This article presents an electrochemical impedance study on the detection of Alkaline Phosphatase (ALP) in phosphate buffer saline (PBS) solution by using anti-ALP functionalized Si nanowires (SiNWs). Electrochemical impedance was measured using a three-electrode cell where the working electrode was a very dense and highly disordered array of Si NWs, eg SiNW-forest. This array was grown using plasma enhanced chemical vapor deposition (PECVD) at 350 C. The NW surface was modified by immobilizing the anti-ALP. A very efficient detection of very low ALP concentrations in PBS solution, ranging from 0.03 U/L to 0.3 U/L (1 nM–10 nM respectively), as well as a high selectivity due to the antibody/antigen interaction were demonstrated. The remarkable sensitivity and selectivity achieved through the combination of SiNW-forest combined electrochemical impedance …