Arsenic Prototype 4.0
- 1 Principles
- 2 Materials
- 3 Step by Step Buildling
- 3.1 Laser Cutting the Inner Scaffold Pieces
- 3.2 Assembling the Electronics
- 3.3 Testing the Electronics
- 3.4 Making the Outer Housing
- 4 Links other References
The basis of the prototype remains the same - the bioreporter, a GMO bacteria expressing eGFP (Green Fluorescent Protein) is incubated with a water sample, where the fluorescence is detected optically and can be quantified in order to measure the concentration of Arsenic in the water
- A vial containing the water sample we want to test is positioned on a socket through which a fluorescent excitation LED (blue 488nm for eGFP) passes.
- GFP absorbs this blue light (λ=475 nm) and emits green light (λ=504 nm) which is detected by a photosensor on which the light is concentrated with the help of two lenses that avoid loss of intensity
- A long-pass filter allow only the signal eGFP fluorescence to reach the photosensor
Moreover, a red LED was added to measure the transmittance, which can be converted into turbidity. The measurement of turbidity will allow us to normalize our results.
- A red LED is placed in-line with the photosensor
We use an arduino to take the measurements, and we have a liquid crystal display (LCD) to read out the numbers.
With these measures, compared against a standard curve of water containing known arsenic concentrations, one can determine the concentration of arsenic in the sample and so know if the water is drinkable or not.
In summary, the most current version consists of:
lasercut chassis blue LED for eGFP fluorescence excitation at 90 degrees from detector red LED for transmittance measurements in line with detector vial holder matching the vial approved by the Swiss authorities light to frequency detector a filter to block excitation light LCD screen read out based on the arduino
You can find the bill of materials here:
Step by Step Buildling
Laser Cutting the Inner Scaffold Pieces
|Groove LED (fluo)||scan||50 (100)||35||x_unilat||0.1|
|Text and Logo||scan||250 (100)||25||x_unilat||0.3|
With these parameters, the 3mm thickness pieces take 5 min to cut, and the 2mm thickness pieces take min.
Assembling the Electronics
There are four PCBs for:
- Blue LED (for the eGFP excitation)
- LCD screen
- Red LED (for the transmittance)
- Light to frequency meter (to detect the light)
Each of the PCBs except the LCD screen PCB are fixed onto the lasercut fiber boards using M3 screws.
Soldering and Mounting
Now mounting onto the particle board:
Soldering the pieces:
light to frequency meter
- LFD 1.JPG
1. place the legs of the light to frequency detector (LFD) in the PCB without bending or soldering
The LCD screen board is ready to be mounted on the arduino.
We are now ready to test it out.
Testing the Electronics
Uploading the arduino code
Reading the measurements
First test the firmware and the hardware without sample.
- When you push the start button, does the blue LED go on?
- Do you see the measurement on the LCD screen?
- Do you then see the red LED go on?
Then try it with sample with fluorescence
- Do you see changes in the measurement?
Now we are ready for the actual bacterial samples.
Transmittance and absorbance
What we actually measure with the red LEDis the transmittance - how much of the red LED light goes through the sample and reaches the other side.
Absorbance (optical density) is then calculated.
A = - log10 T
where T = (Red LED captured by the light to frequency meter with sample / with buffers (no bacteria))
Making the Outer Housing
What we have built so far leaves the samples exposed to ambient light.
So we need to make outer housing that can mount the LCD screen, house the battery, enclose the scaffold inside, and have easy access to put the samples in and out while keeping the light and optics dark.
Links other References