Analysis of microdialysate monoamines, including noradrenaline, dopamine and serotonin, using capillary ultra-high performance liquid chromatography and electrochemical detection

Advancements in Brain Research: The In Vivo/In Vitro Electrochemical Detection of Neurochemicals

Neurochemicals, crucial for nervous system function, influence vital bodily processes and their fluctuations are linked to neurodegenerative diseases and mental health conditions. Monitoring these compounds is pivotal, yet the intricate nature of the central nervous system poses challenges. Researchers have devised methods, notably electrochemical sensing with micro-nanoscale electrodes, offering high-resolution monitoring despite low concentrations and rapid changes. Implantable sensors enable precise detection in brain tissues with minimal damage, while microdialysis-coupled platforms allow in vivo sampling and subsequent in vitro analysis, addressing the selectivity issues seen in other methods. While lacking temporal resolution, techniques like HPLC and CE complement electrochemical sensing's selectivity, particularly for structurally similar neurochemicals. This review covers essential neurochemicals and explores miniaturized electrochemical sensors for brain analysis, emphasizing microdialysis integration. It discusses the pros and cons of these techniques, forecasting electrochemical sensing's future in neuroscience research. Overall, this comprehensive review outlines the evolution, strengths, and potential applications of electrochemical sensing in the study of neurochemicals, offering insights into future advancements in the field.

The High-Precision Liquid Chromatography with Electrochemical Detection (HPLC-ECD) for Monoamines Neurotransmitters and Their Metabolites: A Review

This review highlights the advantages of high-precision liquid chromatography with an electrochemical detector (HPLC-ECD) in detecting and quantifying biological samples obtained through intracerebral microdialysis, specifically the serotonergic and dopaminergic systems: Serotonin (5-HT), 5-hydroxyindolacetic acid (5-HIAA), 3,4-dihydroxyphenylacetic acid (DOPAC), dopamine (DA), 3-metoxytryptamin (3-MT) and homovanillic acid (HVA). Recognized for its speed and selectivity, HPLC enables direct analysis of intracerebral microdialysis samples without complex derivatization. Various chromatographic methods, including reverse phase (RP), are explored for neurotransmitters (NTs) and metabolites separation. Electrochemical detector (ECD), particularly with glassy carbon (GC) electrodes, is emphasized for its simplicity and sensitivity, aimed at enhancing reproducibility through optimization strategies such as modified electrode materials. This paper underscores the determination of limits of detection (LOD) and quantification (LOQ) and the linear range (L.R.) showcasing the potential for real-time monitoring of compounds concentrations. A non-exhaustive compilation of literature values for LOD, LOQ, and L.R. from recent publications is included.

The Development and Full Validation of a Novel Liquid Chromatography Electrochemical Detection Method for Simultaneous Determination of Nine Catecholamines in Rat Brain

Objectives

Chemical neurotransmission, managed by neurotransmitters, has a crucial role in brain processes such as fear, memory, learning, and pain, or neuropathology such as schizophrenia, epilepsy, anxiety/depression, and Parkinson's disease. The measurement of these compounds is used to elucidate the disease mechanisms and evaluate the outcomes of therapeutic interventions. However, this can be quite difficult because of various matrix effects and the problems of chromatographic separation of analysts. In the current study; for the first time, an optimized and fully validated high-performance liquid chromatography-electrochemical detection (HPLC-EC) method according to Food and Drug Administration and European Medicines Agency Bioanalytical Validation Guidance was developed for the simultaneous analysis of nine neurotransmitter compounds, namely dopamine, homovanilic acid, vanilmandelic acid, serotonin (SER), 5-hydroxyindole-3-acetic acid, 4-hydroxy-3-methoxyphenylglycol, norepinephrine, 3,4 dihydroxyphenylacetic acid, and 3-methoxytyramine and simultaneously determined in rat brain samples.

Materials and Methods

Separation was achieved with 150 mm x 4.6 mm, 2.6 μm Kinetex F5 (Phenomenex, USA) column isocratically, and analysis was carried out by HPLC equipped with a DECADE II EC detector.

Results

The method exhibited good selectivity, and the correlation coefficient values for each analyte's calibration curves were > 0.99. The detection and quantification limits ranged from 0.01 to 0.03 ng/mL and 3.04 to 9.13 ng/mL, respectively. The stability of the analyses and method robustness were also examined in detail in the study, and the obtained results are presented statistically.

