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Superoxide Ion: Generation and Chemical Implications Maan Hayyan,*,†,‡ Mohd Ali Hashim,‡,§ and Inas M. AlNashef∥ †

Department of Civil Engineering, ‡University of Malaya Centre for Ionic Liquids (UMCiL), and §Department of Chemical Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia ∥ Department of Chemical and Environmental Engineering, Masdar Institute of Science and Technology, Abu Dhabi, United Arab Emirates ABSTRACT: Superoxide ion (O2•−) is of great significance as a radical species implicated in diverse chemical and biological systems. However, the chemistry knowledge of O2•− is rather scarce. In addition, numerous studies on O2•− were conducted within the latter half of the 20th century. Therefore, the current advancement in technology and instrumentation will certainly provide better insights into mechanisms and products of O2•− reactions and thus will result in new findings. This review emphasizes the state-ofthe-art research on O2•− so as to enable researchers to venture into future research. It comprises the main characteristics of O2•− followed by generation methods. The reaction types of O2•− are reviewed, and its potential applications including the destruction of hazardous chemicals, synthesis of organic compounds, and many other applications are highlighted. The O2•− environmental chemistry is also discussed. The detection methods of O2•− are categorized and elaborated. Special attention is given to the feasibility of using ionic liquids as media for O2•−, addressing the latest progress of generation and applications. The effect of electrodes on the O2•− electrochemical generation is reviewed. Finally, some remarks and future perspectives are concluded.

CONTENTS 1. Introduction 2. Main Characteristics of the Superoxide Ion 3. Generation of the Superoxide Ion 3.1. Electrochemical Reduction of O2 3.2. Chemical Generation of the Superoxide Ion 3.2.1. Synthesis o f S uperoxide Salts (Superoxide Carriers) 3.2.2. Dissolution of Superoxide Salts 3.3. Photochemical and Photocatalytic Generation of the Superoxide Ion 3.4. Generation of O2•− at Oxide Surfaces 3.5. O2•− Generation Media 3.5.1. Aprotic Media 3.5.2. Protic and Aqueous Media 3.5.3. Ionic Liquids 4. Reactions and Potential Applications of the Superoxide Ion 4.1. Reactions 4.1.1. Proton Abstraction 4.1.2. Disproportionation 4.1.3. Nucleophilic Substitution 4.1.4. One-Electron Transfer (Oxidation of the Superoxide Ion) 4.2. Potential Applications of the Superoxide Ion 4.2.1. Destruction of Hazardous Chemicals 4.2.2. Syntheses of Organic Compounds 4.2.3. Other Applications 5. Detection Methods of the Superoxide Ion 5.1. Electrochemical Methods 5.1.1. Redox Properties of O2•− © 2016 American Chemical Society

5.1.2. Direct Detection via O2 Reduction 5.1.3. Biosensor by Immobilizing Enzyme 5.2. Spectrophotometry 5.2.1. UV−Visible 5.2.2. Nitro Blue Tetrazolium Reduction 5.2.3. Cytochrome c Reduction 5.3. Luminescent (Emission) 5.3.1. Chemiluminescence 5.3.2. Electrochemiluminescence 5.3.3. Photoluminescence (Hydroethidine) 5.4. Vibrational Spectroscopy 5.4.1. Raman 5.4.2. Electron Spin Resonance and Spin Trapping 5.4.3. In Situ Fourier Transform Infrared Spectroscopy 6. Superoxide Ion in Ionic Liquids 6.1. Generation of O2•− in ILs 6.1.1. Generation of an Unstable O2•− 6.1.2. Generation of a Stable O2•− 6.1.3. Effects of the IL Structure on O2•− 6.2. Long-Term Stability of O2•− 6.2.1. Phosphonium-Based ILs 6.2.2. Imidazolium-Based ILs 6.3. Applications of O2•− Generated in ILs 6.3.1. Destruction of Hazardous Chemicals by Using O2•− in ILs 6.4. Diffusion Coefficients of the Superoxide Ion

3030 3030 3031 3031 3031 3031 3032 3033 3035 3036 3036 3037 3038 3038 3038 3038 3039 3039 3040 3040 3041 3045 3048 3048 3049 3049

3050 3050 3050 3050 3050 3051 3052 3052 3053 3053 3053 3053 3054 3055 3055 3056 3056 3057 3057 3059 3059 3060 3060 3062 3064

Received: July 14, 2015 Published: February 15, 2016 3029

DOI: 10.1021/acs.chemrev.5b00407 Chem. Rev. 2016, 116, 3029−3085

Chemical Reviews 7. Effect of Electrodes on O2•− Generation 8. Conclusion and Perspective Author Information Corresponding Author Notes Biographies Acknowledgments Nomenclature Abbreviations Greek Symbol References

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A few studies20−22 have reported that O2•− is relatively reactive. In contrast, a previous study23 showed that O2•− or hyperoxide, as some scientists24 name it, is not highly reactive despite being a free radical. The 17-electron O2•− is one of the isolable paramagnetic main-group ions and one of the many intermediate ORR species.25 The term “superoxide” prompted several scientists4,26,27 to presume that O2•− possesses exceptionally high reactivity, particularly as a powerful oxidizing agent and an initiator of radical reactions. Unpaired electrons make free radicals highly reactive, thus allowing them to oxidize various organic pollutants. The quantitative measurement of free radicals is difficult because of their high reactivity and short half-life. Consequently, only a few measurement techniques can be used, including chemical reactions, spin trapping, and direct detection. Table 1 lists the primary physical properties of O2•−.

3065 3066 3067 3067 3067 3067 3067 3067 3067 3068 3068

1. INTRODUCTION Oxygen (O2) molecules have a significant influence on numerous phenomena in the earth’s atmosphere and are active components in diverse chemical reactions and biological processes. The activation of O2 is a crucial factor in various contexts, ranging from biology to material dissolution. In addition, O2 reduction reactions (ORRs) have been recognized as some of the most critical electrocatalytic reactions for their role in industrial processes and electrochemical energy conversion. Therefore, ORRs have been a focal point of electrochemical research for many years. Reactive O2 species (ROS) are potential green oxidants, and previous studies1,2 presented many efficient techniques suitable for preparing ROS. The reduction of O2 in various aqueous and nonaqueous solvents is extremely complicated. Therefore, disproportionation and protonation of intermediate species are unavoidable. Ordinary O2 reacts poorly with most other molecules. However, it can be activated by adding naturally or artificially derived energy that transforms an O2 molecule into ROS. Four oxidation states of O2 are known: (O2)n, where n is 0 (dioxygen, O2); +1 (dioxygen cation, O2+); −1 (superoxide ion, O2•−); and −2 (peroxide dianion, O2•2−). Although maintaining O2•− in a stable state for a long duration is difficult, O2•− has attracted considerable attention. Therefore, O2•− chemistry has been studied throughout the past 50 years. A few studies1−7 have significantly reviewed O2•− chemistry, but these reports are now outdated. Numerous studies8−13 have reported on the organic chemistry of O2•−, and many of these studies14−18 are related to the interaction of O2•− with various organic substances. In addition, O2•− interferes in a wide number of critical reactions and applications, for example, the destruction of hazardous chemicals. Despite these, the O2•− chemistry is still not fully understood. Therefore, O2•− should be examined to broaden our understanding of its elementary reactivity pattern from a pure fundamental chemical view.19 From an engineering perspective, O2•− has seldom been studied because of the difficulty of generating and maintaining O2•− in a stable state. Furthermore, there are no reports on the latest progress in O2•− generation and applications. In this critical review, the generation methods, reactions, applications, and detection methods of O2•− are discussed. This review presents a compilation of extant literature and acts as a reference for the scientific community interested in O2•− chemistry.

Table 1. Physical Properties of O2•− property

28, 29 30, 31 32 33 34 35 36 37, 38 39

In contrast to the more stable peroxide species, O2•− is stable only up to 348 K, as confirmed40 by the disappearance of the Raman band at 1139 cm−1 beyond this temperature. The Raman band is also subjected to the type of reaction, O2 carriers, and catalysts. For instance, O2•− exhibits stretching vibrations at 1128 and 1160−1015 cm−1 when generated on CaO−Al2O3 and MgO−CoO, respectively.41 Although O2•− is considered as a superior nucleophile in aprotic solvents, there is no such reactivity exhibited in H2O, presumably due to the strong solvation and spontaneous disproportionation.4,42,43 The electron-transfer thermodynamic data of O2•− and other radicals were determined and discussed by Sawyer.1,44 Biologically, O2•− can be generated from two major sources: the mitochondrial respiratory chain and phagocytic nicotinamide adenine dinucleotide phosphate oxidase (NADPH oxidase). Thus, it is a byproduct of respiration and a crucial component of the immune defense system.45 The human body generates approximately 5 g of ROS/day46,47 primarily from the leakage of the electron transport chain during oxidative phosphorylation; O2•− and hydrogen peroxide (H2O2) are the two primary products of this leakage. However, the generation of any O2 species essential for life and protection against its toxic effects are dynamically balanced.48−51 These radical species are implicated in several harmful biological processes, such as protein denaturation and lipid peroxidation.52 Under normal circumstances, the biological system releases an enzyme, superoxide dismutase (SOD), which specifically maintains the O2•− concentration at an optimal level.53 The evidence of relating O2•− to human diseases, such as Parkinson’s disease and cancer, is sufficient.54,55 In addition, O2•− is involved in the etiology of aging. Consequently, the

2. MAIN CHARACTERISTICS OF THE SUPEROXIDE ION The monovalent reduction of O2 gives O2•−, and O2•− is considered both a radical and an anion with the radical sign (•) and a charge of −1 (eq 1). O2 + e− ⇄ O2•−

ref

paramagnetic, one unpaired electron O−O bond distance for potassium superoxide (KO2), 1.28 Å UV−vis peak O2•− (aqueous), 245 nm UV−vis peak O2•− (acetonitrile), 255 nm IR absorption spectra, KO2 (O−O stretch; bond order 1.5) 1145 cm−1 1140 cm−1 gas-phase basicity, F− > O2•− > Cl− > Br− O2•− vibrational frequency 1090 cm−1 1108 cm−1 (±20 cm−1)

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findings of studies investigating O2•− reactivity are useful for elucidating the metabolic processes of specific compounds in the biological system. Chen et al.56 and Halliwell57 defined O2•− as an oxygen-derived species that is potentially cytotoxic and causes damage to DNA (Scheme 1).58,59 Furthermore,

(4) peroxy radical decomposition (5) H2O2 decomposition (6) O2•− formation during oxidation of organic compounds (7) O2•− generation at oxide surfaces (8) singlet oxygen one-electron reduction Several studies have reviewed these methods.1,3,85 However, in this review, the focus is on the major generation methods, i.e. electrochemical, chemical, photochemical, and photocatalytic, and generation at oxide surfaces. The methods that have potential industrial applications are preferred, particularly because there are no significant updates on these.

Scheme 1. ROS and Their Possible Cellular Effects (Reprinted from Ref 59. Copyright 2014 American Chemical Society.)

3.1. Electrochemical Reduction of O2

many pathological effects have been attributed to O2•−; therefore, several researchers4,20,60−62 have recommended the use of SOD to counter O2•− in the treatment of certain diseases, for example, degenerative diseases that cause urological disorders, arthritis, and cancer. Moreover, the number of reports showing that O2•− is implicit in the pathology of numerous human diseases are increasing. The excessive production of O2•− can contribute to fibrous alveoli, bronchopneumonia dysplasia, and adult respiratory distress syndrome. In spite of this, O2•− is a crucial biological messenger and an essential antibacterial agent.63−65 According to Huang et al.,66 the growth and spread of cancer cells can be inhibited by O2•−. Human leukemia cells can be selectively destroyed by certain estrogen derivatives that do not harm normal lymphocytes. These derivatives must be modified for SOD inhibition and apoptosis induction. In addition, SOD inhibition results in the accumulation of cellular O2•− and subsequent free-radical-mediated damage to mitochondrial membranes, leading to the release of cytochrome c (Cytc), a small heme protein, from the mitochondria and cancer cell apoptosis. Thus, focusing on SOD is a promising approach to selectively kill cancer cells. Moreover, the mechanism-based combination of SOD inhibitors and freeradical-producing agents may have clinical applications.66

Among all available methods, the electrochemical generation of O2•− by O2 reduction is potentially the most convenient method because no byproducts are formed. The procedure is relatively simple, is time-efficient, and has been used to study the kinetics of O2•− reactions with other substances.1,20,92−95 Haber and Weiss96 proposed that oxidation of ferrous ions by O2 and decomposition of H2O2 leads to O2•− generation. Electrochemical methods have been used for O2•− generation,97−100 which subsequently initiated research on both O2 reduction and energy conversion.101−104 It has been shown that ORR is improved significantly through employing an electrolyte with low viscosity, high O2 solubility, and weak adsorption species.105 Because O2•− has a short half-life, O2•− was generated in aprotic media by using the electrochemical reduction of O2. Sawyer and Roberts100 investigated O2•− reactivity following the discovery that the electrochemical reduction of O2 in dimethyl sulfoxide (DMSO) yields O2•− in a stable state. Thus, a well-defined O2•− source is provided by the electrochemical method. The electrochemical reduction of O2 in aprotic solvents typically occurs at E = ±(−1.0) V versus the standard calomel electrode (SCE; eq 1) in the absence of protonic species or H2O,1,27,94,106 whereas O2•− disproportionates in H2O, forming O2 and hydroperoxide anion (HO2−) (eq 2).94,107,108

3. GENERATION OF THE SUPEROXIDE ION Simple but reliable methods must be used to prepare welldefined radical species for investigating the chemical and biochemical reactivities of O2•−. Ideal sources of O2•− should have no associated reactive substances, and a spectrophotometric observation of the radical itself should be achievable. Previous studies67−71 have examined O2•− as an intermediate. However, reproducing experiments conducted in studies that focus specifically on the generation of O2•− and its subsequent reaction(s) is difficult. Technically, O2•− can be generated from O2 by using various methods such as sonolysis72−76 and pulse radiolysis.77−80 Pulse radiolysis provides a well-defined solution and high O2•− concentrations in the presence of specific radical-scavenging substances. It is widely used in biological studies,81−84 particularly for studying the reaction mechanisms of O2•− with enzymes, namely, SODs and superoxide reductases.85−88 However, specialized equipment is required.89−91 Moreover, because O2•− is a short-lived species in aqueous solutions, it decomposes immediately following pulse radiolysis. Therefore, this method is unsuitable for studying the long-term reactivity of O2•−. Many other methods are available for O2•− generation, such as (1) biological methods (2) chemical and electrochemical methods (3) photochemical and photocatalytic methods

2O2•− + H 2O → O2 + HO2− + OH−

(2)

97

In 1965, Maricle and Hodgson performed the electrolytic reduction of O2 in aprotic solvents and proposed a method for O2•− generation in which O2•− was identified using electron spin resonance (ESR) spectroscopy. A previous study reported that O2•− is stable when a proton source is absent, and reversible voltammetry for O2 reduction was observed in several aprotic media.109 By contrast, O2•− is highly reactive and spontaneously disproportionates in protic solvents. In addition, some supporting electrolytes have been used with aprotic solvents, such as tetra-n-butylammonium perchlorate, tetramethylammonium perchlorate, tetraethylammonium perchlorate (TEAP), tetra-nbutylammonium iodide,35,110−112 and tetraethylammonium tetrafluoroborate.113 3.2. Chemical Generation of the Superoxide Ion

The chemical generation of O2•− consists of two main steps. The initial step is the synthesis of superoxide salts, followed by the solvation of these salts in appropriate media to release O2•−. 3.2.1. Synthesis of Superoxide Salts (Superoxide Carriers). The superoxide ion can be generated chemically from superoxide salts of alkali metals, such as potassium and sodium, and alkaline earth metals, such as strontium and barium. Heavier alkali metals, such as potassium, cesium, and rubidium, react with O2 at atmospheric pressure to produce thermodynamically stable superoxide compounds.4,114,115 3031


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Therefore, they are appropriate accessible systems for studying O2•− in a solid state. However, O2•− interacts significantly with alkali metal superoxides, subsequently causing exchange effects that impact the magnetic properties. Concomitantly, miscellaneous phase transitions occurring because of the adjustments in the superoxide orientation in the crystal structure may complicate structural investigations. Moreover, the O 2 •− structure is susceptible to rotational disorder at sufficiently high temperatures. Although several in-depth studies1,3,25,115−117 have been conducted on the bonding chemistry of O2•−, the O−O distance in the structure of O2•− in the solid state was calculated only imprecisely.25 Potassium superoxide (KO2), rubidium superoxide, and cesium superoxide, which are stable salts of O2•−, are yellow or orange in color.4,116,118 Alkali superoxides are attractive magnetic materials on the basis of p-elements.119,120 They show exciting magnetic properties, such as the multiferroic effect and spin−orbital-coupled phenomena. All these phenomena are observed because of antibonding O2•− complex molecular orbitals that provide new functional possibilities for designing and controlling the properties. However, O2•− molecule rotations still require further investigation. Therefore, there is a need to further explore the magnetic properties and other characteristics of all superoxide salts. Tetrabutylammonium superoxide (TBAS) and trimethylphenylammonium superoxide (TMAS) can be synthesized through an ion-exchange reaction in liquid ammonia.25 The TBAS crystalline structure contains ammonia molecules hydrogen bonded to O2•−, and this may affect the O2•− bonding properties. By contrast, the TMAS crystalline structure contains no solvent molecule and thus it represents the best known approximation to virtually isolate solid-state O2•−. The O−O bond lengths in TMAS and TBAS are 1.332(2) and 1.312(2) Å, respectively. Calcium superoxide (Ca(O2)2) was synthesized by disproportionating calcium peroxide diperoxyhydrate.121,122 However, the highest reported yield ranged from 71 to 73%.122 The main reason for the low yield is the irremovable H2O at the reaction sites. However, these studies are now outdated. Next, KO2 was widely selected as a superoxide salt because its stoichiometry differs from those of the combustion products for most metals (e.g., sodium oxide, sodium peroxide, and sodium ozonide).4 In addition, compared with other superoxide salts, it is highly soluble in organic solvents and is frequently added to the aprotic solvents in the presence of crown ether to bind K.3,123 However, the use of this method had some limitations. The major drawback was the presence of various admixtures in commercial samples. The purity of commercial solid KO2 is approximately 96% (main impurities are potassium peroxide and potassium hydroxide (KOH)). Therefore, basic solutions are always obtained during KO2 dissolution. However, ultrapure KO2 (i.e., 99.9%) is available now. Sodium superoxide (NaO2) exhibits several properties that attracted attention.124−128 Recently, Solovyev et al.119 studied the magnetic properties of NaO2 that are controlled by the relative alignment of O2•− molecules in addition to the state of partially filled antibonding molecular πg orbitals. As shown in Scheme 2, the orbital disorder and degeneracy in the hightemperature pyrite phase increased the weak isotropic antiferromagnetic (AFM) interactions between the molecules. The degeneracy lifted by the transition to the low-temperature marcasite phase led to the orbital order and formation of quasione-dimensional AFM spin chains, which were consistent with

