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Analytica Chimica Acta (v.673, #1)

Editorial Board (pp. iii).

Beyond labels: A review of the application of quantum dots as integrated components of assays, bioprobes, and biosensors utilizing optical transduction by W. Russ Algar; Anthony J. Tavares; Ulrich J. Krull (pp. 1-25).

A comprehensive review of the development of assays, bioprobes, and biosensors using quantum dots (QDs) as integrated components is presented. In contrast to a QD that is selectively introduced as a label, an integrated QD is one that is present in a system throughout a bioanalysis, and simultaneously has a role in transduction and as a scaffold for biorecognition. Through a diverse array of coatings and bioconjugation strategies, it is possible to use QDs as a scaffold for biorecognition events. The modulation of QD luminescence provides the opportunity for the transduction of these events via fluorescence resonance energy transfer (FRET), bioluminescence resonance energy transfer (BRET), charge transfer quenching, and electrochemiluminescence (ECL). An overview of the basic concepts and principles underlying the use of QDs with each of these transduction methods is provided, along with many examples of their application in biological sensing. The latter include: the detection of small molecules using enzyme-linked methods, or using aptamers as affinity probes; the detection of proteins via immunoassays or aptamers; nucleic acid hybridization assays; and assays for protease or nuclease activity. Strategies for multiplexed detection are highlighted among these examples. Although the majority of developments to date have been in vitro, QD-based methods for ex vivo biological sensing are emerging. Some special attention is given to the development of solid-phase assays, which offer certain advantages over their solution-phase counterparts.

Keywords: Abbreviations; A647; Alexa Fluor 647; APTES; aminopropyltriethoxysilane; ATP; adenosine triphosphate; Au NPs; gold nanoparticles; β-CD; β-cyclodextrin; BRET; bioluminescence resonance energy transfer; BSA; bovine serum albumin; CB; conduction band; CNTs; carbon nanotubes; ConA; concanavalin A; CRET; chemiluminescence resonance energy transfer; CT; charge transfer; cTnI; human cardiac troponin I; DHLA; dihydrolipoic acid; ECL; electrochemiluminescence; EDTA; ethylenediaminetetraacetic acid; ELISA; enzyme linked immunosorbent assay; EPR; electron paramagnetic resonance; ER-β; estrogen receptor β; GSH; glutathione; GOX; glucose oxidase; HIV; human immunodeficiency virus; FRET; fluorescence resonance energy transfer; FWHM; full-width-at-half-maximum; PB; Pacific Blue; PCR; polymerase chain reaction; PDDA; poly(diallyldimethylammonium chloride); PEG; polyethylene glycol; PET; photoinduced electron transfer; PIGE; paraffin impregnated graphite electrode; PL; photoluminescence; L-Cys; l; -cysteine; LOD; limit of detection; MAA; mercaptoacetic acid; MBP; maltose binding protein; MB; molecular beacon; MPA; mercaptopropionic acid; MMP; matrix metalloproteinase; MSP; methylation specific polymerase chain reaction; NAD; nicotinamide adenine dinucleotide; _x; QD; _w; quantum dot (peak PL at; w; nm, first exciton peak at; x; nm); RhR; Rhodamine Red; Rluc; Renilla; luciferase; RRE; rev responsive element; SA; Streptavidin; SCE; standard calomel electrode; SMD; single molecule detection; SNP; single nucleotide polymorphism; TAMRA; carboxytetramethylrhodamine; TBA; thrombin binding aptamer; VB; valence bandQuantum dots; Fluorescence resonance energy transfer; Bioluminescence resonance energy transfer; Electrochemiluminescence; Charge transfer; Immobilization

Raman spectroscopic quantification of milk powder constituents by C.M. McGoverin; A.S.S. Clark; S.E. Holroyd; K.C. Gordon (pp. 26-32).

