Bifunctional plasmonic-magnetic nanoparticles (PM-NPs) consist of optical and magnetic components. They are useful for biomedical applications that require optical sensing/imaging/heating, magnetic stimulation/manipulation, or both of these functionalities. An example of these functionalities is that PM-NPs can attach to biological entities and separate them under an external magnetic field while simultaneously detecting their chemical nature via optical sensing. These functionalities are especially useful when studying tissues deep within an organism. Current available bifunctional PM-NPs are limited to quasi-zero-dimensional (0D) nanostructures. There are few reports of quasi-one-dimensional (1D) nanostructures, although 1D PM nanotubes provide advantages for biomedical applications that are not available with 0D nanostructures. A specific type of 1D PM nanotube has applications in targeted single-cell sensing.
Contents
SERS characterization
A laser beam focused on two neighboring Ag nanoparticles can generate a high intensity electric field in the narrow junction due to LSP resonance as a result of coherent electron oscillation in the Ag nanoparticles. This junction is referred to as a “hotspot”. If a molecule is in the vicinity of the hotspot, it’s Raman scattering signals can be significantly amplified. This phenomenon is called surface enhanced Raman scattering (SERS). SERS EF is determined by the electric field intensity. Nanocapsules that have Ag nanoparticles on the inside and outside (double-layer nanotubes) exhibit 2x SERS intensity than those with a single layer of Ag nanoparticles (see below).
Manufacturing SERS nanocapsules and arrays
Researchers at the University of Texas at Austin found a method for economically synthesizing SERS nanocapsules and creating arrays of the nanocapsules capable of ultrasensitive and position-predictable SERS detections. In doing so, the researchers also tackled two major problems previously associated with SERS sensing: 1. Lack of many hotspots with controlled gaps 2. Difficulty of assembling SERS probes at designated locations
The nanocapsules created in this study comprised a 3-layer structure: the Ag/Ni/Ag nano core, a silica capsule, a uniformly distributed Ag nanoparticles on silica. Creating the nanocapsule works as follows: (1) The electrodeposition of a nanoporous anodized aluminum oxide and subsequent deposition of a Cu layer onto this membrane to create a three-electrode electrodeposition system; (2) building the nanowires via electrodeposition, starting at the bottom of the membrane’s nanopores (the amount of electric charge passing through the circuit controls the length of the Ag/Ni/Ag nanowires; (3) dissolving the template membrane in NaOH and subsequently washing the nanowires via sonication and centrifuging in ethanol and deionized water; (4) coating nanocore in silica via hydrolysis of tetraethyl orthosilicate; (5) isolating Ag ions and then incubating with the silica-coated nanocapsule to reduce ionic Ag into metallic Ag nanoparticles on the silica.
In terms of functionality, the nanocore of Ag and Ni allows for manipulating the nanocapsule with electric tweezers. Ag/Ni/Ag nanocores prove necessary for steering/orientation via the AC field. The silica capsule serves to separate the nanocore from the Ag nanoparticles on the outside, preventing the plasmonic quenching effect. The Ag nanodots act as a hotspot layer (with an enhanced electric field) and perform the sensing task.
The researchers arranged the nanocapsules into arrays using magnets of Cr/Ni/Ag. The Cr anchored the magnet in substrate; the Ni worked in concert with the Ni in the nanocapsule core by providing an attractive magnetic field; the Au layer tuned the magnetic interaction force between nanomagnet and nanocapsule. The researchers used a quadruple electrode chip to create an array of nanocapsules. The AC and DC fields could be oriented parallel or perpendicular to each other in order to create the electric tweezers (the mechanism could transport capsules parallel or perpendicular to their orientations depending on whether or not AC and DC fields were directed parallel or perpendicular to one another). The Ag/Ni/Ag nanocores proved necessary for steering/orientation via the AC field (when removed no net torque resulted). The electric tweezers then moved the nanocapsules to their nanomagnet anchors, utilizing the attraction between Ni in capsule and magnet.
These nanocapsule arrays prove consistent, with the variation of Raman intensity between sensors +/-9%. This low error margin comes from the controlled sizing and spacing of Ag nanoparticles and the large number of such hotspots on the nanocapsule. Previously there existed only randomly placed hotspots, which produced more variation between nanocapsule sensitivity and therefore increased the error margin. The nanocapsules can detect Raman spectra at small concentrations (for example, 1,2-bi-(4-pyridyl) ethylene (BPE) at 10-12 M) and as concentration increases, the detection sensitivity increases logarithmically.
Plasmonic simulation
SERS enhancement can be attributed to two factors: electric field (E-field) enhancement due to the plasmonic resonance of nanoparticles and chemical enhancement due to charge transfer between the molecules and metal particles. E-field enhancement is a major contributor to SERS. A group of metallic particles on the same surface can provide stronger E-field enhancement than that of single particles or dimers. A group of nanoparticles with controlled distributions further enhance the E field. Narrower junctions between nanoparticles exhibit a strong E field. For smaller nanoparticles, the normalized electric field increases as size increases. However, for nanoparticles of diameters >40 nm, E-field enhancement reduces. This is because the plasmonic resonance shifts to a longer wavelength. When a double-layer nanotube is placed on top of a glass substrate and excited by a surface normal beam, the hot-spot is at the bottom of the nanotube. This is because the bottom hot-spot is surrounded by silica, while the top is surrounded by air. In addition, the inter-particle coupling through the nanoparticle chain at the outer surface enhances the electric field at the bottom of the nanotube. Not only does the inner layer add more hot-spots for SERS sensing, but it also increases the intensity of the hot spots in the outer layer nanoparticles. However, when the core is filled with platinum, the opposite is observed. The hot spots at the top of the nanotube are enhanced by 2x, while those at the bottom are reduced. This is because at the top of the nanotube, light is reflected to enhance the hot spot, while no light is able to pass through the platinum, which reduces the hot spots on the bottom. A similar phenomenon occurs when the tube is filled with nickel. Thickness of the silica also affects enhancement of SERS. Near-field enhancement can be observed on nanotubes with a silica coating ranging from 70 to 150 nm. However, if the thickness is increased, near-field enhancement will disappear.
Magnetic characterization
These 1D PM nanotubes also offer tunable magnetic properties for controlled manipulation. When the Ni segment is relatively large, the anisotropy direction is along the long axis. When the Ni segment is relatively low, magnetic anisotropy is transverse to the length of the nanotubes with zero remanence. This is a way of controlling the magnetic anisotropy, and can be used to affect the way the tubes align with a magnetic field.
Single-Cell bioanalysis
Because of their various functionalities, 1D PM nanotubes can be used for various applications, including analysis of membrane composition of a single animal cell among many. Because of the bifunctionality of the nanotubes, it is possible to precisely transport the nanotube to a specific living cell, and analyze various features of that cell.