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

Magnetic microbead-based upconversion immunoassay with laser-induced breakdown spectroscopy readout for the detection of prostate-specific antigen

Laser-induced breakdown spectroscopy (LIBS) is a promising technique for the readout of immunochemical assays utilizing indirect detection of labels (Tag-LIBS), typically based on nanoparticles. We have previously demonstrated that Tag-LIBS immunoassay employing yttrium-based photon-upconversion nanoparticles (UCNPs) can reach sensitivity similar to commonly used enzyme and fluorescence immunoassays. In this study, we report on further increasing the sensitivity of UCNP-based Tag-LIBS immunoassay by employing magnetic microbeads (MBs) as the solid phase in the determination of cancer biomarker prostate-specific antigen. Due to the possibility of analyte preconcentration, MBs enabled achieving a limit of detection (LOD) of 4.0 pg·mL-1, representing two orders of magnitude improvement compared with equivalent microtiter plate-based assay (LOD of 460 pg·mL-1). In addition, utilizing MBs opens up the possibility of an internal standardization of the LIBS readout by employing iron spectral lines, which improves the assay robustness by compensating for LIBS signal fluctuations and bead-bound immunocomplexes lost throughout the washing steps. Finally, the practical applicability of the technique was confirmed by the successful analysis of clinical samples, showing a strong correlation with the standard electrochemiluminescence immunoassay. Overall, MB-based Tag-LIBS was confirmed as a promising immunoassay approach, combining fast readout, multiplexing possibilities, and high sensitivity approaching upconversion luminescence scanning while avoiding the requirement of luminescence properties of labels.

Atomic-Level Defect Engineering in GeP Nanoflake Biosensors for Gastric Cancer Diagnosis

Defect engineering offers a promising approach to enhance the sensitivity of biosensing materials by creating abundant chemically active sites. Despite its potential, achieving precise control and modification of these defects remains a significant challenge. Herein, we propose atomic-level defect engineering in GeP two-dimensional (2D) layered materials, following precise in situ growing Au nanoparticles on the single defect active sites for the design of ultrasensitive biosensors. The GeP-based biosensor exhibits notable capabilities for miRNA detection with excellent chemical stability, sensitivity, selectivity, and an extremely low detection limit of 28.6 aM. When applied to clinical tissue samples from gastric cancer patients, the biosensor effectively quantified the miR378c biomarker, enabling accurate stage-specific monitoring. This research not only represents a crucial advancement in the field of biosensing materials through defect engineering but also provides a promising avenue for early cancer diagnosis, staging, and monitoring.

Ultrasensitive and Adjustable Nanothermometers Based on Er3+-Sensitized Core@Shell Nanoparticles for Use in the First Biological Window

In recent years, intensive research has focused on lanthanide-doped nanoparticles (NPs) used as noncontact temperature sensors, particularly in nanomedicine. These NPs must be capable of excitation and emission within biological windows, where biological materials usually show better transparency for radiation. In this article, we propose that NPs sensitized with Er3+ ions can be applied as temperature sensors in biological materials. We synthesized the NPs through a reaction in high-boiling solvents and confirmed their crystal structure and the formation of core@shell NPs by using X-ray diffraction, high-resolution transmission electron microscopy, and element distribution mapping within the NPs. NaErF4@NaYF4, NaYF4:12.5% Er3+, 2.5% Tm3+@NaYF4, NaYF4:7.5% Er3+@NaYF4, and NaYF4:12.5% Er3+, 2.5% Ho3+@NaYF4 exhibited intense upconversion (UC) emission under 1532 nm laser excitation detectable also in the whole human blood. We propose that this UC results from energy transfer between Er3+ ions and from Er3+ to Tm3+ or Ho3+ codopants. To determine the mechanism of UC, we measured the dependence of the emission band intensities on the laser power densities. Importantly, we also analyzed the temperature-dependent emission of the NPs within the 295-360 K range. Based on the collected emission spectra, we calculated the luminescence intensity ratios (LIRs) of the emission bands to assess their potential for optical temperature sensing. The temperature-sensing properties varied with the concentration of Er3+ ions and the presence of additional Tm3+ or Ho3+ codopants. Depending on the NP composition and the emission bands used for luminescence ratio calculations, the maximum relative temperature sensitivity ranged from 4.55%·K-1 to 1.12%·K-1, with temperature resolution between 0.05 and 2.53 K at room temperature. Finally, as proof of using NPs as temperature sensors in biomedicine, we successfully measured the temperature-dependent emission of NaYF4:7.5% Er3+@NaYF4 NPs dispersed in whole blood under 1532 nm excitation. We demonstrated that the ratio of Er3+ ion emission bands changes with temperature, indicating that these NPs have potential applications in temperature sensing within biological environments. We also confirmed the properties of NPs as temperature sensors by measuring the temperature reading uncertainty and the repeatability of the LIR readings during heating-cooling cycles, thereby confirming the excellent properties of the studied systems as temperature sensors.

