Frontiers in Electrochemical Sensors for Neurotransmitter Detection: Towards Measuring Neurotransmitters as Chemical Diagnostics for Brain Disorders

3D-Printed Microelectrodes for Biological Measurement

Microelectrodes are useful electrochemical sensors that can provide spatial biological monitoring. Carbon fiber has been by far the most widely used microelectrode; however, a vast number of different materials and modification strategies have been developed to broaden the scope of microelectrodes. Carbon composite electrodes provide a simple approach to making microelectrodes with a wide range of materials, but manufacturing strategies are complex. 3D printing can provide the ability to make microelectrodes with high precision. We used fused filament fabrication to print single strands of carbon black/polylactic acid (CB/PLA) and multiwall carbon nanotube/polylactic acid (MWCNT/PLA), which were then made into microelectrodes. Microelectrodes ranged from 70 μm in diameter to 400 μm in diameter and were assessed using standard redox probes. MWCNT/PLA electrodes exhibited greater sensitivity, a lower limit of detection, and stability for the measurement of serotonin (5-HT). Both CB/PLA and MWCNT/PLA microelectrodes were able to monitor 5-HT overflow from the ex vivo ileum tissue. MWCNT/PLA microelectrodes were utilized to show differences in 5-HT overflow from ex vivo ileum and colon following exposure to odorants present in spices. These findings highlight that any conductive thermoplastic material can be fabricated into a microelectrode. This simple strategy can utilize a wide range of materials to make 3D-printed microelectrodes for a diverse range of applications.

Biofouling and performance of boron-doped diamond electrodes for detection of dopamine and serotonin in neuron cultivation media

Boron doped diamond has been considered as a fouling-resistive electrode material for in vitro and in vivo detection of neurotransmitters. In this study, its performance in electrochemical detection of dopamine and serotonin in neuron cultivation media Neurobasal™ before and after cultivation of rat neurons was investigated. For differential pulse voltammetry the limits of detection in neat Neurobasal™ medium of 2 µM and 0.2 µM for dopamine and serotonin, respectively, were achieved on the polished surface, which is comparable with physiological values. On oxidized surface twofold higher values, but increased repeatabilities of the signals were obtained. However, in Neurobasal™ media with peptides-containing supplements necessary for cell cultivation, the voltammograms were notably worse shaped due to biofouling, especially in the medium isolated after neuron growth. In these complex media, the amperometric detection mode at +0.75 V (vs. Ag/AgCl) allowed to detect portion-wise additions of dopamine and serotonin (as low as 1-2 µM), mimicking neurotransmitter release from vesicles despite the lower sensitivity in comparison with neat NeurobasalTM. The results indicate substantial differences in detection on boron doped diamond electrode in the presence and absence of proteins, and the necessity of studies in real media for successful implementation to neuron-electrode interfaces.

Surface-Enhanced Raman Scattering Nanosensing and Imaging in Neuroscience

Monitoring neurochemicals and imaging the molecular content of brain tissues in vitro, ex vivo, and in vivo is essential for enhancing our understanding of neurochemistry and the causes of brain disorders. This review explores the potential applications of surface-enhanced Raman scattering (SERS) nanosensors in neurosciences, where their adoption could lead to significant progress in the field. These applications encompass detecting neurotransmitters or brain disorders biomarkers in biofluids with SERS nanosensors, and imaging normal and pathological brain tissues with SERS labeling. Specific studies highlighting in vitro, ex vivo, and in vivo analysis of brain disorders using fit-for-purpose SERS nanosensors will be detailed, with an emphasis on the ability of SERS to detect clinically pertinent levels of neurochemicals. Recent advancements in designing SERS-active nanomaterials, improving experimentation in biofluids, and increasing the usage of machine learning for interpreting SERS spectra will also be discussed. Furthermore, we will address the tagging of tissues presenting pathologies with nanoparticles for SERS imaging, a burgeoning domain of neuroscience that has been demonstrated to be effective in guiding tumor removal during brain surgery. The review also explores future research applications for SERS nanosensors in neuroscience, including monitoring neurochemistry in vivo with greater penetration using surface-enhanced spatially offset Raman scattering (SESORS), near-infrared lasers, and 2-photon techniques. The article concludes by discussing the potential of SERS for investigating the effectiveness of therapies for brain disorders and for integrating conventional neurochemistry techniques with SERS sensing.

Multifunctional Nanomaterials for Advancing Neural Interfaces: Recording, Stimulation, and Beyond

ConspectusNeurotechnology has seen dramatic improvements in the last three decades. The major focus in the field has been to design electrical communication platforms with high spatial resolution, stability, and translatability for understanding and affecting neural pathways. The deployment of nanomaterials in bioelectronics has enhanced the capabilities of conventional approaches employing microelectrode arrays (MEAs) for electrical interfaces, allowing the construction of miniaturized, high-performance neuroelectronics (Garg, R.; et al. ACS Appl. Nano Mater. 2023, 6, 8495). While these advancements in the electrical neuronal interface have revolutionized neurotechnology both in scale and breadth, an in-depth understanding of neurons' interactions is challenging due to the complexity of the environments where the cells and tissues are laid. The activity of large, three-dimensional neuronal systems has proven difficult to accurately monitor and modulate, and chemical cell-cell communication is often completely neglected. Recent breakthroughs in nanotechnology have provided opportunities to use new nonelectric modes of communication with neurons and to significantly enhance electrical signal interface capabilities. The enhanced electrochemical activity and optical activity of nanomaterials owing to their nonbulk electronic properties and surface nanostructuring have seen extensive utilization. Nanomaterials' enhanced optical activity enables remote neural state modulation, whereas the defect-rich surfaces provide an enormous number of available electrocatalytic sites for neurochemical detection and electrochemical modulation of cell microenvironments through Faradaic processes. Such unique properties can allow multimodal neural interrogation toward generating closed-loop interfaces with access to more complete neural state descriptors. In this Account, we will review recent advances and our efforts spearheaded toward utilizing nanostructured electrodes for enhanced bidirectional interfaces with neurons, the application of unique hybrid nanomaterials for remote nongenetic optical stimulation of neurons, tunable nanomaterials for highly sensitive and selective neurotransmitter detection, and the utilization of nanomaterials as electrocatalysts toward electrochemically modulating cellular activity. We highlight applications of these technologies across cell types through nanomaterial engineering with a focus on multifunctional graphene nanostructures applied though several modes of neural modulation but also an exploration of broad material classes for maximizing the potency of closed-loop bioelectronics.

