Photoswitchable lanthanide-doped core-multishell nanoparticles for tunable triple-mode information encryption and dynamic anti-counterfeiting patterns

Highlights

• Photoswitchable upconversion luminescent nanoparticles (PUCNPs) have been successfully designed and synthesized.

• The triple-mode PUCNPs showed dual-color luminescence and photochromic nature.

• The as-prepared PUCNPs can be constructed high-level information encryption and dynamic anti-counterfeiting patterns.

Abstract

Photoresponsive luminescent materials with high-capacity information storage and multilevel anti-counterfeiting capability are attracting much interest in many fields including information encryption and anti-counterfeiting. Herein, a class of novel photoswitchable upconversion luminescent nanoparticles (PUCNPs) with core-multishell structure that shows switchable dual-colored emission is described. The PUCNPs is successively built up from lanthanide-doped NaYF₄ nanoparticles and a photochromic spiropyrane (SP) monomer by grafting of poly(glycidyl methacrylate-co-SPMA) on the nanoparticles via surface reversible addition-fragmentation chain transfer (RAFT) polymerization. The as-prepared PUCNPs with triple-mode optical features displayed green upconversion luminescence under a 980 nm laser, red downconervsion fluorescence and purple color under a 365 nm UV light irradiation and turning off, respectively. Furthermore, they enable dynamic reversible changes in color between colorless and purple states with 365 nm UV and white light stimuli. Their potential in complex information encryption and dynamic anti-counterfeiting patterns is described and its great potential in data storage, and rewritable optical materials is foreseen.

Introduction

The proliferation of fake and counterfeiting is a growing problem, which gained considerable attention on information security and property safety [1,2]. Thus, high-level anti-counterfeiting technology is in urgent demand for the growing field in the protection of important and valuable items, such as money, luxuries, and tickets [3]. Considerable advances have been developed to designing anti-counterfeiting methods, creating many different anti-counterfeiting tools on the current researches [[4], [5], [6], [7]]. However, traditional methods such as water marking is not very effective because the measure is out of date and easy to duplicate [8]. For this purpose, authentication method that employ various markers have been developed to provide high-level security [[9], [10], [11]]. For example, well-designed luminescent materials have become a popular kind of advanced anti-counterfeiting agents, which is trigger to give certain luminescent characteristics upon exposure to a specific wavelength of light [12]. This method has been already employed for anti-counterfeiting of RMB [13]. However, one color is generally limited due to its single mode, which hinders their practical applications.

Tunable multicolor fluorescent materials have a broad range of applications in the color-on-demand fields, including flexible display, bioimaging, chemosensor, security printing, and data encryption [[14], [15], [16], [17], [18], [19]]. Recently, responsive multicolor luminescent materials under different stimuli (e.g., light, temperature, force, and pH) have been investigated [[20], [21], [22], [23], [24], [25], [26], [27]]. Among various stimuli, light exhibits the advantage of remote, nondestructive, high spatial and temporal resolutions, and thus is frequently utilized as the external stimulus [[28], [29], [30], [31]]. To build a photocontrolled property into the smart materials, many advanced concepts have been proposed to fabricate multistate luminescent materials including photoresponsive structures, semiconductor quantum dots, and lanthanide-doped nanoparticles [[32], [33], [34], [35]]. Among these strategies mentioned above, the lanthanide-doped nanoparticles as an easier tool for tunable emission by regulating the concentrations and kinds of lanthanide ions was attractive [[36], [37], [38], [39], [40], [41], [42], [43], [44]]. As a result, a series of upconversion luminescent nanoparticles (UCNPs) with multicolor by incorporating of suitable host matrix and trace lanthanide dopants were prepared via anti-Stokes luminescence process [[45], [46], [47]]. These multicolor systems could be excited by different illumination wavelengths and thus show multistate emission change, which often implies tedious effort in tentative synthesis in such design [[48], [49], [50], [51]]. The situation becomes even worse in the case that the encrypted information is readily identified to obtain upon specific wavelength of light because of invariable emission of materials, which hinders their practical applications for special information encryption and complex anti-counterfeiting [52]. Therefore, dynamic optical materials are highly desired and urgently needed to fabricate advanced information encryption and high-level anti-counterfeiting. Examples of commercial photochromic organic molecules include: spiropyran, azobenzene, diarylethene, fulgide and so on [[53], [54], [55]]. These colorless materials containing photoresponsive groups gradually colored under the increase of UV irradiation time. Among them, only the spiropyran unit show obvious fluorescence upon UV illumination, which could be regarded as a desired photoswitch for advanced anti-counterfeiting application [56].

Here, we purposed a novel strategy for preparing advanced anti-counterfeiting materials by introducing photochromic molecules with dynamic optical property in UCNPs. The photoswitchable upconversion luminescent nanoparticles (PUCNPs) were established by grafting of a photochromic polymer containing photoresponsive spiropyrane (SP) moiety on well-designed lanthanide-doped nanoparticles via surface reversible addition-fragmentation chain transfer (RAFT) polymerization. Specifically, we used the NaYF₄ decorated NaYF₄:Yb/Er with enhanced upconversion luminescence as lanthanide-doped nanoparticles and p(glycidyl methacrylate-co-spiropyran methacrylate) (p(GMA-co-SPMA)) as the photochromic polymer. NaYF₄:Yb/Er and SP moiety are responsive to 980 nm and 365 nm excitation, respectively. Unlike multicolor systems are responsive to single wavelength excitation, the PUCNPs displayed tunable triple-mode optical property of dual-color emission (green for 980 nm laser and red for 365 nm UV light irradiation) and photochromic nature (purple color after turning off UV light). Such careful design can be individually stimulated emission of the photoswitchable nanoparticles to fabricate dynamic optical encryption. Moreover, we further demonstrated that tunable optical color with the increase of UV irradiation time can also be encrypted as dynamic anti-counterfeiting patterns and thus provide more high-level information encryption and anti-fake in many complex applications.