Green Synthesis of Nanomaterials : most cited papers and research trends

A bibliometric analysis using the Web of Science database was performed on the green synthesis of nanomaterials published between 2003 and 2017, obtaining a sample with a total of 159 publications. The number of citations of each paper, thematic areas, sources of publication and countries of origin were quantified. The ten most cited papers were described in further detail, listing the precursor materials, the kind of material produced among other characteristics. There is a variety of precursors listed in the literature, including fungi, bacteria, plants, extracts from plants and fruits. Although United States was a pioneer and is influent in this line of research, India is taking the lead due to government incentives and the presence of natural resources having a total of 44.65% of the analyzed works. Silver oxides is Research, Society and Development, v. 9, n.1, e54911593, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i1.1593 2 the material most produced by this method, according to the most cited works and are commonly applied in antibacterial activity. The main areas of discussion at WoS for this topic are the Chemistry and Science Technology categories with 55 papers each. The main sources of publication are the Colloids and Surfaces B biointerfaces and RSC Advances with 3,774% in each journal in relation to the study sample. Potentialities of the new nanomaterials production method that could be exploited by other researchers from the use of new precursors, including solid agroindustrial wastes, with the opportunity to add value to the biomass and consequently to reduce environmental impact generated by by-products.


Source: Prepared by the authors.
Within the concept of sustainability, the reuse of residues, mainly from fruit peels for production of nanoparticles is according to the guidelines established by legislation in several countries. In Brazil, Federal Law n. 12305/2010 (Lei, 2010), which establishes the National Policy on Solid Waste -PNRS, determines the application of reverse logistics, characterized as an instrument for waste management, enabling its collection, restitution, recycling or adequate disposal (Rogers & Tibben-Lembke, 2001).
To explain the influence of the "green precursors" during the synthesis stages of the nanostructured compounds, several mechanisms are proposed, such as were reported by the authors Makarov and co-workers (Makarov et al., 2014), who consider plant extracts and their functional groups in the role of ligands to the metallic ions responsible for the chemical reduction of the metal atoms and stabilizing the particles. In other studies, the chemical groups carbonyl, thiol, and Amina are associated with the potential for the reduction of metallic ions and the favoring of the growth of the crystals (He et al., 2007). Research, Society and Development, v. 9, n.1, e54911593, 2020(CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i1.1593 Due to the improvement in the resources of freely available databases, the increasing complexity of the research fields and the need for accountability in governmental spending, the use of bibliometric indicators to measure productivity in science and technology is needed. Where bibliometrics is defined as the use of statistical counting of articles, patent documents, citations, words and terms to find trends and patterns in the development of a research field (Milanez, Schiavi, Amaral, Faria, & Gregolin, 2013).
In this paper a bibliometric analysis was used to understand the evolution of the GSN field, including the uses of this technology, which countries are central in its development, if there is any correlation between government policy and a preference for green synthesis, what journals, and papers are the most cited in the area.

Data source
A bibliometric survey was carried out through the Web of Science (WoS) database.
The WoS database was chosen, because of its academic recognition on the world stage, the leading publications of various areas are listed on it. WoS has more than 12000 journals indexed. The Keywords used were "green synthesis" AND "biosynthesis" AND "nanomaterials," the data was checked up to 2017. The reported journal impact factor was taken from the JCR Science Edition 2017.
The empirical parameter Betweenness Centrality (BC) was obtained by CiteSpace (Chen, 2017). High values of BC indicate a dominant influence on the transfer of items through networks, if item transfer follows the shortest paths. Robust BC nodes tend to be part of joints bridging the tightly connected sub-graphs that represent discovered knowledge communities (Zhu & Hua, 2017).

Source: Prepared by the authors.
The top ten articles classified by the number of citations were studied in further detail.
It is understood that these works are the source of reference for the researchers of the area and will allow the greater understanding of the subject under study.

Number of publications and citations.
There were 159 publications in the WoS database that match the search criteria. The

WoS categories
Research, Society and Development, v. 9, n.1, e54911593, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i1.1593 8 The areas with greater relevance to the production of nanomaterials by the methodology of green synthesis or biosynthesis are chemistry, science and technology, and material sciences, as illustrated in Table 1. These results illustrate the striking multidisciplinary characteristic of the theme. The Chemistry category (55) and Science Technology (55) have the highest number of papers on the green synthesis methodology, but Chemistry has a higher parameter BC, and this value can be related to its presence into a more substantial number of journals in WoS. For the entire study sample, the Biotechnology  Overlay maps using CiteSpace shows to the evolutionary relationship of the thematic areas on Green Synthesis Nanoparticles ( Figure 5). In the period evaluated 2013-2017 were Research, Society and Development, v. 9, n.1, e54911593, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i1.1593 9 found 26 nodes, 5% nodes labeled. The results for modularity Q and the mean silhouette scores were 0.4783 and 0.7347, respectively. These two metrics were used to evaluate the overall structural properties of the network. Q value was minor than 50%, meaning that the network is low when divided into loosely coupled clusters. Since the mean silhouette score was 0.7347, suggesting that the homogeneity of these clusters on average are high. It is possible to note that the silhouette plot displays a measure of how close each point in one cluster is to points in the neighboring clusters. Another important fact is that two or more specific areas showed interfaces indicating possible networking.

