This review highlights recent developments and applications of on-line sample preconcentration techniques to enhance the detection sensitivity in microchip electrophoresis (MCE); references are mainly from 2008 and later. Among various developed techniques, we focus on the sample preconcentration based on the changes in the migration velocity of analytes in two or three discontinuous solutions system, since they can provide the sensitivity enhancement with relatively easy experimental procedures and short analysis times. The characteristic features of the on-line sample preconcentration applied to microchip electrophoresis (MCE) are presented, categorized on the basis of “field strength-” or “chemically” induced changes in the migration velocity. The preconcentration techniques utilizing field strength-induced changes in the velocity include field-amplified sample stacking, isotachophoresis and transient-isotachophoresis, whereas those based on chemically induced changes in the velocity are sweeping, transient-trapping and dynamic pH junction.
The first step toward elucidating the mutagenic effects of chemicals and pathways is to determine the specificity of the mutations generated spontaneously or in response to treatment with mutagens. We constructed a set of plasmid-encoded probes for the specific detection of each type of base substitution mutation. Using these probes, we were able to quickly determine both the mutation rate and the specificity of the mutations caused by different types of mutagens and mutagenic conditions. We also developed a PCR-based method to rapidly and robustly determine the mutation spectrum in response to various mutagenic samples in parallel. This system allows one to not only analyze the mutation specificity of various chemicals, but also to search for novel genetic elements that promote the specific mutation events.
Many empirical parameters have been suggested to measure solvent effects in chemical reactions. Gutmann’s donor number has been a successful parameter to quantify the electron-donating property of the solvent molecule; it is defined as the enthalpy change of the addition reaction of solvent molecule to SbCl5 in 1,2-dichloroethane. Calorimetric measurements can be applied to determine the quantity. Because the existence of water is critical for reactions in organic solvents, we have analyzed the enthalpy change using the titration calorimetry while considering the complexation with water. The determined donor numbers of formamide, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and 1,1,3,3-tetramethylurea (TMU) are 22.4, 26.5, 30.0, and 40.4, respectively. The values of DMF and DMSO are in perfect agreement with those of Gutmann. A reliable value for TMU is obtained for the first time on the basis of the enthalpy change for the addition reaction.
A separation system for gold nanoparticles was developed using monolithic silica capillary columns with 50 μm i.d., which were prepared via in-situ sol-gel processes. Gold nanoparticles with five different average sizes were synthesized via reduction of tetrachloroauric acid (HAuCl4) under different synthesis conditions, and were evaluated by UV-visible spectrophotometry, dynamic light scattering as well as transmission electron microscopy before they were separated using the developed system. The results showed that all of the gold nanoparticles had a certain size distribution, and the mean sizes obtained were 13, 17, 33, 43 and 61 nm, with σ = 2.5, 2.7, 5.2, 5.1 and 5.6 nm, respectively. Transmission electron microscopy showed that the samples with mean sizes of 13 and 17 nm were almost spherical, while larger samples were slightly non-uniform. The agglomeration of gold nanoparticles as the sample could be prevented by using a sodium dodecyl sulfate aqueous solution as the mobile phase, and gold nanoparticles were retained by adsorption on the silica surface. Separation with 8 mM sodium dodecyl sulfate as the eluent and a 1000-mm column was successful, and the separation of gold nanoparticles with 61 and 17 nm or 61 and 13 nm was demonstrated. The separation results obtained using a nonporous silica packed column as well as monolithic silica columns with or without mesopore growth were compared. It was found that separation using the mesopore-less monolithic column achieved better resolution. Through the use of a 2000-mm separation column, the mixtures of 61, 43, 17 nm and 61, 33, 13 nm could be separated.
We fabricated a sensitive and selective electrochemical carbon monoxide (CO) sensor for physiological conditions based on the Pt-Ru system. At a bare Pt-Ru electrode, a linear amperometric response to CO concentration was obtained in the range of 0.9 – 9 μM. However, significant current response to model electroactive interferents for physiological conditions, uric acid (UA), ascorbic acid (AA) and hydrogen peroxide (HP), was also recorded at the Pt-Ru electrode. The response to UA and AA was highly suppressed by coating the Pt-Ru electrode surface with a Nafion layer, and the response to HP was almost completely eliminated by the additional coating with a MnO2/chitosan layer. Finally, at the Pt-Ru/Nafion/MnO2 electrode, amperometric CO detection with a sensitivity of 173 nA cm−2 μM−1 was obtained in the concentration range of 0.9 – 9 μM with the UA, AA and HP signal being below 1.7% at the same concentration of CO.
