Synthesis of cysteine-functionalized water-soluble luminescent copper nanoclusters and their application to the determination of chromium(VI)

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.

An electrochemiluminescent dual-signal immunosensor based on a Ru(bpy)32+-doped silica nanoparticle/copper nanocluster composite for okadaic acid detection

In this study, a sensitive dual-signal electrochemiluminescence (ECL) immunosensor was developed for okadaic acid (OA) detection utilizing copper nanoclusters (CuNCs) and Ru(bpy)32+-doped silica nanoparticles (RuSiNPs). Interestingly, the CuNCs could simultaneously enhance both cathodic (-0.95 V) and anodic (+1.15 V) ECL signals of RuSiNPs, forming a dual-signal ECL sensing platform. Further, RuSiNPs@CuNCs were used as immunomarkers by covalently conjugating them with an anti-OA monoclonal antibody (mAb) to form probes. Finally, dual ECL signals of the immunosensor were fabricated and showed good linear relationships with OA concentrations in the range of 0.05-70 ng mL-1, having a median inhibitory concentration (IC50) of 1.972 ng mL-1 and a limit of detection of 0.039 ng mL-1. Moreover, the constant ratio of the cathodic and anodic ECL peaks achieved self-calibration of the detection signal and improved the reliability of the results. Finally, we successfully applied the ECL sensor to detect OA in spiked oyster samples.

An ammonia-sensitive fluorescence sensor based on polyvinyl alcohol-graphene quantum dots/halloysite nanotubes hybrid film for monitoring fish freshness

A fluorescent hybrid film composed of nitrogen-doped graphene quantum dots (N-GQDs) loaded on halloysite nanotubes (HNTs) (N-GQDs/HNTs nanocomposite) as a sensitive element and polyvinyl alcohol (PVA) as a film-forming matrix was designed for freshness detection. The PVA-N-GQDs/HNTs hybrid film exhibited significantly enhanced fluorescence attributed to the loading of N-GQDs onto the surface of HNTs through electrostatic interactions and hydrogen bonding, effectively reducing their aggregation. The fluorescence of the hybrid film could be quenched by ammonia via photoinduced electron transfer (PET), with good linearity in the range of 20 ppm to 500 ppm ammonia and a limit of detection (LOD) of 0.63 ppm. In addition, the hybrid film was applied to monitor the freshness of seawater fish and freshwater fish stored at refrigeration and room temperature to evaluate the practicality of this approach. The developed hybrid film showed promise for nondestructive and on-site monitoring of fish spoilage.

Analytical developments in the synergism of copper particles and cysteine: a review

Cysteine, a sulfur-containing amino acid, is a vital candidate for physiology. Coinage metal particles (both clusters and nanoparticles) are highly interesting for their spectacular plasmonic properties. In this case, copper is the most important candidate for its cost-effectiveness and abundance. However, rapid oxidation destroys the stability of copper particles, warranting the necessity of suitable capping agents and experimental conditions. Cysteine can efficiently carry out such a role. On the contrary, cysteine sensing is a vital step for biomedical science. This review article is based on a comparative account of copper particles with cysteine passivation and copper particles for cysteine sensing. For the deep understanding of readers, we discuss nanoparticles and nanoclusters, properties of cysteine, and importance of capping agents, along with various synthetic protocols and applications (sensing and bioimaging) of cysteine-capped copper particles (cysteine-capped copper nanoparticles and cysteine-capped copper nanoclusters). We also include copper nanoparticles and copper nanoclusters for cysteine sensing. As copper is a plasmonic material, fluorometric and colorimetric methods are mostly used for sensing. Real sample analysis for both copper particles with cysteine and copper particles for cysteine sensing are also incorporated in this review to demonstrate their practical applications. Both cysteine-capped copper particles and copper particles for cysteine sensing are the main essence of this review. The aspect of the synergism of copper and cysteine (unlike other amino acids) is quite promising for future researchers.

