Rapid enzyme-free detection of miRNA-21 in human ovarian cancerous cells using a fluorescent nanobiosensor designed based on hairpin DNA-templated silver nanoclusters

BACKGROUND

Cancer is known as one of the main non-communicable diseases and the leading cause of death in the new era. Early diagnosis of cancer requires the identification of special biomarkers. Currently, microRNAs (miRNAs) have attracted the attention of researchers as useful biomarkers for cancer early detection. Hence, various methods have been recently developed for detecting and monitoring miRNAs. Among all miRNAs, detection of miRNA-21 (miR-21) is important because it is abnormally overexpressed in most cancers. Here, a new biosensor based on silver nanoclusters (AgNCs) is introduced for detecting miR-21.

RESULTS

As a fluorescent probe, a rationally designed hairpin sequence containing a poly-cytosine motif was used to facilitate the formation of AgNCs. A guanine-rich sequence was also employed to enhance the sensing signal. It was found that in the absence of miR-21, adding a guanine-rich sequence to the detecting probe caused only a slight change in the fluorescence emission intensity of AgNCs. While in the presence of miR-21, the emission signal enhanced. A direct correlation was observed between the increase in the fluorescence of AgNCs and the concentration of miR-21. The performance of the proposed biosensor was characterized thoroughly and confirmed. The biosensor detected miR-21 in an applicable linear range from 9 pM to 1.55 nM (LOD: 2 pM).

SIGNIFICANCE

The designed biosensor was successfully applied for detecting miR-21 in human plasma samples and also in human normal and lung and ovarian cancer cells. This biosensing strategy can be used as a model for detecting other miRNAs. The designed nanobiosensor can measure miR-21 without using any enzymes, with fewer experimental steps, and at a low cost compared to the reported biosensors in this field.

EXPAR for biosensing: recent developments and applications

Emerging as a promising novel amplification technique, the exponential amplification reaction (EXPAR) offers significant advantages due to its potent exponential amplification capability, straightforward reaction design, rapid reaction kinetics, and isothermal operation. The past few years have witnessed swift advancements and refinements in EXPAR-based technologies, with numerous high-performance biosensing systems documented. A deeper understanding of the EXPAR mechanism has facilitated the proposal of novel strategies to overcome limitations inherent to traditional EXPAR. Furthermore, the synergistic integration of EXPAR with diverse amplification methodologies, including the use of a CRISPR/Cas system, metal nanoparticles, aptamers, alternative isothermal amplification techniques, and enzymes, has significantly bolstered analytical efficacy, aiming to enhance specificity, sensitivity, and amplification efficiency. This comprehensive review presents a detailed exposition of the EXPAR mechanism and analyzes its primary challenges. Additionally, we summarize the latest research advancements in the biomedical field concerning the integration of EXPAR with diverse amplification technologies for sensing strategies. Finally, we discuss the challenges and future prospects of EXPAR technology in the realms of biosensing and clinical applications.

Stimulus-Responsive Four-Stranded DNA Nanoring Assembly to Host Multiple Nanosilver Clusters for Cooperatively Enhanced Fluorescence Biosensing

Exploring the ability of four-stranded DNA nanorings (fsDNRs) to host multiple nanosilver clusters (NAgCs) for cooperatively amplifiable fluorescence biosensing to a specific initiator (tI*) is fascinating. By designing three DNA single strands and three analogous stem-loop hairpins, we developed a functional fsDNR through sequential cross-opening and overlapped hybridization. Note that a substrate strand (SS) was programmed with six modules: two severed splits (sT and sT') of NAgCs template, two sequestered segments by a middle unpaired spacer, and a partition for tI*-recognizable displacement, while sT and sT' were also tethered in two ends of three hairpins. At first, a triple dsDNA complex with stimulus-responsiveness was formed to guide the specific binding to tI*, while the exposed toehold of the SS activated the forward cascade hybridization of three hairpins, until the ring closure in the tailored self-assembly pathway for forming the fsDNR. The resulting four duplexes forced each pair of sT/sT' to be merged as the parent template in four nicks, guiding the preferential synthesis of four clusters in the shared fsDNR, thereby cooperatively amplifying the green fluorescence signal for sensitive assay of tI*. Meanwhile, the topological conformation of fsDNR can be stabilized by the as-formed cluster adducts to rivet the pair of two splits in the nicks. Benefitting from the self-enhanced effect of multiple emitters, this label-free fluorescent sensing strategy features simplicity, rapidity, and high on-off contrast, without involving complicated nucleic acid amplifiers.

Plasmon-emitter coupling in cytosine-rich hairpin DNA-templated silver nanoclusters: Thermal reversibility, white light emission, and dynamics inside live cells

Bio-templated luminescent noble metal nanoclusters (NCs) have attracted great attention for their intriguing physicochemical properties. Continuous efforts are being made to prepare NCs with high fluorescence quantum yield (QY), good biocompatibility, and tunable emission properties for their widespread practical applications as new-generation environment-friendly photoluminescent materials in materials chemistry and biological systems. Herein, we explored the unique photophysical properties of silver nanoclusters (AgNCs) templated by cytosine-rich customized hairpin DNA. Our results indicate that a 36-nucleotide containing hairpin DNA with 20 cytosine (C20) in the loop can encapsulate photostable red-emitting AgNCs with an absolute QY of ∼24%. The luminescent properties in these DNA-templated AgNCs were found to be linked to the coupling between the surface plasmon and the emitter. These AgNCs exhibited excellent thermal sensitivity and were employed to produce high-quality white light emission with an impressive color rendering index of 90 in the presence of dansyl chloride. In addition, the as-prepared luminescent AgNCs possessing excellent biocompatibility can effectively mark the nuclear region of HeLa cells and can be employed as a luminescent probe to monitor the cellular dynamics at a single molecular resolution.

