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SSuurrffaaccee CChheemmiissttrryy oonn SSeellff--aasssseemmbblleedd
MMoonnoollaayyeerrss
PROEFSCHRIFT
ter verkrijging van de graad van doctor aan de
Technische Universiteit Eindhoven, op gezag van de
rector magnificus, prof.dr.ir. C.J. van Duijn, voor een
commissie aangewezen door het College voor
Promoties in het openbaar te verdedigen
op maandag 26 oktober 2009 om 16.00 uur
door
Claudia Hnsch
geboren te Jena, Duitsland
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Dit proefschrift is goedgekeurd door de promotoren:
prof.dr. U.S. Schubert
en
prof.dr. J.-F. Gohy
Copromotor:
dr. S. Hppener
This research has been financially supported by the Dutch Organization for
Scientific Research, NWO (VICI award for U.S. Schubert).
Cover design: Daan Wouters and Claudia Hnsch
Printed by: PrintPartners Ipskamp, Enschede, The Netherlands
Surface Chemistry on Self-assembled Monolayers by Claudia Hnsch
Eindhoven: Technische Universiteit Eindhoven 2009
A catalogue record is available from the Eindhoven University of Technology
Library
ISBN: 978-90-386-2026-8
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SSuurrffaaccee CChheemmiissttrryy oonn SSeellff--aasssseemmbblleedd
MMoonnoollaayyeerrss
Kerncommissie
prof.dr. U.S. Schubert (Eindhoven University of Technology)
prof.dr. J.-F. Gohy (Eindhoven University of Technology)
dr. S. Hppener (Eindhoven University of Technology)
prof.dr.ir. R.A.J. Janssen (Eindhoven University of Technology)
prof.dr. B.J. Ravoo (Westflische Wilhelms-Universitt Mnster)
prof.dr. N.D. Spencer (Eidgenssische Technische Hochschule Zrich)
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Chapter 1
Chemical modification of self-assembled monolayers by surface reactions 1
1.1. Introduction 2
1.2. Nucleophilic substitution 3
1.3. Click chemistry 8
1.3.1. Click chemistry on azide terminated substrates 9
1.3.2. Click chemistry on acetylene terminated substrates 11
1.4. Metallo-supramolecular modification 15
1.5. Aim and scope of the thesis 21
1.6. References 23
Chapter 2
Chemical surface modification by click chemistry 29
2.1. Introduction 30
2.2. Clicking under conventional reaction conditions 31
2.3. Clicking under microwave heating 36
2.3.1. Stability tests on azide terminated silicon substrates under microwave
irradiation 36
2.3.2. Microwave-assisted surface functionalization by 1,3-dipolar cycloaddition 39
2.3.2.1. Microwave-assisted clicking of propargyl alcohol onto azide
terminated silicon substrates 41
2.3.2.2. Microwave-assisted clicking of an acetylene functionalized
poly(2-ethyl-2-oxazoline) onto azide terminated silicon substrates 43
2.3.2.3. Comparison with reactions under conventional heating 44
2.4. Conclusions 45
2.5. Experimental details 45
2.6. References 49
Chapter 3
Combination of different chemical surface reactions 53
3.1. Introduction 54
3.2. Nucleophilic substitution reactions 54
3.2.1. Nucleophilic substitution of bromine by propargylamine 55
3.2.2. Nucleophilic substitution of bromine by an amine functionalized terpyridine 59
3.3. Supramolecular modification of silicon and glass substrates 63
3.4. Conclusions 703.5. Experimental details 71
3.6. References 77
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Chapter 4
Structuring and functionalization of self-assembled monolayers by electro-oxidation81
4.1. Introduction 82
4.2. Local anodic oxidation 83
4.2.1. Local anodic oxidation of substrates coated with organic films 86
4.2.2. Conclusions 90
4.3. Constructive nanolithography 90
4.3.1. Post-modification of the structured surface areas 94
4.3.2. Conclusions 98
4.4. References 98
Chapter 5
Fabrication and functionalization strategies for nanometer-sized surface templates 107
5.1. Introduction 108
5.2. Local anodic oxidation of bromine terminated silicon substrates 109
5.3. Constructive nanolithography on n-octadecyltrichlorosilane terminated
monolayers 115
5.4. Conclusions 121
5.5. Experimental details 121
5.6. References 123
Chapter 6The preparation of patterned polymer brushes onto chemically active surface
templates 125
6.1. Introduction 126
6.2. Surface-initiated atom transfer radical polymerization 127
6.3. Surface-initiated ring-opening polymerization 132
6.4. Conclusions 137
6.5. Experimental details 137
6.4. References 139
Summary 142
Samenvatting 145
Curriculum vitae 148
List of publications 149
Acknowledgement 151
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Abstract
The functionalization of solid substrates by self-assembly processes provides the possibility to
tailor their surface properties in a controllable fashion. One class of molecules, which
attracted significant attention during the last decades, are silanes self-assembled on hydroxyl
terminated substrates, e.g. silicon and glass. These systems are physically and chemically
robust and can be applied in various fields of technology, e.g. electronics, sensors and others.
The introduction of chemical functionalities can be obtained via two methods. One
preparation method uses the self-assembly of pre-functionalized molecules, which can be
synthesized by different synthetic routes. The second method utilizes chemical surface
reactions for the modification of the monolayer. The following chapter will highlight a
selection of chemical reactions, which have been used for the functionalization of solidsubstrates to introduce a wide range of terminal end groups.
Parts of this chapter will be published: C. Haensch, S. Hoeppener, U. S. Schubert, in
preparation.
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Chapter 1
2
1.1. Introduction
Self-assembled monolayers (SAMs) have attracted significant attention since their
introduction in 1980 by Sagiv.[1] Since then a large variety of self-assembly systems have
been studied, such as thiols on gold[2-5] and alkylsilanes on silicon,[6-10] which represent the
most popular combinations of self-assembling molecules and substrates. In particular, the
self-assembly of silane molecules represents an interesting research area as they form
chemically and physically stable covalently attached monolayers on technologically relevant
surfaces, in particular silicon and glass.[6,11] The functionalization of surfaces provides the
possibility to tailor the surface properties, e.g. wettability, friction, adhesion and conductivity,
in a controlled fashion as the modification of the terminal end groups allows the tuning of
these properties.[12] These surfaces find many applications in various fields, such as
microelectronics, optoelectronics, thin-film technology, protective coatings, chemical sensors,
biosensors, nanotechnology, bioactive surfaces, cell adhesion, protein adsorption and
others.[13-18]
Alkylsiloxane monolayers are usually prepared by a covalent adsorption process of self-
assembling molecules, such as trichloro, trimethoxy or triethoxy silanes, onto the solid
substrate.[5] The self-assembling molecule consists of three parts: the head group, the alkyl
chain and the terminal end group.[16] The head group, i.e., trichloro-, trimethoxy or triethoxy
silane, is responsible for the anchoring of the molecule onto the substrate. The alkyl chain
provides the stability of the monolayer, due to van der Waals interactions, and influences the
ordering of the SAM,[14] the terminal end group introduces chemical functionality into themonolayer system. Suitable molecules are to a certain extent commercially available or can be
prepared by different chemical reactions. The synthesis of trichlorosilanes can be performed
by, e.g., the reaction of bromine functionalized compounds with Mg, followed by the addition
of SiCl4.[19] Furthermore, alkali metal organic compounds can be transformed to RSiCl3 by the
reaction with SiCl4.[20] A third preparation method utilizes a three step synthesis, which starts
from the chlorination of an alcohol with tri-n-butyl or triphenyl phosphine and CCl4, a
Grignard reaction with the chlorinated product and Mg and subsequent addition of SiCl4.[21]
An alternative possibility is the platinum catalyzed addition of HSiCl3 to carbon-carbondouble bonds.[7] The disadvantage of trichlorosilanes is, however, a high reactivity of the head
group and the water sensitivity of these molecules. This limits therefore the possibility for the
presence of a functional terminal end group due to incompatibility and/or interaction of the
chlorosilane with the terminal end group, which can potentially prevent the self-assembly
process and therefore the formation of a well-ordered monolayer. An elegant approach to
overcome this problem is the use of chemical surface reactions on a precursor SAM with
defined organization that is already assembled on a substrate. Surface chemistry represents
here an important tool for the preparation of a large variety of different functional end groups.
