Fast, high-yield synthesis of amphiphilic Ag nanoclusters and the sensing of Hg(2+) in environmental samples

A comprehensive review of atomically precise metal nanoclusters with emergent photophysical properties towards diverse applications

Atomically precise metal nanoclusters (MNCs) composed of a few to hundreds of metal atoms represent an emerging class of nanomaterials with a precise composition. With the size approaching the Fermi wavelength of electrons, their energy levels are well-separated, leading to molecule-like properties, like discrete single electronic transitions, tunable photoluminescence (PL), inherent structural anisotropy, and distinct redox behavior. Extensive synthetic efforts and electronic structure revelation have expanded applicability of MNCs in catalysis, optoelectronics, and biology. This review highlights the intriguing photophysical and electrochemical behaviors of MNCs and their regulatory parameters and applications. Initially, we present a brief discussion on the evolution of MNCs from gas-phase naked metal clusters to monolayer ligand-protected MNCs along with representative studies on their electronic structure. Due to their quantized molecular orbitals, they often exhibit PL, which can be regulated based on their capping ligands, number of atoms, crystal packing, presence of heterometal, and surrounding environment. Apart from PL, the relaxation pathways of MNCs on an ultrafast time scale have been extensively studied, which significantly differ from that of plasmonic metal nanoparticles. Moreover, their interaction with high-intensity light results in unique non-linear optical properties. The synergy between MNCs in a hierarchical self-assembled structure has been exploited to enhance their PL by precisely tuning their non-covalent interactions. Moreover, several NC-based hybrids have been designed to exhibit efficient electron or energy transfer in the photoexcited state. In the next section, we briefly focus on the redox behavior of NCs and facile electron transfer to suitable substrates, which result in enzyme-like catalytic activity. Utilizing these photophysical and electrochemical behaviors, NCs are widely employed in catalysis, optical sensing, and light-harvesting applications, which are also discussed in this review. In the final section, conclusions and open questions for the NC research community are included. This review will provide a comprehensive view of the emerging physicochemical properties of MNCs, thereby enabling an understanding for their precise modulation in future.

Differential Activation of Alkynes between Capped and Naked Ag Nanoclusters Anchored by Highly-Open Mesoporous CeO2 for Two Coupling Reactions with CO2

The cyclization of 3-hydroxy alkynes and the carboxylation of terminal alkynes both with CO2 are two attractive strategies to simultaneously reduce CO2 emission and produce value-added chemicals. Herein, the differential activation of alkynes over atomically precise Ag nanoclusters (NCs) supported on Metal-organic framework-derived highly-open mesoporous CeO2 (HM-CeO2) by reserving or removing their surface captopril ligands is reported. The ligand-capped Ag NCs possess electron-rich Ag atoms as efficient π-activation catalytic sites in cyclization reactions, while the naked Ag NCs possess partial positive-charged Ag atoms as perfect σ-activation catalytic sites in carboxylation reactions. Impressively, via coupling with HM-CeO2 featuring abundant basic sites and quick mass transfer, the ligand-capped Ag NCs afford 97.9% yield of 4,4-dimethyl-5-methylidene-1,3-dioxolan-2-one for the cyclization of 2-methyl-3-butyn-2-ol with CO2, which is 4.5 times that of the naked Ag NCs (21.7%), while the naked Ag NCs achieve 98.5% yield of n-butyl 2-alkynoate for the carboxylation of phenylacetylene with CO2, which is 15.6 times that of ligand-capped Ag NCs (6.3%). Density functional theory calculations reveal the ligand-capped Ag NCs can effectively activate alkynyl carbonate ions for the intramolecular ring closing in cyclization reaction, while the naked Ag NCs are highly affiliative in stabilizing terminal alkynyl anions for the insertion of CO2 in carboxylation reaction.

Recent advances in synthesis and properties of silver nanoclusters

Achieving atomic precision in nanostructured materials is essential for comprehending formation mechanisms and elucidating structure-property relationships. Within the realm of nanoscience and technology, atomically precise ligand-protected noble metal nanoclusters (NCs) have emerged as a rapidly expanding area of interest. These clusters manifest quantum confinement-induced optoelectronic, photophysical, and chemical properties, along with remarkable catalytic capabilities. Among coinage metals, silver distinguishes itself for the fabrication of stable nanoclusters, primarily due to its cost-effectiveness compared to gold. This minireview provides an overview of recent advancements since 2020 in synthetic methodologies and ligand selections toward attaining NCs boasting a minimum of two free valence electrons. Additionally, it explores strategies for fine-tuning optical properties. The discussion extends to surface reactivity, elucidating how exposure to ligands, heat, and light induces transformations in size and structure. Of paramount significance are the applications of silver NCs in catalytic reactions for energy and chemical conversion, supplemented by in-depth mechanistic insights. Furthermore, the review delineates challenges and outlines future directions in the NC field, with an eye toward the design of new functional materials and prospective applications in diverse technologies, including optoelectronics, energy conversion, and fine chemical synthesis.

