Protein-directed synthesis of pH-responsive red fluorescent copper nanoclusters and their applications in cellular imaging and catalysis

Synergism between copper and silver nanoclusters induces fascinating structural modifications, properties, and applications

Among the group 11 transition metal elements, Cu and Ag are widely studied due to their cost effectiveness and easy availability. However, the synergism between copper and silver is also very promising, exhibiting intriguing structures, properties, and applications. Nanoclusters, which are missing links between atoms and nanoparticles, are highly fluorescent due to their discrete energy levels. Their fluorescence can be efficiently tuned because of the synergistic behaviour of copper and silver. Furthermore, their fluorescence can be selectively altered in the presence of various analytes and sensing platforms, as reported by various groups. Moreover, copper clusters can be utilized for sensing silver while silver nanoclusters can be utilized for sensing ionic copper due to the strong interaction between copper and silver. Furthermore, DFT studies have been performed to understand the structural modification due to CuAg synergism. A concise summary of the synergism between copper and silver can open a new window of research for young scientists venturing into the field of environmental nanoscience.

Ultra-small water-soluble fluorescent copper nanoclusters for p-nitrophenol detection

Due to the widespread application of p-nitrophenol (p-NP) across various industries, particularly in the pharmaceutical and chemical sectors, it has emerged as a significant environmental contaminant in both soil and water ecosystems. The development of swift and sensitive detection platforms for p-NP is therefore demanding. Herein, a fluorescence sensor based on ultra-small copper nanoclusters with exterior glutathione ligands determined by electrospray ionization mass spectrometry (ESI-MS) as [Cu14(SG)12]+ (denoted as Cu-SG NCs) has been prepared in high efficiency, and shown high selectivity for p-NP detection. The Cu-SG NCs, synthesized via a facile one-pot chemical reduction technique, exhibit emission maxima at 620 nm. Notably, the introduction of p-NP into the nanocluster system causes a significant quenching of the Cu-SG NCs fluorescence. The quenching phenomenon arises predominantly as a result of the inner filter effect (IFE), which stems from the substantial overlap between the UV-Vis absorption spectrum of p-NP and the excitation wavelength of Cu-SG NCs. The developed fluorescence sensor platform demonstrates a wide determination range for p-NP, ranging from 0.04 to 2000 µM, with a detection limit of 30 nM. Additionally, the sensor efficacy was successfully validated in the analysis of actual water samples. The ease of synthesis, excellent optical properties, and low toxicity of Cu-SG NCs represent significant advantages over the reported noble metal nanomaterials and is highly promising for future practical applications.

Templated Synthesis of Copper Nanoclusters with a Hybrid Lysozyme-Polymer Material for Enhanced Fluorescence

Hybrid materials that combine organic polymers and biomacromolecules offer unique opportunities for precisely controlling 3D chemical environments. Although biological or organic templates have been separately used to control the growth of inorganic nanoclusters, hybrid structures represent a relatively unexplored approach to tailoring nanocluster properties. Here, we demonstrate that a molecularly defined lysozyme-polymer resin material acts as a structural scaffold for the synthesis of copper nanoclusters (CuNCs) with well controlled size distributions. The resulting CuNCs have significantly enhanced fluorescence compared with syntheses based on polymeric or biological templates alone. The synergistic approach described here is appealing for the synthesis of biocompatible fluorescent labels with improved photostability.

Utilizing Copper Nanoclusters as a Fluorescent Probe for Quantitative Monitoring of Doxorubicin Anticancer Drug

Monitoring the amount of chemotherapeutic drugs in biological fluids is extremely important for dose adjustment or control of side effects during the treatment process. In this study, copper nanoclusters (Cu NCs) were synthesized via a one-pot method using ammonium citrate as the reducing agent. Cu NCs exhibited bright blue fluorescence, good optical properties and outstanding photostability. The produced Cu NCs were characterized in detail by UV‒vis absorption, fluorescence spectroscopy and transmission electron microscopy (TEM). The produced Cu NCs showed a high quantum yield of 0.97. A fluorescence system was used for doxorubicin (DOX) determination using Cu NCs as a nanoprobe. The presence of DOX decreased the fluorescence intensity of the CuNCs at 445 nm but increased the fluorescence intensity of the CuNCs at 619 nm. As a result, quantitative detection of DOX can be achieved by measuring the ratio of fluorescence intensities at 445 and 619 nm (F619/F445). The fluorescence quenching activity of the Cu NCs was determined to have a linear relationship with the amount of DOX anticancer drug in the range of 1-15 ppb, and the usability of the Cu NCs as a sensor for detection in biological fluids was demonstrated. It was determined that this method can be used to measure the amount of DOX in biological samples effectively.

Small Copper Nanoclusters Synthesized through Solid-State Reduction inside a Ring-Shaped Polyoxometalate Nanoreactor

Cu nanoclusters exhibit distinctive physicochemical properties and hold significant potential for multifaceted applications. Although Cu nanoclusters are synthesized by reacting Cu ions and reducing agents by covering their surfaces using organic protecting ligands or supporting them inside porous materials, the synthesis of surface-exposed Cu nanoclusters with a controlled number of Cu atoms remains challenging. This study presents a solid-state reduction method for the synthesis of Cu nanoclusters employing a ring-shaped polyoxometalate (POM) as a structurally defined and rigid molecular nanoreactor. Through the reduction of Cu2+ incorporated within the cavity of a ring-shaped POM using H2 at 140 °C, spectroscopic studies and single-crystal X-ray diffraction analysis revealed the formation of surface-exposed Cu nanoclusters with a defined number of Cu atoms within the cavities of POMs. Furthermore, the Cu nanoclusters underwent a reversible redox transformation within the cavity upon alternating the gas atmosphere (i.e., H2 or O2). These Cu nanoclusters produced active hydrogen species that can efficiently hydrogenate various functional groups such as alkenes, alkynes, carbonyls, and nitro groups using H2 as a reductant. We expect that this synthesis approach will facilitate the development of a wide variety of metal nanoclusters with high reactivity and unexplored properties.

