Synthesis and characterisation of silver nanoparticles in viscous solvents and its transfer into non-polar solvents

One-Step Synthesis of Ultrasmall Nanoparticles in Glycerol as a Promising Green Solvent at Room Temperature Using Omega-Shaped Microfluidic Micromixers

Despite innovations in the synthesis protocol of nanoparticles (NPs), the size distribution and uniformity of particles still remain as crucial attributes. Homogeneous and rapid nucleation is a critical phenomenon to obtain monodisperse nanoparticles. Herein, we have carried out the synthesis of metal nanoparticles in a customized microfluidic (MF) chip, with 18 omega-shaped micromixers, by using glycerol as a promising green solvent and reducing agent at various concentrations (10-80%), and simultaneous comparison of the results from batch synthesis. Initially, mixing characterization for 10-80% glycerol was obtained by adjusting the Peclet (Pe) number. Further, the effect of the Pe number, time, and concentrations of polyvinylpyrrolidone, metal source, and glycerol on the NP size was investigated. Interestingly, the experimental findings depicted that by varying different parameters, the spherical nanoparticles with an average ultrasmall particle diameter of <2 nm were obtained at all glycerol concentrations (10-80%), as compared to batch synthesis (giving a yield of ∼10-fold larger particles). The mixing efficiency in this MF chip design was analyzed by using a fluorescent dye in glycerol, while the particle morphology and size were characterized by using dynamic light scattering, transmission electron microscopy, and ultraviolet-visible spectroscopy. Hence, compared to the conventional benchtop-assisted NP synthesis, this study unveils the significant effect of the microfluidic technique on the synthesis of ultrasmall and homogeneous nanoparticles in a single step, using an environmentally friendly solvent.

Silver Nanoparticle Synthesis in Glycerol by Low-Pressure Plasma-Driven Electrolysis: The Roles of Free Electrons and Photons

Low-temperature plasmas in and in contact with liquids have emerged as a catalyst-free approach for the selective, electrode-free, and green synthesis of novel materials. For the synthesis of nanomaterials, short-lived solvated electrons have been proposed to be the critical reducing species, while the role of ultraviolet (UV) photons from plasma is less explored. Here, we demonstrate that UV radiation contributes ∼70% of the integral plasma effect in synthesizing silver (Ag) nanoparticles within a glycerol solution. We suggest that the UV radiation causes C-H bond cleavage of the glycerol molecules, with an experimentally and theoretically determined threshold photon energy of only 5 eV. The photon-induced dissociation leads to the formation of glycerol fragmentation radicals, causing the reduction of Ag+ ions to Ag neutrals, enabling nanoparticle formation in the liquid phase.

Green Synthesis of Ag NPs Using Ustilago maydis as Reducing and Stabilizing Agent

Ustilago maydis (UM) is a fungus that grows naturally on Zea mays; it reduces the corn yields, and thus, it represents huge economic loss; however, it can be used as an exotic food, and in the present work, it is successfully used as a reducing and stabilizing agent for the preparation of silver nanoparticles (Ag NPs) due to its content of amino acids and biosurfactants. The effects of the concentration of UM aqueous extract, pH, and sunlight on the particle size, surface plasmon resonance, stability, and morphology of Ag NPs obtained by green synthesis were evaluated. A green reduction was observed only in presence of UM, and colloidal Ag NPs were obtained with or without the presence of sunlight; nevertheless, continuous sunlight exposure greatly increased the reaction rate. Ag NPs tend to increase in size from 153 nm to 1400 nm at a higher pH and a greater amount of UM, and also, UM tends to stabilize the Ag NPs preventing their agglomeration according to measurement of zeta potential (−10.75 ± 0.84 mV) and SEM observation; furthermore, surface plasmon resonances were more intense between 400 and 480 nm of wavelength adding greater amount of UM. This study concludes that UM not only reduces AgNO3 but also acts as stabilizer of Ag NPs.

