sam chemistry.pdf

7/30/2019 sam chemistry.pdf

1/164

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


2/164

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


3/164

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)


4/164


5/164

FFrrMMuuttttiiuunnddVVaattii


6/164


7/164

DDuuuunnddiicchh:: WWiirrssiinnddeeiinnss..

IIcchhkkaannnnddiirrnniicchhttwweehhttuunn,, oohhnneemmiicchhzzuuvveerrlleettzzeenn..

MMaahhaattmmaaGGaannddhhii


8/164

TTaabbllee ooffCCoonntteennttss

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


3.2. 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


9/164

Chapter 4

Structuring and functionalization of self-assembled monolayers by electro-oxidation81


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


5.2. Local anodic oxidation of bromine terminated silicon substrates 109

5.3. Constructive nanolithography on n-octadecyltrichlorosilane terminated

monolayers 115

5.4. Conclusions 121


5.6. References 123

Chapter 6The preparation of patterned polymer brushes onto chemically active surface

templates 125


6.2. Surface-initiated atom transfer radical polymerization 127

6.3. Surface-initiated ring-opening polymerization 132

6.4. Conclusions 137


6.4. References 139

Summary 142

Samenvatting 145

Curriculum vitae 148

List of publications 149

Acknowledgement 151


10/164


11/164

CChhaapptteerr11

CChheemmiiccaall mmooddiiffiiccaattiioonn ooffsseellff--aasssseemmbblleedd

mmoonnoollaayyeerrss bbyy ssuurrffaaccee rreeaaccttiioonnss

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.


12/164

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


13/164

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.


14/164

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


15/164


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


16/164

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


17/164


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.


18/164

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,


19/164


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


20/164

Chapter 1

10

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.


21/164


11

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


22/164

Chapter 1

12

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


23/164


13

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


24/164

Chapter 1

14

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.


25/164


15

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).


26/164

Chapter 1

16

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


27/164


17

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.


28/164

Chapter 1

18

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


29/164


19

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.


30/164

Chapter 1

20

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


31/164


21

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),


32/164

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.


33/164


23

1.6. References

[1] J. Sagiv,J. Am. Chem. Soc.1980, 102, 9298.

[2] R. G. Nuzzo, D. L. Allara,J. Am. Chem. Soc.1983, 105, 44814483.

[3] M. D. Porter, T. B. Bright, D. L. Allara, C. E. D. Chidsey,J. Am. Chem. Soc.1987,

109, 35593568.

[4] S. E. Creager, G. K. Rowe,Anal. Chim. Acta 1991, 246, 233239.

[5] A. Ulman, Chem. Rev. 1996, 96, 15331554.

[6] L. Netzer, J. Sagiv,J. Am. Chem. Soc.1983, 105, 674676.

[7] S. R. Wasserman, Y.-T. Tao, G. M. Whitesides,Langmuir1989, 5, 10741087.

[8] R. Maoz, J. Sagiv,J. Colloid Interface Sci.1984, 100, 465496.

[9] J. Gun, R. Iscovici, J. Sagiv,J. Colloid Interface Sci.1984, 101, 201213.

[10] J. Gun, J. Sagiv,J. Colloid Interface Sci.1986, 112, 457472.

[11] M. Mrksich, G. M. Whitesides, Trends Biotechnol.1995, 13, 228235.[12] R. K. Smith, P. A. Lewis, P. S. Weiss,Prog. Surf. Sci.2004, 75, 168.

[13] E. Ruckenstein, Z. F. Li,Adv. Colloid Interface Sci.2005, 113, 4363.

[14] N. K. Chaki, K. Vijayamohanan,Biosens. Bioelectron.2002, 17, 112.

[15] A. N. Parikh, D. L. Allara, I. B. Azouz, F. Rondelez,J. Phys. Chem.1994, 98, 7577

7590.