Role of transition metals in coinage metal nanoclusters for the remediation of toxic dyes in aqueous systems

A difficult issue in chemistry and materials science is to create metal compounds with well-defined components. Metal nanoclusters, particularly those of coinage groups (Cu, Ag, and Au), have received considerable research interest in recent years owing to the availability of atomic-level precision via joint experimental and theoretical methods, thus revealing the mechanisms in diverse nano-catalysts and functional materials. The textile sector significantly contributes to wastewater containing pollutants such as dyes and chemical substances. Textile and fabric manufacturing account for about 7 × 105 tons of wastewater annually. Approximately one thousand tons of dyes used in textile processing and finishing has been recorded as being discharged into natural streams and water bodies. Owing to the widespread environmental concerns, research has been conducted to develop absorbents that are capable of removing contaminants and heavy metals from water bodies using low-cost technology. Considering this idea, we reviewed coinage metal nanoclusters for azo and cationic dye degradation. Fluorometric and colorimetric techniques are used for dye degradation using coinage metal nanoclusters. Few reports are available on dye degradation using silver nanoclusters; and some of them are discussed in detailed herein to demonstrate the synergistic effect of gold and silver in dye degradation. Mostly, the Rhodamine B dye is degraded using coinage metals. Silver nanoclusters take less time for degradation than gold and copper nanoclusters. Mostly, H2O2 is used for degradation in gold nanoclusters. Still, all coinage metal nanoclusters have been used for the degradation due to suitable HOMO-LUMO gap, and the adsorption of a dye onto the surface of the catalyst results in the exchange of electrons and holes, which leads to the oxidation and reduction of the adsorbed dye molecule. Compared to other coinage metal nanoclusters, Ag/g-C3N4 nanoclusters displayed an excellent degradation rate constant with the dye Rhodamine B (0.0332 min-1). The behavior of doping transition metals in coinage metal nanoclusters is also reviewed herein. In addition, we discuss the mechanistic grounds for degradation, the fate of metal nanoclusters, anti-bacterial activity of nanoclusters, toxicity of dyes, and sensing of dyes.

Recent Advances of DNA-Templated Metal Nanoclusters for Food Safety Detection: From Synthesis, Applications, Challenges, and Beyond

Food safety concerns have become a significant threat to human health and well-being, catching global attention in recent years. As a result, it is imperative to research conceptually novel biosensing and effective techniques for food matrices detection. Currently, DNA-templated metal nanoclusters (DNA-MNCs) are considered as one of the most promising nanomaterials due to their excellent properties in biosensing. While DNA-MNCs have garnered increasing interest, the reviews of design strategies, applications, and futuristic prospects for biosensing have been hardly found especially in food safety. The synthesis of DNA-MNCs and their use as biosensing materials in food contamination detection, including pathogenic bacteria, toxins, heavy metals, residues of pesticides, and others were comprehensively reviewed. In addition, we summarize the properties of DNA-MNCs briefly and discuss the challenges and future trends. The application of DNA-MNCs powered biosensing has been demonstrated and actively studied, which is a promising paradigm for food safety testing that can supplement or even replace current existing methods. Despite the challenges of difficulty regulating accurately, poor stability, low quantum yield, and difficult commercial transformation, the application prospects of DNA-MNCs biosensors are promising. This review aims to provide insights and directions for the future development of DNA-MNCs based food detection technology.

Fluorescence Biosensing Based on Bifurcated DNA Scaffold-Aggregated Ag Nanocluster via Responsive Conformation Switch of Quasi-Molecular Beacon

To address the limitations of typical hairpin-structural molecular beacons, exploring the ability of a quasi-molecular beacon (qMB) to create label-free fluorescence biosensors is intriguing and remains a challenge. Herein, we propose the first example of modular qMB with the feature of a stimulation-responsive conformation switch to develop an aggregated Ag nanocluster (aAgNC) in a bifurcated DNA scaffold for fluorescently sensing a specific initiator (I*). This qMB was well designed to program four functional modules: I*-recognizable element adopting metastable stem-loop bihairpin structure and two DNA splits (exposed C3GT4 and locked C4AC4T) of aAgNC template that is separated by a tunable hairpin spacer for the customized combination of selective recognition and signaling readout. When presenting I* in an assay route, the specific hybridization induces the directional disassembly of the bihairpin unit, on which the qMB is configurationally switched to liberate the locked split. Thus, the bifurcated parent template pair of C3GT4/C4AC4T is proximal, affording in situ nucleation and clustering of emissive aAgNC. By collecting the fluorescence signal, the quantitative detection of I* is achieved. Benefiting from the ingenious programming of qMB, the recognizing and signaling integration actuates the construction of a facile and convenient fluorescent biosensor featuring rapid reaction kinetics, a wide linear range, high sensitivity, and specificity. This would provide a new paradigm to exploit versatile qMB-based biosensing platforms via stimulation-responsive conformation switches for developing various DNA-scaffolded Ag clusters.

Fluorescence self-assembled DNA hydrogel for the determination of prostate specific antigen by aggregation induced emission

In this study, an aptamer-based, functionalized-DNA hydrogel system is developed for prostate-specific antigen (PSA) detection. A pure DNA hydrogel is constructed using specific DNA building blocks and an aptamer as a cross-linker. Firstly, silver nanoclusters (AgNCs) are constructed on the Y-shaped DNA (Y-DNA) building blocks. Then, the DNA hydrogel was formed via the addition of the cross-linker to the Y-DNA solution. In this case, the fluorescence emission of silver nanoclusters that have accumulated in the hydrogel increases due to aggregation-induced emission (AIE). The presence of PSA and its subsequent interaction with its specific aptamer dissolve the hydrogel structures, which leads to a low emission intensity. A great linear relationship was attained in this assay in the range of 0.05 to 8 ng mL-1 with a detection limit of 4.4 pg mL-1 for the detection of PSA. Additionally, the proposed aptasensor was successfully used to detect PSA in human serum samples. The recovery for different concentrations of PSA was in the range of 96.1% to 99.3%, and the RSD range was from 2.3% to 4.5%. Comparing our method to current ones in the field of PSA detection proves that our platform benefits from a simpler procedure, lower cost, and better efficiency, providing high potential for future clinical applications.

Fluorescent Features and Applicable Biosensing of a Core-Shell Ag Nanocluster Shielded by a DNA Tetrahedral Nanocage

The DNA frame structure as a natural shell to stably shield the sequence-templated Ag nanocluster core (csAgNC) is intriguing yet challenging for applicable fluorescence biosensing, for which the elaborate programming of a cluster scaffold inside a DNA-based cage to guide csAgNC nucleation might be crucial. Herein, we report the first design of a symmetric tetrahedral DNA nanocage (TDC) that was self-assembled in a one-pot process using a C-rich csAgNC template strand and four single strands. Inside the as-constructed soft TDC architecture, the template sequence was logically bridged from one side to another, not in the same face, thereby guiding the in situ synthesis of emissive csAgNC. Because of the strong electron-repulsive capability of the negatively charged TDC, the as-formed csAgNC displayed significantly improved fluorescence stability and superb spectral behavior. By incorporating the recognizable modules of targeted microRNAs (miRNAs) in one vertex of the TDC, an updated TDC (uTDC) biosensing platform was established via the photoinduced electron transfer effect between the emissive csAgNC reporter and hemin/G-quadruplex (hG4) conjugate. Because of the target-interrupted csAgNC switching in three states with the spatial proximity and separation to hG4, an "on-off-on" fluorescing signal response was executed, thus achieving a wide linear range to miRNAs and a limit of detection down to picomoles. Without complicated chemical modifications, this simpler and more cost-effective strategy offered accurate cell imaging of miRNAs, further suggesting possible therapeutic applications.