Nanocluster Surface Microenvironment Modulates Electrocatalytic CO2 Reduction

The catalytic activity and product selectivity of the electrochemical CO2 reduction reaction (eCO2 RR) depend strongly on the local microenvironment of mass diffusion at the nanostructured catalyst and electrolyte interface. Achieving a molecular-level understanding of the electrocatalytic reaction requires the development of tunable metal-ligand interfacial structures with atomic precision, which is highly challenging. Here, the synthesis and molecular structure of a 25-atom silver nanocluster interfaced with an organic shell comprising 18 thiolate ligands are presented. The locally induced hydrophobicity by bulky alkyl functionality near the surface of the Ag25 cluster dramatically enhances the eCO2 RR activity (CO Faradaic efficiency, FECO : 90.3%) with higher CO partial current density (jCO ) in an H-cell compared to Ag25 cluster (FECO : 66.6%) with confined hydrophilicity, which modulates surface interactions with water and CO2 . Remarkably, the hydrophobic Ag25 cluster exhibits jCO as high as -240 mA cm-2 with FECO >90% at -3.4 V cell potential in a gas-fed membrane electrode assembly device. Furthermore, this cluster demonstrates stable eCO2 RR over 120 h. Operando surface-enhanced infrared absorption spectroscopy and theoretical simulations reveal how the ligands alter the neighboring water structure and *CO intermediates, impacting the intrinsic eCO2 RR activity, which provides atomistic mechanistic insights into the crucial role of confined hydrophobicity.

Cocrystallization of Two Negatively Charged Dimercaptomaleonitrile-Stabilized Silver Nanoclusters

Studies on the assembly of atomically precise metal nanoclusters (NCs) are of great significance in the nanomaterial field, which has attracted increasing interest in the last few decades. Herein, we report the cocrystallization of two negatively charged atom-precise silver nanoclusters, the octahedral [Ag₆₂(MNT)₂₄(TPP)₆]^8- (Ag₆₂) and the truncated-tetrahedral [Ag₂₂(MNT)₁₂(TPP)₄]^4- (Ag₂₂) in a 1:2 ratio (MNT^2- = dimercaptomaleonitrile, TPP = triphenylphosphine). As far as we know, a cocrystal containing two negatively charged NCs has seldom been reported. Single-crystal structure determinations reveal that the component Ag₂₂ and Ag₆₂ NCs both adopt core-shell structures. In addition, the component NCs were separately obtained by adjusting the synthetic conditions. This work enriches the structural diversity of silver NCs and extends the family of cluster-based cocrystals.

Photoluminescent Nanoparticles for Chemical and Biological Analysis and Imaging

Research related to the development and application of luminescent nanoparticles (LNPs) for chemical and biological analysis and imaging is flourishing. Novel materials and new applications continue to be reported after two decades of research. This review provides a comprehensive and heuristic overview of this field. It is targeted to both newcomers and experts who are interested in a critical assessment of LNP materials, their properties, strengths and weaknesses, and prospective applications. Numerous LNP materials are cataloged by fundamental descriptions of their chemical identities and physical morphology, quantitative photoluminescence (PL) properties, PL mechanisms, and surface chemistry. These materials include various semiconductor quantum dots, carbon nanotubes, graphene derivatives, carbon dots, nanodiamonds, luminescent metal nanoclusters, lanthanide-doped upconversion nanoparticles and downshifting nanoparticles, triplet-triplet annihilation nanoparticles, persistent-luminescence nanoparticles, conjugated polymer nanoparticles and semiconducting polymer dots, multi-nanoparticle assemblies, and doped and labeled nanoparticles, including but not limited to those based on polymers and silica. As an exercise in the critical assessment of LNP properties, these materials are ranked by several application-related functional criteria. Additional sections highlight recent examples of advances in chemical and biological analysis, point-of-care diagnostics, and cellular, tissue, and in vivo imaging and theranostics. These examples are drawn from the recent literature and organized by both LNP material and the particular properties that are leveraged to an advantage. Finally, a perspective on what comes next for the field is offered.