[16] D. K. Aswal, S. Lenfant, D. Guerin, J. V. Yakhmi, D. Vuillaume, Anal. Chim. Acta

2006, 568, 84108.

[17] B. Pignataro, A. Licciardello, S. Cataldo, G. Marletta,Mater. Sci. Eng. C2003, 23, 712.

[18] S. Morgenthaler, C. Zink, N. D. Spencer, Soft Matter2008, 4, 419438.

[19] R. Maoz, J. Sagiv,Langmuir1987, 3, 10451051.

[20] ABCR catalog, Gelest Inc. 2000.

[21] L. Netzer, R. Iscovici, J. Sagiv, Thin Solid Films1983, 99, 235241.

[22] T. Lummerstorfer, H. Hoffmann,J. Phys. Chem. B2004, 108, 39633966.

[23] T. P. Sullivan, W. T. S. Huck,Eur. J. Org. Chem.2003, 1723.

[24] S. Onclin, B. J. Ravoo, D. N. Reinhoudt,Angew. Chem. Int. Ed.2005, 44, 62826304.

[25] V. Chechik, R. M. Crooks, C. J. M. Stirling,Adv. Mater.2000, 12, 11611171.

[26] X.-M. Li, J. Huskens, D. N. Reinhoudt,J. Mater. Chem.2004, 14, 29542971.

[27] M. A. Fox, J. K. Whitesell, in Organische Chemie: Grundlagen, Mechanismen,

bioorganische Anwendungen, Spektrum Akademischer Verlag GmbH

HeidelbergBerlinOxford 1995.

[28] T. W. G. Solomons, in Organic Chemistry, 6th edition, John Wiley & Sons, Inc. 1996.

[29] H. Finkelstein,Ber. Dtsch. Chem. Ges. 1910, 43, 15281532.

[30] N. Balachander, C. N. Sukenik, Tetrahedron Lett.1988, 29, 55935594.

[31] N. Balachander, C. N. Sukenik,Langmuir1990, 6, 16211627.


34/164

Chapter 1

24

[32] G. E. Fryxell, P. C. Rieke, L. L. Wood, M. H. Engelhard, R. E. Williford, G. L. Graff,

A. A. Campbell, R. J. Wiacek, L. Lee, A. Halverson,Langmuir1996, 12, 50645075.

[33] M. V. Baker, J. D. Watling,Langmuir1997, 13, 20272032.

[34] A. Heise, M. Stamm, M. Rauscher, H. Duschner, H. Menzel, Thin Solid Films 1998,

327-329, 199203.

[35] A. Heise, H. Menzel, H. Yim, M. D. Foster, R. H. Wieringa, A. J. Schouten, V. Erb,

M. Stamm,Langmuir1997, 13, 723728.

[36] M.-T. Lee, G. S. Ferguson,Langmuir2001, 17, 762767.

[37] Y. Wang, J. Cai, H. Rauscher, R. J. Behm, W. A. Goedel, Chem. Eur. J.2005, 11,

39683978.

[38] J. A. Chupa, S. Xu, R. F. Fischetti, R. M. Strongin, J. P. McCauley Jr., A. B. Smith III,

J. K. Blasie, L. J. Peticolas, J. C. Bean,J. Am. Chem. Soc. 1993, 115, 4834384.

[39] Y. Ofir, N. Zenou, I. Goykhman, S. Yitzchaik, J. Phys. Chem. B 2006, 110, 80028009.

[40] R. Sfez, L. De-Zhong, I. Turyan, D. Mandler, S. Yitzchaik,Langmuir2001, 17, 2556

2559.

[41] T. S. Koloski, C. S. Dulcey, Q. J. Haraldson, J. M. Calvert,Langmuir1994, 10, 3122

3133.

[42] A. C. Templeton, M. J. Hostetler, C. T. Kraft, R. W. Murray,J. Am. Chem. Soc.1998,

120, 19061911.

