Measuring response functions of active materials from data.

From flocks of birds to biomolecular assemblies, systems in which many individual components independently consume energy to perform mechanical work exhibit a wide array of striking behaviors. Methods to quantify the dynamics of these so-called active systems generally aim to extract important length or time scales from experimental fields. Because such methods focus on extracting scalar values, they do not wring maximal information from experimental data. We introduce a method to overcome these limitations. We extend the framework of correlation functions by taking into account the internal headings of displacement fields. The functions we construct represent the material response to specific types of active perturbation within the system. Utilizing these response functions we query the material response of disparate active systems composed of actin filaments and myosin motors, from model fluids to living cells. We show we can extract critical length scales from the turbulent flows of an active nematic, anticipate contractility in an active gel, distinguish viscous from viscoelastic dissipation, and even differentiate modes of contractility in living cells. These examples underscore the vast utility of this method which measures response functions from experimental observations of complex active systems.

Limiting pool and actin architecture controls myosin cluster sizes in adherent cells.

The actomyosin cytoskeleton generates mechanical forces that power important cellular processes, such as cell migration, cell division, and mechanosensing. Actomyosin self-assembles into contractile networks and bundles that underlie force generation and transmission in cells. A central step is the assembly of the myosin II filament from myosin monomers, regulation of which has been extensively studied. However, myosin filaments are almost always found as clusters within the cell cortex. While recent studies characterized cluster nucleation dynamics at the cell periphery, how myosin clusters grow on stress fibers remains poorly characterized. Here, we utilize a U2OS osteosarcoma cell line with endogenously tagged myosin II to measure the myosin cluster size distribution in the lamella of adherent cells. We find that myosin clusters can grow with Rho-kinase (ROCK) activity alone in the absence of myosin motor activity. Time-lapse imaging reveals that myosin clusters grow via increased myosin association to existing clusters, which is potentiated by ROCK-dependent myosin filament assembly. Enabling myosin motor activity allows further myosin cluster growth through myosin association that is dependent on F-actin architecture. Using a toy model, we show that myosin self-affinity is sufficient to recapitulate the experimentally observed myosin cluster size distribution, and that myosin cluster sizes are determined by the pool of myosin available for cluster growth. Together, our findings provide new insights into the regulation of myosin cluster sizes within the lamellar actomyosin cytoskeleton.

Swimmers at interfaces enhance interfacial transport.

The behavior of fluid interfaces far from equilibrium plays central roles in nature and in industry. Active swimmers trapped at interfaces can alter transport at fluid boundaries with far reaching implications. Swimmers can become trapped at interfaces in diverse configurations and swim persistently in these surface adhered states. The self-propelled motion of bacteria makes them ideal model swimmers to understand such effects. We have recently characterized the swimming of interfacially trapped Pseudomonas aeruginosa PA01 moving in pusher mode. The swimmers adsorb at the interface with pinned contact lines, which fix the angle of the cell body at the interface and constrain their motion. Thus, swimmers become trapped at interfaces in diverse configurations and swim persistently in these surface adhered states. We observe that most interfacially trapped bacteria swim along circular paths. Fluid interfaces also typically form incompressible two-dimensional layers. These effects influence the flow generated by the swimmers. In our previous work, we have visualized the interfacial flow around a pusher bacterium and described the flow field using two dipolar hydrodynamic modes; one stresslet mode whose symmetries differ from those in bulk, and another bulk mode unique to incompressible fluid interfaces. Based on this understanding, swimmer-induced tracer displacements and swimmer-swimmer pair interactions are explored using analysis and experiment. The settings in which multiple interfacial swimmers with circular motion can significantly enhance interfacial transport of tracers or promote mixing of other swimmers on the interface are identified through simulations and compared to experiment. This study shows the importance of biomixing by swimmers at fluid interfaces and identifies important factors in the design of biomimetic active colloids to enhance interfacial transport.

Motor crosslinking augments elasticity in active nematics.

