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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5632_Библиотеки_им_академика_М_И_Перельмана.pdf
276 K. Tani et al.
distribution was obtained. In addition, antimicrobial tests of the prepared silver
nanoparticles against the model microorganisms Escherichia coli and Bacillus
subtilis showed that the silver nanoparticles have antibacterial activity against
both bacteria. In conclusion, antimicrobial silver nanoparticles can be prepared
using the waste parts of crude drug plants for traditional medicinal use in Nara,
Japan, without the emission of effluents containing organ
ic solvents or strong
chemical reducing agents.
Keywords
Nanoparticles · Silver · Antibacterial · Medicinal plant · Waste · Plant extract ·
Paeonia lactiflora
11.1 Introduction
Nanomaterials, mainly metal nanoparticles, are expected to be applied in various
fields, such as biomedicine (Hang et al. 2024), energy (Tony 2021), and catalysis for
material synthesis (Ishi da et al. 2020), due to their excellent properties. However,
their preparation often involves chemical reactions in the liquid phase (Park et al.
2007), which are often not eco-friendly because they use organic solvents and highly
reactive chemical reagents at high temperatures, resulting in the discharge of large
amounts of liquid waste after preparation and high energy consumption. In order for
our society to continue to take advantage of the wonderful properties of
nanoparticles, it is essential to establish sustainable and eco-friendly nanoparticle
preparation methods.
This paper fi
chemical reactions in liquid-phase systems and then introduces recent topics on the
preparation of eco-friendly metal nanoparticles using plant extracts. Furthermore,
examples of sustainable preparation of metal nanoparticles using waste materials are
presented. In the last section, we present our experimental data on the preparation of
antibacterial Ag nanoparticles using waste products from traditional medicinal plants
in Nara, a world heritage site.
outlines the conventional preparation of metal nanoparticles by
rst
11.2 Eco-Friendly Synthesis of Metal Nanoparticles Using Plant
Extracts
In general, metal nanoparticles are prepared by reducing metal ions with a strong
chemical reducing agent in the presence of a surfactant as a phase transfer agent in an
oil/water two-phase system. The most representative method is that reported by
Brust et al. at Liverpool University in 2004 and is called the Brust–Schiffrin method
(Brust et al. 1994). Using two-phase (water-toluene) reduction of AuC1
sodium borohydride in the presence of an alkanethiol after phase transfer by
ammonium surfactant, solutions of 1–3 nm gold nanoparticles bearing a surface
4-
by
11 Green and Sustainable Synthesis of Silver Nanoparticles Using Wastes… 277
coating of thiol were prepared. The method enabled the room-temperature synthesis
of highly stable functionalized metal nanoparticles of small size (2–2.5 nm). The
proposed method significan tly impacted the pace of subsequent metal nanoparticle
synthesis. A wide variety of functionalized metal nanoparticles have been
synthesized using this method over the years, which has also led to the development
of new scientific fields (Templeton et al.
advantage of accurate
preparation control, but it is not eco-friendly because it
2000; Pileni 2023
). The method has the
produces a large amount of liquid waste including organic solvents, chemical
reducing agents, and surfactants.
On the other hand, for the synthesis of metal nanoparticles using a single phase,
the Turkevich method (Turkevich et al.
1951) is a relatively simple and reproducible
technique for the synthesis of spherical Au nanoparticles between 10 and 30 nm.
However, above 30 nm size the nanoparticles become less spherical, the size
distribution becomes broader, and the results were less reproducible for the synthesis
of Au nanoparticles. In contrast to Au nanoparticles obtained by the Turkevich
method, Ag nanoparticles obtained by this method are much larger (60–200 nm)
with high polydispersity and different morphologies (Wuithschick et al. 2015).
For centuries, traditional extracts from plants, spices, and culinary herbs have
been added to enhance the flavor and extend the shelf life of foods. These extracts are
rich in compounds with high antioxidant activity, such as polyphenols, carotenoids,
and vitamins E and C (Embuscado 2015; López-Pedrouso et al. 2022). Numerous
studies have been conducted on the preparation of metallic nanomaterials by reducing metal ions using the reducing properties of plant components. The preparation
methods of nanomaterials using plant components are eco-friendly because they
produce no liquid waste including organic solvents and chemical reducing agents.
