Peroxidase-like catalytic activity of copper ions and its application for highly sensitive detection of glypican-3

Toward Sensitive and Reliable Immunoassays of Marine Biotoxins: From Rational Design to Food Analysis

Marine biotoxins are metabolites produced by algae that can accumulate in shellfish or fish and enter organisms through the food chain, posing a serious threat to biological health. Therefore, accurate and rapid detection is an urgent requirement for food safety. Although various detection methods, including the mouse bioassay, liquid chromatography-mass spectrometry, and cell detection methods, and protein phosphatase inhibition assays have been developed in the past decades, the current detection methods cannot fully meet these demands. Among these methods, the outstanding immunoassay virtues of high sensitivity, reliability, and low cost are highly advantageous for marine biotoxin detection in complex samples. In this work, we review the recent 5-year progress in marine biotoxin immunodetection technologies such as optical immunoassays, electrochemical immunoassays, and piezoelectric immunoassays. With the assistance of immunoassays, the detection of food-related marine biotoxins can be implemented for ensuring public health and preventing food poisoning. In addition, the immunodetection technique platforms including lateral flow chips and microfluidic chips are also discussed. We carefully investigate the advantages and disadvantages for each immunoassay, which are compared to demonstrate the guidance for selecting appropriate immunoassays and platforms for the detection of marine biotoxins. It is expected that this review will provide insights for the further development of immunoassays and promote the rapid progress and successful translation of advanced immunoassays with food safety detection.

Current methods for the detection of glypican-3

Glypican-3 (GPC3) is a heparan sulfate proteoglycan (HSPG) that binds to the cell membrane via glycosylphosphatidylinositol (GPI), widely expressed in human embryos, and is undetectable in healthy adult liver but overexpressed in human hepatocellular carcinoma (HCC). Therefore, accurate and sensitive detection of GPC3 is critical for disease diagnosis. In recent years, a series of methods have been developed for the highly sensitive detection of GPC3, but there is a lack of reviews on recent advances in GPC3-related assays. In this review, we provide the recent advances in GPC3 detection and GPC3 concentration detection, mainly in terms of various optical sensor-based assays and electrochemical assays, and also provide new insights into the challenges and future directions of the field.

Morphology and Enzyme-Mimicking Activity of Copper Nanoassemblies Regulated by Peptide: Mechanism, Ultrasensitive Assaying of Trypsin, and Screening of Trypsin Inhibitors

To regulate nanostructure synthesis is of crucial importance for developing various applications, including catalysis, bioanalysis, and optical devices. Herein, the morphology and peroxidase (POD)-mimicking activity of peptide-templated copper nanoassemblies (Cu NAs) are regulable with peptide types. The Cu NAs templated with peptide containing single cysteine are uniform nanoclusters with strong POD-like activity. However, the Cu NAs templated with peptide containing two cysteines are fusiform-like with very weak POD-like activity. Unexpectedly, the POD-like activity of Cu NAs templated with peptide containing two cysteines with lysine between the cysteines is significantly enhanced when trypsin is incubated, which is unchanged for the Cu NAs templated with peptide containing two cysteines without lysine between the cysteines. The remarkably enhanced POD-mimicking activity originates from trypsin specifically shearing the peptide bond on the lysine, thereby allowing the aggregated Cu NAs to unravel into individual nanoclusters. Therefore, a robust colorimetric sensing platform was constructed for sensitive and selective detection of trypsin, which showed a linear concentration range of 3-1000 nM and a detection limit of 0.82 nM (S/N = 3). More interestingly, featured by trypsin inhibitor restraining trypsin activity, it enabled us to screen trypsin inhibitors as well. Subsequently, the developed assay was applied to detect trypsin in serum samples with good accuracy and reproducibility. Thus, this strategy shows great potential application in the clinic for diagnosis of trypsin-indicating diseases as well as the screening of trypsin inhibitor-based anti-cancer drugs.