Conclusion

The developed and fully validated method has been successfully applied to actual rat brain samples, and important results have been obtained. In the rat brain sample analysis, the lowest number of SER and the highest amount of noradrenaline were found.

Quantitative detection of dopamine in human serum with surface-enhanced Raman scattering (SERS) of constrained vibrational mode

Dopamine (DA) is a crucial neurotransmitter involved in the hormonal, nervous, and vascular systems being considered as an index to diagnose neurodegenerative diseases, including Parkinson's and Alzheimer's disease. Herein, we demonstrate the quantitative sensing of DA using the peak shift in surface-enhanced Raman scattering (SERS) of 4-mercaptophenylboronic acid (4-MPBA), resulting from the concentration of DA. To enable the signal enhancement of Raman scattering, Ag nanostructure was built with one-step gas-flow sputtering. 4-MPBA was then introduced using vapor-based deposition, acting as a reporter molecule for bonding with DA. The gradual peak-shift from 1075.6 cm^-1 to 1084.7 cm^-1 was observed with the increasing concentration of DA from 1 pM to 100nM. The numerical simulation revealed that DA bonding induced a constrained vibrational mode corresponding to 1084.7 cm^-1 instead of a C-S-coupled C-ring in-plane bending mode of 4-MPBA corresponding to 1075.6 cm^-1. Proposed SERS sensors depicted reliable DA detection in human serum and good selectivity against other analytes, including glucose, creatinine, and uric acid.

Insight into TEMPO-oxidized cellulose-based composites as electrochemical sensors for dopamine assessment

The diagnosis and treatment of many neurological and psychiatric problems depend on establishing simple, inexpensive, and comfortable electrochemical sensors for dopamine (DA) detection. Herein, 2,2,6,6 tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibers (TOC) were successfully loaded with silver nanoparticles (AgNPs) and/or graphite (Gr) and crosslinked by tannic acid, producing composites. This study describes a suitable casting procedure for the composite synthesis of TOC/AgNPs and/or Gr for the electrochemical detection of dopamine. Electrochemical impedance spectra (EIS), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM) were employed to characterize the TOC/AgNPs/Gr composites. In addition, the direct electrochemistry of electrodes treated with the prepared composites was examined using cyclic voltammetry. The TOC/AgNPs/Gr composite-modified electrode improved electrochemical performance towards detecting dopamine compared to TOC/Gr-modified electrodes. Upon employing amperometric measurement, our electrochemical instrument has a wide linear range (0.005-250 μM), a low limit of detection (0.0005 μM) at S/N = 3, and a high sensitivity (0.963 μA μM^-1 cm^-2). Additionally, it was demonstrated that DA detection seemed to have outstanding anti-interference characteristics. The proposed electrochemical sensors meet the clinical criteria regarding reproducibility, selectivity, stability, and recovery. The straightforward electrochemical method utilized in this paper may provide a potential framework for creating dopamine quantification biosensors.

An LC-MS/MS method for simultaneous analysis of up to six monoamines from brain tissues

Monoamines are a class of neuromodulators that are crucial for a variety of brain functions, including control of mood, movement, sleep and cognition. From mammals to insects, the nervous system is enriched in monoamines such as dopamine, serotonin and melatonin, analytes which range from being highly polar to non-polar. Here we developed a method using liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) to quantify in a single run the amounts of six distinct monoamines in extracts from dissected Drosophila and mouse brain tissues. The measured monoamines were dopamine (DA), serotonin (also known as 5-hydroxytryptamine (5-HT)), octopamine (OA, an insect equivalent of norepinephrine), tyramine (TA), melatonin (MT) and N-acetylserotonin (NAS). The analytical range of these monoamines was between 0.25 and 5.0 ng/mL. This quantitative LC-MS/MS methodology has important use for simultaneous measurement of distinct neuroactive monoamines from precious biological specimens.

