Analytica Chimica Acta (v.616, #2)

Contents (iii-iv).

A feed-forward artificial neural network (ANN) learned by error back-propagation is used to generate a retention predictive model for phenoxy acid herbicides in isocratic reversed-phase high-performance liquid chromatography. The investigated solutes (18 compounds), apart from the most common herbicides of this class, include some derivatives of benzoic acid and phenylacetic acid structurally related to phenoxy acids, as a whole covering a pK a range between 2.3 and 4.3. A mixed model in terms of both solute descriptors and eluent attributes is built with the aim of predicting retention in water–acetonitrile mobile phases within a large range of composition (acetonitrile from 30% to 70%, v/v) and acidity (pH of water before mixing with acetonitrile ranging between 2 and 5). The set of input variables consists of solute pK a and quantum chemical molecular descriptors of both the neutral and dissociated form, %v/v of acetonitrile in the mobile phase and pH of aqueous phase before mixing with acetonitrile. After elimination of redundant variables, a nine-dimensional model is identified and its prediction ability is evaluated by external validation based on three solutes not involved in model generation and by cross-validation. A multilinear counterpart in terms of the same descriptors is seen to provide a noticeably poorer retention prediction.
Keywords: Retention models; Artificial neural network; Reversed-phase high-performance liquid chromatography; Phenoxy acids; Mobile phase;

Ensemble preprocessing of near-infrared (NIR) spectra for multivariate calibration by Lu Xu; Yan-Ping Zhou; Li-Juan Tang; Hai-Long Wu; Jian-Hui Jiang; Guo-Li Shen; Ru-Qin Yu (138-143).
Preprocessing of raw near-infrared (NIR) spectral data is indispensable in multivariate calibration when the measured spectra are subject to significant noises, baselines and other undesirable factors. However, due to the lack of sufficient prior information and an incomplete knowledge of the raw data, NIR spectra preprocessing in multivariate calibration is still trial and error. How to select a proper method depends largely on both the nature of the data and the expertise and experience of the practitioners. This might limit the applications of multivariate calibration in many fields, where researchers are not very familiar with the characteristics of many preprocessing methods unique in chemometrics and have difficulties to select the most suitable methods. Another problem is many preprocessing methods, when used alone, might degrade the data in certain aspects or lose some useful information while improving certain qualities of the data. In order to tackle these problems, this paper proposes a new concept of data preprocessing, ensemble preprocessing method, where partial least squares (PLSs) models built on differently preprocessed data are combined by Monte Carlo cross validation (MCCV) stacked regression. Little or no prior information of the data and expertise are required. Moreover, fusion of complementary information obtained by different preprocessing methods often leads to a more stable and accurate calibration model. The investigation of two real data sets has demonstrated the advantages of the proposed method.
Keywords: Ensemble preprocessing; NIR spectra; Multivariate calibration; MCCV stacked regression;

Gold nanoparticle/polyaniline nanotube membranes on the glassy carbon electrode (Au/nanoPAN/GCE) were constructed for the electrochemical sensing of the immobilization and hybridization of DNA. The properties of the Au/nanoPAN/GCE, the characteristics of the immobilization and hybridization of DNA were studied by cyclic voltammetry, differential pulse voltammetry and electrochemical impedance spectroscopy. The synergistic effect of the two kinds of nanomaterials, nanogold and nanoPAN, could enhance dramatically the sensitivity for the DNA hybridization recognition. DNA sequence-specific of phosphinothricin acetyltransferase gene (PAT) existing in some transgenic crops was detected by electrochemical impedance spectroscopic measurement. The dynamic detection range of the sequence-specific DNA was from 1.0 × 10−12 to 1.0 × 10−6  mol L−1, and the detection limit was 3.1 × 10−13  mol L−1. This biosensor had much wider dynamic detection range and lower detection limit for the DNA detection as compared with other DNA biosensors reported by us. The biosensor also has good selectivity, stability and reproducibility.
Keywords: Polyaniline nanotube; Gold nanoparticle; Deoxyribonucleic acid biosensor; Electrochemical impedance spectroscopy; Phosphinothricin acetyltransferase gene;

