Zhi-Qi Zhang

Microwave-assisted on-line derivatization for sensitive flow injection fluorometric determination of formaldehyde in some foods

A rapid and sensitive flow injection fluorometry has been developed for the determination of formaldehyde based on the microwave on-line accelerating its Hantzsch reaction with cyclohexane-1,3-dione. Under the optimized conditions, the fluorescent intensity is proportional to formaldehyde content in the range from 0.05 ng/mL to 2.000 microg/mL. The detection limit (S/N=3) is 0.02 ng/mL and the analytical frequency is 28 injections per hour. The relative standard deviations are 2.2% and 3.1% for eleven injections of 0.100 and 0.001 microg/mL of formaldehyde, respectively. With the assistance of microwave irradiation, a best sensitive fluorometry was established for the determination of formaldehyde at a high analytical frequency. This method was successfully applied to food analysis without requiring any sample pretreatment, and the determination results were correlated well with those obtained by the standard method with a sample pretreatment of steam distillation.

Flow-injection catalytic spectrophotometric determination of oxalic acid using the redox reaction between Victoria blue B and dichromate

Abstract A flow-injection catalytic method is developed for the determination of oxalic acid based on its catalytic effect on the redox reaction between Victoria blue B and potassium dichromate in dilute sulfuric acid medium. The reaction is monitored spectrophotometrically by measuring the decrease in the absorbance of Victoria blue B at the maximum absorption wavelength of 618xa0nm. A calibration graph from 1.0 to 80.0xa0μgxa0ml −1 of oxalic acid and a detection limit of 0.8xa0μgxa0ml −1 was obtained. The sampling rate was about 22 samples per hour. The proposed method is simple and inexpensive. The applicability of the method was demonstrated by the determination of oxalic acid in vegetable food, such as spinach and mushroom.

An efficient ratiometric fluorescence sensor based on metal-organic frameworks and quantum dots for highly selective detection of 6-mercaptopurine

The development of a simple and accurate quantitative method for the determination of 6-mercaptopurine (6-MP) is of great importance because of its serious side effects. Ratiometric fluorescence (RF) sensors are not subject to interference from environmental factors, and exhibit enhanced precision and accuracy. Therefore, a novel RF sensor for the selective detection of 6-MP was developed based on a dual-emission nanosensor. The nanosensor was fabricated by combining a blue-emission metal-organic framework (MOF) NH2-MIL-53(Al) (λem=425nm) with green-emission 3-mercaptopropionic acid-capped CdTe quantum dots (MPA-CdTe QDs) (λem=528nm) under a single excitation wavelength (335nm). Upon addition of 6-MP, the fluorescence of NH2-MIL-53(Al) in the nanohybrid was selectively quenched due to strong inner filter effects, while the fluorescence of the MPA-CdTe QDs was enhanced. The novel RF sensor exhibited higher selectivity towards 6-MP than CdTe QDs alone, and higher sensitivity than MOFs alone. 6-MP could be detected in the range of 0-50μM with a detection limit of 0.15μM (S/N=3). The developed sensor was applied for the determination of 6-MP in human urine samples and satisfactory results were obtained. Overall, a novel and efficient fluorescence-based method was developed for the detection of 6-MP in biosamples.

Flow-injection, on-line concentrating and flame atomic absorption spectrometry for indirect determination of ascorbic acid based on the reduction of iron(III)

Abstract A flow-injection flame atomic absorption spectrometry is proposed for indirect determination of ascorbic acid based on its reducing action to Fe(III). The Fe(II), produced by the reduction of Fe(III), is on-line adsorbed on a mini-column filled with cation-exchange resin to concentrate, then is eluted reversely by 3xa0molxa0l −1 nitric acid to nebulizer and is measured by flame atomic absorption spectrometry, while the excessive Fe(III) reacts with NH 4 F to form anion [FeF 6 ] 3− and passes through the mini-column to be waste. When reaction and concentrating time is 60xa0s, the absorbance of Fe(II) is proportional to the concentration of ascorbic acid in the range from 0.1 to 50xa0mgxa0l −1 . The analytic frequency is 30 samples per hour. This method has been applied to the determination of ascorbic acid in Vitamin C tablets, Vitamin C injection, vegetables and fruit juice. The results obtained in the analysis agreed well with those provided by iodimetry.

Detection of glycoprotein through fluorescent boronic acid-based molecularly imprinted polymer

A novel strategy based on the fluorescent molecularly imprinted polymers (FMIP) for specific recognition and sensitive sensing glycoprotein from biological samples was developed. The FMIP prepared by introducing a fluorescent boronic acid quinoline-based on the PGMA/EDMA bead surface and assembled a glycoprotein molecularly imprinted polymer using surface imprinting technology. The resultant material showed specific recognition to model glycoprotein. At the same time, the change of fluorescence caused by the amount of model glycoprotein could achieve quantitative determination. The method provided a good linear relationship of the concentrations of HRP in the range of 0.05-1xa0μM, and the detection limit was 0.02xa0μM. It is 10 times lower than the previous fluorescence nanosensor for glycoprotein. The established method combined the desired selectivity of molecularly imprinted polymers and high sensitivity of fluorescence spectroscopy. The influence of background impurities could be effectively eliminated. The outstanding features guarantee that it can be successfully applied to detect glycoprotein from biological samples under physiological conditions.

On-line solid-phase extraction-HPLC-fluorescence detection for simultaneous determination of puerarin and daidzein in human serum.

