Fluorescent Ag Research Articles

Methylthiouracil-Templated Formation of the Fluorescent Silver Nanocluster and its Application to Bioimaging

Herein, we report a novel approach to Methylthiouracil-templated formation of fluorescent Ag nanoclusters and its application to bioimaging. We used fluorescence assay and TEM to characterize the silver nanocluster. The complex can be directly employed into bioimaging HeLa cells. The cell toxicity (3-[4,5-dimethylthiazolyl-2]-2,5-diphenyltetrazolium bromide, MTT) assay demonstrated that the silver nanocluster has only little affect on the cytotoxicity to the cells, which further proves the applicability of the method in tumor cell imaging. This work successfully develops the capping Methylthiouracil scaffolds of silver nanoclusters.

Hybridization chain reaction modulated DNA-hosted silver nanoclusters for fluorescent identification of single nucleotide polymorphisms in the let-7 miRNA family

A simple microRNA (miRNA) detection system based on hybridization chain reaction (HCR) has been developed using highly fluorescent DNA-hosted silver (Ag) nanoclusters. In this assay, a new type of hairpin DNA probe (MB1) containing a poly-cytosine nucleotide loop is designed and used as one of the HCR monomers, which is also demonstrated to be an ideal template for in situ synthesis of highly fluorescent Ag nanoclusters. Correspondingly, another HCR monomer (MB2) contains a poly-guanine nucleotide sticky end. Two monomers are stable to coexist in solution until the introduction of the initiator strand (let-7a) triggers a cascade of hybridization events that yields nicked double helices analogous. By taking advantage of HCR, a small amount of let-7a leads to the conformational change of a large amount of MB1, which results in the decrease of fluorescent signal greatly. Overall, this label-free, enzyme-free method allows the sensitive detection of let-7a with high specificity towards single nucleotide polymorphisms in the let-7 miRNA family. In addition, the simple “mix and measure” assay can be extended to detect other types of targets upon slight modification, and thus provides a tool for the early diagnosis and risk assessment of malignancy.

Branched DNA-based Synthesis of Fluorescent Silver Nanocluster

While single strand DNAs have been widely used for the scaffold of brightly fluorescent silver nanoclusters (Ag NCs), double strand DNAs have not been as successful. Herein, we report a novel synthetic approach for bright Ag NCs using branched double strand DNAs as the scaffolds for synthesis. X-shaped DNA (X-DNA) and Y-shaped DNA (Y-DNA) effectively stabilized Ag NCs, and both X-DNA and Y-DNA resulted in brightly fluorescent Ag NCs. The concentration and molar ratio of silver and DNA were found important for the fluorescence efficiency. The brightest Ag NC with the photoluminescence quantum efficiency of 19.8% was obtained for the reaction condition of 10 µM X-DNA, 70 µM silver, and the reaction time of 48 h. The fluorescence lifetime was about 2 ns for the Ag NCs and was also slightly dependent on the synthetic condition. Addition of Cu ions at the Ag NC preparations resulted in the quenching of Ag NC fluorescence, which was different to the brightening cases of single strand DNA stabilized Ag NCs.

Antimicrobial activity of fluorescent Ag nanoparticles

The antimicrobial activity of fluorescent Ag nanoparticles of 1·5 nm (nAg-Fs) is demonstrated and compared with the other Ag nanoparticles of different shapes and size. The antimicrobial activity was evaluated using Gram-positive (Staphylococcus epidermidis NCIM2493 and Bacillus megaterium) and Gram-negative bacteria (Pseudomonas aeruginosa ATCC27853 and Escherichia coli) and fungal strains (Candida albicans and Aspergillus niger). Insights into the possible mechanism were investigated using fluorescence microscope and cytoplasmic materials release assay. The fluorescence microscopic measurements show that the nAg-Fs are localized at the centre of the cell, and 50% decrease in the fluorescence intensity was observed upon 2-h incubation. Maximum cytoplasmic release was observed with spherical Ag nanoparticles of 10 nm. Although the nAg-F shows minimum cytoplasmic release, it has the highest activity. The microbial killing effect of nAg-Fs is actually originates from its intracellular activity. The antimicrobial activity of nAg-Fs is significantly higher than the other synthesized nanoparticles of different shapes and size. The activity of the nanoparticles has been rationalized by considering the shape, size and surface structure of the particles. This study aims to demonstrate the size and shape-dependent antimicrobial activity of Ag nanoparticles. It is shown for the first time that the fluorescent Ag nanoparticles of 1·5 nm have superior antimicrobial activity with respect to the larger particles. The shape and size of the particles actually control their activity. The smaller particles can easily penetrate the cell wall and have pronounced activity. These findings may be useful in the development of potential antimicrobial agents.