An amplified logic gate driven by in situ synthesis of silver nanoclusters for identification of biomarkers

An amplified DNA logic sensor was constructed for the identification of multiple biomarkers, in which the inputs of targets triggered the disassembly of a V-shaped probe (VSP) structure by a strand displacement reaction, leading to the synthesis of silver nanoclusters (AgNCs) for electrocatalytic reduction of H₂O₂. The sensing platform achieved sensitive detection of methylated DNA and microRNA 122 with detection limits down to 3.4 and 4.1 fM, respectively, and can be used for the assay of clinical serum samples from healthy volunteers and liver injury patients with satisfactory results. The DNA logic sensor exhibited the advantages of convenience, low cost, and versatility without the involvement of electroactive label modification, which is helpful for disease diagnosis as well as the fundamental investigation of interfacial electrochemistry and molecular biology.

Detection of pyrimidine-rich DNA sequences based on the formation of parallel and antiparallel triplex DNA and fluorescent silver nanoclusters

In this work, the use of DNA-stabilized fluorescent silver nanoclusters for the detection of target pyrimidine-rich DNA sequences by formation of parallel and antiparallel triplex structures is studied by molecular fluorescence spectroscopy. In the case of parallel triplexes, the probe DNA fragments are Watson-Crick stabilized hairpins, and whereas in the case of antiparallel triplexes, the probe fragments are reverse-Hoogsteen clamps. In all cases, the formation of the triplex structures has been assessed by means of polyacrylamide gel electrophoresis, circular dichroism, and molecular fluorescence spectroscopies, as well as multivariate data analysis methods. The results have shown that it is possible the detection of pyrimidine-rich sequences with an acceptable selectivity by using the approach based on the formation of antiparallel triplex structures.

Ultrasensitive Electrochemiluminescence Biosensor with Silver Nanoclusters as a Novel Signal Probe and α-Fe2O3-Pt as an Efficient Co-reaction Accelerator for Procalcitonin Immunoassay

Herein, a high-efficiency biosensor based on ternary electrochemiluminescence (ECL) system was constructed for procalcitonin (PCT) detection. Specifically, silver nanoclusters (Ag NCs) with stable luminescence properties were prepared with small-molecule lipoic acid (LA) as the ligand, and its ECL emission in persulfate (S₂O₈^2-) was first reported. Meanwhile, the prepared Ag NCs possessed ligand-to-metal charge-transfer characteristics, thus transferring energy from LA to Ag⁺ for luminescence. Based on the small particle size, good biocompatibility, and molecular binding ability, Ag NCs-LA was used as an ideal luminescent probe. In addition, α-Fe₂O₃-Pt was introduced to facilitate the activation of S₂O₈^2-, thereby generating more sulfate radicals to react with the free radicals of Ag NCs to enhance ECL emission. The synergistic effect of the variable valence state of transition metals and high catalytic activity of noble metals endows α-Fe₂O₃-Pt with excellent catalytic ability for S₂O₈^2-. Importantly, the sensing mechanism was systematically demonstrated by UV-vis, fluorescence, and ECL analysis, as well as density functional theory calculations. At last, NKFRGKYKC was designed for specific immobilization of antibodies, thus releasing the antigen binding sites to improve the antigen recognition efficiency. Based on this, the developed biosensor showed high sensitivity for PCT detection, with a wide linear range (10 fg/mL-100 ng/mL) and a low detection limit (3.56 fg/mL), which could be extended to clinical detection of multiple biomarkers.

Label-Free Fluorescent Turn-On Glyphosate Sensing Based on DNA-Templated Silver Nanoclusters

In this work, a label-free fluorescent detection method for glyphosate, based on DNA-templated silver nanoclusters (DNA-Ag NCs) and a Cu²⁺-ion-modulated strategy, was developed. In the presence of Cu²⁺, the fluorescence of the DNA-Ag NCs was quenched. Glyphosate can restore the fluorescence of DNA-Ag NCs. By analyzing the storage stability of the obtained DNA-Ag NCs using different DNA templates, specific DNA-Ag NCs were selected for the construction of the glyphosate sensor. The ultrasensitive detection of glyphosate was achieved by optimizing the buffer pH and Cu²⁺ concentration. The sensing of glyphosate demonstrated a linear response in the range of 1.0-50 ng/mL. The limit of detection (LOD) was 0.2 ng/mL. The proposed method was successfully applied in the detection of glyphosate in a real sample, indicating its high application potential for glyphosate detection.

Modulating the Fluorescence of Silver Nanoclusters Wrapped in DNA Hairpin Loops via Confined Strand Displacement and Transient Concatenate Ligation for Amplifiable Biosensing

It is intriguing to modulate the fluorescence emission of DNA-scaffolded silver nanoclusters (AgNCs) via confined strand displacement and transient concatenate ligation for amplifiable biosensing of a DNA segment related to SARS-CoV-2 (s₂DNA). Herein, three stem-loop structural hairpins for signaling, recognizing, and assisting are designed to assemble a variant three-way DNA device (3WDD) with the aid of two linkers, in which orange-emitting AgNC (oAgNC) is stably clustered and populated in the closed loop of a hairpin reporter. The presence of s₂DNA initiates the toehold-mediated strand displacement that is confined in this 3WDD for repeatable recycling amplification, outputting numerous hybrid DNA-duplex conformers that are implemented for a transient "head-tail-head" tandem ligation one by one. As a result, the oAgNC-hosted hairpin loops are quickly opened in loose coil motifs, bringing a significant fluorescence decay of multiple clusters dependent on s₂DNA. Demonstrations and understanding of the tunable spectral performance of a hairpin loop-wrapped AgNC via switching 3WDD conformation would be highly beneficial to open a new avenue for applicable biosensing, bioanalysis, or clinical diagnostics.

Amplifiable ratiometric fluorescence biosensing of nanosilver multiclusters populated in three-way-junction DNA branches

To explore the fluorescence bio-responsiveness of emissive silver nanoclusters (AgNCs) populated in DNA-branched scaffolds is intriguing yet challenging. In response to a desired targeting model (T*) as a vehicle, herein a customized three-way-junction DNA construct (TWJDC) is assembled via competitive hybridizing cascade among three stem-loop hairpins with specific base sequences, where the repeated recycling of T* enables the exponentially amplifiable output of rigid TWJDC. As designed, these stable hybridization products are highly T*-stimulated responsive and constructing-directional. In the three branched-arms, the unpaired sticky ends provide isotropic binding sites for a signaling hairpin encoded with two C-rich templates of green- and red-AgNCs clustering. The identical ligation of signal probe with three arms of TWJDC liberates its locked stem, enabling the separate growth of red-clusters in three branches. As demonstrated, three clusters of red-AgNCs possess advantageous self-enhancing fluorescent performance relative to single or two cluster(s), good biocompatibility and low cytotoxicity. Utilizing the bicolor AgNCs as dual-emitters with reversely changed emission intensity, we developed an innovative ratiometric strategy displaying sensitively linear dose-dependence on variable T* down to 1.9 pM, which can afford a promising platform for biosensing, bioanalysis, cell imaging, or even clinical theranostics.

