Engineering the Self-Assembly Induced Emission of Cu Nanoclusters by Au(I) Doping

Fluorescent metal nanoclusters: From luminescence mechanism to applications in enzyme activity assays

Metal nanoclusters (MNCs) have outstanding fluorescence property and biocompatibility, which show widespread applications in biological analysis. Particularly, evaluation of enzyme activity with the fluorescent MNCs has been developed rapidly within the past several years. In this review, we first introduced the fluorescent mechanism of mono- and bi-metallic nanoclusters, respectively, whose interesting luminescence properties are mainly resulted from electron transfer between the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels. Meanwhile, the charge migration within the structure occurs through ligand-metal charge transfer (LMCT) or ligand-metal-metal charge transfer (LMMCT). On such foundation, diverse enzyme activities were rigorously evaluated, including three transferases and nine hydrolases, in turn harvesting rapid research progresses within past 5 years. Finally, we summarized the design strategies for evaluating enzyme activity with the MNCs, presented the major issues and challenges remained in the relevant research, coupled by showing some improvement measures. This review will attract researchers dedicated to the studies of the MNCs and provide some constructive insights for their further applications in enzyme analysis.

A novel fluorescent nanoprobe for sensitive detection of 6-thioguanine in human serum based on Cu/Ag nanoclusters

6-Thioguanine (6-TG) is a purine analog anticancer drug used to treat childhood acute leukemia and inflammatory bowel disease; however, the over-dosage use of 6-TG can cause serious adverse effects. Therefore, monitoring the free 6-TG concentration in the human body is critical during drug therapy. In this work, a highly sensitive and rapid fluorescent nanoprobe based on Cu/Ag nanoclusters (NCs) for the detection of 6-TG was developed. The maximum emission wavelength of Cu/Ag NCs was observed at 563 nm with an excitation wavelength of 330 nm. A selective fluorescence quenching effect of 6-TG on the Cu/Ag NCs was found. Under optimum conditions for the determination of 6-TG, a wide linear concentration range from 2.5 to 100 μmol L^-1 was observed with a limit of detection (LOD) of 1.57 μmol L^-1. The characteristics of simple operation, high sensitivity and selectivity make this fluorescent nanoprobe a promising candidate for the detection of 6-TG in biological samples, as demonstrated by the application in spiked human serum with recoveries of 97.6 to 104.8%. In general, this proposed method has good potential for the detection of 6-TG in biological samples.

Gold nanoclusters with enhanced near-infrared emission and its application as sensors for biological molecules

Ultrasmall gold nanoclusters (NCs) have been engineered as a new kind of functional material due to their excellent photoluminescence properties. However, the synthesis of highly luminescent water-soluble nanoclusters with near-infrared (NIR) emission remains limited. Herein, we developed a pH-regulated strategy to facilitate the construction of self-assemblies with enhanced luminescence based on aggregation-induced emission (AIE) strategy. Using 2-mercaptobenzoic acid (MBA) as reductant and stabilizer, the original weakly luminescent AuNCs exhibited intense emission by adjusting pH controllably. The formation of compact organized nanostructures could effectively restrict the rotation and vibration of capping ligands by non-covalent interactions, which reduced the nonradiative relaxation from excited states and finally improved the emission properties of AuNCs. Moreover, the assemblies possess many intriguing features including bright NIR luminescence and excellent biocompatibility, which could be used as luminous probes in biological molecules sensing (tyrosinase (TYR) and dopamine (DA)) and promising candidates for cell imaging. This study provides a simple and feasible strategy for developing metal NCs-based smart optical materials in the field of bioscience.

Experimental and computational insights into luminescence in atomically precise bimetallic Au6-nCun(MPA)5 (n = 0-2) clusters

Bimetallic nanoclusters (NCs) have emerged as a new class of luminescent materials for potential applications in sensing, bio-imaging, and light-emitting diodes (LEDs). Here, we have synthesized gold-copper bimetallic nanoclusters (AuCu NCs) using a one-step co-reduction method and tuned the emission wavelength from 520 nm to 620 nm by changing the [Cu²⁺]/[Au³⁺] molar ratio. The quantum yield (QY) increases from 6% to 13% upon incorporation of the Cu atom in the Au NCs. MALDI-TOF mass spectrometric analysis reveals that the composition of the Au NCs is Au₆(MPA)₅, and the bimetallic nanocluster is Au₄Cu₂(MPA)₅, where 3-mercaptopropionic acid (MPA) is used as the capping ligand. Furthermore, we investigated the optimized structures of the as-synthesized NCs using density functional theory (DFT) along with analysis of the preferable adsorption sites using Fukui functions. We report the HOMO-LUMO gap, which is consistent with the experimentally observed red shift in the UV-Vis absorption features of the Au NCs upon copper doping. XPS studies suggest the formation of intermixing of states between the 5d orbitals of Au and the 3d orbitals of Cu in the AuCu NCs after incorporating Cu atoms into the Au NCs, which is corroborated by the DFT calculations on electronic charge transfer from the Cu to the Au atom in the NCs. The coupling between Au(I) and Cu(I) facilitates the formation of a low-lying mixed Au(I)-Cu(I) energy state. This study elaborates on the impact of Cu doping on the excited-state relaxation dynamics of AuCu NCs.

