Upconversion Nanoparticle-Based Dot-Blot Immunoassay for Quantitative Biomarker Detection.

Dot-blot immunoassays are widely used for the user-friendly detection of clinical biomarkers. However, the majority of dot-blot assays have only limited sensitivity and are only used for qualitative or semiquantitative analysis. To overcome this limitation, we have employed labels based on photon-upconversion nanoparticles (UCNPs) that exhibit anti-Stokes luminescence and can be detected without optical background interference. First, the dot-blot immunoassay on a nitrocellulose membrane was optimized for the quantitative analysis of human serum albumin (HSA), resulting in a limit of detection (LOD) of 0.19 ng/mL and a signal-to-background ratio (S/B) of 722. Commercial quantum dots were used as a reference label, reaching the LOD of 4.32 ng/mL and the S/B of 3, clearly indicating the advantages of UCNPs. In addition, the potential of UCNP-based dot-blot for real sample analysis was confirmed by analyzing spiked urine samples, reaching the LOD of 0.24 ng/mL and recovery rates from 79 to 123%. Furthermore, we demonstrated the versatility and robustness of the assay by adapting it to the detection of two other clinically relevant biomarkers, prostate-specific antigen (PSA) and cardiac troponin (cTn), reaching the LODs in spiked serum of 9.4 pg/mL and 0.62 ng/mL for PSA and cTn, respectively. Finally, clinical samples of patients examined for prostate cancer were analyzed, achieving a strong correlation with the reference electrochemiluminescence immunoassay (recovery rates from 89 to 117%). The achieved results demonstrate that UCNPs are highly sensitive labels that enable the development of dot-blot immunoassays for quantitative analysis of low-abundance biomarkers.

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.

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.

Autumnal migration patterns of hoverflies (Diptera: Syrphidae): interannual variability in timing and sex ratio.

Background: The migration of hoverflies (Diptera: Syrphidae) is a well-known phenomenon, with growing interest due to the ecosystem services provided by migrants. However, we still lack fundamental data on species composition, timing of migration, or sex ratio of migrants. To address this gap, we focused on the southward autumnal migration of hoverflies through central Europe. Methods: To recognize migrating individuals from resident ones, we used a pair of one-side-blocked Malaise traps, exposed in a mountain pass in the Jeseníky mountains, Czech Republic, where a mass migration of hoverflies takes place annually. Traps were set for 4 years, from August to October. Results: In total, we recorded 31 species of migrating hoverflies. The timing of migration differed between the years, taking place from the beginning of September to the end of October. Differences in phenology were observed in the four most common migrant species, where larger species seemed to migrate earlier or at the same time compared to the smaller ones. The sex ratio was strongly asymmetrical in most common species Episyrphus balteatus, Eupeodes corollae, and Sphaerophoria scripta, and varied between years for each species. Weather conditions strongly influenced the migration intensity at ground-level: hoverflies migrate mainly during days with south wind, high temperature, high atmospheric pressure, and low precipitation.

Influence of Label and Solid Support on the Performance of Heterogeneous Immunoassays.

Conventional immunochemical methods used in clinical analysis are often not sensitive enough for early-stage diagnosis, resulting in the need for novel assay formats. Here, we provide a detailed comparison of the effect of different labels and solid supports on the performance of heterogeneous immunoassays. When comparing three types of streptavidin-modified labels─horseradish peroxidase, carboxyfluorescein, and photon-upconversion nanoparticles (UCNPs)─UCNPs led to the most sensitive and robust detection of the cancer biomarker prostate-specific antigen. Additionally, we compared the immunoassay formats based on conventional microtiter plates and magnetic microbeads (MBs). In both cases, the highest signal-to-background ratios and the lowest limits of detection (LODs) were obtained by using the UCNP labels. The MB-based upconversion-linked immunosorbent assay carried out with a preconcentration step provided the lowest LOD of 0.46 pg/mL in serum. The results demonstrate that the use of UCNPs and MBs can significantly improve the sensitivity and working range of heterogeneous immunoassays for biomarker detection.

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).

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.

Effect of Particle Size and Surface Chemistry of Photon-Upconversion Nanoparticles on Analog and Digital Immunoassays for Cardiac Troponin.