Raman spectroscopy has significant potential for the quantification of food products. Milk powder is an important foodstuff and ingredient that is produced on large scale (over 20 million tonnes per annum). Raman spectroscopy, unlike near- and mid-infrared spectroscopies, has not been used extensively to quantify milk powder constituents. The effect of sample presentation on spectroscopic calibrations of protein and fat for 136 New Zealand milk powders was assessed using Raman spectroscopy. Prediction models were produced to quantify a protein concentration range of 32.19–37.65% w/w for skim milk powder, and a protein concentration range of 23.34–25.02% w/w and a fat concentration range of 26.26–29.68% w/w for whole milk powder (where ratios of prediction to deviation exceeded 2.6 with one exception). The resultant calibrations were not influenced by sample orientation; the sample temperature during data collection did affect the calibrations. Calcium fortification in the form of calcium carbonate was identified within a sub-set of samples, reinforcing the efficacy of Raman spectroscopy for identifying both crystalline and non-crystalline constituents within milk powder.

Keywords: Raman spectroscopy; Milk powder; Milk protein; Milk fat; Partial least-squares

Kinetic approach for the enzymatic determination of levodopa and carbidopa assisted by multivariate curve resolution-alternating least squares by Marcos Grünhut; Mariano Garrido; Maria E. Centurión; Beatriz S. Fernández Band (pp. 33-39).

A combination of kinetic spectroscopic monitoring and multivariate curve resolution-alternating least squares (MCR-ALS) was proposed for the enzymatic determination of levodopa (LVD) and carbidopa (CBD) in pharmaceuticals. The enzymatic reaction process was carried out in a reverse stopped-flow injection system and monitored by UV–vis spectroscopy. The spectra (292–600nm) were recorded throughout the reaction and were analyzed by multivariate curve resolution-alternating least squares. A small calibration matrix containing nine mixtures was used in the model construction. Additionally, to evaluate the prediction ability of the model, a set with six validation mixtures was used. The lack of fit obtained was 4.3%, the explained variance 99.8% and the overall prediction error 5.5%. Tablets of commercial samples were analyzed and the results were validated by pharmacopeia method (high performance liquid chromatography). No significant differences were found ( α=0.05) between the reference values and the ones obtained with the proposed method. It is important to note that a unique chemometric model made it possible to determine both analytes simultaneously.

Keywords: Multivariate curve resolution-alternating least squares; Flow injection analysis; Polyphenol oxidase; Levodopa; Carbidopa; Kinetic

Study of pH-dependent solubility of organic bases. Revisit of Henderson-Hasselbalch relationship by Gergely Völgyi; Edit Baka; Karl J. Box; John E.A. Comer; Krisztina Takács-Novák (pp. 40-46).

In this paper the pH-equilibrium solubility profiles of six organic drugs are presented. The equilibrium solubility values were determined using the saturation shake-flask and the Chasing Equilibrium Solubility (CheqSol) methods. Results obtained by the two methods are in good agreement. The aim of the present work was to study the validity of the Henderson-Hasselbalch (HH) relationship in the case of structurally diverse weak bases. The significance of pH control and the effect of the salt form (e.g., fumarate) was also investigated. In the case of monoprotic bases, namely papaverine, promethazine, propafenone and ticlopidine the experimental solubility data precisely follow the HH equation until the limit of salt solubility. The common ion effect on salt solubility was found to be significant at low pHs. Deviation from the HH equation in the case of dibasic quetiapine hydrogen fumarate and the ampholyte desvenlafaxine hydrogen fumarate can be easily interpreted with the formation of different salt compositions. It was concluded that precise pH control is essential in shake-flask solubility measurements. It is also critical that the p K_a value and the intrinsic solubility are accurately determined when the HH relationship is used to predict the pH-dependent aqueous solubility of drugs.

Keywords: pH-dependent solubility; Intrinsic solubility; Henderson-Hasselbalch relationship; Ionizable drugs; Shake-flask method ;

Mixed-mode solid-phase extraction followed by acetylation and gas chromatography mass spectrometry for the reliable determination of trans-resveratrol in wine samples by R. Montes; M. García-López; I. Rodríguez; R. Cela (pp. 47-53).

