There are plenty of decent microphones available for iOS and Android devices. In order to connect professional microphones to your smartphone, you are going to need a XLR mixer. The Saramonic SmartRig+ Di can help. It is a microphone, guitar interface with two XLR mic inputs and two 1/4″ guitar inputs.
You also get two 3.5mm TRS microphone inputs. Gain control is easy using the knobs on the device. +48V phantom power is also available. The device has a MFi certified Lightning connector for iOS devices.[where to get it]