Competitive Upconversion-Linked Immunosorbent Assay for the Sensitive Detection of Diclofenac

Re-assessment of monoclonal antibodies against diclofenac for their application in the analysis of environmental waters

The non-steroidal anti-inflammatory drug (NSAID) diclofenac (DCF) is an important environmental contaminant occurring in surface waters all over the world, because, after excretion, it is not adequately removed from wastewater in sewage treatment plants. To be able to monitor this pollutant, highly efficient analytical methods are needed, including immunoassays. In a medical research project, monoclonal antibodies against diclofenac and its metabolites had been produced. Based on this monoclonal anti-DCF antibody, a new indirect competitive enzyme-linked immunosorbent assay (ELISA) was developed and applied for environmental samples. The introduction of a spacer between diclofenac and the carrier protein in the coating conjugate led to higher sensitivity. With a test midpoint of 3 μg L-1 and a measurement range of 1-30 μg L-1, the system is not sensitive enough for direct analysis of surface water. However, this assay is quite robust against matrix influences and can be used for wastewater. Without adjustment of the calibration, organic solvents up to 5%, natural organic matter (NOM) up to 10 mg L-1, humic acids up to 2.5 mg L-1, and salt concentrations up to 6 g L-1 NaCl and 75 mg L-1 CaCl2 are tolerated. The antibody is also stable in a pH range from 3 to 12. Cross-reactivity (CR) of 1% or less was determined for the metabolites 4'-hydroxydiclofenac (4'-OH-DCF), 5-hydroxydiclofenac (5-OH-DCF), DCF lactam, and other NSAIDs. Relevant cross-reactivity occurred only with an amide derivative of DCF, 6-aminohexanoic acid (DCF-Ahx), aceclofenac (ACF) and DCF methyl ester (DCF-Me) with 150%, 61% and 44%, respectively. These substances, however, have not been found in samples. Only DCF-acyl glucuronide with a cross-reactivity of 57% is of some relevance. For the first time, photodegradation products were tested for cross-reactivity. With the ELISA based on this antibody, water samples were analysed. In sewage treatment plant effluents, concentrations in the range of 1.9-5.2 μg L-1 were determined directly, with recoveries compared to HPLC-MS/MS averaging 136%. Concentrations in lakes ranged from 3 to 4.4 ng L-1 and were, after pre-concentration, determined with an average recovery of 100%.

Single-particle Förster resonance energy transfer from upconversion nanoparticles to organic dyes

Single-particle detection and sensing, powered by Förster resonance energy transfer (FRET), offers precise monitoring of molecular interactions and environmental stimuli at a nanometric resolution. Despite its potential, the widespread use of FRET has been curtailed by the rapid photobleaching of traditional fluorophores. This study presents a robust single-particle FRET platform utilizing upconversion nanoparticles (UCNPs), which stand out for their remarkable photostability, making them superior to conventional organic donors for energy transfer-based assays. Our comprehensive research demonstrates the influence of UCNPs' size, architecture, and dye selection on the efficiency of FRET. We discovered that small particles (∼14 nm) with a Yb3+-enriched outermost shell exhibit a significant boost in FRET efficiency, a benefit not observed in larger particles (∼25 nm). 25 nm UCNPs with an inert NaLuF4 shell demonstrated a comparable level of emission enhancement via FRET as those with a Yb3+-enriched outermost shell. At the single-particle level, these FRET-enhanced UCNPs manifested an upconversion green emission intensity that was 8.3 times greater than that of their unmodified counterparts, while maintaining notable luminescence stability. Our upconversion FRET system opens up new possibilities for developing more effective high-brightness, high-sensitivity single-particle detection, and sensing modalities.

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.

The application of nanoparticles in point-of-care testing (POCT) immunoassays

The Covid-19 pandemic has led to greater recognition of the importance of the fast and timely detection of pathogens. Recent advances in point-of-care testing (POCT) technology have shown promising results for rapid diagnosis. Immunoassays are among the most extensive POCT assays, in which specific labels are used to indicate and amplify the immune signal. Nanoparticles (NPs) are above the rest because of their versatile properties. Much work has been devoted to NPs to find more efficient immunoassays. Herein, we comprehensively describe NP-based immunoassays with a focus on particle species and their specific applications. This review describes immunoassays along with key concepts surrounding their preparation and bioconjugation to show their defining role in immunosensors. The specific mechanisms, microfluidic immunoassays, electrochemical immunoassays (ELCAs), immunochromatographic assays (ICAs), enzyme-linked immunosorbent assays (ELISA), and microarrays are covered herein. For each mechanism, a working explanation of the appropriate background theory and formalism is articulated before examining the biosensing and related point-of-care (POC) utility. Given their maturity, some specific applications using different nanomaterials are discussed in more detail. Finally, we outline future challenges and perspectives to give a brief guideline for the development of appropriate platforms.

