A model for DNA detection by metal-enhanced fluorescence from immobilized silver nanoparticles on solid substrate

A fluorescent aptasensor for H5N1 influenza virus detection based-on the core-shell nanoparticles metal-enhanced fluorescence (MEF)

A fluorescent aptasensor system has been designed for the sensitive detection of recombinant hemagglutinin (rHA) protein of the H5N1 influenza virus in human serum. Guanine-richen anti-rHA aptamers by SELEX were immobilized on the surface of the Ag@SiO2 nanoparticles which performed as a metal-enhanced fluorescence (MEF) sensing platform. Thiazole orange (TO) was used as fluorescent tag which reported to the G-quadruplex secondary structural induced by aptamer-rHA binding event. In the absence of rHA protein, TO was free in the solution with almost no fluorescence emission. When rHA protein was added to the solution, the aptamer strand bound rHA protein to form a stable G-quadruplex complex, which can bind TO and excite the fluorescence emission of TO. Moreover, the excited-state TO captured by the G-quadruplex complex was forced to the surface of the Ag@SiO2 nanoparticles and could experience a surface plasmon resonance enhancement which can be transformed into more efficient fluorescence emission signals, therefore, the fluorescence signal of TO can be amplified largely. This system does not require covalent labeling with fluorophores to the aptamer and the background noise is very low. The detection of rHA protein of the H5N1 influenza virus could be operated both in aqueous buffer and human serum with the detection limit of 2 and 3.5ng/mL respectively. More important, the whole detection process can be finished in a PE tube within 30min, which makes it suitable as a self-contained diagnostic kit for H5N1 influenza virus point-of-care (POC) diagnostic.

Synthesis of silver glyconanoparticles from new sugar-based amphiphiles and their catalytic application

Oligosaccharide-based amphiphiles were readily prepared by click chemistry from ω-azido-hexanoic or dodecanoic acids with propargyl-functionalized maltoheptaose or xyloglucanoligosaccharides. These amphiphilic compounds were used as capping/stabilizer agents in order to obtain highly stable catalytic silver glyconanoparticles (Ag-GNPs) through the in situ reduction of silver nitrate with NaBH4. With a view to long-term storage, the stabilization was optimized using a multivariate approach, and the nanoparticles were characterized by UV-vis, TEM, SAXS, and DLS. In order to explore the functionality of the Ag-GNPs in catalysis, a full kinetic analysis of the reduction of p-nitrophenol by NaBH4 in water and in water/ethanol mixtures was performed under semi-heterogeneous and quasi-homogeneous conditions. A pseudomonomolecular surface reaction was performed, and the kinetic data obtained were treated according to the Langmuir model. The Ag-GNPs were very active, and both substrates adsorbed onto the surface of the nanoparticles. For comparison purposes, the reaction was also performed in the presence of silver-sodium dodecanoate nanoparticles, which showed catalytic activity similar to that of the glyconanoparticles, supporting the choice of the carboxyl group as the stabilizing agent, although it provided much lower temporal stability. Finally, by combining kinetic and water/ethanol surface tension data it was possible to observe the effect of the addition of the less polar solvent (ethanol) to the reaction medium.

Controllable metal-enhanced fluorescence in organized films and colloidal system

In recent years, considerable efforts have been devoted to better understand the unique emission properties of fluorophores enhanced by the localized surface plasmon resonance of metal nanoparticles (NPs), due to the widespread applications of fluorescence techniques. It is demonstrated by experiment and theoretical calculation that the enhancement efficiency strongly depends on the morphology of the metal NPs, the spectral overlap between metal and fluorophores, the separation distance between them, and other factors. Among these aspects to be considered are suitable spacer material and assembling methods to control the spatial arrangement of plasmonic NPs and fluorophore with proper optical properties and interactions. In this contribution, we provide a brief overview on recent progress of metal-enhanced fluorescence in organized films and colloidal systems.