Temperature-Induced Luminescence Intensity Fluctuation of Protein-Protected Copper Nanoclusters: Role of Scaffold Conformation vs Nonradiative Transition

Protein-scaffolded atomically precise metal nanoclusters (NCs) have emerged as a promising class of biofriendly nanoprobes at the forefront of modern research, particularly in the area of sensing. The photoluminescence (PL) intensity of several nanoclusters showed a systematic temperature-dependent fluctuation, but the mechanism remains ambiguous and is poorly understood. We tried to shed some light on this mechanistic aspect by testing a couple of hypotheses: (i) conformational fluctuation of the protein scaffold-mediated PL intensity fluctuation and (ii) PL intensity fluctuation due to the variation in the radiative and nonradiative transition rates. Herein, the PL intensity of the lysozyme-capped copper nanocluster (Lys-Cu NC) showed excellent temperature dependency; upon increasing the temperature, the PL intensity gradually decreased. However, contrasting effects can be seen when the nanocluster is exposed to a chemical denaturant (guanidine hydrochloride (GdnHCl)); the PL intensity increased with the increase in the GdnHCl concentration due to the change in the ionic strength of the medium. This discrepancy clearly suggests that the thermal PL intensity fluctuation cannot be explained by a change in the scaffold conformation. Furthermore, upon closer investigation, we observed a 2-fold increase in the nonradiative decay rate of the Lys-Cu NC at the elevated temperature, which could reasonably explain the decrease in the PL intensity of the nanocluster at the higher temperature. Additionally, from the result, it was evident that the protein scaffold-metal core interaction played a key role here in stabilizing each other; hence, the scaffold structure remained unaffected even in the presence of chemical denaturants.

Advanced Functionalized Nanoclusters (Cu, Ag, and Au) as Effective Catalyst for Organic Transformation Reactions

A considerable amount of research has been carried out in recent years on synthesizing metal nanoclusters (NCs), which have wide applications in the field of optical materials with non-linear properties, bio-sensing, and catalysis. Aside from being structurally accurate, the atomically precise NCs possess well-defined compositions due to significant tailoring, both at the surface and the core, for certain functionalities. To illustrate the importance of atomically precise metal NCs for catalytic processes, this review emphasizes 1) the recent work on Cu, Ag, and Au NCs with their synthesis, 2) the parameters affecting the activity and selectivity of NCs catalysis, and 3) the discussion on the catalytic potential of these metal NCs. Additionally, metal NCs will facilitate the design of extremely active and selective catalysts for significant reactions by elucidating catalytic mechanisms at the atomic and molecular levels. Future advancements in the science of catalysis are expected to come from the potential to design NCs catalysts at the atomic level.

BSA nanoclusters-based sensor for detection of dopamine in schizophrenia from biofluids

OBJECTIVE

To develop nontoxic and stable fluorescent emission B-Cu nanoclusters (NCs) for the specific detection of dopamine at low concentrations in cerebrospinal fluid (CSF).

SIGNIFICANCE

Fluorescent gold and copper NCs conjugated with proteins, such as bovine serum albumin (BSA), offer photostability and healthcare potential. This study focused on fabricating B-Cu NCs that exhibited superior characteristics for sensitive dopamine detection.

METHODS

The study employed various instrumental techniques including attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM), spectrofluorometry, and transmission electron microscopy (TEM) to characterize the formulated B-Cu NCs. The NCs were synthesized, resulting in particle size ∼300 nm. The highest observed fluorescence was recorded at 24542.81 relative fluorescence units (RFU).

RESULTS

The introduction of dopamine at concentrations of 0.1, 0.2, 0.3, and 0.4 ng/mL led to decreased fluorescence in both B-Au and B-Cu NCs due to an electron transport system. This reduction in fluorescence allowed dopamine concentration analysis in phosphate buffer and biological fluids such as blood plasma and CSF. B-Cu NCs showed potential as a biosensing system for point-of-care (POC) applications, specifically for diagnosing schizophrenia.

CONCLUSION

The study successfully synthesized stable and nontoxic B-Cu NCs with enhanced fluorescent emission properties. These NCs exhibited the capacity to detect dopamine at low concentrations in CSF. The study's findings hold promise for future applications, particularly in the development of a B-Cu NCs-based biosensing system for convenient POC detection of schizophrenia by both patients and clinicians. The potential impact of this technology on healthcare and biomedical fields is substantial.

Assembly-Induced Emission of Copper Nanoclusters: Revealing the Sensing Mechanism for Detection of Volatile Basic Nitrogen in Seafood Freshness On-Site Monitoring

