In Vivo Repeatedly Activated Persistent Luminescence Nanoparticles by Radiopharmaceuticals for Long-Lasting Tumor Optical Imaging

Enhancing Persistent Luminescence through Synergy between Optimal Electron Traps and Dye Sensitization

Due to their unique afterglow ability, long-wavelength-light rechargeable persistent luminescence (PersL) nanoparticles (PLNPs) have been emerging as an important category of imaging probes. Among them, ZnGa2O4:0.6% Cr3+ (ZGC) PLNPs have gained widespread recognition due to the ease of synthesis and uniform morphology. Unfortunately, the limited absorption arising from the low molar extinction coefficient of Cr3+ results in relatively low afterglow intensity and rapid decay after long-wavelength LED light irradiation. Herein, we discovered a strategy that boosting dye-sensitization performance was able to effectively amplify the PersL signal under white LED light. Specifically, Dil served as a highly efficient sensitizer for Cr3+, promoting the absorption of the excitation light. By adjusting the Pr dopant concentrations, ZGCP0.5 PLNPs with optimal trap densities were obtained, which showed the highest PersL intensity and dye-sensitized performance. Strikingly, ZGCP0.5-Dil PLNPs exhibited a 24.3-fold enhancement in intensity and a 2-fold prolongation of decay time over bare ZGC PLNPs through the synergy effect of optimal electron traps and dye sensitization. Photostable ZGCP0.5-Dil PLNPs enabled imaging of the HepG2 tumor and effectively guided tumor surgical resection verified by the H&E staining analysis. This strategy could be a significant reference in other dye-sensitization PLNPs to enhance longer-wavelength rechargeable PersL.

Activatable Probes for Ratiometric Imaging of Endogenous Biomarkers In Vivo

Dynamic variations in the concentration and abnormal distribution of endogenous biomarkers are strongly associated with multiple physiological and pathological states. Therefore, it is crucial to design imaging systems capable of real-time detection of dynamic changes in biomarkers for the accurate diagnosis and effective treatment of diseases. Recently, ratiometric imaging has emerged as a widely used technique for sensing and imaging of biomarkers due to its advantage of circumventing the limitations inherent to conventional intensity-dependent signal readout methods while also providing built-in self-calibration for signal correction. Here, the recent progress of ratiometric probes and their applications in sensing and imaging of biomarkers are outlined. Ratiometric probes are classified according to their imaging mechanisms, and ratiometric photoacoustic imaging, ratiometric optical imaging including photoluminescence imaging and self-luminescence imaging, ratiometric magnetic resonance imaging, and dual-modal ratiometric imaging are discussed. The applications of ratiometric probes in the sensing and imaging of biomarkers such as pH, reactive oxygen species (ROS), reactive nitrogen species (RNS), glutathione (GSH), gas molecules, enzymes, metal ions, and hypoxia are discussed in detail. Additionally, this Review presents an overview of challenges faced in this field along with future research directions.

Nanocrystals for Deep-Tissue In Vivo Luminescence Imaging in the Near-Infrared Region

In vivo imaging technologies have emerged as a powerful tool for both fundamental research and clinical practice. In particular, luminescence imaging in the tissue-transparent near-infrared (NIR, 700-1700 nm) region offers tremendous potential for visualizing biological architectures and pathophysiological events in living subjects with deep tissue penetration and high imaging contrast owing to the reduced light-tissue interactions of absorption, scattering, and autofluorescence. The distinctive quantum effects of nanocrystals have been harnessed to achieve exceptional photophysical properties, establishing them as a promising category of luminescent probes. In this comprehensive review, the interactions between light and biological tissues, as well as the advantages of NIR light for in vivo luminescence imaging, are initially elaborated. Subsequently, we focus on achieving deep tissue penetration and improved imaging contrast by optimizing the performance of nanocrystal fluorophores. The ingenious design strategies of NIR nanocrystal probes are discussed, along with their respective biomedical applications in versatile in vivo luminescence imaging modalities. Finally, thought-provoking reflections on the challenges and prospects for future clinical translation of nanocrystal-based in vivo luminescence imaging in the NIR region are wisely provided.

New Opportunities and Old Challenges in the Clinical translation of Nanotheranostics

Nanoparticle-based systems imbued with both diagnostic and therapeutic functions, known as nanotheranostics, have enabled remarkable progress in guiding focal therapy, inducing active responses to endogenous and exogenous biophysical stimuli, and stratifying patients for optimal treatment. However, although in recent years more nanotechnological platforms and techniques have been implemented in the clinic, several important challenges remain that are specific to nanotheranostics. In this Review, we first discuss some of the many ways of 'constructing' nanotheranostics, focusing on the different imaging modalities and therapeutic strategies. We then outline nanotheranostics that are currently used in humans at different stages of clinical development, identifying specific advantages and opportunities. Finally, we define critical steps along the winding road of preclinical and clinical development and suggest actions to overcome technical, manufacturing, regulatory and economical challenges for the safe and effective clinical translation of nanotheranostics.

