We evaluate the sensor sensibility by immersing the sensor in phosphate buffer solution of pH 7.0 and measuring the background noise using a glass substrate for long-term monitoring. We propose a design of asymmetric planar electrodes for long-term monitoring in the phosphate buffer solution at 37°C. The background signals have been observed in cyclic voltammetric measurements, and the surface morphologies of platinum sensors were investigated by the scanning electron microscopy for the long-term experiment. With the same conditions, background oxidize current graphs were obtained over the range from 1.28 to 2.14 μA for the first 18 days. The sensor is intended for use in controlling of blood sugar and tissue sugar in diabetics and is suitable to be employed in a portable diagnostic and therapeutic apparatus.