A papain-based colorimetric catalytic sensing system for immunoassay detection of carcinoembryonic antigen

A colorimetric immunoassay was built for determination of carcinoembryonic antigen (CEA) based on papain-based colorimetric catalytic sensing system through the use of glucose oxidase (GOx). In the presence of GOx, glucose was catalytically oxidized to produce H2O2. Through the assistance of papain (as a peroxide mimetic enzyme), the signal came from the oxidative color development of 3,3',5,5'-tetramethylbenzidine (TMB, from colorless to blue) catalyzed by the generated H2O2. Herein, a sandwich-type immunoassay was built based on GOx as labels. As the concentration of CEA increased, more GOx-labeled antibodies specifically associate with target, which leaded to more H2O2 generation. Immediately following this, more TMB were oxidized with the addition of papain. Accordingly, the absorbance increased further. As a result, the concentration of CEA is positively correlated with the change in absorbance of the solution. Under optimal conditions, the CEA concentration was linear in the range of 0.05-20.0 ng/mL, and the limit of detection (LOD) reached 37 pg/mL. The papain-based colorimetric immunoassay also exhibited satisfactory repeatability, stability, and selectivity.

A novel electrochemical immunosensor that amplifies Poly(o-phenylenediamine) signal by pH-driven cascade reaction used for alpha-foetoprotein detection

The present protocol develops an electrochemical immunosensor with poly(o-phenylene diamine) attached gold nanoparticles (PPD@Au NPs) as the immune platform, polydopamine-loaded cobalt ions (Co²⁺-PDA) as the immune probe, and K₂S₂O₈ as the signal amplifying substance with pH-driven cascade reaction. The application of conventional immunosensors often leads to easy leakage of the current signal and increases the impedance due to assembly. However, this new immunosensor offers the following advantages: (1) The signal substance PPD is modified on the electrode surface, effectively reducing the signal loss and leakage of the immunosensor; (2) The pH response reduces the impedance of the immunosensor while destroying the Co²⁺-PDA secondary antibody label; (3) The pH response releases a small amount of Co²⁺, leading to SO₄^-· generation by K₂S₂O₈ through a cascade reaction, further amplifying the PPD response current signal; (4) The pH response generates excess Co²⁺ and the by-product PDA fragments can consume the SO₄^-· generated by K₂S₂O₈, so that the final response signal decreases with the increasing antigen concentration. The experimental results showed that the immunosensor exhibited good selectivity, long-term stability, and reproducibility for AFP detection in the range of 1 pg/mL-100 ng/mL, with a detection limit of 0.214 pg/mL. Interestingly, it is expected to be used for detecting AFP in actual blood samples.

Solvent assisted size effect on AuNPs and significant inhibition on K562 cells

Herein, the synthesis and characterization of ideal size (∼10 and 40 nm, in diameter) AuNPs (gold nanoparticles) were reported. Two different organic solvents such as DMF (dimethyl formamide) and NMPL (N-methyl-2-pyrrolidone) were used to synthesize AuNPs along with agents reducing agents such as NaBH4 (sodium borohydrate) and Na3C6H5O7 (sodium citrate). The combination of [(HAuCl4)-(DMF)-(NaBH4)] gives AuNPs with an avg. size of 10.2 nm. Similarly, the combination of [(HAuCl4)-(NMPL)-(Na3C6H5O7)] gives AuNPs with an avg. size of 40.4 nm. The morphology of these nanoscale AuNPs has been characterized through TEM and HRTEM imaging followed by SAED for lattice parameters such as d-spacing value (2.6 Å/0.26 nm) of crystalline metal (Au) nanoparticles. Further, these unique and ideal nanoscale AuNPs were used to evaluate the potential working efficacy by using in vitro cell based studies on K562 (leukaemia) blood cancer cells. From the MTT assay results around 88% cell inhibition was measured for ∼10 nm sized AuNPs. The treated cells were stained with different fluorescent dyes such as FITC, DAPI, Rho-6G and their ruptured morphology has been reported in the respective sections. These types of ideal sized metal (Au) nanoparticles are recommended for various theranostics such as to cure breast, colon, lung and liver cancers.

Nanomaterials with enzyme-like characteristics (nanozymes): next-generation artificial enzymes (II)

An updated comprehensive review to help researchers understand nanozymes better and in turn to advance the field.

Branched zinc oxide nanorods arrays modified paper electrode for electrochemical immunosensing by combining biocatalytic precipitation reaction and competitive immunoassay mode

Branched zinc oxide nanorods (BZR) arrays, an array with high charge carries collection efficiency and specific surface area, are grown on the reduced graphene oxide-paper working electrode for the first time to construct a paper-based electrochemical (EC) immunosensor. Typically, the BZR are fabricated via a simple hydrothermal process, which can provide abundant sites for antibodies loading. By combining the large surface area of porous zinc oxide (PZS) and good biocompatibility of gold nanoparticles (AuNPs), PZS-AuNPs (PZS@Au) nanocomposites are designed to label horseradish peroxide (HRP) and antigens. After a competitive reaction between antigens and PZS@Au nanocomposites labeled antigens, the signal labels are introduced into the immunosensor, in which, HRP participate in biocatalytic precipitation process. The produced precipitate reduces the electrode surface area and hinders the electron transfer. With the increase of concentration of antigens, the signal labels introduced into the sensor decrease, thus, a signal-on immunoassay for α-fetoprotein detection is constructed. The proposed paper-based EC immunosensor combines enzymatic biocatalytic precipitation reaction and competitive immunoassay mode for the first time, and possesses a wide linear range from 0.2 pg mL(-1) to 500 ng mL(-1) with a detection limit of 0.08 pg mL(-1). In addition, the proposed method is simple, sensitive and specific and can be a promising platform for other protein detection.

Preparation of magnetic core–shell nanoflower Fe3O4@MnO2 as reusable oxidase mimetics for colorimetric detection of phenol

In this paper, magnetic core–shell nanoflower Fe₃O₄@MnO₂ were fabricated via a solvothermal method.

Multiwalled carbon nanotube@reduced graphene oxide nanoribbon heterostructure: synthesis, intrinsic peroxidase-like catalytic activity, and its application in colorimetric biosensing

The MWCNT@rGONR core–shell heterostructure was proved to be a novel peroxidase mimetic and has been used for colorimetric biosensing of cholesterol.