A carboxylated poly(styrene-b-isoprene-b-styrene) triblock copolymer was synthesized for the construction of an enzymatic fuel cell. Glucose oxidase and bilirubin oxidase were chemically immobilized via the carboxylated functional groups of the polymer. The enzymatic fuel cell working electrodes were modified with graphene to accelerate the electron transfer rate of the system. Essential design and operational parameters were carefully optimized for improving the power of the enzymatic fuel cell. A power density of 20 mu W cm(-2) with only 4 mu g of immobilized bilirubin oxidase was generated from 30 mM glucose at 0.72 V. The improved enzymatic fuel cell was tested in a plant leaf. A power density of 14 nW cm(-2) was generated with glucose produced by photosynthesis reactions conducted during 30 min in the leaf.