Colorimetric immunoassay for human chorionic gonadotropin by using peroxidase-mimicking MnO2 nanorods immobilized in microplate wells

Sensing the invisible: Ultrasensitive and selective colorimetric detection of E. coli O157:H7 based on masking the peroxidase-mimetic activity of aptamer-modified Au/Fe3O4

Escherichia coli O157:H7 (E. coli O157:H7) emerges as a significantly worrisome pathogen associated with foodborne illnesses, emphasizing the imperative for creating precise detection tools. In this investigation, we developed a sensitive colorimetric biosensor for detecting E. coli O157:H7. It was constructed using a nanozyme comprised of Au@Fe3O4 NPs, which was fabricated and subsequently modified with an aptamer (Apt). The nanozyme harnesses its inherent peroxidase-like activity to facilitate the transformation of reduced TMB into its oxidized form in the presence of H2O2, resulting in a noticeable shift to a blue color. However, the presence of E. coli O157:H7 effectively diminished the absorbance of oxidized TMB. Consequently, the normalized absorbance at 652 nm demonstrates a linear decrease corresponding to concentrations of E. coli O157:H7 within the range of 101 to 108 CFU mL-1 with a low limit of detection (LOD, S/N = 3) of 3 CFU mL-1.

A novel test device and quantitative colorimetric method for the detection of human chorionic gonadotropin (hCG) based on Au@Zn-salen MOF for POCT applications

The human chorionic gonadotropin (hCG) hormone is a biomarker that can predict tumors and early pregnancy; however, it is challenging to develop sensitive qualitative-quantitative procedures that are also effective, inventive, and unique. In this study, we used a novel easy in situ reaction of an organic nano-linker with Zn(NO₃)₂·6H₂O and HAuCl₄·3H₂O to produce a gold-zinc-salen metal-organic framework composite known as Au-Zn-Sln-MOF. A wide variety of micro-analytical instruments and spectroscopic techniques were used in order to characterize the newly synthesized Au-Zn-Sln-MOF composite. Disclosure is provided for a novel swab test instrument and a straightforward colorimetric approach for detecting hCG hormone based on an Au-Zn-Sln-MOF composite. Both of these methods are easy. In order to validate a natural enzyme-free immunoassay, an Au-Zn-Sln-MOF composite was utilized in the role of an enzyme; a woman can use this gadget to determine whether or not she is pregnant in the early stages of the pregnancy or whether or not her hCG levels are excessively high, which is a symptom that she may have a tumor. This cotton swab test device is compatible with testing of various biological fluids, such as serum, plasma, or urine, and it can be easily transferred to the market to commercialize it as a costless kit, which will be 20-30% cheaper than what is available on the market. Additionally, it can be used easily at home and for near-patient testing (applications of point-of-care testing (POCT)).

Recent advances in colorimetric sensors based on nanozymes with peroxidase-like activity

Nanozymes have been widely used to construct colorimetric sensors due to their advantages of cost-effectiveness, high stability, good biocompatibility, and ease of modification. The emergence of nanozymes greatly enhanced the detection sensitivity and stability of the colorimetric sensing platform. Recent significant research has focused on designing various sensors based on nanozymes with peroxidase-like activity for colorimetric analysis. However, with the deepening of research, nanozymes with peroxidase-like activity has also exposed some problems, such as weak affinity and low catalytic activity. In view of the above issues, existing investigations have shown that the catalytic properties of nanozymes can be improved by adding surface modification and changing the structure of nanomaterials. In this review, we summarize the recent trends and advances of colorimetric sensors based on several typical nanozymes with peroxidase-like activities, including noble metals, metal oxides, metal sulfides/metal selenides, and carbon and metal-organic frameworks (MOF). Finally, the current challenges and prospects of colorimetric sensors based on nanozymes with peroxidase-like activity are summarized and discussed to provide a reference for researchers in related fields.

