Why Does 2,3,5,6-Tetrachlorophenol Generate the Strongest Intrinsic Chemiluminescence among All Nineteen Chlorophenolic Persistent Organic Pollutants during Environmentally-friendly Advanced Oxidation Process?

Molecular mechanism for the unusual enhancement of the second-step chemiluminescence production from the carcinogenic tetrabromohydroquinone and H2O2

We have found recently that two-step intrinsic hydroxyl radical (·OH)-dependent chemiluminescence (CL) could be produced by carcinogenic tetrahaloquinone and H2O2. However, the first-step CL was too fast to clearly detect the stepwise generation of ·OH and CL, and to distinguish the exact dividing point between the first-step and second-step CL. Here we found that, extremely clear two-step intrinsic CL could be produced by the relative slow reaction of tetrabromohydroquinone (TBHQ) with H2O2, which was directly dependent on the two-step ·OH generation. Interestingly, the second-step, but not the first-step CL production of TBHQ/H2O2 (CRET donor) was markedly enhanced by fluorescein (a typical xanthene dye, CRET acceptor) through a unique chemiluminescence resonance energy transfer (CRET) process. The novel CRET system of TBHQ/H2O2/fluorescein was successfully applied for the sensitive detection of TBHQ with the detection limit as low as 2.5 µmol/L. These findings will help to develop more sensitive and highly efficient CL or CRET systems and specific CL sensor to detect the carcinogenic haloquinones, which may have broad environmental applications.

Electrocatalytic hydro-dehalogenation of halogenated organic pollutants from wastewater: A critical review

Halogenated organic pollutants are often found in wastewater effluent although it has been usually treated by advanced oxidation processes. Atomic hydrogen (H*)-mediated electrocatalytic dehalogenation, with an outperformed performance for breaking the strong carbon-halogen bonds, is of increasing significance for the efficient removal of halogenated organic compounds from water and wastewater. This review consolidates the recent advances in the electrocatalytic hydro-dehalogenation of toxic halogenated organic pollutants from contaminated water. The effect of the molecular structure (e.g., the number and type of halogens, electron-donating or electron-withdrawing groups) on dehalogenation reactivity is firstly predicted, revealing the nucleophilic properties of the existing halogenated organic pollutants. The specific contribution of the direct electron transfer and atomic hydrogen (H*)-mediated indirect electron transfer to dehalogenation efficiency has been established, aiming to better understand the dehalogenation mechanisms. The analyses of entropy and enthalpy illustrate that low pH has a lower energy barrier than that of high pH, facilitating the transformation from proton to H*. Furthermore, the quantitative relationship between dehalogenation efficiency and energy consumption shows an exponential increase of energy consumption for dehalogenation efficiency increasing from 90% to 100%. Lastly, challenges and perspectives are discussed for efficient dehalogenation and practical applications.

Mechanistic Investigation of H2 O2 -dependent Chemiluminescence from Tetrabromo-1,4-Benzoquinone

As a H₂ O₂ -dependent bioluminescent substrate, tetrabromo-1,4-benzoquinone (TBBQ) was first isolated from acorn worm. The mechanism of chemiluminescence (CL) corresponding to the bioluminescence (BL) of acorn worm is largely unknown, let alone the mechanism of BL. In this article, we firstly studied the chemical and physical processes, and mechanism of H₂ O₂ -dependent CL from TBBQ by theoretical and experimental methods. The research results indicate: the CL process is initiated by a nucleophilic substitution reaction, which leads to the formation of an anionic dioxetane through five consecutive reactions; the anionic dioxetane decomposes to the first singlet excited state (S₁ ) via a conical interaction of the potential energy surfaces (PESs) between the ground (S₀ ) and S₁ state; the anionic S₁ -state changes to its neutral form by a proton transfer from the solvent and this neutral product is assigned as the actual luminophore. Moreover, the experimental detection of CL, ^. OH and the identifications of 2,3-dibromo maleic acid and 2-bromo malonic acid as the major final products provide direct evidence of the theoretically suggested mechanism. Finally, this study proves that the activity of the H₂ O₂ -dependent CL from TBBQ is significantly lower than the one from tetrachloro-1,4-benzoquinone (TCBQ), which is caused by the weaker electron withdrawing effect and the stronger heavy atomic effect of bromine.

