Inkjet-printed pH-sensitive QR code labels for real-time food freshness monitoring

In this work, dynamic pH-sensitive quick response code (QR code) labels have been successfully fabricated by inkjet-printing technology. Inks with roselle anthocyanins and curcumin natural dye are prepared, respectively. The rheological property, contact angle, and surface tension of the prepared inks are tested. Both inks show good printability with Newtonian fluid property. The QR code labels with different ratios of inks are fabricated, in which the ink ratio is controlled by automatic computer program. It can be seen that the color of all printed QR code labels is sensitive to the concentrations of ammonia solutions, demonstrating that the pH-sensitive QR code labels are achieved. The ammonia-dependent color difference is analyzed by L*a*b* digital images. It can be found that the color difference strongly depends on the ink ratio, which suggests that the QR code labels with different ink ratios can be applied for diverse food freshness monitoring in real time. Therefore, in future, combined with the QR code application program (QR code APP) of intelligent devices, the developed QR code labels can be used as data carrier and freshness sensor as well, which provides a convenient method to get comprehensive food information, including freshness, expected storage time, date and identification of product, etc.