The document summarizes research on the green synthesis of zinc oxide nanoparticles (ZnO NPs) using plant biomass. It discusses how plant extracts can be used as reducing and capping agents to synthesize ZnO NPs through a simple, non-toxic and environmentally friendly process. The mechanisms of ZnO NP formation and factors that influence their shape, such as plant extract components, pH, temperature and reaction time are described. A variety of plants have been used to successfully synthesize ZnO NPs with different morphologies through this green method.

1. Materials Today Communications 33 (2022) 104747
Available online 22 October 2022
2352-4928/© 2022 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
A review on biogenic green synthesis of ZnO nanoparticles by plant biomass
and their applications
Soumeia Zeghoud a,b
, Hadia Hemmami a,b,c
, Bachir Ben Seghir a,b,d
, Ilham Ben Amor a,b
,
Imane Kouadri b,d,e
, Abdelkrim Rebiai b,f
, Mohammad Messaoudi f,g
, Shakeel Ahmed h,i
,
Pawel Pohl j
, Jesus Simal-Gandara k,*
a
Department of Process Engineering and Petrochemical, Faculty of Technology, University of El Oued, El Oued 39000, Algeria
b
Renewable Energy Development unit in Arid Zones (UDERZA), University of El Oued, El Oued 39000, Algeria
c
Laboratory of Applied Chemistry and Environment, University of El-Oued, El-Oued 39000, Algeria
d
Laboratory of Industrial Analysis and Materials Engineering (LAGIM), University of 8 May 1945, Guelma, P.O. Box 401, Guelma 24000, Algeria
e
Department of Process Engineering, Faculty of Science and Technology, University of 8 May 1945, Guelma, BP 401, Guelma 24000, Algeria
f
Chemistry Department, Faculty of Exact Sciences, University of El Oued, P.O. Box 789, El Oued 39000, Algeria
g
Nuclear Research Centre of Birine, P.O. Box 180, Ain Oussera, Djelfa 17200, Algeria
h
Department of Chemistry, Government Degree College Mendhar, Jammu and Kashmir 185211, India
i
Higher Education Department, Government of Jammu and Kashmir, India
j
Department of Analytical Chemistry and Chemical Metallurgy, Faculty of Chemistry, University of Science and Technology, Wyspianskiego 27, 50-370 Wroclaw, Poland
k
Universidade de Vigo, Nutrition and Bromatology Group, Analytical Chemistry and Food Science Department, Faculty of Science, E32004 Ourense, Spain
A R T I C L E I N F O
Keywords:
ZnO nanoparticles
Green synthesis
Plant extracts
Application
A B S T R A C T
Nanobiotechnology has recently gained prominence as a fundamental branch of modern science and a novel
epoch in the field of material researches. Due to a wide range of applications it attracts attention of many sci-
entists from all over the world. Bionanomaterials are prepared using a variety of physical, chemical, and bio-
logical techniques and methods. Many different metal and metal oxide nanoparticles are reported to be produced
by biological systems, including bacteria, fungi, actinomycetes, yeasts, viruses, and plants. Among all of them,
biocompatible zinc oxide nanoparticles (ZnO NPs), obtained through biosynthesis with the aid of plant-derived
materials, appears to be a highly successful way to create a fast, clean, non-toxic, and environmentally friendly
platform for the production and application of these bionanomaterials. This review focuses on the plant extract-
derived ZnO NPs synthesis, with a special emphasis on the recent advances and applications of these
nanomaterials.
1. Introduction
Nanotechnology is one of the most significant scientific frontiers and
the century’s defining science, which impact on the world economy and
society cannot be overestimated [1,2]. Indeed, nanoparticles (having
diameters between 1 and 100 nm) have a wide range of uses in research
and technology nowadays [3]. That is why techniques and methods of
nanoparticles synthesis and their characterization are crucial owing to
wide applications of these materials [4]. Metal nanoparticles (MNPs)
have numerous applications in sensing, electronics, optoelectronics,
medicine, bioengineering, and information storage [5–7]. Due to
distinctive physical, chemical, and biological characteristics of MNPs,
including their high electrical conductivity, thermal conductivity,
chemical stability, high catalytic activity, and biomedically relevant
antibacterial and anticancer activities [8,9], their applications are sig-
nificant and differentiated. MNPs are mostly fabricated using a variety
of physical and chemical techniques, many of which employ some
harmful and toxic reactive compounds [2]. Therefore, it is vitally
necessary to use more environmentally friendly, ecologically sound, and
greener production methods for them [10]. Plants or plant residues used
as reactive components to synthesize MNPs certainly appear to be
low-cost, and environmentally friendly, while methods applying these
materials are energy-efficient and chemically safe [11].
