Multifunctional Ag@Fe2O3 yolk-shell nanoparticles for simultaneous capture, kill, and removal of pathogen

Article abstract of DOI:10.1039/c1jm13691g

We combined silver and iron oxide nanoparticles to make unique Ag@Fe ₂O₃ yolk-shell multifunctional nanoparticles by the Kirkendall effect. After the surface functionalization using glucose, the Ag@Fe₂O₃-Glu conjugates exhibited both high capture efficiency of bacteria and potent antibacterial activity. The Ag@Fe ₂O₃ yolk-shell nanostructures may offer a unique multifunctional platform for simultaneous rapid detection and capture of bacteria and safe detoxification treatment.

Full text of DOI:10.1039/c1jm13691g

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COMMUNICATION
Multifunctional Ag@Fe2O3 yolk–shell nanoparticles for simultaneous
capture, kill, and removal of pathogen†
Zhanhua Wei,a Zijian Zhou,a Meng Yang,a Chenghong Lin,b Zhenghuan Zhao,a Dengtong Huang,a
b
Zhong Chen and Jinhao Gao
a
*
Received 2nd August 2011, Accepted 2nd September 2011
DOI: 10.1039/c1jm13691g
disinfection in clinics and water purification. The major drawback of
chlorination in water purification is the highly toxic chlorine and the
harmful chemical by-products in water.18 So it is imperative to
develop a safe and multimodal strategy for the rapid detection,
killing, and elimination of bacteria from products. Silver (Ag)
nanoparticles may be a promising alternative as a new potent and
broad-spectrum antibacterial agent due to their ultrasmall size and
unique chemical and physical properties.7–9 In this communication,
we combine advanced features of Ag and magnetic nanoparticles,
design and synthesize a novel core–hollow–shell nanostructure with
Ag nanoparticles as cores and iron oxide as shells, Ag@Fe2O3 yolk–
shell nanoparticles, as a multifunctional platform to aim at detecting,
killing, and eliminating pathogen (e.g., E. coli O157:H7) without any
accessional contaminations in a rapid and efficient manner.
We combined silver and iron oxide nanoparticles to make unique
Ag@Fe2O3 yolk–shell multifunctional nanoparticles by the Kir-
kendall effect. After the surface functionalization using glucose, the
Ag@Fe2O3–Glu conjugates exhibited both high capture efficiency
of bacteria and potent antibacterial activity. The Ag@Fe2O3 yolk–
shell nanostructures may offer a unique multifunctional platform for
simultaneous rapid detection and capture of bacteria and safe
detoxification treatment.
The multidisciplinary developments in the fields of physics, chemistry,
and biology have led to the rational design and use of multifunctional
nanomaterials for biomedical applications, such as bacterial detec-
tion, cell imaging, diagnosis, and therapeutics.1–3 Sensitive detection
and efficient elimination of pathogenic bacteria are vital in human
health and environmental safety, including prevention of infections,
water purification, and biodefense. For example, E. coli O157:H7,
a member of a class of pathogenic E. coli known as enter-
ohemorrhagic Escherichia coli (EHEC), may produce shiga-like
toxins and cause severe illness or even death. Recent advances in
nanotechnology have developed many methods for rapid and sensi-
tive bacterial detection4–6 as well as different types of nanomaterials as
potent antibacterial agents,7–12 while the strategy on integration of
pathogen detection and disinfection was rare. After the evaluation
and confirmation of bacterial infection by sensitive detection, the
powerful antiseptic treatments are needed immediately without the
accessional contaminations, which are crucial in clinics and food
safety.13–15
The successful synthesis of maghemite hollow nanoparticles based
on the oxidization of iron nanoparticles by the Kirkendall effect19–21
speeds up the development of yolk–shell nanostructures using iron
oxide as shells.22–26 Using Ag nanoparticles (ESI, Fig. S1†) as the
seeds, we directly injected Fe(CO)5 into the solution of 1-octadecene
containing oleylamine and Ag nanoparticles under an inert atmo-
sphere, sequentially heated the solution at 250 ꢀC in the presence of
air for 2 hours, and finally obtained Ag@Fe2O3 yolk–shell nano-
particles by centrifugation. As shown in the transmission electron
microscopy (TEM) image (Fig. 1a), the Ag@Fe2O3 yolk–shell
Magnetic nanoparticles have the advantages in bacterial detection
and capture with a quick and sensitive manner.16,17 However, the
relatively moderate capture efficiency5 and the remains of living
bacteria may be unfavorable for the detoxification purpose, especially
aState Key Laboratory of Physical Chemistry of Solid Surfaces, The Key
Laboratory for Chemical Biology of Fujian Province, and Department of
Chemical Biology, College of Chemistry and Chemical Engineering,
Xiamen University, Xiamen, 361005, China. E-mail: jhgao@xmu.edu.cn;
Fax: +86-592-2189959; Tel: +86-592-2180278
bDepartment of Electronic Science and Fujian Key Laboratory of Plasma
and Magnetic Resonance, Xiamen University, Xiamen, 361005, China
† Electronic supplementary information (ESI) available: Detailed
experimental procedures, TEM images, XPS analysis, UV-vis analysis,
and antibacterial results on agar. See DOI: 10.1039/c1jm13691g
Fig. 1 (a) TEM, (b) HRTEM, and (c) STEM-HAADF images of
Ag@Fe2O3 yolk–shell nanoparticles synthesized via the Kirkendall effect.
