A novel approach using plant embryos for green synthesis of silver nanoparticles as antibacterial and catalytic agent

Article abstract of DOI:10.1007/s11164-021-04548-x

The plant extract has been used extensively for biosynthesis of noble metallic nanoparticles due to cost efficiency and simple technology. However, influence of plant growth condition can induce changes in composition and content of bioactive compounds, l

Full text of DOI:10.1007/s11164-021-04548-x

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-021-04548-x
A novel approach using plant embryos for green synthesis
of silver nanoparticles as antibacterial and catalytic agent
Minh‑Trong Tran1,2 · Linh‑Phuong Nguyen3 · Dinh‑Truong Nguyen3 ·
T. Le Cam‑Huong2 · Chi‑Hien Dang1,2 · Tran Thi Kim Chi4 ·
Thanh‑Danh Nguyen1,2
Received: 16 April 2021 / Accepted: 15 July 2021
© The Author(s), under exclusive licence to Springer Nature B.V. 2021
Abstract
The plant extract has been used extensively for biosynthesis of noble metallic nano-
particles due to cost efciency and simple technology. However, infuence of plant
growth condition can induce changes in composition and content of bioactive com-
pounds, leading to difculty in reproducing the metallic nanoparticles. In this work,
we frst utilized the plant embryos (Panax vietnamensis embryos—PVE) cultured
under laboratory condition as a biosource for the synthesis of silver nanoparticles
(AgNPs). This method can reduce unwanted changes of bioactive composition in the
plant. The formation of AgNPs was optimized to aford the best colloidal solution.
The physicochemical properties were characterized using analysis techniques includ-
ing X-ray difraction (XRD), transmission electron microscopy (TEM), selected
area electron difraction (SAED), Fourier transform infrared spectroscopy (FTIR)
and thermal properties. Size of crystalline AgNPs was determined in the range of
3–20 nm with a mean size of 10 nm. The applications were investigated for antibac-
terial and catalytic efect. The biosynthesized AgNPs exhibited high antibacterial
Minh-Trong Tran and Linh-Phuong Nguyen authors contributed equally to this study.
*
*
*
Dinh-Truong Nguyen
truong.nguyen@ttu.edu.vn
Chi-Hien Dang
dangchihien@gmail.com
Thanh-Danh Nguyen
ntdanh@ict.vast.vn
1
2
Graduate University of Science and Technology, Vietnam Academy of Science and Technology,
18 Hoang Quoc Viet, Cau Giay District, Hanoi, Vietnam
Institute of Chemical Technology, Vietnam Academy of Science and Technology, 1A, TL29,
Thanh Loc Ward, District 12 , 70000 Ho Chi Minh City, Vietnam
3
4
School of Biotechnology, Tan Tao University, Long An Province, Vietnam
Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc
Viet, Cau Giay District, Hanoi, Vietnam
Vol.:(0123456789)
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M.-T. Tran et al.
activity against Bacillus subtilis, Staphylococcus aureus and Escherichia coli. The
excellent catalysis behavior of PVE-AgNPs has been investigated for the reduction
of o-, m-, p-nitrophenols and toxic organic dyes including methyl orange, rhodamine
6G and rhodamine B. The efect of catalyst amount on the reaction performance was
studied via kinetic investigation. This work unfolds a potential method for scalable
reproduction of metallic nanoparticles.
Graphic abstract
Keywords Silver nanoparticles · Panax vietnamensis embryos · Antibacterial agent ·
Catalyst · Nitrophenol · Pollutant degradation
Introduction
The outstanding properties of metallic nanoparticles have been used extensively in
various advanced technology innovations ranging from agriculture to environmen-
tal felds [1–3]. Among them, silver nanoparticles (AgNPs) have been explored
widely in optical sensing technology [4, 5], biotechnology [6, 7] and catalysis [8,
9]. Recently, AgNPs are frequently used as antibacterial and antifungal medicines
against a broad range of microbial strains [10–12]. Also, it is well known as an
efective catalyst for the degradation of numerous toxic contaminants via an electron
transfer mechanism [13, 14].
AgNPs can be fabricated using diferent techniques such as γ-radiation [15, 16],
laser ablation technology [17, 18] and chemical reduction [19]. Among them, the
chemical reduction is the most useful and cost-efective method due to scalable
fabrication and easy processing. Several common reductants such as hydrazine
and sodium borohydride have been used frequently for the synthesis of AgNPs.
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A novel approach using plant embryos for green synthesis of…
However, these chemical reductants have numerous disadvantages such as rapid
reduction leading to great size and high toxicity which can limit applicable capac-
ity in pharmaceutical and catalytic felds [20, 21]. Thus, it is highly desirable to
fabricate AgNPs through eco-friendly and sustainable methods for these applica-
tions. Even though various green methods have been reported for the biosynthesis of
AgNPs, the plant-mediated synthesis is well known as an efective technique due to
cost efciency, non-toxic reductants and biocompatible products [22–24]. However,
composition and content of bioactive compounds presented in the plant extract are
strongly dependent on numerous outside factors such as geography, soil condition
and weather, which can lead to low producibility of metallic nanoparticles. Thus, the
usage of the plant embryos grown under laboratory conditions for the fabrication of
metallic nanoparticles is a new approach to address these disadvantages.
