Exceptions to Fourier's Law at the Macroscale

The usual basis to analyze heat transfer within materials is the equation formulated 200 years ago, Fourier's law, which is identical mathematically to the mass diffusion equation, Fick's law. Revisiting this assumption regarding heat transport within translucent materials, performing the experiments in vacuum to avoid air convection, we compare the model predictions to infrared-based measurements with nearly mK temperature resolution. After heat pulses, we find macroscale non-Gaussian tails in the surface temperature profile. At steady state, we find macroscale anomalous hot spots when the sample is topographically rough, and this is validated by using two additional independent methods to measure surface temperature. These discrepancies from Fourier's law for translucent materials suggest that internal radiation whose mean-free-path is millimeters interacts with defects to produce small heat sources that by secondary emission afford an additional, non-local mode of heat transport. For these polymer and inorganic glass materials, this suggests unique strategies of heat management design.

The ergodicity question when imaging DNA conformation using liquid cell electron microscopy

Assessing the ergodicity of graphene liquid cell electron microscope measurements, we report that loop states of circular DNA interconvert reversibly and that loop numbers follow the Boltzmann distribution expected for this molecule in bulk solution, provided that the electron dose is low (80-keV electron energy and electron dose rate 1-20 e- Å-2 s-1). This imaging technique appears to act as a "slow motion" camera that reveals equilibrated distributions by imaging the time average of a few molecules without the need to image a spatial ensemble.

Volatile Droplets on Water are Sculpted by Vigorous Marangoni-Driven Subphase Flow

The shapes of highly volatile oil-on-water droplets become strongly asymmetric when they are out of equilibrium. The unsaturated organic vapor atmosphere causes evaporation and leads to a strong Marangoni flow in the bath, unlike that previously seen in the literature. Inspecting these shapes experimentally on millisecond and submillimeter time and length scales and theoretically by scaling arguments, we confirm that Marangoni-driven convection in the subphase mechanically stresses the droplet edges to an extent that increases for organic droplets of smaller contact angle and accordingly smaller thickness. The viscous stress generated by the subphase overcomes the thermodynamic Laplace pressure. The oil droplets develop copious regularly spaced fingers, and these fingers develop spike-shaped and branched treelike structures. Unlike this behavior for single-component (surfactant-free) oil droplets, droplets composed of two miscible (surfactant-free) organic liquids develop a rim of the less volatile component along the droplet perimeter, from which jets of monodisperse smaller droplets eject periodically due to the Rayleigh-Plateau instability. When evaporation shrinks droplets to μm size, their shapes fluctuate chaotically, and ellipsoidal shapes rupture into smaller daughter droplets when subphase convection flow pulls them in opposite directions. The shape of the evaporating oil droplets is kneaded and sculpted by vigorous flow in the water subphase.

Solid-body trajectoids shaped to roll along desired pathways

In everyday life, rolling motion is typically associated with cylindrical (for example, car wheels) or spherical (for example, billiard balls) bodies tracing linear paths. However, mathematicians have, for decades, been interested in more exotically shaped solids such as the famous oloids1, sphericons2, polycons3, platonicons4 and two-circle rollers5 that roll downhill in curvilinear paths (in contrast to cylinders or spheres) yet indefinitely (in contrast to cones, Supplementary Video 1). The trajectories traced by such bodies have been studied in detail6-9, and can be useful in the context of efficient mixing10,11 and robotics, for example, in magnetically actuated, millimetre-sized sphericon-shaped robots12,13, or larger sphericon- and oloid-shaped robots translocating by shifting their centre of mass14,15. However, the rolling paths of these shapes are all sinusoid-like and their diversity ends there. Accordingly, we were intrigued whether a more general problem is solvable: given an infinite periodic trajectory, find the shape that would trace this trajectory when rolling down a slope. Here, we develop an algorithm to design such bodies-which we call 'trajectoids'-and then validate these designs experimentally by three-dimensionally printing the computed shapes and tracking their rolling paths, including those that close onto themselves such that the body's centre of mass moves intermittently uphill (Supplementary Video 2). Our study is motivated largely by fundamental curiosity, but the existence of trajectoids for most paths has unexpected implications for quantum and classical optics, as the dynamics of qubits, spins and light polarization can be exactly mapped to trajectoids and their paths16.

Experimental Guidelines to Image Transient Single-Molecule Events Using Graphene Liquid Cell Electron Microscopy

In quest of the holy grail to "see" how individual molecules interact in liquid environments, single-molecule imaging methods now include liquid-phase electron microscopy, whose resolution can be nanometers in space and several frames per second in time using an ordinary electron microscope that is routinely available to many researchers. However, with the current state of the art, protocols that sound similar to those described in the literature lead to outcomes that can differ. The key challenge is to achieve sample contrast under a safe electron dose within a frame rate adequate to capture the molecular process. Here, we present such examples from different systems─synthetic polymer, lipid assembly, DNA-enzyme─in which we have done this using graphene liquid cells. We describe detailed experimental procedures and share empirical experience for conducting successful experiments, starting from fabrication of a graphene liquid cell, to identification of high-quality liquid pockets from desirable shapes and sizes, to effective searching for target sample pockets under electron microscopy, and to discrimination of sample molecules and molecular processes of interest. These experimental tips can assist others who wish to make use of this method.

