Plasmon-enhanced Fluorescence Research Articles

Surface plasmon-coupled emission on plasmonic Bragg gratings

Surface plasmon-coupled emission (SPCE) from emitters in a close proximity to a plasmonic Bragg grating is investigated. In this study, the directional fluorescence emission mediated by Bragg-scattered surface plasmons and surface plasmons diffraction cross-coupled through a thin metallic film is observed by using the reverse Kretschmann configuration. We show that controlling of dispersion relation of these surface plasmon modes by tuning the refractive index at upper and lower interfaces of a dense sub-wavelength metallic grating enables selective reducing or increasing the intensity of the light emitted to certain directions. These observations may provide important leads for design of advanced plasmonic structures in applications areas of plasmon-enhanced fluorescence spectroscopy and nanoscale optical sources.

Exploiting the light–metal interaction for biomolecular sensing and imaging

The ability of metal surfaces and nanostructures to localize and enhance optical fields is the primary reason for their application in biosensing and imaging. Local field enhancement boosts the signal-to-noise ratio in measurements and provides the possibility of imaging with resolutions significantly better than the diffraction limit. In fluorescence imaging, local field enhancement leads to improved brightness of molecular emission and to higher detection sensitivity and better discrimination. We review the principles of plasmonic fluorescence enhancement and discuss applications ranging from biosensing to bioimaging.

Plasmon-enhanced upconversion fluorescence in NaYF4:Yb/Er/Gd nanorods coated with Au nanoparticles or nanoshells

We investigate plasmon-enhanced upconversion (UC) fluorescence in Yb3+-Er3+-Gd+3 codoped sodium yttrium fluoride (NaYF4:Yb/Er/Gd) nanorods using gold nanoparticles or nanoshells. A simple method was proposed for the preparation of core/shell NaYF4/Au structures, with dispersed Au nanoparticles or uniform Au coating on the surface of the UC nanorod. Pure hexagonal-phase NaYF4:Yb/Er/Gd nanorods were synthesized via a liquid-solid reaction in oleic acid and ethanol solvent. A one-step approach was introduced to modify the hydrophobic surfaces of the as-deposited NaYF4:Yb/Er/Gd nanorods. After this surface modification, Au nanoparticles or nanoshells were successfully attached on the surfaces of NaYF4:Yb/Er/Gd nanorods. The as-deposited UC nanorods showed a strong UC emission in green and red bands under 980 nm laser excitation. The attachment of Au nanoparticles onto NaYF4:Yb/Er/Gd nanorods resulted in a more than three-fold increase in UC emissions, whereas the formation of continuous and compact Au shells around the nanorods suppressed the emissions. The related interaction mechanisms of the UC emission of NaYF4:Yb/Er/Gd nanorods with plasmon modes in Au nanostructures are analyzed and discussed.

Large enhancement of single molecule fluorescence by coupling to hollow silver nanoshells

We investigated the effect of plasmon-enhanced fluorescence using hollow silver nanoshells and observed enhancement of a single probe's fluorescence up to a factor of 3 orders of magnitude. The enhancement of the quantum efficiency is attributed to enhanced absorption and increased radiative decay rate.

Plasmon-Enhanced Emission of a Phthalocyanine in Polyelectrolyte Films Induced by Gold Nanoparticles

We report on the plasmon-enhanced fluorescence emission achieved in an organic/inorganic hybrid multilayered polyelectrolyte film incorporating aluminum tetrasulfonated phthalocyanine (AlPcTS) and ...

Plasmon-Enhanced Fluorescence and Spectral Modification in SHINEF

Shell-isolated gold nanoparticles provide a reliable, portable, and efficient substrate for luminescence enhancement giving surface-enhanced fluorescence, termed SHINEF. The objectives of the present work are 2-fold: first, tuning the size of the gold core and the shell thickness for maximum SHINEF enhancement, and second, to use the shell isolated nanoparticles (SHINs) to demonstrate the spectral profile modification of the fluorescence spectrum. It is shown, using Langmuir–Blodgett monolayers of a dye with two distinct emissions (monomer and excimer), that the far field radiation of the nanostructure modulate the observed emission from the target fluorophore located in the near field zone of the nanostructure sustaining localized surface plasmon resonances (LSPR).

