Switchgrass (<i>Panicum virgatum</i>) Extract Mediated Green Synthesis of Silver Nanoparticles

doi:10.4236/wjnse.2012.22008

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World Journal of Nano Science and Engineering, 2012, 2, 47-52

http://dx.doi.org/10.4236/wjnse.2012.22008 Published Online June 2012 (http://www.SciRP.org/journal/wjnse)

Switchgrass (Panicum virgatum) Extract Mediated Green

Synthesis of Silver Nanoparticles

Cynthia Mason1,2, Singaravelu Vivekanandhan1,2, Manjusri Misra1,2*, Amar Kumar Mohanty1,2

1School of Engineering, Thornborough Building, University of Guelph, Guelph, Canada

2Bioproducts Discovery and Development Centre, Department of Pla nt Agricultu re,

University of Guelph, Guelph, Canada

Email: *[email protected]

Received January 25, 2012; revised February 16, 2012; accepted February 28, 2012

ABSTRACT

A novel switchgrass (Panicum virgatum) extract mediated green process was demonstrated for the synthesis of silver

nanoparticles from silver nitrate solution at ambient temperature. UV-visible spectroscopic analysis indicates the rapid

reduction of silver (Ag+) ions by swithgrass extract. The silver nanoparticles began to form at 15 min and the reduction

reaction was completed within 2 hours. Synthesized silver nanoparticles were subjected to x-ray diffraction (XRD) for

structural characterization, which confirms the FCC symmetry of silver nanoparticles with the lattice parameter of

4.0962 Å. The particle size of bio-synthesized silver nanoparticles was identified through transmission electron

microscopic (TEM) analysis and found to be in the range of 20 - 40 nm.

Keywords: Panicum virgatum; Biosynthesis; Silver Nanoparticles

1. Introduction

Silver nanoparticles receive enormous scientific, techno-

logical, and commercial attention due to their unique size

and shape dependent properties [1,2]. Extensive research

has been devoted to explore the applications of silver

nanoparticles in diversified fields including healthcare/

biomedical [3-5], sensors [6], spectroscopy [7] and cata-

lysis [8]. One of the challengin g tasks in the synthesis of

nanostructured materials is the precise control of size and

shape [9]. Especially, silver nanoparticles exhibit drastic

variation in their physicochemical properties with the

size, shape, and their conjugation with other organic/

biological substances [10-12]. The synthesis processes of

silver nanoparticles play a major role in the control of

their size and shape, thus wide range of physical, chemi-

cal, as well as biological methods have been established

and reported [13-15]. Among them, biological processes

that are based on bacteria, fungus, bio-derived chemicals,

and plant extracts are extensively investigated due their

eco-friendly protocol and better morphological control

[16-18]. Using “green” methods in the synthesis of silver

nanoparticles has increasingly become a topic of interests

as conventional chemical methods are expensive and re-

quire the use of chemical compounds/organic solvents as

reducing agents.

Recently, plant (leaf, flower, seed, tuber, and bark) ex-

tract mediated biological process for the synthesis of

silver nanoparticles has been extensively explored and

compared to other bio-inspired processes [19-26]. A

range of plant extracts have been investigated for their

ability to efficiently synthesize silver nanoparticles, and

are mentioned as follows. Shankar et al., demonstrated a

geranium (Pelargonium graueolens) leaf extract based

biological process for the synthesis of silver nanoparticles

[21]. Song et al., used persimmon (Diopyros kaki) leaf

extract for the synthesis of bimetallic gold/silver

nanoparticles [22]. Sathishkumar et al. has synthesised

silver nanoparticles using Cinnamon zeylanicum bark

extract and reported their bactericidal activity [23]. An-

anda Babu et al. reported the synthesis of silver nanopar-

ticles using Calotropis procera flower extract at room

temperature [24]. In addition, we have also explored the

synthesis of silver nanoparticles using soy (Glycine max)

and curry (Murraya Koengii) leaf extracts [25,26]. Simi-

larly, neem (Azadirachta indica) and mango (Mangifera

indica) leaf extracts were effectively utilized for the syn-

thesis of silver nanoparticles [27,28]. Apart from silver

nanoparticles, plant ex tract mediated biological processes

are also explored for the synthesis of gold and palladium

nanoparticles [29,30]. Detailed literature work on the

plant extract mediated biological synthesis of metal

nanoparticles has been performed by Rai et al. and Ira-

vani [31,32]. Recent accomplishments in the plant leaf

extract mediated biological process include the impreg-

nation of silver nanopartic l e s into carbon nanotubes, which

*Corresponding author.

