Development of a novel micro biosensor for in vivo monitoring of glutamate release in the brain

Characterisation of a microelectrochemical biosensor for real-time detection of brain extracellular d-serine

A modified development protocol and concomitant characterisation of a first generation biosensor for the detection of brain extracellular d-serine is reported. Functional parameters important for neurochemical monitoring, including sensor sensitivity, O2 interference, selectivity, shelf-life and biocompatibility were examined. Construction and development involved the enzyme d-amino acid oxidase (DAAO), utilising a dip-coating immobilisation method employing a new extended drying approach. The resultant Pt-based polymer enzyme composite sensor achieved high sensitivity to d-serine (0.76 ± 0.04 nA mm-2. μM-1) and a low μM limit of detection (0.33 ± 0.02 μM). The in-vitro response time was within the solution stirring time, suggesting potential sub-second in-vivo response characteristics. Oxygen interference studies demonstrated a 1 % reduction in current at 50 μM O2 when compared to atmospheric O2 levels (200 μM), indicating that the sensor can be used for reliable neurochemical monitoring of d-serine, free from changes in current associated with physiological O2 fluctuations. Potential interference signals generated by the principal electroactive analytes present in the brain were minimised by using a permselective layer of poly(o-phenylenediamine), and although several d-amino acids are possible substrates for DAAO, their physiologically relevant signals were small relative to that for d-serine. Additionally, changing both temperature and pH over possible in vivo ranges (34-40 °C and 7.2-7.6 respectively) resulted in no significant effect on performance. Finally, the biosensor was implanted in the striatum of freely moving rats and used to monitor physiological changes in d-serine over a two-week period.

Advancements in Cerebrospinal Fluid Biosensors: Bridging the Gap from Early Diagnosis to the Detection of Rare Diseases

Cerebrospinal fluid (CSF) is a body fluid that can be used for the diagnosis of various diseases. However, CSF collection requires an invasive and painful procedure called a lumbar puncture (LP). This procedure is applied to any patient with a known risk of central nervous system (CNS) damage or neurodegenerative disease, regardless of their age range. Hence, this can be a very painful procedure, especially in infants and elderly patients. On the other hand, the detection of disease biomarkers in CSF makes diagnoses as accurate as possible. This review aims to explore novel electrochemical biosensing platforms that have impacted biomedical science. Biosensors have emerged as techniques to accelerate the detection of known biomarkers in body fluids such as CSF. Biosensors can be designed and modified in various ways and shapes according to their ultimate applications to detect and quantify biomarkers of interest. This process can also significantly influence the detection and diagnosis of CSF. Hence, it is important to understand the role of this technology in the rapidly progressing field of biomedical science.

Sensor-integrated brain-on-a-chip platforms: Improving the predictive validity in neurodegenerative research

Affecting millions of individuals worldwide, neurodegenerative diseases (NDDs) pose a significant and growing health concern in people over the age of 60 years. Contributing to this trend are the steady increase in the aging population coupled with a persistent lack of disease-altering treatment strategies targeting NDDs. The absence of efficient therapeutics can be attributed to high failure rates in clinical trials and the ineptness of animal models in preceding preclinical studies. To that end, in recent years, significant research effort has been dedicated to the development of human cell-based preclinical disease models characterized by a higher degree of predictive validity. However, a key requirement of any in vitro model constitutes the precise knowledge and replication of the target tissues' (patho-)physiological microenvironment. Herein, microphysiological systems have demonstrated superiority over conventional static 2D/3D in vitro cell culture systems, as they allow for the emulation and continuous monitoring of the onset, progression, and remission of disease-associated phenotypes. This review provides an overview of recent advances in the field of NDD research using organ-on-a-chip platforms. Specific focus is directed toward non-invasive sensing strategies encompassing electrical, electrochemical, and optical sensors. Additionally, promising on- and integrable off-chip sensing strategies targeting key analytes in NDDs will be presented and discussed in detail.

Electrochemical and biosensor techniques to monitor neurotransmitter changes with depression

Depression is a common mental illness. However, its current treatments, like selective serotonin reuptake inhibitors (SSRIs) and micro-dosing ketamine, are extremely variable between patients and not well understood. Three neurotransmitters: serotonin, histamine, and glutamate, have been proposed to be key mediators of depression. This review focuses on analytical methods to quantify these neurotransmitters to better understand neurological mechanisms of depression and how they are altered during treatment. To quantitatively measure serotonin and histamine, electrochemical techniques such as chronoamperometry and fast-scan cyclic voltammetry (FSCV) have been improved to study how specific molecular targets, like transporters and receptors, change with antidepressants and inflammation. Specifically, these studies show that different SSRIs have unique effects on serotonin reuptake and release. Histamine is normally elevated during stress, and a new inflammation hypothesis of depression links histamine and cytokine release. Electrochemical measurements revealed that stress increases histamine, decreases serotonin, and leads to changes in cytokines, like interleukin-6. Biosensors can also measure non-electroactive neurotransmitters, including glutamate and cytokines. In particular, new genetic sensors have shown how glutamate changes with chronic stress, as well as with ketamine treatment. These techniques have been used to characterize how ketamine changes glutamate and serotonin, and to understand how it is different from SSRIs. This review briefly outlines how these electrochemical techniques work, but primarily highlights how they have been used to understand the mechanisms of depression. Future studies should explore multiplexing techniques and personalized medicine using biomarkers in order to investigate multi-analyte changes to antidepressants.

