Detection of RNA Hybridization Using a Transistor Integrated Interdigital Capacitor Sensor

This work demonstrates the detection of target single strand RNA in liquid samples using a microwave resonant sensor that integrates a gold interdigital capacitor with a bipolar junction transistor. The sensor utilizes a negative resistance circuit to compensate for the inherent losses of the interdigital capacitor, enabling the generation of a self-sustaining oscillation. The oscillation frequency is responsive to variations in capacitance, allowing detection of nucleic acid level reactions occuring at the gold surface. A detailed design and optimization process is presented, addressing challenges including temperature-induced frequency drift and phase noise. By calibrating for temperature effects and tuning the transistor’s bias voltage, improved resolution, sensitivity, and frequency stability can be achieved. The sensor’s performance is demonstrated through label-free detection of RNA hybridization, using complementary oligomers immobilized on the gold surface. It achieves a detection limit of 1 pM with 0.2 μL of sample, where complementary and non-complementary target RNA sequences can be distinguished. The presented work offers a solution that balances simplicity, sensitivity, and cost-effectiveness.