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.

pH-Switchable phenylalanine-templated copper nanoclusters: CO2 probing and efficient peroxidase mimicking activity

Inter-cluster conversion through the strategic tuning of external stimuli and thereby modulation of the optical properties of metal nanoclusters (MNCs) is an emerging domain for exploration. Herein, we report the preparation of blue-emitting CuNCs using phenylalanine (Phe) as a template under acidic conditions (pH ∼ 4). The as-prepared CuNCs exhibit a sequential tuning of the photophysical properties upon varying the pH of the solution from pH ∼4 to pH ∼12. Blue-emitting CuNCs (B-CuNCs, λem = 410 nm) are systematically converted to cyan-emitting CuNCs (C-CuNCs, λem = 490 nm) with a large red-shifted emission maximum by 80 nm as a function of pH. Our present investigation delineates an unprecedented switchability of the photoluminescence (PL) properties of the CuNCs with the variations of the pH from pH ∼4 to pH ∼12. Both the Phe-templated CuNCs (B-CuNCs and C-CuNCs) were broadly characterized by various spectroscopic and morphological techniques. The X-ray photoelectron spectroscopy (XPS) studies reveal the presence of different oxidation states in the metallic core of B-CuNCs and C-CuNCs. These results in turn substantiate the pH-induced intercluster conversion of CuNCs through the substantial change in their core composition as well as valence states. Owing to the pH sensitivity, the CuNCs act as an efficient and highly sensitive probe for CO2, and quantitative estimation of the dissolved CO2 in the form of bicarbonate ions has been achieved through the enhancement of the PL intensity, wherein a very low value of the limit of detection (LOD) of ∼60 μM was obtained. Furthermore, we demonstrated that the CuNCs act as an efficient bio-catalyst with peroxidase mimicking enzymatic activity which has been investigated using OPD as a substrate under physiological conditions (pH ∼7.4 and temperature ∼37 °C). The mechanistic investigations confirmed that the oxidation of OPD mainly proceeds through the generation of hydroxyl radicals (˙OH). We hope the present investigations shed light on a multidimensional aspect of MNCs and uncover an upsurging recent interest in MNCs to act as an artificial enzyme.

Perspectives of different colour-emissive nanomaterials in fluorescent ink, LEDs, cell imaging, and sensing of various analytes

In the past 2 decades, multicolour light-emissive nanomaterials have gained significant interest in chemical and biological sciences because of their unique optical properties. These materials have drawn much attention due to their unique characteristics towards various application fields. The development of novel nanomaterials has become the pinpoint for different application areas. In this review, the recent progress in the area of multicolour-emissive nanomaterials is summarized. The different emissions (white, orange, green, red, blue, and multicolour) of nanostructure materials (metal nanoclusters, quantum dots, carbon dots, and rare earth-based nanomaterials) are briefly discussed. The potential applications of different colour-emissive nanomaterials in the development of fluorescent inks, light-emitting diodes, cell imaging, and sensing devices are briefly summarized. Finally, the future perspectives of multicolour-emissive nanomaterials are discussed.

Delayed luminescence guided enhanced circularly polarized emission in atomically precise copper nanoclusters

Metal nanoclusters, owing to their intriguing optical properties, have captivated research interest over the years. Of special interest have been chiral nanoclusters that display optical activity in the visible region of the electromagnetic spectrum. While the ground state chiral properties of metal nanoclusters have been reasonably well studied, of late research focus has shifted attention to their excited state chiral investigations. Herein, we report the synthesis and chiral investigations of a pair of enantiomerically pure copper nanoclusters that exhibit intense optical activity, both in their ground and excited states. The synthesis of nanoclusters using l- and d-isomers of the chiral ligand led to the formation of metal clusters that displayed mirror image circular dichroism and circularly polarized luminescence signals. Structural validation using single crystal XRD, powder XRD and XPS in conjunction with chiroptical and computational analysis helped to develop a structure-property correlation that is unique to such clusters. Investigations on the mechanism of photoluminescence revealed that the system exhibits long excited state lifetimes. A combination of delayed luminescence and chirality resulted in circularly polarized delayed luminescence, a phenomenon that is rather uncommon to the field of metal clusters. The chiral emissive properties could be successfully demonstrated in free-standing polymeric films highlighting their potential for use in the field of data encryption, security tags and polarized light emitting devices. Moreover, the fundamental understanding of the mechanism of excited state chirality in copper clusters opens avenues for the exploration of similar effects in a variety of other clusters.

