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Analytica Chimica Acta (v.687, #1)

Editorial Board (pp. iii).

A novel automated electrochemical ascorbic acid assay in the 24-well microtiter plate format by Sireerat Intarakamhang; Christian Leson; Wolfgang Schuhmann; Albert Schulte (pp. 1-6).

Automatic ascorbic acid (AA) voltammetry was established in 24-well microtiter plates. The assay used a movable assembly of a pencil rod working, an Ag/AgCl reference and a Pt counter electrode with differential pulse voltammetry (DPV) for concentration-dependent current generation. A computer was in command of electrode ( z) and microtiter plate ( x, y) positioning and timed potentiostat operation. Synchronization of these actions supported sequential approach of all wells and subsequent execution of electrode treatment procedures or AA voltammetry at defined intervals in a measuring cycle. DPV in well solutions offered a linear current/concentration range between 0.1 and 8.0mM, a sensitivity of about 1μAmM⁻¹ AA, and a detection limit of 50μM. When used with a calibration curve or standard addition, automated voltammetry of samples with added known amounts of AA demonstrated good recovery rates. Also, the assay achieved the accurate determination of the AA content of vitamin C tablets, a fruit juice and an herbal tea extract. Robotic AA voltammetry has the advantage of conveniently handling multiple samples in a single measuring run without the continuous attention of laboratory personnel. It is a good option when the goal is cost-effective AA screening of sample libraries and has potential for applications in health care and the food processing, cosmetic and pharmaceutical industries.

Keywords: Robotic electrochemistry; Electroanalysis; Voltammetry; Ascorbic acid; Vitamin C; Microtiter plates

Integrated electrochemical transistor as a fast recoverable gas sensor by Ulrich Lange; Vladimir M. Mirsky (pp. 7-11).

A new design of conductometric chemical sensors based on conducting polymers as chemosensitive elements was suggested. The sensor includes six electrodes. Four inner electrodes coated by chemosensitive polymer are used for simultaneous two- and four-point resistance measurements thus providing information on the bulk polymer resistance and on the resistance of the polymer/electrode contacts. Two outer electrodes wired to inner electrodes by polymeric electrolyte are used for electrical control of redox state of the chemosensitive polymer. The outer electrodes are connected to potentiostat as reference and counter electrodes. It allows us to control redox state of the inner (working) electrodes. This new measurement configuration, resembling chemosensitive electrochemical transistors, provides an internal test of the sensor integrity and an electrically driven sensor regeneration. It was tested as a sensor for the detection of nitrogen dioxide. Polythiophene or polyaniline was used as receptors. Cyclic voltammograms of these polymers on the sensor surface measured in air atmosphere were very similar to that measured in aqueous electrolyte. A control of conductivity of these chemosensitive polymers by electrical potential applied vs. incorporated reference electrode was demonstrated. This effect was used for the regeneration of the chemosensitive material after exposure to nitrogen dioxide: in comparison to usual chemiresistors displaying an irreversible behavior in such test even in the time scale of hours, a completely reversible sensor regeneration within few minutes was observed.

Keywords: Gas sensor; Conducting polymer; Electrochemical transistor; Six-point resistance Measurement; Sensor recovery

Microfluidic DNA microarray analysis: A review by Lin Wang; Paul C.H. Li (pp. 12-27).

Microarray DNA hybridization techniques have been used widely from basic to applied molecular biology research. Generally, in a DNA microarray, different probe DNA molecules are immobilized on a solid support in groups and form an array of microspots. Then, hybridization to the microarray can be performed by applying sample DNA solutions in either the bulk or the microfluidic manner. Because the immobilized probe DNA binds and retains its complementary target DNA, detection is achieved through the read-out of the tagged markers on the sample target molecules. The recent microfluidic hybridization method shows the advantages of less sample usage and reduced incubation time. Here, sample solutions are confined in microfabricated channels and flow through the probe microarray area. The high surface-to-volume ratio in microchannels of nanolitre volume greatly enhanced the sensitivity as obtained with the bulk solution method. To generate nanolitre flows, different techniques have been developed, and this including electrokinetic control, vacuum suction and syringe pumping. The latter two are pressure-driven methods which are more flexible without the need of considering the physicochemical properties of solutions. Recently, centrifugal force is employed to drive liquid movement in microchannels. This method utilizes the body force from the liquid itself and there are no additional solution interface contacts such as from electrodes or syringes and tubing. Centrifugal force driven flow also features the ease of parallel hybridizations. In this review, we will summarize the recent advances in microfluidic microarray hybridization and compare the applications of various flow methods.

