Energy Transfer Highway in Nd3+-Sensitized Nanoparticles for Efficient near-Infrared Bioimaging

Portable Near-Infrared to Near-Infrared Platform for Homogeneous Quantification of Biomarkers in Complex Biological Samples

Förster resonance energy transfer (FRET)-based homogeneous immunoassay obviates tedious washing steps and thus is a promising approach for immunoassays. However, a conventional FRET-based homogeneous immunoassay operating in the visible region is not able to overcome the interference of complex biological samples, thus resulting in insufficient detection sensitivity and poor accuracy. Here, we develop a near-infrared (NIR)-to-NIR FRET platform (Ex = 808 nm, Em = 980 nm) that enables background-free high-throughput homogeneous quantification of various biomarkers in complex biological samples. This NIR-to-NIR FRET platform is portable and easy to operate and is mainly composed of a high-performance NIR-to-NIR FRET pair based on lanthanide-doped nanoparticles (LnNPs) and a custom-made microplate reader for readout of NIR luminescence signals. We demonstrate that this NIR-to-NIR FRET platform is versatile and robust, capable of realizing highly sensitive and accurate detection of various critical biomarkers, including small molecules (morphine and 1,25-dihydroxyvitamin D), proteins (human chorionic gonadotropin), and viral particles (adenovirus) in unprocessed complex biological samples (urine, whole blood, and feces) within 5-10 min. We expect this NIR-to-NIR FRET platform to provide low-cost healthcare for populations living in resource-limited areas and be widely used in many other fields, such as food safety and environmental monitoring.

Recent advances in near-infrared I/II persistent luminescent nanoparticles for biosensing and bioimaging in cancer analysis

Photoluminescent materials (PLNs) are photoluminescent materials that can absorb external excitation light, store it, and slowly release it in the form of light in the dark to achieve long-term luminescence. Developing near-infrared (NIR) PLNs is critical to improving long-afterglow luminescent materials. Because they excite in vitro, NIR-PLNs have the potential to avoid interference from in vivo autofluorescence in biomedical applications. These materials are promising for biosensing and bioimaging applications by exploiting the near-infrared biological window. First, we discuss the biomedical applications of PLNs in the first near-infrared window (NIR-I, 700-900 nm), which have been widely developed and specifically introduce biosensors and imaging reagents. However, the light in this area still suffers from significant light scattering and tissue autofluorescence, which will affect the imaging quality. Over time, fluorescence imaging technology in the second near-infrared window (NIR-II, 1000-1700 nm) has also begun to develop rapidly. NIR-II fluorescence imaging has the advantages of low light scattering loss, high tissue penetration depth, high imaging resolution, and high signal-to-noise ratio, and it shows broad application prospects in biological analysis and medical diagnosis. This critical review collected and sorted articles from the past 5 years and introduced their respective fluorescence imaging technologies and backgrounds based on the definitions of NIR-I and NIR-II. We also analyzed the current advantages and dilemmas that remain to be solved. Herein, we also suggested specific approaches NIR-PLNs can use to improve the quality and be more applicable in cancer research.

Nanomaterials for light-mediated therapeutics in deep tissue

Light-mediated therapeutics, including photodynamic therapy, photothermal therapy and light-triggered drug delivery, have been widely studied due to their high specificity and effective therapy. However, conventional light-mediated therapies usually depend on the activation of light-sensitive molecules with UV or visible light, which have poor penetration in biological tissues. Over the past decade, efforts have been made to engineer nanosystems that can generate luminescence through excitation with near-infrared (NIR) light, ultrasound or X-ray. Certain nanosystems can even carry out light-mediated therapy through chemiluminescence, eliminating the need for external activation. Compared to UV or visible light, these 4 excitation modes penetrate more deeply into biological tissues, triggering light-mediated therapy in deeper tissues. In this review, we systematically report the design and mechanisms of different luminescent nanosystems excited by the 4 excitation sources, methods to enhance the generated luminescence, and recent applications of such nanosystems in deep tissue light-mediated therapeutics.

Preparation of rare earth-doped nano-fluorescent materials in the second near-infrared region and their application in biological imaging

Second near-infrared (NIR-II) fluorescence imaging (FLI) has gained widespread interest in the biomedical field because of its advantages of high sensitivity and high penetration depth. In particular, rare earth-doped nanoprobes (RENPs) have shown completely different physical and chemical properties from macroscopic substances owing to their unique size and structure. This paper reviews the synthesis methods and types of RENPs for NIR-II imaging, focusing on new methods to enhance the luminous intensity of RENPs and multi-band imaging and multi-mode imaging of RENPs in biological applications. This review also presents an overview of the challenges and future development prospects based on RENPs in NIR-II regional bioimaging.

