Green synthesis of silver nanoparticles using tea leaf extract and evaluation of their stability and antibacterial activity

Abstract

A simple, environmentally friendly and cost-effective method has been developed to synthesize silver nanoparticles (AgNPs) using tea leaf extract. We have studied the effects of the tea extract dosage, reaction time and reaction temperature on the formation of AgNPs. The AgNPs were synthesized using silver nitrate and tea extract, and the reaction was carried out for 2 h at room temperature. The synthesized AgNPs were characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analyzer, and zeta potential analyzer. The synthesized AgNPs were nearly spherical, with the sizes ranging from 20 to 90 nm. FT-IR spectral analysis indicated the tea extract acted as the reducing and capping agents on the surface of AgNPs. Furthermore, the study of silver ion release from the tea extract synthesized AgNPs showed a good stability in terms of time-dependent release of silver ions. In addition, the antibacterial activity of AgNPs was determined by monitoring the growth curve and also by the Kirby-Bauer disk diffusion method. Due to the larger size and less silver ion release, the AgNPs synthesized by tea extract showed low antibacterial activity against Escherichia coli.

Full text

Colloids
and
Surfaces
A:
Physicochem.
Eng.
Aspects
444 (2014) 226–
231
Contents
lists
available
at
ScienceDirect
Colloids
and
Surfaces
A:
Physicochemical
and
Engineering
Aspects
jo
ur
nal
ho
me
p
ag
e:
www.elsevier.com/locate/colsurfa
Green
synthesis
of
silver
nanoparticles
using
tea
leaf
extract
and
evaluation
of
their
stability
and
antibacterial
activity
Qian
Suna,
Xiang
Caia,b,
Jiangwei
Lia,
Min
Zhengb,
Zuliang
Chenb,c,
Chang-Ping
Yua,∗
aKey
Laboratory
of
Urban
Environment
and
Health,
Institute
of
Urban
Environment,
Chinese
Academy
of
Sciences,
Xiamen
361021,
China
bSchool
of
Chemistry
and
Materials
Science,
Fujian
Normal
University,
Fuzhou
350007,
China
cCentre
for
Environmental
Risk
Assessment
and
Remediation,
University
of
South
Australia,
Mawson
Lakes,
SA
5095,
Australia
h
i
g
h
l
i
g
h
t
s
•A
simple
and
green
way
was
devel-
oped
to
synthesize
AgNPs
using
tea
extract.
•The
synthesized
AgNPs
was
char-
acterized
by
TEM,
XRD,
FT-IR,
and
ICP-MS.
•Ag+release
from
the
synthesized
AgNPs
was
lower
indicating
the
high
stability.
•The
synthesized
AgNPs
showed
slight
antibacterial
activity
against
E.
coli.
g
r
a
p
h
i
c
a
l
a
b
s
t
r
a
c
t
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
24
September
2013
Received
in
revised
form
10
December
2013
Accepted
24
December
2013
Available online 7 January 2014
Keywords:
Green
synthesis
Silver
nanoparticles
Tea
leaf
extract
Silver
ion
release
Antibacterial
effect
a
b
s
t
r
a
c
t
A
simple,
environmentally
friendly
and
cost-effective
method
has
been
developed
to
synthesize
silver
nanoparticles
(AgNPs)
using
tea
leaf
extract.
We
have
studied
the
effects
of
the
tea
extract
dosage,
reaction
time
and
reaction
temperature
on
the
formation
of
AgNPs.
The
AgNPs
were
synthesized
using
silver
nitrate
and
tea
extract,
and
the
reaction
was
carried
out
for
2
h
at
room
temperature.
The
synthesized
AgNPs
were
characterized
using
transmission
electron
microscopy
(TEM),
X-ray
diffraction
(XRD),
Fourier
transform
infrared
spectroscopy
(FT-IR),
thermogravimetric
analyzer,
and
zeta
potential
analyzer.
The
synthesized
AgNPs
were
nearly
spherical,
with
the
sizes
ranging
from
20
to
90
nm.
FT-IR
spectral
analysis
indicated
the
tea
extract
acted
as
the
reducing
and
capping
agents
on
the
surface
of
AgNPs.
Furthermore,
the
study
of
silver
ion
release
from
the
tea
extract
synthesized
AgNPs
showed
a
good
stability
in
terms
of
time-dependent
release
of
silver
ions.
In
addition,
the
antibacterial
activity
of
AgNPs
was
determined
by
monitoring
the
growth
curve
and
also
by
the
Kirby-Bauer
disk
diffusion
method.
