Clinical implications and electrochemical biosensing of monoamine neurotransmitters in body fluids, in vitro, in vivo, and ex vivo models

Shifting paradigm in electrochemical biosensing matrices comprising metal organic frameworks and their composites in disease diagnosis

Metal Organic Frameworks (MOFs) are an evolving category of crystalline microporous materials that have grabbed the research interest for quite some time due to their admirable physio-chemical properties and easy fabrication methods. Their enormous surface area can be a working ground for innumerable molecular adhesions and site for potential sensor matrices. They have been explored in the last decade for incorporation in electrochemical sensor matrices as diagnostic solutions for a plethora of diseases. This review emphasizes on some of the recent advancements in the area of MOF-based electrochemical biosensors with focus on various important diseases and their significance in upgrading the sensor performance. It summarizes MOF-based biosensors for monitoring biomarkers relevant to diabetes, viral and bacterial sepsis infections, neurological disorders, cardiovascular diseases, and cancer in a wide range of real matrices. The discussion has been supplemented with extensive tables elaborating recent trends in the field of MOF-composite probe fabrication strategies with their respective sensing parameters. The article sums up the future scope of these materials in the field of biosensors and enlightens the reader with recent trends for future research scope. This article is categorized under: Diagnostic Tools > Biosensing Diagnostic Tools > Diagnostic Nanodevices.

Disposable paper electrodes for detection of changes in dopamine concentrations in rat brain homogenates

Dopamine, the main catecholamine neurotransmitter plays an important role in renal, cardiovascular, central nervous systems, and pathophysiological processes. The abnormal dopamine levels can result in neurological disorders such as Parkinson's, Alzheimer's, schizophrenia, acute anxiety, neuroblastoma and also contribute to cognitive dysfunctions. Given the widespread importance of dopamine concentration levels, it is imperative to develop sensors that are able to monitor dopamine. Herein, we have developed pre-anodized disposable paper electrode modified with 1-pyrenebutyric acid, for the selective and sensitive determination of dopamine. The sensor was characterized with Fourier transform infrared spectroscopy, Energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and electrochemical techniques for addressing the robust formation and electrochemical activity. The modified electrode exhibited excellent electrocatalytic activity towards dopamine without the common interference from ascorbic acid. The calibration plot for the dopamine sensor resulted linear range from 0.003 μM to 0.5 μM with a detection limit of 0.11 nM. The sensor's potential utility was tested by monitoring dopamine concentration changes in rat brain homogenates when subjected to neurotoxicity. The developed sensor was validated with gold-standard UV-Vis spectroscopy studies and computational studies were performed to understand the interaction between 1-pyrenebutyric acid and dopamine.

All-MXene electronic tongue for neurotransmitters detection

Neurotransmitters (NTs) are molecules produced by neurons that act as the body's chemical messengers. Their abnormal levels in the human system have been associated with many disorders and neurodegenerative diseases, which makes the monitoring of NTs fundamentally important. Specifically for clinical analysis and understanding of brain behavior, simultaneous detection of NTs at low levels quickly and reliably is imperative for disease prevention and early diagnosis. However, the methods currently employed are usually invasive or inappropriate for multiple NTs detection. Herein, we developed a MXene-based impedimetric electronic tongue (e-tongue) for sensitive NT monitoring, using Nb2C, Nb4C3, Mo2C, and Mo2Ti2C3 MXenes as sensing units of the e-tongue, and Principal Component Analysis (PCA) as the data treatment method. The high specific surface area, distinct electrical properties, and chemical stability of the MXenes gave rise to high sensitivity and good reproducibility of the sensor array toward NT detection. Specifically, the e-tongue detected and differentiated multiple NTs (acetylcholine, dopamine, glycine, glutamate, histamine, and tyrosine) at concentrations as low as 1 nmol L-1 and quantified NTs present in a mixture. Besides, analyses performed with interferents and actual samples confirmed the system's potential to be used in clinical diagnostics. The results demonstrate that the MXene-based e-tongue is a suitable, rapid, and simple method for NT monitoring with high accuracy and sensitivity.

Fabrication of Multiple-Channel Electrochemical Microneedle Electrode Array via Separated Functionalization and Assembly Method

Real-time monitoring of physiological indicators inside the body is pivotal for contemporary diagnostics and treatments. Implantable electrodes can not only track specific biomarkers but also facilitate therapeutic interventions. By modifying biometric components, implantable electrodes enable in situ metabolite detection in living tissues, notably beneficial in invasive glucose monitoring, which effectively alleviates the self-blood-glucose-managing burden for patients. However, the development of implantable electrochemical electrodes, especially multi-channel sensing devices, still faces challenges: (1) The complexity of direct preparation hinders functionalized or multi-parameter sensing on a small scale. (2) The fine structure of individual electrodes results in low spatial resolution for sensor functionalization. (3) There is limited conductivity due to simple device structures and weakly conductive electrode materials (such as silicon or polymers). To address these challenges, we developed multiple-channel electrochemical microneedle electrode arrays (MCEMEAs) via a separated functionalization and assembly process. Two-dimensional microneedle (2dMN)-based and one-dimensional microneedle (1dMN)-based electrodes were prepared by laser patterning, which were then modified as sensing electrodes by electrochemical deposition and glucose oxidase decoration to achieve separated functionalization and reduce mutual interference. The electrodes were then assembled into 2dMN- and 1dMN-based multi-channel electrochemical arrays (MCEAs), respectively, to avoid damaging functionalized coatings. In vitro and in vivo results demonstrated that the as-prepared MCEAs exhibit excellent transdermal capability, detection sensitivity, selectivity, and reproducibility, which was capable of real-time, in situ glucose concentration monitoring.

