Analytica Chimica Acta (v.744, #C)

Sodium dodecyl sulfate-modified electrochemical paper-based analytical device for determination of dopamine levels in biological samples by Poomrat Rattanarat; Wijitar Dungchai; Weena Siangproh; Orawon Chailapakul; Charles S. Henry (1-7).
Display Omitted► Selective detection of dopamine over uric acid and ascorbic acid in serum. ► Multilayer electrochemical paper-based analytical device constructed for preconcentration and detection of biological molecules in complex matrix. ► Novel preconcentration mechanism based on electrostatic interactions between dopamine and sodium dodecyl sulfate.We report the development of an electrochemical paper-based analytical device (ePAD) for the selective determination of dopamine (DA) in model serum sample. The ePAD device consists of three layers. In the top layer, SU-8 photoresist defines a hydrophilic sample application spot on the filter paper. The middle layer was made from transparency film and contained two holes, one for sample preconcentration and the other for the surfactant to allow transfer to the third layer. A screen-printed carbon electrode formed the bottom layer and was used for electrochemical measurements. In the absence of the anionic surfactant, sodium dodecyl sulfate (SDS), the oxidation peaks of DA, ascorbic acid (AA) and uric acid (UA) overlapped. With the addition of SDS, the DA oxidation peak shifted to more negative values and was clearly distinguishable from AA and UA. The oxidation potential shift was presumably due to preferential electrostatic interactions between the cationic DA and the anionic SDS. Indeed, whilst the SDS-modified paper improved the DA current five-fold, the non-ionic Tween-20 and cationic tetradecyltrimethylammonium bromide surfactants had no effect or reduced the current, respectively. Furthermore, only the SDS-modified paper showed the selective shift in oxidation potential for DA. DA determination was carried out using square-wave voltammetry between −0.2 and 0.8 V vs. Ag/AgCl, and this ePAD was able to detect DA over a linear range of 1–100 μM with a detection limit (S/N = 3) of 0.37 μM. The ePAD seems suitable as a low cost, easy-to-use, portable device for the selective quantitation of DA in human serum samples.
Keywords: Sodium dodecyl sulfate; Dopamine; Paper-based analytical device; Electrochemical detection; Human serum; Ascorbic acid; Uric acid;

Display Omitted► Analytical evaluation of nanostructured drug delivery systems prepared by scCO2. ► Physicochemical characterization by chromatography and spectroscopy. ► Particle characterization by microscopy and thermal analysis. ► Release assessment by batch, continuous and diffusion devices.The analytical characterization of drug delivery systems prepared by means of green manufacturing technologies using CO2 as a processing fluid is here reviewed. The assessment of the performance of nanopharmaceuticals designed for controlled drug release may result in a complex analytical issue and multidisciplinary studies focused on the evaluation of physicochemical, morphological and textural properties of the products may be required. The determination of the drug content as well as the detection of impurities and solvent residues are often carried out by chromatography. Assays on solid state samples relying on X-ray, vibrational and nuclear magnetic resonance spectroscopies are of great interests to study the composition and structure of pharmaceutical forms. The morphology and size of particles are commonly checked by microscopy and complementary chemical information can be extracted in combination with spectroscopic accessories. Regarding the thermal behavior, calorimetric and thermogravimetric techniques are applied to assess the thermal transitions and stability of the samples. The evaluation of drug release profiles from the nanopharmaceuticals can be based on various experimental set-ups depending on the administration route to be considered. Kinetic curves showing the evolution of the drug concentration as a function of time in various physiological conditions (e.g., gastric, plasmatic or topical) are recorded commonly by UV–vis spectroscopy and/or chromatography. Representative examples are commented in detail to illustrate the characterization strategies.
Keywords: Drug delivery systems; Nanopharmaceuticals; Green manufacturing; Supercritical fluid processing; Drug analysis; Drug release characterization;

