Label-free colorimetric sensor for sensitive detection of choline based on DNAzyme-choline oxidase coupling

Construction of Mn-decorated zeolitic imidazolate framework-90 nanostructure as superior oxidase-like mimic for colorimetric detection of glucose and choline

A Mn decorated zeolitic imidazolate framework-90 (ZIF-90) nanozyme (Mn/ZIF-90) was constructed through an effective and rapid post-synthetic strategy for the first time. The Mn in Mn/ZIF-90 exists in mixed valence states, which is doped to the ZIF-90 through the formation of Mn-O bond. The Zn-N coordination structure of ZIF-90 may change the electronic arrangement of oxygen atoms in the free carbonyl groups (-CHO), allowing the coordination of Mn with O. The prepared Mn/ZIF-90 possesses outstanding oxidase-like activity and remarkable stability. Besides, the catalytic activity of Mn/ZIF-90 can be inhibited in the presence of H2O2. Therefore, using the Mn/ZIF-90-triggered chromogenic reaction of 3,3',5,5'-tetramethylbenzidine (TMB) as an amplifier, a versatile enzyme cascade-based colorimetric method for the detection of glucose and choline with good sensitivity and selectivity was developed. The linear ranges for glucose and choline are 6.25-500 μM and 5-1000 μM, respectively. Furthermore, the developed method was applied in the detection of glucose and choline in rabbit plasma samples, and the recoveries are 89.5-107.3 % and 96.0-109.3 %, respectively. In short, the simple and efficient post-synthetic doping method may provide a new thought for the rational designs of enzyme mimics with improved catalytic performance. Moreover, the colorimetric method based on the excellent catalytic activity of Mn/ZIF-90 may be extended to detect other H2O2-generating or consuming molecules and evaluate the activity of bio-enzymes that can catalyze the generation of glucose or choline.

Mo Single-Atom Nanozyme Anchored to the 2D N-Doped Carbon Film: Catalytic Mechanism, Visual Monitoring of Choline, and Evaluation of Intracellular ROS Generation

Single-atom nanozymes (SANs) have attracted great attention in constructing devices for instant biosensing due to their excellent stability and atom utilization. Here, Mo atoms were immobilized in 2D nitrogen-doped carbon films by cascade-anchored one-pot pyrolysis to obtain Mo single-atom nanozyme (Mo-SAN) with high atomic loading (4.79 wt %) and peroxidase-like activity. The coordination environment and enzyme-like activity mechanism of Mo-SAN were studied by combining synchrotron radiation and density functional theory. The strong oxophilicity of single-atom Mo makes the catalytic center more capable of transferring electrons to free radicals to selectively generate ^•OH in the presence of H₂O₂. Choline oxidase and Mo-SAN were used as signal opening unit and signal amplification unit, respectively. Combining the portability and visualization functions of smartphone and test strips, a paper-based visual sensing platform was constructed, which can accurately identify choline at a concentration of 0.5-35 μM with a limit of detection as low as 0.12 μM. The recovery of human serum samples was 96.4-102.2%, with an error of less than 5%. Furthermore, the potential of Mo-SAN to efficiently generate toxic ^•OH in tumor cells was intuitively confirmed. This work provides a technical and theoretical basis for designing highly active SANs and detecting neurological markers.

Colorimetric Nanobiosensor Design for Determining Oxidase Enzyme Substrates in Food and Biological Samples

