Binding of metal ions to the curcumin mediated methoxy polyethylene glycol thiol conjugated greenly synthesized gold nanoparticles: A fluorescence spectroscopic study

Curcumin, a nontoxic molecule obtained from plant extract, was used to reduce gold salt to produce gold nanoparticles (AuNPs). First time the particles were functionalized with methoxy polyethylene glycol thiol (mPEGT), which acts as a stabilizing agent in such a manner that the methoxy group is pointed toward the aqueous solution to serve as anchor point to many metal ions. The greenly synthesized gold nanoparticles have a size between similar to 40 and similar to 83 nm. The prepared material was characterized in detail by SEM, XRD, TGA and FT-IR spectroscopy. These AuNPs were found to be fluorescence active similar to free curcumin suggesting that curcumin is conjugated at the surface of AuNPs. Exposure of the mPEGT functionalized AuNPs to different concentrations of metal cations in the range between 0 and 100 mu M resulted in the decrease in the fluorescence intensity of the AuNPs unlike free curcumin in aqueous solution. Excited state lifetime measurements further validated quenching mechanism to be static in nature. The binding constant and number of binding sites for such interaction were evaluated. Based on observed bimolecular quenching rate constants, k(Q) values, the order of association of the metals studied is Pb (II) > Hg (II) >Hg (I) > Cd (II)>A1 (III) >Ni (II) >Cu (II) >Na2HAsO4. These interactions between the mPEGT conjugated AuNPs and the metal ions in solution contribute a proof-ofconcept that mPEGT functionalized curcumin mediated AuNPs can be used as a simple, cheap and straightforward, on-site detection system for toxic and even essential metal ions in solution.