Silver Nanoclusters Research Articles

Updated Review on Recent Advances in Silver Nanoclusters in Bioanalytical and Biomedical Applications

Abstract Silver nanoclusters (AgNCs) have emerged as highly adaptable nanomaterials with vast potential in theranostic applications, by integrating therapeutic and diagnostic capabilities within a single platform. This review summarizes current developments in the synthesis, characterization, and use of AgNCs for theranostics. AgNC synthesis has substantially advanced, and a variety of techniques such as chemical reduction, green synthesis, and templated methods are being used to manage stability, size, and form. AgNCs’ optical characteristics, including high fluorescence and surface-enhanced Raman scattering (SERS) signals, make them ideal for bioimaging and diagnostic applications. Furthermore, AgNCs’ surface chemistry enables simple functionalization with therapeutic drugs and targeting ligands, thus improving effectiveness and selectivity. AgNCs have been used in several diagnostic imaging modalities, including photoacoustic imaging, fluorescence imaging, and SERS-based sensing. They are suitable for both in vitro and in vivo imaging applications because of their exceptional photostability and biocompatibility, which enables real-time tracking of disease progression and therapy response.

A hydrogel-functionalized silver nanocluster for bacterial-infected wound healing.

The ever-growing challenges of traditional antibiotic therapy and chronic wound healing have created a hot topic for the development and application of new antimicrobial agents. Silver nanoclusters (Ag NCs) with ultrasmall sizes (<2 nm) and antibacterial effects are promising candidates for next-generation antibiotics, particularly against multi-drug resistant strains. However, the biosafety in the clinical application of Ag NCs remains suboptimal despite some existing studies of Ag NCs for biomedical applications. Considering this, an ultrasmall Ag NC with excellent water solubility was synthesized by a two-phase ligand-exchange method, which exhibits broad-spectrum antibacterial performance. The minimum inhibitory concentrations of Ag NCs against MRSA, S. aureus, P. aeruginosa and E. coli were evaluated as 50, 80, 5 and 5 μg mL-1, respectively. Furthermore, a carbomer hydrogel was prepared to be incorporated into the Ag NCs for achieving excellent biocompatibility and biosafety. In vitro experiments demonstrate that the Ag NC-gel exhibits good antibacterial properties with lower cytotoxicity. Finally, in vivo experiments suggest that this ultrasmall Ag NC functionalized with the hydrogel can serve as an effective and safe antimicrobial agent to aid in wound healing.

Effects of bromine-containing counterion salts in directing the structures of medium-sized silver nanoclusters.

The preparation and structural determination of silver nanoclusters (especially the medium-sized Ag clusters) remain more challenging relative to those of their gold counterparts because of the comparative instability of the former. In this work, three medium-sized Ag clusters were controllably synthesized and structurally determined, namely, [Ag52(S-Adm)30Br4H20]2- (Ag52 for short), Ag54(S-Adm)30Br4H20 (Ag54 for short), and [Ag58(S-Adm)30Br4(NO3)2H22]2+ (Ag58 for short) nanoclusters. Specifically, the introduction of PPh4Br gave rise to the generation of Ag52 and Ag54 nanoclusters with homologous compositions and configurations, while the TOABr salt selected Ag58 as the sole cluster product, whose geometric structure was completely different from those of Ag52 and Ag54 nanoclusters. In addition, the optical absorptions and emissions of the three medium-sized silver nanoclusters were compared. The findings in this work not only provide three uniquely medium-sized nanoclusters to enrich the silver cluster family but also point out a new approach (i.e., changing the counterion salt) for the preparation of new nanoclusters with novel structures.

Localized surface plasmon resonances of size-selected large silver nanoclusters (n = 70-100) soft-landed on a C60 organic substrate.

Silver nanoclusters (Agn NCs) exhibit a remarkable optical property known as localized surface plasmon resonance (LSPR) in the visible to ultraviolet wavelengths. In this study, we address the size gap in LSPR responses between small NCs and nano-islands by synthesizing large Agn NCs with a countable number of atoms (n = 70-100) using a magnetron sputtering method, which were precisely size-selected and soft-landed onto substrates. The monodispersed Agn NCs were immobilized on a pre-decorated substrate with fullerene (C60) molecules, and their LSPR behaviors were characterized using two-photon photoemission (2PPE) spectroscopy. Due to the distinct polarization selectivity of incident light associated with LSPR, the intensity ratio between p- and s-polarized lights (Ip/Is) in 2PPE spectroscopy serves as a reliable indicator of LSPR and its structural correlations. From n = 70 to 100, the Ip/Is value gradually decreases as the cluster size increases. This decrease is attributed to the enhancement of s-polarized light (Is), indicating that large Agn NCs on a C60 substrate undergo a deformation from spherical to flattened geometries, particularly above approximately n = 55.

