Surface Plasmonic Sensors: Sensing Mechanism and Recent Applications

The Biopolymer Active Surface for Optical Fibre Sensors

Optical fibre sensors have the potential to be overly sensitive and responsive, making them useful in various applications to detect the presence of pollutants in the environment, toxic gasses, or pesticides in soil. Deoxyribonucleic acid (DNA) as biopolymer active surfaces for fibre sensors can be designed to detect specific molecules or compounds accurately. In the article, we propose to use an optical fibre taper and DNA complex with surfactant-based sensors to offer a promising approach for gas detection, including ammonia solution, 1,4 thioxane, and trimethyl phosphate imitating hazardous agents. The presented results describe the influence of the adsorption of evaporation of measured agents to the DNA complex layer on a light leakage outside the structure of an optical fibre taper. The DNA layer with additional gas molecules becomes a new cladding of the taper structure, with the possibility to change its properties. The process of adsorption causes a change in the layer's optical properties surrounding a taper-like refractive index and increasing layer diameter, which changes the boundary condition of the structure and interacts with light in a wide spectral range of 600-1200 nm. The research's novelty is implementing a DNA complex active surface as the biodegradable biopolymer alignment for optical devices like in-line fibre sensors and those enabled for hazardous agent detection for substances appearing in the environment as industrial or even warfare toxic agents.

MXene Supported Surface Plasmon Polaritons for Optical Microfiber Ammonia Sensing

The properties of surface plasmons are notoriously dependent on the supporting materials system. However, new capabilities cannot be obtained until the technique of surface plasmon enabled by advanced two-dimensional materials is well understood. Herein, we present the experimental demonstration of surface plasmon polaritons (SPPs) supported by single-layered MXene flakes (Ti3C2Tx) coating on an optical microfiber and its application as an ammonia gas sensor. Enabled by its high controllability of chemical composition, unique atomistically thin layered structure, and metallic-level conductivity, MXene is capable of supporting not only plasmon resonances across a wide range of wavelengths but also a selective sensing mechanism through frequency modulation. Theoretical modeling and optics experiments reveal that, upon adsorbing ammonia molecules, the free electron motion at the interface between the SiO2 microfiber and the MXene coating is modulated (i.e., the modulation of the SPPs under applied light), thus inducing a variation in the evanescent field. Consequently, a wavelength shift is produced, effectively realizing a selective and highly sensitive ammonia sensor with a 100 ppm detection limit. The MXene supported SPPs open a promising path for the application of advanced optical techniques toward gas and chemical analysis.

Side-Opened Hollow Fiber-Based SPR Sensor for High Refractive Index Detection

To facilitate the sensor fabrication and sensing operation in microstructured optical fiber-based surface plasmon resonance (SPR) sensors for high refractive index (RI) detection, we propose a special hollow fiber-based SPR sensor that comprises an opening on its body side and a thin gold layer coated on its outer surface. The analyte is able to flow into the hollow core through the side-opening to form new fiber core, with the Gaussian-like mode propagating in it. We investigate the sensing performance of the proposed sensor in a higher RI range of 1.48 to 1.54 at two feasible schemes: one is to only fill the fiber core with analyte (Scheme A), and the other is to directly immerse the sensor in the analyte (Scheme B). The results demonstrate that our sensor exhibits higher wavelength sensitivity at Scheme A with a maximum wavelength sensitivity of 12,320 nm/RIU, while a greater amplitude sensitivity was found at Scheme B with a maximum amplitude sensitivity of 1146 RIU-1. Our proposed sensor features the advantages of simple fabrication, flexible operation, easy analyte filling and replacing, enhanced real-time detection capabilities, high RI detection, and very high wavelength sensitivity and amplitude sensitivity, which makes it more competitive in SPR sensing applications.

