surface chemistry on self-assembled monolayers · surface chemistry on self-assembled monolayers...

Surface chemistry on self-assembled monolayers

Wouters, C.

DOI:10.6100/IR652748

Published: 01/01/2009

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https://doi.org/10.6100/IR652748https://research.tue.nl/en/publications/surface-chemistry-on-selfassembled-monolayers(0b9e7d84-12fa-4ad5-904c-091946a90f44).html

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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

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

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 70 3.5. Experimental details 71 3.6. References 77

Chapter 4 Structuring and functionalization of self-assembled monolayers by electro-oxidation 81 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 6 The 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 solid substrates 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.

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 the monolayer 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-carbon double 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

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 primary carbon 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 signal in 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.

Chapter 1

4

Si

Br

O OO

C16

SiO OO

N3

C16

SiO OO

SCN

C16

a. b.

d. c. Si

S

OOSi

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

OOSi

S

O OO

C16

SiOO

SiO OO

S

C16

Figure 1.1. Schematic representation of nucleophilic substitution reactions on bromine terminated 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 S2- 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 disordered monolayers. 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


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 hidden amine 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 OOSi

S

O OO

3OS-

Na+

b. b. c.a.

Si

Br

O OO

Cn

N OO

SiO OO

Cn

NH2

SiO OO

CnSi

I

O OOSi

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

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 SO3Na (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 nucleophilic displacement. 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 length of 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


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.

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, such as 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,


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

NN

N

RR

C11

R = COOMe, R= HR = R= COOEt

a. b.

Si

Br

O OO

C11

SiO OO

N3

C11

SiO OO

NN

N

RR

C11

R = COOMe, R= HR = R= COOEt

a. b. Figure 1.4. Schematic representation of the 1,3-dipolar cycloaddition on bromine terminated monolayers 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

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 charge transfer 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 were prepared 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.


11

a. b. c.

Si

NH2

O OOSi

NH

O OO

OO

N OO

NH

O

NH

O

O

O

NN

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 OOSi

NH

O OO

OO

N OO

NH

O

NH

O

O

O

NN

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 prepared by 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

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 NN

O O

O

H

H

a. b. c.

Cl

NH

N NN

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, the acetylene 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 treatment of 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 CCH terminated 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


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 of N,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.

NR

NN Br

O

O O O OH

R =

H

a. b.

NR

NN Br

O

O O O OH

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 and acetylene terminated dendrimers resulted in the formation of dendritic multilayers. This

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 and contact 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 surfaces with 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.


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, including cobalt 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).

Chapter 1

16

NNN

NNN

Os

N

Si(OMe)3

+

2+

3 X- (X = I, PF6)

S

NH

NH2

O

OO

CoOO

ON

O

OO

2

S

NH

N

O

S

NH

N

O

RuO

RuO

OO

RuO O

O

OO

OO

OC

O

O

N

a. b. c.

NNN

NNN

Os

N

Si(OMe)3

+

2+

3 X- (X = I, PF6)

S

NH

NH2

O

OO

CoOO

ON

O

OO

2

S

NH

N

O

S

NH

N

O

RuO

RuO

OO

RuO O

O

OO

OO

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 pH values 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 the electron 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


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 alternating oxidation/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 solid substrates 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.

Chapter 1

18

a. b. c.

OHO

S

ClO

S

ON

NRuH3N

H3N NH3NH3

NH3

S

+

2+

a. b. c.

OHO

S

ClO

S

ON

NRuH3N

H3N NH3NH3

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 and acetylenes.[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 was identified 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


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.

Chapter 1

20

a. b.

S

N

N NN

N

S

N

NNN

NNN

Fe

N

N NN

2+

2 BF4-

N

N

N

N

N

N

NNN

NNN

Fe

S

NN

2+

2 BF4-

a. b.

S

N

N NN

N

S

N

NNN

NNN

Fe

N

N NN

2+

2 BF4-

S

N

NNN

NNN

Fe

N

N NN

2+

2 BF4-

N

N

N

N

N

N

NNN

NNN

Fe

S

NN

2+

2 BF4-

N

N

N

N

N

N

NNN

NNN

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 other via 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 of dendritic wedges by the insertion of pyridine terminated dendritic structures with a terminal


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 active area 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 surface chemistry 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),

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 patterns were 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.


