Chiral On-Chip Separations of Neurotransmitters

Machine-learning assisted multiplex detection of catecholamine neurotransmitters with a colorimetric sensor array

Catecholamine neurotransmitters (CNs), such as dopamine (DA), epinephrine (EP), norepinephrine (NEP), and levodopa (LD), are recognized as the primary biomarkers of a variety of neurological illnesses. Therefore, simultaneous monitoring of these biomarkers is highly recommended for clinical diagnosis and treatment. In this study, a high-performance colorimetric artificial tongue has been proposed for the multiplex detection of CNs. Different aggregation behaviors of gold nanoparticles in the presence of CNs under various buffering conditions generate unique fingerprint response patterns. Under various buffering conditions, the distinct acidity constants of CNs, and consequently their predominant species at a given pH, drive the aggregation of gold nanoparticles (AuNPs). The utilization of machine learning algorithms in this design enables classification and quantification of CNs in various samples. The response profile of the array was analyzed using the linear discriminant analysis algorithm for classification of CNs. This colorimetric sensor array is capable of accurately distinguishing between individual neurotransmitters and their combinations. Partial least squares regression was also applied for quantitation purposes. The obtained analytical figures of merit (FOMs) and linear ranges of 0.6-9 μM (R² = 0.99) for DA, 0.1-10 μM (R² = 0.99) for EP, 0.1-9 μM (R² = 0.99) for NEP and 1-70 μM (R² = 0.99) for LD demonstrated the potential applicability of the developed sensor array in precise and accurate determination of CNs. Finally, the feasibility of the array was validated in human urine samples as a complex biological fluid with LODs of 0.3, 0.5, 0.2, and 1.9 μM for DA, EP, NEP, and LD, respectively.

Experimental and theoretical study of the inclusion complexes of epinephrine with β-cyclodextrin, 18-crown-6 and cucurbit[7]uril

Experimental and molecular dynamics techniques suggested that stable complexes of epinephrine with 18C6, βCD and CB7 might enhance aggregation.

Colorimetric Fingerprints of Gold Nanorods for Discriminating Catecholamine Neurotransmitters in Urine Samples

Catecholamine neurotransmitters, generally including dopamine (DA), epinephrine (EP) and norepinephrine (NE) are known as substantial indicators of various neurological diseases. Simultaneous detection of these compounds and their metabolites is highly recommended in early clinical diagnosis. To this aim, in the present contribution, a high performance colorimetric sensor array has been proposed for the detection and discrimination of catecholamines based on their reducing ability to deposit silver on the surface of gold nanorods (AuNRs). The amassed silver nanoshell led to a blue shift in the longitudinal localized surface plasmon resonance (LSPR) peak of AuNRs, creating a unique pattern for each of the neurotransmitters. Hierarchical cluster analysis (HCA) and linear discriminate analysis (LDA) pattern recognition techniques were employed to identify DA, EP and NE. The proposed colorimetric array is able to differentiate among individual neurotransmitters as well as their mixtures, successfully. Finally, it was shown that the sensor array can identify these neurotransmitters in human urine samples.

Three-dimensional graphene networks as a new substrate for immobilization of laccase and dopamine and its application in glucose/O2 biofuel cell

We report here three-dimensional graphene networks (3D-GNs) as a novel substrate for the immobilization of laccase (Lac) and dopamine (DA) and its application in glucose/O2 biofuel cell. 3D-GNs were synthesized with an Ni(2+)-exchange/KOH activation combination method using a 732-type sulfonic acid ion-exchange resin as the carbon precursor. The 3D-GNs exhibited an interconnected network structure and a high specific surface area. DA was noncovalently functionalized on the surface of 3D-GNs with 3,4,9,10-perylene tetracarboxylic acid (PTCA) as a bridge and used as a novel immobilized mediating system for Lac-based bioelectrocatalytic reduction of oxygen. The 3D-GNs-PTCA-DA nanocomposite modified glassy carbon electrode (GCE) showed stable and well-defined redox current peaks for the catechol/o-quinone redox couple. Due to the mediated electron transfer by the 3D-GNs-PTCA-DA nanocomposite, the Nafion/Lac/3D-GNs-PTCA-DA/GCE exhibited high catalytic activity for oxygen reduction. The 3D-GNs are proven to be a better substrate for Lac and its mediator immobilization than 2D graphene nanosheets (2D-GNs) due to the interconnected network structure and high specific surface area of 3D-GNs. A glucose/O2 fuel cell using Nafion/Lac/3D-GNs-PTCA-DA/GCE as the cathode and Nafion/glucose oxidase/ferrocence/3D-GNs/GCE as the anode can output a maximum power density of 112 μW cm(-2) and a short-circuit current density of 0.96 mA cm(-2). This work may be helpful for exploiting the popular 3D-GNs as an efficient electrode material for many other biotechnology applications.

