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Title: Conductivity studies of single protein molecules
Author: Zaki, Athraa J.
ISNI:       0000 0004 5989 271X
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2016
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A fundamental step for future uses of biomolecules in electronics is the study of the bonding, orientation and conductance of a single molecule attached to a conductive substrate, which is the building block of electronic materials and devices based on molecular conduction. This work provides an in-depth examination of morphology and electrical properties of different molecules anchored to Au(111) and to sustainable carbon materials (graphite and graphene). Cytochrome b562 (Cyt b562), TEM beta-lactamase and the superfolded green uorescent protein engineered with phenyl azide were exposed to UV irradiation to transform the azide compound into the nitrene radical, which enabled successful molecule linking to graphene. The UV-based approach was tested on the above molecules to ascertain its robustness against the specificity of the protein used. The efficiency of the procedure was inspected by imaging via atomic force microscopy (AFM) and scanning tunnelling microscopy (STM). By repeated sample preparation and imaging, we established suitable protein concentrations to enable single-molecule measurements on the resulting samples (e.g., the concentration range optimal for cyt b562 on gold was 0.025-0.5 μM). We used a home-built environmental cell in combination with STM to study the conductance of differently engineered cyt b562 proteins on Au(111), as well as the conductance of oligothiophene on gold, under different humidity and temperature conditions. We found that the conductance of cyt b562 is smaller at lower relative humidity and further decreased when also temperature is reduced. Measuring the conductance as a function of the tip-substrate distance in both tip approaching and retracting modes revealed the occurrence of hysteresis. The engineered cyt b562 with two thiols in the long axis led to less hysteresis in the conductance and larger protein height on gold (from AFM) compared to the protein with thiols in the short axis. Our results stress the importance of protein engineering to control the electrical properties of functionalized surfaces. This study meets the growing demand for achieving more efficient molecule linking to conductive substrates, and studying environmental effects on the electrical response of functionalized surfaces (which is relevant, e.g., to sensing applications).
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics