Overview of the characteristics of micro- and nano-structured surface plasmon resonance sensors

Voltage-induced modulation of interfacial ionic liquids measured using surface plasmon resonant grating nanostructures

We have used surface plasmon resonant metal gratings to induce and probe the dielectric response (i.e., electro-optic modulation) of ionic liquids (ILs) at electrode interfaces. Here, the cross-plane electric field at the electrode surface modulates the refractive index of the IL due to the Pockels effect. This is observed as a shift in the resonant angle of the grating (i.e., Δϕ), which can be related to the change in the local index of refraction of the electrolyte (i.e., Δnlocal). The reflection modulation of the IL is compared against a polar (D2O) and a non-polar solvent (benzene) to confirm the electro-optic origin of resonance shift. The electrostatic accumulation of ions from the IL induces local index changes to the gratings over the extent of electrical double layer (EDL) thickness. Finite difference time domain simulations are used to relate the observed shifts in the plasmon resonance and change in reflection to the change in the local index of refraction of the electrolyte and the thickness of the EDL. Simultaneously using the wavelength and intensity shift of the resonance enables us to determine both the effective thickness and Δn of the double layer. We believe that this technique can be used more broadly, allowing the dynamics associated with the potential-induced ordering and rearrangement of ionic species in electrode-solution interfaces.

Advancements and Perspectives in Optical Biosensors

Optical biosensors exhibit immense potential, offering extraordinary possibilities for biosensing due to their high sensitivity, reusability, and ultrafast sensing capabilities. This review provides a concise overview of optical biosensors, encompassing various platforms, operational mechanisms, and underlying physics, and it summarizes recent advancements in the field. Special attention is given to plasmonic biosensors and metasurface-based biosensors, emphasizing their significant performance in bioassays and, thus, their increasing attraction in biosensing research, positioning them as excellent candidates for lab-on-chip and point-of-care devices. For plasmonic biosensors, we emphasize surface plasmon resonance (SPR) and its subcategories, along with localized surface plasmon resonance (LSPR) devices and surface enhance Raman spectroscopy (SERS), highlighting their ability to perform diverse bioassays. Additionally, we discuss recently emerged metasurface-based biosensors. Toward the conclusion of this review, we address current challenges, opportunities, and prospects in optical biosensing. Considering the advancements and advantages presented by optical biosensors, it is foreseeable that they will become a robust and widespread platform for early disease diagnostics.

Surface plasmonic biosensors: principles, designs and applications

Recently, surface plasmon resonance (SPR) biosensors have been widely used in environmental monitoring, food contamination detection and diagnosing medical conditions due to their superior sensitivity, label-free detection and rapid analysis speed. This paper briefly elaborates on the development history of SPR technology and introduces SPR signal sensing principles. A summary of recent applications of SPR sensors in different fields is highlighted, including their figures of merit and limitations. Finally, the personal perspectives and future development trends about sensor preparation and design are discussed in detail, which may be critical for improving the performance of SPR sensors.

Quantitative Galactose Colorimetric Competitive Assay Based on Galactose Dehydrogenase and Plasmonic Gold Nanostars

We describe a competitive colorimetric assay that enables rapid and sensitive detection of galactose and reduced nicotinamide adenine dinucleotide (NADH) via colorimetric readouts and demonstrate its usefulness for monitoring NAD+-driven enzymatic reactions. We present a sensitive plasmonic sensing approach for assessing galactose concentration and the presence of NADH using galactose dehydrogenase-immobilized gold nanostars (AuNS-PVP-GalDH). The AuNS-PVP-GalDH assay remains turquoise blue in the absence of galactose and NADH; however, as galactose and NADH concentrations grow, the reaction well color changes to a characteristic red color in the presence of an alkaline environment and a metal ion catalyst (detection solution). As a result, when galactose is sensed in the presence of H2O2, the colored response of the AuNS-PVP-GalDH assay transforms from turquoise blue to light pink, and then to wine red in a concentration-dependent manner discernible to the human eye. This competitive AuNS-PVP-GalDH assay could be a viable analytical tool for rapid and convenient galactose quantification in resource-limited areas.

Dielectric metasurfaces for next-generation optical biosensing: a comparison with plasmonic sensing

In the past decades, nanophotonic biosensors have been extended from the extensively studied plasmonic platforms to dielectric metasurfaces. Instead of plasmonic resonance, dielectric metasurfaces are based on Mie resonance, and provide comparable sensitivity with superior resonance bandwidth, Q factor, and figure-of-merit. Although the plasmonic photothermal effect is beneficial in many biomedical applications, it is a fundamental limitation for biosensing. Dielectric metasurfaces solve the ohmic loss and heating problems, providing better repeatability, stability, and biocompatibility. We review the high-Q resonances based on various physical phenomena tailored by meta-atom geometric designs, and compare dielectric and plasmonic metasurfaces in refractometric, surface-enhanced, and chiral sensing for various biomedical and diagnostic applications. Departing from conventional spectral shift measurement using spectrometers, imaging-based and spectrometer-less biosensing are highlighted, including single-wavelength refractometric barcoding, surface-enhanced molecular fingerprinting, and integrated visual reporting. These unique modalities enabled by dielectric metasurfaces point to two important research directions. On the one hand, hyperspectral imaging provides massive information for smart data processing, which not only achieve better biomolecular sensing performance than conventional ensemble averaging, but also enable real-time monitoring of cellular or microbial behaviour in physiological conditions. On the other hand, a single metasurface can integrate both functions of sensing and optical output engineering, using single-wavelength or broadband light sources, which provides simple, fast, compact, and cost-effective solutions. Finally, we provide perspectives in future development on metasurface nanofabrication, functionalization, material, configuration, and integration, towards next-generation optical biosensing for ultra-sensitive, portable/wearable, lab-on-a-chip, point-of-care, multiplexed, and scalable applications.

Recent Advances in Metaphotonic Biosensors

Metaphotonic devices, which enable light manipulation at a subwavelength scale and enhance light-matter interactions, have been emerging as a critical pillar in biosensing. Researchers have been attracted to metaphotonic biosensors, as they solve the limitations of the existing bioanalytical techniques, including the sensitivity, selectivity, and detection limit. Here, we briefly introduce types of metasurfaces utilized in various metaphotonic biomolecular sensing domains such as refractometry, surface-enhanced fluorescence, vibrational spectroscopy, and chiral sensing. Further, we list the prevalent working mechanisms of those metaphotonic bio-detection schemes. Furthermore, we summarize the recent progress in chip integration for metaphotonic biosensing to enable innovative point-of-care devices in healthcare. Finally, we discuss the impediments in metaphotonic biosensing, such as its cost effectiveness and treatment for intricate biospecimens, and present a prospect for potential directions for materializing these device strategies, significantly influencing clinical diagnostics in health and safety.

