DNA Conductance MeasurementsOutlineMotivation for Studying DNA DNA ChemistryDNA StructureOutlineDNA Charge Transport TheoriesSuperexchange and HoppingPolaron-Like DistortionsSequential TunnelingOutlineStilbene Capped DBA systemsSteady State InvestigationsDNA Charge Transport SummaryOutlineDNA as a ConductorDNA as an InsulatorDNA as a SemiconductorProximity Induced SuperconductivitySmall-Scale MeasurementsSmall-Scale MeasurementsContention in DNA ConductivityOutlineGold ContactsSTM Break JunctionOther LabsDNA and STM Break Jxn.DNA Length DependenceDNA Sequence DependenceDifferent Mechanisms?Conduction MechanismTemperature ControlSingle Molecule Gate EffectElectrochemical Gate SystemEC Gate ControlDNA Mismatches2 Base-pair MismatchesSingle Mismatch DetectionControl ExperimentsSummaryDNA BJ SummaryDNA Conductance SummaryDNA Conductance MeasurementsMolecular ElectronicsSeptember 04, 2008Outline• Interest in DNA Charge Transport• DNA Structure• Introduction to Theoretical Models• Photoinduced Charge Transport• History of DNA Conductance Studies• Break-Junction Based DNA Measurements• Summary and ConclusionsMotivation for Studying DNA • Nanoscale Materials:– Electronic Component– Structural Template•Biological Studies:–The number of molecules in a single cell– Non-optical methods of biological study– New Characterization ToolsDNA ChemistryDNA StructureEndres et. al Rev. Mod. Phys. 76 195 Walet et al. cond-mat 0402059Outline• Interest in DNA Charge Transport• DNA Structure• Introduction to Theoretical Models• Photoinduced Charge Transport• History of DNA Conductance Studies• Break-Junction Based DNA Measurements• Summary and ConclusionsDNA Charge Transport Theories•Superexchange• Thermally Activated Hopping• Large Polaron-Like Distortions• Sequential Tunneling•System Complexities– Base-pair overlap– Statistical mechanics– Base-pair Sequence–Length– Interstrand vs. IntrastrandTransport– Donor-Bridge-Acceptor OverlapSuperexchange and Hopping• Superexchange is Bridge Mediated Coherent Transport• A:T Tunneling Barrier• Experiments Show Long-Range Transport Phenomenon• Thermally Assisted Hopping AlternativeJortner et al. PNAS95, 12759DADAPolaron-Like Distortions• Energy minimization between electrical structural energy• Results in a local distortion• Possible distortions and delocalization in DNA• 1-D organic conductors typically result in polaron distortionsHenderson, P.T., et al. PNAS, 1999. 96(15): p. 8353-8358.Sequential TunnelingOutline• Interest in DNA Charge Transport• DNA Structure• Introduction to Theoretical Models• Photoinduced Charge Transport• History of DNA Conductance Studies• Break-Junction Based DNA Measurements• Summary and ConclusionsStilbene Capped DBA systems• System with StilbeneAcceptor and G Donor• Time resolved Spectroscopy• Small-Scale SystemsLewis et al. Acc. Chem. Res. 2001, 34, 159-170Steady State Investigations• Photolysis of Modified Backbone• Strand Cleavage Assays using Electrophorisis•Long-Range Transport over A:T basepairsGiese et al. Nature. 412 p.319DNA Charge Transport Summary• A variety of Chemical CT measurements have shown that DNA is capable of CT• Different Models exist, but lack predictive capabilities• Typical Transfer Rates on the order of 109/s• Yield interest in Direct contact conductance measurementsOutline• Interest in DNA Charge Transport• DNA Structure• Introduction to Theoretical Models• Photoinduced Charge Transport• History of DNA Conductance Studies• Break-Junction Based DNA Measurements• Summary and ConclusionsDNA as a Conductor• Fink & Schönenberger• Low Energy Electron Point Source (70eV)• Conductor –hopping activated• ρ –41.7Ω/cmFink et al Nature 398 407DNA as an Insulator• de Pablo et al. 2000• DFT SIESTA Theoretical Model • AFM measurements many strands 10V no current• λ-DNA many strands on surface•Show that Fink and Schönenberger results affected by LEEPS de Pablo et al PRL 85, 4992DNA as a Semiconductor• Cai et al. 2000•Conducting AFM•Minimum 40nm• Reticulated Networks on surface• Poly(G):Poly(C) • Poly(A):Poly(T)Cai et al. APL77(19) 3105Proximity Induced Superconductivity• λ-DNA placed across Rhenium/Carbon Electrodes • ~3-40 strands across 100’s of nanometers• Conductance near G0• Re is a superconductor at 1K• Superconductivity found through DNA at this temp.Kasumov et al Science 291, 280Small-Scale Measurements• 10.4nm (30 base-pairs) poly(G:C) single DNA • Electrostatic trapping molecules between nanoelectrodes• IV measurements yield large band-gap semiconductingbehavior• Same group 2001 found insulating behavior beyond 40nmPorath et al. Nature 403, 635Small-Scale Measurements• Triple Probe Conducting AFM station•Random Sequence DNA•CNT AFM tipShigematsu et al. J. Chem Phys. 118, 4245Contention in DNA ConductivityYear Author ResultSample TypeLength # of DNA1998 Braun et al. Insulator λ-DNA 12 μm Many1999Fink & ShönenbergerConductor λ-DNA 600 nm Many2000Porath & Dekkeret al.Semiconductor poly(G:C) 10.4 nm One2001 Kasumov et al.Proximity Induced Superconductorλ-DNA 500 nm Several2003 Shigematsu et al.Hopping MechanismSalmon-sperm DNA<20 nm OneMany othersOutline• Interest in DNA Charge Transport• DNA Structure• Introduction to Theoretical Models• Photoinduced Charge Transport• History of DNA Conductance Studies• Break-Junction Based DNA Measurements• Summary and ConclusionsGold ContactsCounts0.0 0.2 0.4 0.6 0.8 1.00-2-4-6-8 012345678 Strecthing (nm)Force (nN)Conductance (G0)0123 Conductance (G0)0.00 1.45 2.90 4.35 5.80Force (nN)Agrait et al. PRLF=1.45 ± 0.1 nNCountsG0=77.5μS, 2e2/hSTM Break Junction0123Conductance (10-5G0)Step length (nm)0.0 1.3 2.6 3.9G (10 -5G0)CountsXu et al. Science 2003Other LabsWandlowski et al., 2006Lindsay et al., 2005Venkataraman et al., 2006DNA and STM Break Jxn.1234Conductance (10-3G0)8-bp DNA-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8-150-100-50050100150Current (nA)Bias Voltage (V) e8-bp DNAR=20 MΩ0.0 0.5 1.0 1.5 2.0 2.50.01.53.04.5Stretching (nm)Conductance (10-3G0)8-bp DNAConductance (G/Go) x10-3Counts5’--3’-SHC G C G C G C GG C G C G C G CHS-3’--5’DNA Length Dependence•Systematically Adjust Length•Use 8, 10, 12, 14 base-pairs•Demonstrates Linear Response with 1/L•Suggests Hopping 5’--3’-SHC G C G C G C GG C G C G C G CHS-3’--5’DNA