In a first aspect, the present invention relates to an isolated aqueous enzymatic preparation obtained from the reaction of at least one oxidoreductase enzyme, preferably a laccase, and at least one natural or synthetic product, preferably a natural compound, said natural or synthetic product comprising, in the structure thereof, at least one phenol or alcohol group, which optionally has one or more hydrophobic chains, or at least one sterol group. In a second aspect, the present invention relates to the use of said isolated aqueous enzymatic preparation in the functionalization of the surface of paper or cellulosic substrates.