Showing papers on "Surface plasmon resonance published in 2015"

Mid-infrared plasmonic biosensing with graphene

Abstract: Infrared spectroscopy is the technique of choice for chemical identification of biomolecules through their vibrational fingerprints. However, infrared light interacts poorly with nanometric-size molecules. We exploit the unique electro-optical properties of graphene to demonstrate a high-sensitivity tunable plasmonic biosensor for chemically specific label-free detection of protein monolayers. The plasmon resonance of nanostructured graphene is dynamically tuned to selectively probe the protein at different frequencies and extract its complex refractive index. Additionally, the extreme spatial light confinement in graphene—up to two orders of magnitude higher than in metals—produces an unprecedentedly high overlap with nanometric biomolecules, enabling superior sensitivity in the detection of their refractive index and vibrational fingerprints. The combination of tunable spectral selectivity and enhanced sensitivity of graphene opens exciting prospects for biosensing.

Surface Plasmon Resonance: A Versatile Technique for Biosensor Applications

Abstract: Surface plasmon resonance (SPR) is a label-free detection method which has emerged during the last two decades as a suitable and reliable platform in clinical analysis for biomolecular interactions. The technique makes it possible to measure interactions in real-time with high sensitivity and without the need of labels. This review article discusses a wide range of applications in optical-based sensors using either surface plasmon resonance (SPR) or surface plasmon resonance imaging (SPRI). Here we summarize the principles, provide examples, and illustrate the utility of SPR and SPRI through example applications from the biomedical, proteomics, genomics and bioengineering fields. In addition, SPR signal amplification strategies and surface functionalization are covered in the review.

Plasmon-enhanced optical sensors: a review

Abstract: Surface plasmon resonance (SPR) has found extensive applications in chemi-sensors and biosensors. Plasmons play different roles in different types of optical sensors. SPR transduces a signal in a colorimetric sensor through shifts in the spectral position and intensity in response to external stimuli. SPR can also concentrate the incident electromagnetic field in a nanostructure, modulating fluorescence emission and enabling plasmon-enhanced fluorescence to be used for ultrasensitive detection. Furthermore, plasmons have been extensively used for amplifying a Raman signal in a surface-enhanced Raman scattering sensor. This paper presents a review of recent research progress in plasmon-enhanced optical sensing, giving emphasis on the physical basis of plasmon-enhanced sensors and how these principles guide the design of sensors. In particular, this paper discusses the design strategies for nanomaterials and nanostructures to plasmonically enhance optical sensing signals, also highlighting the applications of plasmon-enhanced optical sensors in healthcare, homeland security, food safety and environmental monitoring.

Plasmon-induced resonance energy transfer for solar energy conversion

Abstract: In Forster resonance energy transfer (FRET), energy non-radiatively transfers from a blue-shifted emitter to a red-shifted absorber by dipole–dipole coupling. This study shows that plasmonics enables the opposite transfer direction, transferring the plasmonic energy towards the short-wavelength direction to induce charge separation in a semiconductor. Plasmon-induced resonance energy transfer (PIRET) differs from FRET because of the lack of a Stoke's shift, non-local absorption effects and a strong dependence on the plasmon's dephasing rate and dipole moment. PIRET non-radiatively transfers energy through an insulating spacer layer, which prevents interfacial charge recombination losses and dephasing of the plasmon from hot-electron transfer. The distance dependence of dipole–dipole coupling is mapped out for a range of detuning across the plasmon resonance. PIRET can efficiently harvest visible and near-infrared sunlight with energy below the semiconductor band edge to help overcome the constraints of band-edge energetics for single semiconductors in photoelectrochemical cells, photocatalysts and photovoltaics. Plasmon-induced resonance energy transfer is revealed and explored for solar energy harvesting from visible and near-infrared light.

Review of plasmonic fiber optic biochemical sensors: improving the limit of detection.

