Silver nanowires are nanostructured tubes comprising silver. These are known as nanowires due to their minute thickness and diameter which is present in ratios more than 1000 therefore these are found to be extremely thin.

Silver is an excellent element owing to the remarkable properties that it exhibits in various forms and categories. Depending upon the characteristics and properties that these silver nanowires exhibit, a lot of different applications of these nanowires come into existence. Although, their applications can be found in almost all the fields most prominent ones are present in the biomedical field. The most common and some of the best applications of silver nanowires are explained in this article.

Introduction

Nanowires are structures comprised of their diameter and thickness which is equivalent to tens of nanometers or even lesser. Their properties are quite vast and exhibit very exciting features. Bulk materials do not play any role in these as the nanowires are confined to quantum and the energy levels that these exhibits do not match up to the bulk materials. These are also known as one-dimensional nanomaterials because they comprise ratios that are more than 1000.

Silver Nanowires

Silver nanowires are nanostructured wires comprising silver. It is an excellent material to be utilized in these wires due to the strong characteristics that silver possesses. They have vast optical, electrical, and thermal properties which help in the better functioning of these silver nanowires. These have an excellent feature of absorbing and scattering light. Interaction of molecules increases when silver nanowires are embedded in a solution. Perhaps this is carried out by different analytical techniques.

Manufacturing of Silver Nanowires

To manufacture silver nanowires, a series of processes are carried out all of which are chemical methods. A few of these are described below:

Polyol method – In a polyol method, the silver nanowires are manufactured while making the use of an aqueous solvent. This process is carried out in an autoclave where the temperature is kept at 120° C for 8h.

Rapid synthesis – In rapid synthesis, the silver nanowires are manufactured by the mixture of two compounds, polyvinyl pyrrolidone, and copper chloride. This process is carried out in disposable glass vials. Perhaps, ethylene glycol works as a precursor in this method.

Template method – In the template method, silver nanowires are manufactured by embedding nanotubes of an amphiphilic cyanine dye in an aqueous solution because of these work as the chemically active templates.

Electroless deposition – In an electroless deposition, silver nanowires are manufactured by carrying out the process of deposition of silver into polycarbonate membranes. This process is carried out via metal amplification.

Demonstration of antibacterial activity

Silver is being used for a lot of years now as it has exhibited antibacterial activity which includes lesser toxicity to a human cell. This enhances the ranges and areas of applications for silver including burn treatment and water disinfection. Due to the advanced technology, scientists have shifted their focus on the production and utilization of silver nano species including silver nanowires as they exhibit unique and excellent properties in various categories. As explained earlier, these are manufactured through various processes. However, all the freshly manufactured silver nanowires are slightly different from each other but their main property remains the same which is their antibacterial activity.

Common silver nano species

There are several nano species of silver, a few of which are very common such as silver nanowires, silver nanorods, and silver nanospheres. They possess a greater surface area in regards to the volume ratio and that is why the silver nano species are capable of exhibiting better features in comparison to the bulk silver. Due to all these reasons, silver nanowires are highly popular as they work as an antibacterial arsenal. There are a lot of bacterial species against which these silver nanowires authentically work including E.coli, S. aureus, K. pneumonia, and a methicillin-resistant S. aureus. Among all the silver nano species, silver nanowires are gaining a lot of attention as their research has massively increased. This is due to the excellent physicochemical properties that these silver nanowires are capable of exhibiting which are very different from the rest of the silver nano species.

Different synthesis methods

This article includes two different methods which are applied for the synthesis of silver nanowires owing to their respective lengths and diameters.

Synthesis of AgNWs

Silver nanowires are most commonly written as AgNWs. It has been thirty years since the development of the synthesis of AgNWs has started taking place. A lot of solution methods were studied and then later on reviewed by Tang and Tsuji. The two major methods known as hard and soft template methods are explained below.

Hard-Template Synthesis

One of the methods of synthesis of AgNWs is the hard-template synthesis method which exhibits the involvement of nanoporous membranes including carbon nanotubes, silica in nanoporous form, and some membranes of alumina, zeolites, and polycarbonate. All these membranes are included so that they can play a role in the growth of AgNWs. The most beneficial thing about this synthesis is that it is capable of being carried out in a controlled manner as a result of which AgNWs are obtained that have uniformity in size. Another feature of the newly synthesized silver nanowires is that they get immobilized on the said substrate.

Nanoporous membranes

Nanoporous membranes are the ones that are said to be utilized on hard templates as these are commercially available and are comparatively easier to make use of. This enables the formation of silver nanowires in the desired manner in regards to length and diameter. However, this formation is carried out by the synthesis of wires inside the cylindrical pores of the membrane. AgNWs need to go through the purification methods at times and for that dissolution of the nanoporous membranes is certainly carried out.