Controlling the Nature of Photoluminescence of Emissive Metal Nanoclusters

Photoluminescence (PL) serves as one of the most attractive chemical-physical properties of metal nanoclusters. However, the control over the PL nature of metal nanoclusters as fluorescence or phosphorescence remains challenging. Basically, the PL nature control concerns the transition regulation of excited electrons in nanoclusters from their excited state to the ground state. Up to the present, some cases have been reported on adjusting the PL nature of emissive nanoclusters via different means, including the composition regulation, the isomerization, the aggregation, and the temperature variation. At the same time, theoretical calculations have been performed to thoroughly understand the PL nature transformation of these emissive nanoclusters in terms of their electronic structures and transition pathways. This Concept highlights and reviews the recent progress in controlling the PL nature of emissive nanoclusters as fluorescence or phosphorescence, which hopefully paves the way for fabricating novel nanoclusters or cluster-based nanomaterials with customized PL properties.

Highly Bright Gold Nanowires Arrays for Sensitive Detection of Urea and Urease

In this work, highly fluorescent gold nanowire arrays (Au NWs) are successfully synthesized by assembling Zn²⁺ ions and non-emissive oligomeric gold-thiolate clusters using mercaptopropionic acid both as a reducing agent and a growth ligand. The synthesized Au NWs exhibited strong bluish green fluorescence with an absolute quantum yield up to 32% and possessed ultrasensitive pH stimuli-responsive performance in the range of 7.0-7.8. Based on the excellent properties of the as-prepared nanowire arrays, we developed a facile, sensitive, and selective fluorescent method for quantitative detection of urea and urease. The fabricated nanoprobe showed superior biosensing response characteristics with good linearities in the range of 0-100 μM for urea concentration and 0-12 U/L for urease activity. In addition, this fluorescent probe afforded relatively high sensitivity with the detection limit as low as 2.1 μM and 0.13 U/L for urea and urease, respectively. Urea in human urine and urease in human serum were detected with satisfied results, exhibiting a promising potential for biomedical application.

Real-Time Fluorescent Monitoring of Kinetically Controlled Supramolecular Self-Assembly of Atom-Precise Cu8 Nanocluster

Kinetically stable and long-lived intermediates are crucial in monitoring the progress and understanding of supramolecular self-assembly of diverse aggregated structures with collective functions. Herein, the complex dynamics of an atomically precise Cu^I nanocluster [Cu₈ (^t BuC₆ H₄ S)₈ (PPh₃ )₄ ] (Cu8a) is systematically investigated. Remarkably, by monitoring the aggregation-induced emission (AIE) and electron microscopy of the kinetically stable intermediates in real time, the directed self-assembly (DSA) process of Cu8a is deduced. The polymorphism and different emission properties of Cu NCs aggregates were successfully captured, allowing the structure-optical property relationship to be established. More importantly, the utilization of a mathematical "permutation and combination" ideology by introducing a heterogeneous luminescent agent of a carbon dot (CD) to Cu8a aggregates enriches the "visualization" fluorescence window, which offers great potential in real time application for optical sensing of materials.

Self-Assembled Metal Nanoclusters: Driving Forces and Structural Correlation with Optical Properties

Studies on self-assembly of metal nanoclusters (MNCs) are an emerging field of research owing to their significant optical properties and potential applications in many areas. Fabricating the desired self-assembly structure for specific implementation has always been challenging in nanotechnology. The building blocks organize themselves into a hierarchical structure with a high order of directional control in the self-assembly process. An overview of the recent achievements in the self-assembly chemistry of MNCs is summarized in this review article. Here, we investigate the underlying mechanism for the self-assembly structures, and analysis reveals that van der Waals forces, electrostatic interaction, metallophilic interaction, and amphiphilicity are the crucial parameters. In addition, we discuss the principles of template-mediated interaction and the effect of external stimuli on assembly formation in detail. We also focus on the structural correlation of the assemblies with their photophysical properties. A deep perception of the self-assembly mechanism and the degree of interactions on the excited state dynamics is provided for the future synthesis of customizable MNCs with promising applications.