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.

Research Progress on Up-Conversion Fluorescence Probe for Detection of Perfluorooctanoic Acid in Water Treatment

Perfluorooctanoic acid (PFOA) is a new type of organic pollutant in wastewater that is persistent, toxic, and accumulates in living organisms. The development of rapid and sensitive analytical methods to detect PFOA in environmental media is of great importance. Fluorescence detection has the advantages of high efficiency and low cost, in which fluorescent probes have excellent fluorescence properties, excellent bio-solubility, and remarkable photostability. It is necessary to review the fluorescence detection routes for PFOA. In addition, the up-conversion of fluorescent materials (UCNPs), as fluorescent materials to prepare fluorescent probes with, has significant advantages and also attracts the attention of researchers, however, reviews related to their application in detecting PFOA and comparing them with other routes are rare. Furthermore, there are many strategies to improve the performance of up-conversion fluorescent probes including SiO₂ modification and amino modification. These strategies can enhance the detection effect of PFOA. Thus, this work reviews the types of fluorescence detection, the design, and synthesis of UCNPs, their recognition mechanism, properties, and their application progress. Moreover, the development trend and prospects of these detection probes are given.

An Improved Multiple Competitive Immuno-SERS Sensing Platform and Its Application in Rapid Field Chemical Toxin Screening

Improving the signal-to-noise ratio (SNR) by amplifying the outputting signal or reducing nonspecific binding (NSB) are the key techniques in multiple immunoassay. Aiming at these issues, this paper presents an improved multiple indirect competitive immune surface-enhanced Raman scattering (ci-SERS) assay for the rapid screening of highly toxic rodenticides in food and biological samples, which ensured remarkable accuracy, ultra-sensitivity and reproducibility. The non-fouling polymer brush grafted magnetic beads (the MB@P-CyM) were prepared as multiple competitive recognition substrates after conjugating triplex haptens (the MB@P-CyM-hap). It was demonstrated that the particular 3D hair-like structures of P-CyM not only facilitate conjugate high-density hapten but reduce the steric hindrance from SERS probes recognition, thus enhancing SNB. On the other hand, Au nanoflowers (AuNFs) of high SERS activity were synthesized using a simple one-pot hydrazine reduction. For simultaneously detecting three highly toxic rodenticides, i.e., diphacinone (DPN), bromadiolone (BRD) and tetramine (TET), the obtained AuNFs were fabricated as a SERS-encoded nanoprobe cocktail after successively labeling mono-antibodies/Raman probes. By integrating the MB@P-CyM-hap with the SERS-encoded cocktail, a highly sensitive multiple SERS assay was achieved in less than 2 h with a limit of detection of 0.62 ng mL^-1 for BRD, 0.42 ng mL^-1 for TET and 1.37 ng mL^-1 for DPN, respectively. The recoveries of these rodenticides in spiked food and biological samples were determined and ranged from 72 to 123%. Above all, the proposed modifications show remarkable improvements for high efficient multiple chemical toxin immunoassay.

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).

Liquid Crystal Droplet-Based Biosensors: Promising for Point-of-Care Testing

The development of biosensing platforms has been impressively accelerated by advancements in liquid crystal (LC) technology. High response rate, easy operation, and good stability of the LC droplet-based biosensors are all benefits of the long-range order of LC molecules. Bioprobes emerged when LC droplets were combined with biotechnology, and these bioprobes are used extensively for disease diagnosis, food safety, and environmental monitoring. The LC droplet biosensors have high sensitivity and excellent selectivity, making them an attractive tool for the label-free, economical, and real-time detection of different targets. Portable devices work well as the accessory kits for LC droplet-based biosensors to make them easier to use by anyone for on-site monitoring of targets. Herein, we offer a review of the latest developments in the design of LC droplet-based biosensors for qualitative target monitoring and quantitative target analysis.

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.

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.

