Enhanced fluorescence of Cy5-labeled oligonucleotides near silver island films: a distance effect study using single molecule spectroscopy

Engineering Electromagnetic Hot-Spots in Nanoparticle Cluster Arrays on Reflective Substrates for Highly Sensitive Detection of (Bio)molecular Analytes

Intense electromagnetic (EM) hot-spots arising at the junctions or gaps in plasmonic nanoparticle assemblies can drive ultrahigh sensitivity in molecular detection by surface-enhanced spectroscopies. Harnessing this potential however requires access to the confined physical space at the EM hot-spots, which is a challenge for larger analytes such as biomolecules. Here, we demonstrate self-assembly derived gold nanoparticle cluster arrays (NCAs) on gold substrates exhibiting controlled interparticle (<1 nm wide) and intercluster (<10 nm wide) hot-spots as highly promising in this direction. Sensitivity of the NCAs toward detection of small (<1 nm) or large (protein-receptor interactions) analytes in surface-enhanced Raman and metal-enhanced fluorescence assays is found to be strongly impacted by the size of the cluster and the presence of reflective substrates. Experiments supported by numerical simulations attribute the higher sensitivity to higher EM field enhancements at the hot-spots, as well as greater analyte leverage over EM hot-spots. The best-performing arrays could push the sensitivity down to picomolar detection limits for sub-nanometric organic analytes as well as large protein analytes. The investigation paves the way for rational design of plasmonic biosensors and highlights the unique capabilities of a molecular self-assembly approach toward catering to this objective.

Analyte Co-localization at Electromagnetic Gap Hot-Spots for Highly Sensitive (Bio)molecular Detection by Plasmon Enhanced Spectroscopies

Electromagnetic hot-spots at ultranarrow plasmonic nanogaps carry immense potential to drive detection limits down to few molecules in sensors based on surface-enhanced Raman or fluorescence spectroscopies. However, leveraging the EM hot-spots requires access to the gaps, which in turn depends on the size of the analyte in relation to gap distances. Herein, we leverage a well-calibrated process based on self-assembly of block copolymer colloids on a full-wafer level to produce high-density plasmonic nanopillar arrays exhibiting a large number (>10¹⁰ cm^-2) of uniform interpillar EM hot-spots. The approach allows convenient handles to systematically vary the interpillar gap distances down to a sub-10 nm regime. The results show compelling trends of the impact of analyte dimensions in relation to the gap distances toward their leverage over interpillar hot-spots and the resulting sensitivity in SERS-based molecular assays. Comparing the detection of labeled proteins in surface-enhanced Raman and metal-enhanced fluorescence configurations further reveal the relative advantage of fluorescence over Raman detection while encountering the spatial limitations imposed by the gaps. Quantitative assays with limits of detection down to picomolar concentrations are realized for both small organic molecules and proteins. The well-defined geometries delivered by a nanofabrication approach are critical to arriving at realistic geometric models to establish meaningful correlation between the structure, optical properties, and sensitivity of nanopillar arrays in plasmonic assays. The findings emphasize the need for the rational design of EM hot-spots that takes into account the analyte dimensions to drive ultrahigh sensitivity in plasmon-enhanced spectroscopies.

Ultratrace Nitroaromatic Vapor Detection via Surface-Enhanced Fluorescence on Carbazole-Terminated Black Silicon

Detection of nitroaromatic compounds (NACs) is an important applied task for environmental monitoring, medical diagnostics, and forensic analysis. However, detection of NAC vapors is challenging owing to their low vapor pressure and relatively weak sensitivity of the existing detection techniques. Here, we propose a novel concept to design fluorescence (FL) detection platforms based on chemical functionalization of nanotextured dielectric surfaces exhibiting resonant light absorption, trapping, and localization effects. We demonstrate highly-efficient NAC vapor sensor with selective FL-quenching response from monolayers of carbazole moieties covalently bonded to a spiky silicon surface, "black" silicon, produced over the centimeter-scale area using simple reactive ion etching. The sensor is shown to provide unprecedented ppt (10^-12) range limits of detection for several NAC vapors. Easy-to-implement scalable fabrication procedure combined with simple and versatile functionalization techniques applicable to all-dielectric surfaces make the suggested concept promising for realization of various gas sensing systems for social and environmental safety applications.

Chemiplasmonics for high-throughput biosensors

Background

The sensitivity of ELISA for biomarker detection can be significantly increased by integrating fluorescence with plasmonics. In surface-plasmon-coupled emission, the fluorophore emission is generally enhanced through the so-called physical mechanism due to an increase in the local electric field. Despite its fairly high enhancement factors, the use of surface-plasmon-coupled emission for high-throughput and point-of-care applications is still hampered due to the need for expensive focusing optics and spectrometers.

