Target-Induced Stimuli-Responsive Liquid Crystal for Colorimetric Detection of Aflatoxin B1 via MnO 2 Nanosheet- Mediated Catalysis

Food safety is a constant battle against invisible threats, and among the most dangerous are aflatoxins. These are toxic substances produced by certain molds that grow on crops like grains and nuts. One specific type, known as aflatoxin B1, is particularly hazardous because it can cause severe liver damage and is a known cause of cancer. Because these toxins are heat-stable, they survive cooking and processing, making them a persistent risk in the global food supply. Detecting them quickly and accurately is essential, but traditional methods often require expensive laboratory equipment and highly trained scientists, which makes them impractical for checking food in the field or in small markets. There is a growing need for a simple, fast, and inexpensive way to spot these toxins before they reach consumers.

Researchers have been exploring new ways to build sensors that do not require complex chemical labels or elaborate preparation steps. One promising approach involves using tiny droplets of liquid crystal, a state of matter that sits between a liquid and a solid and is sensitive to changes at its surface. By combining these droplets with a material that mimics the behavior of natural enzymes, scientists can create a system that changes color in the presence of a specific target. This new study introduces a method to detect aflatoxin B1 using such a system, relying on a chain reaction that starts with the toxin itself and ends with a visible color change that can be seen with the naked eye.

The team, led by researchers from Hunan University of Chinese Medicine and Hunan Agricultural University, designed a sensor that works like a molecular switch. At the heart of the system are microscopic droplets of liquid crystal, which act as tiny containers. Inside these containers, the researchers placed a substance called glutathione, a molecule that can break down other chemicals. These droplets are stabilized by a layer of surfactant, a type of soap-like molecule that keeps the liquid crystal from mixing with the water around it. To make the sensor work, the researchers also introduced a specific DNA strand, known as an aptamer, which is programmed to grab onto aflatoxin B1 with high precision. They also added a second molecule, a positively charged surfactant, which acts as the trigger for the system.

In the absence of the toxin, the DNA strand holds onto the positively charged surfactant so tightly that the surfactant cannot reach the liquid crystal droplets. Because the trigger is locked away, the liquid crystal droplets remain intact, keeping the glutathione safely sealed inside. The researchers then added a chemical that changes color when it is oxidized, along with a material called manganese dioxide nanosheets. These nanosheets act like a catalyst, speeding up the reaction that turns the colorless chemical into a bright yellow one. Since the glutathione is trapped inside the droplets and cannot interfere, the nanosheets work freely, and the solution turns a deep yellow.

However, the story changes when aflatoxin B1 is present. The toxin has a stronger attraction to the DNA strand than the surfactant does. When the toxin is introduced, it binds to the DNA, forcing the DNA to let go of the surfactant. Once released, the surfactant rushes to the surface of the liquid crystal droplets. This sudden arrival of the surfactant disrupts the delicate balance at the droplet's surface, causing the droplet to eject its contents. The glutathione, now free in the water, attacks the manganese dioxide nanosheets and breaks them down. Without these nanosheets, the chemical reaction that produces the yellow color cannot happen. The result is a solution that remains clear or colorless. The difference between a bright yellow solution and a clear one provides a simple visual signal: yellow means the food is safe, while clear means the dangerous toxin is present.

The researchers tested this system under various conditions to ensure it worked reliably. They found that the reaction happened quickly, reaching a stable state within ten minutes at a temperature of 35 degrees Celsius. They determined that the sensor could detect aflatoxin B1 in concentrations as low as 0.73 nanograms per milliliter, which is an extremely small amount. The system showed a linear response, meaning the color became progressively lighter as the amount of toxin increased, allowing for accurate measurement within a specific range. Crucially, the sensor was highly specific. When the researchers tested it against other common toxins found in food, such as those that cause mold in corn or wheat, the sensor did not react. It only changed color when the specific target, aflatoxin B1, was present, even when other toxins were mixed in at much higher concentrations.

To prove the method works in the real world, the team tested it on actual food samples, including sunflower seeds, soy milk powder, and milk. They added known amounts of the toxin to these samples and measured how well the sensor could find it. The results showed that the sensor could recover between 92% and 108% of the added toxin, indicating high accuracy. They also compared their results with a standard laboratory technique called high-performance liquid chromatography, which is considered the gold standard for this type of analysis. The sensor's results matched the laboratory measurements almost perfectly, confirming that this simple, visual method is just as reliable as the complex equipment used in professional labs.

This work demonstrates that it is possible to create a powerful detection tool without the need for complex labeling or expensive machinery. By using the natural properties of liquid crystals and the specific binding ability of DNA, the researchers have built a system that translates a microscopic molecular event into a clear, visible signal. The method is fast, cost-effective, and easy to use, offering a practical solution for monitoring food safety in places where advanced laboratories are not available. It represents a significant step toward making the detection of dangerous food contaminants accessible to everyone, ensuring that the food on our tables is safe to eat.