Portable and writable photoluminescent chalk for on-site information protection on arbitrary substrates
Graphical abstract
Introduction
In many spy movies, it is a common scene that the secret agent hides the secret information by a specific encryption technique, which can securely communicate with people on one’s own side and avoid the leakage of the secret information to opponents [1]. Beyond those fictitious movies, the confidential information protection is of great significance for secure communication in the current information era. The strategy for protecting the confidential information involves three components: materials, encryption and decryption techniques. As for materials used to store the secret graphical or textual information, it is desirable that materials are low-cost, and can be fabricated on a large scale [2], and they have superior physical and chemical stability to prevent non-artificial destruction such as photobleaching of organic dyes [3]. As for encryption techniques, it is necessary that the hidden information is difficult to be recognized under normal conditions [4]. In addition, the decryption techniques should be convenient and rapid for the reading of the original information with portable and inexpensive equipment [5].
In the past decades, efforts have been devoted to developing advanced technologies to enhance the capabilities of these three components, such as holograms [6], molecular coding [7], stimuli-responsive color-changing systems [8], and security inks [9], [10]. However, these approaches have drawbacks which limit their practical applications. For example, holograms involve the complex photochemical process, high-cost chemicals and instruments [11]. Although molecular coding can provide multiple levels of information protection, the encrypted information can only be recognized by the highly skilled analytical expertise using the specialized equipments [1], [7], and the direct and straightforward readout of encrypted information is difficult.
Recently, inspired by Nature, stimuli-responsive color-changing systems have attracted much attention owing to their advantages such as durability, absence of pigment photochemical degradation, and special encryption and decryption approaches [8]. The stimuli, such as light [12], [13], vapor [14], [15], solvent [2], mechanical force [16], electrical field [17], [18], and temperature [19], are needed to trigger the change of structure and thus the color. These stimuli can be accurately adjusted to run the color-changing systems in the laboratory. However, the harsh outside environment conditions may not allow them to function well as in the laboratory conditions.
Among various strategies used for the secret information protection in the previous reports, security inks made from photoluminescent (PL) materials are the most widely studied, including those that show down- or up-conversion photoluminescence and phosphorescence [20], [21], [22], [23], [24], [25], [26], [27]. The attractive advantages of the security inks include that: (1) PL materials can be synthesized in environmentally friendly and facile ways, making low-cost and large-scale production possible [28], [29]; (2) security inks are compatible with printing techniques such as screen and inkjet printing [30], [31], [32], [33], [34], [35], the words and patterns can be printed with high resolution; and (3) the hidden information printed with security inks can be decrypted using portable devices such as mobile phone or ultraviolet (UV) light [20], [36], [37]. However, there are two problems associated with security inks. First, the optimization for ink formulations is necessary for high-quality printing [5], [10], [32], and some factors should take into account such as the concentration, viscosity, dispersibility and stability of PL materials in the suitable ink medium [33], [35], [38]. Second, the printing techniques are not universally applicable on various surfaces and substrates, and the printing devices such as the inkjet printer are not convenient to carry for the outside usage [4], [35], [39].
Herein, we report a new kind of PL chalk made from lanthanide (Ln)-doped ultralong hydroxyapatite (HAP) nanowires as a promising new complement to security inks (Scheme 1a). The as-prepared PL chalk is lightweight and miniature. By doping with different lanthanide ions, the fluorescence color of the PL chalk can be adjusted. The users can directly write covert information on arbitrary surfaces and substrates using the PL chalk, including paper sheets, metals, fabrics, plastics, woods, walls, foams, leaves, and even human body (Scheme 1b). The as-written secret information can be rapidly read with the help of a hand-held UV light. In addition, dual and triple encryption strategies are also demonstrated for high-level information protection (Scheme 1c).
Section snippets
Chemicals
CaCl2 and NaH2PO4·2H2O were purchased from Sinopharm Chemical Reagent Co., Ltd. Sodium oleate, TbCl3·6H2O, and EuCl3·6H2O were purchased from Aladdin Industrial Corporation. All chemicals were used as received without further purification. Deionized water was used in all experiments.
Synthesis of ultralong hydroxyapatite nanowires
Ultralong HAP nanowires were synthesized based on the calcium oleate precursor solvothermal method developed by this research group [40]. Briefly, CaCl2 (2.220 g) aqueous solution (250 mL) and NaH2PO4·2H2O (2.808 g)
Results and discussion
Chalk is a common writing tool with a long history and widely used in school education owing to easy writing by hand and low-cost production, which plays an important role in the education of knowledge. In this work, taking advantage of photoluminescent lanthanide-doped HAP nanowires, we demonstrate a new kind of PL chalk that can emit brilliant red or green fluorescence under the UV light (Scheme 1). The chalk is prepared by pressure molding of the Ln-doped HAP nanowires, followed by thermal
Conclusions
In this work, we have demonstrated a new kind of the PL chalk made from lanthanide-doped hydroxyapatite ultralong nanowires. The preparation process of the PL chalk, including the synthesis of lanthanide-doped hydroxyapatite ultralong nanowires, pressure molding and sintering, is facile and efficient, and may be scaled up for a large-scale production. The fluorescence color of the PL chalk can be well controlled by adjusting different doped lanthanide ions. The lightweight and miniature PL
Acknowledgements
Financial support from the National Natural Science Foundation of China (21601199, 21875277), the Science and Technology Commission of Shanghai Municipality (18ZR1445200), Shanghai Sailing Program (16YF1413000) is gratefully acknowledged.
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