Oxidation of ABTS by hydrogen peroxide catalyzed by horseradish peroxidase encapsulated into sol–gel glass

A label-free DNAzyme-based colorimetric sensor for the detection of Leptospira interrogans

Leptospirosis is a neglected zoonosis caused by a pathogenic spirochete Leptospira. Diagnosis of leptospirosis in the early stage is difficult and can be easily confused with other infections. The existing detection methods are considered chronophagous and labor-intensive. Leptospira survives in the kidney tubules of reservoir animals such as rodents and shed into the environment through their urine. In this study, we developed an Aptamer-DNAzyme-based biosensor for detecting pathogenic Leptospira in environmental water samples. The cell-specific aptamer with an extensive affinity binds to the cell surface proteins to detect the Leptospira interrogans. The DNAzyme that mimics as a peroxidase enzyme, acts as a transducing agent in the colorimetric reaction positively conditioned by the presence of L. interrogans. The Leptospira-specific aptamer coupled with DNAzyme is coated onto carbon nanotubes, to provide a cost-effective nanomaterial-based detection platform. L. interrogans contamination in the samples is detected with a color change of a peroxidase substrate, ABTS. The dissociation constant of the aptazyme was found to be 356.6 nM. The aptazyme system was able to detect up to 119 CFU/mL of L. interrogans exhibiting a high range of selectivity towards the pathogenic spirochete. This simple detection methodology makes the system promising for the environmental monitoring of L. interrogans.

Comparison of two peroxidases with high potential for biotechnology applications - HRP vs. APEX2

Peroxidases are essential elements in many biotechnological applications. An especially interesting concept involves split enzymes, where the enzyme is separated into two smaller and inactive proteins that can dimerize into a fully active enzyme. Such split forms were developed for the horseradish peroxidase (HRP) and ascorbate peroxidase (APX) already. Both peroxidases have a high potential for biotechnology applications. In the present study, we performed biophysical comparisons of these two peroxidases and their split analogues. The active site availability is similar for all four structures. The split enzymes are comparable in stability with their native analogues, meaning that they can be used for further biotechnology applications. Also, the tertiary structures of the two peroxidases are similar. However, differences that might help in choosing one system over another for biotechnology applications were noticed. The main difference between the two systems is glycosylation which is not present in the case of APX/sAPEX2, while it has a high impact on the HRP/sHRP stability. Further differences are calcium ions and cysteine bridges that are present only in the case of HRP/sHRP. Finally, computational results identified sAPEX2 as the systems with the smallest structural variations during molecular dynamics simulations showing its dominant stability comparing to other simulated proteins. Taken all together, the sAPEX2 system has a high potential for biotechnological applications due to the lack of glycans and cysteines, as well as due to high stability.

Star-Like Polypeptides as Simplified Analogues of Horseradish Peroxidase (HRP) Metalloenzymes

Peroxidases, like horseradish peroxidase (HRP), are heme metalloenzymes that are powerful biocatalysts for various oxidation reactions. By using simple grafting-from approach, ring-opening polymerization (ROP), and manganese porphyrins, star-shaped polypeptides analogues of HRP capable of catalyzing oxidation reactions with H2O2 is successfully prepared. Like their protein model, these simplified analogues show interesting Michaelis-Menten constant (KM) in the mM range for the oxidant. Interestingly, the polymer structures are more resistant to denaturation (heat, proteolysis and oxidant concentration) than HRP, opening up interesting prospects for their use in catalysis or in biosensing devices.

Arginine-Modified Hemin Enhances G-Quadruplex DNAzyme Peroxidase Activity for High Sensitivity Detection

Hemin/G-quadruplex (hG4) complexes are frequently used as artificial peroxidase-like enzymatic systems (termed G4 DNAzymes) in many biosensing applications, in spite of a rather low efficiency, notably in terms of detection limits. To tackle this issue, we report herein a strategy in which hemin is chemically modified with the amino acids found in the active site of parent horseradish peroxidase (HRP), with the aim of recreating an environment conducive to high catalytic activity. When hemin is conjugated with a single arginine, it associates with G4 to create an arginine-hemin/G4 (R-hG4) DNAzyme that exhibits improved catalytic performances, characterized by kinetic analysis and DFT calculations. The practical relevance of this system was demonstrated with the implementation of biosensing assays enabling the chemiluminescent detection of G4-containing DNA and colorimetry detection of the flap endonuclease 1 (FEN1) enzyme with a high efficiency and sensitivity. Our results thus provide a guide for future enzyme engineering campaigns to create ever more efficient peroxidase-mimicking DNA-based systems.

Ultrarobust stable ABTS radical cation prepared using Spore@Cu-TMA biocomposites for antioxidant capacity assay

Herein, spore@Cu-trimesic acid (TMA) biocomposites were prepared by self-assembling Cu-based metal-organic framework on the surface of Bacillus velezensis spores. The laccase-like activity of spore@Cu-TMA biocomposites was enhanced by 14.9 times compared with that of pure spores due to the reaction of Cu2+ ions with laccase on the spore surface and the microporous structure of Cu-TMA shell promoting material transport and increasing substrate accessibility. Spore@Cu-TMA rapidly oxidized and transformed 2,2'-azino-bis (3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) into ABTS●+ without using H2O2. Under optimum conditions, the ABTS●+ could be stored for 21 days at 4 °C and 7 days at 37 °C without the addition of any stabilizers, allowing for the large-scale preparation and long-term storage of ABTS●+. The ultrarobust stable ABTS●+ obtained with the use of Cu-TMA could effectively reduce the "back reaction" by preventing the leaching of the metabolites released by the spores. On the basis of these findings, a rapid, low-cost, and eco-friendly colorimetric platform was successfully developed for the detection of antioxidant capacity. Determination of antioxidant capacity for several antioxidants such as caffeic acid, glutathione, and Trolox revealed their corresponding limits of detection at 4.83, 8.89, and 7.39 nM, respectively, with linear ranges of 0.01-130, 0.01-140, and 0.01-180 μM, respectively. This study provides a facile way to prepare ultrarobust stable ABTS●+ and presents a potential application of spore@Cu-TMA biocomposites in food detection and bioanalysis.