In t
reparation of nanomaterials using plant components, Easy availability of
he p
plants Not competing with food use are important. A similar example is the issue of
competition with food production in bioethanol production. Bioethanol is made from
sugars and starches, such as corn and sugarcane which are used as food for people
and livestock. As the demand for bioethanol increases, crops that would normally be
used as food are used for bioethanol production, resulting in food shortages and
higher crop prices.
Therefore,
the preparation of metal nanoparticles using easily available plant
extracts has been reported (Song et al. 2009; Song and Kim 2009; Huang et al.
2011; Lu et al. 2014). Kim’s group in Cheongju, Korea prepared Au nanoparticles
using the extracts of Magnolia kobus and Diopyros kaki leaves (Song et al. 2009
Magnol
ia kobus is a species of Magnolia native to Japan and Korea, and is widely
planted for ornamental purposes. Diopyros kaki, the oriental persimmon, is native to
East Asia and is grown in Japan, Korea, and China. Because the ripe fruits are edible,
the tree has been widely grown as a fruit tree in Asia. In modern times, it is also
grown and consumed outside East Asia, such as Europe. The authors proposed a
method for preparing Au nanoparticles using the aqueous extracts of common tree
leaves and reported more than 90% conversion within a few minutes and a higher
reaction rate than conventional chemical methods. The size and shape of the
nanoparticles were controlled to some extent by changing the reaction conditions.
).
278 K. Tani et al.
The aqueous extract of the Acacia ehrenbergiana plant cortex from Saudi Arabia
was used to prepare Ag nanoparticles (Alamier et al.
2023). It is commonly found in
the Sahara, northern Sahel, parts of East Africa, the Arabian Peninsula, and Iran. The
prepared Ag nanoparticles with a particle size distribution ranging from 1 to 40 nm
were used as heterogeneous catalysts to reduce Rhodamine B dye from aqueous
solutions in the presence of sodium borohydride, and 96% catalytic reduction was
accomplished within 32 min . The Ag nanoparticles also exhibited high antimicrobial
activity against various Gram-positive, Gram- negative, and fungal microorganisms.
11.3 Sustainable Synthesis of Metal Nanoparticles Using Waste
Using waste to prepare value-added materials is important not only because it
reduces the cost of disposal but also because it converts waste into useful materials.
Preparation of metal nanoparticles using waste discharged in various fields has been
reported. Wastes are classified according to the sector in which it is generated as
follows; Agri-wastes, E-wastes, and Industrial wastes.
11.3.1 Agri-Wastes
Agri-wastes include not only agricultural wastes but also food wastes. While food
shortages are severe in developing countries because of population growth and
disasters caused by climate change, large amounts of food waste are generated
every year in economically wealthy countries because of overconsumption by their
societies. Many of these Agri wastes contain a large amount of moisture, and
incineration of such wastes consumes a large amount of energy, which is undesirable
from the perspectives of energy conservation and CO
Italian researchers in Rome prepared Ag nanoparticles using extracts from bilberry waste or spent coffee grounds as raw materials (Baiocco et al. 2016). The
wastes are commonly found in Italy. The authors focused on them as raw materials
containing relatively high levels of phenolic compounds. They extracted the
components from them using an aqueous ethanol solution and used them to prepare
Ag nanoparticles. Ag nanoparticles with a spherical shape and an average size of
10–20 nm were successfully prepared. This is a truly local production of raw
materials for nanomaterials.
Using the aqueous extract of cotton boll peel waste, SnO
successfully prepared (Narasaiah et al. 2022). The powder of dried cotton boll peel
was boiled to obtain an aqueous extract. The mixture of the extract and SnCl
aqueous solution was thoroughly stirred at 80 °C for 3 h, and the resulting powder
after centrifugation was calcinated at 200 °C for 3 h to obtain SnO
The SnO
nanoparticles exhibited superior photocatalytic efficiency for the degrada-
2
tion of methylene blue and methyl orange dyes under UV light exposure.
In India
, the preparation of Fe nanoparticles using the aqueous extract of waste
peels of Aegle marmelos (bael) fruit was reported (Srivastava et al. 2023). Bael is
emission control.
2
nanoparticles were
2
nanoparticles.
