The Road to Sustainable Nanotechnology: Challenges, Progress

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The road to sustainable nanotechnology: Challenges, progress and opportunities James E. Hutchison ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02121 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 18, 2016

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ACS Sustainable Chemistry & Engineering

The road to sustainable nanotechnology: Challenges, progress and opportunities James E. Hutchison* Department of Chemistry and Biochemistry and Materials Science Institute, 1253 University of Oregon, Eugene, OR 97403-1253, United States e-mail: [email protected] Abstract The rapid development of new nanomaterials has created new opportunities for high performance applications and new product innovation. However, there are concerns that the new properties found at the nanoscale, or the processes of producing nanomaterials, may result in new, undesirable impacts on health and the environment. At the same time, new capabilities made possible through the use of nanomaterials may help us protect the environment and meet some of society’s needs in a more sustainable fashion. The application of green chemistry to the emerging field of nanoscience and nanotechnology, greener nanoscience, was developed in an attempt to maximize the benefits of nanotechnology to society and the environment while minimizing undesirable impacts on health and the planet. This Feature first outlines the need and opportunity for greener nanotechnology. Next, it describes four attributes of nanomaterials that differ from molecular species that present challenges to achieving the aims of greener nanotechnology and provides examples of the ways in which these challenges are being overcome. Recent approaches to (i) defining nanomaterial compositions and structures, (ii) developing reliable and scalable synthesis methods, (iii) developing guidelines for design of safer nanoparticles and (iv) maximizing the net benefit of the use of nanomaterials in products are described. Finally, some of the ongoing and future challenges are outlined.

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Keywords: Green nanoscience, Green Chemistry, Safer nanomaterials, Life cycle impacts, Net environmental benefit, Nanoparticles

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Introduction Nanotechnology research and development are producing new materials with unprecedented properties to fuel new or enhanced applications. A unique aspect of nanotechnology is that the possible health and environmental impacts were considered early in the rise of this new technological area.1 To address these concerns, my colleagues and I launched efforts in greener nanoscience/nanotechnology2-5 where the aims were to design of safer nanomaterials, develop efficient and safe syntheses of nanomaterials and derive the greatest benefit for nanomaterialbased technologies for society and the environment. The subsequent development of this field has been reviewed.6-13 In some ways nanomaterials were similar to molecular species, but in other ways they were not. In the cases where there are similarities, it is possible to leverage our knowledge of molecular (and elemental) properties to advance nanotechnology. When nanomaterials present novel properties, research is needed to understand and later take advantage of these properties. In this Feature, I will examine the specific challenges that nanotechnology presents relative to molecular species and describe some of the ways that the strategies of green chemistry have successfully been applied to this new field. The promise of nanotechnology, that has driven substantial worldwide investments in nanotechnology research and development, is that numerous societal benefits that might be accrued through new properties and materials from nanoscience.14 New material properties emerge at the nanoscale that were previously unknown in molecular chemistry or in microscale materials. For example, the fluorescence emission of semiconductor quantum dots depends strongly on particle size. New optical, electronic, chemical, mechanical and biological properties of nanoscale materials have the potential to meet many of society’s growing needs. As a result, innovative nanotechnologies have been proposed or realized across nearly every technology sector, including solar energy conversion, electronics, medical devices, medical therapeutics and

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diagnostics, water purification, food additives and packaging, cosmetics and sunscreen, catalysts, antimicrobial coatings, etc. By harnessing the new properties of nanomaterials, it may be possible to significantly enhance the performance of materials and devices, achieving the desired level of product performance while using less material.3 In addition to these societal benefits, nanotechnology has the potential to directly benefit the environment because of the unique properties of nanomaterials. Nanomaterials can be used to directly address environmental challenges. For example, one can harness the optical properties of nanoparticles to convert sunlight to electricity or fuels or to produce energy efficient displays with unprecedented color quality, use nanomaterials to remediate contaminated soil and to purify water or air, or employ nanoscale catalysts to reduce the impacts of chemical manufacturing. There can be indirect environmental benefits as well. In these cases, the incorporation of a nanomaterial into a product reduces impacts on the environment across the product’s lifecycle, often during the use phase.15 Examples include: nanocomposites as stronger, lighter materials to decrease the weight and fuel consumption in vehicles; nano-enabled, self-cleaning windows, materials and textiles that require less frequent cleaning and therefore reduce the amounts of water, energy and cleaning agents used; and nanomaterials that can be used to control the release of agrichemicals to reduce the amount of fertilizer that ends up in runoff and leads to eutrophication of waterways. Nanotechnology is an emerging technology. Although there is significant potential for benefit, many questions about the potential impacts remain unanswered: What about the potential impacts on health and the environment? Will nanomaterials present hazards that make their use unfeasible? On the other hand, will the impacts of the use or production of