Cerenkov radiation-mediated in situ activation of silicon nanocrystals for NIR optical imaging

Cerenkov radiation from radiopharmaceuticals (18F-FDG) serves as an internal light source to excite UV-responsive silicon nanocrystals for near-infrared luminescence imaging that offers deeper tissue penetration and high signal-to-noise ratio.

Preparation of ZnGa2O4 nanoflowers and their full-color luminescence properties

Gallate material, a luminescent matrix with excellent performance is normally prepared by vapor deposition or solid phase sintering method at high temperature. However, it has not been solved to prepare gallate-based fluorescent materials with full-color luminescent properties at low temperature. In this paper, ZnGa2O4 undoped or doped with Cr or Mn nanoflowers composed of nanosheet-level structure were prepared by hydrothermal method at low temperature. Under ultraviolet light irradiation, ZnGa2O4, ZnGa2O4:Mn2+ and ZnGa2O4:Cr3+ display three primary colors of blue, green and red luminescence through self-excitation, Mn2+ and Cr3+ excitation respectively. The solid fluorescence yields of blue, green, and red colors are 32.3, 36.5, and 40.7%, respectively. It is highly expected to be applied to color display, biological imaging, white light devices.

Mesoporous polyacrylic acid/calcium phosphate coated persistent luminescence nanoparticles for improved afterglow bioimaging and chemotherapy of bacterial infection

Coating mesoporous drug carriers on the surface of persistent luminescence nanoparticles (PLNPs) not only allows continuous luminous imaging without spontaneous fluorescence interference, but also provides drug release guidance. However, in most cases, the encapsulation of the drug-loaded shells significantly reduces the luminescence of PLNPs, which is unfavorable for bioimaging. In addition, conventional drug-loaded shells alone, such as silica shells, have difficulty in achieving responsive fast drug release. Herein, we report the fabrication of mesoporous polyacrylic acid (PAA)/calcium phosphate (CaP) shell-coated PLNPs (PLNPs@PAA/CaP) for improved afterglow bioimaging and drug delivery. The encapsulation of the PAA/CaP shell effectively prolonged the decay time and enhanced the sustained luminescence of PLNPs by about three times due to the passivation of the surface defects of PLNPs by the shell, and the energy transfer between the shell and PLNPs. Meanwhile, the mesoporous structure and negative charge of the PAA/CaP shells enabled the prepared PLNPs@PAA/CaP to carry the positively charged drug doxycycline hydrochloride efficiently. Under the acidic conditions of bacterial infection, the degradation of PAA/CaP shells and the ionization of PAA enabled fast drug release for effective killing of bacteria at the infection site. The excellent persistent luminescence properties, outstanding biocompatibility, and rapid responsive release feature make the prepared PLNPs@PAA/CaP a promising nanoplatform for diagnostic and therapeutic applications.

Size Control and Improved Aqueous Colloidal Stability of Surface-Functionalized ZnGa2O4:Cr3+ Bright Persistent Luminescent Nanoparticles

Near-infrared (NIR)-emitting ZnGa₂O₄:Cr³⁺ (ZGO) persistent luminescent nanoparticles (PLNPs) have recently attracted considerable attention for diverse optical applications. The widespread use and promising potential of ZGO material in different applications arise from its prolonged post-excitation emission (several minutes to hours) that eliminates the need for continuous in situ excitation and the possibility of its excitation in different spectral regions (X-rays and UV-vis). However, the lack of precise control over particle size/distribution and its poor water dispersibility and/or limited colloidal stability required for certain biological applications are the major bottlenecks that limit its practical applications. To address these fundamental limitations, herein, we have prepared oleic acid (OA)-stabilized ZGO PLNPs with controlled size (7-12 nm, depending on the type of alcohol used in synthesis) and monodispersity. A further increase in size (8-21 nm), with a concomitant increase in persistent luminescence, could be achieved using a seed-mediated approach, employing the as-prepared ZGO PLNPs from the first synthesis as the seed and growing layers of the same material by adding fresh precursors. To remove their surface oleate groups and make the nanoparticles hydrophilic, two surface modification strategies were evaluated: modification with only poly(acrylic acid) (PAA) as the hydrophilic capping agent and modification with either PAA or cysteamine (Cys) as the hydrophilic capping agent in conjunction with BF₄^- as the intermediate surface modifier. The latter surface modifications involving BF₄^- conferred long-term (60 days and longer) colloidal stability to the nanoparticles in aqueous media, which is related to their favorable ζ potential values. The proposed generalized strategy could be used to prepare different kinds of surface-functionalized PLNPs with control of size, hydrophilicity, and colloidal stability and enhanced/prolonged persistent luminescence for diverse potential applications.

