By Hussain Ahmed
Mar 21 2022Reviewed by Megan Craig, M.Sc.A recent study available as a pre-proof in the journal Carbon examines the plasmonic effects of silver and gold nanoparticles of different geometries to regulate and improve the fluorescence intensity of Single-Walled Carbon Nanotubes (SWCNTs).
Study: Plasmon-induced near-infrared fluorescence enhancement of single-walled carbon nanotubes.
Image Credit: Oselote/Shutterstock.com
It was observed that gold (Au) nanorods increase chirality emissions by 80 % whereas silver (Ag) nano triangles increase emissions by 200 %. Finite element modeling (FEM) was used to demonstrate the plasmon-induced localized increase in electron density as well as radiative recombination of dark excited states from the resultant electromagnetic field.
What are Plasmonic Nanoparticles?Plasmonic nanomaterials are metallic nanoparticles with diameters below the incoming light's wavelength. These nanostructures have enhanced light absorption and scattering capabilities at the resonance frequency.
Optical characteristics vary with particle size, shape, and chemical composition spanning wavelengths from ultraviolet (UV) to near-infrared (NIR-II) radiations. Infrared absorption, surface-enhanced Raman scattering (SERS), and energy harvesting are examples of applications based on plasmonic nanoparticles' optical tunability.
Plasmonic nanoparticles have been employed extensively in the past decade to improve the fluorescence characteristics of optically connected fluorophores. The plasmonic nanostructure's resonant wavelength matches the optical characteristics of the fluorescent dyes. Several studies have shown enhancement for a variety of fluorophores. In particular, the NIR-II range is employed for in vivo optical imaging applications.Figure 1.
Schematic of ssDNA-wrapped SWCNT in the vicinity of metallic nanostructures that contribute to plasmon-enhanced fluorescence. Upon illumination at resonant wavelengths (red), the metallic nanoparticles show shape, size, and composition-specific plasmonic effects (blue) for enhancing NIR-II SWCNT fluorescence (green). © Amirjani, A. et al. (2022).
Characteristics of SWCNTsSWCNTs have unique NIR-II fluorescence for in vivo imaging and sensing applications. The absorption and emission peaks change with nanotube diameter according to the indices of chirality.
SWCNTs emit NIR-II inside the optical transparency window of biological materials, where biofluids and tissues are little absorbed. In addition to the large penetration depths at longer wavelengths, these emissions have unlimited photostability, making them ideal for continuous deep tissue imaging in biological samples.
SWCNTs exhibit modest fluorescence quantum yields (QYs), ranging from 0.1 to 1.5 percent in aqueous environments. The increased sensitivity of optical sensors, which use brighter nanotubes, might be attributed to increased QYs.
Due to the absence of scalable, repeatable, and accessible SWCNT synthesis processes, long, defect-free SWCNTs are difficult to fabricate. Therefore, metal-enhanced fluorescence of SWCNTs mounted on substrate surfaces has been used to increase QY. Despite the 10-fold increase in fluorescence, the SWCNTs' surface immobilization restricts their application for solution-phase imaging. Adding reductive brighteners like Trolox may improve QY, but at the price of cytotoxicity and sensor sensitivity.