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nanomaterials offset the lifecycle benefits that are intended? In the event that there are undesirable impacts due to the materials or their production, can materials and processes be designed that minimize these? The latter consideration is the aim of greener or sustainable nanotechnology: To employ nanotechnology to meet societal needs while reducing the direct and indirect impacts of this emerging technology on the environment and human health.3

Potential concerns with nano As with any emerging technology, the impacts of nanomaterials and their production were unknown as the field emerged, and many remain poorly understood. The promise of nanotechnology rests on the emergence of new properties at the nanoscale. Our collective experience with chemicals and ultrafine particles suggested that some of these properties are likely to be beneficial while others may have undesirable impacts. Based upon this experience, the unknown impacts would likely include impacts on health and the environment and from nanomaterial production. An important area of concern involved potential health effects. Given the complex structures and compositions of nanomaterials it was anticipated that nanomaterials might have effects that were unique to their nanoscale size or structure. For example, concerns were raised that rigid, high aspect ratio materials (such as carbon nanotubes), might have health effects similar to those caused by high aspect ratio asbestos particles.16 There were concerns that nanomaterials containing toxic elements might readily release those as soluble ions.17 Catalytically active nanoparticle surfaces could produce reactive oxygen species that can damage cells.18 Complex, three-dimensional nanomaterials, containing multiple copies of targeting

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ligands, might be transported into different biological compartments19 (for example across the blood-brain barrier). Similar questions have been posed regarding the potential environmental impacts of nanomaterials. Could they be aquatic toxins due to their nanostructure or release of toxic ions? Would they persist in the environment due to a lack of biodegradation pathways? Might they bioaccumulate due to their unusual size and surface chemistry? In addition, it seemed likely that there would be environmental impacts of nanomaterial production. New waste streams might present environmental hazards and the production of nanomaterials would consume resources, water and energy.4 Despite a great deal of progress, many of these questions surrounding the impacts of materials and their manufacture remain unanswered. One significant challenge is the seemingly infinite range of nanomaterial compositions and structures.20 It isn’t feasible to study each variant, thus several strategies have emerged to accelerate progress. Tiered assessments have been implemented that involve high-throughput screening to identify potential materials of concern, followed by more detailed testing. Efforts have been made to understand what levels of exposure are likely for different nanomaterials so that testing can be done at those levels, as opposed to much higher, unrealistic levels. Finally, greater emphasis has been placed on nanomaterial characterization21 so that the results of different studies of nanomaterial impacts can be more readily compared.

Opportunities for greening nanotechnology In parallel to the study of effects of nanomaterials, research has also been conducted to determine how composition and structure dictate nanomaterial impacts, and how molecular level design of

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nanomaterials can minimize these impacts.22 Green chemistry principles and strategies have been developed to reduce the impacts and hazards of chemicals.23 In many cases, we can leverage what we know about chemicals and green chemistry in approaching nanomaterials. In other cases, there may be nano-specific hazards and/or process impacts that are different from those found for molecules or bulk elements. Thus, a strategy that is emerging is to identify how, if at all, nanomaterials are different from molecular or elemental species. With this understanding, we can know when to apply our chemistry knowledge to these materials and when we need to consider new approaches to design safer materials. To proactively guide the development of nanotechnology in a way that provides the potential benefits for society while avoiding harm to health and environment, we and others applied the principles of green chemistry to nanoscience.4, 9, 22 The aim of greener nanoscience (or later sustainable nanotechnology) was to design products and processes at the molecular level to enhance product performance and minimize impacts on health and the environment.5 The aims of greener/sustainable nanotechnology center around reducing hazard of the products whenever possible, designing materials to inherent reduce exposures and minimizing the impacts of nanomaterial production. The application of green chemistry to nanoscience applies across the product lifecycle – from extraction to end-of-life (Figure 1).4, 22 This extension of the green chemistry principles emphasizes the role of design and combines some of the principles that are often implemented together toward the same design goal. Designing greener nanoscale products incorporates a number of the original principles to (i) reduce hazard while maintaining function, (ii) design nanomaterials for end-of-life (reuse, repurpose or degradation), and (iii) incorporate inherent safety. Although green chemistry is typically framed as an approach to reduce the inherent

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hazards of chemicals or materials, inherent safety can also be achieved by designing at the molecular level to prevent exposure.