Response surface methodology (RSM) was applied to the optimization of on-line solid-phase extraction (SPE) parameters, and an automated system of on-line SPE coupled with high-performance liquid chromatography (HPLC) with fluorescence detection was developed for the determination of puerarin and daidzein in human serum. The human serum sample of 50 microL was injected into a conditioned C18 SPE cartridge, and the matrix was washed out with acetonitrile-KH(2)PO(4)-triethylamine buffer (0.01 M, pH 7.4) (3:97, v/v) for 3 min at a flow rate of 0.25 mL/min. Then the target analytes were eluted and transferred to the analytical column. A chromatographic gradient elution was programmed with the mobile phase consisting of acetonitrile and KH(2)PO(4)-triethylamine buffer, and the analytes were determined with a fluorescence detector at excitation wavelength of 350 nm and emission wavelength of 472 nm, respectively. The proposed method presented good linear relations (0.85-170 microg/mL for puerarin and 0.2-40 microg/mL for daidzein), satisfactory precision (RSD<8%), and accredited recovery (92.5-107.8%).

MnO2 nanosheet-assisted ligand-DNA interaction-based fluorescence polarization biosensor for the detection of Ag(+) ions.

Silver (Ag+) ions are highly toxic to aquatic organisms and accumulate in the human body via the food chain. Therefore, the development of sensitive and selective quantitative analytical methods for detecting trace amounts of these ions is necessary. In the present study, a MnO2 nanosheet-assisted, ligand-DNA interaction and fluorescence polarization-based method was developed, for the first time, for sensitive detection of Ag+. The addition of Ag+ to the preformed proflavine-DNA complex induced the release of proflavine, which elicited weak changes in fluorescence polarization. The subsequent addition of MnO2 nanosheets magnified the observed changes, making this a feasible method for the detection of Ag+. The calibration graphs indicated good linearity over the concentration ranges of 30-240nM for Ag+, with a detection limit (S/N=3) of 9.1nM. This method additionally exhibits high selectivity. The mechanism underlying the changes in fluorescence polarization caused by the addition of Ag+ in the presence of MnO2 nanosheets was further explored in this study. These findings demonstrate that the present MnO2 nanosheet-assisted fluorescence polarization biosensor may represent a promising tool for the detection of Ag+ ions. The results for practical detecting Ag+ proved that this biosensor can be applied to environmental water sample analysis.

Selective detection of 2,4,6-trinitrophenol based on a fluorescent nanoscale bis(8-hydroxyquinoline) metal complex.

The reliable and accurate detection of explosives such as 2,4,6-trinitrophenol (TNP) and 2,4,6-trinitrotoluene (TNT) is in high demand for homeland security and public safety. Although extremely high sensitivity towards TNT has been demonstrated, detection of TNP remains a challenge. In this work, a fluorescent nanoscale complex composed of bis(8-hydroxyquinoline) and Al(3+) ions has been prepared, characterized and applied in detection of TNP. This complex exhibits the ability to sense the nitro explosive TNP via a fluorescence quenching mechanism with high selectivity. A simple paper test system for the rapid monitoring of TNP was also investigated. The results show that Bhq-Al is a quite ideal sensing material for trace-level detection of TNP.

Size-selective QD@MOF core-shell nanocomposites for the highly sensitive monitoring of oxidase activities

In this work, we proposed a novel and facile method to monitor oxidase activities based on size-selective fluorescent quantum dot (QD)@metal-organic framework (MOF) core-shell nanocomposites (CSNCPs). The CSNCPs were synthesized from ZIF-8 and CdTe QDs in aqueous solution in 40min at room temperature with stirring. The prepared CdTe@ZIF-8 CSNCPs , which have excellent water dispersibility and stability, displays distinct fluorescence responses to hole scavengers of different molecular sizes (e.g., H2O2, substrate, and oxidase) due to the aperture limitation of the ZIF-8 shell. H2O2 can efficiently quench the fluorescence of CdTe@ZIF-8 CSNCPs over a linearity range of 1-100nM with a detection limit of 0.29nM, whereas large molecules such as substrate and oxidase have very little effect on its fluorescence. Therefore, the highly sensitive detection of oxidase activities was achieved by monitoring the fluorescence quenching of CdTe@ZIF-8 CSNCPs by H2O2 produced in the presence of substrate and oxidase, which is proportional to the oxidase activities. The linearity ranges of the uricase and glucose oxidase activity are 0.1-50U/L and 1-100U/L, respectively, and their detection limits are 0.024U/L and 0.26U/L, respectively. Therefore, the current QD@MOF CSNCPs based sensing system is a promising, widely applicable means of monitoring oxidase activities in biochemical research.

Flow-injection on-line oxidizing fluorimetry and solid phase extraction for determination of thioridazine hydrochloride in human plasma.

A simple, sensitive and specific fluorimetric method has been developed for the determination of thioridazine hydrochloride in human plasma involving solid phase extraction (SPE). In a flow-injection system, thioridazine hydrochloride is on-line oxidized into a strongly fluorescent compound with a lead dioxide solid-phase reactor and the fluorescence intensity is measured with a fluorescence detector (lambda(ex)=349nm, lambda(em)=429nm). A comparison of plasma sample pretreatment between SPE procedure and precipitation method was made and the results showed that SPE procedure was better than precipitation method. Under the optimum conditions, the fluorescence intensity is proportional to the concentration of thioridazine hydrochloride in the range from 0.015 to 2.000microg mL(-1). The detection limit is 5.5ng mL(-1) of thioridazine hydrochloride and the relative standard deviation is 1.06%. This method has been applied to determination of thioridazine hydrochloride in real patients plasma samples with the results compared with those obtained by HPLC method.