Enhanced performance of enzymes confined in biocatalytic hydrogen-bonded organic frameworks for sensing of glutamate in the central nervous system

Glutamate (Glu) is a key excitatory neurotransmitter associated with various neurological disorders in the central nervous system, so its measurement is vital to both basic research and biomedical application. In this work, we propose the first example of using biocatalytic hydrogen-bonded organic frameworks (HOFs) as the hosting matrix to encapsulate glutamate oxidase (GLOD) via a de novo approach, fabricating a cascaded-enzyme nanoreactor for Glu biosensing. In this design, the ferriporphyrin ligands can assemble to form Fe-HOFs with high catalase-like activity, while offering a scaffold for the in-situ immobilization of GLOD. Moreover, the formed GLOD@Fe-HOFs are favorable for the efficient diffusion of Glu into the active sites of GLOD via the porous channels, accelerating the cascade reaction with neighboring Fe-HOFs. Consequently, the constructed nanoreactor can offer superior activity and operational stability in the catalytic cascade for Glu biosensing. More importantly, rapid and selective detection can be achieved in the cerebrospinal fluid (CSF) collected from mice in a low sample consumption. Therefore, the successful fabrication of enzyme@HOFs may offer promise to develop high-performance biosensor for further biomedical applications.

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

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

Glutamate oxidase sheets-Prussian blue grafted amperometric biosensor for the real time monitoring of glutamate release from primary cortical neurons

Glutamate (GLU) is a primary excitatory neurotransmitter, and its dysregulation is associated with several neurodegenerative disorders. A major challenge in GLU estimation is the existence of other biomolecules in the brain that could directly get oxidized at the electrode. Hence, highly selective electroenzymatic biosensors that enable rapid estimation of GLU are needed. Initially, a copolymer, poly(2-dimethylaminoethyl methacrylate- styrene) was synthesized through reversible addition-fragmentation chain transfer polymerization to noncovalently functionalize reduced graphene oxide (rGO), named DS-rGO. Glutamate oxidase macromolecule immobilized DS-rGO formed enzyme nanosheets, which was drop-coated over Prussian blue electrodeposited disposable electrodes to fabricate the GLU biosensor. The interconnectivity between the enzyme nanosheets and the Prussian blue endows the biosensor with enhanced conductivity and electrochemical activity. The biosensor exhibited a linearity: 3.25-250 μM; sensitivity: 3.96 μA mM-1 cm-2, and a limit of detection: 0.96 μM for GLU in the Neurobasal Medium. The biosensor was applied to an in vitro primary rat cortical model to discriminate GLU levels in Neurobasal Medium, before and after KCl mediated depolarization, which provides new insights for elucidating neuronal functioning in the brain.

Challenges and strategies faced in the electrochemical biosensing analysis of neurochemicals in vivo: A review

Our brain is an intricate neuromodulatory network, and various neurochemicals, including neurotransmitters, neuromodulators, gases, ions, and energy metabolites, play important roles in regulating normal brain function. Abnormal release or imbalance of these substances will lead to various diseases such as Parkinson's and Alzheimer's diseases, therefore, in situ and real-time analysis of neurochemical interactions in pathophysiological conditions is beneficial to facilitate our understanding of brain function. Implantable electrochemical biosensors are capable of monitoring neurochemical signals in real time in extracellular fluid of specific brain regions because they can provide excellent temporal and spatial resolution. However, in vivo electrochemical biosensing analysis mainly faces the following challenges: First, foreign body reactions induced by microelectrode implantation, non-specific adsorption of proteins and redox products, and aggregation of glial cells, which will cause irreversible degradation of performance such as stability and sensitivity of the microsensor and eventually lead to signal loss; Second, various neurochemicals coexist in the complex brain environment, and electroactive substances with similar formal potentials interfere with each other. Therefore, it is a great challenge to design recognition molecules and tailor functional surfaces to develop in vivo electrochemical biosensors with high selectivity. Here, we take the above challenges as a starting point and detail the basic design principles for improving in vivo stability, selectivity and sensitivity of microsensors through some specific functionalized surface strategies as case studies. At the same time, we summarize surface modification strategies for in vivo electrochemical biosensing analysis of some important neurochemicals for researchers' reference. In addition, we also focus on the electrochemical detection of low basal concentrations of neurochemicals in vivo via amperometric waveform techniques, as well as the stability and biocompatibility of reference electrodes during long-term sensing, and provide an outlook on the future direction of in vivo electrochemical neurosensing.

Editors' Choice-Review-The Future of Carbon-Based Neurochemical Sensing: A Critical Perspective

Carbon-based sensors have remained critical materials for electrochemical detection of neurochemicals, rooted in their inherent biocompatibility and broad potential window. Real-time monitoring using fast-scan cyclic voltammetry has resulted in the rise of minimally invasive carbon fiber microelectrodes as the material of choice for making measurements in tissue, but challenges with carbon fiber's innate properties have limited its applicability to understudied neurochemicals. Here, we provide a critical review of the state of carbon-based real-time neurochemical detection and offer insight into ways we envision addressing these limitations in the future. This piece focuses on three main hinderances of traditional carbon fiber based materials: diminished temporal resolution due to geometric properties and adsorption/desorption properties of the material, poor selectivity/specificity to most neurochemicals, and the inability to tune amorphous carbon surfaces for specific interfacial interactions. Routes to addressing these challenges could lie in methods like computational modeling of single-molecule interfacial interactions, expansion to tunable carbon-based materials, and novel approaches to synthesizing these materials. We hope this critical piece does justice to describing the novel carbon-based materials that have preceded this work, and we hope this review provides useful solutions to innovate carbon-based material development in the future for individualized neurochemical structures.

