Stabilizer-free silver nanoparticles as efficient catalysts for electrochemical reduction of oxygen

Synthesis and Application of Silver Nanoparticles for Caries Management: A Review

Silver nanoparticles have unique physical, chemical, and biological properties that make them attractive for medical applications. They have gained attention in dentistry for their potential use in caries management. This study reviews the different synthesis methods of silver nanoparticles and the application of them for caries management. Silver nanoparticles are tiny silver and are typically less than 100 nanometres in size. They have a high surface area-to-volume ratio, making them highly reactive and allowing them to interact with bacteria and other materials at the molecular level. Silver nanoparticles have low toxicity and biocompatibility. Researchers have employed various methods to synthesise silver nanoparticles, including chemical, physical, and biological methods. By controlling the process, silver nanoparticles have defined sizes, shapes, and surface properties for wide use. Silver nanoparticles exhibit strong antibacterial properties, capable of inhibiting a broad range of bacteria, including antibiotic-resistant strains. They inhibit the growth of cariogenic bacteria, such as Streptococcus mutans. They can disrupt bacterial cell membranes, interfere with enzyme activity, and inhibit bacterial replication. Silver nanoparticles can inhibit biofilm formation, reducing the risk of caries development. Additionally, nano silver fluoride prevents dental caries by promoting tooth remineralisation. They can interact with the tooth structure and enhance the deposition of hydroxyapatite, aiding in repairing early-stage carious lesions. Silver nanoparticles can also be incorporated into dental restorative materials such as composite resins and glass ionomer cements. The incorporation can enhance the material's antibacterial properties, reducing the risk of secondary caries and improving the longevity of the restoration.

Stability of Carbon Supported Silver Electrocatalysts for Alkaline Oxygen Reduction and Evolution Reactions

Ag-based electrocatalysts are promising candidates to catalyze the sluggish oxygen reduction reaction (ORR) in anion exchange membrane fuel cells (AEMFC) and oxygen evolution reaction (OER) in unitized regenerative fuel cells. However, to be competitive with existing technologies, the AEMFC with Ag electrocatalyst must demonstrate superior performance and long-term durability. The latter implies that the catalyst must be stable, withstanding harsh oxidizing conditions. Moreover, since Ag is typically supported by carbon, the strict stability requirements extend to the whole Ag/C catalyst. In this work, Ag supported on Vulcan carbon (Ag/VC) and mesoporous carbon (Ag/MC) materials is synthesized, and their electrochemical stability is studied using a family of complementary techniques. We first employ an online scanning flow cell combined with inductively coupled plasma mass spectrometry (SFC-ICP-MS) to estimate the kinetic dissolution stability window of Ag. Strong correlations between voltammetric features and the dissolution processes are discovered. Very high silver dissolution during the OER renders this material impractical for regenerative fuel cell applications. To address Ag stability during AEMFC load cycles, accelerated stress tests (ASTs) in O2-saturated solutions are carried out in rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) setups. Besides tracking the ORR performance evolution, an ex situ long-term Ag dissolution study is performed. Moreover, morphological changes in the catalyst/support are tracked by identical-location transmission electron microscopy (RDE-IL-TEM). Voltammetry analysis before and after AST reveals a smaller change in ORR activity for Ag/MC, confirming its higher stability. RRDE results reveal a higher increase in the H2O2 yield for Ag/VC after the ASTs. The RDE-IL-TEM measurements demonstrate different degradation processes that can explain the changes in the long term performance. The results in this work point out that the stability of carbon-supported Ag catalysts depends strongly on the morphology of the Ag nanoparticles, which, in turn, can be tuned depending on the chosen carbon support and synthesis method.

Using an ionomer as a size regulator in γ-radiation induced synthesis of Ag nanocatalysts for oxygen reduction reaction in alkaline solution

