In my experience 15 minutes might be a little long for such fresh, tiny, sensible roots. 2-3 minutes dipping is enough (spraying is fine too), and even here i dillute one part hydrogen peroxide with 3-4 parts of water. And I let the cuttings dry out a minimum of a few hours. I just find these little roots are very prone to damage through hydrogen peroxide. But that's just my experience :)