Particle surface characteristics may play an important role in phytotoxicity of alumina nanoparticles

Abstract

The phytotoxicity of alumina nanoparticles loaded with and without phenanthrene (Phen) was investigated by means of root elongation (RE) experiments in this study. Five plant species, Zea mays (corn), Cucumis sativus (cucumber), Glycine max (soybean), Brassica oleracea (cabbage), and Daucus carota (carrot) were used in our study of phytotoxicity by root elongation experiments. The surface characteristics of Phen-loaded and Phen-nonloaded nanoparticles were investigated using the Fourier transformed infrared (FTIR) spectroscopy technique. It was found that when loaded with 10.0%, 100.0%, or 432.4% monomolecular layer (MML) of Phen, the degree of the root elongation inhibition caused by the particles was reduced. The loading of Phen leads to the appearance of a vibrational mode in the region of 850–1050 cm⁻¹, which was assigned to the surface characteristics of the particles and arises from the disappearance of free hydroxyl groups according to an earlier study. When mixed with a known free hydroxyl radical scavenger, DMSO (0.5% and 1.0%), the non-loaded particles also showed decreased inhibition of root elongations. We supposed that the surface characteristics of the particles play an important role in the phytotoxicity of alumina nanoparticles.

Introduction

The developing nanotechnology has brought enormous amount of manufactured nanoparticles into the work place as well as into the ambient air (Oberdorster, 2003). To protect the individuals in the work place as well as the public in the ambient environment from the possible harmful effect of the industrial particles, it is necessary to determine the toxicity and toxicological mechanisms of the manufactured nanoparticles.

Evidence from epidemiological studies suggest that fine particles (particles with aerodynamic diameter smaller than 2.5 μm, PM_2.5) have an association with various adverse human health effects including premature mortality, exacerbation of asthma and other respiratory-tract diseases, decreased lung function (Dockery and Pope, 1994, Schwartz et al., 1996, Pope, 1999, Pope, 2000), and cardiovascular diseases (Dockery, 2001, Donaldson et al., 2001). Studies are currently ongoing to investigate the human health related issues of particles with nanoscale sizes (Spurny, 1998), including the tracheal uptake of 21-nm titania (Churg et al., 1998), the impairment of alveolar macrophage phagocytosis by 29-nm titania and 14.3-nm carbon black (Renwick et al., 2001), and the endocytosis of 50-nm titania particles by human airway cells (Stearns et al., 2001).

Nanometer-sized particles have special toxicity and are usually more toxic than the same material of larger size (Donaldson et al., 1999). When inhaled as single particles, particles with diameter smaller than 50 nm can be highly toxic (Oberdorster, 1996). These particles can be internalized by type II lung epithelium cells (Stearns et al., 2001) because of their nanometer sizes. They have higher deposition efficiency in the lower respiratory tract and slower clearance rates (Spurny, 1998). Intrinsically toxic nanoparticles may damage macrophages that are essential to clear inhaled particles (Donaldson et al., 1998, Renwick et al., 2001), thus, leads to failed clearance of these particles. The failure clearance of these particles may also come from the overload of the particle clearance system, which is caused by the large numbers per unit mass of nanoparticles (Donaldson et al., 1998). The characteristic large specific surface area (surface area per unit mass) of nanoparticles would allow increased interaction between particles and epithelium cells (Donaldson et al., 1998), and results in greater toxicity of the particles (Oberdorster et al., 1992, Oberdorster et al., 2000).

Based on the evidence from the studies published so far, it is typically proposed that the toxicity of particles increases with the decrease of the particle size, because smaller particles have smaller sizes, larger particle numbers and larger surface area per unit mass. However, Oberdorster (Oberdorster et al., 1994) found in their studies that 800-nm SiO₂ particles are as toxic as 20-nm TiO₂ particles. The more intensive and extensive toxicity of nanoparticles, thus, cannot only be explained by their larger specific surface areas, larger number/mass ratios, and smaller sizes.

The surface characteristics of nanoparticles are much different than the large particles (Roco, 1999). Few works up to date have been contributed to the study on the roles of the surface characteristics of nanoparticles taken in their toxicity. To study the surface characteristics of nanoparticles, vibrational spectroscopy is a powerful technique, which has often been used in catalysis studies to determine the surface nature of catalyst carriers using suitable probe molecules (Lavalley and Benaissa, 1985).

In this work, we studied the phytotoxicity of alumina nanoparticles, which is currently one of the two US market leaders for nano-sized materials according to a recent report (Abraham, 2000). The phytotoxicity of pure particles was investigated by means of root elongation (RE) tests. Phenanthrene (Phen), a major constituent of polycyclic aromatic hydrocarbons (PAHs) associated with the atmospheric particulate matters, was loaded onto the pure particles, and the phytotoxicity of Phen-loaded particles were investigated by root elongation tests. The surface characteristics of particles were determined by a Fourier transformed infrared (FTIR) spectroscopy.