In this study, we frst describe the fabrication of AgNPs from aqueous extract of
Panax vietnamensis embryos which are cultured under controllable laboratorial con-
dition. Its antibacterial activity is evaluated for inhibition of three bacterial strains.
The catalytic activity is investigated for the reduction of nitrophenol derivatives and
toxic dyes.
Experimental
Chemicals
All chemicals used were analytical grade without further purifcation. Chemi-
cals including silver nitrate (AgNO3), o-nitrophenol (o-NP), m-nitrophenol (m-
NP), p-nitrophenol (p-NP), sodium borohydride (NaBH4), methyl orange (MO),
rhodamine 6G (Rh6G) and rhodamine B (RhB) were purchased from Acros Co.
(Belgium).
In vitro culture of Panax vietnamensis embryos
Production of Callus samples P. vietnamensis roots were cut into thin cell layers with
approximately 1 mm thick and cultured in MS medium supplemented with 2,4-dini-
trophenylhydrazine (1.0 mg L−1), sucrose (30.0 g L−1) and agar (8.0 g L−1) to develop
the calli according to previous reports [25, 26]. The callus samples were cultured in
fuorescent light of 45 μmol s−1 m−2, temperature of 22 1 °C and relative humidity
of 50–60% in a photoperiod of 16 h. The pH medium was maintained in the range of
5.7–5.8 using NaOH solution (1 M) or HCl solution (1 M) for modifcation before
autoclaving for 20 min at 121 °C. The white callus tissues were explanted to develop
in 10 weeks for the formation of callus samples.
Production of embryo samples P. vietnamensis calli were cut into small pieces and
cultured in SH medium supplemented with benzylaminopurine (1.0 mg L−1), naph-
thaleneacetic acid (0.5 mg L−1), hydrolysate casein (0.2 g L−1), sucrose (30.0 g L−1)
and agar (8.0 g L−1) [27]. The samples were cultured in the same conditions of the
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callus procedure during 12 weeks to obtain the somatic embryos for the biosynthesis
of silver nanoparticles.
Preparation of PVE extract
The PVE was dried at 60 °C for 24 h. The dried powder (10 g) was refuxed with
100 mL of distilled water for 1 h. The mixture was fltered, and the fltrate was
stored in a refrigerator (4 °C) for further studies.
Optimization of AgNPs biosynthesis
The PVE extract was added to AgNO3 solution by stirring in dark condition. Change
in the color of solutions confrmed the AgNPs formation. The reaction parameters
including concentration of Ag+ ion (0–25 mM), reaction temperature (30–100 °C)
and the reaction time (10–120 min) were optimized via measurement of UV–Vis
spectra in the range of 250 to 600 nm. The reduction of silver ions induced an
increase in absorbance at the peak of around 410–450 nm. The nanoparticles bio-
synthesized in the optimized condition were used for their physicochemical charac-
terizations and applications. The solid AgNPs were obtained by centrifugation for
20 min at 4400 rpm and washed with water to remove silver ion and then acetone
to remove the organic impurities. Finally, the nanoparticles were dried in an oven at
60 °C for 6 h.
Physicochemical characterization
For determination of the crystalline structure and composition of AgNPs, powder
X-ray difraction (XRD) analysis was carried out on a Shimadzu 6100 X-ray dif-
fractometer (Japan). The FTIR spectra measured on a Bruker Tensor 27 FTIR spec-
trophotometer (Germany) in the range of 4000–500 cm−1 were applied to detect
covalent bonds of possible functional groups presented in the PVE extract and pow-
dered PVE-AgNPs. For evaluation of the size distribution and morphology of PVE-
AgNPs, TEM (JEOL JEM-1400) and SEM (S-4800 HI-9057–0006) spectroscopy
was used. The energy-dispersive X-ray spectroscopy (EDX) data were analyzed by
an analyzer Horiba (EMAX ENERGY EX-400). The particle size and zeta potential
of the AgNPs solutions were measured by using nanoPartica Horiba SZ-100 (Japan).
Thermogravimetry analysis (TGA) and diferential scanning calorimetry (DSC)
were measured using a LabSys Evo 1600 thermal analyzer (SETARAM, France)
in a temperature range from room temperature to 800 °C in the air atmosphere at a
heating rate of 10 °C min−1.
Antibacterial activity
Antibacterial test of the nanoparticles was carried out by using the disk difu-
sion method as previously reported [28]. Bacillus subtilis and Staphylococcus
aureus as Gram-positive bacteria and Escherichia coli as Gram-negative bac-
terium were utilized to evaluate the antibacterial activity of the PVE-AgNPs
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(1 µL) at diferent concentrations. Standard antibiotic ampicillin (0.01 mg/
mL) and Luria–Bertani (LB) broth were used as positive and negative controls,
respectively. Bacteria were taken out from storage and were cultured in LB broth
at 37 °C for 6 h. After that, a sterile inoculating loop was used to streak bacteria
on agar plates. The plates were incubated at 37 °C for 24 h. The colonies on the
plate were picked up and transferred to liquid media and cultured at 37 °C for
6 h. The culture mixture was gently mixed to create a homogenous suspension.
