Surface Plasmon Resonance-Based Fiber-Optic Hydrogen Gas Sensor Utilizing Indium–Tin Oxide (ITO) Thin Films

We report experimental study on an indium?Ctin oxide (ITO)-coated surface plasmon resonance-based fiber-optic hydrogen gas sensor operating at room temperature. The sensor works on intensity modulation interrogation. Indium?Ctin oxide (In₂O₃?+?SnO₂) films were grown on unclad core of the fiber by thermal evaporation technique. The surface plasmon resonance (SPR) spectra for 100?% nitrogen gas as well as for a mixture of 4?% hydrogen gas and 96?% nitrogen gas were obtained. In the case of mixture of hydrogen and nitrogen gases, a sharp dip in the SPR spectrum was observed implying that the hydrogen gas changes the dielectric properties of ITO. The performance of the sensor has been studied for different percentages of tin oxide in indium oxide and for different thicknesses of ITO film. Both the parameters have been optimized for the best performance of the sensor.     

Optimizing the Tin Selenide (SnSe) Allotrope/Gold-Based Surface Plasmon Resonance Sensors for Enhanced Sensitivity

The 2D material tin selenide monolayer (SnSe) has attracted a lot of attention due to its excellent optoelectronic properties. This study focuses on the investigation of the potential improvement of the response of surface plasmon resonance (SPR) sensors by coating the gold layer with SnSe allotrope (α, δ, ε) monolayers. Using an optimization algorithm along with the transfer matrix method (TMM), we determined the optimal thickness of the gold layer as a function of the number of monolayers added to significantly increase the sensor’s response in terms of reflectivity and phase. With respect to reflectivity, sensitivity increased by 20% in comparison with the optimal bare gold structure, whilst with respect to phase, sensitivity was approximately two orders of magnitude greater than the bare gold structure. Our results demonstrate that SPR sensors modified with SnSe monolayers could be used in diagnostic applications where both high sensitivity and small concentration of analyte are required.

     

Optical Magnetism in Surface Plasmon Resonance–Based Sensors for Enhanced Performance

Surface plasmon resonance (SPR)–based structures are finding important applications in sensing biological as well as inorganic samples. In SPR techniques, an angle-resolved reflection (R) profile of the incident light from a metal-dielectric interface is measured and the resonance characteristics are extracted for the identification of the target sample. However, the performance, and hence the applicability of these structures, suffers when the weight and concentration of the target samples are small. Here, we show that SPR-based sensors can create strong magnetism at optical frequency, which can be used for the detection of target samples instead of using the conventional R profiles, as the magnetic resonance varies depending on the refractive index of the target sample. Using scattering parameters retrieval method, we computationally find out the effective permeability (μ_eff) of a SPR sensor with a structure based on Kretschmann configuration, and use it to calculate the performance of the sensor. A comparison with the conventional technique that uses R profile to detect a target sample shows a significant increase in the sensor performance when μ_eff is used instead.

     

A Polarization-independent SPR Sensor Based on Photonic Crystal Fiber for Low RI Detection

A surface plasmon resonance (SPR) sensor based on a photonic crystal fiber (PCF) is proposed for low refractive index (RI) detection. The core of PCF is formed by two-layer air walls and either layer is composed of six identical sector rings with negative curvature. Plasmonic material gold (Au) is coated on the external cladding surface. Finite element method (FEM) is applied to investigate the performance of the SPR sensor. Results show that the sensor is independent of polarization due to the coincident coupling properties of the two polarized modes. Additionally, in low RI ranging from 1.20 to 1.33, the sensor keeps a high spectral sensitivity with an average value of 7738 nm/RIU. When RI varies from 1.32 to 1.33, the resolution reaches to its maximum of 8.3 × 10⁻⁶. The proposed sensor shows much significance in low RI detection, which is promising in real-time measurement for medical, water pollution, and humidity.

