Strong Emission Enhancement via Dual-Wavelength Coexcitation in YbTm-Doped Upconverting Nanoparticles for Near-Infrared and Subdiffraction Imaging.

Upconversion (UC) emission in lanthanide-doped nanoparticles is typically excited by a single near-infrared (NIR) wavelength, most commonly around 975 nm, which promotes ground-state absorption by Yb sensitizer ions and subsequent energy transfer to activator ions such as Tm. However, due to the presence of multiple long-lived excited states in lanthanide ions, additional excitation wavelengths can activate or modulate further energy-transfer pathways, leading to enrichment or depletion of specific electronic level populations. Despite their significant potential, such possibilities remain underexplored.

Luminescent Nanocrystal Probes for Monitoring Temperature and Thermal Energy Dissipation of Electrical Microcircuit.

In this work, we present an experimental approach for monitoring the temperature of submicrometric, real-time operating electrical circuits using luminescence thermometry. For this purpose, we utilized lanthanide-doped up-converting nanocrystals as nanoscale temperature probes, which, combined with a highly sensitive confocal photoluminescence microscope, enabled temperature monitoring with spatial resolution limited only by the diffraction of light. To validate our concept, we constructed a simple model of an electrical microcircuit based on a single silver nanowire with a diameter of approximately 100 nm and a length of about 50 µm, whose temperature increase was induced by electric current flow.

Advances in the photon avalanche luminescence of inorganic lanthanide-doped nanomaterials.

Photon avalanche (PA)-where the absorption of a single photon initiates a 'chain reaction' of additional absorption and energy transfer events within a material-is a highly nonlinear optical process that results in upconverted light emission with an exceptionally steep dependence on the illumination intensity. Over 40 years following the first demonstration of photon avalanche emission in lanthanide-doped bulk crystals, PA emission has been achieved in nanometer-scale colloidal particles. The scaling of PA to nanomaterials has resulted in significant and rapid advances, such as luminescence imaging beyond the diffraction limit of light, optical thermometry and force sensing with (sub)micron spatial resolution, and all-optical data storage and processing.

Unlocking Single-Particle Multiparametric Sensing: Decoupling Temperature and Viscosity Readouts through Upconverting Polarized Spectroscopy.

Upconverting particles (UCPs), renowned for their capability to convert infrared to visible light, serve as invaluable imaging probes. Furthermore, their responsiveness to diverse external stimuli holds promise for leveraging UCPs as remote multiparametric sensors, capable of characterizing medium properties in a single assessment. However, the utility of UCPs in multiparametric sensing is impeded by crosstalk, wherein distinct external stimuli induce identical alterations in UCP luminescence, hindering accurate interpretation, and yielding erroneous outputs.

The mechanisms behind the extreme susceptibility of photon avalanche emission to quenching.

The photon avalanche (PA) process that emerges in lanthanide-doped crystals yields a threshold and highly nonlinear (of the power law order >5) optical response to photoexcitation. PA emission is the outcome of the excited-state absorption combined with a cross-relaxation process, which creates positive and efficient energy looping. In consequence, this combination of processes should be highly susceptible to small perturbations in energy distribution and can thus be hindered by other competitive "parasitic" processes such as energy transfer (ET) to quenching sites.

Understanding Yb-sensitized photon avalanche in Pr co-doped nanocrystals: modelling and optimization.

Among different upconversion processes where the emitted photon has higher energy than the one absorbed, photon avalanche (PA) is unique, because the luminescence intensity increases by 2-3 orders of magnitude in response to a tiny increase in excitation intensity. Since its discovery in 1979, PA has been observed in bulk materials but until recently, obtaining it at the nanoscale has been a significant challenge. In the present work, the PA phenomenon in β-NaYF colloidal nanocrystals co-doped with Pr and Yb ions was successfully observed at 482 nm (P → H) and 607 nm (P → H) under excitation at 852 nm.

All-Optical Data Processing with Photon-Avalanching Nanocrystalline Photonic Synapse.

Data processing and storage in electronic devices are typically performed as a sequence of elementary binary operations. Alternative approaches, such as neuromorphic or reservoir computing, are rapidly gaining interest where data processing is relatively slow, but can be performed in a more comprehensive way or massively in parallel, like in neuronal circuits. Here, time-domain all-optical information processing capabilities of photon-avalanching (PA) nanoparticles at room temperature are discovered.

