A Detailed Model of Electroenzymatic Glutamate Biosensors To Aid in Sensor Optimization and in Applications in Vivo

Improving Sensitivity and Longevity of In Vivo Glutamate Sensors with Electrodeposited NanoPt

In vivo glutamate sensing has provided valuable insight into the physiology and pathology of the brain. Electrochemical glutamate biosensors, constructed by cross-linking glutamate oxidase onto an electrode and oxidizing H2O2 as a proxy for glutamate, are the gold standard for in vivo glutamate measurements for many applications. While glutamate sensors have been employed ubiquitously for acute measurements, there are almost no reports of long-term, chronic glutamate sensing in vivo, despite demonstrations of glutamate sensors lasting for weeks in vitro. To address this, we utilized a platinum electrode with nanometer-scale roughness (nanoPt) to improve the glutamate sensors' sensitivity and longevity. NanoPt improved the GLU sensitivity by 67.4% and the sensors were stable in vitro for 3 weeks. In vivo, nanoPt glutamate sensors had a measurable signal above a control electrode on the same array for 7 days. We demonstrate the utility of the nanoPt sensors by studying the effect of traumatic brain injury on glutamate in the rat striatum with a flexible electrode array and report measurements of glutamate taken during the injury itself. We also show the flexibility of the nanoPt platform to be applied to other oxidase enzyme-based biosensors by measuring γ-aminobutyric acid in the porcine spinal cord. NanoPt is a simple, effective way to build high sensitivity, robust biosensors harnessing enzymes to detect neurotransmitters in vivo.

Cortico-striatal action control inherent of opponent cognitive-motivational styles

Turning on cue or stopping at a red light requires the detection of such cues to select action sequences, or suppress action, in accordance with cue-associated action rules. Cortico-striatal projections are an essential part of the brain's attention-motor interface. Glutamate-sensing microelectrode arrays were used to measure glutamate transients in the dorsomedial striatum (DMS) of male and female rats walking a treadmill and executing cued turns and stops. Prelimbic-DMS projections were chemogenetically inhibited to determine their behavioral necessity and the cortico-striatal origin of cue-evoked glutamate transients. Furthermore, we investigated rats exhibiting preferably goal-directed (goal trackers, GTs) versus cue-driven attention (sign trackers, STs), to determine the impact of such cognitive-motivational biases on cortico-striatal control. GTs executed more cued turns and initiated such turns more slowly than STs. During turns, but not missed turns or cued stops, cue-evoked glutamate concentrations were higher in GTs than in STs. In conjunction with turn cue-evoked glutamate spike levels, the presence of a single spike rendered GTs to be almost twice as likely to turn than STs. In contrast, multiple glutamate spikes predicted GTs to be less likely to turn than STs. In GTs, but not STs, inhibition of prelimbic-DMS projections attenuated turn rates, turn cue-evoked glutamate peaks, and increased the number of spikes. These findings suggest that turn cue-evoked glutamate release in GTs is tightly controlled by cortico-striatal neuronal activity. In contrast, in STs, glutamate release from DMS glutamatergic terminals may be regulated by other striatal circuitry, preferably mediating cued suppression of action and reward tracking.

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.

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.

Neuroligin-1-Modified Electrodes for Specific Coupling with a Presynaptic Neuronal Membrane

Coordination of synapses onto electrodes with high specificity and maintaining a stable and long-lasting interface have importance in the field of neural interfaces. One potential approach is to present ligands on the surface of electrodes that would be bound through a protein-protein interaction to specific areas of neuronal cells. Here, we functionalize electrode surfaces with genetically engineered neuroligin-1 protein and demonstrate the formation of a nascent presynaptic bouton upon binding to neurexin-1 β on the presynaptic membrane of neurons. The resulting synaptically connected electrode shows an assembly of presynaptic proteins and comparable exocytosis kinetics to that of native synapses. Importantly, a neuroligin-1-induced synapse-electrode interface exhibits type specificity and structural robustness. We envision that the use of synaptic adhesion proteins in modified neural electrodes may lead to new approaches in the interfacing of neural circuity and electronics.