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

To test the biological properties of a novel non-restorative treatment method for arresting dentin caries based on silver nanoclusters (AgNCls) synthesized in polymethacrylic acid (PMAA). Synthesis of AgNCls was performed by photoreduction of AgNO3 in PMAA with 355 nm/wavelength light. AgNCls/PMAA was characterized by absorption/fluorescence spectroscopy and optical and atomic force microscopy. The stability of the clusters in an aerated PMAA solution was evaluated by means of fluorescence spectroscopy. Cytotoxicity was assessed using the MTT assay and antibacterial effect was determined for minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC) and colony forming (CFU) of Streptococcus mutans (S. mutans) and Lactobacillus acidophilus (L. acidophilus). 38% Silver Diammine Fluoride (SDF) was used for the control groups. Chemical and structural identity of the clusters did not change within 9 months; Cell viability of 92%-89% was found after 24-48 h respectively. MIC and MBC were determined from 1:16 and 1:8 dilutions, respectively. Log CFU counts of S. mutans, and L. acidophilus treated with AgNCls/PMAA (3.4 ppm of silver) were significantly lower than in the control groups and even lower than when the same bacterial strains were treated with SDF (15,525 ppm of silver). AgNCls/PMAA presented chemical stability, acceptable cytotoxicity, and a potential antibacterial effect for strains associated with caries lesions at very low concentrations of silver.

Atomically precise Au and Ag nanoclusters doped with a single atom as model alloy catalysts.

Gold and silver nanoclusters (NCs) composed of <200 atoms are novel catalysts because their catalytic properties differ significantly from those of the corresponding bulk surface and can be dramatically tuned by the size (number of atoms). Doping with other metals is a promising approach for improving the catalytic performance of Au and Ag NCs. However, elucidation of the origin of the doping effects and optimization of the catalytic performance are hampered by the technical challenge of controlling the number and location of the dopants. In this regard, atomically precise Au or Ag (Au/Ag) NCs protected by ligands or polymers have recently emerged as an ideal platform because they allow regioselective substitution of single Au/Ag constituent atoms while retaining the size and morphology of the NC. Heterogeneous Au/Ag NC catalysts doped with a single atom can also be prepared by controlled calcination of ligand-protected NCs on solid supports. Comparison of thermal catalysis, electrocatalysis, and photocatalysis between the single-atom-doped and undoped Au/Ag NCs has revealed that the single-atom doping effect can be attributed to an electronic or geometric origin, depending on the dopant element and position. This minireview summarizes the recent progress of the synthesis and catalytic application of single-atom-doped, atomically precise Au/Ag NC catalysts and provides future prospects for the rational development of active and selective metal NC catalysts.

Validating the reconstruction of DNA silver nanoclusters induced by different DNA sequence via HPLC and exploring the optical properties of reconstructed DNA silver nanoclusters

We investigated the influence of redundant DNA templates on the optical properties of DNA-stabilized silver nanoclusters (DNA AgNCs).

An insight, at the atomic level, into the structure and catalytic properties of the isomers of the Cu22 cluster.

The study of structural isomerism in copper nanoclusters has been relatively limited compared to that in gold and silver nanoclusters. In this work, we present the controlled synthesis and structures of two isomeric copper nanoclusters, denoted as Cu22-1 and Cu22-2, whose compositions were determined to be Cu22(SePh)10(Se)6(P(Ph-4F)3)8 through single-crystal X-ray diffraction (SCXRD). The structural isomerism of Cu22-1 and Cu22-2 arises from the different arrangements of a few Cu(SeR)(PR3) motifs on the surface structure. These subtle changes in the surface structure also influence the distortion of the core and the spatial arrangement of the clusters, and affect the electronic structure. Furthermore, due to their distinct structures, Cu22-1 and Cu22-2 exhibit different catalytic properties in the copper-catalyzed [3 + 2] azide-alkyne cycloaddition (CuAAC). Notably, Cu22-1 demonstrates efficient catalytic activity for photoinduced AAC, achieving a yield of 90% within 1 hour. This research contributes to the understanding of structural isomerism in copper nanoclusters and offers insights into the structure-function relationship in these systems.

