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Review

Is ‘Green’ Gold and Silver Nanoparticle Synthesis Environmentally Friendly?

by
Lucas Reijnders
IBED (Institute for Biodiversity and Ecosystem Dynamics), Faculty of Science, University of Amsterdam, Science Park 904, 1090 GE Amsterdam, The Netherlands
Nanomaterials 2025, 15(14), 1095; https://doi.org/10.3390/nano15141095
Submission received: 25 June 2025 / Revised: 8 July 2025 / Accepted: 10 July 2025 / Published: 14 July 2025
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Abstract

In scientific literature biosynthesis of gold and silver nanoparticles and synthesis of these nanoparticles using small organic molecules such as citrate have been called: ‘green’. It has also been often stated that ‘green’ synthesis of gold and silver nanoparticle is environment(ally) friendly or ecofriendly. The characterization environment(ally) friendly or ecofriendly is commonly comparative. The comparison is between ‘green’ and ‘chemical’ synthesis. The few available comparative life cycle assessments addressing the environmental impacts of ‘green synthesis’ of Ag and Au nanoparticles, if compared with ’chemical’ synthesis, strongly suggest that a ‘green’ synthesis should not be equated with being environment(ally) friendly or ecofriendly. The term ‘green’ for Au and Ag nanoparticles obtained by ‘green’ synthesis is a misnomer. There is a case for only using the terms ecofriendly or environment(ally) friendly for nanoparticle synthesis when there is a firm basis for such characterization in comprehensive comparative cradle-to-nanoparticle life cycle assessment, taking into account the uncertainties of outcomes.

Graphical Abstract

1. Introduction

One of the ways to synthesize gold or silver nanoparticles is based on the reduction of a gold or silver salt (commonly HAuCl4, AgNO3) by strong reductants such as hydrazine (N2H4) and sodium borohydride (NaBH4). The negative impacts of these reductants and of chemical substances used ln the coating of silver and gold nanoparticles has led to much research focussing on the use of more benign reductants and coatings in gold and silver nanoparticle synthesis [1,2]. The results thereof are rather often described in terms of ‘green nanoparticle synthesis’. This is illustrated by Table 1 which includes the results of the title search in the Core Collection of the Web of Science for the ‘green synthesis’ of silver and gold nanoparticles. In the title search about 36% of the scientific papers dealing with the synthesis of silver nanoparticles regarded ‘green synthesis’ and the corresponding percentage regarding gold nanoparticles was about 20%. ‘Green’ synthesis of Ag and Au nanoparticles is an active field. From 30 May 2024 to 30 May 2025, in the Core Collection of the Web of Science, the average monthly increase of titles with ‘green synthesis of silver nanoparticles’ was 36, and of titles with ‘green synthesis of gold nanoparticles’ 5. There are occasionally titles of scientific papers dealing with ‘green’ synthesis of silver or gold nanoparticles in which the product is characterized as ‘green silver nanoparticles’ [3,4,5,6,7,8,9,10,11,12,13,14,15,16] or ‘green gold nanoparticles’ [17,18,19,20,21,22,23,24].
There are two main strands in ‘green’ synthesis of gold and silver nanoparticles. Firstly, there is the use of small organic molecules serving as reductant. This type of synthesis is occasionally called ‘green’ in scientific literature. Secondly, and as to the number of published studies more importantly, there is biological synthesis or biosynthesis of gold and silver nanoparticles. This type of synthesis is often called ‘green’.
Research regarding nanoparticle synthesis, using small organic molecules started long before the term ‘green’ emerged in scientific papers about nanoparticle synthesis. The use of citrate as a reductant for HAuCl4 was pioneered by Turkevich et al. [25] and published in 1951. It served the synthesis of (near-)spherical gold colloids coated with citrate in an aqueous solution near boiling point. The Turkevich method can also be used to generate Ag nanoparticles from AgNO3 [26]. In scientific literature, citrate used in the synthesis of silver or gold nanoparticles has been characterized as ‘green’ [1,27,28,29], but also as ‘chemical’ [30,31]. This shows that the terms in practice lack precision. The citrate-based synthesis of gold nanoparticles has been developed and modified to generate a variety of shapes and sizes [29,32,33]. Other small organic molecules used as reductants in the synthesis of silver or gold nanoparticles are various amino acids [34,35], vitamin B [1], glucose [36,37], gallic acid [38,39] and ascorbate [40,41]. Amino acids, ascorbic acid, gallic acid and glucose can also serve as coating or capping agents for nanoparticles [38,42,43].
Biosynthesis of Ag and Au nanoparticles may utilize bacteria, fungi, microalgae, viruses, plants and substances derived from bacteria, fungi, algae and plants [2,5,9,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58]. In biosynthesis, reductants present in, or derived from, organisms are used to generate Au and Ag nanoparticles from respectively HAuCl4 and AgNO3. Additionally, in the biosynthesis of gold and silver nanoparticles substances present in, or derived from, organisms can serve as nanoparticle coatings [53,57,59,60]. Within the field of nanoparticle biosynthesis, studies using plant extracts are by now dominant [61].
In scientific publications ‘green’ Ag and Au nanoparticle synthesis is often characterized as ecofriendly, respectively environment(ally) friendly (see the all fields search in Table 1). The terms ecofriendly and environment(ally) friendly are commonly considered to be synonyms [62]. Their use is not precise: the terms may indicate that the synthesis has no environmental burden or has a minimal environmental burden [62]. The term may also be used in a comparative sense [62]. For instance, a synthesis is environmentally friendly because it has, in comparison with other ways to synthesize the product, a lower environmental burden [62]. In Section 2, a brief description is given of the method used in preparing this manuscript. Section 3 will deal with the questions in what sense the terms environment(ally) friendly and ecofriendly are used, and whether ‘green’ synthesis of Ag and Au nanoparticles is indeed ecofriendly or environment(ally) friendly. Section 4 will present the conclusions of this paper and addresses future directions.

