A new sensor platform is based on so-called phoxonic crystals. Phoxonic crystals are structures designed for simultaneous control of photon and phonon propagation and interaction. They are characterized by a periodic spatial modulation of the dielectric constant as well as elastic properties on a common wavelength scale. Multiple scattering of photons and phonons results in a band gap where propagation of both waves is prohibited. The existence of photonic and phononic band gaps opens up opportunities for novel devices and functional materials. The usage of defect modes is an advantageous concept for measurement. The defect also acts as point of measurement. We show theoretically that the properties of the defect mode can be tailored to provide very high sensitivity to optical and acoustic properties of matter confined within a defect cavity or surrounding the defect or being adsorbed at the cavity surface. In this paper, we introduce the sensor platform and analyze the key features of the sensor transduction scheme. Experimental investigations using a macroscopic device support the theoretical findings.