Gold electrodes were modified with DNA by adsorption. The DNA-modified electrodes were electrochemically characterized with Co(bpy)3+, a electroactive DNA-binding complex, as an indicator. It is interesting that the pair of redox peaks of Co(bpy)3+ split into two pairs at dsDNA-modified gold electrodes. One pair of peaks shifts negative, and the peak current increases notably; another pair of peaks shifts positive. These suggest that dsDNA has been immobilized onto gold electrode surfaces and the layer of dsDNA on the surfaces can bind Co(bpy)3+ in two different ways. Gold electrodes can be modified also with ssDNA by adsorption but only one pair of peaks of Co(bpy)3+ appears at ssDNA-modified gold electrodes. The amount of Co(bpy)3+ enriched by the layers of dsDNA or ssDNA adsorbed at gold electrodes was estimated from the peak charge of Co(bpy)3+ reduction at the electrodes obtained by CV. The stability of the DNA-modified electrodes was investigated. The DNA modification layer on gold surfaces is unstable to alkali and to heat, but stable to acid solutions and very stable in long stock in a dry. state. A comparison of modifications of gold, platinum and glassy carbon with DNA was carried out.

The modification of glassy carbon and gold electrodes with DNA by adsorption or covalent immobilization in a mono-or submonolayer has been investigated using the couple Co(bpy)3+/2+ as an indicator. It has been found that when the solution containing double stranded or single stranded DNA is evaporated to dryness, dehydrated DNA molecules can be irreversibly adsorbed on the surfaces of glassy carbon electrodes, in an amount close to that of the saturated adsorptive monolayer. The DNA-adsorbed layer on glassy carbon electrodes is unstable to bases, but stable to 1 M HCl solution. The adsorption of DNA on the electrodes can be evaluated from the increase in the peak current, the decrease in the value of △Ep, and the negative shift in the value of E for the Co(bpy)3+/2+ couple. DNA is very strongly adsorbed on the oxidized surfaces of glassy carbon electrodes, and the adsorptive layer is very stable towards heating. The covalent immobilization of DNA directly onto the electrode surfaces is impossible due to considerable steric hindrance; but if the active groups (sites) on the electrode surfaces are elongated with other suitable molecules, the covalent immobilization of DNA becomes possible on the electrode surfaces. The quantity of covalently immobilized DNA at the electrodes reported in the paper is about 31% of the saturated monolayer.

A novel electrochemical micromethod for the investigation of the interactions between DNA and non-electroactive species is described. The method was developed using the system of double-stranded DNA (dsDNA) modified gold electrodes (dsDNA/Au), a synthesized water-soluble C60 derivative as a model, and [Co(phen)3]3+/2+ (phen=1, 10-phenanthroline) as an electroactive indicator. Electrochemical studies with dsDNA-modified gold electrodes suggest that the C60 derivative can interact strongly with dsDNA, with binding sites of the major groove of the double helix and phosphate backbone of dsDNA, a binding constant of (1.6

A visual gene-detecting technique using nanoparticle-supported gene probes is described. With the aid of gold nanoparticle-supported 30-end-mercapto-derivatized oligonucleotide serving as detection probe, and 50-end-amino-derivatized oligonucleotide immobilized on glass surface acting as capturing probe, target DNA was detected visually by sandwich hybridization based on highly sensitive "nano-amplification" and silver staining. Different genotypes of Hepatitis B and C viruses in the serum samples from infected patients were detected using homemade HBV, HCV, and HBV/HCV gene chips by the gold/silver nanoparticle staining amplification method. The present visual gene-detecting technique may avoid limitations with the reported methods, for its high sensitivity, good specificity, simplicity, speed, and cheapness. This technique has potential applications in many fields, especially in multi-gene detection gene chips coupled with the detection will find applications in clinic. Additionally, resonance Rayleigh light scattering (RLS) spectroscopy is used, for the first time, to judge and monitor the immobilization of gene probes on gold nanoparticle surfaces.

Gold nanoparticle/streptavidin conjugates covered with 6-ferrocenylhexanethiol were attached onto a biotinylated DNA detection probe of a sandwich DNA complex. Due to the elasticity of the DNA strands, the ferrocene caps on gold nanoparticle/streptavidin conjugates are positioned in close proximity to the underlying electrode modified with a mixed DNA capture probe/hexanethiol self-assembled monolayer and can undergo reversible electron-transfer reactions. A detection level, down to 2.0pM (10amol for the 5μL of sample needed) for oligodeoxynucleotide samples was obtained. The amplification of the voltammetric signals was attributed to the attachment of a large number of redox (ferrocene) markers per DNA duplex formed. The ferrocene oxidation current increased with the target concentration and began to level off at a target concentration of 10nM. An Excellent linearity was found within the range between 6.9 and 150.0pM and reasonable relative standard deviations (between 3.0 and 13.0%) were obtained. The amenability of this method to the analyses of polynucleotides (i.e., PCR products of the pre-S gene of hepatitis B virus in serum samples) was also demonstrated. The method is shown to be simple, selective, reproducible, and costeffective and does not require labeling of the DNA targets.

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.