Search

Nanoparticles (NPs) and exosomes used to transport therapeutic drug molecules to cells have attracted considerable attention in biopharmaceuticals. However, soft NPs entering cells with different angles are hard to be simulated by traditional theoretical endocytic models, let alone clustering of NPs with varied shapes. Here, the endocytic model based on the co-rotational grid method is implemented. The endocytosis of a single soft NP and a nanoparticle cluster (NPC) are investigated. Both the actin force and the dynamic assembly of the clathrin coat have been considered. The results show that NP size is the main factor affecting the internalization efficiency, followed by the aspect ratio, and finally the entry angle. The increase in NP size, aspect ratio, and entry angle will lead to a decrease in internalization efficiency. However, the final configuration of the endocytic vesicle and the internalization efficiency are both independent of the entry angle for NPs with an equivalent radius greater than 15 nm. Besides, a large entry angle can assist a large soft NP in successfully entering cells. The increase in NP stiffness also reduces the internalization efficiency considering the actin force required during endocytosis. The transformation of the NP from the inclined configuration to the vertical configuration is mainly due to NP deformation. Compared with a single NP, NPC has a lower internalization efficiency. The results provide new mechanistic insights into the endocytosis of soft NPs and NPCs. [ABSTRACT FROM AUTHOR]

Nanoparticles have been extensively adopted to deliver therapeutic drug molecules to cells through clathrin-mediated endocytosis (CME). The size and shape of nanoparticles are important factors in the design of a drug delivery system. Both the clathrin coat and actin force induce the bending of the membrane during CME. However, due to the complex coupled effects of size, shape, and surface properties, nanoparticle shape effects are difficult to elucidate through experiments. Herein, we establish a comprehensive framework considering both the actin force and the dynamic assembly of the clathrin coat. To explore the effect of the nanoparticle size and shape on CME, we construct a clathrin coat growth model with actin force feedback. The clathrin coat growth model, nanoparticle internalization efficiency, and transportation efficiency are discussed through numerical analysis. The transportation efficiency is defined by the energy cost of the cell absorbing unit dose target drug. Numerical results illustrate that the proposed clathrin coat growth model is consistent with the actual physiological process, especially for CME considering receptor-mediated effects. The elliptical nanoparticle exhibits higher internalization and transportation efficiencies. A larger nanoparticle has lower internalization efficiency but higher transportation efficiency. Our results demonstrate that the internalization and transportation efficiencies of nanoparticles with an intermediate aspect ratio are higher than those with low or high aspect ratios. Our model provides insight into the intrinsic mechanism of CME and useful guidance for the practical design of the size and shape of nanoparticles for biopharmaceutical research. [ABSTRACT FROM AUTHOR]

We investigate the structural properties of binary polymer brushes, composed of functional 4-armed star polymers and chemically identical linear polymers of different molecular weights. The molecular dynamics simulations confirm recent self-consistent field studies, in which a considerable potential of these systems for the design of switchable surfaces has been claimed. The length of the linear chains serves as a control parameter, which, while passing over a critical value, induces a sharp transition of the molecular conformation. We investigate these transitions at different grafting densities and summarize our findings in a phase diagram. The temperature dependence of the brush structure is investigated in a non-selective solvent, and non-trivial variations of the surface composition are observed. The quantity of these latter effects would be insufficient to build switchable systems, and we argue that a minor quantity of solvent selectivity would suffice to enable the desired feature of an environment-responsive coating. [ABSTRACT FROM AUTHOR]

Upon ultraviolet (UV) light excitation, an intense near-infrared (NIR) emission of Yb3+ (2F5/2→2F7/2) around 980 nm is observed in YVO4:Yb3+ phosphors. Owing to host absorption of YVO4, a broad excitation band ranging from 250 to 350 nm is recorded when the Yb3+ emission was monitored, which suggests an efficient energy transfer from host to Yb3+ ions. The Yb3+ concentration dependence of the visible vanadate emission as well as the Yb3+ emission is investigated. The decay curve of vanadate emission is measured under the excitation of a 266 nm pulsed laser. The decay time of the vanadate emission at 500 nm is remarkably reduced by introducing Yb3+ ions, further verifying that the energy transfer from the vanadate host to the Yb3+ ions is very efficient. Cooperative energy transfer (CET) is discussed as a possible mechanism for the NIR emission. The YVO4:Yb3+ phosphor can convert each UV photon into two NIR photons via CET, which has potential application in the high efficiency silicon-based solar cells. [ABSTRACT FROM AUTHOR]

