Spontaneous modification of glassy carbon surface with anthraquinone from the solutions of its diazonium derivative: An oxygen reduction study

Anthraquinones-based photocatalysis: A comprehensive review

In recent years, there has been significant interest in photocatalytic technologies utilizing semiconductors and photosensitizers responsive to solar light, owing to their potential for energy and environmental applications. Current efforts are focused on enhancing existing photocatalysts and developing new ones tailored for environmental uses. Anthraquinones (AQs) serve as redox-active electron transfer mediators and photochemically active organic photosensitizers, effectively addressing common issues such as low light utilization and carrier separation efficiency found in conventional semiconductors. AQs offer advantages such as abundant raw materials, controlled preparation, excellent electron transfer capabilities, and photosensitivity, with applications spanning the energy, medical, and environmental sectors. Despite their utility, comprehensive reviews on AQs-based photocatalytic systems in environmental contexts are lacking. In this review, we thoroughly describe the photochemical properties of AQs and their potential applications in photocatalysis, particularly in addressing key environmental challenges like clean energy production, antibacterial action, and pollutant degradation. However, AQs face limitations in practical photocatalytic applications due to their low electrical conductivity and solubility-related secondary contamination. To mitigate these issues, the design and synthesis of graphene-immobilized AQs are highlighted as a solution to enhance practical photocatalytic applications. Additionally, future research directions are proposed to deepen the understanding of AQs' theoretical mechanisms and to provide practical applications for wastewater treatment. This review aims to facilitate mechanistic studies and practical applications of AQs-based photocatalytic technologies and to improve understanding of these technologies.

Anthraquinone-2-sulfonate immobilized to conductive polypyrrole hydrogel as a bioanode to enhance power production in microbial fuel cell

In this study, anthraquinone-2-sulfonate (AQS), a redox mediator, was covalently bound to conductive polypyrrole hydrogel (CPH) via electrochemical reduction of the in-situ-generated AQS diazonium salts. The porous structure and hydrophilic surface of this CPH/AQS anode enhanced biofilm formation while the AQS bound on the CPH/AQS anode worked as a redox mediator. The CPH/AQS bioanode reduced the charge transfer resistance from 28.3Ω to 4.1Ω while increased the maximum power density from 762±37mW/m² to 1919±69mW/m², compared with the bare anode. These results demonstrated that the facile synthesis of the CPH/AQS anode provided an efficient route to enhance the power production of microbial fuel cell (MFC).

Superoxide Ion: Generation and Chemical Implications

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-of-the-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.

Electrocatalytic behavior of freely-diffusing and immobilized synthetic flavins in aqueous media

The electrocatalytic activity of three synthetic flavins toward the oxygen reduction reaction was compared to establish structure–property relationships.

Bifunctional redox tagging of carbon nanoparticles

Despite extensive work on the controlled surface modification of carbon with redox moieties, to date almost all available methodologies involve complex chemistry and are prone to the formation of polymerized multi-layer surface structures. Herein, the facile bifunctional redox tagging of carbon nanoparticles (diameter 27 nm) and its characterization is undertaken using the industrial dye Reactive Blue 2. The modification route is demonstrated to be via exceptionally strong physisorption. The modified carbon is found to exhibit both well-defined oxidative and reductive voltammetric redox features which are quantitatively interpreted. The method provides a generic approach to monolayer modifications of carbon and carbon nanoparticle surfaces.

Application of voltammetric techniques at microelectrodes to the study of the chemical stability of highly reactive species

The application of voltammetric techniques to the study of chemical speciation and stability is addressed both theoretically and experimentally in this work. In such systems, electrode reactions are coupled to homogeneous chemical equilibria (complexations, protonations, ion associations, ...) that can be studied in a simple, economical, and accurate way by means of electrochemical methods. These are of particular interest when some of the participating species are unstable given that the generation and characterization of the species are performed in situ and on a short time scale. With the above aim, simple explicit solutions are presented in this article for quantitative characterization with any voltammetric technique and with the most common electrode geometries. From the theoretical results obtained, it is pointed out that the use of square-wave voltammetry in combination with microelectrodes is very suitable. Finally, the theory is applied to the investigation of the ion association between the anthraquinone radical monoanion and the tetrabutylammonium cation in acetonitrile medium.

Spontaneous modification of graphite anode by anthraquinone-2-sulfonic acid for microbial fuel cells

In this study, anthraquinone-2-sulfonic acid (AQS), an electron transfer mediator, was immobilized onto graphite felt surface via spontaneous reduction of the in situ generated AQS diazonium cations. Cyclic voltammetry (CV) and energy dispersive spectrometry (EDS) characterizations of AQS modified graphite demonstrated that AQS was covalently grafted onto the graphite surface. The modified graphite, with a surface AQS concentration of 5.37 ± 1.15 × 10(-9)mol/cm(2), exhibited good electrochemical activity and high stability. The midpoint potential of the modified graphite was about -0.248 V (vs. normal hydrogen electrode, NHE), indicating that electrons could be easily transferred from NADH in bacteria to the electrode. AQS modified anode in MFCs increased the maximum power density from 967 ± 33 mW/m(2) to 1872 ± 42 mW/m(2). These results demonstrated that covalently modified AQS functioned as an electron transfer mediator to facilitate extracellular electron transfer from bacteria to electrode and significantly enhanced the power production in MFCs.

