Ionic 3 AlertController

How can i pass class variable inside the Alert Controller Button Handler. I tried to use the class varibales inside but the are undefined.

class BluetoothPrinterPage {

  unpairedDevices: any;
  pairedDevices: any;
  gettingDevices: Boolean;

constructor(public navCtrl: NavController, public navParams: NavParams, private bluetoothSerial: BluetoothSerial, private alertCtrl: AlertController, public viewCtrl: ViewController) {
    bluetoothSerial.enable();
  }

disconnect() {
    
    let alert = this.alertCtrl.create({
      title: 'Disconnect?',
      message: 'Do you want to Disconnect?',
      buttons: [
        {
          text: 'Cancel',
          role: 'cancel',
          handler: () => {
            console.log('Cancel clicked');
          }
        },
        {
          text: 'Disconnect',
          handler: () => {
            this.bluetoothSerial.disconnect();
          }
        }
      ]
    });
    alert.present();
  }
}


But here inside the handler this.bluetoothSerial is undefined.