Effective medium theory is introduced into a three-layer model to study the anomalous IR properties of nanostructured Pt films. A composite system is set up for the nanostructured film together with adsorbates and water around it. The anomalous IR spectral features, which exhibit a transition from enhanced (or normal) IR absorption to Fano-type bipolar line shape and, finally, to enhanced anomalous IR absorption (the abnormal infrared effects) along with the change in structure and size of nanomaterials, as observed through experiments for CO molecule adsorption, are elucidated by an increase in the volume fraction of metal in the composite system and the effective thickness of the composite system. The theoretical simulation results illustrate that the spectral line shape of IR absorption depends strongly on the volume fraction of metal, while the intensity of the IR band is directly proportional to the effective thickness. This study has revealed, through a physical optical aspect of interaction of CO molecules with nanostructured metal films, one of the possible origins of anomalous IR properties and has shed light on interpreting the peculiar properties of nanomaterials. [ABSTRACT FROM AUTHOR]

Nanocrystal Y2O3 powders with different grain sizes and various doping concentrations of Tb3+ were prepared by an autocombustion reaction. The size and surface effects on the 4f-5d transitions and energy transfers between Tb3+ ions were studied by using x-ray diffraction, transmission electron microscopy, fluorescent spectra, and luminescent decay. It was found that the excitation spectra are composed of two parts: one is the contribution from the Tb3+ at/near the nanoparticle surfaces; another is from the Tb3+ inside the nanoparticles. The study on the concentration quenching and luminescent decay indicated that the energy transfers depopulating the 5D3 and 5D4 level were assigned to the mechanisms of electric dipole-dipole and exchange interaction, respectively. The size confinement greatly affects the energy transfer quenching the emission from the 5D3 level, but slightly affects the energy transfer quenching the emission from the 5D4 level. [ABSTRACT FROM AUTHOR]

In this paper a lattice model for the diffusional transport of particles in the interphase cell nucleus is proposed. Dense networks of chromatin fibers are created by three different methods: Randomly distributed, noninterconnected obstacles, a random walk chain model, and a self-avoiding random walk chain model with persistence length. By comparing a discrete and a continuous version of the random walk chain model, we demonstrate that lattice discretization does not alter particle diffusion. The influence of the three dimensional geometry of the fiber network on the particle diffusion is investigated in detail while varying the occupation volume, chain length, persistence length, and walker size. It is shown that adjacency of the monomers, the excluded volume effect incorporated in the self-avoiding random walk model, and, to a lesser extent, the persistence length affect particle diffusion. It is demonstrated how the introduction of the effective chain occupancy, which is a convolution of the geometric chain volume with the walker size, eliminates the conformational effects of the network on the diffusion, i.e., when plotting the diffusion coefficient as a function of the effective chain volume, the data fall onto a master curve. [ABSTRACT FROM AUTHOR]

The recently introduced method of excess collisions to estimate reaction times of protein-DNA systems in the presence of facilitated diffusion (“sliding”) requires a cell of full system size. This bottleneck is avoided with a modification, by which a set of empirical parameters is calibrated using numerical simulations of a small test system. Once this is done, reaction times for systems of arbitrary dimensions are derived by extrapolation. It is shown that at physiological sliding lengths a test system of the order of 100 nm radius suffices to extract accurate reaction times for realistic cell dimensions. The achieved speedup, when compared to explicit simulations of the reaction process, is increasing in third order of the extrapolated radius of the cell. [ABSTRACT FROM AUTHOR]

Using molecular dynamics simulations, we study the phase separation in a binary two-dimensional core-softened fluid with different size ratios and concentrations. The correlation functions for both components are analyzed to show the dependence of the configurational structure of the binary fluid on size ratio and concentration. A phase separation diagram is obtained and the structural features of the phase separation are further investigated using the direct imaging method. [ABSTRACT FROM AUTHOR]

In this paper, a new method to efficiently simulate diffusion controlled second order chemical reactions is derived and applied to site-specific DNA-binding proteins. The protein enters a spherical cell and propagates via two competing modes, a free diffusion and a DNA-sliding mode, to search for its specific binding site in the center of the cell. There is no need for a straightforward simulation of this process. Instead, an alternative and exact approach is shown to be essentially faster than explicit random walk simulations. The speed-up of this novel simulation technique is rapidly growing with system size. [ABSTRACT FROM AUTHOR]

