Luminescent Metal Nanoclusters with Aggregation-Induced Emission

A new luminescent nickel nanocluster with solvent and ion induced emission enhancement toward heavy metal analysis

Expanding the family of fluorescent metal clusters beyond gold, silver, and copper has always been an issue for researchers to solve. In this study, a novel type of cysteine-capped nickel nanoclusters (Cys-Ni NCs) with bright turquoise emission was developed. The as-synthesized Ni NCs showed aggregation-induced emission enhancement (AIEE) properties across Cd2+ and various polar organic solvents. Concurrently, solvents with different viscosities were used to explore the principle of solvent-induced AIEE of Cys-Ni NCs, revealing a positive correlation between fluorescence intensity and solution viscosity. In addition, the concentration of Cd2+ that induced the AIEE effect was reduced by nearly two orders of magnitude in highly viscous solvents, indicating the possibility of Cys-Ni NCs as a promising nanomaterial platform for Cd2+ sensing analysis. Moreover, we propose a novel fluorescent sensing method for rapid detection of Cu2+ based on the carboxyl group of Cys-Ni NCs coupling with Cu2+. Further, validation of Cu2+ detecting methodologies in environmental water samples with the accuracy up to 93.94% underscores their potential as robust and efficient sensing platforms. This study expands the repertoire of fluorescent metal nanoclusters for highly sensitive and selective sensing of hazardous ions and paves the way for further exploration and wide applications in Cu2+ detection in biological and medicine fields.

Superstructures of copper nanoclusters as NIR TADF emitters: solvent-dependent optical and morphological modulation

Herein, we report 2-mercaptopyridine-templated copper nanoclusters (CuNCs) which display near infra-red (NIR) emission, both in the solid and colloidal states. Interestingly, the NIR emission can be modulated to orange emission by preparing the CuNCs in a mixed solvent system of chloroform and methanol instead of water. The drastic change in the photo-physical properties of the CuNCs when prepared in two different solvent systems is accompanied by a unique morphological tuning. Further studies reveal that the strong NIR emission is the result of thermally activated delayed fluorescence (TADF) which is confirmed by the long excited state lifetime (∼4 μs at room temperature), time resolved emission spectroscopy (TRES) measurements, temperature-dependent photoluminescence studies, temperature-dependent lifetime studies, and excitation-transmittance dependent TRES intensity measurements. The CuNCs exhibit an exceptionally small singlet-triplet energy gap of 58.2 meV, indicating a highly efficient TADF in the system. Moreover, the solvent-dependent morphological tuning of the nanocluster superstructures rendering a drastic change in the photo-physical signatures is the consequence of different ΔE(S1-T1) values for the CuNCs in different solvent environments. Further findings corroborate that the electronic structure of the surface ligands can also help us to tune the ΔE(S1-T1) energy gap for these nanoclusters.

Modulating the photodynamic modality of Au22 nanoclusters through surface conjugation of arginine for promoted healing of bacteria-infected wounds

Developing novel antibacterial agents without drug resistance is highly desired but challenging. In this study, an Au nanocluster (NC)-based photodynamic antibacterial agent with aggregation-induced emission (AIE) has been designed to promote the healing of bacteria-infected wounds by conjugating arginine (Arg) on the surface of Au22 NCs. The conjugation of Arg not only endows the NCs with enhanced visible light absorption, increased photoluminescence (PL) intensity, and prolonged PL lifetime, but it also enables switching the photodynamic production mode of reactive oxygen species (ROS) and extra production of reactive nitrogen species (RNS). These enhancements allow the Arg-Au22 NCs to combine ROS/RNS-mediated antibacterial action with the enhanced inherent antibacterial properties of Au NCs, resulting in outstanding antibacterial efficacy against both Gram-negative and Gram-positive bacteria. In vivo experiments demonstrate the effective treatment of bacteria-infected wounds by the Arg-Au22 NCs, leading to the photodynamic eradication of bacterial infections and reduced inflammation in the wound area without causing systemic harm or impairing blood and liver functions. This study introduces a novel approach to designing metal NC-based photodynamic antibacterials with multiple antibacterial actions, contributing to deeper understanding of ROS/RNS-mediated antibacterial mechanisms, and future utilization of metal NCs in antibacterial therapies.

Luminescent metal nanoclusters and their application in bioimaging

Owing to their unique optical properties and atomically precise structures, metal nanoclusters (MNCs) constitute a new generation of optical probe materials. This mini-review provides a brief overview of luminescence mechanisms and modulation methods of luminescent metal nanoclusters in recent years. Based on these photophysical phenomena, the applications of cluster-based optical probes in optical bioimaging and related sensing, disease diagnosis, and treatment are summarized. Some challenges are also listed at the end.

White Light Emission from Zn(II) and DMSO-Induced Copper Nanocluster Assembly

An assembly of metal nanoclusters driven by appropriate surface ligands and solvent environment may engender entirely new photoluminescence (PL). Herein, we first synthesize histidine (His) stabilized copper nanoparticles (CuNPs) and, subsequently, copper nanoclusters (CuNCs) from it using 3-mercaptopropionic acid (MPA) as an etchant. The CuNCs originally emit bluish-green (λem=470 nm) PL with a low quantum yield (QY∼1.8 %). However, it transformed into a dual-emissive nanocluster assembly (Zn-CuNCs) in the presence of Zn(II) salt, having a distinct blue emission band (λem=420 nm) and a red emission band (λem=615 nm) with eight times QY (∼9.1 %) enhancement. The temperature-dependent emission spectra of Zn-CuNCs depicted that the blue emission band persists for all the temperature ranges (0-80 °C) while the red emission band vanishes at high temperatures (70-80 °C). Thus, the blue emission may originate from the locally excited state (LES) emission of the nanoclusters, while the red emission originates from through-space interaction (TSI) and Cu(I)…Cu(I) interaction within the assembly. Adding dimethyl sulfoxide (DMSO) further modifies the emission intensities; the red band was amplified four times, while the blue band was diminished by 2.5 times. The transmission electron microscopy (TEM) images unveiled that the Zn-CuNCs are a large assembly of tiny nanoclusters, which become more compact in DMSO. The blue emission possesses steady-state fluorescence anisotropy, while the red emission shows no anisotropy. Further, near-perfect white light emission(WLE) was rendered with CIE coordinates of (0.33, 0.32) by combining the dual emission of the Zn-CuNCs with the original green emission of the CuNCs.

Application of carbon quantum dots as fluorescent probes in the detection of antibiotics and heavy metals

Carbon quantum dots (CQDs) are ideal fluorescent probes for rapid detection. This paper reviews the synthesis methods of CQDs, their application in the rapid detection of antibiotics and heavy metals in the environment and food, and the underlying detection mechanisms. The hydrothermal method is the most commonly used for synthesis, and CQDs doped with heteroatoms (such as N, P and S) exhibit superior fluorescence performance. In the presence of antibiotics and heavy metals, the fluorescence of CQDs can be quenched or enhanced. Single-signal and dual-signal probes can be developed using the fluorescence, phosphorescence and absorbance of CQDs, enabling rapid detection of various antibiotics (e.g., tetracycline, quinolone and beta-lactam antibiotics) and heavy metals (e.g., Cd2+, Cr6+, Fe3+, Hg2+, and Pb2+). With the combination of smartphones and fluorescent probe test strips developed based on CQDs, on-the-spot rapid detection can be realized. This review offers new insights into the rapid detection of CQDs.

