No abstract

Herein we addressed a study to determine the enhancement factor (EF) of the Raman signal reached by composite films with two main components, Ag nanoparticles and SiO2 spheres. The study involves the synthesis, structural composition and optical response by using experimental techniques and theoretical-numerical modeling. A colloid with single NPs and agglomerates of them, with a tannic acid layer on its surface, was produced. Separately, porous SiO2 spheres were obtained. A mixture of both, Ag NPs and SiO2 particles was used to produce the films by solvent evaporation method. It is shown that single or agglomerated Ag NPs are preferentially located at the interstices of the SiO2 spheres. Using discrete dipole approximation, the SERS EF has been estimated considering the agglomeration and tannic acid layer. Both, the dielectric spheres and tannic acid layer diminish the electric field intensity and therefore the SERS EF. When a Ag NP with/without a dielectric shell is touching a SiO2 sphere, the EF is as high as 1 × 103, the zones where this value is reached are smaller when the dielectric layer is present. With a cluster of 3 nude Ag NPs surrounded by SiO2 spheres an EF of 2.4 × 103 is obtained.

Detecting low concentrations of thyroxine hormone (T4) is of utmost importance to prevent diseases and medical issues caused by hypothyroidism. In this work, we propose the use of a simple technique for T4 sensing, the well-known surface-enhanced Raman spectroscopy (SERS). Spherical Au and Ag nanoparticles (NPs) and functionalized with ascorbic acid and tannic acid, respectively. They synthesized by a seed-mediated growth method, were used as SERS substrates to quantify T4 at different concentrations. The NPs were characterized by UV-VIS spectroscopy and scanning electron microscopy. Both, Au and Ag NPs, have an average diameter of 50 nm, the first ones have a surface plasmon resonance at 537 nm, while the latter have it at 421 nm. It was noticed the SERS substrates are composed of isolated and agglomerated NPs. From theoreti-cal-numerical calculations, we identified the hot spots |E| of the agglomerate NPs is at least 7 times more intense than that of the isolated NPs. We show the substrates can detect up to 0.01 mM and, for the range of concentration studied, the Ag substrate is more sensitive than Au. Thus, the proposed substrates and the SERS technique constitute a potential prospect for sensing hormones and organic molecules at low concentrations.