Upconversion in Nanostructured Materials: From Optical Tuning to Biomedical Applications

A fluorescent sensor based on single band bright red luminescent core-shell UCNPs for the high-sensitivity detection of glucose and glutathione

As the reliable biomarkers to evaluate the diabetes and neurological disease, sensitive and accurate detection of glucose and glutathione (GSH) in biological samples is necessary for early precaution and diagnosis of related-diseases. The single red upconversion nanoparticles (UCNPs) especially with core-shell structure can penetrate deeper biological tissues and cause less energy loss and thus have higher sensitivity and accuracy. Additionally, an enzyme-controlled cascade signal amplification (ECSAm) strategy will further enhance sensitivity. Herein, using single red UCNPs with core-shell structure as the luminescent material, a fluorescent sensor based on ECSAm was developed for the highly sensitive and accurate detection of glucose and GSH. Under the optimal conditions, the limits of detection for glucose and GSH by fluorescent method were 0.03 μM and 0.075 μM, separately. This assay was used to analyze the content of glucose and GSH in serum samples, and the obtained data was close to that of commercial blood glucose and GSH detection kit. The developed sensor platform based on single red UCNPs with core-shell structure and ECSAm can be a promising method for the accurate and sensitive detection of glucose and GSH in biological samples.

Fluorescence immunoassay for simultaneous detection typical β-agonists in animal derived food using blue-green upconversion nanoparticles as labels

Common typical β-agonists mainly include ractopamine (RAC), salbutamol (SAL), and clenbuterol (CLB). In view of the harm to human health causes by the ingestion of animal derived food containing β-agonists, and a series of regulations have been issued to restrict the usage of β-agonists as growth promoters. In this work, a fluorescence immunoassay is developed for the simultaneous detection of typical β-agonists based on blue-green upconversion nanoparticles (UCNPs) combine with magnetic separation. Here, blue-green UCNPs act as a signal amplification source, and magnetic polystyrene microspheres (MPMs) act as an ideal separation medium. Based on a competitive form, capture probe competes (RAC-OVA@MPMs and SAL-OVA@MPMs) with targets to bind corresponding signal probe (anti-RAC antibody@NaYF4:Yb, Tm UCNPs and anti-SAL antibody@NaYF4:Yb, Er UCNPs). The fluorescence difference values of the competitive immune-complex obtained via magnetic separation at 483 nm and 550 nm are proportional to concentrations of RAC and SAL, respectively. The immunoassay has the wide detection linear range from 0.001 to 100 μg L-1, and the low limit of detection (LOD) is 5.04 × 10-4 μg L-1 for RAC, 1.97 × 10-4 μg L-1 for SAL, respectively. Meanwhile, use of antibody with same recognition ability for SAL and CLB makes that the fluorescence immunoassay can achieve simultaneous detection of three typical β-agonists (RAC, SAL, and CLB). This fluorescence immunoassay has good application value and practicability for simultaneous detection of typical β-agonists in animal derived food.

Functional material-mediated wireless physical stimulation for neuro-modulation and regeneration

Nerve injuries and neurological diseases remain intractable clinical challenges. Despite the advantages of stem cell therapy in treating neurological disorders, uncontrollable cell fates and loss of cell function in vivo are still challenging. Recently, increasing attention has been given to the roles of external physical signals, such as electricity and ultrasound, in regulating stem cell fate as well as activating or inhibiting neuronal activity, which provides new insights for the treatment of neurological disorders. However, direct physical stimulations in vivo are short in accuracy and safety. Functional materials that can absorb energy from a specific physical field exerted in a wireless way and then release another localized physical signal hold great advantages in mediating noninvasive or minimally invasive accurate indirect physical stimulations to promote the therapeutic effect on neurological disorders. In this review, the mechanism by which various physical signals regulate stem cell fate and neuronal activity is summarized. Based on these concepts, the approaches of using functional materials to mediate indirect wireless physical stimulation for neuro-modulation and regeneration are systematically reviewed. We expect that this review will contribute to developing wireless platforms for neural stimulation as an assistance for the treatment of neurological diseases and injuries.

