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Synthesis of copper–silver bimetallic nanopowders

for a biomedical approach; study of their
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Cite this: RSC Adv., 2016, 6, 50933

antibacterial properties
Céline Rousse,a Jérôme Josse,b Valérie Mancier,a Samuel Levi,a Sophie C. Gangloff*b
and Patrick Fricoteaux*a

Copper–silver nanopowders (NPs) are synthesized using a combination of sonoelectrodeposition for the
inner core and galvanic replacement reaction for the outer shell. These combined techniques provide
pure copper core NPs and allow the replacement of the surface copper atoms by silver atoms. The
different Cu–Ag NPs are characterized by transmission electron microscopy, centrifugal liquid
sedimentation, X-ray diffraction, X-ray photoelectron spectroscopy and energy dispersive X-ray
spectroscopy. The diameter of NPs is approximately equal to 7 nm. X-ray photoelectron spectroscopy
results tend to support a core–shell structure. The bactericidal properties of these NPs are tested against
both Staphylococcus aureus (Gram-positive) and Escherichia coli (Gram-negative) bacteria. The
Received 16th March 2016
Accepted 18th May 2016
efficiency of these NPs seems essentially due to the release of Cu2+ and Ag+ ions. The study of different
compositions of Cu–Ag NPs exhibits a good compromise against both S. aureus and E. coli when the
DOI: 10.1039/c6ra07002g
silver atomic percentage is superior to 40%. Finally, tests of wound dressings impregnated with Cu50Ag50
www.rsc.org/advances NP are performed and exhibit successfully their bactericidal properties.

silver presents the best properties against both E. coli and S.

1. Introduction aureus. For Mat Zain et al.,20 the antibacterial effect increased
Due to their unique nanostructuration, nanostructures increase according to the following sequence: Cu < Cu and Ag mixture <
the active surface and may therefore enhance the initial elec- Ag < Cu–Ag alloys. On the contrary, Valodkar et al.21 have found
trical, optical and chemical properties of a material. They can be a more important bacterial growth inhibition in the presence of
employed for example in magnetic recording,1,2 catalysis3,4 or copper as compared to silver; Tamayo et al.22 reported the same
medicine (diagnosis or treatment).5–7 conclusion concerning Listeria monocytogenes. They attributed
Metallic elements such as Ag or Cu, known for their anti- this phenomenon to a better release of copper ions compared to
bacterial properties, were oen cited as being very active against silver ions. Ruparelia et al.19 have found the opposite, i.e. better
both Gram-positive and Gram-negative bacteria.8–10 At the action of silver compared to copper, and explained their results
nanometric scale, the properties of metal or oxide are generally by the presence of a copper oxide layer that decreases the copper
amplied as reviewed by Hajipour et al.11 Nevertheless, since the ions release. Ren et al.23 have compared the action of Cu and
experiments were performed on different bacteria and/or CuO and have found Cu to have the best antibacterial activity.
different nanoparticles (composition, size, etc.), the individual The latter result seems to corroborate those of Ruparelia et al.19
results are difficult to generalize. The silver nanoparticles were For numerous authors, the bactericidal effects of Cu and Ag
certainly the most studied and were tested on different nanoparticles are due to the release of metal ions19,21–23 and to
bacteria12,13 mostly against Escherichia coli (E. coli, Gram- their penetration through the bacterial cell wall.22 The release of
negative) and Staphylococcus aureus (S. aureus, Gram-posi- metal directly within the bacteria accelerating its destruction is
tive).14–17 For example, Kim et al.15 have reported a better effi- bacteria type dependent and could be due to the difference
ciency of silver against E. coli than S. aureus. The comparison between the bacteria cell walls. For Mat Zain et al.,20 Gram-
between the antibacterial activity of Ag and Cu or Ag–Cu positive bacteria are more susceptible to nanoparticles than
(bimetal, alloy or mixture) was also studied and did not always Gram-negative bacteria owing to their more complex cell-wall
give the same conclusions. Some authors18,19 have found that structure. Other studies reported an adsorption impact of
nanoparticles on bacteria surface. The nanoparticles with
a
positive zeta potential have an electrostatic attraction on
LISM, EA 4695, UFR Sciences Exactes et Naturelles, Université de Reims
bacteria with negative charge surface that enhances the total
Champagne-Ardenne, Reims, France. E-mail: patrick.fricoteaux@univ-reims.fr
b
BIOS, EA 4691, UFR Pharmacie, Université de Reims Champagne-Ardenne, Reims,
effect of metals.12,24,25 Another important parameter that can
France. E-mail: sophie.gangloff@univ-reims.fr explain the difference in bacterial susceptibility is the size of the

