Reduction of Cofed Carbon Dioxide Modifies the Local Coordination Environment of Zeolite-Supported, Atomically Dispersed Chromium to Promote Ethane Dehydrogenation

The reduction of CO2 is known to promote increased alkene yields from alkane dehydrogenations when the reactions are cocatalyzed. The mechanism of this promotion is not understood in the context of catalyst active-site environments because CO2 is amphoteric, and even general aspects of the chemistry, including the significance of competing side reactions, differ significantly across catalysts. Atomically dispersed chromium cations stabilized in highly siliceous MFI zeolite are shown here to enable the study of the role of parallel CO2 reduction during ethylene-selective ethane dehydrogenation. Based on infrared spectroscopy and X-ray absorption spectroscopy data interpreted through calculations using density functional theory (DFT), the synthesized catalyst contains atomically dispersed Cr cations stabilized by silanol nests in micropores. Reactor studies show that cofeeding CO2 increases stable ethylene-selective ethane dehydrogenation rates over a wide range of partial pressures. Operando X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) spectra indicate that during reaction at 650 °C the Cr cations maintain a nominal 2+ charge and a total Cr-O coordination number of approximately 2. However, CO2 reduction induces a change, correlated with the CO2 partial pressure, in the population of two distinct Cr-O scattering paths. This indicates that the promotional effect of parallel CO2 reduction can be attributed to a subtle change in Cr-O bond lengths in the local coordination environment of the active site. These insights are made possible by simultaneously fitting multiple EXAFS spectra recorded in different reaction conditions; this novel procedure is expected to be generally applicable for interpreting operando catalysis EXAFS data.

Reversible Intrapore Redox Cycling of Platinum in Platinum-Ion-Exchanged HZSM-5 Catalysts

Isolated platinum(II) ions anchored at acid sites in the pores of zeolite HZSM-5, initially introduced by aqueous ion exchange, were reduced to form platinum nanoparticles that are stably dispersed with a narrow size distribution (1.3 ± 0.4 nm in average diameter). The nanoparticles were confined in reservoirs within the porous zeolite particles, as shown by electron beam tomography and the shape-selective catalysis of alkene hydrogenation. When the nanoparticles were oxidatively fragmented in dry air at elevated temperature, platinum returned to its initial in-pore atomically dispersed state with a charge of +2, as shown previously by X-ray absorption spectroscopy. The results determine the conditions under which platinum is retained within the pores of HZSM-5 particles during redox cycles that are characteristic of the reductive conditions of catalyst operation and the oxidative conditions of catalyst regeneration.

Developing Cheap but Useful Machine Learning-Based Models for Investigating High-Entropy Alloy Catalysts

This work aims to address the challenge of developing interpretable ML-based models when access to large-scale computational resources is limited. Using CoMoFeNiCu high-entropy alloy catalysts as an example, we present a cost-effective workflow that synergistically combines descriptor-based approaches, machine learning-based force fields, and low-cost density functional theory (DFT) calculations to predict high-quality adsorption energies for H, N, and NHx (x = 1, 2, and 3) adsorbates. This is achieved using three specific modifications to typical DFT workflows including: (1) using a sequential optimization protocol, (2) developing a new geometry-based descriptor, and (3) repurposing the already-available low-cost DFT optimization trajectories to develop a ML-FF. Taken together, this study illustrates how cost-effective DFT calculations and appropriately designed descriptors can be used to develop cheap but useful models for predicting high-quality adsorption energies at significantly lower computational costs. We anticipate that this resource-efficient philosophy may be broadly relevant to the larger surface catalysis community.

Screening Cu-Zeolites for Methane Activation Using Curriculum-Based Training

Machine learning (ML), when used synergistically with atomistic simulations, has recently emerged as a powerful tool for accelerated catalyst discovery. However, the application of these techniques has been limited by the lack of interpretable and transferable ML models. In this work, we propose a curriculum-based training (CBT) philosophy to systematically develop reactive machine learning potentials (rMLPs) for high-throughput screening of zeolite catalysts. Our CBT approach combines several different types of calculations to gradually teach the ML model about the relevant regions of the reactive potential energy surface. The resulting rMLPs are accurate, transferable, and interpretable. We further demonstrate the effectiveness of this approach by exhaustively screening thousands of [CuOCu]2+ sites across hundreds of Cu-zeolites for the industrially relevant methane activation reaction. Specifically, this large-scale analysis of the entire International Zeolite Association (IZA) database identifies a set of previously unexplored zeolites (i.e., MEI, ATN, EWO, and CAS) that show the highest ensemble-averaged rates for [CuOCu]2+-catalyzed methane activation. We believe that this CBT philosophy can be generally applied to other zeolite-catalyzed reactions and, subsequently, to other types of heterogeneous catalysts. Thus, this represents an important step toward overcoming the long-standing barriers within the computational heterogeneous catalysis community.