Total volatile basic nitrogen (TVB-N) is a vital indicator for assessing seafood freshness and edibility. Rapid on-site detection of volatile basic nitrogen (VBN) is of significant importance for food safety monitoring. In this study, highly luminescent self-assembled copper nanoclusters (Cu NCs@p-MBA), synthesized using p-mercaptobenzoic acid (p-MBA) as the ligand, were utilized for the sensitive detection of VBNs. Under acidic conditions, Cu NCs@p-MBA formed compact and well-organized nanosheets through noncovalent interactions, accompanied by intense orange fluorescence emission (651 nm). The benzene carboxylic acid part of Cu NCs@p-MBA provided the driving force for supramolecular assembly and exhibited a strong affinity for amines, particularly low-molecular-weight amines such as ammonia (NH3) and trimethylamine (TMA). The quantitative determination of NH3 and TMA showed the detection limits as low as 0.33 and 0.81 ppm, respectively. Cu NCs@p-MBA also demonstrated good responsiveness to putrescine and histamine. Through density functional theory (DFT) calculations and molecular dynamics (MD) simulations, the precise atomic structure, assembly structure, luminescent properties, and reaction processes of Cu NCs@p-MBA were studied, revealing the sensing mechanism of Cu NCs@p-MBA for highly sensitive detection of VBNs. Based on the self-assembled Cu NCs@p-MBA nanosheets, portable fluorescent labels were developed for semiquantitative, visual, and real-time monitoring of seafood freshness. Therefore, this study exemplified the high sensitivity of self-assembly induced emission (SAIE)-type Cu NCs@p-MBA for VBNs sensing, offering an efficient solution for on-site monitoring of seafood freshness.

Ligand "switching on" fluorescence of HIV-1 RNA-templated copper nanoclusters for ligand-RNA interaction assays

Ligand-RNA interaction assay provides the basis for developing new RNA-binding small molecules. In this study, fluorescent copper nanoclusters (CuNCs) were first prepared using two kinds of HIV-1 RNA targets, rev-responsive element (RRE) and transactivator response element (TAR) RNA, as new templates, and it was found that the fluorescence of the single RNA-templated CuNCs was negligible. Using neomycin as a model drug, the fluorescence could be augmented (approximately 6 times) for the neomycin/RNA-templated CuNCs. Thus, a novel method was developed for ligand-RNA interactions by observing the fluorescence changes in CuNCs prepared using RNA before and after the addition of ligands. The preparation parameters of neomycin/RNA-CuNCs were optimized. The as-prepared CuNCs were characterized using UV-vis spectroscopy, fluorescence spectroscopy, and high-resolution transmission electron microscope. Circular dichroism spectral analysis showed that RRE and TAR were inclined to form a double-stranded structure after interaction with neomycin, which was more conducive to the formation of CuNCs. The interactions of neomycin and three test drugs (amikacin, gentamicin, and tobramycin) with RNA were investigated using the proposed method, and the binding constants and number of binding sites were obtained through theoretical calculations. This study provides a novel approach for ligand-RNA interaction assays.

Cu(I)-thioether coordination complexes based on a chiral cyclic β-amino acid ligand

Coordination complexes, particularly metalloproteins, highlight the significance of metal-sulfur bonds in biological processes. Their unique attributes inspire efforts to synthetically reproduce these intricate metal-sulfur motifs. Here, we investigate the synthesis and characterization of copper(I)-thioether coordination complexes derived from copper(I) halides and the chiral cyclic β-amino acid trans-4-aminotetrahydrothiophene-3-carboxylic acid (ATTC), which present distinctive structural properties and ligand-to-metal ratios. By incorporating ATTC as the ligand, we generated complexes that feature a unique chiral conformation and the capacity for hydrogen bonding, facilitating the formation of distinct geometric structures. Through spectroscopic analyses and density functional theory (DFT) calculations, we studied the complexes' optical properties, including their emission bands and variable second-harmonic generation (SHG) efficiencies, which vary based on the halide used. Our findings underscore the potential of the ATTC ligand in creating unusual coordination complexes and pave the way for further investigations into their potential applications, particularly within materials science.

Synthesis of copper nanoclusters from Bacopa monnieri leaves for fluorescence sensing of dichlorvos

In this work, a facile one-step green synthesis was developed for the fabrication of blue fluorescent copper nanocluster (Brahmi-CuNCs) from the extract of Bacopa monnieri (common name is Brahmi) via a microwave method. The as-prepared Brahmi-CuNCs emitted blue fluorescence at 452 nm when excited at 352 nm and showed a quantum yield of 31.32%. Brahmi-derived blue fluorescent CuNCs acted as a probe for fluorescence sensing of dichlorvos. Upon the addition of dichlorvos, the blue emission for Brahmi-CuNCs was gradually turned off, favouring establishment of a calibration graph in the range 0.5-100 μM with a detection limit of 0.23 μM. The as-synthesized Brahmi-CuNCs exhibited marked sensitivity and selectivity towards dichlorvos, favourable for assaying dichlorvos in various samples (cabbage, apple juice, and rice).

Advances in Cu nanocluster catalyst design: recent progress and promising applications

The quest for cleaner pathways to the production of fuels and chemicals from non-fossil feedstock, efficient transformation of raw materials to value-added chemicals under mild conditions, and control over the activity and selectivity of chemical processes are driving the state-of-the-art approaches to the construction and precise chemical modification of sustainable nanocatalysts. As a burgeoning category of atomically precise noble metal nanoclusters, copper nanoclusters (Cu NCs) benefitting from their exclusive structural architecture, ingenious designability of active sites and high surface-to-volume ratio qualify as potential rationally-designed catalysts. In this Minireview, we present a detailed coverage of the optimal design strategies and controlled synthesis of Cu NC catalysts with a focus on tuning of active sites at the atomic level, the implications of cluster size, shape and structure, the ligands and heteroatom doping on catalytic activity, and reaction scope ranging from chemical catalysis to emerging photocatalysis and electrocatalysis.