Engineering highly efficient NIR-II FRET platform for Background-Free homogeneous detection of SARS-CoV-2 neutralizing antibodies in whole blood

Förster or fluorescence resonance energy transfer (FRET) enables to probe biomolecular interactions, thus playing a vital role in bioassays. However, conventional FRET platforms suffer from limited sensitivity due to the low FRET efficiency and poor anti-interference of existing FRET pairs. Here we report a NIR-II (1000-1700 nm) FRET platform with extremely high FRET efficiency and exceptional anti-interference capability. This NIR-II FRET platform is established based on a pair of lanthanides downshifting nanoparticles (DSNPs) by employing Nd³⁺ doped DSNPs as an energy donor and Yb³⁺ doped DSNPs as an energy acceptor. The maximum FRET efficiency of this well-engineered NIR-II FRET platform reaches up to 92.2%, which is much higher than most commonly used ones. Owing to the all-NIR advantage (λ_ex = 808 nm, λ_em = 1064 nm), this highly efficient NIR-II FRET platform exhibits extraordinary anti-interference in whole blood, and thus enabling background-free homogeneous detection of SARS-CoV-2 neutralizing antibodies in clinical whole blood sample with high sensitivity (limit of detection = 0.5 μg/mL) and specificity. This work opens up new opportunities for realizing highly sensitive detection of various biomarkers in biological samples with severe background interference.

Handheld NIR-to-NIR Platform for on-site evaluating protective neutralizing antibody against SARS-CoV-2 ancestral strain and Omicron variant after vaccination or infection

Lateral flow assays (LFAs) are promising points-of-care tests, playing a vital role in diseases screening, diagnosis and surveillance. However, development of portable, cheap, and smart LFAs platform for sensitive and accurate quantification of disease biomarkers in complex media is challenging. Here, a cheap handheld device was developed to realize on-site detection of disease biomarkers by Nd³⁺/Yb³⁺ co-doped near-infrared (NIR)-to-NIR downconversion nanoparticles (DCNPs) based LFA. Its sensitivity is at least 8-fold higher for detecting NIR light signal from Nd³⁺/Yb³⁺ co-doped nanoparticles than conventional expensive InGaAs camera based detection platform. Additionally, we enhance NIR quantum yield of Nd³⁺/Yb³⁺ co-doped nanoparticles up to 35.5% via simultaneous high dopant of sensitizer ions Nd³⁺ and emitter ions Yb³⁺. Combination of NIR-to-NIR handheld detection device and ultra-bright NIR emitting NaNbF₄:Yb60%@NaLuF₄ nanoparticle probe allows the detection sensitivity of SARS-CoV-2 ancestral strain and Omicron variants specific neutralizing antibodies LFA up to the level of commercial enzyme linked immunosorbent assay kit. Furthermore, by this robust method, enhanced neutralizing antibodies against SARS-CoV-2 ancestral strain and Omicron variants are observed in healthy participants with Ad5-nCoV booster on top of two doses of inactivated vaccine. This NIR-to-NIR handheld platform provides a promising strategy for on-site evaluating protective humoral immunity after SARS-CoV-2 vaccination or infection.

Boltzmann- and Non-Boltzmann-Based Thermometers in the First, Second and Third Biological Windows for the SrF2:Yb3+, Ho3+ Nanocrystals Under 980, 940 and 915 nm Excitations

Spectrally determination of temperature based on the lanthanide-doped nanocrystals (NCs) is a vital strategy to noninvasively measure the temperature in practical applications. Here, we synthesized a series of SrF₂:Yb³⁺/Ho³⁺ NCs and simultaneously observed the efficient visible upconversion luminescence (UCL) and near-infrared (NIR) downconversion luminescence (DCL) under 980, 940 and 915 nm excitations. Subsequently, these NCs were further utilized for thermometers based on the Boltzmann (thermally coupled levels, TCLs) and non-Boltzmann (non-thermally coupled levels, NTCLs) of Ho³⁺ ions in the first (~ 650 nm), second (~ 1012 nm) and third (~ 2020 nm) biological windows (BW-I, BW-II and BW-III) under tri-wavelength excitations. The thermometric parameters including the relative sensitivity ([Formula: see text]) and temperature uncertainty ([Formula: see text]) are quantitatively determined on the I₆₄₈/I₅₄₁ (BW-I), I₁₁₈₆/I₁₀₁₂ (BW-II), and I₁₉₅₀/I₂₀₂₀ (BW-III) transitions of Ho³⁺ ions in the temperature range of 303-573 K. Comparative experimental results demonstrated that the thermometer has superior performances.