The consideration of biological methods for the synthesis of NPs has
* Corresponding author.
E-mail address: jsimal@uvigo.es (J. Simal-Gandara).
Contents lists available at ScienceDirect
Materials Today Communications
journal homepage: www.elsevier.com/locate/mtcomm
https://doi.org/10.1016/j.mtcomm.2022.104747
Received 29 July 2022; Received in revised form 5 October 2022; Accepted 20 October 2022

2. Materials Today Communications 33 (2022) 104747
2
grown recently [12]because they reduce the harmful environmental
effects related with the production of nanomaterials [13]. A metal salt, a
reducing agent, and a stabilizer or capping agent are three primary
components needed to prepare NPs chemically [2]. For the large-scale
production of green NPs with dual functionalities (reducing and
capping agents), the biological and green approach makes use of a va-
riety of intriguing biological materials, including bacteria, yeasts, fun-
gus, algae, and plants [14,15]. Due to their diversity and sustainability,
plants are the best candidates among other raw materials identified
viable for the green synthesis of NPs[16].
Recent research demonstrates a huge relevance of the green syn-
thesis methods to produce metal oxide NPs [17], including Zn, Ag, Cu,
Au, Ni, and others [18–21]. ZnO NPs stand out among other metal ox-
ides due to their abundance, stability, electric conductivity, piezoelec-
tricity, nontoxicity, and optical transparency [22,23]. These
nanomaterials are willingly used as important components of paints,
varnishes, polymers, gas sensors, solar cells, medicines, and laser and
optoelectronic devices, which are just a few examples of their successful
applications [24–26]. In addition, for a long time ZnO NPs are utilized as
protective agents in the cosmetics and sunscreen products [20]. The
antibacterial and antifungal properties of ZnO NPs are well known too
[27,28].
Much research has been conducted on the green production of ZnO
NPs, particularly in few recent years. For that reason, this review surveys
the green synthesis of ZnO NPs utilizing the plant extracts, with a special
focus on the recent advancements in the production of this
nanomaterial.
2. Methods of ZnO NPs synthesis
Methods of the synthetic ZnO NPs production include physical,
chemical, and biological approaches as shown in Fig. 1 [29]. Sol-gel
[30], co-precipitation, microemulsion, in addition to hydrothermal
[31], and chemical reduction processes are some examples of chemical
methods applied for the production of these NPs [32]. The sol-gel
technique is a commonly utilized strategy for manufacturing ZnO NPs,
among other chemical synthesis processes, since it brings large amounts
of a required product. In addition, it is simple in use, and requires low
processing temperatures. Physical processes like vapor deposition,
plasma irradiation, and ultrasonic irradiation are also used to fabricate
ZnO NPs [33]. Unfortunately, these processes often need significant
amounts of energy to be applied and the well-built equipment. For that
reason, the biological (green) synthesis of ZnO NPs is an ideal alternative
for above mentioned procedures because it is easy to carry out,
cost-effective and environmentally friendly [34].
3. Positive aspects of eco-friendly synthesis
Since potentially hazardous compounds are required to produce and
stabilize the resultant ZnO NPs with the aid of chemicals methods, they
pose a high risk to both people and animals. On the other hand, bio-
logically derived components, being environmentally friendly and
chemically safe, can be used for the synthesis of ZnO NPs. As such, ZnO
NPs can be synthesized by using a single-step and pollution-free method
that needs less energy to start the reaction, and take place in a shorter
preparation time as compared to other methods. The most important
benefit of such green synthesis is its cost-effectiveness. This is because
reducing agents used in this process are biological species or plants
readily available in large quantities, there is also no need to dispose any
harmful and often toxic wastes accompanying such process [35]. In
addition, the green synthesis method can be easily applied on a large
scale while their potential applications are huge because of a high
availability of plant species and similar materials.
