Among the emerging nanotechnology, nanoparticles get much attention due to their unique physicochemical, optical, electrical, and thermal activities. Nowadays, extensive research on silver nanoparticles is going on due to their wide applicability in different fields. Silver nanoparticles possess excellent anticancer as well as antimicrobial efficacy (hence found major and wide applications as antimicrobial, wound healing, antidiarrheal, and antifungal agents). A huge and advanced perspective of silver nanoparticles is found in environmental hygiene and sterilization due to their magnificent disinfectant properties. The other major applications of silver nanoparticles include diagnostic (as biological tags in biosensors, assays, and quantitative detection), conductive (in conductive inks, pastes, and fillers), optical (metal-enhanced fluorescence and surface-enhanced Raman scattering), and household (pesticides and wastewater treatment) applications. The present review consists of an exhaustive detail about the biological and physical applications of silver nanoparticles along with the analysis of historical evolution, the present scenario, and possible future outcomes.
- silver nanoparticles
- environmental hygiene
In this modern era, pharmaceutical research associated with nano-sized products is rapidly growing. Nanoscience/technology has changed the way of diagnosing, treating, and curing the diseases which proves to be a great change in human life. Nano-sized formulations/products include nano-emulsion, ethosomes, liposomes, nanoparticles, etc. Nanoparticles ranging from 1 to 100 nm are in trend nowadays due to its size-depending optical, thermal, electrical, and biological properties . Nano-sized metallic particles are unique because they can considerably change their chemical, physical, and biological properties because of their surface-to-volume ratio. Silver nanoparticles have unique physical and chemical properties among other metallic nanoparticles; besides this, its wide applications in different fields make them the most catchy and different from all other nano-formulations. Silver nanoparticles are well recognized for their diagnostic (as biological tags in biosensors, assays, and quantitative detection), conductive (in conductive inks, pastes, and fillers), optical (metal-enhanced fluorescence and surface-enhanced Raman scattering), and household (pesticides and wastewater treatment) applications. Silver nanoparticles gained their immense attraction due to its magnificent role in cancer treatment. The biological activity of silver nanoparticles depends upon various factors like surface morphology, surface chemistry, size, size distribution, cell type, cell agglomeration, and reducing agent used for the synthesis of nanoparticles. Silver nanoparticles were firstly recorded by M.C. Lea; by citrate reduction method, he produced stabilized silver colloids. Many methods are there for the synthesis of silver nanoparticle which include a physical method, chemical method, biological method, etc. Physical and chemical methods are somewhat hazardous and costly, whereas biological methods are safe and are simpler to apply for the synthesis of silver nanoparticles. After synthesis and before applying it for any purpose, silver nanoparticles must pass all the characteristic parameters like size, shape, size distribution, surface area, solubility, aggregation, toxicity, and biocompatibility. Many techniques have been used to evaluate all these parameters like UV-Vis spectroscopy, differential screening calorimetry (DSC), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), dynamic light scattering (DLS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) [2, 3, 4, 5, 6].
Advantages of silver nanoparticles :
There is a possibility of high-scale production of silver nanoparticles.
Silver nanoparticles possess long-term stability.
Controlled drug delivery of silver nanoparticles can be achieved.
Silver nanoparticles can be freeze-dried and lyophilized to get powder formulation.
Disadvantages of silver nanoparticles :
Less drug loading capacity.
Dispersion of silver nanoparticles includes some amount of water.
The less capacity to load lipophobic drugs.
2. Methods of preparation
2.1 Physical methods
Physical methods use physical energies to produce the silver nanoparticles with narrow size distribution. Physical methods produce a large quantity of silver nanoparticles in a single process. These methods are also able to give silver nanoparticle powder (Figure 1) .
2.1.1 Evaporation-condensation method
In this method, the metallic (silver-organic) source is kept in the boat with the heat center in a tube furnace. Center heat is enough to evaporate the non-silver particles which get eliminated with the carrier gas leaving behind the silver nanoparticles. The more the temperature of the furnace, the more the concentration of silver nanoparticles formed. But this method takes a quite large time to reach stabilized temperature .
