Scanning probe technology in metalloprotein and biomolecular electronics

Abstract

The interfacing of man-made electronic components with specifically-folded biomacromolecules lies central not only to the development of sensory interfaces and potential new molecular-scale devices, but also enables us to analyse processes of great biological importance in a refined and controllable manner. Recent advances in both available technology, most notably optical and scanning probes in nature, and our understanding of suitable methodologies, have led us to the point where the characteristics of single biological molecules can be interrogated with good levels of reproducibility. We review here the application of scanning probe microscopy to the analysis of and experimentation on biological redox systems. Within this paper the tunnel transport characteristics, as assayed by both scanning tunnelling microscopy (STM) and conducting probe atomic force microscopy (AFM), of single metalloproteins are discussed. In a specific case study the electron transfer characteristics of the blue copper metalloprotein, azurin, are reported. The modulation of these properties under the influence of calibratable compressional force has also been examined in some detail. Work such as this enables one to reproducibly establish the conductance, barrier height, environmental sensitivity and electromechanical properties of these molecules.

Near-Metallic Behavior of Warm Holoferritin Molecules on a Gold(111) Surface

“…A clear distinction could be made between the I − V curves for bare gold(111), holoferritin molecules, and apoferritin molecules at room temperature (25 °C) under ambient conditions (humidity 40−50%) (Figure ). The I − V curves generated from a bare gold substrate were generally symmetric and linear (i.e., ohmic in nature), consistent with earlier reports . The I − V response for the proteins was, however, nonlinear, and the observation of a well-defined electronic band gap clearly suggested a stable barrier to conduction, which is an indication of electron transport across the protein molecules.…”

“…The I-V curves generated from a bare gold substrate were generally symmetric and linear (i.e., ohmic in nature), consistent with earlier reports. 40 The I-V response for the proteins was, however, nonlinear, and the observation of a well-defined electronic band gap clearly suggested a stable barrier to conduction, which is an indication of electron transport across the protein molecules. The I-V curve characteristics of the holo and apo forms of ferritin were quite different from each other.…”

Yeast cytochrome c integrated with electronic elements: a nanoscopic and spectroscopic study down to single-molecule level

“…An analogous increment in the protein vertical size, as compared to bare gold, has been detected from STM images of azurin chemically anchored to SATP-cysteamine-modified gold [56]. Indeed, an underestimation of physical height is a general feature of STM images of biomolecules directly assembled on conductive substrates [44,47,[61][62][63][64][65], and it is likely related to the low conductivity of biomolecules. However, in STM images, obtained operating in constant current mode, an increment in the molecular height (namely, in the registered tunnelling current) of metalloproteins adsorbed on spacers as compared to bare gold can reasonably reflect differences in electronic conductivity.…”

Functional Metalloproteins Integrated with Conductive Substrates: Detecting Single Molecules and Sensing Individual Recognition Events

“…Above all, chemisorption, ensuring a direct and specific linking of the biomolecule with the conductive substrate, provides immobilization of the protein with preferential orientation, also favoring the electronic connection with the substrate. In the case of gold electrodes, thiols of cysteine residues naturally available − or genetically engineered , have often been exploited: thanks to the high affinity of the thiol group for gold, − the proteins are directly linked to the substrate via S−Au bonds. However, in some cases, the strong interaction of the protein with the metallic surface can result in a loss of electrical signal − and partial protein denaturation. ,,− In order to shelter the molecule from possible destructive interactions with the metal, a spacer of suitable length can be introduced.…”