Colloidal Systems

Magnetic separation is a versatile technique used in sample preparation for diagnostic purpose. For such application, an externalmagnetic field is applied to drive the separation of target entity (e.g. bacteria, viruses, parasites and cancer cells) from a complex raw sample in order to ease the subsequent task(s) for disease diagnosis. This separation process not only can be achieved via the utilization of high magnetic field gradient, but also, in most cases, low magnetic field gradient with magnitude less than 100 T m21 is equally feasible. It is the aim of this review paper to summarize the usage of both high gradient magnetic separation and low gradient magnetic separation (LGMS) techniques in this area of research. It is noteworthy that effectiveness of the magnetic separation process not only determines the outcome of a diagnosis but also directly influences its accuracy aswell as sensing time involved. Therefore, understanding the factors that simultaneously influence the efficiency of both magnetic separation process and target detection is necessary. Moreover, for LGMS, there are several important considerations that should be taken into account in order to ensure its successful implementation. Hence, this review paper aims to provide an overview to relate all this crucial information by linking the magnetic separation theory to biomedical diagnostic applications.

This paper exposes an extension of an activation model previously published by the authors. When particles arranged along the compression axis of a sheared suspension, they may overcome the electrostatic repulsion and form force chains associated with shear thickening. A percolation based consideration, allows an estimation of the impact of the force chains on a flowing suspension. It suggests that, similar to mode-coupling models, the suspension becomes unstable before the critical stress evaluated from the activation model is reached. The theory is applicable only to discontinuous shear thickening, and the predictions are compared with results from two experimental studies on aqueous suspensions of inorganic oxides; in one of them hydration repulsion and in the other hydrophobic attraction can be expected. It is shown that the incorporation of non-DLVO forces greatly improve predictions of the shear thickening instability.

This paper adopts a previously developed activation model of shear thickening, published by the authors to sterically stabilized colloidal suspensions. When particles arranged along the compression axis of a sheared suspension, they may overcome the repulsive interaction and form hydroclusters associated with shear thickening. Taking advantage of the total interaction potential of polymeric brush coating and van der Waals attraction, the applicability of the activation model is shown within the validity range of a continuum theory. For the comparison with an extensive experimental investigation, where some parameters are not available, the onset of shear thickening can be predicted with realistic assumptions of the model parameters.

Polymers dilutely adsorbed in colloidal crystals play an underappreciated role in determining the stability of the crystal phase. Recent work has shown that tailoring the size and shape of the adsorbing polymer can help tune the relative thermodynamic stability of the face-centered cubic (FCC) and hexagonal close-packed (HCP) polymorphs [N. A. Mahynski, A. Z. Panagiotopoulos, D. Meng, and S. K. Kumar, Nat. Commun., 2014, 5, 4472]. This is a consequence of how different polymorphs uniquely distribute their interstitial voids. By engineering an adsorbent's morphology to be complementary to the interstices in a desired crystal form, other competing forms may be thermodynamically suppressed. Previous investigations into this effect focused solely on linear polymers, while here we investigate the effects of more complex polymer architectures, namely that of star polymers. We find that even small perturbations to an adsorbing polymer's architecture lead to significant, qualitative changes in the relative stability of close-packed colloidal crystal polymorphs. In contrast to the linear homopolymer case, the FCC phase may be re-stabilized over the HCP with sufficiently large star polymers, and as a result, solvent quality may be used as a polymorphic "switch" between the two forms. This suggests that star polymers can be engineered to stabilize certain crystal phases at will using experimentally accessible parameters such as temperature.

Magnetic separation is a versatile technique used in sample preparation for diagnostic purpose. For such application, an externalmagnetic field is applied to drive the separation of target entity (e.g. bacteria, viruses, parasites and cancer cells) from a complex raw sample in order to ease the subsequent task(s) for disease diagnosis. This separation process not only can be achieved via the utilization of high magnetic field gradient, but also, in most cases, low magnetic field gradient with magnitude less than 100 T m21 is equally feasible. It is the aim of this review paper to summarize the usage of both high gradient magnetic separation and low gradient magnetic separation (LGMS) techniques in this area of research. It is noteworthy that effectiveness of the magnetic separation process not only determines the outcome of a diagnosis but also directly influences its accuracy aswell as sensing time involved. Therefore, understanding the factors that simultaneously influence the eff...

