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Biosensors based on localized surface plasmons (LSPs) in gold nanoparticles (NPs) have received a large amount of attention, because it provides label-free DNA and protein detection without using bulky optics. In many cases, colloidal NPs in solution have been used and they are deposited onto a substrate through a wet process. However we have difficulty in depositing the NPs homogeneously with high surface density. This is because the surface of NPs is negatively charged in order to disperse them in the solution, and the repulsive force between the NPs prevents to form such a nanostructure. For practical application of LSP biosensors, therefore, we need to establish a reliable way to make such structure that provides gold nanostructures that show LSPs. Reported here is fabrication of vacuum-evaporated gold NPs optimized for the LSP biosensing applications. The thin gold films were formed on a fused quartz substrate by thermal-evaporation of gold with the average thicknesses less than 10 nm. The subsequent thermal annealing at 500°C for two hours provided an excellent LSP band at 530 nm observed in UV-vis spectroscopy when the average thickness was 2-5 nm. Scanning electron microscopy revealed that the average size of the NPs is 19 nm in diameter, formed from the 2 nm-thick gold film, and that the average size of 31 nm from 5 nm-thick thin gold film. The highest sensitivity was obtained from the 2 nm-thick film for detection of small molecules such as alkamethiols, whereas the 5 nm-thick film was optimized for protein detection. This is because the size of an evanescent field produced by LSPs depends on the size of NPs. For the measurement of large molecules such as proteins (~10 nm in diameter), larger NPs (30-50 nm in diameter) are suitable. The amounts of protein bound for the NPs as a function of concentration gives the limit of detection (LOD) value of 5 x 10Λ(-8) g/mL. This LOD value is similar to that of the colloidal gold NP LSP sensing. Therefore this method can be used for the reliable way to fabricate the LPS biosensors. |