A Reusable Impedimetric Aptasensor for Detection of Thrombin Employing a Graphite-Epoxy Composite Electrode

Here, we report the application of a label-free electrochemical aptasensor based on a graphite-epoxy composite electrode for the detection of thrombin; in this work, aptamers were immobilized onto the electrodes surface using wet physical adsorption. The detection principle is based on the changes of the interfacial properties of the electrode; these were probed in the presence of the reversible redox couple [Fe(CN)6]3−/[Fe(CN)6]4− using impedance measurements. The electrode surface was partially blocked due to formation of aptamer-thrombin complex, resulting in an increase of the interfacial electron-transfer resistance detected by Electrochemical Impedance Spectroscopy (EIS). The aptasensor showed a linear response for thrombin in the range of 7.5 pM to 75 pM and a detection limit of 4.5 pM. The aptasensor was regenerated by breaking the complex formed between the aptamer and thrombin using 2.0 M NaCl solution at 42 °C, showing its operation for different cycles. The interference response caused by main proteins in serum has been characterized

Selection of DNA aptamers using atomic force microscopy

Atomic force microscopy (AFM) can detect the adhesion or affinity force between a sample surface and cantilever, dynamically. This feature is useful as a method for the selection of aptamers that bind to their targets with very high affinity. Therefore, we propose the Systematic Evolution of Ligands by an EXponential enrichment (SELEX) method using AFM to obtain aptamers that have a strong affinity for target molecules. In this study, thrombin was chosen as the target molecule, and an ‘AFM-SELEX’ cycle was performed. As a result, selected cycles were completed with only three rounds, and many of the obtained aptamers had a higher affinity to thrombin than the conventional thrombin aptamer. Moreover, one type of obtained aptamer had a high affinity to thrombin as well as the anti-thrombin antibody. AFM-SELEX is, therefore, considered to be an available method for the selection of DNA aptamers that have a high affinity for their target molecules

Evaluation of the structure–activity relationship of thrombin with thrombin binding aptamers by voltammetry and atomic force microscopy

The structure–activity relationship of the complex between thrombin and thrombin binding aptamers (TBA) was evaluated by differential pulse voltammetry at a glassy carbon electrode and atomic force microscopy at a highly oriented pyrolytic graphite electrode. The effects on the interaction with thrombin of TBA primary and secondary structures as well as of its folding properties in the presence of alkaline metals were investigated. The complex between thrombin and single stranded aptamers involved the coiling of the single stranded molecules around thrombin structure leading to the formation of a robust TBA–thrombin complex that maintains the symmetry and conformation of the thrombin molecule. Monitoring both thrombin and TBA oxidation peaks, showed that the thrombin oxidation peaks occur at more positive potentials after TBA–thrombin complex formation. In the presence of K+ ions, the aptamers fold into quadruplex structures that facilitate the interaction with thrombin molecules. The TBA–thrombin complex adsorbs at the surface with the aptamer quadruplex always in preferential contact with the surface, and the thrombin molecules on top of the aptamer quadruplex structure, thus being less accessible to the electrode surface leading to the occurrence of thrombin oxidation peaks at less positive potentials

Metal-related gate sinking due to interfacial oxygen layer in Ir/InAlN high electron mobility transistors

We report on an annealing-induced "gate sinking" effect in a 2-nm-thin In0.17Al0.83N/AlN barrier high electron mobility transistor with Ir gate. Investigations by transmission electron microscopy linked the effect to an oxygen containing interlayer between the gate metal and the InAlN layer and revealed diffusion of oxygen into iridium during annealing. Below 700 degrees C the diffusion is inhomogeneous and seems to occur along grain boundaries, which is consistent with the capacitance-voltage analysis. Annealing at 700 degrees C increased the gate capacitance over a factor 2, shifted the threshold voltage from +0.3 to +1 V and increased the transconductance from 400 to 640 mS/mm. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3458700

Characterization of Plasma-Induced Damage of Selectively Recessed GaN/InAlN/AlN/GaN Heterostructures Using SiCl4 and SF6

We have investigated an inductively coupled plasma etching recipe using SiCl4 and SF6 with a resulting selectivity >10 for GaN in respect to InAlN. The formation of an etch-resistant layer of AlF3 on InAlN required about 1 min and was noticed by a 4-times-higher initial etch rate on bare InAlN barrier high electron mobility transistors (HEMTs). Comparing devices with and without plasma-treatment below the gate showed no degradation in drain current and gate leakage current for plasma exposure durations shorter than 30s, indicating no plasma-induced damage of the InAlN barrier. Devices etched longer than the required time for the formation of the etch-resistant barrier exhibited a slight decrease in drain current and an increase in gate leakage current which saturated for longer etching-time durations. Finally, we could prove the quality of the recipe by recessing the highly doped 6 nm GaN cap layer of a GaN/InAlN/AlN/GaN heterostructure down to the 2 nm thin InAlN/AlN barrier layer. (C) 2010 The Japan Society of Applied Physic