2.1.2 Laser ablation method
In this method, metallic/silver plate is dispersed in a liquid medium and illuminated with a laser beam. The metal plate absorbs the laser beam and forms a hot plasma which contains silver particles in maximum concentration. The liquid medium lowers down the temperature and cools the vicinity which initiates the formation of silver nanoparticles. The nature of the silver nanoparticles formed and the ablation efficiency depends upon many factors such as the wavelength of the laser impinging the metallic target, the duration of the laser pulses (in the femto-, pico-, and nanosecond regime), the laser fluency, the ablation time duration, and the effective liquid medium, with or without the presence of surfactants .
The major advantage of both the methods is that it does not include any chemical/reducing and stabilizing agent; therefore, the silver nanoparticles produced by these methods are contamination free and do not need to be purified for further application. However, the major disadvantage is that they consume high energy and costly. Due to these drawbacks, some methods were adopted which are also based on this physical approach but overcome these limitations. These adopted methods are like using ceramic heater which uses less energy and produces continuous heat without any fluctuation and where there is a steep temperature gradient in the vicinity. The second method adopted is thermal decomposition method which produces the silver nanoparticles in solid form. This method works on the principal of complexation between silver and oleate ions and gives silver nanoparticles with 10 nm size. The arch dispersion method was also adopted to overcome the abovementioned limitations and involves the formation of silver nanoparticles in deionized water and does not include the incorporation of any surfactant; it yields silver nanoparticles with less than 10 nm size and hence proves to be a very efficient method [1, 9].
2.2 Chemical methods
These methods are most employed in synthesizing the silver nanoparticles. These methods are based on the reduction of silver ions to the silver atoms which get agglomerated to form the oligomeric clusters which lead to form silver nanoparticles. Various precursors are used in these methods like silver nitrate (AgNO3), silver acetate, and silver chlorate. In these precursors reducing agents like ascorbate, borohydride, and compounds with the hydroxyl and carboxyl group like alcohol, aldehyde, and carbohydrates are incorporated which reduce the silver ion in the precursor and form the silver atom followed by formation of silver nanoparticles. The silver nanoparticles formed are greatly influenced by the nature and properties of reducing agents. The reducing agents are categorized into strong and mild reductants. The strong reductants like borohydrides give large-sized monodispersed nanoparticles, whereas ascorbates and citrates produce small-sized nanoparticles with wide dispersion. Besides this, the morphology (size and shape) of nanoparticles depends upon the type of dispersion medium. The dispersion mediums are a solvent system which acts as the protective or stabilizing agent and is absorbed on the particle surface to prevent agglomeration. Various solvent systems used are mostly polymers like polyvinylpyrrolidone (PVP), polymethylmethacrylate (PMMA), polymethyl acrylic acid (PMAA), and polyethylene glycol (PEG). Polymers are the best candidate as stabilizing agents [10, 11].
2.3 Biological methods
The chemical methods involve a large number of chemical agents like stabilizers (PVP, PMMA, PMAA, and PEG), reducing agents (borohydrides, citrates, and ascorbates) which turns the final product (silver nanoparticles) contaminated. To overcome these limitations, the natural reducing agents are being used nowadays, and this method refers to a biological or green method which is eco-friendly, gives contamination-free product, and consumes less energy. The natural reducing agents like biological microorganisms such as bacteria, fungus, and plant extract are used. The basic principle of this method is that all the natural reducing agents like flavonoids, oils, terpenoids, carbohydrates, enzymes, etc. give an electron to reduce silver ions to silver atoms. This method proves to be a simpler viable alternative to the complex chemical methods to obtain silver nanoparticles. Bacteria are known to be very effective natural reducing agents which give organic and inorganic material, intracellularly and extracellularly. There is a wide range of biological reducing agents available which hence gives a wide choice of precursors for this method (Figure 2) [12, 13].
2.4 Other methods
2.4.1 Photochemical method
Photochemical method uses light especially UV light to transform solution of colloidal silver nanoparticles to stable formulation with different sizes and shapes (Figure 3). In this method, the precursor source is a silver colloidal solution (silver nitrate, silver perchlorate, etc.) which gets photochemically reduced to form the silver nanoparticles in the presence of polymer stabilizers such as PVP, PMMA, and PMAA. The growth of the nanoparticles formed by this method can be controlled by choosing the concentration of stabilizers and type of light source [8, 9].
