Effects of Experimental Conditions on the Signaling Fidelity of Impedance-Based Nucleic Acid Sensors.

Electrochemical impedance spectroscopy (EIS), an extremely sensitive analytical technique, is a widely used signal transduction method for the electrochemical detection of target analytes in a broad range of applications. The use of nucleic acids (aptamers) for sequence-specific or molecular detection in electrochemical biosensor development has been extensive, and the field continues to grow. Although nucleic acid-based sensors using EIS offer exceptional sensitivity, signal fidelity is often linked to the physical and chemical properties of the electrode-solution interface. Little emphasis has been placed on the stability of nucleic acid self-assembled monolayers (SAMs) over repeated voltammetric and impedimetric analyses. We have studied the stability and performance of electrochemical biosensors with mixed SAMs of varying length thiolated nucleic acids and short mercapto alcohols on gold surfaces under repeated electrochemical interrogation. This systematic study demonstrates that signal fidelity is linked to the stability of the SAM layer and nucleic acid structure and the packing density of the nucleic acid on the surface. A decrease in packing density and structural changes of nucleic acids significantly influence the signal change observed with EIS after routine voltammetric analysis. The goal of this article is to improve our understanding of the effect of multiple factors on EIS signal response and to optimize the experimental conditions for development of sensitive and reproducible sensors. Our data demonstrate a need for rigorous control experiments to ensure that the measured change in impedance is unequivocally a result of a specific interaction between the target analyte and nucleic recognition element.

Susceptibility of the Algal Toxin Microcystin-LR to UV/Chlorine Process: Comparison with Chlorination.

Microcystin-LR (MC-LR), an algal toxin (cyanotoxin) common in sources of drinking water, poses a major human health hazard due to its high toxicity. In this study, UV/chlorine was evaluated as a potentially practical and effective process for the degradation of MC-LR. Via mass spectrometry analysis, fewer chlorinated-MC-LR products were detected with UV/chlorine treatment than with chlorination, and a transformation pathway for MC-LR by UV/chlorine was proposed. Different degrees of rapid degradation of MC-LR were observed with varying pH (6-10.4), oxidant dosage (0.5-3 mg L-1), natural organic matter (0-7 mg L-1), and natural water sources. In contrast to the formation of primarily chloroform and dichloroacetic acid in deionized water where MC-LR serves as the only carbon source, additional chlorinated disinfection byproducts were produced when sand filtered natural water was used as a background matrix. The UV/chlorine treated samples also showed quantitatively less cytotoxicity in vitro in HepaRG human liver cell line tests than chlorination treated samples. Following 16 min (96 mJ cm-2) of UV irradiation combined with 1.5 mg L-1 chlorine treatment, the cell viability of the samples increased from 80% after exposure to 1 mg L-1 MC-LR to 90%, while chlorination treatment evidenced no reduction in cytotoxicity with the same reaction time.

Use of Selected Scavengers for the Determination of NF-TiO2 Reactive Oxygen Species during the Degradation of Microcystin-LR under Visible Light Irradiation.

