Highly size- and shape-controlled synthesis of silver nanoparticles via a templated Tollens reaction

Silver Nanoparticles: A Comprehensive Review of Synthesis Methods and Chemical and Physical Properties

Recently, silver nanoparticles (NPs) have attracted significant attention for being highly desirable nanomaterials in scientific studies as a result of their extraordinary characteristics. They are widely known as effective antibacterial agents that are capable of targeting a wide range of pathogens. Their distinct optical characteristics, such as their localized surface plasmon resonance, enlarge their utilization, particularly in the fields of biosensing and imaging. Also, the capacity to control their surface charge and modify them using biocompatible substances offers improved durability and specific interactions with biological systems. Due to their exceptional stability and minimal chemical reactivity, silver NPs are highly suitable for a diverse array of biological applications. These NPs are produced through chemical, biological, and physical processes, each of which has distinct advantages and disadvantages. Chemical and physical techniques often encounter issues with complicated purification, reactive substances, and excessive energy usage. However, eco-friendly biological approaches exist, even though they require longer processing times. A key factor affecting the stability, size distribution, and purity of the NPs is the synthesis process selected. This review focuses on how essential it is to choose the appropriate synthesis method in order to optimize the characteristics and use of silver NPs.

Directing the size and dispersity of silver nanoparticles with kudzu leaf extracts

Kudzu is an abundant and invasive species in the Southeastern United States. The prospective use of kudzu as a non-toxic, green and biocompatible reducing and stabilizing agent for one-pot Ag nanoparticle synthesis was investigated. Ag nanoparticles were synthesized using aqueous and ethanolic kudzu leaf and stem extracts. The size and dispersity of the synthesized nanoparticles were found to depend on the extract used. Ultraviolet-visible and Fourier transform infrared spectroscopies were used to characterize the extracts. Surface-enhanced fluorescence and Raman scattering were used to characterize the surface species on synthesized Ag nanoparticles. The primary reducing and stabilizing agents in aqueous kudzu leaf extracts were determined to be reducing sugars and saponins which result in Ag nanoparticles with average diameters of 21.2 ± 4.8 nm. Ethanolic kudzu leaf extract was determined to be composed of chlorophyll, reducing sugars and saponins, producing Ag nanoparticles with average diameters of 9.0 ± 1.6 nm. Control experiments using a chlorophyllin standard as the reducing and stabilizing agent reveal that chlorophyll has a key role in the formation of small and monodisperse Ag nanoparticles. Experiments carried out in the absence of light demonstrate that reducing sugars and saponins also contribute to the formation of Ag nanoparticles in ethanolic kudzu leaf extracts. We propose a mechanism by which reducing sugars donate electrons to reduce Ag+ leading to the formation of Ag nanoparticles, forming carboxylic acid sugars which stabilize and partially stabilize Ag nanoparticles synthesized with aqueous and ethanolic kudzu leaf extracts, respectively. In the ethanolic extract, photoexcited chlorophyll serves as a co-reducing and co-stabilizing agent, leading to small and monodisperse Ag nanoparticles.

Time-domain Tollens reaction: synthesising silver nanoparticles with the formaldehyde clock

The addition of silver(i) ions to the methylene glycol-sulphite (MGS) clock reaction results in the sudden formation of metallic silver nanoparticles. Stable suspensions are obtained in the presence of poly(vinylpyrrolidone). The time delay before the appearance of the particles, as well as their size, decreases with the initial methylene glycol concentration while their monodispersity increases.