[43] Y. W. Lee, J. Reed-Mundell, C. N. Sukenik, J. E. Zull,Langmuir1993, 9, 30093014.[44] W. H. Binder, R. Sachsenhofer,Macromol. Rapid Commun.2007, 28, 1554.

[45] W. H. Binder, R. Sachsenhofer,Macromol. Rapid Commun.2008, 29, 952981.

[46] J.-F. Lutz,Angew. Chem. Int. Ed.2007, 46, 10181025.

[47] H. C. Kolb, M. G. Finn, K. B. Sharpless,Angew. Chem. Int. Ed.2001, 40, 20042021.

[48] R. Huisgen, in 1,3-Dipolar Cycloaddition Chemistry, Wiley: New York1984.

[49] R. Huisgen,Angew. Chem. Int. Ed. 1963, 2, 565632.

[50] C. W. Torne, C. Christensen, M. Meldal,J. Org. Chem. 2002, 67, 30573064.

[51] V. V. Rostovtsev, L. G. Green, V. V. Fokin, K. B. Sharpless,Angew. Chem. Int. Ed.

2002, 41, 25962599.

[52] N. K. Devaraj, J. P. Collman, QSAR Comb. Sci.2007, 26, 12531260.

[53] I. S. Choi, Y. S. Chi,Angew. Chem. Int. Ed.2006, 45, 48944897.

[54] M. V. Gil, M. J. Arvalo, . Lpez, Synthesis2007, 11, 15891620.

[55] S. Prakash, T. M. Long, J. C. Selby, J. S. Moore, M. A. Shannon,Anal. Chem.2007,

79, 16611667.

[56] S. Sawoo, P. Dutta, A. Chakraborty, R. Mukhopadhyay, O. Bouloussa, A. Sarkar,

Chem. Commun.2008, 59575959.

[57] A. G. Marrani, E. A. Dalchiele, R. Zanoni, F. Decker, F. Cattaruzza, D. Bonifazi, M.Prato,Electrochim. Acta2008, 53, 39033909.


35/164


25

[58] R. Vestberg, M. Malkoch, M. Kade, P. Wu, V. V. Fokin, K. B. Sharpless, E.

Drockenmuller, C. J. Hawker,J. Polym. Sci.: Part A: Polym. Chem.2007, 45, 2835

2846.

[59] C. Gauchet, G. R. Labadie, C. D. Poulter,J. Am. Chem. Soc. 2006, 128, 92749275.

[60] T. Govindaraju, P. Jonkheijm, L. Gogolin, H. Schroeder, C. F. W. Becker, C. M.

Niemeyer, H. Waldmann, Chem. Commun.2008, 37233725.

[61] P.-C. Lin, S.-H. Ueng, M.-C. Tseng, J.-L. Ko, K.-T. Huang, S.-C. Yu, A. K. Adak, Y.-

J. Chen, C.-C. Lin,Angew. Chem. Int. Ed. 2006, 45, 42864290.

[62] R. D. Rohde, H. D. Agnew, W.-S. Yeo, R. C. Bailey, J. R. Heath,J. Am. Chem. Soc.

2006, 128, 95189525.

[63] R.-V. Ostaci, D. Damiron, S. Capponi, G. Vignaud, L. Lger, Y. Grohens, E.

Drockenmuller,Langmuir2008, 24, 27322739.

[64] C. M. Yam, A. K. Kakkar,J. Chem. Soc., Chem. Commun. 1995, 907909.[65] C. M. Yam, A. J. Dickie, A. K. Kakkar,Langmuir2002, 18, 84818487.

[66] S. Ciampi, T. Bcking, K. A. Kilian, M. James, J. B. Harper, J. J. Gooding,Langmuir

2007, 23, 93209329.

[67] S. Ciampi, G. Le Saux, J. B. Harper, J. J. Gooding,Electroanalysis 2008, 20, 1513

1519.

[68] S. Ciampi, T. Bcking, K. A. Kilian, J. B. Harper, J. J. Gooding,Langmuir2008, 24,

58885892.