Wavelength Division Multiplexing-Based High-Sensitivity Surface Plasmon Resonance Imaging Biosensor for High-Throughput Real-Time Molecular Interaction Analysis

In this study, we report the successful development of a novel high-sensitivity intensity-based Surface Plasmon Resonance imaging (SPRi) biosensor and its application for detecting molecular interactions. By optimizing the excitation wavelength and employing a wavelength division multiplexing (WDM) algorithm, the system can determine the optimal excitation wavelength based on the initial refractive index of the sample without adjusting the incidence angle. The experimental results demonstrate that the refractive index resolution of the system reaches 1.77×10-6 RIU. Moreover, it can obtain the optimal excitation wavelength for samples with an initial refractive index in the range of 1.333 to 1.370 RIU and accurately monitor variations within the range of 0.0037 RIU without adjusting the incidence angle. Additionally, our new SPRi technique realized real-time detection of high-throughput biomolecular binding processes, enabling analysis of kinetic parameters. This research is expected to advance the development of more accurate SPRi technologies for molecular interaction analysis.

Research progress of microfluidics-based food safety detection

High demands for food safety detection and analysis have been advocated with people's increasing living standards. Even though numerous analytical testing techniques have been proposed, their widespread adoption is still constrained by the high limit of detection, narrow detection ranges, and high implementation costs. Due to their advantages, such as reduced sample and reagent consumption, high sensitivity, automation, low cost, and portability, using microfluidic devices for food safety monitoring has generated significant interest. This review provides a comprehensive overview of the latest microfluidic detection platforms (published in recent 4 years) and their applications in food safety, aiming to provide references for developing efficient research strategies for food contaminant detection and facilitating the transition of these platforms from laboratory research to practical field use.

Novel Multifunctional Meta-Surface Plasmon Resonance Chip Microplate for High-Throughput Molecular Screening

The utilization of surface plasmon resonance (SPR) sensors for real-time label-free molecular interaction analysis is already being employed in the fields of in vitro diagnostics and biomedicine. However, the widespread application of SPR technology is hindered by its limited detection throughput and high cost. To address this issue, this study introduces a novel multifunctional MetaSPR high-throughput microplate biosensor featuring 3D nanocups array structure, aiming to achieve high-throughput screening with a reduced cost and enhanced speed. Different types of MetaSPR sensors and analytical detection methods have been developed for accurate antibody subtype identification, epitope binding, affinity determination, antibody collocation, and quantitative detection， greatly promoting the screening and analysis of early-stage antibody drugs. The MetaSPR platform combined with nano-enhanced particles amplifies the detection signal and improves the detection sensitivity, making it more convenient, sensitive, and efficient than traditional ELISA. The findings demonstrate that the MetaSPR biosensor is a new practical technology detection platform that can improve the efficiency of biomolecular interaction studies with unlimited potential for new drug development.

Localized Surface Plasmon Resonance Sensing and its Interplay with Fluidics

In this Feature Article, we discuss the interplay between fluidics and the localized surface plasmon resonance (LSPR) sensing technique, primarily focusing on its applications in the realm of bio/chemical sensing within fluidic environments. Commencing with a foundational overview of LSPR principles from a sensing perspective, we subsequently showcase the development of a streamlined LSPR chip integrated with microfluidic structures. This integration opens the doors to advanced experiments involving fluid dynamics, greatly expanding the scope of LSPR-based research. Our discussions then turn to the practical implementation of LSPR and microfluidics in real-time biosensing, with a specific emphasis on monitoring DNA polymerase activity. Additionally, we illustrate the direct sensing of biological fluids, exemplified by the analysis of urine, while also shedding light on a unique particle assembly process that occurs on LSPR chips. We not only discuss the significance of LSPR sensing but also explore its potential to investigate a plethora of phenomena at liquid-liquid and solid-liquid interfaces. This is particularly noteworthy, as existing transduction methods and sensors fall short in fully comprehending these interfacial phenomena. Concluding our discussion, we present a futuristic perspective that provides insights into potential opportunities emerging at the intersection of fluidics and LSPR sensing.

