Chemical surface modification of self-assembled monolayers by radical nitroxide exchange reactions

Article abstract of DOI:10.1002/chem.201100543

This article describes the application of nitroxide exchange reactions of surface-bound alkoxyamines as a tool for reversible chemical modification of self-assembled monolayers (SAMs). This approach is based on radical chemistry, which allows for introduction of various functional groups and can be used to reversibly introduce functionalities at surfaces. To investigate the scope of this surface chemistry, alkoxyamines with different functionalities were synthesized and were then applied to the immobilization of, for example, dyes, sugars, or biotin. Surface analysis was carried out by contact angle, X-ray photoelectron spectroscopy, and fluorescence microscopy measurements. The results show that this reaction is highly efficient, reversible, and mild and allows for immobilization of various sensitive functional groups. In addition, Langmuir-Blodgett lithography was used to generate structured SAMs. Site-selective immobilization of a fluorescent dye could be achieved by nitroxide exchange reactions. A radical exchange: Chemical modification of silicon surfaces with alkoxyamine-based self-assembled monolayers (SAMs) was achieved by reversible radical nitroxide exchange reactions.

Full text of DOI:10.1002/chem.201100543

FULL PAPER
DOI: 10.1002/chem.201100543
Chemical Surface Modification of Self-Assembled Monolayers by Radical
Nitroxide Exchange Reactions
Hendrik Wagner,[a] Marion K. Brinks,[a] Michael Hirtz,[b] Andreas Schꢀfer,[c]
Lifeng Chi,[b] and Armido Studer*[a]
Abstract: This article describes the ap-
plication of nitroxide exchange reac-
tions of surface-bound alkoxyamines as
a tool for reversible chemical modifica-
tion of self-assembled monolayers
(SAMs). This approach is based on
radical chemistry, which allows for in-
troduction of various functional groups
and can be used to reversibly introduce
functionalities at surfaces. To investi-
gate the scope of this surface chemistry,
alkoxyamines with different functional-
ities were synthesized and were then
applied to the immobilization of, for
example, dyes, sugars, or biotin. Sur-
face analysis was carried out by contact
angle, X-ray photoelectron spectrosco-
py, and fluorescence microscopy meas-
urements. The results show that this re-
action is highly efficient, reversible,
and mild and allows for immobilization
of various sensitive functional groups.
In addition, Langmuir–Blodgett lithog-
raphy was used to generate structured
SAMs. Site-selective immobilization of
a fluorescent dye could be achieved by
nitroxide exchange reactions.
Keywords:
monolayers · exchange reactions ·
radicals self assembly
surface chemistry
alkoxyamines
·
·
·
Introduction
Recently, we reported the use of thermal radical carboa-
minoxylations at olefin-terminated SAMs for chemical con-
jugation of silicon surfaces with different functionalized al-
koxyamines.[3] In these processes, the generation of a radical
Self-assembled monolayers (SAMs) with specific functional
groups as head groups are of high interest for many research
fields. Surface properties such as wettability, friction, adhe-
sion, or biocompatibility can be tuned as a function of the
surface functionalities.[1] Thiols on gold and silanes on sili-
con have been shown to form stable self-assembled mono-
layers. A broad variety of standard organic reactions have
been conducted at SAMs including alkene oxidations, amino
acylations, imine bond formations, alcohol acylations, and
epoxide openings with alcohols and amines.[1d,f,g] Other
methods for chemical surface modification have been stud-
ied, such as electrochemical and photochemical transforma-
tions, which allow the in-situ-modulation of SAMs to pro-
vide dynamic surfaces.[1c,d,2] Patterning of SAMs is crucial
for various applications and can be achieved by different
lithographic techniques, for example, photolithograpy, mi-
crocontact printing, or scanning-probe lithography.[1f,g]
À
occurs by thermal C O bond homolysis of a starting alkoxy-
amine. Subsequent addition of the carbon-centered radical
onto the olefin-containing SAM is followed by an irreversi-
ble nitroxide-trapping to afford the carboaminoxylation
product.[4,5] This approach allows the efficient introduction
of various functional groups at SAMs. Importantly, the radi-
cal approach is complementary to commonly used ionic-
bond-forming reactions at surfaces.
