CNT transporters

Fig. 11.6. Predicted amino acid sequence homologies of human CNT1, CNT2, and CNT3. Shaded box indicates a similar amino acid match and a black box indicates an identical amino acid match between the CNT transporters. The alignments were done with Clustal W and shaded with Boxshade.

The method for theoretical calcrdation of the semiconducting zigzag CNT transport properties with adsorbed hydrogen atoms developed. Analytical expressions for the conductivity and the electron diffusion coefficient in zigzag CNT with hydrogen adatoms in the presence of an electric field. 

CNTs transported proteins into cells in a similar maimer to that of DNA- and peptide-functionalized CNTs mentioned above [153,154], SWNTs non-covalently conjugated with various proteins such as streptavidine, cyc-tochrome c, protein A and bovin serum albumin were investigated for carriage into mammalian cells like HeLa, NIH-3T3 fibroblast, HL60 and Jurkats cells [173,174],... 

The same k p scheme has been extended to the study of transport properties of CNTs. The conductivity calculated in the Boltzmann transport theory has shown a large positive magnetoresistance [18], This positive magnetoresistance has been confirmed by full quantum mechanical calculations in the case that the mean free path is much larger than the circumference length [19]. When the mean free path is short, the transport is reduced to that in a 2D graphite, which has also interesting characteristic features [20]. 

The k p scheme has been used also for the study of transport across junctions connecting tubes with different diameters through a region sandwiched by a pentagon-heptagon pair [25]. In Junctions systems, the conductance was predicted to exhibit a universal power-law dependence on the ratio of the circumference of two CNTs [26]. An intriguing dependence on the magnetic-field direction was predicted also [27]. These newer topics will be discussed elsewhere. 

We will discuss below the reeent experimental observations relative to the eleetrieal resistivity and magnetoresistance of individual and bundles of MWCNTs. It is interesting to note however that the ideal transport experiment, i.e., a measurement on a well eharacterised SWCNT at the atomic scale, though this is nowadays within reaeh. Nonetheless, with time the measurements performed tended gradually eloser to these ideal eonditions. Indeed, in order to interpret quantitatively the eleetronie properties of CNTs, one must eombine theoretieal studies with the synthesis of well defined samples, which structural parameters have been precisely determined, and direet electrical measurements on the same sample. 

In conclusion, wc have shown the interesting information which one can get from electrical resistivity measurements on SWCNT and MWCNT and the exciting applications which can be derived. MWCNTs behave as an ultimate carbon fibre revealing specific 2D quantum transport features at low temperatures weak localisation and universal conductance fluctuations. SWCNTs behave as pure quantum wires which, if limited in length, reduce to quantum dots. Thus, each type of CNT has its own features which are strongly dependent on the dimensionality of the electronic gas. We have also briefly discussed the very recent experimental results obtained on the thermopower of SWCNT bundles and the effect of intercalation on the electrical resistivity of these systems. 

The schematic model is depicted in Fig. 8. As the bias voltage increases, the number of the molecular orbitals available for conduction also increases (Fig. 8) and it results in the step-wise increase in the current. It was also found that the conductance peak plotted vs. the bias voltage decreases and broadens with increasing temperature to ca. 1 K. This fact supports the idea that transport of carriers from one electrode to another can take place through one molecular orbital delocalising over whole length of the CNT, or at least the distance between two electrodes (140 nm). In other words, individual CNTs work as coherent quantum wires. 

Fig. 8. Schematic illustration of the tunnelling in a CNT-based device (a) under no bias voltage, there are no orbitals available for conduction, (b) with small bias voltage, only one molecular orbital of a CNT contributes to the carrier transport and (c) when the next molecular orbital enters the bias window, current increases stepwise. Gate voltage can shift all the orbitals upward or downward. AE indicates the energy separation of molecular orbitals.

Figure 12.2 Adenosine metabolism. Intracellular adenosine concentrations depend on the balance between energy storage and breakdown. The most important enzymes catalyzing the reactions are indicated. SAH, S-adenosyl-homocysteine ENTs equilibrative nucleoside transporters CNTs, concentrating nucleoside transporters.

A new approach to improve the performance of solar devices using natural pigments is to employ carbon nanotube (CNT)-based counter-electrodes. As previously reported, the excited dye transfers an electron to Ti02 and so it acquires a positive charge. Then, the cationic molecule subtracts an electron from the counterelectrode which is transported by the electrolyte. This reaction is usually catalyzed by means of conductive and electrocatalytically active species for triiodide reduction of carbon coatings. CNTs have a high superficial area, which represents a very... 

Nucleoside analogues are widely used for the treatment of cancers and viral infections. Although there have been considerable advances in the development of new nucleoside analogs, little is known about the transport mechanisms involved in the intestinal absorption of these compounds. Nucleoside transporters have been subdivided into two major classes by Na+-independent equilibrative transporters (ENT family) and Na+-dependent concentrative transporters (CNT family) [77,100-103],... 

Wang, J., et al. Functional and molecular characteristics of Na(+)-dependent nucleoside transporters. Pharm. Res. 1997, 14, 1524-1532. Ritzel, M. W., et al. Recent molecular advances in studies of the concen-trative Na+-dependent nucleoside transporter (CNT) family identification and characterization of novel human and mouse proteins (hCNT3 and mCNT3) broadly selective for purine and pyrimidine nucleosides (system rib). Mol. Membr. Biol. 2001, 18, 65-72. 

CNTs own excellent materials properties. DNA is an excellent molecule to construct macromolecular networks because it is easy to synthesize, with a high specificity of interaction, and is conformationally flexible. The complementary base-paring properties of DNA molecules have been used to make two-dimensional crystals and prototypes of DNA computers and electronic circuits (Yan et al., 2002 Batalia et al., 2002). Therefore functionalization of CNTs with DNA molecules has great potential for applications such as developing nanodevices or nanosystems, biosensors, electronic sequencing, and gene transporters. 

The biocompatibility and toxicological studies of CNTs have become a controversial hotspot. Although several reports showed that CNTs are non-toxic and could be used as in vivo multifunctional biological transporters (Kam et al., 2005), there is no doubt that the possible hazards associated with CNTs are significant, and that concern is valid the key to address the challenge is to clarify the range and mechanism of nanohazards of CNTs, and possible measurements to reduce CNTs toxicity. 

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