Mechanistic study of the electrochemical oxidation of fluoroquinolones: Ciprofloxacin, danofloxacin, enoxacin, levofloxacin and lomefloxacin.

The fluoroquinolones ciprofloxacin, danofloxacin, enoxacin, levofloxacin and lomefloxacin, occur in water bodies worldwide and therefore pose a threat to the aquatic environment. Advanced purification procedures, such as electrochemical oxidation, may act as a remedy since they contribute to eliminating contaminants and prevent micropollutants from entering open water bodies. By electrochemical treatment in a micro-flow reactor equipped with a boron-doped diamond (BDD) electrode, the fluoroquinolones were efficiently degraded. A total of 15 new products were identified using high-performance high-resolution chromatography coupled with high-resolution multifragmentation mass spectrometry. The ecotoxicity of the emerging transformation products was estimated through in silico quantitative structure activity relationship analysis. Almost all transformation products were predicted less ecotoxic than the initial compounds. The fluoroquinolone degradation followed three major mechanisms depending on the voltage during the electrochemical oxidation. At approximately 1 V, the reactions started with the elimination of molecular hydrogen from the piperazine moiety. At approx. 1.25 V, methyl and methylene groups were eliminated. At 1.5 V, hydroxyl radicals, generated at the BDD electrode, led to substitution at the piperazine ring. This novel finding of the three reactions depending on voltage contributes to the mechanistic understanding of electrochemical oxidation as potential remedy against fluoroquinolones in the aquatic environment.

Electrochemical degradation of antibiotic enoxacin using a novel PbO2 electrode with a graphene nanoplatelets inter-layer: Characteristics, efficiency and mechanism.

A novel PbO2 electrode was fabricated by adding graphene nanoplatelets (GNP) inter-layer into ß-PbO2 active layer (called GNP-PbO2) and utilized to degradation of antibiotic enoxacin (ENO). The GNP-PbO2 electrode had a much rougher surface than the typical PbO2 electrode, with smaller crystal size and lower charge-transfer resistance at the electrode/electrolyte interface. Notably, the GNP inter-layer increased the oxygen evolution potential of PbO2 electrode (2.05 V vs. SCE), which was very beneficial to inhibit oxygen evolution and promote ·OH production. The relatively best operating parameters for ENO removal and energy efficiency were current density of 20 mA cm-2, initial pH of 7, initial ENO concentration of 100 mg L-1 and electrode distance of 4 cm. Furthermore, indirect radical oxidation was found to be the main way during electrolysis process. Based on the observed analysis of intermediate products, the main reaction pathways of ENO included hydroxylation, defluorination and piperazine ring-opening. Finally, combinating with the electro-oxidation capability, stability and safety evaluation, we can conclude that GNP-PbO2 is a promising anode for treatment of various organic pollutants in wastewater.

The influence mechanism of the molecular structure on the peak current and peak potential in electrochemical detection of typical quinolone antibiotics.

Antibiotic pollution in water has become an increasingly serious problem, posing a potentially huge threat to human health. Ofloxacin (OFL), norfloxacin (NOR), and enoxacin (ENX) are typical broad-spectrum quinolone antibiotics, which are frequently detected in various water environments. An electrochemical sensor is a rapid and effective tool to detect antibiotics in the aquatic environment. The molecular structure of target pollutants is an important factor affecting the detection performance of electrochemical sensors. Based on the electrochemical detection results of antibiotics (OFL, NOR, and ENX), we first used the molecular structure analysis method based on quantum chemistry to accurately identify the electronegativity and the electrocatalytic degree of the oxidizable (and non-oxidizable) functional groups of pollutants. We also clarified the influence mechanism of the molecular structure on the peak current and peak potential. These results can provide theoretical support for rapidly selecting electrodes with a suitable electrochemical window to efficiently detect trace organic pollutants (such as antibiotics) in water based on the molecular structure of the target pollutant.

Chlorination of enoxacin (ENO) in the drinking water distribution system: Degradation, byproducts, and toxicity.