Optical color routing enabled by deep learning

Nano-color routing has emerged as an immensely popular and widely discussed subject in the realms of light field manipulation, image sensing, and the integration of deep learning. The conventional dye filters employed in commercial applications have long been hampered by several limitations, including subpar signal-to-noise ratio, restricted upper bounds on optical efficiency, and challenges associated with miniaturization. Nonetheless, the advent of bandpass-free color routing has opened up unprecedented avenues for achieving remarkable optical spectral efficiency and operation at sub-wavelength scales within the area of image sensing applications. This has brought about a paradigm shift, fundamentally transforming the field by offering a promising solution to surmount the constraints encountered with traditional dye filters. This review presents a comprehensive exploration of representative deep learning-driven nano-color routing structure designs, encompassing forward simulation algorithms, photonic neural networks, and various global and local topology optimization methods. A thorough comparison is drawn between the exceptional light-splitting capabilities exhibited by these methods and those of traditional design approaches. Additionally, the existing research on color routing is summarized, highlighting a promising direction for forthcoming development, delivering valuable insights to advance the field of color routing and serving as a powerful reference for future endeavors.

Plasmonic nanosensors for pharmaceutical and biomedical analysis

The detection and identification of clinical biomarkers with related sensitivity have become a source of considerable concern for biomedical analysis. There have been increasing efforts toward the development of single-molecule analytical platforms to overcome this concern. The latest developments in plasmonic nanomaterials include fascinating advances in energy, catalyst chemistry, optics, biotechnology, and medicine. Nanomaterials can be successfully applied to biomolecule and drug detection in plasmonic nanosensors for pharmaceutical and biomedical analysis. Plasmonic-based sensing technology exhibits high sensitivity and selectivity depending on surface plasmon resonance (SPR) or localized surface plasmon resonance (LSPR) phenomena. In this critical paper, we offer an overview of the methodology of the SPR, LSPR, surface-enhanced Raman scattering (SERS), surface-enhanced infrared absorption (SEIRA), surface-enhanced fluorescence (SEF), and plasmonic nanoplatforms advanced for pharmaceutical and biomedical applications. First of all, we present here a brief discussion of the above trends. We have devoted the last section to the explanation of SPR, LSPR, SERS, SEIRA, and SEF platforms, which have found a wide range of applications, and reviewed recent advances for biomedical and pharmaceutical analysis.

Graphene-based biosensors for detecting coronavirus: a brief review

The coronavirus (SARS-CoV-2) disease has affected the globe with 770 437 327 confirmed cases, including about 6 956 900 deaths, according to the World Health Organization (WHO) as of September 2023. Hence, it is imperative to develop diagnostic technologies, such as a rapid cost-effective SARS-CoV-2 detection method. A typical biosensor enables biomolecule detection with an appropriate transducer by generating a measurable signal from the sample. Graphene can be employed as a component for ultrasensitive and selective biosensors based on its physical, optical, and electrochemical properties. Herein, we briefly review graphene-based electrochemical, field-effect transistor (FET), and surface plasmon biosensors for detecting the SARS-CoV-2 target. In addition, details on the surface modification, immobilization, sensitivity and limit of detection (LOD) of all three sensors with regard to SARS-CoV-2 were reported. Finally, the point-of-care (POC) detection of SARS-CoV-2 using a portable smartphone and a wearable watch is a current topic of interest.

Silicon-tapered waveguide for mode conversion in metal-insulator-metal waveguide-based plasmonic sensor for refractive index sensing

In this study, we have undertaken a comprehensive numerical investigation of a refractive index sensor designed around a metal-insulator-metal (MIM) plasmonic waveguide. Our approach utilizes the finite element method to thoroughly analyze the sensor's performance. The sensor's configuration utilizes a ring resonator design, which has been slightly modified at the coupling segment. This modification enhances the efficiency of light coupling between a bus waveguide and the ring resonator, particularly at the resonance wavelength. This strategic adjustment significantly improves the device's extinction ratio, a critical factor in its functionality. Remarkably, the sensitivity of this sensor is determined to be approximately 1155.71 nm/RIU, while it possesses a figure of merit of 25.9. Furthermore, our study delves into the intricate mechanism governing the injection of light into the nanoscale MIM waveguide. We achieve this through the incorporation of silicon-tapered waveguides, which play a pivotal role in facilitating the transformation of a dielectric mode into a plasmonic mode, and vice versa. Ultimately, the findings of this research hold significant promise for advancing the field of plasmonic sensing systems based on MIM waveguide technology. The insights gained here pave the way for the practical realization and optimization of highly efficient and precise plasmonic sensors.