The dmsEFABGH operon encodes an essential and modular electron transfer pathway for extracellular iodate reduction by Shewanella oneidensis MR-1

Extracellular iodate reduction by Shewanella spp. contributes to iodide generation in the biogeochemical cycling of iodine. However, there is a disagreement on whether Shewanella spp. use different extracellular electron transfer pathways with dependence on electron donors in iodate reduction. In this study, a series of gene deletion mutants of Shewanella oneidensis MR-1 were created to investigate the roles of dmsEFABGH, mtrCAB, and so4357-so4362 operons in iodate reduction. The iodate-reducing activity of the mutants was tested with lactate, formate, and H2 as the sole electron donors, respectively. In the absence of single-dms gene, iodate reduction efficiency of the mutants was only 12.9%-84.0% with lactate at 24 hours, 22.1%-85.9% with formate at 20 hours, and 19.6%-57.7% with H2 at 42 hours in comparison to complete reduction by the wild type. Progressive inhibition of iodate reduction was observed when the dms homolog from the so4357-so4362 operon was deleted in the single-dms gene mutants. This result revealed complementation of dmsEFABGH by so4357-so4362 at the single-gene level, indicating modularity of the extracellular electron transfer pathway encoded by dmsEFABGH operon. Under the conditions of all electron donors, significant inhibition of iodate reduction and accumulation of H2O2 were detected for ΔmtrCAB. Collectively, these results demonstrated that the dmsEFABGH operon encodes an essential and modular iodate-reducing pathway without electron donor dependence in S. oneidensis MR-1. The mtrCAB operon was involved in H2O2 elimination with all electron donors. The findings in this study improved the understanding of molecular mechanisms underlying extracellular iodate reduction.IMPORTANCEIodine is an essential trace element for human and animals. Recent studies revealed the contribution of microbial extracellular reduction of iodate in biogeochemical cycling of iodine. Multiple reduced substances can be utilized by microorganisms as energy source for iodate reduction. However, varied electron transfer pathways were proposed for iodate reduction with different electron donors in the model strain Shewanella oneidensis MR-1. Here, through a series of gene deletion and iodate reduction experiments, we discovered that the dmsEFABGH operon was essential for iodate reduction with at least three electron donors, including lactate, formate, and H2. The so4357-so4362 operon was first demonstrated to be capable of complementing the function of dmsEFABGH at single-gene level.

Exploring the potential anti-thyroid activity of Acetyl-L-carnitine: Lactoperoxidase inhibition profile, iodine complexation and scavenging power against H2O2. Experimental and theoretical studies

L-Acetylcarnitine (ALC), a versatile compound, has demonstrated beneficial effects in depression, Alzheimer's disease, cognitive impairment, and other conditions. This study focuses on its antithyroid activity. The precursor molecule, L-carnitine, inhibited the uptake of triiodothyronine (T3) and thyroxine (T4), and it is possible that ALC may reduce the iodination process of T3 and T4. Currently, antithyroid drugs are used to control the excessive production of thyroid hormones (TH) through various mechanisms: (i) forming electron donor-acceptor complexes with molecular iodine, (ii) eliminating hydrogen peroxide, and (iii) inhibiting the enzyme thyroid peroxidase. To understand the pharmacological properties of ALC, we investigated its plausible mechanisms of action. ALC demonstrated the ability to capture iodine (Kc = 8.07 ± 0.32 x 105 M-1), inhibit the enzyme lactoperoxidase (LPO) (IC50 = 17.60 ± 0.76 µM), and scavenge H2O2 (39.82 ± 0.67 mM). A comprehensive physicochemical characterization of ALC was performed using FTIR, Raman, and UV-Vis spectroscopy, along with theoretical DFT calculations. The inhibition process was assessed through fluorescence spectroscopy and vibrational analysis. Docking and molecular dynamics simulations were carried out to predict the binding mode of ALC to LPO and to gain a better understanding into the inhibition process. Furthermore, albumin binding experiments were also conducted. These findings highlight the potential of ALC as a therapeutic agent, providing valuable insights for further investigating its role in the treatment of thyroid disorders.

Accelerated cascade melanoma therapy using enzyme-nanozyme-integrated dissolvable polymeric microneedles

Dissolvable polymeric microneedles (DPMNs) have emerged as a powerful technology for the localized treatment of diseases, such as melanoma. Herein, we fabricated a DPMN patch containing a potent enzyme-nanozyme composite that transforms the upregulated glucose consumption of cancerous cells into lethal reactive oxygen species via a cascade reaction accelerated by endogenous chloride ions and external near-infrared (NIR) irradiation. This was accomplished by combining glucose oxidase (Gox) with a NIR-responsive chloroperoxidase-like copper sulfide (CuS) nanozyme. In contrast with subcutaneous injection, the microneedle system highly localizes the treatment, enhancing nanomedicine uptake by the tumor and reducing its systemic exposure to the kidneys and spleen. NIR irradiation further controls the potency and toxicity of the formulation by thermally disabling Gox. In a mouse melanoma model, this unique combination of photothermal, starvation, and chemodynamic therapies resulted in complete tumor eradication (99.2 ± 0.8 % reduction in tumor volume within 10 d) without producing signs of systemic toxicity. By comparison, other treatment combinations only resulted in a 42-76.5 % reduction in tumor growth. The microneedle patch design is therefore not only highly potent but also with regulated toxicity and improved safety.

ABTS as Both Activator and Electron Shuttle to Activate Persulfate for Diclofenac Degradation: Formation and Contributions of ABTS•+, SO4•-, and •OH

The activation of peroxydisulfate (PDS) by organic compounds has attracted increasing attention. However, some inherent drawbacks including quick activator decomposition and poor anti-interference capacity limited the application of organic compound-activated PDS. It was interestingly found that 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonate) (ABTS) could act as both activator and electron shuttle for PDS activation to enhance diclofenac (DCF) degradation over a pH range of 2.0-11.0. Multiple reactive species of ABTS•+, •OH, and SO4•- were generated in the PDS/ABTS system, while only ABTS•+ and •OH directly contributed to DCF degradation. ABTS•+, generated via the reactions of ABTS with PDS, SO4•-, and •OH, was the dominant reactive species of DCF degradation. No significant decomposition of ABTS was observed in the PDS/ABTS system, and ABTS acted as both activator and electron shuttle. Four possible degradation pathways of DCF were proposed, and the toxicity of DCF decreased after treatment with the PDS/ABTS system. The PDS/ABTS system had good anti-interference capacity to common natural water constituents. Additionally, ABTS was encapsulated into cellulose to obtain ABTS@Ce beads, and the PDS/ABTS@Ce system possessed excellent performance on DCF degradation. This study proposes a new perspective to reconsider the mechanism of activating PDS with organic compounds and highlights the considerable contribution of organic radicals on contaminant removal.

