Ag cluster-aptamer hybrid: specifically marking the nucleus of live cells

Fluorescence Enhancement Method for Aptamer-Templated Silver Nanoclusters and Its Application in the Construction of a β-Amyloid Oligomer Sensor

DNA-templated silver nanoclusters (DNA-AgNCs) have attracted significant attention due to their unique fluorescence properties. However, so far, the relatively low quantum yields of the DNA-AgNCs and the complex design of DNA-AgNC-based sensors have limited their application in biosensing or bioimaging. Herein, we report a novel fluorescence enhancement method. The β-Amyloid Oligomer (AβO) aptamer (Apt_AβO) with A₁₀/T₁₀ at its 3' end can be directly used as the template to fabricate the AgNCs. When the AgNCs were hybridized with the complementary strand that has 12 bases suspended at its 3' terminal, being the same or complementary to the A/T at the 3' end of the Apt_AβO, and two-base mismatches in the complementary region of the aptamer excluded A₁₀/T₁₀, a dramatic fluorescence enhancement (maximum: ∼500-fold; maximum quantum yield: 31.5%) can be realized. The fluorescence enhancement should result from the aggregation-induced emission of the AgNCs, which can be attributed to forming the reticular structure of the hybridized product. To some extent, the method developed in this work is extendable. The fluorescence enhancement was also realized from the thrombin aptamer-templated AgNCs through designing the aptamer and the corresponding complementary strand according to the method. Based on the fluorescence enhancement of the Apt_AβO-templated AgNCs, an "on-off" fluorescence sensor was constructed for the sensitive and selective detection of AβO. This work provides a rational strategy to realize fluorescence enhancement for the aptamer-templated AgNCs and design an aptamer-based fluorescence sensor.

DNA Templated Silver Nanoclusters for Bioanalytical Applications: A Review

Due to their unique programmability, biocompatibility, photostability and high fluorescent quantum yield, DNA templated silver nanoclusters (DNA Ag NCs) have attracted increasing attention for bioanalytical application. This review summarizes the recent developments in fluorescence properties of DNA templated Ag NCs, as well as their applications in bioanalysis. Finally, we herein discuss some current challenges in bioanalytical applications, to promote developments of DNA Ag NCs in biochemical analysis.

Dual-Functional Nanocluster Probe-Based Single-Cell Analysis of RNA Splice Variants

Differential expression of RNA splice variants among individual cells accounts for cell heterogeneity of gene expression, which plays a key role in the regulation of the immune system. However, currently available techniques face difficulties in achieving single-cell analysis of RNA splice variants with high base resolution, high spatial resolution and accurate quantification. Herein, we constructed DNA-templated dual-functional nanocluster probes to achieve in situ imaging and accurate quantification of RNA splice variants at the single-cell level. By designing ultrasmall nanocluster labeled probes to directly target the splicing junction sequence of RNA splice variants, the base recognition resolution is significantly improved. Benefit from the controllable fluorescence of nanoclusters, in situ imaging and genotyping of RNA splice variants are achieved. Due to the atom-precise nanocluster, RNA splice variants can be accurately quantified by laser ablation inductively coupled plasma mass spectrometry at the single-cell level. We further applied the probes to explore the function of MyD88 splice variants in mononuclear macrophages under immune activation. This strategy provides a novel single-cell analysis tool for studying the functional diversity of the immune system and splicing-related immune diseases.

Synergistic integration of metal nanoclusters and biomolecules as hybrid systems for therapeutic applications

