The Importance of Essential-Oils in the Green Synthesis of Silver Nanoparticles

In addition to the above described possibilities, recent studies have evaluated the efficiency of essential-oils (Fig. 2) from different plant extracts using green synthesis method, as these oils are complexes and responsible for bio reductive reaction.

Figure2.

Photograph of some plants produce essential-oils.

Essential-oils are typically attained through vapor-distillation of aromatic plants, particularly those utilized as colognes and flavorings in the perfume and food manufacturing, correspondingly, and in recent times for herbal medicines and biosynthesis of metallic NPs. Chemical composition of some of the essential-oil elements are shown in Fig. 3 and several of them have strong biological activity and they are responsible for reducing, capping and stabilizing metallic NPs. These essential-oils are normally composed of multifaceted mixtures of monoterpenes, genetically related phenols, and sesquiterpenes. In fact, synthesized metallic NPs from plant essential-oils possess numerous significant advantages. Owing to their volatile-nature, there is a much lower level of danger to the surroundings than with present synthesizing methods.

Figure3.

Chemical structures of essential oil constituents.⁵⁹

Alfuraydi et al.⁶⁰ showed that it is possible to synthesize Ag NPs through incubating AgNO₃ with sesame essential-oil. This process can provide spherical NPs with diameters between 6.6-14.8 nm. Brindhadevi et al.⁶¹ studied the antimicrobial activity of wound fabrics treated with Ag NPs, sodium alginate and labdanum essential-oil.

It can be stated that, the essential-oils are utilized to reduce metal ions into NPs due to their complexity and diversity of the available reducing biomolecules.⁶² In addition, some functional groups of essential-oils may interact differently to neutralize ions as well as forming bonds that are necessitate in the nucleation process.⁶³ de Melo et al.⁶⁴ described the mechanism of biosynthesis of Ag NPs using essential-oil of thyme, Thymus vulgaris. This method is environmentally friendly method and it can be deliberated as a replacement approach to produce Ag NPs with optimum stability and extraordinary antibacterial activity. According to de Melo et al.,⁶⁴ the reduction process from Ag⁺ to Ag⁰ occurred through the donation of H⁺ protons from borneol, a compound with higher concentration found in the essential-oil, with the simultaneous stabilization of borneol through the Na⁺ existing in the solution precursor. The reducing reaction of the essential-oil of thyme, Thymus vulgaris, was investigated throughout the Ag NPs formation, and its antibacterial activity against Escherichia coli and Staphylococcus aureus were evaluated.⁶⁴ The nanoparticles had a spherical shape with an average size of 90 nm, and absorbance peak between 415 and 440 nm. Seventeen different compounds, by means of gas chromatography, were identified in the essential-oil. It is found that, most likely, barneol and α-terpineol are the major compounds, and perhaps they are responsible for formation of Ag NPs with potential antibacterial activity. Azizi et al.⁶⁵ using essential-oil ginger, Zingiber zerumbet synthesized zinc oxide-silver, ZnO-Ag core-shell nanocomposite through utilizing the green method. In this study six gram-positive and gram-negative pathogens were utilized to study the antibacterial effect of these samples. It was found that, through the in vitro tests, Ag doping improves the bactericidal activity of ZnO NPs. This is, perhaps, due to that the essential-oil of ginger was efficient in the biosynthesis process and producing comparable nano-composites to the conventional methods, which use hazardous and expensive chemical agents.⁶⁵ Qaralleh et al.⁶⁶ stated that Ag NPs can be synthesized by means of the culture supernatant of the fungal strain Tritirachium oryzae. Then the antibacterial activity of Ag NPs independently and in combination with some antibiotics and essential-oil of the Centaurea damascene plant extract were investigated. Ga’al et al.⁶⁷ using essential-oil of Aquilaria sinensis extracts produced Ag NPs from green, quick and one-pot process. Ag NPs with average sizes ranging between 15 and 87 nm were obtained, and they exhibited significant antibacterial action. In addition, Ga’al et al.⁶⁷ indicated that Ag NPs shows toxicity against the mosquito even at low doses. It is important to highlight the principle of the essential-oil reaction in reducing silver. Vilas⁶⁸ and the coworkers utilized essential-oil present in plant parts as the bio-reducing agent for synthesizing Ag NPs. The chromatography analysis showed that the compounds in higher concentration were terpenes and terpene alcohols. At high temperatures terpene hydrocarbons are capable of cleaving double bonds, then perform the oxidation and dehydrogenation process. This mechanism leads to the formation of free radicals. These free radicals, in turn, are capable participate in reducing heavy metal ions such as Ag. In general, the silver reduction reaction involves balancing electron donation. The formation of Ag⁰ and its clustering sequence is known as the nucleation process. The combination of numerous Ag⁰ through the nucleation step forms a nanoparticle. The essential-oil biomolecules, in addition to participating in the bio-reduction of AgNO₃, can link to the surface of the NPs making them more stable.⁶⁴ Table 1 summarizes the main products obtained from the reducing action of essential-oils in metallic Ag NPs.

