A colorimetric immunoassay is described for the pregnancy marker human chorionic gonadotropin (HCG). The assay is based on the use of MnO 2 nanorods acting as a peroxidase mimic. The nanorods were prepared by a hydrothermal method using EDTA as a template. They exhibit excellent peroxidase-like activity and stability. The nanorods were immobilized in a chitosan matrix in the wells of a BSA-modified microplate which then were further modified with streptavidin and biotinylated capture antibodies. The specific recognition between HCG and the antibodies in wells inhibit the peroxidase-like activity of the nanorods. Hence, the substrate 3,3′,5,5′-tetramethylbenzidine is less efficiently catalytically oxidized by H 2 O 2 to form a blue colored product. This results in a decrease in the absorbance at 652 nm. Response is linear in the 0.5 to 400 mIU·mL −1 HCG concentration range, and the detection limit is 0.36 mIU·mL −1 under optimized conditions. The method is highly specific, acceptably reproducible and stable. It was applied to the determination of HCG concentration in serum samples, and results were in good agreement with data obtained by the reference method.”
Schematic presentation of colorimetric immunoassay for human chorionic gonadotropin (HCG) by using MnO 2 nanorods with peroxidase-like activity immobilized in microplate wells. The specific recognition between HCG and the antibodies in wells inhibits the peroxidase-like activity of the nanorods. TMB: 3,3′,5,5′-tetramethylbenzidine.
Zr-metal–organic frameworks (Zr-MOFs) have received increasing interest for their use as the signal marker in the development of sandwich-structured aptasensors for the detection of exosomes. However, Zr4+ ions of the Zr-MOFs can interact with not only the exosomes, but also the aptamers, leading to possible false positives and a large background response. In the present study, we report for the first time aptasensors with Pd nanoparticle (NP)-decorated and hemin-embedded UiO-66 MOFs serving as the signal amplification marker to eliminate false positives and decrease the background response of aptasensors. To construct aptasensors for detection of exosomes, CD63-specific aptamers were tethered onto magnetic Fe3O4 particles coated with polydopamine (PDA) and UiO-66-NH2 using glutaraldehyde crosslinking for capturing the exosomes. To prepare highly catalytic Zr-MOF-based signal markers, UiO-66 MOFs were modified with hemin followed by Pd NPs. The as-prepared Pd-decorated hemin-embedded MOFs showed high catalytic activity towards the chromogenic oxidation reaction of TMB by H2O2. Moreover, the decoration with Pd NPs led to the change of the surface charge state of the catalytic hemin-embedded UiO-66 MOFs from positive to negative, weakening the interaction between the signal marker and the negatively charged aptamers. Therefore, the as-prepared aptasensors showed an improved sensing performance towards exosomes with a linear concentration range from 4.28 × 102 to 4.28 × 105 and a limit of detection (LOD) of 86.2 particles per μL. The as-prepared aptasensors also showed high sensitivity and selectivity to the exosomes from different origins including the HeLa cell line and MCF-7 cell line.
The level of uric acid is crucial to human health. Octahedral oxygen vacancy MnCo 2 O 4 /Ag (V O -MnCo 2 O 4 /Ag) nanozyme was successfully prepared by simple hydrothermal, calcination and self-reduction methods. V O -MnCo 2 O 4 /Ag nanozyme is rich in Mn 2+ /Mn 3+ and C O 2+ /C O 3+ redox electron pairs, large specific surface area and oxygen vacancies. V O -MnCo 2 O 4 /Ag nanozyme showed high uricase-like activity and peroxidase-like activity. At the same time, the SERS signal of the detected molecule could be significantly enhanced after the catalytic reaction of the V O -MnCo 2 O 4 /Ag nanozyme. The K m values of V O -MnCo 2 O 4 /Ag nanozyme for H 2 O 2 and TMB were 0.04 mM and 0.027 mM respectively. Based on the uric acid oxidase-like and peroxidase-like activities of V O -MnCo 2 O 4 /Ag, we developed a label-free, sensitive, and reliable SERS uric acid detection system. The detection linear range of uric acid is 0.01 μM–1000 μM and the detection of limit is 7.8 × 10 −9 M. The results show that the sensing system has good accuracy, sensitivity, selectivity , and stability. It can be applied to the determination of samples under different conditions. This study provides profound insights into the design of enzyme-like activity regulation and SERS properties regulation of nanozymes, provides guidance for the study of reaction kinetics and catalytic mechanism of nanozymes, and has broad application prospects in the field of nanozymes and SERS sensing analysis.
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