It may sound stupid but are you sure that it is dust not a reflection? I fooled myself once thinking that I got dust inside my new lenses what turned out to be just a reflection. You may turn the lenses 180 degree to see if the dust particle is moving with the lenses. Primes are less likely to be affected with dust that zooms which suck in the air everytime you zoom in. And even if it is dust such a small particle shouldn't bother at all.