Elucidating the role of media nitrogen in augmenting the production of lignin-depolymerizing enzymes by white-rot fungi

Indigenous white-rot fungal isolates Schizophyllum commune, Phanerochaete chrysosporium, Ganoderma racenaceum, and Lentinus squarrosulus, demonstrating the ability to depolymerize lignin of the crop residues, were studied for their potential to produce ligninolytic enzymes using modified production media under conditions of limiting and excess nitrogen for higher enzymatic expressions. Secretome-rich media on the investigation confirmed the successful production of lignin-depolymerizing enzymes, viz. laccase, lignin peroxidase, manganese peroxidase, and versatile peroxidase. Production of laccases and peroxidases was statistically significant in nitrogen-limiting media with and without the substrate, across all white-rot fungal cultures at 95% confidence interval. Nitrogen-limiting media with the substrate on analysis extracellularly expressed 99.27 U of laccase and 68.48 U of manganese peroxidase in Schizophyllum commune, while 195.14 U of lignin peroxidase was produced by Phanerochaete chrysosporium. Lentinus squarrosulus expressed 455.34 U of laccase and 357.13 U of versatile peroxidase with 250.09 U of laccase and 206.95 U of manganese peroxidase produced by Ganoderma racenaceum for every milliliter of the media used. Nitrogen-limiting media triggered the production of laccase during the initial stages of growth while the expression of peroxidases was predominant at a later stage. Also, this media evinced increased enzymatic yields with low biomass content compared to nitrogen-excess conditions. The extant study confirmed the positive influence of nitrogen-limiting media in the efficient production of ligninolytic enzymes and their suggestive degradation potential for environmental pollutants, making these enzymes a safe, clean alternative to the use of chemicals and the media to be effective for large-scale production of ligninolytic enzymes. IMPORTANCE Lignin on account of its high abundance, complex polymeric structure, and biochemical properties is identified as a promising candidate in renewable energy and bioproduct manufacturing. However, depolymerization of lignin remains a major challenge in lignin utilization, entailing the employment of harsh treatments representing not only an environmental concern but also a waste of economic potential. Developing an alternative green technology to minimize this impact is imperative. Methods using enzymes to depolymerize lignin are the focus of recent studies. Current research work emphasized the efficient expression of the major lignin-depolymerizing enzymes: laccases, lignin peroxidases, manganese peroxidases, and versatile peroxidases from native isolates of white-rot fungus for several biotechnological applications as well as treatment of crop residues for use as ruminant feed in improving productivity. The importance of nitrogen in augmenting the expression of lignin-depolymerizing enzymes and providing a media recipe for the cost-effective production of ligninolytic enzymes is highlighted.

Chimeric Biocatalyst Combining Peptidic and Nucleic Acid Components Overcomes the Performance and Limitations of the Native Horseradish Peroxidase

Chimeric peptide-DNAzyme (CPDzyme) is a novel artificial peroxidase that relies on the covalent assembly of DNA, peptides, and an enzyme cofactor in a single scaffold. An accurate control of the assembly of these different partners allows for the design of the CPDzyme prototype G4-Hemin-KHRRH, found to be >2000-fold more active (in terms of conversion number k_cat) than the corresponding but non-covalent G4/Hemin complex and, more importantly, >1.5-fold more active than the corresponding native peroxidase (horseradish peroxidase) when considering a single catalytic center. This unique performance originates in a series of gradual improvements, thanks to an accurate selection and arrangement of the different components of the CPDzyme, in order to benefit from synergistic interactions between them. The optimized prototype G4-Hemin-KHRRH is efficient and robust as it can be used under a wide range of non-physiologically relevant conditions [organic solvents, high temperature (95 °C), and in a wide range of pH (from 2 to 10)], thus compensating for the shortcomings of the natural enzymes. Our approach thus opens broad prospects for the design of ever more efficient artificial enzymes.

Ferric heme b in aqueous micellar and vesicular systems: state-of-the-art and challenges

Ferric heme b (= ferric protoporphyrin IX = hemin) is an important prosthetic group of different types of enzymes, including the intensively investigated and widely applied horseradish peroxidase (HRP). In HRP, hemin is present in monomeric form in a hydrophobic pocket containing among other amino acid side chains the two imidazoyl groups of His170 and His42. Both amino acids are important for the peroxidase activity of HRP as an axial ligand of hemin (proximal His170) and as an acid/base catalyst (distal His42). A key feature of the peroxidase mechanism of HRP is the initial formation of compound I under heterolytic cleavage of added hydrogen peroxide as a terminal oxidant. Investigations of free hemin dispersed in aqueous solution showed that different types of hemin dimers can form, depending on the experimental conditions, possibly resulting in hemin crystallization. Although it has been recognized already in the 1970s that hemin aggregation can be prevented in aqueous solution by using micelle-forming amphiphiles, it remains a challenge to prepare hemin-containing micellar and vesicular systems with peroxidase-like activities. Such systems are of interest as cheap HRP-mimicking catalysts for analytical and synthetic applications. Some of the key concepts on which research in this fascinating and interdisciplinary field is based are summarized, along with major accomplishments and possible directions for further improvement. A systematic analysis of the physico-chemical properties of hemin in aqueous micellar solutions and vesicular dispersions must be combined with a reliable evaluation of its catalytic activity. Future studies should show how well the molecular complexity around hemin in HRP can be mimicked by using micelles or vesicles. Because of the importance of heme b in virtually all biological systems and the fact that porphyrins and hemes can be obtained under potentially prebiotic conditions, ideas exist about the possible role of heme-containing micellar and vesicular systems in prebiotic times.

Tumor-Specific NIR-Activatable Nanoreactor for Self-Enhanced Multimodal Imaging and Cancer Phototherapy

Responsive nanosystems for tumor treatment with high specificity and sensitivity have aroused great attention. Herein, we develop a tumor microenvironment responsive and near-infrared (NIR)-activatable theranostic nanoreactor for imaging-guided anticancer therapy. The nanoreactor (SnO2-x@AGP) is comprised of poly(vinylpyrrolidine) encapsulated hollow mesoporous black SnO2-x nanoparticles coloaded with glucose oxidase (GOx) and 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS). The constructed nanoreactor can be specifically activated through endogenous H2O2 by an NIR-mediated "bursting-like" process to enhance its imaging and therapeutic functions. Black SnO2-x with abundant oxygen vacancies expedites effective separation of electron-hole pairs from energy-band structure and endows them with strong hyperthermia effect upon NIR laser irradiation. The generating toxic H2O2 with the assistance of GOx provides SnO2-x@AGP with the capacity of oxidative stress therapy. Ascended H2O2 can activate ABTS into ABTS•+. ABTS•+ not only possesses significant NIR absorption properties, but also disrupts intracellular glutathione to generate excessive reactive oxygen species for improved phototherapy, leading to more effective treatment together with oxidative stress therapy. Thus, SnO2-x@AGP with NIR-mediated and H2O2-activated performance presents tumor inhibition efficacy with minimized damage to normal tissues. These outstanding characteristics of SnO2-x@AGP bring an insight into the development of activatable nanoreactors for smart, precise, and non-invasive cancer theranostics.

