  Rapid progress in plasmonics biosensors and immunosensors brings many
methodologies to employ resonant subwavelength structures associated with the
physics of multipoles, Fano resonances, and analogous moments. Here, we review
a broad range of traditional and novel techniques that have been utilized for
developing advanced biosensing tools including colloidal nanoparticles
(NPs)-based systems to well-engineered arrays of meta-atoms and metamolecules.
By describing some of the important and practical label-free biosensing methods
based on subwavelength plasmonic technology, we discuss the emerging and
excitation of toroidal multipoles concept, as a third family of multipolar
modes, has made a revolution in enhancing the sensing performance of ultra-low
weight diseases proteins and viruses as biological targets. Taking the
exquisite and untraditional advantages of toroidal plasmonic metasurfaces, we
comprehensively evaluated the detection properties and high-quality of toroidal
immunosensors for the sensing of signature of bioproteins at picomolar (pM)
concentrations with excellent limit of detection (LOD). This understanding
clearly shows how toroidal metasensors have emerged as promising clinical and
pharmacology tools, can be considered as alternatives and big competitors for
conventional plasmonic sensors and transducers.
