Localized synthesis of gold nanoparticles in anisotropic alginate structures

We present a method to create alginate nanocomposite objects having two regions with distinct chemical composition by spatially confining the synthesis of gold nanoparticles (Au NPs) in the polysaccharide matrix. Controlling the floating of sodium alginate drops on calcium chloride solution containing specific concentrations of gold ions, we demonstrate the formation of bicolour structures with a mushroom-like shape. Their immersed portion (the cap) was exclusively involved in the uptake and reduction of gold precursor, acquiring the typical purple colour of Au NPs; whereas, the emerged portion (the stem) did not participate in the in situ synthesis and retained the white colour of calcium alginate. Moreover, we noted that the localized growth of Au NPs was strongly related to the initial amount of gold precursor. In fact for high concentrations, the gold ions were just encapsulated inside the gel matrix and no reduction was observed in the wet structures neither in the cap nor in the stem. The reported procedure is simple and capable of directing the synthesis of Au NPs into selected areas of the alginate matrix, making possible the fabrication of a novel class of alginate structures with an anisotropic character and potential applications in the area of drug delivery and biosensors.