Reversible covalent bond formation has been applied in-
tensively in supramolecular chemistry, yet it is not investi-
gated well in surface chemistry.[6] Reversible radical ex-
change reactions, which belong to the class of dynamic cova-
lent chemistry, have not been applied at SAMs to date.[7]
Thermal nitroxide exchange reactions have already been uti-
lized for the synthesis and functionalization of polymers and
polymer brushes.[8] Additionally, these reactions have been
applied to the generation of dynamic covalent polymers as
well as to the synthesis of dynamic covalent macrocy-
cles.[8d,e,g] Recently, we reported the use of nitroxide ex-
change reactions for reversible assembly of organic–inorgan-
ic hybrid microcrystals.[8n] Herein, we describe the applica-
tion of radical nitroxide exchange reactions at SAMs as a re-
versible process to introduce a broad variety of functional
groups at silicon surfaces (Scheme 1).
[a] H. Wagner, Dr. M. K. Brinks, Prof. Dr. A. Studer
Organic Chemistry Department, University of Mꢀnster
Corrensstrasse 40, 48149 Mꢀnster (Germany)
Fax : (+49)251-83-36523
E-mail: studer@uni-muenster.de
[b] Dr. Dr. M. Hirtz, Prof. Dr. L. Chi
Department of Physics, University of Mꢀnster
Wilhelm-Klemm-Strasse 10, 48149 Mꢀnster (Germany)
À
Thermal C O-bond homolysis in alkoxyamines leads to
[c] Dr. A. Schꢁfer
transient carbon-centered radicals and persistent nitroxides.
In these processes, the radicals diffuse out of the solvent
cage and, controlled by the persistent radical effect (PRE),
undergo selective cross-coupling to reform the alkoxy-
Nanoanalytics GmbH
Heisenbergstrasse 11, 48149 Mꢀnster (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100543.
Chem. Eur. J. 2011, 17, 9107 – 9112
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9107

face nitroxide exchange reactions were conducted by sub-
jecting these alkoxyamine terminated SAMs to a solution of
ClCH2CH2Cl containing an additional alkoxyamine, which
contained a different nitroxide moiety under heating. Func-
tionalized 2,2,6,6-tetramethylpiperidin-N-oxyl (TEMPO) ni-
troxide derivatives, generated in situ by homolysis of the
corresponding soluble alkoxyamines, should undergo nitro-
xide exchange with the surface-bound alkoxyamines. As the
concentration of functionalized nitroxides in solution should
be intrinsically much higher compared to the concentration
of unfunctionalized TEMPO nitroxide that is liberated from
the surface bound alkoxyamine 1, quantitative nitroxide ex-
change at the surface should be achieved. For our studies,
alkoxyamine and nitroxide conjugates 2–9 were prepared
(see the Supporting Information) and used in surface nitro-
xide exchange reactions to generate chemically modified
wafers B–H (Scheme 2). Initial reactions were conducted by
placing a wafer of type A in a solution of alkoxyamine 2 in
ClCH2CH2Cl (10 mmolLÀ1) under an argon atmosphere.
The reaction time was systematically varied and analysis of
the modified surface was performed by contact angle (CA)
measurements. The starting SAM (wafer A) showed a CAadv
(CAadv =advancing contact angle) of 87Æ28. We found that
after a reaction time of 1 h, a significant change of the CA
was observed (Table 1, entry 2). As expected for a successful
reaction with a tetraethyleneglycol (TEG) alkoxyamine, the
surface polarity increased.
Scheme 1. Surface nitroxide exchange reaction (a) and nitroxide ex-
À
change reaction by C O-bond homolysis of alkoxyamines (b).
amine.[9] Thermolysis in the presence of an additional nitro-
xide or alkoxyamine leads to crossover of the nitroxide moi-
eties in the alkoxyamines (Scheme 1).
Results and Discussion
The modification of silicon surfaces by nitroxide exchange
reactions demands the synthesis of silanes with alkoxyamine
head groups for subsequent SAM formation. We synthesized
the alkoxyamine 1 with a silane functionality and prepared
alkoxyamine-terminated SAMs using oxidized silicon
wafers, as previously reported (Scheme 2, wafer A).[10] Sur-
Table 1. Radical nitroxide exchange reaction using wafer A and alkoxy-
amine 2 in ClCH2CH2Cl at 1258C.