2
2
11 Green and Sustainable Synthesis of Silver Nanoparticles Using Wastes… 279
native to India and Bangladesh, and it is cultivated throughout India, Sri Lanka,
Thailand, and Malaysia. Fruits are eaten either fresh from trees or dried and
produced into candy, toffee, pulp powder, or nectar. Thus, a large amount of peel
is discarded after eating and cooking. Peels are rich in polyphenols, which can act
both as a reducing agent and capping agent. Fe nanoparticles were easily prepared by
simply mixing 0.1 M FeCl
solution with waste peel extract at room temperature and
2
adjusting the pH. The nanoparticles as a Fenton catalyst showed catalytic degradation of the anionic dye Eosin Yellow and the cationic dye Fuchsin Basic, with
removal efficiencies as high as 97% within 20 min and 95% within 25 min at 318 K.
In the Czech Republic, bimetallic nanoparticles were produced using extracts
derived from Cannabis sativa and Vitis vinifera waste (Michailidu et al.
2024).
Cannabis sativa is cultivated and used as a source of industrial fiber, seed oil, food,
and medicine. As for Vitis vinifera grape vines, which are common, grapes are eaten
fresh or dried to produce raisins, etc. The fresh grapes are also processed into juice
that is fermented to make wine and vinegar. The extracts were obtained from dried
Vitis vinifera canes collected during the dormancy period and waste material
extracted from C. sativa cannabinoid extracts. The prepared bimetallic nanoparticles
exhibited strong antimicrobial activity and biofilm inhibition against several strains
of Pseudomonas aeruginosa, a common pathogen responsible for hospital-acquired
infections and major contributor to antibiotic resistance. It is expected that the
toxicity of the bimetallic nanoparticles will be lowered while their antibacterial
activity is preserved, as Ag nanoparticles are characterized by higher toxicity for
mammalian cells compared to Au nanoparticles. The authors also prepared Ag
nanoparticles using the extract of Vitis vinifera canes (Miskovska et al. 2024).
Solmaz et al. used the waste of tropical fruit Citrus fortunella for fruit juice,
liqueur, or jam production and used it to synthesize selenium nanoparticles (Solmaz
et al. 2024). Recently, Se nanoparticles are expected to be applied in the biomedical
field due to their antibacterial (Zhang et al. 2021), biofilm-inhibiting (Ullah et al.
2023), anticancer (Geoffrion et al. 2020), and anti-inflammatory (Sun et al. 2023)
effects. The authors investigated the adsorption behavior of the Se nanoparticles
onto a crystal violet dye. A higher maximum adsorption capacity of 23.55 mg/g was
achieved compared with the other nanoparticles.
11.3.2 E-Wastes
E-waste is becoming one of the fastest-growing waste categories globally. In 2019,
53.6 million tons of E-waste were generated worldwide. Only 17% of the E-waste
was collected and recycled (Forti et al. 2020
platinum, and other high-value, recoverable materials wer e mostly dumped or
burned rather than collected for treatment and reuse. However, it is important to
consider the safety of recovering and recycling processes. E-waste recycling processes themselves are also not necessarily sustainable. Typically, they use toxic
substances such as acids and organic solvents, emit greenhouse gases, and often
require significant energy.
This means that gold, silver, copper,
).
280 K. Tani et al.
In the case of the preparation of copper oxide nanoparticles from copper recovered from discarded electrical circuits (Gautam et al.
2023) and Fe
from waste toner and their application as catalyst support (Kouser et al.
nanoparticles
2O3
2022), the
use of organic solvents, treatment with strong acids and alkalis, and sintering at high
temperatures are required.
11.3.3 Industrial-Wastes
In addition, nanomaterials have also been created in recent years from large
quantities of industrial waste. Ag nanoparticles were prepared using wastes from
paper and textile industries, and their antibacterial, antioxidant, and cytotoxic potential were evaluated (Paosen et al.
collected as waste from a paper manufacturer and sericin extracted from silkworm
cocoons. The prepared Ag nanoparticles exhibited antibacterial activity against
foodborne pathogens (Bacillus cereus, Listeria monocytogenes, Staphylococcus
aureus, Escherichia coli O157:H7, Klebsiella pneumoniae, Salmonella
Typhimurium, Shigella sonnei, Vibrio cholerae, and Vibrio parahaemolyticus). In
addition, the Ag nanoparticles provided antioxidant and antibiofilm activities without cytotoxic effects on Caco-2 and human erythrocytes.
