Two-electron Oxidation Reaction Research Articles

ER Oxidoreductin 1-Like Activity of Cyclic Diselenides Drives Protein Disulfide Isomerase in an Electron Relay System.

Disulfide formation generally involves a two-electron oxidation reaction between cysteine residues. Additionally, disulfide formation is an essential post-translational modification for the structural maturation of proteins. This oxidative folding is precisely controlled by an electron relay network constructed by protein disulfide isomerase (PDI), with a CGHC sequence as the redox-active site, and its family enzymes. Creating reagents that mimic the functions of these enzymes facilitates folding during chemical protein synthesis. In this study, we aimed to imitate a biological electron relay system using cyclic diselenide compounds as surrogates for endoplasmic reticulum oxidoreductin 1 (Ero1), which is responsible for the re-oxidation of PDI. Oxidized PDI (PDIox) introduces disulfide bonds into substrate proteins, resulting in its conversion to reduced PDI (PDIred). The PDIred is then re-oxidized to PDIox by a coexisting cyclic diselenide compound, thereby restoring the function of PDI as a disulfide-forming agent. The produced diselenol state is readily oxidized to the original diselenide state with molecular oxygen, continuously sustaining the PDI catalytic cycle. This artificial electron relay system regulating enzymatic PDI function effectively promotes the oxidative folding of disulfide-containing proteins, such as insulin-a hypoglycemic formulation-by enhancing both yield and reaction velocity.

Advances in Two-Electron Water Oxidation Reaction for Hydrogen Peroxide Production: Catalyst Design and Interface Engineering.

Hydrogen peroxide (H2O2) is a versatile and zero-emission material that is widely used in the industrial, domestic, and healthcare sectors. It is clear that it plays a critical role in advancing environmental sustainability, acting as a green energy source, and protecting human health. Conventional production techniques focused on anthraquinone oxidation, however, electrocatalytic synthesis has arisen as a means of utilizing renewable energy sources in conjunction with available resources like oxygen and water. These strides represent a substantial change toward more environmentally and energy-friendly H2O2 manufacturing techniques that are in line with current environmental and energy goals. This work reviews recent advances in two-electron water oxidation reaction (2e-WOR) electrocatalysts, including design principles and reaction mechanisms, examines catalyst design alternatives and experimental characterization techniques, proposes standardized assessment criteria, investigates the impact of the interfacial milieu on the reaction, and discusses the value of in situ characterization and molecular dynamics simulations as a supplement to traditional experimental techniques and theoretical simulations, as shown in Figure 1. The review also emphasizes the importance of device design, interface, and surface engineering in improving the production of H2O2. Through adjustments to the chemical microenvironment, catalysts can demonstrate improved performance, opening the door for commercial applications that are scalable through tandem cell development.

Novel electrocatalyst with abundant oxygen vacancies enabling efficient two-electron water oxidation reaction for H2O2 synthesis

In the realm of electrocatalytic two-electron water oxidation reaction, the quest for efficient and selective catalysts poses a significant challenge in balancing productivity and activity, hindering the decentralized generation of hydrogen peroxide (H2O2) through electrochemical means. Addressing this hurdle, our study introduces a novel approach to crafting an effective and stable electrocatalyst for the two-electron water oxidation reaction (2e-WOR). Through the synthesis of magnesium stannite tungstate from a mixture of MgSnO3 and WO3, the engineer of a material with abundant oxygen vacancies is crucial for catalytic activity. This catalyst demonstrates exceptional performance, ascribed to its distinctive oxygen vacancies proximal to the active center Sn and electronic modulation by tungsten. Notably, Mg1-xSnWO6-x700 exhibits a remarkably low overpotential of 70 mV at 10 mA cm−2, coupled with a high faradic efficiency of 84 % for 2e- WOR at 2.6 V vs. RHE, maintaining consistent performance over 35 h. Furthermore, the formation of oxygen vacancies and the associated electronic transfer underscore the novelty and promise of our material in advancing the field of two-electron WOR. In sum, our electrocatalyst presents a cost-efficient and selective solution for water oxidation, offering potential avenues for enhancing H2O2 production.

Pairing Oxygen Reduction and Water Oxidation for Dual-Pathway H2O2 Production.

