Novel Corrugated Long Period Grating Surface Balloon-Shaped Heterocore-Structured Plastic Optical Fibre Sensor for Microalgal Bioethanol Production

Abstract

A novel long period grating (LPG) inscribed balloon-shaped heterocore-structured plastic optical fibre (POF) sensor is described and experimentally demonstrated for real-time measurement of the ultra-low concentrations of ethanol in microalgal bioethanol production applications. The heterocore structure is established by coupling a 250 μm core diameter POF between two 1000 μm diameter POFs, thus representing a large core-small core-large core configuration. Before coupling as a heterocore structure, the sensing region or small core fibre (SCF; i.e., 250 μm POF) is modified by polishing, LPG inscription, and macro bending into a balloon shape to enhance the sensitivity of the sensor. The sensor was characterized for ethanol-water solutions in the ethanol concentration ranges of 20 to 80 %v/v, 1 to 10 %v/v, 0.1 to 1 %v/v, and 0.00633 to 0.0633 %v/v demonstrating a maximum sensitivity of 3 × 10⁶ %/RIU, a resolution of 7.9 × 10^-6 RIU, and a limit of detection (LOD) of 9.7 × 10^-6 RIU. The experimental results are included for the intended application of bioethanol production using microalgae. The characterization was performed in the ultra-low-level ethanol concentration range, i.e., 0.00633 to 0.03165 %v/v, that is present in real culturing and production conditions, e.g., ethanol-producing blue-green microalgae mixtures. The sensor demonstrated a maximum sensitivity of 210,632.8 %T/%v/v (or 5 × 10⁶ %/RIU as referenced from the RI values of ethanol-water solutions), resolution of 2 × 10^-4%v/v (or 9.4 × 10^-6 RIU), and LOD of 4.9 × 10^-4%v/v (or 2.3 × 10^-5 RIU). Additionally, the response and recovery times of the sensor were investigated in the case of measurement in the air and the ethanol-microalgae mixtures. The experimentally verified, extremely high sensitivity and resolution and very low LOD corresponding to the initial rate of bioethanol production using microalgae of this sensor design, combined with ease of fabrication, low cost, and wide measurement range, makes it a promising candidate to be incorporated into the bioethanol production industry as a real-time sensing solution as well as in other ethanol sensing and/or RI sensing applications.

Keywords: bioethanol production using microalgae; ethanol sensor; heterocore structure; long period grating; macro bending; plastic optical fibre.

Figure 1

Chemical process and integration of the pdc/adh cassette into the Synechocystis PCC6803 model. Adapted from [7].

Figure 2

Schematic of the corrugated LPG surface, balloon-shaped heterocore-structured POF sensor.

Figure 3

Arrangement of the LPG-inscribed, balloon-shaped heterocore-structured sensor using an in-house designed mechanical jig. (a) Photograph; (b) schematic representation.

Figure 4

LPG fabrication, (a) LPG fabrication system. (b) Decreasing spectrum intensity at 694 nm wavelength, monitored during LPG fabrication.

Figure 5

LPG images. (a) Microscope image of LPG on the small core (250 μm) POF. (b) Photo of LPG showing the light leakage from the corrugated LPG surface.

Figure 6

Microscope image of the LPG on the small core (250 μm) POF following a non-use period of three months.

Figure 7

Measured spectra during the fabrication stages of the corrugated LPG surface, balloon-shaped heterocore-structured POF sensor in an air medium. (a) Spectrum of straight heterocore POF structure without polishing and LPG on the SCF and spectrum of straight heterocore POF structure following polishing on the SCF. (b) Spectum of straight and balloon-shaped heterocore POF structure with polishing and LPG on the SCF.

Figure 8

(a) Normalized intensity spectra during the fabrication stages of corrugated LPG surface balloon-shaped heterocore-structured POF Sensor in air medium. (b) Zoomed section of the spectra of Straight heterocore POF structure and polished straight heterocore POF structure in the 600 nm to 700 nm region. (c) Zoomed section of spectra of straight heterocore POF structure, polished straight heterocore POF structure, straight and balloon-shaped heterocore POF structure with polishing and LPG in the 750 nm to 790 nm region.

Figure 9

Illustration of the experimental setup.

Figure 10

Sensor calibration. (a) Spectral responses of the sensor for air, water, and ethanol–water solutions at 20 %v/v, 40 %v/v, 60 %v/v, and 80 %v/v concentration in the wavelength range of 450 nm to 800 nm. (b) Sensor’s calibration curve for the RIs of ethanol–water solutions at 20 %v/v, 40 %v/v, 60 %v/v, and 80 %v/v concentration at wavelength of 691.45 nm.

Figure 11

Average of repeated measurements in ethanol–water solutions at 691.45 nm presented as the shift in transmittance % of the sensor versus RI values of different ethanol concentration ranges. (a) 1 to 10 %v/v ethanol concentration range, (b) 0.1 to 1 %v/v ethanol concentration range, (c) 0.00633 to 0.0633 %v/v ethanol concentration range.

Figure 12

Life cycle of microalgae cells.

Figure 13

Average of repeated measurements in ethanol–microalgae mixtures at 694 nm for the range of 0.00633 to 0.03165 %v/v ethanol concentration. (a) Sensor’s response as the shift in transmittance % versus ethanol–water RI values that were used as a reference. (b) Sensor’s response as the shift in transmittance % versus %v/v ethanol concentration in microalgae mixtures.

Figure 14

Real-time response of sensor at different stages of measurement as intensity counts at 694 nm versus time in seconds.

Figure 15

Real-time analysis of sensor’s response and recovery time in the ethanol–microalgae solution presented in Figure 13. (a) Analysis for trs1—response time from air to microalgae solution. (b) Analysis of trs2—rise time while the sensor is withdrawn from microalgae solution to the air. (c) Analysis of trc1—recovery/fall time from air to the 0.03165% ethanol–microalgae solution. (d) Analysis of trs3—response time to reach 90% response when the sensor is inserted in 0.03165% ethanol–microalgae solution. (e) Analysis of trc2—recovery time from 0.03165% ethanol–microalgae solution to air.

Figure 15

Real-time analysis of sensor’s response and recovery time in the ethanol–microalgae solution presented in Figure 13. (a) Analysis for trs1—response time from air to microalgae solution. (b) Analysis of trs2—rise time while the sensor is withdrawn from microalgae solution to the air. (c) Analysis of trc1—recovery/fall time from air to the 0.03165% ethanol–microalgae solution. (d) Analysis of trs3—response time to reach 90% response when the sensor is inserted in 0.03165% ethanol–microalgae solution. (e) Analysis of trc2—recovery time from 0.03165% ethanol–microalgae solution to air.