![]() The term ‘surface’ in LSPR arises due to the fact that the special interactions between metallic nanoparticles and light incident upon them, these interactions generally being the absorption of a photon (Klabunde Nanoscale Materials in Chem), produce charge density oscillations which resonate at optical frequencies only for particle boundaries. These optical properties were the reason behind gold-silver alloy colloids being use as coloring agents for stained glass windows such as those displayed in the Sainte Chapelle in Paris, decades before the physics behind their optical properties were first described. Spherical gold (Au) nanoparticle colloids of similar sizes appear red, absorbing light maximally in the green region (Stockman Physics Today 2011). For example, light passing through a typical 30nm spherical silver (Ag) colloid appears yellow-green due to the fact that silver particles of this size absorb light in the violet-blue region. Plasmon resonance produces the brilliant colors observed when light passes through metallic colloidal solutions. ![]() The phenomenon is known as or Localized Surface Plasmon Resonance ( LSPR) when the effect is localized to the surfaces of nano-scale particles. These enhancements result from the phenomenon of Surface Plasmon Resonance ( SPR), the optical effects of which were first accurately described mathematically as solutions to Maxwell’s equations for spherical nano-objects by Gustav Mie (Mie 1908). In this case, gold nanoparticles dotted with antibodies (bio-markers) are used to create the characteristic red bands for yes/no readouts.Īlthough the colorimetric properties of metallic nanoparticles are used in many other bio-sensing applications, the greater interest in their usage arises due to the fact that nanostructures of various sizes and shapes can strongly enhance the local electromagnetic field at distances very near their surfaces. One that may be familiar is the dip-stick assay, or lateral-flow immunoassay, commonly used for home pregnancy tests. colorimetric assays, and many other medical and sensing techniques. Metallic nanoparticles have applications far and wide for optical imaging, photo-acoustic (light combined with ultrasound) imaging, nuclear medicine imaging, assays based on color changes, i.e. By attractive, I am not referring to how ‘pretty’ the colorful colloids look arranged in delicate glass bottles for a nanochemistry display. Ironically, it is due to interactions with the very medium by which we have vision at all – electromagnetic energy (light) – that nanoparticles derive some of their most unique and interesting properties.Ĭolloidal solutions of metallic nanoparticles, of gold and silver especially, display optical properties, or interactions with light in the visible to near-infrared ( NIR) regions (Link 2000), which make these particles very attractive for imaging and sensing applications. Indeed, there is more here than meets the eye. ![]() However, there is more to very small metallic particles, or nanoparticles, than their small size and defiance of our macroscopic ideas of gravity. It might help to think of the similar way in which large sugar cubes might fall to the bottom of your iced tea, but smaller sugar crystals will disperse and dissolve if you warm up and stir that same drink of tea. In this state, these nanoparticles, named after their dimensions on the nanometer scale (1/billionth of a meter), will stay dispersed instead of precipitating out of solution. Metallic particles can be synthesized which are adequately small to be suspended in a liquid phase, where the particles exist on a size scale such that buoyancy in the medium and forces of gravity are balanced, and the particle solution, known as a colloid, is stable. ![]()
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