QR-Code-Integrated Colorimetric Sensors for Simultaneous Temperature Threshold and Oxygen Detection

Sensor-integrated Quick Response (QR) codes are a promising route for combining product traceability with sensing capabilities. However, most reported platforms rely on binary read/no-read mechanisms in which sensor activation affects QR readability. To overcome this, we propose a QR architecture that preserves the information-storage and decoding functions of the code while enabling simultaneous environmental monitoring. The approach exploits the intrinsic error-correction capability of QR codes by replacing specific modules of a QR code (with error-correction level H) with colorimetric sensing elements, thereby decoupling sensor response from QR readability. A thermochromic polydimethylsiloxane (PDMS) composite and a photoactivatable ZnO-alginate hydrogel containing methylene blue (MB) were integrated into the QR structure to simultaneously detect a temperature threshold and oxygen presence while maintaining decoding functionality. Both sensors exhibit red-channel responses centered at 610-660 nm, enabling grayscale-based image analysis. A smartphone-based workflow combining red-channel extraction, contrast correction and binarization decodes the embedded identifiers and detects sensor activation through module-level comparison. In a smart-packaging demonstration, the platform successfully identified different combinations of refrigeration and vacuum-sealing states. Furthermore, sensor layouts can be uniquely linked to the encoded identifier, providing additional tamper resistance, traceability, and product authentication. The proposed strategy establishes a versatile and low-cost platform for multiparameter monitoring within existing QR-based infrastructures.