Surface plasmon resonance: theoretical evolutionary design optimization for a model analyte sensitive absorbing-layer system.
Surface plasmon resonance (SPR) has been widely used in a Kretschmann configuration to study optical thickness changes of layers on a Au surface in response to an analyte. The method has been popularized and optimized for protein layers, but has also been used in the same format for other layers without further optimization including those absorbing at the incident wavelength. In this paper, we examine whether SPR remains the "best"attenuated reflectivity format for absorbing overlayers. Experimental data from the SPR response of a copper phthalocyanine film to nitrogen dioxide are used as an input example for a design process using an evolutionary algorithm. The data showed a trend toward thinner gold layer systems ( approximately 25 nm gave an contrast-enhancement of 42.9% compared with approximately 50-nm Au) or Au-free solutions including a layer with low refractive index. From the evolutionary design predictions, further modification could be tested based on available materials and "redundant layers" could be eliminated from the final selection. By inclusion of the external optics, a design could be selected to accommodate poor precision (+/-0.5 degrees ) in the incident angle and a possible multilayer solution was shown using Teflon AF 1600, with refractive index approximately 1.3. The predicted NO(2) response showed an improvement compared with the classical SPR configuration, and the incident angle chosen by the SGA for the interrogation of these layers was close to a stationary point in the absolute response curve, thus offering very good tolerance to automatic position referencing to the reflectivity minimum.