Chitosan stabilized copper iodide nanoparticles enabled nano-bio-engineered platform for efficient electrochemical biosensing of dopamine

Neurodegenerative disorders are one of the significant challenges to the aging society, as per the United Nations, where 1 in 6 people globally over 65 years of age are expected to suffer by 2050. The exact pathophysiological root of these disorders is although not known adequately, but reduced dopamine (most significant neurotransmitters) levels have been reported in people affected by Parkinson's disease. Sensitive detection and effective monitoring of dopamine can help to diagnose these neurodegenerative disorders at a very early stage, which will help to properly treat these disorders and slow down their progression. Therefore, it is crucial to detect physiological and clinically acceptable amounts of dopamine with high sensitivity and selectivity in basic pathophysiology research, medication, and illness diagnosis. Here in this present investigation, nano-bio-engineered stable chitosan stabilized copper iodide nanoparticles (CS@CuI NPs) were synthesized to engineer the active biosensing platform for developing dopamine biosensors. Initially, the as-synthesized nano-bio-engineered CS@CuI NPs were subjected to its drop-casting onto an Indium tin oxide (ITO) conducting glass substrate. This substrate platform was then utilized to immobilize tyrosinase (Tyr) enzyme by drop-casting to fabricate Tyr/CS@CuI NPs/ITO bioelectrode for the ultrasensitive determination of dopamine. Several techniques were used to characterize the structural, optical, and morphological properties of the synthesized CS@CuI NPs and Tyr/CS@CuI NPs/ITO bioelectrode. Further, the as-prepared bioelectrode was evaluated for its suitability and electrocatalytic behaviour towards dopamine by cyclic voltammetry. A perusal of the electroanalytic results of the fabricated biosensor revealed that under the optimized experimental conditions, Tyr/CS@CuI NPs/ITO bioelectrode exhibits a very high electrochemical sensitivity of 11.64 μA μM-1 cm-2 towards dopamine with the low limit of detection and quantification of 0.02 and 0.386 μM, respectively. In addition, the fabricated bioelectrode was stable up to 46 days with only 4.82 % current loss, reusable till 20 scans, and it also performed effectively while real sample analysis. Therefore, the nano-bio-engineered biosensor platform being reported can determine deficient dopamine levels in a very selective and sensitive manner, which can help adequately manage neurodegenerative disorders, further slowing down the disease progression.

Acetylcholinesterase biosensors for electrochemical detection of neurotoxic pesticides and acetylcholine neurotransmitter: A literature review

Neurotoxic pesticides are a group of chemicals that pose a severe threat to both human health and the environment. These molecules are also known to accumulate in the food chain and persist in the environment, which can lead to long-term exposure and adverse effects on non-target organisms. The detrimental effects of these pesticides on neurotransmitter levels and function can lead to a range of neurological and behavioral symptoms, which are closely associated with neurodegenerative diseases. Hence, the accurate and reliable detection of these neurotoxic pesticides and associated neurotransmitters is essential for clinical applications, such as diagnosis and treatment. Over the past few decades, acetylcholinesterase (AchE) biosensors have emerged as a sensitive and reliable tool for the electrochemical detection of neurotoxic pesticides and acetylcholine. These biosensors can be tailored to utilize the high specificity and sensitivity of AchE, enabling the detection of these chemicals. Additionally, enzyme immobilization and the incorporation of nanoparticles have further improved the detection capabilities of these biosensors. AchE biosensors have shown tremendous potential in various fields, including environmental monitoring, clinical diagnosis, and pesticide residue analysis. This review summarizes the advancements in AchE biosensors for electrochemical detection of neurotoxic pesticides and acetylcholine over the past two decades.

Advances in MXene-Based Electrochemical (Bio)Sensors for Neurotransmitter Detection

Neurotransmitters are chemical messengers that play an important role in the nervous system's control of the body's physiological state and behaviour. Abnormal levels of neurotransmitters are closely associated with some mental disorders. Therefore, accurate analysis of neurotransmitters is of great clinical importance. Electrochemical sensors have shown bright application prospects in the detection of neurotransmitters. In recent years, MXene has been increasingly used to prepare electrode materials for fabricating electrochemical neurotransmitter sensors due to its excellent physicochemical properties. This paper systematically introduces the advances in MXene-based electrochemical (bio)sensors for the detection of neurotransmitters (including dopamine, serotonin, epinephrine, norepinephrine, tyrosine, NO, and H₂S), with a focus on their strategies for improving the electrochemical properties of MXene-based electrode materials, and provides the current challenges and future prospects for MXene-based electrochemical neurotransmitter sensors.

Polysaccharides for Medical Technology: Properties and Applications

Over the past decade, the use of polysaccharides has gained tremendous attention in the field of medical technology. They have been applied in various sectors such as tissue engineering, drug delivery system, face mask, and bio-sensing. This review article provides an overview and background of polysaccharides for biomedical uses. Different types of polysaccharides, for example, cellulose and its derivatives, chitin and chitosan, hyaluronic acid, alginate, and pectin are presented. They are fabricated in various forms such as hydrogels, nanoparticles, membranes, and as porous mediums. Successful development and improvement of polysaccharide-based materials will effectively help users to enhance their quality of personal health, decrease cost, and eventually increase the quality of life with respect to sustainability.