Chlorine is widely used as a drinking water disinfectant to ensure water security. However, the transformation mechanisms of its degradation of emerging pollutants within the water distribution system (WDS) is insufficiently understood. Thus, the kinetics, degradation byproducts, and toxicity of the chlorination of enoxacin (ENO, a type of emerging pollutant) were explored in a pilot-scale WDS for the first time. It was found that the chlorination rate of ENO was higher in deionized water (DW) than in the pilot-scale WDS, and the degradation followed second-order kinetics in DW. The degradation efficiency was found to be sensitive to pH, and was highest at a pH of 7.4. The chlorination rate of ENO increased with increasing temperature in both DW and WDS. For different pipe materials, the relative performance of ENO chlorination efficiency followed the order of steel pipe > ductile iron pipe > polyethylene (PE) pipe. Seven intermediates were identified during ENO chlorination, and the primary oxidation reaction involved the cleavage of the piperazine group. Finally, it was found that the potential for chlorine toxicity in treated drinking water in the presence of ENO is higher than it is without this pollutant.

Direct and indirect photolysis of the antibiotic enoxacin: kinetics of oxidation by reactive photo-induced species and simulations.

The purpose of this study was to investigate the aqueous phase photochemical behavior of enoxacin (ENO), an antibiotic selected as a model pollutant of emerging concern. The second-order reaction rate constants of ENO with hydroxyl radicals (HO●) and singlet oxygen (1O2) were determined at pH 3, 7, and 9. Also, the rate constants of the electron transfer reaction between ENO and triplet states of chromophoric dissolved organic matter (3CDOM*) are reported for the first time, based on anthraquinone-2-sulfonate (AQ2S) as CDOM proxy. The sunlight-driven direct and indirect ENO degradation in the presence of dissolved organic matter (DOM) is also discussed. The results show that direct photolysis, which occurs more rapidly at higher pH, along with the reactions with HO● and 3AQ2S*, is the key pathway involved in ENO degradation. The ENO zwitterions, prevailing at pH 7, show kENO, HO●, kENO,1O2, and kENO,3AQ2S* of (14.0 ± 0.8) × 1010, (3.9 ± 0.2) × 106, and (61.5 ± 0.7) × 108 L mol-1 s-1, respectively, whose differences at pH 3, 7, and 9 are due to ENO pH-dependent speciation and reactivity. These k values, along with the experimental ENO photolysis quantum yield, were used in mathematical simulations for predicting ENO persistence in sunlit natural waters. According to the simulations, dissolved organic matter and water depth are expected to have the highest impacts on ENO half-life, varying from a few hours to days in summertime, depending on the concentrations of relevant waterborne species (organic matter, NO3-, NO2-, HCO3-).

Upconversion particle@Fe3O4@molecularly imprinted polymer with controllable shell thickness as high-performance fluorescent probe for sensing quinolones.

Localized photo-polymerization was ingeniously applied to prepare a multifunctional molecularly imprinted polymer (MIP) fluorescent probe using the "layer-by-layer" assembly of MIP and Fe3O4 nanoparticles on NaYF4: Yb3+, Er3+ upconversion particles (MUCPs@MIP). Enrofloxacin was used as the template and chosen as the target molecular during the investigation of the adsorption property. This ternary probe has magnetic and broad-spectrum molecular recognition capability, fast response, and upconversion fluorescence. The results of the fluorescence quenching analysis showed good linear ranges of 1.03nmol/L to 0.28µmol/L for enrofloxacin, 1.69nmol/L to 0.22µmol/L for fleroxacin, 6.92nmol/L to 0.28µmol/L for levofloxacin, 7.54nmol/L to 0.30µmol/L for ciprofloxacin, and 3.90nmol/L to 0.25µmol/L for enoxacin. This probe was further used to determine five quinolones in fish tissues and the recoveries ranging from 90.33% to 108.43% were obtained with relative standard deviation below 5.53%. This work offers a new and general strategy to synthesize a MUCPs@MIP upconversion fluorescence probe with magnetic and selective molecular recognition capability for rapid and accurate sensing of multiple chemical residues in the environment and agri-food products.