A Hemicyanine-Assembled Upconversion Nanosystem for NIR-Excited Visualization of Carbon Monoxide Bio-Signaling In Vivo

Carbon monoxide (CO) is considered as the second gasotransmitter involved in a series of physiological and pathological processes. Although a number of organic fluorescent probes have been developed for imaging CO, these probes display excitation within the ultraviolet or visible range, which restrict their applications in the complex biosystems. In the present work, a strategy is developed to construct an upconversion nanoparticles-based nanosystem for upconversion luminescent (UCL) sensing CO. This nanosystem exhibits a fast response to CO with high sensitivity and selectivity in aqueous solution by a near-infrared-excited ratiometric UCL detection method. Meanwhile, laser scanning upconversion luminescence microscope experiments demonstrate that this nanosystem can visualize the endogenous CO bio-signaling in living cells, deep tissues, zebrafish, and living mice by ratiometric UCL imaging. In particular, this nanosystem has been successfully employed in visualization of the endogenous CO bio-signaling through up-regulation of heme oxygenase-1 (HO-1) in the progression of hypoxia, acute inflammation, or ischemic injury. This work demonstrates that the outstanding performance of the nanosystem not only can provide an effective tool for further understanding the role of CO in the physiological and pathological environment, but also may have great potential ability for tracking the expression of HO-1 in living systems.

Controlling upconversion in emerging multilayer core-shell nanostructures: from fundamentals to frontier applications

Lanthanide-based upconversion nanomaterials have recently attracted considerable attention in both fundamental research and various frontier applications owing to their excellent photon upconversion performance and favourable physicochemical properties. In particular, the emergence of multi-layer core-shell (MLCS) nanostructures offers a versatile and powerful tool to realize well-defined matrix compositions and spatial distributions of the dopant on the nanometer length scale. In contrast to the conventional nanomaterials and commonly investigated core-shell nanoparticles, the rational design of MLCS nanostructures allows us to deliberately introduce more functional properties into an upconversion system, thus providing unprecedented opportunities for the precise manipulation of energy transfer channels, the dynamic control of upconversion processes, the fine tuning of switchable emission colours and new functional integration at a single-particle level. In this review, we present a summary and discussion on the key aspects of the recent progress in lanthanide-based MLCS nanoparticles, including the manipulation of emission and lifetime, the switchable multicolour output and the lanthanide ionic interactions on the nanoscale. Benefitting from the multifunctional and versatile luminescence properties, the MLCS nanostructures exhibit great potential in diversities of frontier applications such as three-dimensional display, upconversion laser, optical memory, anti-counterfeiting, thermometry, bioimaging, and therapy. The outlook and challenges as well as perspectives for the research in MLCS nanostructure materials are also provided. This review would be greatly helpful in exploring new structural designs of lanthanide-based materials to further manipulate the upconversion phenomenon and expand their application boundaries.

Theragnostic nanomotors: Successes and upcoming challenges

The idea of "fantastic voyagers" carrying out medical tasks within the human body has existed as part of popular culture for many decades. The concept revolved around a miniaturized robot that can travel inside the human body and perform complicated functions such as surgery, navigation of otherwise inaccessible biological environments, and delivery of therapeutics. Since the last decade, significant developments have occurred in this arena that are yet to enter mainstream biomedical practises. Here, we define the challenges to make this fiction into reality. We begin by chalking the journey from pills, nanoparticles, and then to micro-nanomotors. The review describes the principles, physicochemical contexts, and advantages that micro-nanomotors provide. The article then describes micro-nanomotors' obstacles such as maneuverability, in vivo imaging, toxicity, and biodistribution. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Nanotechnology Approaches to Biology > Nanoscale Systems in Biology.

Short-Pulse Lasers: A Versatile Tool in Creating Novel Nano-/Micro-Structures and Compositional Analysis for Healthcare and Wellbeing Challenges

Driven by flexibility, precision, repeatability and eco-friendliness, laser-based technologies have attracted great interest to engineer or to analyze materials in various fields including energy, environment, biology and medicine. A major advantage of laser processing relies on the ability to directly structure matter at different scales and to prepare novel materials with unique physical and chemical properties. It is also a contact-free approach that makes it possible to work in inert or reactive liquid or gaseous environment. This leads today to a unique opportunity for designing, fabricating and even analyzing novel complex bio-systems. To illustrate this potential, in this paper, we gather our recent research on four types of laser-based methods relevant for nano-/micro-scale applications. First, we present and discuss pulsed laser ablation in liquid, exploited today for synthetizing ultraclean "bare" nanoparticles attractive for medicine and tissue engineering applications. Second, we discuss robust methods for rapid surface and bulk machining (subtractive manufacturing) at different scales by laser ablation. Among them, the microsphere-assisted laser surface engineering is detailed for its appropriateness to design structured substrates with hierarchically periodic patterns at nano-/micro-scale without chemical treatments. Third, we address the laser-induced forward transfer, a technology based on direct laser printing, to transfer and assemble a multitude of materials (additive structuring), including biological moiety without alteration of functionality. Finally, the fourth method is about chemical analysis: we present the potential of laser-induced breakdown spectroscopy, providing a unique tool for contact-free and space-resolved elemental analysis of organic materials. Overall, we present and discuss the prospect and complementarity of emerging reliable laser technologies, to address challenges in materials' preparation relevant for the development of innovative multi-scale and multi-material platforms for bio-applications.

