Metal-enhanced Fluorescence Effect Research Articles

Highly sensitive metal-enhanced fluorescence biosensor prepared on electrospun fibers decorated with silica-coated silver nanoparticles

A new metal-enhanced fluorescence (MEF)-based biosensor platform was prepared on fibrous substrates. First, polycaprolactone (PCL) fibers were obtained via electrospinning process. PCL fibers were decorated with photoreduced silver nanoparticles followed by silica coating, generating silver-decorated PCL fibers with a silica layer (Ag@SiO2-PCL). The silica layer acted as a spacer between the silver nanoparticles and the fluorescent molecules to optimize the MEF effect. The MEF was obtained from the silver-decorated PCL fibers, the extent of which could be controlled by the thickness of the silica layer. The fibrous structure of Ag@SiO2-PCL had higher protein loading capacity than conventional two-dimensional glass slides due to its large surface area, while the presence of silica-coated silver nanoparticles resulted in higher fluorescence intensity than silica-coated PCL fibers (SiO2-PCL) without silver nanoparticles via the MEF effect. Both features of Ag@SiO2-PCL were combined to produce combined effects in improving the performance of fluorescence-based biosensing. According to immunobinding assays between fluorescently labeled anti-IgG and IgG immobilized onto different substrates, Ag@SiO2-PCL had much better sensing performance in terms of sensitivity and detection limit than glass slides and SiO2-PCL due to the combined effects of a large surface area and the MEF effect.

Visualization of endogenous hydrogen sulfide in living cells based on Au nanorods@silica enhanced fluorescence

Hydrogen sulfide as a gas indicator molecule plays an important role in various human physiological processes. However, due to the high volatility and diffusivity of H2S in biological systems, it is very difficult to implement a precise assay for H2S detection. Compared with the destructive instrumental methods, assays based on fluorescence probes provide noninvasive and real-time detections of H2S in living cells. In this work, we presented a fluorescent nanoprobe based on dye-functionalized Au nanorods (NRs)@silica for sensitive and selective detection of H2S in vitro and living cells. With the metal enhanced fluorescence effect, the fluorescence turn-on and turn-off were controlled by the formation and disassembly of coordination compound between dyes and copper ions. Silica matrix was used to coat the Au NRs to prevent them from the biological cytotoxicity. The effects of the different distances between Au NRs and fluorophores on fluorescent enhancement were explored and approximately 5-fold fluorescence enhancement was obtained with a distance of 22 nm. A detection of limit of 17 nM was achieved. In addition, visualization of exogenous and endogenous H2S in living cells was validated.

Metal-enhanced fluorescence emission and quenching protection effect with a host–guest nanophotonic-supramolecular structure

The functionalization of the nanoparticle’s (NP) surface is one method for tuning their overall properties to fit targeted applications. We developed a nanosensor based on the specific supramolecular interactions between s-cyclodextrin (sCD) nanocavities and organic molecules of biological interests using the metal-enhanced fluorescence effect (MEF) as the detection signal. We grafted sCD, a typical macrocyclic host molecule that interacts specifically with different organic molecules and changes their physical properties (such as their fluorescence emission intensity), on gold NPs. To evaluate this nanosensor and the effect of the metallic core, we worked with a typical organic molecule, Rhodamine B (RhB), that has a strong association constant with sCD (5700 M − 1) and is well-known to be quenched in the presence of cyclodextrins (CDs). The results show that, by grafting sCD on gold NPs, it is possible to increase the sensitivity of RhB detection by 70%, 80%, and 294% when compared with solutions in (1) a phosphate buffer, (2) with sCD, and (3) with Au NPs, respectively. These results show that the use of a supramolecular system attached to a metallic NP can interact specifically with a dye to enhance its fluorescence emission through the MEF effect. Moreover, this type of nanosystem can overcome the quenching of the signal by the matrix, such in the case of RhB with CDs. Eventually, this concept could be extended to other dyes with different quenching effects. For this reason, this type of nanosensor system could be used in the future to protect and enhance the dye emission of fluorophores in different biological media.

Nanowire-Intensified Metal-Enhanced Fluorescence in Hybrid Polymer-Plasmonic Electrospun Filaments.