2.4.2 Electrochemical method
In this method, the silver nanoparticles are formed in a special electrochemical cell in which the silver acts as an anode and the platinum acts as a cathode. The external electrical field is applied to the silver anode which in turn forms the silver clusters followed by the formulation of silver nanoparticles that get deposits on the platinum cathode. This process is conducted at the room temperature, and current density can control the size of silver nanoparticles .
2.4.3 Microemulsion technique
The silver nanoparticles of controllable and uniform size can be synthesized by this technique. The metal precursor and the reducing agent are firstly separated in the two immiscible liquids; the intensity at the interphase and interphase transporters which are mediated by the quaternary ammonium sulfate affects the rate of interaction between metal precursors and reducing agents. The silver nanoparticle clusters when formed at the interphase get stabilized by the stabilizers at the interphase and then transported to the organic solvents by interphase transporter. The major disadvantages of this method are that the organic solvents which are used are deleterious in nature and that the final product is contaminated in nature and must be separated from the surfactants and organic solvents for further applications which are quite difficult .
2.4.4 Microwave-assisted synthesis
In this method, unlike conventional oil bath heating method, microwave heating is used to synthesize the silver nanoparticles. It is a promising method nowadays because microwave heating has a shorter reaction time, reduced energy consumption, and better yield of product which prevent the agglomeration of particles formed. This synthesis involves the carboxymethyl cellulose sodium as a stabilizer. The nanoparticles formed by this have the stability of 2 months without any visual change. Microwave-heated starch is used as a stabilizer which also serves as a template. Polyols like polyethylene glycol and N-vinylpyrrolidine are used as reducing agents as well as stabilizers in which inorganic salt is reduced to form nanoparticles .
2.4.5 Tollens method
In this method, the Ag(NH3)2+ (Tollens reagent) is reduced by saccharides in the presence of ammonia which yields silver nanoparticle films of size 20–50 nm and silver nanoparticles of different sizes. The pH is usually between the 11.5 and 13.0. pH also influences the particle size as at low pH the size of nanoparticles is comparatively small. The polydispersity of the silver nanoparticles can be achieved by lowering the pH .
3.1 UV-Vis spectroscopy
The absorbance of plasmon is responsible for giving a specific color to the nanoparticles. The electromagnetic radiations and the conduction electron are absorbed by the incident light oscillations and hence produce a specific color. The plasmon sample is diluted with the distilled water generally, and silver nanoparticles show peak near about 400 nm. The lambda max of the plasmon resonance solution is responsible for indicating the size of the formulation (Figure 4) [2, 3].
3.2 Fourier transform infrared spectroscopy
3.3 X-ray diffraction
The XRD depicts the crystalline structure of nanoparticles. When X-rays reflect on the sample (crystal structure), it reflect different diffracted patterns. From these patterns, various physicochemical properties of the sample can be predicted. The X-ray diffraction pattern is matched with the standard/reference pattern of the sample, and from this impurities can be detected easily. There is interplanar spacing in the diffraction pattern which is also called d values; these d values are matched with standard silver values. The average crystalline size of nanoparticles can be calculated using Debye-Scherrer formula:
where D is the average crystalline size of the nanoparticles, k is the geometric factor (0.9), λ is the wavelength of X-ray radiation source, and b is the angular full-width at half maximum (FWHM) of the XRD peak at the diffraction angle. From this formula the average size of the silver nanoparticles can be calculated [3, 14].
3.4 Atomic force microscopy
Atomic force microscopy characterizes not only the size shape and sorption but also the dispersion and aggregation of the nanoparticles. AFM helps in the measurement of real-time interactions of nanoparticles with the lipid biological layers, which cannot be achieved by current electron microscopy techniques. No conductive surface or oxide-free surface is required for the measurement in the atomic force microscopy. In addition to this, the major advantage of AFM is that it does not cause any destruction to the native surface and can measure sub-nanometer scale in aqueous fluids. However, the major drawback is the overestimation of the lateral dimensions of the sample due to the size of the cantilever. The operating mode (no contact or contact) is a very crucial factor in sample analysis .