Although UV-induced TiO2 photocatalysis involves the generation of several reactive oxygen species (ROS), the formation of hydroxyl radicals are generally associated with the degradation of persistent organic contaminants in water. In this study, a variety of radical scavengers were employed to discriminate the roles of different ROS during visible light activated (VLA) photocatalysis using nitrogen and fluorine doped TiO2 (NF-TiO2) in the degradation of the hepatotoxin, microcystin-LR (MC-LR) in water. The addition of hydroxyl radical scavengers, methanol and tert-butyl alcohol to the reaction mixture resulted in negligible inhibition of VLA NF-TiO2 photocatalytic degradation of MCLR at pH 3.0 and only partial inhibition at pH 5.7. While hydroxyl radicals generally play the primary role in UV TiO2 photocatalysis, the minimal influence of MeOH and t-BuOH on the degradation process under these experimental conditions indicates hydroxyl radicals (•OH) do not play the primary role in VLA NF-TiO2 photocatalysis. However, strong inhibition was observed in VLA NF-TiO2 photocatalytic degradation of MC-LR in the presence of superoxide dismutase, benzoquinone and catalase at pH 3.0 and 5.7 indicating O2•- and H2O2 play critical roles in the degradation process. Similar degradation rates were observed in the presence of singlet oxygen scavenger, deuterium oxide, which enhances singlet oxygen mediated processes further suggesting singlet oxygen does not play a key role in the degradation of MCLR in these system. Formic acid and cupric nitrate were added to probe the roles of the valence band holes and conduction band electrons, respectively. Under UV+vis light irradiation, almost complete inhibition of MC-LR removal is observed with NF-TiO2 in the presence of •OH scavengers at pH 5.7. These results demonstrate that solution pH plays a major role in the formation and reactivities of ROS during VLA NF-TiO2 photocatalysis. The adsorption strength of the scavengers and MCLR onto NF-TiO2 as well as the speciation of the ROS as a function of pH need to be carefully considered since they also play a key role in the efficiency of the process. These results indicate the reduction of molecular oxygen by photo-generated electrons rather than hydroxyl radicals produced by oxidative reactions of photo-generated holes play a key role in the of VLA NF-TiO2 photocatalytic degradation of MC-LR.

Degradation mechanism of cyanobacterial toxin cylindrospermopsin by hydroxyl radicals in homogeneous UV/H2O2 process.

The degradation of cylindrospermopsin (CYN), a widely distributed and highly toxic cyanobacterial toxin (cyanotoxin), remains poorly elucidated. In this study, the mechanism of CYN destruction by UV-254 nm/H2O2 advanced oxidation process (AOP) was investigated by mass spectrometry. Various byproducts identified indicated three common reaction pathways: hydroxyl addition (+16 Da), alcoholic oxidation or dehydrogenation (-2 Da), and elimination of sulfate (-80 Da). The initiation of the degradation was observed at the hydroxymethyl uracil and tricyclic guanidine groups; uracil moiety cleavage/fragmentation and further ring-opening of the alkaloid were also noted at an extended reaction time or higher UV fluence. The degradation rates of CYN decreased and less byproducts (species) were detected using natural water matrices; however, CYN was effectively eliminated under extended UV irradiation. This study demonstrates the efficiency of CYN degradation and provides a better understanding of the mechanism of CYN degradation by hydroxyl radical, a reactive oxygen species that can be generated by most AOPs and is present in natural water environment.

Influence of air quality on the composition of microbial pathogens in fresh rainwater.

In this study, the microbiological quality of fresh rainwater was assessed from 50 rain events under tropical weather conditions for a year. The levels of four major opportunistic waterborne pathogens, namely, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Aeromonas hydrophila, in rainwater samples were quantified by using a robust and sensitive quantitative PCR (qPCR) method. Of the 50 rainwater samples, 25 were found to be positive for at least one pathogen: 21 for E. coli, 16 for P. aeruginosa, 6 for K. pneumoniae, and 1 for A. hydrophila. In addition to the microbiological assessment of rainwater samples, we also studied the influence of prevailing air quality on the microbial quality of rainwater over the sampling period. A significant change in the diversity and relative abundance of the basic microbial indicator organisms in rainwater was observed during a major regional air pollution episode in Southeast Asia due to biomass-burning emissions.

Sensitive Electrochemical Detection of Microcystin-LR in Water Samples Via Target-Induced Displacement of Aptamer Associated [Ru(NH3)6]3.