A boronic acid fluorophore (C1-APB)/boronic acid-modified γ-cyclodextrin (3-PB-γ-CyD) complex as a supramolecular sensor has been designed for selective glucose recognition in water. The fluorescent response behavior of the C1-APB/3-PB-γ-CyD complex under various pH conditions revealed that a C1-APB/3-PB-γ-CyD complex solution containing glucose showed a large increase in the fluorescence intensity under alkaline pH conditions. In contrast, only small increases in the fluorescence intensity were noted for fructose and without sugar solutions. The observed response selectivity for the C1-APB/3-PB-γ-CyD complex was on the order of glucose >> galactose, mannose > fructose. The evidence on a large value of the inclusion constant (KL·CyD = 6.5 × 103 M−1), a marked broadening of the 1H NMR spectra, and an enhancement of induced circular dichloism (ICD) intensity for the C1-APB/3-PB-γ-CyD complex by glucose binding supported the multi-point interaction of the C1-APB/3-PB-γ-CyD complex with glucose. These results demonstrated that the C1-APB/3-PB-γ-CyD complex functioned as an efficient supramolecular sensor for selective glucose recognition in water.
Electrochemical detection of sugar-related compounds was conducted using a boron-doped diamond (BDD) electrode as a detector for flow-injection analysis (FIA). Sugar-related compounds oxidize at high applied potentials, for which the BDD electrode is suitable for electrochemical measurements. Conditions for an FIA system with a BDD detector were optimized, and the following detection limits were achieved for sugar-related compounds: monosaccharides, 25 – 100 pmol; sugar alcohols, 10 pmol; and oligosaccharides, 10 pmol. The detection limit for monosaccharide D-glucose (Glu) was 105 pmol (S/N = 3). A linear range was acquired from the detection limit to 50 nmol, and the relative standard deviation was 0.65% (20 nmol, n = 6). A high-performance liquid chromatography (HPLC) column was added to the system between the sample injector and the detector and detection limits to the picomole level were achieved, which is the same for the HPLC system and the FIA system. The electrochemical oxidation reaction of Glu was examined using cyclic voltammetry with the BDD detector. The reaction proved to be irreversible, and proceeded according to the following two-step mechanism: (1) application of a high potential (2.00 V vs. Ag/AgCl) to the electrode causes water to electrolyze on the electrode surface with the simultaneous generation of a hydroxyl radical on the surface, and (2) the hydroxyl radical indirectly oxidizes Glu. Thus, Glu can be detected by an increase in the oxidation current caused by reactions with hydroxy radicals.
An inkjet printing method is described to fabricate hydrogen peroxide (H2O2) sensors. Insoluble Prussian blue (PB) nanoparticles were dispersed in aqueous solvent, and were printed on screen printed carbon electrodes with a piezoelectric inkjet printer for H2O2 detection. The electrochemical behavior of the printed sensors was studied by using cyclic voltammetry and chronoamperometry. The printed sensors showed great electrocatalytic activity toward H2O2 and can be used for amperometric detection of H2O2. The calibration curves for H2O2 determination showed a linear range from 0.02 to 0.7 mM with a sensitivity of 164.82 μA M−1 cm−2 for the printed PB film. The results showed the feasibility of applying inkjet printing technology on surface modification; the results also provide an alternative way for manufacturing electrochemical sensors.
