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Surface modification is important because it allows the tuning of surface properties, thereby enabling new applications of a material. It can change physical properties such as wettability and friction, but can also introduce chemical functionalities and binding specificity. Several techniques are available to modify the surface. Of these, organic monolayers have the advantages that they are easily tunable, fairly stable, and do not change the structural properties of the surface.
In the last two decades studies on the coupling of unsaturated hydrocarbons to inorganic surfaces have emerged. These compounds (also referred to as 1-alkenes and 1-alkynes) form covalently coupled monolayers on a variety of substrates, which is shortly reviewed in Chapter 1. This class of surface modification also forms the basis of the studies described in the following chapters.
In Chapter 2, a method for the direct patterning of 1-alkynes onto hydrogen-terminated silicon is presented. The method combines microcontact printing with visible light illumination through the stamp. Since the surface modification is clearly enhanced by the illumination, the method was named light-enhanced microcontact printing (LE-µCP). It results in the local formation of an alkenyl monolayer at the areas where the stamp is in contact with the surface. The method is compatible with functional inks and also allows the preparation of chemically heterogeneous surfaces by backfilling of the uncontacted areas with a second functional ink.
In Chapter 3, a method is introduced to photochemically modify fused silica substrates with 1-alkenes. This yields highly hydrophobic surfaces with high thermal stability, whereas the adsorbed layer provides proper chemical passivation of the underlying surface. The alkenes initially bind to the surface hydroxyl groups in Markovnikov fashion, but at prolonged reaction times oligomerization takes place. Since the reaction is photochemically initiated, it enables the use of photolithography to constructively pattern the silica surfaces. Because of this, the newly developed method forms a valuable addition to the existing modification methods.
The method developed in Chapter 3 is applied in Chapter 4 to locally furnish silica surfaces with a functional linker. This has allowed the selective attachment of single-stranded DNA onto the modified areas. In addition to plain surfaces, the surface reaction is also demonstrated on onto curved, enclosed surfaces, i.e. the inner surface of a microchannel. The surface-bound DNA has been selectively and reversibly hybridized with the complementary DNA. These experiments show that ~ 7 ´1011 fluorescently labeled DNA molecules can be hybridized per cm2. By furnishing target compounds with the complementary DNA strand, this hybridization approach allows the selective, localized binding of proteins, antibodies and other biomolecules to the surface.
In Chapter 5, a new method for the organic modification of porous anodic alumina (PAA) is presented, which is based on the reaction of terminal alkynes with the alumina surface. The reaction results in the formation of a monolayer within several hours at 80 °C and is dependent on both oxygen and light. These monolayers are well-defined and consist of an oxidation product of the 1-alkyne, i.e. its a-hydroxy carboxylate. The obtained monolayers are fairly stable in water and at elevated temperatures. Modification with 1,15-hexadecadiyne results in a surface with available alkyne endgroups, which can be used for further surface chemistry.
In Chapter 6, the biofunctionalization of PAA is explored. To this aim, lactosyl-terminated surfaces are prepared and the subsequent adsorption of peanut agglutinin (PNA) is studied. The PNA binds selectively and reversibly to these surfaces. Moreover, PNA adsorption is higher on surfaces that expose the b-lactoside than on those that display the a-anomer, which is attributed to surface-associated steric hindrance. The adsorption of the pathogens Neisseria gonorrhoeae and Candida albicans onto the lactosylated PAA surfaces is also investigated. Whereas quantification of N. gonorrhoeae adsorption is hindered by high background staining, C. albicans shows increased colonization onto lactosylated surfaces. Thus, this chapter shows that aluminum oxide surfaces can be modified to induce selective adsorption of proteins and microorganisms.
The studies in this thesis show that there is much to be explored in the surface modification of inorganic surfaces. Future studies could focus on the mechanism of the coupling reaction, but also on the reactivity of 1-alkenes and 1-alkynes towards other relevant inorganic materials. In addition, the surface modification with living cells and biofilms is still largely unexplored and may be a research topic of prime interest for the coming years!
|Qualification||Doctor of Philosophy|
|Award date||4 Apr 2012|
|Place of Publication||S.l.|
|Publication status||Published - 2012|
- surface chemistry