New advancements in the management of type I diabetic patients (T1DM) call for a new generation of monitoring systems based upon an array of metabolic biosensors including sensors for insulin detection. Blood insulin values are always less than 270 pM; therefore, specifically designed insulin sensors have to provide a limit of detection in the range of few pico-molar units. At present, electrochemical sensors are one of the best options to provide low-cost technologies for diabetes mellitus monitoring and treatment. Moreover, modern nanotechnology permits to improve further their performances by providing new materials, structured at the nanoscale (i.e., carbon nanotubes), which alter the original sensor properties when used for both in vivo and in vitro biosensing applications. A recent article in literature has proposed a solution based on electrochemical sensors that promise performances in line with above-mentioned insulin detection requirements. However, data presented in this paper show that such a promise is still overestimated, and an electro-chemical sensor capable of insulin detection within the physiological range is still beyond the present state of the art even when adopting nanostructured working electrodes.