Hybrid polymer-plasmonic nanostructures might combine high enhancement of localized fields from metal nanoparticles with light confinement and long-range transport in subwavelength dielectric structures. Here, the complex behavior of fluorophores coupling to Au nanoparticles within polymer nanowires, which features localized metal-enhanced fluorescence (MEF) with unique characteristics compared to conventional structures, is reported. The intensification effect when the particle is placed in the organic filaments is remarkably higher with respect to thin films of comparable thickness, thus highlighting a specific, nanowire-related enhancement of MEF effects. A dependence on the confinement volume in the dielectric nanowire is also indicated, with MEF significantly increasing upon reduction of the wire diameter. These findings are rationalized by finite element simulations, predicting a position-dependent enhancement of the quantum yield of fluorophores embedded in the fibers. Calculation of the ensemble-averaged fluorescence enhancement unveils the possibility of strongly enhancing the overall emission intensity for structures with size twice the diameter of the embedded metal particles. These new, hybrid fluorescent systems with localized enhanced emission, and the general nanowire-enhanced MEF effects associated to them, are highly relevant for developing nanoscale light-emitting devices with high efficiency and intercoupled through nanofiber networks, highly sensitive optical sensors, and novel laser architectures.

Surface enhanced fluorescence effect of SiO2 coated silver nanoparticles and clusters

Composite Ag@SiO2 nanoparticles are prepared for investigating a metal-enhanced fluorescence effect. Due to the competition between enhanced electromagnetic field intensity and fluorescence emission decay, the distance between the molecules and metal surface is an important surface-enhanced fluorescence factor for enhancing or quenching. This distance can be controlled by the size of the separation-layer, and thus the surface-enhanced fluorescence enhancement can be controlled. To avoid fluorescence quenching and obtain a considerable fluorescence enhancement, SiO2 shell is used to separate the molecules from the metal surface. The surface modification of metallic nanoparticles by SiO2 can not only improve the dispersibility and stability of metal particles, but also expanding its application effectively in the biological field. For fluorophores located around metal nanostructures, many factors would affect a fluorescence enhancement effect. Investigation of the enhancement mechanism usually becomes difficult and complicated due to too many variables and factors involved in the enhancement process. In order to systematically study the influence of these factors on fluorescence intensity, single polymer nanoparticles are chosen to be an enhanced substrate. The particle distribution is labeled through electron beam lithography and the surface-enhanced fluorescence spectroscopy of single nanoparticle or cluster is achieved by in - situ measurement method. The influence of metal particle size, cluster property and excitation polarization on the surface enhanced fluorescence is studied by using Rh6G as the probe molecule. Obvious fluorescence enhancement effect of Rh6G molecules at the surface of Ag@SiO2 is observed with nanocomposite, and the enhancement effect is controlled by the metal core particle size, aggregation property, and the polarization direction of the excitation light.

Signal-amplifying nanoparticle/hydrogel hybrid microarray biosensor for metal-enhanced fluorescence detection of organophosphorus compounds

In this study, we developed an enzyme-based miniaturized fluorescence biosensor to detect paraoxon, one of the most well-known neurotoxic organophosphorus compounds. The biosensor was fabricated with poly(ethylene glycol) (PEG) hydrogel microarrays that entrapped acetylcholinesterase (AChE) and quantum dots (QDs) as fluorescence reporters. Metal-enhanced fluorescence (MEF) was utilized to amplify the fluorescence signal, which was achieved by decorating QDs on the surface of silica-coated silver nanoparticles (Ag@Silica). The MEF effects of Ag@Silica were optimized by tuning the thickness of the silica shells, and under the optimized conditions, the fluorescence intensity was shown to be increased 5 fold, compared with the system without MEF. PEG hydrogel microarray entrapping QD-decorated Ag@Silica and AChE was prepared via photopatterning process. The entrapped AChE hydrolyzed paraoxon to produce p-nitrophenol within the hydrogel microstructure, which subsequently quenched the fluorescence of the QDs on the surface of Ag@Silica. The MEF-assisted fluorescence detection resulted in a significant enhancement of paraoxon detection. The detection limit was approximately 1.0 × 10−10 M and 2.0 × 10−7 M for sensing with and without MEF, respectively. The successful integration of a hydrogel microarray system with a microfluidic system was demonstrated to be a potential application for the MEF-based micro-total-analysis-system.

