Error: Paths must be non-empty strings and can't contain ".", "#", "$", "[", or "]" (ionic 2 and firebase)

saveDetails(){
  this.afAuth.authState.take(1).subscribe(auth => {
    this.af.object('request/${auth.uid}').set(this.request)
    .then();
  })

After executing this method, an error: Firebase.child failed: First argument was an invalid path: "request/${auth.uid}". Paths must be non-empty strings and can't contain ".", "#", "$", "[", or "]" is being shown. Tried removing $ but that doesn't work. Pretty sure i am not passing in empty strings too.