Optical biosensors fall into two major groups: those based on molecular recognition and those based on chemical reactivity, known as activity-based sensing.55 While traditional optical biosensors typically rely on a lock-and-key type molecular recognition process, activity-based sensors detect molecular reactivity between the sensor and analyte. Regarding ssDNA-SWCNT catecholamine sensors, our data indicated a strong correlation between catechol redox activity and optical response, suggesting an activity-based model may be a fit. However, the absence of detectable oxidized catechol products was not fully consistent with this model.