Necklace‐like Noble‐Metal Hollow Nanoparticle Chains: Synthesis and Tunable Optical Properties

Abstract

The sizeand shape-dependent physical properties of inorganic nanoparticles provide tunable materials with broad potential applications, and the fabrication of complex nanostructures further enhances their functionality. For example, noble metal nanoparticles exhibit localized surface plasmon resonances (LSPRs), resulting in strong absorbance at visible wavelengths. The LSPRs enable applications such as biological and chemical sensing, biological imaging labels, and nanometer-scale optical waveguides. The formation of more complex metal nanostructures, such as metallodielectric gold nanoshells and gold nanocages, shifts the LSPR to the near-infrared (NIR), enabling significant diagnostic and therapeutic biomedical applications. On the other hand, self-assembly, an attractive and practical methodology, allows the formation of a wide range of nanostructures for promising applications such as nanoparticle arrays for new optical band-gap materials, high-density magnetic recording media, self-assembled monolayers for nanometer-thick films on a variety of substrates, and nanofibers of self-organized small molecules or oligopeptides for drug delivery and tissue engineering. 1D chains of metal nanoparticles exhibit unique, coherent optical properties when the distance between the particles is short enough to allow for nearfield coupling of their surface plasmon resonances. For example, 1D nanoparticle chains can serve as a plasmon waveguide to overcome the diffraction limit of the optical image. Surface lithography, solution-phase templated assembly, oriented aggregation, micelle transition, electric–dipolar interactions, and magnetic dipole moments have all been used to fabricate plasmon-coupled nanoparticle chains, with the goal of controlling precisely the shape and size of the particles as well as the chain geometry. Despite extensive investigations on 1D systems, there are very few reports of the fabrication of 1D chains of structurally complex nanoparticles, such as hollow nanoparticles. Therefore, we describe in this Communication the synthesis methodology and the tunable optical properties of necklace-like noble-metal hollow nanoparticle chains (HNPCs) from templated Co nanoparticle chains formed under a magnetic field. We will demonstrate that the magnetic-field-guiding and -dependent strategy, which is seldom used in nanostructure fabrication, may provide a useful toolkit for the design of advanced nanoparticle devices. Herein, we show that an in situ magnetic field can induce the formation of a sacrificial necklace-like chain template composed of multicrystalline Co nanoparticles with diameter of ca. 15–20 nm. That Co chains could be found on a substrate after removal of solvent from a dispersion demonstrates that they are sufficiently stable to be used as “hard templates” for the production of advanced 1D nanostructures in solution. By using a novel and potentially general galvanic displacement reaction based on the template, similar to the recently reported formation of hollow metal nanoparticles, nanocages, and nanotubes, Au, Pt, and Pd HNPCs can be formed without loss of the preassembled 1D structure of the template. Because the hollow nanostructures exhibit novel properties that differ from their solid counterparts, we demonstrate that the morphology, as well as the LSPR properties, of Au HNPCs can be tuned by the Co-templated chain through symmetrical variation of the magnetic fields. The one-pot synthesis of noble-metal HNPCs is straightforward. Jiang and co-workers have successfully prepared isolated Au hollow nanoparticles using Co nanoparticles as sacrificial templates. Here, we use the same starting materials to synthesize noble-metal HNPCs while an external field is applied. CoCl2·6 H2O and poly(vinyl pyrrolidone) were dissolved in ultrapure H2O (18.2 MX), sonicated for 15 min, and purged with Ar for 15 min. After placing the system into a defined symmetrical magnetic field, a freshly prepared solution of NaBH4 was added dropwise with stirring. Immediately after all of the NaBH4 had been added, an aqueous solution of HAuCl4/KOH (pH = 7.0) was added dropwise with stirring. After 30 min, the product was collected by centrifugation, C O M M U N IC A TI O N