Unveiling the Mystery of Plasmons

Localized surface plasmon resonance (LSPR) around plasmonic nanoparticles enables very high SERS enhancement and SPR sensitivity.

Surface plasmon resonance (SPR) and surface-enhanced Raman scattering (SERS) are two key surface plasmon technologies that ultimately enable single-molecule-level chemical and biological sensors. Due to the tremendous progress in solution-based synthesis methods, plasmonic nanoparticles with various complex shapes (e.g., spheres, rods, and prisms) have been widely used for surface plasmon resonance and surface-enhanced Raman scattering sensing. Localized surface plasmon resonance (LSPR) around plasmonic nanoparticles enables very high SERS enhancement and SPR sensitivity.

What are Plasmonic Nanoparticles?

Materials such as nanoparticles, nanorods, and nanotubes that exhibit optical properties at the nanometer scale are called plasmonic nanoparticles. Plasmons can be defined as when a metal nanoparticle interacts with light whose size is smaller than or equal to the wavelength, and the free electrons begin to resonate at a certain frequency, called the plasmon resonance frequency. Plasmon resonance is a material property that depends on the shape, size, composition and environment of the nanoparticle. Another viable option for modifying plasmon resonances is the synthesis of multimetallic nanoparticle composites.

Metal nanoparticles of gold (Au), platinum (Pt), and silver (Ag) exhibit high light absorption and light scattering behaviors. Gold and silver nanoparticles display unique spectral responses due to their specific wavelengths of light that can drive collective oscillations of conducting electrons. At the resonance of these plasmonic nanoparticles, their absorption and scattering intensity can be 40 times higher than that of non-plasmonic particles. The tunability and brightness of these nanoparticles make them useful in molecular tracking, imaging, and detection.

Applications of Plasmonic Nanoparticles

• In vitro biosensors

Detection of biomolecules at nanomolar concentrations is now common in biological laboratories due to the high specificity of antibodies working in conjunction with a variety of labels, including radioisotopes, enzymes, and phosphorus. Single-antibody or sandwich-based detection methods (i.e., ELISA) have been successfully used in combination with plasmonic nanoparticle-labeled antibodies, particularly in areas where low detection sensitivity is required. Antibodies can be covalently bonded to plasmonic nanoparticles using various methods, such as EDC/NHS chemistry-based amide bonding, maleimide groups, and disulfide bonds.

• Intracellular detection and ex vivo/in vivo imaging

SERS can provide very rich information about the intracellular environment and organelles. However, a challenging issue in applying SERS to cellular systems is how to successfully inject NPs into living cells and maintain effective and detectable SERS signals. The composition of cells and their ability to take up NPs varies greatly depending on the cell type or tissues from which the cells are obtained. Yashchenok et al. used functionalized colloidal particles to probe the intracellular environment. The designed probes are based on the SERS proximity effect or “hotspot” effect of gold nanoparticles and carbon nanotubes to respond to biomolecules in cells, and both are chosen with both functionality and biocompatibility in mind.

• Therapeutic applications

The applications of plasmonic nanoparticles are not limited to sensing and imaging but also exhibit excellent performance in photothermal systems. Because cancer cells lack a general blood supply, delivering drugs to tumor cells for testing or treatment is a major problem. To reduce damage to healthy cells, ligands capable of targeting target tumor cells are attached to the surface of the nanoparticles. The El-Sayed group reported the use of gold nanorods (Au NRs) for near-infrared-triggered photothermal therapy of HSC-3 tumor cells. First, antibody-labeled Au NRs were used for selective labeling of tumor cells in vitro. When these Au-NRs serve as photothermal contrast agents attached to the cell membrane, near-infrared laser irradiation causes hyperthermia effect and subsequently induces cell apoptosis.

Accessing Plasmonic NPs

Plasmonic nanoparticles have unique optical and physical properties that enable higher sensitivity for in vitro detection and intracellular or in vivo imaging at the nanoscale. These plasmonic nanomaterials in biology and medicine offer new and exciting approaches. CD Bioparticles offers a series of novel gold nanoparticles, silver nanoparticles, and gold nanostars, as a plasmonic core, for SERS-based lateral flow immunoassay.