Long-persistent far-UVC light emission in Pr³⁺-doped Sr₂P₂O₇ phosphor for microbial sterilization

Abstract

With the occurrence of global health events such as COVID-19, the development of high-performance ultraviolet-C (UVC) light sources for efficient and convenient sterilization applications holds promising scientific and engineering prospects. However, discovering reliable and stable luminescent materials capable of converting incident excitation energy to UVC radiation remains a challenge. Herein, we report a new Sr₂P₂O₇:Pr³⁺ phosphor that exhibits exceptional capabilities in X-ray energy absorption, storage, UVC-light conversion, and persistent UVC light emission. Specifically, minutes of X-ray excitation can result in more than 24 h of continuous UVC persistent luminescence when monitored at 222 nm emission, realizing far-UVC afterglow for the first time. Intense UVC persistent light emission could be detected by a UVC corona camera in bright indoor and outdoor environments without interference from artificial light or natural sunlight, in consideration of the avoidance of the spectral overlap with ambient light. The self-sustained UVC persistent luminescence form was demonstrated as an optical-conversion strategy for sterilization application, whereby the charged Sr₂P₂O₇:Pr³⁺ persistent phosphor could effectively inactivate infectious methicillin-resistant Staphylococcus aureus (MRSA) within 30 min in an excitation-free manner, offering new insights into developing UVC light sources for sterilization applications. This work expands the field of UV persistent luminescence research to the far-UVC spectral region and is expected to bring new and power-free solutions to some important applications where far-UVC light is needed, such as sterilization and photodynamic therapy.

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Introduction

The development of ultraviolet (UV, 200–400 nm) light-emitting materials is attracting widespread attention due to the distinct wavelength features of UV light in realizing unprecedented applications, including photocatalysis, phototherapy, sterilization, anti-counterfeiting, and covert optical tagging.^1–7 By far, in contrast to the extensive research on luminescent materials emitting in the UVA (320–400 nm) and UVB (280–320 nm) spectral regions, the available materials capable of emitting UVC (200–280 nm) light are relatively lacking, even though there are growing demands for applications in microbial inactivation.^8–11 Currently, the most widely used UV-light sources are mercury lamps, xenon lamps, and excimer lamps, while these gaseous lamps suffer great limitations due to the unfavorable factors of environmental pollution, poor portability, and fragility.^12,13 Thanks to the rapid development of the light-emitting-diode (LED) industry in recent years, high-performance solid-state deep UV LEDs with the advantages of being more portable and durable, energy-efficient, and environmentally friendly, are increasingly considered a promising alternative to conventional gaseous UV-light sources.^14,15 However, to produce UV LEDs with shorter emission wavelengths, the Al component of the AlGaN layer should be increased accordingly, which means a great increase in the development difficulties, from material epitaxial growth to device fabrication.^16,17 On the other hand, with the continuous development of lanthanide-based luminescent materials recently, UVC luminescence can also be realized in some inorganic phosphors, providing new options for deep UV luminescence technology.^18–21 Especially, UVC persistent luminescence is becoming an indispensable complement to conventional UVC light sources.^22,23

For UVC persistent luminescence phosphors, the trivalent praseodymium (Pr³⁺) ion is considered a promising emitter due to its high-energy 4f5d state and effective 4f5d → 4f interconfigurational transitions.^24–26 Moreover, the Stokes shift of the 4f5d → 4f transitions should be less than ∼3000 cm⁻¹ (0.37 eV),²⁷ raising special requirements for suitable host materials. A series of encouraging works on Pr³⁺-doped deep UV persistent phosphors have been reported recently. For example, Yang et al. reported UVC persistent luminescence in Pr³⁺-doped Cs₂NaYF₆ with an emission peak at 250 nm and a duration time of ∼2 h after X-ray irradiation.²⁸ Li et al. reported an X-ray-activated LaPO₄:Pr³⁺ phosphor that exhibited UVC persistent luminescence at 231 nm for ∼2 h.²⁹ Another Pr³⁺-doped phosphate, LuPO₄:Pr³⁺, was investigated by Zhao et al., showing an enhanced UVC afterglow under sunlight stimulation.³⁰ Besides, the multi-responsive persistent phosphor Li₂CaGeO₄:Pr³⁺ was reported by Zhou et al., realizing multimode UVC emission.³¹ Pan and coworkers reported a series of Pr³⁺-doped silicate UVC persistent phosphors that featured hours of afterglow in the range of 265–270 nm after irradiation by a standard 254 nm UV lamp.³² Yan et al. also achieved intense UVC persistent luminescence in Pr³⁺-doped silicates, e.g., Lu₂SiO₅:Pr³⁺ and LiLuSiO₄:Pr³⁺,^33,34 and a considerable enhancement was achieved by Sm³⁺ co-doping in the LiLuSiO₄:Pr³⁺ system. However, the UVC-light emission from the aforementioned persistent phosphors mainly lay in the wavelength range above 250 nm, except for the (La,Lu)PO₄:Pr³⁺ phosphors with shorter emission wavelengths at ∼230 nm but a very short afterglow time.

