Plant growth inhibitory effect of alumina nanoparticles has recently been reported (Ling Y, Watts D, 2004) but the mechanisms of such an effect are yet to be established. The phytotoxicity of aluminum and some of its compounds is well known, but the rapid expansion of nanotechnology resulting in the introduction of new sets of materials in the nanometer range has led to the development of new approaches, experimental methods and modes of investigation.

In this study, the observed phytotoxic effects of alumina nanoparticles suspension on five plant species (Zea mays, Cucumis sativus, Daucus carota, Brassica oleracea, Lactuca sativa) were investigated. Factors that were examined, which are thought to potentially contribute to the observed inhibitory root growth effect included; presence of hydrogen peroxide in Alumina nanoparticles suspension, mechanical contacts between root cell walls and particles, surface characteristics, the presence of residual aluminum in alumina nanoparticles, and movement of very small particles directly through the cell wall. The study of the latter possibility was made possible by the use of ultra-filtration techniques utilizing both 0.025μm and 0.05μm pore size membranes from Millipore®, in addition to the use of both Alumina nanoparticles and fumed Silica nanoparticles. Significant differences exist between the two pore sizes at the highest concentration of 20mg/ml of Alumina nanoparticles permeate for all the plant species used in this investigation, except L.sativa.

To investigate the surface characteristics, nanoparticle supernatants of different concentrations were obtained through centrifugation and used to treat plant species seedlings.

The presence (or absence) of Aluminum in alumina nanoparticles was established through the help of Spectrophotometric technique using Morin as a florescence agent, and the phytotoxicity of dilutions of Aluminum standard solution was compared to that of Alumina supernatants of varying concentrations. A trace of Aluminum was found in the Alumina nanoparticles supernatants from the highest concentration of 20mg/ml of Alumina nanoparticles suspension, with an absorbance of 0.2 AU compared to 4 AU from Aluminum standard solution. There was no statistical difference between the phytotoxicity from the 20mg/ml Alumina nanoparticles supernatants and that from the undiluted, 0.0371 M Aluminum standard solution, with p values for D.carota, L.sativa, B.oleracea and C.sativus being; 0.7, 0.64, 0.05 and 0.32, respectively, while the p value for Z.mays was < 0.0001, as a result of Aluminum resistance from this plant species, suggesting a common source of phytotoxicity.

This investigation answers questions raised by the previous researchers by using the same source of Alumina and Silica nanoparticles, similar experimental methods, data analysis but different approaches.