  In this work, optimized size distribution and optical properties in colloidal
synthesis of gold nanoparticles (GNPs) were obtained using a proposed
ultrasonic Tuerkevich-Frens method. The effect of three ultrasound (20 kHz)
irradiation powers has been analyzed as size and shape control parameter. The
GNPs colloidal solutions were obtained from chloroauric acid (HAuCl$_{4}$) and
trisodium citrate ($\rm C_{6}H_{5}Na_{3}O_{7}\cdot 2H_{2}O$) under continuous
irradiation for 1 hour without any heat or stirring. The surface plasmon
resonance (SPR) was monitored in the UV-Vis spectra every 10 minutes to found
the optimal time for localized SPR wavelength ($\lambda_{\rm LSPR}$) and the
210 sample procedure reduces the $\lambda_{\rm LSPR}$ localization to 20
minutes, while 150 and 60 samples show $\lambda_{\rm LSPR}$ in 60 minutes. The
nucleation and growth of GNPs showed changes in shape and size distribution,
which were associated with physical (cavitation, temperature) and chemical
(radical generation, pH) conditions in the solution. The results showed
quasispherical GNPs as pentakis dodecahedron ($\lambda_{\rm LSPR}$=560 nm),
triakis icosahedron ($\lambda_{\rm LSPR}$=535 nm), and tetrakis hexahedron
($\lambda_{\rm LSPR}$= 525 nm) in a size range from 12-16 nm. US irradiation
induced a disproportionation process, electrons of AuCl$_{2}^-$ rapidly
exchanged through the gold surface. After AuCl$_{4}^-$ and Cl$^-$ are desorbed
and a complex tetrachloroaurate is recycled for the two-electron reduction by
citrate, aurophilic interaction between complexes AuCl$_{2}^-$, electrons
exchange and gold seeds, the deposition of new gold atoms on the surface
promoting the growth of GNPs. These mechanisms are enhanced by the cavitation
effects and transmitted energy into the solution, showing that the plasmonic
response from our nanoparticles can be tuned with this simple method and
minimum intrumentation.