Gold nanostar and graphitic carbon nitride nanocomposite for serotonin detection in biological fluids and human embryonic kidney cell microenvironment

A nanosensor comprising of gold nanostars (Au-Nstars)-graphitic carbon nitride (g-C₃N₄) nanocomposite layered on a glassy carbon electrode (GCE) to detect serotonin (ST) in various body fluids has been fabricated. The nanocomposite and the sensing platform have been thoroughly characterized with UV-visible spectroscopy (UV-vis), transmission electron microscopy (TEM), selected area electron diffraction (SAED), energy dispersive X-ray photoelectron spectroscopy (EDX), and electrochemical techniques such as cyclic voltammetry (CV), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS). The designed ST detection probe has achieved a linear dynamic range (LDR) in the range 5 × 10^-7 and 1 × 10^-3 M with a limit of detection (LOD) of 15.1 nM (RSD < 3.3%). The ST detection capability of the fabricated sensor ranges between the normal and several abnormal pathophysiological situations. The sensor effectively detects ST in real matrices such as urine and blood serum, thus, showing its direct diagnostic applicability. Additionally, the sensor has been tested in the microenvironment of human embryonic kidney (HEK) cells to assess the possibility of ST secretion in cell lines. Interferences because of co-existing molecules have been evaluated, and the shelf-life of the fabricated sensor has been obtained as 8 weeks.

Design and Fabrication of α-MnO2-Nanorods-Modified Glassy-Carbon-Electrode-Based Serotonin Sensor

Serotonin is a very important monoamine neurotransmitter, which takes part in biological and psychological processes. In the present scenario, design and fabrication of a serotonin electrochemical sensor is of great significance. In this study, we have synthesized α-MnO₂ via a hydrothermal synthesis method using potassium permanganate as a precursor. The physiochemical properties, such as structural and phase-purity of the prepared α-MnO₂, were investigated by various characterization techniques and methods (powder X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy). Furthermore, the serotonin sensor was fabricated using α-MnO₂ as an electrode modifier or electro-catalyst. The bare glassy carbon electrode (GCE) was adopted as a working substrate, and its active carbon surface was modified with the synthesized α-MnO₂. This modified GCE (α-MnO₂/GCE = MGCE) was explored as a serotonin sensor. The electrochemical investigations showed that the MGCE has excellent electro-catalytic properties towards determination of serotonin. The MGCE exhibits an excellent detection limit (DL) of 0.14 µM, along with good sensitivity of 2.41 µAµM^-1 cm^-2. The MGCE also demonstrated excellent selectivity for determination of serotonin in the presence of various electro-active/interfering molecules. The MGCE also exhibits good cyclic repeatability, stability, and storage stability.

Design of optoelectrodes for the remote imaging of cells and in situ electrochemical detection of neurosecretory events

Optical fibers have opened avenues for remote imaging, bioanalyses and recently optogenetics. Besides, miniaturized electrochemical sensors have offered new opportunities in sensing directly redox neurotransmitters. The combination of both optical and electrochemical approaches was usually performed on the platform of microscopes or within microsystems. In this work, we developed optoelectrodes which features merge the advantages of both optical fibers and microelectrodes. Optical fiber bundles were modified at one of their extremity by a transparent ITO deposit. The electrochemical responses of these ITO-modified bundles were characterized for the detection of dopamine, epinephrine and norepinephrine. The analytical performances of the optoelectrodes were equivalent to the ones reported for carbon microelectrodes. The remote imaging of model neurosecretory PC12 cells by optoelectrodes was performed upon cell-staining with common fluorescent dyes: acridine orange and calcein-AM. An optoelectrode placed by micromanipulation at a few micrometers-distance from the cells offered remote images with single cell resolution. Finally, in situ electrochemical sensing was demonstrated by additions of K⁺-secretagogue solutions near PC12 cells under observation, leading to exocytotic events detected as amperometric spikes at the ITO surface. Such dual sensors should pave the way for in vivo remote imaging, optogenetic stimulation, and simultaneous detection of neurosecretory activities.

Advances in Metal-Organic Framework (MOFs) based biosensors for diagnosis: An update

Metal-organic frameworks (MOFs) have significant advantages over other candidate classes of chemo-sensory materials owing to their extraordinary structural tunability and characteristics. MOF-based biosensing is a simple, and convenient method for identifying various species. Biomarkers are molecular or cellular processes that link environmental exposure to a health outcome. Biomarkers are important in understanding the links between environmental chemical exposure and the development of chronic diseases, as well as in identifying disease-prone subgroups. Until now, several species, including nanoparticles (NPs) and their nanocomposites, small molecules, and unique complex systems, have been used for the chemical sensing of biomarkers. Following the overview of the field, we discussed the various fabrication methods for MOFs development in this review. We provide a thorough overview of the previous five years of progress to broaden the scope of analytes for future research. Several enzymatic and non-enzymatic sensors are offered, together with a mandatory measuring method that includes detection range and dynamic range. In addition, we reviewed the comparison of enzymatic and non-enzymatic biosensors, inventive edges, and the difficulties that need to be solved. This work might open up new possibilities for material production, sensor development, medical diagnostics, and other sensing fields.