For each possible application, there is a different requirement for the emission wavelength.³⁵ Recently, considerable attention has been paid to those luminescent materials whose emission wavelength is in the far-UVC (200–230 nm) region. This is because far-UV radiation has been demonstrated to be seriously detrimental to bacteria but without skin-damaging effects.^36,37 For example, Brenner's group utilized 222 nm UV light to kill bacteria efficiently but without damaging human cells in skin tissue (a bandpass filter was used to remove the lower- and higher-wavelength components of a krypton-chlorine (Kr-Cl) excimer lamp).^38,39 They demonstrated that 222 nm light is safe for mammalian skin because it cannot penetrate typical human cells, while it has enough penetration to efficiently kill bacteria and viruses that are physically very small (less than 1 μm in diameter).⁴⁰ Inspired by this, far-UVC persistent luminescence can also be realized through a combination of UVC persistent phosphors and suitable bandpass filters, which is expected to bring solutions to some critical applications where a far-UVC afterglow is needed. However, discovering high-performance UVC persistent phosphors with some or all of their emission band in the far-UVC spectral region is highly challenging.

In this work, we report a new Pr³⁺-doped Sr₂P₂O₇ persistent phosphor, whose UVC afterglow at 222 nm could last for more than 24 h after the cessation of X-ray irradiation. The UVC persistent luminescence signal could be detected without background interference by a solar-blind ultraviolet camera, and the visualized UVC emission could be sustained for several hours with high resolution in bright environments, thus enabling interference-free optical tagging applications in both indoor and outdoor ambient light environments. More importantly, the UVC persistent luminescence from the charged Sr₂P₂O₇:Pr³⁺ phosphor could kill MRSA efficiently in vitro within 30 min. This work is expected to inspire the discovery of more excellent Pr³⁺-activated far-UVC persistent phosphors for sterilization applications.

Experimental

Materials synthesis

The Sr_2-xP₂O₇:xPr³⁺ (x = 0, 0.001, 0.002, 0.005, 0.01, 0.02) phosphors were synthesized using a high-temperature solid-state reaction method. To synthesize Sr₂P₂O₇:Pr³⁺ phosphors, stoichiometric amounts of SrCO₃ (Aladdin 99.95%), NH₄H₂PO₄ (Aladdin 99.99%), and Pr(NO₃)₃·6H₂O (Aladdin 99.99%) powders were mixed and ground homogeneously in an agate mortar. All the chemicals were used as received without further purification. Then the obtained powders were pre-sintered at 600 °C in air for 2 h (heating rate, 10 °C min⁻¹). The pre-fired material was reground to fine powders and pressed into preformed disks with a diameter of ∼11 mm using a 30 T dry pressing machine. Finally, the disks were sintered at 1100 °C in air for 3 h (heating rate, 10 °C min⁻¹) to form the final Sr₂P₂O₇:Pr³⁺ solid ceramic disks.

Characterization

The crystal structure and phase composition of the samples were measured by a powder X-ray diffractometer (DMAX-2500PC) with Cu Kα1 radiation (λ = 1.5406 Å). Rietveld refinements of the XRD patterns were performed by utilizing general structure analysis system (GSAS) software. Scanning electron microscopy (SEM) mapping images of the samples were performed on an FEI Nova NanoSEM 430 microscope. The spectral properties, including photostimulated luminescence and persistent luminescence properties, were measured using an FLS1000 spectrofluorometer (Edinburgh Instrument) equipped with a 450 W xenon arc lamp and a photomultiplier tube (measurement range, 200–900 nm). The low-temperature experiments were measured with an OptistatDN cryostat (Oxford Instruments) equipped with a MercuryiTC temperature-controlled system. The thermoluminescence (TL) spectra were recorded using an SL18 thermoluminescence instrument (Guangzhou Rongfan Science and Technology Co., Ltd; heating rate, 4 K s⁻¹). Before all the measurements, the samples were thermally cleaned at 450 °C to empty the traps thoroughly, followed by irradiation with an X-ray tube (Moxtek, tungsten target). UVC persistent luminescence images were obtained by using a solar-blind ultraviolet camera (WNZW-01, Suzhou Micro-Nano Laser & Photonic Technology Co., Ltd). This camera could capture dual-channel luminescence images for both visible and UVC emissions (240–280 nm), which superimpose the UVC image onto the visible image. The UVC radiation was denoted as an area of red color, whose area was proportional to the emission intensity. The UVC persistent luminescence power intensity was measured using a Newport 1936-R optical power meter and a Newport 918D-UV-OD3R UV-enhanced silicon photodetector.

