Recent Advances in Plasmonic Sensor-Based Fiber Optic Probes for Biological Applications

Optical fiber sensors for posture monitoring, ulcer detection and control in a wheelchair: a state-of-the-art

BACKGROUND

In the last ten years, the design and implementation of Optical Fiber Sensors (OFS) in biomedical applications have been discussed, with a focus on different subareas, such as body parameter monitoring and control of assistive devices.

MATERIALS AND METHODS

A scoping review was performed including scientific literature (PubMed/Scopus, IEEE and Web of Science), patents (WIPO/Google Scholar), and commercial information.

RESULTS

The main applications of OFS in the rehabilitation field for preventing future postural diseases and applying them in device controllers were discussed in this review. Physical characteristics of OFS, different uses, and applications of Polymer Optical Fiber pressure sensors are mentioned. The main postures used for posture monitoring analysis when the user is sitting are normal position, crooked back, high lumbar pressure, sitting on the edge of the chair, and crooked back, left position, and right position. Additionally, it is possible to use Machine Learning (ML) algorithms for posture classification, and device control such as Support Vector Machine, k-Nearest Neighbors, etc., obtaining accuracies above 97%. Moreover, the literature mentions wheelchair controllers and Graphical User Interfaces using pressure maps to provide feedback to the user.

CONCLUSIONS

OFS have been used in several healthcare applications as well as postural and preventive applications. The literature showed an effort to implement and design accessible devices for people with disabilities and people with specific diseases. Alternatively, ML algorithms are widely used in this direction, leaving the door open for further studies that allow the application of real-time systems for posture monitoring and wheelchairs control.

Optical fiber immunosensors based on surface plasmon resonance for the detection of Escherichia coli

Every year, millions of people suffer some form of illness associated with the consumption of contaminated food. Escherichia coli (E. coli), found in the intestines of humans and other animals, is commonly associated with various diseases, due to the existence of pathogenic strains. Strict monitoring of food products for human consumption is essential to ensure public health, but traditional cell culture-based methods are associated with long waiting times and high costs. New approaches must be developed to achieve cheap, fast, and on-site monitoring. Thus, in this work, we developed optical fiber sensors based on surface plasmon resonance. Gold and cysteamine-coated fibers were functionalized with anti-E. coli antibody and tested using E. coli suspensions with concentrations ranging from 1 cell/mL to 105 cells/mL. An average logarithmic sensitivity of 0.21 ± 0.01 nm/log(cells/mL) was obtained for three independent assays. An additional assay revealed that including molybdenum disulfide resulted in an increase of approximately 50% in sensitivity. Specificity and selectivity were also evaluated, and the sensors were used to analyze contaminated water samples, which verified their promising applicability in the aquaculture field.

Recent Developments in the Detection of Organic Contaminants Using Molecularly Imprinted Polymers Combined with Various Analytical Techniques

Molecularly imprinted polymers (MIPs) encompass a diverse array of polymeric matrices that exhibit the unique capacity to selectively identify a designated template molecule through specific chemical moieties. Thanks to their pivotal attributes, including exceptional selectivity, extended shelf stability, and other distinct characteristics, this class of compounds has garnered interest in the development of highly responsive sensor systems. As a result, the incorporation of MIPs in crafting distinctive sensors and analytical procedures tailored for specific analytes across various domains has increasingly become a common practice within contemporary analytical chemistry. Furthermore, the range of polymers amenable to MIP formulation significantly influences the potential utilization of both conventional and innovative analytical methodologies. This versatility expands the array of possibilities in which MIP-based sensing can be employed in recognition systems. The following review summarizes the notable progress achieved within the preceding seven-year period in employing MIP-based sensing techniques for analyte determination.

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.

Plasmonic Biosensor on the End-Facet of a Dual-Core Single-Mode Optical Fiber

Optical biosensors target widespread applications, such as drug discovery, medical diagnostics, food quality control, and environmental monitoring. Here, we propose a novel plasmonic biosensor on the end-facet of a dual-core single-mode optical fiber. The concept uses slanted metal gratings on each core, interconnected by a metal stripe biosensing waveguide to couple the cores via the propagation of surface plasmons along the end facet. The scheme enables operation in transmission (core-to-core), thereby eliminating the need to separate the reflected light from the incident light. Importantly, this simplifies and reduces the cost of the interrogation setup because a broadband polarization-maintaining optical fiber coupler or circulator is not required. The proposed biosensor enables remote sensing because the interrogation optoelectronics can be located remotely. In vivo biosensing and brain studies are also enabled because the end-facet can be inserted into a living body, once properly packaged. It can also be dipped into a vial, precluding the need for microfluidic channels or pumps. Bulk sensitivities of 880 nm/RIU and surface sensitivities of 1 nm/nm are predicted under spectral interrogation using cross-correlation analysis. The configuration is embodied by robust and experimentally realizable designs that can be fabricated, e.g., using metal evaporation and focused ion beam milling.