The abnormal IR effects (AIREs) characterized by a positive-going peak of platinum (Pt) nanostructured surface generated in a square-wave potential treatment upon CO molecule chemisorption was observed and analyzed with a consideration of the interparticle interaction and electron-hole damping between nanoislands and CO molecules. A theoretical simulation shows that the islanded nanostructured Pt surfaces, which gives rise to interparticle interaction, coupling with electron-hole mechanism, may contribute to the origins of positive-going peak (AIREs) observed by in situ Fourier transformation IR (FTIR) experiments. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A series of niobate-phosphate and niobate-silicate glasses doped with Eu[sup 3+] ions were prepared. The emissions, phonon-side band spectra, and fluorescence lifetimes of these glasses were studied and the intensity parameters of Eu[sup 3+] were obtained. The temperature dependence of emission intensity of the Eu[sup 3+] ion in these niobate glasses was investigated. The temperature-quenching rates were determined. The results indicate that in niobate glasses, as the concentration of Nb[sub 2]O[sub 5] increases, the covalence becomes stronger, the symmetry becomes lower and the electron-phonon coupling strength becomes stronger. Thus, the lifetimes become shorter, and the nonradiative transition processes and the temperature-quenching effect become stronger. [ABSTRACT FROM AUTHOR]

The temperature dependence of emission intensity of the Eu[sup 3+] ion in several silicate glasses was investigated between 77 and 670 K, under the excitation of a 488-nm laser. It was observed that in some glasses the total emission intensity of the Eu[sup 3+] ion had a maximum at a certain temperature below 670 K, while in the other glasses increased monotonically with the increasing temperature. A unified theoretical model was given considering thermal activation, phonon-assisted absorption and nonradiative energy transfer processes. Based on the model, all of the experimental data were well fitted. The temperature-quenching channels were discussed, and the temperature-quenching rates were determined. [ABSTRACT FROM AUTHOR]

Studies the physicochemical properties of chiral and racemic phospholipid monolayers at the air-water interface. Development of the model used in the study; Interactions working on monolayer molecules; Mean electrical field of the monolayer; Calculation of the orientational order parameters.

Examines the polar orientational phase transition in smectic monolayer films. Analysis on the change of the first orientational order parameter; Details on the differential dielectric constant for monolayer films; Implication of changes of the first orientational order parameter for dielectric constant.

Focuses on a study which investigated the spectroscopic properties of lead fluoride+germanium dioxide+tungsten oxide glass ceramics doped with thulium and codoped with thulium and ytterbium ions. Measurement of the absorption, up-conversion emission, excitation, and time-resolved spectra; Observations from the codoped sample; Comparison between the experimental and theoretical results.

We examine the effects of the external force, torque, temperature, confinement, and excluded volume interactions (EVIs) on the stability of the helical configuration of an intrinsically straight semiflexible biopolymer inside a cylindrical cell.We find that to stabilize a helix, the confinement from both ends of the cell is more effective than a uniaxial force. We show that under a uniaxial force and in absence of confinement from bottom of the cell, a stable helix is very short. Our results reveal that to maintain a low pitch helix, a torque acting at both ends of the filament is a necessity, and the confinement can reduce the required torque to less than half making it much easier to form a stable helix. Moreover, we find that thermal fluctuations and EVIs have little impact on the stability of a helix. Our results can help understand the existence of the helix and ring configurations of some semiflexible biopolymers, such as MreB homologs, inside a rod-shaped bacteria. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) transition metal dichalcogenides (TMDs) receive significant attention due to their intriguing physical properties for both fundamental research and potential applications in electronics, optoelectronics, and catalysis. A high-quality 2D film of NiSe2, a TMD material, is grown epitaxially by a single step direct selenization of a Ni(111) substrate. X-ray photoemission spectroscopy, low-energy electron diffraction, scanning tunneling microscopy, and density functional theory calculations are combined to confirm the formation and structure of the film, revealing a (√3 x √3) superlattice of the NiSe2 film formed on the (√7 x √7) superlattice of the substrate. Fabrication of this 2D NiSe2 film opens opportunities to research its applications, especially for electrocatalysis and energy storage devices. [ABSTRACT FROM AUTHOR]

Simultaneous Stokes and anti-Stokes ultralow-frequency (ULF) Raman measurement down to ~2 cm-1 or 60 GHz is realized by a single-stage spectrometer in combination with volume-Bragg-grating-based notch filters. This system reveals its excellent performance by probing Brillouin signal of acoustic phonons in silicon, germanium, gallium arsenide, and gallium nitride. The deduced sound velocity and elastic constants are in good accordance with previous results determined by various methods. This system can shorten the integration time of the Brillouin signal with a good signal-to-noise ratio by more than 2000-fold compared to a Fabry-Perot interferometer (FPI). This study shows how a filter-based ULF Raman system can be used to reliably achieve Brillouin spectroscopy for condensed materials with high sensitivity and high signal-to-noise ratio, stimulating fast Brillouin spectrum measurements to probe acoustic phonons in semiconductors. [ABSTRACT FROM AUTHOR]

