Large-Scale Purification of Photon-Upconversion Nanoparticles by Gel Electrophoresis for Analogue and Digital Bioassays

Spectrophotometric determination for green hydrophobic deep eutectic solvent-based microextraction of Brilliant Blue FCF (E133) from beverages

In this study, a simple, sensitive, and rapid method called green hydrophobic deep eutectic solvent-based liquid-liquid microextraction was developed to extract Brilliant Blue FCF dye from beverages. This method utilizes hydrophobic DES obtained by forming tetrabutylammonium bromide and 1-octanol in a 1:5 ratio as green extraction solvent. The transition of Brilliant Blue FCF to the DES phase occurred on its own, without the need for any reagents such as added salt or tetrahydrofuran. Several crucial factors were tried to get the best extraction efficiency, including species, DES volume and molar ratio, solution pH, ultrasonication, and centrifugation time. Under optimum conditions, extraction recoveries were achieved in the range of 95.1-101.3 % with the method developed for Brilliant Blue FCF. The detection and determination limits were observed to be 4.1 μg l-1 and 12.1 μg l-1, respectively. In addition, the relative standard deviation values for the method's accuracy were found to be 2.23 % and 3.48 % within and between days, respectively. It has been established that the developed method is highly environmentally friendly thanks to the application of the Analytical GREENness (AGREE) and Green Analytical Procedure Index (GAPI) tools. This study shows that DES applications can be carried out without the use of emulsifiers and dispersants by prioritizing the use of hydrophobic DES compounds as environmentally friendly and green extraction solvents in food samples.

Artificial Intelligence-Aided Massively Parallel Spectroscopy of Freely Diffusing Nanoscale Entities

Massively parallel spectroscopy (MPS) of many single nanoparticles in an aqueous dispersion is reported. As a model system, bioconjugated photon-upconversion nanoparticles (UCNPs) with a near-infrared excitation are prepared. The UCNPs are doped either with Tm3+ (emission 450 and 802 nm) or Er3+ (emission 554 and 660 nm). These UCNPs are conjugated to biotinylated bovine serum albumin (Tm3+-doped) or streptavidin (Er3+-doped). MPS is correlated with an ensemble spectra measurement, and the limit of detection (1.6 fmol L-1) and the linearity range (4.8 fmol L-1 to 40 pmol L-1) for bioconjugated UCNPs are estimated. MPS is used for observing the bioaffinity clustering of bioconjugated UCNPs. This observation is correlated with a native electrophoresis and bioaffinity assay on a microtiter plate. A competitive MPS bioaffinity assay for biotin is developed and characterized with a limit of detection of 6.6 nmol L-1. MPS from complex biological matrices (cell cultivation medium) is performed without increasing background. The compatibility with polydimethylsiloxane microfluidics is proven by recording MPS from a 30 μm deep microfluidic channel.

Revisiting carboxylic group functionalization of silica sol-gel materials

The carboxylic chemical group is a ubiquitous moiety present in amino acids, a ligand for transition metals, a colloidal stabilizer, and a weak acidic ion-exchanger in polymeric resins and given this property, it is attractive for responsive materials or nanopore-based gating applications. As the number of uses increases, subtle requirements are imposed on this molecular group when anchored to various platforms for the functioning of an integrated chemical system. In this context, silica stands as an inert and multipurpose platform that enables the anchoring of multiple chemical entities combined through several orthogonal synthesis methods on the interface. Surface chemical modification relies on the use of organoalkoxysilanes that must meet the demand of tuned chemical properties; this, in turn, urges for innovative approaches for having an improved, but simple, organic toolbox. Starting from commonly available molecular precursors, several approaches have emerged: hydrosilylation, click thiol-ene additions, the use of carbodiimides or the reaction between cyclic anhydrides and anchored amines. In this review, we analyze the importance of the COOH groups in the area of materials science and the commercial availability of COOH-based silanes and present new approaches for obtaining COOH-based organoalkoxide precursors. Undoubtedly, this will attract widespread interest for the ultimate design of highly integrated chemical platforms.