Silver nanoclusters stabilized with PVP-BSA conjugate: Optical properties approach

&lt;abstract&gt; &lt;p&gt;The objective of this research was to synthesize fluorescent silver nanoclusters (NC Ag-BSA/PVP) using polyvinylpyrrolidone polymer (PVP) as a stabilizer in conjunction with bovine serum albumin protein (BSA). The nanoclusters were prepared using a wet chemistry reduction technique with two distinctive pathways: the addition of PVP after BSA and the addition of PVP after the metal precursor. The optical properties of the materials were studied in samples with different BSA/PVP molar ratios and varying amounts of metal/reductant. The impact of protein and polymer amounts on fluorescence was determined. The materials were characterized using X-ray diffraction (XRD), infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), X-ray energy dispersive spectroscopy (EDS), dynamic light scattering (DLS), fluorescence spectroscopy, and UV-Vis-NIR spectroscopy.&lt;/p&gt; &lt;p&gt;It was observed that increasing the BSA/PVP ratio resulted in higher fluorescence intensity at λ = 450 nm and a decrease at λ = 600 nm. Regarding the metal/reductant ratio, the amount of metal ions impacted the intensity obtained at λ = 600 nm. The experiments revealed that BSA had the largest effect on fluorescence intensity at λ = 450 nm, with little effect on fluorescence intensity regardless of the amount of polymer used. Due to its one-step synthesis and favorable reaction conditions, the NC Ag-BSA/PVP obtained under the proposed methodology holds promise as an optical marker material. The use of the stabilizing duo BSA-PVP, as well as the proposed amounts in this research, serves as a precedent for developing new experimental syntheses of colloidal nanoparticles.&lt;/p&gt; &lt;/abstract&gt;

Boosting the oxygen reduction activity of silver nanoclusters via selective exposure of solvent-coordinated sites

A versatile strategy to enhance the electrocatalytic activity of oxygen reduction reactions (ORR) has been developed by activating metal sites occupied by weakly coordinating solvent molecules as catalytic active centers.s

Role of silver nanoparticles and silver nanoclusters for the detection and removal of Hg(ii).

Silver metal, being a 3d transition metal in group 11 in the periodic table, is widely used in material science for its distinguished plasmonic properties. Nanoparticles (NPs) and nanoclusters (NCs) are widely used in sensing applications having a surface plasmon band and emissive properties, respectively. Mercury is one of the detrimental toxins and threats to various ecosystems. The distinction between nanoparticles and nanoclusters, the utility and toxicity of heavy metal mercury, fluorometric and colorimetric approaches to the recognition of mercury ions with NPs and NCs, the mechanism of detection, spot detection, and natural water sample analyses were illustrated in detail in this review article. Moreover, the sensing platform and analyte (Hg2+) fate were described for substantiating the mechanism. It was observed that NCs are mostly utilized for fluorometric approaches, while NPs are mostly employed for colorimetric approaches. Fluorometric detection is mainly quenching-based. However, sensing with enhancement was found in a few reports. Adulteration of other metals with silver particles often modifies the sensing platform.

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.

Quantitative analysis of charge transfer plasmons in silver nanocluster dimers using semiempirical methods.

Plasmonic metal nanoclusters are widely used in chemistry, nanotechnology, and biomedicine. In metal nanocluster dimers, coupling of the plasmons leads to the emergence of two distinct types of modes: (1) bonding dipole plasmons (BDP), which occurs when charge oscillates synchronously within each nanocluster, and (2) charge transfer plasmons (CTP), which occurs when charge oscillates between two conductively linked nanoclusters. Although TDDFT-based modeling has uncovered some trends in these modes, it is computationally expensive for large dimers, and quantitative analysis is challenging. Here, we demonstrate that the semiempirical quantum mechanical method INDO/CIS enables us to quantify the CTP character of each excited state efficiently. In end-to-end Ag nanowire dimers, the longitudinal states have CTP character that decreases with increasing gap distance and nanowire length. In side-by-side dimers, the transverse states have CTP character and generally larger than in the end-to-end dimers, particularly for the longer nanowires. In side-by-side dimers where one nanowire is shifted along the length of the other, the CTP character of the longitudinal states peaks when the dimer is shifted by two Ag-Ag bond lengths, while the transverse states show decreasing CTP character as displacement increases. In the larger Ag31+ nanorod dimers, CTP character follow a similar distance dependence to that seen in the small nanowire but have smaller overall CTP character than the nanowires. Our study demonstrates that INDO/CIS is capable of modeling metal nanocluster dimers at a low computational cost, making it possible to study larger dimers that are difficult to analyze using TDDFT.

