Synthesis of GdAlO3:Mn4+,Ge4+@Au Core-Shell Nanoprobes with Plasmon-Enhanced Near-Infrared Persistent Luminescence for in Vivo Trimodality Bioimaging

pH-Responsive Persistent Luminescent Nanosystem with Sensitized NIR Imaging and Ratiometric Imaging Modes for Tumor Surgery Navigation

Owing to autofluorescence-free feature, persistent luminescent (PersL) nanoparticles (PLNPs) become potential materials for tumor surgical navigation. However, it is still challenging to enhance PersL intensity, contrast ratio, and imaging stability so as to meet clinical demand and avoid missed detection of microlesions. Herein, integrating a tumor microenvironment (TME)-responsive strategy, sensitization enhancement, and internal-standard ratiometric method, a dual-mode PersL imaging strategy is proposed: After loading pH-responsive fluorescent molecule Rh-ADM on PLNPs ZnGa2O4:Cr3+,Mn2+ (ZGCM-Rh8), the fluorescence resonance energy transfer (FRET) pathways between Cr3+ and Rh-ADM, as well as Mn2+ and Rh-ADM, could sensitize the NIR PersL emitted by Cr3+ and quench the green PersL from Mn2+ at acidic TME, respectively. As a result, ZGCM-Rh8 is endowed with WLED (white light LED)-excited NIR imaging mode and UV-excited ratiometric imaging mode. Under WLED, ZGCM-Rh8 realizes 4.5-fold PersL enhancement and 97.9 as the maximum tumor contrast after precise control of Rh-ADM contents, helping with the preoperative diagnosis of deep lesions. Under UV, ZGCM-Rh8 conducts ratiometric PersL imaging steadily, and the "NIR/Vis" ratios at the tumor keep larger than 110, succeeding in detecting out a 1.5 mm small lesion and serving thorough surgical elimination of H22 ectopic intramuscular tumor in balb/c mice. To our knowledge, ZGCM-Rh8 is the first to realize pH-responsive PersL sensitization and apply ratiometric PersL imaging technology to surgical navigation.

X-ray activated near-infrared persistent luminescence nanoparticles for trimodality in vivo imaging

As promising luminescence nanoparticles, near-infrared (NIR) persistent luminescence nanoparticles (PLNPs) have received extensive attention in the field of high-sensitivity bioimaging in recent years. However, NIR PLNPs face problems such as short excitation wavelengths and single imaging modes, which limit their applications in in vivo reactivated imaging and multimodal imaging. Here, we report for the first time novel Gd2GaTaO7:Cr3+,Yb3+ (GGTO) NIR PLNPs that integrate X-ray activated NIR persistent luminescence (PersL), high X-ray attenuation and excellent magnetic properties into a single nanoparticle (NP). In this case, Cr3+ is used as the luminescence center. The co-doped Yb3+ and coating effectively enhance the X-ray activated NIR PersL. At the same time, the presence of the high-Z element Ta also makes the GGTO NPs exhibit high X-ray attenuation performance, which can be used as a CT contrast agent to achieve in vivo CT imaging. In addition, since the matrix contains a large amount of Gd, the GGTO NPs show remarkable magnetic properties, which can realize in vivo MR imaging. GGTO NPs combine the trimodal benefits of X-ray reactivated PersL, CT and MR imaging and are suitable for single or combined applications that require high sensitivity and spatial resolution imaging.

pH-Responsive Plasmon-Enhanced Persistent Luminescent ZnGa2O4:Cr3+ Nanopomegranate for Tumor Imaging

Noble metal compositing is a promising method to enhance radiance intensity of persistent luminescent (PersL) nanoparticles (NPs) via surface plasmon resonance (SPR) for better tumor imaging, but it rarely unites with the pH-response strategy due to the challenge of realizing rigorous pH-responsive spatial distance control as a "button switch" of SPR. Here, ZnGa2O4:Cr3+ (ZGC) NPs as "pomegranate seeds" are cladded with sodium alginate to form nanoclusters (ZGC-SA), subsequently coated with carboxyl-rich polymers to acquire "pomegranate rind" (ZSPB) and finally decorated with 10 nm gold NPs (AuNPs) on the surface to obtain nanopomegranate structure (ZSPB@AuNPs). Though without deliberate distance control, there are plenty of "seeds" inside ZSPB@AuNPs fortunately at appropriate positions, which could be plasmon-enhanced by AuNPs. Furthermore, triggered by carboxyl protonation in subacid tumor, ZSPB@AuNPs aggregate and subsequently facilitate such plasmon enhancement effect, resulting in 4.4-fold PersL promotion at pH 5.5 (tumor microenvironment, TME) over pH 7.4 and in a maximum "tumor to normal tissue ratio" of PersL imaging signals of 125.9. Under surgical navigation of ZSPB@AuNPs, intramuscular tumors of mice could be resected without residue signals left. This nanopomegranate achieves TME pH-responsive plasmon-enhanced PersL for the first time and broadens the way for designing plasmon-enhanced PersL nanosystems.