An Updated Review on Electrochemical Nanobiosensors for Neurotransmitter Detection

Neurotransmitters are chemical compounds released by nerve cells, including neurons, astrocytes, and oligodendrocytes, that play an essential role in the transmission of signals in living organisms, particularly in the central nervous system, and they also perform roles in realizing the function and maintaining the state of each organ in the body. The dysregulation of neurotransmitters can cause neurological disorders. This highlights the significance of precise neurotransmitter monitoring to allow early diagnosis and treatment. This review provides a complete multidisciplinary examination of electrochemical biosensors integrating nanomaterials and nanotechnologies in order to achieve the accurate detection and monitoring of neurotransmitters. We introduce extensively researched neurotransmitters and their respective functions in biological beings. Subsequently, electrochemical biosensors are classified based on methodologies employed for direct detection, encompassing the recently documented cell-based electrochemical monitoring systems. These methods involve the detection of neurotransmitters in neuronal cells in vitro, the identification of neurotransmitters emitted by stem cells, and the in vivo monitoring of neurotransmitters. The incorporation of nanomaterials and nanotechnologies into electrochemical biosensors has the potential to assist in the timely detection and management of neurological disorders. This study provides significant insights for researchers and clinicians regarding precise neurotransmitter monitoring and its implications regarding numerous biological applications.

Fibrous wearable and implantable bioelectronics

Fibrous wearable and implantable devices have emerged as a promising technology, offering a range of new solutions for minimally invasive monitoring of human health. Compared to traditional biomedical devices, fibers offer a possibility for a modular design compatible with large-scale manufacturing and a plethora of advantages including mechanical compliance, breathability, and biocompatibility. The new generation of fibrous biomedical devices can revolutionize easy-to-use and accessible health monitoring systems by serving as building blocks for most common wearables such as fabrics and clothes. Despite significant progress in the fabrication, materials, and application of fibrous biomedical devices, there is still a notable absence of a comprehensive and systematic review on the subject. This review paper provides an overview of recent advancements in the development of fibrous wearable and implantable electronics. We categorized these advancements into three main areas: manufacturing processes, platforms, and applications, outlining their respective merits and limitations. The paper concludes by discussing the outlook and challenges that lie ahead for fiber bioelectronics, providing a holistic view of its current stage of development.

Nanoelectrochemistry reveals how soluble Aβ42 oligomers alter vesicular storage and release of glutamate

Glutamate (Glu) is the major excitatory transmitter in the nervous system. Impairment of its vesicular release by β-amyloid (Aβ) oligomers is thought to participate in pathological processes leading to Alzheimer's disease. However, it remains unclear whether soluble Aβ42 oligomers affect intravesicular amounts of Glu or their release in the brain, or both. Measurements made in this work on single Glu varicosities with an amperometric nanowire Glu biosensor revealed that soluble Aβ42 oligomers first caused a dramatic increase in vesicular Glu storage and stimulation-induced release, accompanied by a high level of parallel spontaneous exocytosis, ultimately resulting in the depletion of intravesicular Glu content and greatly reduced release. Molecular biology tools and mouse models of Aβ amyloidosis have further established that the transient hyperexcitation observed during the primary pathological stage is mediated by an altered behavior of VGLUT1 responsible for transporting Glu into synaptic vesicles. Thereafter, an overexpression of Vps10p-tail-interactor-1a, a protein that maintains spontaneous release of neurotransmitters by selective interaction with t-SNAREs, resulted in a depletion of intravesicular Glu content, triggering advanced-stage neuronal malfunction. These findings are expected to open perspectives for remediating Aβ42-induced neuronal hyperactivity and neuronal degeneration.

Electrochemical study of the effect of radiofrequency on glutamate oxidase activity using a glutamate oxidase-based biosensor

A biosensor based on glutamate oxidase (GluOx) was developed to measure glutamate concentration. The main function of this type of biosensor is related to the structure and catalytic activity of GluOx. Since radiofrequency, as the widest spectrum of electromagnetic fields, can affect the catalytic activity and structure of GluOx, in this study, the effect of these fields on the analytical parameters of the fabricated biosensor was investigated. To build the biosensor a sol-gel solution of chitosan and native GluOx were prepared and then immobilized on the surface of the platinum electrode. Similarly, to investigate the effect of radiofrequency fields on the analytical parameters of the biosensor, instead of the native GluOx, irradiated GluOx was used to build the biosensor. To evaluate the biosensor responses, cyclic voltammetry experiments were performed and voltammograms were considered as biosensor responses. To determine the analytical parameters including detection limit, linear range, and saturation region of the responses, calibration curves were drawn for each of the biosensors. Also the long-term stability and selectivity of the fabricated biosensor were evaluated. Thereafter, the optimum pH and temperature for each of these two biosensors were examined. The results showed that radiofrequency waves harmed the detection and response of biosensors in the saturation region, while they had little effect on the linear region. Such results could be due to the effect of radiofrequency waves on the structure and function of glutamate oxidase. In general, the results indicate that when a glutamate oxidase-based biosensor is used to measure glutamate in radiofrequency fields, corrective coefficients for this type of biosensor should be considered to accurately measure glutamate concentration.