Unveiling the Long-Lived Emission of Copper Nanoclusters Embedded in a Protein Scaffold

Protein-conjugated coinage metal nanoclusters have become promising materials for optoelectronics and biomedical applications. However, the origin of the photoluminescence, especially the long-lived excited state emission in these metal nanoclusters, is still elusive. Here, we unveiled the underlying mechanism of long-lived emission in albumin protein-conjugated copper nanoclusters (Cu NCs) using steady state and time-resolved spectroscopic techniques. Our findings reveal room-temperature phosphorescence (RTP) in protein-conjugated Cu NCs. Time-resolved area-normalized spectra distinguished short- and long-lived components, where the former arises from the singlet state and the latter from the triplet state, thus resulting in RTP. The similarity of the emission spectra at room (298 K) and cryogenic (77 K) temperature ascertains the RTP phenomenon by harvesting the higher-lying triplet states. Time-gated bioimaging of A549 cells using the long-lived emission not only supports RTP emission in the cellular environment but also provides exciting avenues in long-term bioimaging using bovine serum albumin-conjugated Cu NCs.

Protein-Templated Metal Nanoclusters: Molecular-like Hybrids for Biosensing, Diagnostics and Pharmaceutics

The use of proteins as biomolecular templates to synthesize atomically precise metal nanoclusters has been gaining traction due to their appealing properties such as photoluminescence, good colloidal- and photostability and biocompatibility. The synergistic effect of using a protein scaffold and metal nanoclusters makes it especially attractive for biomedical applications. Unlike other reviews, we focus on proteins in general as the protective ligand for various metal nanoclusters and highlight their applications in the biomedical field. We first introduce the approaches and underlined principles in synthesizing protein-templated metal nanoclusters and summarize some of the typical proteins that have been used thus far. Afterwards, we highlight the key physicochemical properties and the characterization techniques commonly used for the size, structure and optical properties of protein-templated metal nanoclusters. We feature two case studies to illustrate the importance of combining these characterization techniques to elucidate the formation process of protein-templated metal nanoclusters. Lastly, we highlight the promising applications of protein-templated metal nanoclusters in three areas-biosensing, diagnostics and therapeutics.

Dual-emissive phenylalanine dehydrogenase-templated gold nanoclusters as a new highly sensitive label-free ratiometric fluorescent probe: heavy metal ions and thiols measurement with live-cell imaging

Phenylalanine dehydrogenase (PheDH) has been proposed as an ideal protein scaffold for the one-step and green synthesis of highly efficient multifunctional gold nanoclusters. The PheDH-stabilized fluorescent gold nanoclusters (PheDH-AuNCs) with dual emission/single excitation exhibited excellent and long-term stability, high water solubility, large Stokes shift and intense photoluminescence. Selectivity studies demonstrated that the red fluorescence emission intensity of PheDH-AuNCs was obviously decreased in less than 10 min by the addition of mercury, copper, cysteine or glutathione under the single excitation at 360 nm, without significant change in the blue emission of the PheDH-AuNCs. Therefore, the as-prepared PheDH-AuNCs as a new excellent fluorescent probe were successfully employed to develop a simple, rapid, low cost, label- and surface modification-free nanoplatform for the ultrasensitive and selective detection of Hg2+, Cu2+, Cys and GSH through a ratiometric fluorescence system with wide linear ranges and detection limits of 1.6, 2.4, 160 and 350 nM, respectively which were lower than previous reports. In addition, the results showed that PheDH-AuNCs can be used for the detection of toxic heavy metal ions and small biomarker thiols in biological and aqueous samples with acceptable recoveries. Interestingly, PheDH-AuNCs also displayed a promising potential for live-cell imaging due to their low toxicity and great chemical- and photo-stability.

Surface Ligand Influences the Cu Nanoclusters as a Dual Sensing Optical Probe for Localized pH Environment and Fluoride Ion

Functional metal nanomaterials, especially in the nanocluster (NC) size regime, with strong fluorescence, aqueous colloidal stability, and low toxicity, necessitate their application potential in biology and environmental science. Here, we successfully report a simple cost-effective method for red-/green-color-emitting protein/amino-acid-mediated Cu NCs in an aqueous medium. As-synthesized Cu NCs were characterized through UV-Vis absorption spectroscopy, fluorescence spectroscopy, time-resolved photoluminescence, dynamic light scattering, zeta potential, transmission electron microscopy and X-ray photoelectron spectroscopy. The optical properties of both Cu NCs responded linearly to the variation in pH in the neutral and alkaline ranges, and a robust pH reversible nature (between pH 7 and 11) was observed that could be extended to rapid, localized pH sensor development. However, a contrasting pH response nature between protein-Cu NCs and amino acid-Cu NCs was recorded. The alteration in protein secondary structure and strong binding nature of the surfactants were suggested to explain this behavior. Furthermore, we investigated their use as an efficient optical probe for fluoride ion detection. The limit of detection for protein-Cu NCs is 6.74 µM, whereas the limit of detection for amino acid-Cu NCs is 4.67 µM. Thus, it is anticipated that ultrasmall Cu NCs will exhibit promise in biological and environmental sensing applications.

Engineering colloidally stable, highly fluorescent and nontoxic Cu nanoclusters via reaction parameter optimization

Metal nanoclusters (NCs) composed of the least number of atoms (a few to tens) have become very attractive for their emerging properties owing to their ultrasmall size. Preparing copper nanoclusters (Cu NCs) in an aqueous medium with high emission properties, strong colloidal stability, and low toxicity has been a long-standing challenge. Although Cu NCs are earth-abundant and inexpensive, they have been comparatively less explored due to their various limitations, such as ease of surface oxidation, poor colloidal stability, and high toxicity. To overcome these constraints, we established a facile synthetic route by optimizing the reaction parameters, especially altering the effective concentration of the reducing agent, to influence their optical characteristics. The improvement of the photoluminescence intensity and superior colloidal stability was modeled from a theoretical standpoint. Moreover, the as-synthesized Cu NCs showed a significant reduction of toxicity in both in vitro and in vivo models. The possibility of using such Cu NCs as a diagnostic probe toward C. elegans was explored. Also, the extension of our approach toward improving the photoluminescence intensity of the Cu NCs on other ligand systems was demonstrated.