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nanoparticles used in the studies. It was oen reported that The potentiostat and the ultrasonic system were indepen-
antibacterial activity increases as the size of particles dent and work alternately. A rest period was also applied
decreases.13,20,26,27 between ultrasounds generation and electrolysis in order to
Overall, silver seems to have the broader antibacterial spec- reduce acoustic streaming in solution, diffusion layer and
trum and this explains the interest to functionalize different possible electronic interactions. In order to produce the copper
material with Ag nanoparticles in order to enhance their NPs, a pulse of polarization (
1 V vs. SSE; pulse duration ¼ 100
bactericidal properties. For example, for food packaging appli- ms) was applied to produce nuclei at the surface of the sono-
cations, Esteban-Tejeda et al.28 prepared a soda-lime glass trode. Those nuclei were immediately pulled off the surface
containing silver nanoparticles and Azlin-Hasim et al.29 have using a 100 W cm
2 power ultrasonic pulse for 100 ms. Applied
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tested silver nanodots in polystyrene-b-polyethylene. For the potential for electrolysis and ultrasound pulses were used out of
textile industry, to create healing clothing, Ag nanoparticles phase, without any latency between them. Then a free time of
insertions tests in various cloths were performed.27,30–33 At the 100 ms was applied to let the diffusion of electrochemical
medical level, the impregnation of silver nanoparticles into species. The sequence of these pulses was repeated to produce
cellulose for antimicrobial wound dressings were tested against a large amount of Cu NP.
E. coli et S. aureus by Maneerung et al.30 Aer the sonoelectrochemical synthesis, the copper nano-
One of the issues concerning these nanoparticles is their particle suspension was ltered under vacuum using a hydro-
cost, especially when large amounts are needed, for example in philic polyethersulfone membrane. Several washings in large
the domain of water purication. To solve this problem and volumes of deionized water and with deoxygenated ethanol
reduce the cost, two strategies have emerged: the use of were performed. The NP was removed from the lter by
a mixture of Ag and Cu particles18,34 or the creation of core–shell immersion in degassed ethanol and sonication in an ultrasonic
particles. Recently, Chen et al.35 have studied the anti-oxidation tank. Finally, the obtained copper NP was dried aer sedi-
and antibacterial properties of Cu–Ag core–shell microparticles mentation and placed under vacuum to avoid oxidation.
using commercial micrometric Cu particles that they have
coated with silver by chemical reduction. They demonstrated 2.2. Silver coating elaboration
that the antibacterial activity is linked to the silver percentage in
the particles. For the silver coating elaboration, the copper nanoparticles
In the present study, Cu–Ag bimetallic nanostructured were dipped into a stirred solution of silver(I) ions in order to
particles were produced by sonoelectrodeposition, followed by cover them with a shell of metallic silver. As described by Fri-
galvanic replacement reaction. The rst method was chosen to coteaux et al.,43 the silver solution was prepared with silver oxide
synthesize the copper core as it presented numerous advan- (Ag2O from Fluka with [Ag(I)] ¼ 10
3 mol L
1), sulphamic acid
tages: simplicity, versatility, low cost and adaptable to an (H3NO3S from Chimie-Plus; 0.2 mol L
1), ethyl-
industrial scale,36 high yield of production,37 absence of by- enediaminetetraacetic acid (EDTA from Chimie-Plus; 0.1 mol
products and possibility to work in aqueous media38 without L
1) and sodium hydroxide pellets (NaOH from Chimie-Plus;
any supplementary chemical products. Moreover, numerous until pH ¼ 9.3). Silver oxide and sulphamic acid were rst dis-
studies showed that the NPs obtained by sonoelectrodeposition solved in deionized water. EDTA and NaOH were slowly added
have very small diameters (less than 10 nm).39–42 The Cu–Ag together to the previous solution. This order of insertion
bimetallic NPs were analyzed for their antibacterial activities prevents the formation of unsolvable silver complexes.
and for their ability to be active aer wound dressings The galvanic replacement reaction of silver (eqn (1)) was
impregnation. For the rst time, the effects against S. aureus performed at ambient temperature.
and E. coli of such bimetallic nanostructures (obtained by
2Ag(I) + Cu / Cu(II) + 2Ag (1)
sonoelectrodeposition) were reported, supporting their suit-
ability for use as bactericidal materials. The obtained silver atomic percentage increases with the
duration of copper immersion inside the silver solution.
2. Experimental section
2.1. Copper core elaboration 2.3. Nanopowder characterization
The copper NPs were synthesized using an out-of-phase pulsed X-ray diffraction (XRD) analyses were carried out using
sonoelectrochemical setup42 with a classical three-electrode a Brucker D8 Advance X-ray diffractometer equipped with
electrochemical cell. The working electrode consists of a tita- a copper anticathode (lCuKa ¼ 1.54056 Å). XRD diffractograms
nium cylinder (called sonotrode or horn) connected to a poten- were recorded in Bragg Brentano conguration. Particle size
tiostat and to an ultrasonic generator. The counter electrode distribution was obtained by centrifugal liquid sedimentation
was a pure copper rod (99.9%) and the reference electrode was (CLS) of a powder suspension in ethanol with a CPS Disc
a saturated mercury sulphate electrode (SSE). The aqueous Centrifuge DC20000 particle size analyser (CPS Instruments)
solution for copper electrolysis was made with copper sulphate with a rotation speed equal to 20 000 rpm.
pentahydrate (CuSO4$5H2O from Panreac) at 0.1 mol L
1 and The zeta potential was measured by a Delsa™ Nano C
sulfuric acid (H2SO4 from Chimie-Plus) at 0.9 mol L
1. The particle analyser (BeckMan Coulter) computer driven with the
temperature of the copper electrolyte was regulated at 30  C. soware Delsa™ Nano 3.73 (BeckMan Coulter Inc.). The