Ag nanoparticles (Ag NPs) are among the most promising candidates to replace Pt as the catalyst for the oxygen reduction reaction (ORR) in anion exchange membrane fuel cells (AEMFCs). However, synthesizing size-controlled Ag NPs with efficient catalytic performance is still challenging. Herein, uniform Ag NPs are produced through a γ-radiation induced synthesis route in aqueous solutions, using the ionomer PTPipQ100 as both an efficient size regulator in the synthesis and a conductor of hydroxide ions during the ORR process. The origin of the size control is mainly attributed to the affinity of the ionomer to metallic silver. The resulting Ag NPs covered with ionomer layers can be applied as model catalysts for ORR. The nanoparticles that were prepared using 320 ppm ionomer in the reaction solution turned out to be coated with a ∼ 1 nm thick ionomer layer and exhibited superior ORR activity as compared to other Ag NPs of similar size studied here. The improved electrocatalytic performance can be attributed to the optimal ionomer coverage that enables fast oxygen diffusion, as well as interactions at the Ag-ionomer interface which promote the desorption of OH intermediates from the Ag surface. This work demonstrates the advantage of using an ionomer as the capping agent to produce efficient ORR catalysts.

Green synthesis of silver nanoparticles using Eupatorium adenophorum leaf extract: characterizations, antioxidant, antibacterial and photocatalytic activities

The green synthesis of metallic nanoparticles has tremendous impacts in various fields as found in recent years due to their low cost, easy and environmentally friendly synthesis. In this article, we report a simple and eco-friendly method for the synthesis of silver nanoparticles (AgNPs) using an aqueous Eupatorium adenophorum (E. adenophorum) leaf extract as a bioreductant. Interestingly, Fourier transform infrared (FTIR) spectroscopy analysis established that the E. adenophorum extract not only served as a bioreductant but also acted as a capping agent to stabilize the nanoparticles by functionalizing the surfaces. Various characterization techniques were adopted, such as X-ray powder diffraction (XRD), FTIR, ultraviolet-visible absorption (UV-Vis) spectroscopy, dynamic light scattering, scanning electron microscopy and energy-dispersive X-ray spectroscopy (EDX) to analyze the biosynthesized AgNPs. Biosynthesized nanoparticles were also explored for antioxidant, antibacterial and photocatalytic activities. The AgNPs showed improved free radical scavenging activity (IC₅₀ 48.96 ± 0.84 µg/mL) and bacterial inhibitory effects against both gram-positive (Staphylococcus aureus; 64.5 µg/mL) and gram-negative (Escherichia coli; 82.5 µg/mL) bacteria. Photocatalytic investigation showed AgNPs were effective at degrading rhodamine dye (78.69% in 90 min) when exposed to sunlight. These findings collectively suggest that E. adenophorum AgNPs were successfully prepared without the involvement of any hazardous chemical and it may be an effective antibacterial, antioxidant and promising agent for the removal of hazardous dye from waste water produced by industrial dyeing processes.

Nitrite reduction over Ag nanoarray electrocatalyst for ammonia synthesis

Electrochemical reduction of nitrite to ammonia can simultaneously achieve ammonia synthesis and N-contaminant removal under mild conditions, which has attracted widespread attention but still lacks efficient catalysts. In this work, Ag nanoarray using NiO nanosheets array on carbon cloth as support is reported as an efficient electrocatalyst to selectively reduce nitrite to ammonia. In 0.1 M NaOH with 0.1 M NO₂^-, such catalyst exhibits a maximum ammonia yield of 5,751 μg h^-1 cm^-2 (57,510 μg h^-1 mg_Ag^-1) and high Faradaic efficiency up to 97.7 %. Density functional theory calculations applied to uncover the catalytic mechanism of NO₂^- reduction reaction on Ag.

Highly stable silver nanohybrid electrocatalysts for the oxygen reduction reaction

Silver nanoparticles (AgNPs) were deliberately functionalized via aryl diazonium chemistry with a monolayer of calix[4]arenes. The resulting nanohybrids show high efficiency and high selectivity toward the ORR in alkaline media along with an exceptional durability and a high methanol tolerance.

Oxygen reduction on silver catalysts electrodeposited on various nanocarbon supports

In this work, Ag particles were electrodeposited onto nitrogen-doped graphene oxide, graphene, multi-walled carbon nanotube (MWCNT), and Vulcan carbon XC-72R supports by varying the upper potential limit. The surface morphology of the resulting Ag-based catalysts was examined by scanning electron microscopy. The electrochemical oxygen reduction reaction (ORR) was tested in alkaline media employing the rotating disk electrode method. The variation of the upper potential limit influenced the size of silver nanoparticles and their number density on the substrate surface. All the Ag-based electrocatalysts studied in this work showed remarkable ORR activity in terms of half-wave potentials. The ORR results combined with hydrogen peroxide reduction results prove that all Ag catalysts tested are suitable for both reactions. Ag/NGO2 catalyst possesses the highest mass activity for ORR, which indicates a relationship between the Ag loading and electrocatalytic activity. The electroreduction of oxygen on all the electrodeposited silver catalysts follows a four-electron pathway in alkaline environment. These materials are promising alternatives for Pt/C catalyst to be used as alkaline membrane fuel cell cathodes.