Essential-oils are considered as suitable candidates in green synthesis metallic NPs.⁶⁹ The possible mechanism of formation metallic NPs involve three possible complexation reactions as shown in Fig. 4. The first reaction occurs between the main components of essential-oils, such as: phenolic acids, flavonoids, terpenoids and anthocyanin; and the selected metal ions.¹ The second possible reaction is occurring between the hydroxyl groups of anthocyanin, flavonoids and phenolic acids. Essential-oils, in turn, can directly cause the reduction of metallic ions. So the substances, the essential-oils, through their hydroxyl groups, combine with the metal ions resulting in formation of some compounds before producing the final metallic NPs through the reduction process.⁸⁰ These transition compounds form NPs through electron transfer and the produced NPs will be coated with anthocyanin and phenolic acids. In the third stage of reaction, which is much slower, the other biomolecules present in the essential-oils, which do not possess reducing capacity in the solution, they diffuse continuously to the solid-liquid interfaces formed in the core of metallic NPs.⁸¹

Figure4.

Complexation reaction as a possible mechanism for green synthesis of metallic NPs mediated by essential-oils.

The non-reducing surface coating derived from the essential-oils can be absorbed by the metallic NPs, contributing to greater stability in the synthesized NPs.⁷⁷ Thus, as described in Fig. 5 the NPs formation process can be generalized by three main steps: the oxidation-reduction, nucleation and growth.⁸²

Figure5.

General steps of metallic NPs formation.

In the redox step, the functional groups of essential-oils, i.e. reducing agent, containing oxygen in the ligand they donate π electrons to the transition metal, i.e. oxidizing agent. In this process, the metal ions suffer from reduction as it oxidizes the chemical components of the essential-oils. In some cases, the thermal decomposition of the ionic complex of the precursor metal salts increasing the reaction rate.⁸³ In the nucleation and growth stages, a series of metastable phases are actively promoted by the action of essential-oils, until the colloidal solution reaches a thermodynamically spatial stable stage.⁸⁴ Within the thermodynamically stable stage, the atoms that experienced reduction in the reaction medium, give rise to groups that reaching a certain characteristic size form a stable particle. The smallest radius of stable nucleolar ions is characterized by its critical radius, whose properties are given by the capacitive ability of the essential-oil to maintain surface free-energy stability with respect to the surface area of the nucleation group.⁸⁵ Thus, the greater the balance between these two factors, the greater the size of the critical core stable.⁸⁶ Regarding the nucleation studies, there are two types of processes: homogeneous nucleation and heterogeneous nucleation.⁸⁷ Homogeneous nucleation occurs in a single step, where the concentration reaches a critical supersaturating condition, and allows uniform diffusion growth of the reduced atoms to the formed core. In heterogeneous nucleation, the formation of nuclei occurs by the action of nucleating agents, these being the growth promoters of the particles.⁸⁸ The next step after nucleation is called growth, a process in which the nuclei migrate to the surface of the solute. In this process, the transport of reactive species to the surface of the particles occurs, coalescence and aggregation that can compete with the growth process.⁸⁹ At this point the stabilizing action of the essential-oil can balance the growth of the NPs by stabilizing them. The electrostatic potential of the electrical double layer on the surface of the NPs, i.e. Columbic repulsion, decreasing the NPs Polydispersity index.⁸⁹