In the present research, stable suspension of β-brominated manganese(III) meso-tetraphenylporphyrin nanoaggregates, Mn(TPPBr X)OAc (X = 0, 2, and 4) were prepared by a host-guest procedure in which DMSO and water are used as solvents and triethylene glycol monomethyl ether is used as stabilizing agent. The organic nanoparticles (ONPs) were studied by dynamic light scattering (DLS) and UV/Vis spectroscopy. Based on the results , Mn(TPPBr 2)OAc and Mn(TPP)OAc have the highest and the lowest particle size respectively. Oxidation of styrene with tert-butyl hydroperoxide (TBHP) in the presence of these organic nanoparticles was investigated and the results indicate that the best catalytic activity (with excellent turnover No. (2000)) was obtained in the case of Mn(TPPBr 2)OAc. Organic and inorganic nanoparticles which supported on various mesoporous materials, polymers or carbon based materials widely used in science research and different publications have been investigated in the literature [1-3]. There is a growing interest in colloidal nanoparticles because of the ease of preparation and good quality which historically were produced by various chemical techniques [4-5]. Recently, self-organization of the porphyrins as nanoscale structures which can mimic related biological structures of photosynthetic organisms has been reported. These nanostructures arrange by self-assembly and other new synthetic methods [6-7]. There is a wide range of practicable applications of porphyrin NPs that derive from the photonic effects of both the specific dye molecule and the dimensions of particle. Furthermore, these synthetic porphyrin-based structures not only have certain potential for solar energy technologies, but also have possible application as optoelectronic and photonic substance for nano and microscale electronic devices [8]. The important of these self-organized nanoparticles arise from the simple methods of preparation of pigments, and high stability. Another application of these ONPs is in the catalytic reactions. When porphyrin contains a redox-active transition metal (e.g., Mn, Fe, Cu), many of these nanoparticles are more stable and active catalysts than the solvated-metalloporphyrins in different reaction media [9-12]. In the present work, three brominated Mn-por-phyrin nanoparticles were prepared and their catalytic activities in the oxidation of styrene have been studied. The free base porphyrins and manganese complexes were prepared and purified as reported previously [13]. The colloidal metalloporphyrins nanoparticles were prepared according to the host-guest procedure reported by Drain et al. [12]. Metalloporphyrin nanoparticles were prepared in 10 mL vials by adding deionized water (5 mL, pH = 6-6.5) to a mixture of triethylene glycol monomethyl ether (PEG 164) (0.20 mL) as stabilizer and a stock solution of Mn(TPPBr X)OAc (X = 0, 2, and 4) in DMSO (1.2 mM, 0.05 mL) which means the molar ratio of water/DMSO is 100/1. The solution was soni-cated during the addition (3 min) and was sonicated for an additional 5 min. The procedure is shown in Scheme 1. The prepared nanoaggregated metalloporphyrins were stable for more than two month and were refrigerated at ca. 4 °C. The size of the prepared nanoparticles was studied by dynamic light scattering (DLS). The UV/Vis spectrum of a colloidal of Mn(TPP)OAc, Mn(TPPBr 2)OAc and Mn(TPPBr 4)OAc nanoaggregate in water/DSMO were recorded and compared with those obtained with the DMSO-dissolved counterpart (Fig. 1). To show such a comparison, all the samples were adjusted to the same concentration value. In the spectrum of the dispersed metalloporphyrin nanoparticles in the all cases, the Soret and Q bands broadened and the absorbance decreased in comparison to those of the DMSO-solvated metalloporphyrin. Different parameters may affect the wavelength and intensity of these peaks. For the nanoaggregate systems, intermo-lecular interactions such as π stacking have powerful effects on electronic structure of the macrocycle as shown by mainly broadened UV/ Vis spectra. Two types of aggregation (J and H) have been reported for porphyrin macrocycles which have distinctive spectral features [9]. Based on the comparison data of the wavelength of Soret band in the solution and dispersed metalloporphyrin nanoparticles for each case, existence of a

Recent work [Mahynski et al., Nat. Commun., 2014, 5, 4472] has demonstrated that the addition of long linear homopolymers thermodynamically biases crystallizing hard-sphere colloids to produce the hexagonal close-packed (HCP) polymorph over the closely related face-centered cubic (FCC) structure when the polymers and colloids are purely repulsive. In this report, we investigate the effects of thermal interactions on each crystal polymorph to explore the possibility of stabilizing the FCC crystal structure over the HCP. We find that the HCP polymorph remains at least as stable as its FCC counterpart across the entire range of interactions we explored, where interactions were quantified by the reduced second virial coefficient,
1.50 < B2 < 1.01. This metric conveniently characterizes the crossover from entropically to energetically dominated systems at B2 ~ 0. While the HCP relies on its octahedral void arrangement for enhanced stability when B2 > 0, its tetrahedral voids produce a similar effect when B2 < 0 (i.e. when energetics dominate). Starting from this, we derive a mean-field expression for the free energy of an infinitely-dilute polymer adsorbed in the crystal phase for nonzero B2. Our results reveal that co-solute biasing of a single polymorph can still be observed in experimentally realizable scenarios when the colloids and polymers have attractive interactions, and provide a possible explanation for the experimental finding that pure FCC crystals are elusive in these binary mixtures.

A theory is presented for the onset of shear thickening in colloidal suspensions of particles, stabilized by an electrostatic repulsion. Based on an activation model a critical shear stress can be derived for the onset of shear thickening in dense suspensions for a constant potential and a constant charge approach of the spheres. Unlike previous models the total interaction potential is taken into account (sum of attraction and repulsion). The critical shear stress is related to the maximum of the total interaction potential scaled by the free volume per particle. A comparison with experimental investigations shows the applicability of the theory.

The authors previously introduced an activation model for the onset of shear thickening in electrically stabilized colloidal suspensions. It predicts that shear thickening occurs, when particles arranged along the compression axis in a sheared suspension do overcome the electrostatic repulsion at a critical shear stress, and are captured in the primary minimum of the DLVO interaction potential. A comparison with an experimental investigation on non-aqueous silica suspensions, carried out by Maranzano and Wagner, is performed. For particle systems that fall into the applicability range of the theory, a good coincidence between the experimental data and the model predictions can be found.

A second-order perturbation theory based on statistical thermodynamics is employed to calculate the phase diagram of suspensions of charge-stabilized, monodisperse, spherical, colloidal particles that are assumed to interact by a standard DLVO potential. Taking into account van der Waals forces, the phase diagram consists of an ordered colloidal crystal, a disordered fluid phase, and a liquid phase that is formed by particles flocculated into the secondary minimum.