Phase-transfer and size-dependent film formation of gold nanoparticles

In this article, prefunctionalized polyoxometalate, γ-[SiW10O36(HSC3H6Si)2O]4−, {[POM(SH)2]4−} capped Au nanoparticles were synthesized successfully in N,N′-dimethylformamide (DMF). The thiol group of [POM(SH)2]4− provides a capping shell around Au nanoparticles. Zeta potential measurements showed that [POM(SH)2]4− coated nanoparticles were negatively charged in DMF. A novel method for transferring Au nanoparticles from DMF solution into non-polar organic solvent cyclohexane was developed. Charge neutralization using CTAB enables concentrating nanoparticle dispersion in cyclohexane which is a prerequisite for transfer and for successful three-dimensional self-assembly. The dispersed particles were stable for at least 45 days. After the phase-transfer, Au nanoparticles could be separated out from the cyclohexane phase in the form of a powder. The hydrophobic particles thus prepared are stable and are readily dispersed in solvent like toluene without further surface treatment. We also report extension of this wet-chemical method to make films by spontaneous assembly of passivated Au nanoparticles at the DMF/H2O–cyclohexane liquid interface. The particle size information and collective self-assembling properties of the [POM(SH)2]4−/CTAB capped Au nanoparticles and film were evaluated by TEM, XRD and FTIR spectroscopy.