Hundred microliters of cell suspension was taken out and mixed with 100 µL of
trypan blue (0.4%). Trypan blue cell suspension was transferred to the chambers
of the hemocytometer by a micropipette. The cells were counted under a micro-
scope at 100 × of magnifcation. The number of live cells should be 100 to 200
in the counting chamber (live cells appear colorless and dead cells stain blue).
If the number of live cells were less than 100 or more than 200, concentrating
or diluting the initial sample needed. Subsequently, the total number of living
cells in the culture mixture was calculated based on the number of cells counted.
Then, the original culture mixture was diluted to a solution that had 107 cells per
mL. Hundred microliters of diluted culture mixture (equivalent 106 cells) were
spread on the agar plates to evaluate the antibacterial activity of nanoparticles.
The plates were incubated for 24 h at 37 °C. Antibacterial activity was recorded
via diameters of inhibition zone around the paper disks in millimeter.
Catalytic activity
The catalytic activity of AgNPs was evaluated by reduction of nitrophenols
including o-NP, m-NP and p-NP and degradation of toxic dyes including MO,
Rh6G and RhB with an excess amount of NaBH4 in aqueous medium. The
reduction of nitrophenols was carried out by adding NaBH4 solution (0.5 mL,
0.05 M) into corresponding nitrophenols (2.5 mL, 0.1 mM), and the degrada-
tion of toxic dyes was carried out by adding NaBH4 solution (0.5 mL, 0.025 M)
into the corresponding toxic dyes (2.5 mL, 0.05 mM). Then, solution of PVE-
AgNPs catalyst (1, 2, 3, 4, 5 μL) was added into the reaction mixture. The reac-
tions were performed in the quartz cuvette and monitored via UV–Vis spectro-
photometers. A decrease in the absorption intensity of relative peaks of o-NP
(414 nm), m-NP (389 nm), p-NP (400 nm), MO (464 nm), Rh6G (524 nm) and
RhB (549 nm) confrmed the reactions occurring. Because this work used a very
high concentration of the borohydride with respect to the pollutants concentra-
tion and very low amount of nanocatalyst, the rate equation of pseudo-frst-order
reaction, ln (At/A0) = − kt, should be considered for reduction of nitrophenols
and degradation of organic dyes. In this equation, k is the rate constant, t is the
reduction time, [A0] is the pollutant concentration at t = 0 and [At] is the pollut-
ant concentration at time t. A ratio of [At]/[A0] was calculated from the absorp-
tion intensity of the maximum peaks of the relative compounds. The value k was
determined from slope value of the straight line generated by plots of ln(At/A0)
according to the reduction time.
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Results and discussion
Biogenic synthesis
A schematic route for green synthesis of AgNPs is illustrated in Fig. 1. The dried
powder of PVE, which was cultured via two steps and collected after 22 culturing
weeks, was refuxed with distilled water. It is well known that the plant extract con-
tains a variety of active compounds (e.g., favonoids, protein, polysaccharide, etc.)
that can be responsible for reduction of silver ions to AgNPs. The mechanism of
metallic nanoparticles formation includes mainly three stages: reduction of metal-
lic ions, clustering and further nanoparticle growth [29]. The features of each
stage depend upon the nature and concentration of reducing agents. For instance,
the hydroxyl groups present in favonoids can be responsible for the formation of
AgNPs. The tautomeric transformation in the favonoid structure from enol form to
keto form can create reactive hydrogen atoms that reduce silver ions to AgNPs [30].
Thus, the present work investigated the formation of AgNPs to determine suitable
condition for the synthetic process. The concentration and morphology of PVE-
AgNPs were evaluated using UV–Vis measurement. The nanoparticles synthesized
in the optimized condition were used to investigate physicochemical characteriza-
tions and applied for antibacterial and catalytic efects.
It is well known that the formation of AgNPs can afect signifcantly their physic-
ochemical characterization and biochemical properties. Thus, the reaction condition
of the biosynthesis should be explored carefully to evaluate morphology and con-
centration of formed AgNPs based on changes in surface plasmon resonance (SPR)
of the particles in UV–Vis spectra. In the present work, three parameters of the bio-
synthesis process including Ag+ ion concentration, temperature and time were used
to study the optimization.
The UV–Vis spectra obtained with a change in the factors are shown in Fig. 2.
The infuence of Ag+ ion concentrations on the formation of PVE-AgNPs (Fig. 2A)
can be observed at peaks of around 435 nm. The exploration was carried out in the
concentration range of 0–25 mM. The result revealed that the SPR peaks did not
appear clearly at the low concentration range of Ag+ ion (<20 mM). An increase
in the absorption intensity with concentrated Ag+ ion indicates PVE-AgNPs
Fig. 1 Schematic illustration of biogenic synthesis of PVE-AgNPs
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Fig. 2 UV–Vis spectra for efect of reaction condition on synthesis of PVE-AgNPs: A concentration of
Ag+ ion, B reaction temperature and C reaction time
concentration formed steadily increasing in the colloidal solution. Moreover, an
increase in the Ag+ ion concentration did not induce a change in absorption wave-
length, which confrmed the stable morphology of the nanoparticles.