Scheme 2. Distribution of Unoccupied Antibonding Molecular Orbitals with the pz Symmetry (Orbital Ordering) in the NaO2 Marcasite Phasea (Reprinted with Permission from Ref 119. Copyright 2014 Royal Society of Chemistry.)

a

Positive and negative lobes of the pz orbitals are in different colors. Two sublattices of O2•− are denoted by indices “1” and “2.”

the experimental magnetic susceptibility data. Moreover, the type of the long-range magnetic order and magnetic transition temperature were evaluated in the marcasite phase. The magnetic order is determined by the behavior of weak isotropic, anisotropic, and Dzyaloshinskii−Moriya exchange interactions between the molecules. At last, a multiferroic phase was predicted, whereby the inversion symmetry was broken by the long-range magnetic order, thus leading to substantial ferroelectric polarization. Recently, an in situ method was proposed for preparing superoxides by integrating H2O2 with aqueous KOH or sodium hydroxide (NaOH);129 NaO2 was generated by mixing NaOH and H2O2 in aqueous media under ambient conditions (eq 3). NaO2 is generated by the rapid formation of alkali peroxide, followed by its decomposition in excess H2O2. However, this method did not provide alkali superoxides in a powder form. By contrast, it was stated that, in this in situ method, O2•− can be generated in solutions for diverse applications. In addition, compared with other technologies, this method requires moderate conditions with low-cost raw materials as there are no temperature gradients and H2O2 stabilizers are not required. 2NaOH + 3H 2O2 → 2NaO2 + 4H 2O

(3)

3.2.2. Dissolution of Superoxide Salts. The superoxide ion can be generated by dissolving superoxide salts in aprotic solvents. Several studies9,10,130 have reported that the solubilities of KO2 and NaO2 in DMSO are extremely low and they are almost insoluble in all other organic solvents. However, their solubilities can be increased by adding tetraalkylammonium salts, which are typically used as supporting electrolytes in electrochemical methods.130 In addition to increasing the stability of the electrochemically generated O2•−, several studies9,130,131 have demonstrated that quaternary ammonium salts can be used to increase the nucleophilicity of O2•−. To address the low solubility of alkali metal superoxides in these solvents, many researchers132−137 have stimulated the solubility by using crown ethers for the O2•− reaction with organic molecules. However, it was demonstrated that salts with higher solubility, such as tetramethylammonium superoxide, can be used.138,139 Of note, NaO2 is expected to be less reactive than KO2 because of its reduced solubility.140 Many research groups3,141,142 have investigated the reactivity, toxicity, and activity of O2•− in biological systems by using the chemical and electrochemical methods for generating O2•− in aprotic media. Table 2 shows a comparison of the electrochemical reduction of O2 and dissolution of superoxide salts. The difficulty in the reduction of O2 originates from the exceptionally strong OO 3032


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in this system are rapidly converted to CH3Ċ HOH radicals. The alcoholic solvent systems have some inherent advantages over other systems (e.g., KO2/DMSO, aqueous solutions), such as easy solvent handling, high purity, the avoiding of possible cation interference supporting electrolytes, in investigating water-insoluble compounds, and the feasibility of maintaining the solution at an extremely low temperature for certain reactions.149,151

Table 2. Comparison of the Electrochemical Reduction of O2 and Chemical Generation chemical generation simple process needs a superoxide salt to be added as a solid oxidant less energy is required superoxide salts can drastically reduce the weight of required dose, reactor volume, storage, and transport or shipment requirements144

electrochemical reduction of O2 moderately complicated process needs high-purity O2 more energy is required for the process O2 as gas requires more precautions and safety procedures to be followed

CH3CH 2OH + hv → CH3CHO• + H

(4)

̇ CH3CH 2OH + hv → CH3CHOH +H

(5)

̇ CH3CH 2OH + H → CH3CHOH + H2

(6)

143

bond (498 kJ/mol). Therefore, the activation of this bond is kinetically slow. Thus, generation of O2•− from its salts might be a better option for some industrial processes, such as the destruction of chlorinated hydrocarbons (CHCs). Nevertheless, electrochemical generation is more favorable for some processes, particularly those dealing with fine chemicals and biological systems that demand ultrahigh purity.

ionizing radiation or VUV photolysis

H 2O ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ O2•− oxygen, KOH, and HCOONa or ethanol

(9)

3.3. Photochemical and Photocatalytic Generation of the Superoxide Ion

Photochemically generated O2•− can play a primary role as a reactant in photochemistry and photobiology. This method has been widely used to investigate the effect of O2•− in various disciplines, particularly in aquatic ecosystems and biotherapy.145,146 O2•− exists in naturally sunlit surface waters. It can be photochemically initiated by electron transfer from an excited state substrate to oxygen.147,148 Alkaline aqueous and ethanolic superoxide solutions can be prepared by either vacuum-UV photolysis or high-energy ionizing radiations.149 Moreover, they have crucial features such as long-term stability and storage at low temperatures. They can be stored at −196 °C for a long period of time with a minor consumption of O2•−; only 10% of O2•− was consumed after 30 days (Table 3). The observed decay rates of O2•− decreased

McDowell et al.152 reported a convenient route to generate O2•− in aqueous solution. The n−π* triplet state of a ketone (benzophenone, acetophenone, or acetone) can be quenched through reaction with a primary or secondary alcohol (e.g., methanol, ethanol, and 2-propanol). Equations 10−13 demonstrate the chemical reactions used in air or O2-saturated solutions to generate O2•−, in which 2-propanol and acetone are used. The conditions are adjustable within broad limits to suit the subsequent application of the solution containing O2•−. However, the prolonged photolysis is expected to reduce the yield of O2•− because of the possible reaction of O2•− with alcohol radicals in oxygen-depleted solutions. hv

Table 3. Stability of O2•− in Ice at −196 °C (Reprinted with permission from Ref 149. Copyright 1983 Elsevier.) period

O2•− (μM)

0.5 h 1.0 h 4.0 h 20.0 h 24.0 h 3.0 days 7.0 days 30.0 days

48.8 49.9 47.8 46.2 46.8 48.3 47.8 43.6

3

(CH3)2 CO → (CH3)2 Ċ −Ȯ

(10)

3 (CH3)2 Ċ −Ȯ + (CH3)2 CHOH → 2•C(CH3)2 OH

(11)

O2 + •C(CH3)2 OH → •OOC(CH3)2 OH

(12)

•

OOC(CH3)2 OH + OH− → (CH3)2 CO + H 2O + O2•− (13)

Another photochemical method generated O2•− in aqueous solutions of p-cresol, acetophenone, and aromatic amines such as anthranilic acid, aniline, and N-acetylanthranilic acid, as well as amino acids such as tyrosine, tryptophan, and kynurenine.153 The superoxide ion or HO2• formed during the near-UV photooxidation of tryptophan into N′-formylkynurenine (eq 14) was confirmed by adding SOD, which increased the yield of H2O2 (eq 15).154

while using alkali metal hydroxides in the order rubidium hydroxide > lithium hydroxide > cesium hydroxide > NaOH > KOH, implying that a KOH-containing solution was the optimal medium. These solutions can be used to investigate the reactivity of O2•− and perhydroxyl radical (HO2•) with numerous compounds over the entire pH range. They can also be used for studies related to the reactivity of O2•−/HO2• with oxygen-sensitive compounds under virtually anaerobic conditions because they can exist without molecular oxygen. The photolyses of ethanolic solutions containing oxygen have been illustrated in eqs 4−9.149,150 Through rearrangement or reaction with excess ethanol, the CH3CH2O• radicals formed

SOD

2O2•− + 2H+ ⎯⎯⎯→ H 2O2 + O2

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The p-cresol was found to be the most efficient superoxide generator.153 The formation of O2•− and H2O2 in sensitizer solutions is influenced by the scavenging of hydrated electrons by using dissolved O2. However, there are alternative mechanisms for O2•− production that are crucial for other classes of sensitizers. For example, the charge-transfer complexes formed by a ground-state electron donor molecule and an excited acceptor may decay through partial or complete electron transfer in polar solvents (e.g., the charge-transfer quenching of triplet dyes by amines). Two distinct photochemical mechanisms can be responsible for univalent oxygen reduction. A photochemically excited sensitizer may be reduced to a radical anion, with O2•− formed by the autoxidation of the sensitizer radical or with the photochemically generated singlet oxygen (1O2) quenched through complete electron transfer, yielding O2•− (eq 16). However, further exploration on these proposed mechanisms is required.

Figure 1. Formation of ROS from EfOM. (Reprinted from ref 147. Copyright 2014 American Chemical Society.)

O2 to generate O2•−. Figure 2 shows a positive and linear correlation between the phenolic concentrations and the steady-state

The direct electron transfer from substrates to 1O2 to yield a radical cation and O2•− by either quenching or reaction has been introduced. These substrates include enamines, azines, amines, phenols, sulfides, and azide anions.155−161 Saito et al.162,163 reported the observation of the generation of O2•− from the reaction of 1O2 with aromatic amines by using SOD, whereas Peters and Rodgers164 observed the O2•− formation in the reaction of NADH with 1O2 by using a laser flash technique to monitor 1,4-benzoquinone (PBQ) reduction, eq 17. The O2•− was produced from the reaction between 1O2 and N,N-dimethylp-anisidine in aqueous media. O2 + NADH → O2•− + NAD• + H+

1

Figure 2. Steady-state concentrations of O2•− in seven solutions of organic matter: Suwannee River humic acid (SRHA); Suwannee River fulvic acid (SRFA); Pony Lake fulvic acid (PLFA); hydrophobic (HPO), transphilic (TPI), and hydrophilic (HPI) fractions; and effluent as a function of the phenolic moieties content. (Reprinted from ref 147. Copyright 2014 American Chemical Society.)

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•−

It was also reported that O2 was produced as an intermediate from the 1O2 oxidation of some alkaloid tertiary amines by single electron transfer (eq 18).165 The 1O2, and

O2•− concentrations (R2 = 0.94). This result indicates that phenolic moieties may play a crucial role in generating O2•− in surface water via a photochemical pathway.147 In photocatalysis, electrons react with molecular oxygen via the reductive pathway to generate O2•−.170−173 Goto et al.174 quantitatively studied the reduction products obtained from molecular oxygen in titanium dioxide (TiO2) photocatalyzed reactions by using aqueous solutions containing 2-propanol. O2•− was the primary product when rutile particles were employed. By contrast, H2O2 was the main product when anatase particles were used. This disparity is crucial because it is associated with the photocatalytic activity of anatase powders being higher than that of rutile powders in numerous photocatalytic reactions when molecular oxygen is used as the electron acceptor. In the presence of ambient air, hydroxyl radical (OH•) was produced by the photogenerated hole reacting with H2O via the oxidative pathway. Hirakawa et al.175 examined the relationship between the properties of the TiO2 powder and the photocatalytic products of O2•− and H2O2 by applying the luminol chemiluminescent probe method. The photocatalytic activity of the photocatalyst can be improved using different methods such as cocatalyst loading, foreign element and self-element doping, and heterojunction structure formation. However, the introduction of foreign

subsequently O2•−, appeared to be powerful, selective, and efficient oxidizing reagents for tertiary amines and particularly for fragile alkaloids. The studies on 1O2 and O2•− have received substantial attention because of their role in photosensitized oxidation reactions.166−169 In biological systems containing polar aqueous media, the single-electron-transfer processes are expected to have a crucial role in the interaction of electron-rich substrates with 1O2.163 The interaction of natural organic matter and photoactive metals with sunlight leads to the generation of a number of reactive species including carbon centered radicals, ROS (e.g., 1 O2, O2•−, and H2O2), and various organoperoxides (Figure 1). Recently, the formation of ROS from effluent organic matter (EfOM) has been examined under simulated solar irradiation. The phenolic moieties in EfOM acted as electron-donating species after photochemical excitation, transferring electrons to 3034


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Figure 3. Photocatalytic degradation (a) and pseudo-first-order kinetic constants (b) of NaPCP over Bi2+xWO6 under visible light irradiation. (Reprinted from ref 176. Copyright 2014 American Chemical Society.)

surface intermolecular electron transfer, and the decomposition of H2O2. Numerous types of catalysts such as nanoparticles,181−183 metal complexes,184,185 metal oxides,186,187 enzymes,188−191 quinones,192−194 and viologens195 have been studied to increase the reduction efficiency of O2. The transition metals and metal ion complexes that induce molecular oxygen activation also have been used extensively to produce O2•−, particularly for the degradation of hazardous chemicals such as herbicides and dyes.196−200 Zerovalent iron (ZVI) and nanoscale ZVI (nZVI) can activate molecular oxygen in air to produce ROS, including O2•−, H2O2, and OH•, which are capable of oxidizing contaminants.197,201−205 nZVI is used principally as a reductant in anoxic media. However, when nZVI is exposed to an aerobic environment, the core−shell characteristic of nZVI imparts oxidative chemistry to generate ROS.206 nZVI produces more ROS compared to bulk iron, making them useful for remediation applications.206−209 The oxidative pathway involving nZVI starts with a two-electron oxidation of dissolved O2 by Fe0 to form H2O2, as shown in eq 19. This H2O2 may form H2O by accepting two more electrons from Fe0 (eq 20), react with Fe2+ under acidic conditions (eq 21) to produce a potent oxidant, OH•, or react with Fe4+ under neutral to alkaline pH conditions (eq 22). Under neutral pH conditions, Fe2+ may react with O2 directly to produce O2•− (eq 23).206,210

impurities may cause undesirable thermal instability and difficulty in tuning the oxidative and reductive species during photocatalysis. Consequently, self-doping can attract attention to enhance the activity of photocatalysts because of its ability to tune the electronic structures without introducing foreign elements.176−178 O2•− can also be photogenerated by Bi2+xWO6 (x = 0, 0.05, 0.1, 0.15, and 0.2) under visible light irradiation with selfdoping.176 The four self-doped x showed significantly higher photocatalytic activities than the pristine Bi2WO6 (Figure 3). The enhancement of the photocatalytic ability was increased with a bismuth self-doping amount and then was gradually reduced, indicating that x = 0.1 was the optimal bismuth doping amount. Further increase of bismuth doping (e.g., x = 0.15) is expected to cause increased defects, which subsequently would act as carrier recombination centers to affect the photocatalytic activity. By contrast, lower bismuth doping (e.g., x = 0.05) may not induce the internal electric-field enhancement. The degradation of the sodium pentachlorophenate (NaPCP) constant over the optimal self-doped catalyst Bi2.1WO6 was approximately 12 times higher compared with Bi2WO6 (Figure 3b). The characterization results and density functional theory (DFT) calculations confirmed that bismuth self-doping did not alter the redox power of photogenerated carriers but acted as a promoter for the separation and transfer of photogenerated electron−hole pairs of Bi2WO6, leading to increased O2•−. This was verified by using photocurrent generation and ESR spectra as well as O2•− detection methods. Bi2WO6 is a potential semiconductor photocatalyst because of its photostability, chemical inertness, and environmentally friendly feature. 3.4. Generation of O2

•−

O2 + Fe 0 + 2H+ → Fe 2 + + H 2O2

(19)

H 2O2 + Fe0 + 2H+ → Fe 2 + + H 2O

(20)

H 2O2 + Fe2 + → Fe3 + + OH• + OH−

at Oxide Surfaces

(acidic pH) (21)

Reactive oxygen species adsorbed at oxide surfaces are essential intermediates for selective catalytic oxidation and in various phenomena occurring at gas−solid and liquid−solid interfaces.179 Superoxide ion, being paramagnetic, is one of these ROS that has been studied widely and can be used to probe the electric field at the surface of ionic solids, particularly oxides. The features of the O2•− formed during a catalytic process provide insight into the nature of the active sites.180 In other cases, O2•− has been generated at the surface of ionic solids or in the framework of zeolites to obtain information on the cationic fields present in the system. A stable O2•− can be generated on oxide surfaces by using methods179 such as photoinduced electron transfer, direct surface−oxygen electron transfer,

H 2O2 + Fe2 + → Fe 4 +O2 + + H 2O

(pH > 5) (22)

Fe2 + + O2 → Fe3 + + O2•−

(pH ∼7)

(23)

Adding ligands (e.g., oxalate, nitrilotriacetic acid, and ethylenediaminetetraacetic acid (EDTA)) to nZVI may lead to substantially higher ROS generation compared with normal nZVI. This could enhance oxidant yield and rates during the reaction of nZVI with O2.211−214 However, these ligands may have undesirable features. For example, EDTA may cause unfavorable environmental consequences because of its poor biodegradability and superior heavy-metal chelating ability.199,215 3035


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hydrogen evolution on the cathode surface (eq 29) and therefore affect the H2O2 electrogeneration. A low pH may also cause a fast corrosion and reduction of surface active sites for the foam nickel particle electrode and cause less O2•− generation through the activation of molecular oxygen with foam nickel. Alkaline media may reduce the OH• oxidation capacity, producing increased iron sludge that covers the surface of the foam nickel particle electrode and blocking the molecular oxygen activation with the foam nickel.

As a result, inorganic ligands may be a preferable alternative to enhance ROS production in the nZVI/O2 system because of high stability and low cost compared with organic ligands.196 Recently, tetrapolyphosphate (TPP) has been investigated as an inorganic ligand for Fe@Fe2O3 core−shell nanowires to promote molecular oxygen activation and increase ROS.196,204 Scheme 3 shows a possible mechanism of enhanced molecular Scheme 3. Illustration for Enhanced Molecular Oxygen Activation with Fe@Fe2O3 Nanowires in the Presence of TPP (Reprinted from Ref 196. Copyright 2014 American Chemical Society.)

