Review
Nanomaterials and the environment: A review for the biennium 2008–2010

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Abstract

Applications of nanotechnology are touching almost every aspect of modern life. The increased use of engineered nanomaterials (ENMs) in consumer products, chemical and medical equipment, information technology, and energy, among others, has increased the number of publications (informative and scientific) on ENMs. By the 1950s, very few papers were committed to nanomaterials (NMs), but in 2009, more than 80,000 journal articles included the concept nanotechnology. The objective of this review is to compile and analyze publications on NMs in the biennium 2008–2010. This review includes the most recent publications in risk assessment/toxicity, characterization and stability, toxicity, fate and transport of NMs in terrestrial ecosystems, and new ENMs. Carbon nanotubes, metallic, metal oxides and hydroxides nanoparticles, quantum dots, and polystyrene NPs are included.

Introduction

As time passes, the debate concerning the nomenclature linked to nanoscience and nanotechnology is becoming less and less voiced. Conversely, day after day it is becoming popular that the term nanomaterials (NMs) includes natural or manmade particles with at least one dimension of 100 nm or less, while nanoparticles (NPs) include those with at least two dimensions between 1 and 100 nm [1].

Nanotechnology has advanced in all fronts: instruments, goods, literature, and profits. According to the Project on Engineering Nanotechnologies [2], since March 2006–August 2009, 212 new nanotechnology-based products (increase of 379%) have been added to the consumer products list, totaling 1025 items produced by 485 companies situated in 24 countries. Pertaining to the literature, an analysis of publications available in electronic data bases indicates that the number of articles on NMs has increased from a very few papers at the beginning of the NM studies in the 1950s, to more than 80,000 journal articles in 2009 that include the concept nanotechnology. Likewise, revenues for nanotechnology and nanomaterials in consumer products were approximately US$1545 million in 2009 [3]. This is expected to increase to $5335million by 2015, driven by the demand for consumer electronics and household cleaning products segments [3].

A search using as engines Sciencedirect.com, Google, and Scifinder has shown that in the biennium 2008–2010, 15 outstanding articles reviewing risk assessment, safety surveys, and NMs toxicity were published (Table 1). As seen in this table, most of the articles refer to uncertainties on NMs toxicity and the needs for more information on NMs handling to prevent environmental and human health effects after long exposure. Pertaining to risk assessment, a study including 40 industries from Switzerland and Germany showed that 26 companies (65.0%) did not perform any risk assessments and 13 (32.5%) performed risk assessments ‘sometimes’ or ‘always’ [4]. In addition, Conti et al. [5] surveyed the safety practices at 82 organizations that either manufacture, handle, research, or use NMs in North America, Asia, Europe, and Australia. Results showed that almost 75% of the organizations did not have a nanospecific health and safety program but most of them (89%) implemented a general environmental health and safety program. Also, a high percentage of the organizations with more than 1000 employees did not provide safe use guidance for the nanoproducts [5]. A feature article by Hoyt and Manson [6] has described the advantages and risk associated to nanotechnology, pinpointing people at risk due to their connection to NMs. At first level are researchers and workers – and their family members – in NM-connected industries, followed by consumers and the general public. Hoyt and Manson [6] have stated that despite the rapid pace of nanotechnology related research, development, and commercial exploitation, by the end of 2006, there were no standards or regulations to govern these activities. This gives an idea of the risk perception at the NM-related organizations and the risks the environment would be exposed to in case of an accidental or intentional NPs spill. Another feature article [7] describes the best practices used at universities and other enterprises to prevent exposure to NMs. The authors remark that academic researchers manage NMs as dangerous substances. For personal protection, they work in the fume hood not in lab benches, transport NMs in sealed containers, and use personal protection equipment (gloves, lab coats, goggles). To prevent lab contamination, wipe hood and other possible contact sources and have spill kit on hand to prevent exposure on spill. They manage NMs and NM-contaminated lab materials as hazardous waste until specific regulations appear. In this way, academic institutions contribute to a better handling of nanomaterials.

Governmental agencies in the United States are taking actions to decrease uncertainties in assessing the exposure and risks derived from nanotechnology. The National Science Foundation jointly with the Environmental Protection Agency created two centers to determine the fate and interactions of NMs in complex environments. These two centers, the Center for the Environmental Implications of Nanotechnology (CEIN) at the University of California – Los Angeles (The University of Texas at El Paso is part of the CEIN) and the Center for the Environmental Implications of NanoTechnology (CEINT) at Duke University were established to generate basic knowledge on NPs toxicity at organismal and community levels and to generate models to predict the NP effects in ecosystems [8]. CEINT is specifically focused on the determination of properties and conditions influencing the distribution of nanoparticles in the environment, the differences between natural and fabricated NMs, possible “nano” effects beyond the surface area effect, and the way we can do risk assessments on an emerging technology.

According to the Nanotechnology Law Report [9], the United States Government Accountability Office (GAO-10-549, June 27, 2010) has stated: “Nanomaterials are widely used in commerce, but EPA faces challenges in regulating risk.” The Law Report [9] highlighted that one of the handicaps is that EPA lacks the technology to monitor and characterize these materials.