2. Method

Databases of major publishers of scientific literature dealing with nanomaterials (ACS, Elsevier, Frontiers, IOP Publishing, MDPI, RSC, Sage, Springer-Nature, Taylor and Francis, Wiley), the Web of Science core collection and Google Scholar have been searched for publications since 1950 relevant to the subjects raised in this paper.

3. Is ‘Green’ Synthesis of Gold and Silver Nanoparticles Ecofriendly or Environment(ally) Friendly?

In recent scientific papers claiming that biosynthesis of gold and silver nanoparticles is environment(ally) friendly or ecofriendly [8,9,19,30,53,56,57,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94] the terms are used in a comparative sense, comparing ‘green ’synthesis with ‘chemical’ synthesis. A recurrent theme is that substances used in ‘green’ synthesis are preferable if compared with substances used in ‘chemical’ synthesis. The latter are characterized as hazardous, having a negative impact on the environment or the ecosystem, dangerous, not environmentally benign, harmful, polluting, respectively toxic or having toxic by-products or wastes.
Replacing substances used in ‘chemical’ synthesis by biological substances, or small organic molecules may be conducive to applications in biomedicine, but is not the only determinant of environmental burden relevant to comparisons with ‘chemical’ synthesis. The following matters may also be important to such comparisons. The first is the input of energy in ‘green’ nanoparticle synthesis. ‘Chemical’ synthesis with the reductants N2H4 and NaBH4 can take place at room temperature [95,96,97]. Synthesis at room temperature may also apply to ‘green’ synthesis, but longer reaction times for ‘green’ synthesis, if compared with ‘chemical’ synthesis, may lead to an increase in the cradle-to-nanoparticle environmental burden linked to energy use for agitation (stirring), lighting and temperature control [1,98]. Also, temperatures may be higher than room temperature. The ‘green’ citrate-based method to generate Au nanoparticles developed by Turkevich et al. [25] used boiling water. Elevated temperatures (up to 121 °C) are furthermore used in ‘green’ synthesis of gold and silver nanoparticles using plant-derived substances [85,91,99,100,101,102,103] and fungal extracts or filtrates [104,105,106]. Also, when (macro)algal extracts are used, ‘green’ synthesis of gold and silver nanoparticles at elevated temperatures has been reported [50,107,108,109]. Reaction times in ‘green’ synthesis may be shortened by ultrasound- and microwave-assisted synthesis [90,110,111]. Such shortening can require substantial energy inputs [112,113]. Papers dealing with the ‘green’ biosynthesis of silver nanoparticles [114,115] have included energy-intensive calcination [116]. Use of energy-intensive liquid phase plasma technology [117] has been reported for the ‘green’ synthesis of silver and gold nanoparticles [118,119], and (high-energetic) gamma radiation for the biosynthesis of gold and silver nanoparticles [120,121].
Secondly, the synthesis of gold and silver nanoparticles results in non-product outputs, containing Au and Ag [122]. If compared with ‘chemical’ synthesis, ‘green’ synthesis of silver and gold nanoparticles may have relatively large non-product outputs of Ag and Au [1,28,123]. Lower yields in ‘green’ synthesis if compared with ‘chemical’ synthesis would, ceteris paribus, lead to a relatively large environmental burden per unit of mass for nanoparticles produced by ‘green’ synthesis, if compared with ‘chemical’ synthesis. This is not necessarily the case, as shown by the high percentage of product yield from HAuCl4 for citrate-based Au nanoparticle synthesis [33], see also Table 2. As to biosynthesis of Ag and Au nanoparticles quantified product yields (% of Ag or Au input) are rarely reported. However, recent publications have been found giving percentual yields for optimized laboratory-scale biosynthesis of gold and silver nanoparticles (see Table 2), which are lower than reported percentual yields for optimized ‘chemical’ synthesis.
