Spectroscopic properties of colloidal indium phosphide quantum wires

Colloidal InP quantum wires are grown by the solution-liquid-solid (SLS) method, and passivated with the traditional quantum dots surfactants 1-hexadecylamine and tri-n-octylphosphine oxide. The size dependence of the band gaps in the wires are determined from the absorption spectra, and compared to other experimental results for InP quantum dots and wires, and to the predictions of theory. The photoluminescence behavior of the wires is also investigated. Efforts to enhance photoluminescence efficiencies through photochemical etching in the presence of HF result only in photochemical thinning or photooxidation, without a significant influence on quantum-wire photoluminescence. However, photooxidation produces residual dot and rod domains within the wires, which are luminescent. The results establish that the quantum-wire band gaps are weakly influenced by the nature of the surface passivation and that colloidal quantum wires have intrinsically low photoluminescence efficiencies.     

Spectroscopic study of visible-light effects on hypericin-lens proteins systems

Molecular interactions between hypericin and alpha-, beta- and gamma-crystallin proteins have been studied by means of absorption and steady-state fluorescence spectroscopy, aiming to clarify if and how the pigment binds to the proteins and to investigate the effects of visible-light irradiation on these molecular systems. Such a study is a prerequisite for assessing the possibility of using hypericin as a mild antidepressant and/or as a photodynamic agent for the treatment of eye tumors and eye viral and bacterial diseases without side injuries to the lens. We have shown that in dark-kept samples, with increasing alpha-crystallin concentration, both the fluorescence emission intensity and the ratio of the absorption maxima around 590 and 550 nm of hypericin increase. These effects have been attributed to the monomerization of nonfluorescent hypericin aggregates caused by the binding of the pigment to alpha-crystallin. The binding constant of hypericin has been evaluated to be of the order of 3.0 (mg/mL)-1, corresponding to a dissociation constant of the order of 0.3 mg/mL. Following irradiation with light of wavelengths over 400 nm, at an irradiance of 20 mW/cm2, both tryptophan and hypericin fluorescence emission intensities decrease. These effects are suggested to be the consequence of a spatial rearrangement of the protein framework which takes place following the alpha-crystallin photopolymerization sensitized by hypericin itself described in the literature. For the sake of comparison hypericin has been studied also in the presence of beta H-, beta L- and gamma-crystallins at the same concentration.     

Ordering in colloidal systems

The role played by statistical mechanics in the ordering of colloidal systems is examined. This role is made explicit through a study of phase (order-disorder) transitions in 1, 2 and 3 dimensions which exist in the examples provided by clay plate, tobacco mossaic virus and latex sphere suspensions. Essentially the statement is that the extrapolation of DLVO theory which attributes stability to a balance between attractive and repulsive two-body forces is not always a panacea. The ideas of Langmuir and Onsager who dealt with statistical mechanics and repulsive forces and considered that attractive long-range forces are not always important are paramount for some systems and must be built into existing theories.     

Nonequilibrium inertial dynamics of colloidal systems

We consider the properties of a one-dimensional fluid of Brownian inertial hard-core particles, whose microscopic dynamics is partially damped by a heat bath. Direct interactions among the particles are represented as binary, instantaneous elastic collisions. Collisions with the heat bath are accounted for by a Fokker-Planck collision operator, whereas direct collisions among the particles are treated by a well known method of kinetic theory, the revised Enskog theory. By means of a time multiple time-scale method we derive the evolution equation for the average density. Remarkably, for large values of the friction parameter and/or of the mass of the particles we obtain the same equation as the one derived within the dynamic density functional theory (DDF). In addition, at moderate values of the friction constant, the present method allows to study the inertial effects not accounted for by DDF method. Finally, a numerical test of these corrections is provided.     

Spectroscopic and crystallographic studies on the stability of self-assembled coordination nanotubes

On complexation with (en)Pd(NO3)2, tetrakis(3,5-pyridine) ligand gives two isomeric coordination nanotubes, which are in slow equilibrium despite the presence of sixteen Pd-N bonds in the tube framework.     

Osmotic compressibility of soft colloidal systems

A turbidimetric analysis of particle interaction of model pH-responsive microgel systems consisting of methacrylic acid-ethyl acrylate cross-linked with diallyl phthalate in colloidal suspensions is described. The structure factor at zero scattering angle, S(0), can be determined with good precision for wavelengths greater than 500 nm, and it measures the dispersion's resistance to particle compression. The structure factor of microgels at various cross-linked densities and ionic strengths falls onto a master curve when plotted against the effective volume fraction, phi(eff) = kc, which clearly suggests that particle interaction potential and osmotic compressibility is a function of effective volume fraction. In addition, the deviation of the structure factor, S(0), of our microgel systems with the structure factor of hard spheres, S(PY)(0), exhibits a maximum at phi(eff) approximately 0.2. Beyond this point the osmotic de-swelling force exceeds the osmotic pressure inside the soft particles resulting in particle shrinkage. Good agreement was obtained when the structural properties of our microgel systems obtained from turbidimetric analysis and rheology measurements were compared. Therefore, a simple turbidimetric analysis of these model pH-responsive microgel systems permits a quantitative evaluation of factors governing particle osmotic compressibility.     

