ZrO2/CeO2/polyacrylic acid nanocomposites with alkaline phosphatase-like activity for sensing

Applications of cerium-based materials in food monitoring

With the rapid development of society, food safety to public health has been a topic that cannot be ignored. In recent years, lanthanide-based materials are studied to be potential candidates in the detection of food samples. Cerium (Ce)-based materials (such as Ce ions, CeO2, Ce-metal organic framework (Ce-MOF), etc.) have also attracted more attention in food detection by virtue of colorimetric, fluorescence, sensing, and other methods. This is because the mixed valence of Ce (Ce3+ and Ce4+), the formation of oxygen vacancies, and their optical and electrochemical properties. In this review, Ce-based materials will be introduced and discussed in the field of food detection, including biogenesis, construction, catalytic mechanisms, combination, and applications. In addition, the current challenges and future development trend of these Ce-based materials in food safety detection are also proposed and discussed. Therefore, it is meaningful to explore the Ce-based materials for detection of biomarkers in food samples.

Surface Chemistry of Biologically Active Reducible Oxide Nanozymes

Reducible metal oxide nanozymes (rNZs) are a subject of intense recent interest due to their catalytic nature, ease of synthesis, and complex surface character. Such materials contain surface sites which facilitate enzyme-mimetic reactions via substrate coordination and redox cycling. Further, these surface reactive sites are shown to be highly sensitive to stresses within the nanomaterial lattice, the physicochemical environment, and to processing conditions occurring as part of their syntheses. When administered in vivo, a complex protein corona binds to the surface, redefining its biological identity and subsequent interactions within the biological system. Catalytic activities of rNZs each deliver a differing impact on protein corona formation, its composition, and in turn, their recognition, and internalization by host cells. Improving the understanding of the precise principles that dominate rNZ surface-biomolecule adsorption raises the question of whether designer rNZs can be engineered to prevent corona formation, or indeed to produce "custom" protein coronas applied either in vitro, and preadministration, or formed immediately upon their exposure to body fluids. Here, fundamental surface chemistry processes and their implications in rNZ material performance are considered. In particular, material structures which inform component adsorption from the application environment, including substrates for enzyme-mimetic reactions are discussed.

Ceria-Based Nanozymes in Point-of-Care Diagnosis: An Emerging Futuristic Approach for Biosensing

In recent years, there has been a notable increase in interest surrounding nanozymes due to their ability to imitate the functions and address the limitations of natural enzymes. The scientific community has been greatly intrigued by the study of nanoceria, primarily because of their distinctive physicochemical characteristics, which include a variety of enzyme-like activities, affordability, exceptional stability, and the ability to easily modify their surfaces. Consequently, nanoceria have found extensive use in various biosensing applications. However, the impact of its redox activity on the enzymatic catalytic mechanism remains a subject of debate, as conflicting findings in the literature have presented both pro-oxidant and antioxidant effects. Herein, we creatively propose a seesaw model to clarify the regulatory mechanism on redox balance and survey possible mechanisms of multienzyme mimetic properties of nanoceria. In addition, this review aims to showcase the latest advancements in this field by systematically discussing over 180 research articles elucidating the significance of ceria-based nanozymes in enhancing, downsizing, and enhancing the efficacy of point-of-care (POC) diagnostics. These advancements align with the ASSURED criteria established by the World Health Organization (WHO). Furthermore, this review also examines potential constraints in order to offer readers a concise overview of the emerging role of nanoceria in the advancement of POC diagnostic systems for future biosensing applications.