Mechanistic elucidation of the catalytic activity of silver nanoclusters: exploring the predominant role of electrostatic surface.

The core and the ligand shell of metal nanoclusters (MNCs) have an influential role in modulating their spectroscopic signatures and catalytic properties. The aspect of electrostatic interactions to regulate the catalytic properties of MNCs has not been comprehensively addressed to date. Our present work conclusively delineates the role of the metal core and the electrostatic surface of MNCs involved in the reduction of nitroarenes. A facile surface modification of mercaptosuccinic acid (MSA)-templated AgNCs has been selectively achieved through Mg2+ ions (Mg-AgNCs). Microscopic studies suggest that the size of Mg-AgNCs is ∼3.3 nm, which is considerably higher than that of MSA-templated AgNCs (∼1.75 nm), confirming the formation of a nano-assembled structure. Our spectroscopic and microscopic experiments revealed that the negatively charged AgNCs efficiently catalyze the reduction of 4-nitrophenol (4-NP) with a rate constant of 0.23 ± 0.01 min-1. However, upon surface modification, the catalytic efficiency almost doubles due to the formation of Mg-AgNCs. Catalysis through AgNCs and Mg-AgNCs collectively portrays the role of the core and electrostatic surfaces. Furthermore, the role of electrostatic interaction has been substantiated by varying the ionic strength of the medium, as well as employing different molecular systems. A quantitative assessment of the Debye screening length asserts the correlation between the ionic strength of the medium and the role of electrostatic interactions involved herein. This highly enhanced catalytic aspect has been utilized for the real sample analysis, wherein AgNCs unexpectedly outperform Mg-AgNCs. This approach of real sample analysis also emanates the role of electrostatics involved. This comprehensive investigation represents the influential role of the core and ligand shell of MNCs as well as the role of electrostatics on its catalytic activities, which is relevant for the rational design of highly efficient catalysts.

An electrochemiluminescence microsensor based on DNA-silver nanoclusters amplification for detecting cellular adenosine triphosphate.

Adenosine triphosphate (ATP), as the primary energy source, plays vital roles in many cellular events. Developing an efficient assay is crucial to rapidly evaluate the level of cellular ATP. A portable and integrated electrochemiluminescence (ECL) microsensor array based on a closed bipolar electrode (BPE) was presented. In the BPE unit, the ECL chemicals and oxidation/reduction were separated from the sensing chamber. The ATP aptamer was assembled with single-stranded DNA (ssDNA) in the sensing chamber. ATP capture made the aptamer disassemble from the ssDNA and facilitated DNA-templated silver nanocluster (Ag NC) generation by the target-rolling circle amplification (RCA) reaction. The guanine-rich padlock sequence produced tandem periodic cytosine-rich sequences by the RCA, inducing Ag NC generation in the cytosine-rich region of the produced DNA strands through Ag+ reduction. The in situ Ag NC generation enhanced the circuit conductivity of the BPE and promoted the ECL reaction of [Ru(bpy)2dppz]2+/tripropylamine in the anodic reservoir. On this ECL microsensor, a good linear relationship of ATP was achieved ranging from 30 to 1000 nM. The ATP content in HepG2 cells was selectively and sensitively determined without complex pretreatment. The ATP amount of 25 cells could be successfully detected when a sub-microliter sample was loaded.

Turn-on fluorescent nanoprobe for ATP detection based on DNA-templated silver nanoclusters.

A turn-on fluorescence nanoprobe was constructed for the determination of adenosine 5'-triphosphate (ATP) based on DNA-templated silver nanoclusters (DNA-AgNCs). The significant enhancement fluorescence intensity of DNA-AgNCs in the presence of ATP is due to the high special binding affinity between ATP and the aptamer, resulting in the environment of DNA-AgNCs with darkish fluorescence lying at one terminus of DNA slightly altering owing to the change of ATP aptamer conformation. A good linear range runs from 9 to 24 mM with a satisfactory detection limit of 3 μM. Furthermore, the proposed nanoprobe exhibited good performance for ATP detection in diluted fetal bovine serum.