Synthesis of Xylan-Click-Quaternized Chitosan via Click Chemistry and Its Application in the Preparation of Nanometal Materials

For the high-valued utilization of hemicelluloses and for realizing the controllable synthesis of NPs, this paper's aim is to combine xylan, chitosan and nanometal materials at the same time. In this research study, firstly, propargyl xylan was synthesized via nucleophilic substitution reaction between xylan and propargyl bromide in NaOH solution. On the other hand, a tosyl group was introduced onto the 6th position of synthesized quaternized chitosan (QCS), and the azide group replaced the tosyl group to obtain 6-amido-QCS (QCS-N3). The synthesis conditions of the above reactions were optimized. Subsequently, the novel xylan-click-QCS polymer was obtained via click reaction between terminal alkyne groups on the xylan chains and azide groups on QCS. Then, AgNPs and AuNPs were synthesized by adopting the xylan-click-QCS polymer as the reducing and stabilizing agent, and the reaction conditions were optimized to obtain well-dispersed and highly stable nanoparticles. There were two kinds of Ag nanomaterials, with diameters of 10~20 nm and 2~5 nm, respectively, indicating the formation of Ag nanoclusters, except for Ag nanoparticles, in this reaction. The diameter of the synthesized AuNPs was 20~30 nm, which possessed a more uniform size distribution. The Ag nanoclusters with a smaller size (2~5 nm) could inhibit MCF-7 cell proliferation effectively, indicating their application potential in cancer therapy. The study gives a new approach to the high-value utilization of biopolymers.

Noble Metal Promoted TiO2 from Silver-Waste Valorisation: Synergism between Ag and Au

Wastewaters from precious metal industries contain high amounts of noble metals, but their efficient recycling is hindered by the wastewater complex composition. Here, we propose an innovative approach for the efficient recovery of noble metals contained in these metal-enriched wastewaters as precursors for the synthesis of noble metal nanoparticles (NPs) and supported metal catalysts. Silver NPs were synthesized from Ag-enriched wastewater and then deposited on TiO2 to prepare photocatalysts. Then, further promotion of the photocatalytic activity of Ag-modified TiO2 was achieved by the addition of as little as 0.5 wt.% of Au. STEM-EDS analyses proved that Au NPs were located on Ag or AgOx nanoparticles. The contact between the two metal-containing NPs results in charge transfer effects, appreciable both in terms of oxidation states determined by XPS and of optical properties. In particular, the plasmon band of Au NPs shows photochromic effects: under UV light irradiation, bimetallic samples exhibit a blue-shift of the plasmon band, which is reversible under dark storage. The activity of the materials was tested towards ethanol photodegradation under UV light. Adding 0.5 wt.% Au NPs resulted in a promoted activity compared to Ag-TiO2, thus showing synergistic effects between Au and Ag. Ethanol was completely converted already after 1 h of UV irradiation, acetaldehyde was formed as the main oxidation product and fully degraded in less than 180 min. Notably, bimetallic samples showed ethylene formation by a parallel dehydration mechanism.

Green synthesis of silver nanoparticles with algae and the importance of capping agents in the process

BACKGROUND

Nanoparticle synthesis is a very interesting area of research currently due to the wide applications of nanoparticles. The nanoparticles have a diameter ranging between 1 and 100 nm and they are used in different fields like electronics, pharmaceuticals, cosmetics, biotechnology, medicines, etc. Nanoparticles have gained the interest of researchers due to their large surface-to-volume ratio and their capability to interact effectively with other particles. Several different methods can be used for the production of silver nanoparticles (AgNPs) including chemical, physical, and biological. Out of all the methods, the biological method is considered the cleanest and safest as no toxic chemicals are used in the process. The biological method includes the use of bacteria, fungi, algae, and plant extract for the synthesis. Algal synthesis of AgNPs is especially interesting because of the high capacity of the algae to take in metals and reduce metal ions. Algae is a widely distributed organism and its availability is abundant; an added advantage is their growth under laboratory conditions. These organisms can help in large-scale production at a low cost.

SHORT CONCLUSION

This review article explains the different factors that should be considered for the effective synthesis of AgNPs using algae. Capping agents also affect the stability of nanoparticles. It also sheds light on the importance of capping agents in the synthesis of AgNPs. Alga-mediated synthesis of AgNPs along with the use of different capping agents can help in modulating the stability and size of the nanoparticles, thereby improving its cost-effectiveness and environment-friendly production.