Ni + 2O2 → Ni 2 + + 2O2•− O2 + 2H+ + 2e− → H 2O2

2H+ + 2e → H 2

(27)

E 0 = 0.67 V

E0 = 0 V

(28) (29)

3.5. O2•− Generation Media

As mentioned above, the main types of media are aprotic and protic solvents, and the selection of media is primarily dependent on the target reaction and mechanism. However, the purity of these media plays a primary role in generating a stable O2•−. 3.5.1. Aprotic Media. Compared with protic and aqueous solutions, the O2•− lifetime is considerably greater in aprotic media. Therefore, O2•− can be generated and used in aprotic media for studying its reactions with various organic and inorganic substrates.3 Table 4 lists the aprotic solvents used for O2•− generation.

oxygen activation with Fe@Fe2O3 nanowires in the presence of TPP. Environmentally benign polyaminocarboxylic ligand diethylenetriamine pentacetate has also been reported to promote molecular oxygen activation with Fe@Fe2O3 core− shell nanowires.199 A novel electro-Fenton (EF) system containing sodium tetrapolyphosphate (Na6TPP) as electrolyte was introduced.200 The use of Na6TPP as the electrolyte improved the system considerably. This can be attributed to the formation of a ferrous− tetrapolyphosphate (Fe(II)−TPP) complex from the chemical and electrochemical corrosion of an iron electrode in the presence of Na6TPP. The Fe(II)−TPP complex can supply an additional molecular oxygen activation pathway to obtain increased H2O2 and OH• through a series of single-electron transfer processes, which include O2•−, to produce a Fe(III)− TPP complex (eqs 24−26)). Fe(II)−TPP + O2 → Fe(III)−TPP + O2•−

Table 4. Conventional Aprotic Solvents Used as Media for O2•− Generation aprotic solvent

abbrev

ref

dimethylformamide dimethyl sulfoxide acetonitrile propylene carbonate pyridine dichloromethane dimethoxyethane acetone diethyl ether

DMF DMSO AcN − − − DME − −

98, 101, 216−218 98, 101, 110 98, 101, 112, 216, 219, 220 221 101, 222 10, 101 133 101 133

(24)

For biochemical studies,223 suitable media for O2•− generation are AcN and DMSO, even though a number of previous studies101,131 have reported that O2•− is unstable in these solvents. By contrast, O2•− was reported10,133 to be reactive in aprotic solvents, such as DMF. An electrolyzed solution of O2− DMF probably contains only trace amounts of O2•− and substantial quantities of other unknown substances, at least one of which is a free radical. Fee and Hildenbrand223 investigated O2•− generation in DMF; DMF containing O2•− was colorless, although a longer period of electrolysis yielded an intense green solution (this was because of a higher degree of degradation of DMF). Therefore, the use of such solutions as an O2•− source in enzymatic studies was believed to cause difficulties in interpreting the results.223−225 However, for using DMSO as an O2•− medium for alkyl halide destruction, several studies10,11,133,134,226 have suspected that R−O2− reacts with DMSO to yield alcohol and dimethyl sulfone (eqs 30−32).

Fe(II)−TPP + O2•− + 2H 2O → Fe(III)−TPP + H 2O2 + 2OH−

(25) •

Fe(II)−TPP + H 2O2 → Fe(III)−TPP + OH + OH

−

(26)

The three-dimensional EF system (3D-E-Fenton) with foam nickel as the particle electrode was developed for organic pollutant degradation.198 Molecular oxygen was adsorbed on the surface of the foam nickel particle electrode and activated to produce O2•− via a one-electron-transfer pathway, as shown in eq 27, for increasing H2O2 and OH• radicals. Concurrently, the dissolved O2 was also adsorbed on the surface of an active carbon fiber cathode and subsequently reduced electrochemically to generate H2O2 via a two-electron-transfer pathway (eq 28). As a result, more H2O2 was generated in the 3D-E-Fenton system compared with the traditional electro-Fenton system. However, the nickel usage has environmental risks. Furthermore, the acidity and alkalinity of the solutions may affect the performance of the system. For example, a low working pH would result in a

O2•− + RX → RO2• + X− 3036

(RX: alkyl halide)

(30)


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RO2• + O2•− → RO2− + O2

(31)

ROO− + (CH3)2 SO → RO− + (CH3)2 SO2

(32)

reduction. The reversible one-electron process was achieved with approximately 1 M OH−. A quasi-reversible response was observed at other OH− concentrations. This is probably caused by O2•− disproportionation.48,86 The ORR performance is expected to decrease and O2•− protonation is suppressed when the alkaline concentration increases. Consequently, the ORR will be shifted from a two-electron-reduction pathway to a oneelectron-reduction pathway in NaOH and KOH solutions. Another possible reason is related to O2•− solvation; an increase in the alkaline concentration results in the formation of a more stable O2•−, which is hard to reoxidize, and subsequently leads to a quasi-reversible response.

By contrast, Goolsby and Sawyer227 reported that the reduction of O2•− on Au or Hg electrodes in DMSO containing TEAP produced the corresponding hydroxide ion (OH−), ethylene, triethylamine, and dimethyl sulfone products. However, compared with other solvents, O2•− is relatively stable in aprotic solvents because the disproportionation required to produce O2•2− is highly unfavorable.4 In addition, certain studies3 reported that the O2•− stability can be significantly improved if aprotic solvents are carefully purified and dried over phosphorus pentoxide and potassium carbonate (K2CO3). For example, the O2•− lifetime was 5−10 h when AcN was distilled once over K2CO3 and 25−28 h when AcN was distilled twice or thrice over K2CO3. Of note, the contact of superoxide solutions with even a trace amount of water will drastically reduce the lifetime of the superoxide. The presence of impurities can cause a decrease in the true O2•− concentration. In addition, it can change the reaction mode between O2•− and the compound of interest. However, compared with the situation of the 1960s and the 1970s, the risk of O2•− reaction with impurities has been drastically reduced because solvents with ultrahigh purity (e.g., 99.9%) are now available. The wider negative potential window is one of the main advantages of using nonaqueous media instead of aqueousbased systems. For instance, compared with H2O, DMSO has a wider cathodic electrochemical window, thereby avoiding the breakdown of the solvent in competing reactions and achieving a significantly higher solubility of O2 and CO2.228 Among aprotic solvents, DMSO is preferred because it deactivates H2O.229,230 In addition, DMSO is popular because of its medical applications.228,231 A stable O2•− can be generated in aprotic media containing tetraalkylammonium salts, and O2•− undergoes several reactions in which it functions as a one-electron donor, base, and nucleophile.35 Further reduction of O2•− can lead to O2•2− formation232 although its presence requires further studies in these nonaqueous solvents. 3.5.2. Protic and Aqueous Media. In aqueous solutions, O2•− has a very short lifetime.3 Therefore, pulse methods, such as pulse radiolysis or flash photolysis, are widely used to study the mechanisms and kinetics of O2•− reactions and applications. Next, KOH and NaOH are the most crucial aqueous alkaline electrolytes and have been regarded as protic solvents for the electroreduction of O2.233 Because the atomic sizes of Na+ and K+ ions are different, their physicochemical properties, such as O2 solubility and diffusion coefficient,234−236 viscosity of the solution,237 and electrode interfacial phenomena,238 are relatively different. Consequently, the electrochemical behavior of ORR in these electrolytes was anticipated to vary.105 A reversible one-electron reduction of O2 in 1 M NaOH produced a stable O2•− state.239 Cobalt(II) 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexadecafluoro-29H,31H-phthalocyanine (CoIIHFPC) adsorbed on a graphite electrode catatlyzed this O2 electroreduction. The possible mechanism for this reduction in a strong alkaline solution is proposed in eqs 33−35. The OH− concentration had a dominant effect on the reduction, and a high OH− concentration led to the formation of a stable O2•−. Cyclic voltammograms of the rotating disk electrode in 0.1, 0.5, 1, and 2 M NaOH solutions showed that the OH− concentration had a dominant effect on the

[HO−CoIIIHFPC] + e− ⇄ [HO−CoIIHFPC]−

(33)

[HO−CoIIHFPC]− + O2 → [CoIIHFPC−O2 ] + OH− (34)

[CoIIHFPC−O2 ] + e− + OH− → [HO−CoIIHFPC]− + O2•−

(35)

A previous study105 used cyclic voltammetry (CV) to analyze the ORR on a polycrystalline Pt surface. In addition, the influence of the reaction media was considered by comparing the ORRs in KOH and NaOH solutions with concentrations ranging from 0.5 to 14 M at 298 K. The ORR was found to be a quasi-reversible diffusion-controlled reaction, which is mainly dependent on the conditions of the electrolyte. The ORR was thermodynamically and kinetically favorable in KOH. In addition, O2•− was generated in alkaline media that contained an electroinactive surfactant similar to quinoline, and this surfactant formed a compact hydrophobic film.240 In these media, a Hg electrode was typically used as the working electrode.8,240−242 A few previous studies3,179,243−245 have shown that O2•− can be generated in H2O by photochemical and electrochemical processes. With the use of more economical methods to generate O2•− in H2O, the contact between O2•− and contaminants in the nonaqueous phase should take place.245 This will make the usage of O2•− more practical in the destruction of organic pollutants if the O2•− is a better choice compared with its subsequent generated species through disproportionation. Dissolved organic matter in naturally occurring water absorbs ultraviolet (UV) radiation and generates an excited triplet state. This state then reacts with O2 to form O2•−.246 O2•− associated with the hydrogen ion (H+) then forms a conjugated acid of the HO2•. Chen et al.247 showed that peroxymonocarbonate (HCO4−) was formed after the reaction between H2O2 and sodium bicarbonate. Subsequently, HCO4− decomposition generated O2•− and OH•. However, the O2•− lifetime in an aqueous solution with high pH was approximately 1 min. The O2•− reactivity was significantly less than OH• reactivity because the nucleophilicity of O2•− is greater than that of other ROS.248 In a strong alkaline solution, periodate reacted with the dissolved O2 to produce O2•−.249 Furthermore, O2•− is generated in H2O modified Fenton’s reagent, such as catalyzed H2O2 propagations (CHP) through the catalyzed decomposition of peroxygens.250 In general, the CHP are based on the standard reaction of Fenton where dilute H2O2 is decomposed by iron(II) to generate OH•.251,252 However, to enhance CHP, Fenton’s reagent was modified by using high concentrations of H2O2 and initiators (e.g., iron 3037


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the generated O2•− with protic impurities present in the ionic liquid. The presence of impurities had a significant effect on the O2•− stability in ILs.108 Previous studies279−281 have stated that the proton source existence leads to a rapid and spontaneous disproportionation of O2•−. The generation of a stable O2•− was reported for the first time in 2001, whereby the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate [BMIm][HFP] was used as a medium.108 Following this, numerous studies109,279,282−288 have used ILs by varying the structures of the cations and anions. The generation of O2•− is important especially due to the transient nature. Therefore, this subject will be further elaborated in section 6.

chelates, soluble iron(III), or minerals). These initiate propagation reactions and thus generate ROS, including OH•, HO2•, O2•−, and HO2− (eqs 36−39). Fe3 + + H 2O2 → Fe2 + + HO2• + H+ •

•

(36)

OH + H 2O2 → HO2 + H 2O

(37)

HO•2

(38)

⇌ O2

•−

+H

+

(pK a = 4.8)

HO2• + Fe 2 + → Fe3 + + HO2−

(39)

•−

The reactivity of O2 in H2O is significantly increased in the presence of solids, including birnessite,253 and in the presence of H2O2 concentrations characteristic of CHP reactions (0.3− 4.0 M)254 as will be explained in section 4.2.1.3. This indicates that the proper selection for a generation method of O2•− plays a considerable role in obtaining a better efficacy for the O2•− in the targeted applications. A new medium system95 was developed on the basis of alkaline solutions containing the long-chain surfactant dimethyldistearylammonium bromide and a Hg-free solid electrode system. This system was used for generating and detecting O2•−. O2•− can also be generated through the reaction of xanthine oxidase (XOD), which is obtained from cow’s milk, with hypoxanthine in the presence of O2 in a phosphate buffer solution at room temperature (eq 40).56 Vitamin K3, similar to other quinones,255 can be reduced electrochemically and was identified as an effective catalyst for O2 reduction to form O2•− in aerated vitamin K3 solutions.256 In addition, O2•− can be generated by the hydroxide-induced reductions of O2 via primary and secondary aromatic amines, hydrazine, and hydroxylamines.257

4. REACTIONS AND POTENTIAL APPLICATIONS OF THE SUPEROXIDE ION In this section, the main reactions of the superoxide ion in vitro and its potential applications reported in the literature are discussed. 4.1. Reactions

The superoxide ion is capable of undergoing various reactions, such as disproportionation, one-electron transfer, deprotonation, and nucleophilic substitution.1,118,289 As mentioned in section 2, the O2•− reactivity is determined on the basis of its chemical characteristics, such as nucleophilicity, free radical and redox properties, and basicity. In this section, these reactions and some of the corresponding proposed mechanisms are discussed in detail. 4.1.1. Proton Abstraction. The superoxide ion O2•− reacts with a proton (eq 41) or proton donor (eq 42) to form HO2•. Diverse organic and inorganic compounds can act as a proton source in a wide number of reactions.3

XOD

hypoxanthine + O2 ⎯⎯⎯⎯→ uric acid + O2•− + H 2O2

(40)

The hydration effects were considered, and the difference in the solvation enthalpies between aprotic and aqueous solutions was 2.5 kJ/mol.218,258,259 The first, second, and third gas-phase hydration enthalpies were 52.3, 40.6, and 29.3 kJ/mol, respectively. Therefore, it is likely that the hydration degree has a considerable effect on the properties of ions, specifically nucleophilicity and reduction−oxidation (redox) potential.258,260 Previous studies261,262 showed that O2•− is unstable in aqueous media; consequently, it disproportionates with fast second-order rate constants in the range 107−1010 M−1 s−1. 3.5.3. Ionic Liquids. Ionic liquids (ILs) are widely recognized as unique and successful solvents, media, and extractants, and as catalysts for various reactions.263−267 ILs have numerous favorable properties in comparison to volatile organic solvents. For example, they have negligible vapor pressure and their nonvolatility minimizes the environmental impact.263,268,269 In addition, the physicochemical properties of ILs can be modified by altering the cation or the anion.270−276 Owing to their exceptional properties, ILs can be used as potential media for generating radical ions. Marcinek et al.277 used ILs as media for radiolytic generation and characterized the radical ions by using UV−vis and near-IR spectroscopies. They found ILs to be ideal media for the simultaneous generation of radical cations and anions. Attempts to generate O2•− in ILs as media were started in 1991 by Carter et al.278 They used imidazolium chloride−aluminum chloride for O2 reduction to generate O2•−. However, the generated O2•− was unstable. This can be attributed to the irreversible reaction of

O2•− + H+ ⇄ HO2•

(41)

O2•− + HX ⇄ HO2• + X−

(42)

Because O2•− is crucial in electrochemical applications and biological systems, deprotonation by O2•− and its subsequent intermediate species has been thoroughly studied in different solutions containing proton sources.290,291 The formation of HO2• from O2•− is relevant in both chemical and biological systems. For example, in simple chemical systems, HO2• is produced from O2•− by a proton transfer reaction from phenol or a one-electron reduction of O2 in the presence of perchloric acid.281 The reaction of O2•− with H2O, methanol, ethanol, 2-propanol, and sugarsparticularly fructose, sucrose, and glucosein several aprotic solvents was investigated.20 The results showed that O2•− was not as reactive in the examined media as reported previously; O2•− reacted relatively rapidly with H2O (1.0 × 105 M−1 s−1) and at an extremely fast rate with methanol (of the order of 107 M−1 s−1). However, the pseudo-second-order rate constant of the reaction of O2•− with ethanol was of the order of 102 M−1 s−1, whereas that of the reaction of O2•− with 2-propanol was DMF > DMSO. This is in accordance with the results of a previous study304 that proposed O2•− protonation by H2O or ethanol in AcN and DMF. Rate constants have been reported for the reactions of O2•− with various inorganic and organic compounds and even certain free radicals.305,306 In these reactions, O2•− acted either as a reductant or as an oxidant.4 In addition, O2•− reduced or oxidized transition metal ions depending on their reduced and oxidized states, respectively.305−307 4.1.3. Nucleophilic Substitution. Nucleophilicity is one of the main exceptional properties that describe how super is O2•−. This was investigated and covered in diverse studies pertaining to O2•− reactivity.3,4,308−310 There are primarily three main factors for the nucleophilic reactivity which include solvation, basicity, polarizability, and α-effect.311 The α-effect, introduced by Edwards and Pearson,312 accounts for the enhanced reactivity of nucleophiles, which bear an unshared electronic pair at the atom adjacent to the nucleophilic site.313 In the case of O2•−, the α-effect was assumed to be the main responsible factor for this phenomenon. However, this assumption was prior to the latest explorations of nucleophilic chemistry. For instance, Hoz and Buncel314,315 defined the α-effect as the positive deviation from a Brønsted plot which was then adopted by the IUPAC Glossary of Terms used in

Scheme 4. Proton-Induced Activation of O2•− in DMSO (Reprinted from Ref 302. Copyright 1988 American Chemical Society.)

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reaction of sulfur dioxide (SO2) with O2•− in DMF (eq 48).9,320 However, this reaction is completed because SO2•− forms a complex with SO2 and dimerizes to form the dithionite ion. This indicates that, in aprotic solvents, O2•− is a more effective reducing agent than the dithionite ion.301

Physical Organic Chemistry in order to specify a reference nucleophile for nucleophilicity comparison purposes. Danen and Warner310 pointed out that the “super” nucleophilicity of O2•− can plausibly be ascribed to a considerable electron-transfer contribution in the transition state (eq 45). O2•− + AX → [O2•− ··· A ··· X ⇄ O2 ··· A ··· X−] •

→ AO2 + X

−

SO2 + O2•− ⇄ O2 + SO2•− (45)

Transition metal complexes can be effectively reduced by O2•−, for example, copper(II),19,294 manganese(III),4,321 and iron(III).322 In addition, O2•− reduces ferricenium ion, MnIV2O2(o-phen)44+, CoIII(o-phen)33+, and IrIVCl62− by oneelectron processes.323 In addition to the aforementioned reactions, there are some other crucial reactions of O2•−.102,125 For example, the complexation of O2•− with various chemicals was investigated extensively. Based on the DFT calculations, decamethylferrocene (DMFc) complexes with O2 in the organic phase have been proposed to generate O2•−, which was protonated by H2O. Once produced, HO2• was subsequently reduced by a second DMFc molecule to generate H2O2, which diffused across the interface into the aqueous phase.289,324 Nagano et al.325 reported the oxidation of O2•− using polyhalides, CO2, phosphates, and acyl halides. The corresponding peroxy intermediates were then used for the conversion of olefins and sulfides to the corresponding oxides and sulfones, respectively, in high yields. Boujlel et al.326 showed that the O2•− can be used for the electrochemical reduction of α-diketones. The authors indicated that the products depended on the potential difference. The chemical oxidation of O2•− by lucigenin has been used in chemiluminescence methods for the detection of O2•−, as will be discussed in section 5.327 This method is based on the measurement of the intensity of the fluorescence radiation emitted after the reaction with O2•−. By carefully designing the electrode/solution interface of a hanging mercury drop electrode with hydrophobic surfactants such as quinoline and isoquinoline, Tian et al.327 used direct electrochemical oxidation of O2•− to determine a variety of biomolecular reactivities of O2•− in aqueous media. Flamm et al.328 reported the development of a new electrochemical sensor system for reliable and continuous detection of O2•− release from cell culture utilizing direct oxidation of O2•− on polymer covered Au microelectrodes. Direct superoxide oxidation was demonstrated to provide a robust measurement principle for sensitive and selective ROS quantification without the need for bio component supported conversion. A few studies1,308,329 were reported for the gas-phase ion/ molecule reactions involving O2•−. This was possible after the development of Fourier transform mass spectrometers with trapped ion cells.330 In this direction, Johlman et al.329 investigated the gas-phase hydrolysis of phenyl benzoate and phenyl acetate by O2•−. However, there is no recent progress on the O2•− gas phase reactions implementing the latest technology.

