PPD and BB are classical ELISA reactants but in most current assays other molecules are used. Therefore, we tested next if it would be possible to use a substrate system with a broader use. Today, TMB is the most widely used substrate in ELISAs. Its oxidation processes over a radical intermediate that can be followed at 370 nm/650 nm (TMB-I) and 450 nm (TMB-II) respectively (Figure ). To test how SWCNTs with different surface modifications would respond to the formation of oxidized TMB products, we incubated TMB for 30 min with HRP and H2O2, with H2O2, as well as without HRP and H2O2 and stopped the reaction with H2SO4 (50 µL, 1M). Addition of 2 µL of the reaction mixtures without HRP generally led to a fluorescence increase, while a fluorescent decrease was observed for reactions in which the enzyme was present (Figure ). Based on the fluorescent changes, we selected (C)30-modified SWCNTs for our next experiments.