Synthesis of Fluorescent Titanium Nanoclusters at ambient temperature for highly sensitive and selective detection of Creatine Kinase MM in myocardial infarction

Fluorescent-based biosensing in Photoluminescence nanomaterials has emerged as a new sensing platform commonly used for disease diagnosis. However, the synthesis of Titanium nanoclusters is highly challenging since Titanium is easily oxidized into TiO₂ at ambient temperature. To overcome this problem, we used an acidic medium and simple and robust protocol to synthesize the Titanium nanoclusters of 3-4 nm diameter, which could report the first fluorescent Titanium nanoclusters. New approaches for the novel synthesis of TiNCs can be used for rapid sensing of myocardial infarction (cardiac arrest). In converting creatine to phosphocreatine, CK-MM activates the reaction to convert ATP to ADP, thereby releasing the phosphate groups. Titanium nanoclusters bind strongly to the phosphate group and then quench the Fluorescence. Thus, this phenomenon can be further applied for quantification approaches. The quenching of fluorescence intensity with CK-MM concentration is linear with R² = 0.9829. The current approach can be applied for CK-MM sensing for a wide concentration range (0.625 U/L - 10 U/L). The detection limit was 0.2513 ng/ml in aqueous medium and 0.3465 ng/ml in human serum with high sensitivity when compared with the previous reported methods. Also, this is the first fluorescent-based sensing method to detect CK- MM. The fluorescent TiNCs is a novel platform to be widely applied for the phosphopeptide and phosphoprotein analysis due to the strong and covalent bondings between Ti with P atoms in the near future in medicine, biomedicine, and biological fields.

Near-Infrared Electrogenerated Chemiluminescence from Simple Copper Nanoclusters for Sensitive Alpha-Fetoprotein Sensing

Exploiting low cost, water-soluble, and near-infrared (NIR) emissive electrochemiluminophores (ECLphores) is significantly important for biological applications. In this study, bright and NIR electrogenerated chemiluminescence (ECL) emissive copper nanoclusters (Cu NCs) were synthesized through a facile one-pot wet chemical reduction method. ECL properties of obtained Cu NCs were examined in the presence of potassium persulfate, resulting in maximum intensity at 735 nm, at least 135 nm red-shifted with respect to all other Cu NCs. Electrochemistry, photoluminescence (PL), and spooling ECL spectroscopies were used to track NIR ECL emission of Cu NCs ascribed to the monomeric excited states. Due to the abundant binding sites of bovine serum albumin (BSA) to anchor target biomolecules, a sandwich-type ECL immunosensor was thus fabricated using such BSA-templated Cu NCs as tags and alpha fetoprotein antigen (AFP) as a model protein for the first time. Without assisting any signal amplification strategies, the proposed NIR ECL biosensor exhibited a wide linear range (1-400 ng mL^-1) and low detection limit (0.02 ng mL^-1) as well as superior selectivity and reproducibility and was successfully applied in real human serum sample determination. This work sets the stage for the development of novel non-noble metal nanoclusters for large-scale and emerging nanotechnology applications.

The Multifarious Applications of Copper Nanoclusters in Biosensing and Bioimaging and Their Translational Role in Early Disease Detection

Nanoclusters possess an ultrasmall size, amongst other favorable attributes, such as a high fluorescence and long-term colloidal stability, and consequently, they carry several advantages when applied in biological systems for use in diagnosis and therapy. Particularly, the early diagnosis of diseases may be facilitated by the right combination of bioimaging modalities and suitable probes. Amongst several metallic nanoclusters, copper nanoclusters (Cu NCs) present advantages over gold or silver NCs, owing to their several advantages, such as high yield, raw abundance, low cost, and presence as an important trace element in biological systems. Additionally, their usage in diagnostics and therapeutic modalities is emerging. As a result, the fluorescent properties of Cu NCs are exploited for use in optical imaging technology, which is the most commonly used research tool in the field of biomedicine. Optical imaging technology presents a myriad of advantages over other bioimaging technologies, which are discussed in this review, and has a promising future, particularly in early cancer diagnosis and imaging-guided treatment. Furthermore, we have consolidated, to the best of our knowledge, the recent trends and applications of copper nanoclusters (Cu NCs), a class of metal nanoclusters that have been gaining much traction as ideal bioimaging probes, in this review. The potential modes in which the Cu NCs are used for bioimaging purposes (e.g., as a fluorescence, magnetic resonance imaging (MRI), two-photon imaging probe) are firstly delineated, followed by their applications as biosensors and bioimaging probes, with a focus on disease detection.

Copper nanocluster-Ru(dcbpy)32+ as a cathodic ECL-RET probe combined with 3D bipedal DNA walker amplification for bioanalysis

In this study, an intra-molecular ECL-RET probe combining the 3D bipedal DNA walker amplification strategy was exquisitely designed to monitor platelet-derived growth factor BB (PDGF-BB).

Copper nanoclusters promote tomato (Solanum lycopersicum L.) yield and quality through improving photosynthesis and roots growth

The innovative and sustainable technologies are highly needed to decrease serious environmental problems from current agriculture. Herein, the green and biosafe copper-based nano-agriculture was described for tomato production. Prepared Cu nanoclusters (NCs) showed small size (3.0 ± 0.5 nm) and high bioavailability. At low concentration (1 mg kg^-1) in soil, Cu NCs improved the activities of antioxidant enzymes (superoxide dismutase, catalase and peroxidase) in the tomato plants, which could help to slow down leaf aging, increase photosynthesis and carbohydrates content by 19.4 % and 14.9 %, respectively. Cu NCs promoted the roots' growth, especially increasing the root tip' number, which might contribute to the increase in absorption of macronutrients (K, Mg and P) and micronutrients (B, Mn, Cu and Zn). The Cu NCs (1 mg kg^-1) promoted tomato growth and increased the tomato fruit yields by 12.2 % compared to the control. Moreover, the tomato fruit qualities had been improved meanwhile the accumulation of Cu in fruits was not observed. These findings indicate that the Cu NCs have potential to be safely applied for tomato production.