Several criteria, such as high yields obtained under mild reaction conditions, the compatibility
with different functional groups, no required catalytic activation and the absence of
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Chemical modification of self-assembled monolayers by surface reactions
3
byproducts, are advantageous for such chemical surface modification reactions.[22] Several
recent reviews highlight the use of different modification sequences to introduce chemical
functionalities into monolayer systems.[23-26] The aim of this chapter is to present a selection
of chemical surface reactions, such as nucleophilic substitutions, the click chemistry approach
and metallo-supramolecular modification of, mainly, silicon and glass surfaces, to introduce
versatile terminal end groups.
1.2. Nucleophilic substitution
Nucleophilic substitution reactions are an important class of chemical reactions in organic and
inorganic chemistry.[27] Thereby, a nucleophile attacks an atom with a positive or partially
positive charge, which is connected to a leaving group. Nucleophiles are electron rich species,
which are electrically neutral or negatively charged and can be either ions or molecules. [28]
Common nucleophiles are hydroxide, cyanide and azide ions, water, ammonia and others. The
nucleophilic substitution follows two main mechanisms, the SN1 and SN2 reactions.[27] SN1
reactions proceed in two steps; the first one involves the elimination of the leaving group and
the formation of a carbenium ion, which reacts in a second step with the nucleophile. In
comparison, the elimination of the leaving group and the addition of the nucleophile in SN2
reactions occur simultaneously. While in organic chemistry in solution both reaction
mechanisms occur, the nucleophilic substitution on solid substrates is mostly reported to
follow the SN2 mechanism. The preference for the SN2 reaction mechanism could be due to
the structure of the terminal end group of the monolayer, which is in most cases a primarycarbon atom. Typically reported nucleophlic substitution reactions in solution are the
exchange of halogen atoms in alkyl halides by various nucleophiles, such as NH3, OH, N3,
CN, etc., the Williamson ether synthesis, the Finkelstein reaction and others.[27-29] In 1988
Balachander and Sukenik recognized the potential to use nucleophilic displacement reactions
for the modification of solid substrates.[30] The authors described the preparation of amine
terminated surfaces by the nucleophilic displacement of bromine terminated substrates with
sodium azide and subsequent reduction of the N3 groups to NH2 functionalities. The
complete exchange of bromine by azide was confirmed by the disappearance of the Br signalin the X-ray photoelectron spectroscopy (XPS) spectrum and the appearance of an absorption
peak at 2098 cm-1 in the infrared (IR) spectrum. Two years later the authors published a series
of nucleophilic transformations on alkyl bromine terminated substrates to introduce a large
variety of functional groups into the monolayer.[31] Bromine terminated surfaces were reacted
with various nucleophiles, such as SCN, S2-, S22- and N3, to generate different surface
functionalities (Figure 1.1). IR spectroscopy and XPS as well as contact angle analysis were
performed to investigate the conversion of the reactions and indicated a quantitative
conversion of the bromine functionalities. Subsequent treatment with LiAlH4 and H2O2
yielded SH, NH2 and SO2 moieties, respectively.
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Chapter 1
4
Si
Br
O OO
C16
SiO OO
N3
C16
SiO OO
SCN
C16
a. b.
d. c.Si
S
OO
Si
S
O OO
C16
SiOO
SiO OO
S
C16
Si
Br
O OO
C16
SiO OO
N3
C16
SiO OO
SCN
C16
a. b.
d. c.Si
S
OO
Si
S
O OO
C16
SiOO
SiO OO
S
C16
Figure 1.1. Schematic representation of nucleophilic substitution reactions on bromineterminated monolayers on glass substrates and Si ATR crystals. (a) KSCN, (b) Na2S, (c)
Na2S2 and (d) NaN3.
Fryxell et al. described the nucleophilic displacement reactions on mixed monolayers,
terminated with bromine and methyl groups.[32] The authors showed the substitution of Br by
sodium azide, thiocyanate, and cysteine. In contrast to the results of Balachander and Sukenik
the required reaction times were significantly longer. The authors explained the prolonged
reaction times by the difference in the quality of the prepared monolayers, as dense, highly
rigid monolayers were used for the chemical transformations. Additionally, the poor reactivity
of bromine terminated alkyl SAMs was assigned to the fact that the nucleophile needs to
penetrate below the surface to attack the carbon atom from the backside, resulting in a
substantial kinetic barrier of the nucleophilic substitution. Baker and Watling demonstrated
the functionalization of alkyl terminated monolayers via free radical bromination and
subsequently modified the surface further.[33] In their report the authors treated a monolayer of
hexadecylsiloxane with bromine in CCl4 to obtain bromine terminated SAMs. Thereby,
bromination occurred at the methyl and the methylene groups in the monolayer. The bromine
terminated monolayers were afterwards treated with different nucleophiles, such as azide,sulfide and thiocyanate. Thereby, a quantitative exchange of bromine by N3 and S
2- was
observed, whereas thiocyanates could be incorporated into the SAM with yields of 80 to 90%.
The nucleophilic substitution of Br by N3 represents an important chemical surface reaction
as azide moieties can be used for further post-modification reactions. Heise and co-workers
described the in-situ modification of mixed silane monolayers by nucleophilic transformation
of Br by N3 and subsequent reduction to NH2,[34] which could be used for the grafting of
polypeptides onto the substrates.[35] The use of amine terminated trimethoxysilanes was not
favorable, as the self-assembly of these compounds resulted in the formation of disorderedmonolayers. Therefore, the previously mentioned two step procedure was chosen to obtain
amine terminated monolayers of good quality. Thereby, a reaction yield of 80% was achieved
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Chemical modification of self-assembled monolayers by surface reactions
5
for the nucleophilic substitution of Br by N3 and 100% for the reduction step to obtain the
amine moieties. The fabrication of well-defined polyimide functionalized surfaces was
reported by Lee and Ferguson.[36] Thereby, bromine terminated monolayers served as pre-
functionalized SAMs, which were reacted with sodium azide, followed by treatment with
LiAlH4 to yield NH2 functionalities. The nucleophilic transformation was characterized by
contact angle measurements, IR spectroscopy and XPS and revealed the full exchange of Br
by N3. The contact angle decreased from 81-85 for the Br terminated surface to 75-79 for
the N3 moieties. The IR spectrum revealed the appearance of an absorption peak at
2097 cm-1 for the azide groups and the preservation of the CH2 vibrations, indicating that no
significant damage of the monolayer occurred. XPS revealed the disappearance of the Br(3d)
signal at 71 eV and the appearance of two peaks at 402 and 405 eV in the N(1s) region, which
could be assigned to the N3 moieties present on the surface. Wang et al. reported a study on
the preparation of maleimido terminated SAMs via different synthetic routes.[37] One method
included the use of nucleophilic substitution to obtain azide moieties, which were reduced to
NH2 groups and subsequently reacted with sulfosuccinimidyl 4-(N-maleimidomethyl)
cyclohexane-1-carboxylate to yield the desired surface functionalities.
An interesting surface modification route via nucleophilic substitution was demonstrated by
Chupa et al.[38] A bromine terminated silicon oxide substrate was treated with anions, which
were formed by the reaction of 4-methylpyridine with lithium diisopropylamide. The resulting
pyridyl moieties were coupled with OsO4 and C60 to introduce buckminsterfullerene groups
for the modification of the electronic and optical responses of the solid substrates. The
introduction of amine functionalities onto silicon oxide surfaces was realized via the hiddenamine route reported by Ofir and co-workers (Figure 1.2a).[39] It involves the nucleophilic
displacement of bromine moieties by potassium phthalimide after three hours of reaction time,
followed by cleavage of the protecting group after one minute to yield NH2 groups. The
authors emphasized the advantage of this reaction pathway as the functionalization of surfaces
with amine moieties is faster compared to the exchange by NaN3 and the subsequent
reduction.