Effects of Genetic Mutation Sites in ADR Genes on Modern Chickens Produced and Domesticated by Artificial Selection

Associations between neurotransmitters, adrenergic receptor (ADR) mutations, and behaviors in chickens produced and domesticated by artificial selection remain unclear. This study investigates the association of neurotransmitters and ADR mutations with egg laying and cockfighting-behaviors associated with significantly different breeding backgrounds-in Shaver Brown and Shamo chickens. Accordingly, the whole sequences of nine ADR genes were determined, and nine amino acid-specific mutation sites from five genes (ADRα1A: S365G, ADRα1D: T440N, ADRα2A: D273E, ADRβ1: N443S, S445N, ADRβ3: R342C, Q404L, and P406S) were extracted. Evolutionary analysis showed that these mutations were not ancestrally derived. These results confirm that the mutations at these sites were artificially selected for domestication and are breed specific. N_ST population analysis confirmed a difference in the degree of genetic differentiation between the two populations in seven genes. The results further confirm differences in the degree of genetic differentiation between the two populations in Shaver Brown (ADRA1B and ADRA1D) and Shamo (ADRA1A and ADRA2B) chickens, indicating that the ADR gene differs between the two breeds. The effects of artificial selection, guided by the human-driven selection of desirable traits, are reflected in adrenaline gene mutations. Furthermore, certain gene mutations may affect domestication, while others may affect other traits in populations or individuals.

Current Sample Preparation Methodologies for Determination of Catecholamines and Their Metabolites

Catecholamines (CAs) and their metabolites play significant roles in many physiological processes. Changes in CAs concentration in vivo can serve as potential indicators for the diagnosis of several diseases such as pheochromocytoma and paraganglioma. Thus, the accurate quantification of CAs and their metabolites in biological samples is quite important and has attracted great research interest. However, due to their extremely low concentrations and numerous co-existing biological interferences, direct analysis of these endogenous compounds often suffers from severe difficulties. Employing suitable sample preparation techniques before instrument detection to enrich the target analytes and remove the interferences is a practicable and straightforward approach. To date, many sample preparation techniques such as solid-phase extraction (SPE), and liquid-liquid extraction (LLE) have been utilized to extract CAs and their metabolites from various biological samples. More recently, several modern techniques such as solid-phase microextraction (SPME), liquid-liquid microextraction (LLME), dispersive solid-phase extraction (DSPE), and chemical derivatizations have also been used with certain advanced features of automation and miniaturization. There are no review articles with the emphasis on sample preparations for the determination of catecholamine neurotransmitters in biological samples. Thus, this review aims to summarize recent progress and advances from 2015 to 2021, with emphasis on the sample preparation techniques combined with separation-based detection methods such capillary electrophoresis (CE) or liquid chromatography (LC) with various detectors. The current review manuscript would be helpful for the researchers with their research interests in diagnostic analysis and biological systems to choose suitable sample pretreatment and detection methods.

Carbon dots-decorated hydroxyapatite nanowires–lanthanide metal–organic framework composites as fluorescent sensors for the detection of dopamine

A ratiometric composite fluorescent probe (HAPNWs-CDs-Tb/MOF) with hydroxyapatite carrier and the fluorescence ratio of carbon dots and lanthanide metal organic framework as the response signal was prepared for the detection of dopamine.

A novel electrochemical sensing platform for detection of dopamine based on gold nanobipyramid/multi-walled carbon nanotube hybrids

Dopamine homeostasis is an important clinical diagnostic index, because an abnormal level in the human body is closely related to certain serious diseases. Herein, a novel electrochemical sensing platform based on gold nanobipyramid/multi-walled carbon nanotube hybrids (AuNBP/MWCNTs) is developed to detect dopamine in human fluids. Using field emission scanning electron microscopy, it is observed that AuNBPs of about 60 nm with two pyramids are well dispersed on the surface of MWCNTs. Energy-dispersive X-ray spectrometry, X-ray diffraction and X-ray photoelectron spectroscopy confirm that AuNBPs are self-assembled onto the surface of MWCNTs to form the hybrids. Cyclic voltammetry reveals that the AuNBP/MWCNTs exhibit good electrocatalytic activity toward dopamine oxidation owing to the synergistic effects of AuNBPs and MWCNTs. In addition, both cyclic voltammetry and differential pulse voltammetry display three well-resolved and distinct oxidation peaks on the AuNBP/MWCNT-modified glassy carbon electrode. Based on AuNBP/MWCNTs, the newly developed electrochemical sensor is used to detect dopamine in the presence of ascorbic acid and uric acid over a wide linear range from 50 nM to 2.7 mM and a low detection limit of 15 nM (at S/N = 3). The electrochemical sensor can also be applied for the quantitative analysis of dopamine in real samples. Graphical abstract A novel electrochemical sensing platform based on gold nanobipyramid/multi-walled carbon nanotube hybrids (AuNBP/MWCNTs) was proposed to detect dopamine in the presence of ascorbic acid and uric acid.

