New concepts with surface plasmons and nano-biointerfaces

In classical surface plasmon-based optical biosensors, a surface plasmon mode is resonantly excited on the metallic sensor surface to probe any analyte-binding-induced refractive index changes. The field of the surface plasmon mode evanescently decays from the metal into an adjacent analyte solution with a typical penetration depth of 200 nm. In order to maximize the sensitivity of SPR biosensors, interfacial polymer architectures with binding site densities that considerably exceed planar arrangements through the use of three-dimensional microstructures were introduced. For biosensors based on surface plasmon fluorescence spectroscopy, this type of matrix offers the additional advantage of preventing fluorescence quenching, which is caused by the proximity of the chromophore label to the acceptor states of the noble metal. By means of probing the binding events with long range surface plasmon modes of which field extend much farther into the analyte solution (up to the micrometer range), substantially thicker sensor matrix layers can be used. Into such matrices larger amounts of ligands can be loaded, which enables one to increase the surface density of binding sites and thus to enhance the sensitivity of the biosensor. We present results which show that functionalized hydrogels are very well suited for meeting the demands of these novel biosensor platforms.