In Situ Growth of Vertically Aligned Gold Nanoparticles within Functionalized Polyvinylidene Fluoride Nanochannels...

A solid synthetic pathway for the localized formation of vertically aligned and well-distributed individual gold nanoparticles (AuNPs) in transparent ion-track-etched polyvinylidene fluoride membranes is herein reported. After a successful preconcentration of the Au-(III) precursor within the functionalized cylindrical nanochannels of these membranes, i.e., ion-track-etched radiografted with poly-(4-vinylpyridine) (P4VP), a Au-(IIII)-to-Au(0) chemical reduction was performed to in situ grow AuNPs inside the nanopores. Four classical reducing agents [ascorbic acid, hydroquinone, sodium citrate, and sodium borohydride (NaBH₄)] were studied, which led, at first glance, to similar composites. The reducing power of each reducing agent has been shown to affect the AuNP nucleation and growth processes. At room temperature, the reduction led to the synthesis of well-dispersed AuNPs along the whole pore length, with the exception of sodium citrate, which was found to be too weak to reduce efficiently the gold precursor. When energy was provided to the system by operating the reaction at 70 °C, the reduction was boosted and the synthesis with sodium citrate gave similar results to those obtained with the other reducing agents at 20 °C, i.e., well-distributed AuNPs of around 30 nm of mean size. Increasing the number of reduction cycles resulted in an increase in AuNP size and, in the case of hydroquinone, in the elongation of the gold nanocrystals. This phenomenon was attributed to template-assisted AuNP growth in the nanopores. Despite the low amount of gold in the material (less than 0.2 %), the alignment of individual AuNPs all along the high aspect ratio nanopores (50:10,000) had a significant effect on the optical properties of the whole material, with the appearance of plasmonic properties for the nanocomposite membranes (λ_plasmon around 530 nm) and a further decrease of the effective refractive index of the nanoporous membranes.