Ultra-small Silver Nanoparticles Research Articles

Ultrasmall AgNP-Impregnated Biocompatible Hydrogel with Highly Effective Biofilm Elimination Properties.

Ultrasmall silver nanoparticles (AgNPs; size < 3 nm) have attracted a great deal of interest as an alternative to commercially available antibiotics due to their ability to eliminate a wide range of microbial pathogens. However, most of these ultrasmall AgNPs are highly reactive and unstable, as well as susceptible to fast oxidation. Therefore, both the stability and toxicity remain major shortcomings for their clinical application and uptake. To circumvent these problems, we present a novel strategy to impregnate ultrasmall AgNPs into a biocompatible thermosensitive hydrogel that enables controlled release of silver alongside long-term storage stability and highly potent antibacterial activity. The advantage of this strategy lies in the combination of a homogenous dispersion of AgNPs in a hydrogel network, which serves as a sustained-release reservoir, and the unique feature of ultrasmall AgNP size, which provides an improved biofilm eradication capacity. The superior biofilm dispersion properties of the AgNP hydrogel is demonstrated in both single-species and multispecies biofilms, eradicating ∼80% of established biofilms compared to untreated controls. Notably, the effective antibacterial concentration of the formulation shows minimal toxicity to human fibroblasts and keratinocytes. These findings present a promising novel strategy for the development of AgNP hydrogels as an efficient antibacterial platform to combat resistant bacterial biofilms associated with wound infections.

Ultra-Small Silver Nanoparticles Immobilized in Mesoporous SBA-15. Microwave-Assisted Synthesis and Catalytic Activity in the 4-Nitrophenol Reduction

Despite the continuous developments in the synthesis of noble metal nanoparticles, the uniformity of particle size distribution still represents a critical aspect. A fast and homogeneous nucleation is a key requirement to achieve a monodisperse particle size distribution and in this scenario, the application of alternative energy sources may constitute a winning strategy for the development of highly active nanocatalysts with unique properties. Here we present several approaches to control the synthesis of Ag nanoparticles stabilized by an anionic template, and the results evidence the advantages of adopting unconventional heating techniques such as microwave heating. The fast and selective electromagnetic heating strongly reduced the nucleation and growth times, impacting on the homogeneity of the resulting particle size distribution. In this work, we have carried out the microwave-assisted synthesis of Ag nanoparticles and the resulting nanoparticles were compared to those synthesized under conventional heating using an oil bath, showing that the differences in temperature profile and heating rates between the two synthesis pathways had a clear effect on the size distribution of the resulting nanoparticles as well as on their stability under long term storage. Finally, the synthesized ultra-small Ag nanoparticles were deposited on a mesoporous substrate, reducing undesired Ostwald ripening and facilitating their reusability. This nanocatalyst was adopted for the abatement of 4-nitrophenol, a well-known carcinogenic pollutant with adverse effects on human beings and aquatic life. The catalytic results confirm the high activity of the catalyst thanks to the high dispersion achieved afforded by ultra-small Ag nanoparticles and the accessibility provided by the wide SBA-15 mesoporous channels.

Supported Ultra-Thin Alumina Membranes with Graphene as Efficient Interference Enhanced Raman Scattering Platforms for Sensing.

The detection of Raman signals from diluted molecules or biomaterials in complex media is still a challenge. Besides the widely studied Raman enhancement by nanoparticle plasmons, interference mechanisms provide an interesting option. A novel approach for amplification platforms based on supported thin alumina membranes was designed and fabricated to optimize the interference processes. The dielectric layer is the extremely thin alumina membrane itself and, its metallic aluminum support, the reflecting medium. A CVD (chemical vapor deposition) single-layer graphene is transferred on the membrane to serve as substrate to deposit the analyte. Experimental results and simulations of the interference processes were employed to determine the relevant parameters of the structure to optimize the Raman enhancement factor (E.F.). Highly homogeneous E.F. over the platform surface are obtained, typically 370 ± (5%), for membranes with ~100 nm pore depth, ~18 nm pore diameter and the complete elimination of the Al2O3 bottom barrier layer. The combined surface enhanced Raman scattering (SERS) and interference amplification is also demonstrated by depositing ultra-small silver nanoparticles. This new approach to amplify the Raman signal of analytes is easily obtained, low-cost and robust with useful enhancement factors (~400) and allows only interference or combined enhancement mechanisms, depending on the analyte requirements.

