A localized surface plasmon resonance based immunosensor for the detection of casein in milk

Immunosensors have a high potential for many applications, because various compounds of interest can be detected at a very high sensitivity [1]. Optical [2–4],piezoelectric [5], and electrochemical [6,7] immunosensors in connection with microfluidic systems [8] have successfully been applied to monitor antigen–antibody binding reactions. In recent years, immunosensors based on surface plasmon resonance (SPR) were developed for the measurement of antigens, which could bind to antibodies immobilized on the biosensor surface [9–11]. Immunosensors based on localized surface plasmon resonance (LSPR) have become strong candidates for the development of miniaturized high-throughput devices [12].LSPR is a resonance phenomenon of free electron waves in a metal [13]. It occurs in metallic nanostructures, such as nanoparticles, nanotriangles [14], and nanoisland [15].LSPR is observed when the frequencies of incident photon match the collective oscillations of the conductive electrons of metal nanoparticles. The nanoparticles exhibit unique optical responses within the UV–Vis region [16], where the absorbance shows an exponential decay with decreasing photon energy (the so-called Mie scattering) onto which an LSPR band, specific for the particle material, is superimposed. The frequency and intensity of the surface plasmon absorption bands highly depend on the type of the material (gold, silver, platinum), the size and the shape of nanostructures as well as on their surrounding environment [17–20]. LSPR-based device can be set up without using the specific configurations, for example, the attenuated total reflection (ATR) optical setup or waveguide coupling, it is possible to fabricate very small devices based on the LSPR technique with a simple optical setup.

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