Therapeutic nanoparticles are designed to enhance efficacy, real-time monitoring, targeting accuracy, biocompatibility, biodegradability, safety, and the synergy of diagnosis and treatment of diseases by leveraging the unique physicochemical and biological properties of well-developed bio-nanomaterials. Recently, bio-inspired metal nanoclusters (NCs) consisting of several to roughly dozens of atoms (<2 nm) have attracted increasing research interest, owing to their ultrafine size, tunable fluorescent capability, good biocompatibility, variable metallic composition, and extensive surface bio-functionalization. Hybrid core-shell nanostructures that effectively incorporate unique fluorescent inorganic moieties with various biomolecules, such as proteins (enzymes, antigens, and antibodies), DNA, and specific cells, create fluorescently visualized molecular nanoparticle. The resultant nanoparticles possess combinatorial properties and synergistic efficacy, such as simplicity, active bio-responsiveness, improved applicability, and low cost, for combination therapy, such as accurate targeting, bioimaging, and enhanced therapeutic and biocatalytic effects. In contrast to larger nanoparticles, bio-inspired metal NCs allow rapid renal clearance and better pharmacokinetics in biological systems. Notably, advances in nanoscience, interfacial chemistry, and biotechnologies have further spurred researchers to explore bio-inspired metal NCs for therapeutic purposes. The current review presents a comprehensive and timely overview of various metal NCs for various therapeutic applications, with a special emphasis on the design rationale behind the use of biomolecules/cells as the main scaffolds. In the different hybrid platform, we summarize the current challenges and emerging perspectives, which are expected to offer in-depth insight into the rational design of bio-inspired metal NCs for personalized treatment and clinical translation.

The spatial organization of trace silver atoms on a DNA template

DNA with programmable information can be used to encode the spatial organization of silver atoms. Based on the primary structures of a DNA template containing a controllable base arrangement and number, the surrounding environment and cluster together can induce the folding of the DNA template into an appropriate secondary structure for forming AgNCs with different fluorescence emissions, such as i-motif, G-quadruplex, dimeric template, triplex, monomeric or dimeric C-loop, emitter pair, and G-enhancer/template conjugate. Stimuli can induce the dynamic structural transformation of the DNA template with a recognition site for favourably or unfavourably forming AgNCs, along with varied fluorescence intensities and colours. The array of several or more of the same and different clusters can be performed on simple and complex nanostructures, while maintaining their original properties. By sorting out this review, we systematically conclude the link between the performance of AgNCs and the secondary structure of the DNA template, and summarize the precise arrangement of nanoclusters on DNA nanotechnology. This clear review on the origin and controllability of AgNCs based on the secondary structure of the DNA template is beneficial for exploring the new probe and optical devices.

Functional Nucleic Acid-Based Live-Cell Fluorescence Imaging

Cell is the structural and functional unit of organism. It serves as a key research object in various biological processes, such as growth, ontogeny, metabolism and stress. Studying the spatiotemporal distribution and functional activity of specific biological molecules in living cells is crucial for exploring the mechanism governing life. It also facilitates the elucidation of pathogenesis, clinical prevention and disease theranostics. In recent years, the fluorescence imaging technique has been greatly exploited for live-cell imaging. However, the development of molecular probes has lagged far behind. Functional nucleic acids (FNAs), for example, aptamer and DNAzyme, possess special chemical and/or biological functions, hence severing as promising molecular tools for cellular imaging. The current mini review focuses on the applications of FNAs in live-cell fluorescence imaging.

Micro RNA Sensing with Green Emitting Silver Nanoclusters

Micro RNA (miR) are regulatory non-coding RNA molecules, which contain a small number of nucleotides ~18-28 nt. There are many various miR sequences found in plants and animals that perform important functions in developmental, metabolic, and disease processes. miRs can bind to complementary sequences within mRNA molecules thus silencing mRNA. Other functions include cardiovascular and neural development, stem cell differentiation, apoptosis, and tumors. In tumors, some miRs can function as oncogenes, others as tumor suppressors. Levels of certain miR molecules reflect cellular events, both normal and pathological. Therefore, miR molecules can be used as biomarkers for disease diagnosis and prognosis. One of these promising molecules is miR-21, which can serve as a biomarker with high potential for early diagnosis of various types of cancer. Here, we present a novel design of miR detection and demonstrate its efficacy on miR-21. The design employs emissive properties of DNA-silver nanoclusters (DNA/AgNC). The detection probe is designed as a hairpin DNA structure with one side of the stem complimentary to miR molecule. The binding of target miR-21 opens the hairpin structure, dramatically modulating emissive properties of AgNC hosted by the C12 loop of the hairpin. "Red" fluorescence of the DNA/AgNC probe is diminished in the presence of the target miR. At the same time, "green" fluorescence is activated and its intensity increases several-fold. The increase in intensity of "green" fluorescence is strong enough to detect the presence of miR-21. The intensity change follows the concentration dependence of the target miR present in a sample, which provides the basis of developing a new, simple probe for miR detection. The detection strategy is specific, as demonstrated using the response of the DNA/AgNC probe towards the scrambled miR-21 sequence and miR-25 molecule. Additionally, the design reported here is very sensitive with an estimated detection limit at ~1 picomole of miR-21.