Galactose Grafted Two-Dimensional Nanosheets as a Scaffold for the In Situ Synthesis of Silver Nanoparticles: A Potential Catalyst for the Reduction of Nitroaromatics

Two major hurdles in NP-based catalysis are the aggregation of the NPs and their recycling. Immobilization of NPs onto a 2D support is the most promising strategy to overcome these difficulties. Herein, amphiphilicity-driven self-assembly of galactose-hexaphenylbenzene-based amphiphiles into galactose-decorated 2D nanosheet is reported. The extremely dense decoration of reducing sugar on the surface of the sheets is used for the in situ synthesis and immobilization of ultrafine catalytically active AgNPs by using Tollens' reaction. The potential of the system as a catalyst for the reduction of various nitroaromatics is demonstrated. Enhanced catalytic activity is observed for the immobilized AgNPs when compared to the corresponding discrete AgNPs. Recovery of the catalytic system from the reaction mixture by ultrafiltration and its subsequent recycling for several cycles without dropping its activity is shown. This is the first report demonstrating the in situ synthesis and immobilization of ultrafine AgNPs onto a 2D nanosheet that exhibits excellent catalytic performance for the reduction of nitroaromatics.

Silver nanoparticles: Synthesis, medical applications and biosafety

Silver nanoparticles (AgNPs) have been one of the most attractive nanomaterials in biomedicine due to their unique physicochemical properties. In this paper, we review the state-of-the-art advances of AgNPs in the synthesis methods, medical applications and biosafety of AgNPs. The synthesis methods of AgNPs include physical, chemical and biological routes. AgNPs are mainly used for antimicrobial and anticancer therapy, and also applied in the promotion of wound repair and bone healing, or as the vaccine adjuvant, anti-diabetic agent and biosensors. This review also summarizes the biological action mechanisms of AgNPs, which mainly involve the release of silver ions (Ag+), generation of reactive oxygen species (ROS), destruction of membrane structure. Despite these therapeutic benefits, their biological safety problems such as potential toxicity on cells, tissue, and organs should be paid enough attention. Besides, we briefly introduce a new type of Ag particles smaller than AgNPs, silver Ångstrom (Å, 1 Å = 0.1 nm) particles (AgÅPs), which exhibit better biological activity and lower toxicity compared with AgNPs. Finally, we conclude the current challenges and point out the future development direction of AgNPs.

Incorporated plant extract fabricated silver/poly-D,l-lactide-co-glycolide nanocomposites for antimicrobial based wound healing

Polymeric nanocomposites have gained extensive attention in modern nanotechnology by reason of its design, flexibility, sole applications and lower life cycle costs. Preparation of composites using spreading of inorganic metal nanoparticles in organic polymeric matrices has plenty of scope and applications in the biomedical field. Poly-D,l-lactide-co-glycolide (PLGA) is an appreciated polymer for composites preparation because of its non-toxic and promising biodistribution. The consideration of metal nanoparticles has extended rapidly with the presence of new nanocomposites into a range of products and technologies. Compared to bulk materials the synthesized metal nanoparticles have unique character and biomedical uses due to its shape, size, and huge surface to volume ratio. Among different inorganic metal nanoparticles, silver nanoparticles (Ag NPs) have dominated in the biomedical field owing to its diverse potential applications including imaging, sensor, diagnosis and disease treatment. Further, medicinal plant extract mediated Ag NPs shown superior advantages and its antimicrobial based wound healing prospective has been established. However, not much information on plant extract mediated Ag NPs integrated PLGA nanocomposites wound healing applications. In the present review, we discussed necessity, preparation, characterization and antimicrobial based wound healing mechanism of incorporated plant extract mediated silver/PLGA nanocomposites.