Research progress on near-infrared long persistent phosphor materials in biomedical applications

After excitation is stopped, long persistent phosphor materials (LPPs) can emit light for a long time. The most important feature is that it allows the separation of excitation and emission in time. Therefore, it plays a vital role in various fields such as data storage, information technology, and biomedicine. Owing to the unique mechanism of storage and luminescence, LPPs can avoid the interference of sample autofluorescence, as well as show strong tissue penetration ability, good afterglow performance, and rich spectral information in the near-infrared (NIR) region, which provides a broad prospect for the application of NIR LPPs in the field of biomedicine. In recent years, the development and applications in biomedical fields have been advanced significantly, such as biological imaging, sensing detection, and surgical guidance. In this review, we focus on the synthesis methods and luminescence mechanisms of different types of NIR LPPs, as well as their applications in bioimaging, biosensing detection, and cancer treatment in the field of biomedicine. Finally, future prospects and challenges of NIR LPPs in biomedical applications are also discussed.

Design of need-based phosphors and scintillators by compositional modulation in the ZnGa2-xAlxO4:Cr3+ spinel: pure compound versus solid solutions

Materials that can depict persistent deep red light under both ultraviolet (UV) and X-ray illumination can be a boon to sustainable economy, particularly for optical imaging, solid state lighting, and anticounterfeiting applications. Herein, we have made a series of compounds starting from ZnGa₂O₄:Cr³⁺ to ZnAl₂O₄:Cr³⁺ (individual spinel) by substituting the varied concentration of Al³⁺ in place of Ga³⁺ in ZnGa_2-xAl_xO₄:Cr³⁺ (solid solution). By virtue of the structural and defect engineering doping strategy, the photo and radioluminescence are expected to be improved. Both Cr and Al doping was found to be energetically favorable in ZnGa₂O₄, where the same does not hold true for Ga doping in ZnAl₂O₄, as indicated by the DFT-calculated defect formation energies. There seems to be ordering around the dopant ion in the solid solutions compared to either ZnGa₂O₄ or ZnAl₂O₄ and is also reflected to as lower persistent luminescence (PerL) lifetimes. PerL under UV, in general. was found to be lower with the enhancement in the Al³⁺ content endowed by the formation of Cr-Cr ion pair, lower probability of antisite formation, and widening band gap. On the other hand, X-ray excited emission enhances in the solid solution due to the decrease in cation inversion and associated defects. Confocal Microscopy showed that larger particles depicted much brighter deep red emission but failed to percolate to the human cells to a detectable limit; hence, future work is needed for the functionalization of the ZnGa_2-xAl_xO₄:Cr³⁺ spinel. This work could be of great implication in designing need-based materials, where UV and X-ray excitation is required, for deep red emission with persistent characteristics from chromium-doped spinels.

Synthesis and characterization of a near-infrared persistent luminescent Cr-doped zinc gallate-calcium phosphate composite

Near-infrared (NIR)-emitting persistent luminescence (PersL) nanoparticles have attracted great attention as a novel optical probe for bioimaging and biosensing applications. These nanoparticles emit long-lasting luminescence after the removal of the excitation source, which effectively eliminates the interference from tissue autofluorescence. Cr-doped zinc gallate (ZnGa₂O₄:Cr³⁺, CZGO) is a representative NIR-emitting PersL material. On the other hand, amorphous calcium phosphate (ACP) is a widely used drug carrier due to its high biocompatibility. In this work, we present a design of an ACP-based drug carrier with PersL properties, by forming a CZGO-ACP composite. The PersL properties of CZGO were preserved by composite formation, while it is found that the Zn²⁺ could migrate from CZGO to ACP during composite formation, leading to different luminescence mechanisms between pure CZGO and the CZGO-ACP composite. The electronic structure of the composite was analyzed by synchrotron X-ray absorption spectroscopy, and a structure-luminescence correlation was proposed.

