Synthesis of Persistent Luminescent Nanoparticles for Rewritable Displays and Illumination Applications

Summary

A protocol is presented for the synthesis of persistent luminescent nanomaterials (PLNPs) and their potential applications in rewritable displays and artistic processing utilizing the afterglow effect under ultraviolet light (365 nm) irradiation.

Abstract

Persistent luminescent nanoparticles (PLNPs) possess the capabilities to maintain extended longevity and robust emission even after the excitation has ceased. PLNPs have been widely used across various domains, including information displays, data encryption, biological imaging, and artistic decoration with sustained and vivid luminosity, providing boundless possibilities for a variety of innovative technology and artistic projects. This protocol focuses on an experimental procedure for the hydrothermal synthesis of PLNPs. The successful synthesis of enduring luminescent nanomaterials with Mn²⁺ or Cr³⁺ serving as a luminescent center in Zn₂GeO₄: Mn (ZGO: Mn) or ZnGa₂O₄: Cr highlights the universality of this synthetic method. On the other hand, the optical properties of ZGO: Mn can be changed by adjusting the pH of precursor solutions, demonstrating the tunability of the protocol. When charged with ultraviolet (UV) at a wavelength of 365 nm for 3 min and then stopped, PLNPs exhibit the remarkable capacity to generate afterglow efficiently and consistently, which makes them ideal for making two-dimensional rewritable displays and three-dimensional transparent, luminous artworks. This protocol outlined in this paper provides a feasible method for the synthesis of persistent luminescent nanoparticles for further illumination and imaging applications, opening up novel prospects for the fields of science and art.

Introduction

Persistent luminescence (PL) is a unique optical process that can store energy from ultraviolet light, visible light, X-rays, or other excitation sources and then release it in the form of photon emission for seconds, minutes, hours, or even for days¹. The discovery of continuous luminous phenomenon originated from the Song dynasty in ancient China 1000 years ago when a painter accidentally discovered a painting that glowed in the dark. It was later found that some natural raw materials and minerals could absorb sunlight and then glow in the dark and can even be made into fascinating glowing pearls². However, the first adequate record of persistent phosphors needed to be traced back to the discovery of PL emission from Bologna stone in the early 17^th century, which gave off a yellow to orange afterglow in the dark^1,2,3,4. Later, it was discovered that the natural impurities of Cu⁺ in BaS played an important role in this persistent luminescence phenomenon^1,4. Until the mid-1990s, the production of persistent phosphors was largely limited to sulfides⁵. In 1996, Matsuzawa et al. reported a new metal oxide (SrAl₂O₄:Eu²⁺, Dy³⁺) phosphor showing extremely bright afterglow, which greatly stimulated the expansion of persistent luminescence research⁶.

The unique properties of persistent luminescent materials are mainly derived from two kinds of active centers: emission centers and trap centers^1,7,8. Among them, the former determines the emission wavelength, while the sustained intensity and time are mainly determined by the trap centers. Therefore, the design of PL materials should take both aspects into consideration in order to achieve the desired emission wavelength and long-lasting luminescence^9,10. The emission centers can be lanthanide ions with 5d to 4f or 4f to 4f transitions, transition metal ions with d to d transitions, or post-transition metal ions with p to s transitions^1,11,12,13. On the other hand, trap centers are formed by lattice defects or various co-dopants^14,15, which usually do not emit radiation but instead store the excitation energy for a while and then gradually release it to the emitting center through thermal or other physical activation^16,17. Many phosphors with different hosts and dopant ions have been reported. So far, inorganic metal compounds¹⁸, metal-organic frameworks⁸, certain organic composites¹⁹, and polymers²⁰ have been found to have PL properties. In recent years, deep trap persistent luminescent materials with controllable energy storage and photon release properties have shown great potential applications in information storage²¹, multi-layer anti-counterfeiting²², and advanced displays²³.