4. Green synthesis of ZnO nanoparticles
Researchers have been interested in the production of ZnO NPs using
biological approaches for the past decade [36]. The development and
relevance of this green synthesis approach is primarily stimulated by a
possibility of the use of fewer chemicals, its cost-effectiveness, and
environmental friendliness. The biological synthesis of ZnO NPs is
certainly more convenient than traditional chemical or physical
methods [37]. Nevertheless, the large-scale synthesis of ZnO NPs uti-
lizing green methods is always a challenge, and processes involved in
them are only carried out on a laboratory scale. However, the
laboratory-scale processing without the powerful equipment will be
achievable shortly, thanks to breakthroughs in understanding the nature
of the biological extracts composition and their reactivity with the metal
ions. In place of chemical solvents and stabilizers, biological substrates,
including bacteria, plants, fungus, and algae, are frequently used to
lessen hazardous effects of the resultant products [38,39].
Fig. 1. Different methods of the nanoparticle synthesis.
S. Zeghoud et al.

3. Materials Today Communications 33 (2022) 104747
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5. Synthesis of ZnO NPs using plant extracts in an
environmentally friendly manner
Plants are the most common biological substrates for the synthesis of
ZnO NPs because of their low production and handling costs, low impact
on the environment, ease of the manufacturing, and the fact that they
are less likely to be harmed by microorganisms. Furthermore, distilled
water and ethanol are the solvents that are the most frequently used to
prepare plant extracts; there are fewer health risks than the microbially
aided ZnO NPs synthesis [40]. ZnO NPs are produced using plant ex-
tracts from various plant parts, including the bark, roots, fruit pulps,
leaves, peels, flowers, and so on. It is believed that plant extracts contain
large amounts of active chemicals such as methylxanthines, phenolic
acids, flavonoids, and saponins. These substances are all together
referred to as antioxidants. Free radicals, reactive oxygen species, and
chelated metal structures are all rendered harmless by these antioxi-
dants [41]. Because of this, it should be no surprise that plant extracts
can function both as bioreductiors and stabilizers [33].
6. Conditions necessary for preparation
Before beginning of the manufacturing process, plants are
completely washed with distilled or pure water. The plant material can
then be dried, ground into a powder, dissolved in a solvent, or simply
soaked to produce the respective plant extract. The resulting extract is
then mixed with a Zn(II) salt solution, which acts as the NPs precursor,
from which a precipitate is produced following a reaction. The precip-
itate is then calcined to produce ZnO NPs (Fig. 2).
7. The mechanism of formation of ZnO NPs by utilizing plant
extracts
Polysaccharides, flavonoids, polyphenols, alkaloids, tannins, amino
acids, and saponins are all reductive antioxidants found in plants. Ter-
penoids, alkaloids, and alkaloids are also present in these materials. As a
result, Zn(II) ions in respective salts solutions can be reduced and capped
using plant extracts, and then, after oxidizing, stable and well dispersed
ZnO NPs can be produced [42]. The Moringa oleifera extract was used in
the production of ZnO NPs, and it was presumed that free radicals
converted L-ascorbic acid, the main component of the plant extract, to
L-dehydro ascorbic acid. The electrostatic attractions between the Zn2+
ions and L-hydro ascorbic acid anions likely resulted in the formation of
a Zn-ascorbic acid complex, which could be subsequently used to
generate ZnO NPs by the calcination process at high temperatures [43].
Karnan and Selvakumar used extracts from the Nephelium lappaceum L.
[44] peels and studied the production process for ZnO NPs. They found
that at pH 5–7, the aromatic hydroxyl groups in polyphenolic ellagic
acid derived from Nephelium lappaceum L. could combine with Zn(II)
ions to form a stable Zn-ellagate complex, which resulted in the pro-
duction of ZnO NPs by the calcination at 450 ◦
C. In addition, Mayedwa
et al. demonstrated that aromatic hydroxyl groups could form stable
complexes with metal ions, enabling the calcination of metal oxide NPs
[45]. This was accomplished by demonstrating that aromatic hydroxyl
groups can form stable complexes with the metal ions.