Keywords: Microfluidic DNA microarray; DNA hybridization; Centrifugal pumping; DNA diagnostics; Radial channel; Spiral channel; Pressure-driven flow

Biosensors and rapid diagnostic tests on the frontier between analytical and clinical chemistry for biomolecular diagnosis of dengue disease: A review by Fernando Sérgio Rodrigues Ribeiro Teles (pp. 28-42).

The past decades have witnessed enormous technological improvements towards the development of simple, cost-effective and accurate rapid diagnostic tests for detection and identification of infectious pathogens. Among them is dengue virus, the etiologic agent of the mosquito-borne dengue disease, one of the most important emerging infectious pathologies of nowadays. Dengue fever may cause potentially deadly hemorrhagic symptoms and is endemic in the tropical and sub-tropical world, being also a serious threat to temperate countries in the developed world. Effective diagnostics for dengue should be able to discriminate among the four antigenically related dengue serotypes and fulfill the requirements for successful decentralized (point-of-care) testing in the harsh environmental conditions found in most tropical regions. The accurate identification of circulating serotypes is crucial for the successful implementation of vector control programs based on reliable epidemiological predictions. This paper briefly summarizes the limitations of the main conventional techniques for biomolecular diagnosis of dengue disease and critically reviews some of the most relevant biosensors and rapid diagnostic tests developed, implemented and reported so far for point-of-care testing of dengue infections. The invaluable contributions of microfluidics and nanotechnology encompass the whole paper, while evaluation concerns of rapid diagnostic tests and foreseen technological improvements in this field are also overviewed for the diagnosis of dengue and other infectious and tropical diseases as well.

Keywords: Abbreviations; CDC; Centers for Disease Control; DENV1–4; dengue virus serotypes (1–4); ssRNA; single-stranded ribonucleic acid; ORF; open-reading frame; NS1; non-structural 1; DHF; dengue hemorrhagic fever; DSS; dengue shock syndrome; WHO; World Health Organization; HI; hemagglutination-inhibition; MAC-EIA; monoclonal antibody capture-enzyme linked immunosorbent assay; RT-PCR; reverse transcription-polymerase chain reaction; 3′-NR; 3′noncoding region; RNA; ribonucleic acid; IgG; immunoglobulin G; IgM; immunoglobulin M; DNA; deoxyribonucleic acid; QCM; quartz-crystal microbalance; MIP; molecularly imprinted polymer; GNP; gold nanoparticle; SAM; self-assembled monolayer; BSA; bovine serum albumin; SPR; surface plasmon resonance; NASBA; nucleic acid sequence-based amplification; S/N; signal-to-noise ratio; CMOS; complementary metal oxide semiconductor; FIA; flow-injection analysis; FCCS; fluorescence cross-correlation spectroscopy; FCS; fluorescence correlation spectroscopy; EIS; electrochemical impedance spectroscopy; BST; barium strontium titanate; FET; field-effect transistor; PNA; peptide nucleic-acid; LOD; limit of detection; cDNA; complementary DNA; TDR; Special Programme for Research and Training in Tropical Diseases; UNDP; United Nations Development Programme; PDVI; Pediatric Dengue Vaccine Initiative; STARD; Standards for Reporting of Diagnostic Accuracy; FIOCRUZ; Fundação Oswaldo Cruz; DPP; ^®; Dual-Path Platform; BLM; bilayer lipid membrane; QD; quantum dot; CNT; carbon nanotube; MS; mass spectrometry; SARS; severe acute respiratory syndromeBiosensor; Dengue; Diagnosis; Evaluation; Rapid test; Tropical disease

Highly sensitive detection of silybin based on adsorptive stripping analysis at single-sided heated screen-printed carbon electrodes modified with multi-walled carbon nanotubes with direct current heating by Shao-Hua Wu; Fa-Hui Nie; Qi-Zhen Chen; Jian-Jun Sun (pp. 43-49).