Based on the above composition, PLNPs with various matrices have been successfully designed and synthesized, such as BaZrSi₃O₉⁷, Y₂O₂S²⁴, Ca₁₄Mg₂(SiO₄)₈²⁵, CaAl₂O₄²⁶, SrAl₂O₄^26,27 , and Sr₂MgSi₂O₇²⁸ with multi-doped luminescent centers, in which the luminescence centers strongly depend on the crystal field effect of the host lattice, while the defects generated or improved by different doping serve as auxiliary centers to control the afterglow intensity and duration. In addition to co-doping, long-lasting emission can also be observed in the case of only one activator, such as heterogeneous PLNPs with the matrix of Y₃Al₂Ga₃O₁₂²⁹, BaGa₂O₄³⁰, Ca₂SnO₄³¹, CdSiO₃³² , and Zn₃Ga₂Ge₂O₁₀³³. Germanate-based ternary oxides include Ca₂Ge₇O₁₆, Zn₂GeO₄, BaGe₄O₉, etc., which are typical wide-bandgap semiconductor materials with tunable emission, reproducible and stable luminescence, high quantum yield, environmental friendliness and wide availability^34,35,36. These advantages make it a good activator-type photoluminescent carrier. In the past few years, germanates with various microstructures^35,37, have been prepared by conventional solid-state reactions or chemical solution methods, and these characteristics make Zn₂GeO₄ useful in sterilize³⁸, anti-counterfeiting³⁹, catalysis⁴⁰, light diodes⁴¹ , biosensing⁴², battery anodes⁴³, detectors^44,45, etc.

In order to expand the application of PL materials, the controllable synthesis of uniform and persistent luminescent nanoparticles has been developed. A decade ago, persistent phosphors were synthesized by solid-state synthesis⁴⁶. However, the long reaction time and high annealing temperature during the synthesis process resulted in large and irregular phosphors, which limited their application in other fields such as biomedicine. In 2007, Chermont et al. used sol-gel approach to synthesize nanoparticles for the first time and prepared Ca_0.2Zn_0.9Mg_0.9Si₂O₆: Eu²⁺, Dy³⁺, Mn²⁺, which opened the era of PLNPs⁴⁷. However, the top-down synthesis strategy is accompanied by problems such as uncontrollable size and morphology, so researchers have done a lot of work in the development of controllable bottom-up synthesis of PLNPs. Since 2015, various synthesis methods have emerged one after another, such as the template synthesis method, hydrothermal/solvent thermal method, sol-gel method and other wet chemical synthesis methods for the synthesis of uniform and controllable PLNPs^47,48,49,50. Among them, hydrothermal synthesis is one of the most commonly used methods for preparing nanomaterials, which can provide an adjustable and mild synthetic method to prepare compounds or materials with special structures and properties⁵¹.

Here, we present a detailed experimental procedure for synthesizing Zn₂GeO₄: Mn PLNPs with 1D nanorods morphology via the hydrothermal method and providing them with a rigid environment for further illumination applications. It was found that the luminescence properties of PLNPs, including emission wavelength and afterglow decay curve, can be changed by adjusting the pH value of the precursor. On the other hand, to emphasize the versatility of this method, we also synthesize PLNPs with Cr as the luminescent center using ZnGa₂O₄ as the matrix (ZnGa₂O₄: Cr), which exhibits afterglow emission (697 nm) in the near-infrared region after being excited by ultraviolet light (365 nm). This article mainly focuses on Zn₂GeO₄: Mn whose pH value of precursor solution is 9.4 for two-dimensional and three-dimensional artworks production and visualization. Zn₂GeO₄: Mn is a type of nanomaterial with Mn ions as the luminescent center which obtains strong green light emission (~ 537 nm) under the excitation of 365 nm ultraviolet light. At the same time, the continuous green light can still be seen after stopping excitation. In order to promote the polymerization of PLNPs in methyl methacrylate, ligands (Poly-ethylene glycol) were added during the hydrothermal synthesis process, and then PLNPs were polymerized with methyl methacrylate (MMA) in a two-dimensional or three-dimensional mold so that it can form glowing artwork while smoothly demolding.

This protocol provides a feasible method for the hydrothermal synthesis, polymerization reactions, and luminescent applications of PLNPs in advanced color rendering. Any differences in pH, temperature, and chemical reagents during nanocrystal growth will affect the size and optical properties of PLNP nanostructures. This detailed protocol aims to help new researchers in the field to improve the reproducibility of PLNPs using a hydrothermal method for further wider applications.