Si
I
O OO
Si
S
O OO
3OS-
Na+
b. b. c.a.
Si
Br
O OO
Cn
N OO
SiO OO
Cn
NH2
SiO OO
Cn
Si
I
O OO
Si
S
O OO
3OS-
Na+
b. b. c.a.
Si
Br
O OO
Cn
N OO
SiO OO
Cn
NH2
SiO OO
Cn
Figure 1.2.Schematic representation of nucleophilic substitution reactions on bromine and
iodine terminated monolayers on glass, quartz, indium-tin oxide and silicon oxide substrates.
(a) Nucleophilic substitution with potassium phthalimide, (b) cleavage of the phthalimide
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Chapter 1
6
groups by CH3NH2 and (c) nucleophilic substitution with mercapto ethanesulfonic acid
sodium salt.
The formation of polyaniline films via a four step modification sequence was demonstrated by
Sfez et al.[40] Monolayers of iodopropyltrimethoxysilane were treated with mercapto
ethanesulfonic acid sodium salt and resulted in the substitution of I by SO 3Na (Figure
1.2b). Chemical proof for the successful substitution was obtained by XPS analysis, which
revealed the complete disappearance of the iodine signal after substitution and the appearance
of a sodium signal at ~ 1070 eV. These surfaces were furthermore utilized for the attachment
of aniline monomers via electrostatic interactions, which were polymerized by either chemical
or electrochemical oxidation. Koloski and co-workers performed nucleophilic displacement
reactions on benzyl halide (Cl and I) terminated substrates.[41] For this purpose, monolayers
of benzyl chloride were treated with sodium iodide, which resulted in an exchange of Cl by
I with yields between 45 and 60%. This reaction is also known as Finkelstein reaction. The
incomplete exchange of Cl by I was explained by the larger size of the iodine atom and the
differences in the C-I and C-Cl bond lengths. The benzyl chloride and iodide terminal groups
were subsequently used afterwards for substitution reactions with N-lithiodiaminoethane and
3-lithiopyridine. UV/vis spectroscopy and XPS were performed to investigate the conversion
as well as the rate of the reaction and revealed a higher reactivity for the iodine terminated
species.
As already mentioned earlier, not only negatively charged ions can act as nucleophiles, but
also neutral molecules, such as water, amines and thiols, can be used for the nucleophilicdisplacement. The reaction of 16-bromohexadecyl and brominated hexadecylsiloxane
monolayers with mercury(II) perchlorate in a water-dimethoxyethane mixture for 3 and 24 h
resulted in a quantitative exchange of Br by OH, respectively. [33] Studies on the reactivity
of gold clusters, protected with mixed monolayers of bromine and methyl groups, in SN2
reactions with primary amines were reported by Templeton and co-workers.[42] The authors
used n-propylamine, isopropylamine and tert-butylamine as nucleophiles and three different
chain lengths of the bromoalkanethiolate chains were investigated. The selectivity and rate of
the reaction was found to be controlled by the size of the nucleophile as well as by the lengthof the bromoalkanethiolate and the surrounding alkanethiolate chains. Lee and co-workers
investigated the selective attachment of peptides onto halide terminated surfaces.[43] This
approach involved the nucleophilic substitution of halide terminated substrates by tri- and
nonapeptides via the thiol group of cysteine moieties. The surfaces, consisting of alkyl and
benzyl halides and -haloacetyl monolayers, showed differences in electrophilicity depending
on the halide atoms. The reactivity towards the exchange by thiols was found to decrease
from iodine to bromine and chlorine and from -haloacetyl over benzyl to alkyl. This
methodology represents a powerful tool for the controlled attachment of peptides and proteins
to various substrates and could furthermore be used for the fabrication of biocompatible
inorganic materials and biosensors. Besides of previously described nucleophilic
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Chemical modification of self-assembled monolayers by surface reactions
7
displacement reactions on halide terminated monolayers also the use of other functional end
groups for this reaction mechanism has been reported. Fryxell and co-workers demonstrated
the nucleophilic substitution on mixed monolayers of trifluoroethyl ester and methyl moieties
by primary amines, e.g. N,N-dimethylethylenediamine, hydrazine, and hydroxylamine.[32] A
low amount of trifluoroethyl ester groups on the surface resulted in the exchange of the
trifluoroethyl functionalities and the formation of amide groups. With increasing percentage
of the trifluoroethyl moieties a competitive cross-linking reaction occurred, which resulted in
the formation of imide functionalities. Maleimido terminated silicon substrates were exposed
to a large variety of different nucleophilic compounds, e.g. decylthiol, octadecylthiol,
aliphatic amines, isoindole, thioacetamide and thiol-tagged DNA oligonucleotides (Figure
1.3).[37] Characterization of the substitution reactions with decylthiol and octadecylthiol was
performed by attenuated total reflection infrared (ATR-IR) spectroscopy and revealed an
increase in the absorption intensities of the methylene vibrations. The attachment of aliphatic
amines, isoindole and thioacetamide was investigated by the appearance of an absorption peak
between 2100 and 2300 cm-1 in the IR spectrum for the CN groups, which were introduced
in the nucleophiles as IR spectroscopic marker. Biomolecules, such as oligonucleotides, could
also be successfully introduced into the monolayer system. XPS revealed the appearance of a
signal at 133.9 eV, which was assigned to the phosphates in the DNA backbone.
N
SiO OO
OO
Cn
N
SiO OO
OO
NH(CH2)6CN
Cn
a. b. c. d. e.
N
SiO OO
OO
S(CH2)
Cn
xCH3
N
SiO OO
OO
N
CN
Cn
SN
N
SiO OO
O CN
Cn
N
SiO OO
OO
S-DNA
Cn
N
SiO OO
OO
Cn
N
SiO OO
OO
NH(CH2)6CN
Cn
a. b. c. d. e.
N
SiO OO
OO
S(CH2)
Cn
xCH3
N
SiO OO
OO
N
CN
Cn
SN
N
SiO OO
O CN
Cn
N
SiO OO
OO
S-DNA
Cn
Figure 1.3. Schematic representation of nucleophilic substitution reactions on maleimido
terminated monolayers on silicon oxide substrates. (a) Decylthiol, (b) aliphatic amine, (c)
isoindole, (d) thioacetamide and (e) DNA-SH.
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Chapter 1
8
Sawoo and co-workers described the modification of silicon and glass surfaces for protein
immobilization via nucleophilic displacement reactions.[56] These included the nucleophilic
exchange of bromine by azide and subsequent copper-free clicking with an alkoxy Fischer-
type metallocarbene complex. In a second nucleophilic substitution step the SAM was reacted
either with 1-pyrenemethylamine, which acted as a fluorescent marker, or with bovine serum
albumin (BSA). The introduced 1-pyrenemethylamine revealed a broad emission at 480 nm in
the fluorescence spectrum indicating the successful attachment of the Fischer carbene
complex to the substrate. The attachment of the protein was investigated by atomic force
microscopy (AFM), which showed a dense coverage of the surface. This research
demonstrated the covalent linkage of biomolecules onto solid substrates under mild reaction
conditions.
These selected examples demonstrate the potential of nucleophilic substitution reactions
conducted on solid substrates and present an interesting modification sequence for the
introduction of various functional end groups into the monolayer. Thereby, mainly halide
terminated surfaces were used due to a high reactivity of the halides towards the exchange by
nucleophilic molecules. Subsequent surface reactions can be additionally applied to these
monolayers to further expand the range of functional end groups.