Hydrogen peroxide (H2O2) is a crucial chemical applied in various industry sectors. However, the current industrial anthraquinone process for H2O2 synthesis is carbon-intensive. With sunlight and renewable electricity as energy inputs, photocatalysis and electrocatalysis have great potential for green H2O2 production from oxygen (O2) and water (H2O). Herein, we review the advances in pairing two-electron O2 reduction and two-electron H2O oxidation reactions for dual-pathway H2O2 synthesis. The basic principles, paired redox reactions, and catalytic device configurations are introduced initially. Aligning with the energy input, the latest photocatalysts, electrocatalysts, and photo-electrocatalysts for dual-pathway H2O2 production are discussed afterward. Finally, we outlook the research opportunities in the future. This minireview aims to provide insights and guidelines for the broad community who are interested in catalyst design and innovative technology for on-site H2O2 synthesis.

Photoelectrochemical Synthesis of Hydrogen Peroxide from Saline Water via the Two-Electron Water Oxidation Reaction.

Hydrogen peroxide (H2O2) production on the anode is more valuable than oxygen and chlorine evolution for photoelectrochemical saline water splitting. In this work, by the introduction of bicarbonate (HCO3-), H2O2 is produced from saline water (2 M KHCO3 + 0.5 M NaCl aqueous solution) via the two-electron water oxidation reaction by a photoanode of bismuth vanadate (BiVO4). Furthermore, the Faradaic efficiency (FE) and accumulation for H2O2 are improved by coating antimony tetroxide (Sb2O4) on BiVO4. A H2O2 FE of 26% at 1.54 V vs RHE is obtained by Sb2O4/BiVO4 and 49 ppm of H2O2 is accumulated after a 135 min chronoamperometry. Similar to that in KHCO3 pure water solution, infrared spectroscopy and electrochemical analysis confirm that HCO3- plays a surface-mediating role in the formation of H2O2 in KHCO3 saline water solution. The presence of HCO3- in the electrolyte is able to not only increase the photocurrent density but also effectively inhibit the chlorine evolution reaction.

Metal Supported Metal Oxide Catalysts for Hydrogen Peroxide Production Via Water Splitting

The two-electron water oxidation reaction (WOR) has drawn recent attention as an efficient, simple method of producing on-site hydrogen peroxide through electrochemical water splitting. However, finding electrocatalysts with high activity and stability in the reaction’s harsh oxidizing environment is challenging. This is made especially difficult due to the competing four-electron and one-electron WOR, which necessitate a catalyst material with high selectivity towards the hydrogen peroxide production pathway. Here, we utilize metal/metal oxide coupling interactions to tune metal oxide catalysts to be active for 2 e- WOR via the creation of a Mott-Schottky junction. Metal/metal oxide interactions have been well studied for supported metal catalysts, but their behavior has been less investigated for catalysts where the metal oxide provides the active sites, called inverse catalysts. In this case, the difference in work functions between the metal support and metal oxide semiconductor catalyst drives electron transfer between the two, facilitating electronic interactions which can modulate the binding energy of reaction intermediates at the interface and thus tune the catalytic ability of the metal oxide. In this work, we investigate a variety of metal oxide catalyst and metal support materials and find that a thin layer of indium tin oxide (ITO) supported by a layer of platinum (Pt) has excellent catalytic ability towards 2 e- WOR. The activity, selectivity, and stability of the ITO/Pt catalyst shows significant improvement over the unsupported ITO catalyst and yields hydrogen peroxide production rates greater than those reported in literature using other standard catalysts for this reaction. This method of catalyst engineering also provides a future pathway to create new catalysts for this electrochemical reaction. Figure 1

Stable production of hydrogen peroxide over zinc oxide @ zeolitic imidazolate Framework-8 composite catalysts

