In Vitro Targeting of Avidin-Expressing Glioma Cells with Biotinylated Persistent Luminescence Nanoparticles

The develop of persistent luminescence nanoparticles with excellent performances in cancer targeted bioimaging and killing: a review

The use of fluorescent nanomaterials in tumor imaging and treatment effectively avoids the original limitations of traditional tumor clinical diagnostic methods. The PLNPs emitted persistent luminescence after the end of excitation light. Owing to their superior optical properties, such as a reduced laser irradiation dose, spontaneous fluorescence interference elimination, and near-infrared imaging, PLNPs show great promise in tumor imaging. Moreover, they also achieve excellent anti-tumor therapeutic effects through surface modification and drug delivery. However, their relatively large size and limited surface modification capacity limit their ability to kill tumors effectively enough for clinical applications. Thus, this article reviews the synthesis and modification of PLNPs and the research progress in targeted tumor imaging and tumor killing. We also discuss the challenges and prospects of their future applications in these fields. This review has value for accelerating the design of PLNPs based platform for cancer diagnosis and treatment.

Recent Advances in NIR or X-ray Excited Persistent Luminescent Materials for Deep Bioimaging

Due to their persistent luminescence, persistent luminescent (PersL) materials have attracted great interest. In the biomedical field, the use of persistent luminescent nanoparticles (PLNPs) eliminates the need for continuous in situ excitation, thereby avoiding interference from tissue autofluorescence and significantly improving the signal-to-noise ratio (SNR). Although persistent luminescence materials can emit light continuously, the luminescence intensity of small-sized nanoparticles in vivo decays quickly. Early persistent luminescent nanoparticles were mostly excited by ultraviolet (UV) or visible light and were administered for imaging purposes through ex vivo charging followed by injection into the body. Limited by the low in vivo penetration depth, UV light cannot secondary charge PLNPs that have decayed in vivo, and visible light does not penetrate deep enough to reach deep tissues, which greatly limits the imaging time of persistent luminescent materials. In order to address this issue, the development of PLNPs that can be activated by light sources with superior tissue penetration capabilities is essential. Near-infrared (NIR) light and X-rays are widely recognized as ideal excitation sources, making persistent luminescent materials stimulated by these two sources a prominent area of research in recent years. This review describes NIR and X-ray excitable persistent luminescence materials and their recent advances in bioimaging.

Persistent luminescent nanophosphors for applications in cancer theranostics, biomedical, imaging and security

The extraordinary and unique properties of persistent luminescent (PerLum) nanostructures like storage of charge carriers, extended afterglow, and some other fascinating characteristics like no need for in-situ excitation, and rechargeable luminescence make such materials a primary candidate in the fields of bio-imaging and therapeutics. Apart from this, due to their extraordinary properties they have also found their place in the fields of anti-counterfeiting, latent fingerprinting (LPF), luminescent markings, photocatalysis, solid-state lighting devices, glow-in-dark toys, etc. Over the past few years, persistent luminescent nanoparticles (PLNPs) have been extensively used for targeted drug delivery, bio-imaging guided photodynamic and photo-thermal therapy, biosensing for cancer detection and subsequent treatment, latent fingerprinting, and anti-counterfeiting owing to their enhanced charge storage ability, in-vitro excitation, increased duration of time between excitation and emission, low tissue absorption, high signal-to-noise ratio, etc. In this review, we have focused on most of the key aspects related to PLNPs, including the different mechanisms leading to such phenomena, key fabrication techniques, properties of hosts and different activators, emission, and excitation characteristics, and important properties of trap states. This review article focuses on recent advances in cancer theranostics with the help of PLNPs. Recent advances in using PLNPs for anti-counterfeiting and latent fingerprinting are also discussed in this review.

H2 O2 -Induced Persistent Luminescence Signal Enhancement Applied to Biosensing

Persistent luminescence nanoparticles (PLNPs) are innovative materials able to emit light for a long time after the end of their excitation. Thanks to this property, their detection can be separated in time from the excitation, making it possible to obtain images with a high signal-to-noise ratio. This optical property can be of particular interest for the development of in vitro biosensors. Here, we report the unexpected effect of hydrogen peroxide (H2 O2 ) on the signal intensity of ZnGa2 O4 :Cr3+ (ZGO) nanoparticles. In the presence of H2 O2 , the signal intensity of ZGO can be amplified. This signal amplification can be used to detect and quantify H2 O2 in various media, using non-functionalized ZGO nanoparticles. This small molecule can be produced by several oxidases when they react with their substrate. Indeed, the quantification of glucose, lactic acid, and uric acid is possible. The limit of detection could be lowered by modifying the nanoparticles synthesis route. These optimized nanoparticles can also be used as new biosensor to detect larger molecules such as antigen, using the appropriate antibody. This unique property, i.e., persistent luminescence signal enhancement induced by H2 O2 , represents a new way to detect biomolecules which could lead to a very large number of bioassay 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.

