Electronic Properties of Functional Biomolecules at Metal/Aqueous Solution Interfaces

Abstract

Monolayers of molecules, which retain their function in the adsorbed state on solid surfaces, are important in materials science, analytical detection, and other technology approaching the nanoscale. Molecular monolayers, including layers of functional biological macromolecules, offer new insight in electronic properties and stochastic single-molecule features and can be probed by new methods which approach the single-molecule level. One of these is in situ scanning tunneling microscopy (STM) in which single-molecule electronic properties directly in aqueous solution are probed. In situ STM combined with physical electrochemistry, single-crystal electrodes, and spectroscopic methods is now a new dimension in interfacial bioelectrochemistry. We overview first some approaches to spectroscopic single-molecule imaging, including fluorescence spectroscopy, chemical reaction dynamics, atomic force microscopy, and electrochemical single-electron transfer. We then focus on in situ STM. In addition to high structural resolution, in situ STM offers a single-molecule spectroscopic perspective. This emerges most clearly when adsorbate molecules contain accessible redox levels, and the tunneling current decomposes into successive single-molecule interfacial electron transfer (ET) steps. Theories of electrochemical ET and in situ STM of redox molecules as well as specific cases are addressed. Two-step in situ STM represents different molecular mechanisms and even new ET phenomena, related to coherent many-electron transfer. A number of systems are noted to accord with these views. The discussion is concluded by attention to one of the still very few redox proteins addressed by in situ STM, the blue copper protein Pseudomonas aeruginosa azurin. Use of comprehensive electrochemical techniques has ascertained that well-defined protein monolayers in two opposite orientations can be formed and interfacial tunneling patterns disclosed. P. aeruginosa azurin emerges as by far the most convincing case where in situ STM of functional metalloproteins to single-molecule resolution has been achieved. This comprehensive approach holds promise for broader use of in situ STM as a single-molecule spectroscopy of metalloproteins and illuminates prerequisites and limitations of in situ STM of biological macromolecules.

Two-dimensional assembly and local redox-activity of molecular hybrid structures in an electrochemical environment

“…Comparing the potentials of the characteristic redox-processes V 21 2 V 1d and V 1d 2 V for HS-6V6-SH and HS-6V6-H under the present experimental conditions with literature data [51][52][53][54][56][57][58][59][60]63 indicates that co-adsorbed bromide ions are partially replaced by perchlorate ions, which are in excess in the supporting electrolyte chosen in this study.…”

“…The solid trace in Fig. 2 reveals, as an example, a well-behaved redox-response V 21 2 V 1d of surface immobilized HS-6V6-H. The oxidation and reduction peaks were found at E ¼ À0.428 V and E ¼ À0.450 V. The peak height scales rather linearly with scan rate.…”

“…4A and 4B) and HS-6V6-SH (Fig. 4D and 4E) assemble on Au(111)-(1 Â 1) in À0.70 V r E r À0.20 V, i.e., the stability ranges of V 21 and V 1d (V 1d ) 2 , in regular arrays of parallel stripes, which cover the entire substrate surface. Typical results for E ¼ À0.20 V are summarized in Fig.…”

“…[78][79][80][81][82][83] The IR spectrum of the electrochemically generated dimer species is interpreted by the vibronic coupling in a one-dimensional charge transfer complex. 82 The negative band around 1100 cm À1 indicates the release of perchlorate ions upon the reduction V 21 2 V 1d . The radical cation monomer is stabilized in a hydrophobic environment 80 and by electrostatic attraction to co-adsorbed ClO 4 À anions.…”

Electrochemically controlled self-assembled monolayers characterized with molecular and sub-molecular resolution

“…136 The SAM serves not only as a linker to connect protein molecules but also as an electronic wire for promoting electron transport. A number of protein monolayers including the blue copper protein azurin, 95,96 the heme protein cytochrome c 4, 91 the iron-sulfur protein ferredoxin, 69 and the blue copper enzyme nitrite reductase (NiR) 137 have been successfully addressed and mapped to single-molecule resolution by employing this strategy together with the combination of single-crystal electrochemistry and in situ STM.…”