Reflective Fiber Temperature Probe Based on Localized Surface Plasmon Resonance towards Low-Cost and Wireless Interrogation

Reflection fiber temperature sensors functionalized with plasmonic nanocomposite material using intensity-based modulation are demonstrated for the first time. Characteristic temperature optical response of the reflective fiber sensor is experimentally tested using Au-incorporated nanocomposite thin films deposited on the fiber tip, and theoretically validated using a thin-film-optic-based optical waveguide model. By optimizing the Au concentration in a dielectric matrix, Au nanoparticles (NP) exhibit a localized surface plasmon resonance (LSPR) absorption band in a visible wavelength that shows a temperature sensitivity ~0.025%/°C as a result of electron-electron and electron-phonon scattering of Au NP and the surrounding matrix. Detailed optical material properties of the on-fiber sensor film are characterized using scanning electron microscopy (SEM) and focused-ion beam (FIB)-assisted transmission electron microscopy (TEM). Airy's expression of transmission and reflection using complex optical constants of layered media is used to model the reflective optical waveguide. A low-cost wireless interrogator based on a photodiode transimpedance-amplifier (TIA) circuit with a low-pass filter is designed to integrate with the sensor. The converted analog voltage is wirelessly transmitted via 2.4 GHz Serial Peripheral Interface (SPI) protocols. Feasibility is demonstrated for portable, remotely interrogated next-generation fiber optic temperature sensors with future capability for monitoring additional parameters of interest.

Recent discovery of tyrosinase inhibitors in traditional Chinese medicines and screening methods

ETHNOPHARMACOLOGICAL RELEVANCE

Tyrosinase, the key rate-limiting enzyme for melanogenesis, is one of the main targets for skin senescence and some pigmented skin diseases, such as albinism and melanoma. Tyrosinase inhibitors are capable of reducing melanin generation and deposition in the skin through blocking the reaction chain of formation. Thus, it has been used for anti-melanoma and showed the potential to be developed into novel skin whitening and spot removing products. With the trend of back-to-nature, natural tyrosinase inhibitors are receiving more and more attention. Traditional Chinese medicines (TCMs) as the promising source of novel chemotypes and pharmacophores, are huge treasures for the discovery of natural tyrosinase inhibitors characterized with green, safe, and highly efficient.

AIM OF THIS REVIEW

This review aims to provide a systematic overview of natural tyrosinase inhibitors and a detailed summary of the types of TCMs from which they originate. In addition, this paper also highlights the screening methods developed for exploring tyrosinase inhibitors in recent years, compares the advantages and disadvantages of various methods under the guidance of different screening principles, and predicts their applications in the future.

MATERIALS AND METHODS

Relevant literature have been obtained using the keywords "tyrosinase inhibitors", "traditional Chinese medicines", "whitening", and "screening" in scientific databases, such as "PubMed", "SciFinder", "Web of Science", "Elsevier", "China Knowledge Resource Integrated databases". Information was also collected from Chinese pharmacopoeia, Chinese herbal classics books, "Google Scholar", "Baidu Scholar", and other literature sources, etc. RESULTS: An overview about the tyrosinase inhibitors derived from TCMs since 2002 has been compiled via the above-mentioned sources. Up to now, 186 components, mainly belonging to flavonoids, lignans, terpenoids, Diels-Alder adducts, simple phenylpropanoids and stilbenes, from 61 kinds of TCMs have been reported to possess tyrosinase inhibitory activity, among which flavonoids are mainly focused on. Furthermore, on the basis of bioactive detection strategies, the screening methods for tyrosinase inhibitors have been classified into bioaffinity-based, intrinsic enzymatic-based, and computer-aided drug design (CADD). Precisely because screening approaches are essential for rapid identification of tyrosinase inhibitors from TCMs, the principles, advantages and disadvantages, and specific applications of each method are presented along with a comparison of applicability.

CONCLUSIONS

The summary of TCMs-derived inhibitors gives a clue on the discovery of candidates with the property to whiten the skin. Meanwhile, the outlook of developed screening methods provides technical references for the efficient exploration of safer and more effective tyrosinase inhibitors from TCMs.

Quantitative Insights into a Plasmonic Ruler Equation from the Perspective of Enhanced Near Field

The plasmonic shift of resonance wavelength induced by near-field coupling enables one to measure nanoscale distances optically. Empirically, the well-known ruler equation correlating plasmon shift with interparticle spacing was proposed. Though it has been widely used in analyzing simulation and experimental outcomes, little is known about the underlying physical mechanism of the characteristic exponential form of the plasmon ruler equation and the universal decay constant therein. In this work, we attempt to decrypt these from the perspective of plasmon near-field enhancement. Based on an analytical quasi-normal mode formula for plasmon shifts, we proved that the exponential decaying electric field is the critical reason that results in the exponential form of the plasmon ruler equation and quantitatively, we found that the universal decay constant in the plasmon ruler equation actually reflects the range of the enhanced near field. This work hopefully helps to deepen the understanding of the mechanism of light-matter interaction in corresponding plasmonic processes.

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.

Research on Dual-Technology Fusion Biosensor Chip Based on RNA Virus Medical Detection

In recent years, the emergence of COVID-19 and other epidemics caused by RNA(ribonucleic acid)-type genetic viruses has aroused the close attention of governments around the world on emergency response to public safety and health emergencies. In this paper, an electrodeless biosensing detection chip for RNA virus medical detection is designed using quartz crystal microbalance technology and local surface plasmon resonance technology. The plasmonic resonance characteristic in the nanostructures of gold nanorods-quartz substrates with different parameters and the surface potential distribution of the quartz crystal microbalance sensing chip were studied by COMSOL finite element simulation software. The results show that the arrangement structure and spacing of gold nanorod dimers greatly affect the local surface plasmon resonance of nanorods, which in turn affects the detection results of biomolecules. Moreover, high concentrations of "hot spots" are distributed between both ends and the gap of the gold nanorod dimer, which reflects the strong hybridization of the multiple resonance modes of the nanoparticles. In addition, by simulating and calculating the surface potential distribution of the electrode area and non-electrode area of the biosensor chip, it was found that the biosensor chip with these two areas can enhance the piezoelectric effect of the quartz chip. Under the same simulation conditions, the biochip with a completely electrodeless structure showed a better sensing performance. The sensor chip combining QCM and LSPR can reduce the influence of the metal electrode on the quartz wafer to improve the sensitivity and accuracy of detection. Considering the significant influence of the gold nanorod dimer plasma resonance mode and the significant advantages of the electrodeless biosensor chip, an electrodeless biosensor combining these two technologies is proposed for RNA virus detection and screening, which has potential applications in biomolecular measurement and other related fields.