O[c]
[%]
N[c]
[%]
C[c]
[%]
Si[c]
[%]
C C/
À
C O
À
Entry
t
CAadv
CArec
[h]
[8][a]
[8][b]
1
2
3
4
0
1
5
10
20
20
87Æ2
64Æ1
64Æ1
66Æ2
63Æ4
92Æ3
68Æ2
45Æ1
44Æ1
45Æ1
44Æ2
38Æ2
21.8
24.2
24.6
24.5
24.5
25.9
0.6
–
–
–
–
19.7
26.1
22.9
23.5
23.8
22.9
57.8
49.7
52.5
52.0
51.7
55.0
7:1
5:1
4:1
4:1
3:1
5:1
5
6[d]
0.1
[a] CAadv =advancing contact angle; [b] CArec =receding contact angle.
[c] Photoelectron spectroscopy (XPS). [d] Wafer (resulting from
B
entry 4) and alkoxyamine 3 were used in the nitroxide exchange reaction.
Analysis of the CA after 5 h, 10 h and 20 h did not indi-
cate any further change (Table 1, entries 3–5). Since the CA
is not sensitive enough to properly evaluate the success of
the surface nitroxide exchange reaction, we used X-ray pho-
toelectron spectroscopy (XPS) to obtain more detailed in-
formation on the reaction progress. In particular, we used
À
À
high resolution XPS C 1 s signals to determine the C C/C
O bond relations before and after surface modification. Un-
fortunately, it was impossible to relate the amount of carbon
and nitrogen by using XPS, due to the large error (see the
À
À
Supporting Information). XPS revealed a C C/C O bond
À
À
ratio of 7:1 (calculated C C/C O-bond ratio: 9:1, see the
Supporting Information) for the starting SAM A (Table 1,
entry 1).
Scheme 2. Chemically modified silicon surfaces A–H and molecular
structures of surface bound alkoxyamine 1, alkoxyamines 2–8 and nitro-
xide 9 used in nitroxide exchange reactions.
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Chemical Surface Modification of Self-Assembled Monolayers
FULL PAPER
In contrast to the CA measurements, in which no change
was detected by extending reaction time from 1 h to 20 h,
À
À
the C C/C O-bond ratio measured by XPS changed from
5:1 after 1 h to 3:1 after 20 h. A further extension of the re-
À
action time did not lead to any significant change of the C
À
C/C O bond ratio. This value agrees well with the calculat-
ed ratio of a fully exchanged wafer (Table 1, entry 5). All
following experiments were therefore conducted for 20 h.
The nice feature of our approach is the dynamic character
of the nitroxide exchange reaction. To document reversibili-
ty we subjected wafer B to a large excess of 3 under the op-
timized conditions (Table 1, entry 6). The values of the XPS-
and CA measurements are in qualitative agreement to those
of wafer A. This indicates that the reversible nitroxide ex-
change reactions can be applied to switch surface properties,
for example, from hydrophobic (A) to hydrophilic (B) and
back to hydrophobic (A).
To investigate the scope of the nitroxide exchange reac-
tion at surfaces we synthesized alkoxyamines 4 and 5 at-
tached to fluorescent dyes (see the Supporting Information).
We used wafers containing a 300 nm oxide layer to prevent
fluorescence quenching for the formation of A and exposed
them to alkoxyamine 4 under the optimized reaction condi-
tions. The success of the reaction was readily confirmed by
fluorescence microscopy, after extensive washing of the
modified wafer with CH2Cl2 and water to remove the phys-
iadsorbed dye (Figure 1). Using alkoxyamine 5 under identi-
cal conditions we also observed fluorescence at the silicon
surfaces, which provides further support for the success of
the reaction. Furthermore, we analyzed the resulting surfa-
ces by using XPS, which revealed a significant change of sur-
face composition (see the Supporting Information).
Figure 1. Structure of rhodamine (a), bodipy (c) and site-selective with
rhodamine (e) modified silicon surfaces. Fluorescence images of rhoda-
mine (b), bodipy (d) and site-selective rhodamine (f) modified SAMs.
We also performed an exchange reaction on wafer A with
the rhodamine conjugate 4 at room temperature to confirm
that immobilization of the fluorescent dye is caused by ther-
mal nitroxide exchange reaction and not as a result of phys-
iadsorption. In fact, in that case we did not observe any
fluorescence at the surface after careful washing of the
wafer (see the Supporting Information).
A further important issue was if the surface nitroxide ex-
change reactions could be applied to prestructured SAMs.
