Plasmonic metal nanoparticles have unique optical, electrical, and thermal properties that make them useful in a wide variety of applications ranging from antimicrobial coatings to molecular diagnostics. One application that takes advantage of the unique optical properties of metal nanoparticles is color engineering – where custom nanoparticle formulations are designed to absorb and scatter specific wavelengths of light to generate a color. Plasmonically generated colors are unique in that the optical properties of metal nanoparticles can be tuned by changing size, shape, and material composition (see our Plasmonics Knowledge Base for more details). While researchers have known that nanoparticles can be used in color engineering applications for many years (in fact, one of the first examples of this application is the gold nanoparticles that were used to color stained glass red in medieval times), commercial applications have been limited by the cost of plasmonics materials. Advances in scaled manufacturing processes now make it possible to incorporate these materials into paints, plastics, cosmetics, and other coatings and composites to impart unique optical properties to a wide range o f materials. NanoComposix has partnered with a number of commercial partners to fabricate OEM nanoparticle formulations for specific applications in which a precise color effect is required.
The vibrant colors of plasmonic nanoparticles occur because the conduction electrons on the surface of each nanoparticle vibrate when excited by light at a specific wavelength. These vibrations result in extremely bright colors that can be tuned by changing the particle size and shape (Figure 1). For a brief tutorial describing this effect, and how nanoparticle colors can be tuned, please refer to our Plasmonics Knowledge Base article.
Standard commercial pigments and dyes either absorb or scatter (reflect) wavelengths of light. In order to change the color of a pigment, the chemical formula must be modified. This reduces the tunability of pigments, and makes subtle changes of color difficult to engineer (typically a new molecule must be used for each new color). Additionally, pigments tend to either scatter or absorb light, but rarely do both.
Nanoparticles, on the other hand, can be engineered to absorb, scatter, or both absorb and scatter. Since the particle colors depend upon size and shape, very subtle changes can be made without requiring a new formulation to be developed. For instance, if a customer desires a green-blue color (like a mix of the two right-most vials depicted in Figure 1 which correspond to 120 nm and 60 nm silver nanoplates), we can fabricate 90 nm silver nanoplates for the application. Likewise, if only very small changes in color are required, the dimension of the nanoparticles can be increased or decreased by just a few nm. This level of control provides an unprecedented level of tunability which cannot be achieved using standard dye and pigment technology.
Nanoparticles appear certain colors because they reflect (scatter) and absorb specific wavelengths of visible light. Light which appears white is a roughly equal mixture of the wavelengths of light that can be perceived by the human eye (roughly 400-700 nm, Figure 2).
When light interacts with a nanoparticle formulation, light is either absorbed or scattered, allowing the light that continues on to reach an observers eye to appear a specific color – often referred to as the “perceived color”. The following sections describe how absorbing and scattering nanoparticles interact with light.
When nanoparticles absorb light, the observer sees the light that is transmitted through the formulation, causing the observer to perceive light that is the opposite color than the absorbed color. For example, small gold nanoparticles absorb blue and green light (Figure 3).