Effect of DNA flanking sequence on charge transport in short DNA duplexes

Several factors can influence charge transport (CT)-mediated DNA, such as sequence, distance, base stacking, base pair mismatch, conformation, tether length, etc. However, the DNA context effect or how flanking sequences influence redox active drugs in the DNA CT reaction and later in DNA enzymatic repair and synthesis is still not well understood. The set of seven DNA molecules in this study have been characterized well for the study of flanking sequence effects. These DNA duplexes are formed from self-complementary strands and contain the common central four-base sequence 5'-A-G-C-T-3', flanked on both sides by either (AT)(n) or (AA)(n) (n = 2, 3, or 4) or AA(AT)(2). UV-vis, fluorescence, UV melting, circular dichroism, and cyclic voltammetry experiments were used to study the flanking sequence effect on CT-mediated DNA by using daunomycin or adriamycin cross-linked with these seven DNA molecules. Our results showed that charge transport was related to the flanking sequence, DNA melting free energy, and ionic strength. For (AA)(n) or (AT)(n) species of the same length, (AA)(n) series were more stable and more efficient CT was observed through the (AA)(n) series. The same trend was observed for (AA)(n) and (AT)(n) series at different ionic strengths, further supporting the idea that flanking sequence can result in different base stacking and modulate charge transport through these seven DNA molecules.

Syntheses of fully sulfonated polyaniline nano-networks and its application to the direct electrochemistry of cytochrome c

Fully sulfonated polyaniline nano-particles, nano-fibrils and nano-networks have been achieved for the first time by electrochemical homopolymerization of orthanilic acid using a three-step electrochemical deposition procedure in a mixed solvent of acetonitrile (ACN) and water. The diameter of the uniform nano-particles is about 60nm, and the nano-fibrils can be organized in two-dimensional (21)) or three-dimensional (313) non-periodic networks with good electrical contact. Average distance between contacts is about 850 and 600 nm for a 2D and 3D system, respectively. The details of the poly(orthanilic acid) (POA) nano-structure were examined with a field emission scanning electron microscope (SEM). The structure and properties of POA were characterized with FTIR, UV-vis and electrochemical methods. The 3D POA nano-networks coated platinum electrode gave a direct electrochemical behavior of horse heart cytochrome c (Cyt c) immobilized on this electrode surface, a pair of well-defined redox waves with formal potential (E-ol) of -0.032 V (versus Ag/AgCl) was achieved. The interaction between Cyt c and POA makes the formal potential shift negatively compared to that of Cyt c in solution. Spectrophotometric and electrochemical methods were used to investigate the interaction of Cyt c with POA.

Simple and convenient preparation of Au-Pt core-shell nanoparticles on surface via a seed growth method

Au-Pt core-shell nanoparticles were prepared on glass surface by a seed growth method. Gold nanoparticles were used as seeds and ascorbic acid-H2PtCL6 solutions as growth solutions to deposit Pt shell on the surface of gold nanoparticles. These core-shell nanoparticles and their growth process were examined by UV-Vis spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy and field-emission environmental scanning electron microscopy and the results indicated that the deposition speed was fast and nanoparticles with obvious core-shell structure could be obtained after 2 min. Moreover, this seed growth method for preparation of the core-shell nanoparticles is simple and convenient compared with other seed growth methods with NH4OH as a mild reductant. In addition, electrochemical experiments indicated that these Au-Pt core-shell nanoparticles had similar electrochemical properties to those of the bulk Pt electrode.

Selective penetration of liquid-phase organic probe molecules into SAM of 2-mercapto-3-n-octylthiophene

An inherently disorganized self-assembled monolayer (SAM) of 2-mercapto-3-n-octylthiophene (MOT) has been formed on a gold bead electrode from its dilute ethanolic solution. The disorganization of the monolayer is attributed to the loose packing of the aliphatic chains of the MOT adsorbates, which results from a large difference in dimension/or cross-sectional area between the head (thiophene thiolate) and the tail (alkane chain) groups. Electrochemical measurements including ac impedance spectroscopy and metal underpotential deposition have shown that the monolayer is almost pinhole free. However, the MOT SAM can be penetrated by an organic probe molecule with affinity for the alkane chain part of the monolayer. Some typical probe molecules with different size and hydrophilicity have been employed to assess the permselectivity of the monolayer. Measurement results demonstrate that the ability of the employed probe molecules to penetrate into the monoalyer is mainly dominated by their hydrophilicity/or hydrophobicity. The results presented here suggest the potential application of MOT monoalyer to effectively modify the electrode surface for several research areas such as electrochemical sensors, electrocatalysis, electroanalysis, and supported hybrid bilayer membranes.

