Usually,I prepared a full coverage of 1mg/mL GO solution on 2 cm x 2 cm substrate and spin-coat for 30 sec at 2000 rpm. You will notice the difference in color of SiO2 substrate after put the GO coating. No need to heat the substrate, room-temp is fine.
Article A practical carbon dioxide gas sensor using room-temperature...
i don´t understand of your paper if you put an unic 3 microliter droplet with a 0,1 mg/ml concentration and then you spin the substrate 30 secs at 2000 rpm or the wafer is rotating when you put the droplet?