We combine the plasmonic properties of metallic nanoparticles enabling a label-free bioanalytical approach with the cavity ring-down method (CRD) in a miniaturized fluidic device as a sensitive and fast optical photon lifetime measurement. Nanoparticles (Ag triangles and Au nanodots) are immobilized within a specially designed microfluidic chip (MFC) that is situated inside an optical cavity. Our approach allows to measure absorption / loss changes of 0.02% – well below the typical detection limit (< 1%) of conventional spectrometers. The proof of principle is demonstrated by sensing refractive index changes of the solution inside the chip. Furthermore, a first bioanalytical application example is presented. Therefore, the binding of 2x10^8 DNA sequences from the phytopathogene microorganism Phytophthora kernoviae onto nanoparticles functionalized with complementary DNA molecules could be detected.