Ratiometric colorimetric detection of nitrite using CoMnO3 nanofibers as an oxidase-like enzyme to induce diazotization reaction

Nitrite is a typical food additive and preservative used in the food industry, which has attracted considerable attention due to its severe adverse effects on human health. Herein, a sensitive and highly selective ratiometric colorimetric sensing platform for the detection of nitrite was created based on a polymetallic oxide nanozyme, CoMnO3 nanofibers (CMO) catalysis integrated with the particular diazotization reaction. The nanozyme has superior oxidase-like activity (Km was 0.105 mM and Vmax was 63.7 × 10-8 M S-1) and could catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to oxidized TMB (oxTMB), as CMO could achieve the conversion of oxygen in the solution to superoxide anion (O2˙-). In addition, it is interesting to note that oxTMB can be diazotized in the presence of nitrite under acidic conditions, causing a shift in the ratio of nitrite concentration to the absorbance peaks at 450 and 652 nm (A450/A652). The ratio of A450/A652 exhibited a positive linear relationship with the concentration of nitrite within the concentration range of 0.2-200 μM, with a detection limit of 0.094 μM. Simultaneously, this method was also successful in quantifying the nitrite produced by brined and pickled foods and the dynamic tracking of the nitrite levels in various types of dishes. The analysis method not only offers dual-signal ratio sensing with high sensitivity but also holds the benefit of outstanding selectivity for the use of the particular reaction, which has a wide range of application prospects in food safety management.

Overview of nanozymes with phosphatase-like activity

Nanomaterials with intrinsic enzyme activity, referred to as nanozymes, have attracted substantial attention in recent years. Among them, phosphatase-mimicking nanozymes have become an increasingly important focus for future research, considering that phosphatase is not only one of key enzymes for phosphorous metabolism, which is essential for many biological processes (e.g., cellular regulation and signaling), but also one of extensively used biocatalytic labels in the enzyme-linked assays as well as a powerful tool enzyme in molecular biology laboratories. Nevertheless, compared with extensive oxidoreductase-mimicking nanozymes, there are a very limited number of nanozymes with phosphatase-like activity have been explored at present. The increasing demand of complex and individualized phosphatase-involved catalytic behaviors is pushing the development of more advanced phosphatase-mimicking nanozymes. Thus, we present an overview on recently reported phosphatase-like nanozymes, providing guidelines and new insights for designing more advanced phosphatase-mimicking nanozyme with superior properties.

A triple amplification strategy using GR-5 DNAzyme as a signal medium for ultrasensitive detection of trace Pb2+ based on CRISPR/Cas12a empowered electrochemical biosensor

Lead ions (Pb²⁺) are a well-known toxic heavy metal that poses a significant threat to human health. Therefore, the development of a simple and ultrasensitive technique for detecting Pb²⁺ is essential. With their trans-cleavage properties, the newly discovered CRISPR-V effectors have become a potential high-precision biometric tool. In this regard, a CRISPR/Cas12a-based electrochemical biosensor (E-CRISPR) has been developed, which is combined with the GR-5 DNAzyme that can specifically recognize Pb²⁺. In this strategy, the GR-5 DNAzyme acts as a signal-mediated intermediary, which can convert Pb²⁺ into nucleic acid signals, thereby becoming single-stranded DNA that triggers strand displacement amplification (SDA) reaction. This is coupled with following activated CRISPR/Cas12a cleavage of the electrochemical signal probe, enabling cooperative signal amplification for ultrasensitive Pb²⁺ detection. The proposed method has a detection limit as low as 0.02 pM. Therefore, we have developed an E-CRISPR detection platform with GR-5 DNAzyme as a signal medium (called SM-E-CRISPR biosensor). This provides a method for the CRISPR system to specifically detect non-nucleic substances by converting the signal using a medium.

Peroxidase-like Activity of CeO2 Nanozymes: Particle Size and Chemical Environment Matter

The enzyme-like activity of metal oxide nanoparticles is governed by a number of factors, including their size, shape, surface chemistry and substrate affinity. For CeO₂ nanoparticles, one of the most prominent inorganic nanozymes that have diverse enzymatic activities, the size effect remains poorly understood. The low-temperature hydrothermal treatment of ceric ammonium nitrate aqueous solutions made it possible to obtain CeO₂ aqueous sols with different particle sizes (2.5, 2.8, 3.9 and 5.1 nm). The peroxidase-like activity of ceria nanoparticles was assessed using the chemiluminescent method in different biologically relevant buffer solutions with an identical pH value (phosphate buffer and Tris-HCl buffer, pH of 7.4). In the phosphate buffer, doubling CeO₂ nanoparticles' size resulted in a two-fold increase in their peroxidase-like activity. The opposite effect was observed for the enzymatic activity of CeO₂ nanoparticles in the phosphate-free Tris-HCl buffer. The possible reasons for the differences in CeO₂ enzyme-like activity are discussed.

