integral intensities of N 1s,
Br 3d, Si 2p, O 1s and C 1s signals and taking into account the tabulated
[32]
atomic sensitivity factors and the instrument transmission. The spectra
were referenced by setting the peak of the saturated hydrocarbon C1s to
284.5 eV to compensate for residual charging effects.
Preparation of maleimido-terminated SAMs via MUTS (method 4) and
mixed monolayers: Similarly to the formation of bromo-terminated
SAMs, the freshly O -plasma-cleaned silicon ATR crystal was immersed
2
in a solution of MUTS (3 mL, 1 mm) in toluene for 16 h at 258C. The
crystal was then washed with dry toluene and sonicated with acetone,
THF.
Ellipsometry: The thicknesses of the prepared monolayers from meth-
ods 1–4 were obtained with a spectroscopic ellipsometer MM301 (OMT,
Germany). The measurements were performed with an incident angle of
7
08 and a wavelength range of 350–900 nm. For each sample, 3–6 mea-
surements were analyzed with the proprietary software of OMT. The re-
Mixed monolayers were prepared with the solution composed of MUTS
and DTS in dry toluene. The molar MUTS:DTS mixing ratios were 1:0,
fractive indices of the organic monolayers and the SiO
assumed to be 1.45.
2
layers were both
Chem. Eur. J. 2005, 11, 3968 – 3978
www.chemeurj.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3977

W. A. Goedel et al.
Atomic force microscopy: AFM was carried out with a Digital Instru-
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The tips used were Olympus (DI NanoSensors) Tapping Mode Etched
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[
[
[
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5
2
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Acknowledgements
We thank M. Mçller, B. Rieger, K. Landfester (University of Ulm), H.
Auweter, and R. Iden (BASF-Aktiengesellschaft) for supporting these in-
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3978
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