Monitoring rapid chemical communication in the brain

Recent advances of electrochemical and optical point-of-care biosensors for detecting neurotransmitter serotonin biomarkers

Since its discovery in 1984, the monoamine serotonin (5-HT) has been recognized for its critical role as a neuromodulator in both the central and peripheral nervous systems. Recent research reveals that serotonin also significantly influences various neuronal activities. Historically, it was believed that peripheral serotonin, produced by tryptophan hydroxylase in intestinal cells, functioned primarily as a hormone. However, new insights have expanded its known roles, necessitating advanced detection methods. Biosensors have emerged as indispensable tools in biomedical diagnostics, enabling the rapid and minimally invasive detection of target analytes with high spatial and temporal resolution. This review summarizes the progress made in the past decade in developing optical and electrochemical biosensors for serotonin detection. We evaluate various sensing strategies that optimize performance in terms of detection limits, sensitivity, and specificity. The study also explores recent innovations in biosensing technologies utilizing surface-modified electrodes with nanomaterials, including gold, graphite, carbon nanotubes, and metal oxide particles. Applications range from in vivo studies to chemical imaging and diagnostics, highlighting future prospects in the field.

Wideband ratiometric measurement of tonic and phasic dopamine release in the striatum

Reward learning, cognition, and motivation are supported by changes in neurotransmitter levels across multiple timescales. Current measurement technologies for various neuromodulators (such as dopamine and serotonin) do not bridge timescales of fluctuations, limiting the ability to define the behavioral significance, regulation, and relationship between fast (phasic) and slow (tonic) dynamics. To help resolve longstanding debates about the behavioral significance of dopamine across timescales, we developed a novel quantification strategy, augmenting extensively used carbon-fiber Fast Scan Cyclic Voltammetry (FSCV). We iteratively engineered the FSCV scan sequence to rapidly modify electrode sensitivity within a sampling window and applied ratiometric analysis for wideband dopamine measurement. This allowed us to selectively eliminate artifacts unrelated to electrochemical detection (i.e., baseline drift), overcoming previous limitations that precluded wideband dopamine detection from milliseconds to hours. We extensively characterize this approach in vitro, validate performance in vivo with simultaneous microdialysis, and deploy this technique to measure wideband dopamine changes across striatal regions under pharmacological, optogenetic, and behavioral manipulations. We demonstrate that our approach can extend to additional analytes, including serotonin and pH, providing a robust platform to assess the contributions of multi-timescale neuromodulator fluctuations to cognition, learning, and motivation.

Multiplexed neurochemical sensing with sub-nM sensitivity across 2.25 mm2 area

Multichannel arrays capable of real-time sensing of neuromodulators in the brain are crucial for gaining insights into new aspects of neural communication. However, measuring neurochemicals, such as dopamine, at low concentrations over large areas has proven challenging. In this research, we demonstrate a novel approach that leverages the scalability and processing power offered by microelectrode array devices integrated with a functionalized, high-density microwire bundle, enabling electrochemical sensing at an unprecedented scale and spatial resolution. The sensors demonstrate outstanding selective molecular recognition by incorporating a selective polymeric membrane. By combining cutting-edge commercial multiplexing, digitization, and data acquisition hardware with a bio-compatible and highly sensitive neurochemical interface array, we establish a powerful platform for neurochemical analysis. This multichannel array has been successfully utilized in vitro and ex vivo systems. Notably, our results show a sensing area of 2.25 mm2 with an impressive detection limit of 820 pM for dopamine. This new approach paves the way for investigating complex neurochemical processes and holds promise for advancing our understanding of brain function and neurological disorders.

Visual Detection of Dopamine with CdS/ZnS Quantum Dots Bearing by ZIF-8 and Nanofiber Membranes

Dopamine (DA) is a widely present, calcium cholinergic neurotransmitter in the body, playing important roles in the central nervous system and cardiovascular system. Developing fast and sensitive DA detection methods is of great significance. Fluorescence-based methods have attracted much attention due to their advantages of easy operation, a fast response speed, and high sensitivity. This study prepared hydrophilic and high-performance CdS/ZnS quantum dots (QDs) for DA detection. The waterborne CdS/ZnS QDs were synthesized in one step using the amphiphilic polymer PEI-g-C14, obtained by grafting tetradecane (C14) to polyethyleneimine (PEI), as a template. The polyacrylonitrile nanofiber membrane (PAN-NFM) was prepared by electrospinning (e-spinning), and a metal organic frame (ZIF-8) was deposited in situ on the surface of the PAN-NFM. The CdS/ZnS QDs were loaded onto this substrate (ZIF-8@PAN-NFM). The results showed that after the deposition of ZIF-8, the water contact angle of the hydrophobic PAN-NFM decreased to within 40°. The nanofiber membrane loaded with QDs also exhibited significant changes in fluorescence in the presence of DA at different concentrations, which could be applied as a fast detection method of DA with high sensitivity. Meanwhile, the fluorescence on this PAN-NFM could be visually observed as it transitioned from a blue-green color to colorless, making it suitable for the real-time detection of DA.