Due
to
the
larger
size
and
less
silver
ion
release,
the
AgNPs
synthesized
by
tea
extract
showed
low
antibacterial
activity
against
Escherichia
coli.
© 2014 Elsevier B.V. All rights reserved.
1.
Introduction
In
recent
years,
silver
nanoparticles
(AgNPs)
have
been
widely
used
in
many
consumer
goods,
such
as
medical
devices,
cleaning
agents,
and
clothing,
due
to
its
unique
antimicrobial
properties.
Generally,
the
method
for
the
AgNP
preparation
involves
the
reduc-
tion
of
silver
ions
in
the
solution
or
in
high
temperature
in
gaseous
∗Corresponding
author.
Tel.:
+86
592
6190768;
fax:
+86
592
6190768.
E-mail
address:
cpyu@iue.ac.cn
(C.-P.
Yu).
environments
[1].
However,
the
reducing
reagents,
such
as
sodium
borohydride,
may
increase
the
environmental
toxicity
or
biological
hazards
[1,2].
Moreover,
the
capping
agents
like
polyvinyl
alcohol
(PVA)
or
gelatin,
have
to
be
used
to
protect
the
AgNPs
from
aggre-
gation.
On
the
other
hand,
the
high
temperature
may
also
increase
the
cost.
Hence,
the
development
of
a
green
synthesis
of
AgNP
by
using
environment-friendly
solvents
and
nontoxic
reagents
is
of
great
interest.
Huang
et
al.
described
the
AgNP
synthesis
using
a
leaf
extract
of
Cinnamomum
camphora,
while
the
reduction
was
considered
due
to
the
phenolics,
terpenoids,
polysaccharides
and
flavonoids
present
0927-7757/$
–
see
front
matter ©
2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.colsurfa.2013.12.065

Q.
Sun
et
al.
/
Colloids
and
Surfaces
A:
Physicochem.
Eng.
Aspects
444 (2014) 226–
231 227
in
the
extract
[3].
Moreover,
the
extracts
of
various
plants,
including
Eucalyptus
hybrid
[4],
Syzygium
cumini
[5],
Sesuvium
portulacastrum
[6],
Boswellia
ovalifoliolata
[7],
Calotropis
procera
[8],
Musa
para-
disiacal
[9],
Acalypha
indica
[10]
were
successfully
used
for
AgNP
synthesis.
In
addition,
tea
leaf
extract
was
used
for
the
AgNP
synthe-
sis.
Begum
et
al.
reported
the
AgNP
synthesized
by
the
ethyl
acetate
extract
of
tea
leaves
[11].
Nadagouda
et
al.
showed
the
synthesized
AgNP
with
the
size
range
of
20–60
nm
[12].
However,
the
reac-
tion
conditions,
including
the
temperature
or
tea
extract
dosage,
the
synthesis
mechanism,
the
AgNP
stability,
and
the
antibacterial
activity
have
not
been
fully
investigated.
Previous
studies
showed
that
AgNPs
would
likely
release
silver
ions
after
entering
the
aquatic
environment
[13,14],
which
would
reduce
the
stability
of
AgNPs.
In
addition,
silver
ions
exhibited
different
physiochemical
properties
and
biological
toxicity
from
AgNPs
[13,15,16].
Therefore,
understanding
of
the
silver
ion
release
from
AgNPs
is
necessary.
Liu
et
al.
reported
more
than
10%
(w/w)
silver
ions
were
released
from
citrate
coated
AgNPs
(2
mg/L)
in
the
air-saturated
(8.3
mg/L)
water
at
pH
5.6
after
24
h
[14].
Lee’s
study
indicated
that
the
silver
ion
release
kinetics
followed
first-order
kinetics
[15].
In
addition,
the
release
rates
of
silver
ions
were
mainly
dependent
on
the
particle
sizes,
the
environmental
factors
(e.g.,
dis-
solved
oxygen,
pH,
temperature)
[13,17],
and
the
capping
agents
[18].
However,
quantitative
data
on
the
silver
ion
release
from
the
green
synthesized
AgNPs
are
limited.
The
present
study
attempts
to
fill
the
knowledge
gap
by
inves-
tigating
the
synthesis,
stability,
and
antimicrobial
ability
of
AgNPs
synthesized
by
tea
extract.
Tea
extract
solution
was
used
as
a
reduc-
ing
and
capping
reagent
for
the
AgNP
synthesis,
and
distilled
water
served
as
the
reaction
medium.
