3,3′–diaminobenzidine (DAB)–H2O2–HRP voltammetric enzyme-linked immunoassay for the detection of carcionembryonic antigen

Redox-mediated dsDNA-dye photooxidase mimic enable catalytic oxidation of 3,3',5,5'-tetramethylbenzidine by dissolved O2 at neutral pH for improved biosensing

Catalytic oxidation of 3,3',5,5'-Tetramethylbenzidine (TMB, an excellent chromogenic substrate) at neutral pH is critically important for amplified bioanalysis. Although some nanozymes exhibited the peroxidase activity at neutral pH, it is difficult to modulate their activity for homogeneous detection of biomolecules. In this work, we developed a redox-mediated dsDNA-dye photooxidase mimic that enables catalytic oxidation of TMB by dissolved O2 at neutral pH for improved biosensing. During illumination, the double-stranded DNA-SYBR Green I (dsDNA-SG) photogenerated singlet oxygen (1O2) can oxidized Mn2+ to Mn3+ that can efficiently oxidize TMB to produce a distinct blue within 4 min under neutral conditions. The catalytic oxidation of TMB can be readily modulated by the formation or dissociation of dsDNA during the sensing. After investigating a series of redox mediators, we found that only the Mn3+/Mn2+ redox mediator can lead to the oxidation of TMB at neutral pH. The maximum reaction rate of Mn2+-mediated dsDNA-SG photooxidase mimic under neutral conditions (pH 7.0) was 1.7 × 10-4 mM/s, even higher than that of horseradish peroxidase (HRP, 8.0 × 10-5 mM/s). The redox-mediated dsDNA-SG photooxidase mimic was used for detection of APE1 at pH 7.0 with over 130-fold higher sensitivity than that at 4.0, owing to the high enzymatic activity of APE1 at neutral pH. Meanwhile, we further extended this photooxidase mimic for the sensitive detection of DNA (LOD, 8 pM) and heavy metal ions at neutral pH. The redox-mediated dsDNA-dye photooxidase mimic with the ease of modulating its enzymatic activity and working at neutral pH is quite appealing for biosensing.

Using the High-Entropy Approach to Obtain Multimetal Oxide Nanozymes: Library Synthesis, In Silico Structure-Activity, and Immunoassay Performance

High-entropy nanomaterials exhibit exceptional mechanical, physical, and chemical properties, finding applications in many industries. Peroxidases are metalloenzymes that accelerate the decomposition of hydrogen peroxide. This study uses the high-entropy approach to generate multimetal oxide-based nanozymes with peroxidase-like activity and explores their application as sensors in ex vivo bioassays. A library of 81 materials was produced using a coprecipitation method for rapid synthesis of up to 100 variants in a single plate. The A and B sites of the magnetite structure, (AA')(BB'B'')2O4, were substituted with up to six different cations (Cu/Fe/Zn/Mg/Mn/Cr). Increasing the compositional complexity improved the catalytic performance; however, substitutions of single elements also caused drastic reductions in the peroxidase-like activity. A generalized linear model was developed describing the relationship between material composition and catalytic activity. Binary interactions between elements that acted synergistically or antagonistically were identified, and a single parameter, the mean interaction effect, was observed to correlate highly with catalytic activity, providing a valuable tool for the design of high-entropy-inspired nanozymes.

A Novel ssDNA Aptamer Targeting Carcinoembryonic Antigen: Selection and Characterization

