A sensitive competitive ELISA for 2,4-dinitrophenol using 3,6-fluorescein diphosphate as a fluorogenic substrate

Recent Developments in Lateral Flow Assays for Salmonella Detection in Food Products: A Review

Salmonellosis is a disease transmitted by contaminated food and is one of the leading causes of infections worldwide, making the early detection of Salmonella of crucial importance for public health. However, current detection methods are laborious and time-consuming, thus impacting the entire food supply chain and leading to production losses and economic sanctions. To mitigate these issues, a number of different biosensors have been developed, including lateral flow assays (LFAs), which have emerged as valuable tools in pathogen detection due to their portability, ease of use, time efficiency, and cost effectiveness. The performance of LFAs has been considerably enhanced by the development of new nanomaterials over the years. In this review, we address the principles and formats of the assay and discuss future prospects and challenges with an emphasis on LFAs developed for the detection of different Salmonella serovars in food.

Advancement in Salmonella Detection Methods: From Conventional to Electrochemical-Based Sensing Detection

Large-scale food-borne outbreaks caused by Salmonella are rarely seen nowadays, thanks to the advanced nature of the medical system. However, small, localised outbreaks in certain regions still exist and could possess a huge threat to the public health if eradication measure is not initiated. This review discusses the progress of Salmonella detection approaches covering their basic principles, characteristics, applications, and performances. Conventional Salmonella detection is usually performed using a culture-based method, which is time-consuming, labour intensive, and unsuitable for on-site testing and high-throughput analysis. To date, there are many detection methods with a unique detection system available for Salmonella detection utilising immunological-based techniques, molecular-based techniques, mass spectrometry, spectroscopy, optical phenotyping, and biosensor methods. The electrochemical biosensor has growing interest in Salmonella detection mainly due to its excellent sensitivity, rapidity, and portability. The use of a highly specific bioreceptor, such as aptamers, and the application of nanomaterials are contributing factors to these excellent characteristics. Furthermore, insight on the types of biorecognition elements, the principles of electrochemical transduction elements, and the miniaturisation potential of electrochemical biosensors are discussed.

A Versatile Platform for the Development of Radiolabeled Antibody-Recruiting Small Molecules

Building on clinical case reports of the abscopal effect, there has been considerable interest in the synergistic effects of radiation and immunotherapies for the treatment of cancer. Here, the first radiolabeled antibody-recruiting small molecule that can chelate a variety of cytotoxic radionuclides is described. The platform consists of a tunable antibody-binding domain against a serum antibody of interest (e.g., dinitrophenyl hapten) to recruit endogenous antibodies that activate effector cell function, a chelate capable of binding diagnostic and therapeutic radiometals, and a tetrazine for bioorthogonal coupling with trans-cyclooctene-modified targeting vectors. The dinitrophenyl-tetrazine ligand was shown to both affect dose-dependent antibody recruitment and immune cell function (phagocytosis) in vitro, and the bisphosphonate ¹⁷⁷Lu-complex was shown to accumulate at sites of calcium accretion in vivo, which was achieved using both active and pretargeting strategies.

Micro-nanoparticles magnetic trap: Toward high sensitivity and rapid microfluidic continuous flow enzyme immunoassay

In this work, we developed a microfluidic system for immunoassays where we combined the use of magnetic nanoparticles as immunosupport, a microfluidic magnetic trap, and a fluorogenic substrate in continuous flow for detection which, together with the optimization of the functionalization of surfaces to minimize nonspecific interactions, resulted in a detection limit in the order of femtomolar and a total assay time of 40 min for antibiotin antibody detection. A magnetic trap made of carbonyl-iron microparticles packaged inside a 200  μ m square microchannel was used to immobilize and concentrate nanoparticles. We functionalized the surface of the iron microparticles with a silica-polyethylene glycol (PEG) shell to avoid corrosion and unspecific protein binding. A new one-step method was developed to coat acrylic microchannels with an organofunctional silane functionalized with PEG to minimize unspecific binding. A model immunoassay was performed using nanoparticles decorated with biotin to capture antibiotin rabbit Immunoglobulin G (IgG) as target primary antibody. The detection was made using antirabbit IgG labeled with the enzyme alkaline phosphatase as a secondary antibody, and we measured fluorescence with a fluorescence microscope. All steps of the immunoassay were performed inside the chip. A calibration curve was obtained in which a detection limit of 8 pg/ml of antibiotin antibody was quantified. The simplicity of the device and the fact that it is made of acrylic, which is compatible with mass production, make it ideal for Point-Of-Care applications.