Chiral capillary electrophoresis-mass spectrometry of 3,4-dihydroxyphenylalanine: evidence for its enantioselective metabolism in PC-12 nerve cells

A fully automated chiral capillary electrophoresis-tandem mass spectrometry (CE-MS/MS) method was developed for enantiomeric quantification of 3,4-dihydroxyphenylalanine (DOPA) and its precursors, phenylalanine (Phe) and tyrosine (Tyr). To avoid MS source contamination, a negatively charged chiral selector, sulfated β-cyclodextrin (sulfated β-CD), that migrated away from the detector was used in combination with the partial filling technique. The six stereoisomers were simultaneously quantified in less than 12 min. Detection limits were 0.48 and 0.51 μM for l- and d-DOPA enantiomers, respectively. Assay reproducibility values (relative standard deviations [RSDs], n=6) were 4.43, 3.15, 4.91, 5.16, 3.96, and 3.25% for l- and d-DOPA, l- and d-Tyr, and l- and d-Phe at 10 μM, respectively. Thanks to the high enantioseparation efficiency, detection of trace d-DOPA in l-/d-DOPA mixtures could be achieved. The assay was employed to study the metabolism of DOPA, a well-known therapeutic drug for treating Parkinson's disease. It was found that l-DOPA was metabolized effectively in PC-12 cells. Approximately 88% of l-DOPA disappeared after incubation at a cell density of 2×10(6)cells/ml for 3 h. However, d-DOPA coexisting with l-DOPA in the incubation solution remained intact. The enantiospecific metabolism of DOPA in this neuronal model was demonstrated.

Chiral separations by CE employing CDs

This paper summarizes the history of chiral separations done by using electromigration methods with CDs. Several enantioresolution mechanisms and a wide number of chiral selectors have been applied to the separation of optical isomers by CE. Among them inclusion-complexation with CDs or their derivatives played a very important role in CE. Since the beginning our group was involved in studying method optimization for enantiomer resolution by using these chiral selectors. One of our publications was the basis for further development in the field, at least for us. New chiral selectors, development of theory, new methodological approaches and a wide number of practical applications are the main results achieved in the last almost 25 years using CE as an enantioseparative technique.

Detection of neurotransmitters by a light scattering technique based on seed-mediated growth of gold nanoparticles

A simple light scattering detection method for neurotransmitters has been developed, based on the growth of gold nanoparticles. Neurotransmitters (dopamine, L-dopa, noradrenaline and adrenaline) can effectively function as active reducing agents for generating gold nanoparticles, which result in enhanced light scattering signals. The strong light scattering of gold nanoparticles then allows the quantitative detection of the neurotransmitters simply by using a common spectrofluorometer. In particular, Au-nanoparticle seeds were added to facilitate the growth of nanoparticles, which was found to enhance the sensing performance greatly. Using this light scattering technique based on the seed-mediated growth of gold nanoparticles, detection limits of 4.4 × 10(-7) M, 3.5 × 10(-7) M, 4.1 × 10(-7) M, and 7.7 × 10(-7) M were achieved for dopamine, L-dopa, noradrenaline and adrenaline, respectively. The present strategy can be extended to detect other biologically important molecules in a very fast, simple and sensitive way, and may have potential applications in a wide range of fields.

