Fig. 3. Concentration detection of the molecules based on high-resolution RF beat notes.

a Measured spectral shifts of the sensor with function 1, function 2, and function 3, for DA, nicotine, and ssDNA detection, respectively. b Performance of the sensors. For the three types of target molecules, Allen deviation shows that the RF measurement uncertainty is ≈10 Hz, limited by the electronic noise. For 2 mM DA, 0.31 μM nicotine, and 20 nM ssDNA, our measured sensitivity reaches 0.505, 158, and 8800 kHz/μM, respectively. With increasing concentration, the sensitivity goes down due to the adsorption saturation, thus the dynamic range of the three sensors can approach 10 mM (DA), 0.7 mM (Nicotine), and 0.2 mM (ssDNA), respectively. We note that the higher concentration enables stronger FRET-based restoration.