Polymer dots as a novel probe for fluorescence sensing of dopamine and imaging in single living cell using droplet microfluidic platform

We report here simple synthetic method for preparing polymer dots (Pdots) via hydrothermal treatment of organic dye (neutral red), urea and trisodium citrate. The prepared Pdots with enhanced quantum yield (quantum yield: 30.2%) was used as a selective and sensitive probe for fluorescent sensing of dopamine (DA) with high selectivity and sensitivity. The as-synthesized Pdots exhibited strong fluorescence intensity at 435 nm, which DA can trigger remarkable fluorescence quenching of such luminescent Pdots on the basis of inner filter effect (IFE) and static quenching effect (SQE). A wide linearity range (0.001 μM-900 μM) for DA detection was obtained with lower DL (3 S/N) of 0.28 nM, and no interference from other molecules such as ascorbic acid, urine acid, glutathione, glucose, epinephrine, arginine, cysteine, proline, creatinine, serine; alanine, L-therionine, Hg²⁺, Mg²⁺, K⁺, Ca²⁺ and Na⁺. The designed sensor was successfully applied in the imaging of DA in single living PC12 cells using droplet microfluidic approach, indicating its acceptable practicability of the proposed assay for DA detection with ultrahigh sensitivity in biological samples.

Organic Electrochemical Transistors/SERS-Active Hybrid Biosensors Featuring Gold Nanoparticles Immobilized on Thiol-Functionalized PEDOT Films

In this study we immobilized gold nanoparticles (AuNPs) onto thiol-functionalized poly(3,4-ethylenedioxythiophene) (PEDOT) films as bioelectronic interfaces (BEIs) to be integrated into organic electrochemical transistors (OECTs) for effective detection of dopamine (DA) and also as surface-enhanced Raman scattering (SERS)-active substrates for the selective detection of p-cresol (PC) in the presence of multiple interferers. This novel PEDOT-based BEI device platform combined (i) an underlying layer of polystyrenesulfonate-doped PEDOT (PEDOT:PSS), which greatly enhanced the transconductance and sensitivity of OECTs for electrochemical sensing of DA in the presence of other ascorbic acid and uric acid metabolites, as well as amperometric response toward DA with a detection limit (S/N = 3) of 37 nM in the linear range from 50 nM to 100 μM; with (ii) a top interfacial layer of AuNP-immobilized three-dimensional (3D) thiol-functionalized PEDOT, which not only improved the performance of OECTs for detecting DA, due to the signal amplification effect of the AuNPs with high catalytic activity, but also enabled downstream analysis (SERS detection) of PC on the same chip. We demonstrate that PEDOT-based 3D OECT devices decorated with a high-density of AuNPs can display new versatility for the design of next-generation biosensors for point-of-care diagnostics.

Hierarchical Au nanoclusters electrodeposited on amine-terminated ITO glass as a SERS-active substrate for the reliable and sensitive detection of serotonin in a Tris-HCl buffer solution

In this study, a SERS-active substrate was fabricated by electrodepositing hierarchical Au nanostructures on amine-terminated ITO (indium tin oxide) glass to achieve an enhanced Raman signal of 5-HT.

Poly(glycine)/graphene oxide modified glassy carbon electrode: Preparation, characterization and simultaneous electrochemical determination of dopamine, uric acid, guanine and adenine

A novel poly(glycine) (p-GLY)/graphene oxide (GO) composite based sensor (p-GLY/GO) was successfully prepared on glassy carbon electrode by simple electropolymerization. Electrochemical responses of analytes on p-GLY/GO modified electrode were studied by cyclic voltammetry and differential pulse voltammetry. The results demonstrated that p-GLY/GO modified electrode showed a favorable application for the simultaneous determination of dopamine (DA), uric acid (UA), guanine (GU) and adenine (AD). Owing to the synergistic effect of p-GLY and GO, the oxidation peaks of four analytes separated well from each other, and the potential separations of oxidation peaks of DA-UA, UA-GU and GU-AD were large up to 170, 350 and 300 mV, respectively. As-prepared p-GLY/GO modified electrode offered wide linear responses for DA, UA, GU and AD over the ranges of 0.20-62, 0.10-105, 0.15-48 and 0.090-103 μM with detection limits of 0.011, 0.061, 0.026 and 0.030 μM (S/N = 3), respectively. Moreover, p-GLY/GO modified electrode presented favorable selectivity, stability and reproducibility, which was a promising candidate as an electrochemical sensor for the simultaneous determination of DA, UA, GU and AD.