Biological enzymes have high catalytic activity and unique substrate selectivity; their immobilization may provide cost reduction and reusability. Using magnetic nanoparticles (MNPs) as support materials for enzymes ensures easy separation from reaction media by an external magnetic field. Thus, MNPs were prepared by the coprecipitation method, coated with silanol groups, then -NH₂-functionalized, and finally activated with glutaraldehyde. Finally, three different oxidase enzymes, i.e., uricase, glucose oxidase, and choline oxidase, were separately immobilized on their surfaces by covalent attachment. Hence, colorimetric nanobiosensors for the determination of three biologically important substrates, i.e., uric acid (UA), glucose (Glu), and choline (Ch), were developed. Hydrogen peroxide liberated from enzyme-substrate reactions was determined by the cupric ion reducing antioxidant capacity (CUPRAC) reagent, bis-neocuproine copper(II) chelate. In addition, UA-free total antioxidant capacity could also be measured via the developed system. Kinetic investigations were carried out for these nanobiosensors to enable the calculation of their Michaelis constants (K _m), revealing no affinity loss for the substrate as a result of immobilization. Enzyme-immobilized MNPs could be reused at least five times. The linear concentration ranges were 5.43-65.22 μM for UA, 11.1-111.1 μM for Glu, and 2.78-44.4 μM for Ch, and the limit of detection (LOD) values with the same order were 0.34, 0.59, and 0.20 μM, respectively. Besides phenolic and thiol-type antioxidants, UA could be determined with an error range of 0.18-4.87%. The method is based on a clear redox reaction with a definite stoichiometry for the enzymatically generated H₂O₂ (which minimizes chemical deviations from Beer's law of optical absorbances), and it was successfully applied to the determination of Glu and UA in fetal bovine serum and Ch in infant formula as real samples.

Choline Oxidase-Integrated Copper Metal-Organic Frameworks as Cascade Nanozymes for One-Step Colorimetric Choline Detection

Choline is an important factor for regulating human health and is widely present in various foods. In this work, a sensor strategy based on a choline oxidase-integrated copper(II) metal-organic framework with peroxidase-like activity is constructed for one-step cascade detection of choline. The one-step cascade strategy can avoid intermediate product transferring in general multi-step reactions, and the multi-enzyme activities can be well exerted under one condition, thus exhibiting excellent catalytic activity and enhanced stability. In the integrated system, choline is catalyzed by ChO_x to produce betaine and H₂O₂, which eventually got converted to hydroxyl radicals by the peroxidase nanozyme, oxidized the chromogenic substrate ABTS, and produced an observable absorption peak at 420 nm. A new choline detection method was thus established and showed a satisfactory linear relationship at 6-300 μM, which has been used for the choline analysis in milk.

Metalloporphyrin and gold nanoparticles modified hollow zeolite imidazole Framework-8 with excellent peroxidase like activity for quick colorimetric determination of choline in infant formula milk powder

Metalloporphyrin and gold nanoparticles (AuNPs) were fixed on the surface of hollow zeolite imidazole framework-8 (HZIF-8) by cation exchange and cross-linking reaction. The obtained hybrid nanozyme, Au/HZIF-8@TCPP(Fe), was fully characterized by TEM, HRTEM, EDS element mapping and XPS. Then, its peroxidase-like activity was explored with Km of 1.74 mM and Vmax of 9.60 × 10^-8 M·S^-1 towards H₂O₂, indicating excellent catalytic activity. Based on cascade reaction between choline oxidase and Au/HZIF-8@TCPP(Fe), a quick colorimetric method was established for choline detection in infant formula milk powder. After comprehensive verification, this method presented the merits of simple operation, satisfied detection limit (0.05 mM), wide linear range (0.05-2.0 mM), high accuracy (recovery of 92.2%-105.2%) and nice selectivity. This colorimetric method was applied to the determination of choline in milk powders of five brands. Our study could offer valuable reference for finding highly efficient nanozyme and constructing novel optical biosensors.

Recent Applications of Pillar[n]arene-Based Host-Guest Recognition in Chemosensing and Imaging

Pillar[n]arene is a novel kind of synthetic supramolecular macrocyclic host characterized by its particular pillar-shaped structure consisting of an electron-rich cavity and two finely adjustable rims. Benefiting from its rigid structure, facile synthesis, ease of functionalization, and outstanding host-guest chemistry, pillar[n]arene shows great potential for diverse applications. Significantly, the host-guest recognition of pillar[n]arene provides a novel approach for chemosensing and imaging. Herein, this Review critically and comprehensively reviews the applications of pillar[n]arene-based host-guest recognition in chemosensing and imaging. The sensing and imaging mechanisms as well as the unique roles and advantages of pillar[n]arene-based host-guest recognition are summarized. In addition, preparations of hybrid materials based on pillar[n]arene and inorganic materials are also introduced comprehensively in the light of chemosensing and imaging. Finally, current challenges and perspectives on pillar[n]arene-based host-guest recognition in chemosensing and imaging are outlined.

