Ionic self assembly with porphyrins and chrysotile nanotubes: supporting functional materials

Lately, the interest in nanotubes has been extended to several non-carbonaceous compounds, including conducting, semiconducting and isolating materials that find potential application in materials science, catalysis, electronics, or drug delivery. Geoinspired synthetic chrysotile (Mg3Si2O5(OH)4) is a stoichiometric silicate having a tubular nanostructure phase, and it can be blended to other compounds to yield advanced materials, without the drawbacks for human health typical of the natural chrysotile’s toxicity.1 We tried to use this as a nanostructured inorganic matrix to organize functional chromophores, such as porphyrins and their derivatives,2 and to obtain organic/inorganic hybrids having potential applications in photonics, non-linear optics and as sensors.3 The adsorption of various derivatives of the negatively charged tetra-(4-sulfonatophenyl)porphyrin (TPPS) on chrysotile has been easily achieved taking advantage of the nanotubes being positively charged over a broad range of pH, as determined via Z-potential measurements. These hybrid nanocomposites have been prepared by dropwise addition of concentrated (650 μM) aqueous solutions of the different dyes to the nanotubes aqueous suspensions. If the experiments are carried out at a neutral pH, the TPPS free base spontaneously adsorbs onto the initially white solid until charges neutralization has occurred (∼ 10 nmol TPPS /mg chrysotile). TPPS metalloderivatives (CuII, ZnII, MnIII) have been also adsorbed successfully on the inorganic matrix, allowing to prepare materials having a variety of optical properties, which have been thoroughly characterized by the combined use of UV/Vis extinction, fluorescence and resonance light scattering (RLS) techniques. TPPS/chrysotile hybrids have shown a reversible response to the presence of acids vapors or solutions, with the formation of TPPS J-aggregates after a substantial rearrangement of the dye molecules at the surface, thus pointing to possible sensor applications. The ionic self assembly approach has been also used to prepare stable colloidal dispersions of the nanotubes in chlorinated solvents through addition of an anionic surfactant (SDS), that can then interact with the tetrabutylammonium salt of TPPS, leading to supported H- and J-aggregates. The latter ones possess very narrow extinction and RLS features which are very similar to those normally observed for unsupported TPPS aggregates in water, and that seem promising for NLO applications.4