Sensitive immunoassays are required for troponin, a low-abundance cardiac biomarker in blood. In contrast to conventional (analog) assays that measure the integrated signal of thousands of molecules, digital assays are based on counting individual biomarker molecules. Photon-upconversion nanoparticles (UCNP) are an excellent nanomaterial for labeling and detecting single biomarker molecules because their unique anti-Stokes emission avoids optical interference, and single nanoparticles can be reliably distinguished from the background signal. Here, the effect of the surface architecture and size of UCNP labels on the performance of upconversion-linked immunosorbent assays (ULISA) is critically assessed. The size, brightness, and surface architecture of UCNP labels are more important for measuring low troponin concentrations in human plasma than changing from an analog to a digital detection mode. Both detection modes result approximately in the same assay sensitivity, reaching a limit of detection (LOD) of 10 pg mL-1 in plasma, which is in the range of troponin concentrations found in the blood of healthy individuals.

Flow Field Analysis Inside and at the Outlet of the Abrasive Head.

This paper focuses on the investigation of a multiphase flow of water, air, and abrasive particles inside and at the outlet of the abrasive head with the help of computational fluid dynamics calculations and measurements. A standard abrasive head with a water nozzle hole diameter of 0.33 mm (0.013") and an abrasive nozzle cylindrical hole diameter of 1.02 mm (0.04") were used for numerical modelling and practical testing. The computed tomography provided an exact 3D geometrical model of the cutting head that was used for the creation of the model. Velocity fields of abrasive particles at the outlet of the abrasive head were measured and analysed using particle tracking velocimetry and, consequently, compared with the calculated results. The calculation model took the distribution of the abrasive particle diameters with the help of the Rosin-Rammler function in intervals of diameters from 150 to 400 mm. In the present study, four levels of water pressure (105, 194, 302, 406 MPa) and four levels of abrasive mass flow rate (100, 200, 300, 400 kg/min) were combined. The values of water pressures and hydraulic powers measured at the abrasive head inlet were used as boundary conditions for numerical modelling. The hydraulic characteristics of the water jet were created from the measured and calculated data. The calculated pressure distribution in the cylindrical part of the abrasive nozzle was compared with studies by other authors. The details of the experiments and calculations are presented in this paper.

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.

Competitive upconversion-linked immunoassay using peptide mimetics for the detection of the mycotoxin zearalenone.

Due to increasing food safety standards, the analysis of mycotoxins has become essential in the food industry. In this work, we have developed a competitive upconversion-linked immunosorbent assay (ULISA) for the analysis of zearalenone (ZEA), one of the most frequently encountered mycotoxins in food worldwide. Instead of a toxin-conjugate conventionally used in competitive immunoassays, we designed a ZEA mimicking peptide extended by a biotin-linker and confirmed its excellent suitability to mimic ZEA by nuclear magnetic resonance (NMR) and surface plasmon resonance (SPR) analysis. Upconversion nanoparticles (UCNP, type NaYF4:Yb,Tm) served as background-free optical label for the detection of the peptide mimetic in the competitive ULISA. Streptavidin-conjugated UCNPs were prepared by click reaction using an alkyne-PEG-neridronate linker. The UCNP conjugate clearly outperformed conventional labels such as enzymes or fluorescent dyes. With a limit of detection of 20 pg mL-1 (63 pM), the competitive ULISA is well applicable to the detection of ZEA at the levels set by the European legislation. Moreover, the ULISA is specific for ZEA and its metabolites (α- and ß-zearalenol) without significant cross-reactivity with other related mycotoxins. We detected ZEA in spiked and naturally contaminated maize samples using liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) as a reference method to demonstrate food analysis in real samples.

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 ß-galactosidase and substrate 4-methylumbelliferyl ß-d-galactopyranoside are described. The fluorescent product 4-methylumbelliferone is detected simultaneously with the barcode.

Surface design of photon-upconversion nanoparticles for high-contrast immunocytochemistry.

