Articles you may be interested inAg dendritic nanostructures for rapid detection of polychlorinated biphenyls based on surface-enhanced Raman scattering effect

IntroductionFor applying local field enhancement arising from local plasmons present in noble metal nanoparticles to biochemical sensors, such as surface-enhanced Raman scattering (SERS) sensors, it is important to control both the shape and the arrangement of the nanoparticles.1 It is well known that wellordered 2D periodic arrays of triangular nanoparticles prepared by nanosphere lithography (NSL) show plasma resonance within a rather narrow wavelength region, and the sharp corners of the triangles result in strong SERS.2 Although NSL enables the size of the nanoparticles to be tuned, the number density of the nanoparticles is geometrically fixed. Some applications, such as Raman imaging, may require more SERS-active spots to improve the space resolution. In addition, it is difficult to tune the lowest order of the local plasma resonance to the near-infrared (NIR) region, which is appropriate for biological applications. 3A drastic enhancement in the local field is considered to also occur at the ends of elongated nanoparticles. 4 In fact, for light in the visible to NIR region, SERS has been observed on arrays of elongated Ag nanoparticles, or so-called nanorods, whose widths are in the range of tens of nanometers and whose aspect ratios are 2 -5. 5-7 Liao et al. 5 prepared in-line aligned nanorod arrays on lithographically produced templates, while Martínes et al. 6 and Wachter et al. 7 produced side-by-side aligned nanorod arrays by oblique deposition. Taking into account the gap enhancement in the local field, 8 in-line alignment of the nanorods is more desirable than side-by-side alignment. However, even today, it is difficult to produce sub-micrometer patterns over large areas at low cost.Gold nanorods are also attracting much interest, since chemically stable Au is considered to be preferred to Ag for the material of the SERS substrates. In addition to lithographically produced Au nanorod arrays, 9 random aggregates of Au nanorods produced by chemical routes [10][11][12] are extensively investigated. However, it is not easy to immobilize the Au nanorods uniformly in isolation with each other.Recently, we demonstrated the direct formation of Ag nanorods with quasi-parallel major axes on a template layer of SiO2 having a strongly anisotropic surface morphology by using a dynamic oblique deposition (DOD) technique. 13 These Ag nanorods show excellent SERS properties. 14,15 In this study, we report that physically self-assembled Au nanorod arrays show fairly highly sensitive SERS properties that are comparable with those of Ag. ExperimentalThe preparation method of Au nanorod arrays is basically the same as that of Ag nanorod arrays reported in our previous papers.14,15 Briefly, template layers of SiO2 were prepared on a glass substrate by the serial bideposition (SBD) technique, which is a version of the vacuum evaporation technique. During SBD, the deposition angle, αSiO 2 , measured from the surface normal was fixed to 78.6˚, while the azimuth was rapidly changed by 180˚ with each deposition of a 10-nm-t...

No abstract

Using a dynamic oblique angle deposition technique, we demonstrate the direct formation of Ag nanorods with quasi-parallel major axes on a template layer of oxide having a strongly anisotropic surface morphology. The optical properties of the nanorods are tuned by varying the deposition conditions without any pre- or post-treatment, and the resulting Ag nanorod arrays exhibit high surface-enhanced Raman scattering (SERS) activity. In addition to high macroscopic uniformity over a large area, our nanorod arrays contain a high density of isolated nanorods. Using the optimum Ag nanorod arrays, the SERS imaging of the microdroplets of a rhodamine 6G solution is successfully demonstrated. The space resolution of the imaging is of the order of at least a few µm. These features are suitable for the SERS imaging of biomaterials.

No abstract