Kinetics and optical properties of the silver nanoparticles in aqueous L64 block copolymer solutions

Abstract

Silver nanoparticles were prepared by the chemical reduction of silver nitrate, AgNO 3, with excess of sodium borohydride, NaBH 4, in dilute aqueous solutions containing triblock copolymers poly(ethylene oxide) (PEO)–poly(propylene oxide) (PPO)–poly(ethylene oxide) (PEO) Pluronic L64. The synthesis was completed in ∼24 h and produced highly stable particle dispersion. The growth of nanoparticles was assessed by combining Mie theory, UV–vis spectroscopy and dynamic light scattering, and the average particle size was determined from transmission electron microscopy. The silver nanoparticles were sterically stabilised as a result of the block copolymer adsorption onto the nanoparticle surface. Besides the stabilizing character, the copolymer affected the kinetics of particle growth, the size and optical properties of as-synthesized Ag nanoparticles. It was revealed that the particle formation took place in two distinct stages. The first step was dominated by Ag ion reduction and growth driven by accretion, while in the second stage coagulated clusters of nanoparticles were obtained obtained. The as-synthesized particles were polydisperse, the individual ones had a size of ∼7.6 nm as given by transmission electron microscopy, and the hydrodynamic radius of clusters amounted to 60 nm. The silver nanoparticles exhibited broad and structured emission bands centered at 300 and 330 nm for excitation wavelengths in two different ranges, 220–240 nm and 260–290 nm, respectively. This process may be assigned to two emission components corresponding to two different excitation wavelength domains.