Engineering luminescent metal nanoclusters for sensing applications

Application of gold nanoclusters in fluorescence sensing and biological detection

Gold nanoclusters (Au NCs) exhibit broad fluorescent spectra from visible to near-infrared regions and good enzyme-mimicking catalytic activities. Combined with excellent stability and exceptional biocompatibility, the Au NCs have been widely exploited in biomedicine such as biocatalysis and bioimaging. Especially, the long fluorescence lifetime and large Stokes shift attribute Au NCs to good probes for fluorescence sensing and biological detection. In this review, we systematically summarized the molecular structure and fluorescence properties of Au NCs and highlighted the advances in fluorescence sensing and biological detection. The Au NCs display high sensitivity and specificity in detecting iodine ions, metal ions, and reactive oxygen species, as well as certain diseases based on the fluorescence activities of Au NCs. We also proposed several points to improve the practicability and accelerate the clinical translation of the Au NCs.

Atomically precise copper nanoclusters mediated Fenton-like reaction for cancer chemodynamic therapy

We developed stable luminescent morpholine-appended copper nanoclusters CuNCs@MorMB with an ultra-small size (<3 nm) and a long emission lifetime (577 ns). They mediate a Fenton-like reaction to produce reactive hydroxyl radicals (˙OH), subsequently depleting antioxidant glutathione levels for cancer chemodynamic therapy (CDT).

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.

Non-Enzymatic Detection of Cardiac Troponin-I with Graphene Oxide Quenched Fluorescent Iron Nanoclusters (FeNCs)

Cardiac troponin I (cTnI) is the most resorted biomarker for the detection of cardiovascular disease (CVD). The means of rapid quantification of cTnI levels in the blood can substantially minimize the risk of acute myocardial infarction and heart failure. A sensor for the non-enzymatic evaluation of cardiac troponin-I has been developed using fluorescent iron nanoclusters via a one-pot synthesis employing (BSA) as the template and reducing agent, and hydrogen peroxide as the additive. The fluorescence of Iron Nanocluster is quenched with graphene oxide (GO) via fluorescence resonance energy transfer (FRET) between conjugate iron nanoclusters and graphene oxide. The sensor shows a low detection limit of 0.011 ng/mL. The benefits of utilizing a non-enzymatic probe for detecting cardiac troponin I is that it avoids the need for enzymes and hence is economical, stable, and less impacted by environmental conditions such as temperature and pH. Non-enzymatic probes are more useful for clinical use since they are more stable and have a longer shelf life. The developed non-enzymatic probes are also highly selective and sensitive to the target analyte, making them suitable for the direct detection of cardiac troponin I in actual biological samples.

Embedding AIE-type Ag28Au1 nanoclusters within ZIF-8 for improved photodynamic wound healing through bacterial eradication

Designing antibacterial agents with rapid bacterial eradication performance is paramount for the treatment of bacteria-infected wounds. Metal nanoclusters (NCs) with aggregation-induced emission (AIE) have been considered as novel photodynamic antibacterial agents without drug resistance, but they suffer from poor photostability and low charge carrier separation efficiency. Herein, we report the design of a photodynamic antibacterial agent by encapsulating AIE-type AgAu NCs (Ag28Au1 NCs) into a zeolitic Zn(2-methylimidazole)2 framework (ZIF-8). The encapsulation of AIE-type Ag28Au1 NCs into porous ZIF-8 could not only enhance the photostability of Ag28Au1 NCs by inhibiting their aggregation but also promote the separation of photoinduced charge carriers, resulting in the rapid generation of destructive reactive oxygen species (ROS) for bacterial killing under visible-light irradiation. Consequently, the as-designed photodynamic Ag28Au1 NCs@ZIF-8 antibacterial agent could rapidly eliminate 97.7% of Escherichia coli (E. coli) and 91.6% of Staphylococcus aureus (S. aureus) within 5 min in vitro under visible light irradiation. Furthermore, in vivo experimental results have highlighted the synergistic effect created by AIE-type Ag28Au1 NCs and ZIF-8, enabling Ag28Au1 NCs@ZIF-8 to effectively eradicate bacteria in infected areas, reduce inflammation, and promote the generation of blood vessels, epithelial tissue, and collagen. This synergistic effect promoted the healing of S. aureus-infected wound, with nearly 100% of wound recovery within 11 days. This work may be interesting because it sheds light on the design of metal NC-based photodynamic nanomedicine for bacteria-infected disease treatment.

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.

Elevating nanomaterial optical sensor arrays through the integration of advanced machine learning techniques for enhancing visual inspection of food quality and safety

Food quality and safety problems caused by inefficient control in the food chain have significant implications for human health, social stability, and economic progress and optical sensor arrays (OSAs) can effectively address these challenges. This review aims to summarize the recent applications of nanomaterials-based OSA for food quality and safety visual monitoring, including colourimetric sensor array (CSA) and fluorescent sensor array (FSA). First, the fundamental properties of various advanced nanomaterials, mainly including metal nanoparticles (MNPs) and nanoclusters (MNCs), quantum dots (QDs), upconversion nanoparticles (UCNPs), and others, were described. Besides, the diverse machine learning (ML) and deep learning (DL) methods of high-dimensional data obtained from the responses between different sensing elements and analytes were presented. Moreover, the recent and representative applications in pesticide residues, heavy metal ions, bacterial contamination, antioxidants, flavor matters, and food freshness detection were comprehensively summarized. Finally, the challenges and future perspectives for nanomaterials-based OSAs are discussed. It is believed that with the advancements in artificial intelligence (AI) techniques and integrated technology, nanomaterials-based OSAs are expected to be an intelligent, effective, and rapid tool for food quality assessment and safety control.

Heteroatom Doping Promoted Ultrabright and Ultrastable Photoluminescence of Water-Soluble Au/Ag Nanoclusters for Visual and Efficient Drug Delivery to Cancer Cells

Photoluminescence (PL) metal nanoclusters (NCs) have attracted extensive attention due to their excellent physicochemical properties, good biocompatibility, and broad application prospects. However, developing water-soluble PL metal NCs with a high quantum yield (QY) and high stability for visual drug delivery remains a great challenge. Herein, we have synthesized ultrabright l-Arg-ATT-Au/Ag NCs (Au/Ag NCs) with a PL QY as high as 73% and excellent photostability by heteroatom doping and surface rigidization in aqueous solution. The as-prepared Au/Ag NCs can maintain a high QY of over 61% in a wide pH range and various ionic environments as well as a respectable resistance to photobleaching. The results from structure characterization and steady-state and time-resolved spectroscopic analysis reveal that Ag doping into Au NCs not only effectively modifies the electronic structure and photostability but also significantly regulates the interfacial dynamics of the excited states and enhances the PL QY of Au/Ag NCs. Studies in vitro indicate Au/Ag NCs have a high loading capacity and pH-triggered release ability of doxorubicin (DOX) that can be visualized from the quenching and recovery of PL intensity and lifetime. Imaging-guided experiments in cancer cells show that DOX of Au/Ag NCs-DOX agents can be efficiently delivered and released in the nucleus with preferential accumulation in the nucleolus, facilitating deep insight into the drug action sites and pharmacological mechanisms. Moreover, the evaluation of anticancer activity in vivo reveals an outstanding suppression rate of 90.2% for mice tumors. These findings demonstrate Au/Ag NCs to be a superior platform for bioimaging and visual drug delivery in biomedical applications.