Lanthanide/Cu2-xSe Nanoparticles for Bacteria-Activated NIR-II Fluorescence Imaging of Infection

A material system enabling specific NIR-II fluorescence imaging of Gram-positive bacteria is described. The material system is based on the electrostatic binding of Cu_2-xSe and vancomycin-modified NaGdF₄:Nd,Yb@NaGdF₄ downconversion nanoparticles (DCNPs), the fluorescence of which is weak owing to the spectral overlap of Cu_2-xSe absorption with the DCNP NIR emission. The presence of Gram-positive bacteria precisely disconnects the bond between vancomycin-modified DCNPs and Cu_2-xSe, thus enabling a strong fluorescent signal. In vivo studies show that the material system can be specifically activated at the site of Gram-positive bacterial infection but is essentially nonfluorescent in the area of Gram-negative bacterial infection.

Engineering Er3+-sensitized nanocrystals to enhance NIR II-responsive upconversion luminescence

An Er³⁺-sensitized system with a high response to 1550 nm radiation in the second near-infrared window (NIR II) has been considered for a new class of potential candidates for applications in bio-imaging and advanced anti-counterfeiting, yet the achievement of highly efficient upconversion emission still remains a challenge. Here, we constructed a novel Er³⁺-sensitized core-shell-shell upconversion nanostructure with a Yb³⁺-enriched core as the emitting layer. This designed nanostructure allows the Yb³⁺ emitting layer to more efficiently trap and lock excitation energy by combining the interfacial energy transfer (IET) from the shell (Er³⁺) to the core (Yb³⁺), high activator Yb³⁺ content, and minimized energy back-transfer. As a result, the NIR II emission at 1000 nm is remarkably enhanced with a high quantum yield (QY) of 11.5%. Based on this trap and lock-in effect of the excitation energy in the Yb³⁺-enriched core, highly efficient 1550 nm-responsive visible and NIR upconversion emissions are also achieved by co-doping with other activator ions (e.g., Ho³⁺ and Tm³⁺). Our research provides a new functional design for improving NIR II-responsive upconversion luminescence, which is significant for developing practical applications.

Versatile Types of Inorganic/Organic NIR-IIa/IIb Fluorophores: From Strategic Design toward Molecular Imaging and Theranostics

In vivo imaging in the second near-infrared window (NIR-II, 1000-1700 nm), which enables us to look deeply into living subjects, is producing marvelous opportunities for biomedical research and clinical applications. Very recently, there has been an upsurge of interdisciplinary studies focusing on developing versatile types of inorganic/organic fluorophores that can be used for noninvasive NIR-IIa/IIb imaging (NIR-IIa, 1300-1400 nm; NIR-IIb, 1500-1700 nm) with near-zero tissue autofluorescence and deeper tissue penetration. This review provides an overview of the reports published to date on the design, properties, molecular imaging, and theranostics of inorganic/organic NIR-IIa/IIb fluorophores. First, we summarize the design concepts of the up-to-date functional NIR-IIa/IIb biomaterials, in the order of single-walled carbon nanotubes (SWCNTs), quantum dots (QDs), rare-earth-doped nanoparticles (RENPs), and organic fluorophores (OFs). Then, these novel imaging modalities and versatile biomedical applications brought by these superior fluorescent properties are reviewed. Finally, challenges and perspectives for future clinical translation, aiming at boosting the clinical application progress of NIR-IIa and NIR-IIb imaging technology are highlighted.

Er3+-Ions-Doped Multiscale Nanoprobes for Fluorescence Imaging in Cellular and Living Mice

With the development of biotechnology, luminescent nanoprobes for biological disease detection are widely used. However, the further application in clinic is limited by the reduced penetration depth in the tissues and light scattering. In this work, we have synthesized NaYF4:Yb,Er,Ce@SiO2-OAlg nanomaterials, which have both upconversion and near-infrared (NIR) luminescence. The optimized probes were determined to achieve cell imaging by its upconversion (UCL) luminescence and in vivo imaging through collection of NIR fluorescence signals simultaneously. The research is conducive to developing accurate diagnostic techniques based on UCL and NIR fluorescence imaging by a single nanoparticle.

