The present invention relates to a molecular assembly on a substrate usable for studying and/or performing biological interactions and/or reactions The invention further relates to an in vitro method to provide a substrate with a heparin coating presenting biotinylated biomolecules comprising linking avidin to a surface capable of binding avidin. A heparin conjugate is then linked to the avidin treated surface to form a layer of heparin thereon. Finally, a biotinylated biological probe or biomolecule is linked to the avidin through the layer of heparin thereby forming an essentially biologically inert surface with biotinylated probes extending out from the surface.