Advances in the Use of Green and Sustainable Synthesis to Obtain Nanomaterials

Abstract

The bibliometric analysis by Methodi Ordinatio reveals the impressive increase in the published articles about green chemistry, and specificity in green synthesis of nanomaterials. In the last decade, they have published over 450 articles, most led by India, China, and Iran. The green synthesis is according to the 12 principles of green chemistry (PGCs) to obtain nanoparticles with minimization of waste and toxic emissions, use of green solvents and alternatives to conventional organic solvents, use of renewable and sustainable raw materials, and energy efficiency and use of renewable energy. After synthesis, the green nanoparticles are characterized to know their physical and chemical properties. Green synthesis can contribute to the sustainable development goals (SDGs) until nine goals can be associated with green synthesis and green nanoparticle applications. Among advantages and limitations, the green syntheses of nanoparticles have the potential to grow more by future perspectives gap.

1. Introduction

The published increase of research articles over time about green chemistry in nanotechnology subject enriches the scientific basis with a lot of manuscripts. When the researcher needs to start any search or deepen knowledge about a determined subject, the excess of information can be a big problem. In this case, the Methodi Ordinatio (MO) can be applied to help organize the articles per relevant order [1].

Pagani et al. created a scientific method that considers the three main aspects of each article: impact factor, published year, and the publication’s annual average of citations [1]. This method was used to classify more than 500 articles about the advances in the use of green and sustainable synthesis to obtain nanomaterials. The first hundred more relevant articles were used to write this chapter.

The most significant advances in using green and sustainable synthesis to obtain nanomaterials are not just the trend. The increased uses of microorganisms, plants, and waste in a green synthesis are related to environmental preservation. When a green synthesis of nanoparticles is performed instead of a typical synthesis, the researcher can collaborate with the 12 principles of green chemistry (PGCs) [2] and directly or indirectly to the sustainable development goals (SDGs), agenda 2030 [3].

Several bottom-up chemical or biological methods can be changed or adapted to turn a green synthesis [4]. This way, these processes can decrease waste and toxic emissions, use green and alternative solvents, use renewable and sustainable raw materials, and take advantage of energy sources better, cherishing energy efficiency and use of renewable energy [2, 5].

This chapter aims to analyze the green synthesis methods used in the first hundred articles by MO. As well as the typical characterizations needed to know about the properties of each synthesized material, situate the most application area, and establish the advantages and limitations of green synthesis of nanoparticles. The most important key of this work is compliance with the 12 principles of green chemistry in the so-called green synthesis and the close relationship between green synthesis and the objectives for sustainable development.

2. Bibliometric analysis

The importance of the bibliometric analysis is a panoramic notion about relevant publications over time. Many articles have been published and cited, but for researchers, it is complicated when starting a new work, and they need to organize these articles by importance. The MO can facilitate this first research work [1]. The MO uses one equation to calculate de InOrdinatio: the article’s importance based on your impact factor, published year, and the number of citations (Eq. (1)).

$$\begin{array}{ll}\mathit{\text{InOrdinatio}}=& \{\left(∆\ast \mathit{\text{IF}}\right)-\left[\lambda \ast \left(\frac{\mathit{\text{ResearchYear}}-\mathit{\text{PubYear}}}{\mathit{\text{HalfLife}}}\right)\right]\\ & +\mathrm{\Omega }\ast \frac{\sum \mathit{Ci}}{\left[\left(\mathit{\text{ResearchYear}}+1\right)-\mathit{\text{PubYear}}\right]}\}\end{array}$$E1

In the equation, the symbols represent: IF is the impact factor and corresponds to the selected journal metrics (such as JCR, CiteScore, SNIP, or SJR SCImago); ∆ indicate the importance of the impact factor; λ is the relevance of the publication year; Ω is the significance about number of citations. These values, which indicate the importance of anything, can range from 1 to 10, according to the researcher’s choice when using the MO in their search. The ResearchYear and PubYear are the year the research was conducted and the year the paper was published, respectively. While ∑Ci is the total number of citations found in Google Scholar, and halflife refers to the median cited halflife of journals with JCR 2020 [1].

In the MO, the first step is to define the keywords that will be searched and where it is done. In this case, the combination searched was ((TS = (green chemistry)) AND TS = (synthesis)) AND TS = (nanomaterials) in Web of Science, Science Direct, Scopus, and Science Open websites. These searches resulted in 6, 140, 425, and 366 articles on each site. After filtering articles to exclude duplicates that were not attractive to the work, the InOrdination equation (Eq. (1)) was applied to 509 articles. Despite many of these articles being used in writing this chapter, after the InOrdinatio ranking, the best hundred articles were chosen to base this review.

