Fluorescent DNAs printed on paper: sensing food spoilage and ripening in the vapor phase

The monitoring of food spoilage is important to human health. Here we tested the ability of DNA-based oligodeoxyfluoroside (ODF) dyes to sense volatiles as they arise from spoilage, and employed a new platform for fluorescence sensing by inkjet printing of varied dyes on cellulose paper. ODFs are a class of short DNA-like oligomers with fluorophores replacing DNA bases. Six tetrameric ODFs of distinct sequences were printed on cotton paper using a commercial inkjet printer; printing methods and additives were evaluated for optimal fluorescence and response. ODFs were exposed via headspace volatiles in a closed container to a variety of foods as they spoiled over a period of days. We observed fluorescence intensity and/or color changes during exposure to meats (ground beef and shrimp), dairy (milk and cheese), fruit (orange juice), grain (bread), and vegetable (lettuce) sources. The printed sensors were imaged under an epifluorescence microscope and the signals were quantified using digital values from red (R), blue (B), green (G), and luma (L) channels, creating fingerprints for each headspace volatile mixture. This set of ODF sensors yielded a general fluorescence increase for protein-rich foods (ground beef, shrimp, and milk), while each food could be distinguished by the relative difference among RGB channels. Incubation temperature affected volatiles produced from spoiling ground beef, as storage at 4 degrees C yielded weaker signals. Spoilage of foods with low water activity (bread and cheese) also produced responses distinct from high-moisture foods, but similar to one another. Ripening of fruits (banana, tomato, and peach) was also tested. Monitoring the spoilage of ground beef (room temperature and 4 degrees C) and milk (room temperature) revealed that the composition of volatiles changes dynamically during the fifteen-day period, suggestive of shifts in bacterial populations.