SAMs flexibility

Covalent immobilization of proteins on nricrostmctured gold surfaces was studied in [226]. On Arese substrates, which were prepared by pCP aird etching. Are immobilization sites of proteins could be spatially controlled using air amino-reactive SAM. The whole process, i.e. production of Are micropattemed substrate including SAM exchairge aird protein immobilization, took a reasonably small amount of time ( 24 h), providing some flexibility in the experimental work. 

Another, more direct approach to overcome the problem of surface reconstruction, is the variation of the mesogenic unit of the surface active molecule. Instead of using flexible n-alkyl moieties with high conformational freedom, SAMs formed from molecules featuring a rigid mesogen in which conformational disorder has been eliminated as completely as possible should be suitable for the preparation... 

In the second part of this Chapter the thickness of the organic layer under discussion is slightly increased and a closer look at recent developments of more complex surface-bonded systems involving polymers is outlined. Despite the introduction of flexible polymer chains, the surface coating should still be defined and uncontrolled heterogeneities minimized. Here, especially, polymer brush-type layers where self-assembled monolayers (SAMs) are used as two-dimensional template systems for the preparation of well-defined surface coatings will be subject of a more detailed discussion. 

For many applications such as catalysis and possible functional devices, SAMs are simply too thin, the organized structure not flexible enough or the sterical situation within the layer too confined in order to incorporate a desired function or respond to changes in the environment in a dynamic and reversible way. One approach to increase the layer thickness of well-ordered self-assembled stractures of up to 100 nm is the formation of SAM and LB multilayers by means of consecutive preparation steps (Fig. 9.1 (3)) [5, 108]. This strategy was successfully applied by several research groups, but requires the constant intervention of the experimenter to put one type of monomolecular layer on top of the other. The dynamic behavior of the layer is limited by the crystal-like organization of the system and the extreme confinement of all surface-bonded molecules. Hence, surface... 

The same aplies to polymer brushes. The use of SAMs as initiator systems for surface-initiated polymerization results in defined polymer brushes of known composition and morphology. The different polymerization techniques, from free radical to living ionic polymerizations and especially the recently developed controlled radical polymerization allows reproducible synthesis of strictly linear, hy-perbranched, dentritic or cross-linked polymer layer structures on solids. The added flexibility and functionality results in robust grafted supports with higher capacity and improved accessibility of surface functions. The collective and fast response of such layers could be used for the design of polymer-bonded catalytic systems with controllable activity. 

Since ab initio calculations are non-trivial for alkanethiols with long chains, methylthiols and other short-chain thiols on Au(lll) have recently received considerable attention. While longer chains with their flexibility and non-negligible chain-chain interactions are essentially a defining feature of SAMs, methylthiols at least supposedly exhibit the same headgroup-substrate interaction. However, due to the different balance of the interactions (chain-chain versus headgroup-substrate), it is not possible to extrapolate the results from short-chain thiols to longer-chain thiols. 

SAMs provide the needed design flexibility, both at the individual molecular and at the material levels, and offer a vehicle for investigation of specific interactions at interfaces, and of the effect of increasing molecular complexity on the structure and stability of two-dimensional assemblies. These studies may eventually produce the design capabilities needed for assemblies of three-dimensional structures (109). 

The ability to form patterned SAMs allows us to engineer the interfacial properties of a surface with one more degree of freedom, in addition to the flexibility offered by SAMs themselves. It provides immediate opportunities to prepare systems in which structures can be controlled in the plane of the interface. SAMs can be used to control the nucleation, adsorption, and wetting of other materials, and thus patterned SAMs can be used as templates to direct and control the assemblies of other materials to form useful structures they ean also be used as patterned resists in directing the dissolution of the substrate to form patterns and structures in the underlying substrates (Au, Si02 and Si) [95]. 

---