Further insights into the assessment of cell cycle phases by FTIR microspectroscopy

Cell growth and replication occurs in an orderly manner through a set of tightly coordinated physiological events, classified as G0, G1, S, G2 and M in conformity to their characteristics. In a previous work, by combining the results of flow cytometry (FC) using propidium iodide (PI) staining, PI-FC, and Fourier Transform Infrared Microspectroscopy (FTIRM), we gathered information to classify live B16 cells into three different set of phases (G0/G1, S and G2/M), according to their nucleic acid content measured as the area integral of the Phosphate I band (PhI, 1274–1182 cm⁻¹). In this work, we demonstrate that, once built a calibration dataset for a cell line determining the intervals of the PhI area integral related to each phase of the cell cycle, such data can be used for assigning the stage to which a live cell belongs without the support of FC. In addition, we evaluate the spectral profile of early G1 B16 cells, and compare it with the one of G0 and late G1 cell cycle phases. FTIRM highlights that G0 and G1 phases are a continuum, where the content of RNA of early G1 cells is in between G0 and late G1, and the overall nucleic acid content varies accordingly. In the paper, we also pinpoint the effects on synchronization protocols on cellular biochemistry, further strengthening the potentialities of a totally label-free methodology for cell sorting. Finally, we demonstrate that the general concept behind the proposed approach may be extended to other mammalian cell lines: human bone osteosarcoma (U2OS) cells were tested.     

New insights into the aggregation of silver pyrazolides using sterically hindered bidentate pyrazole ligands

Recrystallisation of Ag[L(1)] (HL(1) = 3{5}-[pyrid-2-yl]-5{3}-tert-butylpyrazole) in the presence of halide anions leads to two polymorphs of [Ag(3)(μ-Br)(μ-L(1))(2)], which differ in their mode of supramolecular association, and the cluster [Ag(10)(μ-L(1))(8)]Cl(2). In contrast, Ag[L(2)] (HL(2) = 3{5}-[isoquinol-1-yl]-5{3}-tert-butyl-pyrazole) crystallises as a cyclic tetrameric molecule.     

Shear history independence in colloidal aggregation

Stimulated by experiments, we have carried out detailed simulations of aggregation in the presence of shear in a model colloidal system with a short-range attractive potential. For weak shear rates, we find that the shear enhanced the aggregation and that the long-time state of the system is independent of the shear history. For strong shear rates, precipitous fragmentation occurred after the shear was turned on and, after an induction period, the aggregation quickly rebounded in a stochastic manner similar to classical nucleation phenomena. However, the long-time state of the system is, once again, independent of the shear history. Thus, for both weak and strong shear cases, the shear rate acts as a state variable of the aggregating system. Shear rates employed in the simulations can be attained in laboratory experiments, as confirmed by computing the dimensionless Péclet numbers.     

Two-dimensional colloidal aggregation: concentration effects

Extensive numerical simulations of diffusion-limited (DLCA) and reaction-limited (RLCA) colloidal aggregation in two dimensions were performed to elucidate the concentration dependence of the cluster fractal dimension and of the different average cluster sizes. Both on-lattice and off-lattice simulations were used to check the independence of our results on the simulational algorithms and on the space structure. The range in concentration studied spanned 2.5 orders of magnitude. In the DLCA case and in the flocculation regime, it was found that the fractal dimension shows a linear-type increase with the concentration phi, following the law: d(f)=d(fo)+aphi(c). For the on-lattice simulations the fractal dimension in the zero concentration limit, d(fo), was 1.451+/-0.002, while for the off-lattice simulations the same quantity took the value 1.445+/-0.003. The prefactor a and exponent c were for the on-lattice simulations equal to 0.633+/-0.021 and 1.046+/-0.032, while for the off-lattice simulations they were 1.005+/-0.059 and 0.999+/-0.045, respectively. For the exponents z and z', defining the increase of the weight-average (S(w)(t)) and number-average (S(n)(t)) cluster sizes as a function of time, we obtained in the DLCA case the laws: z=z(o)+bphi(d) and z'=z'(o)+b'phi(d'). For the on-lattice simulations, z(o), b, and d were equal to 0.593+/-0.008, 0.696+/-0.068, and 0.485+/-0.048, respectively, while for the off-lattice simulations they were 0.595+/-0.005, 0.807+/-0.093, and 0.599+/-0.051. In the case of the exponent z', the quantities z'(o), b', and d' were, for the on-lattice simulations, equal to 0.615+/-0.004, 0.814+/-0.081, and 0.620+/-0.043, respectively, while for the off-lattice algorithm they took the values 0.598+/-0.002, 0.855+/-0.035, and 0.610+/-0.018. In RLCA we have found again that the fractal dimension, in the flocculation regime, shows a similar linear-type increase with the concentration d(f)=d(fo)+aphi(c), with d(fo)=1.560+/-0.004, a=0.342+/-0.039, and c=1.000+/-0.112. In this RLCA case it was not possible to find a straight line in the log-log plots of S(w)(t) and S(n)(t) in the aggregation regime considered, and no exponents z and z' were defined. We argue however that for sufficiently long periods of time the cluster averages should tend to those for DLCA and, therefore, their exponents should coincide with z and z' of the DLCA case. Finally, we present the bell-shaped master curves for the scaling of the cluster size distribution function and their evolution when the concentration increases, for both the DLCA and RLCA cases.     