Dye-sensitized core-shell NaGdF4:Yb,Er@NaGdF4:Yb,Nd upconversion nanoprobe for determination of H2S

The core-shell NaGdF₄:Yb,Er@NaGdF₄:Yb,Nd upconversion nanoparticles (UCNPs) were successfully obtained with the method of co-precipitation, and the water-solubility of UCNPs was improved by the ligand exchange reaction between nitrosyl tetrafluoroborate (NOBF₄) and nanoparticles. The IR-783 dye with negative charge and NOBF₄-UCNPs with positive charge can bind together by electrostatic action to sensitize UCNPs through the energy transfer from IR-783 to UCNPs. However, with the presence of Na₂S (a commonly used H₂S donor), a highly selective reaction between H₂S and IR-783, which destoried the structure of IR-783 and blocked the energy transfer, thus led to the quenching of luminescent intensity. Based on this, a sensing system for determination of H₂S has been constructed successfully. The linear range of H₂S detection by this system is 0.5-15 μM, and the detection limit is 34.17 nM. Furthermore, the dye-sensitized core-shell NaGdF₄:Yb,Er@NaGdF₄:Yb,Nd upconversion nanoprobe was applied to real sample analysis with satisfactory results.

Recent advances of lanthanide-doped upconversion nanoparticles for biological applications

Near infrared (NIR) excited lanthanide-doped upconversion nanoparticles (UCNPs) are emerging as a new type of fluorescent tag for biological applications, which can emit multi-photon ultraviolet, visible or NIR luminescence for imaging or activation of photosensitive molecules. Here, we present a comprehensive review on recent advances of UCNPs for a manifold of biological applications, including upconversion mechanisms, building bright multicolor upconversion nanocrystals, single nanoparticle and super resolution imaging, in vivo optical and multimodal imaging, photodynamic therapy, light-controlled drug release, biosensing, and toxicities. Our perspectives on the future development of UCNPs are also described.

Nanoarchitectonics from Atom to Life

Functional materials with rational organization cannot be directly created only by nanotechnology-related top-down approaches. For this purpose, a novel research paradigm next to nanotechnology has to be established to create functional materials on the basis of deep nanotechnology knowledge. This task can be assigned to an emerging concept, nanoarchitectonics. In the nanoarchitectonics approaches, functional materials were architected through combination of atom/molecular manipulation, organic chemical synthesis, self-assembly and related spontaneous processes, field-applied assembly, micro/nano fabrications, and bio-related processes. In this short review article, nanoarchitectonics-related approaches on materials fabrications and functions are exemplified from atom-scale to living creature level. Based on their features, unsolved problems for future developments of the nanoarchitectonics concept are finally discussed.

Target-modulated sensitization of upconversion luminescence by NIR-emissive quantum dots: a new strategy to construct upconversion biosensors

We herein used Ag₂Se QDs as a target-modulated sensitizer for UCNPs and the target thrombin as the sensitizing switch to construct a biosensor with enhanced SBR and assay sensitivity, circumventing the limited LRET efficiency of UCNPs.

A Review on Artificial Micro/Nanomotors for Cancer-Targeted Delivery, Diagnosis, and Therapy

Micro/nanomotors have been extensively explored for efficient cancer diagnosis and therapy, as evidenced by significant breakthroughs in the design of micro/nanomotors-based intelligent and comprehensive biomedical platforms. Here, we demonstrate the recent advances of micro/nanomotors in the field of cancer-targeted delivery, diagnosis, and imaging-guided therapy, as well as the challenges and problems faced by micro/nanomotors in clinical applications. The outlook for the future development of micro/nanomotors toward clinical applications is also discussed. We hope to highlight these new advances in micro/nanomotors in the field of cancer diagnosis and therapy, with the ultimate goal of stimulating the successful exploration of intelligent micro/nanomotors for future clinical applications.