Ag23Au2 and Ag22Au3: A Model of Cocrystallization in Bimetal Nanoclusters

The field of cocrystalline nanoclusters stabilized by thiolates is in a period of rapid development. However, the types of cocrystallization have been limited to a few reported until now, so it is of great importance to investigate and understand the novel cocrystallographic structures. Herein, we design and synthesize a new type of cocrystallization, [Ag₂₃Au₂(2-EBT)₁₈Ag₂₂Au₃(2-EBT)₁₈]^2-[2(PPh₄)]²⁺, characterized by thermogravimetric analysis, X-ray photoelectron spectroscopy, and single-crystal X-ray crystallography. Interestingly, both of the cocrystallized nanoclusters show the same outer-shell geometric structure but diffenent cores (Ag₁₁Au₂ vs Ag₁₀Au₃). The cocrystal lattice exhibits a multilayer structure in which both of the cocrystallized nanoclusters and the counterion assemble in a layer-by-layer model. Meanwhile, the counterion is found to be critical for formation and stabilization of the target cocrystal. In addition, the target cocrystal shows high thermal stability, and this result possibly originates from the electrostatic and weak interactions in the cocrystals.

Compression-Driven Internanocluster Reaction for Synthesis of Unconventional Gold Nanoclusters

Can the active kernels in ultrasmall metal nanoparticles (nanoclusters, NCs) react with one another, or can the internanocluster reaction occur when they are in close enough proximity? To resolve this fundamental issue, we investigated the solid-state internanocluster reaction of the most studied gold NC Au₂₅ (SR)₁₈ (SR: thiolate). A novel NC was produced by increasing the pressure to 5 GPa, whose composition was determined to be Au₃₂ (SC₂ H₄ Ph)₂₄ by mass spectrometry and thermogravimetric analysis. As revealed by single-crystal X-ray crystallography, the structure, a bicuboid Au₁₄ kernel and three pairs of interlocked trimetric staples, has not been reported and endows the NC with obvious photoluminescence. DFT calculations indicate that the staples contribute substantially to the absorption properties. Further experiments reveal the pressure (internanocluster distance) can tune the internanocluster reaction, and the resulting product is not necessarily the thermodynamic product.

Progress in controlling the synthesis of atomically precise silver nanoclusters

This short review was designed to summarize the advances in synthesis methods of silver nanoclusters.

Direct Laser Writing of Fluorescent Silver Nanoclusters: A Review of Methods and Applications

Metal nanoclusters (NCs) are nanomaterials of size of less than 2 nm that exhibit a set of unique physical, chemical, optical, and electronic properties. Because of recent interest in NCs, a great deal of effort is being made to develop synthetic routes that allow control over the NC size, shape, geometry, and properties. Direct laser writing is one of the few synthesis methods that allow the generation of photostable NCs with high quantum yield in a highly controlled fashion. A key advantage of laser-written NCs is the ability to create easy-to-use solid-state devices for a range of applications. This review will present necessary background and recent advances in laser writing of silver NCs and their applications in different solid-state matrixes such as glass, zeolites, and polymer substrate. This topic will be of interest to researchers in the fields of materials science, optics and photonics, chemistry, and biomedical sciences.

Effect of subtle changes of isomeric ligands on the synthesis of atomically precise water-soluble gold nanoclusters

The subtle structural change of hydrophilic ligands on the size control of metal nanoclusters (NCs) is unclear but critically important for fundamental understanding. Herein, we report our findings that subtle changes of isomeric ligands lead to a dramatic difference in the size of water-soluble Au NCs. By using isomeric para-mercaptobenzoic acid (p-MBA), m-MBA, and o-MBA as model ligands, it was found that both the steric hindrance and the electronic effect of isomeric ligands significantly influences the size of Au NCs, resulting in the formation of different sized Au44(p-MBA)26 NCs, Au25(m-MBA)18 NCs, and Au37/43(o-MBA)22/26 NCs. Besides this, by collocating any two of the isomeric MBAs as ligand pairs to compare their protecting capability for Au NCs, the protecting abilities of such ligands were found to follow the trend: m-MBA > o-MBA > p-MBA. In addition, the growth process of Au44(p-/o-MBA)26 NCs from Au(i)-MBA complexes in the NaBH4 reduction system was also monitored by real-time UV-vis absorption spectroscopy and ESI mass spectrometry, which complies with the 2e- hopping growth principle, indicating the universal applicability of this principle in the synthesis of thiolated metal NCs. This study provides a fundamental understanding of the effect of ligands' steric hindrance and electronic factors on the size control of water-soluble metal NCs and sheds light on the formation of metal NCs in the NaBH4 reduction system.

