Optical sensors for bacteria detection based on nanostructures fabricated by dynamic shadowing growth
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Several optical sensors based on nanostructures for Salmonella detection have been developed by taking the advantage of the dynamic shadowing growth (DSG). Based on fluorescence detection principle and the advantage of high surface area of nanorods, heterostructured silicon/gold nanorod array fabricated by DSG was functionalized with anti-Salmonella antibodies and organic dye molecules, which produced an enhanced fluorescence upon capture and detection of Salmonella. To reduce the multiple functionalization steps, a simpler detection technique is explored on the basis of the localized surface plasmon resonance (LSPR) properties of Ag and Au nanoparticles (NPs). By using DSG technique, one can produce uniform Ag or Au NPs substrate with fine tunable LSPR wavelength. The sensor ability of Ag nanoparticles was demonstrated by detecting 10-10 M NeutrAvidin. Due to the better stability in aqueous solutions, Au nanoparticles were used as a LSPR sensor for the whole cell detection of Salmonella. The results showed a plasmon peak shift due to the Salmonella antigen and anti-Salmonella antibody binding. However, this shift was not sensitive to the concentration of the bacteria, which is due to the rigid structure of the bacteria. A theoretical model based on Mie theory and Effective Medium theory has been proposed for this detection. A more conventional method for bacteria detection is surface plasmon resonance (SPR) sensor. Ag nanorod mediated SPR sensor for sensitivity enhancement was first investigated using four-layer Fresnel equations and effective medium theory. Compared to the conventional thin metal film SPR configuration, the nanorod mediated SPR sensor presented a larger resonance angle shift and the sensitivity increased with increasing refractive index of the target analyte. The experimental results of Ag film and Ag nanorods/film fabricated by DSG in air and distilled water were analyzed by the theoretical results. In summary, the optical sensors, such as fluorescence immunosensor, LSPR sensor and SPR sensor, based on nanorods or NPs fabricated by DSG technique have shown promising results for bio-molecules and bacteria detection. Also, the unique properties of the nanostructures have great potential for further improvement.