pH-dependent size and structural transition in P123 micelle induced gold nanoparticles

Abstract

The influence of the pH value of an aqueous solution of P123 micelles on the growth and formation of gold nanoparticles (AuNPs), which is of immense importance for their controlled growth in a simple, single-step synthesis process, was investigated using time-evolution optical absorption spectroscopy, dynamic light scattering and transmission electron microscopy techniques. The sizes and structures of the AuNPs are found to be pH-dependent, even within the basic region, with a transition near pH ≈ 9.5, though the free P123 micelles remain almost unchanged. Below this pH value, the slow reduction rate of the gold ions creates a lower number of nucleation centers, which, through an autocatalytic thermodynamically controlled reduction (ATCR), initially formed chain-like aggregated small AuNPs (of different chain lengths) and subsequently, through further diffusion and coalescence, formed well-faceted near symmetrical large AuNPs of size ≫19 nm, the size of the free P123 micelles. Above this pH value, the fast reduction rate of the gold ions creates a large number of nucleation centers, the growth of which is restricted by the limited amount of available gold ions for the ATCR and also by the metal–polymer hydrophobic and polymer–water hydrophilic interactions. Accordingly, controlled growth of the majority of the centers takes place through ATCR, diffusion and early capping with near individual micelles to form isolated symmetric small AuNPs of size <19 nm, with a narrow size distribution, which are desirable for different applications and fundamental studies. However, a minority (but not insignificant amount) of the centers still remain in very small sizes and are trapped inside large micellar assemblies or even in the near atomic states, which limit the yield of the isolated small AuNPs.