16344 | J. Mater. Chem., 2011, 21, 16344–16348
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nanoparticles have the uniform structure with an obvious void
between the Ag yolk and iron oxide shell. The size of Ag is about 8
nm in diameter and the thickness of iron oxide is roughly 3 nm. The
size of Ag yolk and the thickness of the outer shell are controllable. It
is noted that the shells are porous23 and even partially broken as
shown in TEM images (Fig. 1a and S1†) probably due to the Kir-
kendall effect and the mild etching by oleic acid in the synthesis.27 The
high-resolution TEM (HRTEM) image of Ag@Fe2O3 yolk–shell
nanoparticles (Fig. 1b) indicates that both Ag yolk and iron
oxide shell are crystalline. The iron oxide nanoshells are maghemite
(g-Fe2O3) phase with high polycrystallinity,20 which was further
confirmed by the selected area electron diffraction (SAED) pattern
(ESI, Fig. S1†). The lattice spacing of 0.236 nm in Ag corresponds to
the (111) lattice plane, and the lattice spacing of 0.295 nm in iron
oxide corresponds to the (220) lattice plane of g-Fe2O3. The scanning
TEM with high angle annular dark field (STEM-HAADF) image
(Fig. 1c) further confirms the uniform structure of Ag@Fe2O3
yolk–shell nanoparticles.
X-Ray photoelectron spectroscopic (XPS) measurement of
Ag@Fe2O3 yolk–shell nanoparticles gives the signals of Fe(III) and
Ag(0) in the spectra (ESI, Fig. S2†). Inductively coupled plasma
atomic emission spectroscopy (ICP-AES) analysis indicates that the
molar ratio of Ag and Fe2O3 is 6 : 5, corresponding to ꢁ448 ng Ag
per mg of Ag@Fe2O3 yolk–shell nanoparticles. The as-prepared Ag
nanoparticles have the intense surface plasmon resonance (SPR)
absorption at around 417 nm (ESI, Fig. S3†), which is the charac-
teristic absorption peak of Ag nanoparticles.28 Because the refractive
index of iron oxide (2.3–3.1) is significantly higher than that of Ag
(0.18), the presence of an iron oxide shell and its thickness should
strongly affect the plasmon band position.24 For example, the coating
of an iron oxide shell (ꢁ3 nm in thickness) shifted the SPR of silver
nanoparticles to about 506 nm (ESI, Fig. S3†).