The results of this bibliometric analysis are essential to identify some data like the subject importance, publication number per year, the country where the search about the subject was happening, the gaps in the issue that need more investigation, etc. From that, insights can appear as new ideas for work, research, articles, and reviews [1].

In this context, we can note that the number of articles increases over time. The published works in the last decade are nine times smaller than the current: Have 456 published articles between 2014 and 2023 versus 50 published articles between 2007 and 2013, and this search was performed without year restriction, Figure 1(a). The relevance and interest in green chemistry and nanoparticle synthesis are very significant, but India, China, and Iran are leading research in this area, Figure 1(b).

Figure 1.

Bibliometric analysis data: (a) increase in publications throughout the years, among the 509 mentioned articles, and (b) countries that published the most articles on the subject, among the 100 most important articles according to Methodi Ordinatio.

The keywords used in these articles, among others, were green chemistry (37), green synthesis (24), nanoparticle (18), nanomaterial (16), photocatalysis (7), nanocomposite (6), silver nanoparticles (6), nanotechnology (6), plant (4), magnetic nanoparticles (4), plant extract (4), gold nanoparticle (4), and green (4). Figure 2 was created using all the keywords of the hundred-first articles; the word size in the figure represents the frequency it appears in keywords.

Figure 2.

Word cloud with keywords used by articles.

However, green chemistry applied to nanomaterial synthesis is a current, meaningful, and exciting subject. In this sense, we need more understanding of what sustains this subject’s relevance, like the principles of green chemistry, the applications of nanomaterials synthesized by green routes, and the contribution to the sustainable development goals (SDGs). These factors direct the movement’s rise and research projects’ political/economic interests.

3. Principles of green chemistry applied to obtaining nanomaterials

Established by Paul Anastas and John Warner in 1998, the 12 principles of green chemistry (PGCs) concern a conscious attitude guide to promoting green chemistry [2], Figure 3. This concept is according to ISO 14001, an international standard about the environmental management system (EMS), which applies to any organization that wishes to demonstrate conformity with this international standard by promoting sustainability and safety in your production [6].

Figure 3.

Representation of the 12 principles of green chemistry proposed by P. Anastas and J. Warner.

Currently, the “green” adjective is frequently used by researchers. This can be attributed to different causes, i.e., promoting your research, funding requirements, and creating new sustainable alternatives to a current problem or opportunity, among others [7].

Nanoscience is a concept that relates to the nanometric scale (10⁻⁹ m), which must be in at least one of the nanomaterials dimensions (design, manufacture, control, characterization, and applications), and the upper limit is 100 nm. The green concept is so used in nanoscience that already appears the term “Green Nano” [7].

The 12 PGCs can be associated with green nanomaterial synthesis, as the perspectives below, and all consist in performing a chemical more safely, minimizing the dangerous products or reagents, as principle 12.

Principle 12, “Inherently Safer Chemistry for Accident Prevention,” talks about safety that is crucial within the general principles of green chemistry since risk reduction and accident prevention aim not only at environmental sustainability but also at the security of processes, workers, and communities. For this reason, PGCs 12 is contained behind all the other principles, which must be implemented by eliminating risks or at least reducing them [2, 7].

3.1 Minimization of waste and toxic emissions

The minimization of the toxicity of the process is mainly connected with PGCs 1, 2, and 11.

• Prevention: It is about reducing waste as much as possible because it can be much more difficult and expensive to solve future problems with generated waste. In synthesizing nanomaterials, it is possible to reduce the number of reagents with optimizations and avoid the generation of by-products [2, 5, 7]. Furthermore, organic residues and residues or by-products of other processes can be used as agro-wastes [8, 9, 10] and biotechnological processes [11, 12, 13, 14].

• Atom Economy: It is not about reaction yield but about the incorporation of reagent atoms in the final product. This is a limitation in green nanomaterial synthesis. These reactions are known to cause low yield and use of several established reactants and other auxiliaries to obtain the specified nanoparticle. Despite it can be seen as an opportunity for an optimization process [2, 5, 7].

• Real-Time Analysis for Pollution Prevention: It is controlling the reaction to not produce the hazardous subproduct; it happens when trying to understand the mechanism behind the reaction. Sometimes, researchers are focused just on the final product, but they can produce any pollutants during the reaction synthesis or the washing of the nanoparticles. It is necessary to analyze the process to monitor the products of the reaction [5, 7].

