A new method to synthesize ZnO nanoparticles with size gradient in PNIPAM polymer matrix

In this study, poly(N-isopropylacrylamide) hydrogel is used as a matrix for synthesizing zinc oxide (ZnO) nanoparticles in the porous regions of the gel. When the gelation is completed, OH^- ions of potassium hydroxide were diffused into the gel in which zinc acetate molecules were trapped previously during the gelation. The size of the nanoparticles formed in the gel changes with changing the pore size of the slab gels or with changing the concentration of OH^- ions along the axis of a long cylindrical gel. The slices from different parts of the cylindrical gel or the gels prepared in slab forms include different size distribution of the nanoparticles. The synthesis of nanocrystals in a gel by these methods presents two innovations: (i) depending on the position of the slice along the cylindrical gel or cross-linker content of the slab gels, the average size can be tuned; and (ii) the visible emission from the surface states of ZnO nanoparticles can be controlled via the swelling degree of the gel and the type of the solvent. Absorbance, XRD, SEM, and HRTEM measurements were performed to characterize the crystal structure and to estimate the size of the nanoparticles. The steady state fluorescence technique is used to compare qualitatively the change in the size of the nanoparticles and to carry out the interactions of ZnO nanoparticles with the polymer matrix both in collapsed and swollen states.