Inferring the Interfacial Reactivity of Gold Nanoparticles by Surface Plasmon Resonance Measurements

Gold nanoparticles (GNPs) require a functionalization step in most cases to be suitable for applications. Optimizing this step in order to maintain both the stability and the plasmonic properties of the GNPs is a demanding process. Indeed, multiple analyses are required to get sufficient information on the grafting rate and the stability of the obtained suspension, leading to material and time waste. In this study, we propose to investigate ligand reactivity on a gold surface with surface plasmon resonance (SPR) measurements as a way to simulate the reactivity in GNP suspensions. We consider two thiolated ligands in this work: thioglycolic acid (TA) and 6-mercaptohexanoic acid (MHA). These thiols are grafted using different conditions on GNPs (monitored by optical absorption) and on a gold surface (monitored by SPR) and the grafting efficiency and stability are compared. The same conclusions are reached in both cases regarding the best protocol to implement, namely, the thiol molecules should be introduced in a water solution at a low concentration. This demonstrates the suitability of SPR to predict the reactivity on a GNP surface.

Deep Learning for Optical Sensor Applications: A Review

Over the past decade, deep learning (DL) has been applied in a large number of optical sensors applications. DL algorithms can improve the accuracy and reduce the noise level in optical sensors. Optical sensors are considered as a promising technology for modern intelligent sensing platforms. These sensors are widely used in process monitoring, quality prediction, pollution, defence, security, and many other applications. However, they suffer major challenges such as the large generated datasets and low processing speeds for these data, including the high cost of these sensors. These challenges can be mitigated by integrating DL systems with optical sensor technologies. This paper presents recent studies integrating DL algorithms with optical sensor applications. This paper also highlights several directions for DL algorithms that promise a considerable impact on use for optical sensor applications. Moreover, this study provides new directions for the future development of related research.

Hybrid plasmonic metasurface as enhanced Raman hot-spots for pesticide detection at ultralow concentrations

A surface-enhanced Raman scattering (SERS) active metasurface composed of metallic nanohole arrays and metallic nanoparticles is developed. The metasurface can operate in aqueous environments, achieves an enhancement factor of 1.83 × 10⁹ for Rhodamine 6G, and enables the detection of malachite green at a concentation of 0.46 ppb.

Optical Processes behind Plasmonic Applications

Plasmonics is a revolutionary concept in nanophotonics that combines the properties of both photonics and electronics by confining light energy to a nanometer-scale oscillating field of free electrons, known as a surface plasmon. Generation, processing, routing, and amplification of optical signals at the nanoscale hold promise for optical communications, biophotonics, sensing, chemistry, and medical applications. Surface plasmons manifest themselves as confined oscillations, allowing for optical nanoantennas, ultra-compact optical detectors, state-of-the-art sensors, data storage, and energy harvesting designs. Surface plasmons facilitate both resonant characteristics of nanostructures and guiding and controlling light at the nanoscale. Plasmonics and metamaterials enable the advancement of many photonic designs with unparalleled capabilities, including subwavelength waveguides, optical nanoresonators, super- and hyper-lenses, and light concentrators. Alternative plasmonic materials have been developed to be incorporated in the nanostructures for low losses and controlled optical characteristics along with semiconductor-process compatibility. This review describes optical processes behind a range of plasmonic applications. It pays special attention to the topics of field enhancement and collective effects in nanostructures. The advances in these research topics are expected to transform the domain of nanoscale photonics, optical metamaterials, and their various applications.