In this study, we demonstrate the successful development of an electrochemical aptamer-based sensor for point-of-use detection and quantification of the highly potent microcystin-LR (MC-LR) in water. The sensor uses hexaammineruthenium(III) chloride ([Ru(NH3)6]3+) as redox mediator, because of the ability of the positively charged (3+) molecule to associate with the phosphate backbone of the nucleic acids. We quantitatively measure the target-induced displacement of aptamer associated, or surface confined, [Ru(NH3)6]3+ in the presence of MC-LR. Upon the addition of MC-LR in the water, surface-confined [Ru(NH3)6]3+ dissociates, resulting in less faradaic current from the reduction of [Ru(NH3)6]3+ to [Ru(NH3)6]2+ Sensing surfaces of highly packed immobilized aptamers were capable of recording decreasing square wave voltammetry (SWV) signals after the addition of MC-LR in buffer. As a result, SWV recorded substantial signal suppression within 15 min of target incubation. The sensor showed a calculated limit of detection (LOD) of 9.2 pM in buffer. The effects of interferents were minimal, except when high concentrations of natural organic matter (NOM) were present. Also, the sensor performed well in drinking water samples. These results indicate a sensor with potential for fast and specific quantitative determination of MC-LR in drinking water samples. A common challenge when developing electrochemical, aptamer-based sensors is the need to optimize the nucleic acid aptamer in order to achieve sensitive signaling. This is particularly important when an aptamer experiences only a small or localized conformational change that provides only a limited electrochemical signal change. This study suggests a strategy to overcome that challenge through the use of a nucleic acid-associated redox label.

Intermediates and reaction pathways from the degradation of microcystin-LR with sulfate radicals.

Degradation of the cyanotoxin microcystin-LR (m/z 995.5) using sulfate radical-based advanced oxidation technologies (AOTs) and identification of reaction intermediates formed during treatment were investigated in this study. To the best of our knowledge this is the first study on the degradation and identification of reaction intermediates for any cyanotoxin with SO(4)(•-). Tandem mass spectrometry designated the formation of nine (as m/z) reaction intermediates with four of them (m/z 1011.5, 1027.5, 1029.5, and 1045.5) having multiple peaks in the TIC chromatogram. New peaks that were not observed with hydroxyl radical formed during photocatalytic oxidation (PCO) have been detected such as m/z 1045.5. The initially formed intermediates involved the oxidation of the unsaturated bonds of MC-LR especially the diene bonds located on the chain of the Adda amino acid. Subsequent intermediates implicated the oxidative cleavage of small functional groups (i.e., -COOH), up to the complete removal of the Adda chain. The electrophilic character of SO(4)(•-) is proven by the multihydroxylation of the aromatic ring. Toward the end of treatment, simultaneous oxidation of the Adda chain and the cyclic structure occurred without the formation of linear products.

A Comprehensive Review: Development of Electrochemical Biosensors for Detection of Cyanotoxins in Freshwater.

Cyanobacteria harmful algal blooms are increasing in frequency and cyanotoxins have become an environmental and public concern in the U.S. and worldwide. In this Review, the majority of reported studies and developments of electrochemical affinity biosensors for cyanotoxins are critically reviewed and discussed. Essential background information about cyanobacterial toxins and electrochemical biosensors is combined with the rapidly moving development of electrochemical biosensors for these toxins. Current issues and future challenges for the development of useful electrochemical biosensors for cyanotoxin detection that meet the demands for applications in field freshwater samples are discussed. The major aspects of the entire review article in a prescribed sequence include (i) the state-of-the-art knowledge of the toxicity of cyanotoxins, (ii) important harmful algal bloom events, (iii) advisories, guidelines, and regulations, (iv) conventional analytical methods for determination of cyanotoxins, (v) electrochemical transduction, (vi) recognition receptors, (vii) reported electrochemical biosensors for cyanotoxins, (viii) summary of analytical performance, and (ix) recent advances and future trends. Discussion includes electrochemical techniques and devices, biomolecules with high affinity, numerous array designs, various detection approaches, and research strategies in tailoring the properties of the transducer-biomolecule interface. Scientific and engineering aspects are presented in depth. This review aims to serve as a valuable source to scientists and engineers entering the interdisciplinary field of electrochemical biosensors for detection of cyanotoxins in freshwaters.

