Plasmonics has provided one of the commercially proven label-free
biosensing platforms to date. Traditional surface plasmon resonance (SPR) sensors,
however, suffer from moderate sensitivity because of the label-free nature. In this
chapter, we review various recent approaches that have attempted to produce improved
detection characteristics with unique strengths and weaknesses. We first explore
plasmonic field localization for self-aligned overlap or colocalization between localized
near-fields and target molecules developed for dramatically enhanced detection
performance. Localized SPR sensors based on metallic nanoparticles for amplification
of plasmonic optical signatures measured by target molecular interactions have also
been reviewed. Phase-sensitive SPR configurations based on optical path difference as
well as temporal difference control have been explored for sensitivity improvement
compared with conventional intensity-based SPR sensors with measurement of angular
and frequency characteristics. Relatively new analytical methods based on whispering
gallery mode sensors coupled to nanostructures that support localized surface plasmon
to achieve ultrahigh sensitivity that may enable single molecular detection were also
described. We have also included the use of SPR imaging for high-throughput labelfree
detection. Finally, surface-enhanced Raman spectroscopy using plasmonic nearfield
enhancement was discussed for the amplification of molecular signals. This
chapter highlights many exciting research directions that have been unraveled to
develop high-performance optical label-free biosensors based on diverse plasmonic
platforms.
Keywords: Field enhancement, Label-free optical biosensor, Plasmonic
localization, Phase-sensitive measurement, Surface plasmon resonance, Surfaceenhanced
Raman spectroscopy.