Biopolymer Filament Entanglement Softens Then Hardens with Shear

It is unsatisfactory that regarding the problem of entangled macromolecules driven out of equilibrium, experimentally based understanding is usually inferred from the ensemble average of polydisperse samples. Here, confronting with single-molecule imaging this common but poorly understood situation, over a wide range of shear rate we use single-molecule fluorescence imaging to track alignment and stretching of entangled aqueous filamentous actin filaments in a homebuilt rheo-microscope. With increasing shear rate, tube "softening" is followed by "hardening." Physically, this means that dynamical localization first weakens from molecular alignment, then strengthens from filament stretching, even for semiflexible biopolymers shorter than their persistence length.

Response of vesicle shapes to dense inner active matter

We consider experimentally the Takatori-Sahu model of vesicle shape fluctuations induced by enclosed active matter, a model till present tested only in the absence of collective motion because few enclosed bacteria were used to generate the desired active motion (S. C. Takatori and A. Sahu, Phys. Rev. Lett., 2020, 124, 158102). Using deformable giant unilamellar vesicles (GUVs) and phase contrast microscopy, we extract the mode-dependence of GUV shape fluctuations when hundreds of E. coli bacteria are contained within each GUV. In the microscope focal plane, patterns of collective bacteria flow include vortex flow, dipolar flow, and chaotic motion, all of which influence the GUV shapes. The Takatori-Sahu model generalizes well to this situation if one considers the moving element to be the experimentally-determined size of the collecively-moving flock.

Molecules, the Ultimate Nanomotor: Linking Chemical Reaction Intermediates to their Molecular Diffusivity

The intellectual community focused on nanomotors has recently become interested in extending these concepts to individual molecules. Here, we study a chemical reaction according to whose mechanism some intermediate species should speed up while others slow down in predictable ways, if the nanomotor hypothesis of boosted diffusion holds. Accordingly, we scrutinize the absolute diffusion coefficient (D) during intermediate steps of the catalytic cycle for the CuAAC reaction (copper-catalyzed azide-alkyne cycloaddition click reaction), using proton pulsed field-gradient nuclear magnetic resonance to discriminate between the diffusion of various reaction intermediates. We observe time-dependent diffusion that is enhanced for some intermediate molecular species and depressed for those whose size increases owing to complex formation. These findings point to the failure of the conventional Stokes-Einstein equation to fully explain diffusivity during chemical reaction. Without attempting a firm explanation, this paper highlights aspects of the physics of chemical reactions that are imperfectly understood and presents systematic data that can be used to assess hypotheses.

Mixed-Charge Nanocarriers Allow for Selective Targeting of Mitochondria by Otherwise Nonselective Dyes

Targeted delivery of molecular cargos to specific organelles is of paramount importance for developing precise and effective therapeutics and imaging probes. This work describes a disulfide-based delivery method in which mixed-charged nanoparticles traveling through the endolysosomal tract deliver noncovalently bound dye molecules selectively into mitochondria. This system comprises three elements: (1) The nanoparticles deliver their payloads by a kiss-and-go mechanism - that is, they drop off their dye cargos proximate to mitochondria but do not localize therein; (2) the dye molecules are by themselves nonspecific to any cellular structures but become so with the help of mixed-charge nanocarriers; and (3) the dye is engineered in such a way as to remain in mitochondria for a long time, up to days, allowing for observing dynamic remodeling of mitochondrial networks and long-term tracking of mitochondria even in dividing cells. The selectivity of delivery and long-lasting staining derive from the ability to engineer charge-imbalanced, mixed [+/-] on-particle monolayers and from the structural features of the cargo. Regarding the former, the balance of [+] and [-] ligands can be adjusted to limit cytotoxicity and control the number of dye molecules adsorbed onto the particles' surfaces. Regarding the latter, comparative studies with multiple dye derivatives we synthesized rationalize the importance of polar groups, long alkyl chains, and disulfide moieties in the assembly of fluorescent nanoconstructs and long-lasting staining of mitochondria. Overall, this strategy could be useful for delivering hydrophilic and/or anionic small-molecule drugs difficult to target to mitochondria by classical approaches.

Reply to Comment on "Using NMR to Test Molecular Mobility during a Chemical Reaction"

Reports of boosted diffusion during chemical and enzymatic reactions have inspired a loyal community of scientists who find them so counterintuitive that they must be artifact. This second Comment on the subject by these authors is about technicalities of how to analyze data we deposited online regarding J. Phys. Chem. Lett. 2021 12, 2370 and Science 2020 369, 537. Now that their own data is also online, one apparent discrepancy can be resolved: we demonstrate that the authors' data agrees with ours because their first Comment on this subject reported only truncated short-time excerpts of the longer time series they deposited online (zenodo.org/record/4628353). This second Comment adds five additional objections, four of which are too technical to change the qualitative conclusion. The fifth objection errs because it omits recognizing intermediate states of the click reaction during which one reactant complexes with the catalyst to form an object of larger size. Elsewhere we analyzed in great detail the respective influences of boosted diffusion and this hydrodynamic effect (DOI: 10.26434/chemrxiv.14740563.v1). The factual evidence and reasoning in this Reply strongly support this laboratory's earlier conclusions regarding boosted diffusion during common chemical reactions.

