# Opportunities and challenges for nanotechnology in the agri-tech revolution

Gregory Lowry, Astrid Avellan, Leanne Gilbertson

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Gregory Lowry, Astrid Avellan, Leanne Gilbertson. Opportunities and challenges for nanotechnology in the agri-tech revolution. Nature Nanotechnology, 2019, 14 (6), pp.517-522. 10.1038/s41565-019-0461-7. hal-03708331

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# Opportunities and challenges for nanotechnology in the agri-tech revolution

Gregory V. Lowry<fnref n="1" />,²*, Astrid Avellan<fnref n="1" />,² and Leanne M. Gilbertson³,<fnref n="4" />

Current agricultural practices, developed during the green revolution, are becoming unsustainable, especially in the face of climate change and growing populations. Nanotechnology will be an important driver for the impending agri-tech revolution that promises a more sustainable, efficient and resilient agricultural system, while promoting food security. Here, we present the most promising new opportunities and approaches for the application of nanotechnology to improve the use efficiency of necessary inputs (light, water, soil) for crop agriculture, and for better managing biotic and abiotic stress. Potential development and implementation barriers are discussed, emphasizing the need for a systems approach to designing proposed nanotechnologies.

A productive and sustainable agricultural system is essential to the existence of humanity. Although current agricultural practices can produce enough food for nearly eight billion people<fnref n="1" />, they cannot continue to do so sustainably without technological intervention². The amount of resources used in agriculture is staggering. Over three billion metric tonnes of crops are produced globally each year<fnref n="1" />, requiring 187 million metric tonnes of fertilizer, nearly four million tonnes of pesticides³, 2.7 trillion cubic metres of water (about 70% of all freshwater consumptive use globally)<fnref n="4" />, and over two quadrillion BTU of embodied energy⁵. The green revolution, starting over six decades ago, has provided the necessary year-over-year yield gains with minimal expansion of land use, by including abundant and inexpensive nitrogen-fertilizers afforded by the Haber-Bosch process, mechanization, drip irrigation and breeding programmes to produce higher-yield cultivars (Fig. 1).

Although the green revolution has thus far allowed us to provide 2,884 kcal per capita per day (about 8 × 10¹⁵ kcal per year) in 2013⁶ (sufficient calories to meet demand — on average), current agricultural practices are inherently inefficient and unsustainable and have led to unacceptable environmental degradation². Poor utilization efficiency of applied agrochemicals⁷ and crop losses due to disease and drought lead to massive losses in embodied energy and water, as well as cascading environmental implications⁷, including over 37,000 km² of eutrophic waters⁸ and persistent pollutants (such as insecticides) in drinking water sources⁹, posing human health risks. Crop agriculture also accounts for between 7% and 15% of global greenhouse gas emissions (for example, N₂O releases into the atmosphere¹⁰). Agricultural intensification using these same methods is unsustainable.

Degraded soils¹¹,¹² and climate change are flattening the yield curves for many crops, when a 50–80% increase in production is needed by 2050¹⁰. Ubiquitous and inefficient pesticide and herbicide use has induced resistance and can decrease biodiversity and impair the ecosystem functions needed for a healthy agro-ecosystem¹³,¹⁴. Mono-cropping, soil degradation, water scarcity and globalization of diseases have led to agriculture systems of low resilience that are vulnerable to collapse⁷. All of these pressures are flattening yield curves, indicating the end of the 'green revolution'<fnref n="1" />.

A new agri-tech revolution is emerging, with the goal of sustainably meeting future demands for food (and fuel and fibre). The revolution, fuelled by information technology, new materials, methodologies (such as gene editing tools like CRISPR-Cas9)¹⁵ and the use of plant-growth-promoting bacteria¹⁶, promises to make agriculture more efficient, resilient and sustainable (Fig. 1).

Engineered nanomaterials (ENMs) have the potential to lead this change, providing ubiquitous sensing opportunities to monitor crop health at high spatial and temporal resolution by making plants the sensors (for example, see Giraldo et al.¹⁷). They can also improve the efficiency of cropping systems with respect to all inputs through more efficient delivery mechanisms, improving the efficiency of nutrient utilization, and better management of disease and crop loss (up to 40% of all food is lost pre-harvest¹⁸). Finally, enabling plant nanobiotechnology will provide plants with augmented functions to allow them to better manage environmental stresses in a rapidly changing climate.

