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Microbial biosynthesis of silver nanoparticles in different culture media Ke Luo, Samuel Jung, Kyu-Hwan Park, and Young-Rok Kim J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b05092 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018
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Journal of Agricultural and Food Chemistry
Microbial Biosynthesis of Silver Nanoparticles in Different Culture Media
Ke Luo,† Samuel Jung,† Kyu-Hwan Park and Young-Rok Kim*
Graduate School of Biotechnology & Department of Food Science and Biotechnology, College of Life Sciences, Kyung Hee University, Yongin 17104, Korea
*
Corresponding author:
[email protected] Phone: +82-31-201-3830; Fax: +82-31-204-8116
† K.L. and S.J. contributed equally to this work.
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ABSTRACT
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Microbial biosynthesis of metal nanoparticles has been extensively studied for the
3
applications in biomedical sciences and engineering. However, the mechanism for
4
their synthesis through microorganism is not completely understood. In this study,
5
several culture media were investigated for their roles in the microbial biosynthesis of
6
silver nanoparticles (AgNPs). The size and morphology of the synthesized AgNPs
7
were analyzed by UV–Vis spectroscopy, FT-IR, TEM, and DLS. The results
8
demonstrated that nutrient broth (NB) and Mueller-Hinton broth (MHB) among tested
9
media effectively reduced silver ions to form AgNPs with different particle size and
10
shape. Although the involved microorganism enhanced the reduction of silver ions,
11
the size and shape of the particles were shown to mainly depend on the culture media.
12
Our findings suggest that the growth media of bacterial culture play an important role
13
in the synthesis of metallic nanoparticles with regard to their size and shape. We
14
believe our findings would provide useful information for further exploration of
15
microbial biosynthesis of AgNPs and their biomedical applications.
16 17
KEYWORDS: silver nanoparticles, microbial biosynthesis, extracellular, culture
18
media, nutrient broth, Mueller-Hinton broth
19 20
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INTRODUCTION
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Metal nanoparticles have attracted considerable attention due to their unique
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physicochemical characteristics including catalytic activity, optical properties,
24
electronic properties, antimicrobial activity and magnetic properties, etc.1 Amongst
25
them, silver nanoparticles (AgNPs) are used extensively in various applications that
26
includes disinfecting medical devices and home appliances to purifying drinking
27
water.2-4 AgNPs are also well-known antimicrobial agent against various pathogenic
28
microorganisms.5 Furthermore, owing to their unique plasmon-resonance optical
29
scattering properties, AgNPs are currently recognized for their potentials in bio-
30
sensing, medical diagnostics and therapeutics, and biological imaging applications.6-8
31
Due to the environmental issues raised over the past decades, the synthesis of
32
metal nanoparticles by biological process has drawn a great deal of attention.9
33
Numerous biosynthesis methods utilizing bacteria, fungi, plant, and plant extract as
34
reducing agents for the synthesis of AgNPs have been exploited in recent years.5, 10-14
35
Amongst them, the synthesis of AgNPs by living microorganisms is a well-known
36
biomimetic approach, since some microorganisms have been found to synthesize
37
inorganic materials, such as gold nanoparticles,15 magnetite nanoparticles,16 silver
38
nanoparticles,17 and zinc sulfide nanoparticles.18 Bacteria is known to reduce metal
39
ions to metal nanoparticles either intra- or extracellularly. However, the mechanism
40
for intra- and extracellular synthesis of nanoparticles differs depending on the
41
biological agents. In the intracellular synthesis of nanoparticles, metal ions are
42
uptaken by certain microorganisms and bind to either a metal ion reductase or similar
43
proteins, resulting in the reduction of the ions to elemental metals in their zero-valent
44
form, followed by the formation of nanoparticles in cytoplasm.19 In this case, an extra
45
step is required for the purification of the produced nanoparticles if they are to be
46
used for further applications. The electrostatic interaction of the positively charged 3
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metal ions and negatively charged cell wall of the bacteria play important role in ion
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transportation in the microbial cell. The intracellularly synthesized metallic
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nanoparticles could eventually be effluxed out of the cell.20 On the other hands, the
50
extracellular synthesis of metal nanoparticles is generally carried out by the reductive
51
components released from the cell. For example, DNA, NADH-dependent reductase
52
and sulfur-contained protein secreted from the cell can induce the reduction of Ag+
53
ion to Ag0 stage leading to the formation of AgNPs.21-23 Since the electronic and
54
optical properties of metal nanoparticles are size- and shape-dependent, the control of
55
size and shape of the particles is one of the most important aspect for their specific
56
applications.24 Moreover, previous studies have reported that antimicrobial nature of
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AgNPs showed to be depend on their size and shape, where smaller nanoparticles
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displayed better antimicrobial activity.23 It has been demonstrated that AgNPs with
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different
60
microorganisms.25 However, the effect of the growth environment, such as culture
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medium, on the synthesis of AgNPs in a way of affecting their size and shape is not
62
fully understood. In addition, Herein, we report the effect of culture media and several
63
media components on the synthesis of AgNPs in terms of the size and morphology of
64
the particles.