3.5 Scanning electron microscopy
It is a high-resolution technique/microscopy used to detect whole morphology and surface characteristics of the nanoparticles. It is based on the reflection of very high energetic electrons to the probe object. It is a very efficient method to resolve different particle sizes, size distributions, and nanomaterial shapes. The surface morphology of the micro- and nanoscale particles can be easily detected by using SEM. By the histogram obtained particles can be counted either manually or using any software. More specifically for the determination of surface morphology and chemical composition of silver nanoparticles, SEM can be combined with the energy-dispersive X-ray spectroscopy (EDX). The major advantage of this technique is that it can identify the morphology of nanoparticles having size below 10 nm; however, the drawback of this technique is that it is not helpful in determination of the internal structure of the nanoparticles .
3.6 Transmission electron microscopy
TEM is a quantitative method for determination of particles, particle size, size distribution, and morphology. In this technique, the resolution is based upon the ratio of distance between the objective lens and specimen and distance between objective lens and image plane. The major advantages of this technique over the SEM are that it has better efficiency of spatial resolution and other analytical measurements can also be done by this technique. However, the major disadvantage of TEM is sample preparation which is a highly crucial step for better imaging and is highly time-consuming; in addition to this, another disadvantage is high-vacuum and very fine and thin sections of sample are required which are quite difficult to maintain and prepare, respectively .
4. Applications of silver nanoparticles
Applications of silver nanoparticles can be classified in two major classes, that is, therapeutic and physical applications (Figure 5).
4.1 Biological applications
Silver nanoparticles have various biological applications (Figure 6) majorly antimicrobial, anticancer, antioxidant, anti-inflammatory, wound healing, antimalarial, etc. Inbathamiz et al. synthesized silver nanoparticles using aqueous extract of
Most of the urinary tract infections are caused by
Exopolysaccharide of the
Boonkaew et al. developed a burn wound dressing that contains silver nanoparticles to treat infection in a 2-acrylamide-2-methylpropane sulfonic acid sodium salt (AMPSNa+) hydrogel and revealed that hydrogels were nontoxic to normal human dermal fibroblast cells as well as had good action against
Ramar et al. synthesized silver nanoparticles using ethanolic extract of rose (
Sankar et al. prepared silver nanoparticles using the aqueous extract of
Rajeswari et al. synthesized silver nanoparticles using
Arun et al. developed silver nanoparticles using a mushroom fungus
Subbaiya and Selvam synthesized silver nanoparticles by
Rajam et al. prepared silver nanoparticles using fungus
Silver nanoparticles were prepared using culture supernatant of
Kalaivani et al. prepared silver nanoparticles using
4.2 Physical applications
4.2.1 Fabrication of antennas
Alshehri et al. have prepared two samples: the first was fabricated from the nano-metallic silver, and the second consists of micrometer-sized grains. Both types were prepared using thick-film fabrication process. The material involved in sample preparation was fine metal powder, an inorganic binder-like metal oxide, and an organic vehicle that evaporates during the initial drying stages. Both the samples were characterized for the electrical performances. They found that in the lower-frequency range, both types of conductors (samples) behave similarly with electrical loss but increase approximately linearly with increased frequency range (from 0.1 dB/mm/GHz up to 80 GHz), but above 80 GHz frequency, the silver nanoparticle-fabricated sample showed lower electrical loss, and this behavior continues up to the above whole frequency range. The lower level of the loss from the silver nanoparticle conductors and the overall trend in loss per wavelength do not depend significantly on frequency. Therefore, it has been concluded that the silver nanoparticle-fabricated conductors show a less electrical loss at high-frequency range which in turn attributed to lower surface roughness found in the nanoparticles due to better packing and may open opportunities for low-temperature fabrication of antennas and for sub-THz metamaterials with improved performance .