Ligation-triggered fluorescent silver nanoclusters system for the detection of nicotinamide adenine dinucleotide

Herein, we demonstrate a novel silver nanocluster-based fluorescent system for the detection of nicotinamide adenine dinucleotide (NAD(+)), an important biological small molecule involved in a wide range of biological processes. A single-stranded dumbbell DNA probe was designed and used for the assay, which contained a nick in the stem, a poly-cytosine nucleotide loop close to 5' end as the template for the formation of highly fluorescent silver nanoclusters (Ag NCs) and another loop close to 3' end. Only in the presence of NAD(+), the probe was linked at 5' and 3' ends by Escherichia coli DNA ligase, which blocked the DNA polymerase-based extension reaction, ensuring the formation of fluorescent Ag NCs. This technique provided a logarithmic linear relationship in the range of 1pM-500nM with a detection limit of as low as 1 pM NAD(+), and exhibited high selectivity against its analogues, and was then successfully used for the detection of NAD(+) level in four kinds of cell homogenates. In addition, this new approach was conducted in an isothermal and homogeneous condition without the need of any thermal cycling, washing, and separation steps, making it very simple. Overall, this label-free protocol offers a promising alternative for the detection of NAD(+), taking advantage of specificity, sensitivity, cost-efficiency, and simplicity.

Facile preparation of fluorescent Ag-clusters–chitosan-hybrid nanocomposites for bio-applications

Fluorescent silver nanoclusters (Ag NCs) are an important kind of luminescent nanomaterial with many practical applications. In this report, a facile route was designed for the preparation of fluorescent Ag NCs–chitosan hybrid nanocomposites based on the reduction of Ag+ with NaBH4. The hybrid nanocomposites had a spherical morphology with a size of 80 ± 10 nm. Moreover, the as-prepared nanospheres displayed high fluorescence with the emission peak center at 562 nm due to the formation of Ag NCs. In addition, chitosan, a natural polymer, worked as a stabilizing agent, and made the nanocomposites biocompatible, biodegradable and have low-toxicity. These properties mean that the nanospheres are promising materials for bio-applications. The methyl thiazolyl tetrazolium assay confirmed that the fluorescent chitosan–Ag NCs hybrid nanospheres had almost negligible toxicity to normal cells of MC3T3-E1. The outstanding low virulence of the as-prepared fluorescent chitosan–Ag NCs hybrid nanospheres was tested and verified by MC3T3-E1 cells and CAL-27 cells for bio-imaging as well.

Single DNA-Shelled Silver Nanoclusters Probed by Tip Enhanced Fluorescence Spectroscopy

Significant advances in nanoimaging have been made by development of aperture-less tip enhanced fluorescent microscopy, in which so called near field effect enhances the absorption and emission rates and allows overcoming the diffraction limit. Tip Enhanced Fluorescent Spectroscopy (TEFS) involves a combination of classical scanning tunneling microscope (STM) with optical confocal microscope where incident light is focused on the tip of metal probe enhancing electromagnetic field nearby.We applied TEFS technique for investigation of DNA-stabilized fluorescent Ag nanoclusters. DNA-stabilized Ag nanoclusters make up a class of water-soluble fluorophores consisted of 1-10 silver atoms. They exhibit high chemical stability, good biocompatibility and high absorbance and quantum yield. An assembled structure of compact globule with silver clusters inside was obtained using calf thymus DNA condensed by synthetic polycation (poly)allylamine. Compact fluorescent globules 100 nm in size containing 1-2 silver clusters were visualized by TEFS technique. Fourfold enhanced fluorescent signal was obtained and fluorescent spectra of single clusters were registered. Such Ag-DNA-polymer assembled structure seems to be promising in creation highly stable and bright nanoparticles containing multiple emitters therein. TEFS technique thus appears to be a powerful approach for single molecule spectroscopy and superresolution bioimaging.

