A batteryless chemical nanosensor has been developed from either vertically (ZnO) or randomly aligned (Si) semiconductor nanowires. The nanosensor makes use of a unique molecule-surface interaction mechanism that induces an electric potential difference between two electrodes. The magnitude of the electric potential depends sensitively on the molecule type and concentration. We demonstrate the sensing ability of two different platforms using ethanol molecules. Quantum mechanical calculations suggest that the batteryless nanosensor described may be universally applicable to other semiconductor materials.