Construction of voltage amplifier (nano volts to millivolts)
A 741 is utterly incapable of doing anything useful at that level. Drift and noise are orders of magnitude too high. It was hot stuff in 1968, not so much now. 

You can use a "zero-drift" chopper amplifier with ~50nV/K drift but noise will be relatively large, depending on bandwidth. I suspect 100nV RMS (not P-P) should be achievable over a few Hz bandwidth.  

Construction techniques need to be very good to live up to the amplifier, including especially minimizing thermal gradients to reduce thermal EMFs. 

If possible, find a way to get more signal. 

Picoammeters and electrometer amplifiers in general tend to have poor DC performance as they are optimized for extremely low bias current. Since you have, I believe, a very low impedance source, you have the opposite problem.