Enhancing the detection of Toxoplasma gondii via an anti-SAG1 scFv-alkaline phosphatase immunoconjugate

The purpose of this study was to implement a fluorometric method for enhancing the detection sensitivity of Toxoplasma gondii in biological fluids. To address this challenge, we designed and produced a recombinant immunoconjugate tool based on a single-chain antibody fragment anti-T. gondii SAG1 antigen (scFvSG15) genetically fused to the bacterial alkaline phosphatase (AP) using 4-methyl-umbelliferyl-phosphate as fluorogenic substrate. The anti-SAG1 scFv-AP conjugate was fully bifunctional and was used successfully in different assays including immunoblot, fluorometric ELISA and direct immunofluorescence. The fluorometric immunoassay afforded an extremely low detection limit (1 tachyzoite/well), which was in agreement with the real-time PCR control test. The immunofluorescence imaging has provided captivating visual evidence of T. gondii detection. These results strongly suggest that the recombinant anti-SAG1-AP conjugate generated here might serve as useful and highly sensitive immunoassay probe to direct detect T. gondii in a one-step procedure, opening up new perspectives for diagnosis of toxoplasmosis.

Photoinduced electron transfer between Fe(III) and adenosine triphosphate-BODIPY conjugates: Application to alkaline-phosphatase-linked immunoassay

Fluorescent boron dipyrromethene (BODIPY) analogs are often used as sensors for detecting various species because of their relatively high extinction coefficients, outstanding fluorescence quantum yields, photostability, and pH-independent fluorescence. However, there is little-to-no information in the literature that describes the use of BODIPY analogs for detecting alkaline phosphatase (ALP) activity and inhibition. This study discovered that the fluorescence of BODIPY-conjugated adenosine triphosphate (BODIPY-ATP) was quenched by Fe(III) ions through photoinduced electron transfer. The ALP-catalyzed hydrolysis of BODIPY-ATP resulted in the formation of BODIPY-adenosine and phosphate ions. The fluorescence of the generated BODIPY-adenosine was insensitive to the change in the concentration of Fe(III) ions. Thus, the Fe(III)-induced fluorescence quenching of BODIPY-ATP can be paired with its ALP-mediated dephosphorylation to design a turn-on fluorescence probe for ALP sensing. A method detection limit at a signal-to-noise ratio of 3 for ALP was estimated to be 0.02 units/L (~6 pM; 1 ng/mL). This probe was used for the screening of ALP inhibitors, including Na3VO4, imidazole, and arginine. Because ALP is widely used in enzyme-linked immunosorbent assays, the probe was coupled to an ALP-linked immunosorbent assay for the sensitive and selective detection of immunoglobulin G (IgG). The lowest detectable concentration for IgG in this system was 5 ng/mL. Compared with the use of 3,6-fluorescein diphosphate as a signal reporter in an ALP-linked immunosorbent assay, the proposed system provided comparable sensitivity, large linear range, and high stability over temperature and pH changes.

Histochemical imaging of alkaline phosphatase using a novel fluorescent substrate