sizes
and
shapes
can
be
synthesized
by
employing
specific
65 66
67
Media, chemicals, and microorganism. Mueller-Hinton broth (MHB) was
68
purchased from Difco (Lawrence, USA). Yeast extract was purchased from Daejung
69
chemical Co. (Siheung, Korea). Peptone A and beef extract were purchased from
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BIOSESANG (Seongnam, Korea). Silver (I) nitrate was purchased from Daejung
71
chemical Co. (Siheung, Korea). Klebsiella pneumoniae KCTC 2242 and Escherichia
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coli DH5α were obtained from the Korean Collection for Type Cultures (Daejeon,
MATERIALS AND METHODS
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Korea).
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Preparation of culture mediums. To synthesize silver nanoparticles, thirteen
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types of culture mediums were prepared as reducing agent. Abbreviation of different
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reducing agents was listed in Table 1. MHB, beef extract, peptone A, and yeast extract
79
solutions were prepared by dissolving 21g of MHB, 3g of beef extract, 5g of peptone
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A, and 5g of yeast extract in 1L distilled water, respectively. Nutrient broth without
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NaCl (NB) was prepared by dissolving 5g of peptone A and 3g of beef extract in 1L
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distilled water. NaCl was excluded when preparing Nutrient broth to prevent Cl- effect
83
that could interact with silver ion and produce AgCl sediment. For preparation of
84
culture supernatant (N-KP S, N-EC S, M-KP S, and M-EC S) and inoculum (N-KP I,
85
N-EC I, M-KP I, and M-EC I), MHB and NB media were inoculated with K.
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pneumoniae or E. coli, and incubated in shaking incubator (HB-201SL, Hanbaek
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Scientific Co., Korea) at 220 rpm and 37 °C until the inoculum reached an O.D600 of
88
1.0, reflecting the number of K. pneumoniae and E. coli were around 7 × 107 CFU/ml
89
by plate counting. The microbial synthesis of silver nanoparticles was carried out in a
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reaction containing either E. coli or K. pneumoniae to a final concentration of 3.5 ×
91
106 CFU/ml.
92 93
Biological synthesis of AgNPs. 500 µl of each reducing agent was added into a
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glass vial containing 10 ml of 1mM AgNO3 and incubated at 25 °C with a gentle
95
rotation (ROTATOR-AG, FINEPCR, KOREA) at 10 rpm. Since light could affect the
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synthesis of AgNPs,26 the reaction was carried out in dark incubator, in which a
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fluorescent lamp (FPL27EX-D, Sigma lamp, Incheon, Korea) was equipped to offer
98
controlled irradiation with 10 cm of distance away from the sample during the 5
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synthesis. To investigate the irradiation effect on the synthesis of AgNPs, we carried
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out the reaction with or without the visible-light irradiation in MHB and NB for 6
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hours. To monitor the yield of AgNPs synthesized from the reactions, an aliquot (100
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µl) of reaction was taken every 0.5 h and its absorbance (O.D430) was measured using
103
UV-vis spectrophotometer (Infinite 200 PRO, TECAN, Switzerland).