Sex differences in cholinergic signaling affect functional outcomes for theta-gamma coordination in hippocampal subcircuits following experimental febrile status epilepticus

OBJECTIVE

Sex determines cognitive outcome in animal models of early life seizure, where males exhibit impaired hippocampal-dependent learning and memory compared with females. The physiological underpinnings of this sex effect are unclear. Cholinergic signaling is essential for the generation of hippocampal oscillations, and supplementation of cholinergic precursors prior to status epilepticus in immature male rats prevents subsequent memory deficits. We hypothesized that there are sex differences in acetylcholine circuits and their response to experimental febrile status epilepticus (eFSE).

METHODS

eFSE was induced in male and female rat pups. We transversed the hippocampus of postnatal day >60 control (CTL) and eFSE rats with a 64-channel laminar silicon probe to assay cholinergic-dependent theta oscillations under urethane anesthesia. Local field potential properties were compared during (1) baseline sensory stimulation, (2) pharmacological stimulation via acetylcholine reuptake blockade, and (3) sensory stimulation after muscarinic acetylcholine receptor block (atropine).

RESULTS

In all groups, a baseline tail pinch could elicit theta oscillations via corticohippocampal synaptic input. Following atropine, a tail pinch response could no longer be elicited in CTL male, CTL female, or eFSE female rats. In contrast, induced slow theta power in eFSE males after atropine was not decreased to spontaneous levels. Analysis of oscillation bandwidths revealed sex differences in acetylcholine modulation of theta frequency and slow gamma frequency and power. This study also identified significant effects of both sex and eFSE on baseline theta-gamma comodulation, indicating a loss of coupling in eFSE males and a potential gain of function in eFSE females.

SIGNIFICANCE

There are differences in cholinergic modulation of theta and gamma signal coordination between male and female rats. These differences may underlie worse cognitive outcomes in males following eFSE. Promoting the efficacy of muscarinic acetylcholine signaling prior to or following early life seizures could elucidate a mechanism for the temporal discoordination of neural signals within and between hippocampus and neocortex and provide a novel therapeutic approach for improving cognitive outcomes.

Monitoring the electroactive cargo of extracellular vesicles can differentiate various cancer cell lines

Extracellular vesicles (EVs) are pivotal in cell-to-cell communication due to the array of cargo contained within these vesicles. EVs are considered important biomarkers for identification of disease, however most measurement approaches have focused on monitoring specific surface macromolecular targets. Our study focuses on exploring the electroactive component present within cargo from EVs obtained from various cancer and non-cancer cell lines using a disk carbon fiber microelectrode. Variations in the presence of oxidizable components were observed when the total cargo from EVs were measured, with the highest current detected in EVs from MCF7 cells. There were differences observed in the types of oxidizable species present within EVs from MCF7 and A549 cells. Single entity measurements showed clear spikes due to the detection of oxidizable cargo within EVs from MCF7 and A549 cells. These studies highlight the promise of monitoring EVs through the presence of varying electroactive components within the cargo and can drive a wave of new strategies towards specific detection of EVs for diagnosis and prognosis of various diseases.

Fabrication and Optimization of a Molecularly Imprinted Carbon Fiber Microelectrode for Selective Detection of Met-enkephalin Using Fast-Scan Cyclic Voltammetry

Methionine-enkephalin (Met-Enk) is an endogenous opioid peptide that is involved in various physiological processes including memory. A technological gap in the understanding of Met-Enk's role in memory is the lack of rapid measurement tools to selectively quantify Met-Enk concentrations in situ. Here, we integrate molecularly imprinted polymers (MIPs) with carbon fiber microelectrodes (CFMs) to selectively detect Met-Enk by using fast-scan cyclic voltammetry (FSCV). We report two MIP conditions that yield 2-fold and 5-fold higher selectivity toward Met-Enk than the tyrosine-containing hexapeptide fragment angiotensin II (3-8). We demonstrate that MIP technology can be combined with FSCV at CFMs to create rapid and selective sensors for Met-Enk. This technology is a promising platform for creating selective sensors for other peptides and biomarkers.

The Role of Catecholamines in the Pathogenesis of Diseases and the Modified Electrodes for Electrochemical Detection of Catecholamines: A Review

Catecholamines (CAs), which include adrenaline, noradrenaline, and dopamine, are neurotransmitters and hormones that critically regulate the cardiovascular system, metabolism, and stress response in the human body. The abnormal levels of these molecules can lead to the development of various diseases, including pheochromocytoma and paragangliomas, Alzheimer's disease, and Takotsubo cardiomyopathy. Due to their low cost, high sensitivity, flexible detection strategies, ease of integration, and miniaturization, electrochemical techniques have been extensively employed in the detection of CAs, surpassing traditional analytical methods. Electrochemical detection of CAs in real samples is challenging due to the tendency of poisoning electrode. Chemically modified electrodes have been widely used to solve the problems of poor sensitivity and selectivity faced by bare electrodes. There are a few articles that provide an overview of electrochemical detection and efficient enrichment of CAs, but there is a dearth of updates on the role of CAs in the pathogenesis of diseases. Additionally, there is still a lack of systematic synthesis with a focus on modified electrodes for electrochemical detection. Thus, this review provides a summary of the recent clinical pathogenesis of CAs and the modified electrodes for electrochemical detection of CAs published between 2017 and 2022. Moreover, challenges and future perspectives are also highlighted. This work is expected to provide useful guidance to researchers entering this interdisciplinary field, promoting further development of CAs pathogenesis, and developing more novel chemically modified electrodes for the detection of CAs.

