I'm no chemist, and there is no emissions testing where I live, so I'm no emissions specialist really, but Nitrous Oxide is laughing gas, which is used by ricers and racers to get more power than is normally available form their engines. Oxides of Nitrogen (NO) is an emission gas I believe, not Nitrous Oxide (NO2), different things. There are many links in the web to indicate both Nox and NO2 are the same thing, but I believe (and maybe there is a chemist out there reading this who can clarify it) but I believe NO is the chemical nomenclature for the "bad" stuff, and NO2 is the stuff dentists and ricers use.
Possibly "NO" is Nitrogen Oxide and NO2 is NitrOUS Oxide?
There is no emission related sensor that analyzes exhaust gases EXCEPT an 02 sensor (Oxygen sensor) and that's all it can sense is the presence of oxygen molecules. It senses "rich" as the lack of oxygen molecules and "lean" as a higher presence of oxygen molecules.
There are now a couple sets of O2 sensors, depending on exhaust system configuration, which senses how efficient the catalytic converters are.
I've never hear of a knock sensor referred to anything other than a "knock sensor", and there is no "NOx" sensor.