Ferroelectric Bi2O2Te-Based Plasmonic Biosensor for Ultrasensitive Biomolecular Detection

Ultrasensitive detection of biomarkers, particularly proteins, and microRNA, is critical for disease early diagnosis. Although surface plasmon resonance biosensors offer label-free, real-time detection, it is challenging to detect biomolecules at low concentrations that only induce a minor mass or refractive index change on the analyte molecules. Here an ultrasensitive plasmonic biosensor strategy is reported by utilizing the ferroelectric properties of Bi2O2Te as a sensitive-layer material. The polarization alteration of ferroelectric Bi2O2Te produces a significant plasmonic biosensing response, enabling the detection of charged biomolecules even at ultralow concentrations. An extraordinary ultralow detection limit of 1 fm is achieved for protein molecules and an unprecedented 0.1 fm for miRNA molecules, demonstrating exceptional specificity. The finding opens a promising avenue for the integration of 2D ferroelectric materials into plasmonic biosensors, with potential applications spanning a wide range.

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.

Molecularly imprinted composite-based biosensor for the determination of SARS-CoV-2 nucleocapsid protein

This article aims to present a comparative study of three polypyrrole-based molecularly imprinted polymer (MIP) systems for the detection of the recombinant severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nucleocapsid protein (rN). The rN is known for its relatively low propensity to mutate compared to other SARS-CoV-2 antigens. The aforementioned systems include screen-printed carbon electrodes (SPCE) modified with gold nanostructures (MIP1), platinum nanostructures (MIP2), and the unmodified SPCE (MIP3), which was used for control. Pulsed amperometric detection (PAD) was employed as the detection technique, offering the advantage of label-free detection without the need for an additional redox probe. Calibration curves were constructed using the obtained data to evaluate the response of each system. Non-imprinted systems were also tested in parallel to evaluate the contribution of non-specific binding and assess the affinity sensor's efficiency. The analysis of calibration curves revealed that the AuNS-based MIP1 system exhibited the lowest contribution of non-specific binding and displayed a better fit with the chosen fitting model compared to the other systems. Further analysis of this system included determining the limit of detection (LOD) (51.2 ± 2.8 pg/mL), the limit of quantification (LOQ) (153.9 ± 8.3 pg/mL), and a specificity test using a recombinant receptor-binding domain of SARS-CoV-2 spike protein as a control. Based on the results, the AuNS-based MIP1 system demonstrated high specificity and sensitivity for the label-free detection of SARS-CoV-2 nucleocapsid protein. The utilization of PAD without the need for additional redox probes makes this sensing system convenient and valuable for rapid and accurate virus detection.

Development of Detection Antibody Targeting the Linear Epitope in SARS-CoV-2 Nucleocapsid Protein with Ultra-High Sensitivity

The COVID-19 pandemic caused by SARS-CoV-2 highlighted the importance of reliable detection methods for disease control and surveillance. Optimizing detection antibodies by rational screening antigens would improve the sensitivity and specificity of antibody-based detection methods such as colloidal gold immunochromatography. In this study, we screened three peptide antigens with conserved sequences in the N protein of SARS-CoV-2 using bioinformatical and structural biological analyses. Antibodies that specifically recognize these peptides were prepared. The epitope of the peptide that had the highest binding affinity with its antibody was located on the surface of the N protein, which was favorable for antibody binding. Using the optimal antibody that can recognize this epitope, we developed colloidal gold immunochromatography, which can detect the N protein at 10 pg/mL. Importantly, this antibody could effectively recognize both the natural peptide antigen and mutated peptide antigen in the N protein, showing the feasibility of being applied in the large-scale population testing of SARS-CoV-2. Our study provides a platform with reference significance for the rational screening of detection antibodies with high sensitivity, specificity, and reliability for SARS-CoV-2 and other pathogens.