From the studies presented in Table 2 percentual non-product outputs of Ag and Au in optimized nanoparticle biosynthesis can be derived. For gold nanoparticles the reported non-product Au outputs are in 27.3–37.5% range [111,124] and for silver nanoparticles the reported Ag non-product output is 46.7% [103]. An environmentally sensible way to deal with Au and Ag in non-product outputs is functional recycling: use of the non-product output as starting material for the synthesis of functional gold and silver nanomaterials [122,128]. No example of such recycling following ‘green’ Ag and Au nanoparticle synthesis could be found in scientific literature, though there are studies regarding the functional recycling of non-product outputs containing Ag and Au following ‘chemical’ Ag and Au nanoparticle synthesis [122]. In the absence of recycling, shortfalls of available bulk Ag and Au have to be made up with Ag and Au originating in mining, which are associated with large cradle-to-metal environmental burdens, if compared with recycled Ag and Au [129].
Thirdly: when microorganisms, or substances derived thereof [129,130,131] are used in nanoparticle synthesis, cultivation and cultivation media can have a substantial impact on the cradle-to-nanoparticle environmental burden [129,132,133,134,135]. Cultivating plants to obtain plant-derived substances for the biosynthesis and coating of nanoparticles may also be associated with substantial environmental burdens [136,137,138,139,140,141]. In view of the variety of factors that may impact the environmental burden, cradle-to-nanoparticle comprehensive comparative assessments of environmental burdens using life cycle assessment (LCA) methodology would seem a proper way to determine whether a ‘green’ synthesis of Ag and Au nanoparticles is indeed ecofriendly or environment(ally) friendly, if compared with a ‘chemical’ synthesis (also: [61]).
From searching, as outlined in Section 2, a few scientific studies emerged evaluating the comparative characterization of ecofriendly or environment(ally) friendly for ‘green’ synthesis of Ag or Au nanoparticles, from-cradle-to-nanoparticle. The studies used environmental life cycle assessment methodology (e.g., [142]). In life cycle assessment, comparisons are made on the basis of using the same functional unit. A functional unit is in practice a quantitative aspect of the product and may for instance be mass-based (e.g., 1 g of product consisting of silver nanoparticles) or regarding functionality (e.g., catalytic activity in terms of 1 katal or 1 mole converted per second). The LCA studies to be discussed here used the synthesis of a specified mass of nanoparticles as functional unit for comparing different syntheses.
Pati et al. [1] studied the laboratory-scale synthesis of Au nanoparticles from HAuCl4 using a variety of reductants. The impact categories of the LCA considered by Pati et al. [1] are in Table 3 and the reductants studied in Table 4. They found that the ‘green’ reductants do not necessarily reduce the cradle-to-nanoparticle environmental burden of Au nanoparticle synthesis, if compared with the chemical reductants [1]. As to the laboratory-scale synthesis of Au nanoparticles using sodium borohydride and 13 ‘green’ reductants, the cumulative energy demand for the synthesis with sodium borohydride was found by Pati et al. [1] to be lower than the corresponding value for the syntheses using citrate or glucose. The use of organism-derived reductants often led to a higher cumulative energy demand than the use of sodium borohydride [1]. In life cycle assessments of products using a wide range of environmental impact categories cumulative energy demand is a major determinant of the overall environmental burden [143]. Pourzahedi and Eckelman [27] compared the laboratory-scale synthesis of Ag nanoparticles from AgNO3 using the reductants borohydride, ethylene glycol, citrate and starch, using cradle-to-nanoparticle environmental assessment covering a range of environmental interventions (see Table 3). They found that the synthesis using borohydride as a reductant had overall a lower environmental burden than the syntheses applying the reductants citrate and soluble starch.
