Multifunctional NIR-Fluorescent Carbon Nanotubes for Imaging and Sensing By Using Defect Chemistry

Abstract

Single-walled carbon nanotubes (SWCNT) are versatile materials for imaging and sensing in the near infrared (nIR) range. Non-covalent adsorption with (bio)polymers such as ssDNA is a powerful approach to functionalize SWCNTs. This approach lead to sensors that detect the release of biomolecules such as neurotransmitters from cells with high spatiotemporal resolution. However, covalent approaches would be beneficial for many applications because of higher stability and photophysical signal transduction. It has been reported that reactions with diazonium salts create covalent defects and a red-shifted peak in the nIR fluorescence spectrum attributed to a formerly dark exciton. Here, we use defect chemistry to covalently modify SWCNTs and at the same time change their fluorescence spectrum in a well-defined way. For that purpose, we synthesized a maleimide-carrying diazonium salt and used a light-catalyzed reaction to introduce defects. The maleimide group can be further functionalized using thiols in a thiol-en click reaction. The modified SWCNTs were additionally wrapped in DNA for solubilization and characterized via nIR spectroscopy and atomic force microscopy (AFM). Our results show that depending on the attached thiol compound these SWCNTs respond differently to small molecules such as the neurotransmitter dopamine. This approach was then used to optimise the selectivity profile and sensitivity for fluorescent sensing applications. Furthermore, we covalently conjugated antibodies to the SWCNTs and used them for nIR labeling in cells. In summary, we present a new approach to covalently functionalize SWCNTs using defect chemistry and demonstrate applications in nIR imaging and sensing.