Experimental study of sensitivity enhancement in SPR biosensors by use of zinc oxide intermediary layers

We report a novel design for the intermediary layer of surface plasmon resonance (SPR) devices that use high refractive index and high-transmittance zinc oxide (ZnO) dielectric layers to enhance the signal quality and improve the full width at half-maximum (FWHM) of the reflectivity curve. We optimized the design of ZnO thin films using different sputtering parameters and performed analytical comparisons with conventional intermediary layers of chromium (Cr) as well as indium tin oxide (ITO). The study is based on application of the Fresnel equation, which provides an explanation and verification for the observed SPR narrow-width curve and optical transmittance spectra displayed by (ZnO/Au)-1, (Cr/Au)-4 and (ITO/Au)-5 devices. On exposure to ethanol, the (ZnO/Au)-1 device showed a two-fold decrease in FWHM and a 4.5-fold larger shift in intensity interrogation. The (ZnO/Au)-1 device exhibits a wider linearity range and much higher sensitivity. They also exhibit a good linear relationship between angle and concentration dependence in the tested range. We show that these advances represent a novel and simple method for preparing high-sensitivity, high-resolution SPR biosensors for accurate and specific bio-molecular detection.