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Carbon electrodes (glassy carbon, GC, screen-printed carbon, SPC, and carbon fiber, CF) were used as substrate transducers to prepare glucose biosensors of different sizes and geometries, based on iron-ruthenium hexacyanoferrate as H 2 O 2 reduction mediator and glucose oxidase immobilized in a poly(1,2-phenylenediamine) membrane. Their response behavior under hydrodynamic amperometric conditions at an operating potential of À 0.02 V vs. Ag/AgCl was studied and compared. While the GC and SPC based conventional size biosensors showed enzymatic catalysis controlled current response with nonlinear concentration dependence, the CF based micro-biosensor exhibited, due to diffusion-controlled current response, extended linear range calibration curves with relatively lower sensitivity and longer response times. Several preparation parameters responsible for the improvement of biosensor performance were also investigated.