Origins Of Materials

Manufacturing

Distribution

Use

End of Life

Design of safer nanomaterials Design for reduced environmental impact

Design for reduced environmental impact

Design for waste reduction Design for process safety Design for materials efficiency Design for energy efficiency

Figure 1: Mapping of the green nanoscience/nanotechnology design principles4 across the stages of the lifecycle of products. Within the category of designing for reduced environmental impacts at the end of life, strategies include design for reuse, recycle or remanufacturing. Collectively the design principles address the challenge of reducing hazards, enhancing material and process efficiency and maximizing the net environmental benefit of nano-based products.

Designing greener syntheses of nanomaterials can be guided by those developed for molecular species because many nanomaterials are synthesized using wet chemical methods. Thus, key strategies include: (i) use of safer solvents and reagents, (ii) reduction or elimination of solvents, (iii) reduction of waste through improved atom economy and yields, and (iv) elimination of the need for purifications.23 In addition to the direct impacts of production, indirect impacts need also be considered, including energy consumption, water use and the use of feedstocks/resources.23

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The ultimate aim of greener nanotechnology is to achieve a net benefit of the technology to society, by enhancing the benefits to society while minimizing the undesirable impacts to health and environment. Designing new nanomaterials to meet these requirements is a challenge that often results in tradeoffs, for example increased energy consumption vs. a more biodegradable product. Such comparisons, and subsequent decision making regarding material selection or design, need to be conducted within the context of the lifecycle of the product.24

What features of nanomaterials present unique challenges compared to molecular species and how can these be addressed by green chemistry? Although there is still a lot to learn about the impacts of molecular species on health and the environment, we still have a large knowledge base to draw from as we seek to design greener chemicals and greener syntheses. We know much less about nanomaterials.20 When the properties and impacts of nanomaterials are analogous to small molecules, we can leverage this knowledge to assess potential impacts and design greener solutions. When such analogies are absent, additional research is needed to understand the ways in which nanomaterials pose potential hazards or impacts. We can accelerate progress toward greener nanotechnology by defining and focusing on the aspects of nanomaterials that are unique to them, and leveraging our knowledge of molecules whenever possible. In this section, the focus will be on four features of nanomaterials that present unique challenges: (i) material definition, (ii) synthesis, mechanisms and scalable production, (iii) material complexity/structure activity relationships, and (iv) life cycle impacts/benefits. Select examples will be used to illustrate some of the strategies used to tackle those challenges and the considerable progress that has been made.

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Material definition. The lack of material definition is a significant difference between molecules and nanomaterials (see Figure 2). One can readily obtain most molecular species in high purity and a well-defined (molecular) formula. Each molecule in the sample is the same and the composition and structure are precisely known. For nanomaterials, on the other hand, there are no naming conventions and materials are often referred to as the core material, e.g. a gold nanoparticle or a carbon nanotube.22 Such descriptors ignore important structural features of the material that will strongly influence the reactivity, properties and impacts of the material, including size, shape and surface chemistry. Furthermore, nanomaterials are rarely single molecular structures, but more typically materials with a considerable dispersity of sizes and properties.20 In this regard, nanomaterials are somewhat like polymers in that they often have an average molecular weight and a determined size distribution. Finally, even highly purified nanoparticles may have some amount of impurities present in them. Owing to the large molecular weight differential between the parent nanoparticles and the impurities, even a small mass percentage of impurity will represent a significant concentration (or dose) in comparison to the nanoparticle.22

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Composition

Core and surface compositions difficult to define, vary from sample to sample

Defined molecular formula

Size/shape

Often mixtures of core sizes and shapes

Defined molecular structure and shape

Dispersity

Distributions of compositions and structural features are the norm, often lead to averaged properties

Single composition and structure

Purity

Small molecule impurities significantly influence properties

Typically obtained in near pure form

Figure 2: Comparison of nanomaterial attributes compared to those of molecular as these attributes relate to material definition. Distributions and variations in nanomaterial properties complicate efforts to define materials and attribute properties to composition and structures.

