Femtomolar Biodetection by a Compact Core-Shell 3D Chiral Metamaterial.

Advanced sensing tools, detecting extremely low concentrations of circulating biomarkers, can open unexplored routes toward early diagnostics and diseases progression monitoring. Here, we demonstrate the sensing capabilities of a chip-based metamaterial, combining 3D chiral geometry with a functional core-shell nanoarchitecture. The chiral metamaterial provides a circular polarization-dependent optical response, allowing analysis in a complex environment without significant background interferences. The functional nanoarchitecture, based on the conformal coating with a polymer shell, modifies the chiral metamaterial near- and far-field optical response because of the energy transfer between dielectric shell polarization charges and plasmonic core free electrons, leading to efficient interaction with biomolecules. The system sensitivity slope is 27 nm/pM, in the detection of TAR DNA-binding protein 43, clinically relevant for neurodegenerative diseases. Measurements were performed in spiked solution and in human serum with concentrations from 1 pM down to 10 fM, which is a range not accessible with common immunological assays, opening new perspectives for next-generation biomedical systems.