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The one-step method for graphene oxide (GO) simultaneous reduction and carboxylation via ultraviolet irradiation in the inert atmosphere has been reported. X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) data revealed that the proposed approach allows to obtain reduced graphene oxide (rGO) films, containing up to 10 at.% of carboxyl groups. The carboxyl groups concentration can be tuned within the range of 3 to 10 at.% by controlling the oxidation degree of the irradiated GO via the preliminary low-temperature air heating. Furthermore, no carboxylation effect is observed in the case of irradiation of the completely reduced GO films. This coincides with our previous results, validating the proposed model of GO carboxylation based on photoinduced conversion of basal-plane hydroxyl groups and ketones into carboxyl ones. Despite a different degree of carboxylation, all the obtained samples demonstrate almost complete elimination of basal plane groups and restoration of the graphene flakes aromatic structure. This fact is emphasized by the sheet resistance measurements, demonstrating that the obtained C-xy graphene exhibits high electrical conductivity. As a net result, the material obtained by the presented method shows promising applications in the manufacturing of biosensor transducers owing to both its conductive nature and presence of carboxyl groups, playing the role of the anchoring points for biomolecules. [ABSTRACT FROM AUTHOR]