Methods

Here, we describe a new chemiplasmonic-sensing paradigm for enhanced emission through the molecular interactions between aromatic dyes and C₆₀ films on Ag substrates.

Results

A 20-fold enhancement in the emission from rhodamine B-labeled biomolecules can be readily elicited without quenching its red color emission. As a proof of concept, we demonstrate two model bioassays using: 1) the RhB–streptavidin and biotin complexes in which the dye was excited using an inexpensive laser pointer and the ensuing enhanced emission was recorded by a smartphone camera without the need for focusing optics and 2) high-throughput 96-well plate assay for a model antigen (rabbit immunoglobulin) that showed detection sensitivity as low as 6.6 pM.

Conclusion

Our results show clear evidence that chemiplasmonic sensors can be extended to detect biomarkers in a point-of-care setting through a smartphone in simple normal incidence geometry without the need for focusing optics. Furthermore, chemiplasmonic sensors also facilitate high-throughput screening of biomarkers in the conventional 96-well plate format with 10–20 times higher sensitivity.

Fluorescence enhancement of europium complexes by core-shell Ag@SiO₂ nanoparticles

Three kinds of core-shell Ag@SiO2 nanoparticles with shell thickness of around 10, 15, and 25 nm, respectively, have been prepared by modified Stöber method and used for fluorescence enhancement. Six kinds of europium complexes with halobenzoic acid have been synthesized. Elemental analysis and lanthanide coordination titration show that the complexes have the compositions of Eu(p-XBA)3·H2O and Eu(o-XBA)3·2H2O (X=F, Cl, Br). The fluorescence spectra investigation indicates that the introduction of Ag@SiO2 nanoparticles into the europium complexes' solution can significantly enhance the fluorescence intensities of the complexes. The sequence of enhancement factors for halobenzoic acid complexes with different halogen atoms is F<Cl<Br, and the fluorescence enhancement factors increase as the excitation wavelength of complexes increase. When the thickness of the SiO2 shell is 25 nm, the fluorescence intensity of the europium complexes can reach a maximum enhancement factor of 5.1. The fluorescence enhancement mechanism may be the metal-enhanced fluorescence resulting from surface plasmon resonance of nanoparticles. And the nanoparticles near the complexes can effectively prevent complexes from the interaction with the solvent molecules, leading to a decrease of nonradiative energy transfer and the suppression of luminescence quench.

Influence of Adhesion Force on icaA and cidA Gene Expression and Production of Matrix Components in Staphylococcus aureus Biofilms

The majority of human infections are caused by biofilms. The biofilm mode of growth enhances the pathogenicity of Staphylococcus spp. considerably, because once they adhere, staphylococci embed themselves in a protective, self-produced matrix of extracellular polymeric substances (EPSs). The aim of this study was to investigate the influence of forces of staphylococcal adhesion to different biomaterials on icaA (which regulates the production of EPS matrix components) and cidA (which is associated with cell lysis and extracellular DNA [eDNA] release) gene expression in Staphylococcus aureus biofilms. Experiments were performed with S. aureus ATCC 12600 and its isogenic mutant, S. aureus ATCC 12600 Δpbp4, deficient in peptidoglycan cross-linking. Deletion of pbp4 was associated with greater cell wall deformability, while it did not affect the planktonic growth rate, biofilm formation, cell surface hydrophobicity, or zeta potential of the strains. The adhesion forces of S. aureus ATCC 12600 were the strongest on polyethylene (4.9 ± 0.5 nN), intermediate on polymethylmethacrylate (3.1 ± 0.7 nN), and the weakest on stainless steel (1.3 ± 0.2 nN). The production of poly-N-acetylglucosamine, eDNA presence, and expression of icaA genes decreased with increasing adhesion forces. However, no relation between adhesion forces and cidA expression was observed. The adhesion forces of the isogenic mutant S. aureus ATCC 12600 Δpbp4 (deficient in peptidoglycan cross-linking) were much weaker than those of the parent strain and did not show any correlation with the production of poly-N-acetylglucosamine, eDNA presence, or expression of the icaA and cidA genes. This suggests that adhesion forces modulate the production of the matrix molecule poly-N-acetylglucosamine, eDNA presence, and icaA gene expression by inducing nanoscale cell wall deformation, with cross-linked peptidoglycan layers playing a pivotal role in this adhesion force sensing.