[69] L. Britcher, T. J. Barnes, H. J. Griesser, C. A. Prestidge,Langmuir2008, 24, 76257627.

[70] D. A. Long, K. Unal, R. C. Pratt, M. Malkoch, J. Frommer, Adv. Mater. 2007, 19,

44714473.

[71] N. D. Gallant, K. A. Lavery, E. J. Amis, M. L. Becker, Adv. Mater.2007, 19, 965

969.

[72] X.-L. Sun, C. L. Stabler, C. S. Cazalis, E. L. Chaikof,Bioconjugate Chem.2006, 17,

5257.

[73] T. S. Seo, X. Bai, H. Ruparel, Z. Li, N. J. Turro, J. Ju, Proc. Natl. Acad. Sci. USA

2004, 101, 54885493.

[74] T. S. Seo, X. Bai, D. H. Kim, Q. Meng, S. Shi, H. Ruparel, Z. Li, N. J. Turro, J. Ju,

Proc. Natl. Acad. Sci. USA2005, 102, 59265931.

[75] J.-C. Meng, G. Siuzdak, M. G. Finn, Chem. Commun.2004, 21082109.

[76] J.-C. Meng, C. Averbuj, W. G. Lewis, G. Siuzdak, M. G. Finn,Angew. Chem. Int. Ed.

2004, 43, 12551260.

[77] Y. Zhang, Y. Tong, M. Abe, K. Uosaki, M. Osaka, Y. Sasaki, S. Ye,J. Mater. Chem.

2009, 19, 261267.

[78] T. Gupta, M. Altman, A. D. Shukla, D. Freeman, G. Leitus, M. E. van der Boom,Chem. Mater. 2006, 18, 13791382.


36/164

Chapter 1

26

[79] T. Gupta, M. E. van der Boom,J. Am. Chem. Soc. 2007, 129, 1229612303.

[80] S. Sortino, S. Di Bella, S. Conoci, S. Petralia, M. Tomasulo, E. J. Pacsial, F. M.

Raymo,Adv. Mater. 2005, 17, 13901393.

[81] S. Onclin, A. Mulder, J. Huskens, B. J. Ravoo, D. N. Reinhoudt,Langmuir2004, 20,

54605466.

[82] A. Mulder, S. Onclin, M. Pter, J. P. Hoogenboom, H. Beijleveld, J. ter Maat, M. F.

Garca-Paraj, B. J. Ravoo, J. Huskens, N. F. van Hulst, D. N. Reinhoudt, Small2005,

1, 242253.

[83] C. A. Nijhuis, J. K. Sinha, G. Wittstock, J. Huskens, B. J. Ravoo, D. N. Reinhoudt,

Langmuir2006, 22, 97709775.

[84] S. Onclin, J. Huskens, B. J. Ravoo, D. N. Reinhoudt, Small2005, 1, 852857.

[85] O. Crespo-Biel, C. W. Lim, B. J. Ravoo, D. N. Reinhoudt, J. Huskens,J. Am. Chem.

Soc. 2006, 128, 1702417032.[86] V. B. Sadhu, A. Perl, X. Duan, D. N. Reinhoudt, J. Huskens, Soft Matter 2009, 5,

11981204.

[87] O. Crespo-Biel, B. J. Ravoo, J. Huskens, D. N. Reinhoudt,Dalton Trans. 2006, 2737

2741.

[88] O. Crespo-Biel, B. J. Ravoo, D. N. Reinhoudt, J. Huskens,J. Mater. Chem. 2006, 16,

39974021.

[89] C. A. Nijhuis, B. J. Ravoo, J. Huskens, D. N. Reinhoudt, Coord. Chem. Rev. 2007,

251, 17611780.[90] D. I. Rozkiewicz, W. Brugman, R. M. Kerkhoven, B. J. Ravoo, D. N. Reinhoudt,J.

Am. Chem. Soc.2007, 129, 1159311599.