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

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

Alternating Array Stacking of Ag26 Au and Ag24 Au Nanoclusters

An assembly strategy for metal nanoclusters using electrostatic interactions with weak interactions, such as C-H⋅⋅⋅π and π⋅⋅⋅π interactions in which cationic [Ag₂₆ Au(2-EBT)₁₈ (PPh₃ )₆ ]⁺ and anionic [Ag₂₄ Au(2-EBT)₁₈ ]^- nanoclusters gather and assemble in an unusual alternating array stacking structure is presented. [Ag₂₆ Au(2-EBT)₁₈ (PPh₃ )₆ ]⁺ [Ag₂₄ Au(2-EBT)₁₈ ]^- is a new compound type, a double nanocluster ion compound (DNIC). A single nanocluster ion compound (SNIC) [PPh₄ ]⁺ [Ag₂₄ Au(2-EBT)₁₈ ]^- was also synthesized, having a k-vector-differential crystallographic arrangement. [PPh₄ ]⁺ [Ag₂₄ Au(2,4-DMBT)₁₈ ]^- adopts a different assembly mode from both [Ag₂₆ Au(2-EBT)₁₈ (PPh₃ )₆ ]⁺ [Ag₂₄ Au(2-EBT)₁₈ ]^- and [PPh₄ ]⁺ [Ag₂₄ Au(2-EBT)₁₈ ]^- . Thus, the striking packing differences of [Ag₂₆ Au(2-EBT)₁₈ (PPh₃ )₆ ]⁺ [Ag₂₄ Au(2-EBT)₁₈ ]^- , [PPh₄ ]⁺ [Ag₂₄ Au(2-EBT)₁₈ ]^- and the existing [PPh₄ ]⁺ [Ag₂₄ Au(2,4-DMBT)₁₈ ]^- from each other indicate the notable influence of ligands and counterions on the self-assembly of nanoclusters.

Trypsin mediated one-pot reaction for the synthesis of red fluorescent gold nanoclusters: Sensing of multiple analytes (carbidopa, dopamine, Cu2+, Co2+ and Hg2+ ions)

Herein, we fabricated fluorescent gold nanoclusters (Au NCs) by using trypsin as a ligand. The fabricated trypsin-Au NCs emit bright red color fluorescence upon the exposure of 365 nm UV light. The trypsin-Au NCs are stable and well dispersed in water, which exhibited strong red emission peak at 665 nm upon excitation wavelength of 520 nm. The red fluorescence of trypsin-Au NCs was greatly quenched by the addition of multiple analytes such as drugs (carbidopa and dopamine) and three divalent metal ions (Cu²⁺, Co²⁺ and Hg²⁺ ion). As a result, a novel fluorescence "turn-off" probe was developed for the detection of the above analytes with good selectivity and sensitivity. This method exhibits the detection limits for carbidopa, dopamine, Cu²⁺, Co²⁺ and Hg²⁺ ions are 6.5, 0.14, 5.2, 0.0078, and 0.005 nM, respectively. The trypsin-Au NCs were successfully applied to detect drugs (carbidopa, and dopamine) in pharmaceutical samples and metal ions (Cu²⁺, Co²⁺ and Hg²⁺ ion) in biofluids and water samples, exhibiting good precision and accuracy, which offers a facile analytical strategy for assaying of the above analytes in pharmaceutical and biological samples.

Ionic organic cage-encapsulating phase-transferable metal clusters

Exploration of metal clusters (MCs) adaptive to both aqueous and oil phases without disturbing their size is promising for a broad scope of applications. The state-of-the-art approach via ligand-binding may perturb MCs' size due to varied metal-ligand binding strength when shuttling between solvents of different polarity. Herein, we applied physical confinement of a series of small noble MCs (<1 nm) inside ionic organic cages (I-Cages), which by means of anion exchange enables reversible transfer of MCs between aqueous and hydrophobic solutions without varying their ultrasmall size. Moreover, the MCs@I-Cage hybrid serves as a recyclable, reaction-switchable catalyst featuring high activity in liquid-phase NH3BH3 (AB) hydrolysis reaction with a turnover frequency (TOF) of 115 min-1.

Photoluminescent Ag nanoclusters for reversible temperature and pH nanosenors in aqueous solution

A facile, straightforward, and green method was reported for the preparation of water-soluble and highly luminescent silver nanoclusters (AgNCs) using captopril (Capt) as a stabilizing agent. The as-prepared Capt@AgNCs exhibited bright red emission with a strong peak centered at 637 nm and showed low toxicity and good stability. Interestingly, the AgNCs displayed temperature sensitivity based on obvious temperature dependence of the fluorescence emission intensity. Furthermore, the AgNCs showed a good reversible and linear response to the environment temperature over the range from 10 °C to 45 °C with a high resolution and activation energy, which allowed its potential application as a fluorescent nanothermometer. In addition, the AgNCs were prepared to monitor pH via the fluorescence intensity of AgNCs responding sensitively to pH fluctuating within a wide range from 2.08 to 6.06. The study provides promising applications as a convenient and eco-friendly fluorescent temperature and pH nanosenser in environmental and biological fields. Graphical abstract Novel silver nanocluster-based fluorescent nanosensors have been successfully constructed for temperature detection. The nanosensors showed a good reversible and linear response to the environment temperature over the range from 10 °C to 45 °C. In addition, the AgNCs described here are employed as pH sensors by virtue of the fluorescence intensity of their sensitive response to fluctuating pH in a linear range of 2.08-6.06.

