After two minutes, stamps were removed. The substrates were left under humidity for 30 mins before subsequent testing to permit covalent attachment. fNAP functionality testing. Inkjet and contact printed arrays were incubated with 0.5 M sodium hydroxide as a positive test condition and deionized water as a negative test condition. Arrays were imaged prior to incubation to establish baseline intensities. Arrays were then incubated with 200 μL of test solution for 2 h and subsequently imaged again. All imaging occurred at 1 s exposure using a Nikon Ti-2 inverted microscope. The visibility of positive state fNAPs when overlaid with food involved imaging with 10 mm samples of the target food products placed on top of the arrays.