Plasmon Enhanced Spectroscopy of N,N′-Dialkylquinacridones Used as Codopants in OLEDs

Plasmon-enhanced Raman scattering and plasmon-enhanced fluorescence of the two molecules N,N′-dimethylquinacridone (DMQA) and N,N′-diisoamylquinacridone (DIQA) used as codopants in OLEDs (with improved stability) are reported. The work includes a complete vibrational characterization using Raman and infrared spectra supported by DFT calculations. Plasmon enhancement is achieved using silver and mixed Ag/Au nanostructures: colloids and metal island films, supporting localized surface plasmon resonances (LSPR). However, fine-tuning of the experimental conditions is necessary to attain surface-enhanced (resonance) Raman scattering (SERS/SERRS) of the two molecules with similar structures. SERRS spectra of DMQA are obtained on silver colloids and from a Langmuir–Blodgett (LB) monolayer deposited onto silver island films (SIF). High quality SERRS spectra of DIQA were only obtained from LB layers on mixed Ag/Au island films. Using spacer LB layers of arachidic acid (AA) to separate the fluorophore from the meta...

Long range surface plasmon-coupled fluorescence emission for biosensor applications

A biosensor scheme that employs long range surface plasmons (LRSPs) for the efficient excitation and collection of fluorescence light from fluorophore-labeled biomolecules captured in a three-dimensional hydrogel matrix is discussed. This new approach to plasmon-enhanced fluorescence (PEF) is experimentally and theoretically investigated by using the Kretschmann configuration of attenuated total reflection (ATR) method. A layer structure supporting LRSPs that consists of a low refractive index fluoropolymer layer, a thin gold film and a large binding capacity N-isopropylacrylamide (NIPAAm)-based hydrogel matrix swollen in an aqueous sample is employed. By using this layer architecture, the extended field of LRSPs probes the binding of biomolecules in the binding matrix at up to micrometer distances from the gold surface. With respect to regular surface plasmon-enhanced fluorescence spectroscopy (SPFS) and surface plasmon-coupled emission (SPCE), a narrower angular distribution of the fluorescence light intensity, a larger peak intensity and the excitation and emission at lower angles were observed.

Experimental Evidence of Plasmophores: Plasmon-Directed Polarized Emission from Gold Nanorod–Fluorophore Hybrid Nanostructures

We show that the fluorescence emission from individual hybrid nanostructures composed of Au nanorod cores and oxazine 725-embedded mesostructured silica shells is strongly polarized, with the degree of polarization being equal to that of the light scattered by the nanorod and varying from 0 to 1 as the longitudinal plasmon resonance wavelength is increased. Our analyses indicate that the interactions of the plasmon resonance of the nanorod with the excitation and emission processes of the fluorophores are temporally separated under unsaturated excitation conditions. The emission polarization is found through electrodynamic calculations to arise from the plasmon-coupled emission instead of the plasmon-enhanced excitation polarization. The emission carries the direction and polarization properties that are essentially determined by the dipolar plasmon of the nanorod antenna. Our results therefore provide direct and concrete evidence for the plasmophore that has been proposed recently for plasmon-enhanced fluorescence.

Long-Range Surface Plasmon-Enhanced Fluorescence Spectroscopy Biosensor for Ultrasensitive Detection of E. coli O157:H7

A new biosensor platform for the detection of bacterial pathogens based on long-range surface plasmon-enhanced fluorescence spectroscopy (LRSP-FS) is presented. The resonant excitation of LRSP modes provides an enhanced intensity of the electromagnetic field, which is directly translated to an increased strength of fluorescence signal measured upon the capture of target analyte at the sensor surface. LRSPs originate from a coupling of surface plasmons across a thin metallic film embedded in dielectrics with similar refractive indices. With respect to regular surface plasmon-enhanced fluorescence spectroscopy, the excitation of LRSPs offers the advantage of a larger enhancement of the evanescent field intensity and a micrometer probing depth that is comparable to the size of target bacterial pathogens. The potential of the developed sensor platform is demonstrated in an experiment in which the detection of E. coli O157:H7 was carried out using sandwich immunoassays. The limit of detection below 10 cfu mL(-1) and detection time of 40 min were achieved.

Excitation and Reemission of Molecules near Realistic Plasmonic Nanostructures

The enhancement of excitation and reemission of molecules in close proximity to plasmonic nanostructures is studied with special focus on the comparison between idealized and realistically shaped nanostructures. Numerical experiments show that for certain applications choosing a realistic geometry closely resembling the actual nanostructure is imperative, an idealized simulation geometry yielding significantly different results. Finally, a link between excitation and reemission processes is formed via the theory of optical reciprocity, allowing a transparent view of the electromagnetic processes involved in plasmon-enhanced fluorescence and Raman-scattering.

