Perspectives and challenges of photon-upconversion nanoparticles - Part I: routes to brighter particles and quantitative spectroscopic studies

Interfacing with the Brain: How Nanotechnology Can Contribute

Interfacing artificial devices with the human brain is the central goal of neurotechnology. Yet, our imaginations are often limited by currently available paradigms and technologies. Suggestions for brain-machine interfaces have changed over time, along with the available technology. Mechanical levers and cable winches were used to move parts of the brain during the mechanical age. Sophisticated electronic wiring and remote control have arisen during the electronic age, ultimately leading to plug-and-play computer interfaces. Nonetheless, our brains are so complex that these visions, until recently, largely remained unreachable dreams. The general problem, thus far, is that most of our technology is mechanically and/or electrically engineered, whereas the brain is a living, dynamic entity. As a result, these worlds are difficult to interface with one another. Nanotechnology, which encompasses engineered solid-state objects and integrated circuits, excels at small length scales of single to a few hundred nanometers and, thus, matches the sizes of biomolecules, biomolecular assemblies, and parts of cells. Consequently, we envision nanomaterials and nanotools as opportunities to interface with the brain in alternative ways. Here, we review the existing literature on the use of nanotechnology in brain-machine interfaces and look forward in discussing perspectives and limitations based on the authors' expertise across a range of complementary disciplines─from neuroscience, engineering, physics, and chemistry to biology and medicine, computer science and mathematics, and social science and jurisprudence. We focus on nanotechnology but also include information from related fields when useful and complementary.

Molecular Upconversion Nanoparticles for Live-Cell Imaging

Precise molecular control has become a highly attractive feature to develop the next generation of upconversion materials for autofluorescence-free deep tissue imaging. However, in aqueous environments, upconversion molecules are orders of magnitude dimmer than inorganic upconversion nanoparticles, thereby strongly limiting their applicability to bioimaging. By encapsulating ca. 1,900 upconversion molecules into sub-40 nm polymer nanoparticles, we show that molecular precision and nanomaterial brightness can be combined into a new type of hybrid nanomaterial. The brightness of these molecular upconversion nanoparticles (UCMol-NPs) is almost on par with widely used inorganic upconversion nanoparticles, permitting the experimental demonstration of live-cell imaging with UCMol-NPs, an important step toward advancing molecular upconversion into the application era. Fabrication, characterization, and modeling of UCMol-NPs with various sizes and loadings reveal that significant brightness enhancement is possible. This will be paramount for advancing upconversion beyond the current limits of inorganic nanoparticles and translating them into clinical applications.

Near-infrared DNA biosensors based on polysulfonate coatings for the sensitive detection of microRNAs

MicroRNAs (miRNAs) play crucial roles in the regulation of immune cell differentiation and the immune response during allergic rhinitis (AR). Studies have shown that miRNA-155 is significantly upregulated in AR pathogenesis. Therefore, miRNA-155 can be used as a biomarker for AR diagnosis. Although fluorescent biosensors based on upconversion nanoparticles (UCNPs) have made significant advances in the detection of miRNAs, developing UCNPs with polymer coatings, efficient surface passivation, and DNA functionalization for hybrid sensing in biological media remains challenging. Herein, hairpin DNA1 (H1) is modified into a thin polysulfonic acid layer on UCNPs by sulfonamide bonds, and the fluorescence of the UCNPs is quenched by the fluorescence resonance energy transfer (FRET) process of BHQ3 carried by H1. When the target miRNA-155 is present, the hairpin structure of H1 is opened, allowing BHQ3 to move away from the UCNP surface, and the fluorescence of UCNP is restored. At the same time, hairpin DNA1 (H2) can combine with H1 to replace the miRNA-155 that is bound to H1 with the help of the opening stem ring structure of H1, and the replaced miRNA-155 can continue to react with H1 to amplify the fluorescence signal. Under the optimal experimental conditions, the linear range of miRNA-155 is 0.01-3 nM, with a detection limit of 1.14 pM. Furthermore, the constructed biosensor has been applied to determine miRNA-155 in serum samples, and the spiked recoveries range from 99.8% to 104.8%, which indicates that the developed assay has potential applications in monitoring allergic rhinitis or other miRNA related diseases.

Photophysics of fluorescent nanoparticles based on organic dyes - challenges and design principles

Fluorescent nanoparticles have become attractive for bioanalysis and imaging, due to their high brightness and photostability. Many different optical materials have been applied in fluorescent nanoparticles with a broad range of properties and characteristics. One appealing approach is the incorporation of molecular organic fluorophores in nanoparticles with the intention of transferring their known attractive solution-state properties directly to the nanoparticles. However, as molecular dyes are packed closely together in the nanoparticles their interactions most often result in fluorescence quenching and change in spectral properties making this approach challenging. In this perspective we will first discuss the origins of quenching and spectral shifts observed in dye based nanoparticles. On this background, we will then describe various designs of dye based NPs and how they address the challenges of dye-dye interactions and quenching. Our aim is to provide a general framework for understanding the supramolecular mechanisms that determine the photophysics of dye based nanoparticles. This framework of molecular photophysics and its relation to the internal structure of dye based nanoparticles can hopefully serve to assist rational design and optimization of new and improved dye based nanoparticles.

