Quantum defect density controls carbon nanotube sensor sensitivity via photophysical cooperativity

Single-walled carbon nanotubes (SWCNTs) fluoresce in the near-infrared (NIR), which overlaps with the biological transparency window. Therefore, SWCNTs are used as versatile materials for (bio)photonics. Controlled covalent functionalization of SWCNTs with quantum defects (QDs) creates color center carbon nanotubes (CCNTs) with red-shifted additional emission features and increased overall emission. QDs are a new chemical dimension for sensing but is unclear how the number of QDs affect sensor properties such as sensitivity or dynamic range. Here, we develop a NIR fluorescent ratiometric pH sensor based on benzoic acid QDs. We vary the amount of QDs in (6,5)-chirality enriched CCNTs and demonstrate that the amount of pH-sensitive QDs does not affect the dynamic range of the nanosensor (pH = 6.0-8.5), but influences the sensitivity (ratiometric response E11*/E11) by more than 100 %. We attribute this impact of defect density on sensing to photophysical positive cooperativity between QDs, while the dissociation constant of the (pKa-value) of the quantum defect determines the dynamic range of the sensor. These pH sensitive CCNTs can be incorporated into agarhydrogels for ratiometric (1130 nm / 990 nm) pH imaging. Overall, we show that the number of QDs is an efficient tool to tune properties of fluorescent sensors.