Opportunities and challenges in energy and electron transfer of nanocluster based hybrid materials and their sensing applications

This feature article highlights the recent advances of luminescent metal nanoclusters (MNCs) for their potential applications in healthcare and energy-related materials because of their high photosensitivity, thermal stability, low toxicity, and biocompatibility.

Tailoring the photoluminescence of atomically precise nanoclusters

Due to their atomically precise structures and intriguing chemical/physical properties, metal nanoclusters are an emerging class of modular nanomaterials. Photo-luminescence (PL) is one of their most fascinating properties, due to the plethora of promising PL-based applications, such as chemical sensing, bio-imaging, cell labeling, phototherapy, drug delivery, and so on. However, the PL of most current nanoclusters is still unsatisfactory-the PL quantum yield (QY) is relatively low (generally lower than 20%), the emission lifetimes are generally in the nanosecond range, and the emitted color is always red (emission wavelengths of above 630 nm). To address these shortcomings, several strategies have been adopted, and are reviewed herein: capped-ligand engineering, metallic kernel alloying, aggregation-induced emission, self-assembly of nanocluster building blocks into cluster-based networks, and adjustments on external environment factors. We further review promising applications of these fluorescent nanoclusters, with particular focus on their potential to impact the fields of chemical sensing, bio-imaging, and bio-labeling. Finally, scope for improvements and future perspectives of these novel nanomaterials are highlighted as well. Our intended audience is the broader scientific community interested in the fluorescence of metal nanoclusters, and our review hopefully opens up new horizons for these scientists to manipulate PL properties of nanoclusters. This review is based on publications available up to December 2018.

Insights into the Synthesis Mechanism of Ag29 Nanoclusters

The current understanding of the synthesis mechanisms of noble metal clusters is limited, in particular for Ag clusters. Here, we present a detailed investigation into the synthesis process of atomically monodisperse Ag₂₉ clusters, prepared via reduction of AgNO₃ in the presence of dithiolate ligands. Using optical spectroscopy, mass spectrometry, and X-ray spectroscopy, it was determined that the synthesis involves a rapid nucleation and growth to species with up to a few hundred Ag atoms. From these larger species, Ag₂₉ clusters are formed and their concentration increases steadily over time. Oxygen plays an important role in the etching of large particles to Ag₂₉. No other stable Ag cluster species are observed at any point during the synthesis.

Discovery, Mechanism, and Application of Antigalvanic Reaction

Among many outstanding findings associated with the quantum size effect, one of the most exciting is the discovery of the antigalvanic reaction (AGR), which is the opposite of the classic galvanic reaction (GR) that has a history of nearly 240 years. The GR, named after Italian scientist Luigi Galvani, involves the spontaneous reduction of a noble-metal cation by a less noble metal in solution driven by the difference in electrochemical potentials. Classic galvanic reduction has been widely applied and has recently received particular interest in nanoscience and nanotechnology. However, the opposite of GR, that is, reduction of metal ions by less reactive (or more noble) metals, has long been regarded as a virtual impossibility until the recent surprising findings regarding atomically precise ultrasmall metal nanoparticles (nanoclusters), which bridge the gap between metal atoms (complexes) and metal nanocrystals and provide opportunities for novel scientific findings due to their well-defined compositions and structures. The AGR is significant not only because it is the opposite of the classic galvanic theory but also because it opens extensive applications in a large range of fields, such as sensing and tuning the compositions, structures, and properties of nanostructures that are otherwise difficult to obtain. Starting with the proposal of the general AGR concept in 2012 by Wu, a new era began, in which AGR received widespread attention and was extensively studied. After years of effort, great advances have been achieved in the research on AGR, which will be reviewed below. In this Account, we first provide a short introduction to the AGR concept and then discuss the driving force of the AGR together with the effecting factors, including the ligand, particle size, solvent, metal ion precursor, and ion dose. Subsequently, the application of the AGR in engineering atomically precise alloy (bimetallic and trimetallic) and monometallic nanoclusters is described, and tuning the properties of the parent nanoclusters is also included. In particular, four alloying modes (namely, (i) addition, (ii) replacement, (iii) replacement and structural transformation, and (iv) nonreplacement and structural transformation) associated with the AGR are discussed. After that, the applications of the AGR in metal ion sensing and antioxidation are reviewed. Finally, future prospects are discussed, and some challenging issues are presented at the end of this Account. It is expected that this Account will stimulate more scientific and technological interests in the AGR, and exciting progress in the understanding and application of the AGR will be made in the coming years.