Challenges and perspectives of multi-virus biosensing techniques: A review

In the context of globalization, individuals have an increased chance of being infected by multiple viruses simultaneously, thereby highlighting the importance of developing multiplexed devices. In addition to sufficient sensitivity and rapid response, multi-virus sensing techniques are expected to offer additional advantages including high throughput, one-time sampling for parallel analysis, and full automation with data visualization. In this paper, we review the optical, electrochemical, and mechanical platforms that enable multi-virus biosensing. The working mechanisms of each platform, including the detection principle, transducer configuration, bio-interface design, and detected signals, are reviewed. The advantages and limitations, as well as the challenges in implementing various detection strategies in real-life scenarios, were evaluated. Future perspectives on multiplexed biosensing techniques are critically discussed. Earlier access to multi-virus biosensors will efficiently serve for immediate pandemic control, such as in emerging SARS-CoV-2 and monkeypox cases.

Real-Time Ellipsometric Surface Plasmon Resonance Sensor Using Polarization Camera May Provide the Ultimate Detection Limit

Ellipsometric Surface Plasmon Resonance (SPR) sensors are known for their relatively simple optical configuration compared to interferometric and optical heterodyne phase interrogation techniques. However, most of the previously explored ellipsometric SPR sensors based on intensity measurements are limited by their real-time applications because phase or polarization shifts are conducted serially. Here we present an ellipsometric SPR sensor based on a Kretschmann-Raether (KR) diverging beam configuration and a pixelated microgrid polarization camera. The proposed methodology has the advantage of real-time and higher precision sensing applications. The short-term stability of the measurement using the ellipsometric parameters tanψ and cos(Δ) is found to be superior over direct SPR or intensity measurements, particularly with fluctuating sources such as laser diodes. Refractive index and dynamic change measurements in real-time are presented together with Bovine Serum Albumin (BSA)-anti-BSA antibody binding to demonstrate the potential of the developed sensor for biological sensing applications with a resolution of sub-nM and down to pM with additional optimization. The analysis shows that this approach may provide the ultimate detection limit for SPR sensors.

Progress in Plasmonic Sensors as Monitoring Tools for Aquaculture Quality Control

Aquaculture is an expanding economic sector that nourishes the world's growing population due to its nutritional significance over the years as a source of high-quality proteins. However, it has faced severe challenges due to significant cases of environmental pollution, pathogen outbreaks, and the lack of traceability that guarantees the quality assurance of its products. Such context has prompted many researchers to work on the development of novel, affordable, and reliable technologies, many based on nanophotonic sensing methodologies. These emerging technologies, such as surface plasmon resonance (SPR), localised SPR (LSPR), and fibre-optic SPR (FO-SPR) systems, overcome many of the drawbacks of conventional analytical tools in terms of portability, reagent and solvent use, and the simplicity of sample pre-treatments, which would benefit a more sustainable and profitable aquaculture. To highlight the current progress made in these technologies that would allow them to be transferred for implementation in the field, along with the lag with respect to the most cutting-edge plasmonic sensing, this review provides a variety of information on recent advances in these emerging methodologies that can be used to comprehensively monitor the various operations involving the different commercial stages of farmed aquaculture. For example, to detect environmental hazards, track fish health through biochemical indicators, and monitor disease and biosecurity of fish meat products. Furthermore, it highlights the critical issues associated with these technologies, how to integrate them into farming facilities, and the challenges and prospects of developing plasmonic-based sensors for aquaculture.