Inflammation-free electrochemical in vivo sensing of dopamine with atomic-level engineered antioxidative single-atom catalyst

Electrochemical methods with tissue-implantable microelectrodes provide an excellent platform for real-time monitoring the neurochemical dynamics in vivo due to their superior spatiotemporal resolution and high selectivity and sensitivity. Nevertheless, electrode implantation inevitably damages the brain tissue, upregulates reactive oxygen species level, and triggers neuroinflammatory response, resulting in unreliable quantification of neurochemical events. Herein, we report a multifunctional sensing platform for inflammation-free in vivo analysis with atomic-level engineered Fe single-atom catalyst that functions as both single-atom nanozyme with antioxidative activity and electrode material for dopamine oxidation. Through high-temperature pyrolysis and catalytic performance screening, we fabricate a series of Fe single-atom nanozymes with different coordination configurations and find that the Fe single-atom nanozyme with FeN4 exhibits the highest activity toward mimicking catalase and superoxide dismutase as well as eliminating hydroxyl radical, while also featuring high electrode reactivity toward dopamine oxidation. These dual functions endow the single-atom nanozyme-based sensor with anti-inflammatory capabilities, enabling accurate dopamine sensing in living male rat brain. This study provides an avenue for designing inflammation-free electrochemical sensing platforms with atomic-precision engineered single-atom catalysts.

DNA tetrahedral nanocages as a promising nanocarrier for dopamine delivery in neurological disorders

Dopamine is a neurotransmitter in the central nervous system that is essential for many bodily and mental processes, and a lack of it can cause Parkinson's disease. DNA tetrahedral (TD) nanocages are promising in bio-nanotechnology, especially as a nanocarrier. TD is highly programmable, biocompatible, and capable of cell differentiation and proliferation. It also has tissue and blood-brain barrier permeability, making it a powerful tool that could overcome potential barriers in treating neurological disorders. In this study, we used DNA TD as a carrier for dopamine to cells and zebrafish embryos. We investigated the mechanism of complexation between TD and dopamine hydrochloride using gel electrophoresis, fluorescence and circular dichroism (CD) spectroscopy, atomic force microscopy (AFM), and molecular dynamic (MD) simulation tools. Further, we demonstrate that these dopamine-loaded DNA TD nanostructures enhanced cellular uptake and differentiation ability in SH-SY5Y neuroblastoma cells. Furthermore, we extended the study to zebrafish embryos as a model organism to examine survival and uptake. The research provides valuable insights into the complexation mechanism and cellular uptake of dopamine-loaded DNA tetrahedral nanostructures, paving the way for further advancements in nanomedicine for Parkinson's disease and other neurological disorders.

Design and Synthesis of a Novel Binuclear Palladium Complex as a Turn on Fluorescent Receptor for Neurotransmitter Dopamine

A novel binuclear palladium complex has been synthesized by the reaction of tetramethylethylenediamine palladium nitrate and sodium salt of pyromellitic acid. UV vis, NMR as well as fluorescent titration techniques show that this binuclear palladium complex interacts with neurotransmitter dopamine. The fluorescence of the palladium complex in aqueous solution gets enhanced with the gradual addition of the neurotransmitter dopamine which makes this complex to act as a turn on fluorescent sensor for neurotransmitter dopamine.

Ultrasensitive Monolithic Dopamine Microsensors Employing Vertically Aligned Carbon Nanofibers

Brain-on-Chip devices, which facilitate on-chip cultures of neurons to simulate brain functions, are receiving tremendous attention from both fundamental and clinical research. Consequently, microsensors are being developed to accomplish real-time monitoring of neurotransmitters, which are the benchmarks for neuron network operation. Among these, electrochemical sensors have emerged as promising candidates for detecting a critical neurotransmitter, dopamine. However, current state-of-the-art electrochemical dopamine sensors are suffering from issues like limited sensitivity and cumbersome fabrication. Here, a novel route in monolithically microfabricating vertically aligned carbon nanofiber electrochemical dopamine microsensors is reported with an anti-blistering slow cooling process. Thanks to the microfabrication process, microsensors is created with complete insulation and large surface areas. The champion device shows extremely high sensitivity of 4.52× 104 µAµM-1·cm-2, which is two-orders-of-magnitude higher than current devices, and a highly competitive limit of detection of 0.243 nM. These remarkable figures-of-merit will open new windows for applications such as electrochemical recording from a single neuron.

Monitoring norepinephrine release in vivo using next-generation GRABNE sensors

Norepinephrine (NE) is an essential biogenic monoamine neurotransmitter. The first-generation NE sensor makes in vivo, real-time, cell-type-specific and region-specific NE detection possible, but its low NE sensitivity limits its utility. Here, we developed the second-generation GPCR-activation-based NE sensors (GRABNE2m and GRABNE2h) with a superior response and high sensitivity and selectivity to NE both in vitro and in vivo. Notably, these sensors can detect NE release triggered by either optogenetic or behavioral stimuli in freely moving mice, producing robust signals in the locus coeruleus and hypothalamus. With the development of a novel transgenic mouse line, we recorded both NE release and calcium dynamics with dual-color fiber photometry throughout the sleep-wake cycle; moreover, dual-color mesoscopic imaging revealed cell-type-specific spatiotemporal dynamics of NE and calcium during sensory processing and locomotion. Thus, these new GRABNE sensors are valuable tools for monitoring the precise spatiotemporal release of NE in vivo, providing new insights into the physiological and pathophysiological roles of NE.