Batch Fabrication of Microelectrode Arrays with Glassy Carbon Microelectrodes and Interconnections for Neurochemical Sensing: Promises and Challenges

Flexible multielectrode arrays with glassy carbon (GC) electrodes and metal interconnection (hybrid MEAs) have shown promising performance in multi-channel neurochemical sensing. A primary challenge faced by hybrid MEAs fabrication is the adhesion of the metal traces with the GC electrodes, as prolonged electrical and mechanical stimulation can lead to adhesion failure. Previous devices with GC electrodes and interconnects made of a homogeneous material (all GC) demonstrated exceptional electrochemical stability but required miniaturization for enhanced tissue integration and chronic electrochemical sensing. In this study, we used two different methods for the fabrication of all GC-MEAs on thin flexible substrates with miniaturized features. The first method, like that previously reported, involves a double pattern-transfer photolithographic process, including transfer-bonding on temporary polymeric support. The second method requires a double-etching process, which uses a 2 µm-thick low stress silicon nitride coating of the Si wafer as the bottom insulator layer for the MEAs, bypassing the pattern-transfer and demonstrating a novel technique with potential advantages. We confirmed the feasibility of the two fabrication processes by verifying the practical conductivity of 3 µm-wide 2 µm-thick GC traces, the GC microelectrode functionality, and their sensing capability for the detection of serotonin using fast scan cyclic voltammetry. Through the exchange and discussion of insights regarding the strengths and limitations of these microfabrication methods, our goal is to propel the advancement of GC-based MEAs for the next generation of neural interface devices.

SmartFSCV: An Artificial Intelligence Enabled Miniaturised FSCV Device Targeting Serotonin

Goal: Dynamically monitoring serotonin in real-time within target brain regions would significantly improve the diagnostic and therapeutic approaches to a variety of neurological and psychiatric disorders. Current systems for measuring serotonin lack immediacy and portability and are bulky and expensive. Methods: We present a new miniaturised device, named SmartFSCV, designed to monitor dynamic changes of serotonin using fast-scan cyclic voltammetry (FSCV). This device outputs a precision voltage potential between -3 to +3 V, and measures current between -1.5 to +1.5 μA with nano-ampere accuracy. The device can output modifiable arbitrary waveforms for various measurements and uses an N-shaped waveform at a scan-rate of 1000 V/s for sensing serotonin. Results: Four experiments were conducted to validate SmartFSCV: static bench test, dynamic serotonin test and two artificial intelligence (AI) algorithm tests. Conclusions: These tests confirmed the ability of SmartFSCV to accurately sense and make informed decisions about the presence of serotonin using AI.

Microinstrumentation for Brain Organoids

Brain organoids are three-dimensional aggregates of self-organized differentiated stem cells that mimic the structure and function of human brain regions. Organoids bridge the gaps between conventional drug screening models such as planar mammalian cell culture, animal studies, and clinical trials. They can revolutionize the fields of developmental biology, neuroscience, toxicology, and computer engineering. Conventional microinstrumentation for conventional cellular engineering, such as planar microfluidic chips; microelectrode arrays (MEAs); and optical, magnetic, and acoustic techniques, has limitations when applied to three-dimensional (3D) organoids, primarily due to their limits with inherently two-dimensional geometry and interfacing. Hence, there is an urgent need to develop new instrumentation compatible with live cell culture techniques and with scalable 3D formats relevant to organoids. This review discusses conventional planar approaches and emerging 3D microinstrumentation necessary for advanced organoid-machine interfaces. Specifically, this article surveys recently developed microinstrumentation, including 3D printed and curved microfluidics, 3D and fast-scan optical techniques, buckling and self-folding MEAs, 3D interfaces for electrochemical measurements, and 3D spatially controllable magnetic and acoustic technologies relevant to two-way information transfer with brain organoids. This article highlights key challenges that must be addressed for robust organoid culture and reliable 3D spatiotemporal information transfer.

Microfluidic flow injection analysis system for the electrochemical detection of dopamine using diazonium-grafted copper nanoparticles on multi-walled carbon nanotube-modified surfaces

In this proof-of-concept study, a microfluidic flow injection analysis (FIA) system was developed using multi-walled carbon nanotube-modified screen-printed carbon electrodes (CNTSPEs) that were modified with copper nanoparticles (CuNPs) following the electrodeposition of the diazonium salt of 4-aminothiophenol to form 4-thiophenol-conjugated CuNPs (CuNPs-CNTSPE). Transmission electron microscopy (TEM), atomic force microscopy (AFM), and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) were used to characterize the size of CuNPs, morphology and elemental analysis of CuNPs-CNTSPE, respectively. Using electrochemical impedance spectroscopy (EIS), the charge-transfer resistance (Rct) of CuNPs-CNTSPE was found to be 20-fold lower than that of CNTSPE. The CuNPs-CNTSPE displayed an oxidation peak for dopamine at -0.08 V which is ∼80 mV lower than the one detected using CNTSPE. The modified electrode was used in microfluidic flow injection analysis and offline systems for sensitive detection of dopamine (DA). The pH, flow rate, loop volume, concentration, and type of surfactant were all optimized for on-chip detection. Under the optimal conditions, using phosphate electrolyte solution (pH 6) containing 0.05% (w/v) Tween 20® as the carrier at a flow rate of 0.6 mL min-1 and a loop volume of 50 μL, the calibration curve was linear from 1.5 to 500 nM with a limit of detection of 0.33 nM. This technique was used for the successful detection of DA in real samples with recovery ranging from 96.5% to 103.8%. The microfluidic FIA system described here has the potential to be used as an electrochemical point-of-care device for rapid DA detection with high sensitivity and reproducibility.

