Fiber Optic Gas Sensors Based on Lossy Mode Resonances and Sensing Materials Used Therefor: A Comprehensive Review

Sensing Approaches Exploiting Molecularly Imprinted Nanoparticles and Lossy Mode Resonance in Polymer Optical Fibers

In this work, two different lossy mode resonance (LMR) platforms based on plastic optical fibers (POFs) are developed and tested in a biochemical sensing scenario. The LMR platforms are based on the combination of two metal oxides (MOs), i.e., zirconium oxide (ZrO2) and titanium oxide (TiO2), and deposited on the exposed core of D-shaped POF chips. More specifically, two experimental sensor configurations were obtained by swapping the mutual position of the Mos films over to the core of the D-shaped POF probe. The POF-LMR sensors were first characterized as refractometers, proving the bulk sensitivities. Then, both the POF-LMR platforms were functionalized using molecularly imprinted nanoparticles (nanoMIPs) specific for human transferrin (HTR) in order to carry out binding tests. The achieved results report a bulk sensitivity equal to about 148 nm/RIU in the best sensor configuration, namely the POF-TiO2-ZrO2. In contrast, both optical configurations combined with nanoMIPs showed an ultra-low detection limit (fM), demonstrating excellent efficiency of the used receptor (nanoMIPs) and paving the way to disposable POF-LMR biochemical sensors that are easy-to-use, low-cost, and highly sensitive.

Various Types of Light Guides for Use in Lossy Mode Resonance-Based Sensors

A comparative study of figure-of-merit fiber sensors of the mass concentration of NaCl solutions based on single-mode and multi-mode fibers was carried out. Lossy mode resonance is realized on chemically thinned sections of optical fibers to various diameters (from 26 to 100 μm) coated with ZnTe. Thin-film coatings were applied using the method of metalorganic chemical vapor deposition (MOCVD). Samples of single-mode and multi-mode fiber sensors were created in such a way that the depth and spectral position of resonances in aqueous NaCl solutions coincided. Sensors implemented on a single-mode fiber have a higher sensitivity (5930 nm/refractive index unit (RIU)) compared to those on a multi-mode fiber (4860 nm/RIU) and a smaller half-width of the resonance in the transmission spectrum. According to the results of experiments, figure-of-merit sensors are in the range of refractive indices of 1.33-1.35 for: multi-mode fiber-25 RIU^-1, single-mode fiber-75 RIU^-1. The sensitivity of the resulting sensors depends on the surface roughness of the ZnTe coating. The roughness of films synthesized on a single-mode fiber is four times higher than this parameter for a coating on a multi-mode fiber. For the first time, in the transmission spectrum during the synthesis of a thin-film coating on a multi-mode fiber, the possibility of separating the first nine orders of resonances into electric and magnetic transverse components has been demonstrated. The characteristics of sensors with the operating wavelength range in the visible (500-750 nm) and infrared (1350-1550 nm) regions of the spectrum are compared. The characteristics of multi-mode lossy mode resonance sensors are demonstrated, which make them more promising for use in applied devices than for laboratory research.

Combustion, Chemistry, and Carbon Neutrality

Combustion is a reactive oxidation process that releases energy bound in chemical compounds used as fuels─energy that is needed for power generation, transportation, heating, and industrial purposes. Because of greenhouse gas and local pollutant emissions associated with fossil fuels, combustion science and applications are challenged to abandon conventional pathways and to adapt toward the demand of future carbon neutrality. For the design of efficient, low-emission processes, understanding the details of the relevant chemical transformations is essential. Comprehensive knowledge gained from decades of fossil-fuel combustion research includes general principles for establishing and validating reaction mechanisms and process models, relying on both theory and experiments with a suite of analytic monitoring and sensing techniques. Such knowledge can be advantageously applied and extended to configure, analyze, and control new systems using different, nonfossil, potentially zero-carbon fuels. Understanding the impact of combustion and its links with chemistry needs some background. The introduction therefore combines information on exemplary cultural and technological achievements using combustion and on nature and effects of combustion emissions. Subsequently, the methodology of combustion chemistry research is described. A major part is devoted to fuels, followed by a discussion of selected combustion applications, illustrating the chemical information needed for the future.