Absolute Counting Method with Multiplexing Capability for Estimating the Number Concentration of Nanoparticles Using Anisotropically Collapsed Gels

Number concentration─the number of nanoparticles in a given volume─is an important characteristic of any nanoparticle dispersion. However, its estimation for small nanoparticles (∼30 nm) is generally challenging. We introduce an absolute and widely applicable method for analyzing aqueous dispersions of nanoparticles. An innovative immobilization of nanomaterials in the anisotropically collapsed agarose gel is pioneered, followed by optical microscopy and nanoparticle counting. The number of counted nanoparticles is inherently coupled with sampled volume (517 pL) and translates to the number concentration. Photon-upconversion, fluorescence, bright-field, and dark-field microscopy techniques have been proven applicable and used for imaging lanthanide-doped photon-upconversion nanoparticles, their bioconjugates with antibodies, silica dye-doped fluorescent nanoparticles, quantum dots, and pure silica submicron particles. The precision and linearity were characterized by constructing a dilution series of photon-upconversion nanoparticles. The limit of detection was 2.0 × 10⁶ mL^-1, and the working range was from 4.4 × 10⁷ to 2.2 × 10¹⁰ mL^-1. The quantification of nanoparticle clusters was achieved by a thorough analysis of the micrographs. The accuracy was confirmed using gravimetric analysis and transmission electron microscopy as a reference. Multiplexed detection of two nanoparticle types in a mixed dispersion was feasibly demonstrated. The low thickness of the collapsed gel (<1 μm) supported extremely sensitive imaging. This was proven by imaging Tm³⁺-doped photon-upconversion nanoparticles (17 nm hydrodynamic diameter) with a nanoparticle emission rate of only ∼900 photons/s at a wavelength of 800 nm (excitation wavelength 976 nm).

Emerging Biosensing Technologies for the Diagnostics of Viral Infectious Diseases

Several viral infectious diseases appear limitless since the beginning of the 21^st century, expanding into pandemic lengths. Thus, there are extensive efforts to provide more efficient means of diagnosis, a better understanding of acquired immunity, and improved monitoring of inflammatory biomarkers, as these are all crucial for controlling the spread of infection while aiding in vaccine development and improving patient outcomes. In this regard, various biosensors have been developed recently to streamline pathogen and immune response detection by addressing the limitations of traditional methods, including isothermal amplification-based systems and lateral flow assays. This review explores state-of-the-art biosensors for detecting viral pathogens, serological assays, and inflammatory biomarkers from the material perspective, by discussing their advantages, limitations, and further potential regarding their analytical performance, clinical utility, and point-of-care adaptability. Additionally, next-generation biosensing technologies that offer better sensitivity and selectivity, and easy handling for end-users are highlighted. An emerging example of these next-generation biosensors are those powered by novel synthetic biology tools, such as clustered regularly interspaced short palindromic repeats (CRISPR) with CRISPR-associated proteins (Cas), in combination with integrated point-of-care devices. Lastly, the current challenges are discussed and a roadmap for furthering these advanced biosensing technologies to manage future pandemics is provided.

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.

Supersensitive photon upconversion based immunoassay for detection of cardiac troponin I in human plasma

BACKGROUND AND AIMS

Upconverting nanoparticles (UCNPs) are attractive reporters for immunoassays due to their excellent detectability. Assays sensitive enough to measure baseline level of cardiac troponin I cTnI in healthy population could be used to identify patients at risk for cardiovascular disease. Aiming for a cTnI assay of such sensitivity, the surface chemistry of the nanoparticles as well as the assay reagents and the protocol were optimized for monodispersity of the UCNP antibody conjugates (Mab UCNPs) and to minimize their non-specific interactions with the solid support.