Microcontact printing of choline oxidase using a polycation-functionalized zwitterionic polymer as enzyme immobilization matrix

Highly sensitive and selective choline microbiosensors were constructed by microcontact printing (μCP) of choline oxidase (ChOx) in a crosslinked, polyamine-functionalized zwitterionic polymer matrix on microelectrode arrays (MEAs). μCP has emerged as a potential means to create implantable, multiplexed sensor microprobes, which requires the targeted deposition of different sensor materials to specific microelectrode sites on a MEA. However, the less than sufficient enzyme loading and inadequate spatial resolution achieved with current μCP approaches has limited adoption of the method for electroenzymatic microsensors. A novel polymer, poly(2-methacryloyloxyethyl phosphorylcholine)-g-poly(allylamine hydrochloride) (PMPC-g-PAH), has been developed to address this challenge. PMPC-g-PAH contributes to a higher viscosity "ink" that enables thicker immobilized ChOx deposits of high spatial resolution while also providing a hydrophilic, biocompatible microenvironment for the enzyme. Electroenzymatic choline microbiosensors with sensitivity of 639 ± 96 nA μM-1 cm-2 (pH 7.4; n = 4) were constructed that also are selective against both ascorbic acid and dopamine, which are potential electroactive interfering compounds in the mammalian brain. The high sensitivities achieved can lead to smaller MEA microprobes that minimize tissue damage and make possible the monitoring of multiple neurochemicals simultaneously in vivo with high spatial resolution.

A facile approach to create sensitive and selective Cu(ii) sensors on carbon fiber microelectrodes

A facile platform derived from deposition of ethynyl linkers on carbon fiber microelectrodes has been developed for sensitive and selective sensing of Cu(ii). This study is the first to demonstrate the successful anodic deposition of ethynyl linkers, specifically 1,4-diethynylbenzene, onto carbon fiber microelectrodes. Multi-scan deposition of DEB on these microelectrodes resulted in an increased sensitivity and selectivity towards Cu(ii) that persists amidst other divalent interferents and displays sustained performance over four days while stored at room temperature. This method can be extended to other ethynyl terminal moieties, thereby creating a versatile chemical platform that will enable improved sensitivity and selectivity for a new frontier of biomarker measurement.

Recent Advances in Catecholamines Analytical Detection Methods and Their Pretreatment Technologies

Catecholamines (CAs), including adrenaline, noradrenaline, and dopamine, are neurotransmitters and hormones that play a critical role in regulating the cardiovascular system, metabolism, and stress response in the human body. As promising methods for real-time monitoring of catecholamine neurotransmitters, LC-MS detectors have gained widespread acceptance and shown significant progress over the past few years. Other detection methods such as fluorescence detection, colorimetric assays, surface-enhanced Raman spectroscopy, and surface plasmon resonance spectroscopy have also been developed to varying degrees. In addition, efficient pretreatment technology for CAs is flourishing due to the increasing development of many highly selective and recoverable materials. There are a few articles that provide an overview of electrochemical detection and efficient enrichment, but a comprehensive summary focusing on analytical detection technology is lacking. Thus, this review provides a comprehensive summary of recent analytical detection technology research on CAs published between 2017 and 2022. The advantages and limitations of relevant methods including efficient pretreatment technologies for biological matrices and analytical methods used in combination with pretreatment technology have been discussed. Overall, this review article provides a better understanding of the importance of accurate CAs measurement and offers perspectives on the development of novel methods for disease diagnosis and research in this field.

Measuring tryptophan dynamics using fast scan cyclic voltammetry at carbon fiber microelectrodes with improved sensitivity and selectivity

Despite the fact that tryptophan (Trp) is an essential amino acid that humans typically obtain through diet, there are several interesting tryptophan dynamics at play in the body. Quantifying and understanding these dynamics are crucial in studies of depression, autism spectrum disorder, and other disorders that involve neurotransmitters directly synthesized from tryptophan. Here we detail the optimization of waveform parameters in fast scan cyclic voltammetry at carbon fiber microelectrodes to yield four-fold higher sensitivity and six-fold higher selectivity compared to previously reported methods. We demonstrate the utility of our method in measuring (1) exogenous Trp dynamics from administration of Trp to PC-12 cells with and without overexpression of tryptophan hydroxylase-2 and (2) endogenous Trp dynamics in pinealocyte cultures with and without stimulation via norepinephrine. We observed interesting differences in Trp dynamics in both model systems, which demonstrate that our method is indeed sensitive to Trp dynamics in different applications.

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.

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.

Implantable flexible multielectrode arrays for multi-site sensing of serotonin tonic levels

Real-time multi-channel measurements of tonic serotonin (5-hydroxytryptamine, 5-HT) concentrations across different brain regions are of utmost importance to the understanding of 5-HT’s role in anxiety, depression, and impulse control disorders, which will improve the diagnosis and treatment of these neuropsychiatric illnesses. Chronic sampling of 5-HT is critical in tracking disease development as well as the time course of pharmacological treatments. Despite their value, in vivo chronic multi-site measurements of 5-HT have not been reported. To fill this technological gap, we batch fabricated implantable glassy carbon (GC) microelectrode arrays (MEAs) on a flexible SU-8 substrate to provide an electrochemically stable and biocompatible device/tissue interface. Then, to achieve multi-site detection of tonic 5-HT concentrations, we incorporated the poly(3,4-ethylenedioxythiophene)/functionalized carbon nanotube (PEDOT/CNT) coating on the GC microelectrodes in combination with a new square wave voltammetry (SWV) approach, optimized for selective 5-HT measurement. In vitro , the PEDOT/CNT coated GC microelectrodes achieved high sensitivity towards 5-HT, good fouling resistance in the presence of 5-HT, and excellent selectivity towards the most common neurochemical interferents. In vivo , our PEDOT/CNT-coated GC MEAs were able to successfully detect basal 5-HT concentrations at different locations of the CA2 hippocampal region of mice in both anesthetized and awake head-fixed conditions. Furthermore, the implanted PEDOT/CNT-coated MEA achieved stable detection of tonic 5-HT concentrations for one week. Finally, histology data in the hippocampus shows reduced tissue damage and inflammatory responses compared to stiff silicon probes. To the best of our knowledge, this PEDOT/CNT-coated GC MEA is the first implantable flexible multisite sensor capable of chronic in vivo multi-site sensing of tonic 5-HT. This implantable MEA can be custom-designed according to specific brain region of interests and research questions, with the potential to combine electrophysiology recording and multiple analyte sensing to maximize our understanding of neurochemistry.