Deposition of Graphene Oxide on an SPR Fiber Refractometer for Sensor Applications

Graphene-based materials have been increasingly incorporated to optical fiber plasmonic sensors due to the peculiar physical and chemical properties of these materials (hardness and flexibility, high electrical and thermal conductivity, and very good adsorption for many substances, etc.). In this paper, we theoretically and experimentally showed how the addition of graphene oxide (GO) to optical fiber refractometers permits the development of surface plasmon resonance (SPR) sensors with very good characteristics. We used doubly deposited uniform-waist tapered optical fibers (DLUWTs) as supporting structures because of their already proven good performance. The presence of GO as an effective third layer is useful to tune the wavelength of the resonances. In addition, the sensitivity was improved. We depict the procedures for the production of the devices and characterize the GO+DLUWTs produced in this way. We also showed how the experimental results are in agreement with the theoretical predictions and used these to estimate the thickness of deposited GO. Finally, we compared the performance of our sensors with other ones that have been recently reported, showing that our results are among the best reported. Using GO as the medium in contact with the analyte, in addition to the good overall performance of devices, permit consideration of this option as an interesting possibility for the future development of SPR-based fiber sensors.

Plasmonic Sensors beyond the Phase Matching Condition: A Simplified Approach

The conventional approach to optimising plasmonic sensors is typically based entirely on ensuring phase matching between the excitation wave and the surface plasmon supported by the metallic structure. However, this leads to suboptimal performance, even in the simplest sensor configuration based on the Otto geometry. We present a simplified coupled mode theory approach for evaluating and optimizing the sensing properties of plasmonic waveguide refractive index sensors. It only requires the calculation of propagation constants, without the need for calculating mode overlap integrals. We apply our method by evaluating the wavelength-, device length- and refractive index-dependent transmission spectra for an example silicon-on-insulator-based sensor of finite length. This reveals all salient spectral features which are consistent with full-field finite element calculations. This work provides a rapid and convenient framework for designing dielectric-plasmonic sensor prototypes-its applicability to the case of fibre plasmonic sensors is also discussed.

Plasmonic Refractive Index and Temperature Sensor Based on Graphene and LiNbO3

A high-efficiency dual-purpose plasmonic perfect absorber sensor based on LiNbO₃ and graphene layers was investigated in this paper for the refractive index and thermal sensing. The sensor design was kept simple for easy fabrication, comprising a LiNbO₃ substrate with a quartz layer, thin layer of graphene, four gold nanorods, and a nanocavity in each unit cell. The nanocavity is located in the middle of the cell to facilitate the penetration of EM energy to the subsurface layers. The proposed sensor design achieved an output response of 99.9% reflection, which was easy to detect without having any specialized conditions for operability. The performance of the device was numerically investigated for the biomedical refractive index range of 1.33 to 1.40, yielding a sensitivity value of 981 nm/RIU with a figure-of-merit of 61.31 RIU^-1. By including an additional polydimethylsiloxane polymer functional layer on the top, the device was also tested as a thermal sensor, which yielded a sensitivity level of -0.23 nm/°C.

Green-Graphene Protective Overlayer on Optical Microfibers: Prolongs the Device Lifetime

Optical microfibers find new applications in various fields of industry, which in turn require wear resistance, environmental friendliness and ease of use. However, optical microfibers are fragile. Here we report a new method to prolong the microfiber lifetime by modifying its surface with green-extracted graphene overlayers. Graphene films were obtained by dispergation of shungite mineral samples in an aqueous medium. For this, we tapered optical fibers and sculptured them with graphene films mixed with gold nanoparticles. We observed that due to the surface modification the lifetime and survivability of the microfiber increased 5 times, as compared to the bare microfiber. The embedded gold nanoparticles can also be utilized for enhanced sensitivity and other applications.

