Therefore, it's of great interest to design fluorophores with emission wavelengths in the NIR range. Quantum dots , upconversion nanoparticles or single-walled carbon nanotubes (SWCNTs) can serve this purpose. SWCNTs can be conceptualized as a rolled-up single layer of graphene. Their emission peak can be tweaked in a range between 800 nm -2400 nm and depends on their structure described by the chirality (n,m). Furthermore, the SWCNT emission is characterized by a large Stokes shift (> 300 nm) and a high photostability without bleaching. By chemical surface functionalization, e.g. with non-covalent adsorption of surfactants or nucleic acids, SWCNTs can be tailored to become optical biosensors. Depending on the interaction with a specific analyte, the fluorescence intensity changes as a function of the analyte concentration. This kind of biosensor is capable of detecting neurotransmitters , proteins , enzymes or pathogens .