We perform first-principles calculation to study the transport properties of the acid dopped on C(4,4) carbon nanotube coupled to two C(4,4) carbon nanotube electrodes using the non-equilibrium Green formalism combined with the density-functional theory. In particular, we just consider the acid of glycine and alanine molecule dopped on 8 layers C(4,4) carbon nanotube sandwiched between two C(4,4) carbon nanotube electrodes. It is found that the equilibrium conductance of the pure C(4,4) carbon nanotube at Fermi energy is close to 2 G0, two eigenchannels which contribute to the equilibrium conductance are fully open. Both of the two types dopping reduce the conductivity of the carbon nanotube. Compared with the alanine molecule, the alanine influence the transport properties of the C(4,4) carbon nanotube much more greatly. At non-equilibrium, with the increase of the applied bias voltage, more visible current differences appear between the glycine and alanine molecule, which may have the potential use to distuguish the acid of glycine and alanine molecules. A detailed analysis of the transmission coefficient of the eigenchannels, the projected density of states are made to reveal the mechanism of the differences.
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