Commentary pubs.acs.org/accounts
Nanomaterials and Global Sustainability Published as part of the Accounts of Chemical Research special issue “Holy Grails in Chemistry”. Robert J. Hamers* Department of Chemistry, University of Wisconsin, 1101 University Avenue, Madison, Wisconsin 53706, United States ABSTRACT: Nanomaterials provide tremendous opportunities to advance human welfare in many areas including energy storage, catalysis, photovoltaic energy conversion, environmental remediation, and agriculture. As nanomaterials become incorporated into commercial processes and consumer products in increasing amounts, it will be essential to develop an understanding of how these materials interact with the environment. The broad spectrum and complexity of nanomaterials drive a need for molecular-level design rules. Ultimately a grand challenge is to use the power of chemistry to ensure that nanoenabled technologies can come to fruition in an environmentally benign manner.
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INTRODUCTION Over the last 30 years, nanotechnology and nanomaterials have revolutionized the field of chemistry. The “nanorevolution” began with the ability to characterize materials at the atomic scale,1,2 but rapidly evolved as scientists learned that the ability to visualize matter at the nanometer length scale could also be used to manipulate matter,3,4 ultimately leading to the ability to synthesize nanomaterials in a wide range of compositions, sizes, shapes, and morphologies.5−8 When the last “Holy Grail” issue of Accounts of Chemical Research was published in 1995, nanoparticles were generally considered laboratory curiosities synthesized in small quantities. In the years since, the intrinsic properties of nanoparticles have generally become well understood, and nanomaterials are now being rapidly incorporated into a wide range of consumer products and commercial processes. As the scientific foundations of the nanoworld have matured, new factors have emerged associated with how to harness the properties of nanomaterials in a way that maximizes their potential to achieve the overall goal of enhancing the sustainability of our planet. These factors include development of sustainable syntheses of nanomaterials, integration of nanomaterials into key technologies that directly yield societal benefit, and development of an understanding of the behavior of nanomaterials in the environment. Ultimately, a grand challenge of chemistry is how to exploit the properties of nanomaterials to achieve new functionality while also minimizing any potential adverse environmental impact associated with synthesis, use, and disposal. As the widespread use of nanomaterials increases, so does the potential for unintentional release into the environment, resulting in exposure of the life forms therein. Environmental impact is generally considered to be the product of exposure and toxicity. A nanomaterial present in small quantities, for example, likely poses little overall environmental risk. But as nanomaterials become more broadly accepted, the likelihood of © 2017 American Chemical Society
exposure increases, and even materials with relatively low toxicity may have unexpected impact. For example, TiO2 is generally regarded as being relatively benign; however, in the presence of sunlight, TiO2 nanoparticles can be toxic to some freshwater organisms even at parts-per-billion levels due to uptake and in situ photogeneration of reactive oxygen species.9,10 Studies to date have shown that even simple nanoparticles can interact by a number of different mechanisms. A grand challenge is then, “Can one predict the biological impact of nanomaterials?”. If so, then a logical extension would be, “How can the power of chemistry be used to ensure that nanoenabled technologies can come to fruition in an environmentally benign manner?” In order to address this challenge, it is essential to understand what barriers to understanding currently exist. One barrier is the overall complexity of the problem. Nanomaterials used in emerging applications are likely to have complex chemical structures and highly specialized physical structures (e.g., core−shell structures or nonspherical structures) and may be embedded into polymers or other matrices.11 As depicted in Figure 1, even relatively simple, single-component nanomaterials can interact with biological systems through multiple pathways. Nanoparticles smaller than ∼25 nm can be internalized into cells by passing through cell membranes, initiating a cascade of subsequent biochemical processes. However, nanoparticles of all sizes can also have significant effects without entering the cell, including disruption of the cell by binding to its surface, dissolution and release of free metal ions that can be transported into the cell,12,13 and chemical or photochemical production of reactive oxygen species.9,14 Received: December 19, 2016 Published: March 21, 2017 633
DOI: 10.1021/acs.accounts.6b00634 Acc. Chem. Res. 2017, 50, 633−637
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Figure 1. Some of the possible pathways of initial interaction of nanoparticles with biological cells.
Figure 2. Scanning electron microscope image of LiFePO4 nanoparticles within the cathode of a commercial lithium-ion battery.
