Surface Plasmon Coupling Effect of Gold Nanoparticles with Different Shape and Size on Conventional Surface Plasmon Resonance Signal

The labeling strategy with gold nanoparticles for the conventional surface plasmon resonance (SPR) signal enhancement has been frequently used for the sensitive determination of small molecules binding to its interaction partners. However, the influence of gold nanoparticles with different size and shape on SPR signal is not known. In this paper, three kinds of gold nanoparticles, namely nanorods, nanospheres, and nanooctahedrons with different size, were prepared and used to investigate their effects on the conventional SPR signal at a fixed excitation wavelength 670 nm. It was found that the SPR signal (i.e., resonant angle shift) was varied with the shapes and sizes of gold nanoparticles in suspension at a fixed concentration due to their different plasmon absorbance bands. For gold nanorods with different longitudinal absorbance bands, three conventional SPR signal regions could be clearly observed when the gold nanorod suspensions were separately introduced onto the SPR sensor chip surface. One region was the longitudinal absorbance bands coinciding with or close to the SPR excitation wavelength that suppressed the SPR angle shift. The second region was the longitudinal absorbance bands at 624 to 639 and 728 to 763 nm that produced a moderate increase on the SPR resonant angle shift. The third region was found for the longitudinal absorbance bands from 700 to 726 nm that resulted in a remarkable increase in the SPR angle shift responses. This phenomenon can be explained on the basis of calculation of the correlation of SPR angle shift response with the gold nanorod longitudinal absorbance bands. For nanospheres and nanooctahedrons, the SPR angle shift responses were found to be particle shape and size dependent in a simple way with a sustaining increase when the sizes of the nanoparticles were increased. Consequently, a guideline for choosing gold nanoparticles as tags is suggested for the SPR determination of small molecules with binding to the immobilized interaction partners.     

Long Range Surface Plasmons for Observation of Biomolecular Binding Events at Metallic Surfaces

A long range surface plasmon (LRSP) is an electromagnetic wave propagating along a thin metal film with an order of magnitude lower damping than conventional surface plasmon (SP) waves. Thus, the excitation of LRSP is associated with a narrower resonance and it provides larger enhancement of intensity of the electromagnetic field. In surface plasmon resonance (SPR) biosensors, these features allow a more precise observation of the binding of biomolecules in the proximity to the metal surface by using the (label-free) measurement of refractive index (RI) variations and by SP-enhanced fluorescence spectroscopy. In this contribution, we investigate LRSPs excited on a layer structure consisting of a fluoropolymer buffer layer, a thin gold film, and an aqueous sample. By implementing such structure in an SPR sensor, we achieved a 2.4- and 4.4-fold improvement of the resolution in the label-free and fluorescence-based detection, respectively, of the binding of biomolecules in the close proximity to the surface. Moreover, we demonstrate that the sensor resolution can be improved by a factor of 14 and 12 for the label-free and fluorescence-based detection, respectively, if the biomolecular binding events occur within the whole evanescent field of LRSP.     

Bragg-Scattered Surface Plasmon Microscopy: Theoretical Study

We present a new approach to surface plasmon microscopy with high refractive index sensitivity and spatial resolution that is not limited by the propagation length of surface plasmons. It is based on a nanostructured metallic sensor surface supporting Bragg-scattered surface plasmons. We show that these non-propagating surface plasmon modes are excellently suited for spatially resolved observations of refractive index variations on the sensor surface owing to their highly confined field profile perpendicular to as well as parallel to the metal interface. The presented theoretical study reveals that this approach enables reaching similar refractive index sensitivity as regular surface plasmon resonance (SPR) microscopy and offers the advantage of improved spatial resolution when observing dielectric features with lateral size <10???m for the wavelength around 800?nm and gold as the SPR-active metal. This paper demonstrates the potential of Bragg-scattered surface plasmon microscopy for high-throughput SPR biosensing with high-density microarrays.     

