###Let's abstract your task to determine dissolved oxygen (DO) to something more general:
- You have to tie an observable to a concentration.
- It would be nice if the method would work over a broad range of concentrations.
- The method should be specific, or, alt least, very selective.
- If possible, this relation should be linear.
- It would be nice if the method could be packed into a sensor that works on its own (permanently or in cycles) and does not require a lot of personal interaction (= autonomous probe)
###What properties of molecular oxygen dissolved in water can be measured?
- In water, $\ce{O2}$ can be reduced electrochemically: $\ce{O2 + 4e- + 2 H2O -> 4 OH-}$
→ Have a look for galvanic and polarographic oxygen sensors - Molecular oxygen is known to quench the luminescence of electronically excited states of dyes. The observed luminescence intensity (and lifetime) is reciprocally proportional to the the concentration of the quencher(Stern-Volmer equation)
→ Have a look for luminescence-based oxygen sensors
