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 (E-11 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 E-11 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 sp(3) defects through local functionalization (lf). By lf of SWCNTs, new PL assignable to E-11* and E-112* emission is generated at a longer wavelength (>1100 nm) than E-11 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 E-11* and E-112* emission intensities, and as polymerization progresses, E-11* and E-112* emission become brighter, while E-11 emission becomes darker. Interestingly, such behavior also depends on the hydrophobicity of the monomer. A hydrophobic monomer exhibits more rapid increase of E-11* and E-112* emission. At the optimum condition, the E-11* emission intensity is about 6 times brighter than the initial E-11 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.