Sterilization experiment

Methicillin-resistant Staphylococcus aureus (MRSA) suspensions (10⁶ cfu mL⁻¹) were inoculated in 48-well plates with 1 mL of strain solution added to each well. Subsequently, the sterilized Sr₂P₂O₇:Pr³⁺ disks (∼10.7 mm diameter) were divided into two groups: control samples, which were not irradiated and were placed directly on top of the well plates, and the experimental group samples, which were irradiated by an X-ray for 20 min and placed on top of the well plates, then followed by the start of timing. After 30 min, the Sr₂P₂O₇:Pr³⁺ disks were removed and the bacterial solution was stained using a Live & Dead Bacterial Staining Kit (100 T, Yeasen), which could stain live and dead bacteria green and red, respectively. Next, 10 μL of the prepared dye solution (60 μL of DMAO, 120 μL of EthD-III, and 80 μL of 0.85% NaCl solution) was mixed thoroughly with 1 mL of the bacterial suspension and then incubated in the darkness for 15 min. The MRSA suspension was imaged using a confocal laser scanning microscope, where the live (green fluorescent) and dead (red fluorescent) bacteria could be observed with the FITC and Cy3 channels, respectively.

Biocompatibility evaluation

C57 mouse dermal fibroblasts (mDF) suspensions (100 μL per well) were inoculated in 24-well plates, which were subsequently divided into a control group and an experimental group (with sterilized Sr₂P₂O₇:Pr³⁺ disk). After incubation in a cell culture incubator (37 °C, 5% CO₂) for different times (24, 48, and 72 h), the cell activity was detected using the CCK-8 kit (Solarbio).

First-principles calculations

Using the Vienna Ab initio simulation package (VASP),^41,42 theoretical simulations were carried out based on density functional theory (DFT) using the generalized density approximation (GGA)–Perdew–Burke–Ernzerhof (PBE) functional for relaxation of the atomic positions and the self-consistent calculations. Also, the screened Coulomb hybrid functional suggested by Heyd, Scuseria, and Ernzerhof (HSE06) was used to calculate the electronic structures of Sr₂P₂O₇, as shown in the equation below:where E^HF_x and E^PBE_x are the exact Hartree–Fock (HF) and PBE exchange energies, E^PBE_c is the PBE correlation energy, the mixing parameter α is set to 0.25, and the screening parameter ω is 0.20 Å⁻¹.^43,44 The initial atomic positions and crystal symmetry group information of the crystal structure were taken from the Inorganic Crystal Structure Database (ICSD) and the primitive cell of the Sr₂P₂O₇ contained 88 atoms (16 Sr atoms, 16 P atoms, and 56 O atoms). The Sr (4s²4p⁶5s²), P (3s²3p³), and O (2s²2p⁴) were treated as valence electrons, and their interactions with the cores were described by the projector augmented wave (PAW) method. The energy change and the Hellmann–Feynman forces on the atoms were set to 10⁻⁶ eV and 0.01 eV Å⁻¹, respectively. Convergence of the total energy was assured by setting the plane-wave cutoff energy to 550 eV. For k-point integration within the first Brillouin zone, a 3 × 4 × 4 Monkhorst–Pack grid was selected for the self-consistent calculations, while the high symmetric K-paths with zero weight were generated for the HSE06 calculations. Based on the static states mentioned above, the band structure and densities of states (DOS) of the Sr₂P₂O₇ host were calculated.