1.3. Click chemistry
A different reaction mechanism also matches the criteria for efficient surface reactions. The
click chemistry approach has attracted significant attention in various parts of chemistry, suchas in materials science and polymer chemistry during the last years,[44-46] since it was
introduced in 2001 by Sharpless.[47] The concept addresses several criteria. The reaction has to
be modular and wide in scope, provides furthermore very high yields, generates inoffensive
byproducts, is stereospecific, can be performed under mild reaction conditions and with easily
available starting materials.[47] The purification can be preferably achieved by
nonchromatographic methods. A large variety of chemical transformations are part of this
approach, e.g. 1,3-dipolar cycloaddition reactions and Diels-Alder transformations,
nucleophilic substitution chemistry, formation of ureas, thioureas, aromatic heterocycles,oxime ethers, hydrazones, amides, and additions to carbon-carbon multiple bonds, e.g.
epoxidation, dihydroxylation, aziridination, sulfenyl halide addition and Michael additions.
The most perfect click reaction up to date is the Huisgen 1,3-dipolar cycloaddition of organic
azides and acetylenes.[48,49] Thereby, a mixture of 1,4- and 1,5-disubstituted 1,2,3-triazole
systems is formed. A variant of this reaction is the copper catalyzed coupling of azides and
terminal acetylenes, which selectively results in the formation of the 1,4-disubstituted
triazole.[50,51] The general characteristics of the click chemistry approach fit very well with the
requirements of chemical reactions performed on surfaces. This is documented by a large
number of literature examples, which demonstrate the use of click chemistry for the
introduction of functional groups into the monolayer system on different substrates, e.g. gold,
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Chemical modification of self-assembled monolayers by surface reactions
9
silicon and glass.[52-54] Thereby, two synthetic preparation methods are used to introduce
1,2,3-triazole moieties into the monolayer. Whereas the first method uses azide terminated
substrates for the coupling with functional acetylenes, the second modification sequence
involves the generation of surfaces with terminal acetylene moieties. Both modification
sequences will be discussed in more detail in the following sections.
1.3.1. Click chemistry on azide terminated substrates
The first preparation method to obtain 1,2,3-triazoles utilizes azide terminated substrates,
which can be obtained by the nucleophilic displacement of Br by N3, and subsequent
cycloaddition reaction. The first report on the Huisgen 1,3-dipolar cycloaddition on silicon
substrates, covered with a layer of native oxide, was published by Lummerstorfer and
Hoffmann.[22] 11-Bromoundecylsiloxane monolayers were substituted by sodium azide and
N3 functionalities were obtained, which were reacted with different acetylenes (Figure 1.4).
Methoxycarbonyl and bis-(ethoxycarbonyl) acetylene could be quantitatively coupled to the
N3 terminated surfaces after reaction times of 24 hours at 70 C. Surface-enhanced IR
reflection spectroscopy was performed to characterize the substrates and revealed the
complete disappearance of the azide absorption peak, which was found at 2102 cm -1. The
authors used no catalyst, which resulted most likely in the formation of a mixture of the 1,4-
and 1,5-disubstituted products; however, the authors favored the 1,4-disubstituted triazole due
to steric reasons.
Si
Br
O OO
C11
SiO OO
N3
C11
SiO OO
N
N
N
RR
C11
R = COOMe, R= HR = R= COOEt
a. b.
Si
Br
O OO
C11
SiO OO
N3
C11
SiO OO
N
N
N
RR
C11
R = COOMe, R= HR = R= COOEt
a. b.
Figure 1.4.Schematic representation of the 1,3-dipolar cycloaddition on bromine terminatedmonolayers on silicon oxide substrates. (a) Nucleophilic substitution with sodium azide and
(b) cycloaddition with methoxycarbonyl or bis-(ethoxycarbonyl) acetylene.
Prakash et al. demonstrated the cycloaddition of linear and branched polymers, e.g. poly-
(amido amine) dendrimers, poly(2-ethyloxazoline), poly(ethylene glycol) and poly(ethylene
imine), to N3 terminated silica and microfluidic glass channels.[55] The substrates were
characterized by Fourier transform infrared (FT-IR) spectroscopy and XPS as well as by
contact angle and electroosmotic flow (EOF) measurements. Investigations by XPS revealed
the disappearance of the Br peak after the nucleophilic substitution and the appearance of a
characteristic split peak in the N(1s) region for the N3 moieties. After cycloaddition the
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double peak disappeared and a single peak was observed in the N(1s) region. FT-IR ATR
spectroscopy furthermore indicated the successful cycloaddition of the alkyne functionalized
polymers to N3 surfaces by the disappearence of the absorption peak at 2100 cm-1, which is
characteristic for the azide moiety. EOF measurements were performed to classify the
electroosmotic velocity in the modified microchannels. The modification with triazoles
resulted in a change of the zeta potential and therefore the EOF in the channels could be
changed in a systematic fashion. The fabrication of bio-active surfaces via the copper-free
click reaction was demonstrated by Sawoo and co-workers.[56] Azide terminated glass and
silicon surfaces were reacted with a Fischer carbene complex for the immobilization of
proteins, which could find potential applications in the fabrication of biosensors. Ellipsometry
revealed an increase in height of the monolayer of about 0.66 nm after the cycloaddition.
Immobilization tests on this surface were performed with bovine serum albumin. AFM and
high resolution scanning electron microscopy were performed to analyze the immobilized
proteins on the surface. The amount of molecules was calculated to be ~ 330 molecules per
m2. Reaction with dansyl chloride allowed furthermore to investigate the substrate by
fluorescent microscopy. The chemical derivatization of hydrogenated silicon substrates was
demonstrated by Marrani and co-workers.[57] The silicon surfaces were treated with
11-bromo-1-undecene, followed by substitution with NaN3 and subsequent cycloaddition of
ethynylferrocene under Cu(I) catalysis. Next to the characterization by XPS, which revealed
the successful attachment, also cyclic voltammetry was performed to investigate the
functionalized substrates. The electrochemical properties of the ferrocene terminated
substrates revealed the formation of a well-defined SAM as well as an efficient chargetransfer reaction to the ferrocene redox head group. Vestberg et al. presented the formation of
multilayer films via click chemistry.[58] Thereby, NH2 terminated silicon substrates were
reacted with 4-azidobutanoic anhydride to obtain azide functionalities onto the surface. These
moieties were further modified by alternating cycloaddition reactions with acetylene and
azide terminated dendrimers to obtain multilayers. Ellipsometry was used to measure the
thickness of each layer and XPS revealed structural information about the multilayer. Another
example of regio- and chemoselective immobilization of proteins onto glass surfaces was
presented by Gauchet et al. (Figure 1.5).
[59]
Thereby, azide functionalized substrates wereprepared via a multi step procedure, utilizing the transformation of NH2 surfaces to
carbamates followed by the attachment of azide terminated PEG-containing amine linkers and
subsequent deactivation of the residual carbamates. Afterwards a green fluorescent protein
was attached via click chemistry and was analyzed by phosphorimaging, which revealed the
attachment of the protein due to the appearance of a fluorescent signal.
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a. b. c.
Si
NH2
O OO
Si
NH
O OO
OO
N OO
NH
O
NH
O
O
O
N
N
N
protein
Si
NH
O OO
NHO
NH
O
NH
O
O
O
N3
Si
NH
O OO
NHO
8 8
a. b. c.
Si
NH2
O OO
Si
NH
O OO
OO
N OO
NH
O
NH
O
O
O
N
N
N
protein
Si
NH
O OO
NHO
NH
O
NH
O
O
O
N3
Si
NH
O OO
NHO
8 8
Figure 1.5.Schematic representation of the 1,3-dipolar cycloaddition on amine terminated
monolayers on glass substrates. (a) Reaction with N,N-disuccinimidyl carbonate, (b)
attachment of an azide terminated PEG-containing amine linker and (c) cycloaddition with a
green fluorescent protein.