Cell viability and drug evaluation biosensing system based on disposable AuNPs/MWCNT nanocomposite modified screen-printed electrode for exocytosis dopamine detection

Cell viability, as an important index to evaluate drug effects, usually was measured by tetrazolium colorimetric assay, playing a key role in drug development and drug screening. Tedious operating procedures, unsatisfactory sensitivity and abominable environments perplex researchers to acquire more detailed in vivo-relevant biological information. Herein, a simple and low-cost cell viability and drug evaluation biosensing system-based on multiwalled carbon nanotubes, gold nanoparticles and Nafion modified screen-printed electrode (SPE) biosensor was constructed for detection of dopamine (DA) released from living cells to evaluate cytotoxicity of antineoplastic drugs such as cisplatin and resveratrol. The biosensing system was demonstrated to display exceptional selectivity, excellent flexibility and good stability toward DA measurement in complex bio-samples. Additionally, the satisfactory recoveries of DA in real samples revealed the reliability and accuracy of the biosensing system in practical application. The IC₅₀ curves respectively obtained by the biosensing system and tetrazolium colorimetric assay provided similar IC₅₀ value but distinctly different dose-effect relationship, which confirmed the enormous potential of the biosensor in cell viability and described drug efficacy profiles in cell function. In short, the cell viability and drug evaluation system using SPE biosensor paves a new way in drug screening and pharmaceutical application to measure bioactive molecule such as DA.

Development of highly sensitive, flexible dual L-glutamate and GABA microsensors for in vivo brain sensing

Real-time tracking of neurotransmitter levels in vivo has been technically challenging due to the low spatiotemporal resolution of current methods. Since the imbalance of cortical excitation/inhibition (E:I) ratios are associated with a variety of neurological disorders, accurate monitoring of excitatory and inhibitory neurotransmitter levels is crucial for investigating the underlying neural mechanisms of these conditions. Specifically, levels of the excitatory neurotransmitter L-glutamate, and the inhibitory neurotransmitter GABA, are assumed to play critical roles in the E:I balance. Therefore, in this work, a flexible electrochemical microsensor is developed for real-time simultaneous detection of L-glutamate and GABA. The flexible polyimide substrate was used for easier handling during implantation and measurement, along with less brain damage. Further, by electrochemically depositing Pt-black nanostructures on the sensor's surface, the active surface area was enhanced for higher sensitivity. This dual neurotransmitter sensor probe was validated under various settings for its performance, including in vitro, ex vivo tests with glutamatergic neuronal cells and in vivo test with anesthetized rats. Additionally, the sensor's performance has been further investigated in terms of longevity and biocompatibility. Overall, our dual L-glutamate:GABA sensor microprobe has its unique features to enable accurate, real-time, and long-term monitoring of the E:I balance in vivo. Thus, this new tool should aid investigations of neural mechanisms of normal brain function and various neurological disorders.

Functional Fiber Materials to Smart Fiber Devices

The development of fiber materials has accompanied the evolution of human civilization for centuries. Recent advances in materials science and chemistry offered fibers new applications with various functions, including energy harvesting, energy storing, displaying, health monitoring and treating, and computing. The unique one-dimensional shape of fiber devices endows them advantages to work as human-interfaced electronics due to the small size, lightweight, flexibility, and feasibility for integration into large-scale textile systems. In this review, we first present a discussion of the basics of fiber materials and the design principles of fiber devices, followed by a comprehensive analysis on recently developed fiber devices. Finally, we provide the current challenges facing this field and give an outlook on future research directions. With novel fiber devices and new applications continuing to be discovered after two decades of research, we envision that new fiber devices could have an important impact on our life in the near future.

Real-Time Monitoring of Neurotransmitters in the Brain of Living Animals

Neurotransmitters, as important chemical small molecules, perform the function of neural signal transmission from cell to cell. Excess concentrations of neurotransmitters are often closely associated with brain diseases, such as Alzheimer's disease, depression, schizophrenia, and Parkinson's disease. On the other hand, the release of neurotransmitters under the induced stimulation indicates the occurrence of reward-related behaviors, including food and drug addiction. Therefore, to understand the physiological and pathological functions of neurotransmitters, especially in complex environments of the living brain, it is urgent to develop effective tools to monitor their dynamics with high sensitivity and specificity. Over the past 30 years, significant advances in electrochemical sensors and optical probes have brought new possibilities for studying neurons and neural circuits by monitoring the changes in neurotransmitters. This Review focuses on the progress in the construction of sensors for in vivo analysis of neurotransmitters in the brain and summarizes current attempts to address key issues in the development of sensors with high selectivity, sensitivity, and stability. Combined with the latest advances in technologies and methods, several strategies for sensor construction are provided for recording chemical signal changes in the complex environment of the brain.

Multi-Spatiotemporal Probing of Neurochemical Events by Advanced Electrochemical Sensing Methods

Neurochemical events involving biosignals of different time and space dimensionalities constitute the complex basis of neurological functions and diseases. In view of this fact, electrochemical measurements enabling real-time quantification of neurochemicals at multiple levels of spatiotemporal resolution can provide informative clues to decode the molecular networks bridging vesicles and brains. This Minireview focuses on how scientific questions regarding the properties of single vesicles, neurotransmitter release kinetics, interstitial neurochemical dynamics, and multisignal interconnections in vivo have driven the design of electrochemical nano/microsensors, sensing interface engineering, and signal/data processing. An outlook for the future frontline in this realm will also be provided.