Results and discussion

Crystal structure and phase identification

The Sr₂P₂O₇ crystal has an orthorhombic structure belonging to the space group Pnma with the cell parameters of a = 8.9104 Å, b = 5.4035 Å, c = 13.1054 Å, α = β = γ = 90°, V = 631 ų, and Z = 4, which forms a three-dimensional network by linking diphosphate ions with strontium ions.⁴⁵ As shown in Fig. 1a, each strontium atom coordinates nine terminal oxygens from diphosphate groups to form two crystallographically different [SrO₉] decahedrons, named [Sr1O₉] and [Sr2O₉], respectively. Given the closeness of the effective ionic radii, the Pr³⁺ ion (1.179 Å, CN = 9) most probably occupies the Sr²⁺ site (1.31 Å, CN = 9) rather than the P⁵⁺ site (0.17 Å, CN = 4) in the Sr₂P₂O₇ lattice.⁴⁶Fig. 1b depicts the XRD patterns of the as-synthesized Sr_2-xP₂O₇:xPr³⁺ (x = 0, 0.001, 0.002, 0.005, 0.01, 0.02) phosphors. All the diffraction peaks of Sr₂P₂O₇:Pr³⁺ were consistent with the Sr₂P₂O₇ crystal (JCPDS no. 75-1490), demonstrating that no impurity phase was formed with the introduction of Pr³⁺ ions. The Rietveld refinements of the Sr_1.995P₂O₇:0.005Pr³⁺ (SPO:0.005Pr³⁺) phosphor in Fig. 1c were well convergent with the reliability parameters of R_wp = 7.94%, R_p = 5.53%, and χ² = 2.74, indicating the successful formation of the target Sr₂P₂O₇ phase with high purity. The detailed crystallographic data of the representative SPO:0.005Pr³⁺ phosphor is provided in Table S1.† From the SEM images of the sample in Fig. 1e, one can see that the morphology of the particle was irregular and the size ranged from 2–15 μm. The elemental mapping images of a randomly selected particle show that the elements Sr, P, O, and Pr were distributed evenly throughout the whole particle without any traceable element aggregation or phase separation, indicating that Pr³⁺ had been well dispersed in the Sr₂P₂O₇ host.

Fig. 1 (a) Crystal structure of Sr₂P₂O₇ and the local coordination environment of Sr²⁺ ions. (b) XRD patterns of the Sr_2-xP₂O₇:xPr³⁺ (x = 0, 0.001, 0.002, 0.005, 0.01, 0.02) phosphors. (c) Rietveld refinement of the SPO:0.005Pr³⁺ phosphor. (d) EDS spectrum, (e) SEM and EDS mapping images of the SPO:0.005Pr³⁺ phosphor.

UVC photoluminescence and persistent luminescence properties

Based on the fact that the luminescence properties of Pr³⁺ are susceptible to the host material, the band structure and densities of states (DOS) of the Sr₂P₂O₇ host were first calculated by density functional theory (DFT). As depicted in Fig. 2a, the valence band (VB) maximum and the conduction band (CB) minimum were located at the same point, which indicates that the Sr₂P₂O₇ host had a direct bandgap (E_g). The band gap of the Sr₂P₂O₇ was calculated to be 6.89 eV, which was large enough to accommodate massive energy traps and enabled the feasibility of deep UV and even far-UVC luminescence. Besides, it was observed from the total and partial DOS of Sr₂P₂O₇ in Fig. 2b that the VB mainly consisted of the p orbital of O atoms, while the CB was composed mostly of the hybridization between the d orbital of Sr atoms and the p orbital of O atoms, in which the heavy element Sr could favorably capture high-energy irradiation. The calculated results indicate that the Sr₂P₂O₇ host holds great potential for Pr³⁺ to realize deep UV persistent luminescence under high-energy X-ray irradiation.

Fig. 2 (a) Band structure and (b) total DOS and partial DOS of the Sr₂P₂O₇ host. (c) VUV excitation (λ_em = 260 nm) and emission spectra (λ_ex = 193 nm) of the SPO:0.005Pr³⁺ phosphor. (d) Radioluminescence spectra of the Sr_2-xP₂O₇:xPr³⁺ (x = 0.001, 0.002, 0.005, 0.01, 0.02) phosphors.

Fig. 2c shows the vacuum ultraviolet (VUV) photoluminescence emission and excitation spectra of the Sr₂P₂O₇:Pr³⁺ phosphor at room temperature. When excited with VUV light at 193 nm, the Sr₂P₂O₇:Pr³⁺ phosphor exhibited broad emission bands distributed in the UV-spectral region with four distinct emission peaks at 222, 233, 248, and 262 nm, which could be attributed to the interconfigurational transitions from the lowest 4f5d state to various 4f² states (³H₄, ³H₅, ³H₆, and ³F₂) of the Pr³⁺ emitter.^47,48 By monitoring the photoluminescence emission at 260 nm, the VUV excitation spectrum of the Sr₂P₂O₇:Pr³⁺ phosphor was obtained. The lowest 4f5d level of the Sr₂P₂O₇:Pr³⁺ phosphor was measured at 216 nm. The other excitation bands peaking at 196, 175, and 165 nm were attributed to the absorption of the P₂O₇²⁻ complex.⁴⁹ To further study the steady-state radioluminescence of the Sr₂P₂O₇:Pr³⁺ phosphor, the X-ray excited luminescence spectra were also measured, as shown in Fig. 2d. The emission spectra of the Sr_2-xP₂O₇:xPr³⁺x = 0.001, 0.002, 0.005, 0.01, 0.02) phosphor upon X-ray excitation were consistent with that of the VUV emission spectrum in the ultraviolet region, which also showed an obvious far-UVC emission peak at 222 nm. The radioluminescence intensity showed an increasing trend as the Pr³⁺-doping concentration increased from 0.001 to 0.002, then it decreased with further increasing the Pr³⁺ content. The visible emission peaks at ∼479 and ∼596 nm were ascribed to the ³P₀ → ³H₄ and ¹D₂ → ³H₄ intraconfigurational transitions of Pr³⁺ ion, respectively.⁵⁰

