Rare-Earth Doping in Nanostructured Inorganic Materials

Impurity doping is a promising method to impart new properties to various materials. Due to their unique optical, magnetic, and electrical properties, rare-earth ions have been extensively explored as active dopants in inorganic crystal lattices since the 18th century. Rare-earth doping can alter the crystallographic phase, morphology, and size, leading to tunable optical responses of doped nanomaterials. Moreover, rare-earth doping can control the ultimate electronic and catalytic performance of doped nanomaterials in a tunable and scalable manner, enabling significant improvements in energy harvesting and conversion. A better understanding of the critical role of rare-earth doping is a prerequisite for the development of an extensive repertoire of functional nanomaterials for practical applications. In this review, we highlight recent advances in rare-earth doping in inorganic nanomaterials and the associated applications in many fields. This review covers the key criteria for rare-earth doping, including basic electronic structures, lattice environments, and doping strategies, as well as fundamental design principles that enhance the electrical, optical, catalytic, and magnetic properties of the material. We also discuss future research directions and challenges in controlling rare-earth doping for new applications.

A ratiometric luminescence pH sensor based on YAG:V3+,V5+ nanoparticles

A new type of ratiometric luminescence-based pH sensor is described.

Optical Sensing and Imaging of pH Values: Spectroscopies, Materials, and Applications

This is the first comprehensive review on methods and materials for use in optical sensing of pH values and on applications of such sensors. The Review starts with an introduction that contains subsections on the definition of the pH value, a brief look back on optical methods for sensing of pH, on the effects of ionic strength on pH values and pKa values, on the selectivity, sensitivity, precision, dynamic ranges, and temperature dependence of such sensors. Commonly used optical sensing schemes are covered in a next main chapter, with subsections on methods based on absorptiometry, reflectometry, luminescence, refractive index, surface plasmon resonance, photonic crystals, turbidity, mechanical displacement, interferometry, and solvatochromism. This is followed by sections on absorptiometric and luminescent molecular probes for use pH in sensors. Further large sections cover polymeric hosts and supports, and methods for immobilization of indicator dyes. Further and more specific sections summarize the state of the art in materials with dual functionality (indicator and host), nanomaterials, sensors based on upconversion and 2-photon absorption, multiparameter sensors, imaging, and sensors for extreme pH values. A chapter on the many sensing formats has subsections on planar, fiber optic, evanescent wave, refractive index, surface plasmon resonance and holography based sensor designs, and on distributed sensing. Another section summarizes selected applications in areas, such as medicine, biology, oceanography, bioprocess monitoring, corrosion studies, on the use of pH sensors as transducers in biosensors and chemical sensors, and their integration into flow-injection analyzers, microfluidic devices, and lab-on-a-chip systems. An extra section is devoted to current challenges, with subsections on challenges of general nature and those of specific nature. A concluding section gives an outlook on potential future trends and perspectives.

In vivo deep-tissue microscopy with UCNP/Janus-dendrimers as imaging probes: resolution at depth and feasibility of ratiometric sensing

Lanthanide-based upconverting nanoparticles (UCNPs) are known for their remarkable ability to convert near-infrared energy into higher energy light, offering an attractive platform for construction of biological imaging probes. Here we focus on in vivo high-resolution microscopy - an application for which the opportunity to carry out excitation at low photon fluxes in non-linear regime makes UCNPs stand out among all multiphoton probes. To create biocompatible nanoparticles we employed Janus-type dendrimers as surface ligands, featuring multiple carboxylates on one 'face' of the molecule, polyethylene glycol (PEG) residues on another and Eriochrome Cyanine R dye as the core. The UCNP/Janus-dendrimers showed outstanding performance as vascular markers, allowing for depth-resolved mapping of individual capillaries in the mouse brain down to a remarkable depth of ∼1000 μm under continuous wave (CW) excitation with powers not exceeding 20 mW. Using a posteriori deconvolution, high-resolution images could be obtained even at high scanning speeds in spite of the blurring caused by the long luminescence lifetimes of the lanthanide ions. Secondly, the new UCNP/dendrimers allowed us to evaluate the feasibility of quantitative analyte imaging in vivo using a popular ratiometric UCNP-to-ligand excitation energy transfer (EET) scheme. Our results show that the ratio of UCNP emission bands, which for quantitative sensing should respond selectively to the analyte of interest, is also strongly affected by optical heterogeneities of the medium. On the other hand, the luminescence decay times of UCNPs, which are independent of the medium properties, are modulated via EET only insignificantly. As such, quantitative analyte sensing in biological tissues with UCNP-based probes still remains a challenge.

Recent Advances on Functionalized Upconversion Nanoparticles for Detection of Small Molecules and Ions in Biosystems

Significant progress on upconversion-nanoparticle (UCNP)-based probes is witnessed in recent years. Compared with traditional fluorescent probes (e.g., organic dyes, metal complexes, or inorganic quantum dots), UCNPs have many advantages such as non-autofluorescence, high chemical stability, large light-penetration depth, long lifetime, and less damage to samples. This article focuses on recent achievements in the usage of lanthanide-doped UCNPs as efficient probes for biodetection since 2014. The mechanisms of upconversion as well as the luminescence resonance energy transfer process is introduced first, followed by a detailed summary on the recent researches of UCNP-based biodetections including the detection of inorganic ions, gas molecules, reactive oxygen species, and thiols and hydrogen sulfide.

Upconversion Luminescence Sensitized pH-Nanoprobes

Photon upconversion materials, featuring excellent photophysical properties, are promising for bio-medical research due to their low autofluorescence, non-cytotoxicity, low photobleaching and high photostability. Upconversion based pH-nanoprobes are attracting considerable interest due to their superiority over pH-sensitive molecular indicators and metal nanoparticles. Herein, we review the advances in upconversion based pH-nanoprobes, the first time in the seven years since their discovery in 2009. With a brief discussion on the upconversion materials and upconversion processes, the progress in this field has been overviewed, along with the toxicity and biodistribution of upconversion materials for intracellular application. We strongly believe that this survey will encourage the further pursuit of intense research for designing molecular pH-sensors.

Multifunctional MWCNTs–NaGdF4:Yb3+,Er3+,Eu3+ hybrid nanocomposites with potential dual-mode luminescence, magnetism and photothermal properties

Novel multifunctional MWCNTs–NaGdF₄:Yb³⁺,Er³⁺,Eu³⁺ hybrid nanocomposites can simultaneously take advantage of up- and down-conversion luminescence, magnetism and photothermal properties.