High-Sensitivity Refractive Index Sensor Based on D-Shaped Photonic Crystal Fiber with Rectangular Lattice and Nanoscale Gold Film

We propose and investigate a D-shaped photonic fiber refractive index sensor with rectangular lattice based on surface plasmon resonance. In such sensor, the nanoscale gold metal film is deposited on the flat surface where it is side polished. Numerical results show that the average sensitivity of Au-metalized surface plasmon resonance (SPR) sensor could reach as high as 8,129 nm/refractive index unit (RIU) in the dynamic index range from 1.35 to 1.41 as well as 2,000 nm/RIU from 1.33 to 1.35. Compared to conventional Au-metalized SPR sensors, the performance of our device is obviously better, and the production process is greatly simplified.     

A miniature fiber optic surface plasmon resonance sensor for fast detection of Staphylococcal enterotoxin B

A fiber optic surface plasmon resonance (SPR) biosensor for detection of Staphylococcal enterotoxin B (SEB) is reported. The sensor is based on spectral interrogation of surface plasmons in a miniature sensing element based on a side-polished single-mode optical fiber with a thin metal overlayer. For specific detection of SEB, the SPR sensor is functionalized with a covalently crosslinked double-layer of antibodies against SEB. The SPR biosensor is demonstrated to be able to detect ng/ml concentrations of SEB in less than 10 min.     

Enhanced SPR Sensitivity with Nano-Micro-Ribbon Grating—an Exhaustive Simulation Mapping

In this study, we theoretically investigate the sensing potential of 2D nano- and micro-ribbon grating structuration on the surface of Kretschmann-based surface plasmon resonance (SPR) biosensors when they are employed for detection of biomolecular binding events. Numerical simulations were carried out by employing a model based on the hybridization of two classical methods, the Fourier modal method and the finite element method. Our calculations confirm the importance of light manipulation by means of structuration of the plasmonic thin film surfaces on the nano- and micro-scales. Not only does it highlight the geometric parameters that allow the sensitivity enhancement compared with the response of the conventional SPR biosensor based on a flat surface but also describes the transition from the regime where the propagating surface plasmon mode dominates to the regime where the localized surface plasmon mode dominates. An exhaustive mapping of the biosensing potential of the 2D nano- and micro-structured biosensors surface is presented, varying the structural parameters related to the ribbon grating dimensions, i.e., the widths and thicknesses. The nano- and micro-structuration also leads to the creation of regions on biosensor chips that are characterized by strongly enhanced electromagnetic (EM) fields. New opportunities for further improving the sensitivity are offered if localization of biomolecules can be carried out in these regions of high EM fields. The continuum of nano- and micro-ribbon structured biosensors described in this study should prove a valuable tool for developing sensitive and reliable 2D-structured plasmonic biosensors.     

Nanostructuring of Continuous Gold Film by Laser Radiation Under Surface Plasmon Polariton Resonance Conditions

The physical mechanisms of metallic nanoparticles formation by laser technology were studied. The system air/Au film/glass was irradiated by laser at the conditions of surface plasmon resonance. A surface electromagnetic wave was excited in Kretchmann configuration by the fundamental and second harmonics of the Q-switched YAG/Nd⁺³ laser with pulse power density close to the threshold of melting. Nanostructuring of Au film was observed only for the second harmonic (λ = 0.532 μm) irradiation at the surface plasmon polariton resonance (SPR) conditions. Estimations were done using the interference model of the differently directed plasmon polariton waves excited by a surface electromagnetic wave on the metal surface. It was shown that a regular pattern of locally heated spots can be formed in a metallic film by pulsed laser irradiation. The spatial distribution of this pattern is close to the period of interference. The observed effect of laser nanofragmentation is explained by the self-organization of plasmon polariton subsystem in the process of Au nanoparticles formation at high laser intensity levels. These methods open new possibilities for nanostructured surfaces formation utilizing simple self-organization processes.     

Plasmonic and Energy Studies of Ag Nanoparticles in Silica-Titania Hosts

The present work reports an investigation of surface plasmon resonance (SPR) of silver nanoparticles in SiO₂–TiO₂ hosts. The surface plasmon resonance of silver nanoparticles was observed in the wavelength range 300–400 nm. Numerical calculation of SPR of silver nanoparticles with spherical morphology was done on the basis of discrete dipole approximation (DDA) method. The observed fluorescence spectrum fits well with the theoretically calculated one. The luminescence enhancement is attributed to the strong local electric field which increases the exciting and emitting photons coupled to SPR. An effort has been made to study the surface plasmon mediated excitation energy transfer (EET) between two spherical metal nanoparticles. The van der Waals (vdW) energy between plasmonic silver nanoparticles in the present hosts has been estimated.     