8. Factors affecting shape of ZnO NPs synthesized by plant
extracts
The morphology of ZnO NPs and their characteristics are related to
one another in some way [45]. It is crucial to prepare ZnO NPs with such
morphology that is suitable for their intended usage. The production of
ZnO NPs with the aid of the plant extracts can be controlled more pre-
cisely than in the case of physical and chemical methods, hence, it re-
sults in the production of NPs having the appropriate size and
morphology [46]. However, because different plant species contain
varying amounts of active, reducing compounds [46], their reducing
capability can be likewise modified, fundamentally influencing the
synthesis of ZnO NPs. Additional factors that affect the morphology of
NPs are the concentration of the plant extracts and the concentration of
the precursors, the duration of the reaction, the pH level, and the
calcination temperature.
Nevertheless, the size of ZnO NPs tends to decrease when the con-
centrations of the plant extract and the precursor rise. On the other
hand, it tends to grow up as the reaction time and the calcination tem-
perature rise as well. In general, the shape of ZnO NPs is determined by a
combination of six different parameters [47,48]. In the following sec-
tion, the recent reports covering this aspect are given in Table 2, and in
Fig. 3, which illustrates various morphologies that can be obtained for
ZnO NPs.
Fig. 2. Process of green synthesis of ZnO NPs from plant extracts.
S. Zeghoud et al.

4. Materials Today Communications 33 (2022) 104747
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8.1. Effect of plant extract
Plant extracts have two functions in ZnO NPs synthesis: one is to
reduce Zn(II) ions and the other is to stabilize the resultant nano-
structures [49,50]. For ZnO NPs production, various plant sources are
utilized (Table 1) [39,43]. Unless the plant extract is devoid of the
bioactive chemicals, the plant species employed have a little impact on
the production of NPs and their appearance [51]. The extract concen-
tration affects the form, homogeneity, and size of synthesized ZnO NPs
[52].
8.2. pH effect
The pH value of 12 was found to be the ideal alkalinity for the pro-
duction of ZnO NPs by Abdullah et al. [72] and Shabaani et al. [72]
using the extracts prepared from Musa acuminata and Eriobotrya
japonica, respectively. Umamaheswari et al. [58] tried to produce ZnO
NPs by using extract of Raphanus sativus var. longipinnatus at pH 8, 10,
and 14. The resulting reaction mixtures were then examined using
UV-Vis spectra. It was discovered that there was no absorption peak at
pH 14 or pH 8–10, but when the solution was made up to pH 12, a
distinct absorption peak at 369 nm was seen and related to ZnO NPs.
Using the extract made from Nyctanthes arbor-tristis, Jamdagni et al. [66]
tried to synthesize ZnO NPs at a pH range of 9–13. The UV-Vis spectra
revealed no discernible absorption peak at pH 9, indicating that the
absorption lines were nearly linear. The absorption peaks with the
recognizable characteristics were observed at pH 12 and 13. At pH 12,
however, both the absorbance and the sharpness were of a higher
quality, indicating a higher synthesis efficiency and a lower size distri-
bution of resultant ZnO NPs. Therefore, it was postulated that pH 12 was
the most suitable for the synthesis of ZnO NPs with any plant extract and
that the influence of pH on the synthesis of ZnO NPs was not highly
related to the applied plant species. It was likely that when pH was 12,
the ratio of the hydroxyl radicals to hydrogen ions was optimal. The
positively charged Zn2+
ions exerted in these conditions a powerful pull
on the negatively charged OH, encouraging the creation of ZnO bonds
inside the structure [58]. Hence, the conditions that are not favorable for
this process include both the low pH values (a lesser quantity of the OH
ions), and the high pH values (an excessively high number of the OH
ions).
Table 2 summarizes six variables that affect the shape of ZnO NPs.
However, because different plant species have varying concentrations of
active reducing chemicals, their reducing capability is also changed and
has a significant impact on the synthesis of ZnO NPs and their further
applications.