Using NMR to Test Molecular Mobility during a Chemical Reaction

We evaluate critically the use of pulsed gradient spin-echo nuclear magnetic resonance to measure molecular mobility during chemical reactions. With raw NMR spectra available in a public depository, we confirm the boosted mobility during the click chemical reaction (Wang et al. Science 2020 369, 537-541) regardless of the order of magnetic field gradient (linearly increasing, linearly decreasing, random sequence). We also confirm boosted mobility for the Diels-Alder chemical reaction. The conceptual advantage of the former chemical system is that a constant reaction rate implies a constant catalyst concentration, whereas that of the latter is the absence of a paramagnetic catalyst, precluding paramagnetism as an objection to the measurements. The data and discussion in this paper show the reliability of experiments when one avoids convection, allows decay of nuclear spin magnetization between successive pulses and recovery of its intensity between gradients, and satisfies quasi-steady state during the time window to acquire each datum. Especially important is to make comparisons on the time scale of the actual chemical reaction kinetics. We discuss possible sources of mistaken conclusions that are desirable to avoid.

Reincarnations of the phase separation problem

Phase separation is familiar and useful, yet opportunities to manipulate it are surprisingly subtle and complex.

Response to Comment on “Boosted molecular mobility during common chemical reactions”

Günther et al. report that their control experiment using randomized magnetic field gradient sequences disagreed with findings we had reported using linear gradients. However, we show that measurements in our laboratory are consistent using both methods.

Adaptive architecture and mechanoresponse of epithelial cells on a torus

Curvature is a geometric feature widely observed in the epithelia and critical to the performance of fundamental biological functions. Understanding curvature-related biophysical phenomena remains challenging partly owing to the difficulty of quantitatively tuning and measuring curvatures of interfacing individual cells. In this study, we prepared confluent wild-type Madin-Darby canine kidney cells on a torus structure presenting positive, zero, and negative Gaussian curvatures with a tubule diameter of 2-7 cells and quantified the mechanobiological characteristics of individual cells. Cells on the torus surface exhibited topological sensing ability both as an individual cell and collective cell organization. Both cell bodies and nuclei, adapted on the torus, exhibited local Gaussian curvature-dependent preferential orientation. The cells on the torus demonstrated significant adjustment in the nuclear area and exhibited asymmetric nuclear position depending on the local Gaussian curvature. Moreover, cells on top of the torus, where local Gaussian curvature is near zero, exhibited more sensitive morphological adaptations than the nuclei depending on the Gaussian curvature gradient. Furthermore, the spatial heterogeneity of intermediate filament proteins related to mechanoresponsive expression of the cell body and nucleus, vimentin, keratin and lamin A, revealed local Gaussian curvature as a key factor of cellular adaptation on curved surfaces.

Rapid-prototyping a Brownian particle in an active bath

Particles kicked by external forces to produce mobility distinct from thermal diffusion are an iconic feature of the active matter problem. Here, we map this onto a minimal model for experiment and theory covering the wide time and length scales of usual active matter systems. A particle diffusing in a harmonic potential generated by an optical trap is kicked by programmed forces with time correlation at random intervals following the Poisson process. The model's generic simplicity allows us to find conditions for which displacements are Gaussian (or not), how diffusion is perturbed (or not) by kicks, and quantifying heat dissipation to maintain the non-equilibrium steady state in an active bath. The model reproduces experimental results of tracer mobility in an active bath of swimming algal cells. It can be used as a stochastic dynamic simulator for Brownian objects in various active baths without mechanistic understanding, owing to the generic framework of the protocol.

Boosted molecular mobility during common chemical reactions

Mobility of reactants and nearby solvent is more rapid than Brownian diffusion during several common chemical reactions when the energy release rate exceeds a threshold. Screening a family of 15 organic chemical reactions, we demonstrate the largest boost for catalyzed bimolecular reactions, click chemistry, ring-opening metathesis polymerization, and Sonogashira coupling. Boosted diffusion is also observed but to lesser extent for the uncatalyzed Diels-Alder reaction, but not for substitution reactions SN1 and SN2 within instrumental resolution. Diffusion coefficient increases as measured by pulsed-field gradient nuclear magnetic resonance, whereas in microfluidics experiments, molecules in reaction gradients migrate “uphill” in the direction of lesser diffusivity. This microscopic consumption of energy by chemical reactions transduced into mechanical motion presents a form of active matter.