Superhydrophobic surfaces from sustainable colloidal systems

In recent decades, sustainable superhydrophobic surfaces from natural materials and sustainable processes have attracted increased interest due to their lower environmental footprint and potential applications in self-cleaning surfaces and biomedical devices. Although there is significant progress on selecting suitable nano and micro particles to prepare superhydrophobic surfaces, a comprehensive review on the direct use of sustainable colloidal particles (SCPs) is lacking. In this review, we highlight the recent advances on sustainable superhydrophobic surfaces using SCPs. The composition and properties, extraction methods, and chemical modifications are described, including cellulose nanocrystals, chitin/chitosan nanoparticles, and lignin nanoparticles. In addition to the physico–chemical properties and tunable dimensionality, the fabrication methodologies of superhydrophobic surfaces using modified colloids are described. Finally, the potential applications of these sustainable superhydrophobic surfaces ranging from oil/water separation, biomedical, water harvesting, biofabrication, microfluidic reactor, and food packaging are discussed together with a future perspective on the advances made.     

Spectroscopic investigation of a new hybrid glass formed by the interaction between croconate ion and calcium polyphosphate

In this work, a new organic-inorganic hybrid material has been synthesized by the incorporation of croconate ion into a calcium polyphosphate coacervate. The hybrid so obtained was characterized by means of electronic and vibrational spectroscopies. The material is a homogeneous mixture described by a structural model, which includes helical chains of polyphosphate ions, where the calcium ion occupies the internal vacancies of the structure. The croconate ion appears to be occupying the regions outside the polymeric structure, surrounded by several water molecules. The electronic spectrum of the incorporated material shows a broad band peaking at the same wavelength region (363 nm) observed for the aqueous solution of croconate ion, and manifesting the Jahn-Teller effect as evidenced by the doublet structure of the band. The infrared spectrum is widely dominated by the absorption bands of the polyphosphate ion and the appearance of the carbonyl stretching band at ca. 1550 cm(-1) indicates the presence of croconate ion incorporated in the structure. The Raman spectrum of the material shows several vibrational bands related to the oxocarbon moiety; most of them are shifted in comparison with the free ion. These shifts can be understood in terms of strong hydrogen bonding interactions between water molecules and the oxocarbon moiety. The low temperature methodology proposed here can be well used in the preparation of new phosphate glasses containing organic moieties opening the route to an entirely new class of hybrid glasses.     

Calculation of interparticle spacing in colloidal systems

Interparticle spacing (IPS) is a very important parameter for estimation of the viscosity of a suspension. A new equation for calculating IPS was derived on the assumption that each particle is surrounded by a virtual cell, which is the free volume that each particle can occupy. This idea was originated by Kuwabara and widely used for calculating electrophoretic mobility in concentrated suspensions. Our new IPS equation was evaluated and compared with a pre-existing IPS equation proposed by Dinger and Funk using experimental data from three commercial sources of titania. Our equation was found to give reasonable values, including cases where the equation of Dinger and Funk gave anomalous values.     

Mesoscale modeling for self-organization of colloidal systems

This article reviews recent contributions to mesoscale modeling for self-organization of colloidal systems. The review covers both off-lattice models and off-lattice–on-lattice hybrid models that are able to describe the motion of colloidal particles with hydrodynamic interaction. All the models are compared with one another in terms of accuracy, applicable scope, robustness and efficiency.     

Tetraethanolammonium counterions in surfactant and classical colloidal systems

Interaction forces between mica surfaces as a function of added tetraethanolammonium bromide, (EtOH)₄NBr, were determined using surface forces measurements. Critical micelle concentrations and aggregation numbers were determined for tetraethanolammonium dodecylsulfate, (EtOH)₄NDS, using surface tension and fluorescence probe-quencher measurements. The properties of the large hydrophilic counterion (EtOH)₄N⁺ are compared to those of its hydrophobic analogue Pr₄N⁺.     

Specific ion effects in colloidal and biological systems

During the last ten years significant progress has been made in the understanding of specific ion effects. On the one hand new ideas about the origin of these effects came up, and on the other hand new experimental techniques were developed so that now even the ion concentration profile near surfaces can be measured with some confidence. In the present review some of the most important new progresses are summarised and critically discussed, especially in the context of colloidal and biological systems.     

Polyelectrolyte complexes: bulk phases and colloidal systems

When aqueous solutions of polycations and polyanions are mixed, polyelectrolyte complexes form. These are usually insoluble in water, so that they separate out as a new concentrated polymer phase, called a complex coacervate. The behavior of these complexes is reviewed, with emphasis on new measurements that shed light on their structural and mechanical properties, such as cohesive energy, interfacial tension, and viscoelasticity. It turns out that stoichiometric complexes can be considered in many respects as pseudo-neutral, weakly hydrophobic polymers, which are insoluble in water, but become progressively more soluble as salt is added. In fact, the solubility-enhancing effect of salt is quite analogous to that of temperature for polymers in apolar solvents. Since two-phase systems can be prepared in colloidal form, we also discuss several kinds of colloids or 'microphases' that can arise due to polyelectrolyte complexation, such as thin films, 'zipper' brushes, micelles, and micellar networks. A characteristic feature of these charge-driven two-phase systems is that two polymeric ingredients are needed, but that some deviation from strict stoichiometry is tolerated. This turns out to nicely explain how and when the layer-by-layer method works, how a 'leverage rule' applies to the density of the 'zipper brush', and why soluble complexes or micelles appear in a certain window of composition. As variations on the theme, we discuss micelles with metal ions in the core, due to incorporation of supramolecular coordination polyelectrolytes, and micellar networks, which form a new kind of physical gels with unusual properties.