The
reaction
conditions
on
the
synthesis
of
AgNPs
were
studied.
The
obtained
particles
were
ana-
lyzed
by
transmission
electron
microscopy
(TEM),
X-ray
diffraction
(XRD),
Fourier
transform
infrared
(FT-IR)
spectroscopy,
thermo-
gravimetric
analyzer,
and
zeta
potential
analyzer
to
understand
the
morphology
and
capping
of
AgNPs.
The
AgNP
stability
was
eval-
uated
via
the
time-dependent
release
of
silver
ions
from
the
tea
extract
synthesized
AgNPs.
In
addition,
the
antibacterial
activity
by
tea
extract
synthesized
AgNPs
was
also
investigated.
2.
Materials
and
methods
2.1.
Synthesis
of
AgNPs
by
tea
extract
Tea
leaves
extract
was
used
as
a
reducing
agent
for
the
AgNP
syn-
thesis.
16
g
of
dried
green
tea
leaves
(Richun
Tea
Company,
Fujian)
was
added
to
100
mL
ultrapure
water
in
250
mL
Erlenmeyer
flask.
The
mixer
was
boiled
(5
min),
cooled,
filtered,
and
the
filtrate
was
stored
at
4◦C
as
the
stock
solution
and
was
used
within
1
week.
The
total
organic
carbon
(TOC)
content
of
tea
extract
analyzed
by
TOC
analyzer
(TOC-VCPH,
Shimadzu,
Japan),
was
approximately
20
g/L.
The
stock
solution
of
tea
extract
was
diluted
to
1%,
5%,
10%,
25%,
50%
and
100%
(v/v)
as
reducing
and
capping
solution.
750
␮L
silver
nitrate
(10
mM)
was
injected
at
the
rate
of
one
drop
per
second
to
14.25
mL
tea
extract
working
solution
with
vigorously
stirring.
The
working
solution
was
stirred
(700
rpm)
for
120
min
at
25,
40
and
55 ◦C,
respectively.
AgNPs
were
concentrated
and
purified
by
centrifugal
ultrafiltration
(Millipore,
Amicon
Ultra-15
3k,
USA),
and
rinsed
with
Milli-Q
water
(Millipore,
18.2
M
cm,
USA).
2.2.
Characterization
of
AgNPs
The
morphology
of
AgNPs
was
determined
by
TEM
at
100
kV
(Hitachi
H-7600,
Japan).
Samples
were
prepared
by
placing
a
drop
of
fresh
suspension
on
the
TEM
copper
grids,
followed
by
sol-
vent
evaporation
at
room
temperature
overnight.
The
configuration
of
AgNPs
was
determined
by
XRD
(PANalytical,
X’
Pert
Pro,
Netherlands),
operated
at
a
voltage
of
40
kV
and
a
current
of
30
mA
with
Cu
K␣
radiation.
Thermogravimetric
analysis
(TGA)
of
the
tea
extract
synthesized
AgNPs
was
conducted
in
nitrogen
atmosphere
on
a
thermogravimetric
analyzer
(TG
209
F3
Tarsus,
Germany
Net-
zsch
Instruments,
Inc.)
in
the
temperature
range
of
40–1000 ◦C
at
a
scanning
rate
of
10 ◦C/min.
Sample
was
prepared
by
adding
2
mL
of
tea
extract
synthesized
AgNPs
into
petri
dish
and
drying
for
72
h
in
the
freeze
dryer.
The
zeta-potential
(Malvern
Instruments,
Zeta-
PALS,
UK)
of
AgNPs
produced
by
tea
extract
was
analyzed
in
order
to
recognize
the
surface
charge
of
AgNPs.
In
addition,
the
hydrate
par-
ticle
size
was
also
determined
by
Zetasizer
(Malvern
Instruments,
ZetaPALS,
UK).
The
quantification
of
AgNP
stock
suspensions
was
analyzed
by
inductively
coupled
plasma
optical
emission
spectrometry
(ICP-
OES)
(PerkinElmer
Optima
7000
DV,
USA)
after
nitric
acid
digestion.
Silver
concentrations
in
the
solution
were
analyzed
by
inductively
coupled
plasma
mass
spectrometry
(Agilent
7500cx,
USA).
The
dis-
solved
silver
ion
was
isolated
by
removing
AgNPs
using
centrifugal
ultrafilter
devices
(Millipore
Amicon
Ultra-4
3
K,
USA),
subjected
to
centrifugation
for
30
min
at
4000
rpm
[14],
whereas
the
total
silver
concentration
was
analyzed
after
nitric
acid
digestion.
