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Microchip electrophoresis-single wall carbon nanotube press-transferred electrodes for fast and reliable electrochemical sensing of melatonin and its precursors: Nanoanalysis
Abstract
In the current work, single wall carbon nanotube press-transferred electrodes (SW-PTEs) were used for detection of melatonin (MT) and its precursors tryptophan (Trp) and serotonin (5-HT) on microchip electrophoresis (ME). SW-PTEs were simply fabricated by press transferring a filtered dispersion of single wall carbon nanotubes (SWCNTs) on a non-conductive PMMA substrate, where SWCNTs act as exclusive transducers. The coupling of ME-SW-PTEs allowed the fast detection of MT, Trp and 5-HT in less than 150 s with excellent analytical features. It exhibited an impressive anti fouling performance with RSD values of ≤2% and ≤4% for migration times and peak heights, respectively (n=12). In addition, sample analysis was also investigated by analysis of 5-HT, MT and Trp in commercial samples obtaining an excellent quantitative and reproducible recoveries with values of 96.2 ± 1.8 %, 101.3 ± 0.2 % and 95.6 ± 1.2 % for 5-HT, MT and Trp, respectively. The current novel application reveals the analytical power of the press-transfer technology where the fast and reliable determination of MT and its precursors were performed directly on the nanoscale carbon nanotube detectors without the help of any other electrochemical transducer. This article is protected by copyright. All rights reserved.
This article is protected by copyright. All rights reserved.
... Various analytical methods like spectrophometry [6,7], high performance liquid chromatography [8,9], fluorescence [10], chemiluminescence [11], and capillary electrophoresis [12], have been employed for the detection of L-tryp in food, plasma, urine, and other biological media. Some drawbacks have been formulated in mentioned methods such as time-consuming, instrument handling, and high cost of reagents. ...
The main objective of this research work is to develop a low-cost sensor to detect l-tryptophan (L-tryp) in real sample medium based on a modified glassy carbon electrode. For this, copper oxide nanoflowers (CuONFs) and poly-l-glutamic acid (PGA) were used to modify GCE. The prepared NFs and PGA coated electrode was characterized using field emission scanning electron microscope (FE-SEM) with energy dispersive X-ray (EDX) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. Furthermore, the electrochemical activity was performed by cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS). The modified electrode showed excellent electro-catalytic activity towards L-tryp detection in PBS solution at neutral pH 7.0. Based on the physiological pH condition, the proposed electrochemical sensor can detect L-tryp concentration with a linear range of 1.0 × 10−4-8.0 × 10−8 molL−1 with having a detection limit of 5.0 × 10−8 molL−1 and sensitivity of 0.6μA/μMcm2. The selectivity of L-tryp was tested with a mixture of salt and uric acid solution at the above conditions. Finally, this strategy demonstrated excellent recovery value in real sample analysis like milk and urine.
... Sensor methods based on CNT electrodes have many virtues including great sensitivity, selectivity and rapidness. Gomez, Martin, Silva, and Escarpa (2015) built a CNT-based electrode for microfluidic sensing to detect MLT form its herb extract capsules within 150 s using 20 mM di-sodium tetraborate buffer (pH = 9.2) with 15 % (v/v) of methanol, while the separation and injection voltage were both +1500 V, detection potential was +0.70 V. After optimization, great resolutions R 5-HT-MLT = 2.0, R MLT-Trp = 1.5 were obtained corresponding to MT and their two precursors, and the LOD of MLT was 4 µM. ...
Melatonin (MLT) plays a significant role on maintaining the basic physiological functions and regulating various metabolic processes in plentiful organisms. Recent years have witnessed an increase in MLT's share in global market with its affluent functions. However, the worrisome quality issues and inappropriate or excessive application of MLT take place inevitably. In addition, its photosensitive properties, oxidation, complex substrate concentration and trace levels leave exact detection of MLT doubly difficult. Therefore, it is essential to exploit precise, sensitive and stable extraction and detection methods to resolve above questions. In this study, we reviewed the distribution and bioactivities of MLT and conducted a comprehensive overview of the developments of pretreatment and analysis methods for MLT in food samples since 2010. Commonly used pretreatment methods for MLT include not only traditional techniques, but also novel ones, such as solid-phase extraction, QuEChERS, microextraction by packed sorbent, solid phase microextraction, liquid phase microextraction, and so on. Analysis methods include liquid chromatography coupled with different detectors, GC methods, capillary electrophoresis, sensors, and so on. The advantages and disadvantages of different techniques have been compared and the development tendency was prospected.
... Hence, the presence of the organic modifiers in bulk helps solubilisation of hydrophobic analytes in water, and thus can enhance their interactions with micellar phase in the case of micellar electrokinetic chromatography (MEKC; Ghosh & Dey, 2008). The increase of separation efficiency of the biogenic amines obtained with the approach based on addition of alcohol into BGE of capillary zone electrophoresis (CZE) was presented by Male and Loung (2001), Bacaloni et al. (2013), and Gomez et al. (2015). The presence of such organic solvent effects decreased conductivity of the BGE, decreased thermal diffusion, and improved analyte solubility (Landers, 2007). ...
The effects of organic modifiers—alcohols (methanol, isopropanol), acetonitrile, and tetrahydrofuran—and β-cyclodextrins in capillary zone electrophoresis were investigated using a test mixture containing five biogenic amines important from the human health point of view—serotonin, dopamine, adrenaline, noradrenaline, and tyramine—and two B vitamins—thiamine and pyridoxine. The simultaneous addition of tetrahydrofuran and isopropanol was found to improve the resolution of determined analytes and enable effective separation of analytes with very similar electromigration characteristics migrating as one peak (dopamine and serotonin). The developed and optimised separation method based on capillary zone electrophoresis and ultraviolet detection was capable to achieve detection limits at the concentration level in the range of 0.15 to 1.25 μg/mL ⁻¹ . The developed method was also characterised by other favourable validation parameters, such as linearity ( r ² > 0.99), accuracy (82.9–117.8% for the intraday measurements, and 87.6–119.2% for the interday measurements), and precision (intraday relative standard deviation in the range of 0.4–15.5%, interday relative standard deviation in the range of 0.9–18.3%). The method was finally applied to investigate the stability of the analytes in model water matrix samples under various storage conditions.
... Different techniques have been employed for MEL quantification, such as liquid chromatography, [13][14][15][16] capillary electrophoresis, 17 ELISA 18 and RIA. 19 In a dynamic scenario of urgency of clinical reports, electrochemical methods are of great interest for hormone determination due to their portability, easy-to-handle devices, low cost, and fast analysis response. ...
This work reports the characterization and application of a new electrochemical platform modified with gold nanoparticles (AuNP) and functionalized multi-walled carbon nanotubes (f-MWCNT) film to determine melatonin (MEL), a critical sleep inductor neurohormone categorized by the U.S. Food and Drug Administration as a nutritional supplement. The AuNP were synthesized and stabilized by 3-n-propyl-(4-methylpyridinium) silsesquioxane chloride (Si4Pic+Cl−), presenting an electrostatic stabilization and a material with a positive surface charge. Thermogravimetric analysis, Fourier transform-infrared spectroscopy, Raman spectroscopy, and zeta (ζ) potential indicated the insertion of oxygenated functional groups and a superficial negative charge for f-MWCNT. The GCE/f-MWCNT-AuNP-Si4Pic+Cl− platform showed an electrocatalytic profile and a current increase of ca. 7.5 times. Voltammograms were obtained by square wave voltammetry; the current increased with successive additions of MEL in the linear range of 4.9 to 55.5 μmol L−1. The values of LOD and LOQ were 1.6 and 4.7 μmol L−1, respectively. The standard addition was used to quantify MEL in simulated saliva, blood serum, and urine samples. Statistical tests showed no significant differences between the data collected by voltammetry and the comparative technique, molecular absorption spectrometry. It can be concluded that the new method provides novel perspectives for MEL sensing
... Some new strategies have been reported such as the use of the carbon nanomaterial as exclusive material to improve the analytical response and allow reliable studies of the pure nanomaterial. For instance, Martin and Escarpa [88] developed SWCNT press-transferred, as a unique conductive material, for EC detection in MCE. The new carbon nanomaterial is produced by filtration of SWCNTs on a Teflon filter. ...
Microchip capillary electrophoresis (MCE) with integrated electrodes to perform electrochemical (EC) sensing is a powerful tool for fundamental research and practical applications that are increasingly in demand. In this critical review, we discuss the main strategies developed over the last twenty years to design complete MCE-EC devices. First, we outline all the questions raised by the pioneering research groups regarding the coupling of these two techniques in microfluidic devices. Then, we present all the key-solutions, in the form of strategies, that our colleagues have cleverly elaborated to make these devices evolve into elaborate and relevant tools. These evolutions have mainly focused on the nature of the electrodes used, their configuration, the process of their fabrication, their integration and positioning in the microfluidic systems and their performance within the integrated devices. Particular attention is paid to the adequacy of the materials of the microsystems and the electrodes and to the resolution of the electrical interference between the electric fields necessary for the electrophoretic separation and the electrochemical detection. Finally, we describe significant examples of methodologies for the analysis of complex biological, food and environmental samples, highlighting the analytical improvements brought by these MCE-EC systems.