A fluorescent aptasensor based on copper nanoclusters for optical detection of CD44 exon v10, an important isoform in metastatic breast cancer

Recent studies suggest that breast cancer cells express various CD44 isoforms. CD44 is an integral transmembrane protein encoded by a single 20-exon gene. Exon v10 of CD44 plays a critical role in promoting cancer metastasis, so sensitive detection of this isoform helps in early diagnosis of metastatic breast cancer and facilitates the treatment process. This study aimed to use v10-specific aptamers to set up an optical aptasensor based on fluorescent metal nanoclusters. For this purpose, nanoclusters of silver, gold, and copper were prepared by different CD44 v10 DNA aptamers as molecular templates. UV-vis, TEM, and fluorescence spectrometer results confirmed the accuracy and quality of the synthesized aptamer-templated nanoclusters (Apt-NCs). Finally, we compared the performance of the as-prepared Apt-NCs in response to different cultured cell lines. According to the results, the optical response of M-Apt4-CuNCs was more efficient and correlated well with the concentrations of CD44 v10-enriched cells. The detection limit of the aptasensor was 40 ± 5 cells per mL.

Antibody-Responsive Ratiometric Fluorescence Biosensing of Biemissive Silver Nanoclusters Wrapped in Switchable DNA Tweezers

Exploring the ratiometric fluorescence biosensing of DNA-templated biemissive silver nanoclusters (AgNCs) is significant in bioanalysis, yet the design of a stimuli-responsive DNA device is a challenge. Herein, using the anti-digoxin antibody (anti-Dig) with two identical binding sites as a model, a tweezer-like DNA architecture is assembled to populate fluorescent green- and red-AgNCs (g-AgNCs and r-AgNCs), aiming to produce a ratio signal via specific recognition of anti-Dig with two haptens (DigH). To this end, four DNA probes are programmed, including a reporter strand (RS) dually ended with a g-/r-AgNC template sequence, an enhancer strand (ES) tethering two same G-rich tails (G18), a capture strand (CS) labeled with DigH at two ends, and a help strand (HS). Initially, both g-AgNCs and r-AgNCs wrapped in the intact RS are nonfluorescent, whereas the base pairing between RS, ES, CS, and HS resulted in the construction of DNA mechanical tweezers with two symmetric arms hinged by a rigid "fulcrum", in which g-AgNCs are lighted up due to G18 proximity ("green-on"), and r-AgNCs away from G18 are still dark ("red-off"). When two DigHs in proximity recognize and bind anti-Dig, the conformation switch of these tweezers resultantly occurs, taking g-AgNCs away from G18 for "green-off" and bringing r-AgNCs close to G18 for "red-on". As such, the ratiometric fluorescence of r-AgNCs versus g-AgNCs is generated in response to anti-Dig, achieving reliable quantization with a limit of detection at the picomolar level. Based on the fast stimulated switch of unique DNA tweezers, our ratiometric strategy of dual-emitting AgNCs would provide a new avenue for a variety of bioassays.

The Application of Nucleic Acids and Nucleic Acid Materials in Antimicrobial Research

Due to the misuse of antibiotics, multiple drug-resistant pathogenic bacteria have increasingly emerged. This has increased the difficulty of treatment as these bacteria directly affect public health by diminishing the potency of existing antibiotics. Developing alternative therapeutic strategies is the urgent need to reduce the mortality and morbidity related to drug-resistant bacterial infections. In the past 10 to 20 years, nanomedicines have been widely studied and applied as an antibacterial agent. They have become a novel tool for fighting resistant bacteria. The most common innovative substances, metal and metal oxide nanoparticles (NPs), have been widely reported. Until recently, DNA nanostructures were used alone or functionalized with specific DNA sequences by many scholars for antimicrobial purposes which were alternatively selected as therapy for severe bacterial infections. These are a potential candidate for treatments and have a considerable role in killing antibiotic-resistant bacteria. This review involves the dimensions of multidrug resistance and the mechanism of bacteria developing drug resistance. The importance of this article is that we summarized the current study of nano-materials based on nucleic acids in antimicrobial use. Meanwhile, the current progress and the present obstacles for their antibacterial and therapeutic use and special function of stem cells in this field are also discussed.

DNA-templated fluorescent silver nanoclusters on-off switch for specific and sensitive determination of organic mercury in seafood

Organic mercury including methyl-mercury and ethyl-mercury (CH₃Hg⁺ and C₂H₅Hg⁺) has high toxicity and bio-accumulation, and thus is easy to generate bio-amplification in food chain. Hence, the specific detection of organic mercury has great significance for objectively assessing the health risk of mercury in seafood. We herein designed an aptamer (A_S-T7), which consists of a silver nanoclusters (AgNCs) scaffold sequence (A_S) and a T-rich sequence (A_T7), for simultaneously synthetizing DNA-templated AgNCs and recognizing organic mercury, and further developed a label-free fluorescent method for the sensitive and specific determination of organic mercury (CH₃Hg⁺ and C₂H₅Hg⁺ total concentration) by using DNA-templated AgNCs as signal. Without organic mercury, Ag⁺ in the mixture of aptamer and Ag⁺ was bond on A_S of aptamer to form A_S-templated AgNCs after reduction, and thus emitted strong fluorescence. Whereas, in the presence of organic mercury, CH₃Hg⁺/C₂H₅Hg⁺ was bond on A_T7 of aptamer to generate photoinduced electron transfer (PET) between CH₃Hg⁺/C₂H₅Hg⁺ and A_S-templated AgNCs, and thus results in fluorescence quenching of A_S-templated AgNCs. The fluorescent method could be used to rapidly detect organic mercury with a detection limit of 5.0 nM (i.e. 1.01 ng Hg/g), which meets the U.S. EPA standard of 0.3 mg/kg (wet). The method was successfully used to detect organic mercury in water and fish muscle with a recovery of 96%-104% and an inter-days RSD (n = 5) < 7%. The success of the study promised a reliable method for rapid and specific detection of organic mercury in environmental and biological samples.