The atomic-scale spatial relationship of a two-impurity Kondo system has been determined at varying lateral distance by scanning tunneling microscopy (STM) and spectroscopy. The localized spins of two cobalt magnetic adatoms that are placed on different electrodes of an STM form two individual Kondo singlet states, each showing quite different Kondo coupling, i.e., the tip-Kondo with low Kondo temperature and the sample-Kondo with high Kondo temperature. The differential conductance dI/dV spectra show the continuous changes of the resonance peak feature when approaching the Kondo tip laterally to the local sample-Kondo impurity on the surface. The result indicates a notable interplay between these two Kondo systems. We propose a convolution model based on the q factor of the sample-Kondo (qs) and tip-Kondo (qt) to interpret the change of various tunneling channels and the evolution of the experimental spectra. [ABSTRACT FROM AUTHOR]

The C-I bond dissociation enthalpies (BDE) of various organic iodides were calculated using high-level theoretical methods including MP2 and CCSD(T) with extrapolated basis set as well as a number of density functional theory methods. After systematic evaluation of the theoretical results against available experimental C-I BDEs, it was found that the MPW-LYP1M method gave the lowest root mean square error. We, therefore, used this method to examine the substituent effects on different categories of C(sp3)-I and C(sp2)-I bonds. Furthermore, the remote substituent effects on the C-I BDEs of substituted iodobenzenes and substituted (iodomethyl)benzenes were also investigated at the same level. The C-I BDEs of typical heteroaromatic iodides including five-membered and six-membered heterocyclic iodides were also examined. [ABSTRACT FROM AUTHOR]

The authors present an efficient method to fabricate Ni tips for spin-polarized scanning tunneling microscopy (SP-STM) via electrochemical etching of Ni wires in a constant-current mode. Instead of applying constant voltages to trigger the electrochemical etching of Ni wires in previous reports, here a constant current is applied, which ensures a stable etching process and favors a high yield of Ni tips with good quality. The prepared Ni tips have been applied to obtain atomic resolution images on various surfaces in conventional STM measurements and to resolve magnetic-state-dependent contrast of Co islands grown on a Cu(111) surface in SP-STM experiments. [ABSTRACT FROM AUTHOR]

An organic thin-film transistor (OTFT) having a low-dielectric polymer layer between gate insulator and source/drain electrodes is investigated. Copper phthalocyanine (CuPc), a well-known organic semiconductor, is used as an active layer to test performance of the device. Compared with bottom-contact devices, leakage current is reduced by roughly one order of magnitude, and on-state current is enhanced by almost one order of magnitude. The performance of the device is almost the same as that of a top-contact device. The low-dielectric polymer may play two roles to improve OTFT performance. One is that this structure influences electric-field distribution between source/drain electrodes and semiconductor and enhances charge injection. The other is that the polymer influences growth behavior of CuPc thin films and enhances physical connection between source/drain electrodes and semiconductor channel. Advantages of the OTFT having bottom-contact structure make it useful for integrated plastic electronic devices. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Mixed Langmuir films composed of the left- and right-handed chiral enantiomers are analyzed using a model based on the Bragg-Williams theory for binary mixtures, and the contribution of the mixing effect to the generation of Maxwell displacement current (MDC) and surface pressure-area isotherm is discussed. It is revealed that the formation of the homogeneous and intimate mixture of the left- and right-handed chiral enantiomers gives rise to the additional MDC generation due to the dipole-dipole interaction between the left- and right-handed chiral enantiomer molecules by the molecular twist transition during monolayer compression. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We give a general expression of the dielectric constant tensor in orientational orders in the applied static or low-frequency electric field for the semi-two-dimensional monolayer films with C[sub ∞v] symmetry. The dielectric anisotropic orientational tensor and orientational polarization tensor are derived in orientational order parameters. The nonlinear electro-optical Pockels effect including the local-field influence is also discussed using the orientational orders. A numerical calculation result shows that the first orientational order parameter S[sub 1] is a destructive order while the second orientational orientational order parameter S[sub 2] is a constructive one to the dielectric constant in the normal direction of monolayer films. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Compares the up-conversion emission spectra of thulium-doped and thulium, ytterbium co-doped fluorophosphate glasses. Observation of emission excitation at room temperature; Dependence of emission intensity on excitation power.