Detection of biomolecules using quantum inference in a functionalized carbon nanoring with magnetic flux.
M. Jack, M. Encinosa
Florida A&M University, US
Keywords: carbon nanotorus, electron transport, functionalization, biopolymer, magnetic flux, biosensor, tightbinding, nonequilibrium Green’s function method, density functional theory
Abstract:Abstract: The goal of this theoretical study is to examine the dependence of electronic conduction through a carbon nanotorus , when it is functionalized with a bio-molecule, for biosensor applications [2-3]. Conduction through carbon nanotube structures is highly sensitive to applied electric and magnetic fields and to defects and changes in the binding within the hexagonal carbon lattice. A small bias voltage drives currents through the nanotorus with attached metallic leads and with axially symmetric magnetic flux threaded through the torus to create well-known quantum interference in electronic transport such as Aharonov-Bohm oscillations etc. [4-6]. Biomolecules are prepared for weak attachment to the torus with an anthracene tether, which forms with its polycylic aromatic structure non-covalent, van-der-Waals bonds to the hexagonal lattice of the torus . The non-covalent binding only weakly perturbs the electronic states on the torus (sp2-hybridization maintained). This also simplifies the experimental separation of the biological structure from the nanoring later [2-3]. Changes in the current amplitude or to the magnetic-field dependent quantum interference pattern allow for two alternative biosensor detection modes. Due to flat lead attachment below the torus surface many more contact sites and thus larger current amplitudes may be achieved at similar source-drain bias than for equally wide carbon nanotubes for which metallic leads are traditionally contacted at the tube ends [4-6]. Changes to quantum interference when the biomolecule attaches create a highly sensitive, alternative detection mode. Moreover, these modifications to the interference signature could also permit a location-sensitive detection mode: Different relative positions of the biomolecule on the torus surface with respect to the left and right metallic leads should shift the location of interference minima and maxima as a function of magnetic flux in a characteristic fashion. Under small bias, density-of-states, transmission function and conductivity are calculated in tight-binding approximation. All quantities can be derived from the retarded and advanced (non-equilibrium) Green’s functions describing the torus as device region [4-6]. Results are computed for a (3,3) armchair torus with 1800, 3600 and 5400 carbon atoms respectively and for different lead attachments. A corrected electronic hopping parameter g’ is employed in tightbinding description at the atomic contact sites where the anthracene tether couples weakly to the torus lattice. A corrected electronic hopping parameter g’ is employed in tight-binding description at the atomic contact sites where the bio-molecule attaches (non-) covalently. Results produced by this fast tight-binding algorithm are compared with DFT-based transport calculations using the code ‘Quantum Espresso’ (QE)  available on NSF TeraGrid computer resources. Successive comparisons of the two approaches shall allow a realistic self-consistent modeling of the I-V characteristics after biopolymer-functionalization of the nanotorus or -tube. QE delivers the accurate ab-initio calculation of g’ as necessary parameter input for the fast tight-binding code. The DFT results may then be easily scaled to larger device dimensions. References: 1. S. Iijima, Nature (London) 354, 56 (1991). 2. M. Simmons et al., Phys. Rev. Lett. 98, 086802 (2007). 3. B.R. Goldsmith et al., Science 315, 77 (2007). 4. M.P. Anantram and T.R. Govindan, Phys. Rev. B 58 (8), 4882 (1998). 5. H.-K. Zhao and Phy. Lett. A 338, 425 (2005). 6. M. Encinosa and M. Jack, J. Comp.-Aided Mat. Des. 14 (1), 65 (2007); J. Mol. Simul. 34 (1), 9 (2008). 7. P. Giannozzi et al., http://www.quantum-espresso.org.