Plastic optical fibre sensors for in-situ and real-time ultra low-level ethanol concentration measurement in microalgal bioethanol production applications

Bioethanol production using the metabolic engineering of microalgae is considered a promising technique in terms of environmental and economic sustainability. Monitoring this fourth-generation biofuel production process in real-time and in-situ at critical initial stages is crucial for the optimization of process parameters and effectiveness, improving bioethanol yield as well as avoiding sample loss and offline analyses. The design and implementation of optical fibre sensors for real-time measurement of ultra-low level ethanol concentration focused on microalgal bioethanol production as a biofuel was investigated in this research. The sensing objectives were aimed to optimize the production process and enhance bioethanol yield from a metabolically engineered model cyanobacterium, Synechocystis UL004 for which the initial rate of bioethanol production was low (0.1 – 0.5 gL-1 per day or 0.01266 %v/v to 0.0633 %v/v).

Three sensor designs using plastic optical fibres (POFs), namely, a U-bend sensor, a side?polished balloon-shaped heterocore structured sensor, and a corrugated long period grating (LPG) surface balloon-shaped heterocore structured sensor were investigated and experimentally demonstrated for real-time measurement of ethanol in an experimental microalgal bioethanol production system. The sensors’ working principle was based on the evanescent field interaction at the optical fibre-liquid region, resulting in intensity modulation at the output defined in terms of absorbance and transmittance % at different ethanol concentrations in water and in a laboratory-based culture media as a microalgal production environment. The U-bend sensor demonstrated excellent performance with a high sensitivity and a high limit of detection (LOD) in ethanol-water solutions with ethanol detections at concentrations below the initial rate of microalgal bioethanol production. However, when tested for ethanol concentration measurement during actual culturing and production with active microalgal mixtures, this sensor lacked performance in the initial days of the prepared microalgal mixture.

Hence, the concept of using POFs for heterocore structured sensor designs was investigated. Two sensor designs namely, side-polished and corrugated LPG surface balloon-shaped heterocore structured POF sensors were designed to enhance the sensitivity of ethanol measurement further and reduce the effect of cross-sensitivity from microalgal mixtures while measuring ultra-low level ethanol concentration. Both the heterocore structured variants were fabricated using two large core multimode POFs and one small core multimode POF. The small core fibre (SCF) was inserted between two large core fibres (LCFs), forming a large core—small core—large core heterocore structure where small core fibre (SCF) acted as a sensing region, whereas the large core fibres (LCFs) were used as input and output light waveguides as well as to introduce the light leakage in the cladding of SCF at the heterocore structure’s input interface and generate the significant evanescent field. Further sensitivity enhancement of these sensor designs was performed by modification of the sensing region, i.e., side-polishing and/or LPG inscription and macro bending in the balloon shape. The side?polished balloon-shaped heterocore structured sensor revealed high sensitivity and resolution at relatively high concentrations but could not effectively detect the ultra-low level ethanol concentration that corresponds to the initial bioethanol production rate. The corrugated LPG surface balloon-shaped heterocore structured sensor, on the other hand, demonstrated extremely high sensitivity (3 × 106 %/RIU and 5 × 106 %/RIU), excellent resolution (10-6 RIU) and a very low LOD (10-6 RIU and 10-5 RIU) for ethanol-water solutions and ethanol-microalgae mixtures, coupled with a fast response time (~12 seconds) even under microalgal culture conditions.

This study thus demonstrated real-time, repeatable, in situ, and very high-resolution POF sensing solutions for ethanol measurement vis-à-vis ethanol sensors made of Glass Optical Fibres (GOFs). The research shows the feasibility of POF as low cost ethanol sensors owing to low-fabrication cost requiring no specialized equipment and clean room for production. The RI-based sensing principle means that these POF sensors can be engineered for broad concentration range measurements, which is important in biochemical and biomedical applications. We also investigated the alignment of these innovative POF sensors with Industry 5.0, in terms of sustainability, personalisation of sensing solutions, and technological transformation.