In addition to UVC photoluminescence, the Sr₂P₂O₇:Pr³⁺ phosphor also exhibited long-lasting persistent luminescence properties after ceasing high-energy X-ray irradiation. The optimal X-ray (50 kV, 80 μA) excitation duration to fully charge the Sr₂P₂O₇:Pr³⁺ phosphor was determined to be 20 min, as shown in Fig. S1.† Besides, it was found that there was a direct relationship between the UVC afterglow performance and Pr³⁺-doping concentration. Based on the persistent luminescence decay curves and thermoluminescence spectra in Fig. S2 and S3,† the optimal Pr³⁺-doping concentration was determined to be 0.005. Fig. 3a depicts the far-UVC persistent luminescence decay curve of the SPO:0.005Pr³⁺ phosphor monitored at 222 nm. The far-UVC afterglow intensity was still an order of magnitude higher than the background after 24 h of natural decay in the darkness, realizing long-lasting far-UVC persistent luminescence for the first time to the best of our knowledge. The far-UVC persistent luminescence intensity of the charged phosphor disk dropped quite rapidly in the first 3 h, then it exhibited a slow decreasing trend until the end of the measurement. The upper insert shares a consistent emission profile with the photoluminescence emission spectrum, indicating that the UVC persistent luminescence originated from the Pr³⁺ emitting center. The persistent luminescence emission spectra at different decay instants were also recorded in Fig. 3b. The spectral shape remained unchanged while the UVC emission intensity gradually decreased within 12 h natural decay after ceasing X-ray excitation. Moreover, the UVC persistent luminescence decay curves of the SPO:0.005Pr³⁺ phosphor in an indoor LED lighting environment were also recorded to evaluate its afterglow performance in different light conditions, as presented in Fig. 3c. The emission spectrum of a white LED was recorded, as shown in Fig. S4.† Compared to the persistent luminescence emission in the darkness, an obvious enhancement of the UVC afterglow intensity was observed after the initial 1 hour decay, as the illuminance increased from 100 to 500 lux. Subsequently, crossover started to appear for the afterglow decay curves, but the UVC afterglow intensity in the bright ambient light environment was still higher than that in the darkness due to the continuous photostimulation of polychromic ambient light,^34,51,52 indicating that the SPO:0.005Pr³⁺ phosphor has promising application prospects in both dark and bright environments. The absolute UVC afterglow intensity at 222 nm was also measured by an optical power meter with a UV silicon photodetector. As presented in Fig. S5† and Table S2,† the far-UVC persistent luminescence power density at 30 s after the cessation of X-ray excitation was measured and calculated to be 35.42 mW m⁻², and the recording lasted for 30 min. Detailed measurements and calculations were made according to the procedures described in ref. 28. As displayed in Table S3,† the Sr₂P₂O₇:Pr³⁺ phosphor showed the best UVC afterglow performance compared to other previously reported UVC persistent luminescence phosphors.

Fig. 3 (a) Far-UVC persistent luminescence decay curve of the SPO:0.005Pr³⁺ phosphor monitored at 222 nm. (b) Persistent luminescence emission spectra of the SPO:0.005Pr³⁺ phosphor at different decay instants. (c) Persistent luminescence decay curves of the SPO:0.005Pr³⁺ at different light conditions. (d) UVC persistent luminescence images of the charged SPO:0.005Pr³⁺ phosphor in an indoor LED lighting environment (200 lux). (e) UVC persistent luminescence images of the charged SPO:0.005Pr³⁺ phosphor in an outdoor sunlight environment (∼13 000 lux). The sample was pre-irradiated by X-ray for 20 min.