Plasmon-Enhanced Resonance Raman Scattering and Fluorescence in Langmuir−Blodgett Monolayers

Localized surface plasmon resonances in silver and gold nanostructures are engaged to enhance the inelastic Raman scattering and the fluorescence of a phopholipid containing a sulforhodamine 101 acid chloride dye known as Texas Red. The most efficient coupling and enhancement are attained when the excitation laser line is in resonance with both the chromophore and the plasmon absorption of the nanostructure, the case of surface-enhanced resonance Raman scattering, allowing single-molecule detection. The tagged phospholipid was incorporated into a single fatty acid Langmuir monolayer at varying concentrations and transferred onto an evaporated Ag nanoparticle film. Surface-enhanced fluorescence is achieved using shell-isolated silica-coated gold nanoparticles, an emission enhancement named SHINEF.

Localized surface plasmon-enhanced fluorescence spectroscopy for highly-sensitive real-time detection of DNA hybridization

Versatile and highly-sensitive detection of DNA hybridization is described using metal nanostructures-enhanced fluorescence (MEF) emission intensity when fluorescently-labeled DNA oligomers are covalently immobilized on a nanometer-thin amorphous silicon–carbon layer capping the metal nanostructures. The MEF structures are formed by thermal deposition of silver, gold or silver/gold thin films on glass surfaces and post-annealing at 500 °C. The choice of the metal film allows for tuning the optical properties of the interface. The metallic nanostructures are subsequently coated with an amorphous thin silicon–carbon alloy ( a-Si 0.80C 0.20: H) layer deposited by PECVD. Carboxydecyl groups are attached on these surfaces through hydrosilylation then reacted with amine-terminated single-stranded DNA oligomers, forming a covalent link. The immobilized DNA is hybridized with its complementary strand carrying a fluorescent label. Through optimization of the thickness of the a-Si 0.80C 0.20: H alloy overlayer and by working close to resonance conditions for plasmon and fluorophore excitation, the hybridization of very dilute oligomers (5 fM) is easily detected, and the hybridization kinetics can be monitored in situ and in real-time.

Plasmon-enhanced fluorescence from single fluorophores end-linked to gold nanorods.

We reported a method to fabricate fluorescent probes preferentially end-linked to Au nanorods. In comparison with organic dyes, the single nanocomplexes provide significant improvement in signal brightness. The bioconjugated nanoantenna structure implicates an approach that is more convenient and amenable to integration for photonic, optoelectronic, and biotechnological applications.

Optimization of layer structure supporting long range surface plasmons for surface plasmon-enhanced fluorescence spectroscopy biosensors

Long range surface plasmons (LRSPs) are optical waves that propagate along thin metallic films with up to orders of magnitude lower damping compared to regular surface plasmons. Therefore, LRSPs attracted a great deal of attention for development of ultrasensitive biosensors based on surface plasmon resonance and surface plasmon-enhanced fluorescence spectroscopy (SPFS). In this article, the authors investigate the excitation of LRSPs on a biosensor-compatible layer structure consisting of a Cytop fluoropolymer, thin gold film modified by thiol self-assembled monolayer for coupling of receptor biomolecules, and an aqueous sample on its top. The morphology, and optical and electrical properties of the layer structure are determined and related to the performance of a SPFS biosensor. Through increasing the surface energy of Cytop fluoropolymer by O2 plasma, more compact gold films that exhibit lower roughness were prepared which resulted in a higher binding capacity, decreased nonspecific adsorption of biomolecules to the biosensor surface, and in a larger enhancement of electromagnetic field intensity accompanied with the excitation of LRSPs. The authors show that by improving the quality of a gold film supporting LRSPs, the fluorescence signal can be enhanced up to sixfold with respect to regular SPFS biosensors.

High-throughput fabrication of nanoantennae over large areas for biosensing and nanospectroscopy

We report on a simple and high-throughput method for the fabrication of gold and silver nanoparticle dimers with extremely small gaps and extending over large areas, using colloidal lithography and shadow thermal-evaporation techniques. The plasmon resonances of the individual dimers with various structural parameters were studied, as well as their coupling phenomena in the nearly touching regime. The method presented enables fabrication of efficient and low-cost plasmonics structures for applications such as surface-enhanced Raman scattering, plasmon-enhanced fluorescence, and biosensing.