Highlights

PEDOT/CNT-coated GC microelectrodes enabled sensitive and selective tonic detection of serotonin (5-HT) using a new square wave voltammetry (SWV) approach PEDOT/CNT-coated GC MEAs achieved multi-site in vivo 5-HT tonic detection for one week. Flexible MEAs lead to reduced tissue damage and inflammation compared to stiff silicon probes.

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.

Epitaxial Self-Assembly of Interfaces of 2D Metal-Organic Frameworks for Electroanalytical Detection of Neurotransmitters

This paper identifies the electrochemical properties of individual facets of anisotropic layered conductive metal-organic frameworks (MOFs) based on M₃(2,3,6,7,10,11-hexahydroxytriphenylene)₂ (M₃(HHTP)₂) (M = Co, Ni). The electroanalytical advantages of each facet are then applied toward the electrochemical detection of neurochemicals. By employing epitaxially controlled deposition of M₃(HHTP)₂ MOFs on electrodes, the contribution of the basal plane ({001} facets) and edge sites ({100} facets) of these MOFs can be individually determined using electrochemical characterization techniques. Despite having a lower observed heterogeneous electron transfer rate constant, the {001} facets of the M₃(HHTP)₂ systems prove more selective and sensitive for the detection of dopamine than the {100} facets of the same MOF, with the limit of detection (LOD) of 9.9 ± 2 nM in phosphate-buffered saline and 214 ± 48 nM in a simulated cerebrospinal fluid. Langmuir isotherm studies accompanied by all-atom MD simulations suggested that the observed improvement in performance and selectivity is related to the adsorption characteristics of analytes on the basal plane versus edge sites of the MOF interfaces. This work establishes that the distinct crystallographic facets of 2D MOFs can be used to control the fundamental interactions between analyte and electrode, leading to tunable electrochemical properties by controlling their preferential orientation through self-assembly.

Voltammetric Approach for Characterizing the Biophysical and Chemical Functionality of Human Induced Pluripotent Stem Cell-Derived Serotonin Neurons

Depression is quickly becoming one of the world's most pressing public health crises, and there is an urgent need for better diagnostics and therapeutics. Behavioral models in animals and humans have not adequately addressed the diagnosis and treatment of depression, and biomarkers of mental illnesses remain ill-defined. It has been very difficult to identify biomarkers of depression because of in vivo measurement challenges. While our group has made important strides in developing in vivo tools to measure such biomarkers (e.g., serotonin) in mice using voltammetry, these tools cannot be easily applied for depression diagnosis and drug screening in humans due to the inaccessibility of the human brain. In this work, we take a chemical approach, ex vivo, to introduce a human-derived system to investigate brain serotonin. We utilize human induced pluripotent stem cells differentiated into serotonin neurons and establish a new ex vivo model of real-time serotonin neurotransmission measurements. We show that evoked serotonin release responds to stimulation intensity and tryptophan preloading, and that serotonin release and reuptake kinetics resemble those found in vivo in rodents. Finally, after selective serotonin reuptake inhibitor (SSRI) exposure, we find dose-dependent internalization of the serotonin reuptake transporters (a signature of the in vivo response to SSRI). Our new human-derived chemical model has great potential to provide an ex vivo chemical platform as a translational tool for in vivo neuropsychopharmacology.

An Insight of Nanomaterials in Tissue Engineering from Fabrication to Applications

Tissue engineering is a research domain that deals with the growth of various kinds of tissues with the help of synthetic composites. With the culmination of nanotechnology and bioengineering, tissue engineering has emerged as an exciting domain. Recent literature describes its various applications in biomedical and biological sciences, such as facilitating the growth of tissue and organs, gene delivery, biosensor-based detection, etc. It deals with the development of biomimetics to repair, restore, maintain and amplify or strengthen several biological functions at the level of tissue and organs. Herein, the synthesis of nanocomposites based on polymers, along with their classification as conductive hydrogels and bioscaffolds, is comprehensively discussed. Furthermore, their implementation in numerous tissue engineering and regenerative medicine applications is also described. The limitations of tissue engineering are also discussed here. The present review highlights and summarizes the latest progress in the tissue engineering domain directed at functionalized nanomaterials.

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.

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.

Pushing the frontiers: tools for monitoring neurotransmitters and neuromodulators

Neurotransmitters and neuromodulators have a wide range of key roles throughout the nervous system. However, their dynamics in both health and disease have been challenging to assess, owing to the lack of in vivo tools to track them with high spatiotemporal resolution. Thus, developing a platform that enables minimally invasive, large-scale and long-term monitoring of neurotransmitters and neuromodulators with high sensitivity, high molecular specificity and high spatiotemporal resolution has been essential. Here, we review the methods available for monitoring the dynamics of neurotransmitters and neuromodulators. Following a brief summary of non-genetically encoded methods, we focus on recent developments in genetically encoded fluorescent indicators, highlighting how these novel indicators have facilitated advances in our understanding of the functional roles of neurotransmitters and neuromodulators in the nervous system. These studies present a promising outlook for the future development and use of tools to monitor neurotransmitters and neuromodulators.