Local pH-responsive diazoketo-functionalized photoresist for multicomponent protein patterning

Selective surface immobilization of multiple biomolecule components, under mild conditions where they do not denature, is attractive for applications in biosensors and biotechnology. Here, we report on a biocompatible and pH-responsive photoresist containing diazoketo-functionalized methacrylate, methacrylic acid, and poly(ethylene glycol) methacrylate monomers, where the photolithographic process may be carried out in a local pH range to minimize biomolecular denaturation. The polymer is insoluble or sparsely soluble in pH 6.4 or more acidic solution or deionized water, but soluble in a basic solution, pH 7.9 or more. After UV exposure, however, carboxylic acid groups are generated by Wolff rearrangement and photodissociation of the diazoketo groups in the polymer chain, leading to dissolution of UV-exposed polymer at pH 6.4. Using the property of the pH-solubility switching, we demonstrate dual streptavidin patterning using only biological buffers, pH 6.4 and 7.9 solutions, and double exposure patterning to confirm the sustainability of the diazoketo groups in unexposed regions despite carrying out several wet processes.

Metal-enhanced fluorescence of nano-core-shell structure used for sensitive detection of prion protein with a dual-aptamer strategy

Metal-enhanced fluorescence (MEF) as a newly recognized technology is widespread throughout biological research. The use of fluorophore-metal interactions is recognized to be able to alleviate some of fluorophore photophysical constraints, favorably increase both the fluorophore emission intensity and photostability. In this contribution, we developed a novel metal-enhanced fluorescence (MEF) and dual-aptamer-based strategy to achieve the prion detection in solution and intracellular protein imaging simultaneously, which shows high promise for nanostructure-based biosensing. In the presence of prion protein, core-shell Ag@SiO2, which are functionalized covalently by single stranded aptamer (Apt1) of prions and Cyanine 3 (Cy3) decorated the other aptamer (Apt2) were coupled together by the specific interaction between prions and the anti-prion aptamers in solution. By adjusting shell thickness of the pariticles, a dual-aptamer strategy combined MEF can be realized by the excitation and/or emission rates of Cy3. It was found that the enhanced fluorescence intensities followed a linear relationship in the range of 0.05-0.30 nM, which is successfully applied to the detection of PrP in mice brain homogenates.

Development of gold nanoparticle-enhanced fluorescent nanocomposites

A gold nanoparticle-enhanced fluorescent nanocomposite was developed. The designed nanocomposite contained a spherical gold nanoparticle core, a thin PVP coating layer, a silica spacer, and a fluorescent dye layer in the silica matrix. The dye molecules were conjugated to a polymer to be effectively doped in the nanocomposites. Different sized gold nanoparticle cores were used while the spacer thickness was varied. The function of the PVP layer in the fabrication of the nanocomposites was discussed. The fluorescence enhancement effects of the metal core size (gold nanoparticles) and the distance between the fluorescent molecules and the metal core were systematically studied. A series of control experiments were conducted to ensure the accuracy of the fluorescence enhancement measurement. The results showed that the developed nanocomposite can effectively enhance the fluorescence signal of the doped dye conjugates. An enhancement factor of 9.2 was obtained when the nanocomposite contained a 13.7 ± 1.3 nm gold nanoparticle core and a 36.6 ± 4.4 nm silica spacer. It is expected that the developed nanocomposite could be an effective model for studying various effects and the mechanism of metal-enhanced fluorescence at the nanoscale.

Bimetallic Nanoshells for Metal - Enhanced Fluorescence with Broad Band Fluorophores

In this article, we reported the near-field interactions between the Ru(bpy)(3) (2+) complexes and plasmon resonances from the bimetallic nanoshells. The metallic nanoshells were fabricated on 20 nm silica spheres as cores by depositing 10 nm monometallic or bimetallic shells. There were approx. 15 Ru(bpy)(3) (2+) complexes in the silica core. The metal shells were constituted of silver or/and gold. The bimetallic shells could be generated in homogeneous or heterogeneous geometries. The homogeneous bimetallic shells contained 10 nm silver-gold alloys. The heterogeneous bimetallic shells contained successive 5 nm gold and 5 nm silver shells, or alternatively, 5 nm silver and 5 nm gold shells. Optical properties of metal nanoshells were studied on both the ensemble spectra and single nanoparticle imaging measurements. The heterogeneous bimetallic shells were found to have a large scale of metal-enhanced emission relative to the monometallic or homogeneous bimetallic shells. It is because the heterogeneous bimetallic shells may display split dual plasmon resonances which can interact with the excitation and emission bands of the Ru(bpy)(3) (2+) complexes in the silica cores leading to more efficient near-field interactions. The prediction can be demonstrated by the lifetimes. Therefore, it is suggested that both the compositions and geometries of the metal shells can influence the interactions with the fluorophores in the cores. This observation also offers us an opportunity for developing plasmon-based fluorescence metal nanoparticles as novel nanoparticle imaging agents which have high performances in fluorescence cell or tissue imaging.