Aptamer-Based Potentiometric Sensor Enables Highly Selective and Neurocompatible Neurochemical Sensing in Rat Brain

Selective and nondisruptive in vivo neurochemical monitoring within the central nervous system has long been a challenging endeavor. We introduce a new sensing approach that integrates neurocompatible galvanic redox potentiometry (GRP) with customizable phosphorothioate aptamers to specifically probe dopamine (DA) dynamics in live rat brains. The aptamer-functionalized GRP (aptGRP) sensor demonstrates nanomolar sensitivity and over a 10-fold selectivity for DA, even amidst physiological levels of major interfering species. Notably, conventional sensors without the aptamer modification exhibit negligible reactivity to DA concentrations exceeding 20 μM. Critically, the aptGRP sensor operates without altering neuronal activity, thereby permitting real-time, concurrent recordings of both DA flux and electrical signaling in vivo. This breakthrough establishes aptGRP as a viable and promising framework for the development of high-fidelity sensors, offering novel insights into neurotransmission dynamics in a live setting.

Suppression of Resistive Coupling in Nanogap Electrochemical Cell: Resolution of Dual Pathways for Dopamine Oxidation

A nanogap cell involves two working electrodes separated by a nanometer-wide solution to enable unprecedented electrochemical measurements. The powerful nanogap measurements, however, can be seriously interfered with by resistive coupling between the two electrodes to yield erroneous current responses. Herein, we employ the nanogap cell based on double carbon-fiber microelectrodes to suppress resistive coupling for the assessment of intrinsic current responses. Specifically, we modify a commercial bipotentiostat to compensate the Ohmic potential drop shared by the two electrodes through the common current pathway with a fixed resistance in the solution. Resistive coupling through both non-Faradaic and Faradaic processes is suppressed to eliminate erroneous current responses. Our approach is applied to investigate the mechanism of dopamine oxidation at carbon-fiber microelectrodes as important electrochemical sensors for the crucial neurotransmitter. Resistive coupling is suppressed to manifest the intrinsic current responses based on the oxidation of both adsorbed and non-adsorbed forms of dopamine to the respective forms of dopamine-o-quinone. The simultaneous dual oxidation pathways are observed for the first time and can be mediated through either non-concerted or concerted mechanisms of adsorption-coupled electron transfer. The two mechanisms are not discriminated for the two-electron oxidation of dopamine because it can not be determined whether the intermediate, dopamine semi-quinone, is adsorbed on the electrode surface. Significantly, our approach will be useful to manifest intrinsic current responses without resistive coupling for nanogaps and microgaps, which are too narrow to eliminate the common solution resistance by optimizing the position of a reference electrode.

Microdialysis coupled with droplet microfluidics and mass spectrometry for determination of neurotransmitters in vivo with high temporal resolution

Monitoring the concentration fluctuations of neurotransmitters in vivo is valuable for elucidating the chemical signals that underlie brain functions. Microdialysis sampling is a widely used tool for monitoring neurochemicals in vivo. The volume requirements of most techniques that have been coupled to microdialysis, such as HPLC, result in fraction collection times of minutes, thus limiting the temporal resolution possible. Further the time of analysis can become long for cases where many fractions are collected. Previously we have used direct analysis of dialysate by low-flow electrospray ionization-tandem mass spectrometry (ESI-MS/MS) on a triple quadrupole mass spectrometer to monitor acetylcholine, glutamate, and γ-amino-butyric acid to achieve multiplexed in vivo monitoring with temporal resolution of seconds. Here, we have expanded this approach to adenosine, dopamine, and serotonin. The method achieved limits of detection down to 2 nM, enabling basal concentrations of all these compounds, except serotonin, to be measured in vivo. Comparative analysis with LC-MS/MS showed accurate results for all compounds except for glutamate, possibly due to interference for this compound in vivo. Pairing this analysis with droplet microfluidics yields 11 s temporal resolution and can generate dialysate fractions down to 3 nL at rates up to 3 fractions per s from a microdialysis probe. The system is applied to multiplexed monitoring of neurotransmitter dynamics in response to stimulation by 100 mM K+ and amphetamine. These applications demonstrate the suitability of the droplet ESI-MS/MS method for monitoring short-term dynamics of up to six neurotransmitters simultaneously.

Additive Blending Effects on PEDOT:PSS Composite Films for Wearable Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) employing conductive polymers (CPs) have gained remarkable prominence and have undergone extensive advancements in wearable and implantable bioelectronic applications in recent years. Among the diverse arrays of CPs, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a common choice for the active-layer channel in p-type OECTs, showing a remarkably high transconductance for the high amplification of signals in biosensing applications. This investigation focuses on the novel engineering of PEDOT:PSS composite materials by seamlessly integrating several additives, namely, dimethyl sulfoxide (DMSO), (3-glycidyloxypropyl)trimethoxysilane (GOPS), and a nonionic fluorosurfactant (NIFS), to fine-tune their electrical conductivity, self-healing capability, and stretchability. To elucidate the intricate influences of the DMSO, GOPS, and NIFS additives on the formation of PEDOT:PSS composite films, theoretical calculations were performed, encompassing the solubility parameters and surface energies of the constituent components of the NIFS, PEDOT, PSS, and PSS-GOPS polymers. Furthermore, we conducted a comprehensive array of material analyses, which reveal the intricacies of the phase separation phenomenon and its interaction with the materials' characteristics. Our research identified the optimal composition for the PEDOT:PSS composite films, characterized by outstanding self-healing and stretchable capabilities. This composition has proven to be highly effective for constructing an active-layer channel in the form of OECT-based biosensors fabricated onto polydimethylsiloxane substrates for detecting dopamine. Overall, these findings represent significant progress in the application of PEDOT:PSS composite films in wearable bioelectronics and pave the way for the development of state-of-the-art biosensing technologies.