One of the major causes of a drastically shorter life expectancy and one of the most prevalent diseases in the world today is cancer. Given the data on the rise in cancer cases throughout the world, it is obvious that, despite the diagnostic techniques currently being used, there is a pressing need to develop precise and sensitive techniques for early diagnosis of the disease. A high degree of affinity and specificity towards particular targets is maintained by the short nucleic acid molecules known as aptamers. Aptamers outperform antibodies due to their unique benefits, such as their simplicity in synthesis and modification, lack of toxicity, and long-term stability. Utilizing an accurate recognition element and a robust signal transduction mechanism, molecular diagnostics can be extremely sensitive and specific. In this study, development of new single-stranded DNA aptamers against CEA for use in cancer diagnostics was accomplished using SELEX and NGS methods. As a result of 12 iterative SELEX rounds, nine aptamer candidates against CEA were developed. NGS comparative analysis revealed that round twelve had an enriched number of aptamers that were specifically bound, as opposed to round eight. Among the selected nine sequences characterized by bioinformatics analysis and ELONA, an aptamer sequence with the highest specificity and affinity for the target protein was identified and further examined. Aptamer sequence (6) was screened in a concentration-dependent assay, specificity analysis was performed, and its potential secondary and tertiary structures were predicted, which enabled us to test one of the possible putative interactions with CEA. Finally, aptamer sequence (6) labelled with a Cy5 fluorescent tag was used in confocal microscopy to observe its binding towards the CEA expressed in HT-29 human colon adenocarcinoma cell line.

Application of Peroxidase-Mimic Mn2BPMP Boosted by ADP to Enzyme Cascade Assay for Glucose and Cholesterol

The Mn2BPMP complex has an intrinsic peroxidase-like activity in the pH range of 5 to 8, especially a maximum activity at pH 7, while most peroxidase mimics operate at an acidic pH (mainly pH 4). Its peroxidase-like activity is high among small-molecule-based peroxidase mimics with a high reproducibility. In addition, we recently revealed that adenosine mono/diphosphate (AMP and ADP) significantly boosted the peroxidase-like activity of Mn2BPMP. These advantages imply that Mn2BPMP is suitable for biosensing as a substitute for horseradish peroxidase (HRP). Herein, we established a colorimetric one-pot assay system using the enzyme cascade reaction between analyte oxidase and ADP-boosted Mn2BPMP. The simple addition of ADP to the Mn2BPMP-based assay system caused a greater increase in absorbance for the same concentration of H2O2, which resulted in a higher sensitivity. It was applied to one-pot detection of glucose and cholesterol at 25 °C and pH 7.0 for a few minutes.

Rationale of 3,3',5,5'-Tetramethylbenzidine as the Chromogenic Substrate in Colorimetric Analysis

Horseradish peroxidase (HRP)-based assays feature particular interests because of the simple colorimetric readout. In these assays, 3,3',5,5'-tetramethylbenzidine (TMB) is the most widely used chromogenic substrates for HRP. The later research in nanozyme and DNAzyme also used TMB (the chosen substrate) because they are both HRP-mimics. It should be noted that the substrate of HRP is not just limited to TMB but, in fact, a broad range of benzidine derivatives. However, except decreased carcinogenicity due to tetrasubstitution of benzidine, the rationale for the chosen substrate TMB is not clear yet. Here, we addressed such a fundamental issue from the chemistry point of view. Nine benzidine derivatives featuring varied properties (different substitution groups and varied number of substitutions) were selected and investigated with four typical TMB-involved chromogenic systems. Among the existing benzidine substrates that are used for peroxidase-based assays, TMB exhibited the highest sensitivity, better color purity of colored products, and reasonable stability of oxidation products. Moreover, two tetrasubstituted benzidine derivatives other than TMB (4OCH₃ and 2OCH₃2CH₃) were synthesized for comparison. It turned out that the performances (sensitivity, color purity, and stability of the colored products) of TMB are still superior, thus chemically confirming its status of "the chosen substrate" in colorimetric assays.

