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Source: Environmental Nanotechnology Chapter 12 Ecotoxicological Impacts of Nanomaterials Delina Y. Lyon Antoine Thill Jerome Rose Rice University, Houston, Texas Commissariat de l’Energy Atomique, Saclay (Paris), France CNRS-University of Aix-Marseille, Aix-en-Provence, France Rice University, Houston, Texas Pedro J. J. Alvarez Keywords: ecotoxicology, toxicology, antibacterial, reactive oxygen species (ROS), bacteria, developmental toxicity, antimicrobial, uptake, biotransformation. Introdu
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  445 Chapter 12 Ecotoxicological Impactsof Nanomaterials Delina Y.Lyon Rice University, Houston, Texas Antoine Thill Commissariat de l’Energy Atomique, Saclay (Paris), France Jerome Rose CNRS-University of Aix-Marseille, Aix-en-Provence, France Pedro J.J.Alvarez Rice University, Houston, Texas Keywords: ecotoxicology, toxicology, antibacterial, reactive oxygenspecies (ROS), bacteria, developmental toxicity, antimicrobial, uptake,biotransformation. Introduction The widespread production of engineered nanomaterials started in the1980s, and their rapid incorporation into a variety of consumer productsand applications is outpacing the development of appropriate regulationsto mitigate potential risks associated with their release to the environ-ment. Therefore, several research initiatives have been recently startedto improve our understanding of the transport, fate, reactivity, andecotoxicity of several nanomaterials that have a relatively high proba-bility of environmental release.Many of the inorganic nanomaterials, such as TiO 2 , ZnO, and quan-tum dots, are likely to be found in the environment due to their manu-facture or intended application. Nano-sized titanium dioxide (a.k.a.anatase, TiO 2 ), a good opacifier, is used as a pigment in paints, paper,inks, and plastics. In electronics, crystalline SiO 2 works as both a semi-conductor and an electrical insulator. The ceramic nature of ZnO allowsits use as a pigment and a semiconductor. Nano-scale TiO 2 , SiO 2 , and ZnO Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright © 2007 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.Source: Environmental Nanotechnology  offer greater surface area than their bulk counterparts, allowing forimproved performance in established applications. Quantum dots (QDs)are semiconductors that display narrow fluorescence or absorption bandsdue to the quantum constraints imposed on electrons by the finite sizeof the material. Applications of QDs include medical imaging and sen-sors. Some nanomaterials are intentionally dispensed into the envi-ronment, such as zerovalent iron nanoparticles, which have been appliedat more than 20 sites for the in situ remediation of groundwater con-taminated with chlorinated solvents. Commercial applications of suchinorganic nanomaterials currently or will soon include nano-engineeredtitania particles for sunscreens and paints, silica nanoparticles as solidlubricants, and other reagents for groundwater remediation.Organic nanomaterials, such as fullerenes and carbon nanotubes, arealso being produced in increasing amounts. For example, buckminster-fullerene (C 60 ) is being used in applications ranging from cosmetics todrug delivery vectors to semiconductors while carbon nanotubes com-posites are used in tires. Frontier Carbon built a plant to mass produceC 60 on the scale of tons per year [9]. The economy of fullerene produc-tion indicates that fullerene-containing products will soon become widelyavailable. Although C 60 is relatively insoluble in water, it does not pre-cipitate completely when coming into contact with the aquatic envi-ronment. C 60 can form stable nanoscale suspended aggregates (nC 60 ),whose concentration can reach up to 100 mg/L[4, 10]. Fullerols (hydrox-ylated fullerenes) are highly photosensitive and generate ROS thatmay be used for bio-oxidations [11]. Both fullerols and carboxyfullerenescan be used in medical applications as drugs or for diagnostic drugdelivery [12]. These derivatized molecules are more soluble in waterthan their parent fullerene, implying greater potential interaction withorganisms.In the environmental technology industry alone, nanotechnologieshold great promise for reducing waste production, cleaning up industrialcontamination, providing potable water, and improving the efficacy of energy production and use. On the other hand, the environment will beincreasingly prone to suffer pollution from nanomaterials in consumerproducts such as sunscreens, detergents, and cosmetics, as well fromaccidental releases during production, transportation, and disposaloperations. The manufacture, use, and disposal of engineered nanoma-terials are not currently regulated by any government, although the USHouse Scientific Committee has prioritized legislation of nanotechnol-ogy research [13–15]. There has also been movement toward includingenvironmental and health issues in the European Union and Japaneseresearch budgets for nanotechnology. The current European budget forresearch in these areas is approximately $7.5 million, a much smallershare of their total nanotechnology research budget. 