Protection of Ag Clusters by Metal-Oxo Modules

Monodisperse and atomically precise Ag nanoclusters have attracted considerable recent research interest. A conventional silver cluster usually consists of a silver metallic kernel and an organic peripheral ligand shell. Nevertheless, the present inevitable problem is the unsatisfied stability of such nanoclusters. In this concept, we will give an introduction to Ag clusters protected by metal-oxo modules, which exhibit enhanced stability and unique properties. Accordingly, three different types of clusters are summarized: (1) Ag clusters protected by mononuclear oxometallates; (2) Ag clusters protected by block-like metal-oxo clusters; (3) Ag clusters protected by hollow-like metal-oxo clusters. The aim of this concept is to offer possible general guidance and insight into future rational design of more metal-oxo clusters protected silver clusters or even other coinage metal nanoclusters.     

Recent Progress in Inorganic Anions Templated Silver Nanoclusters: Synthesis,Structures and Properties

Over recent years, research on the ligand‐protected silver clusters have gained significant interest owing to their unique potential applications in catalysis, organic optoelectronics, and luminescent materials. However, the synthesis of structurally precise high‐nuclearity silver nanoclusters is still challenging and become one of the prime interests of chemists. The controllable synthesis of high‐nuclearity silver nanoclusters involves the ingenious use of capping ligands or/and templating agents. Thereinto, the main role of the templating agents is to promote the order arrangement of silver ions around them to form discrete molecules. Our lab has performed comprehensive studies on the ligand‐protected silver clusters in the past eight years. This review highlights recent progress in the use of inorganic template anions, silver precursors, solvents, and the ligand types in synthesizing high‐nuclearity silver nanoclusters. Furthermore, some interesting photo‐ and electrochemical properties revealed by silver clusters including luminescent thermochromism, electrical conductivity, and electrochemical reduction of H₂O₂ have been also summarized.     

GAG-containing nucleotides as mediators of DNA-silver clusters and iron-DNA interplay

New types of fluorescence DNA-based silver nanoclusters(DNAn-Ag NCs, n = 1, 2, 3c, 4c, 5c) were synthesized by C3T-rich nucleotides as templates. It is found that the assembly of DNAn-Ag NCs with nucleotides containing GAG sequences produce silver clusters with an enhanced red emission. Results indicate that GAG is the good enhancer of DNAn-Ag NCs constructed by C3T-rich nucleotides. The fluorescence titration reveals that enhanced red emission is sensitive to Fe(III/II) ions with the formation of non-emission nanoclusters. Thus, the GAG-containing nucleotide can be an enhancer for the emission of silver clusters with C3T-rich nucleotide and a mediator of the iron-cluster interplay.     

Site-specific DNA-programmed growth of fluorescent and functional silver nanoclusters

Precise organization of metallic nanoclusters on DNA scaffolds holds great interest for nanopatterned materials that may find uses in electronics, sensors, medicine, and many other fields. Herein, we report the site-specific growth of fluorescent silver nanoclusters by using a mismatched double-stranded DNA template. Few-atom, molecular-scale Ag clusters are found to localize at the mismatched site and the metallized DNA retains its integrity. The DNA-encapsulated nanoclusters can be utilized as functional biological probes to identify single-nucleotide polymorphisms by taking advantage of the very bright fluorescence and excellent photostability of the nanoclusters. This approach offers the possibility of constructing novel DNA-based nanomaterials and nanomechanical devices with more sophisticated functions and will be highly beneficial in future biochemical, pharmaceutical, nanomechanical, and electronic applications.     

Facile and Direct Preparation of Ultrastable Mesoporous Silica with Silver Nanoclusters: High Surface Area

We report the successful direct synthesis of an ultrastable mesoporous silicon dioxide framework containing silver nanoclusters using a modified true liquid crystal templating method. Our modification produced an extraordinary material with a high average Brunauer-Emmett-Teller specific surface area of 1785 m² g⁻¹ – the highest reported surface area to date – and an ultrastable mesoporous structure, which has been stable for nine years so far. This method eliminates the need for reduction of silver oxide into metallic silver and restricts the growth of silver clusters. The silver nanoclusters, with an average size of 1 nm, occupy the pores and walls of the framework. Analysis of the material using nitrogen adsorption/desorption method, high-resolution transmission electron microscopy, X-ray diffractometry, energy-dispersive X-ray diffractometry, X-ray photoelectron spectroscopy, and scanning electron microscopy is discussed herein.     

