Testing gold nanostructures fabricated by hole-mask colloidal lithography as potential substrates for SERS sensors: sensitivity, signal variability, and the aspect of adsorbate deposition.

Gold nanoplasmonic substrates with high sensitivity and spectral reproducibility are key components of molecular sensors based on surface-enhanced Raman scattering (SERS). In this work, we used a confocal Raman microscope and several types of gold nanostructures (arrays of nanodiscs, nanocones and nanodisc dimers) prepared by hole-mask colloidal lithography (HCL) to determine the sources of variability in SERS measurements. We demonstrate that significant variations in the SERS signal can originate from the method of deposition of analyte molecules onto a SERS substrate. While the method based on incubation of SERS substrates in a solution containing the analyte yields a SERS signal with low variability, the droplet deposition method produces a SERS signal with rather high variability. Variability of the SERS signal of a single nanoparticle was determined from the statistical analysis of the SERS signal in short-range Raman maps recorded using different sized laser spots produced by means of different objectives. We show that the number of nanoparticles located within the laser spot can be a source of substantial SERS signal variability, especially for high-magnification objectives. We demonstrate that SERS substrates prepared by HCL exhibit high SERS enhancement and excellent homogeneity (about 20% relative standard deviation from short-range maps). The nanocone arrays are shown to provide the highest SERS enhancement, the lowest relative level of fluorescence background, and also slightly better homogeneity when compared with arrays of nanodisc dimers or single nanodiscs.

Plasmonic nanostructures for surface enhanced spectroscopic methods.

A comprehensive review of theoretical approaches to simulate plasmonic-active metallic nano-arrangements is given. Further, various fabrication methods based on bottom-up, self-organization and top-down techniques are introduced. Here, analytical approaches are discussed to investigate the optical properties of isotropic and non-magnetic spherical or spheroidal particles. Furthermore, numerical methods are introduced to research complex shaped structures. A huge variety of fabrication methods are reviewed, e.g. bottom-up preparation strategies for plasmonic nanostructures to generate metal colloids and core-shell particles as well as complex-shaped structures, self-organization as well as template-based methods and finally, top-down processes, e.g. electron beam lithography and its variants as well as nanoimprinting. The review article is aimed at beginners in the field of surface enhanced spectroscopy (SES) techniques and readers who have a general interest in theoretical modelling of plasmonic substrates for SES applications as well as in the fabrication of the desired structures based on methods of the current state of the art.

Quantitative SERS analysis of azorubine (E 122) in sweet drinks.

Considering both the potential effects on human health and the need for knowledge of food composition, quantitative detection of synthetic dyes in foodstuffs and beverages is an important issue. For the first time, we report a fast quantitative analysis of the food and drink colorant azorubine (E 122) in different types of beverages using surface-enhanced Raman scattering (SERS) without any sample preparation. Seven commercially available sweet drinks (including two negative controls) with high levels of complexity (sugar/artificial sweetener, ethanol content, etc.) were tested. Highly uniform Au "film over nanospheres" (FON) substrates together with use of Raman signal from silicon support as internal intensity standard enabled us to quantitatively determine the concentration of azorubine in each drink. SERS spectral analysis provided sufficient sensitivity (0.5-500 mg L(-1)) and determined azorubine concentration closely correlated with those obtained by a standard HPLC technique. The analysis was direct without the need for any pretreatment of the drinks or Au surface. Our SERS approach is a simple and rapid (35 min) prescan method, which can be easily implemented for a field application and for preliminary testing of food samples.