Interplay of kernel shape and surface structure for NIR luminescence in atomically precise gold nanorods

It is challenging to attain strong near-infrared (NIR) emissive gold nanoclusters. Here we show a rod-shaped cluster with the composition of [Au28(p-MBT)14(Hdppa)3](SO3CF3)2 (1 for short, Hdppa is N,N-bis(diphenylphosphino)amine, p-MBT is 4-methylbenzenethiolate) has been synthesized. Single crystal X-ray structural analysis reveals that it has a rod-like face-centered cubic (fcc) Au22 kernel built from two interpenetrating bicapped cuboctahedral Au15 units. 1 features NIR luminescence with an emission maximum at 920 nm, and the photoluminescence quantum yield (PLQY) is 12%, which is 30-fold of [Au21(m-MBT)12(Hdppa)2]SO3CF3 (2, m-MBT is 3-methylbenzenethiolate) with a similar composition and 60-fold of Au30S(S‑t‑Bu)18 with a similar structure. time-dependent DFT(TDDFT)calculations reveal that the luminescence of 1 is associated with the Au22 kernel. The small Stokes shift of 1 indicates that it has a very small excited state structural distortion, leading to high radiative decay rate (kr) probability. The emission of cluster 1 is a mixture of phosphorescence and thermally activated delayed fluorescence(TADF), and the enhancement of the NIR emission is mainly due to the promotion of kr rather than the inhibition of knr. This work demonstrates that the metal kernel and the surface structure are both very important for cluster-based NIR luminescence materials.

Molecular Packing-Driven Manipulation of Aggregation Induced Emission in Gold Nanoclusters

A key limitation of supramolecular force-driven molecular assembly in aggregation-induced emission (AIE) materials is the need to precisely regulate molecular interactions within the assembly. Achieving such assemblies with in situ manipulable molecular arrangements could provide valuable insights into the role of molecular forces in AIE. Herein, by using glutathione-protected gold nanoclusters (AuNCs) as a model AIE material and a naturally occurring polyphenol, tannic acid (TA), as the assembling agent, we demonstrate that assemblies dominated by covalent bonds and hydrogen bonding show enhanced AIE, while those dominated by π-π stacking promote charge transfer, resulting in significant photoluminescence (PL) quenching. This phenomenon primarily stems from the oxidation of TA into smaller aromatic ring structures, leading to an increase in π-π interactions. The complete in situ oxidation of TA within the assembly induces a morphological transition from 3-D spherical to 2-D sheet-like structures due to the dominance of π-π interactions, consequently resulting in complete PL quenching of AuNCs. These findings highlight the critical role of molecular packing in modulating the AIE properties of AuNCs.

Cu superparticle-based aggregation induced enhancement strategy with PVDF-HFP/CeVO4 NP sensing interface for miR-103a detection

Aggregation-induced emission (AIE) has been widely used in research on electrochemiluminescence (ECL) due to its excellent luminescence intensity. In this work, copper superparticles (Cu SPs) were used to construct ECL biosensor to detect the microRNA-103a (miRNA-103a) in triple-negative breast cancer (TNBC) tumor tissues. Firstly, GSH-capped copper clusters were used as precursors to prepare Cu SPs by the AIE effect. Compared with clusters, Cu SPs possessed higher luminescence performance and energy stability, making them an ideal choice for ECL nanoprobe. The film of PVDF-HFP/CeVO4 NPs was constructed and modified with CPBA and GSH as the sensing interface (PCCG). The PCCG film displayed good conductivity and hydrophilicity, and desirable mechanical stability. Moreover, the PCCG film can induce high carrier mobility rates and dissociate large amounts of the co-reactant K2S2O8 to enhance the ECL intensity of Cu SPs. As a result, the prepared ECL sensor with the catalyzed hairpin assembly (CHA) strategy was employed to quantify miRNA-103a in the range of 100 fM to 100 nM. The biosensor provided a novel analytical approach for the clinical diagnosis of TNBC.

A molecular dynamics study on the ion-mediated self-assembly of monolayer-protected nanoclusters

We studied the effects of metal and molecular cations on the aggregation of atomically precise monolayer-protected nanoclusters (MPCs) in an explicit solvent using atomistic molecular dynamics simulations. While divalent cations such as Zn2+ and Cd2+ promote aggregation by forming ligand-cation-ligand bridges between the MPCs, molecular cations such as tetraethylammonium and cholinium inhibit their aggregation by getting adsorbed into the MPC's ligand shell and reducing the ligand's motion. Here, we studied the aggregation of Au25(SR)18 nanoclusters with two types of ligands, para-mercaptobenzoic acid and D-penicillamine, as prototypical examples.

Luminescent carbon dots versus quantum dots and gold nanoclusters as sensors

Ultra-small nanoparticles, including quantum dots, gold nanoclusters (AuNCs) and carbon dots (CDs), have emerged as a promising class of fluorescent material because of their molecular-like properties and widespread applications in sensing and imaging. However, the fluorescence properties of ultra-small gold nanoparticles (i.e., AuNCs) and CDs are more complicated and well distinguished from conventional quantum dots or organic dye molecules. At this frontier, we highlight recent developments in the fundamental understanding of the fluorescence emission mechanism of these ultra-small nanoparticles. Moreover, this review carefully analyses the underlying principles of ultra-small nanoparticle sensors. We expect that this information on ultra-small nanoparticles will fuel research aimed at achieving precise control over their fluorescence properties and the broadening of their applications.

Biological properties of a novel solution based on silver nanoclusters for arresting dentin caries

Objectives

To test the biological properties of a novel non-restorative treatment method for arresting dentin caries based on silver nanoclusters (AgNCls) synthesized in polymethacrylic acid (PMAA).

Methods

Synthesis of AgNCls was performed by photoreduction of AgNO3 in PMAA with 355 nm/wavelength light. AgNCls/PMAA was characterized by absorption/fluorescence spectroscopy and optical and atomic force microscopy. The stability of the clusters in an aerated PMAA solution was evaluated by means of fluorescence spectroscopy. Cytotoxicity was assessed using the MTT assay and antibacterial effect was determined for minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC) and colony forming (CFU) of Streptococcus mutans (S. mutans) and Lactobacillus acidophilus (L. acidophilus). 38% Silver Diammine Fluoride (SDF) was used for the control groups.

Results

Chemical and structural identity of the clusters did not change within 9 months; Cell viability of 92%-89% was found after 24-48 h respectively. MIC and MBC were determined from 1:16 and 1:8 dilutions, respectively. Log CFU counts of S. mutans, and L. acidophilus treated with AgNCls/PMAA (3.4 ppm of silver) were significantly lower than in the control groups and even lower than when the same bacterial strains were treated with SDF (15,525 ppm of silver).