An LSPR Sensor Integrated with VCSEL and Microfluidic Chip

The work introduces a localized surface plasmon resonance (LSPR) sensor chip integrated with vertical-cavity surface-emitting lasers (VCSELs). Using VCSEL as the light source, the hexagonal gold nanoparticle array was integrated with anodic aluminum oxide (AAO) as the mask on the light-emitting end face. The sensitivity sensing test of the refractive index solution was realized, combined with microfluidic technology. At the same time, the finite-difference time- domain (FDTD) algorithm was applied to model and simulate the gold nanostructures. The experimental results showed that the output power of the sensor was related to the refractive index of the sucrose solution. The maximum sensitivity of the sensor was 1.65 × 106 nW/RIU, which gives it great application potential in the field of biomolecular detection.

Smartphone-based Surface Plasmon Resonance Sensors: a Review

The surface plasmon resonance (SPR) is a phenomenon based on the combination of quantum mechanics and electromagnetism, which leads to the creation of charge oscillations on a metal-dielectric interface. The SPR phenomenon creates a signal which measures refractive index change at the metal-dielectric interface. SPR-based sensors are being developed for real-time and label-free detection of water pollutants, toxins, disease biomarkers, etc., which are highly sensitive and selective. Smartphones provide hardware and software capability which can be incorporated into SPR sensors, enabling the possibility of economical and accurate on-site portable sensing. The camera, screen, and LED flashlight of the smartphone can be employed as components of the sensor. The current article explores the recent advances in smartphone-based SPR sensors by studying their principle, components, application, and signal processing. Furthermore, the general theoretical and practical aspects of SPR sensors are discussed.

Development of a surface plasmon resonance sensor using an optical fiber prepared by electroless displacement gold plating and its application to immunoassay

A simple and low-cost method of fabricating an optical fiber for a surface plasmon resonance (SPR) sensor was proposed. The method is based on the electroless nickel plating and subsequent displacement gold plating of the core of the optical fiber. The thickness of the nickel and gold thin films deposited on the core of the optical fiber could be controlled by measuring the reflected light intensity from the tip of the optical fiber during the plating processes. The sensitivity and resolution of the SPR sensor with the fabricated optical fiber in the refractive index range from 1.333 to 1.348 were 1324.3 nm/RIU and 7.6 × 10^-4 RIU, respectively. The developed SPR sensor was successfully used in the determination of immunoglobulin A (IgA) in human saliva. The IgA quantification results obtained by the SPR sensor were in excellent agreement with those obtained by conventional enzyme-linked immunosorbent assay using a 96-well microtiter plate.

Highly sensitive refractive index sensor based on plastic optical fiber balloon structure

A novel, to the best of our knowledge, design of plastic optical fiber (POF) balloon-based refractive index sensor for the detection of different concentrations of sodium chloride is proposed and experimentally investigated. The experimental characterization supports the finding that the transmission loss is sensitive to the external environment's targeted refractive index changes of the analyte. The proposed sensor achieves a maximum intensity-based sensitivity of 3105 RIU^-1, resolution of 3.22 ×10^-7, and the figure of merit (FOM) is 326 RIU^-1 from 2 to 2.5 Mol/L of the analyte with the chosen refractive index changes at 680 nm for a diameter D = 0.1 cm of the POF balloon structure. Furthermore, a high linear performance of 0.9896 is achieved with good robustness against the fabrication imperfection. The ultra-sensitiveness to the refractive index with a simple demonstration of the POF balloon-based structure has potential applications in the chemical, biological, and food safety sensing fields.

Relevance of the Spectral Analysis Method of Tilted Fiber Bragg Grating-Based Biosensors: A Case-Study for Heart Failure Monitoring

Optical fiber technology has rapidly progressed over the years, providing valuable benefits for biosensing purposes such as sensor miniaturization and the possibility for remote and real-time monitoring. In particular, tilted fiber Bragg gratings (TFBGs) are extremely sensitive to refractive index variations taking place on their surface. The present work comprises a case-study on the impact of different methods of analysis applied to decode spectral variations of bare and plasmonic TFBGs during the detection of N-terminal B-type natriuretic peptide (NT-proBNP), a heart failure biomarker, namely by following the most sensitive mode, peaks of the spectral envelopes, and the envelopes’ crossing point and area. Tracking the lower envelope resulted in the lowest limits of detection (LOD) for bare and plasmonic TFBGs, namely, 0.75 ng/mL and 0.19 ng/mL, respectively. This work demonstrates the importance of the analysis method on the outcome results, which is crucial to attain the most reliable and sensitive method with lower LOD sensors. Furthermore, it makes the scientific community aware to take careful attention when comparing the performance of different biosensors in which different analysis methods were used.