Micromotor That Carries Its Own Fuel Internally

Micromotors enjoy burgeoning interest but a limitation of their design is to require continuous supply of new fuel. The preponderance of extant micromotors depend, for their motion, on irradiation by light or exposure to acid in their environment. Here we demonstrate a motor that carries its own fuel internally, in this sense representing an analogue, in micron-sized objects, of the internal combustion engine. The fuel is DPCP (diphenylcyclopropenone) microcrystal, a solid-state chemical that after ignition by UV light requires no further irradiation to sustain a chemical reaction that emits carbon monoxide gas that can be used to propel the particle on which this chemical resides. It is loaded asymmetrically onto inexpensive microparticles to produce internally fueled propulsion with speed up to ∼20 μm/s over distances up to 15 times the capsule length in water. Once ignited, the motors maintain their direction of motion and move without need for light to follow their path. Possible strategies to extend the idea beyond the current proof of concept are discussed.

Robustness of FCS (Fluorescence Correlation Spectroscopy) with Quenchers Present

Inspired by recent publications doubtful of the FCS technique, we scrutinize how irreversible ("static") and reversible ("dynamic") quenching can influence the interpretation of such data. Textbook presentations often emphasize only how to analyze data in extremes, the absence of quenching or the presence of substantial quenching. Here, we consider intermediate cases where the assessment of photophysics (static quenching, blinking-like triplet-state relaxation) influence on autocorrelation curves can be delicate if dye-labeled objects diffuse on comparably rapid time scales. We used the amino acid, tryptophan, as the quencher. As our example of small-molecule dye that diffuses rapidly, we mix the quencher with the fluorescence dye, Alexa 488. The translational diffusion coefficient, inferred from fit to the standard one-component Fickian diffusion model, speeds up without the loss of quality of fit, but quenching is reflected in the fact that the data become exceptionally noisy. This reflects the bidisperse population of quenched and unquenched dyes when the time scales overlap between the processes of translational diffusion, quenching, and blinking. As our example of the large-molecule dye-labeled object that diffuses relatively slowly, we mixed the quencher with dye-labeled BSA, bovine serum albumin. Diffusion, static quenching, and blinking time scales are now separated. In spite of quenching contribution to the autocorrelation function when the delay time is relatively short, the inferred translational diffusion coefficient now depends weakly on the presence of a quencher. We conclude that when the diffusing molecule is substantially slower to diffuse than the time scale of photophysical processes of the fluorescent dye to which it is attached, the influence of quenching is self-evident and the FCS autocorrelation curves give an appropriate diffusion coefficient if correct fitting functions are chosen in the analysis.

Synthetic asters as elastic and radial skeletons

The radial geometry with rays radiated from a common core occurs ubiquitously in nature for its symmetry and functions. Herein, we report a class of synthetic asters with well-defined core-ray geometry that can function as elastic and radial skeletons to harbor nano- and microparticles. We fabricate the asters in a single, facile, and high-yield step that can be readily scaled up; specifically, amphiphilic gemini molecules self-assemble in water into asters with an amorphous core and divergently growing, twisted crystalline ribbons. The asters can spontaneously position microparticles in the cores, along the radial ribbons, or by the outer rims depending on particle sizes and surface chemistry. Their mechanical properties are determined on single- and multiple-aster levels. We further maneuver the synthetic asters as building blocks to form higher-order structures in virtue of aster-aster adhesion induced by ribbon intertwining. We envision the astral structures to act as rudimentary spatial organizers in nanoscience for coordinated multicomponent systems, possibly leading to emergent, synergistic functions.

Nanosystems capable of organisation and the formation of structures are of interest in nanotechnology. Here, the authors report on synthetic asters made of gemini surfactant which are able to position microparticles based on size and chemical composition which can also be organised into higher order structures.

Colloidal Flatlands Confronted with Urge for the Third Dimension

Two-dimensional sheets are a relatively neglected form of soft matter, interesting because of their capability to deform into the third dimension with little energy cost. Here, we confront colloidal sheets with an abruptly imposed potential tending to produce strings normal to the plane. Experimentally, this is implemented first by using ultrasound-induced acoustic levitation to produce planar sheets and then by abruptly imposing AC electric fields that introduce dipolar interactions. Seeking to identify the microscopic mechanisms underlying the observed collective behavior, we find that the patterns quantified from our fast confocal experimental imaging are reproduced by our Brownian dynamics simulations. We follow the evolution of these patterns, including their structure factor, from start to final steady state, and from successful parametrization we predict simulation phases not yet observed in experiment. The transient-state evolution toward final outcome includes monocrystalline hexagonal lattice, polycrystalline body-centered tetragonal lattice with grain boundaries, interconnected rings, serpentine zigzag chains, and columns vertical to the plane, and a "fat worm" serpentine pattern. To explain the counterintuitive findings presented here, we map dependence on softness of the confining potential.