Overcoming metal-induced fluorescence quenching on plasmo-photonic metasurfaces coated by a self-assembled monolayer

A schematic energy diagram of the present fluorescence (FL)-enhancing process including nonradiative (NR) paths that a self-assembled monolayer (SAM) blocks is presented.

Residence-time dependent cell wall deformation of different Staphylococcus aureus strains on gold measured using surface-enhanced-fluorescence

Bacterial adhesion to surfaces is accompanied by cell wall deformation that may extend to the lipid membrane with an impact on the antimicrobial susceptibility of the organisms. Nanoscale cell wall deformation upon adhesion is difficult to measure, except for Δpbp4 mutants, deficient in peptidoglycan cross-linking. This work explores surface enhanced fluorescence to measure the cell wall deformation of Staphylococci adhering on gold surfaces. Adhesion-related fluorescence enhancement depends on the distance of the bacteria from the surface and the residence-time of the adhering bacteria. A model is forwarded based on the adhesion-related fluorescence enhancement of green-fluorescent microspheres, through which the distance to the surface and cell wall deformation of adhering bacteria can be calculated from their residence-time dependent adhesion-related fluorescence enhancement. The distances between adhering bacteria and a surface, including compression of their extracellular polymeric substance (EPS)-layer, decrease up to 60 min after adhesion, followed by cell wall deformation. Cell wall deformation is independent of the integrity of the EPS-layer and proceeds fastest for a Δpbp4 strain.

Nanostructured surfaces and detection instrumentation for photonic crystal enhanced fluorescence

Photonic crystal (PC) surfaces have been demonstrated as a compelling platform for improving the sensitivity of surface-based fluorescent assays used in disease diagnostics and life science research. PCs can be engineered to support optical resonances at specific wavelengths at which strong electromagnetic fields are utilized to enhance the intensity of surface-bound fluorophore excitation. Meanwhile, the leaky resonant modes of PCs can be used to direct emitted photons within a narrow range of angles for more efficient collection by a fluorescence detection system. The multiplicative effects of enhanced excitation combined with enhanced photon extraction combine to provide improved signal-to-noise ratios for detection of fluorescent emitters, which in turn can be used to reduce the limits of detection of low concentration analytes, such as disease biomarker proteins. Fabrication of PCs using inexpensive manufacturing methods and materials that include replica molding on plastic, nano-imprint lithography on quartz substrates result in devices that are practical for single-use disposable applications. In this review, we will describe the motivation for implementing high-sensitivity fluorescence detection in the context of molecular diagnosis and gene expression analysis though the use of PC surfaces. Recent efforts to improve the design and fabrication of PCs and their associated detection instrumentation are summarized, including the use of PCs coupled with Fabry-Perot cavities and external cavity lasers.

Largely enhanced single-molecule fluorescence in plasmonic nanogaps formed by hybrid silver nanostructures

It has been suggested that narrow gaps between metallic nanostructures can be practical for producing large field enhancement. We design a hybrid silver nanostructure geometry in which fluorescent emitters are sandwiched between silver nanoparticles and silver island film (SIF). A desired number of polyelectrolyte layers are deposited on the SIF surface before the self-assembly of a second silver nanoparticle layer. Layer-by-layer configuration provides a well-defined dye position. It allows us to study the photophyical behaviors of fluorophores in the resulting gap at the single molecule level. The enhancement factor of a fluorophore located in the gap is much higher than those on silver surfaces alone and on glass. These effects may be used for increased detectability of single molecules bound to surfaces which contain metallic structures for either biophysical studies or high sensitivity assays.

A novel aptasensor based on silver nanoparticle enhanced fluorescence

In the present study, we report a novel aptasensor based on silver nanoparticle enhanced fluorescence for the detection of adenosine. First, the distance dependence nature of silver nanoparticle enhanced fluorescence was investigated through fluorescent dyes modified oligonucleotides to control the spacing distance between dyes and AgNP. The results showed that the fluorescence intensity reached the maximum value with the spacing distance of dyes about 8 nm from AgNP surface. The fluorescence intensity decreases when the spacing distance is either above or below this value. Based on this result, a fluorescence switch is constructed. In the "OFF" state, without the target molecules, there is a greater spacing distance between the Cy3 dyes and the AgNP giving comparatively lower fluorescence intensity. While in the "ON" state, in the presence of target molecules, the fluorescence signals increased for the conformation structure change of the aptamer which shorten the spacing distance between the Cy3 dyes and the AgNP to 8 nm. Using adenosine as target, the aptasensor produced a linear range from 200 nM to 200 μM with a correlation coefficient of 0.9949 and the detection limit was 48 nM estimated using 3σ. The aptasensor was also found to be specific in targeting adenosine. The presented method shows a new strategy of combining aptamer recognition and silver nanoparticle for fluorescence signal enhancement and increasing sensitivity.