Delivery of antibacterial silver nanoclusters to Pseudomonas aeruginosa using species-specific DNA aptamers

Introduction. The use of silver as an antimicrobial therapeutic is limited by its toxicity to host cells compared with that required to kill bacterial pathogens.Aim. To use aptamer targeting of DNA scaffolded silver nanoclusters as an antimicrobial agent for treating Pseudomonas aeruginosa infections.Methodology. Antimicrobial activity was assessed in planktonic cultures and in vivo using an invertebrate model of infection.Results. The aptamer conjugates that we call aptabiotics have potent antimicrobial activity. Targeted silver nanoclusters were more effective at killing P. aeruginosa than the equivalent quantity of untargeted silver nanoclusters. The aptabiotics have an IC₅₀ of 1.3-2.6 µM against planktonically grown bacteria. Propidium iodide staining showed that they rapidly depolarize bacterial cells to kill approximately 50 % of the population within 10 min following treatment. In vivo testing in the Galleria mellonella model of infection prolonged survival from an otherwise lethal infection.Conclusion. Using P. aeruginosa as a model, we show that targeting of DNA-scaffolded silver nanoclusters with an aptamer has effective fast-acting antimicrobial activity in vitro and in an in vivo animal model.

Enzyme-free amplified detection of miRNA based on target-catalyzed hairpin assembly and DNA-stabilized fluorescent silver nanoclusters

A simple, cost-effective, sensitive, and selective strategy was developed for microRNA analysis using target-catalyzed hairpin assembly and fluorescent silver nanoclusters.

Tumor Targeting Strategies of Smart Fluorescent Nanoparticles and Their Applications in Cancer Diagnosis and Treatment

Advantages such as strong signal strength, resistance to photobleaching, tunable fluorescence emissions, high sensitivity, and biocompatibility are the driving forces for the application of fluorescent nanoparticles (FNPs) in cancer diagnosis and therapy. In addition, the large surface area and easy modification of FNPs provide a platform for the design of multifunctional nanoparticles (MFNPs) for tumor targeting, diagnosis, and treatment. In order to obtain better targeting and therapeutic effects, it is necessary to understand the properties and targeting mechanisms of FNPs, which are the foundation and play a key role in the targeting design of nanoparticles (NPs). Widely accepted and applied targeting mechanisms such as enhanced permeability and retention (EPR) effect, active targeting, and tumor microenvironment (TME) targeting are summarized here. Additionally, a freshly discovered targeting mechanism is introduced, termed cell membrane permeability targeting (CMPT), which improves the tumor-targeting rate from less than 5% of the EPR effect to more than 50%. A new design strategy is also summarized, which is promising for future clinical targeting NPs/nanomedicines design. The targeting mechanism and design strategy will inspire new insights and thoughts on targeting design and will speed up precision medicine and contribute to cancer therapy and early diagnosis.

Fluorescent Ag clusters conjugated with anterior gradient-2 antigen aptamer for specific detection of cancer cells

Anterior gradient protein 2 homolog (AGR) is a potential tumor biomarker and plays an important role in tissue development and regeneration. The intracellular detection of AGR is rarely reported. By conjugating the AGR aptamer with a cytosine base sequence as Ag cluster template, a highly fluorescent probe (MA@AgNCs) was synthesized for targeting intracellular AGR. The MA@AgNCs display a maximum fluorescence peak at 565 nm, and possess an excellent quantum yield (QY = 87.43%), small size, great biocompatibility, low toxicity, and good stability. Moreover, the as synthesized MA@AgNCs show high specificity on recognizing breast cancer cells.

