In situ formation of stable gold nanoparticles in polymer inclusion membranes

Abstract

Gold nanoparticles (Au nps) were synthesized in the matrix of a plasticized anion-exchange membrane. The membrane was prepared by solvent casting of the solution containing a liquid anion exchanger trioctylmethylammonium chloride (Aliquat-336), a matrix-forming polymer cellulose triacetate (CTA), and a plasticizer dioctyl phthalate (DOP) dissolved in CH 2 Cl 2 . For in situ synthesis of Au nps, the membrane samples were equilibrated with a well-stirred solution containing 0.01 mol L - 1 HAuCl 4 . AuCl 4 - ions were transferred to membrane matrix as an ion pair with Aliquat-336 by an ion-exchange mechanism. In a second step, AuCl 4 - ion-loaded membrane samples were placed in a well-stirred 0.1 mol L - 1 aqueous solution of NaBH 4 for reduction. It was observed that 80% of the anion-exchange sites were readily available for the exchange process after formation of the Au nps. The content of Au nps in the membrane was increased either by increasing the concentration of the Aliquat-336 in membrane or by repeating sequential cycles of loading of AuCl 4 - ions followed by reduction with BH 4 - in the membrane matrix. TEM images of a cross section of the membrane showed that Au nps were dispersed throughout the matrix of the membrane but excluded from the surface. The size distribution of the nps was found to be dependent on Au content in the membrane. For example, 7- to 16-nm Au nps with average size 10 nm were observed in the membrane after the first cycle of synthesis. On increasing the Au content in the membrane by repeating the cycle of synthesis, the size dispersion of nps broadened from 5 to 20 nm without affecting the average size. The λ max (530 nm) and intensity of the surface plasmon band of Au nps embedded in the matrix of membrane were found to remain unaltered over a testing period of a month in the samples kept in water as well as in air under ambient conditions. This indicated that Au nps were quite stable in the membrane matrix. The experimental information obtained by the radiotracers and energy-dispersive X-ray fluorescence (EDXRF) analyses has been used to understand the process of Au nps formation in the membrane matrix.