Using bimetallic Au/Cu nanoplatelets for construction of facile and label-free inner filter effect-based photoluminescence sensing platform for sarcosine detection

Herein, we report a facile and label-free method for sensitive and specific determination of prostate cancer biomarker sarcosine via using photoluminescent bimetallic Au/Cu nanoplatelets (AuCu NPs) to construct an inner filter effect (IFE)-based photoluminescence (PL) sensing platform. The AuCu NPs were formed by the cysteine-induced co-reduction reaction, which displayed bright PL with an emission peak at 560 nm. Meanwhile, the Cu(I) doping caused a maximum 25-fold enhancement of quantum yield (QY), compared with the native Au(I) complexes, i.e., from 0.85 to 21.5%. By integrating the AuCu NPs with p-phenylenediamine (PPD) oxidation reaction, an IFE-based sensor for sarcosine detection was constructed. In this method, sarcosine is oxidized under the catalysis of sarcosine oxidase (SOx) to yield H₂O₂. The latter further oxidizes PPD to form 2,5-diamino-N,N'-bis(p-aminophenyl)-l,4-benzoquinone di-imine (PPDox) in the presence of horseradish peroxidase (HRP). The UV-vis absorption spectrum of the PPDox can overlap well with the excitation and emission spectra of the AuCu NPs, resulting in the efficient quenching of the AuCu NPs via the IFE effect. Therefore, this IFE-based AuCu NPs/SOx/PPD/HRP sensing platform can be used for highly sensitive and specific sensing of sarcosine. The sensing platform showed two linear regions of the PL intensity of the AuCu NPs versus the concentration of sarcosine in the range of 0.5-5 μM and 5-100 μM with a detection limit (LOD) of 0.12 μM (S/N = 3). Furthermore, this IFE-based sensing platform could be developed into a paper-based biosensor for simple, instrument-free, and visual detection of sarcosine.

Multifunctional Host Polymers Assist Au Nanoclusters Achieve High Quantum Yield and Mitochondrial Imaging

The high biocompatibility and excellent photostability of Au nanoclusters (AuNCs) make them stand out in the bioimaging of nanoparticles. However, the low quantum yield and inferior targeting ability of water-soluble AuNCs greatly limit their biological applications. In this study, we designed and synthesized multifunctional host polymers PolySC₄AP and FGGC@AuNCs to fabricate PolySC₄AP/FGGC@AuNC assemblies via a host-guest interaction based on SC₄ (sulfonatocalix[4]arene) and positively charged FGGC ligands (phenylalanine-glycine-glycine-cysteine). Owing to the host-guest assembly strategy and rigid polymer matrix, the quantum yield of FGGC@AuNCs was significantly promoted from 7.0 to 35.3%, accompanied by considerable morphological changes of FGGC@AuNCs. Moreover, PolySC₄AP/FGGC@AuNCs could monitor the location of mitochondria along with R (Pearson's correlation coefficients) value for the co-localization as high as 0.9605, which provided a novel strategy for targeted bioimaging with luminophore.

Multistimuli-Responsive and Antifreeze Aggregation-Induced Emission-Active Gels Based on CuNCs

Multistimuli-responsive fluorescent gelsbased small molecular gelator by supramolecular assembly, possessing excellent dynamic and reversible characteristic, have caused much concern. In this article, aggregation-induced emission-active fluorescence gels (AIE-gels) with chirality were developed by combining Cu nanoclusters (CuNCs) and natural amino acids, l-tryptophan (l-Trp) or d-Tryptophan (d-Trp). In DMSO/H₂O mixed solvents, CuNCs can self-assemble to form intertwined fibersbased nanoparticles with numerous pores by introducing Zn²⁺. Fibers as second networks of heteronetwork structures are characterized with the participation of l-Trp or d-Trp for cross-linking to reinforce mechanical strength and chiral regulation of gel networks. Aggregation-induced emission enhancement (AIEE) of CuNCs endows the gels with excellent fluorescent properties by introducing solvents and gelation process. The fluorescent gels exhibit sufficient fluorescence intensity (FI) at -20 °C to -80 °C and possess sensitive responsibility including gel-sol transition and fluorescence behavior for stimuli of mechanical force, heating, pH, H₂O₂, and ethylene diamine tetraacetic acid (EDTA).