Electroenzymatic choline sensing at near the theoretical performance limit

A high performance, electroenzymatic microsensor for choline based on choline oxidase (ChOx) immobilized on Pt coated with permselective polymer layers has been created that exhibits sensitivity approaching the theoretical performance limit. Sensor construction was guided by simulations performed with a detailed mathematical model. Implantable microsensors with an array of electroenzymatic sensing sites provide a means to record concentration changes of choline, an effective surrogate for acetylcholine due to its very rapid turnover in the brain, and other neurochemicals in vivo. However, electroenzymatic sensors generally have insufficient sensitivity and response time to monitor neurotransmitter signaling on the millisecond timescale with cellular-level spatial resolution. Model simulations suggested that choline sensor performance can be improved significantly by optimizing immobilized ChOx layer thickness and minimizing the thicknesses of permselective polymer coatings as well. Electroenzymatic choline sensors constructed with a ∼5 μm-thick crosslinked ChOx layer atop 200 nm-thick permselective films (poly(m-phenylenediamine) and Nafion) exhibited unprecedented sensitivity and response time of 660 ± 40 nA μM^-1 cm^-2 at 37 °C and 0.36 ± 0.05 s, respectively, while maintaining excellent selectivity. Such performance characteristics provide greater flexibility in the design of microelectrode array (MEA) probes with near cellular-scale sensing sites arranged in more dense arrays. Also, faster response times enable better resolution of transient acetylcholine signals and better correlation of these events with electrophysiological recordings so as to advance study of brain function.

Electrochemistry of Single-Vesicle Events

Neuronal transmission relies on electrical signals and the transfer of chemical signals from one neuron to another. Chemical messages are transmitted from presynaptic neurons to neighboring neurons through the triggered fusion of neurotransmitter-filled vesicles with the cell plasma membrane. This process, known as exocytosis, involves the rapid release of neurotransmitter solutions that are detected with high affinity by the postsynaptic neuron. The type and number of neurotransmitters released and the frequency of vesicular events govern brain functions such as cognition, decision making, learning, and memory. Therefore, to understand neurotransmitters and neuronal function, analytical tools capable of quantitative and chemically selective detection of neurotransmitters with high spatiotemporal resolution are needed. Electrochemistry offers powerful techniques that are sufficiently rapid to allow for the detection of exocytosis activity and provides quantitative measurements of vesicle neurotransmitter content and neurotransmitter release from individual vesicle events. In this review, we provide an overview of the most commonly used electrochemical methods for monitoring single-vesicle events, including recent developments and what is needed for future research.

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

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

A Highly Sensitive Amperometric Glutamate Oxidase Microbiosensor Based on a Reduced Graphene Oxide/Prussian Blue Nanocube/Gold Nanoparticle Composite Film-Modified Pt Electrode

A simple method that relies only on an electrochemical workstation has been investigated to fabricate a highly sensitive glutamate microbiosensor for potential neuroscience applications. In this study, in order to develop the highly sensitive glutamate electrode, a 100 µm platinum wire was modified by the electrochemical deposition of gold nanoparticles, Prussian blue nanocubes, and reduced graphene oxide sheets, which increased the electroactive surface area; and the chitosan layer, which provided a suitable environment to bond the glutamate oxidase. The optimization of the fabrication procedure and analytical conditions is described. The modified electrode was characterized using field emission scanning electron microscopy, impedance spectroscopy, and cyclic voltammetry. The results exhibited its excellent sensitivity for glutamate detection (LOD = 41.33 nM), adequate linearity (50 nM-40 µM), ascendant reproducibility (RSD = 4.44%), and prolonged stability (more than 30 repetitive potential sweeps, two-week lifespan). Because of the important role of glutamate in neurotransmission and brain function, this small-dimension, high-sensitivity glutamate electrode is a promising tool in neuroscience research.

Transcranial photoacoustic imaging of NMDA-evoked focal circuit dynamics in the rat hippocampus

OBJECTIVE

We report the transcranial functional photoacoustic (fPA) neuroimaging of N-methyl-D-aspartate (NMDA) evoked neural activity in the rat hippocampus. Concurrent quantitative electroencephalography (qEEG) and microdialysis were used to record real-time circuit dynamics and excitatory neurotransmitter concentrations, respectively.