The lack of material definition in nanomaterials presents several significant challenges.20 First, the lack of specificity about naming leads to sweeping generalizations about the properties of materials classes that ignore the structural differences within the class. For example, depending upon the surface coating, gold nanoparticles can be dispersed in water or nonpolar organic solvents and can be either non-toxic or quite toxic. Those new to the field should be aware that specific nanomaterials have often been referred to in the literature as the class that they represent, not their specific structure. This is analogous to attributing an observed effect to “aromatic hydrocarbons” or “polypeptides”. Second, the lack of definition makes it difficult to categorize and attribute properties to specific structural attributes which, in turn, make it difficult

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to establish predictive structure/activity relationships (SARs). Finally, material descriptors and definition are critical to the design or selection of a nanomaterial with desired properties. Within the last decade considerable effort has been placed on improved nanomaterial characterization.21 The use of multiple, complementary techniques25 has enhanced what we know about core materials, including their composition, crystallinity, phase, size and shape. There is also a greater appreciation that the surface chemistry26 must be defined in order to understand the properties and impacts of nanomaterials. It is now typical for nanomaterial cores to be characterized by TEM (size, crystallinity, and shape), SAXS (size), XRD (phase and size).25 The overall size in solution is often characterized by DLS or DOSY NMR.27 The surface coatings (ligands or polymers) are typically characterized by XPS (coating composition) or TGA (coating density).21 Increasingly, multiple corroborative techniques25 are used to assess similar attributes, e.g. the use of TEM and SAXS together to evaluate particle size. In addition to more detailed characterization, there has been greater emphasis placed on more complete and specific descriptions of nanomaterials.22 A minimum description of a nanomaterial should include the core composition, core size and size dispersity, the surface chemistry or ligand shell (or coating) composition, and the purity of the sample. In the event that the nanomaterials are non-spherical, the shape should be stated. While there has been good progress, there are still reports that describe the impacts of “gold nanoparticles” or “carbon nanotubes”. These are inadequate descriptors because it is now clear that the sizes, shapes, structures, surfaces and purities of these materials have significant influence on their properties, impacts and hazards. Due to the characterization challenges inherent to nanomaterials, it has been difficult to quantify the purity of nanomaterials. However, it has been recognized that small amounts of

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impurity may have significant influence on nanomaterial properties, especially if those impurities are toxic. As an example, toxic ions released from nanoparticles may overshadow any toxic effects of the parent nanoparticle.28 During the last decade, new purification methods such as dialfiltration and methods to detect impurities (e.g. NMR to assess ligand contamination) have led to reduced contamination and better quantification of any remaining impurities.29 Synthesis, mechanisms and scalable production. The structures of nanomaterials are, in general, more complex than molecular species. As a result, syntheses of these materials are similarly complex. The synthesis of a typical nanoparticle or nanotube requires the (nearly) simultaneous construction of thousands of bonds. Often the mechanisms involved in nanomaterial synthesis are unknown and it is therefore difficult to predict the products of such reactions or design new syntheses. While syntheses of certain nanomaterials have been optimized through painstaking trial and error, general approaches to synthesis of nanomaterials with well-defined size, shape and surface chemistry have not been developed. Many syntheses depend upon rapid nucleation of particles30 which, in turn, requires highly reactive precursors and careful control of reactions conditions, e.g. concentration of all reagents, reaction temperature and reagent mixing. The combination of this strong dependence on reaction conditions, coupled with the lack of mechanistic understanding has made it difficult to scale production of these materials beyond the milligram quantities needed for fundamental research. The complexity, unknown mechanisms of construction and challenges to scaling differentiate nanomaterials from their molecular analogs. Compared to the “brute force” syntheses common during the discovery phase of nanoscience wherein highly reactive (and toxic) reagents were used to produce low yields of nanomaterials often through irreproducible methods, there has been significant progress on

