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Nanoresonator systems have been fully integrated on pre-processed complementary metal oxide semiconductor (CMOS) chips. The systems have been used for high sensitivity mass sensing in air and vacuum. The resonator system, which consists of a cantilever and structures for electrostatic actuation and capacitive read-out, has been defined by low energy electron beam lithography (EBL) combined with direct write laser lithography (DWL) on top of a radiation sensitive CMOS layer. The fabrication of the nanoresonator system has been conducted as a post-process step. CMOS integration radically decreases the parasitic capacitance, enabling detection and amplification of the resonance signal directly on the chip. Fabricated resonator systems have been designed to have resonance frequencies in the range of 1-1.6 MHz. A mass resolution of 3 ag/Hz has been determined in air by placing a single glycerine drop at the apex of a cantilever and subsequently measuring a frequency shift of 14.8 kHz. The frequency shift corresponds to an added mass of 50 fg, which is close to the estimated weight of 41 fg for the glycerine drop.