Does phasic dopamine release cause policy updates?

Phasic dopamine activity is believed to both encode reward-prediction errors (RPEs) and to cause the adaptations that these errors engender. If so, a rat working for optogenetic stimulation of dopamine neurons will repeatedly update its policy and/or action values, thus iteratively increasing its work rate. Here, we challenge this view by demonstrating stable, non-maximal work rates in the face of repeated optogenetic stimulation of midbrain dopamine neurons. Furthermore, we show that rats learn to discriminate between world states distinguished only by their history of dopamine activation. Comparison of these results to reinforcement learning simulations suggests that the induced dopamine transients acted more as rewards than RPEs. However, pursuit of dopaminergic stimulation drifted upwards over a time scale of days and weeks, despite its stability within trials. To reconcile the results with prior findings, we consider multiple roles for dopamine signalling.

An ultra-sensitive platinized nanocavity electrode for analysis of cytosolic catecholamines in one living cell

The catecholamines, mainly dopamine (DA), are present in the cellular cytosol with low abundance, while, play key roles in various neurodegenerative disorders. Here, platinized nanocavity carbon electrodes are employed to analyze cytosolic catecholamines in a single living PC12 cell, which is not easily quantified using the classic electrodes. The confined structure and excellent conductivity in the platinized nanocavity accelerate the electron transfer of the DA, resulting in a low detection limit down to 50 nM. The sensitivity of DA detection is improved to be 10.73 pA mM-1 nm-1 in the response range of 50 nM-100 μM, which guarantees quantitative analysis of cytosolic catecholamines with low abundance. Eventually, the platinized nanocavity electrode is employed to detect cytosolic catecholamines in a single PC12 cell without an obvious interruption of cellular catecholamine level. The cytosolic catecholamines in a single PC12 cell is measured in situ to be 0.1 μM, which is achieved for the first time at the single cell level using the electrochemical method. The results demonstrate that the nanocavity electrode with a high sensitivity could offer a promising means to dynamically track catecholamines in a single cell.

Nanozyme catalyzed-SERRS sensor for the recognition of dopamine based on AgNPs@PVP with oxidase-like activity

Dopamine (DA), as one of the most significant neurotransmitters, is closely related to several diseases. Achieving rapid and sensitive detection of DA remains a challenge. Herein, we proposed a simple, fast, and sensitive method for DA recognition based on surface-enhanced resonance Raman scattering (SERRS) technique. The synthesized silver nanoparticles coated with polyvinylpyrrolidone (AgNPs@PVP) with oxidase activity could not only oxidize 3,3',5,5'-tetramethylbenzidine (TMB) directly to produce a blue oxidation state TMB (oxTMB) but also could be used as the SERS substrate to generate a strong SERRS signal. When DA was added to the above system, the blue color faded along with the decrease in the SERRS signal. The change value of SERRS intensity was in proportion to the concentration of DA in the range of 0.1-10 μM with a limit of detection of 40 nM. This method presented great potential for the recognition of DA-related diseases.

A colorimetric detection of dopamine in urine and serum based on the CeO2 @ZIF-8/Cu-CDs laccase-like nanozyme activity

This study reports a sensitive and selective colorimetric approach for the analysis of dopamine (DA) based on CeO2 @ZIF-8/Cu-CDs laccase-like nanozymes activity. The CeO2 @ZIF-8/Cu-CDs was synthesized using cerium oxide (CeO2 ) and copper-doped carbon dots (Cu-CDs) with 2-methylimidazole by a facilely hydrothermal approach. The CeO2 @ZIF-8/Cu-CDs exhibited excellent laccase-like nanozymes activity and can oxidize the colorless substrate (DA) to red product with 4-aminoantipyrine as the chromogenic agent. The Michaelis-Menten constant (Km ) and the maximal velocity (Vmax ) of CeO2 @ZIF-8/Cu-CDs are 0.20 mM and 1.48 μM/min, respectively. The detection method has a linear range of 0.05-7.5 μg/mL and a detection limit as low as 8.5 ng/mL with good reproducibility. The developed colorimetric sensor was applied to rapid and precise quantitative evaluation of DA levels in serum and urine samples. This study presents a new approach for detecting biological molecules by utilizing the controlled regulation of nanozymes' laccase-like activity.

Development of a Smart Wireless Multisensor Platform for an Optogenetic Brain Implant

Implantable cell replacement therapies promise to completely restore the function of neural structures, possibly changing how we currently perceive the onset of neurodegenerative diseases. One of the major clinical hurdles for the routine implementation of stem cell therapies is poor cell retention and survival, demanding the need to better understand these mechanisms while providing precise and scalable approaches to monitor these cell-based therapies in both pre-clinical and clinical scenarios. This poses significant multidisciplinary challenges regarding planning, defining the methodology and requirements, prototyping and different stages of testing. Aiming toward an optogenetic neural stem cell implant controlled by a smart wireless electronic frontend, we show how an iterative development methodology coupled with a modular design philosophy can mitigate some of these challenges. In this study, we present a miniaturized, wireless-controlled, modular multisensor platform with fully interfaced electronics featuring three different modules: an impedance analyzer, a potentiostat and an optical stimulator. We show the application of the platform for electrical impedance spectroscopy-based cell monitoring, optical stimulation to induce dopamine release from optogenetically modified neurons and a potentiostat for cyclic voltammetry and amperometric detection of dopamine release. The multisensor platform is designed to be used as an opto-electric headstage for future in vivo animal experiments.

