Photon-Upconverting Nanoparticles for Optical Encoding and Multiplexing of Cells, Biomolecules, and Microspheres

Direct Zeptomole Detection of RNA Biomarkers by Ultrabright Fluorescent Nanoparticles on Magnetic Beads

Nucleic acids are important biomarkers in cancer and viral diseases. However, their ultralow concentration in biological/clinical samples makes direct target detection challenging, because it leads to slow hybridization kinetics with the probe and its insufficient signal-to-noise ratio. Therefore, RNA target detection is done by molecular (target) amplification, notably by RT-PCR, which is a tedious multistep method that includes nucleic acid extraction and reverse transcription. Here, a direct method based on ultrabright dye-loaded polymeric nanoparticles in a sandwich-like hybridization assay with magnetic beads is reported. The ultrabright DNA-functionalized nanoparticle, equivalent to ≈10 000 strongly emissive rhodamine dyes, is hybridized with the magnetic bead to the RNA target, providing the signal amplification for the detection. This concept (magneto-fluorescent sandwich) enables high-throughput detection of DNA and RNA sequences of varied lengths from 48 to 1362 nt with the limit of detection down to 0.3 fm using a plate reader (15 zeptomoles), among the best reported for optical sandwich assays. Moreover, it allows semi-quantitative detection of SARS-CoV-2 viral RNA directly in clinical samples without a dedicated RNA extraction step. The developed technology, combining ultrabright nanoparticles with magnetic beads, addresses fundamental challenges in RNA detection; it is expected to accelerate molecular diagnostics of diseases.

Water-stable perovskite-silica nanocomposites for encoded microbeads construction and multiplexed detection

All-inorganic lead halide perovskite nanocrystals exhibiting bright luminescence have great potential as fluorescence elements for optical encoding. However, their limited stability in water hinders the application in biosensing. In this study, novel optical encoded microbeads based on CsPbX3 (X = Cl, Br) nanocrystals are developed and applied in bead-based suspension arrays for the first time. Through the in-situ crystallization of CsPbX3 nanocrystals within mesoporous silica nano-templates (MSNs), accompanied by mesopores collapse after sintering, CsPbX3@MSNs (X3M) nanocomposites with uniform morphology and stable fluorescence intensity in aqueous solutions for up to 50 days are obtained. By assembling X3M with microspheres to form a host-guest structure, an optical encoding microbead (MX3M) library is established by varying the X3M ratio, halide composition, and the size of host microspheres, which can be easily decoded under multi-channel flow cytometer. As a result, MX3M exhibits outstanding capacity for specific target capture and negligible nonspecific absorption performance in the multiplex nucleic acid detection of respiratory viruses, with a low limit of detection (10 copies/rxn). This result highlights the tremendous potential of MX3M encoded microbeads constructed based on CsPbX3 nanocrystals for multiplexed bioassays.

Molecularly Engineered Room-Temperature Phosphorescence for Biomedical Application: From the Visible toward Second Near-Infrared Window

Phosphorescence, characterized by luminescent lifetimes significantly longer than that of biological autofluorescence under ambient environment, is of great value for biomedical applications. Academic evidence of fluorescence imaging indicates that virtually all imaging metrics (sensitivity, resolution, and penetration depths) are improved when progressing into longer wavelength regions, especially the recently reported second near-infrared (NIR-II, 1000-1700 nm) window. Although the emission wavelength of probes does matter, it is not clear whether the guideline of "the longer the wavelength, the better the imaging effect" is still suitable for developing phosphorescent probes. For tissue-specific bioimaging, long-lived probes, even if they emit visible phosphorescence, enable accurate visualization of large deep tissues. For studies dealing with bioimaging of tiny biological architectures or dynamic physiopathological activities, the prerequisite is rigorous planning of long-wavelength phosphorescence, being aware of the cooperative contribution of long wavelengths and long lifetimes for improving the spatiotemporal resolution, penetration depth, and sensitivity of bioimaging. In this Review, emerging molecular engineering methods of room-temperature phosphorescence are discussed through the lens of photophysical mechanisms. We highlight the roles of phosphorescence with emission from visible to NIR-II windows toward bioapplications. To appreciate such advances, challenges and prospects in rapidly growing studies of room-temperature phosphorescence are described.

Communicating Supraparticles to Enable Perceptual, Information-Providing Matter

Materials are the fundament of the physical world, whereas information and its exchange are the centerpieces of the digital world. Their fruitful synergy offers countless opportunities for realizing desired digital transformation processes in the physical world of materials. Yet, to date, a perfect connection between these worlds is missing. From the perspective, this can be achieved by overcoming the paradigm of considering materials as passive objects and turning them into perceptual, information-providing matter. This matter is capable of communicating associated digitally stored information, for example, its origin, fate, and material type as well as its intactness on demand. Herein, the concept of realizing perceptual, information-providing matter by integrating customizable (sub-)micrometer-sized communicating supraparticles (CSPs) is presented. They are assembled from individual nanoparticulate and/or (macro)molecular building blocks with spectrally differentiable signals that are either robust or stimuli-susceptible. Their combination yields functional signal characteristics that provide an identification signature and one or multiple stimuli-recorder features. This enables CSPs to communicate associated digital information on the tagged material and its encountered stimuli histories upon signal readout anywhere across its life cycle. Ultimately, CSPs link the materials and digital worlds with numerous use cases thereof, in particular fostering the transition into an age of sustainability.

Composition Control in Molecular Cluster-Aggregates: A Toolbox for Optical Output Tunability via Energy Transfer Pathways

Composition control is a powerful tool for obtaining high-performance lanthanide (Ln) luminescent materials with adjustable optical outputs. This strategy is well-established for hierarchically structured nanoparticles, but it is rarely applied to molecular compounds due to the limited number of metal centers within a single unit. In this work, we present a series of molecular cluster-aggregates (MCAs) with an icosanuclear core {Ln2Eu2Tb16} (Ln = Ce, Pr, Nd, Sm, Gd, Dy, Ho, Er, Tm, and Yb) in which we explore composition control, akin to nanoparticles, to modulate the optical output. More specifically, we target to understand how the presence of a third LnIII doping ion would impact the well-known TbIII → EuIII energy transfer and the ratiometric optical thermometry performance based on the TbIII/EuIII pair. Photophysical properties at room and at varying temperatures were investigated. Based on experimental data and well-established intrinsic features, such as spin-orbit coupling strength and LnIII 4f energy levels' structure, we discuss the possible luminescent processes present in each MCA and provide insight into qualitative trends that can be rationally correlated throughout the series.

