Multimetallic nanoparticles decorated metal-organic framework for boosting peroxidase-like catalytic activity and its application in point-of-care testing.

Metal-organic frameworks (MOFs) are a sort of promising peroxidase-like nanozyme but face the challenge that the inorganic nodes in most of the MOF structures are generally blocked by the organic linkers. Further enhancement or activation of their peroxidase-like activity plays an important role in developing MOF-based nanozymes. Herein, a multimetallic nanoparticle (NP) decorated-MOF, Cu/Au/Pt NP decorated-Cu-TCPP(Fe) nanozyme (CuAuPt/Cu-TCPP(Fe)) was synthesized in situ and served as a peroxidase-like nanozyme. The peroxidase-like activity of this stable CuAuPt/Cu-TCPP(Fe) nanozyme was enhanced due to the decreased potential barriers for *OH generation in the catalytic process. Owing to the remarkable peroxidase-like activity, a CuAuPt/Cu-TCPP(Fe)-based colorimetric assay was established for the sensitive determination of H2O2 and glucose with the limit of detection (LOD) of 9.3 µM and 4.0 µM, respectively. In addition, a visual point-of-care testing (POCT) device was developed by integrating the CuAuPt/Cu-TCPP(Fe)-based test strips with a smartphone and was employed for a portable test of 20 clinical serum glucose samples. The results determined by this method agree well with the values deduced by clinical automatic biochemical analysis. This work not only represents an inspiration for the usage of MNP/MOF composite as a novel nanozyme for POCT diagnosis, but also provides a deeper insight and understanding into the enhanced enzyme-mimic effect of MNP-hybrid MOF composites, which in turn will guide the engineering of MOF-based functional nanomaterials. Graphical Abstract.

Aptamer-AuNP-conjugated carboxymethyl chitosan-functionalized graphene oxide for colorimetric identification of Salmonella typhimurium.

A novel aptamer-AuNP-conjugated carboxymethyl chitosan-functionalized graphene oxide (CMC/GO@Apt-Au NP) probe was for the first time developed for the determination of Salmonella typhimurium (S. typhimurium). Owing to the conformational change of the aptamers in the presence of S. typhimurium, the Au NPs, which were pre-adsorbed on the aptamers through van der Waals forces, were released into the solution phase and induced the color change of the solution. As a result, S. typhimurium ranging from 102 to 107 CFU/mL was successfully identified using the designed assay with a limit of detection (LOD) of 10 CFU/mL. This low detection level allowed the sensitive recognition of S. typhimurium in milk samples within 40 min without sample pretreatment, a conclusion that agreed well with the traditional plate counting method. The developed method not only provides a rapid way for the determination of S. typhimurium with simplicity and sensitivity but also shows potential universality in the quantification of other pathogenic microorganisms.

Sensitive Identification of Microcystin-LR via a Reagent-Free and Reusable Electrochemical Biosensor Using a Methylene Blue-Labeled Aptamer.

We report a methylene blue (MB)-modified electrochemical aptamer (E-AB) sensor for determining microcystin-LR (MC-LR). The signal transduction of the sensor was based on changes in conformation and position of MB induced by the binding between MC-LR and the modified aptamer probe. In the absence of MC-LR, an aptamer probe was considered partially folded. After combining aptamer and MC-LR, the configuration of the aptamer probe changed and facilitated the electron transfer between MB and the electrode surface. As a result, an increased current response was observed. We optimized the parameters and evaluated the electrochemical performance of the sensor using square wave voltammetry (SWV). MC-LR was measured from 1.0 to 750.0 ng/L with a detection limit of 0.53 ng/L. The reliability of the method was verified by the determination of MC-LR in environmental real samples, such as pond water and tap water. Moreover, we demonstrated that this reagent-less biosensor could be regenerated and reused after rinsing with deionized water with good accuracy and reproducibility. As a reusable and regenerable E-AB sensor, this rapid, reagent-free, and sensitive sensing platform will facilitate routine monitoring of MC-LR in actual samples.

A novel CRISPR/Cas14a system integrated with 2D porphyrin metal-organic framework for microcystin-LR determination through a homogeneous competitive reaction.