Colorimetric Systems for the Detection of Bacterial Contamination: Strategy and Applications

Bacterial contamination is a public health concern worldwide causing enormous social and economic losses. For early diagnosis and adequate management to prevent or treat pathogen-related illnesses, extensive effort has been put into the development of pathogenic bacterial detection systems. Colorimetric sensing systems have attracted increasing attention due to their simple and single-site operation, rapid signal readout with the naked eye, ability to operate without external instruments, portability, compact design, and low cost. In this article, recent trends and advances in colorimetric systems for the detection and monitoring of bacterial contamination are reviewed. This article focuses on pathogen detection strategies and technologies based on reaction factors that affect the color change for visual readout. Reactions used in each strategy are introduced by dividing them into the following five categories: external pH change-induced pH indicator reactions, intracellular enzyme-catalyzed chromogenic reactions, enzyme-like nanoparticle (NP)-catalyzed substrate reactions, NP aggregation-based reactions, and NP accumulation-based reactions. Some recently developed colorimetric systems are introduced, and their challenges and strategies to improve the sensing performance are discussed.

Copper–carbon dot aerogel: a high-performance mimetic peroxidase and its application for versatile colorimetric bioassays

Benefiting from the electron transfer process and generation of hydroxyl radical, copper–carbon dot aerogels exhibit high-performance peroxidase-like activity and are applied for versatile colorimetric bioassays based on multienzyme cascade reactions.

Eco-Friendly Synthesis of SnO2-Cu Nanocomposites and Evaluation of Their Peroxidase Mimetic Activity

The enzyme mimetic activity of nanomaterials has been applied in colorimetric assays and point-of-care diagnostics. Several nanomaterials have been exploited for their peroxidase mimetic activity toward 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of hydrogen peroxide. However, an efficient nanomaterial for the rapid and strong oxidation of TMB remains a strategic challenge. Therefore, in this study, we developed copper-loaded tin oxide (SnO2-Cu) nanocomposites that rapidly oxidize TMB. These nanocomposites have strong absorption at 650 nm and can be used for highly sensitive colorimetric detection. An environmentally friendly (green), rapid, easy, and cost-effective method was developed for the synthesis of these nanocomposites, which were characterized using ultraviolet-visible, energy-dispersive X-ray, and Fourier-transform infrared spectroscopy, as well as scanning electron microscopy. This is the first green synthesis of SnO2-Cu nanocomposites. Their enzyme mimetic activity, which was first studied here, was found to be strongly dependent on the temperature and pH value of the solution. The synthesized nanocomposites have the advantages of low cost, high stability, and ease of preparation for enzyme mimetic applications. Hence, SnO2-Cu nanocomposites are a promising alternative to peroxidase enzymes in colorimetric point-of-care diagnostics.

Highly sensitive electrochemical aptasensor for Glypican-3 based on reduced graphene oxide-hemin nanocomposites modified on screen-printed electrode surface

Glypican-3 (GPC3) is a highly specific tumor marker for hepatocellular carcinoma (HCC), and plays an important role in reflecting the existence, therapeutic evaluation, monitoring and prognosis of HCC. Herein, an electrochemical aptasensor was designed for GPC3 detection with the reduced graphene oxide-hemin nanocomposites (RGO-Hemin) modified on the screen-printed electrode surface as the sensing platform and GPC3 aptamer as recognize molecule. In the existence of GPC3, the aptamer can specifically bind with the target GPC3 and form GPC3-aptamer conjugations on the sensing surface, which would increase the resistance of the electron transfer on the electrode and make the decrease of electrochemical signals of Hemin in RGO-Hemin nanocomposites. The electrochemical current change was recorded by differential pulse voltammetry (DPV). Scanning electron microscopy (SEM), Raman microscope (RM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to characterize the GPC3 electrochemical aptasensor. Under the optimum conditions, the current response of the electrochemical aptasensor is linearly correlated with the concentration of GPC3 (0.001-10.0 μg/mL) with the detection limit of 2.86 ng/mL (S/N = 3) and the sensitivity of 0.134 μA/μM/cm². In addition, the aptasensor was applied to the determination of GPC3 in spiked human plasma and the recoveries fluctuated from 102.68% to 117.29%. All these results show that the aptasensor has good specificity, sensitivity, stability and reproducibility for GPC3 detection.