A novel colorimetric sensor for naked-eye detection of cysteine and Hg2+ based on "on-off" strategy using Co/Zn-grafted mesoporous silica nanoparticles

In an attempt to explore the significance of inorganic mimetic enzymes as sensors, this study introduces a naked-eye analytical sensing platform for the detection of L-cysteine (cys), mercury ions (Hg²⁺) based on (turn off/turn-on) catalytic activity of zinc and cobalt grafted mesoporous silica nanoparticles (MSNs). To this end, Zn-MSN and Co/Zn-MSN catalysts were synthesized and characterized using XRD, FT-IR, FESEM, TEM, and nitrogen adsorption-desorption methods. Then, using the intrinsic peroxidase-like activity of as-synthesized samples, the oxidation reactions of the chromogenic substrate (2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS)) was designed using H₂O₂, which produced green colored cation radical of ABTS. Considering the high peroxidase-like activity of Co/Zn-MSN in comparison to Zn-MSN, it was employed to detect cys and then Hg²⁺. The results indicated that the strong interaction between cys and Co/Zn-MSN was proved by a limit of detection (LOD) down to 0.24 nM and the linear relationship from 0.8-50 nM (turn off). Given the fact that Hg²⁺ has a high-affinity tendency to combine with cys, we were suggested a novel colorimetric path for sensing of Hg²⁺ in the presence of cys (turn on). Based on this method, LOD was found 0.17 nM with the linear range of 0.57-50 nM. Taken together, results showed that the as-prepared catalysts are superior to other nanoparticles as a sensor to measure the target molecules in biological monitoring and clinical diagnostics.

Modification of a nitrocellulose membrane with nanofibers for sensitivity enhancement in lateral flow test strips

Lateral-flow analysis (LFA) is a convenient, low-cost, and rapid detection method, which has been widely used for screening of diseases. However, sensitivity enhancement in LFA is still a focus in this field and remains challenging. Herein, we propose an electrospinning coating method to modify the conventional nitrocellulose (NC) membrane and optimize the liquid flow rate for enhancing the sensitivity of the NC based LFA strips in the detection of human chorionic gonadotropin (HCG) and luteinizing hormone (LH). It can be seen that coating the NC membrane with nitrocellulose fibers could obtain a NC based strip with HCG and LH detection limits of 0.22 and 0.36 mIU mL-1 respectively, and a quantitative linear range of 0.5-500 mIU mL-1. The results show that electrospinning is effective in modifying conventional NC membranes for LFA applications.

One-step colorimetric detection of Staphylococcus aureus based on target-induced shielding against the peroxidase mimicking activity of aptamer-functionalized gold-coated iron oxide nanocomposites

Staphylococcus aureus (S. aureus) is one of the most threatened food-borne pathogens. Thus, it is necessary to establish fast, portable and reliable tools to realize the identification of S. aureus. Herein, the authors describe an effective colorimetric-based biosensor for the detection of S. aureus in multiple types of samples. Initially, a nanozyme composed of gold and iron oxide nanoparticles was synthesized and further modified with S. aureus-specific aptamer via Au-S bond. By utilizing the intrinsic peroxidase-like activity of the above magnetic conjugates, 3,3',5,5'-tetramethylbenzidine (TMB) can be transferred to oxTMB by oxidation of hydrogen peroxide (H₂O₂), resulting in a visible blue color. However, the introduction of S. aureus can turn off the UV-vis absorbance signals of TMB-H₂O₂ system, due to the identification property of the nanozyme probe. Consequently, the optical density of the mixed solution measured at 652 nm decreased linearly as the concentration of S. aureus increased from 10 to 10⁶ CFU mL^-1, with the visible limit of detection as low as 10 CFU mL^-1. The as-prepared sensor can detect S. aureus in spiked water, milk and urine samples quantitatively during 12 min without any pre-enrichment, separation or washing steps. In our perception, the one-step colorimetric assay show promise in practical on-site detection of S. aureus.