Cholesterol Assay Based on Recombinant Cholesterol Oxidase, ABTS, and Horseradish Peroxidase

Cholesterol determination by cholesterol oxidase reaction is a fast, convenient, and highly specific approach with widespread use in clinical diagnostics. Routinely, endpoint measurements with 4-aminophenazone or 4-aminoantipyrine as chromogens and sodium cholate, surfactants, or alcohols as solubilizing agents are used. Here we describe a novel kinetic method to determine cholesterol in 0.05-0.75 mM range in neutral or acidic buffers by use of recombinant cholesterol oxidase from Nocardioides simplex in a coupled reaction with horseradish peroxidase, ABTS as a chromogen, and methyl-β-cyclodextrin as a solubilizing agent.

Design of a H2 O2 -generating P450SPα fusion protein for high yield fatty acid conversion

Sphingomonas paucimobilis' P450SPα (CYP152B1) is a good candidate as industrial biocatalyst. This enzyme is able to use hydrogen peroxide as unique cofactor to catalyze the fatty acids conversion to α-hydroxy fatty acids, thus avoiding the use of expensive electron-donor(s) and redox partner(s). Nevertheless, the toxicity of exogenous H2 O2 toward proteins and cells often results in the failure of the reaction scale-up when it is directly added as co-substrate. In order to bypass this problem, we designed a H2 O2 self-producing enzyme by fusing the P450SPα to the monomeric sarcosine oxidase (MSOX), as H2 O2 donor system, in a unique polypeptide chain, obtaining the P450SPα -polyG-MSOX fusion protein. The purified P450SPα -polyG-MSOX protein displayed high purity (A417 /A280  = 0.6) and H2 O2 -tolerance (kdecay  = 0.0021 ± 0.000055 min-1 ; ΔA417  = 0.018 ± 0.001) as well as good thermal stability (Tm : 59.3 ± 0.3°C and 63.2 ± 0.02°C for P450SPα and MSOX domains, respectively). The data show how the catalytic interplay between the two domains can be finely regulated by using 500 mM sarcosine as sacrificial substrate to generate H2 O2 . Indeed, the fusion protein resulted in a high conversion yield toward fat waste biomass-representative fatty acids, that is, lauric acid (TON = 6,800 compared to the isolated P450SPα TON = 2,307); myristic acid (TON = 6,750); and palmitic acid (TON = 1,962).

Temporally Controlled Multienzyme Catalysis Using a Dissipative Supramolecular Nanozyme

The development of superior functional enzyme mimics (nanozymes) is essential for practical applications, including point-of-care diagnostics, biotechnological applications, biofuels, and environmental remediation. Nanozymes with the ability to control their catalytic activity in response to external fuels offer functionally valuable platforms mimicking nonequilibrium systems in nature. Herein, we fabricated a supramolecular coordination bonding-based dynamic vesicle that exhibits multienzymatic activity. The supramolecular nanozyme shows effective laccase-like catalytic activity with a K_M value better than the native enzyme and higher stability in harsh conditions. Besides, the nanostructure demonstrates an efficient peroxidase-like activity with NADH peroxidase-like properties. Generation of luminescence from luminol and oxidation of dopamine are efficiently catalyzed by the nanozyme with high sensitivity, which is useful for point-of-care detections. Notably, the active nanozyme exhibits dynamic laccase-mimetic activity in response to pH variation, which has never been explored before. While a neutral/high pH leads to the self-assembly, a low pH disintegrates the assembled nanostructures and consequently turns off the nanozyme activity. Altogether, the self-assembled Cu²⁺-based vesicular nanostructure presents a pH-fueled dissipative system demonstrating effective temporally controlled multienzymatic activity.

Simultaneous quantification of peroxidase/ascorbate in vegetables with slope and intercept of kinetic curves in a flow-injection recirculating-catalysis system

In using a flow-injection recirculating-catalysis system developed by us to research the simultaneous quantification for peroxidase and ascorbate, it was discovered that the concentrations of peroxidase activity and ascorbate are correlative with the slope and the negative intercept of the linear response curve during a peroxidase-catalyzed kinetic course. Therefore, based on this finding, a new analytical method and a simplified equation for quantifying the peroxidase activity concentration were proposed, Then, test conditions were optimized, finally the use of the method has realized the simultaneous determination for peroxidase of 2-40 U/L and ascorbate of 0.4-12 mg/L in various vegetables (60 μL). The assayed results were consistent with the comparison method, in which the repeatability (RSD < 1.43%, n = 11) was satisfactory. Another important conclusion obtained in this study is that the determination of the peroxidase activity in biosamples must use the kinetic curve method for fear of the influence from the ascorbate's lag phase.

A holistic carrier-bound immobilization approach for unspecific peroxygenase

Unspecific peroxygenases (UPOs) are among the most studied enzymes in the last decade and their well-deserved fame owes to the enzyme’s ability of catalyzing the regio- and stereospecific hydroxylation of non-activated C–H bonds at the only expense of H₂O₂. This leads to more direct routes for the synthesis of different chiral compounds as well as to easier oxyfunctionalization of complex molecules. Unfortunately, due to the high sensitivity towards the process conditions, UPOs’ application at industrial level has been hampered until now. However, this challenge can be overcome by enzyme immobilization, a valid strategy that has been proven to give several benefits. Within this article, we present three different immobilization procedures suitable for UPOs and two of them led to very promising results. The immobilized enzyme, indeed, shows longer stability and increased robustness to reaction conditions. The immobilized enzyme half-life time is 15-fold higher than for the free AaeUPO PaDa-I and no enzyme deactivation occurred when incubated in organic media for 120 h. Moreover, AaeUPO PaDa-I is proved to be recycled and reused up to 7 times when immobilized.

The Roles of DmsEFAB and MtrCAB in Extracellular Reduction of Iodate by Shewanella oneidensis MR-1 with Lactate as the Sole Electron Donor

To investigate their roles in extracellular reduction of iodate (IO₃ ^- ) with lactate as an electron donor, the gene clusters of dmsEFAB, mtrCAB, mtrDEF, and so4360-4357 in Shewanella oneidensis MR-1were systematically deleted. Deletions of dmsEFAB and/or mtrCAB gene clusters diminished the bacterial ability to reduce IO₃ ^- . Furthermore, DmsEFAB and MtrCAB worked collaboratively to reduce IO₃ ^- of which DmsEFAB played a more dominant role than MtrCAB. MtrCAB was involved in detoxifying the reaction intermediate hydrogen peroxide (H₂ O₂ ). The reaction intermediate hypoiodous acid (HIO) was also found to inhibit microbial IO₃ ^- reduction. SO4360-4357 and MtrDEF, however, were not involved in IO₃ ^- reduction. Collectively, these results suggest a novel mechanism of extracellular reduction of IO₃ ^- at molecular level, in which DmsEFAB reduces IO₃ ^- to HIO and H₂ O₂ . The latter is further reduced to H₂ O by MtrCAB to facilitate the DmsEFAB-mediated IO₃ ^- reduction. The extracellular electron transfer pathway of S. oneidensis MR-1is believed to mediate electron transfer from bacterial cytoplasmic membrane, across the cell envelope to the DmsEFAB and MtrCAB on the bacterial outer membrane.