Fabrication of a polypropylene immunoassay platform by photografting reaction

The technology of an immunoassay detection platform is critical to clinical disease diagnoses, especially for developing a medical diagnostic system. A polymer-based immunoassay platform was fabricated on nonwoven fabric polypropylene (PP) using a photografting reaction to graft 2-hydroxyethyl methacrylate (HEMA) and sulfobetaine (SBMA). The antifouling properties of PP-g-P(HEMA-co-SBMA) were investigated by fibrinogen adsorption and platelet adhesion. Carbonyldiimidazole was employed to activate the pendant hydroxyl groups in HEMA moieties and covalently coupled antibody molecules. The detection of the limit of the immunoassay platform was as low as 10 pg/mL. Antibody amount and bioactivity affected the availability of antibody and the sensitivity of immunoassay. The immune efficiency was dependent on the strategies of antibody immobilization. The immune efficiency of Au-g-P(SBMA-co-HEMA) and Au-SH surfaces measured by QCM-D was 165% and 35.7%, respectively. The covalently binding antibody via hydrophilic polymer chains as spacers could retain fragment antigen-binding up orientation, maintain the bioactivity of antibody, and mainly improve the accessibility of antibody molecules via adjusting the conformations of polymer chains when the antibodies recognized the antigens. Therefore, grafting hydrophilic polymers, such as zwitterionic PSBMA and reactive PHEMA onto nonwoven fabric PP, and binding antibody by covalent strategy had the potential to be developed as a commercial immunoassay platform.

Photoelectrochemical biosensor for CEA detection based on SnS2-GR with multiple quenching effects of Au@CuS-GR

A novel signal on-off type photoelectrochemical (PEC) biosensing system was designed for sensitive detection of carcinoembryonic antigen (CEA) based on tin disulfide nanosheets loaded on reduced graphene cxide (SnS₂-GR) as the photoactive material and gold nanoparticles coated on reduced graphene oxide-functionalized copper sulfide (Au@CuS-GR) for signal amplification. It's the first time for SnS₂-GR was exploited as a sensing matrix. Here, the photocurrent signals of SnS₂ were amplified attributed to the sensitization effect of graphene. As signal amplifier, Au@CuS-GR could quench the photocurrents of SnS₂-GR not only through the p-n type semiconductor quenching effect as well as the steric hindrance effect, but also as peroxidase mimetics to catalyze the oxidation of 4-Chloro-1-naphthol (4-CN) to produce insoluble product on the electrode surface. Based on the multiple signal amplification ability of Au@CuS-GR, CEA was detected sensitively with a linear range from 0.1 pg mL^-1 to 10 ng mL^-1 and limit of detection down to 59.9 fg mL^-1 (S/N = 3). Meanwhile, the PEC biosensor displayed excellent performance in the assay of human serum sample, showing good application prospects for various target analysis.

A novel colorimetric immunosensor based on platinum colloid nanoparticles immobilized on PowerVision as signal probes and Fe3 O4 @β-cyclodextrin as capture probes for ractopamine detection in pork

BACKGROUND

A novel colorimetric immunosensor was developed for the simple, sensitive and selective detection of ractopamine (RAC) based on using β-cyclodextrin-modified Fe₃ O₄ particles (Fe₃ O₄ @β-CD) as capture probes and complex platinum colloid nanoparticles (PtNPs-PV) composed of platinum colloid nanoparticles (PtNPs) and polymerase chelate PowerVision (PV) as signal probes.

RESULTS

PtNPs-PV double catalyzed the chromogenic substrate 3,3'-diaminobenzidine (DAB), which induced changes in the color of DAB and chromogenic absorbance. Incubation temperature, pH and incubation time were systematically optimized and, under optimum conditions, the measured absorbance values showed a linear relationship with the RAC concentrations in the range 0.03-8.1 ng mL^-1 . The detection limit was 0.01 ng mL^-1 . The sensor exhibited high sensitivity and specificity, as demonstrated by testing structurally similar organic compounds such as salbutamol, clenbuterol and dopamine. The practicality of the developed colorimetric immunosensor was supported by the successful detection of RAC in pork samples with recovery ranging from 94.00% to 106.00%.

CONCLUSION

We designed a novel sandwich-type noncompetitive colorimetric immunoassay for the detection of trace levels of RAC in pork. The proposed method can also be used for the detection of toxins in food products via PtNPs-PV amplification. © 2018 Society of Chemical Industry.