Quantum optical immunoassay: upconversion nanoparticle-based neutralizing assay for COVID-19

In a viral pandemic, a few important tests are required for successful containment of the virus and reduction in severity of the infection. Among those tests, a test for the neutralizing ability of an antibody is crucial for assessment of population immunity gained through vaccination, and to test therapeutic value of antibodies made to counter the infections. Here, we report a sensitive technique to detect the relative neutralizing strength of various antibodies against the SARS-CoV-2 virus. We used bright, photostable, background-free, fluorescent upconversion nanoparticles conjugated with SARS-CoV-2 receptor binding domain as a phantom virion. A glass bottom plate coated with angiotensin-converting enzyme 2 (ACE-2) protein imitates the target cells. When no neutralizing IgG antibody was present in the sample, the particles would bind to the ACE-2 with high affinity. In contrast, a neutralizing antibody can prevent particle attachment to the ACE-2-coated substrate. A prototype system consisting of a custom-made confocal microscope was used to quantify particle attachment to the substrate. The sensitivity of this assay can reach 4.0 ng/ml and the dynamic range is from 1.0 ng/ml to 3.2 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upmu$$\end{document}μg/ml. This is to be compared to 19 ng/ml sensitivity of commercially available kits.

Next generation luminol derivative as powerful benchmark probe for chemiluminescence assays

Chemiluminescence (CL) provides outstanding analytical performance due to its independence from external light sources, background-free nature and exceptional sensitivity and selectivity. Yet, ultra-sensitive (bio)analysis is impeded by low hydrophilicity, poor quantum yields, fast kinetics or instability of most CL reagents such as luminol, acridinium esters, dioxetanes or peroxyoxalic derivatives. Photophysical studies show that m-carboxy luminol overcomes these limitations as its hydrophilic design provides a 5-fold increase in relative quantum yield resulting in superior performance in H₂O₂-dependent bioassays with 18-fold higher sensitivity for the quantification of its co-reactant H₂O₂, and 5-times lower detection limits for the luminophore. Studies with CL enhancers suggest its significance for mechanistic investigations in tandem with peroxidases. Finally, its integration into enzymatic and immunoassay applications demonstrates that m-carboxy luminol will provide signal enhancement, lower detection limits, and increased dynamic ranges for any other luminol-based CL assay, thus comprising the potential to replace luminol as benchmark probe.

Attomolar analyte sensing techniques (AttoSens): a review on a decade of progress on chemical and biosensing nanoplatforms

Detecting the ultra-low abundance of analytes in real-life samples, such as biological fluids, water, soil, and food, requires the design and development of high-performance biosensing modalities. The breakthrough efforts from the scientific community have led to the realization of sensing technologies that measure the analyte's ultra-trace level, with relevant sensitivity, selectivity, response time, and sampling efficiency, referred to as Attomolar Analyte Sensing Techniques (AttoSens) in this review. In an AttoSens platform, 1 aM detection corresponds to the quantification of 60 target analyte molecules in 100 μL of sample volume. Herein, we review the approaches listed for various sensor probe design, and their sensing strategies that paved the way for the detection of attomolar (aM: 10^-18 M) concentration of analytes. A summary of the technological advances made by the diverse AttoSens trends from the past decade is presented.

Design and Analytical Performances of a Diclofenac Biosensor for Water Resources Monitoring

Because the broadly consumed pain killer diclofenac (DCF) is a recognized pollutant, monitoring of its concentration is routinely performed in surface waters. As a valuable alternative to chromatographic and immunochemical assays, we developed a piezoelectric immunosensor to quantify DCF, first in buffer (PBS) and then in river water samples. A sensing layer comprising DCF was built up on the surface of silica-coated quartz sensors using a robust coupling chemistry. Binding of a highly affine monoclonal anti-DCF antibody was monitored in real time by quartz crystal microbalance with dissipation (QCM-D) measurements from which were determined a dissociation constant K_D of 0.24 nM and an acoustic antibody surface coverage of 1120 ng/cm² at saturation. On the other hand, an optical antibody surface coverage of 260 ng/cm² was determined by combined nanoplasmonic sensing measurement, giving a hydration percentage of 75% for the antibody monolayer. DCF assay was further set up following a competitive format for which binding of antibody to the sensing layer is inhibited by DCF in solution. The piezoelectric sensor response expressed as frequency shift ΔF was inversely related to the concentration of DCF with a dynamic range of 15-46 nM and a limit of detection (LoD) of 9.5 nM (2.8 μg/L) in PBS. This piezoelectric immunosensor was eventually applied to the assay of DCF in surface water samples taken at three different locations in the Seine and Marne rivers. The calculated concentration of DCF in these samples was in good agreement with official data published by the French center of water analysis eaufrance.