O2•−

Nucleophilic substitution by was extensively used for the reaction with alkyl halides (eq 30).9,131 Sawyer1 indicated that the O2•− nucleophilicity toward primary alkyl halides results in an SN2 displacement of the halide ion from the carbon center. The reactivity followed the sequence benzyl > primary > secondary > tertiary, and the leaving group order was I > Br > OTs > Cl. This is in accordance with the expected inversion and stereoselectivity at the carbon center. The spin-trapping method was used to detect the peroxy radical (ROO•), which was produced in the primary step.316 This radical oxidant was then readily reduced to form the peroxy ion (ROO−). ROO− converts DMSO to its sulfone, yielding alcohol as the main product. 4.1.4. One-Electron Transfer (Oxidation of the Superoxide Ion). The superoxide ion acts as a moderate oneelectron reducing agent.301 The oxidation of O2 can be achieved via a variety of oxidants, ferrocinium ion, high valence Mn complexes, diacyl peroxides, and thianthrene cation radical.317 Using this feature, a wide variety of reactions with organic and inorganic compounds have been reported,3,5,94,318 as illustrated in Figure 4. There are two mechanisms proceeding

Figure 4. One-electron-transfer reactions.

these reactions which are outer-sphere one-electron transfer without forming intermediates (eq 46) and inner-sphere oneelectron transfer where the O2•− adducts form as intermediates (eq 47). O2•− + A ⇄ O2 + A•−

(46)

O2•− + A ⇄ A−O2•− → A•− + O2

(47)

(48)

4.2. Potential Applications of the Superoxide Ion

In section 4.1, we found that O2•− reacts with many substrates through different mechanisms. Various studies have been conducted to observe the reaction between O2•− and various substrates involved in different applications, such as metabolic intermediates (e.g., α-dicarbonyls and α-diketones),331 p-creosol, p-ethoxyphenol, p-tert-butylphenol,107 carbonyl compounds,19 antioxidation agents,22,52,332 tryptophan 2,3-dioxygenase,224 tosylates, 140 2-phenylenediamine, 333 phenylhydrazine, 334

For instance, in a previous study, in DMF, 3,5-di-tert-butylquinone (DTBQ) reacted with O2•−, yielding the semiquinone anion radical (DTBSQ−) as a main product, and the equilibrium constant K was 8.0 × 106.319 Another example is the 3040


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Table 5. Potential Applications of O2•− O2•− source

role study of biological redox chemistry of flavoproteins reduction of Cu(II) salicylate complexes to Cu(I) derivatives implication of O2•− in study of interaction between O2 species and porphyrins (e.g., heme proteins) source to generate HO2• and study its reactivity with 1,4-CHD O2•− as intermediate to study oxidative drug metabolism activation of O2 by radical coupling between O2•− and reduced methyl viologen redox reactions of tris(3,5-di-tert-butylcatecholato)manganate(IV) and bis(3,5-di-tertbutylcatecholato)manganate(III) complexes condensation of nitromethane and malononitrile with a variety of aromatic aldehydes base-catalyzed autoxidation of enones and α-keto enols reactions with carbohydrates

O2 reduction KO2 O2 reduction, tetramethylammonium superoxide O2 reduction, tetramethylammonium superoxide O2 reduction O2 reduction O2 reduction, tetramethylammonium superoxide tetraethylammonium superoxide KO2 KO2

medium/media

ref

DMF DMSO DMSO, DMF, AcN, pyridine

342 343 344−346

DMSO, DMF, AcN

302

AcN DMF AcN, DMSO, DMF

347 348 349

DMF

350

aprotic solvents DMSO:DMF (1:1)

351−353 354

Scheme 5. Nucleophilic Degradation of C6Cl6 by O2•− (Reprinted from Ref 371. Copyright 1987 American Chemical Society.)

nitroaromatic amines,335 fluorinated β-ketoamines and their metal chelates,336 ferrates V and VI,337−339 nitrobenzenes, nitrogen heterocycles, and chloroform.340,341 Table 5 lists some potential applications of O2•− that have been used in many studies. However, the reactions of O2•− are highly recommended to be further explored, and in some cases repeated. With the advent of numerous sophisticated analytical equipment, new findings are expected. In this section, the main potential applications of O2•− are described. 4.2.1. Destruction of Hazardous Chemicals. 4.2.1.1. Destruction of Halogenated Hydrocarbons. The concern for stocks, stores, and environmental reservoirs of obsolete chemicals, and persistent organic pollutant contaminated waste is increasing in the public sector. These substances

must be appropriately identified, collected, and destroyed to stem their continued migration into the environment. Governmental and nongovernmental organizations and the scientific community are taking necessary steps for this purpose. Persistent organic pollutants are highly stable organic compounds. They are used in various industrial applications, such as heat exchanger fluids, degreasers, transformer oils, dry cleaners, and pesticides. In addition, they are produced unintentionally as the byproducts of industrial processes, primarily through incineration and/or other human activities. In general, several methods are available for degrading CHCs, including incineration and oxidation by using various biological, chemical, photochemical, and electrochemical methods.355−364 However, these methods are disadvantageous 3041


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because230 they are associated with high energy consumption, production of toxic byproducts (for example, acidic gases, such as HCI, HF, and HBr), the release of free halogen gases (for example, Cl2, F2, and Br2), economic viability, and the high degree of technological sophistication required. The ideal destruction process comprises the following features:365 (1) the use of inexpensive reagents, (2) reaction conditions that are as mild as possible, (3) minimal use of catalysts and any spent reagent, and (4) environmentally innocuous byproducts. In 1970, the O2•− reactivity with alkyl halides through nucleophilic substitution was reported.9,131 The O2•− technologies that have been developed offer a safe and efficient alternative for the destruction of halogenated hydrocarbons.229,366−370 The generation of O2•− chemically or electrochemically is appealing because O2•− can be transported to or conducted on sites where hazardous chemical waste is stored or discharged. Trichlorobenzene, tetrachlorobenzene, hexachlorobenzene (C6Cl6), pentachlorobenzene, decachlorobiphenyl, and other heavy polychlorinated biphenyls (PCBs) are rapidly oxygenated by O2•− in DMF, AcN, or DMSO.371 Although these substrates were degraded by O2•− in both AcN and DMSO, the reaction rates were about 1/10 as great in AcN and 20 times slower in DMSO. This clearly indicates the impact of the medium on the reaction rate. A reasonable initial step for these oxygenations is the nucleophilic addition of O2•− to C6Cl6. Scheme 5, as proposed by Sawyer et al.,1,371 shows a possible mechanism. However, the fragmentation steps are speculative and are not supported by the detection of any intermediate species. A subsequent loss of chloride (Cl−) ions yields a benzoperoxy radical, which is then reduced by a second O2•− molecule to become a peroxo nucleophile. This nucleophile then attacks the adjacent carbochloro center, thus displacing Cl− ions and forming orthoquinone. The orthoquinone then undergoes facile reactions with O2•− to yield peroxy dicarbonate (C2O62−) and Cl− ions. The C2O62− ions are then hydrolyzed by H2O to form HCO3− and O2. Thus, C6Cl6 is completely degraded by O2•−. In addition to single PCBs, multicompounds, such as Arochlor 1268a commercially available PCB fraction that contains a mixture of Cl7, Cl8, Cl9, and Cl10 polychlorobiphenylshave been examined.1,3 When Arochlor 1268 was combined with excess O2•−, all of Arochlor 1268 was degraded. The most heavily chlorinated compounds in the mixture were the first to react, and all components in the mixture were completely dehalogenated. This showed that PCB mixtures that contain components with ≥3 chlorine atoms per phenyl ring could be degraded entirely, within several hours, by O2•−. The superoxide ion promotes dechlorination to favor the subsequent benzene ring cleavage and final mineralization of NaPCP during bismuth self-doped Bi2WO6 and Bi2.1WO6 photocatalysis by producing easily decomposable quinone intermediates. Figure 5 shows the total organic carbon (TOC) and Cl− determination of NaPCP. The TOC removal and dechlorination efficiency was higher using Bi2.1WO6 (i.e., 85 and 90%, respectively) than using Bi2WO6 (i.e., 45 and 57%, respectively) in 5 h. It was noteworthy to find that, in the presence of PBQ as a detector, the photodegradation of NaPCP was not affected over Bi2WO6 but its rate was drastically decreased from 2.15 to 1.04 h−1 (i.e., 51.6% of depression ratio) in the case of Bi2.1WO6, suggesting the involvement of O2•− in the process. Scheme 6 illustrates the significant change of the

Figure 5. (A) TOC removal during photocatalytic degradation of NaPCP with Bi2WO6 and Bi2.1WO6 under visible light irradiation. (B) Generation of Cl− during photocatalytic degradation of NaPCP with Bi2WO6 and Bi2.1WO6 as the photocatalysts under visible light irradiation. (Reprinted from ref 176. Copyright 2014 American Chemical Society.)

NaPCP degradation pathway as a result of generation of more O2•− by bismuth self-doping; O2•− promoted reductive dechlorination to produce more reductive dechlorination intermediates besides major oxidative dechlorination induced by the direct hole oxidation. With further reductive dechlorination by O2•−, the dechlorinated ring-opening products without strong electron-withdrawing Cl− can be oxidized, by electrophilic holes and OH•, to small-molecule acids. These small-molecule acids are ultimately mineralized to H2O and CO2.176 4.2.1.2. Conversion of Sulfur Compounds. Seiders and Ward372 demonstrated a method of decontaminating articles and/or structures contaminated with mustard gas by using a transition metal complex of tetrasulfonated or tetraaminophthalocyanine as a catalyst which binds oxygen from the air and converts O2 to O2•−; O2•− then dehydrochlorinates the mustard gas to form divinylsulfide. However, this method is not suitable for mustard gas stored as a liquid in containers. In another study conducted by Katori et al.,373 chemically and electrochemically generated O2•− was used, as a model of metabolic reactions, to simulate the conditions of bioorganic systems and study the desulfurization of thiocarbonyl compounds. Several types of thioamides and thioureas, including thiouracils, were readily desulfurized by O2•− to form carbonyl compounds; O2•− was generated by KO2 and 18-crown-6 or O2 reduction in aprotic solvents at room temperature (eqs 49 and 50). This shows the feasibility of O2•−, that was generated 3042


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Scheme 6. Possible Mechanism of the Photocatalytic Decomposition of NaPCP by Bi2.1WO6 under Visible Light. (Reprinted from Ref 176. Copyright 2014 American Chemical Society.)

chemically or electrochemically, for a better understanding of the reactions and mechanisms in biological processes.

Scheme 7. Structures of the Stabilizers Malonate, Citrate, and Phytate

4.2.1.3. Environmental Chemistry of the Superoxide Ion. The superoxide ion is one of the generated ROS that may have a substantial role in modified Fenton systems. It has been widely used for the remediation of contaminated soil and groundwater as a part of CHP reactions. The O2•− generated in CHP, the modified Fenton’s reagent used for in situ chemical oxidation (ISCO), might have superior reactivity with highly oxidized contaminants. This contrasts with what was previously believed. The CHP ISCO technology can be effectively used for treating groundwater in contaminated sites. The application of CHP remediation varies widely. In some sites, contaminants have been treated to undetectable concentrations, whereas significant reduction has been accomplished in other sites.251,374 This can be attributed to several factors including an ineffective mixture of ROS, deactivated catalysis, the minimum contact of pollutants with ROS, and the inhibitory effects of soil organic matter. The instability of H2O2 may be another disadvantage of using CHP ISCO in the subsurface. H2O2 releases O2 that may generate excessive heat, volatilize specific contaminants, and pose a considerable explosion risk. However, the stabilization of H2O2 can be attained by the addition of organic acids such as malonate, citrate, and phytate to reduce the generation of heat and O2 while maintaining an effective production of ROS (Scheme 7).375,376 The stabilizers lead to efficient CHP ISCO by improving the H2O2 transport and contact with contaminants. Additional studies on new potential stabilizers are necessary. An increase in temperature can also influence the reactivity of contaminant destruction. For example, it was observed that the increased destruction of PCB by using 50% H2O2 and 10 mM phytate was due to an opti-

mum balance between the heat generated (80 °C) and the generation of ROS, including OH•, O2•−, and HO2−.376 O2•− plays a primary role in the degradation of hydrophobic contaminants and nonaqueous-phase liquids (NAPLs). NAPLs cannot be oxidized by OH• generated by dilute Fenton’s reactions in the aqueous phase. Therefore, an increased concentration of H2O2 (i.e., >0.3 M) enhances the oxidative treatment of sorbed and NAPL contaminants because the vigorous Fenton-like reactions in the presence of high concentrations of H2O2 generate non-hydroxyl radical species (i.e., O2•− and HO2−), which contribute to the treatment of these contaminants at a faster rate compared with the natural rates of desorption and dissolution.377 In addition, the ROS generated using high concentrations of H2O2 may provide the additional benefit of mineralizing the contaminant degradation products, which is a focal point in the development of sustainable technologies. This is essential to avoid the formation of toxic intermediates that may migrate downstream.378 The effect of O2•− on contaminants is varied. For example, O2•− was reported to be the responsible ROS for the destruction of the carbon tetrachloride dense NAPL (DNAPL) in modified Fenton’s reactions.123 This could be ascribed to the overcoming of mass transfer limitations, possibly O2•− diffusion into the DNAPL. The phosgene was detected by GC−MS analysis after destruction, which was consistent with the destruction of carbon tetrachloride by using O2•− in aprotic media. This shows that O2•− reactions in aqueous and aprotic media may share similar patterns. However, further exploration 3043


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using the CHP system. Likewise, the mineralization of sorbed and NAPL-phase benzo[a]pyrene by CHP through reactions with O2•− and HO2− was postulated because these compounds are unreactive with OH•.377 The conditions of the CHP process can be modified to increase the O2•− and HO2− fluxes for PFOA treatment, resulting in an effective technology.251 As mentioned previously, O2•− has a role in the degradation of hydrophobic contaminants. For example, the effect of contaminant hydrophobicity on the dose of H2O2 in the Fenton-like treatment of soils was reported by Quan et al.380 The increase in the H2O2 concentration has a considerable role in the treatment of more hydrophobic normal alcohols, namely 1-hexanol, 1-heptanol, 1-octanol, 1-nonanol, and 1-decanol. This is attributable to the formation of transient oxygen species other than OH•, such as O2•− or HO2−, which contributes to enhanced contaminant desorption. O2•− was reported to react with various halocarbons to produce 1O2.381 However, the mechanism of 1O2 formation from systems containing O2•−, H2O, and halocarbons has been controversial. Khan382 and Corey et al.383 have proposed that 1 O2 results from the H2O-induced disproportionation of O2•−, whereas Arudi et al.384 and Kanofsky385 have asserted that 1O2 is a product of the reaction of O2•− with halocarbons, with the “X” variable in eqs 51−53 representing either Br or Cl. Several studies226,340,386−391 have supported the hypothesis that 1O2 is the product of the halocarbon−O2•− reaction.

is necessary for validation because several factors can alter the outcomes. These are discussed later. In contrast to the carbon tetrachloride DNAPL, both O2•− and OH• ROS have been involved in chloroform DNAPL destruction. This indicates the need for thoroughly exploring the destruction of DNAPLs because it cannot be generalized. Smith et al.254 reported the degradation of carbon tetrachloride in the presence of 1 M of each of the five solvents and HO2− in KO2 systems in the order acetone > 2-propanol > ethanol > HO2− > methanol > ethylene glycol. This indicates that the reactivity of O2•− increased considerably in the presence of a cosolvent that was less polar than H2O. The increased reactivity of O2•− in H2O solvent systems was likely due to the changes in the solvent shell surrounding O2•− after mixing H2O with less polar molecules. The mixed solvent shell is expected to have the characteristics of both the H2O and the less polar compound, resulting in an increase in the reactivity of O2•− relative to pure aqueous solutions. The reactivity of O2•− can also be increased significantly in aqueous KO2 systems after adding H2O2 at concentrations similar to those used in CHP systems. Some parameters may affect the reactivity of O2•− and its destruction of NAPLs and other contaminants, such as the generation method, cosolvent polarity, contaminant type, and pH of the aqueous solution. Recently, it has been reported that high H2O2 concentrations (i.e., >0.3 M) are not required when using manganese oxide catalyzed CHP systems.253,379 For example, with H2O2 higher than 100 mM, O2•− exhibited a considerably enhanced reactivity with oxidized organic compounds (e.g., highly chlorinated aliphatic compounds).253 The heterogeneous birnessite-catalyzed decomposition of H2O2 at a concentration of 7.5 mM enhanced the degradation of O2•− hexachloroethane (HCE). By contrast, HCE degradation was not detectable in parallel homogeneous Fe(III)−EDTA−H2O2 systems at H2O2 concentrations of L-cysteine > reduced glutathione. Conversely, compounds that were protected by sulfur, L-cystine, S-carboxymethyl-L-cysteine, and oxidized glutathione, neither reacted with O2•− nor formed compounds with a greater S-oxidation number, such as sulfoxide or sulfone. In other applications, Henry et al.423 used chemically generated O2•− (i.e., KO2, DMSO, and 18-crown-6) as a cofactor of dopamine-β-hydroxylase; O2•− acted as a reducing agent for tyramine hydroxylation by dopamine-β-hydroxylase. 4.2.3. Other Applications. The O2•− salts can be used as a solid source for releasing O2 for breathing in space flights and emergency self-rescuers.424 In addition, KO2 can be exploited as an O2 source by firefighters, in mine rescue operations, and for early missions of space programs. As mentioned earlier, KO2 as a O2•− carrier has the unique property to release O2 when it reacts with CO2 and exhaled moisture. Similar results were observed when NaO2 was tested for breathing apparatus applications. Other heavier metal superoxides, such as Ca(O2)2, offer a slight advantage over NaO2 and KO2 because a higher amount of O2 per unit weight is released by Ca(O2)2. In a relevant study,425 a single-pass flow-system test was used to

5. DETECTION METHODS OF THE SUPEROXIDE ION Numerous methods have been used for detecting O2•− generated in different media. These methods are used in biological and chemical systems and are mostly indirect. The detection can be based on O2•− redox properties, binding, trapping, or the reaction of O2•− to produce easily detectable and relatively stable products that accumulate with time.434−437 The detection methods can be classified into spectrophotometric and nonspectrophotometric methods.438 In addition, the detection method depends on how O2•− is generated. For instance, if O2•− is generated by O2 reduction, electrochemical detection is more suitable. In biological systems, the extracellularly released O2•− can be detected and measured using Cytc reduction or chemiluminescence assays. In addition, the intracellularly produced O2•− can be detected using the microscopic nitro blue tetrazolium (NBT) assay, fluorometric assay, oxidation of Fc OxyBurst, and chemiluminescence assay. The difference in O2•− concentrations being produced intracellularly may lead to a difficulty in understanding and identifying O2•− because of the limited sensitivity of existing detection methods.439 Figure 8 shows a flowchart of the known O2•− detection methods. This review is probably the first to give an overview of

Scheme 13. KO2 as an Oxidation Reagent for the Winterfeldt Oxidation of β-Carbolines (Reprinted from Ref 426. Copyright 2003 American Chemical Society.)

