Unique optical properties and applications of hollow gold nanospheres (HGNs)

• The surface plasmon resonance (SPR) absorption of hollow gold nanospheres (HGNs) can be tuned by controlling the diameter and shell thickness to cover the entire visible to near IR (NIR) region of the spectrum.
• HGNs show enhanced performance photothermal therapies (PTTs) of cancer due to their ability to generate strong and narrow SPR, small size, and ease of bioconjugation.
• HGNs strongly enhance imaging applications due to their resistance to photobleaching and strong absorption as well as scattering of light in the desired wavelength regions.

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.