Current Analytical Chemistry - Volume 11, Issue 1, 2015

Since the discovery of their benign (non-toxic) and excellent fluorescent properties, carbon dots (or C-dots) have attracted extensive attentions and presented a clear demonstration of great potential in bioanalytical applications. This intrinsic and somewhat exceptional nontoxic property of C-dots makes them potential alternatives to the conventional semiconductor quantum dots counterpart containing heavy metals that pose serious health and environmental concerns. After a brief discussion of their earlier and current synthetic strategies, a comprehensive summary of the synthetic routes together with their corresponding advantages and disadvantages is reviewed. Functional surface manipulations of Cdots aiming at improving quantum yield and stability for more excellent fluorescent sensing are summarized. This paper provides a concise review of recent significant advances in C-dots, focusing on the synthesis, the optical properties and the analytical applications.

We developed a method that uses electrochemical impedance spectroscopy (EIS) to choose an electrode to determine photosensitizing dye No. 201 (quaternium-73), a thiazolium derivative, in cosmetics. The method is based on nanoparticle metal oxide (tin, cerium, cobalt, iron, and nickel)-modified carbon paste electrodes. Using differential pulse voltammetry (DPV), we found that the electrocatalytic property of tin oxide was better than that of the other metal oxides tested. When the proposed method was used to determine photosensitizing dye No. 201 in commercial cosmetics, the results were were comparable to those obtained using high-performance liquid chromatography with ultraviolet detection.

A novel polyvinyl chloride (PVC) membrane sensor that is highly selective to Fe3+ ions was prepared by using 5-chloro-3-[4-(trifluoromethoxy)phenylimino]indolin-2-one (CFMEPI) ionophore. The sensor exhibits a Nernstian response for Fe3+ ions over a wide concentration range (1.0 ×10-2 -1.0×10-6M) with a slope of 46.7±0.5 mV per decade. The sensor has a response time of 20s and can be used for at least 3 months without any measurable divergence in potential. It was concluded that the sensor response was pH independent in the range of 4.0-8.61. The sensor has some advantages such as short analysis time, particularly high selectivity towards iron (III). The sensor was used successfully for direct determination of Fe3+ in several synthetic and real samples with satisfactory results.

An efficient liquid-liquid extraction method was developed for the removal of Cd2+, Ni2+, Pb2+, and Zn2+ from environmental waste using n-benzoyl-n-phenyl hydroxylamine (BPA). The effects of solvents, pH, stripping agents, extraction time, and interference of other ions on the recoveries were investigated. Study shows chloroform is the most efficient solvent for BPA and maximum extraction percentage of metals can be achieved by it. These metals can be quantitatively extracted between the pH 6.5-10. 1M HNO3, 4M HNO3, 1M HCl, and 5M HNO3 can be used as stripping agents to achieve the maximum percentage recovery of Cd2+, Ni2+, Pb2+, and Zn2+, respectively.

This paper proposes a method using slurry sampling and cold vapor atomic absorption spectrometry (CV AAS) for the determination of mercury in iron supplement used for the treatment of anemia. The optimization step of the slurry preparation was performed using a two-level full factorial design involving the experimental factors: hydrochloric acid concentration, thiourea concentration and sonication time, having as chemometric response the absorbance. Employing the optimized conditions, the method allows the determination of mercury, using external calibration technique, with limits of detection and quantification of 30 and 102 ng L-1, respectively. The precision expressed as a relative standard deviation (RSD) was 10.89 and 6.84% for supplement samples with mercury content of 4.82 and 9.61 ng g-1, respectively. The limits of detection and quantification, considering a sample mass of 1.2 g, were 0.63 a 2.10 ng g-1, respectively. There is no certified reference material of ferrous supplement for evaluation of the accuracy. Thus, addition/recovery tests were performed to evaluate the accuracy of the method. The recovery values achieved varied from 85 to 106%, confirming the applicability of this method for mercury quantification in these iron matrices. The method was satisfactorily applied for the determination of mercury in twelve iron supplement samples with different chemical compositions. In six samples, the mercury content was lower than the limit of quantification (2.10 ng g-1) and in the other six samples, the mercury content varied from 3.17 to 34.86 ng g-1.

In this work is suggested a Clark type electrode with disposable enzyme membrane for phenols determination. This approach allows avoiding the electrode fouling by recording the oxygen reduction current, and obtaining a stable and reproducible sensor response. An appropriate method for enzyme membrane fabrication by cellulose acetate filters coating with tyrosinase through the use of biocompatible hydrogels was proposed and tested. The sensitivity of the catechol determination, as an example, was found to be 0.89 µA mmol-1 L. The detection limit equaled 1 µmol L-1. The achieved RSD was 1.22%.

Two different enzyme electrodes for the determination of ethanol in alcoholic beverages were developed by immobilizing alcohol oxidase or catalase in a k-Carrageenan gel layer overlapping an amperometric gaseous diffusion Clark type oxygen electrode. The response of biosensors toward standard solutions of ethanol,methanol, n-propanol, n-butanol, ethylenglycol and glycerol was recorded, compared and discussed. In the characterization studies of the biosensors several parameters such as pH, operational stability, response time, analysis time and calibration repeatability, between-days and between-electrodes, calibration reproducibility, linearity, and sensitivity, were studied. Finally by using the developed biosensors, the ethanol concentration of several commercial wine and beer samples was determined. The results, obtained with the two enzyme electrodes, were compared and the correlation evaluated by statistic tests. The biosensor method using catalase enzyme seemed to be the most suitable of the two enzyme devices for a selective and less expensive determination of ethanol in alcoholic beverages. The catalase enzyme electrode actually showed the greatest selectivity to ethanol, of all the alcohols tested. The response of the catalase biosensor was not influenced by the presence of the methanol in particular and its operational stability was found to be significantly higher than that of the alcohol oxidase sensor. The only drawback associated with the use of the catalase biosensor, compared to the method based on alcohol oxidase, is the slightly increased time required to perform the ethanol analysis in each sample of alcoholic beverage.

Separation of 5-(dimethylamino) naphthalene-1-sulphonyl (DNS-) derivatives of some amino acids (alanine, leucine, valine, phenylalanine) D- and L-enantiomers with high-performance thin layer chromatography (HPTLC) and pressurized planar electrochromatography (PPEC) both with β-cyclodextrin as the component of the mobile phase is presented. The separation mechanism in the PPEC technique involves two effects (partition and electrophoresis) whereas HPTLC system employs only one (partition). The separation process in both modes was carried out with the HPTLC RP-18 W plates. Influence of variables such as: β-cyclodextrin concentration in the mobile phase and composition of the mobile phase on the migration distance of the solute zones is investigated. Although HPTLC and PPEC systems enable to separate D- and L-DNS- amino acids the electrophoretic effect in PPECsystem is responsible for different separation selectivity in the latter one in comparison to that in first one. The retention of L-DNS- amino acids is stronger regarding their D-antipodes with both separation techniques. The repeatability of the results from separation of DNS-Ala enantiomers with PPEC and HPTLC technique is compared. Slightly worse standard deviation and RSD values for more retained enantiomer, researched with PPEC, are characteristics of this electromigrational technique. In addition a new mode of β-cyclodextrin zone determination on the chromatographic plate is presented.