Hi Oscar , You need to do more study on the nature of electrical noise.
Basically the intrinsic Boltzmann (Johnson) noise of a 1kohm resistor at 1Hz measuring bandwidth is 4nV at room temperature.
For 100Hz bandwidth, the noise is sqrt(100) or 10 times higher e.g. 40nV.
To measure 5nV signal at 100Hz or more you need to
(a) lower the operating temperature to liquid nitrogen or lower
(b) lower the source resistance (difficult with bio signals)
(c) take multiple measurements and average them, this only works if the biosignal is an evoked potential, i.e. a response to a signal you inject somewhere else. The improvement goes as the square root of number of measurements, so you can get noise down to 4nV at 100Hz by stacking 100 samples.
So once you have a measurement setup where the signal is more than Johnson noise , you can start thinking about op-amps. This is a fundamental limit of the laws of physics.
If you want a starting point , look at the OP27 , thare are a lot of low noise appnotes published that refer to this device.