Accumulation of Gold Nanoparticles in Brassic Juncea

Abstract

Enzymatic digestion is proposed as a method for concentrating gold nanoparticles produced in plants. The mild conditions of digestion are used in order to avoid an increase in the gold particle size, which would occur with a high-temperature process, so that material suitable for catalysis may be produced. Gold nanoparticles of a 5–50-nm diameter, as revealed by transmission electron microscopy (TEM), at concentrations 760 and 1120 ppm Au, were produced within Brassica juncea grown on soil with 22–48 mg Au kg⁻¹. X-ray absorption near edge spectroscopy (XANES) reveals that the plant contained approximately equal quantities of Au in the metallic (Au⁰) and oxidized (Au⁺¹) states. Enzymatic digestion dissolved 55–60 wt% of the plant matter. Due to the loss of the soluble gold fraction, no significant increase in the total concentration of gold in the samples was observed. However, it is likely that the concentration of the gold nanoparticles increased by a factor of two. To obtain a gold concentration suitable for catalytic reactions, around 95 wt% of the starting dry biomass would need to be solubilized or removed, which has not yet been achieved.

KEY WORDS:

• X-ray absorption near edge spectroscopy (XANES)
• Brassica juncea
• catalysis
• enzyme
• phytomining
• concentration

ACKNOWLEDGMENTS

This work was supported in part by U.S. Airforce grant AOARD054068, by The MacDiarmid Institute of Advanced Materials and Nanotechnology, and by National Institutes of Health grant S06 GM8012-33. Portions of this research were carried out at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy. Dr. Gardea-Torresdey also acknowledges the Dudley family for the Endowed Research Professorship in Chemistry. The authors also would like to thank the referees who provided valuable comments and insights.