Kazuyoshi Ogawa

Effect of charge inhomogeneity of polyelectrolyte gels on their swelling behavior

Abstract The purpose of this paper is to discuss the role of ionic charges in the swelling of polyelectrolyte gels. According to the classical Florys theory, osmotic pressure arising from mobile ions is regarded as a swelling force due to ionic charges; in other words, the gel was assumed to be surrounded by Donnan potential barriers. Although this assumption was criticized by Katchalsky et al., many studies have accepted it to account for the nature of the swelling or the phase transition in polyelectrolyte gels. Thus, we have tried to verify whether the ‘concept’ of osmotic pressure due to mobile ions is adequate or not. For this purpose, we have studied the swelling behavior of ionic gels with inhomogeneous charge distributions. A gel sample was obtained via inhomogeneous binding of anionic surfactants (e.g. sodium dodecylbenzene sulfate) to lightly crosslinked polymer of N -isopropylacrylamide (NIPA), while the other was based on the physical entrapment of poly(acrylic acid) within the NIPA chain network. Using both NIPA-based ionic gels, we observed a strong effect of the charge inhomogeneity on the degree of swelling. It is therefore indicated that our observations can no longer be explained in terms of the concept of osmotic pressure arising from mobile counterions within the gel phase.

Effects of salt on intermolecular polyelectrolyte complexes formation between cationic microgel and polyanion

The study of interpolyelectrolyte complex (IPEC) formation between cationic microgel and polyanion was presented. The size and molecular weight of cationic microgel are much larger than those of linear anionic polyelectrolyte. The resulting IPEC was divided by dynamic light scattering (DLS), static light scattering (SLS), and turbidity or spectrometry; (i) water-soluble intra-particle complexes consisting of one microgel to which linear polyelectrolytes bind; (ii) complex coacervates (inter-particle complexes composed of aggregated intra-particle complexes); and (iii) insoluble amorphous precipitates. These types depended on not only the mixing ratio of polyanion to cationic microgel but also salt concentration. This trend was discussed from IPECs composition, thermodynamics of IPEC formation and the salt effect on intermolecular interactions which were expected in IPEC formation. The results obtained from the use of microgel in IPECs study suggested that not only electrostatic interaction but also hydrophobic interaction play an important role in the aggregation or association of IPEC.

Stable antibacterial silver nanoparticles produced with seed-derived callus extract of Catharanthus roseus

Abstract Biocompatibility and ecotoxicity concerns associated with chemically produced metallic nanoparticles have led to an increasing interest in the development of environmentally benign alternatives for nanoparticle synthesis using biological platforms. Herein, we report the utilization of an extract of seed-derived callus of Catharanthus roseus for the production of stable silver nanoparticles (Ag NPs). The bioreduction of silver ions was evident from UV–Vis spectroscopy results: the absorption maxima were observed at 425 nm, indicative of elemental silver. Transmission electron micrographs revealed that the Ag NPs were well-dispersed and predominantly spherical with particle sizes in the range of 2–15 nm. The synthesized Ag NPs exhibited colloidal stability in an aqueous dispersion for a period of 120 days, as indicated by UV–Vis absorbance spectra and zeta potential measurements. Fourier transform infrared spectroscopy revealed the possible utilization of hydroxyl groups and amides in the reduction of silver ions and surface stabilization of the Ag NPs, respectively. Notably, the synthesized Ag NPs showed considerable antibacterial action against Escherichia coli even after 8 weeks of storage under ambient conditions. Thus, cell extracts of cultured callus of Catharanthus roseus could be explored as an ecofriendly platform for the synthesis of stable and functional nanoparticles.