MATERIALS AND METHODS

UCNPs were coated with poly(acrylic acid) via two-step ligand exchange and conjugated with monoclonal antibodies. The conjugates were applied in a microplate-based sandwich immunoassay using a combination of two capture antibodies to detect cTnI. Assay was evaluated according to guidelines of Clinical & Laboratory Standards Institute.

RESULTS

The limit of detection and limit of blank of the assay were 0.13 ng/L and 0.01 ng/L cTnI, respectively. The recoveries were >90% in spiked plasma in the linear range. The within- and between-run imprecisions were <10%.

CONCLUSION

The results demonstrate that UCNPs enable quantification of cTnI concentrations expected in plasma of healthy individuals and could be used to identify patients at risk for cardiovascular disease.

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.

Laser-induced breakdown spectroscopy as a straightforward bioimaging tool for plant biologists; the case study for assessment of photon-upconversion nanoparticles in Brassica oleracea L. plant

The main purpose of this work is to thoroughly describe the implementation protocol of laser-induced breakdown spectroscopy (LIBS) method in the plant analysis. Numerous feasibility studies and recent progress in instrumentation and trends in chemical analysis make LIBS an established method in plant bioimaging. In this work, we present an easy and straightforward phytotoxicity case study with a focus on LIBS method. We intend to demonstrate in detail how to manipulate with plants after exposures and how to prepare them for analyses. Moreover, we aim to achieve 2D maps of spatial element distribution with a good resolution without any loss of sensitivity. The benefits of rapid, low-cost bioimaging are highlighted. In this study, cabbage (Brassica oleracea L.) was treated with an aqueous dispersion of photon-upconversion nanoparticles (NaYF₄ doped with Yb³⁺ and Tm³⁺ coated with carboxylated silica shell) in a hydroponic short-term toxicity test. After a 72-hour plant exposure, several macroscopic toxicity end-points were monitored. The translocation of Y, Yb, and Tm across the whole plant was set by employing LIBS with a lateral resolution 100 µm. The LIBS maps of rare-earth elements in B.oleracea plant grown with 50 μg/mL nanoparticle-treated and ion-treated exposures showed the root as the main storage, while the transfer via stem into leaves was minimal. On the contrary, the LIBS maps of plants exposed to the 500 μg/mL nanoparticle-treated and ion-treated uncover slightly different trends, nanoparticles as well as ions were transferred through the stem into leaves. However, the main storage organ was a root as well.

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.

Atom transfer between precision nanoclusters and polydispersed nanoparticles: a facile route for monodisperse alloy nanoparticles and their superstructures

Reactions between atomically precise noble metal nanoclusters (NCs) have been studied widely in the recent past, but such processes between NCs and plasmonic nanoparticles (NPs) have not been explored earlier. For the first time, we demonstrate spontaneous reactions between an atomically precise NC, Au25(PET)18 (PET = 2-phenylethanethiol), and polydispersed silver NPs with an average diameter of 4 nm and protected with PET, resulting in alloy NPs under ambient conditions. These reactions were specific to the nature of the protecting ligands as no reaction was observed between the Au25(SBB)18 NC (SBB = 4-(tert-butyl)benzyl mercaptan) and the very same silver NPs. The mechanism involves an interparticle exchange of the metal and ligand species where the metal-ligand interface plays a vital role in controlling the reaction. The reaction proceeds through transient Au25-xAgx(PET)n alloy cluster intermediates as observed in time-dependent electrospray ionization mass spectrometry (ESI MS). High-resolution transmission electron microscopy (HRTEM) analysis of the resulting dispersion showed the transformation of polydispersed silver NPs into highly monodisperse gold-silver alloy NPs which assembled to form 2-dimensional superlattices. Using NPs of other average sizes (3 and 8 nm), we demonstrated that size plays an important role in the reactivity as observed in ESI MS and HRTEM.

