Nanoscale analysis of proteins using atomic force microscopy (AFM) requires surfaces with well-defined topologies and chemistries for many experimental techniques.
Chemical modification of surfaces has been successfully applied in several instances to immobilize biomolecules in order to obtain valuable information.
[3] In a recent study by Behrens et al., amine-terminated silicon surfaces were successfully used to immobilize bone morphogenetic protein 2 (BMP2) for medical purposes (cf.
[4] Molecules with amine groups (especially APTES) are important for biological applications, because they allow for simple electrostatic interactions with biomolecules.
It was introduced in 1946 by Bigelow et al.,[6] but it was not until 1983 that it attracted widespread interest, when the formation of SAMs of alkanethiolates on gold was reported by Allara et al.[7] Self-assembly of monolayers can be achieved using several systems.
The basis for self-assembly is the formation of a covalent bond between the surface and the molecule forming the layer; and this requirement can be fulfilled using a variety of chemical groups such as organosilanes at hydroxylated materials (glass, silicon, aluminium oxide, mica) and organosulfur-based compounds species at noble metals .
[7][8][9] While the latter system has been well characterized, much less is known about the behavior of organosilane layers on surfaces and the underlying mechanisms that control monolayer organization and structure.
Finlayson-Pitts et al. investigated the effect of certain treatments on silicon and concluded that both the roughness (3-5 Å) and the presence of scattered large particles were preserved after 1 cycle of plasma-treatment.
Mica bears a noticeable advantage over silicon because it is molecularly smooth and hence better suited for studies of small, flat molecules.
[11] It has a crystalline structure with generic formula K[Si3Al]O10Al2(OH)2 and contains sheets of octahedral hydroxyl-aluminum sandwiched between two silicon tetrahedral layers.
Similar to silicon, the surface of mica does not contain an appreciable density of silanol groups for covalent attachment by silanes.
[9] Solution-phase reaction has historically been the method that has been most studied, and a general consensus that has evolved with regards to the conditions required for the formation of smooth aminosilane films includes the following: (1) an anhydrous solvent such as toluene is required, with a rigidly controlled trace amount of water to regulate the degree of polymerization of aminosilanes at the surface and in solution; (2) formation of oligomers and polymers is favored at higher silane concentrations (>10%); (3) moderate temperatures (60–90 °C) can disrupt non-covalent interactions such as hydrogen bonds, leading to fewer silane molecules that are weakly tethered to the surface.
[9] Since oligomers and polymers of silanes have negligible vapor pressure at the reaction temperatures commonly used, they do not reach the surface of the silicate during deposition.