Green synthesis, characterization, enhanced functionality and biological evaluation of silver nanoparticles based on Coriander sativum

The present study focused on the green synthesis of silver nanoparticles from Coriander sativum (CS) containing structural polymers, phenolic compounds and glycosidic bioactive macromolecules. Plant phenolic compounds can act as antioxidants, lignin, and attractants like flavonoids and carotenoids. Henceforth, silver nanoparticles (AgNPs) were prepared extracellularly by the combinatorial action of stabilizing and reduction of the CS leaf extract. The biologically synthesized CS-AgNPs were studied by UV-spectroscopy, zeta potential determination, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis to characterize and confirm the formation of crystalline nanoparticles. The synthesized nanoparticles demonstrated strong antimicrobial activity against all microbial strains examined with varying degrees. The scavenging action on free radicals by CS-AgNPs showed strong antioxidant efficiency with superoxide and hydroxyl radicals at different concentrations as compared with standard ascorbic acid. The presence of in vitro anticancer effect was confirmed at different concentrations on the MCF-7 cell line as revealed with decrease in cell viability which was proportionately related to the concentration of CS-AgNPs illustrating the toxigenic nature of synthesized nanoparticles on cancerous cells.

Oxygen reduction reaction on Pd nanocatalysts prepared by plasma-assisted synthesis on different carbon nanomaterials

In this work He/H₂ plasma jet treatment was used to reduce Pd ions in the aqueous solution with simultaneous deposition of created Pd nanoparticles to support materials. Graphene oxide (GO) and nitrogen-doped graphene oxide (NrGO) were both co-reduced with the Pd ions to formulate catalyst materials. Pd catalyst was also deposited on the surface of carbon black. The prepared catalyst materials were physically characterized using transmission electron microscopy, scanning electron microscopy and x-ray photoelectron spectroscopy. The plasma jet method yielded good dispersion of small Pd particles with average sizes of particles being: Pd/rGO 2.9 ± 0.6 nm, Pd/NrGO 2.3 ± 0.5 nm and Pd/Vulcan 2.8 ± 0.6 nm. The electrochemical oxygen reduction reaction (ORR) kinetics was explored using the rotating disk electrode method. Pd catalyst deposited on nitrogen-doped graphene material showed slightly improved ORR activity as compared to that on the nondoped substrate, however Vulcan carbon-supported Pd catalyst exhibited a higher specific activity for oxygen electroreduction.

Synthesis of Silver Nanoparticles with Gemini Surfactants as Efficient Capping and Stabilizing Agents

The scientific community has paid special attention to silver nanoparticles (AgNPs) in recent years due to their huge technological capacities, particularly in biomedical applications, such as antimicrobials, drug-delivery carriers, device coatings, imaging probes, diagnostic, and optoelectronic platforms. The most popular method of obtaining silver nanoparticles as a colloidal dispersion in aqueous solution is chemical reduction. The choice of the capping agent is particularly important in order to obtain the desired size distribution, shape, and dispersion rate of AgNPs. Gemini alkylammonium salts are named as multifunctional surfactants, and possess a wide variety of applications, which include their use as capping agents for metal nanoparticles synthesis. Because of the high antimicrobial activity of gemini surfactants, AgNPs stabilized by this kind of surfactant may possess unique and strengthened biocidal properties. The present paper presents the synthesis of AgNPs stabilized by gemini surfactants with hexadecyl substituent and variable structure of spacer, obtained via ecofriendly synthesis. UV-Vis spectroscopy and dynamic light scattering were used as analyzing tools in order to confirm physicochemical characterization of the AgNPs (characteristic UV-Vis bands, hydrodynamic diameter of NPs, polydispersity index (PDI)).