Digital and Analog Detection of SARS-CoV-2 Nucleocapsid Protein via an Upconversion-Linked Immunosorbent Assay

The COVID-19 crisis requires fast and highly sensitive tests for the early stage detection of the SARS-CoV-2 virus. For detecting the nucleocapsid protein (N protein), the most abundant viral antigen, we have employed upconversion nanoparticles that emit short-wavelength light under near-infrared excitation (976 nm). The anti-Stokes emission avoids autofluorescence and light scattering and thus enables measurements without optical background interference. The sandwich upconversion-linked immunosorbent assay (ULISA) can be operated both in a conventional analog mode and in a digital mode based on counting individual immune complexes. We have investigated how different antibody combinations affect the detection of the wildtype N protein and the detection of SARS-CoV-2 (alpha variant) in lysed culture fluid via the N protein. The ULISA yielded a limit of detection (LOD) of 1.3 pg/mL (27 fM) for N protein detection independent of the analog or digital readout, which is approximately 3 orders of magnitude more sensitive than conventional enzyme-linked immunosorbent assays or commercial lateral flow assays for home testing. In the case of SARS-CoV-2, the digital ULISA additionally improved the LOD by a factor of 10 compared to the analog readout.

Assessing the reproducibility and up-scaling of the synthesis of Er,Yb-doped NaYF4-based upconverting nanoparticles and control of size, morphology, and optical properties

Lanthanide-based, spectrally shifting, and multi-color luminescent upconverting nanoparticles (UCNPs) have received much attention in the last decades because of their applicability as reporter for bioimaging, super-resolution microscopy, and sensing as well as barcoding and anti-counterfeiting tags. A prerequisite for the broad application of UCNPs in areas such as sensing and encoding are simple, robust, and easily upscalable synthesis protocols that yield large quantities of UCNPs with sizes of 20 nm or more with precisely controlled and tunable physicochemical properties from low-cost reagents with a high reproducibility. In this context, we studied the reproducibility, robustness, and upscalability of the synthesis of β-NaYF₄:Yb, Er UCNPs via thermal decomposition. Reaction parameters included solvent, precursor chemical compositions, ratio, and concentration. The resulting UCNPs were then examined regarding their application-relevant physicochemical properties such as size, size distribution, morphology, crystal phase, chemical composition, and photoluminescence. Based on these screening studies, we propose a small volume and high-concentration synthesis approach that can provide UCNPs with different, yet controlled size, an excellent phase purity and tunable morphology in batch sizes of up to at least 5 g which are well suited for the fabrication of sensors, printable barcodes or authentication and recycling tags.

Photon Upconversion in Small Molecules

Upconversion (UC) is a process that describes the emission of shorter-wavelength light compared to that of the excitation source. Thus, UC is also referred to as anti-Stokes emission because the excitation wavelength is longer than the emission wavelength. UC materials are used in many fields, from electronics to medicine. The objective of using UC in medical research is to synthesize upconversion nanoparticles (UCNPs) composed of a lanthanide core with a coating of adsorbed dye that will generate fluorescence after excitation with near-infrared light to illuminate deep tissue. Emission occurs in the visible and UV range, and excitation mainly in the near-infrared spectrum. UC is observed for lanthanide ions due to the arrangement of their energy levels resulting from f-f electronic transitions. Organic compounds and transition metal ions are also able to form the UC process. Biocompatible UCNPs are designed to absorb infrared light and emit visible light in the UC process. Fluorescent dyes are adsorbed to UCNPs and employed in PDT to achieve deeper tissue effects upon irradiation with infrared light. Fluorescent UCNPs afford selectivity as they may be activated only by illumination of an area of diseased tissue, such as a tumor, with infrared light and are by themselves atoxic in the absence of infrared light. UCNP constructs can be monitored as to their location in the body and uptake by cancer cells, aiding in evaluation of exact doses required to treat the targeted cancer. In this paper, we review current research in UC studies and UCNP development.

Bioconjugates of photon-upconversion nanoparticles for cancer biomarker detection and imaging

The detection of cancer biomarkers in histological samples and blood is of paramount importance for clinical diagnosis. Current methods are limited in terms of sensitivity, hindering early detection of disease. We have overcome the shortcomings of currently available staining and fluorescence labeling methods by taking an integrative approach to establish photon-upconversion nanoparticles (UCNP) as a powerful platform for cancer detection. These nanoparticles are readily synthesized in different sizes to yield efficient and tunable short-wavelength light emission under near-infrared excitation, which eliminates optical background interference of the specimen. Here we present a protocol for the synthesis of UCNPs by high-temperature co-precipitation or seed-mediated growth by thermal decomposition, surface modification by silica or poly(ethylene glycol) that renders the particles resistant to nonspecific binding, and the conjugation of streptavidin or antibodies for biological detection. To detect blood-based biomarkers, we present an upconversion-linked immunosorbent assay for the analog and digital detection of the cancer marker prostate-specific antigen. When applied to immunocytochemistry analysis, UCNPs enable the detection of the breast cancer marker human epidermal growth factor receptor 2 with a signal-to-background ratio 50-fold higher than conventional fluorescent labels. UCNP synthesis takes 4.5 d, the preparation of the antibody-silica-UCNP conjugate takes 3 d, the streptavidin-poly(ethylene glycol)-UCNP conjugate takes 2-3 weeks, upconversion-linked immunosorbent assay takes 2-4 d and immunocytochemistry takes 8-10 h. The procedures can be performed after standard laboratory training in nanomaterials research.

