Silicon Wafer-Based Platinum Microelectrode Array Biosensor for Near Real-Time Measurement of Glutamate in Vivo

Simultaneous, Real-Time Detection of Glutamate and Dopamine in Rat Striatum Using Fast-Scan Cyclic Voltammetry

Glutamate and dopamine (DA) represent two key contributors to striatal functioning, a region of the brain that is essential to motor coordination and motivated behavior. While electroanalytical techniques can be utilized for rapid, spatially resolved detection of DA in the interferent-rich brain environment, glutamate, a nonelectroactive analyte, cannot be directly detected using electroanalytical techniques. However, it can be probed using enzyme-based sensors, which generate an electroactive reporter in the presence of glutamate. The vast majority of glutamate biosensors have relied on amperometric sensing, which is an inherently nonselective detection technique. This approach necessitates the use of complex and performance-limiting modifications to ensure the desired single-analyte specificity. Here, we present a novel glutamate microbiosensor fabricated on a carbon-fiber microelectrode substrate and coupled with fast-scan cyclic voltammetry (FSCV) to enable the simultaneous quantification of glutamate and DA at single recording sites in the brain, which is impossible when using typical amperometric approaches. The glutamate microbiosensors were characterized for sensitivity, stability, and selectivity by using a voltammetric waveform optimized for the simultaneous detection of both species. The applicability of these sensors for the investigation of neural circuits was validated in the rat ventral striatum. Electrically evoked glutamate and DA release were recorded at single-micrometer-scale locations before and after pharmacological manipulation of glutamatergic signaling. Our novel glutamate microbiosensor advances the state of the art by providing a powerful tool for probing coordination between these two species in a way that has previously not been possible.

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.

Flexible and Implantable Polyimide Aptamer-Field-Effect Transistor Biosensors

Monitoring neurochemical signaling across time scales is critical to understanding how brains encode and store information. Flexible (vs stiff) devices have been shown to improve in vivo monitoring, particularly over longer times, by reducing tissue damage and immunological responses. Here, we report our initial steps toward developing flexible and implantable neuroprobes with aptamer-field-effect transistor (FET) biosensors for neurotransmitter monitoring. A high-throughput process was developed to fabricate thin, flexible polyimide probes using microelectromechanical-system (MEMS) technologies, where 150 flexible probes were fabricated on each 4 in. Si wafer. Probes were 150 μm wide and 7 μm thick with two FETs per tip. The bending stiffness was 1.2 × 10^-11 N·m². Semiconductor thin films (3 nm In₂O₃) were functionalized with DNA aptamers for target recognition, which produces aptamer conformational rearrangements detected via changes in FET conductance. Flexible aptamer-FET neuroprobes detected serotonin at femtomolar concentrations in high-ionic strength artificial cerebrospinal fluid. A straightforward implantation process was developed, where microfabricated Si carrier devices assisted with implantation such that flexible neuroprobes detected physiological relevant serotonin in a tissue-hydrogel brain mimic.

Rapid Conductometric Detection of SARS-CoV-2 Proteins and Its Variants Using Molecularly Imprinted Polymer Nanoparticles

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) biosensors have captured more attention than the conventional methodologies for SARS-CoV-2 detection due to having cost-effective platforms and fast detection. However, these reported SARS-CoV-2 biosensors suffer from drawbacks including issues in detection sensitivity, degradation of biomaterials on the sensor's surface, and incapability to reuse the biosensors. To overcome these shortcomings, molecularly imprinted polymer nanoparticles (nanoMIPs) incorporated conductometric biosensor for highly accurate, rapid, and selective detection of two model SARS-CoV-2 proteins: (i) receptor binding domain (RBD) of the spike (S) glycoprotein and (ii) full length trimeric spike protein are introduced. In addition, these biosensors successfully responded to several other SARS-CoV-2 RBD spike protein variants including Alpha, Beta, Gamma, and Delta. Our conductometric biosensor selectively detects the two model proteins and SARS-CoV-2 RBD spike protein variant samples in real-time with sensitivity to a detection limit of 7 pg mL-1 within 10 min of sample incubation. A battery-free, wireless near-field communication (NFC) interface is incorporated with the biosensor for fast and contactless detection of SARS-CoV-2 variants. The smartphone enabled real-time detection and on-screen rapid result for SARS-CoV-2 variants can curve the outbreak due to its ability to alert the user to infection in real time.