Boosting the Photoluminescent Properties of Protein-Stabilized Gold Nanoclusters through Protein Engineering

This work reports on the use of protein engineering as a versatile tool to rationally design metal-binding proteins for the synthesis of highly photoluminescent protein-stabilized gold nanoclusters (Prot-AuNCs). The use of a single repeat protein scaffold allowed the incorporation of a set of designed metal-binding sites to understand the effect of the metal-coordinating residues and the protein environment on the photoluminescent (PL) properties of gold nanoclusters (AuNCs). The resulting Prot-AuNCs, synthesized by two sustainable procedures, showed size-tunable color emission and outstanding PL properties. In a second stage, tryptophan (Trp) residues were introduced at specific positions to provide an electron-rich protein environment and favor energy transfer from Trps to AuNCs. This modification resulted in improved PL properties relevant for future applications in sensing, biological labeling, catalysis, and optics.

Synthesis of water-soluble, ultrabright Cu nanoclusters with core-shell structure via facile reduction approach for determination of 4-nitrophenol

In this work, we reported a facile reduction approach for fabrication of water-soluble and ultrabright Cu nanoclusters with core-shell structure. A certain amount of reducing agent as NaBH₄was introduced into the polyethyleneimine-stabilized Cu nanoclusters (CuNCs@PEI) system, which exhibited 4-fold fluorescence enhancement along with a blue shift of the emission peak. The variations of morphology, valence states and functional groups demonstrated that a Cu shell was formed surround CuNCs (defined as CuNCs-Cu@PEI), attributable to metal complex (PEI-Cu⁺and PEI-Cu²⁺) reduction. The effect of core-shell morphology on luminous and electron relaxation mechanism of CuNCs-Cu@PEI was investigated via temperature-dependent steady and time-resolved fluorescence measurements. The CuNCs-Cu@PEI with a high fluorescence quantum yields of 22.59% were able to homogeneously disperse in aqueous phase, indicating their potential applications in biological labeling, sensing and invivoimaging. Finally, the CuNCs-Cu@PEI was employed as a fluorescence probe to determine 4-nitrophenol, of which the detection limit was much lower than initial CuNCs@PEI.

Ratiometric fluorescent sensing of ethanol based on copper nanoclusters with tunable dual emission

Copper nanoclusters (Cu NCs) have attracted a surge of interest in fluorescent sensors as their outstanding physicochemical and optical properties. However, most of the reports have focused on single-signal fluorescent sensors, which are susceptible to background interferences and affect accuracy of the results. Herein, we constructed a facile ratiometric fluorescent sensor for monitoring ethanol based on Cu NCs with tunable dual emission. Polyvinylpyrrolidone (PVP)-modified Cu NCs were simply prepared in water, which exhibit ratiometric dual emission, including a strong green emission at 520 nm and a weak blue emission at 450 nm. The PVP-Cu NCs in water with strong green emission display monodisperse state due to the formation of hydration shell around Cu NCs. In ethanol where the hydration shell is destructed, Cu NCs tend to aggregate and show strong blue emission. This emission shift might attribute to the enhancement of Cu-Cu metallophilic interaction with the aggregation of Cu NCs, which induces the excited-state level increasing. Thus, a ratiometric fluorescent probe for ethanol based on the PVP-Cu NCs is fabricated, which possesses rapid response (<1 min), and realize full-range detection from 0 to 100%. In addition, this ratiometric probe is successfully applied to determine the alcohol strength of alcohol beverages, demonstrating the great potential in practical application.

Sensitive detection of S. Aureus using aptamer- and vancomycin -copper nanoclusters as dual recognition strategy

The proposed aptamer- and antibiotic-based dual detection sensor, combines copper nanoclusters (CuNCs) as an effective approach for the recognition and quantification of Staphylococcus aureus (S. aureus) as a pathogenic bacteria. A facile method for CuNCs based on vancomycin as the template using a fluorescence platform was proposed for the recognition of the S. aureus whole cells via antibiotic and aptamer. Using dual receptor functionalized CuNCs linked to vancomycin and a specific aptamer and during aggregation induce emission process enhanced fluorescence signal linearly with S. aureus concentrations between 10²-10⁸ CFU/mL, and the detection limit was 80 CFU/mL after 45 min as the optimum incubation time. Non-target bacteria generated negative results, proving the high specificity of the presented sensor. This strategy showed recoveries ranging 86%-98% in milk as real sample and can be used for the development of universal detection platforms for efficient and specific S. aureus detection with great potential applications for monitoring pathogenic bacteria.

