Silicon nanopillar resonators were fabricated and used for sensing airborne engineered nanoparticles (ENPs) by monitoring resonant frequency shifts induced by the mass of trapped ENPs. Inductively coupled plasma (ICP) cryogenic dry etching and thermal oxidation were utilized in sensor fabrication. An electrostatic aerosol sampler and piezo shear actuator were used to collect the flowing ENPs and excite the nanopillars in resonant mode, respectively. By using the fundamental bending frequency, the sensor can achieve a mass sensitivity of 7.41 Hz/fg, which enables its application to a nanobalance to detect airborne ENPs in the femtogram regime. A simple cleaning method of polluted nanopillars was also performed to remove the adhered ENPs, thus efficiently extend the operating life of the sensor. This developed sensor is targeted for use in airborne ENP sensing applications.