Soft-Template Synthesis

Another method for the synthesis of silver nanowires is the soft-template synthesis method. Everything does come with certain drawbacks and to overcome those drawbacks, a new thing comes into existence. The same is the case with the soft-template method. It is comparatively a faster and better process than the hard-template synthesis as it enhances the production and quality of silver nanowires. This entire process is carried out in a solution and holds great importance in terms of biomedical applications. Sun et al. were the first researchers who opted for the synthesis of silver nanowires via polyvinyl as a soft template and fortunately, this became a huge success for them. This method enabled them to move past the deformities that were being caused by the previous methods.

Further investigations

The overview regarding the synthesis of AgNWs made it clear that further researches and experiments on the said materials can be proved highly beneficial as these are contributing to society at a great level. It is also true that the desired products can be achieved in terms of lengths and diameters by shifting the reaction conditions and environments. This proves to be highly beneficial for the future uses and applications of the desired obtained products in various fields but specifically the biomedical field.

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Mechanism of Antibacterial Activity

Silver Nanoparticles Mechanism of Action

Silver as a nanomaterial is being used in various fields for various purposes owing to the characteristics and properties that it possesses of. It is extremely famous for the antimicrobial activity that it comprises of and its mechanism of action entirely varies from that of bulk silver. Marambio-Jones and Hoek were the ones who presented three hypothesized mechanisms of action for nanomaterials containing silver and all three of these are briefly explained below. All of these vary from each other in slight ways yet the mode of action remains the same as the major and the only purpose is the synthesis of silver nanowires. Different mechanisms are utilized to minimize the errors which can be occurred otherwise.

Oxidation of metallic silver

Oxidation of metallic silver is the first and most common mechanism of action for generating the silver ions by oxidizing metallic silver which ultimately forms silver nanoparticles for carrying out antibacterial activities. This entire process is known to be dependent on two things mainly, reaction with hydrogen peroxide embedded in an acidic medium while being in cell membranes of bacteria or simply by carrying out a reaction with oxygen. When the ions become free for interaction within the cell membranes of bacteria, the chances of cell damage increase. However, due to the presence of high affinity in the silver ions, these are capable of having an interaction among respiration chain proteins. This serves as a favorable condition for the freshly produced products which are responsible for carrying out the further reactions.

Inhibition of phosphate uptake

Silver ions are also capable of inhibiting the phosphate uptake as a result of which efflux of extracellular phosphate is caused which then claims to have a cytoplasmic shrinkage due to which detachment of cell membrane occurs as well. Ultimately, intracellular contents start leaking. The formation of the reactive oxygen species also known as ROS is responsible for the second proposed mechanism which ultimately has certain effects on the bacterial cells as well. Silver ions are excellent in the nature that they are present in resulting in better carrying out of the reactions which results in an increased presence and production of ROS. Silver ions are also responsible for enhancing the generativity of ROS owing to their excellent characteristics.

Silver Nanowires Mechanism of Action

The mechanism of action for silver nanowires is rather a little different but their working effects and mechanisms remain the same. Silver nanowires are capable of exhibiting excellent antimicrobial activities just like silver ions and nanoparticles do so. In the case of gram-positive and gram-negative bacteria, mechanisms of action show changes which will be explained below owing to the variations among their shapes and sizes. Visnapuu et al. was the one who massively worked for incorporating the antibacterial effects of silver nanowires in society. The silver nanoparticles for this process are chosen based on their size and surface charge values.

Use of bioluminescent strain

As the research has progressed so has the use of bioluminescent E. coli strain. Their use has been incorporated to demonstrate the controlling effect of antibacterial activities that are going on. However, the major role is played by the size and shape of silver ions as they are the key controllers of antimicrobial activities which further define their applications and uses in society. It is also suggested that a lot of other factors might be playing a role in the swift carrying out of this process so observation and keen handling of the materials involved is extremely necessary.

Emerging Biomedical Applications of Silver Nanowires

One of the major areas in which the applications of silver nanowires are progressing vastly is said to be the biomedical field however, the researchers are still going on. Silver nanowires are massively being used in textiles, drug delivery, surface coating, and much more. All of these are explained below.

Textiles and Fibrous Membranes

According to the previous research carried out regarding the applications of silver nanowires, it was evident that these nanowires were being massively used for the wound dressing materials but as the research has progressed, the hunt for their applications in the textile industry has also emerged which has proved to be a success. Nateghi et al. became the first one to successfully announce the production of cotton fabric loaded with silver nanowires which also possessed antimicrobial properties considering that it has excellent electrical conductivity and protection against UV. This was also proved to be successful in killing the E.coli and S. aureus very effectively owing to the characteristics that were obtained at this production.

Wound healing applications

The textiles that are obtained via silver nanowires work as an excellent product for wound healing applications which is why so much research and studies are also being carried out in this same regard. Zhang et al. became the first one to use the technique of electrospinning so the silver nanowires in their hybrid membranes can be formed along with polyvinyl alcohol. These showed excellent antimicrobial activities in the case of gram-positive rather than gram-negative. Hence, silver nanowires proved to be excellent in wound healing textiles owing to the excellent properties and characteristics that this exhibit.