A novel M2Ga2GeO7:N3+(M = Ca, Ba, Sr; N = Cr, Nd, Er) sub-micron phosphor with multiband NIR emissions: preparation, structure, properties, and LEDs

Near-infrared (NIR) emission materials can be widely applied in various fields, such as food detection, imaging, treatment, electronic products. With the trend of miniaturization of equipment, smaller materials are needed. In this work, we successfully synthesized a series of M₂Ga₂GeO₇:N³⁺(M = Ca, Ba, Sr; N = Cr, Nd, Er) samples and then focused on the study of Nd³⁺doped Sr₂Ga₂GeO₇(SGGO). A series of SGGO:xNd³⁺sub-micron phosphors were prepared via a microwave-assisted sol-gel process combined with subsequent calcination at 750 ℃, and the structural information and luminescent properties were systematically studied. SGGO is a representative tetragonal crystal and belonging to the space group of P4¯21m (113). The Nd³⁺ions occupy eight-coordinated Sr²⁺sites in the crystal lattice. From SEM analysis, the average particle size distribution is 219.7 ± 41.4 nm. The sub-micron phosphors have rich excitation spectra ranging from 350 nm to 850 nm and can produce multiband NIR emissions of 1331, 1056, and 905 nm when excited by ultraviolet and NIR light. The maximum emission intensity was obtained by optimizing the doping ratio of Nd³⁺ions. A commercial chip was then utilized to fabricate light-emitting diodes (LEDs) to verify its application potential in NIR-II mini-LEDs. Compared with blue light LEDs, the as-prepared LEDs had good imaging penetration depth and could be clearly observed under 10 mm of chicken breast coverage. The maximum imaging penetration depth can be 33 mm.

Fabrication of Nd3+ and Yb3+ doped NIR emitting nano fluorescent probe: A candidate for bioimaging applications

The intentional design of rare earth doped luminescent architecture exhibits unique optical properties and it can be considered as a promising and potential probe for optical imaging applications. Calcium fluoride (CaF₂) nanoparticles doped with optimum concentration of Nd³⁺ and Yb³⁺ as sensitizer and activator, respectively, were synthesized by wet precipitation method and characterized by x-ray diffraction (XRD) and photoluminescence. In spite of the fact that the energy transfer takes place from Nd³⁺ to Yb³⁺, the luminescence intensity was found to be weak due to the lattice defects generated from the doping of trivalent cations (Nd³⁺ and Yb³⁺) for divalent host cations (Ca²⁺). These defect centres were tailored via charge compensation approach by co-doping Na⁺ ion and by optimizing its concentration and heat treatment duration. CaF₂ doped with 5 mol% Nd³⁺, 3 mol% Yb³⁺ and 4 mol% Na⁺ after heat treatment for 2 h exhibited significantly enhanced emission intensity and life time. The ex vivo fluorescence imaging experiment was done at various thickness of chicken breast tissue. The maximum theoretical depth penetration of the NIR light was calculated and the value is 14 mm. The fabricated phosphor can serve as contrast agent for deep tissue near infrared (NIR) light imaging.

Exploring Heterostructured Upconversion Nanoparticles: From Rational Engineering to Diverse Applications

Upconversion nanoparticles (UCNPs) represent a class of optical nanomaterials that can convert low-energy excitation photons to high-energy fluorescence emissions. On the basis of UCNPs, heterostructured UCNPs, consisting of UCNPs and other functional counterparts (metals, semiconductors, polymers, etc.), present an intriguing system in which the physicochemical properties are largely influenced by the entire assembled particle and also by the morphology, dimension, and composition of each individual component. As multicomponent nanomaterials, heterostructured UCNPs can overcome challenges associated with a single component and exhibit bifunctional or multifunctional properties, which can further expand their applications in bioimaging, biodetection, and phototherapy. In this review, we provide a summary of recent achievements in the field of heterostructured UCNPs in the aspects of construction strategies, synthetic approaches, and types of heterostructured UCNPs. This review also summarizes the trends in biomedical applications of heterostructured UCNPs and discusses the challenges and potential solutions in this field.

