Synthesis of DNA-templated copper nanoparticles with enhanced fluorescence stability for cellular imaging

Promising biomedical systems based on copper nanoparticles: Synthesis, characterization, and applications

Copper and copper oxide nanoparticles (CuNPs) have unique physicochemical properties that make them highly promising for biomedical applications. This review discusses the application of CuNPs in biomedicine, including diagnosis, therapy, and theranostics. Recent synthesis methods, with an emphasis on green approaches, are described, and the latest techniques for nanoparticle characterization are critically analyzed. CuNPs, including Cu2O, CuO, and Cu, have significant potential as anti-cancer agents, drug delivery systems, and photodynamic therapy enhancers, among other applications. While challenges such as ensuring biocompatibility and stability must be addressed, the state-of-the-art research reviewed here provides strong evidence for the efficacy and versatility of CuNPs. These multifunctional properties have been extensively researched and documented, showcasing the immense potential of CuNPs in biomedicine. Overall, the evidence suggests that CuNPs are a promising avenue for future research and development in biomedicine. We strongly support further progress in the development of synthesis and application strategies to enhance the effectiveness and safety of CuNPs for clinical purposes.

Preparation of copper nanoparticles fluorescent probes and detection of hydrogen peroxide and glucose

Fluorescent copper nanoparticles (CuNPs) was synthesized by one-step chemical reduction method using ascorbic acid (AA) and copper sulfate (CuSO4⋅5H2O) as raw materials, which had good water solubility and fluorescence properties. A green, simple and safe CuNPs@Fe2+ fluorescence probe was developed for the detection of hydrogen peroxide and glucose using Fe2+ as a bridge. The prepared CuNPs could obtain the maximum fluorescence emission wavelength at 440 nm when the excitation wavelength was 360 nm. The average particle size of CuNPs was 10 nm, which had good photobleach resistance, stability and salt tolerance. The fluorescence intensity was quenched due to electron transfer (ET) process when hydrogen peroxide was added to CuNPs@Fe2+ system. This result was mainly because Fenton reaction occured between hydrogen peroxide and Fe2+, producing hydroxyl free radicals (OH) and Fe3+. Since glucose could be catalyzed by specific glucose oxidase (GOX) to produce H2O2 and corresponding oxidation products, the quantitative analysis of glucose was realized when glucose oxidase was introduced into the CuNPs@Fe2+ sensor system. Therefore, a novel CuNPs@Fe2+ fluorescent probe sensor study was constructed to further achieve quantitative detection of H2O2 and glucose. Under the optimized experimental conditions, the linear ranges for H2O2 and glucose were 28.219-171.562 μM and 1.237-75.771 μM, respectively. And the detection limits for H2O2 and glucose were 7.169 μM and 0.540 μM, respectively. In addition, the mechanism of fluorescence probe quenching caused by the interaction between H2O2 and CuNPs@Fe2+ was also discussed. The proposed sensing system had been applied successfully to the detection of glucose in human serum samples.

Highly Stable Fluorescent-Traffic-Light Sensor for Point-of-Care Detection of Tetracycline

Fluorescent point-of-care (POC) sensors have found great utility in fields like clinical diagnosis, food testing, and environmental monitoring. Herein, we developed a highly stable POC sensor that enabled the visual detection of tetracycline (TC) in a distinct fluorescent-traffic-light manner. In the sensor, a composite material of copper nanoclusters and metal-organic framework (CuNCs@MOF-5) prepared with a facile one-pot synthetic strategy was employed as the core element for target recognition and signal transduction. As evidenced by experiments, the as-prepared CuNCs@MOF-5 exhibited significantly improved fluorescence properties in terms of emission enhancement (about 28-fold) and stability improvement (over 110 days) compared to the CuNCs without confining and protection by MOF-5. More importantly, it was found that TC could uniquely interact with Zn(II) to trigger the disassembly of CuNCs@MOF-5, resulting in green fluorescence emission from the TC-Zn(II) complex and red fluorescence weakening of CuNCs. On the basis of this finding, a simple and stable sensor was proposed for POC detection of TC, which demonstrated high sensitivity, selectivity, and reproducibility. In addition to homogeneous visual detection in a 96-well plate, a CuNCs@MOF-5-contained agarose gel array was easily fabricated to achieve direct detection of TC in milk without any pretreatment, thanks to the size-sieving effect of the gel. Moreover, a test paper array was also put forward for low-cost TC detection, which indicates the extensibility and practicability of this sensing strategy.

