The surface chemistry of 2D materials an intriguing  field, as it goes hand in hand with the modification of the entire 2D layer. However, strategies for the reliable functionalization are still in infancy, in particular because of detrimental polymeric residues on 2D material surfaces. We have identified ubiquitously hydrocarbon adlayers on 2D materials form storage conditions besides residues from transfer processes. [1] This has been revealed by a comprehensive TOF-SIMS investigation of various TMD materials. On the other hand we have developed a robust method for non-covalent functionalization of 2D materials. The functional layer is applied to pristine as-grown 2D materials layers, avoiding detrimental effect due to contaminations. Examples for the modification MoS₂ mono and double layers with these self-assembled monolayers will be shown. As for graphene these well define adlayers allow the attachment of antibodies for the specific sensing of small bio-active molecules. [2] These selective layers are highly attractive selective sensor arrays based on graphene.