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Table 6. Reactions of O2•− with Some Substrates O2•− source KO2 O2 reduction, tetramethylammonium superoxide O2 reduction, tetramethylammonium superoxide KO2 O2 reduction KO2 O2 reduction KO2

reactant/substrate

product(s)

medium/media

ref

vitamin K1 and its related compounds 1,2-disubstituted hydrazines

corresponding 2,3-oxide and phthalic acid anion radical production of 1,2disubstituted azo compound

benzene AcN, DMSO

428 334

ascorbic acid, dihydrophenazine, dihydrolumiflavin, and 3,5-di-tert-butylcatechol

respective anion radicals

AcN, DMF

429

trialkylsilane and dialkylsilane ferricenium ion diacyl peroxides Brønsted acids ebselen

disiloxane and cyclopolysiloxane ferrocene and 1O2 1 O2 HO2• selenonate form and seleninate form of ebselen nitric oxides

AcN AcN benzene DMF, AcN toluene−DMSO (3:2 v/v) DMSO, DMF, AcN DMF

430 298 431 432 52

KO2, tetrabutylammonium superoxide O2 reduction

amines

KO2

CoIII−nitrosyl complexes

benzyl bromide

benzyl alcohol, benzaldehyde, benzene and biphenyl corresponding CoII−nitrito complexes and O2

AcN

421 295 433

Figure 8. Flowchart of O2•− detection methods.

all available O2•− detection methods for both chemical and biological systems. In addition, a notable scientific controversy is reported on the ROS detection methods. Most of these techniques have been criticized for a number of reasons, as will be discussed in the case of O2•−. Therefore, O2•− detection and monitoring methods need further development. The main detection methods are briefly discussed in the following subsections.

For instance, O2•− can quantitatively reduce benzoquinone to benzosemiquinone in aprotic media (eq 64).3 However, this

5.1. Electrochemical Methods

method is relatively nonselective. Therefore, it was improved by the reaction of hydroquinone with O2•− (eq 65). A combined

The use of electrochemical methods for O2•− detection have gained attention because they provide direct and real-time measurement in chemical and biological systems. These methods are easily accessible and easy to handle in comparison to other detection methods. 5.1.1. Redox Properties of O2•−. Although some of the aforementioned methods use redox properties, dedicating a section for discussing such properties is essential because they play a primary role in O2•− detection. In addition, these properties are expected to become one of the key factors to be considered while developing appropriate detection methods to provide a better insight into O2•− behavior.

application of both hydroquinone and benzoquinone is possible and appears to be a more reliable method to detect O2•−. Furthermore, PBQ can be used as O2•− scavenger. Recently, Ding et al.176 used PBQ to identify the O2•− contribution to NaPCP photodegradation. 3049


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Tetranitromethane was also reported to detect O2•− in aqueous solution (eq 66).3,440 Kitamura et al.24 proposed the reaction of iodine with O2•− to determine the O2•− concentration. This reaction proceeds quantitatively because of the difference in the redox potentials of iodine and O2•−. The final iodide concentration was determined photometrically at 400 nm (eq 67). O2•− + C(NO2 )4 → O2 + C(NO2 )3− + NO2

(66)

2O2•− + I 2 → 2O2 + 2I−

(67)

5.2. Spectrophotometry

5.2.1. UV−Visible. A UV−visible (vis) spectrophotometer was employed to detect O2•− in any medium because O2•− has a wavelength close to 255 nm in aprotic solvents1,33 and 245 nm in aqueous solutions.32 This method is suitable for the long-term detection and monitoring of O2•−. However, the main limitation of this method is the interference of the cutoff wavelength of the used media (e.g., aprotic media), causing technical difficulties in O2•− detection and monitoring. However, it is useful to detect and monitor O2•− as long as the spectrum is easily identified and is not overlapped with those of other components. The UV−vis spectroscopy method is convenient for chemical applications as it is relatively practical and easy to use compared with other methods that are used mainly for biological applications (e.g., spin trapping and Cytc reduction). The pH measurement can be used as a quick method for O2•− detection because O2•− is a Brønsted base and can affect the pH of a medium.230,452 Therefore, any increase in the pH of a medium confirms O2•− generation. This simple method can be used for fast detection prior to target reaction. However, this method cannot provide precise evaluation and can only provide an approximated indication. 5.2.2. Nitro Blue Tetrazolium Reduction. Nitro blue tetrazolium chloride is a classical biochemical reagent used for O2•− detection. Several previous studies247,249,453 have reported the frequent use of radical scavengers and organic reagents, such as NBT, for O2•− detection. In such cases, O2•− reduced NBT to its deep-blue diformazan form,454 thereby producing its characteristic purple color (Scheme 14).455 In addition, O2•− attacks ditetrazolium (NBT2+), along with one transferred electron, to convert NBT2+ to a tetrazolinyl radical (NBT•+). In the subsequent step, a transfer of one more electron converts NBT•+ to monoformazan. Of note, NBT or O2•− excess may theoretically produce only mono or diformazan.454,456 The visual results obtained using this method are qualitative or semiquantitative and do not reflect quantitative O2•− accurately. Moreover, the method is complicated to standardize. For instance, the reduction of yellow-colored NBT is extremely sensitive to the presence of an electron donor, such as O2•−. These drawbacks have been tackled using spectrophotometric methods by measuring the absorbance of cells containing NBT deposits (i.e., densitometric NBT assay) or the absorbance of NBT dissolved in organic solvents (i.e., colorimetric NBT assay).439,457−459 Colorimetric NBT assays have been modified to quantify the intracellular generation of O2•− in a range of phagocytic cells.439 The particles of formazan were dissolved in 2 M KOH and DMSO, and a microplate reader was used to determine their absorbance. The modified colorimetric NBT assay was quantitatively convenient and sensitive. In addition, this assay can be easily customized to get rid of the hypersensitivity of common NBT assays with respect to stimulated cells that generate high concentrations of intracellular O2•−. However, diformazan is insoluble in water, and its precipitation leads to an inconsistency in absorbance readings. Therefore, modified NBT derivatives that have water-soluble end products have been used and are commercially available, for example, 4-[3-(4-iodophenyl)-2-(4-nitrophenyl)-2H-5-tetrazolio]-1,3-benzene disulfonate sodium salt (WST-1), 4-[3-(2methoxy-4-nitrophenyl)-2-(4-nitrophenyl)-2H-5-tetrazolio]1,3-benzene disulfonate sodium salt (WST-8), and 3′-{1[(phenylamino)-carbonyl]-3,4-tetrazolium}-bis(4-methoxy-6nitro)benzenesulfonic acid hydrate (XTT).438,460−465 The

The reaction of alkyl bromides with O2•− to form a bromide was proposed to quantitatively detect O2•−. For instance, Afanas’ev et al.441 used butyl bromide (BuBr) to investigate the reactions of quinones with O2•− (eq 68). The formed bromide was titrated potentiometrically to determine the O2•− concentration. However, BuBr is suspected to react with other nucleophiles if they are present (e.g., OH− and HO2−). O2•− + BuBr → BuOO• + Br −

(68)

5.1.2. Direct Detection via O2 Reduction. One of the most efficient detection methods is CV in which O2•− has a reduction potential of ±(−1) V versus SCE.1,94,106 This method has been used to investigate the possibility of electrochemical generation of O2•− in various media. However, the use of this method is limited because of various parameters, such as the type of electrodes, media, humidity, and the environment. In addition, this method can monitor the O2•− stability for a short duration. By contrast, it can be used for specific applications involving O2•−. Furthermore, they can be used for reactions that require particular precautions, such as for pharmaceutics and therapeutic experiments. 5.1.3. Biosensor by Immobilizing Enzyme. The immobilization of certain enzymes on electrodes to act as biosensors for O2•− detection has been described extensively.153,442−446 For example, O2•− can be detected using the SOD enzyme probe. The use of the SOD probe is well-known for O2•− dismutation to H2O2 and O2 with high specificity and strong activity. Recently, an SOD immobilization-based biosensor was developed for O2•− detection by using a thin silica−PVA sol−gel film on a Au electrode.447 A rapid and direct electron transfer of SOD was detected without any mediators or promoters. Chin Quee-Smith et al.448 used SOD to investigate if O2•− had any role in the catalyzed epoxidation. SOD was extracted from bovine liver and added to the reaction mixture under certain conditions. Viologens were used because they change color reversibly upon reduction and oxidation. For example, Barrette et al.449 titrated the iron-containing SOD for Pseudomonas ovalis, Azotobacter vinelandii, and Escherichia coli with reduced benzyl viologen, generated coulometrically in situ, in the presence of a potentiometric monitoring electrode. In addition, the use of a glacial acetic acid−diethyl phthalate solution for the determination and differentiation of O2•− was reported.450 This solution quantitatively converts O2•− to O2 and H2O2. In addition, a medium system containing a watersoluble surfactant, such as sodium dodecyl sulfate, was proposed to investigate the reactions of O2•− in aqueous solutions.242 Some other proteins, such as the redox protein Cytc, were used.328,451 These modified sensors have a promising future, and developments for a better detection with excellent performance are currently underway. 3050


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5.2.3. Cytochrome c Reduction. Cytochrome c has been widely used for detecting O2•− produced in biological systems because of its fast superoxide-mediated reduction to Cytc2+.468−470 According to eq 69, Cytc can be reduced by O2•−, and this results in its color change from red to colorless.468,469,471 This method has been utilized to measure O2•− generation rate by various enzymes, whole cells, and vascular tissues. The spectrophotometric reaction is monitored at 550 nm.

Scheme 14. NBT Reaction Mechanism (Reprinted from Ref 454. Copyright 1980 American Chemical Society.)

Cytc − Fe3 + + O2•− → Cytc − Fe2 2 + + O2

(69)

Quick et al.472 reported a method for determining O2•− scavenging efficiency by using the kinetic analysis of Cytc reduction and a UV−vis microtiter plate reader. In this method, Cytc was reduced as the absorbance was increased to 550 nm, following which O2•− was quantified. Most of the Cytc reduction sites were blocked by SOD, thus indicating that Cytc reduction was almost completely O2•− dependent. The O2•− was generated by reacting xanthine oxidase with hypoxanthine (eq 40). This assay provided a high and fast evaluation of the O2•− scavenging efficiency for small molecules of interest and cell or tissue extracts. However, there are many factors that should be taken into account when using this reaction for O2•−detection.468,470,473 Tarpey et al.468 stated several precautions that should be considered. For instance, Cytc reduction is not entirely specific for O2•−. Cellular reductants (e.g., ascorbate and glutathione) are able to reduce Cytc and within tissue extracts can be present in high concentrations. Moreover, some enzymes (e.g., xanthine oxidase) reduce redox-active dyes or quinones that are available and whose reduced forms are competent to reduce Cytc.474 Therefore, the O2•− specificity of this reaction can be improved by the inhibition extent of Cytc reduction by exogenous SOD. Then, cellular peroxidases, cytochrome oxidases, and oxidants including ONOO− and H2O2 will reoxidize the reduced Cytc. However, these reoxidation reactions are expected to underestimate the O2•− formation rate as the apparent Cytc reduction rate is decreased. Thomson et al.470 indicated that, when quantitating O2•− by using the Cytc reduction method, the simultaneous generation of nitric oxide and ONOO− may lead to an underestimation of the O2•− formation rate. The Cytc reduction assay is a nonspecific reaction because Cytc is speculated to be reduced by many reductive constituents present in herbal supplements and foodstuffs. Therefore, this assay is not recommended for detecting O2•− in samples containing reducing compounds, such as vitamin C or other antioxidants. This shows that the detection method should consider the generation method, the medium composition, and the purpose of application.438 To improve the O2•− specificity of the Cytc assay, Cytc can be acetylated or succinoylated.468,470,475−477 Succinoylated Cytc is preferred because it is reduced by flavin-dependent Cytc reductases to a lesser extent. In addition, Cytc succinoylation is more effective in decreasing the Cytc reduction by NADPHcytochrome P-450 reductase or cytochrome b5 and oxidation by Cytc oxidase. However, it decreases the reaction rate constant between O2•− and succinoylated Cytc (i.e., nearly 90%) compared with original Cytc. Therefore, high concentrations of succinoylated Cytc are required.

water solubility of XTT is limited ( 100 mM) and are pH independent.464,466,467 Although the results obtained using WST have other potential applications, the utilization of WST is still limited to biochemical samples.438 Water-soluble tetrazolium, WST-1, was used for detecting O2•−, which was generated from the reaction between hypoxanthine and xanthine oxidase in the presence of SOD. The use of this assay is expected to avoid pipetting errors and variable xanthine oxidase activity between samples.460 Overall, this method can be further modified to meet the demands of the extensive studies related to biological systems. 3051


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Figure 9. Schematic of a graphene-based CRET platform for the detection of C-reactive protein. (Reprinted with permission from ref 491. Copyright 2012 American Chemical Society.)

Scheme 15. Proposed Electrochemiluminescence Mechanism for Luminol in Alkaline Solution (Reprinted from Ref 499. Copyright 2004 American Chemical Society.)

5.3. Luminescent (Emission)

cence techniques are simple, and the cost of instrumentation and maintenance is low as no background light is required. Because chemiluminescence is directly related to the reactant concentrations, it has been used as an ultrasensitive method for the quantification and localization of analytes that exhibit luminescence by participating in the chemiluminescence reaction. These analytes include chemiluminescence precursors, catalysts, oxidants, cofactors, sensitizers, enhancers, and inhibitors.478,483−485 Lucigenin is the most commonly used chemiluminescent compound for O2•− detection. A luminol-based chemiluminescence assay was used to detect O2•−.59,486 The use of lucigenin is disadvantageous because it can only detect extracellular free radicals, primarily O2•−, whereas luminol can detect both

5.3.1. Chemiluminescence. Chemiluminescence is a phenomenon in which chemically generated molecules emit light in their excited states. Chemiluminescence has recently been implemented in diverse fields, such as biotechnology and medicine.478−480 Chemiluminescent methods are commonly used to detect O2•− for their potential to access intracellular sites of O2•− generation, their low cellular toxicity, and the assumed reaction specificity of O2•− with the chemiluminescent probe compared with other chemical methods.468 In general, assays based on chemiluminescence are advantageous because they are not only highly sensitive and rapid but also based on the consumption of ROS.481,482 In addition, chemilumines3052


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(Scheme 16). The NCNTs functionalized with polyelectrolyte exhibited almost double adsorption efficiency toward dissolved

intracellular and extracellular deoxygenation products, including O2•−, H2O2, and hydroxyl ions. However, the chemiluminescence signal of luminol is relatively short-lived; therefore, the signal measurements should be captured as soon as luminol is added.487 By contrast, the potential for the redox cycling of lucigenin and artifactual generation of O2•− have drawn attention because they can appropriately identify the O2•− quantitative rates. Despite its limitations to precisely estimate the O2•− formation rates, the involvement of lucigenin in low concentrations is practical for obtaining qualitative information on O2•−. Other nonredox-cycling compounds have been introduced as O2•− probes.468,474,488 For example, in previous studies,247,482 2-methyl-6-(4-methoxyphenyl)-3,7-dihydroimidazo[1,2α]pyrazin3-onehydrochloride (MCLA) reacted with ROS, forming an intermediate comprising a high-octane dioxetanone and emitting strong light at 465 nm. Recently, a new era of chemiluminescence has been introduced through the implementation of nanotechnology.489−494 The extraordinary physicochemical characteristics of nanomaterials, including high sensitivity, activity, and large surface area, make them have a high potential to act as chemiluminescence resonance energy transfer (CRET) platforms, enhancers, catalysts, reductants, labels, luminophors, or energy acceptors.478,495−497 In 2004, Poznyak et al.498 observed the band gap chemiluminescence of semiconductor quantum dots in both solution and nanoparticulate layers for the first time. The spectral position of the band gap chemiluminescence of the CdSe/CdS core−shell and InP nanocrystals was particle size dependent, allowing an effective tuning of the emission color with higher color purity intrinsic for monodisperse samples. Chemiluminescence is possibly exhibited by diverse nanocrystals in both close-packed films and colloidal solutions. The efficiency of the nanocrystal chemiluminescence can be significantly increased through optimizing the capping ligands, nanocrystal composition, and chemical fuel, which produces the energy required for the excited state generation. Therefore, nanocrystal chemiluminescence has attracted considerable attention in biomedical and analytical applications. Lee et al.491 reported the CRET between chemiluminescent donors and graphene nanosheets. Within the CRET platform, graphene acted as an energy acceptor, which is more effective than graphene oxide, whereas luminol acted as a donor to graphene, triggering the phenomenon of CRET between luminol and graphene (Figure 9). 5.3.2. Electrochemiluminescence. Electrochemiluminescence (electrogenerated chemiluminescence) is a sensitive technique, integrating the advantages of electrochemistry and chemiluminescence.499,500 The rapid development of both the fundamentals and applications of electrochemiluminescence has demonstrated its potential for analytical and bioanalytical chemistry.501 In alkaline solutions,499 O2•− and HOO− are involved in electrochemiluminescence as products of H2O2 (Scheme 15). This is mainly attributed to the formation of H2O2 during substrate specific enzymatic activity. As a result, selective and sensitive detection in the presence of luminol is possible. Another direction to improve electrochemiluminescent immunoassay using quantum dots (QDs) was investigated by Deng et al.500 A facile signal amplification strategy was reported based on the electrochemiluminescence mechanism of QDs by adsorption-induced catalytic reduction of dissolved O2 on nitrogen-doped carbon nanotubes (NCNTs), forming O2•− and subsequently enhancing electrochemiluminescence emission

Scheme 16. Enhanced Electrochemiluminescence Emission Mode via the Adsorption Induced Electron Transfer of Dissolved O2 on NCNT (Reprinted with Permission from Ref 500. Copyright 2011 Royal Society of Chemistry.)