Protocol

1. Synthesis of Zn₂GeO₄: Mn PLNPs

1. Prepare 2 M/L sodium hydroxide solution by dissolving 10 mM sodium hydroxide in 5 mL of deionized water.
2. Prepare 0.4 M/L sodium germanate solution by adding 2 mM of germanium oxide into 5 mL of sodium hydroxide solution, and then stir at room temperature for about 30 min.
3. Add 4 mM of zinc chloride, 0.01 mM of manganese nitrate and 600 µL of nitric acid (65%-68%, wt) to a 100 mL small beaker containing 22 mL of deionized water.
   CAUTION: The addition of nitric acid should be strictly carried out in a fume hood and ensure that there are no open flames or heating around.
4. Stir vigorously until the solution of step 1.3 is completely dissolved.
5. Slowly add 2 mM sodium germanate solution to the solution of step 1.4. Add 1 mL of polyethylene glycol (PEG; 300, Mw) to the solution.
6. Put the calibrated pH meter probe into the solution to monitor the pH value of the reaction system. Set a relatively gentle stirring to avoid splashing of the solution and collision between the stirring bar and the probe.
7. Add ammonium hydroxide with a mass fraction of 25%-28% to the solution drop by drop, and adjust the pH of the solution to 6.0, 8.0 or 9.4 depending on the luminescence property to be studied. Be sure to add ammonium hydroxide slowly and monitor the pH changes of the solution at all times to prevent the system from being too acidic or too alkaline, so as not to affect the morphology and luminescent properties of nanomaterials.
8. Cover the beaker with sealing film and stir the solution at room temperature for 1 h. Try not to expose the system to air to prevent dust from entering and causing solvent volatilization, while stirring at a constant speed so that the liquid level of the system will not splash when the system is fully mixed.
9. Transfer the solution to a Teflon-lined autoclave and place it in an electric thermostatic drying oven at 220 °C for 4 h.
   NOTE: The appropriate Teflon-lined autoclave should be selected according to the volume of the system, and the reactor should be kept clean. The volume of the added raw materials should not exceed 1/3^rd of the volume of the autoclave. At the same time, make sure the autoclave is completely closed before placing it into the electric thermostatic drying oven.
10. Turn OFF the electric thermostatic drying oven when the reaction is completed and wait for the system to cool down to room temperature to take out the reactor. Be sure to wait until the reactor is completely cooled and the pressure is reduced to a safe range before proceeding to the next step to avoid direct contact between high temperature and skin.
11. Slowly open the reactor and transfer the reaction solution to two 50 mL centrifuge tubes. Rinse the reactor with 40 mL of ethanol, and subsequently transfer the ethanol solution to the same centrifuge tubes.
12. Vortex for 30 s so that the solution can be mixed evenly, then centrifuge the sample at 4000 x g for 15 min at room temperature and remove the supernatant.
13. Add 10 mL of deionized water to each centrifuge tube and sonicate for 5 min (240 W, 40 kHz) to redisperse the product.
14. Add 20 mL of ethanol into each centrifuge tube and vortex for 30 s to mix the solution evenly.
15. Continue to centrifuge the product according to the setting mentioned before (4000 x g, 15 min) at room temperature and discard the supernatant.
16. Ultrasonicate for 5 min to disperse the product in 2 mL of methanol solution, seal the sample with sealing film and store it in a 4 °C refrigerator to prevent sample contamination and solvent evaporation for future illumination applications.

2. Synthesis of ZnGa₂O₄: Cr PLNPs

1. Dissolve 12 mM Ga(NO₃)₃.xH₂O, 7.2 mM ZnCl₂ and 0.024 mM Cr(NO₃)₃.9H₂O in 30 mL of deionized water.
2. Add 1 mL of PEG (300, Mw) to the solution. Add ammonium hydroxide (25%-28% wt) to the solution, stir gently to reach a pH of 9.0-9.4. Be sure to control the stirring speed so that the solution can mix thoroughly without splashing onto the pH meter.
3. Cover the beaker with a sealing film and stir the solution at room temperature for 1 h. Try to minimize the exposure of the system to air to prevent dust from entering and solvent evaporation. At the same time, control the stirring speed so that the system does not splash while mixing thoroughly.
4. Transfer the solution to a Teflon-lined autoclave and run at 220 °C for 6 h. Take out the container after the temperature drops to room temperature. Ensure that the reaction vessel has fully cooled down, and the pressure has dropped to a safe range before proceeding with subsequent operations, as well as avoid direct contact of high temperatures with the skin.
5. Transfer the reaction solution to two 50 mL centrifuge tubes. Rinse the reactor with 40 mL of ethanol, and then transfer the ethanol solution to the same centrifuge tubes.
6. Vortex for 30 s to mix the solution and then centrifuge the sample at 4000 x g for 15 min at room temperature and remove the supernatant.
7. Add 10 mL of deionized water to each centrifuge tube and sonicate for 5 min to redisperse the product.
8. Add 20 mL of ethanol into each centrifuge tube and vortex for 30 s to mix the solution evenly. Continue centrifuging the product at room temperature as previously mentioned (4000 x g, 15 min) and discard the supernatant.
9. Ultrasonicate for 5 min to disperse the product in 2 mL of deionized water and seal the sample with a sealing film for storage.

