Static Assemblies Research Articles

A “Transformers”‐like nanochain for precise navigation and efficient cancer treatment

AbstractIntegrated multimodal imaging in theranostics nanomaterials offers extensive prospects for precise and noninvasive cancer treatment. Precisely controlling the structural evolution of plasmonic nanoparticles is crucial in the development of photothermal agents. However, previous successes have been limited to static assemblies and single‐component structures. Here, an activatable plasmonic theranostics system utilizing self‐assembled 1D silver‐coated gold nanochains (1D nanochains) is presented for precise tumor diagnosis and effective treatment. The absorbance of the adaptable core–shell chain structure can shift from visible to near‐infrared (NIR) regions due to the fusion between nearby Au@Ag nanoparticles induced by elevated H2O2 levels in the tumor microenvironment (TME), resulting in the creation of a novel 3D aggregates with strong NIR absorption. With a high photothermal conversion efficiency of 60.2% at 808 nm, nanochains utilizing the TME‐activated characteristics show remarkable qualities for photoacoustic imaging and significantly limit tumor growth in vivo. This study may pave the way for precise tumor diagnosis and treatment through customizable, optically tunable adaptive plasmonic nanostructures.

Purely STDP-based assembly dynamics: Stability, learning, overlaps, drift and aging.

Memories may be encoded in the brain via strongly interconnected groups of neurons, called assemblies. The concept of Hebbian plasticity suggests that these assemblies are generated through synaptic plasticity, strengthening the recurrent connections within select groups of neurons that receive correlated stimulation. To remain stable in absence of such stimulation, the assemblies need to be self-reinforcing under the plasticity rule. Previous models of such assembly maintenance require additional mechanisms of fast homeostatic plasticity often with biologically implausible timescales. Here we provide a model of neuronal assembly generation and maintenance purely based on spike-timing-dependent plasticity (STDP) between excitatory neurons. It uses irregularly and stochastically spiking neurons and STDP that depresses connections of uncorrelated neurons. We find that assemblies do not grow beyond a certain size, because temporally imprecisely correlated spikes dominate the plasticity in large assemblies. Assemblies in the model can be learned or spontaneously emerge. The model allows for prominent, stable overlap structures between static assemblies. Further, assemblies can drift, particularly according to a novel, transient overlap-based mechanism. Finally the model indicates that assemblies grow in the aging brain, where connectivity decreases.

Turning on hotspots: supracolloidal SERS probes made brilliant by an external activation mechanism.

We achieved external activation of local hot-spot sites in supracolloidal assembly structures. The concept was demonstrated by boosting surface-enhanced Raman scattering (SERS) efficiency by one order of magnitude through a heating-induced process. Our approach involves assembling gold nanoparticles with distinct dimensions, i.e. 16 and 80 nm, into well-defined planet-satellite-type arrangement structures using thermoresponsive (poly(N-isopropylacrylamide)) star polymer linkers. Insights into the assembly process were obtained by calculations within the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory framework. We observe one order of magnitude increase in SERS enhancement by a heating-induced volume-phase transition. This magnification aligns with simulations run using the finite-difference time-domain (FDTD) method. The implications of this adaptive supracolloidal concept are twofold: Firstly, our approach bypasses limitations of existing systems that are associated with the limited accessibility of electromagnetic hot-spot sites in strongly coupled, static assemblies of plasmonic nanoparticles, by providing the capability of dynamic hot-spot re-configuration. Second, these externally activated probes offer promising opportunities for the development of messenger materials and associated sensing strategies.

