Programmable inkjet printing of fluorometric freshness-progress-bar for consumer-level visualization of hot-pot freshness

In this work, an easy-to-use fluorometric sensor array in the form of freshness progress bar was fabricated by programmable inkjet printing, enabling consumer-level freshness visualization of Hot-pot dishes (fish, chicken and beef). Firstly, the freshness progress bar was optimized with 2 spoilage-responsive indicators (fluorescein isothiocyanate and rhodamine B) encapsulated in metal-organic framework zeolitic imidazolate framework-8 (ZIF-8). Subsequently, a 40 mm × 10 mm freshness progress bar was precisely fabricated by programmable inkjet printing, ensuring adequate uniformity and reproducibility. The freshness progress bar showed high sensitivity up to 1.343 mg/kg to the total volatile basic nitrogen (TVB-N), good reproducibility (relative standard deviation (RSD) < 5%) and long-term stability (signals remained 92.6% after 10 days of storage). With chemometric analysis, the freshness status of samples could be easily identified. Furthermore, the visual information of freshness progress bar consistent with the TVB-N stipulated by the national standard, showing consistence coefficient of R² > 0.96. Therefore, the freshness progress bar may enable consumer-level management of the Hot-pot foods, which was particularly suitable for E-commerce sales.