A ratiometric fluorescence sensor based on the inner filtration effect of gold nanoparticles on quantum dots for monitoring dopamine

In this study, a smart phone assisted ratiometric fluorescence sensor was designed for detecting dopamine (DA). The ratiometric fluorescence sensor was prepared by simple physical mixing green quantum dots (GQDS) and red quantum dots (RQDS). DA could induce gold nanoparticles (AuNPs) aggregate via hydrogen-bonding interactions, and further changed the absorption spectrum of gold nanoparticles to overlap with a certain emission spectrum of ratiometric fluorescence sensor. AuNPs had inner filtration effect (IFE) on the ratiometric fluorescence sensor. Due to the IFE, the dispersive AuNPs could quench GQDS, whereas the clustered AuNPs could quench RQDS. With the addition of DA, the color of ratiometric fluorescence changed from orange red to green. To simplify the detection process, a smartphone was employed to detecting DA in human urine by measuring RGB value of fluorescence color changes with a detection limit of 86 nM. This proposed method has the advantages of low cost, easy prevalence and simple operation, thus provides a great promise for rapid detection of biomarker in biological samples.

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

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

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.

Bioinspired CuZn-N/C Single-Atom Nanozyme with High Substrate Specificity for Selective Online Monitoring of Epinephrine in Living Brain

Though many elegant laccase mimics have emerged, these mimics generally have no substrate selectivity as well as low activity, making it difficult to fulfill the demand for monitoring in physiological conditions. Herein, inspired by the Cu-N ligand structure in the active site of natural laccase, we revealed that a carbon nanomaterial with atomically dispersed Cu and Zn atoms (CuZn-N/C) and a well-defined ligand structure could function as an effective laccase mimic for selectively catalyzing epinephrine (EP) oxidation. Catalytic activity of the CuZn-N/C nanozyme was superior to those of Cu-N/C and Zn-N/C and featured a Km value nearly 3-fold lower than that of natural laccase, which indicated that CuZn-N/C has a better affinity for EP. Density functional theory (DFT) revealed the mechanism of the superior catalytic ability of dual-metal CuZn-N/C as follows: (1) the exact distance of the two metal atoms in the CuZn-N/C catalyst makes it suitable for adsorption of the EP molecule, and the CuZn-N/C catalyst can offer the second hydrogen bond that stabilizes the adsorption; (2) molecular orbitals and density of states indicate that the strong interaction between the EP molecule and CuZn-N/C is important for EP catalytic oxidization. Furthermore, a sensitive and selective online optical detection platform (OODP) is constructed for determining EP with a low limit of detection (LOD) of 0.235 μM and a linear range of 0.2-20 μM. The system allows real-time measurement of EP release in the rat brain in vivo following ischemia with dexmedetomidine administration. This work not only provides an idea of designing efficient laccase mimics but also builds a promising chemical platform for better understanding EP-related drug action for ischemic cerebrovascular illnesses and opens up possibilities to explore brain function.

New Opportunities of Electrochemistry for Monitoring, Modulating, and Mimicking the Brain Signals

In vivo electrochemistry is a powerful key for unlocking the chemical consequences in neural networks of the brain. The past half-century has witnessed the technology revolutionization in this field along with innovations in electrochemical concepts, principles, methods, and devices. Present applications of electrochemical approaches have extended from measuring neurochemical concentrations to modulating and mimicking brain signals. In this Perspective, newly reported strategies for tackling long-standing challenges of in vivo electrochemical brain monitoring (i.e., basal level measurement, electroactivity dependence, in vivo stability, neuron compatibility, multiplexity, and implantable device fabrication) are highlighted. Moreover, recent progress on neuromodulation tools and neuromorphic devices in electrochemical frameworks is introduced. A glimpse of future opportunities for electrochemistry in brain research is offered at last.

Implantable Electrochemical Sensors for Brain Research

Implantable electrochemical sensors provide reliable tools for in vivo brain research. Recent advances in electrode surface design and high-precision fabrication of devices led to significant developments in selectivity, reversibility, quantitative detection, stability, and compatibility of other methods, which enabled electrochemical sensors to provide molecular-scale research tools for dissecting the mechanisms of the brain. In this Perspective, we summarize the contribution of these advances to brain research and provide an outlook on the development of the next generation of electrochemical sensors for the brain.

Selective Aptamer Modification of Au Surfaces in a Microelectrode Sensor Array for Simultaneous Detection of Multiple Analytes

Aptamers have been employed as the biorecognition element in electrochemical aptamer-based (E-AB) biosensors, for the detection of a diverse range of analyte molecules, on electrodes with sizescales ranging from a few microns to several millimeters. Simultaneous detection of multiple different analytes requires the selective modification of multiple electrode surfaces with different aptamers. This process is typically achieved by incubating separate macroscale electrodes in a solution with the desired aptamer, which is unsuitable for microelectrode arrays in which the electrodes are closely spaced. In this work, we selectively modified electrode surfaces with thiolated aptamers of different single-stranded DNA sequences, by successive removal and addition of thiol monolayers. This was achieved by electrodesorption of thiol monolayers using controlled potential, to expose unmodified gold electrodes to be modified with a different thiolated aptamer, thus enabling multiple different aptamers to be used on the surfaces of closely spaced microelectrodes. All aptamers were methylene blue terminated, allowing redox currents to be measured and used to monitor aptamer probe packing density on the electrode surface and the selectivity of the sensors. Here, we demonstrate the microscale E-AB sensor multianalyte detection method using aptamers for target analytes, adenosine triphosphate, dopamine, and serotonin, which can ultimately be applied to perform localized simultaneous detection using electrode arrays.