Specific recognition of DNA bulge sites by in situ grown fluorescent Ag nanoclusters with high selectivity

There is much demand for the development of reliable methods of bulge site recognition in DNA in order to find disease-related mutations and identify bulge sequence environments. Herein, we found that the bulge site in the DNA duplex can be used to selectively grow fluorescent silver nanoclusters (Ag NCs). The as-prepared Ag NCs exhibit an emission behavior dependent on the base environment near the bulge site and can be employed in situ as a readout for specific bulge site recognition. Fluorescent signal-on sensing for the bulge site based on this inorganic fluorophore is substantially advantageous over the organic fluorophores previously used as probes, in which a fluorescent signal-off response was usually observed with context guanines flanking the bulge site. The strong dependence of the fluorescent Ag NCs' formation on the bases surrounding the bulge site is successfully used to identify the bulge environment near the codon 177 of cancer suppression gene p53. We expect that bulge site-determined Ag NC formation is very likely to be developed into a simple method for bulge site recognition with high selectivity and sensitivity.

Label-free and ultrasensitive microRNA detection based on novel molecular beacon binding readout and target recycling amplification

A label-free and high-sensitive microRNA (miRNA) detection approach by coupling a metal ion-meditated conformational molecular beacon (MB), using novel fluorescent Ag nanocluster (AgNCs) as fluorophore, with endonuclease-assisted target recycling amplification was developed. The assay comprised an Hg2+ ion-meditated conformational MB probe and an assistant probe that do not hybridize with each other at a specific temperature and can be annealed to each other in the presence of the target to form a Y-shape junction structure and released Hg2+. The target-MB hybridization event with the help of assistant probe can readily be read out based on the efficient fluorescence quenching of AgNCs by released Hg2+, while the Y-shape junction structure consisting of the probe MB, assistant probe and target miRNA could be recognized by the endonuclease Nt.BbvCI. The MB probe was then effectively cleaved by the endonuclease, and the regenerated assistant probe and the target further attended another cleavage cycle to implement the signal amplification. The competition displacing interaction between the target and the Hg2+ endows the biosensor with high sequence discrimination capability, while the high signal-to-noise ratio and target recycling amplification allows the biosensor to detect the target with high sensitivity. Under the optimal conditions, the concentration of target miRNA could be conveniently read out with a linear range from 10pM to 1fM. The proposed approach, avoiding any laborious label, possessing high sensitivity and selectivity, provided significant potential applications in future clinical analysis.

Effects of polymorphic DNA on the fluorescent properties of silver nanoclusters

Silver nanoclusters (Ag NCs) with emission wavelengths from 538 nm to 706 nm were synthesized using a series of G-/C-rich polymorphic DNA templates. Thermodynamic studies on the interactions of Ag(+) with polymorphic DNA indicate that the binding affinities greatly depend on the secondary structures of DNA templates, with a decreasing order: the coiled C-rich strand > the i-motif > the duplex > the G-quadruplex. Fluorescent Ag NCs prepared using these polymorphic DNA templates show that fluorescence stabilities are associated with the binding affinities between DNA templates and Ag(+), e.g., the coiled C-rich strand C4A4C3 with the highest binding constant (40.2 × 10(5) M(-1)) stabilizes Ag NCs for over 300 h, while the G-quadruplex G4T4G4 with the lowest binding constant (0.64 × 10(5) M(-1)) generates Ag NCs with the shortest shelf-life (55 h). The cluster sizes of Ag NCs are associated with the sequence lengths of DNA templates. In addition, the red emitting [C4A4C3]-Ag and the NIR emitting [G4T4G4-C4A4C4]-Ag are successfully used for HeLa cell transfection and exhibit good intracellular emission. These results provide a promising pathway to prepare silver nanomaterials with desirable physicochemical properties through designing suitable DNA templates.

