Synergistic cocatalytic effect of MoO3 and creatinine on Cu–Fenton reactions for efficient decomposition of H2O2

The key scientific problems with conventional Fenton reactions are the acidic pH dependence and low ROS production due to inefficient decomposition of H₂O₂. Although Cu–Fenton reactions can break the pH limitation, there is still an urgent need to improve the overall reaction efficiency, and thus broaden its applicability. Herein, we describe a synergistic strategy by introducing MoO₃ cocatalyst and creatinine (Cr) assistant to enhance the efficiency of Cu–Fenton reactions at near-neutral pH. In this strategy, Cu²⁺ interacts with Cr to form a complex (CuCr₂), which is then mainly linked to MoO₃ via the Cu²⁺ binding site (CuCr₂/MoO₃). Experimental and theoretical calculation results manifest that the CuCr₂/MoO₃ exhibits an excellent cocatalytic activity, which significantly facilitates the rate-limiting step of Cu–Fenton reactions, and enables the efficient decomposition of H₂O₂ for the generation of three reactive oxygen species (ROS, ?OH, ¹O₂, ?O₂^?). More significantly, this cocatalytic system with high oxidation activity can be applied for the detection of Cu²⁺ and ROS-based chemodynamic therapy (CDT), as well as sterilization of Escherichia coli. This study represents a new breakthrough in improving the efficiency of Fenton-based reactions with a facile and promising strategy, and drives great progress in practicality.     

Ferrous ion oxidations by √H, √OH and H2O2 in aerated FBX dosimetry system

In the ferrous ion, benzoic acid and xylenol orange (FBX) dosimetric system, benzoic acid (BA) increases the G(Fe³⁺) value. Xylenol orange (XO) controls the BA sensitized chain reaction as well as forms a complex with Fe³⁺. In the aerated FBX system each √H, √OH and H₂O₂ oxidizes 8.5, 6.6 and 7.6 Fe²⁺ ions, respectively; and these values respectively increase to 11.3, 7.6 and 8.6 in oxygenated solution. About 8% √OH reacts with XO and the remaining with BA. The above fractional values are due to this competition. This √OH reaction with XO oxidizes 1.8% and 2.1% ferrous ions only in aerated and oxygenated solutions, respectively. There is a competition between √H reactions with O₂ and with BA, but both lead to the production of H₂O₂. The oxidation of Fe²⁺ by √OH reactions at different concentrations of H₂O₂ is linear with absorbed dose while the √H reactions make the oxidation of Fe²⁺ non-linear with dose. This is due to competition reaction of H-adduct of BA between O₂ and Fe³⁺.     

Fast and Long-Lasting Iron(III) Reduction by Boron Toward Green and Accelerated Fenton Chemistry

Generation of hydroxyl radicals in the Fenton system (Fe^II/H₂O₂) is seriously limited by the sluggish kinetics of Fe^III reduction and fast Fe^III precipitation. Here, boron crystals (C-Boron) remarkably accelerate the Fe^III/Fe^II circulation in Fenton-like systems (C-Boron/Fe^III/H₂O₂) to produce a myriad of hydroxyl radicals with excellent efficiencies in oxidative degradation of various pollutants. The surface B−B bonds and interfacial suboxide boron in the surface B₁₂ icosahedra are the active sites to donate electrons to promote fast Fe^III reduction to Fe^II and further enhance hydroxyl radical production via Fenton chemistry. The C-Boron/Fe^III/H₂O₂ system outperforms the benchmark Fenton (Fe^II/H₂O₂) and Fe^III-based sulfate radical systems. The reactivity and stability of crystalline boron is much higher than the popular molecular reducing agents, nanocarbons, and other metal/metal-free nanomaterials.     