Sensitive determination of enoxacin in pharmaceutical formulations by its quench effect on the fluorescence of glutathione-capped CdTe quantum dots.

A sensitive and simple method for the determination of enoxacin (ENX) was developed based on the fluorescence quenching effect of ENX for glutathione (GSH)-capped CdTe quantum dots (QDs). Under optimum conditions, a good linear relationship was obtained from 4.333 × 10(-9) mol⋅L(-1) to 1.4 × 10(-5) mol⋅L(-1) with a correlation coefficient (R) of 0.9987, and the detection limit (3σ/K) was 1.313 × 10(-9) mol⋅L(-1). The corresponding mechanism has been proposed on the basis of electron transfer supported by ultraviolet-visible (UV) light absorption, fluorescence spectroscopy, and the measurement of fluorescence lifetime. The method has been applied to the determination of ENX in pharmaceutical formulations (enoxacin gluconate injections and commercial tablets) with satisfactory results. The proposed method manifested several advantages such as high sensitivity, short analysis time, low cost and ease of operation.

An application of Ag(III) complex chemiluminescence system for the determination of enoxacin in capsule and biological fluid.

Ag(III) complex chemiluminescence (CL) system was applied for the determination of enoxacin (ENX). The CL conditions of [Ag(HIO(6))(2)](5-)-H(2)SO(4)-ENX systems without any luminescence reagent were investigated and optimized. Under the optimized conditions, the CL intensity was proportional to the concentration of ENX in the range from 6.6 × 10(-5) to 3.3 × 10(-3) g/L. The limit of detection (s/n = 3) was 2.0 × 10(-5) g/L. The recovery of ENX from the spiked pharmaceutical preparations was in the range of 82.9-108% with a relative standard deviation of 1.9-3.0%. For spiked serum and urine samples the recovery of ENX was in the range of 83.7-110% with a relative standard deviation of 1.1-2.8%. The proposed method was applied successfully to the determination of the drug in capsule, serum and urine samples.

Exploring Mn-doped ZnS quantum dots for the room-temperature phosphorescence detection of enoxacin in biological fluids.

While most research works focus on the development of quantum dots (QDs)-based fluorescence sensors, much less attention is paid to the phosphorescence properties of QDs and their potential for phosphorescence detection. In this work, the phosphorescence property of Mn-doped ZnS QDs is explored to develop a novel room-temperature phosphorescence (RTP) method for the facile, rapid, cost-effective, sensitive, and selective detection of enoxacin in biological fluids. The Mn-doped ZnS QDs-based RTP method reported here does not need the use of deoxidants and other inducers and allows the detection of enoxacin in biological fluids without interference from autofluorescence and the scattering light of the matrix. The Mn-doped ZnS QDs offer excellent selectivity for detecting enoxacin in the presence of the main relevant metal ions in biological fluids, biomolecules, and other kinds of antibiotics. Quenching of the phosphorescence emission due to the addition of enoxacin at 1.0 microM is unaffected by 5000-fold excesses of Na (+) and 10000-fold excesses of K (+), Mg (2+), and Ca (2+). Amino acids such as tryptophan, histidine, and l-cysteine at 1000-fold concentration of enoxacin do not affect the detection of enoxacin. Glucose does not affect the detection at 10000-fold concentration of enoxacin. Typical coadministers (mainly other types of antibiotics) such as ceftezole, cefoperazone, oxacillin, and kalii dehydrographolidi succinas are permitted at 50-, 10-, 100-, and 50-fold excesses, respectively, without interference with the detection of enoxacin. The precision for 11 replicate detections of 0.4 microM enoxacin is 1.8% (RSD). The detection limit for enoxacin is 58.6 nM. The recovery of spiked enoxacin in human urine and serum samples ranges from 94 to 104%. The developed Mn-doped ZnS QDs-based RTP method is employed to monitor the time-dependent concentration of enoxacin in urine from a healthy volunteer after the oral medication of enoxacin. The investigation provides evidence that doped QDs are promising for RTP detection in further applications.