A highly selective sensor of cysteine with tunable sensitivity and detection window based on dual-emission Ag nanoclusters

An effective dual-emission fluorescent Ag nanoclusters (NCs)-based probe have been constructed for rapid and selective detection of cysteine (Cys) with tunable sensitivity. Electrostatically induced reversible phase transfer method is employed to synthesize Ag nanoclusters with tunable emission intensity at 430nm and 630nm by controlling molar ratio between Ag and glutathione. The fluorescence of the Ag nanoclusters could be selectively quenched in the presence of Cys with a detection limit as low as 10nM. Good linear correlations are obtained over the concentration range from 0.5 to 55μM (quenched emission at 630nm), 55 to 120μM and 120 to 220μM (enhanced emission at 555nm) and 120 to 200μM (quenched emission at 430nm), respectively. The long-wavelength emission of the Ag nanoclusters can avoid the interference of the autofluorescence of the biosystems, which facilitated their applications in monitoring Cys in urine.

Preparation of water dispersible, fluorescent Ag–PAA–PVP hybrid nanogels and their optical properties

A simple method of synthesizing hybrid silver–polyacrylic acid–poly(N-vinylpyrrolidone) (Ag–PAA–PVP) nanogels was demonstrated through in situ reducing Ag+ inside PAA–PVP nanogels, which were formed by polymerization of acrylic acid in the PVP solution. Due to the ion exchange between Ag+ and acid protons of PAA, stable Ag+ clusters were formed inside the PAA–PVP nanogels, and hybrid nanogels were obtained by reducing Ag+ by ascorbic acid. Transmission electronic microscopic (TEM) images clearly showed the existence of silver nanoparticles inside the Ag–PAA–PVP nanogels. These hybrid nanogels showed typical surface plasma resonance absorption peak around 420nm, and the size of the silver nanoparticles inside the Ag–PAA–PVP nanogels could be controlled from 9.5±1.6nm to 1.9±0.4nm by increasing the feeding amount of Ag+. In addition, these hybrid nanogels showed photoluminescent properties in fluorescent spectra. Considering the pH sensitive property of these hybrid nanogels, they will have potential application in drug delivery and biomedical imaging systems.

Photochemical synthesis of fluorescent Ag nanoclusters and enhanced fluorescence by ionic liquid

Fluorescent silver nanoclusters with a size of about 2 nm were synthesized within 90 s by irradiating a mixture of silver precursor and polymethacrylic acid sodium salt solution under Xe lamp. The proposed method was well optimized by a systematical investigation of experimental parameters, such as irradiation time, initial concentration, pH of the solution. The obtained nanoclusters were well dispersed in water and exhibited bright emission upon visible light excitation. This approach can be further extended to use solar light for the large scale synthesis. Strikingly, the emission intensity of as-prepared silver nanoclusters could be enhanced up to ∼60% by the addition of 1-(2′-hydroxylethyl)-3-methylimidazolium tetrafluoroborate ([HOEtMIm][BF4]) ionic liquid. Meanwhile, these nanoclusters, in between molecules and normal nanoparticles, intensely absorb light around 510 nm covering the most intense part of solar light, should be a promising visible light sensitizer in solar cell and water splitting applications.

Synthesis of Highly Fluorescent Silver Clusters on Gold(I) Surface

Evolution of fluorescence from a giant core-shell particle is new and synergistic, which requires both gold and silver ions in an appropriate ratio in glutathione (GSH) solution. The formation of highly fluorescent Ag(2)/Ag(3) clusters on the surface of Au(I) assembly results in giant Au(I)(core)-Ag(0)(shell) water-soluble microparticles (~500 nm). Here, Au(I) acts as the template for the generation of fluorescent Ag clusters. The presence of gold under the synthetic strategy is selective, and no other metal supports such synergistic evolution. The core-shell particle exhibits stable and static emission (emission maximum, 565 nm; quantum yield, 4.6%; and stroke shift, 179 nm) with an average lifetime of ~25 ns. The drift of electron density by the Au(I) core presumably enhances the fluorescence. The positively charged core offers unprecedented long-term stability to the microparticles in aqueous GSH solution.