446Potential Impacts of Nanomaterials Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright © 2007 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.Ecotoxicological Impacts of Nanomaterials  Many examples in modern history illustrate the unintended conse-quences of initially promising technologies, including the blind releaseof “beneficial” chemicals into the environment, such as asbestos orDDT. These examples forewarn us of potential environmental impactsof some nanomaterials, which deserve more attention and research[16–18]. Furthermore, the large intellectual and financial investmentsin nanotechnology demand that it be publicly accepted and sustainable[19]. The backlash against genetically modified crops resulted in ahuge setback for agriculture. Asimilar backlash against nanotech-nology would result in the delay of beneficial nanomaterials coming tomarket.The matter of determining whether or not a substance is “dangerous”involves not only determining any hazards presented by the materialsuch as toxicity, but also to what degree the material contacts livingorganisms. Currently, the degree to which cellular processes and ecosys-tem health may be impacted by nanomaterials, let alone specific toxic-ity mechanisms, remain largely unknown. This chapter discusses theknown and postulated interactions between nanomaterials and non-mammalian biological indicators, specifically microbes, and how theserelationships foreshadow the potential effects of nanomaterial releasesinto the environment. Why Study the Effects of Nanomaterialson Microorganisms? Microbes are present in almost every environment on earth, and theirflexibility and adaptability allow them to survive under seemingly unliv-able conditions, such as anaerobic, high heat, or extreme cold conditions.Microbes as a whole produce the majority of the biomass in aquatic sys-tems. While plants are the primary biomass in terrestrial systems, theirsurvival depends on the activity of microbes for the breakdown of deadmatter and the recycling of needed nutrients. Microbes play key rolesin the cycling of carbon, nitrogen, phosphorous, and other minerals.Microbial ecotoxicology is therefore a particularly important consider-ation because microorganisms serve as the basis of food webs and theprimary agents for global biogeochemical cycles. Microorganisms arealso important components of soil health and could serve as potentialmediators of transformations that affect nanoparticle mobility and tox-icity in the environment.One benefit of evaluating microbial toxicity is the ability to extrapo-late the observed effects of chemicals on microbes to other higher levelorganisms. Quantitative structure activity relationships (QSARs) areone way to calculate the impact on other organisms based on chemicalstructure [20]. QSARs incorporate mathematical relationships between Ecotoxicological Impacts of Nanomaterials447 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright © 2007 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.Ecotoxicological Impacts of Nanomaterials  the structure of a chemical and its likelihood to increase the toxicity of a compound. Each chemical component is given a numerical value, andthe sum of its components determines its toxicity. One can also use sim-ilar calculations to extrapolate the toxicity of a chemical to an organismbased on its toxicity to an unrelated organism [20]. Methods to Assess Ecotoxicity  As yet, a comprehensive ecotoxicity study of any nanomaterial has notbeen performed. Most studies have analyzed nanomaterial impacts ona type of organism in isolation. The methods for looking at ecosystemsimpact as a whole are established and are reviewed in several texts[20–22], so this review is by no means exhaustive.There are many levels at which one can analyze the impact of a chem-ical, from a single biochemical reaction up to an entire ecosystem withall of its complexity. Most ecotoxicity tests relate to survival, mutation,and reproduction. The majority of the current research on nanomateri-als has examined their impact on biochemical reactions in a cell up tothe survivability of whole multicellular organisms. In order to make com-parisons between chemicals and organisms for risk analysis, severalbenchmark measurements have been established. The most common isthe LC 50 , or lethal concentration of chemical that kills 50 percent of anexposed population as compared to a control. Another common meas-urement is the EC 50 , or effective concentration of a chemical that elic-its some response in 50 percent of the population. The responseexamined can be reproductive capacity, growth, respiration, or anynumber of endpoints. Tests of both the LC 50 and EC 50 must be carefullycontrolled and performed in a standardized manner to ensure compa-rability between experiments and laboratories.There are numerous methods and organisms at various trophic levelsto examine LC 50 or EC 50 . Part of the challenge in ecotoxicology is to findappropriate biomarkers, or physiological processes that respond in a sen-sitive manner to chemical exposure, and bioindicators, or organisms inan ecosystem that reflect the health of that environment. Most of thestudies in nanomaterial toxicology have only looked at biomarkers andnot bioindicators of any specific ecosystem. Many pollutants have a spe-cific biomarker, since the chemicals elicit a specific response from organ-isms [22]. There are also some established organisms that can berelatively easily cultured and used to assess toxicity. Table 12.1 listssome of the commonly used organisms, their common name, andcommon endpoints.Bacteria are among the easiest and least expensive organisms to cul-ture, and they are relatively sensitive to many of the same compounds 448Potential Impacts of Nanomaterials Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright © 2007 The McGraw-Hill Companies. All rights reserved.Any use is subject to the Terms of Use as given at the website.Ecotoxicological Impacts of Nanomaterials
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