4.2.2 In electronically conductive adhesives
Silver nanoparticles can be used as a silver paste in the electrodes because of their high conductivity. They have also been used as conductive fillers in electronically conductive adhesives (ECAs). Chen et al. have synthesized the silver nanoparticles by reducing the silver nitrate with ethanol in the presence of polyvinylpyrrolidone (PVP). Various reaction conditions have been studied such as PVP concentration, reaction time, and reaction temperature. In this method, PVP prevents the aggregation; in addition to this, the PVP increases the rate of spontaneous nucleation and decreases the mean size of silver nanoparticles. The ethanol used in this has been employed as a reducing agent or diluent to adjust the viscosity of the ECAs. The resulting silver nanoparticles obtained with chemical reduction method had very fine dispersion and narrow size distribution. The ECAs had the silver nanoparticles re-dispersed in the ethanol. The absorption peak was determined at 410 nm which was a clear signature of the quantum size effect occurring in the absorption property of silver nanoparticles. It has also been concluded that the particle size of nanoparticles has been decreased with increasing concentration of silver nitrate and with increasing reaction temperature, but with increasing reaction time, the size of nanoparticles has been increased . Yang et al. have prepared silver nanoparticles, silver nanorods, and epoxy resins containing high-performance electrically conductive adhesives (ECAs) using a novel preparation method. The prepared nanoparticles and nanorods were dispersed well, and there was no agglomerate in the matrix. The volume electrical resistivity tests showed the volume electrical resistivity of the ECA was closely related with the various sintering temperatures and time and time and the ECA could achieve the volume electrical resistivity of (3–4) × 9 10–5 Ω after sintering at 160°C for 20 min. They found that the prepared ECA was able to achieve low-temperature sintering and possessed excellent electrical, thermal, and mechanical properties . This offers the possibility to effectively use these synthesized nanoparticles for improving the conductivity of ECAs.
4.2.3 Ink-jet printing
The silver nanoparticles can be used in ink-jet printing. Wu and Hsu have synthesized the silver nanoparticles by chemical reduction from the silver nitrate using triethylamine as reducing and protecting agent. After that the nanoparticles have been sintered using the process involved cleaning it with acetone and deionized water to remove the particles and organic contaminants on the surface; after cleaning the film, it was treated with ozone by UVO-100 UV ozone for 30 min. The silver nanoparticle suspensions were spin coated (500 rpm, 15 s) on the polyimide substrate and dried at room temperature in order to remove the solvent. The resulting silver nanoparticles on the polyimide substrate were heated from 100 to 200°C and held at 200°C for 1 h in order to convert to silver films. The polyimide substrate was then naturally cooled at room temperature in the glass dish. The above synthesized silver nanoparticles were sintered at different temperatures, and it was found that the resistivity of the silver film sintered at 150°C for 1 h was close to the resistivity of bulk silver. Based on the above data, the synthesized nanoparticles had the low sintering temperature; hence, the silver nanoparticle suspensions could be used to fabricate the flexible electronics by ink-jet printing .
The micro-sized silver particle fillers appear as the full-density silver flakes, and the silver nanoparticles fillers appear to be the highly porous agglomerates (similar to open-cell foams). Ye measured/analyzed the distribution of different sized particles using TEM. The electrical resistivity was also measured which was compared with the different levels of filler loading. The silver nanoparticles were prepared using the nano-sized spheres of size approximately 50–150 nm in diameter, micro-sized particles with a diameter of 5–8 μm, and flakes of silver of 10 μm in length. By TEM studies of the distribution of silver particles in micro-sized particle sea, it was concluded that it is difficult to find the cross-linkage of particles and there are fewer chances of different contact and contact area, and by the resistivity measurements, it has been revealed that the conductivity of micro-sized silver particle-filled adhesive is dominated by constriction resistance, the silver nanoparticle-containing adhesive is controlled by tunneling and even thermionic emission, and hence the respective nanoparticles are used to increase the electrical conductivity .