Transcriptomic analysis of adult zebrafish heart and brain in response to 2, 6-dichloro-1, 4-benzoquinone exposure

Halobenzoquinones (HBQs) are emerging and widespread disinfection byproducts (DBPs), but their toxicological mechanisms to aquatic organisms remain elusive. Herein, we evaluated oxidative stress, cardiac toxicity, and cerebral toxicity after 2, 6-dichloro-1, 4-benzoquinone (2,6-DCBQ) exposure in zebrafish. Adult zebrafish were respectively exposed to 0.25, 0.5, and 1 μM 2,6-DCBQ for 96 h. The mortality rate of 2,6-DCBQ (1 μM) was 10%, while the LC50 value was 1.532 μM. Besides, 2,6-DCBQ exposure caused irregularity and elimination of myocardial fiber in the heart, and the pyknosis of nuclears and the agglutination of chromatin in the brain. We measured the 2,6-DCBQ-induced oxidative stresses in the heart and brain. Additionally, the glutathione (GSH) content, superoxide dismutase (SOD) activity, catalase (CAT) activity, and total antioxidant capacity (T-AOC) were significantly inhibited. To better understand the potential toxicity of 2,6-DCBQ, transcriptomic analysis was performed in the control and 1 μM group after 96 h exposure. As a result, 545 and 1228 differentially expressed genes (DEGs) were detected in the heart and brain, respectively. GO analysis revealed that these DEGs were primarily enriched in blood vessel development, vasculature development, and oxidoreductase activity in the heart; response to stimulus, nervous system development, and oxidoreductase activity in the brain. KEGG enrichment analysis indicated that the DEGs were mainly enriched in VEGF signaling pathway and vascular smooth muscle contraction pathway in the heart; neuroactive ligand-receptor interaction, and NOD-like receptor signaling pathway in the brain. These findings exposed the underlying toxicity mechanism of 2,6-DCBQ exposure on zebrafish cardiovascular and brain systems.

Detecting and Quantifying Polyhaloaromatic Environmental Pollutants by Chemiluminescence-Based Analytical Method

Polyhaloaromatic compounds (XAr) are ubiquitous and recalcitrant in the environment. They are potentially carcinogenic to organisms and may induce serious risks to the ecosystem, raising increasing public concern. Therefore, it is important to detect and quantify these ubiquitous XAr in the environment, and to monitor their degradation kinetics during the treatment of these recalcitrant pollutants. We have previously found that unprecedented intrinsic chemiluminescence (CL) can be produced by a haloquinones/H₂O₂ system, a newly-found ^●OH-generating system different from the classic Fenton system. Recently, we found that the degradation of priority pollutant pentachlorophenol by the classic Fe(II)-Fenton system could produce intrinsic CL, which was mainly dependent on the generation of chloroquinone intermediates. Analogous effects were observed for all nineteen chlorophenols, other halophenols and several classes of XAr, and a novel, rapid and sensitive CL-based analytical method was developed to detect these XAr and monitor their degradation kinetics. Interestingly, for those XAr with halohydroxyl quinoid structure, a Co(II)-mediated Fenton-like system could induce a stronger CL emission and higher degradation, probably due to site-specific generation of highly-effective ^●OH. These findings may have broad chemical and environmental implications for future studies, which would be helpful for developing new analytical methods and technologies to investigate those ubiquitous XAr.

Mechanistic Study on Chemiluminescence of Chloranilic Acid by Co(II)-Mediated Fenton-like System

Reacting with H₂O₂, tetrachloro-1,4-benzoquinone (TCBQ) produces chemiluminescence (CL), but chloranilic acid (CA), the dihydroxylation product of TCBQ, does not. However, an unprecedented strong CL generates from CA/H₂O₂ in the presence of Co(II). Why? We performed quantum chemical calculations on the entire reaction process of CA/H₂O₂ and CA/H₂O₂/Co(II) systems. The computational results indicate: for CA/H₂O₂ system, the reason leading to non-CL as: there is no free ^•OH produced by CA/H₂O₂, which prevents the subsequent reaction from taking place; for CA/H₂O₂/Co(II) system, the chemical process resulting in the CL as: First, a neutral dioxetane is formed via six sequential reactions. Then, the neutral dioxetane decomposes to generate a neutral excited-state (S₁) product via a gradually reversible charge transfer initiated luminescence mechanism. A conical intersection of the ground and the S₁-state potential energy surfaces facilitates the production of the S₁-state product. Ultimately, the neutral S₁-state product emits light as a practical light emitter. The key component for forming dioxetane and the following CL is the intrinsically generated ^•OH, which is roaming around at the region of C₂ atoms of the CA moiety, instead of being free. The quantum chemical calculations supported the experimental observation and conclusion by providing the mechanistic explanation in detail.