Integrated microfluidic device with an electroplated palladium decoupler for more sensitive amperometric detection of the 8-hydroxy-deoxyguanosine (8-OH-dG) DNA adduct

8-hydroxy-deoxyguanosine (8-OH-dG) DNA adduct is one of the most frequently used biomarkers reporting on the oxidative stress that leads to DNA damage. More sensitive and reliable microfluidic devices are needed for the detection of these biomarkers of interest. We have developed a capillary electrophoresis (CE)-based microfluidic device with an electroplated palladium decoupler that provides significantly improved detection limit, separation efficiency, and resolving power. The poly(dimethylsiloxane) (PDMS)/glass hybrid device has fully integrated gold microelectrodes covered in situ with palladium nanoparticles using an electroplating technique. The performance and coverage of the electrodes electroplated with palladium particles were evaluated electrochemically and via scanning electron microscope (SEM) imaging, respectively. The performance of the device was tested and evaluated with different buffer systems, pH values, and electric field strengths. The results showed that this device has significantly improved resolving power, even at separation electric field strengths as high as 600 V cm-1. The detection limit for the 8-OH-dG adduct is about 20 attomoles; the concentration limit is on the order of 100 nM (S/N=3). A linear response is reported for both 8-OH-dG and dG in the range from 100 nM to 150 microM (approximately 100 pA microM-1) with separation efficiencies of approximately 120,000-170,000 plates m-1.

Determination of cationic neurotransmitters and metabolites in brain homogenates by microchip electrophoresis and carbon nanotube-modified amperometry

An electrophoretic method for simultaneous determination of catecholamines and their O-methoxylated metabolites on the microchip as well as in the capillary is presented. A complex separation system employing sodium dodecyl sulfate (SDS) micelles, dendrimers forming a second pseudostationary phase and borate complexation is needed for the satisfactory separation of the selected compounds on the short migration length. A carbon nanotube-modified working electrode has been applied for the sensitive amperometric detection with submicromolar detection limits. The applicability of this new method for the analytics of real samples is demonstrated by analysis of mouse brain homogenate on the microchip and human urine by capillary electrophoresis.

Enantiomeric separation of underivatized small amines in conventional and on-chip capillary electrophoresis with contactless conductivity detection

The determination of the enantiomers of small non-UV-absorbing amines which otherwise can only be achieved with difficulty was possible by using a combination of the chiral crown ether (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (18C6H4) and dimethyl-beta-CD as selectors in CE and contactless conductivity measurement for detection. Alkylamines without any other functional group, amino alcohols, species with ether or ester groups and with a cyclic moiety were investigated. The detection limits were found to be about 1.0 microM and the determination is possible up to at least 1.0 mM. The determination of enantiomeric ratios of up to 99.5:0.5 was also found feasible.

Strategies for enantioseparations of catecholamines and structurally related compounds by capillary zone electrophoresis using sulfated β-cyclodextrins as chiral selectors

Strategies for simultaneous enantioseparations of three catecholamines (DL-norepinephrine, DL-epinephrine, and DL-isoproterenol) and three structurally related compounds (DL-octopamine, DL-synephrine, and DL-norephedrine) by CZE using sulfated beta-CDs as chiral selectors were investigated. Four different separation modes were attempted: (I) using randomly sulfate-substituted beta-CD (MI-S-beta-CD) at relatively low concentrations in a high-concentration phosphate buffer at low pH in the normal polarity mode, (II) using MI-S-beta-CD at high concentrations at low pH in the reversed polarity mode, (III) using MI-S-beta-CD at moderately high concentrations in a phosphate buffer at neutral pH in the normal polarity mode, and (IV) using the single isomer heptakis(2,3-dihydroxy-6-O-sulfo)-beta-CD (SI-S-beta-CD) at low to moderately high concentrations in a high-concentration BGE at low pH in the normal polarity mode. Among them, enantioseparation of these cationic solutes was best achieved under the conditions of mode (II). In mode (II) and mode (III), temperature is an important factor affecting the enantioresolution of norepinephrine. In mode (I) and mode (IV), the use of a high-concentration BGE (150-200 mM) is crucial for effective enantioseparation of these cationic solutes with sulfated beta-CDs. Comparative studies of enantioseparations of these cationic solutes with MI-S-beta-CD and SI-S-beta-CD reveal that the sulfate substituents of MI-S-beta-CD located at the C(2)- position interact strongly with the diol moiety of catecholamines.