Different factors are involved in the synthesizing process of metallic NPs. The reaction and interference of chemical and physical parameters involved in reduction of metal ions has been studied intensively.⁹⁰ As shown in Table 2, temperature, pH and reaction time are among the main factors that directly affect the size and morphologies of NPs. In green synthesis method, the concentration and amount of the bio-reducing agent are also among the factors that have impact on the size, shape and stability of NPs.⁶² The combination between pH and temperature begin to determine the size and shape of these nanostructures given the importance of the role of thermal agitation in the efficiency of the nucleation process and growth of nanostructures.⁹¹ In general, the created Ag NPs are stable under pH 9 and they start agglomeration at pH greater than 9.⁹² Accordingly, most of the pH values in Table 2 are below pH 9. As described before, an increase in temperature leads to an increase in both the plasmonic bands and the rate of formation of NPs. This feature is characteristic of the formation of nanostructures, which leads to decreasing their average size, given the contribution that temperature makes to the nucleation rate of nanostructures faster.⁹³ These consequences specify that the influence of temperature not only impacts the reaction rate but also affects the morphologies of Ag NPs in the plasmonic mediated reaction.

Another important point which brings to our attention from Table 2 is that the majority of the reaction temperature is less than 100 °C. This can be directly related to producing the spherical shape of Ag NPs. This, in turn, might be explained utilizing the Gibbs free energy equation, ΔG = ΔH – TΔS, where ΔH is the enthalpy change and ΔS is the entropy change. The spherical Ag NPs possess a larger surface area to the volume ratio than the other shapes, therefore, it is more likely, to generate smaller entropy and smaller heat of formation. The precise reaction at a low temperature causes spherical NPs since at a lower temperature |ΔH| is greater than |TΔS|. Conversely, the precise reaction at a high temperature causes nonspherical shape formation since |TΔS| is greater than |ΔH| at a higher temperature.⁹⁴

Vilas et al.⁶⁸ reported that essential oil of Myristica fragrans can be used for synthesizing Ag NPs. This preparation method provides highly pure crystalline Ag NPs with average size between 12-26 nm. These biosynthesized Ag NPs possess excellent catalytic and bioactive activity due to their surface modification. Nishanthi⁹⁶ and coworkers stated that Ag NP can be synthesized within a green and environmentally friendly method using the extract of the fruit of Garcinia mangostana. The formation of Ag NPs occurred at 80 °C. This process took place through the reaction between increasing temperature and increasing the rate of reaction as well as the formation of nucleation center.⁹¹ The pH of the reaction medium also plays a key role in the formation of NPs. Just like the temperature, pH regulates the formation of nucleation centers. As the pH increases, the number of nucleation centers also increase.⁹⁶ It is significant to highlight this point since the oxidation of phenolic groups is accompanied by the release of H⁺ ions, and therefore the nucleation of Ag NPs is expected to be preferred in an alkaline medium. This step, in turn, leads to the synthesis of smaller particles and also by the production of Ag₂O, element according to the following reaction:

2Ag⁺ 2OH⁻→Ag₂O+H₂O

Maciel et al.⁶⁹ conducted a study on biosynthesizing Ag NPs using essential-oil of Syzygium aromaticum. They have evaluated the formation of Ag NPs under different pH conditions, i.e. 7, 8, 9 and 10. As anticipated, smaller diameter Ag NPs were formed with the increase of pH values and an increase in the polydispersity index is also observed. Zeta potential standards provide evidence about the surface charge around the NPs and stability of the synthesized Ag NPs.⁹⁷ The zeta potential of the biosynthesized Ag NPs for pH ranging from 7 to 10 also increased in that order. Depending on the pH values Ag NPs possess sizes between 31 and 72 nm, and different shapes, namely triangular, square, hexagonal and mostly spheres. de Melo et al.⁶⁴ carried out an investigation using thyme essential-oil for synthesizing Ag NPs. They evaluated the formation of the NPs under different pH conditions, i.e. pH 7, 8, 9, and 10, as shown in Table 3. However, the highest maximum absorbance from the UV-Vis spectrum obtained at pH 10, the maximum absorbance at 440 nm. Regarding the size of the Ag NPs, the pH of the solution interfered differently in the mean diameter of Ag NPs. For instance, at pH 7 the average diameter for Ag NPs was 40 nm, while at pH 10 the average diameter increased to 90 nm. According to our best knowledge a conceivable reason for this outcome is that once the pH value is increased, the reaction rate will be increased as well, with consequent crystallization into smaller particles, which involved the nucleation and growth procedures of smaller particles from Ag nuclei.⁹⁸ In other words, pH value is critically affects the nucleation kinetics of Ag NPs formation. In another attempt Guimarães et al.⁹⁹ using the extract of Ziziphus joazeiro produced Ag NPs with different sizes. In an alkaline environment,i.e. pH 11, the Ag NPs displayed high polydispersity index with different sizes NPs. However, in the neutral condition, i.e. pH 7, NPs with smaller size and reduced degree of agglomeration were obtained. Along with temperature and pH, reaction time is also described as one of the main factors that influence the morphology and size of NPs.¹⁰⁰ This is more likely due to the fact that prolonged kinetic reaction can be conducted to a process of excessive agglomeration of the nuclei forming nanostructures making Ag aggregates on a microscopic scale. Besides, the variation in the conditions of the medium pH and the favored kinetic reaction that supplied to the system through the thermal energy is determine a characteristic time for the optimized reaction. This, in turn, producing materials with a low polydispersity index.¹⁰¹ In addition to these conditions, i.e. prolonged reaction time, the formation of large aggregates is induced, which affects the antibacterial property of the system. On the other hand, Veisi et al.⁷³ biosynthesized Ag NPs utilizing essential-oil of orange peel. The prepared solutions were monitored at different time intervals, i.e. 0 h, 12 h, 14 h, 36 h and 48 h, at a fixed temperature, 70 °C. The absorbance peaks of Ag NPs, recorded from UV-Vis spectrum, increased with increasing the interaction time of the silver nitrate with the essential-oil. The reaction time proved to be important factor in the reduction of the Ag⁺ ions in Ag⁰. The highest absorbance maximum peak found at 412 nm after 48 hours of reaction. Alfuraydi et al.⁶⁰ using sesame essential-oil, Sesamum indicum, synthesized Ag NPs. In their study, the formation of Ag NPs at different time intervals, i.e. 96, 120, 146 and 170 h, was monitored. However, the color change of the solution from colorless to brown occurred after 48 h, confirming the formation of Ag NPs. From the UV-Vis spectrum, a characteristic Surface Plasmonic Resonance (RPS) band was obtained of Ag NPs. The absorbance peak increased with increasing the interaction time of between Ag NO₃ solution and the utilized essential-oil.⁶⁰

The size, shape, and surface area per unit volume of nanoparticles can significantly influence their applications. Therefore, this review articles aimed to highlight the common size and shape of the utilized Ag NPs using essential-oils. The corresponding size and shape of Ag NPs biosynthesized from essential-oils using the utilized references in Table 2 can be find in Table 4. It can be noticed, from Table 4, that the utilized essential-oils, in general, are able to produce spherical NPs. In terms of the size, it can be seen from Table 4 the size of the biosynthesized Ag NPs using different essential-oils are ranging between 6-92 nm. This a good indicator that the wide range Ag NPs size can be obtained by changing the type of the essential-oils.

Table 4.

TEM, SEM images, size shape of the Ag NPs biosynthesized using essential-oils

The biosynthesized NPs from essential-oils have been applied as alternative antibacterial agents to combat pathogenic bacteria.^{64,66,69,102−104} The selected results found in the literature are presented in Table 3, which show the antibacterial activity by the inhibition halo method where (-) results indicate non-sensitive result with diameter halo less than 8 mm, (+) sensitive with halo between 9-14 mm, (++) very sensitive with halo between 15-19 mm and (+++) extremely sensitive with a halo greater than 20 mm.