A promising method of producing hydrogen peroxide (H2O2) is the electrochemical two-electron water oxidation reaction (2e− WOR). In this process, it is important to design electrocatalysts that are both earth abundant and environmentally friendly, as well as offering high stability and production rates. The research of WOR catalysts, such as the extensively used transition metal oxides, is mainly focused on the modification of transition metal elements. Few studies pay attention to the protective heterostructure of metal oxides. Here, we demonstrate for the first time an organometallic skeleton protection strategy to develop highly stable WOR catalysts for H2O2 generation. Unlike the pure ZnO and zeolite imidazole framework-8 (ZIF-8) catalysts, ZnO@ZIF-8 enabled the production of hydrogen peroxide at high voltages. The experimental results demonstrate that the ZnO@ZIF-8 catalyst stably generates H2O2 even under a high voltage of 3.0 V vs. RHE, with a yield reaching 2845.819 μmolmin−1 g−1. ZnO@ZIF-8 shows a relatively low overpotential, with a current density of 10 mA cm−2 and an overpotential of 110 mV. The ZnO@ZIF-8 catalyst’s maximal FE value was 4.72 %. Moreover, the ZnO@ZIF-8 catalyst exhibits remarkable durability even after an extended 60-hour stability test. Operando Raman and theoretic calculation analyses reveal that the metal–organic skeleton being encapsulated on the metal oxide surface synergizes with each other, not only expanding the electrochemical surface area, but also adjusting the catalyst metal sites’ adsorption capacity. A novel approach to the modification of 2e− WOR metal oxide catalyst is presented in this work.

Tandem Fields Facilitating Directional Carrier Migration in Van der Waals Heterojunction for Efficient Overall Piezo-Synthesis of H2O2.

Piezo-synthesis of H2O2 utilizing sustainable mechanical energy as well as earth-abundant water and oxygen is a green, cost-effective, and promising approach. However, achieving simultaneous two-electron water oxidation reaction (2e- WOR) and two-electron oxygen reduction reaction (2e- ORR) faces huge challenges due to insufficient synergistic active sites and slow/messy carrier transfer. Herein, a novel 2D/2D van der Waals heterojunction consisting of BiOIO3 and carbon nitride (BIO/CN) is elaborately designed for highly efficient overall H2O2 piezo-synthesis. Theoretical/experimental results reveal that a Z-scheme electron transfer is formed and facilitated by the tandem interfacial electric field and the bulk piezo-polarization field. On this basis, the carriers are efficiently separated while the oxidation/reduction capacity is preserved, thus providing the strong driving force for the 2e- WOR and 2e- ORR on BIO and CN, respectively. Furthermore, the kinetic and thermodynamic processes of WOR and ORR for H2O2 synthesis improve remarkably. Therefore, BIO/CN exhibits an excellent H2O2 yield of 259.8µM within 30min in pure water and air atmosphere (without any sacrificial agents and aeration). This study provides a new idea on strategically controlling electron transfer toward high-efficiency H2O2 piezo-synthesis and expands the avenue for developing effective environmental purification materials.

Carbazole-containing covalent triazine frameworks for efficient hydrogen peroxide photosynthesis from natural sunlight

Hydrogen peroxide (H2O2) photosynthesis via polymer photocatalysts is an environmentally friendly and sustainable method for H2O2 production. However, their efficiency is currently unsatisfactory due to limited dissociation and insufficient migration of photogenerated charge carriers. Constructing donor–acceptor (D-A) type polymer photocatalysts has been demonstrated to be a promising approach to improve their optical and electronic properties related to charge carrier separation and migration. Herein, we design two donor–acceptor (D-A) type carbazole-containing covalent triazine frameworks (CTF-PCZs), namely CTF-BPCZ and CTF-TPCZ. These CTF-PCZs are found to reduce the exciton binding energy (Eb) and enhance charge carriers transfer ability compared to non-D-A type CTF-HUST-1. Remarkably, CTF-TPCZ exhibit excellent H2O2 photosynthesis performance, achieving a rate of up to 10331 μmol g−1 h−1 from pure water under solar simulator (AM 1.5G), and 6039 μmol g−1 h−1 from pure water under natural sunlight irradiation in scaled-up photocatalytic experiments, significantly surpass those of all previously reported polymer-based photocatalysts. In-situ experiments provide direct evidence for intermediates involved in the photocatalytic process. Both experiments and DFT calculations reveal that the triazine ring fragment plays a crucial role in H2O2 photosynthesis through a two-step single-electron oxygen reduction reaction (ORR) and the carbazolyl group plays a crucial role in one-step two-electron water oxidation reaction (WOR).

Study on Influence Factors of H2O2 Generation Efficiency on Both Cathode and Anode in a Diaphragm-Free Bath.

