Thermoresponsive silver/polymer nanohybrids with switchable metal enhanced fluorescence

In this communication we describe a new approach to the fabrication of fluorescent silver/polymer nanohybrids with thermo-switchable metal enhanced fluorescence (MEF). By manipulating a soft polymer spacer between the silver nanoparticles and the fluorophores a tunable MEF was achieved.     

FRET Enhancement in Aluminum Zero‐Mode Waveguides

Zero‐mode waveguides (ZMWs) can confine light into attoliter volumes, which enables single molecule fluorescence experiments at physiological micromolar concentrations. Of the fluorescence spectroscopy techniques that can be enhanced by ZMWs, Förster resonance energy transfer (FRET) is one of the most widely used in life sciences. Combining zero‐mode waveguides with FRET provides new opportunities to investigate biochemical structures or follow interaction dynamics at micromolar concentrations with single‐molecule resolution. However, prior to any quantitative FRET analysis on biological samples, it is crucial to establish first the influence of the ZMW on the FRET process. Here, we quantify the FRET rates and efficiencies between individual donor–acceptor fluorophore pairs that diffuse into aluminum zero‐mode waveguides. Aluminum ZMWs are important structures thanks to their commercial availability and the large amount of literature that describe their use for single‐molecule fluorescence spectroscopy. We also compared the results between ZMWs milled in gold and aluminum, and found that although gold has a stronger influence on the decay rates, the lower losses of aluminum in the green spectral region provide larger fluorescence brightness enhancement factors. For both aluminum and gold ZMWs, we observed that the FRET rate scales linearly with the isolated donor decay rate and the local density of optical states. Detailed information about FRET in ZMWs unlocks their application as new devices for enhanced single‐molecule FRET at physiological concentrations.     

Plasmon‐Enhanced Fluorescence Resonance Energy Transfer

In this review, we firstly introduce physical mechanism of fluorescence resonance energy transfer (FRET), the methods to measure FRET efficiency, and the applications of FRET. Secondly, we introduce the principle and applications of plasmon‐enhanced fluorescence (PEF). Thirdly, we focused on the principle and applications of plasmon‐enhanced FRET. This review can promote further understanding of FRET and PE‐FRET.     

Amplified Production of Singlet Oxygen in Aqueous Solution Using Metal Enhancement Effects

Studies involving metal enhancement effects have gained popularity, and enhancement of fluorescence due to the close proximity of a dye molecule to a metal nanoparticle is well documented. Although enhancement of singlet oxygen production by metal has been reported, studies are relatively scarce and so far only stationary silver island films have been proven to be adequate to do so. Herein, we describe the synthesis and characterization of core–shell nanoparticles on which a photosensitizer acting as source of singlet oxygen has been covalently attached to the nanoparticle surface. As a proof of concept, silver nanoparticles with a diameter around 68 nm were chosen as the metallic core, and were coated by a silica shell of about 22 nm in thickness. The silica shell plays a dual role as a spacer and a medium onto which the photosensitizer, rose bengal (RB), has been covalently attached. These novel core–shell nanoparticles allow for the amplification of singlet oxygen production by 3.8 times, which is similar to the amplification found for RB in proximity of silver island films.     

Dual Esterase‐ and Steroid‐Responsive Energy Transfer Modulation of Ruthenium(II) and Rhenium(I) Complex Functionalized Gold Nanoparticles

A number of adamantane‐containing ruthenium(II) and rhenium(I) complexes have been synthesized, characterized, and noncovalently functionalized with β‐cyclodextrin‐capped gold nanoparticles (β‐CD–GNPs) through the host–guest interaction between cyclodextrin and adamantane. The resultant nanoconjugates have been characterized by transmission electron microscopy (TEM), energy‐dispersive X‐ray analysis (EDX), and 2D ROESY ¹H NMR experiments. The Förster resonance energy transfer (FRET) properties of the nanoconjugates can be modulated by both esterase‐accelerated hydrolysis and competitive displacement of steroid, by monitoring the emission intensity and luminescence lifetime. The FRET efficiencies are found to vary with the nature of the chromophores and the length of the spacer between the transition metal complexes and the GNPs. This work constitutes a “proof‐of‐principle” assay method for the dual‐functional detection of important classes of biomolecules, such as enzymes and steroids.     