Robust Colloidal Synthesis of Palladium-Gold Alloy Nanoparticles for Hydrogen Sensing

Metal nanoparticles are currently used in a variety of applications, ranging from life sciences to nanoelectronic devices to gas sensors. In particular, the use of palladium nanoparticles is gaining increasing attention due to their ability to catalyze the rapid dissociation of hydrogen, which leads to an excellent response in hydrogen-sensing applications. However, current palladium-nanoparticle-based sensors are hindered by the presence of hysteresis upon hydride formation and decomposition, as this hysteresis limits sensor accuracy. Here, we present a robust colloidal synthesis for palladium-gold alloy nanoparticles and demonstrate their hysteresis-free response when used for hydrogen detection. The obtained colloidal particles, synthesized in an aqueous, room-temperature environment, can be tailored to a variety of applications through changing the size, ratio of metals, and surface stabilization. In particular, the variation of the viscosity of the mixture during synthesis resulted in a highly tunable size distribution and contributed to a significant improvement in size dispersity compared to the state-of-the-art methods.

Green Synthesis of Silver Nanoparticles with Size Distribution Depending on Reducing Species in Glycerol at Ambient pH and Temperatures

With an increase in biodiesel demand, a large surplus of glycerol is expected, and there is interest regarding the usage of glycerol as a value-added product. One such idea is to use glycerol as a "green solvent" to replace petroleum-based organic solvents. Glycerol is nontoxic to humans, and its vapor pressure is sufficiently high for the chemical reaction to be performed at high temperatures under ambient atmospheric pressures. Its dielectric constant is between those of water and organic solvents, and it dissolves widely varying materials, spanning between salts and organic molecules. Metal nanoparticles have been known to be synthesized in glycerol within limited experimental conditions, including high temperatures, alkaline pH conditions, and the irradiance of ultraviolet light. Herein, we report that silver nanoparticles have been formed in glycerol under completely green conditions (e.g., room temperature, neutral pH conditions, and without irradiance of ultraviolet light). We suggest that aldehydes and free radicals are generated in glycerol, which is operating as reducing species.

Synthesis of Colloidal Metal Nanocrystals: A Comprehensive Review on the Reductants

There is a growing interest in controlling the synthesis of colloidal metal nanocrystals and thus tailoring their properties toward various applications. In this context, choosing an appropriate combination of reagents (e.g., salt precursor, reductant, capping agent, and stabilizer) plays a pivotal role in enabling the synthesis of metal nanocrystals with diversified sizes, shapes, and structures. Here we present a comprehensive review that highlights one of the key reagents for the synthesis of metal nanocrystals via chemical reduction: the reductants. We start with a brief introduction to the compounds commonly employed as reductants in the colloidal synthesis of metal nanocrystals by showing their oxidation half-reactions and the corresponding oxidation potentials. Then we offer specific examples pertaining to the controlled synthesis of metal nanocrystals, followed by some fundamental aspects covering the general mechanisms of metal ion reduction based on the Marcus Theory. Afterwards, we present a case-by-case discussion on a wide variety of reductants, including their major properties, reduction mechanisms, and additional effects on the final products. We illustrate these aspects by selecting key examples from the literature and paying close attention to the underlying mechanism in each case. At the end, we conclude by summarizing the highlights of the review and providing some perspectives on future directions.

Carboxylate Functional Groups Mediate Interaction with Silver Nanoparticles in Biofilm Matrix

