Evaluation of seven cosubstrates in the quantification of horseradish peroxidase enzyme by square wave voltammetry

Synthesis of polyadenine-aptamer-stabilized gold nanoclusters and application to the detection of tobramycin in real samples based on their peroxidase-like activity

Tobramycin (TOB) is a widely used aminoglycoside antibiotic for treating human and animal diseases. However, its overuse poses a threat to human health, necessitating the development of a rapid and simple detection method. In this study, polyadenine-aptamer-stabilized gold nanoclusters (Ax-Apt-AuNCs) were synthesized, to investigate the impact of different polyadenine lengths on their properties. A20-Apt-AuNCs demonstrated efficient catalytic activity in the oxidation of o-phenylenediamine to 2,3-diaminophenazine (DAP) in the presence of hydrogen peroxide, resulting in yellow fluorescence emission. Upon binding specifically to the TOB aptamer on A20-Apt-AuNCs, TOB enhanced both their peroxidase-like activity and the fluorescence intensity of DAP. Based on this mechanism, a fluorescence-enhanced aptasensor was developed for TOB detection. The aptasensor exhibited a linear detection range of 10.0 nM to 1.0 μM, with a detection limit of 2.63 nM. Furthermore, its application in real sample analysis produced satisfactory results.

Carbon dots derived from expired drugs based ratiometric fluorescent sensor for horseradish peroxidase in fruits and vegetables and screening inhibitors

Household storage of pharmaceuticals to extract raw materials synthesized from carbon points facilitates the utilization of solid waste resources. A novel ratiometric fluorescence sensing technique was developed to ascertain the presence of horseradish peroxidase (HRP) in fruits and vegetables. The method employed a fluorescent probe, synthesized from expired amoxicillin (referred to as carbon dots, or A-CDs), serving as a reference fluorophore. Additionally, 2,3-diaminophenazine (DAP) was utilized as a specific response signal. DAP resulted from a catalytic reaction system involving phenylenediamine and hydrogen peroxide under the catalysis of HRP. The fluorescence intensity corresponding to DAP at 562 nm exhibited a substantial increase, simultaneous with the fluorescence quenching of A-CDs at 450 nm. The ratiometric fluorescence nanosensors displayed a broad linear range and high sensitivity for the detection of HRP. Across the concentration range 0.01 to 6 U L-1, the fluorescence intensity ratio between DAP and A-CDs demonstrated a proportional increase with rising HRP concentration, achieving an impressive detection limit of 0.002 U L-1. The recovery of HRP in fruit and vegetable samples ranged from 96.1 to 103%, with an RSD value of less than 3.8%. The proposed method facilitated the screening of inhibitors of HRP enzyme activity, contributing to the preservation of freshness in fruits and vegetables.

Horseradish peroxidase-triggered direct in situ fluorescent immunoassay platform for sensing cardiac troponin I and SARS-CoV-2 nucleocapsid protein in serum

Direct in situ fluorescent enzyme-linked immunosorbent assay (ELISA) is rarely investigated and reported. Herein, a direct in situ high-performance HRP-labeled fluorescent immunoassay platform was constructed. The platform was developed based on a rapid in situ fluorogenic reaction between Polyethyleneimine (PEI) and p-Phenylenediamine (PPD) analogues to generate fluorescent copolymer nanoparticles (FCNPs). The formation mechanism of FCNPs was found to be the oxidation of •OH radicals, which was further proved by nitrogen protection and scavenger of •OH radicals. Meantime, the fluorescence wavelength of FCNPs could be adjusted from 471 to 512 nm by introducing various substitution groups into the PPD structure. Using cardiac troponin I (cTnI) and SARS-CoV-2 nucleocapsid protein (N-protein) as the model antigens, the proposed fluorescent ELISA exhibited a wide dynamic range of 5-180 ng/mL and a low limit of detection (LOD) of 0.19 ng/mL for cTnI, and dynamic range of 0-120 ng/mL and a LOD of 0.33 ng/mL for SARS-CoV-2 N protein, respectively. Noteworthy, the proposed method was successful applied to evaluate the cTnI and SARS-CoV-2 N protein levels in serum with satisfied results. Therefore, the proposed platform paved ways for developing novel fluorescence-based HRP-labeled ELISA technologies and broadening biomarker related clinical diagnostics.