Recent Research Progress of 19 F Magnetic Resonance Imaging Probes: Principle, Design, and Their Application

Visualization of biomolecules, cells, and tissues, as well as metabolic processes in vivo is significant for studying the associated biological activities. Fluorine magnetic resonance imaging (19 F MRI) holds potential among various imaging technologies thanks to its negligible background signal and deep tissue penetration in vivo. To achieve detection on the targets with high resolution and accuracy, requirements of high-performance 19 F MRI probes are demanding. An ideal 19 F MRI probe is thought to have, first, fluorine tags with magnetically equivalent 19 F nuclei, second, high fluorine content, third, adequate fluorine nuclei mobility, as well as excellent water solubility or dispersity, but not limited to. This review summarizes the research progresses of 19 F MRI probes and mainly discusses the impacts of structures on in vitro and in vivo imaging performances. Additionally, the applications of 19 F MRI probes in ions sensing, molecular structures analysis, cells tracking, and in vivo diagnosis of disease lesions are also covered in this article. From authors' perspectives, this review is able to provide inspirations for relevant researchers on designing and synthesizing advanced 19 F MRI probes.

Orthogonal Combinatorial Raman Codes Enable Rapid High-Throughput-Out Library Screening of Cell-Targeting Ligands

High-throughput assays play an important role in the fields of drug discovery, genetic analysis, and clinical diagnostics. Although super-capacity coding strategies may facilitate labeling and detecting large numbers of targets in a single assay, practically, the constructed large-capacity codes have to be decoded with complicated procedures or are lack of survivability under the required reaction conditions. This challenge results in either inaccurate or insufficient decoding outputs. Here, we identified chemical-resistant Raman compounds to build a combinatorial coding system for the high-throughput screening of cell-targeting ligands from a focused 8-mer cyclic peptide library. The accurate in situ decoding results proved the signal, synthetic, and functional orthogonality for this Raman coding strategy. The orthogonal Raman codes allowed for a rapid identification of 63 positive hits at one time, evidencing a high-throughput-out capability in the screening process. We anticipate this orthogonal Raman coding strategy being generalized to enable efficient high-throughput-out screening of more useful ligands for cell targeting and drug discovery.

Biological Activities and Biocompatibility Properties of Eu(OH)3 and Tb(OH)3 Nanorods: Evaluation for Wound Healing Applications

Rare earth elements have shown promising results in both bio-imaging and therapy applications due to their superior magnetic, catalytic, and optical properties. In recent years, since lanthanide-based nanomaterials have effective results in wound healing, it has become necessary to investigate the different properties of these nanoparticles. The aim of this study is to investigate the antimicrobial, antibiofilm, and biocompability of Eu(OH)₃ and Tb(OH)₃ nanorods, which have a high potential by triggering angiogenesis and providing ROS activity, especially in wound healing. For this purpose, nanorods were obtained by the microwave-assisted synthesis method. Structural characterizations of Eu(OH)₃ and Tb(OH)₃ nanorods were performed by FT-IR, XRD, and TG-DTA methods, and morphological characterizations were performed by SEM-EDX. Microorganisms that are likely to be present in the wound environment were selected for the antimicrobial activities of the nanorods. The highest efficiency of nanorods with the disc diffusion method was shown against Pseudomonas aeruginosa ATCC 27,853 and Candida albicans ATCC 10,231 microorganisms. One of the problems frequently encountered in an infected wound environment is the formation of bacterial biofilm. Eu(OH)₃ nanorods inhibited 77.5 ± 0.43% and Tb(OH)₃ nanorods 76.16 ± 0.60% of Pseudomonas aeruginosa ATCC 27,853 biofilms. These results show promise for the development of biomaterials with superior properties by adding these nanorods to wound dressings that will be developed especially for wounds with microbial infection. Eu(OH)₃ nanorods are more toxic than Tb(OH)₃ nanorods on NCTC L929 cells. At concentrations of 500 µg/ml and above, both nanorods are toxic to cells.

Ratiometric upconversion luminescence nanoprobes from construction to sensing, imaging, and phototherapeutics

In terms of the combined advantages of upconversion luminescence (UCL) properties and dual-signal ratiometric outputs toward specific targets, the ratiometric UCL nanoprobes exhibit significant applications. This review summarizes and discusses the recent advances in ratiometric UCL nanoprobes, mainly including the construction of nanoprobe systems for sensing, imaging, and phototherapeutics. First, the construction strategies are introduced, involving different types of nanoprobe systems, construction methods, and ratiometric dual-signal modes. Then, the sensing applications are summarized, involving types of targets, sensing mechanisms, sensing targets, and naked-eye visual detection of UCL colors. Afterward, the phototherapeutic applications are discussed, including bio-toxicity, bio-distribution, biosensing, and bioimaging at the level of living cells and small animals, and biomedicine therapy. Particularly, each section is commented on by discussing the state-of-the-art relevant studies on ratiometric UCL nanoprobe systems. Moreover, the current status, challenges, and perspectives in the forthcoming studies are discussed. This review facilitates the exploration of functionally luminescent nanoprobes for excellent sensing, imaging, biomedicine, and multiple applications in significant fields.

Lanthanide-Doped Upconversion Nanoparticles: Exploring A Treasure Trove of NIR-Mediated Emerging Applications

Lanthanide-doped upconversion nanoparticles (UCNPs) possess the remarkable ability to convert multiple near-infrared (NIR) photons into higher energy ultraviolet-visible (UV-vis) photons, making them a prime candidate for several advanced applications within the realm of nanotechnology. Compared to traditional organic fluorophores and quantum dots (QDs), UCNPs possess narrower emission bands (fwhm of 10-50 nm), large anti-Stokes shifts, low toxicity, high chemical stability, and resistance to photobleaching and blinking. In addition, unlike UV-vis excitation, NIR excitation is nondestructive at lower power intensities and has high tissue penetration depths (up to 2 mm) with low autofluorescence and scattering. Together, these properties make UCNPs exceedingly favored for advanced bioanalytical and theranostic applications, where these systems have been well-explored. UCNPs are also well-suited for bioimaging, optically modulating chemistries, forensic science, and other state-of-the-art research applications. In this review, an up-to-date account of emerging applications in UCNP research, beyond bioanalytical and theranostics, are presented including optogenetics, super-resolution imaging, encoded barcodes, fingerprinting, NIR vision, UCNP-assisted photochemical manipulations, optical tweezers, 3D printing, lasing, NIR-II imaging, UCNP-molecule nanohybrids, and UCNP-based persistent luminescent nanocrystals.