... At present, many Trp determination methods have been developed, such as fluorescence spectrometry [19], chemiluminescence [20], capillary electrophoresis [21] and high performance liquid chromatography [22,23]. However, most of these technologies are labor-intensive, requiring expensive equipment and maintenance, and professional operators, while electrochemical method is rapid, simple, low-cost, highly sensitive, well selective, and miniature. ...
Here we report a facile and cost-effective preparation of nickel nanoparticle S/Nitrogen-carbon nanohybrid (Ni NPS/N-C) based on formamide condensation and carbonization, used for highly sensitive and selective electrochemical determination of tryptophan (Trp) in practical samples. The crystallographic phase, surface morphology, elemental distribution, and chemical state of the Ni NPS/N-C nanohybrid were analyzed by XRD, SEM, TEM, EDS, XPS, FTIR, TGA and Raman spectra. The results show that high content of Ni NPs (6.62 wt%) were distributed in the nanohybrid, and a large number of bamboo-like nanotubes were formed during pyrolysis, which dramatically increases the specific surface area, and remarkably promotes the electrical conductivity as well as the catalytic activity of the nanohybrid. The electrochemical behavior of Trp was investigated on the nanohybrid modified carbon paste electrode (Ni NPS/N-C/CPE) and the measurement parameters were optimized. As expected, the modified electrode can remarkably enhance the electrochemical oxidation signal of Trp. Under the optimal experimental conditions, a linear relationship in the range of 0.01–20 μM and 20–80 μM for Trp was established with detection limit of 5.7 nM (S/N = 3). Finally, the proposed sensor was practically applied by evaluation and determination of Trp from various sourcing samples including human serum and pharmaceutical samples.
... The determination of MT in vegetal matrices is a challenging task due to the very low concentrations. In this sense, several analytical approaches have been proposed for the determination of Melatonin, its isomers and compounds involved in their biosynthesis, including liquid chromatography [8,12], capillary electrophoresis [13,14] and electrochemistry [16]. ...
In the present work, a method based on the Ultrasound-dispersive liquid-liquid microextraction (US-DLLME) for the preconcentration of melatonin in Brazilian and Argentinian wine samples prior to the determination by reverse phase high performance liquid chromatography with diode array detector HPLC-DAD was developed. Sample prep optimization was carried out in two stages. First, a full factorial design was executed to evaluate the variables significance. The second step consisted of significant variables optimization (pH, extraction volume, sonication time, centrifugation time) using a Doehlert matrix to determine the critical values. Optimal US-DLLME extraction conditions were as follows: pH of 4.5, sonication time of 6.8 min, centrifugation time of 2.5 min, extractor volume of 250 µL and 5.0 mL of sample. The organic phase was retrieved with a microsyringe to a glass tube, dried and diluted with the mobile phase prior injection. The method presented a limit of detection (LOD) of 0.23 μg L⁻¹ and a limit of quantification (LOQ) of 0.70 μg L⁻¹. The precision expressed as relative standard deviation (R.S.D.) was 0.8 % and 1.9 % at 10.0 and 1.0 μg L⁻¹, respectively. Accuracy was confirmed by comparison of the results found by the proposed methods with an alternative method, and spike tests with recuperations ranging from 92 to 103%. the analyzed samples showed concentrations varying from 2.2 to 5.0 μg L⁻¹.
... It is fast, economic as consumes low sample and solvents volumes and eco-friendly as it generates low waste. [101] Moreover, portable MCE devices are the optimum choice in case of environmental analysis especially in case of direct sample analysis without tedious or long pre-analysis steps. [102] When applying MCE to environmental analysis, it is important that the detection method is sensitive, selective and reproducible with high stability against different substrates present in the environmental samples especially if these samples are meant to be analyzed without pre-cleaning steps. ...
Microfluidic capillary electrophoresis (MCE) is the novel technique resulted from the CE mininaturization as planar separation and analysis device. This review presents and discusses various application fields of this advanced technology published in the period 2017 till mid-2019 in eight different sections including clinical, biological, single cell analysis, environmental, pharmaceuticals, food analysis, forensic and ion analysis. The need for miniaturization of CE and the consequence advantages achieved are also discussed including high-throughput, miniaturized detection, effective separation, portability and the need for micro- or even nano-volume of samples. Comprehensive tables for the MCE applications in the different studied fields are provided. Also, figure comparing the number of the published papers applying MCE in the eight discussed fields within the studied period is included. The future investigation should put into consideration the possibility of replacing conventional CE with the MCE after proper validation. Suitable validation parameters with their suitable accepted ranges should be tailored for analysis methods utilizing such unique technique (MCE).
... Carbon ink was then deposited on the substrate, dried, and polished to generate a microband electrode [69]. Although many of the applications of ME-EC feature PDMS substrates, some groups have investigated other substrate materials, including PMMA [70] and paper [71]. Further details regarding novel electrode materials that have been used for ME-EC can be found in a review by Randviir and Banks [72]. ...
Microchip electrophoresis with amperometric detection (ME-EC) is a useful tool for the determination of redox active compounds in complex biological samples. In this review, a brief background on the principles of ME-EC is provided, including substrate types, electrode materials, and electrode configurations. Several different detection approaches are described, including dual-channel systems for dual-electrode detection and electrochemistry coupled with fluorescence and chemiluminescence. The application of ME-EC to the determination of catecholamines, adenosine and its metabolites, and reactive nitrogen and oxygen species in microdialysis samples and cell lysates is also detailed. Lastly, approaches for coupling of ME-EC with microdialysis sampling to create separation-based sensors that can be used for near real-time monitoring of drug metabolism and neurotransmitters in freely roaming animals are provided.
Graphical abstract
... So far, there have been many analytical methods for Trp determination, such as highperformance liquid chromatography (HPLC) [4,5], fluorescence [6], capillary electrophoresis [7], chemiluminescence [8], and spectrometry [9]. Although the above methods are useful for the quantitative analysis of Trp, most have the disadvantages of high cost, complicated sample pretreatment process, long response time, and the requirement for human-sized instruments. ...
In this paper, a nanocomposite of cuprous oxide and electrochemically reduced graphene oxide (Cu2O-ERGO) was prepared by a simple and low-cost method; hereby, a new method for the electrochemical determination of tryptophan (Trp) by this composite modified glassy carbon electrode (GCE) is proposed. The prepared materials and modified electrodes were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and cyclic voltammetry (CV). The results showed that Cu2O-ERGO/GCE had good electrocatalytic activity for Trp. The effects of supporting electrolyte, scanning rate, accumulation potential, and accumulation time on the determination of Trp were studied. Under the optimum experimental conditions, Trp was quantitatively analyzed by square-wave voltammetry (SWV). The oxidation peak current of Trp had a good linear relationship with its concentration in the range of 0.02-20 μM, and the detection limit was 0.01 μM (S/N = 3). In addition, the modified electrode has high sensitivity, good repeatability, and long-term stability. Finally, the proposed method has been successfully applied in the determination of Trp concentration in practical samples.
... Several papers deal with the CE analysis of 5-HT in different biological sample matrices, such as urine [19][20][21][22][23][24][25][26][27], plasma, serum [22,28], blood [22,29], brain tissue [30,31], bowel content [32], neurons [33,34], and commercially available food supplements [35,36]. Most of the cited CE methods were accompanied with either external (liquid-liquid extraction-LLE, solid phase extraction-SPE) or online (various stacking methods, e.g., field-amplified sample injection, dynamic pH junction, transient isotachophoresis) sample pretreatment in order to decrease limit of detection (LOD). ...
An advanced two-dimensional capillary electrophoresis method, based on on-line combination of capillary isotachophoresis and capillary zone electrophoresis with cyclodextrin additive in background electrolyte, was developed for effective determination of serotonin in human urine. Hydrodynamically closed separation system and large bore capillaries (300–800 µm) were chosen for the possibility to enhance the sample load capacity, and, by that, to decrease limit of detection. Isotachophoresis served for the sample preseparation, defined elimination of sample matrix constituents (sample clean up), and preconcentration of the analyte. Cyclodextrin separation environment enhanced separation selectivity of capillary zone electrophoresis. In this way, serotonin could be successfully separated from the rest of the sample matrix constituents migrating in capillary zone electrophoresis step so that human urine could be directly (i.e., without any external sample preparation) injected into the analyzer. The proposed method was successfully validated, showing favorable parameters of sensitivity (limit of detection for serotonin was 2.32 ng·mL−1), linearity (regression coefficient higher than 0.99), precision (repeatability of the migration time and peak area were in the range of 0.02–1.17% and 5.25–7.88%, respectively), and recovery (ranging in the interval of 90.0–93.6%). The developed method was applied for the assay of the human urine samples obtained from healthy volunteers. The determined concentrations of serotonin in such samples were in the range of 12.4–491.2 ng·mL−1 that was in good agreement with literature data. This advanced method represents a highly effective, reliable, and low-cost alternative for the routine determination of serotonin as a biomarker in human urine.
... Up to date, many analytical techniques have been employed for the determination of Trp, such as high-performance liquid chro- matography (HPLC) [6,7], fluorometric method [8], capillary elec- trophoresis [9], colorimetry [10] and electroanalysis [11,12]. Among them, electrochemical methods have gained great interest due to their favorable properties, such as accuracy, sensitivity, easy operation and low cost. ...