Plasmon-induced modulation of the emission spectra of the fluorescent molecules near gold nanorods

Both the excitation and emission processes of a fluorescent molecule positioned near a noble metal nanocrystal can interact strongly with the localized surface plasmon resonance of the metal nanocrystal. While the effects of this plasmon-fluorophore interaction on the intensity, polarization, and direction of the fluorescence emission have been intensively investigated, the plasmonic effect on the emission spectrum has barely been explored. We show, on the single-particle level, that the localized surface plasmon resonance of Au nanorods can strongly alter the spectral profile of the emission from adjacent fluorescent molecules. The fluorescent molecules are embedded in a mesostructured silica shell that is uniformly coated on each Au nanorod. The longitudinal plasmon resonance wavelengths of the nanorods are deliberately shifted away from the intrinsic fluorescence emission peak wavelength by synthetically tuning the nanorod aspect ratio. The resultant emission spectra of the fluorescent molecules are found to be remarkably modulated. Besides the intrinsic fluorescence peak, a plasmon-induced new peak emerges at the plasmon resonance wavelength. The intensity of this plasmon-induced fluorescence peak increases as the size of the Au nanorod is increased. This spectral modulation can be understood by depicting the decay process of the fluorophore with multiple vibrational energy levels. The plasmon with a specific resonance energy will enhance the transition rate between the energy levels that have the transition energy approximately equal to the plasmon energy. This plasmon-enhanced transition rate results in a modulated spectral profile of the fluorescence emission.

Peptide-functionalized colloidal graphene via interdigited bilayer coating and fluorescence turn-on detection of enzyme

Synthesis of colloidal functional graphene is challenging because graphene is water-insoluble and its relatively inert surface made the functionalization a difficult task. Here we report interdigited bilayer type coating that provide both colloidal stability and functionalization option for graphene. Colloidal graphene oxide is first converted into interdigited bilayer coated graphene oxide and next they are transformed into colloidal graphene by hydrazine reduction. These coated graphenes can be further transformed into colloidal functional graphene using covalent conjugation chemistry. Functional graphene has been synthesized for optical detection of enzyme where a fluorescent dye is covalently linked through a peptide so that the dye fluorescence is quenched by graphene but switches on once enzymes cleave the peptide bond. The interdigited bilayer coating reported here is unique as it provides coating thickness <3 nm, offering optically responsive graphene-fluorophore substrate with high colloidal stability.

Single plasmonic nanoparticles for biosensing

Along with remarkable progress of nanoplasmonics over the past 10 years, single plasmonic nanoparticle sensors have introduced a completely new dimension to the sensing scale, considering that nanoparticles are comparable in size to biomolecules such as nucleic acids or antibodies. Single particle sensing methods have recently shown the possibility of detecting the adsorption of single biomolecules, and have already provided information about conformational changes of single molecules. For practical application, arrays of such compact sensor units are expected to realize massive multiplexing and high throughput in diagnostics and drug discovery in the near future. In this review, recent achievements and perspectives of this emerging biosensing technique are discussed.

Enhanced Single-Molecule Detection using Porous Silver Membrane

We evaluated a commercial porous silver membrane as a support substrate for direct localization and visualization of single molecule events. We characterized the fluorescence behaviors of fluorescently labeled DNA oligonucleotides on the silver membranes. The fluorescence from the fluorescent probes that were immobilized on the porous silver is greatly enhanced. Additionally, correlated to reflectance contour image, it appears that enhanced fluorescence came from location close to the "valley" of the pore channels (or in the voids). These results are of great interest to increase the effectiveness of fluorescence-based single molecule DNA analysis.

Observation of Surface Plasmon Coupled Emission using Thin Platinum Films

Surface plasmon-coupled emission (SPCE) is the directional radiation of light into a glass substrate due to excited fluorophores above a thin metal film. The sharp angular distribution of SPCE is a striking phenomenon and is in stark contrast with the isotropic fluorescence emission. In this paper, we show that SPCE can occur with thin platinum films at green and red wavelengths and was found to be mostly p-polarized. This SPCE emission is the result of near-field interactions of the excited fluorophores with the thin platinum film, and is not simply a reflective or transmissive phenomenon. Our preliminary observation suggests that platinum nanostructures can be part of several novel bio-analysis surfaces.