104 105
Characterization of synthesized AgNPs. The synthesized AgNPs were washed
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3 times with water and ethanol (20 %) to remove the medium that could possibly
107
remained on the surface of AgNPs. Absorption spectra of the nanoparticles were
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analyzed by the UV-vis spectrophotometer (TECAN) at wavelengths ranging from
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360 nm to 600 nm. One drop of AgNPs suspension was placed onto a carbon-coated
110
copper TEM grid (3.05 mm diameter), and dried at desiccator under vacuum. After
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the sample was completely dried, the synthesized AgNPs were characterized by using
112
field emission transmission electron microscopy (FE-TEM, JEM-2100F, JEOL, USA).
113
The size distribution of synthesized AgNPs were determined by dynamic light
114
scattering (DLS) (Zetasizer Nano ZS, Malvern, England) under the setting of
115
refractive index (RI) of 1.33 with 25 replications.
116
The FTIR absorption spectra of AgNPs synthesized by NB and MHB media were
117
measured using a Perkin-Elmer Spectrum One System spectrometer (Foster City, CA,
118
USA) with KBr pellets in the range of 500 to 4000 cm−1.
119 120 121
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Synthesis of silver nanoparticles in different culture media. As the
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synthesis of AgNPs could be influenced by light irradiation from external
124
environment,27, 28 the reaction solution consisting of silver ions and various culture
RESULTS AND DISCUSSION
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media was placed in a dark incubator with a fluorescent lamp providing a controlled
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irradiation toward the sample with a distance of 10 cm. Abbreviations of different
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culture media used for the reduction of silver ions were listed in Table 1. Figure 1A
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shows the color of reaction after incubating 1 mM AgNO3 solution with various
129
media at 25 °C for 2 days. AgNO3 solution without any culture media was used as a
130
control. We did not observe any notable sediment that is an indicative of silver salt
131
formation during or after the reaction with all test media. The results show that the
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reaction solution containing PA, BE, NB, and MHB exhibited brown color owing to
133
the excitation of surface plasmon vibrations of the synthesized AgNPs,29, 30 while
134
there was no significant color change in the reaction containing YE compared to that
135
of control. The UV–Vis spectra of AgNPs synthesized using different culture media
136
are shown in Figure 1B. AgNPs synthesized by PA, BE, and NB showed a distinct
137
absorbance peaks at the range of 430 nm to 440 nm, which falls between a typical
138
SPR band of spherical AgNPs (400-450 nm),31 whereas no such a characteristic peak
139
was observed in a reaction with YE (Figure 1B). In particular, AgNPs synthesized by
140
MHB showed a remarkable red-shift of absorption peak (480 nm), implying that they
141
would have a different size or shape from those synthesized by other media. The
142
absorption intensity demonstrated that NB and MHB yielded a larger amount of
143
AgNPs, suggesting that catalytic activity in the reduction of silver ions was dependent
144
on the composition of culture media. Since NB is composed of BE and PA, its
145
reducing power would mainly be derived from the synergistic effect of BE and PA on
146
reduction activity. On the other hand, we assume that the casein hydrolysate and
147
starch derivatives in MHB acted as a main reducing agent for the synthesis of AgNPs.
