D-shaped fiber optic plasmonic sensors using planar and grating structures of silver and gold: design and analysis.

In this paper, a D-shaped optical fiber plasmonic sensor using planar and grating structures of silver and gold metals is simulated using the finite element method under the wave optics module of COMSOL Multiphysics. Performance defining parameters are based on (i) the transmittance curve, viz., resonance wavelength (λ r), shift in resonance wavelength (Δ λ r), minimum transmittance (T m i n ), and bandwidth (BW), and (ii) on electric field distribution of a surface plasmon wave, viz., penetration depth (PD) and propagation length (PL) obtained for the considered sensor structures. It is found that gold gives wider BW than silver (e.g., at 1.39 refractive index of the sample: 480% for the planar case and 241% for the grating case), which deteriorates sensor performance by degrading detection accuracy. However, gold gives higher Δ λ r than silver (at 1.40-1.39=0.01 change in refractive index of the sample: 18.33% for the planar case and 16.39% for the grating case), which improves sensor performance and enhances sensitivity. A grating slightly increases the BW and Δ λ r for both gold and silver. Further, with respect to silver, the sensor that contains gold demonstrates higher PD (e.g., 22.32% at 1.39 refractive index of the sample for the planar case) and lower PL (e.g., 22.74% at 1.39 refractive index of sample for the planar case). A grating increases the PD (e.g., 10% for silver at 1.39 refractive index of the sample), whereas it decreases the PL (e.g., 8.73% for silver at 1.39 refractive index of the sample). Lower PL signifies the localization of the field, whereas higher PD enables the sensor to detect larger molecules. Therefore, the sensor with grating metals provides better sensitivity with reduced detection accuracy for the detection of comparatively larger molecules.

COVID-19 Detection Using Contemporary Biosensors and Machine Learning Approach: A Review.

The current global pandemic not only claims countless human lives but also rocks the economies of every country on the planet. This fact needs the development of novel, productive, and efficient techniques to detect the SARS-CoV-2 virus. This review article discusses the current state of SARS-CoV-2 virus detection methods such as electrochemical, fluorescent, and electronic, etc., as well as the potential of optical sensors with a wide range of novel approaches and models. This review provides a comprehensive comparison of various detection methods by comparing the various techniques in depth. In addition, there is a brief discussion of the futuristic approach combining optical sensors with machine learning algorithms. It is believed that this study would prove to be critical for the scientific community to explore solutions for detecting viruses with improved functionality.

Novel Bottom-Side Polished PCF-Based Plasmonic Biosensor for Early Detection of Hazardous Cancerous Cells.

This work presents a single-core bowl-shaped bottom-side polished (BSP) photonic crystal fiber (PCF) sensor based on surface plasmon resonance (SPR) concept for the early detection of hazardous cancer cells in human blood, skin, cervical, breast, and adrenal glands. We have studied liquid samples of cancer-affected and healthy samples with their concentrations/refractive indices in the sensing medium. To induce a plasmonic effect in the PCF sensor, the bottom flat section of a silica PCF fiber is coated with a 40nm plasmonic material, such as gold. To strengthen this effect, a thin TiO2 layer of 5 nm is sandwiched between fiber and gold as it strongly holds gold nanoparticles with smooth fiber surface. When the cancer-affected sample is introduced to the sensor's sensing medium, it produces a different absorption peak in the form of a resonance wavelength than the healthy sample. This reallocation of the absorption peak is used to determine sensitivity. Hence, the obtained sensitivities for blood cancer, cervical cancer, adrenal gland cancer, skin cancer, and breast cancer (type-1and type-2) cells are 22,857nm/RIU, 20000nm/RIU, 20714nm/RIU, 20000nm/RIU, 21428nm/RIU, and 25000nm/RIU, respectively, with highest detection limit 0.024. These strong findings indicate that our proposed cancer sensor PCF is a viable choice for early cancer cell detection.

Design and Analysis of an SPR Sensor Based on Antimonene and Platinum for the Detection of Formalin.

Detection of formalin to prevent food adulteration, especially in tropical countries, is of primary concern for public health issues. Life-threatening diseases such as leukaemia and lymphoma occur due to the regular consumption of formalin with food. Traditionally, spectrophotometry and chromatography-based sensors have been employed to detect formalin, which have limitations related to their ability to achieve high sensitivity, selectivity, and fast response. In this paper, a surface plasmon resonance (SPR) sensor for improved sensing of formalin is proposed. The Kretschmann configured SPR sensor probe is designed using silver (Ag), platinum (Pt), antimonene, and chitosan, which increases the sensitivity and selectivity. The maximum sensitivity achieved for the proposed SPR sensor is 206.86 °/RIU. The distribution of the electric field (Ey) component of the electric field is also evaluated to analyze the field enhancement at different layer interfaces and to calculate the penetration depth (176.75 nm).

Design and analysis of graphene- and germanium-based plasmonic probe with photonic spin Hall effect in THz frequency region for magnetic field and refractive index sensing.

