Synthesis and Stabilization of Support-Free Mesoporous Gold Nanoparticles

Abstract

Porous gold nanoparticles (PGNs) are usually prepared in an immobilized form on a solid substrate, which is not practical in many applications. In this work, a simple method is reported for the preparation and stabilization of mesoporous gold particles of a few hundred nanometers in size in aqueous suspension. Nanoparticles of Ag-Au alloy were fabricated on CaF 2 and Si/SiO 2 substrates by the solid-state dewetting method. Silver was selectively dissolved (dealloyed), and the resulting porous gold nanoparticles were chemically removed from the substrate either in a concerted step with dealloying, or in a subsequent step. Nitric acid was used for the one-step dealloying and detachment of the particles from CaF 2 substrate. The consecutive use of HNO 3 and HF resulted in the dealloying and the subsequent detachment of the particles from Si/SiO 2 substrate. The PGNs were recovered from the aqueous suspensions by centrifugation. The Au content of the suspensions was monitored by using elemental analysis (ICP-OES), and recovery was optimized. The morphology and the optical characteristics of the support-free PGNs were analyzed by scanning electron microscopy (SEM), dynamic light scattering spectroscopy (DLS), and near-infrared spectrophotometry (NIR). The obtained PGNs are spherical disk-shaped with a mean particle size of 765 ± 149 nm. The suspended, support-free PGNs display an ideally narrow dipole plasmon peak at around 1450 nm in the NIR spectral region. Thus, the new colloidal PGNs are ideal candidates for biomedical applications, for instance photothermal therapy.

Keywords: dewetting-dealloying; localized surface plasmon resonance; porous gold nanoparticles; support-free porous noble metal.

Scheme 1

Schematic representation of the different preparation methods of colloidal PGNs.

Figure 1

(a) SEM images of porous gold nanoparticles (PGNs) prepared by the dewetting-dealloying method on Si/SiO${}_2$ substrate. (b) SEM images of support-free PGNs chemically detached from the substrate. The suspension of the support-free particles was desiccated on Si for imaging. It is notable that the support-free PGNs are spherical disk shaped.

Figure 2

Optical absorption spectra of supported PGNs on sapphire substrate and support-free PGNs suspended in water or desiccated in sapphire. (Black and red lines: Spectra of the support-free PGNs in water prepared by the one-step and two-step detachment methods, respectively. Green line: Spectrum of support-free PGNs desiccated on sapphire. Blue line: Reference spectrum of supported PGNs on sapphire.).

Figure 3

Optical absorption spectra of support-free PGNs prepared by using the one-step detachment method starting from particles supported on CaF${}_2$. Spectra were recorded after the first and second centrifugation steps. The two spectra overlap.

Figure 4

Particle size distribution curves of support-free PGNs measured in their suspensions by DLS after each recovery step. The size distribution of PGNs is altered in the first centrifugation step, but the second centrifugation does not affect further the size of the PGNs.

Figure 5

SEM images of colloidal PGNs prepared by using fundamentally different methods: (a) Colloidal PGNs prepared by Pedireddy et al. [31] (b) Colloidal PGNs prepared by Hu et al. [30] (c) Colloidal PGNs prepared by using the dewetting-dealloying method reported in this paper. Figures were reproduced by permission of Springer Nature and RSC Publishing.