Fluorescence Quenchers Manipulate the Peroxidase-like Activity of Gold-Based Nanomaterials

Although the regulation of the enzyme-like activities of nanozymes has stimulated great interest recently, the exploration of modulators makes it possible to enhance the catalytic performance of nanozymes, though doing so remains a big challenge. Herein, we systemically studied the effects of fluorescence quenchers on the peroxidase-like activity of bovine serum albumin-stabilized gold nanoclusters (BSA-AuNCs) based on photoinduced electron transfer (PET). We found that PET quenchers can not only quench the fluorescence of BSA-AuNCs but also regulate their intrinsic peroxidase-like activity. Importantly, both BSA and human serum albumin (HSA) could enhance the peroxidase-like activity of Cu²⁺, which provided a new sensing platform for distinguishing BSA and HSA from other thiol-containing biomolecules. The PET quenchers could also manipulate the peroxidase-like activity of polyvinylpyrrolidone-stabilized gold nanoparticles (PVP-AuNPs), which exhibited some opposite results between PVP-AuNPs and BSA-AuNCs. The opposite effects on BSA-AuNCs and PVP-AuNPs were speculated to highly depend on their surface properties. Our findings offer an efficient strategy for tuning the peroxidase-like activities of gold-based nanozymes.

Glypican-3 electrochemical aptamer nanobiosensor based on hemin/graphene nanohybrids peroxidase-like catalytic silver deposition

A Glypican-3 (GPC3) electrochemical aptamer nanobiosensor based on hemin/graphene nanohybrids (HGNs) peroxidase-like catalytic silver deposition and GPC3 aptamer has been constructed for the determination of GPC3. The HGNs were prepared by an one-step reduction method. Fourier transform infrared spectroscopy (FT-IR), ultraviolet spectroscopy (UV-vis), and transmission electron microscopy (TEM) were used to study the structure and morphological characteristics of the HGNs. The GPC3 electrochemical aptamer nanobiosensor was constructed using HGNs-aptamer (HGNs-Apt) as the signal probe and GPC3 aptamer as the capture probe. With the help of the catalytic action of peroxidase-like properties of HGNs, H₂O₂ reduces the silver (Ag) ions in solution to metallic Ag, which deposit on the surface of the electrode. The amount of deposited Ag, which was derived from the amount of GPC3, was quantified by differential pulse voltammetry (DPV). Under optimal conditions, the current response of Ag had a good positive correlation with the GPC3 concentration in the range 10.0-100.0 μg mL^-1 with a correlation coefficient of 0.9958. The detection limit was 3.16 μg mL^-1 at a signal-to-noise ratio of 3, and the sensitivity was calculated to be 0.807 μA μM^-1 cm^-2. The method is validated by analyzing spiked human serum samples with good recovery ranging from 101 to 122%. In addition, the GPC3 electrochemical aptamer nanobiosensor has acceptable selectivity, stability, and reproducibility. Graphical abstract A Glypican-3 electrochemical aptamer nanobiosensor based on hemin/graphene nanohybrids (HGNs) peroxidase-like catalytic silver deposition and GPC3 aptamer has been constructed for the determination of GPC3. The electrochemical aptamer nanobiosensor exhibits high selectivity, acceptance reproducibility, and good recovery performances.

A colorimetric method for determination of the prostate specific antigen based on enzyme-free cascaded signal amplification via peptide-copper(II) nanoparticles