Selective and sensitive detection of microcystin-LR (MC-LR) is of vital importance because of its high toxicity and broad distribution. Herein, a novel and versatile fluorescence sensor (Cas14-pMOFs fluorescence sensor) was developed by combining the CRISPR/Cas14a system with a 2D porphyrin metal-organic framework nanosheets (2D-pMOFs) for MC-LR determination. The designed CRISPR/Cas14a system was activated by the unbound complementary DNA (cDNA), which was positively correlated with MC-LR concentration. Furthermore, the activated Cas14a protein was utilized to indiscriminately cleave the FAM-labeled single-stranded DNA (ssDNA-FAM), which was pre-absorbed on Cu-TCPP(Fe) nanosheets. Because of the desorption of the cleaved ssDNA-FAM, the pre-quenched fluorescence signal was recovered. Owing to the excellent performance in quantifying cDNA using this Cas14-pMOFs fluorescence sensor with a limit of detection (LOD) of 0.12 nM, this Cas14-pMOFs fluorescence sensor was able to detect MC-LR in a range from 50 pg/mL to 1 µg/mL with the LOD of 19 pg/mL. This work not only provided a new insight for the exploration of fluorescence sensors based on 2D-pMOFs coupled with CRISPR/Cas14a, but also, demonstrated its universality in both nucleic acid and non-nucleic acid targets determination.

Recent advances in immunoassay technologies for the detection of human coronavirus infections.

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is the seventh coronavirus (CoV) that has spread in humans and has become a global pandemic since late 2019. Efficient and accurate laboratory diagnostic methods are one of the crucial means to control the development of the current pandemic and to prevent potential future outbreaks. Although real-time reverse transcription-polymerase chain reaction (rRT-PCR) is the preferred laboratory method recommended by the World Health Organization (WHO) for diagnosing and screening SARS-CoV-2 infection, the versatile immunoassays still play an important role for pandemic control. They can be used not only as supplemental tools to identify cases missed by rRT-PCR, but also for first-line screening tests in areas with limited medical resources. Moreover, they are also indispensable tools for retrospective epidemiological surveys and the evaluation of the effectiveness of vaccination. In this review, we summarize the mainstream immunoassay methods for human coronaviruses (HCoVs) and address their benefits, limitations, and applications. Then, technical strategies based on bioinformatics and advanced biosensors were proposed to improve the performance of these methods. Finally, future suggestions and possibilities that can lead to higher sensitivity and specificity are provided for further research.

Heterojunction photocatalysts for degradation of the tetracycline antibiotic: a review.

Antibiotic pollution is a major health issue inducing antibiotic resistance and the inefficiency of actual drugs, thus calling for improved methods to clean water and wastewater. Here we review the recent development of heterojunction photocatalysis and application in degrading tetracycline. We discuss mechanisms for separating photogenerated electron-hole pairs in different heterojunction systems such as traditional, p-n, direct Z-scheme, step-scheme, Schottky, and surface heterojunction. Degradation pathways of tetracycline during photocatalysis are presented. We compare the efficiency of tetracycline removal by various heterojunctions using quantum efficiency, space time yield, and figures of merit. Implications for the treatment of antibiotic-contaminated wastewater are discussed.

Cell density-dependent regulation of microcystin synthetase genes (mcy) expression and microcystin-LR production in Microcystis aeruginosa that mimics quorum sensing.

As the secondary metabolites of cyanobacterial harmful algal blooms (Cyano-HABs), microcystins (MCs) were generated under various environmental and cellular conditions. The understanding of the causes of MCs generation is of great interest in the field of water treatment and environmental science. In this work, we studied how Microcystis aeruginosa (FACHB-905) cell densities affect the MCs synthetase genes (mcy) expression, microcystin-LR (MC-LR) and quorum sensing molecules (Acyl-homoserine lactones (AHLs)) production. An electrochemical sensor was developed here for sensitive and quantitative detection of MC-LR that cultured at different cell densities. The results showed that mcy expression and MC-LR concentration started to increase when the cell density reached ca. 22 × 106 cells/mL, and was significantly increased with increasing cell densities. Moreover, the up-regulation of AHLs with increasing cell densities revealed that MC-LR is quorum sensing-mediated. Our results undoubtedly confirmed that MC-LR was produced in a cell density-dependent way that mimics quorum sensing, and the minimum cell density (ca. 22 × 106 cells/mL) that was required to produce MC-LR was provided and offered a reference standard for the prevention and control of MCs pollution in the actual water environment.

A regenerable ion-imprinted magnetic biocomposite for selective adsorption and detection of Pb2+ in aqueous solution.