Cubic versus hexagonal - phase, size and morphology effects on the photoluminescence quantum yield of NaGdF4:Er3+/Yb3+ upconverting nanoparticles

Upconverting nanoparticles (UCNPs) are well-known for their capacity to convert near-infrared light into UV/visible light, benefitting various applications where light triggering is required. At the nanoscale, loss of luminescence intensity is observed and thus, a decrease in photoluminescence quantum yield (PLQY), usually ascribed to surface quenching. We evaluate this by measuring the PLQY of NaGdF₄:Er³⁺,Yb³⁺ UCNPs as a function of size (ca. 15 to 100 nm) and shape (spheres, cubes, hexagons). Our results show that the PLQY of α-phase NaGdF₄ Er³⁺,Yb³⁺ surpasses that of β-NaGdF₄ for sizes below 20 nm, an observation related to distortion of the crystal lattice when the UCNPs become smaller. The present study also underlines that particle shape must not be neglected as a relevant parameter for PLQY. In fact, based on a mathematical nucleus/hull volumetric model, shape was found to be particularly relevant in the 20 to 60 nm size range of the investigated UCNPs.

Influence of the synthesis route on the spectroscopic, cytotoxic, and temperature-sensing properties of oleate-capped and ligand-free core/shell nanoparticles

The right choice of synthesis route for upconverting nanoparticles (UCNPs) is crucial for obtaining a well-defined product with a specific application capability. Thus we decided to compare the physicochemical, cytotoxic, and temperature-sensing properties of UCNPs obtained from different rare earth (RE) ions, which has been made for the first time in a single study. The core/shell NaYF₄:Yb³⁺,Er³⁺/NaYF₄ UCNPs were obtained by reaction in a mixture of oleic acid and octadecene, and their highly stable water colloids were prepared using the ligand-free modification method. Both oleate-capped and ligand-free UCNPs exhibited a bright upconversion emission upon 975 nm excitation. Moreover, slope values, emission quantum yields, and luminescence lifetimes confirmed an effective energy transfer between the Yb³⁺ and Er³⁺ ions. Additionally, the water colloids of the UCNPs showed temperature-sensing properties with a good thermal sensitivity level, higher than 1 % K^-1 at 358 K. Evaluation of the cytotoxicity profiles of the obtained products indicated that cell viability was decreased in a dose-dependent manner in the analyzed concentration range.

Influence of Gd3+ doping concentration on the properties of Na(Y,Gd)F4:Yb3+, Tm3+ upconverting nanoparticles and their long-term aging behavior

We present a systematic study on the properties of Na(Y,Gd)F₄-based upconverting nanoparticles (UCNP) doped with 18% Yb³⁺, 2% Tm³⁺, and the influence of Gd³⁺ (10-50 mol% Gd³⁺). UCNP were synthesized via the solvothermal method and had a range of diameters within 13 and 50 nm. Structural and photophysical changes were monitored for the UCNP samples after a 24-month incubation period in dry phase and further redispersion. Structural characterization was performed by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) as well as dynamic light scattering (DLS), and the upconversion luminescence (UCL) studies were executed at various temperatures (from 4 to 295 K) using time-resolved and steady-state spectroscopy. An increase in the hexagonal lattice phase with the increase of Gd³⁺ content was found, although the cubic phase was prevalent in most samples. The Tm³⁺-luminescence intensity as well as the Tm³⁺-luminescence decay times peaked at the Gd³⁺ concentration of 30 mol%. Although the general upconverting luminescence properties of the nanoparticles were preserved, the 24-month incubation period lead to irreversible agglomeration of the UCNP and changes in luminescence band ratios and lifetimes.

NIR laser scanning microscopy for photophysical characterization of upconversion nanoparticles and nanohybrids

Photophysical characterization of upconversion nanoparticles (UCNPs) and nanohybrids (UCNHs) is more challenging than that of down-conversion nanomaterials. Moreover, it is still difficult to gain knowledge about the homogeneity of the sample and colocalization of emissive chromophores and nanoparticles in nanohybrids. Near infrared laser scanning microscopy (NIR-LSM) is a well-known and useful imaging technique, which enables excitation in the NIR region and has been extensively applied to optical fluorescence imaging of organic fluorophores and nanomaterials, such as quantum dots, which exhibit a short-lived emission. NIR-LSM has recently been used to determine the empirical emission lifetime of UCNPs, thus extending its application range to nanomaterials with a long lifetime emission. Here, we review our previous findings and include new measurements and samples to fully address the potential of this technique. NIR-LSM has proved to be extraordinarily useful not only for photophysical characterization of UCNHs consisting of UCNPs capped with a fluorophore to easily visualize the occurrence of the resonance energy transfer process between the UCNH constituents and their homogeneity, but also to assess the colocalization of the fluorophore and the UCNP in the UCNH; all this information can be acquired on the micro-/nano-meter scale by just taking one image.

Biosensing based on upconversion nanoparticles for food quality and safety applications

Food safety and quality regulations inevitably call for sensitive and accurate analytical methods to detect harmful contaminants in food and to ensure safe food for the consumer. Both novel and well-established biorecognition elements, together with different transduction schemes, enable the simple and rapid analysis of various food contaminants. Upconversion nanoparticles (UCNPs) are inorganic nanocrystals that convert near-infrared light into shorter wavelength emission. This unique photophysical feature, along with narrow emission bandwidths and large anti-Stokes shift, render UCNPs excellent optical labels for biosensing because they can be detected without optical background interferences from the sample matrix. In this review, we show how this exciting technique has evolved into biosensing platforms for food quality and safety monitoring and highlight recent applications in the field.