Abstract: This paper presents a brief overview of the technologies used to implement surface plasmon resonance (SPR) effects into fiber-optic sensors for chemical and biochemical applications and a survey of results reported over the last ten years. The performance indicators that are relevant for such systems, such as refractometric sensitivity, operating wavelength, and figure of merit (FOM), are discussed and listed in table form. A list of experimental results with reported limits of detection (LOD) for proteins, toxins, viruses, DNA, bacteria, glucose, and various chemicals is also provided for the same time period. Configurations discussed include fiber-optic analogues of the Kretschmann–Raether prism SPR platforms, made from geometry-modified multimode and single-mode optical fibers (unclad, side-polished, tapered, and U-shaped), long period fiber gratings (LPFG), tilted fiber Bragg gratings (TFBG), and specialty fibers (plastic or polymer, microstructured, and photonic crystal fibers). Configurations involving the excitation of surface plasmon polaritons (SPP) on continuous thin metal layers as well as those involving localized SPR (LSPR) phenomena in nanoparticle metal coatings of gold, silver, and other metals at visible and near-infrared wavelengths are described and compared quantitatively.

Facile Photochemical Synthesis of Au/Pt/g-C3N4 with Plasmon-Enhanced Photocatalytic Activity for Antibiotic Degradation

Abstract: A novel plasmonic photocatalyst, Au/Pt/g-C3N4, was prepared by a facile calcination-photodeposition technique. The samples were characterized by X-ray diffraction, energy-dispersive spectroscopy, transmission electron microscopy, and UV–vis diffuse reflectance spectroscopy, and the results demonstrated that the Au and Pt nanoparticles (7–15 nm) were well-dispersed on the surfaces of g-C3N4. The Au/Pt codecorated g-C3N4 heterostructure displayed enhanced photocatalytic activity for antibiotic tetracycline hydrochloride (TC-HCl) degradation, and the degradation rate was 3.4 times higher than that of pure g-C3N4 under visible light irradiation. The enhancement of photocatalytic activity could be attributed to the surface plasmon resonance effect of Au and electron-sink function of Pt nanoparticles, which improve the optical absorption property and photogenerated charge carriers separation of g-C3N4, synergistically facilitating the photocatalysis process. Finally, a possible photocatalytic mechanism for degr...

Tungsten Oxide Single Crystal Nanosheets for Enhanced Multichannel Solar Light Harvesting

Abstract: Substoichiometric tungsten oxide single-crystal nanosheets are successfully prepared via the exfoliation of layered tungstic acid and subsequent introduction of oxygen vacancies. The combination of different strategies, i.e., 2D-structure construction, the introduction of surface oxygen vacancies, and the creation of localized surface plasmon resonance can promote the light-harvesting performance of tungsten oxide through accumulative and synergistic effects.

Graphene–MoS2 hybrid nanostructures enhanced surface plasmon resonance biosensors

Abstract: In this work, we propose a new configuration of surface plasmon resonance (SPR) sensor that is based on graphene–MoS2 hybrid structures for ultrasensitive detection of molecules. The proposed system displays a phase-sensitivity enhancement factor of more than 500-fold when compared to the SPR sensing scheme without the graphene–MoS2 coating or with only graphene coating. Our hypothesis is that the monolayer MoS2 has a much higher optical absorption efficiency (∼5%) than that of the graphene layer (∼2.3%). Based on our findings, the electron energy loss of MoS2 layer is comparable to that of graphene and this will allow a successful (∼100%) of light energy transfer to the graphene–MoS2 coated sensing substrate. Such process will lead to a significant enhancement of SPR signals. Our simulation shows that a quasi-dark point of the reflected light can be achieved under this condition and this has resulted in a steep phase jump at the resonance angle of our newly proposed SPR system. More importantly, we found that phase interrogation detection approach of the graphene–MoS2 hybrid structures-based sensing system is more sensitive than that of using the regularly angular interrogation method and our theoretical analysis indicates that 45 nm of Au film thickness and 3 coating layers of MoS2 nanosheet are the optimized parameters needed for the proposed SPR system to achieve the highest detection sensitivity range.