For the efect of reduction temperature, the synthesis of PVE-AgNPs was inves-
tigated in the range of 30 to 100 °C. The formation of AgNPs was only observed at
high-temperature region (>60 °C) where the SPR peak of AgNPs appeared clearly
at around 440–450 nm. The results showed that the reducing activity of bioactive
components in the PVE extract is similar to some other plant extracts as previously
reported [28]. An increase in absorption intensity with increasing reaction tempera-
ture and achieving a maximum value at 90 °C indicated the strong dependence of
AgNPs formation on the reduction temperature. The decrease in AgNPs concentra-
tion in colloidal solution at 100 °C relates to aggregation of AgNPs. The reduction
in high-temperature requirement for the fabrication of PVE-AgNPs may be related
to limited phytochemicals in the embryos as compared with fully grown plants [31].
The reduction time of Ag+ ion was investigated in the range of 0–120 min with
heating at 90 °C. The results are shown in Fig. 2C. The increase in absorption inten-
sity with extending reaction time confrmed the higher concentration of the PVE-
AgNPs in the colloidal solution. The formation of AgNPs is not observed for ini-
tial 60 min of stirring process. The SPR peak only appears clearly after stirring for
80 min. It is clear that the formation of AgNPs in the presence of PVE extract occurs
slowly which can relate to the negative ions presented in the extract [32].
For further studies on their physicochemical characterizations and applications,
the PVE-AgNPs were synthesized at the optimized conditions including silver ion
concentration (20 mM), reaction temperature (90 °C) and reaction time (120 min).
Physicochemical characterizations of biosynthesized AgNPs
FTIR spectroscopy was used to identify the functional groups of bioactive com-
pounds presented in the nanoparticles as the stabilization agents. The FTIR spec-
tra of dried PVE extract and PVE-AgNPs are shown in Fig. 3. Both the spectra
indicate absorption band similarly. Accordingly, the extract spectrum shows major
bands at 3413, 2930, 1663, 1576, 1408 and 1072 cm−1. The broad band centered
at 3413 cm−1 is assigned to the stretching vibration of O–H and N–H groups which
can relate to bioactive compounds such as polyphenols, favonoids, polysaccharides,
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M.-T. Tran et al.
Fig. 3 FTIR spectra of PVE
extract (a) and PVE-AgNPs (b)
(b)
(a)
1387
1641
2922
2930
1006
3561
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1408
1072
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1576
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Wavelength (cm-1)
proteins and alkaloids [33, 34]. The band at 2930 cm−1 is associated with C–H
stretching vibration of polysaccharides and saponins. The strong band at 1630 corre-
sponds to the stretch vibration of C=O bond relating to amides and carboxylic acid
groups. The bands at 1408 cm−1 and 1072 cm−1 are associated with the stretching
vibrations of C=C aromatic bond and C–O bond, respectively [35]. The spectrum
of PVE-AgNPs shows the similar bands at 3561, 2922, 1641, 1387 and 1006 cm−1.
It refects that the biomolecules act as the stabilization agents in the biosynthesis of
nanoparticles.
Crystalline structure of PVE-AgNPs can be determined by using XRD analysis
as shown in Fig. 4. The Bragg refections of crystal AgNPs presented in the bio-
synthesized nanoparticles were found at 2θ values of 38.24°, 64.80° and 77.78°
related to (111), (220) and (311) face-centered cubic (fcc) planes of AgNPs crystal,
respectively (card no. 00–004-0783). On the other hand, the appearance of strong
peaks at 27.62°, 32.32°, 46.10°, 54.91° and 57.30° corresponds to refections from
(111), (200), (220), (311) and (222) planes of crystal AgCl (card no. 00–001–1013)
[36, 37]. Additionally, the lattice parameters of AgNPs and AgCl were found to be
4.0 and 5.5 Å, respectively (Table S2), which confrmed the presence of AgCl and
AgNPs in the sample PVE-AgNPs [38, 39]. The AgCl crystal in the biosynthesized
AgNPs can relate to the presence of chloride ion in the PVE extract which can be
originated from culture medium and biosynthesis of PVE [32].
Fig. 4 XRD pattern of PVE-
1600
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800
AgNPs
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*
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For study on morphology and elemental compositions of the PVE-AgNPs,
SEM and EDX analysis of the powder sample was used. Particle size and crys-
talline nature were determined by TEM analysis and SAED pattern, respectively.