Time-resolved single molecule fluorescence spectroscopy of Cy5-dCTP: influence of the immobilization strategy

The carbocyanine derivative Cy5 is one of the most utilized probes for single molecule fluorescence spectroscopy. To investigate time dependent changes in single molecules a number of immobilization techniques are used. In the present study the influence of the immobilization strategy on the fluorescence lifetime of Cy5-dCTP has been investigated using time-resolved single molecule imaging and spectroscopy techniques. The fluorescence lifetime of single dye molecule adsorbed on glass from aqueous solution (1.03 ns) or on a dry surface (1.23 ns) has been found to match very well with that measured in an ensemble aqueous solution (1.07 ns). However, the lifetime increases drastically when embedded in a film (approximately 2 ns). The changes in the rigidity of the medium have been found to be responsible for the unusual increase observed in the single molecule fluorescence lifetime of the dye. These observations were well supported by the results obtained during bulk measurements.

Highly efficient detection of single fluorophores in blood serum samples with high autofluorescence

Single molecule detection (SMD) is usually performed on surface-immobilized molecules with diffraction-limited observation volumes, typically with confocal optics to suppress background from the sample and instrument. In this paper we consider the more difficult task of detecting single fluorophores in the presence of a substantial fluorescence background. We determined that for a readily accessible macroscopic observation volume of 1 pL that the background from undiluted blood serum was approximately equal to 2700 Cy5 molecules, and the background from whole blood equal to about 14 000 Cy5 molecules in whole blood. These high backgrounds appear to preclude the possibility of SMD of Cy5 molecules. However, we show that the signal-to-noise ratio (SNR) in high background samples can be increased dramatically by reduction of the observed volume. We were able to detect single surface-bound Cy5-labeled DNA (Cy5-DNA) oligomers in diluted blood serum with an SNR near 40. We also examined freely diffusing Cy5-DNA in blood serum. The data showed that single Cy5-DNA molecules could be detected even in the undiluted serum. We further investigated the SNR on silver island films. We found that the fluorescence signal was greatly enhanced in the presence of metallic nanostructures showing a larger SNR in the application tested. These results suggest the possibility of clinical assays based on SMD in blood serum and possibly whole blood. Increased SNR near metallic nanostructure could probably overcome the need for diffraction-limited volumes and enhance our ability to do in situ SMD.

Time resolved emission studies of Ag-adenine-templated CdS (Ag/CdS) nanohybrids

Ag-adenine-templated CdS (Ag/CdS) nanohybrids have been synthesized and characterized by transmission electron microscopy, selected area electron diffraction, x-ray diffraction, and optical, fluorescence and time resolved emission spectroscopy. Adenine serves as an effective matrix for the stabilization of Ag/CdS through interaction of N(1), N(3) and -NH(2) with Ag. The amount of Ag in the nanohybrid is observed to influence the organization of the Ag and CdS phase in the composite and also modifies the nature of electronic transition in CdS. For the nanohybrid containing a molar ratio of 0.1 of Ag/ CdS, CdS nanoparticles (2.5 nm) surround the Ag (6.5 nm) core. The excitation of these particles by 340 nm light, where the absorption due to the Ag phase in the nanohybrid is negligibly small, results in the enhancement of fluorescence by a factor of 7 compared to that of bare CdS. For the particles containing a molar ratio of Ag/CdS of unity, bigger clusters (14 nm) are produced causing the quenching of emission of CdS. In time resolved emission spectroscopy the spectral shift from 415 nm (3.0 eV) to 550 nm (2.26 eV) monitored over a period of 1-220 ns is understood by the relaxation of charge within the surface states of varied energy from 180 to 370 eV. The observed changes in fluorescence behavior in terms of intensity, lifetime and spectral shift are understood in terms of electronic interaction between Ag and CdS phases. The manipulation of electronic and fluorescence properties in these nanohybrids could be exploited for optoelectronic, molecular-recognition and sensing applications.

Fluorescent avidin-bound silver particle: a strategy for single target molecule detection on a cell membrane

Cy5-avidin conjugate-bound silver nanoparticles were prepared as a fluorescence molecular reagent for the cell imaging. Compared with the metal-free avidin conjugate, the avidin-metal complex was observed to display a stronger emission intensity, shorter lifetime, and better photostability. The avidin-metal complexes were conjugated with the biotin-sites on the surfaces of PM1 cell lines, and the cell images were recorded using scanning confocal microscopy. It was noticed that the avidin-metal complexes bound on the cell surfaces could be identified as the isolated emission spots distinct from the cellular autofluorescence. The emission intensity over the cell image was increased with an increase of the number of avidin-metal complexes on the cell surface but the lifetime was decreased. A quantitative regression curve was achieved between the amount of avidin-metal complex on the cell surface and the emission intensity or lifetime over the entire cell image. On the basis of this curve, we expect to develop an approach that can be used to quantify the amount of target molecules on the cell surfaces using the cell intensity and lifetime images at the single cell level.