The absence of UVC light on the Earth's surface due to ozone absorption results in zero background interference, which enables the UVC persistent luminescence emission from the Sr₂P₂O₇:Pr³⁺ phosphor to be visually detected by a solar-blind UV camera.^32,53 As the red patterns shown in Fig. 3d, the UVC afterglow signal could be monitored and imaged for more than 6 h with high contrast in an indoor LED lighting environment (∼200 lux), which is much more durable than the other materials reported so far. The UVC persistent luminescence could also be detected by the UVC camera for more than 1 h in an outdoor environment under direct sunlight (∼13 000 lux), as shown in Fig. 3e. The obvious enhancement in UVC persistent luminescence intensity as well as shortened imaging time in an outdoor sunlight environment were due to the strong photostimulation effect of natural sunlight illumination, which induces a much faster release of the stored electrons in energy traps. Based on the photostimulation results, the controllable regulation of the afterglow performance could be achieved by reasonably increasing the ambient illuminance, which is expected to realize a considerable enhancement of UVC persistent luminescence in a variety of complex light conditions. Furthermore, to evaluate the stimulation effect of ambient temperature, the UVC persistent luminescence performance at 77 K (liquid-nitrogen temperature) was also recorded in Fig. S6.† Compared to the UVC afterglow decay curve at room temperature, the persistent luminescence intensity was extremely weak, suggesting that the trapped electrons were not released without external thermal stimulation. The 77 K low-temperature experiments further verified that the ambient temperature plays an enormous role in the long-lasting UVC persistent luminescence process.

Thermoluminescence properties

To gain insight into the reason for the excellent UVC afterglow performance in the Sr₂P₂O₇:Pr³⁺ phosphor, a series of thermoluminescence (TL) measurements were conducted under different indoor LED lighting conditions to evaluate the energy-storage capacity of the Sr₂P₂O₇:Pr³⁺ phosphor, which provides reliable information of the energy traps to analyze the trapping and de-trapping processes during prolonged persistent luminescence.⁵⁴ Compared to the undoped Sr₂P₂O₇ host (Fig. S3†), all the TL curves with different Pr³⁺-doping concentrations exhibited a significant enhancement in TL intensity, implying the formation of new energy traps after the introduction of Pr³⁺. Furthermore, time-dependent TL measurements from 10 min to 24 h were conducted to investigate the de-trapping process of the charged Sr₂P₂O₇:Pr³⁺ phosphor in the darkness. As shown in Fig. 4a, the TL curve after 10 min decay showed a broadband emission ranging from 297 to 620 K with two distinct glow peaks at 392.4 and 459.6 K, which could serve as the shallow trap and deep trap, respectively. The intensities of both TL bands started to decline with the prolonging of the decay time, and the low-temperature (LT) band almost vanished after 3 h of natural decay in the darkness, illustrating the gradual release of the trapped electrons in the energy traps. The trap depth was calculated by the initial rising method, which assumes that the concentration of the trapped carriers is approximately constant.⁵⁵ The equation is approximated to

I(T) = C exp(−ΔE/kT) (2)

where I(T) is the TL intensity at temperature T, C is a fitting constant, ΔE is the requested trap depth, and k is the Boltzmann constant. The trap depths were obtained by calculating the slope of the curves plotted in ln(I) versus 1/kT as coordinates, as depicted in Fig. S7.† As presented in the inset of Fig. 4a, the trap depth for the decay instant of 10 min in the darkness was calculated to be 0.94 eV, gradually increasing to 1.19 eV with the decay time extending to 24 h, which indicates the presence of a continuous trap distribution in the Sr₂P₂O₇:Pr³⁺ phosphor. The TL intensity of the high-temperature (HT) band remained appreciable after 24 h decay, demonstrating that there still existed a large number of stored electrons in deep traps that were released quite slowly under room-temperature thermal stimulation. Meanwhile, TL curves of the Sr₂P₂O₇:Pr³⁺ phosphor under the illumination of a white LED (300 lux) were also recorded to study the stimulation effect of the ambient light on the de-trapping process. As shown in Fig. 4b, the intensities of both TL bands dropped quite rapidly compared to that in the darkness. Especially with the increase in the decay time, the TL curve of the deep traps exhibited a distinct rightward and downward trend. Using the TL spectrum after 10 min of natural decay in the darkness as a reference, the intensity of the deep traps dropped to one-third after 3 h of white LED illumination, and only about 3% remained after 24 h of decay. The trap depths were basically sustained at ∼0.81 eV within 24 h of white LED illumination, which are the suitable trap depths for long-lasting persistent luminescence at room temperature,⁵⁶ which endows this phosphor with excellent UVC afterglow performance in bright environments. Upon the illumination of white LED, the stored electrons that were difficult to release by room-temperature thermal stimulation could obtain enough energy to escape from the deep energy traps, resulting in the redistribution of the stored electrons in the energy traps and the change in the trap depth. Accompanied by the continuous refilling of the shallow traps, the UVC afterglow under ambient light illumination was enhanced, which was in agreement with the persistent luminescence decay curves in Fig. 3c. Because the TL curves of the Sr₂P₂O₇:Pr³⁺ phosphor consisted of two overlapping glow peaks, a series of thermal cleaning experiments were performed at different de-trapping temperatures. The pre-irradiated Sr₂P₂O₇:Pr³⁺ phosphor disk was heated to the de-trapping temperature from 298 to 473 K with an interval of 25 K, and then rapidly cooled down to room temperature. Fig. 4c depicts the temperature-dependent TL curves after partial thermal cleaning. With the de-trapping temperature increasing, the intensity of the low-temperature band started to gradually decrease and disappeared at 373 K, whereas the high-temperature band only decreased by ∼10%. The high-temperature band disappeared completely until the de-trapping temperature rose to 448 K. The trap depths were calculated to increase from 0.85 eV to 1.43 eV during the thermal cleaning process, which encompassed two distinct energy trap regions, i.e., 0.85–0.97 eV for shallow traps and 1.18–1.43 eV for deep traps, as shown in Fig. 4d. As for Sr₂P₂O₇:Pr³⁺ phosphor, the oxygen vacancies caused by high-temperature sintering may serve as the deep trapping centers, while the defect complexes due to the cation size mismatch and charge imbalance between Pr³⁺ and Sr²⁺ may act as shallow traps. Considering the time-dependent TL curves in Fig. 4a, the low-temperature band was sustained for about 3 h, and its trap depth was calculated to be 1.10 eV after 3 h of natural decay, which approaches the trap depth after thermal cleaning with the de-trapping temperature at 423 K (1.18 eV). In conclusion, the shallow traps were mainly responsible for the intense UVC persistent luminescence in the initial stage (0–3 h), while the deep energy traps at a depth of 1.18–1.43 eV contributed to the long-lasting UVC afterglow (3–24 h).