Self-assembly of conjugated polymer on hybrid nanospheres for cellular imaging applications

A new kind of hybrid core-shell nanosphere was fabricated by combining the in situ formation of Au nanoparticles and covalent cross-linking of biocompatible carboxymethyl starch dialdehyde (CMSD) and chitosan (CTS). When the fluorescent dye poly[9,9'-bis(6″-(N,N,N-trimethylammonium)-hexyl)fluorene-2,7-ylenevinylene-co-alt-1,4-phenylene dibromide] (PFV) was assembled on the surface of the hybrid nanospheres through electrostatic attraction, these biocompatible hybrid nanospheres exhibited metal-enhanced fluorescence effects. The fluorescence intensity of (CTS-Au)@CMSD/PFV hybrid nanosphere is 1.43 times that of CTS-CMSD/PFV hybrid nanospheres lacking Au nanoparticle. In addition, the (CTS-Au)@CMSD/PFV hybrid nanospheres exhibit excellent biodegradability upon exposure to enzymatic aqueous solution and good biocompatibility when cocultured with HeLa cervical carcinoma cells; these advantages make them attractive for cellular imaging and biological analysis and detection.

Recent advancements in optical DNA biosensors: exploiting the plasmonic effects of metal nanoparticles

The emerging field of plasmonics, the study of electromagnetic responses of metal nanostructures, has revealed many novel signal enhancing phenomena. As applied to the development of label-free optical DNA biosensors, it is now well established that plasmon-based surface enhanced spectroscopies on nanostructured metal surfaces or metal nanoparticles can markedly improve the sensitivity of optical biosensors, with some showing great promise for single molecule detection. In this review, we first summarize the basic concepts of plasmonics in metal nanostructures, as well as the characteristic optical phenomena to which plasmons give rise. We will then describe recent advances in optical DNA biosensing systems enabled by metal nanoparticle-derived plasmonic effects, including the use of surface enhanced Raman scattering (SERS), colorimetric methods, "scanometric" processes, and metal-enhanced fluorescence (MEF).

Quantum dot-based DNA hybridization by electrochemiluminescence and anodic stripping voltammetry

Simple and convenient assays with quantum dots (QDs) as the labels for DNA detection are developed. The probe DNA modified with thiol was first immobilized on a pretreated Au electrode, and then the complementary DNA (cDNA) oligonucleotides were hybridized with the immobilized probes by immersing the probe-modified Au electrode into the cDNA oligonucleotide solution. Finally, the avidin-modified QDs were bound to the biosensor in the presence of biotin-modified cDNA. The fabrication process for the biosensor was monitored by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). Different from the traditional sandwich-structure strategy, the QDs bind to the target DNA directly via the biotin-avidin-system. By observing the ECL signal and determination of the cadmium component in QDs, the DNA hybridization event was detected by ECL and square wave anodic stripping voltammetric technique (SWASV) respectively. For SWASV detection, the signal linearly increased with the increase of the logarithm of the cDNA concentration over the range of 50 nM-5 microM. The minimum detectable concentration is 50 pM. For ECL, it showed wider linearity range over 5 nM-5 microM and lower detectable concentration of 10 pM. This indicated that the ECL assay could be comparable to the conventional electrochemical assay. Furthermore, this biosensor possesses high selectivity over different sequences of target DNA oligonucleotides.

One-pot fabrication of various silver nanostructures on substrates using electron beam irradiation

The one-pot fabrication of various silver nanostructures on substrates is achieved through electron beam irradiation of the surface of silver dodecanethiolate (Ag(I)- SC(12)). The Ag(I)-SC(12) films are simply prepared by spin-coating silver salt and dodecanethiol solution. Various silver nanostructures such as particles, rods, cubes, and networks are prepared from the Ag(I)- SC(12) film with an electron beam voltage from 0.3 to 2 MV, current from 0.06 to 0.24 mA, and/or irradiation time from 30 to 80 s. The morphology and chemical composition of the irradiated samples are characterized by scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), and x-ray diffraction (XRD).

