Nanoarchitectonics of Ln³⁺ (Eu³⁺/Tb³⁺)-induced polyelectrolyte aggregates (LIPAs) for multicolor luminescence in 3D-printed poly(Lactic acid)

Highlights

• Novel nanocomposite based on Ln³⁺-induced polyelectrolyte aggregates (LIPAs) is synthesized.

• Novel luminescent polylactic acid (PLA) composite fiber based on LIPAs is successfully fabricated.

• The functional anti-counterfeiting pattern is prepared by 3D printing using luminescent LIPAs/PLA fiber.

• Multicolor luminescent patterns are invisible, adjustable and ultra-stable.

Abstract

Reversible bright-dark luminescence nanomaterials have been developed for information storage and anti-counterfeiting in recent years, however, their monochrome characteristic and 2D patterning impose limits on the effective information storage density. In this work, a new strategy for producing ultraviolet switchable and stable luminescent patterns is proposed, based on Ln³⁺ (Eu³⁺/Tb³⁺) induced-polyelectrolyte aggregates (LIPAs) with sharp multi-color optical properties. Under 365 nm excitation, a multicolor pattern can be observed using both red and green, which is not visible when this excitation light source is off. The LIPAs enable the luminescent lanthanide complexes to be compatibilized with poly(lactic acid) (PLA). Functional anti-counterfeiting patterns were prepared by 3D printing using luminescent PLA containing by Ln³⁺ (Eu³⁺/Tb³⁺) LIPAs. Invisible, adjustable and ultra-stable multicolor luminescent patterns well enable information storage and enhance information security for anti-counterfeiting applications.

Introduction

The emerging needs for safety and security in many areas of information storage and communication have led to development of authentication and verification technologies, especially those which enable encoding of “hidden” messages [1]. Application areas include information safety, prevention of smuggling, protection of commercial secrets, and verification of information sources. Among these technologies, anti-counterfeiting is becoming increasingly important [2], [3]. However, conventional anti-counterfeiting technologies such as water markings, laser holographics, QR codes and radio frequency identification techniques have the disadvantage of being easy to replicate. During recent years, considerable efforts have been devoted to searching for functional materials which can increase information security and storage density [4], [5]. Luminescent materials have drawn extensive attention because of their exceptional luminescent properties under a specific wavelength of light excitation. Lanthanide-doped up- and down-conversion materials [6], [7], [8], carbon dots [9], [10], [11], [12], inorganic perovskite materials [13], quantum dots [14], [15] and conventional semiconductor nanoparticles [16], [17] can all be used to provide such luminescence.

Among these luminescent materials, the strong luminescence metal center of Ln³⁺ complexes can be observed after the coordination of lanthanide ions (Ln³⁺) with organic ligands [18], [19]. Since organic ligands have strong absorption bands, which absorb more light energy than lanthanide ions themselves. After excitation of the organic ligands, the energy obtained from the excitation source is transferred from the organic ligand to the lanthanide ion [20], [21], [22]. showing the "antenna effect"[23], [24]. In addition, the luminescent lanthanide complexes also exhibit the advantages of long luminescence lifetime, high quantum yield, large Stokes shift, and good luminescence monochromaticity [25]. Typically, Eu³⁺ and Tb³⁺ have been used as the central ion of the luminescent complexes, because they emit light in the visible region [26], [27]. Their luminescence properties have been widely reported.

The application of these complexes in advanced materials still presents technical challenges. Firstly, the Ln³⁺ complexes are difficult to directly disperse in host matrix materials, as they readily form aggregates and are incompatible with many synthetic polymers [28], [29]. Secondly, common processing methods, such as melt extrusion solution coating, and printing may not achieve the target patterned or designed structures [30]. Thirdly, their luminescent lifetimes and quantum yields, both of which are important, are frequently affected by their incorporation in blends and composite materials [31].

The use of luminescent lanthanide complexes to print or pattern information into materials requires optimized dispersion of the lanthanide complex in a host matrix polymer. The blending and processing should also preserve or enhance the photophysical properties (luminescence, quantum yield, and lifetime) of the lanthanide complex, while enabling resolution of and fidelity of encoded information patterns.