Display Omitted► Applicability of surfactants in constant-current coulometry is shown for the first time. ► Reactions of antioxidants with electrogenerated titrants in surfactant media are investigated. ► Water insoluble antioxidants can be determined in water media with addition of surfactants. ► Coulometric determination of antioxidants in pharmaceutical dosage forms using surfactants media is developed.Effect of surfactant presence on electrochemical generation of titrants has been evaluated and discussed for the first time. Cationic (1-dodecylpyridinium and cetylpyridinium bromide), anionic (sodium dodecyl sulfate) and nonionic (Triton X100 and Brij® 35) surfactants as well as nonionic high molecular weight polymer (PEG 4000) do not react with the electrogenerated bromine, iodine and hexacyanoferrate(III) ions. The electrogenerated chlorine chemically interact with Triton X100 and Brij® 35. The allowable range of surfactants concentrations providing 100% current yield has been found. Chain-breaking low molecular weight antioxidants (ascorbic acid, rutin, α-tocopherol and retinol) were determined by reaction with the electrogenerated titrants in surfactant media. Nonionic and cationic surfactants can be used for the determination of antioxidants by reaction with the electrogenerated halogens. On contrary, cationic surfactants gives significantly overstated results of antioxidants determination with electrogenerated hexacyanoferrate(III) ions. The use of surfactants in coulometry of α-tocopherol and retinol provides their solubilization and allows to perform titration in water media. Simple, express and reliable coulometric approach for determination of α-tocopherol, rutin and ascorbic acid in pharmaceuticals using surfactant media has been developed. The relative standard deviation of the measurements does not exceed of 5%.
Keywords: Surfactant media; Constant-current coulometry; Electrogenerated oxidants; Antioxidants; Pharmaceutical analysis;

Display Omitted► Electroanalytics in non-conducting media. ► Chemiresistors based on conducting polymers as quantitative redox sensors. ► Simultaneous conductometry with chemiresistor and redox potentiometry. ► A unique possibility to measure redox-potential in gases and nonconducting liquids. ► An application for redox-titration in non-conducting organic phase is demonstrated.Electroanalytical methods have been applied only in conducting media. An application of conducting polymers allows to overcome this limitation. If such material is in electrochemical equilibrium with dissolved redox active species, its electrical conductivity depends on the redox potential of these species. Therefore, conductometric measurements with conducting polymers can provide about the same information as classical redox electrodes. The approach was applied for redox titration. Equivalent points obtained by this titration in aqueous and organic electrolytes were identical. Then the approach was applied for determination of bromine number by redox titration in non-conducting organic phase.
Keywords: Conducting polymers; Redox titration; Conductometric sensors; Non-conductive media;

Visible light induced photoelectrochemical biosensing based on oxygen-sensitive quantum dots by Wenjing Wang; Lei Bao; Jianping Lei; Wenwen Tu; Huangxian Ju (33-38).
Display Omitted► The near-infrared QDs are synthesized in an aqueous solution. ► QDs-based biosensor exhibits visible-light induced cathodic photocurrent. ► The oxygen dependency of the photocurrent is verified. ► A photoelectrochemical strategy is established by coupling with enzymatic reaction. ► Photoelectrochemical sensor shows high upper detection limit, acceptable stability and accuracy.A visible light induced photoelectrochemical biosensing platform based on oxygen-sensitive near-infrared quantum dots (NIR QDs) was developed for detection of glucose. The NIR QDs were synthesized in an aqueous solution, and characterized with scanning electron microscopy and X-ray photoelectron spectroscopy. The as-prepared NIR QDs were employed to construct oxygen-sensitive photoelectrochemical biosensor on a fluorine-doped tin oxide (FTO) electrode. The oxygen dependency of the photocurrent was investigated at as-prepared electrode, which demonstrated the signal of photocurrent is suppressed with the decreasing of oxygen. Coupling with the consumption of oxygen during enzymatic reaction, a photoelectrochemical strategy was proposed for the detection of substrate. Using glucose oxidase (GOx) as a model enzyme, that is, GOx was covalently attached to the surface of CdTe QDs, the resulting biosensor showed the sensitive response to glucose. Under the irradiation of visible light of a wavelength at 505 nm, the proposed photoelectrochemical method could detect glucose ranging from 0.1 mM to 11 mM with a detection limit of 0.04 mM. The photoelectrochemical biosensor showed a good performance with high upper detection limit, acceptable stability and accuracy, providing an alternative method for monitoring biomolecules and extending the application of near-infrared QDs.
Keywords: Photoelectrochemistry; Near-infrared quantum dots; Biosensing; Enzyme; Glucose;