Waste pickling acid, an iron-rich effluent produced worldwide as a result of
pretreatment of steel surfaces in galvanizing plants, is a potential source of iron
ions for many industrial processes despite its high pollutant loads. Approximately
300,000 tons of waste pickling acid are generated annually in the EU alone, resulting
in a large amount of sludge containing metals for conventional treatment, which
usually involves precipitation with limestone. It is important to make effective use of
large amounts of waste metals to produce new nanomaterials. Polenz et al. (2022)
p
repare
d magnetic Fe
nanoparticles using waste pickling acid as an iron source.
3O4
The nanoparticles were then silica-coated and tested for biocompatibility. The tests
showed no detection of hazardous contaminants (heavy metals) from waste-based
precursors and a high ability to immobilize spermatozoon cells, indicating the
biocompatibility of the magnetic nanoparticles for applications in biomedical fields.
However, the conventional chemical coprecipitation and sol-gel methods were used
to prepare the magnetic nanoparticles, which are not environmentally friendly.
above sections, we review recent studies on eco-friendly and sustainable
In the
preparations of metal nanoparticles using plant extracts and waste. The optical
properties and catalytic activity of metal nanoparticles vary greatly depending on
their size and shape (Sardar et al.
Therefor
e, precise control of particle size and shape during preparation is important
for nanoparticle applications. On the other hand, in the eco-friendly and sustainable
preparation of metal nanoparticles, it is difficult to precisely control the size and
shape of the nanoparticles during preparation. This is due to the large variation in the
components of the raw material, which makes the composition of the extract
unstable. This is especially true for the preparation of nanoparticles using
bio-waste materials.
2021). Using Eucalyptus camaldulensis leaves
2009; Ishida et al. 2020; Hang et al. 2024
).
11 Green and Sustainable Synthesis of Silver Nanoparticles Using Wastes… 281
Therefore, any measures on the eco-friendly methods are needed to overcome this
drawback. The measures include the following:
Pretreatment (e.g., purification) of the raw materials or their extracts.
Post-treatment by size selection of the generated nanoparticles (Taleb et al. 1997)or
by direct chemical modification of the size and shape of the generated
nanoparticles (Naoe et al.
2007; Shimpi et al. 2017).
However, using only such measures, it is difficult to achieve the quality of
nanoparticles produced by eco-friendly methods to reach the level of nanoparticles
prepared by chemical methods. In other words, using current technology, it is
difficult to replace chemical preparation methods with eco-friendly preparation
methods completely. Therefore, it is better to select a preparation method that yields
nanoparticles that are suitable for the given application. It is effective to use
chemically prepared nanoparticles and eco-friendly prepared nanoparticles separately or mixed together (hybrid use) depending on the application. For example,
in the aviation industry, Sustainable Aviation Fuel (SAF) is mixed into jet fuel to
reduce CO
emissions (SAF blending limit: up to 50%) (ICAO 2020).
2
In the preparation of nanopar ticles using plant extracts, it is also necessary to
evaluate the shelf life of the extract and to improve its shelf life. In addition, it is
important to establish methods for selecting plants and bio-waste suitable for
nanoparticle synthesis. For plant parts and bio-waste, it is necessary to develop
simple evaluation methods to determine the following
1. Does it have reducing properties? (containing reducing substances such as
polyphenols?)
2. Does it have a coating capability for nanoparticles? (containing biomolecules
such as proteins acting as coating agents?)
11.4 Preparation of Antibacterial Silver Nano particles Using
Wastes of Traditional Medicinal Plants from the World
Heritage Site Nara
Nara, the ancient capital of Japan approximately 1300 years ago, has many valuable
historical heritages in the prefecture and has three UNESCO World Heritage sites:
“Buddhist buildings in the Horyu-ji area”, “Historic Monuments of Ancient Nara”,
and “Sacres Sites and Pilgrimage Routes in the Kii Mountain Range” (Nara
Prefecture’s World Heritage Properties HP 2024). At the time, Nara enjoyed great
prosperity, emerging as the fountainhead of Japanese culture.
Kampo is
With its roots in ancien t Chinese medicine, this antecedent form of empirical
medicine was introduced to Japan in approximately the fifth to sixth century.
Subsequently, it developed into a unique form of medicine by adapting to the climate
and culture of Japan and was further re fined to suit the constitutions of the Japanese
a traditional Japanese herbal medicine that originated in ancient China.