Aldehyde N,N-dimethylhydrazone-based fluorescent substrate for peroxidase-mediated assays

Numerous assays based on peroxidase activity have been developed for the detection of analytes due to the various optical peroxidase substrates. However, most substrates are sensitive to light and pH and are over-oxidized in the presence of excess H₂O₂. In this study, 2-((6-methoxynaphthalen-2-yl)methylene)-1,1-dimethylhydrazine (MNDH), a fluorescent peroxidase substrate prepared from naphthalene-based aldehyde N,N-dimethylhydrazone, was developed. MNDH showed quantitative fluorescence changes with respect to the H₂O₂ concentration in the presence of horseradish peroxidase (HRP), and the MNDH/HRP assay showed no changes in fluorescence caused by over-oxidation in the presence of excess H₂O₂. Further, MNDH was thermo- and photostable. Additionally, the assay could be operated over a considerably wide pH range, from acidic to neutral. Moreover, MNDH can be used to detect glucose quantitatively in human serum samples by using an enzyme cascade assay system.

Removal of Persistent Sulfamethoxazole and Carbamazepine from Water by Horseradish Peroxidase Encapsulated into Poly(Vinyl Chloride) Electrospun Fibers

Enzymatic conversion of pharmaceutically active ingredients (API), using immobilized enzymes should be considered as a promising industrial tool due to improved reusability and stability of the biocatalysts at harsh process conditions. Therefore, in this study horseradish peroxidase was immobilized into sodium alginate capsules and then trapped into poly(vinyl chloride) electrospun fibers to provide additional enzyme stabilization and protection against the negative effect of harsh process conditions. Due to encapsulation immobilization, 100% of immobilization yield was achieved leading to loading of 25 μg of enzyme in 1 mg of the support. Immobilized in such a way, enzyme showed over 80% activity retention. Further, only slight changes in kinetic parameters of free (K_m = 1.54 mM) and immobilized horseradish peroxidase (K_m = 1.83 mM) were noticed, indicating retention of high catalytic properties and high substrate affinity by encapsulated biocatalyst. Encapsulated horseradish peroxidase was tested in biodegradation of two frequently occurring in wastewater API, sulfamethoxazole (antibiotic) and carbamazepine (anticonvulsant). Over 80% of both pharmaceutics was removed by immobilized enzyme after 24 h of the process from the solution at a concentration of 1 mg/L, under optimal conditions, which were found to be pH 7, temperature 25 °C and 2 mM of H₂O₂. However, even from 10 mg/L solutions, it was possible to remove over 40% of both pharmaceuticals. Finally, the reusability and storage stability study of immobilized horseradish peroxidase showed retention of over 60% of initial activity after 20 days of storage at 4 °C and after 10 repeated catalytic cycles, indicating great practical application potential. By contrast, the free enzyme showed less than 20% of its initial activity after 20 days of storage and exhibited no recycling potential.

ABTS as an electron shuttle to accelerate the degradation of diclofenac with horseradish peroxidase-catalyzed hydrogen peroxide oxidation

Horseradish peroxidase (HRP)-catalyzed hydrogen peroxide (H₂O₂) oxidation could degrade a variety of organic pollutants, but the intrinsic drawback of slow degradation rate limited its widespread application. In this study, 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) was introduced into HRP/H₂O₂ system as an electron shuttle to enhance diclofenac degradation under neutral pH conditions. The green-colored ABTS radical (ABTS^•+), generated by the oxidation of ABTS with HRP-catalyzed H₂O₂ oxidation, was proved to be the main reactive species for the rapid degradation of diclofenac in HRP/H₂O₂/ABTS system. There was no destruction of ABTS/ABTS^•+ in HRP/H₂O₂/ABTS system, and ABTS was verified as an ideal electron shuttle. The reaction conditions including solution pH (4.5-10.5), HRP concentration (0-8 units mL^-1) and H₂O₂ concentration (0-500 μM) would impact the formation of ABTS^•+, and affect the degradation of diclofenac in HRP/H₂O₂/ABTS system. Moreover, compared with Fenton and hydroxylamine/Fenton systems, HRP/H₂O₂/ABTS system had better diclofenac degradation efficiency, higher H₂O₂ utilization efficiency and stronger anti-interference capacity in actual waters. Overall, the present study provided a meaningful and promising way to enhance the degradation of organic pollutants in water with HRP-catalyzed H₂O₂ oxidation.

Colorimetric assay based on horseradish peroxidase/reduced graphene oxide hybrid for sensitive detection of hydrogen peroxide in beverages

We reported a simple and sensitive colorimetric assay for detection of hydrogen peroxide (H₂O₂) based on the oxidation of 2,2׳-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) by UV-Vis spectroscopy method. The reduced graphene oxide (rGO) was prepared using green tea extract as bio-reducing and stabilizer agent and decorated by horseradish peroxidase (HRP). The surface of Au interface was modified with HRP-rGO hybrid. The formation of HRP-rGO hybrid was confirmed by cyclic voltammetry, scanning electron microscopy (SEM), energy-dispersive X-ray Spectroscopy (EDX) and Raman spectroscopy·H₂O₂ can be catalysed by HRP-rGO hybrid and converted into water and oxygen. The ABTS substrate takes up oxygen to form a green coloured product that has absorption peaks at 421, 655 nm and 737 nm. The colour development is linearly dependent on HRP in the range of 4-50 µg/L. The color of the green product solution is stable for 20 min. The absorption intensity is strongly related to the hydrogen peroxide concentration. The absorption intensity of the formed product scaled linearly with the hydrogen peroxide concentration in the ranges of 0.3-20 µM and 20-8000 µM with a detection limit of ≈15 nM could be achieved. The biosensor with excellent limit detection and wide linear ranges was adapted to monitor H₂O₂ in different beverages.

