A colorimetric paper based on curcumin-loaded CD-MOFs and CNFs for visual monitoring of strawberry freshness

• An acid-responsive colorimetric indicator based on curcumin and CD-MOFs is constructed. • CD-MOFs/CNFs composite paper is successfully synthesized in ethanol medium. • The colorimetric paper exhibited high color stability at room-temperature. • The colorimetric paper was capable for monitoring the freshness of strawberries. Curcumin is a natural yellow colorant, but the color variation occurs only under alkaline conditions, limiting the monitoring range for food freshness. In this study, curcumin obtained the property of acid-responsive color transition from red to yellow by encapsulating in alkaline CD-MOFs. Then, the curcumin-loaded CD-MOFs (CD-MOFs-Cur) with red color was incorporated in cellulose nanofibrils (CNFs)/ethanol suspensions forming a colorimetric paper through the wet-forming process. Results showed that the cubic CD-MOFs-Cur was evenly distributed in the network of CNFs. With the increase amount of CD-MOFs-Cur, the paper exhibited a redder color, but all exhibited high color stability. In pH 5–10 buffer solutions, the paper showed obvious color change from red to yellow with the pH decreased, but the pH that induced the color of the paper turn to yellow was lower with the increase of CD-MOFs-Cur amount. Similar color alteration also occurred when CD-MOFs-Cur was exposed to CO 2 . Based on this, the CNFs/CD-MOFs-Cur composite paper was successfully used for visual monitoring of strawberry freshness. This work provides the possibility for curcumin to detect the freshness of foods that release acid gases during spoilage.