Electrosynthesis of H2O2 via both pathways of anodic two-electron water oxidation reaction (2e-WOR) and cathodic two-electron oxygen reduction reaction (2e-ORR) in a diaphragm-free bath can not only improve the generation rate and Faraday efficiency (FE), but also simplify the structure of the electrolysis bath and reduce the energy consumption. The factors that may affect the efficiency of H2O2 generation in coupled electrolytic systems have been systematically investigated. A piece of fluorine-doped tin oxide (FTO) electrode was used as the anode, and in this study, its catalytic performance for 2e-WOR in Na2CO3/NaHCO3 and NaOH solutions was compared. Based on kinetic views, the generation rate of H2O2 via 2e-WOR, the self-decomposition, and the oxidative decomposition rate of the generated H2O2 during electrolysis in carbonate electrolytes were investigated. Furthermore, by choosing polyethylene oxide-modified carbon nanotubes (PEO-CNTs) as the catalyst for 2e-ORR and using its loaded electrode as the cathode, the coupled electrolytic systems for H2O2 generation were set up in a diaphragm bath and in a diaphragm-free bath. It was found that the generated H2O2 in the electrolyte diffuses and causes oxidative decomposition on the anode, which is the main influent factor on the accumulated concentration in H2O2 in a diaphragm-free bath.

Oxygen vacancy based WO3/SnO2-x promote electrochemical H2O2 accumulation by two-electron water oxidation reaction and toxic uniform dimethylhydrazine degradation

The key to constructing an anodic electro-Fenton system hinges on two pivotal criteria: enhancing the catalyst activity and selectivity in water oxidation reaction (WOR), while simultaneously inhibiting the decomposition of hydrogen peroxide (H2O2) which is on-site electrosynthesized at the anode. To address the issues, we synthesized novel WO3/SnO2-x electrocatalysts, enriched with oxygen vacancies, capitalize on the combined activity and selectivity advantages of both WO3 and SnO2-x for the two-electron pathway electrocatalytic production of H2O2. Moreover, the introduction of oxygen vacancies plays a critical role in impeding the decomposition of H2O2. This innovative design ensures that the Faraday efficiency and yield of H2O2 are maintained at over 80 %, with a noteworthy production rate of 0.2 mmol h−1 cm−2. We constructed a novel electro-Fenton system that operates using only H2O as its feedstock and applied it to treat highly toxic uniform dimethylhydrazine (UDMH) from rocket launch effluent. Our experiments revealed a substantial total organic carbon (TOC) removal, achieving approximately 90 % after 120 mins of treatment. Additionally, the toxicity of N-nitrosodimethylamine (NDMA), a byproduct of great concern, was shown to be effectively mitigated, as evidenced by acute toxicity evaluations using zebrafish embryos. The degradation mechanism of UDMH is predominantly characterized by the advanced oxidative action of H2O2 and hydroxyl radicals, as well as by complex electron transfer processes that warrant further investigation.

Rational engineering WO3-X/CNTs@carbon fiber membrane for the high-efficient produce H2O2 via electrochemical two-electron water oxidation route

The electrochemical two-electron water oxidation reaction (2e-WOR) offers a promising avenue for hydrogen peroxide (H2O2) production. It is crucial to design electrocatalysts with high selectivity, yield, resource abundance, and environmental friendliness. In this paper, as a new generation 2e-WOR electrocatalyst, the WO3-X/CNTs@carbon fiber membrane nanostructure with abundant oxygen-rich vacancy (OV) has been discovered by a series of test techniques including XRD, FE-SEM, TEM, FT-IR, Raman and XPS, comprising interconnected carbon nanotubes and carbon fibers enveloped in WO3 nanorods with oxygen-rich vacancies. By comparing the specific surface area of WO3-x/CNTs@carbon fiber membrane at different temperatures, it was observed that the maximum value is 44.09 m2/g when the mass ratio of AMT to PAN was 3:2 at 600 °C. Furthermore, the WO3-X/CNTs@carbon fiber membrane can be directly utilized as an independent electrode. The electrochemical performance test revealed that the Faraday efficiency of the WO3-X/CNTs @CF-600-2 can reach 54 % (2.48 V vs RHE (reversible hydrogen electrode)), while the H2O2 yield can achieve 10.3 μmol· min−1·cm−2, nearly ten times higher than that of pure WO3. Furthermore, the stability testing demonstrated that after 12 h, the WO3-X/CNTs @CF-600-2 exhibited excellent stability with minimal change in its Faraday efficiency. This enhancement is attributed to the improved selectivity of WO3, excellent electrical conductivity, and mass transfer performance of carbon fiber. Finally, we proposed a mechanism for 2e-WOR and provided some perspectives on this reaction along with summarizing recent discoveries.

