Sensitive Water Probing through Nonlinear Photon Upconversion of Lanthanide-Doped Nanoparticles

Lanthanide-doped upconversion nanoparticles have received growing attention in the development of low-background, highly sensitive and selective sensors. Here, we report a water probe based on ligand-free NaYF₄:Yb/Er nanoparticles, utilizing their intrinsically nonlinear upconversion process. The water molecule sensing was realized by monitoring the upconversion emission quenching, which is mainly attributed to efficient energy transfer between upconversion nanoparticles and water molecules as well as water-absorption-induced excitation energy attenuation. The nonlinear upconversion process, together with power function relationship between upconversion emission intensity and excitation power density, offers a sensitive detection of water content down to 0.008 vol % (80 ppm) in an organic solvent. As an added benefit, we show that noncontact detection of water can be achieved just by using water attenuation effect. Moreover, these upconversion nanoparticle based recyclable probes should be particularly suitable for real-time and long-term water monitoring, due to their superior chemical and physical stability. These results could provide insights into the design of upconversion nanoparticle based sensors

Mechanistic Investigation of Photon Upconversion in Nd³⁺-Sensitized Core–Shell Nanoparticles

A new type of core–shell upconversion nanoparticles which can be effectively excited at 795 nm has been designed and synthesized through spatially confined doping of neodymium (Nd³⁺) ions. The use of Nd³⁺ ions as sensitizers facilitates the energy transfer and photon upconversion of a series of lanthanide activators (Er³⁺, Tm³⁺, and Ho³⁺) at a biocompatible excitation wavelength (795 nm) and also significantly minimizes the overheating problem associated with conventional 980 nm excitation. Importantly, the core–shell design enabled high-concentration doping of Nd³⁺ (∼20 mol %) in the shell layer and thus markedly enhanced the upconversion emission from the activators, providing highly attractive luminescent biomarkers for bioimaging without autofluorescence and concern of overheating

Applications and Advances in Machine Learning Force Fields

Force fields (FFs) form the basis of molecular simulations and have significant implications in diverse fields such as materials science, chemistry, physics, and biology. A suitable FF is required to accurately describe system properties. However, an off-the-shelf FF may not be suitable for certain specialized systems, and researchers often need to tailor the FF that fits specific requirements. Before applying machine learning (ML) techniques to construct FFs, the mainstream FFs were primarily based on first-principles force fields (FPFF) and empirical FFs. However, the drawbacks of FPFF and empirical FFs are high cost and low accuracy, respectively, so there is a growing interest in using ML as an effective and precise tool for reconciling this trade-off in developing FFs. In this review, we introduce the fundamental principles of ML and FFs in the context of machine learning force fields (MLFF). We also discuss the advantages and applications of MLFF compared to traditional FFs, as well as the MLFF toolkits widely employed in numerous applications

Controlled Synthesis, Evolution Mechanisms, and Luminescent Properties of ScF_<i>x</i>:Ln (<i>x</i> = 2.76, 3) Nanocrystals

Kinetic or thermodynamic control has been employed to guide the selective synthesis of conventional organic compounds, and it should be a powerful tool as well for accessing unusual inorganic nanocrystals, particularly when a series of members with similar chemical compositions and phase structures exist. Indeed, a comprehensive mapping of the energy barrier distribution of each nanocrystal in a predefined reaction system will enable not only the precise synthesis of nanocrystals with expected sizes, morphologies, phase structures, and ultimately functionalities, but also disclosure of the evolution details of nanocrystals from one structure to another. Using ScF_<i>x</i>:Ln (<i>x</i> = 2.76, 3) series as a proof-of-concept, we have successfully mapped out the energy barriers that correspond to each of the ScF_<i>x</i>:Ln nanocrystals, unraveled suitable temperatures for each type of nanocrystal formation, recorded their phase transition procedures, and also discovered the relationships of the products at each reaction stage. To testify how this approach allows one to tailor the structure-related optical properties, different lanthanide-doped ScF_<i>x</i> nanocrystals were synthesized and a wide-range of luminescence fine-tuning was achieved, which not only showcases high quality of the nanocrystals, but also provides more candidates for various luminescence applications, especially when single-particle upconversion emission is required

Intracellular Adenosine Triphosphate Deprivation through Lanthanide-Doped Nanoparticles

Growing interest in lanthanide-doped nanoparticles for biological and medical uses has brought particular attention to their safety concerns. However, the intrinsic toxicity of this new class of optical nanomaterials in biological systems has not been fully evaluated. In this work, we systematically evaluate the long-term cytotoxicity of lanthanide-doped nanoparticles (NaGdF₄ and NaYF₄) to HeLa cells by monitoring cell viability (mitochondrial activity), adenosine triphosphate (ATP) level, and cell membrane integrity (lactate dehydrogenase release), respectively. Importantly, we find that ligand-free lanthanide-doped nanoparticles induce intracellular ATP deprivation of HeLa cells, resulting in a significant decrease in cell viability after exposure for 7 days. We attribute the particle-induced cell death to two distinct cell death pathways, autophagy and apoptosis, which are primarily mediated via the interaction between the nanoparticle and the phosphate group of cellular ATP. The understanding gained from the investigation of cytotoxicity associated with lanthanide-doped nanoparticles provides keen insights into the safe use of these nanoparticles in biological systems

Black Phosphorus Nanosheets Immobilizing Ce6 for Imaging-Guided Photothermal/Photodynamic Cancer Therapy