Enhancement of the SPR Effect in an Optical Fiber Device Utilizing a Thin Ag Layer and a 3092A Liquid Crystal Mixture

This paper is a continuation of previous work and shows the enhancement of the surface plasmon resonance effect in a tapered optical fiber device. The study investigated liquid crystal cells containing a tapered optical fiber covered with a silver nanolayer, surrounded by a low refractive index liquid crystal in terms of the properties of light propagation in the taper structure. Silver films with a thickness of d = 10 nm were deposited on the tapered waist area. Measurements were performed at room temperature; liquid crystal steering voltage U from 0 to 200 V, with and without any amplitude modulation with a frequency of f = 5 Hz, and the wavelength λ ranged from 550 to 1200 nm. A significant influence of the initial arrangement of liquid crystals molecules on light propagation was observed. Three types of liquid crystal cells—orthogonal, parallel, and twist—were considered. During the measurements, resonant peaks were obtained—the position of which can also be controlled by the type of liquid crystal cells and the steering voltage. Based on the obtained results, the best parameters, such as highest peak’s width reduction, and the highest SNR value were received for twisted cells. In addition, the present work was compared with the previous work and showed the possibility of improving properties of the manufactured probes, and consequently, the surface plasmon resonance effect. In the presented paper, the novelty is mainly focused on the used materials as well as suitable changes in applied technological parameters. In contrast to gold, silver is characterized by different optic and dielectric properties, e.g., refractive index, extension coefficient, and permittivity, which results in changes in the light propagation and the SPR wavelengths.

(INVITED)Quantitative detection of SARS-CoV-2 virions in aqueous mediums by IoT optical fiber sensors

The COVID-19 pandemic has emphasized the need for portable, small-size, low-cost, simple to use, and highly sensitive sensors able to measure a specific substance, with the capability of the transmission over the Internet of statistical data, such as in this specific case on the spread of the SARS-CoV-2 virions. Moreover, to resolve the COVID-19 emergency, the possibility of making selective SARS-CoV-2 measurements in different aqueous matrices could be advantageous. Thus, the realization of rapid and innovative point-of-care diagnostics tests has become a global priority. In response to the current need for quick, highly sensitive and on-site detection of the SARS-CoV-2 virions in different aqueous solutions, two different nanolayer biorecognition systems separately combined with an adaptable optical fiber sensor have been reported in this work. More specifically, two SARS-CoV-2 sensors have been developed and tested by exploiting a plasmonic plastic optical fiber (POF) sensor coupled with two different receptors, both designed for the specific recognition of the SARS-CoV-2 Spike protein; one is aptamer-based and the other one Molecular Imprinted Polymer-based. The preliminary tests on SARS-CoV-2 virions, performed on samples of nasopharyngeal (NP) swabs in universal transport medium (UTM), were compared with data obtained using reverse-transcription polymerase chain reaction (RT-PCR). According to these preliminary experimental results obtained exploiting both receptors, the sensitivity of the proposed SARS-CoV-2 optical fiber sensors proved to be high enough to detect virions. Furthermore, a relatively fast response time (a few minutes) to detect virions was obtained without additional reagents, with the capability to transmit the data via the Internet automatically.

Surface-Enhanced Raman Sensing of Semi-Volatile Organic Compounds by Plasmonic Nanostructures

Facile detection of indoor semi-volatile organic compounds (SVOCs) is a critical issue to raise an increasing concern to current researchers, since their emissions have impacted the health of humans, who spend much of their time indoors after the recent incessant COVID-19 pandemic outbreaks. Plasmonic nanomaterial platforms can utilize an electromagnetic field to induce significant Raman signal enhancements of vibrational spectra of pollutant molecules from localized hotspots. Surface-enhanced Raman scattering (SERS) sensing based on functional plasmonic nanostructures has currently emerged as a powerful analytical technique, which is widely adopted for the ultra-sensitive detection of SVOC molecules, including phthalates and polycyclic aromatic hydrocarbons (PAHs) from household chemicals in indoor environments. This concise topical review gives updated recent developments and trends in optical sensors of surface plasmon resonance (SPR) and SERS for effective sensing of SVOCs by functionalization of noble metal nanostructures. Specific features of plasmonic nanomaterials utilized in sensors are evaluated comparatively, including their various sizes and shapes. Novel aptasensors-assisted SERS technology and its potential application are also introduced for selective sensing. The current challenges and perspectives on SERS-based optical sensors using plasmonic nanomaterial platforms and aptasensors are discussed for applying indoor SVOC detection.

