Nanoparticles have the potential to improve the efficiency of agrochemicals while minimizing their environmental impact. Chuanxin Ma and colleagues have now demonstrated that foliar delivery of nanoscale copper to young seedlings stimulates innate plant defenses in the roots, yielding resistance to sudden soybean death syndrome and enhanced crop growth under pathogen stress. The degree of enhanced immunity was controlled by material properties and was computationally predictable, demonstrating the significance of material tunability as a powerful tool for nano-enabled agriculture and for combatting food insecurity. The cover image is an artist’s impression of the interaction between nanoparticles and plants.

See Ma et al.

Cells and organisms adapt to signals by processing them through reaction networks that ultimately provide downstream functional responses and structural morphogenesis. Now Samanta, Walther and co-workers re-enact this signal processing in DNA protocells. They use genetically improved artificial metalloenzymes to convert signalling molecules into DNA interacting metabolites that induce downstream growth, functional adaptation and fusion processes inside and between protocells. The image is an artist's depiction of signal-molecule-triggered cellullar processes.

See Samanta et al.

It is increasingly apparent that a wide range of proteins present as functional condensates in living cells can undergo a liquid-to-solid transition to form pathological solid protein aggregates associated with disease. Yet, whether such condensates can undergo a liquid-to-solid transition in vitro driven by physical factors remains unexplored. Y. Shen, T. Knowles and co-workers report a general shear response of peptides and proteins that can form solid fibres from liquid–liquid phase separated condensates. During this process, shear forces favour molecular alignment, promote inter-molecular interactions and trigger a liquid-to-solid transition driven by hydrogen bonding. The cover image shows an artistic impression of fibre formation from a condensate.

See Shen et al.

Meeting the growing demand for food requires careful consideration of our finite resources. Agriculture is notoriously resource intensive with system-wide inefficiencies, particularly on-farm application of agrochemicals. Recent developments of nano-enabled agrochemicals show promise for agriculture intensification. Gilbertson et al. analyse these nano solutions with cautious optimism and provide a holistic perspective that goes beyond the immediate nano-induced effects. By considering the performance and environmental trade-offs of using nanomaterials to offset or replace conventional agrochemicals, they identify opportunities to sustainably advance crop production. The cover art is an artistic impression representing how their results define the design space that ensures sustainable enhancement of food production.

See Gilbertson et al.

Nanotechnology expertise and tools provide valuable contributions to the development of therapeutic and diagnostic solutions for COVID-19. In this Focus issue, nanotechnologists depict the key roles that nanotechnology plays in preclinical and clinical SARS-CoV-2 research. In COVID-19 vaccine development, nanoparticles enable the delivery of antigens, in particular nucleic acids, which are important vaccine candidates currently in clinical trials. Moreover, researchers can build on long-standing expertise in nanoparticle-mediated drug delivery to improve the treatment efficacy of drugs currently repurposed for COVID-19. Nanomaterial-based sensors can be rapidly adapted for the sensitive early diagnosis of COVID-19, and expertise in immunoengineering and cancer vaccine development provides important insight into immune-mediated approaches against COVID-19, as illustrated on the cover, which shows some of the crucial players of the immune response against SARS-CoV-2. The time is ripe for nanotechnology to make a real clinical impact.

See Florindo et al.

A number of non-von Neumann architectures have been proposed to circumvent the processing bottlenecks and power consumption limitations hampering further growth of classical computing. Neuromorphic computing based on the idea of the collocation of logic and memory, hyper-connectivity and parallel processing strives to emulate the neural structure of the human brain and can potentially offer substantially lower power consumption. Another brain-inspired approach is in-memory computing where computational tasks are performed within the confines of a computational memory. The cover is the artist depiction of an abacus, the oldest example of the in-memory computing concept. The abacus in the cover image features some elements of a modern in-memory computer made of nanoscale memory devices that derive their functionality from 'atomic organization', 'charge' and 'magnetic spin'.

See Sebastian et al.

Pure polymers are usually bad conductors because of a sizeable electronic band gap around the Fermi level. However, one-dimensional (1D) π-conjugated polymers can become conducting when their π-electron system is tweaked to the transition from one resonant structure to another. Concomitantly, this change in the conjugation constitutes a topological phase transition from a trivial to a non-trivial phase. Borja Cirera et al. now approach this transition point by on-surface synthesis. They produce polymers of both phases, but close to the phase transition and experimentally determine the topology of the π-electrons. The polymers closest to the transition then indeed show very narrow band gaps.