Galactose-Grafted 2D Nanosheets from the Self-Assembly of Amphiphilic Janus Dendrimers for the Capture and Agglutination of Escherichia coli

High aspect ratio, sugar-decorated 2D nanosheets are ideal candidates for the capture and agglutination of bacteria. Herein, the design and synthesis of two carbohydrate-based Janus amphiphiles that spontaneously self-assemble into high aspect ratio 2D sheets are reported. The unique structural features of the sheets include the extremely high aspect ratio and dense display of galactose on the surface. These structural characteristics allow the sheet to act as a supramolecular 2D platform for the capture and agglutination of E. coli through specific multivalent noncovalent interactions, which significantly reduces the mobility of the bacteria and leads to the inhibition of their proliferation. Our results suggest that the design strategy demonstrated here can be applied as a general approach for the crafting of biomolecule-decorated 2D nanosheets, which can perform as 2D platforms for their interaction with specific targets.

AMROBS: All-Metal Replicas of Biological Surfaces-A Novel Approach Combining Established Techniques

Biomimetic work often concerns to biological surfaces and their interaction with the environment. Liquid handling, barrier function and protection against heat, pathogens and predators, to name just a few, require biological surfaces to exhibit specific material properties-properties that often are not suited for specific measurements under lab conditions. In particular, the lack of the necessary sample toughness or conductivity can prove difficult to perform certain experiments. Hence, we present a novel approach to achieve all-metal replicas from biological surfaces (AMROBS). Resulting replicas exhibit microscale accurate replication of morphological topography while providing tough, conductive subjects for investigation and easy chemical surface modification. Combining established techniques like polymer casting (e.g., silicone), chemical silver precipitation and electroplating, all-metal replicas of several technical and biological surfaces (e.g., diffraction foils, lizard skin, flat bug surface) were produced and compared to their original counterparts with regard to morphology and functionality. By using scanning electron microscopy and video analysis, we show that a high degree of replication accuracy is achievable, and conclude the future possibilities of AMROBS in a comprehensive discussion, including the general "do's" and "do nots" of metal replication following this approach.

Silver nanoparticles as antimicrobial therapeutics: current perspectives and future challenges

Utility of silver metal in antimicrobial therapy is an accepted practice since ages that faded with time because of the identification of a few silver resistant strains in the contemporary era. A successive development of antibiotics soon followed. However, due to an indiscriminate and unregulated use coupled with poor legal control measures and a dearth of expertise in handling the critical episodes, the antibiotics era has already seen a steep decline in the past decades due to the evolution of multi-drug resistant 'superbugs' which pose a sizeable challenge to manage with. Due to limited options in the pipeline and no clear strategy in the forefront, the aspirations for novel, MDR focused drug discovery to target the 'superbugs' arose which once again led to the rise of AgNPs in antimicrobial research. In this review, we have focused on the green routes for the synthesis of AgNPs, the mode of microbial inhibition by AgNPs, synergistic effect of AgNPs with antibiotics and future challenges for the development of nano-silver-based therapeutics.

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.

In vitro studies of nanosilver-doped titanium implants for oral and maxillofacial surgery

The addition of an antibacterial agent to dental implants may provide the opportunity to decrease the percentage of implant failures due to peri-implantitis. For this purpose, in this study, the potential efficacy of nanosilver-doped titanium biomaterials was determined. Titanium disks were incorporated with silver nanoparticles over different time periods by Tollens reaction, which is considered to be an eco-friendly, cheap, and easy-to-perform method. The surface roughness, wettability, and silver release profile of each disc were measured. In addition, the antibacterial activity was also evaluated by using disk diffusion tests for bacteria frequently isolated from the peri-implant biofilm: Streptococcus mutans, Streptococcus mitis, Streptococcus oralis, Streptococcus sanguis, Porphyromonas gingivalis, Staphylococcus aureus, and Escherichia coli. Cytotoxicity was evaluated in vitro in a natural human osteoblasts cell culture. The addition of nanosilver significantly increased the surface roughness and decreased the wettability in a dose-dependent manner. These surfaces were significantly toxic to all the tested bacteria following a 48-hour exposure, regardless of silver doping duration. A concentration of 0.05 ppm was sufficient to inhibit Gram-positive and Gram-negative species, with the latter being significantly more susceptible to silver ions. However, after the exposure of human osteoblasts to 0.1 ppm of silver ions, a significant decrease in cell viability was observed by using ToxiLight™ BioAssay Kit after 72 hours. Data from the present study indicated that the incorporation of nanosilver may influence the surface properties that are important in the implant healing process. The presence of nanosilver on the titanium provides an antibacterial activity related to the bacteria involved in peri-implantitis. Finally, the potential toxicological considerations of nanosilver should further be investigated, as both the antibacterial and cytotoxic properties may be observed at similar concentration ranges.