The
AgNP
concentration
was
calculated
by
deducting
dissolved
silver
ion
from
total
silver.
FT-IR
spectroscopy
measurements
were
carried
out
to
identify
the
functional
groups
which
are
bound
distinctively
on
the
AgNP
surface
and
involved
in
the
synthesis
of
AgNPs.
Samples
for
the
FT-
IR
analysis
were
prepared
by
drying
the
tea
extract
taken
before
and
after
synthesis
of
AgNPs.
Samples
for
FT-IR
measurement
were
prepared
by
mixing
1%
(w/w)
specimens
with
100
mg
of
potas-
sium
bromide
powder
and
pressing
the
mixture
into
a
sheer
slice.
Hand-ground
samples
were
measured
by
a
FT-IR
spectrometer
(FT-
IR
Nicolet
5700,
Thermo
Corp.
USA).
The
average
of
9
scans
was
collected
for
each
measurement
using
a
resolution
of
2
cm−1.
2.3.
Silver
ion
release
test
The
dissolution
kinetics
of
AgNPs
synthesized
by
the
tea
extract
in
air-saturated
(8.2
mg
O2/L)
deionized
water
was
investigated.
PVA-coated,
uncoated,
and
commercial
AgNPs
were
also
applied
for
comparison,
for
which
the
preparation
processes
are
described
in
supplementary
information
(SI).
The
AgNP
stock
suspension
was
diluted
with
deionized
water
to
1.0
mg/L.
The
initial
pH
values
of
tea
extract
AgNPs,
PVA-coated
AgNPs,
uncoated
AgNPs,
and
com-
mercial
AgNPs
were
6.9,
6.8,
6.9,
and
7.2,
respectively.
Silver
ion
release
experiments
were
carried
out
in
triplicate
on
dark
shaker
(120
rpm)
at
28 ◦C.
2.4.
Antibacterial
susceptibility
test
The
antibacterial
test
was
carried
out
via
a
growth
inhibition
assay.
Escherichia
coli
K12
strain
MPAO1
(Coli
Genetic
Stock
Center
[CGSC;
Yale
University])
was
grown
on
Luria-Bertani
(LB)
medium
at
37 ◦C
for
overnight.
The
cultures
were
diluted
in
fresh
LB
medium
to
get
an
initial
0.05
absorbance
at
OD600.
150
␮L
of
AgNPs
solu-
tion
under
target
concentrations
were
pipetted
into
eight
parallel
wells
of
a
96-well
microplate
(8
replicates),
and
150
␮L
of
E.
coli
cells
were
inoculated
in
each
well.
The
final
concentrations
of
AgNPs
were
50.0,
25.0,
12.5,
6.25,
3.12,
1.56,
0.78,
0.39,
0.195
mg/L,
respectively.
The
absorbance
was
measured
at
OD600 through
a
96-
well
microplate
with
a
SpectraMax
M5
Multi-detection
Microplate
Reader
(Molecular
Devices
Inc.,
USA)
at
predetermined
time
inter-
vals.
In
addition,
the
antimicrobial
susceptibility
test
was
also
per-
formed
according
to
a
modified
Kirby-Bauer
disk
diffusion
method

228 Q.
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et
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Colloids
and
Surfaces
A:
Physicochem.
Eng.
Aspects
444 (2014) 226–
231
90
92
94
96
98
100
100%50%25%10%5%1%
AgNP production efficiencies
(%)
Tea e xtr act dilute
rate
s
-30
-25
-20
-15
-10
-5
0
100%50%25%10%
5%1%
zeta potential (mV)
Tea e xtract
dil
ute rates
B
A
Fig.
1.
Effects
of
tea
extract
dilute
rates
on
the
AgNP
production
efficiencies
(A)
and
zeta
potential
(B)
(±standard
error).
as
described
by
Liu
et
al.
[17].
Details
of
the
experiment
were
described
in
SI.
3.
Results
and
discussion
3.1.
Effect
of
the
tea
extract
dosage
The
effects
of
the
initial
concentrations
of
the
tea
extract
on
the
AgNPs
productivity
were
studied
at
25 ◦C.
The
stock
solution
of
tea
extract
was
diluted
to
1%,
5%,
10%,
25%,
50%
and
100%
(v/v)
and
was
used
as
working
solution,
with
the
TOC
of
1.0,
2.0,
5.0,
10.1,
and
20.2
g/L,
respectively.
