China Science

The burgeoning field of nanoscience has stimulated an intense interest in properties that depend on particle size. For transition metal particles, one important property that depends on size is catalytic reactivity, in which bonds are broken or formed on the surface of the particles. Decreased particle size may increase, decrease, or have no effect on the reaction rates of a given catalytic system. This Account formulates a molecular theory of the structure sensitivity of catalytic reactions based on the computed activation energies of corresponding elementary reaction steps on transition metal surfaces. Recent progress in computational catalysis, surface science, and nanochemistry has significantly improved our theoretical understanding of particle-dependent reactivity changes in heterogeneous catalytic systems. Reactions that involve the cleavage or formation of molecular ?-bonds, as in CO or N2, must be distinguished from reactions that involve the activation of ?-bonds, such as CH bonds in methane. The activation of molecular ?-bonds requires a reaction center with a unique configuration of several metal atoms and step-edge sites, which can physically not be present on transition metal particles less than 2 nm. This is called class I surface sensitivity, and the rate of reaction will sharply decrease when particle size decreases below a critical size. The activation of ? chemical bonds, in which the activation proceeds at a single metal atom, displays a markedly different size relationship. In this case, the dependence of reaction rate on coordinative unsaturation of reactive surface atoms is large in the forward direction of the reaction, but the activation energy of the reverse recombination reaction will not change. Dissociative adsorption with cleavage of a CH bond is strongly affected by the presence of surface atoms at the particle edges. This is class II surface sensitivity, and the rate will increase with decreasing particle size. Reverse reactions such as hydrogenation typically show particle-size-independent behavior. The rate-limiting step for these class III reactions is the recombination of an adsorbed hydrogen atom with the surface alkyl intermediate and the formation of a ?-type bond. Herein is our molecular theory explaining the three classes of structure sensitivity. We describe how reactions with rates that are independent of particle size and reactions with a positive correlation between size and rate are in fact complementary phenomena. The elucidation of a complete theory explaining the size dependence of transition metal catalysts will assist in the rational design of new catalytic systems and accelerate the evolution of the field of nanotechnology.

Rutger A. Van Santen
Schuit Institute of Catalysis, Laboratory of Inorganic Chemistry and Catalysis, P.O Box 513, 5600 MB Eindhoven, Eindhoven University of Technology, The Netherlands

The selective detection of the anion pyrophosphate (PPi) is a major research focus. PPi is a biologically important target because it is the product of ATP hydrolysis under cellular conditions, and because it is involved in DNA replication catalyzed by DNA polymerase, its detection is being investigated as a real-time DNA sequencing method. In addition, within the past decade, the ability to detect PPi has become important in cancer research. In general, the sensing of anions in aqueous solution requires a strong affinity for anions in water as well as the ability to convert anion recognition into a fluorescent or colorimetric signal. Among the variety of methods for detecting PPi, fluorescent chemosensors and colorimetric sensors for PPi have attracted considerable attention during the past 10 years. Compared with the recognition of metal ions, it is much more challenging to selectively recognize anions in an aqueous system due to the strong hydration effects of anions. Consequently, the design of PPi sensors requires the following: an understanding of the molecular recognition between PPi and the binding sites, the desired solubility in aqueous solutions, the communicating and signaling mechanism, and most importantly, selectivity for PPi over other anions such as AMP and ADP, and particularly phosphate and ATP. This Account classifies chemosensors for PPi according to topological and structural characteristics. Types of chemosensors investigated and reported in this study include those that contain metal ion complexes, metal complexes combined with excimers, those that function with a displacement approach, and those based on hydrogen-bonding interaction. Thus far, the utilization of a metal ion complex as a binding site for PPi has been the most successful strategy. The strong binding affinity between metal ions and PPi allows the detection of PPi in a 100% aqueous solution. We have demonstrated that carefully designed receptors can distinguish between PPi and ATP based on their different total anionic charge densities. We have also demonstrated that a PPi metal ion complex sensor has a bioanalytical application. This sensor can be used in a simple and quick, one-step, homogeneous phase detection method in order to confirm DNA amplification after polymerase chain reaction (PCR).

Sook Kyung Kim#8224?Dong Hoon Lee#8225?Jong-In Hong#8225?Juyoung Yoon#8224?