ENMs are ideal platforms to lead the agri-technology revolution. The small size of these objects (<100 nm) allows them to cross biological barriers, diffusing into the plant vasculature after root or foliar applications, and enabling new efficient routes of delivery of nutrients and pesticides. Many such nanomaterials are metastable, and therefore can provide controlled release of micronutrients or antibiotics to promote plant resilience against disease (see Kah et al.¹⁹). Importantly, the structure and the surface chemistry of ENMs can be modified to provide the properties and functionalities needed to direct them to the right locations within plants or soils, and to offer smart release and delivery strategies. Combined, these features make ENMs a versatile resource for plant nanobiotechnology.

This work provides a forward-looking perspective on (1) the most promising opportunities for nanomaterials and nanotechnology approaches to fuel the agri-tech revolution, (2) the fundamental challenges currently preventing their broad development and application for sustainable agriculture, and (3) the research needed to address these challenges. Finally, we stress the importance of taking a systems perspective on all proposed agri-tech solutions to ensure that they are sustainable, rather than shifting the environmental burden for one environmental compartment to another.

## Opportunities in the agri-tech revolution

ENMs can improve the overall use efficiency of agrochemical inputs (water, light and chemicals) by affording resource recovery options, developing 'smart plants' as sensors, increasing agrochemical efficiency through targeted delivery, improving the health and functions of both soil and plants through microbiome enhancement, and decreasing losses by managing crop diseases better and by making crops more resilient (Fig. 2). These opportunities can increase yields using fewer resources and leading to less collateral damage to the environment.

<footnote n="1">¹Civil and Environmental Engineering Department, Carnegie Mellon University, Pittsburgh, PA, USA. ²Center for Environmental Implications of Nanotechnology, Pittsburgh, PA, USA. ³Department of Civil and Environmental Engineering, University of Pittsburgh, Pittsburgh, PA, USA.</footnote>

<footnote n="4">⁴Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA, USA. *e-mail: glowry@cmu.edu</footnote>
<figure><img src="image_2.png" /><figcaption>**Fig. 1** | The green revolution and the new agri-tech revolution.</figcaption></figure>

Technologies of the green revolution, such as the Haber-Bosch process, mechanization and drip irrigation have increased crop yields to exceed global food demands without massive changes in land use. More recently, flattening yield curves and increasing demand, declining resilience (for example, from climate change and soil degradation) and unacceptably increasing environmental damage indicate that the green revolution has ended. A new agri-tech revolution, driven in part by nanotechnology, is needed to align yields with growing demands, increase resilience and lower environmental impacts to make agriculture sustainable. The curves are illustrative only, rather than based on data, and are intended to demonstrate relative trends in environmental impact, yield versus demand, and resilience.

**Improve the use efficiency of water, light and agrochemicals.** Water availability is a key predictor of yield in food crops<sup>20</sup>. Inadequate water availability already limits crop yields in arid, agriculturally productive regions, which is predicted to intensify with future changes in climate<sup>21</sup>. New nanomaterials and treatment technologies can provide water of sufficiently high quality for agriculture productivity by tapping into non-traditional agriculture water sources (municipal wastewater, or oil and gas wastewater, for example) in water-scarce regions<sup>22</sup>. For example, nano-enabled solar distillation and crystallization technologies can make high-salinity wastewater suitable for crop agriculture<sup>22,23</sup>. Nanomaterials can enhance water-use efficiency in crop agriculture by improving water retention in crops<sup>24,25</sup>, and can be used to develop 'smart plants' that can convey their water needs to growers. As discussed by Giraldo et al.<sup>17</sup>, inexpensive plant-based sensors to monitor water requirements at the individual plant level can enable precision watering approaches that saves water. This approach can also minimize crop losses from water-related stresses (drought and flooding). Nanotechnology can also increase water productivity, defined as the economic value gained from each cubic metre of water used. For example, coupling fish production with plant growth allows the reuse of aquaponics water for food production, as well as the recovery of nitrogen and phosphorus produced from fish. Nanomaterials could make these modes of production possible. Their surface properties can be tuned to provide selective recovery and reuse of nitrogen and phosphorus from these waste streams, as well as effective destruction of pathogens present in the water streams<sup>26,27</sup>. Combinations of these technologies will ultimately have to be deployed to maximize both water use efficiency and water productivity.