Surface-Defect-Involved and Eye-Visible Electrochemiluminescence of Unary Copper Nanoclusters for Immunoassay

A single-stabilizer-capped strategy is proposed for achieving highly efficient and surface-defect-involved electrochemiluminescence (ECL) from unary copper nanoclusters (NCs) via employing l-cysteine (Cys) as a capping agent of luminophore. The Cys-capped CuNCs (Cys-CuNCs) can be electrochemically injected with valence band (VB) holes and exhibit eye-touchable ECL processes around +0.95 and +1.15 V upon employing TPrA as a coreactant. Both accumulated ECL spectra and spooling ECL spectra demonstrated that the two ECL processes are of the same single waveband and spectrally identical to each other with the same maximum emission wavelength of 640 nm. Promisingly, ECL of the Cys-CuNCs/TPrA system is obviously red-shifted for ∼150 nm to PL of Cys-CuNCs, indicating that the bandgap-engineered routes for ECLs of Cys-CuNCs are completely blocked. The oxidative-reduction ECL process of the Cys-CuNCs/TPrA system is a kind of highly efficient, eye-visible, and single-color emission in surface-defect-involved route. The capping agent of Cys can enable the CuNCs/TPrA system with a stronger ECL than other thiol capping agents, so that Cys-CuNCs are utilized as ECL tags for sensitive and selective immunoassays, which exhibit a wide linear response range from 0.05 pg/mL to 0.5 ng/mL and a limit of detection of 0.01 pg/mL (S/N = 3) with carcinoembryonic antigen as the analyte. Moreover, both the luminophore Cys-CuNCs and conjugates Ab₂-CuNCs can be safely stored in aqueous media without any protector, which is promising for the evolution and clinic application of metal NC ECL in the surface-defect-involved route.

Sulfuric acid induced-synthesis coupled with ethanol extraction-water precipitation purification method for orange fluorescent carbon dots with dual-emission: Application for methyl blue detection and cell imaging

In this work, by adjusting the sulfuric acid content in reaction solvent of ethanol, orange fluorescent carbon dots (O-FCDs) with dual-emission wavelength and blue fluorescent carbon dots (B-FCDs) with single-emission wavelength were successfully prepared using 1,3-dihydroxynaphthalene as precursor. Coupling with ethanol extraction-water precipitation purification method, pure O-FCDs and B-FCDs with yields of 9.0 % and 21.3 %, quantum yields (QYs) of 43.0 % and 13.7 % were obtained, respectively. The structures and optical properties of O-FCDs and B-FCDs were investigated by TEM, AFM, Raman, FT-IR, XPS, UV-vis, fluorescence analysis etc. The results revealed that sulfuric acid promoted the carbonization and the oxidation of precursor in the reaction process. In comparison with the B-FCDs, O-FCDs showed narrower lattice spacing and band gap, demonstrating the important role of sulfur-doping in fluorescence tuning. Additionally, O-FCDs showed good sensitivity for methyl blue with a linear response range of 0.05-100 μM (LOD was 20 nM) and the satisfactory results were obtained when O-FCDs were applied to the detection of methyl blue in real fish sample. Moreover, two FCDs showed good biocompatibility and negligible cytotoxicity proved by MTT experiment, while, O-FCDs showed better cell imaging effects than that of B-FCDs. Therefore, the O-FCDs had a broad application prospect as sensing platform in detection of methyl blue and for imaging in biological field.

Cu Nanoparticle-Based Solution and Paper Strips for Colorimetric and Visual Detection of Heavy Metal Ions

The intrinsic toxicity of heavy metal ions to human health or other species calls for the need to develop an analytical tool for the easy and rapid detection of these ions based on inexpensive and stable nanomaterials. This article describes the potential utility of stable Cu nanoparticles (CuNPs) in the detection of toxic metal ions by solution and paper strip-based methods. For this, first, a dodecyl sulfate ion-stabilized CuNP (DS-CuNP) colloid was synthesized by a chemical reduction method. This was followed by treating the dispersion with heavy metal ions and monitoring the spectral change by spectrophotometric and colorimetric techniques. Among a host of metal ions, Hg²⁺, Cd²⁺, and Pb²⁺ have been found to significantly affect the surface plasmon resonance band of CuNPs by concomitantly altering the color of its solution. Notably, the brownish color of CuNP solution changed readily to milky white in the presence of Hg²⁺. Furthermore, the fabricated brownish-yellow test paper strips containing DS-CuNPs transformed to a prominent white color in the presence of a few drops of Hg²⁺ solution. This change in color of the paper strips could be visually detected by the naked eye. The experiments involving the detection of the various ions were carried out by optimizing the experimental conditions qualitatively as well as quantitatively. The limit of detection of the analytes (metal ions) has been found to be 10 μM. Routine analytical techniques like UV-vis spectroscopy, dynamic light scattering, transmission electron microscopy, and Fourier transform infrared spectroscopy formed part of the experiments.