Persistent luminescence nanoparticles functionalized by polymers bearing phosphonic acid anchors: synthesis, characterization, and in vivo behaviour

Optical in vivo imaging has become a widely used technique and is still under development for clinical diagnostics and treatment applications. For further development of the field, researchers have put much effort into the development of inorganic nanoparticles (NPs) as imaging probes. In this trend, our laboratory developed ZnGa_1.995O₄Cr_0.005 (ZGO) nanoparticles, which can emit a bright persistent luminescence signal through the tissue transparency window for dozens of minutes and can be activated in vivo with visible irradiation. These properties endow them with unique features, allowing us to recover information over a long-time study with in vivo imaging without any background. To target tissues of interest, ZGO must circulate long enough in the blood stream, a phenomenon which is limited by the mononuclear phagocyte system (MPS). Depending on their size, charge and coating, the NPs are sooner or later opsonized and stored into the main organs of the MPS (liver, spleen, and lungs). The NPs therefore have to be coated with a hydrophilic polymer to avoid this limitation. To this end, a new functionalization method using two different polyethylene glycol phosphonic acid polymers (a linear one, later named lpPEG and a branched one, later named pPEG) has been studied in this article. The coating has been optimized and characterized in various aqueous media. The behaviour of the newly functionalized NPs has been investigated in the presence of plasmatic proteins, and an in vivo biodistribution study has been performed. Among them ZGOpPEG exhibits a long circulation time, corresponding to low protein adsorption, while presenting an effective one-step process in aqueous medium with a low hydrodynamic diameter increase. This new method is much more advantageous than another strategy we reported previously that used a two-step PEG silane coating performed in an organic solvent (dimethylformamide) for which the final hydrodynamic diameter was twice the initial diameter.

A pretargeting nanoplatform for imaging and enhancing anti-inflammatory drug delivery

This work details a newly developed “sandwich” nanoplatform via neutravidin-biotin system for the detection and treatment of inflammation. First, biotinylated- and folate-conjugated optical imaging micelles targeted activated macrophages via folate/folate receptor interactions. Second, multivalent neutravidin proteins in an optimal concentration accumulated on the biotinylated macrophages. Finally, biotinylated anti-inflammatory drug-loaded micelles delivered drugs effectively at the inflammatory sites via a highly specific neutravidin-biotin affinity. Both in vitro and in vivo studies have shown that the “sandwich” pretargeting platform was able to diagnose inflammation by targeting activated macrophages as well as improve the therapeutic efficacy by amplifying the drug delivery to the inflamed tissue. The overall results support that our new pretargeting platform has the potential for inflammatory disease diagnosis and treatment.

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A “sandwich” nanoplatform system is developed for the improved detection and treatment of inflammation.

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Biotinylated- and folate-conjugated optical imaging micelles are designed to pre-target activated macrophages.

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Multivalent neutravidins accumulate on the biotinylated macrophages via neutravidin-biotin reactions.

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Biotinylated micelles can deliver drugs effectively at the inflammatory sites via specific neutravidin/biotin affinity.

Background-free cell surface glycan analysis using persistent luminescence nanoparticle as an optical probe

In this work, we report a novel cell surface glycan analysis method based on persistent luminescence nanoparticle (PLNP) ZnGa₂O₄: Cr³⁺ (ZGC) as an optical probe. ZGC was first silanized by (3-Aminopropyl) triethoxysilane (APTES), followed by PEGylation with NHS-P EG-Biotin, which not only introduces biotin, but significantly improves the dispersibility and stability of the nanoparticles. Neutral-avidin was then coupled on ZGC surface through the specific biotin-avidin interaction, producing a ZGC-PEG-avidin nanoprobe. As for cell surface glycan detection, different surface glycans are recognized with their corresponding biotinylated lectins, which are then traced by ZGC-PEG-avidin. The persistent luminescence signal is recorded by a microtiter plate reader in time-resolved fluorescence mode. Glycans expression profiling on prostate cancer cell DU145 and normal prostate cell RWPE-1 was analyzed by the proposed detection platform. Similar results were observed from the conventional horseradish peroxidase (HRP)-catalyzed absorbent assay and confocal microscope-based fluorescence imaging, demonstrating the applicability of the proposed platform. The approach based on the long afterglow property of ZGC efficiently eliminates the background noise from cells and substrate, resulting in the best signal-to-noise ratio and high detection sensitivity.

Recent Advances of Persistent Luminescence Nanoparticles in Bioapplications

Persistent luminescence phosphors are a novel group of promising luminescent materials with afterglow properties after the stoppage of excitation. In the past decade, persistent luminescence nanoparticles (PLNPs) with intriguing optical properties have attracted a wide range of attention in various areas. Especially in recent years, the development and applications in biomedical fields have been widely explored. Owing to the efficient elimination of the autofluorescence interferences from biotissues and the ultra-long near-infrared afterglow emission, many researches have focused on the manipulation of PLNPs in biosensing, cell tracking, bioimaging and cancer therapy. These achievements stimulated the growing interest in designing new types of PLNPs with desired superior characteristics and multiple functions. In this review, we summarize the works on synthesis methods, bioapplications, biomembrane modification and biosafety of PLNPs and highlight the recent advances in biosensing, imaging and imaging-guided therapy. We further discuss the new types of PLNPs as a newly emerged class of functional biomaterials for multiple applications. Finally, the remaining problems and challenges are discussed with suggestions and prospects for potential future directions in the biomedical applications.