Tunable surface plasmon resonance sensor based on graphene-coated photonic crystal fiber in terahertz

A terahertz surface plasmon resonance (SPR) sensor is designed based on photonic crystal fiber (PCF). Graphene is selectively coated in the cladding hole of the PCF and used as plasmonic material. The coupling mechanism, loss properties, tunability, and refractive index sensing performance of the designed SPR sensor are investigated using the finite element method. The peak of the loss spectrum corresponding to the SPR frequency can be dynamically tuned by adjusting graphene's chemical potential, and a tuning sensitivity of 767.5 GHz/eV is obtained. The SPR frequency red shifts linearly with an increase in the refractive index of analyte from 1.0 to 1.5. An average frequency sensitivity of 208.14 GHz/RIU is obtained. This research provides theoretical guidance for the design of terahertz in-fiber SPR sensors and filters.

Optical barcoding using polarisation sensitive plasmonic biosensors for the detection of self-assembled monolayers

Periodic subwavelength apertures have the ability to passively detect variations in the dielectric properties of the local sample environment through modification of the plasmon resonances associated with these structures. The resulting resonance peak can effectively provide a ‘fingerprint’ indicative of the dielectric properties of the medium within the near-surface region. Here we report on the use of bimodal silver-based plasmonic colour filters for molecular sensing. Firstly, by exploring the optical output of these devices as a function of the incident polarisation for a range of different analytes of known refractive index, we were able to both maximise and quantify their sensitivity. We then apply this concept to the real-time monitoring of the formation of self-assembled monolayers based on detection of the optical output using a spectrometer. This highlights the potential for bimodal plasmonic devices to be able to dynamically monitor variations in the local environment down to the level of single molecules without the need for specific functionalisation or labelling. Advantages of using this technique include the ability for these devices to be miniaturised and to dynamically tailor their optical output permitting the analysis of very small sample volumes and maximise their dynamic range for a specific analyte.

SPR Based Biosensing Chip for COVID-19 Diagnosis-A Review

Surface Plasmon Resonance (SPR) techniques are highly accurate in detecting biomolecular like blood group measurement, food adulteration, milk adulteration and recently developing as a rapid detection for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus. In order to validate the clinical diagnosis, Real-time reverse transcriptase-polymerase chain reaction (RT-PCR) of nasopharyngeal swabs has been utilized, which is time consuming and expensive. For fast and accurate detection of the SARS-CoV-2 virus, SPR based biosensing chips are described in this review article. SPR sensors have the potential to be employed for fast, accurate, and portable SARS-CoV-2 virus diagnosis. To combat the SARS-CoV-2 pandemic, there is considerable interest in creating innovative biosensors that are quick, reliable, and sensitive for COVID-19 diagnosis.

SPR Based Biosensing Chip for COVID-19 Diagnosis-A Review

Surface Plasmon Resonance (SPR) techniques are highly accurate in detecting biomolecular like blood group measurement, food adulteration, milk adulteration and recently developing as a rapid detection for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus. In order to validate the clinical diagnosis, Real-time reverse transcriptase-polymerase chain reaction (RT-PCR) of nasopharyngeal swabs has been utilized, which is time consuming and expensive. For fast and accurate detection of the SARS-CoV-2 virus, SPR based biosensing chips are described in this review article. SPR sensors have the potential to be employed for fast, accurate, and portable SARS-CoV-2 virus diagnosis. To combat the SARS-CoV-2 pandemic, there is considerable interest in creating innovative biosensors that are quick, reliable, and sensitive for COVID-19 diagnosis.

Repeatable detection of Ag+ ions using a DNA aptamer-linked hydrogel biochemical sensor integrated with microfluidic heating system

This paper describes repeatable detection of Ag⁺ ions using a DNA aptamer-linked hydrogel biochemical sensor integrated with a microfluidic heating system. Biochemical sensors that respond to chemical compounds and produce detectable signals have a critical role in many aspects of modern society. In particular, the repeatable measurement of environmental information such as toxic substances including Ag⁺ ions could be expected to improve the environment. The DNA aptamer is an attractive candidate because of the stability and the selectivity of binding to chemicals. However, previous DNA aptamer biochemical sensors could not measure repeatedly because those sensors did not have initializing functions. To overcome this challenge, we proposed a DNA aptamer-linked hydrogel biochemical sensor integrated with the microfluidic heating system enabling repeatable detection of Ag⁺ ions. The binding Ag⁺ ions are dissociated by heating and flushing through the integrated microfluidic heating device. The DNA aptamer-linked hydrogel had the capability to detect a wide range of Ag⁺ ion concentrations (10^-5-10 mM) including a toxic range for various aquatic organisms. Finally, we demonstrated the repeatable detection of the Ag⁺ ions. These results indicated that our proposed biochemical sensor is expected to use for long-term monitoring with high stability in ambient temperature and low power consumption.

Analogous plasmon-induced absorption based on an end-coupled MDM structure with area-cost-free cavities

We theoretically investigate an end-coupled metal-dielectric-metal (MDM) structure that achieves analogous plasmon-induced absorption (APIA) in an area-cost-free manner. First, a squared ring is set to end-couple with MDM input and output waveguides, generating three Lorentzian-like peaks in the spectrum. Then, two APIA windows as well as two Fano resonances can be induced via appropriately arranging two area-free cavities. Numerous numerical results demonstrate that the proposed structure has remarkable sensing and phase characteristics. Our proposed PIA-based MDM structure is promising in potential applications of bio-chemical sensing, slow light devices, optical switching, and chip-scale plasmonic devices.

Quantitative near-field characterization of surface plasmon polaritons on monocrystalline gold platelets

Near-field microscopy allows for visualization of both the amplitude and phase of surface plasmon polaritons (SPPs). However, their quantitative characterization in a reflection configuration is challenging due to complex wave patterns arising from the interference between several excitation channels. Here, we present near-field measurements of SPPs on large monocrystalline gold platelets in the visible. We study systematically the influence of the incident angle of the exciting light on the SPPs launched by an atomic force microscope tip. We find that the amplitude and phase signals of these SPPs are best disentangled from other signals at grazing incident angle relative to the edge of the gold platelet. Furthermore, we introduce a simple model to extract the wavelength and in particular the propagation length of the tip-launched plasmons. Our experimental results are in excellent agreement with our theoretical model. The presented method allows the quantitative analysis of polaritons occurring in different materials at visible wavelengths.