     

Surface Plasmon Resonance on the Antimonene–Fe2O3–Copper Layer for Optical Attenuated Total Reflection Spectroscopic Application

In this paper, we explore a highly sensitive surface plasmon resonance (SPR) structure. The configuration fabricated by the antimonene-Fe₂O₃-copper (Cu) is theoretically analyzed. Fe₂O₃ work as dielectric nanosheets to enhance the sensitivity. Besides, the x components of the electric field also can be improved. As promising two-dimensional (2D) material with a stronger interaction with biomolecules and higher chemical stability, antimonene exhibits potential applications in sensing. By optimizing the configuration parameters, the highest angular sensitivity of 398°/RIU. The result displays that the sensitivity is enhanced by 79.3% compared with the conventional configuration with a single Cu film. We hope that the simple configuration will find the suitable application value.

     

Combination of Digital Image Processing and Statistical Data Segmentation to Enhance SPR and SPRi Sensor Responses

The work reports the combination of basic digital image processing (DIP) techniques and statistical segmentation strategy (SDS) to improve surface plasmon resonance curve (SPRc) and SPR imaging (SPRi) sensors' performance. The SPR image is used for sensing and monitoring biological events in the so-called SPR imaging process. In the traditional SPR process based on the attenuated total reflection (ATR) method, the image is used to create the SPR curve, and the curve features tracking is employed on sensing applications. The SPR curve features are enhanced after the pixels of the SPR image have been processed with low-complexity filters in the spatial domain (brightness, contrast, threshold, and morphological). The bootstrap was used as a statistical processing approach, selecting lines and columns from the image that was less affected by imperfections and noises in the image detector, and consequently reducing the SPR sensor instrumentation disturbances. Experimental tests with reversible binding water-mixture were performed, and both image and statistical processing were reported. The combination of DIP and SDS approaches improves the extraction of the curve features, increasing the performance in terms of resonance position sensitivity to 81%.

     

Inverse identification of region-specific hyperelastic material parameters for human brain tissue

The identification of material parameters accurately describing the region-dependent mechanical behavior of human brain tissue is crucial for computational models used to assist, e.g., the development of safety equipment like helmets or the planning and execution of brain surgery. While the division of the human brain into different anatomical regions is well established, knowledge about regions with distinct mechanical properties remains limited. Here, we establish an inverse parameter identification scheme using a hyperelastic Ogden model and experimental data from multi-modal testing of tissue from 19 anatomical human brain regions to identify mechanically distinct regions and provide the corresponding material parameters. We assign the 19 anatomical regions to nine governing regions based on similar parameters and microstructures. Statistical analyses confirm differences between the regions and indicate that at least the corpus callosum and the corona radiata should be assigned different material parameters in computational models of the human brain. We provide a total of four parameter sets based on the two initial Poisson’s ratios of 0.45 and 0.49 as well as the pre- and unconditioned experimental responses, respectively. Our results highlight the close interrelation between the Poisson’s ratio and the remaining model parameters. The identified parameters will contribute to more precise computational models enabling spatially resolved predictions of the stress and strain states in human brains under complex mechanical loading conditions.

     

The Effect of Molecular Adsorption on Electro-Optical Properties of Graphene-Based Sensors

Extraordinary electrical and optical features of graphene-based materials attract researchers to improve sensing center of different sensors using them. In this research, the effects of sensing molecules on electro-optical features of graphene-based sensors are modeled. The adsorption effect on the Hamiltonian of the system based on tight-binding model is explored, and also the system band structure is investigated analytically. Then, refractive index deviations based on band gap variations are discovered which are used in response modeling of a graphene-based surface plasmon resonance (SPR) sensor.