Development of Non-ionic Surfactant and Protein-Coated Ultrasmall Silver Nanoparticles: Increased Viscoelasticity Enables Potency in Biological Applications.

To enhance the interactivity with biological cells, we developed ultrasmall (5 nm in diameter) Ag NPs coated with a mixture of Tween-20 (Tw-20) surfactant and human serum albumin (HSA) or hemoglobin (Hb) proteins. These were tested with cancerous and healthy cell lines to investigate the therapeutic applicability. Using the established concept of generation of reactive oxygen species (ROS) and the ROS-induced oxidative stress in carcinogenic cells by Ag NPs, we found that the presently synthesized Ag NPs selectively destroyed the cancerous cells. A mixture of Tw-20 with protein, where the surfactant was in large excess, created a coating over the Ag NPs resulting weaker protein–protein interactions and facilitating interfacial protein–surfactant interactions, which leads to an increase in the film viscoelasticity to enhance the stability of the Ag NPs and cell viability. Moreover, this concept has been applied to drug delivery using a model fluorophore (fluorescein) on Ag NPs to explore the prospects in photodynamic therapy. The results are encouraging and deserve further investigation.

Controlling self-assembly of ultra-small silver nanoparticles: Surface enhancement of Raman and fluorescent spectra

Resorcinol based reduction and capping of ultra-small colloidal silver nanoparticles (Ag NPs) optimally formed hydrogen bond driven clusters within the water-based solution. Large-area self-assembled fractal structures of Ag NPs aggregates were fabricated by solvent evaporation technique on silicon substrate. The final dried surface structures were found to be pH dependent which was adjusted to particular values after the optimal reduction of silver salt, capping of Ag NPs, followed by cluster formation at an optimum pH in the solution phase. These self-assembled surface structures show controlled enhancement of various surface optical phenomena such as Raman spectra of a model dye and luminescence of Ag NPs as a function of pH dependent fractal dimension of the resulting structures. The structure, morphology and various relevant optical properties of self-assembled fractals of Ag NPs were further established by combined use of X-Ray diffractometry (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), UV-Visible, photoluminescence and Raman spectroscopy analysis.

β-Chitin nanofiber hydrogel as a scaffold to in situ fabricate monodispersed ultra-small silver nanoparticles

β-chitin nanofibers (β-ChNFs) were efficiently prepared from squid pens by a combination of ball milling and ultrasonic techniques. The gellation of the β-ChNFs dispersion was achieved through a dialysis neutralization process, and hydrogels, even at 0.2% solid content, had high stiffness, as indicated by the dynamic viscoelasticity measurements. The β-ChNFs hydrogels were used as scaffolds to fabricate ultra-small silver nanoparticles (AgNPs) by in situ ultraviolet (UV) photo-reduction. High-angle annular dark-field scanning transmission energy microscopy (HAADF-STEM) analysis showed that 4 nm monodispersed AgNPs were produced in the hydrogel framework, and X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy analysis revealed the strong interaction between AgNPs and the chitin nanofiber matrix. Furthermore, biological evaluation showed no cytotoxicity and excellent bactericidal activity of the silver nanoparticle-impregnated β-chitin nanofiber hydrogels, thus suggesting that they are promising materials to heal refractory wounds.

Unusual fcc-structured Ag10 kernels trapped in Ag70 nanoclusters.

Controlled trapping atom-precise ultrasmall silver nanoparticles into silver nanoclusters is challenging; thus only limited progress has been made in this area. We are therefore inspired to isolate two novel silver nanoclusters, Ag10@Ag70 (SD/Ag80a and SD/Ag80b; SD = SunDi), where a novel fcc-structured Ag10 kernel built from two single-edge opened Ag6 octahedra by sharing one edge is trapped. The bioctahedral Ag10 kernel is locked by a pair of Mo7O26 10- anions to form an inner Ag10@(Mo7O26)2 core which is further encapsulated by an outer Ag70 shell to form three-shell Ag10@(Mo7O26)2@Ag70 nanoclusters. Notably, the bioctahedral Ag10 kernel has not been observed in silver nanoclusters ever before, thus representing a new embryo state of silver nanoparticles. SD/Ag80a emits in the near infrared (NIR) region (λ em = 730 nm) at low temperature. This work will deepen our understanding on the atomic-level growth of silver nanoparticles and complicated three-shell self-assembly involving polyoxometalate (POM) and two different silver nanoclusters.