Perspective and Prospects on persistent luminescent nanoparticles for biological imaging and tumor therapy

Persistent luminescent nanoparticles (PLNPs) are photoluminescent materials that can still emit luminescence after the cessation of the excitation light source. In recent years, due to their unique optical properties, the PLNPs have attracted extensive attention in the biomedical field. Since the PLNPs effectively eliminate autofluorescence interference from biological tissues, many researchers have contributed a lot of work in the fields of biological imaging and tumor therapy. This article mainly introduces the synthesis methods of the PLNPs and their progress in the application of biological imaging and tumor therapy, as well as the challenges and development prospects.

Multimodal channel cancer chemotherapy by 2D functional gadolinium metal-organic framework

2D nanomaterials generally exhibit enhanced physiochemical and biological functions in biomedical applications due to their high surface-to-volume ratio and surface charge. Conventional cancer chemotherapy based on nanomaterials has been hindered by their low drug loading and poor penetration in tumor tissue. To overcome these difficulties, novel materials systems are urgently needed. Hereby, the lanthanide-based porphyrin metal-organic framework (MOF) nanosheets (NSs) with promising cancer imaging/chemotherapy capacities are fabricated, which display superior performance in the drug loading and tumor tissue penetration. The biodegradable PPF-Gd NSs deliver an ultrahigh drug loading (>1500%) and demonstrate the stable and highly sensitive stimuli-responsive degradation/release for multimodal tumor imaging and cancer chemotherapy. Meanwhile, PPF-Gd NSs also exhibit excellent fluorescence and magnetic resonance imaging capability in vitro and in vivo. Compared to the traditional doxorubicin (DOX) chemotherapy, the in vivo results confirm the evident suppression of the tumor growth by the PPF-Gd/DOX drug delivery system with negligible side effects. This work further supports the potential of lanthanide-based MOF nanomaterials as biodegradable systems to promote the cancer theranostics technology development in the future.

Multimodal channel cancer chemotherapy by 2D functional gadolinium metal-organic framework

2D nanomaterials generally exhibit enhanced physiochemical and biological functions in biomedical applications due to their high surface-to-volume ratio and surface charge. Conventional cancer chemotherapy based on nanomaterials has been hindered by their low drug loading and poor penetration in tumor tissue. To overcome these difficulties, novel materials systems are urgently needed. Hereby, the lanthanide-based porphyrin metal-organic framework (MOF) nanosheets (NSs) with promising cancer imaging/chemotherapy capacities are fabricated, which display superior performance in the drug loading and tumor tissue penetration. The biodegradable PPF-Gd NSs deliver an ultrahigh drug loading (>1500%) and demonstrate the stable and highly sensitive stimuli-responsive degradation/release for multimodal tumor imaging and cancer chemotherapy. Meanwhile, PPF-Gd NSs also exhibit excellent fluorescence and magnetic resonance imaging capability in vitro and in vivo. Compared to the traditional doxorubicin (DOX) chemotherapy, the in vivo results confirm the evident suppression of the tumor growth by the PPF-Gd/DOX drug delivery system with negligible side effects. This work further supports the potential of lanthanide-based MOF nanomaterials as biodegradable systems to promote the cancer theranostics technology development in the future.

Fabrication of G-quadruplex/porphyrin conjugated gold/persistent luminescence theranostic nanoprobe for imaging-guided photodynamic therapy

