Plasmonic nanoparticles for environmental analysis

Abstract

Gold and silver nanoparticles have unique optical properties. For instance, the intense colour of nanoparticle suspensions results from the excitation of a collective oscillation of surface conduction electrons, named surface plasmons. This excitation is done using an electromagnetic radiation that interacts with nanoparticles having a negative real and small positive imaginary dielectric constant, such as nanoparticles of gold or silver. The plasmonic optical properties of metal nanostructures are dependent on their shape and size, the dielectric properties of the metal and the surroundings and on the possible electromagnetic coupling with the localized surface plasmons in nearby other plasmonic objects. The other important consequence of the excitation of surface plasmons is a local significant enhancement of the electromagnetic field at some places of the illuminated nanoparticles. Specific plasmonic properties of gold and silver nanoparticles have allowed the development of many sensors for chemical analysis, including sensors dedicated for environmental analysis. Some of these sensors are so sensitive that recording of the reliable analytical signal of a single molecule is possible. Here, we review analytical techniques based on plasmonic properties of metallic nanoparticles for environmental analysis. We present the theory and mechanism of interaction of the electromagnetic radiation with the plasmonic nanoparticles. We detail analytical techniques including methods utilizing local enhancement of the intensity of the electromagnetic field induced by plasmons, and hence increase in the efficiency of some optical processes in the proximity of the plasmonic nanoparticles. Those techniques are surface-enhanced Raman scattering, surface-enhanced infrared absorption and metal-enhanced fluorescence and methods based on the change in the optical properties of plasmonic nanoparticles caused by the analyte-induced aggregation or by analyte-influenced growth or etching of plasmonic nanostructures. Environmental compounds include heavy metal cations, metallo-organic compounds, polycyclic aromatic hydrocarbons, pesticides, nitrite ions, bacterial cells and bacterial pathogens.