Immunoassay based on Au-Ag bimetallic nanoclusters for colorimetric/fluorescent double biosensing of dicofol

The high toxicity of dicofol (DICO) to nontarget organisms has resulted in the contamination of food materials and caused a threat to human health. Developing a rapid and sensitive detection method of DICO in food samples is essential and still pursued. Fluorescent nanomaterials have been widely applied in biosensors to improve the sensitivity of detection. Herein, glutathione-capped Au-Ag bimetallic nanoclusters (Au-Ag NCs) exhibited the outstanding fluorescence characteristic with the average fluorescence lifetime of 1971.08 ns and photoluminescence quantum yield of 9.84% when the molar ratio of Au to Ag was 5:1. Polyethyleneimine modified gold nanoparticles (PEI-Au NPs) with the positive charge were prepared to generate a strong colorimetric signal. A dual-model colorimetric/fluorescent immune probe based on the Au-Ag NCs and PEI-Au NPs was successfully constructed by electrostatic force, and could be applied in both ic-ELISA and LFIA methods for rapid and ultrasensitive detection of DICO. In the ic-ELISA method, the introduction of fluorescence signal significantly increased the sensitivity of detection with the limit of detection (LOD) of 0.62 ng/mL and exhibited an excellent linear relationship within the range of 1.36 ng/mL-19.92 ng/mL. In the LFIA method, the fluorescence signal of Au-Ag NCs was accumulated on the test line and control line for the fluorescence model detection with a quantitative LOD at the level of 1.59 ng/mL. Such a dual-model colorimetric/fluorescent immunoassay serves as a promising candidate to develop new approaches in field detection.

Strongly phosphorescent and water-soluble gold(I)-silver(I)-cysteine nanoplatelets via versatile small biomolecule cysteine-assisted synthesis for intracellular hypochlorite detection

In biological systems, abnormal levels of hypochlorite (ClO^-) could result in cell dysfunctions. Herein, we report a facile, one-step and green approach based on the versatile small biomolecule cysteine both serving as reducing agent and ligand for synthesizing the strongly photoluminencent and water-soluble Au(I)-Ag(I)-cysteine complexes nanoplatelets (Au(I)-Ag(I)-Cys nanoplatelets) for intracellular hypochlorite detection. Multiple spectroscopic and microscopical tools have been used to characterize the resultant Au(I)-Ag(I)-Cys nanoplatelets. It was found that with the cysteine-assisted synthesis approach, the Ag(I) doping to the Au(I) complexes could form the supramolecular organometallic nanoplatelets. Inside, the Au(I)-Ag(I) metallophilic interactions showing an Au to Ag charge transfer property were formed, thereby enhancing the photoluminescence (PL) intensity via the charge transfer from the bioligand's S to the metal-metal center. The quantum yield (QY) was measured to show a maximum 16-fold enhancement (i.e., from 0.85 to 13.8%). Interestingly, in the presence of ClO^-, the metal-thiolate ligand structure of the as-synthesized Au(I)-Ag(I)-Cys nanoplatelets could be oxidatively damaged, causing the PL quenching, thereby producing the effect of biorecognition towards ClO^- anions. The ClO^--induced PL quenching produced two linear regions at ClO^- concentrations of 0.01-5.0 μM and 5.0-1000 μM with a limit of detection (LOD) of 8.0 nM (S/N = 3). The ClO^--induced PL quenching was specific over the other typical reactive oxygen species (ROS) and the potential interfering substances in biological samples. In addition, the Au(I)-Ag(I)-Cys nanoplatelets had good biocompatibility. Thus, they could be further developed as a biosensor for detecting endogenous ClO^- anions in living cells.

Intrinsic-to-extrinsic emission tuning in luminescent Cu nanoclusters by in situ ligand engineering

Enhancement of the emission quantum yield and expansion of the emission tunability spectrum are the key aspects of an emitter, which direct the evolution of future generation light harvesting materials. In this regard, small molecular ligand-protected Cu nanoclusters (SLCuNCs) have emerged as prospective candidates. Herein, we report the broadband emission tunability in a SLCuNC system, mediated by in situ ligand replacement. 1,6-Hexanedithiol-protected blue emissive discrete Cu nanoclusters (CuNCs) and red emissive CuNC assemblies have been synthesized in one pot. The red emissive CuNC assemblies were characterized and found to be covalently-linked nanocluster superstructures. The blue emissive CuNC was further converted to a green-yellow emissive CuNC over time by a ligand replacement process, which was mediated by the oxidized form of the reducing agent used for synthesizing the blue emissive nanocluster. Steady-state emission results and fluorescence dynamics studies were used to elucidate that the ligand replacement process not only modulates the emission color but also alters the nature of emission from metal-centered intrinsic to ligand-centered extrinsic emission. Moreover, time-dependent blue to green-yellow emission tunability was demonstrated under optimized reaction conditions.