A Turn-On Detection of DNA Sequences by Means of Fluorescence of DNA-Templated Silver Nanoclusters via Unique Interactions of a Hydrated Ionic Liquid

Nucleic acid stability and structure, which are crucial to the properties of fluorescent DNA-templated silver nanoclusters (DNA-Ag NCs), significantly change in ionic liquids. In this work, our purpose was to study DNA-Ag NCs in a buffer containing the hydrated ionic liquid of choline dihydrogen phosphate (choline dhp) to improve fluorescence for application in DNA detection. Due to the stabilisation of an i-motif structure by the choline cation, a unique fluorescence emission—that was not seen in an aqueous buffer—was observed in choline dhp and remained stable for more than 30 days. A DNA-Ag NCs probe was designed to have greater fluorescence intensity in choline dhp in the presence of a target DNA. A turn-on sensing platform in choline dhp was built for the detection of the BRCA1 gene, which is related to familial breast and ovarian cancers. This platform showed better sensitivity and selectivity in distinguishing a target sequence from a mutant sequence in choline dhp than in the aqueous buffer. Our study provides new evidence regarding the effects of structure on properties of fluorescent DNA-Ag NCs and expands the applications of fluorescent DNA-Ag NCs in an ionic liquid because of improved sensitivity and selectivity.

Silk fibroin-derived peptide directed silver nanoclusters for cell imaging

Fluorescent silver nanoclusters (Ag NCs) that are capable of emitting green light have been synthesized using a peptide derived from the C terminal of silk fibroin heavy chain (CSH) via a one-pot, green, and facile synthesis method. The emission was also found to be stable at the excitation wavelength and the fluorescence quantum yield of Ag NCs was measured to be 1.1%. Matrix assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS) indicated the presence of a range of Ag species that correspond to Ag1, Ag2, Ag3 and Ag4. Transmission electron microscopic analyses suggested that the formed particles are uniform and well dispersive with an average diameter of 2.5 nm. The Ag NCs were successfully applied to cell imaging in murine preosteoblast MC3T3-E1 cells. Finally, Ag NCs observed by MTT exhibited distinct cytotoxicity at CSH-Ag NCs concentrations of 600 μM. Based on the concept of utilizing a functional peptide from nature, this study demonstrates a novel approach to fabricate aqueous metal nanoclusters for tracking applications in bioimaging.

In situ fluorescence activation of DNA-silver nanoclusters as a label-free and general strategy for cell nucleus imaging

A facile, general and turn-on nucleus imaging strategy was first developed based on in situ fluorescence activation of C-rich dark silver nanoclusters by G-rich telomeres. After a simple incubation without washing, nanoclusters could selectively stain the nucleus with intense red luminescence, which was confirmed using fixed/living cells and several cell lines.

The potential of aptamers for cancer research

Aptamers are promising alternatives to antibodies and can be used as high affinity agents for the cancer detection and the targeted drug transportation. In this manuscript, we highlight the advantages of aptamers, such as high affinities, specificity and excellent chemical stabilities, which are likely to benefit for the diagnosis of cancer in its early stages and then achieve molecular-level treatment. Also, we discuss the challenges and problems in the current application of aptamers.

DNA metallization: principles, methods, structures, and applications

This review summarizes the research activities on DNA metallization since the concept was first proposed in 1998, covering the principles, methods, structures, and applications.

Histone–DNA interaction: an effective approach to improve the fluorescence intensity and stability of DNA-templated Cu nanoclusters

The histone–DNA interaction is found to greatly improve the fluorescence intensity and stability of DNA-templated Cu nanoclusters (CuNCs).

Stable, polyvalent aptamer-conjugated near-infrared fluorescent nanocomposite for high-performance cancer cell-targeted imaging and therapy

A novel nanocomposite with improved biostability has been designed and used for cancer cell-specific imaging and targeted therapy.