Plasmonic enhancement of cyanine dyes for near-infrared light-triggered photodynamic/photothermal therapy and fluorescent imaging

Near-infrared (NIR) triggered cyanine dyes have attracted considerable attention in multimodal tumor theranostics. However, NIR cyanine dyes used in tumor treatment often suffer from low fluorescence intensity and weak singlet oxygen generation efficiency, resulting in inadequate diagnostic and therapy efficacy for tumors. It is still a great challenge to improve both the photodynamic therapy (PDT) and fluorescent imaging (FLI) efficacy of cyanine dyes in tumor applications. Herein, a novel multifunctional nanoagent AuNRs@SiO2-IR795 was developed to realize the integrated photothermal/photodynamic therapy (PTT/PDT) and FLI at a very low dosage of IR795 (0.4 μM) based on metal-enhanced fluorescence (MEF) effects. In our design, both the fluorescence intensity and reactive oxygen species of AuNRs@SiO2-IR795 nanocomposites were significantly enhanced up to 51.7 and 6.3 folds compared with free IR795, owing to the localized surface plasmon resonance band of AuNRs overlapping with the absorption or fluorescence emission band of the IR795 dye. Under NIR laser irradiation, the cancer cell inhibition efficiency in vitro with synergetic PDT/PTT was up to 82.3%, compared with 10.3% for free IR795. Moreover, the enhanced fluorescence intensity of our designed nanocomposites was helpful to track their behavior in tumor cells. Therefore, our designed nanoagents highlight the applications of multimodal diagnostics and therapy in tumors based on MEF.

Preparation of optical functional composite films and their application in protein detection

In this work, an optical functional composite film was prepared through self-assembly technique for specific protein detection. The Au nanostructures with metal enhanced fluorescence (MEF) effect were prepared via the in situ reduction of Au ions in a filter paper. Then, a multilayer film was prepared on the Au nanostructures by layer-by-layer adsorption of poly-(ethylenimine), poly-(glycolic acid) and biotinylated poly(l-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG-biotin). Through adjusting the structure of multilayer film, an optimum interaction distance between Au nanostructures and fluorophores for the MEF effect was achieved. Meanwhile, the surface PLL-g-PEG-biotin layer allows the film to capture specific streptavidin through biotin-streptavidin interaction. Owing to the MEF effect of Au nanostructures, a significant enhancement in the fluorescence of fluorescein isothiocyanate (FITC) that labeled streptavidin was successfully obtained. This optical functional composite film with enhanced fluorescence could be used to recognize specific protein in a facile and efficient way.

Dye-Labeled Polyacryloyl Hydrazide-Ag Nanoparticle Fluorescent Probe for Ultrasensitive and Selective Detection of Au Ion.

The efficiency of a fluorescence sensing device based on metal-enhanced fluorescence (MEF) is dependent on the optimization of interaction between the fluorophore and the metal nanoparticle (NP). Herewith, ultrasensitive and selective turn-on sensing of Au3+ is achieved by using a suitable combination of fluorophore and metal NP system through sequential MEF effect. Dansyl hydrazide-tagged Ag NPs in the polyacryloyl hydrazide cavity are utilized to sense the picomolar concentration of Au3+ in aqueous media. We demonstrated that the selective Au3+ sensing is due to the selective deposition of Au on the Ag NP surface over the 16 other metal ions studied. The sensitivity is assigned to the strong overlapping of the emission band of the fluorophore with the surface plasmon band of the Au and improvement of fluorescence signal through successive MEF by Ag and Au colloids. The sensing is associated with a fivefold increase in fluorescence intensity and appearance of violet color of the solution. These luminescent Ag–Au bimetallic NPs may be utilized to trace cancer cells in biological systems and for cell imaging applications.

Bivalent Aptasensor Based on Silver-Enhanced Fluorescence Polarization for Rapid Detection of Lactoferrin in Milk.