Aptamers as the powerhouse of dot blot assays

Dot blot assays have always been associated with antibodies as the main molecular recognition element, which are widely employed in a myriad of diagnostic applications. With the rising of aptamers as the equivalent molecular recognition elements of antibodies, dot blot assays are also one of the diagnostic avenues that should be scrutinized for their amenability with aptamers as the potential surrogates of antibodies. In this review, the stepwise procedures of an aptamer-based dot blot assays are underscored before reviewing the existing aptamer-based dot blot assays developed so far. Most of the applications center on monitoring the progress of SELEX and as the validatory assays to assess the potency of aptamer candidates. For the purpose of diagnostics, the current effort is still languid and as such possible suggestions to galvanize the move to spur the aptamer-based dot blot assays to a point-of-care arena are discussed.

Revelation of Fe(V)/Fe(IV) Involvement in the Fe(VI)-ABTS System: Kinetic Modeling and Product Analysis

To quantitatively probe iron intermediate species [Fe(V)/Fe(IV)] in Fe(VI) oxidation, this study systematically investigated the reaction kinetics of Fe(VI) oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic)acid (ABTS) at different ratios of [ABTS]₀/[Fe(VI)]₀ (i.e., >1.0, =1.0, and <1.0) in pH 7.0 phosphate (10 mM)-buffered solution. Compared to the literature, a more comprehensive and robust kinetic model for the Fe(VI)-ABTS system including interactions between high-valent iron species [Fe(VI), Fe(V), and Fe(IV)], ABTS, and the ABTS^•+ radical was proposed and validated. The oxidation of ABTS by Fe(VI) (k = (5.96 ± 0.9%) × 10⁵ M^-1 s^-1), Fe(V) (k = (2.04 ± 0.0%) × 10⁵ M^-1 s^-1), or Fe(IV) (k = (4.64 ± 13.0%) × 10⁵ M^-1 s^-1) proceeds via one-electron transfer to generate ABTS^•+, which is subsequently oxidized by Fe(VI) (k = (8.5 ± 0.0%) × 10² M^-1 s^-1), Fe(V) (k = (1.0 ± 40.0%) × 10⁵ M^-1 s^-1), or Fe(IV) (k = (1.9 ± 17.0%) × 10³ M^-1 s^-1), respectively, via two-electron (oxygen atom) transfer to generate colorless ABTS_ox. At [ABTS]₀/[Fe(VI)]₀ > 1.0, experimental data and model simulation both indicated that the reaction stoichiometric ratio of Fe(VI)/ABTS^•+ increased from 1.0:1.0 to 1.0:1.2 as [ABTS]₀ was increased. Furthermore, the Fe(VI)-ABTS-substrate model was developed to successfully determine reactivity of Fe(V) to different substrates (k = (0.7-1.42) × 10⁶ M^-1 s^-1). Overall, the improved Fe(VI)-ABTS kinetic model provides a useful tool to quantitatively probe Fe(V)/Fe(IV) behaviors in Fe(VI) oxidation and gains new fundamental insights.

Regulation of the Peroxidase-Like Activity of nGO, MoS2 and WS2 Nanozymes by Using Metal Cations

Two dimensional nanoparticles (2D-NPs) along with other nanoscale materials have been deemed to be the next generation of artificial enzymes (nanozymes). The low-cost bulk-scale production, ease of storage and modification of such nanomaterials have given nanozymes an advantage over traditional enzymes. Many studies have been aimed at developing methods to increase the performance of these nanozymes, and also identify interfering agents. To investigate the interference of a number of metal cations, we studied the effect of Ti²⁺ , Fe²⁺ , Ag⁺ , Hg²⁺ , Co²⁺ , Cu²⁺ , Ni²⁺ , Pb²⁺ , Ca²⁺ , Zn²⁺ and Mn²⁺ in a nanozyme assays of 2D-NPs using ABTS radical formation. Ti²⁺ , Co²⁺ , Cu²⁺ , Ni²⁺ , Ca²⁺ , Zn²⁺ and Mn²⁺ ions did not display any notable effect on the peroxidase-like activity of nGO, MoS₂ and WS₂ 2D-NPs. However, Fe²⁺ , Ag⁺ , Hg²⁺ and Pb²⁺ ions' effects on the overall ABTS reaction were significant enough to be visualised by partial least square discriminant analysis (PLSDA). We report that, similar to that of many natural enzymes, the nanozyme activity of 2D-NPs is regulated by a number of metal cations allowing their identification and discrimination by using a statistical analysis tool.

Test-System for Bacteria Sensing Based on Peroxidase-Like Activity of Inkjet-Printed Magnetite Nanoparticles

Rapid detection of bacterial contamination is an essential task in numerous medical and technical processes and one of the most rapidly developing areas of nano-based analytics. Here, we present a simple-to-use and special-equipment-free test-system for bacteria detection based on magnetite nanoparticle arrays. The system is based on peroxide oxidation of chromogenic substrate catalyzed by magnetite nanoparticles, and the process undergoes computer-aided visual analysis. The nanoparticles used had a pristine surface free of adsorbed molecules and demonstrated high catalytic activities up to 6585 U/mg. The catalytic process showed the Michaelis–Menten kinetic with K_m valued 1.22 mmol/L and V_max of 4.39 µmol/s. The nanoparticles synthesized were used for the creation of inkjet printing inks and the design of sensor arrays by soft lithography. The printed sensors require no special equipment for data reading and showed a linear response for the detection of model bacteria in the range of 10⁴–10⁸ colony-forming units (CFU) per milliliter with the detection limit of 3.2 × 10³ CFU/mL.

Exploration and quantification of ascorbate affecting peroxidase-catalyzed chromogenic reactions with a recirculating-flow catalysis detection system

This research used a recirculating-flow catalysis detection system to explore the ascorbate interference with peroxidase-catalyzed reactions and simultaneously determined peroxidase and ascorbate with a kinetic curve.

Mitochondria-Targeting Thermosensitive Initiator with Enhanced Anticancer Efficiency

As one of the most important organelles in cells, the mitochondrion has been reported to exhibit higher temperatures and is vulnerable to free radicals, especially in cancer cells. Here, we report on the use of a mitochondria-targeted thermosensitive radical initiator for cancer cell killing. The thermal-sensitive radical initiator, V044 (2,2'-azobis [2-(2-imidazolin-2-yl)propane]dihydrochloride), was applied as a radical source, which was linked with a mitochondrial targeting moiety, triphenylphosphine (TPP), to construct the mitochondria-targeting radical initiator (TPPV). Mitochondria were applied as the endogenous thermal source of cells, which accelerated the free radical generation of TPPV. Results showed that TPPV could effectively generate free radicals in the mitochondrial area, and the released free radicals effectively damaged mitochondria, exhibiting an enhanced anticancer efficiency. This therapy based on endogenous mitochondrial heat avoids tissue penetration limits and offers a target for mitochondria-targeting systems.