Degradation of ZnGa2O4:Cr3+ luminescent nanoparticles in lysosomal-like medium

The ultimate goal of in vivo imaging is to provide safe tools to probe the inside of a body in order to obtain pathological information, monitor activities, and examine disease progression or regression. In this context zinc gallate doped with chromium III (ZGO) nanoparticles with persistent luminescence properties have been previously developed, and their biodistribution as well as in vitro toxicity were evaluated. However, to date, nothing is known about their potential transformations in biological media, which may hinder their biomedical applications. In order to know if these nanoparticles could degrade, the present work consists of studying their fate over time depending on both their coating and the aqueous media in which they are dispersed. ZGO nanoparticles have been dispersed in three different aqueous solutions for up to 90 days and characterized by numerous techniques. Among the evaluated dispersion media, Artificial Lysosomal Fluid (ALF) mimicking the intracellular lysosome environment elicited significant degradation of ZGO nanoparticles. The chelating agents present in ALF have proved to play a major role in the degradation of the ZGO, by stabilizing the nanoparticles and increasing the contact. An important time decrease of the luminescence properties has also been observed, which correlated with the release of ions from ZGO nanoparticles as well as their decreasing size. This information is valuable since it indicates, for the first time, the long-term degradation of persistent luminescent nanoprobes in an in vivo like model medium. Therefore, possible elimination of the imaging probes after in vivo preclinical applications could be foreseen.

Radioluminescence in biomedicine: physics, applications, and models

The electromagnetic spectrum contains different frequency bands useful for medical imaging and therapy. Short wavelengths (ionizing radiation) are commonly used for radiological and radionuclide imaging and for cancer radiation therapy. Intermediate wavelengths (optical radiation) are useful for more localized imaging and for photodynamic therapy (PDT). Finally, longer wavelengths are the basis for magnetic resonance imaging and for hyperthermia treatments. Recently, there has been a surge of interest for new biomedical methods that synergize optical and ionizing radiation by exploiting the ability of ionizing radiation to stimulate optical emissions. These physical phenomena, together known as radioluminescence, are being used for applications as diverse as radionuclide imaging, radiation therapy monitoring, phototherapy, and nanoparticle-based molecular imaging. This review provides a comprehensive treatment of the physics of radioluminescence and includes simple analytical models to estimate the luminescence yield of scintillators and nanoscintillators, Cherenkov radiation, air fluorescence, and biologically endogenous radioluminescence. Examples of methods that use radioluminescence for diagnostic or therapeutic applications are reviewed and analyzed in light of these quantitative physical models of radioluminescence.

Imaging and therapeutic applications of persistent luminescence nanomaterials

The development of probes for biomolecular imaging and diagnostics is a very active research area. Among the different imaging modalities, optics emerged since it is a noninvasive and cheap imaging technique allowing real time imaging. In vitro, this technique is very useful however in vivo, fluorescence suffers from low signal-to-noise ratio due to tissue autofluorescence under constant excitation. To address this limitation, novel types of optical nanoprobes are actually being developed and among them, persistent luminescence nanoparticles (PLNPs), with long lasting near-infrared (NIR) luminescence capability, allows doing optical imaging without constant excitation and so without autofluorescence. This review will begin by introducing the physical phenomenon associated to the long luminescence decay of such nanoprobes, from minutes to hours after ceasing the excitation. Then we will show how this property can be used to develop in vivo imaging probes and also more recently nanotheranostic agents. Finally, preliminary data on their biocompatibility will be mentioned and we will conclude by envisioning on the future applications and improvements of such nanomaterials.

Semiconducting Photosensitizer-Incorporated Copolymers as Near-Infrared Afterglow Nanoagents for Tumor Imaging

The fact that cancer metastasis is the main cause of death for most cancer patients necessitates the development of imaging tools for sensitive detection of metastases. Although optical imaging has high temporospatial resolution, tissue autofluorescence compromises the sensitivity for in vivo imaging of cancer metastasis. Herein, the synthesis of a series of photosensitizer-incorporated poly(p-phenylenevinylene)-based semiconducting copolymers and their utility as near-infrared (NIR) afterglow imaging nanoagents that emit light after cessation of light irradiation are reported. As compared with nondoped nanoparticles, the nanoparticles derived from the photosensitizer-incorporated copolymers have red-shifted NIR luminescence and amplified afterglow signals, allowing the detection of tiny peritoneal metastatic tumors almost invisible to naked eye. Moreover, the intrinsic oxygen-sensitive nature of afterglow makes those nanoagents potentially useful for in vivo imaging of oxygen levels. Thus, this study introduces a generation of light-excitation-free background-minimized optical imaging agents for the sensitive detection of diseased tissues in vivo.

Light sheet microscopy and SrAl2 O4 nanoparticles codoped with Eu2+ /Dy3+ ions for cancer cell tagging

Light sheet optical microscopy on strontium aluminate nanoparticles (SrAl₂ O₄ NPs)1 codoped with Eu²⁺ and Dy³⁺ was used for cancer cell tagging and tracking. The nanoparticles were synthesized by urea-assisted combustion with optimized percentage values of the 2 codoping rare-earth ions for cell viability and for lower cytotoxic effects. The optical properties of these materials showed an excitation wide range of wavelengths (λ_exc = 254-460 nm), a broad emission band (λ_em = 475-575 nm) with the maximum centered wavelength at 525 nm and a half lifetime within the seconds regime. The feasibility to measure the nanoparticle luminescence under the selective plane illumination configuration was studied by immersing the nanoparticles in 1% Agarose. The potential applicability of the synthesized nanophosphors for cancer cell tagging was demonstrated by using in vitro experiments with human breast adenocarcinoma MCF-7 cells. A single MCF-7 cell observed by the use of light sheet microscopy with UV excitation. The cell has been bio-labeled with FA-SrAl₂ 0₄ : Eu²⁺ , Dy³⁺ NPs and 4',6-diamidino-2-phenylindole, dihydrochloride for nucleus identification.