Govindaraju and co-workers introduced the functionalization of glass substrates via the click
sulfonamide reaction.[60] Sulfonylazide terminated surfaces were reacted with alkyne modified
biomolecules, followed by the immobilization of fluorescently labelled proteins. The
presented results demonstrated the site- and chemoselectivitely immobilization of alkyne
terminated molecules under mild reaction conditions. The fabrication of a protein microarray
was demonstrated by Lin and co-workers.[61]
Azide terminated glass surfaces were preparedby a two step synthesis starting from amine modified slides, which were reacted with glutaric
acid bis-N-hydroxysuccinimide ester and subsequent treatment with 3-azidopropanylamine.
Further reaction with enhanced green fluorescent proteins under Cu(I) catalysis resulted in
protein functionalized substrates, which were analyzed by fluorescence microscopy and
revealed the preservation of the tertiary structure of the protein.
1.3.2. Click chemistry on acetylene terminated substrates
The second possibility to implement click chemistry into surface modification is the
functionalization of silicon surfaces with terminal acetylene moieties, which can be achieved
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by different approaches. Acetylenylation can be obtained by chlorination of hydrogenated
silicon and subsequent reaction with NaCCH.[62] An electroactive benzoquinone was
coupled to the CCH functionalities and resulted in a coverage of 7% of the molecule on the
surface (Figure 1.6). The low yield was explained by the assumption that effective click
reactions could only take place at the step edges of the Si(111) substrate. Subsequently
applied surface chemistry resulted in the immobilization of ferrocene and biotin onto these
surfaces. The presented results are supposed to find applications in the selective
biopassivation of arrays of various types of sensor devices.
H
a. b. c.
Cl
NH
N N
N
O O
O
H
H
a. b. c.
Cl
NH
N N
N
O O
O
H
Figure 1.6. Schematic representation of the 1,3-dipolar cycloaddition on hydrogenated
silicon substrates. (a) PCl5, (b) Na-CCH and (c) cycloaddition with benzoquinone.
Ostaci et al. reported the use of ethynyldimethylchlorosilane to obtain acetylene terminated
surfaces on silicon substrates covered with a 525 m thick layer of SiO2.[63] Thereby, theacetylene group is directly attached to the silicon atom, which prevents the formation of a
well-ordered monolayer. These surfaces were used to perform click reactions with different
azide terminated polymers, e.g. polystyrene, poly(ethylene oxide) and poly(methyl
methacrylate). Polymer brushes with heights of ~ 6 nm could be obtained by this approach,
which offers therefore the possibility to graft a large variety of different polymer systems onto
the surface. Yam and co-workers demonstrated the functionalization of silicon substrates with
acetylene moieties by the reaction of terminal NEt2 groups with organic molecules
containing acidic protons.[64,65]
The NEt2 terminated surfaces were obtained by the treatmentof OH functionalized substrates with SiCl4 or SnCl4 followed by the reaction with HNEt2.
[64]
Subsequent reaction with different modified dialkynes, containing acidic protons, led to the
fabrication of various terminal CCH moieties. An improvement of the procedure
concerning the amount of reaction steps was published a few years later. [65] Herein, OH
functionalized surfaces were reacted with Sn(NEt2)4 to obtain the NEt2 groups in one steps.
The formation of well-ordered acetylene terminated monolayers was reported by different
groups via the reaction of hydrogenated Si with 1,8-nonadiyne[66-68] or 1,6-heptadiyne.[69]
Ciampi and co-workers presented the use of 1,8-nonadiyne for the fabrication of CCHterminated silicon and demonstrated subsequent click chemistry reactions (Figure 1.7). [66-68]
The acetylene monolayers were characterized by contact angle measurements, XPS and X-ray
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reflectrometry and revealed well-formed and densely packed monolayers. These CCH
moieties were afterwards used for the 1,3-dipolar cycloaddition with various azides, including
1-azidobutane, 4-azidophenacyl bromide, 11-azido-3,6,9-trioxaundecan-1-ol, azidomethyl-
ferrocene and others. XPS was utilized to calculate the chemical conversion of the reaction
and revealed yields between 35 and 80% using various reaction conditions. The conditions for
the cycloaddition of azidomethylferrocene onto Si were optimized, using non-aqueous as well
as aqueous conditions and different copper species as catalyst.[67] The use of copper(II) sulfate
pentahydrate and sodium ascorbate resulted in the formation of triazole terminated silicon
with a 45% yield without the contamination of the substrate with Cu, which was observed for
the use of CuI-P(OEt)3 and CuI. The XPS spectra revealed in these cases the presence of Cu.
Cyclic voltammetry was performed and showed the characteristic behavior of the ferrocene
molecules. These modified surfaces could find possible applications for the preparation of
biosensors and memory devices. Experiments concerning the modification of porous silicon
substrates by the click chemistry approach were performed under Cu(I) catalysis, using
copper(II) sulfate pentahydrate and sodium ascorbate, and revealed a non-quantitative
cycloaddition.[68] The addition ofN,N,N,N-tetramethylethane-1,2-diamine, which acted as a
ligand for the copper ions, resulted in almost quantitative conversions. The functionalization
of these porous silicon substrates with oligomers of ethylene oxide could be interesting for the
fabrication of biosensing interfaces, as these surfaces revealed antifouling properties.
H
a. b.
N
R
N
N Br
O
OO
OOH
R =
H
a. b.
N
R
N
N Br
O
OO
OOH
R =
Figure 1.7. Schematic representation of the 1,3-dipolar cycloaddition on hydrogenated
silicon substrates. (a) Reaction with 1,8-nonadiyne and (b) cycloaddition with acetylene
functionalized molecules.
Britcher et al. also demonstrated the functionalization of porous silicon wafers by click
chemistry.[69] 1,6-Heptadiyne was reacted with hydrogenated Si, followed by the coupling of
an azide terminated PEG. The successful attachment of the polymer was proven by XPS.
Treatment of the surface with human serum albumin revealed their resistance towards protein
adsorption and the substrates were suggested to be useful for targeted protein delivery
applications. Another possibility to obtain acetylene terminated surfaces is the use of chemical
surface reactions. Vestberg et al. presented the formation of acetylene moieties by reaction of
NH2 surface groups with 4-pentynoic anhydride.[58] Alternating cycloaddition with azide andacetylene terminated dendrimers resulted in the formation of dendritic multilayers. This
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modification sequence allowed the growth of extremely regular and defect-free dendritic thin
films, which thicknesses could be controlled by the generations of the dendrimer. A similar
approach to obtain acetylene moieties was presented by Long and co-workers, who reacted
NH2 functionalized silicon supports with 4-pentynoic acid, which resulted in the formation
of acetylenes moieties.[70] These groups were furthermore used for cycloaddition reactions
with an azide functionalized dendron. This surface modification was not only shown for an
entire substrate, but was also performed with an AFM tip to obtain localized triazoles. Several
literature examples demonstrated the use of click chemistry for the preparation of active
biosurfaces. The introduction of CCH moieties via reaction of carboxylic acid terminated
silicon or glass substrates with a bifunctional propargyl-derivatized linker was described by
Gallant and co-workers.[71] This was followed by cycloaddition of the acetylene moieties with
an azide functionalized peptide by using CuSO4 and sodium ascorbate as catalytic species.