O2 than carbon nanotubes, leading to faster formation of O2•− and thus improving the electrochemiluminescence emission of the immunosensor. This method showed acceptable accuracy and high sensitivity. In addition, it was proposed that the functionalized NCNTs will offer new opportunities to investigate new QD-based electrochemiluminescence systems and explore new applications of carbon nanomaterials in bioassays. 5.3.3. Photoluminescence (Hydroethidine). Recently, many protocols502−509 have been reported for O2•− detection by using the hydroethidine (HE) (also known as dihydroethidium) assay. Hydroethidine is a cell-permeant compound that is used as a fluorescence probe (Scheme 17). The HE assay based on high-performance liquid chromatography was utilized to independently evaluate O2•− generation.59,502 This assay can be used in various biological systems to detect O2•− in cells, tissues, organisms, microorganisms, and soils. Subsequently, the HE conversion to 2-hydroxyethidium (2-OH-E+) is indicative of O2•− generation. However, other reactions can affect the conversion of HE to ethidium as a quantitative marker for O2•− generation in biological systems.468,507,510−513 In addition, HE can increase the O2•− dismutation rates toward H2O2, leading to an imprecise quantitative evaluation of O2•−. Therefore, the exploitation of HE for in vivo O2•− determination appears to have significant limitations. On the other hand, staining by the oxidative fluorescent probe dihydroethidine, which is freely permeable in cell membranes, is suitable to monitor the in situ production of O2•− and provide a reliable marker of its intracellular presence. 5.4. Vibrational Spectroscopy

5.4.1. Raman. Raman spectroscopy has been used for O2•− detection and studying the related enzymes in biological systems. Both resonance Raman (RR) and surface-enhanced Raman scattering (SERS) can be used for O2•− detection. The RR elucidated both vibrational and electronic transitions because their coupling increases the intensity of Raman bands. This technique is promising for biological systems, where vibrations of chromophoric groups can be monitored in a macromolecular matrix.514 The in situ Raman spectroscopic technique was introduced by using the SERS method. The SERS technique provides high sensitivity to detect a monolayer level species and is expected to verify the presence of ROS in the melts at high temperatures.515 In situ experiments were conducted with KO2 solution in molten (62 + 38) mol % (Li + K)2CO3 eutectic at 650 °C. The vibrational frequency of peak (A) in Figure 10 observed before the addition, 1065 cm−1, is assigned to the stretching vibration of carbonate ions. Before the addition (t = 0), no peak was observed at the (B) position 3053


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Scheme 17. Proposed Mechanism of 2-OH-E+ Formation from the Reaction between O2•− and HE (Reprinted with Permission from Ref 503. Copyright 2008 Nature Publishing Group.)

py is a valuable nondestructive method to probe MnSOD conformation. 5.4.2. Electron Spin Resonance and Spin Trapping. Harbour et al.518 introduced the spin-trapping method and later used it in biological and chemical systems to identify and detect the short-lived free radicals through the use of ESR.176,519 For instance, ESR spectroscopy detects paramagnetic species, such as the unpaired electrons of free radicals. ESR spectroscopy has become one of the main techniques used for detecting O2•− that is biologically generated in in vitro and in vivo models. O2•− has been widely investigated using ESR because this technique provides information on its structure, location, stability, surface mobility, and on-site dynamics.179,520 In addition, ESR is a nondestructive technique, which is an advantage over other chemical techniques that deal with biological systems.521 Because the half-life of these radicals is too short to be detected using ESR spectroscopy, compounds known as spin traps are used to react covalently with the radical products, forming more stable adducts that can be further analyzed. The toxicity and efficiency of these spin traps for O2•− identification are of interest. Spin traps 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and 5-tert-butoxycarbonyl 5-methyl-1-pyrroline-N-oxide (BMPO) are commonly used for O2•− detection.522−524 In general, DMPO has low toxicity, rapidly penetrates lipid bilayers, and can be used for both in vitro and in vivo studies.525,526 However, the main parameters that may limit the use of this method are the pH range and detection limit. BMPO is an analogue of DMPO and is suitable for in vitro and in vivo studies. However, BMPO forms considerably more stable radical adducts with O2•−. Compared with other techniques, the use of the spin-trapping method was limited because of its expensive instrumentation, low sensitivity and selectivity with respect to other radicals present, low rate constants for spin trapping, low adduct stability, and lack of spin trap specificity, and it is inconvenient to use. Therefore, a few studies468,502,527 have reported that the spin-trapping method is

Figure 10. Time variation of Raman spectra after adding KO2 into the carbonate melt in a gas mixture of O2/CO2 with a ratio of 9/1. (Reprinted with permission from ref 515. Copyright 1999 Electrochemical Society.)

(Figure 10). However, a new line was observed at 1047 cm−1 immediately after the addition, and this signal was detectable after a few minutes and developed gradually with time. This signal is ascribed to the stretching vibration of O2•−. Recently, Bonnot et al.516 reported that the non-heme mononuclear site of superoxide reductase can accommodate a high-valent iron− oxo species. This was possible by RR spectroscopy. In a relevant study, David et al.517 presented the combined IR and Raman broad-band vibrational study of manganese superoxide dismutase (MnSOD), a pathological biomarker, for the first time. The combined Raman−Fourier transform IR spectrosco3054


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Table 7. Variation in Wavenumber Range of Identified O2•− by Using IR Spectroscopy wavenumber (cm−1)

application

method

ref

1013 1126 1090−1180 1210 1220 1020−1220 1124 (normal), 1236 (perturbed)

water oxidation catalyzed by cobalt oxide nanoparticle catalyst adsorption of O2 on well-outgassed and partially reduced cerium oxide LaOF and BaF2/LaOF catalysts for methane oxidative coupling electrochemical ORR on model semiconductor n-Ge(100) surface in 0.1 M HClO4 electroreduction of O2 on Au in acidic media adsorption of O2 onto COO−MgO dilute solid solutions O2 reduction under practical operating conditions of intermediate-temperature solid oxide fuel cells

FTIR-ATR FTIR FTIR ATR-IR SEIRAS IR pd-FTIRES

559 544 560 545 537 561 535

inefficient for O2•− detection. In addition, the cost concern of large quantities of the chemicals used as spin traps, such as DMPO, should be considered. Consequently, the ESR spintrapping method is not convenient for a large-scale measurement, and its broad applicability is limited.438 On the other hand, recent developments and extensive efforts have been devoted to making this technique highly sensitive and selective to detect radical species, including O2•−. For instance, the ESR spin-trapping detection of O2•− is 40-fold more sensitive than spectrophotometric analysis with Cytc.528 In addition, spin traps are useful for O2•− detection under conditions in which more conventional methods (e.g., Cytc reduction) cannot be used. Further studies are highly recommended for developing the instrumentation and spin-trapping adducts. 5.4.3. In Situ Fourier Transform Infrared Spectroscopy. Infrared (IR) spectroscopy is a prolific method with extraordinary advantages that provides insight into the structure and bonding of molecules.529−531 Numerous in situ IR instruments have been modified, such as in situ attenuated total reflection Fourier transform IR (FTIR-ATR),532−534 in situ potential-dependent FTIR emission spectroscopy (pd-FTIRES),535 in situ diffuse reflectance FTIR,536 surfaceenhanced IR reflection absorption spectroscopy (SEIRAS),537 and synchrotron-enhanced FTIR.538−540 Compared with IR, ESR is less applicable to the studies relevant to oxygen species adsorbed on numerous paramagnetic oxides. By contrast, IR and FTIR have been widely used for the in situ analysis of adsorbed species and surface reactions. The IR method has been widely used to characterize oxide surfaces.531,535 It has been often used also to characterize the O2 species in the matrix and complexes, and also demonstrated to be useful in determining the oxygen species formed on oxides.541−544 Stoin et al.129 monitored the in situ O2•− formation by using online FTIR. The major advantage of online IR detection is that it is unaffected by the short half-life and low concentration of the reagent (i.e., superoxide ion). Unlike standard FTIR, online FTIR can allow product identification even if it is a short-lived intermediate. The peak at 1108 cm−1 is a clear indication of the presence of O2•− in the reaction mixture. The Raman spectrum of the NaO2 species was expected to be complementary to the IR spectrum. However, Raman is a more sensitive method than FTIR is because it allows for the detection of the symmetric O−O stretch, whereas FTIR functions mainly in the asymmetric mode.129,545 However, the main limit of the Raman detection method is the extremely short lifetime of O2•− species. The suitability of using hyphenated techniques by coupling FTIR to other analytical instruments, such as electrochemistry, thermogravimetry (TG), gas chromatography, mass spectrometry, and HPLC, is another advantage for conducting studies related to chemical, electrochemical, and biochemical reactions.546−555 These techniques can provide improved analysis.

For example, TG coupled with FTIR can identify decomposition products and evolved gases.556 Free superoxide species absorb IR frequency in the range 1070−1200 cm−1.557 However, as shown in Table 7, a variation can be observed in the stretching mode of O2•−. This can be attributed to the O−O stretching. The frequency of the perturbed O−O stretching in the gas phase is 100−200 cm−1 higher than that of an unperturbed one.535,558 There are several influential factors such as the type and phase of the application in which O2•− is being formed, the fact that other ROS may share close wavenumbers (e.g., HO2•),545 and the IR method because different in situ IR instruments used would affect the analytical outputs. For example, the rapid-scan FTIR-ATR mode of visible-light-sensitized catalysis in aqueous solution provides a clear, informative analysis on the kinetics of reaction intermediates.559 Shao et al.558 observed O2•− in the ORR on a Pt thin-film electrode by using surface-enhanced infrared reflection absorption spectroscopy with attenuated total reflection (ATR-SEIRAS). There are two crucial advantages of the ATR-SEIRAS technique compared with other in situ IR techniques because of its substantially higher surface sensitivity and lack of transport limitation for reactants. By using SEIRAS with a modified experimental setup, the adsorbed O2•− in the ORR on Pt in an aqueous solution at pH 11 can be detected. The feasibility of IR spectroscopy is also subject to the media used for O2•− such as the gas or liquid phase and an aqueous or a nonaqueous solution. For example, complexes with O−O stretching frequencies in the range 1050−1200 cm−1 were assigned as superoxides.562 However, the observed bands at 860−880 cm−1 for molecular oxygen on a Pt(111) surface were assigned to a bridge-bonded peroxide or superoxide.563,564 The frequency (1005−1016 cm−1) of the band observed by Shao et al.558 was higher than what was detected in the gas phase, and this difference can be attributed to the effect of the solvent. The acidity and alkalinity may also have a role because of the rapid protonation of the superoxide at low pH values. For example, no band was observed for the solution with 0.1 M HClO4 (pH 1), probably because of the short lifetime of O2•− in the acidic solution.

6. SUPEROXIDE ION IN IONIC LIQUIDS In previous studies on ILs, various gases, including O2,108,283 CO2,565−571 H2,572−574 NH3,575,576 SO2,577 H2S,578,579 and NO2,580−582 were involved. In these studies, the following points were considered:583 (1) significant physicochemical differences may occur by adding or removing a solute from viscous ILs; (2) in addition to removing O2, degassing ILs with dry N2 removes H2O, which may have implications on the structure and viscosity of a medium; (3) certain precautions should be considered when deoxygenation is used in IL media as part of the electrochemical experiment protocol; and (4) gases, such as O2, Ar, and N2, that dissolve in ILs can alter the 3055


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Table 8. Summary of ILs Investigated for O2•− Generation IL 1−3-methoxypropyl-1-methylpiperidinium bis (trifluoromethylsulfonyl)imide 1-(2-methoxyethyl)-1-methylpyridinium tris (pentafluoroethyl)trifluorophosphate N-(3-hydroxypropyl)pyridinium bis (trifluoromethylsulfonyl)imide N-hexylpyridinium bis(trifluoromethylsulfonyl) imide trihexyl(tetradecyl)phosphonium tris (pentafluoroethyl)trifluorophosphate trihexyl(tetradecyl)phosphonium chloride tris(n-hexyl)tetradecylphosphonium trifluorotris (pentafluoroethyl)phosphate tris(n-hexyl)tetradecylphosphonium bis (trifluoromethylsulfonyl)imide trihexyl(tetradecyl)phosphonium dicyanamide triethylsulfonium bis(trifluoromethylsulfonyl)imide N-methoxyethyl-N-methylmorpholinium bis (trifluoromethylsulfonyl)imide 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate N-butyl-N-methylpyrrolidinium bis (trifluoromethanesulfonyl)imide 1-hexyl-1-methyl-pyrrolidinium bis (trifluoromethylsulfonyl)imide 1-butyl-1-methylpyrrolidinium trifluoroacetate 1-ethyl-3-methylimidazolium tetracyanoborate 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide 1,2-dimethyl-3-N-butylimidazolium hexafluorophosphate 1,2-dimethyl-3-propylimidazolium bis (trifluoromethylsulfonyl)imide 1,3-dimethylimidazolium diphosphate 1,3-dimethylimidazolium trifluoromethanesulfonate 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide 1-ethyl-3-methylimidazolium chloride mixed with AlCl3

ref

IL

230,452, 589

ref

1-ethyl-3-methylimidazolium ethylsulfate 1-ethyl-3-methylimidazolium tetrafluoroborate 1-n-propyl-3-methylimidazolium tetrafluoroborate 1-butyl-3-methylimidazolium tetracyanoborate 1-butyl-2,3-dimethylimidazolium trifluoromethanesulfonate 1-butyl-2,3-methylimidazolium bis (trifluoromethanesulfonyl)imide 1-butyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide 1-butyl-3-methylimidazolium hexafluorophosphate

43 452 291 43, 590 591, 592 584, 593

1-butyl-3-methylimidazolium tetrafluoroborate 1-n-butyl-3-methylimidazolium tetrafluoroborate 1-hexyl-3-methylimidazolium chloride 1-hexyl-3-methylimidazolium trifluorotris (pentafluoroethyl)phosphate 1-hexyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide 1-hexyl-3-methylimidazolium tetracyanoborate 1-octyl-3-methylimidazolium chloride trimethylbutylammonium bis (trifluoromethylsulfonyl)imide N-ethyl-N,N-dimethyl-2-methoxyethylammonium bis(trifluoromethylsulfonyl)imide n-hexyltriethylammonium bis (trifluoromethylsulfonyl)imide trimethyl-n-hexylammonium bis (trifluoromethylsulfonyl)imide triethylbutylammonium bis (trifluoromethylsulfonyl)imide tetrabutylammonium hexafluorophosphate (TBAHFP)/AcN lithium hexafluorophosphate (LiHFP)/AcN tetrabutylammonium perchlorate (TBAClO4)/AcN potassium hexafluorophosphate (KHFP)/AcN sodium hexafluorophosphate (NaHFP)/AcN

230, 287, 584, 590, 592, 594 592 291 291 595 27, 109, 283, 584−586, 590, 596−599 230,452 600 594 594 108, 601 283 288 288 282, 283, 288, 565, 590, 594, 596 278

288 284, 602−604 602, 604 594 595 109, 279, 285, 577, 597 109, 286, 288, 590, 597, 605 108, 109, 112, 288, 577, 590 590 109, 583, 602−604, 606 288 590, 594 594 594 288 607 291 109, 279, 282, 565, 584 283 605, 608 455 455 455 455 455

in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [BMPyrr][TFSI].283 Table 8 lists the ILs that have been used as media for O2•− generation. 6.1.1. Generation of an Unstable O 2 •− . Many researchers108,109,282,283,565,584,586,608 have reported the successful generation of O2•− in tested ILs, although CV showed that the O2•− generated in many of these ILs was unstable. The O2•− generation potential among all previously tested ILs was approximately ±(−1 V). It was observed108,452 the absence of the oxidation peak in the backward sweep of CV, indicating that the generated O2•− was unstable in these ILs, for example, pyridinium-based ILs. This instability may be caused by the reaction of O2•− with either the IL cations or cation decomposition products because the voltage at which O2 reduction occurs is proximal to the negative limit of the electrochemical window of certain ILs, such as pyridinium-based ILs. Martiz et al.607 showed that grafting a pyridinium-based IL to a perfluorinated aliphatic chain [C5H5N+C8F18][TFSI] increases O2 solubility by 105%. Many previous studies43,282,595,602,603 have shown that the process of O2 reduction is complex and its outcome is dependent upon the media properties, for example, acidity. Therefore, a miniscule quantity of KO2 was added to ILs to neutralize their acidity.230,291 This amount of KO2 did not affect

properties. In addition, vacuum-based techniques are recommended to remove volatile solutes, such as H2O and O2. 6.1. Generation of O2•− in ILs

Pure O2 has been frequently used to generate O2•−.108,109,282,584 However, air was also used to generate O2•−. For instance, the current density of a cathodic peak was observable in air sparge,585 and this current density was approximately 1/10 of the value reported by another study586 in which experiments were conducted using pure O2. This ratio is plausible by considering the O2 concentration in air. Huang et al.109 employed CV and potential step CA to study the reduction of O2 at various temperatures for ILs comprising pyrrolidinium, ammonium, and imidazolium cations with tetrafluoroborate [BF4]−, bis(trifluoromethylsulfonyl)imide, and hexafluorophosphate [HFP]− anions; O2•− is considerably stable in trimethyl-n-hexylammonium bis(trifluoromethylsulfonyl)imide [TMHAm][TFSI]. The high O2•− stability in ammonium-based ILs was anticipated as ammonium ions are used as superoxide carriers for O2•− (e.g., tetramethylammonium superoxide).4,283,587,588 In addition, being weak Lewis acids, large alkylammonium ions are capable of stabilizing O2•−. This indicates that ORR is a one-electron process, which is reversible and involves the O2/O2•− redox couple in alkylammonium ion based electrolytes. Furthermore, O2•− is stable 3056


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applications of O2•−. However, until now, most efforts have been directed only toward the identification of a suitable medium in which a stable O2•− can be generated. To implement O2•− in reactions and applications, we should consider designing proper ILs that play multiple roles in the process because their contribution to chemical reactions may yield completely different products.612 In addition, ILs can steer the reactions in a particular direction.613 Therefore, ILs have been recognized as efficient catalysts, and they can be successfully used in various organic reactions. Thus, ILs can act as simultaneous multitask media, extractants (e.g., for hazardous materials), catalysts to accelerate the reactions and applications of O2•−, and media for a generating a stable O2•−. This would shift the research focus from the superoxide chemistry stage to the superoxide engineering stage, thus fulfilling the demands of the real world. 6.1.3.1. Effects of Cations. The cation properties play a primary role in the association of ions with charged substrates, particularly with O2•− because of its small size.284,608 Laoire et al.455 found that O2 electrochemistry is strongly influenced by the cation nature and less by the anion in the conducting salt. Consequently, ILs consisting of high cation concentrations are susceptible to nucleophilic attack by species, such as O2•−.279 Therefore, the structure of the cation has a paramount effect on O2•− generation. However, O2•− is highly speculated to react with specific cations, such as imidazolium118,283,288 and quaternary phosphonium,287,581 as will be discussed in section 6.2.287,584 Katayama and co-workers283 estimated the electron distribution in organic cations, consisting of ammonium, imidazolium, and pyrrolidinium, and Scheme 18 depicts their ab initio

the obtained results because the reaction between the acidic impurities and O2•− canceled each other out. This can be verified by checking the absorbance of the DMSO-containing IL following neutralization. In addition, the presence of impurities may significantly influence the O2•− stability in an IL (this will be discussed in the following sections).595 Therefore, current studies should focus on controlling the impurity levels in ILs to obtain pure mixtures. 6.1.2. Generation of a Stable O2•−. The redox potential of O2/O2•− depends predominantly on the solvation degree of O2•− based on the assumption that electrically neutral O2 has approximately constant solvation energy for different media.1 Because O2•− has a negative charge, the O2•− solvation degree is affected by the acceptor number of a medium, that is, the redox potential of O2/O2•− becomes more positive when the acceptor number of the medium increases. For instance, the acceptor numbers of DMF, DMSO, AcN, and H2O are 16.0, 19.3, 19.3, and 54.8, respectively.27,609 Similarly, Katayama et al.283 reported that the O2/O2•− redox potential in [TMHAm][TFSI] was approximate to those in AcN and DMSO. This was anticipated as the acceptor number of [TMHAm][TFSI] is very close to those of AcN and DMSO. The variation in peak potential separation (ΔEp) values for the O2•− generation cyclic voltammograms was interpreted on the basis of the kinetics of heterogeneous electron transfer.1 The surface reactions and uncompensated resistance, particularly for metal-based electrodes, were the most reasonable explanation for these variations. O’Toole et al.610 attributed the slightly greater ΔEp values in their cyclic voltammograms to the residual uncompensated solution resistance (iR). Figure 11

Scheme 18. Mulliken Charges on C and N Atoms in (a) [TMHAm]+, (b) [BMPyrr]+, (c) [EMIm]+, and (d) [DMPIm]+ (Reprinted with Permission from Ref 283. Copyright 2004 Electrochemical Society.)