Synthesis of Novel One-Walled meso-Phenylboronic Acid-Functionalized Calix[4]pyrrole: A Highly Sensitive Electrochemical Sensor for Dopamine

Facile access to new one-walled meso-substituted phenylboronic acid-functionalized calix[4]pyrrole (C4P) has been revealed for the first time, starting from cost-effective and easily accessible materials. The structures of both the intermediate dipyrromethane (DPM) and the targeted functionalized C4P have been confirmed by means of ¹H-NMR, ¹³C-NMR, IR, and HRMS spectral data. The voltammetric investigations of the functionalized C4P films cast over a glassy carbon electrode (C4P-GCE) clearly establish the redox stability and redox accessibility of the boronic acid functional moiety present in the C4P framework. We demonstrate that the presence of the unique boronic acid functionality in the C4P endows it with an excellent potential for the highly sensitive electrochemical sensing of the neurotransmitter dopamine (DA). A linear correlation between the strength of the Faradaic signals corresponding to the electro-oxidation of DA over C4P-GCE and the concentration of DA was observed in a concentration range as wide as 0.165-2.302 μM. The C4P-GCE has revealed exceptional stability and reproducibility in the electrochemical sensing of DA, with a nanomolar level limit of detection as low as 15 nM.

Marine Biological Macromolecules as Matrix Material for Biosensor fabrication

The Ocean covers two-third of our planet and has great biological heterogeneity. Marine organisms like algae, vertebrates, invertebrates, and microbes are known to provide many natural products with biological activities as well as potent sources of biomaterials for therapeutic, biomedical, biosensors, and climate stabilization. Over the years, the field of biosensors have gained huge attention due to their extraordinary ability to provide early disease diagnosis, rapid detection of various molecules and substances along with long term monitoring. This review aims to focus on the properties and employment of various biomaterials (Carbohydrate polymers, proteins, polyacids etc) of marine origin such as Alginate, Chitin, Chitosan, Fucoidan, Carrageenan, Chondroitin Sulfate (CS), Hyaluronic acid (HA), Collagen, marine pigments, marine nanoparticles, Hydroxyapatite (HAp), Biosilica, lectins, and marine whole cell in the design and development of biosensors. Further, this review also covers the source of such marine biomaterials and their promising evolution in the fabrication of biosensors that are potent to be employed in the biomedical, environmental science and agricultural sciences domains. The use of such fabricated biosensors harness the system with excellent specificity, selectivity, biocompatibility, thermally stable and minimal cost advantages. This article is protected by copyright. All rights reserved.

Ultrasensitive Aptasensors for the Detection of Viruses Based on Opto-Electrochemical Readout Systems

Viral infections are becoming the foremost driver of morbidity, mortality and economic loss all around the world. Treatment for diseases associated to some deadly viruses are challenging tasks, due to lack of infrastructure, finance and availability of rapid, accurate and easy-to-use detection methods or devices. The emergence of biosensors has proven to be a success in the field of diagnosis to overcome the challenges associated with traditional methods. Furthermore, the incorporation of aptamers as bio-recognition elements in the design of biosensors has paved a way towards rapid, cost-effective, and specific detection devices which are insensitive to changes in the environment. In the last decade, aptamers have emerged to be suitable and efficient biorecognition elements for the detection of different kinds of analytes, such as metal ions, small and macro molecules, and even cells. The signal generation in the detection process depends on different parameters; one such parameter is whether the labelled molecule is incorporated or not for monitoring the sensing process. Based on the labelling, biosensors are classified as label or label-free; both have their significant advantages and disadvantages. Here, we have primarily reviewed the advantages for using aptamers in the transduction system of sensing devices. Furthermore, the labelled and label-free opto-electrochemical aptasensors for the detection of various kinds of viruses have been discussed. Moreover, numerous globally developed aptasensors for the sensing of different types of viruses have been illustrated and explained in tabulated form.

Breaking the barrier to biomolecule limit-of-detection via 3D printed multi-length-scale graphene-coated electrodes

Sensing of clinically relevant biomolecules such as neurotransmitters at low concentrations can enable an early detection and treatment of a range of diseases. Several nanostructures are being explored by researchers to detect biomolecules at sensitivities beyond the picomolar range. It is recognized, however, that nanostructuring of surfaces alone is not sufficient to enhance sensor sensitivities down to the femtomolar level. In this paper, we break this barrier/limit by introducing a sensing platform that uses a multi-length-scale electrode architecture consisting of 3D printed silver micropillars decorated with graphene nanoflakes and use it to demonstrate the detection of dopamine at a limit-of-detection of 500 attomoles. The graphene provides a high surface area at nanoscale, while micropillar array accelerates the interaction of diffusing analyte molecules with the electrode at low concentrations. The hierarchical electrode architecture introduced in this work opens the possibility of detecting biomolecules at ultralow concentrations.

