Spectral and AFM characterization of trimethylammoniophenylporphyrin and concanavalin A associate in solution and monolithic SiO2 gels obtained by the sol–gel method

Abstract The associate between water-soluble cationic tetrakis[4-(trimethylammonio)phenyl] porphyrin (H2TTMePP) and concanavalin A (Con A) has been studied in the tris-buffer solution by absorption and emission electron spectroscopy. The porphyrin and porphyrin concanavalin associate has been incorporated into the monolithic pure silica gels obtained by polycondensation of tetraethoxysilane. The optically transparent dried gels were studied using absorption and fluorescence spectroscopic techniques and also by the tapping mode of atomic force microscopy (AFM). Complex formation between porphyrin and concanavalin takes place in both solution and gel. In these media porphyrin and its lectin associate exhibit luminescence emission in the vis-ir range when excited with visible light. Upon binding to concanavalin A the increase in porphyrin fluorescence intensity and the red-shift in the absorption and emission maxima have been observed. AFM visualisation of porphyrin and the porphyrin–concanavalin conjugate shows significant differences between nanostructures of the pure porphyrin and complex doped gels. It has been found that the ‘‘smooth’’ surfaces of silica gels prepared by the sol–gel technique are an excellent medium for the AFM visualisation of biomolecules.