Figures of merit of plasmon lattice resonance sensors: shape and material matters

The figure of merit (FOM) of plasmon lattice resonance (PLR) sensors based on the array of metal/Si/SiO₂nanoparticles has been investigated. We demonstrate the shape and material of metal nanoparticles have remarkable effects on the PLR and FOM. FOM is governed by full-widths at half maximum (FWHM) and sensitivity of the PLR. Three different types of PLR can be generated by changing Ag nanoparticles' shapes (pillars, cubes, spheres). One (named PLR₁) is mainly originated from the coupling between Mie resonance of individual Si nanopillars and diffraction waves. PLR₁of Ag/Si/SiO₂nanoparticle arrays is limited in sensing applications due to lower intensity (for Ag pillars and Ag cubes), or smaller FOM (for Ag spheres). The other two are named PLR₂. PLR₂of Ag/Si/SiO₂nanoparticle array with Ag pillars (or Ag cubes) is mainly originated from the coupling between the quadrupole resonance of individual Ag nanopillars (or Ag cubes) and diffraction waves. While PLR₂of Ag/Si/SiO₂nanoparticle array with Ag spheres is mainly originated from the coupling between dipole resonance of individual Ag nanospheres and diffraction waves. The optimal Ag nanoparticles' shape in FOM is pillar due to the smallest FWHM of PLR₂of Ag/Si/SiO₂nanoparticle array with Ag pillars. Meanwhile, a comparison of FOM between Au, Ag and Al nanopillars of fixed size is made. The optimal material of metal nanopillars to obtain a high FOM is Ag due to higher sensitivity and narrower FWHM.

Shape Modulation of Plasmonic Nanostructures by Unconventional Lithographic Technique

Conventional nano-sphere lithography techniques have been extended to the fabrication of highly periodic arrays of sub-wavelength nanoholes in a thin metal film. By combining the dry etching processes of self-assembled monolayers of polystyrene colloids with metal physical deposition, the complete transition from increasing size triangular nanoprism to hexagonally distributed nanoholes array onto thin metal film has been gradually explored. The investigated nano-structured materials exhibit interesting plasmonic properties which can be precisely modulated in a desired optical spectral region. An interesting approach based on optical absorbance measurements has been adopted for rapid and non-invasive inspections of the nano-sphere monolayer after the ion etching process. By enabling an indirect and accurate evaluation of colloid dimensions in a large area, this approach allows the low-cost and reproducible fabrication of plasmonic materials with specifically modulated optical properties suitable for many application in biosensing devices or Raman enhanced effects.

Plasmonic Metasurfaces for Medical Diagnosis Applications: A Review

Plasmonic metasurfaces have been widely used in biosensing to improve the interaction between light and biomolecules through the effects of near-field confinement. When paired with biofunctionalization, plasmonic metasurface sensing is considered as a viable strategy for improving biomarker detection technologies. In this review, we enumerate the fundamental mechanism of plasmonic metasurfaces sensing and present their detection in human tumors and COVID-19. The advantages of rapid sampling, streamlined processes, high sensitivity, and easy accessibility are highlighted compared with traditional detection techniques. This review is looking forward to assisting scientists in advancing research and developing a new generation of multifunctional biosensors.

Nanostructured plasmonic chips employing nanopillar and nanoring hole arrays for enhanced sensitivity of SPR-based biosensing

We present a theoretical analysis of the different nanostructured plasmonic sensor chips-consisting of plasmonic nanostructures present on the surface of plasmonic thin films-interrogated using the Kretschmann configuration for highly sensitive localized sensing, with high tunability from the visible to the infrared regions. Rigorous coupled-wave analysis is performed to analyze all the proposed nanostructured sensor chips and compare their sensing performance. The sensitivity parameters are defined to focus on the detection of a thin layer of biomolecules on the surface of nanostructures. The dimensions of the nanostructures and the incident angle shift the plasmon resonance wavelengths and can be used to tune the operating wavelength. The nanostructured films create local regions of high electric fields, which results in enhanced sensitivity of the proposed structures. The proposed sensors can be used in surface plasmon resonance imaging to detect multiple biomolecules in a single measurement. An extremely high surface sensitivity and figure of merit (FOMS) of 91 nm nm-1 and 0.59 nm-1 has been found, respectively, for one of the proposed nanostructured sensing platforms. Moreover, we demonstrate a very high differential reflectance of 55% per nm thickness of the biolayer.

Technological advances in electrochemical biosensors for the detection of disease biomarkers

With an increasing focus on health in contemporary society, interest in the diagnosis, treatment, and prevention of diseases has grown rapidly. Accordingly, the demand for biosensors for the early diagnosis of disease is increasing. However, the measurement range of existing electrochemical sensors is relatively high, which is not suitable for early disease diagnosis, requiring the detection of small amounts of biocomponents. Various attempts have been made to overcome this and amplify the signal, including binding with various labeling molecules, such as DNA, enzymes, nanoparticles, and carbon materials. Efforts are also being made to increase the sensitivity of electrochemical sensors, and the combination of nanomaterials, materials, and biotechnology offers the potential to increase sensitivity in a variety of ways. Recent studies suggest that electrochemical sensors can be a powerful tool in providing comprehensive insights into the targeting and detection of disease-associated biomarkers. Significant advances in nanomaterial and biomolecule approaches for improved sensitivity have resulted in the development of electrochemical biosensors capable of detecting multiple biomarkers in real time in clinically relevant samples. In this review, we have discussed the recent studies on electrochemical sensors for detection of diseases such as diabetes, degenerative diseases, and cancer. Further, we have highlighted new technologies to improve sensitivity using various materials, including DNA, enzymes, nanoparticles, and carbon materials.

Homogenous overexpression of the extracellular matrix protein Netrin-1 in a hollow fiber bioreactor

The production of recombinant proteins for functional and biophysical studies, especially in the field of structural determination, still represents a challenge as high quality and quantities are needed to adequately perform experiments. This is in part solved by optimizing protein constructs and expression conditions to maximize the yields in regular flask expression systems. Still, work flow and effort can be substantial with no guarantee to obtain improvements. This study presents a combination of workflows that can be used to dramatically increase protein production and improve processing results, specifically for the extracellular matrix protein Netrin-1. This proteoglycan is an axon guidance cue which interacts with various receptors to initiate downstream signaling cascades affecting cell differentiation, proliferation, metabolism, and survival. We were able to produce large glycoprotein quantities in mammalian cells, which were engineered for protein overexpression and secretion into the media using the controlled environment provided by a hollow fiber bioreactor. Close monitoring of the internal bioreactor conditions allowed for stable production over an extended period of time. In addition to this, Netrin-1 concentrations were monitored in expression media through biolayer interferometry which allowed us to increase Netrin-1 media concentrations tenfold over our current flask systems while preserving excellent protein quality and in solution behavior. Our particular combination of genetic engineering, cell culture system, protein purification, and biophysical characterization permitted us to establish an efficient and continuous production of high-quality protein suitable for structural biology studies that can be translated to various biological systems. KEY POINTS: • Hollow fiber bioreactor produces substantial yields of homogenous Netrin-1 • Biolayer interferometry allows target protein quantitation in expression media • High production yields in the bioreactor do not impair Netrin-1 proteoglycan quality.