Accelerating the development of implantable neurochemical biosensors by using existing clinically applied depth electrodes

In this study, an implantable stereo-electroencephalography (sEEG) depth electrode was functionalised with an enzyme coating for enzyme-based biosensing of glucose and L-glutamate. This was done because personalised medicine could benefit from active real-time neurochemical monitoring on small spatial and temporal scales to further understand and treat neurological disorders. To achieve this, the sEEG depth electrode was characterised using cyclic voltammetry (CV), differential pulse voltammetry (DPV), square wave voltammetry (SWV), and electrochemical impedance spectroscopy (EIS) using several electrochemical redox mediators (potassium ferri/ferrocyanide, ruthenium hexamine chloride, and dopamine). To improve performance, the Pt sensors on the sEEG depth electrode were coated with platinum black and a crosslinked gelatin-enzyme film to enable enzymatic biosensing. This characterisation work showed that producing a useable electrode with a good electrochemical response showing the expected behaviour for a platinum electrode was possible. Coating with Pt black improved the sensitivity to H₂O₂ over unmodified electrodes and approached that of well-defined Pt macro disc electrodes. Measured current showed good dependence on concentration, and the calibration curves report good sensitivity of 29.65 nA/cm²/μM for glucose and 8.05 nA/cm²/μM for L-glutamate with a stable, repeatable, and linear response. These findings demonstrate that existing clinical electrode devices can be adapted for combined electrochemical and electrophysiological measurement in patients and obviate the need to develop new electrodes when existing clinically approved devices and the associated knowledge can be reused. This accelerates the time to use and application of in vivo and wearable biosensing for diagnosis, treatment, and personalised medicine.

In Vivo Detection of Glutamate in Tomatoes by an Enzyme-Based Electrochemical Biosensor

The in vivo and on-site detection of key physiology parameters in plants will be of great relevance for precision agriculture and food technology. In this work, a sensitive enzymatic glutamate sensor was successfully developed. To enhance the conductivity and catalytic ability and to fix the glutamate oxidase, Au-Pt nanoparticles were first deposited on screen-printed electrodes, and then carboxylated graphene oxide and carboxylated multiwalled carbon nanotubes were fabricated for the synthesis of the electrode. The detection range of the glutamate sensor is widest (2 μM to 16 mM) up to date, and its detection limit is relatively low (0.14 μM). A number of standard curves were built in the pH range of 3.5-7.5, which can be applied in various plants and fruits. Using this sensor, the glutamate level in tomatoes was determined in vivo. This glutamate sensor has important practical value in precision agriculture. Our strategy also provides a way to establish the detection modes for other biomolecules in plants.

Echem methods and electrode types of the current in vivo electrochemical sensing

For a long time, people have been eager to realize continuous real-time online monitoring of biological compounds. Fortunately, in vivo electrochemical biosensor technology has greatly promoted the development of biological compound detection. This article summarizes the existing in vivo electrochemical detection technologies into two categories: microdialysis (MD) and microelectrode (ME). Then we summarized and discussed the electrode surface time, pollution resistance, linearity and the number of instances of simultaneous detection and analysis, the composition and characteristics of the sensor, and finally, we also predicted and prospected the development of electrochemical technology and sensors in vivo.

Neurotransmitters-Key Factors in Neurological and Neurodegenerative Disorders of the Central Nervous System

Neurotransmitters are molecules that amplify, transmit, and convert signals in cells, having an essential role in information transmission throughout the nervous system. Hundreds of such chemicals have been discovered in the last century, continuing to be identified and studied concerning their action on brain health. These substances have been observed to influence numerous functions, including emotions, thoughts, memories, learning, and movements. Thus, disturbances in neurotransmitters’ homeostasis started being correlated with a plethora of neurological and neurodegenerative disorders. In this respect, the present paper aims to describe the most important neurotransmitters, broadly classified into canonical (e.g., amino acids, monoamines, acetylcholine, purines, soluble gases, neuropeptides) and noncanonical neurotransmitters (e.g., exosomes, steroids, D-aspartic acid), and explain their link with some of the most relevant neurological conditions. Moreover, a brief overview of the recently developed neurotransmitters’ detection methods is offered, followed by several considerations on the modulation of these substances towards restoring homeostasis.

Current Approaches to Monitor Macromolecules Directly from the Cerebral Interstitial Fluid

Gaining insights into the pharmacokinetic and pharmacodynamic properties of lead compounds is crucial during drug development processes. When it comes to the treatment of brain diseases, collecting information at the site of action is challenging. There are only a few techniques available that allow for the direct sampling from the cerebral interstitial space. This review concerns the applicability of microdialysis and other approaches, such as cerebral open flow microperfusion and electrochemical biosensors, to monitor macromolecules (neuropeptides, proteins, …) in the brain. Microdialysis and cerebral open flow microperfusion can also be used to locally apply molecules at the same time at the site of sampling. Innovations in the field are discussed, together with the pitfalls. Moreover, the ‘nuts and bolts’ of the techniques and the current research gaps are addressed. The implementation of these techniques could help to improve drug development of brain-targeted drugs.

Perspective-Electrochemical Sensors for Neurotransmitters and Psychiatrics: Steps toward Physiological Mental Health Monitoring

Therapeutic monitoring of neurotransmitters (NTs) and psychiatric medications is essential for the diagnosis and treatment of mental illness. However, in-vivo monitoring of NTs in humans as well as continuous physiological monitoring of psychiatrics have yet to be realized. In pursuit of this goal, there has been a plethora of work to develop electrochemical sensors for both in-vivo NT monitoring as well as in-vitro detection of psychiatric medications. We review these sensors here while discussing next steps needed to achieve concurrent, continuous physiological monitoring of NTs and psychiatric medications as part of a closed-loop feedback system that guides medication administration.

