Photoresponsive surfaces

FIGURE 5.5.3 Control of cell adhesion on photoresponsive surfaces, (a) Access control of GRGDS peptides for biospecific cell adhesion by photoinduced cis-trans Isomerization of terminal azobenzene residues in grafted polymers (54). 

Wang, S.T., Song, Y.L, and Jiang, L. (2007) Photoresponsive surfaces with controllable wettability. J. Photochem. Photobiol. C-Photochem. Rev., 8, 18-29. 

In principle, the oxidation of proceeds at an electrode potential that is more negative by about 0.7 V than the anodic decomposition paths in the above cases however, because of the adsorption shift, it is readily seen that practically there is no energetic advantage compared to CdX dissolution in competing for photogenerated holes. Similar effects are observed with Se and Te electrolytes. As a consequence of specific adsorption and the fact that the X /X couples involve a two-electron transfer, the overall redox process (adsorption/electron trans-fer/desorption) is also slow, which limits the degree of stabilization that can be attained in such systems. In addition, the type of interaction of the X ions with the electrode surface which produces the shifts in the decomposition potentials also favors anion substitution in the lattice and the concomitant degradation of the photoresponse. 

In the case of undoped PVCa films, impurities and surface states dominate the photoconduction mechanism (6) leading one to question any study of intrinsic pKotoconduction in organic polymers. Poly(N-vinylcarbazole) films yellow under ambient laboratory conditions. Work in our laboratory (7) has shown that ageing of a purified sample of PVCa leads to an increase in photoresponse in the 350-450 nm region while there is an initial drop in photoresponse in the 250-... 

Figure 11.44 (Left) Structures of photoresponsive poly(propylene imine) dendrimers. (Right) The isomerization behavior of pahnitoyl-functionalized amphiphilic dendrimers at the water surface.

Novel Methodology for Immobilization of Biomolecules on the Surface of a Photoresponsible Polymer Containing Azobenzene Moiety... 

The electron transfer from Ru(bpy)2+ to MV2+ is very rapid, the order being 10s—10 M-1 s . But the irradiation of an aqueous solution of both the compounds does not give a photoresponse at the electrode dipped in that solution, because the back electron transfer from MV+ to Ru(bpy)j+ is also very rapid thereby no photochemical products are accumulated in the photostationary state (Scheme 6), However, when Ru(bpy)2 + is modified on the electrode surface by utilizing polymer coating, the electrode shows photoresponse 95), Such a modification of the electrode makes it photoresponsive, to afford a new type of photodiode. 

The surface of a carbon electrode was at first coated with a thin film of an anionic polymer such as sodium poly(styrene-sulfonate) 95) or nafion 96) (thickness thousand A) then the cationic Ru(bpy)2+ was adsorbed in the anionic layer electrostatically. The modification was also made by coating water insoluble polymer pendant Ru(bpy)2 + ( ) from its DMF solution 97). These Ru(bpy) +/polymer modified electrode gave a photoresponse in the MV2+ solution with the Pt counter electrode 95-97) The time-current behaviours induced by irradiation and cutoff of the light under argon are shown in Fig. 28. It is interesting to see that the direction of the photocurrent reversed at the electrode potential of ca. 0.4 V (vs. Ag—AgCl) under... 

Semiconductor electrodes can be used in galvanic cells like metal electrodes and a controlled electrode potential can be applied by means of a potentiostat, if the electrode can be contacted with a suitable metal without formation of a barrier layer (ohmic contact). Suitable techniques for ohmic contacts have been worked out in connection with semiconductor electronics. Surface treatment is important for the properties of semiconductor electrodes in all kind of charge transfer processes and especially in the photoresponse. Mechanical polishing generates a great number of new electronic states underneath the surface 29> which can act as quenchers for excited molecules at the interface. Therefore, sufficient etching is imperative for studying photocurrents caused by excited dyes. 

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