Introduction to dissolved oxygen
Why is oxygen important? For us humans, if we have oxygen, we survive...yay! If not, we don't...boo. So, superficially, this may not seem like a very important parameter to know - you either have oxygen, or you don't. However, for many microorganisms, there are a lot of shades of gray.
For a bacteria or a yeast, different amounts of oxygen produce different results. For instance, starving a yeast cell of oxygen produces ethanol as a metabolite product instead of carbon dioxide. Starving a lake of oxygen not only prevents fishies from living in it, but also promotes the formation of large algae surfaces. Cool, right?
The biggest problem with measuring dissolved oxygen currently is the cost of the equipment available to do it. Typically, dissolved oxygen probes run well into the $400+ range, thus placing them well out of the realm of hobbyists. The cost is not wholly unwarranted - dissolved oxygen meters used a platinum catalyzed reaction with very specific membranes to measure oxygen response. By cutting out the platinum catalyst and the specialized membrane, the cost of a DO meter could drop considerably...enter the optode!
Building a dissolved oxygen probe
How an optode works
In order to reduce cost, we'll be building a dissolved oxygen optode instead of the more common dissolved oxygen electrode.
In an electrode, a small change in a voltage or current is used to detect a change in oxygen concentration. In an optode, a small change in reflected light intensity is used to detect changes in oxygen concentration:
There are advantages and disadvantages to each system.
Properties of commercial dissolved oxygen electrodes:
- Very robust - easily waterproofed
- Very accurate
- Need to recalibrate is rare due to non-reactive nature of the membrane and platinum
- Very small amperages are produced - an amplifier circuit must be built at the amp meter position
- Very, very expensive - $400 and up
Properties of commercial dissolved oxygen optodes:
- Film must be intact for proper sensing - not as robust
- Film must be permeable to oxygen, but impermeable to media (i.e. water)
- Calibration is difficult - more frequent recalibration necessary due to film degradation
- Chemicals in sensing foil respond at visual wavelengths, so background light can interfere with accuracy and precision
- Very cheap components needed
- No need for an amplifier circuit!
- Commercial probes are very, very expensive - $400 and up as well, but based on the design components, could it be made for cheap?
Although the optode has several drawbacks that make it impractical for some uses, there are enough benefits in its simplistic design to make it a potential probe that can handle many situations for $20 or less!
Background & plans to overcome technical hurdles
Flourescent Film Material
The sensor film material must not only fluoresce when exposed to light, but it must also fluoresce at different intensity levels depending on the amount of oxygen in contact with the film. Thankfully, some really smart chemists have already thought of a material to do this...get ready...its a mouthful! The chemical is formally named Tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II). Since this is such a mouthful, we'll be referring to this chemical in the rest of the document as Ru(dppf) which is chemistry-speak for the above name.
Because ruthenium is a metal missing 2 valence electrons (that's the II in the full name for Ru(dppf)), you can only purchase this chemical in one of many complexes. A complex in chemistry is when ions of an opposite charge are associated to an already charged, but unstable molecule. This creates a bond that stabilizes the unstable molecule making it safe and/or easy to handle. Typically, the complexing agent doesn't change the properties of the molecule its bonded to - it just holds on for the ride.
For our application, choice of a complexing molecule isn't very important, so we found the cheapest complex of Ru(dppf) that we could, and ordered some! It happens to be the Ru(dppf) complex (shown to the right).
Our particular part is product number 76886 from Sigma-Aldrich, which at the time of this writing was selling for $59/mg. Yes. You read that correctly - per milligram! If this material is so expensive? How do we expect to make a dO sensor for cheap? Thankfully, we don't need much of it...1mg was enough for us to make 5-6 films to test, thus bringing the price of this chemical to only $10 - $12 per film. Not too bad when you consider a commercial probe costs in the $400 range!
Detecting Film Fluorescence
Now that we have identified a film material which we know will change color when exposed to varying oxygen levels, how will we be able to detect its change in color? Thankfully, the optical properties of Ru(dppf) are very well known and the manufacturer is very happy to provide technical information on their data sheet:
The graph to the left is showing us two very valuable pieces of information. The curve near the left part of the graph is the absorbance curve. The higher this curve travels on the y-axis, the more light is absorbed by the Ru(dppf). When Ru(dppf) absorbs light, it immediately re-radiates it at a different color. The color that the Ru(dppf) emits is given in the right hand part of the graph known as the emission curve. Note that it does not emit a single wavelength of light - instead it is a range starting at ~550nm, and extending into the infrared spectrum.
What does it all mean? What this graph means is that if we can illuminate our Ru(dppf) film with a bright light which has a wavelength near 455nm, our film will respond by emitting quite a bit of light ranging from 550nm (orange) - infrared. Cool! If we take a quick peek into our electronics catalogue, we quickly find several high intensity blue LEDs with 470nm wavelengths for $1.40/each. Additionally, because infrared is such a popular frequency for use in remote controls, it is very simple to find phototransistors that are very sensitive to light near 940nm for only $0.35!
A phototransistor varies the amount of current flowing through the collector/emitter junction based on the amount of light impacting on the phototransistor's light sensing surface. The amount of current produced can very simply, and reliably be read by a using a voltage divider circuit plugged into one of our Arduino's A-D converters. Additionally, if we take a quick peek at the datasheet for the mentioned phototransistor, we can see that the listed phototransistor is extremely insensitive to light emitting at wavelengths lower than 500nm. Therefore, with a proper LED mount design, it may be possible to detect the orange light given off by the Ru(dppf) film without having to implement an optical filter...neato!
Film Permeability to Atmosphere
Now that a material has been selected to
In attempting to build a light tight structure, its important to consider where light could potentially leak into your device. It will help in driving performance requirements for the housing. For the optode, light can enter the structure from 3 main points. Remember, for the optode to work, light in the visible spectrum must be blocked!
- If the optode film is clear, ambient light from the room in which you are measuring