Rapid synthesis of gold nanostructures with cyclic and linear ketones

Colloidal gold nanoparticles were synthesised in aqueous solution by reaction of chloroauric acid with a range of simple aliphatic cyclic (cyclopentanone, cyclohexanone, cycloheptanone and cyclohexanedione) and linear (acetone and 3-hexanone) ketone reagents, at room temperature. The rate of reaction and particle morphology was found to be controlled by the enol content and solubility of the ketone. Cyclohexanediones produced a variety of small 20 nm particles in under 5 minutes, or larger gold nanostars, depending on the ketone isomer. Cyclopentanone was shown to produce near monodisperse 20 nm particles after 13 hours, and cycloheptanone gave polydisperse particles, but in only 50 minutes. However the linear ketones, 3-hexanone and acetone, did not produce stable colloidal suspensions. The mechanism of gold nanoparticle formation via reaction of ketones with chloroauric acid is discussed.

section_content: The cyclopentanone experiment showed a triplet, consistent with the molecular structure of 2-chlorocyclopentanone (Fig. 1 ). Acetone showed singlets, again consistent with the NMR of chloropropan-2-one (Fig. 3 ). 

Cycloheptanone and 3-hexanone present far more complex structures and hence spectra for analysis. No peaks were evident in the cycloheptanone spectra, however the very low solubility of cycloheptanone may have been a contributing factor in this (Fig. 2 ). Although some peaks are visible in the correct region of the 3-hexanone reaction-mixture spectrum, di↵erent peaks are visible in the specturm of pure 3-hexanone, which are not visible in the reactionmixture, throwing some doubt on whether a good scan was achieved, and thus the peaks may be artefacts (Fig. 4 ). 

The diketones studied did not show any particularly obvious changes to their NMRs from starting material to product, that could not be explained by the acidification/dilution on addition of the acid (Fig. 5 ). This indicates that NMR is not an adequate technique to study these systems in detail.

section_content: XPS data were collected from freeze dried powders, as described in the main body of the paper. The 4f peaks are given here, confirming the presence of gold in every sample, and the presence of unreacted Au 3+ in those from linear ketones, suggesting an incomplete reaction at 24 hours (Fig. 7 ). 

For samples produced with cyclohexanone, cycloheptanone and the cyclohexanediones there was good agreement between the 4f 7/2 and 4f 5/2 peaks with expected values (83. 9 and 87. 7 eV). Similarly the linear ketones, acetone and 3-hexanone had a good match to the expected Au 0 and Au 3+ peaks, however the acetone residue contained a far larger proportion of Au 3+ than the 3-hexanone sample, in keeping with its lower pK E. 

However an oddity was the cyclopentanone produced AuNPs, showing 4f 7/2 and 4f 5/2 peaks similar to those samples containing Au 3+. The peaks were also very broad. As the reaction time for this ketone was so long (13 hours) it is likely that there was still trace Au 3+ present and this lead to the shit and the broadened peaks. HRTEM images were analysed by onscreen measurement and Fourier transform. close up of the nanostars from reaction with 1,3-cyclohexanedione shows the sharp tips at the end of each arm (Fig. 8 ). These will localise the SPR, making nanostars an excellent prospect for SERS measurements. Close examination of plates produced by various ketones show a lattice spacing of 0. 235 nm, corresponding to the (111)-plane. (Fig. 9 ).
section_name: XPS analysis