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, Langmuir 1989, 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. C 2003, 23, 7

12. [18] S. Morgenthaler, C. Zink, N. D. Spencer, Soft Matter 2008, 4, 419438. [19] R. Maoz, J. Sagiv, Langmuir 1987, 3, 10451051. [20] ABCR catalog, Gelest Inc. 2000. [21] L. Netzer, R. Iscovici, J. Sagiv, Thin Solid Films 1983, 99, 235241. [22] T. Lummerstorfer, H. Hoffmann, J. Phys. Chem. B 2004, 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, Langmuir 1990, 6, 16211627.

Chapter 1

24

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

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83418349.

CChhaapptteerr 22

CChheemmiiccaall ssuurrffaaccee mmooddiiffiiccaattiioonn bbyy cclliicckk cchheemmiissttrryy

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 properties over 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, Nanotechnology 2008, 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, Langmuir 2009, 25, 80198024.

Chapter 2

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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 to introduce 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 a simplified, 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 the fabrication of functional materials that would benefit from faster processing times. The 1,3-dipolar cycloaddition in organic chemistry was proven to be supported by microwave irradiation. Up to now several examples are reported in literature, where microwave irradiation could be used to decrease the reaction times of the coupling of organic azides with terminal acetylenes in organic synthesis.[33-45] Appukkuttan et al. were one of the first groups reporting the use of microwave irradiation for the preparation of a series of 1,4-disubstituted 1,2,3-triazoles via a three-component reaction in a regioselective fashion.[33] Other examples included the synthesis of a set of oligo-triazoles by using mono-, di-, tetra- and

Chemical surface modification by click chemistry

31

hexaalkynes,[34] the coupling of 2(1H)-pyrazinone with various saccharides to yield glycopeptidomimetics[35] and the solvent-free synthesis of - and -2deoxy-1,2,3-triazolyl-nucleosides.[36] The labeling of oligonucleotides with azide functionalized galactosides was reported by Bouillon et al.[37] Liskamp and Pieters et al. utilized the 1,3-dipolar cycloaddition for the synthesis of triazole-linked glycodendrimers,[43] multivalent dendrimeric peptides,[44] and 1,4,7,10-tetraazadodecane-N,N,N,N-tetraacetic acid conjugated multivalent cyclic-Arg-Gly-Asp (RGD) peptide dendrimers.[45] All these reactions were performed under microwave irradiation and showed an acceleration of the reaction kinetics. The following chapter describes the functionalization of silicon and glass substrates via the copper catalyzed cycloaddition. Thereby, conventional as well as microwave heating were applied to investigate the behavior of the monolayers under different reaction conditions. Due to the large amount of available functional acetylene derivatives a small selection of interesting molecules were chosen to be attached to the surface. Thereby, small as well as large molecules were tested. In addition, stability tests on chemically active monolayers were performed to study the thermal and chemical stability of these systems. 2.2. Clicking under conventional reaction conditions Click chemistry has proven to be a powerful tool for the modification of various surfaces. Thereby, it is necessary to modify the substrates with either terminal azide or acetylene functionalities. These end groups can react with organic azide compounds or functional acetylene derivatives under copper(I) catalysis to form 1,4-disubstituted 1,2,3-triazoles. The essential part of the present investigation was the choice of a suitable, commercially available precursor molecule, which allowed the reproducible and reliable formation of a chemically active monolayer. In particular, 11-bromoundecyltrichlorosilane was found to have suitable properties for the formation of thermal and physical stable monolayers. Moreover, the terminal bromine moiety provided the possibility of serving as a functional group that could be converted into the desired azide functions. As an interesting clickable molecule an acetylene functionalized coumarin derivative was chosen due to its fluorescent properties. The entire sequence of the modification steps required for functionalizing silicon and glass substrates with the coumarin compound is schematically outlined in Figure 2.1.

Chapter 2

surface chemistry on self-assembled monolayers · surface chemistry on self-assembled monolayers...

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