Design and synthesis of fluorescent core–shell nanoparticles with tunable lower critical solution temperature behavior and metal‐enhanced fluorescence

In this article, the preparation of fluorescent nanohybrids with core–shell structure and metal‐enhanced fluorescence (MEF) effect was presented. The fluorescent core–shell nanohybrids were prepared using silver nanoparticles (AgNPs) as cores and fluorophore tethered thermoresponsive copolymers with tunable lower critical solution temperature (LCST) from 15 to 90 °C as shells. These thermoresponsive copolymers were synthesized by the random copolymerization of oligo(ethylene oxide) acrylate and di(ethylene oxide) ethyl ether acrylate using reversible addition–fragmentation chain transfer polymerization and grafted on to AgNPs surface via Ag–S coordination interaction. By thermal manipulation of polymer spacer between AgNPs and fluorophores, the tunable MEF was achieved. It was also revealed that the fluorescent nanohybrids would exhibit maximal MEF when the polymerization degree was tuned to 350. The manipulation of the solution temperatures below and above LCST resulted in switchable MEF behavior. In addition, the phase transition process of the thermoresponsive copolymer was also studied by MEF effect using this fluorescent core–shell nanohybrid design. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 87–95     

G‐Tetraplex‐Induced FRET within Telomeric Repeat Sequences Using PyA‐PerA as Energy Donor–Acceptor Pair

G‐tetraplex induced fluorescence resonance energy transfer (FRET) within telomeric repeat sequences has been studied using a nucleoside‐tethered FRET pair embedded in the human telomeric G‐quadruplex forming sequence (5′‐A GGG TT ^Py A GGG TT ^Per A GGG TTA GGG‐3′, Py=pyrene, Per=perylene). Conformational change from a single strand to an anti‐parallel G‐quadruplex leads to FRET from energy donor ( ^Py A ) to acceptor ( ^Per A ). The distance between the FRET donor/acceptor partners was controlled by changing the number of G‐quartet spacer units. The FRET efficiency decreases with increase in G‐quartet units. Overall findings indicate that this could be further used for the development of FRET‐based sensing and measurement techniques.     

Efficient Excitation‐Energy Transfer in Ion‐Based Organic Nanoparticles with Versatile Tunability of the Fluorescence Colours

Organic nanoparticles consisting of 3,3′‐diethylthiacyanine (TC) and ethidium (ETD) dyes are synthesized by ion‐association between the cationic dye mixture (10 % ETD doping) and the tetrakis(4‐fluorophenyl)borate (TFPB) anion, in the presence of a neutral stabilizing polymer, in aqueous solution. Doping with ETD makes the particle size smaller than without doping. Size tuning can also be conducted by varying the molar ratio (ρ) of the loaded anion to the cationic dyes. The fluorescence spectrum of TC shows good overlap with the absorption of ETD in the 450–600 nm wavelength region, so efficient excitation‐energy transfer from TC (donor) to ETD (acceptor) is observed, yielding organic nanoparticles whose fluorescence colours are tunable. Upon ETD doping, the emission colour changes significantly from greenish‐blue to reddish or whitish. This change is mainly dependent on ρ. For the doped nanoparticle sample with ρ=1, the intensity of fluorescence ascribed to ETD is ～150‐fold higher than that from pure ETD nanoparticles (efficient antenna effect). Non‐radiative Förster resonance‐energy transfer (FRET) is the dominant mechanism for the ETD fluorescence enhancement. The organic nanoparticles of a binary dye system fabricated by the ion‐association method act as efficient light‐harvesting antennae, which are capable of transferring light energy to the dopant acceptors in very close proximity to the donors, and can have multi‐wavelength emission colours with high fluorescence quantum yields.     

