Facile Two-Step Size-Controlled Synthesis of Gold Nanoparticles Functionalized with Triethylamine-Cyclodextrins

A two-step, size-controlled method is described for synthesizing uniform, quasi-spherical, and monodisperse gold nanoparticles (AuNPs) with diameters ranging from 15 to 22 nm, using a combination of triethylamine (TEA) and cyclodextrins (α, β, and γCD). The first step of sonication was conducted in a modified ultrasonic cleaner equipped with a dummy beaker to absorb excess energy from the adjacent transducer and a reaction beaker for the reduction process. Lower sonication energy settings (frequency = 37 kHz, P _acoustic = 30%) enable more predictable and controlled surface plasmon resonance (SPR) in solution by avoiding the purple-to-blue color associated with the larger AuNP size. Hence, the emergence of a light-purple reaction mixture in the first step indicates the optimal time to transfer the solution to a water bath for subsequent heating in the second step. The resulting AuNPs typically exhibit a red wine color (λ = 515-525 nm). Among the systems evaluated, the ternary AuNPs-TEA-γCD system yielded the smallest nanoparticles, high stability, and noninclusion interactions between TEA and γCD, as evidenced by FT-IR and 2D-ROESY. More importantly, observation of AuNPs-TEA exhibited completely different SPR responses if subjected to only one step of the procedure. This procedure demonstrates high repeatability in producing smaller and more predictable aminated gold nanoparticles from the reactants used.