Biofilms causing medical conditions or interfering with technical applications can prove undesirably resistant to silver nanoparticle (AgNP)-based antimicrobial treatment, whereas beneficial biofilms may be adversely affected by the released silver nanoparticles. Isolated biofilm matrices can induce reduction of silver ions and stabilization of the formed nanosilver, thus altering the exposure conditions. We thus study the reduction of silver nitrate solution in model experiments under chemically defined conditions as well as in stream biofilms. Formed silver nanoparticles are characterized by state-of-the art methods. We find that isolated biopolymer fractions of biofilm organic matrix are capable of reducing ionic Ag, whereas other isolated fractions are not, meaning that biopolymer fractions contain both reducing agent and nucleation seed sites. In all of the investigated systems, we find that silver nanoparticle-biopolymer interface is dominated by carboxylate functional groups. This suggests that the mechanism of nanoparticle formation is of general nature. Moreover, we find that glucose concentration within the biofilm organic matrix correlates strongly with the nanoparticle formation rate. We propose a simple mechanistic explanation based on earlier literature and the experimental findings. The observed generality of the extracellular polymeric substance/AgNP system could be used to improve the understanding of impact of Ag+ on aqueous ecosystems, and consequently, to develop biofilm-specific medicines and bio-inspired water decontaminants.

α-((4-Cyanobenzoyl)oxy)-ω-methyl poly(ethylene glycol): a new stabilizer for silver nanoparticles

The article describes the synthesis and properties of α-((4-cyanobenzoyl)oxy)-ω-methyl poly(ethylene glycol), the first poly(ethylene glycol) stabilizer for metal nanoparticles that is based on a cyano rather than a thiol or thiolate anchor group. The silver particles used to evaluate the effectiveness of the new stabilizer typically have a bimodal size distribution with hydrodynamic diameters of ca. 13 and ca. 79 nm. Polymer stability was evaluated as a function of the pH value both for the free stabilizer and for the polymers bound to the surface of the silver nanoparticles using ¹H NMR spectroscopy and zeta potential measurements. The polymer shows a high stability between pH 3 and 9. At pH 12 and higher the polymer coating is degraded over time suggesting that α-((4-cyanobenzoyl)oxy)-ω-methyl poly(ethylene glycol) is a good stabilizer for metal nanoparticles in aqueous media unless very high pH conditions are present in the system. The study thus demonstrates that cyano groups can be viable alternatives to the more conventional thiol/thiolate anchors.

Performance of Ag/Al2O3 catalysts in the liquid phase oxidation of glycerol – effect of preparation method and reaction conditions

A series of silver supported catalysts have been developed and proposed for mild oxidation of glycerol in the liquid phase. High selectivity to glycolic acid, stability in both continuous-flow and periodic mode of reaction, and good resistance to crude glycerol impurities have been achieved.

New insights into the formation mechanism of Ag, Au and AgAu nanoparticles in aqueous alkaline media: alkoxides from alcohols, aldehydes and ketones as universal reducing agents

Alkoxide from alcohols, aldehydes and ketones in alkaline medium is the actual and universal reducing agent of silver and gold ions.

Green synthesis of gold nanoparticles using glycerol-incorporated nanosized liposomes

There has been enormous interest in the last decade in development methods for the inorganic synthesis of metallic nanoparticles of desired sizes and shapes because of their unique properties and extensive applications in catalysis, electronics, plasmonics, and sensing. Here we report on an environmentally friendly, one-pot synthesis of metallic nanoparticles, which avoids the use of organic solvents and requires mild experimental conditions. The developed method uses liposomes as nanoreactors, where the liposomes were prepared by encapsulating chloroauric acid and exploited the use of glycerol, incorporated within the lipid bilayer as well as in its hydrophilic core, as a reducing agent for the controlled preparation of highly homogeneous populations of gold nanoparticles. The effects of temperature, the presence of a capping agent, and the concentration of glycerol on the size and homogeneity of the nanoparticles formed were investigated and compared with solution-based glycerol-mediated nanoparticle synthesis. Well-distributed gold nanoparticle populations in the range of 2-8 nm were prepared in the designed liposomal nanoreactor with a clear dependence of the size on the concentration of glycerol, the temperature, and the presence of a capping agent whereas large, heterogeneous populations of nanoparticles with amorphous shapes were obtained in the absence of liposomes. The particle morphology and sizes were analyzed using transmission electron microscopy imaging, and the liposome size was measured using photon correlation spectroscopy.