Thin films grown in vacuums are how you get graphene layers and FETs that appear to be the basis of the technology here. Someone with a vacuum deposition system would be the prime candidate to develop a custom thin film to target adsorption of the molecules that they're trying to sense.
This part of the study describes what they implemented:
>Instead of using the transistors as the sensors, which need to be disposed of after each use, a system with a reusable printed circuit board (PCB) containing a MOSFET and disposable test strips were employed. In this approach, synchronized double-pulses were applied at the gate and drain terminals of the transistor to ensure that the channel charge does not accumulate, and there is no need to reset the drain and gate paths to mitigate the charge accumulation at the gate and drain of the sensing transistor for sequential testing. With the double-pulse approach, it only takes a few seconds to show the result of the test, due to the rapid response of the functionalized test strips and resulting electrical signal output. As an example, the LoD has been demonstrated to reach 10−15 g/ml and the sensitivity to 78/dec for COVID-19 detection. Similar approaches have been used to detect cerebrospinal fluid (CSF), cardiac troponin I, and Zika virus.27–30
> In this work, use of this double-pulse measurement approach to detect HER2 and CA15-3 in saliva samples collected from healthy volunteers and breast cancer patients was investigated. The voltage output responses of the transistor correlated to the HER2 and CA15-3 concentrations, detection limits, and sensing sensitivity were determined.