Formation of a Surficial Bifunctional Nanolayer on Nb2 O5 for Ultrastable Electrodes for Lithium-Ion Battery

Safe and long cycle life electrode materials for lithium-ion batteries are significantly important to meet the increasing demands of rechargeable batteries. Niobium pentoxide (Nb₂ O₅ ) is one of the highly promising candidates for stable electrodes due to its safety and minimal volume expansion. Nevertheless, pulverization and low conductivity of Nb₂ O₅ have remained as inherent challenges for its practical use as viable electrodes. A highly facile method is proposed to improve the overall cycle retention of Nb₂ O₅ microparticles by ammonia (NH₃ ) gas-driven nitridation. After nitridation, an ultrathin surficial layer (2 nm) is formed on the Nb₂ O₅ , acting as a bifunctional nanolayer that allows facile lithium (Li)-ion transport (10-100 times higher Li diffusivity compared with pristine Nb₂ O₅ microparticles) and further prevents the pulverization of Nb₂ O₅ . With the subsequent decoration of silver (Ag) nanoparticles (NPs), the low electric conductivity of nitridated Nb₂ O₅ is also significantly improved. Cycle retention is greatly improved for nitridated Nb₂ O₅ (96.7%) compared with Nb₂ O₅ (64.7%) for 500 cycles. Ag-decorated, nitridated Nb₂ O₅ microparticles and nitridated Nb₂ O₅ microparticles exhibit ultrastable cycling for 3000 cycles at high current density (3000 mA g^-1 ), which highlights the importance of the surficial nanolayer in improving overall electrochemical performances, in addition to conductive NPs.

Distinct metal-exchange pathways of doped Ag25 nanoclusters

Atomically precise metal nanoclusters (NCs) containing more than one type of metal atom (i.e., doped or alloyed), due to synergistic effects, open new avenues for engineering the catalytic and optical properties of NCs in a manner that homometal NCs cannot. Unfortunately, it is still a major challenge to controllably introduce multimetallic dopants in NCs, understanding the dopants' positions, mechanism, and synergistic effects. To overcome these challenges, we designed a metal-exchange approach involving NCs as molecular templates and metal ions as the source of the incoming dopant. In particular, two structurally similar monodoped silver-rich NCs, [MAg₂₄(SR)₁₈]^2- (M = Pd/Pt and SR: thiolate), were synthesized as templates to study their mechanistic transformation in response to the introduction of gold atoms. The controllable incorporation of Au atoms into the MAg₂₄ framework facilitated the elucidation of distinct doping pathways through high-resolution mass spectrometry, optical spectroscopy and elemental analysis. Interestingly, gold replaced the central Pd atom of [PdAg₂₄(SR)₁₈]^2- clusters to produce predominantly bimetallic [AuAg₂₄(SR)₁₈]^- clusters along with a minor product of an [Au₂Ag₂₃(SR)₁₈]^- cluster. In contrast, the central Pt atom remained intact in [PtAg₂₄(SR)₁₈]^2- clusters, and gold replaced the non-central Ag atoms to form trimetallic [Au_xPtAg_24-x(SR)₁₈]^2- NCs, where x = 1-2, with a portion of the starting [PtAg₂₄(SR)₁₈]^2- NCs remaining. This study reveals some of the unusual metal-exchange pathways of doped NCs and the important role played by the initial metal dopant in directing the position of a second dopant in the final product.