Biologically-active unilamellar vesicles from red blood cells

We demonstrate a method to prepare giant unilamellar vesicles (GUVs) with biologically-active protein activity, by mixing erythrocyte (red blood cell) membrane extract with phospholipids and growing their mixture in a porous hydrogel matrix. This presents a pathway to retain protein biological activity without prior isolation and purification of the protein, though only the activity of the membrane protein GLUT1 is investigated to date. Using the cascade enzymatic reaction glucose oxidase and horseradish peroxidase to assay glucose concentration specifically within the GUV interior, we show that glucose is internalized by GLUT1 whereas adding cytochalasin B, a GLUT1 inhibitor, blocks glucose transport. The method presented here operates at biological ionic strength and is both simple and potentially generalizable.

Steering Coacervation by a Pair of Broad-Spectrum Regulators

Coacervation is liquid-liquid phase separation ubiquitous in industrial applications and cellular biology. Inspired by cellular manipulation of coacervate droplets such as P granules, we report here a regulatory strategy to manipulate synthetic coacervation in a spatiotemporally controllable manner. Two oppositely charged small molecules are shown to phase separate into coacervate droplets in water as a result of electrostatic attraction, hydrophobic effect, and entropy. We identify a down regulator, β-cyclodextrin, to disrupt the hydrophobic effect, thus dissolving the droplets, and an up regulator, amylase, to decompose β-cyclodextrin, thus restoring the droplets. The regulation kinetics is followed in real time on a single-droplet level, revealing diffusion-limited dissolution and reaction-limited condensation, respectively, taking ∼4 s and 2-3 min. Versatility of this strategy to manipulate the coacervation is demonstrated in two aspects: spatially distributed coacervation in virtue of amylase-grafted hydrogel frameworks and coacervate transportation across membranes and hydrogel networks via a disassemble-to-pass strategy. The current regulatory pairs and strategies are anticipated to be general for a wide variety of synthetic self-assembly systems.

Comparing Geometry and Chemistry When Confined Molecules Diffuse in Monodisperse Metal-Organic Framework Pores

The monodisperse pore structure of MOFs (metal-organic frameworks) is advantageous for investigating how porosity influences diffusion. Here we report translational and rotational diffusion using fluorescence correlation spectroscopy and time-correlated single-photon counting, using the three-dimensional pores of the zeolitic-like metal-organic framework family. We compare the influence of size and electric charge as well as dependence on pore size that we controlled through postsynthetic cation-exchange modifications. Charge-charge interactions with the MOF appeared to produce transient adsorption, manifested as a relatively fast and a slower diffusion process, but diffusants without net electric charge displayed a single diffusion process. Obtained from this family of guest molecules selected to be fluorescent, these findings suggest potentially useful general design rules to predict how pore size, guest size, and host-guest interaction control guest mobility within nanopores. With striking fidelity, diffusion coefficient scales with the ratio of cross-sectional areas of diffusant and host pores when charge is taken into account.

Deep line-temporal focusing with high axial resolution and a large field-of-view using intracavity control and incoherent pulse shaping

Line-temporal focusing has been recognized as an elegant strategy that provides two-photon microscopy with an effective means for fast imaging through parallelization, together with an improved resilience to scattering for deep imaging. However, the axial resolution remains sub-optimal, except when using high NA objectives and a small field-of-view. With the introduction of an intracavity control of the spectral width of the femtosecond laser to adaptively fill the back aperture of the objective lens, line-temporal focusing two-photon microscopy is demonstrated to reach near-diffraction-limited axial resolution with a large back-aperture objective lens, and improved immunity to sample scattering. In addition, a new incoherent flattop beam shaping method is proposed which provides a uniform contrast with little degradation of the axial resolution along the focus line, even deep in the sample. This is demonstrated in large volumetric imaging of mouse lung samples.

Substrate curvature affects the shape, orientation, and polarization of renal epithelial cells

The unique structure of kidney tubules is representative of their specialized function. Because maintaining tubular structure and controlled diameter is critical for kidney function, it is critical to understand how topographical cues, such as curvature, might alter cell morphology and biological characteristics. Here, we examined the effect of substrate curvature on the shape and phenotype of two kinds of renal epithelial cells (MDCK and HK-2) cultured on a microchannel with a broad range of principal curvature. We found that cellular architecture on curved substrates was closely related to the cell type-specific characteristics (stiffness, cell-cell adherence) of the cells and their density, as well as the sign and degree of curvature. As the curvature increased on convex channels, HK-2 cells, having lower cell stiffness and monolayer integrity than those of MDCK cells, aligned their in-plane axis perpendicular to the channel but did not significantly change in morphology. By contrast, MDCK cells showed minimal change in both morphology and alignment. However, on concave channels, both cell types were elongated and showed longitudinal directionality, although the changes in MDCK cells were more conservative. Moreover, substrate curvature contributed to cell polarization by enhancing the expression of apical and basolateral cell markers with height increase of the cells. Our study suggests curvature to be an important guiding principle for advanced tissue model developments, and that curved and geometrically ambiguous substrates can modulate the cellular morphology and phenotype.