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.

Facile preparation of silver nanocluster self-assemblies with aggregation-induced emission by equilibrium shifting

Aggregation-induced emission is a promising pathway to get high photoluminescence from metal nanocluster assemblies. The self-assembly of metal nanoclusters with regular morphologies can restrict the rotation and vibration modes of capping ligands, reduce nanoclusters' non-radiative decay, and finally result in an aggregation-induced strong emission. In this study, silver nanocluster self-assemblies stabilized by thiosalicylic acid (TSA) were prepared in water by equilibrium shifting, which exhibit nanofiber-like morphologies. The resulting silver nanocluster self-assemblies exhibit aggregation-induced emission in solid or aggregated state with a decent quantum yield i.e., 13.05%. The obtained silver nanocluster self-assemblies were thoroughly characterized by fluorescence spectroscopy, UV-visible absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), matrix assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF), powder X-ray diffraction (PXRD) and high-resolution transmission electron microscopy (HRTEM). These silver nanocluster self-assemblies with high photoluminescence in aggregated state may have potential use in light emitting devices and bioapplications.

Heterometallation of Photoluminescent Silver(I) Sulfide Nanoclusters Protected by Octahedral Iridium(III) Thiolates

The recently-increasing interest in coinage metal clusters stems from their photophysical properties, which are controlled via heterometallation. Herein, we report homometallic Ag^I ₄₆ S₁₃ clusters protected by octahedral fac-[Ir(aet)₃ ] (aet=2-aminoethanethiolate) molecules and their conversion to heterometallic Ag^I ₄₃ M^I ₃ S₁₃ (M=Cu, Au) clusters. The reactions of fac-[Ir(aet)₃ ] with Ag⁺ and penicillamine produced [Ag₄₆ S₁₃ {Ir(aet)₃ }₁₄ ]²⁰⁺ ([1]²⁰⁺ ), where a spherical Ag^I ₄₆ S₁₃ cluster is covered by fac-[Ir(aet)₃ ] octahedra through thiolato bridges. [1]²⁰⁺ was converted to [Ag₄₃ M₃ S₁₃ {Ir(aet)₃ }₁₄ ]²⁰⁺ ([1^M ]²⁰⁺ ) with an Ag^I ₄₃ M^I ₃ S₁₃ cluster by treatment with M⁺ , retaining its overall structure. [1]²⁰⁺ was photoluminescent and had an emission band ca. 690 nm that originated from an S-to-Ag charge transfer. While [1^Cu ]²⁰⁺ showed an emission band with a slightly higher energy of ca. 650 nm and a lower quantum yield, the emission band for [1^Au ]²⁰⁺ shifted to a much higher energy of ca. 590 nm with an enhanced quantum yield.

Self-Assembly of Precision Noble Metal Nanoclusters: Hierarchical Structural Complexity, Colloidal Superstructures, and Applications

Ligand protected noble metal nanoparticles are excellent building blocks for colloidal self-assembly. Metal nanoparticle self-assembly offers routes for a wide range of multifunctional nanomaterials with enhanced optoelectronic properties. The emergence of atomically precise monolayer thiol-protected noble metal nanoclusters has overcome numerous challenges such as uncontrolled aggregation, polydispersity, and directionalities faced in plasmonic nanoparticle self-assemblies. Because of their well-defined molecular compositions, enhanced stability, and diverse surface functionalities, nanoclusters offer an excellent platform for developing colloidal superstructures via the self-assembly driven by surface ligands and metal cores. More importantly, recent reports have also revealed the hierarchical structural complexity of several nanoclusters. In this review, the formulation and periodic self-assembly of different noble metal nanoclusters are focused upon. Further, self-assembly induced amplification of physicochemical properties, and their potential applications in molecular recognition, sensing, gas storage, device fabrication, bioimaging, therapeutics, and catalysis are discussed. The topics covered in this review are extensively associated with state-of-the-art achievements in the field of precision noble metal nanoclusters.

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).

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.