Biological characteristics and biomarkers of novel SARS-CoV-2 facilitated rapid development and implementation of diagnostic tools and surveillance measures

Existing coronavirus named as a severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has speeded its spread across the globe immediately after emergence in China, Wuhan region, at the end of the year 2019. Different techniques, including genome sequencing, structural feature classification by electron microscopy, and chest imaging using computed tomography, are primarily used to diagnose and screen SARS-CoV-2 suspected individuals. Determination of the viral structure, surface proteins, and genome sequence has provided a design blueprint for the diagnostic investigations of novel SARS-CoV-2 virus and rapidly emerging diagnostic technologies, vaccine trials, and cell-entry-inhibiting drugs. Here, we describe recent understandings on the spike glycoprotein (S protein), receptor-binding domain (RBD), and angiotensin-converting enzyme 2 (ACE2) and their receptor complex. This report also aims to review recently established diagnostic technologies and developments in surveillance measures for SARS-CoV-2 as well as the characteristics and performance of emerging techniques. Smartphone apps for contact tracing can help nations to conduct surveillance measures before a vaccine and effective medicines become available. We also describe promising point-of-care (POC) diagnostic technologies that are under consideration by researchers for advancement beyond the proof-of-concept stage. Developing novel diagnostic techniques needs to be facilitated to establish automatic systems, without any personal involvement or arrangement to curb an existing SARS-CoV-2 epidemic crisis, and could also be appropriate for avoiding the emergence of a future epidemic crisis.

Structure and luminescence of DNA-templated silver clusters

DNA serves as a versatile template for few-atom silver clusters and their organized self-assembly. These clusters possess unique structural and photophysical properties that are programmed into the DNA template sequence, resulting in a rich palette of fluorophores which hold promise as chemical and biomolecular sensors, biolabels, and nanophotonic elements. Here, we review recent advances in the fundamental understanding of DNA-templated silver clusters (Ag N -DNAs), including the role played by the silver-mediated DNA complexes which are synthetic precursors to Ag N -DNAs, structure-property relations of Ag N -DNAs, and the excited state dynamics leading to fluorescence in these clusters. We also summarize the current understanding of how DNA sequence selects the properties of Ag N -DNAs and how sequence can be harnessed for informed design and for ordered multi-cluster assembly. To catalyze future research, we end with a discussion of several opportunities and challenges, both fundamental and applied, for the Ag N -DNA research community. A comprehensive fundamental understanding of this class of metal cluster fluorophores can provide the basis for rational design and for advancement of their applications in fluorescence-based sensing, biosciences, nanophotonics, and catalysis.

Fluorescent platinum nanoclusters as correlative light electron microscopy probes

Correlative Imaging (CI) visualizes a single sample/region of interest with two or more imaging modalities. The technique seeks to elucidate information that may not be discernible by using either of the constituent techniques in isolation. Correlative Light Electron Microscopy (CLEM) can be employed to streamline workflows, i.e., using fluorescent signals in the light microscope (LM) to inform the user of regions which should be imaged with electron microscopy (EM). The efficacy of correlative techniques requires high spatial resolution of signals from both imaging modalities. Ideally, a single point should originate from both the fluorescence and electron density. However, many of the ubiquitously used probes have a significant distance between their fluorescence and electron dense portions. Furthermore, electron dense metal nanoparticles used for EM visualization readily quench any proximal adjacent fluorophores. Therefore, accurate registration of both signals becomes difficult. Here we describe fluorescent nanoclusters in the context of a CLEM probe as they are composed of several atoms of a noble metal, in this case platinum, providing electron density. In addition, their structure confers them with fluorescence via a mechanism analogous to quantum dots. The electron dense core gives rise to the fluorescence which enables highly accurate signal registration between epifluorescence and electron imaging modalities. We provide a protocol for the synthesis of the nanoclusters with some additional techniques for their characterization and finally show how they can be used in a CLEM set up.

Studies on the interactions of Ag(i) with DNA and their implication on the DNA-templated synthesis of silver nanoclusters and on the interaction with complementary DNA and RNA sequences

Silver nanoclusters (AgNCs) prepared by the reduction of silver ions in the presence of DNA oligonucleotides have attracted great interest as potential diagnostic tools for their tunable and high fluorescent properties. In this work, three DNA sequences that consist of a 12-nucleotide long probe sequence at the 5′-end linked to the complementary sequence to three miRNAs are studied. First, the interaction of these sequences with Ag(i) was characterized by means of circular dichroism spectroscopy. By applying multivariate methods to the analysis of spectroscopic data, two complexes with different Ag(i) : DNA ratios were resolved. Secondly, the impact of several experimental variables, such as temperature, borohydride concentration and reaction time, on the formation of AgNCs templated by these three sequences was studied. Finally, the fluorescence properties of the duplexes formed by DNA probes with complementary DNA or miRNA sequences were studied. The results presented here highlight the role of the secondary structure adopted by the DNA probe on the fluorescence properties of DNA-stabilized AgNCs which, in turn, affect the development of methods for miRNA detection.

Variables affecting the fluorescent properties of DNA-stabilized silver nanoclusters are studied. The secondary structure of the AgNC-stabilizing DNA sequence dramatically affects the analytical signal behind the hybridization reaction.

Fluorometric determination of the breast cancer 1 gene based on the target-induced conformational change of a DNA template for copper nanoclusters

The breast cancer 1 (BRCA1) gene is a tumor suppressor gene, whose mutation is closely related to breast cancer. Therefore, the sensitive detection of the BRCA1 gene is extremely important for human health, particularly for women. In this study, a label-free fluorescent method based on hairpin DNA-templated copper nanoclusters (CuNCs) was for the first time developed for the detection of the BRCA1 gene. In the absence of target DNA, the detection system showed a strong red emission and produced a high emission peak. However, in the presence of the BRCA1 gene, the DNA probe hybridized with the BRCA1 gene and conformation of the DNA probe changed. As a result, the amount of produced CuNCs decreased and a low emission peak was obtained. The fluorescence intensity of the detection system was linearly correlated with the concentration of the BRCA1 gene ranging from 2 nM to 600 nM. The detectable limit was 2 nM for the BRCA1 gene assay, which was comparable with those reported by other non-amplifying sensors. Moreover, the developed method showed satisfactory recoveries for the BRCA1 gene assay in the bovine serum. The DNA-templated CuNC-based fluorescent assay thus offered a promising platform for the diagnosis of a breast cancer biomarker.

