Interaction between LDL-mimetic liposomes and acid-treated carbon nanotube electrode during Cu2+-mediated oxidation.

Oxidation of low-density lipoproteins (LDL) causes atherosclerosis. Detection of oxidation of LDL-mimetic liposomes using an electrode might serve as a convenient tool in the search of antioxidants for the prevention of atherosclerosis. This report proposes a reaction mechanism between LDL-mimetic liposomes and an acid-treated carbon nanotube (CNT) electrode. Oxidation of the liposomes, mediated by Cu2+, was monitored by the change in electrode potential, and the fluorescence intensity generated by diphenyl-1-pyrenylphosphine (DPPP) as control. The electrode potential and fluorescence intensity increased concomitantly during oxidation, followed by a gradual decrease. Although the electrical potential peaked faster than the fluorescence intensity, addition of CNT to the DPPP reaction accelerated the latter, suggesting the role of CNT as an accelerator of liposome oxidation. Atomic force microscopy showed increased binding of liposomes to CNT along with liposomal deformation. Further, binding of Cu2+ to the liposome-bound CNT surface was observed by quartz crystal microbalance. In conclusion, the interaction of liposomes with Cu2+ and CNT surface explains the rapid response of the electrode in liposome oxidation.

Evaluation of various electrode materials for detection of oxidized low-density lipoproteins.

Measurement of oxidized low-density lipoprotein (LDL) generated by oxidative stress of various kinds might be useful for evaluating the risk of cardiovascular disease. We evaluated some electrode materials to detect oxidized LDL electrochemically. Some carbon nanotube dispersions were studied as electrode materials. Native LDL was isolated from normal human serum using ultracentrifugation. Oxidized LDL was prepared by treating the native LDL with CuSO4. Electrodes were fabricated by depositing the nanotube dispersion on a gold electrode, with subsequent drying. The potential change of the electrode against a reference electrode was monitored before and after adding native LDL or oxidized LDL. Only acid-treated carbon nanotubes were able to discriminate both LDL preparations, perhaps because of the carboxylic acid groups introduced on the nanotube by acid treatment.

Detection of human adenovirus hexon antigen using carbon nanotube sensors.

Human adenoviruses (HAdVs) have been implicated in a wide range of diseases affecting primarily the respiratory, ocular and gastrointestinal systems. A rapid and efficient method for the detection of HAdV hexon antigen is described using carbon nanotube (CNT) sensors. Anti-HAdV antibody was immobilised on the reverse surface of a CNT sensor. As a control, non-specific mouse IgG was immobilised on another CNT sensor. I-V(gate) curves were measured after incubation of various concentrations of recombinant HAdVs hexon antigen with anti-HAdVs antibody-immobilised or non-specific mouse IgG-immobilised sensors. The curves showed a positive shift that was dependent on the hexon antigen concentration in the anti-HAdV antibody-immobilised sensor, whereas no such shift was observed in the non-specific mouse IgG-immobilised sensor. The sensitivity of the CNT sensor method was greater than that of enzyme-linked immunosorbent assay. Hence, this method offers a new tool for HAdV detection by analysing antigen-antibody interactions.

Detection of influenza virus hemagglutinin with randomly immobilized anti-hemagglutinin antibody on a carbon nanotube sensor.

Carbon nanotube sensors were capable of detecting hemagglutinin binding to anti-hemagglutinin antibody immobilized on the sensor. The carbon nanotube sensors were fabricated by chemical vapor deposition method and it showed field effect transistor property. Anti-hemagglutinin antibody was immobilized by cross-linking on the reverse surface of the carbon nanotube sensor. The current between the source and the drain was measured after incubation of various concentration of hemagglutinin antigen with immobilized anti-hemagglutinin antibody. I-Vgate curve was obtained by plotting the current as a function of the potential applied to the back gate. The I-Vgate curve showed a positive shift in a manner dependent on the hemagglutinin antigen concentration after immobilization of anti-hemagglutinin while no shift was observed without immobilization of anti-hemagglutinin antibody on the surface. The sensitivity of the CNT sensor was higher than that of the QCM method even without controlling the orientation of the anti-hemagglutinin antibody. This method constitutes a new tool to analyze interactions among biomolecules on a substrate.

A pH sensor based on electric properties of nanotubes on a glass substrate.

We fabricated a pH-sensitive device on a glass substrate based on properties of carbon nanotubes. Nanotubes were immobilized specifically on chemically modified areas on a substrate followed by deposition of metallic source and drain electrodes on the area. Some nanotubes connected the source and drain electrodes. A top gate electrode was fabricated on an insulating layer of silane coupling agent on the nanotube. The device showed properties of ann-type field effect transistor when a potential was applied to the nanotube from the top gate electrode. Before fabrication of the insulating layer, the device showed that thep-type field effect transistor and the current through the source and drain electrodes depend on the buffer pH. The current increases with decreasing pH of the CNT solution. This device, which can detect pH, is applicable for use as a biosensor through modification of the CNT surface.

Application of carbon nanotubes for detecting anti-hemagglutinins based on antigen-antibody interaction.

Carbon nanotube sensors detected anti-hemagglutinin binding to immobilized hemagglutinins. An ultra-sensitive detection method for antibodies or antigens in serum is required. Hemagglutinins were immobilized on the reverse side of a carbon nanotube, thereby producing a source and a drain. Electrode pads covered each edge of the nanotube. The I-V curves between the source and the drain were measured after incubation of anti-hemagglutinins with immobilized hemagglutinins in a buffered solution on the reverse side of the nanotube. The sensitivity of the CNT sensor was higher than that of an ELISA system. This method constitutes a new tool to analyze interaction among biomolecules on a substrate.