4.2.5 Water treatment
Dankovich prepared silver nanoparticles in a paper using microwave irradiation. Antibacterial activity and silver release from the silver nanoparticle sheets were assessed for model
4.2.6 Solar cell optimization
Plasmonic effects in thin film silicon solar cell are an emerging technology and area of rigorous research for the researchers from the past couple of years. It has promising application in solar cell fabrication industries where it uses nanoscale properties of silver nanoparticles incorporated in the interface between the metal and dielectric contacts that enhance the light-trapping properties of thin film silicon solar cells by increase absorbance capacity and generation of hot electrons that enhance the photocurrents in the solar cell. Sangno et al. had taken two different thicknesses of the silver thin film (made of silver nanoparticles) of 5.9 and 7.8 nm in 2 × 10–4 (Torr) and 2.5 × 10−4 (Torr) pressure environment for investigation purpose. Samples were annealed at different temperature ranges for a definite time period under vacuum condition of 4.5 × 10−6 Torr. They found that reflectance reduces 13–11% due to plasmonic effect and enhancement in the conversion efficiency of the solar cell .
4.2.7 Biosensor fabrication
Li et al. fabricated nanoenzymatic glucose biosensors by depositing silver nanoparticles using in situ chemical reduction method on TiO2 nanotubes which were synthesized by the anodic oxidation process. The structure, morphology, and mechanical behaviors of the electrode were examined by scanning electron microscopy and nanoindentation. It was found that silver nanoparticles remained both inside and outside of TiO2 nanotubes whose length and diameter were about 1.2 μm and 120 nm. The composition was constructed as an electrode of a nonenzymatic biosensor for glucose oxidation. The electrocatalytic properties of the prepared electrodes for glucose oxidation were investigated by cyclic voltammetry (CVs) and differential pulse voltammetry (DPV). When compared with bare TiO2 and silver-fresh TiO2 nanotube, Ag-TiO2/(500°C) nanotube exhibited the best electrochemical properties from cyclic voltammetry (CVs) results. In addition, the nonenzymatic glucose sensors exhibited excellent selectivity, stability, and repeatability. Nanoenzymatic glucose biosensors have potential application in catalysis and sensor areas . Ruth et al. has synthesized the oligonucleotide-silver nanoparticle (OSN) conjugates and revealed their use with magnetic beads as a biosensor for
4.2.8 Protein sensing
Tung N.H reported that silver nanoparticles labeling could be used in protein sensing studies by liquid electrode plasma-atomic emission spectrometry (LEP-AES). This technique is suitable for on-site portable analysis because plasma gas and the high-power source are not required. Proposed detection method could have a wide variety of promising applications in metal nanoparticle-labeled biomolecule detection .
Duran et al. prepared silver nanoparticles by using
Lipids are the major components of cell membrane and abnormal cellular metabolism-induced lipid changes. Hua et al. investigate silver nanoparticle-induced lipid changes on the surface of macrophage cells using time-of-flight secondary ion mass spectrometry (ToF-SIMS). By using this technique, one can understand the mechanism of cell-nanoparticle interactions at the molecular level and characterize the changes in lipids on the single cell surface . Citrate- and polyethyleneimine-coated silver nanoparticles can be used to understand how the type of capping agents and surface charge affects their colloidal stability, dissolution, and ecotoxicity in the absence/presence of Pony Lake fulvic acid (PLFA). On the basis of this, Jung et al. demonstrate that the differences in colloidal stability, ecotoxicity, and dissolution may be attributed to different capping agents, surface charge, and natural organic matter concentration as well as to the formation of dissolved silver complexes with natural organic matter .
4.2.11 Agricultural and marine
Silver nanoparticles synthesized by Guilger et al. using fungus
Chen prepared silver nanoparticles from filamentous fungus
It is revealed that silver nanoparticles have potential applications in therapeutics as well as in other physical fields. In therapeutics, researchers are seemed to be more focused on anticancer and antimicrobial evaluations. Green synthesis makes them eco-friendly and nonhazardous. Applications of silver nanoparticles are not limited to therapeutics only, they are equally covering physical fields too such as biosensors and antenna fabrication, conductive adhesives, in ink-jet printing, water treatment, solar cell optimization, protein sensing, etc. Rigorous research has been carried out and continued on this nanostructure. Therefore, the silver nanoparticle has the ability to be a lead nanoparticle of the future due to its wide variety of applications.