Predicting Structural Properties of Pure Silica Zeolites Using Deep Neural Network Potentials

Machine learning potentials (MLPs) capable of accurately describing complex ab initio potential energy surfaces (PESs) have revolutionized the field of multiscale atomistic modeling. In this work, using an extensive density functional theory (DFT) data set (denoted as Si-ZEO22) consisting of 219 unique zeolite topologies (350,000 unique DFT calculations) found in the International Zeolite Association (IZA) database, we have trained a DeePMD-kit MLP to model the dynamics of silica frameworks. The performance of our model is evaluated by calculating various properties that probe the accuracy of the energy and force predictions. This MLP demonstrates impressive agreement with DFT for predicting zeolite structural properties, energy-volume trends, and phonon density of states. Furthermore, our model achieves reasonable predictions for stress-strain relationships without including DFT stress data during training. These results highlight the ability of MLPs to capture the flexibility of zeolite frameworks and motivate further MLP development for nanoporous materials with near-ab initio accuracy.

Elucidating correlated defects in metal organic frameworks using theory-guided inelastic neutron scattering spectroscopy

Metal organic frameworks (MOFs) that incorporate metal oxide cluster nodes, exemplified by UiO-66, have been widely studied, especially in terms of their deviations from the ideal, defect-free crystalline structures. Although defects such as missing linkers, missing nodes, and the presence of adventitious synthesis-derived node ligands (such as acetates and formates) have been proposed, their exact structures remain unknown. Previously, it was demonstrated that defects are correlated and span multiple unit cells. The highly specialized techniques used in these studies are not easily applicable to other MOFs. Thus, there is a need to develop new experimental and computational approaches to understand the structure and properties of defects in a wider variety of MOFs. Here, we show how low-frequency phonon modes measured by inelastic neutron scattering (INS) spectroscopy can be combined with density functional theory (DFT) simulations to provide unprecedented insights into the defect structure of UiO-66. We are able to identify and assign peaks in the fingerprint region (<100 cm^-1) which correspond to phonon modes only present in certain defective topologies. Specifically, this analysis suggests that our sample of UiO-66 consists of predominantly defect-free fcu regions with smaller domains corresponding to a defective bcu topology with 4 and 2 acetate ligands bound to the Zr₆O₈ nodes. Importantly, the INS/DFT approach provides detailed structural insights (e.g., relative positions and numbers of acetate ligands) that are not accessible with microscopy-based techniques. The quantitative agreement between DFT simulations and the experimental INS spectrum combined with the relative simplicity of sample preparation, suggests that this methodology may become part of the standard and preferred protocol for the characterization of MOFs, and, in particular, for elucidating the structure defects in these materials.

Interconversion of Atomically Dispersed Platinum Cations and Platinum Clusters in Zeolite ZSM-5 and Formation of Platinum gem-Dicarbonyls

Catalysts composed of platinum dispersed on zeolite supports are widely applied in industry, and coking and sintering of platinum during operation under reactive conditions require their oxidative regeneration, with the platinum cycling between clusters and cations. The intermediate platinum species have remained only incompletely understood. Here, we report an experimental and theoretical investigation of the structure, bonding, and local environment of cationic platinum species in zeolite ZSM-5, which are key intermediates in this cycling. Upon exposure of platinum clusters to O₂ at 700 °C, oxidative fragmentation occurs, and Pt²⁺ ions are stabilized at six-membered rings in the zeolite that contain paired aluminum sites. When exposed to CO under mild conditions, these Pt²⁺ ions form highly uniform platinum gem-dicarbonyls, which can be converted in H₂ to Pt^δ+ monocarbonyls. This conversion, which weakens the platinum-zeolite bonding, is a first step toward platinum migration and aggregation into clusters. X-ray absorption and infrared spectra provide evidence of the reductive and oxidative transformations in various gas environments. The chemistry is general, as shown by the observation of platinum gem-dicarbonyls in several commercially used zeolites (ZSM-5, Beta, mordenite, and Y).