Plasmonic properties of aluminium nanowires in amorphous silicon

Plasmonic structures can help enhance optical activity in the ultraviolet (UV) region and therefore enhancing photocatalytic reactions and the detection of organic and biological species. Most plasmonic structures are composed of Ag or Au. However, producing structures small enough for optical activity in the UV region has proved difficult. In this study, we demonstrate that aluminium nanowires are an excellent alternative. We investigated the plasmonic properties of the Al nanowires as well as the optoelectronic properties of the surroundinga - Simatrix by combining scanning transmission electron microscopy imaging, electron energy loss spectroscopy and electrodynamic modelling. We have found that the Al nanowires have distinct plasmonic modes in the UV and far UV region, from 0.75 eV to 13 eV. In addition, simulated results found that the size and spacing of the Al nanowires, as well as the embedding material were shown to have a large impact on the type of surface plasmon energies that can be generated in the material. Using electromagnetic modelling, we have identified the modes and illustrated how they could be tuned further.

Wedged Fiber Optic Surface Plasmon Resonance Sensor for High-Sensitivity Refractive Index and Temperature Measurements

Here, we experimentally demonstrate a wedged fiber optic surface plasmon resonance (SPR) sensor enabling high-sensitivity temperature detection. The sensing probe has a geometry with two asymmetrical bevels, with one inclined surface coated with an optically thin film supporting propagating plasmons and the other coated with a reflecting metal film. The angle of incident light can be readily tuned through modifying the beveled angles of the fiber tip, which has a remarkable impact on the refractive index sensitivity of SPR sensors. As a result, we measure a high refractive index sensitivity as large as 8161 nm/RIU in a wide refractive index range of 1.333-1.404 for the optimized sensor. Furthermore, we carry out a temperature-sensitivity measurement by packaging the SPR probe into a capillary filled with n-butanol. This showed a temperature sensitivity reaching up to -3.35 nm/°C in a wide temperature range of 20 °C-100 °C. These experimental results are well in agreement with those obtained from simulations, thus suggesting that our work may be of significance in designing reflective fiber optic SPR sensing probes with modified geometries.

Comprehensive field pattern analysis for tailoring of reflectance in a hybrid subwavelength plasmonic grating refractive index sensor and its potential for noninvasive salivary glucose monitoring

A compact hybrid two-dimensional plasmonic subwavelength grating composed of gold and semiconductor ZnS is proposed. By implementing the finite-difference time-domain numerical technique, detailed field pattern analysis and reflectance characteristics of the grating structure are comprehensively investigated, tailored, and improved. An unfamiliar phenomenon of exponential decrease in resonance wavelength with an increase in groove width is observed, validated, and empirically modeled. This confirms that the reflectance resonance dip is because of the surface plasmon resonance in the grating structure, unlike the resonance dip obtained in the diffraction grating because of the Fabry-Perot resonance. A rigorous sensitivity analysis is performed for both generalized bulk and surface analyte detection. The surface sensitivity is observed to be 100.5 nm/RIU at dip 1 for 10-nm-surface analyte thickness. The bulk sensitivity for dip 1 and dip 2 was 104.3 nm/RIU and 800 nm/RIU, respectively. The refractive index range variation of dip 1 for the surface analyte is correlated with the refractive index of the blood by using the linear refractive index model and Gladstone-Dale law for blood. A linear regression analysis correlating blood glucose and salivary glucose with a surface analyte is used. The proposed sensor is observed to be promising for noninvasive salivary glucose monitoring with high surface sensitivity of 1.104 nm/mg/dl with a compact footprint of about 0.5µm×0.2µm in x-z dimensions.

Double Fano Resonance and Independent Regulation Characteristics in a Rectangular-like Nanotetramer Metasurface Structure

Fano resonance, which is based on a plasmonic metasurface, has many potential applications in various fields, such as biochemical sensors, slow light effect, and integrated optical circuits. In this study, a rectangular-like nanotetramer metasurface structure composed of four round-head nanorods was designed. The transmission spectrum, surface charge, and electrical field distributions of the proposed structure were simulated using the finite element method. A double Fano resonance profile was observed in the transmission spectrum. One of the Fano resonances was caused by the symmetry breaking and plasmon hybridization between the horizontal double rods, whereas the other resonance was due to the plasmonic modes' hybridization among four nanorods. These resonances could be independently tuned because of different formation mechanisms. The number of Fano resonances could be adjusted by changing the coupling distance between the horizontal and vertical rods. The results contributed to designing the highly sensitive sensors based on the plasmonic metasurface.