Quantitative SERS Detection of Dopamine in Cerebrospinal Fluid by Dual-Recognition-Induced Hot Spot Generation

Reliable profiling of the extracellular dopamine (DA) concentration in the central nervous system is essential for a deep understanding of its biological and pathological functions. However, quantitative determination of this neurotransmitter remains a challenge because of the extremely low concentration of DA in the cerebrospinal fluid (CSF) of patients. Herein, on the basis of the specific recognition of boronate toward diol and N-hydroxysuccinimide ester toward the amine group, a simple and highly sensitive strategy was presented for DA detection by using surface-enhanced Raman scattering (SERS) spectroscopy as a signal readout. This was realized by first immobilizing 3,3'-dithiodipropionic acid di( N-hydroxysuccinimide ester) on gold thin film surfaces to capture DA, followed by introducing 3-mercaptophenylboronic acid (3-MPBA)-functionalized silver nanoparticles to generate numerous plasmonic "hot spots" with the nanoparticle-on-mirror geometry. Such a dual-recognition mechanism not only avoids complicated bioelement-based manipulations but also efficiently decreases the background signal. With the direct use of the recognition probe 3-MPBA as a Raman reporter, the "signal-on" SERS method was employed to quantify the concentration of DA from 1 pM to 1 μM with a detection limit of 0.3 pM. Moreover, our dual-recognition-directed SERS assay exhibited a high resistance to cerebral interference and was successfully applied to monitoring of DA in CSF samples of patients.

Synergistic electron transfer effect-based signal amplification strategy for the ultrasensitive detection of dopamine

The selective and sensitive detection of dopamine (DA) is of great significance for the identification of schizophrenia, Huntington's disease, and Parkinson's disease from the perspective of molecular diagnostics. So far, most of DA fluorescence sensors are based on the electron transfer from the fluorescence nanomaterials to DA-quinone. However, the limited electron transfer ability of the DA-quinone affects the level of detection sensitivity of these sensors. In this work, based on the DA can reduce Ag⁺ into AgNPs followed by oxidized to DA-quinone, we developed a novel silicon nanoparticles-based electron transfer fluorescent sensor for the detection of DA. As electron transfer acceptor, the AgNPs and DA-quinone can quench the fluorescence of silicon nanoparticles effectively through the synergistic electron transfer effect. Compared with traditional fluorescence DA sensors, the proposed synergistic electron transfer-based sensor improves the detection sensitivity to a great extent (at least 10-fold improvement). The proposed sensor shows a low detection limit of DA, which is as low as 0.1 nM under the optimal conditions. This sensor has potential applicability for the detection of DA in practical sample. This work has been demonstrated to contribute to a substantial improvement in the sensitivity of the sensors. It also gives new insight into design electron transfer-based sensors.

A simple and sensitive high-performance liquid chromatography-electrochemical detection assay for the quantitative determination of monoamines and respective metabolites in six discrete brain regions of mice

A rapid, sensitive, and reproducible assay is described for the quantitative determination of the monoamine neurotransmitters dopamine, norepinephrine and serotonin, their metabolites, and the internal standard 3,4-dihydroxybenzlyamine hydro-bromide in mouse brain homogenate using high-performance liquid chromatography with electrochemical detection. The method was validated in the following brain areas: frontal cortex, striatum, nucleus accumbens, hippocampus, substantia nigra pars compacta and ventral tegmental area. Biogenic amines and relevant metabolites were extracted from discrete brain regions using a simple protein precipitation procedure, and the chromatography was achieved using a C₁₈ column. The method was accurate over the linear range of 0.300-30 ng/mL (r = 0.999) for dopamine and 0.300-15 ng/mL (r = 0.999) for norepinephrine, 3,4-dihydroxybenzlyamine hydro-bromide, homovanillic acid and 5-hydroxyindolacetic acid, with detection limits of ~0.125 ng/mL (5 pg on column) for each of these analytes. Accuracy and linearity for serotonin were observed throughout the concentration range of 0.625-30 ng/mL (r = 0.998) with an analytical detection limit of ~0.300 ng/mL (12 pg on column). Relative recoveries for all analytes were approximately ≥90% and the analytical run time was <10 min. The described method utilized minimal sample preparation procedures and was optimized to provide the sensitivity limits required for simultaneous monoamine and metabolite analysis in small, discrete brain tissue samples.