Determination of enoxacin using Tb composite nanoparticles sensitized luminescence method.

In our study, terbium-acetylacetone (Tb-acac) composite nanoparticles have been prepared under vigorous ultrasonic irradiation. The nanoparticles are water soluble, stable and have extremely narrow emission bands and high internal quantum efficiencies. They were used as fluorescence probes in the determination of enoxacin (Enox) based on the fluorescence enhancement of nanoparticles through fluorescence resonance energy transfer (FRET). The influence of buffer solution on the fluorescence intensity was investigated. Under the optimum conditions, the fluorescence intensity of the Tb-acac-Enox system is linearly proportional to the Enox concentration in the Enox concentration range of 2 x 10(-7)-1 x 10(-4) M. The correlation coefficient for the calibration curve was 0.9976. The limit of detection as defined by IUPAC, C (LOD) = 3S (b)/m (where S (b) is the standard deviation of the blank signals and m is the slope of the calibration graph) was found to be 3 x 10(-8) M. The relative standard deviation (RSD) for six repeated measurements of 1 x 10(-4) M Enox was 1.35%. The method was applied to the determination of Enox in pharmaceutical formulation and recovery results were obtained from urine samples.

[Determination of 16 quinolone residues in animal tissues using high performance liquid chromatography coupled with electrospray ionization tandem mass spectrometry].

A confirmative method was developed with high performance liquid chromatography coupled with electrospray ionization tandem mass spectrometry (HPLC-MS/MS) to simultaneously detect 16 quinolone residues in animal tissues, which included nalidixinic acid, oxolinic acid, flumequine, norfloxacin, enoxacin, ciprofloxacin, lomefloxacin, danofloxacin, enrofloxacin, ofloxacin, sarafloxacin, difloxacin, marbofloxacin, pefloxacin, sparfloxacin and orbifloxacin. In the method, the 16 residues were extracted with acidified acetonitrile, cleaned-up with hexane, and concentrated with a rotary evaporator. Then the reconstituted sample solution was analyzed using HPLC-MS/MS in positive mode, with a Inertsil C8-3 column as the analytical column. The method was validated at 10, 50 and 100 microg/kg. The validation results were as follows: the linear ranges were from 10 to 100 microg/kg; the overall recoveries were from 62.4% to 102% with the relative standard deviations of 1.4%-11.9%. The method is simple, rapid, and accurate, and its performance could meet the requirements of the domestic and international legislation. The method is applicable to simultaneously confirm multi-residues of quinolones in animal tissues such as chicken muscle, chicken liver and fish muscle.

A rapid CE-potential gradient detection method for determination of quinolones.

A rapid CE coupled with potential gradient detection (PGD) for the separation and detection of four quinolones, namely, enoxacin, ofloxacin (OFL), fleroxacin, and pazufloxacin, was described. Separation was performed in a fused-silica capillary (75 microm x 8.5 cm) using a buffer of 30 mM Tris and 4 mM phosphoric acid at pH 8.9. Under the separation voltage of 3 kV, the quinolones were separated within 2.8 min with good linearity (r(2) >or= 0.985). The method was successfully applied in determining OFL in a pharmaceutical formulation. Also, a liquid-liquid extraction (LLE) method was developed and coupled to CE-PGD in determining quinolones that spiked in milk samples. With dichloromethane and hexane for enrichment and purification, the LLE recoveries of the four quinolones were in the range of 77-106%. The detection limits of the quinolones with LLE-CE-PGD were from 23 to 65 ng/mL. The proposed CE-PGD method was validated with an HPLC method, and the results indicated consistency between the two methods.

A batch chemiluminescence determination of enoxacin using a tris-(1,10-phenanthroline)ruthenium(II)-cerium(IV) system.