The magnetic measurement reveals the superparamagnetism of
Ag@Fe2O3 nanoparticles at room temperature. Standard zero-field-
cooling (ZFC) and field-cooling (FC) measurements give the esti-
mated blocking temperature of 110 K (Fig. 2a). The field-dependent
magnetization measurement shows that Ag@Fe2O3 nanoparticles are
superparamagnetic and the saturated magnetization (Ms) is about
24.8 emu gÀ1 particles (64.1 emu gÀ1 in terms of Fe) at room
temperature (Fig. 2b), which is sufficient to allow the nanoparticles to
be suitable for magnetic separation using a small magnet.29–31 To test
the MR transverse relaxation rate enhancement effects of Ag@Fe2O3
nanoparticles, we acquired multi-echo gradient echo images at a 7
tesla (T) MRI scanner. T2-weighted MR images (Fig. 2c) showed that
Ag@Fe2O3 nanoparticles have strong MR relaxation enhancement
with the relaxivity value (r2) of 58.1 mMÀ1 SÀ1 (ESI, Fig. S4†),
indicating that Ag@Fe2O3 nanoparticles can be used as a T2 MRI
contrast agent.
Fig. 2 (a) Temperature-dependence of the ZFC/FC magnetization (at
a magnetic field of 100 Oe) and (b) room-temperature field-dependent
magnetization measurement of Ag@Fe2O3 yolk–shell nanoparticles. (c)
T2-weighted MR images of Ag@Fe2O3 yolk–shell nanoparticles from
a 7.0 T MRI system at different concentrations in water (containing 1%
agarose gel).
E. coli ER2566 cells (107 colony forming units per mL, CFU mLÀ1)
with different amounts of the conjugates for 30 min. Because E. coli
ER2566 was constructed of the recombinant plasmids encoding
enhanced green fluorescent protein (EGFP), we measured the fluo-
rescent intensity of each supernatant after the magnetic capture and
separation (within 5 min) to evaluate the capture efficiency of
bacteria. As shown in Fig. 3b, the fluorescent intensity of superna-
tants decreased dramatically as the concentration of Ag@Fe2O3–Glu
conjugates increased, indicating that the number of captured bacteria
increased. The capture efficiency could reach 97% when we use 64 mg
mLÀ1 of Ag@Fe2O3–Glu conjugates and the use of 128 mg mLÀ1 of
Ag@Fe2O3–Glu conjugates can remove almost all of the bacteria
($99%; Fig. 3c). The high capture efficiency of Ag@Fe2O3–Glu
conjugates indicated the strong interaction between the bacterial cell
walls and the conjugates, which is promising for the rapid and effi-
cient elimination of pathogenic bacteria from products. We used the
TEM technique to detect the captured bacteria and observed the rod
We then did surface modification and functionalization of the
nanoparticles using the conjugate of dopamine (DA) and carbohy-
drates (e.g., glucose, galactose) because many bacteria use mamma-
lian cell surface carbohydrates as anchors for attachments5 and DA
can be used as a robust anchor to present functional molecules on the
surface of iron oxide nanostructures.32,33 The conjugation of as-
prepared Ag@Fe2O3 yolk–shell nanoparticles and monosaccharide
molecules makes a new multifunctional nanomaterial in one nano-
architecture, which could endow it with the ability of rapid detection
and capture of bacteria as well as high antibacterial activity. Using
Ag@Fe2O3–Glu conjugates as an example (Fig. 3a), we incubated
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Table 1 MIC values of glucose-modified Ag@Fe2O3 yolk–shell and g-
Fe2O3 hollow nanoparticles on Gram-negative (E. coli O157:H7) and
Gram-positive (B. subtilis) bacteria
MIC/mg mLÀ1
In terms of Ag for
Ag@Fe2O3
g-Fe2O3
hollow
Ag@Fe2O3
E. coli O157:H7
B. subtilis
15.2
21.8
6.6
9.5
>150
>150
inhibit bacterial growth (Table 1 and Fig. S6†), the Ag cores should
play a key role in killing bacteria. Ag@Fe2O3–Glu conjugates showed
high antibacterial activity with low MIC values in terms of Ag (6.6 mg
mLÀ1 and 9.5 mg mLÀ1 for E. coli O157:H7 and B. subtilis, respec-
tively), which is comparative to previous reports on the antibacterial
study of Ag nanoparticles.34,35 According to the presence of multiple
antibacterial mechanisms of Ag nanoparticles8 and the unique yolk–
shell structure22,23 of Ag@Fe2O3–Glu conjugates, we proposed the
probable mechanism as follows. Ag@Fe2O3–Glu conjugates can
specifically target the surfaces of bacteria because of the strong
binding between glucose and bacterial cell wall, which results in the
high attachment of nanoparticles on bacterial surfaces. The porous
and partially broken iron oxide shells may facilitate the silver ions
and/or silver nanoparticles to release from shells, interact with
bacteria, disrupt the bacterial cell walls and membranes, and lead to
the death of bacteria. We indeed observed iron oxide hollow nano-
structures without Ag cores in the captured bacterial samples (ESI,
Fig. S5†), which directly supports our proposed mechanism. Core–
shell nanostructures, normally synthesized by sequential growth
methods at high temperature (e.g., FePt@Fe3O4 and Pt@Fe3O4
core–shell nanoparticles23), have a stable and compact shell to prevent
the core from contacting with the outside environment, which results
in the inert property of core. Unlike the core–shell nanostructures,
yolk–shell nanoparticles via the Kirkendall effect have three advanced
and important features: porous shells, hollow structure between the
core and shell, and ‘‘naked’’ cores without surface protection,22,23
which allow Ag yolks to interact with the outside species and ensure
Ag@Fe2O3 yolk–shell nanoparticles to have strong antibacterial
properties.