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.

Semiconducting Polymers for Neural Applications

Electronically interfacing with the nervous system for the purposes of health diagnostics and therapy, sports performance monitoring, or device control has been a subject of intense academic and industrial research for decades. This trend has only increased in recent years, with numerous high-profile research initiatives and commercial endeavors. An important research theme has emerged as a result, which is the incorporation of semiconducting polymers in various devices that communicate with the nervous system—from wearable brain-monitoring caps to penetrating implantable microelectrodes. This has been driven by the potential of this broad class of materials to improve the electrical and mechanical properties of the tissue–device interface, along with possibilities for increased biocompatibility. In this review we first begin with a tutorial on neural interfacing, by reviewing the basics of nervous system function, device physics, and neuroelectrophysiological techniques and their demands, and finally we give a brief perspective on how material improvements can address current deficiencies in this system. The second part is a detailed review of past work on semiconducting polymers, covering electrical properties, structure, synthesis, and processing.

A review of chromatographic methods for bioactive tryptophan metabolites, kynurenine, kynurenic acid, quinolinic acid, and others, in biological fluids

sKynurenine (KYN) is synthesized from an essential amino acid, tryptophan by tryptophan 2,3-dioxygenase or indoleamine 2,3-dioxygenase via N-formyl- KYN in vivo. Subsequently, KYN acts as a precursor of some neuroactive metabolites such as kynurenic acid, quinolinic acid, and an important enzyme co-factor, nicotine adenine dinucleotide. These metabolites of tryptophan are a part of the "kynurenine pathway." In addition, KYN functions as an endogenous ligand for the aryl hydrocarbon receptor, which acts as a transcription factor. The levels of tryptophan metabolites are important for the assessment of the stage of neurological disorders, and hence, have garnered significant interest for clinical diagnosis. In this review, the detection of kynurenine, kynurenic acid, quinolinic acid, and other tryptophan metabolites performed via chromatographic methods such as HPLC using UV absorbance, fluorescence, and chromatographic-mass spectrometric detection is summarized.

Application of optogenetics and in vivo imaging approaches for elucidating the neurobiology of addiction

The neurobiology of addiction has been an intense topic of investigation for more than 50 years. Over this time, technological innovation in methods for studying brain function rapidly progressed, leading to increasingly sophisticated experimental approaches. To understand how specific brain regions, cell types, and circuits are affected by drugs of abuse and drive behaviors characteristic of addiction, it is necessary both to observe and manipulate neural activity in addiction-related behavioral paradigms. In pursuit of this goal, there have been several key technological advancements in in vivo imaging and neural circuit modulation in recent years, which have shed light on the cellular and circuit mechanisms of addiction. Here we discuss some of these key technologies, including circuit modulation with optogenetics, in vivo imaging with miniaturized single-photon microscopy (miniscope) and fiber photometry, and how the application of these technologies has garnered novel insights into the neurobiology of addiction.

Deep brain stimulation of the "medial forebrain bundle": a strategy to modulate the reward system and manage treatment-resistant depression

The medial forebrain bundle-a white matter pathway projecting from the ventral tegmental area-is a structure that has been under a lot of scrutinies recently due to its implications in the modulation of certain affective disorders such as major depression. In the following, we will discuss major depression in the context of being a disorder dependent on multiple relevant networks, the pathological performance of which is responsible for the manifestation of various symptoms of the disease which extend into emotional, motivational, physiological, and also cognitive domains of daily living. We will focus on the reward system, an evolutionarily conserved pathway whose underperformance leads to anhedonia and lack of motivation, which are key traits in depression. In the field of deep brain stimulation (DBS), different "hypothesis-driven" targets have been chosen as the subject of clinical trials on efficacy in the treatment-resistant depressed patient. The "medial forebrain bundle" is one such target for DBS, and has had remarkably rapid success in alleviating depressive symptoms, improving anhedonia and motivation. We will review what we have learned from pre-clinical animal studies on defining this white matter tract, its connectivity, and the complex molecular (i.e., neurotransmitter) mechanisms by which its modulation exerts its effects. Imaging studies in the form of tractographic depictions have elucidated its presence in the human brain. Such has led to ongoing clinical trials of DBS targeting this pathway to assess efficacy, which is promising yet still lack in sufficient numbers. Ultimately, one must confirm the mechanism of action and validate proof of antidepressant effect in order to have such treatment become mainstream, to promote widespread improvement in the quality of life of suffering patients.

Construction of a Au@MoS2 composite nanosheet biosensor for the ultrasensitive detection of a neurotransmitter and understanding of its mechanism based on DFT calculations

MoS₂ nanosheets can be applied as electrochemical biosensors to selectively and sensitively respond to the surrounding environment and detect various biomolecules due to their large specific surface area and unique physicochemical properties. In this paper, single-layer or few-layer MoS₂ nanosheets were prepared by an improved liquid phase stripping method, and then combining the unique material characteristics of MoS₂ and the metallic property of Au nanoparticles (AuNPs), Au@MoS₂ composite nanosheets were synthesized based on MoS₂ nanosheets. Then, the structure and properties of MoS₂ nanosheets and Au@MoS₂ composite nanosheets were comprehensively characterized. The results proved that AuNPs were successfully loaded on MoS₂ nanosheets. At the same time, on the basis of the successful preparation of Au@MoS₂ composite nanosheets, an electrochemical biosensor targeting dopamine was successfully constructed by cyclic voltammetry. The linear detection range was 0.5–350 μM, and the detection limit was 0.2 μM. The high-sensitive electrochemical detection of dopamine has been achieved, which provides a new idea for the application of MoS₂-based nanomaterials in the biosensing of neurotransmitters. In addition, density functional theory (DFT) was used to explore the electrochemical performance of Au@MoS₂ composite nanosheets. The results show that the adsorption of Au atoms on the MoS₂ 2D structure improves the conductivity of MoS₂ nanosheets, which theoretically supports the possibilities of its application as a platform for the ultrasensitive detection of neurotransmitters or other biomolecules in the field of disease diagnosis.

