Nanocomposites

The final approach to be reviewed involves the use of preformed nanosized structures to assist in the formation of ICPs with nanodimensions. For example, Pope and co-workers introduced the concept of using silica nanoparticles to stabilize inherently conducting polymers [160]. We subsequently used this approach to produce polyaniline based silica nanocomposites [161] using an electrohydrodynamic synthesis technique. The electrochemical approach allows direct incorporation of a wide range of dopants, including in this case the optically active camphor sulfonate ion to produce chiral particles with nanodimensional ICPs incorporated therein (Fig. 17).

Applications of conducting polymers to nanosized semiconductor particles, commonly referred to as quantum dots, have also been made to produce nanocomposite materials [162]. These materials are typically spherical semiconductor crystals of 5 to 10 nm in size, such that quantum confinement effects predominate within these structures. At this size, these nanocrystals have special optical and electronic properties resulting from these quantum effects. A prime application of these materials is that tight size control will influence the onset of absorption and the fluorescence wavelength, thereby affecting the color of the observed particles in solution. CdS and Cu2S quantum dots were coated by mixing into an N-methylpyrrolidinone (NMP) solution containing the EB form of polyaniline. Extended exposure of the EB to these nanoparticles resulted in the oxidation of the EB to the pernigraniline base form. Co-precipitation of the EB and nanocrystals in NMP was achieved by the addition of ethanol. For CdS, electron transfer from the polymer to the nanocrystals is energetically favorable. Similarly, excitons created on the nanocrystals can also be transferred to the polymer resulting in an electron on the nanoparticle and an electron hole in the polymer. This creates a potential to control the performance of photovoltaic devices by altering the nanocrystal's size and concentration.

In another example, Bremer and co-workers [163] polymerized polyaniline onto nanodimensional polyurethane particles dispersed using a steric stabilizer (Fig. 18). We

Figure 17. High magnification transmission electron micrograph of PAn•(+)-HCSA/silica nanocomposites. Adapted with permission from [16], V. Aboutanos et al., Synth. Met. 106, 89 (1999). © 1999, Elsevier Science.

Conducting polymer "shell"

Outer stabiliser layer

Colloidal material "core" (eg. polyurethane)

Figure 18. Cor-shell nanoparticles. Reprinted with permission from [4], G. G. Wallace and P. C. Innis, J. Nanosci. Nanotech. 2, 441 (2002). © 2002, American Scientific Publishers.

Conducting polymer "shell"

Outer stabiliser layer

Colloidal material "core" (eg. polyurethane)

Figure 18. Cor-shell nanoparticles. Reprinted with permission from [4], G. G. Wallace and P. C. Innis, J. Nanosci. Nanotech. 2, 441 (2002). © 2002, American Scientific Publishers.

adapted this approach to produce highly optically active nanoparticles (20 nm) based on polyaniline doped with (+)-camphor sulfonate coated onto polyurethane particles [164].

A major disadvantage of using this approach is that these structures commonly aggregate to form raspberry-like structures during the polymerization process [161]. In an attempt to minimize this affect, Xia and Wang synthesized polyaniline nanosilica [165] and nanocyrstalline titanium oxide composites [166] while applying ultrasonic irradiation during the chemical polymerization process. Although aggregation was evident, the resultant nanoclusters of 60 to 120 nm were obtained from the initial 15 to 40 nm SiO2 cores. Similar results were observed for PAn-TiO2 nanocomposites. Other morphologies that resulted from the ultrasonication included thread-like and sandwich-like aggregated structures.

Nanocomposites have also been fabricated utilizing aqueous vanadium or iron oxide sols and polyaniline to produce particles characterized by X-ray crystallography as ca. 9 and 13 nm in diameter, respectively [167]. Strong interaction between the inorganic V2O5 core and its polyaniline shell indicates that the PAn-V2O5 nanoparticle functions as an intercalation compound, with electron transfer from the unshared electron pair of the nitrogen atom to the V5+ ion evidenced by electron paramagnetic resonance (EPR) spectroscopy. In the case of a Fe3O4 core, the ferromagnetic character of the Fe3O4 was the dominating feature of the EPR spectrum. However, infrared spectroscopy indicated that electronic interactions are occurring between the Fe3O4 core and the conducting polymer shell.

Recent work aimed at other objectives may also provide an interesting route to the production of ICP-containing nanocomposites: this is in the use of CNTs as host "particles." CNTs in their own right possess interesting electronic properties and when combined with other conjugated polymers synergistic effects have been observed [168, 169]. Our studies have focused on the use of aligned carbon nano-tubes as templates for ICP deposition. Our recent work in this area [170] has revealed significant improvements when PPy glucose oxidase is coated onto aligned CNTs for use as biosensors (Fig. 19). We have also shown that ICPs can be highly effective dispersants for CNTs, presumably attaching

Figure 19. Aligned carbon nanotubes with a bioactive conducting polymer. (a) Pure CNT array before treatment. (b) Aligned CP-CNT coaxial nanowires; inset shows clear image of single tube coated with PPY. (c) PPY only deposited on the top of CNT surface due to high density of tubes. (d) Polymer formed on both outside of walls and the top of the surface of the CNT array. Reprinted with permission from [170], M. Gao et al., Electroanalysis, 15, 1089 (2003). © 2003, Wiley-VCH.

Figure 19. Aligned carbon nanotubes with a bioactive conducting polymer. (a) Pure CNT array before treatment. (b) Aligned CP-CNT coaxial nanowires; inset shows clear image of single tube coated with PPY. (c) PPY only deposited on the top of CNT surface due to high density of tubes. (d) Polymer formed on both outside of walls and the top of the surface of the CNT array. Reprinted with permission from [170], M. Gao et al., Electroanalysis, 15, 1089 (2003). © 2003, Wiley-VCH.

themselves to the CNT structure. The properties of these novel dispersants of nanomaterials are yet to be investigated.

The molecular template concept used to produce polyaniline nanotubes [136] (as discussed) has also been used prepare to sulfonated multiwalled carbon nanotubes [171]. In this instance, the sulfonated nanotube surface serves a dual role acting as an in-situ dopant for the polyaniline as well as providing a supramolecular template onto which the anilinium ions in solution bind. Polymerization therefore occurs predominantly at the sulfonated nanotube surface. The resultant hybrid structure consists of a 10 nm thick polyaniline coating on the CNT surface, providing a novel way of introducing and functionality of to CNTs.

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