Antibiotic Alternatives: Multifunctional Ultra-Small Metal Nanoclusters for Bacterial Infectious Therapy Application

The prevalence of major bacterial infections has emerged as a significant menace to human health and life. Conventional treatment methods primarily rely on antibiotic therapy, but the overuse of these drugs has led to a decline in their efficacy. Moreover, bacteria have developed resistance towards antibiotics, giving rise to the emergence of superbugs. Consequently, there is an urgent need for novel antibacterial agents or alternative strategies to combat bacterial infections. Nanoantibiotics encompass a class of nano-antibacterial materials that possess inherent antimicrobial activity or can serve as carriers to enhance drug delivery efficiency and safety. In recent years, metal nanoclusters (M NCs) have gained prominence in the field of nanoantibiotics due to their ultra-small size (less than 3 nm) and distinctive electronic and optical properties, as well as their biosafety features. In this review, we discuss the recent progress of M NCs as a new generation of antibacterial agents. First, the main synthesis methods and characteristics of M NCs are presented. Then, we focus on reviewing various strategies for detecting and treating pathogenic bacterial infections using M NCs, summarizing the antibacterial effects of these nanoantibiotics on wound infections, biofilms, and oral infections. Finally, we propose a perspective on the remaining challenges and future developments of M NCs for bacterial infectious therapy.

Atomically Precise Nanometer-Sized Pt Catalysts with an Additional Photothermy Functionality

Atomically precise ~1-nm Pt nanoparticles (nanoclusters, NCs) with ambient stability are important in fundamental research and exhibit diverse practical applications (catalysis, biomedicine, etc.). However, synthesizing such materials is challenging. Herein, by employing the mixture ligand protecting strategy, we successfully synthesized the largest organic-ligand-protected (~1-nm) Pt23 NCs precisely characterized with mass spectrometry and single-crystal X-ray diffraction analyses. Interestingly, natural population analysis and Bader charge calculation indicate an alternate, varying charge -layer distribution in the sandwich-like Pt23 NC kernel. Pt23 NCs can catalyze the oxygen reduction reaction under acidic conditions without requiring calcination and other treatments, and the resulting specific and mass activities without further treatment are sevenfold and eightfold higher than those observed for commercial Pt/C catalysts, respectively. Density functional theory and d-band center calculations interpret the high activity. Furthermore, Pt23 NCs exhibit a photothermal conversion efficiency of 68.4 % under 532-nm laser irradiation and can be used at least for six cycles, thus demonstrating great potential for practical applications.

Unlocking Arene Phosphorescence in Bismuth-Organic Materials

Three novel bismuth-organic compounds, with the general formula [Bi2(HPDC)2(PDC)2]·(arene)·2H2O (H2PDC = 2,6-pyridinedicarboxylic acid; arene = pyrene, naphthalene, and azulene), that consist of neutral dinuclear Bi-pyridinedicarboxylate complexes and outer coordination sphere arene molecules were synthesized and structurally characterized. The structures of all three phases exhibit strong π-π stacking interactions between the Bi-bound PDC/HPDC and outer sphere organic molecules; these interactions effectively sandwich the arene molecules between bismuth complexes and thereby prevent molecular vibrations. Upon UV irradiation, the compounds containing pyrene and naphthalene displayed red and green emission, respectively, with quantum yields of 1.3(2) and 30.8(4)%. The emission was found to originate from the T1 → S0 transition of the corresponding arene and result in phosphorescence characteristic of the arene employed. By comparison, the azulene-containing compound displayed very weak blue-purple phosphorescence of unknown origin and is a rare example of T2 → S0 emission from azulene. The pyrene- and naphthalene-containing compounds both display radioluminescence, with intensities of 11 and 38% relative to bismuth germanate, respectively. Collectively, these results provide further insights into the structure-property relationships that underpin luminescence from Bi-based materials and highlight the utility of Bi-organic molecules in the realization of organic emission.

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.

Three species multiplexing of fluorescent dyes and gold nanoclusters recovered with fluorescence lifetime correlation spectroscopy

Biosensor applications often require the simultaneous detection of multiple analytes, with a clear need to go beyond the traditional multiplexing relying on distinct fluorescent dyes across the visible spectrum. Fluorescence lifetime correlation spectroscopy (FLCS) is a powerful approach taking advantage of the fluorescence lifetime information to separate the contributions of different fluorescent species with overlapping emission spectra. However, so far FLCS detection has been demonstrated only on binary mixtures of two fluorescent dyes, limiting its multiplexing capabilities. Here, we report the first quantitative FLCS measurements within a ternary mixture composed of three different fluorescent emitters with near-identical emission spectra. Two organic fluorescent dyes, Alexa Fluor 647 and CF640R, are combined with water-soluble Au18(SG)14 gold nanoclusters. Our experimental data establish that FLCS allows to accurately determine individual concentrations within intricate ternary mixtures. Another major aspect of interest concerns the assessment of the suitability of gold nanoclusters for FLCS multiplexing applications. With their microsecond lifetime and stable emission characteristics, gold nanoclusters add a valuable new aspect to the array of FLCS probes. Extending FLCS multiplexing beyond binary mixtures paves the way for further progress in the simultaneous highly parallel biosensing of multiple species.

Photostimulus-Responsive Peptide Dot-Centered Covalent Organic Polymers: Effective Pesticide Sensing via Enhancing Accessibility

Pesticide detection and monitoring are necessary for human health as the overapplication has serious consequences for environmental pollution. Herein, a proper modulation strategy was implemented to construct the photostimulus-responsive peptide-dot-centered covalent organic polymer (P-PCOP) nanoarchitecture for selective sensing of pesticides. The as-constructed P-PCOP was prepared at room temperature by using amino-containing peptide dots as a building block instead of common organic molecules, and the merits of P-PCOP enable it to reduce the steric hindrance of recognition, enhance the interfacial contact of the target, and facilitate the accessibility of sites, which promises to improve the sensitivity. The P-PCOF exhibited a low detection limit of 0.38 μg L-1 to cartap over the range of 1-80 μg L-1 (R2 = 0.9845), and the recoveries percentage in real samples was estimated to be 93.39-105.82%. More importantly, the DFT calculation confirmed the selective recognition ability of P-PCOP on chemical pesticides. In conjunction with a smartphone-integrated portable reading device, on-site chemical sensing is achieved. The proper modulation strategy of fixing a functional guest on the COP system contributes to the advanced structure-chemical properties that are conducive to their applications in chemical sensing.