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greener nanomaterials synthesis.4 There is a growing number of greener syntheses being reported that address one or more of the opportunities to reduce impacts of synthesis or shed light on the mechanistic aspects of nanoparticle formation or functionalization.31 Successful strategies to apply green chemistry to nanoparticle synthesis are illustrated in Figure 3. Thiol-stabilized gold nanoparticles are a good system to illustrate these strategies because green chemistry principles have been applied to an especially broad set of challenges within this class of materials. The first example of greening AuNP syntheses involved replace highly toxic reagents and carcinogenic solvents in the synthesis process.32 Less toxic/hazardous reagents replaced diborane gas and benzene, while enhancing the scalability and cost effectiveness of the process. Next, yields were enhanced for the reaction producing this nanoparticle core and chemistry was developed for quantitative ligand exchange reactions to introduce desired functionality on the nanoparticle surface.33 Next, we recognized that nanoparticle purification was a major source of waste, so we developed dialfiltration methods to remove small molecule impurities from nanoparticle samples.29 Diafiltration filters can be used many times, produce cleaner nanoparticle products and nearly eliminate the use of volatile organic solvents. Because syntheses based upon ligand exchange involve multiple steps, a next step toward greener nanosyntheses involves fewer (ideally single) steps that reduce the amount of reagents and solvents used.34 Finally, to improve the yield of well-defined nanoparticles and to achieve high yields in a reproducible fashion, flow reaction chemistry was implemented that controlled mixing of reagents and led to reproducibly high yields of the target material.35 Although these steps were pioneered on gold nanoparticles, most are readily adaptable to other nanoparticle core materials.

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Eliminate reaction steps, increase yields Metal precursor

Direct synthesis

Surfactant stabilizer Reducing agent + byproducts

Use safer reagents

Ligand exchange

Solvent

Reduce impacts of purification (e.g. diafiltration)

Reduce, use safer solvents + byproducts

Figure 3: Strategies for greening nanoparticle synthesis. The strategies illustrated here have been implemented for thiol-stabilized gold nanoparticles, but are more widely adaptable to a variety of other inorganic core materials.

The success of a greener synthetic method also depends upon the quality of the product produced. There are a number of reports in the literature that claim a greener method to prepare various nanomaterials.36 Many of these syntheses employ a safer reducing agent, surfactant or solvent. But what about the performance of these materials? If the intention is to produce a material with a desired property, a greener approach is only useful if the property of the material produced equals or exceeds the target attribute. Thus, it is important to assess the performance of the material and weigh the impacts of production in the context of the quality and quantity of material produced.36 Material complexity/structure activity relationships. Two structural features (size and multifunctionality) result in greater complexity of structure/activity relationships for nanomaterials relative to molecular species. A fundamental difference between nanomaterials and their molecular counterparts is that nanomaterial properties depend upon the size of the

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nanostructure, whereas the properties of molecular species depend upon the three-dimensional arrangement of functional groups. Structures within the size range of 1-100 nm are generally accepted as nanostructures, and the size of the structure can influence the optical properties (e.g. size dependent fluorescence in semiconductor quantum dots or size dependent plasmonic effects in gold nanoparticles and nanorods). Size can also influence the reactivity and impacts of nanomaterials in biological and environmental settings. At these small sizes, the surface to volume ratio is high, leading to differences in surface reactivity compared to bulk materials. Within a single class of materials, changes in the diameter of a nanoparticle will also change the radius of curvature of the surface and the surface structure in ways that may significantly influence the reactivity of the nanoparticle. Nanomaterials are typically constructed of a core material that is often a metal, metal oxide, carbon framework or other organic structure. These cores vary in composition, shape and surface chemistry as well as size. Differences in core attributes and the surface chemistries can result in a wide array of different functions. Relative to molecular species, a wider range of functions will be displayed by nanomaterials. Thus, a challenge for nanomaterials is determining the relationships between reactivity, function or activity (with respect to both performance and impacts) and the size and structure.22 Because many nanomaterial functions are not features that small molecules share, there is much to understand about how size and structure influence the reactivity and properties of nanomaterials. There has been progress in recent years in developing basic design principles for safer NPs.4, 9, 22, 37 In addition to a growing body of research, two advances have been important steps to correlate nanoparticle structure with effects as needed to establish more predictive SARs. The first involves access to a wider variety of well-defined nanoparticle samples with good material

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definition.38 The second is the development of high throughput screening methods that permit rapid collection of data on the biological effects of the nanoparticle samples.12, 39 Nanoparticles that elicit a strong effect can be prioritized for more extensive testing that may help determine the mechanism of action. These approaches used in combination are illustrated in Figure 4.