Exploring the diverse biomedical applications of programmable and multifunctional DNA nanomaterials

DNA nanoparticles hold great promise for a range of biological applications, including the development of cutting-edge treatments and diagnostic tests. Their subnanometer-level addressability enables precise, specific modifications with a variety of chemical and biological entities, making them ideal as diagnostic instruments and carriers for targeted delivery. This paper focuses on the potential of DNA nanomaterials, which offer scalability, programmability, and functionality. For example, they can be engineered to provide highly specific biosensing and bioimaging capabilities and show promise as a platform for disease diagnosis and treatment. Successful operation of various biomedical nanomaterials has been demonstrated both in vitro and in vivo. However, there are still significant challenges to overcome, including the need to improve the scalability and reliability of the technology, and to ensure safety in clinical applications. We discuss these challenges and opportunities in detail and highlight the progress and prospects of DNA nanotechnology for biomedical applications.

Label-Free, High-Throughput, Sensitive, and Logical Analysis Using Biomimetic Array Based on Stable Luminescent Copper Nanoclusters and Entropy-Driven Nanomachine

The development of an array for high-throughput and logical analysis of biomarkers is significant for disease diagnosis. DNA-templated copper nanoclusters (CuNCs) have a strong potential to serve as a label-free photoluminescence source in array platforms, but their luminescent stability and sensitivity need to be improved. Herein, we report a facile, sensitive, and robust biomimetic array assay by integrating with stable luminescent CuNCs and entropy-driven nanomachine (EDN). In this strategy, the luminescent stability of CuNCs was improved by adding fructose in CuNCs synthesis to offer a reliable label-free signal. Meanwhile, the DNA template for CuNCs synthesis was introduced into EDN with excellent signal amplification ability, in which the reaction triggered by target miRNA would cause the blunt/protruding conformation change of 3'-terminus accompanied by the production or loss of luminescence. In addition, a biomimetic array fabricated by photonic crystals (PCs) physically enhanced the emitted luminescent signal of CuNCs and achieved high-throughput signal readout by a microplate reader. The proposed assay can isothermally detect as low as 4.5 pM of miR-21. Moreover, the logical EDN was constructed to achieve logical analysis of multiple miRNAs by "AND" or "OR" logic gate operation. Therefore, the proposed assay has the advantages of label-free property, high sensitivity, flexible design, and high-throughput analysis, which provides ideas for developing a new generation of facile and smart platforms in the fields of biological analysis and clinical application.

Micron-Scale Fabrication of Ultrathin Amorphous Copper Nanosheets Templated by DNA Scaffolds

Two-dimensional (2D) amorphous materials could outperform their crystalline counterparts toward various applications because they have more defects and reactive sites and thus could exhibit a unique surface chemical state and provide an advanced electron/ion transport path. Nevertheless, it is challenging to fabricate ultrathin and large-sized 2D amorphous metallic nanomaterials in a mild and controllable manner due to the strong metallic bonds between metal atoms. Here, we reported a simple yet fast (10 min) DNA nanosheet (DNS)-templated method to synthesize micron-scale amorphous copper nanosheets (CuNSs) with a thickness of 1.9 ± 0.4 nm in aqueous solution at room temperature. We demonstrated the amorphous feature of the DNS/CuNSs by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Interestingly, we found that they could transform to crystalline forms under continuous electron beam irradiation. Of note, the amorphous DNS/CuNSs exhibited much stronger photoemission (∼62-fold) and photostability than dsDNA-templated discrete Cu nanoclusters due to the elevation of both the conduction band (CB) and valence band (VB). Such ultrathin amorphous DNS/CuNSs hold great potential for practical applications in biosensing, nanodevices, and photodevices.

A Fluorescent Biosensor for Streptavidin Detection Based on Double-Hairpin DNA-Templated Copper Nanoparticles

In this paper, we developed a sensitive, label-free and facile fluorescent strategy for detecting streptavidin (SA) based on double-hairpin DNA-templated copper nanoparticles (CuNPs) and terminal protection of small molecule-linked DNA. Herein, a special DNA hairpin probe was designed and synthesized, which contained two poly T single-stranded loops and a nick point in the middle of the stem. Inspired by the concept of the terminal protection interaction, the specific binding of SA to the biotinylated DNA probe can prevent the exonuclease degradation and keep the integrity of DNA probe, which can be used for synthesizing fluorescent CuNPs as a template. Conversely, the DNA probe would be digested by exonucleases and therefore, would fail to form CuNPs without SA. After systematic optimization, the detection range of SA concentration is from 0.5 to 150 nM with a low detection limit of 0.09 nM. Additionally, the proposed method was also successfully applied in the biological samples. Finally, the proposed method is sensitive, effective and simple, and can be potentially applied for predicting diseases and discovering new drugs.