Upconversion nanoparticles as an immunocomplexing agent for selective detection of caspases via sandwich-like supracomplexes

In this study, a nanoparticle-based sandwich-like immunoassay was designed in dispersion medium to precisely detect apoptosis over caspase antibodies in order to overcome the disadvantages of traditional apoptosis determination methods such as high cost, large sampling requirement, and appropriate laboratory and equipment conditions. For this purpose, a complementary particulate system including magnetic (MNPs) and upconversion silica (UC-SiNPs) nanoparticles while immobilizing antibodies (primary antibody to MNPs, secondary antibody to UC-SiNPs) were synthesized and characterized. Optimization and selectivity studies of the complex formed by primary antibody immobilized MNPs with standard caspase proteins were examined by the HPLC system. Within the scope of optimization studies, protein concentrations, optimal duration, and temperature parameters were evaluated. Optimal conditions were determined for pH, initial concentration, time, and temperature as 7.4, 5.6 μg/mL, 45 min, and room temperature, respectively. Furthermore, the adsorption of competitive proteins was investigated in selectivity studies as well. Moreover, the primary antibody immobilized MNPs were treated with standard caspase proteins under optimal conditions; subsequently, they were interacted with secondary antibody immobilized UC-SiNPs to demonstrate the supracomplex formation meanwhile zeta potential/size measurements and fluorescence emission spectrometry analyses were performed. As a result of these analyses, it was observed that the sandwich-like supracomplexes were successfully formed that significantly varied upconversion emission intensities of UC-SiNPs in dependence on the amounts of caspase proteins. Because this approach enabled a quantitative result, the nanoparticle-based sandwich-like immunoassay should be classified as an easy-to-handled, fast, and promising alternative to benchmark apoptosis assays.

Advanced techniques for performing photodynamic therapy in deep-seated tissues

Photodynamic therapy (PDT) is a promising localized cancer treatment modality. It has been used successfully to treat a range of dermatological conditions with comparable efficacy to conventional treatments. However, some drawbacks limit the clinical utility of PDT in treating deep-seated tumors. Notably, the penetration limitation of UV and visible light, commonly applied to activate photosensitizers, makes PDT incompetent in treating deep-seated tumors. Development in light delivery technologies, especially fiber optics, led to improved clinical strategies for accessing deep tissues for irradiation. However, PDT efficacy issues remained partly due to light penetration limitations. In this review, we first summarized the current PDT applications for deep-seated tumor treatment. Then, the most recent progress in advanced techniques to overcome the light penetration limitation in PDT, including using functional nanomaterials that can either self-illuminate or be activated by near-infrared (NIR) light and X-rays as transducers, and implantable light delivery devices were discussed. Finally, current challenges and future opportunities of these technologies were discussed, which we hope may inspire the development of more effective techniques to enhance PDT efficacy against deep-seated tumors.

Towards highly efficient NIR II response up-conversion phosphor enabled by long lifetimes of Er3

The second near-infrared (NIR II) response photon up-conversion (UC) materials show great application prospects in the fields of biology and optical communication. However, it is still an enormous challenge to obtain efficient NIR II response materials. Herein, we develop a series of Er³⁺ doped ternary sulfides phosphors with highly efficient UC emissions under 1532 nm irradiation. β-NaYS₂:Er³⁺ achieves a visible UC efficiency as high as 2.6%, along with high brightness, spectral stability of lights illumination and temperature. Such efficient UC is dominated by excited state absorption, accompanied by the advantage of long lifetimes (⁴I_9/2, 9.24 ms; ⁴I_13/2, 30.27 ms) of excited state levels of Er³⁺, instead of the well-recognized energy transfer UC between sensitizer and activator. NaYS₂:Er³⁺ phosphors are further developed for high-performance underwater communication and narrowband NIR photodetectors. Our findings suggest a novel approach for developing NIR II response UC materials, and simulate new applications, eg., simultaneous NIR and visible optical communication.

Rose Bengal-Modified Upconverting Nanoparticles: Synthesis, Characterization, and Biological Evaluation

High-quality upconverting NaYF₄:Yb³⁺,Er³⁺ nanoparticles (UCNPs; 26 nm in diameter) based on lanthanides were synthesized by a high-temperature coprecipitation method. The particles were modified by bisphosphonate-terminated poly(ethylene glycol) (PEG) and Rose Bengal (RB) photosensitizer. The particles were thoroughly characterized using transmission electron microscopy, dynamic light scattering, thermogravimetric analysis, FTIR, and X-ray photoelectron and upconversion luminescence spectroscopy in terms of morphology, hydrodynamic size, composition, and energy transfer to the photosensitizer. Moreover, the singlet oxygen generation from RB-containing UCNPs was investigated using 9,10-diphenylanthracene probe under 980 nm excitation. The cytotoxicity of UCNPs before and after conjugation with RB was evaluated on highly sensitive rat mesenchymal stem cells (rMSCs) and significant differences were found. Correspondingly, consi-derable variations in viability were revealed between the irradiated and non-irradiated rat glioma cell line (C6) exposed to RB-conjugated UCNPs. While the viability of rMSCs was not affected by the presence of UCNPs themselves, the cancer C6 cells were killed after the irradiation at 980 nm due to the reactive oxygen species (ROS) production, thus suggesting the potential of RB-conjugated PEG-modified UCNPs for applications in photodynamic therapy of cancer.