A new disposable multi-walled carbon nanotubes modified single-sided heated screen-printed carbon electrode (MWNT/ss-HSPCE) was fabricated. The electrochemical behavior of silybin was investigated by cyclic voltammetry and the probable electrode reaction mechanism was proposed. A simple and cheap direct current heating supplier was used to heating the electrode for adsorptive accumulation of silybin. The square wave voltammetric stripping peak current of silybin at MWNT/ss-HSPCE with an elevated electrode temperature of 50°C only during accumulation step was dramatically improved compared with that at bare single-sided heated screen-printed carbon electrode (ss-HSPCE) without heating. This enhancement was mainly contributed to the combination of the advantages of multi-walled carbon nanotubes and electrically heated electrodes. Under optimum conditions, two detection linear ranges of silybin were from 1.0×10⁻⁹ to 1.0×10⁻⁷M and 3.0×10⁻⁷ to 1.0×10⁻⁶M. A detection limit of 5.0×10⁻¹⁰M could be obtained (S/N=3), which was more than two magnitudes lower than that at bare ss-HSPCE without heating. To the best of our knowledge, this was also at least two magnitudes lower than any others for electrochemical detection of silybin in the literature. Finally, the method was successfully applied to the determination of silybin in pharmaceutical tablets.

Keywords: Silybin; Multi-walled carbon nanotube; Single-sided heated screen-printed carbon electrode; Direct current heating; Adsorptive stripping analysis

Liquid-phase microextraction with in-drop derivatization combined with microvolume fluorospectrometry for free and hydrolyzed formaldehyde determination in textile samples by María Sáenz; José Alvarado; Francisco Pena-Pereira; Sonia Senra-Ferreiro; Isela Lavilla; Carlos Bendicho (pp. 50-55).

A new miniaturized methodology based on the combination of headspace single drop microextraction and microvolume fluorospectrometry is proposed in this work for the determination of free and hydrolyzed formaldehyde in textile samples. The proposed method is based on the extraction and in-drop derivatization of free and hydrolyzed formaldehyde using the Hantzsch reaction. The effect of experimental variables affecting the performance of the proposed method, such as fluorescence parameters, nature of the extractant phase composition (including acetylacetone concentration, pH, ammonium acetate concentration and presence of an organic solvent), sample temperature, NaCl concentration and microextraction time was carefully investigated. Under optimized conditions, instrumental detection and quantification limits were 26 and 87μgL⁻¹, respectively, whereas procedural detection and quantification limits were 1.0 and 3.5mgkg⁻¹, respectively. Repeatability, expressed as relative standard deviation, was 4.6% ( n=9). The method was successfully applied to the determination of free and hydrolyzed formaldehyde in several textile samples, the found results being in good agreement with those obtained with the EN ISO 14184-1:1998 method.

Keywords: Liquid-phase microextraction; Microvolume fluorospectrometry; Free and hydrolyzed formaldehyde; Textile analysis

Application of porous membrane protected micro-solid-phase-extraction combined with gas chromatography–mass spectrometry for the determination of estrogens in ovarian cyst fluid samples by Sivarajan Kanimozhi; Chanbasha Basheer; Kothandaraman Narasimhan; Lin Liu; Stephen Koh; Feng Xue; Mahesh Choolani; Hian Kee Lee (pp. 56-60).