Field-controlled magnetoelectric core-shell CoFe2O4@BaTiO3 nanoparticles as effective drug carriers and drug release in vitro

The targeted drug release at tumor cells while sparing normal cells is a huge challenge. Core-shell magnetoelectric (ME) nanoparticles have addressed this problem using shape-dependent magneto-electric attributes. The colloidally stable, core-shell cobalt ferrite@barium titanate (CFO@BTO) ME nanoparticles (NPs) used for in vitro study were synthesized using sonochemical method. The structural characteristics and core-shell morphology were analyzed by X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM) respectively. Further magnetic and exchange coupling between two phases of ME nanostructures were studied at room temperature. Colloidal stability was studied in different suspension solutions (Water, SBB, PBS, and DMEM) using dynamic light scattering. Subsequently, the synthesized nanoparticles were functionalized with anticancer drugs including doxorubicin and methotrexate up to 80% via (EDC) chemistry. In vitro cytotoxicity studies carried out on human hepatocellular carcinoma (HepG2) and human malignant melanoma (HT144), cells validated the magneto-electric property of CFO@BTO nano-carriers in the presence of external magnetic field (5 mT), with significantly enhanced cytotoxicity when compared to free drugs and without field replicates. The resulted IC₅₀ values ranging from 5.3-7.3 μg/ml compared to 30.1-43.1 μg/ml in the absence of a magnetic field also confirmed the involved physical attributes of magnetoelectric nanostructures. The fluorescent microscopy results also indicated the increased apoptosis in magnetic field-assisted samples. Finally, hemolysis assay indicated the suitability of CFO@BTO nano-carriers for intravenous applications at IC₅₀ concentration.

Helix Shape Power-Dependent Properties of Single Upconversion Nanoparticles

Nonblinking, nonbleaching, and superbright single upconversion nanoparticles have been recently discovered with nonlinear power-dependent properties and can be switchable under dual-beam excitations, which are ideal for super-resolution microscopy, single-molecule tracking, and digital assays. Here, we report that the brightness of Nd³⁺-Yb³⁺-Er³⁺-doped nanoparticles displays a pair of unusual double helix shapes as the function of power densities of 976 and 808 nm excitations. We systemically analyze the power-dependent emission spectra, lifetimes, and power-intensity double-log slopes of single upconversion nanoparticles, which reveal that the dynamic roles of Nd³⁺ ions in the tridoped nanosystem with underlining electron population pathways are power dependent. That is, at high power 808 nm excitation, Nd³⁺ ions can directly emit upconverted luminescence, with their conventional role of sensitization saturated in the Nd³ → Yb³⁺ → Er³⁺ energy transfer systems. Moreover, we confirm that the universal helix shape phenomena commonly exist in a set of eight batches of core-shell nanoparticles regardless of the doping concentrations of Nd³⁺, Yb³⁺, and Er³⁺ ions in the sensitization shell, migration shell, and active core, though the crossing nodes occur at different excitation power ranges. This study emphasizes the important role of power-dependent properties in both improving the upconversion emission efficiency and the design of nonlinear responsive probes for imaging and sensing.

Unravelling the Aptamer-Analyte Interaction Dynamics through Fluorescence Quenching in Graphene Quantum Dots (GQDs) Based Homogeneous Assays

Graphene quantum dots (GQDs) are used here as a biosensing platform for the recognition of the major food contaminant ochratoxin A (OTA), with a fluorescently labelled DNA aptamer (FAM OTA aptamer) functioning as the biorecognition element. The detection principle lies in the formation of noncovalent interactions between the FAM OTA aptamer and the GQD surface, and the consequent fluorescence quenching. The further change in the fluorescence signal, induced by the formation of the FAM OTA Aptamer/OTA conjugate during the detection step, could then be correlated to the presence and concentration of the target analyte. Upon tuning the concentration of GQDs, a switch in the biorecognition mechanism occurred. Specifically, while a lower GQD concentration (0.060 mg/mL) resulted in a restoration of the fluorescence intensity upon incubation with OTA, a higher GQD concentration (0.150 mg/mL) provided a further quenching of the final fluorescence intensity. Upon further calibration study, it was discovered that the latter mechanism provided a better option in terms of linearity of response, detection limit and selectivity.