Sandwich Immuno-RCA Assay with Single Molecule Counting Readout: The Importance of Biointerface Design

The analysis of low-abundance protein molecules in human serum is reported based on counting of the individual affinity-captured analyte on a solid sensor surface, yielding a readout format similar to digital assays. In this approach, a sandwich immunoassay with rolling circle amplification (RCA) is used for single molecule detection (SMD) through associating the target analyte with spatially distinct bright spots observed by fluorescence microscopy. The unspecific interaction of the target analyte and other immunoassay constituents with the sensor surface is of particular interest in this work, as it ultimately limits the performance of this assay. It is minimized by the design of the respective biointerface and thiol self-assembled monolayer with oligoethylene (OEG) head groups, and a poly[oligo(ethylene glycol) methacrylate] (pHOEGMA) antifouling polymer brush was used for the immobilization of the capture antibody (cAb) on the sensor surface. The assay relying on fluorescent postlabeling of long single-stranded DNA that are grafted from the detection antibody (dAb) by RCA was established with the help of combined surface plasmon resonance and surface plasmon-enhanced fluorescence monitoring of reaction kinetics. These techniques were employed for in situ measurements of conjugating of cAb to the sensor surface, tagging of short single-stranded DNA to dAb, affinity capture of the target analyte from the analyzed liquid sample, and the fluorescence readout of the RCA product. Through mitigation of adsorption of nontarget molecules on the sensor surface by tailoring of the antifouling biointerface, optimizing conjugation chemistry, and by implementing weak Coulombic repelling between dAb and the sensor surface, the limit of detection (LOD) of the assay was substantially improved. For the chosen interleukin-6 biomarker, SMD assay with LOD at a concentration of 4.3 fM was achieved for model (spiked) samples, and validation of the ability of detection of standard human serum samples is demonstrated.

Signal Amplification and Near-Infrared Translation of Enzymatic Reactions by Nanosensors

Enzymatic reactions are used to detect analytes in a range of biochemical methods. To measure the presence of an analyte, the enzyme is conjugated to a recognition unit and converts a substrate into a (colored) product that is detectable by visible (VIS) light. Thus, the lowest enzymatic turnover that can be detected sets a limit on sensitivity. Here, we report that substrates and products of horseradish peroxidase (HRP) and β-galactosidase change the near-infrared (NIR) fluorescence of (bio)polymer modified single-walled carbon nanotubes (SWCNTs). They translate a VIS signal into a beneficial NIR signal. Moreover, the affinity of the nanosensors leads to a higher effective local concentration of the reactants. This causes a non-linear sensor-based signal amplification and translation (SENSAT). We find signal enhancement up to ≈120x for the HRP substrate p-phenylenediamine (PPD), which means that reactions below the limit of detection in the VIS can be followed in the NIR (≈1000 nm). The approach is also applicable to other substrates such as 3,3'-5,5'-tetramethylbenzidine (TMB). An adsorption-based theoretical model fits the observed signals and corroborates the sensor-based enhancement mechanism. This approach can be used to amplify signals, translate them into the NIR and increase sensitivity of biochemical assays.