Conclusions

AgNCls/PMAA presented chemical stability, acceptable cytotoxicity, and a potential antibacterial effect for strains associated with caries lesions at very low concentrations of silver.

Strategic framework for harnessing luminescent metal nanocluster assemblies in biosensing applications

The distinctive physicochemical attributes of ultra-small metal nanoclusters (MNCs) resembling those of molecules make them versatile constituents for self-assembled frameworks. This critical review scrutinizes the influence of assembly on the photoluminescence (PL) properties of MNCs and investigates their utility in biosensing applications. The investigation is initiated with an assessment of the shift from individual MNCs to assemblies and its repercussions on PL efficacy. Subsequently, two distinct biosensing modalities are explored: assembly-driven detection mechanisms and detection predicated on structural modifications in assembled MNCs. Through meticulous examination, we underscore the potential of self-assembly methodologies in tailoring the PL behavior of MNCs for the detection of diverse biological analytes and diseases.

From synthesis to applications of biomolecule-protected luminescent gold nanoclusters

Gold nanoclusters (AuNCs) are a class of novel luminescent nanomaterials that exhibit unique properties of ultra-small size, featuring strong anti-photo-bleaching ability, substantial Stokes shift, good biocompatibility, and low toxicity. Various biomolecules have been developed as templates or ligands to protect AuNCs with enhanced stability and luminescent properties for biomedical applications. In this review, the synthesis of AuNCs based on biomolecules including amino acids, peptides, proteins and DNA are summarized. Owing to the advantages of biomolecule-protected AuNCs, they have been employed extensively for diverse applications. The biological applications, particularly in bioimaging, biosensing, disease therapy and biocatalysis have been described in detail herein. Finally, current challenges and future potential prospects of bio-templated AuNCs in biological research are briefly discussed.

Enzyme-activatable charge transfer in gold nanoclusters

Surface-protecting ligands, as a major component of metal nanoclusters (MNCs), can dominate molecular characteristics, performance behaviors, and biological properties of MNCs, which brings diversity and flexibility to the nanoclusters and largely promotes their applications in optics, electricity, magnetism, catalysis, biology, and other fields. We report herein the design of a new kind of water-soluble luminescent gold nanoclusters (AuNCs) for enzyme-activatable charge transfer (CT) based on the ligand engineering of AuNCs with 6-mercaptopurine ribonucleoside (MPR). This elaborately designed cluster, Au5(MPR)2, can form a stable intramolecular CT state after light excitation, and exhibits long-lived color-tunable phosphorescence. After the cleavage by purine nucleoside phosphorylase (PNP), the CT triplet state can be easily directed to a low-lying energy level, leading to a bathochromic shift of the emission band accompanied by weaker and shorter-lived luminescence. Remarkably, these ligand-engineered AuNCs show high affinity towards PNP as well as decent performance for analyzing and visualizing enzyme activity and related drugs. The work of this paper provides a good example for diversifying physicochemical properties and application scenarios of MNCs by rational ligand engineering, which will facilitate future interest and new strategies to precisely engineer solution-based nanocluster materials.

Expanding the Palette of SWIR Emitting Nanoparticles Based on Au Nanoclusters for Single-Particle Tracking Microscopy

Single-molecule localization microscopy has proved promising to unravel the dynamics and molecular architecture of thin biological samples down to nanoscales. For applications in complex, thick biological tissues shifting single-particle emission wavelengths to the shortwave infrared (SWIR also called NIR II) region between 900 to 2100 nm, where biological tissues are more transparent is key. To date, mainly single-walled carbon nanotubes (SWCNTs) enable such applications, but they are inherently 1D objects. Here, 0D ultra-small luminescent gold nanoclusters (AuNCs, <3 nm) and ≈25 nm AuNC-loaded-polymeric particles that can be detected at the single-particle level in the SWIR are presented. Thanks to high brightness and excellent photostability, it is shown that the dynamics of the spherical polymeric particles can be followed at the single-particle level in solution at video rates for minutes. We compared single particle tracking of AuNC-loaded-polymeric particles with that of SWCNT diffusing in agarose gels demonstrating the specificity and complementarity of diffusion properties of these SWIR-emitting nano-objects when exploring a complex environment. This extends the library of photostable SWIR emitting nanomaterials to 0D nano-objects of variable size for single-molecule localization microscopy in the second biological window, opening unprecedented possibilities for mapping the structure and dynamics of complex biological systems.

Loading of AuNCs with AIE effect onto cerium-based MOFs to boost fluorescence for sensitive detection of Hg2. Talanta 2024;273:125843. [PMID: 38492285 DOI: 10.1016/j.talanta.2024.125843] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Ligand-protected gold nanoclusters (AuNCs) have become promising nanomaterials in fluorescence (FL) methods for mercury ions (Hg2+) monitoring, but low FL efficiency hinders their widespread application. Herein, AuNCs/cerium-based metal-organic frameworks (AuNCs/Ce-MOFs) were prepared by loading 6-aza-2-thiothymine-protected AuNCs (ATT-AuNCs) with aggregation-induced emission (AIE) effect on the surface of Ce-MOFs by electrostatic attraction. This strategy improved the FL intensity of AuNCs through two aspects: (i) the AIE effect of ATT-AuNCs and (ii) the confinement effect of Ce-MOFs, which improved the restriction of intramolecular motion (RIM) of ATT-AuNCs. In addition, Ce-MOFs could adsorb and aggregate Hg2+ during detection, which might increase the local concentration. Therefore, based on the high FL signal of AuNCs/Ce-MOFs and enriched Hg2+, sensitive detection of Hg2+ could be achieved. More importantly, the strong specific recognition between AuNCs and Hg2+ could guarantee selectivity. The developed FL sensor exhibited superior detection performances with a wide linear range of 0.2-500 ng mL-1 and a low detection limit of 0.067 ng mL-1. Furthermore, the FL sensor used for sensitive and selective detection of Hg2+ in real samples, and the results agreed well with the standard method. In summary, this work proposed an effective and generalized strategy for improving the FL efficiency of AuNCs, which would greatly facilitate their application in pollutant monitoring.

Photoluminescent Characterization of Metal Nanoclusters: Basic Parameters, Methods, and Applications

Metal nanoclusters (MNCs) can be synthesized with atomically precise structures and molecule formulae due to the rapid development of nanocluster science in recent decades. The ultrasmall size range (normally < 2 nm) endows MNCs with plenty of molecular-like properties, among which photoluminescent properties have aroused extensive attention. Tracing the research and development processes of luminescent nanoclusters, various photoluminescent analysis and characterization methods play a significant role in elucidating luminescent mechanism and analyzing luminescent properties. In this review, it is aimed to systematically summarize the normally used photoluminescent characterizations in MNCs including basic parameters and methods, such as excitation/emission wavelength, quantum yield, and lifetime. For each key parameter, first its definition and meaning is introduced and then the relevant characterization methods including measuring principles and the revelation of luminescent properties from the collected data are discussed. Then, it is discussed in details how to explore the luminescent mechanism of MNCs and construct NC-based applications based on the measured data. By means of these characterization strategies, the luminescent properties of MNCs and NC-based designs can be explained quantitatively and qualitatively. Hence, this review is expected to provide clear guidance for researchers to characterize luminescent MNCs and better understand the luminescent mechanism from the measured results.

