Studies of photoelectric signals & molecular features of bacteriorhodopsin and electric-field induced patterns on polymer surfaces

Abstract

The topics covering the optical activity, photoelectric signals and the effect of local electrostatic environment on the photophysical activity of a protein - retinal complex, Bacteriorhodopsin (bR) are described in the first half of the thesis. The response of soft polymeric films to an applied electric field leading to a surface deformation is described in the latter part of the thesis. The thesis concludes with a section describing the electric field induced deformation of a liquid alloy droplet as soft electrical contacts for studing electrical properties of bR films. The first part involves the measurement of photoelectric signals from bR in mono- and multi-layer forM.S.. Observation of photoelectric signals forM.S. a direct evidence of the bR functionality. Photoelectric signals from monolayers of bR on conducting polymers in two terminal liquid cells were measured and correlated to the photoinduced process occurring within bR. Local photoelectric measurements were done using wide-field photocurrent imaging to provide insight on the functionality of a single monolayer patch. Further, photoelectric signals from dried bR patches were studied by fabricating a three terminal device, with two lateral electrodes across the polymer (PEDOT: PSS or PANI:DBSA) layer and multilayers of bR in the central region. The underlying polymer becomes electronically active upon photoexcitation of the bR region with the spectral and temporal characteristics of the signal corresponding to that of the bR molecule. The changes in conductance of the underlying polymer layer were correlated to the various doping processes. The temporal features across the lateral electrodes were attributed to the light-induced dipole fluctuations within bR that gets translated as local conductance changes in the polymer layer. In case of underlying polyaniline vii (PANI:DBSA) films, which can be doped by protonic acids, a resistive increase in the current was observed. This was attributed to transient doping of the polymer surface by the bR-protons. The next section involves, bR in the form of PM can be organized as patches with a large areal coverage on specifically treated substrates. Individual bR molecules within the patch are observed to be functionally active. The optical properties of the monolayers of bR oriented on different substrates such as quartz and conducting polymers were studied. The optical activity of a bR monolayer was probed by near-field microscopy in the transmission mode. In aperture based near-field microscopy, sample is typically illuminated via a small aperture (100 nm) which can provide an optical resolution in the scale of ∼ 40 nm. Optical constants were estimated from these single molecule measurements and were found to be higher than the bulk measurements. These single molecule effects were attributed to an additional interaction between the near-field and the transition dipole moment of the retinal chromophore. Upon, introducing an additional pump (corresponding to the excited state absorption) on the probing region, the absorption corresponding to the probe-wavelength is enhanced. This increased absorption was attributed to the dynamics of the photocycle and the quantum efficiencies of the photoconversion process. Further, the effect of local electrostatic environment on the optical activity and photocycle of bR were greatly modified when these protein molecules were oriented on a thin layer of polyaniline. Using this method of pump-probe near-field microscopy, the changes in bR optical activity and photocycle were followed. The latter part deals with electric-field induced effects on polymer films, whose viscosity and elasticity was modified from a viscous like to elastic solid like films. Upon, application of electric field in parallel plate geometry, the film surface deformed to give patterns which characteristic hexagonal ordering whose wavelength depended on various factors. For viscous films, the wavelength depended on the applied electric field and the surface tension whereas for solid like films, the wavelength was found to be independent of the applied electric field as long as it was above the critical value. For higher shear moduli films, inclined plane geometry was used to study the electric field induced deformation. The competition between the elastic strain energy and the van der waals interactions gives rise to a fingering pattern at the contact zone prior to the application of electric field. The distinct electric field induced morphological changes, leading to the formation of two-dimensional hexagonally arranged pillars, large-amplitude fingers, and straightening of contact edge were studied comprehensively. The last chapter deals with deformation of a liquid alloy when an electric field was applied and this deformed alloy was used as contacts to measure the bR photoelectric signals. The thesis concludes with a brief summary and a section on future outlook.