Precision nanomaterial libraries

Characterization

Precise synthesis and purification

Rapid in vivo evaluation and dosimetry

In-depth mechanistic studies of effects Guidelines for Sustainable Molecular Design of Nanoparticles

Figure 4: Strategy for developing guidelines for sustainable design of nanoparticles. Rapid evaluation of precision nanomaterials provides rapid feedback regarding the biological effects of these materials. The feedback assists in further refine design guidelines, guides the selection of new precision nanomaterials for study or suggests priority materials for in-depth mechanistic study.

Several types of biological effects have now been correlated with the reactivity of inorganic cores of nanoparticles. For those that contain soluble toxic ions (zinc, silver, or heavy metals), that ion typically dictates the toxicity of the nanoparticle.40, 41 This led to one of the earliest design principles – avoid incorporation of toxic ions4 – although it is understood that for applications such as antimicrobial action, the toxic ions are essential to function. More recently, the toxicity of metal oxide NPs has been correlated with the materials band gap and the ability of the NP to generate reactive oxygen species (ROS), suggesting that different metal oxides

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produce variable toxic effects.42 However, both dissolution and ROS generation are surface based phenomena; thus, the surface coatings will absolutely affect the toxic potential. Some studies have shown that the release of toxic ions from NPs may be attenuated by specific surface coatings.17 In the case of ROS generation, surface coatings attenuate the reactivity.43 These studies support a complementary design principle – controlling surface chemistry can prevent or reduce unintentional reactivity. However, no systematic studies on precision NPs have been done to examine the relative importance of the design principles outlined here. In so far as surface coatings are stable and the cores release no toxic ions, the surface coatings may dictate toxicity; however, this remains to be experimentally examined. Composition of the surface chemistry can have a strong influence on toxicity. For example, positively charged NPs exhibit higher toxicity, whereas negatively charged or neutral (e.g. polyether) functionalities exhibit some of the lowest toxicities.38 The resulting design principle – use polyether stabilizers instead of cationic groups – is useful, but may unnecessarily limit the design parameters for future applications. Adjusting the composition of mixed ligand shells may be a solution because it is known that NPs can interact with biological structures through multiple attachment points in the ligand shell, leading to an enhanced avidity compared with a single ligand. Determining the relationship between avidity and toxicity will inform the use of the mixtures of ligands likely to appear in next generation applications. It is unclear how the identity of the functional groups and ligand density (dictated by the surface chemistries used and/or the curvature of the NP core) will influence toxicity. Product integration, nanomaterial releases, life cycle impacts/benefits. Nanomaterials are increasingly being integrated into other products, e.g. as fillers in nanocomposites and as surface coatings, as alternatives to small molecule ingredients.24 As with any new material or

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product, there are potential health and environmental impacts related to the use of nanomaterials. Although not unique to nanomaterials, determination of the relative merits of possible alternatives requires examining impacts across the lifecycle, considering product use scenarios and considering tradeoffs in impacts. Little is known about the lifecycle impacts of nanomaterials or their integration into products, making such comparisons are more difficult than comparing two molecular components. When nanomaterials are incorporated into a product, there is the possibility that they may be released into the environment, leading to exposures to humans and other organisms. Among the several challenges faced in determining what exposures occur and the possible impacts are detection and identification of released nanomaterials in the environment (often against a background of naturally occurring nanomaterials)20, 21 and the determination of likely exposures under realistic use scenarios.44 In addition to improved analytical approaches to detect and quantify released nanomaterials,45 mechanisms have been discovered that reduce the likelihood of possible exposure because of environmental transformations.46 For example, sulfidation of silver,47 self-agglomeration of nanomaterials and their adsorption to other surfaces effectively remove nanomaterials from the environment.48 Still, there is much to be learned about how nanomaterials are released and about testing the possible impacts of nanomaterials in the form that they are released. Such studies would also inform green chemistry approaches to design nanomaterial-containing products to prevent exposures. The lack of knowledge regarding the releases and impacts of nanomaterials, the paucity of data on nanomaterial production and the evolving understanding of nanomaterial use scenarios are some of the issues that make these comparisons with non-nanomaterial alternatives challenging. Further, when the integration of a nanomaterial into a product is intended to

