Technological and social development has led to numerous frauds based on information leakage or forgery, which have become increasingly rampant, seriously endangering the security of private properties, and disrupting the normal production and living order [1], [2]. Hence, advanced information security technologies need to be explored urgently to protect essential documents and securely transmit confidential information. Smart fluorescent materials that respond instantly to external stimuli have attracted considerable attention and are used in various applications including data anticounterfeiting [3], [4], encryption [5], recording [6], [7], storage [8] and interaction [9]. External-stimulus fluorescent materials, the best candidates for future information protection areas, exhibit fluorescent signals that can be additionally modulated by external stimulus sources (light [10], force [11], electricity [12], heat [13], and solvents [14]) which provides an additional defense layer, compared to the intrinsic luminescence in conventional structures. Consequently, the leakage, imitation, or forgery of confidential data can be more effectively prevented, which further enhances the security of information storage and transmission. In the past decades, numerous categories of smart fluorescent materials, such as organic dyes [8], metal–organic complexes [15], carbon quantum dots(CQDs) [16], rare-earth-doped up-conversion nanoparticles [2], [17] and inorganic quantum dots [18], [19], have been discovered, studied, and widely utilized for high-level information storage and security protection. Despite the notable achievements, almost all smart fluorescent materials exhibit nonnegligible flaws, such as low luminescence efficiency, poor response to external stimuli, complex processes, harsh preparation conditions, and high cost, which largely hinder their industrial application and progress.
In addition to the excellent optical properties such as high quantum yields, small emission peak widths, and tunable colors, perovskite nanocrystals (PNCs)/polymer composites exhibit solvent, optical, and thermal stabilities induced by encapsulation [20], [21], [22], [23], [24]. Additionally, they can be easily processed into fibers [25], [26], films [27], [28], patterns [29], [30] and other products [31], [32] through simple, in situ electrospinning, spin coating, casting, printing, and brushing methods. This emerging category of materials has attracted extensive attention and has been incorporated into information security [33], [34], [35], [36], [37]. Shan et al. prepared ultrathin, stable, and tunable PNCs/poly(vinylidene fluoride) (PVDF) composite films via hydrophobic–hydrophilic treatment and, based on them, created nonperceptible multi-color fluorescent anticounterfeit tags [36]. Using an emulsion method, Zhang et al. prepared PNCs/polymer nanosphere molecules for storage of higher dimensions of information by modifying their fluorescence lifetimes [37]. The above studies demonstrated the effectiveness of PNCs/polymer composites, novel information security fluorescent materials, to overcome stability problems of perovskites and block external attacks and interference. However, the encryption systems mentioned above rely on single intrinsic optical properties of materials, such as color, strength, lifetime, pattern, or shape, to achieve encryption protection, which has relatively low-security levels and can be easily substituted and imitated by other materials.
Recently, several studies have been carried out on smart perovskite nanocrystal fluorescent composites for use in data encryption and decryption [38], [39], [40], [41], [42], [43], [44]. For example, Hu et al. modulated the switching of CQDs-cetyltrimethyl ammonium bromide(CTAB) @SiO2@polymethyl methacrylate (PMMA) fluorescence through removal and retention of water-assisted Fe3+ using the internal filtration effect between Fe3+ and PNCs to successfully achieve encryption of confidential information [38]. Li et al. converted the invisible Pb-metal–organic framework (MOF) in situ by introducing halide salts and generated a highly fluorescent perovskite/MOF composite, which could be quenched by water impregnation with halide salts to achieve reversible ON/OFF switching of optical signals and encrypted/decrypted information [43]. These findings show that the reversible transformation of the perovskite structure utilizing space limitation can control the smart fluorescent switch to achieve controllable information protection, which is an effective approach to increase the data security level. However, most of these materials consist of perovskites and inorganic porous composites with low yields, cumbersome preparation, and complex polymorphic and high-cost processing, which restricts their industrial use and further development. PNCs/polymer composites, as mentioned above, are inexpensive and processable in large quantities with high industrial potential. However, to the best of our knowledge, there are few reports on smart fluorescence based on these materials, probably due to the ultra-stable and dense encapsulation that makes them inaccessible to chemical agents [45]. Therefore, their development into intelligent fluorescent materials that respond to external stimuli while retaining traditional means of fluorescent protection is an extremely challenging task that requires a more detailed analysis.
The provision of a reversible transformation of the internal perovskite structure without affecting the polymer morphology is crucial to achieving a smart fluorescent switch. For this purpose, we used the mechanism of “weak solvent swelling polymer” to construct a weak solvent by mixing the compatible N,N-dimethylformamide (DMF) and water as an external stimulus to achieve a smart fluorescence of the PNCs/polymer. We directly mixed the perovskite precursor solution with polyvinylidene difluoride (PVDF) polymer, brushing and annealing on the substrate to obtain patterned composite films. With rapid immersion and rapid evaporation of the weak co-solvent, the fluorescence signal of the composite film is reversibly vanished and regenerated. Such reversible switches are valid after multiple cycles and are accompanied by a negligible loss of photoluminescence intensity in practice. On this basis, three-dimensionally (3D) printed MAPbBr3 nanocrystal composite security labels based on an in situ method are successfully obtained. The reversible process is not damaging the polymer matrix, thus the security effectiveness of the printed labels can be verified and maintained even under extreme conditions such as prolonged water immersion.