The purpose of this paper is to describe the use of LGC Promochem AEA 1001 to AEA 1003 monosized fiberanalog shape standards in the study of the effect of particle shape on laser diffraction (LD) particle size analysis (psa). The psa of the AEA standards was conducted using LD psa systems from Beckman Coulter, Horiba,; Malvern Instruments. Flow speed settings, sample refractive index values,; sample cell types were varied to examine the extent to which the shape effect on LD psa results is modified by these variables. The volume; number probability plots resulting from these measurements were each characterized by a spread in the particle size distribution that roughly extended from the breadth to the longest dimension of the particles. For most of the selected sample refractive index values, the volume probability plots were characterized by apparent bimodal distributions. The results, therefore, provide experimental verification of the conclusions from theoretical studies of LD psa system response to monosized elliptical particles in which this apparent bimodality was the predicted result in the case of flow-oriented particles. The data support the findings from previous studies conducted over the past 10 years that have called into question the verity of the tenets of,; therefore the value of the application of, the equivalent spherical volume diameter theory; the random particle orientation model to the interpretation of LD psa results from measurements made on nonspherical particles.

Richard N. Kelly2Email:RKelly@prdus.jnj.com?Jacqueline Kazanjian1
[1] Yves Rocher North America Inc, Exton, PA ;[2] Johnson & Johnson Pharmaceutical Research & Development, LLC, Room P-1029 McKean & Welsh Roads, 19477-0776 Spring House, PA

The purpose of this investigation was to prepare; evaluate the colon-specific microspheres of 5-fluorouracil for the treatment of colon cancer. Core microspheres of alginate were prepared by the modified emulsification method in liquid paraffin; by cross-linking with calcium chloride. The core microspheres were coated with Eudragit S-100 by the solvent evaporation technique to prevent drug release in the stomach; small intestine. The microspheres were characterized by shape, size, surface morphology, size distribution, incorporation efficiency,; in vitro drug release studies. The outer surfaces of the core; coated microspheres, which were spherical in shape, were rough; smooth, respectively. The size of the core microspheres ranged from 22 to 55 ?m,; the size of the coated microspheres ranged from 103 to 185 ?m. The core microspheres sustained the drug release for 10 hours. The release studies of coated microspheres were performed in a pH progression medium mimicking the conditions of the gastrointestinal tract. Release was sustained for up to 20 hours in formulations with core microspheres to a Eudragit S-100 coat ratio of 1?7,; there were no changes in the size, shape, drug content, differential scanning calorimetry thermogram,; in vitro drug release after storage at 40°C/75% relative humidity for 6 months.

Ziyaur Rahman1Email:ziyaur_rahman2@rediffmail.com?Kanchan Kohli1?Roop K. Khar1?Mushir Ali1?Naseem A. Charoo1?Areeg A. A. Shamsher1
[1] Department of Pharmaceutics, F/O Pharmacy, Hamdard University, 110063 New Delhi, India ;[2] D-615. Gali No 1, Chauhan Bangar, 110053 Delhi, India

This research was aimed to characterize microemulsion systems of isopropyl palmitate (IPP), water,; 2?1 Brij 97; 1-butanol by different experimental techniques. A pseudoternary phase diagram was constructed using water titration method. At 45% wt/wt surfactant system, microemulsions containing various ratios of water; IPP were prepared; identified by electrical conductivity, viscosity, differential scanning calorimetry (DSC), cryo-field emission scanning electron microscopy (cryo-FESEM); nuclear magnetic resonance (NMR). The results from conductivity; viscosity suggested a percolation transition from water-in-oil (water/oil) to oil-in-water (oil/water) microemulsions at 30% wt/wt water. From DSC results, the exothermic peak of water; the endothermic peak of IPP indicated that the transition of water/oil to oil/water microemulsions occurred at 30% wt/wt water. Cryo-FESEM photomicrographs revealed globular structures of microemulsions at higher than 15% wt/wt water. In addition, self-diffusion coefficients determined by NMR reflected that the diffusability of water increased at higher than 35% wt/wt water, while that of IPP was in reverse. Therefore, the results from all techniques are in good agreement; indicate that the water/oil; oil/water transition point occurred in the range of 30% to 35% wt/wt water.