The results are shown in Fig. 5. The SEM images confrm that PVE-AgNPs were
Fig. 5 A SEM image, B EDX pattern and elemental composition (inset), C TEM image and particle size
distribution (inset), D selected area electron difraction (SAED) pattern, E scanning transmission elec-
tron microscopy (STEM) and F–J elemental mapping of PVE-AgNPs
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M.-T. Tran et al.
in a cluster manner formed from spherical particles with diameters below 25 nm
(Fig. 5A). The EDX data reveal silver element presented in the solid sample that is
confrmed by strongly elemental signals of 2.6 and 3.0 keV corresponding to bind-
ing energy of silver [40]. The bioactive organic composition in the sample is rep-
resented by carbon and oxygen elements. An intense peak at 2.7 keV is related to
binding energy of chloride, which demonstrated the presence of crystalline AgCl in
the sample [41]. Moreover, the EDX analysis shows the predominant composition
of silver in the synthesized nanoparticles (76.23%, w/w) that promises to use efec-
tively as bioactive and catalytic agents.
TEM analysis of PVE-AgNPs reveals an even geometry with the spherical parti-
cles (Fig. 5C) and narrow size distribution of nanoparticles is observed in the range
of 3–20 nm with an average size of 10 nm. It indicates that the bioactive compounds
in the PVG extract can stabilize efectively to form the nanoparticles with small size
which can enhance activities of AgNPs. The SAED pattern shows bright rings cor-
responding to the crystalline nature of both AgNPs and AgCl. The circular rings are
assigned to refection planes of respective crystalline AgCl ((111), (200), (220) and
(222)) and AgNPs ((111) and (311)), which is in agreement with the XRD data. The
formation of composite Ag/AgCl has been observed in the biogenic AgNPs reported
previously [42, 43]. The STEM and EDX mapping analysis (Fig. 5E–J) showed that
elements, including silver, chloride, carbon and oxygen, indicated the presence of
AgNPs, AgCl and organic components in the nanoparticles.
The thermal properties of PVE extract and the biosynthesized AgNPs were stud-
ied using TGA–DSC analysis. The result reveals the diferent thermal behaviors
between the solid samples as shown in Fig. 6. The PVE extract showed the mass
loss of 10% in temperature region below 100 °C that relates to the evaporation of
water and volatile compounds and about 50% in temperature region of 200 °C, while
the PVE-AgNPs sample is almost stable in this temperature region. It refects that
the biosynthesized nanoparticles do not possess volatile components. The extract is
lost about 96% of weight at 800 °C, while the PVE-AgNPs showed about 40% of
weight loss at this temperature region. The result of extract DSC showed endother-
mic peaks at low-temperature region, which confrmed evaporation process of the
Fig. 6 TGA and DSC curves of
PVE extract and PVE-AgNPs
in air fow of 20 mL min−1 at a
heating rate of 10 °C min−1
90
60
30
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exo
PVE extract
PVE-AgNPs
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volatile compositions. Both DSC curves of PVE extract and PVE-AgNPs showed
the similar exothermic peaks at 280–300 °C, 445 °C and 540–550 °C, which are
related to the oxidation of organic compositions. It confrms the organic molecules
in the extract as stabilizing agents in the biosynthesized AgNPs.
Antibacterial assay
The antibacterial activity of PVE-AgNPs is evaluated against three microbial strains
including B. subtilis and S. aureus (Gram-positive bacteria) and E. coli (Gram-nega-
tive bacterium) at various concentrations. The PVE extract also tested at the highest
concentration exhibited no bioactivity against all the tested strains. The bioactiv-
ity of the biosynthesized PVE-AgNPs and plots of inhibition zone versus various
concentrations are illustrated in Fig. 7. The result shows that PVE-AgNPs exhibit
a good antibacterial activity against all tested bacteria and an increase in sample
concentrations enhances their activity. The diference of bioactivity among the bac-
terial strains can be recognized at each test concentration. It can relate to antibacte-
rial mechanism of AgNPs which strongly depend on the cell wall of each bacterial
strain and membrane generation through hydrophobic and bioaccumulation mecha-
nism [44]. The samples tested at concentration of 2.5 mM did not display any activ-
ity on all the strains, and the zones of bacterial inhibition grow when increasing
Fig. 7 Antibacterial activity of PVE-AgNPs tested at 1.0 μL of solutions with the diferent concentra-
tions (2.5, 5.0, 10.0, 15.0 and 20.0 mM) against E. coli (A), B. subtilis (B), S. aureus (C) and plotting
inhibition zone of various bacteria versus the diferent concentrations (D)
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M.-T. Tran et al.
the concentration of PVE-AgNPs. Minimum inhibition concentration (MIC) values
are found to be 5.0 mM for all the tested strains. PVE-mediated green synthesis
of AgNPs showed similar activity to AgNPs biosynthesized from the plant extract
[45–48]. Therefore, the PVE extract is suggested to use as a green source for the
synthesis of metallic nanoparticles with potential applications in biotechnology.
Catalysis activity
The reduction of toxic organic pollutants including nitrophenols and dyes which
pose serious threat to the living environment is widely considerable in environmen-
tal felds because they are not biodegraded in normally aqueous medium [13, 49].
Treatment of these toxic compounds can be performed by using various methods.