Interestingly, Pourzahedi and Eckelman [27] also found that the production of bulk silver starting with mining, the main source of bulk silver [129], dominated all impact categories of the cradle-to-silver-nanoparticles environmental burden, contributing up to >90% of the burden. Similar conclusions were drawn by Bafana et al. [112] and by Han et al. [129]. In view thereof, it seems farfetched to characterize Ag nanoparticles as ‘green’ [3,4,5,6,7,8,9,10,11,12,13,14,15,16] because the conversion of AgNO3 into Ag nanoparticles, which has only a small overall contribution to the life cycle environmental burden of Ag nanoparticles, is characterized as ‘green’. Also, when ’green’ is attributed to products the term is commonly considered to be interchangeable with environment(ally) friendly or eco-friendly [62], which is at variance with the findings of Pourzahedi and Eckelman {27]. In view thereof, it can be concluded that characterizing silver nanoparticles obtained by ’green synthesis as ‘green’ is a misnomer. The characterization ‘green gold nanoparticles’ [17,18,19,20,21,22,23,24] would appear to be even more of a misnomer because the life cycle environmental burden of bulk gold obtained from mining is more than a factor 10 larger than the lifecycle environmental burden of silver obtained from mining [144].
Sierra et al. [123] performed a comparative environmental LCA (with the impact categories presented in Table 3) for the ‘ecofriendly’ laboratory-scale synthesis of silver nanoparticles using leaf extracts from agricultural plants and the corresponding chemical synthesis using sodium borohydride. The assessment was partly cradle-to-washed-nanoparticles. The environmental burden of agricultural production for the supply of leaves was not included. The overall environmental burden of the ‘ecofriendly’ synthesis of Ag nanoparticles, as found by Sierra et al. [123], was larger than the burden of the synthesis using sodium borohydride. Han et al. [129] compared emerging methods for the synthesis of Ag nanoparticles from AgNO3 using polyethyleneimine, leaf extract from Annona glabra and Rhodococcus bacteria, applying a prospective LCA considering the industrial scale and a large range of environmental impacts (see Table 3). They found that the prospective industrial-scale synthesis using polyethyleneimine was linked to a lower cradle-to-nanoparticle greenhouse gas emission than the syntheses using leaf extract from Annona glabra and Rhodococcus, whereas leaf extract was linked with a relatively high cradle-to-nanoparticle marine eutrophication impact, and Rhodococcus with a relatively high cradle-to-nanoparticle use of resources. Han et al. [129] concluded that using leaf extract from Annona glabra and Rhodococcus bacteria does not guarantee an environmentally friendlier outcome.
Two comments may be made about the comparative life cycle assessments presented here. Firstly, per unit of product mass, the environmental burdens of laboratory-scale syntheses, as studied by Pati et al. [1], Pourzahedi and Eckelman [27] and Sierra et al. [123], may differ substantially from the environmental burdens of industrial-scale production. This is suggested for Ag nanoparticles by the study of Han et al. [129] comparing laboratory-scale and prospective industrial syntheses and applying sensitivity analysis. This is also suggested for gold nanoparticles by comparing the results of Pati et al. [1] for laboratory-scale synthesis of gold nanoparticles using citrate with the prospective LCA study of Grimaldi et al. [145] regarding an industrial-scale synthesis of gold nanoparticles using citrate. It should be noted, though, that anticipatory or prospective life cycle assessments are characterized by relatively large uncertainties [146], a matter not addressed in the study of Grimaldi et al. [145]. Secondly, it may be argued that for comparative studies a functional unit for life cycle assessment that focuses on functionality per unit of mass of the nanoparticles synthesized (e. g. catalytic activity as katal) would be more suitable than a functional unit purely based on mass [61,147]. It should be noted, though, that in nanoparticle biosynthesis the reproducibility of functionality (and size, morphology and coating) is as yet problematical [54,93,148,149,150].
Still, the comparative studies of Pati et al. [1], Pourzahedi and Eckelman [27], Sierra et al. [123] and Han et al. [129] strongly suggest that ’green’ syntheses of Ag and Au nanoparticles should not be equated with environment(ally) friendly or ecofriendly. There is a case for only using the terms ecofriendly or environment(ally) friendly as to Ag and Au nanoparticle synthesis in scientific papers when there is a firm basis for such characterization in comprehensive cradle-to-nanoparticle environmental assessment, taking into account the uncertainties of outcomes.

