Active Sites Derived from Heteroatom Doping in Carbon Materials for Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is a key cathode reaction in fuel cells. Due to the sluggish kinetics of the ORR, various kinds of catalysts have been developed to compensate for the shortcomings of the cathode reaction. Carbon materials are considered ideal cathode catalysts. In particular, heteroatom doping is essential to achieve an excellent ORR activity. Interestingly, doping trace amounts of metals in carbon materials plays an important role in enhancing the electrocatalytic activities. This chapter describes the recent advancements with regard to heteroatom-doped carbons and discusses the active sites decorated in the carbon matrix in terms of their configurations and contents, as well as their effectiveness in boosting the ORR performance. Furthermore, trace metal residues and metal-free catalysts for the ORR are clarified

The Role of Sulfur-Related Species in Oxygen Reduction Reactions

Heteroatom (metal and nonmetal) doping is essential to achieve excellent oxygen reduction reaction (ORR) activity of carbon materials. Among the heteroatoms that have been studied to date, sulfur (S) doping, including metal sulfides and sulfur atoms, has attracted tremendous attention. Since S-doping can modify spin density distributions around the metal centers as well as the synergistic effect between S and other doped heteroatoms, the S-C bond and metal sulfides can function as important ORR active sites. Furthermore, the S-doped hybrid sample shows a small charge-transfer resistance. Therefore, S-doping contributes to the superior ORR performance. This chapter describes the recent advancements of S-doped carbon materials, and their development in the area of ORR with regard to components, structures, and their ORR activities of S-related species

Uniform Chitosan Microparticles Prepared by a Novel Spray-Drying Technique

Particle size and morphology are important properties of pharmaceutical particles. Preparation of microparticles with uniform particle size and morphology is necessary in order to systematically relate these properties to the release behavior and other functionalities such as drug encapsulation and dissolution. In this study, we successfully prepared monodisperse, nonagglomerated chitosan microparticles in a single step by a novel spray-drying technique. The control of particle size and morphology of spray-dried microparticles was investigated experimentally. Microparticles with larger particle size can be produced when chitosan precursor of higher concentration was used. Storage time of chitosan precursor, drying temperature, and addition of lactose were shown to be crucial parameters that affect the particle morphology. Appropriate choice of the drying temperature and precursor storage time permitted control of the particle morphology, ranging from nearly spherical to cap-shaped. Surface characteristics of the particles can be finely tuned by the amount of lactose added into the chitosan precursor

On spray drying of uniform silica-based microencapsulates for controlled release

Although spray drying is a scalable route for particle formation with easy product recovery, a typical spray drier produces broad distributions of particles with various morphologies in a single batch, due to the non-uniform formation of droplets, wide spray trajectories, and various residence times experienced by the droplets during drying. Thus any attempt to interpret the resulting particle functionality directly in relation to their physico-chemical properties is difficult. Here, uniform silica-based microencapsulates encapsulating vitamin B(12) homogenously distributed within their matrix were synthesised in a single step via a micro-fluidic-jet-spray-dryer (MFJSD), utilising a micro-fluidic-aerosol-nozzle (MFAN) for continuous generation of monodisperse droplets. We investigated the effects of lactose and Na-alginate to the properties of the silica matrix, as well as the overall particle shapes. The uniform nature of the particles allowed direct correlations between the matrix properties and the release behaviour of vitamin B(12) to be observed without the complications of wide size distribution or variety of shapes. Spherical particles with relatively smooth surface were obtained with lactose addition, while incorporation of Na-alginate resulted in increasing surface roughness. Lactose accelerated the release of the encapsulated vitamin B(12) (VB12), due to the relatively fast lactose dissolution that allowed buffer to penetrate deep into the matrix to facilitate diffusion and silica erosion. On the contrary, Na-alginate slowed down the release considerably by serving as an additional barrier to decelerate the matrix erosion, as well as due to ionic attraction to the VB12 molecules. Release kinetics data indicated diffusion as the main release mechanism independent of the microencapsulate composition. The release profiles from different compositions of the synthesized particles demonstrated good agreements with the computational predictions, highlighting the ability to modulate the release behaviour directly from the precursor compositions.Australian Research Council (ARC)[DP0773688

Aerosol-Assisted Fast Formulating Uniform Pharmaceutical Polymer Microparticles with Variable Properties toward pH-Sensitive Controlled Drug Release

Microencapsulation is highly attractive for oral drug delivery. Microparticles are a common form of drug carrier for this purpose. There is still a high demand on efficient methods to fabricate microparticles with uniform sizes and well-controlled particle properties. In this paper, uniform hydroxypropyl methylcellulose phthalate (HPMCP)-based pharmaceutical microparticles loaded with either hydrophobic or hydrophilic model drugs have been directly formulated by using a unique aerosol technique, i.e., the microfluidic spray drying technology. A series of microparticles of controllable particle sizes, shapes, and structures are fabricated by tuning the solvent composition and drying temperature. It is found that a more volatile solvent and a higher drying temperature can result in fast evaporation rates to form microparticles of larger lateral size, more irregular shape, and denser matrix. The nature of the model drugs also plays an important role in determining particle properties. The drug release behaviors of the pharmaceutical microparticles are dependent on their structural properties and the nature of a specific drug, as well as sensitive to the pH value of the release medium. Most importantly, drugs in the microparticles obtained by using a more volatile solvent or a higher drying temperature can be well protected from degradation in harsh simulated gastric fluids due to the dense structures of the microparticles, while they can be fast-released in simulated intestinal fluids through particle dissolution. These pharmaceutical microparticles are potentially useful for site-specific (enteric) delivery of orally-administered drugs