Size controlled biogenic silver nanoparticles as antibacterial agent against isolates from HIV infected patients

Silver nanoparticles (AgNPs) are synthesized using biological sources due to its high specificity in biomedical applications. Herein, we report the size and shape controlled synthesis of AgNPs using the aqueous extract of blue green alga, Spirulina platensis. Size, shape and elemental composition of AgNPs were characterized using UV-vis spectroscopy, Fluorescence spectroscopy, FT-IR (Fourier Transform-Infrared Spectroscopy), FT-RS (Fourier Transform-Raman Spectroscopy), SEM-EDAX (Scanning Electron Microscopy-Energy Dispersive X-ray analysis) and HR-TEM (High Resolution Transmission Electron Microscopy). AgNPs were stable, well defined and monodispersed (spherical) with an average size of 6 nm. The synthesized AgNPs were tested for its antibacterial potency against isolates obtained from HIV patients.

Green synthesis of silver nanoparticles in xylan solution via Tollens reaction and their detection for Hg(2+)

This work reported a facile and green method to prepare highly stable and uniformly distributed Ag nanoparticles (AgNPs), in which a biopolymer xylan was used as the stabilizing and reducing agent via the Tollens reaction under microwave irradiation. Different variables were evaluated to optimize the reaction conditions. Complete characterization was performed using UV-Vis, XRD, TEM, size distribution analysis and XPS. The results revealed that AgNPs were well dispersed with diameters of 20-35 nm due to the packing of xylan. The optimal conditions were as follows: microwave irradiation temperature was 60-70 °C, microwave power was 800 W, microwave time was 30 min, the ratio of xylan to AgNO3 was 50 mg: 0.13 mmol, and ammonia concentration was 2%. In addition, the AgNPs were collected via high-speed centrifugal separation, and the supernatant was tested by HPAEC, GPC, FT-IR, and NMR. By comparing the structure of xylan before and after the reaction, the reaction mechanism was discussed. It was noted that the xylan-AgNPs composites showed high selectivity and sensitivity for Hg(2+) detection. The other 15 metal ions used had no obvious effect on the detection of Hg(2+), and the limit of detection (LOD) was 4.6 nM, which is lower than the allowed maximum level of 30 nM for drinking water by WHO. In addition, the xylan-AgNPs composites can be applied for Hg(2+) detection in real water samples. This study provides a novel way for the high-value utilization of a rich biomass resource, and a green method for the synthesis of AgNPs for the selective and sensitive detection of harmful heavy metals.

Facile synthesis of size and wavelength tunable hollow gold nanostructures for the development of a LSPR based label-free fiber-optic biosensor

Hollow bimetallic nanostructures have recently emerged as attractive plasmonic materials due to the ease of optical tunability by changing their size/composition.

SERS enhancement of silver nanoparticles prepared by a template-directed triazole ligand strategy

A one-pot method to prepare silver nanoparticle (AgNP) nanotags using the Tollens' reagent is described.

Selective growth of silver nanoparticle arrays on nanoimprinted sol-gel silica patterns

Selective growth of silver nanoparticles on ~100 nm thick silica patterns produced by nanoimprint method has been successfully demonstrated using either (1) thermo-induced reduction or (2) room temperature electroless deposition (ELD) without removing the ~25 nm thick residual layer left by nanoimprint process. This selectivity was achieved by silane additive, (3-mercaptopropyl)trimethoxysilane (3-MTS), which was added to the silica matrix to control nucleation and growth of silver. The presence of silver nanoparticles was confirmed by EDX and UV-vis spectrum, and the density, distribution, and size of silver particles were examined by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Silica film heat-treated between 400 and 600 °C resulted in silver particles of 100-120 nm diameter with a linear density of 2.63-3.36 μm(-1), while the film treated by room temperature ELD produced silver particles of 67 nm diameter with a linear density of 5.65 μm(-1). The selective growth ratio based on particle density on pattern area versus residual layer is 12.92 and 20.31 for high- and room-temperature processes, respectively, whereas the samples without 3-MTS shows low selective growth ratio of 1.22 and 1.04. These results prove that both approaches are fast and effective, suggesting their potential to produce other type of nanoparticle arrays directly on nanoimprinted patterns.