Catalytic radiosensitization: Insights from materials physicochemistry

Radiotherapy is indispensable in clinical cancer treatment, but because both tumor and normal tissues have similar sensitivity to X-rays, their clinical curative effect is intrinsically limited. Advanced nanomaterials and nanotechnologies have been developed for radiotherapy sensitization, typically employing high atomic number (high-Z) materials to enhance the energy deposition of X-rays in tumor tissues, but the efficiency is largely limited by the toxicity of heavy metals. A new and promising approach for radiosensitization is catalytic radiosensitization, which takes advantage of the catalytic activity of nanomaterials triggered by radiation. The efficiency of catalytic radiosensitization can be greatly enhanced by electron modulation and energy conversion of nanocatalysts upon X-ray irradiation, further enhancing the clinical curative effect. In this review, we highlight the challenges and opportunities in cancer radiosensitization, discuss novel approaches to catalytic radiosensitization, and finally describe the development of catalytic radiosensitization based on an in-depth understanding of radio-nano interactions and catalysis-biological interactions.

Radiopharmaceutical and Eu3+ doped gadolinium oxide nanoparticles mediated triple-excited fluorescence imaging and image-guided surgery

Cerenkov luminescence imaging (CLI) is a novel optical imaging technique that has been applied in clinic using various radionuclides and radiopharmaceuticals. However, clinical application of CLI has been limited by weak optical signal and restricted tissue penetration depth. Various fluorescent probes have been combined with radiopharmaceuticals for improved imaging performances. However, as most of these probes only interact with Cerenkov luminescence (CL), the low photon fluence of CL greatly restricted it's interaction with fluorescent probes for in vivo imaging. Therefore, it is important to develop probes that can effectively convert energy beyond CL such as β and γ to the low energy optical signals. In this study, a Eu3+ doped gadolinium oxide (Gd2O3:Eu) was synthesized and combined with radiopharmaceuticals to achieve a red-shifted optical spectrum with less tissue scattering and enhanced optical signal intensity in this study. The interaction between Gd2O3:Eu and radiopharmaceutical were investigated using 18F-fluorodeoxyglucose (18F-FDG). The ex vivo optical signal intensity of the mixture of Gd2O3:Eu and 18F-FDG reached 369 times as high as that of CLI using 18F-FDG alone. To achieve improved biocompatibility, the Gd2O3:Eu nanoparticles were then modified with polyvinyl alcohol (PVA), and the resulted nanoprobe PVA modified Gd2O3:Eu (Gd2O3:Eu@PVA) was applied in intraoperative tumor imaging. Compared with 18F-FDG alone, intraoperative administration of Gd2O3:Eu@PVA and 18F-FDG combination achieved a much higher tumor-to-normal tissue ratio (TNR, 10.24 ± 2.24 vs. 1.87 ± 0.73, P = 0.0030). The use of Gd2O3:Eu@PVA and 18F-FDG also assisted intraoperative detection of tumors that were omitted by preoperative positron emission tomography (PET) imaging. Further experiment of image-guided surgery demonstrated feasibility of image-guided tumor resection using Gd2O3:Eu@PVA and 18F-FDG. In summary, Gd2O3:Eu can achieve significantly optimized imaging property when combined with 18F-FDG in intraoperative tumor imaging and image-guided tumor resection surgery. It is expected that the development of the Gd2O3:Eu nanoparticle will promote investigation and application of novel nanoparticles that can interact with radiopharmaceuticals for improved imaging properties. This work highlighted the impact of the nanoprobe that can be excited by radiopharmaceuticals emitting CL, β, and γ radiation for precisely imaging of tumor and intraoperatively guide tumor resection.

Persistent luminescence nanoparticles for cancer theranostics application

Persistent luminescence nanoparticles (PLNPs) are unique optical materials that emit afterglow luminescence after ceasing excitation. They exhibit unexpected advantages for in vivo optical imaging of tumors, such as autofluorescence-free, high sensitivity, high penetration depth, and multiple excitation sources (UV light, LED, NIR laser, X-ray, and radiopharmaceuticals). Besides, by incorporating other functional molecules, such as photosensitizers, photothermal agents, or therapeutic drugs, PLNPs are also widely used in persistent luminescence (PersL) imaging-guided tumor therapy. In this review, we first summarize the recent developments in the synthesis and surface functionalization of PLNPs, as well as their toxicity studies. We then discuss the in vivo PersL imaging and multimodal imaging from different excitation sources. Furthermore, we highlight PLNPs-based cancer theranostics applications, such as fluorescence-guided surgery, photothermal therapy, photodynamic therapy, drug/gene delivery and combined therapy. Finally, future prospects and challenges of PLNPs in the research of translational medicine are also discussed.

