Formation of Aggregate-Free Gold Nanoparticles in the Cyclodextrin-Tetrachloroaurate System Follows Finke-Watzky Kinetics

Abstract

Cyclodextrin-capped gold nanoparticles are promising drug-delivery vehicles, but the technique of their preparation without trace amounts of aggregates is still lacking, and the size-manipulation possibility is very limited. In the present study, gold nanoparticles were synthesized by means of 0.1% (w/w) tetrachloroauric acid reduction with cyclodextrins at room temperature, at cyclodextrin concentrations of 0.001 M, 0.002 M and 0.004 M, and pH values of 11, 11.5 and 12. The synthesized nanoparticles were characterized by dynamic light scattering in both back-scattering and forward-scattering modes, spectrophotometry, X-ray photoelectron spectroscopy, transmission electron microscopy and Fourier-transform infrared spectroscopy. These techniques revealed 14.9% Au¹⁺ on their surfaces. The Finke-Watzky kinetics of the reaction was demonstrated, but the actual growth mechanism turned out to be multistage. The synthesis kinetics and the resulting particle-size distribution were pH-dependent. The reaction and centrifugation conditions for the recovery of aggregate-free nanoparticles with different size distributions were determined. The absorbances of the best preparations were 7.6 for α-cyclodextrin, 8.9 for β-cyclodextrin and 7.5 for γ-cyclodextrin. Particle-size distribution by intensity was indicative of the complete absence of aggregates. The resulting preparations were ready to use without the need for concentration, filtration, or further purification. The synthesis meets the requirements of green chemistry.

Keywords: Finke–Watzky kinetics; cyclodextrin; drug delivery vehicle; gold nanoparticles; green chemistry; tetrachloroaurate reduction.

Figure 1

TEM images of gold nanoparticles synthesized at 0.1% HAuCl₄ at room temperature: (a) at 0.002 M α-cyclodextrin, pH 11.5 in the presence of NaCl after 54 min; (b) at 0.004 M β-cyclodextrin, pH 11.5, after 44 min and (c) 2 h 17 min; (d) at 0.002 M α-cyclodextrin, pH 11, after 12 h 38 min and (e) 43 h 29 min; (f) at 0.004 M α-cyclodextrin, pH 11, after 18 min and (g) 6 h 38 min; (h) at 0.004 M β-cyclodextrin, pH 11, after 3 h 15 min; (i) at 0.002 M γ-cyclodextrin, pH 11, after 6 h 38 min and (j) 23 h 33 min.

Figure 1

TEM images of gold nanoparticles synthesized at 0.1% HAuCl₄ at room temperature: (a) at 0.002 M α-cyclodextrin, pH 11.5 in the presence of NaCl after 54 min; (b) at 0.004 M β-cyclodextrin, pH 11.5, after 44 min and (c) 2 h 17 min; (d) at 0.002 M α-cyclodextrin, pH 11, after 12 h 38 min and (e) 43 h 29 min; (f) at 0.004 M α-cyclodextrin, pH 11, after 18 min and (g) 6 h 38 min; (h) at 0.004 M β-cyclodextrin, pH 11, after 3 h 15 min; (i) at 0.002 M γ-cyclodextrin, pH 11, after 6 h 38 min and (j) 23 h 33 min.

Figure 1

TEM images of gold nanoparticles synthesized at 0.1% HAuCl₄ at room temperature: (a) at 0.002 M α-cyclodextrin, pH 11.5 in the presence of NaCl after 54 min; (b) at 0.004 M β-cyclodextrin, pH 11.5, after 44 min and (c) 2 h 17 min; (d) at 0.002 M α-cyclodextrin, pH 11, after 12 h 38 min and (e) 43 h 29 min; (f) at 0.004 M α-cyclodextrin, pH 11, after 18 min and (g) 6 h 38 min; (h) at 0.004 M β-cyclodextrin, pH 11, after 3 h 15 min; (i) at 0.002 M γ-cyclodextrin, pH 11, after 6 h 38 min and (j) 23 h 33 min.

Figure 2

XPS spectra of gold nanoparticles: (a) an overview spectrum; (b) Au4f core-level spectrum; (c) O1s core-level spectrum; (d) Na2p core-level spectrum; (e) C1s core-level spectrum; (f) Cl2p core-level spectrum.

Figure 3

¹H NMR spectra of α-cyclodextrin alone (a,c) and in the presence of HAuCl₄ at pH 6.16 (b,d) in D₂O (a,b) or DMSO (c,d).

Figure 3

¹H NMR spectra of α-cyclodextrin alone (a,c) and in the presence of HAuCl₄ at pH 6.16 (b,d) in D₂O (a,b) or DMSO (c,d).

Figure 4

Normalized FTIR spectra of β-cyclodextrin Cavamax W7 and of gold nanoparticles with immobilized β-cyclodextrin synthesized under reflux conditions.