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ABSTRACT Covert patterns have an extra layer of security protection for anti‐counterfeiting labels as compared with the traditional overt ones. To increase the complexity and security of quick response (QR) codes, it would be valuable to make covert QR codes that will be only scannable after a certain decoding process. In this work, the use of surface‐enhanced Raman spectroscopy (SERS) is explored to fabricate covert QR codes. Through developing new Raman‐active security inks, we can prepare covert QR codes using a convenient inkjet printing method. These QR codes will not be revealed directly. They can only be decoded using a confocal Raman microscope. In addition, multiplex QR codes can be accomplished using multiple Raman probes in printing. Our results showed that the printed QR codes are covert, have strong SERS signals, and can be easily recognized after the SERS decoding. It is anticipated that there is great potential for using such covert and multiplexed SERS‐based QR codes for advanced anti‐counterfeiting applications. 

Anticounterfeiting plays an essential role in authenticating genuine documents and combating forged products. To further advance the anticounterfeiting technology, there is a strong demand to design new functional materials with unique properties that will be appropriate for making multimode complex security labels. Recently, dynamic security labels have emerged as a new type of advanced anticounterfeiting method as they can hold a much higher security level than the traditional static ones. In this work, we report that calcium zinc germanate (CZGO) clinopyroxenes doped with lead ions have several interesting optical properties, such as dynamic fluorescence, long persistent luminescence, and photochromism. We find that the concentration of lead dopants can significantly impact the reaction kinetics as well as the crystallinity and luminescence properties of CZGO phosphors. By fully utilizing these unique properties, we have successfully fabricated several security labels with multilevel information encoding and dynamic optical performance. The combination of multimode and dynamic luminescence makes these labels extremely challenging to illegally duplicate. With further optimization, this lead-doped CZGO clinopyroxene can be well-integrated into modern anticounterfeiting techniques that will generate highly secure anticounterfeiting labels to combat fake products.