نانو لوله های کربنی به عنوان جاذب در تحلیل آفت کش ها Carbon nanotubes as sorbents in the analysis of pesticides

۱٫ Introduction The history of carbon nanostructures began in 1985, when the Buckminsterfullerene C60 was discovered by Kroto et al. (1985). Since that time, the number of discovered structures has rapidly increased. The examples of them include nanotubes discovered by Ijima (1991), the family of fullerenes (Kuzuo et al., 1994; Dorset and Fryer, 2001), carbon nanocones (Ge and Sattler, 1994), carbon nanohorns (Nisha et al., 2000) and other different allotropic carbon nanoparticles. Some examples of carbon nanostructures are presented in Fig. 1. The extreme properties of these materials, such as high surface areas, large aspect ratios, remarkably high mechanical strength as well as electrical and thermal conductivities have spurred a broad range of applications. However, carbon nanotubes (CNTs) are presently the hottest carbon nanostructured material. They can be described as a graphite sheet rolled up into a nanoscale-tube. Two structural forms of CNTs exist: single-walled (SWCNTs) and multi-walled (MWCNTs) nanotubes (Fig. 1). CNT lengths can be as short as a few hundred nanometers or as long as several microns. SWCNT have diameters between 1 and 10 nm and are normally capped at the ends. In contrast, MWCNT diameters are much larger (ranging from 5 nm to a few hundred nanometers) because their structure consists of many concentric cylinders held together by van der Waals forces (Wepasnick et al., 2010). At present, the three main methods for CNT synthesis are arcdischarge, laser ablation and chemical vapor deposition (Huczko, 2002; Kingston and Simard, 2003). The last method seems to be the most promising for possible scale-up due to the relatively low growth temperature, high yields and high purities that can be achieved. It should be mentioned, however, that low synthesis temperature often results in high defect density of the obtained CNTs. Because as-prepared CNTs usually contain carbonaceous or metallic impurities, purification is an essential issue to be addressed. Considerable progress in the purification of CNTs has been made and a number of purification methods including chemical oxidation, physical separation, and combinations of chemical and physical techniques have been developed for obtaining CNTs with desired purity (Hou et al., 2008). Extensive reviews covering chemical and structural characterization of various carbon nanostructures have been published recently (Baer et al., 2010; Wepasnick et al., 2010; Zhang and Yan, 2010). The exceptional properties that these materials possess open new fields in science and engineering. In the field of environmental monitoring, the properties of carbon nanostructures offer opportunities for a wide range of applications for detection and remediation of different contaminants and wastewater treatment. Pesticides continue to be studied more than any other environmental contaminant, because they are widely used to protect plants from disease, weeds and insect damage. They could undergo a variety of transformations that provide a complex pattern of metabolites. Because of their toxicity and environmental fate the European Union has included pesticides in their list of priority pollutants and has established the maximum levels for pesticide residues according to the regulation (EC) No. 396/2005 amending Council Directive 91/414/EEC. The Framework Directive 2009/ 128/EC, which became law in November 2009, aims to reduce the risks and impacts on human health and the environment related to the use of pesticides. Thus, the development of analytical methods for their determination in various of environmental media at the required maximum residue limits demands state-of-art techniques for sample preparation, analyte separation, detection and quantification (Lagana et al., 2002; Andreu and Picó, ۲۰۰۴). Taking into account the widespread interest in carbon nanostructures, it is not surprising that they are also found some application in analysis of pesticides. This article illustrates a growing number of application of CNTs in separation techniques; for preconcentration and enrichment using solid-phase extraction and microextraction. This survey will attempt to cover the state of-the art from 2006 to 2010. The general application of carbon nanotubes in analytical sciences has been discussed in the earlier reviews (Merkoçi, 2006; Valcárcel et al., 2007, 2008; Pyrzynska, 2008; Ravelo-Perez et al., 2010).