Histochemical visualization of phosphatase is exclusively required for Western immunoblotting and antigen-positive cell staining using an alkaline phosphatase (AP)-labeled secondary antibody. This detection has been performed by several reagents including 5-bromo-4-chloro-3-indolyl-phosphate (X-Phos), nitro blue tetrazolium (NBT), 3-(2'-spiroadamantane)-4-methoxy-4-(3″-phosphoryloxy)phenyl-1,2-dioxetane and 2-(5'-chloro-2'-phosphoryloxyphenyl)-6-chloro-4-[3H]-quinazolinone (ELF® 97 Phosphate). We previously reported that 2-(benzothiazol-2-yl)-4-bromophenol bonded with N-acetylneuraminic acid (BTP3-Neu5Ac), enabled fluorescent histochemical visualization of sialidase activity. 2-(Benzothiazol-2-yl)-4-bromophenol (BTP3), which is formed from BTP3-Neu5Ac by sialidase reaction, is a crystalline, insoluble and stable fluorogenic compound, deposited at the site of enzyme activity. We developed a BTP3 phosphate ester (BTP3-Phos) for the purpose of fluorescent histochemical visualization of phosphatase activity. BTP3-Phos emitted fluorescence in a manner dependent on the concentration of the AP-labeled antibody. BTP3-Phos also enabled fluorescent histochemical visualization of AP-blotted dots in a manner dependent on the concentration of the AP-labeled antibody. The detection sensitivity of BTP3-Phos was estimated to be greater than that of the conventional method using X-Phos and NBT. Influenza A virus-infected cells were fixed and reacted with anti-influenza A virus antibodies and incubated continuously with an AP-labeled secondary antibody. BTP3-Phos stained the infected cells with distinct green fluorescence. These results indicate that BTP3-Phos can enable fluorescent immunohistochemical staining analysis using an AP-labeled antibody. BTP3-Phos would be beneficial for histochemical staining of AP activity, and may be applicable for multi-color staining or a cell sorter.

Probing enzymatic activity inside single cells

We report a novel approach for determining the enzymatic activity within a single suspended cell. Using a steady-state microfluidic delivery device and timed exposure to the pore-forming agent digitonin, we controlled the plasma membrane permeation of individual NG108-15 cells. Mildly permeabilized cells (~100 pores) were exposed to a series of concentrations of fluorescein diphosphate (FDP), a fluorogenic alkaline phosphatase substrate, with and without levamisole, an alkaline phosphatase inhibitor. We generated quantitative estimates for intracellular enzyme activity and were able to construct both dose-response and dose-inhibition curves at the single-cell level, resulting in an apparent Michaelis contant Km of 15.3 μM ± 1.02 (mean ± standard error of the mean (SEM), n = 16) and an inhibition constant Ki of 0.59 mM ± 0.07 (mean ± SEM, n = 14). Enzymatic activity could be monitored just 40 s after permeabilization, and five point dose-inhibition curves could be obtained within 150 s. This rapid approach offers a new methodology for characterizing enzyme activity within single cells.

Direct access and control of the intracellular solution environment in single cells

Methods that can control and vary the solution environment around single cells are abundant. In contrast, methods that offer direct access to the intracellular proteome and genome in single cells with the control, flexibility, and convenience given by microfluidic methods are both scarce and in great demand. Here, we present such a method based on using a microfluidic device mounted on a programmable scanning stage and cells on-chip permeabilized by the pore-forming glycoside digitonin. We characterized the on-chip digitonin poration, as well as the solution exchange within cells. Intracellular solution exchange times vary with the dose of exposure to digitonin from less than a second to tens of seconds. Also, the degree of permeabilization obtained for cells treated with the same dose varies considerably, especially for low doses of digitonin exposure and low permeabilities. With the use of the presented setup, the degree of permeabilization can be measured during the permeabilization process, which allows for "on-line" optimization of the digitonin exposure time. Using this calibrated permeabilization method, we demonstrate the generation of intracellular oscillations, intracellular gradients, and the delivery of substrate to initiate enzymatic reactions in situ. This method holds the potential to screen and titrate intracellular receptors or enzymes or to generate intracellular oscillations, useful in the study of signaling pathways and oscillation decoding among other applications.

Dried reagents for multiplex genotyping by tag-array minisequencing to be used in microfluidic devices

We present an optimized procedure for freeze-drying and storing reagents for multiplex PCR followed by genotyping using a tag-array minisequencing assay with four color fluorescence detection which is suitable for microfluidic assay formats. A test panel was established for five cancer mutations in three codons (175, 248 and 273) of the tumor protein gene (TP53) and for 13 common single nucleotide polymorphisms (SNPs) in the TP53 gene. The activity of DNA polymerase was preserved for six months of storage after freeze-drying, and the half-life of activities of exonuclease I and shrimp alkaline phosphatase were estimated to 55 and 200 days, respectively. We conducted a systematic genotyping comparison using freeze-dried and liquid reagents. The accuracy of successful genotyping was 99.1% using freeze-dried reagents compared to liquid reagents. As a proof of concept, the genotyping protocol was carried out with freeze-dried reagents stored in reaction chambers fabricated by micromilling in a cyclic olefin copolymer substrate. The results reported in this study are a key step towards the development of an integrated microfluidic device for point-of-care DNA-based diagnostics.