... Alternative strategies are based on applying high pressures to transfer SWCNTs on polymethylmetacrylate (PMMA) surfaces for on-chip applications [26][27][28] and for spectroelectrochemistry using different transfer strategies [29]. Free-standing electrodes fabricated by filtering CNTs (buckypapers) [30,31] and graphene papers [32][33][34] with high applicability as electrode materials have been highly explored for batteries and supercapacitors, however they have been less studied in electroanalytical applications stuck on solid electrodes [35] and as working electrode [36][37][38]. ...
The authors describe tailor-designed electrodes based on the use of films of carbonaceous nanomaterials with demostrated conductivitiy at the microscale. These represent a unique conductive material for deposition on adaptable nonconductive substrates. Single-walled and multi-walled carbon nanotubes as well as graphene scaffold film electrodes (SFEs) were designed, and their electroanalytical performance was evaluated by using two different designs. The first is a 3-electrode configuration for off-chip detection, and the second is one for on-chip detection of the biomarkers dopamine and catechol at +0.70 V detection potential (vs. Ag/AgCl). The SFEs were fabricated by filtration of conducting carbon nanomaterials using a tailored template. In our perception, they pave the way for a wide field of opportunities in off-chip and microfluidic electrochemistry because of their reliability, ease of fabrication and remarkable performance that is based on the exclusive use of the carbon nanomaterial transducers.
Graphical abstractSchematic of tailor-templated electrodes based on films of carbonaceous nanomaterials as unique conductive material on adaptable non-conductive substrates for off-chip and on-chip detection.
The inherent complexity of biological matrices and presence of several interfering substances in biological samples make them unsuitable for direct analysis. An effective sample preparation technique assists in analyte enrichment, improving selectivity and sensitivity of bioanalytical method. Because of several key benefits of employing 3D printed sorbent in sample extraction, it has recently gained popularity across a variety of industries. Applications for 3D printing in the field of bioanalytical research have grown recently, particularly in the areas of miniaturization, (bio)sensing, sample preparation, and separation sciences. Due to the high expense of the solid phase microextraction cartridge, researcher approaches in-lab production of sorbent material for the extraction of analyte from biological samples. Owing to its distinct advantages such as low costs, automation capabilities, capacity to produce products in a variety of shapes, and reduction of tedious steps of sample preparation, 3D printed sorbents are gaining increased attention in the field of bioanalysis. It is also reported to offer high selectivity and assist in achieving a much lower limit of detection. In this review, we have discussed current advancements in different types of 3D printed sorbents, production methods, and their applications in the field of bioanalytical sample preparation.
Hormones regulate several physiological processes in living organisms, and their detection requires accuracy and sensitivity. Recent advances in nanostructured electrodes for the electrochemical detection of hormones are described. Nanostructured electrodes’ high surface area, electrocatalytic activity, and sensitivity make them a strong hormone detection platform. This paper covers nanostructured electrode design and production using MOFs, zeolites, carbon nanotubes, metal nanoparticles, and 2D materials such as TMDs, Mxenes, graphene, and conducting polymers onto electrodes surfaces that have been used to confer distinct characteristics for the purpose of electrochemical hormone detection. The use of aptamers for hormone recognition is producing especially promising results, as is the use of carbon-based nanomaterials in composite electrodes. These materials are optimized for hormone detection, allowing trace-level quantification. Various electrochemical techniques such as SWV, CV, DPV, EIS, and amperometry are reviewed in depth for hormone detection, showing the ability for quick, selective, and quantitative evaluation. We also discuss hormone immobilization on nanostructured electrodes to improve detection stability and specificity. We focus on real-time monitoring and tailored healthcare with nanostructured electrode-based hormone detection in clinical diagnostics, wearable devices, and point-of-care testing. These nanostructured electrode-based assays are useful for endocrinology research and hormone-related disease diagnostics due to their sensitivity, selectivity, and repeatability. We conclude with nanotechnology–microfluidics integration and tiny portable hormone-detection devices. Nanostructured electrodes can improve hormone regulation and healthcare by facilitating early disease diagnosis and customized therapy.
Our investigation aimed to create and manufacture an electrochemical impedance sensor with the purpose of improving the detection efficiency of melatonin (ME). To achieve this objective, we employed gold nanoparticles coated on polydopamine formed in glassy carbon electrodes (AuNPs/PDA/GCE) as a means to enhance the sensor's capabilities. A novel approach employing the signal-off strategy and electrochemical impedance spectroscopy (EIS) technique was utilized to determine ME. When the AuNPs/PDA/GCE electrode was immersed in a buffered solution containing ME, and the oxidation current of AuNPs was recorded, it was observed that the oxidation current of AuNPs decreased upon the introduction of ME molecules. The decrease in electrical current can be ascribed to the inhibitory impact of ME molecule adsorption on the electrode surface with applying -0.2 V for 150 s in acetate buffer solution (ABS) (pH, 5) through various mechanisms, which hinders the electron transfer process crucial for AuNPs oxidation. Consequently, by utilizing EIS, various concentrations of ME were quantified spanning from 1 to 18 pM. Moreover, the ME sensor achieved an impressive detection limit of 0.32 pM, indicating its remarkable sensitivity in detecting low concentrations of ME. Importantly, these novel sensors demonstrated exceptional attributes in terms of sensitivity, specificity, stability, and repeatability. The outstanding performance of these sensors, coupled with their desirable attributes, establishes their considerable potential for a wide range of practical applications. These applications encompass various fields such as clinical diagnostics, pharmaceutical analysis, environmental monitoring, and industrial quality control, where accurate and sensitive detection of ME is of utmost importance.
Melatonin is a natural hormone produced by the pineal gland and generated as a special metabolite in plants from tryptophan as a precursor and serotonin as an intermediate. These indoleamines show intrinsic fluorescence properties due to the presence of the indole group in their chemical structure, which made them suitable for the analysis by fluorescence-based techniques. In this research, it is described the first comprehensive study of the fluorescence of melatonin and its indole derivatives in several natural deep eutectic solvents (NADES). As melatonin exhibits solvent-dependent fluorescence characteristics, we have measured the quantum yields of the molecules in NADES and conventional solvents with the purpose of understanding the fluorescent behavior in these solvents that will offer a new alternative to conventional medium. The results showed differential fluorescence behavior of melatonin and its metabolic precursors in NADES that could be explained in terms of each fluorophore structure and potential hydrogen bond interactions with eutectic systems. Finally, the turn-off observed for melatonin in the NADES system based on fructose-citric acid and water could inspire the development of new analytical approaches.
This reference describes recent advances and applications of capillary electrophoresis in the field of food science. The first two chapters are devoted to the fundamentals of capillary electrophoresis, and to the main sample preparation techniques used for food analysis using this miniaturized separation technique, respectively. These two introductory chapters are followed by several chapters focused on the different strategies for analyzing specific food components, including lipids, carbohydrates, proteins, peptides, amino acids, vitamins, polyphenols, and food additives.
The information provided in these chapters helps readers to understand and develop appropriate methods to carry out a deep characterization of food samples. Relevant concepts such as food authentication, chemical food safety or the control of the quality and safety of dietary supplements, and food metabolomics are also covered, where appropriate. The big potential of capillary electrophoresis to achieve chiral separations and the determination of enantiomers in food samples or to develop targeted and non-targeted metabolomics strategies to ensure food safety and quality is also described. As an additional step towards analytical miniaturization, a chapter devoted to food analysis by microchip electrophoresis is also included in this book. All 14 chapters are contributed by highly experienced researchers in the field.
Capillary Electrophoresis in Food Analysis is a key source of information for food chemists and analytical chemists in industry (quality control laboratories) and academia (research labs and training courses).
Herein we report the simultaneous determination of serotonin (5-HT) and melatonin (MT) by using highly ordered and vertically-oriented mesoporous silica-nanochannel films (VMSF) on highly electrochemically reduced graphene oxide-carbon nanotubes (HErGO-CNT) composite substrate. Such VMSF/HErGO-CNT with compact 2D-2D layered structure is fabricated on indium tin oxide (ITO) electrodes via a two-step electrochemical process, namely the electrodeposition of ErGO-CNT film onto the ITO surface by cyclic voltammetry and subsequently growth of VMSF on the ErGO-CNT/ITO surface by electrochemically assisted self-assembly method, during which graphene oxide (GO) is subjected to the two-time electrochemical reduction to form HErGO. The CNT encased in the GO sheets served as electronic conducting wires, which not only can promote the electrochemical reduction of GO but also contribute to a certain degree of hydrophobicity, facilitating the controllable electrodeposition of ErGO-CNT composites on ITO under its safe use potential window. In addition, doping CNT is able to enlarge the layer gap between graphene sheets and thus improves the electroactive area and mass transfer of the nanocarbon composite substrate. Furthermore, the underlying HErGO-CNT acts as an efficient electroactive layer and the outer VMSF possesses electrostatic preconcentration and anti-fouling functions, synergistically realizing the direct electrochemical analysis of 5-HT and MT in two complex biological fluids (human whole blood and artificial cerebrospinal fluid).