Localized Surface Plasmon Resonance Biosensing: Current Challenges and Approaches

Abstract: Localized surface plasmon resonance (LSPR) has emerged as a leader among label-free biosensing techniques in that it offers sensitive, robust, and facile detection. Traditional LSPR-based biosensing utilizes the sensitivity of the plasmon frequency to changes in local index of refraction at the nanoparticle surface. Although surface plasmon resonance technologies are now widely used to measure biomolecular interactions, several challenges remain. In this article, we have categorized these challenges into four categories: improving sensitivity and limit of detection, selectivity in complex biological solutions, sensitive detection of membrane-associated species, and the adaptation of sensing elements for point-of-care diagnostic devices. The first section of this article will involve a conceptual discussion of surface plasmon resonance and the factors affecting changes in optical signal detected. The following sections will discuss applications of LSPR biosensing with an emphasis on recent advances and approaches to overcome the four limitations mentioned above. First, improvements in limit of detection through various amplification strategies will be highlighted. The second section will involve advances to improve selectivity in complex media through self-assembled monolayers, “plasmon ruler” devices involving plasmonic coupling, and shape complementarity on the nanoparticle surface. The following section will describe various LSPR platforms designed for the sensitive detection of membrane-associated species. Finally, recent advances towards multiplexed and microfluidic LSPR-based devices for inexpensive, rapid, point-of-care diagnostics will be discussed.

Multifunctional Au-ZnO Plasmonic Nanostructures for Enhanced UV Photodetector and Room Temperature NO Sensing Devices

Abstract: In this study we report the enhancement of UV photodetection and wavelength tunable light induced NO gas sensing at room temperature using Au-ZnO nanocomposites synthesized by a simple photochemical process Plasmonic Au-ZnO nanostructures with a size less than the incident wavelength have been found to exhibit a localized surface plasmon resonance (LSPR) that leads to a strong absorption, scattering and local field enhancement The photoresponse of Au-ZnO nanocomposite can be effectively enhanced by 80 times at 335 nm over control ZnO We also demonstrated Au-ZnO nanocomposite's application to wavelength tunable gas sensor operating at room temperature The sensing response of Au-ZnO nancomposite is enhanced both in UV and visible region, as compared to control ZnO The sensitivity is observed to be higher in the visible region due to the LSPR effect of Au NPs The selectivity is found to be higher for NO gas over CO and some other volatile organic compounds (VOCs), with a minimum detection limit of 01 ppb for Au-ZnO sensor at 335 nm

Optical characterization of single plasmonic nanoparticles

Abstract: This tutorial review surveys the optical properties of plasmonic nanoparticles studied by various single particle spectroscopy techniques. The surface plasmon resonance of metallic nanoparticles depends sensitively on the nanoparticle geometry and its environment, with even relatively minor deviations causing significant changes in the optical spectrum. Because for chemically prepared nanoparticles a distribution of their size and shape is inherent, ensemble spectra of such samples are inhomogeneously broadened, hiding the properties of the individual nanoparticles. The ability to measure one nanoparticle at a time using single particle spectroscopy can overcome this limitation. This review provides an overview of different steady-state single particle spectroscopy techniques that provide detailed insight into the spectral characteristics of plasmonic nanoparticles.

Plasmon-Enhanced Formic Acid Dehydrogenation Using Anisotropic Pd–Au Nanorods Studied at the Single-Particle Level

Abstract: Plasmonic bimetal nanostructures can be used to drive the conventional catalytic reactions efficiently at low temperature with the utilization of solar energy. This work developed Pd-modified Au nanorods, which work as the light absorber and the catalytically active site simultaneously, and exhibit efficient plasmon-enhanced catalytic formic acid dehydrogenation even when below room temperature (5 °C). Plasmon-induced interface interaction and photoreaction dynamics of individual nanorods were investigated by single-particle photoluminescence measurement, and a complete quenching phenomenon at the LSPR region was observed for the first time. More importantly, the spatial distribution of the SPR-induced enhancement, analyzed by the finite difference time domain (FDTD) simulation, shows that only tip-coated Pd can be affected for the occurrence of plasmon resonance energy transfer. This finding provides a route to decrease the amount of Pd species by the selective deposition only at the field-enhanced sites.