The use of energy looping between Tm3+ and Er3+ ions to obtain an intense upconversion under the 1208 nm radiation and its use in temperature sensing

The upconversion phenomenon allows for the emission of nanoparticles (NPs) under excitation with near-infrared (NIR) light. Such property is demanded in biology and medicine to detect or treat diseases such as tumours. The transparency of biological systems for NIR light is limited to three spectral ranges, called biological windows. However, the most frequently used excitation laser to obtain upconversion is out of these ranges, with a wavelength of around 975 nm. In this article, we show an alternative - Tm3+/Er3+-doped NPs that can convert 1208 nm excitation radiation, which is in the range of the 2nd biological window, to visible light within the 1st biological window. The spectroscopic properties of the core@shell NaYF4:Tm3+@NaYF4 and NaYF4:Er3+,Tm3+@NaYF4 NPs revealed a complex mechanism responsible for the observed upconversion. To explain emission in the studied NPs, we propose an energy looping mechanism: a sequence of ground state absorption, energy transfers and cross-relaxation (CR) processes between Tm3+ ions. Next, the excited Tm3+ ions transfer the absorbed energy to Er3+ ions, which results in green, red and NIR emission at 526, 546, 660, 698, 802 and 982 nm. The ratio between these bands is temperature-dependent and can be used in remote optical thermometers with high relative temperature sensitivity, up to 2.37%/°C at 57 °C. The excitation and emission properties of the studied NPs fall within 1st and 2nd biological windows, making them promising candidates for studies in biological systems.

A nanogap-enhanced SERS nanotag-based lateral flow assay for ultrasensitive and simultaneous monitoring of SARS-CoV-2 S and NP antigens

A lateral flow assay (LFA) strip based on dual 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB)-encoded satellite Fe3O4@Au (Mag@Au) SERS tags with nanogap is reported for  ultrasensitive and simultaneous diagnosis of two SARS-CoV-2 functional proteins. Composed of Fe3O4 core, satellite gold shell with nanogaps, and double-layer DTNB, the Mag@Au nanoparticles with an average size of 238 nm were designed as multifunctional tags to efficiently enrich the target SARS-CoV-2 protein from complex samples, significantly enhancing the SERS signal of the LFA strip and provide quantitative SERS detection of analyte on test lines. The developed dual DTNB-encoded satellite Mag@Au-based LFA allowed simultaneous quantification of spike (S) protein and nucleocapsid (NP) protein with detection limits of 23 pg mL-1 and 2 pg mL-1, respectively, lower than commercial ELISA kits and reported SERS-LFA detection system-based Au NPs and Fe3O4@3 nm Au MNPs. This magnetic SERS-LFA also showed high performance of multi-variant strain detection and further distinguished clinical samples of Omicron variant infection, demonstrating the potential of in situ detection of respiratory virus diseases.

Influence of surface modification and size of lanthanide-doped upconverting nanoparticles on wheat seedlings

In recent years, nanotechnology has found widespread applications in environmental monitoring, medical applications, plant fertilisers, cosmetics and others. Therefore, it is important to study nanomaterials' influence and subsequent risks to the environment and organisms (from production to disposal). Therefore, in the present study, the toxic effects of two surface modifications (poly (ethylene glycol)-neridronate, PEG-Ner and poly (acrylic acid), PAA) in comparison to unmodified, 26 nm- and 52 nm-sized core@shell lanthanide-doped upconverting nanoparticles (UCNPs, NaYF4:Yb3+,Er3+@NaYF4) were analysed. Wheat seedlings (Triticum aestivum L.) were chosen as a model organism since this species is one of the most widely cultivated crops. The influence of UCNPs (at concentrations of 0, 10, 50, and 100 μg/mL) on germination percentage, germination rate and growth was studied based on morphological parameters such as root number, root and hypocotyl length, and root and hypocotyl mass. In addition, an assay based on Evans blue staining was conducted to analyse damaged cell membranes and cell death. The type, size and concentration of UCNPs influenced the growth but not the germination of wheat. 52-nm-sized ligand-free UCNPs and the 26-nm-sized UCNPs/PAA decreased plant growth. Moreover, the ligand-free 26-nm-sized UCNPs interacted with the root cell membranes of seedlings. No significant changes were observable regarding viability (tetrazolium chloride reduction assay), oxidative stress and electrolyte leakage from root cells in plants incubated with ligand-free 26-nm-sized UCNPs. Overall, we have shown that the ligand-free UCNPs (of both sizes) had the strongest toxic effect; PAA-modified UCNPs were toxic only at smaller sizes and PEG-Ner-modified UCNPs had no toxic impact. Therefore, PEG-Ner was identified as the safest surface compound among the UCNPs investigated in the study, which may neutralise the harmful effects of nanoparticles on plants.

