Preparation of silver nanoparticle thin films for sensing applications

Abstract

Silver nanoparticles were prepared by the reduction of silver nitrate with sodium borohydride in the presence of a polyelectrolyte acting as a capping agent. Different polyelectrolytes were used including poly(methacrylic acid, sodium salt), poly(acrylic acid, sodium salt), poly(4-styrene sulfonic acid-co-maleic acid, sodium salt), sodium alginate and humic acid. Their concentrations were varied from 0.1, 0.5, 1, 5 and 10 mM. All prepared solutions exhibited a strong plasmon band at the wavelength of 400 nm confirming the formation of silver nanoparticles. Layer-by-layer technique was used to deposit silver nanoparticles into thin films. The growth of these films was affected by the dipping time, the dilution and the ionic strength of the solution. Nanoindentation measurements revealed that the film hardness decreased when lower concentrations of capping agents were used in the preparation of the silver nanoparticles. Sensing properties of silver nanoparticles were then investigated in the forms of solution and thin film. It was found that only nanoparticles prepared in the presence of 0.1 mM of capping agents could be used for organic compound sensing. It was also observed that at this concentration, alginate-capped silver nanoparticle solution had the highest sensitivity to organic compounds while its thin film displayed a strong color shift from yellow in water to red in methanol and ethanol. The best detection was achieved when the ratio of alginate:silver nitrate was 0.1:1. In the case of herbicide sensing, it was found that the color of humic acid-capped silver nanoparticle solution changed from yellow to orange red and purple with increasing herbicide concentration. Furthermore, an improvement of the sensing sensitivity of the film could be done by the modification of film surface with a layer of polycationic polymer.