Performance of chitosan polymer as platform during sensors fabrication and sensing applications

Chitosan is an important polymer produced from deacetylation of several sea and insects crusts. Due to its environmental fate and biological biocompatibility, it can be used in several biological and environmental applications. Sensing of biological compounds in human bodies and also in serum, blood, and different body fluids has found an important application instead of direct determination of the body fluids using complicated tools. Sensing process of biological compounds during bio-analysis of the biological systems, especially human fluids lack of several parameters including: high sensitivity, repeatability, speed of analysis and biocompatibility of the used analytical methods, especially in-vivo analysis. That was due to the time between sample handling and sample determination can change various components and concentrations of the bio-compounds. The need for in-situ analysis was directed the researchers for biosensors to overcome the upgrading problems of bio-analysis. Biosensors were the future of this issue. Chitosan can reserve as great platform for fabrication of different sensors to determine the elements, compounds and body bioactive compounds. The presence of different terminal amino and hydroxyl groups within chitosan framework facilitates the immobilization of different biomarkers to be used as sensing elements for the determined compounds. The use of chitosan as sensors platform was enhanced by using chitosan in its nanoforms.

Advanced Selection Methodologies for DNAzymes in Sensing and Healthcare Applications

DNAzymes have been widely explored owing to their excellent catalytic activity in a broad range of applications, notably in sensing and biomedical devices. These newly discovered applications have built high hopes for designing novel catalytic DNAzymes. However, the selection of efficient DNAzymes is a challenging process but one that is of crucial importance. Initially, systemic evolution of ligands by exponential enrichment (SELEX) was a labor-intensive and time-consuming process, but recent advances have accelerated the automated generation of DNAzyme molecules. This review summarizes recent advances in SELEX that improve the affinity and specificity of DNAzymes. The thriving generation of new DNAzymes is expected to open the door to several healthcare applications. Therefore, a significant portion of this review is dedicated to various biological applications of DNAzymes, such as sensing, therapeutics, and nanodevices. In addition, discussion is further extended to the barriers encountered for the real-life application of these DNAzymes to provide a foundation for future research.

Therapeutic Potential of DNAzyme Loaded on Chitosan/Cyclodextrin Nanoparticle to Recovery of Chemosensitivity in the MCF-7 Cell Line

Commonly, acquired resistances to anticancer drug are mediated by overexpression of a membrane-associated protein that encode via multi-drug resistance gene-1 (MDR1). Herein, the mRNA-cleaving DNAzyme that targets the mRNA of MDR1 gene in doxorubicin-resistant breast cancer cell line (MCF-7/DR) loaded on the chitosan β-cyclodextrin complexes was used as a tropical agent. Chitosan/β-cyclodextrin complexes were used to deliver DNAzymes into cancer cells. Determination of the physicochemical characteristics of the particles was done by photon correlation spectroscopy and scanning electron microscopy. The encapsulation efficiency of the complexes was tested by using gel retardation assay. Positively charged nanoparticles interacted with DNAzyme that could perform as an efficient DNAzyme transfection system. The rationale usage of this platform is to sensitize MCF-7/DR to doxorubicin by downregulating the drug-resistance gene MDR1. Results demonstrated a downregulation of MDR1 mRNAs in MCF-7/DR/DNZ by real-time PCR, compared to the MCF-7/DR as control. WST1 assay showed the 22-fold decrease in drug resistance on treated cells 24 h after transfection. Results showed the intracellular accumulation of Rh123 increased in the treated cells with DNAzyme. Results suggested a potential platform in association with chemotherapy drug for cancer therapy and indicated extremely efficient at delivery of DNAzyme in restoring chemosensitivity.