In this article, a refractive index-based biosensor using a microring resonator structure is presented. The sensor configuration consisted of a circular ring resonator placed inside a racetrack ring resonator. Both resonators fabricated from a single-slot core from GaAs on a substrate layer of Al0.5Ga0.5As. The simulations were performed using the variational finite difference time domain (var-FDTD) method. An extended free spectral range of 129 nm was measured for the layout output signals. This sensor stands out with its ability to detect even the smallest changes in refractive index, achieving a sensitivity of 267.7 nm/RIU. This high level of precision makes it highly effective in identifying various cancer cells, including HeLa cells. Such remarkable sensitivity highlights its strong potential for advancing biomedical diagnostics and improving early disease detection.