For the first time memristive biosensors developed by the incorporation of scAbs (single-chain antibody fragments) enabling label-free detection of free Prostate Specific Antigen (fPSA) for early-stage detection of prostate cancer (PC) are reported. In addition, the first ever-published report of a direct relationship between the size of the bio-probes used to transfer specificity to the sensor and the voltage gap is here included. The proposed bio-detection is achieved by means of a voltage gap acquired in the electrical response of memristive sensors obtained by the bio-modification of silicon nanowire arrays. A comparison between memristive biosensors based on full-size antibodies and antibody fragments is also demonstrated by analysis of the surface morphology via Scanning Electron Microscopy. This new kind of biosensors provide excellent analytical sensitivity with a limit-of-detection down to 0.144 pg/mL on PSA.