9. Crystallographic and morphological characteristics that are
inherent to ZnO
Wurtzite describes the crystal structure of ZnO, and its growth that is
most easily facilitated along the c axis. ZnO NPs exhibit substantial
shape anisotropy and grow in a direction parallel to the basal plane.
Because of the surface tension effect, ZnO NPs can either be crystalline
or amorphous; in some cases, they can even exhibit a metastable crys-
tallographic phase [95].
10. Applications of ZnO oxide nanoparticles
ZnO NPs have a wide range of applications, i.e., in agriculture,
photocatalysis, medicine, food packaging, cosmetics, antioxidant pre-
vention, anticancer drug delivery systems, and other activities related to
Fig. 3. Various morphologies of ZnO NPs: (a) spherical [73], (b) triangular [74], (c) flower-shaped [75], (d) spot-like shaped [76](e) cauliflower-shaped [77], (f)
hexagonal [78], (g) needle-like shaped [79], (h) flaky and rod [80], (i) sheet-like shaped [72], and (j) cylindrical shaped[81].
Table. 1
The synthesis of ZnO NPs in an environmentally friendly manner utilizing
various plant extracts.
S.no
Plants Name Size ZnO NPs (nm) Reference
1 Agathosma betulina 15.8 [53]
2 Laurus nobilis leaf 47.3 [54]
3 Calotropis procera 24 [55]
4 Ocimum tenuiflorum 13.8 [56]
5 Salvia officinalis 11.9 [57]
6 Raphanus sativus var. Longipinnatus 66.4 [58]
7 Myristica fragrans 29 [59]
8 Cayratia pedate 2.2 [60]
9 Parthenium hysterophorus 10 [61]
10 Syzygium cumini 16.4 [62]
11 orange fruit peel 12 [63]
12 Solanum nigrum 29.8 [64]
13 Moringa oleifera 24 [65]
14 Nyctanthes arbor-tristis 16.6 [66]
15 Oak Fruit Hull 44 [49]
16 Solanum torvum 28.2 [67]
17 Phoenix dactylifera 29.3 [68]
18 Elaeagnus angustifolia 26 [69]
19 Célosie argentée 22 [70]
20 Punica granatum 20 [71]
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5. Materials Today Communications 33 (2022) 104747
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their antimicrobial and antibacterial properties. The key applications of
commonly used NPs are given in Fig. 4.4This section presents the most
important and recent applications of ZnO NPs.
10.1. Agricultural applications
Due to the ongoing use of commercially available antibiotics the
agricultural animals treatment, multidrug-resistant bacteria and fungi
have evolved [96]. ZnO NPs are found to be an effective alternative to
Table 2
Factors influencing the green synthesis of ZnO NPs for assisted plant extracts.
S.
no
Influencing
factors
Variables Plants Name Plants
parts
Techniques for
characterization
Shape/
morphology
Size ZnO
NPs
Applications Ref
1. Plant species various
plants
Kalopanax
septemlobus
Bark FTIR, EDX, XRD, TEM,
UV
Flower 500 nm Photocatalytic activity [82]
2. Zizyphus jujube Fruit UV, XRD, FTIR, SEM,
TEM
Spherical 29 nm Photocatalytic activity [83]
3. Codonopsis
lanceolata
Root TEM, EDX, XRD, FTIR,
UV
Flower 500 nm Photocatalytic activity [84]
4. Cydonia oblonga Seeds FESEM, EDX, FTIR,
XRD, UV
– 25 nm Photocatalytic activity [85]
5. Musa acuminate Peel XRD, SEM, FTIR, UV Triangular 30–80 nm Photocatalytic activity [72]
6. Coccinia abyssinica Tuber XRD, UV, TEM, FTIR Hexagonal 10 nm Antimicrobial and
Antioxidant activity
[86]
7. Berberis aristata Leaf EDX, XRD, SEM, UV,
FTIR
Needle 20–40 nm Antioxidant and Antibacterial
activity
[79]
8. Cucurbita
andreana naudin
seed XRD, UV, EDAD,
HRTEM
