Sonochemical deposition of gold nano-shells on suspended polymeric spheres

Abstract

Metal nanoparticles have drawn great interest due to their unique properties for applications in the fields of catalysis, biomedicine and environmental science depending on the architecture of the metal nanoparticle composites. Amongst different designing routes, the chemical template deposition offers great flexibility in terms of the template selection and interfacial interactions, giving rise to controllable designs. In order to control over nanoparticle size distribution and deposition efficiency, a sonochemical approach has been systematically followed in this study. Key parameters of the ultrasound-assisted deposition procedures during the seeding step to synthesise gold nanoparticle-coated poly(styrene) beads were investigated. The impact of the solution pH and the ultrasonic frequency on the template deposition was examined at 139, 300, 500 and 1000 kHz. The results, monitored by transmission electron spectroscopic imaging, show that the highest gold deposition was achieved at 300 kHz, revealing the mechanistic details of the nucleation-crystal growth behaviour as a function of ultrasonic frequency and reaction time. In addition, the concentration ratio between gold ions and poly(styrene) beads was varied. The highest deposition coverage and smallest particle size were reached at 0.05 mM and 2.5 mg, respectively. The proposed mechanism of the MNPs formation and deposition behaviour were then discussed based on the tested parameters.

Keywords: Gold nanoparticles; Nanoscale coatings; Porous metals; Reaction time; Sonochemical deposition.

Graphical abstract

Fig. 1

Transmission electron microscopy and scanning electron microscopy images of Au₁₀ at different frequencies. Scale bars: 500 nm (A, B, C and D), 1 μm (E, F, G and H) and size distributions (I, J, K and L) as a function of ultrasound frequency presented for the 4 corresponding samples right above the distributions.

Fig. 2

Transmission electron microscopy images of coverage density changes in first 15 min of Au₁₀ at 139 (a), 300 (b) and 500 (c) kHz. Scale bar: background images 200 nm and inserted images 25 nm (Fig. S7).

Fig. 3

Transmission electron microscopy images of Au-coated PS beads in different initial Au(III) molar amount under 300 kHz ultrasonic irradiation. Scale bar: overall views at 500 nm (a, b, c and d) and higher magnification of the shells at 250 nm (e, f, g, and h).

Fig. 4

Concentration dependence for (a) Au_2.5, (b) Au₅, (c) Au₁₀ and (d, e) Au₂₀ and (f)corresponding size distributions for samples processed at 300 Hz; the scale bars correspond to 200 nm.

Fig. 5

Idealized illustration scheme of potential pathways to achieve Au MNPs surface deposition at early reaction stages (from Au5 to Au20): 1. Electrostatic interactions between Au(III) ions and template surfaces; 2. Pathways of nuclei deposition: (a) heterogeneous surface nucleation from attracted Au(III) ions, and (b) electrostatic attachment of Au nuclei formed by acoustic cavitation from surrounding environment; and 3. Growth of Au MNPs on the basis of the nucleation pathway (c) and (d), respectively.

Fig. 6

TEM images of Au₅₊₁₅ using the seed-and-growth protocol reported in the previous work (a, c), and Au₂₀ using 300 kHz (b, c). Scale bar: 200 nm (a, c) and 100 nm (b, d).

Fig. 7

Transmission electron microscopy images of Au-coated PS beads at pH=3.5 (a, d), 7.0 (b, e) and 11.2 (c, f); and zeta potential of amine-modified PS beads as a function of pH values (g). Scale bars: 200 nm (a-c) and 20 nm (d-f).