Thin layer coulometric determination of nitrate in fresh waters by Manzar Sohail; Roland De Marco; Krystina Lamb; Eric Bakker (39-44).
Display Omitted► A tubular membrane nitrate ion-selective electrode is capable of the coulometric analysis of nitrate in lake water. ► Exhaustive nitrate electrolysis in an ion-selective coulometric sensor yielded a calibration free response. ► The coulometric nitrate ion-selective electrode possesses a comparable precision and accuracy to spectrophotometry.A nitrate ion-selective electrode (ISE) employing a permeable tubular membrane impregnated with a conventional ISE cocktail has been used successfully in the coulometric analysis of nitrate in fresh waters. The liquid ISE membrane comprising a nitrate ionophore [tridodecylmethylammonium nitrate (TDMAN)], lipophilic electrolyte [tetradodecyl-ammoniumtetrakis(4-chlorophenyl)borate (ETH 500)] and plasticizer [bis(3-ethyl-hexyl)sebacate (DOS)] was supported on a porous polypropylene tube. Coulometric analysis with the tubular membrane ISE showed that nitrate could be detected in the range 10–100 μM with a precision of 2.3% relative standard deviation (RSD), limit of detection of 1.1 μM and relative accuracy of 4.4% compared to a certified reference material (CRM) Lake sample.
Keywords: Ion selective electrode; Coulometry; Nutrients; Nitrate; Environmental analysis;

Display Omitted► We developed an electrochemical method to detect phosphorylation/dephosphorylation. ► Peptides were functionalized on carbon nanofiber nanoelectrode arrays. ► Real-time electrochemical impedance spectroscopy (REIS) showed reversible changes. ► The REIS data of phosphatase showed an exponential decay with a consistent k cat /K m. ► The reaction was analyzed using the Michaelis–Menten heterogeneous enzymatic model.We demonstrate the feasibility of a label-free electrochemical method to detect the kinetics of phosphorylation and dephosphorylation of surface-attached peptides catalyzed by kinase and phosphatase, respectively. The peptides with a sequence specific to c-Src tyrosine kinase and protein tyrosine phosphatase 1B (PTP1B) were first validated with ELISA-based protein tyrosine kinase assay and then functionalized on vertically aligned carbon nanofiber (VACNF) nanoelectrode arrays (NEAs). Real-time electrochemical impedance spectroscopy (REIS) measurements showed reversible impedance changes upon the addition of c-Src kinase and PTP1B phosphatase. Only a small and unreliable impedance variation was observed during the peptide phosphorylation, but a large and fast impedance decrease was observed during the peptide dephosphorylation at different PTP1B concentrations. The REIS data of dephosphorylation displayed a well-defined exponential decay following the Michaelis–Menten heterogeneous enzymatic model with a specific constant, k cat /K m, of (2.1 ± 0.1) × 107  M−1  s−1. Consistent values of the specific constant was measured at PTP1B concentration varying from 1.2 to 2.4 nM with the corresponding electrochemical signal decay constant varying from 38.5 to 19.1 s. This electrochemical method can be potentially used as a label-free method for profiling enzyme activities in fast reactions.
Keywords: Real-time electrochemical impedance spectroscopy; Phosphorylation and dephosphorylation; Carbon nanofiber nanoelectrode array; Heterogeneous enzyme kinetics; c-Src kinase; Protein tyrosine phosphatase 1B;

Gas chromatography mass spectrometry chromatogram of 18 amino acids extracted from selenium enriched yeast. Microwave enhanced acid hydrolysis followed by methyl chlorofomate derivatization was employed for sample preparation.Display Omitted► A rapid microwave digestion procedure for protein hydrolysis is described. ► A simple methyl chloroformate derivatization combined with GC–MS determination was employed. ► A total of 17 amino acids were determined. ► Limits of detection in the sub-nmol range, suitable for rapid monitoring of amino acids in yeast.A simple, rapid microwave digestion procedure for protein hydrolysis preceding the determination of amino acids in yeast using gas chromatography–mass spectrometry (GC–MS) is described. Protein hydrolysis was performed in a focused microwave using 4 M methanesulfonic acid (MAS). Amino acids were derivatized with methyl chlorofomate (MCF) and extracted into chloroform prior to GC–MS analysis. The microwave parameters, including power, temperature and heating time, were optimized. It was found that temperature and heating time were the most influential factors. A total of 17 amino acids were determined in selenium-enriched yeast with use of standard addition calibration. Limits of detection and quantitation (LODs/LOQs) of the amino acids measured were in the sub-nmol range, suitable for monitoring of amino acids in yeast and other food products.
Keywords: Amino acids; Microwave-irradiation hydrolysis; Gas chromatography–mass spectrometry; Selenium enriched yeast;