Synthesis of core‐shell silver–polyaniline nanocomposites by gamma radiolysis method

Core‐shell silver (Ag)–polyaniline (PAni) nanocomposites have been synthesized by the in‐situ gamma radiation‐induced chemical polymerization method. Aqueous solution of aniline, a free‐radical oxidant, and/or silver metal salt were irradiated by γ‐rays. Reduction of the silver salt in aqueous aniline leads to the formation of silver nanoparticles which in turn catalyze oxidation of aniline to polyaniline. The resultant Ag‐PAni nanocomposites were characterized by using different spectroscopy analyses like X‐ray photoelectron, UV–visible, and infrared spectroscopy. The optical absorption bands revealed that the bands at about 400 nm are due to the presence of nanosilver and the blue‐shifted peak at ～ 555 nm is due to the presence of metallic silver within the PAni matrix. X‐ray diffraction pattern clearly indicates the broad amorphous polymer and the sharp metal peaks. Scanning electron microscopy and transmission electron microscopy of the nanocomposite showed a uniform size distribution with spherical and granular morphology. Thermogravimetric analysis revealed that the composites have a higher degradation temperature than polyaniline alone. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5741–5747, 2007     

Fluorescent Silver Nanoclusters in Condensed DNA

We study the formation and fluorescent properties of silver nanoclusters encapsulated in condensed DNA nanoparticles. Fluorescent globular DNA nanoparticles are formed using a dsDNA–cluster complex and polyallylamine as condensing agents. The fluorescence emission spectrum of single DNA nanoparticles is obtained using tip‐enhanced fluorescence microscopy. Fluorescent clusters in condensed DNA nanoparticles appear to be more protected against destructive damage in solution compared to clusters synthesized on a linear polymer chain. The fluorescent clusters on both dsDNA and ssDNA exhibit the same emission bands (at 590 and 680 nm) and the same formation efficiency, which suggests the same binding sites. By using density functional theory, we show that the clusters may bind to the Watson–Crick guanine–cytosine base pairs and to single DNA bases with about the same affinity.     

Chemico‐Physical Synthesis of Surfactant‐ and Ligand‐Free Gold Nanoparticles and Their Anti‐Galvanic Reduction Property

Galvanic reduction (GR) is a classic reaction. In simple terms, metals can reduce less reactive (or more noble) metal ions, while the opposite—metals reduce more reactive (or less noble) metal ions—should not occur. However, recently we found that anti‐galvanic reduction (AGR) occurred to thiolated gold and silver nanoparticles. However, the essential issue whether the occurrence of AGR requires the assistance of reductive thiolate ligands or not still remained unanswered. In this work, by using a novel protocol (chemical reduction and physical ablation), we synthesized surfactant‐ and ligand‐free gold nanoparticles. We found that these as‐prepared nanoparticles can reduce silver ions and copper ions, thus illustrating that AGR is not dependent on reductive ligands. Further experiments demonstrated that AGR is applicable to other metal (such as Pt and Pd) nanoparticles and that the AGR process is size‐dependent. Finally, it was found that the Raman scattering signals of Rhodamine 6G are distinctly enhanced on the gold nanoparticles that had been reacted with silver ions, which indicates the use of AGR for tuning the property of nanoparticles.     

Visualization of Protein‐Specific Glycosylation inside Living Cells

Protein glycosylation is a ubiquitous post‐translational modification that is involved in the regulation of many aspects of protein function. In order to uncover the biological roles of this modification, imaging the glycosylation state of specific proteins within living cells would be of fundamental importance. To date, however, this has not been achieved. Herein, we demonstrate protein‐specific detection of the glycosylation of the intracellular proteins OGT, Foxo1, p53, and Akt1 in living cells. Our generally applicable approach relies on Diels–Alder chemistry to fluorescently label intracellular carbohydrates through metabolic engineering. The target proteins are tagged with enhanced green fluorescent protein (EGFP). Förster resonance energy transfer (FRET) between the EGFP and the glycan‐anchored fluorophore is detected with high contrast even in presence of a large excess of acceptor fluorophores by fluorescence lifetime imaging microscopy (FLIM).     

Silver metal nanoparticles: Facile dendrimer‐assisted size‐controlled synthesis and selective catalytic reduction of chloronitrobenzenes

A simple and versatile synthetic methodology to silver metal nanoparticles that utilizes 3,5‐dihydroxybenzyl alcohol‐based dendrimers as templates and does not necessitate the addition of any external reducing agent is reported. An evaluation of the role of addition rate of the silver acetate solution, and the dendrimer to silver acetate molar ratio, as well as the dendritic effect on nanoparticle growth, suggests that the size of these metal nanoparticles can be controlled by simple variations of these parameters. A probe of the mechanism of the nanoparticle formation and growth indicates that the terminal hydroxyl groups of the dendrimers play a major role in metal ion isolation and reduction, in addition to providing stabilization to the growing metal particles. These silver metal nanoparticles are highly active catalysts for the selective reduction of chloronitrobenzenes to chloroanilines. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4482–4493, 2009     