Lanthanide-Based Nanosensors: Refining Nanoparticle Responsiveness for Single Particle Imaging of Stimuli

Lanthanide nanoparticles (LNPs) are promising sensors of chemical, mechanical, and temperature changes; they combine the narrow-spectral emission and long-lived excited states of individual lanthanide ions with the high spatial resolution and controlled energy transfer of nanocrystalline architectures. Despite considerable progress in optimizing LNP brightness and responsiveness for dynamic sensing, detection of stimuli with a spatial resolution approaching that of individual nanoparticles remains an outstanding challenge. Here, we highlight the existing capabilities and outstanding challenges of LNP sensors, en-route to nanometer-scale, single particle sensor resolution. First, we summarize LNP sensor read-outs, including changes in emission wavelength, lifetime, intensity, and spectral ratiometric values that arise from modified energy transfer networks within nanoparticles. Then, we describe the origins of LNP sensor imprecision, including sensitivity to competing conditions, interparticle heterogeneities, such as the concentration and distribution of dopant ions, and measurement noise. Motivated by these sources of signal variance, we describe synthesis characterization feedback loops to inform and improve sensor precision, and introduce noise-equivalent sensitivity as a figure of merit of LNP sensors. Finally, we project the magnitudes of chemical and pressure stimulus resolution achievable with single LNPs at nanoscale resolution. Our perspective provides a roadmap for translating ensemble LNP sensing capabilities to the single particle level, enabling nanometer-scale sensing in biology, medicine, and sustainability.

(Nano)tag-antibody conjugates in rapid tests

Antibodies (Abs) are naturally derived materials with favorable affinity, selectivity, and fast binding kinetics to the respective antigens, which enables their application as promising recognition elements in the development of various types of biosensors/bioassays, especially in rapid tests. These tests are low-cost and easy-to-use biosensing devices with broad applications including medical or veterinary diagnostics, environmental monitoring and industrial usages such as safety and quality analysis in food, providing on-site quick monitoring of various analytes, making it possible to save analysis costs and time. To reach such features, the conjugation of Abs with various nanomaterials (NMs) as tags is necessary, which range from conventional gold nanoparticles to other nanoparticles recently introduced, where magnetic, plasmonic, photoluminescent, or multi-modal properties play a critical role in the overall performance of the analytical device. In this context, to preserve the Ab affinity and provide a rapid response with long-term storage capability, the use of efficient bio-conjugation techniques is critical. Thanks to their prominent role in rapid tests, many studies have been devoted to the design and development of Abs-NMs conjugates with various chemistries including passive adsorption, covalent coupling, and affinity interactions. In this review, we present the state-of-the-art techniques allowing various Ab-NM conjugates with a special focus on the efficiency of the developed probes to be employed in in vitro rapid tests. Challenges and future perspectives on the development of Ab-conjugated nanotags in rapid diagnostic tests are highlighted along with a survey of the progress in commercially available Ab-NM conjugates.

Highly colloidally stable trimodal 125I-radiolabeled PEG-neridronate-coated upconversion/magnetic bioimaging nanoprobes

"All-in-one" multifunctional nanomaterials, which can be visualized simultaneously by several imaging techniques, are required for the efficient diagnosis and treatment of many serious diseases. This report addresses the design and synthesis of upconversion magnetic NaGdF4:Yb3+/Er3+(Tm3+) nanoparticles by an oleic acid-stabilized high-temperature coprecipitation of lanthanide precursors in octadec-1-ene. The nanoparticles, which emit visible or UV light under near-infrared (NIR) irradiation, were modified by in-house synthesized PEG-neridronate to facilitate their dispersibility and colloidal stability in water and bioanalytically relevant phosphate buffered saline (PBS). The cytotoxicity of the nanoparticles was determined using HeLa cells and human fibroblasts (HF). Subsequently, the particles were modified by Bolton-Hunter-neridronate and radiolabeled by 125I to monitor their biodistribution in mice using single-photon emission computed tomography (SPECT). The upconversion and the paramagnetic properties of the NaGdF4:Yb3+/Er3+(Tm3+)@PEG nanoparticles were evaluated by photoluminescence, magnetic resonance (MR) relaxometry, and magnetic resonance imaging (MRI) with 1 T and 4.7 T preclinical scanners. MRI data were obtained on phantoms with different particle concentrations and during pilot long-time in vivo observations of a mouse model. The biological and physicochemical properties of the NaGdF4:Yb3+/Er3+(Tm3+)@PEG nanoparticles make them promising as a trimodal optical/MRI/SPECT bioimaging and theranostic nanoprobe for experimental medicine.

Assessing the protective effects of different surface coatings on NaYF4:Yb3+, Er3+ upconverting nanoparticles in buffer and DMEM

We studied the dissolution behavior of β NaYF₄:Yb(20%), Er(2%) UCNP of two different sizes in biologically relevant media i.e., water (neutral pH), phosphate buffered saline (PBS), and Dulbecco’s modified Eagle medium (DMEM) at different temperatures and particle concentrations. Special emphasis was dedicated to assess the influence of different surface functionalizations, particularly the potential of mesoporous and microporous silica shells of different thicknesses for UCNP stabilization and protection. Dissolution was quantified electrochemically using a fluoride ion selective electrode (ISE) and by inductively coupled plasma optical emission spectrometry (ICP OES). In addition, dissolution was monitored fluorometrically. These experiments revealed that a thick microporous silica shell drastically decreased dissolution. Our results also underline the critical influence of the chemical composition of the aqueous environment on UCNP dissolution. In DMEM, we observed the formation of a layer of adsorbed molecules on the UCNP surface that protected the UCNP from dissolution and enhanced their fluorescence. Examination of this layer by X-ray photoelectron spectroscopy (XPS) and mass spectrometry (MS) suggested that mainly phenylalanine, lysine, and glucose are adsorbed from DMEM. These findings should be considered in the future for cellular toxicity studies with UCNP and other nanoparticles and the design of new biocompatible surface coatings.