[91] S. Sortino, S. Petralia, S. Conoci, S. Di Bella,Mater. Sci. Eng. C2003, 23, 857860.

[92] S. Sortino, S. Petralia, S. Conoci, S. Di Bella, J. Am. Chem. Soc. 2003, 125, 1122

1123.

[93] T. P. Russell, Science2002, 297, 964967.

[94] J. A. Crowe, J. Genzer,J. Am. Chem. Soc. 2005, 127, 1761017611.

[95] T. Gupta, M. E. van der Boom,Angew. Chem. Int. Ed. 2008, 47, 22602262.

[96] K. I. Ozoemena, T. Nyokong, P. Westbroek,Electroanalysis2003, 14, 17621770.

[97] I. Takahashi, T. Inomata, Y. Funahashi, T. Ozawa, K. Jitsukawa, H. Masuda, Chem.

Commun. 2005, 471473.

[98] T. Gupta, R. Cohen, G. Evmenenko, P. Dutta, M. E. van der Boom,J. Phys. Chem. C

2007, 111, 46554660.

[99] T. Gupta, M. E. van der Boom,J. Am. Chem. Soc. 2006, 128, 84008401.

[100] A. Sato, M. Abe, T. Inomata, T. Kondo, S. Ye, K. Uosaki, Y. Sasaki, Phys. Chem.

Chem. Phys.2001, 3, 34203426.


37/164


27

[101] B.-H. Huisman, H. Schnherr, W. T. S. Huck, A. Friggeri, H.-J. van Manen, E.

Menozzi, G. J. Vancso, F. C. J. M. van Veggel, D. N. Reinhoudt, Angew. Chem. Int.

Ed. 1999, 38, 22482251.

[102] A. D. Shukla, A. Das, M. E. van der Boom,Angew. Chem. Int. Ed. 2005, 44, 3237

3240.

[103] P. N. Mashazi, K. I. Ozoemena, D. M. Maree, T. Nyokong,Electrochim. Acta 2006,

51, 34893494.

[104] P. N. Mashazi, K. I. Ozoemena, T. Nyokong,Electrochim. Acta 2006, 52, 177186.

[105] P. N. Mashazi, P. Westbroek, K. I. Ozoemena, T. Nyokong,Electrochim. Acta 2007,

53, 18581869.

[106] N. K. Devaraj, R. A. Decrau, W. Ebina, J. P. Collman, C. E. D. Chidsey, J. Phys.

Chem. B2006, 110, 1595515962.

[107] R. A. Decrau, J. P. Collman, Y. Yang, Y. Yan, N. K. Devaraj,J. Org. Chem. 2007,72, 27942802.

[108] J. P. Collman, N. K. Devaraj, R. A. Decrau, Y. Yang, Y.-L. Yan, W. Ebina, T. A.

Eberspacher, C. E. D. Chidsey, Science2007, 315, 15651568.

[109] N. Tuccitto, N. Giamblanco, A. Licciardello, G. Marletta, Chem. Commun. 2007,

26212623.

[110] Y. Nishimori, K. Kanaizuka, M. Murata, H. Nishihara, Chem. Asian. J. 2007, 2, 367

376.

[111] D. Hobara, S. Kondo, M.-S. Choi, Y. Ishioka, S. Hirata, M. Murata, K. Azuma, J.Kasahara,Phys. Status Solidi A2007, 204, 17061711.

[112] H.-J. van Manen, T. Auletta, B. Dordi, H. Schnherr, G. J. Vansco, F. C. J. M. van

Veggel, D. N. Reinhoudt,Adv. Funct. Mater. 2002, 12, 811818.

[113] M. Wanunu, A. Vaskevich, A. Shanzer, I. Rubinstein,J. Am. Chem. Soc. 2006, 128,

83418349.


38/164


39/164

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.


40/164

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

sam chemistry.pdf

Documents

Transcript of sam chemistry.pdf