The
formation
of
AgNPs
was
indicated
by
the
appearance
of
signature
brown
color
of
the
solution
(SI
Fig.
S1).
To
understand
the
formation
of
AgNPs,
the
total
silver
con-
centrations
and
silver
ion
concentrations
were
analyzed.
As
shown
in
Fig.
1A
the
production
efficiencies
of
AgNPs
were
99.1%,
99.7%,
99.9%,
99.8%,
94.6%,
and
95.3%
(w/w)
with
1%,
5%,
10%,
25%,
50%,
and
100%
(v/v)
tea
extract,
respectively.
The
production
efficiencies
of
AgNPs
were
all
above
94%
(w/w),
while
the
highest
AgNP
produc-
tion
efficiency
was
achieved
with
5%
(v/v)
tea
extract.
In
addition,
the
zeta
potential
is
shown
in
Fig.
1B.
Generally,
a
suspension
that
exhibits
an
absolute
zeta
potential
less
than
20
mV
is
considered
unstable
and
will
result
in
precipitation
of
particles
from
solution
[19],
whereas
the
absolute
zeta
potential
higher
than
20
mV
is
sta-
ble
[20].
In
this
study,
zeta
potentials
of
AgNPs
were
−20.7
and
−21.3
mV
with
1%
and
5%
(v/v)
tea
extracts,
respectively,
indicat-
ing
the
stability
of
AgNP
suspensions.
The
zeta
potentials
of
AgNPs
increased
as
the
tea
extract
concentrations
increased,
and
reached
−12.0
and
−11.3
mV
with
50%
and
100%
(v/v)
tea
extract,
indicating
the
instability
of
AgNP
suspensions
at
high
tea
extract
concentra-
tion.
Therefore,
5%
(v/v)
tea
extract
was
chosen
in
the
following
study.
3.2.
Effect
of
temperature
on
AgNP
synthesis
The
effect
of
temperature
on
the
AgNPs
formation
was
investi-
gated
at
25,
40,
and
55 ◦C
with
5%
(v/v)
tea
extract.
The
previous
study
of
AgNP
synthesis
by
Pulicaria
glutinosa
extract
showed
the
AgNP
production
was
enhanced
by
increasing
temperature
[21].
Our
results
showed
that
the
increase
in
temperature
had
no
sig-
nificant
effect
on
the
production
efficiencies
of
AgNPs
(data
not
shown),
and
this
difference
may
be
due
to
the
production
efficiency
in
the
present
study
was
already
99.7%
(w/w)
at
25 ◦C
and
had
little
space
to
improve.
However,
the
average
hydrate
particle
sizes
of
AgNPs
were
91,
129,
and
175
nm
at
25,
40,
and
55 ◦C,
respectively
(Fig.
2A).
The
increase
of
AgNP
size
with
the
increasing
temperature
was
in
accordance
with
the
previous
study
[1,21,22].
This
is
prob-
ably
due
to
the
reaction
rates
of
AgNP
synthesis
increased
as
the
temperature
increased,
consequently,
the
particle
sizes
increased
[22].
3.3.
Characterization
of
tea
extract
synthesized
AgNPs
TEM
was
employed
to
characterize
the
size,
shape
and
mor-
phology
of
the
synthesized
AgNPs.
The
TEM
images
of
AgNPs
synthesized
by
5%
(v/v)
diluted
tea
extract
(1
g/L
TOC)
are
shown
in
Fig.
2B–D.
The
morphology
of
AgNPs
is
nearly
spherical.
AgNP
sizes
ranged
from
20
to
90
nm.
The
difference
in
the
particle
size
by
TEM
and
in
situ
dynamic
light
scattering
techniques
may
be
due
to
the
aggregation
during
the
sample
preparation
[14].
The
XRD
pattern
of
the
dried
silver
nanoparticles
is
shown
in
Fig.
3.
The
XRD
peaks
at
2
degree
of
38.1,
44.3,
64.4
and
77.4
can
be
attributed
to
the
(1
1
1),
(2
0
0),
(2
2
0),
and
(3
1
1)
crystalline
planes
of
the
face
centered
cubic
crystalline
structure
of
metallic
silver
(JCPDS
file
No.
01-071-4613).
Besides,
the
peak
near
31.9
implied
the
possible
existence
of
Ag2O
[17,23].
FT-IR
spectroscopy
was
used
to
characterize
and
identify
the
chemical
composition
of
the
AgNP
surface.
