Researchers from the University of Minnesota developed graphene-based quantum capacitance wireless vapor sensors. The sensor is made from a metal-oxide-graphene variable capacitor (varactor) coupled to an inductor, creating a resonant oscillator circuit. The resonant frequency is found to shift in proportion to water vapor concentration.

So basically in these sensors, a change in adsorbed water vapor concentration on the graphene surface translates into a shift in the resonant frequency of a resonant oscillator circuit. The sensors show fast response to abrupt changes in the humidity and further show a monotonic frequency shift with relative humidity that is reversible and stable, particularly after conditioning using repetitive humidity cycling.

The researchers say that the capacitance values extracted from the wireless measurements correlate with those determined from capacitance-voltage measurements. This means that the sensing arises from the variable quantum capacitance in graphene. This new sensor transduction mechanism paves the way for graphene quantum capacitance sensors to for a wide range of chemical and biological sensing applications.

This device uses the quantum capacitance effect - a direct, observable manifestation of the Pauli exclusion principle. This effect is particularly prominent in graphene (due to its low density of states), so far there has been few (if any) practical uses for this effect. It has been suggested before that the quantum capacitance effect could be utilized to realize wireless sensors due to graphene's energy-dependent density of states and excellent surface sensitivity. Those new sensors could have significant advantages over alternative techniques (resistance-based sensing and wireless sensing based upon microelectromechanical systems).