Abstract Engineered gold nanoparticle catalysts containing a mixture of metals enable enhanced specificity and reactivity. We report the synthesis by plants of mixed metal nanoparticles suggesting the possibility of using plants to produce catalysts of specific composition, perhaps even those difficult to synthesise by traditional methods. The nanoparticles contain Au, Ag and Cu as an alloy. The structure and composition are confirmed by scanning transmission electron microscopy (STEM) and energy dispersive X-ray analysis (EDX)

A T Marshall

D Van Agterveld

R G Haverkamp

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Brief Communication
Pick your carats: nanoparticles of gold–silver–copper alloy produced in vivo
R. G. Haverkamp1,*, A. T. Marshall1 and D. van Agterveld2
1Institute of Technology and Engineering, Massey University, Palmerston North, 5331 New Zealand;
2Department of Chemicals Analytics and Physics, Microstructure Analysis and Atomic Spectrometry, Akzo
Nobel Research and Technology Chemicals, P.O. Box 9300, 6800 SB, Arnhem, The Netherlands; *Author for
correspondence (Tel.: +6463504167; E-mail: r.haverkamp@massey.ac.nz)
Received 25 July 2006; accepted in revised form 5 December 2006
Key words: catalysis, gold, silver, copper, nanoparticles, phytomining, phytoremediation, alloy, TEM,
biosynthesis, in vivo in plants
Abstract
Engineered gold nanoparticle catalysts containing a mixture of metals enable enhanced specificity and
reactivity. We report the synthesis by plants of mixed metal nanoparticles suggesting the possibility of using
plants to produce catalysts of specific composition, perhaps even those difficult to synthesise by traditional
methods. The nanoparticles contain Au, Ag and Cu as an alloy. The structure and composition are
confirmed by scanning transmission electron microscopy (STEM) and energy dispersive X-ray analysis
(EDX).
Introduction
The synthesis of nanoparticles of specific compo-
sition and size is a burgeoning area of materials
science research. The properties of these particles
in applications as diverse as catalysis, sensors and
medicine depend critically on the size and com-
position of the nanoparticles. New routes to the
manufacture of these materials extend the choice
of properties that can be obtained.
One material that has received increasing
attention is gold. Nanoparticles of gold are cata-
lytically active for reactions such as the water
gas shift reaction and selective oxidation of CO
(Andreeva, 2002; Grisel et al., 2002; Hutchings &
Haruta, 2005). Gold nanoparticles also have other
applications in the field of sensors (Yanez-Sedeno
& Pingarron, 2005; Puckett et al., 2005; Liu & Lu,
2004) and in medicine (Paciotti et al., 2004).
Intracellular and extracellular synthesis of
metallic nanoparticles, including both gold and
silver, by bacteria and fungi have been demon-
strated previously (Gericke & Pinches, 2006; Klaus
et al., 1999; Mukherjee et al., 2002). Plants have
also previously been shown to produce metal
nanoparticles, including gold, with plant cells
(Reeves & Baker, 2000; Gardea-Torresdey et al.,
2002, 2005). The concentration of gold from ores
by plants, or phytomining, has also been proposed
as an environmentally friendly and economic
method of recovering gold (Anderson et al., 1999;
Gardea-Torresdey et al., 2005; Lamb et al.,
2001a).
To induce accumulation of gold into Brassica
juncea application of ammonium thiocyanate has
been reported, increasing the gold concentration to
57 lg g)1 (Anderson et al., 1998). Other solubil-
ising agents such as cyanide, iodide, bromide, and
thiosulfate have been examined in gold accumu-
lation studies (Lamb et al., 2001b).
Increasingly gold catalysts are being specifically
tailored to contain a mixture of metals enabling
Journal of Nanoparticle Research (2007) 9:697–700  Springer 2007
DOI 10.1007/s11051-006-9198-y
enhanced specificity and reactivity (Toshima &
Yonezawa, 1998). We report the production of
such mixed metal nanoparticles by plants opening
up the possibility of using plants to produce
nanoparticles of specific composition, perhaps
even those difficult to synthesise by traditional
methods. The material we report contains Au, Ag
and Cu as an alloy, the combination of which is
likely to be useful catalytically (Liu et al., 2005;
Dimitratos et al., 2005; Kariuki et al., 2004;
Comotti et al., 2006; Bianchi et al., 2005). While,
to our knowledge, this is the first alloy produced
by plants to be reported we believe it should be
possible to produce a large range of alloys from
many metals by a similar route and to have con-
siderable control over the composition of these
alloys. The plant based route could be considered
to be an environmentally favoured or green
chemistry method of production.
Experimental details
Metal rich soil was produced by applying 100 ml
5 g Au l)1 gold chloride solution, silver nitrate
and copper chloride solutions to 5 kg of sieved
and dried agricultural soil. The soil was dried at
100C overnight then diluted with Au-free soil to
give 48 mg Au kg)1, 44 mg Cu kg)1 and 31 mg
Ag kg)1 in the final soil. The soil was distributed
into 0.5 l greenhouse pots, and watered daily to
dampness but not release for two weeks to allow
equilibration of the metals and the soil. Seeds of
Brassica juncea were then added to the pots. The