The oxygen reduction reaction at silver electrodes in high chloride media and the implications for silver nanoparticle toxicity

The oxygen reduction reaction (ORR) at neutral pH in various aqueous chloride-containing solutions was investigated voltammetrically. In particular, the ORR was performed in high chloride containing aqueous media including authentic and synthetic seawater under oxygen saturated conditions and compared with that in aqueous nitrate and perchlorate media. The experimental voltammograms revealed a two-electron process forming hydrogen peroxide in low chloride media. In contrast, high concentration chloride solutions, including both synthetic and authentic seawater showed an increase of overpotential, accompanied by a splitting of the voltammetric peak into two one-electron features indicating the formation of superoxide in the first step and its release from the silver-solution interface. The implications for silver nanoparticle toxicology are discussed given the markedly greater toxicity of superoxide over peroxide and the high levels of chloride in biological media as well as in seawater.

Antimicrobial Potential and Cytotoxicity of Silver Nanoparticles Phytosynthesized by Pomegranate Peel Extract

The phytosynthesis of metal nanoparticles is nowadays attracting the increased attention of researchers and is much needed given the worldwide matter related to environmental contamination. The antimicrobial activity of colloidal and spray formulation of silver nanoparticles (AgNPs) synthesized by pomegranate peel extract against Candida albicans and Staphylococcus aureus, and their cytotoxicity in mammalian cells were tested in the present study. Dry matter, pH, total phenolics, and ellagic acid in the extract were determined. Then, AgNPs were phytosynthesized and characterized by X-ray diffraction, electron transmission microscopy, dynamic light scattering, zeta potential, and Ag⁺ dosage. Spray formulations and respective chemical-AgNP controls were prepared and tested. The peel extract reduced more than 99% of Ag⁺, and produced nanoparticles with irregular forms and an 89-nm mean size. All AgNP presented antimicrobial activity, and the spray formulation of green-AgNP increased by 255 and 4 times the effectiveness against S. aureus and C. albicans, respectively. The cytotoxicity of colloidal and spray green-AgNP was expressively lower than the respective chemical controls. Pomegranate peel extract produced stable AgNP with antimicrobial action and low cytotoxicity, stimulating its use in the biomedical field.

Au/Cu Bimetallic Nanoparticles via Double-Target Sputtering onto a Liquid Polymer

Alloy nanoparticles (NPs) of a bimetal system, Au/Cu, that form intermetallic compounds in a bulk state have been successfully produced using a double-target sputtering technique onto a low-cost and biocompatible liquid polymer (polyethylene glycol, PEG). The formation of an Au/Cu solid solution alloy in individual NPs was revealed by scanning transmission electron microscopy-energy-dispersive X-ray elemental mapping analysis. Altering the sputter currents for Au and Cu targets resulted in a tailored NP composition, but the particle sizes did not significantly vary. We found similar structures, sizes, and optical properties of Au/Cu NPs obtained by double-head sputtering on carbon-coated transmission electron microscopy grids or PEG and by Au/Cu alloy target sputtering. Random alloy formation occurred in matrix sputtering using double-target heads. This method is advantageous for manipulating the alloy composition through highly independent control of sputter parameters for each metal target.

Silver Nanoparticles Stabilised by Cationic Gemini Surfactants with Variable Spacer Length

The present study is focused on the synthesis and investigation of the physicochemical and biological properties of silver nanoparticles stabilized with a series of cationic gemini surfactants having a polymethylene spacer of variable length. UV-VIS spectroscopy, dynamic light scattering, scanning electron microscopy and zeta potential measurements were applied to provide physicochemical characterization of the silver nanoparticles. The mean size values of the nanoparticles were found to be in the 50 to 115 nm range. From the nanoparticle size distributions and scanning electron microscopy images it results that a population of small nanoparticles with the size of several nanometers was confirmed if the nanoparticles were stabilized with gemini molecules with either a short methylene spacer (two or four −CH₂− groups) or a long spacer (12 −CH₂− groups). The average zeta potential value for silver nanoparticles stabilized with gemini molecules is roughly independent of gemini surfactant spacer length and is approx. +58 mV. An interaction model between silver nanoparticles and gemini molecules which reflects the gained experimental data, is suggested. Microbicidal activity determinations revealed that the silver nanoparticles stabilized with gemini surfactants are more efficient against Gram-negative bacteria and yeasts, which has a direct relation to the interaction mechanism of nanoparticles with the bacterial cell membrane and its structural composition.