Optical sensing of biological, chemical and ionic species through aggregation of plasmonic nanoparticles

Plasmonic nanoparticles made of gold and silver have attracted a great deal of research attention in various fields, such as biosensors, imaging, therapy, nanophotonics, catalysis and light harvesting due to their unique optical and electronic properties. Plasmonic nanoparticle colloids may exhibit strong colours in the visible region due to localized surface plasmon resonances, whereas their aggregates exhibit different linear and nonlinear optical properties. Therefore, a smart design of chemical interactions between analytes and the nanoparticles surface may lead to gradual optical changes, which can be probed by various sensing methods, allowing quantitative analyte detection. A significant amount of research has been carried out toward the development of plasmonic sensors based on analyte-induced aggregation of Au or Ag nanoparticles, and the sensitivity and selectivity of such plasmonic biosensors have been greatly improved over the years. In this feature article, we summarize different design strategies that have been employed to induce the aggregation of plasmonic nanoparticles upon the addition of various analytes such as DNA, proteins, organic molecules and inorganic ions. We introduce various optical assays, such as colorimetry, surface-enhanced Raman scattering, two-photon photoluminescence, dynamic light scattering, hyper-Rayleigh scattering and chiroptical activity. From the discussion, it can be concluded that plasmonic sensors based on nanoparticle aggregation offer simple, highly sensitive and selective detection of various analytes. Finally, we discuss some of the future directions of plasmonic nanosensors toward device integration for practical applications.