Optimization of Au:CuO Thin Films by Plasma Surface Modification for High-Resolution LSPR Gas Sensing at Room Temperature

In this study, thin films composed of gold nanoparticles embedded in a copper oxide matrix (Au:CuO), manifesting Localized Surface Plasmon Resonance (LSPR) behavior, were produced by reactive DC magnetron sputtering and post-deposition in-air annealing. The effect of low-power Ar plasma etching on the surface properties of the plasmonic thin films was studied, envisaging its optimization as gas sensors. Thus, this work pretends to attain the maximum sensing response of the thin film system and to demonstrate its potential as a gas sensor. The results show that as Ar plasma treatment time increases, the host CuO matrix is etched while Au nanoparticles are uncovered, which leads to an enhancement of the sensitivity until a certain limit. Above such a time limit for plasma treatment, the CuO bonds are broken, and oxygen is removed from the film's surface, resulting in a decrease in the gas sensing capabilities. Hence, the importance of the host matrix for the design of the LSPR sensor is also demonstrated. CuO not only provides stability and protection to the Au NPs but also promotes interactions between the thin film's surface and the tested gases, thereby improving the nanocomposite film's sensitivity. The optimized sensor sensitivity was estimated at 849 nm/RIU, which demonstrates that the Au-CuO thin films have the potential to be used as an LSPR platform for gas sensors.

Recent Advances in DNA Nanotechnology for Plasmonic Biosensor Construction

Since 2010, DNA nanotechnology has advanced rapidly, helping overcome limitations in the use of DNA solely as genetic material. DNA nanotechnology has thus helped develop a new method for the construction of biosensors. Among bioprobe materials for biosensors, nucleic acids have shown several advantages. First, it has a complementary sequence for hybridizing the target gene. Second, DNA has various functionalities, such as DNAzymes, DNA junctions or aptamers, because of its unique folded structures with specific sequences. Third, functional groups, such as thiols, amines, or other fluorophores, can easily be introduced into DNA at the 5′ or 3′ end. Finally, DNA can easily be tailored by making junctions or origami structures; these unique structures extend the DNA arm and create a multi-functional bioprobe. Meanwhile, nanomaterials have also been used to advance plasmonic biosensor technologies. Nanomaterials provide various biosensing platforms with high sensitivity and selectivity. Several plasmonic biosensor types have been fabricated, such as surface plasmons, and Raman-based or metal-enhanced biosensors. Introducing DNA nanotechnology to plasmonic biosensors has brought in sight new horizons in the fields of biosensors and nanobiotechnology. This review discusses the recent progress of DNA nanotechnology-based plasmonic biosensors.

Plasmonic Biosensors: Review

Biosensors have globally been considered as biomedical diagnostic tools required in abundant areas including the development of diseases, detection of viruses, diagnosing ecological pollution, food monitoring, and a wide range of other diagnostic and therapeutic biomedical research. Recently, the broadly emerging and promising technique of plasmonic resonance has proven to provide label-free and highly sensitive real-time analysis when used in biosensing applications. In this review, a thorough discussion regarding the most recent techniques used in the design, fabrication, and characterization of plasmonic biosensors is conducted in addition to a comparison between those techniques with regard to their advantages and possible drawbacks when applied in different fields.

Label-free detection of single nanoparticles with disordered nanoisland surface plasmon sensor

We report sensing of single nanoparticles using disordered metallic nanoisland substrates supporting surface plasmon polaritons (SPPs). Speckle patterns arising from leakage radiation of elastically scattered SPPs provide a unique fingerprint of the scattering microstructure at the sensor surface. Experimental measurements of the speckle decorrelation are presented and shown to enable detection of sorption of individual gold nanoparticles and polystyrene beads. Our approach is verified through bright-field and fluorescence imaging of particles adhering to the nanoisland substrate.