Radical Cations of Bilayer Nanographenes.

Chemical redox reactions of bilayer nanographene complexes, (C96H24Ar6)2 (Ar = 2,6-dimethylphenyl) (12) and (C42H12R6)2 (R = tBu) (22), were investigated. Upon two-electron oxidation reactions, 12 and 22 were transformed to radical cations 122•+ and 222•+, respectively. SQUID and EPR measurements on 122•+ and 222•+ indicate that they possess an open-shell singlet ground state with antiferromagnetic interactions between two layers. The shortest separation distance between bilayers in 222•+ (3.30 Å) is shorter than that in 22 (3.44 Å) and 22•+ (3.40 Å), illustrating stronger interaction upon loss of electrons.

Effective electrochemical water oxidation to H2O2 based on a bimetallic Fe/Co metal–organic framework

A bimetallic Fe/Co metal–organic framework exhibits high-efficiency catalysis for electrochemical two-electron water oxidation reaction to produce hydrogen peroxide.

The Nexus of Innovation: Electrochemically Synthesizing H2O2 and Its Integration with Downstream Reactions.

Hydrogen peroxide (H2O2) represents a chemically significant oxidant that is prized for its diverse applicability across various industrial domains. Recent innovations have shed light on the electrosynthesis of H2O2 through two-electron oxygen reduction reactions (2e- ORR) or two-electron water oxidation reactions (2e- WOR), processes that underscore the attractive possibility for the on-site production of this indispensable oxidizing agent. However, the translation of these methods into practical utilization within chemical manufacturing industries remains an aspiration rather than a realized goal. This Perspective intends to furnish a comprehensive overview of the latest advancements in the domain of coupled chemical reactions with H2O2, critically examining emergent strategies that may pave the way for the development of new reaction pathways. These pathways could enable applications that hinge on the availability and reactivity of H2O2, including, but not limited to the chemical synthesis coupled with H2O2 and waste water treatment byFenton-like reactions. Concurrently, the Perspective acknowledges and elucidates some of the salient challenges and opportunities inherent in the coupling of electrochemically generated H2O2, thereby providing a scholarly analysis that might guide future research.

Flower-like WO2@C prepared from the pyrolyzation of ionic liquid for effective elctrocatalytic two-electron water oxidation

Flower-like tungsten dioxide@carbon (WO2@C) is prepared through the pyrolysis of 1-octyl-3-methylimidazolium tungstate ionic liquids. The resultant WO2 particles are surrounded by a few of carbon fibers, displaying the unique flower-like morphology. The WO2@C testified efficient electrocatalytic activity for two-electron water oxidation reaction (2e- WOR) to H2O2. The electrocatalytic H2O2 production rate reaches about 6.55 μmol min−1 cm−2, over 2 times higher than WO2. This may be ascribed to the enhanced electron transferring capacities and abundant defects of WO2@C. This work introduces a special ionic liquid-derived metal oxide@C with the improved electrocatalytic activity for 2e- WOR.

Coordination engineering regulating metal single-atom anchored on n-doped carbon as a bifunctional catalyst for H2O2 production via dual channels

Electrochemical synthesis of hydrogen peroxide (H2O2) via two-electron oxygen reduction reaction (2e- ORR) or two-electron water oxidation reaction (2e- WOR), parades great prospects in substituting traditional anthraquinone process. However, the low efficiency and selectivity hinders its practical utilization. Exploration of a bifunctional catalyst capable of simultaneously driving the 2e- WOR and 2e- ORR is crucial for thoroughly improving H2O2 conversion efficiency. Herein, by means of first principles calculation, the 2e- ORR and 2e- WOR activities of metal-nitrogen doped carbon (M−NC, M = Mn, Fe, Co, Ni, Ru, In, and Pt) are investigated. Combining the pyrrole nitrogen coordination regulation, the best sample (RuN(Po)4) is screened from forty single atom configurations, which exhibits ultra-low overpotentials (0.02 V/0.04 V) and excellent inhibition on 4e- ORR/WOR side reactions. In-depth study further verifies its low reaction energy barrier and high thermodynamic stability for H2O2 synthesis in two pathways. Our work is expected to provide useful understanding for designing highly-efficient bifunctional catalysts for H2O2 production.