Cell adhesion tests were performed with smooth muscle cells and were analyzed by
fluorescence microscopy. The presented results could find applications in biomaterials
research. Sun et al. demonstrated the surface modification via the reaction of maleimido
terminated glass with an alkyne-PEG4-cyclodiene linker, which resulted in the formation of
CCH groups.[72] Subsequent clicking with different biomolecules, e.g. biotin, lactose and a
recombinant protein, was successful, which was demonstrated by subsequent labeling with
fluorescent dyes and confocal fluorescence imaging. Seo and co-workers introduced the
reaction of amine terminated glass chips with succinimidyl N-propargyl glutariamidate to
yield terminal CCH moieties.[73,74] The acetylene terminated substrates were afterwards
treated with azido functionalized DNA under CuI catalysis and were analyzed by AFM andcontact angle measurements. Subsequent, DNA polymerase reactions using photocleavable
fluorescent nucleotides were carried out to investigate the functionality, accessibility and
stability of the attached DNA. Meng and co-workers prepared chemical functionalized porous
silicon substrates for the application in desorption/ionization silicon mass spectrometry
(DIOS-MS).[75,76] DIOS-MS provides the possibility to analyze small molecules without the
use of a matrix using standard MALDI instrumentation. For this purpose acetylene
functionalized surfaces were treated with an azide functionalized triazine derivative. These
CCH moieties were either synthesized by the reaction of amine terminated silicon surfaceswith 4-pentynoyl chloride, by the reaction of hydrogenated silicon substrates with
1,6-heptadiyne or by the reaction of maleimide functionalized silicon with an acetylene
modified triazenyl furan dienophile.
The Huisgen 1,3-dipolar cycloaddition of azides and acetylenes allows the modification of
solid substrates under mild reaction conditions as demonstrated by the summarized literature
examples. Furthermore a large variety of functional groups can be incorporated into the SAM
as this reaction mechanism tolerates a wide range of different functionalities. The surface
modification by click chemistry leads to the construction of bioactive surfaces as well as
electrochemically active substrates. The use of copper(I) catalysts results additionally in the
formation of stereoselective reaction products.
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1.4. Metallo-supramolecular modification
The fabrication of smart surfaces with tailor-made functional groups is an important research
area in various fields of technology.[77] Thereby, it is of great importance to control and switch
the surface properties in a reliable fashion. Such systems find many potential applications in,
e.g. sensors, memory elements, optical/electronic switches, nanodevices, displays, data
storage or can be used for the micro-engineering of smart templates for bioseparation.[78-80]
The fabrication of such systems can be achieved by, e.g., the functionalization of surfaces
with supramolecular binding motifs. Interesting literature examples include the modification
of silicon and glass supports with cyclodextrins,[81-89] which can be used for supramolecular
reactions with dendrimers or adamantly functionalized molecules, and the functionalization
with DNA and RNA.[90] Another class of suitable candidates for the fabrication of such
molecular switches and/or responsive surfaces are metal complexes as they allow the
introduction of interesting optical and electrical properties via the metal-ligand interaction
within the monolayer system.[77,91,92] These surface assemblies can change their properties by
triggers in their environment, such as electrical, mechanical, and chemical influences, or
changes in pH value or temperature.[93-95] Besides the functionalization of silicon and glass
supports with metallo-supramolecular building blocks also other interesting metal-based
modification sequences on various surfaces will be presented in the following paragraphs. The
preparation of such metal-based surfaces can be achieved via different chemical routes. One
possibility is the synthesis of self-assembling molecules containing the metal complex.
Literature examples demonstrated the fabrication of various metal complexes, includingcobalt and iron containing phthalocyanines,[96] amino acids coordinated with cobalt,[97]
bipyridine ligands complexed with osmium[78,79,95,98,99] and ruthenium ions,[95] ammonia
terminated ruthenium complexes,[80,91] oxo-centered triruthenium complexes[77,100] and
metallodendrimers[101] on various surfaces (Figure 1.8).
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N
NN
N
NN
Os
N
Si(OMe)3
+
2+
3 X- (X = I, PF6)
S
NH
NH2
O
O
O
CoO
OO
NO
O
O
2
S
NH
N
O
S
NH
N
O
Ru
O
Ru
O
OO
RuO O
O
OO
O
O
OC
O
O
N
a. b. c.
N
NN
N
NN
Os
N
Si(OMe)3
+
2+
3 X- (X = I, PF6)
S
NH
NH2
O
O
O
CoO
OO
NO
O
O
2
S
NH
N
O
S
NH
N
O
Ru
O
Ru
O
OO
RuO O
O
OO
O
O
OC
O
O
N
a. b. c.
Figure 1.8.Schematic representation of the self-assembling molecules, which can be used for
the attachment onto various surfaces. (a) Amino acids coordinated with cobalt ions, (b)
bipyridine ligands complexed with osmium and (c) oxo-centered triruthenium complexes.
Ozoemena and co-workers described the self-assembly of octahydroxyethylthio- and
octabutylthiometallophthalocyanine complexes, containing iron or cobalt ions, onto gold
electrodes.[96] These monolayers revealed electrocatalytic activity towards the oxidation of
thiols and thiocyanates in acidic environment with the best performance observed under pHvalues ranging from 2 to 9. The detection limit was found to be between 10-6 and 10-7 mol
dm-3. Interestingly, these electrodes showed no fouling after storage in acidic pH for a month.
Moreover, increase of the pH value above 10 led to the desorption of the monolayer.
Negatively charged cobalt complexes functionalized with amino acids could be assembled
onto Au electrodes and were used as promoters for the recognition of the electron transfer
between a horse heart cyctochrome c and the substrate electrode (Figure 1.8a).[97]
Additionally, these modified systems were able to recognize the structure of the redox center
of the cyctochrome c. Such electrodes could find additional applications for studies of theelectron transfer path of various electron transfer proteins. Huisman et al. demonstrated the
synthesis of metallo-dendrimers functionalized with sulfide moieties for the attachment onto
gold substrates.[101] A decanethiol monolayer was modified with dendritic sulfide adsorbates
under varying adsorption times. Thereby, the alkylthiol molecules were desorbed from the
surface and the dendrimers were adsorbed onto the substrate. The successful attachment of the
palladium containing dendrimer was proven by secondary ion mass spectrometry
experiments, which revealed a characteristic fragmentation pattern of the palladium ligand.
Gupta and co-workers demonstrated the synthesis of an osmium bipyridyl complex
functionalized with a trimethoxy group, which could be used for the attachment onto silicon
and glass substrates (Figure 1.8b).[78] The reversible switching between Os2+/Os3+ was
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performed in solutions of ammonium hexanitratocerate(IV) and bis(cyclopentadienyl)cobalt
as oxidizing and reducing agents in acetonitrile and revealed at least 25 redox cycles, which
were identified by UV/vis transmission spectroscopy. Furthermore, it could be demonstrated
that these monolayers were suitable candidates for the fabrication of sensor systems
concerning the detection of water in organic solvents, NO+ cations and FeCl3 in organic and
aqueous solutions.[79,98,99] The detection of water was performed by activating the Os2+
complex with Ce(NO3)6 to obtain monolayer-based Os3+ species. These complexes were
completely reduced to Os2+ in the presence of water after 1.5 hour. Experiments concerning
the quantification of the amount of water revealed a water detection limit between 10 and 300
ppm. The treatment of the monolayer modified electrode with solutions of NO+ ions or FeCl3
resulted in an one-electron transfer process, changing the oxidation state from Os2+ to Os3+,
which could be followed by UV/vis spectroscopy. The formed iron(II) ions could be optically
detected by the addition of 2,2-bipyridyl, which resulted in the formation of a [Fe(bipy)3]2+
complex. The reversibility of the sensor systems was shown by the addition of water, which
resulted in the complete reduction of the Os3+ species back to Os2+. Furthermore, the authors
reported the fabrication of a redox-active monolayer system. [95] Thereby, an information
transfer between two interfaces, functionalized with osmium and ruthenium bipyridyl
complexes, was triggered by Fe(II)/(III) metal ions as electron carriers. Such systems could
effectively be used for the fabrication of advanced interfacial communication systems. Sortino
et al. reported the synthesis of a sulfide functionalized Ru(III/II) (NH3)5-(N-undecyl dodecyl
sulfide)-4,4-bipyridinium (PF6)n complex for the self-assembly onto ultrathin platinum
films.[91] The reversible switching between Ru(II) and Ru(III) was demonstrated by alternatingoxidation/reduction cycles with cerium(IV) sulphate and hydrazine as oxidizing and reducing
agents. UV/vis measurements revealed the optical proof of the switching by the disappearance
and restoration of the metal-to-ligand-charge-transfer (MLCT) absorption band at 530 nm of
the Ru(II) complex. Sato and co-workers studied a ligand substitution reaction of oxo-
centered triruthenium complexes, which were assembled onto gold electrodes (Figure
1.8c).[100] This could be of interest for the fabrication of multilayer assemblies prepared by the
layer-by-layer approach.