Achieving full-color emission of Cu nanocluster self-assembly nanosheets by the virtue of halogen effects

Fluorescent Cu nanoclusters (NCs) have shown potential in lighting and display, because Cu is cheap and easily available. Despite recent successes in improving the emission intensity of Cu NCs on the basis of aggregation-induced emission enhancement and self-assembly-induced emission enhancement, the difficulty in tuning the emission color sheds the doubt for achieving high-performance white light-emitting diodes (WLEDs). In this work, halogen effects are utilized to tune the emission color of Cu nanocluster self-assembly nanosheets (NSASs). By altering the adsorbed halogens from Cl, Br to I, the emission peak of Cu NSASs is tunable from 495 to 674 nm. In this context, halogen atoms are capable of improving the charge transfer and molecular spin coupling of Cu NCs, and thereby narrow the S0T1 gap and facilitate the intersystem crossing of excitons from a singlet to triplet state. As a result, emission spectra redshift and the population of the exiton recombination via the triplet state pathway is increased, which leads to the improvement of the photoluminescence quantum yield (PLQY). By simply introducing and/or mixing different types of cuprous halides, Cu nanocluster co-assembly nanosheets (NCASs) with full-color emission are obtained. The as-prepared Cu NSASs and NCASs are further employed to fabricate monochrome and white LEDs.

Fluorescent nanocomposites based on gold nanoclusters for metal ion detection and white light emitting diodes

Gold nanoclusters (AuNCs) are among the most promising organic-inorganic hybrid luminescent materials for various applications. The current development of AuNCs majorly focuses on controlling their luminescence properties. Herein, we report a new strategy to facilely construct two different nanocomposites featuring enhanced photoluminescence based on mercaptopropionic acid-protected AuNCs (MPA-AuNCs). Through co-assembly with Zn2+ and 2-methylimidazole (2M-IM), the weak luminescence of MPA-AuNCs evolved into either intense blue-green or orange emission at different concentration ratios of additives. HR-TEM and spectroscopic characterization studies revealed that the intense blue-green emission was ascribed to the formation of ZnS quantum dots (QDs) on the outer surface of AuNCs (AuNCs@ZnS), while the strong orange emission originated from the primitive MPA-AuNC core encapsulated by a cubic ZIF-8 shell (AuNCs@ZIF-8). The AuNCs@ZnS nanocomposite was further applied as an exceptional chemical sensor for selective detection of Pb2+ and Fe3+via different quenching mechanisms, and the AuNCs@ZIF-8 composite was applied for fabricating light-converting devices. The co-assembly of AuNCs with Zn2+ and imidazole derivatives provides a facile strategy for acquiring differentiated nanomaterials that have versatile potential applications in chemical detection and light-converting devices.

Plasma-induced black bismuth tungstate as a photon harvester for photocatalytic carbon dioxide conversion

The Bi QDs are reduced in situ on the surface of black Bi₂WO₆ nanosheets using a novel plasma treatment, which shows a superior CO₂ conversion performance.

Progressive aggregation-induced emission strategy for imaging of aluminum ions in cellular microenvironment

A progressive aggregation-induced emission (AIE) strategy is established based on two diverse stimulus-responsive patterns of copper nanoclusters (CuNCs) for imaging of aluminum ions (Al³⁺) in cellular microenvironment. The non-emissive CuNCs were facilely synthesized with l-glutathione (GSH) as both stabilizing agent and reducing agent, and demonstrated the excellent AIE characteristics in the ethanol/water mixture. Moreover, the dispersed CuNCs can be aggregated to give the AIE behavior in aqueous solutions by reducing the pH value, and could be further aggregated with 94-fold reinforce by introducing Al³⁺ ascribe to the strong coordination ability between Al³⁺ and the functional groups of GSH, demonstrating the progressive AIE process. Under endocytosis, the progressive AIE strategy can be employed to distinguish the Al³⁺ in the locations of lysosome against other organelles due to the acidic microenvironment of lysosome. The progressive AIE advantages of CuNCs provide a new concept for signal transduction, and have the promising applications in decoding the functions of intracellular biomolecules.

Origin of the Photoluminescence of Metal Nanoclusters: From Metal-Centered Emission to Ligand-Centered Emission

Recently, metal nanoclusters (MNCs) emerged as a new class of luminescent materials and have attracted tremendous interest in the area of luminescence-related applications due to their excellent luminous properties (good photostability, large Stokes shift) and inherent good biocompatibility. However, the origin of photoluminescence (PL) of MNCs is still not fully understood, which has limited their practical application. In this mini-review, focusing on the origin of the photoemission emission of MNCs, we simply review the evolution of luminescent mechanism models of MNCs, from the pure metal-centered quantum confinement mechanics to ligand-centered p band intermediate state (PBIS) model via a transitional ligand-to-metal charge transfer (LMCT or LMMCT) mechanism as a compromise model.