148
The deprotonated form of the hydroxyl and carboxylic groups in these compounds is
149
believed to facilitate the complexation of Ag ions.32 Subsequently, the Ag ions were
150
reduced to form AgNPs through the oxidation of these groups into carbonyl form, 7
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which also modulated the crystal growth of AgNPs. The shape and size of the
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synthesized AgNPs were visualized through TEM (Figure 1C). The TEM results
153
showed that silver ‘seeds’ with diameter of about 5 nm appeared along with large
154
spherical AgNPs of an average particle size of 20 nm when the silver ions were
155
reduced by PA and BE media. We assume that the presence of large number of silver
156
seeds is possibly due to the insufficient reduction of AgNPs in PA and BE media,
157
limiting the transition from the nucleation stage to the growth stage during the
158
synthesis process, which was also evidenced by the decreased absorption of AgNPs in
159
a reaction with PA and BE media (Figure 1B). However, for the reaction with NB
160
media consisting of a mixture of PA and BE, we observed highly monodisperse
161
spherical AgNPs with a diameter of about 20 nm. On the other hand, MHB media led
162
to the formation of large AgNPs with a shape of planar triangle and discal
163
nanostructure. It has been also reported that the silver discal nanoplates showed an
164
absorption maximum at around 480 nm,33 which supports the fact that the red shifted
165
absorption of MHB-AgNPs was associated with the significant difference in
166
morphology from those synthesized by other media. Thus, our findings suggested that
167
the size and shape of AgNPs were shown to be depending on the media compositions,
168
such as protein and carbohydrate, which not only affect the nucleation of Ag0 but also
169
influence the growth and formation of AgNPs.34
170 171
Characteristics of AgNPs synthesized by NB and MHB medium. As MHB
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and NB exhibited higher catalytic activity in the reduction of silver ions among the
173
components of culture media, these two culture media were selected as a model to
174
investigate the effect of media on the synthesis of AgNPs with respect to the size and
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shape. Considering the irradiation effect that can enhance the reduction of silver
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ions,35 AgNPs synthesis using MHB (MHB-AgNPs) and NB (NB-AgNPs) were 8
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performed under two different conditions, with or without light exposure (Figure 2A).
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In both cases, MHB induced a greater increase in absorbance than that of NB. When
179
the reactions were carried out with light, the absorbance (at 480 nm) of the reaction
180
containing MHB increased rapidly within 3 h, followed by a gradual saturation over
181
the rest of reaction time, whereas the reaction with NB media showed a slower
182
increase in the value over the course of 6 h reaction. On the other hand, the reactions
183
without light showed different results. As expected, the absorbance (at 480 nm) of the
184
reaction with MHB was higher than that with NB (at 430 nm), but the overall values
185
were significantly lower than those with light. In addition, the absorption values of the
186
reaction with MHB reached a saturation level after 0.5 h, suggesting that light
187
synergistically participates in the reduction of silver ions together with other reducing
188
components in culture media. In addition, DLS analysis was carried out to investigate
189
the size distribution of the synthesized AgNPs (Figure 2B). According to DLS data,
190
the average hydrodynamic diameter of the AgNPs synthesized by MHB was 78.8 nm,
191
which was approximately four-fold bigger than those synthesized by NB (21.1 nm). In
192
addition, the AgNPs synthesized by NB and MHB medium were subjected to FTIR
193
analysis to investigate the composition of the surface of AgNPs, which could play an
194
important role in determining the sizes and the colloidal stability of the nanoparticles
195
(Figure 2C). FTIR spectrum showed several absorption peaks at 3340, 1645, 1589,
196
1541, 1384, and 1076 cm−1 in the spectrum scanned from 500 to 4000 cm−1. The IR
197
bands observed at 3340, 1075, and 1589 cm-1 in AgNPs synthesized by MHB
198
represent the characteristics of the O-H, C-O-C, and C=C stretching, respectively,36
199
and the band at 1384 cm−1 is derived from the NO3- remained in the solution.37 It
200
should be noted that the absorption peaks of NB-based AgNPs at 1541 cm-1 and 1645
201
cm-1 were originated from the bending vibrations of the amide I and amide II bands of
202
the proteins, respectively, suggesting that the proteins adsorbed on the surface of 9
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synthesized AgNPs as a capping agent could enhance the colloidal stability of the
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particles.38 From the results, the protein adsorbed on the surface of the nanoparticles
205
could inhibit the further coprecipitation of Ag on the surface of AgNPs, resulting in
206
the formation of small and highly monodisperse AgNPs in contrast to those
207
synthesized by MHB. In other words, the morphology of synthesized AgNPs could be
208
influenced by the composition of amino acids in NB and MHB. NB media is
209
comprised of water soluble PA and BE, which are derived from partially hydrolyzed
210
proteins and beef infusion, respectively. On the other hand, the primarily component
211
of MHB media is casein hydrolysate whose amino acid composition and sequence are
212
much different from those from NB media. From the results, we propose that the
213
composition of culture media not only play an important role in the synthesis of
214
AgNPs, but also affect the particles size through the interaction between the surface of
215
AgNPs and the media components.