Chemical sensors based on periodic mesoporous organosilica @NaYF4:Ln3+ nanocomposites

Here, three unique organic-inorganic hybrid nanocomposite materials prepared by combining NaYF₄:Yb³⁺,Ln³⁺ (Ln³⁺ = Er³⁺, Tm³⁺, Ho³⁺) and periodic mesoporous organosilica (PMO) are proposed for both metal ion sensing and solvent sensing. The luminescence properties of the developed hybrid materials, PMO@NaYF₄:Yb³⁺,Ln³⁺, were studied in detail in the solid state and after dispersing in water. It is found that PMO@NaYF₄:Yb³⁺,Er³⁺ showed selective "turn on" luminescence for Hg²⁺ with the detection limit of 24.4 μM in an aqueous solution. Additionally, the above three materials showed different luminescence emission responses towards water and organic solvents. It is worth noting that all three PMO@NaYF₄:Yb³⁺,Ln³⁺ materials showed "turn on" luminescence towards alcohols. PMO@NaYF₄:Yb³⁺,Er³⁺ and PMO@NaYF₄:Yb³⁺,Ho³⁺ were selected and further developed into sensitive sensors for the detection of water in alcohols by taking advantage of their quenching behavior in water. The detection limit for sensing of water was determined to be 0.21%, 0.18% and 0.29%, corresponding to isopropanol (PMO@NaYF₄:Yb³⁺,Er³⁺), n-butanol (PMO@NaYF₄:Yb³⁺,Er³⁺) and ethanol (PMO@NaYF₄:Yb³⁺,Ho³⁺), respectively. The above results illustrate the potential of these hybrid materials for applications in environmental fields as well as in chemical industries.

Paramagnetic encoding of molecules

Contactless digital tags are increasingly penetrating into many areas of human activities. Digitalization of our environment requires an ever growing number of objects to be identified and tracked with machine-readable labels. Molecules offer immense potential to serve for this purpose, but our ability to write, read, and communicate molecular code with current technology remains limited. Here we show that magnetic patterns can be synthetically encoded into stable molecular scaffolds with paramagnetic lanthanide ions to write digital code into molecules and their mixtures. Owing to the directional character of magnetic susceptibility tensors, each sequence of lanthanides built into one molecule produces a unique magnetic outcome. Multiplexing of the encoded molecules provides a high number of codes that grows double-exponentially with the number of available paramagnetic ions. The codes are readable by nuclear magnetic resonance in the radiofrequency (RF) spectrum, analogously to the macroscopic technology of RF identification. A prototype molecular system capable of 16-bit (65,535 codes) encoding is presented. Future optimized systems can conceivably provide 64-bit (~10^19 codes) or higher encoding to cover the labelling needs in drug discovery, anti-counterfeiting and other areas.

Achieving Multicolor Upconversion Emissions without Changing Compositions

It is widely recognized that a proper way of adjusting fluorescence color is meaningful for pushing forward upconversion materials to be utilized in anti-counterfeiting, display and solid-state lightning applications. Traditional routes that apply different host materials and/or doping categories to adjust fluorescence color have shown large color region tunability yet have to rely on complex synthesis process accompanied with time and raw material consumption. In this work, in order to get a wide luminous color gamut without depending on reciprocating synthesis, we desinged and provided a high-sensitizer-concentration upconversion crystals, hexagonal NaLuF₄:Yb³⁺/Er³⁺ (50/2 mol%), whose red-to-green emission intensity ratio can be conveniently tuned from 2.69 to 4.96 by simply modulating excitation power densities. The promoted three-photon-population progress of red emission achieved by using an intensive excitation laser is considered to be responsible for the facile upconversion modulation. The results may provide new ideas for emission color control that based on external parameters in identical host and the greatly amplified excitation power-sensitivity of NaLuF₄:Yb³⁺/Er³⁺ (50/2 mol%) is highly potential for fluorescence anti-fake and colorful display applications.

Unparalleled selectivity and electronic structure of heterometallic [LnLn'Ln] molecules as 3-qubit quantum gates

Heterometallic lanthanide [LnLn'] coordination complexes that are accessible thermodynamically are very scarce because the metals of this series have very similar chemical behaviour. Trinuclear systems of this category have not been reported. A coordination chemistry scaffold has been shown to produce molecules of type [LnLn'Ln] of high purity, i.e. exhibiting high metal distribution ability, based on their differences in ionic radius. Through a detailed analysis of density functional theory (DFT) based calculations, we discern the energy contributions that lead to the unparalleled chemical selectivity of this molecular system. Some of the previously reported examples are compared here with the newly prepared member of this exotic list, [Er2Pr(LA)2(LB)2(py)(H2O)2](NO3) (1) (H2LA and H2LB are two β-diketone ligands). A magnetic analysis extracted from magnetization and calorimetry determinations identifies the necessary attributes for it to act as an addressable, conditional multiqubit spin-based quantum gate. Complementary ab initio calculations confirm the feasibility of these complexes as composite quantum gates, since they present well-isolated ground states with highly anisotropic and distinct g-tensors. The electronic structure of 1 has also been analyzed by EPR. Pulsed experiments have allowed the establishment of the quantum coherence of the transitions within the relevant spin states, as well as the feasibility of a coherent control of these states via nutation experiments.

Binary temporary photo-response of ZnSe:Mn/ZnS quantum dots for visible time-domain anti-counterfeiting

The development of multi-level anti-counterfeiting techniques is of great significance for economics and security issues, particularly the newly emerged temporal-domain techniques based on lifetime coding. However, the intricate reading methods required to obtain temporal-level information are inevitably cumbersome and expensive, which greatly limits the practical applications of these techniques. Herein, we report a novel, unclonable time-domain anti-counterfeiting strategy for the first time, which is achieved using photo-responsive ZnSe:Mn/ZnS quantum dots (QDs) with dynamic luminescence and can be authenticated by the naked eye. Through introducing electron traps and constructing cascade electron channels in the QDs, the binary temporary photo-response is tailored and manifested as distinctive response rates between the band-edge and Mn ⁴T₁-⁶A₁ transition emissions. Impressively, the generated photo-response is instantaneous, is capable of delayed recovery, and can be visibly detected under UV irradiation. The prospective use of colorless, nontoxic aqueous-phase ZnSe:Mn/ZnS QDs provides a new idea and important guidance for developing the next generation of multi-level anti-counterfeiting techniques without the need for complex time-gated decoding instrumentation.

