Torsionally Stabilized Nano Impedance Microscopy
device applications. Bio-based strategies for light-activated electron pumps, solar cells, chemical sensors, and opto- electronics have been suggested. An obvious challenge is to understand and control the properties of the proteins when attached to electrodes and configured in the ambient environments relevant to potential device architectures. Despite the challenges of characterizing biomolecule interfaces that are a consequence of the delicate nature of the structure, probing a complex property such as impedance would provide valuable insight. Te application of TR-NIM is illustrated here to determine
the resistance, capacitance, and dielectric properties of novel optically active biomolecules [19]. Tese molecules consist of tetramers of alpha helix polypeptides and provide a convenient functional alternative to natural proteins. We recently studied the behavior of zinc protoporphyrin IX (ZnPP) [19]. To determine the behavior of these functional biomolecules on electrodes, microcontact printing was used to create stripes that alternate between ZnPP maquettes and bare graphite. Understanding the dielectric constant is of particular interest because it gives information about the polarizability of these molecules. With a view toward understanding the behavior of future devices, optoelectronic properties of the biomolecules should be probed. Tis can be accomplished by combining TR-NIM with in-situ laser excitation.
Idealized protein-electrode junction devices can be
fabricated by microcontact printing self-assembled layers of proteins in patterns on atomically smooth highly oriented pyrolytic graphite (HOPG) [22]. Te first layer of these engineered amphiphilic proteins will orient on the hydrophobic graphite with the hydrophobic domain nearest the graphite due to hydrophobic interactions. Figure 3 presents a typical spatially resolved impedance measurement [19]. Te topographic structure shown in Figures 3a and 3b are typical of a protein-patterned electrode, in this case with a pattern pitch of 1 µm and protein lines varying in height from 6–20 nm. Many regions of the patterns exhibit heights very close to 6.6 nm, the length of a single 4-helix bundle. Tese regions, in which the orientation of the molecule with respect to the substrate can be assigned, are identified by height profiles in the range of 6.6 ± 0.5 nm, and the impedance is analyzed. Figure 3c iden- tifies several such regions. Te component of the impedance related to resistance is mapped in 3d, and the component related to capacitance is mapped in 3e. Measurements made during laser excitation with 425 nm light were similar to Figures 3d and 3e but differed in magnitude. Figure 4 compares several hundred values of the resistance
(a) and capacitance (b) in a region of the ZnPP monolayer with and without laser excitation. Tis type of analysis enables conclusions to be drawn from small signals with statistical
Figure 3: Proteins patterned on an electrode. ZnPP maquettes, 20 μL, 30 μM, 15 s stamping time. (a) topography, (b) zoomed in area of box in (a), (c) profiles of various areas of ZnPP maquettes. The first line profile corresponds to the line in image (b). Red arrows indicate regions of ~6.6 nm, which are one monolayer in height. (d) x-component of impedance of area in (b), (e) y-component of impedance in (b). x and y z-range = 125 mV, height z-range = 90 nm in (a) and 40 nm in (b). 70 kHz, 0.5 V applied signal. Kathan-Galipeau et al., ACS Nano 4835, copyright 2011, with permission of ACS.
2011 November •
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