PH differential instrumentation amplifier
You cannot just connect the probe across inputs to the inamp and expect good results.

Operational amplifiers need a path to ground from each input.  So the second circuit is better.

the 741 is totally unsuitable, but the LMC6041 in the notes should work well for this circuit.  (2 fA is typical rating though, use the guaranteed value of 4 pA for design unless you want your circuit to only mostly work... 4pA through 300Mohm sensor is 1.6mV offset.)

LMC6041 will be happy with a single supply, so if the -5V supply is inconvenient have a look at this circuit: What is the purpose of this op amp?

I suggest you stop simulating and actually build the circuit.  LMC6041 is $2 from a distributor or ask National, they'll give free samples for school projects.  You don't need to hook up the ADC yet, just get the probe and signal conditioning amplifier working.  Read the output with a voltmeter.  Just like software, breaking the problem into small pieces is the way to make progress.