Yeon Seok Kim

Microstructure and Microwave Dielectric Properties of Modified Zinc Titanates (II)

Abstract Zinc metatitanate, ZnTiO 3 , decomposes into zinc orthotitanate and rutile on heating above 945°C. It was found that this material could be a useful candidate for microwave resonator materials. The substitution of Zn by less than 10 mol% of Ba, Ca, or Sr achieves near zero τ f in spite of the fact that BaTiO 3, CaTiO 3 , and SrTiO 3 themselves have positive τ f . The origin of the temperature-compensating characteristics of the modified system was found to be in the reduction or elimination of rutile by the formation of new phases such as Ba 3 Zn 7 Ti 12 O 34 , Ca 2 Zn 4 Ti 15 O 36 , and Sr 2 Zn 4 Ti 15 O 36 polytitanates, which have much lower positive τ f than the rutile. It was revealed that zinc orthotitanate grains containing around 0.33 mol excess rutile have Zn 2 Ti 3 O 8 nanosize precipitates in the grain. The precipitates had a structural coherency with the Zn 2 TiO 4 cubic matrix phase. The modified system has microwave dielectric properties of e r = 25–32, Q factor at 10 GHz = 2300–5400, and τ f = +56 to −22 ppm/°C.

Aptamer cocktails: enhancement of sensing signals compared to single use of aptamers for detection of bacteria.

Microbial cells have many binding moieties on their surface for binding to their specific bioreceptors. The whole-cell SELEX process enables the isolation of various aptamers that can bind to different components on the cell surface such as proteins, polysaccharides, or flagella with high affinity and specificity. Here, we examine the binding capacity of an aptamer mixture (aptamer cocktail) composed of various combinations of 3 different DNA aptamers isolated from Escherichia coli and compare it with one of the single aptamers using fluorescence-tagged aptamers. The aptamer mixtures showed higher fluorescence signal than did any single aptamer used, which suggests that use of aptamer mixtures can enhance the sensitivity of detection of microbial cells. To further evaluate this effect, the signal enhancement and improvement of sensitivity provided by combinatorial use of aptamers were examined in an electrochemical detection system. With regard to current decreases, the aptamer cocktail immobilized on gold electrodes performed better than a single aptamer immobilized on gold electrodes did. Consequently, the detection limit achieved using the aptamers individually was approximately 18 times that when the 3 aptamers were used in combination. These results support the use of aptamer cocktails for detection of complex targets such as E. coli with enhanced sensitivity.

Isolation and characterization of DNA aptamers against Escherichia coli using a bacterial cell-systematic evolution of ligands by exponential enrichment approach.

Aptamers are powerful capturing probes against various targets such as proteins, small organic compounds, metal ions, and even cells. In this study, we isolated and characterized single-stranded DNA (ssDNA) aptamers against Escherichia coli. A total of 28 ssDNAs were isolated after 10 rounds of selection using a bacterial cell-SELEX (systematic evolution of ligands by exponential enrichment) process. Other bacterial species (Klebsiella pneumoniae, Citrobacter freundii, Enterobacter aerogenes, and Staphylococcus epidermidis) were used for counter selection to enhance the selectivity of ssDNA aptamers against E. coli. Finally, four ssDNA aptamers showed high affinity and selectivity to E. coli, The dissociation constants (K(d)) of these four ssDNA aptamers to E. coli were estimated to range from 12.4 to 25.2 nM. These aptamers did not bind to other bacterial species, including four counter cells, but they showed affinity to different E. coli strains. The binding of these four aptamers to E. coli was observed directly by fluorescence microscopy.