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.

A fast and ultrasensitive ELISA based on rolling circle amplification

A highly sensitive ELISA is critical for early diagnosis and biomarker discovery of various diseases. Although various ELISA technologies have been developed with high sensitivity, they are limited by poor repeatability, high cost, the dependence on complex equipment and/or a prolonged reaction time. To this end, we developed a fast and ultrasensitive ELISA (termed RELISA) based on rolling circle amplification (RCA) and enzymatic signal amplification. The RELISA is established on the traditional ELISA, with only one more RCA step that can be accomplished within 10 minutes. The prolonged single strand DNA (ssDNA) from RCA is able to enrich abundant horseradish peroxidase conjugate (HRP) modified detection probes. Consequently, the intensive HRP is able to catalyze TMB-H₂O₂ to produce significantly enhanced colorimetric signals. With CEACAM-7 as a model biomarker, the RELISA achieves the limit of detection as low as 2.82 pg mL^-1, which is ∼50 times higher than that of the traditional ELISA. Therefore, we envision that the developed RELISA would be a powerful tool for the early diagnosis of various major diseases.

Laser-induced breakdown spectroscopy as a readout method for immunocytochemistry with upconversion nanoparticles

Immunohistochemistry (IHC) and immunocytochemistry (ICC) are widely used to identify cancerous cells within tissues and cell cultures. Even though the optical microscopy evaluation is considered the gold standard, the limited range of useful labels and narrow multiplexing capabilities create an imminent need for alternative readout techniques. Laser-induced breakdown spectroscopy (LIBS) enables large-scale multi-elemental analysis of the surface of biological samples, e.g., thin section or cell pellet. It is, therefore, a potential alternative for IHC and ICC readout of various labels or tags (Tag-LIBS approach). Here, we introduce Tag-LIBS as a method for the specific determination of HER2 biomarker. The cell pellets were labeled with streptavidin-conjugated upconversion nanoparticles (UCNP) through a primary anti-HER2 antibody and a biotinylated secondary antibody. The LIBS scanning enabled detecting the characteristic elemental signature of yttrium as a principal constituent of UCNP, thus indirectly providing a reliable way to differentiate between HER2-positive BT-474 cells and HER2-negative MDA-MB-231 cells. The comparison of results with upconversion optical microscopy and luminescence intensity scanning confirmed that LIBS is a promising alternative for the IHC and ICC readout.

Immunosorbent assay based on upconversion nanoparticles controllable assembly for simultaneous detection of three antibiotics

The abuse of antibiotics leads to an increase in resistant strains, which in turn leads to the development of superbugs that pose great difficulties for the treatment of human diseases. A high-throughput and highly sensitive avidin biotin complex immunosorbent assay based on upconversion nanoparticles controllable assembly (ABC-ULISA) for the detection of antibiotics was developed, which enabled accurate quantitative detection in a shorter period of time. Streptavidin and biotin-labeled upconversion nanoparticles form avidin-biotin-upconversion complex, which was then combined with biotinylated antibody to achieve double amplification of the signal, further improving detection sensitivity. Upconversion nanoparticles with 808 nm excitation provide better penetration without the need for an external source. The 96-well enzyme-linked plate was used as a detection platform to meet the high-throughput needs. ABC-ULISA was used to simultaneously detect three antibiotics with a limit of detection of 0.15 ng/mL for sulfamethazine, 0.03 ng/mL for sarafloxacin, and 0.05 ng/mL for tetracycline. The detection limit of ABC-ULISA was much lower than the traditional ELISA and ordinary ULISA. Moreover, ABC-ULISA was also versatile, and the corresponding target can be detected by changing different antibodies. The results were stable and reliable, and the equipment could be miniaturized, which was expected to be commercialized and on-site.