Engineering Persistent Luminescence Nanoparticles for Biological Applications: From Biosensing/Bioimaging to Theranostics

Persistent luminescence nanoparticles (PLNPs) are unique optical materials emitting long-lasting luminescence after ceasing excitation. Such a unique optical feature allows luminescence detection without constant external illumination to avoid the interferences of autofluorescence and scattering light from biological fluids and tissues. Besides, near-infrared (NIR) PLNPs have advantages of deep penetration and the reactivation of the persistent luminescence (PL) by red or NIR light. These features make the application of NIR-emitting PLNPs in long-term bioimaging no longer limited by the lifetime of PL. To take full advantage of PLNPs for biological applications, the versatile strategies for bridging PLNPs and biological system become increasingly significant for the design of PLNPs-based nanoprobes. In this Account, we summarize our systematic achievements in the biological applications of PLNPs from biosensing/bioimaging to theranostics with emphasizing the engineering strategies for fabricating specific PLNPs-based nanoprobes. We take surface engineering and manipulating energy transfer as the major principles to design various PLNPs-based nanoprobes based on the nature of interactions between nanoprobes and targets. We have developed target-induced formation or interruption of fluorescence resonance energy transfer systems for autofluorescence-free biosensing and imaging of cancer biomarkers. We have decorated single or dual targeting ligands on PLNPs for tumor-targeted imaging, and integrated other modal imaging agents into PLNPs for multimodal imaging. We have also employed specific functionalization for various biomedical applications including chemotherapy, photodynamic therapy, photothermal therapy, stem cells tracking and PL imaging-guided gene therapy. Besides, we have modified PLNPs with multiple functional units to achieve challenging metastatic tumor theranostics. The proposed design principle and comprehensive strategies show great potential in guiding the design of PLNPs nanoprobes and promoting further development of PLNPs in the fields of biological science and medicine. We conclude this Account by outlining the future directions to further promote the practical application of PLNPs. The novel protocols for the synthesis of small-size, monodisperse, and water-soluble PLNPs with high NIR PL intensity and superlong afterglow are the vibrant directions for the biomedical applications of PLNPs. In-depth theories and evidence on luminescence mechanism of PLNPs are highly desired for further improvement of their luminescence performance. Furthermore, other irradiations without tissue penetrating depth limit, such as X-ray, are encouraged for use in energy storage and re-excitation of PLNPs, enabling imaging in deep tissue in vivo and integrating other X-ray sensitized theranostic techniques such as computed tomography imaging and radiotherapy. Last but not least, PLNPs-based nanoprobes and the brand new hybrids of PLNPs with other nanomaterials show a bright prospect for accurate diagnosis and efficient treatment of diseases besides tumors.

Self-Assembled Semiconducting Polymer Nanoparticles for Ultrasensitive Near-Infrared Afterglow Imaging of Metastatic Tumors

Detection of metastatic tumor tissues is crucial for cancer therapy; however, fluorescence agents that allow to do share the disadvantage of low signal-to-background ratio due to tissue autofluorescence. The development of amphiphilic poly(p-phenylenevinylene) derivatives that can self-assemble into the nanoagent (SPPVN) in biological solutions and emit near-infrared afterglow luminescence after cessation of light irradiation for ultrasensitive imaging of metastatic tumors in living mice is herein reported. As compared with the counterpart nanoparticle (PPVP) prepared from the hydrophobic PPV derivate, SPPVN has smaller size, higher energy transfer efficiency, and brighter afterglow luminescence. Moreover, due to the higher PEG density of SPPVN relative to PPVP poly(ethylene glycol), SPPVN has a better accumulation in tumor. Such a high sensitivity and ideal biodistribution allow SPPVN to rapidly detect xenograft tumors with the size as small as 1 mm³ and tiny peritoneal metastatic tumors that are almost invisible to naked eye, which is not possible for PPVP. Moreover, the oxygen-sensitive afterglow makes SPPVN potentially useful for in vivo imaging of oxygen levels. By virtue of enzymatic biodegradability and ideal in vivo clearance, these organic agents can serve as a platform for the construction of advanced afterglow imaging tools.

Oral administration of highly bright Cr3+ doped ZnGa2O4 nanocrystals for in vivo targeted imaging of orthotopic breast cancer

Near-infrared (NIR) long lasting persistent luminescence nanoparticles (PLNPs) have attracted considerable attention in the area of in vivo bioimaging, due to their background-free luminescence characteristics and deep tissue penetration. However, the low fluorescence quantum yield and short afterglow of the currently available PLNPs limit their applications. Here, water-soluble Cr³⁺-doped ZnGa₂O₄ PLNPs with the highest quantum yield (η = 20%) ever reported, bright NIR emission, and excellent colloidal stability were successfully prepared by a one-step hydrothermal method. The afterglow of the resultant nanocrystals lasted for more than 5 days and could be repeatedly reactivated by the light (λ = 657 nm) of a portable light emitting diode lamp after decay. These nanocrystals were functionalized with α,ω-dicarboxyl-terminated poly(ethylene glycol) and poly(acrylic acid) to improve their stability and biocompatibility, so that they could be conjugated with a c(RGDyK) peptide and labeled with ^99mTc for targeted imaging of orthotopic breast cancer by afterglow luminescence imaging and single-photon emission computed tomography imaging. Our NIR-PLNP probes can effectively avoid tissue auto-fluorescence and the light scattering caused by continuous excitation during the diagnosis of cancer.

A simple approach for glutathione functionalized persistent luminescence nanoparticles as versatile platforms for multiple in vivo applications

We develop a simple method by constructing glutathione (GSH) conjugated persistent luminescence nanoparticles (PLNPs–GSH) as versatile platforms for multiple biological applications.

