Novel Approach for the Self Assembly

The Objective

I'd like to demonstrate high-yield synthesis of gold nanorods and their self-assembly into ordered structures. The assembly of gold nanorods into 2-D structures was carried out using a pH-sensitive polymer, polyacrylic acid. Because gold nanorods are covered with a cationic surfactant in solution, the pH-dependent assembly of the nanorods is directed by electrostatic interactions between the positively charged nanorods and the negatively charged, deprotonated polyacrylic acid. Because the chemically-directed, spontaneous self-assembly of nanoparticles by manipulating environmental factors such as pH are rare, such modes of assembly can be used as a precursor for synthesizing future nanodevices.

Methods/Materials

Gold nanorods were created via the seed-mediated approach. After their synthesis, nanorod self-assembly was carried out using a pH-sensitive polymer, polyacrylic acid. After the addition of the acid to the gold nanorod solution at pH of 7, the solution was observed using light scattering experiments, zeta-potential measurements, and absorption spectroscopy to see if any gold nanorods had self-assembled into ordered structures in solution, in addition to TEM.

Results

Gold nanorods assembled into 2D ordered structures with polyacrylic acid at pH of 7 but not at a pH of 3 to yield 2D ordered structures. Zeta potential measurements indicated that negatively-charged polyacrylic acid is adsorbed on the surface of gold nanorods, and light scattering experiments indicated that nanorods formed aggregates of 4-5 gold nanoparticles in solution. The absorption peak also shifted 20 nm left, indicating side-to-side assemblies in solution.

Conclusions/Discussion

The self-assembly of gold nanorods in solution prompted by the presence of polyacrylic acid was achieved. While most assembly was induced by solvent evaporation due to particle-particle interactions, light scattering experiments and absorption spectroscopy showed that some self-assembly also occurs in solution. These aggregates are estimated to contain 4-5 nanorods in solution, and upon evaporation, form assemblies containing hundreds of nanorods as observed by TEM. The mechanism for self-assembly is believed to be electrostatic attractions, and as a result, nanorods were aggregated in solution. It is expected that the present studies on the controllable connection of nanoscale building blocks into desired shape might find application in future nanodevices.

I discovered how to self-assemble gold nanorods using electrostatic interactions between nanoparticles.

Science Fair Project done By Saraf Nawar

 

 

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