Hybrid Theranostic Cubosomes for Efficient NIR-Induced Photodynamic Therapy

In recent years, lipid bicontinuous cubic liquid-crystalline nanoparticles known as cubosomes have been under investigation because of their favorable properties as drug nanocarriers useful for anticancer treatments. Herein, we present organic/inorganic hybrid, theranostic cubosomes stabilized in water with a shell of alternate layers of chitosan, single strand DNA (model genetic material for potential gene therapy), and folic acid-chitosan conjugate (the outmost layer), coencapsulating up-converting Er³⁺ and Yb³⁺ codoped NaYF₄ nanoparticles and daunorubicin. The latter acts as a chemotherapeutic drug of photosensitizing activity, while up-converting nanoparticles serve as energy harvester and diagnostic agent. Cellular uptake and NIR-induced photodynamic therapy were evaluated in vitro against human skin melanoma (MeWo) and ovarian (SKOV-3) cancer cells. Results evidenced the preferential uptake of the theranostic cubosomes in SKOV-3 cells in comparison to uptake in MeWo cells, and this effect was enhanced by the folic acid functionalization of the cubosomes surface. Nanocarriers coloaded with the hybrid fluorophores exhibited a superior NIR-induced photodynamic activity, also confirmed by the improved mitochondrial activity and the most affecting f-actin fibers of cytoskeleton. Similar results, but with higher photocytotoxicity, were detected when folic acid-functionalized cubosomes were incubated with SKOV-3 cells. Taken on the whole, these results prove these hybrid cubosomes are good candidates for the photodynamic treatment of tumor lesions.

What Digital Immunoassays Can Learn from Ambient Analyte Theory: A Perspective

Immunoassays are important tools for clinical diagnosis as well as environmental and food analysis because they enable highly sensitive and quantitative measurements of analyte concentrations. In the 1980s, Roger Ekins suggested to improve the sensitivity of immunoassays by employing microspot assays, which are carried out under ambient analyte conditions and do not change the bulk analyte concentration of a sample during a measurement. More recently, the measurement of single analyte molecules has additionally attracted wide research interest. Although the ability to detect a single analyte molecule is not synonymous with the highest analytical sensitivity, single-molecule detection makes new routes accessible to avoiding background noise. This perspective follows the development of solid-phase immunoassays from the design of label techniques to single-molecule (digital) assays against the backdrop of Ekins's fundamental work on immunoassay theory. The essential aspects of both ambient analyte and digital assay approaches are presented as a guideline to finding a balance between the speed, sensitivity, and precision of immunoassays.

Ultrafast synthesized monometallic nanohybrids as an efficient quencher and recognition antenna of upconversion nanoparticles for the detection of xanthine with enhanced sensitivity and selectivity

Upconversion nanoparticles (UCNPs) have shown great promise in bioanalytical applications owing to their excellent optical properties. Generally, most analytical applications are based on the fluorescence resonance energy transfer (FRET) principle to quench the fluorescence of UCNPs. However, each UCNP contains thousands of emission center ions, and most of them exceed the FRET critical distance, which hinders FRET efficiency and leads to a low signal-to-background ratio (SBR). Herein, a novel nanoprobe for the detection of Xanthine (XA) based on inner filter effects (IFE) and cascade signal amplification strategy was constructed by decorating UCNP with trypsin-chymotrypsin-stabilized gold nanoparticles-gold nanoclusters (Try-chy-AuNPs-AuNCs) monometallic nanohybrids. The Try-chy-AuNPs-AuNCs prepared by ultrafast (3 min) and green synthesis method have efficient upconversion fluorescence quenching ability (the quenching efficiency up to 90.9%), which can effectively improve the SBR of the probe, so as to improve the sensitivity. In addition, the Try-chy-AuNPs-AuNCs have a unique spatial structure, which can effectively prevent the interaction between large-size biothiol (glutathione) and the probe, thus improving its selectivity. Besides, combined with the excellent optical performance of UCNPs and cascaded signal amplification strategy, the sensitivity of the probe can be further improved. Under the optimized conditions, the linear response range of the probe was obtained from 0.05 to 50 μM, 0.06-80 μM and with the low detection limit of 22.6 nM and 26.3 nM for H₂O₂ and XA, respectively. Meanwhile, the developed method has been further applied to the detection of XA in human serum with satisfactory results.

PMVEMA-coated upconverting nanoparticles for upconversion-linked immunoassay of cardiac troponin

Surface engineering of upconverting nanoparticles (UCNPs) is crucial for their bioanalytical applications. Here, an antibody specific to cardiac troponin I (cTnI), an important biomarker for acute myocardial infection, was covalently immobilized on the surface of UCNPs to prepare a label for the detection of cTnI biomarker in an upconversion-linked immunoassay (ULISA). Core-shell UCNPs (NaYF₄:Yb,Tm@NaYF₄) were first coated with poly(methyl vinyl ether-alt-maleic acid) (PMVEMA) and then conjugated to antibodies. The morphology (size and uniformity), hydrodynamic diameter, chemical composition, and amount of coating on the of UCNPs, as well as their upconversion luminescence, colloidal stability, and leaching of Y³⁺ ions into the surrounding media, were determined. The developed ULISA allowed reaching a limit of detection (LOD) of 0.13 ng/ml and 0.25 ng/ml of cTnI in plasma and serum, respectively, which represents 12- and 2-fold improvement to conventional enzyme-linked immunosorbent based on the same immunoreagents.

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.

Photoswitch-Based Fluorescence Encoding of Microspheres in a Limited Spectral Window for Multiplexed Detection

Fluorescence barcoding with multicolor fluorophores is limited by spectral crowding. Herein, we propose a fluorescence encoding method in a single-color channel with photoswitches. The photochromic naphthopyran was used to mediate the fluorescence of polystyrene microspheres through resonance energy transfer. The initial fluorescence intensity (F₀) and the fluorescence after UV light activation (F/F₀) were combined to generate hundreds of 2-dimensional barcodes. The coding capacity was further expanded with the different chemical kinetics of the photoswitches. The photoswitch-based fluorescence barcodes were applied to simultaneously and selectively detect the DNA sequences of COVID-19 (with related mutations) as a proof-of-concept for real applications. The compatibility with the state-of-the-art fluorescence microscopes and simple encoding and decoding make the method very attractive for multiplexed and high-throughput analyses.