Unexpected phosphoryl transfer from Asp351 to fluorescein attached to Lys515 in sarcoplasmic reticulum Ca2+-ATPase

Sarcoplasmic reticulum Ca(2+)-ATPase is an ion pump whose catalytic cycle includes the transient formation of an acyl phosphate at Asp(351), and fluorescein isothiocyanate is a covalent inhibitor of ATP binding to this pump, known to specifically derivatize Lys(515) in the nucleotide-binding site. It was previously found that an unusually stable, phosphorylated form of fluorescein-ATPase, with low fluorescence, is obtained following Ca (2+) loading with acetyl phosphate as energy source and then chelation with EGTA of Ca(2+) on the cytosolic side. Here we show that the phospho-linkage in this low fluorescent species is stable at alkaline pH, unlike the acyl phosphate at Asp(351). Moreover, the low fluorescence and stable phosphoryl group track together in primary and secondary tryptic subfragments, separated by SDS-PAGE after denaturation. Finally, normal fluorescence and absorbance are recovered upon treatment with alkaline phosphatase after extensive trypsinolysis. We conclude that the low fluorescent species is the result of the phosphoryl group being transferred from Asp (351) to the fluorescein moiety during pump reversal, yielding fluorescein monophosphate tethered to Ca(2+)-ATPase.

Controlling enzymatic reactions by geometry in a biomimetic nanoscale network

We demonstrate that a transition from a compact geometry (sphere) to a structured geometry (several spheres connected by nanoconduits) in nanotube-vesicle networks (NVNs) induces an ordinary enzyme-catalyzed reaction to display wavelike properties. The reaction dynamics can be controlled directly by the geometry of the network, and such networks can be used to generate wavelike patterns in product formation. The results have bearing for understanding catalytic reactions in biological systems as well as for designing emerging wet chemical nanotechnological devices.

Enhanced sensitivity and precision in an enzyme-linked immunosorbent assay with fluorogenic substrates compared with commonly used chromogenic substrates

Quantitative enzyme-linked immunosorbent assay (ELISA) is a widely used tool for analyzing biopharmaceutical and vaccine products. The superior sensitivity of the ELISA format is conferred by signal amplification through the enzymatic oxidation or hydrolysis of substrates to products with enhanced color or fluorescence. The extinction coefficient for a colored product or the quantum yield of a fluorescent product, coupled with the efficiency of the immobilized enzyme, is the determining factor for the sensitivity and precision of a given ELISA. The enhancement of precision and sensitivity using fluorogenic substrates was demonstrated in a direct-binding ELISA in a low-analyte concentration range compared with commonly used chromogenic substrates. The enhancement in precision was demonstrated quantitatively with lower coefficients of variation in measurements of signal intensities, approximately a five- to six-fold enhancement in signal-to-noise ratio at a given analyte concentration with fluorogenic substrates. Similarly, the amplitude of the enhancement in sensitivity, as reflected by relative limits of detection or quantitation, is approximately two- to five-fold when compared with commonly used chromogenic substrates. Additional advantages of a fluorescence-based ELISA format include the continuous monitoring of initial rates of enzymatic reactions, the measurement of fluorescence changes in the presence of particulate materials, the absence of a quench step, and a larger quantifiable analyte range.

Utilizing ultrafiltration to remove alkaline phosphatase from clinical analyzer water

Alkaline phosphatase (ALP) conjugated to antibodies is often used in enzyme immunoassays (EIAs). These assays are notably sensitive to experimental conditions. A possible source of interference is bacterial ALP, which is released when bacterial contamination occurs in clinical analyzers. Preliminary experiments led to the selection of a detection kit, ALP source, and specific types of tubes for collecting water samples and performing assays. The release of ALP from various strains of bacteria identified in pure water was demonstrated (10–30×10

Monoclonal Antibody-Based Quantitation of Poly(ethylene glycol)-Derivatized Proteins, Liposomes, and Nanoparticles

