In Vivo synthesis of nanomaterials in plants: location of silver nanoparticles and plant metabolism

MeNPs can be formed in living plants by reduction of the metal ions absorbed as soluble salts. It is very likely that plant metabolism has an important role in MeNP biosynthesis. The in vivo formation of AgNPs was observed in Brassica juncea, Festuca rubra and Medicago sativa. Plants were grown in Hoagland solution for 30 days and then exposed for 24 h to a solution of AgNO3 1000 ppm. In leaf extracts of control plants the concentrations of glucose, fructose, ascorbic acid, citric acid and total polyphenols were determined. Total Ag content in plant fractions was determined by inductively coupled plasma atomic emission spectroscopy. Despite the short exposure time, the Ag uptake and translocation to plant leaves was very high, reaching 6156 and 2459 mg kg-1 in B. juncea and F. rubra, respectively. Ultrastructural analysis was performed by transmission electron microscopy and AgNPs were detected by TEM X-ray microanalysis. TEM images of plant fractions showed the in vivo formation of AgNPs in roots, stems and leaves of the plants. In the roots, AgNPs were present in the cortical parenchyma cells, on the cell wall of the xylem vessels and in regions corresponding to the pits. In leaf tissues AgNPs of different size and shape were located close to the cell wall, as well as in the cytoplasm and within chloroplasts. AgNPs were never observed in the phloem of the three plant species. This is the first report of AgNP synthesis in living plants of F. rubra. The content of reducing sugars and antioxidant compounds, proposed as being involved in the biosynthesis of AgNPs, were quite different between the species, thus suggesting that it is unlikely that a single substance is responsible for this process.