Nanotechnology has the potential to improve the efficiency of light use in agriculture. With plants making use of only a few per cent of incident photons, this represents a pinch point in productivity. Indeed, non-optimal photochemical reactions lead to up to 20% lower carbon assimilation than is theoretically possible<sup>28</sup>, and

bioengineered plants presenting an altered photo-protective mechanism increased their yields by up to 15%<sup>29</sup>. However, societal resistance to accepting bioengineered crops has slowed infiltration of these approaches. Foliar-applied ENMs also have considerable potential to increase plant photosynthesis efficiency<sup>30</sup> through (1) protecting photosynthesis from abiotic stress<sup>31</sup>, (2) increasing chlorophyll content<sup>32</sup>, (3) enhancing light absorption<sup>32</sup> and photon use<sup>31</sup>, (4) scavenging reactive oxygen species<sup>30,33</sup> or (5) assisting electron transfer<sup>30</sup>. However, more rapid photosynthesis may lead to lower nutritional content of the foods<sup>34</sup> produced. Foliar-applied nanomaterials made from micronutrients such as Fe-, Zn- or Si-based ENMs may be one approach to ensuring that micronutrient content of foods remains high, but the mechanisms of uptake of ENMs and their distribution in plants, and the bioavailability of ENM micronutrients, must still be determined.

Nanomaterials and nanotechnology can vastly improve the use efficiency of both nutrients and pesticides, decreasing environmental damage and reducing embodied energy losses. All crops require significant external inputs of nitrogen (N) (up to hundreds of kilograms N per acre)<sup>35</sup>, phosphorus (P), potassium (K) and micronutrients (such as Zn, Cu, Fe and B) as fertilizer to maintain prosperous consecutive growth cycles. Over-application of fertilizer and the remarkably low use efficiencies (<50%)<sup>36</sup> result in considerable nutrient emissions and detrimental impacts on the surrounding soil and surface waters. Nanomaterials can improve nutrient-use efficiency by providing new delivery platforms and pathways for nutrients, enabled by their small size and tailorable surface chemistry<sup>37</sup> as well as inherent slow or environmentally responsive release properties of metal and metal oxide ENMs. A wide range of nanomaterials (carbon-based nanomaterials, mineral nanoparticles, metal and metal oxide nanoparticles)<sup>38–43</sup> have demonstrated the potential to deliver nutrients effectively through roots, leaves and seed coatings. The nanomaterial can serve as the nutrient (as in the case of ZnO nanoparticles) or as the platform to deliver the nutrients<sup>44</sup>; examples of platforms are hydroxyapatite, natural polymer nanoparticles such as chitosan, or nanoscale capsules (liposomes, micelles and nanolumsions, for instance). The ability to tailor the surface chemistry of the ENMs can improve their adherence to leaves and enhance uptake through key biological plant barriers such as the cuticle, or translocation through the mesophyll cells<sup>45</sup>. The use of coatings with molecular recognition, for example aptamers or antibodies, may enable precise delivery of ENMs to desired locations in plants<sup>46</sup>. These approaches can both improve yield and decrease the environmental impacts of agriculture, while reducing the inputs.

Although design of solutions to avoid emissions is the preferred approach to mitigating the environmental consequences of inefficient nutrient use in crop production, there are also opportunities for nanomaterials as sorbents or to functionalize membranes or electrodes to enable selective removal and/or recovery of nutrients (such as N and P) from contaminated waters and waste streams for re-use in agriculture<sup>26,47</sup>.

**Improve soil integrity and function.** Soil is essential to crop agriculture. It is where the micro- and macro-flora, (micro)nutrients, organic matter, humic acid and water interact at the interface with plant roots. Soil also stores water and nutrients, as well as contaminants, helping to modulate the environmental impacts of agriculture. Nano-enabled additives could promote soil health by reducing soil contamination and by lowering the amount of inputs via increased nutrient use efficiency, for example, through nanostructures engineered to target and release nutrients or pesticides only in the plant rhizosphere<sup>42</sup>. There is also potential to apply nanomaterials to the soil or utilize foliar delivery to manipulate the rhizosphere root-soil-microbiome interaction, and hence improve crop yields<sup>48</sup>. For instance, the application of metallic ENMs in soil has been shown to modify N, P and C cycles and the bioavailability of macronutrients
a