Kiwifruit-like Persistent Luminescent Nanoparticles with High-Performance and in Situ Activable Near-Infrared Persistent Luminescence for Long-Term in Vivo Bioimaging

Persistent luminescence nanoparticles (PLNPs) have great potential for bioimaging because they can eliminate the tissue autofluorescence and improve the signal-to-noise ratio significantly. High-temperature calcination is a necessary process for the PLNPs to achieve high luminescence intensity and long afterglow time. However, high-temperature calcination usually results in uncontrollable morphology and poor homogeneity of PLNPs, which greatly limit their applications. Therefore, there is still a high demand to find a suitable method for synthesizing PLNPs with high luminescence intensity and long afterglow time while maintaining their monodispersed morphology. Herein, we report a facile silica template method to synthesize PLNPs with a kiwifruit-like structure that can tolerate high-temperature calcination. The as-prepared kiwifruit-like SiO₂@ZnGa₂O₄:Cr³⁺@SiO₂ PLNPs have enhanced near-infrared persistent luminescence, uniform morphology and size, and good biocompatibility. Moreover, the SiO₂@ZnGa₂O₄:Cr³⁺@SiO₂ PLNPs can be repeatedly activated by soft X-rays in situ and emit near-infrared persistent luminescence with long decay time, holding great potential for deep-tissue and long-term in vivo bioimaging. We believe that this study will open new perspectives for synthesizing high-performance PLNPs for optical imaging and diversified applications.

Molecular afterglow imaging with bright, biodegradable polymer nanoparticles

Afterglow optical agents, which emit light long after cessation of excitation, hold promise for ultrasensitive in vivo imaging because they eliminate tissue autofluorescence. However, afterglow imaging has been limited by its reliance on inorganic nanoparticles with relatively low brightness and short-near-infrared (NIR) emission. Here we present semiconducting polymer nanoparticles (SPNs) <40 nm in diameter that store photon energy via chemical defects and emit long-NIR afterglow luminescence at 780 nm with a half-life of ∼6 min. In vivo, the afterglow intensity of SPNs is more than 100-fold brighter than that of inorganic afterglow agents, and the signal is detectable through the body of a live mouse. High-contrast lymph node and tumor imaging in living mice is demonstrated with a signal-to-background ratio up to 127-times higher than that obtained by NIR fluorescence imaging. Moreover, we developed an afterglow probe, activated only in the presence of biothiols, for early detection of drug-induced hepatotoxicity in living mice.

Long-term toxicological effects of persistent luminescence nanoparticles after intravenous injection in mice

The ZnGa_1.995Cr_0.005O₄ persistent luminescence nanoparticles offer the promise of revolutionary tools for biological imaging with applications such as cell tracking or tumor detection. They can be re-excited through living tissues by visible photons, allowing observations without any time constraints and avoiding the undesirable auto-fluorescence signals observed when fluorescent probes are used. Despite all these advantages, their uses demand extensive toxicological evaluation and control. With this purpose, mice were injected with a single intravenous administration of hydroxylated or PEGylated persistent luminescence nanoparticles at different concentrations and then a set of standard tests were carried out 1day, 1 month and 6 months after the administration. High concentrations of hydroxylated nanoparticles generate structural alterations at histology level, endoplasmic reticulum damage and oxidative stress in liver, as well as rising in white blood cells counts. A mechanism involving the endoplasmic reticulum damage could be the responsible of the observed injuries in case of ZGO-OH. On the contrary, no toxicological effects related to PEGylated nanoprobes treatment were noted during our in vivo experiments, denoting the protective effect of PEG-functionalization and thereby, their potential as biocompatible in vivo diagnostic probes.

Recent progress in biomedical applications of persistent luminescence nanoparticles

Persistent luminescence nanoparticles (PLNPs) are an emerging group of promising luminescent materials that can remain luminescent after the excitation ceases. In the past decade, PLNPs with intriguing optical properties have been developed and their applications in biomedicine have been widely studied. Due to the ultra-long decay time of persistent luminescence, autofluorescence interference in biosensing and bioimaging can be efficiently eliminated. Moreover, PLNPs can remain luminescent for hours, making them valuable in bio-tracing. Also, persistent luminescence imaging can guide cancer therapy with a high signal-to-noise ratio (SNR) and superior sensitivity. Briefly, PLNPs are demonstrated to be a newly-emerging class of functional materials with unprecedented advantages in biomedicine. In this review, we summarized recent advances in the preparation of PLNPs and the applications of PLNPs in biosensing, bioimaging and cancer therapy.

Persistent luminescence tomography for small animal imaging

Fluorescence imaging is a widely used in vivo optical imaging technique for preclinical studies, but strong tissue autofluorescence and external excitation light make it suffer from a low signal-to-noise ratio (SNR). Recently, a new optical imaging method using persistent luminescence has become of interest due to its advantage of circumvention of autofluorescence and bleed-through of excitation light during signal acquisition. In this work, we proposed a tomographic imaging method based on persistent luminescence named persistent luminescence tomography (PLT), which can obtain three dimensional distributions of persistent luminescence probes deep inside small animals. Persistent luminescence signals can last several hours after excitation, which makes it possible for PLT to collect signals without interference by autofluorescence and bleed-through of excitation light, and then to reconstruct tomographic images of high quality. Phantom and mouse experiments are implemented to verify the feasiblity of PLT.