Photodynamic therapy (PDT) received great attention in cancer therapy due to the advantages of negligible drug resistance, low side effects, and minimal invasiveness. Development of theranostic nanoprobes with specific imaging-guided PDT is of great significance in the field. Herein we report the fabrication of a novel theranostic nanoprobe porphyrin/G-quadruplex conjugated gold/persistent luminescence nanocomposites for imaging-guided PDT. The developed nanoprobe contains NIR-emitting persistent luminescent nanoparticles (PLNP) as the core for autofluorescence-free bioimaging and Au coating on PLNP for facile subsequent DNA conjugation. The DNA sequence is designed to contain G-rich AS1411 aptamer for recognizing the over-expressed cellular nucleolin of cancer cell and forming a G-quadruplex structure to combine with tetrakis (4-carboxyphenyl) porphyrin (TCPP) to realize PDT. The AS1411 aptamer-contained DNA conjugated Au-coated PLNP is rapidly prepared via a freezing method with high content of DNA and good aqueous stability. Meanwhile, TCPP is easily loaded into the G-quadruplex structure formed from G-rich AS1411 aptamer on the surface of Au/PLNP in presence of K⁺. The theranostic nanoprobe gives integrated merits of PLNP for autofluorescence-free bioimging, TCPP for PDT and AS1411 aptamer-contained DNA for specific binding to cancer cells. This work provides a new specially designed imaging-guided PDT nanoplatform for theranostics.

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.

68Ga-Labeled Magnetic-NIR Persistent Luminescent Hybrid Mesoporous Nanoparticles for Multimodal Imaging-Guided Chemotherapy and Photodynamic Therapy

Featured with a zero-autofluorescence background, superior signal-to-noise ratio, high sensitivity, and deep penetration ability, near-infrared persistent luminescence nanoparticle (NIR-PLNP)-based multimodal nanoprobes show great potential for full-scale noninvasive cancer diagnosis. However, direct synthesis of NIR-PLNP-based multimodal nanoprobes with high drug loading capacity to meet growing cancer theranostic demands remains a challenge. In this work, multifunctional hybrid mesoporous nanoparticles (HMNPs) that integrate NIR-PLNPs (Ga2O3:Cr3+, Nd3+), magnetic nanoparticles (Gd2O3), and radionuclides (68Ga) are designed and constructed via a large-pore (mesoporous silica nanoparticle) MSN-templated strategy. The ingenious composition design endows HMNPs with rechargeable NIR-PL, superior longitudinal relaxivity, and excellent radioactivity, making these versatile nanoparticles available for long-term in vivo NIR-PL imaging, magnetic resonance imaging (MRI), and positron emission tomography (PET) imaging. More importantly, the application of large-pore MSN templates maintains the mesoporous structure of HMNPs, promising excellent drug loading capacity of these nanoparticles. As a proof-of-concept, HMNPs loaded with a high dose of DOX (chemotherapy agent) and Si-Pc (photosensitizer) are rationally designed for chemotherapy and NIR-PL-sensitized photodynamic therapy (PDT), respectively. Studies with mice tumor models demonstrate that the DOX/Si-Pc-loaded HMNPs possess excellent cancer cell killing ability and an outstanding tumor suppression effect without systemic toxicity. This work shows the great potential of HMNPs as an "all-in-one" nanotheranostic tool for multimodal NIR-PL/MR/PET imaging-guided chemotherapy and NIR-PL-sensitized photodynamic cancer therapy and provides an innovative paradigm for the development of NIR-PLNP-based nanoplatforms in cancer theranostic.

Surface Plasmon-Enhanced Short-Wave Infrared Fluorescence for Detecting Sub-Millimeter-Sized Tumors

Short-wave infrared (SWIR, 900-1700 nm) enables in vivo imaging with high spatiotemporal resolution and penetration depth due to the reduced tissue autofluorescence and decreased photon scattering at long wavelengths. Although small organic SWIR dye molecules have excellent biocompatibility, they have been rarely exploited as compared to their inorganic counterparts, mainly due to their low quantum yield. To increase their brightness, in this work, the SWIR dye molecules are placed in close proximity to gold nanorods (AuNRs) for surface plasmon-enhanced emission. The fluorescence enhancement is optimized by controlling the dye-to-AuNR number ratio and up to ≈45-fold enhancement factor is achieved. In addition, the results indicate that the highest dye-to-AuNR number ratio gives the highest emission intensity per weight and this is used for synthesizing SWIR imaging probes using layer-by-layer (LbL) technique with polymer coating protection. Then, the SWIR imaging probes are applied for in vivo imaging of ovarian cancer and the surface coating effect on intratumor distribution of the imaging probes is investigated in two orthotopic ovarian cancer models. Lastly, it is demonstrated that the plasmon-enhanced SWIR imaging probe has great potential for fluorescence imaging-guided surgery by showing its capability to detect sub-millimeter-sized tumors.

