A fast and sensitive method to monitor hydrogen sulfide (H₂S) in ambient air based on a visible color change of a printed disposable sensor has been developed. As gas-sensitive material, an immobilized copper(II) complex of the azo dye 1-(2-pyridylazo)-2-naphtol (H-PAN) was synthesized and prepared in an ethyl cellulose matrix for screen printing. If H₂S is present in ambient air, the gas sensitive layer changes its color from purple to yellow. A pre-primed polyethylene (PE) foil and a coated offset paper served as the printing substrate. The colorimetric response to the target gas was measured by UV/Vis spectroscopy in reflection at H₂S concentrations between 1 to 20 ppm. Possible cross-sensitivities of the printed sensors towards methane (CH₄), formaldehyde (CH₂O), carbon monoxide (CO), ammonia (NH₃), and nitrogen dioxide (NO₂), as well as the long-term stability was investigated. Furthermore, reflection measurements of the Cu-PAN complex on an amorphous silica powder under gas admission served as preliminary test for the subsequent paste development.

Color consistency when reading colorimetric sensors is a key factor for this technology. Here, we demonstrate how the usage of machine-readable patterns, like QR codes, can be used to solve the problem. We present our approach of using back-compatible color QR codes as colorimetric sensors, which are common QR codes that also embed a set of hundreds of color references as well as colorimetric indicators. The method allows locating the colorimetric sensor within the captured scene and to perform automated color correction to ensure color consistency regardless of the hardware used. To demonstrate it, a $C{O}_2$-sensitive colorimetric indicator was printed on top of a paper-based substrate using screen printing. This indicator was formulated for Modified Atmosphere Packaging (MAP) applications. To verify the method, the sensors were exposed to several environmental conditions (both in gas composition and light conditions). And, images were captured with an 8M pixel digital camera sensor, similar to those used in smartphones. Our results show that the sensors have a relative error of 9% when exposed with a $C{O}_2$ concentration of 20%. This is a good result for low-cost disposable sensors that are not intended for permanent use. However, as soon as light conditions change (2500–6500 K), this error increases up to ${\mathsf{ϵ}}_{\mathbf{20}}$ = 440% (rel. error at 20% $C{O}_2$ concentration) rendering the sensors unusable. Within this work, we demonstrate that our color QR codes can reduce the relative error to ${\mathsf{ϵ}}_{\mathbf{20}}$ = 14%. Furthermore, we show that the most common color correction, white balance, is not sufficient to address the color consistency issue, resulting in a relative error of ${\mathsf{ϵ}}_{\mathbf{20}}$ = 90%.

We present a fast method to monitor hydrogen sulfide (H₂S) in ambient air based on a visible color change. Therefore, an immobilized copper(II) complex of the azo dye 1-(2-pyridylazo)-2-naphtol (H-PAN) was synthesized and prepared in a matrix for screen printing. Different materials, reaching from opaque paper to transparent foils served as substrate. The reaction of the copper(II) complex (Cu-PAN) to the target gas H₂S was measured in reflection via UV/VIS spectroscopy.

We present a simple method to monitor the oxygen (O2) concentration within food packages based on a UV resettable colorimetric indicator. The basis is a material combination of the redox color dye methylene blue (MB) with a titanium dioxide (TiO2) nanopowder and glycerol. The work comprises different paste approaches suitable for screen printing and discusses the influencing factors to achieve the desired sensor properties based on investigations in an application-related scenario.

We present our investigation on the gasochromic reaction of E141 (ii) towards the toxic gas nitrogen dioxide (NO2). E141 (ii) is a chlorophyllin-based food additive, typically used as green coloring for nearly all kinds of sweets. In this presentation we show an alternative approach for using E141 (ii) as optical gas indicator. All solid samples are prepared by multi-layer screen printing on different substrates like paper and PE-foil. Gas measurements are performed using an UV/Vis spectrometer. The influence of the substrate and according layer thickness is shown.