First Direct and Unequivocal Electron Spin Resonance Spin-Trapping Evidence for pH-Dependent Production of Hydroxyl Radicals from Sulfate Radicals

Recently, the sulfate radical (SO₄^•-) has been found to exhibit broad application prospects in various research fields such as chemical, biomedical, and environmental sciences. It has been suggested that SO₄^•- could be transformed into a more reactive hydroxyl radical (^•OH); however, no direct and unequivocal experimental evidence has been reported yet. In this study, using an electron spin resonance (ESR) secondary radical spin-trapping method coupled with the classic spin-trapping agent 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and the typical ^•OH-scavenging agent dimethyl sulfoxide (DMSO), we found that ^•OH can be produced from three SO₄^•--generating systems from weakly acidic (pH = 5.5) to alkaline conditions (optimal at pH = 13.0), while SO₄^•- is the predominant radical species at pH < 5.5. A comparative study with three typical ^•OH-generating systems strongly supports the above conclusion. This is the first direct and unequivocal ESR spin-trapping evidence for ^•OH formation from SO₄^•- over a wide pH range, which is of great significance to understand and study the mechanism of many SO₄^•--related reactions and processes. This study also provides an effective and direct method for unequivocally distinguishing ^•OH from SO₄^•-.

Targeted live-cell nuclear delivery of the DNA 'light-switching' Ru(II) complex via ion-pairing with chlorophenolate counter-anions: the critical role of binding stability and lipophilicity of the ion-pairing complexes

We have found recently that nuclear uptake of the cell-impermeable DNA light-switching Ru(II)-polypyridyl cationic complexes such as [Ru(bpy)2(dppz)]Cl2 was remarkably enhanced by pentachlorophenol (PCP), by forming ion-pairing complexes via a passive diffusion mechanism. However, it is not clear whether the enhanced nuclear uptake of [Ru(bpy)2(dppz)]2+ is only limited to PCP, or it is a general phenomenon for other highly chlorinated phenols (HCPs); and if so, what are the major physicochemical factors in determining nuclear uptake? Here, we found that the nuclear uptake of [Ru(bpy)2(dppz)]2+ can also be facilitated by other two groups of HCPs including three tetrachlorophenol (TeCP) and six trichlorophenol (TCP) isomers. Interestingly and unexpectedly, 2,3,4,5-TeCP was found to be the most effective one for nuclear delivery of [Ru(bpy)2(dppz)]2+, which is even better than the most-highly chlorinated PCP, and much better than its two other TeCP isomers. Further studies showed that the nuclear uptake of [Ru(bpy)2(dppz)]2+ was positively correlated with the binding stability, but to our surprise, inversely correlated with the lipophilicity of the ion-pairing complexes formed between [Ru(bpy)2(dppz)]Cl2 and HCPs. These findings should provide new perspectives for future investigations on using ion-pairing as an effective method for delivering other bio-active metal complexes into their intended cellular targets.

Intrinsic chemiluminescence production from the degradation of haloaromatic pollutants during environmentally-friendly advanced oxidation processes: Mechanism, structure-activity relationship and potential applications

The ubiquitous distribution of halogenated aromatic compounds (XAr) coupled with their carcinogenicity has raised public concerns on their potential risks to both human health and the ecosystem. Recently, advanced oxidation processes (AOPs) have been considered as an "environmentally-friendly" technology for the remediation and destruction of such recalcitrant and highly toxic XAr. During our study on the mechanism of metal-independent production of hydroxyl radicals (OH) by halogenated quinones and H₂O₂, we found, unexpectedly, that an unprecedented OH-dependent two-step intrinsic chemiluminescene (CL) can be produced by H₂O₂ and tetrachloro-p-benzoquinone, the major carcinogenic metabolite of the widely used wood preservative pentachlorophenol. Further investigations showed that, in all OH-generating systems, CL can also be produced not only by pentachlorophenol and all other halogenated phenols, but also by all XAr tested. A systematic structure-activity relationship study for all 19 chlorophenolic congeners showed that the CL increased with an increasing number of Cl-substitution in general. More importantly, a relatively good correlation was observed between the formation of quinoid/semiquinone radical intermediates and CL generation. Based on these results, we propose that OH-dependent formation of quinoid intermediates and electronically excited carbonyl species is responsible for this unusual CL production; and a rapid, sensitive, simple, and effective CL method was developed not only to detect and quantify trace amount of XAr, but also to provide useful information for predicting the toxicity or monitoring real-time degradation kinetics of XAr. These findings may have broad chemical, environmental and biological implications for future studies on halogenated aromatic persistent organic pollutants.