3.2 Use of green solvents and alternatives to conventional organic solvents

In green nanomaterial synthesis, water-based extracts are usually used to substitute the organic solvents. It is according to three PGCs, are them:

• Less Hazardous Chemical Synthesis: This PGC means that synthesis needs to be especially careful not to harm human and planet health. In the green, nanomaterial synthesis is increasing over time, the alternative synthesis that substitutes hazardous reagents to “natural reagents” originating in plants, algae, fungi, and bacteria, among other biological entities [12, 13, 15, 16, 17].

• Designing Safer Chemicals: This PGC is complementary to the before item. Still, this one is focused on products that need safety when applied to specified functions, be it medical, chemical, industrial, or pharmaceutical, among others. The efficacy has a strict relation to security and low toxicity [5, 7].

• Safer Solvents and Auxiliaries: It is about substituting conventional organic solvents with other less dangerous/contaminants. Green nanomaterial synthesis usually can use aqueous plant extracts [18, 19, 20] and ionic liquids [21, 22, 23, 24, 25] for this substitution with more benefits to desired products.

3.3 Energy efficiency and use of renewable energy

The energy efficiency in green nanomaterial synthesis can be the differential about the nanomaterial synthesized; the same product can be obtained in several ways, and the economy of the energy and material can be more accessible when need expand to an industrial scale, for example.

• Design for Energy Efficiency: The economy of the energy minimizes the environmental and financial impacts. This economy can be around the reactants chosen, conditions of the temperature and pressure, and reaction way. It is common for green nanomaterial synthesis to be conducted in microwave [26, 27, 28] and ultrasound [29, 30, 31, 32] to try to be more easily, quickly, and economized energy.

• Reduce Derivatives: This is a fragility of green nanomaterial synthesis because it is expected that derivatives (or co-products) can be created during the process [7]. It can get more waste and derivatives, especially when it needs so many steps to obtain the final product with the aim of having specific shells in its structure. The way to reduce by-products is to minimize the steps of the process. Another specific way is using enzymes [33], which are highly specific and can eliminate the use of other protecting groups or derivatives.

• Catalysis: the green nanomaterial synthesis can be catalyzed for more energy efficiency and to reduce derivatives. But we need to be careful to avoid happening the opposite. It is better that the catalyst chosen be environmentally friendly and ideally can be recovered after the reaction, and not be one more waste after the synthesis.

3.4 Use of renewable and sustainable raw materials

The use of renewable sources of raw materials and concern for the degradation of products and by-products generated are principles of green chemistry strongly related to the synthesis of new nanomaterials.

• Use of Renewable Feedstocks: As renewable raw materials, several bio-based materials have been used in nanomaterial synthesis [15, 34, 35]. The nanomaterials obtained by bio-based synthesis can be diverse, with particular physical and chemical properties.

• Design for Degradation: This can be a problem if, during the biosynthesis, a subproduct is obtained or a feedstock is hard to degrade after the reaction. So, it is necessary to design the synthesis and think about the products. The nanoparticle needs special attention because, after your use, it can be recycled, reused, recovered, or biodegraded and not be another problem to the environment [7].

4. Advances in green synthesis methods for nanomaterials

Nanomaterial synthesis can be divided into two principles: top-down and bottom-up methods [36]. In the top-down method, the bulk materials are transformed into nanoparticles by physical, chemical, and mechanical processes, which reduce the material mechanically. The main processes are spray pyrolysis, sonication, arc discharge, pulse wire discharge, pulsed laser ablation, radiation, electro-deposition, evaporation-condensation, vapor and gas phase, chemical, etching, ball milling, and lithography [13, 36, 37, 38, 39].

The bottom-up method is the transformation of atoms/molecules into nanoparticles. This can involve the dissolution of salts in a solvent, reducing ions to their element with a reducing agent, and stabilizing the resulting neutral nanoparticles with stabilizing agents to prevent agglomerate accumulation. This process can occur by chemical methods (sol-gel technology, co-precipitation, redox processes, pyrolysis, microemulsion, microwave, photochemical, electrochemical, sonochemical, and hydrothermal activity) and biological methods (using fungi, plants, yeast, bacteria, viruses, and biomolecules/biopolymers) [13, 36, 37, 38, 39].

Several methods can be adapted to be “green” by applying the PGCs. Currently, biological base materials are used in the synthesis process, like plants, biomass, microorganisms (bacteria, fungi, and yeast), and enzymes, among others, like additives, stabilizers, or substitutes for other harmful compounds. The aim is the same: obtain the better synthesis method, cheapest, quickly executed, and most sustainable.

