Integration of rhodamine-based compounds into 3D-Printed structures for selective formaldehyde detection and food spoilage monitoring

Formaldehyde (FA) is a widely used yet highly toxic volatile organic compound that poses significant health hazards, necessitating the development of rapid, sensitive, and selective detection platforms. Here, we report a novel colorimetric sensor that integrates a rhodamine-based compound (RhB-HZ) into 3D printed structures for the visual and optical detection of FA. A spirolactam-structured rhodamine derivative was synthesized and embedded within a photocurable PEGDA/HEMA resin, allowing direct fabrication of sensor discs and optical probes using digital light processing 3D printing. Upon exposure to FA, the probe undergoes a selective ring-opening reaction, yielding a vivid pink color and reduced transmission at 562 nm, with high specificity and a linear response across relevant concentrations and potential interferents. The lower detection limit (LOD) of RhB-HZ for FA detection is 0.0261 ppm, below the World Health Organization (WHO) limits. Furthermore, smartphone-based readout enabled portable, cost-effective detection through image analysis of transmitted laser light. This work presents the first demonstration of a 3D-printed sensor specifically designed for trace-level formaldehyde detection related to food spoilage, an unexplored approach in the field. The discs visually indicate freshness in meats like chicken, sardines, and prawns inside sealed packages, offering a scalable, customizable, and adaptable platform for FA and other analyte detection in smart food monitoring. Further, a colorimetric INHIBIT logic gate was constructed using RhB-HZ, enabling binary detection of formaldehyde and sulfide through input-dependent absorbance changes at 562 nm.