Fig. 3 (a) The schematic cartoon of glucose-modified Ag@Fe2O3 yolk–
shell nanostructures (Ag@Fe2O3–Glu conjugates) using dopamine as an
anchor. (b) The fluorescence analysis of the supernatants from E. coli
ER2566 samples after treatment with different concentrations of
Ag@Fe2O3–Glu conjugates (from top to down: 0, 0.5, 1.0, 2.0, 4.0, 8.0,
16.0, 32.0, 64.0, and 128.0 mg mLÀ1). (c) The analysis of bacterial capture
efficiency based on fluorescence measurements.
Because Ag@Fe2O3–Glu conjugates have both excellent capture
efficiency of bacteria and high antibacterial activity, we did the
bacterial elimination experiment from drinking water to investigate
the disinfection effect. After the incubation of contaminated water
(107 CFU mLÀ1 of E. coli ER2566 or E. coli O157:H7 as testing
examples, Fig. 4a and d) with Ag@Fe2O3–Glu conjugates (60 mg
mLÀ1) for about 30 min, the supernatants and precipitates were
collected by magnetic separation immediately and imprinted on the
agar plates. After 24 h of incubation at 37 ꢀC, we did not observe any
bacterial strains on the agar (Fig. 4), indicating that there is not any
living bacterium in the supernatants and the captured bacteria in
precipitates are also dead. The results demonstrate that the
Ag@Fe2O3–Glu conjugate is an excellent candidate as novel and
potent biocides to kill the pathogenic bacteria efficiently and remove
them simultaneously. The combination of these two capabilities may
dramatically improve the disinfection treatment (i.e., not only killing
the bacteria but also reducing the amount of toxins generated by
bacteria after magnetic separation) and warrant the biosafety against
pathogenic bacteria. Moreover, we observed a trace amount of Ag
shape of bacteria (ꢁ2 mm in size) with many aggregations of nano-
particles on the bacterial cell walls (ESI, Fig. S5†), which agrees with
the binding of Ag@Fe2O3–Glu conjugates with bacteria. The detec-
tion limit was approximately 60 CFU mLÀ1 using Ag@Fe2O3–Glu
conjugates.
To test the antibacterial activity, we chose two representative types
of strains as examples: Gram-negative bacteria E. coli O157:H7 and
Gram-positive bacteria Bacillus subtilis (B. subtilis). We evaluated the
in vitro antimicrobial activity by the serial two-fold agar dilution
method and determined the minimum inhibitory concentration
(MIC) of Ag@Fe2O3–Glu conjugates. The MIC values of
Ag@Fe2O3–Glu conjugates for E. coli O157:H7 and B. subtilis are
15.2 mg mLÀ1 and 21.8 mg mLÀ1, respectively (Table 1). Since even 150
mg mLÀ1 of glucose-modified g-Fe2O3 hollow nanoparticles cannot
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platform for simultaneous rapid detection of bacteria and safe
decontamination treatment, which may have attractive applications
in water purification and food safety.
We are thankful to National Science Foundation of China
(21021061, 81000662), the Fundamental Research Funds for the
Central Universities (2010121012), and Program for New Century
Excellent Talents in University (NCET-10-0709) for the financial
support.
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