This work presents an advantageous analytical procedure for the accurate determination of free trans-resveratrol in red and white wines. The proposed method involves solid-phase extraction (SPE), acetylation of the analyte in aqueous media and further determination by gas chromatography (GC) with mass spectrometry detection (MS). The use of a mixed-mode SPE sorbent provides an improvement in the selectivity of the extraction step; moreover, the presence of several intense ions in the electron impact mass spectra of its acetyl derivative guarantees the unambiguous identification of trans-resveratrol. Considering a sample intake of 10mL, the method provides a limit of quantification (LOQ) of 0.8ngmL⁻¹ and linear responses for concentrations up to 2.5μgmL⁻¹, referred to wine samples. The average recovery, estimated with samples fortified at different concentrations in the above range, was 99.6% and the inter-day precision stayed below 8%. Trans-resveratrol levels in the analyzed wines varied from 3.4 to 1810ngmL⁻¹. Cis-resveratrol was also found in all samples. In most cases, equal or higher responses were measured for this latter form than for the trans-isomer. The reduced form of resveratrol, dihydro-resveratrol, was systematically identified in red wines.

Keywords: Resveratrol; Wine; Acetylation; Mixed-mode solid-phase extraction; Gas chromatography; Mass spectrometry

Amperometric sensor for cyanide utilizing cyanidase and formate dehydrogenase by Lothar Ketterer; Michael Keusgen (pp. 54-59).

An amperometric sensor for the detection of cyanide integrated in a flow injection analysis (FIA) system is presented. The specificity of the sensor for cyanide is achieved by a reaction cascade involving two enzymes, cyanidase (EC 3.5.5.1) and formate dehydrogenase (FDH, EC 1.2.1.2). The limit of detection (LOD) and the limit of quantification (LOQ) were determined to be in the low micromolar range. The sensor showed very good long term stability and the linear range of detection extends from 0.7 up to 800μM. The sensor’s ability to measure cyanide in complex matrices, such as plant extracts, is demonstrated. Although the sensor is susceptible to formate and to a lesser extent to thiocyanate, sulfide does not interfere significantly.

Keywords: Cyanide; Cyanidase; Formate dehydrogenase; Amperometry; Flow injection analysis (FIA)

Comprehensive comparison of liquid chromatography selectivity as provided by two types of liquid chromatography detectors (high resolution mass spectrometry and tandem mass spectrometry): “Where is the crossover point?” by A. Kaufmann; P. Butcher; K. Maden; S. Walker; M. Widmer (pp. 60-72).

The selectivity of mass traces obtained by monitoring liquid chromatography coupled to high resolution mass spectrometry (LC-HRMS) and liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) was compared. A number of blank extracts (fish, pork kidney, pork liver and honey) were separated by ultra performance liquid chromatography (UPLC). Detected were some 100 dummy transitions respectively dummy exact masses (traces). These dummy masses were the product of a random generator. The range of the permitted masses corresponded to those which are typical for analytes (e.g. veterinary drugs). The large number of monitored dummy traces ensured that endogenous compounds present in the matrix extract, produced a significant number of detectable chromatographic peaks. All obtained chromatographic peaks were integrated and standardized. Standardisation was done by dividing these absolute peak areas by the average response of a set of 7 different veterinary drugs. This permitted a direct comparison between the LC-HRMS and LC-MS/MS data. The data indicated that the selectivity of LC-HRMS exceeds LC-MS/MS, if high resolution mass spectrometry (HRMS) data is recorded with a resolution of 50,000 full width at half maximum (FWHM) and a corresponding mass window. This conclusion was further supported by experimental data (MS/MS based trace analysis), where a false positive finding was observed. An endogenous matrix compound present in honey matrix behaved like a banned nitroimidazole drug. This included identical retention time and two MRM traces, producing an MRM ratio between them, which perfectly matched the ratio observed in the external standard. HRMS measurement clearly resolved the interfering matrix compound and unmasked the false positive MS/MS finding.

Keywords: High resolution mass spectrometry; tandem mass spectrometry; selectivity; comparison; trace analysis

Determination of phosphate compounds in meat products by 31-Phosphorus Nuclear Magnetic Resonance spectroscopy with methylenediphosphonic acid after alkaline extraction by P. Hrynczyszyn; A. Jastrzębska; E. Szłyk (pp. 73-78).

