Sensing with Chirality-Pure Near-Infrared Fluorescent Carbon Nanotubes

Semiconducting single-wall carbon nanotubes (SWCNTs) fluoresce in the near-infrared (NIR) region, and the emission wavelength depends on their chirality (n,m). Interactions with the environment affect the fluorescence and can be tailored by functionalizing SWCNTs with biopolymers such as DNA, which is the basis for fluorescent biosensors. So far, such biosensors have been mainly assembled from mixtures of SWCNT chiralities with large spectral overlap, which affects sensitivity as well as selectivity and prevents multiplexed sensing. The main challenge to gain chirality-pure sensors has been to combine approaches to isolate specific SWCNTs and generic (bio)functionalization approaches. Here, we created chirality-pure SWCNT-based NIR biosensors for important analytes such as neurotransmitters and investigated the effect of SWCNT chirality/handedness as well as long-term stability and sensitivity. For this purpose, we used aqueous two-phase extraction (ATPE) to gain chirality-pure (6,5)-, (7,5)-, (9,4)-, and (7,6)-SWCNTs (emission at ∼990, 1040, 1115, and 1130 nm, respectively). An exchange of the surfactant sodium deoxycholate (DOC) to specific single-stranded (ss)DNA sequences yielded monochiral sensors for small analytes (dopamine, riboflavin, ascorbic acid, pH). DOC residues impaired sensitivity, and therefore substantial removal was necessary. The assembled monochiral (6,5)-SWCNTs were up to 10 times brighter than their nonpurified counterparts, and the ssDNA sequence determined the absolute fluorescence intensity as well as colloidal (long-term) stability and selectivity for the analytes. (GT)₄₀-(6,5)-SWCNTs displayed the maximum fluorescence response to the neurotransmitter dopamine (+140%, K_d = 1.9 × 10^-7 M) and a long-term stability of >14 days. The specific ssDNA sequences imparted selectivity to the analytes mostly independent of SWCNT chirality and handedness of (±) (6,5)-SWCNTs, which allowed a predictable design. Finally, multiple monochiral/single-color SWCNTs were combined to achieve ratiometric/multiplexed sensing of the important analytes dopamine, riboflavin, H₂O₂, and pH. In summary, we demonstrated the assembly, characteristics, and potential of monochiral (single-color) SWCNTs for NIR fluorescence sensing applications.