Europium-Doped Hydroxyapatite Nanorods: Influence of Silver Doping

In this work, we report the influence of silver nanoparticles of different size on the optical behavior of the europium-doped hydroxyapatite nanorods. The presence of ionic, plasmonically visible and silent (ultra-small) silver nanoparticles on the surface of europium-doped apatite and its influence on its luminescent behavior were discussed. It was found that ionic silver exhibited a concentration-dependent enhancement in emission at the excitation of europium. For plasmonically visible and silent silver nanoparticles, the emission observed was due to the contribution of the ligand passivating the europium-doped hydroxyapatite nanoparticle surface. These nanoparticles due to its luminescent and biocompatible nature can be used for bioimaging applications.

Photoreduction of carbon dioxide under visible light by ultra-small Ag nanoparticles doped into Co-ZIF-9

Metal–organic frameworks (MOFs) are well-known porous materials able to adsorb CO2, and their performance in CO2 reduction has attracted much attention from researchers. A classical Co-MOF, Co-ZIF-9, has been proposed as a novel photocatalyst for reducing CO2 into chemical feedstocks. Herein, Co-ZIF-9 with a rod-like structure was obtained through reflux. Ultra-small silver nanoparticles (Ag NPs, smaller than 5 nm) were doped into Co-ZIF-9 by the photodeposition method. With the assistance of a photosensitizer, the resultant composite Ag@Co-ZIF-9 shows catalytic reactivity in converting CO2 into CO under visible light irradiation. Compared with bare Co-ZIF-9, the photocatalytic performance of Ag@Co-ZIF-9 increases by more than twofold (around 28.4 μmol CO) and the selectivity is enhanced by about 20% (22.9 μmol H2) for 0.5 h of irradiation. This demonstrates that Ag NPs doping may provide a possible way to promote the efficiency and selectivity of MOF materials in CO2 photoreduction.

High-Speed but Not Magic: Microwave-Assisted Synthesis of Ultra-Small Silver Nanoparticles.

Reaction procedures have been improved to achieve higher yields and shorter reaction times: one possibility is the usage of microwave reactors. In the literature, this is under discussion, for example, nonthermal effects resulting from the microwave radiation are claimed. Especially for the synthesis of nanomaterials, it is of crucial importance to be aware of influences on the reaction pathway. Therefore, we compare the syntheses of ultra-small silver nanoparticles via conventional and microwave heating. We employed a versatile one-pot polyol synthesis of poly(acrylic acid)-stabilized silver nanoparticles, which display superior catalytic properties. No microwave-specific effects in terms of particle size distribution characteristics, as derived by small-angle X-ray scattering and dynamic light scattering, are revealed. Because of the characteristics of a closed system, microwave reactors give access to elevated temperatures and pressures. Therefore, the speed of particle formation can be increased by a factor of 30 when the reaction temperature is increased from 200 to 250 °C. The particle growth process follows a cluster coalescence mechanism. A postsynthetic incubation step at 250 °C induces a further growth of the particles while the size distribution broadens. Thus, utilization of microwave reactors enables an enormous decrease of the reaction time as well as the opportunity of tuning the particle size. Possibly, decomposition of the stabilizing ligand at elevated temperatures results in reduced yields. A compromise between short reaction times and high yields can be found at a temperature of 250 °C and a corresponding reaction time of 30 s.

Multi-functional bio-compatible luminescent apatite with fatty acid passivated nano silver covers and its theranostics potential

Europium doped hydroxyapatite (EDA) nanorods with linoleic acid passivated silver ions on their surfaces were synthesized using facile, one-step hydrothermal route. Annealing the samples at 250 °C resulted in formation of ultra-small silver (USS) nanoparticles on the surface by nucleation through diffusion process. EDA exhibited luminescence properties due to the presence of europium ions doped on the calcium sites of hydroxyapatite. These EDA nanorods exhibited a different luminescent behavior in the presence of silver ions and USS nanoparticles. This report also demonstrates excellent biocompatibility and cytotoxicity of EDA nanorods with silver ions towards fibroblast cell lines (F929) and breast cancer cells (MCF-7). Visible and near infra-red (NIR) emissions in EDA, induced by silver ions and USS nanoparticles makes it a potential system for deep tissue imaging applications. The arrangement of USS over the EDA was tunable and hence the selectivity and enhancement of the Eu3+ ions emission can also be tuned. The multifunctional properties of this system such as its active luminescence over a wide range, its cell proliferation towards normal cells and cytotoxicity towards cancer cells shows its potential for application in cancer theranostics.