LC/MS/MS structure elucidation of reaction intermediates formed during the TiO(2) photocatalysis of microcystin-LR.

Microcystin-LR (MC-LR), a cyanotoxin and emerging drinking water contaminant, was treated with TiO(2) photocatalysts immobilized on stainless steel plates as an alternative to nanoparticles in slurry. The reaction intermediates of MC-LR were identified with mass spectrometry (MS) at pH of Milli-Q water (pH(sq)=5.7). Eleven new [M+H](+) were observed in the liquid chromatography mass spectrometry (LC/MS) chromatogram with some of them giving multiple peaks. Most of these reaction intermediates have not been reported from previous studies employing TiO(2) nanoparticles at acidic conditions (pH=4.0). Investigating the effects of pH (for 3.0

Solar photo-Fenton treatment of microcystin-LR in aqueous environment: Transformation products and toxicity in different water matrices.

Transformation products and toxicity patterns of microcystin-LR (MC-LR), a common cyanotoxin in freshwaters, during degradation by solar photo-Fenton process were studied in the absence and presence of two major water components, namely fulvic acid and alkalinity. The transformation products m/z 795, 835, 515/1030 and 532 can be formed through attack of OH on the conjugated carbon double bonds of Adda. Transformation products with m/z 1010, 966 and 513 can be generated through the attack of OH on the methoxy group of Adda. The transformation products m/z 783, 508 and 1012 can be originated from the attack of OH on the cyclic structure of MC-LR. Transformation products (m/z 522, 1028, 1012, 1046 and 514) formed after hydroxylation of the aromatic ring with OH were also identified in this study. The toxicity study revealed that fulvic acid and alkalinity strongly influence the toxicity profiles of solar photo-Fenton treated MC-LR. Fulvic acid enhanced the detoxification whereas low level total alkalinity (1.8 mg L-1 CaCO3) inhibited the detoxification of MC-LR by solar photo-Fenton process as assessed by protein phosphatase-1 (PP-1) inhibition assay. This work provides insights on the utility of solar photo-Fenton destruction of MC-LR in water based on transformation products and toxicity data.

Harmful Algae Bloom Occurrence in Urban Ponds: Relationship of Toxin Levels with Cell Density and Species Composition.

Retention ponds constructed within urban watershed areas of high density populations are common as a result of green infrastructure applications. Several urban ponds in the Northern Kentucky, USA area were monitored for algal community (algae and cyanobacteria) from October 2012 to September 2013. Many of the harmful algal blooms observed during this study were composed primarily of the cyanobacteria genus, Microcystis. No correlations were observed between basic water quality parameters (dissolved oxygen, pH, conductivity, temperature, nitrate and soluble reactive phosphate) and the presence of cyanobacteria and/or microcystin cyanobacterial toxin levels. Furthermore, levels of microcystin toxins did not always coincide with high Microcystis cell counts. Harmful algal blooms in small urban ponds are common which pose risk to human and ecological health due to proximity of dense human population including pets and wild animals. Because harmful algal blooms were detected throughout the year in this study, adaptation of universal guidelines for the design, construction and maintenance of urban ponds may be necessary to protect watershed aquatic ecosystems, and lower health risks from exposure to such harmful blooms.

Identification of Microcystis aeruginosa Peptides Responsible for Allergic Sensitization and Characterization of Functional Interactions between Cyanobacterial Toxins and Immunogenic Peptides.

