Prosthetic hands have greatly evolved in mechatronic, robotic and control aspects. However, occasional accidents might happen due to excessive grip force or the breaking of contact due to slip. Fast transient slip events can be properly handled by a low-level controller that can behave like a reflex to maintain grasp stability in a shared control manner between the user and the prosthetic hand itself. Here we propose the use of a reflective optic sensor to capture slip events and evaluate the performance of a monotonic PI (MPI) control law that acts as to suppress slip. We have characterized the response of the sensor to motion and noted that transparent surfaces generate smaller responses. The proof-of-concept experiment demonstrated the effectiveness of the MPI controller where slip events were properly suppressed by an increase in grip force.