Ultrasmall silver nanoparticles loaded in alginate–hyaluronic acid hybrid hydrogels for treating infected wounds

ABSTRACTNowadays, silver nanoparticles are in the limelight to control infection during wound healing process, and a vast variety of antimicrobial dressings based on colloidal silver have been marketed to fight wound invasion of pathogen bacteria, which represents one of the main adverse effects limiting the repair process. Here we propose a biofunctional hydrogel based on alginate (ALG) and hyaluronic acid (HA) embedding ultrasmall silver nanoparticles (usSN, <1 nm) as antimicrobial component. The hydrogels were fabricated in different size by a straightforward internal gelation method using CaCO3 and glucono-δ-lactone. To follow usSN release from the hydrogels in aqueous media, catalytic activity of usSN-loaded hydrogels was evaluated. Results suggested that catalytic activity was low in intact hydrogels and high when hydrogels dissolved, which suggests that usSN firmly interact with polymer chains and are available in the medium depending on the extent of hydrogel degradation. HA-containing hydrogels showed faster dissolution in simulated physiological conditions and higher antibiofouling properties as compared to hydrogels made only of ALG. Free usSN were not toxic toward human mesenchymal stem cells (Ad-MSCs), previously isolated from subcutaneous adipose tissue biopsies, up to 50 µg/mL. At this concentration, viability of Ad-MSCs was unaffected whereas their motility was significantly higher as compared to control (p<0.01) for both free usSN and hydrogel integrating. Antimicrobial activity on clinical isolates of both Gram-positive and Gram-negative bacteria demonstrated that usSN at 50 µg/mL were able to kill all the bacteria tested after 24 and 48 h of contact time. In the case of hydrogels, a matrix effect was found and bacterial killing was significant only at 24 h and dependent on bacterial strain, being Gram-negative bacteria more susceptible. These results clearly indicate that usSN interaction with polymer network and exposure time can strongly affect usSN antimicrobial profile in the hydrogel and, in turn, timing of hydrogel change at injured site in a clinical setting. On the whole, ALG/HA hydrogels integrating usSN can be considered a suitable option to fabricate biofunctional dressings for hospitalized patients and worth of further in vivo investigation.

Ultra-small silver nanoparticles induced ROS activated Toll-pathway against Staphylococcus aureus disease in silkworm model

The present study investigated the therapeutic action of flavonoids loaded silver nanoparticles (FLV-Ag NPs) on a silkworm, Bombyx mori L., larvae infected by the S. aureus, where an in vivo model system was used. FLV-Ag NPs were synthesized using a Ricinus communis L. leaf extracted flavonoid mixture in various concentrations. The reduction reaction was performed under a simple sunlight exposure condition. In the reduction process, quercetin and kaempferol loadings were also found. They were confirmed by UV–visible, TEM, XRD, XPS, DLS and FTIR spectroscopic techniques. TEM confirmed that the synthesized NPs were monodispersed and with an average size of 5.8nm±0.04. Initially, those synthesized FLV-Ag NPs were tested against S. aureus in in vitro by comparing their potential inhibitory activity with only flavonoids mixture (FLVs) and pure silver nanoparticles (Ag NPs). Furthermore, these FLV-Ag NPs were used to treat S. aureus infected B. mori larvae and the therapeutic efficacy was measured. Our results demonstrate that the FLV-Ag NPs are effective in curing the S. aureus infection through the induced expression of antimicrobial peptide genes, the enhanced oxidative enzyme levels, and the promoted phagocytosis of S. aureus by the haemocytes in silkworm larvae. On the basis of these results, it is proposed that the mechanism of the antimicrobial activity of FLV-Ag NPs involves the NPs-insect fat body cell association and a reactive oxygen species-mediated Toll-pathway.

Optical Observation of Plasmonic Nonlocal Effects in a 2D Superlattice of Ultrasmall Gold Nanoparticles.

