Deducing the role of functionalizing macromolecules in the nucleation of colloidal nanoparticles

Official URL: http://risc01.sabanciuniv.edu/record=b1534423 (Table of Contents)

Although polymers are widely employed to impart specific function to surfaces, techniques to characterize the adsorbed molecule are limited and usually indirect. We present two independent dynamic NMR studies to deduce the manner in which adsorbed polyvinylpyrrolidone (PVP) attaches to the surface of ZnO colloidal nanoparticles and directs particle precipitation. In our colloidal system, the conformation of the polymer molecule on the particle surface—loosely described as trains, loops, or tails—can be elucidated by probing the nuclear response of the solvent or the polymer to pulsed magnetic fields. The dynamic 1H-NMR signal of polymer-juxtaposed solvent molecules are monitored in our first approach, in which we monitor both spin-lattice (also known as T1) and spin-spin (also known as T2) relaxation behavior of the solvent, as the ease of solvent relaxation varies with proximity to the nanoparticle surface. In a second approach, the proton signal of the polymer is monitored over time. The rationale of this approach is that the signal of particle-bound polymer moieties will be lost faster than that of unbound polymer ones. Determining the signal of spin-echo and solid-echo processes enables calculation of polymer bound fraction. The ZnO nanoparticle platforms are synthesized by the hydrolysis and condensation of zinc oxide precursors in PVP-containing solutions. Particle size distribution is determined by dynamic light scattering (DLS), as well as extrapolation from UV-visible absorption spectra. Three specific PVP concentrations are chosen in producing ZnO particles. Synthesis in solutions of high PVP concentration produces comparable results between the hydrodynamic radius and the extrapolated UV-visible radius, whereas at low polymeric concentrations, the difference between the two radii increases, suggesting a change in the conformation of adsorbed polymer on the nanoparticle surface under varying concentration. Dynamic NMR T1 analysis reveals a loss of solvent mobility at higher polymer concentrations. In a separate experiment, we investigate the effect of the Zn precursor cations on the solvation of PVP, by using DLS to measure the hydrodynamic radius of the interpenetrating PVP in propanol, in the presence and absence of the zinc cation precursor. The results suggest that the zinc precursor ions pull the PVP chains closer, most likely due to the pyrrolidone rings on the polymers, and reduce the radius of gyration of polymer globules at high PVP concentrations. These results validate a model of highly dense polymer globules serving as reactors for ZnO nanoparticle precipitation, in which the high density of pyrrolidone rings in the globule hinders the diffusion of Zn+2by electrostatic interaction. At high concentrations, large PVP globules appear to trap the reactant species and adsorb in a train conformation on the surface of precipitating ZnO nanoparticles. At low concentrations, the sizes of PVP globules are comparable to that of the evolving ZnO nanoparticles, and PVP adsorbs in looped and tail-only conformation.