Third, in order to drive the covalent grafting of the transaminase on the epoxy linker arm, the PP_PDA_GDE membrane was partially functionalized with polyethyleneimine (prior to enzyme immobilization; Figure , step 2). Thus, the PP_PDA_GDE was immersed into 50 mL of a 5 mg/mL PEI solution in carbonate/bicarbonate 0.1 M buffer at pH 9.5 and stirred for 90 minutes (unless stated otherwise) at room temperature.