Effects of polymer micelles of alkylated polyethylenimines on generation of gold nanoparticles

Mono- and di-alkylated polyethylenimines (PEI-1R, PEI-2R) were synthesized and used as both reductants, by exploiting the functionality of the polyethylenimine's (PEI) amino groups, and stabilizers able to protect nascent gold nanoparticles generated from hydrogen tetrachloroaurate (HAuCl ₄). From TEM images of the stained polymers, it is clear that the polymer micelles are round and well-structured when formed from PEI-2R, fused and less well-structured when formed from PEI-1R, and totally nonstructured when formed from PEI. These findings coincide with the results found by using pyrene as a probe to investigate aggregation behavior, where PEI-2R with a fluorescence intensity ratio (I₁/I₃) of 1.48 forms the more closely packed polymer micelles than PEI-1R (I₁/I₃ = 1.64) and PEI (I₁/I₃ = 1.72). The use of the highly alkylated polymer micelle (PEI-2R) results in the fastest reduction of HAuCl₄, and gives the most effective protection to the generated gold nanoparticles. When used at higher polymer concentrations than required for micelle formation, it was found that polymer hydrophobicity was highly influential in directing the nanoparticle's morphology, i.e., the resulting polymer micelles were labeled with perfect and round necklace-like gold nanoparticles when PEI-2R was used, and imperfect and less round gold nanoparticles when PEI-1R was employed. These structures were totally absent when PEI was used. The use of alkylated PEI, being able to act simultaneously as both a reductant and as a very effective protective agent, greatly simplifies the process used for preparing gold nanoparticles.