Accurate and stable chronic in vivo voltammetry enabled by a replaceable subcutaneous reference electrode

In vivo sensing of neurotransmitters has provided valuable insight into both healthy and diseased brain. However, chronically implanted Ag/AgCl reference electrodes suffer from degradationgradation, resulting in errors in the potential at the working electrode. Here, we report a simple, effective way to protect in vivo sensing measurements from reference polarization with a replaceable subcutaneously implanted reference. We compared a brain-implanted reference and a subcutaneous reference and observed no difference in impedance or dopamine redox peak separation in an acute preparation. Chronically, peak background potential and dopamine oxidation potential shifts were eliminated for three weeks. Scanning electron microscopy shows changes in surface morphology and composition of chronically implanted Ag/AgCl electrodes, and postmortem histology reveals extensive cell death and gliosis in the surrounding tissue. As accurate reference potentials are critical to in vivo electrochemistry applications, this simple technique can improve a wide and diverse assortment of in vivo preparations.

Implantable aptamer-field-effect transistor neuroprobes for in vivo neurotransmitter monitoring

While tools for monitoring in vivo electrophysiology have been extensively developed, neurochemical recording technologies remain limited. Nevertheless, chemical communication via neurotransmitters plays central roles in brain information processing. We developed implantable aptamer–field-effect transistor (FET) neuroprobes for monitoring neurotransmitters. Neuroprobes were fabricated using high-throughput microelectromechanical system (MEMS) technologies, where 150 probes with shanks of either 150- or 50-μm widths and thicknesses were fabricated on 4-inch Si wafers. Nanoscale FETs with ultrathin (~3 to 4 nm) In₂O₃ semiconductor films were prepared using sol-gel processing. The In₂O₃ surfaces were coupled with synthetic oligonucleotide receptors (aptamers) to recognize and to detect the neurotransmitter serotonin. Aptamer-FET neuroprobes enabled femtomolar serotonin detection limits in brain tissue with minimal biofouling. Stimulated serotonin release was detected in vivo. This study opens opportunities for integrated neural activity recordings at high spatiotemporal resolution by combining these aptamer-FET sensors with other types of Si-based implantable probes to advance our understanding of brain function.

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.

Miniaturisation of a peptide-based electrochemical protease activity sensor using platinum microelectrodes

Proteases are ideal target biomarkers as they have been implicated in many disease states, including steps associated with cancer progression. Electrochemical peptide-based biosensors have attracted much interest in recent years. However, the significantly large size of the electrodes typically used in most of these platforms has led to performance limitations. These could be addressed by the enhancements offered by microelectrodes, such as rapid response times, improved mass transport, higher signal-to-noise and sensitivity, as well as more localised and less invasive measurements. We present the production and characterisation of a miniaturised electrochemical biosensor for the detection of trypsin, based on 25 μm diameter Pt microelectrodes (rather than the ubiquitous Au electrodes), benchmarked by establishing the equivalent Pt macroelectrode response in terms of quantitative response to the protease, the kinetics of cleavage and the effects of non-specific protein binding and temperature. Interestingly, although there was little difference between Au and Pt macroelectrode response, significant differences were observed between the responses of the Pt macroelectrode and microelectrode systems indicative of increased reproducibility in the microelectrode SAM structure and sensor performance between the electrodes, increased storage stability and a decrease in the cleavage rate at functionalised microelectrodes, which is mitigated by measurement at normal body temperature. Together, these results demonstrate the robustness and sensitivity of the miniaturised sensing platform and its ability to operate within the clinically-relevant concentration ranges of proteases in normal and disease states. These are critical features for its translation into implantable devices.

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.

Cellular-Scale Microelectrode Arrays to Monitor Movement-Related Neuron Activities in the Epileptic Hippocampus of Awake Mice

OBJECTIVE

Epilepsy affects 50 million people worldwide and its pathogenesis is still unknown. In particular, the movement-related neural activities involving glutamate (Glu) and electrophysiological signals at cellular level remains unclear.

METHODS

A cellular-scale implantable microelectrode array (MEA) was fabricated to detect the movement-related neural activities involving Glu concentration and electrophysiological signals. Platinum and reduced graphene oxide nanocomposites were deposited to enhance the surface area. Glu oxidase (Gluox) were coated to effectively recognize Glu molecule.

RESULTS

Neural activities in the hippocampus of normal and epileptic mice is different, and the changes are closely connected with movement. Glu concentration and spike firing rate in the epileptic mice were much higher than those in the normal ones. And the neural activities with significant synchronization were detected in the epileptic mice even without seizure occurrence. Meanwhile, the spikes fire more intensively and Glu level became much higher during the movement of the mice compared to the stationary state.

CONCLUSION

The existing abnormality of neural activities in the epileptic mice are potential factors to induce a seizure. Movement may impact the neural activities and the duration of seizure.

