In this work, the mass response of a resonant, CMOS (Complementary MOS) compatible MEMS sensor, oriented at the detection of diagnostic markers, is presented. The sensor is fabricated with a MEMS (Micro-electro-mechanical System) post-processing method on a standard, CMOS-based VLSI technology, retaining maximum compatibility with the CMOS process flow. The mechanical resonator is based on inductive actuation and detection, and the sensing is based on the microbalance principle. A protocol for covalent bonding of organo-functional silanes (to be used as link sites for biomolecular probes) on the resonator surface is presented. The effect on the mechanical frequency response of a test mass attached to the surface is demonstrated by grafting of gold nanoparticles (NPs) to the amino- terminated surface silanes. The measured mass sensitivity compares favorably both with standard Quartz Crystal Microbalances (QCM) and with existing MEMS-based approaches.