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.
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.
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 17th century, which gave off a yellow to orange afterglow in the dark1,2,3,4. Later, it was discovered that the natural impurities of Cu+ in BaS played an important role in this persistent luminescence phenomenon1,4. 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 centers1,7,8. 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 luminescence9,10. 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 transitions1,11,12,13. On the other hand, trap centers are formed by lattice defects or various co-dopants14,15, 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 activation16,17. 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, SrAl2O426,27 , 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 availability34,35,36. These advantages make it a good activator-type photoluminescent carrier. In the past few years, germanates with various microstructures35,37, 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, detectors44,45, 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 Ca0.2Zn0.9Mg0.9Si2O6: 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 PLNPs47,48,49,50. 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.
1. Synthesis of Zn2GeO4: Mn PLNPs
2. Synthesis of ZnGa2O4: Cr PLNPs
3. Purification for raw materials
4. Copolymerization of methyl methacrylate (MMA)
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 scattering (DLS), zeta potential results and X-ray diffraction (XRD) of ZGO: Mn are done (Figure 2). The afterglow spectra and the time-dependent decay curves of ZGO: Mn with different pH (pH = 6.0, 8.0, 9.4) in aqueous solution excited at 365 nm for 3 min, along with photoluminescence images and afterglow images are characterized (Figure 3). The TEM, XRD, afterglow spectra and afterglow attenuation curve of ZnGa2O4: Cr are obtained (Figure 4). The copolymerization diagram of ZGO: Mn whose pH is 9.4 and MMA is shown in Figure 5. The afterglow emission spectra and afterglow intensity as a function of time for the luminescent material based on PMMA (ZGO: Mn-PMMA) can be obtained after irradiating the targets with 365 nm for 3 min and then ceasing excitation. Rewritable display and glow-in-the-dark art processing materials in 2D or 3D molds can be captured by camera upon excitation with UV light at 365 nm for 3 min (Figure 5), which indicates successful polymerization of PLNPs into PMMA matrix for illumination applications.
For obtaining high-resolution, transmission electron microscopy (TEM) images, measurements were performed on transmission electron microscope operated at an acceleration voltage of 200 kV. An XRD system was used to characterize the X-ray diffraction (XRD) data of PLNPs. The afterglow emission spectra and afterglow intensity decay curves as a function of time were carried out using spectrofluorometer. A digital camera was used to record photoluminescence and persistent luminescence images of PLNPs, 2D and 3D luminescent materials.
Figure 1: Synthesis of Zn2GeO4: Mn (ZGO: Mn) PLNPs. (A) The synthesis diagram of ZGO: Mn nanorods. (B) Transmission electron microscopy (TEM) of ZGO: Mn nanostructures whose pH is 9.4. Scale bar: 50 nm. (C) High-resolution transmission electron microscopy (HRTEM) image of ZGO: Mn whose pH is 9.4. Scale bar: 6 nm. Please click here to view a larger version of this figure.
Figure 2: Synthesis of functional Zn2GeO4: Mn nanostructures. (A) Dynamic light scattering (DLS) analysis of Zn2GeO4: Mn-Poly-ethylene glycol (ZGO: Mn-PEG) dispersed in deionized water and a log-normal function fit to obtain the size distribution. (B) Zeta potential results of ZGO: Mn-PEG. (C) X-ray diffraction (XRD) analysis of Zn2GeO4: Mn. Please click here to view a larger version of this figure.
Figure 3: Characterization of ZGO: Mn in aqueous solution. (A) The afterglow emission spectra of ZGO: Mn-PEG with different pH (pH = 6.0, 8.0, 9.4) excited by 365 nm for 3 min. (B) Time-dependent afterglow decay curve of ZGO: Mn in water at 537 nm (excited with 365 nm UV light for 3 min). (C) Photoluminescence images of ZGO: Mn nanorods. (D) Afterglow luminescence decay images of ZGO: Mn. Please click here to view a larger version of this figure.
Figure 4: Characterization of ZnGa2O4: Cr in aqueous solution. (A) TEM images of ZnGa2O4: Cr nanoparticles. Scale bar: 50 nm. (B) XRD analysis of ZnGa2O4: Cr. (C) The afterglow emission spectrum of ZnGa2O4: Cr after 3 min of excitation at 365 nm. (D) Time-dependent afterglow decay curve of ZnGa2O4: Cr in water at 697 nm. Please click here to view a larger version of this figure.
Figure 5: PLNPs for rewritable display and 3D illumination applications. (A) Schematic diagram of the copolymerization reaction between ZGO: Mn whose pH is 9.4 and methyl methacrylate (MMA). (B) The afterglow emission spectrum of ZGO: Mn-PMMA excited by 365 nm for 3 min. (C) The variation of afterglow intensity over time of ZGO: Mn-PMMA at 537 nm (excited with 365 nm UV light for 3 min). (D) Photoactivated afterglow for rewritable display and data storage (written by UV irradiation). (E) Luminescence photographs of transparent 3D duck with ZGO: Mn-PMMA after UV irradiation. Please click here to view a larger version of this figure.
