In this article, we compared different silicon oxynitride (SiON) microring-based photonic structures for biosensing applications. We varied the refractive index of SiON as well as the dimensions of the resonators, and analyzed their volumetric sensitivity and limit of detection by using glucose-water solutions. By functionalizing the surface of these sensors, we also investigated the specific detection of Aflatoxin M1. We observed binding of aflatoxin at concentrations as low as 12.5nM, and regeneration of a single functionalized sample up to 17 times by using glycine solution. The results open up the path for designing cost effective biosensors for a fast and reliable sensitive analysis of AFM1 in milk.