Photoluminescence Quenching of Hydrophobic Ag29 Nanoclusters Caused by Molecular Decoupling during Aqueous Phase Transfer and EmissionRecovery through Supramolecular Recoupling

Exploiting emissive hydrophobic nanoclusters for hydrophilic applications remains a challenge because of photoluminescence (PL) quenching during phase transfer. In addition, the mechanism underlying PL quenching remains unclear. In this study, the PL-quenching mechanism was examined by analyzing the atomically precise structures and optical properties of a surface-engineered Ag29 nanocluster with an all-around-carboxyl-functionalized surface. Specifically, phase-transfer-triggered PL quenching was justified as molecular decoupling, which directed an unfixed cluster surface and weakened the radiative transition. Furthermore, emission recovery of the quenched nanoclusters was accomplished by using a supramolecular recoupling approach through the glutathione-addition-induced aggregation of cluster molecules, wherein the restriction of intracluster motion and intercluster rotation strengthened the radiative transition of the clusters. The results of this work offer a new perspective on structure-emission correlations for atomically precise nanoclusters and hopefully provide insight into the fabrication of highly emissive cluster-based nanomaterials for downstream hydrophilic applications.

Metal Ion-Regulated Fluorescent Sensor Array Based on Gold Nanoclusters for Physiological Phosphate Sensing

The detection of physiological phosphates (PPs) is of great importance due to their essential roles in numerous biological processes, but the efficient detection of different PPs simultaneously remains challenging. In this work, we propose a fluorescence sensor array for detecting PPs based on metal-ion-regulated gold nanoclusters (AuNCs) via an indicator-displacement assay. Zn2+ and Eu3+ are selected to assemble with two different AuNCs, resulting in quenching or enhancing their fluorescence. Based on the competitive interaction of metal ions with AuNCs and PPs, the fluorescence of AuNCs will be recovered owing to the disassembly of AuNC-metal ion ensembles. Depending on different PPs' distinct fluorescence responses, a four-channel sensor array was established. The array not only exhibits good discrimination capability for eight kinds of PPs (i.e., ATP, ADP, AMP, GTP, CTP, UTP, PPi, and Pi) via linear discriminant analysis but also enables quantitative detection of single phosphate (e.g., ATP) in the presence of interfering PPs mixtures. Moreover, potential application of the present sensor array for the discrimination of different PPs in real samples (e.g., cell lysates and serum) was successfully demonstrated with a good performance. This work illustrates the great potential of a metal ion-regulated sensor array as a new and efficient sensing platform for differential sensing of phosphates as well as other disease-related biomolecules.

N-Heterocyclic Carbene-Stabilized Atomically Precise Metal Nanoclusters

This perspective highlights advances in the preparation and understanding of metal nanoclusters stabilized by organic ligands with a focus on N-heterocyclic carbenes (NHCs). We demonstrate the need for a clear understanding of the relationship between NHC properties and their resulting metal nanocluster structure and properties. We emphasize the importance of balancing nanocluster stability with the introduction of reactive sites for catalytic applications and the importance of a better understanding of how these clusters interact with their environments for effective use in biological applications. The impact of atom-scale simulations, development of atomic interaction potentials suitable for large-scale molecular dynamics simulations, and a deeper understanding of the mechanisms behind synthetic methods and physical properties (e.g., the bright fluorescence displayed by many clusters) are emphasized.

Micelle-mediated chemiluminescence of 6-aza-2-thiothymine-protected gold nanoclusters for carbazochrome sodium sulfonate detection

Chemiluminescence (CL) intensity of luminol-H2O2 system was dramatically enhanced by cetyltrimethylammonium bromide (CTAB) micelle-mediated 6-aza-2-thiothymine-protected gold nanoclusters (ATT-AuNCs). It is proved that spherical micelles of CTAB in aqueous solution improved the dispersity of ATT-AuNCs, thus enhancing their catalytic activity, which brought in the increased CL intensity of luminol-H2O2 system. Carbazochrome sodium sulfonate (CSS) with a hemostatic containing tetrahydroindole structure broke the spherical micelles and notably quenched the CL intensity of luminol-H2O2-CTAB-ATT AuNCs system. Based on these results, a simple, fast, and sensitive CL method has been developed for the detection of CSS with a linear range of 0.25-25 μM and a detection limit of 0.11 μM. The method has also been successfully applied to the determination of CSS in serum with satisfied recoveries in the range of 89.6 % to 103.7 %. This study not only provides an effective approach for CSS detection but also paves the way for AuNCs-based CL applications.

General Gel-sol Method to Synthesize Various Highly Fluorescent Nanoclusters and Assay of Nuclease with the Near Infrared-emitting Gold Nanoclusters

A general egg white gel-sol strategy for fabrication of highly fluorescent Au, Ag, Cu, and Pt nanoclusters (NCs) and the first example of using Au NCs for assay of nuclease activity and inhibition were described. The Au NCs enabled bright red fluorescence, and the other Ag, Cu, and Pt NCs have highly blue emission. The red-emitting Au NCs were further applied in assay of S1 nuclease activity and inhibition. Free hemin efficiently quenches the emission of Au NCs by photoinduced electron transfer due to the formation of Au NCs-hemin conjugates. However, G-quadruplex/hemin exerts negligible effect on its fluorescence due to no Au NCs-hemin conjugate formed. There are stronger electrostatic repulsion effects between both negatively charged G-quadruplex and Au NCs. Therefore, a novel G-quadruplex/hemin-based Au NCs fluorescent sensor for S1 nuclease was designed. A known G-rich oligonucleotide (ODN) serves as not only substrate for S1 nuclease but also for the construction of G-quadruplex/hemin. The G-rich ODN is hydrolyzed into fragments by S1 nuclease resulting in no G-quadruplex/hemin formation. Therefore, the free hemin quenches Au NCs fluorescence remarkably and the assay of S1 nuclease activity and inhibition has accomplished. Both the fluorescent NCs syntheses and the detection of S1 nuclease are facile and efficient.

Proposed Structural Model for Chiral Au40(SC2H4Ph)24 Nanoclusters

The structural configuration of thiolate-protected gold nanoclusters plays a pivotal role in elucidating the correlation between their structure and properties, comprehending their stability, and guiding experimental synthesis. In this study, utilizing the grand unified model and the ring model, we employed an innovative strategy of fusing triangular Au3 and tetrahedral Au4 elementary blocks by sharing a gold atom to design the gold core, predicting the structure of the Au40(SR)24 nanoclusters. Density functional theory calculations indicate that with the protective ligands simplified to methyl groups the energy of the predicted Au40(SR)24 is 0.45 eV lower than that of the experimentally reported Au40(o-MBT)24 nanoclusters, implying its substantial stability. Furthermore, the calculated UV absorption spectrum and circular dichroism spectrum of predicted Au40(SR)24 are consistent with the experimental results of Au40(SC2H4Ph)24 nanoclusters, suggesting that the predicted structure is a likely candidate for the structure of Au40(SC2H4Ph)24 nanoclusters.