A regenerable ion-imprinted magnetic biocomposite (IIMB) was successfully synthesized for simultaneous removal of Pb2+ using Serratia marcescens and carboxymethyl chitosan (CMC) as functional carriers, Pb2+ was utilized as the imprinted ion, while Fe3O4 served as the magnetic component. The structure and properties of IIMB were characterized by various techniques. The adsorption kinetics, isotherms and thermodynamics were applied to interpret the Pb2+ adsorption process on IIMB. The results showed the IIMB possessed prominent uptake ability toward Pb2+. The pseudo-second-order kinetic (R2 = 0.9989) and Langmuir models (R2 = 0.9555) fitted the data well. Adsorption thermodynamics revealed that the adsorption was a spontaneous endothermic reaction. The possible adsorption mechanisms involved physical adsorption, electrostatic attraction and complexing. Moreover, because Pb2+ can be specifically and strongly adsorbed on IIMB, a simple method for detection of Pb2+ was established by coupling IIMB with flame atomic absorption spectrometry (IIMB-FAAS). The developed IIMB-FAAS assay can sensitively detect Pb2+ with a linear range from 5.0 to 500.0 µg/L. The detection limit (LOD) of 0.95 µg/L as well as a quantification limit (LOQ) of 3.20 µg/L were obtained. This work proved that the IIMB could selective and efficient adsorb Pb2+, which provided some insights into wastewater treatment, water quality inspection and environmental remediation.

The research of aptamer biosensor technologies for detection of microorganism.

The activities and transmissions of microorganisms are closely related to human, and all kinds of diseases caused by pathogenic microorganisms have attracted attention in the world and brought many challenges to human health and public health. The traditional microbial detection technologies have characteristics of longer detection cycle and complicated processes, therefore, which can no longer meet the detection requirements in the field of public health. At present, it is the focus to develop and design a novel, rapid, and simple microbial detection method in the field of public health. Herein, this article summarized the development of aptamer biosensor technologies for detection of microorganism in the aspect of bacteria, viruses, and toxins in detail, including optical aptamer sensors such as fluorometry and colorimetry, electrochemical aptamer sensors, and other technologies combined with aptamer. KEY POINTS: • Aptamer biosensor is a good platform for microbial detection. • Aptamer biosensors include optical sensors and electrochemical sensors. • Aptamer sensors have been widely used in the detection of bacteria, viruses, and other microorganisms.

Extraordinary Dielectric Properties at Heterojunctions of Amorphous Ferroelectrics.

Materials having a high dielectric constant are needed for a variety of electrical applications from transistors to capacitors. Ferroelectric amorphous-oxide (glass) alkali-ion electrolytes of composition A2.99Ba0.005ClO (A = Li, Na) are shown by two different types of measurement and different consistent analyses to have extraordinarily high dielectric constants, varying from 109 at 25 °C to 1010 at 220 °C if the glass is properly conditioned. These anomalously high dielectric properties coexist with alkali-ion conductivities at 25 °C that are equivalent to those of the best organic-liquid electrolytes of a Li-ion cell, and cyclic voltammetry (CV) in a Au/glass electrolyte/Au cell is stable from -10 to +10 V. A model to interpret microscopically all the key features of the CV curves shows that the electric-double-layer capacitors that form at the gold/electrolyte interfaces in the Au/glass electrolyte/Au heterojunction reverse polarization at an applied voltage V = ±2.1 V, resulting in three almost equivalent discharging capacitances for a single physical capacitor from -10 to +10 V.

Direct Observation of C2O4•- and CO2•- by Oxidation of Oxalate within Nanogap of Scanning Electrochemical Microscope.

Oxalate oxidation in the presence of different oxidized luminophores leads to the emission of light and has been studied extensively in electrogenerated chemiluminescence (ECL). The proposed mechanism involves the initial formation of the oxalate radical anion, C2O4•-. The ensuing decomposition of C2O4•- produces a very strong reductant, CO2•-, which reacts with the oxidized luminophores to generate excited states that emit light. Although the mechanism has been proposed for decades, the experimental demonstration is still lacking, because of the complexity of the system and the short lifetimes of both radical anions. To address these issues, we studied oxalate oxidation at platinum ultramicroelectrodes (UMEs) in anhydrous N, N-dimethylformamide (DMF) solution by nanoscale scanning electrochemical microscopy (SECM) with the tip generation/substrate collection (TG/SC) mode. A Pt nanoelectrode was utilized as the SECM generator for oxalate oxidation, while another Pt UME served as the SECM collector and was used to capture the generated intermediates. We studied the influence of the gap distance, d, on the substrate current ( is). The results indicate that, when 73 nm < d < 500 nm, the species captured by the substrate were primarily CO2•-, while C2O4•- was the predominant intermediate measured when d was below 73 nm. A half-life of 1.3 µs for C2O4•- was obtained, which indicates a stepwise mechanism for oxalate oxidation. The relevance of these observations to the use of oxalate as the coreactant in ECL systems is also discussed.