A simple polymerization strategy has been used to produce a monolithic solid phase micro extraction (SPME) fiber on the basis of molecularly imprinted polymer able to couple with GC and GC–MS for selective extraction and analysis of triazine herbicides. A fiber was produced by copolymerization of methacrylic acid–ethylene glycol dimethacrylate imprinted with atrazine.The effective factors influencing the polymerization have been investigated and are detailed here. At the optimum conditions the prepared fiber is firm, inexpensive, durable and thermally stable up to 280 °C which has vital importance in SPME coupled with GC or GC/MS. In addition, the influences of pH, extraction time and temperature on the extraction efficiency of analytes were optimized. Selectivity of prepared fibers in relation to triazine herbicides and some of the other pesticide has been investigated. The high extraction efficiency was obtained for atrazine, simazine, propazine, cyanazine, ametryn, terbutryn and prometryn yielding the detection limits of 20, 70, 80, 81, 69, 88 and 68 ng mL−1, respectively and the high quantities of recoveries. The reliability of prepared fiber to extraction of atrazine and other analogues in real samples has been investigated and proved by implementation of SPME in spiked samples such as tap water, onion and rice.
Keywords: Gas chromatography; Solid-phase microextraction; Molecularly imprinted polymer; Atrazine; Triazine herbicides;

In the microchannels made of hydrophobic materials, the fluid velocity is determined by the zeta potential and velocity slip, both of which may be inhomogeneous due to the adsorption of protein to the channel wall. The inhomogeneity of zeta potential and slip coefficient sometimes causes recirculating flows which in turn affect the transport and mixing of solutes through the microchannels. In the present investigation we devise a method for the simultaneous estimation of inhomogeneous zeta potential and inhomogeneous slip coefficient using velocity measurements. A conjugate gradient method supplemented by the adjoint variable method is adopted in the solution of the relevant inverse problem to reduce the computational burden. The present method is found to estimate the inhomogeneous zeta potential and the slip coefficient simultaneously even with noisy velocity measurements. This method is expected to contribute to the optimal design and robust operation of various microfluidic devices, where the flow patterns and the volumetric flow rates are critically influenced by the profiles of inhomogeneous zeta potential and inhomogeneous slip coefficient.
Keywords: Electroosmotic flow; Inhomogeneous zeta potential; Inhomogeneous slip coefficient; Simultaneous estimation; Adjoint variable method;

Ultrasensitive determination of silver ion based on synchronous fluorescence spectroscopy with nanoparticles by Lun Wang; A.-Ni Liang; Hong-Qi Chen; Yan Liu; Bin-bin Qian; Jie Fu (170-176).
Based on the characteristics of synchronous fluorescence spectroscopy (SFS), a new method with high sensitivity and selectivity was developed for rapid determination of silver ion with functional cadmium sulphide (CdS) nanoparticles as a fluorescence probe. When Δλ (λ em  −  λ ex) = 215 nm, maximum synchronous fluorescence is produced at 304 nm. Under optimal conditions, functional cadmium sulphide displayed a calibration response for silver ion over a wide concentration range from 0.8 × 10−10 to 1.5 × 10−8  mol L−1. The limit of detection was 0.4 × 10−10  mol L−1 and the relative standard deviation of seven replicate measurements for the lowest concentration (0.8 × 10−10  mol L−1) was 2.8%. Compared with several fluorescence methods, the proposed method had a wider linear range and improved the sensitivity. Furthermore, the concentration dependence of the synchronous fluorescence intensity is effectively described by a Langmuir-type binding isotherm.
Keywords: Synchronous fluorescence; Cadmium sulphide nanoparticles; Silver ion; Determination;