Fluid-particle heat transfer in static assemblies: Effect of particle shape

The heat transfer in fluid flow through static assemblies of randomly distributed spheroids, including spheres, prolates, and oblates, is investigated using direct numerical simulation (DNS). Constant heat fluxes are imposed at individual particle surface by the Neumann boundary condition for temperature. The superficial Reynolds number (Re) and the solid volume fraction (φ) range from 2.5 to 200, and from 0.1 to 0.3, respectively. The simulation results for spheres and prolates agree well with the DNS data and correlations in literature and new correlations for prolates and oblates are proposed. With these results, the particle shape effect on the average Nusselt number (Nuavg) is described more comprehensively: At low φ (e.g., 0.1), the Nuavg for prolates and oblates are higher than that of spheres, and as Re increases, the Nuavg of oblates gradually catches up that of prolates and becomes comparable at Re≈200. Moreover, oblates attenuate and intensify the heat transfer at low and high Re at high φ, respectively.

Hydrodynamic forces on monodisperse assemblies of axisymmetric elongated particles: Orientation and voidage effects

We investigate the average drag, lift, and torque on static assemblies of capsule‐like particles of aspect ratio 4. The performed simulations are from Stokes flow to high Reynolds numbers (0.1 ≤ Re ≤ 1,000) at different solids volume fraction (0.1 ≤ ɛ s ≤ 0.5). Individual particle forces as a function of the incident angle ϕ with respect to the average flow are scattered. However, the average particle force as a function of ϕ is found to be independent of mutual particle orientations for all but the highest volume fractions. On average, a sine‐squared scaling of drag and sine‐cosine scaling of lift holds for static multiparticle systems of elongated particles. For a packed bed, our findings can be utilized to compute the pressure drop with knowledge of the particle‐orientation distribution, and the average particle drag at ϕ = 0° and 90°. We propose closures for average forces to be used in Euler–Lagrange simulations of particles of aspect ratio 4.

Active Patchy Colloids with Shape-Tunable Dynamics.

Controlling the complex dynamics of active colloids-the autonomous locomotion of colloidal particles and their spontaneous assembly-is challenging yet crucial for creating functional, out-of-equilibrium colloidal systems potentially useful for nano- and micromachines. Herein, by introducing the synthesis of active "patchy" colloids of various low-symmetry shapes, we demonstrate that the dynamics of such systems can be precisely tuned. The low-symmetry patchy colloids are made in bulk via a cluster-encapsulation-dewetting method. They carry essential information encoded in their shapes (particle geometry, number, size, and configurations of surface patches, etc.) that programs their locomotive and assembling behaviors. Under AC electric field, we show that the velocity of particle propulsion and the ability to brake and steer can be modulated by having two asymmetrical patches with various bending angles. The assembly of monopatch particles leads to the formation of dynamic and reconfigurable structures such as spinners and "cooperative swimmers" depending on the particle's aspect ratios. A particle with two patches of different sizes allows for "directional bonding", a concept popular in static assemblies but rare in dynamic ones. With the capability to make tunable and complex shapes, we anticipate the discovery of a diverse range of new dynamics and structures when other external stimuli (e.g., magnetic, optical, chemical, etc.) are employed and spark synergy with shapes.

Results of Three Neutron Diagnosed Subcritical Experiments

Neutron diagnosed subcritical experiments (NDSEs) aim to measure the fission chain decay of subcritical test objects initiated by neutrons from an external source. The ultimate goal of future NDSEs is to make such measurements on dynamic subcritical objects as they are imploded. As a step toward that goal, we made measurements on three static subcritical assemblies containing highly enriched uranium. Specifically, we measured the die-off of both fission gamma rays and neutrons, with nanosecond resolution over several hundred microseconds, relative to the emission time of neutrons from a source in close proximity to the subcritical objects. As simulations are expected to play a key role in interpreting future NDSEs, we compared our data to detailed MCNP® calculations. There was good agreement between the data and MCNP over die-off times expected to be most important to future NDSEs, but there were significant differences at both earlier and later times. We attempted to reconcile these differences by several changes to the simulations. In particular, we found that reducing the 235U(n,n’) cross section to 80% of the current ENDF7.1 evaluation resulted in much better agreement.

The nanotechnology of life-inspired systems.