SIGNIFICANCE

The MEA can monitor changes of movement, Glu and neuron discharges synchronously and provides us an effective technology to understand the neuronal disease.

An implantable multifunctional neural microprobe for simultaneous multi-analyte sensing and chemical delivery

A multifunctional chemical neural probe fabrication process exploiting PDMS thin-film transfer to incorporate a microfluidic channel onto a silicon-based microelectrode array (MEA) platform, and enzyme microstamping to provide multi-analyte detection is described. The Si/PDMS hybrid chemtrode, modified with a nano-based on-probe IrOx reference electrode, was validated in brain phantoms and in rat brain.

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.

Multimaterial and multifunctional neural interfaces: from surface-type and implantable electrodes to fiber-based devices

Development of neural interfaces from surface electrodes to fibers with various type, functionality, and materials.

High frequency, real-time neurochemical and neuropharmacological measurements in situ in the living body

The beautiful and complex brain machinery is perfectly synchronized, and our bodies have evolved to protect it against a myriad of potential threats. Shielded physically by the skull and chemically by the blood brain barrier, the brain processes internal and external information so that we can efficiently relate to the world that surrounds us while simultaneously and unconsciously controlling our vital functions. When coupled with the brittle nature of its internal chemical and electric signals, the brain's "armor" render accessing it a challenging and delicate endeavor that has historically limited our understanding of its structural and neurochemical intricacies. In this review, we briefly summarize the advancements made over the past 10 years to decode the brain's neurochemistry and neuropharmacology in situ, at the site of interest in the brain, with special focus on what we consider game-changing emerging technologies (eg, genetically encoded indicators and electrochemical aptamer-based sensors) and the challenges these must overcome before chronic, in situ chemosensing measurements become routine.

Nucleus Accumbens Cholinergic Interneurons Oppose Cue-Motivated Behavior

BACKGROUND

Environmental reward-predictive stimuli provide a major source of motivation for adaptive reward pursuit behavior. This cue-motivated behavior is known to be mediated by the nucleus accumbens (NAc) core. The cholinergic interneurons in the NAc are tonically active and densely arborized and thus well suited to modulate NAc function. However, their causal contribution to adaptive behavior remains unknown. Here we investigated the function of NAc cholinergic interneurons in cue-motivated behavior.

METHODS

We used chemogenetics, optogenetics, pharmacology, and a translationally analogous Pavlovian-to-instrumental transfer behavioral task designed to assess the motivating influence of a reward-predictive cue over reward-seeking actions in male and female rats.

RESULTS

The data show that NAc cholinergic interneuron activity critically opposes the motivating influence of appetitive cues. Chemogenetic inhibition of NAc cholinergic interneurons augmented cue-motivated behavior. Optical stimulation of acetylcholine release from NAc cholinergic interneurons prevented cues from invigorating reward-seeking behavior, an effect that was mediated by activation of β2-containing nicotinic acetylcholine receptors.

CONCLUSIONS

NAc cholinergic interneurons provide a critical regulatory influence over adaptive cue-motivated behavior and therefore are a potential therapeutic target for the maladaptive cue-motivated behavior that marks many psychiatric conditions, including addiction and depression.

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.

Drift Subtraction for Fast-Scan Cyclic Voltammetry Using Double-Waveform Partial-Least-Squares Regression

Background-subtracted fast-scan cyclic voltammetry (FSCV) provides a method for detecting molecular fluctuations with high spatiotemporal resolution in the brain of awake and behaving animals. The rapid scan rates generate large background currents that are subtracted to reveal changes in analyte concentration. Although these background currents are relatively stable, small changes do occur over time. These changes, referred to as electrochemical drift, result in background-subtraction artifacts that constrain the utility of FSCV, particularly when quantifying chemical changes that gradually occur over long measurement times (minutes). The voltammetric features of electrochemical drift are varied and can span the entire potential window, potentially obscuring the signal from any targeted analyte. We present a straightforward method for extending the duration of a single FSCV recording window. First, we have implemented voltammetric waveforms in pairs that consist of a smaller triangular sweep followed by a conventional voltammetric scan. The initial, abbreviated waveform is used to capture drift information that can serve as a predictor for the contribution of electrochemical drift to the subsequent full voltammetric scan using partial-least-squares regression (PLSR). This double-waveform partial-least-squares regression (DW-PLSR) paradigm permits reliable subtraction of the drift component to the voltammetric data. Here, DW-PLSR is used to improve quantification of adenosine, dopamine, and hydrogen peroxide fluctuations occurring >10 min from the initial background position, both in vitro and in vivo. The results demonstrate that DW-PLSR is a powerful tool for evaluating and interpreting both rapid (seconds) and gradual (minutes) chemical changes captured in FSCV recordings over extended durations.