Detection of cellular metabolites by redox enzymatic cascades

Detection of cellular metabolites that are disease biomarkers is important for human healthcare monitoring and assessing prognosis and therapeutic response. Accurate and rapid detection of microbial metabolites and pathway intermediates is also crucial for the process optimization required for development of bioconversion methods using metabolically engineered cells. Various redox enzymes can generate electrons that can be employed in enzyme-based biosensors and in the detection of cellular metabolites. These reactions can directly transform target compounds into various readout signals. By incorporating engineered enzymes into enzymatic cascades, the readout signals can be improved in terms of accuracy and sensitivity. This review critically discusses selected redox enzymatic and chemoenzymatic cascades currently employed for detection of human- and microbe-related cellular metabolites including, amino acids, d-glucose, inorganic ions (pyrophosphate, phosphate, and sulfate), nitro- and halogenated phenols, NAD(P)H, fatty acids, fatty aldehyde, alkane, short chain acids, and cellular metabolites.

Highly sensitive and selective nanostructured microbiosensors for glucose and lactate simultaneous measurements in blood serum and in vivo in brain tissue

Highly sensitive and selective nanostructured lactate and glucose microbiosensors for their in vivo simultaneous determination in rat brain were developed based on carbon fiber microelectrodes (CFM) modified with nanoporous gold (NPG) using the Dynamic Hydrogen Bubble Template (DHBT) method. Electrodeposition of platinum nanoparticles (PtNP) onto the NPG film enhances the sensitivity and the electrocatalytic properties towards H₂O₂ detection. The nanostructured microelectrode platform was modified by glucose oxidase (GOx) and lactate oxidase (LOx) enzyme immobilization. High selective measurements were achieved by covering with a perm-selective layer of electropolymerized m-phenylenediamine, deposition of a Nafion® film and by using a null sensor. The morphological characteristics and electroanalytical performance of the microbiosensors were assessed, by scanning electron microscopy and electrochemical techniques, respectively. The PtNP/NPG/CFM shows a high sensitivity to H₂O₂ (5.96 A M^-1 cm^-2) at 0.36 V vs. Ag/AgCl, with a linear range from 0.2 to 200 μM, and an LOD of 10 nM. The microbiosensors were applied to the simultaneous determination of lactate and glucose in blood serum samples. Moreover, the basal extracellular concentrations of lactate and glucose were measured in vivo in four different rat brain structures. These results support the potential of the microbiosensor to be used as a valuable tool to investigate brain neurochemicals 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.

Quality control methods in musculoskeletal tissue engineering: from imaging to biosensors

Tissue engineering is rapidly progressing toward clinical application. In the musculoskeletal field, there has been an increasing necessity for bone and cartilage replacement. Despite the promising translational potential of tissue engineering approaches, careful attention should be given to the quality of developed constructs to increase the real applicability to patients. After a general introduction to musculoskeletal tissue engineering, this narrative review aims to offer an overview of methods, starting from classical techniques, such as gene expression analysis and histology, to less common methods, such as Raman spectroscopy, microcomputed tomography, and biosensors, that can be employed to assess the quality of constructs in terms of viability, morphology, or matrix deposition. A particular emphasis is given to standards and good practices (GXP), which can be applicable in different sectors. Moreover, a classification of the methods into destructive, noninvasive, or conservative based on the possible further development of a preimplant quality monitoring system is proposed. Biosensors in musculoskeletal tissue engineering have not yet been used but have been proposed as a novel technology that can be exploited with numerous advantages, including minimal invasiveness, making them suitable for the development of preimplant quality control systems.

On-Chip Glucose Detection Based on Glucose Oxidase Immobilized on a Platinum-Modified, Gold Microband Electrode

We report the microfabrication and characterization of gold microband electrodes on silicon using standard microfabrication methods, i.e., lithography and etching techniques. A two-step electrodeposition process was carried out using the on-chip platinum reference and gold counter electrodes, thus incorporating glucose oxidase onto a platinum-modified, gold microband electrode with an o-phenylenediamine and ß-cyclodextrin mixture. The as-fabricated electrodes were studied using optical microscopy, scanning electron microscopy, and atomic force microscopy. The two-step electrodeposition process was conducted in low sample volumes (50 µL) of both solutions required for biosensor construction. Cyclic voltammetry and electrochemical impedance spectroscopy were utilised for electrochemical characterization at each stage of the deposition process. The enzymatic-based microband biosensor demonstrated a linear response to glucose from 2.5-15 mM, using both linear sweep voltammetry and chronoamperometric measurements in buffer-based solutions. The biosensor performance was examined in 30 µL volumes of fetal bovine serum. Whilst a reduction in the sensor sensitivity was evident within 100% serum samples (compared to buffer media), the sensor demonstrated linear glucose detection with increasing glucose concentrations (5-17 mM).

Quantitative Nano-amperometric Measurement of Intravesicular Glutamate Content and its Sub-Quantal Release by Living Neurons

Quantitative measurements of intravesicular glutamate (Glu) and of transient exocytotic release contents directly from individual living neurons are highly desired for understanding the mechanisms (full or sub-quantal release?) of synaptic transmission and plasticity. However, this could not be achieved so far due to the lack of adequate experimental strategies relying on selective and sensitive Glu nanosensors. Herein, we introduce a novel electrochemical Glu nanobiosensor based on a single SiC nanowire that can selectively measure in real-time Glu fluxes released via exocytosis by large Glu vesicles (ca. 125 nm diameter) present in single hippocampal axonal varicosities as well as their intravesicular content before exocytosis. These measurements revealed a sub-quantal release mode in living hippocampal neurons, viz., only ca. one third to one half of intravesicular Glu molecules are released by individual vesicles during exocytotic events. Importantly, this fraction remained practically the same when hippocampal neurons were pretreated with L-Glu-precursor L-glutamine, while it significantly increased after zinc treatment, although in both cases the intravesicular contents were drastically affected.