The advances in recent nanofabrication techniques have facilitated explorations of metal structures into nanometer scales, where the traditional local-response Drude model with hard-wall boundary conditions fails to accurately describe their optical responses. The emerging nonlocal effects in single ultrasmall silver nanoparticles have been experimentally observed in single-particle spectroscopy enabled by the unprecedented high spatial resolution of electron energy loss spectroscopy (EELS). However, the unambiguous optical observation of such new effects in gold nanoparticles has yet not been reported, due to the extremely weak scattering and the obscuring fingerprint of strong interband transitions. Here we present a nanosystem, a superlattice monolayer formed by sub-10 nm gold nanoparticles. Plasmon resonances are spectrally well-separated from interband transitions, while exhibiting clearly distinguishable blueshifts compared to predictions by the classical local-response model. Our far-field spectroscopy was performed by a standard optical transmission and reflection setup, and the results agreed excellently with the hydrodynamic nonlocal model, opening a simple and widely accessible way for addressing quantum effects in nanoplasmonic systems.

Monitoring the fate of small silver nanoparticles during artificial digestion

We report on the results of an in vitro digestion study of silver nanoparticles in presence and absence of food. The particles were poly(acrylic acid) stabilized ultra-small silver nanoparticles with a radius of 3.1nm and a relative size distribution width of 0.2. As food components oil, starch, skimmed milk powder and a mixture thereof were chosen. Aggregation of the particles was quantified with small-angle X-ray scattering in terms of log-normal radii distributions. Complete aggregation of the primary particles was determined in the absence of food. In contrast, the presence of oil and starch initiates a disaggregation in the intestine. Only small aggregates of 6nm radii and aggregation numbers of 7 were found in the presence of milk powder. It prevents primary particles from etching in the gastric and intestinal juice. Our results indicate that the silver nanoparticles can pass the digestion process in a nanoscale form but undergo a strong and food-dependent transformation in their state of aggregation.

Positive homotropic allosteric binding of silver(i) ions in multidentate azacalixpyridine macrocycles: effect on the formation and stabilization of silver nanoparticles

We report herein the occurrence of a positive homotropic allosteric binding effect of two macrocyclic azacalixpyridines. This effect was firstly found to be crucial in the formation and stabilization of ultra-small silver nanoparticles.

Ultrasmall silver nanoparticles supported on silica and their catalytic performances for carbon monoxide oxidation

Abstract The ultrasmall Ag nanoparticles supported on silica were prepared by a novel and facile method, which was based on the introduction of the functional monomer salicylaldimine Schiff base into a SiO2 matrix. The ultrasmall silica-supported Ag nanoparticles were characterized by means of XRD, TEM, UV–Vis, XPS and BET, and tested for CO oxidation. It was found that the catalytic oxidation of CO over the ultrasmall Ag nanoparticle catalysts was a strong size dependent reaction and the Ag particle size in a range of 3–5 nm was favorable for CO oxidation.

Study on optical properties of aggregated ultra-small metal nanoparticles

We study the optical properties of dimer and septamer aggregates (in forms of being separated or merged) of ultra-small silver nanoparticles (a few nanometers in diameter) by calculating their extinction spectra and optical field distributions using the discrete dipole approximation method. Characteristics of their extinction spectra are identified due to different resonance modes dependent on the incidence states of light. Specifically, as polarization of the incidence light is perpendicular to the center-to-center lines of the nanoparticles, interaction of neighboring nanoparticles is in the “repulsive coupling mode”, which results in blue-shift of the resonance peak with decrease of the nanoparticle interspacing. While, whenever there is a projection of the polarization of incident light on the center-to-center lines of the nanoparticles, the “attractive coupling mode” between the nanoparticles dominates in their resonant interaction, which results in red-shift of the resonance peak with decrease of the nanoparticle interspacing. It is also shown that optical interaction of the ultra-small nanoparticles is only effective when their interspacings are approximately less than 15 nm, and particularly prominent when Coulomb interactions of the electronic charges of plasmons are active within a few nanometers.

Novel Synthesis of Ultra-Small Silver Nanoparticles with High Antimicrobial Activity

Ultra-small and well dispersed silver nanoparticles were prepared by mixing AgNO3 in PEI (polyethylenimine) solution. Particle size analysis and transmission electron microscopy confirmed that the average diameters of PEI/silver nanoparticles are 20-40 nm. The ultra-small silver nanoparticles had an antimicrobial activity in the aqueous solution at the concentration over 0.0105% by M.I.C test. Suggestively, the PEI/silver nanoparticles have potentials for development of wound dressings.