Rational design of a novel MOF-based ternary nanocomposite for effectively monitoring harmful organophosphates in foods and the environment

Methyl parathion (MP) is a widely used organophosphate insecticide that is extremely toxic due to its ability to irreversibly inhibit acetylcholinesterase in the body and persistently accumulate in the environment. Timely detection of MP can prevent harmful residue exposure to humans. Therefore, the development of fast, efficient electrochemical methods to detect trace MP has been highly beneficial for monitoring harmful residues in foods and environment to ensure food safety and ecological conservation. Herein, a novel hybrid metal-organic framework (MOF) nanocomposite composed of Pt nanoparticles (PtNPs), multi-walled carbon nanotubes (MWCNTs), and UiO-66-NH₂ (PtNPs/UiO-66-NH₂/MWCNTs) was rationally designed and prepared by a facile two-step strategy for the sensitive determination of MP. The synergistic effects are illustrated in detail using XRD, XPS, FTIR, TEM, and SEM studies as well as electrochemical technologies such as CV, EIS, and DPV. In addition, the performance of the ternary nanocomposite for detecting MP was investigated by comparing it with the binary-component one. The results showed that the PtNPs/UiO-66-NH₂/MWCNT-based electrochemical sensor exhibited outstanding sensitivity of 21.9 μA μM^-1 cm^-2, satisfactory low detection limit of 0.026 μM and wide linear range of 0.11-227.95 μM for MP analysis. Furthermore, the fabricated sensor delivered distinguished freedom from interferences, outstanding regeneration ability, and adequate recoveries for fresh foods and river water samples. In conclusion, the proposed PtNPs/UiO-66-NH₂/MWCNT-based sensor provides a potentially useful analytical tool for determining hazardous residues of OPs in foods and the environment.

Polymer based nanocomposites: A strategic tool for detection of toxic pollutants in environmental matrices

A large fraction of population is suffering from waterborne diseases due to the contaminated drinking water. Both anthropogenic and natural sources are responsible for water contamination. Revolution in industrial and agriculture sectors along with a huge increase in human population has brought more amount of wastes like heavy metals, pesticides and antibiotics. These toxins are very harmful for human health, therefore, it is necessary to sense their presence in environment. Conventional strategies face various problems in detection and quantification of these pollutants such as expensive equipment and requirement of high maintenance with limited portability. Recently, nanostructured devices have been developed to detect environmental pollutants. Polymeric nanocomposites have been found robust, cost effective, highly efficient and accurate for sensing various environmental pollutants and this is due to their porous framework, multi-functionalities, redox properties, great conductivity, catalytic features, facile operation at room temperature and large surface area. Synergistic effects between polymeric matrix and nanomaterials are responsible for improved sensing features and environmental adaptability. This review focuses on the recent advancement in polymeric nanocomposites for sensing heavy metals, pesticides and antibiotics. The advantages, disadvantages, operating conditions and future perspectives of polymeric nanocomposites for sensing toxic pollutants have also been discussed.

Nanobody and Nanozyme-Enabled Immunoassays with Enhanced Specificity and Sensitivity

Immunoassay as a rapid and convenient method for detecting a variety of targets has attracted tremendous interest with its high specificity and sensitivity. Among the commonly used immunoassays, enzyme-linked immunosorbent assay has been widely used as a gold standard method in various fields that consists of two main components including a recognition element and an enzyme label. With the rapid advances in nanotechnology, nanobodies and nanozymes enable immunoassays with enhanced specificity and sensitivity compared with conventional antibodies and natural enzymes. This review is focused on the applications of nanobodies and nanozymes in immunoassays. Nanobodies advantage lies in their small size, high specificity, mass expression, and high stability. Nanozymes with peroxidase, phosphatase, and oxidase activities and their applications in immunoassays are highlighted and discussed in detail. In addition, the challenges and outlooks in terms of the use of nanobodies and the development of novel nanozymes in practical applications are discussed.