To address future nanotechnology needs, it is critical to look forward to what emerging technologies are likely to be using nanomaterials in large volumes and thereby present potential for significant exposure during synthesis, use, or disposal. Here, I describe four classes of materials whose use is likely to increase greatly and that therefore may be especially important to understand from the standpoint of overall environmental impact. These areas are (1) energy storage nanomaterials, (2) two-dimensional nanomaterials, (3) agricultural nanomaterials, and (4) water purification nanomaterials. This list is necessarily incomplete but will give the reader of glimpse of how the maturation of nanotechnology poses new and interesting chemistry questions that connect to issues surrounding global sustainability.
understand the environmental impact of these materials and to develop strategies to minimize routes of potential exposure. Nearly all lithium ion battery cathode materials include transition metals, typically in high oxidation states. Many transition metals induce toxicity across broad classes of organisms, suggesting that many cathode materials have potential for adverse biological impact if released into the environment. For example, in recent work we showed that the NMC class of battery cathode nanomaterials can induce significant toxicity in Shewanella oneidensis, a common soil bacterium used as a model system.13 While we found that the biological response could be largely replicated using Ni2+ and Co2+ ions in their water-stable (2+) oxidation states, in the parent LiNixMnyCo(1−x−y)O2 nanoparticles, the transition metals are in higher oxidation states (e.g., Co3+). The interaction of metals in high oxidation states with water has the potential to create hydroxyl radical and other reactive oxygen species via reactions such as Co3+ + OH− → Co2+ + • OH.14 Ultimately the choice of materials involves a complex interplay of performance, cost, and environmental safety. By understanding what the environmental interactions are, it should be possible to integrate environmental safety into a more complete life-cycle analysis of the benefits provided by these emerging nanomaterials.
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EXAMPLE 1: NANOMATERIALS AND THE ENERGY STORAGE REVOLUTION The revolution in mobile electronics, a global push to replace internal combustion engines with electric vehicles, and a desire to make the best use of renewable sources such as wind and solar energy are all based on the ability to store energy electrochemically. While the ubiquitous lithium ion battery used in today’s consumer electronics uses anodes of graphite and cathodes of LiCoO2, lithium and cobalt are costly and their projected use constitutes a significant fraction of the world’s supply,15 leading to efforts to replace Co with other redoxactive elements such as Fe, Ni, Al, and Mn. These emerging materials, such as LiFePO4 (“LFP”), LiNixMnyCo(1−x−y)O2 (0 < x, y < 1) (“NMC”) and LiNixCoyAl(1−x−y) (“NCA”) provide high performance with reduced cost, with nanosized materials leading to improved electrical and Li-ion conductivity and improved mechanical properties. For example, Figure 2 shows a scanning electron microscope image of the cathode from a commercial lithium-ion battery, showing a preponderance of nanoparticles (in this case, LiFePO4) ∼40−50 nm in diameter. Energy storage requires substantial amounts of materials, and even a modest electric vehicle will have >30 kg of NMC, LiFePO4, or other active cathode materials, leading to the potential for exposure to lifeforms during manufacturing, use, and most importantly, disposal. While recycling can greatly reduce environmental impact, at present a large fraction of battery materials are not recycled,16 and the low cost of the emerging materials provides little economic incentive for recycling. Much of the world’s electronic waste is shipped to China, India, and other countries with less stringent environmental regulation, increasing potential for release into the environment.17 Thus, it becomes especially important to
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EXAMPLE 2: TWO-DIMENSIONAL NANOMATERIALS In contrast to energy storage materials where the end utility dictated specific composition of specific classes of materials, the emerging interest in two-dimensional nanomaterials is driven by a myriad of applications centered around the unique properties of two-dimensional materials. Starting with the scalable synthesis of graphene by chemical vapor deposition onto copper foil,18 many two-dimensional materials including metal chalcogenides19,20 and other electronic and structural materials have recently become available.21 In practice, it is simpler to make a two-dimensional material extremely thin and uniform than it is to make spherical particles of comparable diameter. For example, the fabrication of few-layer graphene sheets by liquid-phase exfoliation22 and single-layer graphene by CVD18 are now taking place on a large scale.18 Similar preparation methods apply to other two-dimensional materials, and there is currently rapid growth in the ability to synthesize two-dimensional nanomaterials in diverse compositions.20 634
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location. Ideally, proper design of such materials could include the ability to have built-in self-destruct mechanisms that would be triggered by an appropriate stimulus such as light, heat, or pH change. The net result would be improvement in efficiency of application and reduced exposure to nontargeted organisms.