Silver nanowires garments

Furthermore, silver nanowires are capable of protecting against the cold which is promoted through the silver nanowire garments. This is one of the latest inventions of silver nanowires and is most commonly known as personal thermal management or PTM. In this case, garments are provided with a coating of silver nanowires and then they perform the desired function which is protection against cold. The nanowires are said to be conductors so once the garments start conducting the energy it also starts producing heat which is responsible for the entire task.

Good insulator

Owing to the conductivity, silver nanowires enable the garments in becoming good insulators which is why they start reflecting ninety percent of the body heat. This is an excellent feature as even the wool sweaters are not capable of reflecting this much body heat rather they reflect only twenty percent of body heat. However, these are very much breathable garments and do not alter the comfort level of consumers that is why their production has massively increased.

Medical devices' surface Coating

The usage of AgNWs is also explored as the components of the surface coating materials by various researchers for lessening the hospital-produced infections linked to the usage of invasive devices like catheters and contamination of work surfaces. There have been dispersions of AgNWs in various polymeric materials for applications in flexible electronics for future applications in biomedicine. AgNWs dispersions effects are investigated by Shahzadi et al. in chitosan films. Chitosan has been utilized in various applications in biomedicine. It is a renewable origin's safe and biocompatible polysaccharide. Thus, exploration of its physicochemical compatibility with AgNWs can result in new ways of using the metal nanostructures in medical devices, wound healing, and drug delivery, etc. Followed by casting, soft-template produced AgNWs were mixed with chitosan solution for forming the bio-based hybrid films.

Disc diffusion experiments

According to disc diffusion experiments, these hybrid films were active in front of B. subtilis and E. coli, ensuring that AgNWs can bestow antibacterial characteristics to the biocompatible composite films. Blending the AgNWs with konjac glucomannan (KGM) active against S. aureus and E. coli produces environmentally friendly composite films. Activity is shown in both of these studies against both gram-negative and gram-positive bacteria. Also, as compared to the zones of inhibition reported for silver nanowire–chitosan-based hybrid films, the zones of inhibition for silver nanowires-KGM films for E. coli were way more, which suggests that film's different components are capable of altering or changing the silver nanowires effectiveness against Gram-negative bacteria.

Hybrid films mechanical characteristics

Moreover, there was a major increase in the hybrid film's mechanical characteristics in both of the studies. When thinking of applications like wound dressing in which tensile strength and flexibility are important; or in tissue regeneration like bone or/and cartilage, this is very interesting as the environment's mechanics have a major function in the tissue's regeneration. The usage of AgNWs loaded on polydimethylsiloxane (PDMS) films was explored by Jiang and Teng along with their excellent antibacterial activity and human cell compatibility for making effective antibacterial coatings for addressing the main problem in the secondary care settings of the surface microbial contamination above all. Before being embedded in the PDMS, the produced-AgNWs are drop-coated on a silicon wafer for producing a uniformly coated matrix.

The killing of bacteria

In comparison with a PDMS-only control in the short term, extremely high killing rates were displayed by the AgNW-coated PDMS films against S. aureus and E. coli. Also, the number of bacterial cells (dead and living) that can adhere to the AgNW-containing films was considerably more as compared to the PDMS control. This was the reason why authors start hypothesizing the PVP molecule's hydrophobic moiety, left after synthesis on the AgNWs, was capable of making a hydrophobic interaction with the bacterial cell wall.

Drug Delivery

In the drug delivery area, the constant requirement for enhanced therapeutical efficacy and safety has caused an increase in the amount of research in nanotechnology-based systems. AgNWs can offer new munitions in fighting against kind of diseases like tumors. AgNWs cytocompatibility was investigated by Singh et al. for their usage as the theranostic agents. AgNWs can be easily modified, chemically, making them good for targeted delivery; once they are joined at the site of the tumor, they can be exploited as thermal ablation agents because of their thermal conductivity; drug carriers by chemical conjugation; and tunable contrast agents because of their localized surface plasmon modes.

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Ultra-light aerogel

In the United States, a researchers group from the Lawrence Livermore National Laboratory (LLNL) made a new kind of ultra-light aerogel for the rose to hold light with no need of bending or lowering it. It is a metal foam that is grafted into that material's new class with extremely light rare characteristics that can be utilized specifically in the electronic energy industries.

Device performance

Conductive and ultra-light silver nanowires are used by this material in particular. A lead author of a published study, Fang Qian said that better performance of the device can result because of these silver nanotube aerogels' excellent electrical/mechanical characteristics and high porosity and can also create new possibilities for sensors, catalysis, medical devices, energy storage, and cell fuel.

Conclusion

Silver nanowires are one of the excellent production of nanostructured materials. They have various manufacturing techniques depending upon the category that they need to be manufactured for. Depending on their characteristics, a lot of biomedical applications of silver nanowires come into existence. All these applications are highly authentic and add up to the production of silver nanowires maintaining their credibility and enhancing their quality. 

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https://www.hindawi.com/journals/jnm/2014/193201/\

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https://www.sciencedirect.com/science/article/abs/pii/S092151071730123X