BACKGROUND: The cyanobacterium species Microcystis aeruginosa produces microcystin and an array of diverse metabolites believed responsible for their toxicity and/or immunogenicity. Previously, chronic rhinitis patients were demonstrated to elicit a specific IgE response to nontoxic strains of M. aeruginosa by skin-prick testing, indicating that cyanobacteria allergenicity resides in a non-toxin-producing component of the organism. OBJECTIVES: We sought to identify and characterize M. aeruginosa peptide(s) responsible for allergic sensitization in susceptible individuals, and we investigated the functional interactions between cyanobacterial toxins and their coexpressed immunogenic peptides. METHODS: Sera from patients and extracts from M. aeruginosa toxic [MC(+)] and nontoxic [MC(-)] strains were used to test IgE-specific reactivity by direct and indirect ELISAs; 2D gel electrophoresis, followed by immunoblots and mass spectrometry (MS), was performed to identify the relevant sensitizing peptides. Cytotoxicity and mediator release assays were performed using the MC(+) and MC(-) lysates. RESULTS: We found specific IgE to be increased more in response to the MC(-) strain than the MC(+) strain. This response was inhibited by preincubation of MC(-) lysate with increasing concentrations of microcystin. MS revealed that phycocyanin and the core-membrane linker peptide are the responsible allergens, and MC(-) extracts containing these proteins induced ß-hexosaminidase release in rat basophil leukemia cells. CONCLUSIONS: Phycobiliprotein complexes in M. aeruginosa have been identified as the relevant sensitizing proteins. Our finding that allergenicity is inhibited in a dose-dependent manner by microcystin toxin suggests that further investigation is warranted to understand the interplay between immunogenicity and toxicity of cyanobacteria under diverse environmental conditions. CITATION: Geh EN, Ghosh D, McKell M, de la Cruz AA, Stelma G, Bernstein JA. 2015. Identification of Microcystis aeruginosa peptides responsible for allergic sensitization and characterization of functional interactions between cyanobacterial toxins and immunogenic peptides. Environ Health Perspect 123:1159-1166; http://dx.doi.org/10.1289/ehp.1409065.

Destruction of microcystins (cyanotoxins) by UV-254 nm-based direct photolysis and advanced oxidation processes (AOPs): influence of variable amino acids on the degradation kinetics and reaction mechanisms.

Hepatotoxic microcystins (MCs) are the most frequently detected group of cyanobacterial toxins. This study investigated the degradation of common MC variants in water, MC-LR, MC-RR, MC-YR and MC-LA, by UV-254 nm-based processes, UV only, UV/H2O2, UV/S2O8(2-) and UV/HSO5(-). Limited direct photolysis of MCs was observed, while the addition of an oxidant significantly improved the degradation efficiency with an order of UV/S2O8(2-) > UV/HSO5(-) > UV/H2O2 at the same initial molar concentration of the oxidant. The removal of MC-LR by UV/H2O2 appeared to be faster than another cyanotoxin, cylindrospermopsin, at either the same initial molar concentration or the same initial organic carbon concentration of the toxin. It suggested a faster reaction of MC-LR with hydroxyl radical, which was further supported by the determined second-order rate constant of MCs with hydroxyl radical. Both isomerization and photohydration byproducts were observed in UV only process for all four MCs; while in UV/H2O2, hydroxylation and diene-Adda double bond cleavage byproducts were detected. The presence of a tyrosine in the structure of MC-YR significantly promoted the formation of monohydroxylation byproduct m/z 1061; while the presence of a second arginine in MC-RR led to the elimination of a guanidine group and the absence of double bond cleavage byproducts. It was therefore demonstrated in this study that the variable amino acids in the structure of MCs influenced not only the degradation kinetics but also the preferable reaction mechanisms.

Kinetics and mechanisms of cylindrospermopsin destruction by sulfate radical-based advanced oxidation processes.

