This article presents a label-free platform for protein characterization and quantitation based on localized surface plasmon resonance (LSPR) on a nanostructured metallic film. With recent advances in manufacturing techniques, the reproducibility of nanostructured thin films has allowed the transition of LSPR from academic interest into the first system commercialization for practical use in research, bioprocess, and diagnostic applications. The nanostructured gold films exhibit a distinct color caused by the absorption of certain wavelengths in the spectrum of white light. The film color changes as biomolecules or other chemicals come into contact with the LSPR surface, enabling precise quantitation of biomolecular interactions. This article reviews the sensitivity and dynamic range of LSPR. It also explains how LSPR signals can be amplified through enzymatic reactions to achieve greater and faster sensitivities than published results for ELISA end-point analyses.
Stained glass has been displayed in the windows of churches all across Western Europe since Medieval times. It was only in 1857 that Faraday1, and later Rayleigh and Mie,2 formulated a scientific explanation for the coloration of the stained glass. As it turns out, the stained glass colors are based on the sands used by the ancient artisans. Unknown to them, different sands contained various quantities of metal salts or minerals (such as gold chloride, gold oxides, cobalt, and silver compounds) which, when melted into glass, resulted in distinct colors ranging from reds and blues to yellow. We now understand that the metal minerals decomposed in the melt and the metal ions aggregated into nanometer-size inclusions. These glass colors are based on wavelength absorbtion of light by the electronic oscillation modes of the metal nanoparticles, called localized surface plasmons.3
Basic studies using colloidal solutions showed that colors and wavelengths absorbed can be tailored by the metal's nature, size, and local environment surrounding the metal colloids.4 Using colloidal solutions to monitor binding events for label-free bioanalyses has become routine.5 However, the transition from the academic laboratory to commercialization has been awaiting manufacturing techniques that can replicate what researchers have achieved in Eppendorf tubes.6 With the emergence of powerful metrology tools that can explore the nanoworld, the relationship between the nano and the macro realm finally could be mastered. Today, stable metal films with precise nanostructuring and tunable absorption can be routinely manufactured using a wide range of surfaces. These films are at the core of a new commercial technology, termed localized surface plasmon resonance (LSPR). This article aims to introduce the salient features of LSPR technology and show how LSPR is useful for protein characterization and quantitation.