Analysis of human tear fluid by Raman spectroscopy by J. Filik; N. Stone (177-184).
Tear fluid is a complex aqueous solution containing proteins, metabolites, electrolytes and lipids. This study uses Raman spectroscopy to analyse the composition of human tear fluid from three healthy volunteers. Two different methods are used to obtain Raman spectra from the 3 μL tear samples: (i) solution-phase Raman spectroscopy, and (ii) drop coating deposition Raman spectroscopy (DCDRS). Tear samples were either basal fluid, or yawn reflex secreted fluid. Calibration of the solution technique with standard protein solutions (5–15 mg mL−1) showed that this method could predict the protein concentration (cross-validation) with an error of less than 1 mg mL−1. The Raman signals from the tear fluid were very weak but signals due to protein and urea were clearly observable in all samples. The drop coating deposition technique was shown to produce very high signal-to-noise spectra for relatively short acquisition times, and small sample volumes. Raman point mapping combined with principal components analysis showed that the protein, urea, bicarbonate and lipid could all be detected in the tear samples and that the distribution of these components was inhomogeneous. Their position within the drying pattern was shown to depend on their relative solubilities. The results of this study suggest that solution Raman measurements may be calibrated to give the total tear protein concentration and DCDRS could be used to give a fingerprint of the tear protein (and lipid) composition.
Keywords: Raman spectroscopy; Tear fluid; Lacrimal fluid; Drop coating deposition Raman; Spectral mapping;

Determination of total sterols in brown algae by Fourier transform infrared spectroscopy by Naïma Bouzidi; Yasmina Daghbouche; Mohamed El Hattab; Zahia Aliche; Gérald Culioli; Louis Piovetti; Salvador Garrigues; Miguel de la Guardia (185-189).
A procedure is proposed for the determination of the total amount of sterols in brown algae Bifurcaria bifurcata, Cladostephus hirsitus, Dictyota dichotoma and Cystoseira sedoides, globally determined as fucosterol, which is the major sterol contained in these algae. The method involves the use of cholesterol as reference standard and a correction factor of 1.259 ± 0.003, which represents the ratio between the slopes of calibration lines obtained from fucosterol and cholesterol. The method provides precise and accurate results for the IR analysis of real samples.
Keywords: Fourier transform infrared; Solid-phase extraction; Sterol determination; Fucosterol; Cholesterol; Brown algae;

A weak chemiluminescence (CL) emission was observed upon mixing peroxynitrite (ONOO) with dihydralazine sulfate (DHZS). Further experiments showed that carbonate media could enhance the CL emission significantly. Based on these observations, a novel flow injection CL method for the determination of DHZS is developed. The CL signal is linearly with DHZS concentration in the range of 0.01–3.0 μg mL−1 with a detection limit of 3.6 ng mL−1. The method was applied to the analysis of DHZS in pharmaceutical preparations and compared well with the high-performance liquid chromatography (HPLC) method. The CL mechanism is discussed and it is postulated that it involves nitrosoperoxocarboxylate (ONOOCO2 ), which is an unstable adduct and can rapidly decompose into •NO2 and •CO3 radical. The latter can then oxidize DHZS to give out strong CL emission.
Keywords: Peroxynitrite; Carbon dioxide; Chemiluminescence; Dihydralazine sulfate;

Hydroxyl radicals (•OH) generated in the human body may play an important role in tissue injury at sites of inflammation in oxidative stress-originated diseases. As a more convenient, efficient, and less costly alternative to HPLC/electrochemical detection techniques and to the nonspecific, low-yield deoxyribose (TBARS) test, we used a salicylate probe for detecting •OH generated by the reaction of iron(II)–EDTA complex with H2O2. The produced hydroxyl radicals attack both the salicylate probe and the hydroxyl radical scavengers that are incubated in solution for 10 min. Added radical scavengers compete with salicylate for the •OH produced, and diminish chromophore formation from Cu(II)–neocuproine. At the end of the incubation period, the reaction was stopped by adding catalase. With the aid of this reaction, a kinetic approach was adopted to assess the hydroxyl radical scavenging properties of polyphenolics, flavonoids and other compounds (e.g., ascorbic acid, glucose, mannitol). A second-order rate constant for the reaction of the scavenger with •OH could be deduced from the inhibition of colour formation due to the salicylate probe. In addition to phenolics and flavonoids, five kinds of herbs were evaluated for their •OH scavenging activity using the developed method. The modified CUPRAC (cupric ion reducing antioxidant capacity) assay proved to be efficient for ascorbic acid, gallic acid and chlorogenic acid, for which the deoxyribose assay test is basically nonresponsive. An important contribution of this developed assay is the inhibition of the Fenton reaction with catalase degradation of hydrogen peroxide so that the remaining H2O2 would neither give a CUPRAC absorbance nor involve in redox cycling of phenolic antioxidants, enabling the rapid assay of polyphenolics.
Keywords: Hydroxyl radical detection; Radical scavenging; Cupric reducing antioxidant capacity (CUPRAC) assay; Phenolics; Flavonoids;

