Label-Free and Ultrasensitive Detection of Butyrylcholinesterase and Organophosphorus Pesticides by Mn(II)-Based Electron Spin Resonance Spectroscopy with a Zero Background Signal.

Mn(II)-based electron spin resonance (ESR) spectroscopy was used for detecting butyrylcholinesterase (BChE) and organophosphorus pesticides (OPs). MnO2 nanosheets were synthesized with manganese chloride and hydrogen peroxide. With the catalysis of BChE, S-butyrylthiocholine iodide (BTCh) was hydrolyzed into thiocholine which has a reducing -SH group. In the presence of thiocholine, MnO2 nanosheets were broken down and Mn(IV) in MnO2 nanosheets was reduced into Mn(II). Mn2+ is a paramagnetic ion and gives a good ESR signal. In contrast, MnO2 nanosheets have no ESR signal and need not be separated from Mn2+. Mn2+ can be determined directly by ESR spectroscopy, and no further sensing probe is needed. ESR spectroscopy based on directly detecting Mn2+ is much simpler than those using other probes besides MnO2. The ESR signal of Mn2+ is proportional to the catalytic activity of BChE. OPs which inhibit the activity of BChE can also be detected by probing the ESR signal of Mn2+. Since there is no ESR signal of MnO2 nanosheets, the background signal in the absence of BChE was close to zero. The limit of detection (LOD) of BChE was as low as 0.042 U L-1. The standard curve for determining the OP paraoxon was established by measuring the inhibition of BChE by paraoxon, and the LOD of paraoxon was found to be 0.076 ng mL-1. The spiked Chinese cabbage extract samples were analyzed, and the experimental results indicated that the recoveries were from 96.5 to 102.8%. The planted Chinese cabbage was sprayed with the paraoxon solution, and the residue amount of paraoxon in the extract was estimated by the method. The result obtained by the present method was consistent with that obtained by HPLC, which proved the practicability of this new method.

Application of Cu2+-based electron spin resonance spectroscopy in measurement of antioxidant capacity of fruits.

The antioxidant capacity of 22 kinds of fruits was measured by the developed electron spin resonance (ESR) method based on Cu2+ sensor. Cu2+ is reduced to Cu+ by the antioxidants in the fruits, and the remaining Cu2+ was determined by ESR and UV-Vis spectroscopy. Cu2+ can give an ESR signal whereas Cu+ cannot, and the loss of the ESR signal was used to quantify the antioxidant capacity of various fruits. The results were shown as vitamin C equivalent antioxidant capacity (VCEAC). The VCEAC values obtained by ESR and UV-Vis methods ranged from 24.23 to 688.61 mg/100 g and from 24.12 to 677.79 mg/100 g, respectively. Cupric ion reducing antioxidant capacity (CUPRAC) and 1,1-diphenyl-2-picryl-hydrazyl (DPPH) methods were employed for comparison. Based on Pearson's correlation test, the results obtained by CUPRAC and DPPH methods were both significantly correlated with these obtained by the present method, which indicated that the novel method was reliable. Total phenolic content for all kinds of fruits was measured with the Folin-Ciocalteu reagent, and VCEAC values obtained by the ESR method were significantly correlated with total phenolic contents. Graphical abstract.

Electron spin resonance spectroscopy for immunoassay using iron oxide nanoparticles as probe.

With the help of iron oxide nanoparticles, electron spin resonance spectroscopy (ESR) was applied to immunoassay. Iron oxide nanoparticles were used as the ESR probe in order to achieve an amplification of the signal resulting from the large amount of Fe3+ ion enclosed in each nanoparticle. Rabbit IgG was used as antigen to test this method. Polyclonal antibody of rabbit IgG was used as antibody to detect the antigen. Iron oxide nanoparticle with a diameter of either 10 or 30 nm was labeled to the antibody, and Fe3+ in the nanoparticle was probed for ESR signal. The sepharose beads were used as solid phase to which rabbit IgG was conjugated. The nanoparticle-labeled antibody was first added in the sample containing antigen, and the antigen-conjugated sepharose beads were then added into the sample. The nanoparticle-labeled antibody bound to the antigen on sepharose beads was separated from the sample by centrifugation and measured. We found that the detection ranges of the antigen obtained with nanoparticles of different sizes were different because the amount of antibody on nanoparticles of 10 nm was about one order of magnitude higher than that on nanoparticles of 30 nm. When 10 nm nanoparticle was used as probe, the upper limit of detection was 40.00 µg mL-1, and the analytical sensitivity was 1.81 µg mL-1. When 30 nm nanoparticle was used, the upper limit of detection was 3.00 µg mL-1, and the sensitivity was 0.014 and 0.13 µg mL-1 depending on the ratio of nanoparticle to antibody. Graphical abstract Schematic diagram of procedure and ESR spectra.