Nanoscale potassium sensing based on valinomycin-anchored fluorescent gold nanoclusters

Gold nanoclusters are a smart platform for sensing potassium ions (K+). They have been synthesized using bovine serum albumin (BSA) and valinomycin (Val) to protect and cap the nanoclusters. The nanoclusters (Val-AuNCs) produced have a red emission at 616 nm under excitation with 470 nm. In the presence of K+, the valinomycin polar groups switch to the molecule's interior by complexing with K+, forming a bracelet structure, and being surrounded by the hydrophobic exterior conformation. This structure allows a proposed fluorometric method for detecting K+ by switching between the Val-AuNCs' hydrophilicity and hydrophobicity, which induces the aggregation of gold nanoclusters. As a result, significant quenching is seen in fluorescence after adding K+. The quenching in fluorescence in the presence of K+ is attributed to the aggregation mechanism. This sensing technique provides a highly precise and selective sensing method for K+ in the range 0.78 to 8 µM with LOD equal to 233 nM. The selectivity of Val-AuNCs toward K+ ions was investigated compared to other ions. Furthermore, the Val-AuNCs have novel possibilities as favorable sensor candidates for various imaging applications. Our detection technique was validated by determining K+ ions in postmortem vitreous humor samples, which yielded promising results.

Bright near-infrared emission from the Au39(SR)29 nanocluster

The synthesis of atomically precise gold nanoclusters with high photoluminescence quantum yield (PLQY) in the near-infrared (NIR) region and understanding their photoluminescence mechanism are crucial for both fundamental science and practical applications. Herein, we report a highly luminescent, molecularly pure Au39(PET)29 (PET = 2-phenylethanethiolate) nanocluster with PLQY of 19% in the NIR range (915 nm). Steady state and time-resolved PL analyses, as well as temperature-dependent PL measurements reveal the emission nature of Au39(PET)29, which consists of prompt fluorescence (weak), thermally activated delayed fluorescence (TADF), and phosphorescence (predominant). Furthermore, strong dipole-dipole interaction in the solid-state (e.g., Au39(PET)29 nanoclusters embedded in a polystyrene thin-film) is found to narrow the energy gap between the S1 and T1 states, which results in faster intersystem crossing and reverse intersystem crossing; thus, the ratio of TADF to phosphorescence varies and the total PLQY is increased to 32%. This highly luminescent nanocluster holds promise in imaging, sensing and optoelectronic applications.

Solvent-Mediated Hetero/Homo-Phase Crystallization of Copper Nanoclusters and Superatomic Kernel-Related NIR Phosphorescence

Atomically precise superatomic copper nanoclusters (Cu NCs) have been the subject of immense interest for their intriguing structures and diverse properties; nonetheless, the variable oxidation state of copper ions and complex solvation effects in wet synthesis systems pose significant challenges for comprehending their synthesis and crystallization mechanism. Herein, we present a solvent-mediated approach for the synthesis of two Cu NCs, namely, superatomic Cu26 and pure-Cu(I) Cu16. They initially formed as a hetero-phase and then separated as a homo-phase via modulating binary solvent composition. In situ UV/vis absorption and electrospray ionization mass spectra revealed that the solvent-mediated assembly was determined to be the underlying mechanism of hetero/homo-phase crystallization. Cu26 is a 2-electron superatom with a kernel-shell structure that includes a [Cu20Se12]4- shell and [Cu6]4+ kernel, containing two 1S jellium electrons. Conversely, Cu16 is a pure-Cu(I) Cu/Se nanocluster that features a [Cu16Se6]4+ core protected by extra dimercaptomaleonitrile ligands. Remarkably, Cu26 exhibits unique near-infrared phosphorescence (NIR PH) at 933 nm due to the presence of a superatomic kernel-related charge transfer state (3MM(Cu)CT). Overall, this work not only showcases the hetero/homo-phase crystallization of Cu NCs driven by a solvent-mediated assembly mechanism but also enables the rare occurrence of NIR PH within the 2-electron copper superatom family.

Manipulation of aurophilicity in constructed clusters of gold(I) complexes with boosted luminescence and smart responsiveness

High-performance luminescent gold(I) complexes have attracted considerable attention due to their potential applications in various fields, but their construction is a significantly challenging task. Herein, we designed and synthesized a series of novel dinuclear gold(I) complexes 1-4 based on 1,2-bis(diphenylphosphino)benzene and 1,4-bis(diphenylphosphino)benzene frameworks, where para-substitutions of benzene ring were employed for comparison and bulky t-butyl groups were introduced into carbazole ligands to assist flexibly regulating the aurophilicity. Among them, the structure of complex 1 was confirmed by single-crystal X-ray diffraction, and all the complexes exhibited typical aggregation-induced emission characteristics. Due to the construction of intramolecular aurophilicity and the formation of molecular clusters, noticeable enhancement of the luminescent efficiency was achieved for the core complex 1. Together with the introduction of flexible t-butyl groups, good responsiveness towards external mechanical force and solvent vapors were also realized. Moreover, the specific bioimaging ability of complex 1 towards cancer cells was demonstrated. Thus, this work presents the crucial capability of aurophilic manipulation in tuning the luminescence and smart behaviors of gold complexes, and it will open a new route to developing high-performance luminescent materials.

Fluorescent metal nanoclusters: prospects for photoinduced electron transfer and energy harvesting

Research on noble metal nanoclusters (MNCs) (elements with filled electron d-bands) is progressing forward because of the extensive and extraordinary chemical, optical, and physical properties of these materials. Because of the ultrasmall size of the MNCs (typically within 1-3 nm), they can be applied in areas of nearly all possible scientific domains. The greatest advantage of MNCs is the tunability that can be imposed, not only on their structures, but also on their chemical, physical, and biological properties. Nowadays, MNCs are very effectively used as energy donors and acceptors under suitable conditions and hence act as energy harvesters in solar cells, semiconductors, and biomarkers. In addition, ultrafast photoinduced electron transfer (PET) can be practised using MNCs under various circumstances. Herein, we have focused on the energy harvesting phenomena of Au-, Ag-, and Cu-based MNCs and elaborated on different ways to apply them.

