Ultrasonic Dispersion of Nanoparticles for Environmental, Health and Safety Assessment – Issues and Recommendations
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Nanotoxicology, 2010; Early Online, 1–19 Ultrasonic dispersion of nanoparticles for environmental, health and safety assessment – issues and recommendations JULIAN S. TAUROZZI1, VINCENT A. HACKLEY1, & MARK R. WIESNER2 1Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland, and 2Department of Civil and Environmental Engineering, Duke University, Durham, North Carolina, USA (Received 9 July 2010; accepted 28 September 2010) Abstract Studies designed to investigate the environmental or biological interactions of nanoscale materials frequently rely on the use of ultrasound (sonication) to prepare test suspensions. However, the inconsistent application of ultrasonic treatment across laboratories, and the lack of process standardization can lead to significant variability in suspension characteristics. At present, there is widespread recognition that sonication must be applied judiciously and reported in a consistent manner that is quantifiable and reproducible; current reporting practices generally lack these attributes. The objectives of the present work were to: (i) Survey potential sonication effects that can alter the physicochemical or biological properties of dispersed nanomaterials (within the context of toxicity testing) and discuss methods to mitigate these effects, (ii) propose a method for standardizing the measurement of sonication power, and (iii) offer a set of reporting guidelines to facilitate the reproducibility of studies involving engineered nanoparticle suspensions obtained via sonication. Keywords: Environmental assessment, toxicology, nanoparticle, nanomaterial, ultrasonics, sonication, dispersion, suspension Introduction in biological and environmental systems (Maynard 2006; Barnard 2006). Nanotechnology, and more specifically the emer- Moreover, the assessment of potential risks associ- For personal use only. gence of engineered nanoscale particles (ENPs), ated with ENPs requires the evaluation of their behav- shows promising potential for the development of ior in environmentally and biologically relevant advanced materials and devices in the fields of med- matrices (e.g., cell media, serum, whole blood, stan- icine, biotechnology, energy, and environmental dardized seawater). When ENPs are not already in a remediation, among others (Zhang 2003; Salata fully dispersed form, the test suspensions are typically 2004; Raimondi et al. 2005). Similarly, ENPs are obtained by dispersing dry powders or stock suspen- migrating into a wide range of consumer products, sions of the source material into the desired test many of which are now in the global market place medium prior to in vitro or in vivo evaluation (Project on Emerging Nanotechnologies 2010). How- (Murdock et al. 2008; Jiang et al. 2009; Labille ever, some of the properties that make ENPs attractive et al. 2009). In these situations, the use of ultrasonic may also pose environmental and health hazards energy, a process commonly referred to as ‘soni- (Colvin 2003; Gwinn and Vallyathan 2006; cation’, is arguably the most widely used procedure. Kreyling et al. 2006; Dobrovolskaia and McNeil Sonication refers to the application of sound energy 2007; Kahru and Dubourguier 2010). As both public at frequencies largely inaudible to the human ear Nanotoxicology Downloaded from informahealthcare.com by NIST National Institiutes of Standards & Technology on 12/03/10 and private funding for application-oriented nanoma- (higher than » 20 kHz), in order to facilitate the terial research continues to grow and nanoparticle- disruption of particle agglomerates through a process based consumer products reach the market, there is known as cavitation. Ultrasound disruption is more an increasing demand for a more comprehensive energy efficient and can achieve a higher degree of scientific understanding of the interactions of ENPs powder fragmentation, at constant specific energy, Correspondence: Dr. Vincent A. Hackley, National Institute of Standards and Technology, 100 Bureau Dr. Mail Stop 8520, Gaithersburg, 20899, MD, USA. E-mail: [email protected] ISSN 1743-5390 print/ISSN 1743-5404 online Ó 2010 Informa UK, Ltd. DOI: 10.3109/17435390.2010.528846 2 J. S. Taurozzi et al. than other conventional dispersion techniques (Park dispersion, but their consideration is beyond the et al. 1993; Hielscher 2005; Mandzy et al. 2005). scope of the present work. Sonication is a convenient, relatively inexpensive tool that is simple to operate and maintain. Consequently, Terminology sonication has found extensive application in toxico- logical and environmental studies, where it is used to We define at the outset several critical terms, which break down powders or re-disperse