Thank you very much @CrossRoads for replying,
I don't want to switch between 1.8V and 3.3V, I want to supply voltage to LVC125A buffer chip using the IO pin, I want to power it down when ever I need, I think I need to add a Transistor switch to switch the buffer chip ON and OFF, The reason I want 1.8V is : To power the AD8232 which uses only 140uA to run.
Thank you very much for explaining me this, I will use 3.3V regulator from a 5V source and add another voltage regulator to make it 1.8V