A process for the isolation of cellulose nanofibers, in particular carboxylic acid functionalized cellulose nanofibers (MxG-CNF-CO2H) from the sustainable grass hybrid Miscanthus x. giganteus (MxG). The process includes the steps of performing bleach treatment on a quantity of MxG followed by ammonium persulfate oxidation. The resulting MxG-CNF-CO2Hs have good dispersibility in aqueous and polar organic solvents. They are also able to form gels at relatively low concentrations. The MxG-CNF-CO2H show significantly higher reinforcement effect when compared to carboxylic acid functionalized cellulose nanocrystals at the same weight percent in a composite composition.