Fast and Long‐Lasting Iron(III) Reduction by Boron Toward Green and Accelerated Fenton Chemistry

Generation of hydroxyl radicals in the Fenton system (Fe^II/H₂O₂) is seriously limited by the sluggish kinetics of Fe^III reduction and fast Fe^III precipitation. Here, boron crystals (C‐Boron) remarkably accelerate the Fe^III/Fe^II circulation in Fenton‐like systems (C‐Boron/Fe^III/H₂O₂) to produce a myriad of hydroxyl radicals with excellent efficiencies in oxidative degradation of various pollutants. The surface B?B bonds and interfacial suboxide boron in the surface B₁₂ icosahedra are the active sites to donate electrons to promote fast Fe^III reduction to Fe^II and further enhance hydroxyl radical production via Fenton chemistry. The C‐Boron/Fe^III/H₂O₂ system outperforms the benchmark Fenton (Fe^II/H₂O₂) and Fe^III‐based sulfate radical systems. The reactivity and stability of crystalline boron is much higher than the popular molecular reducing agents, nanocarbons, and other metal/metal‐free nanomaterials.     

Simultaneous Fenton‐like Ion Delivery and Glutathione Depletion by MnO2‐Based Nanoagent to Enhance Chemodynamic Therapy

Chemodynamic therapy (CDT) utilizes iron‐initiated Fenton chemistry to destroy tumor cells by converting endogenous H₂O₂ into the highly toxic hydroxyl radical (^.OH). There is a paucity of Fenton‐like metal‐based CDT agents. Intracellular glutathione (GSH) with ^.OH scavenging ability greatly reduces CDT efficacy. A self‐reinforcing CDT nanoagent based on MnO₂ is reported that has both Fenton‐like Mn²⁺ delivery and GSH depletion properties. In the presence of HCO₃^?, which is abundant in the physiological medium, Mn²⁺ exerts Fenton‐like activity to generate ^.OH from H₂O₂. Upon uptake of MnO₂‐coated mesoporous silica nanoparticles (MS@MnO₂ NPs) by cancer cells, the MnO₂ shell undergoes a redox reaction with GSH to form glutathione disulfide and Mn²⁺, resulting in GSH depletion‐enhanced CDT. This, together with the GSH‐activated MRI contrast effect and dissociation of MnO₂, allows MS@MnO₂ NPs to achieve MRI‐monitored chemo–chemodynamic combination therapy.     

COD reduction of waste water streams of active pharmaceutical ingredient – Atenolol manufacturing unit by advanced oxidation-Fenton process

Active pharmaceutical intermediates (API) in waste waters have adverse effects on aquatic life and environment. The API have high COD value and low BOD₃ and hence difficult to treat biologically. In this study, advanced oxidation processes (AOPs) utilizing the H₂O₂/Fe⁺², Fenton reactions were investigated in lab-scale experiments for the degradation of Atenolol containing waste water streams. The experimental results showed that the Fenton process using H₂O₂/Fe⁺² was the most effective treatment process. With Fenton processes, COD reduction of wastewater can be achieved successfully. It is suggested that Fenton processes are viable techniques for the degradation of Atenolol from the waste water stream with relatively low toxic by-products in the effluent which can be easily biodegraded in the activated sludge process. Hence, the Fenton process with H₂O₂/Fe⁺² is considered a suitable pretreatment method to degrade the active pharmaceutical molecules and to improve the biodegradability of waste water.     

Fe3O4 Anisotropic Nanostructures in Hydrogels: Efficient Catalysts for the Rapid Removal of Organic Dyes from Wastewater

Fe₃O₄ anisotropic nanostructures that exhibit excellent catalytic performance are rarely used to catalyze Fenton‐like reactions because of the inevitable drawbacks resulting from traditional preparation methods. In this study, a facile, nontoxic, water‐based approach is developed for directly regulating a series of anisotropic morphologies of Fe₃O₄ nanostructures in a hydrogel matrix. In having the advantages of both the catalytic activity of Fe₃O₄ and the adsorptive capacity of an anionic polymer network, the hybrid nanocomposites have the capability to effect the rapid removal of cationic dyes, such as methylene blue, from water samples. Perhaps more interestingly, hybrid nanocomposites loaded with Fe₃O₄ nanorods exhibit the highest catalytic activity compared to those composed of nanoneedles and nanooctahedra, revealing the important role of nanostructure morphology. By means of scanning electrochemical microscopy, it is revealed that Fe₃O₄ nanorods can efficiently catalyze H₂O₂ decomposition and thus generate more free radicals (^.OH, ^.HO₂) for methylene blue degradation, which might account for their high catalytic activity.     