3. Purification for raw materials

1. Purify methyl methacrylate (MMA) by column chromatography as described below.
   1. Fill half of the column with alkaline aluminum oxide (100-200 mesh) and compact lightly with a glass rod. When filling the column with aluminum oxide, pay attention to even distribution and uniform compaction of the filler to improve separation efficiency.
   2. Add a small amount of MMA and open the PTFE throttle piston below. Once the solvent layer wets the entire aluminum oxide and liquid flows out, add more MMA and repeat this process multiple times. The time when mass ratio of the entire MMA added to the basic aluminum oxide is: 1:50 represents the end of the process.
   3. Place the final collected MMA sample into a glass bottle, seal it with a sealing film and store at 4 °C.
      CAUTION: The entire process should be carried out in a fume hood due to the strong volatility of MMA. At the same time, operators should wear masks and lab coats.
2. Purify azobisisobutyronitrile (AIBN) by recrystallization as described below.
   1. Prepare a 50 mL of mixed solution with a volume ratio of 7:3 of ethanol and distilled water and heat the solution.
   2. Add 5 g of AIBN when the solution is boiling and stir to mix the solution evenly.
   3. Remove insoluble impurities by hot filtration as described below.
      1. Place the filter paper snugly against the inner wall of the triangular funnel and ensure that the filter paper is below the edge of the funnel.
      2. Place the glass rod against the three-layer section of the filter paper. If the glass rod is not placed against the three-layer section of the filter paper, it may puncture the filter paper, leading to inefficient filtration.
      3. Place the tip of the beaker containing the solution close to the glass rod and pour it while it is hot. The purpose of this step is to prevent splashing liquid droplets.
      4. Rinse the beaker with 10 mL of cold distilled water, and again perform the above filtration process; repeat 3x.
   4. The solution becomes supersaturated due to the decrease in solubility during cooling, resulting in the precipitation of crystals. Put the collected solution in a refrigerator at 4 °C for cooling and crystallization. The sample will appear in the state of white needle-like crystals.
   5. Seal the sample with aluminum foil and store at 4 °C.
      CAUTION Protective measures should be taken during the operation due to the toxicity of AIBN, while also avoiding contact with open flames, high temperatures, and oxidizing agents.

