Morphology-Tailored Gold Nanoraspberries Based on Seed-Mediated Space-Confined Self-Assembly

Abstract

Raspberry-like structure, providing a high degree of symmetry and strong interparticle coupling, has received extensive attention from the community of functional material synthesis. Such structure constructed in the nanoscale using gold nanoparticles has broad applicability due to its tunable collective plasmon resonances, while the synthetic process with precise control of the morphology is critical in realizing its target functions. Here, we demonstrate a synthetic strategy of seed-mediated space-confined self-assembly using the virus-like silica (V-SiO₂) nanoparticles as the templates, which can yield gold nanoraspberries (AuNRbs) with uniform size and controllable morphology. The spikes on V-SiO₂ templates serve dual functions of providing more growth sites for gold nanoseeds and activating the space-confined effect for gold nanoparticles. AuNRbs with wide-range tunability of plasmon resonances from the visible to near infrared (NIR) region have been successfully synthesized, and how their geometric configurations affect their optical properties is thoroughly discussed. The close-packed AuNRbs have also demonstrated huge potential in Raman sensing due to their abundant "built-in" hotspots. This strategy offers a new route towards synthesizing high-quality AuNRbs with the capability of engineering the morphology to achieve target functions, which is highly desirable for a large number of applications.

Keywords: gold nanoraspberries (AuNRbs); space-confined effect; surface-enhanced Raman scattering (SERS); tunable plasmon resonances.

Figure 1

(a) A schematic diagram of the fabrication process of gold nanoraspberries (AuNRbs). Transmission electron microscopy (TEM) images of (b) virus-like silica (V-SiO₂) nanoparticles; (c) gold nanocomposites (AuNCs) formed by gold nanoseeds-attached V-SiO₂ nanoparticles; and (d) AuNRbs after the growth process. (e) The scanning electron microscopy (SEM) image of AuNRbs. The inset figure shows a single AuNRb. (f) A closeup photograph of a single raspberry fruit. (g) The TEM image of a single AuNRb.

Figure 2

A TEM image of (a) AuNCs, and AuNRbs synthesized using a HAuCl₄ solution with a concentration of (b) 0.025 mM, (c) 0.0375 mM, (d) 0.075 mM and (e) 0.225 mM, respectively. The scale bar is 100 nm. (f–j) The size distribution of assembled gold nanoseeds and gold nanoparticles corresponding to (a–e), respectively. (k–o) The extinction spectrum with an inset showing the photo of colloid solution corresponding to (a–e), respectively.

Figure 3

(a) The calculated absorption cross-sections, scattering cross-sections and extinction cross-sections of AuNRb-8 (red), AuNRb-11 (magenta), AuNRb-15 (blue) and AuNRb-22 (cyan). The extinction cross-section is the sum of the absorption cross-section and the scattering cross-section. (b) The dependence of cross-sections as a function of the nanoparticle size when the interparticle distance is kept at 4 nm. The sizes of the nanoparticles used in simulation increased from 6 nm (red) to 8 nm (magenta), 13 nm (blue) and 16 nm (cyan). (c) The dependence of cross-sections as a function of the interparticle distance when the nanoparticle size is kept at 10 nm. The interparticle distances of the nanoparticles used in simulation decreased from 5.2 nm (red) to 3.2 nm (magenta), 1.9 nm (blue) and 0.9 nm (cyan). The models used in simulation are displayed in boxes with border colors representing different conditions.

Figure 4

(a) A schematic diagram illustrating the formation mechanism of the AuNRb structure. (b) The TEM image of a single AuNC. The red ellipse highlights a spike covered with gold nanoseeds. (c) The close-up TEM image of a AuNRb-8. Inside the dotted blue boxes are the silica spikes. (d) The close-up TEM image of a AuNRb-15. Inside the dotted blue box is a silica spike.

Figure 5

(a) A schematic diagram of surface enhanced Raman scattering (SERS) detection. (b) The SERS performance of AuNC, AuNRb-15, AuNRb-22 and AuNP-20 using Rhodamine 6G (R6G) as the probe molecules with a concentration of 10⁻⁵ M excited by a 633 nm laser.