A colorimetric paper-based smart label soaked with a deep-eutectic solvent for the detection of malondialdehyde

Malondialdehyde (MDA) is a mutagenic secondary product of the peroxidation of polyunsaturated fatty acids, often used as a target for the food oxidation state, whose determination is frequently performed by its slow reaction with thiobarbituric acid (TBA), dissolved in acidified aqueous-ethanol medium, to produce a pink colored MDA-TBA adduct. In this investigation, we have verified that this reaction also occurs in comparable times in a deep eutectic solvent (DES) such as choline chloride-malonic acid (ChCl-MA) displaying intrinsic acid properties. This finding has allowed us to set up a MDA detection procedure suitable for simply assembling colorimetric smart labels of possible use in packaged food. It is based on the use of paper discs soaked with a controlled concentration of TBA dissolved in the mentioned DES, which are exposed to the vapors released from controlled amounts of analyzed samples, whose temperature is of course a constraint for the formation of the pink colored adduct. It profits from the negligible volatility of DESs which makes them ideal candidates to replace volatile organic and aqueous media in colorimetric sensing and, moreover, it allows any preliminary sample extraction to be avoided. In order to avoid only rough MDA determinations by color comparison with standard discs, also a device allowing MDA quantification by smartphone-based reflectance measurements was assembled. The application of this procedure to real samples consisting of fried seed oil gave results in good agreement with values previously reported in literature for similar samples. Finally, it deserves to be underlined that this approach could be easily applied to the colorimetric sensing of other several analytes.