Near-infrared probes for luminescence lifetime imaging

Biomedical luminescence imaging in the near-infrared (NIR, 700-1700 nm) region has shown great potential in visualizing biological processes and pathological conditions at cellular and animal levels, owing to the reduced tissue absorption and scattering compared to light in the visible (400-700 nm) region. To overcome the background interference and signal attenuation during intensity-based luminescence imaging, lifetime imaging has demonstrated a reliable imaging modality complementary to intensity measurement. Several selective or environment-responsive probes have been successfully developed for luminescence lifetime imaging and multiplex detection. This review summarizes recent advances in the application of luminescence lifetime imaging at cellular and animal levels in NIR-I and NIR-II regions. Finally, the challenges and further directions of luminescence lifetime imaging are also discussed.

Unravelling the benefits of transition-metal-co-doping in lanthanide upconversion nanoparticles

In this review we provide an overview of the current knowledge on lanthanide upconversion materials co-doped with transition metals. We focus on how the co-dopants affect the host lattice and the energy transfer processes.

A tunable full-color lanthanide noncovalent polymer based on cucurbituril-mediated supramolecular dimerization

The construction of lanthanide multicolor luminescent materials with tunable photoluminescence properties has been developed as one of the increasingly significant topics and shown inventive applications in miscellaneous fields. However, fabricating such materials based on synergistically assembly-induced emission rather than simple blending of different fluorescent dyes together still remains a challenge. Herein, we report a europium-based noncovalent polymer with tunable full-color emission, which is constructed from the 2,6-pyridinedicarboxylic acid-bearing bromophenylpyridinium salt. This rationally designed bifunctional component can concurrently serve as a guest molecule and a chelating ligand to associate with cucurbit[8]uril and europium ions, thus leading to the formation of a trichromatic (red–green–blue, RGB) photoluminescent polypseudorotaxane-type noncovalent polymer in aqueous solution. Meanwhile, the full-color emission enclosed within the RGB color triangle could be readily produced by simply tuning the molar ratio of cucurbit[8]uril and europium ions. The lanthanide supramolecular polymer featuring tricolor emission, long lifetime, high photoluminescence efficiency and low cytotoxicity could be further applied in multicolor imaging in a cellular environment. These results provide a new and feasible strategy for the construction of full-color single lanthanide self-assembled nanoconstructs.

A lanthanide noncovalent polymer is constructed by integrating host–guest complexation and metal–ligand coordination, and can exhibit tunable trichromatic emission and multiple excited-state lifetimes under single wavelength excitation.

Rational Design of Surface-State Controlled Multicolor Cross-Linked Carbon Dots with Distinct Photoluminescence and Cellular Uptake Properties

We disclose for the first time a facile synthetic methodology for the preparation of multicolor carbon dots (CDs) from a single source barring any chromatographic separations. This was achieved via sequential intraparticle cross-linking of surface abundant carboxylic acid groups on the CDs synthesized from a precursor to control their photoluminescence (PL) spectra as well as affect their degree of cellular internalization in cancer cells. The change in PL spectra with sequential cross-linking was projected by theoretical density functional theory (DFT) studies and validated by multiple characterization tools such as X-ray photoelectron spectroscopy (XPS), PL spectroscopy, ninhydrin assay, etc. The variation in cellular internalization of these cross-linked CDs was demonstrated using inhibitor assays, confocal microscopy, and flow cytometry. We supplemented our findings with high-resolution dark-field imaging to visualize and confirm the colocalization of these CDs into distinct intracellular compartments. Finally, to prove the surface-state controlled PL mechanisms of these cross-linked CDs, we fabricated a triple-channel sensor array for the identification of different analytes including metal ions and biologically relevant proteins.

Resonance Energy Transfer to Track the Motion of Lanthanide Ions—What Drives the Intermixing in Core-Shell Upconverting Nanoparticles?

The imagination of clearly separated core-shell structures is already outdated by the fact, that the nanoparticle core-shell structures remain in terms of efficiency behind their respective bulk material due to intermixing between core and shell dopant ions. In order to optimize the photoluminescence of core-shell UCNP the intermixing should be as small as possible and therefore, key parameters of this process need to be identified. In the present work the Ln(III) ion migration in the host lattices NaYF₄ and NaGdF₄ was monitored. These investigations have been performed by laser spectroscopy with help of lanthanide resonance energy transfer (LRET) between Eu(III) as donor and Pr(III) or Nd(III) as acceptor. The LRET is evaluated based on the Förster theory. The findings corroborate the literature and point out the migration of ions in the host lattices. Based on the introduced LRET model, the acceptor concentration in the surrounding of one donor depends clearly on the design of the applied core-shell-shell nanoparticles. In general, thinner intermediate insulating shells lead to higher acceptor concentration, stronger quenching of the Eu(III) donor and subsequently stronger sensitization of the Pr(III) or the Nd(III) acceptors. The choice of the host lattice as well as of the synthesis temperature are parameters to be considered for the intermixing process.

A near-infrared light triggered fluormetric biosensor for sensitive detection of acetylcholinesterase activity based on NaErF4: 0.5 % Ho3+@NaYF4 upconversion nano-probe

Acetylcholinesterase (AChE), as an important neurotransmitter, is widely present in the peripheral and central nervous systems. The aberrant expression of AChE could cause diverse neurodegenerative diseases. Herein, we developed a facile and interference-free fluorimetric biosensing platform for highly sensitive AChE activity determination based on a NaErF₄: 0.5 % Ho³⁺@NaYF₄ nano-probe. This nano-probe exhibits a unique property of emitting bright monochromic red (650 nm) upconversion (UC) emission under multiband (~808, ~980, and ~1530 nm) near-infrared (NIR) excitations. The principle of this detection relies on the quenching of the strong monochromic red UC emission by oxidization products of 3,3',5,5'-tetramethylbenzidine generated through AChE-modulated cascade reactions. This system shows a great sensing performance with a detection limit (LOD) of 0.0019 mU mL^{- 1} for AChE, as well as good specificity and stability. Furthermore, we validated the potential of the nano-probe in biological samples by determination of AChE in whole blood with a LOD of 0.0027 mU mL^-1, indicating the potential application of our proposed platform for monitoring the progression of AChE-related disease.