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measurements were done in distilled water at constant compared with dry NPs, copper salts in solution and massive
temperature (25  C) by means of a Peltier device. The chemical copper (Goodfellow 99.9%). To follow the calibrated wound
composition was investigated by energy dispersive X-ray spec- dressing antibacterial activity, the samples (1 cm2) were laid
troscopy (EDXS) using a JEOL 1300 microprobe coupled with directly on the surface of the agar plates. Each time, a positive
a JEOL JSM 6440LA scanning electron microscope (SEM). All the (bleach disposed in a well) and a negative (bare wound dressing
EDXS atomic percentages are given with an accuracy of 4%. laid on agar) control were included in the test. The plates were
Surface composition was analyzed with an X-ray photoelectron then incubated at 37  C and the radius of the growth inhibition
spectroscopy (XPS) system. The apparatus used was a SSX-100 zones from the center of the deposit were measured aer 24
system using Al Ka X-rays, with spectra recorded at 35 take- hours of incubation. Each assay was repeated at least 3 times.
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off angle. Transmission electron microscopy (TEM) was per-

formed using a Philips CM20 ultra twin SEM.

2.4. Wound dressing functionalization 3. Results

The commercial wound dressing used in this study 3.1. Materials characterization
(Leukomed®, BSN Medical) is sterile, permeable to air and
Cu and Cu–Ag NPs with different compositions were charac-
water. It is composed of a white polyurethane non-weaved
terized by different methods. Except for zeta potential
support (Hypax®) and a neutral compress, and it does not
measurements, the Cu50–Ag50 NP was chosen as an example to
have any intrinsic antibacterial activity. To insert nanoparticles
exhibit the different results when numerous compositions of
into the wound dressing, a sample (1 cm2) was immersed into
Cu–Ag nanoparticles were analyzed. TEM analyses revealed that
deionized water containing 0.5 g L
1 or 1.0 g L
1 of Cu–Ag NP.
Cu and Cu–Ag NPs are roughly spherical with heterogeneous
Ultrasonic horn was employed at high power (204 W cm
2) to
diameters (Fig. 1a1 and b1). Using the centrifugal liquid sedi-
shake the solution during 20 min. The wound dressing piece
mentation method, the particle size distribution was deter-
was then removed from the aqueous solution and dried under
mined and the hydrodynamic diameter is nearly the same
sterile controlled atmosphere at 18  C during 12 h. The quantity
before and aer galvanic replacement reaction. It varies
of impregnated powder inside the wound dressing was deduced
between 5 and 10 nm with predominance for a diameter equal
by comparison between the quantity of silver introduced and
to 7 nm (Fig. 1a2 and b2).
the quantity remaining in the solution aer impregnation. The
The XRD diffractogram of the copper NPs (Fig. 2a) exhibits
rst one was obtained by weighing the introduced silver NP; the
diffraction peaks corresponding mainly to the metallic copper
second one by inductively coupled plasma atomic emission