Preparation of Pd/Bacterial Cellulose Hybrid Nanofibers for Dopamine Detection

Palladium nanoparticle-bacterial cellulose (PdBC) hybrid nanofibers were synthesized by in-situ chemical reduction method. The obtained PdBC nanofibers were characterized by a series of analytical techniques. The results revealed that Pd nanoparticles were evenly dispersed on the surfaces of BC nanofibers. Then, the as-prepared PdBC nanofibers were mixed with laccase (Lac) and Nafion to obtain mixture suspension, which was further modified on electrode surface to construct novel biosensing platform. Finally, the prepared electrochemical biosensor was employed to detect dopamine. The analysis result was satisfactory, the sensor showed excellent electrocatalysis towards dopamine with high sensitivity (38.4 µA·mM⁻¹), low detection limit (1.26 µM), and wide linear range (5–167 µM). Moreover, the biosensor also showed good repeatability, reproducibility, selectivity and stability and was successfully used in the detection of dopamine in human urine, thus providing a promising method for dopamine analysis in clinical application.

Sensitive determination of norepinephrine, epinephrine, dopamine and 5-hydroxytryptamine by coupling HPLC with [Ag(HIO6 )2 ](5-) -luminol chemiluminescence detection

Based on the enhancing effects of norepinephrine (NE), epinephrine (EP), dopamine (DA) and 5-hydroxytryptamine (5-HT) on the chemiluminescence (CL) reaction between [Ag(HIO6 )2 ](5-) and luminol in alkaline solution, a high-performance liquid chromatography (HPLC) method with CL detection was explored for the sensitive determination of monoamine neurotransmitters for the first time. The UV-visible absorption spectra were recorded to study the enhancement mechanism of monoamine neurotransmitters on the CL of [Ag(HIO6 )2 ](5-) and luminol reaction. The HPLC separation of NE, EP, DA and 5-HT was achieved with isocratic elution using a mixture of aqueous 0.2% phosphoric acid and methanol (5:95, v/v) within 11.0 min. Under the optimized conditions, the detection limits of NE, EP, DA, and 5-HT were 4.8, 0.9, 1.9 and 2.3 ng/mL, respectively, corresponding to 17.6-96.0 pg for 20 μL sample injection. The recoveries of monoamine neurotransmitters in rat brain were >95.6% with the precisions expressed by RSD <5.0%. The validated HPLC-CL method was successfully applied for the quantification of NE, EP, DA and 5-HT in rat brain. This method has promising potential for some biological and clinical investigations focusing on the levels of monoamine neurotransmitters. Copyright © 2016 John Wiley & Sons, Ltd.

Sensitive and accurate determination of neurotransmitters from in vivo rat brain microdialysate of Parkinson's disease using in situ ultrasound-assisted derivatization dispersive liquid–liquid microextraction by UHPLC-MS/MS

In situ UA-DDLLME coupled with UHPLC-MS/MS has been developed for simultaneous determination of neurotransmitters and baicalein from Parkinson's disease rats.

Analytical approach to determining human biogenic amines and their metabolites using eVol microextraction in packed syringe coupled to liquid chromatography mass spectrometry method with hydrophilic interaction chromatography column

