We report the optimization of the length of a D-shaped plastic optical fiber (POF) sensor for refractive index (RI) sensing from a numerical and experimental point of view. The sensing principle is based on total internal reflection (TIR). POFs with 1 mm in diameter were embedded in grooves, realized in planar supports with different lengths, and polished to remove the cladding and part of the core. All D-shaped POF sensors were tested using aqueous medium with different refractive indices (from 1.332 to 1.471) through intensity-based configuration. Results showed two different responses. Considering the refractive index (RI) range (1.33–1.39), the sensitivity and the resolution of the sensor were strongly dependent on the sensing region length. The highest sensitivity (resolution of 6.48 × 10−3 refractive index units, RIU) was obtained with 6 cm sensing length. In the RI range (1.41–1.47), the length of the sensing region was not a critical aspect to obtain the best resolution. These results enable the application of this optical platform for chemical and biochemical evanescent field sensing. The sensor production procedure is very simple, fast, and low-cost.
We present a very simple approach for the detection of the Perfluorinated Alkylated Substances (PFAs) in water solution. Perfluorooctanesulfonate (PFOS) and Perfluorooctanoate (PFOA) are the most extensively investigated perfluoroalkyl and polyfluoroalkyl substances in water because human exposition can occur through different pathways, even if the dietary intake seems to be their main route of exposure. The developed sensor is based on a specific Molecularly Imprinted Polymer (MIP) receptor deposited on a simple D-shaped Plastic Optical Fiber (POF) platform. This novel chemical sensor has been characterized using a very simple and low-cost experimental setup based on an LED and two photodetectors. This optical sensor system is an alternative method to monitor the presence of contaminants with an MIP receptor, instead of a surface plasmon resonance (SPR) sensor in D-shaped POFs. For the sake of comparison, the results obtained exploiting the same MIP for PFAs on a classic SPR-POF sensor have been reported. The experimental results have shown that the actual limit of detection of this new configuration was about 0.5 ppb. It is similar to the one obtained by the configuration based on an SPR-POF with the same MIP receptor.
In this study the influence of the surface roughness on the transmission capacities of D-shaped plastic optical fibers (POFs) and sensors performance was investigated. Five D-shaped POF sensors were produced and characterized for refractive index sensing between 1.33 and 1.41. The sensors were characterized using a low-cost optical sensing system based on the variation of the transmitted light though the POF with refractive index changes (RI). Higher surface roughness increases the scattering losses through the POF and influences the sensors’ performance; therefore, a balance must be attained. Generally, the best performance was achieved when the sensing region was polished with P600 sandpaper as a final polishing step. Polishing with sandpapers of lower grit size resulted in lower scattering, higher linearity of the sensor response and generally lower performance for RI sensing. A sensor resolution of 10−3–10−4 RIU, dependent on the value of the external refractive index, was obtained through simple and low-cost manufacturing procedures. The obtained results show the importance of surface roughness in the development of POF sensors which can be used in several applications, such as for water quality assessment.
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