Orange-red emitting copper nanoclusters for endogenous GSH, temperature sensing, and cellular imaging

Glutathione (GSH) plays an important role in the biochemical defense system of the human body. Designing an exceptional probe to detect trace amounts of GSH is of great significance for studying the oxidative stress reaction and related diseases. In this study, a selective and sensitive orange-red emitting copper nanocluster(CuNC)-based fluorescent probe for the detection of GSH was devised in the matrix of polyvinylpyrrolidone (PVP) using hydrazine hydrate (HYD) as the reducing agent and 2-mercaptobenzothiazole (MBT) as the stabilizer. A peaceable without external assistance method (room temperature reaction) was employed to synthesize fluorescent CuNCs with orange-red luminescence emission at 606 nm upon excitation at 377 nm. The fluorescence intensity of CuNCs was found to decrease or even quench by the addition of GSH, which indicated the stronger binding ability of -SH in GSH with the CuNCs losing the protection of PVP. Based on this principle, the present sensor system exhibits a good linear response towards GSH ranging from 0.10 to 40 μM, and the limit of detection was found to be 12.4 nM. Moreover, due to the excellent selectivity and high sensitivity of the GSH sensor, it might act as a potential probe for the detection of GSH in the lysosomal environment of tumor cells. Thus, this strategy has a promising application potential for the early identification and prevention of cancer with low toxicity and good stability.

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.

Recent advances in templated synthesis of metal nanoclusters and their applications in biosensing, bioimaging and theranostics

As a kind of promising nanomaterials, metal nanoclusters (MNCs) generally composed of several to hundreds of metal atoms have received increasing interest owing to their unique properties, such as ultrasmall size (<2 nm), fascinating physical and chemical properties, and so on. Recently, template-assisted synthesis of MNCs (e.g., Au, Ag, Cu, Pt and Cd) has attracted extensive attention in biological fields. Up to now, various templates (e.g., dendrimers, polymers, DNAs, proteins and peptides) with different configurations and spaces have been applied to prepare MNCs with the advantages of facile preparation, controllable size, good water-solubility and biocompatibility. Herein, we focus on the recent advances in the template-assisted synthesis of MNCs, including the templates used to synthesize MNCs, and their applications in biosensing, bioimaging, and disease theranostics. Finally, the challenges and future perspectives of template-assisted synthesized MNCs are highlighted. We believe that this review could not only arouse more interest in MNCs but also promote their further development and applications by presenting the recent advances in this area to researchers from various fields, such as chemistry, material science, physiology, biomedicine, and so on.

Metal-Protein Hybrid Materials with Desired Functions and Potential Applications

Metal nanohybrids are fast emerging functional nanomaterials with advanced structures, intriguing physicochemical properties, and a broad range of important applications in current nanoscience research. Significant efforts have been devoted toward design and develop versatile metal nanohybrid systems. Among numerous biological components, diverse proteins offer avenues for making advanced multifunctional systems with unusual properties, desired functions, and potential applications. This review discusses the rational design, properties, and applications of metal-protein nanohybrid materials fabricated from proteins and inorganic components. The construction of functional biomimetic nanohybrid materials is first briefly introduced. The properties and functions of these hybrid materials are then discussed. After that, an overview of promising application of biomimetic metal-protein nanohybrid materials is provided. Finally, the key challenges and outlooks related to this fascinating research area are also outlined.

Probing the Interaction of Bovine Serum Albumin with Copper Nanoclusters: Realization of Binding Pathway Different from Protein Corona

With an aim to understand the interaction mechanism of bovine serum albumin (BSA) with copper nanoclusters (CuNCs), three different types CuNCs having chemically different surface ligands, namely, tannic acid (TA), chitosan, and cysteine (Cys), have been fabricated, and investigations are carried out in the absence and presence of protein (BSA) at ensemble-averaged and single-molecule levels. The CuNCs, capped with different surface ligands, are consciously chosen so that the role of surface ligands in the overall protein-NCs interactions is clearly understood, but, more importantly, to find whether these CuNCs can interact with protein in a new pathway without forming the "protein corona", which otherwise has been observed in relatively larger nanoparticles when they are exposed to biological fluids. Analysis of the data obtained from fluorescence, ζ-potential, and ITC measurements has clearly indicated that the BSA protein in the presence of CuNCs does not attain the binding stoichiometry (BSA/CuNCs > 1) that is required for the formation of "protein corona". This conclusion is further substantiated by the outcome of the fluorescence correlation spectroscopy (FCS) study. Further analysis of data and thermodynamic calculations have revealed that the surface ligands of the CuNCs play an important role in the protein-NCs binding events, and they can alter the mode and thermodynamics of the process. Specifically, the data have demonstrated that the binding of BSA with TA-CuNCs and Chitosan-CuNCs follows two types of binding modes; however, the same with Cys-CuNCs goes through only one type of binding mode. Circular dichroism (CD) measurements have indicated that the basic structure of BSA remains almost unaltered in the presence of CuNCs. The outcome of the present study is expected to encourage and enable better application of NCs in biological applications.

Aggregation-Induced Emission of Au/Ag Alloy Nanoclusters for Fluorescence Detection of Inorganic Pyrophosphate and Pyrophosphatase Activity

The development of sensitive and accurate detection of inorganic pyrophosphate (PPi) and pyrophosphatase activity (PPase) is important as they play vital roles in biological systems. However, it is still not satisfactory for most of the analytical methods for PPi and PPase because of their Cu²⁺-dependence and poor accuracy. Although the metal ion triggered aggregation-induced emission (AIE) of metal nanoclusters (NCs) offers a new approach to design a Cu²⁺-free strategy for the accurate determination of PPi and PPase recently, current methods are all focused on utilizing pure metal NCs. Alloy NCs incorporating the advantages of diverse metal usually can achieve improved behaviors in the application, such as enhanced sensitivity and stability. In this work, glutathione stabilized alloy Au/Ag NCs were synthesized via a simple method and used for the fluorescence detection of PPi and PPase based on a Zn²⁺-regulated AIE strategy. The controlled release of Zn²⁺ by PPi and PPase could regulate the AIE of Au/Ag NCs and be employed to response PPi concentration and PPase activity. This method processes simple procedure, high sensitivity and stability, and low toxicity. In addition, we also studied the AIE behaviors of this Au/Ag NCs and offer some fundamental understanding of the AIE properties of water-soluble alloy NCs. This study not only provides a straightforward and new approach for PPi and PPase determination but a basis for further study on the AIE properties of alloy NCs and their application.