An Overview of Artificial Olfaction Systems with a Focus on Surface Plasmon Resonance for the Analysis of Volatile Organic Compounds

The last three decades have witnessed an increasing demand for novel analytical tools for the analysis of gases including odorants and volatile organic compounds (VOCs) in various domains. Traditional techniques such as gas chromatography coupled with mass spectrometry, although very efficient, present several drawbacks. Such a context has incited the research and industrial communities to work on the development of alternative technologies such as artificial olfaction systems, including gas sensors, olfactory biosensors and electronic noses (eNs). A wide variety of these systems have been designed using chemiresistive, electrochemical, acoustic or optical transducers. Among optical transduction systems, surface plasmon resonance (SPR) has been extensively studied thanks to its attractive features (high sensitivity, label free, real-time measurements). In this paper, we present an overview of the advances in the development of artificial olfaction systems with a focus on their development based on propagating SPR with different coupling configurations, including prism coupler, wave guide, and grating.

Metal-enhanced sensing platform for the highly sensitive detection of C-reactive protein antibody and rhodamine B with applications in cardiovascular diseases and food safety

The potential applications of metal-enhanced fluorescence (MEF) devices include biosensors for the detection of trace amounts in biosciences, biotechnology, and pathogens that are relevant to medical diagnostics and food control. In the present study, the silver (Ag) film thickness (56 nm) of an MEF system was calibrated to maximize the depth-to-width ratio (Γ) of the surface plasmon resonance (SPR) active metal from reflectance dip curves. Upon plasmon coupling with thermally evaporated Ag, we demonstrated a 2.21-fold enhancement compared to the pristine flat substrate with the coefficient of variation (CV) ≈0.22% and the limit of detection (LOD) 0.001 mg L-1 of the concentration of an Alexa Fluor 488-labeled anti-C-reactive protein antibody (CRP@Alexa fluor 488). The structure was developed to simplify the in situ generation of biosensors for the surface-enhanced Raman spectroscopy (SERS) to determine Rhodamine B (RhB) with a highly robust performance. The procedure presented a simple and rapid sample pretreatment for the determination of RhB with a limit of quantification of 10-10 M and a satisfactory linear response (0.98). The results showed the excellent performance of the surface plasmon coupled emission (SPCE), which opens up possibilities for the accurate detection of small-volume and low-concentration target analytes due to the improved sensitivity and signal-to-noise ratio (SNR).

Optical Sensing Using Fiber-Optic Multimode Interference Devices: A Review of Nonconventional Sensing Schemes

We review fiber-based multimode interference (MMI) devices with a particular focus on optical fiber-based sensing applications. The present review complements a recently published, extensive review where the sensing of conventional physical variables such as refractive index, temperature, displacement, and strain was covered. This review focuses on MMI fiber sensors for nonconventional physical variables, including mechanical, electromagnetic, chemical, and optical, covering around fifteen years of work in the field. Finally, by the end of this paper, we also review some new trends of MMI-based schemes based on polymer fibers, for wavelength-locking applications, for retrieving the thermo-optic coefficient of liquid samples, and for measuring the dynamics of complex fluids.

A comprehensive review on plasmonic-based biosensors used in viral diagnostics

The proliferation and transmission of viruses has become a threat to worldwide biosecurity, as exemplified by the current COVID-19 pandemic. Early diagnosis of viral infection and disease control have always been critical. Virus detection can be achieved based on various plasmonic phenomena, including propagating surface plasmon resonance (SPR), localized SPR, surface-enhanced Raman scattering, surface-enhanced fluorescence and surface-enhanced infrared absorption spectroscopy. The present review covers all available information on plasmonic-based virus detection, and collected data on these sensors based on several parameters. These data will assist the audience in advancing research and development of a new generation of versatile virus biosensors.

Nanostructured and Spiky Gold Shell Growth on Magnetic Particles for SERS Applications

Multifunctional micro- and nanoparticles have potential uses in advanced detection methods, such as the combined separation and detection of biomolecules. Combining multiple tasks is possible but requires the specific tailoring of these particles during synthesis or further functionalization. Here, we synthesized nanostructured gold shells on magnetic particle cores and demonstrated the use of them in surface-enhanced Raman scattering (SERS). To grow the gold shells, gold seeds were bound to silica-coated iron oxide aggregate particles. We explored different functional groups on the surface to achieve different interactions with gold seeds. Then, we used an aqueous cetyltrimethylammonium bromide (CTAB)-based strategy to grow the seeds into spikes. We investigated the influence of the surface chemistry on seed attachment and on further growth of spikes. We also explored different experimental conditions to achieve either spiky or bumpy plasmonic structures on the particles. We demonstrated that the particles showed SERS enhancement of a model Raman probe molecule, 2-mercaptopyrimidine, on the order of 10⁴. We also investigated the impact of gold shell morphology—spiky or bumpy—on SERS enhancements and on particle stability over time. We found that spiky shells lead to greater enhancements, however their high aspect ratio structures are less stable and morphological changes occur more quickly than observed with bumpy shells.

Narrowband Light Reflection Resonances from Waveguide Modes for High-Quality Sensors

Designing various nanostructures to achieve narrowband light reflection resonances is desirable for optical sensing applications. In this work, we theoretically demonstrate two narrowband light reflection resonances resulting from the excitations of the zero-order transverse magnetic (TM) and transverse electric (TE) waveguide modes, in a waveguide structure consisting of an Au sphere array on an indium tin oxide (ITO) spacer on a silica (SiO₂) substrate. The positions of the light reflection resonances can be tuned easily, by varying the array periods of gold (Au) spheres or by changing the thickness of the ITO film. More importantly, the light reflection resonances have a very narrow bandwidth, the full width at half maximum (FWHM) of which can be reduced to only several nanometers for the zero-order TM and TE waveguide modes. The conventionally defined performance parameters of sensors, sensitivity (S) and figure of merit (FOM), have quite high values of about 80 nm/RIU and 32, respectively, in the visible wavelength range.

A Spellbinding Interplay Between Biological Barcoding and Nanotechnology

Great scientific research with improved potential in probing biological locales has remained a giant stride. The use of bio-barcodes with the potential use of nanotechnology is a hallmark being developed among recent advanced techniques. Biobarcoding is a novel method used for screening biomolecules to identify and divulge ragbag biodiversity. It establishes successful barcoding projects in the field of nanomedical technology for massively testing disease diagnosis and treatment. Biobarcoding and nanotechnology are recently developed technologies that provide unique opportunities and challenges for multiplex detection such as DNAs, proteins and nucleic acids of animals, plants, viruses, and various other species. These technologies also clump drug delivery, gene delivery, and DNA sequencing. Bio-barcode amplification assay (BCA) is used at large for the detection and identification of proteins and DNAs. DNA barcoding combined with nanotechnology has been proven highly sensitive rendering fast uniplex and multiplex detection of pathogens in food, blood, and other specimens. This review takes a panoramic view of current advances in nano bio-barcodes which have been summarized to explore additional applications such as detection of cytokines, neurotransmitters, cancer markers, prostate-specific antigens, and allergens. In the future, it will also be possible to detect some fungi, algae, protozoa, and other pollutants in food, agriculture, and clinical samples. Using these technologies, specific and efficient sensors would possibly be developed that can perform swift detections of antigens, allergens, and other specimens.