Tryptophan and melatonin are pleiotropic molecules, each capable of influencing several cellular, biochemical, and physiological responses. Therefore, sensitive detection of tryptophan and melatonin in pharmaceutical and human samples is crucial for human well-being. Mass spectrometry, high-performance liquid chromatography, and capillary electrophoresis are common methods for both tryptophan and melatonin analysis; however, these methods require copious amounts of time, money, and manpower. Novel electrochemical and optical detection tools have been subjects of intensive research due to their ability to offer a better signal-to-noise ratio, high specificity, ultra-sensitivity, and wide dynamic range. Recently, researchers have designed sensitive and selective electrochemical and optical platforms by using new surface modifications, microfabrication techniques, and the decoration of diverse nanomaterials with unique properties for the detection of tryptophan and melatonin. However, there is a scarcity of review articles addressing the recent developments in the electrochemical and optical detection of tryptophan and melatonin. Here, we provide a critical and objective review of high-sensitivity tryptophan and melatonin sensors that have been developed over the past six years (2015 onwards). We review the principles, performance, and limitations of these sensors. We also address critical aspects of sensitivity and selectivity, limit and range of detection, fabrication process and time, durability, and biocompatibility. Finally, we discuss challenges related to tryptophan and melatonin detection and present future outlooks.
Nano-CeO2/reduced graphene oxide (RGO) composite were successfully prepared using a simple electrochemical technique. The structure and morphology of the composite were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). Numerous of CeO2 nanocrystals are found to be wrapped inside uniformly by gossamer like RGO. The electrochemical behaviors of tryptophan (Trp) on the glassy carbon electrode modified with nano-CeO2/RGO (nano-CeO2/RGO/GCE) were investigated by second derivative linear sweep voltammetry (SDLSV)and cyclic voltammetry (CV). It was found that high electrocatalytic activity was exhibited on the modified electrode for Trp oxidation. The peak current increased about 76-fold and the peak potential shifted 88 mV negatively at the nano-CeO2/RGO/GCE compared with that of the bare GCE, demonstrating the synergistic effect of nano-CeO2 and RGO on the electrode surface. The measurement parameters are optimized. Under the optimum conditions, a good linear relationship was observed between Trp oxidation peak current and its concentration in the range of 0.01-10 μM, with a sensitivity of 4.7306 μA μM⁻¹ and the detection limit was obtained as 6.0 nM (S/N=3) after accumulating at 0 V for 90 s. The developed method also exhibited good reproducibility, stability and anti-interference ability, and has been successfully applied to Trp detection in commercial compound amino acid injections, human serum and human urine samples with recoveries of 98.5 %-106.0 %.
This paper describes the development of a novel, simple and inexpensive electrochemical device containing an integrated and disposable three‐electrode system for detection. The base of this platform consists on a polydimethylsiloxane (PDMS) structure containing microchannels which were prototyped using 3D‐printed molds. Pencil graphite leads were inserted into these microchannels and utilized as working, counter and reference electrodes in a novel design. Morphological analysis and electrochemical experiments with benchmark redox probes were carried out in order to evaluate the performance and characterize the miniaturized device proposed. Even using inexpensive materials and a simple fabrication protocol, the electrochemical platform developed provided good repeatability and reproducibility over a low cost (ca. $2 per device), acceptable lifetime (ca. 250 voltammetric runs) and extremely reduced consumption of samples and reagents (order of µL). As proof of concept, the analytical feasibility of the platform was investigated through the simultaneous determination of dopamine (DOPA) and acetaminophen (AC). The two analytes showed linear dependence on the concentration range from 1 to 15 µM and the limits of detection (LODs) achieved were 0.21 µM for DOPA and 0.29 µM for AC. Moreover, the platform was successfully applied on the determination of DOPA and AC in spiked blood serum and urine samples. The results obtained with the device described here were better than some reports in literature that use more costly electrodic materials and complex modification steps for the detection of the same analytes. This article is protected by copyright. All rights reserved
Metabolic rare diseases present a serious clinical manifestation causing several problems such as mental retardation, liver damage, hepatocarcinoma and even death without a diagnosis in the first days of life. Additionally, there is not any final treatment for these diseases but a dietary restriction, so patients have to control their biomarkers level during their whole life. Current methods for diagnosis and monitoring are tedious, very time-consuming, not portable and need high amount of sample, so they are not suitable for a point-of-care (POC) application. In this sense, electrochemistry presents the required characteristics to be used as POC for metabolic rare diseases. Specifically, this review explores the current methods for amino acids and carbohydrates sensing as biomarkers of metabolic rare diseases using electrochemical (bio)-sensing approaches, especially those based on nanomaterials in both, off and on-chip detection, as well as their suitability in metabolic rare diseases diagnosis and monitoring using clinical samples.
The fabrication of PDMS microfluidic structures through soft lithography is widely reported. While this well‐established method gives high precision microstructures and has been successfully used for many researchers, it often requires sophisticated instrumentation and expensive materials such as clean room facilities and photoresists. Thus, we present here a simple protocol that allows the rapid molding of simple linear microchannels in PDMS substrates aiming microfluidics‐based applications. It might serve as an alternative to researchers which do not have access to sophisticated facilities such as clean rooms. The method developed here consists on the use of pencil graphite leads as template for the molding of PDMS channels. It yields structures that can be used for several applications, such as housing support for electrochemical sensors or channels for flow devices. Here, the microdevices produced through this protocol were employed for the accommodation of carbon black paste, which was utilized for the first time as amperometric sensor in microchip electrophoresis. This platform was successfully used for the separation and detection of model analytes. Ascorbic acid and iodide were separated within 45 s with peak resolution of 1.2 and sensitivities of 198 pA/μM and 492 pA/μM, respectively. The background noise was ca. 84 pA. The analytical usefulness of the system developed was successfully tested through the quantification of iodide in commercial pharmaceutical formulations. It demonstrates good efficiency of the microfabrication protocol developed and enables its use for the easy and rapid prototyping of PDMS structures over a low fabrication cost.
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Presenting the most relevant advances for employing carbon-based nanostructured materials for analytical purposes, this book serves as a reference manual that guides readers through the possibilities and helps when selecting the most appropriate material for targeted analytical applications. It critically discusses the role these nanomaterials can play in sample preparation, separation procedures and detection limit improvements whilst also considering the future trends in this field.
Useful to direct initiatives, this book fills a gap in the literature for graduate students and professional researchers discussing the advantages and limitations across analytical chemistry in industry and academia.
Alpha-1-acid glycoprotein (AGP) is a serum glycoprotein whose levels are increased two or three times during disease or injury. This makes it a potential biomarker for inflammatory bowel diseases and sepsis. Consequently, fast, simple, and cheap analytical methods for prognosis, diagnosis, and follow-up of these diseases are demanded. In this work, we propose a simple electrochemical approach based on carbon nanotubes scaffold films (CNSFs) for total AGP determination in serum samples. Firstly, AGP is labeled with an electrochemical tag (osmium(VI) complex), and then the total amount of AGP is quantified by adsorptive transfer stripping square wave voltammetry (AdTSWV). Multi-walled carbon nanotubes scaffold films (MWSFs) yielded the best analytical performance in terms of sensitivity with a good limit of detection of 0.6 mg L−1 for AGP determination in serum samples, in less than 20 min. A simplified AGP calibration and its sequential serum sample analysis strategy with good accuracy (81%) and excellent reproducibility (RSD < 1%) was additionally proposed to meet the point-of-care/needs requirements.
Multi-walled carbon nanotubes scaffold films for total AGP determination on disposable platforms integrating single-point calibration and sequential sample analysis.
In this work, vertically aligned nickel nanowires‐based electrochemical sensors (v‐NiNWs) for rapid detection of branched‐chain amino acids for a non‐invasive screening of maple syrup urine disease (MSUD), are proposed. v‐NiNWs meet the required features for decentralized clinical diagnosis: easy to operate, fast sensor response and short total analysis times (5 min), as well as low sample consumption (250 ml). The analytical characteristics (for leucine as representative branched‐chain amino acid in MSUD) such as detection limit (8 μM) and linear range (25–700 μM which includes as midpoint the amino acids cut‐off of 250 μM) clearly showed the suitability of v‐NiNWs as new and disposable analytical tool for rapid screening of branched‐chain amino acids in MSUD because of their high capabilities for reliable differentiation between healthy and ill clinical sample profiles. In addition, intra and inter‐electrodes repeatability were excellent with values of RSD≤4 and 9 %, respectively (n=5). This approach opens novel avenues not only for ultrafast diagnosis of MSUD using miniaturized and even disposable devices but also for a future point‐of‐care testing; providing novel alternatives for on‐site decentralized monitoring.
In this research, a dispersive solid-phase extraction (dSPE) with graphene oxide@SiO2 (GO@SiO2) nanocomposites as the adsorbent followed by high-performance liquid chromatography analysis was developed for simultaneous determination of melatonin, l-tryptophan, and two l-tryptophan-derived esters in food (black sesame seed (Sesamum indicum L.) was selected in this case). The GO@SiO2 nanocomposite was prepared by one-pot aggregation in aqueous phase with sol-gel technique. The structure and morphology of the GO@SiO2 were characterized by scanning electron microscopy, transmission electron microscopy, Fourier transform-infrared spectroscopy, and X-ray diffraction. The extraction conditions of dSPE including the ratio of material to liquid, adsorption and desorption time, desorption temperature, and desorption solvents were investigated, respectively. The detection limits of the developed method for the analysis of melatonin, l-tryptophan, l-tryptophan methyl ester, and l-tryptophan ethyl ester were achieved below 0.1 μg mL⁻¹. The established method was successfully applied to the analysis of the target analytes in black sesame seed, which provided a simple, low-cost, and sensitive approach for the determination of trace compounds in complex samples.