Zinc oxide nanoparticles-chitosan composite film for cholesterol biosensor by Raju Khan; Ajeet Kaushik; Pratima R. Solanki; Anees A. Ansari; Manoj K. Pandey; B.D. Malhotra (207-213).
Zinc oxide nanoparticles (NanoZnO) uniformly dispersed in chitosan (CHIT) have been used to fabricate a hybrid nanocomposite film onto indium-tin-oxide (ITO) glass plate. Cholesterol oxidase (ChOx) has been immobilized onto this NanoZnO–CHIT composite film using physiosorption technique. Both NanoZnO-CHIT/ITO electrode and ChOx/NanoZnO-CHIT/ITO bioelectrode have been characterized using Fourier transform-infrared (FTIR), X-ray diffraction (XRD), cyclic voltammetry (CV), scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS) techniques, respectively. The ChOx/NanoZnO-CHIT/ITO bioelectrode exhibits linearity from 5 to 300 mg dl−1 of cholesterol with detection limit as 5 mg dl−1, sensitivity as 1.41 × 10−4  A mg dl−1 and the value of Michaelis–Menten constant (K m) as 8.63 mg dl−1. This cholesterol biosensor can be used to estimate cholesterol in serum samples.
Keywords: Zinc oxide nanoparticles-Chitosan film; Cholesterol oxidase; Cholesterol; Biosensor;

A porphyrin derivative containing 2-(oxymethyl)pyridine units showing unexpected ratiometric fluorescent recognition of Zn2+ with high selectivity by Chun-Yan Li; Xiao-Bing Zhang; Yan-Yan Dong; Qiu-Juan Ma; Zhi-Xiang Han; Yan Zhao; Guo-Li Shen; Ru-Qin Yu (214-221).
A porphyrin derivative (1), containing two 2-(oxymethyl)pyridine units has been designed and synthesized as chemosensor for recognition of metal ions. Unlike many common porphyrin derivatives that show response to different heavy metal ions, compound 1 exhibits unexpected ratiometric fluorescence response to Zn2+ with high selectivity. The response of the novel chemosensor to zinc was based on the porphyrin metallation with cooperating effect of 2-(oxymethyl)pyridine units. The change of fluorescence of 1 was attributed to the formation of an inclusion complex between porphyrin ring and Zn2+ by 1:1 complex ratio (K  = 1.04 × 105), which has been utilized as the basis of the fabrication of the Zn2+-sensitive fluorescent chemosensor. The analytical performance characteristics of the proposed Zn2+-sensitive chemosensor were investigated. The sensor can be applied to the quantification of Zn2+ with a linear range covering from 3.2 × 10−7 to 1.8 × 10−4  M and a detection limit of 5.5 × 10−8  M. The experiment results show that the response behavior of 1 to Zn2+ is pH-independent in medium condition (pH 4.0–8.0) and show excellent selectivity for Zn2+ over transition metal cations.
Keywords: Fluorescent recognition; Zinc(II) ions; Porphyrin; Ratiometric;

Selective adenosine-5′-monophosphate uptake by water-compatible molecularly imprinted polymer by Florent Breton; Raphaël Delépée; Damien Jégourel; Dominique Deville-Bonne; Luigi A. Agrofoglio (222-229).
Molecularly imprinted polymers (MIPs) were prepared for adenosine-5′-monophosphate (AMP), a substrate of AMP-activated protein kinase. The template molecule was formed by the vinylphenylboronate diester of adenosine on which 5′-free hydroxide was protected by tert-butyldimethylsilyl group in order to mimic the steric hindrance of the phosphate moiety of AMP. Molecular imprinting was performed by complexing acrylamide and the template in a highly cross-linked polymer. MIPs were tested in batch experiments with aqueous samples of nucleotides and a number of parameters were investigated. The use of tetrabutylammonium hydroxide (TBAH) was necessary to obtain a rebinding of nucleotides on MIP. The adsorption of AMP was optimal in 5 mM ammonium acetate buffer solution pH 9.5 for 30 min, with 30 mM of TBAH. The imprinted polymer was selective for AMP towards others nucleotides or deoxi analogues.
Keywords: Molecularly imprinted polymer; Adenosine monophosphate; Boronic acid; Water-compatible polymer;