stock suspensions are used throughout the text, in order to avoid ambi- to their nanoscale constituents and allow for the guity, as these terms can have different meanings in evaluation of the properties and behavior of the dis- different scientific and technical circles. For the most persed nanoparticles in relevant systems. part, we follow definitions relevant to nanotechnology However, the inconsistent application of ultrasonic as set forth in the standard E2456 (2006) by ASTM treatment across laboratories and between individual International (formally the American Society for operators, combined with the lack of accepted process Testing and Materials). Additional guidance is standardization, has likely contributed to variability in derived from recommendations of the International observed results. Furthermore, the complex physical Union of Pure and Applied Chemistry (IUPAC and chemical phenomena that occur during sonica- 2009). tion – and which may significantly alter the dispersed For the purposes of this publication, a nanoparticle material’s properties – are frequently underappreci- is defined as a sub-classification of ultrafine particle ated, perhaps resulting in the use of suspensions that that is characterized by dimensions in the nanoscale may not meet the intended experimental design, or (i.e., between 1 and 100 nm) in at least two dimen- potentially leading to artifacts that could compromise sions, with the recognition that the more rigorous conclusions as to how the observed ENP’s ecological definition of nanoparticle also requires that these or toxicological behavior relates to the material’s particles exhibit novel size-dependent properties. inherent properties. It is the authors’ contention A primary particle is the smallest discrete identifiable that the inconsistent application and reporting of entity associated with a particle system; in this con- sonication is a potential leading contributor to vari- text, larger particle structures (e.g., aggregates and ability in test results in which sonication is utilized; agglomerates) may comprise primary particles. An this conclusion has been echoed by the results of aggregate is a discrete assemblage of primary particles recent interlaboratory studies (Roebben et al. 2010). strongly bonded together (i.e., fused, sintered, or Moreover, while not the main focus of this work, it metallically bonded), which are not easily broken is important to note that many of the sonication- apart. While the distinction is often blurred, aggre- induced physicochemical effects and recommenda- gates might also be differentiated from agglomerates, For personal use only. tions addressed herein are equally relevant to other which are assemblages of particles (including primary ENP research areas where sonication is utilized, in particles and/or smaller aggregates) held together by particular those involving the fate and transport of relatively weak forces (e.g., Van der Waals, capillary, nanomaterials in environmental systems. or electrostatic), that may break apart into smaller The objectives of the present work were to: (i) particles upon processing (e.g., using sonication or Highlight salient aspects of the ultrasonic disruption high intensity mixing). Commonly, agglomerates process and survey potential sonication-induced exhibit the same overall specific surface area (SSA) effects that may alter the physical, chemical or bio- of the constituent particles, while aggregates have a logical properties and behavior of aqueous nanopar- SSA less than that of the constituent primary particles. ticle suspensions (within the context of toxicological In a related fashion, agglomeration and aggregation testing), (ii) propose a method for standardizing the refer, respectively, to the process by which agglom- measurement of sonication power, and (iii) offer a set erates or aggregates form and grow. It should be noted of reporting guidelines to facilitate the reproducibility that, although here we define them as distinct entities, of studies involving engineered nanoparticle suspen- the terms aggregate and agglomerate are often used Nanotoxicology Downloaded from informahealthcare.com by NIST National Institiutes of Standards & Technology on 12/03/10 sions obtained via sonication. interchangeably to denote particle assemblies. Our The authors aimed not only to indentify and discuss usage is consistent with previous recommendations key sonication issues relevant to ENP dispersion, but published in this journal (Oberdörster et al. 2007), also to address them by offering practical recommen- but is by no means universally adopted. dations. For this reason, some discussions will follow Additionally, in the present work, a suspension is a traditional literature review format, while others are defined as a liquid (typically aqueous) in which par- more