Metallic Nanowire Array–Polymer Hybrid Film for Surface Plasmon Resonance Sensitivity Enhancement and Spectral Range Enlargement

In this paper, the coupling interaction is investigated between a metallic nanowire array and a metal film under the Kretschmann condition. The plasmonic multilayer is composed of a metallic nanowire array embedded in a polymer layer positioned above a metal film, exploiting the classical surface plasmon resonance (SPR) configuration. We analyze the influence of various structural parameters of the metallic nanowire array on the SPR spectrum of thin metal film. The results show that the coupling interactions of nanowires with the metal film can greatly affect SPR resonance wavelength and increase SPR sensitivity. The coupling strength of metallic nanowire array and metal film also impacts resonance wavelength, which can be used to adjust SPR range but have little effect on its sensitivity. The results are confirmed using a dipole coupling resonance model of metallic nanowire. We demonstrated that this nanostructured hybrid structure can be used for high sensitivity SPR monitoring in a large spectral range, which is important for advanced SPR measurement including fiber-optic SPR sensing technology.     

Sensitivity enhancement of optical immunosensors by the use of a surface plasmon resonance fluoroimmunoassay

Optical immunosensors employing evanescent wave techniques have the potential to address the requirements of the 'alternative site' market; however, this potential has yet to be realised. The development of 'direct' sensors, such as those using surface plasmon resonance (SPR), has been hampered by problems of non-specific binding and poor sensitivity to small molecules. 'Indirect' sensors (for example, those employing a fluorescently labelled reagent) overcome many of the problems of direct sensors but require more sophisticated instrumentation because of the low light levels detected. In an attempt to combine the best features of the two techniques, an indirect SPR fluoroimmunoassay (SPRF) technique has been investigated. The surface field intensity enhancement produced by SPR is used to boost the emission from a fluorescently labelled immunoassay complex at a metal surface. The potential of the method is demonstrated by assaying for human Chorionic Gonadotrophin (hCG) in serum. Enhanced sensitivity over conventional total internal reflection fluorescence (TIRF) and SPR techniques was achieved.     

Modulation of Visible and Near-Infrared Surface Plasmon Resonance of Au Nanoparticles Based on Highly Doped Graphene

We study an active modulation of surface plasmon resonance (SPR) of Au nanoparticles based on highly doped graphene in visible and near-infrared regions. We find that compared to the traditional metal SPR, the SPR of Au nanoparticles based on graphene causes a remarkable blue shift. The field intensity in the gap is redistributed to standing wave. The field intensity of standing wave is about one order of magnitude higher than the traditional model. Moreover, the SPR of Au nanoparticles can be actively modulated by varying the graphene Fermi energy. We find the maximum modulation of field intensity of absorption spectra is more than 21.6 % at λ?=?822?nm and the amount of blue shift is 17.4 nm, which is about 2.14 % of the initial wavelength λ ₀?=?813.4?nm, with increasing monolayer graphene Fermi energy from 1.0 to 1.5 ev. We find that the SPR sensitivity to the refractive index n of the environment is about 642 nm per refractive index unit (RIU). The SPR wavelengths have a big blue shift, which is about 33 nm, with increasing number of graphene layers from 1 to 3, and some shoulders on the absorption spectra are observed in the models with multilayer graphene. Finally, we study the Au nanorod array based on monolayer graphene. We find that the blue shift caused by the graphene increases from 14 to 24 nm, with increasing gap g _y from 10 to 20 nm. Then, it decreases from 24 to 14 nm, with increasing gap g _y from 20 to 50 nm. This study provides a new way for actively modulating the optical and optoelectronic devices.     