4. Conclusions and Future Directions

The few available comparative life cycle assessments addressing the environmental impacts linked to ’green synthesis’ and ’chemical’ synthesis of Ag and Au nanoparticles strongly suggest that being ’green’ should not be equated with being environment(ally) friendly or ecofriendly. In view thereof, there is a case for using the terms ecofriendly or environment(ally) friendly regarding gold and silver nanoparticle synthesis only when there is a firm basis for this characterization in comprehensive cradle-to-nanoparticle environmental assessment, while taking account of uncertainties. Characterizing gold and silver nanoparticles as ’green’ when they are obtained by ’green’ synthesis is a misnomer. There is a case for researchers to focus on what can actually be done to lower the life cycle environmental burden of Ag and Au nanoparticles, making them more environment(ally) friendly. Implementing options to reduce the life cycle environmental burden may be conducive to the social acceptance of nanoparticle applications (e.g., [151]). Comprehensive life cycle assessment may be helpful in reducing the life cycle environmental burden of nanoparticles [1,27,128,132,147,152,153]. Higher product yields, recycling of non-product outputs, improved resource-efficiency and substitution of fossil fuel-based energy by solar energy are examples of options that may reduce life cycle environmental burdens. To facilitate comprehensive cradle-to-nanoparticle assessment, improvement of the database for nanoparticle biosynthesis is a matter of importance. In view thereof it is preferable that future publications presenting biosynthesis of Ag and Au nanoparticles include (so far often unreported) quantitative data about product yields (as % of noble metal input), product functionality per unit of mass, reproducibility and fate of non-product outputs.

Funding

The research for this paper received no external funding.

Data Availability Statement

Data supporting the findings are available in the references of the paper.

Acknowledgments

The author thanks three anonymous reviewers for their comments.

Conflicts of Interest

There are no conflicts of interest relevant to the content of the paper to declare.