A low-cost thin-layer electrochemical flow-through cell based on a carbon paste electrode (CPE), was constructed for the highly sensitive determination of cadmium(II) (Cd2+) and lead(II) (Pb2+) ions. The sensitivity of the proposed cell for Cd2+ and Pb2+ ion detection was improved by using the smallest channel height without the need for any complicated electrode modification. Under the optimum conditions, the detection limits of Cd2+ and Pb2+ ions (0.08 and 0.07 μg dm−3, respectively) were 13.8- and 11.4-fold lower than that of a commercial flow cell (1.1 and 0.8 μg dm−3, respectively). Moreover, the percentage recoveries of Cd2+ and Pb2+ for the in-house designed thin-layer flow cell were higher than those for the commercially available cell in all tested water samples, and within the acceptable range. The proposed flow cell is promising as an inexpensive and alternative one for the highly sensitive monitoring of heavy metal ions.
A graphene-modified glassy carbon electrode was fabricated via a drop-casting method, and applied to the electrochemical detection of epinephrine. The capacity of the graphene-modified glassy carbon electrode for the selective detection of epinephrine was confirmed in a sufficient amount of ascorbic acid (2 mmol L−1) by cyclic voltammetry. The modified electrode showed an excellent electrocatalytical effect on the oxidation of epinephrine. A pair of well-defined redox waves were observed in voltammograms of epinephrine in a phosphate buffered solution (pH 4.0). The peak potentials of epinephrine remained unchanged, and the oxidation peak currents showed a linear relation versus the epinephrine concentration in the range of 3.85 × 10−7 – 1.31 × 10−5 mol L−1 and 1.31 × 10−5 – 1.09 × 10−4 mol L−1 with a correlation coefficient as follows: ipa1 = −4.25 × 10−6 – 1.99c (mol L−1), R1 = 0.9953; ipa2 = −4.31 × 10−5 − 0.315c (mol L−1), R2 = 0.9988. Detection limit is estimated to be 8.9 × 10−8 mol L−1. Graphene-modified glassy carbon electrode was applied to epinephrine sample analysis, and the results were in good agreement with the standard values.
A nanostructure fiber based on conducting polypyrrole synthesized by an electrochemical method has been developed, and used for electrochemically switching solid-phase microextraction (ES-SPME). The ES-SPME was prepared by the doping of eriochrome blue in polypyrrole (PPy-ECB) and used for selectively extracting the Ni(II) cation in the presence of some transition and heavy metal ions. The cation-exchange behavior of electrochemically prepared polypyrrole on stainless-steel with and without eriochrome blue (ECB) dye was characterized using ICP-OES analysis. The effects of the scan rate for electrochemical synthesis, uptake and the release potential on the extraction behavior of the PPy-ECB conductive fiber were studied. Uptake and release time profiles show that the process of electrically switched cation exchange could be completed within 250 s. The results of the present study point concerning the possibility of developing a selective extraction process for Ni(II) from waste water was explored using such a nanostructured PPy-ECB film through an electrically switched cation exchange.
Hydrophilic oxygen radical absorbance capacity (H-ORAC) is a method for evaluating antioxidant capacities of solutions of hydrophilic compounds. In this study, we improved the original method for H-ORAC determination, and evaluated the precision of the two improved methods (methods A and B) by interlaboratory studies using 5 antioxidant solutions and 5 food extracts as test samples. An interlaboratory study of method A, in accordance with the harmonized protocol, demonstrated satisfactory performance (intermediate precision relative standard deviations (RSDint) ranging from 4.6 to 18.8%; the reproducibility relative standard deviations (RSDR) ranging from 7.0 to 21.1%, and the HorRat values ranging from 0.40 to 1.93). However, methodological problems remained, and a further improved method, method B, was thus developed. An interlaboratory study of method B by 5 participating laboratories showed better intermediate precision and reproducibility (RSDint and RSDR ranging from 1.8 to 9.4%, and from 4.4 to 13.8%, respectively), and all HorRat values for the test samples were less than 1.3, suggesting good performance for the H-ORAC measurement.
