Hello Harold,
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As I pointed out in another LMP7721 thread, you do not really need the guard buffer (U4) if you are just guarding locally around the input pins and not driving large guard areas or cable shields.
You can just connect the "guard" trace to the negative input. Your impedance is low enough at that point (<1k) that the extra capacitance should not cause any peaking.
You should also have a current-limiting resistor, say 100-470 ohms, in series with the output of the bias driver U3 in case the output gets shorted (similar to U4 output). Anything that goes off-board should be protected from accidents...
How stable is your 5V supply? The PH sensor only puts out ±400mV - so any variation in the bias voltage will appear in series with your sensor, causing it to "wander" with the supply. You may want to consider using a reference (ideally the internal ADC reference if you can get at it). I would recommend using one of the ADC channels to monitor the bias voltage to keep an eye on it...