A colorimetric and ratiometric photometric sequential assay for ascorbic acid and alkaline phosphatase in serum based on valence states modulation

The photometric method is widely used in real clinical tests due to its simple operation, low cost and convenient. Many of the reported colorimetric ALP assays so far are non- ratiometric because the detection was based on changes in absorbance at a single wavelength. The development of novel colorimetric and ratiometric assay is of importance for quantitatively measuring target with high accuracy. The challenge in the design of ratiometric photometric assay is that the chromophore must have a significant spectral shift before and after binding to the target. Here, we report a colorimetric and ratiometric photometric sequential assay of AA and ALP based on the complexation between ARS and Cu²⁺ and redox reaction between AA and Cu²⁺. The absorption band of ARS centered at 425 nm (yellow color), which could be shifted to 510 nm (red color) upon Cu²⁺ binding. However, as far as we know, this classic color reaction has not been used to develop a ratiometric photometric method to sequentially detect AA and ALP, although photometric methods based on the regulation of other color reagents with oxidizing metal ions have been reported. The proposed sensing system shows a limit of detection for ALP at 0.24 U L^-1 and could be applied for detecting ALP in newborn calf serum. The established sensing system makes a useful contribution to the detection of ALP in complex clinical samples.

A direct competitive nanozyme-linked immunosorbent assay based on MnO2 nanosheets as a catalytic label for the determination of fumonisin B1

A direct competitive nanozyme-linked immunosorbent assay (dcNLISA) based on MnO₂ nanosheets (MnO₂ NSs) as a nanozyme label was developed for the highly sensitive determination of fumonisin B₁ (FB₁). MnO₂ NS-labeled fumonisin B₁-bovine serum albumin was easily synthesized as a competing antigen for the dcNLISA. And color changes derived from the MnO₂-3,3',5,5'-tetramethylbenzidine (TMB) system were exploited as the output signals of the dcNLISA. Several experimental parameters including the concentrations of the coating antibody, pH values, ionic strength and methanol concentration were optimized. Under the optimal conditions, the proposed method demonstrated a linear range (1.17-20.74 ng mL^-1) with a reliable correlation coefficient (R² = 0.9989), a satisfactory limit of detection (0.63 ng mL^-1) and high selectivity for the detection of FB₁. The recoveries of FB₁ in spiked corn and wheat samples were in the range of 85.31-108.16% with coefficients of variation (CVs) ranging from 6.14% to 9.23%. Meanwhile, the testing results showed good consistency (R² = 0.9892) between the developed dcNLISA and the reference method, liquid chromatography/mass spectrometry/mass spectrometry (LC-MS/MS) method. The proposed method was proven to be simple, sensitive, cost-effective and reliable for the screening of FB₁ in cereals.

Nanozyme-Participated Biosensing of Pesticides and Cholinesterases: A Critical Review

To improve the output and quality of agricultural products, pesticides are globally utilized as an efficient tool to protect crops from insects. However, given that most pesticides used are difficult to decompose, they inevitably remain in agricultural products and are further enriched into food chains and ecosystems, posing great threats to human health and the environment. Thus, developing efficient methods and tools to monitor pesticide residues and related biomarkers (acetylcholinesterase and butylcholinesterase) became quite significant. With the advantages of excellent stability, tailorable catalytic performance, low cost, and easy mass production, nanomaterials with enzyme-like properties (nanozymes) are extensively utilized in fields ranging from biomedicine to environmental remediation. Especially, with the catalytic nature to offer amplified signals for highly sensitive detection, nanozymes were finding potential applications in the sensing of various analytes, including pesticides and their biomarkers. To highlight the progress in this field, here the sensing principles of pesticides and cholinesterases based on nanozyme catalysis are definitively summarized, and emerging detection methods and technologies with the participation of nanozymes are critically discussed. Importantly, typical examples are introduced to reveal the promising use of nanozymes. Also, some challenges in the field and future trends are proposed, with the hope of inspiring more efforts to advance nanozyme-involved sensors for pesticides and cholinesterases.