Solution effect on diazonium-modified Au(111): reactions and structures

Surface modifications of a Au(111) electrode with 4-bromobenzenediazonium tetrafluoroborate (BBD) in acetonitrile (ACN) and 0.1 M HClO4 have been characterized by scanning tunneling microscopy (STM). In ACN, STM results reveal the formation of disordered thin organic films. The involvement of the radical as an intermediate is evidenced by the negative effect of radical scavengers on organic thin film formation. In contrast, the 4,4'-dibromobiphenyl monolayer is observed when the aqueous solution is used as a medium to carry out the grafting experiment. The biphenyl compound is considered to be generated by a radical-radical coupling reaction.

Electrochemical Reduction of Oxygen on Anthraquinone/Carbon Nanotubes Nanohybrid Modified Glassy Carbon Electrode in Neutral Medium

The electrochemical behaviors of monohydroxy-anthraquinone/multiwall carbon nanotubes (MHAQ/MWCNTs) nanohybrid modified glassy carbon (MHAQ/MWCNTs/GC) electrodes in neutral medium were investigated; also reported was their application in the electrocatalysis of oxygen reduction reaction (ORR). The resulting MHAQ/MWCNTs nanohybrid was characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). It was found that the ORR at the MHAQ/MWCNTs/GC electrode occurs irreversibly at a potential about 214 mV less negative than at a bare GC electrode in pH 7.0 buffer solution. Cyclic voltammetric and rotating disk electrode (RDE) techniques indicated that the MHAQ/MWCNTs nanohybrid has high electrocatalytic activity for the two-electron reduction of oxygen in the studied potential range. The kinetic parameters of ORR at the MHAQ/MWCNTs nanohybrid modified GC electrode were also determined by RDE and EIS techniques.

Edge plane pyrolytic graphite electrode covalently modified with 2-anthraquinonyl groups: theory and experiment

An edge plane pyrolitic graphite (EPPG) electrode was modified by electrochemical reduction of anthraquinone-2-diazonium tetrafluoroborate (AQ2-N(2)(+)BF(4)(-)), giving an EPPG-AQ2-modified electrode of a surface coverage below a monolayer. Cyclic voltammograms simulated using Marcus-Hush theory for 2e(-) process assuming a uniform surface gave unrealistically low values of reorganisation energies, λ, for both electron transfer steps. Subsequently, two models of surface inhomogeneity based on Marcus-Hush theory were investigated: a distribution of formal potentials, E', and a distribution of electron tunneling distances, r(0). The simulation of cyclic voltammograms involving the distribution of formal potentials showed a better fit than the simulation with the distribution of tunneling distances. Importantly the reorganization energies used for the simulation of E' distribution were similar to the literature values for adsorbed species.

Ferrocene-functionalized carbon nanoparticles

Carbon nanoparticles were synthesized from natural gas soot and functionalized with ferrocenyl moieties by using 4-ferrocenylphenyldiazonium as the reactive precursor. The incorporation of the ferrocenyl units onto the carbon nanoparticle surface was confirmed by varied spectroscopic measurements. For instance, in FTIR measurements the characteristic vibrational bands of the ferrocenyl and phenyl moieties could be clearly identified. XPS measurements showed that there were approximately 31.9 ferrocenyl units per nanoparticle. UV-vis spectroscopic measurements displayed an absorption band at ca. 465 nm which was consistent with the optical characteristics of ferrocenyl derivatives. Furthermore, with surface functionalization by the ferrocenyl moieties, the photoluminescence of the carbon nanoparticles was found to diminish in intensity and red-shift in energy with the addition of NOBF(4). This was accounted for by the formation of varied electron-accepting moieties on the particle surface, such as positively charged ferrocenium, quinone-like derivatives, and nitrosation of the aromatic rings of the graphitic cores. Interestingly, in electrochemical studies the nanoparticle-bound ferrocenyl moieties were found to exhibit two pairs of voltammetric waves with a difference of their formal potentials at about 78 mV, suggesting nanoparticle-mediated intraparticle charge delocalization at mixed valence as a result of the strong core-ligand covalent bonds and the conductive sp(2) carbon matrix of the graphitic cores. Consistent behaviors were observed in near-infrared measurements, indicating that the particles behaved analogously to a Class I/II mixed-valence compound.

Direct electrochemistry and electrocatalysis of anthraquinone-monosulfonate/polyaniline hybrid film synthesized by a novel electrochemical doping-dedoping-redoping method on pre-activated spectroscopically pure graphite surface

Polyaniline (PANI) film in the form of emeraldine salt (ES) doped with anthraquinone-2-sulfonate (AQS) was synthesized by a novel electrochemical doping-dedoping-redoping method on pre-activated spectroscopically pure graphite electrode surface. SEM showed the highly porous microstructure with sponge-like morphology of the AQS/PANI hybrid film, which has more available active sites for facilitating electron transfer and energy efficiency of redox reactions. FTIR and UV-vis spectra demonstrated the incorporation of AQS into the conductive PANI matrix. Cyclic voltammetric (CV), electrochemical impedance spectroscope (EIS), rotating ring-disk electrode (RRDE) and chronoamperometry (CA) techniques indicated that the AQS/PANI composite has high electrocatalytic activity and remarkable stability for the two-electron reduction of oxygen via an electrochemical-chemical mechanism at the base of the porous PANI film. The acid centers of PANI (protonated imine group) played an important role not only in two internal redox transform processes, but also in an external charge transfer reaction during the electrocatalytic reduction of oxygen. The incorporation of anionic AQS groups into PANI matrix to prepare electroactive PANI using the electrochemical doping-dedoping-redoping method is conceptually new, and may be extended to the development of new functional materials from many other conducting polymers and quinonoid compounds for wide applications in catalysis, sensors, molecular electronics, and so on.