Cantilever-based mass sensors have recently emerged as an effective device for label-free detection of ultra small masses. To improve their sensitivity, a new concept based on delayed-position feedback is proposed and implemented. The proposed approach utilizes feedback delays and inherent system nonlinearities to produce stable limit-cycle oscillations. These limit cycles are generated as a result of the trivial solutions loosing stability via a supercritical Hopf bifurcation. The amplitude of the limit cycles is shown to be ultra sensitive to variations in the sensor natural frequency. This creates an ultra-sensitive cantilever-based mass sensing platform which does not require any changes or additions to the current sensor geometry and can be implemented in real time.