Immunosensing Based on Optical Fiber Technology: Recent Advances

The evolution of optical fiber technology has revolutionized a variety of fields, from optical transmission to environmental monitoring and biomedicine, given their unique properties and versatility. For biosensing purposes, the light guided in the fiber core is exposed to the surrounding media where the analytes of interest are detected by different techniques, according to the optical fiber configuration and biofunctionalization strategy employed. These configurations differ in manufacturing complexity, cost and overall performance. The biofunctionalization strategies can be carried out directly on bare fibers or on coated fibers. The former relies on interactions between the evanescent wave (EW) of the fiber and the analyte of interest, whereas the latter can comprise plasmonic methods such as surface plasmon resonance (SPR) and localized SPR (LSPR), both originating from the interaction between light and metal surface electrons. This review presents the basics of optical fiber immunosensors for a broad audience as well as the more recent research trends on the topic. Several optical fiber configurations used for biosensing applications are highlighted, namely uncladded, U-shape, D-shape, tapered, end-face reflected, fiber gratings and special optical fibers, alongside practical application examples. Furthermore, EW, SPR, LSPR and biofunctionalization strategies, as well as the most recent advances and applications of immunosensors, are also covered. Finally, the main challenges and an outlook over the future direction of the field is presented.

Ultrasensitive nanoplasmonic biosensor based on interferometric excitation of multipolar plasmonic modes

We propose a nanoplasmonic interferometric biosensor, which exploits the selective excitation of multipolar plasmonic modes in a nanoslit to provide a novel scheme for highly-sensitive biosensing. In this design, two counter-propagating surface plasmon polaritons interfere at the location of the nanoslit, selectively exciting the dipolar and quadrupolar modes of the structure depending on the phase relationship induced by the analyte. The contrasting radiation patterns produced by these modes result in large changes in the angular distribution of the transmitted light that depends on the analyte concentration. The resultant far-field is numerically modeled and the sensing performance of the structure is assessed, resulting in maximum bulk and surface sensitivities of S_B = 1.12 × 10⁵ deg/RIU and S_S = 302 deg/RIU, respectively, and a bulk-sensing resolution of the order of 10^-8 RIU. The design allows ample control over the trade-off between operating range and resolution through the slit's width, making this platform suitable for a broad range of sensing requirements.

Development of a U-bent plastic optical fiber biosensor with plasmonic labels for the detection of chikungunya non-structural protein 3

This study presents a novel plasmonic fiber optic sandwich immunobiosensor for the detection of chikungunya, an infectious mosquito-borne disease with chronic musculoskeletal pain and acute febrile illness, by exploiting non-structural protein 3 (CHIKV-nsP3) as a biomarker. A plasmonic sandwich immunoassay for CHIKV-nsP3 was realized on the surface of a compact U-bent plastic optical fiber (POF, 0.5 mm core diameter) with gold nanoparticles (AuNPs) as labels. The high evanescent wave absorbance (EWA) sensitivity of the U-bent probes allows the absorption of the light passing through the fiber by the AuNP labels, upon the formation of a sandwich immunocomplex of CHIKV-nsP3 on the core surface of the U-bent probe region. A simple optical set-up with a low-cost green LED and a photodetector on either end of the U-bent probe gave rise to a detection limit of 0.52 ng mL-1 (8.6 pM), and a linear range of 1-104 ng mL-1 with a sensitivity of 0.1043A530 nm/log(CnsP3). In addition, the plasmonic POF biosensor shows strong specificity towards the CHIKV-nsP3 analyte in comparison with Pf-HRP2, HIgG, and dengue whole virus. The results illustrate the potential of plasmonic POF biosensors for direct and sensitive point-of-care detection of the chikungunya viral disease.

Biosensing Applications Using Nanostructure-Based Localized Surface Plasmon Resonance Sensors

Localized surface plasmon resonance (LSPR)-based biosensors have recently garnered increasing attention due to their potential to allow label-free, portable, low-cost, and real-time monitoring of diverse analytes. Recent developments in this technology have focused on biochemical markers in clinical and environmental settings coupled with advances in nanostructure technology. Therefore, this review focuses on the recent advances in LSPR-based biosensor technology for the detection of diverse chemicals and biomolecules. Moreover, we also provide recent examples of sensing strategies based on diverse nanostructure platforms, in addition to their advantages and limitations. Finally, this review discusses potential strategies for the development of biosensors with enhanced sensing performance.