In active materials, uncoordinated internal stresses lead to emergent long-range flows. An understanding of how the behavior of active materials depends on mesoscopic (hydrodynamic) parameters is developing, but there remains a gap in knowledge concerning how hydrodynamic parameters depend on the properties of microscopic elements. In this work, we combine experiments and multiscale modeling to relate the structure and dynamics of active nematics composed of biopolymer filaments and molecular motors to their microscopic properties, in particular motor processivity, speed, and valency. We show that crosslinking of filaments by both motors and passive crosslinkers not only augments the contributions to nematic elasticity from excluded volume effects but dominates them. By altering motor kinetics we show that a competition between motor speed and crosslinking results in a nonmonotonic dependence of nematic flow on motor speed. By modulating passive filament crosslinking we show that energy transfer into nematic flow is in large part dictated by crosslinking. Thus motor proteins both generate activity and contribute to nematic elasticity. Our results provide new insights for rationally engineering active materials.

Efficient Gamma Ray Detection Using CdTe/CdS Core/Shell Quantum Dots: A Simple and Rapid Fluorescence Approach.

Gamma rays, as hazardous nuclear radiation, necessitate effective and rapid detection methods. This paper introduces a low-cost, fast, and simple fluorescence method based on CdTe/CdS core/shell quantum dots for gamma-ray detection. CdTe/CdS quantum dots, subjected to gamma irradiation from a 60Co source under various conditions, were investigated to assess their fluorescence sensor capabilities. The obtained results showed that an increase in CdTe/CdS nanoparticle size was associated with decreased sensitivity, while a reduction in CdTe/CdS concentration correlated with increased sensitivity. To further validate the practicality of CdTe/CdS core/shell quantum dots in gamma-ray detection, the structural properties of the quantum dots were meticulously studied. Raman spectroscopy, X-ray diffraction (XRD), and Fourier-transform infrared (FT-IR) analysis were conducted before and after gamma-ray radiation. The results demonstrated the crystalline stability of CdTe/CdS core/shell quantum dots under gamma irradiation, highlighting their robust structural integrity. In conclusion, the experimental findings underscore the exceptional potential of CdTe/CdS quantum dots as an off-fluorescence probe for simple, low-cost, fast, and on-site detection of gamma rays. This research contributes to the advancement of efficient and practical methods for gamma-ray sensing in various applications.

Effect of dry and wet finishing and polishing on color change and opacity of nanofill and nanohybrid composites.

BACKGROUND: As superior esthetic is one of the main reasons for using composite resins, it is very important to be familiar with factors and techniques affecting their optical properties and appearance. AIM: The aim of this study was comparing the effect of finishing and polishing with and without water coolant, on the color change and opacity of composite resin materials. METHODS: Composites used for preparing samples were Z250 (microhybrid), Z350XT (nanofilled), and Z550 (nanohybrid). Then divided into 4 groups of 5 depending on finishing and polishing technique (dry or wet) and time (immediate and after twenty-four hours). After polishing, samples were assessed using a spectrophotometer. Color change and opacity were determined. Data was analyzed using Kolmogorov-Smirnov, ANOVA and Tukey HSD tests. RESULTS: Type of material at both time had a significant effect on ΔE and opacity. Our results in dry and wet technique immediately(T0) showed that the highest and lowest ΔE and opacity belong to Z350XT (p < 0.001). After Twenty-four hours (T24), opacity of Z250 in wet condition was higher than dry condition (p < 0.001). CONCLUSIONS: Wet or dry technique was only effective on color in immediate polishing. Regarding opacity, technique was only effective in case of delayed polishing.

Experimental observations of fractal landscape dynamics in a dense emulsion.

Many soft and biological materials display so-called 'soft glassy' dynamics; their constituents undergo anomalous random motions and complex cooperative rearrangements. A recent simulation model of one soft glassy material, a coarsening foam, suggested that the random motions of its bubbles are due to the system configuration moving over a fractal energy landscape in high-dimensional space. Here we show that the salient geometrical features of such high-dimensional fractal landscapes can be explored and reliably quantified, using empirical trajectory data from many degrees of freedom, in a model-free manner. For a mayonnaise-like dense emulsion, analysis of the observed trajectories of oil droplets quantitatively reproduces the high-dimensional fractal geometry of the configuration path and its associated local energy minima generated using a computational model. That geometry in turn drives the droplets' complex random motion observed in real space. Our results indicate that experimental studies can elucidate whether the similar dynamics in different soft and biological materials may also be due to fractal landscape dynamics.