A Reversible Dual‐Response Fluorescence Switch for the Detection of Multiple Analytes

This paper reports a reversible dual fluorescence switch for the detection of a proton target and 2,4,6‐trinitrotoluene (TNT) with opposite‐response results, based on fluorophore derivatization of silica nanoparticles. Fluorescent silica nanoparticles were synthesized through modification of the surface with a nitrobenzoxadiazole (NBD) fluorophore and an organic amine to form a hybrid monolayer of fluorophores and amino ligands; the resultant nanoparticles showed different fluorescence responses to the proton target and TNT. Protonation of the amino ligands leads to fluorescence enhancement due to inhibition of photoinduced electron transfer (PET) between the amine and fluorophore. By contrast, addition of TNT results in fluorescence quenching because a fluorescence resonance energy transfer (FRET) happens between the NBD fluorophore and the formed TNT–amine complex. The fluorescence signal is reversible through washing with the proper solvents and the nanoparticles can be reused after centrifugal separation. Furthermore, these nanoparticles were assembled into chips on an etched silicon wafer for the detection of TNT and the proton target. The assembled chip can be used as a convenient indicator of herbicide (2,4‐dichlorophenoxyacetic acid) and TNT residues with the use of only 10 μL of sample. The simple NBD‐grafted silica nanoparticles reported here show a reversible signal and good assembly flexibility; thus, they can be applied in multianalyte detection.     

Surface‐Enhanced Fluorescence from Fluorophore‐Assembled Monolayers by Using Ag@SiO2 Nanoparticles

A new and simple procedure to enhance the fluorescence of analytes on the surfaces of a solid substrate is demonstrated based on Ag@SiO₂ nanoparticles. Two kinds of silver–silica core–shell nanoparticles with shell thicknesses of around 3 and 15 nm have been prepared and used as enhancing agents, respectively. By simply pipetting drops of the enhancing agents onto substrate surfaces with Rose Bengal monolayers, an enhancement of about 27 times, compared with the control sample, is achieved by using the Ag@SiO₂ nanoparticles with shells of about 3 nm, whereas an enhancement of around 11.7 times is obtained when using those with thicker shells. The effects of shell thickness and surface density of the enhancing agents on the enhancement have been investigated experimentally. The results show that this method can be potentially helpful in fluorescence‐based surface analysis.     

A highly sensitive “turn‐on” fluorescent sensor for the detection of human serum proteins based on the size exclusion of the polyacrylamide gel

A highly sensitive “turn‐on” fluorescent sensor based on the size exclusion of the polyacrylamide gel was developed for the on‐gels detection of human serum proteins after PAGE. The possible mechanism of this fluorescence sensor was illustrated and validated by utilizing five kinds of colloidal silver nanoparticles with different particle size distribution and six kinds of polyacrylamide gels with different pore size. It was attributed to that silver nanoparticles (<5 nm in diameter) had been selectively absorbed into the gel and formed the small silver nanoclusters, resulting in the red fluorescence. Using this new technique for the detection of human serum proteins after PAGE, a satisfactory sensitivity was achieved and some relatively low‐abundance proteins (e.g. zinc‐alpha‐2‐glycoprotein), which are the significant proteinic markers of certain diseases can be easily detected, but not with traditional methods. Furthermore, it was also successfully applied to distinguish between serums from hepatoma patient and healthy people. As a new protein detection technique, the colloidal silver nanoparticles based “turn‐on” fluorescent sensor offers a rapid, economic, low background, and sensitive way for direct detection of human serum proteins, showing available potential and significance in the development of nanobiotechnology and proteome research.     

