Quantum Defect Density Controls Carbon Nanotube Sensor Sensitivity via Photophysical Cooperativity

Single-walled carbon nanotubes (SWCNTs) fluoresce in the near-infrared (NIR), overlapping 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 represent a new chemical dimension for sensing. However, it is unclear how the number of QDs affects sensor properties such as sensitivity or dynamic range. Here, we develop an 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 approximately 6.8–7.9), but influences the sensitivity (ratiometric response E11*/E11) by more than 100%. We attribute this impact of defect density on sensing to positive photophysical cooperativity between QDs, while the dissociation constant of the (pKa) of the quantum defect determines the dynamic range of the sensor. These pH-sensitive CCNTs can be incorporated into agar hydrogels for ratiometric (1130 nm/990 nm) pH imaging. Overall, we show that the number of QDs is an efficient tool to tune the sensitivity of fluorescent sensors.