Biotinylated peptide-Cu²⁺ nanoparticles (Cu-P NPs) were synthesized via "one-pot" self-assembly. The peptide P conststs of a hydrophobic dipeptide (FF), a tripeptide (KGH), and a biotin moiety attached to the terminal amino group of the Lys residue. The Cu-P NPs contain abundant catalytically active Cu²⁺ ions which are liberated by acid-induced dissolution. The released Cu²⁺ ions catalyze the oxidization of 3,3',5,5'-tetramethylbenzidine (TMB) by H₂O₂ because of their intrinsic peroxidase activity, and this results in the formation of a blue-green coloration. Based on the streptavidin-biotin interaction, the Cu-P NPs were employed to establish an enzyme-free colorimetric method for determination of prostate-specific antigen (PSA) as a model biomarker. Under optimal conditions, the linear response range is 0.001-1 ng mL^-1, with a limit of detection as low as 1 pg mL^-1. Graphical abstract Schematic illustration of a colorimetric immunoassay for the prostate specific antigen (PSA) with biotinylated peptide-Cu²⁺ nanoparticle (Cu-P NP) as the signal label based on the streptavidin (SA)-biotin interaction. The signal was produced by Cu²⁺-catalyzed oxidization of 3,3',5,5'-tetramethylbenzidine (TMB). P: KGHFF.

Nanomaterials-Based Colorimetric Immunoassays

Colorimetric immunoassays for tumor marker detection have attracted considerable attention due to their simplicity and high efficiency. With the achievements of nanotechnology and nanoscience, nanomaterials-based colorimetric immunoassays have been demonstrated to be promising alternatives to conventional colorimetric enzyme-linked immunoassays. This review is focused on the progress in colorimetric immunoassays with the signal amplification of nanomaterials, including nanomaterials-based artificial enzymes to catalyze the chromogenic reactions, analyte-induced aggregation or size/morphology change of nanomaterials, nanomaterials as the carriers for loading enzyme labels, and chromogenic reactions induced by the constituent elements released from nanomaterials.

Highly efficient redox reaction between potassium permanganate and 3,3′,5,5′-tetramethylbenzidine for application in hydrogen peroxide based colorimetric assays

Potassium permanganate (KMnO₄) is one of the most important oxidants, which plays important roles in many fields. Here, we found that KMnO₄ could directly induce the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) to generate an oxidized product with a color change. This redox reaction is highly efficient, and 1 μM KMnO₄ is enough to cause detectable changes in the absorbance signal. Meanwhile, this reaction is very fast and the generated blue product can stabilize for a relatively long period, which has great advantages in practical applications. Since hydrogen peroxide (H₂O₂) is able to react with KMnO₄ under acidic conditions, the KMnO₄-TMB system can be used for the detection of H₂O₂; the absorbance signal induced by 5 μM H₂O₂ can be easily detected in this method. Meanwhile, the KMnO₄-TMB system can also be used for the detection of glucose by monitoring the generation of H₂O₂, which is the main product of glucose oxidation; this method permits detection of concentrations as low as 10 μM glucose, and the sensitivity is comparable to or higher than most peroxidase mimetic based methods, but avoiding the preparation and storage of the nanomaterials. Furthermore, the KMnO₄-TMB system can even be used for analyzing glucose in serum samples, which can also be expected to be used in immunoassays.

The redox reaction between potassium permanganate and 3,3′,5,5′-tetramethylbenzidine is fast and highly efficient, which can be used for different biosensing.

A Colorimetric Enzyme-Linked Immunosorbent Assay with CuO Nanoparticles as Signal Labels Based on the Growth of Gold Nanoparticles In Situ

A colorimetric immunoassay has been reported for prostate-specific antigen (PSA) detection with CuO nanoparticles (CuO NPs) as signal labels. The method is based on Cu²⁺-catalyzed oxidation of ascorbic acid (AA) by O₂ to depress the formation of colored gold nanoparticles (AuNPs). Specifically, HAuCl₄ can be reduced by AA to produce AuNPs in situ. In the presence of target, CuO NPs-labeled antibodies were captured via the sandwich-type immunoreaction. After dissolving CuO nanoparticles with acid, the released Cu²⁺ catalyzed the oxidation of AA by O₂, thus depressing the generation of AuNPs. To demonstrate the accuracy of the colorimetric assay, the released Cu²⁺ was further determined by a fluorescence probe. The colorimetric immunoassay shows a linear relationship for PSA detection in the range of 0.1~10 ng/mL. The detection limit of 0.05 ng/mL is comparable to that obtained by other CuO NPs-based methods. The high throughput, simplicity, and sensitivity of the proposed colorimetric immunoassay exhibited good applicability for assays of serum samples.