Light‐Harvesting Nanoparticle Probes for FRET‐Based Detection of Oligonucleotides with Single‐Molecule Sensitivity

Controlling the emission of bright luminescent nanoparticles by a single molecular recognition event remains a challenge in the design of ultrasensitive probes for biomolecules. Herein, we developed 20‐nm light‐harvesting nanoantenna particles, built of a tailor‐made hydrophobic charged polymer poly(ethyl methacrylate‐co‐methacrylic acid), encapsulating circa 1000 strongly coupled and highly emissive rhodamine dyes with their bulky counterion. Being 87‐fold brighter than quantum dots QDots 605 in single‐particle microscopy (with 550‐nm excitation), these DNA‐functionalized nanoparticles exhibit over 50 % total FRET efficiency to a single hybridized FRET acceptor, a highly photostable dye (ATTO665), leading to circa 250‐fold signal amplification. The obtained FRET nanoprobes enable single‐molecule detection of short DNA and RNA sequences, encoding a cancer marker (survivin), and imaging single hybridization events by an epi‐fluorescence microscope with ultralow excitation irradiance close to that of ambient sunlight.     

Unmodified "GNP-oligonucleotide" nanobiohybrids: a simple route for emission enhancement of DNA intercalators

We present herein a simple method for enhancing the emission of DNA intercalators in homogeneous nanobiohybrids of unlabeled oligonucleotides and unmodified gold nanoparticles (GNPs). Pristine single‐stranded DNA (ss‐DNA) has been wrapped around unmodified GNPs to induce metal‐enhanced fluorescence (MEF) of DNA intercalators, such as ethidium bromide and propidium iodide. The thickness of the ss‐DNA layer on the gold nanosurface determines the extent of MEF, since this depends on the position of the intercalator in relation to the metal surface. Presumably, at a suitable thickness of this DNA layer, more of the intercalator is localized at the optimum distance from the nanoparticle to give rise to MEF. Importantly, no external spacer or coating agent was needed to induce the MEF effect of the GNPs. The concentration ratios of Au to DNA in the nanohybrids, as well as the capping agents applied to the GNPs, play key roles in enhancing the emission of the intercalators. The dimensions of both components of the nanobiohybrids, that is, the size of the GNPs and the length of the oligonucleotide, have considerable influences on the emission enhancement of the intercalators. Emission intensity increased with increasing size of the GNPs and length of the oligonucleotide only when the DNA efficiently wrapped the nanoparticles. An almost 100 % increment in the quantum yield of ethidium bromide was achieved with the GNP–DNA nanobiohybrid compared with that with DNA alone (in the absence of GNP), and the fluorescence emission was enhanced by 50 % even at an oligonucleotide concentration of 2 nM . The plasmonic effect of the GNPs in the emission enhancement was also established by the use of similar nanobioconjugates of ss‐DNA with nonmetallic carbon nanoparticles and TiO₂ nanoparticles, with which no increase in the fluorescence emission of ethidium bromide was observed.     

Dual‐Luminophore‐Labeled Gold Nanoparticles with Completely Resolved Emission for the Simultaneous Imaging of MMP‐2 and MMP‐7 in Living Cells under Single Wavelength Excitation

Although considerable effort has been devoted to the design of various nanoprobes for the fluorescent detection of multiple biomarkers in a single assay, they often suffer from emission‐overlapping, owing to small Stokes shifts and wide emission spectra, which results in cross‐talk and inaccurate quantification. Herein, we report the design and synthesis of a new nanoprobe for multienzyme detection with completely resolved emission peaks under single‐wavelength excitation. The probe was assembled by attaching a cleavable peptide spacer, which was comprised from a matrix metalloproteinase‐2 (MMP‐2) substrate and a MMP‐7 substrate, onto the surface of gold nanoparticles (AuNPs) through cysteine residues. A lanthanide complex, BCTOT‐Eu^III (BCTOT=1,10‐bis(5′‐chlorosulfo‐thiophene‐2′‐yl)‐4,4,5,5,6,6,7,7‐octafluorodecane‐1,3,8,10‐tetraone), and 7‐amino‐4‐methylcoumarin (AMC) were attached to the N terminus and the C terminus of the peptide, respectively. In the presence of one or both targeting enzymes, the substrate was cleaved and fluorescence resonance energy transfer (FRET) between the dyes and AuNPs was prohibited, thereby resulting in the dramatic fluorescence emission of dyes. Importantly, there was no emission cross‐talk between the two dyes, thereby ensuring accurate detection of each enzyme. Based on this, the simultaneous fluorescence image of MMP‐2 and MMP‐7 was accomplished in living cells under single wavelength excitation. The apparent differences in the fluorescence imaging indicated distinct differences between the expression levels of MMPs between the human normal liver cells and the human hepatoma cells.