1.1. Synthesis of Ag NPs by Chemical Methods
Chemical methods are the most extensively investigated route for developing Ag NPs. The chemical methods can be divided into four different categories: (1) chemical reduction, (2) electrochemical techniques, (3) pyrolysis, and (4) irradiation-assisted methods
[3]. Among these methods, the chemical reduction of Ag
+ species to Ag
0 in solution using reducing agents is the most common and widely reported synthesis method of Ag NPs, usually no aggregation, high yield, and low preparation cost
[2].
The formation mechanism of nanoparticles from solutions has been investigated, including nucleation and growth process
[4]. The nucleation process requires more activation energy than the growth process, which is controlled by diffusion. The relative rates of nucleation and growth process are governed by varying reaction parameters, such as concentration of the reactants, the potency of the reducing agent, pH, or temperature. These parameters can be adjusted to fine-tune the nanoparticle synthesis achieving a single nucleation step and a steady growth process following a typical formation pattern of a LaMer model
[5].
Figure 1 shows a plot of the LaMer model in which the monomer concentration is shown as a function of time
[6]. In Stage I, the concentration of free monomers in solution increases sharply until a supersaturation (C
s) point is obtained. The concentration of monomer is high enough to overcome the energy barrier for nucleation. In Stage II, burst nucleation leads to a significant decrease of the monomer concentration in solution, which reaches below the level of minimum supersaturation (C
min), where no additional nucleation occurs. In Stage III, growth continues without further nucleation to form nanocrystals. The separation of nucleation and growth process gives rise to identical growing times of all the nanoparticles, and therefore, uniformity in sizes. As an alternative to this synthesis, a very high uniformity can be achieved using seed-mediated-growth synthesis instead of seedless synthesis. Small nanoparticles, seeds, are added to a growing solution and act as the nucleus for continuous growth. The reduction kinetics, in this case, is maintained to be slow enough not to achieve saturation.
The chemical reduction method usually involves three main components, i.e., metal precursors, reducing agents, and stabilizing/capping agents
[7]. Different reducing agents, such as sodium citrate, ascorbate, sodium borohydride (NaBH
4), elemental hydrogen, polyol, Tollen’s reagent, N, N-dimethylformamide (DMF), or polyethylene glycol (PEG)-block copolymers, are used. The choice of reducing agents and solvents gives rise to a different type of synthesis and offers an enormous number of possibilities.
Among the various possibilities, Turkevich’s and Brust’s methods, using citrate and NaBH
4, are the most used ones and generate very different nanoparticles in terms of sizes and solvent solubility
[8][9]. Together with the choice of metal salt, reduction agent, and experimental factors, the capping agent is the main factor in determining the size and physicochemical properties of the nanoparticles
[10]. This capping provides colloidal stability to the nanoparticle, either electrostatically or sterically, and controls the nanoparticle growth due to interactions with different facets, generating different morphologies. Capping agents contain polymers, such as poly (N-vinyl-2-pyrrolidone) (PVP), PEG, polymethacrylic acid (PMAA) and polymethylmethacrylate (PMMA) or small molecules such as citrate, oleilamine, alkanethiols and so on. Different morphologies (nanospheres
[11], nanorods
[12], nanocubes
[13], nanodisks
[14], nanoprims/nanotriangles
[15], truncated octahedron
[16], nanobipyramids
[17], nanoshells
[18][19], nanobars
[20], nanorice
[20], nanowires
[21] and nanostars
[22]) have been achieved by selecting capping agents and controlling experimental parameters, as shown in
Figure 2. The nanoparticle shape controls, among others, the plasmonic modes and offers the possibility to manipulate and respond to a light stimulus that goes from 400 nm to the middle infrared
[23][24]. The morphology is also relevant in their catalytic activity and the creation of electrical paths in conductive composites and inks, where high aspect ratio morphologies are preferred.
The synthesis parameters (reaction temperature, pH of precursors, reducers and stabilizing agents) are crucial factors in determining nanoparticles formation and nucleation process. Many different examples of Ag NPs synthesis were proposed. Wiley et al.
[25] obtained Ag NPs with three different shapes (nanocubes, bipyramids, nanowires) using ethylene glycol as a reducing agent. In their experiment, PVP was added as a stabilizing agent and reducing agent, which also played a role in controlling the shape. In addition, during the reaction, the seeds with district crystal structures were selected via etching, which produced the different morphologies of Ag NPs. Xue et al.
[26] reported pH-switchable silver nanoprism growth pathways. They found that excellent control over the growth of Ag nanoprisms can be achieved by adjusting solution pH. Jiang et al.
[27] investigated the reaction temperature effect on the formation and growth of Ag NPs using two or three reducing agents, including citric acid,
l-ascorbic acid, and NaBH
4. They found that there was a significant jump in the particle size at around 32 °C. The size of plates increases from 90 to 180 nm and from 25 to 48 nm for spheres. Yoo et al.