From agro-waste to tool: biotechnological characterization and application of Ganoderma lucidum E47 laccase in dye decolorization

The culture of fungal species from agro-waste allows for the sustainable preparation of valuable biotechnological products and contributes to establish the Circular Economy concept. The Ganoderma lucidum species is well known as producer of laccases (EC 1.10.3.2), which serves as a tool to oxidize chemicals. When producing G. lucidum E47 basidiomes with edible purposes out of rice crop residues, its laccase remains as by-product. In this work, we report the biotechnological characterization and application of the laccase recovered from spent cultures of the G. lucidum E47 strain. We detected at least one polypeptide (ca. 59 kDa) which displays attractive activity and stability values when used in the range of 18-45 °C in mildly acidic environment (pH 4.8-5.8). These parameters can be enhanced in the presence of organic cosolvents such as butyl acetate and methyl iso-butyl ketone, but the opposite effect is observed with solvents of lower log P. The best activity-stability performance is reached when the biocatalyst is used in pH 4.8 buffer with 5% (v/v) butyl acetate at 37 °C. The laccase was capable of decolorizing xanthene, azo and triarylmethane dyes, exhibiting excellent selectivity on bromocresol green and bromocresol purple. Furthermore, the biocatalyst displayed an attractive activity when assessed for the decolorization of bromocresol green in a proof-of-concept effluent biotreatment.

Self-Rolling Refillable Tubular Enzyme Containers Made of Recombinant Spider Silk and Chitosan

Encapsulation of enzymes is often necessary to stabilize them against environmental conditions or to protect them from other harmful enzymes such as proteases. Here, a refillable spatial confinement system was produced using a fully degradable self-rolling biopolymer bilayer. The enzyme containers comprise spider silk and chitosan and enable one-pot reactions in the micro- to milliliter regime by trapping the enzyme inside the semipermeable tube and allow the substrate and/or product either to diffuse freely or to be entrapped. The tubes are stable toward several organic and aqueous solvents. A one-tube system with esterase-2 was used to establish the system. Further, a two-tube system was applied to mimic enzymatic cascades, where the enzymes have to be separated, because they, for example, inhibit each other. The entrapment mode was also tested in the two-tube system, which is beneficial for toxic products or for obtaining high concentrations of the desired product.

Quantification of carbonate radical formation by the bicarbonate-dependent peroxidase activity of superoxide dismutase 1 using pyrogallol red bleaching

Carbonate radicals (CO₃•^-) are generated by the bicarbonate-dependent peroxidase activity of cytosolic superoxide dismutase (Cu,Zn-SOD, SOD-1). The present work explored the use of bleaching of pyrogallol red (PGR) dye to quantify the rate of CO₃•^- formation from bovine and human SOD-1 (bSOD-1 and hSOD-1, respectively). This approach was compared to previously reported methods using electron paramagnetic resonance spin trapping with DMPO, and the oxidation of ABTS (2,2-azino-bis(3-ethylbenzothiazoline)-6-sulfonic acid). The kinetics of PGR consumption elicited by CO₃•^- was followed by visible spectrophotometry. Solutions containing PGR (5–200 μM), SOD-1 (0.3–3 μM), H₂O₂ (2 mM) in bicarbonate buffer (200 mM, pH 7.4) showed a rapid loss of the PGR absorption band centered at 540 nm. The initial consumption rate (R_i) gave values independent of the initial PGR concentration allowing an estimate to be made of the rate of CO₃•^- release of 24.6 ± 4.3 μM min⁻¹ for 3 μM bSOD-1. Both bSOD-1 and hSOD-1 showed a similar peroxidase activity, with enzymatic inactivation occurring over a period of 20 min. The single Trp residue (Trp³²) present in hSOD-1 was rapidly consumed (initial consumption rate 1.2 ± 0.1 μM min⁻¹) with this occurring more rapidly than hSOD-1 inactivation, suggesting that these processes are not directly related. Added free Trp was rapidly oxidized in competition with PGR. These data indicate that PGR reacts rapidly and efficiently with CO₃•^- resulting from the peroxidase activity of SOD-1, and that PGR-bleaching is a simple, fast and cheap method to quantify CO₃•^- release from bSOD-1 and hSOD-1 peroxidase activity.

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CO₃•^- are released during the bicarbonate-dependent peroxidase activity of SOD-1.

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The rate and extent of CO₃•^- release can be determined by pyrogallol red bleaching.

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Inactivation of bSOD-1 and hSOD-1 occurs rapidly during the reaction.

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SOD-1 inactivation is independent of the presence of pyrogallol red.

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This assay should help elucidate protein oxidation/crosslinking mediated by SOD-1.

Free Heme and Amyloid-β: A Fatal Liaison in Alzheimer's Disease

While the etiology of Alzheimer's disease (AD) is still unknown, an increased formation of amyloid-β (Aβ) peptide and oxidative processes are major pathological mechanism of the disease. The interaction of Aβ with free heme leads to the formation of peroxidase-active Aβ-heme complexes. However, enzyme-kinetic data and systematic mutational studies are still missing. These aspects were addressed in this study to evaluate the role of Aβ-heme complexes in AD. The enzyme-kinetic measurements showed peroxidase-specific pH- and H2O2-dependencies. In addition, the enzymatic activity of Aβ-heme complexes constantly increased at higher peptide excess. Moreover, the role of the Aβ sequence for the named enzymatic activity was tested, depicting human-specific R5, Y10, and H13 as essential amino acids. Also by studying Y10 as an endogenous peroxidase substrate for Aβ-heme complexes, ratio-specific effects were observed, showing an optimal dityrosine formation at an about 40-fold peptide excess. As dityrosine formation promotes Aβ fibrillation while free heme disturbs protein aggregation, we also investigated the effect of Aβ-heme complex-derived peroxidase activity on the formation of Aβ fibrils. The fluorescence measurements showed a different fibrillation behavior at strong peroxidase activity, leading also to altered fibril morphologies. The latter was detected by electron microscopy. As illustrated by selected in vivo measurements on a mouse model of AD, the disease is also characterized by Aβ-derived microvessel destructions and hemolytic processes. Thus, thrombo-hemorrhagic events are discussed as a source for free heme in brain tissue. In summary, we suggest the formation and enzymatic activity of Aβ-heme complexes as pathological key features of AD.