Prostate Specific Antigen Biosensor Based on Long Range Surface Plasmon-Enhanced Fluorescence Spectroscopy and Dextran Hydrogel Binding Matrix

A new biosensor based on surface plasmon-enhanced fluorescence spectroscopy (SPFS), which employs long-range surface plasmons (LRSP) and a photo-cross-linkable carboxymethyl dextran (PCDM) hydrogel binding matrix, is reported. LRSPs are surface plasmon modes that propagate along a thin metallic film with orders of magnitude lower damping compared to regular surface plasmons. Therefore, their excitation provides strong enhancement of the intensity of the electromagnetic field and a greatly increased fluorescence signal measured upon binding of fluorophore-labeled molecules on the sensor surface. In addition, these modes exhibit highly extended evanescent fields penetrating up to micrometers in distance from the metallic sensor surface. Therefore, a PCDM hydrogel with approximately micrometer thickness was anchored on the sensor surface to serve as the binding matrix. We show that this approach provides large binding capacity and allows for the ultrasensitive detection. In a model experiment, the developed biosensor platform was applied for the detection of free prostate specific antigen (f-PSA) in buffer and human serum by using a sandwich immunoassay. The limit of detection at the low femtomolar range was achieved, which is approximately 4 orders of magnitude lower than that for direct detection of f-PSA based on the monitoring of binding-induced refractive index changes.

Optical Properties of Micro-patterned Silver Nanoparticle Substrates

In this paper, we describe a novel technique for depositing metal nanoparticles (NPs) on a planar substrate whereby the NPs are micro-patterned on the surface by a simple stamp-printing procedure. The method exploits the attractive force between negatively charged colloidal metal NPs and positively-charged polyelectrolyte layers which have been selectively deposited on the surface. Using this technique, large uniform areas of patterned metal NPs, with different plasmonic properties, were achieved by optimisation of the stamping process. We report the observation of unusual fluorescence emission from these structures. The emission was measured using epifluorescence microscopy. Fluorescence lifetime behaviour was also measured. Furthermore, the mu-patterned NPs exhibited blinking behaviour under 469 nm excitation and the fluorescence spectrum was multi-peaked. It has been established that the fluorescence is independent of the plasmon resonance properties of the NPs. As well as optimising the novel NP mu-patterning technique, this work discusses the origin and characteristics of the anomalous fluorescence behaviour in order to characterise and minimise this unwanted background contribution in the use of metal NPs for plasmonic enhancement of fluorescence for optical biochip applications.

Strong Polarization Dependence of Plasmon-Enhanced Fluorescence on Single Gold Nanorods

We report on the strong polarization dependence of the plasmon-enhanced fluorescence on single gold nanorods. The fluorescence from the organic fluorophores that are embedded in a mesostructured silica shell around individual gold nanorods is enhanced by the longitudinal plasmon resonance of the nanorods. Our electrostatic calculations show that under an off-resonance excitation, the electric field intensity contour around a nanorod rotates away from the length axis as the excitation polarization is varied. The polarization dependence of the plasmon-enhanced fluorescence is ascribed to the dependence of the averaged electric field intensity enhancement within the silica shell on the excitation polarization. The measured fluorescence enhancement factor is in very good agreement with that obtained from the electrostatic calculations. The fluorescence enhancement factor increases as the longitudinal plasmon wavelength is synthetically tuned close to the excitation wavelength. In addition, the polarization dependence is used to determine the orientation angle of the gold nanorods. The results are in excellent agreement with the actual measurements. Furthermore, the emission spectrum of the fluorophore is modified by the longitudinal plasmon resonance of the gold nanorods. A linear correlation between the emission peak wavelength and the longitudinal plasmon wavelength is obtained.

Core-shell Au-Ag nanoparticles in dielectric nanocomposites with plasmon-enhanced fluorescence: A new paradigm in antimony glasses

The nano era demands the synthesis of new nanostructured materials, if possible by simplified techniques, with remarkable properties and versatile applications. Here, we demonstrate a new single-step reproducible melt-quench methodology to fabricate core-shell bimetallic (Au0-Ag0) nanoparticles (28–89 nm) embedded glasses (dielectrics) by the use of a new reducing glass matrix, K2O-B2O3-Sb2O3 (KBS) without applying any external reducing agent or multiple processing steps. The surface plasmon resonance (SPR) band of these nanocomposites embedded in KBS glass is tunable in the range 554–681 nm. More remarkably, taking advantage of the selective reduction capability of Sb2O3, this single-step methodology is used to fabricate inter-metallic: rare-earth ions co-embedded (Au-Ag:Sm3+) dielectric (glass)-based-dnanocomposites and study the effect of enhanced local field on the red upconversion fluorescence of Sm3+ ions at 636 nm. The enhancement is found to be about 2 folds. This single-step in-situ selective reduction approach can be used to fabricate a variety of hybrid-nanocomposite devices for laser based applications (see supplementary information).