Interaction Mechanisms and Interface Configuration of Cysteine Adsorbed on Gold, Silver, and Copper Nanoparticles

Cysteine-protected metal nanoparticles (NPs) have shown interesting physicochemical properties of potential utility in biomedical applications and in the understanding of protein folding. Herein, cysteine interaction with gold, silver, and copper NPs is characterized by Raman spectroscopy and density functional theory calculations to elucidate the molecular conformation and adsorption sites for each metal. The experimental analysis of Raman spectra upon adsorption with respect to free cysteine indicates that while the C-S bond and carboxyl group are similarly affected by adsorption on the three metal NPs, the amino group is sterically influenced by the electronegativity of each metal, causing a greater modification in the case of gold NPs. A theoretical approach that takes into consideration intermolecular interactions using two cysteine molecules is proposed using a S-metal-S interface motif anchored to the metal surface. These interactions generate the stabilization of an organo-metallic complex that combines gauche (P_H) and anti (P_C) rotameric conformers of cysteine on the surface of all three metals. Similarities between the calculated Raman spectra and experimental data confirm the thiol and carboxyl as adsorption groups for gold, silver, and copper NPs and suggest the formation of monomeric "staple motifs" that have been found in the protecting monolayer of atomic-precise thiolate-capped metal nanoclusters.

Recent Advances in the Application and Mechanism of Carbon Dots/Metal-Organic Frameworks Hybrids in Photocatalysis and the Detection of Environmental Pollutants

Metal-organic frameworks (MOFs) are a class of crystalline porous materials with simple synthesis conditions, large specific surface area, structural diversity, and a wide range of interesting properties. The integration of MOFs with other materials can provide new multifunctional composites that exhibit both component properties and new characteristics. In recent years, the integration of carbon dots (CDs) into MOFs to form composites has shown improved optical properties and fascinating new characteristics. This review focuses on the design and synthesis strategies of CDs@MOFs composites (including pore-confined synthesis, in situ encapsulation, post-synthesis modification and impregnation method) and their recent research progress in photocatalysis and detection of environmental pollutants. Both the achievements and problems are evaluated and proposed, and the opportunities and challenges of CDs@MOF composite are discussed.

Development of a paper printed colorimetric sensor based on Cu-Curcumin nanoparticles for evolving point-of-care clinical diagnosis of sodium

The homeostatic control of Sodium (Na⁺) ion in the human body assumes paramount relevance owing to its physiological importance. Any deviation from the normal level causes serious health problems like hypernatremia, hyponatremia, stroke, kidney problems etc. Therefore, quantification of Na⁺ levels in body fluids has significant diagnostic and prognostic importance. However, interfering ions like Potassium ion (K⁺) is the major hurdle in sodium detection. In this work, we synthesized the clusters of 3-9 nm-sized highly stable and pure Copper nanoparticles surface functionalised with curcumin, through chemical reduction method. Each cluster of particles is encapsulated in a curcumin layer which is clearly visible in TEM images. The results show that these curcumin functionalized Cu NPs (CuC) are highly selective to the colorimetric detection of Na⁺. The ions like K⁺, Mg²⁺ and Zn²⁺ did not interfere with the Na⁺ in this sensing technique. Low-cost paper-based sensor strips are fabricated and calibrated for the sensing of sodium in the physiological range and shade cards were developed as a calorimetric guide for estimation of Na⁺ which makes them ideal point of care diagnostic platform. We demonstrate that the proposed CuC paper strip can be used for detecting Na⁺ concentration within the whole physiological range in both blood serum and urine.

Preparation of fluorescent bimetallic silver/copper nanoparticles and their utility of dual-mode fluorimetric and colorimetric probe for Hg2

A dual-mode colorimetric and fluorimetric probe was successfully established based on silver/copper bimetallic nanoparticles (AgCu-BNPs). The AgCu-BNPs were confirmed as individually bimetallic nanoparticles with a mean size of 7.7 ± 0.2 nm, as characterized by high resolution transmission electron microscopy. Intriguingly, the AgCu-BNPs possess both surface plasmon resonances (SPR) and fluorescence emission. AgCu-BNPs emanate bright blue fluorescence with optical emission centered at 442 nm with high quantum yield of 30.3%, and AgCu-BNPs were attenuated or even quenched by Hg²⁺ via both static and dynamic quenching, coincidently accompanied by a visible color change, which endow AgCu-BNPs a unique utility as dual-mode colorimetric and fluorimetric probes. The detection limits as low as 89 nM and 9 nM were determined by dual-mode of AgCu-BNPs, respectively. The recovery rates in real samples were found to be 97.3-118.8%, and 89.5-112.7% by colorimetric and fluorescent methods separately, demonstrates the good environmental tolerance of the dual-mode probe.

