OD/F Device

Measuring Optical Density (OD) is a central element in microbiological work and synthetic biology. One question that has to be answered often is how many cells are in a suspension. Here, the OD can give you a hint. Unfortunately, commercially available OD meters cost several hundred dollars ([http://www.laboratory-equipment.com/laboratory-equipment/cell-density-meter.php OD meter]), and can limit the spread of synthetic biology.

Therefore, we wanted to devenlop an alternative for measuring OD, specifically designed for Biohackspaces, DIY and community laboratories and schools. With our OD/F device, we want to enable many people to do good, precise and inexpensive science research.

Especially for the Interlab Study fluorescence, too, has been of importance. One aim of this study was to measure the correlation between OD and fluorescence. Since the taks of measuring OD and fluorescence are often performed at the same time, we want to present a device that can measure both fluorescence and OD with just some easy adjustments. This way, we can measure how much fluorescence there is per amount of cells.

In fact, you can find some DIY posts for turbidity meters such as [http://www.thingiverse.com/thing:74415 turbidity sensors]. However, a proper assessment of their linearity as well as a calculated OD-value are missing.

Regarding fluorescence, we are of course not re-inventing the wheel (well, not totally). The 2010 iGEM Cambridge team actually built a very similar device, the E.glometer. However, there's no data available showing an actual comparison of the data from their device and some proven commercial system to, for example, assess linearity of the measurement.

Measuring Principle

The measuring principle for both optical density (OD) and fluorescence measurement is depicted below. For OD measurement we shine through the sample with an LED and a fixed width. A filter blocks any other light but 600 nm. This way, the sensor mainly senses the 600 nm light which is needed for OD₆₀₀ measurement.

For fluorescence measurement a similar approach is chosen. The filter again is used to block the exciting light from being sensed. That way only the emitted light from the fluorescence protein is measured.

Measuring principle for OD/F device The left image shows the measurement approach for the optical density. The light shines through the sample with a fixed width. The right image shows the fluorescence measurement approach, exciting the fluorescence proteins from below and measuring from the side.

Our OD/F Device

Further details about selecting filters, code, a construction manual and evaluation can be found here.

Application

Using the presented device is easy, and it works just like any other commerically available spectrophotometer. First, you have to take a blank reference, the pure medium without cells. After placing the cuvette in the device, press the red button to take the blank and take out your blank cuvette again. Subsequently, take all measurements you would like to compare to your medium. To do this, fill a cuvette with sample, put it into the device. The measured value will be displayed. Do not press the red button again until you want to take another blank/reference.

How to use our OD/F device

Achievements

When building the OD/F Device, our goal was to develop a system that

• is straightforward to use
• is at least as accurate as commercially available systems but at the same time costs significantly less
• produces stable, reproducible results
• weights little and is easy to carry around
• uses widely available parts and is easy to build
• can measured both optical density and fluorescence

Commercially available equipment uses lasers and a set of two fine filters, one between laser and sample and one between sample and sensor. To beat down the price, our OD/F Device uses a simpler measuring principle: It has just one rather low-quality filter, between sample and sensor, and illuminates with an LED instead of a laser. However, the LED is not as accurate as a laser and has a non-optimal spectrum. On top of that, due to the missing filter between laser and sample, stray light might influence the measurement. Nevertheless, one main goal was to produce an inexpensive device. Given that we therefore had to compromise some of the measurement quality, were we still able to produce stable, precise and good data?

The answer is: Yes! With the optimal design of our cuvette holder we achieved good-quality results albeit using the cheap filter.

[graphs showing the awesomeness of our OD/F device]

On top of that, we also achieved all other goals we set in the beginning: The device works with one button, the blank button, only and is thus really straightforward to use. With a cost of just $60, the OD/F beats any commercially available system by far. Due to the use of light plexiglass for the casing and the compact design, one unit weighs less than 500 g and can be easily connected to any power adapter via USB. We also managed to build the device with common, inexpensive and easily available parts, and published all technical details and a construction manual on our wiki. At least, though we actually built two separate devices for OD and fluorescence measurement, both work really well and putting the two units into one case would just require to laser cut a new top.