GSH and H2O2 Co‐Activatable Mitochondria‐Targeted Photodynamic Therapy under Normoxia and Hypoxia

Currently, photosensitizers (PSs) that are microenvironment responsive and hypoxia active are scarcely available and urgently desired for antitumor photodynamic therapy (PDT). Presented herein is the design of a redox stimuli activatable metal‐free photosensitizer (aPS), also functioning as a pre‐photosensitizer as it is converted to a PS by the mutual presence of glutathione (GSH) and hydrogen peroxide (H₂O₂) with high specificity on a basis of domino reactions on the benzothiadiazole ring. Superior to traditional PSs, the activated aPS contributed to efficient generation of reactive oxygen species including singlet oxygen and superoxide ion through both type 1 and type 2 pathways, alleviating the aerobic requirement for PDT. Equipped with a triphenylphosphine ligand for mitochondria targeting, ^mito aPS showed excellent phototoxicity to tumor cells with low light fluence under both normoxic and hypoxic conditions, after activation by intracellular GSH and H₂O₂. The ^mito aPS was also compatible to near infrared PDT with two photon excitation (800 nm) for extensive bioapplications.     

GSH and H2O2 Co-Activatable Mitochondria-Targeted Photodynamic Therapy under Normoxia and Hypoxia

Currently, photosensitizers (PSs) that are microenvironment responsive and hypoxia active are scarcely available and urgently desired for antitumor photodynamic therapy (PDT). Presented herein is the design of a redox stimuli activatable metal-free photosensitizer (aPS), also functioning as a pre-photosensitizer as it is converted to a PS by the mutual presence of glutathione (GSH) and hydrogen peroxide (H₂O₂) with high specificity on a basis of domino reactions on the benzothiadiazole ring. Superior to traditional PSs, the activated aPS contributed to efficient generation of reactive oxygen species including singlet oxygen and superoxide ion through both type 1 and type 2 pathways, alleviating the aerobic requirement for PDT. Equipped with a triphenylphosphine ligand for mitochondria targeting, ^mito aPS showed excellent phototoxicity to tumor cells with low light fluence under both normoxic and hypoxic conditions, after activation by intracellular GSH and H₂O₂. The ^mito aPS was also compatible to near infrared PDT with two photon excitation (800 nm) for extensive bioapplications.     

An Enzyme-Mimicking Single-Atom Catalyst as an Efficient Multiple Reactive Oxygen and Nitrogen Species Scavenger for Sepsis Management

Sepsis, characterized by immoderate production of multiple reactive oxygen and nitrogen species (RONS), causes high morbidity and mortality. Despite progress made with nanozymes, efficient antioxidant therapy to eliminate these RONS remains challenging, owing largely to the specificity and low activity of exploited nanozymes. Herein, an enzyme-mimicking single-atom catalyst, Co/PMCS, features atomically dispersed coordinatively unsaturated active Co-porphyrin centers, which can rapidly obliterate multiple RONS to alleviate sepsis. Co/PMCS can eliminate O₂^.− and H₂O₂ by mimicking superoxide dismutase, catalase, and glutathione peroxidase, while removing ^.OH via the oxidative-reduction cycle, with markedly higher activity than nanozymes. It can also scavenge ^.NO through formation of a nitrosyl–metal complex. Eventually, it can reduce proinflammatory cytokine levels, protect organs from damage, and confer a distinct survival advantage to the infected sepsis mice.     

