Coupling reagents activate pre-formed SAMs (made of thiols, disulfides or silanes) toward further (bio)chemical functionalization. The linkers we sell are heterobifunctional - that is, they have two different ends that are specific towards different chemical groups (figure below).
Coupling reagents are usually an alternative to making a mixed self-assembled monolayer, and the degree to which they modify SAMs is determined by the concentration of the linker used.