4. Copolymerization of methyl methacrylate (MMA)

1. Set the water bath temperature to 80 °C.
   NOTE: The temperature of the water has a severe influence on the rate of polymerization and thus affects the final product formation. Therefore, the temperature of the water bath should be strictly guaranteed not to be too high.
2. Weigh 20 g of MMA into a 100 mL eggplant-shaped bottle. Keep the container dry before the experiment.
   NOTE: The eggplant-shaped bottle is chosen to facilitate the water bath heating and the replacement of air with nitrogen in the system. Try to weigh the sample in a well-ventilated environment while wearing a mask.
3. Add the pre-prepared methanol solution of Zn₂GeO₄: Mn into the reaction vessel.
4. Thoroughly dissolve the sample in MMA with the help of ultrasound for about 10 min (240 W, 40 kHz) at room temperature. Keep the reaction vessel sealed to prevent solvent evaporation and avoid excessively high temperatures during the ultrasonication process.
5. Add 0.012 g of AIBN to the solution and mix the solution completely.
   NOTE: AIBN should be used under anhydrous conditions and ensure that there is no open flame around the experimental operation. Be sure to wear protective gear.
6. Place the flask in an 80 °C water bath and purge the air from the reaction system with N₂ for approximately 35 min. When the reaction is about to end, gently shake the reaction container. If the solution does not shake vigorously, it proves that the reaction is successful.
   NOTE: The reaction time will change with the temperature of the water bath. Make sure that the temperature of the water bath reaches 80 °C and start timing for 35 min.
7. After the reaction is over, quickly transfer the reaction vessel to an ice bath to cool it down quickly.
   NOTE: This process should be as fast as possible to avoid excessive pre-polymerization of MMA, and the ice bath can be prepared in advance during the reaction interval.
8. Slowly pour the solution into a two-dimensional or three-dimensional mold, put the mold into an electric thermostatic drying oven at 40 °C for 10 h, 70 °C for 8 h, and 100 °C for another 2 h to obtain target material.
9. Close the electric thermostatic drying oven after the reaction stops, and let it cool down to room temperature. Open the electric thermostatic drying oven to take out the mold after the reaction system is sufficiently cooled to avoid skin burns caused by direct contact between high temperature and body.
10. Carefully remove the mold and expose the polymerized PMMA sample (ZGO: Mn-PMMA) to a UV lamp for about 3 min. For example, when exposing a transparent ZGO: Mn-PMMA film to ultraviolet light through a black cardboard cutout in the shape of the letter H, a corresponding pattern of green phosphorescent emission is obtained. The pattern can be erased after 5 min. Subsequently, the process can be repeated by using another black cardboard cutout in the shape of different letters, generating new luminescent patterns.

Results

The synthesis diagram of Zn₂GeO₄: Mn (ZGO: Mn) PLNPs is shown in Figure 1. The amphiphilic polymer Poly-ethylene glycol (PEG) is added to modify the ligand-free Zn₂GeO₄: Mn (ZGO: Mn) nanorods to better dissolve in MMA medium. First, the transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) images of ZGO: Mn whose pH is 9.4 are collected (Figure 1), and then dynamic light scatt...

Discussion

This article introduces a synthesis method for persistent luminescent nanomaterials and polymerization for color rendering applications. The materials showed extremely stable optical properties and a visible afterglow after ceasing excitation of ultraviolet light. A persistent luminescent nanomaterial (Zn₂GeO₄: Mn) was prepared using a hydrothermal method with different pH (Figure 1A). The TEM image showed that ZGO: Mn PLNPs whose pH is 9.4 were rod-shaped with an avera...

Materials

Name	Company	Catalog Number	Comments
azobisisobutyronitrile (99%)	Macklin	A800354	Further purification required
methyl methacrylate(99%)	Sigma-Aldrich	M55909	Further purification required
deionized water	Merck	ZEQ7016T0C	Milli-Q Direct Water Purification System
alkaline aluminum oxide (100-200 mesh)	Macklin	A800033
ammonium hydroxide  (25%-28%, wt)	Macklin	A801005
beaker	Synthware	B220100
chromium(III) nitrate nonahydrate (99.95%)	Aladdin	C116448
centrifuge	ThermoFisher Scientific	75004250
column	Synthware	C184464CR
digital camera	Canon	EOS M50 Mark II
electric thermostaticdrying oven	Longyue	LDO-9036A
ethanol (99.7%)	Greagent	1158566
gallium nitrate hydrate(99.9%)	Aladdin	G109501
germanium oxide (99.99%)	Sinopharm Chemical ReagentCo., Ltd	51009860
glass rod	Sinopharm Chemical ReagentCo., Ltd	91229401
powder X-Ray Diffractometer	D2 PHASER DESKTOP XRD	BRUKER
manganese nitrate (98%)	Macklin	M828399
methanol (99.5%)	Greagent	1226426
nitric acid (65.0-68.0%, wt)	Sinopharm Chemical ReagentCo., Ltd	10014508
pH meter	Shanghai Leici Sensor Technology Co., Ltd	PHS-3C
polyethylene glycol (300, Mw)	Adamas	01050882(41713A)
sealing film	Parafilm	2025722
sodium hydroxide (GR)	Sinopharm Chemical ReagentCo., Ltd	10019764
spectrometer	Horiba	Fluorolog-3
transmission electron microscope	JEOL	JEM-1400 Plus
transmission electron microscope	JEOL	2100 Plus
triangular funnel	Synthware	F181975
ultrasound machine	centrifuge	JP-040S
zinc chloride (98%)	Greagent	01113266/G81783A