Self-Referenced Optical Fiber Sensor Based on LSPR Generated by Gold and Silver Nanoparticles Embedded in Layer-by-Layer Nanostructured Coatings


In this work, an optical fiber sensor based on the localized surface plasmon resonance (LSPR) phenomenon has been designed for the detection of two different chemical species (mercury and hydrogen peroxide) by using Layer-by-Layer Embedding (LbL-E) as a nanofabrication technique. In the first step, silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs) have been synthesized by using a chemical protocol as a function of the strict control of three main parameters, which were polyelectrolyte concentration, a loading agent, and a reducing agent. In the second step, their incorporation into nanometric thin films have been demonstrated as a function of the number of bilayers, which shows two well-located absorption peaks associated to their LSPR in the visible region at 420 nm (AgNPs) and 530 nm (AuNPs). Finally, both plasmonic peaks provide a stable real-time reference measurement, which can be extracted from the spectral response of the optical fiber sensor, which shows a specific sensing mechanism as a function of the analyte of study.

2.3. Fabrication of the Layer-by-Layer Films

The Layer-by-Layer nanoassembly technique was used for the fabrication of the multilayer structure. In this work, the presence of PAH and PAA were used as the positive and negative charged polyelectrolytes for the build-up of the polyelectrolyte structure film. In addition, as it has been demonstrated in the previous section, these charged structures also played a key role in stabilizing the synthesized metallic nanoparticles. The nanofabrication process was based on the sequential exposition of the substrate (glass slide and optical fiber core) to the polycationic solution PAH-capped AgNPs (PAH-AgNPs) and to the polyanion PAA-capped AuNPs (PAA-AuNPs) with an immersion time of 5 min. Rinsing for 1 min in deionized water was performed for the two polyelectrolytes baths, and drying for 30 s was performed after each rinsing step. The combination of a cationic monolayer with an anionic monolayer is called a bilayer. The LbL process was carried out using a 3-axis cartesian robot from Nadetech Innovations. Finally, an important aspect to remark is that no atmospheric oxidation of the LbL films was observed using this experimental process, which shows the long-term stability of the resultant films.

Keywords: Fiber optic sensor; Gold nanoparticles; Silver nanoparticles; Localized surface plasmon resonance; Layer-by-Layer Embedding; Mercury; Hydrogen peroxide.