APPROACH

We hypothesized that location-specific fPA voltage-sensitive dye (VSD) contrast would identify neural activity changes in the hippocampus which correlate with NMDA-evoked excitatory neurotransmission.

MAIN RESULTS

Transcranial fPA VSD imaging at the contralateral side of the microdialysis probe provided NMDA-evoked VSD responses with positive correlation to extracellular glutamate concentration changes. qEEG validated a wide range of glutamatergic excitation, which culminated in focal seizure activity after a high NMDA dose. We conclude that transcranial fPA VSD imaging can distinguish focal glutamate loads in the rat hippocampus, based on the VSD redistribution mechanism which is sensitive to the electrophysiologic membrane potential.

SIGNIFICANCE

Our results suggest the future utility of this emerging technology in both laboratory and clinical sciences as an innovative functional neuroimaging modality.

Electroenzymatic glutamate sensing at near the theoretical performance limit

The sensitivity and response time of glutamate sensors based on glutamate oxidase immobilized on planar platinum microelectrodes have been improved to near the theoretical performance limits predicted by a detailed mathematical model. Microprobes with an array of electroenzymatic sensing sites have emerged as useful tools for the monitoring of glutamate and other neurotransmitters in vivo; and implemented as such, they can be used to study many complex neurological diseases and disorders including Parkinson's disease and drug addiction. However, less than optimal sensitivity and response time has limited the spatiotemporal resolution of these promising research tools. A mathematical model has guided systematic improvement of an electroenzymatic glutamate microsensor constructed with a 1-2 μm-thick crosslinked glutamate oxidase layer and underlying permselective coating of polyphenylenediamine and Nafion reduced to less than 200 nm thick. These design modifications led to a nearly 6-fold improvement in sensitivity to 320 ± 20 nA μM-1 cm-2 at 37 °C and a ∼10-fold reduction in response time to 80 ± 10 ms. Importantly, the sensitivity and response times were attained while maintaining a low detection limit and excellent selectivity. Direct measurement of the transport properties of the enzyme and polymer layers used to create the biosensors enabled improvement of the mathematical model as well. Subsequent model simulations indicated that the performance characteristics achieved with the optimized biosensors approach the theoretical limits predicted for devices of this construction. Such high-performance glutamate biosensors will be more effective in vivo at a size closer to cellular dimension and will enable better correlation of glutamate signaling events with electrical recordings.

Molecular Crowding and a Minimal Footprint at a Gold Nanoparticle Support Stabilize Glucose Oxidase and Boost Its Activity

Enzymes conjugated to nanomaterials are used in the design of various biotechnologies. In the development of biosensors, surface modifications with the enzyme glucose oxidase (GOx) serve to aid the detection of blood glucose. In order to optimize sensor effectiveness, the enzyme tertiary structure needs to be preserved upon immobilization to retain the enzyme's catalytic activity. Because of the nature of GOx, it suffers from a tendency to denature when immobilized at a solid surface; hence, methods to optimize enzyme stability are of great importance. Here, we introduce the study of the interaction of GOx to the highly curved surface of 20 nm gold nanoparticles (AuNP) with an absorbed monolayer coating of enzyme as determined by flocculation assays and quantification of immobilized GOx at the nanoparticle surface. Enzyme crowding was determined by comparing the number of enzymes that bind to how many can physically fit. These measurements show how placing a monolayer of enzyme where the enzyme spreads thin at the AuNP surface still provides stable catalytic performance of up to 14 days compared to enzymes free in solution. Moreover, by the increasing enzyme density via increasing the amount of GOx present in solution during the GOx/AuNP conjugation step creates a molecularly crowded environment at the highly curved nanoparticle surface. This limits the size of the enzyme footprint for attachment and shows that the activity per enzyme can be enhanced up to 300%. This is of great importance for implementing stable and sensitive sensor technologies that are constructed by enzyme-based nanoparticle scaffolds. Here, we show by using the conditions that maintain GOx structure and function when limiting the enzyme coating to an ultrathin layer, the design and construction of an ultrafast responding diagnostic sensor technology for glucose can be achieved, which is crucial for monitoring rapid fluctuations of, for instance, glucose in the brain.