Nucleic Acids Analysis

Nucleic acids are natural biopolymers of nucleotides that store, encode, transmit and express genetic information, which play central roles in diverse cellular events and diseases in living things. The analysis of nucleic acids and nucleic acids-based analysis have been widely applied in biological studies, clinical diagnosis, environmental analysis, food safety and forensic analysis. During the past decades, the field of nucleic acids analysis has been rapidly advancing with many technological breakthroughs. In this review, we focus on the methods developed for analyzing nucleic acids, nucleic acids-based analysis, device for nucleic acids analysis, and applications of nucleic acids analysis. The representative strategies for the development of new nucleic acids analysis in this field are summarized, and key advantages and possible limitations are discussed. Finally, a brief perspective on existing challenges and further research development is provided.

Controllable Frequency Dependence of Resonance Energy Transfer Coupled with Localized Surface Plasmon Polaritons

We investigate the intrinsic characteristics of resonance energy transfer (RET) coupled with localized surface plasmon polaritons (LSPPs) from the perspective of macroscopic quantum electrodynamics. To quantify the effect of LSPPs, we propose a numerical scheme that allows us to accurately calculate the rate of RET between a donor-acceptor pair near a nanoparticle. Our study shows that LSPPs can be used to enhance the RET rate significantly and control its frequency dependence by modifying a core/shell structure, which indicates the possibility of RET rate optimization. Moreover, we systematically explore the angle (distance) dependence of the RET rate and analyze its origin. According to different frequency regimes, the angle dependence of RET is dominated by different mechanisms, such as LSPPs, surface plasmon polaritons (SPPs), and anti-resonance. For the proposed core/shell structure, the characteristic distance of RET coupled with LSPPs (approximately 0.05 emission wavelength) is shorter than that of RET coupled with SPPs (approximately 0.1 emission wavelength), which may provide promising applications in energy science.

Persistent luminescence-polypyrrole nanocomposite for dual-modal imaging and photothermal therapy of mammary cancer

Multifunctional nanocomposites that possess imaging and high-performance therapeutic features are experiencing a surge in interest in the precision clinical anticancer treatment. In this work, we reported the fabrication and bio-application of a novel persistent luminescence-polypyrrole nanocomposite (LPLNP@SPP) for photoacoustic/persistent luminescence (PA/PL) dual-modal imaging guided photothermal therapy (PTT). The construction of LPLNP@SPP avoids the PL quenching of LPLNP-OH by the polypyrrole-coating, and thus enables the combination of PL and PTT. The LPLNP@SPP shows excellent biocompatibility, long lasting near-infrared (NIR) PL emitting without in situ excitation and high-contrast PA signals. Meanwhile, this nanocomposite exhibits strong NIR absorbance and exceptional photothermal conversion capability, which provides notable potential for imaging-guided antitumor therapy. Thus, our work highlights the dual-functional core-shell LPLNP@SPP as a feasible theranostic nanoplatform for cancer diagnosis and therapy.

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.

A novel universal nano-luciferase-involved reporter system for long-term probing food-borne probiotics and pathogenic bacteria in mice by in situ bioluminescence imaging

Food-borne bacteria have received increasing attention due to their great impact on human health. Bioimaging makes it possible to monitor bacteria inside the living body in real time and in situ. Nano-luciferase (NLuc) as a new member of the luciferase family exhibits superior properties than the commonly used luciferases, including small size, high stability and improved luminescence. Herein, NLuc, CBRLuc and FLuc were well expressed in varied food-borne bacteria. Results showed that the signal intensity of E. coli-NLuc was about 41 times higher than E. coli-CBRLuc, L. plantarum-NLuc was nearly 227 times that of L. plantarum-FLuc in vitro. Moreover, NLuc was applied to trace L. plantarum and E. coli in vivo through the whole body and separated digestive tract imaging, as well as the feces bacterium counting and probing. The persistence of bioluminescent strains was predominantly localized in colon and cecum of mice after oral administration. The NLuc system showed its incomparable superiority, especially in the application of intestinal imaging and the universality for food-borne bacteria. We demonstrated that the NLuc system was a brilliant alternative for specific application of food-borne bacteria in vivo, aiming to collect more accurate and real-time information of food-borne bacteria from the living body for further investigation of their damage mechanism and nutrition effect.

Schematic illustration of the preparation of bioluminescent bacteria and the experimental design of tracing of the foodborne bacteria in vivo.