DNAzyme-mediated fluorescence signal variation of DNA-Ag nanoclusters and construction of an aptasensor for ATP.

DNA-templated silver nanoclusters (DNA-AgNCs) are novel nanomaterials with unique fluorescence characteristics. DNAzyme is a functional oligonucleotide that can catalyze the disruption of nucleic acid substrates. In this research, the effect of DNAzyme digestion on the fluorescence property of DNA-AgNCs was explored for the first time. A significant reduction in the fluorescence intensity of DNA-AgNCs after cleavage by DNAzyme was discovered. Further research found that the DNAzyme-catalyzed cleavage reduced the stability of DNA-AgNCs and led to their aggregation, accounting for a decline in fluorescence intensity up to 84%. Inspired by the above finding, a fluorescent aptasensor that integrates the benefits of DNA-AgNCs, exonuclease III (Exo III)-assisted signal amplification and DNAzyme was developed for sensitive detection of adenosine triphosphate (ATP). Under optimal conditions, the linear range was from 25 μM to 1000 μM and the detection limit was estimated to be 4.46 μM. Furthermore, this fluorescent aptasensor was effectively employed to quantify ATP levels in human serum samples, demonstrating its practicality in detecting ATP in biological matrices. The elucidation of DNAzyme-based fluorescence characteristic variation of DNA-AgNCs may provide insights into the interactions between DNAzyme and nanomaterials and has great prospects in the construction of fluorescent biosensors.

Fragmentation patterns of DNA-stabilized silver nanoclusters under mass spectrometry.

DNA-stabilized silver nanoclusters (AgN-DNAs) are emitters with tuneable structures and photophysical properties. While understanding of the sequence-structure-property relationships of AgN-DNAs has advanced significantly, their chemical transformations and degradation pathways are far less understood. To advance understanding of these pathways, we analysed the fragmentation products of 21 different red and NIR AgN-DNAs using negative ion mode electrospray ionization mass spectrometry (ESI-MS). AgN-DNAs were found to lose Ag+ under ESI-MS conditions, and sufficient loss of silver atoms can lead to a transition to a lesser number of effective valence electrons, N0. Of more than 400 mass spectral peaks analysed, only even values of N0 were identified, suggesting that solution-phase AgN-DNAs with odd values of N0 are unlikely to be stable. AgN-DNAs stabilized by three DNA strands were found to fragment significantly more than AgN-DNAs stabilized by two DNA strands. Moreover, the fragmentation behaviour depends strongly on the DNA template sequence, with diverse fragmentation patterns even for AgN-DNAs with similar molecular formulae. Molecular dynamics simulations, with forces calculated from density functional theory, of the fragmentation of (DNA)2(Ag16Cl2)8+ with a known crystal structure show that the 6-electron Ag16Cl2 core fragments into a 4-electron Ag10 and a 2-electron Ag6, preserving electron-pairing rules even at early stages of the fragmentation process, in agreement with experimental observation. These findings provide new insights into the mechanisms by which AgN-DNAs degrade and transform, with relevance for their applications in sensing and biomedical applications.

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.

Assembly of anionic silver nanoclusters with controlled packing structures through site-specific ionic bridges.

The assembly of metal nanoclusters (NCs) into crystalline lattice structures is of interest in the development of NC-based functional materials. Here we demonstrate that the assembled structures of tri-anionic tetrahedral symmetric [Ag29(BDT)12]3- (Ag29 NC, BDT: 1,3-benzenedithiol) NCs are controlled into a polyethylene-like zigzag chain and a "poly-ring-fused-cyclohexane"-like honeycomb arrangement through ionic interactions with alkali metal cations such as K+ and Cs+. The site-specific binding of alkali metal ions on the tetrahedrally arranged binding sites of Ag29 NCs successfully connects the adjacent NCs into various packing modes. The number and type of bridges between NCs determine the Ag29 NC packing structures, which are affected by the solvent species, enabling the transformation of packing modes in the single-crystalline state. The photoluminescence (PL) properties of the crystals responded to the packing modes of the NCs in terms of anisotropy and bridge linkage style inducing a varied degree of relaxation of the excited state depending on the relocation mobility of alkali metal ions in the crystals.