Abstract
Cellular and tissue imaging in the second near-infrared window (NIR-II, ~1000–1350 nm) is advantageous for in vivo studies because of low light extinction by biological constituents at these wavelengths. However, deep tissue imaging at the single molecule sensitivity has not been achieved in the NIR-II window due to lack of suitable bio-probes. Single-walled carbon nanotubes have emerged as promising near-infrared luminescent molecular bio-probes; yet, their inefficient photoluminescence (quantum yield ~1%) drives requirements for sizeable excitation doses (~1–10 kW/cm2) that are significantly blue-shifted from the NIR-II region (<850 nm) and may thus ultimately compromise live tissue. Here, we show that single nanotube imaging can be achieved in live brain tissue using ultralow excitation doses (~0.1 kW/cm2), an order of magnitude lower than those currently used. To accomplish this, we synthesized fluorescent sp3-defect tailored (6,5) carbon nanotubes which, when excited at their first order excitonic transition (~985 nm) fluoresce brightly at ~1160 nm. The biocompatibility of these functionalized nanotubes, which are wrapped by encapsulation agent (phospholipid-polyethylene glycol), is demonstrated using standard cytotoxicity assays. Single molecule photophysical studies of these biocompatible nanotubes allowed us to identify the optimal luminescence properties in the context of biological imaging.
Highlights
SWCNTs have low luminescence quantum yield (QY)[8] due to non-radiative mechanisms and photoluminescence quenching by structural defects and environmental quencher[16,17,18,19,20]
Successful incorporation of the fluorescent sp[3] defects in (6,5)-SWCNTs is directly confirmed from the observation of the intense defect PL (E11−) at 1160 nm, which is redshifted from the native PL (E11) from unfunctionalized segments of the (6,5) nanotubes (Fig. 1a)
We suspended the f-SWCNT in phospholipid-polyethylene glycol (PL-PEG), which improves the biocompatibility of SWCNTs30,31
Summary
SWCNTs have low luminescence quantum yield (QY)[8] due to non-radiative mechanisms and photoluminescence quenching by structural defects and environmental quencher[16,17,18,19,20]. In order to improve this low luminescence QY, an elegant approach consists in covalently attaching low density chemical functional groups to the SWCNTs, e.g., alkyl and aryl groups, leading to the creation of sp[3] defects that strongly localize the band-edge excitons into quantum states (E11−) located 100–300 meV below the first bright band-edge exciton level (E11)[21,22,23,24,25,26,27,28]. The f-SWCNTs enable high signal-to-noise ratio imaging within the NIR-II region (985 nm excitation, 1160 nm emission) using excitation intensities that are one order of magnitude less than those required for the biocompatible unf-SWCNTs (845 nm excitation and 985 nm emission)
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