Bloch Surface Wave Resonance Based Sensors as an Alternative to Surface Plasmon Resonance Sensors

We report on a highly sensitive measurement of the relative humidity (RH) of moist air using both the surface plasmon resonance (SPR) and Bloch surface wave resonance (BSWR). Both resonances are resolved in the Kretschmann configuration when the wavelength interrogation method is utilized. The SPR is revealed for a multilayer plasmonic structure of SF10/Cr/Au, while the BSWR is resolved for a multilayer dielectric structure (MDS) comprising four bilayers of TiO2/SiO2 with a rough termination layer of TiO2. The SPR effect is manifested by a dip in the reflectance of a p-polarized wave, and a shift of the dip with the change in the RH, or equivalently with the change in the refractive index of moist air is revealed, giving a sensitivity in a range of 0.042-0.072 nm/%RH. The BSWR effect is manifested by a dip in the reflectance of the spectral interference of s- and p-polarized waves, which represents an effective approach in resolving the resonance with maximum depth. For the MDS under study, the BSWRs were resolved within two band gaps, and for moist air we obtained sensitivities of 0.021-0.038 nm/%RH and 0.046-0.065 nm/%RH, respectively. We also revealed that the SPR based RH measurement is with the figure of merit (FOM) up to 4.7 × 10-4 %RH-1, while BSWR based measurements have FOMs as high as 3.0 × 10-3 %RH-1 and 1.1 × 10-3 %RH-1, respectively. The obtained spectral interferometry based results demonstrate that the BSWR based sensor employing the available MDS has a similar sensitivity as the SPR based sensor, but outperforms it in the FOM. BSW based sensors employing dielectrics thus represent an effective alternative with a number of advantages, including better mechanical and chemical stability than metal films used in SPR sensing.

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.

Hot Electron Driven Photocatalysis on Plasmon-Resonant Grating Nanostructures

We demonstrate the hot electron injection of photoexcited carriers in an Ag-based plasmon resonant grating structure. By varying the incident angle of irradiation, sharp dips are observed in the reflectance with p-polarized light (electric field perpendicular to grating lines) when there is wavevector matching between the incident light and the plasmon resonant modes of the grating and no angle dependence is observed with s-polarized light. This configuration enables us to compare photoelectrochemical current produced by plasmon resonant excitation with that of bulk metal interband absorption simply by rotating the polarization of the incident light while keeping all other parameters of the measurement fixed. With 633 nm light, we observed a 12-fold enhancement in the photocurrent (i.e., reaction rate) between resonant and nonresonant polarizations at incident angles of ±7.6° from normal. At 785 nm irradiation, we observed similar resonant profiles to those obtained with 633 nm wavelength light but with a 44-fold enhancement factor. Using 532 nm light, we observed two resonant peaks (with approximately 10× enhancement) in the photocurrent at 19.4° and 28.0° incident angles, each corresponding to higher order modes in the grating with more nodes per period. The lower enhancement factors observed at shorter wavelengths are attributed to interband transitions, which provide a damping mechanism for the plasmon resonance. Finite difference time domain (FDTD) simulations of these grating structures confirm the resonant profiles observed in the angle-dependent spectra of these gratings and provide a detailed picture of the electric field profiles on and off resonance.

Simultaneous Real-Time Detection of Pregnancy-Associated Plasma Protein-A and -A2 Using a Graphene Oxide-Based Surface Plasmon Resonance Biosensor

Background

Pregnancy-associated plasma protein-A and -A2 (PAPP-A and -A2) are principally expressed in placental trophoblasts and play a critical role in the regulation of fetal and placental growth. PAPP-A2 shares 45% amino acid similarity with PAPP-A. This study aimed to investigate the efficacy of real-time detection of PAPP-A and PAPP-A2 using a novel surface plasmon resonance (SPR) biosensor based on graphene oxide (GO).

Methods

Traditional SPR and GO-based SPR chips were fabricated to measure PAPP-A and PAPP-A2 concentrations. We compared SPR response curves of PAPP-A and PAPP-A2 between traditional SPR and GO-SPR biosensors. We also performed interference tests and specificity analyses among PAPP-A, PAPP-A2, and mixed interference proteins.

Results

The time to detect PAPP-A and PAPP-A2 was about 150 seconds with both traditional SPR and GO-SPR biosensors. Approximately double SPR angle shifts were noted with the GO-SPR biosensor compared to the traditional SPR biosensor at a PAPP-A and PAPP-A2 concentration of 5 μg/mL. The limit of detection of the GO-SPR biosensor was as low as 0.5 ng/mL for both PAPP-A and PAPP-A2. Interference testing revealed that almost all of the protein bonded on the GO-SPR biosensor with anti-PAPP-A from the mixture of proteins was PAPP-A, and that almost no other proteins were captured except for PAPP-A2. However, the SPR signal of PAPP-A2 (5.75 mdeg) was much smaller than that of PAPP-A (13.76 mdeg). Similar results were noted with anti-PAPP-A2, where almost all of the protein bonded on the GO-SPR biosensor was PAPP-A2. The SPR signal of PAPP-A (5.17 mdeg) was much smaller than that of PAPP-A2 (13.94 mdeg).

Conclusion

The GO-SPR biosensor could distinguish PAPP-A and PAPP-A2 from various mixed interference proteins with high sensitivity and specificity. It could potentially be used to measure PAPP-A and PAPP-A2 in clinical blood samples during pregnancy.

Vertically Coupled Plasmonic Racetrack Ring Resonator for Biosensor Applications

Plasmonic chemical and biological sensors offer significant advantages such as really compact sizes and extremely high sensitivity. Biosensors based on plasmonic waveguides and resonators are some of the most attractive candidates for mobile and wearable devices. However, high losses in the metal and complicated schemes for practical implementation make it challenging to find the optimal configuration of a compact plasmon biosensor. Here, we propose a novel plasmonic refractive index sensor based on a metal strip waveguide placed under a waveguide-based racetrack ring resonator made of the same metal. This scheme guarantees effective coupling between the waveguide and resonator and low loss light transmittance through the long-range waveguide. The proposed device can be easily fabricated (e.g., using optical lithography) and integrated with materials like graphene oxide for providing adsorption of the biomolecules on the sensitive part of the optical elements. To analyze the properties of the designed sensing system, we performed numerical simulations along with some analytical estimations. There is one other interesting general feature of this sensing scheme that is worth pointing out before looking at its details. The sensitivity of the considered device can be significantly increased by surrounding the resonator with media of slightly different refractive indices, which allows sensitivity to reach a value of more than 1 μm per refractive index unit.

