1
Abstract—Gold nanoplates are promising for optical and
electronic applications; but their synthesis is complex, often requiring a seeded growth process or spherical to triangle morphology transformation. We have discovered a biological protocol to promote the anisotropic growth of different crystal planes under ambient conditions. Thin, flat, single-crystalline gold nanoplates were produced when aqueous chloroaurate ions reacted with the mycelia-free spent medium. While the exact mechanism for this shape-controlled synthesis is not clear at this time, the possibility of achieving nanoparticle shape control in a fungal based system is exciting.
Index Terms—Gold, Nanoparticle, Nanoplates, Triangular,
Aspergillus niger, Mycelia-free spent media.
I. INTRODUCTION
Much effort has been devoted to the synthesis of inorganic nanoparticles with different shapes. This includes zero-dimensional nanospheres,1 one dimensional nanorods,2 and two-dimensional nanoplates.3 Compared to zero and one dimensional nanostructures, the synthesis of two-dimensional nanostructures in high yield has always been a challenge, often requiring complex and time-consuming steps such as morphology transformation from the nanospheres,4 or the seeded growth process.5 Meanwhile, biological organisms have evolved the ability to direct the synthesis and assembly of crystalline inorganic materials under environmentally benign conditions with good control of chemical composition, size and morphology. Examples include the formation of iron oxides by bacteria, silica deposition in diatoms, calcification in cyanobacteria,
Manuscript received November 17, 2006.
Prof. Jim Yang Lee is with Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore-119260 and also a Fellow of Singapore-MIT Alliance, National University of Singapore, Singapore-117576. (phone: 65-6874-2899; fax: 65-6779-1936; e-mail: cheleejy@nus.edu.sg).
Prof. Daniel I.C. Wang is with Department of Chemical Engineering, M.I.T., 77 Massachusetts Ave., Cambridge, MA 02139, USA and also a Fellow of Singapore-MIT Alliance, National University of Singapore, Singapore-117576.
Prof. Yen Peng Ting is with Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore-119260.
Jianping Xie is with Singapore-MIT Alliance, National University of Singapore, Singapore-117576.
magnetic nanoparticles in magnetotactic bacteria, and the formation of various other biogenic minerals by bacteria, algae, and fungi.6 The growing interest in the biomimetic synthesis of nanoparticles is inspired by nature’s effectiveness and the selectivity of the biological systems.7 Herein we report the discovery that the mycelia-free spent medium of Aspergillus niger, when reacted with chloroaurate ions, could form gold nanoplates at high yield. It is possible to develop a biological protocol for the size- and shape- controlled synthesis of nanoparticles using this fungal based biological system.
II. EXPERIMENTAL
Aspergillus niger was obtained from Dr H. Brandl (University of Zurich, Switzerland) and was cultured according to the protocol of Bosshard et al8. The fungus was grown in 250mL flasks, each containing 100mL media with the following composition (g/L): 100 sucrose (Biorad), 1.5 NaNO3 (Merck), 0.5 KH2PO4 (Merck), 0.025 MgSO4.7H2O (Merck), 0.025 KCl (Merck), and 1.6 yeast extract (Difco). The culture was incubated in an incubator at 30oC with rotary shaking at 120rpm. After 4 days of fermentation, the mycelia-free spent medium was separated from the culture broth by filtration through Whatman Autovials (0.45μm). HAuCl4 was added to 10mL mycelia-free spent medium to a an overall Au(III) concentration of 1
×
10-3M. The final pH was 3. The mixture was gently agitated with a magnetic stirrer, and the reaction was carried out at room temperature (25oC). Aliquots of the reaction mixture were removed at regular intervals and analyzed by UV-vis spectroscopy and TEM imaging. UV-vis spectroscopy was performed on a Shimadzu UV-2450 spectrophotometer operating at 1 nm resolution. TEM analysis was carried out on a JEOL JEM2010 microscope operating at 200kV.III. RESULTS AND DISCUSSION
On mixing the mycelia-free spent medium with the aqueous solution of chloroaurate ions, the color of the latter changed from yellow to red purple in 3 hours, signaling the formation of gold nanoparticles. The gold nanoparticles obtained were characterized by TEM and UV-visible
Biological Routes to Gold Nanoplates
Jianping Xie
a, Jim Yang Lee*
a,c, Daniel I.C. Wang
a,band Yen Peng Ting
ca
Singapore-MIT Alliance, 4 Engineering Drive 3, National University of Singapore, Singapore 117576.
bDepartment of Chemical Engineering, M.I.T., 77 Massachusetts Ave., Cambridge, MA 02139, USA.
cDepartment of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent
Ridge Crescent, Singapore 119260.
2 spectroscopy. A representative TEM image is shown in Fig.
1. Gold nanoparticles were formed in several different shapes, ranging from polydisperse small spheres to large polygons (triangles and hexagons). The low contrast from the polygons suggests that they were flat, unlike the spherical nanoparticles surrounding them. The percentage of particles with the platelet morphology was about 50%. The lateral size of the triangles was between 50 and 200nm, and the thickness was approximately 10-20nm. The selected-area electron diffraction pattern (Fig.1B) from one of the gold nanotriangles in Fig.1A clearly suggests single crystallinity. The array of spots (Fig.1B) corresponds well with a beam direction perpendicular to the [111] plane. The [111] plane is the large flat surface of the gold crystal. Crystal shapes are determined by the relative growth rates in different crystallographic directions. Metabolic products secreted by the fungus, such as proteins, organic acids and polysaccharides, are expected to interact with the crystal faces differently, thereby changing the surface energies of the latter in due course. Based on the experimental results the [111] surface had interacted stronger with the biomolecules secreted by the fungus, resulting in crystal growth biased by the inhibition of gold atom accumulation on the [111] surface.
A UV-visible spectrum of the gold nanoparticles formed in mycelia-free spent medium is given in Fig.2. The two distinct absorptions centering at 533 and 856 nm are due to the surface plasmon resonance (SPR) of nanospheres and to the in-plane SPR of nanoplates respectively. The broadness of the absorption at ca. 856nm reflects the polydispersivity in the shape and size distributions of the anisotropic Au nanoparticles.
pH was important to the morphology of the nanoparticles formed in the mycelia-free spent medium. At pH 3, ca. 40% (51 out of 125) of the crystals were thin and flat. The percentage reduced to 4% (3 out of 67) at pH 5, and no platelike structures were found at pH 8. Since reduction was much slower under acidic condition (3 hours to complete the reaction at pH 3, 30 minutes at pH 5 and a few seconds at pH 8), the biomolecules in the mycelia-free spent medium had more time to interact with the growing nuclei at low pH, enabling the anisotropic growth into different final shapes.
Fig. 1 A) Representative TEM image of gold nanoparticles formed in mycelia-free spent medium. B) Electron diffraction pattern from a single Au triangular with the electron beam perpendicular to the [111] plane.
500 600 700 800 900 856nm 533nm A b sorba n ce ( a .u .) Wavelength (nm)
Fig. 2 UV-visible spectrum of gold nanoparticles formed in mycelia-free spent medium.
IV. CONCLUSION
In conclusion, a biological protocol for the synthesis of gold nanoplates using mycelia-free spent medium of Aspergillus niger was demonstrated. The metabolic products secreted by A.niger such as proteins, organic acids and polysaccharides, are believed to be able to differentiate different crystal faces and directed the growth into extended flat crystals. The strong absorption at ~856nm due to SPR is close to the femtosecond laser pulse wavelength (720-780 nm) and we expect these nanoplates to have promising non-linear optical properties.
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