Fluorescence-tunable Ag-DNA biosensor with tailored cytotoxicity for live-cell applications

DNA-stabilized silver clusters (Ag-DNA) show excellent promise as a multi-functional nanoagent for molecular investigations in living cells. The unique properties of these fluorescent nanomaterials allow for intracellular optical sensors with tunable cytotoxicity based on simple modifications of the DNA sequences. Three Ag-DNA nanoagent designs are investigated, exhibiting optical responses to the intracellular environments and sensing-capability of ions, functional inside living cells. Their sequence-dependent fluorescence responses inside living cells include (1) a strong splitting of the fluorescence peak for a DNA hairpin construct, (2) an excitation and emission shift of up to 120 nm for a single-stranded DNA construct, and (3) a sequence robust in fluorescence properties. Additionally, the cytotoxicity of these Ag-DNA constructs is tunable, ranging from highly cytotoxic to biocompatible Ag-DNA, independent of their optical sensing capability. Thus, Ag-DNA represents a versatile live-cell nanoagent addressable towards anti-cancer, patient-specific and anti-bacterial applications.

Biostable L-DNA-Templated Aptamer-Silver Nanoclusters for Cell-Type-Specific Imaging at Physiological Temperature

The high susceptibility of the natural D-conformation of DNA (D-DNA) to nucleases greatly limits the application of DNA-templated silver nanoclusters (Ag NCs) in biological matrixes. Here we demonstrate that the L-conformation of DNA (L-DNA), the enantiomer of D-DNA, can also be used for the preparation of aptamer-Ag NCs. The extraordinary resistance of L-DNA to nuclease digestion confers much higher biostability to these NCs than those templated by D-DNA, thus making cell-type-specific imaging possible at physiological temperatures, using at least 100-times lower Ag NC concentration than reported D-DNA-templated ones. The L-DNA-templated metal NC probes with enhanced biostability might promote the applications of metal nanocluster probes in complex biological systems.

DNA-stabilized silver nanoclusters and carbon nanoparticles oxide: A sensitive platform for label-free fluorescence turn-on detection of HIV-DNA sequences

Based on the remarkable difference between the interactions of carbon nanoparticles (CNPs) oxide with single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA), and the fact that fluorescence of DNA-stabilized silver nanoclusters (AgNCs) can be quenched by CNPs oxide, DNA-functionalized AgNCs were applied as label-free fluorescence probes and a novel fluorescence resonance energy transfer (FRET) sensor was successfully constructed for the detection of human immunodeficiency virus (HIV) DNA sequences. CNPs oxide were prepared with the oxidation of candle soot, hence it is simple, time-saving and low-cost. The strategy of dual AgNCs probes was applied to improve the detection sensitivity by using dual- probe capturing the same target DNA in a sandwich mode and as the fluorescence donor, and using CNPs oxide as the acceptor. In the presence of target DNA, a dsDNA hybrid forms, leading to the desorption of the ssDNA-AgNCs probes from CNPs oxide, and the recovering of fluorescence of the AgNCs in a HIV-DNA concentration-dependent manner. The results show that HIV-DNA can be detected in the range of 1-50nM with a detection limit of 0.40nM in aqueous buffer. The method is simple, rapid and sensitive with no need of labeled fluorescent probes, and moreover, the design of fluorescent dual-probe makes full use of the excellent fluorescence property of AgNCs and further improves the detection sensitivity.

Metal nanoclusters: novel probes for diagnostic and therapeutic applications

Metal nanoclusters, composed of several to a few hundred metal atoms, have received worldwide attention due to their extraordinary physical and chemical characteristics. Recently, great efforts have been devoted to the exploration of the potential diagnostic and therapeutic applications of metal nanoclusters. Here we focus on the recent advances and new horizons in this area, and introduce the rising progress on the use of metal nanoclusters for biological analysis, biological imaging, therapeutic applications, DNA assembly and logic gate construction, enzyme mimic catalysis, as well as thermometers and pH meters. Furthermore, the future challenges in the construction of biofunctional metal nanoclusters for diagnostic and therapeutic applications are also discussed. We expect that the rapidly growing interest in metal nanocluster-based theranostic applications will certainly not only fuel the excitement and stimulate research in this highly active field, but also inspire broader concerns across various disciplines.