Among the principal articles according to InOrdinatio classification, the plants are the feedstocks more used in nanomaterial synthesis, allowed by fungi, bacteria, and yeast (Figure 4). It can be explained by the diversity of the plants and part of it (leaf, stem, flower, fruit, peel, and seed) that can be used in synthesis.

Figure 4.

Representativeness of the types of raw materials used in the first hundred articles with the best InOrdinatio.

4.1 Nanomaterial synthesis using plant extracts

The increase in the use of plant extracts is explained by possible variations with different plants and parts of it. This variation is related to the influence caused by the secondary metabolites, a particular characteristic of each species [38, 40]. Usually, different metallic nanoparticles with different sizes can be synthesized using plant extracts, Table 1, in which cadmium, chromium, copper, gold, iron, silver, and zinc nanoparticles are the most common.

Nanomaterial obtained	Size (nm)	Plant used	Reference
Cadmium oxide	113	Hibiscus Sabdariffa flower extract	[41]
Chromium Oxide	17–42	Abutilon indicum (L.) leaf extract	[42]
Copper nanoparticles	5–20	Ziziphus spina-christi (L.) fruit extract	[43]
Copper oxide	61.48	Momordica charantia fruit extract	[44]
Gold; Silver	15–25; 15–20	Memecylon umbellatum leaf extract	[45]
Gold; Silver	5.82; 5.54	Clerodendrum inerme leaves extract	[46]
Gold nanoparticles	10–100	Nerium oleander stem bark extract	[47]
Gold nanoparticles	5–20	Gnidia glauca flower extract	[48]
Gold nanoparticles	20–140	Rind of watermelon aqueous extract	[49]
Iron oxide	13.42	Garcinia mangostana fruit peel extract	[50]
Iron oxide	20–30	Excoecaria cochinchinensis extract	[51]
Silver nanoparticles	18.2	Vitex negundo L. extract	[52]
Silver nanoparticles	37.71–71.99	Chrysanthemum indicum L.	[53]
Silver nanoparticles	21–173	Conocarpus Lancifolius fruits extract	[54]
Silver nanoparticles	25.2	Nigella sativa extract	[55]
Silver nanoparticles	45–110	Brillantaisia patula extract; Crossopteryx febrífuga extract; Senna siamea extract	[56]
Silver oxide; Zinc oxide	50; 60	Moringa oleifera gum	[57]
Zinc Oxide	52.24	Cayratia pedata leaves extract	[58]
Zinc oxide	480	Panax ginseng extract; Acanthopanax senticosus extract; Kalopanax septemlobus extract Dendropanax morbifera extract	[59]
Zinc oxide	9–38	Azadirachta indica extract	[60]

Table 1.

Metallic nanoparticles synthesized with different plant extracts.

Another typical process is using biomass from any part of the plant in nanomaterial synthesis to obtain the nanoparticles, like the seed, fruit peel, fallen leaves, and bagasse.

4.2 Use of microorganisms for the nanomaterial synthesis

It is increasing the use of microorganisms in nanomaterial synthesis [12, 15, 34, 61, 62]. Bacteria, fungi, and yeast, when added have more expressive results than plants in nanomaterials synthesis (Figure 5). The bacteria application is related to the bacteria resistance with variations in temperature, pH, incubation time, and oxygenation, among other environmental stresses [15, 34, 61]. Another cause of the success of the bacteria is the handling, manipulation, and genetic modification facilitated [61]. Usually, these adjustments depend on the size, shape, and composition of interest [15]. Usually, metallic nanoparticles are synthesized with bacteria (Au, Ag, CdS, Co₃O_4, Fe₃O_2, Hg, MnO₂, PbS, Pd, Pt, Sb, Se, Te, Ti, ZnS, and ZrO₂ nanoparticles) [13, 34, 61, 62], which can be synthesized by either intracellular or extracellular mechanisms [12, 15].

Figure 5.

Where the nanomaterials obtained by green synthesis are applied in the first hundred articles with greater relevance to this research.