(JCPDS le 004-0836) with only very low traces of copper oxide (I)
spectrometry analysis (ICP-AES, Varian-spectrometer) of the
(Cu2O) (JCPDS le 005-0667). Aer galvanic replacement reac-
solution aer residual silver dissolution with nitric acid. The
tion (Fig. 2b), only metallic silver diffraction peaks (JCPDS le
deduced nal impregnated wound dressing concentrations
004-0783) and copper diffraction peaks (JCPDS le 004-0836) are
were 3 mg cm
2 (for an initial concentration at 0.5 g L
1 of Cu–
present, indicating that the NPs are not a Cu–Ag alloy but two
Ag NP) and 6 mg cm
2 (for an initial concentration at 1.0 g L
1
distinct metals.
of Cu–Ag NP).
To study the repartition of silver on copper core, the Cu50–
Ag50 NP was characterized by XPS and EDXS. The comparative
2.5. Antibacterial assays results are exhibited in Table 1. It shows that the silver
The antibacterial assays were evaluated on S. aureus 8325-4 percentage values obtained by XPS is larger than the ones ob-
(prophage-free NCTC 8325) and E. coli DH5-a (Invitrogen). The tained by EDXS. Given that XPS is a surface analysis technique,
bacteria, taken from a frozen aliquot, were rst stripped on this highlights that the silver percentage is higher on the
a Trypto-Casein-Soy (TCS, Biomérieux) agar plate to isolate surface than in the center of the particle. This proves that the
colonies. Two colonies were inoculated into TCS broth and particles obtained are richer in silver at the surface suggesting
cultured during 12 hours at 37  C. The bacteria were then a core–shell structure. Such Cu–Ag core–shell structures were
inoculated into fresh TCS broth to obtain a suspension with an already obtained for submicro and microparticles.35,42,44,45,47 As
optical density equal to 0.1 (A620 nm). The bacteria were then shown in a previous work,42 the silver shell widely modies the
cultured at 37  C up to a culture absorbance (A620 nm) between conversion reaction kinetic that becomes very slow from about
0.2 and 0.4, so that the bacteria were in a growing phase. 40 at% of silver. This was explained by the presence of the silver
Thereaer, 2 mL of the culture was spread on the surface of coat limiting the access to the subjacent copper. This point
fresh agar plates. The excess was removed and the plates were rules out the formation of Janus structures in which copper
prepared for the assay. would be easily accessible until the total conversion. In this
To detect the NPs antibacterial efficiency, the principle of the case, no slow-down of kinetic would then be observed.
antibiograms was used. In the present study, the impregnated Finally, the zeta potential of the NPs was measured. Fig. 3
disks were replaced by wells. The 6 mm diameter wells were exhibits its evolution versus the atomic silver percentage. The
punched at equal distance in the agar plate and calibrated obtained zeta potential is mainly negative for NPs with
aqueous solutions of NPs (2 mg in 30 mL) was deposited in the a composition of 0 to 75 atomic silver percentages. It becomes
well. The bactericidal effects of aqueous solution of NPs were positive when the silver atomic percentage is over 75%.