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.

Unveiling the photoluminescence dynamics of gold nanoclusters with fluorescence correlation spectroscopy

Gold nanoclusters (AuNCs) have captured significant interest for their photoluminescent properties; however, their rapid photodynamics remain elusive while probed by ensemble-averaging spectroscopy techniques. To address this challenge, we use fluorescence correlation spectroscopy (FCS) to uncover the photoluminescence dynamics of colloidal Au18(SG)14 nanoclusters. Our FCS analysis reveals the photoluminescence (PL) brightness per nanocluster, elucidating the impact of photoexcitation saturation and ligand interactions. Unlike DNA-encapsulated silver nanoclusters, their gold counterparts notably exhibit minimal blinking, with moderate amplitudes and 200 μs characteristic times. Our data also clearly reveal the occurrence of photon antibunching in the PL emission, showcasing the quantum nature of the PL process, with each AuNC acting as an individual quantum source. Using zero-mode waveguide nanoapertures, we achieve a 16-fold enhancement of the PL brightness of individual AuNCs. This constitutes an important enabling proof-of-concept for tailoring emission properties through nanophotonics. Overall, our study bridges the gap between ensemble-averaged techniques and single-molecule spectroscopy, offering new insights into AuNC photodynamics for biosensing and imaging applications.

Optical nanosensors based on noble metal nanoclusters for detecting food contaminants: A review

Food contaminants present a significant threat to public health. In response to escalating global concerns regarding food safety, there is a growing demand for straightforward, rapid, and sensitive detection technologies. Noble metal nanoclusters (NMNCs) have garnered considerable attention due to their superior attributes compared to other optical materials. These attributes include high catalytic activity, excellent biocompatibility, and outstanding photoluminescence properties. These features render NMNCs promising candidates for crafting nanosensors for food contaminant detection, offering the potential for the development of uncomplicated, swift, sensitive, user-friendly, and cost-effective detection approaches. This review investigates optical nanosensors based on NMNCs, including the synthesis methodologies of NMNCs, sensing strategies, and their applications in detecting food contaminants. Furthermore, it involves a comparative assessment of the applications of NMNCs in optical sensing and their performance. Ultimately, this paper imparts fresh perspectives on the forthcoming challenges. Hitherto, optical (particularly fluorescent) nanosensors founded on NMNCs have demonstrated exceptional sensing capabilities in the realm of food contaminant detection. To enhance sensing performance, future research should prioritize atomically precise NMNCs synthesis, augmentation of catalytic activity and optical properties, development of high-throughput and multimode sensing, integration of NMNCs with microfluidic devices, and the optimization of NMNCs storage, shelf life, and transportation conditions.

Molecularly Engineered Room-Temperature Phosphorescence for Biomedical Application: From the Visible toward Second Near-Infrared Window

Phosphorescence, characterized by luminescent lifetimes significantly longer than that of biological autofluorescence under ambient environment, is of great value for biomedical applications. Academic evidence of fluorescence imaging indicates that virtually all imaging metrics (sensitivity, resolution, and penetration depths) are improved when progressing into longer wavelength regions, especially the recently reported second near-infrared (NIR-II, 1000-1700 nm) window. Although the emission wavelength of probes does matter, it is not clear whether the guideline of "the longer the wavelength, the better the imaging effect" is still suitable for developing phosphorescent probes. For tissue-specific bioimaging, long-lived probes, even if they emit visible phosphorescence, enable accurate visualization of large deep tissues. For studies dealing with bioimaging of tiny biological architectures or dynamic physiopathological activities, the prerequisite is rigorous planning of long-wavelength phosphorescence, being aware of the cooperative contribution of long wavelengths and long lifetimes for improving the spatiotemporal resolution, penetration depth, and sensitivity of bioimaging. In this Review, emerging molecular engineering methods of room-temperature phosphorescence are discussed through the lens of photophysical mechanisms. We highlight the roles of phosphorescence with emission from visible to NIR-II windows toward bioapplications. To appreciate such advances, challenges and prospects in rapidly growing studies of room-temperature phosphorescence are described.

Thermostable protein-stabilized gold nanoclusters as a peroxidase mimic

Protein-stabilized gold nanoclusters (AuNCs) are fascinating nanostructures with exciting properties owing to their ultra-small sizes and functional shell. However, their applications under extreme conditions are still complicated, waiting for programmable solutions. Therefore, the design of a multi-functional protein stabilizer for specific purposes gains attention to improve the stability and functionality of AuNCs. Herein, we exploited the thermostability of genetically engineered KlenTaq DNA polymerase containing five cysteine residues (KTQ5C) to synthesize heat-stable AuNCs (AuNC@KTQ5C) and characterize optical, structural, and hydrodynamic properties. Besides their excellent photophysical properties, AuNC@KTQ5C also exhibit superior peroxidase-like (POD-like) catalytic activity following typical Michaelis-Menten kinetics together with a high affinity towards the POD substrate 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)-diammonium salt (ABTS). Moreover, FTIR and relative catalytic activity analysis of AuNC@KTQ5C reveal that KTQ5C is resistant to changes in protein secondary structure while the AuNCs conserve 70-80% of their catalytic performance after heat treatments up to more than 80 °C. Our findings show that stabilizing AuNCs with thermostable KTQ5C not only preserves the advantages of protein-stabilized AuNCs but can also promote the resistance of AuNCs against aggregation due to protein denaturation under extreme reaction temperatures, protecting their fluorescent emission or catalytic activity.

Engineering GaN/AuNC core-shell nanowire heterojunctions by gold nanoclusters with excitation-dependent behavior for enhancing the responsivity and stability of self-driven photodetectors

Self-driven broadband photodetectors (PDs) with low-power consumption have great potential applications in the wide range of next-generation optoelectronic devices. In this study, a self-driven broadband PD responding to an ultraviolet-visible range based on gallium nitride/gold nanocluster (GaN/AuNC) core-shell nanowire heterojunctions is fabricated for the first time. By introducing the AuNCs onto the GaN nanowire surfaces, the GaN/AuNC core-shell nanowire heterojunctions can be formed efficiently. It is crucial that AuNCs have the functions of light collectors and hole conductors in heterojunctions due to the suitable energy level alignment. Under the optimized conditions of AuNCs, it is found that GaN/AuNC core-shell nanowires can significantly increase the photocurrent and responsivity of PDs, mainly resulting from the light interreflection within the heterojunctions and the effective improvement of carrier transport. Owing to the excitation-dependent emission behavior of AuNCs, the responsivity of PD with GaN/AuNC core-shell nanowire heterojunctions can be enhanced by around 330% compared with that of PD without AuNCs under visible illumination. Furthermore, GaN/AuNC hybrid nanowires with excitation-dependent fluorescence behavior can modulate the enhanced amplitude performance of broadband PDs. Owing to the high stability of AuNCs, the photocurrent of the PD with AuNCs is still quite stable after continuous operation for more than 20 000 s. Therefore, this study provides an effective method for developing new broadband PDs with high performance and low energy consumption.

