Biosensing applications of clay-modified electrodes: a review

Two-dimensional layered inorganic solids, such as cationic clays and layered double hydroxides (LDHs), also defined as anionic clays, have open structures which are favourable for interactions with enzymes and which intercalate redox mediators. This review aims to show the interest in clays and LDHs as suitable host matrices likely to immobilize enzymes onto electrode surfaces for biosensing applications. It is meant to provide an overview of the various types of electrochemical biosensors that have been developed with these 2D layered materials, along with significant advances over the last several years. The different biosensor configurations and their specific transduction procedures are discussed.      

Electrochemical deposition of silicate–cetyltrimethylammonium bromide nanocomposite film on glassy carbon electrode for sensing of methyl parathion

A novel electrochemical sensor for methyl parathion based on silicate– cetyltrimethylammonium bromide nanocomposite film has been fabricated by electro-assisted deposition onto glassy carbon electrode in one-step via an electrochemical modulation of pH at the electrode/solution interface to promote controlled gelification of tetraethylorthosilicate sol, and was characterized with scanning electron microscopy, X-ray diffraction, and electrochemical impedance spectroscopy. The electrochemical sensing of methyl parathion on the film-modified electrode was investigated applying cyclic voltammetry and square wave voltammetry. Compared to the unmodified electrode, the shapes of the redox peaks were improved and the peak currents significantly increased. Experimental parameters such as deposition time, pH value, and accumulation conditions have been optimized. A linear relationship between the peak current and methyl parathion concentration was obtained in the range from 1.0 × 10⁻⁷ to 1.0 × 10⁻⁴ mol L⁻¹ with a detection limit of 1.04 × 10 ⁻⁸ mol L⁻¹ (S/N = 3) after accumulation at 0 V for 120 s. The film electrode shows great promise for determination of methyl parathion in real samples.      

Playing tag with quantitative proteomics

There is steady need for new proteomic strategies on quantitative measurements that provide essential components for detailing dynamic changes in many cellular functions and processes. Stable isotope labeling is a rapidly evolving field, which can be used either after protein extraction with chemical labeling, or in cell culture with metabolic incorporation. In this review, we explore the most frequently utilized quantitation techniques with particular attention paid to chemical labeling using different isotopic tags, including a recent labeling strategy—soluble polymer-based isotopic labeling (SoPIL)—that achieves efficient labeling in homogeneous conditions. Special care should be devoted to the selection of appropriate quantitation approaches according to the needs of the sample and overall experimental design. We evaluate recent advances in quantitative proteomics using stable isotope labeling and their applications to current insightful biological inquiries. Figure Chemical modules of isotopic tags for quantitative proteomics.     

Combining silica-based adsorbents and SPME fibers in the extraction of the volatiles of beer: an exploratory study

A series of silica-based materials were employed as sorbents within solid-phase microextraction vials. The aim of the study was to evaluate the effect of an additional phase on the distribution of the volatile and less volatile analytes. The adsorption of six probe molecules, namely isoamyl acetate, ethyl hexanoate (ethyl caproate), phenylethyl alcohol, ethyl octanoate (ethyl caprilate), 2-phenylethyl acetate, and ethyl decanoate, was monitored by detecting the desorbed amount on a DVD–CAR–PDMS fiber from Pilsen beer. The microextraction process involved the presence of different silica-based phases produced via different methods: xerogel produced by hydrolytic and non-hydrolytic routes, aerogel, pyrogenic, and precipitated silica. The resulting data are discussed in correlation with sorbent texture properties (specific area and pore diameter). The modification of silica with alkyl groups also affects the preconcentrated amount of the target molecules in the headspace. The presence of sorbents was shown to affect the analyte signal more than the addition of NaCl or the use of ultrasound. Analyte’s equilibrium between fiber and sorbent     

Electrochemical studies of (−)-epigallocatechin gallate and its interaction with DNA