Rectangular, rod 45–65 nm Antibacterial activity [87]
Cytotoxicity study
Antioxidant activity
Antifungal activity
9. Plant extract 1 % Hibiscus sabdariffa flower FESEM, EDX, FTIR,
XRD, HRTEM, UV
Spherical 20–40 nm Photocatalytic activity [88]
4 % Spherical 12 nm
8 % Spherical 5 nm
10. Concentration 1.96 % C. halicacabum leaves XRD, Zeta potential,
UV
Hexagonal 62 nm Antibacterial activity [89]
3.85 % Hexagonal 55 nm
7.41 % Hexagonal 48 nm
11. Precursor 0.005 mol/
kg
Aloe vera leaf SEM, XRD, TEM, UV Spherical 63 nm Antimicrobial and
¨
Photocatalytic activities
[81]
Concentration 0.01 mol/
kg
Spherical 65 nm
0.05 mol/
kg
Cylindrical
shaped
40–45 nm
12. 0.01 mol/L Banana peel EDX, FTIR, XRD, TEM,
UV
hexagonal
wurtzite shape
128 nm – [90]
13. 0.05 mol/L hexagonal
wurtzite shape
74.19 nm
14. 0.1 mol/L hexagonal
wurtzite shape
59.59 nm
15. Reaction time 0.5 h Cassia auriculata leaves EDX, FTIR, XRD, TEM,
UV
Rod 20–30 nm Antimicrobial activities [91]
1 h Flower shaped
2 h Flower shaped
16. 0.33 h Aloe vera level SEM, EDX, FTIR, and
XRD
Flaky and rod 18 µm – [80]
48 h Flaky and rod 618 µm
17. PH value 4 Eclipta alba leaves TEM, XRD, UV spherical 112 nm Antimicrobial activity [92]
5 110 nm
6 103 nm
7 100 nm
8 5 nm
18. 8 Musa acuminata peel XRD, SEM, FTIR, UV sheet-like
structure
79.9 nm Photocatalytic activity [72]
9 sheet-like 66.6 nm
10 leaf-like 40.0 nm
11 triangular-like
shape
33.3 nm
12 triangular 30.7 nm
19. 7 Veronica multifida leaf XRD, SEM, TEM, FTIR,
UV
hexagonal 11.5 nm Antimicrobial activity [93]
12 spherical 29.5 nm
20. Calcination 250 ◦
C Ocimum
gratissimum
leaf XRD, SEM, TEM, FTIR,
UV
Spherical 14 nm – [94]
temperature 400 ◦
C Spherical 29 nm
21. 400 ◦
C Camellia sinensis
L.
leaf FESEM, EDX, FTIR,
XRD, HRTEM, UV
Spherical 19 nm Cytotoxicity; Antibacterial,
Hemolytic, Anti-Oxidant
activities
[48]
550 ◦
C Spherical 21.41 nm
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6. Materials Today Communications 33 (2022) 104747
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the traditional antibiotics for treating the fungal and other microbial
infections in agricultural animals and plants. ZnO NPs have exceptional
pesticide efficacy against the Artemia salina larvae [97].
The potential of biosynthesized ZnO NPs as antifungal and antibac-
terial agents for the agriculture purposes is traceable to the biomolecules
found in the plant extracts used for their biosynthesis [66]. According to
the study on the effect of ZnO NPs on the Solanum lycopersicum’s
reproductive system, their application tend to increase the germination
rate and the proteins content. A number of studies [51]. ZnO NPs are
also established to boost the food crop productivity, according to several
researchs [98,99].
10.2. Photocatalytic activity
The photocatalytic activity of ZnO NPs shows the improved electron
mobility, which accelerates the rate at which ZnO electrons are photo-
generated, preventing photogenerated holes and electrons from the
recombination and increasing the lifetime of the photogenerated charge
carriers. The photocatalytic reaction rate may be increased by various
means, including decreasing the bandgap, increasing the defect con-
centration, and increasing surface area [100]. As the pollutant concen-
tration rises, so does the photocatalytic activity, and as a result, the
likelihood of the lit light beam reaching the catalyst particles decreases.