A second method for the construction of supramolecular metal assemblies onto solidsubstrates relies on the attachment of the metal complex to a functional substrate via surface
chemistry. Sortino et al. reported the three step synthesis of a redox-switch dipolar
ruthenium(III/II) (4,4-bipyridinium) complex bearing five NH3 ligands onto platinum
substrates (Figure 1.9).[92] This modification sequence included the chemisorption of
11-mercaptoundecanoic acid onto Pt, reaction with SOCl2 to yield reactive acyl chlorides and
subsequent quaternization with Ru(II) (NH3)5-4,4-bipyridine (PF6)2. XPS was performed and
revealed a quantitative acylation. UV/vis spectroscopy was performed to analyze the complex
attached to the substrate and showed an absorption band at 588 nm, which could be assigned
to the quarternized Ru complex.
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a. b. c.
OHO
S
ClO
S
ON
N
RuH
3N
H3N
NH3
NH3
NH3
S
+
2+
a. b. c.
OHO
S
ClO
S
ON
N
RuH
3N
H3N
NH3
NH3
NH3
S
+
2+
Figure 1.9.Schematic representation of the functionalization of platinum substrates with a
dipolar ruthenium(III/II) (4,4-bipyridinium) complex bearing five NH3 ligands. (a)
Chemisorption of 11-mercaptoundecanoic acid, (b) reaction with SOCl2 and (c)
quaternization with Ru(II) (NH3)5-4,4-bipyridine (PF6)2.
Shukla and co-workers demonstrated the formation of a ruthenium based monolayer onto
glass, indium tin oxide modified glass and silicon supports.[102] The functional monolayer was
obtained by the coupling of surface bounded benzyl halides with a phenol terminated
tris(bipyridyl)-ruthenium complex. Surface sensitive characterization tools, e.g. AFM, XPS,UV/vis spectroscopy and cyclic voltammetry, were performed to characterize the resulting
monolayer and revealed the reversible switching of the oxidation state of the metal center.
Mashazi et al. described the fabrication of metal phthalocyanine modified sensors for the
detection of cysteine and hydrogen peroxide.[103-105] Therefore, these surfaces were modified
with a monolayer of metal (Co, Fe, Mn) tetracarboxylic acid phthalocyanines via a two step
synthesis starting from the self-assembly of mercaptoethanol onto gold, followed by the
reaction with metal coordinated tetracarboxylic acid chloride phthalocyanine. Another
convenient coupling procedure uses the 1,3-dipolar cycloaddition of azides andacetylenes.[47,48] Collman and co-workers used this modification sequence for the synthesis of
acetylene modified porphyrin and ferrocene species and subsequent attachment to mixed
monolayers consisting of terminal azide and methyl groups onto gold substrates.[106] The
authors furthermore investigated the electron transfer rates to the acetylene functionalized
redox species, which revealed a dependence on the length and degree of conjugation of the
N3 terminated thiol and on the length of the surrounding thiol. The authors additionally
presented the synthesis of alkyne functionalized cytochrome c oxidase models for the
immobilization onto Au.
[107,108]
The successful attachment of the metal complexes wasidentified by cyclic voltammetry and IR spectroscopy by the disappearance of the absorption
band at 2100 cm-1. Such surfaces could be of importance to understand the influence of the
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Chemical modification of self-assembled monolayers by surface reactions
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redox-active components in the active site of cytochrome c oxidase during the reduction of
dioxygen to water. Crespo-Biel and co-workers investigated the multivalent binding of copper
and nickel ions containing supramolecular complexes onto cyclodextrin terminated gold
surfaces.[85] The complexes were obtained by reaction of an adamantly functionalized
ethylenediamine with CuCl2 and NiCl2, respectively. The binding of the complexes was
studied depending on the pH value by using surface plasmon resonance spectroscopy, which
revealed a divalent binding for the Cu(II) and the Ni(II) ion systems.
Next to the previously described methods another possibility to functionalize solid substrates
with metal complexes relies on the covalent binding of functional ligand systems onto the
surface and subsequent complexation reactions. Tuccitto and co-workers described the
patterned attachment of 4-(4-mercaptophenyl)-2,2:6,2-terpyridine onto gold substrates via
Au-S bonding.[109] Subsequent reaction with a Fe(II) salt resulted in the formation of iron
terpyridine complexes, which were used for the specific non-covalent interaction with
lactoferrin. Nishimori et al. reported the construction of molecular wires of linear and
branched bis-terpyridine complexes, which is shown in Figure 1.10. [110] For this purpose, a
thiol terminated terpyridine molecule was assembled onto gold and was subsequently
complexed with Fe(II) ions. Stepwise addition of a two-way-bridging or three-way-bridging
ligand and metal ions resulted in the formation of linear or branched oligomer wires. This
research could be used for the investigation and the fabrication of molecular electronic
devices.
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a. b.
S
N
NNN
N
S
N
N
NN
N
NN
Fe
N
NNN
2+
2 BF4-
N
N
N
N
N
N
N
NN
N
NN
Fe
S
NN
2+
2 BF4-
a. b.
S
N
NNN
N
S
N
N
NN
N
NN
Fe
N
NNN
2+
2 BF4-
S
N
N
NN
N
NN
Fe
N
NNN
2+
2 BF4-
N
N
N
N
N
N
N
NN
N
NN
Fe
S
NN
2+
2 BF4-
N
N
N
N
N
N
N
NN
N
NN
Fe
S
NN
2+
2 BF4-
Figure 1.10. Schematic representation of the construction of molecular wires of (a) linear
and (b) branched bis-terpyridine complexes on gold substrates.
The construction of a two-dimensional network consisting of gold nanoparticles attached to
each othervia supramolecular interactions was demonstrated by Hobara and co-workers.[111]
Four differently modified thiol terpyridine systems were used for the attachment onto Au
particles, followed by the transfer onto a silicon substrate covered with a layer of SiO2. The
complexation with iron(II) ions led to the formation of a network with a conductivity, which
was 4 to 5 times higher then the non-conjugated derivative. Such devices represent potential
candidates for the development of high-performance molecular devices. The functionalization
of gold surfaces with dendritic structures via metal coordination was demonstrated by van
Manen and Wanunu.[112,113] Van Manen and co-workers prepared monolayers consisting ofdendritic wedges by the insertion of pyridine terminated dendritic structures with a terminal
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Chemical modification of self-assembled monolayers by surface reactions
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sulfide group into decanethiol terminated gold surfaces.[112] Tapping mode AFM
characterization revealed an increase of height of 3.3 nm after the functionalization. The
pyridine moieties were afterwards coordinated with second generation dendritic wedges
functionalized with a focal SCS (sulfur-carbon-sulfur) Pd(II) pincer moiety. AFM studies
showed nano-sized features ranging from 3.1 to 7.5 nm indicating that not all pyridine
moieties were coordinated. Wanunu et al. reported the divergent growth of coordination
dendrimers on surfaces.[113] A hydroxamate ligand functionalized with disulfide moieties was
self-assembled onto gold and was subsequently reacted with Zr4+ ions, followed by the
addition of linear or branched hydroxamate ligands for the preparation of dendritic structures.
The introduction of functional groups into the ligand systems could be used for the
amplification of the surface properties as well as for multiple derivatization routes via
dendritic growth.
The functionalization of solid substrates via metallo-supramolecular interactions leads to the
tailoring of the surface properties in a controllable fashion. The modification with metal
complexes allows the introduction of interesting optical and electrical properties. These
systems can change their properties by changes in the environment, such as temperature or pH
value. As a consequence, it is possible to use these substrates for the construction of smart
surfaces and, in particular, for sensor systems.