The
FT-IR
spectra
of
control
dried
tea
extract
(before
reaction
without
AgNO3)
and
syn-
thesized
AgNPs
(after
reaction
with
AgNO3)
are
shown
in
Fig.
4.
Both
of
them
showed
a
shift
in
peaks:
3420–3371
(due
to
N–H
stretching,
amides),
2931–2925
(due
to
C–H
stretching,
alkanes),
1383–1371
(characteristic
of
hydroxyl
groups,
phenolic
hydroxyl),
1051–1044
cm−1(due
to
C-stretching,
ether
groups).
In
addition,
the
synthesized
AgNPs
showed
other
peaks
at
1695,
1452,
1241,
and
926
cm−1related
to
alkene
groups
(C
C
stretching),
tertiary
ammonium
ions,
poly
phenols,
aliphatic
amines
(C–N
stretch-
ing
vibrations),
and
alkene
groups
(C–H
stretching),
respectively.
The
FT-IR
analysis
indicated
the
involvement
of
amides,
carboxyl,
amino
groups
and
poly
phenols
in
the
synthesized
AgNPs.
It
is
well
known
that
there
are
tea
polyphenols,
protein,
and
amino
acid
in
tea.
The
organic
compounds
in
tea
extract
could
attribute
to
the
reduction
of
AgNO3and
the
stabilization
of
AgNPs
by
the
surface
bound
by
the
organic
compounds
[24].
Similar
observations
were
noticed
in
the
green
synthesis
of
AgNPs
using
plant
extract
[25,26].
The
synthesis
of
AgNPs
was
demonstrated
by
the
tea
polyphenol
(Baicao
Co.
China)
of
5.0
mg/L
(TEM
image
is
shown
in
SI
Fig.
S2),
which
supported
AgNPs
could
be
synthesized
by
the
reduction
of
silver
ion
via
tea
polyphenols.
The
capping
organic
groups
on
the
surface
of
tea
extract
synthe-
sized
AgNPs
was
further
confirmed
by
TGA.
Fig.
5
shows
the
TGA
curve
with
three
weight
losses.
The
first
weight
loss
was
observed
at

Q.
Sun
et
al.
/
Colloids
and
Surfaces
A:
Physicochem.
Eng.
Aspects
444 (2014) 226–
231 229
Fig.
2.
The
average
hydrate
particle
sizes
(±standard
error)
(A),
and
TEM
image
of
AgNPs
formed
at
25 ◦C
(B),
40 ◦C
(C),
and
55 ◦C
(D).
around
100 ◦C
due
to
the
loss
of
adsorbed
water.
The
second
weight
loss,
which
accounted
for
19%
of
the
total
AgNPs
weight,
appeared
at
180–380 ◦C.
The
degradation
of
organic
compounds,
including
ferulic
acid,
ascorbic
acid,
and
quercetins,
might
cause
the
weight
loss
[27].
The
combustion
of
carbohydrates
and
the
less
condensed
structures
of
the
lignin
molecules
would
also
cause
the
weight
loss
[28].
In
addition,
the
degradation
or
glycosylation
of
the
catechins
might
contribute
to
the
weight
loss
[29].
There
is
a
steady
weight
loss
appeared
at
380–1000 ◦C,
which
accounted
for
19%
of
the
total
weight
loss.
This
was
probably
due
to
the
thermal
degradation
of
Fig.
3.
XRD
pattern
of
5%
(v/v)
tea
extract
synthesized
silver
nanoparticles
AgNPs.
resistant
aromatic
structures
and
the
decomposition
of
biogenic
salt,
such
as
carbonates
[28].
Data
from
TGA
indicated
the
content
of
tea
component
estimated
from
the
weight
loss
was
39%
of
the
tea
extract
synthesized
AgNPs.
3.4.
Release
of
silver
ion
Since
the
silver
ion
release
is
an
important
behavior
of
AgNPs,
the
characterization
of
silver
ion
release
is
necessary
to
under-
stand
the
environmental
fate
of
synthesized
AgNPs.
In
this
study,
500100015002000250030003500
30
60
90
120
150
180
210
240
1383
.11
1370.29
1451.62
1050.59
1603.16
2925.20
3370.53
1043.55
926.28
832.24
1627
.01
1240
.91
2930.57
% Transmittance
Wavenumbe
rs (c
m-1)
Tea ex
trac
t synthes
ize
d AgNPs
Tea
extract
1694.78
3419.70
Fig.
4.