Flexible, multifunctional neural probe with liquid metal enabled, ultra-large tunable stiffness for deep-brain chemical sensing and agent delivery

Flexible neural probes have been pursued previously to minimize the mechanical mismatch between soft neural tissues and implants and thereby improve long-term performance. However, difficulties with insertion of such probes deep into the brain severely restricts their utility. We describe a solution to this problem using gallium (Ga) in probe construction, taking advantage of the solid-to-liquid phase change of the metal at body temperature and probe shape deformation to provide temperature-dependent control of stiffness over 5 orders of magnitude. Probes in the stiff state were successfully inserted 2 cm-deep into agarose gel "brain phantoms" and into rat brains under cooled conditions where, upon Ga melting, they became ultra soft, flexible, and stretchable in all directions. The current 30 μm-thick probes incorporated multilayer, deformable microfluidic channels for chemical agent delivery, electrical interconnects through Ga wires, and high-performance electrochemical glutamate sensing. These PDMS-based microprobes of ultra-large tunable stiffness (ULTS) should serve as an attractive platform for multifunctional chronic neural implants.

Distinct cortical-amygdala projections drive reward value encoding and retrieval

The value of an anticipated rewarding event is a crucial component of the decision to engage in its pursuit. But little is known of the networks responsible for encoding and retrieving this value. By using biosensors and pharmacological manipulations, we found that basolateral amygdala (BLA) glutamatergic activity tracks and mediates encoding and retrieval of the state-dependent incentive value of a palatable food reward. Projection-specific, bidirectional chemogenetic and optogenetic manipulations revealed that the orbitofrontal cortex (OFC) supports the BLA in these processes. Critically, the function of ventrolateral and medial OFC→BLA projections is doubly dissociable. Whereas lateral OFC→BLA projections are necessary and sufficient for encoding of the positive value of a reward, medial OFC→BLA projections are necessary and sufficient for retrieving this value from memory. These data reveal a new circuit for adaptive reward valuation and pursuit and provide insight into the dysfunction in these processes that characterizes myriad psychiatric diseases.

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.

High-Density, Long-Lasting, and Multi-region Electrophysiological Recordings Using Polymer Electrode Arrays

The brain is a massive neuronal network, organized into anatomically distributed sub-circuits, with functionally relevant activity occurring at timescales ranging from milliseconds to years. Current methods to monitor neural activity, however, lack the necessary conjunction of anatomical spatial coverage, temporal resolution, and long-term stability to measure this distributed activity. Here we introduce a large-scale, multi-site, extracellular recording platform that integrates polymer electrodes with a modular stacking headstage design supporting up to 1,024 recording channels in freely behaving rats. This system can support months-long recordings from hundreds of well-isolated units across multiple brain regions. Moreover, these recordings are stable enough to track large numbers of single units for over a week. This platform enables large-scale electrophysiological interrogation of the fast dynamics and long-timescale evolution of anatomically distributed circuits, and thereby provides a new tool for understanding brain activity.

Wireless resonant circuits for the minimally invasive sensing of biophysical processes in magnetic resonance imaging

Biological electromagnetic fields arise throughout all tissue depths and types, and correlate with physiological processes and signalling in organs of the body. Most of the methods for monitoring these fields are either highly invasive or spatially coarse. Here, we show that implantable active coil-based transducers that are detectable via magnetic resonance imaging enable the remote sensing of biological fields. These devices consist of inductively coupled resonant circuits that change their properties in response to electrical or photonic cues, thereby modulating the local magnetic resonance imaging signal without the need for onboard power or wired connectivity. We discuss design parameters relevant to the construction of the transducers on millimetre and submillimetre scales, and demonstrate their in vivo functionality for measuring time-resolved bioluminescence in rodent brains. Biophysical sensing via microcircuits that leverage the capabilities of magnetic resonance imaging may enable a wide range of biological and biomedical applications.

Microbiosensor fabrication by polydimethylsiloxane stamping for combined sensing of glucose and choline