Efficient Fe(III)/Fe(II) cycling triggered by MoO2 in Fenton reaction for the degradation of dye molecules and the reduction of Cr(VI)

There is a relatively low efficiency of Fe(III)/Fe(II) conversion cycle and H₂O₂ decomposition (<30%) in conventional Fenton process, which further results in a low production efficiency of OH and seriously restricts the application of Fenton. Herein, we report that the commercial MoO₂ can be used as the cocatalyst in Fenton process to dramatically accelerate the oxidation of Lissamine rhodamine B (L-RhB), where the efficiency of Fe(III)/Fe(II) cycling is greatly enhanced in the Fenton reaction meanwhile. And the L-RhB solution could be degraded nearly 100% in 1 min in the MoO₂ cocatalytic Fenton system under the optimal reaction condition, which is apparently better than that of the conventional Fenton system (～50%). Different from the conventional Fenton reaction where the OH plays an important role in the oxidation process, it shows that ¹O₂ contributes most in the MoO₂ cocatalytic Fenton reaction. However, it is found that the exposed Mo⁴⁺ active sites on the surface of MoO₂ powders can greatly promote the rate-limiting step of Fe³⁺/Fe²⁺ cycle conversion, thus minimizing the dosage of H₂O₂ (0.400 mmol/L) and Fe²⁺ (0.105 mmol/L). Interestingly, the MoO₂ cocatalytic Fenton system also exhibits a good ability for reducing Cr(VI) ions, where the reduction ability for Cr(VI) reaches almost 100% within 2 h. In short, this work shows a new discovery for MoO₂ cocatalytic advanced oxidation processes (AOPs), which devotes a lot to the practical water remediation application.     

Monitoring Singlet Oxygen and Hydroxyl Radical Formation with Fluorescent Probes During Photodynamic Therapy

Singlet oxygen (¹O₂) is the primary oxidant generated in photodynamic therapy (PDT) protocols involving sensitizers resulting in type II reactions. ¹O₂ can give rise to additional reactive oxygen species (ROS) such as the hydroxyl radical (^?OH). The current study was designed to assess 3′‐p‐(aminophenyl) fluorescein (APF) and 3′‐p‐(hydroxyphenyl) fluorescein (HPF) as probes for the detection of ¹O₂ and ^?OH under conditions relevant to PDT. Cell‐free studies indicated that both APF and HPF were converted to fluorescent products following exposure to ¹O₂ generated by irradiation of a water‐soluble photosensitizing agent (TPPS) and that APF was 35‐fold more sensitive than HPF. Using the ¹O₂ probe singlet oxygen sensor green (SOSG) we confirmed that 1 mm NaN₃ quenched ¹O₂‐induced APF/HPF fluorescence, while 1% DMSO had no effect. APF and HPF also yielded a fluorescent product upon interacting with ^?OH generated from H₂O₂ via the Fenton reaction in a cell‐free system. DMSO quenched the fluorogenic interaction between APF/HPF and ^?OH at doses as low as 0.02%. Although NaN₃ was expected to quench ^?OH‐induced APF/HPF fluorescence, co‐incubating NaN₃ with APF or HPF in the presence of ^?OH markedly enhanced fluorescence. Cultured L1210 cells that had been photosensitized with benzoporphyhrin derivative exhibited APF fluorescence immediately following irradiation. Approximately 50% of the cellular fluorescence could be suppressed by inclusion of either DMSO or the iron‐chelator desferroxamine. Combining the latter two agents did not enhance suppression. We conclude that APF can be used to monitor the formation of both ¹O₂ and ^?OH in cells subjected to PDT if studies are performed in the presence and absence of DMSO, respectively. That portion of the fluorescence quenched by DMSO will represent the contribution of ^?OH. This procedure could represent a useful means for evaluating formation of both ROS in the context of PDT.     

Highly Efficient FeIII-initiated Self-cycled Fenton System in Piezo-catalytic Process for Organic Pollutants Degradation

Piezo-catalytic self-Fenton (PSF) system has been emerging as a promising technique for wastewater treatment, while the competing O₂ reductive hydrogen peroxide (H₂O₂) production and Fe^III reduction seriously limited the reaction kinetics. Here, we develop a two-electron water oxidative H₂O₂ production (WOR−H₂O₂) coupled with Fe^III reduction over a Fe^III/BiOIO₃ piezo-catalyst for highly efficient PSF. It is found that the presence of Fe^III can simultaneously initiate the WOR−H₂O₂ and reduction of Fe^III to Fe^II, thereby enabling a rapid reaction kinetics towards subsequent Fenton reaction of H₂O₂/Fe^II. The Fe^III initiating PSF system offers exceptional self-recyclable degradation of pollutants with a degradation rate constant for sulfamethoxazole (SMZ) over 3.5 times as that of the classic Fe^II-PSF system. This study offers a new perspective for constructing efficient PSF systems and shatters the preconceived notion of Fe^III in the Fenton reaction.     

