Functionalization of probe tips and supports for single-molecule recognition force microscopy.

The measuring tip of a force microscope can be converted into a monomolecular sensorif one or few "ligand" molecules are attached to the apex of the tip while maintainingligand function. Functionalized tips are used to study fine details of receptor-ligand interactionby force spectroscopy or to map cognate "receptor" molecules on the sample surface. Thereceptor (or target) molecules can be present on the surface of a biological specimen; alternatively,soluble target molecules must be immobilized on ultraflat supports. This review describes the methodsof tip functionalization, as well as target molecule immobilization. Silicon nitride tips, siliconchips, and mica have usually been functionalized in three steps: (1) aminofunctionalization,(2) crosslinker attachment, and (3) ligand/receptor coupling, whereby numerous crosslinkersare available to couple widely different ligand molecules. Gold-covered tips and/or supports haveusually been coated with a self-assembled monolayer, on top of which the ligand/receptor moleculehas been coupled either directly or via a crosslinker molecule. Apart from these general strategies,many simplified methods have been used for tip and/or support functionalization, even single-stepmethods such as adsorption or chemisorption being very efficient under suitable circumstances. Allmethods are described with the same explicitness and critical parameters are discussed. In conclusion,this review should help to find suitable methods for specific problems of tip and support functionalization.