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provide a benefit to the environment (for example, reduced energy usage during the use of the product), that benefit must be weighed against the potential impacts of the production, integration and use of the nanomaterial. In the early development of nanotechnology, most of the focus was placed on understanding the health/environmental impacts due to release or disposal of nanomaterials or the direct impacts related to nanomaterial production. With respect to production, it was found that the impacts of the byproducts of production, wastes generated during purification7 and poor performance of materials produced by bioderived reagents36 were targets for the application of green chemistry. Today, there is a growing realization that indirect impacts or benefits through the lifecycle, for example, environmental benefits accrued in the use phase, may dominate those that are specific to the nanomaterial. Two examples that illustrate the need to examine the use of nanomaterials within a lifecycle context involve the use of CdSe nanocrystals in lighting fixtures and the use of nanosilver to render textiles antimicrobial. In the first instance, the use of a cadmium-containing material within a lighting fixture could be cause for concern.49 Through leaching studies it was shown that nanoparticles are not released from the fixture. Life cycle analysis showed that energy savings for the use of quantum dot optics in televisions during the use-phase reduced cadmium emissions (based upon EU Cd air emissions due to electricity generation in 2010) by more than ten times the amount of Cd in the fixture.50 In the case of nanosilver for antimicrobial (or antiodor) applications on textiles, the question of net environmental benefit depends upon the loading of nanosilver on the textile. Life cycle analysis51 suggests that if the loading of nanosilver on the textile is sufficiently high, 1000s to 10000s ppm of silver, it may not be possible to accrue a net benefit during the normal lifetime

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of the garment. However, product performance also depends upon loading.51 Through a series of studies, it has been shown antimicrobial activity can be achieved at significantly lower loadings than are often used in products.52 At these lower loadings, the lifecycle impacts of the use of silver may be readily offset by reductions in other lifecycle impacts if textiles are washed somewhat less frequently51 or if nanosilver coated hospital sheets can be used instead of disposable sheets.53 Opportunities for the future Nanomaterials are now widely used in a number of applications and number of applications is growing.54 There has been considerable progress toward the molecular-level design of safer nanomaterials, more efficient and safer syntheses of nanomaterials and incorporation of nanomaterials into products that provide a net environmental benefit. However, owing to the complexity of nanomaterials, there are still significant challenges to be addressed through research and product development. With respect to synthesis and processing, there is still a need for improved understanding of mechanisms that control material formation and functionalization. Mechanistic understanding is the key to rational synthesis and predictable synthesis and is essential to identify future production methods that permit more sophisticated and multiple functionalities without increasing waste of production. Our understanding of transformations and surface reactivity of nanomaterials is also in its infancy, and will be important for understanding, and predicting, product performance, as well as impacts in organisms and the environment. With respect to design of nanomaterials with specific, desired properties, there is still much to learn. “Design rules” that help us predict properties, hazards and routes of exposure are essential for developing the next generations of nanomaterials. Moreover, as nanomaterials are more widely used in materials of commerce,54 design that enables recycling/recovery and/or

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safe disposal will be needed. Finally, methods to assess the benefits and impacts of nanoenabled products across the product lifecycle are needed to compare their performance to other nanomaterial-based and non-nano alternatives. Such methods will be important for material selection in new products, but will also illuminate deficiencies in nanomaterial attributes that will inform re-design of those materials to improve their performance.

Acknowledgments Thanks to Julie Haack for helpful discussions and input on this Feature. This work was supported by EPA grant no. RD83558001.

Corresponding Author [email protected] References 1. 2. 3. 4. 5. 6. 7. 8. 9.

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For Table of Contents Only: Origins Of Materials

Manufacturing

Distribution

Use

End of Life

Design of safer nanomaterials Design for reduced environmental impact Design for waste reduction Design for process safety Design for materials efficiency Design for energy efficiency

The road to sustainable nanotechnology: Challenges, progress and opportunities James E. Hutchison Synopsis: The application of nanomaterial design strategies across the lifecycle of nanoscale products is a powerful approach to advance sustainable nanotechnologies

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The Road to Sustainable Nanotechnology: Challenges, Progress

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