Real-time immunoassay of ferritin using surface plasmon resonance biosensor

A surface plasmon resonance (SPR) biosensor was used for the first time to determine the concentration of ferritin in both HBS-EP buffer and serum. The monoclonal antibody was immobilized on the carboxymethyl dextran-modified gold surface by an amine coupling method. The interaction of antibody with antigen was monitored in real-time. The signal was enhanced by sandwich amplification strategy to improve the sensitivity and specificity of the immunoassay, especially in serum. The linear range of the assay in serum is over 30-200 ng ml with the detection limit of 28 ng ml(-1). The sensitivity, specificity, and reproducibility of the assay are satisfactory. The analyte and enhancement antibody-binding surface could be regenerated by pH 2.0 glycine-HCl buffer and the same antibody-immobilized surface could be used for more than 50 cycles of ferritin binding and regeneration.

Amplified DNA detection sensitivity using streptavidin-biotinylated protein complex: Characterization by electrochemical impedance spectroscopy

Thiol-terminated oligonucleotide was immobilized to gold surface by self-assembly method. A novel amplification strategy was introduced for improving the sensitivity of DNA. hybridization using biotin labeled protein-streptavidin network complex. This complex can be formed in a cross-linking network of molecules so that the amplification of the response signal will be realized due to the big molecular size of the complex. It could be proved from the impedance technique that this amplification strategy caused dramatic improvement of the detection sensitivity. These results give significant advances in the generality and sensitivity as it is applied to biosensing.

Interaction of low-molecular-weight chitosan with mimic membrane studied by electrochemical methods and surface plasmon resonance

Chitosan has shown its potential as a non-viral gene carrier and an adsorption enhancer for subsequent drug delivery to cells. These results showed that chitosan acted as a membrane perturbant. However, there is currently a lack of direct experimental evidence of this membrane perturbance effect, especially for chitosans with low molecular weight (LMW). In this report, the interaction between a lipid (didodecyl dimethylammonium bromide; DDAB) bilayer and chitosan with molecular weight (MW) of 4200 Da was studied with cyclic voltammetry (CV), electrochemical impedance spectroscopy and surface plasmon resonance (SPR). A lipid bilayer was formed by-fusion of oppositely charged lipid vesicles on a mercaptopropionic acid (MPA)-modified gold surface to mimic a cell membrane. The results showed that the LMW chitosan could disrupt the lipid bilayer, and the effect seemed,to be in a concentration-dependent manner.

Studies of electrochemical quantized capacitance charging of surface ensembles of silver nanoparticles

CTAB-stabilized silver nanoparticles were synthesized by NaBH4 reduction. The as-prepared nanoparticles can be self-assembled on 3-mercaptopropionic acid (MPA) modified gold electrode, which was supported strongly by XPS measurements. Exceptional long-term stability of the as-prepared colloidal silver aqueous solution and the desorption of silver nanoparticle ensemble on MPA after alcohol rinsing proved that these CTAB molecules adsorbed on silver core formed interdigitated bilayer structure. DPV and differential capacitance measurements were performed to characterize the as-prepared silver nanoparticle ensemble. and the interesting quantized capacitance charging behaviors were observed.

Characterization of organic-inorganic multilayer films by cyclic voltammetry, UV-Vis spectrometry, X-ray photoelectron spectroscopy, small-angle X-ray diffraction and electrochemical impedance spectroscopy

We have employed several techniques, including cyclic voltammetry, UV-Vis spectrometry, small-angle X-ray diffraction, X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy, to characterize the formation processes and interfacial features of ultrathin multilayer films of silicotungstate and a cationic redox polymer on cysteamine-coated Au electrodes self-assembled monolayers. All of these techniques confirm that the multilayer films are built up stepwise as well as uniformly in a layer-by-layer fashion. In particular, the electrochemical impedance spectroscopy is successfully used to monitor the multilayer deposition processes. It has been proved that the electrochemical impedance spectroscopy is a very useful technique in characterization of multilayer films because it provides valuable information about the interfacial impedance features.