In situ synthesis of stretchable and highly stable multi-color carbon-dots/polyurethane composite films for light-emitting devices

Multi-color-emissive fluorescent polymer nanocomposite films have potential applications in optoelectronic devices. Herein, stretchable, mechanically stable multi-color carbon-dots-based films are in situ fabricated by condensation and aging of carboxylated polyurethane in the presence of various carbon sources. As-prepared CDs/PU films emit different colors covering from blue (414 nm) to red (620 nm) by tuning reaction conditions. Moreover, CDs are fixed and have good dispersion in the PU matrix due to the interactions of amine groups from the carbon sources with the carboxylate group of PU. Thus, phase separation of composite films can be avoided. And, more than 90% of their emission intensity is preserved after soaking in water for 30 days, aging for up to 6 h at 100 °C, and subjecting to several cycles of stretching and natural recovery. These advantages are encouraging for the use of CDs/PU composite films in solid-state lighting applications. Remote multi-color LEDs have been fabricated by placing a down-conversion layer of CDs/PU films separated through coating them on the same chips (emission at 365 nm), with Commission Internationale de l'Eclairage color coordinates of (0.22, 0.23), (0.43, 0.53), (0.49, 0.46), and (0.41, 0.28), respectively.

Stretchable, mechanically stable multi-color carbon-dots-based polymer films are in situ fabricated, and showed potential for application in optoelectronic devices.

Structure-dependent luminescent copper nanoclusters: self-assembly and morphological evolution

Highly ordered metal nanoclusters (NCs) have attracted much attention due to their controllability and regularity.

An overview on the current understanding of the photophysical properties of metal nanoclusters and their potential applications

Photophysics of atomically precise metal nanoclusters (MNCs) is an emerging area of research due to their potential applications in optoelectronics, photovoltaics, sensing, bio-imaging and catalysis. An overview of the recent advances in the photophysical properties of MNCs is presented in this review. To begin with, we illustrate general synthesis methodologies of MNCs using direct reduction, chemical etching, ligand exchange, metal exchange and intercluster reaction. Due to strong quantum confinement, the NCs possess unique electronic properties such as discrete optical absorption, intense photoluminescence (PL), molecular-like electron dynamics and non-linear optical behavior. Discussions have also been carried out to unveil the influence of the core size, nature of ligands, heteroatom doping, and surrounding environments on the optical absorption and photophysical properties of metal clusters. Recent findings reveal that the excited-state dynamics, nonlinear optical properties and aggregation induced emission of metal clusters offer exciting opportunities for potential applications. We discuss briefly about their versatile applications in optoelectronics, sensing, catalysis and bio-imaging. Finally, the future perspective of this research field is given.

Self-Assembly of Metal Nanoclusters for Aggregation-Induced Emission

Aggregation-induced emission (AIE) is an intriguing strategy to enhance the luminescence of metal nanoclusters (NCs). However, the morphologies of aggregated NCs are often irregular and inhomogeneous, leading to instability and poor color purity of the aggregations, which greatly limit their further potential in optical applications. Inspired by self-assembly techniques, manipulating metal NCs into well-defined architectures has achieved success. The self-assembled metal NCs often exhibit enhancing emission stability and intensity compared to the individually or randomly aggregated ones. Meanwhile, the emission color of metal NCs becomes tunable. In this review, we summarize the synthetic strategies involved in self-assembly of metal NCs for the first time. For each synthetic strategy, we describe the self-assembly mechanisms involved and the dependence of optical properties on the self-assembly. Finally, we outline the current challenges to and perspectives on the development of this area.

Stimuli-Responsive Luminescent Copper Nanoclusters in Alginate and Their Sensing Ability for Glucose

Visually observable pH-responsive luminescent materials are developed by integrating the properties of aggregation-induced emission enhancement of Cu nanoclusters (NCs) and the Ca²⁺-triggered gelatin of alginate. Sodium alginate, CaCO₃ nanoparticles, and Cu NCs are dispersed in aqueous solution, which is in a transparent fluid state, showing weak photoluminescence (PL). The introduced H⁺ can react with the CaCO₃ nanoparticles to produce free Ca²⁺, which can cross-link the alginate chains into gel networks. Meanwhile, a dramatic increase in the PL intensity of Cu NCs and a blue shift in the PL peak appeared, assigned to the Ca²⁺-induced enhancement and gelatin-induced enhancement, respectively. Their potential application as a sensor for glucose is also demonstrated based on the principle that glucose oxidase can recognize glucose and produce H⁺, which further triggers the above-mentioned two-stage enhancement. A linear relationship between the PL intensity and the concentration of glucose in the range of 0.1-2.0 mM is obtained, with the limit of detection calculated as 3.2 × 10^-5 M.