The nanomaterial synthesis using fungi has interesting advantages because of the presence of enzymes, proteins, and/or reducing components on their cell surfaces [13, 15]. Fungi have unique characteristics such as their resistance to toxicity, ease of handling, large surface areas that lead to increased production rates, easy and simple downstream processing, economic viability, and the larger spectrum of nanoparticle morphologies [12, 34, 61]. Furthermore, the synthesis process can be facilitated because they endure harsh synthesis conditions such as flow pressure or agitation in the bioreactor, and have accelerated growth in controllable ways, and almost all fungi are better resistant to genetic or environmental mutations when compared with bacteria [12, 61]. Both extracellular and intracellular syntheses of metallic nanoparticles using fungi have been investigated as Au, Ag, TiO_2, ZnO, and ZrO₂ nanoparticles [13, 34, 62].

Yeasts, which are eukaryotic microorganisms, like other microorganisms, have been explored in nanomaterial synthesis [13, 15, 34]. Their ability to absorb and accumulate toxic metals enables them to be used in the synthesis of nanomaterials [15, 34]. Diverse yeast species are employed for the preparation of innumerable metallic nanoparticles, such as Ag, Au, CdS, Fe₃O₄, PbS, and Sb₂O₃ [13, 34]. During the synthesis, the mechanism used by yeast to form and stabilize the nanoparticle defines its size, shape, and properties [15].

4.3 Application of green methods in nanoparticle synthesis

Several eco-friendly techniques can be used for nanomaterial synthesis: sonochemical, microwave, hydrothermal, solvothermal, and electrochemical, among others. But two of them are the most used: sonochemical [29, 36, 63, 64] and microwave [26, 27, 28, 36, 63] methods.

The sonochemical method uses ultrasound, an inaudible sound wave with a frequency higher than 20 kHz [63]. The ultrasound possesses high energy that can generate the acoustic cavitation effect, which results consecutively in the formation, growth, and implosive collapse of these bubbles in a liquid environment. During the collapse of a cavity, high local temperatures and pressures arise within a very short period, which leads to an increase in the rate of reactions [29, 36, 63, 64]. It is an eco-friendly, green, fast, and easy method of nanostructure synthesis. It is used for nanostructure material preparation and unusual nanostructured inorganic materials such as carbonyl compounds (Fe(CO)₅, Co(CO)₃NO, Mo(CO)₆, and W(CO)₆) [36].

In microwave-assisted synthesis, electromagnetic irradiation can reduce the energy needed for the synthesis process, providing rapid heating and facilitating greener preparation of nanoparticles [26, 27]. The main advantages of using microwave-assisted in nanomaterial synthesis are reduced reaction time (causes the reduction in activation energy), high product yield (a shorter reaction time reduces the chance of undesirable side products), high product purity, high reproducibility (a uniform microwave field around the reaction mixture), and reaction conditions can be easily optimized [26]. This technique is used to synthesize various metal nanoparticles [36].

5. Characterization of nanomaterials obtained by green synthesis

The nanomaterials need characterizations to confirm your structure and chemical and physical properties. Among them are the most common to observe about the structure, such as transmission electron microscopy (TEM), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Other characterizations can be needed to evaluate specific chemical and physical properties according to the components and applications of the synthesized nanomaterials, such as tests to assess the stability and reactivity.

5.1 Structural characterization techniques

5.1.1 Transmission electron microscopy

The transmission electron microscopy (TEM) characterization is the most popular among the nanoparticles [65, 66, 67]. With this analysis, the size and shape of the nanomaterials can be evaluated. And, if it has a coating or void, it can be observed.

The TEM is a powerful electron microscope (the magnification can be 2 million times better than that of the light microscope) and uses a beam of electrons to focus on a sample, producing a highly detailed image with morphological features, compositions, and crystallization information. The principle of TEM is the beam of electrons, which has a wavelength shorter than light. When the electron illuminates the sample, the resolution power increases, consequently increasing the wavelength of the electron transmission [68].

TEM analysis has applications in nanotechnology to study nanoparticles and detect and identify fractures and damaged microparticles, enabling subsequent repair mechanisms for these particles. Among other fields, TEM can be applied in biology, microbiology, and forensic studies to examine the cell structures of plants, animals, and microorganisms and analyze bacteria flagella, plasmids, and the shapes/sizes of microbial cell organelles [68].

The TEM have limitations, such as the price, the extensive equipment, the tedious sample preparation, and laborious operation and maintenance because of its sensibility. But, despite the limitations, it has important advantages to researchers, like significant and power magnification, the variety of fields that can be applied, and the production of very efficient, high-quality images with high clarity [68].

5.1.2 X-ray diffraction

X-ray diffraction (XRD) is a quick and nondestructive analytical technique that can be applied to powder, solid, and liquid samples to analyze their physical properties. With this characterization it is possible to know about the composition, cell dimensions, and crystalline structure [69, 70].

