ToF-SIMS investigation of FIB-patterning of lactoferrin by using self-assembled monolayers of iron complexes

Geometrically well-defined patterns of surface-immobilized proteins can be produced with several methods. We developed a method for patterning of proteins by means of specific, non-covalent interactions between a protein and a metal complex immobilized at the surface. In particular, reproducible patterns of lactoferrin have been obtained by exploiting the different adsorption properties of this protein on a OH-terminated self-assembled monolayer (SAM) or onto an iron-containing SAM present in certain regions of the pattern. The OH-terminated SAM was etched with a focused ion beam (FIB) in order to produce square regions of bare gold. These regions were selectively covered with a SAM of iron-terpyridine complex, formed via a stepwise procedure involving the initial formation of a mixed component SAM (containing the terpyridine ligand) and the subsequent reaction with an iron(II) salt in order to produce the complex. The patterned substrate was finally allowed to interact with a lactoferrin solution. It is shown that lactoferrin selectively and stably adsorbs on iron-containing layers, whereas it is not retained on the OH-terminated regions of the surface. The use of ToF-SIMS was crucial for obtaining this information, as well as for monitoring each sequential step necessary for the preparation of the patterns.