PHYS01200704032 Debes Ray - Homi Bhabha National Institute

Chapter 2: Block Copolymer-mediated Synthesis of Gold Nanoparticles
2.3.2. Role of Block Copolymer
The PEO-PPO-PEO block copolymers participate in different roles in the reduction,
nucleation and growth, and stabilization in the synthesis of gold nanoparticles from its salts in
aqueous solution [94,97,98,146].
(i) Gold Ion Reduction by PEO-PPO-PEO Block Copolymers
The following three possibilities exists by which AuCl - 4 ions can be reduced by the PEO-
PPO-PEO block copolymers based on reduction of metal ions reported for the PEO-type
surfactant, PEO homopolymer and PEO containing polymers [97,98].
(a) The alcohol (hydroxyl) functionality at the two ends of a PEO-PPO-PEO block copolymer
molecule could act as reductant for the metal ions [147,148]. Alcohols are often used as
reductants for the synthesis of metal nanoparticles [147,148]. However, alcohols (e.g.
methanol, ethanol, 1-propanol, 2-propanol, ethylene glycol and glycerol) at lower
concentrations (consistent with the typical PEO-PPO-PEO block copolymer concentration
used) do not lead to observable reduction of gold ions.
(b) Hydroperoxides (ROCOOH) that can be formed by polyethers such as PEO-PPO-PEO
block copolymers upon reaction with oxygen from the air has both oxidizing and weakly
reducing properties [148-151]. In the case even if PEO-PPO-PEO block copolymers form
hydroperoxides in air-saturated water, this effect is found significantly small.
(c) The PEO in PEO-PPO-PEO block copolymers forms cavities (pseudo-crown ether
structure) that can bind metal ions [152-156] and reduction of bound AuCl - 4 ions can proceed
via oxidation of the oxyethylene and oxypropylene segments by the metal center [157]. PEO
in aqueous solution is known to form a conformation similar to crown ethers (pseudo-crown
ether structure) that is able to bind with metal ions [153,154]. The induced cyclization is
caused by ion-dipole interactions between the templating ion and the electron lone pairs of
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