Cylindrospermopsin (CYN) is a potent cyanobacterial toxin frequently found in water bodies worldwide raising concerns over the safety of drinking and recreational waters. A number of technologies have been investigated to remove and/or degrade cyanotoxins with advanced oxidation processes (AOPs) being among the most promising and effective for water detoxification. In this study, the degradation of CYN by sulfate radical-based UV-254 nm-AOPs was evaluated. The UV/S2O8(2-) (UV/peroxydisulfate) was more efficient than UV/HSO5(-) (UV/peroxysulfate) and UV/H2O2 (UV/hydrogen peroxide) processes when natural water samples were used as reaction matrices. The observed UV fluence based pseudo-first-order rate constants followed the expected order of radical quantum yields. The presence of 200 µM natural organic matter (NOM) as carbon slightly inhibited the destruction of CYN; 1.24 mg L(-1)NO3(-) (nitrate) had no significant influence on the removal efficiency and 50 µg L(-1) Fe(2+) [iron (2+)] or Cu(2+) [copper (2+)] improved the performance of UV/S2O8(2-). The addition of tert-butyl alcohol (t-BuOH; hydroxyl radical scavenger) in the reaction yielded byproducts that indicated specific sites in CYN preferentially attacked by sulfate radicals (SRs). The predominant CYN degradation byproduct was P448 consistent with fragmentation of the C5C6 bond of the uracil ring. The subsequent formation of P420 and P392 through a stepwise loss of carbonyl group(s) further supported the fragmentation pathway at C5C6. The byproduct P432 was identified exclusively as mono-hydroxylation of CYN at tricyclic guanidine ring, whereas P414 was detected as dehydrogenation at the tricyclic ring. The elimination of sulfate group and the opening of tricyclic ring were also observed. The possible degradation pathways of CYN by SR-AOP were presented.

A review on cylindrospermopsin: the global occurrence, detection, toxicity and degradation of a potent cyanotoxin.

Cylindrospermopsin is an important cyanobacterial toxin found in water bodies worldwide. The ever-increasing and global occurrence of massive and prolonged blooms of cylindrospermopsin-producing cyanobacteria poses a potential threat to both human and ecosystem health. Its toxicity is associated with metabolic activation and may involve mechanisms that adversely affect a wide variety of targets in an organism. Cylindrospermopsin has been shown to be cytotoxic, dermatotoxic, genotoxic, hepatotoxic in vivo, developmentally toxic, and may be carcinogenic. Human exposure may occur through drinking water, during recreational activities and by consuming foods in which the toxin may have bioaccumulated. Drinking water shortages of sufficient quality coupled with growing human pressures and climate variability and change necessitate an integrated and sustainable water management program. This review presents an overview of the importance of cylindrospermopsin, its detection, toxicity, worldwide distribution, and lastly, its chemical and biological degradation and removal by natural processes and drinking water treatment processes.

Biochemical response of diverse organs in adult Danio rerio (zebrafish) exposed to sub-lethal concentrations of microcystin-LR and microcystin-RR: a balneation study.

The present study was carried out to examine the dose-response of microcystin-LR (MC-LR) and microcystin-RR (MC-RR) toxicity in adult Danio rerio (zebrafish) under balneation conditions at various time points. The differential responses of superoxide dismutase (SOD), glutathione peroxidase (GPx), glutathione reductase (GR), and glutathione-S-transferase (GST) as biomarkers were assessed for oxygen mediated toxicity in liver, gills, intestine and brain tissues of zebrafish exposed to dissolved MC-LR and MC-RR (0.1-10.0 µgl(-1)). To investigate the time related response of biomarkers, fish were sampled after 4, 7 and 15 days of exposure. Responses varied (i) between MC-LR and MC-RR (for certain groups), (ii) for different enzymes at all time points, and (iii) for different tissues. In general, most of the enzymes followed a bell shaped curve, with an abrupt increase in activity at a particular concentration. It was observed that upon exposure to MC-LR and MC-RR, some enzymes showed an adaptive response after the first time point wherein the enzyme activity increased in some tissues. The increase in enzyme activity is suggestive of their cellular and metabolic adaptations to the continued stress and toxin exposure. Enzyme activities in general increased at lower concentrations (≤ 5.0 µgl(-1)) and decreased at higher concentrations (≥ 5.0 µgl(-1)). An abrupt change in enzyme activities was observed at a particular concentration in all the tissue enzymes. For GPx and GR, there was a differential response in the case of fish exposed to MC-LR and MC-RR, which could be due to the difference in toxicity potentials of these cyanotoxins. In general, initial stress conditions were observed in most of the tissue enzymes following the exposure to microcystins (MCs). This observation suggests that MCs found in trace levels are likely to have deleterious effects on aquatic organisms and can trigger a variety of biochemical responses depending on their specific toxicity.