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 average diameter of about 65 nm (Figure 1B). The high-resolution TEM images showed good crystallinity of nanomaterials, and the spacing of adjacent lattice fringes parallel to the rod direction was 0.70 nm, which was in good agreement with the (110) plane spacing of Zn2GeO452, indicating that all Zn2GeO4: Mn was highly crystalline (Figure 1C).
In addition, dynamic light scattering (DLS) data of ZGO: Mn in deionized water fitted by a log-normal function showed that the calculated hydrodynamic size was 63 nm in diameter and well dispersed (Figure 2A). The zeta potential results also showed a negative surface of ZGO: Mn (Figure 2B), indicating that the PLNPs had good solubility and stability in aqueous solution. The XRD spectrum of ZGO: Mn can be assigned to the rhombohedral phase of Zn2GeO4 (Figure 2C).
The optical properties of PLNPs could be changed by adjusting the pH of the precursor solution. The afterglow visible emission spectra of ZGO: Mn solution excited at 365 nm for 3 min showed that the PLNPs exhibited a red shift of the main emission peak when the precursor solution changed from acidic to alkaline (Figure 3A). The decay curve of afterglow intensity over time was also affected by the pH value, but in any case, the emission intensity can still be detected when the afterglow time reached 300 s, which verified the successful synthesis of PLNPs with different optical properties (Figure 3B). The photoluminescence images (Figure 3C) and afterglow images (Figure 3D) of PLNPs with different pH levels were captured by the camera. The results revealed that the solution exhibited a brighter green luminescence as the alkalinity of the solution increased.
PLNPs with ZnGa2O4 as the matrix and Cr as the luminescence center (ZnGa2O4: Cr) were synthesized in order to highlight the versatility of this hydrothermal method. The TEM image showed that ZnGa2O4: Cr was smaller in size and uniformly dispersed (Figure 4A). The XRD results indicated that all the diffraction peaks matched well with those of cubic spinel ZnGa2O4 (Figure 4B). The afterglow emission spectrum showed that ZnGa2O4: Cr exhibited obvious near-infrared emission after being excited by ultraviolet light for 3 min (Figure 4C). The emission intensity of ZnGa2O4: Cr at 697 nm was monitored over time, and the results showed that the afterglow emission could still be collected after 300 s (Figure 4D).
There are several key steps in the hydrothermal synthesis of PLNPs. First, it is necessary to strictly control the pH of the aqueous solution during the synthesis of PLNPs. The acidity and alkalinity of the solution can have a dramatic effect on the persistence of the afterglow and even the color of the afterglow. Secondly, when monitoring the pH of the solution, it's important to adjust the appropriate stirring speed to ensure that the stirrer does not collide with the pH probe while the solution is being mixed evenly. Finally, when the solution is stirred for 1 h in an environment with a target pH, keep the reaction system sealed as much as possible to avoid the influence of volatilization of ammonia water.
Furthermore, two-dimensional and three-dimensional luminous artworks are copolymerized by PLNPs and MMA to make PLNPs more functional (Figure 5A). It was found that whether it is a film or a three-dimensional duck, the material shows good afterglow ability (Figure 5B,C). The 2D and 3D materials display bright green luminescence after the cessation of ultraviolet light excitation, and the green afterglow images can still be seen after 5s (Figure 4D,E), which indicates that PLNPs can generate effective afterglow under the activation of the UV lamp.
Some processes need to be paid attention to when performing MMA polymerization. First, the raw materials, i.e., MMA and AIBN, need to be purified so as not to affect the polymerization. The second, when adding methanol dispersed Zn2GeO4: Mn in the MMA solution, ultrasonically disperse the whole system uniformly. Third, remember to observe the state of the solution all the time. It means that the pre-polymerization is successful if the swaying of the flask only causes a slight shaking of the solution. Fourth, the flask should be ice-bathed immediately after the pre-polymerization to prevent excessive polymerization. Finally, the temperature should be programmed during the polymerization reaction in the electric thermostatic drying oven to form a homogeneous matrix.
In summary, we expect that this protocol not only provides a detailed experimental procedure for the hydrothermal synthesis of long-lasting emission nanomaterials but also introduces a method for the copolymerization of PLNPs and MMA to further achieve UV-mediated rewritable and luminescent applications. More importantly, the optical properties of PLNPs can be further tuned to have various colors of afterglow by adjusting different defects, luminescent centers and pH values based on this method. In addition, the method of copolymerization of PLNPs and polymers can be further used in art production. These advantages make PLNPs ideal materials for luminescence and imaging applications. On the other hand, the hydrothermal synthesis method may limit the application of certain materials, especially temperature-sensitive substances since the reaction requires relatively high temperatures. Therefore, the appropriate synthesis method needs to be selected based on specific applications and requirements.
The authors have nothing to disclose.
The authors thank the funding of the National Natural Science Foundation of China (82001945), the Shanghai Pujiang Program (20PJ1410700), and the starting grant of ShanghaiTech University. The authors thank the Centre for High-resolution Electron Microscopy (ChEM), School of Physical Science and Technology, ShanghaiTech University (No. EM02161943) for the material characterization support. The authors thank the Analytical Instrumentation Center (#SPST-AIC10112914), School of Physical Science and Technology, ShanghaiTech University for the spectral test support and XRD test support. The authors also thank Prof. Jianfeng Li for the help with the material characterizations.
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 |