Among them, the catalytic reduction using oxidizing and reducing agents such as
H2O2 and NaBH4 is reported frequently in the last decade. In the catalysis mecha-
nism of the degradation using NaBH4, the surface of AgNPs plays an important role
as an absorbent of pollutant and BH4− ion where the electron transfer process occurs
−
from BH4 (electron donor) to pollutant (electron acceptor) (Fig. 8). The catalytic
performance of the nanoparticles is signifcantly dependent on several principle fac-
tors such as the surface of nanocatalyst, difusion of the reactant, the binding abil-
ity between the catalyst and the reactant and desorption of the reaction products
Fig. 8 Proposed mechanism of AgNPs catalysis in reduction of nitrophenols and organic dyes by NaBH4
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A novel approach using plant embryos for green synthesis of…
[32]. Otherwise, the catalytic behavior of the metallic nanoparticles is strongly
dependent on the substrates and the amount of catalysts. Thus, the present study
has investigated diferent substrates including o-, m-, p-nitrophenols, methyl orange
(MO), rhodamine 6G (Rh6G) and rhodamine B (RhB) with the various doses of the
nanocatalyst.
Reduction of nitrophenols
Reduction of nitrophenols into aminophenols (AP) not only pays considerable atten-
tions in environmental feld, but also possesses the potential applications in pharma-
ceutical technology. The AP derivatives are well known as key intermediates and
precursors for numerous drugs such as pain treatment drugs (e.g., phenacetin, par-
acetamol, etc.) or sulfonamide drugs (sulfanilamide, protosil, etc.) [50]. The reduc-
tion kinetics of nitrophenols in the presence of PVE-AgNPs with a time-dependent
manner of concentration were monitored in situ by measuring UV–Vis absorption
from the cuvette. The reactions were confrmed by discoloration of the pollutant
solution and decreasing absorbance values at relative peaks in the UV–Vis spectra.
Figure 9 illustrates the reduction of o-NP into o-AP in the presence of diferent
doses of PVE-AgNPs solution (1–5 μL) which shows the degradation of absorp-
tion intention at peak 414 nm in UV–Vis spectra. An increasing reaction rate with
the catalyst dose was confrmed via reduction of reaction time. The reduction of
o-NP at 1 μL dose of the colloidal solution was completed in 27 min. However,
reaction performance was increasing with 18 min in the presence of 3 μL PVE-
AgNPs and even only taken 9 min at 5 μL PVE-AgNPs. For investigation of the
kinetic, the plots of ln(A0/At) values versus reaction time showed the good linear
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L
400
500
600
1
2
3
4
5
0
400 800 1200 1600
Time (s)
Wavelength (nm)
Catalyst volume ( L)
Fig. 9 UV–Vis spectra of o-nitrophenol reduction at diferent volumes of catalyst: 1–5 μL (A–E), plots
of frst-order kinetic (F) and rate constant values (G)
1 3

M.-T. Tran et al.
correlations for reduction of o-NP that coincided with the pseudo-frst-order reac-
tion (Fig. 9E). Furthermore, the rate constants calculated are quickly increased with
high concentration of the nanocatalyst (Fig. 9F). The value calculated at 1 μL PVE-
AgNPs is 1.14×10–3 s−1, and this value is approximately double at 3 μL AgNPs
(2.18×10–3 s−1) and up to 3.01×10–3 s−1 at 5 μL PVE-AgNPs. It is clear that the
total surface area of the nanocatalyst infuences considerably the reduction behavior
of o-NP.
For the reduction of m-NP, the absorption intensity at peak 398 nm is gradually
reduced according to the reaction time after completing the addition of nanocata-
lyst. The slight infuence of catalyst concentration on improvement in the reduction
performance of m-NP is observed in Fig. 10. The result showed that the reduction
yield was slowly increased from about 85% within 21 min in the presence of 1 μL
PVE-AgNPs to 90% within 15 min at 3 μL PVE-AgNPs and achieved 90% within
12 min at 5 μL PVE-AgNPs. The study on the reaction kinetics showed the rate
constants increasing with the nanocatalyst concentrations (Fig. 10F, G). The values
were found to be 0.97×10–3 s−1, 1.72×10–3 s−1 and 2.03×10–3 s−1at 1 μL, 3 μL and
5 μL doses of PVE-AgNPs solution, respectively.
The similar trend is also found for the reduction of p-NP as shown in Fig. 11.