Atomically Dispersed Platinum in Surface and Subsurface Sites on MgO Have Contrasting Catalytic Properties for CO Oxidation

Atomically dispersed metals on metal oxide supports are a rapidly growing class of catalysts. Developing an understanding of where and how the metals are bonded to the supports is challenging because support surfaces are heterogeneous, and most reports lack a detailed consideration of these points. Herein, we report two atomically dispersed CO oxidation catalysts having markedly different metal-support interactions: platinum in the first layer of crystalline MgO powder and platinum in the second layer of this support. Structural models have been determined on the basis of data and computations, including those determined by extended X-ray absorption fine structure and X-ray absorption near edge structure spectroscopies, infrared spectroscopy of adsorbed CO, and scanning transmission electron microscopy. The data demonstrate the transformation of surface to subsurface platinum as the temperature of sample calcination increased. Catalyst performance data demonstrate the lower activity but greater stability of the subsurface platinum than of the surface platinum.

A Theory-Guided X-ray Absorption Spectroscopy Approach for Identifying Active Sites in Atomically Dispersed Transition-Metal Catalysts

Atomically dispersed supported metal catalysts offer new properties and the benefits of maximized metal accessibility and utilization. The characterization of these materials, however, remains challenging. Using atomically dispersed platinum supported on crystalline MgO (chosen for its well-defined bonding sites) as a prototypical example, we demonstrate how systematic density functional theory calculations for assessing all the potentially stable platinum sites, combined with automated analysis of extended X-ray absorption fine structure (EXAFS) spectra, leads to unbiased identification of isolated, surface-enveloped platinum cations as the catalytic species for CO oxidation. The catalyst has been characterized by atomic-resolution imaging and EXAFS and high-energy resolution fluorescence detection X-ray absorption near edge spectroscopy. The proposed platinum sites are in agreement with experiment. This theory-guided workflow leads to rigorously determined structural models and provides a more detailed picture of the structure of the catalytically active site than what is currently possible with conventional EXAFS analyses. As this approach is efficient and agnostic to the metal, support, and catalytic reaction, we posit that it will be of broad interest to the materials characterization and catalysis communities.

Cation-exchanged zeolites for the selective oxidation of methane to methanol

Development of an ideal methane activation catalyst presents a trade-off between stability and reactivity of the active site that can be achieved by tuning the transition metal cation, active site motif and the zeolite topology.

Understanding trends in C-H bond activation in heterogeneous catalysis

While the search for catalysts capable of directly converting methane to higher value commodity chemicals and liquid fuels has been active for over a century, a viable industrial process for selective methane activation has yet to be developed. Electronic structure calculations are playing an increasingly relevant role in this search, but large-scale materials screening efforts are hindered by computationally expensive transition state barrier calculations. The purpose of the present letter is twofold. First, we show that, for the wide range of catalysts that proceed via a radical intermediate, a unifying framework for predicting C-H activation barriers using a single universal descriptor can be established. Second, we combine this scaling approach with a thermodynamic analysis of active site formation to provide a map of methane activation rates. Our model successfully rationalizes the available empirical data and lays the foundation for future catalyst design strategies that transcend different catalyst classes.

DFT-Derived Force Fields for Modeling Hydrocarbon Adsorption in MIL-47(V)

Generic force fields such as UFF and DREIDING are widely used for predicting molecular adsorption and diffusion in metal-organic frameworks (MOFs), but the accuracy of these force fields is unclear. We describe a general framework for developing transferable force fields for modeling the adsorption of alkanes in a nonflexible MIL-47(V) MOF using periodic density functional theory (DFT) calculations. By calculating the interaction energies for a large number of energetically favorable adsorbate configurations using DFT, we obtain a force field that gives good predictions of adsorption isotherms, heats of adsorption, and diffusion properties for a wide range of alkanes and alkenes in MIL-47(V). The force field is shown to be transferable to related materials such as MIL-53(Cr) and is used to calculate the free-energy differences for the experimentally observed phases of MIL-53(Fe).

Role of amine structure on carbon dioxide adsorption from ultradilute gas streams such as ambient air

A fundamental study on the adsorption properties of primary, secondary, and tertiary amine materials is used to evaluate what amine type(s) are best suited for ultradilute CO(2) capture applications. A series of comparable materials comprised of primary, secondary, or tertiary amines ligated to a mesoporous silica support via a propyl linker are used to systematically assess the role of amine type. Both CO(2) and water adsorption isotherms are presented for these materials in the range relevant to CO(2) capture from ambient air and it is demonstrated that primary amines are the best candidates for CO(2) capture from air. Primary amines possess both the highest amine efficiency for CO(2) adsorption as well as enhanced water affinity compared to other amine types or the bare silica support. The results suggest that the rational design of amine adsorbents for the extraction of CO(2) from ambient air should focus on adsorbents rich in primary amines.