In preclinical and clinical research, to destroy cancers, particularly those located in deep tissues, is still a great challenge. Photodynamic therapy and photothermal therapy are promising alternative approaches for tissue cancer curing. Black phosphorus (BP)-based nanomaterials, with broad UV–vis near-infrared absorbance and excellent photothermal effect, have shown great potential in biomedical applications. Herein, a biocompatible therapeutic platform, chlorin e6 (Ce6)-decorated BP nanosheets (NSs), has been developed for fluorescence and thermal imaging-guided photothermal and photodynamic synergistic cancer treatment. Taking advantage of the relatively high surface area of exfoliated BP NSs, the PEG-NH₂-modified BP NSs (BP@PEG) are loaded with a Ce6 photosensitizer. The resulted BP@PEG/Ce6 NSs not only have good biocompatibility, physiological stability, and tumor-targeting property but also exhibit enhanced photothermal conversion efficiency (43.6%) compared with BP@PEG NSs (28.7%). In addition, BP@PEG/Ce6 NSs could efficiently generate reactive oxygen species because of the release of the Ce6 photosensitizer, which is also verified by in vitro studies. In vivo fluorescence imaging suggests that BP@PEG/Ce6 NSs can accumulate in the tumor targetedly through the enhanced permeability and retention effect. Both in vitro and in vivo studies suggest that BP@PEG/Ce6 can be a promising nanotheranostic agent for synergetic photothermal/photodynamic cancer therapy

Templating C₆₀ on MoS₂ Nanosheets for 2D Hybrid van der Waals <i>p</i>–<i>n</i> Nanoheterojunctions

C₆₀ and single-layer MoS₂ nanocomposites were facilely prepared via a combined solvent transfer and surface deposition (STSD) method by templating C₆₀ aggregates on 2D MoS₂ nanosheets to construct hybrid van der Waals heterojunctions. The electronic property of the hybrid nanomaterials was investigated in a direct charge transport diode device configuration of ITO/C₆₀–MoS₂ nanocomposites/Al; rewritable nonvolatile resistive switching with low SET/RESET voltage (∼3 V), high ON/OFF resistance ratio (∼4 × 10³), and superior electrical bistability (>10⁴ s) of a flash memory behavior was observed. This particular electrical property of C₆₀–MoS₂ nanocomposites, not possessed by either C₆₀ or MoS₂ nanosheets, was supposed to be due to the efficiently established C₆₀/MoS₂ <i>p</i>–<i>n</i> nanojunction, which controls the electron tunneling via junction barriers modulated by electric-field-induced polarization. Thus, our 2D templating method through STSD is promising to massively allocate van der Waals <i>p</i>–<i>n</i> heterojunctions in 2D nanocomposites, opening a window for important insights into the charge transport across the interface of organic/2D-semiconductors

The Effect of Surface Coating on Energy Migration-Mediated Upconversion

Lanthanide-doped upconversion nanoparticles have been the focus of a growing body of investigation because of their promising applications ranging from data storage to biological imaging and drug delivery. Here we present the rational design, synthesis, and characterization of a new class of core–shell upconversion nanoparticles displaying unprecedented optical properties. Specifically, we show that the epitaxial growth of an optically inert NaYF₄ layer around a lanthanide-doped NaGdF₄@NaGdF₄ core–shell nanoparticle effectively prevents surface quenching of excitation energy. At room temperature, the energy migrates over Gd sublattices and is adequately trapped by the activator ions embedded in host lattices. Importantly, the NaYF₄ shell-coating strategy gives access to tunable upconversion emissions from a variety of activators (Dy³⁺, Sm³⁺, Tb³⁺, and Eu³⁺) doped at very low concentrations (down to 1 mol %). Our mechanistic investigations make possible, for the first time, the realization of efficient emissions from Tb³⁺ and Eu³⁺ activators that are doped homogeneously with Yb³⁺/Tm³⁺ ions. The advances on these luminescent nanomaterials offer exciting opportunities for important biological and energy applications

Interdiffusion Reaction-Assisted Hybridization of Two-Dimensional Metal–Organic Frameworks and Ti₃C₂T_<i>x</i> Nanosheets for Electrocatalytic Oxygen Evolution

Two-dimensional (2D) metal–organic framework (MOF) nanosheets have been recently regarded as the model electrocatalysts due to their porous structure, fast mass and ion transfer through the thickness, and large portion of exposed active metal centers. Combining them with electrically conductive 2D nanosheets is anticipated to achieve further improved performance in electrocatalysis. In this work, we <i>in situ</i> hybridized 2D cobalt 1,4-benzenedicarboxylate (CoBDC) with Ti₃C₂T_<i>x</i> (the MXene phase) nanosheets <i>via</i> an interdiffusion reaction-assisted process. The resulting hybrid material was applied in the oxygen evolution reaction and achieved a current density of 10 mA cm^–2 at a potential of 1.64 V <i>vs</i> reversible hydrogen electrode and a Tafel slope of 48.2 mV dec^–1 in 0.1 M KOH. These results outperform those obtained by the standard IrO₂-based catalyst and are comparable with or even better than those achieved by the previously reported state-of-the-art transition-metal-based catalysts. While the CoBDC layer provided the highly porous structure and large active surface area, the electrically conductive and hydrophilic Ti₃C₂T_<i>x</i> nanosheets enabled the rapid charge and ion transfer across the well-defined Ti₃C₂T_<i>x</i>–CoBDC interface and facilitated the access of aqueous electrolyte to the catalytically active CoBDC surfaces. The hybrid nanosheets were further fabricated into an air cathode for a rechargeable zinc–air battery, which was successfully used to power a light-emitting diode. We believe that the <i>in situ</i> hybridization of MXenes and 2D MOFs with interface control will provide more opportunities for their use in energy-based applications