Mg2+- or Ca2+-regulated aptamer adsorption on polydopamine-coated magnetic nanoparticles for fluorescence detection of ochratoxin A.

It has been observed that polyvalent metal ions can mediate the adsorption of DNA on polydopamine (PDA) surfaces. Exploiting this, we used two divalent metal ions (Mg2+ or Ca2+) to promote the adsorption of fluorescence-labelled ochratoxin A (OTA) aptamers on PDA-coated magnetic nanoparticles (Fe3O4@PDA). Based on the different adsorption affinities of free aptamers and OTA-bound aptamers, a facile assay method was established for OTA detection. The aptamers adsorbed on Fe3O4@PDA were removed via simple magnetic separation, and the remaining aptamers in the supernatant exhibited a positive correlation with the OTA concentration. The concentrations of Mg2+ and Ca2+ were finely tuned to attain the optimal adsorption affinity and sensitivity for OTA detection. In addition, other factors, including the Fe3O4@PDA dosage, pH, mixing order, and incubation time, were studied. Finally, under optimized conditions, a detection limit (3σ/s) of 1.26 ng/mL was achieved for OTA. Real samples of spiked red wine were analysed with this aptamer-based method. This is the first report of regulating aptamer adsorption on the PDA surface with polyvalent metal ions for OTA detection. By changing the aptamers, the method can be easily extended to other target analytes.

Online micro solid phase extraction coupled with ultra-performance liquid chromatography-tandem mass spectrometry for trace analysis of endogenous plant hormones in Ulva linza.

OBJECTIVE: Ulva linza (L.) is a species of green algae widely distributed in China. We aimed to establish a sensitive online analytical method for quantification of endogenous phytohormones in fresh minute seaweed samples. METHOD: The method for quantification of endogenous plant hormones in fresh minute samples was developed based on a homemade online micro solid phase extraction (m-SPE) system coupled with an ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) platform. The online m-SPE instrument injected the eluent of m-SPE directly onto the LC separation column, improving the utilization of samples and saving time. The m-SPE column, of which the effective size was 9.6 × 2 mm i.d., was filled with 19 mg of C18 (10 µm). RESULTS: Under optimized conditions, the limits of detection were 0.002-0.060 ng ml-1 for five plant hormones. The actual sample recoveries of phytohormones were 76.4-103.4% and the coefficients of variance were below 14.1%. The temporal distribution of these endogenous plant hormones of U. linza during different growth periods is described. CONCLUSION: The proposed online m-SPE method was successfully applied to quantification of endogenous acidic and alkaline plant hormones in U. linza. It provides important information for the further study of the physiological and ecological effects of plant hormones in lower algal species.

Enhancement of Chemiluminescence Intensity of S2* in Non-premixed Hydrogen Microjet Flame in the Photometric Detector for Sulfur Detection.

A transparent quartz rod (q) placed vertically on top of a non-premixed hydrogen microjet flame in a flame photometric detector (qFPD) was developed and evaluated for sulfur detection. The microjet flame burned around the quartz rod because of Coanda effect, forming an extended downstream flame zone with a relatively low temperature between 550 and 650 °C, which is favorable to the formation of S2*. The emission intensity of S2* and the signal-to-noise ratio (SNR) of sulfur response were enhanced 2.6- and 2.1-fold, respectively. It was found that the quartz rod of diameter 4 mm with a tip shape of semicircle placed 6 mm above the nozzle yielded the highest SNR. The limits of detection (LOD) for seven kinds of tested sulfur-containing compounds of qFPD were 0.3-0.5 pg S s-1, which is 5-7 times better than that of commercially available FPD detectors (LOD: 1.6-2.8 pg S s-1). The selectivity of sulfur over carbon was 105 on qFPD when the SNR for the mass flow rate of S and C atoms was ∼3 times. It was the first time that a quartz rod was used vertically on top of a microjet hydrogen-rich flame in FPD to enhance the chemiluminescence of S2* and improve the LOD down to 0.3-0.5 pg S s-1.

A flame photometric detector with a silicon photodiode assembly for sulfur detection.

A flame photometric detector with a silicon photodiode assembly instead of a photomultiplier tube for sulfur detection was developed and evaluated. The photosensitive area of photodiode, the optical design, and band-pass filters, were optimized. It was found that the optimal photosensitive area of the photodiode was 100 (mm)2, and three focus lenses combined with a broad band-pass filter of 378/52 nm and a QB21 glass yielded the best result. This design fully utilized the wide emission spectrum of S2*, the response characteristics of silicon photodiode, and effective absorption of strong emission spectrums of OH* at wavelength around 310 nm by QB21 glass. The limits of detection for nine kinds of sulfur containing compounds were between 5.8 × 10-12 to 9.5 × 10-12 g s-1. This mode provided a linear response of 3 orders of magnitude for compounds being tested and a selectivity of sulfur over carbon of 105. It is demonstrated for the first time that the overall performance of the flame photometric detector integrated with a silicon photodiode assembly work at room temperature was comparable to a conventional detector coupled with a photomultiplier tube, with advantages of short equilibration time, robust to electromagnetic interference and vibration, and low cost. The new detector can find wide application in gas chromatography and on-line monitoring instruments for sulfur measurement.

Dispersive Matrix Solid-Phase Extraction Method Coupled with High Performance Liquid Chromatography-Tandem Mass Spectrometry for Ultrasensitive Quantification of Endogenous Brassinosteroids in Minute Plants and Its Application for Geographical Distribution Study.

An ultrasensitive analysis method for quantification of endogenous brassinosteroids in fresh minute plants was developed based on dispersive matrix solid-phase extraction coupled with high performance liquid chromatography-tandem mass spectrometry. During the dispersive matrix solid-phase extraction, plant samples were first ground with solid sorbent (dispersant) in one microcentrifuge tube and then centrifuged after adding extraction solvent and cleanup materials (another type of sorbent). Three protocols based on dispersive matrix solid-phase extraction were compared and discussed for plant samples with different matrix complexity. The choice of any protocol was a compromise of increasing purification efficiency and decreasing sample loss. Under optimized conditions, the limits of detection were 1.38-6.75 pg mL-1 for five brassinosteroids in the oilseed rape samples. The intraday and interday precisions were in the range of 0.8%-9.8% and 4.6%-17.3%, respectively. The proposed method was successfully applied to detection of endogenous brassinosteroids in milligram oilseed rape (2.0 mg) and submilligram Arabidopsis thaliana seedlings (0.5 mg). Finally, the geographical distribution of five endogenous brassinosteroids of Brassica napus L. oilseed rape in different provinces of origin in the Yangtze River basin was described.