C. MASON ET AL.

indicates new opportunities for this process in the

development of novel multifunctional materials [33,34].

Present research has prompted for further exploration in

the use of plant extracts for the synthesis of silver nano-

particles from switchgrass extract. Switchgrass is a war m-

season perennial plant that requires minimal agriculture in-

puts (including pesticides, energy, and fertilizer), with the

ability to survive on marginal lands, providing economic

and envi r onm ent al a dv ant a ges. S witc h gr ass ha s be e n wi del y

used as fuel for generating energy and is currently used as

feedstock for bio-ethanol production. Despite these devel-

opments in its ma ny u ses, th ere are currently no r eports that

show the bio-reduction mechanism of switchgrass extract

for the synthesis of silver nanoparticles. This report outlines

the use of switchgrass extract as the reducing agent in the

reaction that converts silver ions into silver nanoparticles.

Bioreduction mechanism of switchgrass extract for the syn-

thesis of silver nanoparticles was investigated trough UV-

visible, XRD, TEM, and XRD techniques.

2. Experimental

2.1. Preparation of Switchgrass Extract

The switchgrass extract was made using 20 g of fresh

switchgrass, which was obtained from the Elora Research

Station, University of Guelph, Ontario. A photograph of

switchgrass is shown in Figure 1. Prior to extract pre-

paration, the switchgrass was cleaned thoroughly using

deionized water and then cut into small pieces. The swit-

chgrass sample was then added into 125 mL of boiling

deionized water, and left to boil for 3 minutes. The so lu-

tion was then removed from the heat source and left to

cool to ambient temperature (approximately 23˚C). Fol-

lowing this step, the extract was then filtered through a

course sieve to remove any leaf matter and the resultant

filtrate was then refrigerated.

2.2. Synthesis of Silver Nanoparticles

The silver nitrate (AgNO3) used in this experiment was

Figure 1. Photograph of switchgrass.

obtained from Sigma Aldrich. 3 mL of switchgrass ex-

tract was added to 60 mL of 10–3 M AgNO3 solution and

the reaction was left to take place at ambient conditions.

The observed change in color from colorless to transpar-

ent yellow and finally to a dark brown with time, indi-

cating the formation of silver nanoparticles. Reduction of

the Ag+ ions was monitored with respect to time using

UV-visible spectral analysis. Once the reaction mixture

had reached a dark brown color, it was then centrifuged

in order to collect the silver nanoparticles. The nanoparticles

were washed an additional two times using deionized

water, and were then re-suspended in 95% ethanol (Fisher

Scientific) prior to characterization.

2.3. Characterization

Optical absorbance of the synthesized silver nanoparticles

was performed using a UV-visible spectrophotometer

(Varian Cary 300 Bio) between the wavelengths of 300

and 700 nm at a resolution of 1 nm. The reaction mixture

was first diluted 15 times with distilled water and used

for UV-visible analysis. Transmission electron micro-

scopy (TEM) was performed on the silver nanoparticles

us in g a LE O mo d el 912 AB instrument at the acceleratin g

voltage of 100 k. A drop of the silver nanoparticle-ethanol

dispersion was placed on a carbon coated copper grid,

which allowed the ethanol to evaporate before analysis

began. The phase purity and the crystalline structure of

bio-systhesized silver nanoparticles were investigated

through x-ray diffraction technique using a Rigaku

Mul-tiflex x-ray powder diffractometer employing CuKα

radiation. The silver nanoparticle dispersion was placed

on a glass slide and the solution (ethanol) was allowed to

evaporate such that a thin film of silver nanoparticles

remained. This thin silver film was subjected to x-ray

diffraction operating between 10˚ and 80˚, with a scan-

ning rate of 2˚ per minute. The average crystallite size of

the silver nanoparticles was calculated using a line

broadening profile of (111) peak at 38˚ and Sherrer’s

formula as follows,

d = 0.9 (λ)/β Cos θ

where λ is the wavelength (1.5418 Å), β is the full width

half maximum (FWHM) of corresponding peek, and θ is

the angle of the diffra ction pe ek .

3. Results and Discussion

Figure 2 shows the color change of reaction mixture

( silver n itrate solu tion and switchg rass extr act) with respect

to time. Color change can be observed at 15 minutes from

colorless to faint yellow, indicating the formation of sil-

ver nanoparticles. As time elapsed, the yellow colored

solution eventually became dark brown at 120 minutes,

which is due to the increasing concentration of silver

nanoparticles as well as the particles’ growth in size. Ther e

C. MASON ET AL. 49

0 15 30 60 75 90 120 180

(In minutes)

Figure 2. Photograph of reaction mixture (switchgrass extract

and silver nitrate solution) as a function of time.

is no significant change beyond 180 minutes, therefore

indicating the completion of the reduction reaction. This

was further confirmed by UV-vis spectroscopic analysis.