Molecular dynamics simulation of the aggregation of colloidal particles

The spontaneous time evolution of systems containing N colloidal particles (N = 12, 24, 100) in a spherical cell of volume V at a constant volume fraction φ=0.1 was studied by a molecular dynamics method in the NVT ensemble. The starting velocities of the particles are allocated according to the Maxwell distribution at T=273 K.

Pairwise interaction of the particles was specified by molecular, electrostatic and elastic forces. The changes in the potential energy of the systems were calculated during the establishment of dynamic equilibrium. Coagulation takes place at sufficiently high values of the Hamaker constant. The value of the coefficient of Brownian diffusion, which is calculated from the half-time of coagulation, is found to be close to the known value for aqueous dispersions. The inclusion of electrostatic forces prevents coagulation.

The results obtained are in agreement with those obtained using theories of aggregate formation. Some structural characteristics of aggregates and stable systems are discussed.     

Effect of aggregation on viscosity of colloidal suslension

This experimental study of viscosity of colloidal suspensions was performed using monodisperse polystyrene latex with particle diameter of 1.15 μm and a pH dependent negative zeta potential of up to 120 mV in aqueous solutions. The range of electrostatic repulsion between the particles was controlled by varying the concentration of potassium chloride. Suspensions under investigation were either in a stable, coagulated, or gelated, state depending on the salt concentration. Shear thinning behaviour was observed for all the samples studied. The dependence of viscosity on shear rate imposed was found to depend substantially on the salt concentration.     

Pyrolytic alkylation-gas chromatography-mass spectrometry of model polymers Further insights into the mechanism and scope of the technique

The mechanism of the high-temperature hydrolysis and alkylation with tetraalkylammonium hydroxides of bio- and geopolymers has been approached mainly by studying the behaviour of single standard compounds. In the present work, we have applied this technique to three polymers of known structure, i.e. suberin, polycitraconic acid (PCA) and a lignin dehydrogenase polymer (DHP), related respectively to natural polyesters, fulvic acids and lignins, in order to get new insight into the reaction mechanisms. As further application of the technique, the case study of the lignin signature during the coalification process has been analyzed by pyrolysis-butylation of humic acids extracted from two peat and lignite samples.     

Molecular insights into the surface morphology, layering structure, and aggregation kinetics of surfactant-stabilized graphene dispersions

