SANS STUDIES INSIGHT INTO IMPROVING OF YIELD OF BLOCK COPOLYMER-STABILIZED GOLD NANOPARTICLES

Abstract

Triblock copolymer poly(ethylene oxide)‐poly(propylene oxide)‐poly(ethylene oxide) (PEO‐PPO‐PEO) are well known as dispersion stabilizers. It has also been recently found that they can act as reducing agents along with stabilizers and these two properties of block copolymers together have provided a single‐step synthesis and stabilization of gold nanoparticles at ambient temperature. We have studied the synthesis of stable gold nanoparticle solutions using block copolymer P85. Gold nanoparticles are prepared from 1 wt% aqueous solution of P85 mixed with varying concentration of HAuCl4.3H2O salt in the range 0.001 to 0.1 wt%. Surface plasmon resonance (SPR) band in UV‐visible absorption spectra confirm the formation of the gold nanoparticles and the maximum yield of the nanoparticles is found to be quite low at 0.005 wt% of the salt solution. Small‐angle neutron scattering (SANS) measurements in these systems suggest that a very small fraction of the block copolymers (<1%) is only associated with the gold nanoparticles and remaining form their own micelles, which probably results in the low yield. This can be explained as on an average a high block copolymer‐to‐gold ion ratio r0 (22) is required for 1 wt% P85 in the reduction reaction to produce gold nanoparticles. Based on this understanding, a step‐addition method is used to enhance the yield of gold nanoparticles by manifold where the gold salt is added in small steps to maintain higher value of r(>r0) and therefore continuous formation of nanoparticles.