This study establishes the applicability of using high-performance liquid chromatography (HPLC) with fluorescence detection for the simultaneous determination of malondialdehyde (MDA) and ofloxacin (OFL). The MDA and OFL were separated through a reverse-phase C18 column (250 mm × 4.6 mm) at a flow rate of 1.0 mL min−1 and then detected using a fluorescence detector (excitation: 532 nm; emission: 553 nm). The separation conditions were optimized by varying the concentration and pH of the phosphate buffer and the percentage of organic solvent; the optimal mobile phase was a mixture of 50 mM phosphate buffer (adjusted to pH 5.8 with potassium hydroxide) and methanol (45:55, v/v). The retention times of MDA and OFL were 3.6 and 5.9 min, respectively, with detection limits (at a signal-to-noise ratio of 3) of 0.015 and 4.0 μM, respectively. This method afforded linear responses between the MDA and OFL concentrations and the HPLC peak areas within the ranges 0.15–2.43 μM and 0.06–1.0 mM, respectively. The precisions of the determinations of MDA and OFL, measured in terms of relative standard deviations, were 1.6–5.0% and 1.9–3.6%, respectively, for intra-day assays and 1.0–4.3% and 0.3–1.8%, respectively, for inter-day assays. The average recoveries of MDA and OFL spiked in plasma were 100.4% and 98.8%, respectively. To the best of our knowledge, this paper describes the first practical analytical approach toward simultaneously monitoring the levels of MDA and OFL in plasma. The OFL-induced oxidative stress measured using this method indicated that OFL treatment did not markedly increase the level of MDA.
Keywords: Malondialdehyde; Ofloxacin; Oxidative stress; Antimicrobial agent; Liquid chromatography;

Lateral-flow colloidal gold-based immunoassay for the rapid detection of deoxynivalenol with two indicator ranges by Anna Yu. Kolosova; Liberty Sibanda; Frédéric Dumoulin; Janet Lewis; Etienne Duveiller; Carlos Van Peteghem; Sarah De Saeger (235-244).
A lateral-flow immunoassay using a colloidal gold-labelled monoclonal antibody was developed for the rapid detection of deoxynivalenol (DON). Different parameters, such as the amount of immunoreagents, type of the materials, composition of the blocking solution and of the detector reagent mixture, were investigated to provide the optimum assay performance. The experimental results demonstrated that such a visual test had an indicator range rather than a cut-off value. Thus, tests for DON determination with two different indicator ranges of 250–500 and 1000–2000 μg kg−1 were designed. The method allowed detection of DON at low and high concentration levels, which could be useful for research and practical purposes. The assay applied to spiked wheat and pig feed samples demonstrated accurate and reproducible results. The applicability of the developed lateral-flow test was also confirmed under real field conditions. The test strips prepared in Belgium were sent to Mexico, where they were used for the screening of DON contamination in different bread wheat entries from Fusarium Head Blight inoculated plots. The results were compared with those obtained by ELISA and LC–MS/MS. A poor correlation between ELISA and LC–MS/MS was observed. Visual results of the dipstick tests were in a good agreement with the results of the LC–MS/MS method. Coupled with a simple and fast sample preparation, this qualitative one-step test based on the visual evaluation of results did not require any equipment. Results could be obtained within 10 min. The described assay format can be used as a simple, rapid, cost-effective and robust on-site screening tool for mycotoxin contamination in different agricultural commodities.
Keywords: Deoxynivalenol; Lateral-flow immunoassay; Colloidal gold; Rapid method; Wheat;