STATEMENT OF SIGNIFICANCE

In many tissues, such as renal tubules or intestinal villi, epithelial layers exist in naturally curved forms, a geometry that is not reproduced by flat cultures. Because maintaining tubular structure is critical for kidney function, it is important to understand how topographical cues, such as curvature, might alter cell morphology and biological characteristics. We found that cellular architecture on curved substrates was closely related to cell type and density, as well as the sign and degree of the curvature. Moreover, substrate curvature contributed to cell polarization by enhancing the expression of apical and basolateral cell markers with height increase. Our results suggested that substrate curvature might contribute to cellular architecture and enhance the polarization of kidney tubule cells.

Longer-Lasting Electron-Based Microscopy of Single Molecules in Aqueous Medium

Use of electron-based microscopy in aqueous media has been held back because aqueous samples tend to suffer from water radiolysis and other chemical degradation caused by the high energy of incident electrons. Here we show that aqueous liquid pockets in graphene liquid cells at room temperature display significantly improved stability when using deuterated water, D₂O. Reporting transmission electron microscopy (TEM) experiments based on common imaging conditions, we conclude that use of D₂O outperforms adding radical scavengers to H₂O regardless of imaging details; it increases the lifetime of dissolved organic macromolecules by a factor of 2-5, and it delays by even longer the appearance of radiolysis-induced bubbles, by a factor of time up to 10. We quantify statistically the consequences of minimizing the electron voltage and dose and conclude that the D₂O environment increases sample longevity without noticeable sacrifice of contrast that is critical for direct imaging of weakly scattering organic macromolecules and biomolecules.

Cloaked Exosomes: Biocompatible, Durable, and Degradable Encapsulation

Exosomes-nanosized extracellular vesicles (EVs) naturally secreted from cells-have emerged as promising biomarkers and potential therapeutic vehicles, but methods to manipulate them for engineering purposes remain elusive. Among the technical obstacles are the small size and surface complexity of exosomes and the complex processing steps required, which reduce the biocompatibility of currently available methods. The encapsulation of exosomes with a nanofilm of supramolecular complexes of ferric ions (Fe³⁺ ) and tannic acid is demonstrated here. The resulting natural polyphenol, ≈10 nm thick, protects exosomes from external aggressors such as UV-C irradiation or heat and is controllably degraded on demand. Furthermore, gold nanoparticles can be covalently attached for single-exosome level visualization. To fully exploit their therapeutic potential, chemotherapeutic drug-loaded EVs are functionalized to achieve the targeted, selective killing of cancer cells preferentially over normal cells. This nanofilm not only preserves the native size and chemical makeup of the intrinsic exosomes, but also confers new capabilities for efficient tumor targeting and pH-controlled release of drugs. Demonstrating a scalable method to produce biocompatible, durable, on-demand degradable, and chemically controllable shields for exosome modification and functionalization, the methods introduced here are expected to bring the potential of exosome-based nanomedicine applications closer to reality.

How to better focus waves by considering symmetry and information loss

We amend the general belief that waves with extended spherical wavefront focus at their center of curvature. Instead, when the spherical symmetry of waves is broken by propagating them through a finite aperture along an average direction, the forward/backward symmetry is broken and the focal volume shifts its center backward along that direction. The extent of this focal shift increases as smaller apertures are used, up to the point that the nominal focal plane is out of focus. Furthermore, the loss of axial symmetry with noncircular apertures causes distinct focal shifts in distinct axial planes, and the resulting astigmatism possibly degrades the axial focusing resolution. Using experiments and simulations, focal shift with noncircular apertures is described for classical and temporal focusing. The usefulness of these conclusions to improve imaging resolution is demonstrated in a high-resolution optical microscopy application, namely line-temporal focusing microscopy. These conclusions follow from fundamental symmetries of the wave geometry and matter for an increasing number of emerging optical techniques. This work offers a general framework and strategy to understand and improve virtually any wave-based application whose efficacy depends on optimal focusing and may be helpful when information is transmitted by waves in applications from electromagnetic communications, to biological and astronomical imaging, to lithography and even warfare.

From dynamic self-assembly to networked chemical systems

Although dynamic self-assembly, DySA, is a relatively new area of research, the past decade has brought numerous demonstrations of how various types of components - on scales from (macro)molecular to macroscopic - can be arranged into ordered structures thriving in non-equilibrium, steady states. At the same time, none of these dynamic assemblies has so far proven practically relevant, prompting questions about the field's prospects and ultimate objectives. The main thesis of this Review is that formation of dynamic assemblies cannot be an end in itself - instead, we should think more ambitiously of using such assemblies as control elements (reconfigurable catalysts, nanomachines, etc.) of larger, networked systems directing sequences of chemical reactions or assembly tasks. Such networked systems would be inspired by biology but intended to operate in environments and conditions incompatible with living matter (e.g., in organic solvents, elevated temperatures, etc.). To realize this vision, we need to start considering not only the interactions mediating dynamic self-assembly of individual components, but also how components of different types could coexist and communicate within larger, multicomponent ensembles. Along these lines, the review starts with the discussion of the conceptual foundations of self-assembly in equilibrium and non-equilibrium regimes. It discusses key examples of interactions and phenomena that can provide the basis for various DySA modalities (e.g., those driven by light, magnetic fields, flows, etc.). It then focuses on the recent examples where organization of components in steady states is coupled to other processes taking place in the system (catalysis, formation of dynamic supramolecular materials, control of chirality, etc.). With these examples of functional DySA, we then look forward and consider conditions that must be fulfilled to allow components of multiple types to coexist, function, and communicate with one another within the networked DySA systems of the future. As the closing examples show, such systems are already appearing heralding new opportunities - and, to be sure, new challenges - for DySA research.