Methyl parathion (MP) is a highly toxic organophosphate and its exposure may lead to substantial adverse effects to human health. The existence of 4-nitrophenol (4-NP) in the form of free phenol, glucuronide (4-NP-G) or as a sulfate ester (4-NP-S) can be used as biomarkers to assess the duration and extent of MP exposure. In this work, a MC-CE device incorporating post-CE amperometric detection using multi-walled carbon nanotubes (MWNTs) modified carbon fiber microelectrode (CFME) was fabricated and assessed for simultaneous determination of 4-NP, 4-NP-G and 4-NP-S in human urine. The detection sensitivity and stability was greatly enhanced by the modification of MWNTs. The capability of the MC-CE device with dual MWNTs modified CFME for detecting impurity was assessed and reliability established by high recoveries from 95 to 97% for spiked MP biomarkers. The method developed is shown to provide a simple, sensitive and reliable means for monitoring 4-NP, 4-NP-G and 4-NP-S in human urine. This article is protected by copyright. All rights reserved
Advanced nanomaterials such as carbon nanotubes (CNTs) display unprecedented properties such as strength, electrical conductance, thermal stability, and intriguing optical properties. These properties of CNT allow construction of small microfluidic devices leading to miniaturization of analyses previously conducted on a laboratory bench. With dimensions of only millimeters to a few square centimeters, these devices are called lab-on-a-chip (LOC). A LOC device requires a multidisciplinary contribution from different fields and offers automation, portability, and high-throughput screening along with a significant reduction in reagent consumption. Today, CNT can play a vital role in many parts of a LOC such as membrane channels, sensors and channel walls. This review paper provides an overview of recent trends in the use of CNT in LOC devices and covers challenges and recent advances in the field. CNTs are also reviewed in terms of synthesis, integration techniques, functionalization and superhydrophobicity. In addition, the toxicity of these nanomaterials is reviewed as a major challenge and recent approaches addressing this issue are discussed.
In this work, we demonstrate for the first time the use of inexpensive commercial pencil graphite leads as simple amperometric sensors for microchip electrophoresis. A PDMS support containing one channel was fabricated through soft lithography and sanded pencil graphite leads were inserted into this channel to be used as working electrodes. The electrochemical and morphological characterization of the sensor was carried out. The graphite electrode was coupled to PDMS microchips in end-channel configuration and electrophoretic experiments were performed using nitrite and ascorbate as probe analytes. The analytes were successfully separated and detected in well-defined peaks with satisfactory resolution using the microfluidic platform proposed. The repeatability of the pencil graphite electrode was satisfactory (RSD values of 1.6% for nitrite and 12.3 % for ascorbate regarding the peak currents) and its lifetime was estimated to be ca. 700 electrophoretic runs over a cost of ca. $ 0.05 per electrode. The limits of detection achieved with the proposed system were 2.8 μM for nitrite and 5.7 μM for ascorbate. For proof of principle, the pencil graphite electrode was employed for the real analysis of well water samples and nitrite was successfully quantified at levels below its maximum contaminant level established in Brazil and US. This article is protected by copyright. All rights reserved
A glassy carbon electrode (GCE) modified with electropun tricobalt tetroxide nanoparticles decorated carbon nanofibers (Co3O4-CNF) were first applied for the determination of L-tryptophan (L-Trp). The synthesized Co3O4-CNF were characterized by scanning electron microscopy, Raman, X-ray diffraction and electrochemical impedance spectroscopy. The electrochemical detection of L-try was successfully conducted in 0.1 M phosphate solution (pH 2). The linear relationship of L-Trp was in the range of 0.005–40 μM and the detection limit reached 0.002 μM (S/N = 3) with little interference from other amino acids.
Carbon and metallic-based nanostructures have been progressively implemented as innovative electrochemical detectors in CE and ME. For both type of nanomaterials and toward selected examples, this review details the impact of these nanomaterials for enhanced detection performance in CE, ME and paper-based microfluidic devices. The analytical performance and the analytical potential in real world applications is also presented and discussed. This article is protected by copyright. All rights reserved.
A rapid, reliable and sensitive LC-MS/MS method for the determination of melatonin in milk was developed and validated. Sample was extracted with dichloromethane and cleaned by passing through Chem Elut solid phase extraction cartridge. The solvent was evaporated to dryness, reconstituted with methanol and analysed by LC-MS/MS on Agilent zorbax Eclipse XDB C-18 rapid resolution analytical column. The analytical procedure was found to be accurate, precise and linear. The method accuracy was 92.2 % (range 90.06-94.58) and the mean precision was 1.55 % and the calibration was linear for 1 to 150 pg mL(-1) (R(2) > 0.99), the lowest limit of quantification (LLOQ) was 1 pg mL(-1). 7-D Melatonin (7-DM) was used as an internal standard. This method was proved to be a promising method for the determination of melatonin for market milk and human milk samples.
"Lab-on-a-chip" approaches based on the novel marriage between an electrokinetic microfluidic platform and nanotechnology is proposed for analytical domains. Conceptually, the analytical challenges are linked with the analytical promises offered from the integration of lab-on-a-chip and nanotechnologies. The analytical suitability of the electrokinetic microfluidic platform with multi-walled carbon nanotubes as detectors is proposed based on its dual format/use as a flow and separation system, independently. Two relevant applications of high significance, determination of total isoflavones and fast detection of antioxidant profiles were chosen to demonstrate their analytical potential. For both analytical uses, the target challenges, the strategy proposed, the expected role of microfluidics and carbon nanotubes and future prospects are discussed and demonstrated. A good analytical performance of the proposed microfluidic platform in terms of reliability, versatility and fast analytical solutions is demonstrated.
This survey summarizes the findings, accumulated within the last 2 years, concerning melatonin's role in defending against toxic free radicals. Free radicals are chemical constituents that have an unpaired electron in their outer orbital and, because of this feature, are highly reactive. Inspired oxygen, which sustains life, also is harmful because up to 5% of the oxygen (O2) taken in is converted to oxygen-free radicals. The addition of a single electron to O2 produces the superoxide anion radical (O2-.); O2-. is catalytic-reduced by superoxide dismutase, to hydrogen peroxide (H2O2). Although H2O2 is not itself a free radical, it can be toxic at high concentrations and, more importantly, it can be reduced to the hydroxyl radical (.OH). The .OH is the most toxic of the oxygen-based radicals and it wreaks havoc within cells, particularly with macromolecules. In recent in vitro studies, melatonin was shown to be a very efficient neutralizer of the .OH; indeed, in the system used to test its free radical scavenging ability it was found to be significantly more effective than the well known antioxidant, glutathione (GSH), in doing so. Likewise, melatonin has been shown to stimulate glutathione peroxidase (GSH-Px) activity in neural tissue; GSH-PX metabolizes reduced glutathione to its oxidized form and in doing so it converts H2O2 to H2O, thereby reducing generation of the .OH by eliminating its precursor. More recent studies have shown that melatonin is also a more efficient scavenger of the peroxyl radical than is vitamin E. The peroxyl radical is generated during lipid peroxidation and propagates the chain reaction that leads to massive lipid destruction in cell membranes. In vivo studies have demonstrated that melatonin is remarkably potent in protecting against free radical damage induced by a variety of means. Thus, DNA damage resulting from either the exposure of animals to the chemical carcinogen safrole or to ionizing radiation is markedly reduced when melatonin is co-administered. Likewise, the induction of cataracts, generally accepted as being a consequence of free radical attack on lenticular macromolecules, in newborn rats injected with a GSH-depleting drug are prevented when the animals are given daily melatonin injections. Also, paraquat-induced lipid peroxidation in the lungs of rats is overcome when they also receive melatonin during the exposure period. Paraquat is a highly toxic herbicide that inflicts at least part of its damage by generating free radicals.(ABSTRACT TRUNCATED AT 400 WORDS)
An original analytical method has been developed for the determination of the antioxidants trans-resveratrol (t-RSV) and cis-resveratrol (c-RSV) and of melatonin (MLT) in red and white wine. The method is based on HPLC coupled to fluorescence detection. Separation was obtained by using a RP column (C8, 150 mm x 4.6 mm id, 5 mum) and a mobile phase composed of 79% aqueous phosphate buffer at pH 3.0 and 21% ACN. Fluorescence intensity was monitored at lambda = 386 nm while exciting at lambda = 298 nm, mirtazapine was used as the internal standard. A careful pretreatment of wine samples was developed, using SPE with C18 cartridges (100 mg, 1 mL). The calibration curves were linear over the following concentration ranges: 0.03-5.00 ng/mL for MLT, 3-500 ng/mL for t-RSV and 1-150 ng/mL for c-RSV. The LOD values were 0.01 ng/mL for MLT, 1 ng/mL for t-RSV and 0.3 ng/mL for c-RSV. Precision data, as well as extraction yield and sample purification results, were satisfactory. Thus, the method seems to be suitable for the analysis of MLT and resveratrol isomers in wine samples. Moreover, wine total polyphenol content and antioxidant activity were evaluated.
Melatonin (N-acetyl-5-methoxytryptamine) is the chief secretory product of the pineal gland and synthesized enzymatically from serotonin (5-hydroxytryptamine). These indoleamine derivatives play an important role in the prevention of oxidative damage. In the present study, DMPD radical scavenging and cupric ion (Cu(2+)) reducing ability of melatonin and serotonin as trolox equivalent antioxidant activity (TEAC) was investigated. Melatonin and serotonin demonstrated 73.5 and 127.4 microg/mL trolox equivalent DMPD( radical+) scavenging activity at the concentration of 100 microg/mL. Also, at the same concentration, melatonin and serotonin showed 14.41 and 116.09 microg/mL trolox equivalent cupric ion (Cu(2+)) reducing ability. These results showed that melatonin and serotonin had marked DMPD(radical+) radical scavenging and cupric ions (Cu(2+)) reducing ability. Especially, serotonin had higher DMPD radical scavenging and cupric ions (Cu(2+)) reducing activity than melatonin because of its phenolic group.
