Novel Immunosensor Based on Metal Single-Atom Nanozymes with Enhanced Oxidase-Like Activity for Capsaicin Analysis in Spicy Food

Dual FeCo single-atom nanozymes with specific oxidase-like activity for sensitive detection of aflatoxin B1

Single-atom nanozymes (SAzymes) with well-defined metal-nitrogen-carbon coordination structures are of great interest for the development of colorimetric biosensing. However, their catalytic efficiency and specificity is restricted due to the limited number of single metal atoms that can serve as catalytic centre. Therefore, the construction of SAzymes with high activity and specificity is vital but remains challenging. To address these issues, we prepared a bimetallic SAzymes with an independent iron and cobalt structure (FeCo/NC), and the oxidase-like activity was enhanced by >112.8% relative to Fe/NC. This preparation strategy increased the amount of single metal atoms loaded, resulting in a strong synergistic effect and proximity-orientation effects due to the unique structure of single Co and Fe atoms coexisting on graphene. The oxidase-mimicking activity of FeCo/NC was specifically enhanced by co doping, although the activities of peroxidase-, superoxide dismutase-, or catalase-like were not significantly affected. In light of these discoveries, as a symbol of the proof-of-concept, FeCo/NC-based colorimetric immunoassays were developed for sensitive detection of aflatoxin B1 (AFB1), achieving a linear range of 0.01-10 ng/mL and a detection limit of 0.005 ng/mL. This study provides a convenient strategy for promoting the catalytic activity and specificity of SAzymes, thereby enhancing their potential in biosensing.

Magnetic relaxation switching sensor based on self-generated porous Fe3O4 with controllable apparent diffusion coefficient for highly sensitive detection of caffeine

Magnetic relaxation switch (MRS) sensors based on magnetic nanoparticles have received much attention in the field of environmental monitoring due to their rapid response and resistance to complex matrix interference. However, conventional MRS (cMRS) sensors constructed using magnetic nanoparticles have low sensitivity due to the lower relaxation rate of the magnetic probe. In this study, self-generated porous Fe3O4 were employed in the MRS sensors for detection of caffeine, and the regulation of transverse relaxation performance by porous structure has been explored. The abundant porous structure of self-generated porous Fe3O4 restricted the diffusion of the surrounding water molecules, and the controllable pore size can effectively adjust the apparent diffusion coefficient (ADC), as a result, improving the transverse relaxation properties. The developed MRS sensor exhibits a wide linear detection range from 0.5 to 100 ng/mL and a low limit of detection (LOD) of 0.047 ng/mL for caffeine. The LOD is 12 times lower than that of cMRS sensors. Consequently, the MRS sensor was further applied to the detection of caffeine in surface water samples. The results were consistent with those detected using high-performance liquid chromatography, demonstrating its superior anti-interference ability and the significant potential in environmental monitoring and food safety domains.

Stable Magnetic Relaxation Switch Sensor Based on Fe3O4@Gel for Ultrafast Detection of Cd2

To overcome the dual challenges of signal instability and prolonged detection in conventional magnetic relaxation switching (MRS) systems, a novel Fe3O4-encapsulated alginate hydrogel nanocomposite (Fe3O4@Gel) sensor was designed for rapid screening of the cadmium ion. Compared with the traditional Fe3O4-based sensors, the Fe3O4 was embedded in the gel network framework to avoid magnetic field-induced aggregation, which helped to improve the stability of MRS. On the other hand, compared with MRS based on gel, the Fe3O4 accelerated the relaxation process of water molecules inside the gel, obtaining a fast detection time of the sensor within 38 s, which is one-fifth of the detection time of the traditional magnetic relaxation switch sensor with pure hydrogel of 191 s. Mechanistically, target-induced immunocomplex formation modulates alkaline phosphatase activity, triggering cascade enzymatic reactions that precisely regulate hydrogel swelling dynamics. This stimuli-responsive behavior translates quantitative Cd2+ concentrations into reproducible transverse relaxation time (T2) signal shifts (R2 = 0.987), achieving sub-ppt sensitivity (6 pg/mL) across linearity (0.01-10 ng/mL). Practical validation in complex matrices demonstrated 96.62%-109.97% spike recoveries. This multifunctional nanoplatform establishes a new paradigm for high-fidelity, field-deployable hazard screening in complex systems.

