Hollow gold nanospheres (HGNs) with nearinfrared (NIR) surface plasmon resonance (SPR) absorption are highly desired for many applications including photothermal ablation therapy (PTA) of cancer; however, they are challenging to synthesize at relevant resonant wavelengths in a reproducible manner. In this work, we have systematically varied the reaction parameters to determine the origin of the irreproducibility of synthesis. This allows for much finer control of the synthesis, including homogeneous NIR absorbing HGNs that were characterized using UV−vis spectroscopy and electron microscopy (EM) techniques. We have found that cobalt seed particle growth time plays a more critical role than previously realized and is one of the most important parameters for high synthetic reproducibility. The results also provide new insight into the mechanism of cobalt seed and HGN growth, which further aids the successful synthesis of high quality HGNs with strong and tunable NIR SPR absorption.

The synthesis, structural and optical characterization, and application of superparamagnetic and water‐dispersed Fe3O4‐Au core‐shell nanoparticles for surface enhanced Raman scattering (SERS) is reported. The structure of the nanoparticles was determined by scanning transmission electron microscopy (STEM) and high‐resolution transmission electron microscopy (HRTEM). STEM images of the Fe3O4‐Au core‐shell nanoparticles reveal an average diameter of 120 nm and a high degree of surface roughness. The nanoparticles, which display superparamagnetic properties due to the core Fe3O4 material, exhibit a visible surface plasmon resonance (SPR) peaked at 580 nm due to the outer gold shell. The nanoparticles are used as a substrate for surface enhanced Raman scattering (SERS) with rhodamine 6G (R6G) as a Raman reporter molecule. The SERS enhancement factor is estimated to be on the order of 106, which is ∼ 2 times larger than that of conventional gold nanoparticles (AuNPs) under similar conditions. Significantly, magnetically‐induced aggregation of the Fe3O4‐Au core‐shell nanoparticles substantially enhanced SERS activity compared to non‐magnetically‐aggregated Fe3O4‐Au nanoparticles. This is attributed to both increased scattering from the aggregates as well as “hot spots” due to more junction sites in the magnetically‐induced aggregates. The magnetic properties of the Fe3O4 core, coupled with the optical properties of the Au shell, make the Fe3O4‐Au nanoparticles unique for various potential applications including biological sensing and therapy.

The field of plasmonics is driven by the investigation of the interaction between the electromagnetic (EM) field (light) and metal nanostructures. In particular, noble metal nanoparticles have been studied extensively due to their interesting surface plasmon resonance (SPR) properties and related applications. Tuning of the SPR position in energy is possible through synthetic variation in size, shape, aspect ratio, the dielectric constant of the surrounding media, surface morphology and whether particles are aggregated. One unique metal structure capable of meeting a wide range of criteria for multiple applications calling for enhanced EM field is the hollow gold nanosphere (HGN). HGNs have hollow solvent filled dielectric cores and polycrystalline gold shells that, due to the two surfaces or interfaces, can generate enhanced EM field. They possess a unique combination of properties that include small size (20-125 nm), large surface to volume (S/V) ratios, spherical shape, narrow and tunable SPR (~520-1000 nm) and biocompatibility. Their surfaces can also be easily functionalized to target and deliver biomolecules and are resistant to photobleaching. Additionally their scattering and absorption cross-sections can be tailored, making them excellent candidates for a variety of applications including surface enhanced Raman scattering (SERS), sensing, imaging, drug delivery, site specific silencing and photothermal therapies (PTTs). This review will provide a perspective on the continued investigation of the plasmonic properties associated with HGNs and how these properties can be refined and harnessed for emerging applications.

Much progress has been made in using hematite (α‐Fe2O3) as a potentially practical and sustainable material for applications such as solar‐energy conversion and photoelectrochemical (PEC) water splitting; however, recent studies have shown that the performance can be limited by a very short charge‐carrier diffusion length or exciton lifetime. In this study, we performed ultrafast studies on hematite nanoparticles of different shapes to determine the possible influence of particle shape on the exciton dynamics. Nanorice, multifaceted spheroidal nanoparticles, faceted nanocubes, and faceted nanorhombohedra were synthesized and characterized by using SEM and XRD techniques. Their exciton dynamics were investigated by using femtosecond transient absorption (TA) spectroscopy. Although the TA spectral features differ for the four samples studied, their decay profiles are similar, which can be fitted with time constants of 1–3 ps, approximately 25 ps, and a slow nanosecond component extending beyond the experimental time window that was measured (2 ns). The results indicate that the overall exciton lifetime is weakly dependent on the shape of the hematite nanoparticles, even though the overall optical absorption and scattering are influenced by the particle shape. This study suggests that other strategies need to be developed to increase the exciton lifetime or to lengthen the exciton diffusion length in hematite nanostructures.

Large Fe3O4@SiO2 nanoparticles (∼200 nm) functionalized with gold and poly­(vinylpyrrolidone) have been synthesized, characterized, and evaluated for bioseparation and sensing applications. The particles have been characterized using a combination of experimental techniques including ultraviolet visible spectroscopy, energy-dispersive spectroscopy, powder X-ray diffraction, Fourier transform infrared spectroscopy, electron microscopy, superconducting quantum interference device magnetrometry, and surface-enhanced Raman sensing (SERS) spectroscopy. The particles have a unique surface morphology comprised of roughened gold nodules. The surface coatings prevent oxidation and render the particles easy to functionalize in order to target a wide range of moieties. The gold coverage is not only uniform across the entire particle surface but also ultrathin so as to maintain a high percentage of the cores magnetic saturation (∼68%) when compared to that of bare magnetite. The gold nodules facilitate the generation of hot spots that enhance the electromagnetic field associated with the particle surface and are therefore useful in sensing applications like SERS, and the strong magnetic core allows for rapid separation (∼30 s) of target molecules from solution once they are bound to the particles.