282 K. Tani et al.
people before evolving into a distinct form of traditional medicine (Watanabe et al.
2011).
In Nara, formerly known as “Yamato”, where Japan’s oldest imperial court was
established, there had been a close relationship with crude drugs for traditional
medicinal use since ancient times. To prepare for epidemics, medicinal plants were
cultivated in the Kinki region centering on Yamato, and crude drugs from China and
other countries were also gathered in Yamato. In addition to these historical factors,
Nara is surrounded by mountains with ample rainfall, hot summers, cold winters,
and little snowfall, making it geologically suitable for the cultivation of various
plants for crude drugs. In the Edo period (1603–1867), demand for Kampo
medicines increased, and as a measure for self-sufficiency in Japan, many attempts
were made to import seeds and seedlings of medicinal plants from China, survey and
collect medicinal herbs and trees growing wild in the mountains and fields in Japan,
and cultivate them. In particular, the eighth Shogun, Yoshimune TOKUGAWA,
encouraged the cultivation of medicinal plants throughout Japan. Due to these
circumstances, Nara has been one of the most important areas where medicinal
plants have been cultivated since ancient times. Even nowadays, there are many
small pharmaceutical companies in Nara that manufacture Kampo medicines. For
example, Daranisuke (陀羅尼助) is a gastrointestinal drug produced from the inner
bark extra ct of yellow cedar (Phellodendron amurense Rupr) and other herbal
extracts. It improves unpleasant conditions, such as weak stomach, overeating,
loss of appetite, excessive drinking, heavy stomach, stomach or abdominal bloating,
heartburn, pressure on the chest, and nausea and vomiting (ex. Sankogan Co., Ltd.
(HP)
2024).
The p
s a perennial plant of the genus Paeonia. It is native to Siberia, China,
eony i
and Mongolia. Its scientific name is Paeonia lactiflora. The peony came to Japan
from China during the Nara period (710–794). The beautiful looking peony has a
large number of varieties as horticultural species. Its roots have sedative, painrelieving, antitussive, and antihypertensive effects and are used as an ingredient in
Kampo medicine. The varieties suitable for medicinal use are currently being
studied. In Japan, Ya mato peony from Nara, which has been used for medicinal
purposes for many years, is considered the highest quality (Fig. 11.1).
For medi
cinal plants, dried roots and bark are used as raw materials for crude
drugs, whereas other parts are discarded. In the case of the peony, the roots of the
peony are used as crude drugs, and the flowers are distributed as ornamental,
whereas the leaves and buds are discarded because they are no longer used. In
Japan, more than 1600 tons of peony are used annually as raw material for crude
drugs (Yamamoto et al. 2021). In other words, a large number of non-root parts of
the peony are discarded. However, it has been reported that discarded parts of peony,
such as leaves, contain reducing components (Tatsumoto
3), and if these
202
discarded parts can be used as reducing agents, an eco-friendly large-scale production method for metallic nanoparticles can be developed. In this study, we focused
on peony (Paeonia lactiflora) grown in Nara as a crude drug plant for traditional
medicinal use and prepared antimicrobial silver nanoparticles using discarded parts
(leaves) of the plant.
11 Green and Sustainable Synthesis of Silver Nanoparticles Using Wastes… 283
Fig. 11.1 Photos of (a) flower and (b) roots (after washing and drying) of Yamato peony (Paeonia
lactiflora) in Nara
Fig. 11.2 (a) UV-Vis spectra of the mixture of Paeonia lactiflora leaf extract and Ag nitrate
solution after heat treatment and (b) particle size distribution of the reaction mixture after heating.
The spectrum of the mixture after the reaction without Ag ions is also shown for comparison.
Reaction temp. = 90 °C, Reaction time = 1h
Dried Paeonia lactiflora leaves were ground in a grinder, and hot water extraction
was performed for 10 min. After centrifugation, the supernatant was filtered through
a filter with a pore diameter of 4 μm, and the resulting filtrate was used as the extract.