Kinetics of lipid raft formation at lipid monolayer-bilayer junction probed by surface plasmon resonance

A label-free, non-dispruptive, and real-time analytical device to monitor the dynamic features of biomolecules and their interactions with neighboring molecules is an essential prerequisite for biochip- and diagonostic assays. To explore one of the central questions on the lipid-lipid interactions in the course of the liquid-ordered (l_o) domain formation, called rafts, we developed a method of reconstituting continuous but spatially heterogeneous lipid membrane platforms with molayer-bilayer juntions (MBJs) that enable to form the l_o domains in a spatiotemporally controlled manner. This allows us to detect the time-lapse dynamics of the lipid-lipid interactions during raft formation and resultant membrane phase changes together with the raft-associated receptor-ligand binding through the surface plasmon resonance (SPR). For cross-validation, using epifluorescence microscopy, we demonstrated the underlying mechanisms for raft formations that the infiltration of cholesterols into the sphingolipid-enriched domains plays a crucial roles in the membrane phase-separation. Our membrane platform, being capable of monitoring dynamic interactions among lipids and performing the systematic optical analysis, will unveil physiological roles of cholesterols in a variety of biological events.

Correlating Nanostructure, Optical and Electronic Properties of Nanogranular Silver Layers during Polymer-Template-Assisted Sputter Deposition

Tailoring the optical and electronic properties of nanostructured polymer-metal composites demonstrates great potential for efficient fabrication of modern organic optical and electronic devices such as flexible sensors, transistors, diodes, or photovoltaics. Self-assembled polymer-metal nanocomposites offer an excellent perspective for creating hierarchical nanostructures on macroscopic scales by simple bottom-up processes. We investigate the growth processes of nanogranular silver (Ag) layers on diblock copolymer thin film templates during sputter deposition. The Ag growth is strongly driven by self-assembly and selective wetting on the lamella structure of polystyrene-block-poly(methyl methacrylate). We correlate the emerging nanoscale morphologies with collective optical and electronic properties and quantify the difference in Ag growth on the corresponding homopolymer thin films. Thus, we are able to determine the influence of the respective polymer template and observe substrate effects on the Ag cluster percolation threshold, which affects the insulator-to-metal transition (IMT). Optical spectroscopy in the UV-vis regime reveals localized surface plasmon resonance for the metal-polymer composite. Their maximum absorption is observed around the IMT due to the subsequent long-range electron conduction in percolated nanogranular Ag layers. Using X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy, we identify the oxidation of Ag at the acrylate side chains as an essential influencing factor driving the selective wetting behavior in the early growth stages. The results of polymer-templated cluster growth are corroborated by atomic force microscopy and field emission scanning electron microscopy.

Fabrication of a Bare Optical Fiber-Based Biosensor

A bare optical fiber-based biosensor is proposed for measuring the refractive index of different liquids and the binding kinetics of biomolecules to the sensor surface. This optical fiber sensor is based on the Kretschmann's configuration to attain total internal reflection (TIR) for surface plasmon resonance (SPR) excitation. One end of the bare optical fiber is coated with a gold film. By guiding the light source from the other end into the optical fiber, the light is reflected from the gold-deposited end and the surface evanescent wave is excited in the gold film-transparent material interface. Methanol and ethanol solutions with different refractive indices are used for measuring the corresponding changes in the peak values of the spectra and calculating the corresponding sensitivities. These values are experimentally determined to be in the order of 10^-4~10^-5 refractive index unit (RIU). Binding of proteins onto the sensor surface is also monitored in real time to obtain the binding kinetics. We believe that, in the future, this optical fiber sensor can serve as a useful biosensor for in situ measurement of allergens, antibody-antigen interactions, and even circulating tumor cells in the blood.

Molecularly Imprinted Polymer Materials as Selective Recognition Sorbents for Explosives: A Review

Explosives are of significant interest to homeland security departments and forensic investigations. Fast, sensitive and selective detection of these chemicals is of great concern for security purposes as well as for triage and decontamination in contaminated areas. To this end, selective sorbents with fast binding kinetics and high binding capacity, either in combination with a sensor transducer or a sampling/sample-preparation method, are required. Molecularly imprinted polymers (MIPs) show promise as cost-effective and rugged artificial selective sorbents, which have a wide variety of applications. This manuscript reviews the innovative strategies developed in 57 manuscripts (published from 2006 to 2019) to use MIP materials for explosives. To the best of our knowledge, there are currently no commercially available MIP-modified sensors or sample preparation methods for explosives in the market. We believe that this review provides information to give insight into the future prospects and potential commercialization of such materials. We warn the readers of the hazards of working with explosives.

Towards Portable Nanophotonic Sensors

A range of nanophotonic sensors composed of different materials and device configurations have been developed over the past two decades. These sensors have achieved high performance in terms of sensitivity and detection limit. The size of onchip nanophotonic sensors is also small and they are regarded as a strong candidate to provide the next generation sensors for a range of applications including chemical and biosensing for point-of-care diagnostics. However, the apparatus used to perform measurements of nanophotonic sensor chips is bulky, expensive and requires experts to operate them. Thus, although integrated nanophotonic sensors have shown high performance and are compact themselves their practical applications are limited by the lack of a compact readout system required for their measurements. To achieve the aim of using nanophotonic sensors in daily life it is important to develop nanophotonic sensors which are not only themselves small, but their readout system is also portable, compact and easy to operate. Recognizing the need to develop compact readout systems for onchip nanophotonic sensors, different groups around the globe have started to put efforts in this direction. This review article discusses different works carried out to develop integrated nanophotonic sensors with compact readout systems, which are divided into two categories; onchip nanophotonic sensors with monolithically integrated readout and onchip nanophotonic sensors with separate but compact readout systems.

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

The survey focuses on the most significant contributions in the field of fiber optic plasmonic sensors (FOPS) in recent years. FOPSs are plasmonic sensor-based fiber optic probes that use an optical field to measure the biological agents. Owing to their high sensitivity, high resolution, and low cost, FOPS turn out to be potential alternatives to conventional biological fiber optic sensors. FOPS use optical transduction mechanisms to enhance sensitivity and resolution. The optical transduction mechanisms of FOPS with different geometrical structures and the photonic properties of the geometries are discussed in detail. The studies of optical properties with a combination of suitable materials for testing the biosamples allow for diagnosing diseases in the medical field.