On‐surface Fenton and Fenton‐like reactions appraised by paper spray ionization mass spectrometry

On‐surface degradation of sildenafil (an adequate substrate as it contains assorted functional groups in its structure) promoted by the Fenton (Fe²⁺/H₂O₂) and Fenton‐like (Mⁿ⁺/H₂O₂; Mⁿ⁺ = Fe³⁺, Co²⁺, Cu²⁺, Mn²⁺) systems was investigated by using paper spray ionization mass spectrometry (PS‐MS). The performance of each system was compared by measuring the ratio between the relative intensities of the ions of m/z 475 (protonated sildenafil) and m/z 235 (protonated lidocaine, used as a convenient internal standard and added to the paper just before the PS‐MS analyzes). The results indicated the following order in the rates of such reactions: Fe²⁺/H₂O₂ ≫ H₂O₂ ≫ Cu²⁺/H₂O₂ > Mⁿ⁺/H₂O₂ (Mⁿ⁺ = Fe³⁺, Co²⁺, Mn²⁺) ~ Mⁿ⁺ (Mⁿ⁺ = Fe²⁺, Fe³⁺, Co²⁺, Cu²⁺, Mn²). The superior capability of Fe²⁺/H₂O₂ in causing the degradation of sildenafil indicates that Fe²⁺ efficiently decomposes H₂O₂ to yield hydroxyl radicals, quite reactive species that cause the substrate oxidation. The results also indicate that H₂O₂ can spontaneously decompose likely to yield hydroxyl radicals, although in a much smaller extension than the Fenton system. This effect, however, is strongly inhibited by the presence of the other cations, ie, Fe³⁺, Co²⁺, Cu²⁺, and Mn²⁺. A unique oxidation by‐product was detected in the reaction between Fe²⁺/H₂O₂ with sildenafil, and a possible structure for it was proposed based on the MS/MS data. The on‐surface reaction of other substrates (trimethoprim and tamoxifen) with the Fenton system was also investigated. In conclusion, PS‐MS shows to be a convenient platform to promptly monitor on‐surface oxidation reactions.     

Oxidation of fast green FCF by the solar photo-Fenton process

This study was conducted to assess the removal efficiency of fast green FCF (a dye) from aqueous medium using the photo-Fenton process. Fenton’s reagent, a mixture of hydrogen peroxide (H₂O₂) and ferric ions (Fe^3t+), used to generate hydroxyl radicals (·OH), was used to attack the target contaminant and degrade it. A visible light source was used to provide the radiation needed in the photo-Fenton method (i.e. H₂O₂/Fe³⁺). The effects of varying the parameters of ferric ion, fast green FCF and hydrogen peroxide concentrations, as well as pH, and light intensity on the reaction rate were determined. More effective and faster than Fenton’s reagent in removing fast green FCF, the results show that the photo-Fenton method completely oxidizes and degrades fast green FCF into CO₂ and H₂O. A tentative mechanism for photobleaching of the dye is proposed.     

Co-delivery of enzymes and photosensitizers via metal-phenolic network capsules for enhanced photodynamic therapy

The intrinsic hypoxic tumor microenvironment and limited accumulation of photosensitizers（PSs） result in unsatisfied efficiency of photodynamic therapy（PDT）.To enhance the PDT efficiency against solid tumors,a functional oxygen self-supplying and PS-delivering nanosystem is fabricated via the combination of catalase（CAT）,chlorin e6（Ce6） and metal-phenolic network（MPN） capsule.It is demonstrated that the CAT encapsulated in the capsules（named CCM capsules） could catalyze the degradation of hydrog...     