Optically Active Nanomaterials for Bioimaging and Targeted Therapy

Non-invasive tracking for monitoring the selective delivery and transplantation of biotargeted agents in vivo has been employed as one of the most effective tools in the field of nanomedicine. Different nanoprobes have been developed and applied to bioimaging tissues and the treatment of diseases ranging from inflammatory and cardiovascular diseases to cancer. Herein, we will review the recent advances in the development of optics-responsive nanomaterials, including organic and inorganic nanoparticles, for multimodal bioimaging and targeted therapy. The main focus is placed on nanoprobe fabrication, mechanistic illustrations, and diagnostic, or therapeutical applications. These nanomedicine strategies have promoted a better understanding of the biological events underlying diverse disease etiologies, thereby facilitating diagnosis, illness evaluation, therapeutic effect, and drug discovery.

Turning solid into gel for high-efficient persistent luminescence-sensitized photodynamic therapy

Bioavailable persistent luminescence material is an ideal internal light source for long-term photodynamic therapy, but inevitably suffers from low utilization efficiency and weak persistent luminescence due to corrosion and screening processes. Herein, we show a facile and smart "turning solid into gel" strategy to fabricate persistent luminescence hydrogel for high-efficient persistent luminescence-sensitized photodynamic therapy. The homogeneous persistent luminescence hydrogel was synthesized via dispersing high-temperature calcined persistent luminescence material without corrosion and screening into a biocompatible alginate-Ca²⁺ hydrogel. The simple synthesis strategy allows 100% of utilization efficiency and intact persistent luminescence of persistent luminescence material. The persistent luminescence hydrogel possesses favorable biocompatibility, bright persistent luminescence, red light renewability, good syringeability, and strong fixing ability in tumors. The persistent luminescence hydrogel can be easily injected in vivo as a powerful localized light source for superior persistent luminescence-sensitized photodynamic therapy of tumors. The "turning solid into gel" strategy enables taking full advantages of persistent luminescence for biological applications, and shows great potential in utilizing diverse theranostic agents regardless of hydrophilicity and hydrophobicity.

Chromium-Doped Zinc Gallogermanate@Zeolitic Imidazolate Framework-8: A Multifunctional Nanoplatform for Rechargeable In Vivo Persistent Luminescence Imaging and pH-Responsive Drug Release

Multifunctional theranostic nanoplatforms greatly improve the accuracy and effectiveness in tumor treatments. Much effort has been made in developing advanced optical imaging-based tumor theranostic nanoplatforms. However, autofluorescence and irradiation damage of the conventional fluorescence imaging technologies as well as unsatisfied curative effects of the nanoplatforms remain great challenges against their wide applications. Herein, we constructed a novel core-shell multifunctional nanoplatform, that is, chromium-doped zinc gallogermanate (ZGGO) near-infrared (NIR) persistent luminescent nanoparticles (PLNPs) as a core and zeolitic imidazolate framework-8 (ZIF-8) as a shell (namely ZGGO@ZIF-8). The ZGGO@ZIF-8 nanoplatform possessed dual functionalities of the autofluorescence-free NIR PersL imaging as well as the pH-responsive drug delivery, thus it has high potential in tumor theranostics. Notably, the loading content of doxorubicin (DOX) in ZGGO@ZIF-8 (LC = 93.2%) was quite high, and the drug release of DOX-loaded ZGGO@ZIF-8 was accelerated in an acidic microenvironment such as tumor cells. The ZGGO@ZIF-8 opens up a new material system in the combination of PLNPs with metal-organic frameworks and may offer new opportunities for the development of advanced multifunctional nanoplatforms for tumor theranostics, chemical sensing, and optical information storage.

In-Taken Labeling and in Vivo Tracing Foodborne Probiotics via DNA-Encapsulated Persistent Luminescence Nanoprobe Assisted Autofluorescence-Free Bioimaging

An in vivo probing strategy that can real-time and in situ trace target probiotics inside the living body is herein proposed by employing plasmid-like DNA as in-taken assistance, persistent luminescence nanophosphors (PLNPs) as optical labeling, and background-free fluorescence bioimaging as signal readout. PLNPs with superlong afterglow and excellent biocompatibility and stability were surface-modified by DNA molecules with a specific sequence, which greatly promoted the nanoparticle penetration into the bacteria and facilitated the in vivo bioimaging with high sensitivity and signal-to-noise ratio. Compared with the previous surface-labeling strategy by antibody recognition, the in-taken optical labeling demonstrated improved stability, and reached ideal results of real-time and in situ monitoring the in vivo behaviors of target probiotics, supporting the further development of in vivo investigation methodology for foodborne probiotics. Moreover, such a strategy offers a promising platform that leverage nanoscience to food nutrition as well as food-safety research, aiming to collect more accurate and fresh information from the living body.