Bi2O3 boosts brightness, biocompatibility and stability of Mn-doped Ba3(VO4)2 as NIR-II contrast agent

Deep-tissue fluorescence imaging remains a major challenge as there is limited availability of bright biocompatible materials with high photo- and chemical stability. Contrast agents with emission wavelengths above 1000 nm are most favorable for deep tissue imaging, offering deeper penetration and less scattering than those operating at shorter wavelengths. Organic fluorophores suffer from low stability while inorganic nanomaterials (e.g. quantum dots) are based typically on heavy metals raising toxicity concerns. Here, we report scalable flame aerosol synthesis of water-dispersible Ba₃(VO₄)₂ nanoparticles doped with Mn⁵⁺ which exhibit a narrow emission band at 1180 nm upon near-infrared excitation. Their co-synthesis with Bi₂O₃ results in even higher absorption and ten-fold increased emission intensity. The addition of Bi₂O₃ also improved both chemical stability and cytocompatibility by an order of magnitude enabling imaging deep within tissue. Taken together, these bright particles offer excellent photo-, chemical and colloidal stability in various media with cytocompatibility to HeLa cells superior to existing commercial contrast agents.

Ultrabright NIR-II Emissive Polymer Dots for Metastatic Ovarian Cancer Detection

Intraoperative diagnosis of metastatic tumors is of significant importance to the treatment of ovarian cancer. NIR-II fluorescence imaging holds great promise for facile detection of tumor in situ with high sensitivity and resolution. Herein, a kind of NIR-II fluorescent polymer dots (NIR-II Pdots) with high brightness is developed for real-time detection of metastatic ovarian cancer via NIR-II fluorescence imaging. The NIR-II Pdots are constructed via the self-assembly of NIR-II emissive aggregation induced emission luminogens (NIR-II AIEgens) and poly (styrene)-graft-poly(ethylene glycol) in water. Such NIR-II Pdots show very high fluorophore contents of nearly 30% and high quantum yield of 5.4% at emission maximum near 1020 nm. Further modification of the NIR-II Pdots with targeting peptides yields NIR-II Pdots-GnRH, which can afford enhanced affinity of NIR-II Pdots to ovarian cancer. Upon intravenous injection of the NIR-II Pdots, whole-body organs and vessels, peritoneal and lymphatic metastases of ovarian cancer are clearly visualized by NIR-II fluorescence imaging. Under the guidance of NIR-II fluorescence imaging, the metastatic foci with the diameter down to ≈2 mm can be facilely eliminated. The results indicate preclinical potential value of the NIR-II Pdots for metastatic ovarian cancer detection.

A review on the structural dependent optical properties and energy transfer of Mn4+ and multiple ion-codoped complex oxide phosphors

The tetravalent manganese Mn4+ ions with a 3d3 electron configuration as luminescence centers in solid-state inorganic compounds have been widely investigated because they emit bright light in the red to far-red region when they are excited by light with a wavelength in the UV to blue light region. Herein, we present an overview of the recent developments of Mn4+ and multiple ion such as Bi3+ and rare earth ion Dy3+, Nd3+, Yb3+, Er3+, Ho3+, and Tm3+ codoped complex oxide phosphors. Most of the specified host lattices of these complex oxide phosphors possess multiple metallic cations, which provide possible substitutions with different codopants and form various luminescence centers with diverse spectra. The luminescence of Mn4+ and multiple ion-codoped materials spans almost the whole visible light to near infrared (NIR) region. The crystal structures of complex oxide phosphors, the spectroscopic properties of Mn4+, and the energy transfer between Mn4+ and multiple ions are introduced and summarized in detail with regard to their practical applications. This review provides an insight into the optical properties of Mn4+ and the energy transfer process in multiple ion-codoped luminescence materials, which will be helpful in the development of novel excellent materials for applications in the lighting industry.

Bright Blue, Green, and Red Luminescence from Dye-Sensitized Core@Shell Upconversion Nanophosphors under 800 nm Near-Infrared Light

In this study, Li-based blue- and green-emitting core@shell (C@S) upconversion nanophosphors (UCNPs) and NaGdF4-based red-emitting C@S UCNPs were synthesized, and IR-808 dyes were conjugated with the C@S UCNPs to enhance upconversion (UC) luminescence. The surface of the as-synthesized C@S UCNPs, which was originally capped with oleic acid, was modified with BF4- to conjugate the IR-808 dye having a carboxyl functional group to the surface of the UCNPs. After the conjugation with IR-808 dyes, absorbance of the UCNPs was significantly increased. As a result, dye-sensitized blue (B)-, green (G)-, and red (R)-emitting UCNPs exhibited 87-fold, 10.8-fold, and 110-fold enhanced UC luminescence compared with B-, G-, and R-emitting Nd3+-doped C@S UCNPs under 800 nm near-infrared (NIR) light excitation, respectively. Consequently, dye-sensitized UCNPs exhibiting strong UC luminescence under 800 nm NIR light excitation have high applicability in a variety of biological applications.