Kernel Tuning and Nonuniform Influence on Optical and Electrochemical Gaps of Bimetal Nanoclusters

Fine tuning nanoparticles with atomic precision is exciting and challenging and is critical for tuning the properties, understanding the structure-property correlation and determining the practical applications of nanoparticles. Some ultrasmall thiolated metal nanoparticles (metal nanoclusters) have been shown to be precisely doped, and even the protecting staple metal atom could be precisely reduced. However, the precise addition or reduction of the kernel atom while the other metal atoms in the nanocluster remain the same has not been successful until now, to the best of our knowledge. Here, by carefully selecting the protecting ligand with adequate steric hindrance, we synthesized a novel nanocluster in which the kernel can be regarded as that formed by the addition of two silver atoms to both ends of the Pt@Ag₁₂ icosohedral kernel of the Ag₂₄Pt(SR)₁₈ (SR: thiolate) nanocluster, as revealed by single crystal X-ray crystallography. Interestingly, compared with the previously reported Ag₂₄Pt(SR)₁₈ nanocluster, the as-obtained novel bimetal nanocluster exhibits a similar absorption but a different electrochemical gap. One possible explanation for this result is that the kernel tuning does not essentially change the electronic structure, but obviously influences the charge on the Pt@Ag₁₂ kernel, as demonstrated by natural population analysis, thus possibly resulting in the large electrochemical gap difference between the two nanoclusters. This work not only provides a novel strategy to tune metal nanoclusters but also reveals that the kernel change does not necessarily alter the optical and electrochemical gaps in a uniform manner, which has important implications for the structure-property correlation of nanoparticles.

Is the kernel-staples match a key-lock match?

Metal nanoclusters provide excellent references for understanding metal nanoparticle surfaces, which remain mysterious due to the difficulty of atomically precise characterization. Although some remarkable advances have been achieved for understanding the structure of metal nanoclusters, it is still unknown if the inner kernel-outer staples match is a key-lock match and how the surface staples influence some of the properties of metal nanoclusters. Herein, we have developed an acid-induction method for synthesizing a novel gold nanocluster whose composition is determined to be Au₄₂(TBBT)₂₆ (TBBT: 4-tert-butylbenzenelthiolate) by ESI-MS and single-crystal X-ray crystallography (SCXC). SCXC also reveals that Au₄₂(TBBT)₂₆ has an identical kernel but different staples with an existing gold nanocluster Au₄₄(TBBT)₂₈, indicating that the kernel-staples match is not a key-lock match and the existence of homo-ligand-homo-kernel-hetero-staples phenomenon in metal nanoclusters provides some reference for understanding the growth or transformation of metal nanoclusters. Further experiments reveal that the staples greatly contribute to the stability of gold nanoclusters and influence their photoluminescence intensity and that minute differences in the interfacial structure can lead to enhanced stability and photoluminescence.

Two-Way Transformation between fcc- and Nonfcc-Structured Gold Nanoclusters

Precisely tuning the structure of nanomaterials, especially in a two-way style, is challenging but of great importance for regulating properties and for practical applications. The structural transformation from nonfcc to fcc (face center cubic) in gold nanoclusters has been recently reported; however, the reverse process, that is, the structural transformation from fcc to nonfcc, not to mention the two-way structural transformation between fcc and nonfcc, remains unknown. We developed a novel synthesis method, successfully fulfilled the two-way structure transformation, and studied the stability of gold nanoclusters with different structures. Additionally, a novel gold nanocluster was synthesized and structurally resolved by single-crystal X-ray crystallography. This work has important implications for structure and property tuning of gold nanoclusters and might open up some new potential applications for gold nanoclusters.

The fcc structure isomerization in gold nanoclusters

Structural isomerization is an important concept in organic chemistry and it is recently found to be applicable to thiolated gold nanoparticles. However, to the best of our knowledge, the isomerization with the kernel structure of the cluster changed while maintaining fcc packing was not previously found. Here, we report such a structural isomerization by synthesizing a novel gold nanocluster and solving its atomic structure. The as-obtained novel gold nanocluster Au₅₂(PET)₃₂ (PET = phenylethanethiolate) has completely the same Au/S molar ratio as a well-known gold nanocluster Au₅₂(TBBT)₃₂ (TBBT = 4-tert-butyl-benzenethiolate) but an essentially different fcc structure. As a result of fcc structure isomerization, Au₅₂(PET)₃₂ has remarkably different UV/vis/NIR absorption from Au₅₂(TBBT)₃₂. Another interesting finding in this work is that the kernel of Au₅₂(PET)₃₂ has high-indexed (311)-like facets, which is not previously reported in the structures of gold nanoclusters to the best of our knowledge.