Recent evidence has shown that neurologically active compounds play an important role in the physiology of higher plants. The human neurotransmitter melatonin (N-acetyl-5-methoxytryptamine) has been found in more than 140 different plant species and is associated with flower and seed development, reproductive capacity and root growth. Hyperforin, once thought to be characteristic of St. John's wort (Hypericum perforatum L.), has now been found in several other plant species and preliminary findings indicate that it may function as a modulator of plant cell polarity and ion flow. The excitotoxin, β-methylamino-L-alanine, has been used to selectively isolate glutamate receptor mutants in Arabidopsis, and these studies are leading to new understandings of plant cell development. The study of these and other plant neurochemicals may lead to a new understanding of plant signal transmission and the mechanisms by which plant cells sense, interpret and respond to environmental cues.
The main multidimensional carbon allotropes could be classified into carbon nanotubes as one dimension (1D) material, graphene as (2D) material as well as graphite and diamond as (3D) carbon materials. Along this review, a discussion using these four structures as electrochemical detectors in capillary and microchip electrophoresis will permit us to explore the recent advances in this field.This article is protected by copyright. All rights reserved
These insights attempt to share with the community the lights and shadows of one emerging and exciting topic, Food Microfluidics, defined as microfluidic technology for food analysis and diagnosis in important areas such as food safety and quality. The reader is invited to question non-easy interrogations such as why Food Microfluidics, what is the next step and what could we do with the available technology. This article invites food analysts to be seduced by this technology and then to take an interesting trip departing from the main gained achievements, having a look at the crossing bridges over Food Microfluidic challenges or having a look at available technology to start. Finally, this trip arrives at a privileged place to gaze the horizons. A wonderful landscape - full of inspiration - for Food Microfluidics is anticipated. These insights have also been written wishing to give improved conceptual and realistic solutions for food analysis, with the additional hope to attract the community with exciting technology, in order to get novel and unexpected achievements in this field.
Single-walled carbon nanotube press-transferred electrodes (SWPTEs) are new disposable electrodes where carbon nanotubes act as exclusive electrochemical transducers, being an excellent alternative to common approaches in the field. In the current work, these pioneering SWPTEs coupled to microfluidic chips (MCs) have been employed to develop their first real application. A class-selective electrochemical isoflavone index determination has been proposed for fast and reliable qualitative and quantitative assessment of class-isoflavones based on the co-migration of the total glycosides (TG) and total aglycones (TA) in less than 250 s with very good intra-SWPTE repeatability (RSDs ≤ 8%, n = 5) and inter-SWPTE reproducibility (RSDs ≤ 9%, n = 3). These novel SWPTEs are entering with important roles into the micro and nanotechnology scenes expanding new frontiers in the food analysis and health field.
A simple method for the fabrication of optically transparent electrodes (OTE) from commercial single-walled carbon nanotubes (SWCNT) is presented. We disperse SWCNT in organic solvents, avoiding surfactants. Films obtained by filtering are transferred to non-conductive polymer substrates only by pressure, avoiding the chemical removing of the filter. The electro-optical properties of the electrodes have been studied and optimized using an experimental design strategy, concluding that both properties depend on the total mass of SWCNT transferred to the support. In general, well-interconnected SWCNT networks yield little differences in the electrochemical behavior. However, it has been observed that electrodes with a high mass of SWCNT produce significant changes in the voltammograms, obtaining unexpected ratios between the anodic and cathodic peak currents. SWCNT-OTE have been used to study the electropolymerization of aniline.
Melatonin has recently been detected in various plants and foods. However, data regarding the food composition of melatonin are too scarce to evaluate dietary intake. This paper aims to identify melatonin unequivocally using LC–MS in a wide set of varieties of tomatoes (Lycopersicon esculentum) and strawberry (Fragariaananassa). Furthermore, a validated LC fluorescence was developed.This is the first time melatonin has been identified in Bond, Borsalina, Catalina, Gordal, Lucinda, Marbone, Myriade, Pitenza, Santonio, Perlino, Platero, and RAF varieties of tomatoes, as well as in strawberry (Fragaria ananassa): Camarosa, Candonga, Festival, and Primoris. Melatonin concentration was shown to vary greatly depending on the tomato varieties and harvests (2009, 2010), ranging from 4.11ng/g to 114.52ng/g fresh weight. However, the four varieties of strawberries collected during the two harvests showed greater similarity in melatonin (1.38–11.26ng/g fresh weight).
Melatonin (MT) presence in higher plants was recently discovered and the knowledge of its function in vivo is limited. Several studies have recently shown the occurrence of MT and related compounds in grapes and wines. The analysis of MT in plants and foods represents a highly challenging task due to its wide concentration range, the difficulty in the selection of the extraction solvents because of its amphipathic nature, and the fact that it reacts quickly with other matrix components. Thus, sample processing factors; preparation/cleanup procedures; and chromatographic/detection parameters, such as HILIC and reverse phase (C8 and C18 ) chromatographic modes, ESI, and atmospheric pressure chemical ionization (APCI) in both negative and positive modes were evaluated. Taken together, we have demonstrated that optimal conditions were quite different for each of the matrices under study. A sonication-mediated extraction step was necessary for grape skin (100% v/v methanol) and plant tissues (50% v/v methanol), while wine and must required a SPE preconcentration step. HILIC-(+) APCI ionization was better for MT standards, while C8 -(+) APCI was the best choice for grape skin and C18 -(+ESI) was suitable for wine. On the other hand, C8 -(+)ESI was the most appropriate for vegetal tissues of Arabidopsis thaliana. Proposed methods were validated and the LODs were in the low picogram levels range. The optimized approaches were applied to the determination of MT and its isomer in different vegetal/food samples; levels found within the range: 4.9-440 ng/g.
Although polyphenols represent the paradigm of the health-promoting effects ascribed to grape products, recently, attention has been paid to dietary melatonin, significantly present in Mediterranean foods. In this work, we measured melatonin, its isomers, stilbenes (trans- and cis-resveratrol and their glucosides, piceids) and total polyphenols in some different grape products (red, white and dessert wines, grape juices and Modena balsamic vinegars) of distinct Italian areas. We also evaluated their antiradical activity by DPPH(·) and ABTS(·+) assays. For indoleamine analysis, the separation was carried out on a 1.7-μm C(18) BEH column and the detection performed by means of mass spectrometry with electrospray ionization in positive ion mode with multiple reaction monitoring. The confirmation of the peak identity was accomplished by injection into the high-resolution system (Orbitrap) using accurate mass measurements (error below 1.0 ppm). Mass spectrometry analyses revealed, for the first time, the presence of melatonin in dessert wines and balsamic vinegars, as well as the occurrence of three different melatonin isomers in grape products.
A novel single-walled carbon nanotubes press-transfer electrodes (SW-PTEs) for microfluidic sensing is proposed. In this approach, carbon nanotubes are press-transferred on poly-methyl-methacrylate (PMMA) substrates and easily coupled to microfluidic chips and acting as the exclusive transducer in the electrochemical sensing. The detector design consisted of a press-transferred SW-film (7×1 mm) positioned and centred on the PMMA substrate (33×9 mm). The analytical performance of the SW-PTEs was deeply evaluated using two commercial SWs sources and employing a mixture of dopamine and catechol as model analytes. Analyte detection was influenced by the volume of commercial SW dispersion used in the fabrication of SW-PTEs, being 5 mL taken from a dispersion of 0.5mg/100mL as the most favorable volume. In addition, excellent repeatability (RSDs≤7%, n=5), inter-electrodes reproducibility (RSDs≤9%, n=5), and an extreme resistance to fouling were obtained even after 1h of microchip analysis with RSDs≤4% and RSDs≤9% (n=15) for migration times and peak heights, respectively. A good sensitivity, remarkable signal-to-noise characteristics and a well-defined linear concentration dependence (r≥0.990) was also obtained, being able these novel detectors to be considered as valuable tools for quantitative analysis. Analytical characterization of the SW-PTEs by Field Emission Scanning Electron Microscopy (FESEM) revealed the individual bundles of SWs highly ordered over the PMMA at the background where the SW bundles were embedded on the PMMA substrate giving to the electrode a high stability. Furthermore, the lab-fabricated SW-PTEs can be afforded in any laboratory since it does not require clean-room facilities, are highly compatible with microfluidic scale, mass-production and disposability. In addition, the proposed approach draws new and exciting horizons for electrochemical microfluidic sensing such as the use of other pure or hybrid nanomaterials and also the possibilities to incorporate biomolecules for high selective sensing.