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Fig. 1 Characterization of Cu (a1 and a2) and Cu50–Ag50 (b1 and b2) NPs: (1) transmission electron microphotograph, (2) particle size distribution
by CLS method.

Fig. 3 Zeta potential of Cu and Cu–Ag NPs versus silver atomic

percentage of NPs.

All bactericide assays were conducted in nanoparticles

dispersed in aqueous solution because with dry nanoparticles
Fig. 2 X-ray-diffractograms recorded in Bragg-Brentano conditions the inhibition zones were not homogenous around the wells
for Cu (a) and Cu50–Ag50 (b) NPs. due to the powders landing in the well resulting in a various
diffusion concentration in the agar. Different concentrations of
nanopowders were tested before choosing an ideal quantity of
Table 1 Comparison of at% of Ag in nanoparticles between EDXS introduced nanoparticles. A very weak antibacterial activity is
(4%) and XPS (2%) method discernable for 0.1 mg of NPs (Fig. 4a). For higher quantities,
the inhibition zone is marked and a quantity equal to 2 mg was
EDXS results (at% Ag) XPS results (at% Ag)
chosen for the following tests. As seen in Fig. 4b, S. aureus
20 58 bacteria present susceptibility to copper and to Cu50–Ag50 NPs
50 88 (the choice of this particular composition will be justied at the
end of this paragraph 3.2.). The inhibition zone is slightly larger
with copper NPs compared to Cu50–Ag50 NPs. This can be
explained by the fact that copper is directly accessible in sample
3.2. Antibacterial activities of the core shell particles 2. However, as seen in sample 3, the silver coating does not
Antibacterial assays on agar plates were performed on S. aureus hamper the antibacterial activity of copper against S. aureus.
and E. coli to evaluate their sensitivity against the different NPs. This is explained by the porosity of the silver shell.

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Fig. 4 Diffusion antibacterial assay against S. aureus (a) with 0.1 mg of NP (b) with 2 mg of NP (c) comparison between massive and ionic copper.
1: bleach, 2: Cu NP, 3: Cu50–Ag50 NP, 4: massive Cu (0.1 g and 30 mL of Trypto-Casein-Soy (TCS)), 5: Cu(NO3)2 (30 mL of 1 mol L
1).

In order to verify that the release of ions inside the agar has
a preponderant impact on the bacteria in the diffusion assay,
the effect of Cu2+ ions (Cu(NO3)2 solution) and massive copper
were also tested for their antibacterial effects (Fig. 4c). The
growth inhibition zone observed with sample 5 corresponding
to Cu2+ ions is much larger than the one corresponding to the
massive copper (sample 4). This result indicates clearly a greater
activity of ions as compared to metallic copper.
The different core–shell NPs produced were also tested
against E. coli. The impact of the silver atomic percentage on S.
Fig. 6 Diffusion antibacterial assay with impregnated wound dressing
aureus and E. coli growth inhibition is summarized in Fig. 5. S.
by Cu50–Ag50 NPs. (a) S. aureus, (b) E. coli. 1: bleach, 2: bare wound
aureus is sensitive for each tested NP. The response against S. dressing, 3: impregnated wound dressing of Cu50–Ag50 (3 mg cm
2),
aureus can be considered as constant whatever the silver atomic 4: impregnated wound dressing of Cu50–Ag50 (6 mg cm
2).
percentage. For each tested composition, S. aureus is more
sensitive than E. coli. Note that no bactericide properties against
E. coli were observed for an atomic silver percentage inferior to for Cu20–Ag80.42 Secondly, its response again E. coli is satisfac-
40% (the powder was deposited in well of a 6 mm diameter). tory and allows using a minimum of silver.
Beyond 40 at% of silver, the activity becomes effective and
increases with silver percentage. Consequently, to be efficient
against both S. aureus and E. coli, the atomic silver percentage of 3.3. Wound dressing antibacterial activity
theses NPs needs to be superior to 40%. With a composition of
Wound dressings impregnated aer immersion inside NP
Cu50–Ag50, the response again E. coli is correct even if it is not
solution under ultrasound (containing 3 or 6 mg cm
2 of NP)
the best one (compared to one obtained with Cu20–Ag80). For the
were laid on the agar covered with bacteria; the face intended to
following assays, the composition of Cu50–Ag50 was selected to
be in contact with the skin was in contact with the agar. With
impregnate wound dressings for different reasons. Firstly, the
this setting, the impregnated samples show antibacterial
time of Cu50–Ag50 preparation was about ve times shorter than
activities on both S. aureus and E. coli (Fig. 6) in comparison to
the plain wound dressing that serves as the negative control for
antibacterial activity. A gradual effect of the Cu50–Ag50 NP
impregnated wound dressing can be seen between 3 mg cm
2
and 6 mg cm
2 both against S. aureus (Fig. 6a) and E. coli
(Fig. 6b). In parallel to the increased concentrations of the NP
on the wound dressing, the measured inhibition zone increased
from 19.5 to 21.0 mm for S. aureus (Fig. 6a) and for E. coli, from
18.7 to 27.4 mm (Fig. 6b).