Optical Properties and Plasmonic Performance of Titanium Nitride

Abstract: Titanium nitride (TiN) is one of the most well-established engineering materials nowadays. TiN can overcome most of the drawbacks of palsmonic metals due to its high electron conductivity and mobility, high melting point and due to the compatibility of its growth with Complementary Metal Oxide Semiconductor (CMOS) technology. In this work, we review the dielectric function spectra of TiN and we evaluate the plasmonic performance of TiN by calculating (i) the Surface Plasmon Polariton (SPP) dispersion relations and (ii) the Localized Surface Plasmon Resonance (LSPR) band of TiN nanoparticles, and we demonstrate a significant plasmonic performance of TiN.

Fano Resonance and Spectrally Modified Photoluminescence Enhancement in Monolayer MoS2 Integrated with Plasmonic Nanoantenna Array

Abstract: The manipulation of light-matter interactions in two-dimensional atomically thin crystals is critical for obtaining new optoelectronic functionalities in these strongly confined materials. Here, by integrating chemically grown monolayers of MoS2 with a silver-bowtie nanoantenna array supporting narrow surface-lattice plasmonic resonances, a unique two-dimensional optical system has been achieved. The enhanced exciton-plasmon coupling enables profound changes in the emission and excitation processes leading to spectrally tunable, large photoluminescence enhancement as well as surface-enhanced Raman scattering at room temperature. Furthermore, due to the decreased damping of MoS2 excitons interacting with the plasmonic resonances of the bowtie array at low temperatures stronger exciton-plasmon coupling is achieved resulting in a Fano line shape in the reflection spectrum. The Fano line shape, which is due to the interference between the pathways involving the excitation of the exciton and plasmon, can be tuned by altering the coupling strengths between the two systems via changing the design of the bowties lattice. The ability to manipulate the optical properties of two-dimensional systems with tunable plasmonic resonators offers a new platform for the design of novel optical devices with precisely tailored responses.

Photonic Crystal Fiber-Based Surface Plasmon Resonance Sensor with Selective Analyte Channels and Graphene-Silver Deposited Core

Abstract: We propose a surface plasmon resonance (SPR) sensor based on photonic crystal fiber (PCF) with selectively filled analyte channels. Silver is used as the plasmonic material to accurately detect the analytes and is coated with a thin graphene layer to prevent oxidation. The liquid-filled cores are placed near to the metallic channel for easy excitation of free electrons to produce surface plasmon waves (SPWs). Surface plasmons along the metal surface are excited with a leaky Gaussian-like core guided mode. Numerical investigations of the fiber’s properties and sensing performance are performed using the finite element method (FEM). The proposed sensor shows maximum amplitude sensitivity of 418 Refractive Index Units (RIU−1) with resolution as high as 2.4 × 10−5 RIU. Using the wavelength interrogation method, a maximum refractive index (RI) sensitivity of 3000 nm/RIU in the sensing range of 1.46–1.49 is achieved. The proposed sensor is suitable for detecting various high RI chemicals, biochemical and organic chemical analytes. Additionally, the effects of fiber structural parameters on the properties of plasmonic excitation are investigated and optimized for sensing performance as well as reducing the sensor’s footprint.

Surface plasmon resonance sensor based on D-shaped microstructured optical fiber with hollow core.

Abstract: To solve the phase matching and analyte filling problems in the microstructured optical fiber (MOF)-based surface plasmon resonance (SPR) sensors, we present the D-shaped hollow core MOF-based SPR sensor. The air hole in the fiber core can lower the refractive index of a Gaussian-like core mode to match with that of a plasmon mode. The analyte is deposited directly onto the D-shaped flat surface instead of filling the fiber holes. We numerically investigate the effect of the air hole in the core on the SPR sensing performance, and identify the sensor sensitivity on wavelength, amplitude and phase. This work allows us to determine the feasibility of using the D-shaped hollow-core MOFs to develop a high-sensitivity, real-time and distributed SPR sensor.