Polyacrylic Acid/Polyaniline-Coated Multimode Interferometer for Ammonia Detection

A coaxial optical fiber interferometer (COFI) is proposed here for ammonia sensing, which comprises two light-carrying single-mode fibers (SMF) fused to a section of no-core fiber (NCF), thus forming an optical interferometer. The outer surface of the COFI is coated with a layer of polyacrylic acid (PAA)/polyaniline (PAni) film. The refractive index (RI) of the sensitive layer varies when PAA/PAni interacts with ammonia, which leads to the resonance wavelength shift. The surface morphology and structure of the PAA/PAni composites were characterized by using a scanning electron microscope (SEM) and Fourier-transform infrared (FTIR) spectroscopy. When the sensor was exposed to an ammonia atmosphere of different concentrations at room temperature, the sensing performance of the PAA/PAni composite film was superior to that of a sensitive film formed by single-component PAA or PAni. According to the experimental results, the composite film formed by 5 wt% PAA mixed with 2 wt% PAni shows better performance when used for ammonia sensing. A maximum sensitivity of 9.8 pm/ppm was obtained under the ammonia concentration of 50 ppm. In addition, the sensor shows good performance in response time (100 s) and recovery time (180 s) and has good stability and selectivity. The proposed optical fiber ammonia sensor is adapted to monitor leakage in its production, storage, transportation, and application.

Gas Sensor Based on Lossy Mode Resonances by Means of Thin Graphene Oxide Films Fabricated onto Planar Coverslips

The use of planar waveguides has recently shown great success in the field of optical sensors based on the Lossy Mode Resonance (LMR) phenomenon. The properties of Graphene Oxide (GO) have been widely exploited in various sectors of science and technology, with promising results for gas sensing applications. This work combines both, the LMR-based sensing technology on planar waveguides and the use of a GO thin film as a sensitive coating, to monitor ethanol, water, and acetone. Experimental results on the fabrication and performance of the sensor are presented. The obtained results showed a sensitivity of 3.1, 2.0, and 0.6 pm/ppm for ethanol, water, and acetone respectively, with a linearity factor R² > 0.95 in all cases.

Ratiometric Optical Fiber Dissolved Oxygen Sensor Based on Fluorescence Quenching Principle

In this study, a ratiometric optical fiber dissolved oxygen sensor based on dynamic quenching of fluorescence from a ruthenium complex is reported. Tris(4,7-diphenyl-1,10-phenanthrolin) ruthenium(II) dichloride complex (Ru(dpp)₃²⁺) is used as an oxygen-sensitive dye, and semiconductor nanomaterial CdSe/ZnS quantum dots (QDs) are used as a reference dye by mixing the two substances and coating it on the plastic optical fiber end to form a composite sensitive film. The linear relationship between the relative fluorescence intensity of the ruthenium complex and the oxygen concentration is described using the Stern–Volmer equation, and the ruthenium complex doping concentration in the sol-gel film is tuned. The sensor is tested in gaseous oxygen and aqueous solution. The experimental results indicate that the measurement of dissolved oxygen has a lower sensitivity in an aqueous environment than in a gaseous environment. This is due to the uneven distribution of oxygen in aqueous solution and the low solubility of oxygen in water, which results in a small contact area between the ruthenium complex and oxygen in solution, leading to a less-severe fluorescence quenching effect than that in gaseous oxygen. In detecting dissolved oxygen, the sensor has a good linear Stern–Volmer calibration plot from 0 to 18.25 mg/L, the linearity can reach 99.62%, and the sensitivity can reach 0.0310/[O₂] unit. The salinity stability, repeatability, and temperature characteristics of the sensor are characterized. The dissolved oxygen sensor investigated in this research could be used in various marine monitoring and environmental protection applications.