Combination of gold nanoclusters and silicon quantum dots for ratiometric fluorometry: One system, two mechanisms

A ratiometric fluorometry based on silicon quantum dots (SiQDs) and gold nanoclusters (AuNCs) is constructed for detecting activity of butyrylcholinesterase (BChE) in human serum. By using thiobutyrylcholine iodide (BTCh) as the substrate of BChE-catalyzed hydrolysis reaction, variation of fluorescence emission from AuNCs is employed as an indicator of BChE activity since one of the hydrolysis products, thiocholine (TCh), would influence the aggregation state of AuNCs and consequently led to the change of fluorescence quantum efficiency of AuNCs. It is interesting that there are two mechanisms working for the fluorescence emission of aggregated AuNCs: aggregation-induced emission enhancement (AIEE) and aggregation-caused quenching (ACQ) with the presence of TCh at very low and higher concentration levels, respectively. Although both of these mechanisms can be utilized for sensing BChE, their opposite influence on the fluorescence emission of aggregated AuNCs should be worthy of attention, especially in the process of developing fluorescence methods for detecting trace targets by using AuNCs. In order to eliminate the fluctuation of fluorophotometer, SiQDs is chosen as the fluorophore to develop by ratiometric fluorescence methods in this work. Additionally, obvious aggregation of AuNCs induces significant decrease of inner filter effect (IFE) on the fluorescence emitted from SiQDs, while mild aggregation of AuNCs demonstrates little IFE. The linear ranges for detecting activity of BChE are 0.004 - 0.05 U/L and 0.5 - 20 U/L by ratiometric fluorometry based on the AIEE and ACQ, respectively. The very different responses originated from AIEE and ACQ of AuNCs would respectively make their own contributions to the determination of BChE activities at very low or high levels, which facilitate the developments of enhanced or quenched fluorescence methods. However, the detection of BChE activities at medium levels might suffer from the combination of AIEE and ACQ with ambiguous fractions. Therefore, it must be careful during the processes of developing and applying fluorescence methods based on the AIEE and ACQ of AuNCs, as well as the process of evaluating their analytical performance.

Al8 Cluster-Based Metal Halide Frameworks: Balancing Singlet-Triplet Excited States to Achieve White Light and Multicolor Luminescence

Luminescent metal clusters have attracted great interest in current research; however, the design synthesis of Al clusters with color-tunable luminescence remains challenging. Herein, an [Al8 (OH)8 (NA)16 ] (Al8 , HNA = nicotinic acid) molecular cluster with dual luminescence properties of fluorescence and room-temperature phosphorescence (RTP) is synthesized by choosing HNA ligand as phosphor. Its prompt photoluminescence (PL) spectrum exhibits approximately white light emission at room temperature. Considering that halogen atoms can be used to regulate the RTP property by balancing the singlet and triplet excitons, different CdX2 (X- = Cl- , Br- , I- ) are introduced into the reactive system of the Al8 cluster, and three new Al8 cluster-based metal-organic frameworks, {[Al8 Cd3 Cl5 (OH)8 (NA)17 H2 O]·2HNA}n (CdCl2 -Al8 ), {[Al8 Cd4 Br7 (OH)8 (NA)16 CH3 CN]·NA·HNA}n (CdBr2 -Al8 ) and {[Al8 Cd8 I16 (OH)8 (NA)16 ]}n (CdI2 -Al8 ) are successfully obtained. They realize the color tunability from blue to yellow at room temperature. The origination of fluorescence and phosphorescence has also been illustrated by structure-property analysis and theoretical calculation. This work provides new insights into the design of multicolor luminescent metal cluster-based materials and develops advanced photo-functional materials for multicolor display, anti-counterfeiting, and encryption applications.

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.

Visible to NIR-II Photoluminescence of Atomically Precise Gold Nanoclusters

Atomically precise gold nanoclusters (NCs) have emerged as a new class of precision materials and attracted wide interest in recent years. One of the unique properties of such nanoclusters pertains to their photoluminescence (PL), for it can widely span visible to near-infrared-I and -II wavelengths (NIR-I/II), and even beyond 1700 nm by manipulating the size, structure, and composition. The current research efforts focus on the structure-PL correlation and the development of strategies for raising the PL quantum yields, which is nontrivial when moving from the visible to the near-infrared wavelengths, especially in the NIR-II regions. This review summarizes the recent progress in the field, including i) the types of PL observed in gold NCs such as fluorescence, phosphorescence, and thermally activated delayed fluorescence, as well as dual emission; ii) some effective strategies that are devised to improve the PL quantum yield (QY) of gold NCs, such as heterometal doping, surface rigidification, and core phonon engineering, with double-digit QYs for the NIR PL on the horizons; and iii) the applications of luminescent gold NCs in bioimaging, photosensitization, and optoelectronics. Finally, the remaining challenges and opportunities for future research are highlighted.

A Comprehensive Analysis of Luminescent Crystallized Cu Nanoclusters

Photoluminescence (PL) emission is an intriguing characteristic displayed by atomically precise d10 metal nanoclusters (NCs), renowned for their meticulous atomic arrangements, which have captivated the scientific community. Cu(I) NCs are a focal point in extensive research due to their abundance, cost-effectiveness, and unique luminescent attributes. Despite similar core sizes, their luminescent characteristics vary, influenced by multiple factors. Progress hinges on synthesizing new NCs and modifying existing ones, with postsynthetic alterations impacting emission properties. The rapid advancements in this field pose challenges in discerning essential points for excelling amidst competition with other d10 NCs. This Perspective explores the intricate origins of PL emission in Cu(I) NCs, providing a comprehensive review of their correlated structural architectures. Understanding the mechanistic origin of PL emission in each cluster is crucial for correlating diverse characteristics, contributing to a deeper comprehension from both fundamental and applied scientific perspectives.

Prolonged near-infrared fluorescence imaging of microRNAs and proteases in vivo by aggregation-enhanced emission from DNA-AuNC nanomachines

Developing a comprehensive strategy for imaging various biomarkers (i.e., microRNAs and proteases) in vivo is an exceptionally formidable task. Herein, we have designed a deoxyribonucleic acid-gold nanocluster (DNA-AuNC) nanomachine for detecting tumor-related TK1 mRNA and cathepsin B in living cells and in vivo. The DNA-AuNC nanomachine is constructed using AuNCs and DNA modules that incorporate a three component DNA hybrid (TD) and a single-stranded fuel DNA (FD). Upon being internalized into tumor cells, the TK1 mRNA initiates the DNA-AuNC nanomachine through DNA strand displacement cascades, leading to the amplified self-assembly and the aggregation-enhanced emission of AuNCs for in situ imaging. Furthermore, with the aid of a protease nanomediator consisting of a mediator DNA/peptide complex and AuNCs (DpAuNCs), the DNA-AuNC nanomachine can be triggered by the protease-activated disassembly of the DNA/peptide complex on the nanomediator, resulting in the aggregation of AuNCs for in vivo protease amplified detection. It is worth noting that our study demonstrates the impressive tumor permeability and accumulation capabilities of the DNA-AuNC nanomachines via in situ amplified self-assembly, thereby facilitating prolonged imaging of TK1 mRNA and cathepsin B both in vitro and in vivo. This strategy presents a versatile and biomarker-specific paradigm for disease diagnosis.