Enhanced visible photoluminescence in ZnO quantum dots by promotion of oxygen vacancy formation

We report on the synthesis of ZnO quantum dots (QDs) rich in oxygen vacancies by inducing an oxygen deficient environment. The precise tunability of particle size is achieved by counter ion capping of the precursor used for synthesis. The prepared QDs show size tunable visible emission with high quantum yield.

pH-Sensitive Mebeverine Microspheres for Colon Delivery

Mebeverine hydrochloride is known to suffer from extensive first pass effect. In an attempt to improve its oral bioavailability and possibility to restrict its absorption only to the colon, mebeverine microspheres were prepared by emulsion solvent evaporation method. Four formulations were prepared with varying drug and polymer ratio. These formulations were subjected to various evaluation parameters like percent practical yield, entrapment efficiency, particle size, in vitro drug release, in vivo activity. Practical yield of the microspheres was up to 89.59% with encapsulation efficiency up to 79.4%. Scanning electron microscopy confirmed that the microsphere structures were smooth, spherical, and discrete and the particles were of the size range 200 to 300 μm. In vitro release of the drug showed biphasic release pattern with non-Fickian diffusion release in 12 h. On the basis of drug content, particle size, in vitro release and in vivo studies, formulation F-3 was found to be optimal. Antiirritable bowel syndrome activity was performed in colorectal distention in rat, which is a model for constipation-induced irritable bowel syndrome. The formulations F-2 and F-3 showed significant effect in fecal output when compared to the control as well as the marketed preparation in the constipation-induced irritable bowel syndrome in rats.

Plant system: nature's nanofactory

Three categories of plants growing under three different extreme conditions were taken for assaying their promises to undertake nano-transformation. It was found that all of them successfully synthesize silver nanoparticles. The synthesis was performed akin to room temperature. X-ray and transmission electron microscopy analyses were performed to ascertain the formation of silver nanoparticles. X-ray analysis indicated that silver nanoparticles have FCC unit cell structure. Individual nanoparticles having the particle sizes of 2-5 nm were found. Possible involved mechanisms for the synthesis of silver nanoparticles from above plant systems have also been proposed.

In vitro and in vivo evaluation of ranitidine hydrochloride ethyl cellulose floating microparticles

The real issue in the development of oral controlled release dosage forms is not just to prolong the delivery of drugs but also to prolong the presence of dosage forms in the stomach in order to improve the bioavailability of drugs with a 'narrow absorption window'. In the present study, an anti-ulcer drug, ranitidine hydrochloride, is delivered through a gastroretentive ethyl cellulose-based microparticulate system capable of floating on simulated gastric fluid for > 12 h. Preparation of microparticles is done by solvent evaporation technique with modification by using an ethanol co-solvent system. The formulated microspheres were free flowing with good packability and encapsulation efficiencies were up to 96%. Scanning electron microscopy confirmed porous, spherical particles in the size range 300-750 microm. Microspheres showed excellent buoyancy and a biphasic controlled release pattern with 12h. In vivo bioavailability studies performed on rabbits and T(max), C(max), AUC were calculated and confirmed significant improvement in bioavailability. The data obtained thus suggests that a microparticulate floating delivery system can be successfully designed to give controlled drug delivery, improved oral bioavailability and many other desirable characteristics.

Lactobacillusassisted synthesis of titanium nanoparticles

An eco-friendlylactobacillussp. (microbe) assisted synthesis of titanium nanoparticles is reported. The synthesis is performed at room temperature. X-ray and transmission electron microscopy analyses are performed to ascertain the formation of Ti nanoparticles. Individual nanoparticles as well as a number of aggregates almost spherical in shape having a size of 40–60 nm are found.

