Nanoplasmonic Approaches for Sensitive Detection and Molecular Characterization of Extracellular Vesicles

All cells release a multitude of nanoscale extracellular vesicles (nEVs) into circulation, offering immense potential for new diagnostic strategies. Yet, clinical translation for nEVs remains a challenge due to their vast heterogeneity, our insufficient ability to isolate subpopulations, and the low frequency of disease-associated nEVs in biofluids. The growing field of nanoplasmonics is poised to address many of these challenges. Innovative materials engineering approaches based on exploiting nanoplasmonic phenomena, i.e., the unique interaction of light with nanoscale metallic materials, can achieve unrivaled sensitivity, offering real-time analysis and new modes of medical and biological imaging.

Researchers from the University of California, Davis begin with an introduction into the basic structure and function of nEVs before critically reviewing recent studies utilizing nanoplasmonic platforms to detect and characterize nEVs. For the major techniques considered, surface plasmon resonance (SPR), localized SPR, and surface enhanced Raman spectroscopy (SERS), they introduce and summarize the background theory before reviewing the studies applied to nEVs. Along the way, the researchers consider notable aspects, limitations, and considerations needed to apply plasmonic technologies to nEV detection and analysis.

Label-free detection of nEVs using the nPLEX biosensor

(A) The well-defined biogenesis route of exosomes from cancer cells via multivesicular body (MVB) formation, consequent fusion with the cell membrane, and excretion of exosomes. The upper inset displays membrane proteins on the shell of the exosome; these proteins act as common recognition sites in biosensing applications. The lower inset shows a transmission electron micrograph of exosomes isolated from human ovarian cancer cell (CaOV3) culture. (B) Near a periodic nanohole surface the electromagnetic fields are clearly enhanced and accumulated as shown by finite-difference time-domain (FDTD) simulation. The enhanced field spreads over the nanostructure from one hole to another, which enables high sensitivity for the nPLEX assay. (C) The nPLEX sensor imaged by a scanning electron microscope (SEM). The nanoholes of diameter 200 nm are distributed evenly over the surface with periodicity of 450 nm (the inset demonstrates a closer view of the surface). (D) The nPLEX system is equipped with a metal-oxide-semiconductor (CMOS) detection unit for acquiring transmitted light intensity from the sensor chip. (E) The binding of nEVs to a periodic nanohole structure induces a change in the local refractive index and thus a spectral shift in transmission spectral peak. Wavelength shifts, or intensity changes at fixed wavelength, are monitored for nEV quantification. (F) An SEM image demonstrating surface-adsorbed exosomes on the nPLEX sensor.