Label-free, chip-scale biosensors that exploit periodic nanophotonic and nanoplasmonic structures convert binding-induced refractive-index changes into sharp spectral signatures, enabling compact biochemical and hormone testing. Advances in metasurface and nanograting architectures have improved spectral quality factors and bulk sensitivity; however, for small molecules, pushing label-free limits of detection into the zeptomolar regime in fiber-compatible, low-cost formats remains difficult because of the…