Upconversional Nanoprobes with Highly Efficient Energy Transfer for Ultrasensitive Detection of Alkaline Phosphatase

Sensitive detection of alkaline phosphatase (ALP) activity in human serum is important for diagnosis of various diseases. In this work, a novel sandwich-structured upconversion nanoparticle, NaYF₄:Yb/Er@NaErF₄@NaYF₄, is fabricated to construct an upconversional nanoprobe for ultrasensitive detection of phosphate and ALP activity. The inner shell of NaErF₄ bridges the emitters in the core with the external luminescence quenchers to greatly improve the energy transfer efficiency. The quencher, herein, is a coordination complex formed between sulfosalicylic acid and ferric ions. Owing to the higher affinity for phosphate, ferric ions dissociate from the complex and potently combine with phosphate ions, thus interrupting the energy transfer process and recovering the luminescence. This upconversional nanoprobe shows rapid and sensitive detection of phosphate with a limit of detection of 2.5 nM. Because ALP catalyzes the hydrolysis of p-nitrophenyl phosphate to form p-nitrophenol and inorganic phosphate ions, the nanoprobe is further utilized to achieve sensitive detection of ALP with a limit of detection of 0.5 μU/mL. This novel strategy offers a new opportunity for developing sensitive upconversional nanoprobes and many other energy transfer-based applications.

Near-Infrared Excited Orthogonal Emissive Upconversion Nanoparticles for Imaging-Guided On-Demand Therapy

Photodynamic therapy (PDT) has been considered as a promising and noninvasive strategy for clinical cancer treatment. Nonetheless, building a smart "off-on" theranostic PDT platform to spatiotemporally control the generation of reactive oxygen species in the PDT treatment still remains challenging. Here, we have rationally developed photoswitching upconversion nanoparticles (UCNPs) with orthogonal emissive properties in response to two distinct near-infrared (NIR) emissions at 808 and 980 nm, i.e., red emission with 980 nm excitation and green emission with 808 nm excitation. Unlike traditional photoswitching UCNPs, these specially designed core-shell-shell structured UCNPs do not require complicated multilayer doping as their red and green upconversion luminescence both originate from the same activator Er³⁺ ions in the core structure. As a proof of concept, we have demonstrated the capability of these orthogonal emissive UCNPs for imaging-guided PDT in a real-time manner, where the red emission excited by 980 nm light is used to trigger PDT and the green emission with 808 nm excitation is to diagnose and monitor the therapeutic treatment. Our study suggests that such specially designed UCNPs with orthogonal emissions hold great promise for NIR light-targeted and imaging-guided therapy under precisely spatiotemporal control.

Lanthanide-Doped Photoluminescence Hollow Structures: Recent Advances and Applications

Lanthanide-doped nanomaterials have attracted significant attention for their preeminent properties and widespread applications. Due to the unique characteristic, the lanthanide-doped photoluminescence materials with hollow structures may provide advantages including enhanced light harvesting, intensified electric field density, improved luminescent property, and larger drug loading capacity. Herein, the synthesis, properties, and applications of lanthanide-doped photoluminescence hollow structures (LPHSs) are comprehensively reviewed. First, different strategies for the engineered synthesis of LPHSs are described in detail, which contain hard, soft, self-templating methods and other techniques. Thereafter, the relationship between their structure features and photoluminescence properties is discussed. Then, niche applications including biomedicines, bioimaging, therapy, and energy storage/conversion are focused on and superiorities of LPHSs for these applications are particularly highlighted. Finally, keen insights into the challenges and personal prospects for the future development of the LPHSs are provided.

Determination of Fe3+ upon Special "Upconversion Luminescence" of Dopamine

A promising technique based on the luminescence with long wavelength excitation and short wavelength emission (LExL, λ_ex-L > λ_em) is developed. This LExL is different from traditional upconversion luminescence (UCL). The LExL, namely, special "UCL", is realized by a xenon light source of a common spectrofluorometer. In this work, we found that dopamine (DA) has this LExL phenomenon. The LExL of DA is mainly caused by the excitations of second-order diffraction light (λ_ex-L/2). The two-photon absorption properties of DA have been calculated employing the density functional response theory. The LExL and Stokes luminescence (SL, λ_ex-S < λ_em) of DA both showed static quenching upon the addition of Fe³⁺. Dual-mode luminescence methods upon LExL (λ_ex-L/λ_em at 565/317 nm) and SL (λ_ex-S/λ_em at 282/317 nm) of DA were applied for the selective determination of Fe³⁺. The detection limits are 0.30 and 0.52 μmol L^-1 for LExL and SL, respectively. In addition, their linear ranges for Fe³⁺ determination are both from 0.70 to 30 μmol L^-1. The LExL method of DA not only meets the basic determination criteria for Fe³⁺ but also offers additional advantages in resisting more interferences and shows satisfactory feasibility performances.