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Table 1. Results of searches in the Core Collection of the Web of Science (consulted 30 May 2025) referred to in this paper.
Table 1. Results of searches in the Core Collection of the Web of Science (consulted 30 May 2025) referred to in this paper.
Scope of SearchWords Used in SearchNumber of Hits
Title‘green synthesis silver nanoparticles’2184
Title‘green synthesis gold nanoparticles’.648
All fields‘ecofriendly green synthesis silver nanoparticles’607
All fields‘environmentally friendly green synthesis silver nanoparticles’1411
All fields‘ecofriendly green synthesis gold nanoparticles’279
All fields‘environmentally friendly green synthesis gold nanoparticles’690
Table 2. Percentual product yields for optimized laboratory-scale synthesis of gold and silver nanoparticles from HAuCl4 and AgNO3.
Table 2. Percentual product yields for optimized laboratory-scale synthesis of gold and silver nanoparticles from HAuCl4 and AgNO3.
Type of Synthesis Nanoparticle Product Yield (% of Ag or Au Input)References
BiosynthesisAu (mostly spherical)62.5–72.7%[111,124]
‘Chemical’ synthesisAu (various shapes)Up to 95%[125]
Citrate-based synthesis, with traces tanninsAu (quasi-spherical)>90%[33]
BiosynthesisAg (roughly spherical)53.3%[103]
‘Chemical’ synthesisAg (crystalline nanoparticles, nanowires)>95%, 99%[126,127]
Table 3. Impact categories studied in the comparative life cycle assessments discussed in this paper.
Table 3. Impact categories studied in the comparative life cycle assessments discussed in this paper.
Study Environmental Impact CategoriesNanoparticles and Scale
Pati et al. [1]Cumulative Energy Demand
Global Warming Potential
Metal Depletion Potential
Agricultural Land Occupation
Fresh Water Ecotoxicity		Gold nanoparticles, laboratory-scale
Pourzahedi and Eckelman [27]Global Warming Potential
Ozone Depletion Potential
Oxidizing Smog Formation
Respiratory Effects
Acidification
Human Toxicity
Ecotoxicity		Silver nanoparticles, laboratory-scale
Sierra et al. [123]Abiotic depletion
Global Warming
Ozone layer Depletion
Human Toxicity
Fresh Water Toxicity
Terrestrial Ecotoxicity
Photochemical Oxidation
Acidification
Eutrophication		Sliver nanoparticles, laboratory-scale
Han et al. [129]Fossil fuel extraction
Ore extraction
Water consumption
Agricultural land occupation
Global warming
Ozone layer depletion
Ionizing radiation
Fresh water eutrophication
Fresh water ecotoxicity
Human carcinogenicity
Non-carcinogenic human toxicity
Marine eutrophication
Marine ecotoxicity
Particulate matter formation
Photochemical oxidant formation
Terrestrial eutrophication
Terrestrial ecotoxicity		Silver nanoparticles, prospective industrial scale and laboratory-scale
Table 4. Reductants considered in the lifecycle assessment studies of Pati et al. [1], Pourzahedi and Eckelman [27], Sierra et al. [123] and Han et al. [129].
Table 4. Reductants considered in the lifecycle assessment studies of Pati et al. [1], Pourzahedi and Eckelman [27], Sierra et al. [123] and Han et al. [129].
StudyChemical ReductantsSmall Organic Molecules (‘Green’)Organism-Derived Reductants and Organisms (‘Green’)
Pati et al. [1]Sodium borohydride
Hydrazine		Citrate
Vitamin B
D-glucose		C. alba extract
C. camphora
Cinnamon
Coriander
Cypress leaf extract
Ginseng
Grape pomace
Mushroom extract
Soybean seed extract
Sugarbeet pulp
Pourzahedi and Eckelman [27]Sodium borohydride
Ethylene glycol		CitrateSoluble Starch
Sierra et al. [123]Sodium borohydride Plant extract
Han et al. [129]Polyethyleneimine Plant extract from Annona glabra
Rhodococcus
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Reijnders, L. Is ‘Green’ Gold and Silver Nanoparticle Synthesis Environmentally Friendly? Nanomaterials 2025, 15, 1095. https://doi.org/10.3390/nano15141095

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Reijnders L. Is ‘Green’ Gold and Silver Nanoparticle Synthesis Environmentally Friendly? Nanomaterials. 2025; 15(14):1095. https://doi.org/10.3390/nano15141095

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Reijnders, Lucas. 2025. "Is ‘Green’ Gold and Silver Nanoparticle Synthesis Environmentally Friendly?" Nanomaterials 15, no. 14: 1095. https://doi.org/10.3390/nano15141095

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Reijnders, L. (2025). Is ‘Green’ Gold and Silver Nanoparticle Synthesis Environmentally Friendly? Nanomaterials, 15(14), 1095. https://doi.org/10.3390/nano15141095

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