The cover art shows the distribution of an electronic edge state located at the termini of the 1D polymer on the gold surface. This state is a fingerprint of the topologically non-trivial nature of the polymer.

Article Ecija; N&Vs by Corso

The development of efficient memory devices and powerful spintronic applications requires a detailed understanding of the dynamic response of magnetic materials. Claire Donnelly et al. now present a time-resolved imaging technique to record the magnetization dynamics of bulk magnets, which often display a complex texture. In a stroboscopic measurement, the authors periodically excite the magnetization of a microdisc with a radio-frequency magnetic field and then, for a number of different time delays, probe the three-dimensional magnetization distribution via X-ray laminography, a synchrotron-based technique that maps the bulk magnetic texture. With this pump–probe experiment, they obtain a series of freeze frames of the magnetization with a time resolution of 70 ps and a spatial resolution of 50 nm.

The cover art shows a snapshot of the transient magnetization in a microdisc that is excited with an alternating magnetic field. The swirling streamlines represent the local magnetization within the disc, including magnetic vortices that move from side to side.

Letter by Donnelly

Nanoparticles are usually endocytosed by cells and digested by lysosomes. Therefore, these waste disposal organelles, which are characterized by a low pH as compared to the cytoplasm, offer a potential target for nanoparticle-mediated cancer cell killing. Borkowska et al. designed mixed-charge nanoparticles that, upon entering the acidic tumour microenvironment, aggregate into clusters on the surface of the cells. The nanoparticle clusters are then endocytosed and form even larger clusters in the more acidic lysosomes, as shown in the cover image. Nanoparticle aggregation then causes lysosome swelling and rupture and ultimately leads to cell death. Importantly, the distinct charge decoration of the nanoparticles prevents aggregation in the less acidic extracellular environment of healthy tissues and thus offers a cancer-cell selective targeting strategy.

Article by Kandere-Grzybowska; N&Vs by Xia

When a magnetic field exerts a force on a moving electron it creates direction-dependent phases; hence, it breaks time-reversal symmetry. A nonreciprocal flow of sound is much harder to achieve and requires artificial gauge potentials that give rise to a magnetic field for sound. Mathew, del Pino and Verhagen now implement such a gauge field for vibrations in an on-chip optomechanical system. They use dynamic modulation of strong laser light to induce multi-mode interactions between two nanoscale resonators of different resonance frequencies. The laser light’s radiation pressure force mediates phonon transport between the resonators, but with a phase difference depending on the direction of the phonon transfer. These experiments establish a synthetic gauge field for nanomechanical transport, such that vibrations feel an effective magnetic field. The cover art displays one of the vibrating nanomechanical resonators under strong laser light illumination.

Letter by Verhagen

In healthy tissue, dead cells are usually rapidly cleared by macrophages to ensure tissue homeostasis. However, in atherosclerosis, phagocytic clearance is impaired, and thus, large necrotic cores are formed. Flores et al. report a nanotherapy that restores the phagocytic capacity of macrophages and promotes inflammation resolution in atherosclerosis. Single-walled carbon nanotubes are first loaded with a chemical inhibitor of a key anti-phagocytic signalling pathway. Upon injection, they accumulate in macrophages within atherosclerotic plaques and reactivate phagocytosis. This Trojan horse strategy shows efficient plaque reduction in a mouse model of atherosclerosis.

Article by Leeper et al N&Vs by Fredman

Clar’s goblet is a polycyclic aromatic hydrocarbon with an unconventional source of magnetism. This bow tie shaped nanographene, which was predicted by Erich Clar in the 1970s, possesses an even number of carbon atoms and π-electrons, but the connectivity of the carbon atoms prohibits pairing of all electrons into π-bonds. The resulting diradical character makes the molecule very reactive and, thus, unstable under standard conditions. Mishra, Beyer and colleagues have now synthesized Clar’s goblet on a gold surface under ultra-high vacuum conditions. By means of low-temperature scanning tunnelling microscopy and spectroscopy, they have unveiled the structure of the molecule and its antiferromagnetic ground state, and controllably manipulated the radical character. The cover art is based on a scanning probe image revealing the carbon–carbon bond structure of Clar’s goblet and shows an artificial ensemble of nine molecules.

Letter by Fasel et al.; N&Vs by Melle-Franco