216 217
Microbial biosynthesis of AgNPs. To investigate the effect of bacterial
218
metabolites on the synthesis of AgNPs, NB and MHB media were inoculated with K.
219
pneumoniae and E. coli, and their whole bacterial culture and the culture supernatant,
220
which represents the cellular and extracellular matrices, respectively, was examined.
221
In this study, we tested Klebsiella pneumoniae since this microorganism has
222
intensively been investigated for the cell-associated biosynthesis of AgNPs.17, 26, 39, 40
223
Escherichia coli is also widely used model microorganism in biological study, and we
224
used it as a control microorganism. Reduction of silver ions was not observed in the
225
reaction without NB or MHB media (Figure 3A), indicating that the intrinsic reducing
226
power of the microorganisms at test concentration were not strong enough to reduce
227
silver ions to form AgNPs. On the other hand, regardless of the types of media, the
228
intensity of absorption band was increased by the introduced microorganisms (Figure 10
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3B and 3C), which clearly indicates that the bacterial components or metabolites, such
230
as NADH-dependent reductase and nitrate reductase, could facilitate the synthesis of
231
AgNPs along with media components.13,
232
culture of K. pneumoniae showed higher absorbance compared to that of E. coli,
233
which agrees well with the previous observation.41 This might be due to the higher
234
metabolic activity of K. pneumoniae, because we observed a rapid growth of K.
235
pneumoniae in both culture media comparing to E. coli (see S1 in Supporting
236
Information). Moreover, the absorption amplitude of MHB-AgNPs was greater than
237
that of NB-AgNPs (Figure 3c), regardless of the presence of bacteria or bacterial
238
metabolites, indicating that culture media play a major role for the yield of AgNPs. In
239
addition, we observed a slight blue-shift in the absorption peak of AgNPs synthesized
240
by MHB with bacteria from 480 nm to 475 nm comparing to that without bacteria. On
241
the other hand, no shift of absorption peak was observed for the AgNPs synthesized
242
from NB containing bacterial culture or culture supernatant. TEM and DLS analysis
243
revealed that the presence of bacteria or types of bacteria had no effect on the size and
244
shape of the synthesized AgNPs (Figure 4A and 4B). We speculate that the resulting
245
blue-shift observed in the reaction with MHB and bacteria might be due to the
246
interaction of proteins released from the introduced microorganism with
247
biosynthesized AgNPs.42 Moreover, we found that the diameter of AgNPs synthesized
248
from NB media were significantly smaller than those synthesized from MHB media.
249
The size differences of AgNPs synthesized from NB media with K. pneumoniae and E.
250
coli were insignificant (Figure 4A and 4B). TEM results also showed that the shapes
251
of AgNPs obtained from NB-based group were independent of the type of
252
microorganism. Crystalline nature of the AgNPs synthesized was further examined by
253
the SAED pattern with bright circular spots corresponding to (1 1 1), (2 0 0), (2 2 0),
254
(3 1 1), and (2 2 2) Bragg reflection planes.43 SAED pattern of MHB-AgNPs with
22
Note that AgNPs synthesized with a
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bright circular rings corresponding to (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2)
256
planes indicated that the nanoparticles are highly crystalline. In contrast, NB-AgNPs
257
showed weak diffraction spots probably due to the interaction with proteins during the
258
synthesis process, which could affect superstructure of the nanoparticles.44 These
259
findings demonstrate that the use of different culture media predetermined the
260
morphology, size and crystal nature of biosynthesized AgNPs (Figure 4A and 4B).