Self-Trapped, Thermally Equilibrated Delayed Fluorescence Enables Low-Reabsorption Luminescent Solar Concentrators Based on Gold-Doped Silver Nanoclusters

Reabsorption-free luminescent solar concentrators (LSCs) are crucial ingredients for photovoltaic windows. Atomically precise metal nanoclusters (NCs) with large Stokes-shifted photoluminescence (PL) hold great promise for applications in LSCs. However, a fundamental understanding of the PL mechanism, particularly on the excited-state interaction and exciton kinetics, is still lacking. Herein, we studied the exciton-phonon coupling and singlet/triplet exciton dynamics for gold-doped silver NCs in a solid matrix. Following photoexcitation, the excitons can be self-trapped via strong exciton-phonon coupling. Subsequently, rapid thermal equilibration between the singlet and triplet states occurs due to the coexistence of small energy splitting and spin-orbit coupling. Finally, broadband delayed fluorescence with a large Stokes shift can be generated, namely, self-trapped, thermally equilibrated delayed fluorescence (ST-TEDF). Benefiting from superior ST-TEDF, we demonstrated efficient LSCs with minimized reabsorption.

Synthesis of Greenish-Yellow Fluorescent Copper Nanocluster for the Selective and Sensitive Detection of Fipronil Pesticide in Vegetables and Grain Samples

In this paper, a new synthetic route is introduced for the synthesis of high-luminescent greenish-yellow fluorescent copper nanoclusters (PVP@A. senna-Cu NCs) using Avaram senna (A. senna) and polyvinylpyrrolidone (PVP) as templates. A. senna plant extract mainly contains variety of phytochemicals including glycosides, sugars, saponins, phenols, and terpenoids that show good pharmacological activities such as anti-inflammatory, antioxidant, and antidiabetic. PVP is a stable and biocompatible polymer that is used as a stabilizing agent for the synthesis of PVP@A. senna-Cu NCs. The size, surface functionality, and element composition of the fabricated Cu NCs were confirmed by various analytical techniques. The as-prepared greenish-yellow fluorescent Cu NCs exhibit significant selectivity towards fipronil, thereby favoring to assay fipronil pesticide with good linearity in the range of 3.0-30 μM with a detection limit of 65.19 nM. More importantly, PVP@A. senna-Cu NCs are successfully applied to assay fipronil in vegetable and grain samples.

Strategies for Improving Selectivity and Sensitivity of Schiff Base Fluorescent Chemosensors for Toxic and Heavy Metals

Toxic cations, including heavy metals, pose significant environmental and health risks, necessitating the development of reliable detection methods. This review investigates the techniques and approaches used to strengthen the sensitivity and selectivity of Schiff base fluorescent chemosensors designed specifically to detect toxic and heavy metal cations. The paper explores a range of strategies, including functional group variations, structural modifications, and the integration of nanomaterials or auxiliary receptors, to amplify the efficiency of these chemosensors. By improving selectivity towards targeted cations and achieving heightened sensitivity and detection limits, consequently, these strategies contribute to the advancement of accurate and efficient detection methods while increasing the range of end-use applications. The findings discussed in this review offer valuable insights into the potential of leveraging Schiff base fluorescent chemosensors for the accurate and reliable detection and monitoring of heavy metal cations in various fields, including environmental monitoring, biomedical research, and industrial safety.

Engineering Au44 Nanoclusters for NIR-II Luminescence Imaging-Guided Photoactivatable Cancer Immunotherapy

Immunotherapy is an advanced therapeutic strategy of cancer treatment but suffers from the issues of off-target adverse effects, lack of real-time monitoring techniques, and unsustainable response. Herein, an ultrasmall Au nanocluster (NC)-based theranostic probe is designed for second near-infrared window (NIR-II) photoluminescence (PL) imaging-guided phototherapies and photoactivatable cancer immunotherapy. The probe (Au44MBA26-NLG for short) is composed of atomically precise and NIR-II emitting Au44MBA26 NCs (here MBA denotes water-soluble 4-mercaptobenzoic acid) conjugated with immune checkpoint inhibitor 1-cyclohexyl-2-(5H-imidazo[5,1-a]isoindol-5-yl)ethanol (NLG919) via a singlet oxygen (1O2)-cleavable linker. Upon NIR photoirradiation, the Au44MBA26-NLG not only enables NIR-II PL imaging of tumors in deep tissues for guiding tumor therapy but also allows the leverage of photothermal property for cancer photothermal therapy (PTT) and the photogenerated 1O2 for photodynamic therapy (PDT) and releasing NLG919 for cancer immunotherapy. Such a multiple effect modulated by Au44MBA26-NLG prompts the proliferation and activation of effector T cells, upshifts systemic antitumor T-lymphocyte (T cell) immunity, and finally suppresses the growth of both primary and distant tumors in living mice. Overall, this study may provide a promising theranostic nanoplatform toward NIR-II PL imaging-guided phototherapies and photoactivatable cancer immunotherapy.

In Situ Depletion-Guided Engineering of Nanoshell-like Gold Nanocluster Assemblies with Enhanced Peroxidase-like Nanozyme Activity

Functional superstructures constructed from metal nanoclusters (MNCs) hold great promise in providing highly tunable photoluminescence (PL), catalytic activity, photothermal stability, and biological functionality. However, their controlled synthesis with well-defined size, structure, and properties remains a significant challenge. Herein, we introduce a novel approach that combines depletion attraction and thermal activation to induce the in situ formation of spherical superclusters (AuSCs) from Au(I)-thiolate complexes within the assembly. Extensive characterization and electron tomographic reconstruction reveal that Au(I)-thiolate complexes can be sequentially transitioned into metallic Au0, resulting in hollow nanoshell-like structures with consistent size (∼110 nm) and diverse shell configurations. Our results demonstrate that AuSCs with thinner shells, containing a high concentration of Au(I)-thiolate complexes, exhibit the highest PL, while AuSCs with thicker shells, containing high concentrations of metallic gold atoms and low ligand density, show remarkable peroxidase-like nanozyme activity in the 3,3',5,5'-tetramethylbenzidine (TMB) oxidation reaction.

Solvent-Induced Modulation in the Optical Properties of Copper Nanoclusters and Revealing the Isomeric Effect of Templates

The solvent plays an influential role in controlling the nucleation process of metal nanoclusters (MNCs) and thereby significantly modulates their optical signatures. Herein, we have demonstrated the solvent-induced modulation in the optical properties of copper nanoclusters (CuNCs), primarily governed by the solvent polarity. During the preparation of para-mercaptobenzoic acid (p-MBA)-templated CuNCs, the simultaneous formation of blue-emitting CuNCs (B-CuNCs) and red-emitting CuNCs (R-CuNCs) were observed up to 7 h of reaction time, reflected from the systematic increment in the photoluminescence (PL) intensity at 420 nm and 615 nm, respectively. However, after 7 h of reaction time, the exclusive formation of B-CuNCs was observed. Such simultaneous growth and depletion dynamics of CuNCs result in a significant modulation in their optical properties. The variation of the solvent from water to less polar solvents such as DMSO and DMF restricts this inter-cluster dynamics by stabilizing both the CuNCs (B-CuNCs and R-CuNCs). Thereby, a single-component White Light Emission (WLE) was realized in DMSO with CIE coordinates (0.37, 0.36). The isomeric effect of the templates has also been investigated which extensively controls the optical and catalytic properties of the CuNCs.

