When it comes to airline safety, one technical detail most people don’t know about is the 5-ppm limit of biodiesel fuel contamination in jet fuel. This industry standard is needed because the fatty acid methyl esters derived from animal fat or vegetable oil that make up biodiesel have a higher freezing point than the alkanes in petroleum-derived fuels. Because different types of fuels are transported and stored with the same equipment, some biodiesel can end up in jet fuel and increase the viscosity of the fuel at the cold temperatures of high altitude, affecting engine performance or even causing an engine to stall. Current analytical techniques to test fuel quality often require costly and time-consuming off-site lab tests. Jonathan K. Fong and Zi-Ling (Ben) Xue at the University of Tennessee, Knoxville, have now devised a disposable optical sensor to signal unsafe biodiesel levels. The researchers dissolved the dye Nile blue chloride in methanol and then incorporated the mixture into an ethyl cellulose film made in ethanol. As the biodiesel methyl esters permeate the film, they displace the more polar alcohols, with the solvation difference causing the sensor to change color from blue to pink. The sensors the Tennessee team prepared detect biodiesel to 0.5 ppm concentration (Chem. Commun. 2013, DOI: 10.1039/c3cc43958e). By developing calibration curves and using a visible spectrometer, the fuel sensor could also be used by fuel distributors to verify the fraction of biodiesel blended with diesel, up to 200,000 ppm, or 20%. The University of Tennessee is patenting the sensor, Xue told C&EN.