A batch type chemiluminescence (CL) determination of enoxacin is described. In this work, it was observed that enoxacin could enhance the chemiluminescence (CL) emission Ru(phen)3(2+)-Ce(IV) system and this enhancement effect was dependent on the concentration of enoxacin, based on which, CL system was established for the determination of enoxacin. Under the optimum experimental conditions, the linear range and detection limit are 0.6406-64.06 microg/ml and 0.0210 microg/ml, respectively. The R.S.D. is 1.75%. (n = 10). The proposed method has been applied to detect the content of enoxacin in pharmaceutical formulation and human serum with satisfactory results. The possible mechanism of the CL reaction was discussed.

A new approach to quantitative NMR: fluoroquinolones analysis by evaluating the chemical shift displacements.

Quantitative NMR spectroscopy is always an attractive goal as the identity and quantity could be simultaneously determined. Although significant advancements have been achieved in this field it is common that all reported quantitative NMR methods perform the analysis by utilizing the average integral intensities of selected signals. During the calculation of the area under NMR peaks several response problems can occur which should always be treated carefully to overcome inaccuracies. In the method proposed in this work the quantitative information is obtained utilizing the measurement of selected protons chemical shift displacements which is a quite straightforward and highly reproducible process. The (1)H NMR spectra of multiple fluoroquinolone (FQ) solutions revealed that the chemical shifts of protons, especially the aromatic ones, were concentration dependent for all tested compounds, as a result of extensive self-association phenomena. In the present work a novel methodology is described for the quantitation of several FQs based on this dependence. The proposed method was applied to Ciprofloxacin solutions over a wide range of concentrations. Evaluation of the obtained data presented acceptable characteristics regarding accuracy, precision, and robustness. The applicability limitations of this method were found to be posed by current instrumentation, mainly by the magnetic field frequency e.g. the slope of the response function achieved with a 400MHz instrument was twice the one achieved at 200MHz. The pH effect was negligible from pD 2.5 to 5.5. The phenomenon appeared in a pattern that can be applied for a plethora of drug categories revealing self-association phenomena in a range of concentration determined by the magnet strength of the instrument.

[Simultaneous determination of quinolones in foods by LC/MS/MS].

A simple method was developed for the simultaneous determination of seven quinolones (enoxacin, ofloxacin, ciprofloxacin, danofloxacin, lomefloxacin, enrofloxacin and sarafloxacin) in foods using liquid chromatography coupled with electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS). The seven quinolones were extracted with acetonitrile containing 0.2% formic acid, and the extracted solution was cleaned up on a C18 cartridge. The extract was diluted with 5 mmol/L IPCC-MS3 for injection into the LC-ESI-MS/MS. The LC separation was carried out on an ODS column with gradient elution of 5 mmol/L IPCC-MS3-acetonitrile as the mobile phase. Mass spectral acquisition was done in the positive ion mode by applying selected reaction monitoring (SRM). The recoveries of the seven quinolones were mostly greater than 60% from foods fortified at 10 ng/g. The detection limits in foods were 2 ng/g for enoxacin and ciprofloxacin, and 1 ng/g for the other drugs. Twenty cattle muscle, 7 swine muscle, 9 chicken muscle, 16 milk, 19 prawn and 20 broiled eel samples from retail markets were analyzed by this method. Enrofloxacin and its metabolite ciprofloxacin were detected in 9 broiled eel at the level of trace (tr)-34 ng/g and tr-10 ng/g, respectively.

[Analysis of the response factors of different quinolones detected by evaporative light-scattering detector].