Modification of single molecule fluorescence near metallic nanostructures

In recent years there has been a growing interest in the interactions of fluorophores with metallic nanostructures or nanoparticles. The spectra properties of fluorophores can be dramatically modified by near-field interactions with the electron clouds present in metals. Near-field interactions are those occurring within a wavelength distance of an excited fluorophore. These interactions modify the emission in ways not seen in ensemble fluorescence experiments. In this review we provide an insightful description of the photophysics of metal plasmons and near-field interactions. Additionally, we summarize recent works on single-molecule studies on metal-fluorophore interactions and suggest how these effects will result in new classes of experimental procedures, novel probes, bioassays and devices. The spectral properties of single fluorophores can be dramatically altered by near-field interactions with the electron clouds presented in metals.

Plasmon-controlled fluorescence towards high-sensitivity optical sensing

Fluorescence spectroscopy is widely used in chemical and biological research. Until recently most of the fluorescence experiments have been performed in the far-field regime. By far-field we imply at least several wavelengths from the fluorescent probe molecule. In recent years there has been growing interest in the interactions of fluorophores with metallic surfaces or particles. Near-field interactions are those occurring within a wavelength distance of an excited fluorophore. The spectral properties of fluorophores can dramatically be altered by near-field interactions with the electron clouds present in metals. These interactions modify the emission in ways not seen in classical fluorescence experiments. Fluorophores in the excited state can create plasmons that radiate into the far-field and fluorophores in the ground state can interact with and be excited by surface plasmons. These reciprocal interactions suggest that the novel optical absorption and scattering properties of metallic nanostructures can be used to control the decay rates, location, and direction of fluorophore emission. We refer to these phenomena as plasmon-controlled fluorescence (PCF). An overview of the recent work on metal-fluorophore interactions is presented. Recent research combining plasmonics and fluorescence suggest that PCF could lead to new classes of experimental procedures, novel probes, bioassays, and devices.

Plasmon-controlled fluorescence: a new paradigm in fluorescence spectroscopy

Fluorescence spectroscopy is widely used in biological research. Until recently, essentially all fluorescence experiments were performed using optical energy which has radiated to the far-field. By far-field we mean at least several wavelengths from the fluorophore, but propagating far-field radiation is usually detected at larger macroscopic distances from the sample. In recent years there has been a growing interest in the interactions of fluorophores with metallic surfaces or particles. Near-field interactions are those occurring within a wavelength distance of an excited fluorophore. The spectral properties of fluorophores can be dramatically altered by near-field interactions with the electron clouds present in metals. These interactions modify the emission in ways not seen in classical fluorescence experiments. In this review we provide an intuitive description of the complex physics of plasmons and near-field interactions. Additionally, we summarize the recent work on metal-fluorophore interactions and suggest how these effects will result in new classes of experimental procedures, novel probes, bioassays and devices.

Metal-enhanced fluorescence of single green fluorescent protein (GFP)

The green fluorescent protein (GFP) has emerged as a powerful reporter molecule for monitoring gene expression, protein localization, and protein-protein interaction. However, the detection of low concentrations of GFPs is limited by the weakness of the fluorescent signal and the low photostability. In this report, we observed the proximity of single GFPs to metallic silver nanoparticles increases its fluorescence intensity approximately 6-fold and decreases the decay time. Single protein molecules on the silvered surfaces emitted 10-fold more photons as compared to glass prior to photobleaching. The photostability of single GFP has increased to some extent. Accordingly, we observed longer duration time and suppressed blinking. The single-molecule lifetime histograms indicate the relatively heterogeneous distributions of protein mutants inside the structure.

Effect of the polarization and incident angle of excitation light on the fluorescence enhancement observed with a multilayered substrate fabricated by Ag and Al2O3

The fluorescence from a fluorophore on a multilayered substrate fabricated by a metal and a dielectric is known to be enhanced by more than 100-fold. In the course of this study, we prepared a multilayered substrate with Ag as the metal and Al(2)O(3) as the dielectric and then investigated the effects of the polarization of the excitation light on the enhancement of the multilayered substrate. It was found that the enhancement was attributed to an electric field oscillating parallel to the substrate. Maximum 200-fold enhancement could be achieved with 80 nm thick Al(2)O(3) when an unpolarized excitation light was used with an incident angle of 20 degrees.