Glutathione-Protected Silver Nanoclusters as Cysteine-Selective Fluorometric and Colorimetric Probe

The integration of the unique thiol-Ag chemistry and the specific steric hindrance from the organic layer of fluorescent Ag nanoclusters (AgNCs) was first developed in this work to achieve a simple detection of cysteine (Cys) with high selectivity and sensitivity. The key design is a strongly red-emitting AgNC protected by the interference biothiol, glutathione, or GSH (hereafter referred to as GSH-AgNCs), where both the physicochemical properties (Ag surface chemistry and fluorescence) of the NC core and the physical properties (e.g., steric hindrance) of the organic shell were fully utilized for Cys detection with three major features. First, owing to the specific thiol-Ag interaction, the fluorescent GSH-AgNCs showed superior selectivity for Cys over the other 19 natural amino acids (nonthiol-containing). Second, the GSH protecting layer on the NC surface made possible the differentiation of Cys from GSH (or other large-sized thiol molecules) simply by their size. Third, the ultrasmall size of GSH-AgNCs and the high affinity of the thiol-Ag interaction provided high sensitivity for Cys detection with a detection limit of <3 nM. The assay developed in this study is of interest not only because it provides a simple Cys sensor with high selectivity and sensitivity but also because it exemplifies the utilization of the physical properties of organic ligands on the nanomaterial surface to further improve the sensor performance, which could open a new design strategy for other sensor development.

Sensitive and selective detection of Hg2+ and Cu2+ ions by fluorescent Ag nanoclusters synthesized via a hydrothermal method

An easily prepared fluorescent Ag nanoclusters (Ag NCs) probe for the sensitive and selective detection of Hg(2+) and Cu(2+) ions was developed here. The Ag NCs were synthesized by using polymethacrylic acid sodium salt as a template via a convenient hydrothermal process. The as-prepared fluorescent Ag NCs were monodispersed, uniform and less than 2 nm in diameter, and can be quenched in the presence of mercury (Hg(2+)) or copper (Cu(2+)) ions. Excellent linear relationships existed between the quenching degree of the Ag NCs and the concentrations of Hg(2+) or Cu(2+) ions in the range of 10 nM to 20 μM or 10 nM to 30 μM, respectively. By using ethylenediaminetetraacetate (EDTA) as the masking agent of Cu(2+), Hg(2+) was exclusively detected in coexistence with Cu(2+) with high sensitivity (LOD = 10 nM), which also provided a reusable detection method for Cu(2+). Furthermore, the different quenching phenomena caused by the two metals ions such as changes in visible colour, shifts of UV absorbance peaks and changes in size of Ag NCs make it easy to distinguish between them. Therefore the easily synthesized fluorescent Ag NCs may have great potential as Hg(2+) and Cu(2+) ions sensors.

A novel ratiometric fluorescence probe based on BSA assembled silver nanoclusters for mercuric ion selective sensing

A novel ratiometric fluorescence probe based on protein–silver nanoclusters (NCs) for mercuric ion (Hg2+) has been synthesized and characterized. A facile and simple one-step method for the synthesis of fluorescent Ag NCs, using bovine serum albumin (BSA) without preprocessing as a scaffold was developed. BSA assembled Ag NC (BSA-Ag NCs) were first found to exhibit ratiometric fluorescence with blue emission at 480 nm and red emission at 634 nm. Upon the addition of Hg2+, the emission of BSA-Ag NCs at 480 nm gradually increased and at 634 nm steadily decreased. The presence of Hg2+ induced a color change from light yellow to colorless and a bright red fluorescence to blue fluorescence. The determination of Hg2+ based on ratiometric fluorescence was developed. The linear dynamic range was from 5.0 × 10−8 M to 2.5 × 10−5 M with a detection limit of 4.87 × 10−8 M. The relative standard deviation (RSD) at 0.30 μM for Hg2+ of six replicates determination was 2.8%. The BSA-Ag NCs were conveniently and easily synthesized, so could serve as a “naked-eye” fluorescent probe for Hg2+. The probe may be a good candidate for the application of sensing Hg2+ in environmental analysis and fluorescence imaging.