FT-IR
spectra
of
the
tea
extract
and
tea
extract
reduced
AgNPs.

230 Q.
Sun
et
al.
/
Colloids
and
Surfaces
A:
Physicochem.
Eng.
Aspects
444 (2014) 226–
231
120010008006004002000
60
70
80
90
100
TG %
Temperature
(oC)
Fig.
5.
TGA
curve
of
tea
extract
synthesized
AgNPs.
time-dependent
release
of
silver
ion
from
the
tea
extract
synthe-
sized
AgNPs
was
measured
using
centrifugal
ultrafiltration
and
ICP-MS.
For
comparison,
the
silver
ion
releases
by
AgNPs
pre-
pared
by
other
processes,
including
PVA-coated
AgNPs,
uncoated
AgNPs,
and
commercial
AgNPs,
were
also
tested.
Fig.
6
shows
time-
resolved
concentrations
of
silver
ion
released
from
the
AgNPs.
The
released
silver
ion
concentrations
of
tea
extract
synthesized
AgNPs
was
6.73
␮g/L
at
2
h,
and
was
increased
to
8.94
␮g/L
at
24
h,
while
the
released
silver
ion
concentrations
of
commercial
AgNPs,
PVA-coated
AgNPs,
and
uncoated
AgNPs
at
24
h
were
28.3,
35.2,
83.2
␮g/L,
respectively.
The
lowest
silver
ion
release
rate
was
achieved
by
tea
extract
synthesized
AgNPs.
The
reason
for
this
is
because
the
surface
of
AgNPs
might
be
sufficiently
covered
by
the
groups
from
the
tea
extract,
including
the
amides,
carboxyl,
phenols,
etc.,
as
shown
in
Fig.
4.
These
functional
groups
might
inhibit
the
dissolution
of
AgNPs
by
oxygen
to
release
silver
ions
[30].
In
addition,
the
released
silver
ions
could
be
reduced
to
AgNPs
due
to
the
reducing
capacity
of
tea
extract.
Furthermore,
TEM
analysis
showed
the
average
sizes
of
the
tea
extract
synthesized,
PVA-coated,
uncoated,
and
commercial
AgNPs
were
45,
12,
15,
and
40
nm,
respectively.
Previous
study
showed
the
silver
ion
releasing
rate
decreased
with
an
increased
particle
size
[14].
AgNPs
could
keep
release
silver
ions
in
the
aquatic
environment.
In
the
present
study,
the
release
rates
of
silver
ions
were
2.8%,
3.5%,
and
8.3%
(w/w)
at
pH
around
7
in
the
presence
of
dissolved
oxygen
after
24
h
for
commercial
AgNPs,
PVA-coated
AgNPs,
and
uncoated
AgNPs,
respectively.
In
comparison,
the
release
rate
of
silver
ions
from
AgNPs
synthesized
by
tea
extract
was
less
than
0.9%
(w/w).
The
1400120010008006004002000
0
20
40
60
80
100
120
Silver ion concentration ( g/L)
Tim
e (mi
n)
Tea
ex
tratct
synthes
ized
AgNP s
PV
A coated
AgNPs
Uncoa
ted
AgNPs
Commerc
ial
AgNP s
Fig.
6.
Silver
ion
release
kinetics
by
different
AgNPs
(±standard
error).
lower
ion
release
rate
of
tea
extract
synthesized
AgNPs
indicated
a
good
stability
and
the
probably
longer
preservation
time.
In
addi-
tion,
AgNPs
is
known
to
show
the
antibacterial
activity,
which
to
some
extent
is
through
the
release
of
silver
ions
[31].
Therefore,
the
antimicrobial
activity
of
tea
extract
synthesized
AgNPs
is
expected
to
be
less
than
that
of
other
AgNPs.
3.5.
Antibacterial
activity
The
antibacterial
activity
of
AgNPs
against
E.
coli
was
investi-
gated.
Silver
ion
was
found
to
be
the
most
toxic
species
to
inhibit
the
growth
of
E.
coli.
The
E.
coli
growth
was
completely
inhibited
with
the
silver
ion
concentration
at
or
higher
than
1.56
mg/L.
The
inhibi-
tion
concentrations
were
12.5
and
25.0
mg/L
for
PVA-coated
AgNPs
and
uncoated
AgNPs,
respectively.
Whereas
the
tea
extract
synthe-
sized
AgNPs
and
commercial
AgNPs
did
not
show
E.
coli
growth
inhibition
even
at
50.0
mg/L.