Single-cell fluorescence imaging using metal plasmon-coupled probe 2: single-molecule counting on lifetime image

Multiple Alexa Fluor 647-conjugated concanavalin A (conA) molecules were covalently bound to a single 20 nm silver particle to synthesize metal plasmon-coupled probes (PCPs). The fluorescence images were recorded by scanning confocal microscopy in both intensity and lifetime. The brightness of PCPs was 30-fold brighter than those of free conA and the lifetime of PCPs was shortened dramatically. PCPs were used to label T-lymphocytic ( PM1) cells. The emission spots by PCPs bound on the cell surfaces were separated clearly from the cell images by autofluorescence due to the brighter signal and shorter lifetime of PCPs. The emission spots by PCPs were also scanned in three dimensions to count the distribution of bound fluorophores on the cell surfaces. The metal-associated fluorophores thus are suggested using as novel molecular imaging agents to quantify the components and describe their distributions on the cell surfaces.

Reduced blinking and long-lasting fluorescence of single fluorophores coupling to silver nanoparticles

The fluorescence signal of single organic fluorophores is characterized by random blinking and irreversible photobleaching. Photoinduced blinking of Cy5 has posed various limitations of this popular near-infrared (NIR) probe in biological applications. Here we show that fluorophore-metal nanoparticle (NP) complexes greatly suppress Cy5 blinking and noticeably reduce photobleaching events. The blinking behavior of single Cy5 molecules was investigated and compared in the absence and the presence of silver nanostructures. A power-law distribution of off time population was observed for single Cy5 molecules. Average off times were compared to evaluate the plasmonic effect of silver nanoparticles on the triplet decay rates. We furthermore demonstrate enhanced photostability in the presence of silver NPs. The results show that plasmonic-controlled fluorescence can lead to a novel physical mechanism to enhance fluorescence intensity, reduce blinking, and increase photostability.

Fluorescence blinking dynamics of silver nanoparticle and silver nanorod films

Fluorescence blinking of silver nanoparticle films is observed when illuminated with red light (635 nm). The observed power-law off-time distribution is attributed to random surface diffusion and subsequent agglomeration of atomic Ag leading to the formation of photo-active Ag nanoclusters. These nanoclusters can in turn diffuse randomly to form non-emitting Ag clusters after aggregation with another Ag species. This is revealed in the power-law on-time distribution. Silver oxides found on the surfaces of Ag nanostructures are important for photoblinking to take place since nanostructures with a protective layer of polymeric citrate or cetyltrimethylammonium bromide (CTAB) against atmospheric O(2) do not display obvious emission intermittency.

Single molecule photophysics near metallic nanostructures

Metal-enhanced fluorescence (MEF) is useful in single molecule detection (SMD) by increasing the photostability, brightness and increase in radiative decay rates of fluorophores. We have investigated MEF from an individual fluorophore tethered to a single silver nanoparticle and also a single fluorophore between a silver dimer. The fluorescence lifetime results revealed a near-field interaction mechanism of fluorophore with the metal particle. Finite-difference time-domain (FDTD) calculations were employed to study the distribution of electric field near the metal monomer and dimer. The coupling effect of metal particles on the fluorescence enhancement was studied. We have also investigated the photophysics of FRET near metal nanoparticles and our preliminary results suggest an enhanced FRET efficiency in the presence of a metal nanoparticle. In total, our results demonstrate improved detectability at the single molecule level for a variety of fluorophores and quantum dots in proximity to the silver nanoparticles due to the near-field metal-fluorophore interactions.

Single-Molecule Studies on Fluorescently Labeled Silver Particles: Effects of Particle Size

We studied the dependence of single-molecule fluorescence on the size of nearby metal particles. The silver particles were synthesized with average diameters of metal cores being 5, 20, 50, 70, and 100 nm, respectively. A single-stranded oligonucleotide was chemically bound to a single silver particle and a Cy5-labeled complementary single-stranded oligonucleotide was hybridized with the particle-bound oligonucleotide. The space between the fluorophore and metal core was separated by a rigid hybridized DNA duplex of 8 nm length. The single fluorescence images and intensity traces were recorded by scanning confocal microscopy. The single fluorophore-labeled 50 nm silver particles displayed the most enhanced intensity, a factor of 17-fold increase relative to the free fluorophores in the absence of metal. Numerical simulations by the finite-difference time-domain (FDTD) method and results from Mie theory were used to compare with the experimental results. The 50 nm silver particles were also labeled by multiple fluorophores. The fluorescence intensity of multiple fluorophore-labeled metal particles increases dramatically with the loading number and reached 400-fold relative to the free single fluorophore when the loading number of fluorophore per metal particle was 50. The fluorophore also displayed better photostability when binding on the metal particle. These results can aid us to develop novel nanoscale fluorophores for clinical diagnostics and bioassay.