An electrochemical biosensor based on Au@MoS₂ composite nanosheets was successfully prepared for the high-sensitivity detection of dopamine.

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.

Simulated Performance of Electroenzymatic Glutamate Biosensors In Vivo Illuminates the Complex Connection to Calibration In Vitro

Detailed simulations show that the relationship between electroenzymatic glutamate (Glut) sensor performance in vitro and that modeled in vivo is complicated by the influence of both resistances to mass transfer and clearance rates of Glut and H₂O₂ in the brain extracellular space (ECS). Mathematical modeling provides a powerful means to illustrate how these devices are expected to respond to a variety of conditions in vivo in ways that cannot be accomplished readily using existing experimental techniques. Through the use of transient model simulations in one spatial dimension, it is shown that the sensor response in vivo may exhibit much greater dependence on H₂O₂ mass transfer and clearance in the surrounding tissue than previously thought. This dependence may lead to sensor signals more than double the expected values (based on prior sensor calibration in vitro) for Glut release events within a few microns of the sensor surface. The sensor response in general is greatly affected by the distance between the device and location of Glut release, and apparent concentrations reported by simulated sensors consistently are well below the actual Glut levels for events occurring at distances greater than a few microns. Simulations of transient Glut concentrations, including a physiologically relevant bolus release, indicate that detection of Glut signaling likely is limited to events within 30 μm of the sensor surface based on representative sensor detection limits. It follows that important limitations also exist with respect to interpretation of decays in sensor signals, including relation of such data to actual Glut concentration declines in vivo. Thus, the use of sensor signal data to determine quantitatively the rates of Glut uptake from the brain ECS likely is problematic. The model is designed to represent a broad range of relevant physiological conditions, and although limited to one dimension, provides much needed guidance regarding the interpretation in general of electroenzymatic sensor data gathered 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.

Carbon-based neural electrodes: promises and challenges

Neural electrodes are primary functional elements of neuroelectronic devices designed to record neural activity based on electrochemical signals. These electrodes may also be utilized for electrically stimulating the neural cells, such that their response can be simultaneously recorded. In addition to being medically safe, the electrode material should be electrically conductive and electrochemically stable under harsh biological environments. Mechanical flexibility and conformability, resistance to crack formation and compatibility with common microfabrication techniques are equally desirable properties. Traditionally, (noble) metals have been the preferred for neural electrode applications due to their proven biosafety and a relatively high electrical conductivity. Carbon is a recent addition to this list, which is far superior in terms of its electrochemical stability and corrosion resistance. Carbon has also enabled 3D electrode fabrication as opposed to the thin-film based 2D structures. One of carbon's peculiar aspects is its availability in a wide range of allotropes with specialized properties that render it highly versatile. These variations, however, also make it difficult to understand carbon itself as a unique material, and thus, each allotrope is often regarded independently. Some carbon types have already shown promising results in bioelectronic medicine, while many others remain potential candidates. In this topical review, we first provide a broad overview of the neuroelectronic devices and the basic requirements of an electrode material. We subsequently discuss the carbon family of materials and their properties that are useful in neural applications. Examples of devices fabricated using bulk and nano carbon materials are reviewed and critically compared. We then summarize the challenges, future prospects and next-generation carbon technology that can be helpful in the field of neural sciences. The article aims at providing a common platform to neuroscientists, electrochemists, biologists, microsystems engineers and carbon scientists to enable active and comprehensive efforts directed towards carbon-based neuroelectronic device fabrication.

Microelectrode Sensor for Real-Time Measurements of Nitrite in the Living Brain, in the Presence of Ascorbate

The impaired blood flow to the brain causes a decrease in the supply of oxygen that can result in cerebral ischemia; if the blood flow is not restored quickly, neuronal injury or death will occur. Under hypoxic conditions, the production of nitric oxide (^●NO), via the classical L-arginine–^●NO synthase pathway, is reduced, which can compromise ^●NO-dependent vasodilation. However, the alternative nitrite (NO₂⁻) reduction to ^●NO, under neuronal hypoxia and ischemia conditions, has been viewed as an in vivo storage pool of ^●NO, complementing its enzymatic synthesis. Brain research is thus demanding suitable tools to probe nitrite’s temporal and spatial dynamics in vivo. In this work, we propose a new method for the real-time measurement of nitrite concentration in the brain extracellular space, using fast-scan cyclic voltammetry (FSCV) and carbon microfiber electrodes as sensing probes. In this way, nitrite was detected anodically and in vitro, in the 5–500 µM range, in the presence of increasing physiological concentrations of ascorbate (100–500 µM). These sensors were then tested for real-time and in vivo recordings in the anesthetized rat hippocampus; using fast electrochemical techniques, local and reproducible transients of nitrite oxidation signals were observed, upon pressure ejection of an exogenous nitrite solution into the brain tissue. Nitrite microsensors are thus a valuable tool for investigating the role of this inorganic anion in brain redox signaling.

A concise review: the synergy between artificial intelligence and biomedical nanomaterials that empowers nanomedicine

Nanomedicine has recently experienced unprecedented growth and development. However, the complexity of operations at the nanoscale introduces a layer of difficulty in the clinical translation of nanodrugs and biomedical nanotechnology. This problem is further exacerbated when engineering and optimizing nanomaterials for biomedical purposes. To navigate this issue, artificial intelligence (AI) algorithms have been applied for data analysis and inference, allowing for a more applicable understanding of the complex interaction amongst the abundant variables in a system involving the synthesis or use of nanomedicine. Here, we report on the current relationship and implications of nanomedicine and AI. Particularly, we explore AI as a tool for enabling nanomedicine in the context of nanodrug screening and development, brain-machine interfaces and nanotoxicology. We also report on the current state and future direction of nanomedicine and AI in cancer, diabetes, and neurological disorder therapy.

