The sensitive and selective detection of signaling biomolecules is an important component of medical research, with applications ranging from developmental biology to degenerative diseases. Several neurotransmitters, such as serotonin, dopamine, and norephinephrine, are redox active catecholamines that can be electrically detected. However, electrochemical detection of biological samples is limited since interfering species, such as ascorbic acid, mask the signal of the target molecules. We have developed a nanofluidic device that amplifies and selectively detects the electrochemical signal of catechols. The detection is based on rapid redox-cycling between electrodes inside of a nanochannel. This device is capable of detecting the signal generated by only a few hundred molecules residing in the active area of the nanofluidic sensor. The nanochannel design of the sensor eliminates interference by molecules undergoing irreversible redox reactions. We demonstrate the selectivity of the device by detecting catechol in the presence of ascorbic acid, whose oxidized form is only stable on the order of milliseconds.