It is significant to note that, the measurement of the halo of inhibition, shown in Table 3, signifies a qualitative analysis of the antibacterial action of the compounds, characterizing the diffusion capacity of the species with high toxicity against bacteria. It can be seen that, there is no direct relationship between the essential-oil and the size of the halo of inhibition, since the antibacterial action of the biosynthesized NPs depends on multiple factors, such as interaction of NPs with cell walls that vary according to the nature of the bacterium, i.e. gram-positive or gram-negative, and its own characteristics of each organism.¹⁰⁷ However, the most important aspect refers to the antibacterial activity of all synthesized NPs, demonstrating the potential of green synthesis of NPs towards the antibacterial activity.¹⁰⁸ A practical challenge of the reported research works is to understand what is the actual mechanisms of NPs reaction against the utilized bacteria. Vilas et al.⁶⁸ in their investigation showed that how the differences in cell wall of Gram-negative and Gram-positive bacteria interfered with the action of the utilized NPs synthesized from essential-oil of Coleus aromaticus. In fact, NPs were more efficient in controlling growth bacterial strain of E. coli (Gram-negative) than that of S. aureus (Gram-positive), which has a structure able to restrict the action and permeability of NPs in cells, called peptidoglycan. A similar result was found by Alfuraydi et al.⁶⁰ in their research using biosynthesized Ag NPs prepared form the essential-oil of Sesamum indicum. Ag NPs inhibited the growth of Gram-negative bacteria, i.e. P. aeruginosa, K. pneumoniae and E. coli, however they were not efficient in combating Gram-positive bacteria, i.e. B. subtilis and S. aureus. This result can be associated with the ability of Ag NPs to bind with the surface of the cell membrane of Gram-negative bacteria, causing changes in respiration and cell permeability, in addition to the protective biological barrier found in Gram-positive bacteria, which hinders the reaction of NPs.⁶⁰ In contrast, Manju and coworkers¹⁰⁹ evaluated the reaction of the NPs biosynthesized from essential-oil of Nigella sativa, obtaining greater activity against the S. aureus bacteria, Gram-positive, with halos of inhibition 16 mm, than against Vibrio harveyi, Gram-negative, with 5 mm inhibition halos. The size and shape of metallic nanoparticles directly interfere with their antibacterial activity.¹¹⁰ The high surface/volume ratio of NPs increases their affinity with the bacterial cell membrane.¹¹¹ Furthermore, the attraction between positive and negative charges of NPs and bacterial cell membrane, respectively, increases due to the presence of lipopolysaccharide or techoic acid.¹¹² In a study conducted by de Melo et al.⁶⁴ spherical shape Ag NPs with mean diameters of 40 nm and predominance were more efficient in controlling Gram-negative bacteria, E. coli, and Gram-positive, S. aureus than bigger Ag NPs, i.e. 90 nm, and they are mainly stem shaped. Maciel et al.⁶⁹ discussed the influence of the shape and size of NPs with the ability to inhibit bacterial growth. Metallic NPs can impair exopoly-saccharide synthesis, making the possibility of this reaction to be zero interactions between bacteria and host cells. With decreased exopolysaccharide synthesis, hydrophobicity of the cell surface is reduced, directly affecting biofilm production in different microorganisms.¹¹³ Investigations on metallic NPs against bacterial biofilm formation showed favorable results against different bacteria. Depending on the dose, the metallic NPs are able to disintegrate and inhibit the hydrophobicity of S. aureus and V. harveyi colonies, in 78% and 46%, respectively.¹⁰⁹ This response, perhaps, associated with inhibition of exopolysaccharide synthesis confirming the efficiency of NPs as bacterial biofilm inhibitory agents.¹⁰⁹ The production of metallic NPs as antibacterial agents is a very essential technology. mentioned in the literature, mainly with Ag and Au NPs. The influence of essential-oils on biosynthesized NPs process is a promising strategy both from the view point of morphology, i.e. how much of the antibacterial role for the resulting system.