Fabrication and bioconjugation of BIII and CrIII co-doped ZnGa2O4 persistent luminescent nanoparticles for dual-targeted cancer bioimaging

Persistent luminescent nanoparticles (PLNPs) show great potential in realizing precision imaging due to the absence of in situ excitation and no background interference. However, the current PLNP-based tumour imaging is usually achieved by single targeting or passive targeting strategies, and thus it lacks high specificity and affinity for efficient persistent luminescence imaging in vivo. Herein we report the bioconjugation of multiple targeting ligands on the surface of PLNPs for dual-targeted bioimaging to improve the specificity and affinity of the PLNP nanoprobe for in vitro and in vivo bioimaging. The PLNPs were prepared by co-doping Cr^III and B^III into ZnGa₂O₄via a hydrothermal-calcination method. While Cr^III doped ZnGa₂O₄ PLNPs possess excellent near-infrared luminescence along with long afterglow and red light renewable near-infrared luminescence, doping of B^III into the PLNPs further improves the persistent luminescence. Conjugation of two targeting ligands, hyaluronic acid and folic acid, which have specificity toward the cluster determinant 44 receptor and folic acid receptor in tumour cells, respectively, provides synergistic targeting effects to enhance the specificity and affinity toward tumour cells. This work provides a dual-targeting strategy for fabricating PLNP-based nanoprobes to realize precision tumour-targeted bioimaging.

Chemically engineered persistent luminescence nanoprobes for bioimaging

Imaging nanoprobes are a group of nanosized agents developed for providing improved contrast for bioimaging. Among various imaging probes, optical sensors capable of following biological events or progresses at the cellular and molecular levels are actually actively developed for early detection, accurate diagnosis, and monitoring of the treatment of diseases. The optical activities of nanoprobes can be tuned on demand by chemists by engineering their composition, size and surface nature. This review will focus on researches devoted to the conception of nanoprobes with particular optical properties, called persistent luminescence, and their use as new powerful bioimaging agents in preclinical assays.

Long persistent phosphors—from fundamentals to applications

We present multidisciplinary research on synthetic methods, afterglow mechanisms, characterization techniques, material kinds, and applications of long persistent phosphors.

A persistent luminescence microsphere-based probe for convenient imaging analysis of dopamine

SrMgSi₂O₆:Eu_0.01,Dy_0.02 persistent luminescence microspheres have been synthesized via a simple template method and a new probe was established based on turn-off of the persistent luminescence emission for detection and optical imaging of dopamine.

Persistent luminescence nanoprobe for biosensing and lifetime imaging of cell apoptosis via time-resolved fluorescence resonance energy transfer

Time-resolved fluorescence technique can reduce the short-lived background luminescence and auto-fluorescence interference from cells and tissues by exerting the delay time between pulsed excitation light and signal acquisition. Here, we prepared persistent luminescence nanoparticles (PLNPs) to design a universal time-resolved fluorescence resonance energy transfer (TR-FRET) platform for biosensing, lifetime imaging of cell apoptosis and in situ lifetime quantification of intracellular caspase-3. Three kinds of PLNPs-based nanoprobes are assembled by covalently binding dye-labeled peptides or DNA to carboxyl-functionalized PLNPs for the efficient detection of caspase-3, microRNA and protein. The peptides-functionalized nanoprobe is also employed for fluorescence lifetime imaging to monitor cell apoptosis, which shows a dependence of cellular fluorescence lifetime on caspase-3 activity and thus leads to an in situ quantification method. This work provides a proof-of-concept for PLNPs-based TR-FRET analysis and demonstrates its potential in exploring dynamical information of life process.

Controlling aminosilane layer thickness to extend the plasma half-life of stealth persistent luminescence nanoparticles in vivo

Therapeutics and diagnostics both initiated the development and rational design of nanoparticles intended for biomedical applications. Yet, the fate of these nanosystems in vivo is hardly manageable and generally results in their rapid uptake by the mononuclear phagocyte system, i.e. liver and spleen. To overcome this essential limitation, efforts have been made to understand the influence of physico-chemical parameters on the behaviour of nanoparticles in vivo and on their ability to be uptaken by phagocytic cells. Notably, polyethylene glycol grafting and precise control of its density have not only been shown to prevent protein adsorption on the surface of nanoparticles, but also to significantly reduce macrophage uptake in vitro. In this article, we suggest the use of persistent luminescence to study the influence of another parameter, aminosilane layer thickness, on both in vitro protein adsorption and in vivo biodistribution of stealth persistent nanophosphors.

Optically stimulated persistent luminescence of europium-doped LaAlO3 nanocrystals

Persistent luminescence associated with Eu³⁺ in LaAlO₃:Eu³⁺ nanocrystals can be observed and enhanced by optical stimulation.

A novel NIR long phosphorescent phosphor:SrSnO3:Bi2+

A novel phosphor with near-infrared (NIR) long persistent luminescence, SrSnO₃:Bi²⁺ was successfully synthesized by traditional solid-state reaction.