Synthesis of Iron Nanometallic Glasses and Their Application in Cancer Therapy by a Localized Fenton Reaction

Metallic glasses and cancer theranostics are emerging fields that do not seem to be related to each other. Herein, we report the facile synthesis of amorphous iron nanoparticles (AFeNPs) and their superior physicochemical properties compared to their crystalline counterpart, iron nanocrystals (FeNCs). The AFeNPs can be used for cancer theranostics by inducing a Fenton reaction in the tumor by taking advantage of the mild acidity and the overproduced H₂O₂ in a tumor microenvironment: Ionization of the AFeNPs enables on‐demand ferrous ion release in the tumor, and subsequent H₂O₂ disproportionation leads to efficient ^.OH generation. The endogenous stimuli‐responsive ^.OH generation in the presence AFeNPs enables a highly specific cancer therapy without the need for external energy input.     

A Supramolecular Photosensitizer System Based on Nano-Cu/ZIF-8 Capped with Water-Soluble Pillar[6]arene and Methylene Blue Host–Guest Complexations

Photodynamic therapy (PDT) as a safe, non-invasive modality for cancer therapy, in which the low oxygen and high glutathione in the tumor microenvironment reduces therapeutic efficiency. In order to overcome these problems, we prepared a supramolecular photosensitive system of O₂-Cu/ZIF-8@ZIF-8@WP6–MB (OCZWM), which was loaded with oxygen to increase the oxygen concentration in the tumor microenvironment, and the Cu²⁺ in the system reacted with glutathione (GSH) to reduce the GSH concentration to generate Cu⁺. It is worth noting that the generated Cu⁺ can produce the Fenton reaction, thus realizing the combination therapy of PDT and chemodynamic therapy (CDT) to achieve the purpose of significantly improving the anti-cancer efficiency.     

Enzyme-Triggered Chemodynamic Therapy via a Peptide-H2S Donor Conjugate with Complexed Fe2+

Inducing high levels of reactive oxygen species (ROS) inside tumor cells is a cancer therapy method termed chemodynamic therapy (CDT). Relying on delivery of Fenton reaction promoters such as Fe²⁺, CDT takes advantage of overproduced ROS in the tumor microenvironment. We developed a peptide-H₂S donor conjugate, complexed with Fe²⁺, termed AAN - PTC – Fe²⁺ . The AAN tripeptide was specifically cleaved by legumain, an enzyme overexpressed in glioma cells, to release carbonyl sulfide (COS). Hydrolysis of COS by carbonic anhydrase formed H₂S, an inhibitor of catalase, an enzyme that detoxifies H₂O₂. Fe²⁺ and H₂S together increased intracellular ROS levels and decreased viability in C6 glioma cells compared with controls lacking either Fe²⁺, the AAN sequence, or the ability to generate H₂S. AAN - PTC – Fe²⁺ performed better than temezolimide while exhibiting no cytotoxicity toward H9C2 cardiomyocytes. This study provides an H₂S-amplified, enzyme-responsive platform for synergistic cancer treatment.     

Electrocatalytic Reduction of Oxygen at Anthraquinonedisulfonate/Polypyrrole Composite Film Modified Electrodes and Its Application to the Electrochemical Oxidation of Azo Dye

We report here the electrocatalytic reduction of oxygen on thin anthraquindisulfonate (AQDS)/poplypyrrole (PPy) composite film modified electrodes and its application to the electrooxidation of azo dye‐amaranth. The polymer‐coated cathode exhibited good electrocatalytic activity towards oxygen reduction reaction (ORR), and allowed the formation of strong oxidant hydroxyl radical (^.OH) in the medium via Electro‐Fenton's reaction between cathodically generated H₂O₂ and added or regenerated Fe²⁺. The electrochemical behaviors of ORR in various pH solutions were described using cyclic voltammetry (CV), rotating disk electrode (RDE) and chronoamperometric (CA) techniques. The effect of solution pH on amaranth mineralization by the Fe²⁺/H₂O₂ and Fe³⁺/H₂O₂ electrooxidation systems was studied. In addition, the long‐term electrocatalytic activity and stability of the AQDS/PPy composite film during multiple experimental runs were also examined electrochemically.