Colocalized Particle Counting Platform for Zeptomole Level Multiplexed Quantification

For multiplexed detection, it is important yet challenging to simultaneously meet the requirement of sensitivity, throughput, and implementation convenience for practical applications. Using the detection of DNAs and miRNAs for illustration, we present a colocalized particle counting platform that can realize the separation-free multiplexed detection of 6 nucleic acid targets with a zeptomole sensitivity and a dynamic range of up to 5 orders of magnitude. The presence of target induces the formation of a sandwich nanostructure via hybridization; thus, there is an occurrence of colocalization of two microbeads with two different colors. The sequence specific coding is realized by an arbitrary combination of two fluorescence channels with different emitting colors. The platform presents robustness in detecting multiple nucleic acid targets with a minimal cross talk and matrix effect as well as the ability to distinguish the specific miRNA from members of the same family. The results of simultaneous detection of 3 miRNAs in 3 different cell lines present straight consistency with that of the standard qRT-PCR. This platform can be adapted to other multiplexing designs such as the "turn-off" mode, in which the proportion of colocalized microbeads is decreased due to the strand-displacement reaction initiated by the specific target. This separation-free platform offers the possibility to achieve the on-site multiplexed detection with compatibility to different experimental designs and extensibility to other signal sources for enumeration.

Aqueous biphasic systems formed by hydrophilic and hydrophobic deep eutectic solvents for the partitioning of dyes

Two kinds of hydrophobic deep eutectic solvents (DESs), including hexafluoroisopropanol (HFIP)-based DESs and polypropylene glycol 400 (PPG400)-based DESs, were synthesized to construct aqueous biphasic systems (ABSs) with choline chloride (ChCl)-based hydrophilic DESs for the first time, respectively. Three kinds of dyes with different hydrophobicity, including tartrazine, methylene blue and sudan Ⅲ, were chosen as the target analytes to evaluate the partitioning behaviors of the DES/DES ABSs. The effect of the types of hydrophilic DESs on dyes extraction were investigated and ChCl-glycol (ChCl-G) was selected as the phase-forming component. Then the partitioning of dyes in diverse DES/DES ABSs with different hydrophobicity was addressed by altering the carbon chain length of hydrogen bond accepter (HBA) in hydrophobic DES and changing the molar ratio of HBA: hydrogen bond donor (HBD) in hydrophilic DES. The results proved that in the ABSs of HFIP-based DES/DES, the sudan Ⅲ tended to migrate to the hydrophobic DES-rich phase with the increased carbon chain length of hydrophobic DES and was inclined to transfer to the hydrophilic DES-rich phase with the increasing proportion of G in ChCl-G. Afterwards, the methylene blue was chosen to explore the influence factors of the extraction process. And the results showed that the adjusting of pH value could achieve a complete opposite distribution of methylene blue in PPG400-based DES/DES ABSs. Besides, extraction of dyes in real samples were evaluated and recoveries of 92.3%-106.1% were achieved. Moreover, the analysis of mixed samples demonstrated that 88.64% of tartrazine and 92.63% of methylene blue were enriched into the hydrophilic phase, while nearly all of the sudan Ⅲ was moved into the hydrophobic phase. In addition, the phase-forming components could be reused according to the regeneration studies. Method validation proved the good precision, repeatability and stability of the established method. At last, the extraction mechanism was further investigated by dynamic light scattering (DLS) and transmission electron microscope (TEM). It turned out the formation of DES-dye aggregates might be responsible for the separation process. Above all, the results highlighted the possibility of the DES/DES ABSs as tunable systems for the partitioning of dyes with different hydrophobicity.