Controlling Plasmon-Induced Resonance Energy Transfer and Hot Electron Injection Processes in Metal@TiO2 Core–Shell Nanoparticles

Abstract: Plasmonic metals can excite charge carriers in semiconductors through plasmon-induced resonance energy transfer (PIRET) and hot electron injection processes. Transient absorption spectroscopy reveals that the presence of plasmon-induced charge separation mechanisms in metal@TiO2 core–shell nanoparticles can be controlled by tailoring the spectral overlap and the physical contact between the metal and the semiconductor. In Ag@SiO2@TiO2 sandwich nanoparticles, the localized surface plasmon resonance band is overlapped with the absorption band edge of TiO2, enabling PIRET, while the SiO2 barrier prevents hot electron transfer. In Au@TiO2, hot electron injection occurs, but the lack of spectral overlap disables PIRET. In Ag@TiO2, both hot electron transfer and PIRET take place. In Au@SiO2@TiO2, photoconversion in TiO2 is not enhanced by the plasmon despite strong light absorption by Au.

Fano Resonant Aluminum Nanoclusters for Plasmonic Colorimetric Sensing.

Abstract: Aluminum is an abundant and high-quality material for plasmonics with potential for large-area, low-cost photonic technologies. Here we examine aluminum nanoclusters with plasmonic Fano resonances that can be tuned from the near-UV into the visible region of the spectrum. These nanoclusters can be designed with specific chromaticities in the blue-green region of the spectrum and exhibit a remarkable spectral sensitivity to changes in the local dielectric environment. We show that such structures can be used quite generally for colorimetric localized surface plasmon resonance (LSPR) sensing, where the presence of analytes is detected by directly observable color changes rather than through photodetectors and spectral analyzers. To quantify our results and provide a metric for optimization of such structures for colorimetric LSPR sensing, we introduce a figure of merit based on the color perception ability of the human eye.

Sensing using plasmonic nanostructures and nanoparticles.

Abstract: Nanoparticles are widely used in various fields of science and technology as well as in everyday life. In particular, gold and silver nanoparticles display unique optical properties that render them extremely attractive for various applications. In this review, we focus on the use of noble metal nanoparticles as plasmonic nanosensors with extremely high sensitivity, even reaching single molecule detection. Sensors based on plasmon resonance shifts, as well as the use of surface-enhanced Raman scattering and surface-enhanced fluorescence, will be considered in this work.

Laser Trapping of Colloidal Metal Nanoparticles

Abstract: Optical trapping using focused laser beams (laser tweezers) has been proven to be extremely useful for contactless manipulation of a variety of small objects, including biological cells, organelles within cells, and a wide range of other dielectric micro- and nano-objects. Colloidal metal nanoparticles have drawn increasing attention in the field of optical trapping because of their unique interactions with electromagnetic radiation, caused by surface plasmon resonance effects, enabling a large number of nano-optical applications of high current interest. Here we try to give a comprehensive overview of the field of laser trapping and manipulation of metal nanoparticles based on results reported in the recent literature. We also discuss and describe the fundamentals of optical forces in the context of plasmonic nanoparticles, including effects of polarization, optical angular momentum, and laser heating effects, as well as the various techniques that have been used to trap and manipulate metal nanoparticles. We conclude by suggesting possible directions for future research.