A concise guide to chemical reactions of atomically precise noble metal nanoclusters

Nanoparticles (NPs) with atomic precision, known as nanoclusters (NCs), are an emerging field in materials science in view of their fascinating structure-property relationships. Ultrasmall noble metal NPs have molecule-like properties that make them fundamentally unique compared with their plasmonic counterparts and bulk materials. In this review, we present a comprehensive account of the chemistry of monolayer-protected atomically precise noble metal nanoclusters with a focus on the chemical reactions, their diversity, associated kinetics, and implications. To begin with, we briefly review the history of the evolution of such precision materials. Then the review explores the diverse chemistry of noble metal nanoclusters, including ligand exchange reactions, ligand-induced structural transformations, and reactions with metal ions, metal thiolates, and halocarbons. Just as molecules do, these precision materials also undergo intercluster reactions in solution. Supramolecular forces between these systems facilitate the creation of well-defined hierarchical assemblies, composites, and hybrid materials. We conclude the review with a future perspective and scope of such chemistry.

Selective fluorescence turn-on detection of combination cisplatin-etoposide chemotherapy based on N-CDs/GSH-CuNCs nanoprobe

Cisplatin (CIS) and etoposide (ETP) combination therapy is highly effective for treating various cancers. However, the potential for pharmacokinetic interactions between these drugs necessitates selective sensing methods to quantitate both CIS and ETP levels in patient's plasma. This work develops a dual fluorescence probe strategy using glutathione-capped copper nanoclusters (GSH-CuNCs) and nitrogen-doped carbon dots (N-CDs) for the simultaneous analysis of CIS and ETP. The fluorescence signal of GSH-CuNCs at 615 nm increased linearly with CIS concentration while the N-CD emission at 480 nm remained unaffected. Conversely, the N-CD fluorescence was selectively enhanced by ETP with no interference with the CuNC fluorescence. Extensive materials characterization including UV-vis, fluorescence spectroscopy, XRD, and TEM confirmed the synthesis of the nanoprobes. The sensor showed high sensitivity with limits of detection of 6.95 ng mL-1 for CIS and 7.63 ng mL-1 for ETP along with excellent selectivity against potential interferences in rabbit plasma. Method feasibility was demonstrated with application to real rabbit plasma samples. The method was further applied to estimate the pharmacokinetic parameters of CIS before and after ETP coadministration. The dual nanoprobe sensing strategy enables rapid and selective quantitation of CIS and ETP levels to facilitate therapeutic drug monitoring and optimization of combination chemotherapy regimens.

Ratiometric fluorescence sensing of hazardous ciprofloxacin based on aggregation induced emission enhancement of thiolate-protected gold nanoclusters induced by La3+ ion

In this research work, ultrasensitive and reliable ratiometric sensor was designed for assay of ciprofloxacin (CIPRO). The platform consists of thiolate-protected gold nanoclusters (GSH@AuNCs) with high fluorescence quantum yield and long-term stability. In the existence of lanthanum (La3+) ion, the GSH@AuNCs emission was sharply raised owing to the formation of La3+/GSH@AuNCs system accompanied with aggregation-induced emission enhancement (AIEE). Addition of CIPRO, a good La3+ ion coordinator, decreased the fluorescence emission of La3+/GSH@AuNCs system at 610 nm significantly as a result of disaggregation caused by the removal of La3+ ion from GSH@AuNCs surface. Furthermore, the blue emission of La3+-CIPRO coordination complex appears at 440 nm. Under optimized conditions, the fluorescence ratios (F440/F610) were linearly increased with increasing the CIPRO amount within the range of 0.003-200 µM with a limit of detection equal to 0.2 nM (S/N = 3). The as-fabricated La3+/GSH@AuNCs system possesses outstanding reliability and sensitivity for the detection of CIPRO. The system was effectively used to assay CIPRO in human urine and milk specimens with recoveries % of 97.6-102.3 % and 96.7-105.7 % for urine and milk samples, respectively and RSD % did not exceed 2.5 %, suggesting the accuracy of the method.

Physicochemical, electrochemical, and biological characterization of field assisted gold nanocluster-coated barium titanate nanoparticles for biomedical applications

Fluorescence-based bioimaging is an imperative approach with high clinical relevance in healthcare applications and biomedical research. The field of bioimaging plays an indispensable role in gaining insight into the internal architecture of cells/tissues and comprehending the physiological functions associated with biological systems. With the utility of piezoelectric nanomaterials, the bioelectric interface has been significantly investigated, leading to remarkable clinical relevance. Herein, we have developed barium titanate nanoparticle (BT) coated gold nanoclusters (AuNCs) in the presence and absence of an electromagnetic field (EMF). In this work, the effect of low (0.6 G) and high (2.0 G) EMFs on the structural arrangement of these piezoelectric nanocomposites (ABT) has been extensively studied with the help of X-ray diffraction (XRD), high diffraction resolution transmission electron microscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS). Furthermore, the two derivatives of ABT i.e. 0.6 ABT and 2.0 ABT have been evaluated for electrochemical behavior for their applicability as a candidate for exploring the bioelectric interface. Additionally, ABT, 0.6 ABT, and 2.0 ABT have been explored for cytocompatibility and bioimaging applications. The proposed piezoelectric nanocomposite, as a multifunctional platform, has enormous proficiency in the field of bioimaging and the capability to be utilized across the bioelectric interface.

White light emission from helically stacked humin-mimic based H-aggregates in heteroatom free carbon dots

White light emission (WLE), particularly from heteroatom free carbon dots (CDs), is unusual. Besides, deciphering the origin of WLE from a H-aggregated molecular fluorophore in such kinds of CDs is a challenging task due to their non-fluorescent character resulting from a forbidden transition from a lower-energy excitonic state. Therefore, rigorous investigation on their elusive excited state photophysical properties along with their steady-state optical phenomena has to be carried out to shed light on the nature of distinct emissive states formed in the CDs. Herein, for the first time, we report WLE from imperfect H-aggregates of co-facially π-π stacked humin-like structures comprising furfural monomer units as a unique molecular fluorophore in CDs, as revealed from combined spectroscopic and microscopic studies, synthesized through hydrothermal treatment of the single precursor, dextrose. H-aggregates in CDs show a broad range of excitation-dependent emission spectra with color coordinates close to pure white light, i.e., CIE (0.35, 0.37) and a color temperature of 6000 K. Imperfect orientation between the transition dipole moments of adjacent monomer units in the H-aggregate's molecular arrangement is expected to cause ground state symmetry breaking, as confirmed by Circular Dichroism (CD) studies, which established helically stacked nature in molecular aggregates and produced significant oscillatory strength at lower energy excitonic states to enable fluorescence. TRES and TAS investigations have been performed to minimise the intricacies associated with excited state photophysics, which is regarded as an essential step in gaining a grasp on emissive states. Based on the observation of two isoemissive spots in the time-resolved area normalized emission spectra (TRANES), the existence of three oligomeric species in the excited state equilibrium of the pure/hybrid H-aggregates has been established. The exciton dynamics through electron relaxation from the higher to the lower excitonic states, charge transfer (CT) states, and surface trap mediated emission in excimer states of H-aggregates have also been endorsed as three distinct emissive states from femtosecond transient absorption spectroscopy (TAS) studies corroborating with their steady-state absorption and emission behavior. The results would demonstrate the usage of CDs as a cutting-edge fluorescent material for creating aggregate-induced white light emission.

