CE with sequential light-emitting diode-induced fluorescence and electro-chemiluminescence detections for the determination of amino acids and alkaloids

Mechanistic Insight into a Novel Ultrasensitive Nicotine Assay Base on High-Efficiency Quenching of Gold Nanocluster Cathodic Electrochemiluminescence

Monitoring nicotine concentrations in human fluids is extremely crucial owing to the harmful effect of nicotine on human health. Herein, it is shown that nicotine could quench the cathodic electrochemiluminescence (ECL) of gold nanoclusters (AuNCs) with high efficiency. The ECL quenching mechanism of nicotine was studied in detail using various experimental tools and theoretical calculations. It was concluded that the strongly oxidizing intermediate SO₄^•-, produced from K₂S₂O₈, could oxidized nicotine, resulting in ECL emission quenching. On the basis of this high-efficiency ECL quenching of the AuNCs/K₂S₂O₈ system, a recyclable, ultrasensitive, and selective ECL sensing platform for nicotine detection was proposed. Even in the absence of any complex signal amplification techniques, the ECL sensor for nicotine detection showed an unprecedentedly low detection limit of 7.0 × 10^-13 M (S/N = 3) and a wide linear range over 8 orders of magnitude. Most remarkably, it could be successfully used for nicotine detection in human urine samples. This is expected to promote the investigations and applications on nicotine-related diseases. We believe that the proposed ECL platform can hold great prospects for commercialization in biomedical fields and tobacco industries.

Selectivity enhancement in capillary electrophoresis by means of two-dimensional separation or dual detection concepts

For the identification and quantification of analytes in complex samples, highly selective analytical strategies are required. The selectivity of single separation techniques such as gas chromatography (GC), liquid chromatography (LC), or capillary electrophoresis (CE) with common detection principles can be enhanced by hyphenating orthogonal separation techniques but also by using complementary detection systems. In this review, two-dimensional systems containing CE in at least one dimension are reviewed, namely LC-CE or 2D CE systems. Particular attention is paid to the aspect of selectivity enhancement due to the orthogonality of the different separation mechanisms. As an alternative concept, dual detection approaches are reviewed using the common detectors of CE such as UV/VIS, laser-induced fluorescence, capacitively coupled contactless conductivity (C⁴D), electrochemical detection, and mass spectrometry. Special emphasis is given to dual detection systems implementing the highly flexible C⁴D as one detection component. Selectivity enhancement can be achieved in case of complementarity of the different detection techniques.

Baseline separation of amino acid biomarkers of hepatocellular carcinoma by polyvinylpyrrolidone-filled capillary electrophoresis with light-emitting diode-induced fluorescence in the presence of mixed micelles

Separation of amino acid biomarkers could be performed by polyvinylpyrrolidone-filled capillary electrophoresis in the presence of mixed micelles.

Modification of indium tin oxide with dendrimer-encapsulated nanoparticles to provide enhanced stable electrochemiluminescence of Ru(bpy)3(2+)/tripropylamine while preserving optical transparency of indium tin oxide for sensitive electrochemiluminescence-based analyses

Here, we report highly enhanced stable electrogenerated chemiluminescence (ECL) of Ru(bpy)3(2+) (bpy = 2,2'-bipyridyl) with tripropylamine (TPrA) coreactant on indium tin oxide (ITO) electrodes modified with amine-terminated dendrimers encapsulating catalytic nanoparticles while maintaining optical transparency of ITO and feasibility of the modified ITOs to sensitive ECL-based assays. As model systems, we prepared Pt and Au dendrimer-encapsulated nanoparticles (DENs) using amine-terminated sixth-generation poly(amido amine) dendrimers and subsequently immobilized the DENs onto ITO surfaces via electrooxidative grafting of the terminal amines of dendrimers to the surfaces. The resulting DEN-modified ITOs preserved good optical transparency of ITO and exhibited highly catalyzed electrochemical oxidation of Ru(bpy)3(2+)/TPrA, leading to significantly increased ECL emission. Especially, the Pt DEN-modified ITO electrode provides negligible transmittance drop, i.e., only ∼1.99% over the entire visible region, and exhibited not only much enhanced (i.e., ∼213-fold increase compared to ECL obtained from bare ITO) but also stable ECL emission under consecutive potential scans from 0.00 to 1.10 V for 10 cycles, which allowed ∼329 times more sensitive ECL-based analysis of nicotine using the Pt DEN-modified ITO compared with the use of bare ITO.