A Highly Sensitive and Selective Fluorescent Probe Using MPA-InP/ZnS QDs for Detection of Trace Amounts of Cu2+ in Water

Detection of copper (II) ions (Cu²⁺) in water is important for preventing them from entering the human body to preserve human health. Here, a highly sensitive and selective fluorescence probe that uses mercaptopropionic acid (MPA)-capped InP/ZnS quantum dots (MPA-InP/ZnS QDs) was proposed for the detection of trace amounts of Cu²⁺ in water. The fluorescence of MPA-InP/ZnS QDs can be quenched significantly in the presence of Cu²⁺, and the fluorescence intensity shows excellent linearity when the concentration of Cu²⁺ varies from 0–1000 nM; this probe also exhibits an extremely low limit of detection of 0.22 nM. Furthermore, a possible fluorescence-quenching mechanism was proposed. The MPA-InP/ZnS QDs probes were further applied to the detection of trace Cu²⁺ in real water samples and drink samples, showing good feasibility.

Copper nanocluster composites for analytical (bio)-sensing and imaging: a review

As an ideal substitute for traditional organic fluorescent dyes or up-conversion nanomaterials, copper nanoclusters (CuNCs) have developed rapidly and have been involved in exciting achievements in versatile applications. The emergence of novel CuNCs composites improves the poor stability and fluorescence intensity of CuNCs. With this in mind, great efforts have been made to develop a wide variety of CuNCs composites, and impressive progress has been made in the past few years. In this review, we systematically summarize absorption, fluorescence, electrochemiluminescence, and catalytic properties and focus on the multiple factors that affect the fluorescence properties of CuNCs. The fluorescence properties of CuNCs are discussed from the point of view of core size, surface ligands, self-assembly, metal defects, pH, solvent, ions, metal doping, and confinement effect. Especially, we illustrate the research progress and representative applications of CuNCs composites in bio-related fields, which have received considerable interests in the past years. Additionally, the sensing mechanism of CuNCs composites is highlighted. Finally, we summarize current challenges and look forward to the future development of CuNCs composites. Schematic diagram of the categories, possible sensing mechanisms, and bio-related applications of copper nanoclusters composites.

Self-Assembly of Amphiphilic Copper Nanoclusters Driven by Cationic Surfactants

Amphiphilicity is an excellent physicochemical property, which is yet to be explored from traditional surfactants to nanoparticles. This article shows that the amphiphilicity of copper nanoclusters (CuNCs) can be readily tuned by electrostatic interactions with cationic surfactants and cetyltrimethylammonium cations (CTA⁺) with counterions Br^-, Cl^-, and C₇H₈O₃S^-. Due to the role of surface ligands, the complexes of glutathione-capped CuNCs (GSH-CuNCs) and the surfactants exhibit good amphiphilicity, which enables them to self-assemble like a molecular amphiphile. This could significantly increase the utility of metal nanoclusters in basic and applied research. As the concentration of the surfactant changes, the aggregates change from nanoparticles to network-like structures. After the formation of supramolecular self-assemblies by hydrophobic interactions, the enhancement of fluorescence intensity was observed, which can be ascribed to the suppression of intramolecular vibrations based on aggregation-induced emission (AIE) and combined with the compactness of GSH-CuNCs in self-assemblies. Our study provides a facile way to generate solid fluorescent materials with excellent fluorescence performance, which may find applications in light-emitting diodes (LEDs).

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.

A novel"turn-off"fluorescence assay based on acid-copper nanoclusters in deep eutectic solvent micelles for co-aggregation inducing fluorescence enhancement and its application