For the spatially controlled attachment of alkoxyamine 1 to
a silicon wafer we applied Langmuir–Blodgett (LB) lithog-
raphy. It was shown that mixed monolayers of l-a-dipalmi-
toyl-phosphatidylcholin (DPPC) and dyes can be transferred
by the LB technique onto a surface in regular stripes with
sub-micrometer lateral dimension.[11] As reported, regular
stripes of covalently on-wafer-bound alkoxyamine 1 can be
obtained by LB-transfer of a mixed monolayer of DPPC
and 1 with subsequent covalent attachment of 1 by trans-si-
lyletherification and removal of the structure-determining
non-covalently-bound DPPC (wafer I, Scheme 3).[10a]
Scheme 3. Formation of patterned alkoxyamine SAMs (LC=liquid con-
densed phase, LE=liquid expanded phase) and nitroxide exchange reac-
tion at LB prestructured SAMs with surface-bound alkoxyamine 1 and
soluble alkoxyamine 4.
It is important to note that our nitroxide exchange reac-
tion is based on radical chemistry that tolerates many func-
tional groups. Moreover, radical chemistry is orthogonal to
ionic chemistry. The introduction of sensitive functionalities,
for example, unprotected alcohols, should be possible by
this methodology because the surface nitroxide exchange re-
action can be conducted under neutral conditions. In con-
Pleasingly, we observed site-selective immobilization of
rhodamine at the prestructured SAM (wafer J) by treatment
of I with alkoxyamine 4 at 1258C for 20 h. Success of the ex-
change process was readily confirmed by fluorescence mi-
croscopy (see Figure 1 and the Supporting Information)
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9109

A. Studer et al.
trast to ionic reactions, no protecting group strategies are re-
quired applying our approach. For this purpose, we investi-
gated the immobilization of unprotected sugar conjugates 6
and 7 using surface nitroxide exchange reactions.
The success of the surface reaction was followed by CA
measurements and XPS studies (Table 2). The immobiliza-
We were also interested in the immobilization of other
biologically important functionalities. Along this line, we
were able to immobilize a hydrophobic dipeptide using al-
koxyamine 8, as confirmed by CA measurements and XPS.
The CA changed from 87Æ2 to 73Æ28 and the high resolu-
À
tion XPS C 1 s revealed an increase of the C O bond con-
tent (wafer G; Table 2) at the surface. We also generated
biotinylated surfaces using the nitroxide exchange reaction.
However, in that case we had to use the free nitroxide 9 in-
stead of the corresponding alkoxyamine, due to decomposi-
tion of the alkoxyamine during surface reaction. The mea-
surement of the CA and analysis by XPS indicate the suc-
cess of the surface reaction (wafer H; Table 2). This experi-
ment shows that nitroxide exchange reactions can be con-
ducted either with soluble alkoxyamines or directly with the
corresponding free nitroxides, which was even superior to
alkoxyamines in the latter case.
Table 2. Radical nitroxide exchange reaction using wafer A and biocon-
jugates 6, 7, 8, and 9 (wafers E, F, G, and H) in ClCH2CH2Cl at 1258C.
À
Wafer
CAadv
CArec
O
[%]
N
[%]
C
[%]
Si
[%]
C C/
À
C O
[8][a]
[8][a]
A
E
F
G
H
87Æ2
40Æ3
42Æ4
73Æ2
68Æ4
68Æ2
21.8
27.3
30.0
29.3
27.5
0.6
0.9
0.8
0.6
0.6
19.7
42.3
35.6
16.3
20.5
57.8
28.3
30.3
53.1
49.8
7:1
1:1
1:1
5:1
5:1
<10
<10
34Æ1
34Æ1
tion of sugar conjugates 6 and 7 caused a significant de-
crease of the CA (wafer E and F; Table 2, Figure 2), and
Biotinylated surfaces are of high interest as they can be
used for the immobilization of streptavidin and, more gener-
ally, for the generation of protein biochips. To this end, we
treated wafer H with a fluorescent-dye-tagged streptavidin
(Oyster-488 conjugate) and observed successful protein im-
mobilization as analyzed by fluorescence microscopy (see
the Supporting Information). The control experiment on the
unmodified surface revealed no protein binding after careful
washing.