Solvent-Induced Aggregation of Self-Assembled Copper-Cysteine Nanoparticles Reacted with Glutathione: Enhancing Linear and Nonlinear Optical Properties

Copper-thiolate self-assembly nanostructures are a unique class of nanomaterials because of their interesting properties such as hierarchical structures, luminescence, and large nonlinear optical efficiency. Herein, we synthesized biomolecule cysteine (Cys) and glutathione (GSH) capped sub-100 nm self-assembly nanoparticles (Cu-Cys-GSH NPs) with red fluorescence. The as-synthesized NPs show high emission enhancement in the presence of ethanol, caused by the aggregation-induced emission. We correlated the structure and optical properties of Cu-Cys-GSH NPs by measuring the mass, morphology, and surface charge as well as their two-photon excited fluorescence cross-section (σ2PEPL), two-photon absorption cross-section (σTPA) and first hyperpolarizability (β) of Cu-Cys-GSH NPs in water and water-ethanol using near-infrared wavelength. We found a high β value as (77 ± 10) × 10-28 esu (in water) compared to the reference medium water. The estimated values of σ2PEPL and σTPA are found to be (13 ± 2) GM and (1.4 ± 0.2) × 104 GM, respectively. We hope our investigations of linear and nonlinear optical properties of copper-thiolate self-assemblies in water and its solvent-induced aggregates will open up new possibilities in designing self-assembled systems for many applications including sensing, drug delivery, and catalysis.

Precision nanoengineering for functional self-assemblies across length scales

As nanotechnology continues to push the boundaries across disciplines, there is an increasing need for engineering nanomaterials with atomic-level precision for self-assembly across length scales, i.e., from the nanoscale to the macroscale. Although molecular self-assembly allows atomic precision, extending it beyond certain length scales presents a challenge. Therefore, the attention has turned to size and shape-controlled metal nanoparticles as building blocks for multifunctional colloidal self-assemblies. However, traditionally, metal nanoparticles suffer from polydispersity, uncontrolled aggregation, and inhomogeneous ligand distribution, resulting in heterogeneous end products. In this feature article, I will discuss how virus capsids provide clues for designing subunit-based, precise, efficient, and error-free self-assembly of colloidal molecules. The atomically precise nanoscale proteinic subunits of capsids display rigidity (conformational and structural) and patchy distribution of interacting sites. Recent experimental evidence suggests that atomically precise noble metal nanoclusters display an anisotropic distribution of ligands and patchy ligand bundles. This enables symmetry breaking, consequently offering a facile route for two-dimensional colloidal crystals, bilayers, and elastic monolayer membranes. Furthermore, inter-nanocluster interactions mediated via the ligand functional groups are versatile, offering routes for discrete supracolloidal capsids, composite cages, toroids, and macroscopic hierarchically porous frameworks. Therefore, engineered nanoparticles with atomically precise structures have the potential to overcome the limitations of molecular self-assembly and large colloidal particles. Self-assembly allows the emergence of new optical properties, mechanical strength, photothermal stability, catalytic efficiency, quantum yield, and biological properties. The self-assembled structures allow reproducible optoelectronic properties, mechanical performance, and accurate sensing. More importantly, the intrinsic properties of individual nanoclusters are retained across length scales. The atomically precise nanoparticles offer enormous potential for next-generation functional materials, optoelectronics, precision sensors, and photonic devices.

Size and charge effects of metal nanoclusters on antibacterial mechanisms

Nanomaterials, specifically metal nanoclusters (NCs), are gaining attention as a promising class of antibacterial agents. Metal NCs exhibit antibacterial properties due to their ultrasmall size, extensive surface area, and well-controlled surface ligands. The antibacterial mechanisms of metal NCs are influenced by two primary factors: size and surface charge. In this review, we summarize the impacts of size and surface charge of metal NCs on the antibacterial mechanisms, their interactions with bacteria, and the factors that influence their antibacterial effects against both gram-negative and gram-positive bacteria. Additionally, we highlight the mechanisms that occur when NCs are negatively or positively charged, and provide examples of their applications as antibacterial agents. A better understanding of relationships between antibacterial activity and the properties of metal NCs will aid in the design and synthesis of nanomaterials for the development of effective antibacterial agents against bacterial infections. Based on the remarkable achievements in the design of metal NCs, this review also presents conclusions on current challenges and future perspectives of metal NCs for both fundamental investigations and practical antibacterial applications.

Ligand-protected metal nanoclusters as low-loss, highly polarized emitters for optical waveguides

Photoluminescent molecules and nanomaterials have potential applications as active waveguides, but such a use has often been limited by high optical losses and complex fabrication processes. We explored ligand-protected metal nanoclusters (LPMNCs), which can have strong, stable, and tunable emission, as waveguides. Two alloy LPMNCs, Pt1Ag18 and AuxAg19-x (7 ≤ x ≤ 9), were synthesized and structurally determined. Crystals of both exhibited excellent optical waveguide performance, with optical loss coefficients of 5.26 × 10-3 and 7.77 × 10-3 decibels per micrometer, respectively, lower than those demonstrated by most inorganic, organic, and hybrid materials. The crystal packing and molecular orientation of the Pt1Ag18 compound led to an extremely high polarization ratio of 0.91. Aggregation enhanced the quantum yields of Pt1Ag18 and AuxAg19-x LPMNCs by 115- and 1.5-fold, respectively. This photonic cluster with low loss and high polarization provides a generalizable and versatile platform for active waveguides and polarizable materials.

Restriction of intramolecular rotation for functionalizing metal nanoclusters

The restriction of intramolecular rotation has been extensively exploited to trigger the property enhancement of nanocluster-based materials. However, such a restriction is induced mainly by intermolecular aggregation. The direct restriction of intramolecular rotation of metal nanoclusters, which could boost their properties at the single molecular level, remains rarely explored. Here, ligand engineering was applied to activate intramolecular interactions at the interface between peripheral ligands and metallic kernels of metal nanoclusters. For the newly reported Au4Ag13(SPhCl2)9(DPPM)3 nanocluster, the molecule-level interactions between the Cl terminals on thiol ligands and the Ag atoms on the cluster kernel remarkably restricted the intramolecular rotation, endowing this robust nanocluster with superior thermal stability, emission intensity, and non-linear optical properties over its cluster analogue. This work presents a novel case of the restriction of intramolecular rotation (i.e., intramolecular interaction-induced property enhancement) for functionalizing metal clusters at the single molecular level.