Fluorescence polarization immunoassay for the determination of diclofenac in wastewater

Pharmacologically active compounds are often detected in wastewater and surface waters. The nonsteroidal anti-inflammatory drug diclofenac (DCF) was included in the European watch list of substances that requires its environmental monitoring in the member states. DCF may harmfully influence the ecosystem already at concentrations ≤ 1 μg L-1. The fast and easy quantification of DCF is becoming a subject of global importance. Fluorescence polarization immunoassay (FPIA) is a homogeneous mix-and-read method which does not require the immobilization of reagents. FPIA can be performed in one phase within 20-30 min, making it possible to analyse wastewater without any complicated pre-treatment. In this study, new tracer molecules with different structures, linking fluorophores to derivatives of the analyte, were synthesized, three homologous tracers based on DCF, two including a C6 spacer, and one heterologous tracer derived from 5-hydroxy-DCF. The tracer molecules were thoroughly assessed for performance. Regarding sensitivity of the FPIA, the lowest limit of detection reached was 2.0 μg L-1 with a working range up to 870 μg L-1. The method was validated for real wastewater samples against LC-MS/MS as reference method with good agreement of both methods. Graphical abstract.

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.

Recent progress in the development of upconversion nanomaterials in bioimaging and disease treatment

Multifunctional lanthanide-based upconversion nanoparticles (UCNPs), which feature efficiently convert low-energy photons into high-energy photons, have attracted considerable attention in the domain of materials science and biomedical applications. Due to their unique photophysical properties, including light-emitting stability, excellent upconversion luminescence efficiency, low autofluorescence, and high detection sensitivity, and high penetration depth in samples, UCNPs have been widely applied in biomedical applications, such as biosensing, imaging and theranostics. In this review, we briefly introduced the major components of UCNPs and the luminescence mechanism. Then, we compared several common design synthesis strategies and presented their advantages and disadvantages. Several examples of the functionalization of UCNPs were given. Next, we detailed their biological applications in bioimaging and disease treatment, particularly drug delivery and photodynamic therapy, including antibacterial photodynamic therapy. Finally, the future practical applications in materials science and biomedical fields, as well as the remaining challenges to UCNPs application, were described. This review provides useful practical information and insights for the research on and application of UCNPs in the field of cancer.

Hydrogel Matrix-Grafted Impedimetric Aptasensors for the Detection of Diclofenac

Driven by the growing concern about the release of untreated emerging pollutants and the need for determining small amounts of these pollutants present in the environment, novel biosensors dedicated to molecular recognition are developed. We have designed biosensors using a novel class of grafted polymers, surface-attached hydrogel thin films, on conductive transducers as a biocompatible matrix for biomolecule immobilization. We showed that they can be dedicated to the molecular recognition of diclofenac (DCL). The immobilization of the aptamer onto surface-attached hydrogel thin films by covalent attachment provides a biodegradable shelter, providing the aptamer with excellent environments to preserve its active and functional structure while allowing the detection of DCL. The grafting of the aptamer is obtained using the formation of amide bonds via the activation of carboxylic acid groups of the poly(acrylic acid) hydrogel thin film. For improved sensitivity and higher stability of the sensor, a high density of the immobilized aptamer is enabled. The aptamer-modified electrode was then incubated with DCL solutions at different concentrations. The performances of the aptasensor were investigated by electrochemical impedance spectroscopy. The change in charge-transfer resistance was found to be linear with DCL concentration in the 30 pM to 1 μM range. The detection limit was calculated to be 0.02 nM. The improvement of the limit of detection can be mainly attributed to the three-dimensional environment of the hydrogel matrix which improves the grafting density of the aptamer and the affinity of the aptamer to DCL.

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.

An Aggregation-Induced Emission-Based Indirect Competitive Immunoassay for Fluorescence "Turn-On" Detection of Drug Residues in Foodstuffs

A new fluorescent "turn-on" probe-based immunosensor for detecting drug residues in foodstuffs was established by combining the mechanism of aggregation-induced emission (AIE) and an indirect competitive enzyme-linked immunosorbent assay (ELISA). In this study, a luminogen, with negligible fluorescence emission (TPE-HPro), aggregated in the presence of H2O2, and exhibited astrong yellow emission based on its AIE characteristics. This AIE process was further configured into an immunoassay for analyzing drug residues in foodstuffs. In this approach, glucose oxidase (GOx) was used as an enzyme label for the immunoassay and triggered GOx/glucose-mediated H2O2 generation, which caused oxidation of TPE-HPro and a "turn-on" fluorescence response at 540 nm. To quantitatively analyze the drug residues in foodstuffs, we used amantadine (AMD) as an assay model. By combining the AIE-active "turn-on" fluorescent signal generation mechanism with conventional ELISAs, quantifying AMD concentrations in chicken muscle samples was realized with an IC50 (50% inhibitory concentration) value of 0.38 ng/mL in buffer and a limited detection of 0.06 μg/kg in chicken samples. Overall, the conceptual integration of AIE with ELISA represents a potent and sensitive strategy that broadens the applicability of the AIE-based fluorometric assays.