For some decades now, nanotechnology has been touted as the 'next big thing' with potential impact comparable to the steam, electricity or Internet revolutions - but has it lived up to these expectations? While advances in top-down nanolithography, now reaching 10-nm resolution, have resulted in devices that are rapidly approaching mass production, attempts to produce nanoscale devices using bottom-up approaches have met with only limited success. We have been inundated with nanoparticles of almost any shape, material and composition, but their societal impact has been far from revolutionary, with growing concerns over their toxicity. Despite nebulous hopes that making hierarchical nanomaterials will lead to new, emergent properties, no breakthrough applications seem imminent. In this Perspective, we argue that the time is ripe to look beyond individual nano-objects and their static assemblies, and instead focus on systems comprising different types of 'nanoparts' interacting and/or communicating with one another to perform desired functions. Such systems are interesting for a variety of reasons: they can act autonomously without external electrical or optical connections, can be dynamic and reconfigurable, and can act as 'nanomachines' by directing the flow of mass, energy or information . In thinking how this systems nanoscience approach could be implemented to design useful - as opposed to toy-model - nanosystems, our choice of applications and our nanoengineering should be inspired by living matter.

A dynamin-like protein involved in bacterial cell membrane surveillance under environmental stress.

In ever-changing natural environments, bacteria are continuously challenged with numerous biotic and abiotic stresses. Accordingly, they have evolved both specific and more general mechanisms to counteract stress-induced damage and ensure survival. In the soil habitat of Bacillus subtilis, peptide antibiotics and bacteriophages are among the primary stressors that affect the integrity of the cytoplasmic membrane. Dynamin-like proteins (DLPs) play a major role in eukaryotic membrane re-modelling processes, including antiviral activities, but the function of the corresponding bacterial homologues was so far poorly understood. Here, we report on the protective function of a bacterial DLP, DynA from B. subtilis. We provide evidence that DynA plays an important role in a membrane surveillance system that counteracts membrane pore formation provoked by antibiotics and phages. In unstressed cells, DynA is a highly dynamic membrane-associated protein. Upon membrane damage, DynA localizes into large and static assemblies, where DynA acts locally to counteract stress-induced pores, presumably by inducing lipid bilayer fusion and sealing membrane gaps. Thus, lack of DynA increases the sensitivity to antibiotic exposure and phage infection. Taken together, our work suggests that DynA, and potentially other bacterial DLPs, contribute to the innate immunity of bacteria against membrane stress.

The Bacterial SMC Complex Displays Two Distinct Modes of Interaction with the Chromosome

The bacterial SMC (structural maintenance of chromosomes) complex binds nonspecifically to DNA invitro and forms two discrete subcellular centers invivo, one in each cell half. How this distribution is maintained is unclear. We show by time-lapse imaging of single molecules that the localization is achieved through limited, yet rapid movement of the SMC subunits through the nucleoid. Accessory ScpAB subunits mediate the arrest of 20% of SMC molecules at the center of a cell half and do not move together with the 80% mobile SMC molecules. Only free SMC, but not the preformed SMC/ScpAB complex, was able to bind to DNA invitro, revealing distinct functions of SMC fractions. Thus, whereas SMC alone dynamically interacts with many sites on the chromosome, it forms static assemblies together with ScpAB complex partners. Our findings reveal two distinct modes of interaction of SMC with the chromosome and indicate that limited diffusion within a confined space and transient arrest may be a general mechanism for positioning proteins within a chromosome and within a noncompartmentalized cell.

Mechanical masking of films deposited by atomic layer deposition

In this work a new method to selectively deposit films by atomic layer deposition (ALD) is presented. It was found that polished silicon masks pressed against silicon substrates were able to mask ALD deposition with submicron diffusion under the mask. Static and dynamic assemblies were fabricated to realize the benefits of mechanical masking. The static assemblies demonstrated the ability to block deposition on the back sides of transmission electron microscopy grids, as well as the back sides of 100 mm silicon wafers. The dynamic masking assembly was able to selectively deposit platinum, and then passivate the metal region with zirconium oxide in situ, resulting in a fully embedded metal in dielectric structure.