Separation of amino acids by capillary electrophoresis with light-emitting diode-induced fluorescence in the presence of electroosmotic flow

In this chapter, we describe a method to identify amino acids (AA) by capillary electrophoresis in conjunction with light-emitting diode-induced fluorescence (LEDIF). First, amino acids labeled with naphthalene-2,3-dicarboxaldehyde (NDA) in the presence of cyanide are converted to highly fluorescent cyanobenz[f]isoindole (CBI) derivatives. Next, they are separated by gel electrophoresis in the presence of EOF. In the process, the CBI products were excited by a violet LED to produce green fluorescence. In addition to the optical setup of light-emitting diode-induced fluorescence, the preparation of poly(ethylene) oxide for amino acid separation is also described in this chapter.

Determination of arecoline in areca nut based on field amplification in capillary electrophoresis coupled with electrochemiluminescence detection

A sensitive capillary electrophoresis-electrochemiluminescence (CE-ECL) assay with an ionic liquid (IL) was developed for the determination of arecoline in areca nut. The IL, 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF(4) ), was an effective additive improved not only the separation selectivity but also the detection sensitivity of the analyte. BMImBF(4) in the separation electrolyte made the resistance of the separation buffer much lower than that of the sample solution, which resulted in an enhanced field amplified electrokinetic injection CE. ECL intensity of arecoline is about two times higher than that of the analyte with phosphate-IL buffer system. Resolution between arecoline and other unknown compounds in real samples was improved. Under the optimized conditions (ECL detection at 1.2 V, 16 kV separation voltage, 20 mmol/L phosphate with 10 mmol/L BMImBF(4) buffer at pH 7.50, 5 mmol/L Ru(bpy)(3)(2+) and 50 mmol/L phosphate buffer in the detection reservoir), a detection limit of 5 × 10(-9) mol/L for arecoline was obtained. Relative standard deviations of the ECL intensity and the migration time were 4.51% and 0.72% for arecoline. This method was successfully applied to determination of the amount of arecoline in areca nut within 450 s.

Determination of nicotine and its metabolite cotinine in urine and cigarette samples by capillary electrophoresis coupled with electrochemiluminescence

In this paper, CE coupled with electrochemiluminesence (ECL) detection using a 76-μm Pt disk as working electrode was developed for nicotine (NIC) determination. The major metabolite of NIC is cotinine (COT), which has a similar tertiary amine structure to NIC. However, there is a carbonyl group attached in the structure of COT, which leads to the great decrease in ECL response. In order to improve the ECL response of COT, NaBH(4) was used for carbonyl reduction. After reduction, NIC and COT were separated and detected by CE-ECL. ECL response plotted with NIC concentration was linear between 5.0×10(-7) and 5.0×10(-5)  mol/L (81-8100 μg/L), with LOD of 5.0×10(-8)  mol/L (8.1 μg/L). The developed CE-ECL method was applied for NIC determination in urine and cigarette samples.

Capillary electrophoresis with LED-induced native fluorescence detection for determination of isoquinoline alkaloids and their cytotoxicity in extracts of Chelidonium majus L

In this study, we introduced a simple and sensitive method of capillary electrophoresis with ultraviolet light-emitting diode-induced native fluorescence (UV-LEDIF) detection for the determination of isoquinoline alkaloids in extracts of Chelidonium majus L. Samples were extracted with acidic methanol and the extracts were directly analysed by CE. Simultaneous determination of protopine, chelidonine, coptisine, sanguinarine, allocryptopine, chelerythrine and stylopine was performed in 20mM phosphate buffer (pH 3.1). The baseline separation of these alkaloids was finished within 20 min. As these alkaloids have native fluorescence, they were directly detected using the commercially available UV light emitting diode without troublesome fluorescent derivatisation. Satisfactory LOD values were obtained for the studied compounds considering their appearance in natural extracts. Lower limits of detection were 0.05 μg/mL for protopine, 0.06 μg/mL for stylopine and allocryptopine, 0.07 μg/mL for chelidonine, 0.22 μg/mL for sanguinarine, 1.7 μg/mL for chelerythrine and 5.5 μg/mL for coptisine. The developed method was successfully applied to determine the contents of seven alkaloids in the aerial parts of Chelidonium majus L, which varied from 0.025 to 0.763% (w/w). Also, to demonstrate the potential of the proposed CE method, an estimation of the cytotoxic properties of selected Celandine alkaloids in a natural extract was carried out.

