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We show that interfering surface plasmon polaritons can be excited with a focused laser beam at normal incidence to a plane metal film. No protrusions or holes are needed in this excitation scheme. Depending on the axial position of the focus, the intensity distribution on the metal surface is either dominated by interferences between counterpropagating plasmons or by a two-lobe pattern characteristic of localized surface plasmon excitation. Our experiments can be accurately explained by use of the angular spectrum representation and provide a simple means for locally exciting standing surface plasmon polaritons.

The extinction spectra and TEM images show that the silver nanoprisms would undergo shape transformation in the presence of 5 × 10 -6 M bromide ions. In this study, HS(CH 2 ) 15 CO 2 H (MHA) were introduced to modify the surface of silver nanoprisms. The MHA-modified silver nanoprisms can prevent the shape transformation in the presence of bromide ions with a concentration as high as 0.1 M. Furthermore, it was found that the etching process on the unmodified silver nanoprisms caused by the bromide ions can be stopped immediately by adding MHA into the colloid solution. However, the rate of the etching process in the initial stage was too fast to be stopped by adding MHA to control the silver colloid with the desired SPR wavelength. PVP-modified silver nanoprisms cannot prevent the shape evolution in high concentration bromide ions, such as 10 -3 M, but can slow down the etching rate. The silver nanostructures with the desired SPR wavelength can be obtained by the following simple method: KBr was added into the PVP-modified silver nanoprism colloid to initiate the shape evolution, and after a period of time, MHA was added to terminate the etching reaction. The PVP-KBr-MHA treated silver nanostructures with various colors produced by this simple method not only have long-term stability stored at room temperature but also can prevent shape transformation in highly concentrated chloride solutions.

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Disulfide bond is relevant to many protein folding/unfolding functions and conformational diseases. To elucidate the effects of disulfide bonds on protein folding, unfolding, and misfolding, we performed Fourier transform-Raman measurements on serial chemical-induced denaturations of bovine serum albumin (BSA). By directly monitoring Raman stretching at S-S (~507 cm À1 ), S-H (~2566cm À1 ), amide I (1655 cm À1 for α-helix; 1667 cm À1 for β-sheet structure), and amide III (>1300 cm À1 for α-helix; 1246 cm À1 for β-sheet structure), the status of disulfide bonds and secondary structure of BSA at different states were elucidated. Both disulfide bonds and secondary structure (mostly in α-helix) of BSA appeared relatively stable even when the protein was unfolded by urea solution. However, disulfide bonds were completely reduced and protein secondary structure changed from α-helix to a relatively β-sheet dominant when the protein was modified by the mixed solution of urea and dithiothreitol (urea/ DTT). Adhering to these structural changes, the protein proceeded to different degrees of polymerization. BSA would aggregate into a high molecular mass (over 700 kDa) of protein ensemble when it was exposed to the mixed urea/DTT solution. An irreversible change in S-S/S-H conversion and secondary structure was responsible for protein misfolding. We demonstrate here that Fourier transform-Raman directly probe S-S/S-H conversion and secondary structural change of BSA at different states, and these results clearly indicate that disulfide bonds and secondary structure of BSA serve as concrete frameworks to stabilize protein structure. As the frameworks collapse, the protein undergoes an irreversible structural change and results in protein misfolding.