The addition of 10 µM substrate/product modified the fluorescence of the modified SWCNTs by up to + 164 % in the case of PPD (substrate) and -87 % for BB (respective product). Furthermore, the emission maxima shifted up to 5.8 nm towards higher and 1.7 nm towards lower wavelengths. Generally, the addition of HRP substrates/products (Figure , Figure ) affected the fluorescence to a higher degree than the addition of β-galactosidase substrates (Figure and). The most sensitive pair was PPD and BB. Contrary to benzidine substrates, which form radical products when converted with HRP, this system allowed us to characterize the fluorescence modulation of SWCNTs via a substrate/product system in an enzymatically independent way before moving to an enzymatic reaction. Therefore, some of the modified SWCNTs can serve as nanosensors to monitor enzymatic reactions.