Brain neurochemical monitoring

Brain neurochemical monitoring aims to provide continuous and accurate measurements of brain biomarkers. It has enabled significant advances in neuroscience for application in clinical diagnostics, treatment, and prevention of brain diseases. Microfabricated electrochemical and optical spectroscopy sensing technologies have been developed for precise monitoring of brain neurochemicals. Here, a comprehensive review on the progress of sensing technologies developed for brain neurochemical monitoring is presented. The review provides a summary of the widely measured clinically relevant neurochemicals and commonly adopted recognition technologies. Recent advances in sampling, electrochemistry, and optical spectroscopy for brain neurochemical monitoring are highlighted and their application are discussed. Existing gaps in current technologies and future directions to design industry standard brain neurochemical sensing devices for clinical applications are addressed.

Addressing the Selectivity of Enzyme Biosensors: Solutions and Perspectives

Enzymatic biosensors enjoy commercial success and are the subject of continued research efforts to widen their range of practical application. For these biosensors to reach their full potential, their selectivity challenges need to be addressed by comprehensive, solid approaches. This review discusses the status of enzymatic biosensors in achieving accurate and selective measurements via direct biocatalytic and inhibition-based detection, with a focus on electrochemical enzyme biosensors. Examples of practical solutions for tackling the activity and selectivity problems and preventing interferences from co-existing electroactive compounds in the samples are provided such as the use of permselective membranes, sentinel sensors and coupled multi-enzyme systems. The effect of activators, inhibitors or enzymatic substrates are also addressed by coupled enzymatic reactions and multi-sensor arrays combined with data interpretation via chemometrics. In addition to these more traditional approaches, the review discusses some ingenious recent approaches, detailing also on possible solutions involving the use of nanomaterials to ensuring the biosensors' selectivity. Overall, the examples presented illustrate the various tools available when developing enzyme biosensors for new applications and stress the necessity to more comprehensively investigate their selectivity and validate the biosensors versus standard analytical methods.

Recent Advances in In Vivo Neurochemical Monitoring

The brain is a complex network that accounts for only 5% of human mass but consumes 20% of our energy. Uncovering the mysteries of the brain's functions in motion, memory, learning, behavior, and mental health remains a hot but challenging topic. Neurochemicals in the brain, such as neurotransmitters, neuromodulators, gliotransmitters, hormones, and metabolism substrates and products, play vital roles in mediating and modulating normal brain function, and their abnormal release or imbalanced concentrations can cause various diseases, such as epilepsy, Alzheimer's disease, and Parkinson's disease. A wide range of techniques have been used to probe the concentrations of neurochemicals under normal, stimulated, diseased, and drug-induced conditions in order to understand the neurochemistry of drug mechanisms and develop diagnostic tools or therapies. Recent advancements in detection methods, device fabrication, and new materials have resulted in the development of neurochemical sensors with improved performance. However, direct in vivo measurements require a robust sensor that is highly sensitive and selective with minimal fouling and reduced inflammatory foreign body responses. Here, we review recent advances in neurochemical sensor development for in vivo studies, with a focus on electrochemical and optical probes. Other alternative methods are also compared. We discuss in detail the in vivo challenges for these methods and provide an outlook for future directions.

Electrochemical sensing: A prognostic tool in the fight against COVID-19

The COVID-19 pandemic has devastated the world, despite all efforts in infection control and treatment/vaccine development. Hospitals are currently overcrowded, with health statuses of patients often being hard to gauge. Therefore, methods for determining infection severity need to be developed so that high-risk patients can be prioritized, resources can be efficiently distributed, and fatalities can be prevented. Electrochemical prognostic biosensing of various biomarkers may hold promise in solving these problems as they are low-cost and provide timely results. Therefore, we have reviewed the literature and extracted the most promising biomarkers along with their most favourable electrochemical sensors. The biomarkers discussed in this paper are C-reactive protein (CRP), interleukins (ILs), tumour necrosis factor alpha (TNFα), interferons (IFNs), glutamate, breath pH, lymphocytes, platelets, neutrophils and D-dimer. Metabolic syndrome is also discussed as comorbidity for COVID-19 patients, as it increases infection severity and raises chances of becoming infected. Cannabinoids, especially cannabidiol (CBD), are discussed as a potential adjunct therapy for COVID-19 as their medicinal properties may be desirable in minimizing the neurodegenerative or severe inflammatory damage caused by severe COVID-19 infection. Currently, hospitals are struggling to provide adequate care; thus, point-of-care electrochemical sensor development needs to be prioritized to provide an approximate prognosis for hospital patients. During and following the immediate aftermath of the pandemic, electrochemical sensors can also be integrated into wearable and portable devices to help patients monitor recovery while returning to their daily lives. Beyond the COVID-19 pandemic, these sensors will also prove useful for monitoring inflammation-based diseases such as cancer and cardiovascular disease.