Carbon fiber microelectrode array loaded with the diazonium salt-single-walled carbon nanotubes composites for the simultaneous monitoring of dopamine and serotonin in vivo

Carbon fiber microelectrode arrays based on diazonium salt and single-walled carbon nanotubes composites (DS-SWCNT/CFMEA) have been fabricated, and it developed for the simultaneous monitoring of dopamine (DA) and serotonin (5-HT) with differential pulse voltammary (DPV). The diazonium salt can improve the water-solubility of single-walled carbon nanotubes and show good selectivity to DA, thus DS-SWCNT/CFMEA exhibits enhanced electrocatalytic activity for the oxidation of DA and 5-HT, and well antifouling ability to the other biomolecules. Moreover, DS-SWCNT/CFMEA shows the wider liner range, and the good performance of precision, reproducibility and biocompatibility. The excellent characteristics of the prepared microsensor array make it to be used to monitor the release of DA and 5-HT in the mouse brain striatum of different group over time. Meanwhile, the results of in vivo on line assay further confirmed the pharmacological effects of Uncaria alkaloid extract solution on DA and 5-HT. This research may provide a new method for monitoring the release of neurobiomolecules, and the microsensor array are expected to be a tool for the study of pharmacological and physiological processes on line in vivo.

Carbon-Based Electrochemical Sensors for In Vivo and In Vitro Neurotransmitter Detection

As essential neurological chemical messengers, neurotransmitters play an integral role in the maintenance of normal mammalian physiology. Aberrant neurotransmitter activity is associated with a range of neurological conditions including Parkinson's disease, Alzheimer's disease, and Huntington's disease. Many studies to date have tested different approaches to detecting neurotransmitters, yet the detection of these materials within the brain, due to the complex environment of the brain and the rapid metabolism of neurotransmitters, remains challenging and an area of active research. There is a clear need for the development of novel neurotransmitter sensing technologies capable of rapidly and sensitively monitoring specific analytes within the brain without adversely impacting the local microenvironment in which they are implanted. Owing to their excellent sensitivity, portability, ease-of-use, amenability to microprocessing, and low cost, electrochemical sensors methods have been widely studied in the context of neurotransmitter monitoring. The present review, thus, surveys current progress in this research field, discussing developed electrochemical neurotransmitter sensors capable of detecting dopamine (DA), serotonin (5-HT), acetylcholine (Ach), glutamate (Glu), nitric oxide (NO), adenosine (ADO), and so on. Of these technologies, those based on carbon nanostructures-modified electrodes including carbon nanotubes (CNTs), graphene (GR), gaphdiyne (GDY), carbon nanofibers (CNFs), and derivatives thereof hold particular promise owing to their excellent biocompatibility and electrocatalytic performance. The continued development of these and related technologies is, thus, likely to lead to major advances in the clinical diagnosis of neurological diseases and the detection of novel biomarkers thereof.

Selective Separation and Analysis of Catecholamines in Urine Based on Magnetic Solid Phase Extraction by Mercaptophenylboronic Acid Functionalized Fe3O4-NH2@Au Magnetic Nanoparticles Coupled with HPLC

A novel magnetic solid phase extraction based on mercaptophenylboronic acid (MPBA)-functionalized Fe3O4-NH2@Au nanomaterial (Fe3O4-NH2@Au-MPBA) was developed for selective separation and enrichment of catecholamines (including dopamine, norepinephrine and adrenaline). Fe3O4-NH2@Au-MPBA nanoparticles were achieved by self-assembly-anchoring MPBA molecules on the surface of Fe3O4-NH2@Au nanocomposites, which were synthesized via a facial ultrasonic auxiliary in situ reduction process. The interaction between cis-diol from catecholamines and boronic acid was reversible and could be flexibly controlled by adjusting pH value. The catecholamines could be quickly adsorbed by Fe3O4-NH2@Au-MPBA in weak alkaline solution (pH 8.0–9.0) and subsequently released in acid solution (pH 1.0–2.0). The process of adsorption and dissociation was very fast. Furthermore, the three catecholamines could be detected in urine from children by high performance liquid chromatography (HPLC) with electrochemical detector. Under optimal conditions, norepinephrine (NE), epinephrine (EP) and dopamine (DA) were separated very well from internal standard and exhibited a good linearity in the range of 2.5–500.0 ng mL−1, with correlation coefficients of r2 > 0.9907. Limits of detection (LOD) (signal to noise = 3) were 0.39, 0.27 and 0.60 ng mL−1 for NE, EP and DA, respectively. Recoveries for the spiked catecholamines were in the range of 85.4–105.2% with the relative standard deviation (RSD) < 11.5%.

Microscope-FTIR Spectrometry Based Sensor for Neurotransmitters Detection

In this work, we present a new sensing approach for aqueous samples based on the microscope-FTIR spectrometer and applied for neurotransmitters. Our contribution in this work consists of a new sample handling system for the microscope-FTIR spectrometer based on a total reflective mirror, a heated hydrophobic layer for solvent removal/evaporation and sample confinement and a microfluidic system that handles sample injection unlike standard sample handling system which was based only on a total reflective mirror. In addition, another part of our contribution consists of proposing a new algorithm to extract molecular composition of the solution with high estimation ratios and based on the analysis of detected peaks on IR spectra. The data acquired from the microscope-FTIR spectrometer was analyzed by a newly developed algorithm to identify each neurotransmitter in homogeneous and non-homogeneous solutions with high selectivity. We used six neurotransmitter molecules (Dopamine hydrochloride, L-Ascorbic acid, Acetylcholine chloride, y-Aminobutyric, Glycine and L-Glutamic acid). The results obtained based on the algorithm developed showed that, using the new system, the six neurotransmitters can be identified in homogeneous and mixture solutions with an estimation ratio range of 88.8%-100% for Dopamine hydrochloride, 80%-100% for L-Ascorbic acid, 75%-100% for Acetylcholine chloride, 75%-100% for L-Glutamic, 77.7%-100% for y-Aminobutyric and 75%-100% for Glycine.

