The self-assembly and redox properties of viologen derivatives [N-methyl-N 0 -di(10-mercaptodecyl)-4,4 0 -bipyridinium; HSC 10 VC 10 SH] immobilized on a Au(111) substrate were investigated by ultrahigh vacuum scanning tunneling microscopy (UHV-STM) and cyclic voltammetry. We demonstrate here a novel self-assembly monolayer (SAM) matrix appropriate for the isolation of viologen molecules. An octanethiol was used as the SAM matrix, in which the HSC 10 VC 10 SH was inserted at molecular lattice defects. The isolated single molecules of viologen derivatives inserted in the SAM matrix were observed as protrusions in STM topography using a constant current mode. We measured the conformational change of the viologen molecular protrusions using STM and investigated the changes in the width and height of the alkyl group, which were due to the change in the polarity of viologen molecules caused by the electron charge.
A synthesized 21-mer single-stranded DNA (ssDNA) was covalently immobilized onto a self-assembled aminoethanethiol monolayer modified gold electrode onto quartz crystal microbalance (QCM). The covalently immobilized ssDNA was hybridized with complementary ssDNA. The interaction between surface-immobilized ssDNA and complementary 21-mer DNA in solution was also examined. Each step was followed by monitoring changes in the QCM frequency over time. From the results, the measured frequency change was about 256 Hz, 160 Hz and 141 Hz, respectively. As a result, the adsorption mass was about 273 ng/cm2, 171 ng/cm2 and 151 ng/cm2, according to the Sauerbrey equation. Also, PBS with pH 7.0 was selected as a supporting electrolyte to achieve maximum sensitivity and good bioactivity. The anodic peak current was linearly related to the concentrations between 1 x 10(-5) and 9 x 10(-5) M with a detection limit of 1.0 x 10(-6) M.
The structure and dynamic organization of a mixed Langmuir film of glucose oxidase (GOx) and lipid at the air-water interface were studied. The film was transferred onto the Prussian Blue (PB)-modified Pt electrode for biosensor preparation. The PB modified electrode showed well defined redox peaks in 0.1 M PBS electrolyte. The Langmuir film was characterized at the air-water interface by pi-A isotherms. The mixed monolayer was formed by spreading GOx on the LB trough covered with lipid. Time-pressure results show that at least 90 minutes are required to reach the equilibrium state of GOx-lipid film. The monolayer was transferred onto the PB-modified electrode when surface pressure was 40 mN/m. This sensor was characterized by a very low detection limit and a wide linear range. The optimal conditions for both fabricating and response of the sensor were investigated. The proposed biosensor showed a linear calibration range from 5 x 10(-6) to 6 x 10(-5) M. The detection limit was determined to be 1.5 x 10(-6) M.
A low-temperature ultrahigh-vacuum scanning tunneling microscope (UHV-STM) was used to image viologen (N-methyl-N'-di (8-mercaptooctyl)-4,4'-bipyridinium; HSC8VC8SH) molecules and to perform local spectroscopic measurements on these molecules. Self-assembly of viologen molecules was conducted on Au (111), which had been thermally deposited onto freshly cleaved, heated mica. Here, we demonstrate a novel SAM matrix appropriate for the isolation of viologen molecules composed of octanethiol (C8) in which HSC8VC8SH was inserted at defects in the molecular lattice. The isolated single molecules of viologen inserted in the SAM matrix were observed as protrusions in STM topography using a constant current mode. STM images at 298 K showed protrusions with a topographic height of about 2.71 nm (HSC8VC8SH) with viologen molecules that self-assembled on the substrate. The current-voltage (I-V) characteristics were measured while the electrical properties of the formed monolayer were scanned using scanning tunneling spectroscopy (STS). We found the high peak current-like rectification at +1.14 V (HSC8VC8SH). The rectification ratios, RR = J (at +2.5 V)/J (at -2.5 V), are in the range of 4.47.
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