Synthesis of gold nanoplates by aspartate reduction of gold chloride

Article abstract of DOI:10.1039/b315732f

Single crystal nanoplates with thickness less than 30 nm, characterized by hexagonal and truncated triangular shapes bounded mainly by {111} facets, were obtained in large quantities by aspartate reduction of gold chloride.

Full text of DOI:10.1039/b315732f

Synthesis of gold nanoplates by aspartate reduction of gold chloride†
Yong Shao, Yongdong Jin and Shaojun Dong*
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese
Academy of Sciences, Changchun, 130022, P. R. of China. E-mail: dongsj@ns.ciac.jl.cn
Received (in Cambridge, UK) 3rd December 2003, Accepted 25th February 2004
First published as an Advance Article on the web 5th April 2004
Single crystal nanoplates with thickness less than 30 nm,
characterized by hexagonal and truncated triangular shapes
bounded mainly by {111} facets, were obtained in large
quantities by aspartate reduction of gold chloride.
randomly distributed pairs of lines can be seen across the faces of
the flat crystals and have also been observed in gold crystals
synthesized by repeating polypeptides.11 These lines, always
starting from spots and radiating down to the edges of the flat
crystals, may be ascribed to bending of the thin crystals12 or the
presence of multiply twinned structures.9,11 The insets of Fig. 1c
and 1d show the electron diffraction patterns obtained by aligning
the electron beam perpendicular to the planar surface of the
nanoplates. The hexagonal symmetry of these pattern spots
indicates that these nanoplates are single crystals bounded mainly
by {111} facets.13 On the other hand, the X-ray diffraction (XRD)
A growing interest is being shown in biomineralization and
biomimetic synthesis1 for the creation of nanoscale materials
because environmentally benign, ‘green’ conditions can be used
rather than the extreme conditions needed for conventional
chemical synthesis.2 Crystal morphology and size have often been
biologically regulated by biomolecules3,4 or organisms.5,6 Some
fundamental investigations have revealed that biomolecules and
organisms can selectively recognize inorganic surfaces or serve as
matrices for inorganic growth and nucleation.7 Bacteria,8 silver-
binding peptides9 and silver-tolerant yeast,10 for example, have
been used to fabricate silver nanoparticles, and the morphology of
gold crystals has also been influenced by polypeptides.11 It is
natural that constituent units from biomacromolecules or organisms
should pose a basis for genuine biological nanofabrication.
Therefore, amino acid moieties from an organic matrix may exert
an influence on the specific interactions between biomolecules and
inorganic materials. Here we show that biologically related small
molecules, -amino acids, can control the size and morphology of
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the resultant gold nanostructures.
Reactions were carried out at 25 °C under mild stirring in
aqueous solution containing 0.01% tetrachloroaurate and 0.5 mg
mL21 amino acids for 12 h. No additional reducing agent and
surfactant were needed for the synthesis of gold particles.
Ultraviolet-visible (UV/Vis) spectroscopy was first used to screen
crystals of gold nanostructures (Fig. S1†). The solution incubated
with
L
-tryptophan quickly turned red in color with a very
characteristic surface plasmon absorption centered at 560 nm,
showing the formation of gold nanoparticles. The solution of
aspartate gave two characteristic plasmon peaks centered at 570 nm
and 750 nm, indicating special gold nanostructures. The gold
particles prepared using amino acids were further analyzed by
transmission electron microscopy (TEM) and electron diffraction.
It is very interesting that the aspartate-prepared particles show a
distinct shape. From TEM, the product is dominated by nanoplates
(Fig. 1). The X-ray photoelectron spectroscopy (XPS) analysis
reveals that the prepared nanoplates consist of elemental gold (Fig.
2). The nanoplates mainly assembled in a randomly overlapping
fashion when the sample was mounted on carbon-coated copper
grids, but the shape of the nanoplates can be clearly distinguished
(Fig. 1a). The nanoplates, with thickness less than 30 nm measured
by the comparison of the TEM images from simultaneously formed
spherical and plate-shape nanoparticles, are of hexagonal and
truncated triangular morphology, as clearly shown in Fig. 1b. The
nanoplates have very sharp and smooth edges with average edge
length 590 nm for hexagonal nanoplates (Fig. 1c) and 840 nm for
truncated triangular nanoplates (Fig. 1d). To our knowledge, this is
the first report on the production of large planar gold nanostructures
based on biologically related small molecules. The continuous and
Fig. 1 Typical TEM images of a sample prepared from a reaction solution
containing aspartate, showing the high- (a) and slightly lower- (b) degree