Metal-Enhanced Fluorescence of Phycobiliproteins from Heterogeneous Plasmonic Nanostructures

We report here the use of plasmonic metal nanostructures in the form of silver island films (SiFs) to enhance the fluorescence emission of five different phycobiliproteins. Our findings clearly show that the phycobiliproteins display up to a 9-fold increase in fluorescence emission intensity, with a maximum 7-fold decrease in lifetime when they are assembled as a monolayer above SiFs, as compared to a monolayer assembled on the surface of amine-terminated glass slides of the control sample. The study was also repeated with a thin liquid layer of the phycobiliproteins sandwiched between two glass substrates (and a SiFs and a glass substrate) clamped together. Similarly, the results show a maximum 10-fold increase in fluorescence emission intensity coupled with a 2-fold decrease in lifetime of the phycobiliproteins in the SiF-glass setup as compared to the glass control sample, implying that near-field enhancement of phycobiliprotein emission can be attained both with and without chemical linkage of the proteins to the SiFs. Hence, our results clearly show that metal-enhanced fluorescence (MEF) can potentially be employed to increase the sensitivity and detection limit of the plethora of bioassays that employ phycobiliproteins as fluorescence labels, such as in fluoro-immunoassays where the assay can be tethered on the surface of SiFs, and also in flow cytometry where analytes in the liquid phase could potentially flow through channels coated with SiFs without actually being attached to the silver.

Fluorescence images of DNA-bound YOYO between coupled silver particles

Oligonucleotide-bound silver particles were coupled through hybridization with target complementary oligonucleotides. YOYO molecules were intercalated into DNA duplexes bound between the coupled metal particles. Fluorescence images of YOYO molecules were monitored by scanning confocal microscopy. Relative to the free single YOYO, the emission brightness of the image was enhanced 80-fold after intercalating the fluorophores into the DNA duplexes between the coupled silver particles. Some images of the labeled metal particle dimers were observed to be dumbbell-shaped, suggesting that the stretching of intercalated YOYO molecules was restricted because of the orientation effect of fluorophores. The shortened lifetime of YOYO molecules between the coupled metal particles indicated that the fluorescence was enhanced via a near-field interaction mechanism between the fluorophore and the metal nanoparticle.

Enhanced Förster Resonance Energy Transfer on Single Metal Particle. 2. Dependence on Donor-Acceptor Separation Distance, Particle Size, and Distance from Metal Surface

We studied the effect of metal particles on Förster resonance energy transfer (FRET) between nearby donor-acceptor pairs. The studies included the effect of donor-acceptor distance, silver particle size, and distance from the metal surface. The metal particles were synthesized with average diameters of 15, 40, and 80 nm, respectively. A Cy5-labeled oligonucleotide was chemically bound to a single silver particle with a distance of 2 or 10 nm from the surface of metal core. A Cy5.5-labeled complementary oligonucleotide was bound to the particle-conjugated oligonucleotide by hybridization. The spacer length between donor-acceptor was adjusted by the number of base pairs. FRET between the donor-acceptor pair was investigated by dual-channel single-molecule fluorescence detection. Both the emission intensities and lifetimes indicated that FRET was enhanced efficiently by the metal particles. The results showed an increase of apparent energy transfer distance with the size of silver particle and distance from the metal core. Simulations by finite-difference time-domain (FDTD) calculations were used to compare with the experimental results. The local fields at the location of the donor-acceptor pair appeared to correlate with the FRET efficiency. These results will aid in the design of metal particles for using FRET to determine biomolecule proximity at distances beyond the usual Förster distance.

Metal-enhanced single-molecule fluorescence on silver particle monomer and dimer: coupling effect between metal particles

We prepared silver particle dimers with single Cy5 molecules localized between coupled metal particles. The silver particles with a 20 nm diameter were chemically bound with single-stranded oligonucleotides. The dimers were formed by hybridization with double-length single-stranded oligonucleotides that contained single Cy5 molecules. The image analysis revealed that the single-molecule fluorescence was enhanced 7-fold on the metal monomer and 13-fold on the metal dimer relative to the free Cy5-labeled oligonucleotide in the absence of metal. The lifetimes were shortened on the silver monomers and further shortened on the silver dimers, demonstrating the near-field interaction mechanism of fluorophore with the metal substrate. Finite-difference time-domain (FDTD) calculations were employed to study the distribution of electric field near the metal monomer and dimer. The coupling effect of metal particle on the fluorescence enhancement was discussed.