title: Metalloprotein tunnel junctions: compressional modulation of barrier height and transport mechanism.
abstract: Though the incorporation of sensory or potentially-switchable biological entities into electronic devices brings with it a number of complicating issues associated with hydration, structural complexity/delicacy, and low conductance, the possibility of resolving properties of fundamental importance (such as the influence of protein fold on conductance) at a molecularly-resolved level, are exciting. Our ability to analyse charge transport through a biological macromolecule remains, though, a significant practical and theoretical challenge. Though much information can be gained by carrying out such examinations at a molecular level, there exist few methods where such controlled analyses are, in fact, feasible. Here we report on the electron transport characteristics of a blue copper metalloprotein as characterized by conductive-probe atomic force microscopy. At very low imposed force, contact resistance is high, electrical contact unstable, and the junction undergoes dielectric breakdown at 1.1-1.5 GV m(-1). At increased applied force, the current-voltage characteristics are entirely reproducible and well-described by a Simmons (non-resonant) tunnelling model. Though highly resistive, observations demonstrate the ability of the protein matrix to mediate appreciable tunnelling current. Non-resonant behaviour is consistent with observations of bias-independent tunnelling imaging. In fitting observed transport characteristics to this model, it is possible to deconvolute barrier height and length at specific experimental conditions and, specifically, to monitor the modulation of these parameters by imposed compressional force. At higher field spectroscopic features assignable to metal based density of states are reproducibly observed. These vanish in a force regime where the tunnel barrier to direct tip-sample communication decreases.
authors.affiliation: Central Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3TA.
authors.forename: Jason J
authors.initials: JJ
authors.lastname: Davis
authors.forename: Nan
authors.initials: N
authors.lastname: Wang
authors.forename: Ashley
authors.initials: A
authors.lastname: Morgan
authors.forename: Tiantian
authors.initials: T
authors.lastname: Zhang
authors.forename: Jianwei
authors.initials: J
authors.lastname: Zhao
mesh_terms: D001400:Azurin / Q000032:analysis* / Q000737:chemistry*; D019245:Compressive Strength; D003198:Computer Simulation; D004553:Electric Conductivity; D004563:Electrochemistry / Q000379:methods*; D004579:Electron Transport; D008956:Models, Chemical; D011550:Pseudomonas aeruginosa / Q000737:chemistry*; D012666:Semiconductors; D013314:Stress, Mechanical
chemical_list: D001400:Azurin