Printed sensor labels for colorimetric detection of ammonia, formaldehyde and hydrogen sulfide from the ambient air

We present a novel method to monitor the exposure to toxic gases in ambient air based on a visible color change of a colorimetric gas sensor, which can be evaluated by the camera of a smartphone, stationary reader or even naked eye. This sensor consists of a disposable paper tag or plastic card and a gas sensitive material, which has been deposited by a screen-printing process. The integration of the gas sensitive layer into the machine-readable pattern of a QR-like code that incorporates color references for color calibration; makes it suitable for quantitative as well as illumination and camera independent evaluation of the color change due to the reaction with the target gas. This work focusses on the development of printable pastes for the detection of ammonia (NH3), formaldehyde (CH2O) and hydrogen sulfide (H2S) for the application in a colorimetric sensor label. NH3 and CH2O are detected with commercially available pH sensitive color dyes. The alkaline gas NH3 is tracked by the reaction with a pH indicator directly, while CH2O is detected by an intermediate reaction of CH2O with a primary amine through nucleophilic addition. For the detection of H2S, an immobilized copper(II) complex of the azo dye 1-(2-pyridylazo)-2-naphtol (H-PAN) is synthesized. The color change of the printed sensors labels is characterized by the evaluation of RGB values taken with an in-situ readout station using a smartphone camera.