Solar Panel Boost converter (40v to 100V)
You need a ground connection between 0V of the arduino and the source of the MOSFET - there is no arguament about this - it won't work without it.

Also how can your sensors be measuring current - something is needed in series with the wires coming from the solar panel (or a hall effect sensor).

When you do connect the ground start with the smallest duty cycle you can because, with no load you'll just get a massive voltage at the step-up point and make a mess of the output capacitors. 

Each cycle of on-off of the FET transfers an amount of energy to the output and this might be (off the top of my head) an energy of about 50 uJ. This energy will lift the 220uF output capacitor 0.5 volts (ish) per cycle. You don't need to be clever to understand that a 100V is reached in a very short period of time and, without control this will continue until the MOSFET fries on over-voltage.

If you want, put a dummy load on to simulate the real load.