Decreasing absorption intensity at peak 400 nm in UV–Vis spectra was observed
according to the reaction time. The reduction of p-NP in the presence of PVE-
AgNPs was completed more rapidly in comparison with the other nitrophenols. The
reduction process was completed within 18 min at 1 μL and 2 μL doses, and the
reaction time was achieved a maximum value of 12 min at 3 μL dose. Infuence
of catalyst amount on the reaction performance was not observed obviously when
higher volume of the colloidal solution used. The similar result was also observed
A
B
D
C
0.3
0.2
0.1
0.0
0.3
0.2
0.1
0.0
0.3
0.2
0.1
0.0
0.3
0.2
0.1
0.0
2
L
3
L
4 L
1
L
12 min
17 min
17 min
15 min
350
400
450
500
550
350
400
450
500
550
350
400
450
500
550
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
E
F
G
0.3
0.2
0.1
0.0
6
4
2
0
0
-1
-2
-3
-4
5
L
12 min
1
2
3
4
5
L
L
L
L
L
350
400
450
500
550
1
2
3
4
5
0
400
800
1200
Wavelength (nm)
Time (s)
Catalyst volume ( L)
Fig. 10 UV–Vis spectra of m-nitrophenol reduction at diferent volumes of catalyst: 1–5 μL (A–E) plots
of frst-order kinetic (F) and rate constant values (G)
1 3

A novel approach using plant embryos for green synthesis of…
A
B
C
D
2.5
2.0
1.5
1.0
0.5
0.0
2.5
2.0
1.5
1.0
0.5
0.0
2.5
2.0
1.5
1.0
0.5
0.0
2.5
2.0
1.5
1.0
0.5
0.0
1
L
3
L
2
L
4
L
18 min
12 min
18 min
12 min
400
500
350
400
450
500
550
350
400
450
500
550
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
E
G
F
2.0
1.5
1.0
0.5
0.0
6
4
2
0
0
-1
-2
-3
-4
5
L
12 min
1
2
3
4
5
L
L
L
L
L
350
400
450
500
550
1
2
3
4
5
0
400
800
1200
Wavelength (nm)
Catalyst volume ( L)
Time (s)
Fig. 11 UV–Vis spectra of p-nitrophenol reduction at diferent volumes of catalyst: 1–5 μL (A–E), plots
of frst-order kinetic (F) and rate constant values (G)
for the rate constants which were increased from 2.1×10–3 s−1 at 1 μL dose to
4.2×10–3 s−1 at 3 μL dose. In general, the best performance for reduction of p-NP
was found at 3 μL dose of PVE-AgNPs used.
As given in Table 1, the reduction ability using PVE-AgNPs is increasing in the
order of m-NP<o-NP<p-NP at each catalyst amount used. It is clear that the cata-
lytic reduction of nitrophenols is dependent on the structure of substrates, which can
be related to the difusion ability of nitrophenol and desorption ability of aminophe-
nol from the surface of catalyst. In comparison with AgNPs synthesized from plant
extracts, plant embryo-mediated AgNPs in this work showed good efciency for the
reduction of nitrophenols [51–54].
Degradation of organic dyes
PVE-AgNPs catalyst was evaluated for degradation of three water-soluble dyes
including MO, Rh6G and RhB which is used widely and become popular pollut-
ants in industrial wastewater. The dyes with diferent structural characters possess
various absorption peaks of 464 nm (MO), 524 nm (Rh6G) and 549 nm (RhB) in
UV–Vis spectra even after the addition of NaBH4 solution the peaks remained as
same. The weak degradation of dyes without the catalyst can be observed after
30 min and reported elsewhere [55].
Methyl orange is an organic azo dye used commonly in various felds. MO is
one important contaminant of wastewater that induces several environmental and
health problems. Indeed, its degradation with low-cost and simple technique is
paying attentions. The result revealed that the catalyst amount clearly infuenced
the reaction rate (Fig. 12). The completion of MO degradation was taken about
1 3

M.-T. Tran et al.
1 3

A novel approach using plant embryos for green synthesis of…
A
D
B
C
1.5
1.2
0.9
0.6
0.3
0.0
1.5
1.2
0.9
0.6
0.3
0.0
1.5
1.2
0.9
0.6
0.3
0.0
1.5
1.2
0.9
0.6
0.3
0.0
1
L
2 L
3
18 min
L
4
L
21 min
18 min
15 min
400
500
600
400
500
600
400
500
600
400
500
600
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
E
F
G
1.5
1.2
0.9
0.6
0.3
0.0
15
10
5
0
-1
-2
-3
-4
-5
5
L
12 min
1
2
3
4
5
L
L
L
L
L
0
400
500
600
1
2
3
4
5
0
500
1000
Time (s)
Wavelength (nm)
Catalyst volume ( L)
Fig. 12 UV–Vis spectra of methyl orange degradation at diferent volumes of catalyst: 1–5 μL (A–E),
plots of frst-order kinetic (F) and rate constant values (G)
21 min at 1 μL PVE-AgNPs, 18 min at 3 μL PVE-AgNPs and 12 min at 5 μL PVE-
AgNPs. Good linear correlations between ln(A0/At) values and the reaction times
confrmed that the degradation of organic dye occurred via the pseudo-frst-order
reaction. Moreover, the increasing rate constants with catalyst amount were found
to be 2.09×10–3 s−1 at 1 μL PVE-AgNPs, 2.82×10–3 s−1 at 3 μL PVE-AgNPs and
4.15×10–3 s−1 at 5 μL PVE-AgNPs.
Rhodamine 6G, a fuorescent dye, is used extensively in biotechnology felds.