Because ZnO NPs have a larger surface area, a narrower bandgap, and a
smaller particle size, they absorb more the UV radiation and decompose
more quickly. As a result of the photocatalytic activity, the production of
smaller NPs is boosted [101]while in solvent-free conditions [102],
various acridine and xanthene derivatives were produced. In the latter
case, some physiologically active heterocyclic compounds were suc-
cessfully synthesized with the aid of a gentle and efficient ZnO NPs
catalyst, which was recyclable and could be used again with a signifi-
cantly little loss of its catalytic activity.
10.3. Medicinal uses
Through the modulation of the neuronal excitability or even the
neurotransmitter release, ZnO NPs may have a role in the CNS and even
throughout the disease development. ZnO NPs were shown to alter the
cell or tissue functioning, the biocompatibility, and the brain tissue
engineering in several investigations [103,104]. Unfortunately, little
information is present about the influence of ZnO NPS on CNS and the
CNS-related diseases. It was demonstrated that the ZnO NPs affect the
spatial cognition in rats and synaptic transmission in vitro by enhancing
the long-term potentiation (LTP). It is also believed that the exposure to
ZnO NPs may be genotoxic due to the oxidative stress and the lipid
peroxidation [105,106]. However, because of their ability to target, ZnO
NPs may be helpful in cancer and/or autoimmune therapies [107].
10.4. Food packaging
In polymer science, composites comprise a continuous polymeric
matrix and a discontinuous polymeric filler [108]. Thanks to recent
advancements, nanotechnology may now be used to create new mate-
rials with better qualities. The inclusion of ZnO NPs can provide several
benefits. This is owing to a widespread usage of ZnO in the food industry
as a Zn supplement, with the ZnO degrading into the Zn2+
ions after
entering the human body [109]. Because the polymeric matrix contains
ZnO NPs, the packaging may interact with food and play a dynamic role
in preserving it, which is one of the main applications of ZnO NPs in the
food packaging. Additionally, ZnO NPs improve the packaging attri-
butes, including its mechanical strength, barrier properties, and stability
[110].
10.5. Cosmetics
NPs are now widely used in a variety of sectors, including industry,
cosmetics, engineering, agriculture, and medicine. They are employed
more frequently in cosmetics and dermal-based products because of
their enhanced surface area and distinctive physiochemical properties.
Because of their ability to offer the enhanced UV protection, ZnO NPs are
frequently used in cosmetics and skin applications. Although their use is
growing in popularity, few questions are raised concerning some po-
tential negative consequences. Despite being used in a variety of the
dermatological therapies, ZnO NPs are not adequately investigated
using the alternative in vitro test approaches for their propensity to
produce the skin sensitization (SS). The Human Cell Line Activation Test
(h-CLAT), which analyses a substance’s capacity to upregulate the
expression of CD86 and CD54 in the THP-1 cell line [111], was used to
evaluate the skin sensitizing potential of ZnO NPs.
10.6. Antioxidant activity
Because of the electron density transfer at the O atoms, ZnO NPs have
antioxidant properties, which depend on the above-mentioned O atoms
structural arrangement [47]. The naturally produced material demon-
strates a significant natural antioxidant activity from higher plants
Fig. 4. Different applications of ZnO NPs.
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7. Materials Today Communications 33 (2022) 104747
7
against chronic disorders caused by oxidative processes. Zinc acts as an
antioxidant by reducing the cell membrane damage caused by free
radicals. Several enzymes involve this element as an important cofactor
or a component in the oxidative processes. The persistent action of an-
tioxidants causes the increased susceptibility to the specific types of the
oxidative stress. The antioxidant enzyme catalase removes H2O2 from
the body, and that is why the mitochondrial membrane structure is
preserved from damages [112].
10.7. Anticancer drug delivery
ZnO NPs made from plant extracts were utilized for the past 20 years
to stop the growth of cancer cells. It is shown that ZnO NPs can cause the
leukemic cells to die while having no negative impact on the healthy
cells [113]. Additionally, it was demonstrated that ZnO NPs can
significantly increase the selective toxicity towards the tumor T cells
while having no negative effect on the healthy body cells [107]. ZnO
NPs were also demonstrated to have a selective cytotoxic effect against
the brain tumor cells, with no negative effects on the healthy human
astrocytes [102]. Recently, it was discovered that ZnO NPs photo-
synthesized using the Mangifera indica leaf extract were the effective
anticancer drug with the cytotoxic effect comparable to cyclophospha-
mide at low dosages against the lung cancer (A549). The effectiveness of
biosynthesized ZnO NPs as an anticancer drug was also inferred to be
dose-dependent, suggesting that the ZnO NPs’ anticancer activity
peaked when the higher dosages were administered [114]. Compared to
other NPs, those of ZnO expanded their use in the cancer treatment
delivery due to their biodegradable features and low toxicity. When
medications including baicalin, curcumin, doxorubicin, and paclitaxel
are placed onto ZnO NPs as the delivery vehicles, they show the
improved solubility and the increased toxicity [115,116].
