By combining peptide design, monolayer patterning, and a new probe of bio-optoelectronic function to characterize the dielectric and optoelectronic properties of an ambient protein-electrode system, we report the simultaneous detection of electron transport and the effect of optical absorption on dielectric polarizabilityin oriented peptide single molecular layers.
66 Å amphiphilic alpha helices self assemble in detergent solution to form 4-helix bundles, as illustrated in Fig a and b. 5 ZnPP (red) are added per 4-helix bundle on the histidine site (dark blue). Self-assembled protein layers were patterned on atomically smooth HOPG using microcontact printing. The helices are oriented perpendicular to the underlying substrate. A 425 nm LED (blue) is directed at the tip-sample junction (Fig. c).
Torsionally stabilized nano-impedance microscopy was developed to simultaneously probe transport and impedance of single molecule layers at interfaces, and was used to investigate the sample impedance. Protein patterns are shown in the topography image, Fig. d. Cross-sectional line profiles are taken and red arrow indicates regions of one monolayer in height in Fig. e. Fig. f and g gave X and Y components of impedance, which can be converted to resistance and capacitance of the protein molecular layer.
Resistance (R) and capacitance (C) of monolayer ZnPP maquettes at 70 kHz in the presence and absence of optical illumination are compared in Fig. h and i. The 2D/3D histograms show the clear reduction of resistance and increase in capacitance under light. Optical absorption causes an increase of charge carriers that reside in delocalized states in ZnPP, thus the decrease of the resistance. Most interesting is the optical dependence of capacitance, which is related to molecular polarizability. The photo induced increase of 117%-267% in dielectric constant corresponds to an excited state polarization volume increase and agrees quantitatively with that found in porphyrin molecules in liquid.