Laravel Notification - Call to a member function routeNotificationFor() on string

Laravel 5.5

Controller

public function sendBookingSms(){
  $checkState = session()->get('checkState');
  $staffs = Staff::whereIn('staffId',$checkState)->get();
  foreach ($staffs as $staff) {
    $email = str_replace(" ","","44".substr($staff->mobile, 1)).'@mail.mightytext.net';
    Notification::send($email, new NewBooking($email));
  }
  return $staffs;
  session()->forget('checkState');
  return redirect(route('booking.current'))->with('message','Succesfully Send SMS to selected staffs !!');
}

NewBooking.php (Notification)

public function toMail($notifiable)
{
    return (new MailMessage)
                ->line('The introduction to the notification.')
                ->action('Notification Action', url('/'))
                ->line('Thank you for using our application!');
}

When calling this controller I am getting this error.

$staffs.

{"staffId":45,"forname":"Eldhose","surname":"John","categoryId":2,"email":"devhelloworld@gmail.com","mobile":"07588593278","whatsappNumber":"57656578658","gender":1,"address":"Poole","pincode":null,"modeOfTransport":1,"pickupLocation":"Office","branchId":0,"zoneId":1,"bandId":1,"paymentMode":1,"payRateWeekday":10,"payRateWeekNight":20,"payRateWeekendDay":10,"payRateWeekendNight":20,"payRateSpecialBhday":11,"payRateSpecialBhnight":15,"payRateBhday":11,"payRateBhnight":15,"status":1,"deleted_at":null,"created_at":"2018-02-26 22:16:44","updated_at":"2018-02-26 22:16:44"}

Please help me on this.... Thanks

via Chebli Mohamed