Embedding Carbon Quantum Dots into Crystalline Polyimide Covalent Organic Frameworks to Enhance Water Oxidation for Achieving Dual-Channel Photocatalytic H2O2 Generation in a Wide pH Range.

Photocatalytic technique is regarded as the cleanest approach for producing H2O2. Herein, two kinds of novel polyimide COFs decorated with CQDs (namely, MPa-COFs/CQDs and MNd-COFs/CQDs) were constructed by using the one-pot hydrothermal method. Due to the electron donor role of CQDs, the recombination of photoinduced electrons and holes was suppressed after the combination of polyimide COFs with CQDs. Importantly, the introduction of CQDs not only boosted the absorbing ability of polyimide COFs toward visible light but also reduced the impedance and improved the charge transfer efficiency. After CQDs were embedded into polyimide COFs, the surface hydrophilicity of catalysts was significantly improved, which provided convenience for the water oxidation reaction. Benefiting from the electron donor-acceptor interaction between polyimide COFs and CQDs, a step-by-step two-electron oxygen reduction reaction over polyimide COFs was enhanced. More interestingly, the embedding of CQDs can create a direct two-electron water oxidation reaction pathway, which played an important role in photocatalytic H2O2 generation. Meanwhile, H+ generated from water oxidation can also be used for the reduction of oxygen to form H2O2. Under the synergistic effects of water oxidation and oxygen reduction, as-prepared MPa-COFs/CQDs-2 displayed excellent performance in photocatalytic H2O2 generation, and its yield was as high as 540 μmol/g within 60 min. In short, the current work shared an effective strategy to improve the performance of polyimide COFs in photocatalytic H2O2 production.

Phtotoelectrochemical water oxidation to H2O2 based on N-TiO2 derived from NH2-MIL-125 and in-situ application on degradation dye

Electrochemical two-electron water oxidation reaction (2e− WOR) to produce H2O2 has been widely concerned. However, four-electron competition reaction causes the large overpotential and low production rates of 2e− WOR, which restrict its’ development and application. Herein, we prepared N-TiO2 derived from NH2-MIL-125 by the hydrothermal combined with calcination method. We found that N-TiO2 as photoanode exhibited the well properties of photoelectrocatalysis water oxidation, which gave an overpotential of 630 mV at 1 mA cm−2. The overpotentials of N-TiO2 was approximately 130 mV (at 1 mA cm−2) lower than NH2-MIL-125, and even lower than the previous reported TiO2, which may attribute to the increased oxygen vacancy with the calcination process and N doping. In addition, we investigated the degradation performance of the prepared catalysts to degrade methylene blue by photoelctrocatalysis on-site the preparation of H2O2. It was shown that N-TiO2 performed high degradation efficiency (∼91%) and excellent stability. The possible mechanism was speculated due to the increased oxygen vacancy and N doping. This work provides a new idea for photoelectrocatalysis water oxidation materials and points out a new way for on-site H2O2 production for direct use.

ZnO/Co-doped polyhistidine as an efficient bioelctrocatalyst for hydroxyl radical generation: Fabrication, characterization and application for microbial inactivation

The hydroxyl radical (•OH) is a well-known powerful oxidizing agent that can react unselectively with the environmental contaminants. Electrochemical radical generation has been regarded as a promising choice to enable H2O2 formation and its converting to •OH. Here, by modifying of zinc oxide with cobalt doped polyhistidine (Co:pHis), we succeeded in fabricating a new, efficient, and stable bioelectrocatalyst for converting the two-electron water oxidation reaction (2e-WOR) product to •OH. The structural, morphological and electrochemical properties of fabricated bioelectrocatalyst were investigated by different characterization techniques. The functional groups of pHis increased the rate of 2e-WOR due to the formation of hydrogen bonding with H2O2. The presented bioelectrocatalyst gives an overpotential of 2.6 V (vs. RHE) at 0.2 mA/cm2 and peak Faradaic efficiency of 57% toward •OH generation. The electrogenerated •OH by the proposed new bioelectrocatalyst was successfully applied for the inactivation of E. coli (>95%) as a typical microbial contaminant.