Manganese-Mediated Growth of ZnS Shell on KMnF3:Yb,Er Cores toward Enhanced Up/Downconversion Luminescence

Epitaxially growing a semiconductor shell on the surface of upconversion nanocrystals to form a core/shell structure is believed to be a promising strategy to improve the luminescent efficiency of lanthanide ions doped in particle cores and, meanwhile, enriches the optical properties of the resulting nanocrystals. However, liquid-phase synthesis of such core/shell-structured nanocrystals comprised of a lanthanide ion-doped core and semiconductor shell remains challenging because of the chemical incompatibilities between lanthanides and the most intermediate gap semiconductors. In this context, the successful growth of ZnS shell on a KMnF₃ core codoped with Yb³⁺/Er³⁺ ions is reported to enhance the upconversion luminescence of Er³⁺ ions. The underlying core/shell formation mechanism is elucidated in detail combining the hard-soft acid-base theory with structural analysis of the resulting nanocrystals. Quite unexpectedly, Mn²⁺ diffusion across the core/shell interface occurs during ZnS shell growth, giving rise to Mn²⁺ emission from the ZnS shell. Thus, the resulting core/shell particles exhibited unique up/downconversion luminescence from doped lanthanide metal ions and transition-metal ions, respectively. By manipulating the ion diffusion and shell growth kinetics, the upconversion and downconversion luminescent performance of KMnF₃:Yb,Er@ZnS nanocrystals are further optimized and the related mechanisms are discussed. Further, temperature-dependent upconversion and downconversion photoluminescence properties of KMnF₃:Yb,Er@ZnS nanocrystals show potential for ratiometric luminescence temperature sensing.

Near-Infrared Lanthanide-Doped Nanoparticles for a Low Interference Lateral Flow Immunoassay Test

The lateral flow immunoassay test (LFT), as a method of a point of care test, is widely used in disease diagnosis, food security, and environment observation due to its portability and testing rapidity. A fluorescence lateral flow immunoassay was developed recently to enhance the sensitivity and accuracy of the LFT. However, for most fluorescence reporters, their emission and excitation wavelengths are located in the ultraviolet or visible region. Serum or whole blood significantly absorbs and scatters light of this region, and this will result in background signal interference. In this study, we replace traditional fluorescence reporters with near-infrared lanthanide-doped nanoparticles (NIR-RENPs) to establish a NIR-LFT platform. Blood and other biological samples scatter and absorb less near-infrared light than visible light, and the autofluorescence of biological samples is rarely located in this region. Therefore, using NIR light as a signal can diminish the interference of background noise and suffer from less signal attenuation. In addition, compared with commonly used NIR organic dye, NIR-RENPs have better stability. It is promising that lateral flow immunoassays based on NIR lanthanide-doped nanoparticles are able to acquire a lower detection limit and better accuracy, and they are more suitable for application in commercial settings.

Neodymium-Sensitized Nanoconstructs for Near-Infrared Enabled Photomedicine

Neodymium (Nd³⁺ )-sensitized nanoconstructs have gained increasing attention in recent decades due to their unique properties, especially optical properties. The design of various Nd³⁺ -sensitized nanosystems is expected to contribute to medical and health applications, due to their advantageous properties such as high penetration depth, excellent photostability, non-photobleaching, low cytotoxicity, etc. However, the low conversion efficiency and potential long-term toxicity of Nd³⁺ -sensitized nanoconstructs are huge obstacles to their clinical translations. This review article summarizes three energy transfer pathways of all kinds of Nd³⁺ -sensitized nanoconstructs focusing on the properties of Nd³⁺ ions and discusses their recent potential applications as near-infrared (NIR) enabled photomedicine. This review article will contribute to the design and fabrication of novel Nd³⁺ -sensitized nanoconstructs for NIR-enabled photomedicine, aiming for potentially safer and more efficient designs to get closer to clinical usage.

Efficient X-ray excited short-wavelength infrared phosphor

X-ray combined with short-wavelength infrared (SWIR) phosphors, that can make full advantage of the deeper tissue penetration of SWIR light, can be used as a fluorescent probe to realize biological imaging of deep tissues; they are, however, limited by their lower luminescence efficiency. Here, we describe a strategy to synthesize highly efficient SWIR luminescence phosphor based on the efficient energy transfer process between charge transfer state (CTS) of Yb³⁺ and the ⁶I_J levels of Gd³⁺ as well as Gd³⁺-Gd³⁺. This allows us to achieve 813.8 mW/m² of SWIR luminescence power in Yb³⁺-BaGd_0.6Y_0.4ZnO₅ (BGYZ). Our results highlight that this approach to enhance SWIR luminescence may provide new opportunities for the deep-tissue biological imaging.