<figure><img src="image_3.png" /></figure>

b

<figure><img src="image_4.png" /></figure>

c

<figure><img src="image_5.png" /></figure>

d

<figure><img src="image_6.png" /><figcaption>**Fig. 2** | Agricultural uses of nanotechnology. **a**, Transforming plants into sensors that enable monitoring of plant needs at high spatial and temporal resolution. **b**, Enabling targeted delivery of ENMs to selected locations in plants, similar to drug delivery in humans. **c**, Better management of both biotic and abiotic environmental stresses. **d**, Improving nutrient utilization efficiency, or recovering nutrients from wastewater, to minimize N and P inputs for agriculture. Panel **d** adapted from ref. ⁷², RSC.</figcaption></figure>

N and P through the modification of microbial activity, networking and/or abundance⁴⁹,⁵⁰. Microorganisms are also known to induce ENM biotransformation and ion release, and this might be used for slow delivery strategies³¹,⁵². Metal and metal oxide ENMs can be used to alter cell signalling and the composition and function of the soil microbiome to enhance the availability of micronutrients and improve crop performance⁵³. ENMs could potentially be used to increase the number of beneficial plant endophytes, or to encourage plant root colonization by N₂-fixing rhizobia, but achieving this would require better understanding of the role of soil and plant microbiome in plant physiology. These strategies offer great opportunities to improve the sustainability of agriculture while increasing crop yields and plant nutritional values.

**Better manage environmental stressors to minimize crop loss.** Both biotic and abiotic stresses can lower crop yield and food quality. Biotic (pathogens, weed competition) and abiotic stressors (including extreme temperatures, water stress or flooding, and high salinity) are estimated to reduce global agriculture productivity and average yields of major plant species yields by about 50% worldwide³⁴,⁵⁵. Plant responses to these stresses are complex; for example, many abiotic stress conditions can enhance plant susceptibility to pathogen infection (and vice versa), and the agricultural losses due to this combined stress are high⁵⁶.

Targeted, smart delivery of low doses of ENMs to crops can modify the plant's response to abiotic stresses such as drought³⁷, extreme heat³³, heavy metals in soils⁵⁸ or elevated salinity⁵⁹, making them more resilient against these threats. Nanotechnology has also demonstrated the potential for more efficiently and sustainably managing pathogen, pests and weeds (see Kah et al.¹⁹). Leveraging technologies developed for drug delivery in humans, surface functionalization and rational design of ENMs can lead to new agrochemical products that can readily cross biological barriers⁶⁰, are more efficiently used than larger-sized materials⁵⁷ and can help to sense biomarkers related to soil and plant health⁴⁶. Stimuli-responsive platforms could deliver agrochemicals when needed in response to elevated temperature, to manage heat stress; in response to low pH, to deliver materials into the root zone; in response to high pH (≥7)⁶¹, to deliver antimicrobials in phloem; or in response to pathogen presence. These strategies, which can increase plant resistance to biotic and abiotic stresses, will make the agriculture system more resilient and less sensitive to climate change.

**Implementation barriers and research needs**

There are many challenges to overcome to realize the full potential of ENM-enabled technologies for agriculture. Critical barriers to rapid and widespread deployment of these technologies include a lack of mechanistic understanding of plant-ENM interactions,
limited routes for the ENM delivery into plant vasculature and organelles, and incomplete understanding of the environmental and human health risks from ENMs intentionally placed into the food chain. In addition to technological and engineering challenges, the successful adoption of new technologies within the low profit margins of crop agriculture depends on their ability to balance system costs and benefits.

**ENM-plant interactions and mechanisms of action are unknown.**
The first demonstrations of root uptake of relatively insoluble ENMs by plant and their translocation to plant leaves were made less than a decade ago⁶². Current development of nano-enabled solutions is largely restricted to phenomenological observations, and the underlying mechanisms remain to be elucidated. This limits our ability to harness the numerous design handles (such as size, shape and surface chemistry) to manipulate properties (solubility or surface charge, for example) that can enhance or inhibit their performance in their intended use. In the context of nano-enabled agrochemicals for augmenting plant functions, this includes the biophysics controlling nanomaterial-plant interfacial interactions, for example with the leaf cuticle, chloroplast cell walls and phloem sieve plates.