Here we report a novel type of bivalent aptasensor based on silver-enhanced fluorescence polarization (FP) for detection of lactoferrin (Lac) in milk powder with high sensitivity and specificity. The novel two split aptamers were obtained from the aptamer reported in our previous SELEX (systematic evolution of ligands by exponential enrichment) selection, and their minimal structural units were optimized on the basis of their affinity and specificity. Also, dual binding sites of split aptamers were verified. The bivalent aptamers were modified to be linked with signal-molecule fluorescein isothiocyanate (FITC) and enhancer silver decahedral nanoparticles (Ag10NPs). The split aptamers could bind to different sites of Lac and assemble into a split-aptamers-target complex, narrowing the distance between Ag10NPs and FITC dye. As a result, Ag10NPs could produce a mass-augmented and metal-enhanced fluorescence (MEF) effect. In general, ternary amplification based on Ag10NPs, split aptamers, and the MEF effect all contributed to the significant increase of FP values. It was proved that the sensitivity of this assay was about 3 orders of magnitude over traditional aptamer-based homogeneous assays with a detection limit of 1.25 pM. Furthermore, this design was examined by actual milk powder with rapid and high-throughout detection.

Metal-Enhanced Ratiometric Fluorescence/Naked Eye Bimodal Biosensor for Lead Ions Analysis with Bifunctional Nanocomposite Probes.

A novel metal-enhanced ratiometric fluorescence/naked eye bimodal biosensor based on ZnFe2O4@Au-Ag bifunctional nanocomposite and DNA/CeO2 complex for lead ions (Pb2+) has been successfully developed. The nanocomposite probe was composed of a magnetic ZnFe2O4 core and a Au-Ag hollow nanocube shell. Upon bioconjugation, bifunctional magnetic nanocomposites could not only make the probe possess excellent recyclability but also provide an enrichment of "hot spots" for surface enhanced fluorescence detection of Pb2+ by a metal-enhanced fluorescence effect. Typically, the bifunctional nanocomposites conjugated with double-stranded DNA (included Pb2+-specific DNAzyme strand and corresponding substrate strand) to form a Pb2+ biosensor. Nanoceria as a fluorescence quencher strongly adsorbed DNA. Therefore, the formation of double-stranded DNA brought the labeled nitrogen sulfur doped carbon dots (N,S-CDs) and CeO2 into close proximity, which significantly quenched the fluorescence of N,S-CDs. The presence of Pb2+ led to the breakage of the DNAzyme strand, resulting in the fluorescence signal of Cy3 decreasing, while the fluorescence intensity of N,S-CDs aggrandized. First, a preliminary test of Pb2+ was performed by the naked eye. The disengaged DNA/CeO2 complex could result in color change after adding H2O2 because of autocatalysis of CeO2, resulting in real-time visual detection of Pb2+. If further accurate determination was required, the fluorescence intensity ratio of these two fluorescence indicators was measured at 562 and 424 nm (I562/I424). A good linear correlation exists between the log(I562/I424) and the logarithm of Pb2+ concentrations ranging from 10-12 to 3 × 10-6 M. Remarkably, the detection limit of this ratiometric biosensor was 3 × 10-13 M, which ascribed to its superior fluorescence enhancement. Interestingly, the developed bifunctional nanocomposite probe manifests good recyclability and selectivity. More importantly, the biosensor provided potential application of on-site and real-time unknown Pb2+ ions in real systems.

Near-Infrared Dye Immobilized in Porous Silica Layer on Gold Nanorod and Its Fluorescence Enhancement by Strengthened Electromagnetic Field Based on Surface Plasmon Resonance

We report immobilizing Nile Blue A, which is a cationic fluorescent dye emitting in the near-infrared region, in the porous silica layer on gold nanorod and its fluorescence enhancement by strengthened electromagnetic field based on surface plasmon resonance. The effect of the spacer corresponding to the silica layer on the metal-enhanced fluorescence effect is also discussed in detail. Hollow silica nanorod was in advance prepared, and then the silica layer was partly etched to increase the porosity for the improvement of the mass transfer. Subsequently, gold nanorod was fabricated in the restricted space of hollow silica nanorod. Finally, Nile Blue A was physically immobilized in the porous silica layer on gold nanorod through electrostatic interactions. The fluorescence enhancement of Nile Blue A based on surface plasmon resonance was semi-quantified by comparative experiments using hollow silica nanorod, which is exactly the same structure except for gold as silica-coated gold nanorod. Since our results demonstrated that the porosity degree of the silica layer significantly affected the fluorescence enhancement of Nile Blue A, it is hopeful that our design concept, distinct from the conventional one, can lay a foundation for further development of near-infrared fluorescence nanomaterials.