Structure, optical properties, and catalytic applications of alkynyl-protected M4Rh2 (M = Ag/Au) nanoclusters with atomic precision: a comparative study

We report two atomically precise alloy nanoclusters of Ag₄Rh₂(CCAr^F)₈(PPh₃)₂ and Au₄Rh₂(CCAr^F)₈(PPh₃)₂ (Ar = 3,5-(CF₃)₂C₆H₃, abbreviated as Ag₄Rh₂ and Au₄Rh₂, respectively) co-protected by alkynyl and phosphine ligands. Both clusters have identical octahedral metal core configurations and can be termed superatoms with two free electrons. However, they possess different optical features, manifested by totally different absorbance peaks, and drastically different emission peaks, and also, Ag₄Rh₂ has a much higher fluorescence quantum yield (18.43%) than Au₄Rh₂ (4.98%). Moreover, Au₄Rh₂ exhibited markedly superior catalytic performance in the electrochemical hydrogen evolution reaction (HER), manifested by a much lower overpotential at 10 mA cm^-2 and better stability. Density functional theory (DFT) calculations revealed that the free energy change of Au₄Rh₂ for the adsorption of two H* (0.64 eV) is lower than that of Ag₄Rh₂ for the adsorption of one H* (-0.90 eV) after stripping a single alkynyl ligand from the cluster. In contrast, Ag₄Rh₂ demonstrated much stronger catalytic capability for catalyzing 4-nitrophenol reduction. The present study provides an exquisite example to understand the structure-property relationship of atomically precise alloy nanoclusters, and emphasizes the importance of fine-tuning of the physicochemical properties and catalytic performance of the metal nanoclusters through modulating the metal core and beyond.

Amorphous Copper-Based Nanoparticles with Clusterization-Triggered Phosphorescence for Ultrasensing 2,4,6-Trinitrotoluene

Amorphous metal-based nanostructures have attracted great attention recently due to their facilitative electron transfer and abundant reactive sites, whereas it remains enigmatic as to whether amorphous copper-based nanoparticles (CuNPs) can be achieved. Here, for synthesizing amorphous CuNPs, glutathione is adopted as a ligand to inhibit the nucleation and crystallization process via its electrostatic repulsion. By subtly tailoring the solvent polarity, not only can amorphous glutathione-functionalized CuNPs (GSH-CuNPs) with phosphorescent performance be achieved after transferring the non-conjugation of GSH ligand to through-space conjugation, namely clusterization-triggered emission, but also the phosphorescence-off of GSH-CuNPs toward 2,4,6-trinitrotoluene (TNT) can be realized by the photoinduced electron-transfer process through the hydrogen bond channel, which is established between carboxyl and amino groups of GSH-CuNPs with the nitryl group of TNT. Benefitting from the intrinsic superiorities of the amorphous CuNPs, desired phosphorescence and detection performances of GSH-CuNPs toward airborne TNT microparticulates are undoubtedly realized, including high quantum yield (13.22%), excellent specificity in 33 potential interferents, instantaneous response, and ultralow detection limit (1.56 pg). The present GSH-CuNPs are expected to stretch amorphous metal-based nanostructures and deepen the insights into amorphous materials for optical detection.

Ultrasmall Coinage Metal Nanoclusters as Promising Theranostic Probes for Biomedical Applications

Ultrasmall coinage metal nanoclusters (NCs, <3 nm) have emerged as a novel class of theranostic probes due to their atomically precise size and engineered physicochemical properties. The rapid advances in the design and applications of metal NC-based theranostic probes are made possible by the atomic-level engineering of metal NCs. This Perspective article examines (i) how the functions of metal NCs are engineered for theranostic applications, (ii) how a metal NC-based theranostic probe is designed and how its physicochemical properties affect the theranostic performance, and (iii) how metal NCs are used to diagnose and treat various diseases. We first summarize the tailored properties of metal NCs for theranostic applications in terms of biocompatibility and tumor targeting. We focus our discussion on the theranostic applications of metal NCs in bioimaging-directed disease diagnosis, photoinduced disease therapy, nanomedicine, drug delivery, and optical urinalysis. Lastly, an outlook on the challenges and opportunities in the future development of metal NCs for theranostic applications is provided.

Photoluminescent gold nanoclusters as two-photon excited ratiometric pH sensor and photoactivated peroxidase

A two-photon excited ratiometric fluorescent pH sensor is reported by combining L-cysteine-protected AuNCs (Cys@AuNCs) with fluorescein isothiocyanate (FITC). Cys@AuNCs were synthesized through a one-step self-reduction route and showed pH-responsive photoluminescence at 650 nm. Benefiting from the opposite pH response of Cys@AuNCs and FITC, the fluorescence ratio (F_{515 nm}/F_{650 nm}) of FITC&Cys@AuNCs provided a large dynamic range of 200-fold for pH measurement in the response interval of pH 5.0-8.0. Based on the excellent two-photon absorption coefficient of Cys@AuNCs, the sensor was expected to achieve sensitive quantitation of pH in living cells under two-photon excitation. In addition, colorimetric biosensing based on enzyme-like metal nanoclusters has attracted wide attention due to their low-cost, simplicity, and practicality. It is crucial to develop high catalytic activity nanozyme from the viewpoint of practical application. The synthesized Cys@AuNCs exhibited excellent photoactivated peroxidase-like activity with high substrate affinity and catalytic reaction rate, promising for rapid colorimetric biosensing of field analysis and the control of catalytic reactions by photostimulation.

Luminescent [CO2@Ag20(SAdm)10(CF3COO)10(DMA)2] nanocluster: synthetic strategy and its implication towards white light emission

Owing to the quantized size and associated discrete energy levels, atomically precise silver nanoclusters (Ag NCs) hold great potential for designing functional luminescent materials. However, the thermally activated non-radiative transition of Ag(I)-based NCs has faded the opportunities. To acquire the structurally rigid architecture of cluster nodes for constraining such transitions, a new synthetic approach is unveiled here that utilizes a neutral template as a cluster-directing agent to assemble twenty Ag(I) atoms that ensure the maximum number of surface-protecting ligand attachment possibilities in a particular solvent medium. The solvent polarity triggers the precise structural design to circumvent the over-reliance of the templates, which results in the formation of [CO₂@Ag₂₀(SAdm)₁₀(CF₃COO)₁₀(DMA)₂] NC (where SAdm = 1-adamantanethiolate and DMA = N,N-dimethylacetamide) exhibiting an unprecedented room-temperature photoluminescence emission. The high quantum yield of the generated blue emission ensures its candidature as an ideal donor for artificial light-harvesting system design, and it is utilized with the two-step sequential energy transfer process, which finally results in the generation of ideal white light. For implementing perfect white light emission, the required chromophores in the green and red emission regions were chosen based on their effective spectral overlap with the donor components. Due to their favorable energy-level distribution, excited state energy transfers occurred from the NC to β-carotene at the initial step, then from the conjugate of the NC and β-carotene to another chromophore, Nile Blue, at the second step via a sequential Förster resonance energy transfer pathway.