A worthwhile reading review on the chemical stability of metallic nanoparticles appeared in 2009 [10]. In that review, Auffan et al. [10] have pointed out that the fate of NMs in the environment and within organisms may be dissolution dependent. The authors gathered information about the comparative toxicity of ZnO and TiO2 NPs to in vitro-cultured mammalian cells. Higher toxicity was observed in cells exposed to ZnO NPs, which exhibits higher dissolution patterns compared to TiO2 NPs. Another example is the lower toxicity of coated compared to uncoated quantum dots (QD) CdSe. The toxicity was related to the Cd ions released to the medium by the uncoated QD. Apparently, the toxicity exhibited on cell culture can be extrapolated to organisms. However, there is no data so far to corroborate this thought. In addition, it seems that the phase transformations of NPs are size related. For instance, TiO2 NPs shows three crystalline structures, anatase, rutile, and brookite, which are of different size. These size-dependent phase transformations are involved in the toxicity of TiO2 towards single cell organisms [10]. As pointed out by Lubick [11] such variability “can make reference materials particularly hard to develop for agencies like the U.S. National Institute of Standards and Technology. That variability makes comparisons between studies difficult and lab protocols tricky to universalize.” The Gardea–Torresdey research group at the University of Texas at El Paso has reported that CeO2 NPs are comparatively more toxic to soybean plants (Glycine max L.) than ZnO NPs [12]. This complicates the panorama about toxicity because the nanoceria are less soluble than ZnO NPs. There is a lack of information for several aspects of NMs such as use and handling of the nanowaste [13], life cycle of most of the NM in use [14], toxicity of NMs to human tissues [15], or how to do risk assessment [16]. Other aspects about NMs in need of study are: determination of bioavailability in different environments [17], levels of toxicity to tissues and individuals [18], and the scarcity of test materials for ecotoxicology studies [19].

Pertaining to nanomaterials synthesis and applications, 22 review papers were published in the biennium of 2008–2010 (Table 2). As seen in this table two detailed reviews described the application of QD in nanoelectronics [20], [21]. The synthesis of NMs describing formation methods of oxide NPs, inorganic–organic NMs, carbon nanotubes, and chalcogenide NMs was also reviewed [22], [23], [24], [25], [26]. Catalysis was also extensively reviewed. The use of carbon nanotubes (CNTs) – inorganic materials [27], catalyst in fuel cell reaction [28], catalytic power of metal NPs [29] and silica-coated NMs [30] were worthwhile reviews to read. The use of NMs to determine soil properties [31] and detect persistent organic pollutants [32] was also summarized. Another area extensively reviewed in 2008–2010 was nanobioscience. Reviews on functionalization of plant viruses to form electroactive NPs [33], use of NMs in controlling the biochemical microenvironment of cells [34], in vivo nanocarriers for RNAi delivery [35], and starch NP formation [36] appeared in the biennium 2008–2010. Also, subjects like nano-optic [37], the fabrication and application of boron nitrite nanotubes [38], electron-beam induced deposition [39], the use of NMs in lithographic techniques [40], and the use of NPs in medical applications [41] were also reviewed. Twenty-two reviews in 2 years is a clear indication of the number of publications and the activity in the field of nanotechnology.

Section snippets

Engineered nanomaterials composition

Engineered nanomaterials (ENMs) encompass those NPs synthesized and modified in order to enhance their performance in several technological and industrialized processes [42]. A number of ENMs are currently manufactured from different sources depending on their potential applications. Nanomaterial composition differs according to their formulation. Fullerenes and CNTs are NMs classified as carbon-based materials, QD as semiconductors, and metal oxides are considered as inorganic NPs [43]. ENMs

Toxicity of nanomaterials

What makes NPs different from their bulk counterparts is mainly related to their high surface/volume ratio. It has been estimated that in a 20 nm NP only 20% of the atoms would be located on the outside, compared to 40% in a 10 nm NP [66]. As a consequence, physicochemical, optical, reactive, and electrical properties change. Besides, the milieu where NMs are present determines their behavior, reactivity, and their potential toxicity.

Essays to determine NPs toxicity involve their dispersion in

Fate and transport of NPs in terrestrial ecosystems

Due to the rapid development of nanotechnology, there has been a significant increase in the amount of various engineered ENMs annually released into the environment. It is estimated that a significant fraction of these ENMs will enter terrestrial ecosystems through both direct and indirect modes: through zero-valent metal for remediation of contaminated soil [95], [96], [97], through photocatalyst for water treatment [98], [99], or via human activities such as biomass burning, fossil fuel

New nanoproducts

Several new ENMs were produced in the biennium 2008–2010. Table 4 includes those NMs with specific properties or applications. NMs reported in the literature but with no definite applications were not included in this review. From the listed NMs, cobalt ferrites [125], organically modified iron oxide NPs [126], colloidal-supported metal NPs [127], cryptomelane-type manganese dioxide NMs [128], TiO2 nanorods [129], and magnetically recyclable Au@Co core–shell NPs [130] show catalytic activity.

Acknowledgements

This material is based upon work supported by the National Science Foundation and the Environmental Protection Agency under Cooperative Agreement Number DBI-0830117. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation or the Environmental Protection Agency. This work has not been subjected to EPA review and no official endorsement should be inferred. The

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