High performance microprobes for combined sensing of glucose and choline were fabricated using microcontact printing (μCP) to transfer choline oxidase (ChOx) and glucose oxidase (GOx) onto targeted sites on microelectrode arrays (MEAs). Most electroenzymatic sensing sites on MEAs for neuroscience applications are created by manual enzyme deposition, which becomes problematic when the array feature size is less than or equal to ∼100 μm. The μCP process used here relies on use of soft lithography to create features on a polydimethylsiloxane (PDMS) microstamp that correspond to the dimensions and array locations of targeted, microscale sites on a MEA. Precise alignment of the stamp with the MEA is also required to transfer enzyme only onto the specified microelectrode(s). The dual sensor fabrication process began with polyphenylenediamine (PPD) electrodeposition on all Pt microelectrodes to block common interferents (e.g., ascorbic acid and dopamine) found in brain extracellular fluid. Next, a chitosan film was electrodeposited to serve as an adhesive layer. The two enzymes, ChOx and GOx, were transferred onto different microelectrodes of 2 × 2 arrays using two different PDMS stamps and a microscope for stamp alignment. Using constant potential amperometry, the combined sensing microprobe was confirmed to have high sensitivity for choline and glucose (286 and 117 μA mM cm-2, respectively) accompanied by low detection limits (1 and 3 μM, respectively) and rapid response times (≤2 s). This work demonstrates the use of μCP for facile creation of multianalyte sensing microprobes by targeted deposition of enzymes onto preselected sites of a microelectrode array.

Organic Electrochemistry and a Role Reversal: Using Synthesis To Optimize Electrochemical Methods

Diblock copolymers are excellent coatings for microelectrode arrays because they provide a stable surface that can support both synthetic and analytical electrochemistry. However, the surfaces that are optimal for synthetic studies are not the same as the surfaces that are optimal for analytical studies. Hence, no one surface provides an ideal platform for both building and analyzing a molecular library. Fortunately, the synthetic chemistry available on a microelectrode array allows a surface that is ideal for synthesis can be converted into one that is ideal for signaling studies; a scenario that allows for the use of an optimized synthetic and analytical surface on a single microelectrode array.

Electrochemical Probes of Microbial Community Behavior

Advances in next-generation sequencing technology along with decreasing costs now allow the microbial population, or microbiome, of a location to be determined relatively quickly. This research reveals that microbial communities are more diverse and complex than ever imagined. New and specialized instrumentation is required to investigate, with high spatial and temporal resolution, the dynamic biochemical environment that is created by microbes, which allows them to exist in every corner of the Earth. This review describes how electrochemical probes and techniques are being used and optimized to learn about microbial communities. Described approaches include voltammetry, electrochemical impedance spectroscopy, scanning electrochemical microscopy, separation techniques coupled with electrochemical detection, and arrays of complementary metal-oxide-semiconductor circuits. Microbial communities also interact with and influence their surroundings; therefore, the review also includes a discussion of how electrochemical probes optimized for microbial analysis are utilized in healthcare diagnostics and environmental monitoring applications.

Microfabricated Probes for Studying Brain Chemistry: A Review

Probe techniques for monitoring in vivo chemistry (e.g., electrochemical sensors and microdialysis sampling probes) have significantly contributed to a better understanding of neurotransmission in correlation to behaviors and neurological disorders. Microfabrication allows construction of neural probes with high reproducibility, scalability, design flexibility, and multiplexed features. This technology has translated well into fabricating miniaturized neurochemical probes for electrochemical detection and sampling. Microfabricated electrochemical probes provide a better control of spatial resolution with multisite detection on a single compact platform. This development allows the observation of heterogeneity of neurochemical activity precisely within the brain region. Microfabricated sampling probes are starting to emerge that enable chemical measurements at high spatial resolution and potential for reducing tissue damage. Recent advancement in analytical methods also facilitates neurochemical monitoring at high temporal resolution. Furthermore, a positive feature of microfabricated probes is that they can be feasibly built with other sensing and stimulating platforms including optogenetics. Such integrated probes will empower researchers to precisely elucidate brain function and develop novel treatments for neurological disorders.

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

Simulations conducted with a detailed model of glutamate biosensor performance describe the observed sensor performance well, illustrate the limits of sensor performance, and suggest a path toward sensor optimization. Glutamate is the most important excitatory neurotransmitter in the brain, and electroenzymatic sensors have emerged as a useful tool for the monitoring of glutamate signaling in vivo. However, the utility of these sensors currently is limited by their sensitivity and response time. A mathematical model of a typical glutamate biosensor consisting of a Pt electrode coated with a permselective polymer film and a top layer of cross-linked glutamate oxidase has been constructed in terms of differential material balances on glutamate, H₂O₂, and O₂ in one spatial dimension. Simulations suggest that reducing thicknesses of the permselective polymer and enzyme layers can increase sensitivity ∼6-fold and reduce response time ∼7-fold, and thereby improve resolution of transient glutamate signals. At currently employed enzyme layer thicknesses, both intrinsic enzyme kinetics and enzyme deactivation likely are masked by mass transfer. However, O₂-dependence studies show essentially no reduction in signal at the lowest anticipated O₂ concentrations for expected glutamate concentrations in the brain and that O₂ transport limitations in vitro are anticipated only at glutamate concentrations in the mM range. Finally, the limitations of current biosensors in monitoring glutamate transients is simulated and used to illustrate the need for optimized biosensors to report glutamate signaling accurately on a subsecond time scale. This work demonstrates how a detailed model can be used to guide optimization of electroenzymatic sensors similar to that for glutamate and to ensure appropriate interpretation of data gathered using such biosensors.

