Room-Temperature Turkevich Method: Formation of Gold Nanoparticles at the Speed of Mixing Using Cyclic Oxocarbon Reducing Agents

We demonstrate a facile and reproducible means of producing quasi-spherical, colloidally stable gold nanoparticles (AuNPs) on the basis of rapid room-temperature mixing of aqueous solutions of HAuCl₄ and a cyclic oxocarbon diacid (squaric acid, SA; croconic acid, CA; or rhodizonic acid, SR) or ascorbic acid (AA) as dual reducing and capping agent. Although these reducing agents generally produced larger particles than those derived from the classical Turkevich method (using citrate in boiling water) and achieved a lower nanoparticle size uniformity in our hands (i.e., 30.4 ± 8.6, 33.1 ± 9.3, 29.9 ± 6.3, and 29.7 ± 7.6 nm for SA, AA, CA, and SR, respectively, compared with 15.8 ± 3.7 nm for citrate), the method is versatile and exceptionally convenient as fairly monodisperse AuNPs can be made “on-demand” within seconds by simple mixing in the absence of heating. A preliminary investigation into the effects of reaction parameters, such as synthesis temperature and the molar ratio of reducing agent to HAuCl₄, was carried out. The reagent molar ratio was found to play a pivotal role in the mean AuNP size and size distribution, whereas reaction temperature (e.g., 5, 20, or 100 °C) only played a very minor role. Interestingly, CA- and SR-mediated reduction generated AuNPs displaying bimodal size distributions, with a large fraction of the total nanoparticle count being represented by small AuNPs in the 3.5 ± 1.9 nm (CA) and 5.1 ± 1.0 nm (SR) size regimes. Cyclic and differential pulse voltammetry procedures were conducted to gain insight into the redox chemistry of the cyclic oxocarbons as prospective reducing agents for general metal nanoparticle synthesis as well as to furnish additional evidence in support of a proposed mechanism for the overall oxidation process using squaric acid as a representative cyclic oxocarbon acid. Finally, the catalytic activities of the prepared AuNPs were evaluated using the borohydride-assisted reduction of 4-nitrophenol as a model reaction, exhibiting apparent rates of 2.0 × 10^–3, 3.6 × 10^–3, 1.9 × 10^–3, and 13.8 × 10^–3 s^–1 for SA-, AA-, CA-, and SR-derived AuNPs, respectively (5 mol % catalyst). Notably, AuNPs generated using SR boasted a catalytic rate twice as high as that of Turkevich (citrate)-derived AuNPs at the same Au catalyst loading, an outcome we attribute to the prevalence of ultrasmall (∼5 nm) AuNPs produced in that sample. Overall, these findings open the possibility for “on-the-fly” nanomanufacturing methods (e.g., “glow stick”-inspired preparation) that allow the expedient, reproducible, and low-cost synthesis of metal nanoparticles with minimal environmental impact