Document Type


Publication Date

Summer 2015


Biosensors enable scientists to learn more about biomolecular interactions, and are often used as detectors to indicate the presence of specific analytes. Currently, many detection methods require what are called labels, molecules that bind to an analyte of interest and can be easily detected. Radioactive isotopes are often used for this purpose, but labels such as these have the potential to interfere with the processes and molecules being studied. This poses a problem for medical screening, as labels may lead to an incorrect diagnosis. Therefore, label-free bio sensing techniques are in demand. Surface Plasmon Resonance Imaging (SPR), is one such method. A phenomenon derived from Maxwell’s Equations, SPR occurs at the interface between a dielectric and a metal thin film (Fig. 1). In the metal surface, electrons are not tied to particular atoms and are free to move throughout the material. This “sea” of free electrons can be modeled as a simple harmonic oscillator. In the presence of a drive force—in this case, the electric field in a light wave—electrons will oscillate. Unless the drive force is very close to the resonance frequency, little energy will be transferred to the oscillator. If the driving force matches the resonant frequency however, total energy transfer and SPR will occur. In other words, when a light beam has the correct wavelength to excite the plasmon, it will be absorbed by the metal. One important property of SPR is that the resonant frequencies can be found in the visible light spectrum. The particular frequency required depends largely on the dielectric constant of the metal used. Most importantly, this constant is sensitive to the refractive index of the metal’s surroundings. Consider a glass-gold interface submerged in water (Fig. 1). This system will oscillate at some frequency. If foreign particles are introduced, they raise the refractive index of the water and redshift the plasmon frequency. Thus, the plasmon surface can “detect” refractive index changes and foreign particles with high levels of sensitivity.