Chiral functionalization of optically inactive monolayer-protected silver nanoclusters by chiral ligand-exchange reactions

We report the ligand-exchange reaction between the optically inactive racemic penicillamine monolayer on a silver nanocluster surface and enantiopure D- or L-penicillamine dissolved in solution. Emergence of the identical band positions in the gel electrophoresis separation assures the presence of size-invariant silver nanoclusters (1.05 and 1.30 nm in core diameter) during the ligand-exchange reaction and allows us to further examine the optical/chiroptical properties of these nanoclusters. Consequently, chiral functionalization of the achiral silver nanoclusters has been demonstrated, yielding large Cotton effects in metal-based electronic transitions with an almost mirror-image relationship between the enantiomeric compounds. The ligand-exchange experiments as well as the normal syntheses of the silver nanoclusters revealed that their absorption profiles and anisotropy factors were strongly dependent on the enantiomeric purity (or enantiomeric excess) of surface chiral penicillamine, so that (several-fold) larger chiroptical responses of the silver nanoclusters as compared to those of the analogous gold clusters with a comparable size could be induced by the metal core deformation or rearrangement along with a universally influential vicinal contribution from the chiral ligand field.     

Formation of bimetallic Ag-Pd nanoclusters via the reaction between Ag nanoclusters and Pd2+ ions

We have investigated systematically the mechanistic aspects of the Ag-Pd bimetallic cluster formation within sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles by using in-situ X-ray absorption spectroscopy (XAS). A two-step sequential reduction method is employed for the synthesis of Ag-Pd bimetallic clusters. The first step involves preparation of Ag nanoclusters, by mixing the Ag+ ions containing the AOT microemulsion system with a reducing agent hydrazine (N2H4) containing the AOT microemulsion system. In the second step, the addition of Pd2+ ions to Ag nanoclusters led to the formation of Ag-Pd bimetallic clusters via the reaction between Ag nanoclusters and Pd2+ ions in AOT reverse micelles. The reduction of silver ions and the formation of corresponding Ag nanoclusters are monitored as a function of the dosage of the reducing agent, hydrazine. In-situ XAS allowed probing of the reaction between Ag nanoclusters and Pd2+ ions during the formation of Ag-Pd bimetallic clusters. Analysis of Ag and Pd K-edge XAS spectra reveals that in the final stage Ag-Pd clusters, in which "Ag" atoms prefer to be surrounded by "Pd" and "Pd" atoms prefer to be surrounded by "Pd", were formed. On the basis of XAS results presented here, we are able to propose a structural model for each step so that this work provides a detailed insight into the mechanism of nucleation and growth of Ag-Pd bimetallic clusters. We also discussed the atomic distribution of Ag and Pd atoms in Ag-Pd bimetallic clusters based on the calculated XAS structural parameters.     

Electrochemical detection of DNA hybridization based on DNA-templated assembly of silver cluster

The growth of metals on DNA templates has generated considerable interest in connection to the design of metallic nanostructures. Here we exploit the DNA-induced generation of metal clusters for developing an electrical biosensing protocol. The new hybridization assay employs a probe-modified gold surface, and is based on the electrostatic ‘collection’ of silver cations along the DNA duplex, the reductive formation of silver nanoclusters along the DNA backbone, dissolution of the silver aggregate and stripping potentiometric detection of the dissolved silver at a thick-film carbon electrode. The new protocol thus combines the inherent signal amplification of stripping analysis with effective discrimination against nonhybridized DNA.     

In situ depositing silver nanoclusters on silk fibroin fibers supports by a novel biotemplate redox technique at room temperature

Evolvement of bioinspired approaches for the construction of well-ordered nanostructures is a crucial intersection of branches of materials science and biotechnology. In this paper, floriated clusters of silver nanocrystallites, which consist of polycrystalline grains about 5 nm in diameter, have been successfully prepared on silk fibroin fibers (SFFs) through an in situ biotemplate redox approach at room temperature. The reductive amino acid tyrosine of SFFs mainly provided both reduction and location functions under alkaline conditions and could reduce Ag(I) ions to Ag(0). Finally, stable silver nanoclusters were generated on SFF substrates. The morphologies of silver nanoclusters were mostly attributed to the concentration of silver nitrate solution as well as special configurations and structures of silk fibroin macromolecules. A possible mechanism was explored intensively for tyrosine-residue-based silver nanocrystal formation.     