Efficient removal of microcystin-LR by UV-C/H2O2 in synthetic and natural water samples.

The destruction of the commonly found cyanobacterial toxin, microcystin-LR (MC-LR), in surface waters by UV-C/H(2)O(2) advanced oxidation process (AOP) was studied. Experiments were carried out in a bench scale photochemical apparatus with low pressure mercury vapor germicidal lamps emitting at 253.7 nm. The degradation of MC-LR was a function of UV fluence. A 93.9% removal with an initial MC-LR concentration of 1 µM was achieved with a UV fluence of 80 mJ/cm(2) and an initial H(2)O(2) concentration of 882 µM. When increasing the concentration of MC-LR only, the UV fluence-based pseudo-first order reaction rate constant generally decreased, which was probably due to the competition between by-products and MC-LR for hydroxyl radicals. An increase in H(2)O(2) concentration led to higher removal efficiency; however, the effect of HO scavenging by H(2)O(2) became significant for high H(2)O(2) concentrations. The impact of water quality parameters, such as pH, alkalinity and the presence of natural organic matter (NOM), was also studied. Field water samples from Lake Erie, Michigan and St. Johns River, Florida were employed to evaluate the potential application of this process for the degradation of MC-LR. Results showed that the presence of both alkalinity (as 89.6-117.8 mg CaCO(3)/L) and NOM (as ∼2 to ∼9.5 mg/L TOC) contributed to a significant decrease in the destruction rate of MC-LR. However, a final concentration of MC-LR bellow the guideline value of 1 µg/L was still achievable under current experimental conditions when an initial MC-LR concentration of 2.5 µg/L was spiked into those real water samples.

Effects of water parameters on the degradation of microcystin-LR under visible light-activated TiO2 photocatalyst.

A study was performed to determine the effect of pH, alkalinity, natural organic matter (NOM) and dissolved oxygen in the performance of nitrogen and fluorine doped TiO(2) (NF-TiO(2)) for the degradation of hepatotoxin microcystin-LR (MC-LR) in synthetic and natural water under visible light irradiation. The initial degradation rate of MC-LR was fastest under acidic conditions (3.50 ± 0.02 × 10(-3) µM min(-1) at pH 3.0) and decreased to 2.29 ± 0.07 × 10(-3) and 0.54 ± 0.02 × 10(-3) µM min(-1) at pH 5.7 and 7.1, respectively. Attractive forces between the opposite charged MC-LR and NF-TiO(2) are likely responsible for the enhancement in the photocatalytic decomposition of MC-LR resulting from increased interfacial adsorption. For carbonate buffered solutions, the photocatalytic activity of NF-TiO(2) was reduced when increasing the carbonate concentration up to 150 mg CaCO(3) L(-1). The scavenging of radical species by the bicarbonate ion at pH 7.1 is discussed. In the presence of NOM, the degradation rates decreased as pH and initial concentration of the NOM increased. The inhibition was higher with fulvic acid than humic acid under alkaline conditions. Oxygenated solution yields higher NF-TiO(2) photocatalytic degradation of MC-LR compared to nitrogen sparged solution at pH 5.7. The involvement of specific reactive oxygen species implicated in the photodegradation is proposed. Finally, no significant degradation is observed with various natural waters spiked with MC-LR under visible light (λ > 420 nm) but high removal was achieved with simulated solar light. This study provides a better understanding of the interactions and photocatalytic processes initiated by NF-TiO(2) under visible and solar light. The results indicate solar photocatalytic oxidation is a promising technology for the treatment of water contaminated with cyanotoxins.