Active colloids with collective mobility status and research opportunities

The collective mobility of active matter (self-propelled objects that transduce energy into mechanical work to drive their motion, most commonly through fluids) constitutes a new frontier in science and achievable technology. This review surveys the current status of the research field, what kinds of new scientific problems can be tackled in the short term, and what long-term directions are envisioned. We focus on: (1) attempts to formulate design principles to tailor active particles; (2) attempts to design principles according to which active particles interact under circumstances where particle-particle interactions of traditional colloid science are augmented by a family of nonequilibrium effects discussed here; (3) attempts to design intended patterns of collective behavior and dynamic assembly; (4) speculative links to equilibrium thermodynamics. In each aspect, we assess achievements, limitations, and research opportunities.

Enzyme leaps fuel antichemotaxis

There is mounting evidence that enzyme diffusivity is enhanced when the enzyme is catalytically active. Here, using superresolution microscopy [stimulated emission-depletion fluorescence correlation spectroscopy (STED-FCS)], we show that active enzymes migrate spontaneously in the direction of lower substrate concentration ("antichemotaxis") by a process analogous to the run-and-tumble foraging strategy of swimming microorganisms and our theory quantifies the mechanism. The two enzymes studied, urease and acetylcholinesterase, display two families of transit times through subdiffraction-sized focus spots, a diffusive mode and a ballistic mode, and the latter transit time is close to the inverse rate of catalytic turnover. This biochemical information-processing algorithm may be useful to design synthetic self-propelled swimmers and nanoparticles relevant to active materials. Executed by molecules lacking the decision-making circuitry of microorganisms, antichemotaxis by this run-and-tumble process offers the biological function to homogenize product concentration, which could be significant in situations when the reactant concentration varies from spot to spot.

Cell migration in microengineered tumor environments

Recent advances in microengineered cell migration platforms are discussed critically with a focus on how cell migration is influenced by engineered tumor microenvironments, the medical relevance being to understand how tumor microenvironments may promote or suppress the progression of cancer. We first introduce key findings in cancer cell migration under the influence of the physical environment, which is systematically controlled by microengineering technology, followed by multi-cues of physico-chemical factors, which represent the complexity of the tumor environment. Recognizing that cancer cells constantly communicate not only with each other but also with tumor-associated cells such as vascular, fibroblast, and immune cells, and also with non-cellular components, it follows that cell motility in tumor microenvironments, especially metastasis via the invasion of cancer cells into the extracellular matrix and other tissues, is closely related to the malignancy of cancer-related mortality. Medical relevance of forefront research realized in microfabricated devices, such as single cell sorting based on the analysis of cell migration behavior, may assist personalized theragnostics based on the cell migration phenotype. Furthermore, we urge development of theory and numerical understanding of single or collective cell migration in microengineered platforms to gain new insights in cancer metastasis and in therapeutic strategies.

Metal-Organic Framework "Swimmers" with Energy-Efficient Autonomous Motility

Placed at a water/air interface, particles of porphyrin-based MOFs (metal-organic frameworks) cut from large-area films display efficient, multiple-use autonomous motility powered by release of solvents incorporated in the MOF matrix and directionality dictated by their shapes. The particles can be refueled multiple times and can achieve speeds of ca. 200 mm·s^-1 with high kinetic energy per unit of chemical "fuel" expended (>50 μJ·g^-1). Efficiency of motion depends on the nature of the fuel used as well as the microstructure and surface wettability of the MOF surface. When multiple movers are present at the interface, they organize into "open" structures that exhibit collective, time-periodic motions.

Liquid-Cell Electron Microscopy of Adsorbed Polymers

Individual macromolecules of polystyrene sulfonate and poly(ethylene oxide) are visualized with nanometer resolution using transmission electron microscopy (TEM) imaging of aqueous solutions with and without added salt, trapped in liquid pockets between creased graphene sheets. Successful imaging with 0.3 s per frame is enabled by the sluggish mobility of the adsorbed molecules. This study finds, validating others, that an advantage of this graphene liquid-cell approach is apparently to retard sample degradation from incident electrons, in addition to minimizing background scattering because graphene windows are atomically thin. Its new application here to polymers devoid of metal-ion labeling allows the projected sizes and conformational fluctuations of adsorbed molecules and adsorption-desorption events to be analyzed. Confirming the identification of the observed objects, this study reports statistical analysis of datasets of hundreds of images for times up to 100 s, with variation of the chemical makeup of the polymer, the molecular weight of the polymer, and the salt concentration. This observation of discrete polymer molecules in solution environment may be useful generally, as the findings are obtained using an ordinary TEM microscope, whose kind is available to many researchers routinely.