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.

Hydrogel-Coated Microelectrode Resists Protein Passivation of In Vivo Amperometric Sensors

Passivation of electrodes caused by nonspecific adsorption of protein can dramatically reduce sensing sensitivity and accuracy, which is a great challenge for in vivo neurochemical monitoring. However, most antipassivation strategies are not suitable to carbon fiber microelectrodes (CFMEs) for in vivo measurement, and these methods also do not work on electrochemical biosensors that fix biometric elements. In this study, we demonstrate that chitosan hydrogel-coated microelectrodes can avoid the current passivation caused by protein adsorption on the surface of carbon fiber because the chitosan hydrogel prepared by local pH gradient caused by hydrogen evolution reaction has three-dimensional networks containing large amounts of water. The highly hydrophilic three-dimensional structure of hydrogel not only forms a biocompatible interface to confine enzymes but also keeps the fast mass transfer of analytes, such as dopamine, ascorbic acid, and glucose. The consistency of the precalibration and postcalibration of the prepared sensor enables in vivo amperometric detection of both electroactive species based on their redox property and electroinactive species based on the enzyme. This study provides a simple and versatile strategy to constitute an amperometric sensor interface to resist passivation of protein adsorption in a complex biological environment such as the brain.

Conductive Metal-Organic Framework Microelectrodes Regulated by Conjugated Molecular Wires for Monitoring of Dopamine in the Mouse Brain

Herein, we demonstrated a strategy to regulate the conductive metal-organic framework (MOF) surface, by the conjugated molecule wires for selective and sensitive determination of dopamine (DA) in the live brain. The MOFs were decorated at the carbon fiber electrode deposited by Au nanoleaves as the upper electric transducer to provide rich electrocatalytic sites for electron transfer of neurochemicals at the electrode surface, leading to greatly enhanced sensitivity for detection of neurochemicals. On the other hand, the conjugated molecular wire, 4-(thiophen-3-ylethynyl)-benzaldehyde (RP1), was synthesized and assembled as an underlying bridge to regulate the electrochemical processes at the MOF-based electrode, specifically decreasing the reaction Gibbs free energy of DA oxidation, thus selectively promoting the heterogeneous electron transfer of DA from the MOF layer to the electrode surface. Owing to the electrocatalytic activity for DA oxidation, the present microsensor exhibited high selectivity for real-time tracking of DA in a good linear relationship in the range of 0.004-0.4 μM with a detection limit of 1 nM. Eventually, this functionalized electrode was successfully applied for in vivo monitoring of DA in mouse brains with Parkinson's disease (PD) model. The results indicated that the levels of DA were obviously decreased in both acute and subacute PD models. Moreover, the level of DA strongly depended on the amount of uric acid (UA), a physiological antioxidant, which rose as the UA amount was lower than 200 mg kg^-1 but was downregulated again after treatment by a higher amount of UA.

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.

In Vivo Electrochemical Biosensors: Recent Advances in Molecular Design, Electrode Materials, and Electrochemical Devices

Electrochemical biosensors provide powerful tools for dissecting the dynamically changing neurochemical signals in the living brain, which contribute to the insight into the physiological and pathological processes of the brain, due to their high spatial and temporal resolutions. Recent advances in the integration of in vivo electrochemical sensors with cross-disciplinary advances have reinvigorated the development of in vivo sensors with even better performance. In this Review, we summarize the recent advances in molecular design, electrode materials, and electrochemical devices for in vivo electrochemical sensors from molecular to macroscopic dimensions, highlighting the methods to obtain high performance for fulfilling the requirements for determination in the complex brain through flexible and smart design of molecules, materials, and devices. Also, we look forward to the development of next-generation in vivo electrochemical biosensors.

Theoretical and Cyclic Voltammetric Analysis of Asparagine and Glutamine Electrocatalytic Activities for Dopamine Sensing Applications

The molecular dynamics and density functional theory (DFT) can be applied to discriminate electrocatalyst’s electron transfer (ET) properties. It will be interesting to discriminate the ET properties of green electrocatalysts such as amino acids. Here, we have used DFT to compare the electrocatalytic abilities of asparagine and glutamine at the carbon paste electrode interface. Cyclic voltammetric results reveal that the electrocatalytic activities of aspargine are higher than glutamine for dopamine sensing. Dopamine requires less energy to bind with asparagine when compared to glutamine. Additionally, asparagine has higher electron-donating and accepting powers. Therefore, asparagine has a higher electrocatalytic activity than glutamine—the ability for the asparagine and glutamine carbon electrodes to detect dopamine in commercial injection, and to obtain satisfactory results. As a part of the work, we have also studied dopamine interaction with the modified carbon surface using molecular dynamics.

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.

