Raman spectra and DFT calculations of thiophenol molecules adsorbed on a gold surface

Improving 5-halouracils SERS detection driven by Watson & Crick pairing recognition. A spectroscopic & DFT study

The halogenated C5-substituted uracil derivatives (5-fluor-, 5-chloro, and 5-bromouracil) have drawn attention recently due to their pharmacological uses, properties, and importance as biomarkers and water pollutants. From an analytical point of view, these species are expected to be at very low levels in biological and environmental samples, and the development of methodologies for their determination is a central goal in research. The Raman technique and one of its special effects, Surface-Enhanced Raman Scattering (SERS), is a very suitable approach for detecting and quantifying these compounds. In practice, enhancing Raman scattering requires a nanostructured noble metal surface with the species of interest attached to it. Still, to maximize the effect, a deeper comprehension of the nature of the analyte-metal surface interaction is desirable. The structural information SERS spectra provide can be complemented by theoretical approaches, such as the Density Functional Theory (DFT) calculations. This work studied three 5-halouracils attached to silver nanoparticles (AgNPs) from experimental and theoretical perspectives. The observed patterns in the spectroscopic behavior showed a trend related to the electronegativity at the halogenated moieties, suggesting their direct influence in enhancing CC and CO stretching modes. Then, the formation of base pairs with adenine through hydrogen bonding was studied as a strategy to improve the detectability through SERS, supported by the well-known high affinity of adenine towards metal nanoparticles. We show that adenine favors the orientation of the 5-halouracils, reaching an additional signal enhancement that is very useful for analytical purposes, as demonstrated for 5-FU, reaching a limit of detection (LOD) of 2.36 nmol L-1. Wavenumber shifts and intensification of NH modes observed in the SERS spectra, along with DFT calculations, strongly suggest that forming hydrogen bonding (NH----N) upon the interaction of the base pairs with an Ag20 cluster is key for improving the halouracils LOD through SERS.

Ultrahighly Sensitive Surface-Enhanced Raman Spectroscopy Film of Silver Nanoparticles Dispersed in Three Dimensions on a Thin Alumina Nanowire Framework

To develop highly sensitive surface-enhanced Raman spectroscopy (SERS) films, various types of aggregated Ag nanowire (NW) and nanoparticle (NP) complex structures were fabricated using anodic aluminum oxide (AAO) templates and thermal evaporation. Aggregated AgNW structures with numerous tapered nanogaps were fabricated via Ag deposition on aggregated thin alumina nanowires of different lengths. AgNP complex structures were obtained by collapsing vertically aligned thin alumina nanowires 1 μm in length and depositing AgNPs on their tops and sides using surface tension during ethanol drying after functionalization. The Raman signal enhancement factors (EFs) of the samples were evaluated by comparing the SERS signal of the thiophenol (TP) self-assembled monolayer (SAM) on the nanostructures with the Raman signal of neat TP. EFs as high as ~2.3 × 107 were obtained for the optimized aggregated AgNW structure (NW length of 1 μm) and ~3.5 × 107 for the optimized AgNP complex structure. The large EF of the AgNP complex film is attributed mainly to the AgNPs dispersed in three dimensions on the sides of the thin alumina nanowires, strongly implying some important, relevant physics yet to be discovered and also a very promising nanostructure scheme for developing ultrahighly sensitive SERS films with EF > 108.