The production of graphene with open band gaps for the manufacturing of graphene-based electronic and optical devices requires synthesis methods to either control the number of layers to enrich AB-stacked bilayer or trilayer graphene or control the extent of functionalization of monolayer graphene. Solution-phase dispersion of graphene is promising for both methods to create printable electronics and nanocomposites. However, both methods face common challenges, including controlling the surface morphology, reducing the turbostratic layering, and enhancing the dispersion stability. To address these challenges at the molecular level, we successfully combined molecular simulations, theoretical modeling, and experimental measurements. First, we probed the surface structure and electrostatic potential of monolayer graphene dispersed in a sodium cholate (SC) surfactant aqueous solution, which exhibits 2D sheets partially covered with a monolayer of negatively charged cholate ions. Similar to the case of carbon nanotube functionalization, one may regulate the binding affinity of charged reactants for graphene functionalization by manipulating the surface morphology. Subsequently, we quantified the interactions between two graphene-surfactant assemblies by calculating the potential of mean force (PMF) between two surfactant-covered graphene sheets, which confirmed the existence of a metastable bilayer graphene structure due to the steric hindrance of the confined surfactant molecules. The traditional Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was found to be adequate to explain the long-range electrostatic repulsions between the ionic surfactant-covered graphene sheets but was unable to account for the dominant, short-range steric hindrance imparted by the confined surfactant molecules. Interestingly, one faces a dilemma when using surfactants to disperse and stabilize graphene in aqueous solution: on the one hand, surfactants can stabilize graphene aqueous dispersions, but on the other hand, they prevent the formation of new AB-stacked bilayer and trilayer graphene resulting from the reaggregation process. Finally, the lifetime and time-dependent distribution of various graphene layer types were predicted using a kinetic model of colloid aggregation, and each graphene layer type was further decomposed into subtypes, including the AB-stacked species and various turbostratic species. The kinetic model of colloid aggregation developed here can serve as a useful tool to evaluate the quality of graphene dispersions for subsequent substrate-transferring or functionalization processes.     

Mesoscopic structure formation in colloidal aggregation and gelation

Recent small-angle light scattering experiments have revealed that diffusively aggregating spherical particles develop structure on a mesoscopic length scale (∼ tens of particles). The mesoscopic structural length scale persists even when the aggregation proceeds to the formation of a space-spanning network (a gel). We review the technique of small-angle light scattering, survey the experimental evidence for mesoscopic structure formation, discuss attempts at understanding these experimental observations by computer simulation of irreversible and reversible diffusion-limited cluster aggregation (DLCA), and propose a coherent picture for the understanding of non-equilibrium aggregation in the context of phase transitions.     

Coupling between polymer adsorption and colloidal particle aggregation

Studies of the adsorption of high molecular weight polymers on colloidal latex and silica particles and their subsequent flocculation were carried out. Neutral polyethylene oxide samples with both a narrow and a broad molecular weight distribution were used together with low charged cationic copolymers. The influence of the particle concentration and polymer dose on the flocculation were systematically investigated under quiescent conditions.Equilibrium bridging only occurred with polyelectrolyte, even in very dilute suspensions, at high particle coverage. In contrast to this, non-equilibrium bridging occurred with both neutral polymer and polyelectrolytes but only for more concentrated suspensions and small amounts of adsorbed polymer. Polymer adsorption in dilute suspensions, which did not show particle aggregation was measured an electrophoretic technique. In more concentrated suspensions, where flocculation takes place, we found that aggregation prevents further polymer adsorption and induces both an excluded volume and a surface effect. The consequences on the shape of the isotherms differ according to the aggregation mechanism.A significant decrease of the amount, , of adsorbed polymer is observed with non-equilibrium bridging. When both mechanisms simultaneously contribute to the aggregation, the value of depends on their relative importance. In the intermediate range of copolymer dose their respective contributions are critically sensitive to the details of the mixing step and stirring, leading to non reproducible experimental results.     

Triglyceride nanocrystal aggregation into polycrystalline colloidal networks: Ultra-small angle X-ray scattering,models and computer simulation

Triacylglycerols (TAGs) are the majority molecules present in edible fats and oils. Many of the functional characteristics of fat products depend on the colloidal fat crystal network present. Identifying the hierarchies of these colloidal networks and how they spontaneously self-assemble is important to understand their functionality and the oil binding capacity, and new insights into the nano- to meso-scale structure in these colloidal fat networks have been reported in recent years. Ultra small angle X-ray scattering (USAXS) is a technique new to the study of edible oil structures and, when combined with modelling and computer simulation, has enabled significant advances to be made in understanding the nano- to micro-scale crystalline structures of edible oils. In the four years since crystalline nanoplatelets (CNPs) were characterized, models have been made of these highly anisotropic nanoscale structures in which they were treated as the primary unit. In those models, CNPs were represented as close-packed rigid layers of spheres, so chosen because the van der Waals sphere–sphere interaction is known. The intent of the models was to predict the hierarchy of colloidal fat networks that would self-assemble from the components in edible oils. Initially, CNP aggregation was modelled under the assumption that all CNPs are present before aggregation begins and that their solubility in liquid oil is very low. The models successfully predicted the fractal dimensions subsequently measured using USAXS. This brief review reports on some of the latest models and simulations together with the results of USAXS experiments carried out on binary lipid systems, such as SSS in OOO, as well as certain complex systems that contain many different TAG molecules. The excellent agreement between the two approaches has established that USAXS is a powerful tool in the elucidation of the nano- to meso-length scales in fats and oils.     