DNA assembled metal nanoclusters: synthesis to novel applications

In this review, we have discussed the emergence of promising environmental-benign DNA assembled fluorescent metal nanoclusters and their unique electronic structures, unusual physical and chemical properties.

One-pot one-cluster synthesis of fluorescent and bio-compatible Ag14 nanoclusters for cancer cell imaging

Small-molecule-protected silver nanoclusters have smaller hydrodynamic diameter, and thus may hold greater potential in biomedicine application compared with the same core-sized, macromolecule (i.e. DNA)-protected silver nanoclusters. However, the live cell imaging labeled by small-molecule-protected silver nanoclusters has not been reported until now, and the synthesis and atom-precise characterization of silver nanoclusters have been challenging for a long time. We develop a one-pot one-cluster synthesis method to prepare silver nanoclusters capped with GSH which is bio-compatible. The as-prepared silver nanoclusters are identified to be Ag14(SG)11 (abbreviated as Ag14, SG: glutathione) by isotope-resolvable ESI-MS. The structure is probed by 1D NMR spectroscopy together with 2D COSY and HSQC. This cluster species is fluorescent and the fluorescence quantum yield is solvent-dependent. Very importantly, Ag14 was successfully applied to label lung cancer cells (A549) for imaging, and this work represents the first attempt to image live cells with small-molecule-protected silver nanoclusters. Furthermore, it is revealed that the Ag14 nanoclusters exhibit lower cytotoxicity compared with some other silver species (including silver salt, silver complex and large silver nanoparticles), and the explanation is also provided. The comparison of silver nanoclusters to state-of-the-art labeling materials in terms of cytotoxicity and photobleaching lifetime is also conducted.

G-quadruplex enhanced fluorescence of DNA-silver nanoclusters and their application in bioimaging

Guanine proximity based fluorescence enhanced DNA-templated silver nanoclusters (AgNCs) have been reported and applied for bioanalysis. Herein, we studied the G-quadruplex enhanced fluorescence of DNA-AgNCs and gained several significant conclusions, which will be helpful for the design of future probes. Our results demonstrate that a G-quadruplex can also effectively stimulate the fluorescence potential of AgNCs. The major contribution of the G-quadruplex is to provide guanine bases, and its special structure has no measurable impact. The DNA-templated AgNCs were further analysed by native polyacrylamide gel electrophoresis and the guanine proximity enhancement mechanism could be visually verified by this method. Moreover, the fluorescence emission of C3A (CCCA)4 stabilized AgNCs was found to be easily and effectively enhanced by G-quadruplexes, such as T30695, AS1411 and TBA, especially AS1411. Benefiting from the high brightness of AS1411 enhanced DNA-AgNCs and the specific binding affinity of AS1411 for nucleolin, the AS1411 enhanced AgNCs can stain cancer cells for bioimaging.

Protein-DNA interactions: a novel approach to improve the fluorescence stability of DNA/Ag nanoclusters

Protein-DNA interactions are known to play an important role in a variety of biological processes. We have shown here that protein-DNA binding events can also be used to greatly improve the fluorescence stability of DNA-templated Ag nanoclusters (Ag NCs), which would be highly beneficial for Ag NCs in applications of biosensing/imaging.

A novel biosensor for copper(ii) ions based on turn-on resonance light scattering of ssDNA templated silver nanoclusters

A new ultrasensitive biosensor for copper(ii) ions was first developed based on turn-on resonance light scattering (RLS) of ssDNA templated silver nanoclusters through anti-galvanic reduction (AGR).

Novel conjugated Ag@PNIPAM nanocomposites for an effective antibacterial wound dressing

It is well known that nanosilver or silver ions could act as an effective antibacterial agent without the development of bacterial resistance but long term exposure may induce in vivo toxicity.

