Quantification of carboxylic binding sites on functionalized silicon nitride surface through X-ray photoelectron signal and fluorescence labelling

The wet-chemical functionalization of inorganic surfaces by organic bifunctional interlayers is a viable method towards the realization of optical (bio-)sensing devices. A detailed description of the organic-inorganic interface and a quantification of the surface functional group density is desirable, as they will determine the interaction of the transducer surface with the bioreceptor, thus final sensor efficiency. By characterizing surfaces at different sample preparation stages, we demonstrate that X-ray photoelectron spectroscopy (XPS) can be used to identify different coexisting carbon species at the interface as well as to quantify the packing density of the bifunctional interlayer. As a model system for proof-of-concept, the silicon nitride surface was functionalized with a dodecanoic acid molecule bearing a carboxylic end group, and the molecular packing density was quantified both via standard fluorescence-based labelling and XPS attenuation. Our combined approach identifies the fraction of bioreceptor-accessible and -inaccessible binding sites on the functionalized surface. This information will help to optimize the formation of dense layers of immobilized bioreceptors at the transducer surface.