Application of solvent floatation to separation and determination of triazine herbicides in honey by high-performance liquid chromatography.

Based on the foaming property of the honey, a rapid, simple, and effective method solvent floatation (SF) was developed and firstly applied to the extraction and separation of triazine herbicides in honey. The analytes were determined by high-performance liquid chromatography. Some parameters affecting the extraction efficiencies, such as the type and volume of extraction solvent, type of salt, amount of (NH4)2SO4, pH value of sample solution, gas flow rate, and floatation time, were investigated and optimized. The limits of detection for analytes are in the range of 0.16-0.56 µg kg-1. The recoveries and relative standard deviations for determining triazines in five real honey samples are in the range of 78.2-112.9 and 0.2-9.2%, respectively.

Magnetical hollow fiber bar collection of extract in homogenous ionic liquid microextraction of triazine herbicides in water samples.

The homogeneous ionic liquid microextraction combined with magnetical hollow fiber bar collection was developed for extracting triazine herbicides from water samples. These analytes were separated and determined by high performance liquid chromatography. The triazines were quickly extracted into ionic liquid microdroplets dispersed in solution, and then these microdroplets were completely collected with magnetical hollow fiber bars; the pores of which were impregnated with hydrophobic ionic liquid, which makes the phase separation simplified with no need of centrifugation. Some experimental parameters, such as the type of ionic liquid, ultrasonic immersion time of hollow fiber, pH of sample solution, volume of hydrophilic ionic liquid, amount of ion-pairing agent NH4PF6, NaCl concentration, number of magnetical hollow fiber bar, stirring rate, and collection time were investigated and optimized. When the present method was applied to the analysis of real water samples, the precision and recoveries of six triazine herbicides vary from 0.1 to 9.2% and 73.4 to 118.5%, respectively. The detection limits for terbumeton, ametryn, prometryn, terbutryn, trietazine, and dimethametryn were 0.48, 0.15, 0.15, 0.14, 0.35, and 0.16 µg L-1, respectively.

Determination of antioxidant capacity of thiol-containing compounds by electron spin resonance spectroscopy based on Cu2+ ion reduction.

Electron spin resonance spectroscopy was applied to determining the antioxidant capacity of eight thiol-containing compounds, including reduced glutathione, N-acetyl-L-cycsteine, methimazole, captopril, and tiopronin with one thiol group, 1,4-dithioerythritol and 2,3-dimercapto-1-propanol with two thiol groups, as well as L-cystine with no free thiol group. Cu2+ ion gives an electron spin resonance signal and is reduced to Cu+ ion with no electron spin resonance signal by the free thiol group in the compounds. Trolox equivalent antioxidant capacity (TEAC) was used to evaluate the reducing ability of the thiol-containing compounds and the TEAC values were found to be relevant to the number of thiol groups contained in the compounds. For the purpose of comparison, the UV-vis spectrophotometry, cupric reducing antioxidant capacity (CUPRAC) method, and Ellman assay were applied to the determination of the antioxidant capacity of the thiol-containing compounds. The TEAC values obtained by the present method were very close to those obtained by UV-vis method. However, compared with CUPRAC method, for methimazole the present method gave a more reasonable TEAC value. The present method was also applied to the quantification of N-acetyl-L-cycsteine, methimazole, captopril, and tiopronin in their pharmaceutical formulations.

Determination of antioxidant capacity of diverse fruits by electron spin resonance (ESR) and UV-vis spectrometries.

Twenty-one kinds of fruits including strawberry, mulberry, lemon, banana, etc. were measured for antioxidant capacity based on their ability to scavenge 1,1-diphenyl-2-picryl-hydrazyl (DPPH) radical. Vitamin C equivalent antioxidant capacity (VCEAC) was used to quantify antioxidant capacity of the studied fruits. The results were expressed as mg of ascorbic acid equivalent per 100g fruit. Each fruit was divided into two parts: harvest part (fresh fruit analyzed immediately), and liquid nitrogen frozen part (fruit frozen and pulverized in liquid nitrogen). Antioxidant capacities of both fresh and frozen fruits were determined, and VCEAC values were proved to have no significant difference. For the frozen fruits, the antioxidant capacities were measured by electron spin resonance spectroscopy (ESR) and UV-vis spectrometry. VCEAC values obtained with UV-vis and ESR range from 11.48 to 345.75mg/100g and 7.01 to 366.26mg/100g. Experimental results indicated that VCEAC values obtained by two methods were highly correlated.