Mid-Infrared Plasmonic Biosensing with Graphene

Abstract: Infrared spectroscopy is the technique of choice for chemical identification of biomolecules through their vibrational fingerprints. However, infrared light interacts poorly with nanometric size molecules. Here, we exploit the unique electro-optical properties of graphene to demonstrate a high-sensitivity tunable plasmonic biosensor for chemically-specific label-free detection of protein monolayers. The plasmon resonance of nanostructured graphene is dynamically tuned to selectively probe the protein at different frequencies and extract its complex refractive index. Additionally, the extreme spatial light confinement in graphene, up to two orders of magnitude higher than in metals, produces an unprecedentedly high overlap with nanometric biomolecules, enabling superior sensitivity in the detection of their refractive index and vibrational fingerprints. The combination of tunable spectral selectivity and enhanced sensitivity of graphene opens exciting prospects for biosensing.

Production of Monodisperse Gold Nanobipyramids with Number Percentages Approaching 100% and Evaluation of Their Plasmonic Properties

Abstract: Gold nanobipyramids (NBPs) and nanorods (NRs) are two common types of elongated colloidal plasmonic metal nanocrystals, with their longitudinal plasmon wavelengths synthetically tunable over a wide spectral range. Au NBPs have sharper tips and narrower shape and size distributions than Au NRs. However, the number percentages of Au NBPs have been limited below ≈60%. Herein, a method for producing Au NBPs with number percentages approaching 100% and longitudinal plasmon resonance wavelengths synthetically tuned from ≈700 to ≈1200 nm is reported. This method relies on a stepwise combination of seed-mediated growth, Ag overgrowth, depletion force-induced self-separation, and final chemical etching of Ag. The obtained Au NBPs have the same shapes and sizes as the directly grown ones. Systematic comparisons of the plasmonic properties between the purified Au NBP and high-yield single-crystalline Au NR samples show unambiguously that Au NBPs are superior to Au NRs in terms of the plasmon peak width, refractive index sensitivity, figure of merit, two-photon photoluminescence, and surface-enhanced Raman scattering.

Gold Nanocluster-Assisted Fluorescent Detection for Hydrogen Peroxide and Cholesterol Based on the Inner Filter Effect of Gold Nanoparticles.

Abstract: We developed a simple, sensitive inner filter effect (IFE)-based fluorescent assay for sensing H2O2 and cholesterol. In the process, poly(vinylpyrrolidone)-protected gold nanoparticles (PVP-AuNPs) and fluorescent BSA-protected gold nanoclusters (BSA-AuNCs) were used as an IFE absorber/fluorophore pair. PVP-AuNPs can be a powerful absorber to influence the emission of the fluorophore, BSA-AuNCs, in the IFE-based fluorescent assays. That is due to the high extinction coefficient of AuNPs and the complementary overlap between the surface plasmon resonance (SPR) absorption of PVP-AuNPs and the excitation of BSA-AuNCs. The PVP-Au seeds, produced by directly mixing PVP with HAuCl4, were able to catalyze H2O2 to enlarge AuNPs. The SPR absorption of PVP-AuNPs was enhanced with an increased concentration of H2O2 and, subsequently, induced significant fluorescence quenching of BSA-AuNCs. The IFE-based fluorescent assay enabled the detection of H2O2 and generation of H2O2 in the presence of O2/cholesterol and choles...

Hierarchical Porous Plasmonic Metamaterials for Reproducible Ultrasensitive Surface‐Enhanced Raman Spectroscopy

Abstract: Hierarchical porous plasmonic metamaterials consisting of periodic nanoholes with tunable diameter and uniformly distributed mesopores over the bulk are developed as a new class of 3D surface-enhanced Raman spectroscopy (SERS) substrates. This multiscale architecture not only facilitates efficient cascaded electromagnetic enhancement but also provides an enormous number of Raman-active binding sites, exhibiting excellent reproducibility and ultrasensitive detection of aromatic molecules down to 10(-13) M.

Noncovalently functionalized monolayer graphene for sensitivity enhancement of surface plasmon resonance immunosensors.