One-step synthesis of uniform silver nanoparticles capped by saturated decanoate: direct spray printing ink to form metallic silver films

The one-step synthesis and spectroscopic characterizations of size-controlled silver nanoparticles are described. The transmission electron microscopy (TEM), nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FT-IR), thermal gravimetric-mass analysis (TGA-MS) and X-ray photoelectron spectroscopy (XPS) techniques were used to characterize the decanoate-protected silver nanoparticles. TEM analysis showed that spherical nanoclusters of 7.52 +/- 0.57 nm were produced. Furthermore, the particle sizes are uniform with a narrow size distribution. For all samples, Ag 3d(5/2) and 3d(3/2) components appeared at 368.5 and 374.5 eV, respectively, in the XPS spectrum; these values compare very well with the typical values of carboxylate-protected Ag nanoparticles. A thermal analysis mass spectrometer was used to analyze the desorption behavior of decanoate-protected nanoparticles. From the desorption maximum temperatures of 181 and 263 degrees C, activation energies of 27.2 and 32.2 kcal mol(-1) for the desorption processes in the Ag MPCs were obtained, assuming a first-order reaction and using a pre-exponential factor of 1 x 10(13) s(-1). A specific resistivity of 6.097 microOmega cm for the silver metal film (0.7 microm) on a Si wafer can be produced simply by thermal annealing of an Ag monolayer-protected clusters film under an atmosphere of 90% N(2)-10% H(2) at 300 degrees C for 1 h.

Fluorescence signals of core-shell quantum dots enhanced by single crystalline gold caps on silicon nanowires

We use nanoscale (20-300 nm in diameter) single crystalline gold (Au)-caps on silicon nanowires (NWs) grown by the vapor-liquid-solid (VLS) growth mechanism to enhance the fluorescence photoluminescence (PL) signals of highly dilute core/shell CdSeTe/ZnS quantum dots (QDs) in aqueous solution (10(-5) M). For NWs without Au-caps, as they appear, for example, after Au etching in aqua regia or buffered KI/I(2)-solution, essentially no fluorescence signal of the same diluted QDs could be observed. Fluorescence PL signals were measured using excitation with a laser wavelength of 633 nm. The signal enhancement by single crystalline, nanoscale Au-caps is discussed and interpreted based on finite element modeling (FEM).

A simple gold nanoparticle-mediated immobilization method to fabricate highly homogeneous DNA microarrays having higher capacities than those prepared by using conventional techniques

A simple, highly efficient immobilization method to fabricate DNA microarrays, that utilizes gold nanoparticles as the mediator, has been developed. The fabrication method begins with electrostatic attachment of amine-modified DNA to gold nanoparticles. The resulting gold-DNA complexes are immobilized on conventional amine or aldehyde functionalized glass slides. By employing gold nanoparticles as the immobilization mediator, implementation of this procedure yields highly homogeneous microarrays that have higher binding capacities than those produced by conventional methods. This outcome is due to the increased three-dimensional immobilization surface provided by the gold nanoparticles as well as the intrinsic effects of gold on emission properties. This novel immobilization strategy gives microarrays that produce more intense hybridization signals for the complementary DNA. Furthermore, the silver enhancement technique, made possible only in the case of immobilized gold nanoparticles on the microarrays, enables simple monitoring of the integrity of the immobilized DNA probe.

Toward improved biochips based on rolling circle amplification--influences of the microenvironment on the fluorescence properties of labeled DNA oligonucleotides

Microarrays have become an increasingly important tool for biotechnology and molecular diagnostics. Despite many advantages, their sensitivity is still insufficient for such tasks as the analysis of small sample quantities and for the detection of alterations in gene expression of low-abundance genes. Accordingly, amplification strategies are necessary. Approaches to amplify the signal intensity include the increase of the number of dye molecules per target through either particle labels or rolling circle amplification, as used for this study.

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.