Recent Advances in Fluorescent Probes for Zinc Ions Based on Various Response Mechanisms

Zinc is a vital metal element with extensive applications in various fields such as industry, metallurgy, agriculture, food, and healthcare. For living organisms, zinc ions are indispensable, and their deficiency can lead to physiological and metabolic abnormalities that cause multiple diseases. Hence, there is a significant need for selective recognition and effective detection of free zinc ions. As a probe method with high sensitivity, high selectivity, real-time monitoring, safety, harmlessness and ease of operation, fluorescent probes have been widely used in metal ion identification studies, and many convenient, low-cost and easy-to-operate fluorescent probes for Zn²⁺ detection have been developed. This article reviews the latest research advances in fluorescent chemosensors for Zn²⁺ detection from 2019 to 2023. In particular, sensors working through photo-induced electron transfer (PET), excited state intramolecular proton transfer (ESIPT), intramolecular charge transfer (ICT), fluorescence resonance energy transfer (FRET), chelation-enhanced fluorescence (CHEF), and aggregation-induced emission (AIE) mechanisms are described. We discuss the use of various recognition mechanisms in detecting zinc ions through specific cases, some of which have been validated through theoretical calculations.

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.

Intrinsic Optical Properties and Emerging Applications of Gold Nanostructures

The collective oscillation of free electrons at the nanoscale surface of gold nanostructures is closely modulated by tuning the size, shape/morphology, phase, composition, hybridization, assembly, and nanopatterning, along with the surroundings of the plasmonic surface located at a dielectric interface with air, liquid, and solid. This review first introduces the physical origin of the intrinsic optical properties of gold nanostructures and further summarizes stimuli-responsive changes in optical properties, metal-field-enhanced optical signals, luminescence spectral shaping, chiroptical response, and photogenerated hot carriers. The current success in the landscape of nanoscience and nanotechnology mainly originates from the abundant optical properties of gold nanostructures in the thermodynamically stable face-centered cubic (fcc) phase. It has been further extended by crystal phase engineering to prepare thermodynamically unfavorable phases (e.g., kinetically stable) and heterophases to modulate their intriguing phase-dependent optical properties. A broad range of promising applications, including but not limited to full-color displays, solar energy harvesting, photochemical reactions, optical sensing, and microscopic/biomedical imaging, have fostered parallel research on the multitude of physical effects occurring in gold nanostructures.

Structural water molecules dominated p band intermediate states as a unified model for the origin on the photoluminescence emission of noble metal nanoclusters: from monolayer protected clusters to cage confined nanoclusters

In the past several decades, noble metal nanoclusters (NMNCs) have been developed as an emerging class of luminescent materials due to their superior photo-stability and biocompatibility, but their luminous quantum yield is relatively low and the physical origin of the bright photoluminescence (PL) of NMNCs remain elusive, which limited their practical application. As the well-defined structure and composition of NMNCs have been determined, in this mini-review, the effect of each component (metal core, ligand shell and interfacial water) on their PL properties and corresponded working mechanism were comprehensively introduced, and a model that structural water molecules dominated p band intermediate state was proposed to give a unified understanding on the PL mechanism of NMNCs and a further perspective to the future developments of NMNCs by revisiting the development of our studies on the PL mechanism of NMNCs in the past decade.

Zeolite-Encaged Luminescent Silver Nanoclusters

Silver nanoclusters (Ag NCs) are nanoscale aggregates that possess molecular-like discrete energy levels, resulting in electronic configuration-dependent tunable luminescence spanning the entire visible range. Benefiting from the efficient ion exchange capacity, nanometer dimensional cages, and high thermal and chemical stabilities, zeolites have been employed as desirable inorganic matrices to disperse and stabilize Ag NCs. This paper reviewed the recent research progresses on the luminescence properties, spectral manipulation, as well as the theoretical modelling of electronic structure and optical transition of Ag NCs confined inside various zeolites with different topology structures. Furthermore, potential applications of the zeolite-encaged luminescent Ag NCs in lighting, gas monitoring and sensing were presented. This review concludes with a brief comment on the possible future directions in the study of zeolite-encaged luminescent Ag NCs.

Terbium-triggered aggregation-induced emission of bimetallic nanoclusters for anticancer drugs sensing via the inner filter effect

The development of accurate and sensitive detection methods of anticancer drugs is of significant importance because they play vital roles in biological systems. In recent years, bimetallic nanoclusters (BMNCs) incorporating the advantages of two metals have gained more and more attention, and can be widely applied in sensing applications. In this work, for the first time, we designed a sensing platform based on terbium ion (Tb³⁺) triggered aggregation-induced emission (AIE) of BMNCs. Tb³⁺ hybrid glutathione (GS) protected Ag/Cu nanoclusters (Tb³⁺@GS-AgCuNCs) were facilely fabricated according to the complexation reaction between Tb³⁺ and the carboxyl group of GS. Due to the inner filter effect (IFE), the fluorescence of Tb³⁺@GS-AgCuNCs decreased significantly in the presence of anticancer drugs with 6-thioguanine and methotrexate as representatives. In addition, the sensing platform was applied to monitor 6-thioguanine and methotrexate in real serum samples, indicating that it has great potential in anticancer drugs related applications.

Regulating the Electronic Structure of Metal Nanoclusters by Longitudinal Single-Dithiolate Substitution

It is significant but challenging to understand the property evolution of metal nanoclusters by orientated regulation of the electronic structure. Previous research has demonstrated that the optical properties of metal nanoclusters with anisotropic structures are greatly impacted by their longitudinal electronic structure. However, the manipulation of optical properties of metal nanoclusters by regulating their electronic structure through longitudinal dithiolate substitutions has not yet been reported. In this study, we first achieved the longitudinal single-dithiolate replacement of metal nanoclusters and obtained two novel nanoclusters: Au₂₈(SPh-^tBu)₁₈(SCH₂SCH₂S) and Au₂₈(SPh-^tBu)₁₈(SCH₂CH₂CH₂S). Both experimental and theoretical results demonstrated the regulation of the electronic structure (dipole moment) in the z (longitudinal) and x directions, resulting in absorption redshift and photoluminescence (polarity) enhancement. These findings not only deepen the understanding of the property-electronic structure correlation of metal nanoclusters but also provide guidance for their subtle property tuning.

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.

Preparation of a Red-Emitting, Chitosan-Stabilized Copper Nanocluster Composite and Its Application as a Hydrogen Peroxide Detection Probe in the Analysis of Water Samples

Hydrogen peroxide (H₂O₂) is an important reactive oxygen species that mediates a variety of physiological functions in biological processes, and it is an essential mediator in food, pharmaceutical, and environmental analysis. However, H₂O₂ can be dangerous and toxic at certain concentrations. It is crucial to detect the concentration of H₂O₂ in the environment for human health and environmental protection. Herein, we prepared the red-emitting copper nanoclusters (Cu NCs) by a one-step method, with lipoic acid (LA) and sodium borohydride as protective ligands and reducing agents, respectively, moreover, adding chitosan (CS) to wrap LA-Cu NCs. The as-prepared LA-Cu NCs@CS have stronger fluorescence than LA-Cu NCs. We found that the presence of H₂O₂ causes the fluorescence of LA-Cu NCs@CS to be strongly quenched. Based on this, a fluorescent probe based on LA-Cu NCs@CS was constructed for the detection of H₂O₂ with a limit of detection of 47 nM. The results from this research not only illustrate that the as--developed fluorescent probe exhibits good selectivity and high sensitivity to H₂O₂ in environmental water samples but also propose a novel strategy to prepare red-emitting copper nanoclusters (Cu NCs) by a one-step method.