AIM: To analyze the response factors of different quinolone antibiotics detected by evaporative light-scattering detector (ELSD). METHODS: The response factors of five different quinolones (enoxacin, levofloxacin, ciprofloxacin, lomefloxacin and gatifloxacin) detected by ELSD were determined by using a YMC-Pack ODS-AM cloumn (150 mm x 4.6 mm ID, 5 microns) as analytical column and 0.5% triethylamine (adjusting pH 2.5 with trifluoroacetic acid)-acetonitrile (48:12) as mobile phase at a flow rate of 0.6 mL.min-1, the temperature of the drift tube was set at 117 degrees C, and the flow of carrier gas at 3.0 L.min-1. Detector responses (A) and the amount of injection of each substance (m) were fitted to the logarithmic regression: log A = b log m + log a. RESULTS: The linear regression equation obtained were: enoxacin: Y = 1.0799X + 2.7611, r2 = 0.9996; levofloxacin: Y = 1.0913X + 2.7235, r2 = 0.9997; ciprofloxacin: Y = 1.0828X + 2.7523, r2 = 0.9994; lomefloxacin: Y = 1.0891X + 2.7391, r2 = 0.9993; gatifloxacin: Y = 1.0878X + 2.7392, r2 = 0.9995. The differences between them were negligible. CONCLUSION: Different quinolones can give the same responses with ELSD detection. So, the HPLC-ELSD methods can be applied to the determination of new substances by using another substance as reference.

Spectrophotometric determination of enoxacin as ion-pairs with bromophenol blue and bromocresol purple in bulk and pharmaceutical dosage form.

Three simple, accurate and sensitive spectrophotometric methods were developed for determination of enoxacin. The methods based on extraction of this drug into chloroform as ion pairs with sulphonphthalein dyes as bromophenol blue and bromocresol purple. The optimum conditions of the reactions were studied and optimized. The absorbance of yellow products was measured at 412 nm for enoxacin-bromophenol blue and 410 nm for enoxacin-bromocresol purple. Linearity ranges were found to be 2.0-20.0 microg ml(-1) for enoxacin-bromophenol blue and 0.77-17.62 microg ml(-1) for enoxacin-bromocresol purple. The detection limits were found to be 0.084 microg ml(-1) and 0.193 microg ml(-1) for enoxacin-bromophenol blue and enoxacin-bromocresol purple, respectively. The composition of the ion pairs was found 1:1 by Job's method. The developed methods were applied successfully for the determination of this drug in pharmaceutical preparation. The data obtained by developed methods were compared with the spectrophotometric method in literature. No differences were found statistically.

Selective separation and simultaneous determination of trace levels of five types of fluorinated quinolone drugs by thin-layer chromatography/fluorescence densitometry.

This paper reports the determination of trace levels of 5 types of fluorinated quinolone drugs, i.e., ciprofloxacin, norfloxacin, enoxacin, pefloxacin, and ofloxacin, by thin-layer chromatography (TLC)/fluorescence densitometry. The new analytical method uses 2-step TLC development, selective separation, and simultaneous determination of the 5 drugs. The method was also applied to the determination of recoveries of standards of the 5 drugs in plasma and urine samples. The results show that the method has a wide linear range, high repeatability, and good stability.

[Terbium sensitized chemiluminescence of enoxacin].

AIM: To determine the enoxacin by means of terbium sensitized chemiluminescence in pharmaceutical preparations as well as urine samples. METHODS: Through six-way injection valve Ce4+, H2SO3, Tb3+ and enoxacin (ENX) standard solutions were injected into the flow system in a certain order, the chemiluminescence signal was detected by weak luminescence analyzer. RESULTS: The chemiluminescence intensity was linearly related to the concentration of ENX in the range of 2.0 x 10(-9)-6.0 x 10(-8) mol.L-1 with a detection limit of 4.0 x 10(-10) mol.L-1. Relative standard deviation was less than 1.6% (n = 11). CONCLUSION: It provides a sensitive rapid, simple and accuracy measurement of enoxacin for the pharmacokinetic studies.

Application of derivative UV spectrophotometry for the determination of enoxacin and nalidixic acid in tablets.

First-, second-, third- and forth-order derivative spectrophotometric methods, using "peak-zero" (P-O) and "peak-peak" (P-P) techniques of measurement have been developed for the determination of enoxacin and nalidixic acid in tablets. The calibration curves were linear in the concentration range of 2.0-12.0 micrograms ml-1 for the analysed quinolones. The procedure was simple, rapid and the results were reliable.