Dual-emission copper nanoclusters-based ratiometric fluorescent probe for intracellular detection of hydroxyl and superoxide anion species

A fluorescent nanoprobe based on copper nanoclusters (CuNCs) has been developed for ratiometric detection of hydroxyl radicals (^•OH) and superoxide anion radicals (O₂^•-). Two differently luminescent CuNCs, namely cyan-emissive poly(methacrylic acid)-protected copper nanoclusters (PCuNCs) and orange-emissive bovine serum albumin-protected CuNCs (BCuNCs), were conjugated to obtain a hybrid, dual-emission nanoprobe (PCuNCs-BCuNCs) with the corresponding peaks at 445 nm and 652 nm at an excitation wavelength of 360 nm. In particular, the fluorescence peak at 445 nm gradually enhanced with the incremental addition of ^•OH and O₂^•-. However, the fluorescence emission at 652 nm was greatly quenched in the presence of ^•OH, while in case of O₂^•-, the fluorescence intensity remained constant. The differential response of the PCuNCs-BCuNCs towards ^•OH and O₂^•- formed the basis of ratiometric detection. Under optimal conditions, the PCuNCs-BCuNCs exhibited good sensitivity and linearity towards ^•OH and O₂^•- with limits of detection of 0.15 μM and 1.8 μM, respectively. Moreover, the nanoprobe exhibited high selectivity for ^•OH and O₂^•- over other potential ROS interferences. Besides, PCuNCs-BCuNCs were eventually applied for qualitative and quantitative ratiometric assessment of intracellular ^•OH and O₂^•- in L-132 cells. Therefore, this strategy unveils a new potential for copper nanocluster-based sensing of ROS.

Highly Stable Pyrimidine Based Luminescent Copper Nanoclusters with Superoxide Dismutase Mimetic and Nitric Oxide Releasing Activity

Copper nanoclusters (CuNCs) are emerging as an interesting class of materials for various biomedical applications. In this work, we have designed highly stable nucleobase-capped luminescent CuNCs and studied the effect of substituents on the cluster composition and photophysical properties. The NCs exhibit exceptional stability in ambient atmosphere and show significant variation in the emission properties with a change in position of substituents on the ligand, thiouracil. This study represents the first example of a nanocluster that functionally mimics the activity of a major antioxidant enzyme, superoxide dismutase (SOD). In addition to their enzyme-mimetic activity, the CuNCs evince controlled release of nitric oxide (NO), a key gaseous molecule of endothelial system from S-nitrosothiol, S-nitrosoglutathione (GSNO). Further, to a greater significance, these luminescent CuNCs are readily taken up by the mammalian cells and exhibit low toxicity. The superoxide dismutase and NO releasing activity of the fluorescent, biocompatible copper nanoclusters suggest their potential application in both therapeutics and bioimaging.

Comparative Determination of Cytotoxicity of Sub-10 nm Copper Nanoparticles to Prokaryotic and Eukaryotic Systems

Copper nanoparticles demonstrate antibacterial activity, but their toxicity to eukaryotic systems is less understood. Here, we carried out a comparative study to determine the biocompatibility and cytotoxicity of sub-10 nm copper nanoparticles to a variety of biological systems, including prokaryotic cells (Escherichia coli), yeast, mammalian cell lines (HEK293T, PC12), and zebrafish embryos. We determined the bearing threshold for the cell-death-inducing concentration of copper nanoparticles by probing cell growth, viability, as well as embryological features. To exclude the partial toxicity effect from the remnant reactants, we developed a purification approach using agarose gel electrophoresis. Purified CuONP solution inhibits bacterial growth and causes eukaryotic cell death at 170 and 122.5 ppm (w/w) during the 18 h of treatment, respectively. CuONP significantly reduces the pigmentation of retina pigmented epithelium of zebrafish embryos at 85 ppm. The cytotoxicity of CuONP in eukaryotic cells could arise from the oxidative stress induced by CuONP. This result suggests that small copper nanoparticles exert cytotoxicity in both prokaryotic and eukaryotic systems, and therefore, caution should be used to avoid direct contact of copper nanoparticles to human tissues considering the potential use of copper nanoparticles in the clinical setting.

Water-soluble luminescent copper nanoclusters as a fluorescent quenching probe for the detection of rutin and quercetin based on the inner filter effect

In the present study, we proposed a simple, sensitive and selective fluorescence method for the detection of rutin (Rut) and quercetin (Que) based on the inner filter effect (IFE) utilizing water- soluble cysteine-stabilized copper nanoclusters (Cys-CuNCs) as a fluorescent probe. The Cys-CuNCs were successfully synthesized and characterized using UV-visible absorption, emission, Fourier-transform infrared (FT-IR) spectroscopy, fluorescence lifetime, high resolution transmission electron microscopy (HR-TEM) and zeta potential measurements. Cys-CuNCs exhibited bluish-green luminescence under UV light with characteristic emission maxima at 486 nm. Cys-CuNCs was successfully exploited as fluorescent probes for the detection of Rut/Que. The addition of increasing concentrations of Rut/Que led to a gradual decrease in the emission intensity of Cys-CuNCs. The decrease in Cys-CuNC emission intensities were attributed to the strong IFE and static quenching mechanism. The calculated limit of detection values for Rut and Que were as low as 0.021 μM and 0.035 μM, respectively. The Cys-CuNCs sensing probe exhibited excellent selectivity in the presence of other potential interfering compounds. Furthermore, the present method was successfully applied to the analysis of both Rut and Que in biological samples.