New Theoretical Insights into the Crystal-Field Splitting and Transition Mechanism for Nd3+-Doped Y3Al5O12

There has been considerable research interest paid to rare-earth transition-metal-doped Y₃Al₅O₁₂, which has great potential for application as a laser crystal of new-type laser devices because of its unique optoelectronic and photophysical properties. Here, we present new research conducted on the structural evolution and crystal-field characteristics of a rare-earth Nd-doped Y₃Al₅O₁₂ laser crystal by using the CALYPSO structure search method and our newly developed WEPMD method. A novel cage-like structure with a Nd³⁺ concentration of 4.16% is uncovered, which belongs to the standardized C₂₂₂ space group. Our results indicate that the impurity Nd³⁺ ions are likely to substitute the Y³⁺ at the central site of the host Y₃Al₅O₁₂ crystal lattice. The laser emission ⁴F_3/2 → ⁴I_11/2 occurring at 1077 nm is in accord with that of the experimental data. By introducing the proper correlation crystal field, three transitions, ⁴G_5/2 → ⁴I_9/2, ⁴F_7/2 → ⁴I_9/2, and ⁴S_3/2 → ⁴I_9/2, are predicted to be good candidates for laser action. These findings can provide powerful guidelines for further experiments of rare-earth-metal-doped laser crystals.

Time-Gated Ratiometric Detection with the Same Working Wavelength To Minimize the Interferences from Photon Attenuation for Accurate in Vivo Detection

Luminescence imaging, exhibiting noninvasive, sensitive, rapid, and versatile properties, plays an important role in biomedical applications. It is usually unsuitable for direct biodetection, because the detected luminescence intensity can be influenced by various factors such as the luminescent substance concentration, the depth of the luminescent substance in the organism, etc. Ratiometric imaging may eliminate the interference due to the luminescent substance concentration on the working signal. However, the conventional ratiometric imaging mode has a limited capacity for in vivo signal acquisition and fidelity due to the highly variable and wavelength-dependent scattering and absorption process in biotissue. In this work, we demonstrate a general imaging mode in which two signals with the same working wavelength are used to perform ratiometric sensing ignoring the depth of the luminescent substance in the organism. Dual-channel decoding is achieved by time-gated imaging technology, in which the signals from lanthanide ions and fluorescent dyes are distinguished by their different luminescent lifetimes. The ratiometric signal is proven to be nonsensitive to the detection depth and excitation power densities; thus, we could utilize the working curve measured in vitro to determine the amount of target substance (hypochlorous acid) in vivo.

Intense one-band near-infrared upconversion luminescence induced by using spontaneous polarization BiOCl sheet crystals as hosts for Yb3+ and Tm3+ ions

A new scheme that utilizes the anisotropic polarization of layered hosts to easily produce an efficient single-band NIR UC emission of Tm³⁺ ions.

NaYbF4@CaF2 core-satellite upconversion nanoparticles: one-pot synthesis and sensitive detection of glutathione

A new class of core-satellite upconversion nanoparticles (UCNPs) formed through a kinetically controlled oriented attachment is presented. The core-satellite UCNPs comprising an optically active α-NaYbF4 core and several CaF2 satellites are synthesized by a one-pot sequential injection technique. Compared to conventional core-shell UCNPs, these core-satellite UCNPs show larger surface-to-volume ratios and are suitable for further surface modifications. As a proof-of-concept, a biosensing system is constructed by coating MnO2 nanosheets on the α-NaYbF4:Tm@CaF2 core-satellite UCNPs for high-sensitivity biothiol detection. These core-satellite UCNPs show great potential in the development of UCNP-based nanohybrids for biosensing, multimodal imaging and drug delivery.

An efficient dye-sensitized NIR emissive lanthanide nanomaterial and its application in fluorescence-guided peritumoral lymph node dissection

The luminescence intensity of near-infrared (NIR) emitting lanthanide nanoparticles (LnNPs) is usually limited, owing to their small absorption cross section. Although dye sensitization has been proven to be an effective way to improve the luminescence intensity of LnNPs, the sensitization effect is fairly limited, owing to the simplicity of the sensitizers used and the complexity of the energy transfer process, typically involving three steps. In this study, a more efficient sensitizer (Cy7) was chosen to replace a commonly used one (ICG) and the energy transfer process was also optimized through using Yb3+ ions as emitter ions and Nd3+ ions as intermediate ions. With Cy7 as a sensitizer, the sensitization effect was assessed to be better than with ICG, owing to the higher quantum yield of Cy7. Meanwhile, the Cy7-sensitized NIR lanthanide nanomaterial was proven to be good for deep tissue penetration and low-power excitation bioimaging. Furthermore, the highly-enhanced NIR signal was successfully used in blood vessel imaging and fluorescence-guided peritumoral lymph node dissection in a mouse model.

