We’ve all been in this situation: you have an unopened package of fresh fish in the fridge that has just past its expiry date. However, you wonder whether you can still eat it, as often these dates include a certain safety margin. So what do you do? You open the package and give it a sniff. This can be a feasible solution at home, but this technique is not suited for stores or distribution centers since opening the package destroys the preservative atmospheric environment. However, the lack of an individual testing method results in the waste of large amounts of food that are in fact still consumable. In the Terafood project (https://terafood.iemn.fr/), we investigate the development of a sensor to measure food spoilage of each individual package, while still keeping the package unopened and untarnished. The sensor is based on the so-called photo-acoustic technique, where a sensor detects sound that is generated through molecules (volatile organic compounds, VOCs) that are indicative of food spoilage (more info on these VOCs is available in first Terafood newsflash1).
In order to do so, small, cheap, power-efficient sensors are needed that can be placed inside food packages such as a microchip. In the last couple of decades, the technological developments in electronics are unparalleled, think of a smart phone that is equipped with a large number of electronic sensors. Compared to electronic chips, silicon photonic chips use these same fabrication technologies, but they manipulate light instead of electricity. Originally the main application of silicon photonic chips was in telecom and data centers to communicate over optical fibers. However, also sensing applications grow rapidly in photonics, for example for monitoring blood sugar in-situ, detecting sexually transmitted infections, identifying different chemical compounds, measuring temperature, strain, proximity, …
The world of electronics and photonics both operate with electromagnetic waves, although using very different frequencies of these, which has consequences in how they are generated and detected. The world of electronics uses radio signals, which means that they work with antennas and slow moving electric fields. In photonics however, we enter the world of lasers and photodetectors (cameras). In between, we find the terahertz frequencies (0.1-10 THz), which can be seen either as light with a very low energy, or as radio waves with a very high frequency. It is not straightforward to generate and detect these waves, typically cooled sources (and detectors) down operating at a few Kelvins are used.
So why the interest in these cumbersome THz frequencies? Firstly, because a vast amount of gasses have many characteristic absorption lines in this terahertz range (the molecular fingerprint). Secondly, the packaging material typically used for food is transparent to terahertz waves, meaning that we can have an external read-out system that works without opening the packaging (which will have a passive sensor inside). This is also true for e.g. HDPE which is dark black to our naked eye, but transparent to terahertz waves. Moreover, we can overcome the generation issues by making it sensitive enough to operate at low power, making it compatible with cheap, but weak, electronic sources. As such, we can avoid the need for cryo-cooled detectors by transducing the signal to another form of energy, i.e. sound.
This is the photo-acoustic technique. It is a form of optical spectroscopy since it will detect a molecule based on the wavelengths of light it absorbs. But rather than measuring the amount of light not absorbed by the gas, after passing through, we allow the gas to convert the absorbed light into sound and to detect subsequently the sound.
Let’s delve a bit more into the physical mechanisms behind such a photo-acoustic sensor. When the gasses present in a food package for example absorb the terahertz wave, this energy results in a temperature increase and, since we are working with gasses, a pressure increase. When we now turn our terahertz source on and off at a specific frequency, we will get pressure waves with that same frequency. We then only need a microphone to detect this signal to successfully transduce and detect the terahertz absorption and to assess which gasses are present in the food package.
The photo-acoustic technique is already being used for very sensitive measurements. But it still requires preparation of the sample, the set-up and the work in the lab. In contrast, we are developing a sensor that is very small, can operate in-situ and will not require special, labor intensive preparation. Furthermore, the use of microchips ensures a very low cost-price and because we confine the energy to a much smaller space than usual, we get a big enhancement of the signal to allow for a small, cheap and power-efficient device. Lastly the sensor enables us to test samples without opening, and thereby destroying them, thanks to the use of terahertz waves.
We develop an on-chip sensor that is placed inside food package such that we can determine the freshness of food without opening the package.
o More information on the project can be found on the project website: https://terafood.iemn.fr/
o If you want more information on the project, do not hesitate to contact the project coordinator Mathias Vanwolleghem - firstname.lastname@example.org
o If you are interested to become a member of the advisory board of this project, you can contact Isabelle Sioen – Isabelle.Sioen@UGent.be
With support of the European Regional Development Fund