IP-Dip-Based SPR Structure for Refractive Index Sensing of Liquid Analytes

In this paper, we present a two-dimensional surface plasmon resonance structure for refractive index sensing of liquid analytes. The polymer structure was designed with a period of 500 nm and prepared in a novel IP-Dip polymer by direct laser writing lithography based on a mechanism of two-photon absorption. The sample with a set of prepared IP-Dip structures was coated by 40 nm thin gold layer. The sample was encapsulated into a prototyped chip with inlet and outlet. The sensing properties were investigated by angular measurement using the prepared solutions of isopropyl alcohol in deionized water of different concentrations. Sensitivity of 478-617 nm per refractive index unit was achieved in angular arrangement at external angle of incidence of 20°.

Proof of Concept for a Quick and Highly Sensitive On-Site Detection of SARS-CoV-2 by Plasmonic Optical Fibers and Molecularly Imprinted Polymers

The rapid spread of the Coronavirus Disease 2019 (COVID-19) pandemic, caused by the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) pathogen has generated a huge international public health emergency. Currently the reference diagnostic technique for virus determination is Reverse Transcription Polymerase Chain Reaction (RT-PCR) real time analysis that requires specialized equipment, reagents and facilities and typically 3-4 h to perform. Thus, the realization of simple, low-cost, small-size, rapid and point-of-care diagnostics tests has become a global priority. In response to the current need for quick, highly sensitive and on-site detection of the SARS-CoV-2 virus in several aqueous solutions, a specific molecularly imprinted polymer (MIP) receptor has been designed, realized, and combined with an optical sensor. More specifically, the proof of concept of a SARS-CoV-2 sensor has been demonstrated by exploiting a plasmonic plastic optical fiber sensor coupled with a novel kind of synthetic MIP nano-layer, especially designed for the specific recognition of Subunit 1 of the SARS-CoV-2 Spike protein. First, we have tested the effectiveness of the developed MIP receptor to bind the Subunit 1 of the SARS-CoV-2 spike protein, then the results of preliminary tests on SARS-CoV-2 virions, performed on samples of nasopharyngeal (NP) swabs in universal transport medium (UTM) and physiological solution (0.9% NaCl), were compared with those obtained with RT-PCR. According to these preliminary results, the sensitivity of the proposed optical-chemical sensor proved to be higher than the RT-PCR one. Furthermore, a relatively fast response time (about 10 min) to the virus was obtained without the use of additional reagents.

SPR Effect Controlled by an Electric Field in a Tapered Optical Fiber Surrounded by a Low Refractive Index Nematic Liquid Crystal

This paper presents the influence of a thin metal layer deposition on the surface of a tapered optical fiber surrounded by a low liquid crystal, on light propagation inside the taper structure. In this research, three types of liquid crystal cells were under investigation: orthogonal, parallel, and twist. They differed by the rubbing direction of the electrodes in relation to the fiber axis determining the initial molecule arrangement inside the cell. Gold films with thickness d = 30 nm were deposited on the tapered fiber surface in the tapered waist area. Cells including a tapered optical fiber with no metallic layer were also examined and presented as a reference. All measurements were performed at room temperature for a different steering voltage U from 0 to 200 V, with and without any amplitude modulation with a frequency f = 5 Hz, and the wavelength λ range from 550 to 1200 nm. As a result, the resonant peaks were obtained, which depends on a liquid crystal cell type and steering voltage, as well. This paper shows the possibility of sensing the change of applied voltage by the constructed system. During measurements, additional effects as signal overlapping and intermodal interference were observed reducing measured voltage value. In the future, the improved, similar systems that will have a better response could be used as a sensor of factors to which liquid crystal (LC) will be sensitive, especially temperature and electric field.

Detection of Bacterial and Viral Pathogens Using Photonic Point-of-Care Devices

Infectious diseases caused by bacteria and viruses are highly contagious and can easily be transmitted via air, water, body fluids, etc. Throughout human civilization, there have been several pandemic outbreaks, such as the Plague, Spanish Flu, Swine-Flu, and, recently, COVID-19, amongst many others. Early diagnosis not only increases the chance of quick recovery but also helps prevent the spread of infections. Conventional diagnostic techniques can provide reliable results but have several drawbacks, including costly devices, lengthy wait time, and requirement of trained professionals to operate the devices, making them inaccessible in low-resource settings. Thus, a significant effort has been directed towards point-of-care (POC) devices that enable rapid diagnosis of bacterial and viral infections. A majority of the POC devices are based on plasmonics and/or microfluidics-based platforms integrated with mobile readers and imaging systems. These techniques have been shown to provide rapid, sensitive detection of pathogens. The advantages of POC devices include low-cost, rapid results, and portability, which enables on-site testing anywhere across the globe. Here we aim to review the recent advances in novel POC technologies in detecting bacteria and viruses that led to a breakthrough in the modern healthcare industry.

