Regioregular polythiophene/gold nanoparticle hybrid materials

Gold nanoparticles have attracted intense interest in materials and physical chemistry since the development of simple solution-phase methods for their synthesis and stabilization. The unique physical properties of gold nanoparticles have created a wide variety of potential applications including biological markers, DNA sensors, molecular recognition systems and nanoscale electronics. Encapsulation of inorganic nano-particles in core-shell structures with conducting polymers has been attempted with a variety of potentially conductive organic systems. These materials may differ from the pure polymer in that some of the physical and chemical properties, such as the conductivity, are enhanced. The stabilization of nanoparticles with polymers has been investigated by a number of groups. Walker et al. have synthesized gold nanoparticles by using poly(methylphenylphosphazene) (PMPP) where gold nanoparticles were stabilized by the nitrogen lone pair electrons on the polymer backbone. Zhou and coworkers have reported the preparation of novel p-conjugated polydithiafulvene (PDF) protected PbS colloidal nanoparticles. The authors believe that the PbS nanoparticles are stabilized by weak interactions between the sulfur moieties of PDF and the surface unsaturatedly coordinated Pb ions present on the surface of the nanoparticles. Gold nanoparticles inside well-defined polystyrene–oligothiophene–polystyrene triblock copolymer (PS–OT–PS) micelles were fabricated through the reduction of gold salt in an oligothiophene core by the electrons transferred from the oligothiophene. Advincula et al. have used a polyelectrolyte complex (PEC) with water-soluble terthiophene derivatives to reduce HAuCl4 to gold nanoparticles. Since regioregular polythiophene are amongst the most widely studied conjugated polymers, we sought to prepare and characterize polythiophene/gold nanoparticle hybrid materials. The synthesis of regioregular poly(3-hexylthiophene) stabilized gold nanoparticles was accomplished using a room temperature, two-phase, one-pot reaction involving the reduction of tetrachloroauric acid by sodium borohydride in the presence of regioregular poly(3-hexylthiophene)s (Scheme 1).{Although Advincula and coworkers reported that tetrachloroauric acid oxidized the water-soluble terthiophene derivatives, we found that using a large amount of tetraoctylammonium bromide prevented the oxidation of poly(3-hexylthiophene)s. Work by Torsi et al. has shown that conjugated polymers can stabilize inorganic nanoparticles by coordinating or forming complexes with the inorganic nanoparticles. Furthermore, work by Ng et al. has demonstrated that sulfur atoms of polythiophenes can interact with metal ions. Hence, we surmise that the gold nanoparticles are stabilized by the interaction of the sulfur atoms of poly(3-hexylthiophene)s with the gold surface. We have used various amounts of polythiophenes (25, 20, 15 and 10 mg) in preparing the gold nanoparticles. We have found that nanoparticle size distribution was large when 25, 20 and 10 mg of the polythiophene was used. However, a narrow size distribution of nanoparticles was obtained by using 15 mg of regioregular poly(3-hexylthiophene). This surprising result is reproducible. While it has been found that as the concentration of oligothiophenes increases, nanoparticle size also increases, we have found no study that shows that the size distribution of gold nanoparticles can be controlled by tuning the concentration of polythiophenes. At high concentration, e.g. 25 mg, we believe that the aggregation of polythiophenes results in simple nanoparticle fusion or growth, leading to larger gold nanoparticles; while at low concentration, there may not be sufficient polymer to stabilize the nanoparticles. UV-Vis behavior of polythiophene-stabilized nanoparticles has been investigated by comparing the UV-Vis absorptions of regioregular polythiophene, polythiophene-gold stabilized nanoparticles, and dodecanethiol-stabilized nanoparticles (synthesized according to the simple and efficient procedure reported by Brust et al.) in toluene at room temperature. The main absorption band observed at 450 nm in the UV-Vis spectrum of polythiophene-stabilized nanoparticles (Fig. 1b) is attributed to the absorption of free polythiophenes in the solution. This is consistent with the UV-Vis spectrum of pure polythiophene solution which shows a strong absorbance around 450 nm due to the p–p* transition of the conjugated polymer chain (Fig. 1a). In their studies of preparing gold nanoparticles from terthiophene, Youk et al. have reported gold surface plasmon peaks at around 530 nm. Our spectra are quite similar to the oligothiophene gold nanoparticle spectra. Two factors contribute to the broad absorption from 500 to 600 nm in the UV-Vis spectrum of polythiophenestabilized nanoparticles: one is the long-range order of the polythiophene caused by self-assembly of polymers on nanoparticles, and the other is gold surface plasmon peak of gold nanoparticles which can also be observed around 530 nm in the spectrum of dodecanethiol-stabilized nanoparticles (Fig. 1c).