Nanosensors for Environmental Analysis Based on Plasmonic Nanoparticles

Interesting optical properties of gold and silver nanoparticles have been known for a fairly long time. Intensive colour of suspensions of such nanoparticles is a consequence of the excitation of a collective oscillation of surface conduction electrons (called surface plasmons) when electromagnetic radiation interacts with metal nanoparticles with 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. Mentioned above specific plasmonic properties of gold and silver nanoparticles allowed on 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 even from a single molecule of an analyte is possible. This chapter describes the most important analytical techniques based on plasmonic properties of gold and silver nanoparticles, which are used for environmental analysis. The basic theoretical background of these techniques including the mechanism of the interaction of the electromagnetic radiation with the plasmonic nanoparticles is also presented. Analytical techniques presented in this review include methods utilizing local enhancement of the intensity of the electromagnetic field induced by plasmons, and hence increase of the efficiency of some optical processes in the proximity of the plasmonic nanoparticles, such as: surface-enhanced Raman scattering, surface enhanced infrared absorption and metal enhanced fluorescence, and methods based on the change of the optical properties of plasmonic nanoparticles caused by the analyte-induced aggregation or by analyte-influenced growth or etching of plasmonic nanostructures. We focus on description of methods used for the detection of compounds important in the environmental analysis, such as: cations of heavy metals, metallo-organic compounds, polycyclic aromatic hydrocarbons, pesticides, nitrite ions, and bacterial cells and bacterial pathogens. The review ends with a perspective of these methods, description of the main challenges to be overcome, and suggestion of areas where the most promising developments are likely to happen in future.