Cerasome-based gold-nanoshell encapsulating L-menthol for ultrasound contrast imaging and photothermal therapy of cancer

Various nanoformulations of perfluorocarbon have been developed thus far, to achieve ultrasound imaging of tumors and tumor-targeted therapy. However, their application has been greatly limited by their short sonographic duration and large size distribution. A novel theranostic agent was constructed based on gold nanoshell cerasome-encapsulated L-menthol (GNC-LM). Owing to the sustained and controllable generation of L-menthol bubbles under near-infrared laser irradiation, GNC-LM showed good performance in contrast enhancement of ultrasound imaging in vivo. GNC-LM could be imaged for 30 min, which is much longer than the imaging time of SonoVue (commercially used microbubbles). Moreover, photothermal therapy (PTT) based on the light-to-heat conversion of the nanosystem effectively ablated the tumor. Our study demonstrated the promising potential of the obtained GNC-LM to serve as a therapeutic nanoprobe for ultrasound contrast imaging and PTT of tumors.

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.

Afterglow Resonance Energy Transfer Inhibition for Fibroblast Activation Protein-α Assay

Traditional photoluminescence resonance energy transfer (PRET)-based sensors are widely applied, but still suffer from the severe background interference from in situ excitation. The afterglow nature of the persistent luminescence nanoparticles (PLNPs) allows optosensing after the stoppage of in situ illumination, and thus subtly overcomes that interference. We proposed a simple strategy for functionalizing PLNPs for bioanalytical applications and the new afterglow resonance energy transfer (ARET)-based assay for quantitative determination and imaging of fibroblast activation protein-alpha (FAPα) in live cells using Au-decorated Cr³⁺_0.004:ZnGa₂O₄ as donor and Cy5.5-KGPNQC-SH as acceptor. The ARET between the donor and acceptor quenches the afterglow of the donor, and the cleavage of peptide KGPNQC by FAPα inhibits the ARET and restores the afterglow of the donor. The ARET-based assay of FAPα, with the linear range of 0.1-2.0 mg·L^-1 (1.2-22.9 nM), LOD of 11 μg·L^-1 (115 pM), and RSD of 3.9% (for 0.5 mg·L^-1 FAPα, n = 5), displays higher sensitivity, lower limit of detection (LOD), and better anti-interference capability than the corresponding PRET-based assay. Besides, the ARET-based sensors are lighted up by the FAPα-positive U87MG and MDA-MB-435 cells, but kept in the dark when incubated in the FAPα-negative AD293 cells. The proposed ARET-based sensor can detect FAPα of U87MG and MDA-MB-435 living cells in human serum with the spiked recoveries of 95.6-103%. Our data demonstrated a simple and effective strategy for bridging PLNPs to bioanalytical applications, and an attractive ARET assay of FAPα.

Routes to Potentially Safer T1 Magnetic Resonance Imaging Contrast in a Compact Plasmonic Nanoparticle with Enhanced Fluorescence

Engineering a compact, near-infrared plasmonic nanostructure with integrated image-enhancing agents for combined imaging and therapy is an important nanomedical challenge. Recently, we showed that Au@SiO₂@Au nanomatryoshkas (NM) are a highly promising nanostructure for hosting either T₁ MRI or fluorescent contrast agents with a photothermal therapeutic response in a compact geometry. Here, we show that a near-infrared-resonant NM can provide simultaneous contrast enhancement for both T₁ magnetic resonance imaging (MRI) and fluorescence optical imaging (FOI) by encapsulating both types of contrast agents in the internal silica layer between the Au core and shell. We also show that this method of T₁ enhancement is even more effective for Fe(III), a potentially safer contrast agent compared to Gd(III). Fe-NM-based contrast agents are found to have relaxivities 2× greater than those found in the widely used gadolinium chelate, Gd(III) DOTA, providing a practical alternative that would eliminate Gd(III) patient exposure entirely. This dual-modality nanostructure can enable not only tissue visualization with MRI but also fluorescence-based nanoparticle tracking for quantifying nanoparticle distributions in vivo, in addition to a near-infrared photothermal therapeutic response.