Another point of interest was the proper quantification of
the surface nitroxide exchange reactions, since CA and XPS
does not allow for a highly accurate determination of the re-
action yield. To this end, we prepared SAMs using alkoxya-
mine 10 based on a fluorinated nitroxide 11. The resulting
wafer K contains fluorine atoms and was verified by XPS
(1.6 At%, Figure 3). Wafer K was treated with TEMPO
under optimized conditions and the resulting monolayer was
again, after careful washing, analyzed by XPS. In fact, even
fluorine, which is readily detected by XPS, could not be
identified anymore at the surface. Wafer A was reacted with
nitroxide 11 under the optimized conditions as a control ex-
periment.[13] In that case, fluorine was again detected by
XPS at the surface (1.5 At%, see the Supporting Informa-
tion). Therefore, we conclude that, at least for that particu-
lar reaction, a highly efficient nitroxide exchange at the sur-
face occurred. Since we used similar conditions for all other
exchange reactions, we assume that these processes general-
ly occur with high efficiency at the surface (Figure 3).
À
also a significant increase of the C O content at the surface
as measured by high resolution XPS C 1 s (Figure 2). The
À
À
C C/C O bond ratio detected by XPS changed from 7:1
(wafer A) to 1:1 (wafer E and F) after 20 h, which is a much
higher value than theoretically expected for a quantitative
À
exchange reaction. Due to the fact that the C O bonds of
the sugar moiety are at the top of the SAM they are there-
fore close to the analyzed interface. Signals of bonds at the
interface appear in XPS generally stronger, due to a higher
mean free path of the corresponding electrons.[12]
Conclusion
We have reported the first application of nitroxide exchange
reactions at SAMs. Due to its reversible character, function-
alities can be removed and new functionalities can be intro-
duced in the same operation. One important advantage of
this approach is that nitroxide exchange reactions can be
conducted under neutral conditions. They are orthogonal to
ionic reactions and various functional groups are tolerated.
However, reaction temperatures are still relatively high, so
Figure 2. Surface reaction scheme for the immobilization of sugar conju-
gate 7 (a), high resolution XPS C 1 s before (b) and after (c) surface re-
action. Water droplet on a hydrophobic (A) and hydrophilic (F) surface
(d).
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Chemical Surface Modification of Self-Assembled Monolayers
FULL PAPER
The surfaces were rinsed again with
ultrapure water and blown dry with
argon.
General procedure: The oxidized
wafers were placed into a sealed tube
and solutions of 1 or 10 were added
(1.5 mL, 10 mmol in absolute toluene).
The mixture was allowed to stand at
608C for three days. The surfaces were
rinsed with CH2Cl2 followed by soxh-
let extraction for at least 14 h in
CH2Cl2. The surface nitroxide ex-
change reaction was conducted with
alkoxyamines 2–8, TEMPO, or nitro-
xide 9 and 11 in ClCH2CH2Cl (1.5 mL,
10 mmolLÀ1). The reaction mixture
was degassed with argon for 5 min and
alkoxyamine-functionalized
wafers
were added to the solution. The reac-
tion mixture was additionally degassed
for 5 min with argon. The sealed tube
was allowed to stand at 1258C for 20 h
(in the case of alkoxyamine 2 the reac-
tion time was varied from 1, 5, 10 and
20 h). The wafers were then carefully
washed with CH2Cl2 and cleaned by
ultrasonication and continuous extrac-
tion with CH2Cl2 for at least 14 h. The
wafers were then dried in an argon
flow and analyzed by contact angle
measurements, XPS or when possible
by fluorescence microscopy.
Figure 3. Molecular structure of alkoxyamine 10 and nitroxide 11 (a) and surface reaction Scheme for nitro-
xide exchange reaction (b) XPS analysis of K (c) and A (d).
decreasing the reaction temperature remains
a future
task.[8n,14] Moreover, it is possible to apply this reaction to
prestructured SAMs for site-selective immobilization of in-
teresting functionalities. We believe that nitroxide exchange
reactions are very well suited for reversible modification of
surfaces with a broad variety of substrates. As we have dem-
onstrated, the immobilization of various biologically inter-
esting moieties is possible by this method.
Acknowledgements
The authors thank the Deutsche Forschungsgemeinschaft, SFB858 for
funding.
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Experimental Section
General: The experimental procedure for the preparation of alkoxy-
amine 1 and 10 and the preparation of alkoxyamine and nitroxide conju-
gates 2–9 and 11 are described within the Supporting Information. XPS
spectra and all detailed data of the surface characterization are also part
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Received: February 18, 2011
Revised: April 4, 2011
Published online: July 5, 2011
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