To explore the opportunities generated by the sensor-based signal enhancement during an enzymatic reaction, we next monitored the HRP-based oxidation of PPD to BB with different SWCNT surface modifications. As expected, the enzymatic turnover of the reaction led to a decrease of the fluorescence (Figure ). Importantly, the fluorescence decreased only upon addition of all reaction components. At reaction times > 60 min, the effect of H2O2 was negligible suggesting that the observed response is caused by the formation of BB. Sensors which performed better in our initial screening (Figure , Figure ) also allowed a more precise monitoring of the reaction. Furthermore, it was possible to follow the reaction when SWCNTs were immobilized (Figure ). Combined with the 2D excitation-emission spectra (Figure ), this suggests that the mechanism behind the fluorescence decrease is not aggregation based.