A Critical Review of Carbon Quantum Dots: From Synthesis toward Applications in Electrochemical Biosensors for the Determination of a Depression-Related Neurotransmitter

Depression has become the leading cause of disability worldwide and is a global health burden. Quantitative assessment of depression-related neurotransmitter concentrations in human fluids is highly desirable for diagnosis, monitoring disease, and therapeutic interventions of depression. In this review, we focused on the latest strategies of CD-based electrochemical biosensors for detecting a depression-related neurotransmitter. We began this review with an overview of the microstructure, optical properties and cytotoxicity of CDs. Next, we introduced the development of synthetic methods of CDs, including the "Top-down" route and "Bottom-up" route. Finally, we highlighted detecting an application of CD-based electrochemical sensors in a depression-related neurotransmitter. Moreover, challenges and future perspectives on the recent progress of CD-based electrochemical sensors in depression-related neurotransmitter detection were discussed.

Bimodal neural probe for highly co-localized chemical and electrical monitoring of neural activities in vivo

Investigation of the chemical and electrical signals of cells in vivo is critical for studying functional connectivity and brain diseases. Most previous studies have observed either the electrical signals or the chemical signals of cells because recording electrical signals and neurochemicals are done by fundamentally different methods. Herein, we present a bimodal MEMS neural probe that is monolithically integrated with an array of microelectrodes for recording electrical activity, microfluidic channels for sampling extracellular fluid, and a microfluidic interface chip for multiple drug delivery and sample isolation from the localized region at the cellular level. In this work, we successfully demonstrated the functionality of our probe by monitoring and modulating bimodal (electrical and chemical) neural activities through the delivery of chemicals in a co-localized brain region in vivo. We expect our bimodal probe to provide opportunities for a variety of in-depth studies of brain functions as well as for the investigation of neural circuits related to brain diseases.

Determination and Application of Nineteen Monoamines in the Gut Microbiota Targeting Phenylalanine, Tryptophan, and Glutamic Acid Metabolic Pathways

It has been reported that monoamine neurotransmitters can be produced by gut microbiota, and that several related metabolites of amino acids in these pathways are associated with nervous system (NVS) diseases. Herein, we focused on three pathways, namely, phenylalanine (Phe), tryptophan (Trp), and glutamic acid (Glu), and established an underivatized liquid chromatography–tandem mass spectrometry (LC-MS/MS) method for the quantification of nineteen monoamine neurotransmitters and related metabolites in the gut microbiota. The neurotransmitters and related metabolites included Phe, tyrosine (Tyr), l-dopa (Dopa), dopamine (DA), 3-methoxytyramine, Trp, hydroxytryptophan, 5-hydroxytryptamine (5-HT), 5-hydroxyindole-3-acetic acid (5-HIAA), kynurenine (KN), kynurenic acid (KYNA), melatonin, tryptamine (TA), indole-3-lactic acid (ILA), indole-3-acetic acid (IAA), indolyl-3-propionic acid (IPA), Glu, gamma-aminobutyric acid (GABA), and acetylcholine (Ach). A fluoro-phenyl bonded column was used for separation, and the mobile phase consisted of methanol:acetonitrile (1:1) and water, with 0.2% formic acid in both phases. The compounds exhibited symmetric peak shapes and sufficient sensitivity under a total analysis time of 8.5 min. The method was fully validated with acceptable linearity, accuracy, precision, matrix effect, extraction recovery, and stability. The results showed that neurotransmitters, such as Dopa, DA, 5-HT, GABA, and Ach, were present in the gut microbiota. The metabolic pathway of Trp was disordered under depression, with lower levels of 5-HT, 5-HIAA, KN, KYNA, TA, ILA, IAA, IPA, and Glu, and a higher ratio of KYNA/KN. In addition, some first-line NVS drugs, such as sertraline, imipramine, and chlorpromazine, showed regulatory potential on these pathways in the gut microbiota.

Recent advances in enzymeless-based electrochemical sensors to diagnose neurodegenerative diseases

The use of sensitive electrochemical sensors to detect biomarkers is an effective method for the early diagnosis of several neurodegenerative diseases (NDs), such as Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, etc. However, the commercialization of enzyme/aptamer-based sensors is still hampered owing to the historic drawbacks of biorecognition elements including high cost, poor stability, and complex integration technology. Non-enzymatic electrochemical sensors are more attractive compared to their traditional counterparts and can be widely harnessed owing to their low cost, high stability, sensitivity, and ease of miniaturization. This review summarizes recent research progress focusing on the construction of non-enzymatic electrochemical sensors and analyzes their present use in the early diagnosis of NDs. Additionally, this review addresses the limitations and challenges of the use of current non-enzymatic electrochemical sensor technologies for the diagnosis of NDs and highlights the possible directions for future research.