Regulating Catalytic Activity of DNA-Templated Silver Nanoclusters Based on their Differential Interactions with DNA Structures and Stimuli-Responsive Structural Transition

This work reports exquisite engineering of catalytic activity of DNA-templated silver nanoclusters (DNA-AgNCs) based on unique adsorption phenomena of DNAs on DNA-AgNCs and reversible transition between double and triple-stranded DNAs. Four DNA homopolymers exhibit different inhibition effects on the catalytic activity of DNA-AgNCs, poly adenine (polyA) > poly guanine (polyG) > poly cytosine (polyC) > poly thymine (polyT), demonstrating that polyA strands have the strongest adsorption affinity on DNA-AgNCs. Through the formation of T-A•T triplex DNAs, catalytic activity of DNA-AgNCs is restored from the deactivated state by double or single-stranded DNAs, indicating the participation of N7 groups of adenine bases in binding to DNA-AgNCs and blocking active sites. Accordingly, reversibly regulating catalytic activity of DNA-AgNCs can be realized based on DNA input-stimulated transition between duplex and triplex structures. In the end, two low-cost and facile biosensing methods are presented, which are derived from the activity-switchable platform. It is worthy to anticipate that the DNA-AgNCs with controlled catalytic activity will inspire researchers to devise more functionalized nanocatalysts and contribute to the exploration of intelligent biomedicine in the future.

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.

Dual emitting Ag35 nanocluster protected by 2-pyrene imine thiol

In this communication, we present the synthesis of 2-pyrene imine thiol (2-PIT)-protected Ag₃₅ nanoclusters using a ligand exchange-induced structural transformation reaction. The formation of the nanocluster and its composition were confirmed through several spectroscopic and electron microscopic studies. The UV-vis absorption spectrum showed a set of characteristic features of the nanocluster. This nanocluster showed blue emission under UV light due to pyrene to metal core charge-transfer, and NIR emission due to charge-transfer within the metal core. This is the first report on dual emitting pyrene protected atomically precise silver nanoclusters.

Guanine-Lighting-Up Fluorescence Biosensing of Silver Nanoclusters Populated in Functional DNA Constructs by a pH-Triggered Switch

Dark or weak-emissive DNA-harbored silver nanoclusters (AgNCs) can be remarkably lighted up when approaching to guanine bases. The resultant bright AgNCs acting as a fluorescent reporter are fascinating in biosensing. To explore the applicable guanine-enhanced emission of AgNCs for biosensing microRNA-155 (miR-155) as a model, here we designed a unique stem-loop hairpin beacon (HB) encoding with an miR-155-recognizable sequence and a AgNCs-nucleable template, as well as a hairpin helper tethering a partially locked guanine-rich (15-nt) tail (G₁₅H), while two identical cytosine-rich segments were inserted in HB and G₁₅H to merge for folding/unfolding of i-motif at changed pHs. Initially, the intact clusters populated in HB (HB/AgNCs) were almost nonfluorescent in a buffer (pH 7.0). Then, miR-155 was introduced to trigger a repeated hairpin assembly of HB and G₁₅H by competitive strand displacement reactions at decreased pH 5.0 within 10 min, consequently generating numerous duplex DNA constructs (DDCs). With the resultant template of pH-responsive i-motifs incorporated in DDCs, their folding at pH 5.0 brought the proximity of unlocked G₁₅ overhang to the clusters in a crowded environment, remarkably lighting up the red-emitting fluorescence of HB/AgNCs (λ_em = 628 ± 5 nm) for amplified signal readout. About 3.5-fold enhancement of quantum yield was achievable using different variants of i-motif length and G₁₅ position. Simply by adding OH^-, the configuration fluctuation of i-motifs was implemented for switchable fluorescence biosensing to variable miR-155. Based on a one-step amplification and signaling scheme, this subtle strategy was rapid, low-cost, and specific for miR-155 with high sensitivity down to 67 pM.

Comparative study of gold and silver interactions with amino acids and nucleobases

Metal nanoclusters (NCs) have gained much attention in the last decade. In solution, metal nanoclusters can be stabilized by proteins, and, thus, exhibit many advantages in biocatalysis, biosensing, and bioimaging. In spite of much progress in the synthesis of polypeptide-stabilized gold (Au) clusters, their structure, as well as amino acid-cluster and amino acid-Au+ interactions, remain poorly understood. It is not entirely clear which amino acid (AA) residues and sites in the protein are preferred for binding. The understanding of NC-protein interactions and how they evolve in the polypeptide templates is the key to designing Au NCs. In this work, binding of gold ion Au+ and diatomic neutral gold nanocluster Au2 with a full set of α-proteinogenic amino acids is studied using Density Functional Theory (DFT) and the ab initio RI-MP2 method in order to find the preferred sites of gold interaction in proteins. We demonstrated that the interaction of gold cations and clusters with protonated and deprotonated amino acid residues do not differ greatly. The binding affinity of AAs to the Au2 cluster increases in the following order: Cys(-H+) > Asp(-H+) > Tyr(-H+) > Glu(-H+) > Arg > Gln, His, Met ≫ Asn, Pro, Trp > Lys, Tyr, Phe > His(+H+) > Asp > Lys(+H+) > Glu, Leu > Arg(+H+) > Ile, Val, Ala > Thr, Ser > Gly, Cys, which agrees with the available experimental data that gold cluster synthesis occurs in a wide range of pH - amino acid residues with different protonation states are involved in this process. The significant difference in the binding energy of metal atoms with nucleobases and amino acids apparently means that unlike on DNA templates, neutral metal atoms are strongly bound to amino acid residues and can't freely diffuse in a polypeptide globula. This fact allows one to conclude that formation of metal NCs in proteins occurs through the nucleation of reduced Au atoms bound to the neighboring amino acid residues, and the flexibility of the amino acid residue side-chains and protein chain as a whole plays a significant role in this process.

Nucleobase Pair-Metal Dimer/Dinuclear Metal Cation Interaction: A Theoretical Study

Nucleobase pair-metal dimer/dinuclear metal cation interactions play an important role in biological applications because of their highly symmetrical structures and high stabilities. In this work, we have selected five adenine-adenine hydrogen bonding, adenine-thymine (AT), adenine-uracil, adenine-adenine stacking pairs, and Watson-Crick AT stacking pairs and studied their interaction with the coinage metal dimer M2 and M2 2+ metal cations, where M = Ag, Au, and Cu. Quantum chemical calculations have been carried out with density functional theory (DFT) and time-dependent DFT (TDDFT) methods. Electronic structures were analyzed by the partial density of states method. During interactions, we find that M-M distances are shorter than the sum of van der Waals radii of the corresponding two homocoinage metal atoms, which show the existence of significant metallophilic interactions. Results indicated that nucleobase-M2 2+ complexes are stronger as compared to nucleobase-M2 complexes. Also, the replacement of the hydrogen bond by the dinuclear metal cation-coordinated bond forms more stable alternative metallo-DNA sequences in AAST base pairs. TDDFT calculations reveal that nucleobase-Cu2 complexes and nucleobase-Ag2 2+/Au2 2+ complexes can be used for fluorescent markers and logic gate applications. Atom-in-molecules analysis predicted the noncovalent interaction in these complexes.