Scanning electrochemical microscopy at the nanometer level.

This review describes how one can perform nanometer (nm)-scale SECM experiments through advances in tip fabrication and positioning and instrumentation design. Basic SECM methodology including instrumentation and feedback and generation/collection modes are discussed. Aspects of nanoscale SECM including fabrication of nm-sized electrodes and nano SECM instrumentation are also described. State of the art applications related to nanogaps (i.e., rapid homogeneous reactions and short-lived intermediates; heterogeneous electron transfer kinetics; nanoparticles (NPs) and clusters) and nanoscale imaging (e.g., single NPs, single biological samples, combined methods) are described. Future possibilities and prospects are suggested that might lead to even better resolution, thus introducing SECM electrochemical imaging to the single atom level.

Detection of CO2•- in the Electrochemical Reduction of Carbon Dioxide in N,N-Dimethylformamide by Scanning Electrochemical Microscopy.

The electrocatalytic reduction of CO2 has been studied extensively and produces a number of products. The initial reaction in the CO2 reduction is often taken to be the 1e formation of the radical anion, CO2•-. However, the electrochemical detection and characterization of CO2•- is challenging because of the short lifetime of CO2•-, which can dimerize and react with proton donors and even mild oxidants. Here, we report the generation and quantitative determination of CO2•- in N,N-dimethylformamide (DMF) with the tip generation/substrate collection (TG/SC) mode of scanning electrochemical microscopy (SECM). CO2 was reduced at a hemisphere-shaped Hg/Pt ultramicroelectrode (UME) or a Hg/Au film UME, which were utilized as the SECM tips. The CO2•- produced can either dimerize to form oxalate within the nanogap between SECM tip and substrate or collected at SECM substrate (e.g., an Au UME). The collection efficiency (CE) for CO2•- depends on the distance (d) between the tip and substrate. The dimerization rate (6.0 × 108 M-1 s-1) and half-life (10 ns) of CO2•- can be evaluated by fitting the collection efficiency vs distance curve. The dimerized species of CO2•-, oxalate, can also be determined quantitatively. Furthermore, the formal potential (E0') and heterogeneous rate constant (k0) for CO2 reduction were determined with different quaternary ammonium electrolytes. The significant difference in k0 is due to a tunneling effect caused by the adsorption of the electrolytes on the electrode surface at negative potentials.

An improved Bathocuproine assay for accurate valence identification and quantification of copper bound by biomolecules.

Copper is an essential metal in all organisms. Reliably quantifying and identifying the copper content and oxidation state is crucial, since the information is essential to understanding protein structure and function. Chromophoric ligands, such as Bathocuproine (BC) and its water-soluble analog, Bathocuproinedisulfonic acid (BCS), preferentially bind Cu(I) over Cu(II), and therefore have been widely used as optical probes to determine the oxidation state of copper bound by biomolecules. However, the BCS assay is commonly misused, leading to erroneous conclusions regarding the role of copper in biological processes. By measuring the redox potential of Cu(II)-BCS2 and conducting UV-vis absorption measurements in the presence of oxidizable amino acids, the thermodynamic origin of the potential artifacts becomes evident. The BCS assay was improved by introducing a strong Cu(II) chelator EDTA prior to the addition of BCS to prevent interference that might arise from Cu(II) present in the sample. The strong Cu(II) chelator rids of all the potential errors inherent in the conventional BCS assay. Applications of the improved assay to peptides and protein containing oxidizable amino acid residues confirm that free Cu(II) no longer leads to artifacts, thereby resolving issues related to this persistently misused colorimetric assay of Cu(I) in biological systems.

Continuous nanoflow-scanning electrochemical microscopy: voltammetric characterization and application for accurate and reproducible imaging of enzyme-labeled protein microarrays.