Metal enhanced fluorescence on super-hydrophobic clusters of gold nanoparticles

We used optical lithography, electroless deposition and deep reactive ion etching techniques to realize arrays of super-hydrophobic gold nanoparticles arranged in a hierarchical structure. At the micro-scale, silicon-micro pillars in the chip permit to manipulate and concentrate biological solutions, at the nano-scale, gold nanoparticles enable metal enhanced fluorescence (MEF) effects, whereby fluorescence signal of fluorophores in close proximity to a rough metal surface is amplified by orders of magnitude. Here, we demonstrated the device in the analysis of fluorescein derived gold-binding peptides (GBP-FITC). While super-hydrophobic schemes and MEF effects have been heretofore used in isolation, their integration in a platform may advance the current state of fluorescence-based sensing technology in medical diagnostics and biotechnology. This scheme may be employed in protein microarrays where the increased sensitivity of the device may enable the early detection of cancer biomarkers or other proteins of biomedical interest.

Metal-enhanced fluorescence of single shell-isolated alloy metal nanoparticle.

A single silica-shell isolated Au-Ag alloy nanoparticle is used for investigating a metal-enhanced fluorescence effect. Well-dispersed alloy nanoparticles are prepared by the facile chemical method, and the property of local surface plasmon resonance is controlled by adjusting the metal component of the alloy and shell thickness. The distance dependence of fluorescence enhancement for a single Au-Ag alloy nanoparticle is studied systematically with different silica shell thickness ranging from 2 to 35nm. The isolation shell not only adjusts the distance between metal surface and fluorophore emitters but also improves the chemical stability of the metal particle.

Nano-supramolecular complex synthesis: Switch on/off enhanced fluorescence control and molecular release using a simple chemistry reaction

A nanosensor based on β-cyclodextrin (βCD) macrocycles linked to gold nanoparticles for rhodamine B (RhB) sensing was developed applying the metal-enhanced fluorescence effect (MEF). Hence, we have developed many ways to control the distance of supramolecular systems to nanoparticle surface with different bioconjugation strategies in order to optimize signal detection. Different PEG spacer arm lengths were used to cover the nanoparticle surface with molecular spacers. This type of molecular shell is biocompatible, enabling to switch on/off the MEF effect using a dithiane linker by a simple reduction reaction. In the presence of the nanosensor obtained, an increase was observed in RhB fluorescence emission depending on molecular length, that is a characteristic effect of MEF. The major increase measured was 60% compared with RhB emission in buffer at 1nM level, for a spacer length of 3.58nm and an 80% increase as compared with that in the presence of βCD. These differences are ascribed to the fact that, in the presence of macrocycle, we can observe a well-known quenching effect that is overcome by the presence of the metallic core. Even at shorter spacer distances, with PEG lengths of 1.2 and 2.17nm, increases of 25 and 47% respectively allow the analyte detection by the RhB complexation with βCD. The optimal MEF enhancement was measured with the maximal emission signal stabilized after 50min due to plasmonic effects based on inter nanoparticle interactions. Moreover, the emission increase, in the presence of the metallic core, was accompanied with a diminution in the fluorescence lifetime's decay value averages, characteristic of MEF. This fact shows that the excited state is protected from the non-radiative emission decays enhancing the analytical signal.

