How to measure micro and nano amps via microcontroller
Consider a transimpedance amplifier, which will give you zero burden (negligible voltage drop). You have to use an amplifier with very low bias current compared to your measured current to get good accuracy. For example, an amplifier with 1pA bias current and a 10M feedback resistor would give you an accurate 1V out for -100nA in. You can feed that into the ADC and display it. As this (ignore the photodiode). 



You can switch Rf to different values for different ranges (eg. 3nA/30nA/300nA/3uA full scale would require 1G/100M/10M/1M resistors). 

Making circuits to measure nA accurately (pA or fA leakage) requires care. Google electrometer circuits for more details, there is more to it than is appropriate for this answer format.