Biomimetic two-dimensional nanozymes: synthesis, hybridization, functional tailoring, and biosensor applications

Biological enzymes play important roles in mediating the biological reactions in vitro and in vivo due to their high catalytic activity, strong bioactivity, and high specificity; however, they have also some disadvantages such as high cost, low environmental stability, weak reusability, and difficult production. To overcome these shortcomings, functional nanomaterials including metallic nanoparticles, single atoms, metal oxides, alloys, and others have been utilized as nanozymes to mimic the properties and functions of natural enzymes. Due to the development of the synthesis and applications of two-dimensional (2D) materials, 2D nanomaterials have shown high potential to be used as novel nanozymes in biosensing, bioimaging, therapy, logic gates, and environmental remediation due to their unique physical, chemical, biological, and electronic properties. In this work, we summarize recent advances in the preparation and functionalization, as well as biosensor and immunoassay applications of various 2D material-based nanozymes. To achieve this aim, first we demonstrate the preparation strategies of 2D nanozymes such as chemical reduction, templated synthesis, chemical exfoliation, calcination, electrochemical deposition, hydrothermal synthesis, and many others. Meanwhile, the structure and properties of the 2D nanozymes prepared by conjugating 2D materials with nanoparticles, metal oxides, biomolecules, polymers, ions, and 2D heteromaterials are introduced and discussed in detail. Then, the applications of the prepared 2D nanozymes in colorimetric, electrochemical, fluorescent, and electrochemiluminescent sensors are demonstrated.

Electrospun nitrocellulose membrane for immunochromatographic test strip with high sensitivity

The main goal of this work is to develop an economical, portable, disposable, and reliable point of care paper biosensor based on visualization, which can be used to detect viruses, bacteria, and proteins. However, the sensitivity of immunochromatography test (ICT) strips based on nitrocellulose to target detection has always been a problem. Here, we use an electrospun nitrocellulose (ENC) fiber membrane instead of traditional nitrocellulose fiber membrane to construct ICT strips for early pregnancy detection. By proper selection of the diameter of the ENC fiber to adjust the pore size, porosity, and morphology of the membrane, ICT strips with low flow rate and high protein loading were obtained. Based on these properties, a convenient and sensitive method for the colorimetric determination of human chorionic gonadotropin was developed. Under the optimal conditions, the detection limit of ICT based on ENC membrane is 10 mIU mL^-1 (S/N = 3), the linear detection range is 5-1000 mIU mL^-1, and the linear relationship is Y = 0.0434 X - 0.0136 (R² = 0.9802). In addition, the test strip has good specificity and stability, and will not produce false-positive results. Graphical abstract.

Electrochemiluminescence immunoassay of human chorionic gonadotropin using silver carbon quantum dots and functionalized polymer nanospheres

A composite, reduced graphene oxide (rGO) doped with silver nanoparticles (Ag NPs), was prepared by using binary reductants of sodium citrate and hydrazine hydrate. Carbon quantum dots (CQDs) synthesized by papaya peel combined with silver ions to form a CQDs-loaded silver nanoparticle (AgCQDs) nanocomposite. Polymer nanospheres (PNS) were generated via the infinite coordination polymer of ferrocene dicarboxylic acid and employed as carriers to load AgCQDs. The prepared AgCQDs@PNS-PEI has good biocompatibility and electrical conductivity and can be used as a matrix for the immobilization of a secondary antibody (Ab2). A sandwich-type electrochemiluminescence (ECL) immunosensor using AgCQDs@PNS-PEI nanocomposite as probe has been developed for the detection of human chorionic gonadotropin (HCG). The proposed immunosensor exhibits a linear range from 0.00100 to 500 mIU mL^-1 and the detection limit is 0.33 μIU mL^-1 (S/N = 3) under optimal conditions. The sensor exhibits excellent selectivity, good reproducibility, and high stability. These features demonstrate that the proposed method has promising potential for clinical protein detection and displays a new strategy to fabricate an immunosensor. Graphical abstract.

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.

A novel colorimetric immunoassay based on enzyme-regulated instant generation of Turnbull's blue for the sensitive determination of ochratoxin A

The aim of this study was to develop a novel colorimetric sensing method based on enzyme-regulated instant generation of Turnbull's blue, serving as a chromogenic agent, for a sensitive immunoassay for the determination of ochratoxin A (OTA).