Time and pH dependent colon specific, pulsatile delivery of theophylline for nocturnal asthma

In this study, investigation of an oral colon specific, pulsatile device to achieve time and/or site specific release of theophylline, based on chronopharmaceutical consideration. The basic design consists of an insoluble hard gelatin capsule body, filled with eudragit microcapsules of theophylline and sealed with a hydrogel plug. The entire device was enteric coated, so that the variability in gastric emptying time can be overcome and a colon-specific release can be achieved. The theophylline microcapsules were prepared in four batches, with Eudragit L-100 and S-100 (1:2) by varying drug to polymer ratio and evaluated for the particle size, drug content and in vitro release profile and from the obtained results; one better formulation was selected for further fabrication of pulsatile capsule. Different hydrogel polymers were used as plugs, to maintain a suitable lag period and it was found that the drug release was controlled by the proportion of polymers used. In vitro release studies of pulsatile device revealed that, increasing the hydrophilic polymer content resulted in delayed release of theophylline from microcapsules. The gamma scintigraphic study pointed out the capability of the system to release drug in lower parts of GIT after a programmed lag time for nocturnal asthma. Programmable pulsatile, colon-specific release has been achieved from a capsule device over a 2-24h period, consistent with the demands of chronotherapeutic drug delivery.

Phacoemulsification in true exfoliation of the lens capsule: a case series

PURPOSE

We report five cases (seven eyes) of true exfoliation during an 18-month period. Of the two bilateral cases, the first was identified immediately before cataract surgery and the second spontaneously developed a split in the anterior capsule just before capsulorrhexis, mimicking a partial capsulorrhexis. In the three unilateral cases, true exfoliation was noted during the first examination.

METHODS

Vision blue aided uneventful capsulorrhexis differentiating its edge from the true exfoliation edge, and in the first two cases, the anterior capsule was sent for histopathology and ultrasound of the fellow eye was requested.

RESULTS

Ultrasound and histopathology demonstrated lamellar separation of the anterior portion of the lens capsule, confirming the diagnosis of true exfoliation. Cataract surgery by phacoemulsification was uneventful in all cases.

CONCLUSION

True exfoliation of the lens capsule can masquerade as a partial capsulorrhexis and should be looked for before surgery and immediately before capsulorrhexis to avoid creating a partial thickness capsulorrhexis and its related surgical complications. No zonule weakness was appreciated in our cases. To our knowledge, the spontaneous occurrence of a curvilinear lamellar capsular dehiscence with a flap before capsulorrhexis has not been reported before. This series highlights that cataracts associated with true exfoliation of the lens capsule can be safely operated, with the help of vision blue, by routine phacoemulsification without having to convert to the extracapsular technique.

Ocular manifestations of head injury: a clinical study

AIM

This prospective study aimed to clinically correlate the various ocular findings with the neurological status in cases of closed head injury.

METHODS

A total of 200 consecutive cases of closed head injury admitted to a major teaching hospital underwent a thorough ophthalmic assessment. The Glasgow coma scale (GCS) and the Revised trauma score (RTS) were applied to grade the severity of injury and assess the prognosis. Kendall's tau-b and Fisher's exact test were used in the analysis.

RESULTS

The main causes of head injury were road traffic accidents 52.5% followed by assaults in 34%. Ocular involvement was found in 167(83.5%) cases. These included corneal and scleral tears in 2%, subconjunctival haemorrhage or ecchymosis in 46%, orbital fractures 12%, pupillary involvement 6.5%, papilloedema 5.5%, intraocular trauma 5.5%, proptosis 3%, lateral rectus palsy 2%, lacrimal gland prolapse 1%, and optic nerve trauma 0.5%. All 21 patients (10.5%) who died had eye involvement. In all, 150 cases (75%) with a RTS of 12 had a good prognosis. Of these 124 (82.6%) had ocular involvement of no neurological significance.

CONCLUSIONS

Although sophisticated imaging techniques are available to localize lesions, early ophthalmic assessment in correlation with the GCS aids in prognosticating outcomes. Pupillary involvement, papilloedema, and ocular motor paresis pointed to a more severe head injury. To our knowledge, this is the only prospective study recording ocular findings in the first few hours and attempting a correlation with the final outcome.

Spontaneous rupture of spleen in falciparum malaria

Spontaneous rupture of spleen is an extremely rare complication of falciparum malaria. We report a 3 1/2-year-old girl with splenic rupture who was managed successfully with splenectomy and antimalarials.