Magnetic, long persistent luminescent and mesoporous nanoparticles as trackable transport drug carriers

In this paper, Gd₂O₃@mSiO₂@CaTiO₃:Pr nanoparticles with a novel core/shell structure featured by mesoporous, magnetic and long persistent luminescent properties have been synthesized via a facile template method. The as-prepared nanoparticles show ordered mesoporous characteristics, monodisperse spherical morphology and narrow size distribution, so they are expected to be suitable as a drug carrier. These nanoparticles exhibit bright red phosphorescence at 614 nm after UV irradiation and the persistent luminescence can persist for more than one hour. Moreover, they possess a prominent longitudinal relaxivity (r₁) value of 6.43 mM^-1 s^-1, which suggests that these nanoparticles can be used as MR imaging agents. The combination of long persistent luminescent and magnetic properties reveals the high imaging sensitivity, which suggests that these nanoparticles can be used for imaging in vivo with a high signal to noise ratio. Drug release results indicate that the nanoparticles hold excellent drug sustained properties. The cumulative released amount of drugs can also be easily monitored by the change of luminescence intensity. After modification with polyethylene glycol, the resulting nanoparticles demonstrate good biocompatibility and low toxicity. Furthermore, we can realize in vivo imaging for over 20 min using these nanoparticles. These results indicate the promising use of these nanoparticles as multifunctional traceable drug carriers in vivo.

Mesoporous persistent nanophosphors for in vivo optical bioimaging and drug-delivery

Based upon the ambitious idea that one single particle could serve multiple purposes at the same time, the combination and simultaneous use of imaging and therapeutics has lately arisen as one of the most promising prospects among nanotechnologies directed toward biomedical applications. Intended for both therapeutics and diagnostics in vivo, highly complex nanostructures were specifically designed to simultaneously act as optical imaging probes and delivery vehicles. Yet, such multifunctional photonic nanoplatforms usually exploit fluorescence phenomena which require constant excitation light through biological tissues and thus significantly reduce the detection sensitivity due to the autofluorescence from living animals. In order to overcome this critical issue, the present article introduces a novel multifunctional agent based on persistent luminescence mesoporous nanoparticles. Being composed of a hybrid chromium-doped zinc gallate core/mesoporous silica shell architecture, we show that this nanotechnology can be used as an efficient doxorubicin-delivery vehicle presenting a higher cytotoxicity toward U87MG cells than its unloaded counterpart in vitro. In addition, we demonstrate that a persistent luminescence signal from these doxorubicin-loaded mesoporous nanophosphors opens a new way to highly sensitive detection in vivo, giving access to the real-time biodistribution of the carrier without any autofluorescence from the animal tissues. This new persistent luminescence-based hybrid nanotechnology can be easily applied to the delivery of any therapeutic agent, thus constituting a versatile and sensitive optical nanotool dedicated to both therapeutic and diagnostic applications in vivo.

Persistent luminescence strontium aluminate nanoparticles as reporters in lateral flow assays

Demand for highly sensitive, robust diagnostics and environmental monitoring methods has led to extensive research in improving reporter technologies. Inorganic phosphorescent materials exhibiting persistent luminescence are commonly found in electroluminescent displays and glowing paints but are not widely used as reporters in diagnostic assays. Persistent luminescence nanoparticles (PLNPs) offer advantages over conventional photoluminescent probes, including the potential for enhanced sensitivity by collecting time-resolved measurements or images with decreased background autofluorescence while eliminating the need for expensive optical hardware, superior resistance to photobleaching, amenability to quantitation, and facile bioconjugation schemes. We isolated rare-earth doped strontium aluminate PLNPs from larger-particle commercial materials by wet milling and differential sedimentation and water-stabilized the particles by silica encapsulation using a modified Stöber process. Surface treatment with aldehyde silane followed by reductive amination with heterobifunctional amine-poly(ethylene glycol)-carboxyl allowed covalent attachment of proteins to the particles using standard carbodiimide chemistry. NeutrAvidin PLNPs were used in lateral flow assays (LFAs) with biotinylated lysozyme as a model analyte in buffer and monoclonal anti-lysozyme HyHEL-5 antibodies at the test line. Preliminary experiments revealed a limit of detection below 100 pg/mL using the NeutrAvidin PLNPs, which was approximately an order of magnitude more sensitive than colloidal gold.

Hydrophobic sodium fluoride-based nanocrystals doped with lanthanide ions: assessment of in vitro toxicity to human blood lymphocytes and phagocytes

In vitro immunotoxicity of hydrophobic sodium fluoride-based nanocrystals (NCs) doped with lanthanide ions was examined in this study. Although there is already a significant amount of optical and structural data on NaYF4 NCs, data on safety assessment are missing. Therefore, peripheral whole blood from human volunteers was used to evaluate the effect of 25 and 30 nm hydrophobic NaYF4 NCs dissolved in cyclohexane (CH) on lymphocytes, and of 10 nm NaYF4 NCs on phagocytes. In the concentration range 0.12-75 µg cm(-2) (0.17-106 µg ml(-1) ), both 25 and 30nm NaYF4 NCs did not induce cytotoxicity when measured as incorporation of [(3) H]-thymidine into DNA. Assessment of lymphocyte function showed significant suppression of the proliferative activity of T-lymphocytes and T-dependent B-cell response in peripheral blood cultures (n = 7) stimulated in vitro with mitogens phytohemagglutinin (PHA) and pokeweed (PWM) (PHA > PWM). No clear dose-response effect was observed. Phagocytic activity and respiratory burst of leukocytes (n = 5-8) were generally less affected. A dose-dependent suppression of phagocytic activity of granulocytes in cultures treated with 25 nm NCs was observed (vs. medium control). A decrease in phagocytic activity of monocytes was found in cells exposed to higher doses of 10 and 30 nm NCs. The respiratory burst of phagocytes was significantly decreased by exposure to the middle dose of 30 nm NCs only. In conclusion, our results demonstrate immunotoxic effects of hydrophobic NaYF4 NCs doped with lanthanide ions to lymphocytes and to lesser extent to phagocytes. Further research needs to be done, particularly faze transfer of hydrophobic NCs to hydrophilic ones, to eliminate the solvent effect.

