Watching it all fall apart The control of the shape and size of metal nanoparticles can be very sensitive to the growth conditions of the particles. Ye et al. studied the reverse process: They tracked the dissolution of gold nanoparticles in a redox environment inside a liquid cell within an electron microscope, controlling the particle dissolution with the electron beam. Tracking short-lived particle shapes revealed structures of greater or lesser stability. The findings suggest kinetic routes to particle sizes and shapes that would otherwise be difficult to generate. Science , this issue p. 874

Surface topography profoundly influences cell adhesion, differentiation, and stem cell fate control. Numerous studies using a variety of materials demonstrate that nanoscale topographies change the intracellular organization of actin cytoskeleton and therefore a broad range of cellular dynamics in live cells. However, the underlying molecular mechanism is not well understood, leaving why actin cytoskeleton responds to topographical features unexplained and therefore preventing researchers from predicting optimal topographic features for desired cell behavior. Here we demonstrate that topography-induced membrane curvature plays a crucial role in modulating intracellular actin organization. By inducing precisely controlled membrane curvatures using engineered vertical nanostructures as topographies, we find that actin fibers form at the sites of nanostructures in a curvature-dependent manner with an upper limit for the diameter of curvature at ∼400 nm. Nanotopography-induced actin fibers are branched actin nucleated by the Arp2/3 complex and are mediated by a curvature-sensing protein FBP17. Our study reveals that the formation of nanotopography-induced actin fibers drastically reduces the amount of stress fibers and mature focal adhesions to result in the reorganization of actin cytoskeleton in the entire cell. These findings establish the membrane curvature as a key linkage between surface topography and topography-induced cell signaling and behavior.

The serum iron level in humans is tightly controlled by the action of the hormone hepcidin on the iron efflux transporter ferroportin. Hepcidin regulates iron absorption and recycling by inducing ferroportin internalization and degradation 1 . Aberrant ferroportin activity can lead to diseases of iron overload, like hemochromatosis, or iron limitation anemias 2 . Here, we determined cryogenic electron microscopy (cryo-EM) structures of ferroportin in lipid nanodiscs, both in the apo state and in complex with cobalt, an iron mimetic, and hepcidin. These structures and accompanying molecular dynamics simulations identify two metal binding sites within the N- and C-domains of ferroportin. Hepcidin binds ferroportin in an outward-open conformation and completely occludes the iron efflux pathway to inhibit transport. The carboxy-terminus of hepcidin directly contacts the divalent metal in the ferroportin C-domain. We further show that hepcidin binding to ferroportin is coupled to iron binding, with an 80-fold increase in hepcidin affinity in the presence of iron. These results suggest a model for hepcidin regulation of ferroportin, where only iron loaded ferroportin molecules are targeted for degradation. More broadly, our structural and functional insights are likely to enable more targeted manipulation of the hepcidin-ferroportin axis in disorders of iron homeostasis.

Luminescent solar concentrators doped with CdSe/CdS quantum dots provide a potentially low-cost and high-performance alternative to costly highband-gap III−V semiconductor materials to serve as a top junction in multijunction photovoltaic devices for efficient utilization of blue photons. In this study, a photonic mirror was coupled with such a luminescent waveguide to form an optical cavity where emitted luminescence was trapped omnidirectionally. By mitigating escape cone and scattering losses, 82% of luminesced photons travel the length of the waveguide, creating a concentration ratio of 30.3 for blue photons in a waveguide with a geometric gain of 61. Further, we study the photon transport inside the luminescent waveguide, showing unimpeded photon collection across the entire length of the waveguide. L uminescent solar concentrators 1−4 (LSCs) have been studied extensively for the last three decades as low-cost alternatives to single-and multijunction photovoltaic (PV) devices. As silicon prices have fallen, it has become increasingly clear that future solar panels will need to have both low cost and high efficiency. One promising strategy for achieving a higher efficiency is to use different parts of the solar spectrum in photovoltaic materials with varying band gaps to minimize losses associated with carrier thermalization and incomplete photon absorption. For these multijunction (MJ) PV devices, there is a strong need for developing low-cost, high-band-gap solar cells for efficient utilization of the high-energy part of the solar spectrum. A luminescent solar concentrator could provide exactly this function, serving as the top junction in a multijunction architecture by converting blue photons into guided luminescence. Due to the concentration effect, only small amounts of high-performing but expensive III−V photovoltaic materials are needed to collect the light from an inexpensive luminescent waveguide. Such a device requires high concentration factors to reduce the cost of the III−V photovoltaic material. High concentration also allows the Stokes shift of the lumophore to be recovered in the operating voltage of the photovoltaic cell.The concentration factor and collection efficiency achieved by LSCs to date have been limited due to parasitic losses such as nonunity quantum yields of the lumophores, imperfect light trapping within the waveguide, and reabsorption and scattering of propagating photons. 5 Previous studies have sought to solve each of these parasitic losses individually, resulting in modest performance improvements. 6−15 Here we achieve a luminescent concentration ratio greater than 30 with an optical efficiency of 82% for blue photons by simultaneously addressing the materials and optical challenges of the LSC system. These concentration ratios are achieved through the combination of designer quantum dot lumophores and photonic mirrors, and microscale silicon photovoltaic cells are used to detect the concentration of light in the waveguide. To the best of our knowledge, this is the highest ...

Nanoparticle self-assembly has been well studied theoretically, but it remains challenging to directly observe and quantify individual nanoparticle interactions. With our custom image analysis method, we track the trajectories of nanoparticle movement with high precision from a stack of relatively noisy images obtained using liquid cell transmission electron microscopy. In a time frame of minutes, Pt-Fe nanoparticles self-assembled into a loosely packed hcp lattice. The energetics and stability of the dynamic assembly were studied quantitatively. From velocity and diffusion measurements, we experimentally determined the magnitude of forces between single particles and the related physical properties. The results illustrate that long-range anisotropic forces drive the formation of chains, which then clump and fold to maximize close range van der Waals interactions.