OP184 is one of the best AD op amps for low noise purposes with single supply configuration. But, 3.9 nV input spectral density and 300nVpp in 0.1 - 10 Hz noise voltage is still too high for nanovolt measurement. I know paralleling the op amps and lowering amplifier bandwidth can reduce the noise of amplifier effectively, but do you know some other idea for decreasing noise voltage of amplifier?
ADA4084-1 is next generation OP184; higher speed, less power, less money.
Also see AN-940
What is your sensor/source impedance?