Application of CdTe/ZnS Core/Shell Quantum Dots as on Fluorescence Sensor for Detection of Gamma Rays.

Gamma rays are a type of ionizing radiation that are extremely hazardous and dangerous for humans and the environment. The fluorescence method is a simple, useful, and fast method for the detection of gamma rays. In this research, CdTe/ZnS core/shell quantum dots were used as on fluorescence sensor for the detection of gamma rays. CdTe/ZnS core/shell QDs were prepared via a simple and rapid photochemical method. The shell thickness and concentration of CdTe/ZnS core/shell quantum dots were studied as two important factors in the optical behavior of CdTe/ZnS quantum dots. The obtained results showed that the PL intensity of CdTe/ZnS QDs after gamma irradiation was increased and also a slight redshift in the PL spectrum was observed. X-ray diffractions (XRD) and Raman analyses were used to study the effect of gamma irradiation on the structural properties of CdTe/ZnS QDs. The obtained results showed that gamma irradiation couldn't damage the crystalline structure of CdTe/ZnS core/shell QDs.

CdS Semiconductor Quantum Dots; Facile Synthesis, Application as Off Fluorescent Sensor for Detection of Lead (Pb2+) Ions and Catalyst for Degradation of Dyes from Water.

The CdS quantum dots (QDs) were prepared by rapid, one-pot, and novel photochemical method, which used Thioglycolic acid (TGA) molecules as both stabilizer and sulfur source. The structure and morphology of the prepared CdS QDs were characterized by different analyses such as XRD, FT-IR, Raman, EDS, TEM, PL, and absorption. In this work, was used of CdS QDs as off fluorescence sensor for rapid and simple detection of lead (Pb2+) ions in water. The PL intensity of CdS QDs in the presence of lead ions decreased gradually and in the presence of 100 µM lead ions, photo emission completely quenched. The photocatalyst performance of CdS QDs was investigated by methylene blue (MB), methylene orange (MO), and rhodamine b (RB) pollutant dyes under both UV and sun lights. The obtained results showed that CdS QDs had excellent photocatalyst activity with dyes under UV light and 94.9% of MO dye, 94.4% of RB dye, and 81.2% of MB was degraded after 60 min UV irradiation. For understanding about which parameter have a key role in the photodegradation process of MO by CdS QDs under UV illumination, several radical scavengers were used, and results showed that holes have a key role in the degradation process.

Mercury (Hg2+) Detection in Aqueous Media, Photocatalyst, and Antibacterial Applications of CdTe/ZnS Quantum Dots.

In the present work, CdTe/ZnS high luminescence quantum dots (QDs) were synthesized by a facile, fast, one-pot, and room temperature photochemical method. Synthesized QDs were characterized by different structural and optical analyses such as X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Fourier transform-infrared spectroscopy (FT-IR), Raman, photoluminescence (PL) and UV-visible (UV-vis) spectroscopies. The results confirmed the successful growth of the ZnS shell and formation of CdTe/ZnS core/shell structure. CdTe/ZnS prepared QDs indicated a PL quantum yield of about 51%. These high luminescence QDs were used for detection of Hg2+ ions in aqueous media, as catalyst for photodegradation of different organic dyes, and as antibacterial material for the inhibition of bacterial growth. PL intensity of the CdTe/ZnS QDs was completely quenched after addition of 1 m molar Hg2+in to the media. Photocatalyst activity of CdTe/ZnS QDs was studied by rhodamine b, methylene blue, and methylene orange as organic dyes under both the sun and UV illuminations, and results showed that CdTe/ZnS QDs had the best photocatalyst activity for methylene blue degradation under UV irradiation and radical scavenger results indicated that electrons have a main role in photodegradation of methylene blue dye by CdTe/ZnS QDs under UV illumination. Antibacterial effects of CdTe/ZnS QDs evaluated by Minimum Inhibitory Concentration (MIC), and Minimum Bactericidal Concentration (MBC) methods against two strains of bacteria. The results of the antibacterial test showed that CdTe/ZnS could inhibit bacterial growth in Bacillus cereus (Gram-positive) and Escherichia coli (Gram-negative G) bacteria.

Effect of concentration and shell thickness on the optical behavior of aqueous CdTe/ZnSe core/shell quantum dots (QDs) exposed to ionizing radiation.