Prapaporn Boonme1?Karen Krauel3?Anja Graf3?Thomas Rades3?Varaporn Buraphacheep Junyaprasert1Email:pyvbp@mahidol.ac.th
[1] Department of Pharmacy, Faculty of Pharmacy, Mahidol University, 10400 Bangkok, Thailand ;[2] Department of Pharmaceutical Technology, Faculty of Pharmaceutical Sciences, Prince of Songkla University, 90112 Songkhla, Thailand ;[3] School of Pharmacy, University of Otago, PO Box 913, 9001 Dunedin, New Zealand

Fluid dynamics of pellets processed in bottom spray traditional Wurster coating; swirl accelerated air (precision) coating were compared with the intent to understand; facilitate improvements in the coating processes. Fluid dynamics was described by pellet mass flow rate (MFR) obtained using a pellet collection system; images captured using high speed photography. Pellet flow within the partition column was found to be denser; slower in Wurster coating than in precision coating, suggesting a higher tendency of agglomeration during the coating process. The influence of partition gap; load on the MFR indicated that the mechanism of transport of pellets into the coating zone in precision coating depended on a strong suction, whereas in Wurster coating, pellets were transported by a combination of peripheral fluidization, gravity,; weak suction pressure. In precision coating, MFR was found to increase uniformly with air flow rate; atomizing pressure, whereas MFR in Wurster coating did not correlate as well with air flow rate; atomizing pressure. This demonstration showed that transport in precision coating was air dominated. In conclusion, fluid dynamics in precision coating was found to be air dominated; dependent on pressure differential, thus it is more responsive to changes in operational variables than Wurster coating.

L. W. Chan1?Elaine S. K. Tang1?Paul W. S. Heng1Email:phapaulh@nus.edu.sg
[1] Department of Pharmacy, National University of Singapore, 18 Science Drive 4, 117543 Singapore

The purpose of this study was to evaluate DNA degradation upon thermal heating using dielectric relaxation; direct current (DC) conductivity methods. Herring sperm DNA, human growth hormone (HgH) plasmid DNA,; secreted alkaline phosphatase (SEAP) plasmid DNA were used as the examples. DNA was heated at 80°C for 1 hour. The dielectric relaxation spectra as a function of the applied field frequency were measured for HgH DNA at 0.5 hours; at 1 hour. The frequency range covered was from 10 kHz to 100 kHz. The DC conductivity measurements were made for all 3 kinds of DNA at 4 time points: 0 hours, 0.5 hours, 0.75 hours,; 1 hour. At each time point the DC conductivity was measured for each sample as a function of concentration via water dilution. The results show that the dielectric relaxation method is less sensitive in characterizing heat-driven DNA degradation. Conversely, DC conductivity is very sensitive. The semiquantitative dependence of the conductivity upon heating suggests that DNA degradation involves more than plasmid DNA nicking. Double strand; single strand breaks may also occur. In addition, herring sperm DNA, HgH DNA,; SEAP DNA, though similar in their DC conductivity functional forms upon dilution, exhibit significant differences in their responses to sustained heating.

Jonathan I. Sheu1?Eric Y. Sheu2Email:ericsheu@comcast.net
[1] Acalanes High School, 1200 Pleasant Hill Rd, 94549 Lafayette, CA ;[2] 7 Olde Creek Place, 94549 Lafayette, CA

The composition, structure and electrical properties of alumina barrier layers grown by anodic oxidation in F?-containing (FC) and F?-free (FF) oxalic acid solutions were studied using the re-anodizing/dissolution technique, Fourier-transformed infrared and X-ray photoelectron spectroscopy. These results confirmed formation in FC anodizing solutions of films structurally different from ones grown in FF oxalic acid baths. It was found that the barrier layer of FC alumina films is composed of two layers differing in the dissolution rate. These differences are related to the formation in the FC electrolyte of a barrier layer composed of a more microporous outer part and a thin, non-porous and non-scalloped inner part consisting of aluminum oxide and aluminum fluoride.

A. JagminasaEmail:jagmin@ktl.mii.lt?I. Vrublevskyb?J. Kuzmarskyta?V. Jasulaitiena
[a]Institute of Chemistry, A. Gotauto 9, LT-01108 Vilnius, Lithuania;[b]Department of Microelectricals, Belarusian State University of Informatics; Radioelectricals, 6 Brovka Street, Minsk 220013, Belarus