In this paper, an electrochemical investigation of (−)-epigallocatechin gallate (EGCG) and its interaction with DNA is presented. Via an electrochemical approach assisted by ultraviolet–visible (UV–Vis) spectroscopy, we propose that EGCG can intercalate into DNA strands forming a nonelectroactive complex, which results in the decrease of the anodic peak current of EGCG. Meanwhile, an electrochemical study with the DNA–Cu(II)–EGCG system shows that damage to DNA can be recognized electrochemically via the increase in the anodic peak current resulting from the oxidation of guanine and adenine bases. The damage can also be recognized spectrophotometrically via an increase in the 260 nm absorption band. In addition, it was found that EGCG is able to discriminate dsDNA from ssDNA, making a potential electrochemical indicator for the detection of DNA hybridization events. A rapid and convenient method of detecting EGCG was also developed in this work. Figure Interaction of EGCG with DNA and damage to DNA in the presence of Cu(II) Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Gradient HPLC separation of dehydroepiandrosterone (DHEA) from its metabolites and biological congeners: role of tetrahydrofuran in the chromatographic mechanism

A three-step gradient reversed-phase high-performance liquid chromatography (RP-HPLC) method was developed for the separation of dehydroepiandrosterone (DHEA), its sulfate ester (DHEA-S), its three C7-oxidized metabolites (7αOH-DHEA, 7βOH-DHEA, 7-keto-DHEA), and its biosynthetic congeners (androstenedione, testosterone, estradiol, pregnenolone). This new method allows the quantitative characterization of DHEA metabolism and biosynthetic transformation under given physiological, pathological, or therapeutically influenced circumstances. Tetrahydrofuran probably acts as a proton acceptor coadsorbent, while isopropanol behaves as a proton donor during the separation of testosterone, estradiol, and the stereoisomers of 7-OH-DHEA. Figure Optimized gradient RP-HPLC results in full separation of DHEA from its biosynthetic congeners and metabolites     

Preparation and enantioseparation characteristics of three chiral stationary phases based on modified β-cyclodextrin for liquid chromatography

In order to study the effect of the nature and the length of the spacer, three mixed 10-undecenoate/phenylcarbamate derivatives of β-cyclodextrin have been prepared and linked to allylsilica gel by means of a radical reaction. The chiral recognition ability of the resulting materials, when used as liquid chromatography chiral stationary phases (CSPs), was evaluated using heptane and either 2-propanol or chloroform as organic mobile-phase modifiers. A large variety of racemic compounds have been separated successfully on these CSPs (mainly pharmaceuticals and herbicides). Optimization of these separations was discussed in terms of mobile-phase composition and structural patterns of the injected analytes. The efficiencies of the three prepared materials were compared to those of previously described perphenylated-β-cyclodextrin column and to analogous cellulose derivative-based CSPs. Schematic illustration of the b-cyclodextrin/mandelic acid inclusion complex     

Interaction of push–pull <Emphasis Type="Italic">tert-</Emphasis>enamines with phenylglyoxal

Abstract  The reaction of push–pull enamines with 1,2-biselectrophilic phenylglyoxal was investigated. Phenylglyoxal was found to react depending on the structure of the push–pull enamine, affording either a hydroxyalkylation product at the methyl group or the cyclic product via participation of the methyl group and the β-carbon of the enamine. Graphical abstract        

Aptamers as molecular recognition elements for electrical nanobiosensors

Recent advances in nanotechnology have enabled the development of nanoscale sensors that outperform conventional biosensors. This review summarizes the nanoscale biosensors that use aptamers as molecular recognition elements. The advantages of aptamers over antibodies as sensors are highlighted. These advantages are especially apparent with electrical sensors such as electrochemical sensors or those using field-effect transistors. Figure Feeling proteins with aptamer-functionalized carbon nanotubes     

Amperometric sensing of hydrogen peroxide vapor for security screening

Rapid detection of the hydrogen peroxide precursor of peroxide explosives is required in numerous security screening applications. We describe a highly sensitive and selective amperometric detection of hydrogen peroxide vapor at an agarose-coated Prussian-blue (PB) modified thick-film carbon transducer. The sensor responds rapidly and reversibly to dynamic changes in the level of the peroxide vapor, with no apparent carry over and with a detection limit of 6 ppbv. The remarkable selectivity of the PB-based screen-printed electrode towards hydrogen peroxide leads to effective discrimination against common beverage samples. For example, blind tests have demonstrated the ability to selectively and non-invasively identify concealed hydrogen peroxide in drinking cups and bottles. The attractive performance of the new microfabricated PB-based amperometric peroxide vapor sensor indicates great potential for addressing a wide range of security screening and surveillance applications. Figure Experimental setup (left) with three electrode electrochemical Hydrogen Peroxide sensor hanging above container of “unknown” liquid. Schematic (right) demonstrating fundamental principles of operation of the sensor.     