A cost effective and environmentally friendly extraction technique using porous membrane protected micro-solid phase extraction (μ-SPE) is described for the extraction of estrogens in cyst fluid samples obtained from cancer patients. A sorbent (ethylsilane (C2) modified silica) (20mg) was packed in a porous polypropylene envelope (2cm×1.5cm) whose edges were heat sealed to secure the contents. The μ-SPE device was conditioned with acetone and placed in a stirred (1:5) diluted cyst fluid sample solution (10mL) to extract estrogens for 60min. After extraction, the analytes were desorbed and simultaneously derivatized with a 5:1 mixture of acetone and N, O-bis(trimethylsilyl)-trifluoroacetamide. The extract (2μL) was analyzed by gas chromatography–mass spectrometry. Various extraction, desorption and derivatization conditions were optimized for μ-SPE. With this simple technique, low limits of detection of between 9 and 22ngL⁻¹ and linear range from the detection limits up to 50μgL⁻¹ were achieved. The optimized method was used to extract estrogens from cyst fluid samples obtained from patients with malignant and benign ovarian tumors.

Keywords: Estrogens; Micro-solid-phase-extraction; Cancer risk; Cyst fluids; Ovarian cancer

Extraction mechanism of sulfamethoxazole in water samples using aqueous two-phase systems of poly(propylene glycol) and salt by Xueqiao Xie; Yun Wang; Juan Han; Yongsheng Yan (pp. 61-66).

Based on the poly(propylene glycol)₄₀₀ (PPG₄₀₀)–salt aqueous two-phase system (ATPS), a green, economical and effective sample pretreatment technique coupled with high performance liquid chromatography was proposed for the separation and determination of sulfamethoxazole (SMX). The extraction yield of SMX in PPG₄₀₀–salt ATPS is influenced by various factors, including the salt species, the amount of salt, pH, and the temperature. Under the optimum conditions, most of SMX was partitioning into the polymer-rich phase with the average extraction efficiency of 99.2%, which may be attributed to the hydrophobic interaction and salting-out effect. This extraction technique has been successfully applied to the analysis of SMX in real water samples with the recoveries of 96.0–100.6%, the detection limits of 0.1μgL⁻¹, and the linear ranges of 2.5–250.0μgL⁻¹.

Keywords: Extraction; Sulfamethoxazole; Aqueous two-phase systems; Poly(propylene glycol); Water samples

Pros and cons of analytical methods to quantify surrogate contaminants from the challenge test in recycled polyethylene terephthalate by Juliana S. Félix; Pilar Alfaro; Cristina Nerín (pp. 67-74).

Different analytical methods were optimized and applied to quantify certain surrogate contaminants (toluene, chlorobenzene, phenol, limonene and benzophenone) in samples of contaminated and recycled flakes and virgin pellets of polyethylene terephthalate (PET) coming from the industrial challenge test. A screening analysis of the PET samples was carried out by direct solid-phase microextraction (SPME) in headspace mode (HS). The methods developed and used for quantitative analysis were a) total dissolution of PET samples in dichloroacetic acid and analysis by HS-SPME coupled to gas chromatography–mass spectrometry (GC–MS) and, b) dichloromethane extraction and analysis by GC–MS. The concentration of all surrogates in the contaminated PET flakes analyzed by HS-SPME method was lower than expected according to information provided by the supplier. Dichloroacetic acid interacted with the surrogates, resulting in a tremendous decrease of limonene concentration. The degradation compounds from limonene were identified. Dichloromethane extraction and GC–MS analysis evidenced the highest values of analytes in these PET samples. Based on the foregoing data, the efficiency of the recycling process was evaluated, whereby the removal of 99.9% of the surrogates proceeding from the contaminated flakes was confirmed.

Keywords: Recycled polyethylene terephthalate; Surrogates; Challenge test; Headspace-solid-phase microextraction; Gas chromatography–mass spectrometry

Quartz crystal microbalance as a sensing active element for rupture scanning within frequency band by F.N. Dultsev; E.A. Kolosovsky (pp. 75-81).

