Science Spin March 2007

Using light diodes to detect poisons

Imagine having a sensor that can monitor your home water supply to ensure that no harmful chemicals, such as rat poison, comes through the tap. As an undergraduate physics student at DCU, this is my aim. Under the Hamilton Scholarship Programme, I am using light emitting diodes (LEDs) to build such a sensor at the National Centre for Sensor Research.

Chemicals in water absorb light, and as this is a selective process it can be used for identification of substances. The absorption pattern is almost like a characteristic fingerprint, and it can be observed using a technique called photometric detection. The technique requires a source of light of known wavelength, and this is where LEDs come into play. The light emitting diodes are small, similar to Christmas tree lights, and apart from being small, they are cheap.

Detecting poison

Suppose an evil super-villain spikes a city's water supply by dissolving a colourless poisonous chemical in the reservoir. In order to save the city, we would need to detect the poison immediately and know exactly how much was present, so we take action.The photometric detection technique can be used to detect such a poison. The technique relies on three key properties, wavelength, intensity, and absorbance.

The wavelength of a light source can be thought of as its colour. For example, violet light has a wavelength of roughly 400nm (10-9metres). Red light, has a wavelength of about 600nm. In fact, as you move from the top of the rainbow to the bottom (from red to violet), you get light of shorter and shorter wavelengths. The intensity is simply how much light is coming from our source (in our case, an LED). The absorbance refers to the liquid we are testing (in our case, the poisoned water).

Every chemical absorbs light of a certain wavelength. For example, Chromate, which causes cancer, absorbs light at about 254nm, which is ultra-violet light. So when we shine an ultra-violet LED through a chromate sample, all the light should be absorbed and none should get through to the other side. But let's say the chromate was dissolved in water. Water itself absorbs practically no light, so, if pure, the UV would shine right through.

However, the dissolved chromate absorbs light, so much less will get through. Comparing the intensity entering to the intensity of light leaving the sample gives us a good measure of chromate. Applying a formula, called the Beer-Lambert Law, we can calculate how much of a particular wavelength has been absorbed, and this is how we can test for the presence of chemicals, such as chromate.
In photometric detection, we can be dealing with water samples which have hundreds of chemicals dissolved.

Yet, detection remains effective because each chemical has its own unique absorbance fingerprint. Instruments, called spectrophotometers, use lights of all different wavelengths so that pretty much any chemical can be detected. If we put our water sample, which we suspect contains poison, into a spectrophotometer, the wavelength absorbed will confirm our suspicions. By using the Beer-Lambert formula we can see just how much light is being absorbed, and if the number is high, we would know there is a lot of poison in the water.

The spectrophotometer is a fairly expensive piece of laboratory equipment, but in the not too distant future, these devices will be smaller and cheaper. You may yet be able to buy these sensors, fit them onto your kitchen tap and measure the amount of poison in your glass of water!