Integrated photonic sensors offer a compact and high-performance solution for detecting low concentrations of biomarkers and gases, which is key to addressing 21st-century health and climate challenges. They work by analyzing changes in the optical properties of guided light in real time. Group IV photonic platforms are compatible with microelectronics manufacturing, enabling scalable production of lab-on-a-chip devices and smart sensor networks. This thesis focuses on developing novel integrated photonic sensor architectures in the NIR and MIR ranges, aiming to improve detection limits, data richness, footprint, and overall system efficiency. Three main contributions support this work. The first is a complex refractive index sensor operating in the NIR, based on a Mach-Zehnder interferometer modified to simultaneously detect absorption and refractive index changes, allowing full material characterization in a single measurement. The second consists of an interferometric sensor in the NIR, based on a bimodal waveguide combined with controlled modal excitation and coherent phase detection. The performance achieved is comparable to that of a conventional interferometer but occupies only half the area. Finally, waveguides for absorption spectroscopy in the MIR, based on different materials, have been rigorously designed and compared in pursuit of optimal performance.