A Refractive Index Sensitive Liquid Level Monitoring Sensor Based on Multimode Interference

According to the beam propagation method, a fiber refractive index-sensitive multimode interference (MMI) structure fabricated by splicing a self-made silica glass rod between two single mode fibers (SMF–NCF (no core fiber)–SMF structure) is proposed for liquid level monitoring. Theoretical and experimental investigation was carried out meticulously using a 4.5 cm and a 9.5 cm long silica glass rod. It is proved that the simple and economical sensor with the shorter length has high sensitivity, satisfactory repeatability, and favorable stability. The sensitivity climbs with the increase in refractive index of the measured liquid, which is 204 pm/mm for pure water, 265.8 pm/mm for 10% glycerin solution, and 352.5 pm/mm for 25% glycerin solution. The proposed sensor can be standardized in certain application circumstances to achieve accurate liquid level monitoring.

Surface Plasmon Resonance-Based Temperature Sensor with Outer Surface Metal Coating on Multi-Core Photonic Crystal Fibre

We report an innovative design of a multi-core photonic crystal fibre-based surface plasmon resonance temperature sensor using ethanol and benzene as temperature-sensitive materials with a segmented outer-surface metal coating scheme. A stable sensing performance for a detection range of 10–80 ∘ C was found while using ethanol as the temperature-sensitive material; while using benzene both blue and red frequency shifts were observed. The maximum temperature sensitivities obtained from this proposed temperature sensor were 360 pm/ ∘ C and 23.3 nm/ ∘ C with resolutions of 2.78 × 10 − 1 ∘ C and 4.29 × 10 − 3 ∘ C, respectively, when using ethanol or benzene as the sensing medium.

Surface Plasmon Resonance Sensor Based on Dual-Side Polished Microstructured Optical Fiber with Dual-Core

A surface plasmon resonance (SPR) sensor based on a dual-side polished microstructured optical fiber (MOF) with a dual core is proposed for a large analyte refractive index (RI; n_a) detection range. Gold is used as a plasmonic material coated on the polished surface, and analytes can be directly contacted with the gold film. The special structure not only facilitates the fabrication of the sensor, but also can work in the n_a range of 1.42–1.46 when the background material RI is 1.45, which is beyond the reach of other traditional MOF-SPR sensors. The sensing performance of the sensor was investigated by the wavelength and amplitude interrogation methods. The detailed numerical results showed that the proposed sensor can work effectively in the n_a range of 1.35–1.47 and exhibits higher sensitivity in the n_a range of 1.42–1.43.

Hybrid Plasmonic Fiber-Optic Sensors

With the increasing demand of achieving comprehensive perception in every aspect of life, optical fibers have shown great potential in various applications due to their highly-sensitive, highly-integrated, flexible and real-time sensing capabilities. Among various sensing mechanisms, plasmonics based fiber-optic sensors provide remarkable sensitivity benefiting from their outstanding plasmon-matter interaction. Therefore, surface plasmon resonance (SPR) and localized SPR (LSPR)-based hybrid fiber-optic sensors have captured intensive research attention. Conventionally, SPR- or LSPR-based hybrid fiber-optic sensors rely on the resonant electron oscillations of thin metallic films or metallic nanoparticles functionalized on fiber surfaces. Coupled with the new advances in functional nanomaterials as well as fiber structure design and fabrication in recent years, new solutions continue to emerge to further improve the fiber-optic plasmonic sensors' performances in terms of sensitivity, specificity and biocompatibility. For instance, 2D materials like graphene can enhance the surface plasmon intensity at the metallic film surface due to the plasmon-matter interaction. Two-dimensional (2D) morphology of transition metal oxides can be doped with abundant free electrons to facilitate intrinsic plasmonics in visible or near-infrared frequencies, realizing exceptional field confinement and high sensitivity detection of analyte molecules. Gold nanoparticles capped with macrocyclic supramolecules show excellent selectivity to target biomolecules and ultralow limits of detection. Moreover, specially designed microstructured optical fibers are able to achieve high birefringence that can suppress the output inaccuracy induced by polarization crosstalk and meanwhile deliver promising sensitivity. This review aims to reveal and explore the frontiers of such hybrid plasmonic fiber-optic platforms in various sensing applications.