The synthesis of gold nanoclusters with high stability and their application in fluorometric detection for Hg2+ and cell imaging

Sensing Hg²⁺ is significant to protecting human health and environmental ecosystems, for its toxicity and genotoxicity. Here, highly stable fluorescent folic acid (FA)-protected Au nanoclusters (FA-AuNCs) were synthesized by optimizing the reactive parameters with high quantum yield of 34.7%. Main components of Au₄L were confirmed by MALDI-TOF, and the electron-rich residues of FA shell enabled FA-AuNCs excellent photostability. FA-AuNCs exhibited sensitive response behavior to Hg²⁺ with a minimum detectability of 1.3 nM, and presented extreme effect to the detection of Hg²⁺ in real water. Notably, the cellular imaging and in-situ detection of Hg²⁺ in cells can be achieved visually. The high selectivity was attributed to the chemical bond formed between Au⁺ (4f¹⁴5d¹⁰) and Hg²⁺ (4f¹⁴5d¹⁰). And the internal filter effect and static quenching effect were proved triggering the quenching of FA-AuNCs. The ultra-stable FA-AuNCs provide a potential promising opportunity for the in-situ tracing Hg²⁺ from environmental and biological samples.

Fluorescent detection of emerging virus based on nanoparticles: From synthesis to application

The spread of COVID-19 has caused huge economic losses and irreversible social impact. Therefore, to successfully prevent the spread of the virus and solve public health problems, it is urgent to develop detection methods with high sensitivity and accuracy. However, existing detection methods are time-consuming, rely on instruments, and require skilled operators, making rapid detection challenging to implement. Biosensors based on fluorescent nanoparticles have attracted interest in the field of detection because of their advantages, such as high sensitivity, low detection limit, and simple result readout. In this review, we systematically describe the synthesis, intrinsic advantages, and applications of organic dye-doped fluorescent nanoparticles, metal nanoclusters, up-conversion particles, quantum dots, carbon dots, and others for virus detection. Furthermore, future research initiatives are highlighted, including green production of fluorescent nanoparticles with high quantum yield, speedy signal reading by integrating with intelligent information, and error reduction by coupling with numerous fluorescent nanoparticles.

Green-Emissive Copper Nanocluster with Aggregation-Enhanced Emission for Selective Detection of Al3

Selective detection of Al³⁺ is of great significance both for the benefit of human health and environmental safety considerations. In this work, a sensitive and selective fluorescence assay for Al³⁺ was proposed based on the green-emissive Cu nanoclusters (Cu NCs). Different from the commonly reported works, the green emissive Cu NCs showed dual emission bands at 450 and 510 nm, attributed to the reaction product between polyvinyl pyrrolidone and ascorbic acid and the Cu core, respectively. Al³⁺ could induce the aggregation of Cu NCs by forming covalent bonds, which results in the enhancement of photoluminescence intensity. This enhancement phenomenon is rather selective to Al³⁺ , which endows the detection in real samples. These results provide new insights for the fluorescence mechanisms of metal NCs, which also provided a functional luminescent material for various applications, such as chemical sensing, bioimaging and photoelectric devices.

Optical Sensing of Toxic Cyanide Anions Using Noble Metal Nanomaterials

Water toxicity, one of the major concerns for ecosystems and the health of humanity, is usually attributed to inorganic anions-induced contamination. Particularly, cyanide ions are considered one of the most harmful elements required to be monitored in water. The need for cyanide sensing and monitoring has tempted the development of sensing technologies without highly sophisticated instruments or highly skilled operations for the objective of in-situ monitoring. Recent decades have witnessed the growth of noble metal nanomaterials-based sensors for detecting cyanide ions quantitatively as nanoscience and nanotechnologies advance to allow nanoscale-inherent physicochemical properties to be exploited for sensing performance. Particularly, noble metal nanostructure e-based optical sensors have permitted cyanide ions of nanomolar levels, or even lower, to be detectable. This capability lends itself to analytical application in the quantitative detection of harmful elements in environmental water samples. This review covers the noble metal nanomaterials-based sensors for cyanide ions detection developed in a variety of approaches, such as those based on colorimetry, fluorescence, Rayleigh scattering (RS), and surface-enhanced Raman scattering (SERS). Additionally, major challenges associated with these nano-platforms are also addressed, while future perspectives are given with directions towards resolving these issues.

Structured copper-hydride nanoclusters provide insight into the surface-vacancy-defect to non-defect structural evolution

Exploring the structural evolution of clusters with similar sizes and atom numbers induced by the removal or addition of a few atoms contributes to a deep understanding of structure-property relationships. Herein, three well-characterized copper-hydride nanoclusters that provide insight into the surface-vacancy-defect to non-defect structural evolution were reported. A surface-defective copper hydride nanocluster [Cu₂₈(S-c-C₆H₁₁)₁₈(PPh₂Py)₃H₈]²⁺ (Cu₂₈-PPh₂Py for short) with only one C ₁ symmetry axis was synthesized using a one-pot method under mild conditions, and its structure was determined. Through ligand regulation, a 29^th copper atom was inserted into the surface vacancy site to give two non-defective copper hydride nanoclusters, namely [Cu₂₉(SAdm)₁₅Cl₃(P(Ph-Cl)₃)₄H₁₀]⁺ (Cu₂₉-P(Ph-Cl)₃ for short) with one C ₃ symmetry axis and (Cu₂₉(S-c-C₆H₁₁)₁₈(P(Ph-^pMe)₃)₄H₁₀)⁺ (Cu₂₉-P(Ph-Me)₃ for short) with four C ₃ symmetry axes. The optimized structures show that the 10 hydrides cap four triangular and all six square-planar structures of the cuboctahedral Cu₁₃ core of Cu₂₉-P(Ph-Me)₃, while the 10 hydrides cap four triangular and six square-planar structures of the anti-cuboctahedral Cu₁₃ core of Cu₂₉-P(Ph-Cl)₃, with the eight hydrides in Cu₂₈-PPh₂Py capping four triangular and four square planar-structures of its anti-cuboctahedral Cu₁₃ core. Cluster stability was found to increase sequentially from Cu₂₈-PPh₂Py to Cu₂₉-P(Ph-Cl)₃ and then to Cu₂₉-P(Ph-Me)₃, which indicates that stability is affected by the overall structure of the cluster. Structural adjustments to the metal core, shell, and core-shell bonding model, in moving from Cu₂₈-PPh₂Py to Cu₂₉-P(Ph-Cl)₃ and then to Cu₂₉-P(Ph-Me)₃, enable the structural evolution and mechanism responsible for their physicochemical properties to be understood and provide valuable insight into the structures of surface vacancies in copper nanoclusters and structure-property relationships.