Spray drying of monodispersed microencapsulates: implications of formulation and process parameters on microstructural properties and controlled release functionality

Monash University; China scholarship councilParticulates for pharmaceutical applications require stringent control over their characteristics to realize the optimal therapeutic performance. By generating uniform spray-dried silica particles encapsulating different model drugs via a microfluidic jet spray drying technique, we demonstrated how the effects of formulation and process parameters on the investigated properties could be directly quantified without the complications of wide particle distributions typical of conventional spray drying. The implemented strategies included incorporating lactose to modify the internal microstructures to regulate release, and increasing drying temperature during synthesis to modify the surface features of particles. The physicochemical properties of encapsulated drugs were shown to influence particle morphologies and release profiles, while the pH of initial precursors influenced the particle morphologies with slight effects on the initial release rates. The outcomes would be useful to indentify appropriate formulations and manufacturing parameters in designing spray-dried silica-based microencapsulates with tailor-made controlled release functionalities

On designing particulate carriers for encapsulation and controlled release applications

A microfluidic jet spray drying technique was used to encapsulate hydrophilic drug in uniform microparticles, while tailoring their controlled release functionalities. The effects of different matrix compositions on the release behaviour of a model drug were conducted by spray drying an aqueous polymeric dispersion of a neutral copolymer based on ethyl acrylate and methyl methacrylate (Eudragit (R) NE) as the main encapsulating matrix. Lactose and silica nanoparticles were used as additives to modify the matrix compositions, with vitamin B-12 as the model drug. Evaporation-induced self-assembly of a model drug (vitamin B-12) and the matrix materials due to their colloidal interactions produced microparticles with specific morphologies for immediate or prolonged releases. Having lactose distributed homogeneously in the matrix resulted in significantly faster and almost complete release due to enhanced swelling of the polymeric matrix with the dissolution of lactose. In contrast, silica nanoparticles existed mainly at the surface of the particles, due to the slower diffusion of nanoparticles within the droplets upon drying, which could be responsible for the initial burst release of vitamin B-12 molecules with erosion of nanoparticles upon contact with the buffer. These outcomes demonstrated the capability to tune the particle response(s) from the knowledge of material properties, with the understanding of release mechanisms elucidated from monodisperse particles of different compositions. (C) 2012 Elsevier B.V. All rights reserved

Spray-drying water-based assembly of hierarchical and ordered mesoporous silica microparticles with enhanced pore accessibility for efficient bio-adsorption

The fast and scalable spray-drying-assisted evaporation-induced self-assembly (EISA) synthesis of hierarchically porous SBA-15-type silica microparticles from a water-based system is demonstrated. The SBA-15-type silica microparticles has bowl-like shapes, uniform micro-sizes (∼90 µm), large ordered mesopores (∼9.5 nm), hierarchical meso-/macropores (20–100 nm) and open surfaces. In the synthesis, soft- and hard-templating approaches are combined in a single rapid drying process with a non-ionic tri-block copolymer (F127) and a water-insoluble polymer colloid (Eudragit RS, 120 nm) as the co-templates. The RS polymer colloid plays three important roles. First, the RS nanoparticles can be partially dissolved by in-situ generated ethanol to form RS polymer chains. The RS chains swell and modulate the hydrophilic-hydrophobic balance of F127 micelles to allow the formation of an ordered mesostructure with large mesopore sizes. Without RS, only worm-like mesostructure with much smaller mesopore sizes can be formed. Second, part of the RS nanoparticles plays a role in templating the hierarchical pores distributed throughout the microparticles. Third, part of the RS polymer forms surface “skins” and “bumps”, which can be removed by calcination to enable a more open surface structure to overcome the low pore accessibility issue of spray-dried porous microparticles. The obtained materials have high surface areas (315–510 m2 g−1) and large pore volumes (0.64–1.0 cm3 g−1), which are dependent on RS concentration, HCl concentration, silica precursor hydrolysis time and drying temperature. The representative materials are promising for the adsorption of lysozyme. The adsorption occurs at a >three-fold faster rate, in a five-fold larger capacity (an increase from 20 to 100 mg g−1) and without pore blockage compared with the adsorption of lysozyme onto spray-dried microparticles of similar physicochemical properties obtained without the use of RS

Particle shrinkage and morphology of milk powder made with a monodisperse spray dryer

The drying of monodisperse droplets can produce uniform powders where every particle has experienced similar, predictable air conditions. For the first time, a single-stream dryer was used with monodisperse milk droplets having solids contents as high as those used in industrial spray dryers (over 40 wt% total solids). With size measurement of droplets and powders, the particle shrinkage was precisely determined. Changes in shrinkage and morphology were observed when varying feed solids contents and drying temperatures. The majority of particles had an inwardly buckled morphology and hollow, inflated particles were produced at higher temperatures due to boiling. Drying was modelled with a numerical simulation using the Reaction Engineering Approach (REA), which is a semi-empirical model of moisture removal rate. This model accurately predicted the moisture contents of the powders collected. Several sub-models were used for particle shrinkage. The most accurate shrinkage assumption was found to be isotropic shrinkage from the removal of water (known as perfect shrinkage). This may suggest that the surface crust (initially spherical) is viscoelastic and deforms in response to drying stresses. (C) 2011 Elsevier B.V. All rights reserved