A Large Detection-Range Plasmonic Sensor Based on An H-Shaped Photonic Crystal Fiber

An H-shaped photonic crystal fiber (PCF)-based surface plasmon resonance (SPR) sensor is proposed for detecting large refractive index (RI) range which can either be higher or lower than the RI of the fiber material used. The grooves of the H-shaped PCF as the sensing channels are coated with gold film and then brought into direct contact with the analyte, which not only reduces the complexity of the fabrication but also provides reusable capacity compared with other designs. The sensing performance of the proposed sensor is investigated by using the finite element method. Numerical results show that the sensor can work normally in the large analyte RI (n_a) range from 1.33 to 1.49, and reach the maximum sensitivity of 25,900 nm/RIU (RI units) at the n_a range 1.47–1.48. Moreover, the sensor shows good stability in the tolerances of ±10% of the gold-film thickness.

Analysis of a highly sensitive side polished hollow fiber plasmonic sensor and its application as a magnetometer

A highly sensitive hollow fiber (HF) sensor based on surface plasmon resonance is proposed and analyzed numerically. The sensor design involves a side polished HF with a nanoscale gold film deposited over the polished surface and an analyte filled core. The hollow portion of the fiber serves simultaneously as an analyte channel and core, when filled with high refractive index liquid. By analyzing the modal characteristics of the sensor using the finite element method, it is found that the wavelength sensitivity of the HF sensor varies from 25,642 nm/RIU (refractive index unit) to 60,000 nm/RIU in the range 1.45 to 1.47 RIU along with highest amplitude sensitivity of ${4231.7}\,\,{{\rm RIU}^{-1}}$4231.7RIU^-1. The minimum measurable alteration by the sensor lies in the order of ${{10}^{-6}}\,\,{\rm RIU}$10^-6RIU. The sensor also exhibits a high value of figure of merit (FOM) up to ${976}\,\,{{\rm RIU}^{-1}}$976RIU^-1 representing a very good overall performance. Moreover, due to the feasibility of design, the specific application of the sensor to magnetic field sensing is also demonstrated and achieved maximum sensitivity of 1361 pm/Oe.

Lossy Mode Resonance Generation on Sputtered Aluminum-Doped Zinc Oxide Thin Films Deposited on Multimode Optical Fiber Structures for Sensing Applications in the 1.55 µm Wavelength Range

In this work, we demonstrated lossy mode resonance (LMR) generation in optical fiber structures based on multimode fibers coated with aluminum-doped zinc oxide (AZO) films. AZO thin films were deposited by using radio frequency magnetron sputtering. In order to exhibit the usefulness of the LMR effect for sensing applications in optical fiber based systems, the deposition conditions of the AZO film coatings were set to obtain the second LMR order within the 1.55 µm wavelength range. An optical transmission configuration setup was used to investigate the LMR effect on fiber structures based on the use of no-core and cladding-removed multimode fibers coated with AZO films synthesized from metallic sputtering targets with different proportions of Zn:Al, 92:8% and 98:2%, at atomic concentrations. The optical and electrical/chemical features of the AZO films were characterized with UV–vis and XPS spectroscopy, respectively. The optical response of the proposed sensing configuration to refractive index (RI) variations was experimentally demonstrated. For the best approach, the sensitivity of wavelength displacement to RI variations on the liquid surrounding media was found to be 1214.7 nm/RIU.

A Temperature Plasmonic Sensor Based on a Side Opening Hollow Fiber Filled with High Refractive Index Sensing Medium

A surface plasmon resonance temperature sensor based on a side opening hollow-core microstructured optical fiber is proposed in this paper. This design employs a gold nanowire to excite the plasmon mode, and can be easily filled with the sensing medium through the side opening of the fiber, which not only simplifies the fabrication of the sensor but can also use the high refractive index sensing medium. The coupling characteristics, sensing performance and fabrication tolerance of the sensor are analyzed by using the finite element method. The simulation results indicate that the maximum sensitivity is 3.21 nm/°C for the x-polarized core mode in the temperature range of 13.27–50.99 °C, and 4.98 nm/°C for the y-polarized core mode in the temperature range of 14.55–51.19 °C, when benzene is used as the sensing medium. The sensor also shows a good stability in the range of ±10% fabrication tolerance.