Development of fluorescence oligonucleotide probes based on cytosine- and guanine-rich sequences

The properties of cytosine- and guanine-rich oligonucleotides contributed to employing them as sensing elements in various biosensors. In this paper, we report our current development of fluorescence oligonucleotide probes based on i-motif or G-quadruplex forming oligonucleotides for cellular measurements or bioimaging applications. Additionally, we also focus on the spectral properties of the new fluorescent silver nanoclusters based system (ChONC12-AgNCs) that is able to anchor at the Langmuir monolayer interface, which is mimicking the surface of living cells membrane.

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.

Highly fluorescent few atoms silver nanoclusters with strong photocatalytic activity synthesized by ultrashort light pulses

While there are conventional chemical synthesis methods to generate metal nanoclusters (NCs), many of them are adversely affected by the unavoidable contamination of the nanoproduct solution, resulting in aggregation, background noise in analytical chemistry, toxicity, and deactivation of the catalyst. In this work, physical method of ultrafast laser ablation as a "green" synthesis approach together with mechanical centrifugation to obtain silver NCs, simplifying widely the chemical synthesis requirements, is proposed. Remarkably, compared with conventional methods for synthesizing Ag NCs, this new approach starts with a colloid that contains nanosized particles as well as smaller species, managing to obtain colloids with few atoms NCs by centrifugation. Those colloids were analyzed by fluorescence spectroscopy observing UV bands corresponding with HOMO-LUMO cluster transitions. Besides, independent HRTEM measurements were made confirming the presence of few atoms Ag NCs, as well as small NPs in different formation stages. Equally important, photocatalytic efficiency of the obtained NCs was studied through degradation of Methylene Blue (MB) when it was mixed with as-prepared or highly centrifuged colloid, showing an enhanced photocatalytic efficiency of 79% as compared to 57% for pure MB after 180 min of illumination. Consequently, this work contributes to establishing a simple approach to synthesize highly fluorescent and photocatalytic NCs.

DNA bioassays based on the fluorescence 'turn off' of silver nanocluster beacon

Recognition and quantification of oligonucleotide sequences play important roles in medical diagnosis. In this study, a new fluorescent oligonucleotide-stabilized silver nanocluster beacon (NCB) probe was designed for sensitive detection of oligonucleotide sequence targets. This probe contained two tailored DNA strands. One strand was a signal probe strand containing a cytosine-rich strand template for fluorescent silver nanocluster (Ag NC) synthesis and a detection sections at each end. The other strand was a fluorescence enhancing strand containing a guanine-rich section for signal enhancement at one end and a linker section complementary to one end of the signal probe strand. After synthesis of the Ag NCs and hybridization of the two strands, the fluorescence intensity of the as-prepared silver NCB was enhanced 200-fold compared with the Ag NCs. Two NCBs were designed to detect two disease-related oligonucleotide sequences, and results indicated that the two target oligonucleotide sequences in the range 50.0-600.0 and 50.0-200.0 nM could be linearly detected with detection limits of 20 and 25 nM, respectively. The developed fluorescence method using NCBs for oligonucleotide sequence detection was sensitive, facile and had potential for use in bioanalysis and diagnosis.

Targeted DNA-driven catalytic assembly light-up ratiometric fluorescence of biemissive silver nanoclusters for amplified biosensing

We report a ratiometric fluorescence strategy using biemissive silver nanoclusters that are harbored in a functional hairpin beacon for rapid, specific and sensitive detection of specific HIV-related DNA as a model.

Protein-protected gold/silver alloy nanoclusters in metal-enhanced singlet oxygen generation and their correlation with photoluminescence

Photoluminescent noble metal nanoclusters (NCs, core size <2 nm) have recently emerged as a new type of photosensitizers advantageous over conventional photosensitizers due to their high singlet oxygen (¹O₂) generation efficiency, excellent photostability and water solubility, as well as good biocompatibility for photodynamic therapy and bioimaging. However, no correlation has been established between the intrinsic ¹O₂ generation and photoluminescence properties of metal NCs with their size, composition, and concentration, which is important to customize the molecule-like properties of NCs for different applications. Herein, we report a systematic study to uncover the rational design of bimetallic NCs with controllable ¹O₂ generation efficiency by tuning their compositions through spontaneous galvanic displacement reaction. A series of ultrasmall gold/silver alloy nanoclusters (AuAgNCs) were synthesized by reacting bovine serum albumin (BSA) protein-protected Ag₁₃NCs (13 Ag atoms/cluster) with varying concentrations of gold precursor at room temperature. It was found that the ¹O₂ generation efficiency of the resultant BSA-protected AuAgNCs were inversely correlated to their photoluminescence intensity. Interestingly, plasmonic gold nanoparticles (>10 nm) were also formed simultaneously by photobleaching of the BSA-AuAgNCs, leading to significant metal enhancement effect to the ¹O₂ generation rate much higher (~45 times) than that of the monometallic BSA-Ag₁₃NC. This versatile two-for-one strategy to develop next generation metal-enhanced bimetallic NC photosensitizers in one pot opens up new opportunities in designing advanced hybrid nanomaterials with complementary and/or enhanced functionalities.

Coenzyme A-aptamer-facilitated label-free electrochemical stripping strategy for sensitive detection of histone acetyltransferase activity

Abnormal histone acetyltransferases (HAT) activity gives rise to all kinds of cellular diseases. Herein, we first report a coenzyme A (CoA)-aptamer-facilitated label-free electrochemical stripping biosensor for sensitive detection of HAT activity via square wave voltammetry (SWV) technique. The presence of HAT can lead to the transfer of the acetyl group from acetyl coenzyme A (Ac-CoA) to lysine residues of substrate peptide, thus generating CoA molecule. Later, CoA, which acts as an initiator, can embrace its aptamer via the typical target-aptamer interaction, then arousing deoxynucleotide terminal transferase (TdT)-induced silver nanoclusters (AgNCs) as signal output. Under optimized conditions, the resultant aptasensor shows obvious electrochemical stripping signal and is employed for HAT p300 analysis in a wide concentration range from 0.01 to 100 nM with a very low detection limit of 0.0028 nM (3δ/slope). The good analytical performances of the biosensor depend on the strong interaction of CoA and its aptamer and abundant stripping resource rooted from AgNCs. Next, the proposed biosensor is used for screening HAT's inhibitors and the practical HAT detection with satisfactory results. Therefore, the new, simple and sensitive HAT biosensor presents a promising direction for HAT-targeted drug discovery and epigenetic research.