3D fuzzy graphene microelectrode array for dopamine sensing at sub-cellular spatial resolution

The development of a high sensitivity real-time sensor for multi-site detection of dopamine (DA) with high spatial and temporal resolution is of fundamental importance to study the complex spatial and temporal pattern of DA dynamics in the brain, thus improving the understanding and treatments of neurological and neuropsychiatric disorders. In response to this need, here we present high surface area out-of-plane grown three-dimensional (3D) fuzzy graphene (3DFG) microelectrode arrays (MEAs) for highly selective, sensitive, and stable DA electrochemical sensing. 3DFG microelectrodes present a remarkable sensitivity to DA (2.12 ± 0.05 nA/nM, with LOD of 364.44 ± 8.65 pM), the highest reported for nanocarbon MEAs using Fast Scan Cyclic Voltammetry (FSCV). The high surface area of 3DFG allows for miniaturization of electrode down to 2 × 2 μm², without compromising the electrochemical performance. Moreover, 3DFG MEAs are electrochemically stable under 7.2 million scans of continuous FSCV cycling, present exceptional selectivity over the most common interferents in vitro with minimum fouling by electrochemical byproducts and can discriminate DA and serotonin (5-HT) in response to the injection of their 50:50 mixture. These results highlight the potential of 3DFG MEAs as a promising platform for FSCV based multi-site detection of DA with high sensitivity, selectivity, and spatial resolution.

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.

The brain in flux: Genetic, physiologic, and therapeutic perspectives on transporters in the CNS

Active and passive transporters constitute a gene family of approximately 2000 members. These proteins are required for import and export across the blood brain barrier, clearance of neurotransmitters, inter-cellular solute transfer, and transport across the membranes of subcellular organelles. Neurologic, neurodevelopmental, and psychiatric diseases have been linked to alterations in function and/or mutations in every one of these types of transporters, and many of the transporters are targeted by therapeutics. This is the 4th biennial special edition of Neurochemistry International that originates from a scientific meeting devoted to studies of transporters and their relationship to brain function and to neurodevelopmental, neurologic, and psychiatric disorders. This meeting provides the only international forum for the presentation and discussion of cutting-edge research on brain transporters covering fundamental aspects of transporter structure, function, and trafficking. Scientists describe the novel approaches being used to link this information to physiology/circuit function and behavior. The meeting also addresses translational topics surrounding mouse models of brain transporter disorders, novel human brain disorders arising from transporter mutations, and innovative therapeutic approaches centered on modification of transporter function. This special issue includes a sampling of review articles that address timely questions of the field and several primary research articles.

Next-Generation Diamond Electrodes for Neurochemical Sensing: Challenges and Opportunities

Carbon-based electrodes combined with fast-scan cyclic voltammetry (FSCV) enable neurochemical sensing with high spatiotemporal resolution and sensitivity. While their attractive electrochemical and conductive properties have established a long history of use in the detection of neurotransmitters both in vitro and in vivo, carbon fiber microelectrodes (CFMEs) also have limitations in their fabrication, flexibility, and chronic stability. Diamond is a form of carbon with a more rigid bonding structure (sp3-hybridized) which can become conductive when boron-doped. Boron-doped diamond (BDD) is characterized by an extremely wide potential window, low background current, and good biocompatibility. Additionally, methods for processing and patterning diamond allow for high-throughput batch fabrication and customization of electrode arrays with unique architectures. While tradeoffs in sensitivity can undermine the advantages of BDD as a neurochemical sensor, there are numerous untapped opportunities to further improve performance, including anodic pretreatment, or optimization of the FSCV waveform, instrumentation, sp2/sp3 character, doping, surface characteristics, and signal processing. Here, we review the state-of-the-art in diamond electrodes for neurochemical sensing and discuss potential opportunities for future advancements of the technology. We highlight our team's progress with the development of an all-diamond fiber ultramicroelectrode as a novel approach to advance the performance and applications of diamond-based neurochemical sensors.

Chemosensing of neurotransmitters with selectivity and naked eye detection of l-DOPA based on fluorescent Zn(ii)-terpyridine bearing boronic acid complexes

A new range of fluorescent Zn(ii)-terpyridine complexes are described and studied in-depth as chemosensors for catecholamine-based neurotransmitters in pure water.

"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.

Real-Time Fast Scan Cyclic Voltammetry Detection and Quantification of Exogenously Administered Melatonin in Mice Brain

Melatonin (MT) has been recently considered an excellent candidate for the treatment of sleep disorders, neural injuries, and neurological diseases. To better investigate the actions of MT in various brain functions, real-time detection of MT concentrations in specific brain regions is much desired. Previously, we have demonstrated detection of exogenously administered MT in anesthetized mouse brain using square wave voltammetry (SWV). Here, for the first time, we show successful detection of exogenous MT in the brain using fast scan cyclic voltammetry (FSCV) on electrochemically pre-activated carbon fiber microelectrodes (CFEs). In vitro evaluation showed the highest sensitivity (28.1 nA/μM) and lowest detection limit (20.2 ± 4.8 nM) ever reported for MT detection at carbon surface. Additionally, an extensive CFE stability and fouling assessment demonstrated that a prolonged CFE pre-conditioning stabilizes the background, in vitro and in vivo, and provides consistent CFE sensitivity over time even in the presence of a high MT concentration. Finally, the stable in vivo background, with minimized CFE fouling, allows us to achieve a drift-free FSCV detection of exogenous administered MT in mouse brain over a period of 3 min, which is significantly longer than the duration limit (usually < 90 s) for traditional in vivo FSCV acquisition. The MT concentration and dynamics measured by FSCV are in good agreement with SWV, while microdialysis further validated the concentration range. These results demonstrated reliable MT detection using FSCV that has the potential to monitor MT in the brain over long periods of time.