Expanding the scope of MS binding assays to low-affinity markers as exemplified for mGAT1

Following a recently developed concept of MS binding assays based on the quantification of a native marker by LC–MS a procedure to study binding of a low-affinity marker in kinetic, saturation, and competition experiments was established. Separation of bound and unbound marker—the most crucial step of the assay—could be effectively achieved by filtration in a 96-well-format. MS binding assays according to this procedure allowed the reliable characterization of NO 711 binding to mGAT1 in presence of physiological NaCl concentrations. Comparing the results obtained in the present study with those from experiments using 1 mol L⁻¹ NaCl in the incubation milieu reveals remarkable differences with respect to the marker’s affinity and kinetics and to the investigated test compound’s potency. Principle of MS binding assays After incubation of a target with a native marker, bound and unbound marker are separated by filtration. Subsequently, the bound native marker is liberated from the target and finally quantified by LC-MS-MS. Dedicated to Prof. Hans-Dietrich Stachel on the occasion of his 80th birthday     

Purity assessment problem in quantitative NMR—impurity resonance overlaps with monitor signal multiplets from stereoisomers

This paper describes the situation that can emerge when the signals to be evaluated in quantitative NMR measurements—so-called “monitor signals”—consist of several resonance lines from the stereoisomers of the analyte in addition to an impurity signal underneath. The monitor signal problem is demonstrated in the purity assessment of two samples of 2-(isopropylamino)-4-(ethylamino)-6-chloro-1,3,5-triazine (atrazine), a common herbizide which served as analyte in a CCQM intercomparison. It is shown that, in DMSO-d₆ solution, a mixture of stereoisomers leads to several individual overlapping singlets, which are further split by spin–spin coupling. A measurement protocol was developed for finding and identifying an impurity that has a signal that is positioned precisely beneath the methyl signal chosen as the monitor signal in one of the samples. Quantitative NMR purity assessment is still possible in this special case, but with higher uncertainty. Electronic Supplementary Material Supplementary material is available for this article at     

Compatibility of quantum dots with immunobuffers, and its effect on signal/background of quantum dot-based immunoassay

In this work, the compatibility of quantum dots (QDs) with immunobuffers was studied by investigating the fluorescence stability of QDs in immunobuffers (in this research immunobuffers were defined as buffers for immunoaffinity binding or separation). Experimentally, the fluorescence signals of QDs with different surface chemistries (amine-terminated, streptavidin-coated, or antibody-conjugated) in commonly used immunobuffers were monitored versus time. The effect of some buffer composition on the compatibility of QDs with these buffers was also explored. Based on experimental data, the QD compatibility with these buffers is summarized, and it is found that a trace amount of bovine serum albumin added to most of these buffers helps QDs to achieve compatibility with them. Moreover, with QD as fluorescence label and C-reactive protein as a model analyte, a magnetic bead-based assay was performed using compatible and incompatible QD–immunobuffer systems. It is shown that compatible QD–immunobuffer systems can be used to achieve a higher assay signal/background ratio.      

Determination of glucosinolates in traditional Chinese herbs by high-performance liquid chromatography and electrospray ionization mass spectrometry

A reversed-phase HPLC method has been developed for determination of twelve intact glucosinolates—glucoiberin, glucocheirolin, progoitrin, sinigrin, epiprogoitrin, glucoraphenin, sinalbin, gluconapin, glucosibarin, glucotropaeolin, glucoerucin, and gluconasturtiin—in ten traditional Chinese plants. The samples were extracted with methanol and the extracts were cleaned on an activated Florisil column. A mobile phase gradient prepared from methanol and 30 mmol L⁻¹ ammonium acetate at pH 5.0 enabled baseline separation of the glucosinolates. Glucosinolate detection was confirmed by quadrupole time-of-flight tandem mass spectrometric analysis in negative-ionization mode. Detection limits ranged from 0.06 to 0.36 μg g⁻¹ when 5 g of dried plant was analyzed. Recoveries of the glucosinolates were better than 85% and precision (relative standard derivation, n = 3) ranged from 5.3 to 14.6%. Analysis of the glucosinolates provided scientific evidence enabling differentiation of three pairs of easily confused plants. Figure Glucosinolates Analysis for the Differentiation of Easily-Confusing Herbs     

