The plasmonic properties of gold nanoparticle clusters formed via applying an AC electric field.

An external electric AC field with a field strength ranging from 10 V mm-1 to 30 V mm-1 and a frequency ranging from 0.1 kHz to 1 MHz was applied to suspensions of gold nanoparticles (Au NPs) to control their plasmonic properties. Apparent differences in the UV-vis spectra of the Au NPs were observed between the spectra with and without the field application. The characteristic red color of the Au NP suspension darkened; this suggested that the application of the AC field caused the aggregation of the Au NPs. In addition, the sizes of the Au NP clusters in suspension formed by the AC field application depended on the frequency of the applied field. The surface-enhanced Raman scattering (SERS) effects of Au NP clusters were examined by comparing the difference in Raman intensities obtained at 30 V mm-1 and in a frequency range of 0.1 kHz to 1 MHz. The application of a low-frequency field at 0.1 kHz caused a rapid aggregation of the Au NPs, resulting in low Raman intensities of the probe molecules. Conversely, high-frequency applications between 1 kHz and 1 MHz successfully enhanced the Raman intensities of the molecules in suspension. The strong correlation of the optical/sensing properties with the Au NP clustering states reveals that the application of an AC electric field is a powerful tool for control over the plasmonic properties.

A Generalized Model for Describing Particle Formation in the Synthesis of Monodisperse Oxide Particles Based on the Hydrolysis and Condensation of Tetraethyl Orthosilicate.

A generalized model considering reaction and diffusion processes as rate determining steps is proposed for the formation of particles from hydrolysis and condensation of tetraethyl orthosilicate (TEOS). Reaction rate constant is assumed to be proportional to contact-surface area of two particles. Diffusion rate constant is given by Fuchs' stability theory with interparticle potential containing van der Waals attraction and electrostatic repulsion. To examine the validity of the model, ammonia-catalyzed reactions of TEOS were carried out in an ethanol-water solution at a water concentration of 11 mol/dm(3), an ammonia concentration of 1.0 mol/dm(3), and TEOS concentrations of 0.2 and 0.4 mol/dm(3) in the presence and in the absence of electrolytes. The experiments included competitive particle growth and seeded and nonseeded reactions. The model quantitatively expressed the results of the competitive growth experiments in which particles with different sizes grew at the same rate. The model gave the expected trend for the effect of ionic strength on particle sizes in the nonseeded reaction and the suppression of secondary particles in the seeded reaction by the addition of an electrolyte KCl. Good estimation was also obtained for transient particle size distributions in the nonseeded reaction and the seeded reaction in which secondary particles were generated. Copyright 2000 Academic Press.

Effect of Varidase (streptokinase) on biofilm formed by Staphylococcus aureus.

Staphylococcus aureus forms a fibrin-rich biofilm in the presence of plasma which is highly resistant to attack by the human immune system and to chemotherapy. Varidase, composed mainly of streptokinase, is used for hydrolyzing clots. In this study, we attempted to destroy the biofilm of S. aureus with Varidase and to apply this drug in the treatment of staphylococcal infections. Four clinical isolates were used in the experiments. These organisms formed a several-millimeter-thick biofilm on type IV collagen coated coverslips in trypticase soy broth containing 50% human plasma. The biofilm was composed of bacterial cell which adhered to fibrillar fibers and of sediment derived from plasma. 10,000 U/ml of Varidase, the dose which is used clinically, removed the sediment and reduced the number of live bacteria in biofilms to less than 20% of control. 200 U/ml of Varidase was also effective against biofilms of the organisms. An equal combination of Varidase and ofloxacin had an additive effect on the bacteria. The results of this study demonstrate that Varidase is highly effective in destroying biofilms of S. aureus in vitro and suggest that this drug would be useful for treating staphylococcal infections.

Social facilitation effects in recognition memory.

This study investigates the effects of observer presence on recognition memory. The number of learning repetitions, the degree of association of category-instance word pairs, and the time interval between acquisition and testing were manipulated. While the basic recognition data conformed well to previous findings in the learning and memory literature, predictions, derived from the drive theory of social facilitation found little support. Instead, study results seem best explained by descriptions of the cognitive processes employed by subjects both during the initial acquisition and later recognition of the category-instance pairs.