The principle of XRD is based on constructive interference between monochromatic X-ray and a crystalline sample. The X-ray is generated by a cathode ray tube. These X-rays are filtered, collimated, and directed to the sample. The X-rays’ wavelength is the same magnitude as the distance between the atoms in a crystalline lattice [69, 70]. The constructive interference and a diffracted ray produce a diffraction pattern, which satisfies Bragg’s law (nλ = 2dsinθ), where n is a natural number, λ is the wavelength of the incident radiation, d is the distance between atomic planes, and θ is the angle of incidence about the considered plane. This law relates the wavelength of electromagnetic radiation to the diffraction angle and the lattice spacing in a crystalline sample, and it is used in the measurement of crystals and their phases [69, 70].

The XRD is essential to develop and improve new materials in laboratories and industries. The main uses are qualitative and quantitative phase analysis of pure substances and mixtures. The most common method for phase analysis is often called X-ray powder diffraction (XRPD). The XRPD is used to characterize crystalline materials, identify fine-grained minerals that are difficult to determine optically, determine unit cell dimensions, and measure sample purity. With enhancements, it is possible to determine crystal structures using Rietveld refinement, determine modal amounts of minerals (quantitative analysis), and characterize thin film samples, among others [69, 70].

The XRD advantages are that, in most cases, it provides an unambiguous determination, sample preparation is minimal, and the data interpretation is relatively straightforward (the operator does not need to be an XRD expert), which makes this technique the most popular and suitable [69, 70]. Usually, the researchers use the standards published by the Joint Committee on Powder Diffraction Standards-International Center for Diffraction Data (JCPDS-ICDD) to help identify the composition of their sample.

5.1.3 Scanning electron microscopy

The scanning electron microscope (SEM) is a type of electron microscope that produces images that possibility study solid particles surfaces. With the increased knowledge about nanotechnology, the materials continue to shrink, making this analysis increasingly necessary for their characterization [71, 72]. The SEM principles consist of using electrons for imaging in a similar way that light microscopes use visible light, but the resolution of SEM is superior to that of a light microscope because the wavelength of electrons is much smaller than that of light [71].

The SEM uses a focused beam of electrons to generate a variety of signals that scan the surface of a solid sample. The signals that derive from electron-sample interactions reveal sample information, such as external morphology, chemical composition, and crystalline structure and orientation of materials making up the sample [71, 72, 73].

Among the advantages of SEM are that no elaborate sample preparation techniques are required and the capability to perform analyses of specific points on the sample [72, 73]. With this technique, it is possible to generate high-resolution images of the shapes of objects and to show spatial variations in chemical compositions by obtaining elemental maps or spot chemical analyses using energy-dispersive spectroscopy (EDS), discrimination of phases based on the mean atomic number using backscattered electrons (BSE), and compositional maps based on differences in trace element activators using cathode-luminescence (CL) [73].

6. Applications of nanomaterials obtained by green synthesis

The nanoparticles synthesized by green routes can have a lot of applications. Among the hundred articles with the higher InOrdinatio, the areas that aroused interest in nanotechnology applications were catalysis [28, 29, 48, 59, 60, 64, 74, 75, 76, 77, 78, 79, 80], medical/biomedical [19, 42, 45, 49, 50, 62, 63, 81, 82, 83, 84, 85, 86], environmental [7, 27, 34, 63, 87, 88, 89], biological [33, 46, 56, 90, 91], water treatment [27, 37, 38, 89, 92, 93, 94], agriculture [18, 34, 95, 96, 97], and food industries [37, 38, 87, 95, 97], among others [98, 99, 100, 101, 102, 103]. Several researches do not establish a specific application but show the characteristics of the nanoparticles, like antimicrobial [61, 104] or antibacterial [43, 46, 49, 53, 57, 60, 105, 106] activity, as a differential between these materials (Figure 5).

The efficiency in a determined application depends on the composition, physical and chemical properties, and sometimes on the particular characteristics of the nanoparticles. It is common for the same nanoparticle to be applied with different functions in different areas, with some needed adjustments. Most can be versatile, while others are specific to be efficient in just one application.

7. Contribution to the sustainable development goals (SDGs)

In 2015, the United Nations adopted the sustainable development goals (SDGs) with social, environmental, economic, and institutional dimensions. The SDGs aim to end poverty, protect the planet, and promote peace and prosperity by 2030. The United Nations General Assembly (UNGA) has set 17 integrated goals, which the results must affect and balance social, economic, and environmental sustainability. The SDGs must be achieved in every context [3].