seedlings were thinned to leave five healthy plants
in each pot. After 9 weeks of growth, 100 ml of
4 g l)1 potassium cyanide solution was irrigated
onto each pot. Fourteen days later the above-
ground biomass was harvested and dried at
110C then ground and sieved to less than
180 lm.
The concentration of gold in the plant material
was measured using AA spectroscopy (GBL
Avanta R, k = 242.8 nm). The biomass was ashed
at 550C for 4 h in silica crucibles and digested in
aqua regia in sealed glass tubes at 150C for 2 h.
The samples were then diluted with 2 mol l)1 HCl
to a Au concentration of 2–10 ppm. A gold stan-
dard solution (1,000 ppm Au, Spectrasol) was
used to prepare standard solutions ranging from 2
to 20 ppm Au in 2 mol l)1 HCl.
For TEM imaging the plant material was
embedded in Spurr’s resin. Sections of about
60 nm thickness were prepared using a Leica EM-
UC6 ultramicrotome, collected on a water surface
and transferred to a slightly etched (Ar plasma) Ni
grid and dried. The thin sections were then covered
with a thin layer of carbon to prevent charging
during analysis. A JEOL JEM-2010F-HR TEM
was used, with a 200 kV electron beam (field
emission gun), equipped with a scanning unit
(STEM) and a Thermo Noran energy dispersive
X-ray (EDX) and the sample held in a Be holder.
We recorded some images with High Angle
Annular Dark Field mode (HAADF) using a dark
field detector where only electrons that are signif-
icantly scattered by the sample material are
detected, which leads to heavier elements in the
sample appearing bright. EDX spectra and maps
were recorded under analytical probe (1.0 nm)
conditions. A particle diameter of more than 5 nm
is required for it to be visible in the STEM-EDX
maps under our measurement conditions.
Results and discussion
The dried plants contained an average of
760 lg Au g)1, 300 lg g)1 Cu, and 730 lg g)1
Ag. Typically in plants containing such a high level
of gold about half of the gold is in the form of
metal and half in the form of a soluble Au+ salt.
The metallic portion of the gold is present in the
form of nanoparticles which appear to range in
size from 5–50 nm (Figure 1) although there may
be some particles smaller that were more difficult
to observe. We confirmed that these were particles
of high atomic mass initially by HAADF imaging.
A STEM-EDX map for Au, Ag and Cu was also
recorded from which it was apparent that all three
elements are present in all particles (Figure 2).
The elemental compositions of 30 particles were
determined by spot analysis. Complete solid solu-
tion is possible between Au, Ag and Cu and this
forms the basis of the common gold alloys for
jewellery. The gold content of the larger nano-
particles varied between 20 and 55 atomic %
with corresponding 80–44 atomic % Ag and up to
1 atomic % Cu. The average analysis for the larger
nanoparticles (for which we were able to get a
more accurate analysis than for the smaller parti-
cles, and we have averaged about half the spot
698
analyses) was 43 atomic % Au, 57 atomic % Ag
with less than 1% Cu. There were variable minor
amounts of Mg, Si, P, S Cl and Ca which are likely
to be from covering or surrounding material and
these analyses we report have excluded these ele-
ments when calculating the Au, Ag and Cu. The
atomic ratios of Au/Ag varied from 0.25–1.2 (with
an average of 0.80) compared with the ratio for the
bulk plant material of 0.57.
We found a lattice spacing of 0.22–0.23 nm
from a Fourier transform (Figure 3a inset) and
also 0.22–0.23 nm from a measurement of lattice
spacing directly off the images (Figure 3). This is
commensurate with a mixture of Au (111) which
has a spacing of 0.235 nm (Akita et al., 2005), Ag
(111) of 0.235 nm and Cu (111) of 0.209 nm
(Somorjai, 1994) although the accuracy of the
measurement is such that we can not make a
definitive statement about composition from the
spacing alone. This combined with the EDX
analysis indicates that the nanoparticles consist of
an alloy and not a mixed phase of separate
elements (Frenkel et al., 2000).
Conclusion
In summary, we have used Brassica juncea to
produce nanoparticles of an alloy. We are not
aware of any other instances of metallic alloys
being produced by plants. This preliminary work
points the way to a new biosynthetic method for
the production of nanoparticles of metal alloys for
a range of nanotechnology applications. We pro-
pose that these nanoparticle alloys can be tailor
made using plants by adjustment of the metal
Figure 2. STEM-EDX maps of Au, Ag and Cu.
Figure 1. (a) STEM micrograph of metallic nanoparticles from Brassica juncea; (b) STEM-HAADF image of nanoparticles in
plant material (bright areas indicate high atomic mass).
699
components in the growth medium. Specifically we
have demonstrated the synthesis of the Au–Ag–Cu
class of alloy.
Acknowledgments
This study was supported in part by the US Air-
force grant AOARD054068 and by The MacDi-
armid Institute for Advanced Materials and
Nanotechnology.
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Pick your carats: nanoparticles of gold–silver–copper alloy produced in vivo

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Pick your carats: nanoparticles of gold–silver–copper alloy produced in vivo

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