Key Role of the Oxidized Citrate-Free Radical in theNucleation Mechanism of the Metal Nanoparticle Turkevich Synthesis

The step-by-step mechanism of citrate oxidation, silver ion reduction into atoms, and nucleation of nanoparticles by the Turkevich method are deduced from the gamma- and pulse-radiolysis yields of dicarboxy acetone (DCA), H-2, CO2, and of silver ion reduction. Our results demonstrate that the stronger reductant is not citrate (Cit) but the oxidized radical Cit(-H)(center dot). The formation yields of DCA and CO2 confirm the decarboxylation process during the Cit(-H)(center dot) oxidation. In the pulse radiolysis of solutions of sodium citrate and silver perchlorate, the transient spectra and the kinetics are observed from 20 ps to 800 ms. In particular, the successive H abstraction from citrate by OH center dot radicals, then the one-electron transfer from the citrate radicals Cit(-H)(center dot) to silver ions initiating the simultaneous nucleation and growth of the reduced silver oligomers are observed. The knowledge of the nuclearity-dependent kinetics and thermodynamics of silver atoms, oligomers, and nanoparticles in solution is used to bracket the standard reduction potentials of the first (>= 0.4 V-NHE) and the second one-electron transfers from citrate (<=-1.2 V-NHE). During the Turkevich synthesis, the Cit(-H)(center dot) radical was shown to be released in the bulk solution from citrate oxidation by Ag+ adsorbed on the walls, or directly by the trivalent Au-III ions present in the bulk, respectively. Then, the strong Cit(-H)(center dot) reductant alone is able, as in radiolysis, to overcome the thermodynamic barrier of the very negative potential for the reduction of the free monovalent ions into atoms that is required to initiate the nucleation and growth. The reduction potentials values of citrate and Cit(-H)(center dot) also explain part of the anti-oxidant properties of citrate.