Crosslinked chitosan microspheres for encapsulation of diclofenac sodium: effect of crosslinking agent

Microspheres of chitosan crosslinked with three different crosslinking agents viz, glutaraldehyde, sulphuric acid and heat treatment have been prepared to encapsulate diclofenac sodium (DS). Chitosan microspheres are produced in a w/o emulsion followed by crosslinking in the water phase by one of the crosslinking methods. Encapsulation of DS has been carried out by soaking the already swollen crosslinked microspheres in a saturated solution of DS. Microspheres are further characterized by FTIR, x-RD and SEM. The in-vitro release studies are performed in 7.4 pH buffer solution. Microspheres produced are spherical and have smooth surfaces, with sizes ranging between 40-230 microm, as evidenced by SEM. The crosslinking of chitosan takes place at the free amino group in all the cases, as evidenced by FTIR. This leads to the formation of imine groups or ionic bonds. Polymer crystallinity increases after crosslinking, as determined by x-RD. The method adopted for drug loading into the microspheres is satisfactory, and up to 28-30% w/w loading is observed for the sulphuric acid-crosslinked microspheres, whereas 23-29 and 15-23% of loadings are obtained for the glutaraldehyde (GA)- and heat-crosslinked microspheres, respectively. Among all the systems studied, the 32% GA crosslinked microspheres have shown the sloxvest release i.e. 41% at 420 min, and a fastest release of 81% at 500 min is shown by heat crosslinking for 3 h. Drug release from the matrices deviates slightly from the Fickian process.

An assessment of solubility profiles of structurally similar hazardous pesticide in water + methanol mixture and co-solvent effect on partition coefficient

This paper reports solubility and partition coefficient data for the structurally similar pesticides, fenvalerate and cypermethrin, measured by UV spectrophotometry in binary mixtures of methanol and water at different temperatures. The solubility of both pesticides is much higher in methanol than in water at all temperatures. Partition coefficients were also measured between water+heptanol immiscible mixtures at 298.15K, and these data show a decrease with increasing composition of methanol in water.

Cellulose acetate microspheres prepared by o/w emulsification and solvent evaporation method

The study is concerned with the development of cellulose acetate microspheres by the o/w emulsification and solvent evaporation method in the presence of polyvinyl alcohol as an emulsifying agent. The influence of process parameters such as solvent mixture (acetone + dichloromethane) composition, concentration of the emulsifying agent and speed of stirring has been examined. The microspheres have been analysed for their size, drug loading capacity and release kinetics. Spherical and smooth surfaced microspheres with encapsulation efficiencies ranging between 73-98%, were obtained. Use of acetone in the oil phase drastically reduced the particle size. Slow drug release from microspheres occurred up to approximately 8 h and the release was found to be non-Fickian. An optimization procedure was employed to investigate and identify the key parameters affecting the properties of the microspheres. A 33 randomized full factorial design was used in the analyses of the data. A linear model with interactive terms was generated using a multiple linear regression approach. The statistical analysis confirms the significant effect of solvent composition and concentration of emulsifying agent on the drug release characteristics.

In vitro release study of verapamil hydrochloride through sodium alginate interpenetrating monolithic membranes

Polymeric sodium alginate interpenetrating network membranes containing verapamil hydrochloride were fabricated for transdermal application. The membranes were evaluated for their physical properties, weight and thickness uniformity, water vapor transmission, as well as drug content uniformity. All the thin patches were transparent, smooth, and flexible. The drug-loaded membranes were analyzed by X-ray diffraction to understand the drug polymorphism inside the membrane. The transdermal patches were permeable to water vapor, indicating the permeability characteristics of the polymers. The in vitro drug release was performed in distilled water using a Keshary-Chien diffusion cell. The release data were analyzed to understand the mechanism of drug release.

Chemically modified polyacrylamide-g-guar gum-based crosslinked anionic microgels as pH-sensitive drug delivery systems: preparation and characterization

New spherically shaped cross-linked hydrogels of polyacrylamide-grafted guar gum were prepared by the emulsification method. These were selectively derivatized by saponification of the -CONH2 group to the -COOH group. The derived microgels were characterized by FTIR and elemental analyses. The derivatized microgels were responsive to pH and ionic strength of the external medium. The swelling of microgels increased when the pH of the medium changed from acidic to alkaline. Transport parameters, viz., solvent front velocity and diffusion coefficients were calculated from a measurement of the dimensional response of the microgels under variable pH conditions. The variation in pH changed the transport mechanism from Case II (in 0.1 N HCl) to non-Fickian (in pH 7.4 buffer), and these processes are relaxation-controlled. Ionic strength exerted a profound influence on the swelling of the microgels. Swelling was reversible and pulsatile with the changing environmental conditions. The pH-sensitive microgels were loaded with diltiazem hydrochloride and nifedipine (both antihypertensive drugs) and their release studies were performed in both the simulated gastric and intestinal pH conditions. The release was relatively quicker in pH 7.4 buffer than observed in 0.1 N HCl; the release followed non-Fickian transport in almost all the cases.

