Dear Dharmesh,

I don't have much experience in electronics, but I think an instrumentation amplifier may do your job, you can feed its output to ADC of Arduino and get digital signal.

First, you won't be able to do it on an Arduino Uno because its inputs are not true analog; they are pulse width modulated. I solved this problem by using an Arduino Due, which has true analog outputs and inputs.

Second, you'll just need an OpAmp to add a gain to the voltage signal, and you can also filter out noise at the same time. Calculate the gain you want (from 200mV to 5V gives a gain of 25), then pick the correct OpAmp circuit for the job - I think a comparator would be appropriate here. Simply refer to elec eng tutorials online (like this one: https://www.electronics-tutorials.ws/opamp/op-amp-comparator.html).

Third, the code for showing a readout to your LCD screen (these are standard with the Arduino kits) is also standard, you can find this online in any Arduino forum. Use C+ in the IDE to write a "if...then" for different levels of input voltage to cause a display of either "10 ppm", "100 ppm", etc. That should be easy to do in 3 lines.

The hardest part is getting your sensor to detect the analyte repeatably!