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New Optoplasmonic Sensor for Quickly Testing Milk and Other Liquids for Quality

Food safety and food quality standards have never been higher in Germany and across the European Union (EU). This is particularly true in the dairy sector. However, in spite of such high standards, traces of pesticides, impurities, and antibiotics can enter milk, and at times can lead to grave consequences for consumer health.

Chip with integrated light source and light detector for analyzing contaminants in milk. Image credit: © Fraunhofer FEP.

In the EU-sponsored project MOLOKO, Fraunhofer scientists have collaborated with partners to design a new optoplasmonic sensor to provide quick, on-site analysis of quality and safety parameters for milk. This early-warning system will offer the industry considerable savings in terms of money and time, as well as a significant reduction in wasted product, helping to enhance performance along the whole supply chain.

Food safety is a crucial factor in the food industry, and more so in the dairy industry. Here, udder infections can result in dangerous organisms entering the milk, and chemical substances such as pesticides or antibiotics can pollute the product via fodder, or due to poor control of equipment and storage facilities. To prevent the entry of adulterated milk into the food chain, checks are carried out during the complete production process and supply chain.

However, these conventional tests are time-consuming and costly. Samples are procured from milk tankers containing a blend of produce collected from any number of dairy farms and then analyzed in the lab. If the milk shows evidence of contamination, the whole load must be destroyed, with high losses for all of the dairies and farmers involved. If there were a test wherein farmers could test their own milk before it is collected by the tanker, such wastage can be prevented.

Quality Check Delivers Results in Five Minutes

In the project MOLOKO (Multiplex phOtonic sensor for pLasmonic-based Online detection of contaminants in milK), 12 collaborators from seven countries—including one dairy—have come up with a quick and inexpensive test for detecting quality factors in milk.

In a test that takes about 5 minutes, the product is analyzed by a new optoplasmonic sensor for a total of six substances, thereby providing an add-on check and an early-warning system within the supply chain, well before the milk is conveyed into a tanker.

The sensor is functionalized with receptors for certain antibodies that act as indicators of many safety and quality parameters for milk. It, thus, permits dairy farms to perform automated, on-site quantitative tests.

Unique Integrated Sensor Architecture

The complete system consists of a sensor containing organic photodetectors (OPDs), a reusable microfluidic chip, a nanostructured plasmonic grating, organic light-emitting transistors (OLETs) or diodes (OLEDs), and the specific antibodies. The organic photodetector is being designed at the Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP, and the microfluidic chip at the Fraunhofer Institute for Electronic Nano Systems (ENAS).

The OLET, meanwhile, is being designed by CNR-ISMN in Bologna, and the photonic grating by the company Plasmore Srl in Pavia, both in Italy. The project’s coordinator is CNR-ISMN.

The unique thing about our chip is that it can be reused. The target molecules are stripped from the immobilized antibodies by a regenerating buffer. This means that the antibodies can be reused for further tests.

Andreas Morschhauser, Researcher, Fraunhofer ENAS

Actually, the chip’s estimated service life is 100 test cycles. In each test, a total of six parameters relating to pollutants and proteins are evaluated. In order to achieve this, Morschhauser and his colleagues have created a micro-fluidic system in the form of an automated, miniaturized cartridge that is replaceable.

Besides supplying information on milk quality and safety, the measured parameters also inform farmers about each cow’s health and condition. This helps them to detect infections at an early stage and start treatment straight away. Timely treatment can result in a more judicious administration of antibiotics and thus a decrease in their use.

A Nanostructured Grating for Surface Plasmon Resonance

But how does the test work?

Light emitted by the transistor falls onto a grating coated with antibodies specific to the various substances being tested for. When milk is flushed over the grating, any target molecules in the milk then bond with the antibodies. This alters the refractive index in the immediate vicinity of the grating, which in turn modifies how this light is reflected. The reflected light is registered by the photodetector, which measures minimal changes in the refractive index.

Dr Michael Törker, Researcher, Fraunhofer FEP

This common phenomenon called surface plasmon resonance occurs on exclusively structured nanogratings. It delivers fast and extremely sensitive readings.

The objective is to employ this biosensor at several points along the value chain—both as a lab device and directly fitted into dairy equipment. Furthermore, it will also be ideal for testing the quality of liquids other than milk, such as water or beer.

The only modification needed is a change to the immobilized capture molecules and to the necessary reaction buffer. This would just involve substituting the capture molecules with ones that are ideally modified for the required purpose.

Preliminary results from the creation of the optoplasmonic chip will be demonstrated at CES 2020 in Las Vegas between January 7th and 10th, 2020 (Sands Expo Center, OE-A booth, no. 40950).

The project was funded by the European Union’s Horizon 2020 research and innovation program under grant agreement No 780839 (MOLOKO).

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