As mixtures, deep eutectic solvent (DES) is designability. By adjusting the long alkyl chain hydrogen bond acceptors (HBAs) or hydrogen bond donors (HBDs), the DES displays surfactant characteristics and can form micelles. Hence, a novel, simple, facile and green natural organic acids capped copper nanoclusters (Aci-CuNCs) was synthesized and the spectrum behavior of Aci-CuNCs in DES micelles was researched. It was found that the surfactant-like DES can form micellar co-aggregation with Aci-CuNCs, resulting in the fluorescence (FL) intensive of Aci-CuNCs increase. Corresponding performance of spectral properties of Aci-CuNCs in DES medium were systematically studied by fourier transform infrared spectrometer, 3D FL spectroscopy, FL emission/excitation spectra, ultraviolet absorption spectroscopy. In the mechanism exploration part, on the one hand, the existence of micellar co-aggregation was confirmed by the conductivity, the mass effect of DES, dynamic light scattering and transmission electron microscopy. On the other hand, the influence of different kinds of DESs (types of HBAs/HBDs, molar ratio) and some possible factors (ionic strength and temperature) were discussed in detail to investigate the main driving forces for the formation of micellar co-aggregates. The results of mechanism exploration prove that the long alkyl chain of DES is amphiphilic which can form micellar co-aggregation with Aci-CuNCs through hydrogen bonding. The DES micelle provides Aci-CuNCs with a relatively stable and closed micro-environment which can effectively prevent collisions with water molecules and weakening of fluorescence intensity. On the basic of the above research, a "turn-off" fluorimetric method based on Aci-CuNCs in DES medium was applied for the determination of Fe³⁺. Under the optimum conditions, the assay worked in the Fe³⁺ concentration ranges from 1 to -20 μM and had a detection limit of 0.0374 μM. Method validation study illustrates the proposed system can provide a good accuracy, repeatability and stability conditions. Furthermore, the real sample analysis result demonstrates that no obvious matrix effect is found. As a consequence, the FL assays (Aci-CuNCs-based DES) composed of natural organic acid capped CuNCs and green solvent DES provides a simple, gentle and environmentally friendly method for the detection of iron ions.

A simple and convenient dry-state SEIRS method for glutathione detection based on citrate functionalized silver nanoparticles in human biological fluids

Graphical representation for determination of glutathione using citrate functionalized AgNPs enriched dry-state SEIRS method.

Deep eutectic solvent assisted synthesis of carbon dots using Sophora flavescens Aiton modified with polyethyleneimine: Application in myricetin sensing and cell imaging

Here, an efficient method for synthesizing carbon dots (CDs) using a deep eutectic solvent (DES) was developed. To investigate the influence of different DESs on the quantum yield of CDs, different hydrogen-bonding acceptors (HBAs) and hydrogen-bonding donors (HBDs) were used to synthesize the DES and prepare CDs. Using Sophora flavescens Aiton as precursor, CDs were prepared using choline chloride (ChCl)/urea based DES as reaction media and doping agent in the presence of water. The CDs showed strong blue fluorescence and were further modified with polyethyleneimine (CDs@PEI). The fluorescence intensity of CDs@PEI was selectively quenched by myricetin with a limit of detection (LOD) of 10 nM. Furthermore, CDs@PEI was used to analyze myricetin in the extracts that were fluorescent by DES with satisfactory performance of Abelmoschus manihot (Linn.) Medicus flowers, vine teas and blueberries. Finally, the bio-imaging application of CDs@PEI was tested and the results confirmed its potential application in bio-imaging.

'Turn-off' fluorescence strategy for determination of hexavalent chromium ions based on copper nanoclusters

Cu nanoclusters (CuNCs) capped by tannic acid (TA) (CuNCs@TA) can be used as a highly sensitive fluorescent probe for Cr(VI) detection. Therefore, a fluorescence detection method for Cr(VI) can be established according to the fluorescence quenching of CuNCs@TA that is caused immediately after the addition of Cr(VI). The fluorescence quenching efficiency of CuNCs@TA was linearly correlated with Cr(VI) concentration within the range 0.03-60 μM, and the detection limit for Cr(VI) was 5 nM. This method was demonstrated to be suitable for detecting Cr(VI) in actual water samples. We found that sodium thiosulfate (ST) can redox with Cr(VI) and therefore restore the fluorescence of CuNCs@TA. The mechanism of CuNCs@TA fluorescence quenching and enhancement by Cr(VI) and ST was investigated in detail. The 'turn-on' fluorescent sensor is of practical significance and has broad application prospects.