Measurement of Carcinoembryonic Antigen in Clinical Serum Samples Using a Centrifugal Microfluidic Device

Carcinoembryonic antigen (CEA) is a broad-spectrum tumor marker used in clinical applications. The primarily clinical method for measuring CEA is based on chemiluminescence in serum during enzyme-linked immunosorbent assays (ELISA) in 96-well plates. However, this multi-step process requires large and expensive instruments, and takes a long time. In this study, a high-throughput centrifugal microfluidic device was developed for detecting CEA in serum without the need for cumbersome washing steps normally used in immunoreactions. This centrifugal microdevice contains 14 identical pencil-like units, and the CEA molecules are separated from the bulk serum for subsequent immunofluorescence detection using density gradient centrifugation in each unit simultaneously. To determine the optimal conditions for CEA detection in serum, the effects of the density of the medium, rotation speed, and spin duration were investigated. The measured values from 34 clinical serum samples using this high-throughput centrifugal microfluidic device showed good agreement with the known values (average relative error = 9.22%). These results indicate that the high-throughput centrifugal microfluidic device could provide an alternative approach for replacing the classical method for CEA detection in clinical serum samples.

A single nanoparticle-based real-time monitoring of biocatalytic progress and detection of hydrogen peroxide

This paper reported a new method to observe the catalytic progress of the natural horseradish peroxidase (HRP) in-situ on single gold nanoparticles (GNPs) by the combination of dark field imaging and plasmonic resonance scattering spectra. The produced single HRP-GNP exhibited localized catalytic property toward H₂O₂-Diaminobenzidine (DAB), which could be used to detect the concentration of H₂O₂ in micro/nanospace. The linear range for H₂O₂ sensing was from 0.01 μM to 5 μM with a detection limit of 10 nM. The new design strategy could be applied for a broader bioanalysis situation by substituting the HRP with other specified biocatalyst.

A label-free amperometric immunosensor for the detection of carcinoembryonic antigen based on novel magnetic carbon and gold nanocomposites

In this work, a novel label-free electrochemical immunosensor was developed for the quantitative detection of carcinoembryonic antigen (CEA).

An ultrasensitive electrochemical immunosensor for CEA using MWCNT-NH2 supported PdPt nanocages as labels for signal amplification

An ultrasensitive sandwich-type immunosensor was proposed by using MWCNT-NH₂ supported PdPt nanocages as labels for signal amplification.

An enzyme-linked immuno-mass spectrometric assay with the substrate adenosine monophosphate

An enzyme-linked immuno-mass spectrometric assay (ELIMSA) with the specific detection probe streptavidin conjugated to alkaline phosphatase catalyzed the production of adenosine from the substrate adenosine monophosphate (AMP) for sensitive quantification of prostate-specific antigen (PSA) by mass spectrometry. Adenosine ionized efficiently and was measured to the femtomole range by dilution and direct analysis with micro-liquid chromatography, electrospray ionization, and mass spectrometry (LC-ESI-MS). The LC-ESI-MS assay for adenosine production was shown to be linear and accurate using internal (13)C(15)N adenosine isotope dilution, internal (13)C(15)N adenosine one-point calibration, and external adenosine standard curves with close agreement. The detection limits of LC-ESI-MS for alkaline phosphatase-streptavidin (AP-SA, ∼190,000 Da) was tested by injecting 0.1 μl of a 1 pg/ml solution, i.e., 100 attograms or 526 yoctomole (5.26E-22) of the alkaline-phosphatase labeled probe on column (about 315 AP-SA molecules). The ELIMSA for PSA was linear and showed strong signals across the picogram per milliliter range and could robustly detect PSA from all of the prostatectomy patients and all of the female plasma samples that ranged as low as 70 pg/ml with strong signals well separated from the background and well within the limit of quantification of the AP-SA probe. The results of the ELIMSA assay for PSA are normal and homogenous when independently replicated with a fresh standard over multiple days, and intra and inter diem assay variation was less than 10 % of the mean. In a blind comparison, ELIMSA showed excellent agreement with, but was more sensitive than, the present gold standard commercial fluorescent ELISA, or ECL-based detection, of PSA from normal and prostatectomy samples, respectively.