Development of hollow microspheres as floating controlled-release systems for cardiovascular drugs: preparation and release characteristics

Hollow microspheres of cellulose acetate loaded with four cardiovascular drugs (nifedipine [NFD], nicardapine hydrochloride [NCD], verapamil hydrochloride [VRP], and dipyridamole [DIP]) were prepared by a novel solvent diffusion-evaporation method. The oil-in-water emulsion prepared in an aqueous solution of 0.05% poly(vinyl alcohol) medium with ethyl acetate, a water-soluble and less toxic solvent, was used as the dispersing solvent. The yield of the microspheres was up to 80%. The microspheres had smooth surfaces, with free-flowing and good-packing properties. Scanning electron microscopy (SEM) confirmed their hollow structures, with sizes in the range 489-350 microm. The microspheres tended to float over the gastric media for more than 12 h. The drug loaded in hollow microspheres was in an amorphous state, as confirmed by differential scanning microscopy (DSC). The release of the drugs was controlled for more than 8 h. The release kinetics followed different transport mechanisms depending on the nature of the drug molecules.

Encapsulation of antihypertensive drugs in cellulose-based matrix microspheres: characterization and release kinetics of microspheres and tableted microspheres

This study is an attempt to prepare microspheres loaded with two antihypertensive drugs viz., nifedipine (NFD) and verapamil hydrochloride (VRP) using cellulose-based polymers viz., ethyl cellulose (EC) and cellulose acetate (CA). Emulsification and solvent evaporation methods were optimized using ethyl acetate as a dispersing solvent. The particles are spherical in shape and have smooth surfaces, as evidenced by the scanning electron microscopy. The microspheres were characterized for their particle size and distribution, tapped density and encapsulation efficiency. Smaller sized particles with a narrow size distribution were produced with EC when compared to CA matrices. Molecular level drug distribution in the microspheres was confirmed by differential scanning calorimetry. The microspheres were directly compressed into tablets using different excipients. The drug release from CA was faster than EC microspheres and, also, the VRP release was faster than NFD. The excipients used in tableting showed an effect on the release as well as the physical properties of the tablets.

Microspheres as floating drug-delivery systems to increase gastric retention of drugs

Gastric emptying is a complex process, which is highly variable and makes in vivo performance of the drug-delivery systems uncertain. In order to avoid this variability, efforts have been made to increase the retention time of the drug-delivery systems for more than 12 h. The floating or hydrodynamically controlled drug-delivery systems are useful in such applications. The present review addresses briefly the physiology of the gastric emptying process with respect to floating drug-delivery systems. In recent years, the multiparticulate drug-delivery systems are used in the oral delivery of drugs. One of the approaches toward this goal is to develop the floating microspheres so as to increase the gastric retention time. Such systems have more advantages over the single-unit dosage forms. The development of floating microspheres involves different solvent evaporation techniques to create the hollow inner core. The present review addresses the preparation and characterization of the floating microspheres for the peroral route of administration of the drug.

In-vitro release kinetics of cefadroxil-loaded sodium alginate interpenetrating network beads

This paper reports the development of new interpenetrating polymeric networks of sodium alginate with gelatin or egg albumin cross-linked with a common cross-linking agent, glutaraldehyde, for the in-vitro release of cefadroxil. The beads formed were characterized by Fourier transform infra-red spectroscopy, scanning electron microscopy and differential scanning calorimetry. Swelling/drying experiments were performed to compute the diffusion coefficients and the molecular mass between cross-links of the beads. The release results were evaluated using an empirical equation to understand the transport mechanism. The extent of cross-linking was studied in terms of the size and release characteristics of the beads. The experimental and derived quantities have been used to study their dependencies on the nature of the polymeric beads, transport mechanism, encapsulation efficiency and drug diffusion, as well as the cross-linking abilities of the polymers.

Biodegradable polymeric nanoparticles as drug delivery devices

This review presents the most outstanding contributions in the field of biodegradable polymeric nanoparticles used as drug delivery systems. Methods of preparation, drug loading and drug release are covered. The most important findings on surface modification methods as well as surface characterization are covered from 1990 through mid-2000.