Surface Plasmon Resonance Intensification by Cavity-Ring Plasma Structure in the Giga-Hertz Regime

In this paper, we propose a novel sub-wavelength plasma structure that can effectively enhance surface plasmon resonance (SPR) to achieve a significant local field. On the basis of a plasma ring structure, we add a slit and two thin plasma layers, working as a metal-insulator-metal (MIM) waveguide at a specific incident wave frequency and generate the Fabry-Perot resonance (FPR). The structure thus couples the incident wave energy to the vicinity of the slit and intensifies the SPR inside the plasma ring. In addition, we also find the coupling and competing between SPR and FPR. For the coupling mode, the average field enhancement in the ring is up to a factor of 9.7. Moreover, the optimized thickness of the plasma layer is much thinner than the skin depth of the plasma to ensure the incident wave easily entering the MIM waveguide. We further calculate the dispersion relationship of surface plasmon polaritons in the waveguide cavity. The simulation results and theoretical dispersion function are in good agreements.

     

SPR biosensing coupled to a digital microfluidic microstreaming system

This article reports on a proof-of-concept system composed of a droplet based surface plasmon resonance (SPR) system coupled to a surface acoustic wave (SAW) microfluidic plateform. It is now well established that surface based binding analyses such as SPR are highly influenced by the transport of analyte to the sensing surface. Further, obtaining reliable equilibrium in flow cells to realize quantification studies is not straightforward. An original solution compared to generally used pressure driven flows is then proposed to favourably cope with these issues. Efficiency of SAW microstreaming coupled to SPR biosensing is considered, in order to improve the accuracy of kinetic parameter estimation in mass transport limited regime and to realize reliable quantification studies. First, the droplet based SPR technique and its advantages are presented. Then, the integration of the microstreaming on the system is discussed. Streptavidin binding is then monitored in static mode and under SAW streaming mode.     

Review of Some Interesting Surface Plasmon Resonance-enhanced Properties of Noble Metal Nanoparticles and Their Applications to Biosystems

Noble metal, especially gold (Au) and silver (Ag) nanoparticles exhibit unique and tunable optical properties on account of their surface plasmon resonance (SPR). In this review, we discuss the SPR-enhanced optical properties of noble metal nanoparticles, with an emphasis on the recent advances in the utility of these plasmonic properties in molecular-specific imaging and sensing, photo-diagnostics, and selective photothermal therapy. The strongly enhanced SPR scattering from Au nanoparticles makes them useful as bright optical tags for molecular-specific biological imaging and detection using simple dark-field optical microscopy. On the other hand, the SPR absorption of the nanoparticles has allowed their use in the selective laser photothermal therapy of cancer. We also discuss the sensitivity of the nanoparticle SPR frequency to the local medium dielectric constant, which has been successfully exploited for the optical sensing of chemical and biological analytes. Plasmon coupling between metal nanoparticle pairs is also discussed, which forms the basis for nanoparticle assembly-based biodiagnostics and the plasmon ruler for dynamic measurement of nanoscale distances in biological systems.     

Metal ion selectivity for formation of the calmodulin-metal-target peptide ternary complex studied by surface plasmon resonance spectroscopy.

Ion selectivities for Ca(2+) signaling pathways of 33 metal ions were examined based on the Ca(2+)-dependent on/off switching mechanism of calmodulin (CaM): Ca(2+) ion-induced selective binding of CaM-Ca(2+) ion complex to the target peptide was observed as an increase in surface plasmon resonance (SPR) signals. As the target peptide, M13 of 26-amino-acid residues derived from skeletal muscle myosin light-chain kinase was immobilized in the dextran matrix, over which sample solutions containing CaM and each metal ion were injected in a flow system. Large changes in SPR signals were also observed for Sr(2+), Ba(2+), Cd(2+), Pb(2+), Y(3+) and trivalent lanthanide ions, thereby indicating that not only Ca(2+) but also these metal ions induce the formation of CaM-M13-metal ion ternary complex. No SPR signal was, however, induced by Mg(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+) and all monovalent metal ions examined. The latter silent SPR signal indicates that these ions, even if they bind to CaM, are incapable of forming the CaM-M13-metal ion ternary complex. Comparing the obtained SPR results with ionic radii of those metal ions, it was found that all cations examined with ionic radii close to or greater than that of Ca(2+) induced the formation of the CaM-metal-M13 ternary complex, whereas those with smaller ionic radii were not effective, or much less so. Since these results are so consistent with earlier systematic data for the effects of various metal ions on the conformational changes of CaM, it is concluded that the present SPR analysis may be used for a simple screening and evaluating method for physiologically relevant metal ion selectivity for the Ca(2+) signaling via CaM based on CaM/peptide interactions.     