Modification of the extraction procedure and application of the³¹P NMR method for the determination of polyphosphates in meat products were studied. In the elaborated procedure threefold water extraction at alkaline pH (borate buffer and 0.1M EDTA) was applied. Furthermore, the new external standard for³¹P NMR determination of phosphates was proposed. Obtained recoveries were between 95 and 99% and variation coefficients (CV) was ≤5%, indicating an increase in accuracy and the precision of the proposed procedure in relation to the spectrophotometric method. The described procedure of sample preparation with³¹P NMR method was applied for the determination of polyphosphate additives in meat products. The satisfactory precision (CV=0.39–3.40%) shows the benefit of the NMR method in the routine analysis of the phosphate ions in meat products.

Keywords: Alkaline extraction; 31-Phosphorus Nuclear Magnetic Resonance; Methylenediphosphonic acid; Phosphate compounds determination; Meat products

Potentiometric determination of cetylpyridinium chloride using a new type of screen-printed ion selective electrodes by Gehad G. Mohamed; Tamer Awad Ali; M.F. El-Shahat; A.M. Al-Sabagh; M.A. Migahed; Elmorsy Khaled (pp. 79-87).

A new type of screen-printed ion-selective electrode for the determination of cetylpyridinium chloride (CPC) is presented. These new electrodes involve in situ, modified and unmodified screen-printed ion-selective electrodes for the determination of CPC. The screen-printed electrodes (SPEs) show a stable, near-Nernstian response for 1×10⁻² to 1×10⁻⁶M CPC at 25°C over the pH range 2–8 with cationic slope 60.66±1.10. The lower detection limit is found to be 8×10⁻⁷M and response time of about 3s and exhibit adequate shelf-life (6 months). The fabricated electrodes can be also successfully used in the potentiometric titration of CPC with sodium tetraphenylborate (NaTPB). The analytical performances of the SPEs are compared with those for carbon paste electrode (CPE) and polyvinyl chloride (PVC) electrodes. The method is applied for pharmaceutical preparations with a percentage recovery of 99.60% and R.S.D.=0.53. The frequently used CPC of analytical and technical grade as well as different water samples has been successfully titrated and the results obtained agreed with those obtained with commercial electrode and standard two-phase titration method. The sensitivity of the proposed method is comparable with the official method and ability of field measurements.

Keywords: Cetylpyridinium chloride ion-selective electrodes; Screen-printed carbon paste electrodes; Carbon paste electrodes; Polyvinyl chloride membrane electrodes; Pharmaceutical preparations; Water samples

Electrochemical sensors based on binuclear cobalt phthalocyanine/surfactant/ordered mesoporous carbon composite electrode by Ling Liu; Li-ping Guo; Xiang-jie Bo; Jing Bai; Xiu-jun Cui (pp. 88-94).

A new ordered mesoporous carbon (OMC) composite modified electrode was fabricated for the first time. Binuclear cobalt phthalocyaninehexasulfonate sodium salt (bi-CoPc) can be adsorbed onto didodecyldimethylammonium bromide (DDAB)/OMC film by ion exchange. UV–vis spectroscopy, scanning electron microscopy (SEM) and electrochemical methods were used to characterize the composite film. The cyclic voltammograms demonstrate that the charge transfer of bi-CoPc is promoted by the presence of OMC. Further study indicated that bi-CoPc/DDAB/OMC film is the excellent electrocatalyst for the electrochemical reduction of oxygen in a neutral aqueous solution and hemoglobin (Hb) at lower concentrations. Additionally, as an amperometric 2-mercaptoethanol (2-ME) sensor, this modified electrode shows a wider linear range (2.5×10⁻⁶ to 1.4×10⁻⁴M), high sensitivity (16.5μAmM⁻¹) and low detection limit of 0.6μM (S/N=3). All these confirm the fact that the new composite film may have wide potential applications in biofuel cells, biological and environmental sensors.