A specific Tween-80-Rhodamine S-MWNTs phosphorescent reagent for the detection of trace calcitonin by Jia-Ming Liu; Xiao-Mei Huang; Li-Hong Zhang; Zhi-Yong Zheng; Xuan Lin; Xiao-Yang Zhang; Li Jiao; Ma-Lin Cui; Shu-Lian Jiang; Shao-Qin Lin (60-67).
A new Tween-80-Rhodamine S-water-soluble multi-walled carbon nanotubes (Tween-80-Rhod.S-MWNTs-EDC-NHS, TRMEN) phosphorescent labelling reagent was developed. High sensitive solid substrate room temperature phosphorescence immunoassay (SSRTPIA) for the determination of calcitonin (CT) in human serum and the prediction of human diseases based on the TRMEN could be used to label anti-calcitonin antibody (AbCT) to form the TRMEN-AbCT labelling product, which could take high specific immunoreaction with CT causing that the ΔI p of the system was linear to the content of CT. Moreover, the reaction mechanisms of both labelling AbCT by TRMEN and SSRTPIA for the determination of trace CT were discussed. This research not only provides a new hormones analysis method, but also expands the application field of MWNTs and promotes the development of SSRTP and IA.Display Omitted► A Tween-80-Rhodamine S-multi-walled carbon nanotubes labelling reagent was developed. ► The phosphorescence immunoassay was established for the determination of calcitonin. ► This method has been applied to determine CT and the prediction of diseases. ► The structure of MWNTs was characterized with SEM and IR. ► The mechanisms for both determining trace CT and labelling AbCT were discussed.The present study proposed a simple sensitive and specific immunoassay for the quantification of calcitonin (CT) in human serum with water-soluble multi-walled carbon nanotubes (MWNTs). The ―COOH group of MWNTs could react with the ―NH― group of rhodamine S (Rhod.S) molecules to form Rhod.S-MWNTs, which could emit room temperature phosphorescence (RTP) on acetate cellulose membrane (ACM) and react with Tween-80 to form micellar compound. Tween-80-Rhod.S-MWNTs (TRM), as a phosphorescent labelling reagent, could dramatically enhance the RTP signal of the system. The developed TRM phosphorescent reagent was used to label anti-calcitonin antibody (AbCT) to form the TRM-AbCT labelling product, which could take high specific immunoreaction with CT, and the ΔI p (= I p2  −  I p1, I p2 and I p1 were the phosphorescence intensity of the test solution and the blank sample, respectively) of the system was linear to the content of CT. Hence, a new solid substrate room temperature phosphorescence immunoassay (SSRTPIA) was established for the determination of CT in human serum. This sensitive (limit of quantification (LOQ) was 8.0 × 10−14  g mL−1), accurate, selective and precise method has been applied to determine CT in human serum and predict primary osteoporosis and fractures, with the results in good agreement with those obtained by chemiluminescence immunoassay (CLIA). Simultaneously, the structure of MWNTs was characterized with scanning electron microscopy (SEM) and infrared spectroscopy (IR), and the reaction mechanisms of both labelling AbCT with TRM and SSRTPIA for the determination of trace CT were discussed.
Keywords: Calcitonin; Tween-80-Rhodamine S-multi-walled carbon nanotubes phosphorescent reagent; Solid substrate room temperature phosphorescence immunoassay; Prediction of human diseases;