Integrated Experimental and Theoretical Studies on an Electrochemical Immunosensor

Electrochemical immunosensors (EIs) integrate biorecognition molecules (e.g., antibodies) with redox enzymes (e.g., horseradish peroxidase) to combine the advantages of immunoassays (high sensitivity and selectivity) with those of electrochemical biosensors (quantitative electrical signal). However, the complex network of mass-transfer, catalysis, and electrochemical reaction steps that produce the electrical signal makes the design and optimization of EI systems challenging. This paper presents an integrated experimental and modeling framework to address this challenge. The framework includes (1) a mechanistic mathematical model that describes the rate of key mass-transfer and reaction steps; (2) a statistical-design-of-experiments study to optimize operating conditions and validate the mechanistic model; and (3) a novel dimensional analysis to assess the degree to which individual mass-transfer and reaction steps limit the EI's signal amplitude and sensitivity. The validated mechanistic model was able to predict the effect of four independent variables (working electrode overpotential, pH, and concentrations of catechol and hydrogen peroxide) on the EI's signal magnitude. The model was then used to calculate dimensionless groups, including Damkohler numbers, novel current-control coefficients, and sensitivity-control coefficients that indicated the extent to which the individual mass-transfer or reaction steps limited the EI's signal amplitude and sensitivity.

Study of horseradish peroxidase and hydrogen peroxide bi-analyte sensor with boronate affinity-based molecularly imprinted film

A novel electrochemical horseradish peroxidase (HRP) sensor was developed based on boronate affinity-based electropolymerized polythionine (PTh) molecularly imprinted polymer (MIP) as specific recognition element for HRP on gold nanoparticles (AuNPs) modified glassy carbon electrode, in which PTh acted as the electrochemical probe for the sensor. The sensor was characterized by scanning electron microscopy and electron dispersive spectroscopy. Electrochemical impedance spectroscopy, cyclic voltammetry, and differential pulse voltammetry were exploited for the study of the properties of the MIP sensor. The MIP sensor exhibited excellent linear response over the range of 2.0 × 10−10 mg/mL ∼ 1.0 × 10−7 mg/mL for HRP. In addition, with MIP film as HRP immobilized matrices, the sensor for the detection of H2O2 was developed with the MIP sensor based on the reduction of H2O2 catalyzed by HRP in the presence of electron mediator PTh. The sensor showed linear relationships between the current response and H2O2 concentration from 6.0 × 10−7 to 2.0 × 10−5 mol/L. HRP and H2O2 bi-analyte sensor based on MIP film was successfully developed in this work. The developed method can also be applicable for enzyme and its enzymatic substrate bi-analyte sensor.

Facile synthesis of boronic acid-decorated carbon nanodots as optical nanoprobes for glycoprotein sensing

In this article, we proposed new nitrogen-doped boronic acid-decorated carbon nanodots (CNDs) for the recognition and detection of glycoproteins. These doped, decorated CNDs were obtained by a one-step hydrothermal carbonization method using phenylboronic acid and ethylenediamine as precursors. Compared to traditional synthesized and then functionalized nanoscale sensing systems, this method is more facile and efficient. The as-prepared nitrogen-doped CNDs possessed a quasi-spherical morphology and a high quantum yield of approximately 14.5%. The added glycoproteins (taking horseradish peroxidase as a model protein) can selectively induce the assembly and fluorescence quenching of CNDs through the formation of cyclic boronate esters, because the boronic acid groups on the CND surfaces can covalently interact with cis-diol-containing glycoproteins. These fluorescence responses can be used to properly quantify horseradish peroxidase in the range of 3.3-333.3 μg mL-1 with a detection limit of 0.52 μg mL-1, and the selectivity assay with functionalized CNDs was further investigated using various proteins with different quantities of glycosylation sites as well as using smaller molecules. The results show that the nanosensing system possesses favorable selectivity. Due to its simplicity and effectiveness, the system has great application prospects as a practical platform for glycoprotein sensing.

InP/ZnS quantum dot-based fluorescent probe for directly sensitive and selective detection of horseradish peroxidase

InP/ZnS quantum dot (QD)-based fluorescent probe for directly sensitive and selective detection of horseradish peroxidase (HRP) was reported herein. Fluorescence of InP/ZnS QDs was statically quenched by HRP, due to the ground state complex formation of InP/ZnS QDs with HRP. Such ground state complex formation between InP/ZnS QDs and HRP reduced both the α-helix content and the melting temperature of HRP. Several key factors including InP/ZnS QDs concentration, buffer pH value, ionic strength, reaction temperature, and reaction time those affected the analytical performance of InP/ZnS QDs in HRP determination were investigated thoroughly. Under the optimal conditions, fluorescence intensity of InP/ZnS QDs was linearly decreased with the increasing of HRP concentration during the range of 1.0 × 10^-9 M ∼ 3.0 × 10^-8 M (0.01 U ml^-1 ∼ 0.3 U ml^-1) with the detection limit as low as 1.2 × 10^-10 M (1.2 mU ml^-1). The present method showed excellent selectivity for HRP over some amino acids, nucleotides, and common proteins. This method was utilized to detect HRP in synthetic samples successfully.