Many two-dimensional nanomaterials exhibit quantum confinement effects or novel catalytic properties as a result of their reduced dimensionality.13 The practical utility of these properties is amplified by the fact that materials only 1−2 atomic layers have enormous surface area, thereby posing increased opportunity for them to display unusual chemical properties. Some two-dimensional materials can combine high in-plane electrical conductivity with relatively little light absorption, making them useful as a transparent conductors. Other materials are of interest because of outstanding mechanical strength.23 or because of the optical, magnetic, or catalytic properties.19,20 As with energy storage materials discussed above, the applications of two-dimensional materials may be inherently three-dimensional, thereby involving enhanced potential for larger volumes. Examples include use of two-dimensional nanosheets as strengthening agents in polymer composites and as enhanced adsorbents. Twodimensional materials are poised for potential application in many areas. At present, it is largely unknown whether the unique quantum confinement effects exhibited by many of these materials will play a significant role in their potential environmental impact. However, the very large surface area suggests that it will be important to investigate the absorptive properties of these materials. As depicted in Figure 3, these
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EXAMPLE 4: NANOMATERIALS FOR CLEAN WATER Nanoparticles will likely play a major role in efforts to ensure global access to clean water. Nanoparticles are able to effectively remove both organic and inorganic contaminants. As one example, zero-valent iron is an effective reducing agent and is able to aid in the transformation of organic pollutants such as chlorinated hydrocarbons into more benign forms.26 The high surface area of metal and metal oxide nanomaterials also makes them excellent sorbents for inorganic anions.27,28 In many parts of the world subsurface waters are contaminated by the highly toxic arsenate ion due to naturally occurring arsenicbearing mineral deposits. The strong affinity of nanoparticles of Fe2O3, SiO2, and Al2O3 for AsO43− and other anions of interest may be very useful in water remediation, as many small ions do not adsorb well to traditional adsorbents such as carbon that are commonly used in water purification systems.27 Surface adsorption processes may also play a negative role by transforming nominally nontoxic materials such as Fe2O3 into species that could now have significant biological impact. For example, if arsenate-coated Fe2O3 nanoparticles were able to penetrate through biological membranes and into various tissues, the transformed nanoparticles could act as a carrier and enhance the transport and biological uptake by organisms in the environment. Ultimately, a key factor is that applications such as water purification may use nanoparticles made from compounds that are considered benign, but the very high surface area of nanoparticles also means that it is essential to understand how nanoparticles transform during and after their intended use.
Figure 3. Two-dimensional nanomaterials may act as strong sorbents for hydrocarbons and small ions, as depicted here.
adsorptive properties could potentially be used in environmental remediation to adsorb contaminants, for example, but could also be unintentional carriers within biological and environmental systems.
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EXAMPLE 3: NANOMATERIALS AND AGRICULTURE Another major growth area for nanomaterials over the next 20 years is likely to be in the field of sustainable agriculture and water purification. As the world’s population grows, there is increased demand for technologies that can provide clean water and that can increase the efficiency of agriculture. The potential use of nanomaterials in agriculture is particularly important because this use could significantly increase food production but would likely involve application of large quantities of materials and be accompanied by comparatively high levels of ancillary nanoparticle exposure to both plant and animal life. Initial studies in this area have shown that use of Cu-based nanoparticles increases the efficiency of Cu-based fungicides.24,25 A longer-term opportunity for nanomaterials is to enhance the efficacy of agricultural chemicals such as fertilizers, pesticides, and micronutrients. Nanoparticles have been shown to have some advantageous effects on crops, although fundamental mechanisms remain poorly understood.24,25 Another potentially advantageous use of nanomaterials in agriculture could be the use of polymers to encapsulate a “payload”, such as nutrient, that could be delivered to the plant. The use of nanomaterials with specific exposed chemical groups could significantly enhance the ability to target the specific
BARRIERS AND OPPORTUNITIES
The use of nanomaterials has the potential to greatly reduce reliance on fossil fuels and can enhance the effective use of scarce resources in many technology fields, presenting many new opportunities for chemists to play a direct and creative role in ensuring the future health of our planet. In doing so, it is also essential that we understand how the synthesis, use, and disposal of nanomaterials impacts the overall cycle of sustainability, including understanding how nanomaterials directly interact with the environment and the life forms therein. The ultimate goal of synthesizing and implementing nanomaterials in an environmentally benign manner is clearly a long-term grand challenge that will required a concerted effort by chemists, biologists, and materials scientists. Given the complexity of the example materials outlined above, it is essential to think about a long-term strategy to address this challenge. The field of organic chemistry can provide a guiding example. Organic compounds pose a rich variety of chemical reactions that, at first glance to an unsophisticated observer, appears daunting. However, by focusing on mechanistic understanding, organic chemists have developed sophisticated sets of design rules that are used to guide synthesis and to predict properties of compounds that have not yet been made. In a similar manner, the challenge of understanding nano−biological interactions is inherently a 635
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insights of nano−bio interactions and the development of science-based design rules provide a pathway to maximize the overall benefits associated with emerging nanomaterials.