Nanoparticle-Based Bioaffinity Assays: From the Research Laboratory to the Market

Advances in the development of new biorecognition elements, nanoparticle-based labels as well as instrumentation have inspired the design of new bioaffinity assays. This review critically discusses the potential of nanoparticles to replace current enzymatic or molecular labels in immunoassays and other bioaffinity assays. Successful implementations of nanoparticles in commercial assays and the need for rapid tests incorporating nanoparticles in different roles such as capture support, signal generation elements, and signal amplification systems are highlighted. The limited number of nanoparticles applied in current commercial assays can be explained by challenges associated with the analysis of real samples (e.g., blood, urine, or nasal swabs) that are difficult to resolve, particularly if the same performance can be achieved more easily by conventional labels. Lateral flow assays that are based on the visual detection of the red-colored line formed by colloidal gold are a notable exception, exemplified by SARS-CoV-2 rapid antigen tests that have moved from initial laboratory testing to widespread market adaption in less than two years.

Designing photon upconversion nanoparticles capable of intense emission in whole human blood

The properties of upconverting nanoparticles (UCNPs) are crucial for their applications in biomedicine. For studies of organisms and biological materials, the penetration depth of excitation light is also essential as the depth from which the luminescence can be detected. Currently, many researchers are trying to obtain UCNPs with intense emission under excitation wavelengths from the biological transparency windows to increase the penetration depth. However, studies comparing the properties of various types of UCNPs in real conditions are rare. This article shows how deep the 808, 975, 1208, and 1532 nm laser radiation penetrates human blood. Moreover, we determined how thick a layer of blood still permits for observation of the luminescence signal. The measured luminescence properties indicated that the near-infrared light could pass through the blood even to a depth of 7.5 mm. The determined properties of core/shell NaErF4/NaYF4 materials are the most advantageous, and their emission is detectable through 3.0 mm of blood layer using a 1532 nm laser. We prove that the NaErF4/NaYF4 UCNPs can be perfect alternatives for the most studied NaYF4:Yb3+,Er3+/NaYF4. Additionally, the setup proposed in this article can potentially decrease reliance on animal testing in initial biomedicine research.

Bright photon upconversion in LiYbF4:Tm3+@LiYF4 nanoparticles and their application for singlet oxygen generation and in immunoassay for SARS-CoV-2 nucleoprotein

Photon upconversion is an intensively investigated phenomenon in the materials sciences due to its unique applications, mainly in biomedicine for disease prevention and treatment. This study reports the synthesis and properties of tetragonal LiYbF4:Tm3+@LiYF4 core@shell nanoparticles (NPs) and their applications. The NPs had sizes ranging from 18.5 to 23.7 nm. As a result of the energy transfer between Yb3+ and Tm3+ ions, the synthesized NPs show intense emission in the ultraviolet (UV) range up to 347 nm under 975 nm excitation. The bright emission in the UV range allows for singlet oxygen generation in the presence of hematoporphyrin on the surface of NPs. Our studies show that irradiation with a 975 nm laser of the functionalized NPs allows for the production of amounts of singlet oxygen easily detectable by Singlet Oxygen Sensor Green. The high emission intensity of NPs at 800 nm allowed the application of the synthesized NPs in an upconversion-linked immunosorbent assay (ULISA) for highly sensitive detection of the nucleoprotein from SARS-CoV-2, the causative agent of Covid-19. This article proves that LiYbF4:Tm3+@LiYF4 core@shell nanoparticles can be perfect alternatives for the most commonly studied upconverting NPs based on the NaYF4 host compound and are good candidates for biomedical applications.

Recent Trends in Lateral Flow Immunoassays with Optical Nanoparticles

Rapid, accurate, and convenient diagnosis is essential for effective disease management. Various detection methods, such as enzyme-linked immunosorbent assay, have been extensively used, with lateral flow immunoassay (LFIA) recently emerging as a major diagnostic tool. Nanoparticles (NPs) with characteristic optical properties are used as probes for LFIA, and researchers have presented various types of optical NPs with modified optical properties. Herein, we review the literature on LFIA with optical NPs for the detection of specific targets in the context of diagnostics.