synthesis of zn2geo4&#58; mn persistent luminescent nanoparticles (plnps)

Hydrothermal Synthesis of Persistent Luminescent Nanoparticles

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.

Author Spotlight: Advancing Bioimaging and Therapy with Functional Nanomaterials

Synthesis of Persistent Luminescent Nanoparticles for Rewritable Displays and Illumination Applications

Our research focuses on developing functional nanomaterials based on precise biotechnology. We aim to achieve bioimaging disease treatment, and physiological function regulation in a minimally invasive and highly selective fashion by using nano composites with luminescence, thermal magnetic, and acoustic properties. We have demonstrated that utilizing a rigid backbone chain polymer, polymethyl methacrylate as the matrix, purely polycyclic aromatic hydrocarbons can exhibit stable and exceptionally long phosphorescent lifetimes, showcasing purely organic room temperature phosphorescent properties.We expect that this protocol not only provide a detailed experimental procedure for the hydrothermal synthesis of long-lasting imaging nanomaterials, but also introduce the method for the copolymerization of persistent luminescent nanoparticles and MMA to further achieve ultraviolet mediated rewriteable and luminescent applications. The hydrothermal synthesis masses of persistent luminescent nanoparticles could restrict the application of certain temperature sensitive substance since they require relatively high reaction temperatures. Consequently, the selection of an appropriate synthesis method should be contingent on specific application and requirement.The main research directions in our laboratory include the development of temperature-sensitive luminescence nanoprobe for microscopic temperature monitoring, immune cells selective nanocomposite for immune imaging and therapy, and the establishment of new acoustic and magnetic responsive materials.

Synthesis of Zn2GeO4&#58; Mn Persistent Luminescent Nanoparticles (PLNPs)

To begin, take five milliliters of deionized water and add 10 millimolars of sodium hydroxide to it. Then add two millimolars of germanium oxide to the sodium hydroxide solution to prepare sodium germinate solution and stir the mixture at room temperature for about 30 minutes. Next, add four millimolars of zinc chloride, 0.01 millimolars of manganese nitrate, and 600 microliters of nitric acid to 22 milliliters of deionized water and stir until completely dissolved.Now, slowly add the sodium germinate solution to the prepared solution and add one milliliter of polyethylene glycol. Then place the calibrated pH meter probe into the solution to monitor the pH value. Add ammonium hydroxide with a mass fraction of 25 to 28%to the solution drop by drop and adjust the pH to 6.0, 8.0, or 9.4.Cover the beaker with sealing film, and stir the solution at room temperature for one hour while maintaining a constant stirring speed. Afterward, transfer the solution to a Teflon lined autoclave and place it in an electric thermostatic drying oven at 220 degrees Celsius for four hours. After completion, turn off the oven and let the system cool to room temperature before taking out the reactor.Open the reactor slowly and transfer the reaction solution to 250 milliliter centrifuge tubes. Rinse the reactor with 40 milliliters of ethanol and transfer the ethanol to the tubes. Then vortex the solution for 30 seconds.Centrifuge at 4, 000 G for 15 minutes at room temperature and remove the supernatant. Next, add 10 milliliters of deionized water to each tube and sonicate for five minutes to redisperse the product. Add 20 milliliters of ethanol to each tube and vortex for 30 seconds for even mixing.Repeat the centrifugation process and discard the supernatant. Ultrasonicate the product for five minutes to disperse the product in two milliliters of methanol solution and store the sample at four degrees Celsius after sealing it with sealing film. The TEM image showed that zinc germinate manganese PLNPs with pH 9.4 were rod shaped and had an average diameter of about 65 nanometers.The high resolution TEM images demonstrated that the nanomaterials had good crystallinity with a lattice spacing of 0.70 nanometers. The afterglow visible emission spectra of the zinc germinate manganese solution showed that the PLNPs exhibited a red shift of the main emission peak when the precursor solution changed from acidic to alkaline. Additionally, the pH value had an impact on the decay curve of afterglow intensity over time.The photo luminescence images and afterglow images of PLNPs revealed that the solution exhibited a brighter green luminescence as the alkalinity of the solution increased.

Research

JoVE Journal

Chemistry

Persistent luminescent nanoparticles (PLNPs) possess the capabilities to maintain extended longevity and robust emission even after the excitation has ...