Spatially marking and quantitatively counting membrane immunoglobulin M in live cells via Ag cluster-aptamer probes

A probe composed of an aptamer and a silver cluster, where the aptamer targets mIgM of live cells and the silver cluster provides fluorescent imaging and mass quantification of mIgM of live cells, is presented. This new probe simultaneously provides accurate spatial and mass information of mIgM in live cells.

Selectively assaying CEA based on a creative strategy of gold nanoparticles enhancing silver nanoclusters' fluorescence

Herein, we have successfully built up connections between nanoparticles and nanoclusters, and further constructed a surface-enhanced fluorescence (SEF) strategy based on the two types of nanomaterials for selectively assaying carcinoembryonic antigen (CEA). Specifically, silver nanoclusters provided the original fluorescence signal, while gold nanoparticles modified with DNA served as the fluorescence enhancer simultaneously. On the basis of this proposed nano-system, the two nanomaterials were linked by CEA-aptamer, thus facilitating SEF occurring. Nevertheless, more competitive interactions between CEA and CEA-aptamer emerged once CEA added, leading to SEF failed and their fluorescence decreased. Significantly, this creative method was further applied to detect CEA, and showed the linear relationship between the fluorescence intensity and CEA concentrations in the range of 0.01-1 ng mL(-1) with a detection limit of 3 pg mL(-1) at a signal-to-noise ratio of 3, demonstrating its sensitivity and promising towards multiple applications. On the whole, this approach we established may broaden potential ways of combining nanoparticles and nanoclusters for detecting trace targets in bioanalytical fields.

Concatemeric dsDNA-templated copper nanoparticles strategy with improved sensitivity and stability based on rolling circle replication and its application in microRNA detection

DNA-templated copper nanoparticles (CuNPs) have emerged as promising fluorescent probes for biochemical assays, but the reported monomeric CuNPs remain problematic because of weak fluorescence and poor stability. To solve this problem, a novel concatemeric dsDNA-templated CuNPs (dsDNA-CuNPs) strategy was proposed by introducing the rolling circle replication (RCR) technique into CuNPs synthesis. In this strategy, a short oligonucleotide primer could trigger RCR and be further converted to a long concatemeric dsDNA scaffold through hybridization. After the addition of copper ions and ascorbate, concatemeric dsDNA-CuNPs could effectively form and emit intense fluorescence in the range of 500-650 nm under a 340 nm excitation. In comparison with monomeric dsDNA-CuNPs, the sensitivity of concatemeric dsDNA-CuNPs was greatly improved with ~10,000 folds amplification. And their fluorescence signal was detected to reserve ~60% at 2.5 h after formation, revealing ~2 times enhanced stability. On the basis of these advantages, microRNA let-7d was selected as the model target to testify this strategy as a versatile assay platform. By directly using let-7d as the primer in RCR, the simple, low-cost, and selective microRNA detection was successfully achieved with a good linearity between 10 and 400 pM and a detection limit of 10 pM. The concatemeric dsDNA-CuNPs strategy might be widely adapted to various analytes that can directly or indirectly induce RCR.

Electrochemical impedance spectroscopy aptasensor for ultrasensitive detection of adenosine with dual backfillers

A highly sensitive and label-free electrochemical impedance spectroscopy (EIS) aptasensor for the detection of adenosine was fabricated by co-assembling thiolated aptamer, dithiothreitol (DTT) and 6-mercaptohexanol (MCH) on gold electrode surface, forming Au/aptamer-DTT/MCH. The interfacial electron transfer resistance (Ret) of the aptasensor using [Fe(CN)6](3-/4-) as the probe increased with adenosine concentration, and the change in Ret (∆Ret) against the logarithm of adenosine concentration was linear over the range from 0.05 pM to 17 pM with a detection limit of 0.02 pM. Compared to that of aptasensors fabricated with MCH or DTT alone as the backfiller, the detection limit was improved dramatically (LOD was 0.03 nM and 0.2 pM for Au/aptamer/MCH and Au/aptamer-DTT, respectively), which was attributed primarily to the coupling of the cyclic- and linear -configuration backfillers. The coupling showed remarkably higher resistance to nonspecific adsorption, leading to low background noise and high response signal. The aptasensor reported herein is applicable for the detection of other kinds of aptamer-binding chemicals and biomolecules.

