Abstract
This paper presents a study of adsorption and vibrational features of folic acid, using surface-enhanced Raman scattering (SERS). A gold-capped silicon nanopillar (Au NP) with a height of 600 nm and a width of 120 nm was utilized to study the vibrational features of FA molecules adsorbed on the nanopillars within the high electromagnetic field areas. The adsorption behaviour of folic acid and the band assignment of the main vibrations together with the optimized geometry of folic acid and folic acid in the presence of a cluster of 10 gold atoms were assessed using the density functional theory (B3LYP(6–31G(d))) and the scalar relativistic effective core potential with a double-zeta basis set (LANL2DZ). The vibrations obtained from the solid-state folic acid and the folic acid on a gold cluster were in accordance with those observed experimentally. The analysis of the main vibrations indicated that the interaction of folic acid with the Au NP occurred primarily through the nitrogen atoms, from their pteridine ring. Finally, the obtained adsorption isotherm for folic acid was deduced from the analysis of the SERS spectra and it followed a negative cooperative binding model.
Highlights
Folic acid (FA), known as vitamin B, is a compound of low molecular weight (441 g/mol), which is formed by a pteridine ring, p-aminobenzoic acid and glutamic acid
The adsorption of FA on Au NPs was carried out using surface-enhanced Raman scattering (SERS) spectroscopy and density functional theory (DFT) calculations
The comparison between SERS and Raman scattering spectra shows that the interaction of the FA with the Au NP surface mainly occurs through the nitrogen in the pteridine ring
Summary
Folic acid (FA), known as vitamin B, is a compound of low molecular weight (441 g/mol), which is formed by a pteridine ring, p-aminobenzoic acid and glutamic acid. Since the early 1990s it has been known that there is a relationship between FA deficiency and an increased risk of birth defects and chronic diseases, as well as congenital heart defects [2]. FA is a high-affinity ligand for the folate receptor (FR) (Kd=1nM), which maintains its strong binding properties when conjugated to other molecules [3]. Different methods have been developed for FA and FR detection, including cytological testing, fluorescent imaging, electro‐ chemical methods and radio-labelled assays [4]. While the effectiveness of these methods has yet to be challenged, certain issues — like the time-consuming nature of radiolabelled assays — have encouraged us to explore new methods and techniques for FA detection
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