Hydrogels Based on Ag+ -Modulated Assembly of 5'-Adenosine Monophosphate for Enriching Biomolecules

Supramolecular hydrogels obtained by combining 5'-adenosine monophosphate (AMP) with Ag⁺ were fabricated in this work. Their gelation capability was enhanced by increasing the concentration of Ag⁺ or decreasing the pH. The gels are very sensitive to light, which endows them with potential applications as visible-light photosensitive materials. Coordination between the nucleobase of AMP and Ag⁺ , as well as π-π stacking of nucleobases, are considered to be the main driving forces for self-assembly. The hydrogels successfully achieved the encapsulation and enrichment of biomolecules. Hydrogen bonding between the amino group of guest molecules and silver nanoparticles along the nanofibers drives the enrichment and is considered to be a crucial interaction.

From Galvanic to Anti-Galvanic Synthesis of Bimetallic Nanoparticles and Applications in Catalysis, Sensing, and Materials Science

The properties of two alloyed metals have been known since the Bronze Age to outperform those of a single metal. How alloying and mixing metals applies to the nanoworld is now attracting considerable attention. The galvanic process, which is more than two centuries old and involves the reduction of a noble-metal cation by a less noble metal, has not only been used in technological processes, but also in the design of nanomaterials for the synthesis of bimetallic transition-metal nanoparticles. The background and nanoscience applications of the galvanic reactions (GRs) are reviewed here, in particular with emphasis on recent progress in bimetallic catalysis. Very recently, new reactions have been discovered with nanomaterials that contradict the galvanic principle, and these reactions, called anti-galvanic reactions (AGRs), are now attracting much interest for their mechanistic, synthetic, catalytic, and sensor aspects. The second part of the review deals with these AGRs and compares GRs and AGRs, including the intriguing AGRs mechanism and the first applications.

A novel double-helical-kernel evolution pattern of gold nanoclusters: alternate single-stranded growth at both ends

Studying the kernel evolution pattern of gold nanoclusters is intriguing but challenging due to the difficulty of precise size control and structure resolution. Herein, we successfully synthesized two novel gold nanoclusters, Au₃₄(S-c-C₆H₁₁)₂₂ and Au₄₂(S-c-C₆H₁₁)₂₆ (S-c-C₆H₁₁: cyclohexanethiolate), and resolved their structures. Interestingly, it was found that the kernel evolves from Au₂₈(S-c-C₆H₁₁)₂₀ to Au₃₄(S-c-C₆H₁₁)₂₂ and Au₄₂(S-c-C₆H₁₁)₂₆ in a novel fashion: alternate single-stranded evolution at both ends, which is remarkably different from the reported double-stranded growth at the bottom for the 4-tert-butylbenzenethiolate (TBBT)-protected nanocluster series. This work illustrates the variety of kernel evolution patterns and the directionality of the ligands with respect to the evolution of the kernel. In addition, differential pulse voltammetry (DPV) revealed that the electrochemical gap between the first oxidation and the first reduction potential decreases as the size increases from Au₂₈(S-c-C₆H₁₁)₂₀ to Au₃₄(S-c-C₆H₁₁)₂₂ and Au₄₂(S-c-C₆H₁₁)₂₆.

Penicillamine-protected Ag20 nanoclusters and fluorescence chemosensing for trace detection of copper ions

We report the synthesis of penicillamine-protected Ag₂₀ nanoclusters (NCs), with properties of high monodispersity, red fluorescence and water solubility. Full characterization of the Ag₂₀ NCs is addressed, along with first-principles optimization calculations, revealing the chemical composition and structure of the as-prepared Ag NCs within a molecular formula [Ag₂₀(DPA)₁₈-H]^-. Moreover, natural bond orbital (NBO) analysis demonstrates the charge-transfer interactions between the ligand and Ag atoms, and helps in understanding the origins of fluorescence of Ag₂₀ NCs related to the ligand-to-metal charge transfer (LMCT) mechanism. Further, fluorescence chemosensing of the Ag₂₀ NCs is demonstrated for tracing copper ions with high sensitivity and selectivity in aqueous solution.

Fluorescent metal quantum clusters: an updated overview of the synthesis, properties, and biological applications

A brief history of metal quantum clusters, their synthesis methods, physical properties, and an updated overview of their applications is provided.