Keywords: Ordered mesoporous carbon; Binuclear cobalt phthalocyaninehexasulfonate sodium salt; Oxygen; Hemoglobin; 2-Mercaptoethanol; Electrocatalysis

The effects of flow type on aptamer capture in differential mobility cytometry cell separations by Yan Liu; Se Won Bae; Kelong Wang; Jong-In Hong; Zhi Zhu; Weihong Tan; Dimitri Pappas (pp. 95-100).

In this work, differential mobility cytometry (DMC) was used to monitor cell separation based on aptamer recognition for target cells. In this device, open-tubular capillaries coated with Sgc8 aptamers were used as affinity chromatography columns for separation. After cells were injected into the columns, oscillating flow was generated to allow for long-term cell adhesion studies. This process was monitored by optical microscopy, and differential imaging was used to analyze the cells as they adhered to the affinity surface. We investigated the capture time, capture efficiency, purity of target and control cells, as well as the reusability of the affinity columns. Capture time for both CCRF-CEM cells and Jurkat T cells was 0.4±0.2s, which demonstrated the high separation affinity between aptamers and target cells. The capture efficiency for CCRF-CEM cells was 95% and purity was 99% in a cell mixture. With the advantage of both high cell capture efficiency and purity, DMC combined with aptamer-based separation emerges as a powerful tool for rare cell enrichment. In addition, aptamer-based DMC channels were found to be more robust than antibody based channels with respect to reuse of the separation device.

Keywords: Differential mobility cytometry; Aptamer; Cell separation; Cell capture time

Flow-through immunomagnetic separation system for waterborne pathogen isolation and detection: Application to Giardia and Cryptosporidium cell isolation by Qasem Ramadan; Lay Christophe; William Teo; Li ShuJun; Feng Han Hua (pp. 101-108).

Simultaneous sample washing and concentration of two waterborne pathogen samples were demonstrated using a rotational magnetic system under continuous flow conditions. The rotation of periodically arranged small permanent magnets close to a fluidic channel carrying magnetic particle suspension allows the trapping and release of particles along the fluidic channel in a periodic manner. Each trapping and release event resembles one washing cycle.The performance of the magnetic separation system (MSS) was evaluated in order to test its functionality to isolate magnetic-labelled protozoan cells from filtered, concentrated tap water, secondary effluent water, and purified water. Experimental protocols described in US Environmental Protection Agency method 1623 which rely on the use of a magnetic particle concentrator, were applied to test and compare our continuous flow cell separation system to the standard magnetic bead-based isolation instruments. The recovery efficiencies for Giardia cysts using the magnetic tube holder and our magnetic separation system were 90.5% and 90.1%, respectively, from a tap water matrix and about 31% and 18.5%, respectively, from a spiked secondary effluent matrix. The recovery efficiencies for Cryptosporidium cells using the magnetic tube holder and our magnetic separation system were 90% and 83.3%, respectively, from a tap water matrix and about 38% and 36%, respectively, from a spiked secondary effluent matrix. Recoveries from all matrices with the continuous flow system were typically higher in glass tubing conduits than in molded plastic conduits.

Keywords: Abbreviations; USEPA; US Environmental Protection Agency; IMS; Immunomagnetic separation; MSS; Magnetic separation system; t; s; ₁; The time required for the particle to travel to and be retained at the first trapping zone; T; ₀; The time required for a particle to travel through the entire fluidic channel and reach the separation chamber without any influence of magnetic force; T; _m; The time required for a magnetic particle to travel through the entire fluidic channel and reach the separation chamber with the present of magnetic force effect from the magnet array; t; _Sn; The time required for a magnetic particle to travel between two adjacent trapping zones; PC; Poly carbonate; PBS; Phosphate buffered saline; S1; Syringe 1; S2; Syringe 2; S3; Syringe 3; V1; Valve 1; V2; Valve 2; V3; Valve3; Pi1; Fluidic inlet 1; Pi2; Fluidic inlet 2; Pi3; Fluidic inlet 3; MA; Magnetic assembly (see supplementary materials); SM; Separation magnet (see supplementary materials)Continuous flow; Washing; Separation; Magnetic particles; Giardia; Cryptosporidium

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Analytica Chimica Acta (v.673, #1)