Single-atom nanozymes for enhanced electrochemical biosensing: A review

Enzyme-based electrochemical biosensors have broad and significant applications in biomedical, environmental monitoring, and food safety fields. However, the application of natural enzymes is limited due to issues such as poor stability, complex preparation, and high cost. Single-atom nanozymes (SAzymes), with their unique catalytic properties and efficient enzyme-like activities, present a promising alternative in the field of electrochemical biosensing. Compared to traditional enzymes, SAzyme offer enhanced stability and controllability, making them particularly effective in complex detection environments. This work presents the first systematic review of the progress made since 2018 in the use of SAzymes as alternatives to natural enzymes in electrochemical biosensors, and presents the latest advancements in this area. The review begins with a discussion of various enzyme-like activities of single-atom materials, including peroxidase (POD)-like, oxidase (OXD)-like, catalase (CAT)-like, and superoxide dismutase (SOD)-like activities. It then explores the advantages of SAzymes in improving the performance of electrochemical biosensors from multiple perspectives. The review also summarizes the applications of SAzyme-based electrochemical sensors for reactive oxygen species (ROS), metabolites, neurotransmitters, and other analytes, highlighting specific examples to elucidate underlying catalytic mechanisms and understand fundamental structure-performance relationships. In the final section, the challenges faced by SAzyme-based electrochemical biosensing are discussed, along with potential solutions.

Tuning Atomically Dispersed Metal Sites in Nanozymes for Sensing Applications

Nanozymes with atomically dispersed metal sites (ADzymes), especially single-atom nanozymes, have attracted widespread attention in recent years due to their unique advantages in mimicking the active sites of natural enzymes. These nanozymes not only maximize exposure of catalytic sites but also possess superior catalytic activity performance, achieving challenging catalytic reactions. These advantages position ADzymes as highly promising candidates in the field of sensing and biosensing. This review summarizes the classification and properties of ADzymes, systematically highlighting some typical regulation strategies involving central metal, coordination environment, etc., to achieve their catalytical activity, specificity, and multifunctionality. Then, we present the recent advances of ADzymes in different sensing fields, including colorimetry, fluorescence, electrochemistry, chemiluminescence, photoelectrochemistry, and electrochemiluminescence. Taking advantage of their unique catalytic performance, the resultant ADzymes show great potential in achieving the goal of sensitivity, selectivity and accuracy for the detection of various targets. Specifically, the underlying mechanisms in terms of signal amplification were discussed in detail. Finally, the current challenges and perspectives on the development of advanced ADzymes are discussed.

Application and Cytotoxicity Evaluation of Fe-MIL-101 Nanozyme in Milk

In this study, we used Fe-MIL-101 nanozyme to convert lactose into lactitol, and it was proved that Fe-MIL-101 nanozyme has lactase-like activity. Due to the potential health effects of nanomaterials, we evaluated the cytotoxicity of Fe-MIL-101 nanozyme. To reduce the potential toxicity of the nanozyme, we applied centrifugation and membrane filtration. When the membrane aperture size was 100 nm, the residual content of Fe-MIL-101 nanozyme was 14.09 μg/mL. The residual content of Fe-MIL-101 nanozyme was reduced by optimizing time, temperature, and Fe-MIL-101 nanozyme-to-substrate ratio. It was showed that the concentration of Fe was 38.47 mg/kg and the concentration of H2BDC was 0 mg/kg under optimized conditions (110℃, 2 h of reaction and the ratio of Fe-MIL-101 nanozyme to substrate is 1:20). The result met the national standard of China. Experiments measuring cytotoxicity, oxidative stress, and cell membrane damage revealed that less than 20 μg/mL Fe-MIL-101 nanozyme had no significant cytotoxicity. Our study findings showed that Fe-MIL-101 nanozyme reduced lactose content in milk.