Photon-Upconversion Barcoding with Multiple Barcode Channels: Application for Droplet Microfluidics

Barcoding facilitates high-throughput analytical methods in complex matrixes with a reduced volume of sample, reagents, time, and cost. Because of orthogonality to fluorescence, photon-upconversion barcodes attracted considerable attention in recent years. We constructed an epiluminescence detector, which, for the first time, demonstrated the reading of photon-upconversion spectra from microdroplets in a microfluidic chip with frequency up to 10 Hz. Non-negative least-squares deconvolution enabled the reading of an unprecedented number of photon-upconversion barcode channels (six) from emission spectra (excitation 980 nm, emission 430-875 nm). The standard deviation of barcode reading from microdroplets was ∼1%. Described barcoding can be, for example, used for multiparameter titrations, multiplexed biological and chemical assays, optimizations on a microfluidic platform, and preparation of barcoded concentration gradients and libraries.

The effects of photon-upconversion nanoparticles on the growth of radish and duckweed: Bioaccumulation, imaging, and spectroscopic studies

In this study, radish (Raphanus sativus L.) and common duckweed (Lemna minor L.) were treated with an aqueous dispersion of carboxylated silica-coated photon-upconversion nanoparticles containing rare-earth elements (Y, Yb, and Er). The total concentration of rare earths and their bioaccumulation factors were determined in root, hypocotyl, and leaves of R. sativus after 72 h, and in L. minor fronds after 168 h. In R. sativus, translocation factors were determined as the ratio of rare earths concentration in hypocotyl versus root and in leaves versus hypocotyl. The lengths of the root and hypocotyl in R. sativus, as well as the frond area in L. minor, were monitored as toxicity endpoints. To distinguish rare earth bioaccumulation patterns, two-dimensional maps of elemental distribution in the whole R. sativus plant and L. minor fronds were obtained by laser-induced breakdown spectroscopy with a lateral resolution of 100 μm. Moreover, the bioaccumulation was inspected using a photon-upconversion laser microscanner. The results revealed that the tested nanoparticles became adsorbed onto L. minor fronds and R. sativus roots, as well as transferred from roots through the hypocotyl and into leaves of R. sativus. The bioaccumulation patterns and spatial distribution of rare earths in nanoparticle-treated plants therefore differed from those of the positive control. Overall, carboxylated silica-coated photon-upconversion nanoparticles are stable, can easily translocate from roots to leaves, and are expected to become adsorbed onto the plant surface. They are also significantly toxic to the tested plants at nominal concentrations of 100 and 1000 μg/mL.

Click-conjugated photon-upconversion nanoparticles in an immunoassay for honeybee pathogen Melissococcus plutonius

European foulbrood (EFB) is an infectious disease affecting honeybee larvae caused by the bacterium Melissococcus plutonius. The enzyme-linked immunosorbent assay (ELISA) is the gold standard for antibody-based bacteria detection, however, its sensitivity is not high enough to reveal early-stage EFB infection. Photon-upconversion nanoparticles (UCNPs) are lanthanide-doped nanomaterials that emit light of shorter wavelength under near-infrared (NIR) excitation and thus avoid optical background interference. After conjugation with specific biorecognition molecules, UCNPs can be used as ultrasensitive labels in immunoassays. Here, we introduce a method for conjugation of UCNPs with streptavidin based on copper-free click chemistry, which involves surface modification of UCNPs with alkyne-modified bovine serum albumin (BSA) that prevents the non-specific binding and provides reactive groups for conjugation with streptavidin-azide. To develop a sandwich upconversion-linked immunosorbent assay (ULISA) for M. plutonius detection, we have prepared a rabbit polyclonal anti-Melissococcus antibody. The specific capture of the bacteria was followed by binding of biotinylated antibody and UCNP-BSA-streptavidin conjugate for a highly sensitive upconversion readout. The assay yielded an LOD of 340 CFU mL-1 with a wide working range up to 109 CFU mL-1, which is 400 times better than the LOD of the conventional ELISA. The practical applicability of the ULISA was successfully demonstrated by detecting M. plutonius in spiked real samples of bees, larvae and bottom hive debris. These results show a great potential of the assay for early diagnosis of EFB, which can prevent uncontrolled spreading of the infection and losses of honeybee colonies.