Automated extraction of direct,reactive, and vat dyes from cellulosic fibers for forensic analysis by capillary electrophoresis

Systematic designed experiments were employed to find the optimum conditions for extraction of direct, reactive, and vat dyes from cotton fibers prior to forensic characterization. Automated microextractions were coupled with measurements of extraction efficiencies on a microplate reader UV–visible spectrophotometer to enable rapid screening of extraction efficiency as a function of solvent composition. Solvent extraction conditions were also developed to be compatible with subsequent forensic characterization of extracted dyes by capillary electrophoresis with UV–visible diode array detection. The capillary electrophoresis electrolyte successfully used in this work consists of 5 mM ammonium acetate in 40:60 acetonitrile–water at pH 9.3, with the addition of sodium dithionite reducing agent to facilitate analysis of vat dyes. The ultimate goal of these research efforts is enhanced discrimination of trace fiber evidence by analysis of extracted dyes. Figure Fitted absorbance response surface for extraction of a direct dye, C. I. yellow 58, using a ternary solvent system.     

Chemical analysis of acoustically levitated drops by Raman spectroscopy

An experimental apparatus combining Raman spectroscopy with acoustic levitation, Raman acoustic levitation spectroscopy (RALS), is investigated in the field of physical and chemical analytics. Whereas acoustic levitation enables the contactless handling of microsized samples, Raman spectroscopy offers the advantage of a noninvasive method without complex sample preparation. After carrying out some systematic tests to probe the sensitivity of the technique to drop size, shape, and position, RALS has been successfully applied in monitoring sample dilution and preconcentration, evaporation, crystallization, an acid–base reaction, and analytes in a surface-enhanced Raman spectroscopy colloidal suspension. Figure We have systematically investigated the analytical potential of Raman spectroscopy of samples in acoustically levitated drops.     

A novel experimental method for potential controlled electrochemical impedance spectroscopy

Abstract  In many cases, impedance spectroscopy on electrochemical systems is performed in combination with a potentiostat. The common way of combining a potentiostat and a frequency response analyzer has a number of restrictions. Bandwidth limitations and artifacts from the setup distort the impedance data in sometimes unpredictable manners, and special care has to be taken in physical interpretation of such data. Therefore, potential controlled measurement of artifact-free impedance spectra in a wide range of frequencies is desirable. In this work, a novel experimental method is presented in which the more reliable two-probe impedance spectroscopy is combined with a potential control by a potentiostat without interfering each other. Results obtained on passive zinc by two-probe EIS, the classical potentiostat—EIS combination and the new setup are compared with each other. For an application of the proposed method in measurements of solid state systems, the specific problems are discussed. Graphical abstract        

Nuclear magnetic resonance of mass-limited samples using small RF coils

Figure Schematic diagram of a typical arrangement used for hyphenating chemical microseparations (e.g. capillary HPLC, CE, or CEC) with microcoil NMR detection     

HPLC-ICP-MS and HPLC-ES-MS/MS characterization of synthetic seleno-arsenic compounds

Various toxicological and metabolic interactions have been reported to exist between arsenic and selenium. In the present study, synthetic seleno-arsenic compounds, potentially suitable for probing metabolic interactions between these two elements, were prepared and tentatively characterized by using high-performance liquid chromatography (HPLC)–electrospray tandem mass spectrometry and HPLC–inductively coupled plasma mass spectrometry. In analogy to the recently identified thio-arsenic species, which can be prepared from their corresponding oxo-arsenic species via reaction with H₂S, the seleno-arsenic compounds were also derived from oxo-arsenic compounds via reaction with H₂Se. Figure H₂Se bubbled into solutions containing oxo‐arsenosugars converts them into their seleno‐arsenosugar analogues.