Electrochemical guided mode resonance biosensor for simultaneous refractive index and electrochemical measurements

A comprehensive miniaturised biosensing platform needs to detect multiple analytes, which often rely on different transduction mechanisms for their detection and quantification. This necessitates the development of multimodal sensors capable of simultaneous measurements without interference between measurement modalities. Guided mode resonance (GMR) biosensors are highly sensitive to refractive index changes and well suited to out-of-plane optical coupling, but have not previously been implemented in a multimodal configuration. Here, we present the electrochemical guided mode resonance (EC-GMR) sensor, a Si3N4 GMR photonic grating integrated with an indium tin oxide (ITO) electrode that enables concurrent optical and electrochemical sensing. Finite difference time domain (FDTD) simulations show that the resonance wavelength of the GMR shifts as a function of ITO thickness and applied bias. These findings are validated experimentally using EC-GMR devices fabricated with a range of ITO thicknesses. We demonstrate that the EC-GMR achieves a refractive index sensitivity of 84.4 nm/RIU while simultaneously measuring the double-layer capacitance. To highlight its multimodal capability, we characterise the optical and electrochemical responses of methylene blue, a widely used redox reporter in biosensing. By integrating photonic and electrochemical modalities in a compact format, the EC-GMR platform provides critical analytical information beyond that which could be obtained from a traditional single-modality sensor. This approach extends the analytical utility of GMR sensors and represents a robust, adaptable solution for diverse biosensing applications, from clinical diagnostics to environmental monitoring.