Neuro-nanotechnology: diagnostic and therapeutic nano-based strategies in applied neuroscience

Artificial, de-novo manufactured materials (with controlled nano-sized characteristics) have been progressively used by neuroscientists during the last several decades. The introduction of novel implantable bioelectronics interfaces that are better suited to their biological targets is one example of an innovation that has emerged as a result of advanced nanostructures and implantable bioelectronics interfaces, which has increased the potential of prostheses and neural interfaces. The unique physical-chemical properties of nanoparticles have also facilitated the development of novel imaging instruments for advanced laboratory systems, as well as intelligently manufactured scaffolds and microelectrodes and other technologies designed to increase our understanding of neural tissue processes. The incorporation of nanotechnology into physiology and cell biology enables the tailoring of molecular interactions. This involves unique interactions with neurons and glial cells in neuroscience. Technology solutions intended to effectively interact with neuronal cells, improved molecular-based diagnostic techniques, biomaterials and hybridized compounds utilized for neural regeneration, neuroprotection, and targeted delivery of medicines as well as small chemicals across the blood-brain barrier are all purposes of the present article.

Advanced theragnostics for the central nervous system (CNS) and neurological disorders using functional inorganic nanomaterials

Various types of inorganic nanomaterials are capable of diagnostic biomarker detection and the therapeutic delivery of a disease or inflammatory modulating agent. Those multi-functional nanomaterials have been utilized to treat neurodegenerative diseases and central nervous system (CNS) injuries in an effective and personalized manner. Even though many nanomaterials can deliver a payload and detect a biomarker of interest, only a few studies have yet to fully utilize this combined strategy to its full potential. Combining a nanomaterial's ability to facilitate targeted delivery, promote cellular proliferation and differentiation, and carry a large amount of material with various sensing approaches makes it possible to diagnose a patient selectively and sensitively while offering preventative measures or early disease-modifying strategies. By tuning the properties of an inorganic nanomaterial, the dimensionality, hydrophilicity, size, charge, shape, surface chemistry, and many other chemical and physical parameters, different types of cells in the central nervous system can be monitored, modulated, or further studies to elucidate underlying disease mechanisms. Scientists and clinicians have better understood the underlying processes of pathologies for many neurologically related diseases and injuries by implementing multi-dimensional 0D, 1D, and 2D theragnostic nanomaterials. The incorporation of nanomaterials has allowed scientists to better understand how to detect and treat these conditions at an early stage. To this end, having the multi-modal ability to both sense and treat ailments of the central nervous system can lead to favorable outcomes for patients suffering from such injuries and diseases.

Ascorbic acid does not necessarily interfere with the electrochemical detection of dopamine

It is widely stated that ascorbic acid (AA) interferes with the electrochemical detection of neurotransmitters, especially dopamine, because of their overlapping oxidation potentials on typical electrode materials. As the concentration of AA is several orders of magnitude higher than the concentration of neurotransmitters, detection of neurotransmitters is difficult in the presence of AA and requires either highly stable AA concentration or highly selective neurotransmitter sensors. In contrast to the common opinion, we show that AA does not always interfere electrochemical detection of neurotransmitters. The decay of AA is rapid in cell culture medium, having a half-time of 2.1 hours, according to which the concentration decreases by 93% in 8 hours and by 99.75% in 18 hours. Thus, AA is eventually no longer detected by electrodes and the concentration of neurotransmitters can be effectively monitored. To validate this claim, we used unmodified single-wall carbon nanotube electrode to measure dopamine at physiologically relevant concentration range (25-1000 nM) from human midbrain organoid medium with highly linear response. Finally, AA is known to affect dopamine oxidation current through regeneration of dopamine, which complicates precise detection of small amounts of dopamine. By designing experiments as described here, this complication can be completely eliminated.

Significance of an Electrochemical Sensor and Nanocomposites: Toward the Electrocatalytic Detection of Neurotransmitters and Their Importance within the Physiological System

A prominent neurotransmitter (NT), dopamine (DA), is a chemical messenger that transmits signals between one neuron to the next to pass on a signal to and from the central nervous system (CNS). The imbalanced concentration of DA may cause numerous neurological sicknesses and syndromes, for example, Parkinson's disease (PD) and schizophrenia. There are many types of NTs in the brain, including epinephrine, norepinephrine (NE), serotonin, and glutamate. Electrochemical sensors have offered a creative direction to biomedical analysis and testing. Researches are in progress to improve the performance of sensors and develop new protocols for sensor design. This review article focuses on the area of sensor growth to discover the applicability of polymers and metallic particles and composite materials as tools in electrochemical sensor surface incorporation. Electrochemical sensors have attracted the attention of researchers as they possess high sensitivity, quick reaction rate, good controllability, and instantaneous detection. Efficient complex materials provide considerable benefits for biological detection as they have exclusive chemical and physical properties. Due to distinctive electrocatalytic characteristics, metallic nanoparticles add fascinating traits to materials that depend on the material's morphology and size. Herein, we have collected much information on NTs and their importance within the physiological system. Furthermore, the electrochemical sensors and corresponding techniques (such as voltammetric, amperometry, impedance, and chronoamperometry) and the different types of electrodes' roles in the analysis of NTs are discussed. Furthermore, other methods for detecting NTs include optical and microdialysis methods. Finally, we show the advantages and disadvantages of different techniques and conclude remarks with future perspectives.