Advances in Optical Single-Molecule Detection: En Route to Supersensitive Bioaffinity Assays

The ability to detect low concentrations of analytes and in particular low-abundance biomarkers is of fundamental importance, e.g., for early-stage disease diagnosis. The prospect of reaching the ultimate limit of detection has driven the development of single-molecule bioaffinity assays. While many review articles have highlighted the potentials of single-molecule technologies for analytical and diagnostic applications, these technologies are not as widespread in real-world applications as one should expect. This Review provides a theoretical background on single-molecule-or better digital-assays to critically assess their potential compared to traditional analog assays. Selected examples from the literature include bioaffinity assays for the detection of biomolecules such as proteins, nucleic acids, and viruses. The structure of the Review highlights the versatility of optical single-molecule labeling techniques, including enzymatic amplification, molecular labels, and innovative nanomaterials.

Contemporary Synthesis of Ultrasmall (sub-10 nm) Upconverting Nanomaterials

Due to their unique photophysical properties, upconverting nanoparticles (UCNPs), i. e. particles capable of converting near-infrared (NIR) photons into tunable emissions in the range of ultraviolet (UV) to NIR, have great potential for use in various biomedical fields such as bioimaging, photodynamic therapy and bioanalytical applications. As far as biomedical applications are concerned, these materials have a number of advantageous properties such as brilliant luminescence and exceptional photostability. Very small "stealth" particles (sub-10 nm), which can circulate in the body largely undetected by the immune system, are particularly important for in vivo use. The fabrication of such particles, which simultaneously have a defined (ultrasmall) size and the required optical properties, is a great challenge and an area that is in its infancy. This minireview provides a concise overview of recent developments on appropriate synthetic methodologies to produce such UCNPs. Particular attention was given to the influence of both surfactants and dopants used to precisely adjust size, crystalline phase and optical properties of UCNPs.

Efficient sub-15 nm cubic-phase core/shell upconversion nanoparticles as reporters for ensemble and single particle studies

Single particle imaging of upconversion nanoparticles (UCNPs) has typically been realized using hexagonal (β) phase lanthanide-doped sodium yttrium fluoride (NaYF₄) materials, the upconversion luminescence (UCL) of which saturates at power densities (P) of several hundred W cm^-2 under 980 nm near-infrared (NIR) excitation. Cubic (α) phase UCNPs have been mostly neglected because of their commonly observed lower UCL efficiency at comparable P in ensemble level studies. Here, we describe a set of sub-15 nm ytterbium-enriched α-NaYbF₄:Er³⁺@CaF₂ core/shell UCNPs doped with varying Er³⁺ concentrations (5-25%), studied over a wide P range of ∼8-10⁵ W cm^-2, which emit intense UCL even at a low P of 10 W cm^-2 and also saturate at relatively low P. The highest upconversion quantum yield (Φ_UC) and the highest particle brightness were obtained for an Er³⁺ dopant concentration of 12%, reaching the highest Φ_UC of 0.77% at a saturation power density (P_sat) of 110 W cm^-2. These 12%Er³⁺-doped core/shell UCNPs were also the brightest UCNPs among this series under microscopic conditions at high P of ∼10²-10⁵ W cm^-2 as demonstrated by imaging studies at the single particle level. Our results underline the potential applicability of the described sub-15 nm cubic-phase core/shell UCNPs for ensemble- and single particle-level bioimaging.

A novel one-pot strategy to rapidly synthesize bright red emitting upconversion nanocrystals with core–shell–shell structure

By growing NaYbF₄ and NaYF₄ on β-NaErF4:0.005Tm in one pot, the upconversion intensity was tremendously enhanced.

Coating of upconversion nanoparticles with silica nanoshells of 5-250 nm thickness

A concept for the growth of silica shells with a thickness of 5-250 nm onto oleate-coated NaYF₄:Yb³⁺/Er³⁺ upconversion nanoparticles (UCNP) is presented. The concept enables the precise adjustment of shell thicknesses for the preparation of thick-shelled nanoparticles for applications in plasmonics and sensing. First, an initial 5-11 nm thick shell is grown onto the UCNPs in a reverse microemulsion. This is followed by a stepwise growth of these particles without a purification step, where in each step equal volumes of tetraethyl orthosilicate and ammonia water are added, while the volumes of cyclohexane and the surfactant Igepal^® CO-520 are increased so that the ammonia water and surfactant concentrations remain constant. Hence, the number of micelles stays constant, and their size is increased to accommodate the growing core-shell particles. Consequently, the formation of core-free silica particles is suppressed. When the negative zeta potential of the particles, which continuously decreased during the stepwise growth, falls below -40 mV, the particles can be dispersed in an ammoniacal ethanol solution and grown further by the continuous addition of tetraethyl orthosilicate to a diameter larger than 500 nm. Due to the high colloidal stability, a coalescence of the particles can be suppressed, and single-core particles are obtained. This strategy can be easily transferred to other nanomaterials for the design of plasmonic nanoconstructs and sensor systems.