A new method has been developed and validated for the simultaneous analysis of 11 polychlorinated biphenyls (PCBs), in water samples at trace levels by gas chromatography coupled to triple quadrupole mass spectrometry (GC-QqQ-MS/MS). Water samples were extracted by the QuEChERS (Quick Easy Cheap Effective Rugged and Safe) method. The QqQ analyzer acquired data in multiple reactions monitoring (MRM), permitting both quantification and confirmation in a single injection with a running time reduced up to 11.0 min. The effect of matrix interferences in extracts on analyte quantification and the identification of PCBs in water samples was deeply studied. The results showed that PCBs were prone to strong matrix interactions in water samples, and the quantification and identification of PCBs were highly affected by a matrix enhancement effect. To evaluate the performance of the method, validation experiments were carried out on water samples at three spiking levels (1.6, 8.0, 40.0 μg L−1). Recovery was in the range of 95 – 109% at 1.6 μg L−1, 90 – 95% at 8.0 μg L−1 and 97 – 102% at 40.0 μg L−1, respectively. Precision values expressed as relative standard deviation (RSD) were lower than 15%. Linearity in the range of 0.5 – 50.0 μg L−1 provided determination coefficients (R2) higher than 0.999 for all compounds. The limits of detection (LODs) for PCBs were ≤0.1 μg L−1 and the limits of quantification (LOQs) ranged from 0.04 to 0.3 μg L−1. The applicability of the proposed method to detect and quantify PCBs has been demonstrated in analyse of 15 real samples.
We present a selective method for simultaneous determination of five polyether ionophores such as salinomycin (SAL), monensin (MON), narasin (NAR), semduramicin (SEM) and lasalocid (LAS) in aquatic samples using a liquid chromatography with one-step fluorescent derivatization of 2-(4-hydrazinocarbonyl-phenyl) 4,5-diphenylimidazole (HCPI) and 4-(4,5-diphenyl-1H-imidazol-2-yl) benzoyl chloride hydrochloride (DIB-Cl). Fluorescent one-step derivatization for SAL, MON, NAR and SEM using HCPI and for LAS using DIB-Cl was monitored by an LC/fluorescence detector (Ex, 340 nm; Em, 465 nm). Chromatographic separation was performed on a TSK-GEL ODS-120T (4.6 × 150 mm, 3 μm) column using a mobile phase of 0.1% formic acid in acetonitrile and 0.5 mM ammonium formate in water (70/30, v/v). The limits of detections were 0.01 μg/mL (50 pg) for LAS, 0.05 μg/mL (250 pg) for SAL, NAR and SEM, and 0.1 μg/mL (500 pg) for MON, respectively. The recoveries for water samples were indicated to be the range of 79.6 ± 6.4 – 99.0 ± 4.4% with associated precision values (between-day for 3 days) for repeatability. Based on solid-phase extraction, the limit of quantitation values indicated 0.1 ng/mL for SAL, MON, NAR and SEM, and 0.01 ng/mL for LAS in water samples.
A simple method was developed to determine histamine, an important compound in chemical food poisoning, by extraction followed by hydrophilic interaction chromatography–tandem mass spectrometry using a hydrophilic column with sulfobetaine-type zwitterion groups. The quantitation range in seafood products was from 0.4 to 200 mg kg−1 for 5 g food samples. Quantitative recoveries were obtained with four types of seafood product. These results agreed well with those from the more complex, conventional HPLC method, which requires sample derivatization with dansyl chloride.
Polydimethylsiloxane has been dominantly employed as the substrate material for microchip capillary electrophoresis. The poor surface chemistry, however, generates inconsistent electroosmotic flow under the electrophoretic condition, limiting its broader applications. In this work, different polyelectrolytes, including polydiallyldimethylammonium chloride, polyvinyl sulfate, and dextran sulfate, were successfully deposited onto polydimethylsiloxane microchannel surfaces. The polyelectrolyte coated polydimethylsiloxane microchannel showed improved consistency and reproducibility in electroosmotic flow under an electric field over the uncoated native microchannel.
In this article, we report on a food-cholesterol monitoring sensor based on a non-enzymatic approach. Amorphous and single-crystal gold electrodes were modified with an alkanethiol self-assembled monolayer to quantify it by voltammetry. We first discuss the basic characteristics of these sensors and provide more information about the instrument developed by JSK Co. This instrument is a battery-operated handheld voltammetric analyzer, which mounts a sensor chip to monitor cholesterol contents in food samples. The sensor showed excellent linearity with the cholesterol concentration; egg-yolk samples were analyzed to give an excellent agreement between the values obtained by the sensor (1.4 mM) and chromatography (1.1 mM).