Towards understanding of poly-guanine activated fluorescent silver nanoclusters

It has been recently reported that the fluorescence of some DNA-templated silver nanoclusters (AgNCs) can be significantly enhanced upon by hybridizing with a partially complementary DNA containing a G-rich overhang near the AgNCs. This discovery has found a number of analytical applications but many fundamental questions remain to be answered. In this work, the photostability of these activated AgNCs is reported. After adding the G-rich DNA activator, the fluorescence intensity peaks in ∼1 h and then starts to decay, where the decaying rate is much faster with light exposure. The lost fluorescence is recovered by adding NaBH4, suggesting that the bleaching is an oxidative process. Once activated, the G-rich activator can be removed while the AgNCs still maintain most of their fluorescence intensity. UV-vis spectroscopy suggests that new AgNC species are generated upon hybridization with the activator. The base sequence and length of the template DNA have also been varied, leading to different emission colors and color change after hybridization. G-rich aptamers can also serve as activators. Our results indicate that activation of the fluorescence by G-rich DNA could be a convenient method for biosensor development since the unstable NaBH4 is not required for the activation step.

One-step DNA-programmed growth of CpG conjugated silver nanoclusters: a potential platform for simultaneous enhanced immune response and cell imaging

We designed a one-pot synthesis that allows CpG-functionalized AgNCs to be prepared, combining attractive features of enhanced immune response and intracellular imaging.

Luminescent silver nanoclusters anchored by oligonucleotides detect human telomerase ribonucleic acid template

Luminescent silver nanoclusters were anchored by designed oligonucleotides. After hybridizing with human telomerase RNA template, the luminescence of the cluster decreased linearly with respect to the concentration of the complementary strand (25-250 nM). The cluster is therefore a potential candidate for human telomerase detection.

Quantum dots for fluorescent biosensing and bio-imaging applications

Quantum dots (QDs) have been facilitating the development of sensitive fluorescence biosensors over the past two decades due to their high quantum yield, narrow and tunable emission spectrum and good photostability. The new emerging QDs with improved biocompatibility further promote their biological applications. In this review, we first briefly introduce the prevalently used QDs and their preparation and bioconjugation approaches. Then we summarize QDs-based fluorescent biosensing for proteins and nucleic acids, and QDs-based applications in cellular and in vivo targeting and imaging. Last but not the least, we envision the potential QDs-based applications in future perspectives.

Synthesis, characterization and optical properties of low nuclearity liganded silver clusters: Ag31(SG)19 and Ag15(SG)11

We report a simple synthesis of silver:glutathione (Ag:SG) clusters using a cyclic reduction under oxidative conditions. Two syntheses are described which lead to solutions containing well-defined Ag31(SG)19 and Ag15(SG)11 clusters that have been characterized by mass spectrometry. The optical properties of silver:glutathione (Ag:SG) cluster solutions have been investigated experimentally. In particular, the solution containing Ag15(SG)11 clusters shows a bright and photostable emission. For Ag31(SG)19 and Ag15(SG)11 clusters, the comparison of experimental findings with DFT and TDDFT calculations allowed us to reveal the structural and electronic properties of such low nuclearity liganded silver clusters.

Temporal techniques: dynamic tracking of nanomaterials in live cells

Temporal analytical techniques to track nanoparticles in live cell would provide rich information to well understand the biologic properties of nanoparticles in molecular level. Significant advances in fluorescence microscopy techniques with high temporal and spatial resolution allow single nanoparticles to label biomolecules, ions, and microstructures in live cells, which will address many fundamental questions in cell biology. This review highlights the real time tracking techniques for monitoring the movement of nanomaterials such as carbon nanotubes (CNTs), quantum dots (QDs), metal clusters, upconver-sional nanomaterials, and polystyrene (PS) nanoparticles etc. in live cells. The biological properties of nanoparticles in live cells are also briefly summarized according to fluorescence microscopy studies.