Sources of publication
The Top 20 journals with more publications in the area under study are listed in Table   2. To classify the journals the Journal Impact Factor (JCR) representing citation activity in  Table 2 it is possible to see that the two journals with higher JCR, "Colloids and Research, Society and Development, v. 9, n.1, e54911593, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i1.1593 Surfaces B-biointerfaces" e "RSC Advances," are also the two with a higher number of publications in the area a strong indication that citations follow publication. There is no predominance of a given journal tough.
Comparing the list in Table 2 with the correlation between areas seen in Figure 5 it is possible to see that at least the seven journals with more publications in the area have words related to the central hub in their titles. They fit in the categories of chemistry, biochemistry and material sciences that appear strongly interrelated in the figure.

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The division of publication by country of origin is represented in Table 3 and Figure 6.
The country that is most prominent in the production of nanomaterials by this new methodology of synthesis is India that was the origin of 44.65% of the articles of the sample studied, corresponding to a total of 71 works. Followed by China and the USA, with a total of 17 publications each. India presents the node with the highest centrality parameter (BC) this characteristic means influence on the researchers of the sample because it connects two or more large groups of nodes. The USA, despite the smallest number of published works, has a high BC value that may be associated with the early start of the researches in the area (2006). India, at least up to 2015 was not a leading country in nanoscience and nanotechnology as a whole. According to Zibareva (Zibareva, 2015), China, United States, France, Germany, South Korea, and Japan are the leading countries in the broader nano research field. The prominence of India in the specific area of green synthesis shows a local preference, probably due to natural resources. The growth of research in India in the field of nanotechnology is justified, since, from the year 2000, significant investments were fomented with the Program on Nanomaterials: Science and Devices by the Department of Science and Technology (DST), with initiatives of support to projects related to nanomaterials and their technologies (A. Kumar, 2014). Figure 7 shows the co-authorship between the countries represented by the nodes, the orange circle's size is related to the number of papers.

Source: Prepared by the authors.
In continental terms, there is a predominance of the Asian continent, as the origin of the works developed and published, in addition to the interaction of researchers from different countries divided into six main clusters. Research, Society and Development, v. 9, n.1, e54911593, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i1.1593 13

Top 10 WoS research papers as classified by the number of citations
The ten most cited articles are displayed in Table 4. As might be expected, the four papers with more citations are reviews that report the experimental procedures and the products obtained by green synthesis of nanomaterials. Most of the ten main Works deal with silver oxide synthesis.  Research, Society and Development, v. 9, n.1, e54911593, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i1.1593 15

Main keywords
The keywords used in this research (biosynthesis, green synthesis, and nanomaterial) are often followed by ones related to gold and silver nanoparticles (Table 5) and Figure 7. The application of these materials in antibacterial activity has a large number of occurrences in the study sample. The same behavior was observed in the parameter of centrality for the corresponding category of WoS (Biochemistry Molecular Biology, see Table 1).

Conclusions
It was observed with the bibliometric analysis that the research related to the subject in the study, green* synthesis and biosynthesis of nanomaterials, has evolved significantly in recent years, the result expressed by the most significant number of published papers and by Research, Society and Development, v. 9, n.1, e54911593, 2020(CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i1.1593 16 the interest of the researchers evidenced in the significant increase in the number of quotes.
The multidisciplinarity of the subject under study is seen from the diversity of areas where it appears, including chemistry, materials science and the environment, and applications focused on the medical area. As for the leading periodicals in which the works are published, the same multidisciplinary behavior is observed, characterized by documents in journals of an environmental nature, as well as with sources of the discussion focused on the area of chemistry and materials.
About 82.0 % of the published material in this subject is comprised of papers. More than forty percent of these papers come from India (44.65 %), proving that this country is the current leader in this production methodology.
Six of the ten most cited Works in this subject review, in which a diversity of precursors is for green synthesis and biosynthesis are presented, including plants, bacteria, fungi, plant extracts, and fruits, illustrating the search for new materials and substrates. The most common papers though are on the synthesis of silver oxides due to their large area of application, mainly in biomedicine.
Finally, it is hoped that through the results presented here, the interest and motivation of new researchers to explore different residues for the synthesis of materials, especially the agroindustrial ones, that need a suitable destination according to the directives of the PNRS. Furthermore, in new review studies, it is necessary to expand the databases during refinement to cover a larger number of articles.