In this work, we analyze the design of a graphene- and germanium-based plasmonic sensor with photonic spin Hall effect (PSHE) for detection of refractive index (RI) of a gas medium and magnetic field (B) applied to the graphene monolayer in THz frequency region. The PSHE phenomenon is studied in both conventional as well as modified weak measurements. The effect of gaseous medium thickness (d4), transverse magnetic (TM) mode's order, and amplified angle parameter (Δ) is studied on the sensor's performance. Parameters such as sensitivity, resolution, and figure of merit have been considered for sensor's performance evaluation. The results indicate that in the conventional weak measurements, for a TM1 mode (with d4 = 20 µm, B = 0, and Δ = 0.1°), an RI resolution of 2.32 × 10-12 RIU is achievable for gas medium in the range 1-1.1 RIU. In the modified weak measurements, for a TM3 mode (with d4 = 100 µm, B = 0, and Δ = 0.1°), the RI resolution close to 1.39 × 10-10 RIU is achievable for gas sensing. The same sensor design was also studied for magnetic field sensing while keeping the value of gaseous medium RI (n4) as 1. The results indicate that for a TM1 mode (with d4 = 20 µm and Δ = 0.1°), in the conventional weak measurements, a magnetic field resolution of 5.31 × 10-4 µT (i.e., 0.53 nT) is achievable for a range 0-1 T of B. Further, it is found that in contrast with the conventional case, the resolutions in the modified weak measurements are improved for large values of the Δ. Some of the results emerge better or comparable with the resolutions of RI and magnetic field measurement (5 × 10-9 RIU and 0.7 µT or 1.22 × 10-11 RIU and 1.46 × 10-2 µT) existing in the literature.

Plasmonics-based gas sensor with photonic spin hall effect in broad terahertz frequency range under variable chemical potential of graphene.

Graphene monolayer of sub-nanometer thickness possesses strong metallic and plasmonic behavior in a broad terahertz (THz) frequency range. This plasmonic effect can be considerably manipulated when graphene layer is subjected to a variable chemical potential (Ef) via chemical doping or electrical gating. The strong adsorption characteristics of graphene layer is another important advantage. In this work, a photonic spin Hall effect (PSHE) based plasmonic sensor consisting of germanium prism, organic dielectric layer, and graphene monolayer is simulated and analyzed in THz range aiming at highly sensitive and reliable gas sensing. Modified Otto configuration and Kubo formulation for graphene at room temperature are considered. The sensor's performance is examined in terms of figure of merit (FOM). The analysis indicates that under angular interrogation scheme of sensor operation, the FOM improves for smaller chemical potential (moderate doping) and higher THz frequency. Moreover, the influence of temperature on gas sensor's performance (FOM) is negligible, which suggests that the sensor is capable of providing stable sensing performance against temperature variation. The sensor design is highly flexible in terms of selection of THz frequency as an alternative interrogation scheme (i.e., measuring the variation in spin-dependent shift peak value of PSHE spectrum upon change in gas medium refractive index) can also be implemented. It is found that there is no need to change the moderate doping of graphene monolayer (i.e., Ef remains around its normal value ~ 0.1 eV) as the sensitivity achievable with this alternative method has considerably greater magnitude at smaller THz frequency (e.g., 2 THz). The magnitudes of FOM (with angular interrogation method) and sensitivity (with alternative method) are found to be significantly greater for rarer gaseous media, which might possibly assist in early detection of airborne viruses such as SARS-Cov-2 (while using appropriate specificity method) and to measure the concentration of a particular gas in a given gaseous mixture. Supplementary Information: The online version contains supplementary material available at 10.1007/s11082-022-03626-7.

Graphene-Based Plasmonic Sensor at THz Frequency with Photonic Spin Hall Effect Assisted by Magneto-optic Phenomenon.

Graphene monolayer of sub-nanometer thickness shows strong metallic and plasmonic behavior in terahertz (THz) frequency range. This plasmonic effect varies considerably when graphene layer is placed under a magnetic field of appropriate strength. The strong adsorption characteristic of graphene layer is another advantage. In this work, a photonic spin Hall effect (PSHE)-based plasmonic sensor consisting of germanium prism, organic dielectric layer, and graphene monolayer is simulated and analyzed in THz aiming at highly sensitive and reliable sensing under variable magnetic field. Modified Otto configuration and magneto-optic effect in graphene are considered. The sensor's performance is examined in terms of sensitivity, limit of detection (LOD), and figure of merit (FOM). The analysis indicates that LOD of the order of 10-5 RIU for gas sensing is achievable, which is finer than recently reported gas sensors based on different techniques. Further, the FOM improves when a larger magnitude of magnetic field is applied. The FOM is even greater for rarer gaseous media, which can make the sensor extremely useful in early detection of airborne viruses such as SARS-Cov-2 (while using appropriate specificity method) and to measure the concentration of a particular gas in a given gaseous mixture. The results further indicate that the same sensor design can be used for magnetic field detection while the FOM of magnetic field detection is significantly greater for rarer gaseous medium (e.g., air), which may enable the probe to be used in early detection of radiation leakage in nuclear reactors. For larger magnitudes of magnetic field, the corresponding LOD becomes finer.

A nanolayered structure for sensitive detection of hemoglobin concentration using surface plasmon resonance.

The objective of the proposed work is to design a biosensor that monitors hemoglobin (Hb) concentration using the combination of nanolayer, i.e., barium titanate (BaTiO3) and antimonene based on surface plasmon resonance (SPR) technique. Antimonene is used here as bio-recognition element (BRE) layer to attach the Hb analyte through physical adsorption due to its hydrophilic nature, higher adsorption energy and larger active surface area. The use of BaTiO3 adlayer (7 nm) just before antimonene is to enhance the refractive index (RI) sensitivity up to 1.90 times for the proposed SPR biosensor. The reason behind sensitivity enhancement is its high dielectric constant which enhances the electromagnetic field with in analyte medium. The performance of the biosensor is demonstrated with performance parameters namely sensitivity, detection accuracy (DA), figure of merit (FOM) and resolution. The proposed biosensor has potential to achieve much higher performance in terms of RI sensitivity of 303.83°/RIU, FOM of 50.39 RIU-1 and resolution of 0.021 g/l in comparison with reported biosensors in the literature for detection of Hb concentration. Thus, based on the obtained results one can say that the proposed work unlocks a reliable sensing in the field of medical science to detect hemoglobin-related diseases in human being.