Synthesis of silver nanoparticles
Silver nanoparticles were synthesized employing sodium borohydride (NaBH4) as a primary reductant and, trisodium citrate (TSC), both as secondary reductant as well as stabilizing agent. The reduction processes were carried out at two different temperatures, i.e. at 60 C and 90 C, mediated predominantly by, sodium borohydride and trisodium citrate, respectively. A typical procedure is as follows: the required volumes of freshly prepared aqueous solutions containing NaBH4 and TSC were mixed and heated to 60 C for 30 minutes in the dark with vigorous stirring to ensure a homogenous solution. At the end of 30 minutes, the required volume of AgNO3 solution was added drop-wise to the mixture and subsequently, the temperature was further raised to 90 C. As the temperature reached 90 C, the pH of the solution was adjusted to 10.5 using 0.1 M NaOH while heating was continued for 20 minutes, until a change of colour was evident. The nanoparticle suspension was allowed to cool at room temperature. In order to remove the unreacted reductants, AgNP suspensions were centrifuged (12 000 rpm, 15 minutes) and washed thrice, followed by redispersion in DI water and were finally stored at 4 C for future use. For determining the yield of AgNPs, the silver nanoparticles formed (collected by centrifugation at 14 000 rpm, 20 minutes) were dissolved in dilute HNO3 and the total mass of silver in the form of AgNPs was determined by inductively coupled plasma-atomic emission spectroscopy (ARCOS ICP-AES spectro, Germany). Subsequently, yield was determined as a ratio of mass of silver in the form of nanoparticles to the mass of silver (Ag+) added to the flask during the synthesis reaction. Table 1 shows the optimized reaction conditions and details pertaining to the synthesis of different sized AgNPs.