Near-Infrared Light-Excited Upconverting Persistent Nanophosphors in Vivo for Imaging-Guided Cell Therapy

Optical imaging for biological applications is in need of more sensitive tool. Persistent luminescent nanophosphors enable highly sensitive in vivo optical detection and almost completely avoid tissue autofluorescence. Nevertheless, the actual persistent luminescent nanophosphors necessitate ex vivo activation before systemic operation, which severely restricted the use of long-term imaging in vivo. Hence, we introduced a novel generation of optical nanophosphors, based on (Zn₂SiO₄:Mn):Y³⁺, Yb³⁺, Tm³⁺ upconverting persistent luminescent nanophosphors; these nanophosphors can be excited in vivo through living tissues by highly penetrating near-infrared light. We can trace labeled tumor therapeutic macrophages in vivo after endocytosing these nanophosphors in vitro and follow macrophages biodistribution by a simple whole animal optical detection. These nanophosphors will open novel potentials for cell therapy research and for a variety of applications in diagnosis in vivo.

Ultra-small nanocluster mediated synthesis of Nd3+-doped core-shell nanocrystals with emission in the second near-infrared window for multimodal imaging of tumor vasculature

In-vivo intravital short wavelength infrared (SWIR, 1000-2300 nm) fluorescence imaging has attracted considerable attention in the imaging of tumor vasculature due to its low background, high sensitivity, and deep penetration. It can noninvasively provide dynamic feedback on the tumorigenesis, growth, necrosis and metastasis. Herein, monodisperse Nd³⁺-doped core-shell downconversion luminescent nanocrystals with strong emission in the second near-infrared (NIR II) window, strong temperature-dependent paramagnetism and fast attenuation to X-rays were prepared from ultra-small nanoclusters. The use of nanoclusters resulted in very uniform bright nanocrystals with a relative quantum yield comparable to the standard dye IR-26. These bright NIR nanocrystals were modified with 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000] to endow with excellent water-solubility, biocompatibility and a blood circulation half-life of 5.9 h. They were then successfully used to demonstrate the variation of tumor vasculature with tumor progression from tumorigenesis, growth, to necrosis in the subcutaneous breast tumor through the NIR II fluorescence imaging. They were also used as contrast agent of magnetic resonance imaging (MRI) and X-ray computed tomography (CT) imaging of tumor to provide complementary anatomic structure. Their great potential in NIR II imaging of tumor was further demonstrated with an orthotopic breast tumor. Their in-vivo biosafety was also investigated by hemanalysis and histological analyses.

Fe₃O₄ Nanoparticles in Targeted Drug/Gene Delivery Systems

Fe₃O₄ nanoparticles (NPs), the most traditional magnetic nanoparticles, have received a great deal of attention in the biomedical field, especially for targeted drug/gene delivery systems, due to their outstanding magnetism, biocompatibility, lower toxicity, biodegradability, and other features. Naked Fe₃O₄ NPs are easy to aggregate and oxidize, and thus are often made with various coatings to realize superior properties for targeted drug/gene delivery. In this review, we first list the three commonly utilized synthesis methods of Fe₃O₄ NPs, and their advantages and disadvantages. In the second part, we describe coating materials that exhibit noticeable features that allow functionalization of Fe₃O₄ NPs and summarize their methods of drug targeting/gene delivery. Then our efforts will be devoted to the research status and progress of several different functionalized Fe₃O₄ NP delivery systems loaded with chemotherapeutic agents, and we present targeted gene transitive carriers in detail. In the following section, we illuminate the most effective treatment systems of the combined drug and gene therapy. Finally, we propose opportunities and challenges of the clinical transformation of Fe₃O₄ NPs targeting drug/gene delivery systems.

Controlled synthesis of upconverting nanoparticles/CuS yolk–shell nanoparticles for in vitro synergistic photothermal and photodynamic therapy of cancer cells

A chemical solution method involving multistep process has been developed to fabricate UCNPs@CuS yolk–shell nanoparticles for synergistic photothermal and photodynamic therapy of cancer cells.