Rare-earth nanoparticles induce depression, anxiety-like behavior, and memory impairment in mice

Rare-earth nanoparticles have been widely studied for disease diagnosis, in vivo optical imaging, biosensing, and drug delivery. However, the effects of rare-earth nanoparticles on a central nervous system remain unclear. Here, we report that the continuous exposure to rare-earth nanoparticles in mice can cause behavioral alterations including cognitive deficits, anxiety, and depression-like behavior. Using an open-field test and a morris water maze, we showed that long-term exposure to rare-earth nanoparticles may lead to significant depression, anxiety-like behavior, and memory impairment. The histopathological investigation on the neurotoxicological effects of nanoparticles indicated a significant decrease in cell viability after seven days' nanoparticle exposure. Western blotting analysis suggested that the changes of ATP-citrate lyase (ACLY) and O-linked N-acetylglucosamine transferase (OGT, a unique glycosyltransferase enzyme) played important roles in neurobehavioral disorders in mice. These findings provide a pathway to understand the cytotoxicity of rare-earth nanoparticles for medial applications and offer insights into the risk of these nanoparticles in biological systems.

Light-Regulated Natural Fluorescence of the PCC 6803@ZIF-8 Composite as an Encoded Microsphere for the Detection of Multiple Biomarkers

The use of color-encoded microspheres for a bead-based assay has attracted increasing attention for high-throughput multiplexed bioassays. A fluorescent PCC 6803@ZIF-8 composite was prepared as a bead-based assay platform by a self-assembled zeolitic imidazolate framework (ZIF-8) on the surface of inactivated PCC 6803 cells. The composite fluorescence owing to the presence of pigment proteins in PCC 6803 could be gradually bleached with the prolongation of the ultraviolet light irradiation time. The composites with different fluorescence intensities were therefore obtained as encoded microspheres for the multiplexed assay. ZIF-8 provides a stable, rigid shell and a large specific surface area for composites, which prevent the composites from breakage during use and storage, simplify the protein immobilization procedure, reduce non-specific adsorption, and enhance the detection sensitivity. The encoded composites were successfully used to detect multiple DNA insertion sequences of Mycobacterium tuberculosis. The presented strategy offers an innovative color-encoding method for high-throughput multiplexed bioassays without the need of using chemically synthesized fluorescent materials.

Upconversion luminescent nanomaterials: A promising new platform for food safety analysis

Foodborne diseases have become a significant threat to public health worldwide. Development of analytical techniques that enable fast and accurate detection of foodborne pathogens is significant for food science and safety research. Assays based on lanthanide (Ln) ion-doped upconversion nanoparticles (UCNPs) show up as a cutting edge platform in biomedical fields because of the superior physicochemical features of UCNPs, including negligible autofluorescence, large signal-to-noise ratio, minimum photodamage to biological samples, high penetration depth, and attractive optical and chemical features. In recent decades, this novel and promising technology has been gradually introduced to food safety research. Herein, we have reviewed the recent progress of Ln³⁺-doped UCNPs in food safety research with emphasis on the following aspects: 1) the upconversion mechanism and detection principles; 2) the history of UCNPs development in analytical chemistry; 3) the in-depth state-of-the-art synthesis strategies, including synthesis protocols for UCNPs, luminescence, structure, morphology, and surface engineering; 4) applications of UCNPs in foodborne pathogens detection, including mycotoxins, heavy metal ions, pesticide residue, antibiotics, estrogen residue, and pathogenic bacteria; and 5) the challenging and future perspectives of using UCNPs in food safety research. Considering the diversity and complexity of the foodborne harmful substances, developing novel detections and quantification techniques and the rigorous investigations about the effect of the harmful substances on human health should be accelerated.

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.

Manipulation of up-conversion emission in NaYF4 core@shell nanoparticles doped by Er3+, Tm3+, or Yb3+ ions by excitation wavelength-three ions-plenty of possibilities

Nanoparticles (NPs) based on host compound NaYF₄ with core@shell structures were synthesised by the precipitation reaction in high-boiling point octadecene/oleic acid solvent. Four laser wavelengths were used (808, 975, 1208, or 1532 nm) for excitation of the obtained NPs. The resulting emission and mechanisms responsible for spectroscopic properties were studied in detail. Depending on NP compositions, i.e. type of doping ion (Er³⁺, Tm³⁺, or Yb³⁺) or presence of dopants in the same or different phases, adjustable up-conversion (UC) could be obtained with emission peaks covering the visible to near-infrared range (475 to 1625 nm). The presented results demonstrated multifunctionality of the prepared NPs. NaYF₄:2%Tm³⁺@NaYF₄ NPs exhibited emission at 700 and 1450 nm under 808 nm laser excitation or 800 and 1625 nm emission under 1208 nm laser radiation, as a result of ground- and excited-state absorption processes (GSA and ESA, respectively). However, NaYF₄:5%Er³⁺,2%Tm³⁺@NaYF₄ NPs showed the most interesting properties, as they can convert all studied laser wavelengths due to the absorption of Tm³⁺ (808, 1208 nm) or Er³⁺ ions (808, 975, 1532 nm), revealing a photon avalanche process under 1208 nm laser excitation, as well as GSA and ESA at other excitation wavelengths. The NaYF₄:2%Tm³⁺@NaYF₄:5%Er³⁺ NPs revealed the resultant emission properties, as the dopant ions were separated within core and shell phases. The NaYF₄:18%Yb³⁺,2%Tm³⁺@NaYF₄ and NaYF₄:18%Yb³⁺,2%Tm³⁺@NaYF₄:5%Er³⁺ samples showed the brightest emission, around 800 nm, under 975 nm excitation, though other laser wavelengths allowed for observation of luminescence, as well, especially in NPs with Er³⁺ in the outer shell, capable of UC under 1532 nm. The presented results highlight the unique and universal properties of lanthanide ions for designing luminescent NPs for a variety of potential applications, such as confocal microscopy.