Biodegradable polymeric nanoparticles as drug delivery devices

This review presents the most outstanding contributions in the field of biodegradable polymeric nanoparticles used as drug delivery systems. Methods of preparation, drug loading and drug release are covered. The most important findings on surface modification methods as well as surface characterization are covered from 1990 through mid-2000.

Solubility study of hazardous pesticide (chlorpyrifos) by gas chromatography

Solubility data of a hazardous pesticide like chlorpyrifos are important in order to determine its residual toxicity from soil and to understand its controlled release characteristics. In this paper, solubility of chlorpyrifos was measured by using gas chromatography in pure water, methanol, and water-methanol mixtures at 298.15, 303. 15, and 308.15K. The results indicate that chlorpyrifos is more soluble in methanol than in water. The solubility of chlorpyrifos in water can be enhanced by adding methanol to water. The effect of temperature on the solubility of chlorpyrifos was observed at higher methanol content in the mixture.

Preparation of cross-linked sodium alginate microparticles using glutaraldehyde in methanol

Polymeric sodium alginate microparticles were prepared by precipitating sodium alginate in methanol, followed by cross-linking with glutaraldehyde. The extent of cross-linking was controlled by the time of exposure to glutaraldehyde. The topology of microparticles was characterized by scanning electron microscopy (SEM), which indicated smooth surfaces. The equilibrium swelling experiments were carried out in water to observe the effect of cross-linking and drug loading for better utility of microparticles. It was found that swelling decreased, but drug loading increased, with an increase in cross-linking of the matrix.

Urea-formaldehyde nanocapsules for the controlled release of diclofenac sodium

Urea-formaldehyde (UF), polymerized in situ in aqueous media, is a rigid polymer that can be used in the controlled release of bioactive molecules. During the process of polymerization, liquid neem seed oil (NSO) was encapsulated at three different loadings, which was then replaced by diclofenac sodium (DS), the presence of which was confirmed by FTIR spectra. The nanocapsules thus prepared were evaluated for percentage loading of the drug, particle size and release characteristics. Thermal analysis and x-ray data were obtained to understand the physical nature of the encapsulated drug. The surface characteristics of the nanocapsules were studied using a scanning electron microscope. Particles in the size range around 500 nm were obtained. The percentage loading efficiency of NSO was approximately 90-95, whereas for DS, the loading efficiency was only 50-60%. Complete release of the drug from the matrices occurred in 24 h, whereas, at 8 h only 60% of the drug was released.

Controlled release of antihypertensive drug from the interpenetrating network poly(vinyl alcohol)-guar gum hydrogel microspheres

Poly(vinyl alcohol)-guar gum interpenetrating network microspheres were prepared by cross-linking with glutaraldehyde. Nifedipine, an antihypertensive drug, was loaded into these matrices before and after cross-linking to study its release patterns. The extent of cross-linking was analyzed by Fourier transform infrared spectroscopy and differential scanning calorimetry. Furthermore, the microspheres were characterized for drug entrapment efficiency, particle size, transport of water into the matrix and drug release kinetics. Scanning electron microscopic photographs confirmed the spherical nature and surface morphology. The mean particle size of the microspheres was found to be around 300 microm. The molecular transport phenomenon, as studied by the dynamic swelling experiments, indicated that an increase in cross-linking affected the transport mechanism from Fickian to non-Fickian. The in vitro release study indicated that the release from these microspheres is not only dependent upon the extent of cross-linking, but also on the amount of the drug loaded as well as the method of drug loading.

Glutaraldehyde crosslinked sodium alginate beads containing liquid pesticide for soil application

This paper presents experimental results on the successful encapsulation of a natural liquid pesticide 'neem (Azadirachta Indica A. Juss.) seed oil' hereafter designated as NSO, using sodium alginate (Na-Alg) as a controlled release (CR) polymer after crosslinking with glutaraldehyde (GA). The NSO-containing beads have been prepared by changing the experimental variables such as the extent of crosslinking and the amount of loading in order to optimize the process variables. The absence of chemical interactions between active ingredients and polymer as well as crosslinking agent was confirmed by FTIR spectral measurements. The SEM data indicated that the structure of the walls of the beads are smooth and nonporous. The swelling results indicated that swelling of the polymeric beads decreases with increasing exposure time to the crosslinking agent. However, no significant variation in swelling was observed with different amounts of NSO loading. In order to understand the crosslinkability and its effect on the NSO release patterns of the beads, an attempt was made to calculate the molar mass between crosslinks using the Flory-Rehner equation. The release data have been fitted to an empirical equation to estimate the kinetic parameters.