Real-Time, In Vivo Molecular Monitoring Using Electrochemical Aptamer Based Sensors: Opportunities and Challenges

The continuous, real-time measurement of specific molecules in situ in the body would greatly improve our ability to understand, diagnose, and treat disease. The vast majority of continuous molecular sensing technologies, however, either (1) rely on the chemical or enzymatic reactivity of their targets, sharply limiting their scope, or (2) have never been shown (and likely will never be shown) to operate in the complex environments found in vivo. Against this background, here we review electrochemical aptamer-based (EAB) sensors, an electrochemical approach to real-time molecular monitoring that has now seen 15 years of academic development. The strengths of the EAB platform are significant: to date it is the only molecular measurement technology that (1) functions independently of the chemical reactivity of its targets, and is thus general, and (2) supports in vivo measurements. Specifically, using EAB sensors we, and others, have already reported the real-time, seconds-resolved measurements of multiple, unrelated drugs and metabolites in situ in the veins and tissues of live animals. Against these strengths, we detail the platform's remaining weaknesses, which include still limited measurement duration (hours, rather than the more desirable days) and the difficulty in obtaining sufficiently high performance aptamers against new targets, before then detailing promising approaches overcoming these hurdles. Finally, we close by exploring the opportunities we believe this potentially revolutionary technology (as well as a few, possibly competing, technologies) will create for both researchers and clinicians.

Brain-Implantable Multifunctional Probe for Simultaneous Detection of Glutamate and GABA Neurotransmitters: Optimization and In Vivo Studies

Imbalances in levels of glutamate (GLU) and gamma-aminobutyric acid (GABA) and their sub-second signaling dynamics occur in several brain disorders including traumatic brain injury, epilepsy, and Alzheimer's disease. The present work reports on the optimization and in vivo testing of a silicon (Si) multifunctional biosensor probe for sub-second simultaneous real-time detection of GLU and GABA. The Si probe features four surface-functionalized platinum ultramicroelectrodes (UMEs) for detection of GLU and GABA, a sentinel site, and integrated microfluidics for in-situ calibration. Optimal enzyme concentrations, size-exclusion phenylenediamine layer and micro spotting conditions were systematically investigated. The measured GLU sensitivity for the GLU and GABA sites were as high as 219 ± 8 nA μM-1 cm-2 (n = 3). The measured GABA sensitivity was as high as 10 ± 1 nA μM-1 cm-2 (n = 3). Baseline recordings (n = 18) in live rats demonstrated a useful probe life of at least 11 days with GLU and GABA concentrations changing at the levels of 100's and 1000's of μM and with expected periodic bursts or fluctuations during walking, teeth grinding and other activities and with a clear difference in the peak amplitude of the sensor fluctuations between rest (low) and activity (higher), or when the rat was surprised (a reaction with no movement). Importantly, the probe could improve methods for large-scale monitoring of neurochemical activity and network function in disease and injury, in live rodent brain.

A sensitive photoluminescent sensor based on highly charged monoruthenium(II) complexes for dopamine detection

A sensitive and selective photoluminescent sensor based on the highly charged monoruthenium(II) complex was designed to detect dopamine (DA) in aqueous samples. Two novel highly charged cationic ruthenium(II) complexes [Ru(bpy)₂(bpy-N)]X₄ (bpy = 2,2'-bipyridine, bpy-N = 4,4'-bis[N,N,N-triethyl-(methylamino)]-2,2'-bipyridine, X^- = [PF₆]^- (1a) or Cl^- (1b) and [Ru(bpy)(bpy-N)₂]X₆ (X^- = [PF₆]^- (2a) or Cl^-(2b)) can be assembled with anionic surfactant sodium dodecylbenzene sulfonate (SDBS), leading to an enhancement of photoluminescence intensity. Upon addition of DA to the system, the photoluminescence intensity of the assembled system was quenched due to the energy transfer effect. It exhibited a wide linear range (0.1-50 μM) and low detection limit (10 nM). The sensor demonstrated a high selectivity toward DA, especially in the presence of adrenaline (Adr) and norepinephrine (NE), whose structures are similar to DA in biological systems. With the merits of simple operation, obvious phenomenon and fast response speed, the sensor had a potential application prospect in human urine sample.

A fluorescent nanosensor paint detects dopamine release at axonal varicosities with high spatiotemporal resolution

SignificanceThe neurotransmitter dopamine controls normal behavior and dopaminergic dysfunction is prevalent in multiple brain diseases. To reach a detailed understanding of how dopamine release and signaling are regulated at the subcellular level, we developed a near infrared fluorescent dopamine nanosensor 'paint' (AndromeDA) to directly image dopamine release and its spatiotemporal characteristics. With AndromeDA, we can ascribe discrete DA release events to defined axonal varicosities, directly assess the heterogeneity of DA release events across such release sites, and determine the molecular components of the DA release machinery. AndromeDA thus provides a new method for gaining fundamental insights into the core mechanisms of dopamine release, which with greatly benefit our knowledge of dopamine biology and pathobiology.

Chemical Imaging and Analysis of Single Nerve Cells by Secondary Ion Mass Spectrometry Imaging and Cellular Electrochemistry

A nerve cell is a unit of neuronal communication in the nervous system and is a heterogeneous molecular structure, which is highly mediated to accommodate cellular functions. Understanding the complex regulatory mechanisms of neural communication at the single cell level requires analytical techniques with high sensitivity, specificity, and spatial resolution. Challenging technologies for chemical imaging and analysis of nerve cells will be described in this review. Secondary ion mass spectrometry (SIMS) allows for non-targeted and targeted molecular imaging of nerve cells and synapses at subcellular resolution. Cellular electrochemistry is well-suited for quantifying the amount of reactive chemicals released from living nerve cells. These techniques will also be discussed regarding multimodal imaging approaches that have recently been shown to be advantageous for the understanding of structural and functional relationships in the nervous system. This review aims to provide an insight into the strengths, limitations, and potentials of these technologies for synaptic and neuronal analyses.