Formulation and Structural Study of a Biocompatible Water-in-Oil Microemulsion as an Appropriate Enzyme Carrier: The Model Case of Horseradish Peroxidase

A novel biocompatible water-in-oil microemulsion was developed using nonionic surfactants and was investigated as a potential enzyme delivery system for pharmaceutical applications. The system was composed of isopropyl myristate/polysorbate 80 (Tween 80)/distilled monoglycerides/water/propylene glycol (PG), had a low total surfactant concentration (8.3% w/w), and was able to incorporate approximately 3% w/w aqueous phase containing horseradish peroxidase (HRP). Structural and activity aspects of the system were studied using a variety of techniques such as dynamic light scattering (DLS), electron paramagnetic resonance (EPR), and dynamic interfacial tension. The apparent hydrodynamic diameter of the empty droplets was calculated at about 37 nm. Different enzyme concentrations, ranging from 0.01 to 1.39 μM, were used for both DLS and EPR studies to effectively determine the localization of the macromolecule in the microemulsion. According to the results, for high enzyme concentrations, a participation of HRP in the surfactant monolayer of the microemulsion is evident. The number of reverse micelles in the microemulsion was defined by a theoretical model and was used to clarify how the enzyme concentration affects the number of empty and loaded reverse micelles. To assure that the system allows the enzyme to retain its catalytic activity, an oxidative reaction catalyzed by HRP was successfully carried out with the use of the model substrate 2,2'-azino-bis[3-ethylbenzothiazoline-6-sulfonic acid]. The influence of several parameters such as temperature, pH, and PG concentration was examined to optimize the reaction conditions, and a kinetic study was conducted revealing an ordered-Bi-Bi mechanism. Values of all kinetic parameters were determined. The release of the encapsulated enzyme was studied using an adequate receiver phase, revealing the effectiveness of the proposed microemulsion not only as a microreactor but also as a carrier for therapeutic biomolecules.

Beyond the Roles in Biomimetic Chemistry: An Insight into the Intrinsic Catalytic Activity of an Enzyme for Tumor-Selective Phototheranostics

Protein-assisted biomimetic synthesis has been an emerging offshoot of nanofabrication in recent years owing to its features of green chemistry, facile process, and ease of multi-integration. As a result, many proteins have been used for biomimetic synthesis of varying kinds of nanostructures. Although the efforts on exploring new proteins and investigating their roles in biomimetic chemistry are increasing, the most essential intrinsic properties of proteins are largely neglected. Herein we report a frequently used enzyme (horseradish peroxidase, HRP) to demonstrate the possibility of enzymatic activity retaining after accomplishing the roles in biomimetic synthesis of ultrasmall gadolinium (Gd) nanodots and stowing its substrate 2,2'-Azinobis (3-ethylbenzothiazoline-6-sulfonic acid ammonium salt) (ABTS), denoted as Gd@HRP^ABTS. It was found that ca. 70% of the enzymatic activity of HRP was preserved. The associated changes of protein structure with chemical treatments were studied by spectroscopic analysis. Leveraging on the highly retained catalytic activity, Gd@HRP^ABTS exerts strong catalytic oxidation of peroxidase substrate ABTS into photoactive counterparts in the presence of intrinsic H₂O₂ inside the tumor, therefore enabling tumor-selective catalytic photoacoustic (PA) imaging and photothermal therapy (PTT). In addition, the MR moiety of Gd@HRP^ABTS provides guidance for PTT and further diagrams that Gd@HRP^ABTS is clearable from the body via kidneys. Preliminary toxicity studies show no observed adverse effects by administration of them. This study demonstrates beyond the well-known roles in biomimetic chemistry that HRP can also preserve its enzymatic activity for tumor catalytic theranostics.

Reactions of Ferrate(VI) with Iodide and Hypoiodous Acid: Kinetics, Pathways, and Implications for the Fate of Iodine during Water Treatment

Oxidative treatment of iodide-containing waters can form toxic iodinated disinfection byproducts (I-DBPs). To better understand the fate of iodine, kinetics, products, and stoichiometries for the reactions of ferrate(VI) with iodide (I^-) and hypoiodous acid (HOI) were determined. Ferrate(VI) showed considerable reactivities to both I^- and HOI with higher reactivities at lower pH. Interestingly, the reaction of ferrate(VI) with HOI ( k = 6.0 × 10³ M^-1 s^-1 at pH 9) was much faster than with I^- ( k = 5.6 × 10² M^-1 s^-1 at pH 9). The main reaction pathway during treatment of I^--containing waters was the oxidation of I^- to HOI and its further oxidation to IO₃^- by ferrate(VI). However, for pH > 9, the HOI disproportionation catalyzed by ferrate(VI) became an additional transformation pathway forming I^- and IO₃^-. The reduction of HOI by hydrogen peroxide, the latter being produced from ferrate(VI) decomposition, also contributes to the I^- regeneration in the pH range 9-11. A kinetic model was developed that could well simulate the fate of iodine in the ferrate(VI)-I^- system. Overall, due to a rapid oxidation of I^- to IO₃^- with short-lifetimes of HOI, ferrate(VI) oxidation appears to be a promising option for I-DBP mitigation during treatment of I^--containing waters.

Fluorescent carbon nanoparticles: mimicking hydrogen peroxide properties in a chemiluminescence system

Fluorescent carbon nanoparticles (FCNs), as novel luminescent reagents exhibiting hydrogen peroxide mimicking properties, can directly react with luminol, NaHCO₃ and NaHSO₃ in alkaline conditions to yield novel chemiluminescence, and show great potential towards further applications of ultra-weak chemiluminescence.

Horseradish peroxidase-mediated in situ synthesis of silver nanoparticles: application for sensing of mercury

Schematic representation for spectrophotometric detection of Hg²⁺ using an enzyme-mediated formation of silver nanoparticles.

The laccase-like reactivity of manganese oxide nanomaterials for pollutant conversion: rate analysis and cyclic voltammetry

Nanostructured manganese oxides, e.g. MnO₂, have shown laccase-like catalytic activities, and are thus promising for pollutant oxidation in wastewater treatment. We have systematically compared the laccase-like reactivity of manganese oxide nanomaterials of different crystallinity, including α-, β-, γ-, δ-, and ɛ-MnO₂, and Mn₃O₄, with 2,2′-azinobis-(3-ethylbenzthiazoline-6-sulfonate) (ABTS) and 17β-estradiol (E2) as the probing substrates. The reaction rate behaviors were examined with regard to substrate oxidation and oxygen reduction to evaluate the laccase-like catalysis of the materials, among which γ-MnO₂ exhibits the best performance. Cyclic voltammetry (CV) was employed to assess the six MnO_x nanomaterials, and the results correlate well with their laccase-like catalytic activities. The findings help understand the mechanisms of and the factors controlling the laccase-like reactivity of different manganese oxides nanomaterials, and provide a basis for future design and application of MnO_x-based catalysts.