Principal factors that determine the extension of detection range in molecular beacon aptamer/conjugated polyelectrolyte bioassays.

A strategy to extend the detection range of weakly-binding targets is reported that takes advantage of fluorescence resonance energy transfer (FRET)-based bioassays based on molecular beacon aptamers (MBAs) and cationic conjugated polyelectrolytes (CPEs). In comparison to other aptamer-target pairs, the aptamer-based adenosine triphosphate (ATP) detection assays are limited by the relatively weak binding between the two partners. In response, a series of MBAs were designed that have different stem stabilities while keeping the constant ATP-specific aptamer sequence in the loop part. The MBAs are labeled with a fluorophore and a quencher at both termini. In the absence of ATP, the hairpin MBAs can be opened by CPEs via a combination of electrostatic and hydrophobic interactions, showing a FRET-sensitized fluorophore signal. In the presence of ATP, the aptamer forms a G-quadruplex and the FRET signal decreases due to tighter contact between the fluorophore and quencher in the ATP/MBA/CPE triplex structure. The FRET-sensitized signal is inversely proportional to [ATP]. The extension of the detection range is determined by the competition between opening of the ATP/MBA G-quadruplex by CPEs and the composite influence by ATP/aptamer binding and the stem interactions. With increasing stem stability, the weak binding of ATP and its aptamer is successfully compensated to show the resistance to disruption by CPEs, resulting in a substantially broadened detection range (from millimolar up to nanomolar concentrations) and a remarkably improved limit of detection. From a general perspective, this strategy has the potential to be extended to other chemical- and biological-assays with low target binding affinity

Hydroxyapatite Humidifier Vibrator Housing Fabrication and Characteristics

The humidifier vibrator housing is difficult to clean and prone to contamination due to its metallic material. To overcome these shortcomings, the humidifier vibrator housing was manufactured using Hydroxyapatite as a raw material. Although hydroxyapatite has excellent antibacterial properties and biocompatibility, it is difficult to manufacture a sintered body due to its weak fracture toughness. Therefore, hydroxyapatite sintered compacts were prepared according to the amount of plasticizer added and their physical properties were compared. The average compressive strength was 395.1 N·mm-2 at 8 % of the amount of added plasticizer, and the average bending strength was 61.8 N·mm-2 at 6 % of the amount of added plasticizer. The hydroxyapatite sintered compact showed the effect of inhibiting the production of bacteria regardless of the amount of plasticizer added. As a result of this physical property study, it was possible to develop a humidifier vibrator housing with excellent antibacterial properties and maintaining mechanical strength

Density Functional Theory Studies of the [2]Rotaxane Component of the Stoddart−Heath Molecular Switch

The central component of the programmable molecular switch recently demonstrated by Stoddart and Heath is [2]rotaxane, which consists of a cyclobis(paraquat-p-phenylene) shuttle (CBPQT^(4+))(PF_6-)_4 (the ring) encircling a finger and moving between two stations, tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP). As a step toward understanding the mechanism of this switch, we report here its electronic structure using two flavors of density functional theory (DFT):  B3LYP/6-31G^(**) and PBE/6-31G^(**). We find that the electronic structure of composite [2]rotaxane can be constructed reasonably well from its parts by combining the states of separate stations (TTF and DNP) with or without the (CBPQT)(PF_6)_4 shuttle around them. That is, the “CBPQT@TTF” state, (TTF)(CBPQT)(PF_6)_4−(DNP), is described well as a combination of the (TTF)(CBPQT)(PF_6)_4 complex and free DNP, and the “CBPQT@DNP” state, (TTF)−(DNP)(CBPQT)(PF_6)_4, is described well as a combination of free TTF and the (DNP)(CBPQT)(PF_6)_4 complex. This allows an aufbau or a “bottom-up” approach to predict the complicated [n]rotaxanes in terms of their components. This should be useful in designing new components to lead to improved properties of the switches. A critical function of the (CBPQT^(4+))(PF_6-)_4 shuttle in switching is that it induces a downshift of the frontier orbital energy levels of the station it is on (TTF or DNP). This occurs because of the net positive electrostatic potential exerted by the CBPQT^(4+) ring, which is located closer to the active station than the four PF_6-'s. This downshift alters the relative position of energy levels between TTF and DNP, which in turn alters the electron tunneling rate between them, even when the shuttle is not involved directly in the actual tunneling process. Based on this switching mechanism, the “CBPQT@TTF” state is expected to be a better conductor since it has better aligned levels between the two stations. A second potential role of the (CBPQT^(4+))(PF_6-)_4 shuttle in switching is to provide low-lying LUMO levels. If the shuttle is involved in the actual tunneling process, the reduced HOMO−LUMO gap (from 3.6 eV for the isolated finger to 1.1 eV for “CBPQT@TTF” or to 0.6 eV for “CBPQT@DNP” using B3LYP) would significantly facilitate the electron tunneling through the system. This might occur in a folded conformation where a direct contact between free station and the shuttle on the other station is possible. When this becomes the main switching mechanism, we expect the “CBPQT@DNP” state to become a better conductor because its HOMO−LUMO gap is smaller and because its HOMO and LUMO are localized at different stations (HOMO exclusively at TTF and LUMO at CBPQT encircling DNP) so that the HOMO-to-LUMO tunneling would be through the entire molecule of [2]rotaxane. Thus an essential element in designing these switches is to determine the configuration of the molecules (e.g., through self-assembled monolayers or incorporation of conformation stabilizing units)

Atomistic Simulations of Corrosion Inhibitors Adsorbed on Calcite Surfaces I. Force field Parameters for Calcite

We report a new force field for calcite suitable to study scale formation and squeeze treatments for oil field applications. The force field reproduces the cell parameters, density, and the compressibility of the calcite crystal as well as the enthalpy of immersion of various solvents. Surface energetics for the various surfaces of interest are predicted

The MS-Q Force Field for Clay Minerals: Application to Oil Production

A Morse-charge equilibration force field (MS-Q FF), originally developed for the bulk oxides SiO_2 and Al_2O_3, has been used to model kaolinite and pyrophyllite clay minerals and their interactions with representative organic molecules. The MS-Q FF reproduces the structural parameters for these clay minerals and gives accurate enthalpies of immersions in water, organic solvents, and hydrocarbons. To form a basis for improving squeeze corrosion treatment strategies, we calculate the adsorption energy of oleic imidazoline, a corrosion inhibitor oil production chemical