assembled nanoplates during the evaporation of solvent. The representative
images of individual hexagonal and truncated triangular nanoplates are
shown in (c) and (d). The insets show the selected-area electron diffraction
patterns. The scale bars are 200 nm.
† Electronic supplementary information (ESI) available: Fig. S1. UV/
Visible-NIR extinction spectra of an aqueous dispersion of gold nano-
particles synthesized by tyrosine (a), phenylalanine (b), lysine (c), aspartate
(d) and tryptophan (e). See http://www.rsc.org/suppdata/cc/b3/b315732f/
Fig. 2 XPS analysis of Au 4f orbitals of the as-prepared gold nanoplates
supported on a glass slide.
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pattern recorded from the batch sample is also displayed in Fig. 3,
and the peaks are assigned to diffraction from the {111}, {200},
{220}, {311}, and {222} planes of fcc gold, respectively. It is
worth noting that the ratio between the intensities of the {200} and
{111} diffraction peaks is much lower than the conventional value
(0.071 versus 0.53). The ratio between the intensities of the {220}
and {111} peaks is also much lower than the conventional value
(0.034 versus 0.33). These observations confirm that our nano-
plates are mainly dominated by {111} facets, and thus their {111}
planes tend to be preferentially oriented parallel to the surface of the
supporting substrate.
However, compared to aspartate, TEM of the lysine-prepared
gold particles shows spherical nanoparticles 6 ± 2 nm in diameter
(Fig. 4a). Similarly, that of the tryptophan-prepared gold particles
shows the presence of spherical nanoparticles 60 ± 5 nm in size with
high monodispersity (Fig. 4b). However, the size distribution of the
arginine-prepared nanoparticles is very wide with diameter 10 ± 5
nm (Fig. 4c). Examination of the tyrosine-prepared gold particles
reveals the presence of spherical and rod-shaped particles (Fig. 4d).
In these cases, the polycrystalline natures of the as-prepared
nanoparticles are revealed by electron diffraction patterns (Fig. 4,
insets). Based on these observations, aspartate may be specific for
binding to facets other than {111}. This specific interaction of the
amino acid with the crystal lattice structure may not only influence
the surface energies but also alter the distribution of surface
concentration of this species. The specific-binding amino acid also
served as reductant for gold(III) at the growing crystals. The
increase in the local concentration of the amino acid at the binding
facets should elevate the chemically reducing environment within
this local region and must bias accretion onto a face other than
{111} thus increasing the area of the {111} facet. Clearly, the
amino acid provides both recognition and reduction. But the facet-
specific recognition may lose at elevated temperature since under
boiling conditions tetrachloroaurate was reduced to yield spherical
gold nanoparticles in different sizes depending on the molar ratio of
aspartate to tetrachloroaurate and no other morphologies were
found.14 Nevertheless, amino acids other than aspartate show an
unspecific binding to gold facets. The fact that aspartate does show
such distinct shape control over the crystal growth reflects that
these biologically related small molecules also have a profound
influence on the gold crystal growth as macromolecules.11
In summary,
nanostructures has been described. Some size and shape controls
can be exercised by different amino acids. Among natural -amino
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-amino acid-based one-pot synthesis of gold
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acids, aspartate shows a very distinct capability of shape control to
produce gold nanoplates. Although not yet optimized for a certain
amino acid by altering the concentration ratio to fabricate other
special gold nanostructures, the experimental protocols herein
serve to emphasize the practical significance of certain amino acid
residues from biomacromolecules or organisms in biomimetic
construction of nanomaterials. This work provides a further insight
into understanding the contribution from these residues for the
natural process of biomineralization.
Fig. 3 XRD pattern of a batch sample synthesized by aspartate.
Financial support from Special Funds for Major State Basic
Research of China (2002CB713803) and National Natural Science
Foundation of China (No.29835120) is acknowledged.
Notes and references
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Fig. 4 TEM images of the gold nanoparticles synthesized by lysine (a),
tryptophan (b), arginine (c), and tyrosine (d). The scale bars are 5 nm (a), 50
nm (b), 20 nm (c), and 100 nm (d), respectively. The insets show their
selected-area-electron-diffraction patterns, respectively.
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