Gold-aluminum-based surface plasmon resonance sensor with a high quality factor and figure of merit for the detection of hemoglobin

A gold (Au) and aluminum (Al)-based surface plasmon resonance (SPR) sensor, with a high-quality factor and figure of merit, for the detection of hemoglobin concentration in human blood is proposed. The sensing performance of a bimetallic Au grating over an Al film-based sensor depends on the thickness of the Al film. The sensor's performance is closely analyzed in terms of well-defined performance parameters: sensitivity, FWHM, and depth of SPR dips in order to optimize the thickness of Al film. With optimized thickness, we could achieve quality factor and figure of merit values of 286.2  RIU^-1 and 0.97  deg^-1, respectively. Al is chemically unstable, and, to address the oxidation problem, we analyzed the sensor with an ultrathin protective Au layer between the Au grating and Al film. We show that the introduction of an ultrathin Au layer gives stability to the sensor without much affecting the performance of the sensor. The obtained values of quality factor and figure of merit of the proposed sensor are 245.2  RIU^-1 and 0.86  deg^-1, respectively. We also discuss the potential of hemoglobin detection of the proposed sensor in the near-infrared region. Reported sensitivity of the sensor is 0.62°/dgL^-1.

Plasmonic Interface Modified with Graphene Oxide Sheets Overlayer for Sensitivity Enhancement

A novel strategy to modify the plasmonic interface by spin-coating an overlayer of graphene oxide sheets (GOSs) on top of the surface plasmon resonance (SPR) sensor is proposed and demonstrated. Thanks to the excellent electrical conductivity, large surface area, and high-refractive index of the GOSs layer, the GOSs-modified SPR (GOSs-SPR) sensor achieves an improved sensitivity in the detection of bulky refractive index solutions and bovine serum albumin (BSA) solutions. The maximum sensitivity of 2715.1 nm/RIU achieved by three spin-coatings shows an enhancement of 20.2% than the case without the modification of the GOSs overlayer. Benefiting from the large surface area and abundant surface functional groups, the GOSs-SPR sensor has a greater sensitivity enhancement (up to 39.35%) in the detection of the BSA molecules. Most importantly, we have firstly experimentally demonstrated that the GOSs overlayer with thickness over hundreds nanometers can still lead to a great enhancement of sensitivity of SPR sensors. Additionally, the proposed modification method for the plasmonic interface is a simple and effective strategy to boost the sensitivity in a chemical-free and environment-friendly manner, without additional chemical or biological amplification steps. These unique features make the proposed GOSs-SPR biosensor a low-cost and biocompatible platform in the fields of biochemical sensing, drug screening, and environmental monitoring.

A Plasmonic Chip-Scale Refractive Index Sensor Design Based on Multiple Fano Resonances

In this paper, multiple Fano resonances preferred in the refractive index sensing area are achieved based on sub-wavelength metal-insulator-metal (MIM) waveguides. Two slot cavities, which are placed between or above the MIM waveguides, can support the bright modes or the dark modes, respectively. Owing to the mode interferences, dual Fano resonances with obvious asymmetrical spectral responses are achieved. High sensitivity and high figure of merit are investigated by using the finite-difference time-domain (FDTD) method. In view of the development of chip-scale integrated photonics, two extra slot cavities are successively added to the structure, and consequently, three and four ultra-sharp Fano peaks with considerable performances are obtained, respectively. It is believed that this proposed structure can find important applications in the on-chip optical sensing and optical communication areas.

Ultra-sensitive detection by metal nanoparticles-mediated enhanced SPR biosensors

Surface plasmon resonance (SPR), as an optical technique, has widely been used for the detection of biomarkers. Various investigations have been conducted to address the impacts of SPR on the kinetics of biological interactions between the ligand and its cognate bio-element. Up until now, different biofunctionalized metal nanoparticles (NPs) have been used for the ultrasensitive detection of biomarkers in the enhanced SPR. The enhancement of plasmonic properties and refractive index by means of metal NPs in SPR-based biosensors have significantly improved the diagnosis and monitoring of molecular markers in different disesaes including malignancies. In all the enhanced SPR systems utilized for the direct/sandwich assay, each NP is covalently modified with the analyte molecules like antibody (Ab) or a nucleic acid such as DNA/RNA aptamer (Ap) capable of interaction with the related biomarker(s). The increasing of density near the gold surface and plasmonic coupling of gold film and NPs can provide a large shift in the refractive index enhancing the plasmonic resonance because the SPR response unit is sensitive to alteration of the refractive index and the mass shifting onto the chip surface. In this study, we review the potential applications of two major NPs for enhancing the SPR signals for the detection of molecular biomarkers, including gold and magnetic NPs.

Dispersion engineering with plasmonic nano structures for enhanced surface plasmon resonance sensing

We demonstrate numerically and experimentally the enhancement of Surface Plasmon Resonance (SPR) sensing via dispersion engineering of the plasmonic response using plasmonic nanograting. Following their design and optimization, the plasmonic nanograting structures are fabricated using e-beam lithography and lift-off process and integrated into conventional prism based Kretschmann configuration. The presence of absorptive nanograting near the metal film, provides strong field enhancement with localization and allows to control the dispersion relation which was originally dictated by a conventional SPR structure. This contributes to the enhancement in Q factor which is found to be 3–4 times higher as compared to the conventional Kretschmann configuration. The influence of the incident angle on resonance wavelength is also demonstrated both numerically and experimentally, where, only a negligible wavelength shift is observed with increasing the incident angles for plasmonic nanograting configuration. This surprising feature may be helpful for studying and utilizing light-matter interaction between plasmons and narrow linewidth media (e.g. Rb atom or molecule) having nonlocalities in their susceptibility-momentum relation. Finally, we analyze the role of plasmonic nanograting in enhancing the performance of an SPR sensor. Our results indicate that the integrated SPR-nanograting device shows a great promise as a sensor for various types of analytes.

Plasmonic metasurface cavity for simultaneous enhancement of optical electric and magnetic fields in deep subwavelength volume

It has been hard to achieve simultaneous plasmonic enhancement of nanoscale light-matter interactions in terms of both electric and magnetic manners with easily reproducible fabrication method and systematic theoretical design rule. In this paper, a novel concept of a flat nanofocusing device is proposed for simultaneously squeezing both electric and magnetic fields in deep-subwavelength volume (~λ³/538) in a large area. Based on the funneled unit cell structures and surface plasmon-assisted coherent interactions between them, the array of rectangular nanocavity connected to a tapered nanoantenna, plasmonic metasurface cavity, is constructed by periodic arrangement of the unit cell. The average enhancement factors of electric and magnetic field intensities reach about 60 and 22 in nanocavities, respectively. The proposed outstanding performance of the device is verified numerically and experimentally. We expect that this work would expand methodologies involving optical near-field manipulations in large areas and related potential applications including nanophotonic sensors, nonlinear responses, and quantum interactions.