Trigger detection

Hi all, im trying to make object detect if they have been hit by an explosion, but having trouble getting the objects to detect if they have been hit.

I have made the explosion a big sphere and anything with a certain tag in it or that touches it should be destroyed.

The explosion object has collision2d has collision added and the objects im trying to destroy have a rigidbody2d applied.

Any help will be greatly appreciated.

         void OnTriggerEnter(Collider2D objecthit)
         {
             if(objecthit.gameObject.tag == "enemy")
             {
                 //objecthit.gameObject.active = false;
                 Destroy (objecthit.gameObject);
             }
 }