High Fructose Concentration Increases the Fluorescence Stability of DNA-Templated Copper Nanoclusters by Several Thousand Times

DNA-templated copper nanoclusters (CuNCs) have limited applications because of their low fluorescence stability (several tens of minutes). In this study, we prepared CuNCs with improved temporal fluorescence stability by introducing fructose into the CuNC synthesis process and optimizing the reaction conditions. The inclusion of fructose increased the operating lifetime of CuNCs by approximately 5200-fold from 30 min to 108 days and improved their stability against heat, acids, and bases compared to CuNCs synthesized under original conditions. In addition, the fluorescence signal of CuNCs was maintained for a significantly longer time when stored at refrigeration (4 °C) and freezing (-20 °C) temperatures. Importantly, this method did not require the addition of substances other than fructose or any additional physicochemical treatment to maintain the fluorescence of DNA-templated CuNCs for more than several tens of days. As such, this study could serve as a basis to improve the stability of CuNCs for various applications.

Nanoscale zerovalent copper (nZVC) catalyzed environmental remediation of organic and inorganic contaminants: A review

Over the past decade, the nano zerovalent copper has emerged as an effective nano-catalyst for the environment remediation processes due to its ease of synthesis, low cost, controllable particle size and high reactivity despite its release during the remediation process and related concentration dependent toxicities. However, the improvised techniques involving the use of supports or immobilizer for the synthesis of Cu⁰ has significantly increased its stability and motivated the researchers to explore the applicability of Cu⁰ for the environment remediation processes, which is evident from access to numerous reports on nano zerovalent copper mediated remediation of contaminants. Initially, this review allows the understanding of the various resources used to synthesize zerovalent copper nanomaterial and the structure of Cu⁰ nanoparticles, followed by focus on the reaction mechanism and the species involved in the contaminant remediation process. The studies comprehensively presented the application of nano zerovalent copper for remediation of organic/inorganic contaminants in combination with various oxidizing and reducing agents under oxic and anoxic conditions. Further, it was evaluated that the immobilizers or support combined with various irradiation sources originates a synergistic effect and have a significant effect on the stability and the redox properties of nZVC in the remediation process. Therefore, the review proposed that the future scope of research should include rigorous focus on deriving an exact mechanism for synergistic effect for the removal of contaminants by supported nZVC.

DNA dendrimer-templated copper nanoparticles: self-assembly, aggregation-induced emission enhancement and sensing of lead ions

Copper nanomaterials based on DNA scaffold (DNA-Cu NMs) are becoming a novel fluorescent material, but it is still challenging to obtain highly fluorescent DNA-Cu NMs with excellent stability. In this work, we report a kind of copper nano-assemblies (Cu NASs) with aggregation-induced emission enhancement (AIEE) property using DNA dendrimers with sticky end as template. The sticky end of the DNA dendrimers induced the formation of much bigger Cu NASs with average size ranging from 131 to 264 nm, depending on the length of the DNA dendrimer sticky end from 6 bases to 27 bases. Compared with complete complementary DNA dendrimer, nearly 6-fold fluorescence enhancement was achieved using DNA dendrimer with 27 bases sticky end. Moreover, the DNA dendrimer-Cu NASs demonstrated excellent stability in serum and could be rapidly quenched by Pb²⁺ ions. Based on the above property, highly sensitive and selective fluorescent detection of Pb²⁺ ions was possible with a linear range of 2.0-100 nM and a detection limit of 0.75 nM. Due to the sensitive and rapid response to Pb²⁺ as well as excellent stability in complex matrix, the proposed fluorescent Cu NASs demonstrated high potential as an excellent fluorescent probe for Pb²⁺ in complex matrix.

Ultrasensitive and turn-on homogeneous Hg2+ sensing based on a target-triggered isothermal cycling reaction and dsDNA-templated copper nanoparticles

In this work, an ultrasensitive and turn-on sensor for homogeneous Hg2+ detection has been constructed based on a target-triggered isothermal cycling reaction and rapid label-free signal output with dsDNA-templated copper nanoparticles (CuNPs). As the key component of the sensor, a hairpin DNA without any labels was designed to contain different functional sequence segments and to resist digestion by exonuclease due to the protruding 3'-terminus. In the presence of Hg2+, the formation of a T-Hg2+-T structure turned the protruding 3'-terminus of the hairpin DNA to a blunt end that could be efficiently digested by Exo III, accompanied by Hg2+ release, followed by another digestion cycle. Hence, the Hg2+-triggered isothermal cycling reaction accumulated numerous dsDNA templates that facilitated fluorescent CuNP generation and finally output an amplified signal used to identify the target. This protocol is capable of Hg2+ sensing in a concentration range of 5 orders of magnitude with a detection limit down to 3.9 pM. The as-constructed sensor also revealed high selectivity, as well as satisfactory results in recovery experiments of Hg2+ detection in real water samples.