In this work CdTe/ZnSe core/shell quantum dots (QDs) were synthesized via a simple, rapid and room temperature photochemical approach. Optical properties of aqueous prepared CdTe/ZnSe QDs were studied systematically under gamma irradiation with dose range of 0 Gy to 20 kGy. The obtained results showed a regular red shift behavior versus gamma irradiation dose, in photoluminescence peak and absorption edge of the CdTe/ZnSe QDs. Structural properties of CdTe/ZnSe QDs before and after gamma irradiation were characterized by means of X-ray diffraction (XRD), Raman and Fourier-transform infrared (FT-IR) analyses. The obtained results showed that the crystalline structure of CdTe/ZnSe core/shell QDs did not change after gamma irradiation. Concentration and shell thickness as two important factors on the sensitivity of CdTe/ZnSe QDs in front of gamma irradiation have been investigated. Based on this study, CdTe/ZnSe QDs are suggested as good candidates for gamma dosimeter.

Interfacial Flow around Brownian Colloids.

Understanding the flow created by particle motion at interfaces is a critical step toward understanding hydrodynamic interactions and colloidal self organization. We have developed correlated displacement velocimetry to measure flow fields around interfacially trapped Brownian particles. These flow fields can be decomposed into interfacial hydrodynamic multipoles, including force monopole and dipole flows. These structures provide key insights essential to understanding the interface's mechanical response. Importantly, the flow structure shows that the interface is incompressible for scant surfactant near the ideal gaseous state and contains information about interfacial properties and hydrodynamic coupling with the bulk fluid. The same dataset can be used to predict the response of the interface to applied, complex forces, enabling virtual experiments that produce higher order interfacial multipoles.

Motile Bacteria at Oil-Water Interfaces: Pseudomonas aeruginosa.

Bacteria are important examples of active or self-propelled colloids. Because of their directed motion, they accumulate near interfaces. There, they can become trapped and swim adjacent to the interface via hydrodynamic interactions, or they can adsorb directly and swim in an adhered state with complex trajectories that differ from those in bulk in both form and spatiotemporal implications. We have adopted the monotrichous bacterium Pseudomonas aeruginosa PA01 as a model species and have studied its motion at oil-aqueous interfaces. We have identified conditions in which bacteria swim persistently without restructuring the interface, allowing detailed and prolonged study of their motion. In addition to characterizing the ensemble behavior of the bacteria, we have observed a gallery of distinct trajectories of individual swimmers on and near fluid interfaces. We attribute these diverse swimming behaviors to differing trapped states for the bacteria in the fluid interface. These trajectory types include Brownian diffusive paths for passive adsorbed bacteria, curvilinear trajectories including curly paths with radii of curvature larger than the cell body length, and rapid pirouette motions with radii of curvature comparable to the cell body length. Finally, we see interfacial visitors that come and go from the interfacial plane. We characterize these individual swimmer motions. This work may impact nutrient cycles for bacteria on or near interfaces in nature. This work will also have implications in microrobotics, as active colloids in general and bacteria in particular are used to carry cargo in this burgeoning field. Finally, these results have implications in engineering of active surfaces that exploit interfacially trapped self-propelled colloids.

Nanoscale Rheology and Anisotropic Diffusion Using Single Gold Nanorod Probes.

The complex rotational and translational Brownian motion of anisotropic particles depends on their shape and the viscoelasticity of their surroundings. Because of their strong optical scattering and chemical versatility, gold nanorods would seem to provide the ultimate probes of rheology at the nanoscale, but the suitably accurate orientational tracking required to compute rheology has not been demonstrated. Here we image single gold nanorods with a laser-illuminated dark-field microscope and use optical polarization to determine their three-dimensional orientation to better than one degree. We convert the rotational diffusion of single nanorods in viscoelastic polyethylene glycol solutions to rheology and obtain excellent agreement with bulk measurements. Extensions of earlier models of anisotropic translational diffusion to three dimensions and viscoelastic fluids give excellent agreement with the observed motion of single nanorods. We find that nanorod tracking provides a uniquely capable approach to microrheology and provides a powerful tool for probing nanoscale dynamics and structure in a range of soft materials.