Electrochemical Selectivity Achieved Using a Double Voltammetric Waveform and Partial Least Squares Regression: Differentiating Endogenous Hydrogen Peroxide Fluctuations from Shifts in pH

Hydrogen peroxide (H₂O₂) is a reactive oxygen species that serves as an important signaling molecule in normal brain function. At the same time, excessive H₂O₂ concentrations contribute to myriad pathological consequences resulting from oxidative stress. Studies to elucidate the diverse roles that H₂O₂ plays in complex biological environments have been hindered by the lack of robust methods for probing dynamic H₂O₂ fluctuations in living systems with molecular specificity. Background-subtracted fast-scan cyclic voltammetry at carbon-fiber microelectrodes provides a method of detecting rapid H₂O₂ fluctuations with high temporal and spatial resolution in brain tissue. However, H₂O₂ fluctuations can be masked by local changes in pH (ΔpH), because the voltammograms for these species can have significant peak overlap, hindering quantification. We present a method for removing ΔpH-related contributions from complex voltammetric data. By employing two distinct potential waveforms per scan, one in which H₂O₂ is electrochemically silent and a second in which both ΔpH and H₂O₂ are redox active, a clear distinction between H₂O₂ and ΔpH signals is established. A partial least-squares regression (PLSR) model is used to predict the ΔpH signal and subtract it from the voltammetric data. The model has been validated both in vitro and in vivo using k-fold cross-validation. The data demonstrate that the double waveform PLSR model is a powerful tool that can be used to disambiguate and evaluate naturally occurring H₂O₂ fluctuations in vivo.

Selective and Mechanically Robust Sensors for Electrochemical Measurements of Real-Time Hydrogen Peroxide Dynamics in Vivo

Hydrogen peroxide (H2O2) is an endogenous molecule that plays several important roles in brain function: it is generated in cellular respiration, serves as a modulator of dopaminergic signaling, and its presence can indicate the upstream production of more aggressive reactive oxygen species (ROS). H2O2 has been implicated in several neurodegenerative diseases, including Parkinson's disease (PD), creating a critical need to identify mechanisms by which H2O2 modulates cellular processes in general and how it affects the dopaminergic nigrostriatal pathway, in particular. Furthermore, there is broad interest in selective electrochemical quantification of H2O2, because it is often enzymatically generated at biosensors as a reporter for the presence of nonelectroactive target molecules. H2O2 fluctuations can be monitored in real time using fast-scan cyclic voltammetry (FSCV) coupled with carbon-fiber microelectrodes. However, selective identification is a critical issue when working in the presence of other molecules that generate similar voltammograms, such as adenosine and histamine. We have addressed this problem by fabricating a robust, H2O2-selective electrode. 1,3-Phenylenediamine (mPD) was electrodeposited on a carbon-fiber microelectrode to create a size-exclusion membrane, rendering the electrode sensitive to H2O2 fluctuations and pH shifts but not to other commonly studied neurochemicals. The electrodes are described and characterized herein. The data demonstrate that this technology can be used to ensure the selective detection of H2O2, enabling confident characterization of the role this molecule plays in normal physiological function as well as in the progression of PD and other neuropathies involving oxidative stress.

Ceftriaxone reduces L-dopa-induced dyskinesia severity in 6-hydroxydopamine parkinson's disease model

BACKGROUND

Increased extracellular glutamate may contribute to l-dopa induced dyskinesia, a debilitating side effect faced by Parkinson's disease patients 5 to 10 years after l-dopa treatment. Therapeutic strategies targeting postsynaptic glutamate receptors to mitigate dyskinesia may have limited success because of significant side effects. Increasing glutamate uptake may be another approach to attenuate excess glutamatergic neurotransmission to mitigate dyskinesia severity or prolong the time prior to onset. Initiation of a ceftriaxone regimen at the time of nigrostriatal lesion can attenuate tyrosine hydroxylase loss in conjunction with increased glutamate uptake and glutamate transporter GLT-1 expression in a rat 6-hydroxydopamine model. In this article, we examined if a ceftriaxone regimen initiated 1 week after nigrostriatal lesion, but prior to l-dopa, could reduce l-dopa-induced dyskinesia in an established dyskinesia model.