Recent Advances in Electrochemical Sensors for the Detection of Biomolecules and Whole Cells

Electrochemical sensors are considered an auspicious tool to detect biomolecules (e.g., DNA, proteins, and lipids), which are valuable sources for the early diagnosis of diseases and disorders. Advances in electrochemical sensing platforms have enabled the development of a new type of biosensor, enabling label-free, non-destructive detection of viability, function, and the genetic signature of whole cells. Numerous studies have attempted to enhance both the sensitivity and selectivity of electrochemical sensors, which are the most critical parameters for assessing sensor performance. Various nanomaterials, including metal nanoparticles, carbon nanotubes, graphene and its derivatives, and metal oxide nanoparticles, have been used to improve the electrical conductivity and electrocatalytic properties of working electrodes, increasing sensor sensitivity. Further modifications have been implemented to advance sensor platform selectivity and biocompatibility using biomaterials such as antibodies, aptamers, extracellular matrix (ECM) proteins, and peptide composites. This paper summarizes recent electrochemical sensors designed to detect target biomolecules and animal cells (cancer cells and stem cells). We hope that this review will inspire researchers to increase their efforts to accelerate biosensor progress-enabling a prosperous future in regenerative medicine and the biomedical industry.

Comparison of electrochemical performance of various boron-doped diamond electrodes: Dopamine sensing in biomimicking media used for cell cultivation

Chemically inert and biocompatible boron-doped diamond (BDD) has been successfully used in neuroscience for sensitive neurochemicals sensing and/or as a growth substrate for neurons. In this study, several types of BDD differing in (i) fabrication route, i.e. conventional microwave plasma enhanced chemical vapour deposition (MW-PECVD) reactor vs. MW-PECVD with linear antenna delivery system, (ii) morphology, i.e. planar vs. porous BDD, and (iii) surface treatment, i.e. H-terminated (H-BDDs) vs. O-terminated (O-BDDs), were characterized from a morphological, structural, and electrochemical point of view. Further, planar and porous BDD-based electrodes were tested for sensing of dopamine in common biomimicking environments of pH 7.4, namely phosphate buffer (PB) and HEPES buffered saline (HBS). In HBS, potential windows are narrowed due to electrooxidation of its buffering component (i.e. HEPES), however, dopamine sensing in HBS is possible. H-BDDs (both planar and porous) outperformed O-BDDs as they provided clearer dopamine signals with higher peak currents. As expected, due to its enlarged surface area and increased sp² content, the highest sensitivity and lowest detection limits of 8 × 10^-8 mol L^-1 and 6 × 10^-8 mol L^-1 in PB and HBS media, respectively, were achieved by square-wave voltammetry on porous H-BDD.

Electrochemical Detection of Neurotransmitters

Neurotransmitters are important chemical messengers in the nervous system that play a crucial role in physiological and physical health. Abnormal levels of neurotransmitters have been correlated with physical, psychotic, and neurodegenerative diseases such as Alzheimer's, Parkinson's, dementia, addiction, depression, and schizophrenia. Although multiple neurotechnological approaches have been reported in the literature, the detection and monitoring of neurotransmitters in the brain remains a challenge and continues to garner significant attention. Neurotechnology that provides high-throughput, as well as fast and specific quantification of target analytes in the brain, without negatively impacting the implanted region is highly desired for the monitoring of the complex intercommunication of neurotransmitters. Therefore, it is crucial to develop clinical assessment techniques that are sensitive and reliable to monitor and modulate these chemical messengers and screen diseases. This review focuses on summarizing the current electrochemical measurement techniques that are capable of sensing neurotransmitters with high temporal resolution in real time. Advanced neurotransmitter sensing platforms that integrate nanomaterials and biorecognition elements are explored.

Multisensor Systems and Arrays for Medical Applications Employing Naturally-Occurring Compounds and Materials

The significant improvement of quality of life achieved over the last decades has stimulated the development of new approaches in medicine to take into account the personal needs of each patient. Precision medicine, providing healthcare customization, opens new horizons in the diagnosis, treatment and prevention of numerous diseases. As a consequence, there is a growing demand for novel analytical devices and methods capable of addressing the challenges of precision medicine. For example, various types of sensors or their arrays are highly suitable for simultaneous monitoring of multiple analytes in complex biological media in order to obtain more information about the health status of a patient or to follow the treatment process. Besides, the development of sustainable sensors based on natural chemicals allows reducing their environmental impact. This review is concerned with the application of such analytical platforms in various areas of medicine: analysis of body fluids, wearable sensors, drug manufacturing and screening. The importance and role of naturally-occurring compounds in the development of electrochemical multisensor systems and arrays are discussed.

Carbon and graphene quantum dots: a review on syntheses, characterization, biological and sensing applications for neurotransmitter determination

Neuro-transmitters have been considered to be essential biochemical molecules, which monitor physiological and behavioral function in the peripheral and central nervous systems. Thus, it is of high pharmaceutical and biological significance to analyze neuro-transmitters in the biological samples. So far, researchers have devised a lot of techniques for assaying these samples. It has been found that electro-chemical sensors possess features of robustness, selectivity, and sensitivity as well as real-time measurement. Graphene quantum dots (GQDs) and carbon QDs (CQDs) are considered some of the most promising carbon-based nanomaterials at the forefront of this research area. This is due to their characteristics including lower toxicity, higher solubility in various solvents, great electronic features, strong chemical inertness, high specific surface areas, plenty of edge sites for functionalization, and versatility, in addition to their ability to be modified via absorbent surface chemicals and the addition of modifiers or nano-materials. Hence in the present review, the synthesis methods of GQDs and CQDs has been summarized and their characterization methods also been analyzed. The applications of carbon-based QDs (GQDs and CQDs) in biological and sensing areas, such as biological imaging, drug/gene delivery, antibacterial and antioxidant activity, photoluminescence sensors, electrochemiluminescence sensors and electrochemical sensors, have also been discussed. This study then covers sensing features of key neurotransmitters, including dopamine, tyrosine, epinephrine, norepinephrine, serotonin and acetylcholine. Hence, issues and challenges of the GQDs and CQDs were analyzed for their further development.

