Photophysical Properties of Tandem Quantum Defects in Carbon Nanotubes

Single-walled carbon nanotubes (SWCNTs) are versatile near-infrared (NIR) fluorophores that can be chemically functionalized to create biosensors. Numerous noncovalent approaches were developed to detect analytes, but these design concepts can be susceptible to nonspecific binding and reduced stability. In contrast, covalent modification of SWCNTs with quantum defects can be utilized to tune their fluorescence properties and enable new molecular recognition concepts. Here, we present and assess four different synthetic pathways/sequences to modify SWCNTs covalently with both sp³ quantum defects and DNA-based guanine defects. We find that it is possible to create two defect types without disrupting the optical properties or chemical stability. Interestingly, the emission peak associated with sp³ defects (E₁₁*) shifts around 3 nm when combined with guanine defects, indicating a coupling between the two defect types. However, it is far lower than the red-shift in bandgap-related emission (E₁₁) by guanine quantum defects (40 nm). We furthermore demonstrate that combinations of defects can be used for (bio)sensing. In summary, the combination of multiple quantum defect types in SWCNTs provides a platform for multifunctional biosensors and a new design space that can be explored.