Proteins modification of poly(dimethylsiloxane) microfluidic channels for the enhanced microchip electrophoresis

This report described proteins modification of poly(dimethylsiloxane) (PDMS) microfluidic chip based on layer-by-layer (LBL) assembly technique for enhancing separation efficiency. Two kinds of protein-coated films were prepared. One was obtained by successively immobilizing the cationic polyelectrolyte (chitosan, Chit), gold nanoparticles (GNPs), and protein (albumin, Albu) to the PDMS microfluidic channels surface. The other was achieved by sequentially coating lysozyme (Lys) and Albu. Neurotransmitters (dopamine, DA; epinephrine, EP) and environmental pollutants (p-phenylenediamine, p-PDA; 4-aminophenol, 4-AP; hydroquinone, HQ) as two groups of separation models were studied to evaluate the effect of the functional PDMS microfluidic chips. The results clearly showed these analytes were efficiently separated within 140 s in a 3.7 cm long separation channel and successfully detected with in-channel amperometric detection mode. Experimental parameters in two protocols were optimized in detail. The detection limits of DA, EP, p-PDA, 4-AP, and HQ were 2.0, 4.7, 8.1, 12.3, and 14.8 microM (S/N=3) on the Chit-GNPs-Albu coated PDMS/PDMS microchip, and 1.2, 2.7, 7.2, 9.8, and 12.2 microM (S/N=3) on the Lys-Albu coated one, respectively. In addition, through modification, the more homogenous channel surface displayed higher reproducibility and better stability.

Enzymatic sensitivity enhancement of biogenic monoamines on a chip

Detection of biogenic monoamines in nanomolar concentrations is of great importance for probing the brain chemistry and for their analytics in biological fluids. The sensitivity enhancement of amperometric detection of neurotransmitters (NTs) and their metabolites after their electrophoretic separation on a microchip is presented and is based on coupled enzymatic reactions. The current response of the analyte is amplified by cyclic oxidation on a gold electrode mediated by reduced nicotinamide dinucleotide coenzyme and glucose oxidase enzyme present in the electrophoresis buffer. Using this approach, detection limits of about 10 nM for NTs and their metabolites can be reached.

Separation and determination of norepinephrine, epinephrine and isoprinaline enantiomers by capillary electrophoresis in pharmaceutical formulation and human serum

A capillary electrophoresis method with ultraviolet (UV) detection was developed and optimized for the enantiomer separation of norepinephrine (NE), epinephrine (EP) and isoprenaline (IP) using dual cyclodextrins (CDs) of 2-hydroxypropyl-beta-CD (HP-beta-CD) and heptakis (2,6-di-o-methyl)-beta-CD (DM-beta-CD) as chiral selectors. Optimal separation was obtained using a running buffer of 50mM phosphate containing 30mM HP-beta-CD and 5mM DM-beta-CD at pH 2.90 and a field strength of 20kV in 45cmx75mum (40cm effective length) uncoated capillary. The UV absorbance detection was set at 205nm. A 0.1% (w/w) polyethylene glycol or 0.1% (v/v) acetonitrile was used to enhance the detection sensitivity. There was a wide and excellent linear calibration graph for each enantiomer in the range 1.0x10(-3) to 1.0x10(-6)M and the detection limit (S/N=3) was found from 8.5x10(-7) to 9.5x10(-7)M. The method has been applied for the determination of isoprenaline in isoprenaline hydrochloride aerosol and to the analysis of serum samples. The recoveries of NE and EP in serum and IP in drug were ranged from 90 to 110%. The relative standard deviations of all the analyte peaks were less than 2.8% for migration time and less than 4.8% for peak area.