Display Omitted► Influence of the porogenic solvent on the selectivity of MIPs was investigated. ► Recognition properties in different porogens were evaluated via chromatography. ► Role of the porogen was studied via complementary molecular dynamics simulations.In molecular imprinting the porogen plays a decisive role, as it not only affects the physical properties of the resulting polymer including its porosity, the specific surface area, and the swelling behavior, but also governs the stability of the prepolymerization complex, which in turn decisively determines the recognition properties of the resulting molecularly imprinted polymer (MIP).In this study, the influence of the porogen on the selectivity of MIPs was investigated. Therefore, bulk MIPs against 4-nitrophenol using 4-vinylpyridine (4-VP) as functional monomer and ethylene glycol dimethacrylate (EDMA) as crosslinker were prepared in acetonitrile and chloroform. The recognition properties of both MIPs were evaluated during chromatographic studies using the respective porogenic solvents as mobile phase for both MIPs. Along with the characterization of the morphology of the obtained polymers via SEM and BET analysis, the beneficial nature of chloroform as porogen for imprinting 4-NP was experimentally demonstrated and verified by findings obtained from complementary molecular dynamics simulations. Moreover, the application of chloroform as mobile phase for the MIP prepared in acetonitrile and vice versa clearly demonstrated the dependence of the resulting recognition properties on the selection of the mobile phase.
Keywords: Molecularly imprinted polymers (MIPs); 4-Nitrophenol; Molecular modeling; HPLC; Selectivity;

The sensor realized the separation and determination of benzenediol isomers.Display Omitted► Determination of benzenediol isomers at the same time. ► A chemiluminescence (CL) array sensor was described. ► Graphene-magnetite-molecularly imprinted polymer was used. ► The adsorption capacity was improved.A chemiluminescence (CL) array sensor for determination of benzenediol isomers simultaneously using the system of luminol–NaOH–H2O2 based on a graphene-magnetite-molecularly imprinted polymer (GM-MIP) is described. Use of graphene in the GM-MIP thus prepared is helpful to improve the adsorption capacity, while use of magnetite nanoparticles can facilitate the isolation of GM-MIP at end of their synthesis, and rendering easier the use of the polymers in the array sensor. The adsorption performance and properties were characterized. The GM-MIP was used to increase the selectivity in CL analysis. In addition, the sensor was reusable and of good selectivity and adsorption capacity. The array sensor was finally used for the determination of hydroquinone, resorcinol and catechol in waste water samples simultaneously.
Keywords: Chemiluminescence; Array sensor; Benzenediol isomers; Graphene; Magnetite; Molecularly imprinted polymers;

Highly sensitive detection of nitroaromatic explosives using an electrospun nanofibrous sensor based on a novel fluorescent conjugated polymer by Yuanyuan Long; Haibo Chen; Huaming Wang; Zhou Peng; Yufei Yang; Guoqing Zhang; Na Li; Feng Liu; Jian Pei (82-91).
Display Omitted► A novel electrospun nanofibrous sensor was constructed for nitroaromatic explosives. ► A new conjugated polymer P was developed as a fluorescent probe of the sensor. ► The sensor exhibited remarkable sensitivity toward nitroaromatic explosive vapors. ► The present strategy shows potentials in fabrication of portable sensing devices.An electrospun nanofibrous explosive sensor was first constructed based on a newly developed fluorescent conjugated polymer P containing heteroatom polycyclic units. Electrospinning by doping polymer P as a fluorescent probe in a polystyrene supporting matrix afforded a fluorescence nanofibrous film with unique porous structures, and effectively avoided the aggregation of polymer P. The novel explosive sensor exhibited stable fluorescence property, satisfactory reversibility with less than 5% loss of signal intensity after four quenching–regeneration cycles, and good reproducibility among three batches with a relative standard deviation of 2.8%. Such fabricated sensor also showed remarkable sensitivity toward a series of trace nitroaromatic explosive vapors, including picric acid (parts-per-trillion level) and 2,4,6-trinitrotoluene vapor (parts-per-billion level), as well as good selectivity with less than 10% response to typical interferents. Therefore, the present strategy extends the application of different kinds of conjugated polymers for the construction of optical chemosensors.
Keywords: Fluorescent sensor; Electrospinning; Nanofibers; Conjugated polymer; Nitroaromatic explosive; Detection;