Stabilized Silver Nanoparticles and Nanoclusters Ag<Subscript>n</Subscript> in Humic-Based Bioactive Nanocomposites

Silver-containing nanocomposites synthesized from the compounds of a humic series have been studied using modern physical–chemical methods (EPR, TEM, IR and XRD, etc.). It is shown that the humic substances with different functional groups and isolated from different sources have also different ability of stabilizing the silver nanoparticles. Long-term stable nanoparticles and silver clusters have been found. A multiplet, observed in the EPR spectra of silver-containing nanocomposites, which are obtained from humic substances isolated from therapeutic muds and shales, is assigned to the formed Ag_n nanoclusters. Formation of the silver molecular clusters depends on the kind of humic substances and depth of their decomposition.     

Mass-spectrometric study of the formation of silver nanoclusters in polyether media: 2. Fast atom bombardment and modeling

Within the problem of the synthesis of silver nanoclusters and nanoparticles in polyether media, systems containing silver nitrate AgNO₃ and low-molecular-weight polyethers, poly(ethylene glycol) PEG-400 or oxyethylated glycerol OEG-5, were studied by fast atom bombardment (FAB) mass spectrometry. The formation of stable clusters of polyether oligomers (M _m ) with silver cations M _m · Ag⁺ was shown, in agreement with the previous data of laser desorption/ionization. Quantum-chemical DFT calculations have shown that the M _m · Ag⁺ clusters are stabilized by wrapping of the polyether chain around the silver cation with the cation coordinating ether oxygen atoms. Silver nanoclusters were not found in the FAB mass spectra of liquid systems, but Ag _n ⁺ clusters were detected for silver nanoparticles separated from the reaction medium. No products of chemical transformations of PEG-400 or OEG-5 were observed by FAB. A plausible mechanism of the reduction of silver cations involving nitrate anions is discussed.     

Novel silver nanostructures from silver mirror reaction on reactive substrates

Novel silver clusters have been prepared by simply carrying out the silver mirror reaction on certain reactive substrates. Leaflike fractal silver microstructures and perpendicularly aligned silver nanosheets were produced on a commercially available copper foil and sandpaper-rubbed copper foil, respectively. The surface features of copper foils and the chemical state of Cu atoms play important roles in regulating the morphological structures of the resulting silver clusters. Silver nanoclusters with various morphologies ranging from the leaflike to flowerlike hierarchical structures can be produced from the silver mirror reaction on commercially available copper foils after being treated with a dilute aqueous HCl solution under different conditions. The aqueous solution of silver nanosheets shows an optical absorption spectrum with a broad light-scattering peak at about 350 nm, compared to a corresponding surface plasmon absorption band around 430 nm for silver nanoparticles from the conventional silver mirror reaction on glass.     

Mass-spectrometric study of the formation of silver nanoclusters in polyethers: I. Laser desorption/ionization

Within the framework of a problem of the synthesis of silver nanoparticles and Ag _n nanoclusters in polyethers, the systems containing silver nitrate AgNO₃ and the low-molecular-weight polyethers poly(ethylene glycol) PEG-400 and oxyethylated glycerol OEG-5, in which silver ions were reduced, were studied by laser desorption/ionization mass spectrometry. The occurrence of Ag _n silver nanoclusters with n up to 35 in the systems was detected. For n > 2, the presence of ??magic numbers?? was observed; that is, positively charged Ag _n ⁺ clusters with predominantly odd values of n were detected. Negatively charged Ag _n clusters with n = 1?C3 were also detected. It was shown that one of the expected processes, namely, the formation of the stable clusters of polyether oligomers (M _m ) with the silver cation M _m · Ag⁺, took place in the test systems.     

Fluorescent Silver Nanoclusters in Condensed DNA

We study the formation and fluorescent properties of silver nanoclusters encapsulated in condensed DNA nanoparticles. Fluorescent globular DNA nanoparticles are formed using a dsDNA–cluster complex and polyallylamine as condensing agents. The fluorescence emission spectrum of single DNA nanoparticles is obtained using tip‐enhanced fluorescence microscopy. Fluorescent clusters in condensed DNA nanoparticles appear to be more protected against destructive damage in solution compared to clusters synthesized on a linear polymer chain. The fluorescent clusters on both dsDNA and ssDNA exhibit the same emission bands (at 590 and 680 nm) and the same formation efficiency, which suggests the same binding sites. By using density functional theory, we show that the clusters may bind to the Watson–Crick guanine–cytosine base pairs and to single DNA bases with about the same affinity.     