This physical appearance of the reaction mixture turning

from yellow to brown is due to the surface plasmon reso-

nance (SPR) of the silver nanoparticles, which is consid-

ered to be the primary signature of nanoparticle formation.

UV-vis spectroscopy is a versatile technique to understand

the bioreduction mechanism of silver ions into silver

nanoparticles by switchgrass extract. The UV-vis spectra

of the reaction mixture recorded as a function of time, is

are shown in Figure 3(a). An observed peak at 435 nm is

assigned to the surface plasmon resonance band (longitu-

dinal vibration) of the silver nanoparticles, which is com-

parable with the literature values and exhibits continuous

rise in intensity without any ch ange in the peak position as

a function of time. During 15 - 60 minutes intervals the

absorption peak was weak and broad, which indicates the

smaller size of silver nanoparticles. Nucleation occurs

between 60 - 90 minutes, which appeared as strong a ab-

sorption peak, as shown in Figure 3(a). Figure 3(b) shows

the absorbance at λmax (i.e. at 435 nm) as a function of time.

From Figure 3(b) it is indentified that the reduction of

silver ions to silver nanoparticles occurs quite rapidly, as

more than 90% of the bioreduction reaction completes

within 90 minutes. Th is is faster than earlier stud ies of the

synthesis of silver nanoparticles usi ng biol ogical sources.

The crystalline structure of the bio-syntheized silver

nanoparticles was investigated by XRD analysis and the

obtained x-ray diffraction pattern is shown in Figure 4.

The obtained diffraction peaks at 38˚, 44˚, 64˚ and 77˚

are respectively assigned to (111), (200), (311) and (222)

plans, which indicates that the synthesized silver nanop ar-

ticles are crystallized in face centered cubic (fcc) sym-

metry. No additional diffraction peaks were observed

other than the characteristic peak of the silver structure

that reflects the purity of synthesized silver nanoparticles.

The lattice parameter (A) of the bio-synthesized silver

nanoparticles was calculated from the diffraction data

and was found to be A = 4.0962 Å, which is comparab le

with the JCPDS value. Th e calculated crystallite size has

been found to be ~10 nm, which is comparable with the

particle size as obtained from TEM analysis.

(a)

(b)

Figure 3. (a) UV-vis absorption spectra of reaction mixture

(switchgrass and silver nitrate) and (b) Absorbance at λmax

(i.e., at 435 nm) as a function of time.

Figure 4. XRD pattern of silver nanoparticles synthesized

using switchgrass extract.

The microscopic structure of the switchgrass extract

mediated bio-synthesized silver nanoparticles was inves-

tigated by TEM analysis. The obtained TEM images are

shown in Figure 5. TEM images confirm the formation

of silver nanoparticles with the size range between 20

and 40 nm. In addition, the TEM images show th e shape

of the nanoparticles are highly diversified, which includes

C. MASON ET AL.

(a) (b)

Figure 5. (a)-(d) TEM images of silver nanoparticles synthesized using switchgrass extract.

5. Acknowledgements

spherical, rod-like, triangular, pentagonal, and hexagonal.

It was found that the synthesized silver nanoparticles have

the tendency to aggregate and form the agglomerations. The authors would lik e to acknowledge the 2009, Ontario

Ministry of Agriculture, Food and Rural Affairs (OMA-

FRA)—New Directions & Alternative Renewable Fuels

Research Program Project number SR9225 and NSERC

NCE AUTO21 project for the financial support to carry

out this research.

The agglomerated silver nanoparticles assemble to-

gether without much physical contact. Thus they can be

separated by physical agitation for the further utilization.

4. Conclusion

Silver nanoparticles were successfully synthesized using

switchgrass extract used at room temperature. Synthesis

of silver nanoparticles through this process was fairly

rapid, with 90% of silver ion reduction completed within

90 minutes. TEM analysis confirms that the synthesized

silver nanoparticles exist between 20 and 40 nm and ex-

hibit the tendency to aggregate. XRD analysis indicates

the formation of phase pure silver nanoparticles with

FCC symmetry. The calculated av erage crystallite size is

found to be of 10 nm, which is consistent with TEM

analysis. The fundamental understanding of switchgrass

mediated biological process for the synthesis of silver

nanoparticles will allow the expansion of this p rocess for

the synthesis of gold and palladium nanoparticles and

their applications.

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