We have demonstrated dispersion of Ln³⁺ complexes by forming coordination complexes with macromolecular chains, which can inhibit aggregation of Ln³⁺ complexes [32], [33]. Specifically we have reported polyelectrolyte-stabilized nano-particle structures with the inclusion of Ln³⁺ complexes, in which the positively charged Ln³⁺ ions interact electrostatically with the negatively charged substituents (such as, -COOH) of polyelectrolyte chains, and induce aggregation to form nanostructures. We refer to the resulting nanoparticles as Ln³⁺ (Eu³⁺/Tb³⁺)-Induced Polyelectrolyte Aggregates (LIPAs).

3D printing is an additive manufacturing technology that deposits materials layer by layer [34], including direct ink writing (DIW) [35], two-photon polymerization (TPP) [36], selective laser melting (SLM) [37] and fused deposition modeling (FDM) [38]. Advanced manufacturing technology and excellent printing materials have largely promoted the development of 3D printing in the fields of supercapacitors, photovoltaic solar cells, dye purification, and anti-counterfeiting information storage [39], [40], [41], [42], [43]. Furthermore, high-precision 3D printing technology is an excellent strategy for forming complex polymer structures with high resolution [44]. For devices and materials that contain imprinted information, the conservation of raw materials, high information storage density, color diversity, and facile information reading or retrieval are all general requirements [45], [46]. Thus, our strategy is to achieve all of these properties within complicated 3D printed structures. Incorporation of LIPAs into 3D polymeric materials through the stable electrostatic interaction between LIPAs and matrix polymers, we will result in polymers for 3D printing. This requires the selection of both the LIPA polyelectrolytes and the printable matrix polymers [47].

Polyelectrolytes containing hydrophilic hydroxyl groups are good candidates materials for LIPAs, because the hydroxyl groups can also serve as ligands for the Ln³⁺ [32], [48]. Chitosan, a natural alkaline polysaccharide produced by deacetylation of chitin, and a cationic biopolymer with complementary electrostatic properties that can readily bind to negatively charged surfaces [49], [50] It has several favorable biological activities and compatibility with mammalian cells and biological systems [51]. The unique cationic charge and biocompatibility of chitosan make it play a vital role in drug carrier and water treatment [52], [53], [54]. Moreover it is suitable for use as a coordinating ligand with transition metals and Ln³⁺ ions [55]. Hyaluronan, also known as hyaluronic acid, is a high molecular weight, anionic polysaccharide consisting of two disaccharide units, d-glucuronic acid and N-acetyl-d-glucosamine. Hyaluronan is found in the extracellular matrix of many mammalian tissues [56]. Both chitosan and hyaluronan can interact with other hydroxyl, ester, and carboxyl groups to form hydrogen bonding or complex interactions [57], [58]. Therefore, polysaccharide-based LIPAs may be readily compounded with matrix polymers containing hydroxyl, carboxyl, and ester groups.

The rapidly growing demand for 3D printing has promoted the application of functional fibers including electrospun fibers, carbon nanofibers, and biodegradable fibers in gas sensors, electrocatalysis, batteries, water purification, and information storage [59], [60], [61], [62], [63], [64], [65]. Among them, Poly(lactic acid) (PLA) is a polyester that has good processability via several methods, including 3D printing [66], [67]. PLA is leading among many biopolymers due to its excellent physical and mechanical properties, reproducibility, biodegradability, and relatively good availability [68], [69]. So, in this work, we choose PLA as the matrix material to form 3D printed LIPA-containing materials.

To preserve the luminescence lifetime and quantum yield, the local environment of the Ln³⁺ complexes in LIPAs should be tuned. The Ln³⁺ complexes should be protected from energy dissipating molecules, and local vibration of complexes should be reduced [31], [70]. Strong interactions between positively charged Ln³⁺ ions and functional groups in polyelectrolytes make polyelectrolytes excellent polymeric materials for use in LIPAs.

In this study, Eu³⁺ and Tb³⁺ were selected as luminescent Ln³⁺ complex components, and chitosan and hyaluronan were chosen as polyelectrolytes for use in LIPAs. Subsequently, we refer to europium- and terbium-containing LIPA as Eu-LIPA Tb-LIPA, respectively. In this work, we optimize the ratios of Ln³⁺ to light antenna ligands and polyelectrolytes, to prepare Tb-LIPA and Eu-LIPA. Then, Tb-LIPA and Eu-LIPA are compounded with PLA by melt extrusion to obtain luminescent fibers as materials for 3D printing. We demonstrate 3D printing of these materials to prepare three-dimensional structures with encoded “hidden” luminescent patterns that can be discovered under illumination with UV light. The photophysical properties are measured and reported.