Fluorescent Double-Stranded DNA-Templated Copper Nanoprobes for Rapid Diagnosis of Tuberculosis

In this work, we investigate highly sensitive fluorescent Cu nanoparticles for use as rapid and specific nucleic acid amplification nanoprobes (NPs) for the diagnosis of tuberculosis. After applying polymerase chain reaction (PCR) to a tuberculosis (TB) sample, we demonstrate that the presence of the targeted IS6110 DNA sequence of TB can be easily and directly detected through the in situ formation of DNA-templated fluorescent Cu NPs and subsequently quantified using only a smartphone. Compared to traditional DNA analysis, this sensing platform does not require purification steps and eliminates the need for electrophoresis to confirm the PCR results. After optimization, this dsDNA-Cu NP-PCR method has the ability to analyze clinical TB nucleic acid samples at a detection limit of 5 fg/μL, and the fluorescent signal can be distinguished in only ∼3 min after the DNA has been amplified. Moreover, with the combination of smartphone-assisted imaging analysis, we can further reduce the instrument size/cost and enhance the portability. In this manner, we are able to eliminate the need for a fluorescent spectrophotometer to measure the clinical sample. These results demonstrate this platform's practical applicability, combining a smartphone and on-site analysis while retaining the detection performance, making it suitable for clinical DNA applications in resource-limited regions of the world.

Fluorometric determination of microRNA using arched probe-mediated isothermal exponential amplification combined with DNA-templated silver nanoclusters

A highly sensitive fluorometric method is described for the determination of microRNA-141. It is based on the use of arched probe-mediated isothermal exponential amplification reaction (EXPAR) and of DNA-templated silver nanoclusters (DNA-AgNCs). The EXPAR utilizes microRNA-141 as the trigger, polymerases and endonucleases as amplification activators, and two arched probes as exponential amplification templates. This enables the conversion of microRNA to a large number of reporter sequences under isothermal conditions within minutes. The generated reporter sequences act as scaffolds for the synthesis of fluorescent DNA-AgNCs by reduction of Ag (I) with NaBH₄. The DNA-AgNCs function as signalling fluorophores with excitation/emission maxima at 540/610 nm. The method exhibits high sensitivity for microRNA-141 with a detection limit as low as 0.87 fM and a dynamic range from 1 fM to 500 fM. The method can distinguish nucleotides in the microRNA-200 family. Graphical abstract Schematic representation of a fluorometric method for sensitive detection of microRNA based on arched probe-mediated isothermal exponential amplification combined with DNA-templated silver nanoclusters.

A Eu3+-inspired fluorescent carbon nanodot probe for the sensitive visualization of anthrax biomarker by integrating EDTA chelation

The rapid and sensitive visualization of 2,6-dipicolinic acid (DPA, a unique anthrax biomarker) is essential to prevent anthrax disease or biological terrorist attack. In this study, a Eu³⁺-labeled ethylenediaminetetraacetic acid loaded hyperbranched polyethyleneimine carbon nanodot (hPEI-CD-EDTA-Eu³⁺) nanoprobe has been proposed for the ratiometric DPA detection. The sensing mechanism is based on the rapid DPA-Eu³⁺ chelation within 30 s and subsequent enhanced fluorescence emission through the antenna effect. With the introduction of EDTA chelating unit, the resulted fluorescence of Eu³⁺-complex is greatly enhanced, which endows sensitive DPA perception. By employing hPEI-CD as the internal reference, ratiometric DPA sensing is realized with a good linearity in the concentration range from 1.0 to 100 nM, with a limit of detection of 190 pM (S/N = 3). The specific chelation affinity between Eu³⁺ and DPA provides satisfying selectivity over other amino acids and ions. Using nanoprobe-loaded polyvinylidene fluoride paper as the analytical device, point-of-care DPA visualization is achieved. Furthermore, the practical application of designed paper device is validated by the visual detection of metabolic DPA-release from Bacillus subtilis spores.

tRNA as a stabilizing matrix for fluorescent silver clusters: photophysical properties and IR study

In this experimental study fluorescent silver clusters on a tRNA matrix were synthesized for the first time. Two types of fluorescent complexes emitting in the green (550 nm) and red (635 nm) regions of the visible spectrum were obtained. Using FTIR spectroscopy, we identified possible binding sites for the clusters, which appeared to be within the helical regions of tRNA. It was also shown that tRNA retained its double helical structure after the cluster formation, which is essential for its functionality.

A Photoluminescent Colorimetric Probe of Bovine Serum Albumin-Stabilized Gold Nanoclusters for New Psychoactive Substances: Cathinone Drugs in Seized Street Samples

Screening of illicit drugs for new psychoactive substances-namely cathinone-at crime scenes is in high demand. A dual-emission bovine serum albumin-stabilized gold nanoclusters probe was synthesized and used for quantitation and screening of 4-chloromethcathinone and cathinone analogues in an aqueous solution. The photoluminescent (PL) color of the bovine serum albumin-stabilized Au nanoclusters (BSA-Au NCs) probe solution changed from red to dark blue during the identification of cathinone drugs when excited using a portable ultraviolet light-emitting diodes lamp (365 nm). This probe solution allows the PL color-changing point and limit of detection down to 10.0 and 0.14 mM, respectively, for 4-chloromethcathinone. The phenomenon of PL color-changing of BSA-Au NCs was attributed to its PL band at 650 nm, quenching through an electron transfer mechanism. The probe solution was highly specific to cathinone drugs, over other popular illicit drugs, including heroin, cocaine, ketamine, and methamphetamine. The practicality of this BSA-Au NCs probe was assessed by using it to screen illicit drugs seized by law enforcement officers. All 20 actual cases from street and smuggling samples were validated using this BSA-Au NCs probe solution and then confirmed using gas chromatography-mass spectrometry. The results reveal this BSA-Au NCs probe solution is practical for screening cathinone drugs at crime scenes.

Ultrafast fluorescence dynamics of DNA-based silver clusters

Atomic-level understanding of the nature of the electronically excited states in ligand-stabilized metal nanoclusters (NCs) is a prerequisite for the design of new NCs with desired properties. In this study, we investigate the emission dynamics of a Ag-DNA complex using the fluorescence up-conversion technique. We show that most of the relaxation from the Franck-Condon state to the emissive state takes place in less than 100 fs, in spite of a relatively large Stokes shift of 4500 cm-1. This relaxation is much faster than typical solvent/DNA relaxation rates. A further small relaxation occurs with time constants ranging from a few to hundreds of picoseconds. We also calculate the Stokes shift for model complexes of a small three-atom Ag3+ cluster with cytosine and guanine. The results of our calculations show that a substantial geometry change of the Ag3+ cluster is observed in the S1 state of both complexes, which results in Stokes shifts comparable with the experimental value. We conclude that the Stokes shift in the Ag-DNA complex arises mostly due to the change in the geometry of the Ag cluster in the excited state rather than to the solvent/DNA reorganization. Also, a different structure of the Ag-DNA complex ("dark cluster"), the excited state of which decays in 200 fs, is observed. The nature of this ultrafast deactivation is unclear, which requires further investigations.