On the role of Gdions in enhancement of UV emission from Yb-Tmup-converting LiYFnanocrystals.

Materials capable of emitting ultraviolet (UV) radiation are sought for applications ranging from theranostics or photodynamic therapy to specific photocatalysis. The nanometer size of these materials, as well as excitation with near-infrared (NIR) light, is essential for many applications. Tetragonal tetrafluoride LiY(Gd)Fnanocrystalline host for up-converting Tm-Ybactivator-sensitizer pair is a promising candidate to achieve UV-vis up-converted radiation under NIR excitation, important for numerous photo-chemical and bio-medical applications.

Upconversion FRET quantitation: the role of donor photoexcitation mode and compositional architecture on the decay and intensity based responses.

Lanthanide-doped colloidal nanoparticles capable of photon upconversion (UC) offer long luminescence lifetimes, narrowband absorption and emission spectra, and efficient anti-Stokes emission. These features are highly advantageous for Förster Resonance Energy Transfer (FRET) based detection. Upconverting nanoparticles (UCNPs) as donors may solve the existing problems of molecular FRET systems, such as photobleaching and limitations in quantitative analysis, but these new labels also bring new challenges.

Quantitative Comparison of the Light-to-Heat Conversion Efficiency in Nanomaterials Suitable for Photothermal Therapy.

Functional colloidal nanoparticles capable of converting between various energy types are finding an increasing number of applications. One of the relevant examples concerns light-to-heat-converting colloidal nanoparticles that may be useful for localized photothermal therapy of cancers. Unfortunately, quantitative comparison and ranking of nanoheaters are not straightforward as materials of different compositions and structures have different photophysical and chemical properties and may interact differently with the biological environment.

Lanthanide-doped heterostructured nanocomposites toward advanced optical anti-counterfeiting and information storage.

The continuously growing importance of information storage, transmission, and authentication impose many new demands and challenges for modern nano-photonic materials and information storage technologies, both in security and storage capacity. Recently, luminescent lanthanide-doped nanomaterials have drawn much attention in this field because of their photostability, multimodal/multicolor/narrowband emissions, and long luminescence lifetime. Here, we report a multimodal nanocomposite composed of lanthanide-doped upconverting nanoparticle and EuSe semiconductor, which was constructed by utilizing a cation exchange strategy.

Engineering the Compositional Architecture of Core-Shell Upconverting Lanthanide-Doped Nanoparticles for Optimal Luminescent Donor in Resonance Energy Transfer: The Effects of Energy Migration and Storage.

Förster Resonance Energy Transfer (FRET) between single molecule donor (D) and acceptor (A) is well understood from a fundamental perspective and is widely applied in biology, biotechnology, medical diagnostics, and bio-imaging. Lanthanide doped upconverting nanoparticles (UCNPs) have demonstrated their suitability as alternative donor species. Nevertheless, while they solve most disadvantageous features of organic donor molecules, such as photo-bleaching, spectral cross-excitation, and emission bleed-through, the fundamental understanding and practical realizations of bioassays with UCNP donors remain challenging.

Laser Refrigeration by an Ytterbium-Doped NaYF Microspinner.

Thermal control of liquids with high (micrometric) spatial resolution is required for advanced research such as single molecule/cell studies (where temperature is a key factor) or for the development of advanced microfluidic devices (based on the creation of thermal gradients at the microscale). Local and remote heating of liquids is easily achieved by focusing a laser beam with wavelength adjusted to absorption bands of the liquid medium or of the embedded colloidal absorbers. The opposite effect, that is highly localized cooling, is much more difficult to achieve.

Standardization of Methodology of Light-to-Heat Conversion Efficiency Determination for Colloidal Nanoheaters.

Localized photothermal therapy (PTT) has been demonstrated to be a promising method of combating cancer, that additionally synergistically enhances other treatment modalities such as photodynamic therapy or chemotherapy. PTT exploits nanoparticles (called nanoheaters), that upon proper biofunctionalization may target cancerous tissues, and under light stimulation may convert the energy of photons to heat, leading to local overheating and treatment of cancerous cells. Despite extensive work, there is, however, no agreement on how to accurately and quantitatively compare light-to-heat conversion efficiency (η) and rank the nanoheating performances of various groups of nanomaterials.