Abstract: A highly efficient surface plasmon resonance (SPR) immunosensor is described using a functionalized single graphene layer on a thin gold film. The aim of this approach was two-fold: first, to amplify the SPR signal by growing graphene through chemical vapor deposition and, second, to control the immobilization of biotinylated cholera toxin antigen on copper coordinated nitrilotriacetic acid (NTA) using graphene as an ultrathin layer. The NTA groups were attached to graphene via pyrene derivatives implying π–π interactions. With this setup, an immunosensor for the specific antibody anticholera toxin with a detection limit of 4 pg mL–1 was obtained. In parallel, NTA polypyrrole films of different thicknesses were electrogenerated on the gold sensing platform where the optimal electropolymerization conditions were determined. For this optimized polypyrrole-NTA setup, the simple presence of a graphene layer between the gold and polymer film led to a significant increase of the SPR signal.

On the Performance of Graphene-Based D-Shaped Photonic Crystal Fibre Biosensor Using Surface Plasmon Resonance

Abstract: We propose a D-shaped photonic crystal fibre-based surface plasmon resonance sensor considering graphene on silver for sensing of refractive index of analyte and thickness of biolayer. The different structural and material parameters associated with sensor have been optimised. Graphene not only helps in adsorption of biomolecules due to π-π stacking interaction but at the same time prevents oxidation of metal-like silver. Numerical simulation shows that amplitude sensitivity of the proposed structure for chemical analytes is 216 RIU−1 (refractive index unit) with a resolution of 4.6 × 10−5 RIU while the wavelength sensitivity of the proposed sensor is found to be as high as 3700 nm RIU−1 with resolution of 2.7 × 10−5 RIU. Further, the proposed sensor can also be used for the detection of biolayer thickness in both amplitude and wavelength interrogations. An amplitude sensitivity of 0.26 nm−1 with resolution of 39 pm and wavelength sensitivity of 2 nm nm−1 with resolution of 50 pm is achievable for the determination of biolayer thickness. The proposed structure is easy to use as there is no need of filling of voids, and the analytes can be placed easily on the flat surface of photonic crystal fibre (PCF).

Surface Plasmon Resonance Photonic Crystal Fiber Biosensor: A Practical Sensing Approach

Abstract: We propose a simple, two rings, hexagonal lattice photonic crystal fiber biosensor using surface plasmon resonance phenomenon. An active plasmonic gold layer and the analyte (sample) are placed outside the fiber structure instead of inside the air-holes, which will result in a simpler and straight forward fabrication process. The proposed sensor exhibits birefringent behavior that enhances its sensitivity. Numerical investigation of the guiding properties and sensing performance are conducted by finite element method. Using wavelength and amplitude interrogation methods, the proposed sensor could provide maximum sensitivity of 4000 nm/RIU and 320 RIU $^{-1}$ , respectively. The resolutions of the sensor are $2.5 \times 10^{-5}$ and $3.125 \times 10^{-5}$ RIU for wavelength and amplitude interrogation modes. The proposed sensor design shows promising results that could be used in biological and biochemical analytes detection.

Distance-Dependent Plasmon-Enhanced Fluorescence of Upconversion Nanoparticles using Polyelectrolyte Multilayers as Tunable Spacers

Abstract: Lanthanide-doped upconversion nanoparticles (UCNPs) have attracted widespread interests in bioapplications due to their unique optical properties by converting near infrared excitation to visible emission. However, relatively low quantum yield prompts a need for developing methods for fluorescence enhancement. Plasmon nanostructures are known to efficiently enhance fluorescence of the surrounding fluorophores by acting as nanoantennae to focus electric field into nano-volume. Here, we reported a novel plasmon-enhanced fluorescence system in which the distance between UCNPs and nanoantennae (gold nanorods, AuNRs) was precisely tuned by using layer-by-layer assembled polyelectrolyte multilayers as spacers. By modulating the aspect ratio of AuNRs, localized surface plasmon resonance (LSPR) wavelength at 980 nm was obtained, matching the native excitation of UCNPs resulting in maximum enhancement of 22.6-fold with 8 nm spacer thickness. These findings provide a unique platform for exploring hybrid nanostructures composed of UCNPs and plasmonic nanostructures in bioimaging applications.