Direct Linearization Method Kinematic Variation Analysis

Velocity and acceleration analysis is an important tool for predicting the motion of mechanisms. The results, however, may be inaccurate when applied to manufactured products due to the process variations that occur in production. Small changes in mechanism dimensions can accumulate and propagate, causing a significant variation in the performance of the mechanism. A new application of statistical analysis is presented for predicting the effects of variation on mechanism kinematic performance. The new method is an extension of the direct linearization method developed for static assemblies. This method provides a solution that is a closed form. It may be applied to two-dimensional mechanisms to predict variation in velocity and acceleration due to dimensional variations. It is also shown how form, orientation, and position variations may be included in the analysis to analyze variations that occur within the joints. Only two assemblies are analyzed to characterize the distribution: The first determines the mean, and the second estimates the variance. The system is computationally efficient and well suited for design iteration.

Transmission of stresses in static and sheared granular beds: The influence of particle size, shearing rate, layer thickness and sensor size

The spatial non-uniformities and temporal fluctuations in the normal stress transmitted across a sheared granular layer have been studied through a combination of experiments in a Jenike shear cell equipped with normal force (stress) transducers imbedded on the bottom shearing surface and discrete element method (DEM) simulations. Experiments were carried out with particles of different sizes and layers of different thicknesses; the normal stress was measured at several different shearing rates and at several positions on the bottom surface. The DEM simulations revealed a direct link between the spatial inhomogeneities and temporal fluctuations in the stress recorded in our measurements. We found that the dependence of the average normal stress on the bottom surface as a function of height mirrored that in Janssen's analysis of stresses in wall-bounded static assemblies.

Internal states of model isotropic granular packings. I. Assembling process, geometry, and contact networks

This is the first paper of a series of three, in which we report on numerical simulation studies of geometric and mechanical properties of static assemblies of spherical beads under an isotropic pressure. The influence of various assembling processes on packing microstructures is investigated. It is accurately checked that frictionless systems assemble in the unique random close packing (RCP) state in the low pressure limit if the compression process is fast enough, higher solid fractions corresponding to more ordered configurations with traces of crystallization. Specific properties directly related to isostaticity of the force-carrying structure in the rigid limit are discussed. With frictional grains, different preparation procedures result in quite different inner structures that cannot be classified by the sole density. If partly or completely lubricated they will assemble like frictionless ones, approaching the RCP solid fraction Phi_{RCP} approximately 0.639 with a high coordination number: z* approximately =6 on the force-carrying backbone. If compressed with a realistic coefficient of friction mu=0.3 packings stabilize in a loose state with Phi approximately 0.593 and z* approximately =4.5 . And, more surprisingly, an idealized "vibration" procedure, which maintains an agitated, collisional regime up to high densities results in equally small values of z* while Phi is close to the maximum value Phi_{RCP}. Low coordination packings have a large proportion (>10%) of rattlers--grains carrying no force--the effect of which should be accounted for on studying position correlations, and also contain a small proportion of localized "floppy modes" associated with divalent grains. Low-pressure states of frictional packings retain a finite level of force indeterminacy even when assembled with the slowest compression rates simulated, except in the case when the friction coefficient tends to infinity. Different microstructures are characterized in terms of near neighbor correlations on various scales, and some comparisons with available laboratory data are reported, although values of contact coordination numbers apparently remain experimentally inaccessible.

Entropy and Temperature of a Static Granular Assembly: AnAb InitioApproach

We construct a statistical framework for static assemblies of deformable grains which parallels that of equilibrium statistical mechanics but with a conservation principle based on the mechanical stress tensor. We define a state function that has all the attributes of entropy. In particular, maximizing this function leads to a well-defined granular temperature and the equivalent of the Boltzman distribution for ensembles of grain packings. Predictions of the ensemble are verified against simulated packings of frictionless, deformable disks.