Distinct metal-exchange pathways of doped Ag25 nanoclusters

Atomically precise metal nanoclusters (NCs) containing more than one type of metal atom (i.e., doped or alloyed), due to synergistic effects, open new avenues for engineering the catalytic and optical properties of NCs in a manner that homometal NCs cannot. Unfortunately, it is still a major challenge to controllably introduce multimetallic dopants in NCs, understanding the dopants' positions, mechanism, and synergistic effects. To overcome these challenges, we designed a metal-exchange approach involving NCs as molecular templates and metal ions as the source of the incoming dopant. In particular, two structurally similar monodoped silver-rich NCs, [MAg₂₄(SR)₁₈]^2- (M = Pd/Pt and SR: thiolate), were synthesized as templates to study their mechanistic transformation in response to the introduction of gold atoms. The controllable incorporation of Au atoms into the MAg₂₄ framework facilitated the elucidation of distinct doping pathways through high-resolution mass spectrometry, optical spectroscopy and elemental analysis. Interestingly, gold replaced the central Pd atom of [PdAg₂₄(SR)₁₈]^2- clusters to produce predominantly bimetallic [AuAg₂₄(SR)₁₈]^- clusters along with a minor product of an [Au₂Ag₂₃(SR)₁₈]^- cluster. In contrast, the central Pt atom remained intact in [PtAg₂₄(SR)₁₈]^2- clusters, and gold replaced the non-central Ag atoms to form trimetallic [Au_xPtAg_24-x(SR)₁₈]^2- NCs, where x = 1-2, with a portion of the starting [PtAg₂₄(SR)₁₈]^2- NCs remaining. This study reveals some of the unusual metal-exchange pathways of doped NCs and the important role played by the initial metal dopant in directing the position of a second dopant in the final product.

Effect of compartmentalization of donor and acceptor on the ultrafast resonance energy transfer from DAPI to silver nanoclusters

The mechanism and dynamics of excitation energy transfer (EET) from photo-excited 4',6-diamidino-2-phenylindole (DAPI) to silver nanoclusters (Ag NCs) and its subsequent modulation in the presence of cationic polymer poly(diallyldimethylammonium chloride) (PDADMAC) and Calf Thymus DNA (CT-DNA) have been demonstrated using steady-state fluorescence and femtosecond fluorescence upconversion techniques. The synthesized Ag NCs were characterized using FTIR, mass spectrometry, XPS, HRTEM, DLS, UV-Vis and PL spectroscopy. Mass spectrometric analysis reveals the formation of ultrasmall Ag4 NCs with a small amount of Ag5 NCs. UV-Vis and PL spectra reveal distinct molecular-like optoelectronic behaviour of these ultrasmall Ag NCs. The dihydrolipoic acid-capped Ag NCs strongly quench the fluorescence of DAPI with concomitant increase in its photoluminescence (PL) intensity at 675 nm. This steady-state fluorescence quenching proceeds with a significant shortening of the fluorescence lifetime of DAPI in the presence of Ag NCs, signifying the nonradiative Förster resonance energy transfer (FRET) from DAPI to Ag NCs. Various energy transfer parameters have been estimated from FRET theory. The present FRET pair shows a characteristic Förster distance of 2.45 nm and can be utilized as a reporter of short-range distances in various FRET based applications. Moreover, this nonradiative FRET is completely suppressed in the presence of both 0.2 wt% PDADMAC and CT-DNA. Our results reveal selective compartmentalization of Ag NCs and DAPI in the presence of 0.2 wt% PDADMAC and CT-DNA, respectively. This selective compartmentalization of donor and acceptor and the subsequent modification of the FRET process may find application in various sensing, photovoltaic, and light harvesting applications.

Copper nanocluster-based fluorescent probe for sensitive and selective detection of Hg(2+) in water and food stuff

In this study, Hg(2+) ions were found to quench the fluorescence of glutathione (GSH)-capped copper clusters (Cu NCs). The Cu NCs were prepared by a simple reduction of CuSO4 in the presence of GSH serving both as a reducing and protecting agents, and characterized by ultraviolet-visible absorption spectroscopy (UV-vis), high resolution scanning electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectrometer (XPS). The GSH-Cu NCs displayed a small size, excellent water-dispersibility, good storage stability, good photostability and were stable in the presence of high concentrations of salt. The GSH-Cu NCs possessed strong blue fluorescence with a quantum yield of 10.6% and exhibited an excitation-independent fluorescence behavior. The zeta potential, TEM, resonance light scattering and dynamic light scattering measurements demonstrated that the Hg(2+) ion-induced aggregation of the Cu NCs contributed to the fluorescence quenching of the dispersed Cu NCs. On these findings, a sensitive and selective fluorescent probe was developed for detecting Hg(2+) in the linear range from 10nM to 10μM with a detection limit of 3.3nM (S/N=3). The proposed method has been successfully applied to determine Hg(2+) content in water sample and food stuff. The results of the proposed method were in good agreement with those obtained by a hydride generation atomic fluorescence spectrometry (HG-AFS).

Multiamino polymeric capping of fluorescent silver nanodots as an effective protective, amphiphilic and pH/thermo-responsive coating

New multiamino thermosensitive polymers based on MEO₂MA have been described and used to the in situ synthesis of polymeric capped silver nanodots. The new highly luminescent hybrids show amphiphilic and pH/thermo-responsiveness.

DNA stabilized Ag–Au alloy nanoclusters and their application as sensing probes for mercury ions

Metal nanoclusters (NCs) have attracted plenty of attention because of their unique properties and great application potentials.