The UV Effect on the Chemiresistive Response of ZnO Nanostructures to Isopropanol and Benzene at PPM Concentrations in Mixture with Dry and Wet Air

Towards the development of low-power miniature gas detectors, there is a high interest in the research of light-activated metal oxide gas sensors capable to operate at room temperature (RT). Herein, we study ZnO nanostructures grown by the electrochemical deposition method over Si/SiO2 substrates equipped by multiple Pt electrodes to serve as on-chip gas monitors and thoroughly estimate its chemiresistive performance upon exposing to two model VOCs, isopropanol and benzene, in a wide operating temperature range, from RT to 350 °C, and LED-powered UV illumination, 380 nm wavelength; the dry air and humid-enriched, 50 rel. %, air are employed as a background. We show that the UV activation allows one to get a distinctive chemiresistive signal of the ZnO sensor to isopropanol at RT regardless of the interfering presence of H2O vapors. On the contrary, the benzene vapors do not react with UV-illuminated ZnO at RT under dry air while the humidity’s appearance gives an opportunity to detect this gas. Still, both VOCs are well detected by the ZnO sensor under heating at a 200–350 °C range independently on additional UV exciting. We employ quantum chemical calculations to explain the differences between these two VOCs’ interactions with ZnO surface by a remarkable distinction of the binding energies characterizing single molecules, which is −0.44 eV in the case of isopropanol and −3.67 eV in the case of benzene. The full covering of a ZnO supercell by H2O molecules taken for the effect’s estimation shifts the binding energies to −0.50 eV and −0.72 eV, respectively. This theory insight supports the experimental observation that benzene could not react with ZnO surface at RT under employed LED UV without humidity’s presence, indifference to isopropanol.

Beyond near-infrared lossy mode resonances with fluoride glass optical fiber

The objective of this Letter consists of the exploration of the lossy mode resonance (LMR) phenomenon beyond the near-infrared region and specifically in the short wave infrared region (SWIR) and medium wave infrared region (MWIR). The experimental and theoretical results show for the first time, to the best of our knowledge, not only LMRs in these regions, but also the utilization of fluoride glass optical fiber associated with this phenomenon. The fabricated devices consist of a nanometric thin-film of titanium dioxide used as LMR generating material, which probed extraordinary sensitivities to external refractive index (RI) variations. RI sensitivity was studied in the SWIR and MWIR under different conditions, such as the LMR wavelength range or the order of resonance, showing a tremendous potential for the detection of minute concentrations of gaseous or biological compounds in different media.

Chemical Surface Adsorption and Trace Detection of Alcohol Gas in Graphene Oxide-Based Acid-Etched SnO2 Aerogels

An acidified SnO₂/rGO aerogel (ASGA) is an attractive contributor in ethanol gas sensing under ultralow concentration because of the sufficient active sites and adsorption pores in SnO₂ and the rGA, respectively. Furthermore, a p-n heterojunction is successfully constructed by the high electron mobility between ASP and rGA to establish a brand-new bandgap of 2.72 eV, where more electrons are released and the surface energy is decreased, to improve the gas sensitivity. The ASGA owns a specific surface area of 256.1 m²/g, far higher than SnO₂ powder (68.7 m²/g), indicating an excellent adsorption performance, so it can acquire more ethanol gas for a redox reaction. For gas-sensing ability, the ASGA exhibits an excellent response of R_a/R_g = 137.4 to 20 ppm of ethanol at the optimum temperature of 210 °C and can reach a response of 1.2 even at the limit detection concentration of 0.25 ppm. After the concentration gradient change test, a nonlinear increase between concentration and sensitivity (S-C curve) is observed, and it indirectly proves the chemical adsorption between ethanol and ASGA, which exhibits charge transfer and improves electron mobility. In addition, a detailed energy band diagram and sensor response diagram jointly depict the gas-sensitive mechanism. Finally, a conversed calculation explains the feasibility of the nonlinear S-C curve from the atomic level, which further verifies the chemical adsorption during the sensing process.