Analysis of biogenic amines (BAs) in different human samples provides insight into the mechanisms of various biological processes, including pathological conditions, and thus may be very important in diagnosing and monitoring several neurological disorders and cancerous tumors. In this work, we developed a simple and fast procedure using a digitally controlled microextraction in packed syringe (MEPS) coupled to liquid chromatography mass spectrometry (LC-MS) method for simultaneous determination of biogenic amines, their precursors and metabolites in human plasma and urine samples. The separation of 12 low molecular weight and hydrophilic molecules with a wide range of polarities was achieved with hydrophilic interaction chromatography (HILIC) column without derivatization step in 12 min. MEPS was implemented using the APS sorbent in semi-automated analytical syringe (eVol(®)) and small volume of urine and plasma samples, 5 0µL and 100 μL, respectively. We evaluated important parameters influencing MEPS efficiency, including stationary phase selection, sample pH and volume, number of extraction cycles, and washing and elution volumes. In optimized MEPS conditions, the analytes were eluted by 3 × 50 μL of methanol with 0.1% formic acid. The chromatographic separation of analytes was performed on XBridge Amide™ BEH analytical column (3.0mm × 100 mm, 3.5 µm) using gradient elution with mobile phase consisting of phase A: 10mM ammonium formate buffer in water pH 3.0 and phase B: 10mM ammonium formate buffer in acetonitrile pH 3.0. The LC-HILIC-MS method was validated and, in optimum conditions, presented good linearity in concentration range within 10-2000 ng/mL for all the analytes with a determination coefficient (r(2)) higher than 0.999 for plasma and urine samples. Method recovery ranged within 87.6-104.3% for plasma samples and 84.2-98.6% for urine samples. The developed method utilizing polar APS sorbent along with polar HILIC column was applied for simultaneous bioanalysis of trace amounts of polar endogenous biogenic amines in real human urine and plasma samples.

Determination of Histamine by High-Performance Liquid Chromatography After Precolumn Derivatization with o-Phthalaldehyde-Sulfite

A fast and sensitive method was developed for in vivo determination of histamine in the brain microdialysate by reverse ion pair chromatography with electrochemical detection. The microdialysates were derivatized with o-phthalaldehyde and sodium sulfite, and separation was achieved using isocratic elution within 10 min. The separation was performed in an Agilent Eclipse Plus C18 column (3.0 × 150 mm, particle size 3.5 μm), and the mobile phase consisted of 100 mM monosodium phosphate (pH 6.0), 500 mg L(-1) OSA and 20% methanol (v/v). The linearity (R(2)) was found to be >0.999, with a range from 2 to 50 nM and excellent repeatability (relative standard deviation, 2.29-6.04%), and the limit of detection was 0.4 nM. This method was successfully applied to analyze the extracellular concentration of histamine in the hypothalamus of rats, with probe recovery calculated in vivo.

SERS Detection of Dopamine Using Label-Free Acridine Red as Molecular Probe in Reduced Graphene Oxide/Silver Nanotriangle Sol Substrate

The reduced graphene oxide/silver nanotriangle (rGO/AgNT) composite sol was prepared by the reduction of silver ions with sodium borohydride in the presence of H2O2 and sodium citrate. In the nanosol substrate, the molecular probe of acridine red (AR) exhibited a weak surface-enhanced Raman scattering (SERS) peak at 1506 cm(-1) due to its interaction with the rGO of rGO/AgNT. Upon addition of dopamine (DA), the competitive adsorption between DA and AR with the rGO took place, and the AR molecules were adsorbed on the AgNT aggregates with a strong SERS peak at 1506 cm(-1) that caused the SERS peak increase. The increased SERS intensity is linear to the DA concentration in the range of 2.5-500 μmol/L. This new analytical system was investigated by SERS, fluorescence, absorption, transmission electron microscope (TEM), and scanning electron microscope (SEM) techniques, and a SERS quantitative analysis method for DA was established, using AR as a label-free molecular probe.

Monoamine oxidase B layer-by-layer film fabrication and characterization toward dopamine detection

In this work nanostructured film composites of the monoamine oxidase B (MAO-B) enzyme, free or encapsulated in liposomes, were fabricated by the layer-by-layer (LbL) self-assembly technique, employing polyethylene imine (PEI) as polycation. Initially, the MAO-B enzyme was incorporated into liposomes in order to preserve its enzymatic structure ensuring their activity and catalytic stability. The LbL film growth was monitored by surface plasmon resonance (SPR) by gold resonance angle shift analysis after each bilayer deposition. Subsequently, the films were applied as amperometric biosensors for dopamine detection using Prussian Blue (PB) as the electron mediator. The biosensor fabricated by MAO-B incorporated into liposomes composed of DPPG:POPG in the ratio (1:4) (w/w) showed the best performance with a sensitivity of 0.86 (μA cm(-2))/(mmol L(-1)) and a detection limit of 0.33 mmol L(-1).