Quantum dot-based nanoprobes for in vivo targeted imaging

Fluorescent semiconductor quantum dots (QDs) have attracted tremendous attention over the last decade. The superior optical properties of QDs over conventional organic dyes make them attractive labels for a wide variety of biomedical applications, whereas their potential toxicity and instability in biological environment have puzzled scientific researchers. Much research effort has been devoted to surface modification and functionalization of QDs to make them versatile probes for biomedical applications, and significant progress has been made over the last several years. This review article aims to describe the current state-of-the-art of the synthesis, modification, bioconjugation, and applications of QDs for in vivo targeted imaging. In addition, QD-based multifunctional nanoprobes are also summarized.

Highly controllable synthesis of near-infrared persistent luminescence SiO2/CaMgSi2O6 composite nanospheres for imaging in vivo

High quality near-infrared (NIR) persistent luminescence nanospheres (PLNPs) were synthesized using a simple mesoporous template method. The as-synthesized NIR persistent luminescence nanoparticles have uniform spherical morphology, tunable sizes, and a nominal composition of SiO(2)/CaMgSi(2)O(6):Eu(2+), Pr(3+), Mn(2+) (denoted as SEPM). Their NIR persistent luminescence at 660 nm can be detected during more than 1 hour. The in vivo distribution of the nanoparticles in the abdomen can be detected in real time after injection into the abdomen of a mouse. The nanoparticles can be metabolized from the lymph circulation and transferred from the abdomen to the bladder. The results indicate an effective method to offer high quality NIR persistent luminescence nanoprobes for imaging.

The in vivo activation of persistent nanophosphors for optical imaging of vascularization, tumours and grafted cells

Optical imaging for biological applications requires more sensitive tools. Near-infrared persistent luminescence nanoparticles enable highly sensitive in vivo optical detection and complete avoidance of tissue autofluorescence. However, the actual generation of persistent luminescence nanoparticles necessitates ex vivo activation before systemic administration, which prevents long-term imaging in living animals. Here, we introduce a new generation of optical nanoprobes, based on chromium-doped zinc gallate, whose persistent luminescence can be activated in vivo through living tissues using highly penetrating low-energy red photons. Surface functionalization of this photonic probe can be adjusted to favour multiple biomedical applications such as tumour targeting. Notably, we show that cells can endocytose these nanoparticles in vitro and that, after intravenous injection, we can track labelled cells in vivo and follow their biodistribution by a simple whole animal optical detection, opening new perspectives for cell therapy research and for a variety of diagnosis applications.

NIR to NIR upconversion in KYb2F7: RE3+ (RE = Tm, Er) nanoparticles for biological imaging

Until recently, many contrast agents widely used in biological imaging have absorbed and emitted in the visible region, limiting their usefulness for deeper tissue imaging. In order to push the boundaries of deep tissue imaging with non-ionizing radiation, contrast agents in the near infrared (NIR) regime, which is not strongly absorbed or scattered by most tissues, are being sought after. Upconverting nanoparticles (UCNPs) are attractive candidates since their upconversion emission is tunable with a very narrow bandwidth and they do not photobleach or blink. The upconversion produced by the nanoparticles can be tailored for NIR to NIR by carefully choosing the lanthanide dopants and dopant ratios such as KYb₂F₇: RE³⁺ (RE = Tm, Er). Spectroscopic characterization was done by analyzing absorption, fluorescence, and quantum yield data. In order to study the toxicity of the nanoparticles Monkey Retinal Endothelial Cells (MREC) were cultivated in 24 well plates and then treated with nanoparticles at different concentrations in triplicate to obtain the optimal concentration for in vivo experiments. It will be shown that these UCNPs do not elicit a strong toxic response such as quantum dots and some noble metal nanoparticles. 3-D optical slices of nanoparticle treated fibroblast cells were imaged using a confocal microscope where the nucleus and cytoplasm were stained with DAPI and Alexa Fluor respectively. These results presented support the initial assumption, which suggests that KYb₂F₇: RE³⁺ would be excellent candidates for NIR contrast agents.

Red and near infrared persistent luminescence nano-probes for bioimaging and targeting applications

Schematic representation of the different processes in persistent luminescence: charging (1), stimulation (2), discharging (3) (PET-persistent energy transfer, QT-quantum tunneling).

Competitive adsorption of thiolated poly(ethylene glycol) and alkane-thiols on gold nanoparticles and its effect on cluster formation

The surface concentration and conformation of thiol-terminated poly(ethylene glycol) (PEG) on gold nanoparticles are studied before and after coadsorption of alkane-thiols. Thermogravimetric analysis (TGA) indicates alkane-thiol ligands will competitively adsorb on gold surfaces of nanoparticles and that the extent of PEG-thiol replacement depends on the specific length of the alkane-thiol molecule. The conformation of the polymer is also affected by the length and packing density of the alkane-thiol. Dynamic light scattering (DLS) shows that the hydrodynamic size of coated particles has an intermediate maximum for the adsorption of octane-thiol, which also forms the most densely packed alkane-thiol monolayers. These two factors greatly impact the formation of clusters by nanoparticle surfactants. Small angle X-ray scattering (SAXS) shows that the largest clusters are formed when particles have a low PEG-thiol surface concentration and an extended PEG conformation.