Figure 11. Currents of reduction peaks (O2•− generation) versus the square root of sweep rates v1/2 in [MOEMPip][TPTP]. (Reprinted with permission from ref 43. Copyright 2012 Elsevier.)

shows the linear relationship that was identified between the square root of sweep rates and reduction peak currents for O2•− generation in 1-(2-methoxyethyl)-1-methylpiperidinium tris(pentafluoroethyl)trifluorophosphate [MOEMPip][TPTP], thus indicating that this reaction was diffusion controlled,452,602,610 which is in accordance with the electrochemistry of an irreversible redox couple.43,108,611 6.1.3. Effects of the IL Structure on O2•−. The structure of ILs has a paramount effect on the generation, reactions, and

molecular orbital calculations. They found that all charges on the C atoms in trimethyl-n-hexylammonium [TMHAm]+ and 1-butyl-1-methylpyrrolidinium [BMPyrr]+ were negative, suggesting that nucleophilic reagents, such as O2•− and OH−, did not attack the C atoms in these aliphatic and alicyclic cations. However, all C atoms at 2-, 4-, and 5-positions in the heterocycle of 1-ethyl-3-methylimidazolium [EMIm]+ and 1,23057


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dimethyl-3-propylimidazolium [DMPIm]+ were positive. The 2-position C atom had the highest positive charge, and thus was the first to be attacked by the nucleophilic reagents. This prospect is supported by previous studies614,615 that reported the nucleophilic addition to various imidazolium [Im]+ ions to occur on the 2-position C atom. Therefore, O2•− reacted with [Im]+ ions, such as [EMIm]+ and [DMPIm]+, leading to the degradation of ILs.283 The charge on the 2-position C atom was higher in [DMPIm]+ than that in [EMIm]+, indicating that the addition of the methyl group to the 2-position C atom resulted in the increase in the reactivity of the nucleophilic reagents toward [Im]+ ions. The substituents attached to the cations of ILs have significant effects on the properties of ILs, particularly the physical properties. For example, a longer alkyl chain indicates more hydrophobicity of the IL.616,617 In addition, viscosity of an IL increases with an increase in the alkyl chain length and/or fluorination of the cationic component because of an increase in van der Waals interactions.282,618 For instance, the solubility of O2 in 1-hexyl-1-methyl-pyrrolidinium [HMPyrr]+-based IL is higher than that in [BMPyrr]+-based IL (14.5 and 9.1 mM at 25 °C, respectively; Scheme 19).27,452 This is in accordance

Figure 12. CAs for O2 reduction at carbon fiber ultramicroelectrode in O2-saturated trihexyl(tetradecyl)phosphonium tris(pentafluoroethyl)trifluorophosphate [P14,666][TPTP] at 25, 35, and 45 °C. (Reprinted with permission from ref 43. Copyright 2012 Elsevier.)

followed a reasonable procedure to purify ILs by mixing with diethyl ether, followed by washing with 10% NaOH solution. The organic phase was then washed many times with H2O until the pH of the H2O phase was neutral. The remaining H2O in the organic phase was then removed using anhydrous sodium sulfate. After filtration, a rotary evaporator can be used to evaporate the organic phase, followed by maintaining the sample under vacuum for 2 days to ensure a low H2O content. In addition to the redox peaks of the O2/O2•− redox couple, small precathodic peaks after O2 sparging have been observed in some CV scans in several studies (Figure 13).43,584,585 These

Scheme 19. Structures of [HMPyrr]+ and [BMPyrr]+

with the data reported by Chapeaux et al.619 They found that the mutual solubilities increased with an increase in the alkyl chain length of the cation. By contrast, the diffusion coefficient of O2 in [HMPyrr]+ is less than that in [BMPyrr]+ (2.5 × 10−10 and 5.9 × 10−10 m2/s at 25 °C, respectively). In addition, a previous study reported that O2•− had a higher consumption percentage of 17.9% and a rate of 4.049 × 10−3 mM/min in [BMPyrr]+ than in [HMPyrr]+ (consumption percentage of 13.3% and a rate of 3.334 × 10−3 mM/min).620 This confirms that increasing the alkyl chain length has a significant effect on the O2•− stability. However, this conclusion is still at an early stage, and further investigations are necessary to tailor cations such that they are compatible with the needs of O2•− reactions. 6.1.3.2. Effects of Anions. Anions might have an indirect effect because anions can influence chemical reactions and their properties.621 Huang et al.109 and Hayyan et al.620 indicated that varying the anions may cause a minimal variation in the limiting currents and kinetics. However, the anions have an effect on the hydrophobicity and hydrophilicity of ILs. Therefore, the careful selection of anions comprising hydrophobic ILs may lead to the avoidance of disproportionation thus resulting in a more stable O2•−. 6.1.3.3. Effect of the Physical Properties of ILs on O2•−. Properties, such as viscosity, conductivity, O2 diffusion coefficient, solubility, and individual ion diffusion coefficients, play a primary role in the ORR mass transport kinetics, current density, and number of electrons exchanged.592 Figure 12 shows that an increase in temperature causes the steady-state current of the CA to increase. This results from the decrease in the IL viscosity and subsequent increase in O2 diffusivity. In addition, similar to aprotic solvents, impurities in ILs play a major role in O2•− stability. Recently, Pozo-Gonzalo et al.592

Figure 13. CVs in [MOEMPip][TPTP] after sparging O2 as a function of temperature at the glassy carbon (GC) macroelectrode. Sweep rate: 100 mV/s. (Reprinted with permission from ref 43. Copyright 2012 Elsevier.)

peaks have been ascribed to the irremovable unknown trace impurities present in ILs. However, no small peaks were observed close to −1 V in background screening after N2 sparging, thus confirming that no detectable impurities can affect O2 reduction.43 These impurities are speculated to be electrochemically inert but reactive after O2 reduction, thus obtaining detectable active compounds. Another possibility is the occurrence of heterogeneous reactions or cation adsorption 3058


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on the surface of the working electrode.43,110,622 Therefore, the occurrence of these precathodic peaks needs to be further explored. This can help in designing more appropriate ILs as media for generating a stable O2•−.

dicyanamide, [BMPyrr][TFSI], 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate, 1-hexyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide [HMPyrr][TFSI], [MOEMPip][TPTP], and 1-butyl-1-methylpyrrolidinium trifluoroacetate [BMPyrr][TFA]. Furthermore, the highest O2•− stability was observed in [MOEMMor][TFSI] and [N112,1O2][TFSI], and the pseudo-second-order rate constants of O2•− were 0.2 × 10−2 and 0.1 × 10−2 M−1 s−1, respectively.620 6.2.1. Phosphonium-Based ILs. Figure 15 shows cyclic voltammograms for O2 reduction in the quaternary

6.2. Long-Term Stability of O2•−

Although many researchers118,230,283,584,623 have agreed that O2•− can be generated in ILs, the O2•− stability is still being explored. All previous studies have presented new potentials for modifying the reactivity of this critical intermediate within the discussed media. The primary characteristics of O2•− that have been studied are basicity and nucleophilicity. In most of the previous studies on ILs,109,112,279,284,584,586 CV was used to investigate the generation and stability of O2•−. However, this does not indicate the actual stability because CV experiments can last from a few seconds to a few minutes, for example, 90 s for 100 mV/s, ranging from 2 to −2.5 V. In contrast, recent studies118,230,288,291,624 have observed the long-term stability of O2•− to investigate the possible reactions of O2•− with various types of ILs. Exploration of reactions at low sweep rates is more feasible because a long-term voltammetric scale provides a better reaction visibility.625 Hence, the long-term stability test is paramount to employ O2•− in diverse applications as major industrial applications are continuous processes. A significant decrease in absorbance was recorded for O2•− stability in DMSO containing triethylsulfonium bis(trifluoromethylsulfonyl)imide,1,3-dimethyl-imidazolium methylsulfate, 1-butyl-2,3-dimethylimidazolium trifluoromethylsulfonate, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIm][TFSI], 1-ethyl-3-methylimidazolium methylsulfate [EMIm][MS], [P14,666][TPTP], trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide [P14,666][TFSI], and pyridinium-based ILs (Figure 14).43,291,595,620 This indicated

Figure 15. Cyclic voltammograms of O2•− generation in [P14,666][TFSI] at 25 °C with a sweep rate of 100 mV/s. (Reprinted with permission from ref 230. Copyright 2012 Elsevier.)

phosphonium-based IL [P14,666][TFSI].230 The forward peak (i.e., reduction) of the cyclic voltammogram of [P14,666][TFSI] was similar to that of quaternary ammonium-based ILs. However, the backward peak (i.e., oxidation) was broader and further shifted from the initial forward peak, thus indicating that the quaternary phosphonium ion reacted with O2•− because the phosphonium ion was the most probable proton source. Therefore, a mechanism that involves R-proton abstraction from the quaternary phosphonium ion by O2•− (eq 70) was proposed.584 +

O2•− + R3PCH 2R′ → HO2• + R3PCHR′

(70)

Equation 71 shows a potential outcome for the protonated O2•− form, which is to be disproportionated, thereby yielding H2O2 with subsequent O2 regeneration: 2HO2• → H 2O2 + O2

•−

Figure 14. Change in the O2 absorbance peak with time in [EMIm][MS]. (Reprinted with permission from ref 620. Copyright 2015 Springer.)

(71)

The presence of a weak acidic trihexyl(tetradecyl)phosphonium cation [P14,666]+ renders O2•− unstable in phosphonium-based ILs because of the formation of a phosphorus ylide and HO2• and the partial regeneration of O2 resulting from follow-up homogeneous reactions.584 On the other hand, Pozo-Gonzalo et al.591 reported the formation of a stable O2•− in trihexyl(tetradecyl)phosphonium chloride in the presence of H2O. They observed a chemically reversible O2/O2•− redox couple rather than the disproportionation reaction. This stability was attributed to a strong ion pairing between the phosphonium ion and O2•−. Pozo-Gonzalo et al.592 in a subsequent study stated that the O2•− instability in [P14,666]+ previously observed by Evans et al.584 was due to the residual

that the generated O2•− was unstable. This is potentially due to the reaction between the cation and O2•−. In contrast, a slight gradual decrease in the intensity of the O2•− absorbance peak with time was observed in DMSO containing N-ethyl-N,Ndimethyl-2-methoxyethylammonium bis(trifluoromethylsulfonyl)imide [N112,1O2][TFSI], N-methoxyethyl-N-methylmorpholinium bis(trifluoromethylsulfonyl)imide [MOEMMor][TFSI], 1-(3-methoxypropyl)-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide [MOPMPip][TFSI], 1-butyl-1-methylpyrrolidinium 3059


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inhibition.629,630 In addition, they possess interesting biological activities. In a subsequent study, Feroci et al.631 investigated CO2 activation by O2•− generated electrochemically in ILs to yield the carboxylating reagent C2O62− at a less negative potential (i.e., − 1.4 V versus Ag) than the direct CO2 cathodic reduction (i.e., − 2.4 V versus Ag). Based on the reaction of O2•− with the IL (i.e., 2-imidazolone), the reaction between O2•− and CO2 was concluded to be relatively selective. Therefore, [Im]+-based ILs are not potential media for applications involving O2•−, and thus other ILs can be a better choice as media for O2•− generation and applications.

acidic impurities present in the IL. However, longer-term stability is necessary to validate the short-term stability. 6.2.2. Imidazolium-Based ILs. Previous studies112,118,284 have shown that O2•− was unstable in [Im]+-based ILs. A number of researchers626−628 have previously reported that there was a reaction between O2•− and [Im]+. This reaction was ascribed to ion pairing, which may cause an attack on C-2 of [Im]+-based ILs by the nucleophilic O2•−, thereby forming a complex or a new product.118 In addition, [Im]+ ions can be attacked by nucleophilic reagents, such as cyanide and hydroxide ions.614,615 Therefore, [Im]+ is expected to react with O2•− because it is a strong nucleophilic reagent.283 The UV−vis absorbance spectrum of O2•− generated in [EMIm][TFSI] shows a significant decrease in absorbance, and no new bands were formed (Figure 16).288,624 This indicates

6.3. Applications of O2•− Generated in ILs

The superoxide ion generated electrochemically was investigated112 in the following two solvent systems: (1) pressurized O2 + CO2 + AcN with TEAP as a supporting electrolyte and (2) O2 + CO2 + [BMIm][HFP]. The results showed that in both solvent systems CO2 and O2•− reacted to yield C2O62−. A similar study631 was conducted to electrochemically activate CO2 in O2/CO2-saturated ILs by using O2•−. This activation was used for bond formation between C and N from amines and CO2 in the organic carbamate synthesis (Scheme 21). The reduction of O2 at the Au working electrode in the presence of CO2 was investigated using [EMIm][TFSI] as an electrolyte.565 The proposed mechanism of the reaction between CO2 and O2•− is illustrated in eqs 72−76. The nucleophilic addition mechanism of O2•− to CO2 requires oneelectron heterogeneous transfer to O2 at the electrode surface. Following this, two homogeneous chemical steps occur, leading to either the further transfer of electron or disproportionation. However, again, [Im]+-based ILs are not suitable media for such reactions.

Figure 16. UV−vis spectrum of O2•− chemically generated in DMSO in the presence of [EMIm][TFSI]. (Reprinted with permission from ref 288. Copyright 2010 Elsevier.)

that the generated O2•− was unstable, thus confirming its reaction. By contrast, a previous study118 reported that the addition of 1-n-butyl-2,3-dimethylimidazolium tetrafluoroborate to DMSO caused the appearance of a new band with λmax > 300 nm. In addition, it caused the O2•− band, which formed an isosbestic point at 305 nm, to disappear. The new band in the spectrum is potentially attributable to the hydrolysis of [BF4]−, which produced HF that reacted with O2•−. However, further analyses are required for a better understanding. AlNashef et al.288,624 successfully identified 2-imidazolone as the product, which was formed by the reaction of O2•− with the [Im]+-based IL under ambient conditions (Scheme 20). Table 9 lists the results of the reactions of O2•− with various types of [Im]+-based ILs. The 2-imidazolones are useful as intermediates for pharmaceutical compounds, agrochemicals and polymers, and they contribute in malignant tumor growth

2O2 + 2e− ⇌ 2O2•−

(72)

O2•− + CO2 → CO4•−

(73)

CO4•− + CO2 → C2O6•−

(74)

C2O6•− + O2•− → C2O6 2 − + O2

(75)

overall: O2 + 2CO2 + 2e− → C2O6 2 −

(76)

O2•−

Next, undergoes a nucleophilic reaction with labile acidic H protons (not H atoms; this occurs in a radical abstraction reaction) on reactive substrates R to form substrate carbanions (R−) and HO2•.632,633 The outcome of HO2• production ranges from dimerization, and subsequent dismutation, that forms H2O2 and O2 to the radical reaction with an additional substrate or even possibly the IL component. The formation of H2O2 by dismutation indicates that the oxidation of an additional organic substrate occurs through H2O2 and not O2•−. Martiz et al.607 showed that the electrogeneration of diorganylsilanones from difunctional precursors in the presence of hexamethyldisiloxane or hexamethylcyclotrisiloxane in selected ILs had a high O2 solubility, and were inert toward O2•−. This allows functionalized siloxanes to be produced selectively in suitable isolated yields. Later, Villagrán et al.596 investigated the bulk electrolysis of O2-saturated [BMPyrr][TFSI] or [EMIm][TFSI] solutions containing 0.31 M phenol or 4-tert-butylphenol at potentials at which O2•− and peroxide

Scheme 20. Reaction of O2•− with [Im]+ (Reprinted with Permission from Ref 288. Copyright 2010 Elsevier.)