A silver nitrate solution was added to the extract, and the reduction reaction was
carried out under stirring at a given temperature. After the reaction, the UV-Vis
spectrum of the reaction mixture was recorded. Figure 11.2a
shows
the UV-Vis
284 K. Tani et al.
spectrum of a mixture of silver nitrate solution and Paeonia lactiflora leaf extract
after the reaction at 90 °C for 1 h. The spectrum of the reaction solution without Ag
ions is also shown for comparison. The UV-Vis spectrum in the presence of Ag ions
shows an absorption peak at around 440 nm. As surface plasmon resonance (SPR)
absorption due to Ag nanoparticles is observed at approximately 400 nm (Solomon
et al.
2007), indicating the formation of Ag nanoparticles using the extract. On the
other hand, no absorption peak was observed near the corresponding wavelength in
the reaction without Ag ions. In the following sections, the absorption peak near
440 nm is used as an indicator of Ag nanoparticles. In addition, the DLS measurement of the solution after the reaction revealed peaks at approximately 10 and
100 nm, respectively (Fig.
nanoparticles was 10
11.2b). The results show that the size of primary
nm, and they aggregated into aggregates of approximately
100 nm.
Next, the effect of reaction time on the formation of Ag nanoparticles was
investigated. The absorbance at the maximum absorption wavelength can be used
as an indicator of the number of nanoparticles produced. The absorbance at the
maximum absorption wavelength λ
increased with the reaction time and became
max
almost constant after a reaction time of 1 h (Fig. 11.3). The DLS profile also changed
little with reaction times greater than 1 h (data not shown). Under the reaction
conditions, the reduction reaction was considered to have reached equilibrium
after a reaction time of 1 h. In the following sections, the reaction time is set at 1 h).
The effect of reaction temperature on the formation of Ag nanoparticles was also
investigated. Reduction of aqueous Ag nitrate solution by the extract from Paeonia
lactiflora leaves at 70–90 °C. The reaction time was 1 h. The absorbance at the
maximum absorption wavelength λ
increased with the reaction
max
Fig. 11.3 Effect of reaction
time on absorbance at λ
the mixture of Paeonia
lactiflora leaf extract and Ag
nitrate solution. Reaction
temperature = 90 °C
max
of
11 Green and Sustainable Synthesis of Silver Nanoparticles Using Wastes… 285
Fig. 11.4 Effect of reaction temperature on (a) UV-Vis spectra and (b) dynamic light scattering
profiles of a mixture of Paeonia lactiflora leaf extract and Ag nitrate solution after heat treatment.
Reaction time = 1h
temperature (Fig. 11.4a), indicating that the number of Ag nanoparticles produced
increased with the reaction temperature. The peak wavelength shifted to lower
wavelengths as the reaction temperature increased. This result implies that the higher
the reaction temperature, the smaller the size of the Ag nanoparticles. In the
preparation of nanoparticles using other plant extracts, nanoparticles with smaller
sizes have been reported to form at higher reaction temperatures (Song and Kim
2009; S
ong et al. 2010).
In
fact,
in
this study, the DLS profile under higher
temperature conditions exhibited peaks at smaller particle sizes and a narrower
particle size distribution (Fig. 11.4b). In the chemical synthesis of metal
nanoparticles, reactions are often conducted under reflux at fairly high temperatures
to obtain monodisperse nanoparticles (Shimizu et al. 2003; Park et al. 2004).
SEM observation and EDS analysis of the prepared Ag nanoparticle powders
obtained after lyophilization were performed. Figure 11.5 shows SEM image and
EDS analysis results for the Ag nanoparticle powder. The red dots in the figure
represent silver. Aggregates of nanoparticles were observed. The elemental mapping
of Ag shows that Ag is homogenously distributed only on the aggregates of the
nanoparticles. The EDS spectrum (Fig. 11.5b) also confirmed the presence of Ag
peaks, confirming the formation of Ag nanoparticles.
Thermogravimetry (TG) measurements of the prepared Ag nanoparticles were
performed. Figure 11.6
s the thermogravimetric profile of the Ag nanoparticles
show
in a nitrogen gas atmosphere. The weight of the sample remained constant above
450 °C. There was a degradation of 53.2% of the sample attributed to the bio-organic
part of the material, indicating 46.8% of the nanoparticle is Ag component. TG
measurements of Ag nanoparticles prepared by other plant extracts have reported
that the silver content is approximately 80% in Ag nanoparticles by propolis extract
2019),
(Barbosa et al.
61% in Ag nanoparticles by tea leaf extract (Sun et al. 2014),