For example, CeO₂ nanoparticles associated with chloroplasts can enhance photosynthesis under salt or heat stress³³. This is hypothesized to be a result of the antioxidant properties of CeO₂ nanoparticles, allowing the chloroplasts to manage oxidative stress from the environmental conditions. However, the spatial and temporal association of the CeO₂ nanoparticles with the chloroplast is unknown. More in situ evidence is needed to determine the range of mechanisms responsible for the beneficial effects, to enable smarter designs for nanomaterials to enhance photosynthesis and manage plant stress. Similarly, treating plants with low doses of an antimicrobial/micronutrient CuO nanoparticle increases their ability to manage biotic stress from fungal root infections. As discussed by Kah et al.¹⁹, the precise mechanisms behind these enhancements are unknown, hampering efforts to improve ENM designs and to optimize delivery and performance.

Efforts are under way to understand the physical-chemical parameters that will influence ENM-plant interactions, ENM uptake and transformation, and the biological effects of ENMs, but the dogma from plant biology regarding size exclusion limits and pathways of ENM transport is only slowly breaking down. A deep integration of disciplines including, for example, plant biology, nanochemistry and plant pathology is necessary to define the full potential of plant nanobiotechnology. This convergence of thought will be a challenge.

**Methods to deliver ENMs efficiently to plants are limited.**
The most promising plant nanobiotechnology approaches will require methods to deliver ENMs to selected locations into millions of plants precisely and cost-effectively. Delivery to plant roots through soil irrigation applications or to plants through foliar spray are both being evaluated.

Delivering ENMs to roots in soils will be especially challenging. Untargeted broadcasting of ENMs into soils, even at low parts per million concentrations, is unlikely to be energy efficient, owing to the sheer volume of soil to be treated and the relatively high embodied energy of many of the ENMs that are proposed for use. Targeted delivery or slow release approaches have been tried, for example using a virus with high affinity for the plant roots to deliver the ENMs⁶³ and chitosan carriers that provide slow release⁶⁴. Targeted approaches such as foliar application of ENMs, followed by uptake, phloem loading and transport to roots and rhizosphere soil¹⁵, or the use of ENMs on seed coatings to improve germination and seedling resilience, are likely to be more efficient than soil broadcasting.

Foliar application of ENMs can easily be done with existing infrastructure (spraying or spray irrigation), but this is hampered by ultraviolet degradation of applied ENMs, weather (wind and rain can remove the applied ENMs) and low uptake of the ENMs into plant leaves. The ability to design ENMs, functionalize their surfaces and tune formulations to promote leaf adhesion and uptake will enable solutions to these challenges⁴⁵. A key challenge will be to design intentionally applied coatings or surface functionality that are not quenched by incidentally adsorbed biomacromolecules in soils or plant cells, such as natural organic matter or proteins⁶⁵.

Delivering ENMs to plants at the right place, at the right time, at the right dose and in the right form remains a considerable challenge. Many climate-related stressors, such as heat or cold, do their damage to crops during a short-term extreme event. ENMs that become activated by environmental triggers such as temperature, pH or salinity could potentially be applied to plants ahead of time and actuate when certain environmental conditions are met, thus deploying precisely when needed to augment plant function and overcome the extreme climate event. Coupling ENMs with temperature- or pH-responsive polymeric materials could make this possible.

## Managing risks and risk perception

As discussed by Lombi et al.⁶⁶, agriculture is a complex socio-ecological system. The use of ENMs in agriculture would make food an important route of exposure to engineered nanomaterials. Unsurprisingly, consumers are sensitive to potential safety concerns with ENM use on food crops. Public perception of safety and regulatory concerns surrounding the use of engineered nanomaterials in food production must be addressed to ensure safety, and to assist the acceptance and adoption of plant nanobiotechnology approaches.

There is a growing acceptance that the size of nanomaterials does not make them inherently toxic⁶⁷. In fact, metal-based ENMs are often less toxic to plants than their ionic counterparts at equivalent doses⁴². However, one benefit of metal and metal oxide ENMs is their ability to access biological compartments that ionic metal species cannot, or to provide prolonged releases of a metal nutrient (such as Zn or Fe). The impact of this spatial and temporal uniqueness of agent delivery afforded by ENMs in plants must be considered when assessing potential safety concerns. Moreover, purposely increasing the amount of ENMs entering plants meant for consumption (lettuce, for example), or concentrating them in grains or fruits, such as corn kernels or tomatoes, as a means to combat the nutrient dilution expected as CO₂ concentrations rise³⁴, requires appropriate risk assessment and regulation. Regulatory guidelines to address specific safety issues related to ENMs in food and agriculture are emerging in the European Union⁶⁸, but developing them is challenging given the complexity of agricultural systems, including multiple organisms (plants, pests and humans), multiple microbiomes (soil, plant), climate variability, soil structures and compositions, ENM design variables and societal engagement.