Covalent attachment of poly(ethylene glycol) (PEG) molecules to drugs, proteins, and liposomes is a proven technology for improving their bioavailability, safety, and efficacy. Qualitative and quantitative analysis of PEG-derivatized molecules is important for both drug development and clinical applications. We previously reported the development of a monoclonal IgM antibody (AGP3) to PEG. We now describe a new IgG1 monoclonal antibody (E11) to PEG and show that it can be used in combination with AGP3 to detect and quantify PEG-derivatized molecules. Both antibodies bound the repeating subunits of the PEG backbone and could detect free PEG and PEG-modified proteins by ELISA, immunoblotting, and flow cytometry. Detection sensitivity increased with the length and the number of PEG chains on pegylated molecules. Both antibodies also efficiently accelerated the clearance of a PEG-modified enzyme in vivo. A sandwich ELISA in which E11/AGP3 were employed as the capture/detection antibodies was developed to detect PEG-modified proteins at concentrations as low as 1.2 ng/mL. In addition, the ELISA could also quantify, in the presence of 10% fetal bovine serum, free methoxy-PEG20,000, PEG2,000-quantum dots, and PEG2,000-liposomes at concentrations as low as 20 ng/mL (1.0 nM), 1.4 ng/mL (3.1 pM), and 2.4 ng/mL (3.13 nM phospholipids), respectively. Finally, we show that the sandwich ELISA could accurately measured the in vivo half-life of a PEG-modified enzyme. These antibodies should be generally applicable to the qualitative and quantitative analysis of all PEG-derivatized molecules.

Fluorometric cell-ELISA for quantifying rabies infection and heparin inhibition

The purpose of the present study was to implement a fluorometric method for detecting and quantifying viral antigens in human meduloblastoma cells infected by two types of fixed rabies virus (CVS-MB and CVS-BHK) and a street virus using a cell-enzyme linked immunosorbent assay (cell-ELISA) technique; alkaline phosphatase was used as the antibody-marker enzyme and 4-methyl-umbelliferyl-phosphate as the fluorogenic substrate. The system was used for detecting up to 1:10,000 viral inoculums, followed by evaluating the effect of heparin on infection. Infected cultures were reliably differentiated from their respective negative controls in both assays allowing data to be analysed statistically. As reported in another study, heparin produces strong inhibition when the CVS-BHK viral strain is used for infection; it has thus been suggested that it binds to the neural cell adhesion molecule and could be blocked by using this drug. This fluorometric method is less time-consuming, has increased reproducibility and useful for quantitation of collected data and can therefore be considered as a useful tool for research.

Controlled Initiation of Enzymatic Reactions in Micrometer-Sized Biomimetic Compartments

We present a technique to initiate chemical reactions involving few reactants inside micrometer-scale biomimetic vesicles (10(-12) to 10(-15) L) integral to three-dimensional surfactant networks. The shape of these networks is under dynamic control, allowing for transfer and mixing of two or several reactants at will. Specifically, two nanotube-connected vesicles were filled with reactants (substrate and enzyme, respectively) by microinjection. Initially, the vesicles are far apart and any diffusive mixing (on relevant experimental time scales) between the contents of the separated vesicles is hindered because of the narrow diameter and long axial extension of the nanotube. To initiate a reaction, the vesicles were brought close together, the nanotube was consumed by the vesicles and at a critical distance, the nanotube-vesicle junctions were dilated leading to formation of one spherical reactor, and hence mixing of the contents. We demonstrate the concept using a model enzymatic reaction, which yields a fluorescent product (two-step hydrolysis of fluorescein diphosphate by alkaline phosphatase), where product formation was measured as a function of time using a FRAP fluorescence microscopy protocol. By comparing the enzymatic activity with bulk measurements, the enzyme concentration inside the vesicle could be determined. Reactions could be followed for systems having as few as approximately 15 enzyme molecules confined to a reactor vesicle. To describe the experiments we use a simple diffusion-controlled reaction model and solve it using a survival probability approach. The agreement with experiment is qualitative, but the model describes the trends well. It is shown that the model correctly predicts (i) single-exponential decay after a few seconds, and (ii) that the substrate decay constant depends on the number of enzymes and geometry of reaction container. The numerical correction factor Lambda is introduced in order to ensure semiquantitative agreement between experiment and theory. It was shown that this numerical factor depends weakly on vesicle radius and number of enzymes, thus it is sufficient to determine this factor only once in a single calibration measurement.