Synthesis and Spectroscopic Characterization of Water-Soluble Fluorescent Ag Nanoclusters

Water-soluble fluorescent Ag nanoclusters (NCs) were synthesized at room temperature with sodium dodecyl sulfonate (SDS) as a protective agent. The effects of synthetic conditions on the fluorescence properties of Ag NCs were investigated. The results show that the fluorescence intensity of Ag NCs strongly depends on the synthetic conditions, such as the molar ratio of AgNO3 versus SDS and sodium borohydride (NaBH4), the reaction time, and the pH value of the reaction solution. Under the optimum conditions, the as-prepared Ag NCs exist in face-centered-cubic phase with an average size of 2 nm. Fluorescence spectra of Ag NCs show emission peaks at 365 nm for different excitation wavelength. Resonant absorptions are observed at 203 nm and 277 nm in the absorption spectrum, which can be used to establish the electronic levels in the Ag NCs system.

Highly sensitive label-free fluorescent detection of Hg2+ ions by DNA molecular machine-based Ag nanoclusters

We present here a highly selective and sensitive label-free method to detect Hg(2+) ions in aqueous solution by using DNA molecular machine-based fluorescent Ag nanoclusters (AgNCs). This mechanism is based on the Hg(2+) ions triggering machine-like operations of DNA and the "product" of the machine being used to stabilize fluorescent AgNCs. In this method, a tailored DNA, containing a sequence for Hg(2+) ions recognition, a sequence-specific nicking site for Nb BbvC I and a sequence complementary to the DNA as a template for the synthesis of fluorescent AgNCs, was firstly designed. In the presence of Hg(2+) ions, the machine's function operations were triggered. A series of machine-like operations, including replication, scission, and displacement then occurred with the addition of polymerase/dNTPs/Nb BbvC I, which manufactured lots of "product" DNA. The "product" DNA could act as a template for the preparation of fluorescent AgNCs. Thus the fluorescence of the AgNCs could be used as a signal transduction of this DNA machine, which was related to the concentration of the Hg(2+) ions. The repeated synthesis of the "product" and its template effect for AgNCs synthesis led to signal amplification in the assay of Hg(2+) ions. A linear response to the concentration of Hg(2+) ions was observed in the range from 0.08 nM to 20 nM and a detection limit of 0.08 nM was obtained. By contrast, the operation of the machine could not be executed in an Hg(2+) ion-free system. Moreover, the detection was not only label-free but also specific for Hg(2+) ions without being affected by other metal ions.

Thiourea-functionalized poly(phenyleneethynylene): fluorescent chemosensors for anions and cations

A novel poly(phenyleneethynylene) containing thiourea groups [poly(1)] was synthesized. The anion sensing ability of poly(1) was assessed using tetra-n-butylammonium (TBA) salts of a series of anions in DMF. The addition of F− changed the color of the polymer solution in DMF from yellow to orange and quenched the fluorescence, whereas the visible color and fluorescence of the solution only varied slightly upon the addition of Cl−, HSO4−, Br−, CH3COO−, C6H5COO−, and NO3−; this indicates that poly(1) exhibits an F− sensing ability. 1H NMR titrations of poly(1) revealed that the colorimetric and fluorescent response of poly(1) is triggered by thiourea/F− hydrogen bonding. Poly(1) also exhibited a turn-off fluorescence Ag+ sensing ability. The addition of Ag+ quenched the fluorescence of the polymer, while the addition of Pb2+, Zn2+, Cu2+, Mn2+, K+, Na+, and Co2+ resulted in only slight changes to the fluorescence. The sensor containing poly(1) and Ag+ shows a turn-on fluorescence adenosine diphosphate (ADP) sensing ability.