In green chemistry, namely, the synthesis of nanomaterials can involve several of the 17 SDGs, such as 3. good health and well-being; 6. clean water and sanitation; 7. affordable and clean energy; 8. decent work and economic growth; 9. industry, innovation, and infrastructure; 11. sustainable cities and communities; 12. responsible consumption and production; 13. climate action; and 14. life below water (Figure 6).

Figure 6.

SDGs related to green chemistry in the synthesis of nanomaterials.

Each objective mentioned will be related below to the goals that the green synthesis of nanomaterials can collaborate directly or indirectly.

• Good health and well-being: The nanoparticle synthesis is aligned with some goal targets, such as: “substantially reduce the number of deaths and illnesses from hazardous chemicals and air, water and soil pollution and contamination”, and “Support the research and development of vaccines and medicines for the communicable and non-communicable diseases that primarily affect developing countries” [3]. Several nanoparticles synthesized by green routes used environmentally friendly feedstock rather than hazardous and contaminant products. Furthermore, many nanoparticles can be applied in medicine or nanomedicine, like a disease treatment.

• Clean water and sanitation: Considering that 80% of the used water does not receive adequate treatment, the majority of nanoparticles synthesized by green chemistry can be applied in water treatment or catalysis to degrade nocive compounds. So, the nanomaterial is closely related to this goal, specifically with the target “improve water quality by reducing pollution, eliminating dumping and minimizing release of hazardous chemicals and materials, halving the proportion of untreated wastewater and substantially increasing recycling and safe reuse globally” [3].

• Affordable and clean energy: When talking about clean energy involved in nanomaterials synthesis, we can talk about the energy used in nanoparticle synthesis or nanomaterials that enable or facilitate the use of renewable energies. In this context, it can establish a relation with the target “to facilitate access to clean energy research and technology, including renewable energy, energy efficiency and advanced and cleaner fossil-fuel technology, and promote investment in energy infrastructure and clean energy technology” [3].

• Decent work and economic growth: The research investment in nanotechnology (or any research area) collaborates by itself with decent work and economic growth because it can “Promote development-oriented policies that support productive activities, decent job creation, entrepreneurship, creativity and innovation” and “reduce the proportion of young people not in employment, education or training” [3]. The green nanoparticles can be associated with goal targets: “endeavor to decouple economic growth from environmental degradation” and “promote safe and secure working environments for all workers” [3].

• Industry, innovation, and infrastructure: With the enhancement of the research is possible to meet the targets: “Support domestic technology development, research and innovation in developing countries” and “Enhance scientific research, upgrade the technological capabilities of industrial sectors in all countries, in particular developing countries” [3]. With green nanotechnology is possible to collaborate to “upgrade infrastructure and retrofit industries to make them sustainable, with increased resource-use efficiency and greater adoption of clean and environmentally sound technologies and industrial processes, with all countries taking action in accordance with their respective capabilities” [3].

• Sustainable cities and communities: Synthesizing nanoparticles by a green process can contribute to targets “Strengthen efforts to protect and safeguard the world’s natural heritage” and “reduce the adverse per capita environmental impact of cities” [3]. It is because the green synthesis aims for the 12 PGCs, to trying not to pollute before, during, and after the process, besides using some green nanoparticles in environmentally friendly applications.

• Responsible consumption and production: The use the environmental-friendly feedstock in green synthesis of nanoparticles, such as food waste, contributes to goal 12 in specific targets such as “achieve the sustainable management and efficient use of natural resources,” “reduce food losses along production and supply chains, including postharvest losses,” “achieve the environmentally sound management of chemicals and all wastes throughout their life cycle,” and “substantially reduce waste generation through prevention, reduction, recycling and reuse” [3]. When a researcher changes one chemical in your synthesis by a waste, your synthesis can be more sustainable and further one or more targets cited.

• Climate action: Green synthesis favors the environment’s health, indirectly affecting climate health. When promoting research based on green synthesis of nanomaterials can promote the target “Improve education, awareness-raising and human and institutional capacity on climate change mitigation, adaptation, impact reduction, and early warning” [3].

• Life below water: Over time, the use of nanotechnology in water treatment is increasing. Consequently, in better water, the capacity of the life quality is better. By green synthesis, the quality of the water can be preserved without discard of hazardous chemicals, and the water quality can be recovered by water treatment using green nanotechnology. This way the targets achieved can be “prevent and significantly reduce marine pollution of all kinds, in particular from land-based activities, including marine debris and nutrient pollution” and “Minimize and address the impacts of ocean acidification, including through enhanced scientific cooperation at all levels” [3].