chemistry problem in which the chemical groups on the outside of the nanoparticle and the chemical functional groups exposed on the exterior surface of a cell control the initial interactions. The challenge then becomes, “How can one develop design rules that have predictive power?”. Many of the core scientific issues associated with nanomaterials in the environment revolve around understanding fundamental interactions at solid−liquid interfaces. Therefore, continuing development of in situ interface characterization techniques, computational method development, and careful mechanistic studies of the chemistry and biology of nanoparticle interactions with organisms will be essential to longterm understanding of nanoparticle impacts on the environment. This simplified picture rapidly becomes complicated by the fact that while nanomaterials are synthesized and engineered with specific composition and structure to achieve specific properties, when released into the environment nanomaterials quickly undergo chemical changes. These changes can involve loss of material via dissolution as well as acquisition of inorganic or organic coatings.29 Nanoparticles that are embedded into polymer matrices may dissolve or release different components at different rates. Consequently, the materials are not static but are dynamic. Because of the dynamic nature of nanoparticle interactions, one of the key challenges in developing molecular-level understanding has been the absence of analytical tools capable of characterizing the atomic-level structure and composition of nanoparticles in relevant matrices, such as at or inside cells. However, recent advances in subdiffraction imaging,30,31 in situ X-ray techniques,32 and nonlinear optical methods33 have markedly improved the ability to characterize the important interfacial phenomena that ultimately control nanoparticle interactions to improve our understanding of nano−bio interactions. Biological organisms are more complex and more variable than typical chemical systems, limiting the ability to glean fundamental insights regarding the fundamental mechanisms of nanoparticle−organism interactions. However, the use of genetic variants and the development of genomic and proteomic methods may be able to overcome these limitations to provide more direct chemical insights. Additionally, the development and implementation of model systems will be important in testing and validating specific hypotheses about mechanisms of interaction. Since the first interaction is typically between a nanomaterial and the cell membrane, model systems such as free-standing and supported lipid bilayers may provide a platform for testing specific hypotheses about mechanisms of interaction. While bilayers have greatly reduced complexity, their use as model system can enable use of experimental measurements that cannot be applied to the full biological system. Understanding the interfacial phenomena associated with nano−bio interactions involves many different length scales. Theory and computation, especially when directly coupled with experiments for validation, provide the opportunity to link detailed atomistic mechanisms with diffusion and related phenomena that occur on longer length scales.34 Ultimately, an important target of research in this area is the development of materials that have technological utility and are environmentally benign. Steps in this direction are already being taken. For example, cellulosic nanofibers35 may provide mechanical properties similar to those of carbon nanotubes, and fluorescent “carbon dots”36 may replace CdSe quantum dots. While in many cases technological needs may limit the ability to use alternative materials, fundamental molecular-level
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CONCLUSION Nanomaterials provide tremendous opportunities to advance human welfare in many areas including energy storage, catalysis, photovoltaic energy conversion, environmental remediation, and agriculture. As nanomaterials become incorporated into commercial processes and consumer products in increasing amounts, it will be essential to develop an understanding of how these materials interact with the environment. The broad spectrum and complexity of nanomaterials drives a need for molecular-level design rules. Continuing advances in analytical instrumentation and methodology, computation and data science, and “-omic” techniques have the potential to provide the necessary information and insights needed to identify how to design and synthesize materials that are technologically useful and environmentally benign. Ultimately, research in this field represents an important platform for chemists to contribute to enhancing global sustainability.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Robert J. Hamers: 0000-0003-3821-9625 Notes
The author declares no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Science Foundation Centers for Chemical Innovation Program, Award No. CHE1503408 to the Center for Sustainable Nanotechnology. R.J.H. thanks Mimi Hang for assistance with electron microscopy.
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