NIR-Light-Driven Generation of Reactive Oxygen Species Using Ru(II)-Decorated Lipid-Encapsulated Upconverting Nanoparticles

The biological application of ruthenium anticancer prodrugs for photodynamic therapy (PDT) and photoactivated chemotherapy (PACT) is restricted by the need to use poorly penetrating high-energy photons for their activation, i.e., typically blue or green light. Upconverting nanoparticles (UCNPs), which produce high-energy light under near-infrared (NIR) excitation, may solve this issue, provided that the coupling between the UCNP surface and the Ru prodrug is optimized to produce stable nanoconjugates with efficient energy transfer from the UCNP to the ruthenium complex. Herein, we report on the synthesis and photochemistry of the two structurally related ruthenium(II) polypyridyl complexes [Ru(bpy)₂(5)](PF₆)₂ ([1](PF₆)₂) and [Ru(bpy)₂(6)](PF₆)₂ ([2](PF₆)₂), where bpy = 2,2-bipyridine, 5 is 5,6-bis(dodecyloxy)-2,9-dimethyl-1,10-phenanthroline, and 6 is 5,6-bis(dodecyloxy)-1,10-phenanthroline. [1](PF₆)₂ is photolabile as a result of the steric strain induced by ligand 5, but the irradiation of [1](PF₆)₂ in solution leads to the nonselective and slow photosubstitution of one of its three ligands, making it a poor PACT compound. On the other hand, [2](PF₆)₂ is an efficient and photostable PDT photosensitizer. The water-dispersible, negatively charged nanoconjugate UCNP@lipid/[2] was prepared by the encapsulation of 44 nm diameter NaYF₄:Yb³⁺,Tm³⁺ UCNPs in a mixture of 1,2-dioleoyl-sn-glycero-3-phosphate and 1,2-dioleoyl-sn-glycero-3-phosphocholine phospholipids, cholesterol, and the amphiphilic complex [2](PF₆)₂. A nonradiative energy transfer efficiency of 12% between the Tm³⁺ ions in the UCNP and the Ru²⁺ acceptor [2]²⁺ was found using time-resolved emission spectroscopy. Under irradiation with NIR light (969 nm), UCNP@lipid/[2] was found to produce reactive oxygen species (ROS), as judged by the oxidation of the nonspecific ROS probe 2',7'-dichlorodihydrofluorescein (DCFH^2-). Determination of the type of ROS produced was precluded by the negative surface charge of the nanoconjugate, which resulted in the electrostatic repulsion of the more specific but also negatively charged ¹O₂ probe tetrasodium 9,10-anthracenediyl-bis(methylene)dimalonate (Na₄(ADMBMA)).

Photophysical Properties of Upconverting Nanoparticle-Phthalocyanine Complexes

Interaction between upconverting nanoparticles and aluminum octacarboxyphthalocyanine was studied. The efficiency of non-radiative energy transfer from the nanoparticles to phthalocyanine increased with the number of phthalocyanine molecules adsorbed on the nanoparticle, but only up to a certain limit. Further increase in the phthalocyanine concentration resulted in a decrease of its sensitized fluorescence due to the dimerization of dye molecules on the nanoparticle surface. When subjected to infrared irradiation, phthalocyanine molecules in the hybrid complex generated singlet oxygen. The observed effects are of interest in regard to the targeted search for new components of efficient third-generation hybrid photosensitizers.

Yb,Nd,Er-doped upconversion nanoparticles: 980 nm versus 808 nm excitation

Yb,Nd,Er-doped upconversion nanoparticles (UCNPs) have attracted considerable interest as luminescent reporters for bioimaging, sensing, energy conversion/shaping, and anticounterfeiting due to their capability to convert multiple near-infrared (NIR) photons into shorter wavelength ultraviolet, visible or NIR luminescence by successive absorption of two or more NIR photons. This enables optical measurements in complex media with very little background and high penetration depths for bioimaging. The use of Nd³⁺ as substitute for the commonly employed sensitizer Yb³⁺ or in combination with Yb³⁺ shifts the excitation wavelength from about 980 nm, where the absorption of water can weaken upconversion luminescence, to about 800 nm, and laser-induced local overheating effects in cells, tissue, and live animal studies can be minimized. To systematically investigate the potential of Nd³⁺ doping, we assessed the performance of a set of similarly sized Yb³⁺,Nd³⁺,Er³⁺-doped core- and core-shell UCNPs of different particle architecture in water at broadly varied excitation power densities (P) with steady state and time-resolved fluorometry for excitation at 980 nm and 808 nm. As a measure for UCNPs performance, the P-dependent upconversion quantum yield (Φ_UC) and its saturation behavior were used as well as particle brightness (B_UC). Based upon spectroscopic measurements at both excitation wavelengths in water and in a lipid phantom and B_UC-based calculations of signal size at different penetration depths, conditions under which excitation at 808 nm is advantageous are derived and parameters for the further optimization of triple-doped UCNPs are given.

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.

Precise cell behaviors manipulation through light-responsive nano-regulators: recent advance and perspective

Nanotechnology-assisted spatiotemporal manipulation of biological events holds great promise in advancing the practice of precision medicine in healthcare systems. The progress in internal and/or external stimuli-responsive nanoplatforms for highly specific cellular regulations and theranostic controls offer potential clinical translations of the revolutionized nanomedicine. To successfully implement this new paradigm, the emerging light-responsive nanoregulators with unparalleled precise cell functions manipulation have gained intensive attention, providing UV-Vis light-triggered photocleavage or photoisomerization studies, as well as near-infrared (NIR) light-mediated deep-tissue applications for stimulating cellular signal cascades and treatment of mortal diseases. This review discusses current developments of light-activatable nanoplatforms for modulations of various cellular events including neuromodulations, stem cell monitoring, immunomanipulation, cancer therapy, and other biological target intervention. In summary, the propagation of light-controlled nanomedicine would place a bright prospect for future medicine.