     

Detection of Staphylococcus aureus enterotoxin B at femtomolar levels with a miniature integrated two-channel surface plasmon resonance (SPR) sensor

Surface plasmon resonance (SPR) biosensors offer the capability for continuous real-time monitoring. The commercial instruments available have been large in size, expensive, and not amenable to field applications. We report here an SPR sensor system based on a prototype two-channel system similar to the single channel Spreeta devices. This system is an ideal candidate for field use. The two-channel design provides a reference channel to compensate for bulk refractive index (RI), non-specific binding and temperature variations. The SPR software includes a calibration function that normalizes the response from both channels, thus enabling accurate referencing. In addition, a temperature-controlled enclosure utilizing a thermo-electric module based on the Peltier effect provides the temperature stability necessary for accurate measurements of RI. The complete SPR sensor system can be powered by a 12V battery. Pre-functionalized, disposable, gold-coated thin glass slides provide easily renewable sensor elements for the system. Staphylococcus aureus enterotoxin B (SEB), a small protein toxin was directly detectable at sub-nanomolar levels and with amplification at femtomolar levels. A regeneration procedure for the sensor surface allowed for over 60 direct detection cycles in a 1-month period.     

Steering of Guided Light with Graphene Metasurface for Refractive Index Sensing with High Figure of Merits

The steering of guided light in surface plasmon resonance (SPR) sensing platforms introduced more than eight decades ago from the first proposed optical sensor in 1983. However, sensing the environmental variation considering transverse modes is still require the attention from the scientist. Here, for the first time, by considering steering of guided light a high-performance SPR sensor base on Otto structure is proposed. By incorporating the graphene and white graphene in to a prism-waveguide configuration, we calculated the excitation of both TE(TM) modes as refractive index is changed from 1 to 1.04. to analysis of the structure finite-difference time-domain (FDTD) is applied. To benchmark of the structure performance parameters including sensitivity, figure of merit, polarization extinction ratio (PER), and insertion loss (IL) are calculated. Numerical results show that maximum sensitivity and figure of merit are obtained for TM modes of 1226 and 27 respectively. In such a case, graphene monolayer is applied. By considering coupling condition, at the μc?=?0.4 eV, the maximum value of PER is 75 dB, and IL is 0.022 dB. Moreover, it is obtained that in all these conditions PER is higher than 8 dB, and IL is less than 0.04 dB.

     

SPR Label-Free Biosensor with Oxide-Metal-Oxide-Coated D-Typed Optical Fiber: a Theoretical Study

In this article, a surface plasmon resonance (SPR) biosensor based on D-typed optical fiber coated by Al₂O₃/Ag/Al₂O₃ film is investigated numerically. Resonance in near infrared with an optimized architecture is achieved. Refractive index sensitivity of 6558 nm/RIU (refractive index unit) and detection limit of 1.5 × 10⁻⁶ RIU, corresponding to 0.4357 nm/μM and detection limit of 23 nM in BSA (bovine serum albumin) concentration sensing, are obtained. The analysis of the performance of the sensor in gaseous sensing indicates that this proposed SPR sensor is much suitable for label-free biosensing in aqueous media.

     

A Computational Study of the Giant Local Electric Field Enhancement in Al-Au-Ag Trimetallic Three-Layered Nanoshells

The surface plasmon resonance (SPR)-induced local field effect in Al-Au-Ag trimetallic three-layered nanoshells has been studied theoretically. Because of having three kinds of metal, three plasmonic bands have been observed in the absorption spectra and the local electric field factor spectra. The local electric field enhancement and the corresponding resonance wavelength for different plasmon coupling modes and spatial positions of the Al-Au-Ag nanoshells with various geometry dimensions are investigated to find the maximum local electric field enhancement. The calculation results indicate that the giant local electric field enhancement could be stimulated by the plasmon coupling in the middle Au shell or the outer Ag shell and could be optimized by increasing the Ag shell thickness and decreasing the Au shell thickness. What is more, the local electric field enhancement also nonmonotonously depends on the dielectric constant of the environment; the local electric field intensity will be weakened when the surrounding dielectric constant is too small or too large.