The effects of sample preparation, cultivar, leaf age and tea processing on melatonin content of mulberry (Morus spp.) leaves were investigated. Sample preparation using ultrasonic technique in combination with solid phase extraction resulted in high recovery rate (76%), when compared to homogenisation in combination with liquid–liquid extraction procedure (12% recovery rate). The melatonin contents in mulberry leaves harvested from three major cultivars (Buriram 60, Sakonnakhon and Khunphai) grown in Thailand were identified and quantified using high-performance liquid chromatography combined with fluorescence detector. The average melatonin content of the mulberry leaves cv. Buriram 60 (279.6 ng/g dry weight (DW)) was higher than those of cv. Sakonnakhon (100.5 ng/g DW) and cv. Khunphai (40.7 ng/g DW). The melatonin contents of all cultivars tested were highest in the tip of the leaves, followed by that in the young leaves, whereas the lowest was found in the old leaves. The melatonin contents of the two types of tea produced from mulberry leaves cv. Buriram 60 were also determined. Heat treatment during tea processing decreased the melatonin content in mulberry leaves cv. Buriram 60 by approximately 87%, when compared to that of the fresh leaves. However, there were no significant differences between the melatonin contents of the mulberry leaf tea produced with blanching (mulberry green tea) and those produced without blanching (mulberry black tea).Highlights► Significant amount of melatonin was identified in mulberry leaves. ► Suitable sample preparation was important factor for accuracy of melatonin analysis. ► Melatonin contents in mulberry leaves depend on cultivar and leaf age. ► Tea processing could decrease melatonin content in mulberry leaves.
Single-walled carbon nanotubes (SWCNTs) should constitute an important natural step towards the improvement of the analytical performance of microfluidic electrochemical sensing. SWCNTs inherently offer lower detection potentials, higher surfaces and better stability than the existing carbon electrodes. However, pristine SWCNTs contain some carbonaceous and metallic impurities that influence their electrochemical performance. Thus, an appropriate processing method is important for obtaining high purity SWCNTs for analytical applications. In this work, a set of 0.1 mg mL(-1) SWCNT dispersions with different degrees of purity and different dispersants (SDBS; pluronic F68 and DMF) was carefully characterized by near infrared (NIR) spectroscopy giving a Purity Index (NIR-PI) ranging from 0.039 to 0.310. The highest purity was obtained when air oxidized SWCNTs were dispersed in SDBS, followed by centrifugation. The SWCNT dispersions were utilized to modify microfluidic chip electrodes for the electrochemical sensing of dopamine and catechol. In comparison with non-SWCNT-based electrodes, the sample with the highest NIR-PI (0.310) exhibited the best analytical performance in terms of improved sensitivity (3-folds higher), very good signal-to-noise ratio, high resistance-to-fouling in terms of relative standard deviation (RSD 7%; n = 15), and enhanced resolution (2-folds higher). In addition, very well-defined concentration dependence was also obtained with excellent correlation coefficients (r ≥ 0.990). Likewise, a good analytical sensitivity, suitable detection limits (LODs) and a very good precision with independence of the concentration assayed (RSDs ≤ 5%) was achieved. These valuable features indicate the suitability of this material for quantitative analysis. NIR-PI and further TEM and XRD characterization demonstrated that the analytical response was driven and controlled by the high NIR-PI of the SWCNTs used. The significance of this work is the demonstration for the first time of the sensitivity-purity relationship in SWCNT microfluidic chips. A novel and valuable analytical tool for electrochemical sensing has been developed: SWCNTs with high purity and a rich surface chemistry with functional groups, both essential for analytical purposes. Also, this work helps to better understand the analytical potency of SWCNTs coupled to microfluidic chips and it opens new gates for using these unique dispersions in real-world applications.
The effect of light on melatonin biosynthesis was examined in detached rice (Oryza sativa cv. Asahi) leaves during the senescence process. The detached leaves were exposed to senescence treatment either in constant darkness or in constant light, and subjected to HPLC analysis for melatonin and its precursors. Higher melatonin levels were detected in rice leaves under constant light while very low levels were observed in constant darkness. Levels of the melatonin intermediates, tryptamine, serotonin, and N-acetylserotonin significantly decreased in the dark compared to those in the light. Furthermore, relative mRNA levels of melatonin biosynthetic genes and their corresponding proteins decreased accordingly in constant darkness. The most striking difference between constant light and dark was observed in levels of the protein tryptamine 5-hydroxylase. These results suggest that melatonin biosynthesis during senescence is dependent on light signals in rice leaves, contrary to the response found in animals.
Several studies have shown the presence of melatonin and related compounds in grapes and wines. The latter provides evidence of the possibility to enhance the nutraceutical properties of premium wines. However, there are many external factors that can influence the levels of this indolamine in grape and wines. In this study, the monitoring of melatonin and its tentatively identified isomer was carried out during the entire winemaking process in Vitis vinifera cv. Malbec by ultra high-performance liquid chromatography-tandem mass spectrometry. Laboratory and pilot studies were carried out to elucidate the role of grape, yeasts, and tryptophan in the evolution of the indolamines during the fermentation process. Melatonin was detected in grape extract within the range 120-160 ng/g while its isomer was found in musts and finished wines. Our results demonstrate that Saccaromyces cervisiae plays a decisive role in contributing to the content of melatonin and its isomer in wine.
With the appearance of advanced approaches such as screen-printed technology, biosensors, microchips and nanotechnology, among others, electroanalysis is undergoing a true Renaissance. Inherent miniaturization of electrochemistry makes it a unique detection and transduction principle, highly compatible with the modern miniaturized analytical chemistry involving micro- and nanotechnologies. It also implies advantages on portability and further disposability. Another very unique feature linked to electrochemistry is the versatility for "selectivity design" towards the suitable selection of (nano)(bio)materials and by the direct manipulation of the electrical properties. Their remarkable sensitivity and low cost are additional valuable features. However, from my personal perspective, these "natural beauties" are underexploited in the analysis of food samples not only because of the complexity of food samples but also because electrochemistry has traditionally been seen as "a difficult thing". From my own experience, electrochemical approaches have been very useful in the evaluation of antioxidant activities in vitro, in the development of screening methods, as high-performance detectors in advanced analytical microsystems such as capillary-electrophoresis microchips and in the development of microfluidic inmunosensors. In consequence, electroanalysis has also demonstrated an important role in fields such as antioxidant sensing, quality control assessment, detection of frauds and food safety. In this personal account, drawing from selected examples of my own work, I illustrate the marriage between electrochemistry and food analysis, food electroanalysis, by sense and simplicity.
A capillary electrophoresis (CE) microsystem, based on the combination of microphotolithographically fabricated separation chips and thick-film electrochemical detector strips, is described. The microsystem consists of a planar screen-printed carbon line electrode mounted perpendicular to the flow direction. Such coupling obviates the need for permanent attachment of the detector, to allow easy and fast replacement of the working electrode. Variables influencing the separation efficiency and amperometric response, including the channel-electrode spacing, separation voltage, or detection potential, are assessed and optimized. The versatility, simplicity, and low-cost advantages of the new design are coupled to an attractive performance, with submicromolar detection limits, and good precision. Applicability for assays of mixtures of nitroaromatic explosives or catecholamines is demonstrated. Such use of screen-printed detectors should also benefit conventional CE systems, particularly in applications requiring a frequent replacement of the working electrode.
Resistance inducers are a class of agrochemicals, including benzothiadiazole and chitosan, which activate the plant own defence mechanisms. In this work, open-field treatments with plant activators were performed on two red grape (Vitis vinifera L.) varieties cultivated in different localities, Groppello (Brescia, Lombardia, Italy) and Merlot (Treviso, Veneto, Italy). Treatments were carried out every 10 days until the véraison and, after harvesting, experimental wines (microvinificates) were prepared. In general, both melatonin and total polyphenol content, determined by mass spectrometry and Folin-Ciocalteu assay, respectively, were higher in wines produced from grapes treated with resistance inducers than in those obtained from untreated control and conventional fungicide-treated grapes. Accordingly, antiradical power of wines derived from plant activator-treated grapes, measured by both DPPH (2,2-diphenyl-1-picrylhydrazyl) and the ABTS [(2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)] radical-scavenging assay, was higher than in their counterparts. To the best of our knowledge, this is the first report on the effects of agrochemicals on the melatonin content of red wine.
Melatonin is a neurohormone produced by the pineal gland of animals. Serotonin is a monoamine neurotransmitter and one of the precursors of melatonin biosynthesis. These two indoleamines have recently been reported to have widespread occurrence in many edible plants. Consuming foodstuffs containing melatonin and serotonin could raise their physiologic concentrations in blood and enhance human health. Literature concerning analytical methods suitable for determination of melatonin and serotonin in edible plants is limited, although several liquid chromatographic (LC) techniques have been used for their quantification. Liquid chromatography-mass spectrometry (LC-MS) methods combine selectivity, sensitivity, and high precision, and enable the simultaneous determination of melatonin and serotonin. This work reviews LC and LC-MS techniques used to determine melatonin and serotonin, and the available data on melatonin and serotonin levels in edible plants.