Fig. 4 (a) TL spectra of the SPO:0.005Pr³⁺ phosphor at different decay instants in the darkness. (b) TL spectra of the SPO:0.005Pr³⁺ phosphor at different decay instants in a bright indoor environment (300 lux). (c) TL curves of the SPO:0.005Pr³⁺ phosphor with thermal cleaning at different de-trapping temperatures from 298 to 473 K with an interval of 25 K in the darkness, and (d) the calculated trap depth using the initial rising method. The sample was pre-irradiated by X-ray for 20 min.

Photostimulated UVC persistent luminescence properties

According to the TL results, a large number of trapped electrons still remained in the deep traps after 24 h of natural decay in the darkness, which means that additional external stimulation is necessary to release the stored energy in the Sr₂P₂O₇:Pr³⁺ phosphor. Therefore, low-energy visible and near-infrared (NIR) light sources were used to facilitate the release of those deep-trapped electrons, enabling the rejuvenation of far-UVC persistent luminescence through portable and feasible optical stimulation.^57,58 As shown in Fig. 5a, the repeated photostimulated persistent luminescence decay curves at 222 nm were recorded on 24 h-decayed samples using a commercial white LED for photostimulation for a total of 8 times. In each cycle, the Sr₂P₂O₇:Pr³⁺ phosphor was illuminated by a white LED (800 lux) for 30 s, followed by 20 min of far-UVC persistent luminescence. After short-time illumination of the white LED, the far-UVC afterglow intensity was noticeably enhanced by more than an order of magnitude. With the increase in the photostimulation cycle, the initial afterglow intensity gradually decreased, but it was still higher than the original afterglow intensity. Besides, such an enhancement of far-UVC persistent luminescence could also be observed when illuminating the Sr₂P₂O₇:Pr³⁺ phosphor with deep-tissue penetrable NIR light, e.g., 808 nm laser (100 mW for 10 s), as presented in Fig. 5b.

Fig. 5 (a) Repeated photostimulated persistent luminescence decay curves obtained on a 24 h-decayed SPO:0.005Pr³⁺ phosphor disc. The decayed sample was illuminated by a white LED (800 lux) for 30 s at every 20 min and (b) 808 nm NIR laser (100 mW) for 10 s at every 10 min for a total of 8 times. (c) Schematic representation of UVC persistent luminescence mechanism in the Sr₂P₂O₇:Pr³⁺ phosphor.

