Figure 2. Surface modification strategies of paper-based enzymatic biosensors: (a) inkjet-printed graphene–enzyme composite; (b) biomolecule drop cast over nanostructured layer; (c) biomolecule and redox mediator mixture drop casting; (d) screen-printed carbon–redox mediator composite with biomolecule drop casting; (e) biomolecule and redox mediator mixture drop casting over nanostructured layer