The detoxifcation of this contaminant is signifcantly considerable for wastewater
treatment. The result indicated the reaction occurring much difcultly in comparison
with other contaminants, which was confrmed by a slow decrease in the absorption
intensity according to time at the same doses of the nanocatalyst (Fig. 13). The deg-
radation was completed within more than 60 min at 1 μL PVE-AgNPs (70% yield
in 60 min), 60 min at 2 μL PVE-AgNPs, 18 min at 3 μL PVE-AgNPs and 15 min
at 5 μL PVE-AgNPs. The reaction kinetics showed that although the rate constants
were increased with the catalyst amount, the rate constants were generally lower
than the values of the other contaminants at the same dose. The values were found to
be 0.38×10–3 s−1, 2.06×10–3 s−1 and 3.54×10–3 s−1at 1 μL, 3 μL and 5 μL doses of
PVE-AgNPs solution, respectively.
Rhodamine B is widely employed in biotechnology and textile felds for vari-
ous purposes and forms an important contaminant in wastewater. Due to great
stability, its removal is highly desirable. The result revealed strong efect of the
nanocatalyst amount on the degradation efciency as shown in Fig. 14. The reac-
tion was completed within 25 min at 1 μL PVE-AgNPs. When the nanoparticles
amount was increased at 3 μL and 5 μL, the reaction time was signifcantly reduced
within only 12 min and 6 min, respectively. The rate constant values were found to
1 3

M.-T. Tran et al.
B
A
C
D
2.5
2.0
1.5
1.0
0.5
0.0
2.5
2.0
1.5
1.0
0.5
0.0
2.5
2.0
1.5
1.0
0.5
0.0
2.5
2.0
1.5
1.0
0.5
0.0
3
L
1
60 min
L
2
60 min
L
4
L
15 min
18 min
450
500
550
600
450
500
550
600
450
500
550
600
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
E
F
G
2.5
2.0
1.5
1.0
0.5
0.0
15
10
5
0
-1
-2
-3
-4
-5
5
L
15 min
1
2
3
4
5
L
L
L
L
L
0
450
500
550
600
1
2
3
4
5
0
1000 2000 3000 4000
Wavelength (nm)
Catalyst volume ( L)
Time (s)
Fig. 13 UV–Vis spectra of rhodamine 6G degradation at diferent volumes of catalyst: 1–5 μL (A–E),
plots of frst-order kinetic (F) and rate constant values (G)
B
C
A
D
4
3
2
1
0
4
3
2
1
0
4
3
2
1
0
4
3
2
1
0
2
L
1
24 min
L
3
L
4 L
12 min
15 min
12 min
400
450
500
550
600
400
450
500
550
600
400
450
500
550
600
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
G
E
F
4
3
2
1
0
15
10
5
0
-1
-2
-3
-4
-5
5
L
6 min
1
2
3
4
5
L
L
L
L
L
0
400
450
500
550
600
1
2
3
4
5
-500
0
500 1000 1500
Wavelength (nm)
Time (s)
Catalyst volume ( L)
Fig. 14 UV–Vis spectra of rhodamine B degradation at diferent volumes of catalyst: 1–5 μL (A–E),
plots of frst-order kinetic (F) and rate constant values (G)
be 2.16×10–3 s−1 at 1 μL dose, 4.08×10–3 s−1 at 3 μL dose and 12.60×10–3 s−1
at 5 μL dose. In general, the degradation of RhB showed the highest catalytic ef-
ciency among the organic dyes for all doses of PVE-AgNPs. Once again, the data
confrmed the important efect of substrate structures on the catalytic degradation
1 3

A novel approach using plant embryos for green synthesis of…
using the metallic nanoparticles (Table 1). Moreover, AgNPs biosynthesized from
plant embryos extract in this work indicated high catalysis performance of dye deg-
radation in comparison with catalysts synthesized from plant extracts [55–58].
Conclusions
The present study demonstrated the plant embryos as a controllable biosource for
eco-friendly synthesis of metallic nanoparticles. This approach can provide a new
method for scalable production of metallic nanoparticles from the controllable labo-
ratory condition. The silver nanoparticles were well characterized by the analysis
technology of nanomaterials. The mean size of crystalline AgNPs was determined
around 10 nm by TEM spectroscopy. The nanoparticles showed high activity against
three tested bacterial strains. The MIC value was found to be 5.0 mM. The AgNPs
catalyst showed good activity for reduction of nitrophenols and degradation of
organic dyes. The reaction performance was improved signifcantly with an increase
in the catalyst amount and infuenced clearly by the substrate structures. The data
showed that the reaction efciency in the presence of nanocatalyst was increasing
in the order of m-nitrophenol<o- nitrophenol<p-nitrophenol for reduction of nitro-
phenols and Rh6G<MO<RhB for degradation of organic dyes. Therefore, plant
embryos-mediated synthesis can unfold a potential green method for the fabrication
of nanometals as bioactive and catalytic agents.
Supplementary Information The online version contains supplementary material available at https://doi.
org/10.1007/s11164-021-04548-x.
Acknowledgements This work was supported by Vietnam Academy of Science and Technology (No.
NCVCC15.01/21-21).
Data availability statement The data used to support the fndings of this study are included within the
article.
Declarations
Conficts of interest The authors declare that they have no conficts of interest.
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