3,6-Fluorescein Diphosphate: A Sensitive Fluorogenic and Chromogenic Substrate for Protein Tyrosine Phosphatases*

A highly sensitive and continuous protein tyrosine phosphatase (PTPase) assay using 3,6-fluorescein diphosphate (FDP) is described. Leukocyte phosphatase CD45 (leukocyte common antigen), protein tyrosine phosphatase-1B, and leukocyte common antigen-related protein LAR preferentially hydrolyze FDP to fluorescein monophosphate (FMP) with V(max) and K(m) values comparable with those of phosphotyrosine peptide substrates. Further hydrolysis of FMP to fluorescein was less efficient because of increased K(m) values compared with those of FDP. FMP absorbs strongly at 445 nm and fluoresces intensely near 515 nm, both of which are insensitive to pH perturbations above pH 6. Its high catalytic efficiency, coupled with the highly sensitive dual detection in the visible wavelength region and wider pH operating range, make FDP the substrate of choice for PTPase inhibitor screening in HTS format and assay miniaturization.

Novel fluorogenic substrates for acid phosphatase

Fluorinated versions of fluorescein diphosphate (FDP) can provide significantly enhanced fluorescence upon hydrolysis by acid phosphatase, as compared with FDP, when measured at the reaction pH.

Partial isolation from intact cells of a cell surface-exposed lysophosphatidylinositol-phospholipase C

A novel cell surface phosphoinositide-cleaving phospholipase C (ecto-PLC) activity was isolated from cultured cells by exploiting its presumed external exposure. Biotinylation of intact cells followed by solubilization of the biotinylated proteins from a membrane fraction and recovery onto immobilized-avidin beads, allowed assay of this cell surface enzyme activity apart from the background of the substantial family of intracellular PLCs. Several cell lines of differing ecto-PLC expression were examined as well as cells stably transfected to overexpress the glycosylphosphatidylinositol (GP) anchored protein human placental alkaline phosphatase (PLAP) as a cell surface enzyme marker. The resulting bead preparations from ecto-PLC positive cells possessed calcium-dependent PLC activity with preference for lysophosphatidylinositol (lysaPI) rather than phosphatidylinositol (PI). The function of ecto-PLC of intact cells evidently is not to release GPI-anchored proteins at the cell surface, as no detectable Ca(2+)-dependent release of overexpressed PLAP from ecto-PLC-positive cells was observed. To investigate the cell surface linkage of the ecto-PLC itself, intact cells were treated with bacterial PI-PLC to cleave simple GPI anchors, but no decrease in ecto-PLC activity was observed. High ionic strength washes of biotinylated membranes prior to the generation of bead preparations did not substantially reduce the lysoPI-PLC activity. The results verify that the ecto-PLC is truly cell surface-exposed, and unlike other members of the PLC family that are thought to be peripheral membrane proteins, this novel lysoPI-PLC is most likely a true membrane protein.

A fluorogenic assay of endogenous phosphatase for assessment of cell adhesion

Assays of cell adhesion generally require prelabeling of cells with radioactive or fluorescent probes. A new fluorogenic phosphatase assay requiring no prelabeling has been developed to quantitate cell number, which can thus serve as the basis for quantitating cell adhesion or migration. The assay uses the non-fluorescent substrate 3,6-fluorescein diphosphate (FDP) whose dephosphorylation generates fluorescein. The fluorescence generated is linear with incubation time and cell number until substrate becomes limiting; the assay easily quantitates cells over a range from 10(3) to 10(6) for a variety of cell types, including resting T cells. It is as sensitive as the 51Cr assay, but has the many advantages of a non-radioactive assay, making more convenient the removal of nonadherent cells by simple 1 x g sedimentation. Unlike most other non-radioactive assays, it requires no pre-incubation; this: (1) reduces cell manipulation; (2) eliminates problems of spontaneous release; and (3) avoids potential dye toxicity. This technique of cell quantitation has been adopted as standard in our laboratory for routine adhesion and migration assays.