[28] synthesized Ag NPs via hydrazine reduction of Tollen’s reagent. AgCl and ammonium hydroxide were mixed in the presence of PVP as a stabilizer, resulting in a clear solution without any precipitate. Then, hydrazine hydrate was injected to reduce [Ag(NH
3)
2]
+ and obtain Ag NPs at room temperature. The size of the obtained Ag NPs was controlled by the concentration of PVP. The diameter of Ag NPs ranged from 68 to 119 nm. In addition, AgCl was retrieved from the electronic scrap and used as a metal precursor. Zhang et al.
[29] prepared colloid Ag NPs by using hyperbranched poly(methylene bisacrylamide-aminoethyl piperazine) with terminal dimethylamine groups (HPAMAM-N(CH
3)
2) as the effective reducing and stabilizing agent. The Ag NPs were synthesized by mixing AgNO
3 aqueous solution with HPAMAM-N(CH
3)
2 aqueous solution at room temperature. A series of Ag NPs were prepared by adjusting the molar ratio of N/Ag in feed. Malassis et al.
[30] used one-step aqueous synthesis of Ag NPs using ascorbic acid (vitamin C) as a reducing and stabilizing agent at the same time. Metal precursor, AgNO
3, is mixed with ascorbic acid under a strong stirring at room temperature for 30 s. It was showed that the size of Ag NPs obtained in the range of 20 to 175 nm was affected by the pH of either the metal salt solution or the ascorbic acid solution. Leng et al.
[31] produced Ag NPs by the reduction of AgNO
3 using the ethylene glycol in the presence of PVP. They also studied the different reaction conditions (i.e., AgNO
3 concentration, AgNO
3 and PVP mass ratio and the reaction temperature) on the size and size distribution of Ag NPs. When the mass ratio of AgNO
3 to PVP was 2:1, Ag NPs with a narrow size distribution from 20 nm to 30 nm were obtained. Sakthivel et al.
[32] prepared isonicotinic acid hydrazide capped Ag NPs by the wet chemical method and used NaBH
4 as a reducing agent. The synthesized Ag NPs distributed in two ranges and the average size of the particles is 35 and 536 nm. Isonicotinic acid hydrazide capped Ag NPs were applied to detect Hg
2+ in an aqueous medium in the nanomolar range. Sreelekha et al.
[33] synthesized Ag NPs using trisodium citrate as reducing and stabilizing agents via the chemical method. As described
in their work, Ag NPs were prepared by mixing the trisodium citrate solution and AgNO
3 solution at a temperature of about 80 °C, followed by separation of the NPs by centrifugation and drying to collect. Kamarudin et al.
[34] fabricated two types of Ag NPs by varying the amount of precursor and synthesis temperature. The higher the reaction temperature, the higher the reaction rate, which increased the process of ions to NPs conversion.
Waqas et al.
[35], for instance, successfully fabricated Ag NPs with three different shapes (i.e., spherical, star and pyramidal) via a simple wet chemical approach. An aqueous solution containing silver nitrate (AgNO
3), sodium hydroxide, oleic acid, and ammonia was used in their synthesis. During the reaction, the concentration of sodium hydroxide and oleic acid played an essential role in controlling the size, shape, and homogeneity of Ag NPs. Among all the three shapes, Ag NPs with star shape showed the highest electrocatalytic sensing efficacy in terms of linear range, the limit of detection, and sensitivity, because of more exposed catalytic active sites and fast diffusion of ions between electrode-electrolyte interfaces.
Meanwhile, Makwana et al.
[36] successfully prepared fluorescent Ag NPs by only applying a single chemical regent calix
[37] resorcinarene polyhydrazide (CPH) which acted as a reduction and dispersant agent. Bearing hydrazide group of CPH on its periphery acted as a reducing agent and its web type of structure as a stabilizing agent for the formation of calix-protected Ag NPs. Their results showed a large quantity of well-dispersed spherical CPH-Ag NPs with an average diameter of 7 nm. In addition, CPH-Ag NPs were not only selective and sensitive fluorescent sensors for Fe
3+ from 0.1 to 10 µM but also exhibited reasonably good antimicrobial activity.
Bare Ag NPs are not stable and can rapidly undergo agglomeration due to their high reactivity. In most of the preparation routes, capping agents are required to control the particle size, from absorbing or attaching with each other surfaces and avoiding the aggregation of nanoparticles; coating is a way to produce electrostatic, steric or electrosteric interactions between particles and helps stabilize the nanoparticles
[38][39]. Conversely, the capping agents often influence various properties of nanoparticles, including their shape and interactions with surrounding solvent, therefore, their dispersibility and size
[10]. For instance, Ajitha et al.
[10] investigated the influences of capping agents (PEG, ethylenediaminetetraacetic acid (EDTA), PVP, and polyvinyl alcohol (PVA)) on Ag NPs using a simple chemical reduction method. As described in their research, the average particle size was the largest for the PEG-capped NPs with a diameter of 44 nm and the smallest for the PVA-capped ones with a diameter of 27 nm. In addition, the PVA-capped ones were observed to be the most stable and to display the highest antibacterial activity.