Luminescent Characteristics of Nanocomposite Films Based on Conjugated Copolymer Superyellow and Gold Nanoparticles

It was established that the insertion of gold nanoparticles (NPs) stabilized by an organic (thiophenol) or inorganic (silicon dioxide) shell into films of conjugated copolymer SuperYellow helps to increase its photoluminescence [the metal-enhanced fluorescence effect (MEF)], which may result from interaction between the components of the nanocomposites. It was shown that the maximum emission intensity of SuperYellow in the obtained nanocomposite films is observed with Au nanoparticle content between 1 and 5 wt.%. It was found that the magnitude of the MEF effect depends on the nature of the shell of the Au nanoparticle, reaching 250%.

Metal-Enhanced Fluorescence of Silver Island Associated with Silver Nanoparticle.

The coupling plasmon of a hybrid nanostructure, silver island (SI) associated with silver nanoparticle (SNP), on metal-enhanced fluorescence (MEF) was studied theoretically. We used the multiple multipole method to analyze the plasmon-mediated enhancement factor on the fluorescence of a molecule immobilized on SNP and located in the gap zone between SI and SNP; herein, the SI was modeled as an oblate spheroid. Numerical results show that the enhancement factor of the hybrid nanostructure is higher than that of a SNP or a SI alone due to the coupled gap mode. This finding is in agreement with the previous experimental results. In addition, the plasmon band of the structure is broadband and tunable, which can be red-shifted and broadened by flattening or enlarging SI. Based on this property, the hybrid nanostructure can be tailored to obtain the optimal enhancement factor on a specific molecule according to its excitation spectrum. Moreover, we found that there is an induced optical force allowing SNP be attracted by SI. Consequently, the gap is reduced gradually to perform a stronger MEF effect.

Facile Synthesis of Biocompatible Fluorescent Nanoparticles for Cellular Imaging and Targeted Detection of Cancer Cells.

In this work, we report the facile synthesis of functional core-shell structured nanoparticles with fluorescence enhancement, which show specific targeting of cancer cells. Biopolymer poly-l-lysine was used to coat the silver core with various shell thicknesses. Then, the nanoparticles were functionalized with folic acid as a targeting agent for folic acid receptor. The metal-enhanced fluorescence effect was observed when the fluorophore (5-(and-6)-carboxyfluorescein-succinimidyl ester) was conjugated to the modified nanoparticle surface. Cellular imaging assay of the nanoparticles in folic acid receptor-positive cancer cells showed their excellent biocompatibility and selectivity. The as-prepared functional nanoparticles demonstrate the efficiency of the metal-enhanced fluorescence effect and provide an alternative approach for the cellular imaging and targeting of cancer cells.

Protein Detection: An Optical Nanoruler Based on a Conjugated Polymer−Silver Nanoprism Pair for Label‐Free Protein Detection (Adv. Mater. 39/2015)

An optical nanoruler system based on a conjugated polyelectrolyte (CP)-silver nanoprism (Ag NPR) pair is developed for label-free biological detection by taking advantage of the metal-enhanced fluorescence (MEF) effect of Ag nanostructures. The system described by L. Li, S. Wang, and co-workers on page 6040 effectively transduces and amplifies the change of the metal–fluorophore distance into the response of the CP fluorescence, which is used to monitor the presence of specific antigens and even mammalian cells with high sensitivity and good selectivity.

Preparation of biocompatible fluorescent core–shell nanoparticles for cellular imaging

Abstract In this work, we report a facile method to prepare biocompatible fluorescent core–shell structured nanoparticles. The as-prepared nanoparticle was composed of a gold core enclosed in a protein shell. An amine-reactive fluorophore, 5-(and-6)-carboxyfluorescein-succinimidyl ester, was assembled onto the outer shell completing the fluorescent nanoparticle. Fluorescence quenching of the fluorophore was efficiently avoided because of the presence of the shell, which served as a spacer layer between the fluorophore and the gold core. There was an increase in fluorescence intensity, when compared to the fluorophore in its free state that can be attributed to the metal-enhanced fluorescence effect of the gold core. Well-dispersed green fluorescent faculae were observed when the as-prepared nanoparticles were studied by the confocal laser scanning microscope, demonstrating the successful assembly of the fluorophores. The nanoparticles were readily uptaken by A549 cancer cells and the results of cell viability assays evidenced their good biocompatibility. Cumulatively, these findings indicate their potential application for cellular imaging.