Enantiomeric separation of 1-phenylethylamine and 1-cyclohexylethylamine in capillary electrophoresis with contactless conductivity detection

Contactless conductivity detection was employed for the detection of the enantiomers of 1-phenylethylamine and 1-cyclohexylethylamine which were separated in capillary electrophoresis with unprecedented high resolutions R(s) of 2.3 and 3.3, respectively, by using a combination of dimethyl-beta-cyclodextrin and the chiral crown ether 18C6H4 as chiral selectors in a citric acid buffer of pH 2.4. The conductivity measurement enabled the direct detection, i.e. without having to derivatize or resort to indirect methods, of all species including the non-UV-absorbing enantiomers of cyclohexylamine. Detection limits of 0.5 microM were achieved and the determination of enantiomeric ratios of up to 99:1 was found possible.

Microchip capillary electrophoresis with a cellulose-DNA-modified screen-printed electrode for the analysis of neurotransmitters

A microfluidic chip based on capillary electrophoresis coupled with a cellulose-single-stranded DNA (cellulose-ssDNA) modified electrode was used for the simultaneous analysis of dopamine (DA), norepinephrine (NE), 3,4-dihydroxy-L-phenylalanine (L-DOPA), 3,4-dihydroxyphenylacetic acid (DOPAC), and ascorbic acid (AA). The modification of the electrode improved the electrophoretic analysis performance by lowering the detection potential and enhancing the signal-to-noise characteristic without surface poisoning of the electrode. The sensitivity of the modified electrode was about 12 times higher than those of the bare ones. The test compounds were separated using a 62 mm long separation channel at the separation field strength of +200 V/cm within 220 s in a 10 mM phosphate buffer (pH 7.4). The most favorable potential for the amperometric detection was 0.7 V (vs. Ag/AgCl). A reproducible response (relative standard deviation of 1.3, 1.3, 2.1, 3.1, 3.4% for DA, NE, L-DOPA, DOPAC, and AA, respectively, for n = 9) for repetitive sample injections reflected the negligible electrode fouling at the cellulose-ssDNA modified electrode. Square-wave voltammetric analyses reflected the sensitivities of the modified electrode for DA, NE, L-DOPA, DOPAC, and AA which were 1.78, 0.82, 0.69, 2.45, and 1.23 nC/microM with detection limits of 0.032, 0.93, 1.13, 0.31, and 0.62 microM, respectively. The applicability of this microsystem to real sample analysis was demonstrated.

Electrochemical techniques for lab-on-a-chip applications

During the last few years there has been a rapid increase in the use of electrochemical reactions in lab-on-a-chip devices. This development, which has so far mainly focussed on electrochemical detection in chip-based capillary electrophoresis, can be explained by the fact that electrochemical techniques and devices are particularly well-suited for inclusion in lab-on-a-chip systems. The most important reason for this is that the required electrodes can readily be manufactured and miniaturised without loss of analytical performance using conventional microfabrication methods. In this Research Highlight article, the developments during the last three years concerning electrochemical techniques for lab on-a-chip applications are discussed, with particular focus on emerging electrochemical methods for sample clean-up and preconcentration, electrochemical derivatisation and electrochemical detection in chip-based capillary electrophoresis.

Recent developments in electrochemical detection for microchip capillary electrophoresis

Significant progress in the development of miniaturized microfluidic systems has occurred since their inception over a decade ago. This is primarily due to the numerous advantages of microchip analysis, including the ability to analyze minute samples, speed of analysis, reduced cost and waste, and portability. This review focuses on recent developments in integrating electrochemical (EC) detection with microchip capillary electrophoresis (CE). These detection modes include amperometry, conductimetry, and potentiometry. EC detection is ideal for use with microchip CE systems because it can be easily miniaturized with no diminution in analytical performance. Advances in microchip format, electrode material and design, decoupling of the detector from the separation field, and integration of sample preparation, separation, and detection on-chip are discussed. Microchip CEEC applications for enzyme/immunoassays, clinical and environmental assays, as well as the detection of neurotransmitters are also described.