4. Discussion
Silver or copper have been used as antimicrobial agents over the
centuries. Currently, there is a renewed interest for inorganic
nanoparticles like silver and copper as bactericide compounds
Fig. 5 Antibacterial activity against S. aureus and against E. coli versus due to their low cytotoxicity, known bacterial killing properties
silver atomic percentage of NPs. and their low ability to induce resistance mechanisms.46

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The antibacterial effects of silver or copper nanoparticles The difference in the cell walls of Gram-positive and Gram-
have been widely observed but their mechanisms of action are negative bacteria could explain the difference of mechanisms
not fully understood, due mainly to bacterial physiology and between copper and silver. Gram-positive bacteria like S. aureus
enzyme mechanisms diversity. Most of the studies have high- present a cell wall that allows the diffusion of small molecule,
lighted that a direct contact between the nanoparticles and the unlike the more complex cell wall of Gram-negative bacteria like
bacteria leads to damages in the bacteria cell wall and/or E. coli, bounded with an outer lipid membrane. Indeed, Gram-
membrane. As a second step, this rupture of the cell-wall negative cell wall also notably differs from the Gram-positive
permeability allows the penetration of nanoparticles into the one by the presence in its outer membrane of lipopolysaccha-
bacteria and an enhanced alteration of biological molecules by rides that enhances the negative charge of the cell wall. The
Published on 19 May 2016. Downloaded by University College London on 27/05/2016 09:52:58.