Janus Particle Synthesis, Assembly, and Application

Janus particles are colloidal particles with more than a single type of surface chemistry or composition, ranging in size from hundreds of nanometers to a few micrometers. Like traditional colloids, they are large enough to be observed under optical microscopy in real time and small enough to diffuse by Brownian motion, but their interesting and useful new properties of directional interaction bring new research opportunities to the fields of soft matter and fundamental materials research as well as to applications in other disciplines and in technologies such as electronic paper and other multiphase engineering. In this review, a variety of methods that have been used to synthesize Janus particles are introduced. Following this, we summarize the use of Janus particles as basic units that assemble into novel structures and tune important material properties. The concluding sections highlight some of the technological applications, including recent progress in using Janus particles as microprobes, micromotors, electronic paper, and solid surfactants.

Giant capsids from lattice self-assembly of cyclodextrin complexes

Proteins can readily assemble into rigid, crystalline and functional structures such as viral capsids and bacterial compartments. Despite ongoing advances, it is still a fundamental challenge to design and synthesize protein-mimetic molecules to form crystalline structures. Here we report the lattice self-assembly of cyclodextrin complexes into a variety of capsid-like structures such as lamellae, helical tubes and hollow rhombic dodecahedra. The dodecahedral morphology has not hitherto been observed in self-assembly systems. The tubes can spontaneously encapsulate colloidal particles and liposomes. The dodecahedra and tubes are respectively comparable to and much larger than the largest known virus. In particular, the resemblance to protein assemblies is not limited to morphology but extends to structural rigidity and crystallinity—a well-defined, 2D rhombic lattice of molecular arrangement is strikingly universal for all the observed structures. We propose a simple design rule for the current lattice self-assembly, potentially opening doors for new protein-mimetic materials.

Designing and synthesizing protein mimetic molecules to form crystalline structures can be a challenge. Here the authors show lattice self-assembly of cyclodextrin complexes into a variety of capsid like structures, such as, lamellae, helical tubes and hollow rhombic dodecahedra.

Vector assembly of colloids on monolayer substrates

The key to spontaneous and directed assembly is to encode the desired assembly information to building blocks in a programmable and efficient way. In computer graphics, raster graphics encodes images on a single-pixel level, conferring fine details at the expense of large file sizes, whereas vector graphics encrypts shape information into vectors that allow small file sizes and operational transformations. Here, we adapt this raster/vector concept to a 2D colloidal system and realize 'vector assembly' by manipulating particles on a colloidal monolayer substrate with optical tweezers. In contrast to raster assembly that assigns optical tweezers to each particle, vector assembly requires a minimal number of optical tweezers that allow operations like chain elongation and shortening. This vector approach enables simple uniform particles to form a vast collection of colloidal arenes and colloidenes, the spontaneous dissociation of which is achieved with precision and stage-by-stage complexity by simply removing the optical tweezers.

Real-Space, in Situ Maps of Hydrogel Pores

We characterize the porosity of hydrogels by imaging the displacement trajectories of embedded tracer particles. This offers the possibility of characterizing the size and projected shape of individual pores as well as direct, real-space maps of heterogeneous porosity and its distribution. The scheme shows that when fluorescent spherical particles treated to avoid specific adsorption are loaded into the gel, their displacement trajectories from Brownian motion report on the size and projected shape in which the pore resides, convoluted by the particle size. Of special interest is how pores and their distribution respond to stimuli. These ideas are validated in agarose gels loaded with latex particles stabilized by adsorbed bovine serum albumin. Gels heated from room temperature produced an increasingly more monodisperse pore size distribution because increasing temperature preferentially enlarges smaller pores, but this was irreversible upon cooling, and shearing agarose gels beyond the yield point destroyed larger pores preferably. The method is considered to be generalizable beyond the agarose system presented here as proof of concept.

Reconfiguring active particles by electrostatic imbalance

Active materials represent a new class of condensed matter in which motile elements may collectively form dynamic, global structures out of equilibrium. Here, we present a general strategy to reconfigure active particles into various collective states by introducing imbalanced interactions. We demonstrate the concept with computer simulations of self-propelled colloidal spheres, and experimentally validate it in a two-dimensional (2D) system of metal-dielectric Janus colloids subjected to perpendicular a.c. electric fields. The mismatched, frequency-dependent dielectric responses of the two hemispheres of the colloids allow simultaneous control of particle motility and colloidal interactions. We realized swarms, chains, clusters and isotropic gases from the same precursor particle by changing the electric-field frequency. Large-scale polar waves, vortices and jammed domains are also observed, with the persistent time-dependent evolution of their collective structure evoking that of classical materials. This strategy of asymmetry-driven active self-organization should generalize rationally to other active 2D and three-dimensional (3D) materials.