261
In conclusion, we present the effect of different culture media on the
262
microorganism-mediated synthesis of AgNPs. The results suggest that the components
263
of culture media can affect the size and shape of synthesized AgNPs, independent of
264
whether the synthesis was carried out with microorganism or not. That is, culture
265
media plays a predominant role in determining the physical characteristics of
266
biosynthesized AgNPs. We hope our findings would provide a meaningful
267
information for further elucidating the exact mechanism of microorganism-mediated
268
synthesis of AgNPs.
269 270
271
This work was supported by the Korea Research Foundation Grant (NRF-
272
2016R1D1A1B03934878) and the Cooperative Research Program for Agricultural
273
Science
274
Administration, Republic of Korea.
ACKNOWLEDGEMENTS
&
Technology
Development
(PJ01199303),
Rural
Development
275 276
277
Growth curve of E. coli and K. pneumoniae in NB and MHB media (Figure S1).
SUPPORTING INFORMATION DESCRIPTION
278 279
Notes
280
The authors declare no competing financial interest. 12
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Figure captions Figure 1 Synthesis of AgNPs in different culture media. (A) digital photograph and (B) UV/Vis absorption spectra of reaction solution containing various reducing agents without (left) and with silver ions (right). 1 mM AgNO3 solution without any reducing agent was used as a control. (C) TEM image of the synthesized AgNPs in different culture media. The type of culture media are indicated on top left coner of each image.
Figure 2. (A) O.D values of the reaction after AgNPs synthesis with NB and MHB media without (top) and with light irradiation (down). (B) Size distribution of AgNPs synthesized by NB (blue) and MHB (black), respectively. The synthesis was carried out with light irradiation. (C) FT-IR spectrum of AgNPs synthesized by NB (blue) and MHB media (black).
Figure 3. Digital photograph and corresponding UV/Vis absorption spectra of the reaction after AgNPs synthesis by using culture supernatant and whole bacterial culture in HEPES (A), NB (B) and MHB media (C).
Figure 4. TEM, SAED patterns and DLS analysis of AgNPs synthesized from bacterial cultures in NB (A) and MHB (B) media. TEM images show AgNPs synthesized by using culture supernatant and whole bacterial culture in NB (A) inoculated with K. pneumoniae and E. coli (N-KP C; N-EC C; NKP W; N-EC W), and using that of MHB (B) media (M-KP C; M-EC C; MKP W; M-EC W). The corresponding SAEC patterns and DLS data are shown below. Histograms show the size distribution of the synthesized AgNPs through the condition described above. Solid line and dash line 18
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represent the culture supernatant and whole bacterial culture, respectively.
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Table Table 1. The composition of reducing agents used for the synthesis of AgNPs. Reducing agents Abbreviation Medium
Bacteria
Fraction
AgNO3
None
Whole solution
Cont
Peptone A
None
Whole solution
PA
Beef extract
None
Whole solution
BE
Yeast extract
None
Whole solution
YE
Nutrient broth
None
Whole solution
NB
Nutrient broth
K. pneumoniae 2242
Culture supernatant
N-KP C
Nutrient broth
K. pneumoniae 2242
Whole bacterial culture
N-KP W
Nutrient broth
E. coli DH5α
Culture supernatant
N-EC C
Nutrient broth
E. coli DH5α
Whole bacterial culture
N-EC W
Muller-Hintone broth
None
Whole solution
MHB
Muller-Hintone broth
K. pneumoniae 2242
Culture supernatant
M-KP C
Muller-Hintone broth
K. pneumoniae 2242
Whole bacterial culture
M-KP W
Muller-Hintone broth
E. coli DH5α
Culture supernatant
M-EC C
Muller-Hintone broth
E. coli DH5α
Whole bacterial culture
M-EC W
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Figure Graphics Figure 1
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Figure 2
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Figure 3
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