nanoparticles. These bactericidal effects of metallic nano- electrostatic interactions between bacteria and nanoparticles
particles are generally attributed to the induction of an oxida- could also explain the higher efficiency of NPs with positive zeta
tive stress through the release of reactive oxygen species (ROS). potential.12,24 Both S. aureus and E. coli have negative global
Hwang et al.47 reported for example that Ag ions induce ROS surface charges and most of the synthesized nanopowders
that consequently alter bacterial compounds. It was also re- compositions have a negative zeta potential. Consequently, the
ported that metallic and ionic forms of copper produce hydroxyl zeta potential alone could not explain the activity of the NPs
radicals that damage both major proteins and DNA and that studied. Nevertheless, a zeta potential effect cannot be totally
silver ions bind to sulfur, oxygen and nitrogen of essential excluded. It is interesting to note that the zeta potential of the
biological molecules thus inhibiting bacterial growth.10,11,25 NPs with more than 40 at% of silver started to be less negative
Considering their respective antibacterial spectrum – copper (Fig. 3) and this percentage corresponds to the beginning of
is very active against S. aureus while silver is more efficient activity against E. coli (Fig. 5). So, even if the zeta potential is
against E. coli – the combination of copper and silver in a single negative above 40 Ag at%, it could lead to a lower repulsion
nanoparticle is therefore interesting. Many efforts have recently force between nanoparticles and bacteria and thus to a better
been made to synthesize specic copper/silver nanostructures, activity of Ag.
with long lasting effects, high chemical stability and low cyto- Finally, the impregnated Cu50–Ag50 NPs wound dressings
toxicity. With such a core–shell arrangement the formation of exhibit bactericidal concentration dependent properties (Fig. 6).
an oxide copper oxidation is avoided thanks to the silver It is also interesting to note that this concentration effect is
coating.35,48 In the present study, Cu–Ag core–shell nano- enhanced against E. coli. Indeed, for these bacteria, the inhi-
structures were obtained using sonoelectrolysis followed by bition zone increases by 50% when the impregnated NPs
galvanic replacement reaction. These core–shell nanostructures concentration doubles. For S. aureus, the effect is already clearly
allowed an increase of the reactive surface and a decrease of the visible at very low concentrations and only a slight increase is
total amount of each metal, specially the covering one, with observable (8%) when the concentration doubles. One explica-
effective low cost methods. tion could be, once again, linked to the necessity to have a tight
As seen in the antibacterial assays, looking at the efficiency contact between the NPs and E. coli, for an ion diffusion
of different copper physical states of matter against S. aureus, it important enough to see the effects on S. aureus. Nevertheless,
appears that the Cu2+ ions are much more effective as compared our impregnated Cu50–Ag50 NPs wound dressings are effective
to massive copper. The growth inhibition zone observed with against both S. aureus and E. coli even with a low concentration
copper NP is more important than with massive copper (Fig. 4). of NPs, which is of medical relevance. The core–shell insertion
The fact that massive copper does not diffuse into the agar on the bers of wound dressing is moreover relatively simple
explains this relative bacterial insensitivity. However the pres- and the NPs present a large advantage compared to Ag+ and/or
ence of a slight inhibition zone shows that part of the bacteria is Cu2+ solutions. The NPs behave indeed as a tank for silver and
destroyed and this necessarily results from the release of Cu2+ copper that only gradually released ions, which confers
ions in the growth medium and proves the preponderant role of a persistence antibacterial efficacy. Furthermore, for the same
released ions against these bacteria. material quantity, the nanometric scale exhibits much larger
Even if the ions release is enhanced by nanometric scale, the active surface compared to the micrometric one thus amplifying
composition (atomic percentage of copper and silver) is never- their properties. The core–shell structures allow using only
theless important to dene for bactericide applications as seen a single type of particle and not a mix of copper and silver
from Fig. 5. Although all the core–shell NPs present a good particles thus avoiding the heterogeneity of the repartition of
activity against S. aureus, due principally to the presence of these two elements in the wound dressing.
copper, these core–shell NPs start to present an interesting
activity against E. coli only when the percentage of atomic silver is 5. Conclusions
over 40 at%, conrming the different action mechanisms of both
metals. It can be noticed that even if the copper is silver coated, Cu–Ag core–shell NPs were obtained using sonoelectrolysis
the covering is not totally compact since the galvanic replacement followed by galvanic replacement reaction. The method chosen
reaction (eqn (1)) continues until reaching values close to 80 Ag allows obtaining very ne NPs in aqueous media without using
at%. This phenomenon is classic in the case of galvanic surfactants and by-products formation. Moreover, they are free
replacement reaction and stands from the presence of porosity in of oxides traces. The Cu50–Ag50 synthesized particles are effec-
the replacement layer49 allowing the release of ions from the core. tive antimicrobial against both S. aureus and E. coli. Their use to

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functionalize wound dressing is of positive interest since they 19 J. P. Ruparelia, A. K. Chatterjee, S. P. Duttagupta and
are already effective at low quantities. Toxicological tests on S. Mukherji, Acta Biomater., 2008, 4, 707.
human cells are underway in order to verify the biocompatibility 20 N. Mat Zain, A. G. F. Stapley and G. Shama, Carbohydr.
between Cu–Ag core–shell NPs and human tissues. Polym., 2014, 112, 195.
21 M. Valodkar, S. Modi, A. Pal and S. Thakore, Mater. Res. Bull.,
Acknowledgements 2011, 46, 384.
22 L. A. Tamayo, P. A. Zapata, N. D. Vejar, M. I. Azócar,
This study was supported by the Conseil Régional de la Marne. M. A. Gulppi, X. Zhou, G. E. Thompson, F. M. Rabagliati
We are also grateful to the URCA PICT IBiSA Imaging Center and M. A. Páez, Mater. Sci. Eng., C, 2014, 40, 24.
Published on 19 May 2016. Downloaded by University College London on 27/05/2016 09:52:58.

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