Tryptophan (TRP), the precursor of the scavenger or immunomodulator molecules melatonin (MLT) and picolinic acid, can be found in the diet; and could be an alternative nutritional supplement used to regulate the immune response in the generation of free radicals. In an experimental model, the systemic administration of lipopolysaccharide (LPS), to promote the synthesis of pro-inflammatory cytokines, reactive oxygen species, and antioxidant enzymes, was performed on adult female, pregnant and lactating rats fed with a diet of TRP content (0.5mg/100g protein). Lung tissue was evaluated for levels of the products of lipoperoxidation (LPO's), malonaldehyde (MDA) and 4-hydroxy alkenals (4-HDA); nitrites (NO2), glutathione peroxidase (Gpx) enzyme activity, and the serum concentration of interferon-gamma (IFN-gamma), which were measured in the following groups: control (CTRL), LPS, MLT, TRP, LPS plus MLT (LPS+MLT), and LPS plus TRP (LPS+TRP). Results showed that the lung tissue levels of MDA and 4-HDA in the LPS+TRP group were significantly lower than in the TRP group. Statistically significant differences were not observed in nitric oxide levels among the groups LPS+MLT and LPS+TRP compared to the group under endotoxic shock (LPS). The Gpx enzyme activity was modified in the LPS+MLT vs the LPS group, but the difference was not statistically significant. The LPS+MLT group showed a smaller serum concentration (98%) of IFN-gamma than the LPS group. Statistically significant differences were not observed among the animals of the LPS+TRP and the LPS groups.
The different approaches for constructing nanomaterial-based detectors for conventional CE and microchip electrophoresis are described in this review. They include three main types of nanomaterials, including carbon nanotubes, nanoparticles, and nanorods in various designs. The fundamental reasons for the enhanced detection performance of nanomaterial-based detectors, such as higher sensitivity, improved limits of detection, and higher peak capacity, are discussed in detail. Various applications for biomedical, food, and environmental analyses are reviewed.
In this paper we show a simple isocratic chromatographic method for the detection of serotonin and its precursors and metabolites from various types of gastrointestinal tissue. The paper measures for the first time basal measurements of melatonin in the gastrointestinal tract, which has recently been shown to be released from the musosal lining of the gut. Tissue samples were stable following sample preparation in either 0.1 m perchloric acid or mobile phase. Analysis was carried out using a mobile phase consisting of 10% acetonitrile-90% acetate acid buffer pH 4.0 with 2 mm decane-sulfonic acid sodium salt at a column temperature of 50 degrees C. Electrochemical detection was utilized at a potential of +850 mV vs Ag/AgCl reference electrode at 10 microA full-scale deflection. The detection limit of 5-HT and melatonin was 241 and 308 nm respectively for a 10 microL injection. As a result of the method optimization, total analysis was reduced to 30 min. Accurate responses of the tissue samples following sample preparation could be obtained following a week after storage at -80 degrees C. This method is capable of preparing and analysing of samples from all regions of the gastrointestinal tract.
This paper examines for the first time the analytical possibilities of fast and simultaneous detection of prominent natural antioxidants including examples of flavonoids and vitamins using a CE microchip with electrochemical detection (ED). Unpinched injection conditions, zone electrophoretic separation and amperometric detection were carefully assayed and optimised. Analysis involved the zone electrophoretic separation of arbutin, (+)-catechin and ascorbic acid in less than 4 min using a borate buffer (pH 9.0, 50 mM), employing 2 kV as the separation voltage and +1.0 V as the detection potential. In addition, the separation of different 'couples' of natural antioxidants of food significance including (+)-catechin and ascorbic acid, (+)-catechin and rutin, as well as arbutin and phlorizdin is proposed. To demonstrate the potential and future role of CE microsystems, analytical possibilities and a new route in the raw sample analysis are presented. The preliminary results obtained allow the proposal of CE-ED microchips as a real gateway to microanalysis in foods.
A micellar electrokinetic chromatography method has been developed for simultaneous determination of melatonin and its precursors and metabolites. A 20 mM borate buffer pH 9.5 with 50 mM SDS served as the electrolyte. Tryptophan, 5-methoxyindoleacetic acid, 6-hydroxymelatonin, melatonin, serotonin, and 5-methoxytryptamine were baseline separated in less than 13 min. The limits of detection for UV detection and fluorometric detection based on native fluorescence of analytes were at the sub-ppm level. The proposed method with UV detection was applied to melatonin content control in pharmaceutical tablets with a precision expressed as RSD (n = 7) = 1.6%. For biological samples extraction with chloroform and ethyl acetate was examined. With ethyl acetate and chloroform recoveries of 87.2% and 82.1% melatonin, respectively, were obtained from plasma samples. The recovery of melatonin from spiked urine samples was 80.0% for ethyl acetate and 82.5% for chloroform. Fluorometric detection provides about two-fold improvement over UV in the detection of melatonin and minor improvements for three other analytes, but is much poorer than UV for tryptophan and 6-hydroxymelatonin in applied conditions.
Determination of melatonin (MT) (N-acetyl-5-methoxytryptamine) and related indole compounds using standard capillary electrophoresis (CE) system with UV detection was investigated. Satisfactory separations of six analytes i.e. l-tryptophan (l-TRP), 5-methoxyindoleacetic acid (5-MIAA), 6-hydroxymelatonin (6-HMT), MT, serotonin (SER) and 5-methoxytryptamine (5-MTRA) were performed employing micellar electrokinetic chromatography (MEKC). The optimal background electrolytes (BGE) used for separations were 20mM tetraborate buffer (pH 9.2) and 20mM phosphate buffer (pH 3.3) when employing techniques with normal and reverse migration of micelles, respectively. Fifty millimolar sodium dodecyl sulfate (SDS) was employed as the pseudostationary phase and voltage of +/-20kV was used throughout the investigation. On-line preconcentration techniques, stacking and sweeping, were applied in order to overcome high detection limits that are a serious drawback of CE with UV detection. A comparison of used techniques, concerning enhancement factors and limits of detection (LOD), is presented. Obtained results show that the use of stacking with reverse migrating micelles (SRMM) as one of preconcentration techniques allows obtaining the lowest estimated LODs for MT at the level of 30ng/mL with injection time of 99s at 0.5psi. Estimated LODs for other analytes in these conditions were, 21, 26 and 100ng/mL for l-TRP, 5-MIAA and 6-HMT, respectively. Signals of 5-MTRA and SER obtainable only with 10s injection allowed reaching estimated LODs of 62.5 and 130ng/mL, respectively. Analysis of spiked, diluted human serum was carried out as a preliminary application illustration of developed procedure.
Currently, the information available on the physiological functions of melatonin in higher plants is rather limited and the role of plant melatonin in human health remains undetermined. Research in this area has been slow due to lack of efficient analytical methods for rapid identification and quantification of the melatonin and related compounds in complex plant matrices. In this communication, we report the development of a rapid, accurate method for extraction, detection and quantification of plant melatonin, serotonin and indole-3-acetic acid (IAA) by Liquid chromatography-tandem mass spectrometry (LC-MS/MS) with electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure photoionization (APPI), respectively. The limit of detection (LOD) of melatonin in the plant extraction was 5 pg/ml and the limit of quantification (LOQ) was 0.02 ng/ml, as well as LOD for serotonin was 100 pg/ml and the LOQ was 5 ng/ml, LOD for IAA was 50 pg/ml and the LOQ was 0.7 ng/ml. There was a linear relationship between melatonin, serotonin, and IAA concentration and peak area over a quantifiable range of 0.02 ng/ml to 0.1 mg/ml, 5 ng/ml to 0.1 mg/ml, and 0.7 ng/ml to 0.1 mg/ml, respectively, in the plant extract.
MEKC was used in conjunction with end-column amperometric detection (AD) at a carbon disc electrode (0.3 mm diameter) for the selective and sensitive determination of melatonin and its five related indoleamines including its precursors and metabolites in the pineal gland. The introduction of a sample stacking technique in injection and the buffer additive SDS in the buffer solution system provided the rapid and sensitive analysis. Optimal buffer conditions (10 mmol/L phosphate containing 20 mmol/L SDS, pH 7.2), detection potential (+1.0 V vs. Ag/AgCl), and electrokinetic injection 10 s with the separation voltage of 24 kV were employed to achieve the baseline separation of six pineal hormones within 15 min. The peak currents and the analyte concentrations have a good linear relationship over the range of 6.0 x 10(-8) 6.0 x 10(-5 )mol/L. The detection limits for six pineal hormones by AD are 9.7 to 41.8 nmol/L (equal to 2.0 to 9.7 ng/mL) (S/N = 3), respectively. It is proved to provide about 30- to 250-fold improvement over UV, and be comparable with the sensitive fluorescence detection, which needs pre-column derivatization. The proposed method has been applied for analysis of melatonin and related indoleamines in rat pineal glands. A very simple sample pretreatment procedure, merely involving the homogenization step in perchloric acid, was enough to achieve recoveries in the range of 71 to 127% for all the analytes in the pineal gland.
Microfluidic devices using carbon nanotube (CNT) materials (single-walled and two multiwalled (MWCNT)) for the analysis of selected analyte groups of significance in foods such as dietary antioxidants, water-soluble vitamins, vanilla flavors, and isoflavones involved in representative food samples have been explored for the first time. Ultrafast separations resulted in well-defined and resolved peaks with enhanced voltammetric current in comparison with those obtained from unmodified screen-printed electrodes, turning MWCNT into an ideal material for electrochemical sensing in food analysis. Resolution was improved by a factor of 2, and sensitivity was dramatically enhanced with amplification factors toward calibration slopes from 4- to 16-fold. In both qualitative and quantitative domains, this impressive performance of CNTs integrated on microfluidics allowed solving specific challenges in food environments such as the direct detection of analytes in complex natural samples and unambiguous analytes in the control of fraud, which was not possible on nonmodified surfaces, avoiding the integration of complex preconcentration steps on these microdevices. The use of these unique materials in microfluidics for food analysis has opened new expectations in "lab-on-a-chip" domains.
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