PK_a determination of graphene-like materials: Validating chemical functionalization

We report a novel pK_a determination for different graphene-like samples: graphene oxide (GO), reduced GO (rGO), graphene nanoribbons (GNR), oxidized GNR (GONR), thiol- and imidazole-functionalized GO (GOSH and GOIMZ, respectively) and thiol-functionalized GONR (GONRSH). Using the specialized computational program BEST7 for treating titration curves, pK_as for different functional groups were discriminated (confirmed by infrared spectra) and their composition quantified. Overall, three equilibria were distinguished, two relative to carboxylic acids exhibiting different acidic degrees (pK_a1~4.0 and pK_a2~6.0) and one relative to alcohols (pK_a4~10.0). Upon functionalization on carboxylate sites, thiol (pK_a^GOSH/GONRSH=6.7) and imidazole (pK_a^GOIMZ=6.6) moieties were discerned, followed by a decrease of their carboxylate percentage (compared to the precursors), thus allowing determining the degree of functionalization (48% and 36% of thiol content for GOSH and GONRSH respectively, and 29% of imidazole for GOIMZ). The proposed method is innovative and simpler when compared to the traditional tools usually employed to quantify chemical functionalization.