Probing and Quantifying the Food-Borne Pathogens and Toxins: From In Vitro to In Vivo

Development of real-time and in situ analytical methods for determination of food-borne pathogens and toxins ingested into the human body would be a promising research direction in the food-safety area. The present perspective starts with summarization of the up-to-date progress of the nanomaterial-assisted in vitro detection methods for pathogens and toxins and finally focuses on application of animal bioimaging to in vivo study, including prospective strategies for in vivo quantification of target pathogens or toxins and in vivo investigation of their behaviors inside the living body, with the assistance of real-time and non-invasive optical bioimaging. This perspective provides the advisory direction for food-safety research, from in vitro to in vivo, along with a prospective discussion of the further development roadmap of the food-safety detection techniques, especially the bioimaging-guided methods for investigation and mediation of the food contamination effect to human health.

Fabrication of an activatable hybrid persistent luminescence nanoprobe for background-free bioimaging-guided investigation of food-borne aflatoxin in vivo

The development of in situ and real-time analytical methods for specifically probing food-borne hazardous substances is promising for clarifying their harmful behaviors and related disease mechanisms inside the living body through in situ investigation of their in vivo behaviors. Herein, optical nanoimaging with the ability of in situ non-damage detection and real-time monitoring was introduced for specific recognition of aflatoxin in cellular levels and in vivo via the fluorescence resonance energy transfer (FRET) protocol. Persistent luminescence nanophosphors (PLNPs) with distinct advantages of improved sensitivity and signal-to-noise ratio were employed in in vivo bioimaging as photoluminescence nanoprobes, while copper sulfide nanoparticles were utilized as the quencher. Due to their long-lasting afterglow, PLNPs do not require external illumination before imaging, effectively eliminating the scattering light and autofluorescence from the biological matrix that can occur during in situ excitation. The proposed FRET imaging assay achieved high sensitivity and specificity as well as improved imaging resolution for the target aflatoxin present in vivo. This study will provide insights towards advanced methodology for the applications of bioimaging in food safety, and could potentially provide an advisory roadmap for bioimaging-guided exploration and mediation of food-borne hazards to human health.

Construction of persistent luminescence nanophosphor-copper sulfide hybrid FRET nanoprobes for background-free bioimaging-guided investigation of food-borne aflatoxin in vivo.

Interconversion between KSc2F7:Yb/Er and K2NaScF6:Yb/Er nanocrystals: the role of chemistry

Interconversion between nanocrystals of orthorhombic KSc₂F₇:Yb/Er and cubic K₂NaScF₆:Yb/Er was realized by adjusting the according chemical reaction conditions.

Plasmon-Induced Selective Enhancement of Green Emission in Lanthanide-Doped Nanoparticles

By introducing an 18 nm thick Au nanofilm, selective enhancement of green emission from lanthanide-doped (β-NaYF₄:Yb³⁺/Er³⁺) upconversion nanoparticles (UCNPs) is demonstrated. The Au nanofilm is deposited on a microfiber surface by the sputtering method and then covered with the UCNPs. The plasma on the surface of the Au nanofilm can be excited by launching a 980 nm wavelength laser beam into the microfiber, resulting in an enhancement of the local electric field and a strong thermal effect. A 36-fold luminescence intensity enhancement of the UCNPs at 523 nm is observed, with no obvious reduction in the photostability of the UCNPs. Further, the intensity ratios of the emissions at 523-545 nm and at 523-655 nm are enhanced with increasing pump power, which is attributed to the increasing plasmon-induced thermal effect. Therefore, the fabricated device is further demonstrated to exhibit an excellent ability in temperature sensing. By controlling the pump power and the UCNP concentration, a wide temperature range (325-811 K) and a high temperature resolution (0.035-0.046 K) are achieved in the fabricated device.

LaAlO₃:Mn4+ as Near-Infrared Emitting Persistent Luminescence Phosphor for Medical Imaging: A Charge Compensation Study

Mn4+-activated phosphors are emerging as a novel class of deep red/near-infrared emitting persistent luminescence materials for medical imaging as a promising alternative to Cr3+-doped nanomaterials. Currently, it remains a challenge to improve the afterglow and photoluminescence properties of these phosphors through a traditional high-temperature solid-state reaction method in air. Herein we propose a charge compensation strategy for enhancing the photoluminescence and afterglow performance of Mn4+-activated LaAlO₃ phosphors. LaAlO₃:Mn4+ (LAO:Mn4+) was synthesized by high-temperature solid-state reaction in air. The charge compensation strategies for LaAlO₃:Mn4+ phosphors were systematically discussed. Interestingly, Cl-/Na⁺/Ca2+/Sr2+/Ba2+/Ge4+ co-dopants were all found to be beneficial for enhancing LaAlO₃:Mn4+ luminescence and afterglow intensity. This strategy shows great promise and opens up new avenues for the exploration of more promising near-infrared emitting long persistent phosphors for medical imaging.