Engineering the Self-Assembly Induced Emission of Copper Nanoclusters as 3D Nanomaterials with Mesoporous Sphere Structures by the Crosslinking of Ce3

Aggregation-induced emission has provided fluorescence enhancement strategies for metal nanoclusters. However, the morphology of the aggregated nanoclusters tended to be irregular due to the random aggregated route, which would result in the formation of an unstable product. Herein, copper nanoclusters were directly synthesized by using l-cysteine as both the reducing and protection ligand. Initially, the structure of the product was irregular. Furthermore, Ce3+ was introduced to re-arrange the aggregates through a crosslinking avenue. It was interesting to find that well-ordered three-dimensional nanomaterials with mesoporous sphere structures were obtained after re-aggregation. On the basis of the stability test at a relatively high temperature and the light-emitting diode fabrication investigation, it revealed that the regulated product demonstrated more promising stability and color purity for practical applications than the random aggregated product with irregular structures.

Self-Assembly Driven Aggregation-Induced Emission of Copper Nanoclusters: A Novel Technology for Lighting

Because of the specific properties including HOMO-LUMO electronic transition, size-dependent fluorescent emission, and intense light absorption, metal nanoclusters (NCs) have been considered to be one of the most competitive color conversion materials in light-emitting diodes (LEDs). However, the monotonous emission color and the low emission stability and intensity of individual metal NCs strongly limit their universal application. Inspired by the concept of "aggregation-induced emission" (AIE), the utilization of highly ordered metal NC assemblies opens a door to resolve these problems. After self-assembly, the emission stability and intensity of metal NC assemblies are enhanced. At the same time, the emission color of metal NC assemblies become tunable. We termed this process as self-assembly driven AIE of metal NCs. In this review, we use Cu NCs as the example to convey the concept that the compact and ordered arrangement can efficiently improve the metal NCs' emission stability, tunability, and intensity. We first introduce the synthesis of 2D Cu NC self-assemblies and their emissions. We further summarize some of the factors that can affect the emissions of 2D Cu NC self-assemblies. We then discuss the utilization of 2D Cu NC self-assemblies as color conversion materials for LEDs. At last, we outline current challenges and our perspectives on the development of this area.

d-Penicillamine-coated Cu/Ag alloy nanocluster superstructures: aggregation-induced emission and tunable photoluminescence from red to orange

The aggregation-induced emission (AIE) behavior of metal nanoclusters has attracted much attention owing to their extensive application prospects in bio-imaging and chemical sensors. However, the intrinsic mechanism of metal nanoclusters' aggregation-induced emission is still not very clear. Herein, Cu nanoclusters S1 and Ag(i)-doped Cu/Ag nanoclusters S2 and S3 coated with d-penicillamine are designed and synthesized by a self-assembly strategy. S1-S3 show strong luminescence properties with luminescence quantum yields as high as 11.4%-14.2%. Moreover, their luminescence peak position shows an obvious hypsochromic shift from 615 (S1) to 570 nm (S3). With the introduction of Ag(i) ions, the assembly morphology also shows clear changes from the irregular assembly structure (S1) to large spherical particles with the average size of 0.18 μm for S2 and 0.47 μm for S3. A detailed investigation of high-resolution transmission electron microscopy (HRTEM) images, X-ray diffraction (XRD) patterns, ESI-TOF-mass spectra, UV-vis absorption spectra, photoluminescence spectra and luminescence lifetimes of Cu/Ag nanoclusters S2 and S3 indicates that Cu/Ag nanoclusters are actually a Cu/Ag alloy nanocluster superstructure and the microscopic arrangement of S2 and S3 is more compact and ordered relative to S1. The more compact and ordered Cu/Ag alloy nanocluster superstructure enhances the metal-metal interaction of inter-nanoclusters and intra-nanoclusters and facilitates the radiative transition of ligand-to-metal charge transfer (LMCT) and/or ligand-to-metal-metal charge transfer (LMMCT), which results in the aggregation-induced emission phenomenon. In addition, an enhanced metal-metal interaction increases the average metal-metal distance of the Cu/Ag nanocluster superstructure, leading to a hypsochromic shift of emission spectra. Furthermore, the Cu/Ag alloy nanoclusters show good stability and reversibility on pH cycling between pH = 3 and 7 and temperature cycling between 5 °C and 48 °C. The Cu/Ag alloy nanoclusters can be used as probes for Ag(i) ion and halide anion detection in real water samples by the ratiometric PL method (I₅₇₀/I₆₁₅).

Modulation of the optical color of Au nanoclusters and its application in ratiometric photoluminescence detection

A novel strategy for the optical color modulation of glutathione stabilized Au nanoclusters is reported and applied in ratiometric photoluminescence detection.