Pyridoxamine-5-phosphate enzyme-linked immune mass spectrometric assay substrate for linear absolute quantification of alkaline phosphatase to the yoctomole range applied to prostate specific antigen

There is a need to measure proteins that are present in concentrations below the detection limits of existing colorimetric approaches with enzyme-linked immunoabsorbent assays (ELISA). The powerful enzyme alkaline phosphatase conjugated to the highly specific bacterial protein streptavidin binds to biotinylated macromolecules like proteins, antibodies, or other ligands and receptors with a high affinity. The binding of the biotinylated detection antibody, with resulting amplification of the signal by the catalytic production of reporter molecules, is key to the sensitivity of ELISA. The specificity and amplification of the signal by the enzyme alkaline phosphatase in ELISA together with the sensitivity of liquid chromatography electrospray ionization and mass spectrometry (LC-ESI-MS) to detect femtomole to picomole amounts of reporter molecules results in an ultrasensitive enzyme-linked immune mass spectrometric assay (ELIMSA). The novel ELIMSA substrate pyridoxamine-5-phosphate (PA5P) is cleaved by the enzyme alkaline phosphatase to yield the basic and hydrophilic product pyridoxamine (PA) that elutes rapidly with symmetrical peaks and a flat baseline. Pyridoxamine (PA) and (13)C PA were both observed to show a linear relationship between log ion intensity and quantity from picomole to femtomole amounts by liquid chromatography-electrospray ionization and mass spectrometry. Four independent methods, (i) internal (13)C isotope PA dilution curves, (ii) internal (13)C isotope one-point calibration, (iii) external PA standard curve, and (iv) external (13)C PA standard curve, all agreed within 1 digit in the same order of magnitude on the linear quantification of PA. Hence, a mass spectrometer can be used to robustly detect 526 ymol of the alkaline phosphatase streptavidin probe and accurately quantify zeptomole amounts of PSA against log linear absolute standard by micro electrospray on a simple ion trap.

Detection of subnanomolar melamine based on electrochemical accumulation coupled with enzyme colorimetric assay

Based on the synergetic effect of the electrochemical accumulation process and the signal amplification of enzymes, a new sensitive method has been developed for the detection of subnanomolar melamine. There are two steps involved in the sensor construction process: (1) accumulation of melamine on an electrode by cyclic voltammetric method and (2) chemical coupling of horseradish peroxidase (HRP) with the accumulated melamine through the linkage of glutaraldehyde. The coupled HRP catalyzes the oxidation of guaiacol to generate an amber-colored product. Quantitative analysis of melamine is performed by measuring the absorption intensities of the colored product. Under the optimal conditions, the method showed a wide linearity in the concentration range from 1.0 × 10(-11) to 1.0 × 10(-8) M for melamine detection. Moreover, it has been successfully applied to detect melamine in different infant formula powders and fish feed samples.

Microbiosensor for Alzheimer's disease diagnostics: detection of amyloid beta biomarkers

Alzheimer's disease (AD) affects about 35.6 million people worldwide, and if current trends continue with no medical advancement, one in 85 people will be affected by 2050. Thus, there is an urgent need to develop a cost-effective, easy to use, sensor platform to diagnose and study AD. The measurement of peptide amyloid beta (Aβ) found in CSF has been assessed as an avenue to diagnose and study the disease. The quantification of the ratio of Aβ1-40/42 (or Aβ ratio) has been established as a reliable test to diagnose AD through human clinical trials. Therefore, we have developed a multiplexed, implantable immunosensor to detect amyloid beta (Aβ) isoforms using triple barrel carbon fiber microelectrodes as the sensor platform. Antibodies act as the biorecognition element of the sensor and selectively capture and bind Aβ1-40 and Aβ1-42 to the electrode surface. Electrochemistry was used to measure the intrinsic oxidation signal of Aβ at 0.65 V (vs. Ag/AgCl), originating from a single tyrosine residue found at position 10 in its amino acid sequence. Using the proposed immunosensor Aβ1-40 and Aβ1-42 could be specifically detected in CSF from mice within a detection range of 20-50 nM and 20-140 nM respectively. The immunosensor enables real-time, highly sensitive detection of Aβ and opens up the possibilities for diagnostic ex vivo applications and research-based in vivo studies.