## Outlook and future directions

Nano-enabled technological solutions will be an important tool for the next agri-tech revolution needed to transform agriculture to be more efficient, resilient and sustainable. Applications of nano-technology in agriculture are just now emerging and will continue to advance with the advance in both nanotechnology and the fundamental understanding of nanomaterial-plant interactions. Better control over matter at atomic scales to improve selectivity of reactivity of nanomaterials, the use of biomolecular targeting approaches and the development of environmentally responsive bioinspired designs of ENMs will ultimately improve the
<figure><img src="image_7.png" /><figcaption>**Fig. 3 | System trade-offs.** Evaluating system trade-offs between utilization hazard and fabrication cost on the one hand, and environmental and financial benefits on the other, is critical to ensure that using ENMs on-farm to improve agriculture sustainability does not introduce or unexpectedly shift burdens to other lifecycle stages.</figcaption></figure>

atom-efficiency of agrochemical delivery, and enable delivery at the right time, place, dose and form. It will lead to 'smart plants' that can help to manage agriculture at high spatial and temporal resolution to promote resource-use efficiency.

The use of ENMs in crop production will require direct application of large quantities of ENMs to the environment and to crops intended for future consumption. It is therefore critical that we approach the development of nano-enabled solutions in a way that considers the entire system, to ensure that we do not shift burden up- or downstream of the farm (Fig. 3). Delivering nutrients more efficiently, for example, can reduce inputs and reduce run-off, but may require a nanomaterial that has a higher embodied energy than the nitrogen fertilizer offset, or that could adversely influence the rhizosphere microbiome through an unin tended residual impurity from the synthesis. At the same time, there may be unexpected or coupled intentional benefits realized through delivering nutrients more efficiently, such as increasing nutritional content in the food or enhancing the beneficial micro bial diversity in the rhizosphere. Considering the full life cycle impacts of a nano-enabled solution enables quantification of the trade-offs across the system — from raw material acquisition to end of life — and benchmarking to conventional alternatives to ensure that development of nano-enabled crop production real izes its potential, sustainably.

Critical to improving nanomaterial design for advancing crop production in the next revolution is the identification of nano-specific properties that initiate beneficial and adverse responses. This will require a range of laboratory and field testing to generate comprehensive data to inform future decisions, regulations, design and development⁶⁹. Although access to such information is a challenge for the nano field at large, progress has been made towards more global design principles for nanomaterials⁷⁰,⁷¹. There is an opportunity to translate these findings and approach into the development of nanomaterials for agriculture crop production.

This Perspective presents the opportunities for nanotechnology in crop agriculture, which is only one part of the agro-food system. There are many additional opportunities to improve the overall agro-food system²⁶, including (bio)chemical sensors, minimizing post-harvest losses or household food waste, enhancing efficiency and water productivity in hydroponics and aquaponics, designing new vaccines for livestock, and better leveraging plant-soil micro biomes for increasing crop efficiency and resilience. As nano-manufacturing capabilities continue to improve, more complex layered or multi-payload ENMs may become feasible to produce as cost-effective delivery systems.

Received: 21 February 2019; Accepted: 18 April 2019;
Published online: 5 June 2019

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## Acknowledgements

G.V.L. and A.A. thank the US National Science Foundation (NSF) and the Environmental Protection Agency (EPA) under NSF Cooperative Agreement EF-1266252, the Center for the Environmental Implications of NanoTechnology (CEINT) and the NSF (CBET-1530563; Nano for Agriculturally Relevant Materials (NanoFARM)) for supporting this effort. L.M.G. acknowledges support from the Gordon and Betty Moore Foundation.

## Competing interests

The authors declare no competing interests.

## Additional information

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Correspondence should be addressed to G.V.L.

Journal peer review information: *Nature Nanotechnology* thanks David Britt and Jason White for their contribution to the peer review of this work.

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