Based on the above results and discussion, a luminescence mechanism is proposed to interpret the UVC persistent luminescence in the Sr₂P₂O₇:Pr³⁺ phosphor, as depicted in Fig. 5c. Upon X-ray irradiation, the high-energy X-ray photons are absorbed by the lattice atoms due to the photoelectric effect, followed by the ejection of energetic electrons and the cascading production of additional ionized electrons. Then the electrons of the Sr in the VB are excited into the CB through low-energy elastic collisions, while the generated holes stay in the VB (process 1).^59,60 Subsequently, the excited electrons and holes are captured by energy traps and Pr³⁺, respectively (process 2).⁶¹ After the cessation of X-ray irradiation, the stored electrons in the shallow traps gradually de-trap into the CB owing to the thermal vibration (process 3) and then re-combine with the excited state of Pr³⁺ (process 5). The spontaneous recombination of electrons and holes in the Pr³⁺ emitters leads to UVC persistent luminescence (process 6). After a long period of natural decay, the stored electrons in the shallow traps are mostly exhausted, whereas there are still a large number of electrons in these deep traps. Under low-energy visible or NIR light stimulation, these deep-trapped electrons can escape back to the CB and part of them can refill the depleted shallow traps (process 4), enhancing the far-UVC persistent luminescence.

Sterilization application of the Sr₂P₂O₇:Pr³⁺ phosphor

Methicillin-resistant Staphylococcus aureus (MRSA), a highly virulent drug-resistant bacterium with high morbidity and mortality, has become one of the major sources of common pathogens resulting in hospital-acquired infections, posing a serious threat to human health.^62,63 As a proof of concept, a series of bactericidal experiments were designed to validate the inactivation effect of the charged Sr₂P₂O₇:Pr³⁺ phosphor on MRSA. A schematic diagram of the sterilization application is presented in Fig. 6a. The disinfected Sr₂P₂O₇:Pr³⁺ phosphor disc was irradiated by an X-ray beam for 20 min and then placed on top of the well plate filled with MRSA suspensions. As shown in Fig. 6b, the MRSA exhibited different degrees of inactivation after undergoing various irradiation durations from 0 to 30 min. The subsequent laser scanning confocal micrographs presented an exciting result showing that the vast majority of the bacteria had died after 30 min direct irradiation, indicating that the MRSA could be effectively inactivated by the persistent UVC light emission from the charged Sr₂P₂O₇:Pr³⁺ phosphor disc. In addition, sterilization experiments were also conducted with Sr₂P₂O₇:Pr³⁺ phosphor disks irradiated by X-ray for 0, 1, 5, 10, and 20 min, respectively. As predicted in Fig. S8,† there was an obvious improvement of sterilization effect with the increase in the X-ray charging time. Furthermore, the Sr₂P₂O₇:Pr³⁺ phosphor also showed very good biocompatibility, as shown in Fig. S9.† The above-mentioned results provide solid evidence that the Sr₂P₂O₇:Pr³⁺ phosphor with a remarkable bactericidal effect and good biocompatibility has great potential for realizing effective sterilization applications under excitation-free conditions.

Fig. 6 (a) Schematic diagram of sterilization application. (b) Confocal laser scanning micrograph of MRSA with different irradiation time by the charged SPO:0.005Pr³⁺ phosphor discs. The discs were pre-irradiated by X-ray for 20 min, then covered on the top of the 48 wells plate for different irradiation durations.

Conclusions

In summary, a new Sr₂P₂O₇:Pr³⁺ persistent phosphor was developed, realizing far-UVC persistent luminescence for the first time and the best UVC afterglow performance compared to other previously reported UVC persistent phosphors. Upon minutes of X-ray irradiation, the as-synthesized Sr₂P₂O₇:Pr³⁺ phosphor emitted intense far-UVC afterglow at 222 nm for more than 24 h. Besides, enhanced UVC persistent luminescence was observed under the continuous photostimulation of an indoor white LED, indicating that this phosphor can work well in bright environments. In contrast to visible or NIR persistent luminescence, which is commonly susceptible to ambient light interference, the long-lasting UVC afterglow from the charged Sr₂P₂O₇:Pr³⁺ phosphor could be clearly detected by a solar-blind corona camera for several hours in bright indoor and outdoor environments, thus providing a potential alternative for interference-free optical tagging applications. More importantly, the unique advantage of UVC light for bacterial inactivation was also demonstrated, which could be used to kill MRSA effectively under excitation-free conditions. Overall, the prepared Sr₂P₂O₇:Pr³⁺ phosphor is expected to find many important applications in sterilization, secret optical tagging, and photodynamic therapy.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by Key Research and Development Program of Shandong Province (Major Scientific and Technological Innovation Project) (Grant No. 2021CXGC011101), National Natural Science Foundation of China (Grant Nos. 51902184 and 82201741), State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences (Grant No. SITP-NLIST-YB-2022-10), and “Qi-Lu Young Scholar Fund” from Shandong University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qi01253k

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