Photoluminescent hydrophilic cyclodextrin-stabilized cysteine-protected copper nanoclusters for detecting lysozyme

Lysozyme (LYZ) sensors have attracted increased attention because rapid and sensitive detection of LYZ is highly desirable for various diseases associated with humans. In this research, we designed L-cysteine-protected ultra small photoluminescent (PL) copper nanoclusters (CuNCs) conjugated with β-cyclodextrin (β-CD) for rapid detection of LYZ in human serum samples at room temperature. The proposed β-CD-CuNCs exhibited excellent water solubility, ultrafine size, good dispersion, bright photoluminescence, and good photostability. The β-CD-CuNCs exhibit an excitation and emission maximum at 370 nm and 492 nm, respectively, with an absolute quantum yield (QY) of 54.6%. β-CD-CuNCs showed a fluorescence lifetime of 12.7 ns. The addition of LYZ would result in PL quenching from β-CD-CuNCs. The lowest detectable LYZ concentration was 50 nM for the naked eye and the limit of detection (LOD) and limit of quantification (LOQ) were 0.36 nM and 1.21 nM, respectively, by emission measurement observed in the LYZ concentration range from 30 to 100 nM. It is important to note that the PL β-CD-CuNC strategy possessed great selectivity toward LYZ relative to other biomolecules. The proposed nanosensor was efficiently applied to determine the LYZ level in human serum sample average recoveries from 96.15 to 104.05% and relative standard deviation (RSD) values lower than 3.0%. Moreover, the proposed sensing system showed many advantages, including high speed, high sensitivity, high selectivity, low cost, and simple preparation.

Novel synthesis of orange-red emitting copper nanoclusters stabilized by methionine as a fluorescent probe for norfloxacin sensing

In the present work, we report a novel chemical approach for the synthesis of orange-red emitting copper nanoclusters (Cu NCs) using L-methionine as stabilizing agent at room temperature for the first time. The synthetic route is facile, economical and viable. The methionine stabilized copper nanoclusters (Cu NCs/Met) were thoroughly characterized by TEM, FT-IR, XPS, UV-Vis, steady state and transient fluorescence spectroscopy. The results show the synthesized Cu NCs/Met with a fluorescence quantum yield of 4.37% possessed high stability and excellent optical features such as large Stokes shift and long fluorescence lifetime (8.3 μs). Significantly, the fluorescence intensity of Cu NCs/Met could be efficiently quenched by norfloxacin (NOR) pharmaceutical. A fast and cost-effective NOR sensor was proposed employing Cu NCs/Met as the fluorescent nanoprobe, and the quenching mechanisms were attributed to inner filter effect and agglomeration-induced quenching. The developed sensor exhibited a high sensitivity and selectivity towards NOR in a wide linear range from 0.05 to 250 μM with a detection limit as low as 17 nM. Moreover, the practicability of the developed NOR sensor for real sample assay was validated with satisfactory recoveries, indicating this sensing platform with great potential for label-free pharmaceutical detection in complex systems.

Fluorescence turn-off-on for highly selective detection of serum l-cysteine based on AuNCs-AuNPs ensembles

The discriminative monitoring of l-cysteine in biological fluids is a great challenge. A fluorescent “turn-off-on” probe based on AuNCs-AuNPs ensembles for l-cysteine detection with high selectivity has been developed.

Recent progress in copper nanocluster-based fluorescent probing: a review

Copper nanoclusters (CuNCs) are an attractive alternative to other metal nanoclusters. The synthesis of CuNCs is highly efficient and fast, with low-cost and without any complicated manipulation. Because of their tunable fluorescence and low toxicity, CuNCs have been highly exploited for biochemical sensing. This review (with 172 refs.) summarizes the progress that has been made in the field in the past years. Following an introduction into the fundamentals of CuNCs, the review first focuses on synthetic methods and the fluorescence properties of CuNCs (with subsections on the use of proteins, peptides, DNA and other molecules as templates). This is followed by a section on the use of CuNCs in fluorometric assays, with subsections on the detection of small molecules, proteins, nucleic acids, various other biomolecules including drugs, and of pH values. A further large chapter summarizes the work related to environmental analyses, specifically on determination of metal ions, anions and pollutants. Graphical abstract Schematic representation of the synthesis and potential applications of copper nanocluster (CuNCs) in biochemical analysis, emphatically reflected in some vital areas such as small molecule analysis, biomacromolecule monitoring, cell imaging, ions detection, toxic pollutant, etc.

Fluorescence-tunable copper nanoclusters and their application in hexavalent chromium sensing

Generally, metal nanoclusters are synthesized using only a single ligand. Thus, the properties and applications of these nanomaterials are limited by the nature of the ligand used. In this study, we have developed a new synthetic strategy to prepare bi-ligand copper nanoclusters (Cu NCs). These bi-ligand Cu NCs are synthesized from copper ions, thiosalicylic acid, and cysteamine by a simple one-pot method, and they exhibit high quantum yields (>18.9%) and good photostability. Most interestingly, the fluorescence intensities and surface properties of the Cu NCs can be tailored by changing the ratio of the two ligands. Consequently, the bi-ligand Cu NCs show great promise as fluorescent probes. Accordingly, the Cu NCs were applied to the inner-filter-effect-based detection of hexavalent chromium in water. A wide linear range of 0.1–1000 μM and a low detection limit (signal-to-noise ratio = 3) of 0.03 μM was obtained. The recoveries for the real sample analysis were between 98.3 and 105.0% and the relative standard deviations were below 4.54%, demonstrating the repeatability and practical utility of this assay.