     

A surface plasmon resonance probe with a novel integrated reference sensor surface

A surface plasmon resonance (SPR) sensor probe with integrated reference surface is described. In order to fabricate the integrated reference surface, two dielectric layers with different thickness were deposited on the single gold SPR sensor surface via plasma polymerization of hexamethyldisiloxane. The working sensor surface was a 34 nm dielectric layer with immobilized bovine serum albumin (BSA) antigen and an adjacent thin 1 nm dielectric layer without BSA provided reference surface. A specific immunoreaction of anti-BSA antibody was detected after immersion of the SPR probe into sample solution. Simultaneous observation of reference and working surface response enabled determination of the immunoreaction without the need for the baseline measurement. Moreover, compensation of nonspecific adsorption could be confirmed using anti-human serum albumin antibody.     

Metallic Grating-Assisted Fiber Optic SPR Sensor with Extreme Sensitivity in IR Region

We theoretically propose a surface plasmon resonance (SPR)-based fiber optic refractive index (RI) sensor. A surface plasmon exciting metallic grating formed with the alternation of indium tin oxide (ITO) and silver (Ag) stripes is considered on the core of the fiber. A thin film of silicon is used as an overlay. Silicon film not only protects the metallic grating from oxidation but also enhances the field to improve the device sensitivity. The sensor is characterized in terms of sensitivity, detection accuracy (DA), figure of merit (FoM), and quality factor (QF). The maximum sensitivity in the RI range 1.33 to 1.38 refractive index unit (RIU) is reported to be?~25 µm/RIU in infra-red region of investigation.

     

Platinum Layers Sandwiched Between Black Phosphorous and Graphene for Enhanced SPR Sensor Performance

Highly sensitive surface plasmon resonance (SPR) sensor consisting of Ag-Pt bimetallic films sandwiched with 2D materials black phosphorus (BP) and graphene over Pt layer in Kretschmann configuration is analyzed theoretically using the transfer matrix method. Numerical results show that upon suitable optimization of thickness of Ag-Pt layers and the number of layers of BP and graphene, sensitivity as high as 412°/RIU (degree/refractive index unit) can be achieved for p-polarized light of wavelength 633 nm. This performance can be tuned and controlled by changing the number of layers of BP and graphene. Furthermore, the addition of graphene and heterostructures of black phosphorus not only improved the sensitivity of the sensor but also kept the FWHM of the resonance curve much smaller than the conventional sensor utilizing Au as plasmonic metal and hence improved the resolution to a significant extent. We expect that this new proposed design will be useful for medical diagnosis, biomolecular detection, and chemical examination.

     

Real Time Measurements of Membrane Protein:Receptor Interactions Using Surface Plasmon Resonance (SPR)

Protein-protein interactions are pivotal to most, if not all, physiological processes, and understanding the nature of such interactions is a central step in biological research. Surface Plasmon Resonance (SPR) is a sensitive detection technique for label-free study of bio-molecular interactions in real time. In a typical SPR experiment, one component (usually a protein, termed ''ligand'') is immobilized onto a sensor chip surface, while the other (the ''analyte'') is free in solution and is injected over the surface. Association and dissociation of the analyte from the ligand are measured and plotted in real time on a graph called a sensogram, from which pre-equilibrium and equilibrium data is derived. Being label-free, consuming low amounts of material, and providing pre-equilibrium kinetic data, often makes SPR the method of choice when studying dynamics of protein interactions. However, one has to keep in mind that due to the method''s high sensitivity, the data obtained needs to be carefully analyzed, and supported by other biochemical methods. SPR is particularly suitable for studying membrane proteins since it consumes small amounts of purified material, and is compatible with lipids and detergents. This protocol describes an SPR experiment characterizing the kinetic properties of the interaction between a membrane protein (an ABC transporter) and a soluble protein (the transporter''s cognate substrate binding protein).     