A Nuclease Protection ELISA Assay for Colorimetric and Electrochemical Detection of Nucleic Acids

Early and accurate diagnosis is crucial to monitor infection outcomes and provide timely interventions. However, gold standard polymerase chain reaction assays (PCR) are labor-intensive and require expensive reagents and instrumentation. Nuclease protection has been used for decades to detect and quantify nucleic acid but has not yet been investigated as a diagnostic tool for infectious disease. In this work, we describe a nuclease protection enzyme-linked immunosorbent assay (NP-ELISA) for accurate and sensitive detection of nucleic acid. Briefly, binding of a nucleic acid target to an oligo probe protects it from digestion of un-hybridized nucleic acid by S1 nuclease. Following the workflow of an ELISA, a horseradish peroxidase (HRP)-conjugated antibody binds the probe and oxidizes its substrate to generate signal. The assay was validated with three HRP substrates for absorbance, chemiluminescence, and electrochemical readouts, demonstrating great versatility. Electrochemical detection with 3,3',5,5'-Tetramethylbenzidine (TMB) gave the highest assay sensitivity with a limit of detection of 3.72×103 molecules mL-1. Furthermore, non-complementary targets did not generate a response, indicating a high degree of specificity. This proof of principle serves as a stepping stone towards developing miniaturized, multiplexed nuclease protection assays for point-of-care diagnosis.

Zymographic approach to determine the intrinsic enzyme specific activity of diamine oxidase in presence of interfering enzymes

The purpose of this investigation was to elaborate a fast zymographic assay of oxidase enzymes in the presence of interfering enzymes as catalase (which disturbs current dosages based on H₂O₂ detection). This method also allows the determination of intrinsic specific activity (ISA) of oxidases, such as diamine oxidase (DAO) or glucose oxidase (GOD). The SDS-PAGE gels with entrapped peroxidase have been obtained by polymerization of acrylamide and bis-acrylamide in the presence of horse-radish peroxidase. The entrapped peroxidase was uniformly distributed in the PolyacrylAmide (PAA) material and did not migrate during electrophoresis. The obtained PAA gels allow the electrophoretic separation of various oxidases from contaminating proteins. As an example, to reveal DAO, the resulting PAA-gel should be incubated after the electrophoretic run in the developing solution containing putrescine (a DAO substrate) and o-phenylenediamine (a HRP substrate) to give coloured bands on the gel in the presence of DAO-generated H₂O₂. The results showed that is possible to determine the DAO in the presence of interfering catalase because they migrate differently. Thus, the H₂O₂ released in situ by DAO is no more decomposed by catalase because of its different mobility. It was also found that the same electrophoretic gel, after zymography, can be restained by Coomassie Blue for quantitation of proteins corresponding to the zymographic bands. With the obtained enzyme units and protein concentration it is also possible to calculate the intrinsic specific activity of DAO directly from the intensities of enzyme bands in zymography and from those of protein bands (Coomassie Blue staining), quantified by densitometry.

Square-wave voltammetry assays for glycoproteins on nanoporous gold

Electrochemical enzyme-linked lectinsorbent assays (ELLA) were developed using nanoporous gold (NPG) as a solid support for protein immobilization and as an electrode for the electrochemical determination of the product of the reaction between alkaline phosphatase (ALP) and p-aminophenyl phosphate (p-APP), which is p-aminophenol (p-AP). Glycoproteins or concanavalin A (Con A) and ALP conjugates were covalently immobilized onto lipoic acid self-assembled monolayers on NPG. The binding of Con A - ALP (or soybean agglutinin - ALP) conjugate to glycoproteins covalently immobilized on NPG and subsequent incubation with p-APP substrate was found to result in square-wave voltammograms whose peak difference current varied with the identity of the glycoprotein. NPG presenting covalently bound glycoproteins was used as the basis for a competitive electrochemical assay for glycoproteins in solution (transferrin and IgG). A kinetic ELLA based on steric hindrance of the enzyme-substrate reaction and hence reduced enzymatic reaction rate after glycoprotein binding is demonstrated using immobilized Con A-ALP conjugates. Using the immobilized Con A-ALP conjugate, the binding affinity of immunoglobulin G (IgG) was found to be 105 nM, and that for transferrin was found to be 650 nM. Minimal interference was observed in the presence of 5 mg mL-1 BSA as a model serum protein in both the kinetic and competitive ELLA. Inhibition studies were performed with methyl D-mannoside for the binding of TSF and IgG to Con A-ALP; IC50 values were found to be 90 μM and 286 μM, respectively. Surface coverages of proteins were estimated using solution depletion and the BCA protein concentration assay.