Chemical functionalisation of graphene and other 2-dimensional materials is a key step in realizing their full potential in various applications. There is a need for non-destructive and unambiguous identification of chemical groups and mapping of their distribution on such materials with nanoscale spatial resolution and at monolayer thicknesses. In this work, AFM-coupled infrared spectroscopy is used to analyse single layer reduced graphene oxide flakes that have been non-covalently functionalized with sulfonated pyrenes. We show this technique to be capable of distinguishing between the different pyrene moieties and mapping the sulfonate groups on a 1.7 nm functionalised monolayer of reduced graphene oxide with 32 nm spatial resolution. This technique is also shown to be sensitive to small changes in the sulfonate absorption spectra arising from chemical and surface effects, enough to distinguish between different functionalizing molecules even on materials with anisotropic thermal conductivity.