Effect of variable periodontal ligament thickness and its non-linear material properties on the location of a tooth’s centre of resistance

In orthodontics, the 3D translational and rotational movement of a tooth is determined by the force–moment system applied and the location of the tooth’s centre of resistance (CR). Because of the practical constraints of in-vivo experiments, the finite element (FE) method is commonly used to determine the CR. The objective of this study was to investigate the geometric model details required for accurate CR determination, and the effect of material non-linearity of the periodontal ligament (PDL). A FE model of a human lower canine derived from a high-resolution µCT scan (voxel size: 50 µm) was investigated by applying four different modelling approaches to the PDL. These comprised linear and non-linear material models, each with uniform and realistic PDL thickness. The CR locations determined for the four model configurations were in the range 37.2–45.3% (alveolar margin: 0%; root apex: 100%). We observed that a non-linear material model introduces load-dependent results that are dominated by the PDL regions under tension. Load variation within the range used in clinical orthodontic practice resulted in CR variations below 0.3%. Furthermore, the individualized realistic PDL geometry shifted the CR towards the alveolar margin by 2.3% and 2.8% on average for the linear and non-linear material models, respectively. We concluded that for conventional clinical therapy and the generation of representative reference data, the least sophisticated modelling approach with linear material behaviour and uniform PDL thickness appears sufficiently accurate. Research applications that require more precise treatment monitoring and planning may, however, benefit from the more accurate results obtained from the non-linear constitutive law and individualized realistic PDL geometry.     

A comparison between a new model and current models for estimating trunk segment inertial parameters

Modeling of the body segments to estimate segment inertial parameters is required in the kinetic analysis of human motion. A new geometric model for the trunk has been developed that uses various cross-sectional shapes to estimate segment volume and adopts a non-uniform density function that is gender-specific. The goal of this study was to test the accuracy of the new model for estimating the trunk's inertial parameters by comparing it to the more current models used in biomechanical research. Trunk inertial parameters estimated from dual X-ray absorptiometry (DXA) were used as the standard. Twenty-five female and 24 male college-aged participants were recruited for the study. Comparisons of the new model to the accepted models were accomplished by determining the error between the models’ trunk inertial estimates and that from DXA. Results showed that the new model was more accurate across all inertial estimates than the other models. The new model had errors within 6.0% for both genders, whereas the other models had higher average errors ranging from 10% to over 50% and were much more inconsistent between the genders. In addition, there was little consistency in the level of accuracy for the other models when estimating the different inertial parameters. These results suggest that the new model provides more accurate and consistent trunk inertial estimates than the other models for both female and male college-aged individuals. However, similar studies need to be performed using other populations, such as elderly or individuals from a distinct morphology (e.g. obese). In addition, the effect of using different models on the outcome of kinetic parameters, such as joint moments and forces needs to be assessed.     

Tuning Plasmonic Properties of Truncated Gold Nanospheres by Coating

The influence of TiO₂ coating on resonant properties of gold nanoisland films deposited on silica substrates was studied numerically and in experiments. The model describing plasmonic properties of a metal truncated nanosphere placed on a substrate and covered by a thin dielectric layer has been developed. The model allows calculating a particle polarizability spectrum and, respectively, its surface plasmon resonance (SPR) wavelength for any given cover thickness, particle radius and truncation parameter, and dielectric functions of the particle, the substrate, the coating layer, and the surrounding medium. Dependence of the SPR position calculated for truncated gold nanospheres has coincided with the measured one for the gold nanoisland films covered with titania of different thicknesses. In the experiments, gold films with thickness of 5 nm were deposited on a silica glass substrate, annealed at 500 °C to form nanoislands of 20 nm in diameter, and covered with amorphous titania layers using atomic layer deposition technique. The resulting structures were characterized with scanning electron microscopy and optical absorption spectroscopy. The measured dependence of the SPR position on titania film thickness corresponded to the one calculated for truncated sphere-shaped nanoparticles with the truncation angle of ~50°. We demonstrated the possibility of tuning the SPR position within ~100 nm range by depositing to 30 nm thick titania layer.