Optical fiber pH sensors based on layer-by-layer electrostatic self-assembled Neutral Red

The layer-by-layer (LbL) electrostatic self-assembly technique was used for fabricating evanescent wave optical fiber pH sensors based on the colorimetric pH indicator Neutral Red (NR). Different strategies for the fabrication of the nanostructured LbL pH-sensitive overlays were studied. In an initial approach poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) were used as matrix materials for immobilizing the active dye onto the core of an optical fiber, this was the sensitive multilayer structure (PAH + NR/PAA). This kind of polymeric structure has been already proposed in the past by several groups to incorporate pH indicators. Although the fabricated devices were sensitive to pH changes, some possible matrix-induced side effects could lead to a non-optimum measurement of pH. In fact, the polymeric matrix without the indicator, the (PAH/PAA) multilayer structure, was also tested against pH variations and, surprisingly, this matrix also showed a high sensitivity to pH changes mainly due to swelling. The results obtained in this work reveal that some important issues, like a high non-linearity, make this matrix a non-optimal solution for the fabrication of optical fiber pH sensors. Due to this, a third approach is proposed and studied in this work: a (NR/PAA) multilayer structure. This structure showed faster response times (below 10 s), high repeatability, low hysteresis, and a dynamic range of 2.5 dB from pH 3 to 9.