Molecular Templates

Distinctly different from the templating techniques described previously, this approach relies on the interaction at the molecular scale of the conducting polymer and a templating dopant or surfactant/polyelectrolyte to form a nanoparticle. The molecular template not only functions to direct morphology but it can also impart other properties to the ICP nanoparticle, as will be discussed.

Others have shown that molecular templates, such as a polyacrylate film predeposited on a carbon electrode, can be used effectively to create polypyrrole nanowires via electrochemical oxidation of pyrrole [127, 128].

Choi and Park [129] have modified surfaces with cyclo-dextrins to assist in the electroformation of polyaniline nanowires. Others have utilized the concept of molecular templates to form polyaniline supramolecular rods with electrical conductivity improved by 2 orders of magnitude [130].

Opal Template

ICP Infiltrated Opal Template

ICP Inverse Opal

Figure 16. Schematic illustrating preparation of an inverse ICP opal using a synthetic opal template. Reprinted with permission from [4], G. G. Wallace and P. C. Innis, J. Nanosci. Nanotech. 2, 441 (2002). © 2002, American Scientific Publishers.

Opal Template

ICP Infiltrated Opal Template

ICP Inverse Opal

Figure 16. Schematic illustrating preparation of an inverse ICP opal using a synthetic opal template. Reprinted with permission from [4], G. G. Wallace and P. C. Innis, J. Nanosci. Nanotech. 2, 441 (2002). © 2002, American Scientific Publishers.

A template guided synthesis of water-soluble chiral conducting polyaniline in the presence of (S)-(-)- and (R)-( + )-2-pyrrolidone-5-carboxylic acid [(S)-PCA and (R)-PCA] has been reported to produce nanotubes [131]. The structures prepared have outer diameters of 80-220 nm with an inner tube diameter of 50-130 nm. It was proposed that the tubular structures form as a result of the interaction of the hydrophobic aniline being templated by the hydrophilic car-boxylic acid groups of the PCA in aqueous media during chiral tube formation. The resultant tubes were shown to be optically active, suggesting that the polyaniline chains possess a preferred helical screw.

McCarthy et al. [132, 133] reported a template-guided synthesis of water-soluble chiral polyaniline nanocomposites. The nanoparticles were prepared by the physical adsorption of aniline monomer onto a templating poly(acrylic acid) in the presence of (+)- or (—)-CSA, followed by chemical oxidation. Using this approach, optically active nanocomposites of approximately 100 nm diameter were formed. Earlier work by Sun and Yang [134] using polyelectrolytes produced similar nonchiral dispersions in which the polyaniline chain is interwound with a water-soluble polymer by electrostatic forces [135]. Similar work by Samuelson et al. utilized DNA as a chiral template for polyaniline [89].

Shen and Wan [136] have shown that chemical oxidation of pyrrole (by APS) in the presence of ^-naphthalene sulfonic acid (S-NSA) results in the formation of tubules down to ca. 200 nm in diameter. The tubular structures had reasonable conductivity (10 S cm-1) and were soluble in m-cresol. In later work [137] it was reported that the morphology was significantly influenced by the concentration of the S-NSA. Granular morphologies were present when [S-NSA] was less than 0.2 M, while on increasing the S-NSA concentration fiber formation becomes the dominant morphology. At S-NSA concentrations above 2.9 M, a block-type morphology predominated with an associated drop in conductivity to 2 mS cm-1. Monomer concentration was shown to have little effect on the resultant morphology and electrical conductivity. The morphological steps in tube formation were described as grain to short tube to long tube. Other morphology types have also been noted using the S-NSA anion, namely hollow microspheres ranging from 450 to 1370 nm (with inner wall thicknesses of 20 to 250 nm, respectively) and conductivities of up to 190 mS cm-1 [138]. A similar approach has been extended to polyaniline [139]. The influence of a secondary inorganic acid codopant has also been demonstrated, with conductivities from 10—1 to 100 S/cm [140]. Others [141] have used lipid tubules as templates during chemical oxidation of pyrrole to form nanofibers with diameters between 10 and 50 nm and lengths reaching up to several hundred hundreds of micrometers.

The concept of using molecular templates has been taken a step further by Zhang and Wan [142] by the incorporation of 10 nm Fe3O4 nanomagnet particles into the S-NSA nanorods and nanotubes that are 80-100 nm in diameter. These PAni-S-NSA/Fe3O4 nanostructures were observed to exhibit superparamagnetic behavior (i.e., hysteresis loop effects). More importantly, both the electrical conductivity and magnetic properties of these nanoparticles could be manipulated by control over the loading level of the nano-magnetic particles. Increased loading of the Fe3O4 nano-particle decreased the conductivity from ca. 70 mS cm-1 for no loading to ca. 10 mS cm-1 at 20 wt%, while inducing superparamagnetic behavior in the nanocomposite.

Self-assembled polyaniline nanofibers and nanotubes, which also exhibit photoisomerization functionality, have been described by Huang and Wan [143] using azobenzene-sulfonic acid (ABSA) as the molecular templating surfactant, dopant, and photoactive agent. Nanostructures formed were similar to those by Wan et al. [131, 136, 138, 140, 142] discussed previously, with diameters of 110-130 nm and fiber lengths of 3-8 /m. The photoinduced isomerization, from trans to cis, of the ABSA dopant was observed by UV-vis spectroscopy at 430 nm (n-w* transition) after irradiation of the nanocomposites at 365 nm (over 0, 2, 6, 10, and 12 min) by UV-vis spectroscopy from 300 to 800 nm. The photoi-somerization of the composite material was observed to be slower than for ABSA due to steric hinderance as a result of the trans-ABSA interacting along the polymer backbone.

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