Determination of isocyanates by capillary electrophoresis with tris(2,2'-bipyridine)ruthenium(II) electrochemiluminescence

CE with tris(2,2'-bipyridyl) ruthenium(II) (Ru(bpy)(3) (2+)) electrochemiluminescence (ECL) detection for the quantitative determination of isocyanates was first reported. Hexamethylene diisocyanate (HDI) and hexyl isocyanate (HI) were used as the model analytes. Commercially available N,N-diethyl-N'-methylethylenediamine was used as the derivatization reagent. It has both a secondary amine group and a tertiary amine group. The secondary amine group can quantitatively react with isocyanate group, and the tertiary amine group can react with Ru(bpy)(3) (2+) to produce strong ECL signal for sensitive detection. The derivatization reaction was almost instantaneous and is much faster than other reported derivative reactions using other derivative reagents. The urea formed was stable. Linear calibration curve was obtained in the range from 0.01 to 10 microM for HDI, and 0.02 to 20 microM for hexyl isocyanate (HI). The detection limit is 0.01 microM for HDI and 0.02 microM for HI. The method is more sensitive than UV-detection and electrochemical detection. For practical application, recovery higher than 90% for HDI and HI was obtained for foam sample.

Enantioseparation of nicotine alkaloids in cigarettes by CE using sulfated beta-CD as a chiral selector and a capillary coated with amino groups

Nicotine (NC) and its related compounds (cotinine (CN), nornicotine (NN), anatabine (AT) and anabasine (AB)) were simultaneously enantioseparated by CE using a capillary with amino groups and sulfated beta-CD as a chiral selector. The optimum running conditions were found to be 30 mM acetate buffer (pH 5.0) containing 8% sulfated beta-CD with an applied voltage of +15 kV at 30 degrees C using direct detection at 260 nm. Using a capillary coated with amino groups, the EOF migrates toward the positive pole. However, when sulfated beta-CD was added to the BGE, it was found that the EOF migrated toward the negative pole due to ionic adsorption of sulfated beta-CD to amino groups on the capillary inner wall. All the cationic analytes migrated as anions, suggesting that they formed stable anionic complexes with sulfated beta-CD. With this system and a simple pretreatment with mini-cartridges, NC alkaloids in five cigarette samples were enantioseparated. As a result, each of the compounds except for CN was detected. In the case of NC, only (S)-NC was detected (more than 99.9%), but in the case of NN, AT and AB, the ratios of (S)-isomer to total isomers were in the ranges 58-70, 81-85 and 59-65%, respectively. On the other hand, only NC was detected in cigarette smoke and the ratio of (S)- and (R)-NCs was 96:4. The amounts of NC alkaloids in cigarettes suggest that the production of (R)-NC resulted from racemization due to the high temperature/burning of the cigarette.

Determination of ascorbic acid in individual rat hepatocyte cells based on capillary electrophoresis with electrochemiluminescence detection

A novel method to detect ascorbic acid (AA) in individual rat hepatocyte cells was developed by combining CE with electrochemiluminescence (ECL) based on tris(2,2'-bipyridine) ruthenium(II) (Ru(bpy)(3)2+). A single cell, followed by 0.1% SDS as cell lysis solution, was injected into the inlet of the separation capillary by electromigration. After optimizing the analytical conditions, the RSDs of migration time and peak height were 0.38% and 2.6% for 1.0x10(-5) M AA (n=10), respectively. The linear range of AA was from 1.0x10(-8) to 5.0x10(-5) M with a correlation coefficient of 0.9979 and the LOD (S/N=3) was estimated to be 1.0x10(-8) M. This method has been successfully applied to determine AA in single rat hepatocytes and the amount of AA in seven rat hepatocytes ranged from 16 to 62 fmol. The above results demonstrated that CE coupled with ECL is convenient, sensitive, and will become an attractive alternative method for single-cell analysis.