Distance-dependent fluorescence modulation under continuous flow in an LTCC-integrated plasmonic competitive immunoassay

Abstract We report an LTCC-integrated optofluidic plasmonic immunoplatform that enables sequential flow-through fluorescence measurements with real-time optical signal acquisition. The system combines graphene quantum dot donors and gold nanoparticle plasmonic quenchers within a competitive microfluidic immunoassay architecture designed to investigate distance-dependent fluorescence modulation at biofunctionalized nanointerfaces. Fluorescence modulation is associated with distance-dependent interactions between biofunctionalized nanoparticles. Competitive binding of cortisol increases donor–acceptor separation, resulting in concentration-dependent fluorescence recovery within the 10–200 µM range (R 2 = 0.96). Spectral and analytical results are consistent with coupled plasmonic, interfacial, and optical attenuation effects contributing to the observed fluorescence modulation. The LTCC-integrated architecture enables stable real-time optical measurements under confined flow conditions. This work provides a proof-of-principle demonstration of the platform; however, before clinical application could be considered, further optimization and validation are required at cortisol concentrations corresponding to physiological conditions and in biological matrices.