Surface sol–gel growth of ultrathin SiO2 films on roughened Au electrodes: Extending borrowed SERS to a SERS inactive material

Robust, reproducible, recyclable SERS substrates: monolayers of gold nanostars grafted on glass and coated with a thin silica layer

We prepared and characterized recyclable surface enhanced Raman spectroscopy (SERS) active glass chips. Gold nanostars were grafted on properly functionalized glasses by means of electrostatic interactions and then they were coated with a silica layer of controllable thickness in the nanometer range. The SERS activity of the obtained substrates were tested in terms of reproducibility and homogeneity intra-samples and inter-samples from different batches using the Raman reporter as the model compound rhodamine 6G. The uncoated substrates were used as reference to evaluate the effect of silica spacers on SERS enhancement factors (EFs). The chemical route to obtain silica-coated SERS chips is described in detail, and the morphology and the optical response of substrates have been characterized. We demonstrate that SERS substrates coated with 1 nm silica conserve a good EF, and that the coating confers to the SERS platform an extreme robustness leading to reusability of the substrates.

A flexible SERS-active film for studying the effect of non-metallic nanostructures on Raman enhancement

Since the discovery of surface enhanced Raman scattering (SERS), the choice of SERS-active materials has been limited mainly to metals, especially gold and silver in the visible spectrum. Although non-metals can also be SERS-active by forming nanostructures or composite structures with SERS-active materials, the mechanism behind it is still unclear and there is no perfect technique to study it. In this work, by constructing a SERS structure on a flexible polydimethylsiloxane film, we provide a way to study the effect of non-metallic nanostructures on Raman enhancement by attaching the above film onto flat and nanostructured surfaces. It was found that a nanoporous silicon surface contributes to an additional, up to five times, Raman enhancement. The pore depth and pore size also influence the observed Raman enhancement. These findings will help us not only to understand the mechanism of SERS involving non-metallic nanostructures, but also to design more efficient SERS structures for various applications.

Ultrathin polydopamine film coated gold nanoparticles: a sensitive, uniform, and stable SHINERS substrate for detection of benzotriazole

In shell-isolated nanoparticle (NP)-enhanced Raman spectroscopy (SHINERS), traditional metal oxide-based shells have inferior chemical inertness, they require strict preparation conditions, and lack specific groups, which lead to their poor selectivity toward target molecules. In this study, ultrathin and compact gold (Au)@polydopamine (PDA) SHINERS NPs were successfully fabricated by a simple self-polymerization technique. High wrapping tendency of PDA, a multifunctional biopolymer, favored the fabrication process. Au@PDA NPs exhibited a typical shell-isolated effect, i.e., Au@PDA NPs with a thick shell (more than 2.3 nm) showed a lower SERS activity, while those with an ultrathin (1.3 nm) shell exhibited higher SERS activity compared to uncoated Au NPs. The Au@PDA SHINERS substrate shows high performance in terms of sensitivity, uniformity, and stability. The relative standard deviations (RSDs) of SERS intensities from ten positions on the same substrate were less than 4%. Their Raman intensities dropped by only 15% over two months. More importantly, the Au@PDA (1.3 nm) SHINERS substrate exhibited high SERS activity for label-free and quantitative detection of benzotriazole (BTA), an important corrosion inhibitor, through utilizing a presumed π-π stacking interaction. A broad linear range from 10^-4 to 10^-8 M was achieved with a low detection limit (LOD) of 1 nM (0.119 μg L^-1). The LOD was not only significantly lower than the maximum allowable level (20 μg L^-1) of the Australian government water guide, but also lower than that of some modern methods such as fluorescence, liquid chromatography, and gas chromatography coupled with mass spectrometry. Furthermore, the substrate showed excellent discrimination against other compounds with a single aromatic ring. It is expected that the Au@PDA SHINERS substrate will offer great potential for analysis application in a complicated environmental system.

Effect of nanostructured silicon on surface enhanced Raman scattering

Non-metallic materials are often employed in SERS systems by forming composite structures with SERS-active metal materials. However, the role of the non-metallic structures in these composites and the effect of them on the SERS enhancement are still unclear. Herein, we studied the effect of silicon morphology on SERS enhancement on silver nanoparticles-coated different structured silicon surfaces. Our finding will help to further understand the SERS mechanism and pave the way for making more efficient SERS systems.

The surface morphology of non-metallic silicon has a big effect on the SERS enhancement of silver nanoparticle-coated silicon surfaces.