A nitric oxide (NO) biosensor based on silver nanoparticles was fabricated with high sensitivity and selectivity as well as stability. Silver nanoparticles could preserve the microstructures of hemoglobin, but the electrochemical reactivity of the protein and its detection sensitivity toward NO could be greatly enhanced. Accordingly, a NO biosensor was developed. The linear concentration range was from 1.0 x 10(-6) to 5.0 x 10(-5) M. Its detection limit was 3.0 x 10(-7) M with a sensitivity of 0.0424 microA microM(-1) NO. The possible co-existing compounds would not interfere with the detection.