Similar
results
were
observed
by
the
Kirby-Bauer
disk
diffu-
sion
method.
As
shown
in
Fig.
S3,
the
biggest
inhibition
zones
are
observed
for
silver
ion
(5
mm).
PVA-coated
and
uncoated
AgNPs
also
showed
clear
antimicrobial
activity
with
inhibition
zone
diam-
eter
of
1.2–1.5
mm.
However,
tea
extract
synthesized
AgNPs
along
with
commercial
AgNPs
showed
little
antibacterial
activity
with
inhibition
zone
diameter
of
0.5–0.8
mm.
It
is
not
surprising
for
the
best
antimicrobial
activity
of
sil-
ver
ion,
since
the
good
antibacterial
activity
of
silver
ions
has
been
reported
[17,32].
Previous
studies
indicated
the
antibacterial
activity
of
AgNPs
by
attachment
to
the
bacterial
cell
wall,
or
the
for-
mation
of
free
radicals
[33,34].
In
addition,
the
silver
ions
released
from
AgNPs
may
play
a
vital
role
of
the
antibacterial
activity
due
to
the
interaction
of
silver
ion
with
the
thiol
groups
of
enzymes
[35].
As
shown
in
Fig.
6,
the
silver
ion
release
rate
of
the
tea
extract
synthesized
AgNPs
was
lower
due
to
the
functional
groups
on
the
surface
of
AgNPs
or
the
reducing
capacity
of
tea
extract.
This
might
result
in
the
lower
antibacterial
activity
of
tea
extract
synthesized
AgNPs.
On
the
other
hand,
smaller
NPs
were
found
to
be
more
toxic
due
to
the
easier
uptake
and
larger
surface
area
[36,37].
Fur-
thermore,
the
potential
for
the
silver
ion
releasing
also
increased
with
a
decreasing
AgNP
size
[38].
Therefore,
the
toxicity
of
AgNPs
was
found
to
be
size
and
ion
release
rate
dependent
[37].
The
less
antibacterial
activity
of
the
tea
extract
synthesized
AgNPs
might
be
due
to
the
larger
size
and
less
silver
ion
release.
4.
Conclusions
The
present
study
described
a
green
and
simple
way
to
syn-
thetize
AgNPs
by
tea
extract.
AgNPs
were
characterized
by
TEM,
XRD,
TGA
and
FT-IR.
The
synthesized
AgNPs
was
crystalline
struc-
ture,
20–90
nm
in
size,
with
functional
groups
from
the
tea
extract
capped
on
the
surface.
The
conditions
such
as
the
dosage
of
the
tea
extract
and
reaction
temperature
showed
an
effect
on
the
produc-
tion
efficiency
and
formation
rate
of
AgNPs.
The
silver
ion
release
from
the
tea
extract
synthesized
AgNPs
was
lower
compared
to
the
PVA-coated
AgNPs,
uncoated
AgNPs,
and
commercial
AgNPs,
which
highlight
a
good
stability
due
to
the
functional
groups
from
the
tea
extract
capped
on
the
AgNPs.
However,
due
to
the
larger
size
and
less
silver
ion
release,
the
biosynthesized
AgNPs
showed
slight
antibacterial
activity
against
E.
coli.
Acknowledgements
We
appreciate
Dr.
Sikandar
I.
Mulla
for
editing
the
manuscript.
This
work
was
supported
by
the
Natural
Science
Foundation
of
Fujian
Province,
China
(2011J05035),
the
National
Science
Founda-
tion
of
China
(41201490),
Science
and
Technology
Innovation
and

Q.
Sun
et
al.
/
Colloids
and
Surfaces
A:
Physicochem.
Eng.
Aspects
444 (2014) 226–
231 231
Collaboration
Team
Project
of
the
Chinese
Academy
of
Sciences,
Technology
Foundation
for
Selected
Overseas
Chinese
Scholar
of
MOHRSS,
China,
Technology
Planning
Project
of
Xiamen,
China
(3502Z20120012),
the
Special
Program
for
Key
Basic
Research
of
the
Ministry
of
Science
and
Technology,
China
(2010CB434802),
and
the
CAS/SAFEA
International
Partnership
Program
for
Creative
Research
Teams
(KZCX2-YW-T08).
The
authors
do
not
have
any
conflict
of
interest
to
declare.
Appendix
A.
Supplementary
data
Supplementary
data
associated
with
this
article
can
be
found,
in
the
online
version,
at
http://dx.doi.org/10.1016/j.colsurfa.
2013.12.065.
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