A new method based on the use of quartz crystal microbalance (QCM) as an active sensing element is developed, optimized and tested in a model system to measure the rupture force and deduce size distribution of nanoparticles. As suggested by model predictions, the QCM is shaped as a strip. The ratio of rupture signals at the second and the third harmonics versus the geometric position of a body on QCM surface is investigated theoretically. Recommendations concerning the use of the method for measuring the nanoparticle size distribution are presented. It is shown experimentally for an ensemble of test particles with a characteristic size within 20–30nm that the proposed method allows one to determine particle size distribution. On the basis of the position and value of the measured rupture signal, a histogram of particle size distribution and percentage of each size fraction were determined. The main merits of the bond-rupture method are its rapid response, simplicity and the ability to discriminate between specific and non-specific interactions. The method is highly sensitive with respect to mass (the sensitivity is generally dependent on the chemical nature of receptor and analyte and may reach 8×10⁻¹⁴gmm⁻²) and applicable to measuring rupture forces either for weak bonds, for example hydrogen bonds, or for strong covalent bonds (10⁻¹¹–10⁻⁹N). This procedure may become a good alternative for the existing methods, such as AFM or optical methods of determining biological objects, and win a broad range of applications both in laboratory research and in biosensing for various purposes. Possible applications include medicine, diagnostics, environmental or agricultural monitoring.

Keywords: Biosensor; Bond-rupture event; Quartz crystal microbalance (QCM)

Quantum dot-based “turn-on” fluorescent probe for detection of zinc and cadmium ions in aqueous media by Hu Xu; Ran Miao; Zheng Fang; Xinhua Zhong (pp. 82-88).

Health or environmental issue caused by abnormal level of metal ions like Zn²⁺ or Cd²⁺ is a worldwide concern. Developing an inexpensive and facile detection method for Zn²⁺ and Cd²⁺ is in urgent demand. Due to their super optical properties, fluorescent quantum dots (QDs) have been developed as a promising alternative for organic dyes in fluorescence analysis. In this study, a CdTe QDs-based sensitive and selective probe for Zn²⁺ and Cd²⁺ in aqueous media was reported. The proposed probe worked in fluorescence “turn-on” mode. The initial bright fluorescence of CdTe QDs was effectively quenched by sulfur anions (S²⁻). The presence of Zn²⁺ (or Cd²⁺) can “turn-on” the weak fluorescence of QDs quenched by S²⁻ due to the formation of ZnS (or CdS) passivation shell. Under optimal conditions, a good linear relationship between the fluorescence response and concentration of Zn²⁺ (or Cd²⁺) could be obtained in the range from 1.6 to 35μM (1.3–25μM for Cd²⁺). The limit of detection (LOD) for Zn²⁺ and Cd²⁺ were found to be 1.2 and 0.5μM, respectively. Furthermore, the present probe exhibited a high selectivity for Zn²⁺ and Cd²⁺ over other metal ions and was successfully used in the detection of Zn²⁺ or Cd²⁺ in real water samples.

Keywords: Quantum dots; Fluorescent probe; Fluorescence enhancement; Metal ions

Label-free capacitive immunosensor based on quartz crystal Au electrode for rapid and sensitive detection of Escherichia coli O157:H7 by Dujuan Li; Yangyang Feng; Ling Zhou; Zunzhong Ye; Jianping Wang; Yibin Ying; Chuanmin Ruan; Ronghui Wang; Yanbin Li (pp. 89-96).

A label-free capacitive immunosensor based on quartz crystal Au electrode was developed for rapid and sensitive detection of Escherichia coli O157:H7. The immunosensor was fabricated by immobilizing affinity-purified anti- E. coli O157:H7 antibodies onto self-assembled monolayers (SAMs) of 3-mercaptopropionic acid (MPA) on the surface of a quartz crystal Au electrode. Bacteria suspended in solution became attached to the immobilized antibodies when the immunosensor was tested in liquid samples. The change in capacitance caused by the bacteria was directly measured by an electrochemical detector. An equivalent circuit was introduced to simulate the capacitive immunosensor. The immunosensor was evaluated for E. coli O157:H7 detection in pure culture and inoculated food samples. The experimental results indicated that the capacitance change was linearly correlated with the cell concentration of E. coli O157:H7. The immunosensor was able to discriminate between cellular concentrations of 10²–10⁵cfumL⁻¹ and has applications in detecting pathogens in food samples. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were also employed to characterize the stepwise assembly of the immunosensor.

Keywords: Escherichia coli; O157:H7; Capacitive immunosensor; Self-assembled monolayers; Cyclic voltammetry; Electrochemical impedance spectroscopy

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Analytica Chimica Acta (v.687, #1)