Surface Modifications for Photon-Upconversion-Based Energy-Transfer Nanoprobes

An emerging class of inorganic optical reporters are near-infrared (NIR) excitable lanthanide-based upconversion nanoparticles (UCNPs) with multicolor emission and long luminescence lifetimes in the range of several hundred microseconds. For the design of chemical sensors and optical probes that reveal analyte-specific changes in their spectroscopic properties, these nanomaterials must be combined with sensitive indicator dyes that change their absorption and/or fluorescence properties selectively upon interaction with their target analyte, utilizing either resonance energy transfer (RET) processes or reabsorption-related inner filter effects. The rational development of UCNP-based nanoprobes for chemical sensing and imaging in a biological environment requires reliable methods for the surface functionalization of UCNPs, the analysis and quantification of surface groups, a high colloidal stability of UCNPs in aqueous media as well as the chemically stable attachment of the indicator molecules, and suitable instrumentation for the spectroscopic characterization of the energy-transfer systems and the derived nanosensors. These topics are highlighted in the following feature article, and examples of functionalized core-shell nanoprobes for the sensing of different biologically relevant analytes in aqueous environments will be presented. Special emphasis is placed on the intracellular sensing of pH.

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.

Förster Resonance Energy Transfer-Activated Processes in Smart Nanotheranostics Fabricated in a Sustainable Manner

Multilayer nanocarriers loaded with optically activated payloads are gaining increasing attention due to their anticipated crucial role for providing new mechanisms of energy transfers in the health-oriented applications, as well as for energy storage and environmental protection. The combination of careful selection of optical components for efficient Förster resonance energy transfer, and surface engineering of the nanocarriers, allowed us to synthesize and characterize novel theranostic nanosystems for diagnosis and therapy of deep-seated tumors. The cargo, constrained within the oil core of the nanocapsules, composed of NaYF₄ :Tm⁺³ , Yb⁺³ up-converting nanoparticles together with a second-generation porphyrin-based photosensitizing agent-Verteporfin, assured requisite diagnostic and therapeutic functions under near-IR laser excitation. The outer polyaminoacid shell of the nanocapsules was functionalized with a ligand-poly(l-glutamic acid) functionalized by PEG-ylated folic acid-to ensure both a "stealth" effect and active targeting towards human breast cancer cells. The preparation criteria of all nanocarrier building blocks meet the requirements for sustainable and green chemistry practices. The multifunctionality of the proposed nanocarriers is a consequence of both the surface-functionalized organic exterior part, which was accessible for selective accumulation in cancer cells, and the hydrophobic optically active interior, which shows phototoxicity upon irradiation within the first biological window.

Engineering Efficient Photon Upconversion in Semiconductor Heterostructures

Photon upconversion is a photophysical process in which two low-energy photons are converted into one high-energy photon. Photon upconversion has broad appeal for a range of applications from biomedical imaging and targeted drug release to solar energy harvesting. Current upconversion nanosystems, including lanthanide-doped nanocrystals and triplet-triplet annihilation molecules, have achieved upconversion quantum yields on the order of 10-30%. However, the performance of these materials is hampered by inherently narrow absorption cross sections and fixed energy levels originating in atomic, ionic, or molecular states. Semiconductors, on the other hand, have inherently wide absorption cross sections. Moreover, recent advances enable the synthesis of colloidal semiconductor nanoparticles with complex heterostructures that can control band alignments and tune optical properties. We synthesize and characterize a three-component heterostructure that successfully upconverts photons under continuous-wave illumination and solar-relevant photon fluxes. The heterostructure is composed of two cadmium selenide quantum dots (QDs), an absorber and emitter, spatially separated by a cadmium sulfide nanorod (NR). We demonstrate that the principles of semiconductor heterostructure engineering can be applied to engineer improved upconversion efficiency. We first eliminate electron trap states near the surface of the absorbing QD and then tailor the band gap of the NR such that charge carriers are funneled to the emitting QD. When combined, these two changes result in a 100-fold improvement in photon upconversion performance.

Perspective: lanthanide-doped upconverting nanoparticles

In this perspective, we aim to present an overview of some important physical and chemical aspects of lanthanide-doped upconverting nanoparticle research to be considered, from synthesis considerations to bioapplications. To this end, we have reviewed several practical considerations and prepared several straightforward recommendations toward improved cohesion in the field, based on observed trends over the last decade of research.

Critical Considerations on the Clinical Translation of Upconversion Nanoparticles (UCNPs): Recommendations from the European Upconversion Network (COST Action CM1403)

The unique photoluminescent properties of upconversion nanoparticles (UCNPs) have attracted worldwide research interest and inspired many bioanalytical applications. The anti-Stokes emission with long luminescence lifetimes, narrow and multiple absorption and emission bands, and excellent photostability enable background-free and multiplexed detection in deep tissues. So far, however, in vitro and in vivo applications of UCNPs are restricted to the laboratory use due to safety concerns. Possible harmful effects may originate from the chemical composition but also from the small size of UCNPs. Potential end users must rely on well-founded safety data. Thus, a risk to benefit assessment of the envisioned combined therapeutic and diagnostic ("theranostic") applications is fundamentally important to bridge the translational gap between laboratory and clinics. The COST Action CM1403 "The European Upconversion Network-From the Design of Photon-Upconverting Nanomaterials to Biomedical Applications" integrates research on UCNPs ranging from fundamental materials synthesis and research, detection instrumentation, biofunctionalization, and bioassay development to toxicity testing. Such an interdisciplinary approach is necessary for a better and safer theranostic use of UCNPs. Here, the status of nanotoxicity research on UCNPs is compared to other nanomaterials, and routes for the translation of UCNPs into clinical applications are delineated.