Synthesis of Exosome-Based Fluorescent Gold Nanoclusters for Cellular Imaging Applications

In recent years, fluorescent metal nanoclusters have been used to develop bioimaging and sensing technology. Notably, protein-templated fluorescent gold nanoclusters (AuNCs) are attracting interest due to their excellent fluorescence properties and biocompatibility. Herein, we used an exosome template to synthesize AuNCs in an eco-friendly manner that required neither harsh conditions nor toxic chemicals. Specifically, we used a neutral (pH 7) and alkaline (pH 11.5) pH to synthesize two different exosome-based AuNCs (exo-AuNCs) with independent blue and red emission. Using field-emission scanning electron microscopy, energy dispersive X-ray microanalysis, nanoparticle tracking analysis, and X-ray photoelectron spectroscopy, we demonstrated that AuNCs were successfully formed in the exosomes. Red-emitting exo-AuNCs were found to have a larger Stokes shift and a stronger fluorescence intensity than the blue-emitting exo-AuNCs. Both exo-AuNCs were compatible with MCF-7 (human breast cancer), HeLa (human cervical cancer), and HT29 (human colon cancer) cells, although blue-emitting exo-AuNCs were cytotoxic at high concentrations (≥5 mg/mL). Red-emitting exo-AuNCs successfully stained the nucleus and were compatible with membrane-staining dyes. This is the first study to use exosomes to synthesize fluorescent nanomaterials for cellular imaging applications. As exosomes are naturally produced via secretion from almost all types of cell, the proposed method could serve as a strategy for low-cost production of versatile nanomaterials.

Copper nanoclusters: designed synthesis, structural diversity, and multiplatform applications

Atomically precise metal nanoclusters (MNCs) have gained tremendous research interest in recent years due to their extraordinary properties. The molecular-like properties that originate from the quantized electronic states provide novel opportunities for the construction of unique nanomaterials possessing rich molecular-like absorption, luminescence, and magnetic properties. The field of monolayer-protected metal nanoclusters, especially copper, with well-defined molecular structures and compositions, is relatively new, about two to three decades old. Nevertheless, the massive progress in the field illustrates the importance of such nanoobjects as promising materials for various applications. In this respect, nanocluster-based catalysts have become very popular, showing high efficiencies and activities for the catalytic conversion of chemical compounds. Biomedical applications of clusters are an active research field aimed at finding better fluorescent contrast agents, therapeutic pharmaceuticals for the treatment and prevention of diseases, the early diagnosis of cancers and other potent diseases, especially at early stages. A huge library of structures and the compositions of copper nanoclusters (CuNCs) with atomic precisions have already been discovered during last few decades; however, there are many concerns to be addressed and questions to be answered. Hopefully, in future, with the combined efforts of material scientists, inorganic chemists, and computational scientists, a thorough understanding of the unique molecular-like properties of metal nanoclusters will be achieved. This, on the other hand, will allow the interdisciplinary researchers to design novel catalysts, biosensors, or therapeutic agents using highly structured, atomically precise, and stable CuNCs. Thus, we hope this review will guide the reader through the field of CuNCs, while discussing the main achievements and improvements, along with challenges and drawbacks that one needs to face and overcome.

Fluorescence resonance energy transfer using DNA-templated copper nanoparticles for ratiometric detection of microRNAs

We examined the effectiveness of a ratiometric method using DNA-templated copper nanoparticles, which can function as a probe for fluorescence resonance energy transfer. This method in combination with PCR successfully detected the target microRNA, which corresponded well with the results obtained by quantitative reverse transcription PCR.