In Vivo Electrochemical Sensors for Neurochemicals: Recent Update

In vivo electrochemical sensing based on implantable microelectrodes is a strong driving force of analytical neurochemistry in brain. The complex and dynamic neurochemical network sets stringent standards of in vivo electrochemical sensors including high spatiotemporal resolution, selectivity, sensitivity, and minimized disturbance on brain function. Although advanced materials and novel technologies have promoted the development of in vivo electrochemical sensors drastically, gaps with the goals still exist. This Review mainly focuses on recent attempts on the key issues of in vivo electrochemical sensors including selectivity, tissue response and sensing reliability, and compatibility with electrophysiological techniques. In vivo electrochemical methods with bare carbon fiber electrodes, of which the selectivity is achieved either with electrochemical techniques such as fast-scan cyclic voltammetry and differential pulse voltammetry or based on the physiological nature will not be reviewed. Following the elaboration of each issue involved in in vivo electrochemical sensors, possible solutions supported by the latest methodological progress will be discussed, aiming to provide inspiring and practical instructions for future research.

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

It is extremely challenging to chemically diagnose disorders of the brain. There is hence great interest in designing and optimizing tools for direct detection of chemical biomarkers implicated in neurological disorders to improve diagnosis and treatment. Tools that are capable of monitoring brain chemicals, neurotransmitters in particular, need to be biocompatible, perform with high spatiotemporal resolution, and ensure high selectivity and sensitivity. Recent advances in electrochemical methods are addressing these criteria; the resulting devices demonstrate great promise for in vivo neurotransmitter detection. None of these devices are currently used for diagnostic purposes, however these cutting-edge technologies are promising more sensitive, selective, faster, and less invasive measurements. Via this review we highlight significant technical advances and in vivo studies, performed in the last 5 years, that we believe will facilitate the development of diagnostic tools for brain disorders.

Ultrafast Glutamate Biosensor Recordings in Brain Slices Reveal Complex Single Exocytosis Transients

Neuronal communication relies on vesicular neurotransmitter release from signaling neurons and detection of these molecules by neighboring neurons. Glutamate, the main excitatory neurotransmitter in the mammalian brain, is involved in nearly all brain functions. However, glutamate has suffered from detection schemes that lack temporal and spatial resolution allowed by electrochemistry. Here we show an amperometric, novel, ultrafast enzyme-based nanoparticle modified sensor, measuring random bursts of hundreds to thousands of rapid spontaneous glutamate exocytotic release events at approximately 30 Hz frequency in the nucleus accumbens of rodent brain slices. Characterizing these single submillisecond exocytosis events revealed a great diversity in spike shape characteristics and size of quantal release, suggesting variability in fusion pore dynamics controlling the glutamate release by cells in this brain region. Hence, this novel biosensor allows recording of rapid single glutamate exocytosis events in the brain tissue and offers insight on regulatory aspects of exocytotic glutamate release, which is critical to understanding of brain glutamate function and dysfunction.

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

L- Glutamate is the main excitatory neurotransmitter in the central nervous system and hyperglutamatergic signaling is implicated in neurological and neurodegenerative diseases. Monitoring glutamate with a glutamate oxidase-based amperometric biosensor offers advantages such as high spatial and high temporal resolution. However, commercially-available glutamate biosensors are expensive and larger in size. Here, we report the development of 50 µm diameter biosensor for real-time monitoring of L-glutamate in vivo. A polymer, poly-o-phenylenediamine (PPD) layer was electropolymerized onto a 50 µm Pt wire to act as a permselective membrane. Then, glutamate oxidase entrapped in a biocompatible chitosan matrix was cast onto the microelectrode surface. Finally, ascorbate oxidase was coated to eliminate interferences from high levels of extracellular ascorbic acid present in brain tissue. L-glutamate measurements were performed amperometrically at an applied potential of 0.6 V vs Ag/AgCl. The biosensor exhibited a linear range from 5 to 150 μM, with a high sensitivity of 0.097 ± 0.001 nA/μM and one-week storage stability. The biosensor also showed a rapid steady state response to L-glutamate within 2 s, with a limit of detection of 0.044 μM. The biosensor was used successfully to detect stimulated glutamate in the subthalamic nucleus in brain slices and in vivo. Thus, this biosensor is appropriate for future neuroscience applications.