As mentioned on Arduino’s forms there are few direct sensors and the only ones I know of are in expensive chemistry equipment. They mentioned using Spectrometry to figure out the concentration of the chemicals, mean your would have to calibrate the sensor to look for a change that would indicate a Nitrate chemical reaction, based on that rate you could detect the concentration, but before you could monitor, you would have to make test solutions for reference.
As for creating something like this you will need, a high power LED/Laser a diffraction grating with about 200 lines per mm, and a lens or mirror to focus the light on a CMOS sensor or a smaller light sensor. The sensor must have an analog output to a OpAmp then to your Micro controller, once you have calibrated data monitoring with a Spark cores simple. Best of Luck.