Ultrasensitive optical detection of trinitrotoluene by ethylenediamine-capped gold nanoparticles by Dongyue Lin; Honglin Liu; Kai Qian; Xia Zhou; Liangbao Yang; Jinhuai Liu (92-98).
Display Omitted► EDA can interact with the citrate-stabilized Au NPs through electrostatic attraction. ► EDA-capped Au NPs can be used as an ultrasensitive optical probe for TNT detection. ► Direct colorimetric assays for TNT have a detection limit of 400 pM. ► DLS assays evidenced the aggregation of the probe induced by TNT in 0.4 pM level. ► The probe has an excellent discrimination against other nitro compounds.This study found that 1,2-ethylenediamine (EDA) as a primary amine could be modified onto the surface of citrate-stabilized gold nanoparticles (Au NPs), and the EDA-capped Au NPs were successfully used as an ultrasensitive optical probe for TNT detection. The strong donor–acceptor (D–A) interactions between EDA and trinitrotoluene (TNT) at the Au NP/solution interface induced significant aggregation of the EDA-capped Au NPs, and enabled to easily realize the direct colorimetric detection of ultratrace TNT. The results showed that such a color change was readily seen by the naked eye, and the colorimetric detection could be down to 400 pM level of TNT with excellent discrimination against other nitro compounds. UV–vis absorption spectroscopy was used to examine the TNT-induced changes in local surface plasmon resonance (LSPR) of EDA-capped Au NPs, and a new LSPR band at ca. 630 nm arose along with the addition of TNT, which produced a detection limit of TNT down to ca. 40 pM. Furthermore, dynamic light scattering measurements evidenced the ultratrace TNT-induced small changes in the size of the EDA-capped Au NPs, and realized the quick and accurate detection of TNT in 0.4 pM level. These results demonstrated the ultrahigh sensitivity of this optical probe for TNT detection. Moreover, this optical probe is sample, stable, low-cost, and these excellent properties make it quite promising for infield and rapid detection of TNT.
Keywords: Ethylenediamine; Trinitrotoluene; Gold nanoparticles; Colorimetric; Dynamic light scattering;

Display Omitted► Testosterone, epitestosterone and epitestosterone glucuronide in urine were analyzed. ► This rLVSI-CE method enhances sensitivity of monitoring of urine samples by UV detection. ► This method could improve sensitivity up to 2500 folds.In this research, a novel stacking capillary electrophoresis method, repetitive large volume sample injection and sweeping MEKC (rLVSI-sweeping MEKC) were developed to analyze the presence of three androgenic steroids considered as sport doping drugs, testosterone (T), epitestosterone (E) and epitestosterone glucuronide (EG) in urine. This method provides better sensitivity enhancement than the traditional large volume sample stacking-sweeping strategies due to sensitivity enhancement by repetitive injections. This multiple sampling method enhances sensitivity of monitoring of urine samples by UV detection (254 nm). Firstly, the phosphate buffer was filled into an uncoated fused silica capillary and the samples were injected into the capillary at 10 psi for 20 s, and then stacked at −10 kV for 1 min using phosphate buffer containing SDS. The above injecting and stacking steps were repeated five times. Finally, separation was performed at −20 kV, using phosphate buffer containing methanol, SDS and (2-hydroxypropyl)-β-cyclodextrin. Method validation showed that calibration plots were linear (r  ≧ 0.997) over a range of 5–200 ng mL−1 for T, 20–200 ng mL−1 for E and 0.5–500 ng mL−1 for EG. The limits of detection were 1.0 ng mL−1 for T, 5.0 ng mL−1 for E and 200.0 pg mL−1 for EG. When evaluating precision and accuracy, values of RSD and RE in intra-day (n  = 3) and inter-day (n  = 5) analysis were found to be less than 10.0%. Compared with the simple LVSS-sweeping, which is also a stacking strategy, this method further improves sensitivity up to 25 folds (∼2500 folds with MEKC without preconcentration). This method was applied to monitor 10 athletes’ urine, and did not detect any analyte. The novel stacking method was feasible for monitoring of doping by sportsmen.
Keywords: Testosterone; Epitestosterone; Epitestosterone glucuronide; Urine; rLVSI-sweeping CE;