Numerical Simulation of Broadband Scattering by Coated and Noncoated Metal Nanostructures Using Discrete Dipole Approximation Method

We suggest numerical method to study the optical response of metal nanostructures. The analysis of optical properties such as scattering and absorption by coated and noncoated nanogeometry has been done using discrete dipole approximation (DDA) method. The core-shell nanogeometry supports surface plasmon resonances, which are highly tunable from 400 to 1100 nm. The tunability of surface plasmon resonance (SPR) highly depends on the structural anisotropy and chosen core-shell material. Further, we have observed that aspect ratio is one of the key parameter to decide the nature and position of the plasmonic peaks and magnitude of optical cross section. We have also shown that coated nanospheroid is a more appropriate geometry as compared to coated nanosphere and noncoated nanospheroid in terms of wide tunability of surface plasmon resonance. The wide tunability in SPR is observed for the effective radii 90 nm core-shell (Au@SiO₂) nanospheroid with aspect ratio 0.1.     

Plasmonic Properties of Silver Nanoparticles on Two Substrates

In this paper, we examine the plasmonic properties of silver nanoparticles, with an emphasis on the sensitivity of the extinction spectra on the supporting substrate: silica (SiO₂) microsphere and indium tin oxide (ITO) coated glass slide, on which silver particles are deposited electroless and electrochemically, respectively. The microstructures and phases of these nanoparticles are characterized by transmission electron microscopy, field emission electron microscopy and X-ray diffraction analysis. The surface plasmon resonance (SPR) properties which are experimentally measured in the ultraviolet-visible-near infrared spectral region are compared to electrodynamics calculations based on the discrete dipole approximation. A wide SPR band ranging from 400 to 800 nm is observed for the silver nanoparticles on a silica microsphere, which is similar to the plasmon resonance characteristics of metal nanoshells. The SPR of a conducting substrate, however, has an effect on the plasmonic properties of silver nanoparticles at longer wavelength.      

Short order nanohole arrays in metals for highly sensitive probing of local indices of refraction as the basis for a highly multiplexed biosensor technology

A small array of subwavelength apertures patterned in a gold film on glass was characterized for use as a biosensor. It is widely believed that such arrays allow the resonance of photons with surface plasmons in the metallic film. Surface plasmon methods (and other evanescent wave methods) are extremely well suited for the measure of real time biospecific interactions. An extremely high sensitivity of 88,000%/refractive index unit was measured on an array with theoretical active area of .09 microm2. The formation of a biological monolayer was monitored. Both sensitivity and resolution were determined through measurement. The measured resolution, for a sensor with an active area of less than 1.5 microm2, is 9.4 x 10(-8) refractive index units which leads to a calculated sensitivity of 3.45E6%/refractive index unit. These values far exceed theoretical and calculated values of other grating coupled surface plasmon resonance (SPR) detectors and prism based SPR detectors. Because the active sensing area can be quite small (.025 microm2) single molecule studies are possible as well as massive multiplexing on a single chip format.     

Sensitivity Enhancement of Ag-ITO-TMDCs-Graphene Nanostructure Based on Surface Plasmon Resonance Biosensors

Plasmonics - The novel configuration of surface plasmon resonance (SPR) based on Ag-indium tin oxide (ITO)-transition metal dichalcogenides (TMDCs)-graphene hybrid structures is proposed for highly...     

Highly Sensitive SPR Sensor Based on Ag-ITO-BlueP/TMDCs-Graphene Heterostructure

Plasmonics - The novel surface plasmon resonance (SPR) sensor based on hybrid structure of Ag-indium tin oxide (ITO)-blue phosphorene (BlueP)/transition metal dichalcogenides (TMDCs)-graphene is...