Rational synthesis of three-dimensional core-double shell upconversion nanodendrites with ultrabright luminescence for bioimaging application

Herein, we rationally fabricated three-dimensional upconversion core–double shell nanodendrites as efficient and safe luminescent probes for in vitro and in vivo bioimaging.

Engineering the morphology of rare-earth doped NaYF₄-based upconversion nanoparticles (UCNPs) can effectively tune their upconversion luminescence emission (UCLE) properties. Herein, we rationally synthesized a new class of three-dimensional upconversion core–double-shell nanodendrites (UCNDs) including an active core (NaYF₄:Yb,Er,Ca) capped by a transition layer (NaYF₄:Yb,Ca) and an active outer shell (NaNdF₄:Yb,Ca). The high concentration of the Nd³⁺ sensitizer in the outer dendritic shell enhances the luminescence intensity, while the transition layer enriched with Yb³⁺ acts as an efficient energy migration network between the outer shell and inner core along with preventing the undesired quenching effects resulting from Nd³⁺. These unique structural and compositional merits enhanced the UCLE of UCNDs by 5 and 15 times relative to NaYF₄:Yb,Er,Ca@NaYF₄:Yb,Ca truncated core–shell UCNPs and NaYF₄:Yb,Er,Ca spherical core UCNPs, respectively, under excitation at 980 nm. The SiO₂–COOH layer coated UCNDs (UCND@SiO₂–COOH) were successfully used as efficient long-term luminescent probes for in vitro and in vivo bioimaging without any significant toxicity. The uptake and retention of UCND@SiO₂–COOH were mostly found in the liver and spleen. This study may open the way towards the preparation of three-dimensional UCND nanostructures for biomedical applications.

A critical comparison of lanthanide based upconversion nanoparticles to fluorescent proteins, semiconductor quantum dots, and carbon dots for use in optical sensing and imaging

The right choice of a fluorescent probe is essential for successful luminescence imaging and sensing and especially concerning in vivo and in vitro applications, the development of new classes have gained more and more attention in the last years. One of the most promising class are upconversion nanoparticles (UCNPs)-inorganic nanocrystals capable to convert near-infrared light in high energy radiation. In this review we will compare UCNPs with other fluorescent probes in terms of (a) the optical properties of the probes, such as their brightness, photostability and excitation wavelength; (b) their chemical properties such as the dispersibility, stability under experimental or physiological conditions, availability of chemical modification strategies for labelling; and (c) the potential toxicity and biocompatibility of the probe. Thereby we want to provide a better understanding of the advantages and drawbacks of UCNPs and address future challenges in the design of the nanocrystals.

Honeycomb-Satellite Structured pH/H2O2-Responsive Degradable Nanoplatform for Efficient Photodynamic Therapy and Multimodal Imaging

The oxygen-deprived environment of a solid tumor is still great restriction in achieving an efficient photodynamic therapy (PDT). In this work, we developed a smart pH-controllable and H₂O₂-responsive nanoplatform with degradable property, which was based on honeycomb manganese oxide (hMnO₂) nanospheres loaded with Ce6-sensitized core-shell-shell structured up-conversion nanoparticles (NaGdF₄:Yb/Er,Tm@NaGdF₄:Yb@NaNdF₄:Yb) (abbreviated as hMUC). In the system, the speedy breakup of the as-prepared hMnO₂ nanostructures results in release of loaded Ce6-sensitized UCNPs under the condition of H₂O₂ in acid solution. When exposed to tissue-penetrable 808 nm laser, up-conversion nanoparticles (UCNPs) emit higher-energy visible photons which would be absorbed by Ce6 to yield cytotoxic reactive oxygen species (ROS), thus triggering PDT treatment naturally. Moreover, the in vitro and in vivo experiments demonstrate that hMUC sample with the honeycomb-satellite structure can serve as multimodal bioimaging contrast agent for self-enhanced upconversion luminescence (UCL), magnetic resonance imaging (MRI) and computed tomography (CT) imaging, indicating that the as-prepared hMUC could be used in imaging-guided diagnosis and treatment, which has a potential application in the PDT treatment of tumor.