Disorder-Induced Broadband Near-Infrared Persistent and Photostimulated Luminescence in Mg2SnO4:Cr3

Materials with near-infrared (NIR) persistent luminescence (PersL) and NIR-to-NIR photostimulated luminescence (PSL) properties are attractive platforms for photonic energy harvesting and release. In this work, we develop Mg₂SnO₄:Cr as a broadband NIR PersL and NIR-to-NIR PSL material (luminescence maxima at ∼800 nm) and reveal the origin of the PersL and PSL properties. The material has an inverse spinel structure with the Mg²⁺ and Sn⁴⁺ disorder at the Wyckoff 16d site based on the Rietveld refinement. Cr K-edge X-ray absorption near-edge structure (XANES) spectra uncover that the doped Cr ions have a +3 valence state and occupy the disordered (Mg,Sn) site with octahedral coordination. The disorder results in multiple Cr³⁺ centers, and the broadband luminescence originates from the ⁴T₂(⁴F) → ⁴A₂ transition of Cr³⁺ at sites with intermediate crystal field strength. The distribution of trap depths is continuous according to the analysis of thermoluminescence (TL) spectra using the initial rising method, which relates to the random distribution of Mg²⁺ and Sn⁴⁺ at the second coordination sphere of the Cr³⁺ centers rather than the oxygen-related defects. Stimulating the material with a NIR laser, the NIR PersL gets significantly enhanced due to a PSL process. The broadband PersL and PSL are detectable beyond 100 h and have good tissue penetrability and therefore the developed Mg₂SnO₄:Cr³⁺ has potential in applications of optical information storage/reading and autofluorescence-free bioimaging. Finally, three crystal and electronic structure factors are proposed for screening new Cr³⁺-activated PersL and PSL materials.

Multiple emission bands NIR-persistent luminescence mSiO2@Zn0.6Ca0.4Ga2O4:Cr3+,Yb3+ nanoparticles for biological applications

Persistent luminescence nanoparticles (PLNPs) emitting in the NIR window (700-1700 nm) have shown great promise in the field of fluorescence imaging due to their unique properties, including the absence of in situ excitation and low optical scattering in tissues. However, they are still facing some challenges, such as irregular shape, wide size distribution and poor persistent luminescence performance. Here, we report a facile mesoporous template method for synthesizing mSiO₂@Zn_0.6Ca_0.4Ga₂O₄:Cr³⁺,Yb³⁺ (mSiO₂@ZCGO) persistent luminescent nanoparticles, which show a regular morphology and a size of about 69 nm. In addition, these nanocrystals exhibit persistent luminescence in multi-NIR windows, the first infrared window (∼696 nm of Cr³⁺ emission) and second infrared window (∼1000 nm of Yb³⁺ emission). Under illumination of a 254 nm UV lamp for 10 min, the persistent time of Cr³⁺ ions and Yb³⁺ ions lasted more than 120 min and 10 min, respectively. In particular, the NIR persistent emission of mSiO₂@ZCGO could be stimulated by soft X-ray, which is beneficial to long-term imaging in deep tissues. The optical penetration length of Yb³⁺ ions persistent luminescence was evaluated to be 2.8 mm. These results demonstrate the great promise of mSiO₂@ZCGO for deep-tissue bio-imaging.

Nanoparticles for Cerenkov and Radioluminescent Light Enhancement for Imaging and Radiotherapy

Cerenkov luminescence imaging and Cerenkov photodynamic therapy have been developed in recent years to exploit the Cerenkov radiation (CR) generated by radioisotopes, frequently used in Nuclear Medicine, to diagnose and fight cancer lesions. For in vivo detection, the endpoint energy of the radioisotope and, thus, the total number of the emitted Cerenkov photons, represents a very important variable and explains why, for example, ⁶⁸Ga is better than ¹⁸F. However, it was also found that the scintillation process is an important mechanism for light production. Nanotechnology represents the most important field, providing nanosctructures which are able to shift the UV-blue emission into a more suitable wavelength, with reduced absorption, which is useful especially for in vivo imaging and therapy applications. Nanoparticles can be made, loaded or linked to fluorescent dyes to modify the optical properties of CR radiation. They also represent a useful platform for therapeutic agents, such as photosensitizer drugs for the production of reactive oxygen species (ROS). Generally, NPs can be spaced by CR sources; however, for in vivo imaging applications, NPs bound to or incorporating radioisotopes are the most interesting nanocomplexes thanks to their high degree of mutual colocalization and the reduced problem of false uptake detection. Moreover, the distance between the NPs and CR source is crucial for energy conversion. Here, we review the principal NPs proposed in the literature, discussing their properties and the main results obtained by the proponent experimental groups.