Polymeric Nanocomposites for Environmental and Industrial Applications

Polymeric nanocomposites (PNC) have an outstanding potential for various applications as the integrated structure of the PNCs exhibits properties that none of its component materials individually possess. Moreover, it is possible to fabricate PNCs into desired shapes and sizes, which would enable controlling their properties, such as their surface area, magnetic behavior, optical properties, and catalytic activity. The low cost and light weight of PNCs have further contributed to their potential in various environmental and industrial applications. Stimuli-responsive nanocomposites are a subgroup of PNCs having a minimum of one promising chemical and physical property that may be controlled by or follow a stimulus response. Such outstanding properties and behaviors have extended the scope of application of these nanocomposites. The present review discusses the various methods of preparation available for PNCs, including in situ synthesis, solution mixing, melt blending, and electrospinning. In addition, various environmental and industrial applications of PNCs, including those in the fields of water treatment, electromagnetic shielding in aerospace applications, sensor devices, and food packaging, are outlined.

High Sensitivity Surface Plasmon Resonance Sensor Based on Periodic Multilayer Thin Films

Surface plasmon resonance (SPR) biosensors consisting of alternate layers of silver (Ag) and TiO₂ thin film have been proposed as a high sensitivity biosensor. The structure not only prevents the Ag film from oxidation, but also enhances the field inside the structure, thereby improving the performance of the sensor. Genetic algorithm (GA) was used to optimize the proposed structure and its maximum angular sensitivity was 384°/RIU (refractive index unit) at the refractive index environment of 1.3425, which is about 3.12 times that of the conventional Ag-based biosensor. A detailed discussion, based on the finite difference time domain (FDTD) method, revealed that an enhanced evanescent field at the top layer–analyte region results in the ultra-sensitivity characteristic. We expect that the proposed structure can be a suitable biosensor for chemical detection, clinical diagnostics, and biological examination.

Differential Refractive Index Sensor Based on Coupled Plasmon Waveguide Resonance in the C-Band

We proposed a differential fiber-optic refractive index sensor based on coupled plasmon waveguide resonance (CPWR) in the C-band. The sensor head is a BK7 prism coated with ITO/Au/ITO/TiO₂ film. CPWR is excited on the film by the S-polarized components of an incident light. The narrow absorption peak of CPWR makes it possible to realize dual-wavelength differential intensity (DI) interrogation by using only one incident point. To implement DI interrogation, we used a DWDM component to sample the lights with central wavelengths of 1529.55 and 1561.42 nm from the lights reflected back by the sensor head. The intensities of the dual-wavelength lights varied oppositely within the measurement range of refractive index, thus, a steep slope was produced as the refractive index of the sample increased. The experimental results show that the sensitivity is 32.15/RIUs within the measurement range from 1.3584 to 1.3689 and the resolution reaches 9.3 × 10⁻⁶ RIUs. Benefiting from the single incident point scheme, the proposed sensor would be easier to calibrate in bio-chemical sensing applications. Moreover, this sensing method is expected to be applied to retro-reflecting SPR sensors with tapered fiber tip to achieve better resolution than wavelength interrogation.

Coherent Surface Plasmon Hole Burning via Spontaneously Generated Coherence

Surface plasmon (SP)—induced spectral hole burning (SHB) at the silver-dielectric interface is investigated theoretically. We notice a typical lamb dip at a selective frequency, which abruptly reduces the absorption spectrum of the surface plasmons polaritons (SPP). Introducing the spontaneous generated coherence (SGC) in the atomic medium, the slope of dispersion becomes normal. Additionally, slow SPP propagation is also noticed at the interface. The spectral hole burning dip is enhanced with the SGC effect and can be modified and controlled with the frequency and intensity of the driving fields. The SPP propagation length at the hole-burning region is greatly enhanced under the effect of SGC. A propagation length of the order of 600 µm is achieved for the modes, which is a remarkable result. The enhancement of plasmon hole burning under SGC will find significant applications in sensing technology, optical communication, optical tweezers and nano-photonics.