METHODS

Ceftriaxone (200 mg/kg, intraperitoneal, once daily, 7 consecutive days) was initiated 7 days post-6-hydroxydopamine lesion (days 7-13) and continued every other week (days 21-27, 35-39) until the end of the study (day 39 postlesion, 20 days of l-dopa).

RESULTS

Ceftriaxone significantly reduced abnormal involuntary movements at 5 time points examined during chronic l-dopa treatment. Partial recovery of motor impairment from nigrostriatal lesion by l-dopa was unaffected by ceftriaxone. The ceftriaxone-treated l-dopa group had significantly increased striatal GLT-1 expression and glutamate uptake. Striatal tyrosine hydroxylase loss in this group was not significantly different when compared with the l-dopa alone group.

CONCLUSIONS

Initiation of ceftriaxone after nigrostriatal lesion, but prior to and during l-dopa, may reduce dyskinesia severity without affecting l-dopa efficacy or the reduction of striatal tyrosine hydroxylase loss. © 2017 International Parkinson and Movement Disorder Society.

Enzyme Deposition by Polydimethylsiloxane Stamping for Biosensor Fabrication

High-performance biosensors were fabricated by efficiently transferring enzyme onto Pt electrode surfaces using a polydimethylsiloxane (PDMS) stamp. Polypyrrole and Nafion were coated first on the electrode surface to act as permselective films for exclusion of both anionic and cationic electrooxidizable interfering compounds. A chitosan film then was electrochemically deposited to serve as an adhesive layer for enzyme immobilization. Glucose oxidase (GOx) was selected as a model enzyme for construction of a glucose biosensor, and a mixture of GOx and bovine serum albumin was stamped onto the chitosan-coated surface and subsequently crosslinked using glutaraldehyde vapor. For the optimized fabrication process, the biosensor exhibited excellent performance characteristics including a linear range up to 2 mM with sensitivity of 29.4 ± 1.3 μA mM^-1 cm^-2 and detection limit of 4.3 ± 1.7 μM (S/N = 3) as well as a rapid response time of ~2 s. In comparison to those previously described, this glucose biosensor exhibits an excellent combination of high sensitivity, low detection limit, rapid response time, and good selectivity. Thus, these results support the use of PDMS stamping as an effective enzyme deposition method for electroenzymatic biosensor fabrication, which may prove especially useful for the deposition of enzyme at selected sites on microelectrode array microprobes of the kind used for neuroscience research in vivo.

Multianalyte Physiological Microanalytical Devices

Advances in scientific instrumentation have allowed experimentalists to evaluate well-known systems in new ways and to gain insight into previously unexplored or poorly understood phenomena. Within the growing field of multianalyte physiometry (MAP), microphysiometers are being developed that are capable of electrochemically measuring changes in the concentration of various metabolites in real time. By simultaneously quantifying multiple analytes, these devices have begun to unravel the complex pathways that govern biological responses to ischemia and oxidative stress while contributing to basic scientific discoveries in bioenergetics and neurology. Patients and clinicians have also benefited from the highly translational nature of MAP, and the continued expansion of the repertoire of analytes that can be measured with multianalyte microphysiometers will undoubtedly play a role in the automation and personalization of medicine. This is perhaps most evident with the recent advent of fully integrated noninvasive sensor arrays that can continuously monitor changes in analytes linked to specific disease states and deliver a therapeutic agent as required without the need for patient action.

Optogenetic excitation of cholinergic inputs to hippocampus primes future contextual fear associations

Learning about context is essential for appropriate behavioral strategies, but important contingencies may not arise during initial learning. A variant of contextual fear conditioning, context pre-exposure facilitation, allows us to directly test the relationship between novelty-induced acetylcholine release and later contextual associability. We demonstrate that optogenetically-enhanced acetylcholine during initial contextual exploration leads to stronger fear after subsequent pairing with shock, suggesting that novelty-induced acetylcholine release primes future contextual associations.

Versatile Flexible Graphene Multielectrode Arrays

Graphene is a promising material possessing features relevant to bioelectronics applications. Graphene microelectrodes (GMEAs), which are fabricated in a dense array on a flexible polyimide substrate, were investigated in this work for their performance via electrical impedance spectroscopy. Biocompatibility and suitability of the GMEAs for extracellular recordings were tested by measuring electrical activities from acute heart tissue and cardiac muscle cells. The recordings show encouraging signal-to-noise ratios of 65 ± 15 for heart tissue recordings and 20 ± 10 for HL-1 cells. Considering the low noise and excellent robustness of the devices, the sensor arrays are suitable for diverse and biologically relevant applications.