Cargo carrying bacteria at interfaces.

The displacements of ensembles of colloids at the interface between oil and suspensions of the bacterium Pseudomonas aeruginosa PA14ΔpelA indicate enhanced colloid mobilities and apparently diffusive motion driven by interactions with the bacteria. However, inspection of individual trajectories of ∼500 particles reveals prolonged, directed displacements inconsistent with purely hydrodynamic interactions between swimming bacteria and colloids. Analysis of the properties of colloid paths indicates trajectories can be sorted into four distinct categories, including diffusive, persistent, curly, and mixed trajectory types. Non-diffusive trajectories are the norm, comprising 2/3 of the observed trajectories. Imaging of colloids in the interface reveals anisotropic assemblies formed by colloids decorated with one or more adhered bacteria that drive the colloids along these paths. The trajectories and enhanced transport result from individual colloids being moved as cargo by these adhered bacteria. The implications of these structures and open questions for interfacial transport are discussed and related to the active colloid literature.

A glassy carbon electrode modified with TiO2(200)-rGO hybrid nanosheets for aptamer based impedimetric determination of the prostate specific antigen.

TiO2(200)-rGO hybrid nanosheets were synthesized starting from TiO2, rGO and NaOH solid powders via a scalable hydrothermal process. The weight ratio of TiO2-GO was found to be crucial on the crystal growth and biosensor properties of the final hybrid nanosheets. They were characterized by means of SEM, FESEM-EDX, XRD, XPS, Raman and FTIR spectroscopies in order to verify the formation of very thin TiO2 anatase nanosheets with an orientation of the anatase crystal structure towards the (200) plane. The free active sites of TiO2 structure and the large surface of the 2D graphene structure strongly facilitate charge transport confirmed by BET-BJH analyses. Compared to pure AuNPs, rGO and TiO2, the hybrid nanosheet modified electrode represents the most sensitive aptasensing platform for the determination of PSA. The detection was based on that the variation of electron transfer resistance (Rct) at the modified electrode surface in a solution containing 3.0 mmol L-1 [Fe(CN)6]3-/4- as a redox probe and 0.1 mol L-1 KCl as supporting electrolyte. The detection limit of the sensor is 1 pg mL-1, and the sensor can be operated up to 30 days. It was applied to the analysis of PSA levels in spiked serum samples obtained from patients with prostate cancer. Data compare well with those obtained by an immunoradiometric assay. Graphical abstract Scalable reduced graphene oxide (rGO)-TiO2(200) mesoporous hybrid nanosheets with large surface area and new crystal growth of anatase (A) are introduced as efficient, durable, selective with low detection limit aptamer based prostate specific antigen biosensor.

Imaging bacterial 3D motion using digital in-line holographic microscopy and correlation-based de-noising algorithm.

Better understanding of bacteria environment interactions in the context of biofilm formation requires accurate 3-dimentional measurements of bacteria motility. Digital Holographic Microscopy (DHM) has demonstrated its capability in resolving 3D distribution and mobility of particulates in a dense suspension. Due to their low scattering efficiency, bacteria are substantially difficult to be imaged by DHM. In this paper, we introduce a novel correlation-based de-noising algorithm to remove the background noise and enhance the quality of the hologram. Implemented in conjunction with DHM, we demonstrate that the method allows DHM to resolve 3-D E. coli bacteria locations of a dense suspension (>107 cells/ml) with submicron resolutions (<0.5 µm) over substantial depth and to obtain thousands of 3D cell trajectories.

Failed escape: solid surfaces prevent tumbling of Escherichia coli.

Understanding how bacteria move close to surfaces is crucial for a broad range of microbial processes including biofilm formation, bacterial dispersion, and pathogenic infections. We used digital holographic microscopy to capture a large number (>10(3)) of three-dimensional Escherichia coli trajectories near and far from a surface. We found that within 20 µm from a surface tumbles are suppressed by 50% and reorientations are largely confined to surface-parallel directions, preventing escape of bacteria from the near-surface region. A hydrodynamic model indicates that the tumble suppression is likely due to a surface-induced reduction in the hydrodynamic force responsible for the flagellar unbundling that causes tumbling. These findings imply that tumbling does not provide an effective means to escape trapping near surfaces.