Exposure of Tillandsia usneoides to silver nanoparticles

Spanish moss (Tillandsia usneoides) is an epiphyte which absorbs nutrients and water from air and rainfall. Spanish moss has threadlike leaves with a length of 2.5-7.5 cm and a rough surface which makes it suitable for atmospheric monitoring. T. usneoides, after pre-treatment with distilled water, was exposed to Ag-NPs through two spraying cycles (Ag-T1, Ag-T2) of a suspension at 1000 mg L-1 in a special greenhouse. Five replicates of treated and untreated plants were harvested, air dried and finely milled. 250 mg were weighted in PTFE vessels and mineralized in microwave oven (Start D 1200, Milestone) with HNO3 Suprapure (Merck, Germany) and H2O2. The mineralized solution was brought to 20 mL with Milli-Q water. The major and trace elements were determined by ICP-OES (Arcos, Ametek). Some samples of leaves were placed in formalin 10% for analysis with ESEM-EDS. Concentration of Ag in plant tissues depended from the time of exposure. In particular 0.1 mg Ag kg-1 was the background level of untreated Tillandsia, while Ag-T1 was 2.7 mg kg-1 and Ag-T2 was 3.2 mg kg-1. The Ag accumulation in Tillandsia tissues were statistically significant with respect to background level (p<0.05). The leaves of Tillandsia treated with Ag-NP had higher amounts of Ca, Na, P and S than the control; at the same time, Al and Fe concentrations were lower. Electron microscopy investigations on the fate of Ag-NPs in T. usneoides leaves revealed the presence of Ag-NP on the leaves tissue.