An infrared IgG immunoassay based on the use of a nanocomposite consisting of silica coated Fe3O4 superparticles

A reliable, rapid and ultrasensitive immunoassay is described for determination of immunoglobulin G (IgG). It is making use of biofunctional magnetite (Fe₃O₄) superparticles coated with SiO₂ and serving as an infrared (IR) probe. The unique IR fingerprint signals originating from the transverse and longitudinal phonon modes, respectively, of the asymmetric stretching of the Si-O-Si bridges display a satisfactory resistance to optical interference from the environment. The adoption of Fe₃O₄ superparticles instead of Fe₃O₄ nanoparticles as the magnetic core warrants a controllable structure and a strong magnetic response. This facilitates the efficient purification of the probes and the alleviation of the interfacial resistance between the liquid-solid interfaces by using a magnet. The gold-coated substrate was used to immobilize goat-anti-human IgG. The analyte (human IgG) was incubated with the IR probes, and then captured by the substrate immobilized antibody with the assistance of an external magnetic field. The integral area of the IR absorption band between 1250 cm^-1 - 900 cm^-1 was chosen for quantitative assay. The limit of detection is 95 fM, which is two orders of magnitude better than that without the magnetic field. The assay time was shortened from 2 h to 1 min. High selectivity, specificity, and long-term stability of the immunoassay were achieved. The performance of the assay when analyzing blood samples confirmed the practicability of the method. Graphical abstract Schematic presentation of the infrared (IR) immunoassay based on Fe₃O₄ superparticle@SiO₂ nanocomposites. The assistance of an external magnetic field reduces the incubation time and improves the detection sensitivity.

Upconversion fluorescent nanoparticles based-sensor array for discrimination of the same variety red grape wines

A fluorescent sensor array composed of upconversion nanomaterials to distinguish the same variety of red grape wines was constructed.

Liquid chromatography-tandem mass spectrometry detection of diclofenac and related compounds in water samples

A frequently studied environmental contaminant is the active substance diclofenac, which is removed insufficiently in sewage treatment plants. Since its inclusion in the watch list of the EU Water Framework Directive, the concentrations in surface waters will be determined throughout Europe. For this, still, more precise analytical methods are needed. As a reference, HPLC-MS is frequently employed. One of the major metabolites is 4'-hydroxydiclofenac (4'-OH-DCF). Also, diclofenac lactam is important for assessing degradation and transformation. Aceclofenac (ACF), the glycolic acid ester of diclofenac is used as a drug, too, and could potentially be cleaved to yield diclofenac again. In various sewage treatment plant influent samples, diclofenac, 4'-OH-DCF, DCF lactam and ACF could be determined with detection limits of 3 μg/L, 0.2 μg/L, 0.17 μg/L and 10 ng/L, respectively.

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.

Sensor arrays made by self-organized nanoreceptors for detection and discrimination of carboxylate drugs

An array of self-organized nanoreceptors based on monolayer-protected gold nanoparticles in combination with different commercially available fluorescent dyes can detect and discriminate nonsteroidal anti-inflammatory drugs.

Sensors and bioassays powered by upconverting materials

In recent years, considerable efforts have been done to better understand the peculiar emission properties of upconverting materials due to their widespread applications in different and important technological fields such as upconversion-based photoactivated cancer therapies, photoactivated drug-delivery, magnetic resonance imaging contrast agents, bioimaging. However, one of the most promising applications of upconverting materials concerns the field of sensing, due to their unique emission properties. In fact, the minimal autofluorescence, blinking, photo-bleaching, and high photostability makes them an excellent alternative to organic dyes or quantum dots. This article reviews the state-of-the-art, design, and sensing strategies of upconversion-based sensing platforms, with special attention to upconverting nanoparticles, as well as how the incorporation of these materials into pre-existing diagnostic tests and bioassays have improved their capabilities for the detection of different kinds of analytes.