1.5. Aim and scope of the thesis
The functionalization of surfaces by means of self-assembled monolayers represents an activearea of research that offers versatile possibilities for the tailoring of surface properties in a
controlled fashion. In particular alkyl silane precursor molecules are interesting, because they
form chemically and physically stable monomolecular layers on various substrates. The
modification of the terminal end groups allows the tuning of the surface properties. This thesis
focuses on the modification and characterization of functional self-assembled monolayers by
means of chemical surface reactions. Furthermore, structuring of the monolayers was
performed to obtain bifunctional surfaces with different functional groups, which were
subsequently used to enable the selective binding of nanomaterials, to perform surfacechemistry on the nanometer scale and to initiate the growth of polymer brushes.
Chapter 1 highlights a selection of chemical surface reactions from literature for the
introduction of functional moieties into the monolayer. Thereby, special interest was placed
on nucleophilic displacement reactions, the click chemistry approach and the modification of
surfaces with metallo-supramolecular binding motifs. Literature examples are discussed to
show the potential of these synthetic pathways for the modification of surfaces with various
functionalities.
Chapter 2 describes the functionalization and characterization of silicon and glass supports by
the copper catalyzed Huisgen 1,3-dipolar cycloaddition. Thereby, differently substituted
acetylene compounds, e.g. a fluorescent coumarin dye, propargyl alcohol and poly(oxazoline),
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Chapter 1
22
were coupled to azide terminated substrates under conventional and microwave-assisted
reaction conditions. The functionalized monolayers were characterized concerning the
chemical conversion as well as the integrity of the modified SAM.
Chapter 3 discusses the surface modification by the combination of different chemical
reaction sequences. The first part demontrates the nucleophilic substitution on bromine
terminated silicon supports with functional amines. The obtained surface moieties were used
in various post-modification reactions, e.g. click chemistry and complexation with a
polymeric ruthenium complex. Supramolecular functionalization with terpyridine metal
complexes is presented in the second part of the chapter. The modification with Fe(II) bis-
terpyridine building blocks resulted in the formation of switchable supramolecular binding
motifs on the surface, which were opened and closed by an external trigger and were
subsequently complexed with various transition metal ions.
Chapter 4 reviews the patterning of self-assembled monolayers by electro-oxidative
lithographic methods to fabricate chemically active surface templates. Thereby, the local
anodic oxidation and the constructive nanolithography will be discussed in more detail. The
first method creates silicon oxide features on a large variety of different substrates. These
features can be used for further modifications, e.g. self-assembly of other self-organizing
molecules, nanoparticles etc. The second patterning technique allows the introduction of
chemically active surface templates, which can be used for the guided assembly of
nanoparticles, precursor molecules and the performance of nanometer-sized surface
chemistry.
Chapter 5 demonstrates the patterning of bromine and n-octadecyltrichlorosilane (OTS)terminated substrates by local anodic oxidation and constructive nanolithography,
respectively. Post-modification reactions on bromine/silicon oxide surface templates were
used to guide the assembly of nanoparticles in a regio- and chemoselective fashion. Surface
patterns of OTS and carboxylic acid functionalities were applied in a second self-assembly
step to yield bromine terminated nanofeatures for subsequent click chemistry reactions on the
nanometer scale.
Chapter 6 deals with the patterning of OTS monolayers by the constructive nanolithography
technique utilizing a metal stamp to create micrometer-sized surface templates. These patternswere used for the grafting from approach of styrene and L-lactide. Two different
polymerization techniques, the atom transfer radical polymerization (ATRP) and the ring-
opening polymerization (ROP), were applied to obtain covalently anchored polymer brushes.
Atomic force microscopy was performed to characterize the height of the brushes as well as
the surface morphology.
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Chemical modification of self-assembled monolayers by surface reactions
23
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CChhaapptteerr22
CChheemmiiccaall ssuurrffaaccee mmooddiiffiiccaattiioonn bbyy cclliicckkcchheemmiissttrryy
Abstract
The functionalization of surfaces and the ability to tailor their properties with desired
physico-chemical functions is an important field of research with a broad spectrum of
potential applications. These applications range from the modification of wetting propertiesover the alteration of optical properties to the fabrication of molecular electronic devices. In
each of these fields it is of specific importance to be able to control the quality of the layers
with high precision. As a potential chemical surface reaction the 1,3-dipolar cycloaddition
attracted significant attention during the last years. Thereby, acetylenes or azide terminated
surfaces led to the formation of triazole terminated monolayers on different substrates. The
1,3-dipolar cycloaddition was performed under conventional heating as well as under
microwave irradiation. Functional acetylene derivatives, such as fluorescent dyes, small
organic compounds and polymers, were attached onto azide terminated silicon and glass
surfaces. The thermal and chemical stability of the precursor monolayers under microwave
irradiation as well as the optimization of the chemical surface reactions were investigated.
Based on this approach, a large variety of functional groups can be potentially introduced
onto various substrates leading to the possibility to tune the surface properties.
Parts of this chapter have been published: C. Haensch, S. Hoeppener, U. S. Schubert,
Nanotechnology2008, 19, 035703/17; M. W. M. Fijten, C. Haensch, B. M. van Lankvelt, R.
Hoogenboom, U. S. Schubert,Macromol. Chem. Phys. 2008, 209, 18871895; C. Haensch, T.Erdmenger, M. W. M. Fijten, S. Hoeppener, U. S. Schubert,Langmuir2009, 25, 80198024.
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Chapter 2
30
2.1. Introduction
Surface modification of solid substrates by self-assembled monolayers is an interesting
research area to functionalize surfaces with tailor-made properties.[1] Extensive work has been
performed on the self-assembly of alkyl thiol and alkyl silane monolayers that are known to
form stable, covalently anchored monomolecular layers on a variety of substrates, e.g. gold,
silver, glass or silicon. In particular, the silane chemistry is of interest for practical
applications, because of the physical and chemical stability of silanes and the technologically
relevant substrates, which can be used for surface modification.[2] These alkylsiloxane
monolayers are generally formed by an adsorption process of surfactant molecules, such as
alkyltrichlorosilanes (R-SiCl3), alkyltrimethoxysilanes (R-Si(OMe)3) or alkyltriethoxysilanes
(R-Si(OEt)3), onto solid substrates.[3] The introduction of other chemical functionalities can be
performed by subsequent surface modifications. One class of chemical reactions that is
compatible with the requirements of chemical surface functionalization routes, such as high
yields, high selectivity and the generation of inoffensive by-products, is the so-called click
chemistry approach.[4-6] This is nowadays a widely used concept that has, since its
introduction in 2001 by Sharpless,[4] found countless applications in various fields, e.g.
polymer chemistry,[7-10] cellulose chemistry,[11] surface modification[12-18] and others. In
particular, the Huisgen 1,3-dipolar cycloaddition reaction[19] of terminal acetylenes and azides
is known as one of the most perfectly working click reactions. The first report on click
chemistry on solid substrates was published by Lummerstorfer and Hoffmann.[19] Since then a
large variety of literature examples showed the potential of this modification sequence tointroduce functional groups into the monolayer under conventional heating conditions.
Whereas the use of microwaves in organic and macromolecular synthesis has developed into
an emerging field of basic and applied chemistry,[20-29] the utilization in surface
functionalization is up to now rarely reported. However, the decrease of the reaction times
from hours to minutes, the improvement of reaction yields, solvent-free reaction conditions,
suppression of the formation of side products and the improvement of the reproducibility are
major advantages of microwave heating compared to conventional heating.[23,30-32] The
implementation of this technique into the field of surface functionalization holds promise for asimplified, fast and straightforward modification of solid substrates. In particular, in the field
of surface chemistry the effective tuning of surface properties represents a key step towards