An amperometric biosensor for adenosine 5 -triphosphate (ATP) was developed applying a competitive assay of two glucose converting enzymes: glucose oxidase (GOD) and hexokinase (HEX). Competition between GOD and HEX for glucose in presence of ATP, lead to a decrease in the current coming from the hydrogen peroxide generated by the GOD, and allows ATP detection. The biosensor was realized on commercial screen-printed electrodes modified with carbon nanotubes (CNTs). Confinement of CNTs in a polycarbonate membrane, Chitosan and Nafion polymers was investigated as possible solutions for implantable sensors. Nafion gave the best performances, with a sensitivity of 25 pA/μM mm-2, and a detection limit of 257 M. The sensor resulted able to measure ATP concentrations in the range of hundred of μmol/l.