When illuminated by near-UV light, titanium dioxide (TiO2) exhibits excellent bactericidal activity. However, there exist some different mechanisms for cell killing via photocatalysis. In the present study, the photocatalytically bactericidal mechanism of TiO2 thin films was investigated by atomic force microscopy (AFM) in conjugation with some other techniques. The decomposition process of the cell wall and the cell membrane was directly observed by AFM for the first time. The resultant change in cell permeability was confirmed by potassium ion (K+) leakage. Quantum dots (QDs) were designed originally as a probe to examine the cell permeability for macromolecules. The corresponding bactericidal activity of TiO2 thin films was examined by cell viability assay. These results suggested that the cell death was caused by the decomposition of the cell wall and the cell membrane and the resultant leakage of intracellular molecules.

The immobilization of thiol-derivatized DNA on a Au (111) single crystal surface by self-assembly has been investigated by electrochemical scanning tunneling microscopy (EC-STM). Continuous potential-dependent orientation changes of double-stranded oligodeoxynucleotides (ODN) have been observed in a certain potential range from 200 to 600mV (versus SCE). It is suggested that the DNA duplexes stand straight on the gold surface at potentials negative of the potential of zero charge (pzc) and then lay down on the surface when the potential shifts positively. These results are in agreement with the expectation based on the Coulombic interaction consideration between negatively charged DNA helices and gold surface. As the applied potential shifts positively, the surface charge changes from negative to positive, that is, the Coulombic force between negatively charged DNA helices and gold surfaces changes from repulsion to attraction. However, for the single-stranded oligodeoxynucleotides, no distinct changes in the surface structure were observed with the applied potential.

Double-stranded DNA(poly(dA)30?poly(dT)30)-modified gold electrodes, prepared by air-drying/adsorption method, have been investigated by various techniques, including cyclic voltammetry (CV), quartz crystal microbalance (QCM), electrochemical scanning tunneling microscopy (EC-STM), and surface-enhanced Raman scattering spectroscopy (SERS). CV and QCM results show that an average surface coverage of (7.5 (0.2)

The interactions of benzyl viologen (BV) with single-and double-stranded calf-thymus DNA immobilized onto gold electrodes have been studied by electrochemical methods. Benzyl viologen interacts electrostatically with both doublestranded (ds) and single-stranded (ss) DNA, and the strength of the interactions is dependent on ionic strength (μ). The dicationic form (BV2+) binds to dsDNA 9 times more strongly than the singly reduced form, BV·+, in a pH 7.4 Tris-HCl buffer solution at μ=8.4mM. BV2+ binds to ssDNA 5 times more strongly than the BV·+ form. From measurements atμ=8.4mM, a binding constant (K2+) of 2.0 ((0.2)×104M-1 and a binding site size (s) of 1 base pair were obtained, respectively, for dsDNA. For ssDNA, at the same ionic strength, the values obtained for K and s were 3.6 ((0.4) ×104M-1 and 2 nucleotides, respectively. The amount of BV bound, whether to dsDNA or ssDNA, decreased with increasing ionic strength. Whereas the binding rate of BV to both dsDNA and ssDNA immobilized onto gold electrodes is relatively low, once immobilized, it dissociates rapidly away from the electrode surface. The electron-transfer rate constant for BV is moderately fast at both dsDNA- and ssDNA-modified gold electrodes. The application of benzyl viologen as an electroactive indicator capable of differentiating between surface-immobilized single- and double-stranded DNA in denaturation/regeneration cycles has been explored.

A novel microscale and surface-based method for the study of the interactions of DNA with other redox-active molecules using DNA-modified electrodes is described. The method is simple, convenient, reliable, reagentsaving, and applicable for DNA studies, especially those involving microsamples. Information such as binding site size (s, in base pairs), binding constant (K), ratio (KOx/KRed) of the binding constants for the oxidized and reduced forms of a bound species, binding free energy (△Gb), and interaction mode, including changes in the mode of interaction, and "limiting" ratio KOx°/KRed° at zero ionic strength can be obtained using only 3-15μg of DNA samples. The method was developed using [Co(Phen)3]3+/2+ (Phen) 1,10-phenanthroline)/doublestranded DNA (dsDNA)-modified gold electrodes and [Co(bpy)3]3+/2+ (2,2'-bipyridyl)/dsDNA-modified gold electrodes as model systems. For the [Co(Phen)3]3+/2+/dsDNA-modified gold electrode system, a K2+ of (2.5(0.3)×105M-1 and an s of 5bp were obtained in 5mM pH 7.1 Tris-HCl buffer solution containing 50mM NaCl. For [Co(bpy)3]3+/2+/dsDNA-modified gold electrodes, K3+ and s values of (1.3(0.3)×105M-1 and 3bp, respectively, were obtained. While the s values are consistent with those reported in the literature obtained by solution methods, the K values are almost an order of magnitude larger. A transition in the nature of the interaction between dsDNA and [Co(Phen)3]3+/2+, from electrostatic to intercalative with increasing ionic strength, was found in our studies. Negative values of △E°' for [Co(bpy)3]3+/2+ bound to dsDNA suggest that its interaction with dsDNA is predominantly electrostatic over the ionic strength range of 5-105mM. The "limiting" ratio K3+°/K2+° of 22 obtained for [Co(Phen)3]3+/2+ bound to dsDNA at zero ionic strength suggests that electrostatic interactions are predominant over intercalative ones under these limiting conditions. The ratio for [Co(bpy)3]3+/2+ of 16 also indicates that the 3+ form binds to dsDNA more strongly than the 2+ form at zero ionic strength. For [Co(Phen)3]3+/2+/single-stranded DNA (ssDNA)-modified gold electrodes, the nonuniformity of the surface structure of ssDNA-modified gold electrodes greatly complicates the analysis. A system consisting of a dsDNA-modified gold electrode and [Co(tppz)2]3+/2+ (tppz) tetra-2-pyridyl-1,4-pyrazine) was studied by this method, with a K2+ value of (5(1)×105M-1 and an s value of 7bp being obtained.