Two-Photon Excitation Microscopy of Individual Single-Walled Carbon Nanotubes

Two-photon fluorescence imaging achieves deep-tissue penetration through long excitation wavelengths and nonlinear excitation confinement. The NIR-IIb transparency window is particularly attractive, as it optimally balances tissue scattering and absorption. However, efficient fluorophores for two-photon excitation in this window remain limited. Moreover, the weak near-infrared emission of individual emitters and the low photon detection efficiency have so far precluded single-particle imaging. Here, we characterize the two-photon excitation properties of chirality-sorted pristine and quantum color center-functionalized single-walled carbon nanotubes under 1630–1700 nm excitation. By measuring and comparing their two-photon action cross sections, we identify quantum color center-functionalized (6,5) nanotubes emitting at 1140 nm as the most promising emitter, with an exceptionally large cross section of (17.1 ± 0.6) × 103 GM/nm. Leveraging these favorable photophysical properties, we image individual nanotubes in the NIR-IIb window, which to our knowledge is the first demonstration of single-particle imaging in this spectral window. These results establish quantum color center-functionalized (6,5) nanotubes as a strong candidate for long-wavelength two-photon imaging and pave the way toward single-particle imaging in scattering tissues.