Synergy between NIR luminescence and thermal emission toward highly sensitive NIR operating emissive thermometry.

There are many figures of merit, which determine suitability of luminescent thermometers for practical applications. These include thermal sensitivity, thermal accuracy as well as ease and cost effectivness of technical implementation. A novel contactless emission thermometer is proposed, which takes advantage of the coexistence of photoluminescence from Nd doping ions and black body emission in transparent Nd doped-oxyfluorotellurite glass host matrix.

Enhancing FRET biosensing beyond 10 nm with photon avalanche nanoparticles.

Förster Resonance Energy Transfer (FRET) between donor (D) and acceptor (A) molecules is a phenomenon commonly exploited to study or visualize biological interactions at the molecular level. However, commonly used organic D and A molecules often suffer from photobleaching and spectral bleed-through, and their spectral properties hinder quantitative analysis. Lanthanide-doped upconverting nanoparticles (UCNPs) as alternative D species offer significant improvements in terms of photostability, spectral purity and background-free luminescence detection, but they bring new challenges related to multiple donor ions existing in a single large size UCNP and the need for nanoparticle biofunctionalization.

Standardizing luminescence nanothermometry for biomedical applications.

Luminescence nanothermometry enables accurate, remote, and all-optically-based thermal sensing. Notwithstanding its fast development, there are serious obstacles hindering reproducibility and reliable quantitative assessment of nanothermometers, which impede the intentional design, optimization and use of these sensors. These issues include ambiguities or absence of established universal rules for quantitative evaluation, incorrect assumptions about the mechanisms behind the thermal response of the sensors as well as the dependence of the nanothermometers readout on external conditions and host materials themselves.

Heterodimers made of metal-organic frameworks and upconversion nanoparticles for bioimaging and pH-responsive dual-drug delivery.

Developing multifunctional nanocomposites for a pH-responsive controlled dual-drug delivery is still a huge challenge. Herein, we report a gentle and simple method for growing metal-organic frameworks (MOFs) that can load two anticancer drugs, namely DOX and 5-FU (doxorubicin and 5-fluorouracil), on the surface of upconversion nanoparticles (UCNPs) by the reactions of Schiff bases and electrostatic adsorption. The resulting pH-responsive UCMOFs@D@5 nanosystem showed effective dual-drug release by the cleavage of chemical bonds and the disruption of the MOF structure under acidic conditions.

Single-Cell Biodetection by Upconverting Microspinners.

Near-infrared-light-mediated optical tweezing of individual upconverting particles has enabled all-optical single-cell studies, such as intracellular thermal sensing and minimally invasive cytoplasm investigations. Furthermore, the intrinsic optical birefringence of upconverting particles renders them light-driven luminescent spinners with a yet unexplored potential in biomedicine. In this work, the use of upconverting spinners is showcased for the accurate and specific detection of single-cell and single-bacteria attachment events, through real-time monitoring of the spinners rotation velocity of the spinner.

Near-infrared excited luminescence and imaging of HeLa cells by using Mn enhanced Tb and Yb cooperative upconversion in NaYF nanocrystals.

Advanced biodetection and bioimaging require fluorescent labels which exhibit many, easily distinguishable colors to identify or study numerous biotargets in a single sample. Although numerous different colors have been demonstrated with lanthanide doped nanoparticles, these colors usually originate from various ratios of overlapping multiple emission bands from activators, which severely limits the number of available labels. As a consequence, different lanthanide doped labels cannot be easily distinguished from each other ( Er from Ho) in a quantitative way, when such labels are co-localized during microscopy wide-field imaging.

A new forum for upconversion research: the UPCON conference.

The Conference and Spring School on Properties, Design and Applications of Upconversion Nanomaterials (UPCON) provides a new forum for all experts and newcomers in the field of upconversion research. On the occasion of the second UPCON 2018 in Valencia (Spain), we are pleased to present a collection of 12 reviews and research articles that reflect recent advances in upconversion materials, their unique luminescent properties and many applications spanning from nanoscale thermometry to biomedicine.