Carbon and graphene quantum dots for biological and sensing applications of neurotransmitters.

Real-time in vivo detection techniques for neurotransmitters: a review

Functional synapses in the central nervous system depend on a chemical signal exchange process that involves neurotransmitter delivery between neurons and receptor cells in the neuro system.

Novel electrochemical biosensor for serotonin detection based on gold nanorattles decorated reduced graphene oxide in biological fluids and in vitro model

Abnormal level of serotonin (ST) in body fluids is related to various clinical conditions including behavioral and psychotic disorders; hence its fast detection in clinically relevant ranges have tremendous importance in medical science. In view of this, we have developed a novel biosensor for ST detection using Au-nanorattles (AuNRTs)- reduced graphene oxide (rGO) nanocomposite coated on to the gold nanoparticles (AuNPs) deposited glassy carbon electrode (GCE). The nanocomposite/sensor probe was characterized using UV-Vis, TEM, SAED, EDX, AFM, and electrochemical techniques including LSV and EIS. Thereafter, the suitability of fabricated GCE/AuNPs/AuNRTs-rGO-Naf sensor probe was applied for ST determination which showed a linear dynamic range (LDR) of 3 × 10^-6 - 1 × 10^-3 M and the detection limit (DL) of 3.87 (±0.02) ×10^-7 (RSD < 4.2%) M, which falls in the ranges of normal as well as various abnormal pathophysiological conditions. The designed sensor is successfully applied to detect ST in various real matrices viz. urine, blood serum, and in vitro model to show its direct clinical/practical applicability. Interferences due to the coexisting molecules were assessed and the long-term stability of the designed sensor was also examined which was found to be 8 weeks.

Electrochemical Nanocomposite Single-Use Sensor for Dopamine Detection

In this work, we report the development of a simple and sensitive sensor based on graphite screen-printed electrodes (GSPEs) modified by a nanocomposite film for dopamine (DA) detection. The sensor was realized by electrodepositing polyaniline (PANI) and gold nanoparticles (AuNPs) onto the graphite working electrode. The sensor surface was fully characterized by means of the cyclic voltammetry (CV) technique using [Fe(CN)6]4-/3- and [Ru(NH3)6]2+/3+ as redox probes. The electrochemical behavior of the nanocomposite sensor towards DA oxidation was assessed by differential pulse voltammetry (DPV) in phosphate buffer saline at physiological pH. The sensor response was found to be linearly related to DA concentration in the range 1-100 μM DA, with a limit of detection of 0.86 μM. The performance of the sensor in terms of reproducibility and selectivity was also studied. Finally, the sensor was successfully applied for a preliminary DA determination in human serum samples.

A facile and label-free ratiometric optical sensor for selective detection of norepinephrine by combining second-order scattering and fluorescence signals

In this work, a facile and label-free ratiometric sensor is constructed for selective determination of norepinephrine (NE) by coupling second-order scattering (SOS) and fluorescence, two different and independent optical signals. Herein, polyethyleneimine (PEI) dilute solution medium shows an intensive SOS signal without any fluorescence response. Interestingly, NE can be selectively induced by PEI to emit bright fluorescence, and meanwhile causes an observable decrease in the SOS signal due to the interactions between NE and PEI. The simultaneous variation of the two independent signals can be used for ratiometric sensing of NE. Under the optimal conditions, the resultant ratiometric sensor displays high sensitivity and selectivity toward NE by simultaneously monitoring fluorescence and SOS signals with the same excitation wavelength. The proposed sensor exhibits a good linear relationship versus NE concentration in the range of 10.0 nM-45.0 μM with a detection limit of 2.0 nM (S/N = 3) and has been successfully applied to the determination of NE in real samples without the use of any extra reagent. The combination of fluorescence and SOS signals provides a new scheme for ratiometric sensor design, greatly simplifying experimental procedure and effectively enhancing detection accuracy. Moreover, the proposed analytical strategy further broadens the application of dilute solutions of polymers in research into optical sensor and green analytical chemistry. Graphical abstract.

Latest Trends in Electrochemical Sensors for Neurotransmitters: A Review

Neurotransmitters are endogenous chemical messengers which play an important role in many of the brain functions, abnormal levels being correlated with physical, psychotic and neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's disease. Therefore, their sensitive and robust detection is of great clinical significance. Electrochemical methods have been intensively used in the last decades for neurotransmitter detection, outclassing more complicated analytical techniques such as conventional spectrophotometry, chromatography, fluorescence, flow injection, and capillary electrophoresis. In this manuscript, the most successful and promising electrochemical enzyme-free and enzymatic sensors for neurotransmitter detection are reviewed. Focusing on the activity of worldwide researchers mainly during the last ten years (2010-2019), without pretending to be exhaustive, we present an overview of the progress made in sensing strategies during this time. Particular emphasis is placed on nanostructured-based sensors, which show a substantial improvement of the analytical performances. This review also examines the progress made in biosensors for neurotransmitter measurements in vitro, in vivo and ex vivo.