Short-peptide-based hydrogel: a template for the in situ synthesis of fluorescent silver nanoclusters by using sunlight

N-terminally Fmoc-protected dipeptide, Fmoc-Val-Asp-OH, forms a transparent, stable hydrogel with a minimum gelation concentration of 0.2% w/v. The gelation property of the hydrogel was investigated by using methods such as transmission electron microscopy, field-emission scanning electron microscopy, atomic force microscopy and Fourier transform infrared spectroscopy. The silver-ion-encapsulating hydrogel can efficiently and spontaneously produce fluorescent silver nanoclusters under sunlight at physiological pH (7.46) by using a green chemistry approach. Interestingly, in the absence of any conventional reducing agent but in the presence of sunlight, silver ions were reduced by the carboxylate group of a gelator peptide that contains an aspartic acid residue. These clusters were investigated by using UV/Vis spectroscopy, photoluminescence spectroscopy, high-resolution transmission electron microscopy (HR-TEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) studies. Mass spectrometric analysis shows the presence of a few atoms in nanoclusters containing only Ag(2). The reported fluorescent Ag nanoclusters show excellent optical properties, including a very narrow emission profile and large Stokes shift (>100 nm). The reported fluorescent Ag nanoclusters within hydrogel are very stable even after 6 months storage in the dark at 4 °C. The as-prepared hydrogel-nanocluster conjugate could have applications in antibacterial preparations, bioimaging and other purposes.     

Sub-nanometre sized metal clusters: from synthetic challenges to the unique property discoveries

Sub-nanometre sized metal clusters, with dimensions between metal atoms and nanoparticles, have attracted more and more attention due to their unique electronic structures and the subsequent unusual physical and chemical properties. However, the tiny size of the metal clusters brings the difficulty of their synthesis compared to the easier preparation of large nanoparticles. Up to now various synthetic techniques and routes have been successfully applied to the preparation of sub-nanometre clusters. Among the metals, gold clusters, especially the alkanethiolate monolayer protected clusters (MPCs), have been extensively investigated during the past decades. In recent years, silver and copper nanoclusters have also attracted enormous interest mainly due to their excellent photoluminescent properties. Meanwhile, more structural characteristics, particular optical, catalytic, electronic and magnetic properties and the related technical applications of the metal nanoclusters have been discovered in recent years. In this critical review, recent advances in sub-nanometre sized metal clusters (Au, Ag, Cu, etc.) including the synthetic techniques, structural characterizations, novel physical, chemical and optical properties and their potential applications are discussed in detail. We finally give a brief outlook on the future development of metal nanoclusters from the viewpoint of controlled synthesis and their potential applications.     

Luminescence of ordered silver iodide nanoclusters inside zeolite host

Ordered silver iodide nanoclusters inside zeolite Y host were prepared by using a thermal diffusion method. The Y-AgI samples were characterized with powder X-ray diffraction, differential thermal analysis, X-ray photoelectron spectroscopy, adsorption technique and chemical analysis. The results show that silver iodide nanoclusters were situated in the ordered cages of the zeolite Y host. The results on the luminescence of the nanocomposites Y-AgI suggest that when the sizes of silver iodide nanoclusters are were very small, non-radiation surface recombination exceeds radiation process and the luminescence efficiency increases as the size of the silver iodide nanoclusters increaseds As the sizes of AgI nanoclusters increased, non-radiative surface recombination effect become equal to the radiation effect of the excitons. As the size of the nanoclusters of AgI in zeolite host further increases, the surface recombination of the nanoclusters becomes a major process.     

Sensitive signal-on fluorescent sensing for copper ions based on the polyethyleneimine-capped silver nanoclusters–cysteine system

In this work, we present a label-free sensor for copper ions. This sensor is composed of silver nanoclusters and cysteine. The fluorescence of the silver nanoclusters was quenched by cysteine, which was recovered in the presence of copper ions. This binding of silver nanoclusters to cysteine promoted agglomeration of silver nanoclusters to yield larger non-fluorescent silver nanoparticles. The presence of copper ions resulted in the oxidation of cysteine to form a disulfide compound, leading to recovery of fluorescence of the silver nanoclusters. The fluorescence of the silver nanoclusters in the presence of cysteine increased with increasing concentration of copper ions in the range of 10–200 nM. The detection limit of this sensor for copper ions was 2.3 nM. The silver nanoclusters–cysteine sensor provides a simple, cost-effective, and sensitive platform for the detection of copper ions.     

Synthesis and characterization of colloidal fluorescent silver nanoclusters

Ultrasmall water-soluble silver nanoclusters are synthesized, and their properties are investigated. The silver nanoclusters have high colloidal stability and show fluorescence in the red. This demonstrates that like gold nanoclusters also silver nanoclusters can be fluorescent.