The role of Li+ in the upconversion emission enhancement of (YYbEr)2O3 nanoparticles

The mechanism of upconversion enhancement for Li+-doped materials is still contentious. Attempting to settle the debate, here the upconversion emission enhancement of (Y0.97-xYb0.02Er0.01Lix)2O3, x = 0.000-0.123, nanoparticles is studied. Li+ incorporation in the Y2O3 host lattice is achieved via co-precipitation and solid-state reaction routes. In contrast to numerous reports, elemental analysis reveals that the former method does not afford Li+-bearing nanoparticles. The solid-state reaction route accomplishes an effective Li+ doping, as witnessed by inductively coupled plasma atomic emission spectroscopy and X-ray photoelectron spectroscopy (XPS). Transmission electron microscopy and powder X-ray diffraction showed an increase in nanoparticle size with increasing Li+ concentration. Rietveld refinement of powder X-ray diffraction data shows that the cubic lattice parameter decreases with increasing Li+ content. The emission quantum yield increases tenfold with increasing Li+ content up to x = 0.123, reaching a maximal value of 0.04% at x = 0.031. XPS and infrared spectroscopy show that the carbonate groups increase with increasing Li+ content, thus not supporting the prevailing view that the upconversion luminescence enhancement observed upon Li+ nanoparticle's doping is due to the decrease of the number of quenching carbonate groups present. Rather, the particle size increment and the decrease in the lattice parameter of the host crystals are shown to be the prime sources of quantum yield enhancement.

Selective non-enzymatic total bilirubin detection in serum using europium complexes with different β-diketone-derived ligands as luminescence probes

Three europium(III) complexes, Eu(ectfd)₃ (Hectfd = 1-(9-ethyl-9H-carbazol-7-yl)-4,4,4-trifluorobutane-1,3-dione), Eu(tta)₃ (Htta = 4,4,4-trifluoro-1-(thiophen-2-yl)-butane-1,3-dione), and Eu(dbt)₃ (Hdbt = 2-(4',4',4'-trifluoro-1',3'-dioxobutyl)dibenzothiophene), were synthesized and employed to detect total bilirubin (BR) in blood-serum samples. UV-visible absorption and fluorescence (FL) spectroscopies were used to evaluate the selectivity of each europium (III) fluorescence probe to BR, which was shown to remarkably reduce the luminescence intensities of the europium(III) complexes at a wavelength of 612 nm. The luminescence intensity of each complex is linearly related to BR concentration. Eu(tta)₃ was shown to be the more-appropriate fluorescence probe for the sensitive and reliable detection of total BR in blood serum samples than either Eu(ectfd)₃ or Eu(dbt)₃. This observation can be ascribed to special σ-hole bonding between Htta and BR. In addition, the optimal pH test conditions for the detection of BR in human serum by the Eu(tta)₃ probe were determined. Sensitivity was shown to be dramatically affected by the pH of the medium. The experimental results reveal that pH 7.5 is optimal for this probe, which coincides with the pH of human serum. Furthermore, BR detection using the Eu(tta)₃ luminescence probe is simple, practical, and relatively free of interference from coexisting substances; it has a minimum detection limit (DL) of 68 nM and is a potential candidate for the routine assessment of total BR in serum samples. Graphical Abstract ᅟ.

More Than a Light Switch: Engineering Unconventional Fluorescent Configurations for Biological Sensing

Fluorescence is a powerful and sensitive tool in biological detection, used widely for cellular imaging and in vitro molecular diagnostics. Over time, three prominent conventions have emerged in the design of fluorescent biosensors: a sensor is ideally specific for its target, only one fluorescence signal turns on or off in response to the target, and each target requires its own sensor and signal combination. These are conventions but not requirements, and sensors that break with one or more of these conventions can offer new capabilities and advantages. Here, we review "unconventional" fluorescent sensor configurations based on fluorescent dyes, proteins, and nanomaterials such as quantum dots and metal nanoclusters. These configurations include multifluorophore Förster resonance energy transfer (FRET) networks, temporal multiplexing, photonic logic, and cross-reactive arrays or "noses". The more complex but carefully engineered biorecognition and fluorescence signaling modalities in unconventional designs are richer in information, afford greater multiplexing capacity, and are potentially better suited to the analysis of complex biological samples, interactions, processes, and diseases. We conclude with a short perspective on the future of unconventional fluorescent sensors and encourage researchers to imagine sensing beyond the metaphorical light bulb and light switch combination.

Upconversion nanoparticle bioconjugates characterized by capillary electrophoresis

Upconversion nanoparticles (UCNPs) are an emerging class of optical materials with high potential in bioimaging due to practically no background signal and high penetration depth. Their excellent optical properties and easy surface functionalization make them perfect for conjugation with targeting ligands. In this work, capillary electrophoretic (CE) method with laser-induced fluorescence detection was used to investigate the behavior of carboxyl-silica-coated UCNPs. Folic acid, targeting folate receptor overexpressed by wide variety of cancer cells, was used for illustrative purposes and assessed by CE under optimized conditions. Peptide-mediated bioconjugation of antibodies to UCNPs was also investigated. Despite the numerous advantages of CE, this is the first time that CE was employed for characterization of UCNPs and their bioconjugates. The separation conditions were optimized including the background electrolyte concentration and pH. The optimized electrolyte was 20 mM borate buffer with pH 8.