Defination : A nanoshell, or rather a nanoshell plasmon, is a type of spherical nanoparticle consisting of a dielectric core which is covered by a thin metallic shell (usually gold). These nanoshells involve a quasiparticle called a plasmon which is a collective excitation or quantum plasma oscillation where the electrons simultaneously oscillate with respect to all the ions.
The simultaneous oscillation can be called plasmon hybridization where the tunability of the oscillation is associated with the mixture of the inner and outer shell where they hybridize to give a lower energy or higher energy. This lower energy couples strongly to incident light, whereas the higher energy is an anti-bonding and weakly combines to incident light. The hybridization interaction is stronger for thinner shell layers, hence, the thickness of the shell and overall particle radius determines which wavelength of light it couples with. Nanoshells can be varied across a broad range of the light spectrum that spans the visible and near-infrared regions. The interaction of light and nanoparticles affects the placement of charges which affects the coupling strength. Incident light polarized parallel to the substrate gives an s-polarization (Figure 1b), hence the charges are further from the substrate surface which gives a stronger interaction between the shell and core. Otherwise, a p-polarization is formed which gives a more strongly shifted plasmon energy causing a weaker interaction and coupling.
Production of Nanoshell
A state of the art method for synthesizing gold nanoshells is the use of the Microfluidic Composite Foams. This method has the potential to replace the standard lithographic method of synthesizing plasmonic nanoshells. The production process described below was an experiment performed by Suhanya Duraiswamy and Saif A. Khan of the Department of Chemical and Biomolecular Engineering in Singapore. Although this method was an experiment, it represents the future of nanoshells synthesis.
The materials required for the production of the nanoshells are the following; Tetraethyl orthosilicate, ammonium hydroxide, hydroxylamine hydrochloride, 3-aminopropyl tris, hydrogentetrachloroaurate(III) trihydrate, tetrakis(hydroxymethyl) phosphonium chloride, sodium hydroxide, potassium carbonate, ethanol, Ultrapure water, and glassware washed in aqua regia and rinsed thoroughly in water.)
The first step in synthesizing nanoshells in this method is by creating the device for the reaction to take place within. Microfluidic device patterns were fabricated onto silicon wafers by standard photolithography using negative photoresist SU-8 2050. Devices were subsequently molded in poly(dimethylsiloxane) (PDMS) using the soft lithography technique. (40) Briefly, PDMS was molded onto the SU-8 masters at 70 °C for 4 h, peeled, cut, and cleaned. Inlet and outlet holes (1/16-in. o.d.) were punched into the device. The microchannels were irreversibly bonded to a glass slide precoated with a thin layer of PDMS after a brief 35 s air plasma treatment. The microchannels have a rectangular cross-section and are 300 μm wide, 155 μm deep, and 0.45 m long.
The actual production of the nanoparticles involves pumping “silicone oil, a mixture of gold-seeded silica particles and gold-plating solution and reducing agent solution to the microfluidic device while nitrogen gas was delivered from a cylinder.” The plating solution was then left to age, in a controlled environment, for longer than 24 hours. After the aging process, the fluid is collected from the Microfluidic Device and placed in a centrifuge. The resulting liquid has a layer of oil on the surface with a solution below that contains the nanoshells.
The reason this method is revolutionary is that the size and relative thickness of the gold nanoshell can be controlled by changing the amount of time the reaction is allowed to take place as well as the concentration of the plating solution. Thereby allowing researchers to tailor the particles to suit their given needs. Albeit for optics or cancer treatment.