Synthesis of Heterodimeric Sphere–Prism Nanostructures via Metastable Gold Supraspheres

Noble-metal nanoparticles of nonspherical shapes are interesting for their sizeand shape-dependent optical properties and for potential applications in hyperthermia of tumors, pathogen detection, and infrared-absorbing coatings. Typically, such particles are grown in the presence of surfactants that stabilize certain crystallographic faces. For example, silver nanocubes can be prepared by stabilizing the Ag {100} faces with poly(4-vinylpyrrolidone) (PVP), while gold nanorods are grown readily in the presence of cetyltrimethylammonium bromide (CTAB), which adsorbs selectively onto Au {100} faces. Other nanostructures prepared by the latter method include gold hexagons, gold triangles, silver disks, and several other shapes. Recently, considerable effort has been devoted to the preparation of hybrid or dimer nanostructures, in which two (or more) domains of different shapes or material properties are permanently connected. Such structures are usually made by epitaxial nucleation and growth on presynthesized nanoparticle seeds 17] or by thermal decomposition of core–shell nanoparticles. Herein, we describe a conceptually different route to a new class of nanoscopic heterodimers composed of spherical and polygonal domains. In our method, individual nanoparticles (NPs) are first assembled into metastable, supraspherical aggregates (SS, Figure 1a,b), and are then thermally decomposed into heterodimers (Figure 2). These composite particles are the result of temperature-induced coalescence of individual NPs accompanied by crystal nucleation. During this process, the relative sizes and dimensions of the SS and crystalline domains change controllably and give rise to pronounced changes in the particles; optical response. In a typical experiment, gold supraspheres (SS, diameter 96 13 nm) were prepared by rapid addition of 1,8octanedithiol dissolved in toluene (2.86 mm, 40 mL) to a stirred solution of gold nanoparticles (cAu= 1.0 mm, 1.75 mL) stabilized in toluene by excess didodecyldimethylammonium bromide (DDAB, 9 mm) and dodecylamine (DDA, 20 mm ; Figure 1a,b). The dithiol molecules displaced a portion of the loosely bound surfactant molecules and simultaneously crosslinked the NPs. The cross-linking continued until all NPs in solution were aggregated into spherical aggregates (SS), each composed of approximately 2500 NPs and with an average of 150 dithiol ligands per NP (for details of the growth mechanism, see reference [21]). When the SS solution was heated at 95 8C, it remained blue for times t< 100 min, then rapidly turned gray; subsequently, its color slowly changed to green (t 100–200 min, Figure 1c). Corresponding UV/Vis spectra showed that at around t= 100 min, the intensity of the SS surface plasmon resonance (SPR) band at lmax= 580 nm decreased dramatically, while a new, strong band at lmax= 920 nm appeared Figure 1. a) Preparation of Au SS by controlled cross-linking of gold nanoparticles. The transformation is accompanied by a pronounced color change. b) SEM side view of several supraspheres resting on a surface of silicon. Scale bar=100 nm. c) Color changes observed during thermal treatment of Au SS at 95 8C. d) UV/Vis spectra of the reaction mixture recorded at different heating times t. The spectra change abruptly at around 100 min.