10.8. Applications for antimicrobials
Exceptional properties of ZnO NPs can be also linked to their anti-
microbial properties [117]. Numerous researches examined the anti-
microbial activity of biosynthesized ZnO NPs against various bacteria
and fungi, and they found them to be quite effective [118,119]. The
Escherichia coli and Staphylococcus aureus development was examined
using the shake flask method at different concentrations of photo-
synthesized ZnO NPs. It was found that ZnO NPs could inhibit the bac-
terial cell growth because it was significantly slower in the presence of
ZnO NPs than that of the bacteria in the control group. For E. coli and
S. aureus, respectively, the bacterial growth declines by 5.1–100 % and
23–99 % when the ZnO NP concentration rises [120].
10.9. Antibacterial properties of synthesized ZnO NPs from plant extracts
Bacteria are diverse, widespread, single-celled organisms that are
almost omnipresent in daily life. Injurious to the human health, they
have a strong capacity for the survival, reproduce swiftly, and adapt to
the shifting environmental conditions. The antibiotic resistance among
bacteria is on the rise, endangering human life seriously [121]. The issue
of the antibiotic resistance brought on by the formation of bacterial
biofilms may be resolved by using plant extracts in the green production
of ZnO NPs [122]. They will become a new topic of studies in the realm
of the antibacterial agents due to their strong antibacterial capabilities,
biocompatibility, non-toxicity, safety, and stability traits [78] as was
shown for ZnO NPs produced with the aid of the Prunus dulcis extract,
which antibacterial properties were studied using a disc diffusion
technique. According to Upadhyaya et al.48. Dobrucka and Dugaszew-
ska, biosynthesized ZnO NPs prevented the growth of S. aureus, E. coli,
and S. paratyphi. ZnO NPs synthesized using the Lawsonia inermis leaf
extract prevented the growth of B. subtilis and P. aeruginosa. Even
gentamicin did not have the same inhibitory effect on P. aeruginosa as
ZnO NPs produced using the extract of Trifolium pratense flowers, as was
discovered by [123]. Stan et al. ZnO NPs produced using the Allium
sativum extract inhibited S. aureus, E. coli, B. subtilis, P. aeruginosa,
L. monocytogenes, and S. typhimurium, all to a greater extent than ZnO
NPs produced chemically. ZnO NPs made from the Dysphania ambro-
sioides extract had an inhibitory effect on S. aureus and S. epidermidis that
was comparable to chlorhexidine, according to [124]. Finally, ZnO NPs
produced from the Aloe vera extract was shown to be efficient in elim-
inating the clinical isolates of methicillin-resistant S. aureus (MRSA)
when combined with antibiotics [45].
11. Conclusion
This review article discusses potential environmental and energy
applications while concentrating on the ecologically friendly or green
synthesis of ZnO NPs. The plant species used for the green synthesis has a
negligible impact on the production and the appearance of these NPs,
unless the extract is lacking in the bioactive compounds. The aim of this
review is to better understand how the green synthesis is developing in
the effective production of ZnO NPs, while these NPs are widely applied
in different potential industrial uses. In general, the fabrication of ZnO
NPs from the natural raw materials is more environmentally friendly,
while the resultant nanoproduct more biologically compatible and
active.
CRediT authorship contribution statement
All authors equally contribute to Conceptualization, Methodology,
Formal analysis, Investigation, Writing, and Visualization, under Su-
pervision of the corresponding author J. Simal-Gandara.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgements
Funding for open access charge: Universidade de Vigo/CISUG.
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