A mono-copper doped undeca-gold cluster with up-converted and anti-stokes emissions of fluorescence and phosphorescence

We have synthesized single crystals of a highly stable Cu-doped undeca-gold cluster protected by both triphenylphosphine (PPh3) and 2-pyridinethiol (-SPy) ligands, formulated as [Au11Cu1(PPh3)7(SPy)3]+. This cluster (Au11Cu1 NCs for short) has a metallic core of C3v Au@Au10 with the Cu atom capped on one of the nine triangular facets and it is triply-coordinated to three N atoms of the SPy ligands of which the sulfur atom simultaneously binds to three adjacent Au atoms via singly-coordinated S-Au bonds, respectively. The other seven gold atoms form a crown structure by a link of three orthogons with common sides and are protected by seven PPh3 ligands. Besides the well-organized coordination, this Au11Cu1 nanocluster is demonstrated to exhibit superatom stability of the metallic core within 8 valence electrons (assuming that the 3 electrophilic-SPy ligands capture 3 electrons from the metal center). More interestingly, this Au11Cu1 nanocluster shows interesting emissions in both ultraviolet visible (UV-Vis) and near infrared (NIR) regions, and the emissions display novel anti-Stokes up-conversion lasing characteristics. TD-DFT calculated UV-vis and emission spectra well reproduce the experimental results, shedding light on the nature of excitation states and underlying mechanism of electronic transitions between diverse energy levels of such a monolayer-protected bimetallic cluster.

Multiplexed optical barcoding of cells via photochemical programming of bioorthogonal host-guest recognition

Modern chemical and biological studies are undergoing a paradigm shift, where understanding the fate of individual cells, in an apparently homogeneous population, is becoming increasingly important. This has inculcated a growing demand for developing strategies that label individual cells with unique fluorescent signatures or barcodes so that their spatiotemporal trajectories can be mapped in real time. Among various approaches, light-regulated methods employing photocaged fluorophores have received particular attention, owing to their fine spatiotemporal control over labelling. However, their multiplexed use to barcode large numbers of cells for interrogating cellular libraries or complex tissues remains inherently challenging, due to the lack of multiple spectrally distinct photoactivated states in the currently available photocaged fluorophores. We report here an alternative multiplexable strategy based on optically controlled host-guest recognition in the cucurbit[7]uril (CB[7]) system that provides spatial control over the positioning of fluorophores to generate distinct barcodes in 'user-defined' cells. Using a combination of three spectrally distinct CB[7]-conjugated fluorophores and by sequentially performing cycles of photoactivation and fluorophore encoding, we demonstrate 10-color barcoding in microtubule-targeted fixed cells as well as 7-color barcoding in cell surface glycan targeted live MCF7 cells.

Cyanine-modified near-infrared upconversion nanoprobe for ratiometric sensing of N2H4 in living cells

Although being as an important chemical material in industry, hydrazine (N₂H₄) is highly toxic to the humans and animals. The development of sensitive methods for the detection of hydrazine is meaningful. Herein, we develop a new organic-inorganic hybrid nanoprobe for the detection of N₂H₄ based on luminescent resonance energy transfer (LRET) process. The nanoprobe contains N₂H₄-responsive NIR cyanine dye (CQM1) and α-cyclodextrin (CD) anchored on the surface of lanthanide-doped upconversion nanophosphors (UCNPs). In the presence of hydrazine, the hybrid materials (CQM1-UCNPs) showed the a large ratiometric luminescent signal change with high sensitivity and selectivity. More importantly, by taking advantage of ratiometric Upconversion luminescent (UCL) signal and the features of NIR emission/excitation, the nanoprobe was successfully applied for visualization of hydrazine in living cells for the first time.

Light-Emitting Diode Excitation for Upconversion Microscopy: A Quantitative Assessment

Lanthanide-based upconversion nanoparticles (UCNPs) generally require high power laser excitation. Here, we report wide-field upconversion microscopy at single-nanoparticle sensitivity using incoherent excitation of a 970 nm light-emitting diode (LED). We show that due to its broad emission spectrum, LED excitation is about 3 times less effective for UCNPs and generates high background compared to laser illumination. To counter this, we use time-gated luminescence detection to eliminate the residual background from the LED source, so that individual UCNPs with high sensitizer (Yb³⁺) doping and inert shell protection become clearly identified under LED excitation at 1.18 W cm^-2, as confirmed by correlated electron microscopy images. Hydrophilic UCNPs are obtained by polysaccharide coating via a facile ligand exchange protocol to demonstrate imaging of cellular uptake using LED excitation. These results suggest a viable approach to bypassing the limitations associated with high-power lasers when applying UCNPs and upconversion microscopy to life science research.

Multiplexed Nanobiosensors: Current Trends in Early Diagnostics

The ever-growing demand for fast, cheap, and reliable diagnostic tools for personalised medicine is encouraging scientists to improve existing technology platforms and to create new methods for the detection and quantification of biomarkers of clinical significance. Simultaneous detection of multiple analytes allows more accurate assessment of changes in biomarker expression and offers the possibility of disease diagnosis at the earliest stages. The concept of multiplexing, where multiple analytes can be detected in a single sample, can be tackled using several types of nanomaterial-based biosensors. Quantum dots are widely used photoluminescent nanoparticles and represent one of the most frequent choices for different multiplex systems. However, nanoparticles that incorporate gold, silver, and rare earth metals with their unique optical properties are an emerging perspective in the multiplexing field. In this review, we summarise progress in various nanoparticle applications for multiplexed biomarkers.

Photon-upconversion barcode for monitoring an enzymatic reaction with a fluorescence reporter in droplet microfluidics

We report luminescent photon-upconversion barcodes for indexing the chemical content of droplets. The barcode is compatible with the simultaneous detection of fluorescence. The encoding and decoding of the initial concentration of enzyme β-galactosidase and substrate 4-methylumbelliferyl β-d-galactopyranoside are described. The fluorescent product 4-methylumbelliferone is detected simultaneously with the barcode.

Metasurface Enhanced Sensitized Photon Upconversion: Toward Highly Efficient Low Power Upconversion Applications and Nanoscale E-Field Sensors

Large-scale nanoimprinted metasurfaces based on silicon photonic crystal slabs were produced and coated with a NaYF₄:Yb³⁺/Er³⁺ upconversion nanoparticle (UCNP) layer. UCNPs on these metasurfaces yield a more than 500-fold enhanced upconversion emission compared to UCNPs on planar surfaces. It is also demonstrated how the optical response of the UCNPs can be used to estimate the local field energy in the coating layer. Optical simulations using the finite element method validate the experimental results and the calculated spatial three-dimensional field energy distribution helps us to understand the emission enhancement mechanism of the UCNPs closely attached to the metasurface. In addition, we analyzed the spectral shifts of the resonances for uncoated and coated metasurfaces and metasurfaces submerged in water to enable a prediction of the optimum layer thicknesses for different excitation wavelengths, paving the way to applications such as electromagnetic field sensors or bioassays.