New insights on the formation of gold nanoparticles and Pluronic nanocomposites: Kinetics and thermodynamics parameters

Abstract

The present contribution reports new insights into the structure, kinetics, and thermodynamics of gold nanoparticles (AuNP) synthesized in the presence of the block copolymer Pluronic F127 (PLU). AuNP-PLU nanocomposite samples were prepared in the form of aqueous colloidal suspensions by using PLU as a reducing agent of Au(III) ions to Au(0) and stabilizer of the formed AuNP. Transmission electron microscopy reveals that typical AuNP-PLU are nearly polygonal (mean diameter = 18.5 nm ± 0.2) and highly crystalline, while dynamic light scattering provides mean hydrodynamic diameter around 50 nm. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) and Raman spectroscopies indicate that AuNP are located at the more hydrophobic (polypropylene oxide) block of PLU by coordination with oxygen atoms of ether groups. Monitoring the optical absorbance of the localized surface plasmon resonance (LSPR) band of AuNP as a function of the reaction time and PLU concentration suggests the formation of AuNP-PLU is of pseudo first order kinetics, with the observed rate constant being dependent on the PLU concentration. Thus, the concentrations of block copolymer required to form the nanoparticles can be precisely determined. Thermodynamics data collected by isothermal calorimetry titration identified two regimes of molecular interaction: electrostatic, between Au(III) and PLU, and hydrophobic, between Au(0) and PLU, whose energies depend on the PLU concentration. It is concluded that optimal conditions for the synthesis of AuNP-PLU nanocomposites are those in which PLU is provided in concentrations above its critical micellar concentration. At this condition, the Au(III) to Au(0) reaction reaches completion and the establishment of hydrophobic interactions between Au(0) nuclei and polypropylene oxide block becomes feasible.