Angew. Chem. Int. Ed. 56, 8094–8098 (2017)

DNA barcoding allows unambiguous species identification through genetic sequencing and can serve as an analytical tool to prevent food fraud. However its routine application is hampered by strict requirements regarding sample treatment and the need for specialized equipment and handling personnel. Valentini et al. have now developed a method for rapid, naked-eye detection of counterfeit food. Similar to barcoding, their technique — called NanoTracer — is based on the identification of species-specific DNA sequences through common molecular biology methods, but the detection is a colorimetric assay that exploits the plasmonic properties of gold nanoparticles, requiring simple laboratory techniques.

The researchers use ordinary DNA extraction kits to isolate the DNA from samples of perch and saffron, two food items that are often mislabelled or contaminated. With a specific polymerase chain reaction, they amplify the DNA regions that are unique to the investigated species and generate single-stranded fragments (amplicons). For authentication, Valentini et al. amplify a DNA region from perch, whereas to detect spice contamination they select the DNA of known saffron contaminants. The generated amplicons contain, by design, not only the DNA sequence of interest but also a universal tag that can hybridize to complementary oligonucleotide sequences attached to gold nanoparticles. DNA coupling triggers nanoparticle aggregation and induces a red-to-violet colour change of the solution — a visual indication of the presence of the desired species.

The entire test, from preparation of the food matrix to the colour-change determination, takes three hours and can detect the presence of contaminants down to 1% by weight.