Highly specific nuclear labeling via in situ formation of fluorescent copper nanoparticles

When imaging cells, nuclear counterstaining is imperative; however, many commercial nuclear-staining dyes based on nucleic acid intercalation result in nonspecific signals in the cytoplasm. Here, we propose a new strategy that stains the nucleus with high specificity by in situ formation of DNA-templated copper nanoparticles (CuNPs). We demonstrated that genomic DNA in the nucleus enabled rapid formation of highly fluorescent CuNPs immediately following addition of a copper ion source and ascorbate as a reducing agent. Moreover, we found that RNA and mitochondrial DNA, largely responsible for nonspecific cytoplasmic signals from commercial nuclear-staining dyes, did not mediate the formation of the highly fluorescent CuNPs, resulting in highly specific nuclear staining at a reduced cost relative to commercially available methods. Furthermore, we verified the compatibility of the proposed method with other fluorescence-labeling techniques. These results demonstrated the efficacy of this method and its promise as a powerful tool for cell imaging.

Photoprotecting Uracil by Coupling with Lossy Nanocavities

We analyze how the photorelaxation dynamics of a molecule can be controlled by modifying its electromagnetic environment using a nanocavity mode. In particular, we consider the photorelaxation of the RNA nucleobase uracil, which is the natural mechanism to prevent photodamage. In our theoretical work, we identify the operative conditions in which strong coupling with the cavity mode can open an efficient photoprotective channel, resulting in a relaxation dynamics twice as fast as the natural one. We rely on a state-of-the-art chemically detailed molecular model and a non-Hermitian Hamiltonian propagation approach to perform full-quantum simulations of the system dissipative dynamics. By focusing on the photon decay, our analysis unveils the active role played by cavity-induced dissipative processes in modifying chemical reaction rates, in the context of molecular polaritonics. Remarkably, we find that the photorelaxation efficiency is maximized when an optimal trade-off between light-matter coupling strength and photon decay rate is satisfied. This result is in contrast with the common intuition that increasing the quality factor of nanocavities and plasmonic devices improves their performance. Finally, we use a detailed model of a metal nanoparticle to show that the speedup of the uracil relaxation could be observed via coupling with a nanosphere pseudomode, without requiring the implementation of complex nanophotonic structures.

Fluorescent detection of Cu (II) ions based on DNAzymatic cascaded cyclic amplification

A fluorescent biosensor based on the cascaded cyclic amplification-lighted copper nanoparticles has been developed, optimized, and validated. In the double-modular cascaded cyclic amplification, a DNAzymatic cyclic amplification unit transforms metal ion signal to specific DNA sequences, and a linear/exponential integrated amplification unit converts as-prepared DNA codes to identical thymine (T)-rich DNA templates. T-rich scaffolds can induce the generation of red fluorescent copper nanoparticles, with fluorescence emission at 625 nm upon the excitation at 340 nm, as signal vehicles for quantitative detection of metal ions. Copper ions, selected as the model target, could be detected in a wide linear range from 10 to 10⁴ nM depending on the increased fluorescent intensity, and the detection limit is 5.6 ± 0.52 nM (n = 3) within 40 min, which is 4 orders of magnitude lower than the limits set in drinking water. In the detection of Cu²⁺ in real tap and lake water, the results between inductively coupled plasma mass spectrometry (ICP-MS) and our proposed biosensor were consistent, illustrating the practicability of the fabricated method. In summary, the established fluorescent biosensor compensates the deficiency of immunoassays failing to analyze metal ions, broadens ranges of biomarkers responding to cleaved DNAzymes, provides an open platform sensing different metal ions, and meets the increasing need for the ultrasensitive detection in the field of food safety, environmental monitoring, and medical diagnosis.

Target-induced in-situ formation of fluorescent DNA-templated copper nanoparticles by a catalytic hairpin assembly: application to the determination of DNA and thrombin

A fluorometric method is described for nucleic acid signal amplification through target-induced catalytic hairpin assembly with DNA-templated copper nanoparticles (Cu NPs). The toehold-mediated self-assembly of three metastable hairpins is triggered in presence of target DNA. This leads to the formation of a three-way junction structure with protruding mononucleotides at the 3' terminus. The target DNA is released from the formed branched structure and triggers another assembly cycle. As a result, plenty of branched DNA becomes available for the synthesis of Cu NPs which have fluorescence excitation/emission maxima at 340/590 nm. At the same time, the branched structure protects the Cu NPs from digestion by exonuclease III. The unreacted hairpins are digested by exonuclease III, and this warrants a lower background signal. The method can detect ssDNA (24 nt) at low concentration (44 pM) and is selective over single-nucleotide polymorphism. On addition of an aptamer, the strategy can also be applied to the quantitation of thrombin at levels as low as 0.9 nM. Graphical abstractSchematic representation of target-induced catalytic hairpin assembly to form branched DNA template for the in situ synthesis of fluorescent Cu nanoparticles.