Recent Progress in Quantitatively Monitoring Vesicular Neurotransmitter Release and Storage With Micro/Nanoelectrodes

Exocytosis is one of the essential steps for chemical signal transmission between neurons. In this process, vesicles dock and fuse with the plasma membrane and release the stored neurotransmitters through fusion pores into the extracellular space, and all of these steps are governed with various molecules, such as proteins, ions, and even lipids. Quantitatively monitoring vesicular neurotransmitter release in exocytosis and initial neurotransmitter storage in individual vesicles is significant for the study of chemical signal transmission of the central nervous system (CNS) and neurological diseases. Electrochemistry with micro/nanoelectrodes exhibits great spatial-temporal resolution and high sensitivity. It can be used to examine the exocytotic kinetics from the aspect of neurotransmitters and quantify the neurotransmitter storage in individual vesicles. In this review, we first introduce the recent advances of single-cell amperometry (SCA) and the nanoscale interface between two immiscible electrolyte solutions (nanoITIES), which can monitor the quantity and release the kinetics of electrochemically and non-electrochemically active neurotransmitters, respectively. Then, the development and application of the vesicle impact electrochemical cytometry (VIEC) and intracellular vesicle impact electrochemical cytometry (IVIEC) and their combination with other advanced techniques can further explain the mechanism of neurotransmitter storage in vesicles before exocytosis. It has been proved that these electrochemical techniques have great potential in the field of neuroscience.

Flexible Electronics for Monitoring in vivo Electrophysiology and Metabolite Signals

Numerous efforts have been made to develop efficient biosensors for detecting analytes in the human body. However, biosensors are often developed on rigid materials, which limits their application on skin, organs, and other tissues in the human body where good flexibility is required. Developing flexible materials for biosensors that can be used on soft and irregularly shaped surfaces would significantly expand the clinical application of biosensors. In this review, we will provide a selective overview of recently developed flexible electronic devices and their applications for monitoring in vivo metabolite and electrophysiology signals. The article provides guidelines for the development of an in vivo signal monitoring system and emphasizes research from various disciplines for the further development of flexible electronics that can be used in more biomedical applications in the future.

"Nano": An Emerging Avenue in Electrochemical Detection of Neurotransmitters

The growing importance of nanomaterials toward the detection of neurotransmitter molecules has been chronicled in this review. Neurotransmitters (NTs) are chemicals that serve as messengers in synaptic transmission and are key players in brain functions. Abnormal levels of NTs are associated with numerous psychotic and neurodegenerative diseases. Therefore, their sensitive and robust detection is of great significance in clinical diagnostics. For more than three decades, electrochemical sensors have made a mark toward clinical detection of NTs. The superiority of these electrochemical sensors lies in their ability to enable sensitive, simple, rapid, and selective determination of analyte molecules while remaining relatively inexpensive. Additionally, these sensors are capable of being integrated in robust, portable, and miniaturized devices to establish point-of-care diagnostic platforms. Nanomaterials have emerged as promising materials with significant implications for electrochemical sensing due to their inherent capability to achieve high surface coverage, superior sensitivity, and rapid response in addition to simple device architecture and miniaturization. Considering the enormous significance of the levels of NTs in biological systems and the advances in sensing ushered in with the integration of nanotechnology in electrochemistry, the analysis of NTs by employing nanomaterials as interface materials in various matrices has emerged as an active area of research. This review explores the advancements made in the field of electrochemical sensors for the sensitive and selective determination of NTs which have been described in the past two decades with a distinctive focus on extremely innovative attributes introduced by nanotechnology.

Novel microwire-based biosensor probe for simultaneous real-time measurement of glutamate and GABA dynamics in vitro and in vivo

Glutamate (GLU) and γ-aminobutyric acid (GABA) are the major excitatory (E) and inhibitory (I) neurotransmitters in the brain, respectively. Dysregulation of the E/I ratio is associated with numerous neurological disorders. Enzyme-based microelectrode array biosensors present the potential for improved biocompatibility, localized sample volumes, and much faster sampling rates over existing measurement methods. However, enzymes degrade over time. To overcome the time limitation of permanently implanted microbiosensors, we created a microwire-based biosensor that can be periodically inserted into a permanently implanted cannula. Biosensor coatings were based on our previously developed GLU and reagent-free GABA shank-type biosensor. In addition, the microwire biosensors were in the same geometric plane for the improved acquisition of signals in planar tissue including rodent brain slices, cultured cells, and brain regions with laminar structure. We measured real-time dynamics of GLU and GABA in rat hippocampal slices and observed a significant, nonlinear shift in the E/I ratio from excitatory to inhibitory dominance as electrical stimulation frequency increased from 10 to 140 Hz, suggesting that GABA release is a component of a homeostatic mechanism in the hippocampus to prevent excitotoxic damage. Additionally, we recorded from a freely moving rat over fourteen weeks, inserting fresh biosensors each time, thus demonstrating that the microwire biosensor overcomes the time limitation of permanently implanted biosensors and that the biosensors detect relevant changes in GLU and GABA levels that are consistent with various behaviors.

In Vivo Glutamate Sensing inside the Mouse Brain with Perovskite Nickelate-Nafion Heterostructures

Glutamate, one of the main neurotransmitters in the brain, plays a critical role in communication between neurons, neuronal development, and various neurological disorders. Extracellular measurement of neurotransmitters such as glutamate in the brain is important for understanding these processes and developing a new generation of brain-machine interfaces. Here, we demonstrate the use of a perovskite nickelate-Nafion heterostructure as a promising glutamate sensor with a low detection limit of 16 nM and a response time of 1.2 s via amperometric sensing. We have designed and successfully tested novel perovskite nickelate-Nafion electrodes for recording of glutamate release ex vivo in electrically stimulated brain slices and in vivo from the primary visual cortex (V1) of awake mice exposed to visual stimuli. These results demonstrate the potential of perovskite nickelates as sensing media for brain-machine interfaces.