Nanomolecular and Supramolecular Paths toward Peptidoglycan Structure

The chemistry of the supramolecular interaction—the study of the noncovalent forces directing intermolecular assembly—is new. In contrast, microbiological study of molecular assemblies is longstanding. Moreover, and in contrast to the static bimolecular assemblies that constitute the chemical supramolecular frontier, the molecular assemblies of the microorganism are dynamic and polymolecular. The difference in complexity between the two is staggering, and it would be understandable were it thought that the intersection of supramolecular chemistry and microbiology is in the distant future. In small but significant ways, however, the paths toward this intersection are being identified.

Analysis of the evolution of granular stress-strain and voidage states based on DEM simulations.

We review and discuss the results of our granular-dynamics simulations of the time evolution of the microstructure of compact granular beds as found in pouring, in hopper filling and discharge, and in a shear cell. These systems are mainly quasi-static. However, it is also common to encounter localized 'shear zones' with significant velocity/voidage fluctuations and high bulk-strain gradients. These narrow-banded zones are separated from near-static regions by sharp, discontinuous changes of bulk stress and voidage. Within these bands the granular assembly undergoes a transition from the quasi-static to the inertial state, where enduring particle contacts are increasingly replaced by collisional ones. We focus on the discrete particle origins of this inhomogeneous yield/flow behaviour. We show the usefulness of analysing the local evolution in terms of relative rotation of the grains which is observed to cause rapid local bulk dilation responsible for setting off avalanches near free-surface boundaries and protracted bulk-failure planes in confined static assemblies. We also present some evidence to suggest that allowing for effective continuous particle-particle interactions could approximate observed effects attributable to particle shape and surface roughness. Wavelet analyses have been applied successfully to generate the variations in periodicity and the relative sequence of evolution of the stress, strain-rate and voidage states in avalanching granular heaps and in the wall region of axially symmetric hopper flows.

Mechanical response of a static granular piling

ABSTRACTWe present the results of different experiments which aim is to investigate the response of a static granular assemblies to various local perturbations. We design a sensitive experimental method to probe the response of a localized stress at the top of a granular piling. The piling is an horizontal 3D granular layer confined in a box and the spatial distribution of loads at the bottom is monitored. This analysis defines some crucial experimental test to inform a currently debated issue on the mechanical status of static granular assemblies.

Analysis of Disjoint Two-Dimensional Particle Assemblies

This paper describes a new discrete algorithm for analyzing the behavior of two-dimensional (2D) assemblies of disjoint particles. The algorithm treats each particle as a 2D polygon, and generates contact normal and shear forces between two contacting particles when inference (overlap) occurs. The key new feature of the algorithm is a very efficient and effective multilevel contact detection scheme that makes it possible to track the behavior of much larger assemblies of polygonal particles than has been possible in the past. Examples involving large assemblies of particles are presented to show the capabilities of the method for (1) analyzing quasi static assemblies of particles subjected to stress (such as soil or rock masses), and (2) investigating dynamic particulate flows (such as flows through hoppers and gates). The algorithm may be used to conduct analyses of slip and failure processes in geologic materials and for investigating fracture and fatigue of metals.

Latest developments on valve seat-seal assembly

The Helicoflex® metallic seal initially designed for static assemblies is presently used as a valve primary seal for specific applications where the elastomeric seal commonly used in such a configuration is totally unsuitable owing to low resistance to radiation and high temperature, and incompatibility with ultrahigh vacuum conditions. The hydrogen injection quick valves of the JET fusion reactor, as well as the aluminium ultrahigh vacuum valves of the TRISTAN accelerator already operate using this technology over a rather narrow range of working conditions. In order to widen the field of applications and establish the basic principles of the sealing function applied to a valve seat-seal assembly, tests were carried out in the Sealing Techniques Laboratory of the French Atomic Energy Commission (CEA). The seat configuration and seal holding device have been reconsidered. Seals have been tested using different grades of sealing lining material, i.e. successively aluminium, silver and copper. In this respect, such an assembly configuration was found to be in full compliance with the requirements of the new types of valve, those which are being considered for applications where ultrahigh vacuum is combined with a leak rate requirement of 10 −10 Pa m 3 s −1 and a high bake-out temperature.