New Methods for the Site-Selective Placement of Peptides on a Microelectrode Array: Probing VEGF-v107 Binding as Proof of Concept

Cu(I)-catalyzed "click" reactions cannot be performed on a borate ester derived polymer coating on a microelectrode array because the Cu(II) precursor for the catalyst triggers background reactions between both acetylene and azide groups with the polymer surface. Fortunately, the Cu(II)-background reaction can itself be used to site-selectively add the acetylene and azide nucleophiles to the surface of the array. In this way, molecules previously functionalized for use in "click" reactions can be added directly to the array. In a similar fashion, activated esters can be added site-selectively to a borate ester coated array. The new chemistry can be used to explore new biological interactions on the arrays. Specifically, the binding of a v107 derived peptide with both human and murine VEGF was probed using a functionalized microelectrode array.

Microfabricated, amperometric, enzyme-based biosensors for in vivo applications

Miniaturized electrochemical in vivo biosensors allow the measurement of fast extracellular dynamics of neurotransmitter and energy metabolism directly in the tissue. Enzyme-based amperometric biosensing is characterized by high specificity and precision as well as high spatial and temporal resolution. Aside from glucose monitoring, many systems have been introduced mainly for application in the central nervous system in animal models. We compare the microsensor principle with other methods applied in biomedical research to show advantages and drawbacks. Electrochemical sensor systems are easily miniaturized and fabricated by microtechnology processes. We review different microfabrication approaches for in vivo sensor platforms, ranging from simple modified wires and fibres to fully microfabricated systems on silicon, ceramic or polymer substrates. The various immobilization methods for the enzyme such as chemical cross-linking and entrapment in polymer membranes are discussed. The resulting sensor performance is compared in detail. We also examine different concepts to reject interfering substances by additional membranes, aspects of instrumentation and biocompatibility. Practical considerations are elaborated, and conclusions for future developments are presented. Graphical Abstract ᅟ.

Exercise-Mediated Increase in Nigral Tyrosine Hydroxylase Is Accompanied by Increased Nigral GFR-α1 and EAAC1 Expression in Aging Rats

Exercise may alleviate locomotor impairment in Parkinson's disease (PD) or aging. Identifying molecular responses immediately engaged by exercise in the nigrostriatal pathway and allied tissue may reveal critical targets associated with its long-term benefits. In aging, there is loss of tyrosine hydroxylase (TH) and the glial cell line-derived neurotrophic factor (GDNF) receptor, GFR-α1, in the substantia nigra (SN). Exercise can increase GDNF expression, but its effect on GFR-α1 expression is unknown. Infusion of GDNF into striatum or GFR-α1 in SN, respectively, can increase locomotor activity and TH function in SN but not striatum in aged rats. GDNF may also increase glutamate transporter expression, which attenuates TH loss in PD models. We utilized a footshock-free treadmill exercise regimen to determine the immediate impact of short-term exercise on GFR-α1 expression, dopamine regulation, glutamate transporter expression, and glutamate uptake in 18 month old male Brown-Norway/Fischer 344 F1 hybrid rats. GFR-α1 and TH expression significantly increased in SN but not striatum. This exercise regimen did not affect glutamate uptake or glutamate transporter expression in striatum. However, EAAC1 expression increased in SN. These results indicate that nigral GFR-α1 and EAAC1 expression increased in conjunction with increased nigral TH expression following short-term exercise.

The neural substrates for the rewarding and dopamine-releasing effects of medial forebrain bundle stimulation have partially discrepant frequency responses

Midbrain dopamine neurons have long been implicated in the rewarding effect produced by electrical brain stimulation of the medial forebrain bundle (MFB). These neurons are excited trans-synaptically, but their precise role in intracranial self-stimulation (ICSS) has yet to be determined. This study assessed the hypothesis that midbrain dopamine neurons are in series with the directly stimulated substrate for self-stimulation of the MFB and either perform spatio-temporal integration of synaptic input from directly activated MFB fibers or relay the results of such integration to efferent stages of the reward circuitry. Psychometric current-frequency trade-off functions were derived from ICSS performance, and chemometric trade-off functions were derived from stimulation-induced dopamine transients in the nucleus accumbens (NAc) shell, measured by means of fast-scan cyclic voltammetry. Whereas the psychometric functions decline monotonically over a broad range of pulse frequencies and level off only at high frequencies, the chemometric functions obtained with the same rats and electrodes are either U-shaped or level off at lower pulse frequencies. This discrepancy was observed when the dopamine transients were recorded in either anesthetized or awake subjects. The lack of correspondence between the psychometric and chemometric functions is inconsistent with the hypothesis that dopamine neurons projecting to the NAc shell constitute an entire series stage of the neural circuit subserving self-stimulation of the MFB.