Fabrication and elimination of PTAA/P4VP layer-by-layer films

Layer-by-layer (LBL) ultra-thin films have been assembled by alternate adsorption of poly(3-thiophene acetic acid) (PTAA) and poly(4-vinyl-pyridine) (P4VP) on planar quartz slides via hydrogen-bonding interaction. Subsequently, the multilayers can be controllably removed by changing the pH values of the aqueous solutions used for film immersion. Our present study is an attempt to reveal the mechanisms of the multilayer film with two-dimensional (2D) ultraviolet-visible (UV-vis) correlation spectroscopy. UV-vis spectroscopy is primarily employed here to monitor the buildup and removal of the ultra-thin films. 2D correlation analysis is performed on the basis of the corresponding spectra for further studies. The morphology of the multilayer film is characterized by atomic force microscopy (AFM). When eliminated in alkaline and acidic aqueous solutions, the polymers in the films show diverse phenomena mainly due to the different extents of dissociation of PTAA and protonation of P4VP at different pH values. In alkaline solution, the elimination of PTAA takes place before P4VP, while in acid solution, the removal of these two polymers adopts a reverse order.

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.

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.

Characterization of cap-shaped silver particles for surface-enhanced fluorescence effects

Surface-enhanced fluorescence has potentially many desirable properties as an analytical method for medical diagnostics, but the effect observed so far is rather modest and only in conjunction with fluorophores with low quantum yields. Coupled with the fact that preparation of suitable surfaces at low costs has been difficult, this has limited its utilities. Here we report a novel method for forming uniform and reproducible surfaces with respectable enhancement ratios even for high-quantum-yield fluorophores. Formation of dense surface-adsorbed latex spheres on a flat surface via partial aggregation, followed by evaporation of silver, results in a film consisting of cap-shaped silver particles at high densities. Binding of fluorescence biomolecules, either through physisorption or antigen-antibody reaction, was performed, and enhancements close to 50 have been observed with fluorophores such as R-phycoerythrin and Alexa 546-labeled, bovine serum albumin, both of which have quantum yields around 0.8. We attribute this to the unique shape of the silver particle and the presence of abundant gaps among adjacent particles at high densities. The effectiveness of the new surface is also demonstrated with IL-6 sandwich assays.

Separation distance dependent fluorescence enhancement of fluorescein isothiocyanate by silver nanoparticles

Nanocomposites consisting of a metal core, a silica-spacer shell with controlled thickness, and a dye-labelled shell were synthesized and separation distance dependent fluorescence enhancement of fluorescein isothiocyanate by silver nanoparticles was studied; the results indicated an optimum enhancement of 4.8 times with a spacer shell thickness of 21 nm.

Distance-dependent emission from dye-labeled oligonucleotides on striped Au/Ag nanowires: effect of secondary structure and hybridization efficiency

When fluorescently tagged oligonucleotides are located near metal surfaces, their emission intensity is impacted by both electromagnetic effects (i.e., quenching and/or enhancement of emission) and the structure of the nucleic acids (e.g., random coil, hairpin, or duplex). We present experiments exploring the effect of label position and secondary structure in oligonucleotide probes as a function of hybridization buffer, which impacts the percentage of double-stranded probes on the surface after exposure to complementary DNA. Nanowires containing identifiable patterns of Au and Ag segments were used as the metal substrates in this work, which enabled us to directly compare different dye positions in a single multiplexed experiment and differences in emission for probes attached to the two metals. The observed metal-dye separation dependence for unstructured surface-bound oligonucleotides is highly sensitive to hybridization efficiency, due to substantial changes in DNA extension from the surface upon hybridization. In contrast, fluorophore labeled oligonucleotides designed to form hairpin secondary structures analogous to solution-phase molecular beacon probes are relatively insensitive to hybridization efficiency, since the folded form is quenched and therefore does not appreciably impact the observed distance-dependence of the response. Differences in fluorescence patterning on Au and Ag were noted as a function of not only chromophore identity but also metal-dye separation. For example, emission intensity for TAMRA-labeled oligonucleotides changed from brighter on Ag for 24-base probes to brighter on Au for 48-base probes. We also observed fluorescence enhancement at the ends of nanowires and at surface defects where heightened electromagnetic fields affect the fluorescence.