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Table 9. 2-Imidazolones Obtained from the Reaction of O2•− with [Im]+-Based ILs (Reprinted with Permission from Ref 288. Copyright 2010 Elsevier.) entry

ionic liquid

product

1 2 3

1-butyl-3-methylimidazolium hexafluorophosphate 1-ethyl-3-methylimidazolium ethylsulfate 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

1-butyl-3-methyl-2-imidazolone 1-ethyl-3-methyl-2-imidazolone 1-ethyl-3-methyl-2-imidazolone

4 5 6 7

1-hexyl-3-methylimidazolium chloride 1-octyl-3-methylimidazolium chloride 1,3-dimethylimidazolium diphosphate 1,3-dimethylimidazolium trifluoromethanesulfonate

1-hexyl-3-methyl-2-imidazolone 1-octyl-3-methyl-2-imidazolone 1,3-dimethyl-2-imidazolone 1,3-dimethyl-2-imidazolone

yield % (mg) 95 97 98 96 96 96 97 97

(1.1) (0.9) (0.8) (723) (1.3) (0.7) (0.7) (0.8)

purity of isolated product (%) 97 97 98 98 98 98 98 98

Scheme 21. Subsequent Reactions of O2•− with CO2 and [Im]+ (Reprinted with Permission from Ref 631. Copyright 2011 Elsevier.)

are generated, that is, −1.35 and −2.05 V, respectively. The corresponding phenyl triflate was formed through electrolysis at −2.05 V. The standard electron-reservoir complex [FeICp-η6-C6Me6)],1,1 can be used conveniently with air to deprotonate weak acids following intermediation by O2•−.627 This was proven when [Im]+ salts were deprotonated, thereby generating norbornene-supported N-heterocyclic carbenes. Synthesis of H2O2 by applying O2 reduction is also a potential application. In a previous study, H2O2 was electrosynthesized using O2 reduction in H2O-containing ILs, such as 1-n-butyl-3-methylimidazolium tetrafluoroborate [BMIm][BF4].606 Furthermore, H2O2 could subsequently be used in situ for alkene epoxidation (Scheme 22). Islam et al.634 applied the redox reaction of the O2/O2•− redox couple to remove H2O. They ascribed the electrolysis of H2O to cathodic and anodic reactions that are recognized as hydrogen evolution reaction (HER) and oxygen evolution reaction (OER; eqs 77 and 78). These reactions were conducted at Au, Pt, and GC

electrodes in N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate and N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide. In these reactions, O2•− is known to react with H2O through a concerted proton transfer mechanism to form HO2−. 2H 2O → O2 + 4H+ + 4e− 4H 2O + 4e− → 2H 2 + 4OH−

(anodic reaction)

(77)

(cathodic reaction) (78)

The O2•− stability in ILs makes O2 detection possible. Therefore, recently, new IL-based sensors for O2 detection were introduced. For instance, Baltes et al.594 reported that the detection limit can be as low as 5 ppm at 25 °C, indicating that the sensor had a high sensitivity and a rapid response time. Toniolo et al.635 fabricated an oxygen amperometric gas sensor based on the electrocatalytic reduction of O2 in ILs. The advantages of using ILs increased by the addition of a small amount of a further low-melting salt bearing a quinone moiety. This allows the reduction of O2 to occur through an electrocatalytic pathway at a lower potential compared with direct reduction. A solid-state O2 gas sensor was fabricated on the basis of porous polyethylene 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIm][BF4]-supported membrane,635−637 and this sensor had a high sensitivity, excellent reproducibility, and wide detection range. However, the performance of these modified IL-based sensors is dependent on the stability and response of the O2/O2•− redox couple. In addition, the decomposition temperature of ILs is another factor that plays a primary role in electrochemical high temperature applications. However, the limitations of using ILs, such as slow response and limited decomposition temperature, can be overcome by designing new ILs that have a wide range of conductivity,

Scheme 22. Epoxidation of Alkenes (Reprinted with Permission from Ref 606. Copyright 2005 Royal Society of Chemistry.)

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contrast to the aprotic organic solvents, in the aforementioned IL, the grain growth and assembly were affected significantly by the IL moieties. In this process, the Zn salt was a key parameter in the O2 reduction. This can lead to numerous possibilities for the electrodeposition of ordered structures by nanocrystal assembly. In addition, O2•− generation and stability play a crucial role in this process, thus making this an efficient reaction.

electrochemical window, thermal stability, gas solubility, and O2•− stability. Zigah et al.608 used the O2/O2•− redox couple as a probe to study the effects of the addition of a cosolvent (DMF) to triethylbutylammonium bis(trifluoromethylsulfonyl)imide on the diffusion of charged and neutral species. In addition, Zhao et al.583 showed that degassing with N2 removes the dissolved O2 and a significant amount of the adventitious H2O present in the studied hydrophilic IL media. This was confirmed by studying [BMIm][BF4] containing the redox probe tetracyanoquinodimethane (TCNQ) and 9% (v/v) deliberately added H2O. A 1-h N2 degassing under benchtop conditions resulted in a significant decrease in the TCNQ reduction current obtained under steady-state conditions. This was deduced through a decrease in the TCNQ diffusion coefficient from 2.6 × 10−7 to 4.6 × 10−8 cm2 s−1. The degassing process almost removed the 9% H2O, as confirmed by the Karl Fischer titrator. Recently, a biosensor based on the copper−zinc SOD immobilization in a Au nanoparticle−chitosan−IL (GNP−CS−IL) biocomposite film was fabricated (Figure 17).638 The CV and

Zn 2 + + 2O2•− ⇄ ZnO4 ⇄ ZnO2 + O2 ⇄ ZnO +

3 O2 2 (79)

Siedlecka et al. proposed that O2•− contributed in the degradation of IL residues in H2O using a Fenton-like system. The IL was oxidized in a dilute aqueous solution of 1-butyl-3methylimidazolium chloride, and O2•− behaved as a reducing agent, contributing to IL degradation. Research in this direction is highly essential as it deals with the chemical degradation of poor or nonbiodegradable ILs. Radical species, including O2•−, have powerful capability to interact in such reactions. As discussed in section 4, superoxide salts can be used as a source for breathing. However, the use of O2•− generated in ILs as an O2 source during space flights requires further examination. 6.3.1. Destruction of Hazardous Chemicals by Using O2•− in ILs. Recently, the solubility of various types of CHCs in diverse ILs at different temperatures has been investigated.641,642 The IL structure significantly affected the CHC solubility. Furthermore, the low-temperature oxidation of CHCs could be achieved using O2•− generated in ILs. However, the generated O2•− was unstable in certain ILs, as mentioned earlier. In a related study, Hayyan et al.230,291,600 showed that O2•− generated in ILs is capable of destroying chloroethanes, chlorobenzenes, and chlorophenols; O2•− was generated chemically under ambient conditions by using KO2 in several ILs, such as [MOPMPip][TFSI] and [HMPyrr][TFSI], for the destruction of chlorobenzenes, including pentachlorobenzene, C6Cl6, 1,2-dichlorobenzene, 1,3-dichlorobenzene, and 1,3,5-trichlorobenzene. In addition, HCE was destroyed by O2•− generated in [BMPyrr][TFA] and 2,4-dichlorophenol in [MOEMMor][TFSI]. Furthermore, KO2 was reported144 as an oxidant for ODS. The removal of sulfur ranged between 90 and 99% for benzothiophene and dibenzothiophene. The study showed that KO2 was not only comparable with H2O2 but also potentially superior for ultrasound-assisted or general ODS. However, KO2 was used to destroy sulfur compounds in [BMIm][HFP]. The results showed that [BMIm][HFP] (both cation ([Im]+) and anion (HF−)) was an unsuitable medium for O2•− generation. This is attributable to the reaction between [Im]+ ions and O2•− to produce the corresponding 2-imidazolone. In addition, the use of [HFP]− is undesirable for such reactions because it produces HF when it comes into contact with H2O. However, more analysis and interpretations are required to provide a clear explanation of the mechanism. Based on the in situ generation of H2O2 from O2•−, the authors of this study144 stated that, compared with H2O2, favorable results were obtained when KO2 was used. According to their report,144 the in situ generated H2O2 attained 100% purity, whereas the studied H2O2 obtained only 30% purity. This explains how 7 mmol of KO2 achieved a rate of sulfur removal similar to that of 44 mmol of H2O2, despite the difference between the sulfur:KO2 (1:28) and sulfur:H2O2 (1:180) ratios. Both reactions achieved >98% sulfur removal under identical conditions. 341

Figure 17. SOD/GNP−CS−IL/GCE assembly process. (Reprinted with permission from ref 638. Copyright 2013 Elsevier.)

CA were used to evaluate the biosensor electrochemical performance. For the real-time measurement of O2•−, the biosensor showed excellent selectivity, rapid amperometric response ( k0(Au) > k0(Pt). The reported αc values ranged from 0.35 to 0.47. In general, it was found that specific surface interactions between O2•− and metallic electrode surfaces may produce smaller k0 values than those produced using the GC electrode.35,281 It was also found that k0 values at the GC electrode were smaller than those obtained at the same GC electrode in DMF, DMSO, and AcN solutions containing 0.1 M TEAP. By contrast, these values were comparable with the k0 values obtained in quinoline containing 0.1 M TEAP. This clearly shows that the solvent has an effect on O2/O2•− ORR. The ORR was also investigated using CV on a MWCNT-modified edge plane pyrolytic graphite (EPPG) electrode in the following three ILs: [EMIm][BF4], [PMIm][BF4], and [BMIm][BF4].604 The O2•− oxidation and reduction peak currents increased significantly following the MWCNT modification of the EPPG electrode, and k0 increased substantially. Under identical conditions in [PMIm][BF4], the MWCNT-modified EPPG electrode showed the most satisfactory electrocatalytic activity for the ORR, in which k0 increased from 2.9 × 10−3 to 10.4 × 10−3 cm s−1, whereas k0 increased from 4.3 × 10−3 to 8.3 × 10−3 in [EMIm][BF4] and from 2.3 × 10−3 to 4.2 × 10−3 cm s−1 in [BMIm][BF4]. The increase in k0 can be mainly attributed to the increase in both the electrode surface area and the number of defect sites on the electrode. The difference between the oxidation and reduction peak potentials significantly depended on the electrode substrate, as discussed in section 6.1.2. Previous studies279,602 have reported the broadening of the cyclic voltammograms. Figure 18 shows the reduction peak at an increased negative potential (i.e., − 1.7 V) and an oxidation peak potential (i.e., − 1.3 V) where the voltammetry is more broad on the Pt electrode compared with the Au electrode, with an increased peak separation ΔEp between the reduction and

8. CONCLUSION AND PERSPECTIVE The superoxide ion (O2•−) is one of the primary links between biology and chemistry. It can be generated biologically and chemically and is crucially implicated in numerous reactions. The generation and application of O2•− has attracted considerable attention from scientists in various disciplines in the second half of the last century. However, a limited number of articles in this field have been published since 1991, indicating that there is a real need to highlight this topic to the scientific community. Therefore, this review presents a summary of different generation and detection methods. Numerous remarkable applications of O2•− have also been covered, including in the synthesis of organic compounds, in the destruction of hazardous chemicals in both aqueous and nonaqueous phases, and in the treatment of contaminated soil and groundwater.

Figure 18. Cyclic voltammograms for the reduction of O2 in [N6222][TFSI] at varying scan rates (50, 100, 200, 400, 700, and 1000 mV s−1) on (a) Pt and (b) Au at 35 °C. (Reprinted from ref 279. Copyright 2009 American Chemical Society.) 3066


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For most of the applications reported using nonaqueous media, aprotic solvents were used for O2•− generation. The analytical instruments had some limitations then. Therefore, the exceptional diversity of O2•− implications should be further investigated to validate the outcome of O2•− reactions by using the current facilities of advanced analytical instruments. In addition, new reactions can be identified using the versatile O2•− characteristics by varying the substrate and media. Furthermore, the efficacy of chemically or photochemically or electrochemically generated O2•− as a model for biotherapy and metabolic reactions in bioorganic systems will be of great interest. In the years following 2001, ILs were used as media for generating O2•− because of their unique properties compared with conventional aprotic solvents. These properties include a wide electrochemical window, high stability, nonvolatility, and negligible vapor pressure. The potential of ILs as highly promising media for the generation and reactions of O2•− warrants recognition in various fields. Therefore, ILs should be used, and great efforts should be directed toward exploring the possible biological and chemical applications of O2•−. Although O2•− has a key role in biological systems, the detection methods and mechanisms of O2•− are still not well understood to identify the cause of many diseases and their treatment. A thorough understanding of O2•− chemistry will significantly fulfill the demands of the 21st century.

Malaya Centre for Ionic Liquids, and he is actively involved in the management of the High Impact Research programs. His work on ionic liquids and deep eutectic solvents has gained international recognition. Inas AlNashef graduated with B.Sc. in chemical engineering from the University of Jordan in 1984. He worked as a teaching and research assistant at Kuwait University and United Arab Emirates University, where he participated actively in different research areas. Dr. AlNashef joined King Saud University, Riyadh, Saudi Arabia, after obtaining his Ph.D. from the University of South Carolina in 2004. In 2011, Dr. AlNashef was promoted to associate professor. Dr. AlNashef was very active in research related to green engineering and sustainability and established collaboration with University of Malaya, Kuala Lumpur, Malaysia, where he was a co-advisor for seven Ph.D. students. Dr. AlNashef moved to Abu Dhabi (UAE), where he is now employed as an associate professor in the Department of Chemical and Environmental Engineering at Masdar Institute of Science and Technology. Dr. AlNashef has coauthored more than 60 peer-reviewed journal publications. In addition, he has received seven patents from the U.S. and EU Patent Offices. He is also a recipient of several prestigious awards including the King Abdullah Award for best invention in 2013.

ACKNOWLEDGMENTS The authors acknowledge the University of Malaya HIRMOHE (D000003-16001) and University of Malaya Centre for Ionic Liquids (UMCiL) for their support.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] or [email protected].

NOMENCLATURE

Notes

Abbreviations

The authors declare no competing financial interest.

[BF4]− [BMIm][BF4]

tetrafluoroborate anion 1-n-butyl-3-methylimidazolium tetrafluoroborate [BMIm][HFP] 1-n-butyl-3-methylimidazolium hexafluorophosphate [BMPyrr]+ 1-butyl-1-methylpyrrolidinium [BMPyrr][TFA] 1-butyl-1-methylpyrrolidinium trifluoroacetate [BMPyrr][TFSI] 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [DMPIm]+ 1,2-dimethyl-3-propylimidazolium [EMIm][BF4] 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIm][MS] 1-ethyl-3-methylimidazolium methylsulfate [EMIm][TFSI] 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [HFP]− hexafluorophosphate anion [HMPyrr]+ 1-hexyl-1-methylpyrrolidinium [HMPyrr][TFSI] 1-hexyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [Im]+ imidazolium cation [MOEMMor][TFSI] N-methoxyethyl-N-methylmorpholinium bis(trifluoromethylsulfonyl)imide [MOPMPip][TFSI] 1-(3-methoxypropyl)-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide [MOEMPip][TPTP] 1-(2-methoxyethyl)-1-methylpiperidinium tris(pentafluoroethyl)trifluorophosphate [N6222][TFSI] triethyl-n-hexylammonium bis(trifluoromethylsulfonyl)imide

Biographies Maan Hayyan received his B.Sc. and M.Sc. degrees in chemical engineering from the University of TechnologyBaghdad in 2004 and 2006, respectively. He then obtained his Ph.D. with Distinction from the University of Malaya in 2012 and was awarded the best student award. Later, he joined the University of Malaya Centre for Ionic Liquids (UMCiL) as a senior researcher for one year. In August 2013, Dr. Hayyan was employed as a senior lecturer at the Department of Civil Engineering/University of Malaya under the Environmental Engineering Programme. Dr. Hayyan has 11 filed and granted patents and authored 47 peer-reviewed journal publications in different fields. He has also been a peer reviewer for a number of ISI indexed journals. In addition, he was awarded several gold medals from international exhibitions for research and innovation. His main interests are the generation and reactions of superoxide ion, in addition to applications pertaining to ionic liquids and deep eutectic solvents. Mohd Ali Hashim is on staff at the Department of Chemical Engineering. He joined the University of Malaya as a lecturer in early 1980, became the Head of the Chemical Engineering Department from 1986 to 1990, and was promoted to a full professor in January 1991. During the period 1991−1996, he was the Dean of the Institute of Advanced Studies, and between 2002 and 2005 he was seconded to the Ministry of Science, Technology and Innovation as its Director of Science and Technology. Prof. Ali is considered an expert in solid− fluid interactions, wastewater treatment, and ionic liquids, for which he has received numerous awards, including the National Science Award, Malaysia. He has published widely in top ranking journals, and he holds several patents in his area of expertise. His h-index in ISI Thompson-Reuters is 25. He is currently the Head of the University of 3067


Chemical Reviews [N112,1O2][TFSI] [P14,666]+ [P14,666][TPTP] [PMIm][BF4] [TMHAm][TFSI] [TMHAm]+ 1,4-CHD 3-MIH AcN AFM BMPO BuBr C2O62− C6Cl6 CHC CHP CA Ca(O2)2 CoIIHFPC CRET CV Cytc Cl− DFT DME DMF DMFc DMPO DMSO DNAPL EDTA EF EfOM E0′ EPPG ESR Fe(II)−TPP FTIR GC GNP−CS−IL H2O2 HE HCE HCO4− HO2• HO2− IL IR ISCO k0 KO2 KOH MWCNT MnSOD

Review

N-ethyl-N,N-dimethyl-2-methoxyethylammonium bis(trifluoromethylsulfonyl)imide trihexyl(tetradecyl)phosphonium cation trihexyl(tetradecyl)phosphonium tris(pentafluoroethyl)trifluorophosphate 1-n-propyl-3-methylimidazolium tetrafluoroborate trimethyl-n-hexylammonium bis(trifluoromethylsulfonyl)imide trimethyl-n-hexylammonium 1,4-cyclohexadiene 3-methylindole acetonitrile antiferromagnetic 5-tert-butoxycarbonyl 5-methyl-1-pyrroline-N-oxide butyl bromide peroxy dicarbonate ion hexachlorobenzene chlorinated hydrocarbon catalyzed H2O2 propagations chronoamperometry calcium superoxide cobalt(II) 1,2,3,4,8,9,10,11,15,16,17, 18,22,23,24,25-hexadecafluoro29H,31H-phthalocyanine chemiluminescence resonance energy transfer cyclic voltammetry cytochrome c chloride density functional theory dimethoxyethane dimethylformamide decamethylferrocene 5,5-dimethyl-1-pyrroline-N-oxide dimethyl sulfoxide dense NAPL ethylenediaminetetraacetic acid electro-Fenton effluent organic matter formal potential edge plane pyrolytic graphite electron spin resonance ferrous−tetrapolyphosphate Fourier transform infrared spectroscopy glassy carbon Au nanoparticle−chitosan−IL hydrogen peroxide hydroethidine hexachloroethane peroxymonocarbonate perhydroxyl radical hydroperoxide anion ionic liquid infrared in situ chemical oxidation standard rate constant potassium superoxide potassium hydroxide multiwalled carbon nanotube manganese superoxide dismutase

NaO2 NaOH NaPCP Na6TPP NAPL NBT NCNTs nZVI O2 O2•− O2+ O2•2− O3 O3•− 1 O2 ODS OH− OH• ORRs ROO− PBQ PCBs PCP PFOA pd-FTIRES QD ROS RR SEIRAS SERS SO2 SOD SPEs TBAS TCNQ TEAP TG TiO2 TMAS TOC TPP VUV ZVI 3D-E-Fenton ΔEp

sodium superoxide sodium hydroxide sodium pentachlorophenate sodium tetrapolyphosphate nonaqueous-phase liquid nitro blue tetrazolium nitrogen-doped carbon nanotubes nanoscale ZVI oxygen superoxide ion dioxygen cation peroxide dianion ozone ozonide singlet oxygen oxidative desulfurization hydroxide ion hydroxyl radical O2 reduction reactions peroxy ion 1,4-benzoquinone polychlorinated biphenyls pentachlorophenol perfluorooctanoic acid in situ potential-dependent FTIR emission spectroscopy quantum dot reactive oxygen species resonance Raman surface-enhanced IR reflection absorption spectroscopy surface enhanced Raman scattering sulfur dioxide superoxide dismutase screen-printed electrodes tetrabutylammonium superoxide tetracyanoquinodimethane tetraethylammonium perchlorate thermogravimetry titanium dioxide trimethylphenylammonium superoxide total organic carbon tetrapolyphosphate vacuum ultraviolet zerovalent iron three-dimensional EF system peak potential separation

Greek Symbol

αc cathodic transfer coefficient

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