In this paper we propose CMOS-compatible Memristive-Biosensors as label-free, highly sensitive sensors for in-air detection of Vascular Endothelial Growth Factor (VEGF) molecules. The memristive behavior of the fabricated devices is strongly affected by molecules in proximity of the wire surface. In this paper, we demonstrate the reproducibility of the measurement based on the memristive voltage gap. We also show the successful sensing of femto-molar amounts of VEGF. Specifically, we demonstrate a correlation between the decreasing behavior of the voltage gap and the increasing concentrations of VEGF. The voltage gap dependence on the pH of the initial solution is also shown as a further proof of the ionic interactions occurring at the SiNW surface. All measurements are performed in air, under controlled humidity; this makes our approach more sensitive thanks to the lowered Debye screening effect of counterions.