Surface plasmon enhanced upconversion luminescence for biosensing applications

Upconversion (UC) luminescence sensing is a technique to improve the detection limit of conventional fluorescence in biosensing that is commonly limited by the autofluorescence-generated background signal. The main limitation of UC materials is their low wavelength conversion efficiency. Many studies have been made to enhance the efficiency of UC materials by optimizing light absorption and energy transfer processes. However, rather low efficiency remains an issue limiting the practical usage of UC materials in biosensors. Plasmon enhancement is a way to improve UC photoluminescence by enhancing the excitation and emission rates. In this study, we modeled and fabricated gold gratings for exciting surface plasmon polaritons (SPPs) at 976-nm wavelength. We aim at increasing the local optical intensity at the locations of UC nanoparticles on a nano-structured plasmonic surface. The UC nanoparticles were adsorbed on the gratings via biomolecule conjugation. UC photoluminescence on the gratings was compared with flat gold surfaces. Experimentally, we achieved UC enhancement up to 70, which is relatively high in comparison with other plasmon-enhanced UC techniques presented in the literature. The results of our work can be applied in various biosensing applications in which low excitation intensity is preferred.