Immunohistochemistry (IHC) and immunocytochemistry (ICC) are routinely employed for the microscopic identification and diagnosis of cancerous cells in histological tissues and cell cultures. The maximally attainable contrast of conventional histological staining techniques, however, is low. While the anti-Stokes emission of photon-upconversion nanoparticles (UCNP) can efficiently eliminate optical background interference, excluding non-specific interactions of the label with the histological sample is equally important for specific immunolabeling. To address both requirements, we have designed and characterized several UCNP-based nanoconjugates as labels for the highly specific detection of the cancer biomarker HER2 on various breast cancer cell lines. An optimized streptavidin-PEG-neridronate-UCNP conjugate provided an unsurpassed signal-to-background ratio of 319, which was 50-fold better than conventional fluorescent labeling under the same experimental conditions. In combination, the absence of optical interference and non-specific binding lays the foundation for computer-based data evaluation in digital pathology.

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.

Measurement of Sub-femtomolar Concentrations of Prostate-Specific Antigen through Single-Molecule Counting with an Upconversion-Linked Immunosorbent Assay.

Single-molecule (digital) immunoassays provide the ability to detect much lower protein concentrations than conventional immunoassays. As photon-upconversion nanoparticles (UCNPs) can be detected without optical background interference, they are excellent labels for so-called single-molecule upconversion-linked immunosorbent assays (ULISAs). We have introduced a UCNP label design based on streptavidin-PEG-neridronate and a two-step detection scheme involving a biotinylated antibody that efficiently reduces nonspecific binding on microtiter plates. In a microtiter plate immunoassay, individual sandwich immune complexes of the cancer marker prostate-specific antigen (PSA) are detected and counted by wide-field epiluminescence microscopy (digital readout). The digital detection is 16× more sensitive than the respective analogue readout and thus expands the limit of detection to the sub-femtomolar concentration range (LOD: 23 fg mL-1, 800 aM). The single molecule ULISA shows excellent correlation with an electrochemiluminescence reference method. Although the analogue readout can routinely measure PSA concentrations in human serum samples, very low concentrations have to be monitored after radical prostatectomy. Combining the digital and analogue readout covers a dynamic range of more than 3 orders of magnitude in a single experiment.

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.

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.

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

The performance of photon-upconversion nanoparticles (UCNPs) as background-free luminescent labels in bioanalytical applications strongly depends on the preparation of well-defined and water-dispersible nanoconjugates. We have exploited the separation power of agarose-gel electrophoresis to purify milligram amounts of homogeneous UCNPs covered with carboxylated silica, biotin, or streptavidin with recovery rates of 30 to 50%. Clusters containing discrete numbers of UCNPs were isolated from the gel and reanalyzed by agarose-gel electrophoresis, single-nanoparticle-upconversion microscopy, and additional complementary methods. The purified nanoconjugates improved conventional (analogue) bioaffinity assays and provided highly monodisperse conjugates for assays that rely on counting individual UCNPs (digital assays).

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.

Prussian Blue Nanoparticles as a Catalytic Label in a Sandwich Nanozyme-Linked Immunosorbent Assay.

Enzyme immunoassays are widely used for detection of analytes within various samples. However, enzymes as labels suffer several disadvantages such as high production cost and limited stability. Catalytic nanoparticles (nanozymes) can be used as an alternative label in immunoassays overcoming the inherent disadvantages of enzymes. Prussian blue nanoparticles (PBNPs) are nanozymes composed of the Fe4[Fe(CN)6]3-based coordination polymer. They reveal peroxidase-like activity and are capable of catalyzing the oxidation of colorless 3,3',5,5'-tetramethylbenzidine in the presence of H2O2 to form intensely blue product. Here, we introduce the method for conjugation of PBNPs with antibodies and their application in nanozyme-linked immunosorbent assay (NLISA). Sandwich NLISA for detection of human serum albumin in urine was developed with limit of detection (LOD) of 1.2 ng·mL-1 and working range up to 1 µg·mL-1. Furthermore, the microbial contamination of Salmonella Typhimurium in powdered milk was detected with LOD of 6 × 103 colony-forming units (cfu)·mL-1 and working range up to 106 cfu·mL-1. In both cases, a critical comparison with the same immunoassay but using native peroxidase as label was realized. The achieved results confirmed the suitability of PBNPs for universal and robust replacement of enzyme labels.