The coupling of scanning electrochemical microscopy (SECM) to a continuous nanoflow (CNF) system is accomplished with the use of a microconcentric ring electrode/injector probe. The gold microring electrode encapsulated by a glass sheath is robust and can be beveled and polished. The CNF system, comprising a precision gas displacement pump and a rotary valve, is capable of delivering solution to the center of the SECM probe in the range of 1-150 nL/min. Major advantages of the CNF-SECM imaging mode over the conventional SECM generation/collection (G/C) mode include higher imaging resolution, immunity from interferences by species in the bulk solution or at other sites of the substrate, elimination of the feedback current that could interfere with the G/C data interpretation, and versatility of initiating surface reactions/processes via introducing different reactants into the flowing stream. Parameters such as flow rates, probe/substrate separations, and collection efficiencies are examined and optimized. Higher resolution, reproducibility, and accuracy are demonstrated through the application of CNF-SECM to horseradish peroxidase (HRP)-amplified imaging of protein microarrays. By flowing H2O2 and ferrocenemethanol through the injector and detecting the surface-generated ferriceniummethanol, human IgG spots covered with HPR-labeled antihuman IgG can be detected in the range of 13 nM-1.333 µM with a detection limit of 3.0 nM. In addition, consistent images of microarray spots for selective and high-density detection of analytes can be attained.

Tabersonine inhibits amyloid fibril formation and cytotoxicity of Aß(1-42).

The misfolding and aggregation of amyloid beta (Aß) peptides into amyloid fibrils are key events in the amyloid cascade hypothesis for the etiology of Alzheimer's disease (AD). Using thioflavin-T (ThT) fluorescence assay, atomic force microscopy, circular dichroism, size exclusion chromatography, surface plasmon resonance (SPR), and cytotoxicity tests, we demonstrate that tabersonine, an ingredient extracted from the bean of Voacanga africana, disrupts Aß(1-42) aggregation and ameliorates Aß aggregate-induced cytotoxicity. A small amount of tabersonine (e.g., 10 µM) can effectively inhibit the formation of Aß(1-42) (e.g., 80 µM) fibrils or convert mature fibrils into largely innocuous amorphous aggregates. SPR results indicate that tabersonine binds to Aß(1-42) oligomers in a dose-dependent way. Molecular dynamics (MD) simulations further confirm that tabersonine can bind to oligomers such as the pentamer of Aß(1-42). Tabersonine preferentially interact with the ß-sheet grooves of Aß(1-42) containing aromatic and hydrophobic residues. The various binding sites and modes explain the diverse inhibitory effects of tabersonine on Aß aggregation. Given that tabersonine is a natural product and a precursor for vincristine used in cancer chemotherapy, the biocompatibility and small size essential for permeating the blood-brain barrier make it a potential therapeutic drug candidate for treating AD.

Microconcentric ring electrode/injector assembly for sensitive voltammetric analysis in single droplets of ultrasmall volumes.

This paper describes the construction of a microring electrode concentric to an inner injection capillary for voltammetric determination of trace analytes in nanoliter- to picoliter-sized droplets. The gold microring is sandwiched between a pulled fused-silica capillary and borosilicate glass tubing. Compared to polymer-coated microring electrodes, the glass-encapsulated electrode is more robust and does not swell in organic solvents. Consequently, the microring electrode is suitable for voltammetric studies of redox-active species and their accompanying ion transfers between two immiscible solvents. Droplets of variable sizes can be conveniently dispensed from front-loaded sample plugs into an immiscible liquid, greatly simplifying the experimental procedure and facilitating analysis of samples of limited availability. The size of the microring and the volume of the droplet deduced from well-defined voltammograms correlate well with those estimated from their geometric dimensions. The thin-layer cell behavior can be attained with well-defined voltammetric peaks and small capacitive current. Exhaustive electrolysis in single droplets can be accomplished in short times (e.g., ∼85 s in a droplet of 1.42 nL at a microring of 11.4 µm in radius). Anodic stripping voltammetry of Ag deposited onto the microring electrode resulted in a detection limit of 0.13 fmol (14 fg) of Ag(+). The microring electrode/injector assembly can be polished repeatedly and is versatile for various applications (e.g., sample plugs can also be back-loaded via a rotary injection valve and an HPLC pump for flow injection analysis).