Hybrid Nanoclusters for Near-Infrared to Near-Infrared Upconverted Persistent Luminescence Bioimaging

Persistent luminescence (PL) bioimaging provides an optimal method of eliminating autofluorescence for a higher resolution and sensitivity because of the absence of excitation light. However, ultraviolet light is still necessary in common energy charging processes, which limits its reactivation in vivo because of its low penetration depth. In the present study, we introduce a type of hybrid nanocluster (UCPL-NC) composed of upconversion nanoparticles, β-NaYbF₄:Tm@NaYF₄, and persistent nanoparticles, Zn_1.1Ga_1.8Ge_0.1O₄:0.5%Cr, which can be activated by a 980 nm laser and exhibits an afterglow at 700 nm to realize near-infrared (NIR) to NIR UCPL bioimaging. The PL of the UCPL-NCs can be reactivated even when covered with a 10 mm pork. We demonstrate that these polyethylene glycol-modified phospholipid-functionalized UCPL-NCs can be reactivated in vivo and applied in the PL lymphatic imaging on small animals.

Persistent Luminescence Nanophosphor Involved Near-Infrared Optical Bioimaging for Investigation of Foodborne Probiotics Biodistribution in Vivo: A Proof-of-Concept Study

Probiotics has attracted great attention in food nutrition and safety research field, but thus far there are limited analytical techniques for visualized and real-time monitoring of the probiotics when they are ingested in vivo. Herein, the optical bioimaging technique has been introduced for investigation of foodborne probiotics biodistribution in vivo, employing the near-infrared (NIR) emitting persistent luminescence nanophosphors (PLNPs) of Cr³⁺-doped zinc gallogermanate (ZGGO) as the contrast nanoprobes. The ultrabrightness, super long afterglow, polydispersed size, low toxicity, and excellent photostability and biocompatibility of PLNPs were demonstrated to be qualified as a tracer for labeling probiotics via antibody (anti-Gram positive bacteria LTA antibody) recognition as well as contrast agent for long-term bioimaging the probiotics. In vivo optical bioimaging assay showed that the LTA antibody functionalized ZGGO nanoprobes that could be efficiently tagged to the probiobics were successfully applied for real-time monitoring and nondamaged probing of the biodistribution of probiotics inside the living body after oral administration. This work presents a proof-of-concept that exploited the bioimaging methodology for real-time and nondamaged researching the foodborne probiotics behaviors in vivo, which would open up a novel way of food safety detection and nutrition investigation.

Tuning Plasmonic Enhancement of Single Nanocrystal Upconversion Luminescence by Varying Gold Nanorod Diameter

Plasmonic enhancement induced by metallic nanostructures is an effective strategy to improve the upconversion efficiency of lanthanide-doped nanocrystals. It is demonstrated that plasmonic enhancement of the upconversion luminescence (UCL) of single NaYF₄ :Yb³⁺ /Er³⁺ /Mn²⁺ nanocrystal can be tuned by tailoring scattering and absorption cross sections of gold nanorods, which is synthesized wet chemically. The assembly of the single gold nanorod and single upconversion nanocrystal is achieved by the atomic force microscope probe manipulation. By selecting two kinds of gold nanorods with similar longitudinal surface plasmon resonance wavelength but different diameters (27.3 and 46.7 nm), which extinction spectra are separately dominant by the absorption and scattering, the maximum UCL enhancement by a factor of 110 is achieved with the 46.7 nm-diameter gold nanorod, while it is 19 for the nanorod with the diameter of 27.3 nm. Such strong enhancement with the larger gold nanorod is due to stronger scattering ability and greater extent of the near-field enhancement. The enhanced UCL shows a strong dependence on the excitation polarization relative to the nanorod long axis. Time-resolved measurements and finite-difference time-domain simulations unveil that both excitation and emission processes of UCL are accelerated by the nanorod plasmonic effect.

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.

Cr3+/Er3+ co-doped LaAlO3 perovskite phosphor: a near-infrared persistent luminescence probe covering the first and third biological windows

We have developed a novel persistent phosphor of LaAlO₃ perovskite doped with Er³⁺, Cr³⁺ and Sm³⁺ (LAO:Er–Cr–Sm), which exhibits long persistent luminescence (PersL) at 1553 nm due to the Er³⁺:⁴I_13/2 → ⁴I_15/2 transition as well as at 734 nm due to the Cr³⁺:²E(²G) → ⁴A₂(⁴F) transition.