Polyphenols play a key role in biological processes thanks to their antioxidant properties. One of the most widespread compounds within this group is chlorogenic acid, which is found in coffee, sunflower seeds, blueberry leaves, and chicory. It reduces the oxidation of low-density lipoproteins, thus reducing the risk of cardiovascular disease. However, an excess of chlorogenic acid in the production of, say, plant-based protein may lead to undesirable coloration of the end product. For this reason, it is highly important to monitor the presence of this acid during production.

Researchers from ITMO have developed high-precision sensors capable of detecting chlorogenic acid in amounts from 10 to 250 micromoles with an accuracy up to 99%. The technology takes the form of silicon substrates covered with a film of gold nanoparticles. The sensors work in tandem with Raman spectroscopy – an analytical method that can identify the structure of a substance and its amount with high precision by analyzing the scattering of a laser beam. As a result, scientists can produce unique “fingerprints” of molecules which, just like in forensic science, may be used to identify specific substances.

A close-up of the substrate coated in gold nanoparticles. Photo by Dmitry Grigoryev / ITMO.NEWS

A close-up of the substrate coated in gold nanoparticles. Photo by Dmitry Grigoryev / ITMO.NEWS

The gold nanoparticles are applied onto the substrates in several stages. First, nanoparticles between 14 and 99 nanometers in size are synthesized in a solution. Next, they are combined like building blocks with the help of a special molecule and then arranged on the border of water and an organic solvent, thus producing an even, super-thin film just a few dozen nanometers thick. The film is transferred onto the substrate using the water transfer method: the substrate is carefully dipped into the solution and the film sticks to the surface in an even layer. The production method is shown in more detail in this documentary film (link in Russian).

“Raman spectroscopy allows us to quickly conduct tests on both stationary and portable devices. Using this method, we can produce a ‘fingerprint’ that allows us to accurately assess the structure of a substance. To do this, we aim a laser beam at the sensor substrate and analyze its scattering upon contact with the sensor’s surface. The spectrometer registers scattered light and we end up with an individual set of spectral lines that’s unique to this specific substance. With our device, chlorogenic acid is detected with an almost 100% accuracy; in addition, we can identify its ratio within the examined material with an accuracy up to 90%,” explains Evgeny Smirnov, DSc, the curator of the research team and a researcher at ITMO’s Infochemistry Scientific Center.

Today, the scientists process the results of Raman spectroscopy manually. They compare the properties of chlorogenic acid extracted from natural raw material with the “fingerprint” of chemically pure chlorogenic acid. In the future, they plan to automate the process, teaching algorithms and software to independently recognize – and gauge the amount of – specific substances within mixtures based on spectral lines.

The sensors may be used in agriculture to select the raw material of highest quality, such as coffee beans, from which chlorogenic acid may then be extracted. The technology can also be used in production to track the desired level of the acid within the end product and to prevent discoloration. This is applicable for medical products for diabetes and cardiovascular disease, wine, or plant-based protein.

“Our main goal is to make the process of detecting polyphenols accessible to manufacturers of every size. To do this, we strive to reduce the cost of substrates as much as possible without affecting their effectiveness. Right now, we’re looking for a suitable substitute for silicon, as the material’s cost ends up being higher than the actual gold nanoparticles, even when accounting for associated workload. We’re also conducting experiments with silver nanoparticles,” says Arina Pavlova, one of the authors of the study and a Bachelor’s student at the Infochemistry Scientific Center.

Arina Pavlova. Photo by Dmitry Grigoryev / ITMO.NEWS

Arina Pavlova. Photo by Dmitry Grigoryev / ITMO.NEWS

The project was conducted with financial support from the Russian Science Foundation’s grant No. 22-73-00206.