Surface plasmon hands

The attachment of protein to the transducer surface for immunoassay is a difficult problem, since it must be achieved without interfering with the active site. If an unlabeled assay is to be performed it must also be such that subsequent non specific interaction with the surface can be inhibited. This requirement is often contrary to those for effective antibody-antigen complex formation. A surface close packed with antibody will be sterically hindered and its reaction with antigen inhibited. On the other hand, a suitably spaced packing allows non-specific interactions to occur and large false positive signals to be recorded. As mentioned earlier, this was demonstrated by Cullen and Lowe who used the surface plasmon resonance technique to probe specific and non-specific protein interactions at metal surfaces. (23). 

Plasmonic hand gaps and trapped plasmons on nanostructured metal surfaces. Physical Review Letters, 95, 116802. 

Fig. la. Schematic showing the optical field (magnetic component) at an interface which supports surface plasmons. The dielectric function in the dielectric medium is the diectric function in the metal can be approximated hy the Drude-Lorentz expression given in the upper right hand corner. Notice that the field extends much farther into the dielectric than the metal, b. The reflectivity in an ATR configuration. The 0 is the critical angle and 0gp is the angle at which the surface plasmon is excited. Reflectivity extends from zero to one. Notice that the reflectivity from s waves, i.e., those waves with their electric vector perpendicular to the plane of incidence do not excite a surface mode.. 

This asymptotic trend is indicated by the dotted continuation of the peak frequency. It becomes more than obvious that the limitation of nuclear sizes inhibits a clear empirical confirmation of this trend. The Mie resonance in the clusters, on the other hand, approaches a finite value in the bulk limit. This is due to the long-range Coulomb force, which generally produces a plasmon dispersion co a q. The overall trends for very large clusters complies with a linear growth of the surface plasmon frequency. 

B.N. Feltis, et al., A hand-held surface plasmon resonance biosensor for the detection of ricin and other biological agents. Biosensors and Bioelectronics 23 (2008) 1131-1136. 

The effect of such metal cluster and cluster related but also plain metal films is explained on one hand by the strong enhancement of the field strength of light due to surface plasmons and Mie plasmons (not for plain metals), which are driven by the irradiation of the cluster layer. The resonant interlayer now is driven by the plasmons, and the fluorophore is exposed to this enhanced light field of the interlayer and has consequently an enhanced absorption probability. On the other hand during emission the fluorophore acts as radiating dipole and oscillator, thereby influenced by the nearby resonant layer. Thus, an additional charge is induced and the emission process is enhanced. 

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