Quantum chemical insights into the aggregation induced emission phenomena: A QM/MM study for pyrazine derivatives

There have been intensive studies on the newly discovered phenomena called aggregation induced emission (AIE), in contrast to the conventional aggregation quenching. Through combined quantum mechanics and molecular mechanics computations, we have investigated the aggregation effects on the excited state decays, both via radiative and nonradiative routes, for pyrazine derivatives 2,3‐dicyano‐5,6‐diphenylpyrazine ( DCDPP ) and 2,3‐dicyanopyrazino phenanthrene ( DCPP ) in condensed phase. We show that for DCDPP there appear AIE for all the temperature, because the phenyl ring torsional motions in gas phase can efficiently dissipate the electronic excited state energy, and get hindered in aggregate; while for its “locked”‐phenyl counterpart, DCPP , theoretical calculation can only give the normal aggregation quenching. These first‐principles based findings are consistent with recent experiment. The primary origin of the exotic AIE phenomena is due to the nonradiative decay effects. This is the first time that AIE is understood based on theoretical chemistry calculations for aggregates, which helps to resolve the present disputes over the mechanism. © 2012 Wiley Periodicals, Inc.     

Hydrodynamic and interparticle potential effects on aggregation of colloidal particles

The effects of both hydrodynamic interaction and the form of the interparticle potential on the aggregation process for dispersed spherical particles are investigated by computational simulation. The simulation methods of Brownian Dynamics (BD) and Stokesian Dynamics (SD) are applied, over a range of solid volume fraction of $0.04???0.12$. The interparticle potential is a combination of a generalized Lennard-Jones form and a Yukawa potential, the latter of which describes a screened electrostatic repulsion at longer range. The combined potential is parameterized to include a roughly constant primary minimum near contact, along with a variable repulsive barrier at slightly larger separation. The microstructure is characterized through the pair distribution function, g(r), and the static structure factor. The repulsive barrier reduces the rate of aggregation and is also seen to affect the structure, with a large repulsion associated with a more tenuous structure. This is reflected in the potential having a strong effect on the evolution of ‘bonds’ per particle. Hydrodynamic interactions were found to reduce the solid fraction required for percolation, with the influence depending upon the form of the potential; the difference in percolation threshold was significant, with ${?}_{c\text{,}\mathit{SD}}?0.06$ and ${?}_{c\text{,}\mathit{BD}}?0.08$ a typical difference for moderate repulsion barriers. These results are for 864 particles in a cubic unit cell. To address the mechanism for this influence of hydrodynamic interactions, a complementary analysis of the evolution of numerous independent three-particle aggregates was performed. The analysis shows that hydrodynamic interaction slows the evolution toward a condensed aggregate of lowest potential energy in a way which cannot be explained by a simple rescaling of the drag due to uncorrelated particle motions.     

Light-induced aggregation of colloidal gold nanoparticles capped by thymine derivatives

The colloid stability of thymine-coated gold nanoparticles under light irradiation as a function of particle size, surface charge, and exposure time was investigated in alkaline, aqueous solutions as well as in a 0.5 vol % of DMF in H(2)O mixture. With increasing exposure to light irradiation at 280 nm, more and more particles coagulated. Light-induced aggregation of colloidal gold nanoparticles was attributed to reorientation of thymine terminal groups tethered on gold particle surfaces. A smaller particle size and negatively charged surface reduced the rate of photodimerization or even inhibited the photoreaction. UV-vis and FTIR spectroscopy confirmed the photodimerization of terminal thymine molecules under 280 nm light irradiation. The reaction kinetics of thymine photodimerization appears to be a combination of first-order reactions, each having different rates, reflecting the inhomogeneity and high curvature of the gold nanoparticle surfaces.     

Model of aggregation of colloidal fine particles in the presence of supersized polymer

A simple model of the process of stabilization and destabilization of fine colloidal suspensions induced by supersized linear polymers has been tested by the direct simulation method. In the model, a single polymer molecule may bind a number of colloidal particles and thus form an aggregate. It is assumed that a simultaneous attachment of a few fine particles to one macromolecule does not necessarily destabilize the suspension. The destabilization of the system (occurring if aggregate sedimentation dominates its diffusion ability) takes place only when the number of the attached particles per macromolecule exceeds the critical value which depends on the polymer coil dimension in the dispersion medium. The model permits interpretation of several experimental observations of the behavior of colloidal sols upon introduction of very high molecularweight polymers. The simulation results have been compared with the experimental data on the effect of polyacrylamide on the stability of AgI sol.     

