Jörn Kasbohm

Chemical synthesis and antibacterial activity of novel-shaped silver nanoparticles

Silver nanoparticles are useful for medical applications due to their strong antibacterial activity. The antibacterial activity can be tuned by controlling the size and shape of the prepared silver nanoparticles. In this work, silver nanoparticles with different sizes and shapes were synthesized by solution phase routes, and their interactions with Escherichia coli were studied. Triangular silver nanoprisms were prepared by the reduction of silver nitrate at room temperature in the presence of polyvinylpyrrolidone, sodium citrate, hydrogen peroxide and sodium borohydride. Spherical silver nanoparticles were also prepared using silver nitrate as metal precursor and sodium citrate as well as sodium borohydride as reducing agents. The morphologies and structures of the nanoparticles were characterized by transmission electron microscopy, UV-visible spectroscopy and X-ray diffraction. The results indicated that spherical silver nanoparticles were obtained with different average sizes of 4, 21 and 40 nm, respectively. The edged silver nanoprisms containing mainly {111} lattice planes were obtained in the range size of 25 to 400 nm. The antibacterial study revealed that the edged triangular silver nanoprisms with {111} lattice planes exhibited the strongest antibacterial property, compared with spherical nanoparticles. Our study demonstrated that triangular silver nanoprisms with sharp edges also display a good antibacterial activity in comparison to other shaped nanoparticles.

EFFECTS OF CHEMICAL STRUCTURE ON THE STABILITY OF SMECTITES IN SHORT-TERM ALTERATION EXPERIMENTS

Because of their isolating capacity, smectite-rich clays have been proposed as buffer and backfill materials in high-level radioactive waste repositories. These repositories have to guarantee long-term safety for ~1 million years. Thermodynamics and kinetics of possible alteration processes of bentonite determine its long-term performance as a barrier material. Smectites in 25 different clays and bentonites were investigated in order to identify possible differences in their rates of alteration. These samples were saturated for 30 days in 1 M NaCl solution and deionized water, and then overhead rotated at speeds of 20 rpm and 60 rpm. Depending on the octahedral and interlayer composition, each of the smectites studied had specific rate of alteration, a so-called specific dissolution potential of smectite. The bentonites were classed as ‘slow-reacting bentonite’, ‘moderate-reacting bentonite’, or ‘fast-reacting bentonite’ corresponding to a relatively low (ΔP specific dissolution potential — <-5%), moderate (-5% < ΔP < -20%), or high specific dissolution potential (ΔP > -20%), respectively. The larger the amount of octahedral Fe and Mg compared to octahedral Al, the greater the specific dissolution potential. The present study found that the interlayer composition has a discernible impact on the rate of alteration. In experiments with rotation speeds of 60 rpm and a 1 M NaCl solution, Na+ was found to be the stabilizing cation in the interlayers of all the smectites. The Na-stabilizing mechanism was identified in only some of the smectites (type A) in experiments with 20 rpm (1 M NaCl solution). A second stabilization mechanism (by interlayer cations; Ca and Mg) was identified for other smectites (type B). Each bentonite has a specific rate of alteration. ‘Slow-reacting bentonite’ and clay with smectite-illite interstratifications are recommended as potential clay barriers in HLW repositories. The experimental and analytical procedures described here could be applied to potential barrier materials to identify ‘slow-reacting bentonite’.