Polyethyleneimine-capped copper nanoclusters for detection and discrimination of 2,4,6-trinitrotoluene and 2,4,6-trinitrophenol

The detection and discrimination of 2,4,6-trinitrotoluene (TNT) and 2,4,6-trinitrophenol (TNP) from analogues are of great importance to global security and are full of challenges in the field of trace sensing. Here, benefitting from the strong electrophilicity of TNT, a sensing strategy is established by synthesizing polyethyleneimine capped copper nanoclusters (PEI-Cu NCs) with abundant -NH₂ groups. By carefully controlling the size and structure of PEI-Cu NCs, Förster resonance energy transfer (FRET) from PEI-Cu NCs to the Meisenheimer complex occurs resulting from their spectral overlap when detecting TNT, while, due to the energy level match of TNP with PEI-Cu NCs, as well as the strong affinity between its -OH and -NH₂ in PEI-Cu NCs, photo-induced electron transfer (PET) is feasibly expected. As a result, TNT and TNP could be detected from 26 types of analogues and cations with a limit of detection (LOD) of 26.57 and 12.82 nM, respectively. Besides, owing to the brown color of the Meisenheimer complex, the discrimination of TNT and TNP could be additionally realized by colorimetric detection. We expect that the proposed methodology would not only shine light on the detection and discrimination of TNT and TNP that mitigate against public security concerns, but also pave a way for the deep understanding of FRET and PET related fluorescence quenching mechanisms from the aspect of controllable sensing material design and synthesis.

Highly Efficient Luminescence from Charge-Transfer Gold Nanoclusters Enabled by Lewis Acid

Understanding the complicated intramolecular charge transfer (ICT) behaviors of nanomaterials is crucial to the development of high-quality nanoluminophores for various applications. However, the ICT process in molecule-like metal nanoclusters has been rarely explored. Herein, a proton binding-induced enhanced ICT state is discovered in 6-aza-2-thiothymine-protected gold nanoclusters (ATT-AuNCs). Such an excited-state electron transfer process gives rise to the weakened and red-shifted photoluminescence of these nanoclusters. By the joint use of this newfound ICT mechanism and a restriction of intramolecular motion (RIM) strategy, a red shift in the emission maxima of 30 nm with 27.5-fold higher fluorescence quantum efficiency is achieved after introducing rare-earth scandium ion (Sc³⁺) into ATT-AuNCs. Furthermore, it is found that upon the addition of Sc³⁺, the photoinduced electron transfer (PET) rate from ATT-AuNCs to minocycline is largely accelerated by forming a donor-bridge-acceptor structure. This paper offers a simple method to modulate the luminescent properties of metal nanoclusters for the rational design of next-generation sensing platforms.

A review on functional nanoarchitectonics nanocomposites based on octahedral metal atom clusters (Nb6, Mo6, Ta6, W6, Re6): inorganic 0D and 2D powders and films

This review is dedicated to various functional nanoarchitectonic nanocomposites based on molecular octahedral metal atom clusters (Nb₆, Mo₆, Ta₆, W₆, Re₆). Powder and film nanocomposites with two-dimensional, one-dimensional and zero-dimensional morphologies are presented, as well as film matrices from organic polymers to inorganic layered oxides. The high potential and synergetic effects of these nanocomposites for biotechnology applications, photovoltaic, solar control, catalytic, photonic and sensor applications are demonstrated. This review also provides a basic level of understanding how nanocomposites are characterized and processed using different techniques and methods. The main objective of this review would be to provide guiding significance for the design of new high-performance nanocomposites based on transition metal atom clusters.

Synthesis of Orange-Red Emissive Au-SG and AuAg-SG Nanoclusters and Their Turn-OFF vs. Turn-ON Metal Ion Sensing

Synthesis of luminescent metal cluster for selective sensing of specific analyte with detail mechanistic understanding is very important for real world applications as well as for developing new emissive materials. In the present work, we have synthesized L-glutathione stabilized gold (Au-SG) and gold-silver bimetallic (AuAg-SG) clusters under identical experimental conditions with orange red emissive characteristics for both. Detail photo physical analysis reveals that both clusters are phosphorescent in nature with moderate quantum yield of 7% and 19% for Au-SG and AuAg-SG respectively and their excited state lifetime values are in the range of 1-2 μs. While Au-SG cluster showed luminescence quenching response (turn-off) in presence of Fe³⁺ and Hg²⁺ ions, AuAg-SG cluster showed turn-off response for Cu²⁺, Fe³⁺ and Hg²⁺, but luminescent enhancement (turn-on) response for Cd²⁺ ions. The highest detection limit obtained for Cu²⁺ ion by AuAg-SG cluster is 20 nM while for Cd²⁺ ion it is 75 nM. From Time Correlated Single Photo Counting (TCSPC) and Dynamic Light Scattering (DLS) measurements we postulated that except Cd²⁺, all other metal ions cause aggregation of clusters through ligation with SG ligands while Cd²⁺ ion does not induce any cluster aggregation but binds to cluster surface atoms. The near constant life time values of both clusters during gradual addition of respective metal ions confirms static quenching/enhancement process through formation of stable ground state adducts.

Gold nanocluster with AIE: A novel photodynamic antibacterial and deodorant molecule

Designing long-lasting yet high-efficiency antimicrobial and deodorant agents is an everlasting goal for environmental and public health. Here we present the design of AIE-featured Au nanoclusters (NCs) for visible-light-driven antibacterial and deodorant applications. Owing to the intriguing AIE traits, the good harvest of visible-light, and rich surface chemistry, the AIE-featured Au NCs unprecedentedly exhibit excellent visible-light-driven antibacterial activities against gram-positive (≥98.5%) and gram-negative bacteria (≥99.94%), which is resulted from their photodynamic producibility of abundant reactive oxygen species including O₂^•-, •OH and H₂O₂ via O₂ reduction and subsequent H₂O₂ oxidation. In addition, the Au NCs are demonstrated to be biocompatible, and easy to be deployed for downstream antibacterial and deodorant applications. For example, the Au NCs-modified domestic materials (e.g., latex, ceramic glaze, organic fiber, and clothings) achieve long-lasting antibacterial efficiency of 99% and deodorant efficiency of >97.9% under visible-light irradiation. This work may shed light on designing novel AIE-featured metal NCs with photodynamic antibacterial and deodorant functions, enabling metal NCs and corresponding downstream materials to step into the photodynamic antibacterial and deodorant era.