Amplified immunoassay of human IgG using real-time biomolecular interaction analysis (BIA) technology

An automated biomolecular interaction analysis instrument (BI-Acore) based on surface plasmon resonance (SPR) has been used to determine human immunoglobulin G (IgG) in real time. Polyclonal anti-human IgG antibody was covalently immobilized to a carboxymethyldextran modified gold film surface. The samples of human IgG prepared in HBS buffer were poured over the immobilized surface. The signal amplification antibody was applied to amplify the response signal. After each measurement, the surface was regenerated with 0.1 mol/L H3PO4. The assay was rapid, requiring only 30 min for antibody immobilization and 20 min for each subsequent process of immune binding, antibody amplification and regeneration. The antibody immobilized surface had good response to human IgG in the range of 0.12-60 nmol/L with a detection limit of 60 pmol/L. The same antibody immobilized surface could be used for more than 110 cycles of binding, amplification and regeneration. The results demonstrate that the sensitivity, specificity and reproducibility of amplified immunoassay using real-time BIA technology are satisfactory.

Enhanced surface plasmon resonance immunoassay for human complement factor 4

Using an enhanced surface plasmon resonance (SPR) immunosensor, we have determined the concentration of human complement factor 4 (C4). Antibody protein was concentrated into a carboxymethyldextran-modified gold surface by electrostatic attraction force and a simultaneous covalent immobilization of antibody based on amine coupling reaction took place. The sandwich method was applied to enhance the response signal and the specificity of antigen binding assay. The antibody immobilized surface had good response to C4 in the range of 0.02-20 mug/ml by this enhanced immunoassay. The regeneration effect by pH 2 glycine-HC1 buffer was also investigated. The same antibody immobilized surface could be used more than 80 cycles of C4 binding and regeneration. In addition, the ability to determinate C4 directly from serum sample without any purification was investigated. The sensitivity, specificity and reproducibility of the enhanced immunoassay are satisfactory. The results clearly demonstrate the advantages of the enhanced SPR technique for C4 immunoassay.

Fabrication and electrochemical behavior of a sol-gel-derived carbon ceramic composite electrode entrapping 2 : 18-molybdodiphosphate

A new type of inorganic-organic hybrid material incorporating carbon powder and alpha -type 2:18-molybdodiphosphate (P2Mo18) in a methyltrimethoxysilane (MTMOS) based gel has been produced by a sol-gel process and used to fabricate a chemically modified electrode. The P2Mo18-doped carbon ceramic composite electrode was characterized using SEM and cyclic voltammetry. Square-wave voltammetry with an excellent sensitivity was exploited to conveniently investigate the dependence of current and half-wave potential (E-1/2) on pH. The chemically modified electrode has some advantages over the modified film electrodes constructed by the conventional methods, such as long-term stability, reproducibility, and especially repeatability of surface-renewal by simple polishing in the event of surface fouling or dopant leaching. In addition, the modified electrode shows a good catalytic activity for the electrochemical reduction of bromate in an acidic aqueous solution. (C) 2000 Elsevier Science B.V. All rights reserved.

Double layer capacitance in electrochemical biosensor

The principle and technique of double layer capacitance and its application in electrochemical biosensor are briefly reviewed with 50 references. The future development of double layer capacitance biosensor is expected.

Mixed ion transfer analysis in redox processes of electroactive thin films

Electrochemical quartz crystal microbalance (EQCM) technique was used to measure the ion transfer in redox processes in electroactive organic thin films, such as self-assembled monolayer (SAM) (4-pyridyl hydroquinone, abbr. 4PHQ), multilayer based on SAM and conducting polymer film (here poly-(3,4-ethylenedioxythiophene), abbr. PEDOT). A mechanism of mixed ion transfer is developed and presented. Analysis of mixed ion transfer during redox processes successfully elucidates the deviation of oscillation frequency of the quartz crystal from theoretical expectation.

The electrochemistry of cytochrome c at a viologen-thiol self-assembled monolayer

At the self-assembled monolayer (SAM) of a thiol-functionalized viologen modified gold electrode, cytochrome c (cyt c) exhibits a quasi-reversible electrochemical reaction. The heterogeneous electron transfer rate constant of cyt c in 0.1 mol/L phosphate buffer solution(pH 6.96) is 0.164 cm.s(-1) at 500 mV/s. The adsorbed cyt c on the viologen SAM forms a closely packed monolayer, whose average electron transfer rate is 4.85 s(-1) in the scan range of 50 to 500 mV/s. These results suggest that the SAM of viologen-thiol is a relatively stable, ordered and well-behaved monolayer from an electrochemical standpoint and it promotes the electron transfer process of biomolecules on electrode surface well.