Self-limiting aggregation leads to long-lived metastable clusters in colloidal solutions

The existence of a metastable state with limited Coulomb-blocked aggregation at the onset of instability in a colloidal solution is proposed and demonstrated both experimentally and theoretically (through Monte Carlo simulations). Such a stable state of small clusters of metallic colloids happens to be extremely important for techniques such as surface-enhanced Raman scattering (SERS), which profits explicitly from collective plasmon resonances in these clusters to boost Raman signals of specific analytes. In fact, SERS provides a unique tool to understand, monitor, and study the onset of aggregation in colloidal silver/gold and to prove the existence of the proposed state at the boundary of colloid coalescence.     

Control over colloidal aggregation in monolayers of latex particles at the oil-water interface

The controlled generation of 2D aggregate networks is studied experimentally using micrometer-sized polystyrene latex particles attached to the oil-water interface. Starting from an initially crystalline monolayer, appropriate combinations of carefully added electrolyte and surfactant enable control over both the fractal dimension and the kinetics of aggregation. Remarkably, the colloidal crystals formed upon first spreading remain stable, even for days, when substantial amounts of electrolyte are added to the aqueous phase. Pressure-area isotherms reveal a slow time evolution of the electrostatic dipole-dipole interaction. When the electrostatic interaction has been sufficiently weakened, aggregation proceeds in well-defined, reproducible manner. The aggregation process is analyzed using quantitative video microscopy. The evolution of the cluster size distribution and its moments is characterized, and static and dynamic scaling exponents are derived to identify the nature of the aggregation process. In the range of concentrations studied here, the kinetics all agree with a "fast", diffusion-limited cluster type of aggregation. However, the fractal dimension strongly depends on the composition of the aqueous subphase. Rather dense structures are found when only electrolyte is used, whereas more open structures are obtained when even small amounts of surfactant are added. It is suggested that this structural dependency is related to the effect of surfactant on the contact angle and its consequences for the anisotropic nature of the capillary interactions.     

Bimodal colloidal mixtures: from fast to slow aggregation regions

A Brownian dynamics simulation has been used to investigate the aggregation kinetics of bimodal colloidal mixtures with similar surface chemistries but different sizes, driven by the DLVO interaction potential. The time evolution of structural formation is examined by the mean number of neighbors under fast and slow aggregation regions. It was found that the electrolyte ionic strength affects the kinetic pattern of colloidal aggregation. Under the high electrolyte ionic strength conditions (fast aggregation), the selective aggregation of the least stable single component can take place in the early stage, while the other component is enriched in this least stable component in the later stage. With the ionic strength decreasing (towards the slow aggregation), the hybrid aggregation (selective aggregation and heteroaggregation) gradually dominates the aggregation kinetics. Also in the early stage, this evolves to the heteroaggregation of different components under lower ionic strength conditions. The volume fraction has no obvious influence on this kinetic pattern in the early stage.     

Responsive colloidal systems: reversible aggregation and fabrication of superhydrophobic surfaces

We report on a method of fabricating stimuli-responsive core-shell nanoparticles using block copolymers covalently bound to a silica nanoparticle surface. We used the "grafting to" approach to graft amphiphilic block copolymer brushes of poly(styrene-b-2-vinylpyridine-b-ethylene oxide) and poly(styrene-b-4-vinylpyridine) onto silica nanoparticles with two different diameters: colloidal silica 200 nm in diameter and fumed silica 15 nm in diameter. We used the pH-responsive properties of the grafted brush to regulate the interactions between the particles, and between the particles and their environment. We show that this behavior can be applied for a reversible formation of particle aggregates, and can be used to tune and stabilize the secondary aggregates of particles of the appropriate size and morphology in an aqueous environment. The suspensions of the particles form a textured hydrophilic coating on various substrates upon casting and the evaporation of water. Heating above the polymer's glass transition temperature or treatment in acidic water result in back and forth switching between superhydrophobic and hydrophilic surfaces, respectively.