Photonic resonator absorption microscopy: why consider metallic and magneto-plasmonic nano-assemblies over bare nanoparticles for digital biosensing?

The unique optical interaction of species such as nanomaterials, proteins, viruses, antibodies, microRNA, and exosomes with the one-dimensional grating-based photonic crystals (PCs) has been leveraged in their detection using photonic crystal absorption microscopy (PRAM). While the principle and fundamental mechanism of such interfacial interactions are well delineated using wavelength and intensity modulations associated with the guided-mode resonance (GMR) of the PC, the effect of nano-assemblies in place of nanoparticles (NPs) has not been reported previously. In this work, the fundamental limitations observed with pristine NPs are overcome through the use of tunable AuNP assemblies synthesized via adiabatic cooling technology, where tunable nano-assemblies are obtained by subjecting the respective NPs to - 196 °C. Moreover, the higher contrast rendered by magneto-plasmonic, Fe₃O₄-Au hybrid nano-assemblies vis-à-vis metallic AuNP assemblies is corroborated with COMSOL Multiphysics simulations using electric and magnetic field hotspots. The high-contrast digital resolution enabled by magneto-plasmonic hybrid nano-assemblies, on account of synergistic coupling between the GMR of the underlying PC, delocalized Bragg, and localized Mie plasmons of dielectric-metal nano-assemblies, demonstrated excellent performance for microRNA-375-3p detection, opening a new window to explore hybrids of tunable "permittivity + permeability" as active probes in the design and development of microscopy-based biosensing modalities.