Generally, metal nanoclusters are synthesized using only a single ligand.

Preparation of nitrogen-doped carbon quantum dots and its application as a fluorescent probe for Cr(vi) ion detection

A facile synthetic method was used to prepare N-CQDs via the hydrothermal treatment of gelatin.

Size-controlled atomically precise copper nanoclusters: Synthetic protocols, spectroscopic properties and applications

Noble metal nanoclusters (NCs) are a new class of nanomaterials which are considered being a missing link between isolated metal atoms and metal nanoparticles (NPs). The sizes of the NCs are comparable to the Fermi wavelength of the conduction electrons, and this renders them to be luminescent in nature. They exhibit size-dependent fluorescence properties spanning almost the entire breath of the visible spectrum. Among all the noble metal NCs being explored, copper NCs (CuNCs) are the most rarely investigated primarily because of their propensity of getting oxidised. In this chapter, we have given a comprehensive understanding as to why these NCs are luminescent in nature. We have also given a detailed overview regarding the various templates used for the synthesis of these CuNCs along with the respective protocols being followed. The various instrumental techniques used to characterize these CuNCs are discussed which provides an in-depth understanding as to how these CuNCs can be properly examined. Finally, we have highlighted some of the most recent applications of these CuNCs which make them unique to serve as the next-generation fluorophores.

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Copper nanoclusters/polydopamine nanospheres based fluorescence aptasensor for protein kinase activity determination

A fluorescence aptasensor was constructed for protein kinase (PKA) activity detection by utilizing copper nanoclusters (CuNCs) and polydopamine nanospheres (PDANS). Through the π-π stacking interactions between adenosine triphosphate (ATP) aptamer and PDANS, the ATP aptamer modified CuNCs (apt-CuNCs) were absorbed onto PDANS surface, thus the fluorescence of apt-CuNCs were quenched through fluorescence resonance energy transfer (FRET) from apt-CuNCs to PDANS. In the presence of ATP, ATP specifically bound to aptamer, causing the dissociation of apt-CuNCs from PDANS surface and restoring the fluorescence of apt-CuNCs. However, PKA translated ATP into adenosine diphosphate (ADP), and ADP had no competence to combine with ATP aptamer, thus, apt-CuNCs were released and absorbed onto the PDANS surface to cause the fluorescence quenching of apt-CuNCs again. Therefore, PKA activity was conveniently detected via the fluorescence signal change. Under the optimal conditions, PKA activity was detected in the range of 0.05-4.5 U mL^-1 with a detection limit of 0.021 U mL^-1. Furthermore, the feasibility of the aptasensor for kinase inhibitor screening was explored via assessment of kinase inhibitor H-89 as one model. This aptasensor was also performed for PKA activity determination in HepG2 cell lysates with satisfactory results.

An ammonia-based etchant for attaining copper nanoclusters with green fluorescence emission

Luminescent copper nanoclusters (CuNCs) constitute a very active research topic due to their unique properties and lower cost than gold and silver NCs. In this study, we report a new, facile, and rapid top-down etching method for synthesizing luminescent CuNCs, using Cu nanoparticles (CuNPs) as the precursor and ammonia (NH3) as the etchant. The etching mechanism is systematically investigated and the optical and structural properties of the obtained CuNCs are carefully studied. The NH3-triggered etching process is very fast and the newly generated CuNCs can emit strong green fluorescence with a high quantum yield. Moreover, by coupling the urease-catalyzed hydrolysis of urea with the NH3-induced etching of CuNPs, we developed a novel fluorescence turn-on assay for urea. The linear range for urea detection is from 0.25 to 5 mM, and the limit of detection is 0.01 mM. This novel sensing approach, with good sensitivity and excellent selectivity, is then successfully utilized to detect urea in human serum samples, demonstrating its great potential in clinical diagnosis. In addition, the proposed coupling method can be extended to monitor other analytes that influence the size-focusing etching process, allowing metal NCs to be used to construct diverse chemosensors and biosensors.