This way, green chemistry should not be seen just as a trend but as an alternative solution for the search for a more sustainable world.

8. Advantages, limitations, and future perspectives of the nanomaterial obtained by green synthesis

8.1 Advantages and limitations

The nanomaterials obtained by green synthesis have several advantages, such as no use of hazardous chemicals, reuse of different wastes, possibility to application in several areas, high specificity, less toxic effect, the feasibility of the approach, eco-friendly, cost-effective, among others [27, 38, 103].

When are using plant extracts the nanoparticles can be synthesized and functionalized by the same compound, decreasing one or more steps to it. The microorganisms can favor the resulting selectivity [27]. Several nanomaterials have excellent antioxidant and antimicrobial action besides body biocompatibility, which can be used in medicine and biomedicine applications. Green synthesis can collaborate with this particular characteristic [27, 38, 103].

Basically, the potential of green nanotechnology includes the “natural” feedstock used, nonuse and nonelimination of hazardous chemicals, easy process that possibility reproducibility, application of the principles of green chemistry, and care with the eco and human health [28, 34].

However, the several advantages of green-synthesized nanomaterials do not exclude their limitation, which can be the costs associated with manufacturing and processing on a large scale, the unknown toxicity, improper management after use, and the unknown reaction with other compounds after discard [34, 38, 93]. The incorrect manipulation of the nanomaterials, independent of their synthesis method, can cause an environmental risk instead of a solution and directly and indirectly affect soil, fauna, and humans [93].

Applying the nanoparticles with security requires several characterizations about their physical and chemical properties and surface and their behavior in different conditions, thus having a high cost. Another limitation is the unregulated application of the nanoparticles, which can pose potential risks to human and environmental health, mainly when applied in agrifood, cosmetic, and medicine industries [34, 93, 107].

The limitations need to be overcome so the nanoparticles collaborate with both principles of green chemistry and sustainable development goals. Otherwise, the synthesis can be green by PGC and SDG, but the materials obtained will go against hand.

8.2 Future perspectives

There has been an undiscussed expressive increase in the use of green synthesis for nanoparticles in the last few years. The more than 456 articles published in the last decade show it. However, there are gaps when discussing green synthesis, which needs more studies, exploration, and scientific knowledge. Future perspectives can collaborate to optimize and use synthesis and nanoparticles obtained, such as [37, 39]:

1. Many nonexplored natural materials can be used in green synthesis (biopolymers, plants, microorganisms, among others), mainly the nearby/regional feedstocks.

2. It needs better control of the size and shape of nanoparticles. Several synthesis methods possibilities the average size between an interval but not a standard size. Some applications require more precision.

3. It is very important know-how the behavior during the synthesis and about nanoparticles. The possible mechanisms can happen and secondary mechanisms if they exist. This makes synthesis methods predictable and consistent, even for producing the same material. Understanding the biological components, chemical agents, and molecular mechanisms involved in synthesis is also required.

4. The green synthesis is still primarily restricted to the laboratory phase. Scientific investigations are required to implement the industrial production of green nanomaterials.

5. Effectiveness risk management is necessary for the whole life cycle of green synthesis (production, handling, storage, and disposal). The toxic nature of nanomaterials is a serious concern.

9. Conclusion

In conclusion, the remarkable increase in published papers on green chemistry, particularly in the context of sustainable synthesis of nanomaterials, highlights the global commitment to more ecologically correct scientific practices. This trend emphasizes a concerted quest to adopt green principles for synthesizing nanomaterials.

By adhering to the 12 principles of green chemistry, researchers have explored the potential of green synthesis techniques, marking a new era in nanoparticle manufacturing characterized by minimal waste generation, reduced toxic emissions, and a shift toward renewable resources and sustainable processes. Integrating green synthesis practices for nanomaterials aligns with the United Nations Sustainable Development Goals (SDGs), significantly contributing to at least nine fundamental goals.

Despite the advantages, it is vital to recognize the limitations and challenges accompanying nanoparticle green synthesis. However, these limitations should clarify this field’s potential for growth and innovation. After all, the scientific community continues to explore, refine, and expand the boundaries of green synthesis. The future of the synthesis of green nanomaterials is promising, inviting us to explore its potential more profoundly and contribute significantly to a more sustainable world.

Acknowledgments

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001.

Conflict of interest

The authors declare no conflict of interest.