A sensor is disclosed, for implantation within a blood vessel to monitor a substance in or property of blood. In one embodiment, the sensor detects nitric oxide or a nitric oxide metabolite. In another embodiment, other substances such as glutamate, aspartate, arginine, citrulline, acetylcholine, calcium, potassium, or dopamine are monitored. The sensor may be attached to a support structure such as a stent, guidewire, or catheter. In a further embodiment, a catheter is disclosed that extracts patient fluid to a sensor outside the body for monitoring a substance or property of the patient fluid. Methods are also disclosed.