Radical Polymer Grafting on the Surface of Single-Walled Carbon Nanotubes Enhances Photoluminescence in the Near-Infrared Region: Implications for Bioimaging and Biosensing

Photoluminescence (PL) of single-walled carbon nanotubes (SWCNTs) in the near-infrared (NIR) region (E11 emission) is attractive, especially for bioimaging and biosensing in vivo because of the good transparency of the body in the NIR region. In this study, we achieved brighter NIR emission of SWCNTs compared with E11 emission by radical polymer grafting on the SWCNT surface, in which radical polymerization using surfactant-dispersed SWCNTs, so-called carbon nanotube (CNT) micelle polymerization, generates emissive sp3 defects through local functionalization (lf). By lf of SWCNTs, new PL assignable to E11* and E112* emission is generated at a longer wavelength (>1100 nm) than E11 emission (<1100 nm), which is preferable for bioimaging to avoid the strong autofluorescence from the body. It was found that the initiator concentration and polymerization time strongly affect the E11* and E112* emission intensities, and as polymerization progresses, E11* and E112* emission become brighter, while E11 emission becomes darker. Interestingly, such behavior also depends on the hydrophobicity of the monomer. A hydrophobic monomer exhibits more rapid increase of E11* and E112* emission. At the optimum condition, the E11* emission intensity is about 6 times brighter than the initial E11 emission. Because a cross-linker is also added in CNT micelle polymerization, polymer cross-linking occurs during polymerization and a very stable coating layer is formed. Therefore, a stable dispersion after the removal of the surfactant is realized while maintaining bright PL emission.